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Chemical Engineering Journal 218 (2013) 39 45 Contents lists available at SciVerse ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej A new nano CaO-based CO 2 adsorbent prepared using an adsorption phase technique Yan Wang a, Yanqing Zhu a,b, Sufang Wu a,c, a Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, PR China b Institute of Environmental and Municipal Engineering, North China University of Water Resources and Electric Power, Zhengzhou, Henan 450011, PR China c Zhejiang Provincial Engineering Research Center of Industrial Boiler and Furnace Flue Gas Pollution Control, Hangzhou 311202, PR China highlights " Using adsorption phase technique, a coating layer of 4.5 nm 11.6 nm was formed with the TiO 2 content increasing. " TiO 2 content played an important role of CO 2 adsorption durability. " Compact factor between 0.8 and 1.3 was tested to relate the adsorption stability. graphical abstract article info abstract Article history: Received 13 August 2012 Received in revised form 19 November 2012 Accepted 21 November 2012 Available online 29 November 2012 Keywords: CO 2 adsorbent Nano CaO Adsorption phase technique This study describes for the first time micro-scale hydrolysis has been used in the adsorption phase to prepare a nano CaO-based CO 2 adsorbent with a highly durable sorption capacity. The hydrolysis of Ti(OC 4 H 9 ) 4 to form TiO 2 was used to prepare TiO 2 -coated nano CaCO 3, which was then calcinated to prepare a nano CaO-based CO 2 adsorbent with a controlled coating layer. The coating compactness was defined for the first time in this study to describe the mole ratio of Ti to Ca on the surface of the nano CaCO 3. The coating compactness and the durability of the sorption capacity of samples with varying TiO 2 content, hydrolysis temperature, and ester concentration were studied in detail. The properties of the reactive adsorption of the prepared nano CaO-based CO 2 adsorbents were tested using a thermogravimetric analyzer. The results showed that, of the conditions tested, the TiO 2 content exerts the most influence on the durability of the sorption capacity. The nano CaCO 3 that was coated with 10 wt.% TiO 2 and prepared under 20 C, which has a corresponding coating compactness of 1.0, exhibited a much more durable CO 2 sorption capacity than the other prepared samples. Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. 1. Introduction The capture of CO 2 through the use of a CaO-based adsorbent plays an important role in the efficient separation of CO 2 from combustion/gasification gases [1 3] and sorption-enhanced hydrogen production processes [4 6]. This capture is based on the reversible carbonation reaction of CaO [7]. CaO is a potential adsorbent because of its high reactive sorption capacity and the abundance of its natural precursors, such as limestone (CaCO 3 ) Corresponding author at: Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, PR China. Tel.: +86 571 87953138. E-mail address: wsf@zju.edu.cn (S. Wu). [8] and dolomites (Ca, Mg(CO 3 ) 2 ) [9,10]. However, the CaO-based adsorbents exhibit a rapid decay in their absorption capacity during multiple carbonation calcination reaction cycles [11]. It is widely believed that the capacity decay is mainly due to the sintering of CaO and CaCO 3 in the regeneration process [12], the physical aggregation of the crystals, which leads to an increased particle size, or the loss of porosity that is caused by the volume reduction of the small pores [13]. Compared with natural adsorbents, nano CaCO 3 has drawn increasing attention [14 17] because of its higher reactive sorption capacity, fast reaction rate and its significant improvement in the durability of the adsorbent. However, because nano CaCO 3 has a high ratio surface area and a high surface energy, it aggregates 1385-8947/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/1016/j.cej.2012.11.095

40 Y. Wang et al. / Chemical Engineering Journal 218 (2013) 39 45 more easily, which makes sintering a serious problem. Several methods with varying degrees of success have been proposed to increase the life cycle performance of CaO-based sorbents by preventing the sintering of CaO. These methods include the incorporation of inert materials, such as Al 2 O 3 [18], SiO 2 [19,20], MgO [21,22], and CaTiO 3 [14] through several chemical synthesis and physical methods, the utilization of potentially sintering-resistant calcium precursors [23,24], the hydration treatment of CaO to regain its reactivity [25,26], and the development of high surface area CaCO 3 [27]. The addition of an inert material is a general way to modify the nano adsorbent to obtain better durability. In previous studies, the combination of an inert material and the adsorbent was usually achieved through immediate wet mixing [28], precipitation [29,30] or surface coating [20]. The coating of a CaO-based adsorbent is an effective way to prevent sintering. The surface coating material behaves as a partition wall that prevents the agglomeration of CaO particles. Therefore, controlling the coating layer is important for improving durability. However, previous research on coating-modified CaO-based adsorbents has not reported the proper preparation of a controlled coating layer [14,20]. The adsorption phase technique [31,32] uses the adsorption phase as a microreactor to coat the surface of the adsorbent. It is a remarkable technique because the particle size is efficiently and easily controlled. In this paper, the adsorption phase technique was first used to prepare TiO 2 -coated nano calcium carbonate. A controlled coating layer was obtained through the micro-scale hydrolysis of Ti(OC 4 H 9 ) 4 in the adsorption phase. The effects of the TiO 2 content, hydrolysis temperature, and ester concentration on the CO 2 adsorption performance and the relationship between the durability and the nano calcium carbonate coating compactness were studied. 2. Experimental 2.1. Reagents and instruments Nano CaCO 3 (>95% purity) with a particle size of 70 nm with water (Huzhou Ling Hua Ltd., China) was used as the CaO-based sorbent precursor. Tetrabutyl titanate (Ti(OC 4 H 9 ) 4 ; Shanpu Shanghai Chemical Co. Ltd.) was used as the source of TiO 2, and ethanol (Wuxi Jingke Chemical Co. Ltd.) was used as the solvent. An A-type zeolite (Sinopharm Chemical Reagent Co. Ltd.) was used to remove the water from the nano CaCO 3. A thermo-gravimetric analyzer (TGA, Pyris1, Perkin-Elmer, USA) was used for the reactive sorption capacity measurements. X-ray photoelectron spectroscopy (XPS, VG ESCALAB MK II, UK) was used to detect the type and relative content of the surface elements. The morphology of the sorbent was investigated using a transmission electron microscope (TEM, JEM-1200EX, USA). 2.2. Preparation of TiO 2 -coated nano CaCO 3 using an adsorption phase technique An 18-g mass of aqueous nano CaCO 3 was dispersed in dehydrated ethanol using an ultrasonic dispersion method to form a suspension and then placed into heat-treated zeolite to dehydrate. A clear dehydrated ethanol solution with a specific concentration of Ti(OC 4 H 9 ) 4 was added dropwise into the dehydrated suspension while stirring. After all of the solution was added, the solution was stirred for an additional 1 2 h. The solids were collected by vacuum filtration, dried and then heat-treated at 500 C for 2 h until the solid sample was obtained. TiO 2 -coated nano CaCO 3 samples with different TiO 2 contents can be obtained by changing the amount of n-butyl titanate. In addition to the TiO 2 content, the hydrolysis temperature and the concentration of the n-butyl titanate ethanol solution were also varied to prepare the different samples. All of the sample preparation conditions are listed in Table 1. The preparation method is based on the adsorption phase technique. The mechanism by which TiO 2 is formed on the nano calcium carbonate is shown in Fig. 1. The nano CaCO 3 with water was dehydrated using zeolite, and only a thin layer of water on the round surface of the nano CaCO 3 particle was maintained. When an ethanol solution of tetrabutyl titanate was added dropwise into the dehydrated suspension, the Ti(OC 4 H 9 ) 4 spread to the water layer and quickly hydrolyzed. Different operating conditions resulted in different distributions of TiO 2 on the surface of the nano CaCO 3 and different compact factors. 2.3. Compactness definition and characterization test The compact factor C is defined in Eq. (1) to quantify the coating compactness. This factor is the relative molar content ratio of the coating material (titanium) and the material to be coated (calcium) on the surface of the nano CaCO 3. It describes the coating status and, in this paper, represents the dispersion of TiO 2 on the surface of the nano CaCO 3. Table 1 Preparation conditions of samples. Sample Nos. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 TiO 2 content/wt.% 0 5 6.5 8 10 12 15 20 10 10 10 10 10 10 T/ C 11.5 15 15 12.5 14.5 12 12.5 0 20 40 70 22 22 Ester concentration/wt.% 9.6 9.6 9.6 9.6 9.6 9.6 9.6 9.6 9.6 9.6 9.6 5 17.5 Fig. 1. Mechanism image of ester hydrolysis of adsorption phase reaction.

Y. Wang et al. / Chemical Engineering Journal 218 (2013) 39 45 41 Intensity (a.u.) C ¼ n Ti n Ca ; In Eq. (1), n refers to the molar number of elements. Based on the locations of the characteristic lines that appear in the XPS energy spectrum, the types of elements that are found on the surface can be identified. The XPS energy spectrum of sample No. 5 is shown in Fig. 2. The elements Ti, Ca, C, and O were observed on the surface. Once a specific energy region of Ti and Ca are chosen for further scanning, a single element spectrum is obtained, as shown in Fig. 3a and b. The photoelectron line intensity (photoelectron peak area) reflects the number of atomic levels; thus, we can obtain the relative molar content of the surface elements. The compact factor can be calculated from the following equation based on the Atomic Sensitivity Factor (ASF) method: C ¼ n Ti n Ca ¼ I Ti=S Ti I Ca =S Ca ; Ca2p3/2&1/2 C1s Ti2p3/2&1/2 Ca2s 0 100 200 300 400 500 600 700 800 900 1000 Binding Energy (ev) where I is the peak area of the XPS test data fitting and S is the sensitivity factor, which is related to the element and equipment used (in this instrument, S Ti = 1.1 and S Ca = 0.71). 2.4. Sorption capacity test method O1s Ti2s Ca LMM Ti LMM O KVV Fig. 2. XPS full spectrum scan of TiO 2 coated CaO-based adsorbent. The cyclic CO 2 sorption capacity was measured with a thermogravimetric analyzer. The tests were conducted under carbonation, with a CO 2 partial pressure of 0.02 MPa in N 2 at 600 C for 10 min, and calcination in N 2 at atmospheric pressure and 725 C for 10 min. The temperature was increased at a rate of 15 C/min and decreased at a rate of 40 C/min. The cyclic CO 2 sorption capacity tests were also conducted in a fixed bed reactor. We used the reactive sorption capacity to analyze the reaction of CaO with CO 2. The reactive sorption capacity and decay ratio were calculated according to the following equations: CO 2 sorption amount Reactive sorption capacity ¼ The mass of CaO in adsorbent ðgco 2 =gcaoþ; Decay ratio ¼ ðscþ 1 ðscþ n ðscþ 1 100%; ð4þ where (SC) 1 is the reactive sorption capacity of the first run and (SC) n is the reactive sorption capacity of the nth run. ð1þ ð2þ ð3þ 3. Results and discussion 3.1. Effects of TiO 2 content on the durability of the CO 2 adsorption cycles To investigate the effect of the TiO 2 content on the cyclic performance, seven samples (from No. 2 to No. 8) with different TiO 2 mass fractions of 5%, 6.5%, 8%, 10%, 12%, 15%, and 20% were prepared. The TEM images of several typical samples are shown in Fig. 4. As shown in Fig. 4, the more dispersed nano calcium carbonate particles were lighter in color. 1 The formation of the TiO 2 layer darkened the colors of the edges. As the TiO 2 content increases, the thickness and distribution of the dark parts become more obvious. The samples with 5% and 10% TiO 2 showed a thin layer and a partial coating around the edges of the particles. The sample with 20% TiO 2 showed a thick and almost complete coating around the edges of the particles. The thickness of the coating layer ranged from 4.5 nm to 11.6 nm as the TiO 2 content increased from 5% to 20%. The reactive sorption capacity test results are shown in Fig. 5. The sample marked 0% was not coated with TiO 2. The initial reactive sorption capacity of each sample was high, decayed rapidly in the initial five to six cycles, and then decayed slowly. After 30 cycles of carbonation calcination, the reactive sorption capacity of most samples remained at a relatively stable value; this was true for all samples except the samples with 5% and 6.5% TiO 2, which continued to decay. The non-tio 2 coated sample continued to decay with a lower reactive sorption capacity than the other samples, which indicates that the coating operation improved the durability of the sorption capacity. A few samples, such as those with 10% and 12% TiO 2, showed an increase in the CO 2 reactive sorption capacity after 20 cycles. We conjecture that this phenomenon is caused by self-reactivation [14,33]. An additional 30 cycles of carbonation calcination of the 10% TiO 2 sample were run after the initial 30 cycles. The sorption capacity was almost stable after the 40th run, as shown in Fig. 6. The results of the BET surface area measurements on samples 1 and 5 before and after several runs are shown in Table 2. As shown in the results, after 12 runs, the surface area and the average pore size of sample No. 5, which was coated with TiO 2, were maintained at almost constant levels, whereas those of sample No. 1 decreased. This finding indicates that the TiO 2 coating may provide a partition wall that prevents the agglomeration of the CaO particles and thus improves the durability of the reaction sorption capacity of the coated samples. Fig. 7 shows the effect of the TiO 2 content on the sorption capacity in the 1st and the 30th runs. With increasing TiO 2 content, the sorption capacity of the initial sorbent decreased and that of the sample in the 30th run first increased and then decreased. In the first run, the increasing content of the coating material led to a reduction in the relative content of CaO, which reduces the number of CO 2 adsorption sites and decreases the CO 2 sorption capacity. As the cycles of adsorption and desorption progressed, the activity of CaO decreased, and thus, the sorption capacity decreased. When the TiO 2 content was greater than 8%, the decay ratio was smaller, which meant that the durability was improved. Taking into account the sorption capacity, a TiO 2 content in the range of 8 10% was considered to be optimal. Among those values, after 30 cycles, the sample with 10% TiO 2 had the highest sorption capacity, with a value of 04 g of CO 2 /g of CaO, and the lowest decay ratio at 28.3%, which results in the best adsorption 1 For interpretation of color in Fig. 4, the reader is referred to the web version of this article.

42 Y. Wang et al. / Chemical Engineering Journal 218 (2013) 39 45 a 600 500 scanning curve of Ca2p FittedCurves FittedCurves b 600 500 scanning curve of Ti2p FittedCurves FittedCurves Intensity (a.u.) 400 300 200 Intensity (a.u.) 400 300 200 100 100 0 0 340 342 344 346 348 350 352 354 356 358 Binding Energy (ev) 454 456 458 460 462 464 466 468 470 Binding Energy (ev) Fig. 3. XPS spectra of element (a) Ca, (b) Ti. Fig. 4. TEM images of samples with different TiO 2 content. (a) Non-TiO 2 coated nano CaCO 3 (sample No. 1), (b) 5% TiO 2 (sample No. 2), (c) 10% TiO 2 (sample No. 5), (d) 20% TiO 2 (sample No. 8). performance. In S.F. Wu s research, the reactive sorption capacity of TiO 2 /CaCO 3 was 4.2 mol/kg after 40 runs, which was 14.3% lower than that of sample No. 5. 3.2. Effects of hydrolysis conditions on the durability of the CO 2 adsorption cycles Because the n-butyl titanate hydrolysis reaction is a fast reaction, the hydrolysis conditions have a great impact on the reaction rate, which in turn affects the formation and growth of the coating layer. Hydrolysis temperatures varying from 0 Cto70 C and ester concentrations in the range of 5 17.5% were studied in this section. Fig. 8 shows the CO 2 reactive sorption capacities of the adsorbent samples No. 5 and 9 12, which were prepared under different temperatures. As shown in the figure, the different hydrolysis temperatures had little influence on the reactive sorption capacity of the initial cycles. As the cycles progressed, the impact of the temperature became apparent. The sorption capacities of the samples prepared under 40 C and 70 C temperature conditions were gradually reduced; these were not as stable as the other three samples, which were prepared under temperatures in the range of 0 20 C. In general, the sorption capacity of the sample prepared under 40 C was higher than that of the sample prepared under 70 C. Thus, a lower temperature is relatively favorable, and a temperature range of 0 20 C is optimal. The reactant concentration of butyl titanate dissolved in the ethanol solution also had some impact on the adsorption performance of the adsorbent. Fig. 9 shows the CO 2 reactive sorption capacities of the adsorbent samples Nos. 10, 13, and 14, which were prepared under the different ester concentrations of 5%, 9.6%, and 17.5%, respectively. As the cycles progressed, when the ester concentration was 17.5%, the sorption capacity gradually

Y. Wang et al. / Chemical Engineering Journal 218 (2013) 39 45 43 0.7 0.0 0 5 10 15 20 25 30 number of cycles 0% 5% 6.5% 8% 10% 12% 15% 20% 0.7 0.0 1st run 30th run 0 5 10 15 20 TiO 2 content/% Fig. 5. CO 2 reactive sorption capacity of sample Nos. 1 8 with different TiO 2 content (carbonation: CO 2 partial pressure of 0.02 MPa at 600 C for 10 min, and calcination with an atmospheric pressure in N 2 at 725 C for 10 min). Fig. 7. Effect of TiO 2 content on sorption capacity of 1st and 30th run. 0.0 10%TiO 2 0%TiO 2 0 10 20 30 40 50 60 number of cycles Fig. 6. CO 2 reactive sorption capacity of sample No. 5. 0 5 10 15 20 25 30 number of cycles Fig. 8. CO 2 reactive sorption capacity of sample Nos. 5, and 9 12 prepared under different temperatures varying from 0 C to 70 C. Table 2 BET analytic results of samples 1 and 5 before and after several runs. Sample BET surface area (m 2 /g) Average pore size (nm) No. 1 No. 5 No. 1 No. 5 Fresh 5.90 20.76 20.80 19.14 After 12 runs 4.35 10.92 18.42 12.42 After 20 runs 3.33 13 16.60 13.18 decayed and did not reach a steady state even after 30 cycles. In contrast, when the ester concentration was relatively low, the sorption capacity was basically stable after only 10 cycles. 3.3. Compactness and durability The adsorption performance of the novel adsorbent was correlated with the compactness of the coating layer. The compact factors of the TiO 2 coating layers of samples with different adsorption performances are shown in Table 3. 5% 9.6% 17.5% 0 5 10 15 20 25 30 number of cycles Fig. 9. CO 2 reactive sorption capacity of sample Nos. 10, and 13,14 prepared under different ester concentrations of 5 17.5%.

44 Y. Wang et al. / Chemical Engineering Journal 218 (2013) 39 45 Table 3 The compact factors of TiO 2 coating layer under different experimental conditions. Sample Nos. 2 4 7 8 10 11 12 C 0.81 1.31 1.52 1.89 1.03 1.09 0.77 0.7 1st run sorption capacity 30th run sorption capacity decay ratio of 30th run 0 0.8 1.0 1.2 1.4 1.6 1.8 2.0 compact factor Fig. 10. The relationship between compact factor and the 30th sorption capacity and decay ratio. The TiO 2 content and the compact factor both increased in samples Nos. 2, 4, 7, and 8. It is easy to understand that an increased TiO 2 content indicates that more TiO 2 is gathered on the surface of the nano calcium carbonate. Thus, the relative content of titanium to calcium on the surface was greater. The hydrolysis temperature is critical for the control of the reaction kinetics that determines the formation rate and the growth pattern of TiO 2, which results in the dispersion of TiO 2 on the surface of the nano calcium carbonate. Higher temperatures were conducive to the growth of the Ti(OH) 4 precipitate crystal, which led to an inferior dispersion of TiO 2 and thus a smaller compact factor. As shown in Fig. 10, the sorption capacity in the 30th run first increases and then decreases with increasing compact factor. A compact factor in the range of 0.8 1.3 resulted in a small decay ratio and a high sorption capacity in the 30th run; this range was thus regarded as an optimal range of C. When C was less than 0.85, the sorption capacity decayed the most throughout the 30- cycles because the coating layer was not dense and the distribution of the coating material on the surface was not uniform. Thus, this coating was not able to prevent the agglomeration of the CaCO 3 or the sintering of the nano particles. When C was greater than 1.3, the coating layer was uniform and sufficiently dense. For example, the compact factor of sample 8 was 1.89, and the sorption capacity of this sample gradually stabilized after the first few initial cycles of decay. However, the increase in the coating material content led to a reduction in the relative content of CaO, which greatly reduced the overall sorption capacity of the sorbent. There were two main factors that affected the compact factor C: the coating content and the operating conditions, which include the temperature and the concentration used in the preparation of the absorbent. The coating content mainly determined the coating thickness, whereas the operating conditions regulated the coating uniformity. When the coating material content was constant, both the sorption capacity and the durability of the sorbent prepared at a low temperature and a low reactant concentration were improved probably because these conditions reduce the reaction rate, thus making the coating material on the surface of the nano calcium carbonate distribute more evenly. In addition, when the 80 60 40 20 decay ratio(%) compact factor was close to 1.0, the compactness was considered to be improved. 4. Conclusion The application of an adsorption phase technique for the preparation of a TiO 2 -coated nano-cao based CO 2 adsorbent with high durability and a controlled coating layer was introduced in this study. A coating layer of 4.5 11.6 nm was formed with increasing TiO 2 content. The TiO 2 coating can prevent the sintering of nano CaCO 3 during multiple carbonation/calcination cycles. Among the TiO 2 content, the hydrolysis temperature, and the ester concentration, the TiO 2 content has the most effect on the durability of resultant absorbent for CO 2 adsorption. The optimal content of TiO 2 ranges from 8% to 12%. The optimal hydrolysis conditions included a relatively low temperature in the range of 0 20 C and a low reactant concentration of 5 10 wt.%. The compact factor is a quantitative description of the status of the TiO 2 coating layer and differs when the TiO 2 -coated nano-caco 3 is prepared under different conditions. When the compact factor is between 0.9 and 1.3, the compactness is considered to be optimal because the adsorption stability of these samples is the best of the samples tested. Acknowledgments The National Natural Science Foundation of China is thanked for its financial support (20876142). Kimberly Braches from McMaster University is also acknowledged for her editing assistance. Appendix A. 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