Characterisation of Materials for Hydrogen Storage

Transcription

1 Characterisation of Materials for Hydrogen Storage A Thesis submitted to The University of Manchester for the degree of MPhil in the School of Materials 2010 Vitor Fernandes School of Materials/UMIST

2 LIST OF CONTENTS List of contents 2 List of figures 4 List of tables 6 Abstract 7 Acknowledgements 8 Declaration 9 Copyright statement 9 1. Introduction Hydrogen Economy Hydrogen Storage Technologies General Hydrogen storage methods Metal Hydrides Introduction Absorption mechanism Thermodynamics properties Different types of metal hydrides Chemical Hydrides Introduction Hydrolysis reaction Kinetics of sodium borohydride hydrolysis Termophysical properties of sodium borohydride hydrolysis Sodium borohydride recycle Experimental Materials Methods Characterization techniques Experimental setup Experimental Self-hydrolysis Catalyzed hydrolysis with powder catalyst Catalyzed hydrolysis with supported catalyst Results Introduction Water-based self-hydrolysis Methanol based self-hydrolysis 65 2

3 3.4 Catalyzed hydrolysis with powder catalyst Catalyst characterization Effect of temperature and activation energy Influence of catalyst concentration Effect of sodium borohydride concentration Effect of water/borohydride ratio Catalyzed hydrolysis with supported catalyst Foam/catalyst system characterization Catalytic activity for hydrogen production 90 4 Discussion 98 5 Conclusions Suggestions for Future Work Reference 115 Word count:

4 LIST OF FIGURES Figure 1. Hydrogen pathways 11 Figure 2. Hydrogen phase diagram 12 Figure 3. Current status in terms of weight, volume and cost of various hydrogen storage 13 technologies. These values are estimates from storage system developers and the R&D community. Figure 4. Schematic representation of hydrogen absorption in metals. 19 Figure 5. Pressure composition isotherms for hydrogen absorption in a typical 21 intermetallics compound and construction of the van t Hoff graphic. The slope of the line represents the enthalpy of formation divided by the gas constant. The intercept represents the entropy of formation divided by the gas constant Figure 6. Van t Hoff plots of some selected hydrides 24 Figure 7. Maximum deliverable hydrogen from hydrolysis or thermolysis of ionic 25 hydrides Figure 8. Scanning electron micrograph of the as-received nickel foam 44 Figure 9. Scanning electron microscope Phillips, Model XL 30 FEG used in this work. 45 Figure 10. Experimental setup used in the self-hydrolysis studies. 49 Figure 11. Experimental setup used in powder catalyst studies 50 Figure 12. Experimental setup used in electrochemical characterization of sodium 51 borohydride hydrolysis. Figure 13. Experimental setup used in kinetics studies of the electrodeposited support 51 catalyst. Figure 14. Hydrogen generation rate as function of time for self-hydrolysis of NaBH 4 in 63 aqueous solutions of different NaBH 4 concentration, [2-20] wt.% at fixed temperature, 318 K, and fixed solution volume, 25 cm 3 Figure 15. Variation of the ph with time for water based self-hydrolysis of NaBH 4 at fixed 63 temperature, 318 K, and fixed solution volume, 25 cm 3, at different NaBH 4 concentrations, [2-20] wt.%. Figure 16. Hydrogen generation rate as function of time for water based self-hydrolysis of 64 NaBH 4 at a fixed concentration, 10 wt.%, and fixed solution volume, 25 cm 3, at two different temperatures, 318 K and 338 K. Figure 17. Volume of gas generated as a function of time in a 10 wt.% NaBH 4 nonstabilised 66 solution for different water /methanol ratios at 318 K. Figure 18. Sodium borohydride conversion (%) and ph solution changes as a function of 66 reaction time for an initial concentration of 10 wt.% NaBH 4 solution at 318 K with (a) 100 wt.% water; (b) methanol with no added water;. Figure 19. Volume of gas generated as a function of time in methanol solution for 68 different sodium borohydride concentrations, at 318 K. Figure 20. Variation of hydrogen generation rate with sodium borohydride concentration, 68 both on logarithmic scales, for hydrolysis in methanol solution, at 318 K. Figure 21. Arrhenius plot for NaBH 4 hydrolysis in methanol solution in the temperature 70 range of 283 and 328 K, starting with fixed initial concentration of NaBH 4 of 10 wt.%. Figure 22. Scanning electron micrograph of the powder catalyst e (a); EDS analysis of 71 the area displayed (b) and high resolution Scanning electron micrograph showing that the catalyst contains two different species (c) Figure 23. Transmission electron microscopy of powder catalyst. 72 Figure 24. Nitrogen adsorption/desorption isotherms. 74 Figure 25. XRD pattern of the synthesized nickel-based powder catalyst. 74 Figure 26. Figure 26. FTIR spectra of nickel, Ni-based1 (before and after reaction with 75 NaBH 4 ) and NaBH 4. Figure 27. XPS spectra for the nickel and Ni-based catalyst. (a); ruthenium catalyst (b) 76 Figure 28. Hydrogen generation rate as a function of time for catalyzed hydrolysis of 78 NaBH 4 at different temperatures between 297 K and 333 K, using 10 wt.% NaBH 4 solution stabilized with 10 wt.% of NaOH and ~50 mg of powder catalyst. Figure 29. Arrhenius plot for the catalyzed hydrolysis of sodium borohydride using powder catalyst. 78 4

5 Figure 30. (a) Hydrogen generation rate as function of time for catalyzed hydrolysis of NaBH 4 at different catalyst loadings between 20.1 mg and 100 mg, using 10 wt.% NaBH 4 solution stabilized with 10 wt.% of NaOH and ~50 mg of powder catalyst, at a temperature of 318 K. (b) Variation of hydrogen generation rate with catalyst loading, both on logarithmic scales, for the hydrolysis reaction. Figure 31. Effect of sodium borohydride concentration on the hydrogen production rate 82 using 50 mg powder catalyst, temperature of 318 K. Solutions were stabilized with 10 wt % NaOH. Figure 32. Rate of hydrogen release for various concentrations of NaBH 4 reaction rate 82 using 50 mg powder catalyst, temperature of 318 K. Solutions were stabilized with 10 wt % NaOH. Figure 33. Hydrogen generation rate as function of time for catalyzed hydrolysis of 84 NaBH 4 at different fraction H 2 O/NaBH 4, using fixed amount of NaBH 4, 1g, stabilized with 10 wt.% of NaOH, at fixed temperature of 318 K. The catalyst loading used in all experiments was ~50 mg of powder catalyst. Figure 34. Rate of hydrogen production for various concentrations of NaBH 4 using a 84 variation of the water fraction for a fixed amount of borohydride (red dots) and a variation of borohydride fraction for a fixed water volume (blue dots). (wnabh 4 /wh 2 O) 30-2%. Figure 35. Hydrogen generation rate of powder catalyst using 10 wt.% solutions at 318 K 85 and 10 wt.% of stabilizer. Solutions containing mixtures of the stabilizer were also used. Figure 36. Nickel-foam used as a catalyst support, as received. 87 Figure 37. Scanning electron micrograph of nickel-based catalyst, supported on nickel 87 foam (deposition of catalyst was done using Doctor Blade technique (a), and respective EDX analysis (b). Figure 38. Scanning electron micrograph of Ni-based catalyst 1 prepared by 89 electrodeposition of ruthenium on nickel foam. (a); Cross section view (b) and EDS analysis showing Ni and Ru within the elemental composition of the catalyst (c). Figure 39. XRD patterns of the catalyst prepared by electrodeposition of Ni and Ru on Ni 90 foam(catalyst 2); catalyst prepared by electrodeposition of ruthenium on nickel foam (catalyst 3); and as-received nickel foam. Figure 40. Hydrogen generation rate as a function of time for catalyzed hydrolysis of 92 NaBH 4 at different temperatures between 303 K and 338 K, using 2 wt.% NaBH 4 solution stabilized with 10 wt.% of NaOH and nickel-ruthenium doctor blade supported catalyst on a nickel foam (a). Arrhenius plot for the catalyzed hydrolysis of borohydride, using the doctor-blade supported catalyst are shown (b). Figure 41. Volume of hydrogen produced with time at 3 different temperatures, between K and 318 K, for catalyst hydrolysis of sodium borohydride, using a solution 2 wt.% NaBH 4 stabilized with 10 wt.% of NaOH, with nickel foam. Figure 42. Hydrogen production rate as function of time for catalyzed hydrolysis of 95 NaBH 4, at two different temperatures, 318 K and 275 K, using 2 wt.% NaBH 4 solution stabilized with 3 wt.% of NaOH with a foam of nickel electrodeposited with ruthenium as catalyst. The experiment was made using dynamic reactor with a fuel circulation velocity of 43 cm3/min. Figure 43. Hydrogen production rate as function of time for catalyzed hydrolysis of 97 NaBH 4, at different NaBH4 circulation rates between 85 and 10 ml/min, at constant solution temperature of 318 K, using 2 wt.% NaBH 4 solution stabilized with 3 wt.% of NaOH. Nickel foam electrodeposited with ruthenium was used as catalyst. Figure 44. Hydrogen production rate as function of time for catalyzed hydrolysis of NaBH 4, at constant solution temperature of 318 K, using 2 wt.% NaBH 4 solutions stabilized with 3 wt.% of NaOH with nickel foam electrodeposited with ruthenium as catalyst, for four successive utilizations

6 Figure 45. Figure 46. Figure 47. X-ray diffraction patterns of products of NaBH 4 hydrolyses in (a) 50% mixture of water/methanol solution; (b) methanol solution with no add water; at 318 K with 10 wt% of sodium borohydride (our result). EDS analysis of the by-product of production of hydrogen reaction in methanol with no added water at a temperature of 318 K (a); comparison with spectra of the by-product after reaction with water. Schematic representation of the reaction of sodium borohydride with water and methanol, and the reaction by-products. The recycling possibilities are also shown LIST OF TABLES Table 1. Composition of the solution used for the electrodeposition. 58 Table 2. Activation energy for sodium borohydride hydrolysis catalyst and non-catalyst in water and methanol solutions. 69 Table 3. Crystal structure suggested by TEM analysis of the powder catalyst. 72 Table 4. Textural properties of the Ni based powder catalyst. 73 Table 5. Comparative analysis of hydrogen produced rates obtain in this work with the results from the open literature. 106 Table 6. Comparative analysis of the activation energy between the powder catalysts 107 synthesized in this work with relevant data find on literature. 6

7 ABSTRACT The University of Manchester, Vitor Ramiro Almeida Fernandes, MPhil degree, Characterization of Materials for Hydrogen Storage, The present dissertation aimed at studying of materials for hydrogen storage. The option of sodium borohydride was found to be of interest, specially for portable and niche applications. A compreenhensive study of the kinetics and thermodynamics aspects of hydrogen generation from sodium borohydride was made. Three different types of sodium borohydride hydrolysis were considered, two of them self-hydrolysis, namely, water and methanolysis, and the other in presence of a stabilized solution and in the presence of a catalyst, referred to as catalyzed hydrolysis. For the catalyzed hydrolysis two types of catalyst were studied: a powder nickel based catalyst and a nickel foam supported catalyst. Sodium borohydride self-hydrolysis was studied under different temperatures and at different sodium borohydride concentrations. The ph variation during the reaction was also studied. It was found that increase on temperature and NaBH 4 concentrations raise the hydrogen production rate. It was also observed that during the hydrolysis reaction occur an increase on ph, which slow down the kinetics of hydrolysis. The reaction of sodium borohydride with methanol (methanolysis) was also analyzed. It was found that the water/methanol rate, temperatures and sodium borohydride concentration exert considerable influence on the hydrogen produced rates. Methanolysis of sodium borohydride show to be feasible method for lowtemperature hydrogen rate and the possibility of methanol regeneration can be used as potential high gravimetric density hydrogen storage system. For catalytic hydrolysis of sodium borohydride a nickel based powder catalyst was prepared by wet chemistry method. The catalyst show small particle size and large superficial area, which give them a high catalytic activity, hydrogen production rates of ~16 l min -1 g -1, the highest value find on literature for this type of catalyst. By the kinetics experiments made was possible estimated two different activation energy, one for high temperatures (31 kjmol -1 ) and another for low temperatures (68 kjmol -1 ) and a reaction order of 1.18 with respect to catalyst concentration. Powder catalysts show good durability without loosing of activity. The possibility of supporting the catalyst was also investigated using as support material nickel foam with large superficial area. Two types of deposition were used: Doctor Blade technique and Electrodeposition method. Both methods proved to have excellent catalytic activity; however on Doctor Blade supported catalyst some detachment of the powder was detected. Electrodeposited support catalyst shows no detachment of particle and no significant loss of activity with the various reutilisations. Support catalyst allows the developing of reactors systems operated in dynamic mode with large autonomy. 7

8 ACKNOWLEDGEMENTS I would like to thank Professor George Thompson for the opportunity to work and deliver my thesis on such an interesting topic, for supervising my work and for all readiness, availability, experience and knowledge that has always provided me during this work. To Dra. Carmen Mireya Rangel, I like to express my gratitude for the possibility of carry out the experimental work at UPCH/LNEG. I m also grateful for all the help and support, knowledge and determination that provided me with additional motivation for making this thesis. To Teruo Hashimoto and Teresa Paiva I would like to give my thanks for the work done on TEM/SEM/EDS analysis. I would also like to thank to all my colleagues at UPCH/LNEG for helping me to carry out this work and also for all the support. I also would like to acknowledge my Family and my friends for the continuous encouragement and support. 8

9 DECLARATION I declare that no portion of the work referred to in the thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institute of learning. COPYRIGHT STATEMENT i. The author of this thesis (including any appendices and/or schedules to this thesis) owns certain copyright or related rights in it (the Copyright ) and s/he has given The University of Manchester certain rights to use such Copyright, including for administrative purposes. ii. Copies of this thesis, either in full or in extracts and whether in hard or electronic copy, may be made only in accordance with the Copyright, Designs and Patents Act 1988 (as amended) and regulations issued under it or, where appropriate, in accordance with licensing agreements which the University has from time to time. This page must form part of any such copies made. iii. The ownership of certain Copyright, patents, designs, trademarks and other intellectual property (the Intellectual Property ) and any reproductions of copyright works in the thesis, for example graphs and tables ( Reproductions ), which may be described in this thesis, may not be owned by the author and may be owned by third parties. Such Intellectual Property and Reproductions cannot and must not be made available for use without the prior written permission of the owner(s) of the relevant Intellectual Property and/or Reproductions. iv. Further information on the conditions under which disclosure, publication and commercialisation of this thesis, the Copyright and any Intellectual Property and/or Reproductions described in it may take place is available in the University IP Policy (see in any relevant Thesis restriction declarations deposited in the University Library, The University Library s regulations (see and in The University s policy on presentation of Theses 9

10 1. INTRODUCTION 1.1 Hydrogen economy Global energy consumption is expected to increase dramatically in the next decades, driven by rising standards of living and a growing population worldwide. Since the 18 th century, fossil fuels have powered the technology and transportation networks that drive Society. The supply of fossil fuels is limited, and the global oil and gas reserves are concentrated in a few regions of world, some of them politically unstable. As a result, security of supply is increasingly difficult to assure. Moreover, the use of fossil fuels produces carbon dioxide and other pollutants that are associated with global warming and health problems. In this context, the demands for clean and abundant energy have resulted in increased attention worldwide to the possibilities of a hydrogen economy as a long-term solution for a secure energy future. However, greater global investment in R&D is required before hydrogen can supply energy in quantities and at a cost that are competitive with fossil fuels. Hydrogen is very plentiful on the Planet, being the third most abundant element on the Earth s surface. One of the most important characteristics of hydrogen is the fact that it has the highest gravimetric energy density of the energy sources known today. Environmental friendliness and more efficient processing are other important features that make hydrogen one of most promising alternatives to fuel fossil. The greatest limitation of hydrogen is that it has to be produced, since on the Earth it does not exist in its free form. This implies that energy must be used to separate it from water or other relevant chemical compound. At present, most of the World's hydrogen is produced from natural gas by steam reforming. However, to achieve the environmental benefits, we must produce hydrogen from non fossil resources, such as water, using a renewable energy source, see Figure 1. 10

11 Figure 1. Hydrogen pathways [1] Barriers to the development of a hydrogen economy include the current cost of both hydrogen fuel and fuel cells, with the problem of storage being recognised as a key enabling technology. Reducing these barriers is one of the driving factors in the Government s involvement in hydrogen and fuel cell research and development. However, this involvement raises concerns, including the cost of such research and the possibility of the Government picking winners among competing technologies Hydrogen storage technologies General The hydrogen molecule can be found in various forms depending on the temperature and the pressure. The phase diagram of hydrogen is shown in Figure 2. At temperatures less than 11 K, hydrogen is a solid with a density of 70.6 kg m -3, and a gas at higher temperatures, with a density of kg m -3 at 273 K and a pressure of 1 bar. Hydrogen is a liquid in a small zone between the triple and critical points, with a density of 70.8 kg m -3 at 20 K The strong repulsive interaction between 11

12 hydrogen atoms is responsible for the low critical temperature (Tc = 33 K) of the gas [2]. Figure 2. Hydrogen phase diagram [2] Hydrogen storage is a key enabling technology for the advancement of the hydrogen economy in transportation, stationary, and portable applications. Two technical challenges need to be overcome for efficient hydrogen storage. First, increase of hydrogen density is required. At ambient temperatures and atmospheric pressure, 1 kg of hydrogen has a volume of ~11 m 3. In order to promote the volume reduction the temperature should be decreased below the critical temperature, the pressure should be increased by compression work or, finally, repulsion has to be reduced by interaction of hydrogen with other molecules. Other important requirement for a hydrogen storage system is the reversibility of hydrogen uptake and release. For stationary applications, the important parameters to consider are the system cost, operating cost, maintenance issues, refuelling time, safety and environmental impact. Storage system volume and storage system weight are important, but not critical. 12

13 For transportation use, the overarching technical challenge is how to store the amount of hydrogen required for a conventional driving range, within the vehicular constraints of weight, volume, efficiency, safety, and cost. Durability over the performance lifetime of these systems must also be verified and validated, and acceptable refuelling times must be achieved. The most important key challenges include: Weight and Volume. In reality, the weight and volume of hydrogen storage systems are too high, see Figure 3, resulting in inadequate vehicle range compared with conventional petroleum fuelled vehicles. New materials and components are needed to develop compact, lightweight, hydrogen storage systems with a good driving range in all light-duty vehicle platforms. Figure 3. Current statuss in terms of weight, volume and cost of various hydrogen storage technologies. These values are estimates from storage system developers and the R&D community. [3] Efficiency. Energy efficiency is a challenge for all hydrogen storage approaches. The energy required to allow hydrogen in and out is an issue for reversible solid-state materials. Life-cycle energy efficiency is a challenge for chemical hydride storage in 13

14 which the by-product is regenerated off-board. In addition, the energy associated with compression and liquefaction must be considered for compressed and liquid hydrogen technologies. The goal to achieve in the future is 90% energy efficiency. Durability. At the present time, durability of hydrogen storage systems is inadequate. New materials and components are needed that provide hydrogen storage systems with lifetimes of 1500 cycles. Refuelling Time. The current-refuelling times are too long. The development of hydrogen storage systems with refuelling times of less than three minutes is essential for implementation of the hydrogen economy. Cost. The cost of on-board hydrogen storage systems is still too high, particularly in comparison with conventional storage systems for petroleum fuels. Low-cost materials and components for hydrogen storage systems are needed, as well as lowcost, high-volume manufacturing methods. Codes and Standards. Applicable codes and standards for hydrogen storage systems and interface technologies, which will facilitate implementation/commercialisation and assure safety and public acceptance, have not been established. Standardized hardware and operating procedures, and applicable codes and standards, are required Hydrogen storage methods Six different methods for hydrogen storages are currently being investigated: These include high-pressure gas cylinders (up to 800 bar), Liquid hydrogen in cryogenic tanks (at 21 K), hydrogen adsorption on solids with large surface areas, metal hydrides and chemical hydrides. Gaseous Hydrogen The most traditional storage system is high-pressure gas cylinders. This type of storage works at pressures greater than 20 MPa, which implies a high cost of compression without reaching an adequate energy density. Since hydrogen behaves very like an ideal gas at ambient temperature, the theoretical work for isothermal compression of hydrogen from pressure p 1 to p 2 is given by 14

15 W=p 1 V 1 log (p 2 /p 1 ) (1) where V 1 is the volume of the gas at pressure p 1. The compression between 0.1 MPa and 80 MPa has a consumption of 2.21 kwh/kg. The real system is not isothermal and the energy consumption is higher than the theoretical value. Because of the logarithmic dependence, the work need is greater in the lower pressures stages. Depending on pressures difference, several stages of compression may be required. Hydrogen can be compressed using a standard piston-type compressor with slight modifications of seals in order to compensate for the high diffusivity of hydrogen. The relatively low hydrogen density requires higher-pressure vessels that are expensive, heavy and bulky. The gravimetric hydrogen density decreases with increase of pressure due to the increasing thickness of the wall of the pressure cylinder. New technologies use lightweight composite cylinders, which are able to withstand pressures to 80 MPa, and therefore the hydrogen gas can reach a volumetric density of 36 kg.m -3, approximately one-half of the liquid form. However, the cost of this new material is high and increases proportionally with the required vessel pressure, due to the amount of the composite material needed. The advantages in using compressed hydrogen are that the process is fairly simple and filling a tank can be achieved in a relatively short time period. The disadvantages are that the tanks can only achieve lower energy densities of about 1.3 kwh kg -1, and a large amount of the energy that hydrogen can carry is lost when acquiring the high hydrogen pressures stated previously. Liquid hydrogen Liquid hydrogen is stored in cryogenic tanks at 21.2 K and ambient pressure. Because of its low critical temperature (33 K), liquid hydrogen should be stored in open systems. The volumetric density of liquid hydrogen is 70.8 kg.m -3. With regard to the technology, the liquid hydrogen is achieved through a Linde cycle, similar to that used for other gases such as N 2. The theoretical work necessary for the liquefaction of hydrogen from ambient temperature is 3.23 kwhkg -1. But in practice the real work is 15.2 kwhkg -1, around 50% of the lower heating value of 15

16 hydrogen (33.3 kwhkg -1 ), i.e using liquefied hydrogen 50% of the energy is spent in the process. The lower heating values is defined as the energy release when the hydrogen undergoes complete combustion with oxygen in standard conditions. The biggest problem associated with liquid hydrogen is the boil-off losses due to thermal conductivity. The boil off is function of the size, shape and thermal insulation of the storage containers. The theoretical best shape is a sphere because it has the least ratio of surface per volume and the stress and strain are uniformly distributed. However, very large sphere containers present manufacturing problems. Proper insulations can reduce the boil off. Normally the containers combine different kinds of insulation: vacuum insulation, vapour cooled radiation shields and multilayer insulation. Although, a large amount of energy is needed for the liquefaction, the continuous boil-off of hydrogen and the relatively complex and expensive containers are problems that limit the practical applications of this technology. However, for applications where cost presents no problem and when the hydrogen is consumed in a short period of time, this storage method is a good option. Hydrogen adsorption on solids with large surface areas Adsorption of hydrogen in material with large surface areas is possible due to weak van der Waal forces. In this process, a molecule of hydrogen interacts with several atoms of the surface of the adsorbent. This interaction depends on the pressure, temperature and nature of the surface of the adsorbent, with two types of force involved: one attractive and other repulsive. The overall effect of these two kinds o force is a weak interaction between the adsorbent and adsorbate. It is because of this weak strength that the physisorption is only observed at low temperatures. This interaction depends on the pressure, temperature and nature of the surface of the adsorbent. Carbon nanotubes may be a promising material for hydrogen storage. Dillon et al. (1997) described a hydrogen desorption experiment in carbon nanotubes. The authors 16

17 estimated a hydrogen storage capacity of carbon nanotubes of 5-10 mass%. However, experimental results in carbon nanotubes have been controversial because of a lack of reproducibility. The impurity and the quantity of different nanotubes contribute to the relatively large scatter in the experimental results. Errors in volumetric and gravimetric measurement methods may also contribute to the wide disparity in the experimental results. Hydrogen can be stored into nanotubes by chemisorptions or physisorption. In the physisorption process, hydrogen molecules are adsorbed inside the nanotubes due to van der Waals attractive forces between carbon and hydrogen. In chemisorption, the hydrogen molecules are covalently bonded to the carbon atoms of the nanotubes. Theoretical calculation shows that the maximum storage is about 3.3 wt.% for physisorption and 7.7 wt.% for chemisorption [5]. The desorption process is more difficult in chemisorption, with increased temperatures required. 1.3 Metal hydrides Introduction As mentioned above, hydrogen storage is clearly one of the most important challenges in the development of the Hydrogen Economy. In previous sections, the characteristics of some storage methods have already been described. Although the storage of hydrogen in the form of high-pressure gas or liquid is possible, however the security issues, especially in the case of mobile applications, and the large amount of energy need for liquefaction or to compress the hydrogen, are important drawbacks. The reversible storage in the form of solids is feasible and it appears the best option from the point of view of safety and economic viability [2]. Some metals and alloys react, under moderate pressure and temperature, with hydrogen to form metal hydrides, which can be used for hydrogen storage with god safety, compact and economic properties. The metal hydrides have a higher density storage volume (6.5 H atoms.cm -3 ) than pressurized hydrogen (0.99 H atoms.cm -3 ) or liquid hydrogen (4.2 H atoms.cm -3 ) [7]. A further important feature of metal hydrides is the reversibility provided by almost all of the metal hydrides [8]. The important constraints include low-density gravity (high volumetric density) for mobile applications, the stability of the material and the cost of metal alloys. 17

18 In recent times, a strong research base has been directed at the design of new materials that have improved properties (capacity, absorption-desorption kinetics, thermal properties, toxicity, cyclability and cost) for the desired applications [9]. Due to these studies, many different metal hydrides have been developed for applications as diverse as hydrogen vehicles, air conditioners, heat pumps, Ni-MH batteries and many other devices [10]. For example, since 1990, Ni-MH batteries have been available with electrodes formed by hydrogen-absorbing alloys (in the Ni-MH battery, the alloy used consists basically of LaNi 5 ). However, the actual storage capacity is not sufficiently high for PEM fuel cells on board vehicles to achieve the objectives proposed by DOE, with major improvements necessary Absorption mechanism Hydrogen storage in the form of metal hydrides occurs from the reaction of metal with hydrogen. The metal atoms constitute the host lattice and the hydrogen atoms are trapped in the interstitial sites that can be a vacancy or a line defect. The hydrogen capacity of metal compounds depends on the surface structure, morphology and purity. There are two different ways of hydriding a metal, namely direct dissociative chemisorption and electrochemical splitting of water. These reactions are, respectively [11] (G. Sandrock 1999) M + x/2h 2 MH x, (2) M + x/2h 2 O + x/2 e - MH x + x/2 OH -, (3) where M represents the metal. Figure 4 shows a schematic representation of the process of hydrogenation by direct dissociative chemisorption. In this process, first the hydrogen gas molecules are 18

19 adsorbed onto the metal surface, where the molecules are split into H atoms. The first hydrogen atoms that penetrate the bulk phase occupy mainly interstitial positions. This phase, in which the hydrogen atom in Ni-MH is in a dilute solid solution, is the so-called α-phase. When hydrogen atoms enter the metal, they obtain extra electron density (Hδ-) and their van der Waals radii increase. The α-phase, where the H atoms can move relatively easy through the metal, is more pronounced when the metal possesses large interstitial sites. When the amount of hydrogen atoms in the metal increases a phase transition is induced. The resulting hydride often has a different structure than the corresponding metal. Since the ß-phase has a larger volume than the metal, the transition can be accompanied by stresses in the material and significant transition- and activation-energies. Further, since the phase has to grow or shrink, the difference in structure and volume can also cause kinetic barriers for hydrogen loading or discharging. Figure 4. Schematic representation of hydrogen absorption in metals. [2] The absorption process usually takes place under isothermal conditions to increase the pressure of hydrogen in the system. 19

20 In the absorption process, for a given temperature, the hydrogen begins to dissolve into the solid. Increase of external pressure increases the content of H 2 in the solid (phase α). At this stage the H 2 content is very low. Subsequently, a point is reached where there is no increase in pressure, but the content of H 2 continues to rise. This is due to hydride formation i.e. the change from phase α to phase β (hydride phase). Continuing increase in pressure yields a fully formed hydride (only β phase exits). After this point is reached, the pressure increases very rapidly due to the proximity of the end point of absorption. Figure 4 shows a schematic representation of this phenomenon. In the discharge, pressure decreases so that heat should be supplied to maintain isotherm reversibility. However, even in the best case, there is an effect of hysteresis. The value of the hysteresis is closely related to the expansion that accompanies the formation of hydride. The formation of metal hydrides by adsorption of hydrogen is determined by both the thermodynamics and kinetics of the reaction. In the next section, the basic mechanism that controls these two aspects is described, along with the possible ways of improving the properties of metal hydrides and exploiting them Thermodynamics properties Thermodynamic aspects of the formation of hydrides are described in pressurecomposition isotherms (PCI). As already mentioned, metal and hydrogen form two different types of hydrides: the α phase, in which only a small part of H 2 is absorbed, and the β phase, in which the hydride is fully formed. Figure 5 shows the pressure-composition isotherms for the absorption of hydrogen in typical intermetallics alloys, showing the solid-solution (phase α), the hydride phase (phase β) and the equilibrium area (region where the two phases coexist). The coexistence region is characterized by a flat plateau and ends at the critical temperature, Tc. The region of stability in the PC curves is due to rapid formation of the hydride at the surface (α phase) followed by a slow diffusion of hydrogen through the surface layer of the hydride. The length of the plateau represents the amount of hydrogen that can be reversibly stored in the compound. 20

21 PCT thermodynamic properties are directly related to the van't Hoff equation (Equation 4) [11]. The equilibrium pressure (P equation ) is related to changes in enthalpy and entropy as a function of temperature. The right side of Figure 5 shows the construction of the graphic van't Hoff isotherm. Figure 5. Pressure composition isotherms for hydrogen absorption in a typical intermetallics compound and construction of the van t Hoff graphic. The slope of the line represents the enthalpy of formation divided by the gas constant. The intercept represents the entropy of formation divided by the gas constant. [2] ln ( ) P eq H = R 1 S T R (4) The enthalpy changes related to hydride formation or dissociation can be obtained experimentally from the slope of the van t Hoff's plots. The slope of the line is related to the enthalpy of formation divided by the gas constant and the intercept gives the entropy of formation divided by the gas constant. While the enthalpy term depends on the metal-hydrogen bond stability, the entropy term corresponds essentially to the transition from molecular hydrogen to atomic hydrogen, necessary for the passage from the gaseous to the solid phase and it is similar for all the known hydrides. The value is approximately S f -130 JK -1 mol -1 H2 [2] for all metal-hydrogen systems. The entropy of formation for metal hydrides leads to a significant evolution of heat Q = T. S (exothermic reaction) during the adsorption of hydrogen. In order 21

22 to desorb hydrogen, the metal hydride must be provided with the same amount of heat (reaction endothermic). If hydrogen is desorbed below RT, the environment can deliver this heat. However, if desorption occurs above of RT, the heat needs to be transferred from an external source. The working temperature of a metal/hydride system is fixed by the thermodynamic equilibrium pressure and by the overall reaction kinetics. In order to make metal hydrides interesting for the use in hydrogen reservoirs, the working pressure and temperature must be in the range 1-10 bar and K, respectively, corresponding to an enthalpy change between 15 and 24 kjmol -1 H Different types of metal hydrides Most of the natural elements adsorb hydrogen under appropriate conditions: however metallic systems that allow absorption and desorption of hydrogen in a reversible way under conditions of temperature and pressure compatible with practical applications (pressure 0-10 bar, temperature K) are focused on intermetallictype systems [2-15]. Especially interesting are intermetallic compounds in the form of a ternary system AB x H n, because the variation of the elements allows desirable PCT properties to be achieved. Element A, normally a rare earth or an alkaline earth metal, forms a stable hydride and element B, usually a transition metal, forms only unstable hydrides. There are three main groups of metal hydrides used for hydrogen storage: AB 5, AB 2 and AB hydrides. The most representative elements of the AB 5 group are LaNi 5. This group presents an extreme versatility because many elements can be substituted in A and B lattice sites. Usual examples of substitutes for element A are Ce, Mn, Ca, Y and Zr. For substitution of element B Al, Mn, Cr, Si, Zn, Cu, Fe and Co, are the most used [12]. With combinations of these elements, improved PCT properties are possible with decrease of the corrosion rate and volume expansion. An improvement of the time cycle is a further important achievement enhancing the versatility of this group. An important property of AB 5 compounds is their relatively high tolerance to O 2 and H 2 O, and they do not readily form oxides that interfere with their ability to absorb 22

23 hydrogen. Hysteresis is usually low [11]. The principal disadvantages of AB 5 compounds are their high cost and the low hydrogen capacity, normally between 0.7 and 1 wt.% (1.28 wt.% is the reversible capacity for LaNi 5 )[12,13]. The AB 2 group, like AB 5, is a large and versatile group with favoravel PCT properties near the ambient temperature. In this group, A element is typically Zr, Ti, Hf, or elements of the group of lanthanides. The element B is generally a metal belonging to groups of transition metals, the most used being Fe, Mn and Cr. The reversible absorption capacity of the compounds is close to AB 5; however, the maximum capacity is normally higher, especially at increased temperatures and pressures. As for AB 5, partial replacements of the A and B elements allow improvements in PCT properties, in corrosion rate, in volume expansion and in the cycle time. The costs of the compounds of this group are normally a little less than for the AB 5 alloys [11]. The group AB is mainly derived from TiFe. This group tends to have two plateaux, the upper of which is less stable [14]. Oxide films are easily formed in contact with air or if the H 2 is not of high purity. These oxides reduce the hydrogen absorption capacity, but decrease the tendency toward self-ignition when in contact with air or humidity. Hysteresis is usually high. In order to improve the properties of this group it is normal to partially replace Fe by other elements, the most used being Mn and Ni. The reversible absorption capacity of this group is about 1.5 wt.% [11,15], with lower costs than previous groups. The PCT properties of various metal hydrides are show in Figure 6. PCT versatility and modification are evident; for example, the stabilisation of the hydride of LaNi 5 by the partial substitution of nickel with aluminium in LaNi 5 is shown as well as with substitution of lanthanum with mischmetal. 23

24 Figure 6. Van t Hoff plots of some selected hydrides. [2] 1.4 Chemical hydrides Introduction Chemical hydrides, hydrogen-containing materials that chemically react with water releasing hydrogen and heat, have emerged as potential hydrogen carriers and media for storage due to their high volumetric and gravimetric efficiency. In contrast to the reversible hydrogen storage described in section 1.3, which requires simultaneous hydrogen discharging and recharging processes, irreversible chemical hydrogen storage allows us to deal with these two processes individually and may enhance the possibility of improving both processes. The chemical hydrides can be classified in two groups: simple binary hydrides and complex hydrides. In simple hydrides (MHn), the hydrogen is bonded covalently or ionically to a metal. In complex hydrides, the hydrogen is combined with two different metals M and M. Normally M is a group IIIA element. 24

25 Several different hydrides have been considered for hydrogen storage. Figure 7 shows the gravimetric density of the most promising and studied chemical hydrides. Figure 7. Maximum deliverable hydrogen from hydrolysis or thermolysis of ionic hydrides. [16] Sodium borohydride (NaBH 4 ) is one most studied chemical hydrides owing to its combined advantage of high hydrogen capacity (with a theoretical value of 10.8 wt.%) [16], good stability in alkaline solutions, easy control of the hydrogen generation rate, friendly operation (low reaction-initiation temperature, stable in air under no pressure and the NaBH 4 solution is nonflammable) and the environmentally benign hydrolysis product (borax, NaBO 2 ). Hydrogen can be released through chemical pathways such as thermolysis, hydrolysis or a combination of the two [17]. However, in practical hydrogen generation system only the hydrolysis approach has been used. This is because in hydrolysis one-half of the hydrogen produced arises from reduction of water, resulting in a high hydrogen storage capacity. The generated H 2 has high purity (no CO, S) and is humidified (heat generates some water vapour), which facilitates its use in fuel cells. The gravimetric density of 25

26 NaBH 4 is better than metal hydrides and the volumetric density is better than liquid hydrogen. In water, the solubility of sodium borohydride is about 35 wt.% at 298 K and a 30 wt.% NaBH 4 fuel (30 wt.% NaBH 4, 3 wt.% NaOH, and 67 wt.% H 2 O) has a theoretical hydrogen content of 6.6 wt.% -equivalent to about 66 g H 2 l -1, compared with 70 g H 2 l -1 for liquid hydrogen and 23 g H 2 l -1 for compressed hydrogen (at 5,000 psi) Hydrolysis reaction As previously considered, the theoretical hydrogen released on NaBH 4 hydrolysis is about 10.8 wt.%, where one-half of the hydrogen is from H 2 O. Ideally, one mole of sodium borohydride reacts with 2 moles of water to liberate 4 moles of hydrogen (equation 5). NaBH 4 + 2H 2 O 4 H 2 + NaBO 2 + heat (5) In practice, an excess of water is needed because the by-products of reaction are normally in the hydrated form. In equation 6, the required excess of water is represented by the factor x. This factor accounts for the fact that the by-product can exist with varying degrees of hydration. NaBH 4 + (2+x) H 2 O 4 H 2 + NaBO 2.xH 2 O + heat (6) Besides this, an excess of water is necessary to pre-dissolve the reactants. Sodium borohydride solutions are a convenient storage method, but have some limitations. The main one is the NaBH 4 solubility. At 298 K, it is 55 g per 100g of water, which correspond at gravimetric hydrogen storage capacity of 7.5 wt.%. This value does not take into account the effect of the by-products of the reaction NaBO 2 solubility that is 28g per 100 g of water. However, the crystallization of sodium metaborate during hydrogen generation may cause irreparable damage to hydrogen storage/generation systems due to the accumulation on liquid flow lines such as valves, pipes and fittings. Preventing large 26

27 amounts of the reaction by-product, sodium metaborate, from solidifying is a crucial. In order to do this an excess of water is required. Taking in account the effect of NaBO 2, the NaBH 4 capacities is only about 16 per 100g of water, which decreases the hydrogen gravimetric capacity to 2.96 wt.% [18]. Furthermore, the addition of a base to stabilize the solution also decreases the hydrogen storage density. In the context of hydrogen storage efficiency, any water in excess proportionally reduces the gravimetric efficiency. This drawback can be minimized for a power generation system based on a PEM fuel cell, where the water produced in the fuel cell can be re-used for the hydrolysis reaction, increasing the overall energy density Kinetics of sodium borohydride hydrolysis Understanding and controlling the kinetics of the hydrolysis of sodium borohydride are important keys to the successful implementation of the technology. The hydrolysis reactions are known to be ph dependent, reacting to completion under acidic conditions and stabilizing under strongly basic conditions. Acids are used as catalyst to improve the reaction kinetics, and bases are used to stabilize the reactions preventing premature release of hydrogen. In order to improve the kinetics of sodium borohydride hydrolysis in stabilized solutions, metal catalysts are used contributing considerably to an increase of hydrogen yield. This effect varies with the type and method of preparation of the catalyst. Some authors have focused they work on the development of cheap and efficient new catalysts. Complete conversion of the sodium borohydride is only possible under catalyzed hydrolysis or ph control using an acid solution. Matthews et al. [19] reported 80% conversion using steam (pure water). In this thesis, it is evident that, with the use of methanol solutions with no added water, it is possible to achieve total conversion of NaBH 4 without catalyst or acid addition and with fast kinetics. Others factors affect the rate of reaction, including temperature, ph, NaBH 4 concentration and phase of water used. Many authors have investigated these effects 27

28 on reaction kinetics and proposed some general mechanisms for the hydrolysis; however, the pathways are not totally understood. Self-hydrolysis Sodium borohydride release hydrogen in contact with pure water. After a few hours, there is hardly any hydrogen production while there is still plenty of unreacted borohydride. Although the reaction is thermodynamically favorable in pure water solutions, the conversion of sodium borohydride is less than 10% at ambient temperature. This is caused by the increase of the ph during the reaction that stabilizes reaction intermediates and slows the hydrolysis. Several studies investigated the decomposition rate of sodium borohydride solutions at various NaBH 4 and NaOH concentrations and at different temperatures. Kreevoy et al. [20] described the stability of sodium borohydride in aqueous solution, equation 7, where t 1/2 is the half-life of NaBH 4 (the time it takes for one-half of NaBH 4 solution to decompose) in minutes and T is temperature in Kelvin. Log 10 (t 1/2 ) = ph (0.034T ) (7) This equation describes the kinetics in a region that is too dilute and with a large concentration of NaOH (up to 40% weight) to be practical for real hydrogen generation systems. More recently, Moon et al. [21] investigated the influence of ph and temperature on the non-catalyzed hydrolysis of sodium borohydride with concentrated solutions between 5 and 25 wt.%. They found a very slow hydrogen evolution that increased with the concentration until 20 wt.%, and then diminished for higher concentrations. They also showed that, with more concentrated solutions, the rates diminished due to the solubility and mobility inhibition by NaBO 2 and reduced water availability. It was concluded that hydrogen evolution depended of both ph and temperature. Minkina et al. [22] investigated the effects of temperature and NaOH concentrations on the degradation of concentrated sodium borohydride solutions. They find that at 28

29 temperatures higher than 30ºC, at least 5 wt.% of NaOH needed be added to stabilize the solution. Kojima et al. [23] reported that, at room temperature, the maximum conversion of sodium borohydride without using catalyst is 7%. Matthews et al. [19] studied the hydrolysis of sodium borohydride with saturated steam at temperatures between 383 to 413 K without a catalyst. At 383 K, they obtained the highest yields of hydrogen. However, on raising the temperature to 413 K, the reaction rate diminished significantly. They observed formation of a solid agglomerated products that was soft and wet at 383 K and hard and dry at 413 K. Therefore, they proposed that the formation of an impermeable by-product shell during reaction at high temperatures limits the mass transfer of the steam to the reactant and reduces the rate of conversion. Catalyzed hydrolysis Sodium borohydride solutions in self-hydrolysis become chemically stabilized and do not generate significant amounts of H 2 under ambient conditions. However, upon addition of some catalysts, the hydrolysis rate of NaBH 4 can be dramatically accelerated, allowing the reaction to proceed to completion. Catalytic hydrolysis of NaBH 4 is a complex process, which involves solid-phase dissolution, liquid-phase transfer of the reactant and by-products, and the reaction occurring at the catalyst surface, at varied temperatures. Two catalytic approaches have been investigated to improve the rate of sodium borohydride reaction: Adding acid additives and use of metal catalyst. Acid catalyst The addition of acid to NaBH 4 solution overcomes the ph stabilization and allows the complete conversion of sodium borohydride at a considerable rate. The addition of acids delays the formation of the metaborate ions by shifting the ph of the 29

30 reaction to lower values which allows improved hydrogen yield. Several studies have reported that reaction rates do not depend of type of acid used. Many mechanisms have been proposed for borohydride hydrolysis catalyzed by addition of acid. Schlesinger et al. [24] presented in the early of 1950s the first study of acid catalyzed hydrolysis of NaBH 4. Using buffers to control the ph, they reported that, at a ph of 7, 90% conversion was observed in only 5 minutes. With an increase in ph to 7.4, in the same time they observed only 67% conversion. Later, Davis et al. [25] proposed a mechanism involving the intermediate BH 3 OH -. H 3 O+ + BH 4 - H 2 + BH 3 + H 2 O (8) BH 3 +3H 2 O H 2 BH 3 + 3H 2 (9) Kreevoy et al. [20] proposed a mechanism with formation of intermediate H 2 BH 3 according to reactions H + + BH - 4 H 2 BH 3 (10) - H 2 O + BH 4 H 2 BH 3 + OH (11) H 2 BH 3 H 2 + BH 3 (12) H 2 O + BH 3 3H 2 + B(OH) 3 (13) - OH + B(OH) 3 B(OH) 4 (14) They suggested that the formation of the H 2 BH 3 intermediate is the limiting step of hydrolysis in acidic conditions and the reaction model proposed by them is first order kinetics in NaBH 4 and H +, across the ph range. In this mechanism, they do not take water into account in the rate equation. Wang and Jolly [26] suggested that borohydride hydrolysis is subject to an acid-base equilibrium between borane and boric acid. In strongly alkaline solutions (ph > 9), the hydrolysis reactions (equation 15) gives borate ions (B(OH) - 4 ). Under more acidic conditions (ph<9), the hydrolysis yields boric acid (equation 16). BH H 2 O B(OH) H 2 (15) 30

31 H + + BH H 2 O B(OH) 3 + 4H 2 (16) Acid catalyzed hydrolysis of sodium borohydride allows the complete conversion of sodium borohydride with good yields of hydrogen. However, this technique requires large amounts of acid, making this method unsafe, heavy and bulky. The difficulty of controlling the reaction is a further drawback. Because of this, the use of acid as a catalyst for NaBH 4 hydrolysis has not been considered as a potential accelerator for hydrogen production. However, most recently, the effects of acid accelerators on hydrogen generation from solid sodium borohydride for small scale portable applications has been studied. Prosini and Gilson [27] designed a hydrogen generator for fuel cell powered cellular phone based on the hydrogen on demand concept and taking advantage of hydrolysis of solid NaBH 4 with HCl/water solution. They also evaluated the optimum HCl/hydride (=1), HCl/H 2 O (=1.6), acid solution/boride (= 3.6 cm 3 g -1 ) ratios and hydrogen mass flow rates upon operation. The estimated an energy density (four times larger than commercial high energy density lithium-ion batteries) and costs make such a device competitive with state-of-the-art commercial batteries. Murugesan and Subramanian [28] also studied hydrogen generation using acidified water and solid NaBH 4 for small scale portable applications. They investigated various acids (mineral acids: HCl, H 2 SO 4, HNO 3, H 3 PO 4 ; organic benign acids: HCOOH, CH 3 COOH) on hydrogen yield from solid NaBH 4. In all the mineral acids studied, they observed an increase in the hydrogen production rate, which increased with increasing acid concentration. In the case of benign organic acids, in general, they are required at higher concentrations to provide hydrogen production rates similar to mineral acids. Metal catalyst Aqueous solutions of sodium borohydride can be safely stored by stabilization of the solution by adding NaOH. As hydrogen is required, the reaction hydrolysis can be greatly accelerated using a metal catalyst. A large number of metal or metal compound have been reported to be active for catalytic hydrolysis of sodium borohydride hydrolysis in alkaline solutions under ambient temperature conditions, 31

32 including Co, Ni, Co and Ni borides, Ru, Pt, Pd, Pt-Ru, Pt-Pd and fluorinated Mg 2 Ni alloy. The efficiency of the catalyst has been investigated in terms of type and forms of the catalyst. The noble metals have been widely studied in the form of metal powder, metal salts and nanoparticles. The non-noble metal catalysts have been intensively studied in the form of metal powder, metal salts, metal borides and their alloys obtained by chemical reduction of their salts, Raney type metals and alloys of Raney type metals, nanoparticles and recently by binary and tertiary metal alloy catalysts. Noble metals are highly active compared with non-noble metals. However, nonnoble metal catalysts, especially nickel and cobalt, are also attractive because of their low cost and comparable catalytic activity. Qualitative study reveals that, in the case of noble metals, the form of the catalyst plays a major role in the catalytic activity. For example, the reaction order decreases in the order: Rh > Pt > Pd > Ru, in metal powder form, while the order is Ru, Rh > Pt >> Pd in the form of their metal salts. The decrease in reaction order with non-noble metals such as Co and Ni in the form of their metal salt is Ru> Rh > Pt > Co > Ni > Os > Ir > Fe >> Pd. Co is more active that Ni in all the forms except in the Raney form in which activity of Raney Ni is close to Raney Co. With respect to the form of the catalyst, the activity decreases in the order metal salts > nanoclusters > alloys of metal borides > metal borides > Raney type metal alloys > Raney type metal > metal powder. The activity of a catalyst is also related to the particle size and dispersion degree. Small particle size and good dispersion promote large catalyst contact with the NaBH 4 solution, which is very important for increasing the reaction rate and reducing the catalyst loading. The structure and catalytic activity of the produced catalyst are generally sensitive to its preparation conditions. The conditions include type of reducing agent, ph of the reduction medium during the preparation of the catalyst, type of precursor, phase of the precursor, ratio of reducing ion/ metal ion and heat treatment. Park et al. [29] compared the use of H 2, sodium borohydride and combination of H 2 and sodium borohydride as reducing agents, and showed that the type of reducing agent can significantly influence the hydrogen generation rate. They found better 32

33 hydrogen production rates when the catalyst was prepared using the combination of H 2 and sodium borohydride as reducing agent. Various researchers [30,31,32] studied the effects of ph of the reduction medium during the preparation of the catalysts. These studies revealed that the catalytic activity was strongly dependent on the ph of the reduction medium during its preparation. In general, the activity decreases in the order: catalyst prepared in acidic medium > catalyst prepared in neutral medium > catalyst prepared in alkaline medium. The effect of type of precursor was studied by various researchers. Liu et al. [33] and Akdim et al. [34] compared the activities of solid cobalt (II) and liquid cobalt (II) salts and found higher hydrogen generation rate in the case of solid cobalt (II) salt. They explained that this effect was related with solid dissolution that led to decrease of ph and implied a lower amount of water and, consequently, a larger sodium borohydride concentration. Jeong et al. [35] evaluated the effect of the borohydride ion/metal ion ratio. The Co- B catalysts were prepared with different mole ratios of borohydride ion to metal ion (NaBH 4 /Co 2+ : 0.67, 1.5, 3.0). They found that metallic Co was formed at higher NaBH 4 /Co 2+ (= 3) ratios due to the high reductive properties of NaBH 4, and the activity of hydrogen generation from alkali stabilized aqueous NaBH 4 solution decreased. They found for this catalyst that the maximum hydrogen generation is achieved with NaBH 4 /Co 2+ equal to 1.5. Heat treatment has a strong effect on the activity of the catalysts for hydrogen generation from the hydrolysis of NaBH 4. Many studies has been made in this area and clearly showed that heat treatment result in excellent hydrogen generation performance of the catalysts. The studies also revealed that an optimum heat treatment temperature exists for the catalyst, which varies for different catalysts. The performance enhancement occurs due to compositional changes, phase transition: from an amorphous phase to a mixture of crystal and metal or, in pure metal, change in BET surface area, change in the particle size which increases and formation of agglomerates with increase in heat treatment temperature. In the case of supported catalyst, the heat treatment temperature can also enhance the adhesion between metal catalysts and the substrate and thereby increase the durability of the catalyst which is also a crucial issue in the development of the catalyst [36]. 33

34 One important approach for increase of catalyst activity is the use of material with high surface areas as supports for the catalyst. The use of a support for catalyst also solves problems such as clogging and fluidization in the use of powder structures in the application of successive hydrogen generators. A current research focus is also on developing supported catalyst to increase the rates and yields of the borohydride hydrolysis reaction. Effect of NaBH 4 concentration The effect of NaBH 4 concentration on the kinetics of sodium borohydride hydrolysis has been studied by various researchers using different catalysts. In general, both noble and non-noble catalysts, either in powder or supported form, exhibited an increase in hydrogen generation rate, which reaches a maximum and then decreases. Amendola et al. [37], explained this effect due to the fact that at higher NaBH 4 concentration, the viscosity of the solution increases due to increased of reaction byproducts. This increases the mass transport resistance and blocks contact of NaBH 4 and H 2 O to catalyst sites, thereby affecting the hydrogen generation rate. Effect of NaOH concentration NaOH was generally added to NaBH 4 solution to make the solutions basic and to prevent NaBH 4 self hydrolysis. Various authors have investigated the effects of NaOH concentration on hydrogen generation and very different, even conflicting, results have been found. In noble metal catalysts, the hydrogen generation rate decreases with increase in NaOH concentration. According to Amendola et al. [31, 37], this is probably due to lowered solubility of the reaction product and hence reduced availability of water at higher NaOH concentration. The effects of NaOH on non-noble metals were dependent on the form of the catalyst. The results showed that metal salts follow the same trend as noble metals, in which the hydrogen generation rate decreases with increase in NaOH concentration. However, metal borides follow a trend in which hydrogen generation rate increase with increase in NaOH concentration. Ni and Co borides alloys showed an entirely different behaviour; the hydrogen generation rate displayed a maximum value and then decreased with further increase in NaOH concentration. For nickel metal powder, it has been found that for a concentration range between 10 % - 30 % of NaOH the hydrogen production rate remained constant with increasing 34

35 NaOH concentration. For increased NaOH concentrations, the hydrogen generation rate decreased with further increase of NaOH. All the studies clearly show that dependence of the hydrogen generation rate of on the concentration of NaOH is complex and not only depends on the type of catalyst but also on the form of catalyst. This demands a better understanding of the kinetic mechanism. Kinetic models Understanding the mechanism of the metal catalyzed hydrolysis is of crucial importance to the design of reactors and the selection process for optimal hydrogen production. The hydrogen generation from an alkaline NaBH 4 solution has been extensively investigated and four kinetic models have been proposed, namely zeroorder, first-order, Langmuir Hinshelwood and Michaelis-Menten models. Zero-order model For a reactor with a volume V and a catalyst mass m cat, the zero-order kinetics described the reaction rate (r A ) of NaBH 4 hydrolysis as: -r A = -dc A /dt = m cat k 0 /V (17) where C A is the NaBH 4 concentration and k 0 is the zero-order constant expressed in mol/(m cat.time). Integrating equation 17 and noting that C A can not be negative, gives equation (18) C A0 - C A = (m cat k 0 /V).t for t < (C A0.V) / (m cat k 0 ) (18) where C A0 is the initial concentration of NaBH 4. Therefore, plotting C A0 - C A as a function of time should give a straight line, and the slope of the line can be used to calculate the zero-order rate constant, k 0. First-order model The first-order model establishes that for a reactor with a volume V and a catalyst mass m cat, the rate of NaBH 4 hydrolysis is: 35

36 -r A = -dc A /dt = (m cat k 1 C A )/V (19) where C A is the NaBH 4 concentration and k 1 is the first-order constant expressed in mol/(m cat.time). Integrating equation 19 gives: - CA0 CA (dc A / C A ) = [(m cat k 1 )/V]- 0 t dt (20) ln (C A0 / C A ) = [(m cat k 1 )/V].t (21) where C A0 is the initial concentration of NaBH 4. Therefore, plotting ln (C A0 / C A ) as a function of time should give a straight line, and the slope of the line can be used to calculate the first-order rate constant k 1. Langmuir Hinshelwood model This model proposes an overall reaction constituted by two steps. The first step is the absorption of the BH 4 - on the surface of the catalyst that follows the reaction (22). BH M BH 4 -.M (22) The surface coverage θ A of the absorbed species on catalyst surface is given by the Langmuir adsorption isotherm (23). θ A = KC A / (1+ K C A ) (23) The second step consists of the reaction of the absorbed species to form hydrogen as shown in the reaction (24). BH - 4.M + 2H 2 O BO - 2.M + 4H 2 (24) With an excess of water, the reaction is assumed to be proportional to the quantity of adsorbed NaBH 4 molecules θ A. Then, for a reactor with volume V, and a catalyst mass m cat, the reaction rate per unit of volume can be written as: -r A = (m cat k L θ A )/V (25) 36

37 with k L expressed in units of mol/(m cat.time). Combination of equations 23 and 25 gives: -r A = - dc A / dt = (m cat /V) k L [(KC A / (1+ K C A )] (26) where K is the adsorption coefficient that normally varies with temperature according to: K= Aexp(- H ads /RT) (27) Separating and integrating equation 26 gives, - CA0 CA [(1+ KC A ) /KC A ] dc A = 0 t [(m cat k L )/V]dt (28) (C A0 -C A ) + 1/K ln(c A0 /C A ) = [(m cat k L )/V].t (29) Plotting (C A0 -C A ) + 1/K ln(c A0 /C A ) as a function of time should give a straight line where the slope of this line (k slope ) used to calculate the k L as follows: k slope = [(m cat k L )/V (30) Observation of equation 29 shows that left side of this equation contains the left side of equation 18 of the zero-order model, the left side of 21 of the first-order model and the coefficient K. Therefore, this model can be considered to be a combination of the zero- and first-order models, and it is the coefficient K that determines which part is predominant. From equation 27 it is possible to establish that when temperature increases K decreases, because the enthalpy is almost always negative. Consequently, at high temperatures, the term KC A is less than 1 and the reaction kinetics are predominantly first-order. At lower T, K becomes larger, and KC A is greater than one, so the reaction becomes zero-order. Michaelis-Menten model This model proposes an overall reaction composed also of two steps: First, the reactant interacts with catalyst and forms an intermediate; in a second step, the intermediate decomposes to produce the reaction products and regenerates the catalyst 37

38 Following this model in NaBH 4 hydrolysis, the metal catalyst (M) reacts with BH - 4, with a rate constant k 1, and forms an intermediate metal borohydride complex (MBH 4 ). Then, the MBH 4 can reversibly dissociate to BH - 4 and M, with a reaction rate of k -1, or react with water to give B(OH) - 4, H 2 and regenerate the metal catalyst at a reaction rate k 2 (equation 31) BH M MBH 4 B(OH) M + 4H 2 (31) In the steady state, the concentration of the intermediate stays the same even if the concentrations of reactants and products change. This occurs when the rates of formation and breakdown of the MBH 4 complex are equal. Then we have d[mbh 4 ]/dt = k 1 [BH 4 - ][M] - k -1 [MBH 4 ]-k 2 [MBH 4 ] = 0 (32) Rearranging equation 32 gives, [MBH 4 ] = {k 1 [BH 4 - ][M]}/ (k -1 + k 2 ) = [BH 4 - ][M]}/ k M (33) where k M is the Michaelis constant, defined as k M = (k -1 + k 2 ) / k 1 (34) The concentration of unoccupied metal sites [M] is equal to the total metal site concentration [M 0 ] minus the concentration of the MBH 4 complex. Substituting equation 35 in equation 33 gives [M] = [M 0 ] [MBH 4 ] (35) [MBH 4 ] = [BH 4 - ][M 0 ]}/ k M [BH 4 - ] (36) According to equation 32, the rate of catalyst hydrolysis of NaBH 4 (r) is given by r = -d[bh - 4 ]/dt = d[h2]/4dt = k 2 [MBH 4 ] (37) By substituting the expression for [MBH 4 ], equation 36, into equation 37 and assuming that sodium borohydride is totally ionized ([BH - 4 ] = [NaBH 4 ], the following is obtained 38

39 r = k 2 [NaBH 4 ][M 0 ]}/ k M [NaBH 4 ] (38) According to this model, the absorption of BH - 4 on the active sites of the catalyst and generation of the intermediate MBH 4 complex are the critical steps in the catalytic hydrolysis of sodium borohydride. When high concentrations of NaBH 4 are used, the active sites of the catalyst are fully occupied and this model predicts that hydrolysis reactions follow zero-order kinetics. In low NaBH 4 concentrations, the model predicts first-order kinetics. The M-M model can therefore be considered to be a combination of zero-order kinetics and first-order kinetics in terms of NaBH 4 concentration. Using a Co-B catalyst, Wang at el. [38] performed a combined experimental study and proposed the Michaelis-Menten (M-M) model for the analysis of the hydrolysis kinetics. Their studies demonstrated that the hydrolysis reaction is of first-order kinetics with respect to the catalyst, and the kinetics order changes from zero- and first-order to NaBH 4 depending on the concentration of sodium borohydride. However, they indicated that this critical value can vary with concentration of NaOH. They established the concentration of approximately 1.5 wt.% as the critical value for distinguishing between first- and zero-order kinetics. Power-law model The previous models do not consider the influence of the stabilizer (NaOH) on hydrolysis, and this can explain some differences between the experimental values and the theoretical values predicted by the model. By the power-law model, the rate of catalytic hydrolysis of NaBH 4 can be determined by equation 39) r NaBH4 = -(dn NaBH4 /w.dt) = k.[nabh 4 ] x [NaOH] y [H 2 O] z (39) k = A 0.exp(E a /RT) (40) where N NaBH4 is the moles of sodium borohydride converted, w, y and z are the reaction order with respect to NaBH 4, NaOH and H 2 O, respectively, k is the rate constant, A 0 is the pre-exponential factor and E a is the activation energy. In the case of hydrolysis rates measured at constant NaBH 4, NaOH and H 2 O concentrations, equation 39 is simplified and can be written as: 39

40 r NaBH4 = k. const. (41) In this case, because the concentrations are constants, plotting the ln of reaction rate versus ln(1/t), the term E a /R is given by the slope Thermophysical properties of sodium borohydride hydrolysis Thermodynamic properties of hydrated borates play a very important role in understanding the sodium borohydride hydrolysis. The determination of these properties is of crucial importance for scientific research and industrial applications of this technology. The Gibbs free energy and the heat of formation determine whether a reaction is thermodynamically favoured and how much energy must be added or removed from the reaction. Thermodynamics properties of the compound involved in the reaction are necessary to estimate the heat of reaction. The amount of hydrogen produced by hydrolysis of sodium borohydride is independent of the water excess, because the excess water in the reaction appears as the hydration factor in borate (equation 6). The hydrated forms of borate that have been detected include NaBO 2.2H 2 O; NaBO 2.4H 2 O and Na 2 BO 7.5H 2 O. A. Li et al [39] developed general equations based on a group contribution method to estimate the thermodynamics properties of the different hydrated borates. They established that the Gibbs free energy of formation ( G 0 f ) and the enthalpy of formation ( H 0 f ) of the hydrated borates phase can be estimated by considering the contributions of the cations in aqueous solution (M + aq) ( H f 0 ) (borate) = a H f 0 (M v+ (aq)) + H f 0 (B x O y (OH) z ) + nh f 0 (H 2 O) (42) ( G f 0 ) (borate) = a G f 0 (M v+ (aq)) + G f 0 (B x O y (OH )z ) + ng f 0 (H 2 O) (43) where, x, y, z, a e n are the stoichiometric coefficients of boron, oxygen, hydroxyl, cation, and structural water in the borate, and v + is the valence of the metal cation (M). 40

41 Using equations 42 and 43 and the properties of the compound involved in the reaction, obtained from the literature, the heat of formation and the Gibbs free energy can be calculated for borates with different degrees of hydration. Marrero et al. [40] used this method to reveal that the Gibbs free energy and the heat of formation decrease with increase of the hydration factor; therefore, the formation of highly hydrated borates is more thermodynamically favoured. This factor became a drawback for this technology because it decreases the gravimetric efficiency. In the same way, increasing the hydration factor gives a more exothermic reaction Metal catalysts are advantageous over acids because they can be recovered and reused. They can be used in an on/off fashion by inserting or removing the catalyst into/from solution Sodium borohydride recycling At least two different processes have been intensively studied in the past few years for NaBH 4 recycling, namely the Mechano-Chemical Process and the Dynamic Hydriding/Dehydriding Process [41,42]. In these processes the by-product of the sodium borohydride hydrolysis, sodium metaborate (NaBO 2 ), is used in its anhydrous form as the starting material to produce NaBH 4. Mechano-Chemical Process MgH 2 hydride plays an important role in the mechano-chemical process where it is used as an H-donor and O-acceptor. In the mechano-chemical process, which applies MgH 2 in a high-density ball mill and is operated under ambient conditions, the mechanical force is converted to chemical energy as shown in Equation 44. NaBO 2 + 2MgH 2 NaBH 4 + 2MgO G 0 (298 K) = 270 [kj (mol NaBH 4 ) 1 ] (44) Dynamic Hydriding/Dehydriding Process In the dynamic hydriding/dehydriding process, which makes use of the transitional state between the hydriding and dehydriding states, thermal energy is rapidly applied 41

42 to a mixture of NaBO 2 and Mg under a hydrogen atmosphere in order to generate a very reactive state of hydrogen, namely, protide: H that combines transitionally with Mg as Mg 2 (H ). NaBO 2 + 2H 2 + 2Mg NaBH 4 + 2MgO G 0 (298 K) = 342 kj (45) The reaction given in equation 45 is initiated at the outer surface of NaBO 2 and Mg particles. Hydrogen is first converted to protide at the outer surface of Mg under lower temperature conditions in the hydriding regions. Then the protide reacts with NaBO 2 to form NaBH 4 under higher temperature conditions in the dehydriding regions where NaBO 2 releases O 2 which is transferred to the surface of Mg to form MgO. The oxidation of Mg particles spreads from the surface toward the centre of a particle and the rate-controlling factor is greatly dependent on the depth of the MgO layer and the particle size. The smaller the particle size, and the larger the specific surface area, the higher the reaction rate and the yield. The conversion rate (recovery rate) of the mechano-chemical process approaches closely to 100%. The dynamic hydriding/dehydriding process reaches a maximum of 70%. The conversion rate can be improved considerably by reducing the size of the Mg particles. 42

43 2. EXPERIMENTAL 2.1 Introduction materials Materials used in all the experimental studies of the sodium borohydride hydrolysis are presented in this section. The material described includes reactant solutions for kinetics studies and the reactants for catalyst synthesis. Descriptions of the material used in the supported catalyst are also reported. 2.2 Materials The following chemicals and materials were used for catalyst synthesis and support Sodium borohydride Commercial, anhydrous sodium borohydride (NaBH 4 ) powder, provided by ROHM and HAAS, was used throughout the study. In alkaline hydrolysis, NaBH 4 was used in solution stabilized with commercial sodium hydroxide, supplied by Pronalab. Deionized water was used in the preparation of all the aqueous solutions.. Nickel Chloride-6-hydrate Extra pure (97% purity) Nickel Chloride-6-hydrate (NiCl 2.6H 2 O), provided by Riedel-de Haën, was used as the source of nickel ions for the nickel-based catalyst production. Ruthenium (III) chloride hydrate Ruthenium (III) chloride hydrate (Cl 3 Ru.xH 2 O), with 35 40% of ruthenium, was provided by ACROS; this was used as the source of ruthenium ions for the nickelbased catalyst production. Nickel foam For preparation of the supported catalyst, a nickel foam, provided by INCO, was used. The morphological characteristics include a density of 380 g.m -2, thickness of 1.6 mm and PPI (pores per inch) of 110. Figure 8 shows a scanning electron micrograph of the as-received foam that was employed in this study 43

44 Figure 8. Scanning electron micrograph of the as-received nickel foam 2.2 Methods Characterization techniques Scanning Electron Microscopy The samples (which) were observed by scanning electron microscopy (SEM). This method is based on the principle of the interaction between electron and matter. An electron gun produces a high energetic and very narrow beam of electrons that is accelerated by an electric field. The electrons will interact with the matter, producing secondary electrons, back-scattered electrons and characteristics X-Rays, which can be detected by specific detectors. The SEM images are particularly useful to obtain a three-dimensional appearance of the surface of the materials examined, to determine the uniformity of the sample, or elemental composition. In this work, SEM analysis was used to determine the morphology and elemental composition of the catalyst. The equipment used was a Philips XL30 FEG coupled with an energy dispersive spectroscopy (EDS) unit operating at 15 kv in a low vacuum mode (Figure 9). Powder catalyst samples were prepared by dispersion on a double size adhesive carbon tape. Supported catalysts were analyzed as-produced. The samples were previously coated with an ultrathin coating of gold, deposited on the sample by a low vacuum sputter coating using a Fine Coat Ion Sputter JFC 1100 operating at 1.2 kv and 10 ma. For high resolution SEM analysis, a Zeiss Ultra 55 FEGSEM, with in-lens backscattered electron (BSE) detection and EDX analysis facilities, was employed. 44

45 Generally, for BSE observation, a low kv (typically 1.5 kv) was used for high resolution near-surface imaging. Conversely, for EDX analysis, an acceleration voltage of 15 kv was used to excite characteristic X-rays. Powder catalyst samples were prepared by placing the powder on a carbon-coated copper grids. Supported catalysts were examined after diamond cutting a section with a diamond knife in an ultramicrotome (Leica Ultracut). In both cases, the prepared specimens were generally examined without coating; where charging arose, the samples were coated with an ultrathin coating of carbon, deposited on the sample in a Edwards (E306) low vacuum sputter coating unit operated at 30 kv and 60 ma. Figure 9. Scanning electron microscope Phillips, Model XL 30 FEG used in this work. Transmission Electron Microscopy For transmission electron microscopy (TEM) a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through. An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or detected by a sensor such as a CCD camera. 45

46 In this work TEM was made using a Tecnai F30 field emission gun instrument was employed; the microscope was operated at an accelerating voltage of 300 kv. For morphologic analysis the TEM was operated in the bright field and high angle angular dark field modes.. Elemental analysis was performed with an energy dispersive system (EDAX instruments) attached to the microscope. Samples were prepared by placing the powder on carbon-coated copper grids. X Ray Diffraction XRD is a non-destructive analytical technique, which reveals information about the crystallographic structure, chemical composition, and physical properties of materials and thin films. This technique is based on observing the scattered intensity of an X-ray beam impinging on a sample as a function of incident and scattered angles, polarization and wavelength or energy. The wave nature of the X-rays means that they are diffracted by the lattice of the crystal to give a unique pattern of peaks of 'reflections' at differing angles and of different intensities, just as light can be diffracted by a grating of suitably spaced lines. The diffracted beams from atoms in successive planes cancel unless they are in phase, and the condition for this is given by the Bragg relationship (equation 46). nλ = 2 d sin θ (46) where, λ is the wavelength of the X-rays d is the distance between different planes of atoms in the crystal lattice. θ is the angle of diffraction. The X-ray detector moves around the sample and measures the intensity of these peaks and the position of these peaks [diffraction angle 2θ]. The highest peak is defined as the 100% * peak and the intensities of all the other peaks are measured as a percentage of the 100% peak. 46

47 In this work the XRD analysis was carried out using a Rigaku Geigerflex X-Ray diffractometer, employing Cu-Kα radiation. The diffracted radiations were measured in a range of 2θ = 3 to 123, operating at 45 kw/20 ma. The usual procedure included placing the sample holder on a glass microscope slide and holding it in place with sticky tape. Recesses were filled with the ground and sieved sample and the holder was tapped lightly to ensure the corners were filled; a second glass microscope slide was drawn over the recess to remove excess material. A third slide was placed on the sample surface and held in place with sticky tape. The whole unit was turned over such that the first glass slide was removed. The sample was inserted into the X-ray instrument. X-ray Photoelectron Spectroscopy X-ray Photoelectron Spectroscopy (XPS) utilizes X-rays (with a photon energy of ev) to examine the core-level kinetic energy distribution of the emitted photoelectrons to study the composition and electronic states of the surface regions of a sample. In this technique, the photon is absorbed by an atom in a molecule or solid, leading to ionization and the emission of a core (inner shell) electron. For each and every element, there is a characteristic binding energy associated with each core atomic orbital i.e. each element will give rise to a characteristic set of peaks in the photoelectron spectrum at kinetic energies determined by the photon energy and the respective binding energies. The presence of peaks at particular energies therefore indicates the presence of a specific element in the sample under study. Furthermore, the intensity of the peaks is related to the concentration of the element within the sampled region. In this work, X-ray photoelectron spectroscopy (XPS) of the catalyst before use was made on a VG Scientific ESCALAB 200 A spectrometer using Mg Kα ( ev) as the radiation source. The photoelectrons were analyzed at a takeoff angle of 0º. A powder sample was made into a pellet of 12 mm diameter, using MPa pressure. 47

48 N 2 Physisorption The determination of the surface area (S BET ) of the catalyst, by N 2 physisorption was undertaken at 77K, using a Quantachrome Instruments Nova 4200e apparatus. Prior to the analysis, the samples (0,127 g) were degassed at 160ºC for 3 h. Pore size distributions were obtained from the desorption branch of the isotherms, using the Barrett, Joyner and Halenda (BHJ) method. The micropore volumes and mesopore surface areas were determined by the t-method Experimental setup Setup 1 Figure 10 presents the experimental setup used in the kinetics study of sodium borohydride self-hydrolysis (water and methanol based). The system consists of a glass reactor, a water gas trap and a volumetric measurement system. The reactor consisted of a tubular glass, with top modified to allow the connection of a thermocouple, a ph meter and a mechanical system for fast injection of the solution. For temperature control, the reactor was partially immersed in a thermostatically-controlled water bath. The temperature of the bath was controlled by a Memmert heater. The reactor was connected to a cool water trap, where the produced gas is washed in order to prevent methanol and water vapour entering the measuring system. The volume of hydrogen released was in this way detected by a water displacement method, using an inverted burette of 2000 cm 3 capacity. Setup 2 The typical experimental setup for powder catalyst hydrolysis studies are show in Figure 11. The reactor chamber consisted of a tubular glass tube, of diameter of 25 mm and height of 200 mm. The reactor was partially immersed in a water bath for temperature control using a bath from Memmert, Schwabach, Germany. The hydrogen produced was measured by the water displacement method, using an inverted burette of 2000 cm 3 capacity. 48

49 Setup 3 The typical setup used for electrochemical studies of the support catalyst is presented in Figure 12. The setup consisted of the reactor chamber, potential measurement, a computer data acquisition and a hydrogen measurement system. When temperature control was required, a thermostatic water bath was used and the reactor was partially immersed in the bath. The control of temperature was made using a Memmert bath. Simultaneous with the hydrogen measurement, the potential was measured using a data logger from Fluke, model Hydra Series III and the values were recorded using computer data acquisition. The Ni-foam supported catalyst was the working electrode and an Ag/AgCl, KCl sat. reference electrode. The hydrogen volumetric measuring system was similar to that described previously. Figure 10. Experimental setup used in the self-hydrolysis studies. 49

50 Figure 11. Experimental setup used in powder catalyst studies Setup 4 Figure 13 shows the typical experimental setup used for the kinetic experiments with the electrodeposited support catalyst. The basic constituents of this setup include a sodium borohydride storage tank, peristaltic pump, reactor chamber, spent fuel chamber and a volumetric measuring device. The hydrogen storage tank is a glass vessel with 500 cm 3 capacity and with two entries, one on the top where the fuel exits the reactor, and the other at the bottom where the liquid reaction products came to the storage tank. When needed, the storage tank was immersed in a temperature controlled water bath provided by Memmert. The reactor chamber consisted of a tubular glass tube with a diameter of 25 mm and a height of 120 mm. The spent fuel chamber is a normal glass vessel, where the reaction products are split into hydrogen (going to the measurement device) and liquid products (returned to the storage tank). The hydrogen volumetric measuring system is similar to the described in the previous setups. The circulation of the fuel is assured by the peristaltic pump provided by Watson Marlow, model

51 Figure 12. Experimental setup used in electrochemical characterization of sodium borohydride hydrolysis. Figure 13. Experimental setup used in kinetics studies of the electrodeposited support catalyst. 51

52 2.3 Experimental procedure Self-hydrolysis Water-based self-hydrolysis It is well known that sodium borohydride undergoes self-hydrolysis when dissolved in aqueous solutions. In this work, we will study how the NaBH 4 concentration and temperature influence the kinetics of hydrolysis. In order to study the effect of sodium borohydride concentration on the hydrogen production rate, a set of experiments was made for an extensive range of concentrations, including 2, 5, 7, 10, 15 and 20 wt.%. The reaction hydrolysis was carried out using setup 1 and, in each experiment, a specific mass of sodium borohydride was stored at the bottom of the reactor chamber; then a volume of 25 cm 3 of water was rapidly injected. Immediately hydrogen production was observed in the volumetric measurement device and the value was recorded. Simultaneously, the ph of the solution was measured using a ph meter, the values were recorded as a function of reaction time A constant temperature of 318 K for all the experiments was achieved by a thermostated bath where the reactor was immersed. The temperature effect on self-hydrolysis of sodium borohydride was also studied by performing a series of experiments at two different temperatures, 318 and 338 K. For these experiments a concentration 10 wt.% of sodium borohydride was used in a volume of 25 cm 3 of water. The setup used in this study was setup 1 (Figure 3) and, for both experiments, 2.5 g of sodium borohydride was placed in the reactor chamber and then closed. Using a syringe, 25 cm 3 of water was rapidly injected. The reactor was immersed in a water bath at the desired temperature. Methanol-based self-hydrolysis Is well known that sodium borohydride is reactive in low molecular weight alcohols such as methanol, ethanol and ethylene glycol. Methanol has the highest reactivity of all; further its low weight makes methanol a good alternative to water in sodium borohydride hydrolysis. 52

53 In this section, the procedure for was intensive analysis of the kinetics of the reaction of sodium borohydride with methanol is given. For this study, the influence of the ratio of water/methanol, the sodium borohydride concentration and the effect of temperature was determined to gain insight into the methanolysis reaction. All the experiments were performed using methanol (99,8%) from Fluka in the absence of stabilizers and catalysts. The experimental setup used was setup 1 (Figure. 3), with a tubular glass reactor as the reactor chamber at ambient pressure. A ph/ion meter, model 25 from Denver Instruments, was used for ph measurement. In order to prevent methanol vapour, produced as a result of the high reaction temperature, influencing the hydrogen volume measured, all experiments used a cold water trap to catch the methanol vapour. To study the effect of different methanol/water ratios on the hydrolysis reaction rate, a set of experiments was made with change of the quantity of methanol used. The experimental procedure consisted of preparation of 10 cm 3 of a water/methanol mixture with methanol concentrations of 100, 90, 50, 10 and 0 v/v %. 1 g of sodium borohydride was stored at the bottom of the reactor chamber. The reactor was them immersed in a thermostatic bath at a temperature of 318 K. Then, the water/methanol solution was rapidly injected and the values of hydrogen produced were measure on the volumetric measuring device with time. At this temperature, the gas produced contained a large amount of methanol gas. In order to evaluate how the ph changes during the sodium borohydride hydrolysis reactions with and without methanol, two experiments were made with the values of ph was measured simultaneously with the hydrogen volume. For these experiments, 1 g of sodium borohydride was placed in the reactor and 20 cm 3 of solution, 100% water in one case and methanol with no added water, was injected and the values of ph and hydrogen produced recorded with time. The temperature used in both cases was 318 K. To study the effect of sodium borohydride concentration on the hydrogen generation rate of the sodium borohydride methanolysis, a series of experiments was made with different amounts of sodium borohydride, namely 2, 5, 7.5, 10, 15 and 20 wt.%. For this a specific quantity of sodium borohydride was placed in the reactor chamber that was immersed in the bath at a temperature of 318 K. 10 cm 3 of methanol was then injected rapidly and the volume of hydrogen was measured with time. 53

54 For investigation of the effect of reaction temperature on the kinetics of sodium borohydride methanolysis, a set of experiments was carried out over an extensive range of temperatures of: 283, 288, 293, 309, 378 and 328 K. The previous included temperatures as low as 283 due to expected higher reaction rates when compared with water. In this study, 1 g of sodium borohydride was placed in the reactor bottom which was placed in a thermostatic bath at the selected temperature. Then, 20 cm 3 of methanol were injected in the reactor and the hydrogen produced was measured over time at each temperature Catalyzed hydrolysis with powder catalyst Catalyst Synthesis and Characterization Since the catalysts available in the literature are mostly based on noble metals, in this work we decided to synthesize a catalyst that may be active for sodium borohydride hydrolysis, but based on a low cost material. Available methods in the laboratory that might be applied to catalyst synthesis include sol-gel, ball milling and wet chemistry methods. All these methods could provide powder catalysts with relatively high surface area. An option for catalyst synthesis that uses wet chemistry was adopted due to the ease of preparation and requirement for relatively unsophisticated equipment, and the possibility of obtaining yields near to 100%. Wet chemistry gives also the opportunity of using the same synthesis method when catalyst fixation is required. The selected method included a reduction of nickel salts using sodium borohydride as a reducing agent. The procedure to obtain the catalyst in the form of powder is now described. Solutions of NiCl 2.6H 2 O and RuCl 3.xH 2 O, dissolved in ultrapure water, were used as precursors. The nickel/ruthenium weight ratio was controlled by using precise quantities of each precursor. As a reducing agent, a solution of 10 wt.% of sodium borohydride, stabilized with 3 wt.% of sodium hydroxide, was used. The synthesis procedure consisted of mixing the precursors in ultrapure water. Under magnetic stirring, the reducing solution was dropped slowly into the salt solution. The reduction process was finished when the solution had no colour. Then, filtration 54

55 was made to separate the catalyst. Finally, the reaction product was washed several times with hot water and the final product was dried for several hours, in air. The morphology and elemental composition of the catalyst was examined by SEM coupled with a EDS, TEM, XRD and FTIR. The textural properties were made by N 2 absorption. Effect of temperature and activation energy For study of the effect of temperature on sodium borohydride catalyzed hydrolysis, a set of kinetic experiments was made at six different temperatures, 297, 303, 308, 318, 328 and 333 K. The temperatures were chosen over a temperature interval from room temperature to 333 K in order to cover the range of temperatures that might develop in utilizations without temperature control; this may be of interest due to the fact that the reaction is exothermic. For each experiment a fixed amount of powder catalyst (50 mg) was stored at the bottom of the reactor chamber that was immersed in a water bath at the selected temperature. A solution with 10 wt.% of NaBH 4 stabilized with 10 wt.% of NaOH was prepared. All the experiments were made by injection of 10 cm 3 of this solution into the reactor chamber. After an induction time, hydrogen was produced and the volume was measured with time. The experimental setup used in this study was setup 2, described above. Influence of catalyst concentration In order to select the optimum catalyst mass to be used in the characterization of the catalyst activity and kinetics of the hydrolysis reaction, experiments were conducted at 318 K in a total solution volume of 10 cm 3 for a 10 wt.% of sodium borohydride concentration. The experimental setup used in this study was similar to the used in the previous section. Samples of the Ni-based powder catalyst, with weights of 0.020, 0.035, 0.050, 0.075and g, were placed at the bottom of the glass reactor (pre-heated). 10 cm 3 sodium borohydride solution was prepared with 10 wt.% and was added to reactor chamber before connecting to the measuring system using an inverted burette at ambient pressure. In order to record the rate of hydrogen production more effectively, the time was recorded for every 100 cm 3 of hydrogen produced. 55

56 Effect of NaBH 4 concentration In order To explore the effect of sodium borohydride concentration on the hydrogen generation rate of the catalytic hydrolysis of sodium borohydride, a series of experiments was made at different concentrations NaBH 4. In this study,10 cm 3 of solution with 1, 2, 5, 10, 15, 20, 30 wt.% of NaBH 4 stabilized with 10wt.% of NaOH, was used. The powder catalyst loading was the same for all samples, 50 mg, and the temperature was kept constant at 318 K. The experimental procedure and setup used were similar to those used in previous sections. Influence water/borohydride ratio In the above section, the influence of borohydride concentration was studied by changing the quantity of sodium borohydride, keeping constant the water content. In order to complement the effect of concentration, a study of the influence of the water/borohydride ratio was made by changing the water loads using a fixed amount of NaBH 4. The values of the water content were selected based on the solubility of sodium borohydride, solubility of NaOH, solubility of Na 2 BO 2 and water necessary for the hydrolysis reaction. The water content is referred as the water/borohydride molar ratio. Following this, kinetics of NaBH 4 hydrolysis was studied with water/borohydride ratios of 8, 16, 42, 63 and 84, using a constant amount of sodium borohydride, 1g. The experimental procedure was similar to that in the previous section and a temperature of 318 K was used in all the experiments. Effect of stabilizer type The concentration and type of the stabilizer is an important aspect in the catalyzed hydrolysis of sodium borohydride because it changes the chemical and physical properties of the sodium borohydride solution. Therefore, to examine the effect of the cation of the stabilizer, a comparative study was made using KOH and NaOH, as well as mixtures of both stabilizers. Experiments were carried out using 1 g sodium borohydride at 318 K and a solution volume of 10 cm 3. The total loading of the stabilizer was 1g, with the following NaOH/KOH proportions 100 wt.% NaOH, 75 wt.% NaOH + 25 wt.% KOH, 25 wt.% 56

57 NaOH + 75 wt.% KOH and 100 wt.% KOH. The experimental procedure was analogous to that described in the above section Catalyzed hydrolysis with supported catalyst Catalyst Synthesis and Characterization Supported catalysts have great advantages in hydrogen production systems due to the easy removal of the working solution; facilitating, the recovery and the re-utilization of the catalyst. The Literature reports on materials with good properties for support catalysts. Most studies include zeolites, alumina and silica-alumina, metallic foams and carbon compounds. All these materials improve the superficial area of the catalyst with increase of the activity. Ceramic compounds do not have the chemical strength necessary to maintain the structural stability with the vigorously evolution of hydrogen. For these reasons, such materials were not considered in this work. Metal foams have good structural stability and electrical conductivity, and allow the use of electrodeposition in order to generate a supported catalyst. In this work, nickel foam was used because it is relatively cheap and easy to work with. Further, when the foam is oxidized it has catalytic properties for sodium borohydride hydrolysis. Two techniques were used for the preparation of the catalyst: the Doctor Blade Technique and the electrodeposition method. The procedure for each is given separately below. Doctor Blade technique A modification of the Doctor Blade technique [43] was used to produce supported catalysts with high surface area and activity. Because this technique does not use complex materials, it is a relatively cheap way to produce good support catalysts. In this technique, a thin film of a prepared slurry containing the powder catalyst is applied to the substrate, in the present case a nickel foam. The powder catalyst was prepared by a wet chemical method, as explained in section A slurry with this powder catalyst, Nafion and isopropyl alcohol was prepared and was then uniformly applied on the foam with successive loadings and 10 minutes drying between them. When all the catalyst was deposited, a heat treatment was 57

58 made at 423 K. The morphological characteristics of the nickel foam employed are described in section 2.1. Electrodeposition method In the electrodeposition method, a current density is applied to the foam immersed in an electrolyte solution that contains the salt of the compound that is to be deposited. This method allows the deposition of a fine layer of catalyst in all the supported materials A 7 x 10 cm nickel foam, previously cleaned, was used as a substrate for electrodeposition. A potential of 6 V was applied to the working electrode in a solution of 3 wt.% NaOH as electrolyte for 10 min. A nickel foam was used as a counter electrode. After this pre-treatment, the nickel foam was washed in pure water several times. Using this treatment, improved adhesion between the electrodeposits and the substrates was anticipated. The ruthenium and nickel were electrodeposited using a two-electrode system, with the nickel foam as the working electrode onto which the deposition is to be made; a simple nickel foam was used as counter electrode. Three different catalysts, named EDRu82.5, EDNiRu30.8 and EDRu29.2, were prepared changing the experimental condition. Table 1 shows the composition of the three solutions used. Table 1 Composition of the solution used for the electrodeposition. Catalyst NH 3 KOH Cl 3 Ru.xH 2 O NiCl 2.6H 2 O EDRu mg - EDNiRu M 5 M 100 mg 379 mg EDRu mg - Electrodeposition was made with a dc current applied using a dc power supply from a ISO-TECH, model IPS-3610D. In each case, electrodeposition was made at ambient temperature (~393 K), using a current density of ~0.05 A.cm 2. 58

59 Catalytic activity for hydrogen production Doctor Blade technique In order to study the catalytic activity of the supported catalyst prepared by the Doctor Blade technique [43], a series of experiments was made using experimental setup 3. The effect of temperature, activation energy and the electrochemical behaviour of the support catalyst were investigated. All the experiments were made in the stationary mode without magnetic stirring and under controlled temperature in a batch reactor. Effect of temperature and activation energy To investigate the effect of temperature on the sodium borohydride catalyzed hydrolysis, a series of experiments were made at four temperatures, 303, 318, 328 and 338 K. As for the powder catalyst study, the temperatures were chosen to comprise part of the real environmental conditions throughout the year and to cover a range of temperatures that might develop in utilizations on reactors without temperature control. The experimental procedure comprises the preparation of a solution with 10 wt.% of NaBH 4 stabilized with 10 wt.% of NaOH that was used in all the experiments. Before utilization, the solutions were pre-heated to the selected temperature for each experiment. The nickel foam catalyst was fixed inside the centre of the reactor chamber. The sodium borohydride solution was then injected and the reactor was rapidly closed. After an induction time, hydrogen was produced and the volume released was measured with time. Electrochemical characterization The use of a supported catalyst allowed recording of the variation of the open circuit potential while spontaneous generation of hydrogen occurred once in contact with the borohydride containing solution. This permitted electrochemical characterization of the sodium borohydride reaction. For the electrochemical study, a set of experiments was made at 3 different temperatures, 318, 328 and 338 K. The experimental setup used in this investigation was setup 3. Two electrodes, the reference electrode (Ag/AgCl sat. KCl) and the working electrode (the nickel foam support catalyst) were located in the reactor 59

60 chamber and connected to the potential measuring device Hydra Series II, Fluke Co Ltd, USA. The variation of potential with time was recorded on a computer. For the sodium borohydride hydrolysis reaction, 130 cm 3 of solution 2 wt.% NaBH 4 and 10 wt.% NaOH were injected in to the reactor that was rapidly closed. Values of open circuit potential and hydrogen production were measured with time immediately after the solution injection. Electrodeposition method The supported catalysts, prepared by the electrodeposition method, were studied in order to evaluate the kinetic properties for sodium borohydride hydrolysis. The experiments were made in the dynamic mode, using experimental setup 4 described in section The effects of temperature and activation energy, fuel velocity circulation and a study of the re-utilization were analyzed and discussed. Experimental details are reported herein. Effect of temperature and Activation Energy The catalyst activity and the effect of reaction temperature on the sodium borohydride catalyzed hydrolysis were studied by performing a set of kinetics experiments at two different temperatures, 295 and 318 K, using the electrodeposited catalyst EDRu82.5 in the dynamic mode. 200 cm 3 of a solution of 2 wt.% sodium borohydride, stabilized with 3 wt.% of sodium hydroxide, was put in the storage tank and the supported catalyst was fixed at the center of the reactor chamber. A circulation of 43 cm 3 per minute was set in the pump and, when the fuel started to fill the reactor chamber, the reaction time started and the values of hydrogen production in were measured with time. The storage tank was immersed in the water bath with the temperature controlled to ensure that the sodium borohydride entered at the temperature under study. All the experiments were conducted up to the total consumption of the sodium borohydride. Effect of fuel velocity circulation Circulation rate has an effect on the reaction kinetics; this is related to the loss of heat during the circulation and the change in NaBH 4 concentration as the reaction 60

61 proceeded. To study this effect, a series of experiments was made at different circulation rates with values of 10, 23, 43 and 85 cm 3 /min selected. All the experiments were made using 200 cm 3 of a solution of 2 wt.% sodium borohydride, stabilized with 3 wt.% of sodium hydroxide, and placed in the storage tank at a fixed temperature of 318 K. After the supported catalyst EDRu82.5 was fixed in the reactor, circulation of the solution at the pre-selected rate was initiated. When the fuel arrived at the reactor, the time of reaction started and the hydrogen produced was measured. All the experiments were conducted up to the total consumption of the sodium borohydride. Re-utilization An objective of the study the influence of successive loading of reactant solution on the catalyst activity; to undertake this, experimental work was performed with successive loadings of solution of 2 wt.% of sodium borohydride, stabilized with 3 wt.% of sodium hydroxide, at a fixed circulation rate of 23 cm 3 /min and with a selected solution temperature, 318 K. For this study supported catalyst EDRu82.5 was re-used four times under the same experimental condition. A fuel loading of 200 cm 3 was employed and all the experiments were performed until total hydrolysis of the sodium borohydride. The experimental procedure was similar to the previously described studies. 61

62 3 RESULTS 3.1 Introduction Results are presented for self-hydrolysis of sodium borohydride in water, in mixtures of methanol-water and for the catalysed hydrolysis using a nickel catalyst synthesized from nickel salts, using wet chemistry in a reducing environment. The main issues regarding reaction efficiency, such as maximum concentration of borohydride usable in the hydrolysis reaction, the roles of water and ph as well catalyst stability and durability, are considered. 3.2 Water-based self-hydrolysis The effect of sodium borohydride concentration on the NaBH 4 hydrolysis rate has been examined by performing a set of experiments with different NaBH 4 concentrations in aqueous solutions in the absence of catalyst at 318 K. Figure 1 shows the hydrogen generation rate obtained for each concentration studied as a function of time. In the Figure 14 it is observed that the reaction occurs in two stages. In the initial stage, an exponential increase of the hydrogen evolution is observed, which followed by second stage where a marked slowing down of the hydrogen production rate is registered. The decrease in the reaction rate is maintained until the end of the experiment. The obtained results imply the following: There is an increase in the hydrogen production rate with an increase in sodium borohydride concentration. Only partial conversion of sodium borohydride by hydrolysis was observed in the tested conditions (until relatively long reaction times). Aqueous solutions of NaBH 4 cannot be used as storage solutions since they are not stable and there is some sodium borohydride degradation. The arrest of the self-hydrolysis reaction rate is attributed to dramatic changes in ph as a function of reaction time, as shown in Figure

63 Vol H2/ml wt% NaBH4 15 wt% NaBH4 10 wt% NaBH4 7 wt% NaBH4 5 wt% NaBH4 2 wt% NaBH4 0 0,00 50,00 100,00 150,00 200,00 250,00 300,00 t /min Figure 14. Hydrogen generation rate as function of time for self-hydrolysis of NaBH 4 in aqueous solutions of different NaBH 4 concentration, [2-20] wt.% at fixed temperature, 318 K, and fixed solution volume, 25 cm 3 11,5 ph 11 10,5 10 9,5 ph 20 wt% ph 15 wt% ph 10 wt% ph 7wt. % ph 5 wt% ph 2 wt% 9 0,00 50,00 100,00 150,00 200,00 250,00 300,00 t /min Figure 15. Variation of the ph with time for water based self-hydrolysis of NaBH 4 at fixed temperature, 318 K, and fixed solution volume, 25 cm 3, at different NaBH 4 concentrations, [2-20] wt.%. 63

64 As expected, a fast increase in the ph in the initial stages of the reaction was observed, followed by a slow increase over the subsequent reaction period. By comparison of this behaviour with the hydrogen production rate, it can be established that, for low ph, fast kinetics of self-hydrolysis are observed and with increase of ph a decrease in the rate of hydrolysis reaction is registered The influence of the temperature on the rate of hydrogen production on NaBH 4 selfhydrolysis was also studied by performing two experiments at two different temperatures of 318 and 338 K. The data obtained are presented in Figure 16. From the results, it is possible to observe that water self-hydrolysis had a large temperature dependence, with a faster increase in the hydrogen production when the temperature is increased. An increase in the reaction rates, taking the borohydride to full conversion, led to consideration of the use of methanol and methanol-water mixtures. Results are presented in section T K T K Vol H2/ml ,00 50,00 100,00 150,00 200,00 t /min Figure 16. Hydrogen generation rate as function of time for water based selfhydrolysis of NaBH 4 at a fixed concentration, 10 wt.%, and fixed solution volume, 25 cm 3, at two different temperatures, 318 K and 338 K. 64

65 3.3 Methanol based self-hydrolysis A set of experiments was conducted in various water-methanol mixtures to evaluate the effect of methanol concentration on sodium borohydride hydrolysis at constant temperature, 45ºC and with a constant concentration of NaBH 4, 10 wt.%. The rate of hydrogen generation is shown in Figure 17. An increase in the rate of hydrogen production was observed with increase of methanol content. For large water contents, 100 and 90%, a slow conversion rate and larger lag time were observed. Increasing the methanol percentage gives a fast increase in hydrolysis kinetics. In the case of no added water, full conversion of the available hydrogen in the sodium borohydride was attained at relatively short times. The reaction lag time decreases with the quantity of methanol and, in the case of methanol solutions with no added water, no lag time was observed and the reaction started immediately when methanol is in contact with the NaBH 4 solution. For the hydrolysis in methanol with no added water and for 100% water, a study of the changes in ph during hydrolysis was undertaken. Figure 18 present the results obtained. Self-hydrolysis of sodium borohydride in 100% water is characterised by a rapid increase of solution ph as the reaction progresses due to the alkaline character of the reaction by-products. This alkalization is the cause of the arrest in the reaction rate observed in Figure 18 (a). In methanol with no added water, the change in ph is closely related with hydrogen production; however, no significant effect of increase of the ph on the hydrolysis rate was observed. 65

66 V /ml H2O:CH3OH = 9:1 H2O:CH3OH = 5:5 H2O:CH3OH = 0:10 H2O:CH3OH = 1:9 H2O:CH3OH = 10: ,00 10,00 20,00 30,00 40,00 50,00 60,00 70,00 80,00 Figure 17. Volume of gas generated as a function of time in a 10 wt.% NaBH 4 nonstabilised solution for different water /methanol ratios at 318 K. t /min [NaBH4] /% [NaBH4] ph ph t /min (a) [NaBH4] / % [NaBH4] ph 10,5 10 ph 20 9, (b) t / min Figure 18. Sodium borohydride conversion (%) and ph solution changes as a function of reaction time for an initial concentration of 10 wt.% NaBH 4 solution at 318 K with (a) 100 wt.% water; (b) methanol with no added water;. 66

67 In order to understand the dependence of the hydrolysis reaction rate on NaBH 4, concentration, a set of experiments was made with initial concentrations of sodium borohydride between 2-20 wt.%. The tests were conducted at constant temperature of 318 K in a solution of methanol with no added water. The rate of hydrogen generation was examined by adding the NaBH 4 solution into the reactor with 10 ml of methanol solution. The rate of hydrogen generated with time of reaction is given in Figure 19. The rate of hydrogen generation increased with increase with sodium borohydride concentration, with both axes on logarithmic scales. A straight line with a slope of 1.05 was obtained, see Figure 20. This result indicates that the hydrolysis is pseudofirst order with respect to the concentration of sodium borohydride. The hydrolysis of NaBH 4 is an exothermic reaction and consequently depends on temperature. In order to study the effect of temperature on the hydrolysis of NaBH 4 in methanol solution, a set of experiments was undertaken at different temperatures. The hydrogen generation reaction was studied by supplying 1 g NaBH 4 into the reactor containing 20 ml of methanol solution. The ranges of temperatures examined were 283, 288, 293, 309, 378 and 328 K. The results show a linear dependence of the hydrogen production rate with temperature. The Arrhenius plot, in which the logarithm of the hydrogen generation rate is plotted against the reciprocal of absolute temperature (1/T), is shown in Figure 21. From the slope of the straight line, the activation energy is calculated as be 13 kj mol 1. Table 2 compares the activation energy obtained in this work with values from the literature regarding methanol mixtures and water. 67

68 V /ml m(nabh4) 0.2g m(nabh4) 0.5 g m(nabh4) 0.75g m(nabh4) 1 g m(nabh4) 1.5g m(nabh4) 2g 0,00 2,00 4,00 6,00 8,00 10,00 12,00 14,00 t /min Figure 19. Volume of gas generated as a function of time in methanol solution for different sodium borohydride concentrations, at 318 K LN k ,5 0 0,5 1 1,5 2 LN [NaBH4] Figure 20. Variation of hydrogen generation rate with sodium borohydride concentration, both on logarithmic scales, for hydrolysis in methanol solution, at 318 K. 68

69 Table 2. Activation energy for sodium borohydride hydrolysis catalyst and noncatalyst in water and methanol solutions. Reaction Activation Energy Reference Self-hydrolysis methanol solution 13 kjmol -1 This work Self-hydrolysis methanol solution kjmol -1 [44] Self-hydrolysis water solution 86.9 kjmol -1 [44] [45] Even though the use of methanol mixtures represents an advantage regarding to water in the conversion of sodium borohydride to hydrogen as far as: reduction by a factor of 5 on the activation energy, in presence of methanol, when compared with values found in 100% solutions water; lowers the freezing temperature of the reactant mixture providing short times for initiation of the reaction and allows high rates even at low temperature. However, more controllable and higher rates are considered to be obtained with the use of a catalyst. The next section reports on the synthesis, characterization and catalytic activity of a nickel-based catalyst. In order to make easier the recovery and reutilization of the catalyst, the study of a supported catalyst is also included in section

70 7,7 LN k 7,6 7,5 7,4 7,3 7,2 7,1 7 6,9 6,8 0,0030 0,0031 0,0032 0,0033 0,0034 0,0035 0,0036 1/T/ºK-1 Figure 21. Arrhenius plot for NaBH 4 hydrolysis in methanol solution in the temperature range of 283 and 328 K, starting with fixed initial concentration of NaBH 4 of 10 wt.%. 3.4 Catalyzed hydrolysis with powder catalyst Catalyst characterization The nickel-based powder catalysts, produced by a wet chemistry method, were analysed by SEM/EDS, TEM, X-Ray, FTIR and XPS. Textural properties were also determined by nitrogen adsorption/desorption isotherms. The catalyst is a powder containing nickel and ruthenium species. Figure 22 shows the nano-sized particles in the nickel-based catalyst as revealed by scanning electron microscopy. The amount of ruthenium is small and was not detectable by EDS. Figure 22c shows a high resolution scanning electron micrograph of the powder catalyst particle. Contrast variation is observed due to atomic number differences between ruthenium and nickel particles. The lighter particles are ruthenium and the grey particles are nickel. Combined with transmission electron microscopy (TEM) results (Figure 23), it is suggested that Ru particles were not oxidized; the particle size is 30 to 50 nm, but the particles are clustered together. Ni particles are already oxidized and the NiO particle size is 30 to 100 nm. Table 3 gives the possible crystal structures suggested by TEM analysis. 70

71 (a) (b) (c) Figure 22. Scanning electron micrograph of the powder catalyst e (a); EDS analysis of the area displayed (b) and high resolution Scanning electron micrograph showing that the catalyst contains two different species (c) 71

72 Table 3. Crystal structure suggested by TEM analysis of the powder catalyst. Compound Crystal structure Structure parameters and Planes Ni NiO Ru Cubic Cubic Hexagonal Tetragonal a=3.513; (111) 2.03, (200) 1.76 a=8.353; (222) 2.41, (400) 2.09 a= c=4.281; (100) 2.34, (002) 2.14, (101) 2.06 RuO 2 a= c=3.106 (110) 3.18, (101) 2.56, (200) A 2.08A Figure 23. Transmissionn electron microscopy of powder catalyst 72

73 The nitrogen (N 2 ) adsorption and desorption data as a function of pressure (at liquid nitrogen temperature) are displayed in Figure 24. The results show that the N 2 isotherm curve is of type IV, validating the using the Barrett, Joyner and Halenda method (BHJ) used for characterization of the textural properties of the powder catalyst. The total specific surface area, as determined by N 2 physisorption, gave a value of 54 m 2 g -1 and a negligible micropore volume. Table 4 summarises the textural properties of the nickel- based powder catalyst. The results of X-ray diffraction data of the synthesized nickel-based powder catalyst are presented in Figure 25. The XRD analysis detected the presence of a mixture of compounds in the catalyst. The principal compounds identified were ruthenium oxide, nickel hydroxide and some nickel boron compounds. Metallic nickel and ruthenium were also detected. Table 4 Textural properties of the Ni based powder catalyst. S BET /m 2 g -1 V µ /cm 3 g -1 /m 2 g -1 V p /cm 3 g -1 Catalyst S BET BET surface area, V µ - micropore volume by t-method; S ext (mesopore+macropore) surface areas by t-method; V p Total pore volume. Figure 26 shows the FTIR spectra the powder catalysts before and after reaction, including reaction products; data are also provided for NaBH 4. Collected data were recorded within a wavelength interval from 4000 to 400 cm -1. The FTIR analysis of the catalyst indicated characteristics bands for Ni OOH and Ni-OH groups at 463 and 651 cm -1 respectively, as well as a wide band at cm -1 and peaks between cm -1 and at 1633 cm -1 that are attributed to stretching and deformation vibration of OH groups and bending vibrations of H O H. 73

74 Volume (cm 3 /g) ,2 0,4 0,6 0,8 1 Relative Pressure (P/P 0 ) Figure 24. Nitrogen adsorption/desorption isotherms. - Ru - RuO 2 - Ni(OH) 2.0,75H 2O - Ni - Ni 3B 2O 6 - NiB Figure 25. XRD pattern of the synthesized nickel-based powder catalyst. 74

75 % Transmittance Ni N-bi 1 +NaBH4+NaOH Ni-bi1 NaBH wavenumbers (cm-1) Figure 26. FTIR spectra of nickel, Ni-based (before and after reaction with NaBH 4 ) and NaBH 4. For catalyst powder catalyst, after reaction with borohydride in NaOH, 3 bands at 532, 564 and 764 cm -1 were attributed to stretching vibrations of the O B O group. Bands at 933 and 1100 cm -1 were associated with the symmetric vibration of B O and B O - groups. Further, several bands, observed at 1180 and 1470 cm -1, correspond to asymmetric vibrations of the B O group. Peaks at 3260 e 3437 cm -1 correspond to O H vibrations. Three peaks were identified for NaBH 4, which were associated with B-H bonds ( cm -1 ). The peak at 1630 cm -1 was attributed to Na H vibrations, and the peak at 1130 cm -1 to deformations of BH 2. Figure 27a) illustrates a typical XPS spectrum for nickel and the nickel-based catalyst. The presence of two peaks at and ev that can be attributed at Ni(0)2p 3/2 and Ni(0)2p 1/2 respectively is observed. The spectrum shows two additional peaks at and 880 ev, corresponding to Ni(II) 2p 3/2 and Ni(II)2p 1/2 respectively; these indicate the presence of other oxidized species of nickel. The data are in agreement with FTIR results. The XPS spectrum of the ruthenium catalyst is shown in Figure 27b); a doublet at and ev was attributed to Ru(0)3d 5/2 and Ru(0)3d 3/2, representative of the electron core level XPS transition for metallic ruthenium. The interpretation of the 75

76 other peaks suggests the presence of ruthenium oxides, i.e.ruo 2, as confirmed by the doublet at and ev and also RuO 3 that has a peak at 285 ev. (a) (b) Figure 27. XPS spectra for the nickel and Ni-based catalyst. (a); ruthenium catalyst (b) Effect of temperature and activation energy In order to study the effect of temperature on the hydrolysis of sodium borohydride, a set of experiments was carried out using a fixed concentration of 10 wt.% NaBH 4, stabilized with 10 wt.% NaOH in the temperature range between 297 to 333 K. Results are presented in Figure 28, where the horizontal line indicates the theoretical hydrogen volume expected according to the total conversion of the amount of NaBH 4 employed. The following comments may be made: 76

77 The variation of the volume with time data shows 3 regions: the initially produced hydrogen volume follows a linear relationship with time, showing faster kinetics compared with the second stage in which the rate slows down in a non-linear fashion, and a plateau indicates the end of the reaction. The hydrogen release rate increases with increasing temperature. In all cases, 100% conversion of NaBH 4 to hydrogen was possible. An induction time for the reaction was observed, which appears to be dependent on temperature and is being smaller ft higher temperatures. Rates up to 16 l min -1 g -1 at 333 K are considered high rates for the nickelbased catalyst when compared with values from the Literature, Taking into account the linear zone of the volume against time data, the values of the initial rate of the reaction were estimated. The results obtained for each temperature are presented in Figure 29 in an Arrhenius plot in order to estimate the activation energy. The following observations were made: two linear regions are evident, an activation energy of 68 kjmol -1 is estimated for low temperatures, an activation energy of 31 kjmol -1 is estimated for high temperatures, the estimated values are lower than the Ea values for self-hydrolysis in the absence of the catalyst (87 kjmol -1 ), as expected, and 77

78 V /ml T K T K T 318,15 K T K T K ,00 10,00 20,00 30,00 40,00 50,00 60,00 70,00 t /min T K Theoretical H2 Production Figure 28. Hydrogen generation rate as a function of time for catalyzed hydrolysis of NaBH 4 at different temperatures between 297 K and 333 K, using 10 wt.% NaBH 4 solution stabilized with 10 wt.% of NaOH and ~50 mg of powder catalyst. Lnk 0-0,5-1 -1,5-2 -2,5-3 -3,5-4 0,0029 0,003 0,0031 0,0032 0,0033 0,0034 0,0035 1/T /K -1 Figure 29. Arrhenius plot for the catalyzed hydrolysis of sodium borohydride using powder catalyst. 78

79 The transition temperature for the change in Ea is 308 K (35ºC); this may be related to the stability of by-products and their ability to retain/release water, which is discussed later Influence of catalyst concentration The influence of the catalyst mass in the hydrogen generation rate was studied by performing a set of experiments at a fixed temperature, 318 K, fixed concentration of NaBH 4 (10 wt.%), and fixed amount of stabilizer (10 wt.% of NaOH), with different loadings of the catalyst in the range between 20,1 and 100 mg. Figure 30a) shows the results obtained, which indicate an increased hydrogen generated volume with increasing catalyst mass. The estimated rate with catalyst amount is presented in Figure 30b) in the log-log plane for catalyst loadings between 35 and 100 mg. From the data, the following conclusions arise: The presence of an induction time for the reaction was detected and was found to be dependent on the catalyst loading, increasing with reduction of the catalyst mass. As expected, the hydrogen production rate increases with increased catalyst loadings within the tested range. The order of the reaction in relation to the catalyst amount data indicated a slope of ~1.18 in the concentration range between 35 and 100 mg. 79

80 l 1500 /m V ,00 5,00 10,00 15,00 20,00 25,00 30,00 t /min (a) 20.1 mg 35 mg 50 mg 75 mg 100 mg 6,5 k N L 6 5,5 y = 1,1835x + 6,7927 R² = 0, ,2-1 -0,8-0,6-0,4-0,2 0 LN /Cat (b) Figure 30. (a) Hydrogen generation rate as function of time for catalyzed hydrolysis of NaBH 4 at different catalyst loadings between 20.1 mg and 100 mg, using 10 wt.% NaBH 4 solution stabilized with 10 wt.% of NaOH and ~50 mg of powder catalyst, at a temperature of 318 K. (b) Variation of hydrogen generation rate with catalyst loading, both on logarithmic scales, for the hydrolysis reaction. 80

81 3.4.4 Effect of sodium borohydride concentration In order to compare the effect of NaBH 4 concentration during hydrolysis, a set of experiments was made at constant temperature, 318 K, and with the sodium borohydride concentration varied between wt.%. Figure 31 presents the data obtained as a function of time. In the inset, data are given for small concentrations of NaBH 4. Figure 32 shows the plot of the reaction rate with concentration of NaBH 4 for the concentration study. Consideration of data reveals the following: for all concentrations tested, a conversion yield of 100% was found, i.e. the maximum theoretical hydrogen volume expected according to equation (5) was obtained. for concentrations up to 5 wt.%, a fast increase of the hydrogen release rate was observed with increase of NaBH 4 concentration. The kinetics of hydrolysis is slow. for concentrations of 5 to 15 wt.%, fast kinetics are registered with values between 460 and 520 ml of hydrogen per minute. The dependence of the hydrogen release rate with concentration is less than that observed for low NaBH 4 concentrations, for concentration higher than 15wt.%, the reaction rate decreases with increase of NaBH 4 concentration and slow kinetics are registered, a maximum is evident at 10 wt %, and the induction time has a fast increase with increase of NaBH 4 concentration to 5 wt.%; thereafter, it is stable for higher concentrations, with no significant changes. 81

82 wt% NaBH4 20 wt% NaBH wt% NaBH4 Vol /ml wt% NaBH4 Vol /ml wt% NaBH4 2 wt% NaBH4 1 wt% NaBH ,00 5,00 10,00 15,00 20,00 t /min 0,00 50,00 100,00 150,00 t /min Figure 31. Effect of sodium borohydride concentration on the hydrogen production rate using 50 mg powder catalyst, temperature of 318 K. Solutions were stabilized with 10 wt % NaOH K /(ml/min) [NaBH 4 ] /wt% Figure 32. Rate of hydrogen release for various concentrations of NaBH 4 reaction rate using 50 mg powder catalyst, temperature of 318 K. Solutions were stabilized with 10 wt % NaOH. 82

83 3.4.5 Effect of water/borohydride ratio In order to study the effect of the fraction H 2 O/NaBH 4 on the hydrogen production rate, a set of experiments was made with a fixed quantity of sodium borohydride, 1 g, and changing the amounts of water to give molar ratios water to NaBH 4 between 8 (R8) and 84 (R84). Figure 33 shows the hydrogen production rate with time for each ratio studied The following observations were made from the Figure: A smallest quantity of water tested, R8, the system has fast initial kinetics when compared with the system with large amounts of water, R84. However, after one-half of the reaction was completed, an inversion was observed and sample R84 had faster kinetics. This may be related to the temperature and water management, which are discussed later. For experiments R16 and R42, initially have the same hydrogen production rates and when two-thirds of the reaction is achieved the R16 experiment have a slow down on the reaction rate, probably due to decrease of available water. A maximum is evident at R42. The induction time shows a stable value of ~1 minute for ratios of H 2 O/NaBH 4 to 42; it then increased with increased water proportion. Figure 34 shows the results of a comparison obtained for different concentrations of NaBH 4 using different methods. Blue dots show the hydrogen production rate obtained for the variation of borohydride fraction for a fixed water volume, and the red dots show the hydrogen production rates for water fraction variation with a fixed amount of borohydride. Similar behaviour is observed for both methods. 83

84 Vol /ml R 8 R 16 R 42 R 63 R ,00 5,00 10,00 15,00 20,00 25,00 30,00 35,00 40,00 t /min Figure 33. Hydrogen generation rate as function of time for catalyzed hydrolysis of NaBH 4 at different fraction H 2 O/NaBH 4, using fixed amount of NaBH 4, 1g, stabilized with 10 wt.% of NaOH, at fixed temperature of 318 K. The catalyst loading used in all experiments was ~50 mg of powder catalyst K /(ml/min) ,0 5,0 10,0 15,0 20,0 25,0 30,0 35,0 [NaBH 4 ] /wt% Figure 34. Rate of hydrogen production for various concentrations of NaBH 4 using a variation of the water fraction for a fixed amount of borohydride (red dots) and a variation of borohydride fraction for a fixed water volume (blue dots). (wnabh 4 /wh 2 O) 30-2%. 84

85 3.4.6 Effect of stabilizer type The effect of the cation of the stabilizer was studied in a comparative study using KOH and NaOH, as well as mixtures of both stabilizers. Experiments were carried out using 1 g sodium borohydride at 318 K and a solution volume of 10 cm 3. The results are presented in Figure 35. The following observations are made from Figure 35: NaOH favours the 1 st half of reaction, and KOH favours the 2 nd half of the reaction. Improved results were obtained for mixtures of both stabilizers, where NaOH is of a large percentage. The induction time shows approximately the same value for all the samples, which indicates that the cation of the stabilizer does not have an influence on the induction time V /ml wt% NaOH 75 wt% NaOH + 25 wt% KOH 25 wt% NaOH + 75 wt% KOH 100 wt% KOH 0 0,00 2,00 4,00 6,00 8,00 10,00 12,00 14,00 t /min Figure 35. Hydrogen generation rate of powder catalyst using 10 wt.% solutions at 318 K and 10 wt.% of stabilizer. Solutions containing mixtures of the stabilizer were also used. 85

86 3.5 Catalyzed hydrolysis with supported catalyst Foam/catalyst system characterization As previously stated, supported catalysts have great advantages in hydrogen production systems since apart of the easy removal of the working solution the recovery and the re-utilization of the catalyst is facilitated. Another advantage of supported catalyst is the large increase on the surface area that can be expected of the final catalyst. In this work, a nickel foam was the chosen material due to its large surface area and because the nickel foam has itself some catalytic properties for hydrogen evolution. Figure 36 shows a SEM view of the nickel foam as received. By analysis was possible confirmed the morphologic characteristic given by material provider (section 2.2). The average pore size calculated SEM picture was 597 µm. Doctor Blade catalyst Using a modification of the Doctor Blade technique, describe previously, a supported catalyst was prepared by fixation of the powder catalyst on the nickel foam. On the Figure 37 are show the SEM analysis of the supported catalyst. Figure 37a shows the surface morphology of the nickel foam with fixed catalyst where is possible observed a layer of the catalyst uniformly distributed by all the nickel foam area. Figure 37b shows corresponding EDS analysis. 86

87 Figure 36. Nickel-foam used as a catalyst support, as received. (a) (b) Figure 37. Scanning electron micrograph of the nickel-based catalyst, supported on nickel foam (deposition of catalyst was done using Doctor Blade technique (a), and respective EDX analysis (b). 87