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Influence of coexisting Al2O3 on the activity of copper catalyst for water–gas-shift reaction
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Influence of coexisting Al2O3 on the activity of copper catalyst for wateregas-shift reaction Kunimasa Sagata 1, Yasuhiro Kaneda, Hiroyuki Yamaura, Sengo Kobayashi, Hidenori Yahiro* Department of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan
abstract Keywords:
Influence of coexisting Al2O3 on the catalytic activity for low-temperature wateregas-shift
Low-temperature wateregas-shift
(LT-WGS) reaction over Cu catalyst was investigated. The catalytic activity of Cu/Al2O3
reaction
catalyst increased with decreasing mesopore size when S/C ratio was 2.2, whereas the
Copper catalyst
catalytic activity with S/C ratio ¼ 4.6 increased with increasing mesopore size. IR mea-
Bidentate-type carbonate
surement combined with kinetic study suggested that the low catalytic activity of Cu/CeO2
Basicity
catalyst comes from the restriction of CO adsorption on Cu0 by bidentate-type carbonate
Pore size
formed on the strong basic site of CeO2 support. On the other hand, it was found that bidentate-type carbonate was not formed on Cu/Al2O3 showing high catalytic activity for LT-WGS reaction. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The hydrogen-production with high efficiency is one of the key technologies for creation of clean energy system. There are three principal methods utilized to produce hydrogen from hydrocarbon fuels: steam reforming, partial oxidation, and autothermal reforming. Since these processes produce large amounts of carbon monoxide, wateregas-shift reaction (WGS reaction: CO þ H2O ¼ CO2 þ H2) is used for hydrogen purification. WGS reaction generally consists of two steps; high-temperature WGS (HT-WGS) reaction performed at 623e723 K and low-temperature WGS (LT-WGS) reaction at 473e523 K, due to restriction of thermodynamics at higher temperature and kinetics at lower temperature. Copper
catalyst is a potential candidate for LT-WGS reaction [1e9]. Up to date, a large number of Cu-based catalysts, such as Cu/ZnO [1], Cu/ZnO/Al2O3 [3], CueMn spinel oxides [5], and Cu/CeO2 [6], have been reported to exhibit high activity for LT-WGS reaction. In particular, Cu/ZnO catalyst has been commercialized as a catalyst active for LT-WGS reaction. Many researchers have pointed out that the WGS activity of CueZnObased catalysts is dependent upon the surface area of metallic copper (Cu0) [3,10]. However, Chinchen and Spencer [11] have reported that the WGS reaction in CueZnOeAl2O3 catalysts is sensitive to parameters other than Cu0 surface area. In our previous study, we reported the influence of metal oxide added to Cu catalysts on the catalytic activity of WGS reaction [12]. Although the catalytic activity was linearly correlated with surface area of Cu0 in the identical copper-
* Corresponding author. Tel.: þ81 89 927 9929; fax: þ81 89 927 9946. E-mail address: [email protected] (H. Yahiro). 1 Present address: Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, Kobe 657-8501, Japan. http://dx.doi.org/10.1016/j.ijhydene.2014.07.021 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Sagata K, et al., Influence of coexisting Al2O3 on the activity of copper catalyst for wateregasshift reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.07.021
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metal oxide system, the activity per Cu0 surface area changed depending on the metal oxide added. This result suggests the presence of alternative factors except for surface area of Cu0. In a series of the researches of copper-metal oxide catalysts, it was found that the basicity of metal oxide added to Cu catalyst has an influence on the catalytic activity of LT-WGS reaction [12]. In this study, we investigated the influence of coexisting Al2O3 on the catalytic activity for WGS reaction over Cu catalysts.
Experimental Catalyst preparation Preparation of mesoporous Cu/Al2O3 catalysts Mesoporous Al2O3 support was prepared by surfactant-assisted synthesis method reported by Cabrera and co-workers [13]. 0.8 g of NaOH dissolved in 4 ml of water was added to 80 ml of triethanolamine (TEA) and then heated at 393 K for 10 min to evaporate water. To this solution, 21.8 ml of aluminum-secbutoxide was slowly added with stirring. The resulting solution was heated at 423 K for 10 min (solution I). 14.56 g of alkyltrimetylammonium bromide (CmH2mþ1N(CH3)3Br, m ¼ 12, 16) was dissolved in 100 or 200 ml of water at 333 K (solution II). Solution I was slowly added to solution II at 333 K with vigorous stirring and aged for 72 h. The obtained solid was filtered, washed with ethanol, dried at 303 K for 1 h, and finally calcined at 773 K for 8 h under air. Alumina support with different pore size was obtained by changing either the ratio of water (solution II) to TEA (solution I) or the kind of surfactant species. Mesoporous alumina-supported Cu catalysts were prepared by a conventional impregnation method using copper nitrate. The resulting impregnated catalysts were dried at 383 K for 1 h, followed by calcination at 773 K for 8 h in air. The loading amount of Cu was unified to be 12 wt%. The mesoporous Cu/Al2O3 catalysts used in the present study are listed in Table 1.
Preparation of Cu/metal oxide catalysts Cu/metal oxide (Al2O3 and CeO2) catalysts were prepared by co-precipitation method using mixed aqueous solutions of Cu(NO3)2$6H2O and each metal nitrate hydrate (Al(NO3)3$9H2O or Ce(NO3)3$6H2O) with sodium carbonate decahydrate (Na2CO3$10H2O) solution as a precipitant. The mixed aqueous solution of metal nitrates (0.1 mol dm3 in total) was added to an aqueous Na2CO3 solution (0.1 mol dm3) under vigorous stirring at room temperature. The pH of solution was controlled at 9e9.5 by adding an aqueous NaOH solution. After aging at 333 K for 24 h, the precipitates were collected by filter, washed with de-ionized water, and then air-dried at 383 K
overnight. The resulting precipitates were finally calcined at 773 K for 8 h in air. The loading amount of Cu was unified to be 33 mol%.
Characterization X-ray powder diffraction (XRD) analysis was performed to determine the crystalline phase of catalyst using a Rigaku MiniFlexII diffractometer with CuKa radiation. BET analysis was conducted to determine the specific surface areas of catalyst using N2 adsorption at 77 K with a BEL Japan Bellsorpmini instrument. The surface area of Cu0 was measured by N2O decomposition method described elsewhere [8]. In-situ DRIFT-IR spectra were recorded with a FT-IR spectrometer (Spectrum One, PerkinElmer) equipped with MCT detector. TEM analysis was performed on a JEM-2010 (JEOL) equipped with EDS.
Evaluation of catalytic performance WGS reaction was carried out in a fixed-bed continuous flow reactor at 473 K. Prior to the activity test, the catalyst was reduced by flowing 20 vol% H2/He gas (30 cm3 min1) at 523 K for 2 h. The reactant gases contained 9.5e10.2 vol% of CO, 22.4e43.9 vol% of H2O, 39.8e60.1 vol% of H2, and 6.8e7.3 vol% of CO2. The total flow rate was 96 cm3 min1 and the volume of catalyst bed was 1.0 cm3 (gas space velocity ¼ 5760 h1). The effluents were analyzed by on-line gas chromatography (Shimadzu, GC-8AIT) using an active carbon column.
Results and discussion Influence of pore size of Al2O3 support on the catalytic activity for WGS reaction Mesoporous Cu/Al2O3 catalysts with different pore size were prepared to investigate the influences of pore size on the catalytic activity for WGS reaction. Fig. 1(A) shows N2 adsorption/desorption isotherms of mesoporous Cu/Al2O3 catalysts. All the Cu/Al2O3 catalysts showed similar IV-type isotherm, demonstrating the presence of mesopores. The pore size distributions of mesoporous Cu/Al2O3 catalysts are shown in Fig. 1(BeD). As can be seen in Fig. 1(BeD), narrow distribution in mesopore size was observed for each Cu/Al2O3 catalysts. Furthermore, the change in the surfactant and the water/TEA molar ratio provided the different averaged size of mesopore: 3.7 nm for CA-1, 4.2 nm for CA-2, and 5.4 nm for CA-3 (Table 1). Crystalline phases of Cu/Al2O3 catalysts were examined by XRD analysis as shown in Fig. 2. All the Cu/Al2O3 catalysts
Table 1 e Information of mesoporous Cu/Al2O3 catalysts. Catalysts CA-1 CA-2 CA-3
Surfactants
Water/TEA
SBET (m2 g1)
Dmeso (nm)
SCu (m2 g1)
[C16H33N(CH3)3]Br [C16H33N(CH3)3]Br [C12H25N(CH3)3]Br
18.4 9.2 9.2
276 273 289
3.7 4.2 5.4
1.4 0.7 0.5
Please cite this article in press as: Sagata K, et al., Influence of coexisting Al2O3 on the activity of copper catalyst for wateregasshift reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.07.021
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Fig. 1 e (A) Nitrogen adsorption (closed symbols)/desorption (open symbols) isotherms and (BeD) distributions of mesopore size. (C, B, (B)) CA-1, (▪, ▫, (C)) CA-2, and (▵, :, (D)) CA-3.
provided the broad diffraction lines assigned to g-Al2O3 (JCPDS file No. 10e425) and no diffraction line from CuO. This suggests that CuO was supported in the form of extremely small crystallites below the detection level for XRD or as an amorphous phase. The morphology of mesoporous Cu/Al2O3 catalyst was observed by TEM measurement. Fig. 3 shows TEM images of Cu/Al2O3 catalysts with different mesopore size. The sample was reduced by H2 at 523 K for 2 h prior to TEM measurement.
Fig. 2 e XRD patterns of (a) CA-1, (b) CA-2, and (c) CA-3 catalysts.
The mesoporous structure of sponge-like Al2O3 support was observed for all the Cu/Al2O3 samples. Metallic copper particles were not detected for all samples at the beginning TEM measurement (Fig. 3(A)e(C)). However, when the samples were further exposed to electron beam, the particles, probably attributed to Cu0, became detectable as shown in Fig. 3(A’), 3(A”), and 3(B’). This result suggests that the undetectable nano-sized Cu0 particles were formed on mesoporous Al2O3 matrix for all samples just after H2 reduction and such small particles grew up by irradiation of electron beam. From N2 adsorption and TEM measurements, it was demonstrated that nano-sized Cu0 particles exist in the pores of mesoporous Al2O3 matrix regardless of mesopore size. The catalytic activities of mesoporous Cu/Al2O3 catalysts were measured with different steam/carbon (S/C) ratio. The results are shown in Fig. 4. The catalytic activity of Cu/Al2O3 catalyst at S/C ¼ 2.2 (white bars) decreased with increasing average mesopore size. In contrast, the catalytic activity at S/ C ¼ 4.6 increased with increasing average mesopore size. This trade-off relation between the catalytic activity and the average mesopore size can be interpreted as follows. The catalytic activity of copper-based catalyst for WGS reaction was reported to depend on the surface area of Cu0 [3,8,10,12,14e16]; the higher catalytic activity, the larger surface area of Cu0. The surface areas of Cu0, measured for CA-1, CA-2, and CA-3 samples, are listed in Table 1. The surface area of Cu0 decreased in the following order: CA-1 > CA-2 > CA-3, being corresponded to the order in the catalytic activity under the experimental condition with low S/C ratio (S/C ¼ 2.2). On
Please cite this article in press as: Sagata K, et al., Influence of coexisting Al2O3 on the activity of copper catalyst for wateregasshift reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.07.021
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Fig. 3 e TEM images of Cu/Al2O3 catalysts after reduction. (A) CA-1, (A’) and (A”) CA-1 after the elapse of 5 (A’) and 15 (A”) min with exposing electron beam for (A), (B) CA-2, (B’) CA-2 after the elapse of 5 min for (B), and (C) CA-3.
the other hand, the influence of excess water on the catalytic activity should be considered under experimental condition with high S/C ratio (S/C ¼ 4.6). Oh et al. [17] reported that the capillary condensation of water vapor on mesoporous materials occurs in pores with smaller size. Taking this literature into consideration, the smaller mesopore size may suppress CO diffusion to Cu0 located in mesopore, probably decreasing catalytic activity.
IR study on Cu/Al2O3 catalysts We have reported that the catalytic activity of Cu-based catalyst for WGS reaction is strongly related to the weak basic sites of support oxide; in the cases of Cu/Al2O3 and Cu/ ZnO catalysts, the catalytic activity was proportional to the number of weak basic sites, while in the case of Cu/CeO2
catalyst with many strong basic sites, there was no correlation between the catalytic activity and the number of weak basic sites [12]. In order to clarify the reason of high catalytic activity of Cu/Al2O3, IR study of Cu/Al2O3 catalyst was carried out and compared with that of Cu/CeO2 catalyst. Fig. 5 shows IR spectra of Cu/Al2O3 and Cu/CeO2 catalysts at 323 K after CO introduction (2000 ppm CO/N2). Cu/Al2O3 and Cu/CeO2 catalysts were prereduced by H2 at 523 K before IR measurement. In the IR spectra of Cu/Al2O3 (Fig. 5(A)), an IR peak was observed at 2095 cm1, being assigned to the stretching band of CO adsorbed on Cu0 [18]. In addition, the absorption bands of carbonate species were observed in the range of 1800e1000 cm1. The peak intensities of these bands were unchanged with adsorption time. As depicted in Fig. 5(B), an IR band due to Cu0eCO was also observed for Cu/CeO2 catalyst. In contrast to Cu/Al2O3, Cu/CeO2 catalyst exhibited
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Fig. 4 e Catalytic activities at S/C (steam/carbon) ¼ 2.2 (white bar) and 4.6 (gray bar) and average pore diameter of mesoporous Cu/Al2O3 catalysts.
intense bands attributed to monodentate-type carbonate at 1061, 1369, and 1516 cm1 formed on weak basic site and bidentate-type carbonate formed on strong basic site at 1045, 1307, and 1609 cm1 [6,12,19,20]. The peak intensity of Cu0eCO band decreased with adsorption time. On the other hand, the peak intensity of bidentate-type carbonate species increased with adsorption time and no change in the peak intensity of monodentate-type carbonate species was observed. Fig. 6 shows the change in the band intensities of Cu0eCO (ICueCO) and bidentate-type carbonate species (I-CO3) with time when Cu/Al2O3 and Cu/CeO2 were exposed to CO(2000 ppm)/ N2. ICueCO for CueAl2O3 catalyst was unchanged (Fig. 6(A)) with time. On the other hand, ICueCO for CueCeO2 catalyst (Fig. 6(B)) decreased with adsorption time and simultaneously I-CO3 increased. On the basis of Fig. 6(B), the apparent rate constants of Cu0eCO reduction (kCueCO) and bidentate-type carbonate formation (k-CO3) were estimated. The changes in peak intensities of Cu0eCO and bidentate-type carbonate are represented by, DI ¼ Ifin Iobs
(1)
where Ifin and Iobs stand for the peak intensities at t ¼ 240 min and t ¼ t min, respectively. Based on the first-order kinetic, kCueCO and k-CO3 are determined by kt ¼ ln
I0 jDIj
(2)
(I0 represents the absorption intensity at t ¼ 0). The results are shown in Fig. 7. The linear correlations for both Cu0eCO reduction and bidentate-type carbonate formation were observed in Fig. 7, indicating that ICueCO decreased and I-CO3 increased according to a pseudo first-order rate equation. The kCueCO and k-CO3 values determined from Fig. 7 were 0.0196 and 0.0144 min1, respectively. In a similar way, the kCueCO and k-CO3 values for Cu/CeO2 with 50 mol% Cu were measured to be 0.0151 and 0.0166 min1, respectively (not shown here). It was found that kCueCO was in almost agreement with k-CO3. This result suggests that CO adsorbed on Cu0
Fig. 5 e DRIFT-IR spectra at 323 K after CO introduction (2000 ppm CO/N2). (A) Cu/Al2O3 and (B) Cu/CeO2 catalysts were prereduced at 523 K for 1 h under 20% H2/N2 before IR measurement. The upward and downward arrows indicate increase and decrease in the peak intensities with time. The double peaks at 2117 and 2170 cm¡1 are due to gaseous CO.
in Cu/CeO2 catalyst is converted into bidentate-type carbonate, probably formed on CeOx with strong basic site, and then the resulting bidentate-type carbonate restricts CO adsorption on Cu0. This conclusion was supported by the measurements of Cu0 surface area. After Cu/Al2O3 catalyst was exposed to 2% CO/He at 323 K for 60 min, followed by flushed with He at 473 K for 10 min, the obtained Cu0 surface area was 9.8 m2 g1, being almost the same as that without exposure to CO (9.2 m2 g1).
Please cite this article in press as: Sagata K, et al., Influence of coexisting Al2O3 on the activity of copper catalyst for wateregasshift reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.07.021
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The present IR study demonstrated that bidentate-type carbonates were not or slightly formed on Cu/Al2O3 showing high catalytic activity for WGS reaction and they were formed on Cu/CeO2 showing low catalytic activity. Several researchers reported that the presence of carbonate species formed on catalyst surface has a large influence on the catalytic property for WGS reaction [6,21,22]; for example, Guo et al. [22] suggested that the deactivation of Cu/ZnO catalyst system was caused by the blockage of active sites existing on Cu and ZnO interface due to carbonate formation. For supported catalyst, two sites important for WGS reaction, the site activating CO on metal and the principal site for water activation on support, were proposed by Grenoble et al. [23] Taking above literature into consideration, the formation of bidentate-type carbonate on strong basic sites of support may suppress the activation of CO and/or H2O on the interface between Cu and support, resulting in low catalytic activity for WGS reaction. On the other hand, the carbonate species formed on weak basic sites may decompose to regenerate active sites on the interface between Cu and support, resulting in high catalytic activity. It is considered that the high catalytic activity of Cu/Al2O3 is attributed to less amount of bidentate-type carbonate formed on Al2O3 support.
Conclusions
Fig. 6 e Time dependence of (-) ICueCO and (B) I-CO3 after CO introduction (2000 ppm CO/N2 at 323 K). (A) Cu/Al2O3 and (B) Cu/CeO2.
On the other hand, Cu0 surface area of Cu/CeO2 was drastically decreased from 10.7 m2 g1 without CO exposure to 5.0 m2 g1 with CO exposure. This result suggests that Cu0 in Cu/CeO2 is partially covered by carbonate species.
Fig. 7 e First order plots of DI of (-) Cu0eCO and (B) eCO3 for Cu/CeO2 catalyst.
Influence of coexisting Al2O3 on the catalytic activity for lowtemperature wateregas-shift (LT-WGS) reaction over Cu catalysts was investigated. An ordered mesoporous Al2O3 prepared by surfactant-assisted synthesis method was used as a support of Cu catalyst to clarify the influence of pore size of support on the catalytic activity of WGS reaction. The catalytic activities of Cu/Al2O3 catalysts increased with decreasing mesopore size when S/C ratio was 2.2, whereas the catalytic activities with S/C ratio ¼ 4.6 increased with increasing mesopore size. In the former case, the increment of the catalytic activity is considered to be attributed to increasing in the surface area of Cu0 as an active site with decreasing mesopore. On the other hand, in the latter case, the decrement of the catalytic activity may come from the capillary condensation of water vapor on mesoporous materials; the smaller mesopore size may suppress CO diffusion to Cu0 located in mesopore. IR studies for Cu/Al2O3 and Cu/CeO2 catalysts were carried out to elucidate the surface species formed after CO introduction. The IR bands assigned to Cu0eCO and carbonate species appeared for both catalysts after CO introduction. No or less change in peak intensities of Cu0eCO and carbonate species with adsorption time was observed for Cu/Al2O3. On the other hand, for Cu/CeO2 catalyst, the peak intensity of Cu0eCO decreased with adsorption time and that of bidentate-type carbonate increased. The kinetic study on the change in IR peak intensities of Cu0eCO and bidentate-type carbonate for Cu/CeO2 catalyst suggests that CO adsorbed on Cu0 is converted into bidentate-type carbonate and then the obtained bidentate-type carbonate restricts CO adsorption on Cu0. The present IR study demonstrated that bidentate-type carbonates were not or slightly formed on Cu/Al2O3 showing high catalytic activity for WGS reaction and they were formed on Cu/CeO2 showing low catalytic activity.
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Influence of coexisting Al2O3 on the activity of copper catalyst for wateregas-shift reaction Kunimasa Sagata 1, Yasuhiro Kaneda, Hiroyuki Yamaura, Sengo Kobayashi, Hidenori Yahiro* Department of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan
abstract Keywords:
Influence of coexisting Al2O3 on the catalytic activity for low-temperature wateregas-shift
Low-temperature wateregas-shift
(LT-WGS) reaction over Cu catalyst was investigated. The catalytic activity of Cu/Al2O3
reaction
catalyst increased with decreasing mesopore size when S/C ratio was 2.2, whereas the
Copper catalyst
catalytic activity with S/C ratio ¼ 4.6 increased with increasing mesopore size. IR mea-
Bidentate-type carbonate
surement combined with kinetic study suggested that the low catalytic activity of Cu/CeO2
Basicity
catalyst comes from the restriction of CO adsorption on Cu0 by bidentate-type carbonate
Pore size
formed on the strong basic site of CeO2 support. On the other hand, it was found that bidentate-type carbonate was not formed on Cu/Al2O3 showing high catalytic activity for LT-WGS reaction. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The hydrogen-production with high efficiency is one of the key technologies for creation of clean energy system. There are three principal methods utilized to produce hydrogen from hydrocarbon fuels: steam reforming, partial oxidation, and autothermal reforming. Since these processes produce large amounts of carbon monoxide, wateregas-shift reaction (WGS reaction: CO þ H2O ¼ CO2 þ H2) is used for hydrogen purification. WGS reaction generally consists of two steps; high-temperature WGS (HT-WGS) reaction performed at 623e723 K and low-temperature WGS (LT-WGS) reaction at 473e523 K, due to restriction of thermodynamics at higher temperature and kinetics at lower temperature. Copper
catalyst is a potential candidate for LT-WGS reaction [1e9]. Up to date, a large number of Cu-based catalysts, such as Cu/ZnO [1], Cu/ZnO/Al2O3 [3], CueMn spinel oxides [5], and Cu/CeO2 [6], have been reported to exhibit high activity for LT-WGS reaction. In particular, Cu/ZnO catalyst has been commercialized as a catalyst active for LT-WGS reaction. Many researchers have pointed out that the WGS activity of CueZnObased catalysts is dependent upon the surface area of metallic copper (Cu0) [3,10]. However, Chinchen and Spencer [11] have reported that the WGS reaction in CueZnOeAl2O3 catalysts is sensitive to parameters other than Cu0 surface area. In our previous study, we reported the influence of metal oxide added to Cu catalysts on the catalytic activity of WGS reaction [12]. Although the catalytic activity was linearly correlated with surface area of Cu0 in the identical copper-
* Corresponding author. Tel.: þ81 89 927 9929; fax: þ81 89 927 9946. E-mail address: [email protected] (H. Yahiro). 1 Present address: Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, Kobe 657-8501, Japan. http://dx.doi.org/10.1016/j.ijhydene.2014.07.021 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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metal oxide system, the activity per Cu0 surface area changed depending on the metal oxide added. This result suggests the presence of alternative factors except for surface area of Cu0. In a series of the researches of copper-metal oxide catalysts, it was found that the basicity of metal oxide added to Cu catalyst has an influence on the catalytic activity of LT-WGS reaction [12]. In this study, we investigated the influence of coexisting Al2O3 on the catalytic activity for WGS reaction over Cu catalysts.
Experimental Catalyst preparation Preparation of mesoporous Cu/Al2O3 catalysts Mesoporous Al2O3 support was prepared by surfactant-assisted synthesis method reported by Cabrera and co-workers [13]. 0.8 g of NaOH dissolved in 4 ml of water was added to 80 ml of triethanolamine (TEA) and then heated at 393 K for 10 min to evaporate water. To this solution, 21.8 ml of aluminum-secbutoxide was slowly added with stirring. The resulting solution was heated at 423 K for 10 min (solution I). 14.56 g of alkyltrimetylammonium bromide (CmH2mþ1N(CH3)3Br, m ¼ 12, 16) was dissolved in 100 or 200 ml of water at 333 K (solution II). Solution I was slowly added to solution II at 333 K with vigorous stirring and aged for 72 h. The obtained solid was filtered, washed with ethanol, dried at 303 K for 1 h, and finally calcined at 773 K for 8 h under air. Alumina support with different pore size was obtained by changing either the ratio of water (solution II) to TEA (solution I) or the kind of surfactant species. Mesoporous alumina-supported Cu catalysts were prepared by a conventional impregnation method using copper nitrate. The resulting impregnated catalysts were dried at 383 K for 1 h, followed by calcination at 773 K for 8 h in air. The loading amount of Cu was unified to be 12 wt%. The mesoporous Cu/Al2O3 catalysts used in the present study are listed in Table 1.
Preparation of Cu/metal oxide catalysts Cu/metal oxide (Al2O3 and CeO2) catalysts were prepared by co-precipitation method using mixed aqueous solutions of Cu(NO3)2$6H2O and each metal nitrate hydrate (Al(NO3)3$9H2O or Ce(NO3)3$6H2O) with sodium carbonate decahydrate (Na2CO3$10H2O) solution as a precipitant. The mixed aqueous solution of metal nitrates (0.1 mol dm3 in total) was added to an aqueous Na2CO3 solution (0.1 mol dm3) under vigorous stirring at room temperature. The pH of solution was controlled at 9e9.5 by adding an aqueous NaOH solution. After aging at 333 K for 24 h, the precipitates were collected by filter, washed with de-ionized water, and then air-dried at 383 K
overnight. The resulting precipitates were finally calcined at 773 K for 8 h in air. The loading amount of Cu was unified to be 33 mol%.
Characterization X-ray powder diffraction (XRD) analysis was performed to determine the crystalline phase of catalyst using a Rigaku MiniFlexII diffractometer with CuKa radiation. BET analysis was conducted to determine the specific surface areas of catalyst using N2 adsorption at 77 K with a BEL Japan Bellsorpmini instrument. The surface area of Cu0 was measured by N2O decomposition method described elsewhere [8]. In-situ DRIFT-IR spectra were recorded with a FT-IR spectrometer (Spectrum One, PerkinElmer) equipped with MCT detector. TEM analysis was performed on a JEM-2010 (JEOL) equipped with EDS.
Evaluation of catalytic performance WGS reaction was carried out in a fixed-bed continuous flow reactor at 473 K. Prior to the activity test, the catalyst was reduced by flowing 20 vol% H2/He gas (30 cm3 min1) at 523 K for 2 h. The reactant gases contained 9.5e10.2 vol% of CO, 22.4e43.9 vol% of H2O, 39.8e60.1 vol% of H2, and 6.8e7.3 vol% of CO2. The total flow rate was 96 cm3 min1 and the volume of catalyst bed was 1.0 cm3 (gas space velocity ¼ 5760 h1). The effluents were analyzed by on-line gas chromatography (Shimadzu, GC-8AIT) using an active carbon column.
Results and discussion Influence of pore size of Al2O3 support on the catalytic activity for WGS reaction Mesoporous Cu/Al2O3 catalysts with different pore size were prepared to investigate the influences of pore size on the catalytic activity for WGS reaction. Fig. 1(A) shows N2 adsorption/desorption isotherms of mesoporous Cu/Al2O3 catalysts. All the Cu/Al2O3 catalysts showed similar IV-type isotherm, demonstrating the presence of mesopores. The pore size distributions of mesoporous Cu/Al2O3 catalysts are shown in Fig. 1(BeD). As can be seen in Fig. 1(BeD), narrow distribution in mesopore size was observed for each Cu/Al2O3 catalysts. Furthermore, the change in the surfactant and the water/TEA molar ratio provided the different averaged size of mesopore: 3.7 nm for CA-1, 4.2 nm for CA-2, and 5.4 nm for CA-3 (Table 1). Crystalline phases of Cu/Al2O3 catalysts were examined by XRD analysis as shown in Fig. 2. All the Cu/Al2O3 catalysts
Table 1 e Information of mesoporous Cu/Al2O3 catalysts. Catalysts CA-1 CA-2 CA-3
Surfactants
Water/TEA
SBET (m2 g1)
Dmeso (nm)
SCu (m2 g1)
[C16H33N(CH3)3]Br [C16H33N(CH3)3]Br [C12H25N(CH3)3]Br
18.4 9.2 9.2
276 273 289
3.7 4.2 5.4
1.4 0.7 0.5
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Fig. 1 e (A) Nitrogen adsorption (closed symbols)/desorption (open symbols) isotherms and (BeD) distributions of mesopore size. (C, B, (B)) CA-1, (▪, ▫, (C)) CA-2, and (▵, :, (D)) CA-3.
provided the broad diffraction lines assigned to g-Al2O3 (JCPDS file No. 10e425) and no diffraction line from CuO. This suggests that CuO was supported in the form of extremely small crystallites below the detection level for XRD or as an amorphous phase. The morphology of mesoporous Cu/Al2O3 catalyst was observed by TEM measurement. Fig. 3 shows TEM images of Cu/Al2O3 catalysts with different mesopore size. The sample was reduced by H2 at 523 K for 2 h prior to TEM measurement.
Fig. 2 e XRD patterns of (a) CA-1, (b) CA-2, and (c) CA-3 catalysts.
The mesoporous structure of sponge-like Al2O3 support was observed for all the Cu/Al2O3 samples. Metallic copper particles were not detected for all samples at the beginning TEM measurement (Fig. 3(A)e(C)). However, when the samples were further exposed to electron beam, the particles, probably attributed to Cu0, became detectable as shown in Fig. 3(A’), 3(A”), and 3(B’). This result suggests that the undetectable nano-sized Cu0 particles were formed on mesoporous Al2O3 matrix for all samples just after H2 reduction and such small particles grew up by irradiation of electron beam. From N2 adsorption and TEM measurements, it was demonstrated that nano-sized Cu0 particles exist in the pores of mesoporous Al2O3 matrix regardless of mesopore size. The catalytic activities of mesoporous Cu/Al2O3 catalysts were measured with different steam/carbon (S/C) ratio. The results are shown in Fig. 4. The catalytic activity of Cu/Al2O3 catalyst at S/C ¼ 2.2 (white bars) decreased with increasing average mesopore size. In contrast, the catalytic activity at S/ C ¼ 4.6 increased with increasing average mesopore size. This trade-off relation between the catalytic activity and the average mesopore size can be interpreted as follows. The catalytic activity of copper-based catalyst for WGS reaction was reported to depend on the surface area of Cu0 [3,8,10,12,14e16]; the higher catalytic activity, the larger surface area of Cu0. The surface areas of Cu0, measured for CA-1, CA-2, and CA-3 samples, are listed in Table 1. The surface area of Cu0 decreased in the following order: CA-1 > CA-2 > CA-3, being corresponded to the order in the catalytic activity under the experimental condition with low S/C ratio (S/C ¼ 2.2). On
Please cite this article in press as: Sagata K, et al., Influence of coexisting Al2O3 on the activity of copper catalyst for wateregasshift reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.07.021
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Fig. 3 e TEM images of Cu/Al2O3 catalysts after reduction. (A) CA-1, (A’) and (A”) CA-1 after the elapse of 5 (A’) and 15 (A”) min with exposing electron beam for (A), (B) CA-2, (B’) CA-2 after the elapse of 5 min for (B), and (C) CA-3.
the other hand, the influence of excess water on the catalytic activity should be considered under experimental condition with high S/C ratio (S/C ¼ 4.6). Oh et al. [17] reported that the capillary condensation of water vapor on mesoporous materials occurs in pores with smaller size. Taking this literature into consideration, the smaller mesopore size may suppress CO diffusion to Cu0 located in mesopore, probably decreasing catalytic activity.
IR study on Cu/Al2O3 catalysts We have reported that the catalytic activity of Cu-based catalyst for WGS reaction is strongly related to the weak basic sites of support oxide; in the cases of Cu/Al2O3 and Cu/ ZnO catalysts, the catalytic activity was proportional to the number of weak basic sites, while in the case of Cu/CeO2
catalyst with many strong basic sites, there was no correlation between the catalytic activity and the number of weak basic sites [12]. In order to clarify the reason of high catalytic activity of Cu/Al2O3, IR study of Cu/Al2O3 catalyst was carried out and compared with that of Cu/CeO2 catalyst. Fig. 5 shows IR spectra of Cu/Al2O3 and Cu/CeO2 catalysts at 323 K after CO introduction (2000 ppm CO/N2). Cu/Al2O3 and Cu/CeO2 catalysts were prereduced by H2 at 523 K before IR measurement. In the IR spectra of Cu/Al2O3 (Fig. 5(A)), an IR peak was observed at 2095 cm1, being assigned to the stretching band of CO adsorbed on Cu0 [18]. In addition, the absorption bands of carbonate species were observed in the range of 1800e1000 cm1. The peak intensities of these bands were unchanged with adsorption time. As depicted in Fig. 5(B), an IR band due to Cu0eCO was also observed for Cu/CeO2 catalyst. In contrast to Cu/Al2O3, Cu/CeO2 catalyst exhibited
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Fig. 4 e Catalytic activities at S/C (steam/carbon) ¼ 2.2 (white bar) and 4.6 (gray bar) and average pore diameter of mesoporous Cu/Al2O3 catalysts.
intense bands attributed to monodentate-type carbonate at 1061, 1369, and 1516 cm1 formed on weak basic site and bidentate-type carbonate formed on strong basic site at 1045, 1307, and 1609 cm1 [6,12,19,20]. The peak intensity of Cu0eCO band decreased with adsorption time. On the other hand, the peak intensity of bidentate-type carbonate species increased with adsorption time and no change in the peak intensity of monodentate-type carbonate species was observed. Fig. 6 shows the change in the band intensities of Cu0eCO (ICueCO) and bidentate-type carbonate species (I-CO3) with time when Cu/Al2O3 and Cu/CeO2 were exposed to CO(2000 ppm)/ N2. ICueCO for CueAl2O3 catalyst was unchanged (Fig. 6(A)) with time. On the other hand, ICueCO for CueCeO2 catalyst (Fig. 6(B)) decreased with adsorption time and simultaneously I-CO3 increased. On the basis of Fig. 6(B), the apparent rate constants of Cu0eCO reduction (kCueCO) and bidentate-type carbonate formation (k-CO3) were estimated. The changes in peak intensities of Cu0eCO and bidentate-type carbonate are represented by, DI ¼ Ifin Iobs
(1)
where Ifin and Iobs stand for the peak intensities at t ¼ 240 min and t ¼ t min, respectively. Based on the first-order kinetic, kCueCO and k-CO3 are determined by kt ¼ ln
I0 jDIj
(2)
(I0 represents the absorption intensity at t ¼ 0). The results are shown in Fig. 7. The linear correlations for both Cu0eCO reduction and bidentate-type carbonate formation were observed in Fig. 7, indicating that ICueCO decreased and I-CO3 increased according to a pseudo first-order rate equation. The kCueCO and k-CO3 values determined from Fig. 7 were 0.0196 and 0.0144 min1, respectively. In a similar way, the kCueCO and k-CO3 values for Cu/CeO2 with 50 mol% Cu were measured to be 0.0151 and 0.0166 min1, respectively (not shown here). It was found that kCueCO was in almost agreement with k-CO3. This result suggests that CO adsorbed on Cu0
Fig. 5 e DRIFT-IR spectra at 323 K after CO introduction (2000 ppm CO/N2). (A) Cu/Al2O3 and (B) Cu/CeO2 catalysts were prereduced at 523 K for 1 h under 20% H2/N2 before IR measurement. The upward and downward arrows indicate increase and decrease in the peak intensities with time. The double peaks at 2117 and 2170 cm¡1 are due to gaseous CO.
in Cu/CeO2 catalyst is converted into bidentate-type carbonate, probably formed on CeOx with strong basic site, and then the resulting bidentate-type carbonate restricts CO adsorption on Cu0. This conclusion was supported by the measurements of Cu0 surface area. After Cu/Al2O3 catalyst was exposed to 2% CO/He at 323 K for 60 min, followed by flushed with He at 473 K for 10 min, the obtained Cu0 surface area was 9.8 m2 g1, being almost the same as that without exposure to CO (9.2 m2 g1).
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The present IR study demonstrated that bidentate-type carbonates were not or slightly formed on Cu/Al2O3 showing high catalytic activity for WGS reaction and they were formed on Cu/CeO2 showing low catalytic activity. Several researchers reported that the presence of carbonate species formed on catalyst surface has a large influence on the catalytic property for WGS reaction [6,21,22]; for example, Guo et al. [22] suggested that the deactivation of Cu/ZnO catalyst system was caused by the blockage of active sites existing on Cu and ZnO interface due to carbonate formation. For supported catalyst, two sites important for WGS reaction, the site activating CO on metal and the principal site for water activation on support, were proposed by Grenoble et al. [23] Taking above literature into consideration, the formation of bidentate-type carbonate on strong basic sites of support may suppress the activation of CO and/or H2O on the interface between Cu and support, resulting in low catalytic activity for WGS reaction. On the other hand, the carbonate species formed on weak basic sites may decompose to regenerate active sites on the interface between Cu and support, resulting in high catalytic activity. It is considered that the high catalytic activity of Cu/Al2O3 is attributed to less amount of bidentate-type carbonate formed on Al2O3 support.
Conclusions
Fig. 6 e Time dependence of (-) ICueCO and (B) I-CO3 after CO introduction (2000 ppm CO/N2 at 323 K). (A) Cu/Al2O3 and (B) Cu/CeO2.
On the other hand, Cu0 surface area of Cu/CeO2 was drastically decreased from 10.7 m2 g1 without CO exposure to 5.0 m2 g1 with CO exposure. This result suggests that Cu0 in Cu/CeO2 is partially covered by carbonate species.
Fig. 7 e First order plots of DI of (-) Cu0eCO and (B) eCO3 for Cu/CeO2 catalyst.
Influence of coexisting Al2O3 on the catalytic activity for lowtemperature wateregas-shift (LT-WGS) reaction over Cu catalysts was investigated. An ordered mesoporous Al2O3 prepared by surfactant-assisted synthesis method was used as a support of Cu catalyst to clarify the influence of pore size of support on the catalytic activity of WGS reaction. The catalytic activities of Cu/Al2O3 catalysts increased with decreasing mesopore size when S/C ratio was 2.2, whereas the catalytic activities with S/C ratio ¼ 4.6 increased with increasing mesopore size. In the former case, the increment of the catalytic activity is considered to be attributed to increasing in the surface area of Cu0 as an active site with decreasing mesopore. On the other hand, in the latter case, the decrement of the catalytic activity may come from the capillary condensation of water vapor on mesoporous materials; the smaller mesopore size may suppress CO diffusion to Cu0 located in mesopore. IR studies for Cu/Al2O3 and Cu/CeO2 catalysts were carried out to elucidate the surface species formed after CO introduction. The IR bands assigned to Cu0eCO and carbonate species appeared for both catalysts after CO introduction. No or less change in peak intensities of Cu0eCO and carbonate species with adsorption time was observed for Cu/Al2O3. On the other hand, for Cu/CeO2 catalyst, the peak intensity of Cu0eCO decreased with adsorption time and that of bidentate-type carbonate increased. The kinetic study on the change in IR peak intensities of Cu0eCO and bidentate-type carbonate for Cu/CeO2 catalyst suggests that CO adsorbed on Cu0 is converted into bidentate-type carbonate and then the obtained bidentate-type carbonate restricts CO adsorption on Cu0. The present IR study demonstrated that bidentate-type carbonates were not or slightly formed on Cu/Al2O3 showing high catalytic activity for WGS reaction and they were formed on Cu/CeO2 showing low catalytic activity.
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