DEVELOPMENT OF MICROPOROSITY IN CARBONS FOR CARBON DIOXIDE ADSORPTION. A dissertation submitted. to Kent State University in partial

Transcription

1 DEVELOPMENT OF MICROPOROSITY IN CARBONS FOR CARBON DIOXIDE ADSORPTION A dissertation submitted to Kent State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy by Jowita E Marszewska May 2017 Copyright All rights reserved Except for previously published materials

2 Dissertation written by Jowita E Marszewska M.Sc., Military University of Technology, Poland, 2010 Ph.D., Kent State University, USA, 2017 Approved by, Chair, Doctoral Dissertation Committee Dr. Mietek Jaroniec, Members, Doctoral Dissertation Committee Dr. Anatoly Khitrin Dr. Songping Huang Dr. Brett Ellman Dr. John Portman Accepted by, Chair, Department of Chemistry Dr. Michael Tubergen, Dean, College of Arts and Sciences Dr. James L. Blank

3 TABLE OF CONTENTS LIST OF FIGURES... VI LIST OF TABLES... X ACKNOWLEDGEMENTS... XI CHAPTER 1 INTRODUCTION Activated carbons Methods used for synthesis of porous carbons Hard-templating synthesis of carbons Soft-templating synthesis of carbons Stӧber-like synthesis of carbon spheres Activation processes Influence of porosity on CO 2 adsorption Microporous carbons used as CO 2 sorbents Zeolite templated carbons Carbide-derived carbons Activated carbons Research objectives and summary CHAPTER 2 CHARACTERIZATION TECHNIQUES AND MATERIALS USED FOR SYNTHESIS OF CARBONS Characterization techniques Nitrogen adsorption Carbon dioxide adsorption iii

4 2.1.3 Thermogravimetric analysis Scanning electron microscopy Reagents and materials CHAPTER 3 EFFECT OF ACTIVATING AGENTS ON THE DEVELOPMENT OF MICROPOROSITY IN POLYMERIC-BASED CARBONS FOR CO 2 ADSORPTION Synthesis Results and discussion Surface area and porosity of the activated carbons studied CO 2 adsorption properties Morphology Conclusions CHAPTER 4 POTASSIUM SALT-ASSISTED SYNTHESIS OF HIGHLY MICROPOROUS CARBON SPHERES FOR CO 2 ADSORPTION Synthesis Results and Discussion Morphology Nitrogen adsorption studies CO 2 adsorption studies Conclusions CHAPTER 5 TAILORING POROSITY IN CARBON SPHERES FOR FAST CARBON DIOXIDE ADSORPTION iv

5 5.1 Synthesis Results and discussion Carbon spheres obtained in the presence of colloidal silica Carbon spheres obtained in the presence of TEOS Conclusions CHAPTER 6 CONCLUSIONS REFERENCES v

6 LIST OF FIGURES Figure 1. CO 2 uptake at 25 C as a function of the BET specific surface area. The values adapted from references Figure 2. CO 2 uptake at 25 C as a function of the ultramicropore volume. The values adapted from references. 42,46,46,48,59, Figure 3. CO 2 uptake at 0 C as a function of the volume of pores with sizes < 1 nm. The values adapted from references. 39,48,69, Figure 4. Classification of adsorption isotherms Figure 5. Classification of hysteresis loops Figure 6. Schematic illustration of the preparation of microporous carbons Figure 7. Nitrogen adsorption-desorption isotherms measured at -196 C (left) and the incremental pore size distributions calculated using DFT method for carbonaceous slit-like pores (right) for the carbon samples studied Figure 8. CO 2 adsorption isotherms measured for all carbon materials at 0 and 25 C.. 50 Figure 9. CO 2 uptakes at 0 C and 1 bar as functions of the specific surface area (A) and the volume of pores smaller than 1 nm (B) Figure 10. Isosteric heat of CO 2 adsorption for carbon materials calculated from CO 2 adsorption isotherms measured at 0 and 25 C Figure 11. Image of non-activated Ambersorb 563 beads on a Petri dish (a), SEM image of a non-activated Ambersorb 563 sphere (b), KOH-activated sphere (d), and low magnification of the KOH activated spheres (c). Scale bars are 500 µm vi

7 Figure 12. Schematic illustration of synthesis of microporous carbon spheres Figure 13. SEM images of the carbon spheres prepared without salt addition (A), with 3:1 (B), 5:1 (C) and 7:1 (D) potassium-carbon weight ratios Figure 14. Nitrogen adsorption-desorption isotherms measured at -196 C (left) and the differential pore size distributions calculated using DFT method (right) for all carbons studied Figure 15. CO 2 adsorption isotherms measured for all carbon materials at 0 C (left) and CO 2 uptake at 0 C as a function of the volume of pores below 1 nm (right) Figure 16. CO 2 adsorption isotherms for C-K-5 sample at 0, 25, 50 and 120 C (left); the dashed line indicates partial pressure in flue gas. CO 2 uptakes at 0.15 atm and 1 atm for C-K-5 material as functions of temperature (right); ordinate axis is in a logarithmic scale Figure 17. Isosteric heat of CO 2 adsorption for C-K-5 sample calculated from CO 2 adsorption isotherms measured at 0, 25, 50, and 120 C Figure 18. Schematic illustration of the synthesis of micro-mesoporous carbon spheres using colloidal silica as the silica source Figure 19. Schematic illustration of the synthesis of microporous carbon spheres using tetraethyl orthosilicate as the silica source Figure 20. SEM images of as-synthesized (A), etched (B), activated (C), and activated and etched (D) carbon spheres prepared in the presence of colloidal silica. All scale bars are 1 µm vii

8 Figure 21. TG profiles in air for C-Si-2.5, C-Si*-2.5 (left panel), C-Si-3, and C-Si*-3 (right panel) samples Figure 22. Nitrogen adsorption-desorption isotherms measured at -196 C and the corresponding pore size distributions (inset) for the carbon materials and the carbonsilica composites obtained using modified Stӧber synthesis in the presence of colloidal silica for the C-Si-2.5 series Figure 23. Nitrogen adsorption-desorption isotherms measured at -196 C and the corresponding pore size distributions (inset) for the carbon materials and the carbonsilica composites obtained using modified Stӧber synthesis in the presence of colloidal silica for the C-Si-3 series Figure 24. KJS mesopore size distributions for the C-Si-2.5-A* and C-Si-3-A* carbon samples Figure 25. CO 2 adsorption isotherms measured for the C-Si-2.5-A* and C-Si-3-A* samples at 0 C and 23 C Figure 26. Dubinin-Radushkievich plots for CO 2 adsorption measured on the C-Si-2.5- A* (left panel) and C-Si-3-A* (right panel) carbons at 0 C Figure 27. Change of the normalized pressure as a function of time during CO 2 adsorption (left panel) and change of the time until 90% pressure drop as a function of CO 2 dose pressure (right panel) Figure 28. SEM images of as-synthesized (A), etched (B), activated (C), and activated and etched (D) carbon spheres prepared in the presence of TEOS. All scale bars are 1 µm viii

9 Figure 29. TG profiles in air for the C-T-2.5, C-T*-2.5 (left panel), C-T-3, and C-T*-3 (right panel) samples Figure 30. Nitrogen adsorption-desorption isotherms measured at -196 C and the corresponding pore size distributions (inset) for the carbon materials and the carbonsilica composites obtained by using modified Stӧber synthesis in the presence of TEOS for the C-T-2.5 series Figure 31. Nitrogen adsorption-desorption isotherms measured at -196 C and the corresponding pore size distributions (inset) for the carbon materials and the carbonsilica composites obtained by using modified Stӧber synthesis in the presence of TEOS for the C-T-3 series Figure 32. CO 2 adsorption isotherms measured at 0 and 23 C for the C-T-2.5-A* and C- T-3-A* samples ix

10 LIST OF TABLES Table 1. Examples of carbon sorbents for CO Table 2. Reagents and materials used in the synthesis of carbons Table 3. Structural parameters of the carbons subjected to different activating agents Table 4. Structural parameters and CO 2 uptakes for all carbon materials Table 5. CO 2 uptakes at 0, 25, 50 and 120 C for C-K-5 carbon Table 6. Labeling scheme for all prepared samples Table 7. Structural parameters of the materials studied Table 8. Comparison of the mesopore structure parameters calculated by DFT and KJS methods Table 9. Comparison of CO 2 uptakes on various porous sorbents Table 10. Structural parameters of carbon and carbon-silica composites obtained by using modified Stӧber synthesis x

11 ACKNOWLEDGEMENTS I would like to thank my advisor Professor Mietek Jaroniec for his support, guidance, help, and positivity throughout these past 5 years. Thank you, Professor for being an understanding but challenging boss, for mentoring me, supporting me, and giving me confidence when I needed it. Thank you for teaching me how to be a researcher, always taking time for discussions, and sharing your knowledge and expertise. I would like to thank you for your kindness and understanding, your personal approach, and for many laughs we shared. I thank my committee Dr. Anatoly Khitrin, Dr. Songping Huang, Dr. Brett Ellmann, and Dr. John Portman for their time, guidance, and assistance with this dissertation. I acknowledge and thank my collaborators, especially Professor Jerzy Choma for his assistance and encouragement to apply for the Ph.D. program at Kent State University, and introducing me to nanoporous materials. I am especially thankful to my loving husband, Michał, for your constant support and encouragement to be the best I can. For believing in me, motivating me, challenging me, cheering me up, and teaching me. For all the ideas and endless discussions we had, for all the corrections, and smart comments you made. Thank you for your patience and all the happy and tough moments. We survived grad school and I could not have done it without you! xi

12 Within the department, I would like to thank Erin Michael-McLaughlin for assisting with all the necessary paperwork as well as answering all the questions and always being positive and happy to help, Kristen Camputaro for all the support and help while teaching as well as dealing with problems in the lab, Catherine Slapnicker for her assistance and support, Larry Maurer for helping with lab issues and glassware. I thank current and former lab members: Nilantha Wickramaratne, Chamila Gunathilake, Alexandre Amormino dos Santos Gonçalves, Liping Zhang, Yuanyuan Liu, Pramila Poudyel Ghimire, Arosha Dassanayake for their help and support. I would like to express my deepest gratitude to my friends Sangeetha Selvam and Jibin Abraham Punnoose for always being there for me, supporting me, understanding me, helping me get through the difficult times, offering me advice, being there for movie nights, and loving strawberry pie, Sriramakrishna Yarabarla for always being positive, supporting, and offering rides when I needed them, Alexandre Amormino dos Santos Gonçalves, Maria Costa, Luiz K. C. de Souza for support, help, all laughs, and Brazilian-Polish nights, Paulina Wasita, Agnieszka Bolibok, Adam Cieślik, Joanna Górka, Mirosław Salamończyk for supporting me and giving me strength to accomplish it all. I would like to acknowledge Jadwiga Jaroniec for her advice, support, and help with adjusting to life in the USA. I am especially grateful to my parents, who always believe in me, want the best for me, and encourage me to reach for the stars. Thank you for always reminding me about the importance of family, love, and education. Thank you to my mother, Barbara Ludwinowicz, for guiding me, trusting me with my choices, and showing me you need to xii

13 love what you do in order to be happy. Thank you to my father, Jarosław Ludwinowicz, for giving me advice, teaching me how to think logically, how to be an independent and strong woman, and always being proud of me. Thank you to my sister, Patrycja Ludwinowicz, for always supporting me, understanding me without words, and keeping my spirits up. Thank you to my late great-grandfather Paweł Wrycz-Rekowski for teaching me so many things, for your wisdom and love. Thank you to Marszewski family: my in-laws, Michalina, Maciej, Klaudia, and Zosia for your love and support. Jowita May 2017, Kent, Ohio xiii

14 CHAPTER 1 INTRODUCTION * 1.1 Activated carbons Nanostructured carbons are by far one of the most studied physical sorbents. Their applications span from the removal of contaminants from gases and liquids, water treatment, catalysis, energy storage, to gas capture, storage, and separation. 1,2 Mesoporous carbons, carbon aerogels, carbons blacks, and activated carbons (ACs) belong to this class of materials. The latter are highly porous, mainly microporous, carbon materials with high specific surface areas up to 3000 m 2 g -1. Micropores (pores < 2 nm), due to their small size, possess an extremely well-developed surface that contribute to a large extent to the total surface area of the AC materials. Microporous carbons exhibit several advantages as compared with other sorbents: 1) easy preparation and control over the pore structure, 2) high chemical and thermal stability, 3) low preparation cost, and 4) efficient regeneration. Preparation of ACs is straight-forward, * Reprinted partially from Carbon, vol. 94, Jowita Ludwinowicz and Mietek Jaroniec, Effect of activating agents on the development of microporosity in polymeric-based carbons for CO 2 adsorption, , Copyright 2015, with permission from Elsevier and from Carbon, vol. 82, Jowita Ludwinowicz and Mietek Jaroniec, Potassium salt-assisted synthesis of highly microporous carbon spheres for CO 2 adsorption, , Copyright 2015, with permission from Elsevier. 1

15 inexpensive, already applied on an industbrial scale, and carbon precursors are widely available - biomass, coal, polymers are only a few examples. Usually, production of ACs involves two steps: 1) thermal treatment of carbon precursor in an inert gas atmosphere (usually nitrogen) and 2) activation of the char. Pores are formed in carbon s framework as carbonization proceeds. At higher temperatures, the structure of the materials undergoes consolidation and densification, which causes stress and strain, leading to cracks, voids, and leaving empty spaces in the framework - pores. Post-synthesis activation of carbons is performed to enlarge the volume of micropores. The process involves thermal treatment of carbon with an activator. At higher temperatures, series of reactions occur that leads to the development of extra micropores and/or small mesopores in a carbon matrix. Drawbacks of using ACs as sorbents include slow diffusion of gases and liquids to active adsorption sites. 1.2 Methods used for synthesis of porous carbons Carbon materials are produced and used commercially in industry, medicine, agriculture, households and so on. Depending on their targeted application, structural properties of carbon materials can be tuned simply by using different synthetic approaches proposed for development of those materials. The main advancement in generating well-defined nanoporous carbons has been achieved via hard-templating and soft-templating methods. 2

16 1.2.1 Hard-templating synthesis of carbons The self-assembly synthesis of ordered porous silica 3 opened new possibilities for the design and development of materials with well-defined structural parameters. Ryoo et al. 4 proposed to use silica with highly ordered pores as templates to obtain carbons. Generally, this approach, referred to as hard-templating, involves the use of templates with well-characterized structures, impregnation of those templates with carbonizable precursors, and thermal treatment during which precursors are transformed to carbon frameworks. The next step involves the removal of templates after which porous carbons are obtained in the form of inverse replicas of the templates used. 5,6 Because porosity of carbons is defined by the structural properties of templates, adsorption properties of the former can be easily predicted. Nowadays, a wide variety of templates such as metalorganic frameworks (MOFs) 7, zeolites 8, clays 9, aluminum oxide 10, are available apart from silica. Replication of those templates affords carbons with finely tuned porosity, including the size of pores (mesopores and/or micropores) and their connectivity, high specific surface areas, and large pore volume. Many factors determine the success of a replication process. Apart from selecting the proper template from a wide pool of candidates, choosing the right precursor, carbonization parameters, and etching conditions seem crucial to generating reliably replicated carbons. The feature that a proper precursor should possess in order to be a candidate for nanocasting process is an ability to carbonize, preferably with high yield; furfuryl alcohol, sucrose, glucose, acetonitrile, and ethylene are among the most studied. 5,8,11 Method of precursor delivery to pores is important as well. So far, impregnation, chemical vapor deposition or a 3

17 combination of those rendered materials with good structural properties. 12 Too high carbonization temperature can affect porosity, especially microporosity, leading to the structural shrinkage and closing of micropores and small mesopores. 13 The last step, template removal, can be achieved by selection of an efficient etching agent (usually NaOH and HF are used for silica removal). Hard-templating is a multi-step, timeconsuming process that requires preparation of a sacrificial template and generates waste in a form of etching agents (such as NaOH or HF); however, it affords carbon structures that often could not be synthesized using other methods. 6 The colloidal imprinting is another route to synthesize mesoporous carbons. It belongs to the templating techniques because it requires the use of a hard-template in the form of solid particles. Their subsequent etching renders materials with spherical mesopores. Specifically, this approach involves imprinting of silica colloids in a carbon precursor, followed by carbonization and dissolution of silica. 14 The uniformity and replication of a true size and shape of mesopores can be achieved by ensuring that a carbon precursor (pitch or carbonizable polymer) completely covers the surface of silica colloids and the proper carbonization parameters are selected. Colloidal silica templating can offer advantages over other methods because it delivers carbons with spherical pores and uniform mesoporosity. Moreover, relatively inexpensive silica colloids with various diameters are available for purchase. Overall, hard-templating affords carbons with high surface areas, high volume of mesopores and micropores (especially after activation processes), suitable for adsorption of various bulk molecules including dyes, 15 proteins, 16,17 as well as gases. 18 4

18 1.2.2 Soft-templating synthesis of carbons Soft-templating utilizes self-assembly of triblock copolymers (soft templates) and carbon precursors (usually, resorcinol and formaldehyde) in an acidic or basic solutions. 19 The resulting organic-organic composites are transformed into carbons during thermal treatment, at the same time a template removal occurs due to thermal decomposition. Specifically, a synthesis involves the organic-organic self-assembly: block copolymers, referred to as structure directing agents, form ordered mesostructure under selected conditions. The synthesis in an acidic medium is more feasible because multiple steps of pre-polymerization, including ph and temperature adjustments, are necessary to achieve a mesostructure in a basic solution. 20 Carbon precursors are incorporated into already formed hydrophilic mesodomains and polymerize. The resulting mesostructured polymeric composites are subjected to thermal treatment in an inert gas; two processes (carbonization and template removal) occur simultaneously yielding a final product - ordered mesoporous carbon. The soft-templating approach is more feasible, inexpensive, and straightforward as compared to hard-templating; as the synthesis is carried in a one-pot fashion, there is no need for a sacrificial template and etching process. Importantly, the self-assembly system can be tuned to tailor the porous structure of the resulting material; the size, shape, and connectivity of pores can be controlled this way. Usually, however, softtemplating affords only mesoporous materials (pore widths 2-50 nm) with smaller pore sizes up to a few nanometers. 5

19 1.2.3 Stӧber-like synthesis of carbon spheres Carbon spheres (CS) are widely researched due to the fact that by controlling their size as well as adsorption properties, it is possible to use these materials in a wide spectrum of applications ranging from gas capture/storage, environmental remediation, sensing to energy storage and conversion. 21 Very versatile strategy for production of monodisperse carbon spheres via extended Stöber method has been reported in 2011 by Liu et al. 22 The original Stöber method, reported in 1968, 23 was proposed for the synthesis of uniform silica spheres by controlled hydrolysis of tetraethyl orthosilicate (TEOS) in ammonia-ethanol-water solution. Recently, this approach was extended to the preparation of carbon spheres with diameters of ca. 500 nm. Synthesis of polymeric spheres was carried under the same synthetic conditions as siliceous spheres, using ammonia-ethanol-water phase. Polymerization of resorcinol (1,3-dihydroxy benzene) and formaldehyde (methanal) occurred similarly to condensation of silicon alkoxides. 24,25 Namely, the reaction proceeds according to the mechanism reported by Pekala. 26 First, resorcinol anion forms, then, the addition of formaldehyde occurs and hydroxymethyl derivatives ( CH 2 OH) are formed. These species condense to methylene ( CH 2 ) and methylene ether ( CH 2 OCH 2 ) bridged compounds which further disproportionate forming methylene bridges and formaldehyde. All these steps lead to the formation of a crosslinked polymer. Curing at higher temperatures can be used to enhance crosslinks formation. The obtained polymer is transformed into carbon by thermal treatment in an inert atmosphere. The advantage of this method is that it delivers monodisperse carbon spheres with substantial microporosity that can be further improved. The size of the 6

20 spheres can also be tuned via adjusting synthesis parameters such as temperature, the composition of solvents, and ammonia concentration Activation processes Post-synthesis activation is performed to enlarge the volume of micropores in carbon materials. The process involves thermal treatment of carbon in either an oxidizing gas (CO 2, H 2 O) or with a solid chemical (KOH, CaCl 2, and ZnCl 2 ). 1,27 The treatment temperature usually varies between 450 and 1000 C and depends on the selected activating agent. At higher temperatures, series of reactions occur that lead to the development of extra micropores in a carbon matrix. When carbon is treated with the stream of activating gas, for instance, CO 2, CO is formed according to the reaction: 28 C + CO 2 2CO (1) As can be seen, CO 2 reacts with carbon atoms causing their removal from carbon atomic layers and as a result, extra pores are formed. Similar process is occurring for other activating agents such as H 2 O. 29 Generally, carbon gasification is performed at temperatures above 700 C and for the time longer than 2 hours. Unfortunately, this implies higher energy consumption and significant losses in final carbon yield. On the other hand, activation in oxidative gases is a simple process which provides good control over porosity. CO 2 is often chosen as an activating agent to develop micropores with sizes < 1 nm, however it is much more effective in improving the volume of pores with sizes from 1 to 2.5 nm. 30 Another activating agent, NH 3, can also develop micropores in the range nm, but the volume of these pores is lower as compared with CO This strategy can be beneficial because apart from micropore development, it might lead to 7

21 doping of nitrogen atoms into carbon framework. The activation process can also be conducted in the presence of steam generating carbons with larger micropores and small mesopores. All discussed activators work by widening of pores (due to reactions such as reaction 1); thus the development of porous structure depends greatly on the structural properties of the pre-activated material. Gasification can be instrumental when dealing with carbon composites or other fragile carbon matrixes, where the use of other chemical activators can destroy either structure or morphology of carbons selected for the activation process. Following reactions occur between carbon and KOH activating agent: 32,33 2KOH K 2 O + H 2 O (2) C + H 2 O CO + H 2 (3) K 2 O + CO 2 K 2 CO 3 (4) K 2 CO 3 K 2 O + CO 2 (5) K 2 O + H 2 2K + H 2 O (6) K 2 O + C 2K + CO (7) As can be seen, steam is generated in reaction 2 and 6, and because it can further react with carbon (reaction 3), it will lead to the removal of carbon in the form of CO, causing the formation of extra pores. Apart from gaseous products, metallic potassium is obtained in reaction 6 and 7. Potassium is inserted between carbon atomic layers, essentially increasing the distance between them and after acidic removal of potassium, this space becomes vacant - a pore is formed. Porosity can be tailored by using this particular method under varying conditions including: 1) temperature, 2) time, 3) amount 8

22 of an activator, 4) impregnation process. Advantages of using chemical activation include: use of lower temperatures and therefore, lower energy cost and higher yield of the product. Additional preparations such as mixing, impregnation of carbon with an activating agent, and washing to remove metal constitute drawbacks of this process. KOH is the best activating agent for the creation of small micropores (below 1 nm). 34,35 Upon longer activation time and/or higher activator ratio these narrow pores could be widened to larger micropores and then to small mesopores. Zinc chloride is another chemical that can be used as an activating agent. Its activation mechanism varies from KOH; ZnCl 2 promotes dehydration reaction by the elimination of water molecule from hydroxyl-rich lignocellulosic precursor molecules which carbonize upon thermal treatment. This activator is effective for development of microporosity in biomass-derived carbons. 36 Apart from the aforementioned activation agents, new activation routes are constantly explored. For instance, Ma et al. prepared carbons via carbonization of polysaccharides in the presence of two acids - HNO 3 and H 3 PO 4 mixed in different compositions. 37 This activation route was successful and obtained materials exhibited high surface area and high volume of micropores. 1.4 Influence of porosity on CO 2 adsorption Characterization of micropores with very narrow sizes can be problematic due to kinetic restrictions of nitrogen gas during adsorption at cryogenic temperatures. On the other hand, CO 2 can be used as an adsorptive at 0 and 25 C. These relatively high temperatures cause faster diffusion of CO 2 molecules to pores that are not easily accessed by N 2 at -196 C resulting in much faster measurement time. Relative pressure of 0.03 for 9

23 CO 2 restricts the sizes of pores that are filled depending on the temperature of measurement. 38 High-surface area activated carbons are commercially available; however, their CO 2 adsorption capacities at 1 bar and 0 or 25 C are low because the micropore structure of these materials is not optimized. Numerous studies showed that broad pore size distribution limits CO 2 and therefore, new carbons with microporosity tailored specifically for CO 2 were developed (see examples in Table 1). Table 1. Examples of carbon sorbents for CO 2. a Carbon precursor Activating agent S BET (m 2 g -1 ) V mi (cm 3 g -1 ) CO 2 adsorption at 25 C and 1 bar (mmol g -1 ) CO 2 adsorption at 0 C and 1 bar (mmol g - 1 ) Ref Porous aromatic framework Poly(vinylidene chloride) KOH CO Lignin waste KOH Saran KOH Phenolic resin ZnCl a Notation: S BET specific surface area calculated using BET equation, V mi volume of pores below 2 nm. 10

24 In order to achieve high CO 2 adsorption capacity a carbon material should possess appropriate structural parameters: high specific surface area, a large volume of pores with the proper size, and favorable surface chemistry. The higher the surface area of the carbon, the higher the adsorption capacity is. High surface areas can be achieved in the materials through the introduction of porosity. As mentioned before, specific surface area of the materials depends greatly on the microporosity present in the structure. This means that while generating a high volume of small micropores in a designed material, specific surface area is simultaneously developed; these two values depend on each other. To investigate how the specific surface area is one of the parameters determining CO 2 uptake at atmospheric pressure and room temperature the CO 2 uptake was plotted as a function of the specific surface area achieved for selected carbons designed for CO 2 adsorption. 11

25 Figure 1. CO 2 uptake at 25 C as a function of the BET specific surface area. The values adapted from references The large data set presented in Figure 1 is for activated carbons designed specifically for CO 2 adsorption. The graph represents the CO 2 uptake values collected at 1 bar and 25 C as a function of the BET specific surface area. Clearly, points are scattered, for instance, the lowest and highest values of CO 2 uptake were obtained for carbon materials exhibiting similar BET specific surface areas. The small value of the correlation coefficient for all points (R² = 0.035) suggests that the specific surface area does not determine the CO 2 uptake at 1 bar and 25 C. Similar results were obtained for CO 2 uptakes at 1 bar and 0 C. For selected single studies 61,66,71 the CO 2 uptakes show 12

26 some correlation with the specific surface areas, however, these fits are usually worse than the ones between CO 2 adsorption and volume of narrow micropores. For a larger data set, the correlation is poor as can be seen in Figure 1. The reasoning behind such results can lay in the use of different pressure ranges for calculation of the BET specific surface area of the carbons studied. Specifically, pressure range where equation exhibits linear behavior varies between materials studied and because of this, the comparison is difficult. Similarly, the comparison between pore sizes from different studies can constitute a problem. Pore size distributions of carbons in the range of small micropores are calculated using density functional theory methods (discussed in the next chapter). Because various models are available and can be used for determination of pore size distributions in carbon materials, a comparison of such data does not give conclusive results and it is hard to expect a high correlation of fits coming from results from different calculation techniques. Despite these disadvantages, the CO 2 uptake dependence on the pore size presented in Figure 2 and Figure 3 exhibit far better correlations (R 2 =0.5517) and (R 2 =0.7563) as compared with the previously presented data for the specific surface area. 13

27 CO 2 uptake (mmol g -1 ) Volume of ultramicropores (cm 3 g -1 ) Figure 2. CO 2 uptake at 25 C as a function of the ultramicropore volume. The values adapted from references. 42,46,46,48,59,64 14

28 CO 2 uptake (mmol g -1 ) Volume of pores < 1nm (cm 3 g -1 ) Figure 3. CO 2 uptake at 0 C as a function of the volume of pores with sizes < 1 nm. The values adapted from references. 39,48,69,72 The data displayed in Figure 2 and Figure 3 show that the CO 2 uptake on activated carbons at 25 C depends on the volume of ultramicropores (pores with sizes below 0.7 nm) and that pores with sizes < 1 nm govern CO 2 adsorption at 0 C. At room temperature, CO 2 molecules possess higher energy and thus, their adsorption requires the presence of smaller pores; ultramicropores exhibit strong adsorption thanks to their small size that leads to overlap of the adsorption potentials from the opposing pore s walls resulting in a strong retention of CO 2 molecules in these small micropores. When the temperature is decreased to 0 C, the energy of molecules is reduced and so slightly 15

29 larger pores come into play and adsorption of CO 2 molecules happens in pores with sizes up to 1 nm. CO 2 adsorption on carbons under pressures up to 1 bar is primarily dependent on the pore size, especially small micropores in the range up to 1 nm. 55 Such pores influence the isosteric heat of CO 2 adsorption and the selectivity of CO 2 over other gases such as N 2. Overall, change in the size of generated micropores is a primary factor that influences the CO 2 uptake at atmospheric pressure and other aspects, for instance, presence of basic dopants, are much less relevant for CO 2 capture under these conditions. 1.5 Microporous carbons used as CO 2 sorbents Strategies that gained the most attention for CO 2 capture are: absorption in liquids, adsorption on solids, membrane separation, and cryogenic methods. 73 Absorption in amine solutions is currently used industrially; however, the amine regeneration is energy-intensive and future storage and/or disposal of those concentrated solutions may turn to be environmentally harmful in a long run. 74 In contrast, adsorption on solids is relatively inexpensive and more environmentally friendly approach. Various porous adsorbents have been developed specifically for CO 2 capture including activated carbons, zeolites, metal-organic frameworks (MOFs), metal oxides, and amine functionalized and impregnated materials (mostly silicas and carbons). 73,74 Interestingly, most of those materials can be tailored to meet requirements of specific applications, such as flue gas scrubbing in power plants or in the lime kiln during cement production. 75,76 Among the aforementioned solid sorbents, microporous carbons and MOFs have attracted the most attention. Since the first report of MOFs use for CO 2 capture in 1998 by Furukawa et al. 77 these materials excel in adsorption of CO 2 under high pressures; however, the 16

30 application of MOFs is limited due to their relatively high production cost and problems with hydrothermal stability. Thus, practical application of MOFs is limited only to storage rather than capture of CO 2. In contrast, microporous carbons exhibit several advantages such as: 1) high chemical stability (in water, alkaline, and acidic media), 2) high thermal stability, 3) low cost, 4) easy preparation and control of the pore structure, and 5) efficient regeneration. These features make porous carbons attractive adsorbents for CO 2 capture. An important advantage of microporous carbons is their high isosteric heat of adsorption (~30 kj mol -1 ). High-surface area activated carbons are commercially available, however, their CO 2 adsorption capacities at ambient conditions are low because their micropore structure is not optimized, containing large micropores and broad pore size distribution. Development of novel carbons with microporosity optimized specifically for CO 2 capture applications is a challenging task but if successful, would be highly beneficial for fabrication of other carbon materials for applications requiring the presence of small pores Zeolite templated carbons Zeolite-templated carbons are appealing materials for CO 2 adsorption. These materials possess appropriate nanostructure: high specific surface area and most importantly, high volume of pores with the proper size. Zeolite-templated carbons can be obtained via hard-templating synthesis. Zeolites have uniform three-dimensional pore channels and depending on the structure of zeolite, the carbon replica possesses nanopores ranging from micropores to mesopores. Zeolite matrix can be selected from a large pool of available materials, thus one can control textural properties, specifically 17

31 pore sizes, just by choosing the suitable template. Zeolites are great candidates for hard templates as these materials exhibit high microporosity, which is crucial for applications such as capture and storage of gases like hydrogen, methane, and carbon dioxide. Xia et al. prepared series of N-doped zeolite-templated carbons via CVD method and showed their CO 2 uptakes. 78 Authors were able to achieve a material with a very high volume of micropores (1.24 cm 3 g -1 ) and high nitrogen content (4.6 wt. %). The group recorded CO 2 uptake capacity of 6.9 mmol g -1 at 0 C and 4.4 mmol g -1 at 25 C under atmospheric pressure. Carbon showed the recyclability over 2 cycles and good selectivity as compared with N 2. Zhuo used the infiltration combined with CVD method as a synthetic technique to obtain carbons with CO 2 adsorption capacity of 2.1 mmol g -1 at 25 C. 79 Group of Tang showed sulfur-doped microporous carbon sorbent with CO 2 uptake capacity of 2.5 mmol g -1 at 25 C and 1 bar. 80 In both publications authors observed lower microporosity than Xia et al. The above results emphasize the importance of porosity on the CO 2 adsorption in carbons. Feasibility of zeolite nanocasting can be questionable because hard-templating synthesis involves multiple preparation steps, which makes the process time consuming and expensive. Infiltration of the precursor into smallest micropores might also constitute a problem. Even though zeolite-templating yields carbons with well-defined porosity, their CO 2 uptakes are smaller than those of activated carbons Carbide-derived carbons Carbide-derived carbons (CDCs) are promising candidates for CO 2 capture. These materials can be obtained as products in reaction between metal carbides (SiC, TiC, etc.) 18

32 and etching agents (usually Cl 2 but also Br 2, F 2 and I 2 or HF, CCl 4, CHCl 3 ) according to general scheme: 81 M x C y(s) + ( z ) Cl 2 2 = xmcl z (g) + yc (s) (8) where M is a metal. These materials exhibit high porosity, which is developed after extraction of metals from selected metal carbides. The pore size range that can be achieved in carbons depends on the carbide precursor and temperature of extraction. Use of lower temperatures favors preservation of an initial carbide structure; treatment at higher temperatures leads to the pore widening which results from material s structural reorganization. Silvestre-Albero et al. prepared series of TiC-derived carbons and showed that a temperature of the chlorine treatment affects the development of microporosity in the samples. 82 CO 2 uptakes of 3.93 mmol g -1 at 25 C and 3.72 mmol g -1 at 0 C were achieved. These authors explained higher adsorption at higher temperatures by the presence of constrictions in narrow pores which could be more accessible to CO 2 molecules. The studied material showed much better selectivity toward CO 2 than N 2 and CH 4. The SiC-derived carbon obtained by Bhatia et al. possessed mainly micropores with sizes up to 1.5 nm. 83 This material adsorbed ca. 4.8 mmol g -1 of CO 2 at 0 C, interestingly, CO 2 capture decreased upon increasing fluorination intensity. This is due to the decreased pore volume and surface area as the groups on the surface block pore entrances. Even though volumes of micropores in CDCs are high, the adsorption properties of these nanostructures can be further improved by utilizing processes such as post-synthesis activation (discussed in section 1.3). Treatments of CDCs with an activating agent at higher temperatures can double surface areas by increasing volume of 19

33 micropores. Extra micropores are created in an activation process and additionally, small micropores with sizes below 0.5 nm that are already present in the carbon s structure are widened to the size which is needed for efficient CO 2 sorption. This way, significant improvement in the sorbents CO 2 capacity can be achieved, specifically up to ca. 7.0 mmol g -1 of CO 2 can be adsorbed at 0 C and 1 bar. 81 CDCs have well-defined porosity; however these materials contain pores with sizes in the range ca. 0.5 nm, which are more suitable for CO 2 adsorption at 0.1 bar Activated carbons Recently, highly microporous carbons excel as sorbents in physisorption processes. Activated carbons have been proposed as good CO 2 sorbents because of their well-developed porous structure. So far, researchers were putting efforts into establishing features that carbon sorbents should possess to achieve high CO 2 uptakes in physisorption processes. The presence of micropores with desired sizes is of a great importance in an efficient CO 2 capture, thus porosity generation and tuning in carbon materials are main aspects while designing practical sorbents. A vast pool of carbon precursors and activation agents, the simplicity of preparation, low cost of production, and established production technology all favor carbons as sorbents for CO 2 at atmospheric pressure. Even though activation is required to enlarge porosity in carbon materials, this process is straightforward and highly effective in the generation of extra micropores with desired sizes. Both established activation routes and novel methods are reported for the preparation of carbons intended for CO 2 adsorption. Especially potassium compounds such as KOH, 84 K 2 CO 3, 85 and K 2 C 2 O 4, 86 have been used for chemical activation of 20

34 carbons. Martin-Jimeno et al. 87 reported KOH-activated carbon xerogels that adsorbed 4.9 mmol g -1 of CO 2 (0 C, 1 bar). Yang et al. 88 and Dziura et al. 42 performed KOH activation of the carbons prepared from polymers and achieved CO 2 uptakes of 6.5 mmol g -1 (0 C and 1.13 bar) and 6.7 mmol g -1 (0 C, 1.07 bar), respectively. Lee et al. 89 reported even higher CO 2 uptake of 7.3 mmol g -1 (0 C and 1 bar) for KOH-activated hierarchically porous organic materials obtained from polymer nanoparticles. In contrast to KOH, ZnCl 2 is rather used for activation of biomass and organics. For instance, Vargas et al. 90 activated a lignocellulosic precursor with ZnCl 2 achieving CO 2 capacity of 2.7 mmol g -1 (0 C, 1 bar), while Meng and Park 91 activated polypyrrole achieving CO 2 uptake of 3.8 mmol g -1 at (25 C, 1 bar). For example, Choma et al. 71 reported CO 2 - activated carbon from waste CDs and DVDs that achieved 4.3 mmol g -1 of CO 2 (0 C, 1.07 bar). Jin et al. 64 performed CO 2 activation of the carbon prepared from a commercial phenolic resin and carbon nanotubes and achieved CO 2 uptake of 5.6 mmol g -1 (0 C, 1 bar). Steam is also a viable activating agent as discussed before. Sui et al. 92 carried out steam activation of graphene aerogels, which adsorbed 2.5 mmol g -1 of CO 2 (0 C, 1 bar). Tseng et al. 93 performed steam activation of the carbons obtained from melaminemodified phenol-formaldehyde resins and achieved 6.7 mmol g -1 of CO 2 (0 C, 1 atm). Similarly to water, gaseous ammonia (NH 3 ) has been used for carbon activation. Przepiorski et al. 94 and Shafeeyan et al. 95 reported that NH 3 -activated carbons adsorb more of CO 2 than commercial activated carbon. Additionally, activation of already microporous carbons will result is higher micropore volume and thus, higher sorbent capacity. Activation of phenolic resin-based carbons can generate materials attractive for 21

35 adsorption of CO 2 because they possess intrinsic microporosity with a high fraction of small micropores (< 1 nm) that enhance CO 2 uptake at ambient conditions. Gorka and Jaroniec 96 used CO 2 and water vapor post-synthesis activation of these carbons, whereas Souza et al. 59 utilized KOH as an activating agent. Wickramaratne and Jaroniec 69 reported that CO 2 activation of carbon spheres obtained by the extended Stöber method can produce materials with CO 2 capacity as high as 8.05 mmol g -1 at 0 C and 1 atm. As can be seen, activation of carbon is a primary technique of choice for production of highly microporous sorbents for CO 2. Substantial progress has been made within the last couple of years in the field of activated carbons, namely, scientists were able to improve the carbon sorbents capacity from 2 to almost 9 mmol g -1 of CO 2 at 1 bar and 0 C. This shows the research is undoubtedly progressing in a good direction and that effort put toward identification of parameters influencing CO 2 at ambient conditions for carbon materials brought a substantial advancement in the CO 2 capacity. Thus, it is essential to further investigate the influence of structural parameters and new activation methods to achieve even better CO 2 uptakes and practical application of solid sorbents. 1.6 Research objectives and summary Microporous carbons are well-known sorbents and prospective materials for capture and storage of carbon dioxide It is difficult to control the size and volume of micropores (pores < 2 nm), yet they are crucial to achieve good performance in most applications, including CO 2 adsorption. The ideal material for CO 2 adsorption under atmospheric pressure would possess a high volume of small micropores, with sizes < 1 nm, that enhance CO 2 uptake under atmospheric pressure. 35,45 To achieve a high volume 22

36 of micropores in carbon sorbents, the post-synthesis activation can be performed. 27 Such treatment facilitates tailoring of pore sizes via selection of proper activating agent and conditions. The objective of this dissertation research is to design, synthesize, and characterize new highly microporous carbon sorbents for CO 2 adsorption. The aim is to improve activation processes to achieve a larger volume of micropores in carbon materials and better control their sizes. Generation of micropores with desired sizes is still a challenge, therefore the plan is to experimentally improve activation processes to create and control micropore sizes to achieve high CO 2 uptake in carbon sorbents. This study is focused on the development of microporosity in carbon spheres (CS) obtained from polymeric precursors. The proposed research is intended to expand knowledge of 1) CO 2 adsorption on nanostructured highly-microporous carbons, 2) microporosity tuning specifically for CO 2 capture, and 3) activation process improvement towards CO 2 capture. This dissertation outlines pathways for the development of porosity, especially microporosity, in carbon-based sorbents for CO 2 adsorption. The structure of this dissertation consists of the introductory chapter describing activated carbon materials, activation processes, and influence of porosity on CO 2 adsorption. Additionally, an overview of characterization techniques including low-temperature nitrogen adsorption, carbon dioxide adsorption, thermogravimetry (TG), and scanning electron microscopy (SEM) is provided. 23

37 The next chapter presents the effect of different activating agents on the development of microporosity in polymeric-based carbons. 101 Even though numerous publications are devoted to the post-synthesis activation of carbons, these efforts focus on different materials, various activating agents, and activation conditions. The results obtained for carbons derived from different precursors and activated under different conditions are not easily comparable and determination of the most effective route is not reliable. The main goal of this project is to establish the performance of different activation methods toward the creation of small micropores and enhancement of CO 2 adsorption. It is shown that activation of one carbon matrix with five most common activating agents can give insight into the development of porosity, especially microporosity, in the carbon s structure. Moreover, the gathered data allows for identification of the most effective activator for generation of small micropores with sizes < 1 nm. Another chapter is devoted to the development of carbon sorbents with high porosity through potassium organic salt addition. 48 KOH is an activating agent that is the most effective in the generation of high surface area and a large volume of small micropores in carbon materials. The main disadvantages of this approach are: 1) the postsynthesis treatment is an additional step in the carbon preparation process, 2) it requires using corrosive chemical agents, 3) it involves either physical mixing of the carbon with solid KOH or its impregnation with KOH solution. Instead of post-synthesis activation, it is proposed to carry out the synthesis of carbon spheres in the presence of potassium organic salt. It is shown that the synthesis of carbons in the presence of potassium organic 24

38 salt represents a novel route for material activation - in situ activation, leading to the generation of an additional fraction of small pores in the carbon framework. The proposed method reduces the number of synthetic steps and eliminates the need for use of aggressive chemicals. The strategy afforded sorbents with high surface area and high volume of pores tailored for CO 2 uptake. Next chapter of the dissertation is devoted to the synthesis of micro- and micromesoporous carbon spheres using the extended Stӧber method in combination with silica templating and controlled post-synthesis activation with CO The extended Stӧber method reported by Liu et al. 38 has been selected because it affords microporous carbon spheres with a substantial amount of ultramicropores. It was shown that a combination of silica incorporation and CO 2 activation resulted in the significant improvement of structural parameters. The latter showed that both, microporosity and mesoporosity are important in the development of prospective CO 2 sorbents. Overall, the studies described above resulted in (1) better understanding of activation process, (2) new activation technique, (3) deeper understanding of CO 2 adsorption on microporous carbons, and (4) improved CO 2 sorbents. The proposed research contributed to the area of nanostructured carbon sorbents but the concepts applied in the development of sorbents for CO 2 can be useful for preparation of sorbents intended for capture of gaseous pollutants, gas storage, or industrial filtration and separation processes. 25

39 This dissertation is based on the following publications: 1) Ludwinowicz, J.; Jaroniec, M. Effect of activating agents on the development of microporosity in polymeric-based carbons for CO 2 adsorption. Carbon 2015, 94, ) Ludwinowicz, J.; Jaroniec, M. Potassium salt-assisted synthesis of highly microporous carbon spheres for CO 2 adsorption. Carbon 2015, 82, ) Marszewska, J.; Jaroniec, M. Tailoring porosity in carbon spheres for fast carbon dioxide adsorption. J. Colloid Interface Sci. 2017, 487, Other articles published: 4) Marszewski, M.; Marszewska, J.; Pylypenko, S.; Jaroniec, M. Synthesis of porous crystalline doped titania photocatalysts using modified precursor strategy. Chem. Mater. 2016, 28, ) Costa, M. J. F.; Marszewska, J.; Gonçalves, A. A. S.; de Souza, L. K. C.; Araujo, A. S.; Jaroniec, M. Microwave-assisted single-surfactant templating synthesis of mesoporous zeolites. RSC Adv., 2016, 6, ) Choma, J.; Jedynak, K.; Fahrenholz, W.; Ludwinowicz, J.; Jaroniec, M. Microporosity development in phenolic resin-based mesoporous carbons for enhancing CO 2 adsorption at ambient conditions. Appl. Surf. Sci. 2014, 289, ) Choma, J.; Fahrenholz, W.; Jamiola, D.; Ludwinowicz, J.; Jaroniec, M. Development of mesoporosity in carbon spheres obtained by Stӧber method. Microporous Mesoporous Mater. 2014, 185,

40 8) Fierro, C. M.; Gorka, J.; Zazo, J. A.; Rodriguez, J. J.; Ludwinowicz, J.; Jaroniec, M. Colloidal templating synthesis and adsorption characteristics of microporousmesoporous carbons from kraft lignin. Carbon 2013, 62, ) Choma, J.; Jedynak, K.; Fahrenholz, W.; Ludwinowicz, J.; Jaroniec, M. Development of microporosity in mesoporous carbons. Ochrona Srodowiska. 2013, 35, ) Choma, J.; Jamiola, D.; Ludwinowicz, J.; Jaroniec, M. Deposition of silver nanoparticles on silica spheres and rods. Colloids Surf., A 2012, 411,

41 CHAPTER 2 CHARACTERIZATION TECHNIQUES AND MATERIALS USED FOR SYNTHESIS OF CARBONS Nanostructured carbons obtained in this research were evaluated using numerous characterization methods. Structural parameters including: specific surface area, total pore volume, pore size and pore size distribution were obtained using low-temperature nitrogen adsorption-desorption measurements. Scanning electron microscopy was used to characterize the morphology of obtained materials. Thermogravimetric analysis was used to assess thermal stability, evaluate the composition of obtained composites, and ensure proper washing of materials after activation processes. 2.1 Characterization techniques Nitrogen adsorption Gas adsorption introduction Low-temperature nitrogen adsorption is the main method for characterization of porosity and structural properties of carbon materials obtained in this dissertation. Development of porosity with desired sizes in carbon materials is one of the goals of this research. Generally, pores with sizes in nano-range (up to 100 nm) are classified by IUPAC into three categories: micropores (pores < 2 nm), mesopores (pores 2-50 nm), 28

42 micropores (pores > 50 nm). Micropores can be further differentiated into ultramicropores (pores < 0.7 nm) and supermicropores (pores nm). 38 Adsorption is a process where molecules, atoms or ions from a gas, liquid, or dissolved solid adhere to a surface. For gas adsorption, gas, referred to as the adsorptive, is adsorbed on the surface of a solid, and referred to as adsorbent. We measure the amount of gas adsorbed and plot it against equilibrium relative pressure (p/p o ) which is a ratio of pressure at the experiment conditions to p o which is the saturation pressure of the pure adsorptive at the experiment temperature. The results are presented in the form of adsorption isotherm. The classification of isotherms is presented in Figure 4. 29

43 Figure 4. Classification of adsorption isotherms. Reprinted with permission from Kruk, M.; Jaroniec, M. Gas Adsorption Characterization of Ordered Organic Inorganic Nanocomposite Materials. Chem. Mater. 2001, 13 (10), Copyright 2001 American Chemical Society. For details related to the IUPAC classification of adsorption isotherms see also references 38 and 111. Type I isotherm is characteristic for microporous solids. This isotherm is very steep in the range of very low pressures which indicates gas uptake in micropores so called micropore filling. When the isotherm flattens, an amount adsorbed is reaching the limiting value, which is governed by the amount of micropores present. Strong adsorption 30

44 in micropores can be explained by a small size of these pores that leads to overlap of the adoption potentials from the opposing pore s walls resulting in strong retention of gas molecules in micropores. According to the updated classification of isotherms (see ref. 38) Type I isotherm can be differentiated into Type I(a) characteristic for materials with narrow micropores and Type I(b) for materials with broader micropore distribution ranging even to small mesopores. In this case, the isotherm does not flatten at a relative pressure ~0.1 but rather its knee is extended. Type II isotherm is characteristic for nonporous or macroporous solids. Adsorption occurs via monolayer formation followed by multilayer formation as the relative pressure increases. Usually the point at which monolayer completion occurs could be easily identified the place where a knee ends and an almost linear portion follows. Type III isotherm is characteristic for nonporous or macroporous solids as well. Adsorption isotherm is similar to Type II; however, the end of monolayer formation is non-distinguishable. Interactions between adsorbent surface and adsorbate are much weaker than between adsorbate molecules themselves; hence, the convex isotherm shape is in the whole pressure range. Type IV isotherm is characteristic for mesoporous solids. Adsorption occurs by monolayer, multilayer formation, followed by instant capillary condensation of adsorptive in mesopores to a liquid-like phase at a pressure lower than saturation pressure of the bulk liquid. Hysteresis is a typical feature of Type IV isotherms and it appears because of capillary condensation. According to the updated classification of isotherms (see ref. 38) Type IV isotherm can be differentiated into Type IV(a) (Type IV in Figure 4) characteristic for materials with pore sizes larger than 4 nm and Type IV(b) (Type IVc in Figure 4) for materials with small mesopores 31

45 with widths below 4 nm. Type V isotherm combines features of Type III and Type IV isotherms: convex shape at the low pressures range due to weak adsorbate-adsorbent interactions and a hysteresis due to capillary condensation. Type VI isotherm, added in the updated classification of isotherms (see ref. 38), exhibits step-by-step adsorption on a non-porous adsorbents with the energetically homogeneous surface. Adsorption-desorption hysteresis loops, depending on their shape, are classified by IUPAC into six categories. Type H1 hysteresis possesses parallel adsorption and desorption branches which is indicative of the presence of mesopores with uniform sizes. Such isotherms are usually obtained for materials with cylindrical pore geometry. Type H2(a) (Type H2 in Figure 5) hysteresis loop resembles a triangle in shape. The desorption branch is very steep and appears at about 0.4 relative pressure range. Type H2(b) hysteresis loop (see ref 38) features softer desorption branch as compared with H2(a). Such isotherms are usually obtained for materials with pore constrictions or cagelike structures. Type H3 and H4 exhibit the common feature: relatively narrow hysteresis stretches over a wide range of relative pressures. Type H3 hysteresis loop is characteristic for materials with pores consisting of aggregated plate-like particles and for materials with only partially filled macropores. H4 loops are characteristic for micro-mesoporous carbons, mesoporous zeolites, and materials with aggregated zeolite crystallites. H5 hysteresis loop, added in the updated classification of isotherms (see ref. 38), is characteristic for materials which contain mesopores with and without constrictions. 32

46 Figure 5. Classification of hysteresis loops. Reprinted with permission from Kruk, M.; Jaroniec, M. Gas Adsorption Characterization of Ordered Organic Inorganic Nanocomposite Materials. Chem. Mater. 2001, 13 (10), Copyright 2001 American Chemical Society. For details related to the IUPAC classification of hysteresis loops see also references 38 and 111. Nitrogen adsorption-desorption isotherms were measured at -196 C on ASAP 2010 and 2020 gas adsorption analyzers manufactured by Micromeritics (Norcross, GA, USA). All the samples were degassed under vacuum at 200 ºC for 2 h prior to the measurements. In a typical nitrogen adsorption measurement, roughly 0.1 g of carbon was placed in a tube of known mass and volume. The tube with a sample was then kept under vacuum at 200 ºC for 2 h to remove all of the adsorbed species from material s surface. After the cool down process was complete, the mass of the tube was measured and the mass of the sample was calculated. Then, the volume of the sample was 33

47 calculated using the density of carbon material (2.1 cm 3 g -1 ). To accommodate the change in volume between room temperature and temperature of the measurement (-196 C), the conversion factor of was used. The volume of the tube at measurement conditions was calculated by subtracting volume occupied by a sample from the volume of an empty tube at -196 C. For adsorption measurement, the tube with the sample was fixed under the vacuum and immersed in a Dewar filled with liquid nitrogen. The measurement proceeded by dosing a known amount of nitrogen gas into the tube of known volume that contained the carbon sorbent, waiting until equilibrium pressure was established, and recording the amount of nitrogen adsorbed at this specific pressure. The adsorption measurement was conducted until the relative pressure reached and was subsequently followed by desorption of adsorbed nitrogen molecules until the relative pressure of 0.3. The collected points were used to plot an adsorption-desorption isotherm. Specific surface area and total pore volume Specific surface area (S BET ) of a material can be evaluated by fitting the measured adsorption data in a relative pressure range of to linear form of Brunnauer- Emmett-Teller (BET) equation: 102 p p0 n (1 p p0 ) = C 1 n m C ( p p 0 ) + 1 n m C (1) where, n is the amount adsorbed at pressure p/p o, C is a BET constant, and n m is the monolayer capacity. The latter is extracted from the equation but before it can be used for 34

48 calculation of S BET the unit needs to be transformed from cm 3 STP g -1 to mol g -1. The following equation can then be used to calculate S BET of desired material: S BET = n m N 0 ω (2) where, N 0 is the Avogadro number (6.023x10 23 molecules mol -1 ) and ω is a nitrogen cross-sectional area (0.162x10-18 m 2 molecule -1 ). Total pore volume The total pore volume (V t ) describes the volume of all pores accessible to nitrogen at -196 C added volumes of micropores and mesopores. Total pore volume can be evaluated by converting the amount of nitrogen adsorbed at a relative pressure of 0.99 to the volume of liquid nitrogen at experiment conditions. 103 The assumption is that under these conditions all the pores are filled with an adsorbate and its density is equal to the density of bulk liquid adsorbate. The conversion from the amount of nitrogen adsorbed (in cm 3 STP g -1 ) to the volume of liquid nitrogen (in cm 3 g -1 ) is by using a conversion factor of which is for N 2 at -196 C. Micropore size distribution Pore size distribution (PSD) is a plot of the volume of pores as a function of their widths. PSDs in the range of micropores can be calculated from adsorption branches of nitrogen adsorption-desorption isotherms using the DFT (density functional theory) method for carbons with pores of slit-like geometry. This is a computational method, which is used to generate multiple theoretical isotherms for a system with the strictly defined type of adsorbent, adsorbate, and pore geometry. These isotherms can be 35

49 regarded as local isotherms for a given type of adsorbent and adsorbate, and these are calculated for pores of different widths and specified pore geometry. To obtain the pore size distribution for a system studied the following integral equation of adsorption is inverted by using regularization method: a t (p) = a(p, w)j(w)dw (3) where, a t is experimentally measured adsorption isotherm, J(w) is pore size distribution, and a(p,w) is a kernel of the theoretical adsorption isotherms for pores of the width w. In this approach, experimental adsorption isotherm is correlated with a local adsorption isotherm, which is generated by DFT calculations and Monte Carlo simulations for pores of the width w. Thus, the DFT pore size analysis should be used only when the system studied is well-matched with the selected DFT kernel selected. Otherwise, the generated pore size distribution may be incorrect resulting in false PSDs and pore volumes. The biggest drawback of the classical DFT method used is that it produces artifacts in the calculated PSDs in the form of minima observed at the 1 nm pore width. The continuity of the peak in this range should be assumed even though the peak dip is usually clearly visible on PSD graphs. 104 For all the carbons, PSDs were calculated using DFT method available in software provided by Micromeritics. The volume of micropores, pores below 1 nm, and 0.7 nm were obtained from the cumulative volume vs pore size graph calculated using the DFT method. 36

50 Mesopore size analysis The pore size distribution functions in the range of mesopores were calculated from the adsorption branch of isotherms using the Kruk-Jaroniec-Sayari (KJS) method 105 based on the Barrett-Joyner-Halenda (BJH) calculation assuming cylindrical pore geometry. 106 To calculate PSD using BJH method the statistical film thickness as a function of pressure and a relation between the pore width corrected for the film thickness and the condensation pressure of adsorbate the following values have to be known a priori. The Kelvin-type equation can then be used in the form: w = 2 γ V L RT ln( p 0 p ) + t ( p p o ) + c (4) where, w is a pore width, γ is a surface tension equal to 8.88 x 10-3 N m -1 for liquid nitrogen at -196 C, V L is the molar volume of adsorbate equal to cm 3 mol -1 for liquid nitrogen at -196 C, R is the gas constant equal to J mol -1 K -1, T is the absolute temperature, t(p/p o ) is the statistical film thickness, and c is a constant equal to 0.3 nm for nitrogen at -196 C. Even though the KJS method was developed for silica materials with cylindrical pores, it works well for carbons by simply using the statistical film thickness curve (t-curve) for a reference carbon surface 107 (obtained by fitting the low-temperature nitrogen adsorption isotherm measured on a reference carbon to the multilayer part of the t-curve for ordered mesoporous silica MCM ) and the experimental Kelvin-type relation presented above, PSDs for carbons with cylindrical mesopores (sizes up to 12 nm) can be successfully calculated. 37

51 The volume of mesopores in the desired range can be evaluated by integration of the area under the PSD curve. Pore widths can be obtained from maxima of the pore size distributions peaks Carbon dioxide adsorption Carbon dioxide adsorption isotherms were measured at 0 and 25 C on ASAP 2020 volumetric adsorption analyzer manufactured by Micromeritics (Norcross, GA, USA). All the samples were outgassed at 200 C for 2 hours prior to measurements. The procedure for CO 2 measurements is similar to the one described in a previous section for a typical nitrogen adsorption measurement with a few changes. CO 2 was used as an adsorbate and there was no need to use a factor for volume change as the measurements were taken at or close to room temperature. The tube with the sample was fixed under the vacuum and immersed in a Dewar filled with water (of 25 C) or water/ice mix (0 C) for measurement. Noteworthy, relatively high temperatures (0 or 25 C) and pressures (p/p o ~ 0.03) for CO 2 adsorption cause faster diffusion of CO 2 molecules to pores that are not easily accessed by N 2 at -196 C resulting in a much faster time of measurement. Isosteric heat of adsorption Isosteric heat of adsorption is related to the strength of adsorbent-adsorbate interactions that play an important role in the uptake of CO 2 on the surface of carbons. The isosteric enthalpy of adsorption, which is the recommended IUPAC term, is equal 38

52 with an opposite sign to the heat of adsorption q st which can be calculated by using the Clausius Clapeyron equation: q st = RT 2 ( lnp/ T) a (5) where R is the universal gas constant, T is the absolute temperature, p is the equilibrium pressure, a is the amount adsorbed. 108 The isosteric heat of adsorption values in the range of CO 2 uptakes can be calculated from at least two adsorption isotherms measured at different temperatures. The collected data can be fitted to the equation (5) provided that the amount adsorbed remains constant for the systems studied. Unfortunately, because the method relies on the measured equilibrium pressures, any experimental errors will result in high uncertainty of the calculated values. Therefore, calculations based on only two isotherms will introduce a high uncertainty of the calculated values as compared with the systems using 3 or more isotherms Thermogravimetric analysis Thermogravimetric measurements (TG) were made using TGA Q500 thermogravimetric analyzer manufactured by TA Instruments (New Castle, DE, USA). Data were recorded from 30 to 700 C under gas flow with a heating rate of 5 C min -1. In a typical experiment, ~10 mg of sample was loaded into the tared platinum pan. This pan was placed on a wire hanger, which is connected to the microbalance at the top of the instrument balance. Then, the furnace tube was lifted to keep the pan inside the furnace. A heating rate was selected and the measurement began under the flow (60 cm 3 min -1 ) of 39

53 a selected gas. The results were presented in the form of a graph where % weight is plotted against temperature Scanning electron microscopy Scanning electron microscopy (SEM) images were taken on a Hitachi S-2600N scanning electron microscope using 25 kv accelerating voltage. In a typical experiment, the sample was applied on an adhesive carbon tape, then, a sputter deposition system was used to coat the sample with gold in order to electrically connect the sample holder with the sample. This is done to prevent image distortion caused by unwanted charging of a sample by the electron beam. Then, the sample holder was placed in the SEM sample chamber and the chamber was evacuated. After achieving the desired vacuum level, the electron beam was introduced. Magnification, brightness, and contrast were adjusted and images were taken and saved. 40

54 2.2 Reagents and materials Table 2. Reagents and materials used in the synthesis of carbons. Chemical Producer Chemical formula Water Purified using IonPure 150 H 2 O Ethanol, 95% Acros Organics C 2 H 5 OH Resorcinol, 98% Acros Organics C 6 H 4 (OH) 2 Formaldehyde, 37 wt% solution in water, stabilized with % methanol Potassium oxalate monohydrate, 99% Ammonium hydroxide, 29 wt% Acros Organics Acros Organics Fischer Scientific HCHO K 2 C 2 O 4 H 2 O NH 4 OH Potassium hydroxide Fisher Scientific KOH Zinc chloride Fisher Scientific ZnCl 2 Tetraethyl orthosilicate (TEOS) Ludox AS-40 colloidal silica, 40 wt% solution in water Fisher Scientific SiC 8 H 20 O 4 Sigma-Aldrich SiO 2 Ambersorb 563 Rohm and Haas Company Dow Chemical Company C 41

55 CHAPTER 3 EFFECT OF ACTIVATING AGENTS ON THE DEVELOPMENT OF MICROPOROSITY IN POLYMERIC-BASED CARBONS FOR CO 2 ADSORPTION * High-surface area activated carbons are commercially available; however, their CO 2 adsorption capacities under atmospheric pressure and room temperature conditions are low because their micropore structure is not optimized and broad pore size distribution limits CO 2 adsorption. The same applies to carbons obtained by using different templates e.g. zeolites, metal carbides or precursors such as polymers, biomass, etc. A lot of effort has been put into designing and optimizing carbon adsorbents that will show high potential for CO 2 capture at industrial scale. Pore optimization for the majority of carbon materials reported so far was achieved by using the post-synthesis activation under carefully controlled conditions. As mentioned in the introduction chapter, there is a vast variety of activation methods available for tuning porosity in carbons. Unfortunately, the reported results are not easily comparable and determination of the best activator is not possible because different carbons, prepared from different precursors and at different conditions, are reported in each case. * Reprinted from Carbon, 94, Jowita Ludwinowicz and Mietek Jaroniec, Effect of activating agents on the development of microporosity in polymeric-based carbons for CO 2 adsorption, , Copyright 2015, with permission from Elsevier 42

56 In this study, we compared the effect of the most common activating agents on the structure development in carbon materials, and indirectly, on the CO 2 adsorption. To facilitate this comparison, a single commercial carbon sorbent was used and activated with: 1) KOH, 2) CO 2, 3) H 2 O, 4) NH 3, and 5) ZnCl 2. Ambersorb 563 was selected because it possesses surface area and pore volume similar to typical not-activated carbons Synthesis Carbon activation was performed according to the previous reports. Temperature and ratio of chemical reagent to carbon were chosen to obtain the highest volume of micropores. 91,96,110 KOH activation was performed by mixing 2 g of potassium hydroxide with 0.5 g of carbon, followed by thermal treatment in an alumina tube furnace at 700 C for 1 h under nitrogen flow (heating rate 10 C min -1 ). The obtained material was washed with 0.01 M HCl solution and deionized water until ph ~7 to remove KOH residue. Finally, the material was dried at 100 C for 12 h. The resulting carbon material was labeled C-KOH. H 2 O, CO 2 and NH 3 activations were performed by heating 0.5 g of carbon to 750 C (H 2 O) or 850 C (CO 2, NH 3 ) under nitrogen flow in a quartz tube furnace using heating rate of 10 C min -1. After reaching desired temperature, the flowing gas was switched to an activating gaseous agent and the sample was kept under these conditions for 4 h. In the case of NH 3 and H 2 O activations gaseous nitrogen (60 cm 3 min -1 ) was purged through a container with ammonia solution or water (both at room temperature) and introduced directly to the quartz tube. After activation, the gas was switched back to 43

57 pure nitrogen. Heating and cooling under nitrogen prevented uncontrolled activation. The resulting carbon materials were labeled C-CO 2, C-NH 3, and C-H 2 O, respectively. ZnCl 2 activation was performed by mixing 2 g of zinc chloride with 0.5 g of carbon, followed by thermal treatment at 500 C for 2 h under nitrogen flow (heating rate 10 C min -1 ). The obtained material was washed with deionized water until ph ~7 to remove ZnCl 2 residue. Finally, the material was dried at 100 C for 12 h. The resulting carbon material was labeled C-ZnCl 2. For the purpose of comparison, the carbon sample without activation was labeled as C. Figure 6. Schematic illustration of the preparation of microporous carbons. 3.2 Results and discussion Surface area and porosity of the activated carbons studied Nitrogen adsorption-desorption isotherms and the corresponding PSD plots for all carbon samples are shown in Figure 7. All isotherms are of Type IV according to the IUPAC classification. 111 The samples show the capillary condensation step at relative 44

58 pressure range above 0.9, which suggests the presence of large mesopores, in agreement with the data presented for the non-activated material by Choma and Jaroniec. 109 The mesoporous structure of the carbons studied improves mass transfer of adsorbate molecules, which is beneficial in sorption applications. 112 Mesoporosity would not be discussed in details because microporosity development is the focus of this study; therefore the emphasis would be placed on the analysis of the pores below 2 nm. The structural parameters of the carbons obtained using different activating agents are listed in Table 3. The S BET surface area of the starting material was 570 m 2 g -1 and increased to 2030 m 2 g -1 after KOH activation, the pore volume increased from 0.65 to 1.45 cm 3 g -1, and the micropore volume from 0.19 to 0.72 cm 3 g -1. These data indicate that activating agents developed an additional porosity, especially microporosity, in the carbon studied. The adsorption capacity of the activated carbons increased as it can be seen from the shift of adsorption isotherms in Figure 7. 45

59 Figure 7. Nitrogen adsorption-desorption isotherms measured at -196 C (left) and the incremental pore size distributions calculated using DFT method for carbonaceous slitlike pores (right) for the carbon samples studied. In this study, the most common activating agents have been compared in terms of their activation performance towards structure development. Adsorption analysis showed that the activating agents used enlarged the values of the surface area and pore volume in the carbon studied in the following order: ZnCl 2 H 2 O NH 3 CO 2 KOH. 46

60 Table 3. Structural parameters of the carbons subjected to different activating agents. a Sample S BET (m 2 g -1 ) V t (cm 3 g -1 ) V me (cm 3 g -1 ) V mi (cm 3 g -1 ) V smi (cm 3 g -1 ) V umi (cm 3 g -1 ) CO 2 uptake (mmol g -1 ) C C-KOH C-CO C-NH C-H 2 O C-ZnCl a Notation: V mi volume of pores below 2 nm obtained by DFT method; V smi volume of pores below 1 nm obtained by DFT method; V umi volume of pores below 0.7 nm obtained by DFT method; CO 2 uptake at 0 C and 1 bar. KOH activation generated the highest volume of micropores; it resulted in almost four-fold increase in the specific surface area (from 566 to 2034 m 2 g -1 ). This extra surface came from micropores created in the carbon s structure by an activator. CO 2 activation showed smaller values of the surface area and total pore volume m 2 g -1 and 1.39 cm 3 g -1 ; the significant increase in the mesopore volume should be noted for this sample. Ammonia generated the same amount of ultramicropores as CO 2 ; however, the micropore volume was smaller and hence, the specific surface area of NH 3 -activated carbon was lower m 2 g -1. Water vapor activated the polymer-based carbon studied 47

61 to a smaller extent as compared with ammonia: both mesopore and micropore volumes increased, resulting in higher specific surface area than that of the starting material. Although activation with ZnCl 2 resulted in a slight increase in S BET, no change was observed for all other structural parameters. ZnCl 2 works quite well for the development of porosity in various biomass precursors because it acts as a dehydrating agent, which facilitates degradation of lignocellulosic materials upon carbonization Ambersorb 563, however, is already a durable carbon material consisting of spherical beads with high thermal stability 116 and unlike biomass, is not susceptible to ZnCl 2 activation. Therefore, ZnCl 2 is not very effective activator for already carbonized materials. The purpose of this study was to examine the efficiency of different activating agents toward creation of micropores, especially those with sizes < 1 nm, which are essential for enhancing the CO 2 uptake at ambient conditions. 27 The PSD curves for the carbons studied show that the C-KOH and C-CO 2 carbons feature one significant peak in the range up to 1 nm and the second one in the range between 1 and 2.5 nm. For C-NH 3 the second peak is in the range from 1 to ~2 nm. These peaks reflect an increase in the micropore volume and the effectiveness of activation. Note that the standard DFT model used produces an artificial deep minimum at ~1 nm pore width on the calculated PSDs (Figure 7); therefore, in this range a continuity of the PSD peak can be assumed. 104 KOH generated the highest volume of pores below 1 nm; almost three-fold increase in the volume of small micropores (from 0.16 cm 3 g -1 to 0.42 cm 3 g- 1 ) is observed for C-KOH and almost four-fold increase in the micropore volume (from 0.19 to 0.72 cm 3 g -1 ). Note that the micropores represent about 50% of the total pore volume. This is not surprising; 48

62 the KOH activation has been used for the development of carbons for various applications 117,118 because of its effectiveness for enhancing their microporosity. KOH is one of the best activating agents suitable for the creation of small micropores; so it is likely that the KOH activated carbons would exhibit the high volume of micropores < 1 nm. For the carbon studied CO 2 was the most effective in creating pores with sizes between 1 and 2.5 nm, it exceeded the efficiency of both KOH and NH 3 in this range. CO 2 activation was not as effective as KOH in developing micropores smaller than 1 nm. CO 2 activator should be used for applications requiring micropores larger than 1 nm. 119 NH 3, similarly to CO 2, can be employed as an activating agent to develop micropores with widths in between 1 and 2.5 nm - this result is in agreement with the study of Yang et al. 31 NH 3 has a lower activating power than CO 2 because both PSD peaks in the micropore range are smaller than those observed for C-CO 2. For the carbon activated with H 2 O the volume of ultramicropores is second to that activated with KOH, but the activator s performance in terms of small micropores is worse than that for the KOH and CO 2 activators, and similar to that achieved with NH 3. As mentioned before, ZnCl 2 does not activate the carbon studied and there is no significant change in the meso- and micropore volume as compared with the starting sample. Overall, the activating agents used enlarged microporosity and consequently surface area and pore volume in the carbon in the following order: ZnCl 2 H 2 O NH 3 CO 2 KOH CO 2 adsorption properties CO 2 adsorption capacity was measured at 0 and 25 C on the carbon samples obtained using different activating agents. CO 2 adsorption on the carbons studied is 49

63 shown in Figure 8. In addition, Table 3 lists the CO 2 uptakes measured at 0 C and 1 bar for all carbon materials. Figure 8. CO 2 adsorption isotherms measured for all carbon materials at 0 and 25 C. As expected, the CO 2 uptakes increase with the S BET surface area and pore volume in the following order C-KOH > C-CO 2 > C-NH 3 > C-H 2 O > C-ZnCl 2 > C. The highest CO 2 uptakes, 6.9 and 5.5 mmol g -1 are observed for the materials with the highest values of the structural parameters: KOH- and CO 2 -activated carbons. Considering the ease of preparation, CO 2 activation is more convenient and better from environmental viewpoint to carry out than KOH treatment, thus a compromise needs to be achieved while choosing the ease of preparation and only somewhat smaller CO 2 capacity. On the other hand, KOH activation is performed at lower temperature and thus, is more energy efficient from industrial viewpoint. Activated carbons proposed for CO 2 adsorption should be highly microporous with high volume of small micropores; therefore we 50

64 studied the effect of an activating agent on its effectiveness to create such pores. KOHactivated carbon is greatly microporous - micropores constitute 50 % of all pores in the material - and more than a half (58 %) of these micropores belong to pores with sizes below 1 nm. CO 2 activation generates almost 10% less micropores in the volume than KOH. CO 2, NH 3 and H 2 O activations produced similar volumes of small micropores but CO 2 capacity was almost 1 mmol g -1 higher for the C-CO 2 carbon. This could be attributed to higher pore volume and surface area of CO 2 activated sample. C-NH 3 exhibits higher CO 2 capacity than C-H 2 O; apart from higher pore volume this could be attributed to the fact that during thermal treatment, nitrogen functional groups could have been introduced to carbon framework by substituting oxygen functional groups present on the surface. Even though nitrogen species are released fast at temperatures higher than 700 C, Yang et al. report ca. 3 wt % of nitrogen was still distributed throughout carbon matrix at 900 C. 31 Nitrogen species act as basic sites and attract acidic CO 2 molecules that are adsorbing on the carbon surface. For ZnCl 2 -activated sample there is almost no change in volume of micropores as compared with the non-activated carbon. C-KOH sample showed significantly higher S BET, V t and V smi, which resulted in the best CO 2 uptake at 0 C and 1 bar mmol g -1, and best low-pressure performance mmol g -1 at 0 C and 0.15 bar - because of its highest volume of small micropores. Further CO 2 sorption studies were conducted at 25 C, 1 bar and 0.15 bar, the uptakes were 4.0 and 1.1 mmol g -1 respectively, which is a fairly good result. 120 In our previous work 48 we reported the exponential correlation of the CO 2 uptake with temperature. Basing on this and CO 2 uptake measured at two temperatures (0 and 25 C) for the C- 51

65 KOH sample, we could estimate the CO 2 capacity for any selected conditions, specifically, temperature in the range C and pressure up to 1 bar. The estimated working capacity is 3.6 mmol g -1 (158 mg g -1 ) for the C-KOH-based pressure adsorption system operating in a cycle (compression at 1 bar and 30 C, and decompression at bar and 60 C). 121 Thus, the C-KOH material obtained by activation of a commercial carbon achieved impressive cycle capacity, which fits well in acceptable range of 3-4 mmol g -1 to be considered competitive with currently used absorption in amine solutions. 76 Figure 9. CO 2 uptakes at 0 C and 1 bar as functions of the specific surface area (A) and the volume of pores smaller than 1 nm (B). Figure 9 shows CO 2 uptakes at 0 C and 1 bar as functions of the S BET surface area and the volume of micropores with sizes below 1 nm for all activated carbons. The CO 2 uptake changes linearly with the specific surface area (correlation coefficient R 2 = 52

66 0.9424) and the volume of pores with sizes up to 1 nm (correlation coefficient R 2 = ). Interestingly, the correlation between the CO 2 uptake and the volume of ultramicropores (not shown) was much lower - correlation coefficient R 2 = Although for the carbon studied the CO 2 uptake depends on the BET specific surface area, its dependence on the volume of small micropores (below 1 nm) is better justified in terms of the statistical analysis. Our data support the previous findings that pores with sizes < 1 nm govern CO 2 adsorption on activated carbons at ambient conditions. 53

67 Figure 10. Isosteric heat of CO 2 adsorption for carbon materials calculated from CO 2 adsorption isotherms measured at 0 and 25 C. Figure 10 shows the isosteric heat of CO 2 adsorption for carbon materials calculated from CO 2 adsorption isotherms measured at 0 and 25 C. Because only two isotherms were used in calculations, the uncertainty of the resulting values is high (~ 10%). 48 For CO 2 uptakes in the range of mmol g -1 the calculated values found are in the range of kj mol -1, which is typical for activated carbons. 48,88 The highest values are for the C-NH 3 carbon, which could be due to the potential interaction between basic nitrogen species and acidic CO 2 molecule. Heat of adsorption for KOH activated 54

68 carbon is changing from kj mol -1, which is close to the value reported previously for potassium-salt activated carbon. 48 Overall, the C-KOH material showed an excellent CO 2 capacity and could be practically used as a sorbent for CO 2 capture Morphology Ambersorb 563 adsorbent consists of spherical carbon beads with diameters of ca. 500 µm (Figure 11a and Figure 11b). All studied materials retained spherical morphology upon treatment with different activating agents. This is expected in the case of CO and ZnCl activating agents but interestingly, the morphology was retained for the carbon sample activated under most harsh conditions - in the presence of KOH excess. Figure 11c shows a lower magnification of KOH-activated spheres and Figure 11d shows SEM image of a single KOH-activated sphere. The surface of the sphere is not smooth as a non-activated one, but it is unevenly etched. Physical mixing of a nonactivated carbon with KOH might have led to its uneven distribution during activation process and therefore, the surface of spheres might be exposed to the chemical agent action at different extents. This can be seen at lower magnification of the KOH-activated spheres (Figure 11c) - some spheres are etched more than the others. Additionally, the sphere is cracked; the observed crack can be caused by weakening the structure of carbon during KOH treatment at high temperature, as well as by evolving gases. 124 Despite the imperfections, the spherical morphology of the carbons studied is preserved contrary to the other KOH activated carbons obtained under the same conditions. 35,110 55

69 Figure 11. Image of non-activated Ambersorb 563 beads on a Petri dish (a), SEM image of a non-activated Ambersorb 563 sphere (b), KOH-activated sphere (d), and low magnification of the KOH activated spheres (c). Scale bars are 500 µm. 56

70 Generally, the activation process did not influence the morphology of carbons studied - all samples possess spherical morphology, which is desired in industrial applications Conclusions The study shows the influence of five activating agents: 1) CO 2, 2) H 2 O, 3) NH 3, 4) KOH, and 5) ZnCl 2 on the porosity and CO 2 adsorption of the resulting carbon materials. Low-temperature nitrogen adsorption analysis showed that the activating agents enlarged the surface area and pore volume in the carbon sorbents in the following order: ZnCl 2 H 2 O NH 3 CO 2 KOH. It is shown that KOH activation yielded the highest volume of micropores and small micropores; also, the CO 2 uptake for the KOHactivated sample was the highest. ZnCl 2 was the least effective - it did not activate the carbon studied. It is more suitable for activation of bio-derived carbons because it does not lead to the development of extra porosity in strongly carbonized polymeric beads and should not be used for chemical activation of already carbonized materials. To sum up, activation of the same carbon with different activating agents gave conclusive results about the effectiveness of these activators on the creation of microporosity. Our data support the previous findings that pores with sizes < 1 nm govern CO 2 adsorption on activated carbons at ambient conditions. Based on this information and CO 2 adsorption measurements, we determined that KOH activation appears to be the most effective route to obtain carbon sorbents intended for CO 2 capture. 57

71 CHAPTER 4 POTASSIUM SALT-ASSISTED SYNTHESIS OF HIGHLY MICROPOROUS CARBON SPHERES FOR CO 2 ADSORPTION * Despite the broad research in the area of porous carbons, there is still interest in controlling both microporosity and morphology of these materials mainly due to the fact that desired applications such as gas capture/storage to energy storage/conversion and separation applications require tailored porosity and certain morphology. The goal of this project was exploration of new avenue for developing extra microporosity in polymericbased carbon materials without disturbing spherical morphology. As mentioned before, post-synthesis activation processes are commonly used to enhance the microporosity in phenolic resin-based carbons; Gorka and Jaroniec 96 used CO 2 and water vapor post-synthesis activation of these carbons, whereas Souza et al. 59 utilized KOH as an activating agent. Wickramaratne and Jaroniec 69 reported that CO 2 activation of carbon spheres obtained by extended Stöber method can produce materials with CO 2 capacity as high as 8.05 mmol g -1 at 0 C and 1 atm. Therefore, activated phenolic resin-based carbon spheres are attractive for adsorption of CO 2 because they possess intrinsic microporosity with high fraction of small micropores (< 1 nm). * Reprinted from Carbon, 82, Jowita Ludwinowicz and Mietek Jaroniec, Potassium salt-assisted synthesis of highly microporous carbon spheres for CO 2 adsorption, , Copyright 2015, with permission from Elsevier. 58

72 Especially potassium compounds such as KOH, 84 K 2 CO 3, 85 and K 2 C 2 O 4, 86 have been used for chemical activation of carbons; however, this process involves post-synthesis impregnation of carbons, which can be time consuming and somewhat difficult due to the hydrophobic nature of these materials. Physical mixing can be used instead of impregnation, but it often leads to an uneven distribution of the activating agent resulting in poor activation. Overall, the post-synthesis activation is an additional step in the preparation of carbons. This work shows a simple strategy to enhance microporosity in the carbon spheres prepared by Stöber method, which involves the addition of potassium oxalate to the ethanol-water-ammonia solution of resorcinol and formaldehyde. This one-pot modified Stöber synthesis produces the salt-containing polymer spheres, the carbonization of which in the presence of potassium species is accompanied by their activation resulting in the development of additional microporosity. This works shows the effect of direct addition of potassium organic salt during formation of phenolic resin spheres, which plays the role of an in-situ activating agent and additional carbon precursor. The proposed method affords carbons with spherical morphology, high surface area and high volume of micropores. 4.1 Synthesis Carbon spheres were prepared by a suitably modified recipe reported by Liu et al. 22 namely, 0.20 g of resorcinol was added to the mixture consisting of 20 ml of water, 8 ml of ethanol, and 0.10 ml of ammonium hydroxide under magnetic stirring for 10 minutes at room temperature. Next, 0.75 g, 1.20 g, or 1.65 g of K 2 C 2 O 4 H 2 O were added 59

73 to the synthesis mixture under stirring for 30 min to achieve the potassium-carbon weight ratio equal to 3:1, 5:1, and 7:1, respectively. Afterwards, 0.28 ml of formaldehyde was added and the mixture was stirred for 24 hours and then subjected to the hydrothermal treatment in an autoclave at 100 C for 24 hours. Subsequently, the solution was transferred to a Petri dish and dried at room conditions overnight. The dried materials were carbonized in nitrogen atmosphere at 350 C for 2 hours (1 C min -1 heating rate); then, temperature was ramped to 800 C (1 C min -1 heating rate) and kept at that temperature for 2 hours. The carbonized materials were washed with 0.01 M HCl solution and deionized water until ph ~7 to remove salt residue. Finally, the materials were dried at 100 C for 12 hours. The resulting carbon materials were labeled C-K-r, where r denotes the potassium- carbon weight ratio. For the purpose of comparison, one carbon sample was prepared without salt addition and labeled as C. 60

74 Figure 12. Schematic illustration of synthesis of microporous carbon spheres. 4.2 Results and Discussion Morphology Morphology of the obtained carbons was studied using scanning electron microscopy. Figure 13 presents SEM images of all carbon materials, which possess spherical morphology with sizes between 0.5 and ~1 µm; however, the carbon spheres prepared without salt addition were more uniform and ca. 500 nm in size, whereas the activated spheres had somewhat disturbed shapes and larger diameters. These irregularities and non-uniformity may have resulted from the salt addition, which affected precipitation, polymerization, and/or condensation rates through ph changes (as 61

75 discussed in details in reference 25 ). Most importantly, the spherical morphology, which is important for some applications, was preserved despite the salt addition and the activation process accompanying carbonization of polymeric spheres. Note that postsynthesis KOH activation of polymeric spheres destroyed the original morphology. 126,127 The recipe proposed in this work allows for simultaneous chemical activation with retention of spherical morphology thanks to uniform distribution of potassium species throughout phenolic resin spheres, which was facilitated by organic nature of oxalate moiety. In addition, neither pre- nor post-activation was required in this strategy because the activating agent (potassium salt) was incorporated directly into polymeric matrix allowing simultaneous carbonization and activation during thermal treatment. Yet, this strategy afforded materials with extremely well-developed porous structure (see the next section). 62

76 Figure 13. SEM images of the carbon spheres prepared without salt addition (A), with 3:1 (B), 5:1 (C) and 7:1 (D) potassium-carbon weight ratios Nitrogen adsorption studies Nitrogen adsorption data were used to evaluate the specific surface area and porosity of the carbons studied. Figure 14 shows low-temperature nitrogen adsorption-desorption isotherms and the corresponding pore size distributions for all carbons. All adsorption 63

77 isotherms are of Type I according to the IUPAC classification, 111 which is characteristic for microporous materials. For the sample C-K-7, the isotherm does not flatten at a relative pressure ca. 0.1, indicating the carbon has large micropores and/or small mesopores as well. 128 Table 4 lists the calculated structural parameters for all carbons. The specific surface area ranged from 460 to 2130 m 2 g -1, pore volume ranged from 0.24 to 1.10 cm 3 g -1, and micropore volume from 0.20 to 0.78 cm 3 g -1. Importantly, all these parameters increased with potassium oxalate amount, showing the effectiveness of structure development in carbon materials obtained in the presence of potassium organic salts. In the case of C-K-7 activation resulted in almost five-fold increase of S BET and V t and almost four-fold increase of V mi. The goal of this study was the development of materials featuring high CO 2 uptake. Thus, the development of microporosity, especially small micropores with sizes below 0.7 and 1 nm, was important. The calculated PSDs confirm that all carbons possessed well-developed microporosity. The volume of ultramicropores (pores < 0.7 nm) ranged from 0.15 to 0.36 cm 3 g -1 and increased throughout the series except for the last two samples C-K-5 and C-K-7. The value peaked for C-K-5 carbon and dropped for C-K-7. The volume of fine micropores (V mi<1nm ) ranged from 0.16 to 0.46 cm 3 g -1 and increased throughout the series. For the C-K-7 sample, however, the fraction of fine pores in the total porosity was only 42 %, whereas the C-K-5 sample showed almost twice higher percentage of fine micropores, 75 %. In addition, the C-K-7 sample possessed 64

78 some additional mesoporosity, as well. The presence of mesoporosity is attributed to a high amount of the activating agent that caused the excessive pore widening during activation. As a result, the ultramicropores grew into supermicropores (pores from 0.7 to 2 nm) and supermicropores grew into small mesopores. 27, Overall, all carbons were highly microporous, which projected well on their CO 2 adsorption properties. Figure 14. Nitrogen adsorption-desorption isotherms measured at -196 C (left) and the differential pore size distributions calculated using DFT method (right) for all carbons studied. 65

79 Table 4. Structural parameters and CO 2 uptakes for all carbon materials. a Sample S BET (m 2 g -1 ) V t (cm 3 g -1 ) V mi (cm 3 g -1 ) V mi<1nm (cm 3 g -1 ) V umi (cm 3 g -1 ) Micro- porosity (%) CO 2 uptake (mmol g -1 ) C C-K C-K C-K a Notation: S BET specific surface area; V t single point (total) pore volume at p/p 0 = 0.99; V mi volume of pores below 2 nm obtained by DFT method; V umi volume of pores below 0.7 nm obtained by DFT method; V mi<1nm volume of pores below 1 nm obtained by DFT method; Microporosity percentage of the volume of micropores in the total pore volume, CO 2 uptake at 0 C and 1 atm. All data show that potassium organic salt is an excellent activating agent. Moreover, the degree of activation can be easily controlled by an amount of the salt added. Interestingly, the volume of ultramicropores reached a maximum at the weight ratio = 5:1 under the conditions used. Further salt addition caused a gradual improvement of the structural parameters (S BET, V t, and V mi ) but, unfortunately, did not improve volume of ultramicropores. Thus, for CO 2 adsorption and other applications requiring large volumes of ultramicropores the 5:1 potassium-carbon ratio is optimal; however, 66

80 higher ratios can be used to achieve the larger values of S BET, V t, and V mi, which are suitable for all other applications CO 2 adsorption studies The synthesized samples were investigated in terms of their performance for CO 2 adsorption. Figure 15 shows CO 2 adsorption isotherms and Table 4 lists CO 2 uptakes measured at 0 C for all carbon materials. The CO 2 uptakes ranged from 2.8 to 6.6 mmol g -1 and increased throughout the series. Other activated carbons achieve comparable results 63,64,124,132 but these carbons required post-synthesis activation. The pre-activated carbons usually achieve up to 2 mmol g -1 uptakes of CO 2. 59,124 Importantly, in the proposed recipe neither pre- nor post-activation of the carbons is required, and the resulting samples show CO 2 uptakes comparable to the carbons obtained by KOH postsynthesis activation. 59,84 These results may be attributed to the high volume of fine micropores created by simultaneous carbonization-activation of polymeric spheres with uniformly distributed potassium species. Previous reports show the strong correlation between ultramicropore volume and CO 2 uptake. 35,45,61,63 The same is observed in the current study. Figure 15 shows CO 2 uptake at 1 atm as a function of volume of fine micropores. The straight line represents the linear regression with correlation coefficient R 2 = As a result, the carbons prepared with salt addition adsorbed more CO 2 because of increase in V mi<1nm, which resulted in over two-fold increase in the CO 2 uptake between C-K-7 and C carbon. 67

81 Figure 15. CO 2 adsorption isotherms measured for all carbon materials at 0 C (left) and CO 2 uptake at 0 C as a function of the volume of pores below 1 nm (right). C-K-7 sample shows significantly higher S BET, V t and V mi which resulted in the best CO 2 uptake at 1 atm; however, it is C-K-5 sample that showed the best low-pressure performance because of its highest volume of ultramicropores. Thus, further CO 2 sorption studies under flue-gas-like conditions were conducted using C-K-5 sample. CO 2 partial pressure of atm 133 was assumed as the representative value for CO 2 post-combustion capture from flue gas. Figure 16 shows CO 2 adsorption isotherms at temperatures: 0, 25, 50 and 120 C, and Table 2 shows CO 2 uptakes at these temperatures and 0.15 atm and 1 atm. The uptakes ranged from 2.4 to 0.2 mmol g -1 at 0.15 atm and from 6.3 to 1.1 mmol g -1 at 1 atm. Understandably, the CO 2 uptake for both series decreased with temperature. Figure 16 shows these uptakes as functions of temperature. Both sets correlated exponentially with temperature; correlation coefficient R 2 =

82 for both fits. Clearly, CO 2 adsorption is dependent on temperature; however low-pressure adsorption is more susceptible to temperature rise than adsorption at high-pressures. Figure 16. CO 2 adsorption isotherms for C-K-5 sample at 0, 25, 50 and 120 C (left); the dashed line indicates partial pressure in flue gas. CO 2 uptakes at 0.15 atm and 1 atm for C-K-5 material as functions of temperature (right); ordinate axis is in a logarithmic scale. The C-K-5 sample achieved impressive results. It adsorbed maximum of 1.5 mmol g -1 at 0.15 atm and 25 C, which is twice more than the best value at low pressure reported in a highly cited review by Samanta et al., which is devoted to post-combustion CO 2 capture by solid sorbents. 76 The C-K-5 sample showed cycle capacity of 0.4 mmol g-1 (17.4 mg g -1 ) if cycled through adsorption at 25 C and 0.15 atm and desorption at 120 C and 1 atm. Clearly, high adsorption properties have to be complemented by good desorption properties (i.e. small uptake at 120 C) to yield a good sorbent. In our case, the C-K-5 material achieved impressive cycle capacity on the order of equilibrium 69

83 adsorption reported for many activated carbons. 76 Further reduction of CO 2 uptake at 120 C and 1 atm could yield even better result. We suspect this could be achieved by reduction of volume of micropores > 1 nm, which as shown in the case of C-K-7 related to higher CO 2 uptake at high pressure. Table 5. CO 2 uptakes at 0, 25, 50 and 120 C for C-K-5 carbon. Temperature ( C) CO 2 uptake at 0.15 atm (mmol g -1 ) CO 2 uptake at 1 atm (mmol g -1 )

84 Figure 17. Isosteric heat of CO 2 adsorption for C-K-5 sample calculated from CO 2 adsorption isotherms measured at 0, 25, 50, and 120 C. Figure 17 shows the isosteric heat of CO 2 adsorption for C-K-5 carbon calculated using adsorption isotherms measured at 0, 25, 50, and 120 C. The calculated values were in the range of kj mol -1 with the CO 2 amount adsorbed in the range of mmol g -1. Similar values for activated carbons were reported by other authors. 59,134,135 The isosteric heat of CO 2 adsorption was calculated using four isotherms (left part) and three isotherms (right part). Clearly, the significantly higher error was observed in the 71

85 case of lower number of measurements. For the range of CO 2 uptakes above 3.1 mmol g - 1, only two isotherms were available and error significantly impacted the results. Overall C-K-5 material showed an excellent equilibrium and practically useful CO 2 sorption performance without need for pre- or post-activation, which is often used to enlarge porosity of various carbons Conclusions A simple one-pot modified Stöber synthesis in the presence of potassium oxalate was proposed to create extra microporosity in carbon spheres. Highly microporous carbon spheres were obtained via thermal treatment of the K-containing phenolic resin spheres. The specific surface area of the resulting carbon spheres ranged from 460 to 2130 m 2 g -1, pore volume varied from 0.24 to 1.10 cm 3 g -1, and micropore volume ranged from 0.20 to 0.78 cm 3 g -1. The adjustment of the amount of salt added was shown to be an effective strategy for tuning the volume of ultramicropores and fine micropores (< 1 nm) and consequently, for improving the CO 2 uptake at low pressures. Simply by controlling the amount of salt added one is able to alter porosity, both in the range of micropores and small mesopores. For CO 2 adsorption and other applications requiring large volumes of ultramicropores the 5:1 potassium-carbon weight ratio is optimal; however, higher ratios can be used to achieve desired values of S BET, V t, and V mi for all other applications. Importantly, introduction of potassium oxalate to the synthesis did not affect the spherical morphology of carbons, which is desired in some industrial applications. The obtained materials showed an excellent CO 2 uptakes ranging from

86 to 6.6 mmol g -1 at 0 C and 1 atm. The optimized material showed high low-pressure CO 2 uptakes of mmol g -1 in temperature range from 0 to 120 C, respectively. The proposed recipe for the preparation of polymeric-based carbons is versatile and opens new possibilities for in-situ activation and pore tuning. Because the latter feature is crucial for applications in gas capture and/or storage, the reported strategy might be useful for production of activated carbons suitable for those purposes. 73

87 CHAPTER 5 TAILORING POROSITY IN CARBON SPHERES FOR FAST CARBON DIOXIDE ADSORPTION * Nanoporous carbon materials are used in applications ranging from gas capture and storage, separation, to energy storage and conversion. 48,55,137,138 Capture and storage of carbon dioxide (CO 2 ) is one of the areas where carbon sorbents are the primary candidates. Prospective CO 2 sorbents should have high thermal stability, recyclability, and a well-developed structure with large surface area and large volume of pores < 1 nm. 48,55,137,138 These structural features of carbon materials can be developed and controlled through different methods such as: hard-templating, soft-templating, and postsynthesis activation. 21 Unfortunately, none of these methods provide good control over the size of micropores, which is important from a viewpoint of CO 2 adsorption. For this reason, researchers have tried to optimize conditions of syntheses and activation processes to obtain small micropores with sizes < 1 nm required for efficient CO 2 adsorption at ambient conditions. So far, KOH and CO 2 activations have generated large volumes of small micropores resulting in high CO 2 uptakes. 39,69,139 The best reported carbons can adsorb more than 4 mmol g -1 of CO 2 (25 C, 1 bar) and more than 8 mmol g - 1 (0 C, 1 bar). 37,69,140 Notably, when the material is highly microporous, it may result in * Reprinted from Journal of Colloid and Interface Science, 487, Jowita Marszewska and Mietek Jaroniec, Tailoring porosity in carbon spheres for fast carbon dioxide adsorption, , Copyright 2017, with permission from Elsevier. 74

88 long equilibration times for CO 2 adsorption, which may impact practical application of such a material. One solution to improve gas diffusion within the material is by introduction of larger pores, such as mesopores. 141 Large mesopores enable faster transfer of gas from the bulk phase to micropores, and thus, result in faster equilibration. One way to introduce mesopores in carbon spheres is through using soft templating, 141 but such mesopores usually have sizes only up to few nanometers. Because of this, these pores only slightly improve the diffusion process. Templating with silica colloids can afford much larger pore sizes and gives an option to easily control the pore size by selecting silica colloids with desired diameters. Overall, quick and effective CO 2 adsorption requires both large volume of small micropores and the presence of large mesopores forming a well-interconnected micro-mesoporous carbon structure. In this work, the porous carbon spheres were synthesized to achieve high CO 2 adsorption capacity and fast equilibration. The porosity was tailored by using two methods: 1) CO 2 activation, to develop microporosity for high CO 2 uptake and 2) silica templating, to develop micro- and mesoporosity for fast CO 2 equilibration. The resulting materials had small micropores and large mesopores resulting in superior CO 2 sorption properties. In addition, the study compares the effect of CO 2 activation on the microporous structure before and after silica etching and between materials with incorporated silica colloids and TEOS-generated silica. Finally, we demonstrate the benefits of microporosity and mesoporosity in CO 2 adsorption on carbon spheres. 75

89 5.1 Synthesis Microporous and micro-mesoporous carbon spheres were prepared using the modified Stöber method 22 combined with either colloidal silica templating 142 or TEOS introduction. 143 In each series, samples with 2.5 and 3 weight ratios of silica to carbon were prepared. The resulting silica-carbon composites were either: 1) first activated with CO 2 and then etched with NaOH solution or 2) first etched and then activated. Briefly, resorcinol (0.60 g) and formaldehyde (0.84 ml) were mixed in a waterethanol solution (60 ml of water and 24 ml of ethanol) in the presence of ammonia (0.3 ml) and colloidal silica nanoparticles or TEOS. The mixture was stirred at 30 C for 24 h and then subjected to hydrothermal treatment in an autoclave at 100 C for another 24 h. Then, the solution was transferred to a Petri dish and dried at 100 C for 24 h. The dried materials were transferred to quartz boats and carbonized in a tube furnace under nitrogen atmosphere at 350 C for 2 h (1 C min -1 ramping rate); after temperature reached 600 C (1 C min -1 ramping rate) the materials were kept at that temperature for 4 h. Silica etching was performed by immersing the composites in 3 M NaOH aqueous solution at 60 C overnight; afterwards, the materials were filtered, washed with deionized water, and dried. CO 2 activation was performed by heating the materials to 850 C under nitrogen atmosphere (10 C min -1 ramping rate) in a tube furnace. After temperature reached 850 C, gas was switched to CO 2 for 4 h, and switched back to nitrogen afterwards, for the cooling period. This technique was used to avoid uncontrolled activation with CO 2 during heating and cooling stages. Table 6 lists all prepared samples 76

90 and explains the labeling scheme. Figure 18 and Figure 19 show schematic of the synthesis using colloidal silica and TEOS as the silica source, respectively. Table 6. Labeling scheme for all prepared samples. Sample Silica precursor Silica to carbon ratio Etched Activated Order of etching and activation C-Si-2.5 colloidal silica 2.5 No No C-Si*-2.5 colloidal silica 2.5 Yes No C-Si-2.5-A colloidal silica 2.5 No Yes C-Si-2.5-A* colloidal silica 2.5 Yes Yes Activated first C-Si*-2.5-A colloidal silica 2.5 Yes Yes Etched first C-Si-3 colloidal silica 3 No No C-Si*-3 colloidal silica 3 Yes No C-Si-3-A colloidal silica 3 No Yes C-Si-3-A* colloidal silica 3 Yes Yes Activated first C-Si*-3-A colloidal silica 3 Yes Yes Etched first C-T-2.5 TEOS 2.5 No No C-T*-2.5 TEOS 2.5 Yes No C-T-2.5-A TEOS 2.5 No Yes C-T-2.5-A* TEOS 2.5 Yes Yes Activated first C-T*-2.5-A TEOS 2.5 Yes Yes Etched first C-T-3 TEOS 3 No No C-T*-3 TEOS 3 Yes No 77

91 Sample Silica precursor Silica to carbon ratio Etched Activated Order of etching and activation C-T-3-A TEOS 3 No Yes C-T-3-A* TEOS 3 Yes Yes Activated first C-T*-3-A TEOS 3 Yes Yes Etched first 78

92 Figure 18. Schematic illustration of the synthesis of micro-mesoporous carbon spheres using colloidal silica as the silica source. 79

93 Figure 19. Schematic illustration of the synthesis of microporous carbon spheres using tetraethyl orthosilicate as the silica source. 80

94 5.2 Results and discussion Carbon spheres obtained in the presence of colloidal silica Micro-mesoporous carbon spheres were produced by a one-pot modified Stöber synthesis in combination with colloidal silica templating. Polymerization of formaldehyde and resorcinol in the presence of ammonia and silica colloids resulted in the silica-phenolic resin composite spheres. After carbonization at 600 C, the phenolic resin was transformed into carbon resulting in the silica-carbon composite spheres. The obtained composite spheres underwent a controlled post-synthesis activation with CO 2 using two different strategies (see routes A and B in Figure 18). In route A, the silicacarbon composites were first activated with CO 2 and then subjected to silica etching. In route B, this order was reversed, silica was etched first, and then the materials were subjected to the CO 2 activation. Both routes resulted in carbon spheres with welldeveloped structures. Their morphology, structural parameters, and adsorption properties are discussed in the following paragraphs Morphology Scanning electron microscopy was used to assess the morphology of the obtained carbons. Figure 20 shows SEM pictures of the as-synthesized, etched, activated, and activated and etched carbon spheres prepared in the presence of colloidal silica. All studied carbons showed spherical morphology with the diameters of nm. Importantly, the addition of colloidal silica to the synthesis did not impact the spherical morphology of the final materials. The silica etching did not change the morphology 81

95 either. Although not visible in these pictures, our previous study showed that etching of silica colloids created spherical mesopores within carbon framework. 142 Figure 20. SEM images of as-synthesized (A), etched (B), activated (C), and activated and etched (D) carbon spheres prepared in the presence of colloidal silica. All scale bars are 1 µm. 82

96 Unsurprisingly, the size of the activated spheres is somewhat smaller, which stems from the oxidative environment of CO 2 at higher temperatures. Namely, when CO 2 gas comes in contact with carbon spheres, not only does it diffuse into the pores, reacting with carbon walls and causing their widening, but it reacts with outer layer of the carbon spheres as well, leading to their oxidation into CO and removal. 28 This explains why the diameters of the activated spheres are smaller than before the CO 2 activation. Overall, spherical morphology of the studied carbons was retained despite the silica addition, NaOH etching, and post-synthesis activation. CO 2 activation is therefore an excellent strategy for preparing carbons with well-developed structure and spherical morphology Thermogravimetric analysis Thermogravimetric analysis under air flow in the temperature range from 30 C to 700 C was performed to determine the amount of silica in 1) the silica-carbon composites and 2) the etched carbons. TG profiles for the silica-carbon composites and the carbons after silica etching are presented in Figure 21. Based on these profiles, the content of silica in the composites was 30 wt% and 34 wt%, for C-Si-2.5 and C-Si-3, respectively. These results show that roughly half of the added colloid was incorporated into the final materials based on theoretical values of ca. 70 wt% and 75 wt% calculated assuming a complete incorporation. Moreover, the results show that a higher amount of silica can be incorporated into a carbon framework when a higher amount of colloid is used during the synthesis; 20% increase in the silica-to-carbon ratio (3 vs 2.5) resulted in 13 wt% increase in the silica content (34 wt% vs. 30 wt%). 83

97 Figure 21. TG profiles in air for C-Si-2.5, C-Si*-2.5 (left panel), C-Si-3, and C-Si*-3 (right panel) samples. The TG profiles for the etched carbon spheres proved the silica removal was successful. The residual silica content was established to be 3 wt% for the C-Si*-2.5 and 4 wt% for the C-Si*-3 sample. The remaining silica could not be removed even after extended washing time. Thermogravimetric curves presented in Figure 21 also show that the silica-carbon composites are more thermally stable than the etched carbons. For instance, the C-Si-2.5 silica-carbon composite starts oxidation at 460 C whereas the C-Si*-2.5 etched carbon starts oxidizing at significantly lower temperate of 370 C (almost 100 C difference). For the C-Si-3 composite, the oxidation again starts at 460 C, while oxidation of the C- Si*-3 etched carbon already proceeds at 340 C. It is clear that the presence of silica improves thermal stability of the studied materials by retardation of carbon combustion. 84

98 Adsorption studies Figure 22 and Figure 23 present low-temperature nitrogen adsorption-desorption isotherms and Table 7 lists the calculated adsorption parameters for the two series of materials prepared in the presence of colloidal silica. Notably, these carbon materials exhibited higher specific surface areas and total pore volumes than the similar materials prepared without CO 2 activation. 142 As expected, the two silica-carbon composites (C-Si- 2.5 and C-Si-3) had the lowest values of specific surface area and total pore volume among all materials in these two series. The isotherms for these samples presented in Figure 22 and Figure 23 are of type I according to the IUPAC classification, indicative of microporous materials. 38 Their PSDs, shown as insets in Figure 22 and Figure 23, exhibit peaks below 2 nm, confirming the presence of micropores. Isotherms for the CO 2 - activated silica-carbon composites (C-Si-2.5-A and C-Si-3-A) are also type I according to the IUPAC classification, but with much higher nitrogen uptakes in the low relative pressure region as compared with the data for non-activated composites. This is reflected by visible differences in the structural parameters between the activated and nonactivated composites. 85

99 Table 7. Structural parameters of the materials studied. a Sample name b BET specific surface area (m 2 g -1 ) DFT specific surface area (m 2 g -1 ) Total pore volume (cm 3 g -1 ) Mesopore volume (cm 3 g -1 ) Micropore volume (cm 3 g -1 ) Ultramicropore volume (cm 3 g -1 ) C-Si C-Si* C-Si-2.5-A C-Si-2.5-A* C-Si*-2.5-A C-Si C-Si* C-Si-3-A C-Si-3-A* C-Si*-3-A - a Notation: BET specific surface area was calculated using the Brunnauer-Emmett-Teller (BET) method in the relative pressure range; DFT specific surface area was calculated using the DFT method for carbons with slit-shaped pores; Total pore volume was calculated as a single-point pore volume at relative pressure 0.99; Mesopore volume denotes the volume of mesopores calculated by subtraction of the micropore volume from the total pore volume; Micropore volume denotes volume of pores below 2 nm calculated by the DFT method; Ultramicropore volume denotes volume of pores 86

100 below 0.7 nm calculated by the DFT method; b Sample notation: C denotes carbon; Si denotes colloidal silica; 2.5 and 3 denotes the ratio of silica to carbon; Si* denotes dissolution of SiO 2 ; A denotes CO 2 activation; A* denotes dissolution of SiO 2 after CO 2 activation. Both BET and DFT specific surface area values were calculated for the samples studied. Note the BET equation represents multilayer localized adsorption on energetically homogeneous surfaces and is not fully applicable to adsorption in micropores, which occurs via micropore filling. Therefore, the BET method is not expected to work well for microporous materials. On the other hand, the DFT method is applicable for micro-mesoporous materials. 144 Even though our materials possess small and large micropores as well as small and large mesopores, the calculated BET and DFT specific surface areas do not differ greatly. The DFT specific surface area values increased after activation from 419 m 2 g -1 to 647 m 2 g -1 (C-Si-2.5 vs. C-Si-2.5-A) and from 472 m 2 g -1 to 780 m 2 g -1 (C-Si-3 vs. C-Si-3-A). The peaks below 2 nm are higher for the activated materials than for the non-activated ones, and the corresponding micropore volumes increased from 0.15 cm 3 g -1 to 0.19 cm 3 g -1 (C-Si-2.5 vs. C-Si-2.5-A) and from 0.14 cm 3 g -1 to 0.29 cm 3 g -1 (C-Si-3 vs. C-Si-3-A). 87

101 Figure 22. Nitrogen adsorption-desorption isotherms measured at -196 C and the corresponding pore size distributions (inset) for the carbon materials and the carbon-silica composites obtained using modified Stӧber synthesis in the presence of colloidal silica for the C-Si-2.5 series. 88

102 Figure 23. Nitrogen adsorption-desorption isotherms measured at -196 C and the corresponding pore size distributions (inset) for the carbon materials and the carbon-silica composites obtained using modified Stӧber synthesis in the presence of colloidal silica for the C-Si-3 series. The etched carbon spheres (C-Si*-2.5 and C-Si*-3) have isotherms of type IV with H2(a) hysteresis loop. Such hysteresis, showing a delayed desorption, is typical for mesoporous materials with constricted pores. 38 The corresponding PSDs, shown as insets in Figure 22 and Figure 23, confirm the presence of mesopores, with the corresponding peaks having maxima at ca. 9.4 nm. Interestingly, the PSDs for the silica-carbon composites do not 89

103 have similar peaks, which directly shows that these mesopores were created due to the removal of silica colloids. The pore size (9.4 nm) is underestimated by ca. 20%, considering that the nominal size of the colloidal particles used is 12 nm. The error stems from the fact that the DFT method used for the calculations of these PSDs was developed assuming slit-like micropores rather than spherical mesopores. It is therefore expected to give some difference in the pore size. As an alternative way to calculate PSDs for these materials, we used the KJS method of PSD calculation. Interestingly, even though the KJS method was developed for cylindrical mesopores, it gave much better representation of mesopore sizes than the DFT method, as can be seen in Figure 24. The peaks in KJS PSDs reach maxima at 12.7 nm, which is very close to the expected value for the pores created by 12 nm spherical particles. The mesopore volumes calculated by the KJS method are larger than the mesopore volumes obtained by the DFT method (see Table 8). The difference reaches almost 20% for C-Si-2.5-A* sample and almost 30% for C-Si-3- A*. All these results show that proper pore size analysis for these materials is challenging due to the presence of both slit-like micropores and spherical mesopores. An ideal solution to this problem would be a model that would allow for concurrent analysis of pores in the range of micropores and mesopores with a selection of various pore sizes and geometries. In terms of other structural parameters, we observed that the higher the silica amount is, the better the structural parameters are, for instance, the pore volume increased from 0.39 cm 3 g -1 to 0.49 cm 3 g -1 between the materials C-Si*-2.5 and C-Si*-3, respectively. All these observations indicate that incorporation of colloidal silica followed by its dissolution resulted in the creation of mesopores and development of 90

104 structural parameters such as surface area and pore volume. The presence of mesopores is especially important because it improves mass transfer properties of materials, which is important in practical applications such as CO 2 sorption. Notably, the size and volume of mesopores created in this way can be easily tailored by selecting the size and amount of silica colloids. Figure 24. KJS mesopore size distributions for the C-Si-2.5-A* and C-Si-3-A* carbon samples. 91

105 Table 8. Comparison of the mesopore structure parameters calculated by DFT and KJS methods. a Sample name DFT method KJS method V me (cm 3 g -1 ) w me (nm) V me (cm 3 g -1 ) w me (nm) C-Si-2.5-A* C-Si-3-A* a Notation: V me volume of mesopores evaluated from the PSD calculated with the indicated method; w me mesopore width at the maximum of PSD in the range of mesopores. The CO 2 activation was performed either before or after silica dissolution (see routes A and B in Figure 18). CO 2 was selected as an activating agent because it is effective in the development of microporosity suitable for CO 2 adsorption, and unlike KOH activation, which is an effective activating agent too, CO 2 activation does not deteriorate spherical morphology of the carbon spheres. 72 Moreover, CO 2 does not readily react with silica, whereas KOH would react with the incorporated silica colloids in uncontrolled manner. All activated and etched materials (prepared through routes A and B and with 2.5 and 3 silica-to-carbon ratios) are of type IV with H2(a) hysteresis loop, indicating the presence of constricted mesopores, similar to the etched materials. Data summarized in Table 7 show that the specific surface areas and total pore volumes for both, the carbons (C-Si*-2.5-A and C-Si*-3-A) and the silica-carbon composites (C-Si-2.5-A and C-Si-3-92

106 A), increased after the CO 2 activation. From the porosity viewpoint, the CO 2 activation significantly enlarged the volume of small and large micropores, and slightly enlarged the mesopore volume, what can be seen on PSDs (Figure 22 and Figure 23). It should be noted that the PSD curves show minima at 1 nm, which is an artificial feature of the DFT model used; therefore, a sharp minimum at 1 nm should be disregarded. 104 CO 2 activation generated a large fraction of ultramicropores (which are crucial pores for CO 2 adsorption), effectively doubling their volume, even in the presence of silica. On the other hand, we observed a slight increase in the mesopore volume, which is apparent by comparing the composite and the activated composite samples, or the etched and the etched and activated carbons, in both series. This is not surprising and stems from the fact that CO 2 activation widens all pores, leading to the large micropores (slightly smaller than 2 nm) present in the structure to become small mesopores (slightly larger than 2 nm). Comparing the final carbon materials, the best structural properties were obtained for the carbons produced via CO 2 activation of the silica-carbon composites followed by the silica dissolution (C-Si-2.5-A* and C-Si-3-A*). We suspect that the silica presence stabilized the carbon framework, allowing for better development of porosity during activation. In contrast, when silica was removed first (C-Si*-2.5-A and C-Si*-3-A), the remaining carbon framework became much more fragile because it lacked the silica support and had more hollow structure due to the voids left after the silica. This was especially apparent for the C-Si*-3-A sample, which had the highest silica content and could not withstand the CO 2 activation after the silica removal. In contrast, the C-Si-3-A* carbon (activated then etched) exhibited the best structural properties: DFT specific 93

107 surface area of 1479 m 2 g -1, total pore volume of 1.22 cm 3 g -1, mesopore volume of 0.74 cm 3 g -1, micropore volume of 0.48 cm 3 g -1, and ultramicropore volume of 0.23 cm 3 g -1. Compared with the starting material (C-Si-3), the combination of colloidal silica templating and CO 2 activation resulted in a three-fold increase of the DFT surface area and the total pore volume, and a two-fold increase in the volumes of micropores and ultramicropores. Overall, the CO 2 activation successfully developed additional microporosity while the addition of colloidal silica followed by its removal with NaOH introduced additional mesoporosity in the prepared carbon spheres. The above discussion of the structural parameters, based on the adsorption analysis, provides a good assessment of porosity in the range of micro and mesopores Carbon dioxide adsorption CO 2 adsorption at 0 C and 23 C was measured on the selected carbons with the best adsorption parameters from each series. Figure 25 shows CO 2 adsorption isotherms for the C-Si-2.5-A* and C-Si-3-A* carbons. These materials were first activated and then etched to achieve the best structural parameters (as discussed in the previous section), which translated into excellent CO 2 sorption properties. Both carbons achieved very high CO 2 uptakes with the C-Si-3-A* material scoring 7.8 mmol g -1 at 0 C and 1 bar. This result is impressive in relation to the common CO 2 adsorbents (see Table 9) and close to 8.05 mmol g -1 reported for the CO 2 -activated highly microporous carbon spheres obtained by Stӧber method. 69 Although slightly smaller, the carbon spheres discussed in 94

108 this work possess mesopores, in addition to micropores, which benefits CO 2 adsorption by improving the mass transfer properties (discussed subsequently). Table 9. Comparison of CO 2 uptakes on various porous sorbents. Material CO 2 uptake at 1 bar 0 C (mmol g -1 ) 25 C (mmol g -1 ) Polymer-derived carbon Biomass-derived carbon Commercial carbon Ambersorb MOF Porous polymer ZIF SBA This work (at 23 C) As compared with the C-Si-3-A* sample, the C-Si-2.5-A* carbon exhibited a smaller CO 2 uptake of 4.5 mmol g -1 (0 C, 1 bar). This value is 42% smaller, which is roughly the difference in the volume of the micropores < 1 nm between these materials. Specifically, the C-Si-2.5-A* carbon had 0.22 cm 3 g -1 while the C-Si-3-A* carbon had 0.32 cm 3 g -1 of pores < 1 nm (31% difference). The remaining difference stems from the substantial difference in the specific surface area between these materials, which although less, will affect the CO 2 uptakes too. Interestingly, the CO 2 uptakes at room temperature are the same for both samples, 4.0 mmol g -1 (23 C, 1 bar). This is because at room 95

109 temperature ultramicropores (< 0.7 nm), not ultramicropores with small supermicropores (< 1 nm) drive CO 2 adsorption. The ultramicropore volumes for these materials are very close: 0.18 cm 3 g -1 for C-Si-2.5-A* and 0.23 cm 3 g -1 C-Si-3-A*, which explains why their room temperature CO 2 uptakes are the same. All these results highlight the importance of a proper pore size for the sorbents designed for CO 2 capture: with the higher temperatures the CO 2 molecules have more energy and thus, smaller micropores with stronger adsorption potential are required to retain them. Still, the 4.0 mmol g -1 uptake at room temperature is in the range of typical values for phenolic-resin derived carbons

110 Figure 25. CO 2 adsorption isotherms measured for the C-Si-2.5-A* and C-Si-3-A* samples at 0 C and 23 C. Isosteric heat of CO 2 adsorption was calculated for the C-Si-3-A* carbon using the Clausius-Clapeyron equation and the CO 2 adsorption isotherms at 0 C and 23 C. The calculated values are in the range kj mol -1. Although the values carry relatively high uncertainty because data at only two temperatures were available for the calculation, they are similar to the other q st values reported in the literature for similar materials. 40,41,49 97

111 The Dubinin-Radushkievich (DR) equation was used as an alternative method to estimate the volume of micropores using carbon dioxide adsorption data. Generally, the DR plots are linear for highly microporous carbons with narrow distribution of micropores. 153 In this study, both investigated carbons (C-Si-2.5-A* and C-Si-3-A*) possessed high volume of pores < 1 nm but also a significant volume of pores with sizes between 1-2 nm, which is evident in the PSDs shown in Figure 22 and Figure 23. As a result, the DR plots displayed in Figure 26 show a slightly non-linear behavior. Figure 26. Dubinin-Radushkievich plots for CO 2 adsorption measured on the C-Si-2.5- A* (left panel) and C-Si-3-A* (right panel) carbons at 0 C. Still, for the C-Si-2.5-A* carbon a limiting value of micropores was estimated as 0.29 cm 3 g -1 (straight line in the range of log (p o /p) 2 ) and for the C-Si-3-A* carbon the volume was estimated as 0.40 cm 3 g -1 (straight line in the range of log (p o /p) 2 ). Because under conditions of 0 C and 1 bar CO 2 adsorption is governed by micropores 98

112 with sizes below 1 nm, we assume that these estimated limiting values of micropores represent the volumes of pores with sizes < 1 nm. These values are somewhat higher than the volumes calculated via the DFT method: 0.29 cm 3 g -1 vs 0.22 cm 3 g -1 for C-Si- 2.5-A* and 0.40 cm 3 g -1 vs cm 3 g -1 for C-Si-3-A*. The differences are significant and amount to 32% and 25%, respectively. This shows the limitations of the DR method when the systems studied possess micropores with different sizes. Even though an extended version of the DR equation for carbons with bimodal micropore size distributions is available, it is difficult to estimate the volumes of the different fractions of micropores. 154 Perhaps the DR plots based on CO 2 adsorption would work well for highly microporous carbons with narrow micropores with sizes below 1 nm. For such carbons, the DR plots could provide great alternative for calculation of the micropore volume over DFT method calculated from nitrogen adsorption isotherm. Its main advantage would be much faster time of measurement of CO 2 isotherm as compared to N 2. In the case of the carbons studied, however, due to their wide pore size distribution over the whole micropore range, the DR analysis of microporosity can be less accurate. Figure 27 presents results of the rate of adsorption measurements that were used to show the effect of mesopores on the improvement of the mass transfer properties and the reduction of equilibration time. Two carbon samples were compared: microporous carbon spheres prepared using extended Stӧber method and reported elsewhere 48 and the micro-mesoporous carbon C-Si-3-A*. The left panel shows the dependence between normalized pressure and time for the first dose of CO 2 on the completely degassed samples. The pressure was normalized with respect to the starting value (normalized 99

113 pressure 1) and the equilibrium value (normalized pressure 0) to adjust for the slightly different initial and equilibrium pressures naturally resulting from the difference in the samples. Figure 27. Change of the normalized pressure as a function of time during CO 2 adsorption (left panel) and change of the time until 90% pressure drop as a function of CO 2 dose pressure (right panel). Clearly, the CO 2 adsorption proceeds faster on the micro-mesoporous carbon than on the solely microporous carbon. To further exemplify this observation, the right panel shows the 90%-pressure drop time at different pressures. The 90%-pressure drop time represents the time taken to reduce the difference between the dose pressure and the equilibrium pressure by 90%. It shows how fast the system can reach a close-toequilibrium condition after a fresh dose of CO 2. The graph shows that the micromesoporous carbon performs better than the microporous one, namely, the equilibration 100

114 time of CO 2 on this sorbent is shorter at any given pressure. The difference amounts to 40% at low pressures and to 30% at the higher pressures. For both samples, at low pressures, the equilibration takes longer, whereas at high pressures, diffusion occurs faster resulting in faster equilibration. This is because the small pores have already been filled with the adsorbate but also because less adsorbate is adsorbed at higher pressures. Overall, the rate of adsorption measurements proved that the presence of mesopores in microporous carbons improves gas diffusion, resulting in faster adsorption and equilibration of CO Carbon spheres obtained in the presence of TEOS A separate set of carbon spheres was also prepared by a modified Stöber synthesis combined with TEOS addition. The synthesis was the same as the one described in the previous section, but instead of silica colloids, TEOS was used as a silica source. The synthesis resulted in polymer-silica composites, carbonization of which led to silicacarbon composites. Similarly to the materials prepared in the presence of colloidal silica, these materials were subjected to CO 2 activation before or after silica dissolution (see routes A and B in Figure 19) Morphology Scanning electron microscopy images presented in Figure 28 show that the spherical morphology of the carbons obtained after synthesis and post-synthesis treatments was retained. Even though the samples studied possess spherical morphology, they are not uniform and have larger diameters as compared with the samples prepared in 101

115 the presence of silica colloids. This could be the result of the addition of TEOS to the synthesis, which perhaps impacted the spheres growth; 26 alternatively, this may be due to the swelling of spheres due to the TEOS-generated silica incorporation. Figure 28, panel A shows that the as-synthesized spheres are compactly aggregated, which is perhaps due to the spider web-like fibers of the TEOS-generated silica between the spheres. Similar effect was observed by Choma et al., 143 where TEOS added to Stӧberlike synthesis caused formation of brush-like silica on the surface of carbon spheres leading to connected silica-carbon spheres. In contrast, the etched spheres (Figure 28, panel B) are more loose and no silica is present between the spheres. Most importantly, spherical morphology of the carbons was preserved despite the silica addition, the NaOH etching, and the CO 2 activation. This again shows that CO 2 activation is a good strategy to develop additional microporosity and simultaneously to preserve the spherical morphology of carbons. 102

116 Figure 28. SEM images of as-synthesized (A), etched (B), activated (C), and activated and etched (D) carbon spheres prepared in the presence of TEOS. All scale bars are 1 µm. 103