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ZnO methanol synthesis catalysts — morphology effect or active site model?
Applied Catalysis A: General 208 (2001) 163–167
The role of ZnO in Cu/ZnO methanol synthesis catalysts — morphology effect or active site model? Y. Choi a , K. Futagami a , T. Fujitani b,∗ , J. Nakamura a a
Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan National Institute for Resources and Environment, Tsukuba, Ibaraki 305-8569, Japan
b
Received 8 May 2000; received in revised form 19 June 2000; accepted 20 June 2000
Abstract We examined whether or not the presence of ZnO changed the morphology of Cu particles in Cu/ZnO methanol synthesis catalysts using physical mixtures of Cu/SiO2 + ZnO/SiO2 . The yield of methanol produced by the CO2 hydrogenation over the physical mixture increased by the reduction with H2 at 573–723 K. The promotion of the methanol synthesis activity was due to migration of Zn from ZnO onto Cu/SiO2 , leading to the Cu–Zn active sites, as previously reported. On the other hand, no activity change was observed upon the reduction treatment for the reverse water gas shift (RWGS) reaction over the physical mixture; this result cannot be explained by the morphology change. The activation energy of the methanol synthesis and the RWGS reaction over the physical mixture did not vary upon reduction, further supporting the absence of any morphology change. It was thus clearly proved that the role of ZnO was not to change the morphology of Cu, but to create the Cu–Zn active sites for methanol synthesis only. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Cu/ZnO catalyst; Methanol synthesis; Reverse water gas shift reaction; Role of ZnO; Morphology of Cu; Active site
1. Introduction The methanol synthesis from CO/CO2 /H2 is an important industrial process, and Cu/ZnO-based catalysts are well known to be active for this reaction. Although numerous investigations of the methanol synthesis catalysts have been carried out over the past few decades, there are still controversies concerning the role of ZnO and the active sites. It is widely accepted that the use of ZnO as a support material leads to a highly dispersed Cu catalyst. The controversial issue is the additional effect of ZnO upon methanol synthesis activity on Cu. The proposed models may be classified into ∗ Corresponding author. Tel.: +81-298-618173; fax: +81-298-530202. E-mail address: [email protected] (T. Fujitani).
three categories: (i) spillover model by Burch et al. and Spencer [1,2] in which ZnO acts as a reservoir of hydrogen for the hydrogenation of CO over Cu surfaces; (ii) the morphology effect proposed by Yoshihara and Campbell [3], Ovesen et al. [4], Hadden et al. [5], and Topsøe and Topsøe [6], in which the morphology of copper particles on a ZnO support is responsible for the effect of ZnO upon the methanol synthesis on Cu; and (iii) the active site model on Cu surfaces proposed by us (Fujitani and Nakamura and coworkers) [7–13]. We have reported the role of ZnO, the active sites and the reaction mechanism of the methanol synthesis by hydrogenation of CO2 over Cu/ZnO-based catalysts using both surface science and powder catalysts [7–16]. In experiments using a physical mixture of Cu/SiO2 + ZnO/SiO2 [8], Zn atoms were found
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to migrate from ZnO onto the Cu particles by H2 reduction at 573–723 K, leading to an increase in methanol synthesis activity. We confirmed that the resulting Zn-containing Cu/SiO2 , or (Zn)Cu/SiO2 , separated from the physical mixture was responsible for the promotional effect [10]. We found that the promotional effect of ZnO could be ascribed to the creation of the Cu–Zn active site on the Cu surface [7,8,10]. The active site model was then proved by the surface science study using a Zn-deposited Cu (1 1 1) model catalyst [15,16]. It has been clearly shown that the deposition of Zn significantly promotes the methanol synthesis activity of Cu (1 1 1). We have thus concluded that the role of ZnO in Cu/ZnO-based catalysts is to create the Cu–Zn active site required for methanol synthesis from CO2 and H2 . The metallic Cu is also necessary for the hydrogenation processes such as formate synthesis, so that the Cu–Zn site and metallic Cu cooperatively catalyze the methanol synthesis from CO2 and H2 . We then studied the elementary steps, formate intermediate and kinetics [16–18]. The active site determination for the methanol synthesis from CO and H2 is now under investigation; results will be reported elsewhere. Waugh [19] recently commented that our data [12] concerning the promotion of Zn upon methanol synthesis over Cu–ZnO catalysts could be explained by the effect of ZnO on the morphology of the Cu particles, i.e. a morphology effect, influencing the catalytic activities of both the methanol synthesis and a parallel reaction of the reverse water gas shift (RWGS) reaction (CO2 + H2 → CO + H2 O) over Cu/ZnO-based catalysts. We replied that the promotion of Zn cannot be explained by the morphology effect, but is well-explained by the creation of the Cu–Zn active sites [20]. He discussed the effects of the Cu morphology on the methanol synthesis and the RWGS reaction using different catalysts. However, the morphology effect on both reactions should be examined using an identical catalyst. In this study, we examined the catalytic activities for both the methanol synthesis by CO2 hydrogenation and the RWGS reaction using selected catalysts of the physical mixture of Cu/SiO2 + ZnO/SiO2 , Cu/SiO2 , the promoted (Zn)Cu/SiO2 [10], and Cu/ZnO/Al2 O3 . We then discussed whether or not the presence of the Zn promoter changed the morphology of Cu particles.
2. Experimental A Cu/SiO2 catalyst was prepared by the impregnation method using Cu(NO3 )2 ·3H2 O and commercial silica (Aerosil 300, 99.9%). The loading of Cu was 30 wt.%. The Cu/SiO2 was dried in an oven at 383 K for 12 h in air, and then calcined at 623 K for 2 h in air. The Cu/SiO2 catalyst was sieved to collect the catalyst particles of <150 m. A ZnO/SiO2 catalyst was prepared by the alkoxide method using Zn(NO3 )2 ·6H2 O and Si(OCH2 CH3 )4 [8]. The loading of ZnO was 80 wt.%. The ZnO/SiO2 catalyst was calcined at 723 K for 2 h in air, and then sieved to collect the catalyst particles of 150–250 m. The catalysts were physically mixed with a weight ratio of Cu/SiO2 : ZnO/SiO2 = 0.25:0.25 g, then the mixture was reduced with H2 in a flow reactor for 2 h at 523–723 K and 50 atm. This leads to the migration of Zn species from ZnO/SiO2 onto the Cu/SiO2 , as previously reported [8]. The hydrogenation of CO2 was carried out at 50 atm with a H2 /CO2 ratio of 3 and a flow rate of 75 cc/min. In some experiments, we used the resulting Zn-containing Cu/SiO2 separated from the physical mixture using a 150 m sieve, which was abbreviated (Zn)Cu/SiO2 [10]. The collected (Zn)Cu/SiO2 catalyst was set in the flow reactor and re-reduced with H2 at 523 K and 50 atm. The weight of the (Zn)Cu/SiO2 catalyst used for reaction was 0.25 g. The reaction conditions were the same as those for physical mixture catalysts. We also examined the catalytic activity of a Cu/ZnO/Al2 O3 catalyst to compare it with those of the physical mixture, Cu/SiO2 and (Zn)Cu/SiO2 . The Cu/ZnO/Al2 O3 catalyst was prepared by a co-precipitation method. Both an aqueous solution of Na2 CO3 (1.1 M) and a mixed solution of copper, zinc and, aluminum nitrates (total metal concentration 1.0 M) were added dropwise to distilled water at 300 K. The precipitate was aged in the mixed solution at 303 K for 72 h, followed by a thorough washing with distilled water. Subsequently, the precipitate was dried overnight in an oven at 383 K, and then calcined in air at 623 K for 3 h. The loading of Cu, ZnO and Al2 O3 of the prepared Cu/ZnO/Al2 O3 was 50:40:10 wt.%. The hydrogenation of CO2 was carried out at a PH2 /PCO2 = 37.5/12.5 atm. The flow rate was 800 cc/min and the weight of the catalyst was 0.125 g.
Y. Choi et al. / Applied Catalysis A: General 208 (2001) 163–167
The reaction products were analyzed using gas chromatographs with a thermal conductivity detector and a flame ionization detector. Catalytic activities were evaluated in terms of mass time yield (MTY) defined as the weight of the product molecules per catalyst weight per time (g molecule/kg-cat/h). In the case of the physically mixed catalyst, MTY was calculated based on the weight of Cu/SiO2 . The copper surface areas of each catalyst were measured with N2 O/He (2.54% N2 O) gas by the reactive frontal chromatography, as previously reported [8].
3. Results and discussion We examined the effect of the reduction pretreatment by H2 on the catalytic activities as previously carried out [8,10]. Fig. 1 shows the yields of CO formed by the RWGS reaction and methanol as a function of the reduction temperature of the physical mixture, Cu/SiO2 , and (Zn)Cu/SiO2 . As for the physical mixture, the methanol yield increased with increasing reduction temperature in the range of 573–723 K,
Fig. 1. The yield of methanol and CO produced by CO2 hydrogenation over a physical mixture of Cu/SiO2 + ZnO/SiO2 , Cu/SiO2 , and a promoted (Zn)Cu/SiO2 as a function of reduction temperature. The CO2 hydrogenation was carried out at PH2 /PCO2 = 37.5/12.5 atm and 523 K. Cu/SiO2 : ZnO/SiO2 = 0.25:0.25 g, Cu/SiO2 and (Zn)Cu/SiO2 = 0.25 g. Methanol (䊏) and CO (䊉) yields over the physical mixture; methanol (䊐) and CO (䊊) yields over Cu/SiO2 ; methanol (4) yield over (Zn)Cu/SiO2 .
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although the methanol yield over Cu/SiO2 was held constant regardless of the reduction temperature. After the reduction at 723 K, the methanol yield of the physical mixture was four times more active than that of Cu/SiO2 . It is also shown in Fig. 1 that the methanol synthesis activity of the (Zn)Cu/SiO2 is in good agreement with that of the physical mixture. These results well reproduced the previous result [8,10]. On the other hand, we observed no change in the CO formation by the RWGS reaction based on the reduction temperature of the physical mixture. The CO yields were the same as those over the Cu/SiO2 catalyst as shown in Fig. 1. That is, the reduction of the physical mixture promoted the methanol synthesis activity, but the reduction did not change the RWGS activity. If the promotion of the methanol synthesis is due to the morphology effect, the catalytic activity of the RWGS reaction should be changed, as pointed out by Waugh [19], because the RWGS reaction on Cu is known to be structure-sensitive [3,15]. Accordingly, the results shown in Fig. 1 cannot be explained by the morphology effect. We measured the Cu surface area in order to examine the effect of the H2 reduction upon the Cu surface area. The Cu surface area, reduction temperature and turnover frequency (TOF) of the catalysts tested are listed in Table 1. The Cu surface areas of all catalysts were almost the same, independent of the reduction temperatures, although TOF of the physical mixture and the (Zn)Cu/SiO2 catalysts increased with increasing reduction temperature. This indicates that the promotion of the methanol synthesis activity observed for the physical mixture and the (Zn)Cu/SiO2 catalysts was not due to the change in Cu surface area, but due to the creation of the Cu–Zn active sites. We also examined the morphology effect from the viewpoint of the activation energy for the methanol synthesis and the RWGS reaction. We thus measured the apparent activation energy over the physical mixture reduced at different reduction temperatures. Fig. 2a and b show the Arrhenius plots for the RWGS reaction and the methanol synthesis rates over the physical mixture. No significant dependence of reduction temperature upon the MTY of CO was observed, indicating no change in the catalytic performance of the physical mixture for the RWGS reaction. As for the methanol synthesis shown in Fig. 2b, the lines of the Arrhenius plot are parallel. The activation
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Table 1 Cu surface areas and TOF of Cu/SiO2 , Cu/SiO2 + ZnO/SiO2 , and (Zn)Cu/SiO2 catalysts Catalysts
Cu/SiO2 Cu/SiO2 +ZnO/SiO2 (Zn)Cu/SiO2
523 K reduction
623 K reduction (10−3
723 K reduction (10−3
Cu surface area (m2 /g-CuSiO2 cat
TOF molecules/ site/sec)
Cu surface area (m2 /g-Cu/SiO2 cat)
TOF molecules/ site/sec)
Cu surface area (m2 /g-Cu/SiO2 cat)
TOF (10−3 molecules/ site/sec)
1.42 1.43 1.40
7.66 9.68 7.48
1.40 1.42 1.38
7.77 16.1 19.5
1.46 1.38 1.36
7.76 25.7 25.6
energies thus derived from the Arrhenius plots are shown in Fig. 3 as a function of the reduction temperature. No dependence of the activation energy on the reduction temperature was observed for either the RWGS reaction (100–110 kJ/mol) or the methanol synthesis (58–62 kJ/mol). It has been reported that the apparent activation energy of CO formation varies depending on the surface structure of Cu [3,15]. We accordingly concluded that the morphology of Cu did not change during the reduction treatment of the physical mixture. As for the methanol synthesis, the quality of the active sites was thus not changed, but the quantity of the active sites increased with increasing reduction temperature. That is, the role of ZnO is to create Cu–Zn active sites on the Cu surface.
Fig. 2. Arrhenius plots of the mass time yield (MTY) for CO formation (a) and CH3 OH formation (b) over a physical mixture of Cu/SiO2 + ZnO/SiO2 reduced at 523 (䊊), 573 (䊐), 623 (4), 673 (䉫) and 723 K (5) prior to reaction. Cu/SiO2 :ZnO/SiO2 = 0.25: 0.25 g, PH2 /PCO2 = 37.5/12.5 atm.
Fig. 3. Activation energy derived from the slopes in Fig. 2. CO (䊉) and CH3 OH (䊏) formation over a physical mixture of Cu/SiO2 + ZnO/SiO2 catalyst. CO (䉫) and CH3 OH (4) formation over a Cu/ZnO/Al2 O3 catalyst. PH2 /PCO2 = 37.5/12.5 atm.
Y. Choi et al. / Applied Catalysis A: General 208 (2001) 163–167
The activation energies of the RWGS reaction and the methanol synthesis over the coprecipitation Cu/ZnO/Al2 O3 catalyst were also measured to be 118 and 56 kJ/mol as shown in Fig. 3, respectively, which were in good agreement with those over the physical mixture of the Cu/SiO2 + ZnO/SiO2 catalysts. We thus consider that industrial Cu/ZnO/Al2 O3 catalysts are basically identical to the model catalyst of the physical mixture in terms of the active site for the RWGS reaction (metallic Cu) and methanol synthesis from CO2 and H2 (Cu–Zn site).
4. Conclusion The reduction treatment of a physical mixture of Cu/SiO2 + ZnO/SiO2 caused an increase in the catalytic activity of the methanol synthesis by the hydrogenation of CO2 . In order to examine the morphology change of Cu particles in the physical mixture during the treatment, we compared the catalytic activity of the RWGS reaction over the physical mixture with those over a Cu/SiO2 catalyst. The obtained results for the measurements of the catalytic activity indicated no morphology change in Cu. It was thus proved that the reduction treatment of the physical mixture leads to the creation of the Cu–Zn active site for the methanol synthesis, as previously proposed.
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References [1] R. Burch, S.E. Golunski, M.S. Spencer, J. Chem. Soc., Faraday Trans. 86 (1990) 2683. [2] M.S. Spencer, Catal. Lett. 50 (1998) 37. [3] J. Yoshihara, C.T. Campbell, J. Catal. 161 (1996) 776. [4] C.V. Ovesen, B.S. Clausen, J. Schiøtz, P. Stoltze, H. Topsøe, J.K. Nørskov, J. Catal. 168 (1997) 133. [5] R.A. Hadden, B. Sakakini, J. Tabatabaei, K.C. Waugh, Catal. Lett. 44 (1997) 145. [6] N.Y. Topsøe, H Topsøe, J. Mol. Catal. A 141 (1999) 95. [7] Y. Kanai, T. Watanabe, T. Fujitani, M. Saito, J. Nakamura, T. Uchijima, Catal. Lett. 27 (1994) 67. [8] Y. Kanai, T. Watanabe, T. Fujitani, T. Uchijima, J. Nakamura, Catal. Lett. 38 (1996) 157. [9] J. Nakamura, I. Nakamura, T. Uchijima, Y. Kanai, T. Watanabe, T. Fujitani, J. Catal. 160 (1996) 65. [10] T. Fujitani, T. Matsuda, Y. Kushida, S. Ogihara, T. Uchijima, J. Nakamura, Catal. Lett. 49 (1997) 175. [11] I. Nakamura, H. Nakano, T. Fujitani, T. Uchijima, J. Nakamura, Surf. Sci. 402-404 (1998) 92. [12] T. Fujitani, J. Nakamura, Catal. Lett. 56 (1998) 119. [13] T. Fujitani, J. Nakamura, Appl. Catal. A 191 (2000) 111. [14] I. Nakamura, T. Fujitani, T. Uchijima, J. Nakamura, J. Vac. Sci. Technol. A14 (1996) 1464. [15] I. Nakamura, T. Fujitani, T. Uchijima, J. Nakamura, Surf. Sci. 400 (1998) 387. [16] T. Fujitani, I. Nakamura, T. Uchijima, J. Nakamura, Surf. Sci. 383 (1997) 285. [17] T. Yatsu, H. Nishimura, T. Fujitani, J. Nakamura, J. Catal. 191 (2000) 423. [18] I. Nakamura, H. Nakano, T. Fujitani, T. Uchijima, J. Nakamura, J. Vac. Sci. Technol. A17 (1999) 1592. [19] K.C. Waugh, Catal. Lett. 58 (1999) 163. [20] T. Fujitani, J. Nakamura, Catal. Lett. 63 (1999) 245.
The role of ZnO in Cu/ZnO methanol synthesis catalysts — morphology effect or active site model? Y. Choi a , K. Futagami a , T. Fujitani b,∗ , J. Nakamura a a
Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan National Institute for Resources and Environment, Tsukuba, Ibaraki 305-8569, Japan
b
Received 8 May 2000; received in revised form 19 June 2000; accepted 20 June 2000
Abstract We examined whether or not the presence of ZnO changed the morphology of Cu particles in Cu/ZnO methanol synthesis catalysts using physical mixtures of Cu/SiO2 + ZnO/SiO2 . The yield of methanol produced by the CO2 hydrogenation over the physical mixture increased by the reduction with H2 at 573–723 K. The promotion of the methanol synthesis activity was due to migration of Zn from ZnO onto Cu/SiO2 , leading to the Cu–Zn active sites, as previously reported. On the other hand, no activity change was observed upon the reduction treatment for the reverse water gas shift (RWGS) reaction over the physical mixture; this result cannot be explained by the morphology change. The activation energy of the methanol synthesis and the RWGS reaction over the physical mixture did not vary upon reduction, further supporting the absence of any morphology change. It was thus clearly proved that the role of ZnO was not to change the morphology of Cu, but to create the Cu–Zn active sites for methanol synthesis only. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Cu/ZnO catalyst; Methanol synthesis; Reverse water gas shift reaction; Role of ZnO; Morphology of Cu; Active site
1. Introduction The methanol synthesis from CO/CO2 /H2 is an important industrial process, and Cu/ZnO-based catalysts are well known to be active for this reaction. Although numerous investigations of the methanol synthesis catalysts have been carried out over the past few decades, there are still controversies concerning the role of ZnO and the active sites. It is widely accepted that the use of ZnO as a support material leads to a highly dispersed Cu catalyst. The controversial issue is the additional effect of ZnO upon methanol synthesis activity on Cu. The proposed models may be classified into ∗ Corresponding author. Tel.: +81-298-618173; fax: +81-298-530202. E-mail address: [email protected] (T. Fujitani).
three categories: (i) spillover model by Burch et al. and Spencer [1,2] in which ZnO acts as a reservoir of hydrogen for the hydrogenation of CO over Cu surfaces; (ii) the morphology effect proposed by Yoshihara and Campbell [3], Ovesen et al. [4], Hadden et al. [5], and Topsøe and Topsøe [6], in which the morphology of copper particles on a ZnO support is responsible for the effect of ZnO upon the methanol synthesis on Cu; and (iii) the active site model on Cu surfaces proposed by us (Fujitani and Nakamura and coworkers) [7–13]. We have reported the role of ZnO, the active sites and the reaction mechanism of the methanol synthesis by hydrogenation of CO2 over Cu/ZnO-based catalysts using both surface science and powder catalysts [7–16]. In experiments using a physical mixture of Cu/SiO2 + ZnO/SiO2 [8], Zn atoms were found
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to migrate from ZnO onto the Cu particles by H2 reduction at 573–723 K, leading to an increase in methanol synthesis activity. We confirmed that the resulting Zn-containing Cu/SiO2 , or (Zn)Cu/SiO2 , separated from the physical mixture was responsible for the promotional effect [10]. We found that the promotional effect of ZnO could be ascribed to the creation of the Cu–Zn active site on the Cu surface [7,8,10]. The active site model was then proved by the surface science study using a Zn-deposited Cu (1 1 1) model catalyst [15,16]. It has been clearly shown that the deposition of Zn significantly promotes the methanol synthesis activity of Cu (1 1 1). We have thus concluded that the role of ZnO in Cu/ZnO-based catalysts is to create the Cu–Zn active site required for methanol synthesis from CO2 and H2 . The metallic Cu is also necessary for the hydrogenation processes such as formate synthesis, so that the Cu–Zn site and metallic Cu cooperatively catalyze the methanol synthesis from CO2 and H2 . We then studied the elementary steps, formate intermediate and kinetics [16–18]. The active site determination for the methanol synthesis from CO and H2 is now under investigation; results will be reported elsewhere. Waugh [19] recently commented that our data [12] concerning the promotion of Zn upon methanol synthesis over Cu–ZnO catalysts could be explained by the effect of ZnO on the morphology of the Cu particles, i.e. a morphology effect, influencing the catalytic activities of both the methanol synthesis and a parallel reaction of the reverse water gas shift (RWGS) reaction (CO2 + H2 → CO + H2 O) over Cu/ZnO-based catalysts. We replied that the promotion of Zn cannot be explained by the morphology effect, but is well-explained by the creation of the Cu–Zn active sites [20]. He discussed the effects of the Cu morphology on the methanol synthesis and the RWGS reaction using different catalysts. However, the morphology effect on both reactions should be examined using an identical catalyst. In this study, we examined the catalytic activities for both the methanol synthesis by CO2 hydrogenation and the RWGS reaction using selected catalysts of the physical mixture of Cu/SiO2 + ZnO/SiO2 , Cu/SiO2 , the promoted (Zn)Cu/SiO2 [10], and Cu/ZnO/Al2 O3 . We then discussed whether or not the presence of the Zn promoter changed the morphology of Cu particles.
2. Experimental A Cu/SiO2 catalyst was prepared by the impregnation method using Cu(NO3 )2 ·3H2 O and commercial silica (Aerosil 300, 99.9%). The loading of Cu was 30 wt.%. The Cu/SiO2 was dried in an oven at 383 K for 12 h in air, and then calcined at 623 K for 2 h in air. The Cu/SiO2 catalyst was sieved to collect the catalyst particles of <150 m. A ZnO/SiO2 catalyst was prepared by the alkoxide method using Zn(NO3 )2 ·6H2 O and Si(OCH2 CH3 )4 [8]. The loading of ZnO was 80 wt.%. The ZnO/SiO2 catalyst was calcined at 723 K for 2 h in air, and then sieved to collect the catalyst particles of 150–250 m. The catalysts were physically mixed with a weight ratio of Cu/SiO2 : ZnO/SiO2 = 0.25:0.25 g, then the mixture was reduced with H2 in a flow reactor for 2 h at 523–723 K and 50 atm. This leads to the migration of Zn species from ZnO/SiO2 onto the Cu/SiO2 , as previously reported [8]. The hydrogenation of CO2 was carried out at 50 atm with a H2 /CO2 ratio of 3 and a flow rate of 75 cc/min. In some experiments, we used the resulting Zn-containing Cu/SiO2 separated from the physical mixture using a 150 m sieve, which was abbreviated (Zn)Cu/SiO2 [10]. The collected (Zn)Cu/SiO2 catalyst was set in the flow reactor and re-reduced with H2 at 523 K and 50 atm. The weight of the (Zn)Cu/SiO2 catalyst used for reaction was 0.25 g. The reaction conditions were the same as those for physical mixture catalysts. We also examined the catalytic activity of a Cu/ZnO/Al2 O3 catalyst to compare it with those of the physical mixture, Cu/SiO2 and (Zn)Cu/SiO2 . The Cu/ZnO/Al2 O3 catalyst was prepared by a co-precipitation method. Both an aqueous solution of Na2 CO3 (1.1 M) and a mixed solution of copper, zinc and, aluminum nitrates (total metal concentration 1.0 M) were added dropwise to distilled water at 300 K. The precipitate was aged in the mixed solution at 303 K for 72 h, followed by a thorough washing with distilled water. Subsequently, the precipitate was dried overnight in an oven at 383 K, and then calcined in air at 623 K for 3 h. The loading of Cu, ZnO and Al2 O3 of the prepared Cu/ZnO/Al2 O3 was 50:40:10 wt.%. The hydrogenation of CO2 was carried out at a PH2 /PCO2 = 37.5/12.5 atm. The flow rate was 800 cc/min and the weight of the catalyst was 0.125 g.
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The reaction products were analyzed using gas chromatographs with a thermal conductivity detector and a flame ionization detector. Catalytic activities were evaluated in terms of mass time yield (MTY) defined as the weight of the product molecules per catalyst weight per time (g molecule/kg-cat/h). In the case of the physically mixed catalyst, MTY was calculated based on the weight of Cu/SiO2 . The copper surface areas of each catalyst were measured with N2 O/He (2.54% N2 O) gas by the reactive frontal chromatography, as previously reported [8].
3. Results and discussion We examined the effect of the reduction pretreatment by H2 on the catalytic activities as previously carried out [8,10]. Fig. 1 shows the yields of CO formed by the RWGS reaction and methanol as a function of the reduction temperature of the physical mixture, Cu/SiO2 , and (Zn)Cu/SiO2 . As for the physical mixture, the methanol yield increased with increasing reduction temperature in the range of 573–723 K,
Fig. 1. The yield of methanol and CO produced by CO2 hydrogenation over a physical mixture of Cu/SiO2 + ZnO/SiO2 , Cu/SiO2 , and a promoted (Zn)Cu/SiO2 as a function of reduction temperature. The CO2 hydrogenation was carried out at PH2 /PCO2 = 37.5/12.5 atm and 523 K. Cu/SiO2 : ZnO/SiO2 = 0.25:0.25 g, Cu/SiO2 and (Zn)Cu/SiO2 = 0.25 g. Methanol (䊏) and CO (䊉) yields over the physical mixture; methanol (䊐) and CO (䊊) yields over Cu/SiO2 ; methanol (4) yield over (Zn)Cu/SiO2 .
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although the methanol yield over Cu/SiO2 was held constant regardless of the reduction temperature. After the reduction at 723 K, the methanol yield of the physical mixture was four times more active than that of Cu/SiO2 . It is also shown in Fig. 1 that the methanol synthesis activity of the (Zn)Cu/SiO2 is in good agreement with that of the physical mixture. These results well reproduced the previous result [8,10]. On the other hand, we observed no change in the CO formation by the RWGS reaction based on the reduction temperature of the physical mixture. The CO yields were the same as those over the Cu/SiO2 catalyst as shown in Fig. 1. That is, the reduction of the physical mixture promoted the methanol synthesis activity, but the reduction did not change the RWGS activity. If the promotion of the methanol synthesis is due to the morphology effect, the catalytic activity of the RWGS reaction should be changed, as pointed out by Waugh [19], because the RWGS reaction on Cu is known to be structure-sensitive [3,15]. Accordingly, the results shown in Fig. 1 cannot be explained by the morphology effect. We measured the Cu surface area in order to examine the effect of the H2 reduction upon the Cu surface area. The Cu surface area, reduction temperature and turnover frequency (TOF) of the catalysts tested are listed in Table 1. The Cu surface areas of all catalysts were almost the same, independent of the reduction temperatures, although TOF of the physical mixture and the (Zn)Cu/SiO2 catalysts increased with increasing reduction temperature. This indicates that the promotion of the methanol synthesis activity observed for the physical mixture and the (Zn)Cu/SiO2 catalysts was not due to the change in Cu surface area, but due to the creation of the Cu–Zn active sites. We also examined the morphology effect from the viewpoint of the activation energy for the methanol synthesis and the RWGS reaction. We thus measured the apparent activation energy over the physical mixture reduced at different reduction temperatures. Fig. 2a and b show the Arrhenius plots for the RWGS reaction and the methanol synthesis rates over the physical mixture. No significant dependence of reduction temperature upon the MTY of CO was observed, indicating no change in the catalytic performance of the physical mixture for the RWGS reaction. As for the methanol synthesis shown in Fig. 2b, the lines of the Arrhenius plot are parallel. The activation
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Table 1 Cu surface areas and TOF of Cu/SiO2 , Cu/SiO2 + ZnO/SiO2 , and (Zn)Cu/SiO2 catalysts Catalysts
Cu/SiO2 Cu/SiO2 +ZnO/SiO2 (Zn)Cu/SiO2
523 K reduction
623 K reduction (10−3
723 K reduction (10−3
Cu surface area (m2 /g-CuSiO2 cat
TOF molecules/ site/sec)
Cu surface area (m2 /g-Cu/SiO2 cat)
TOF molecules/ site/sec)
Cu surface area (m2 /g-Cu/SiO2 cat)
TOF (10−3 molecules/ site/sec)
1.42 1.43 1.40
7.66 9.68 7.48
1.40 1.42 1.38
7.77 16.1 19.5
1.46 1.38 1.36
7.76 25.7 25.6
energies thus derived from the Arrhenius plots are shown in Fig. 3 as a function of the reduction temperature. No dependence of the activation energy on the reduction temperature was observed for either the RWGS reaction (100–110 kJ/mol) or the methanol synthesis (58–62 kJ/mol). It has been reported that the apparent activation energy of CO formation varies depending on the surface structure of Cu [3,15]. We accordingly concluded that the morphology of Cu did not change during the reduction treatment of the physical mixture. As for the methanol synthesis, the quality of the active sites was thus not changed, but the quantity of the active sites increased with increasing reduction temperature. That is, the role of ZnO is to create Cu–Zn active sites on the Cu surface.
Fig. 2. Arrhenius plots of the mass time yield (MTY) for CO formation (a) and CH3 OH formation (b) over a physical mixture of Cu/SiO2 + ZnO/SiO2 reduced at 523 (䊊), 573 (䊐), 623 (4), 673 (䉫) and 723 K (5) prior to reaction. Cu/SiO2 :ZnO/SiO2 = 0.25: 0.25 g, PH2 /PCO2 = 37.5/12.5 atm.
Fig. 3. Activation energy derived from the slopes in Fig. 2. CO (䊉) and CH3 OH (䊏) formation over a physical mixture of Cu/SiO2 + ZnO/SiO2 catalyst. CO (䉫) and CH3 OH (4) formation over a Cu/ZnO/Al2 O3 catalyst. PH2 /PCO2 = 37.5/12.5 atm.
Y. Choi et al. / Applied Catalysis A: General 208 (2001) 163–167
The activation energies of the RWGS reaction and the methanol synthesis over the coprecipitation Cu/ZnO/Al2 O3 catalyst were also measured to be 118 and 56 kJ/mol as shown in Fig. 3, respectively, which were in good agreement with those over the physical mixture of the Cu/SiO2 + ZnO/SiO2 catalysts. We thus consider that industrial Cu/ZnO/Al2 O3 catalysts are basically identical to the model catalyst of the physical mixture in terms of the active site for the RWGS reaction (metallic Cu) and methanol synthesis from CO2 and H2 (Cu–Zn site).
4. Conclusion The reduction treatment of a physical mixture of Cu/SiO2 + ZnO/SiO2 caused an increase in the catalytic activity of the methanol synthesis by the hydrogenation of CO2 . In order to examine the morphology change of Cu particles in the physical mixture during the treatment, we compared the catalytic activity of the RWGS reaction over the physical mixture with those over a Cu/SiO2 catalyst. The obtained results for the measurements of the catalytic activity indicated no morphology change in Cu. It was thus proved that the reduction treatment of the physical mixture leads to the creation of the Cu–Zn active site for the methanol synthesis, as previously proposed.
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