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CeO2 catalyst via Ce(OH)CO3 for the water-gas shift reaction
Renewable Energy xxx (2014) 1e7
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Optimization of a highly active nano-sized Pt/CeO2 catalyst via Ce(OH)CO3 for the water-gas shift reaction Dae-Woon Jeong a, Won-Jun Jang a, Jae-Oh Shim a, Won-Bi Han a, Hak-Min Kim a, Yeol-Lim Lee a, Jong Wook Bae b, Hyun-Seog Roh a, * a b
Department of Environmental Engineering, Yonsei University, 1 Yonseidae-gil, Wonju, Gangwon 220-710, South Korea School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon, Gyeonggi 440-746, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 March 2014 Accepted 22 July 2014 Available online xxx
Crystalline cerium hydroxy carbonate (CHC: Ce(OH)CO3) was prepared by a novel precipitation/digestion method at room temperature in air. The nano-sized CeO2 supports were obtained by the thermal decomposition of CHC and the Pt/CeO2 catalysts were prepared by an incipient wetness impregnation method. The pre-calcination temperature and aging time were optimized to obtain a highly active Pt/ CeO2 catalyst for the water gas shift reaction (WGS). The Pt/CeO2 catalyst exhibited the highest CO conversion (82%) and the lowest activation energy (55 kJ/mol) at a very high gas hourly space velocity (GHSV) of 45,515 h1 when the optimized synthesis parameter (pre-calcined temperature ¼ 400 C and aging time ¼ 4 h) was used in the synthesis of CeO2. This is mainly due to the high BET surface area, nano-sized CeO2, and intimate interaction between Pt and CeO2. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Cerium hydroxy carbonate (CHC: Ce(OH) CO3) Nano-sized CeO2 Pt Pre-calcination temperature Aging time Water gas shift (WGS)
1. Introduction The water-gas shift reaction (WGS) is an important reaction used in the chemical industry for the production of clean hydrogen [13]. Recently, a Pt/CeO2 catalyst has been identified as a promising catalyst for the WGS due to the unique redox property and high oxygen storage capacity (OSC) of CeO2 [46]. It is well known that nano-sized CeO2 supports help to stabilize CePtOx species which are known to be active species for the WGS [710]. It has also been reported that nano-sized Pt stabilizes CeO2 supports against sintering or loss of surface area [710]. As a consequence, various wet chemical synthesis routes have been studied to obtain nano-sized CeO2 supports with controlled physical and chemical properties for the WGS [911]. To obtain high quality nano-sized CeO2 supports, one of the simplest approaches is the thermal decomposition of a molecular precursor like cerium hydroxy carbonate (CHC: Ce(OH)CO3) [9,1216]. The morphology, crystallite size, and physicochemical nature of CeO2 can be easily controlled by using CHC [13]. Several routes have been pursued to synthesize CHC by a chemical
* Corresponding author. Fax: þ82 33 760 2571. E-mail address: [email protected] (H.-S. Roh).
precipitation reaction employing aqueous solutions containing cerium (III) nitrate (Ce(NO3)3∙6H2O) with excess urea [13,14] or ammonium carbonate [15,16] as precipitants. However, it is necessary to employ an excess precipitant (urea or ammonium carbonate) and to increase the temperature up to 80 C for the precipitation of CHC. In a previous study, we developed a novel chemical precipitation/digestion method to synthesize CHC by employing mixed precipitants (KOH þ K2CO3) at room temperature in air [9]. As a result, the Pt/CeO2 catalyst prepared by a developed precipitation/ digestion method shows significant WGS activity [9]. The catalytic performance of Pt/CeO2 is reported to depend on the synthesis parameters. In particular, it is well known that the pre-calcination temperature and aging time have significant effects on the physicochemical properties of catalysts and the catalytic performance [1720]. Rahemi et al. [17] demonstrated that the pre-calcination temperature plays a significant role in determining the crystallite size and the interaction between the metal and support. Koo et al. [18] also reported that a catalyst pre-calcined at low temperature exhibited a high activity and stability due to the small crystallite size and intimate interaction between the metal and support. Zhang et al. [19] reported that an extended precipitation aging time can improve the reducibility of catalysts. Although we reported [9] that a high BET surface area of pre-calcined CeO2 results in a high
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Please cite this article in press as: Jeong D-W, et al., Optimization of a highly active nano-sized Pt/CeO2 catalyst via Ce(OH)CO3 for the water-gas shift reaction, Renewable Energy (2014), http://dx.doi.org/10.1016/j.renene.2014.07.041
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activity for the WGS, the effect of the synthesis parameters on the crystallite size, interaction between metal and support, and reduction property has not been investigated with a Pt/CeO2 catalyst via CHC. The objective of this study is to study the effects of the synthesis parameters (pre-calcination temperature and aging time) on the physicochemical properties and catalytic performance of Pt/CeO2 prepared via CHC in the WGS. The effects of the synthesis parameters on the physicochemical properties and catalytic performance were characterized by BET, XRD, and TPR and were related to the activity results in the WGS. We also attempted to determine the optimum synthesis parameter of the Pt/CeO2 catalyst which resulted in maximized its activity in the WGS. 2. Experimental 2.1. Catalyst preparation The CeO2 supports were prepared by a novel chemical precipitation/digestion method used for the synthesis of the CHC precursor, as reported earlier [9]. The precipitates during the synthesis of CHC were aged at room temperature (RT) in air. The aging time was changed systemically from 0 to 8 h. The precipitates were washed with distilled water to remove Kþ ion impurities and air-dried at RT. The as-dried precursor, CHC, was pre-calcined at various temperatures for 4 h in air to obtain the CeO2 supports. The pre-calcination temperature was varied from 400 to 700 C. Unless otherwise stated, the aging time and precalcination temperature were fixed at 4 h and 400 C. 1 wt% Pt was loaded on the pre-calcined CeO2 supports by an incipient wetness impregnation method where Pt(NH3)4∙(NO3)2 (99%, Aldrich) was used as a precursor. The calcination of the catalysts was carried out at 400 C for 4 h in air.
3. Results and discussion 3.1. Effect of the pre-calcination temperature 3.1.1. Catalyst characterization Fig. 1 shows the XRD patterns of the fresh and used Pt/CeO2 catalysts pre-calcined at various temperatures. Characteristic Pt metal peaks are not detected on account of the low loading content of Pt (1 wt%). All characteristic peaks can be assigned to CeO2 with a fluorite structure [9,10]. The intensity of the CeO2 peak increases due to the increasing crystallite size with increasing pre-calcination temperature from 400 to 700 C. To evaluate the effect of the precalcination temperature on the crystallite size of CeO2, the crystallite size was estimated by the Scherrer equation [27,28]. Table 1 summarizes the CeO2 crystallite size and BET surface area of the fresh and used Pt/CeO2 catalysts pre-calcined at various temperatures. The CeO2 crystallite size of the Pt/CeO2 catalyst pre-calcined at 400 C is the smallest among the prepared catalysts, while that of the catalyst pre-calcined at 700 C is the largest. Clearly, the BET surface area dramatically decreases with increasing pre-calcination temperature, as shown in Table 1. These results indicate that the Pt/
2.2. Characterization The BET surface area was measured by nitrogen adsorption at 196 C using an ASAP 2010 (Micromeritics) instrument. The XRD patterns were recorded using a Rigaku D/MAX-IIIC diffractometer (Ni filtered CueK radiation, 40 kV, 50 mA). Temperature programmed reduction (TPR) experiments were carried out using an Autochem 2910 (Micromeritics). TPR was performed using 10% H2 in Ar from 20 to 600 C at a heating rate of 10 C/min. The sensitivity of the detector was calibrated by reducing a known weight of NiO [2123]. 2.3. Catalytic reaction Activity tests were carried out from 200 to 360 C under atmospheric pressure in a fixed-bed micro-tubular quartz reactor with an inner diameter of 4 mm. The detailed procedure for the WGS reaction has been explained in the literature [2428]. The simulated reformed gas was composed of 6.5 vol% CO, 7.1 vol% CO2, 0.7 vol% CH4, 42.4 vol% H2, 28.7 vol% H2O, and 14.5 vol% N2. The feed H2O/(CH4 þ CO þ CO2) ratio was fixed at 2.0 due to the fact that a H2O/CH4 ratio is typically 3.0 in the steam reforming of methane (SRM: H2O þ CH4 ¼ 3H2 þ CO) to avoid coke formation [2932]. To screen catalysts for the WGS, we tested Pt/CeO2 catalysts at a GHSV of 45,515 h1, which is 15 times higher than that of the typical experimental conditions employed for the WGS. The effluent gases from the reactor were analyzed on-line using a micro gas chromatograph (Agilent 3000).
Fig. 1. XRD patterns of the (a) fresh and (b) used Pt/CeO2 catalysts pre-calcined at various temperatures.
Please cite this article in press as: Jeong D-W, et al., Optimization of a highly active nano-sized Pt/CeO2 catalyst via Ce(OH)CO3 for the water-gas shift reaction, Renewable Energy (2014), http://dx.doi.org/10.1016/j.renene.2014.07.041
D.-W. Jeong et al. / Renewable Energy xxx (2014) 1e7 Table 1 Characteristics of the Pt/CeO2 catalysts obtained as a function of the pre-calcination temperature. Pre-calcination temperature ( C) 400 500 600 700
SBET (m2/g)
Crystallite size (nm)
Fresh
Used
Fresh
Used
136 86 24 7
96 58 15 4
7.9 10.6 17.4 47.1
8.3 11.5 19.4 56.2
(29%Y) (33%Y) (38%Y) (43%Y)
(5%[) (8%[) (11%[) (19%[)
CeO2 catalyst pre-calcined at 400 C still retains the nano-sized crystalline structure of CeO2 and a high BET surface area after calcination. It is reported that the nano-sized CeO2 support stabilizes CePtOx type species on its surface and subsurface layers, which are known to be active species for the WGS [810,33]. The BET surface area of used catalysts also exhibits a similar trend as the fresh catalysts. Moreover, the used Pt/CeO2 catalyst pre-calcined at 400 C showed the lowest growth of the CeO2 crystallite size (5%) and the lowest decrease of the BET surface area (29%). Therefore, it is confirmed that Pt/CeO2 catalyst pre-calcined at 400 C has a high resistance against sintering. The TPR patterns of the Pt/CeO2 catalysts pre-calcined at various temperatures are shown in Fig. 2. It is known that lower temperature peaks are assigned to the reduction of surface PtOx species, while higher temperature peaks are attributed to the reduction of CePtOx species which possess an intimate interaction between Pt and CeO2 [810,33]. For the Pt/CeO2 catalysts pre-calcined at 400 and 500 C, the reduction peaks of the CePtOx species are observed at 275 and 221 C, respectively [810,33]. In the case of the Pt/CeO2 catalysts pre-calcined at 600 and 700 C, the reduction peaks of surface PtOx species appeared at 87 and 69 C, respectively. It is reported that Pt2þ ion stabilized on nano-scale ceria is more active sites for WGS [810,33]. It can be concluded that the precalcination temperature of CHC has a significant effect on converting surface PtOx species into CePtOx, which is known to be an active site for the WGS [810,33]. As a result, the reduction of Pt/ CeO2 pre-calcined at 400 C is possible at 275 C, which is the highest reduction temperature. This result clearly indicates that the catalyst pre-calcined at 400 C results in an intimate interaction between Pt and CeO2, which demonstrates resistance to sintering. It is well known that an intimate interaction between metal and support is effective to mitigate sintering [18,21,27]. Thus, it is
Fig. 2. TPR patterns of the Pt/CeO2 catalysts pre-calcined at various temperatures.
3
expected that the Pt/CeO2 catalyst pre-calcined at 400 C should have a high CO conversion as well as high resistance to sintering. 3.1.2. Reaction results Fig. 3 shows the CO conversion profiles of the WGS over the Pt/ CeO2 catalysts pre-calcined at various temperatures. At a reaction temperature of 200 C, all of the catalysts showed negligible CO conversion. Within the reaction temperature range from 240 to 360 C, the Pt/CeO2 catalysts pre-calcined at 400 and 500 C showed higher CO conversions than the other catalysts. At 320 C, the Pt/CeO2 catalysts pre-calcined at 400 and 500 C resulted in CO conversion close to equilibrium value. The CO conversion of the Pt/ CeO2 catalyst pre-calcined at 400 C is slightly higher than that of Pt/CeO2 catalyst pre-calcined at 500 C. At the reaction temperature of 360 C, the CO conversions of the Pt/CeO2 catalysts pre-calcined at 400 and 500 C decreased due to the exothermic nature of the WGS. These results indicate that the pre-calcination temperature of CHC is an important parameter which influences the catalytic performance of Pt/CeO2 in the WGS. The pre-calcination temperature of CHC strongly affects the BET surface area, crystallite size of CeO2, and interaction between the metal and support of the Pt/ CeO2 catalysts. Therefore, it can be concluded that the highest activity of the Pt/CeO2 catalyst pre-calcined at 400 C is correlated to its high BET surface area, nano-sized CeO2, and an intimate interaction between Pt and CeO2. The Arrhenius plot in Fig. 4 compares the turnover frequency (TOF) of the Pt/CeO2 catalysts pre-calcined at various temperatures. The activation energies (Ea) of the WGS reaction were calculated from the slopes of the straight lines presented in Fig. 4. The Ea value decreases with increasing pre-calcination temperature. Hence, the Ea value of the Pt/CeO2 catalysts pre-calcined at 400 and 500 C are similar but the Ea of the Pt/CeO2 catalyst pre-calcined at 400 C (55 kJ/mol) is slightly lower than that of the Pt/CeO2 catalyst precalcined at 500 C (57 kJ/mol). This result agrees well with the WGS reaction results. Consequently, the pre-calcination temperature of the CHC can be controlled to result in a lower activation energy, resulting in a higher CO conversion. Fig. 5 shows the selectivity to CO2 and CH4 as a function of the reaction temperature over Pt/CeO2 catalysts pre-calcined at various temperatures. At 200 C, the undesirable methanation reaction occurred very slightly. However, almost 100% selectivity to CO2
Fig. 3. CO conversion as a function of the reaction temperature obtained over Pt/CeO2 catalysts pre-calcined at various temperatures (H2O/(CH4 þ CO þ CO2) ¼ 2.0, GHSV ¼ 45,515 h1).
Please cite this article in press as: Jeong D-W, et al., Optimization of a highly active nano-sized Pt/CeO2 catalyst via Ce(OH)CO3 for the water-gas shift reaction, Renewable Energy (2014), http://dx.doi.org/10.1016/j.renene.2014.07.041
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Fig. 4. Arrhenius plots of the turnover frequency of CO conversion obtained over Pt/ CeO2 catalysts pre-calcined at various temperatures (H2O/(CH4 þ CO þ CO2) ¼ 2.0, GHSV ¼ 45,515 h1).
could be achieved above 240 C for all of the catalysts. Therefore, it has been confirmed that the Pt/CeO2 catalysts prepared from CHC are selective toward the WGS. 3.2. Effect of aging time 3.2.1. Catalyst characterization Fig. 6 depicts the XRD patterns of the fresh and used Pt/CeO2 catalysts as a function of the aging time. All of the peaks are consistent with the peaks originating from CeO2 [9,10]. Table 2 summarizes the CeO2 crystallite size and BET surface area of the fresh and used Pt/CeO2 catalysts as a function of the aging time. Clearly, the crystallite size of CeO2 and the BET surface area depend on the aging time employed for the synthesis of CHC. It is expected that the aging time is correlated to the WGS activity. The optimum aging time for the synthesis of CHC is 4 h to obtain a nano-sized CeO2 crystalline and high BET surface area. Moreover, the Pt/CeO2 catalyst prepared with an aging time of 4 h has a high resistance to sintering, resulting in the smallest growth of CeO2 crystallite size (5%) and the smallest decrease of BET surface area (29%).
Fig. 6. XRD patterns of the (a) fresh and (b) used Pt/CeO2 catalysts obtained as a function of the aging time.
Fig. 7 shows the TPR patterns of the Pt/CeO2 catalysts obtained as a function of the aging time. All of the catalysts show a reduction peak assigned to the reduction of Pt species due to its interaction with the CeO2. The reduction peak of the Pt/CeO2 catalyst with an aging time of 4 h is observed at 275 C, which is the highest reduction temperature among the catalysts prepared in this study. This result indicates that the Pt/CeO2 catalyst prepared with an aging time of 4 h has an intimate interaction between Pt and CeO2. This is due to the fact that the nano-sized CeO2 support prepared with an aging time of 4 h helps to stabilize CePtOx species [810,33]. Therefore, it can be concluded that the aging time strongly influences the resistance to sintering of CeO2. The growth of the CeO2 crystallite size agrees well with the reduction property confirmed from the TPR results.
Fig. 5. Selectivity to CO2 and CH4 obtained over Pt/CeO2 catalysts pre-calcined at various temperatures (H2O/(CH4 þ CO þ CO2) ¼ 2.0, GHSV ¼ 45,515 h1).
3.2.2. Reaction results The WGS reaction data obtained from the Pt/CeO2 catalysts prepared at the various aging times are presented in Fig. 8. At 200 and 240 C, all catalysts showed low CO conversions. At the reaction temperature of 280 C, the CO conversions were about 53%, except for the catalyst prepared without aging. At 320 C, the CO conversion nearly reached equilibrium when aging times of 4 and 8 h were
Please cite this article in press as: Jeong D-W, et al., Optimization of a highly active nano-sized Pt/CeO2 catalyst via Ce(OH)CO3 for the water-gas shift reaction, Renewable Energy (2014), http://dx.doi.org/10.1016/j.renene.2014.07.041
D.-W. Jeong et al. / Renewable Energy xxx (2014) 1e7
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Table 2 Characteristics of the Pt/CeO2 catalysts obtained as a function of the aging time. Aging time (h)
0 2 4 8
SBET (m2/g)
Crystallite size (nm)
Fresh
Used
Fresh
Used
110 113 136 123
72 76 96 86
8.9 8.5 7.9 8.2
9.9 9.3 8.3 8.7
(35%Y) (33%Y) (29%Y) (30%Y)
(11%[) (9%[) (5%[) (6%[)
used for the synthesis of CHC. On the contrary, the Pt/CeO2 catalyst prepared without aging showed a CO conversion of 63% at 320 C. The CO conversion at the various aging times decreased in the order of 4 h z 8 h > 2 h > 0 h. This result indicates that the aging time determines not only the characteristics of the catalyst but also the catalytic performance in the WGS. As a result, the optimum aging time was determined to be 4 h due to the high activity of Pt/CeO2 resulting from the highest BET surface area, nano-sized CeO2, and an intimate interaction between Pt and CeO2. Fig. 9 shows the Arrhenius plot of the Pt/CeO2 catalysts as a function of the aging time. Clearly, Ea depends on the aging time
Fig. 7. TPR patterns of the Pt/CeO2 catalysts obtained as a function of the aging time.
Fig. 8. CO conversions at various reaction temperatures obtained over Pt/CeO2 catalysts as a function of aging time (H2O/(CH4 þ CO þ CO2) ¼ 2.0, GHSV ¼ 45,515 h1).
Fig. 9. Arrhenius plots of the turnover frequency of the CO conversion obtained over Pt/CeO2 catalysts as a function of the aging time (H2O/(CH4 þ CO þ CO2) ¼ 2.0, GHSV ¼ 45,515 h1).
applied in the synthesis of CHC, ranging from 55 kJ/mol over Pt/ CeO2 (aging time ¼ 4 h) to 63 kJ/mol over Pt/CeO2 (aging time ¼ 0 h). The calculated trend of the Ea value is good agreement with the reaction results. Fig. 10 depicts the selectivity to CO2 and CH4 for various reaction temperatures over the Pt/CeO2 catalysts as a function of the aging time. The methanation reaction took place very slightly at 200 C. However, 100% selectivity to CO2 was obtained above 240 C over all of the catalysts. The carbon balance is shown in Table S1. The carbon balance is defined as the amount of carbon in the product gas divided by the amount of carbon in the reactant gas. For all of the catalysts, the value of carbon balance is closed up to 99 ± 2%. The carbon balance of the Pt/CeO2 catalysts is within the experimental error in all cases and at all temperatures. This result indicates that the carbonate species are not formed on the Pt/CeO2 catalysts. To evaluate the stability of the optimized Pt/CeO2 catalyst (precalcination temperature ¼ 400 C and aging time ¼ 4 h) in the WGS, CO conversion data was collected at 320 C for 100 h at a GHSV of 45,515 h1. Fig. 11 shows the CO conversion data as a function of the time of operation. The CO conversion was maintained for 100 h
Fig. 10. Selectivity to CO2 and CH4 obtained over Pt/CeO2 catalysts as a function of the aging time (H2O/(CH4 þ CO þ CO2) ¼ 2.0, GHSV ¼ 45,515 h1).
Please cite this article in press as: Jeong D-W, et al., Optimization of a highly active nano-sized Pt/CeO2 catalyst via Ce(OH)CO3 for the water-gas shift reaction, Renewable Energy (2014), http://dx.doi.org/10.1016/j.renene.2014.07.041
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by the Ministry of Science, (2013R1A1A1A05007370).
ICT
and
Future
Planning
Appendix. A Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.renene.2014.07.041. References
Fig. 11. CO conversion as a function of the time of operation (T ¼ 320 C) obtained over the Pt/CeO2 catalyst with the optimized synthesis parameter (pre-calcination temperature ¼ 400 C and aging time ¼ 4 h) (H2O/(CH4 þ CO þ CO2) ¼ 2.0, GHSV ¼ 45,515 h1).
without significant catalyst deactivation. Therefore, it has been confirmed that the optimized Pt/CeO2 catalyst exhibited high activity as well as high stability. The high activity/stability of the optimized Pt/CeO2 catalyst in the WGS can be explained as follows. The characterization results showed that this catalyst has a high BET surface area due to the nano-crystalline nature of the CeO2 particles. The nano-sized CeO2 support can stabilize CeePtOx species at the surface [810,33]. The presence of CePtOx species over the optimized Pt/CeO2 catalyst is confirmed in the TPR results. Moreover, the CePtOx reduction of the optimized Pt/CeO2 catalyst is possible at 275 C, which is the highest reduction temperature. This result indicates that there is an intimate interaction between Pt and CeO2 in the Pt/CeO2 catalyst. It can be concluded that the pre-calcination temperature and aging time applied in the synthesis of the Pt/CeO2 catalyst can be controlled in such a way that more active CeePtOx sites are created at the surface of the catalyst. As a result, the optimized Pt/CeO2 catalyst exhibited stable activity, the highest CO conversion, and the lowest Ea. Consequentially, due to the excellent catalytic activity, low activation energy, and high resistance to sintering, the optimized synthesis parameter is found to be a pre-calcination temperature of 400 C and an aging time of 4 h. 4. Conclusion The catalytic performance of Pt/CeO2 catalysts strongly depends on the BET surface area, crystallite size of CeO2, and reduction property of Pt. The Pt/CeO2 catalyst prepared under the optimized synthesis parameter conditions (pre-calcination temperature ¼ 400 C, aging time ¼ 4 h) had the highest BET surface area, nano-sized CeO2, and intimate interaction between Pt and CeO2. As a result, the Pt/CeO2 catalyst prepared with the optimized synthesis parameter exhibited the highest CO conversion (82%) and the lowest activation energy (55 kJ/mol) among the catalysts tested in this study. Moreover, this catalyst has a high resistance to sintering, due to the stabilized CeePtOx species. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded
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D.-W. Jeong et al. / Renewable Energy xxx (2014) 1e7 [25] Subramanian V, Potdar HS, Jeong D-W, Shim J-O, Jang W-J, Roh H-S, et al. Synthesis of a novel nano-sized Pt/ZnO catalyst for water gas shift reaction in medium temperature application. Catal Lett 2012;142:1075e81. [26] Jeong D-W, Subramanian V, Shim J-O, Jang W-J, Seo Y-C, Roh H-S, et al. Hightemperature water gas shift reaction over Fe/Al/Cu oxide based catalysts using simulated waste-derived synthesis gas. Catal Lett 2013;143:438e44. [27] Jeong D-W, Jang W-J, Shim J-O, Han W-B, Roh H-S, Jung UH, et al. Lowtemperature water-gas shift reaction over supported Cu catalysts. Renew Energy 2014;65:102e7. [28] Jeong D-W, Potdar HS, Roh H-S. Comparative study on nano-sized 1 wt.% Pt/ Ce0.8Zr0.2O2 and 1 wt.% Pt/Ce0.2Zr0.8O2 catalysts for a single stage water gas shift reaction. Catal Lett 2012;142:439e44. [29] Roh H-S, Eum I-H, Jeong D-W. Low temperature steam reforming of methane over Ni-Ce(1-x)Zr(x)O2 catalysts under severe conditions. Renew Energy 2012;42:212e6.
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[30] Jeong D-W, Na H-S, Shim J-O, Jang W-J, Roh H-S, Jung UH, et al. Hydrogen production from low temperature WGS reaction on co-precipitated Cu-CeO2 catalysts: an optimization of Cu loading. Int J Hydrogen Energy 2014;39: 9135e42. [31] Jeong D-W, Jang W-J, Shim J-O, Han W-B, Jeon K-W, Seo Y-C, et al. A comparison study on high-temperature water-gas shift reaction over Fe/Al/ Cu and Fe/Al/Ni catalysts using simulated waste-derived synthesis gas. J Mater Cycles Waste Manag 2014. http://dx.doi.org/10.1007/s10163-014-0272-8. [32] Mane RB, Jeong D-W, Malawadkar AV, Roh H-S, Rode CV. Effect of composition and pretreatment parameters on activity and stability of Cu-Al catalysts for water-gas shift reaction. ChemCatChem 2014;6:1698e706. [33] Pierre D, Deng W, Flytzani-Stephanopoulos M. The importance of strongly bound Pt-CeOx species for the water-gas shift reaction: catalyst activity and stability evaluation. Top Catal 2007;46:363e73.
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Optimization of a highly active nano-sized Pt/CeO2 catalyst via Ce(OH)CO3 for the water-gas shift reaction Dae-Woon Jeong a, Won-Jun Jang a, Jae-Oh Shim a, Won-Bi Han a, Hak-Min Kim a, Yeol-Lim Lee a, Jong Wook Bae b, Hyun-Seog Roh a, * a b
Department of Environmental Engineering, Yonsei University, 1 Yonseidae-gil, Wonju, Gangwon 220-710, South Korea School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon, Gyeonggi 440-746, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 March 2014 Accepted 22 July 2014 Available online xxx
Crystalline cerium hydroxy carbonate (CHC: Ce(OH)CO3) was prepared by a novel precipitation/digestion method at room temperature in air. The nano-sized CeO2 supports were obtained by the thermal decomposition of CHC and the Pt/CeO2 catalysts were prepared by an incipient wetness impregnation method. The pre-calcination temperature and aging time were optimized to obtain a highly active Pt/ CeO2 catalyst for the water gas shift reaction (WGS). The Pt/CeO2 catalyst exhibited the highest CO conversion (82%) and the lowest activation energy (55 kJ/mol) at a very high gas hourly space velocity (GHSV) of 45,515 h1 when the optimized synthesis parameter (pre-calcined temperature ¼ 400 C and aging time ¼ 4 h) was used in the synthesis of CeO2. This is mainly due to the high BET surface area, nano-sized CeO2, and intimate interaction between Pt and CeO2. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Cerium hydroxy carbonate (CHC: Ce(OH) CO3) Nano-sized CeO2 Pt Pre-calcination temperature Aging time Water gas shift (WGS)
1. Introduction The water-gas shift reaction (WGS) is an important reaction used in the chemical industry for the production of clean hydrogen [13]. Recently, a Pt/CeO2 catalyst has been identified as a promising catalyst for the WGS due to the unique redox property and high oxygen storage capacity (OSC) of CeO2 [46]. It is well known that nano-sized CeO2 supports help to stabilize CePtOx species which are known to be active species for the WGS [710]. It has also been reported that nano-sized Pt stabilizes CeO2 supports against sintering or loss of surface area [710]. As a consequence, various wet chemical synthesis routes have been studied to obtain nano-sized CeO2 supports with controlled physical and chemical properties for the WGS [911]. To obtain high quality nano-sized CeO2 supports, one of the simplest approaches is the thermal decomposition of a molecular precursor like cerium hydroxy carbonate (CHC: Ce(OH)CO3) [9,1216]. The morphology, crystallite size, and physicochemical nature of CeO2 can be easily controlled by using CHC [13]. Several routes have been pursued to synthesize CHC by a chemical
* Corresponding author. Fax: þ82 33 760 2571. E-mail address: [email protected] (H.-S. Roh).
precipitation reaction employing aqueous solutions containing cerium (III) nitrate (Ce(NO3)3∙6H2O) with excess urea [13,14] or ammonium carbonate [15,16] as precipitants. However, it is necessary to employ an excess precipitant (urea or ammonium carbonate) and to increase the temperature up to 80 C for the precipitation of CHC. In a previous study, we developed a novel chemical precipitation/digestion method to synthesize CHC by employing mixed precipitants (KOH þ K2CO3) at room temperature in air [9]. As a result, the Pt/CeO2 catalyst prepared by a developed precipitation/ digestion method shows significant WGS activity [9]. The catalytic performance of Pt/CeO2 is reported to depend on the synthesis parameters. In particular, it is well known that the pre-calcination temperature and aging time have significant effects on the physicochemical properties of catalysts and the catalytic performance [1720]. Rahemi et al. [17] demonstrated that the pre-calcination temperature plays a significant role in determining the crystallite size and the interaction between the metal and support. Koo et al. [18] also reported that a catalyst pre-calcined at low temperature exhibited a high activity and stability due to the small crystallite size and intimate interaction between the metal and support. Zhang et al. [19] reported that an extended precipitation aging time can improve the reducibility of catalysts. Although we reported [9] that a high BET surface area of pre-calcined CeO2 results in a high
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Please cite this article in press as: Jeong D-W, et al., Optimization of a highly active nano-sized Pt/CeO2 catalyst via Ce(OH)CO3 for the water-gas shift reaction, Renewable Energy (2014), http://dx.doi.org/10.1016/j.renene.2014.07.041
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activity for the WGS, the effect of the synthesis parameters on the crystallite size, interaction between metal and support, and reduction property has not been investigated with a Pt/CeO2 catalyst via CHC. The objective of this study is to study the effects of the synthesis parameters (pre-calcination temperature and aging time) on the physicochemical properties and catalytic performance of Pt/CeO2 prepared via CHC in the WGS. The effects of the synthesis parameters on the physicochemical properties and catalytic performance were characterized by BET, XRD, and TPR and were related to the activity results in the WGS. We also attempted to determine the optimum synthesis parameter of the Pt/CeO2 catalyst which resulted in maximized its activity in the WGS. 2. Experimental 2.1. Catalyst preparation The CeO2 supports were prepared by a novel chemical precipitation/digestion method used for the synthesis of the CHC precursor, as reported earlier [9]. The precipitates during the synthesis of CHC were aged at room temperature (RT) in air. The aging time was changed systemically from 0 to 8 h. The precipitates were washed with distilled water to remove Kþ ion impurities and air-dried at RT. The as-dried precursor, CHC, was pre-calcined at various temperatures for 4 h in air to obtain the CeO2 supports. The pre-calcination temperature was varied from 400 to 700 C. Unless otherwise stated, the aging time and precalcination temperature were fixed at 4 h and 400 C. 1 wt% Pt was loaded on the pre-calcined CeO2 supports by an incipient wetness impregnation method where Pt(NH3)4∙(NO3)2 (99%, Aldrich) was used as a precursor. The calcination of the catalysts was carried out at 400 C for 4 h in air.
3. Results and discussion 3.1. Effect of the pre-calcination temperature 3.1.1. Catalyst characterization Fig. 1 shows the XRD patterns of the fresh and used Pt/CeO2 catalysts pre-calcined at various temperatures. Characteristic Pt metal peaks are not detected on account of the low loading content of Pt (1 wt%). All characteristic peaks can be assigned to CeO2 with a fluorite structure [9,10]. The intensity of the CeO2 peak increases due to the increasing crystallite size with increasing pre-calcination temperature from 400 to 700 C. To evaluate the effect of the precalcination temperature on the crystallite size of CeO2, the crystallite size was estimated by the Scherrer equation [27,28]. Table 1 summarizes the CeO2 crystallite size and BET surface area of the fresh and used Pt/CeO2 catalysts pre-calcined at various temperatures. The CeO2 crystallite size of the Pt/CeO2 catalyst pre-calcined at 400 C is the smallest among the prepared catalysts, while that of the catalyst pre-calcined at 700 C is the largest. Clearly, the BET surface area dramatically decreases with increasing pre-calcination temperature, as shown in Table 1. These results indicate that the Pt/
2.2. Characterization The BET surface area was measured by nitrogen adsorption at 196 C using an ASAP 2010 (Micromeritics) instrument. The XRD patterns were recorded using a Rigaku D/MAX-IIIC diffractometer (Ni filtered CueK radiation, 40 kV, 50 mA). Temperature programmed reduction (TPR) experiments were carried out using an Autochem 2910 (Micromeritics). TPR was performed using 10% H2 in Ar from 20 to 600 C at a heating rate of 10 C/min. The sensitivity of the detector was calibrated by reducing a known weight of NiO [2123]. 2.3. Catalytic reaction Activity tests were carried out from 200 to 360 C under atmospheric pressure in a fixed-bed micro-tubular quartz reactor with an inner diameter of 4 mm. The detailed procedure for the WGS reaction has been explained in the literature [2428]. The simulated reformed gas was composed of 6.5 vol% CO, 7.1 vol% CO2, 0.7 vol% CH4, 42.4 vol% H2, 28.7 vol% H2O, and 14.5 vol% N2. The feed H2O/(CH4 þ CO þ CO2) ratio was fixed at 2.0 due to the fact that a H2O/CH4 ratio is typically 3.0 in the steam reforming of methane (SRM: H2O þ CH4 ¼ 3H2 þ CO) to avoid coke formation [2932]. To screen catalysts for the WGS, we tested Pt/CeO2 catalysts at a GHSV of 45,515 h1, which is 15 times higher than that of the typical experimental conditions employed for the WGS. The effluent gases from the reactor were analyzed on-line using a micro gas chromatograph (Agilent 3000).
Fig. 1. XRD patterns of the (a) fresh and (b) used Pt/CeO2 catalysts pre-calcined at various temperatures.
Please cite this article in press as: Jeong D-W, et al., Optimization of a highly active nano-sized Pt/CeO2 catalyst via Ce(OH)CO3 for the water-gas shift reaction, Renewable Energy (2014), http://dx.doi.org/10.1016/j.renene.2014.07.041
D.-W. Jeong et al. / Renewable Energy xxx (2014) 1e7 Table 1 Characteristics of the Pt/CeO2 catalysts obtained as a function of the pre-calcination temperature. Pre-calcination temperature ( C) 400 500 600 700
SBET (m2/g)
Crystallite size (nm)
Fresh
Used
Fresh
Used
136 86 24 7
96 58 15 4
7.9 10.6 17.4 47.1
8.3 11.5 19.4 56.2
(29%Y) (33%Y) (38%Y) (43%Y)
(5%[) (8%[) (11%[) (19%[)
CeO2 catalyst pre-calcined at 400 C still retains the nano-sized crystalline structure of CeO2 and a high BET surface area after calcination. It is reported that the nano-sized CeO2 support stabilizes CePtOx type species on its surface and subsurface layers, which are known to be active species for the WGS [810,33]. The BET surface area of used catalysts also exhibits a similar trend as the fresh catalysts. Moreover, the used Pt/CeO2 catalyst pre-calcined at 400 C showed the lowest growth of the CeO2 crystallite size (5%) and the lowest decrease of the BET surface area (29%). Therefore, it is confirmed that Pt/CeO2 catalyst pre-calcined at 400 C has a high resistance against sintering. The TPR patterns of the Pt/CeO2 catalysts pre-calcined at various temperatures are shown in Fig. 2. It is known that lower temperature peaks are assigned to the reduction of surface PtOx species, while higher temperature peaks are attributed to the reduction of CePtOx species which possess an intimate interaction between Pt and CeO2 [810,33]. For the Pt/CeO2 catalysts pre-calcined at 400 and 500 C, the reduction peaks of the CePtOx species are observed at 275 and 221 C, respectively [810,33]. In the case of the Pt/CeO2 catalysts pre-calcined at 600 and 700 C, the reduction peaks of surface PtOx species appeared at 87 and 69 C, respectively. It is reported that Pt2þ ion stabilized on nano-scale ceria is more active sites for WGS [810,33]. It can be concluded that the precalcination temperature of CHC has a significant effect on converting surface PtOx species into CePtOx, which is known to be an active site for the WGS [810,33]. As a result, the reduction of Pt/ CeO2 pre-calcined at 400 C is possible at 275 C, which is the highest reduction temperature. This result clearly indicates that the catalyst pre-calcined at 400 C results in an intimate interaction between Pt and CeO2, which demonstrates resistance to sintering. It is well known that an intimate interaction between metal and support is effective to mitigate sintering [18,21,27]. Thus, it is
Fig. 2. TPR patterns of the Pt/CeO2 catalysts pre-calcined at various temperatures.
3
expected that the Pt/CeO2 catalyst pre-calcined at 400 C should have a high CO conversion as well as high resistance to sintering. 3.1.2. Reaction results Fig. 3 shows the CO conversion profiles of the WGS over the Pt/ CeO2 catalysts pre-calcined at various temperatures. At a reaction temperature of 200 C, all of the catalysts showed negligible CO conversion. Within the reaction temperature range from 240 to 360 C, the Pt/CeO2 catalysts pre-calcined at 400 and 500 C showed higher CO conversions than the other catalysts. At 320 C, the Pt/CeO2 catalysts pre-calcined at 400 and 500 C resulted in CO conversion close to equilibrium value. The CO conversion of the Pt/ CeO2 catalyst pre-calcined at 400 C is slightly higher than that of Pt/CeO2 catalyst pre-calcined at 500 C. At the reaction temperature of 360 C, the CO conversions of the Pt/CeO2 catalysts pre-calcined at 400 and 500 C decreased due to the exothermic nature of the WGS. These results indicate that the pre-calcination temperature of CHC is an important parameter which influences the catalytic performance of Pt/CeO2 in the WGS. The pre-calcination temperature of CHC strongly affects the BET surface area, crystallite size of CeO2, and interaction between the metal and support of the Pt/ CeO2 catalysts. Therefore, it can be concluded that the highest activity of the Pt/CeO2 catalyst pre-calcined at 400 C is correlated to its high BET surface area, nano-sized CeO2, and an intimate interaction between Pt and CeO2. The Arrhenius plot in Fig. 4 compares the turnover frequency (TOF) of the Pt/CeO2 catalysts pre-calcined at various temperatures. The activation energies (Ea) of the WGS reaction were calculated from the slopes of the straight lines presented in Fig. 4. The Ea value decreases with increasing pre-calcination temperature. Hence, the Ea value of the Pt/CeO2 catalysts pre-calcined at 400 and 500 C are similar but the Ea of the Pt/CeO2 catalyst pre-calcined at 400 C (55 kJ/mol) is slightly lower than that of the Pt/CeO2 catalyst precalcined at 500 C (57 kJ/mol). This result agrees well with the WGS reaction results. Consequently, the pre-calcination temperature of the CHC can be controlled to result in a lower activation energy, resulting in a higher CO conversion. Fig. 5 shows the selectivity to CO2 and CH4 as a function of the reaction temperature over Pt/CeO2 catalysts pre-calcined at various temperatures. At 200 C, the undesirable methanation reaction occurred very slightly. However, almost 100% selectivity to CO2
Fig. 3. CO conversion as a function of the reaction temperature obtained over Pt/CeO2 catalysts pre-calcined at various temperatures (H2O/(CH4 þ CO þ CO2) ¼ 2.0, GHSV ¼ 45,515 h1).
Please cite this article in press as: Jeong D-W, et al., Optimization of a highly active nano-sized Pt/CeO2 catalyst via Ce(OH)CO3 for the water-gas shift reaction, Renewable Energy (2014), http://dx.doi.org/10.1016/j.renene.2014.07.041
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Fig. 4. Arrhenius plots of the turnover frequency of CO conversion obtained over Pt/ CeO2 catalysts pre-calcined at various temperatures (H2O/(CH4 þ CO þ CO2) ¼ 2.0, GHSV ¼ 45,515 h1).
could be achieved above 240 C for all of the catalysts. Therefore, it has been confirmed that the Pt/CeO2 catalysts prepared from CHC are selective toward the WGS. 3.2. Effect of aging time 3.2.1. Catalyst characterization Fig. 6 depicts the XRD patterns of the fresh and used Pt/CeO2 catalysts as a function of the aging time. All of the peaks are consistent with the peaks originating from CeO2 [9,10]. Table 2 summarizes the CeO2 crystallite size and BET surface area of the fresh and used Pt/CeO2 catalysts as a function of the aging time. Clearly, the crystallite size of CeO2 and the BET surface area depend on the aging time employed for the synthesis of CHC. It is expected that the aging time is correlated to the WGS activity. The optimum aging time for the synthesis of CHC is 4 h to obtain a nano-sized CeO2 crystalline and high BET surface area. Moreover, the Pt/CeO2 catalyst prepared with an aging time of 4 h has a high resistance to sintering, resulting in the smallest growth of CeO2 crystallite size (5%) and the smallest decrease of BET surface area (29%).
Fig. 6. XRD patterns of the (a) fresh and (b) used Pt/CeO2 catalysts obtained as a function of the aging time.
Fig. 7 shows the TPR patterns of the Pt/CeO2 catalysts obtained as a function of the aging time. All of the catalysts show a reduction peak assigned to the reduction of Pt species due to its interaction with the CeO2. The reduction peak of the Pt/CeO2 catalyst with an aging time of 4 h is observed at 275 C, which is the highest reduction temperature among the catalysts prepared in this study. This result indicates that the Pt/CeO2 catalyst prepared with an aging time of 4 h has an intimate interaction between Pt and CeO2. This is due to the fact that the nano-sized CeO2 support prepared with an aging time of 4 h helps to stabilize CePtOx species [810,33]. Therefore, it can be concluded that the aging time strongly influences the resistance to sintering of CeO2. The growth of the CeO2 crystallite size agrees well with the reduction property confirmed from the TPR results.
Fig. 5. Selectivity to CO2 and CH4 obtained over Pt/CeO2 catalysts pre-calcined at various temperatures (H2O/(CH4 þ CO þ CO2) ¼ 2.0, GHSV ¼ 45,515 h1).
3.2.2. Reaction results The WGS reaction data obtained from the Pt/CeO2 catalysts prepared at the various aging times are presented in Fig. 8. At 200 and 240 C, all catalysts showed low CO conversions. At the reaction temperature of 280 C, the CO conversions were about 53%, except for the catalyst prepared without aging. At 320 C, the CO conversion nearly reached equilibrium when aging times of 4 and 8 h were
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D.-W. Jeong et al. / Renewable Energy xxx (2014) 1e7
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Table 2 Characteristics of the Pt/CeO2 catalysts obtained as a function of the aging time. Aging time (h)
0 2 4 8
SBET (m2/g)
Crystallite size (nm)
Fresh
Used
Fresh
Used
110 113 136 123
72 76 96 86
8.9 8.5 7.9 8.2
9.9 9.3 8.3 8.7
(35%Y) (33%Y) (29%Y) (30%Y)
(11%[) (9%[) (5%[) (6%[)
used for the synthesis of CHC. On the contrary, the Pt/CeO2 catalyst prepared without aging showed a CO conversion of 63% at 320 C. The CO conversion at the various aging times decreased in the order of 4 h z 8 h > 2 h > 0 h. This result indicates that the aging time determines not only the characteristics of the catalyst but also the catalytic performance in the WGS. As a result, the optimum aging time was determined to be 4 h due to the high activity of Pt/CeO2 resulting from the highest BET surface area, nano-sized CeO2, and an intimate interaction between Pt and CeO2. Fig. 9 shows the Arrhenius plot of the Pt/CeO2 catalysts as a function of the aging time. Clearly, Ea depends on the aging time
Fig. 7. TPR patterns of the Pt/CeO2 catalysts obtained as a function of the aging time.
Fig. 8. CO conversions at various reaction temperatures obtained over Pt/CeO2 catalysts as a function of aging time (H2O/(CH4 þ CO þ CO2) ¼ 2.0, GHSV ¼ 45,515 h1).
Fig. 9. Arrhenius plots of the turnover frequency of the CO conversion obtained over Pt/CeO2 catalysts as a function of the aging time (H2O/(CH4 þ CO þ CO2) ¼ 2.0, GHSV ¼ 45,515 h1).
applied in the synthesis of CHC, ranging from 55 kJ/mol over Pt/ CeO2 (aging time ¼ 4 h) to 63 kJ/mol over Pt/CeO2 (aging time ¼ 0 h). The calculated trend of the Ea value is good agreement with the reaction results. Fig. 10 depicts the selectivity to CO2 and CH4 for various reaction temperatures over the Pt/CeO2 catalysts as a function of the aging time. The methanation reaction took place very slightly at 200 C. However, 100% selectivity to CO2 was obtained above 240 C over all of the catalysts. The carbon balance is shown in Table S1. The carbon balance is defined as the amount of carbon in the product gas divided by the amount of carbon in the reactant gas. For all of the catalysts, the value of carbon balance is closed up to 99 ± 2%. The carbon balance of the Pt/CeO2 catalysts is within the experimental error in all cases and at all temperatures. This result indicates that the carbonate species are not formed on the Pt/CeO2 catalysts. To evaluate the stability of the optimized Pt/CeO2 catalyst (precalcination temperature ¼ 400 C and aging time ¼ 4 h) in the WGS, CO conversion data was collected at 320 C for 100 h at a GHSV of 45,515 h1. Fig. 11 shows the CO conversion data as a function of the time of operation. The CO conversion was maintained for 100 h
Fig. 10. Selectivity to CO2 and CH4 obtained over Pt/CeO2 catalysts as a function of the aging time (H2O/(CH4 þ CO þ CO2) ¼ 2.0, GHSV ¼ 45,515 h1).
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by the Ministry of Science, (2013R1A1A1A05007370).
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and
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Planning
Appendix. A Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.renene.2014.07.041. References
Fig. 11. CO conversion as a function of the time of operation (T ¼ 320 C) obtained over the Pt/CeO2 catalyst with the optimized synthesis parameter (pre-calcination temperature ¼ 400 C and aging time ¼ 4 h) (H2O/(CH4 þ CO þ CO2) ¼ 2.0, GHSV ¼ 45,515 h1).
without significant catalyst deactivation. Therefore, it has been confirmed that the optimized Pt/CeO2 catalyst exhibited high activity as well as high stability. The high activity/stability of the optimized Pt/CeO2 catalyst in the WGS can be explained as follows. The characterization results showed that this catalyst has a high BET surface area due to the nano-crystalline nature of the CeO2 particles. The nano-sized CeO2 support can stabilize CeePtOx species at the surface [810,33]. The presence of CePtOx species over the optimized Pt/CeO2 catalyst is confirmed in the TPR results. Moreover, the CePtOx reduction of the optimized Pt/CeO2 catalyst is possible at 275 C, which is the highest reduction temperature. This result indicates that there is an intimate interaction between Pt and CeO2 in the Pt/CeO2 catalyst. It can be concluded that the pre-calcination temperature and aging time applied in the synthesis of the Pt/CeO2 catalyst can be controlled in such a way that more active CeePtOx sites are created at the surface of the catalyst. As a result, the optimized Pt/CeO2 catalyst exhibited stable activity, the highest CO conversion, and the lowest Ea. Consequentially, due to the excellent catalytic activity, low activation energy, and high resistance to sintering, the optimized synthesis parameter is found to be a pre-calcination temperature of 400 C and an aging time of 4 h. 4. Conclusion The catalytic performance of Pt/CeO2 catalysts strongly depends on the BET surface area, crystallite size of CeO2, and reduction property of Pt. The Pt/CeO2 catalyst prepared under the optimized synthesis parameter conditions (pre-calcination temperature ¼ 400 C, aging time ¼ 4 h) had the highest BET surface area, nano-sized CeO2, and intimate interaction between Pt and CeO2. As a result, the Pt/CeO2 catalyst prepared with the optimized synthesis parameter exhibited the highest CO conversion (82%) and the lowest activation energy (55 kJ/mol) among the catalysts tested in this study. Moreover, this catalyst has a high resistance to sintering, due to the stabilized CeePtOx species. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded
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Please cite this article in press as: Jeong D-W, et al., Optimization of a highly active nano-sized Pt/CeO2 catalyst via Ce(OH)CO3 for the water-gas shift reaction, Renewable Energy (2014), http://dx.doi.org/10.1016/j.renene.2014.07.041