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SiO2 catalysts
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
2123
Methanol decomposition on unpromoted and Zn promoted Cu/SiOz catalysts M. Clement, Y. Zhang, D.S. Brands, E.K. Poels and A. Bliek Department of Chemical Engineering, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands The decomposition of methanol over promoted and unpromoted copper containing catalysts has been investigated. The reaction rate over unpromoted Cu/SiO2 is constant with increasing catalyst reduction temperature, whereas the ZnO promoted catalyst showed a strong decrease in activity, accompanied by an enhanced selectivity to methyl formate. FT-IR experiments showed that at reaction temperature methoxy species are stabilised on the high temperature reduced Cu/ZnO/SiO2 catalyst. Since stabilisation of these species at 470 K only occurs on oxidised copper, this suggests that copper cations are present in the high temperature reduced catalysts. Combining these results with earlier findings on methanol synthesis, it is proposed that the newly formed sites are located at the Cu-ZnO interface. At this interface, ionic copper is stabilised. 1. INTRODUCTION Studying promoted Cu/SiO2 systems, a remarkable effect of the reduction temperature on activity in hydrogenation and hydrogenolysis of esters was observed at our laboratory [ 1]. An increase in activity was observed at increasing reduction temperatures, as also reported by Yurieva et al.[2]. A model was proposed to explain this behaviour. In particular it was proposed highly active, epitaxial copper particles are formed upon reduction, while the hydrogen dissolves in the mixed oxide as H § Deactivation was observed upon high temperature treatment in an inert atmosphere, causing the highly active copper to redissolve, releasing the dissolved protons as hydrogen gas. This illustrates the reversible nature of the activation. Decomposition of methanol over copper containing catalysts is closely related to the ester hydrogenolysis and methanol synthesis reactions. It is relevant for industrial hydrogen production [3] or use in fuel cells [4]. Hence, additional information can be obtained studying this reaction. This paper describes a series of methanol decomposition experiments in order to investigate the effect of reduction temperature on the activity of promoted copper catalysts in methanol decomposition, combining reactivity data with FT-IR experiments.
2124
2 EXPERIMENTAL 2.1 Catalyst Preparation Silica supported catalysts were prepared by homogeneous deposition precipitation of copper nitrate (Merck, >99.5% pure) and zinc nitrate hexahydrate (Janssen >98%pure) onto Aerosil 200 silica, according to a method described elsewhere [ 1]. The catalyst codes refer to the theoretical copper and promoter loading respectively; actual loadings are given in table 1 together with specific copper surface areas and the total catalyst surface area. These catalysts are extensively charactefised elsewhere [1 ].
2.2 Catalyst Characterisation The copper metal surface areas were determined using N20 chemisorption according to the method described by Luys et al. [5]. BET surface areas were measured on a Carlo Erba Porosimeter 1800. Actual catalyst metal loadings were determined with Inductively Coupled Plasma - Atomic Emission Spectroscopy (ICP-AES) using a multichannel Thermo Jarrel Ash ICAP 957 spectrometer, upgraded to model ICAP 61.
2.3 Methanol Decomposition The reactivity measurements were carried out in a gas-phase flow set-up. Catalyst samples (100 mg) were pre-treated by calcination in a flow of 60 ml'min ~ air at 750 K for 12 hours. Reduction was performed at 600K-750K for one hour in a flow of 60 ml'min m hydrogen (99.999 % pure and further purified using a copper catalyst and 5/~ molecular sieves). During calcination and reduction, the sample was heated to the required temperature with 72 K.h m. Methanol decomposition activity was measured at 427 K using a flow of 27 ml-min ~ 2.5 vol. % methanol (Aldrich, 99.8 % pure) at 2 bar in Helium (Praxair, 99.996 % pure and further purified using 5 /~ molecular sieves). The product mixture was analysed using a Chrompack 438A gas chromatograph equipped with two Porapack columns and a methanizer with FID detector.
2.4 Fourier-Transform Infrared Spectroscopy FTIR spectra were obtained using a Biorad FTS 45A spectrometer. Catalyst samples of 10 mg were crushed and pressed into self-supporting discs of l cm diameter. The discs were placed in a transmission flow cell with calcium fluoride windows described elsewhere [6]. For each spectrum, with a resolution of 1cm 1, 16 scans were averaged. Preceding the experiment, the samples were pre-treated as in the activity tests. Aider reduction, the temperature was lowered to 470 K, at which temperature the cell was evacuated to a pressure < lmbar in about 30 s. Subsequently, 10 mbar of methanol (Aldrich 99.8 % pure) was admitted to the cell.
Table 1: Metal loading of the dried catalysts; copper specific area ~ d BET specific ~ea. ...... Code Cu loading Zn loading Cu specific areaa SBEr ............................................. (wt%) ...................... (wt %) (m2/go~t).............................(m2/g) . . . . . . . . . . cs15 14.2 15.4 320 CZS1510 14.8 9.0 17.6 434 aider reduction at 600 K
2125 3 RESULTS AND DISCUSSION
3.1 methanol decomposition The methanol decomposition activity of both the promoted and unpromoted catalysts is shown in figure 1. The reactivity experiments show that the methanol decomposition rate over the unpromoted silica supported copper (CS15) is approximately constant with increasing catalyst reduction temperature (600-750 K). At 600 K, the ZnO promoted Cu/SiO2 catalyst (CZS 1510) behaves quite similar to the unpromoted sample when reduced, but upon reduction above 650 K, activity decreases as a function of reduction temperature. This decrease is accompanied by the production of large amounts of methyl formate. The effect of decreasing reaction rate over the promoted catalyst is opposite to the trend observed in methanol synthesis and ester hydrogenolysis reactions as described in the introduction. Figure 2 shows the product selectivities of the decomposition reaction over the CS15 and CZS1510 catalysts as a function of the reduction temperature. Again, at 600 K both catalysts show approximately the same behaviour. However, at higher reduction temperatures, the selectivity of the unpromoted CS15 catalyst towards CO increases, while the promoted CZS 1510 catalyst shows an increasing selectivity towards methyl formate.
50
100
.~40
.~
]
75
,3o "=Z
"E 25
u 10 0 ..... 600
I
I
I
650
700
750
reduction temperature I K
Fig. 1. Activity of copper containing catalysts in methanol decomposition at 423 K as a function of reduction temperature (W/F = 0.1 g.ml ~.s). Solid symbols: Cu/ZnO/SiO2; open symbols: Cu/SiO2.
o 600
i 650
700
750
reduction temperature I K
Fig. 2. Selectivity in methanol decomposition at 423 K vs. Reduction temperature. Solid symbols: Cu/ZnO/SiO2; open symbols: Cu/SiO2, circles: selectivity towards methyl formate; triangles: selectivity towards CO. CO2 production proved minimal.
3.2 FT-IR experiments In figure 3, an overview of measurements on the promoted CZS1510 catalyst is shown. The bands from 2750 to 3050 cmI are showing the C-H stretching regions. The catalyst was reduced at 600 and 700 K. Spectra 1 and 2 are recorded after 10 minutes exposure to 10 mbar of methanol at 470 K. Spectra 3 and 4 were recorded after evacuation to 10s mbar for about 10 minutes at the same temperature.
2127 formate production with increasing reduction temperature, the increasing activity of the promoted catalyst can not be ascribed to unpromoted Cu phases. FT-IR experiments show that methoxy species are stabilised on the high temperature (700 K) reduced Cu/ZnO/SiO2 catalyst. Clarke et al. [7] studied the methanol decomposition over oxidised and reduced Cu/SiO~ catalysts. These authors observed that on the reduced catalyst, methoxy species were only observed below 360 K, but methoxide on oxidised copper catalysts proved to be far more stable. On an oxidised catalyst methoxy species were observed up to 470 K. These peaks are in fact very similar to the ones observed on the high temperature reduced CZS1510 sample (see figure 3), which is surprising because higher reduction temperature should yield a more reduced phase. When the results presented in this paper are compared with results on methanol synthesis over the same catalysts, contrasting behaviour is found. First, the CZS 1510 catalyst reduced at 600 K is fairly similar in activity to the unpromoted CS 15 catalyst. However, after reduction at 700 K the promoted catalyst does not increase in activity, as in the methanol synthesis reaction, but it decreases. These phenomena indicate that methanol adsorption on the 700 K reduced catalyst takes place on a different kind of site than on the 600 K reduced CZS1510 and CS15 catalysts. A low activity in methanol decomposition of a highly active methanol synthesis catalyst (using CO/H 2 feedgas) was observed earlier for promoted palladium and rhodium catalysts as well [ 13,14]. From the FT-IR data, it is evident that the nature of these adsorption sites is essential for the stabilisation of the methoxide at higher temperatures. At the temperature at which the spectra were taken, methoxy species are only observed on oxidised copper [7], and not on zerovalent copper, zinc oxide or silica. However, the occurrence of oxidised copper in a high temperature reduced catalyst is somewhat surprising. It was proposed earlier by our group in conjunction with Yurieva c.s., that upon high temperature reduction of promoted copper catalysts, small Cu particles are being formed which are highly active in methanol synthesis. Combining these results with findings in literature [ 15], where activity is enlarged by a factor 6 when Cu was partially covered by ZnO, we assume highly active sites near the Cu-ZnO interface. Indeed, some authors have suggested that the Cu-ZnO interface plays a crucial role in methanol synthesis [15,16,17]. This activity of interfaces in methanol synthesis is also in agreement with the assertion in literature that unpromoted, metallic copper is inactive in methanol synthesis from CO/H 2 [ 18]. The sites at the Cu-ZnO interface could contain cationic copper, stabilised by the ZnO, or copper that is readily oxidisable by e.g. the decomposition products [ 19]. Interestingly, it was shown by Ernst et al. that in strictly two-dimensional layers of copper at low surface coverage grown on a zinc oxide single crystal face, the copper is cationic in character [20]. Likewise, copper located at the Cu-ZnO interface could also have a cationic character. 5 CONCLUSIONS The ZnO promoted Cu/SiO 2 catalyst shows a decreasing activity with increasing catalyst reduction temperature in the methanol decomposition reaction. Simultaneously, the selectivity towards methyl formate increases. This trend is opposite to previous observations in methanol synthesis and ester hydrogenolysis reactions. It was shown by FT-IR spectroscopy that methoxy species are stabilised on the high temperature reduced promoted catalyst. On the other hand, methoxide is only observed on oxidised copper at the temperatures at which the spectra were recorded (197~
2128 The stabilisation of methoxy species could explain the lower methanol decomposition activity and the higher selectivity to methyl formate of the promoted catalyst at higher reduction temperature. Methoxide stabilisation would also promote the methyl ester dissociation and/or aeyl hydrogenation, thus enhancing reaction rate [21 ]. Combining the present results with the earlier findings on methanol synthesis and methyl ester hydrogenolysis, it is proposed that cationic copper species are formed at the Cu-ZnO interface of small copper particles formed upon high temperature reduction of the Zn promoted Cu/SiO2 catalysts. It is concluded that at the Cu-ZnO interface, cationic copper is stabilised and/or the copper is rendered readily oxidisable. REFERENCES
1. Brands, D.S., Poels, E.K., and Bliek, A., Stud. Surf. Sci. Catal. 101, 1085 (1996). 2. Yurieva, T.M., Plyasova, L.M., Marakova, O.V., and Krieger, T.A., dr. Mol. Catal.A: Chemical 113, 455 (1996). 3. "Ullman's Encyclopedia of Industrial Chemistry", 5th ed. Vol A13, VCH Verlagsgesellschaft, Weinheim, Germany 1986, p.358. 4. Petterson, L., and Sj6str~im, K., Combust. Sci. and Techn. 71, 129 (1990). 5. Luys, M.J., Oefelt, P.H.v., Brouwer, W.G.J., Pijpers, A.P., and Scholten, J.F.F., Appl. Catal. 7, 75 (1983). 6. Bijsterbosch, J.W., Langeveld, A.D.v., Kapteijn, F., and Moulijn, J.A., Vibr. Spectr. 4, 245 (1993). 7. Clarke, D.B., Lee, D.-L., Sandoval, M.J., and Bell, A.T., dr. Catal. 150, 82 (1994). 8. Ueno, B.A., Onishi, T., and Tamaru, K.,J. Chem. Soc. Faraday Trans. 1 67, 3585 (1971) 9. Cheng, W.H., Akhter, S., and Kung, H.H., J. Catal. 82, 341 (1983). 10. Tawarah, K.M., and Hansen, R.S., Jr. Cata187, 305 (1984). 11. Roberts, D.L., and Griffin, G.L., J Catal. 101, 201 (1986). 12. Chadwick, D., and Zheng, K., Catal. Lett. 20, 231 (1993). 13. Poels, E.K., Koolstra, R., Geus, J.W., and Ponce, V., Stud. Surf Sci. Catal. 11, 239 (1982). 14. Poels, E.K., Ph.D. Thesis, Leiden University 1984, p. 124. 15. Nakamura, J., Catal. Lett. 31,325 (1995). 16. Bailie, J.E., and Rochester, C.H., Catal. Lett. 31,333 (1995). 17. Butch, R., Chappell, R.J., and Golunski, S.E., dr. Chem. Soc. Faraday Trans. 1 85 (10), 3569 (1989). 18. Nonneman, L.E.Y., and Ponec, V., Catal. Lett. 7, 213 (1990). 19. Okamoto, Y., Fukino, K. Imanaka, T., and Teranishi, S., J. Phys. Chem. 87, 3747 (1983). 20. Ernst, K.H., Ludviksson, A., Zhang, R., Yoshihara, J., and Campbell, C.T., Phys. Rev. B 47 (20), 13782 (1993). 21. Turek, T., Trimm, D.L., and Cant, N.W., Catal. Rev.-Sci. Eng. 36, 645 (1994).
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Methanol decomposition on unpromoted and Zn promoted Cu/SiOz catalysts M. Clement, Y. Zhang, D.S. Brands, E.K. Poels and A. Bliek Department of Chemical Engineering, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands The decomposition of methanol over promoted and unpromoted copper containing catalysts has been investigated. The reaction rate over unpromoted Cu/SiO2 is constant with increasing catalyst reduction temperature, whereas the ZnO promoted catalyst showed a strong decrease in activity, accompanied by an enhanced selectivity to methyl formate. FT-IR experiments showed that at reaction temperature methoxy species are stabilised on the high temperature reduced Cu/ZnO/SiO2 catalyst. Since stabilisation of these species at 470 K only occurs on oxidised copper, this suggests that copper cations are present in the high temperature reduced catalysts. Combining these results with earlier findings on methanol synthesis, it is proposed that the newly formed sites are located at the Cu-ZnO interface. At this interface, ionic copper is stabilised. 1. INTRODUCTION Studying promoted Cu/SiO2 systems, a remarkable effect of the reduction temperature on activity in hydrogenation and hydrogenolysis of esters was observed at our laboratory [ 1]. An increase in activity was observed at increasing reduction temperatures, as also reported by Yurieva et al.[2]. A model was proposed to explain this behaviour. In particular it was proposed highly active, epitaxial copper particles are formed upon reduction, while the hydrogen dissolves in the mixed oxide as H § Deactivation was observed upon high temperature treatment in an inert atmosphere, causing the highly active copper to redissolve, releasing the dissolved protons as hydrogen gas. This illustrates the reversible nature of the activation. Decomposition of methanol over copper containing catalysts is closely related to the ester hydrogenolysis and methanol synthesis reactions. It is relevant for industrial hydrogen production [3] or use in fuel cells [4]. Hence, additional information can be obtained studying this reaction. This paper describes a series of methanol decomposition experiments in order to investigate the effect of reduction temperature on the activity of promoted copper catalysts in methanol decomposition, combining reactivity data with FT-IR experiments.
2124
2 EXPERIMENTAL 2.1 Catalyst Preparation Silica supported catalysts were prepared by homogeneous deposition precipitation of copper nitrate (Merck, >99.5% pure) and zinc nitrate hexahydrate (Janssen >98%pure) onto Aerosil 200 silica, according to a method described elsewhere [ 1]. The catalyst codes refer to the theoretical copper and promoter loading respectively; actual loadings are given in table 1 together with specific copper surface areas and the total catalyst surface area. These catalysts are extensively charactefised elsewhere [1 ].
2.2 Catalyst Characterisation The copper metal surface areas were determined using N20 chemisorption according to the method described by Luys et al. [5]. BET surface areas were measured on a Carlo Erba Porosimeter 1800. Actual catalyst metal loadings were determined with Inductively Coupled Plasma - Atomic Emission Spectroscopy (ICP-AES) using a multichannel Thermo Jarrel Ash ICAP 957 spectrometer, upgraded to model ICAP 61.
2.3 Methanol Decomposition The reactivity measurements were carried out in a gas-phase flow set-up. Catalyst samples (100 mg) were pre-treated by calcination in a flow of 60 ml'min ~ air at 750 K for 12 hours. Reduction was performed at 600K-750K for one hour in a flow of 60 ml'min m hydrogen (99.999 % pure and further purified using a copper catalyst and 5/~ molecular sieves). During calcination and reduction, the sample was heated to the required temperature with 72 K.h m. Methanol decomposition activity was measured at 427 K using a flow of 27 ml-min ~ 2.5 vol. % methanol (Aldrich, 99.8 % pure) at 2 bar in Helium (Praxair, 99.996 % pure and further purified using 5 /~ molecular sieves). The product mixture was analysed using a Chrompack 438A gas chromatograph equipped with two Porapack columns and a methanizer with FID detector.
2.4 Fourier-Transform Infrared Spectroscopy FTIR spectra were obtained using a Biorad FTS 45A spectrometer. Catalyst samples of 10 mg were crushed and pressed into self-supporting discs of l cm diameter. The discs were placed in a transmission flow cell with calcium fluoride windows described elsewhere [6]. For each spectrum, with a resolution of 1cm 1, 16 scans were averaged. Preceding the experiment, the samples were pre-treated as in the activity tests. Aider reduction, the temperature was lowered to 470 K, at which temperature the cell was evacuated to a pressure < lmbar in about 30 s. Subsequently, 10 mbar of methanol (Aldrich 99.8 % pure) was admitted to the cell.
Table 1: Metal loading of the dried catalysts; copper specific area ~ d BET specific ~ea. ...... Code Cu loading Zn loading Cu specific areaa SBEr ............................................. (wt%) ...................... (wt %) (m2/go~t).............................(m2/g) . . . . . . . . . . cs15 14.2 15.4 320 CZS1510 14.8 9.0 17.6 434 aider reduction at 600 K
2125 3 RESULTS AND DISCUSSION
3.1 methanol decomposition The methanol decomposition activity of both the promoted and unpromoted catalysts is shown in figure 1. The reactivity experiments show that the methanol decomposition rate over the unpromoted silica supported copper (CS15) is approximately constant with increasing catalyst reduction temperature (600-750 K). At 600 K, the ZnO promoted Cu/SiO2 catalyst (CZS 1510) behaves quite similar to the unpromoted sample when reduced, but upon reduction above 650 K, activity decreases as a function of reduction temperature. This decrease is accompanied by the production of large amounts of methyl formate. The effect of decreasing reaction rate over the promoted catalyst is opposite to the trend observed in methanol synthesis and ester hydrogenolysis reactions as described in the introduction. Figure 2 shows the product selectivities of the decomposition reaction over the CS15 and CZS1510 catalysts as a function of the reduction temperature. Again, at 600 K both catalysts show approximately the same behaviour. However, at higher reduction temperatures, the selectivity of the unpromoted CS15 catalyst towards CO increases, while the promoted CZS 1510 catalyst shows an increasing selectivity towards methyl formate.
50
100
.~40
.~
]
75
,3o "=Z
"E 25
u 10 0 ..... 600
I
I
I
650
700
750
reduction temperature I K
Fig. 1. Activity of copper containing catalysts in methanol decomposition at 423 K as a function of reduction temperature (W/F = 0.1 g.ml ~.s). Solid symbols: Cu/ZnO/SiO2; open symbols: Cu/SiO2.
o 600
i 650
700
750
reduction temperature I K
Fig. 2. Selectivity in methanol decomposition at 423 K vs. Reduction temperature. Solid symbols: Cu/ZnO/SiO2; open symbols: Cu/SiO2, circles: selectivity towards methyl formate; triangles: selectivity towards CO. CO2 production proved minimal.
3.2 FT-IR experiments In figure 3, an overview of measurements on the promoted CZS1510 catalyst is shown. The bands from 2750 to 3050 cmI are showing the C-H stretching regions. The catalyst was reduced at 600 and 700 K. Spectra 1 and 2 are recorded after 10 minutes exposure to 10 mbar of methanol at 470 K. Spectra 3 and 4 were recorded after evacuation to 10s mbar for about 10 minutes at the same temperature.
2127 formate production with increasing reduction temperature, the increasing activity of the promoted catalyst can not be ascribed to unpromoted Cu phases. FT-IR experiments show that methoxy species are stabilised on the high temperature (700 K) reduced Cu/ZnO/SiO2 catalyst. Clarke et al. [7] studied the methanol decomposition over oxidised and reduced Cu/SiO~ catalysts. These authors observed that on the reduced catalyst, methoxy species were only observed below 360 K, but methoxide on oxidised copper catalysts proved to be far more stable. On an oxidised catalyst methoxy species were observed up to 470 K. These peaks are in fact very similar to the ones observed on the high temperature reduced CZS1510 sample (see figure 3), which is surprising because higher reduction temperature should yield a more reduced phase. When the results presented in this paper are compared with results on methanol synthesis over the same catalysts, contrasting behaviour is found. First, the CZS 1510 catalyst reduced at 600 K is fairly similar in activity to the unpromoted CS 15 catalyst. However, after reduction at 700 K the promoted catalyst does not increase in activity, as in the methanol synthesis reaction, but it decreases. These phenomena indicate that methanol adsorption on the 700 K reduced catalyst takes place on a different kind of site than on the 600 K reduced CZS1510 and CS15 catalysts. A low activity in methanol decomposition of a highly active methanol synthesis catalyst (using CO/H 2 feedgas) was observed earlier for promoted palladium and rhodium catalysts as well [ 13,14]. From the FT-IR data, it is evident that the nature of these adsorption sites is essential for the stabilisation of the methoxide at higher temperatures. At the temperature at which the spectra were taken, methoxy species are only observed on oxidised copper [7], and not on zerovalent copper, zinc oxide or silica. However, the occurrence of oxidised copper in a high temperature reduced catalyst is somewhat surprising. It was proposed earlier by our group in conjunction with Yurieva c.s., that upon high temperature reduction of promoted copper catalysts, small Cu particles are being formed which are highly active in methanol synthesis. Combining these results with findings in literature [ 15], where activity is enlarged by a factor 6 when Cu was partially covered by ZnO, we assume highly active sites near the Cu-ZnO interface. Indeed, some authors have suggested that the Cu-ZnO interface plays a crucial role in methanol synthesis [15,16,17]. This activity of interfaces in methanol synthesis is also in agreement with the assertion in literature that unpromoted, metallic copper is inactive in methanol synthesis from CO/H 2 [ 18]. The sites at the Cu-ZnO interface could contain cationic copper, stabilised by the ZnO, or copper that is readily oxidisable by e.g. the decomposition products [ 19]. Interestingly, it was shown by Ernst et al. that in strictly two-dimensional layers of copper at low surface coverage grown on a zinc oxide single crystal face, the copper is cationic in character [20]. Likewise, copper located at the Cu-ZnO interface could also have a cationic character. 5 CONCLUSIONS The ZnO promoted Cu/SiO 2 catalyst shows a decreasing activity with increasing catalyst reduction temperature in the methanol decomposition reaction. Simultaneously, the selectivity towards methyl formate increases. This trend is opposite to previous observations in methanol synthesis and ester hydrogenolysis reactions. It was shown by FT-IR spectroscopy that methoxy species are stabilised on the high temperature reduced promoted catalyst. On the other hand, methoxide is only observed on oxidised copper at the temperatures at which the spectra were recorded (197~
2128 The stabilisation of methoxy species could explain the lower methanol decomposition activity and the higher selectivity to methyl formate of the promoted catalyst at higher reduction temperature. Methoxide stabilisation would also promote the methyl ester dissociation and/or aeyl hydrogenation, thus enhancing reaction rate [21 ]. Combining the present results with the earlier findings on methanol synthesis and methyl ester hydrogenolysis, it is proposed that cationic copper species are formed at the Cu-ZnO interface of small copper particles formed upon high temperature reduction of the Zn promoted Cu/SiO2 catalysts. It is concluded that at the Cu-ZnO interface, cationic copper is stabilised and/or the copper is rendered readily oxidisable. REFERENCES
1. Brands, D.S., Poels, E.K., and Bliek, A., Stud. Surf. Sci. Catal. 101, 1085 (1996). 2. Yurieva, T.M., Plyasova, L.M., Marakova, O.V., and Krieger, T.A., dr. Mol. Catal.A: Chemical 113, 455 (1996). 3. "Ullman's Encyclopedia of Industrial Chemistry", 5th ed. Vol A13, VCH Verlagsgesellschaft, Weinheim, Germany 1986, p.358. 4. Petterson, L., and Sj6str~im, K., Combust. Sci. and Techn. 71, 129 (1990). 5. Luys, M.J., Oefelt, P.H.v., Brouwer, W.G.J., Pijpers, A.P., and Scholten, J.F.F., Appl. Catal. 7, 75 (1983). 6. Bijsterbosch, J.W., Langeveld, A.D.v., Kapteijn, F., and Moulijn, J.A., Vibr. Spectr. 4, 245 (1993). 7. Clarke, D.B., Lee, D.-L., Sandoval, M.J., and Bell, A.T., dr. Catal. 150, 82 (1994). 8. Ueno, B.A., Onishi, T., and Tamaru, K.,J. Chem. Soc. Faraday Trans. 1 67, 3585 (1971) 9. Cheng, W.H., Akhter, S., and Kung, H.H., J. Catal. 82, 341 (1983). 10. Tawarah, K.M., and Hansen, R.S., Jr. Cata187, 305 (1984). 11. Roberts, D.L., and Griffin, G.L., J Catal. 101, 201 (1986). 12. Chadwick, D., and Zheng, K., Catal. Lett. 20, 231 (1993). 13. Poels, E.K., Koolstra, R., Geus, J.W., and Ponce, V., Stud. Surf Sci. Catal. 11, 239 (1982). 14. Poels, E.K., Ph.D. Thesis, Leiden University 1984, p. 124. 15. Nakamura, J., Catal. Lett. 31,325 (1995). 16. Bailie, J.E., and Rochester, C.H., Catal. Lett. 31,333 (1995). 17. Butch, R., Chappell, R.J., and Golunski, S.E., dr. Chem. Soc. Faraday Trans. 1 85 (10), 3569 (1989). 18. Nonneman, L.E.Y., and Ponec, V., Catal. Lett. 7, 213 (1990). 19. Okamoto, Y., Fukino, K. Imanaka, T., and Teranishi, S., J. Phys. Chem. 87, 3747 (1983). 20. Ernst, K.H., Ludviksson, A., Zhang, R., Yoshihara, J., and Campbell, C.T., Phys. Rev. B 47 (20), 13782 (1993). 21. Turek, T., Trimm, D.L., and Cant, N.W., Catal. Rev.-Sci. Eng. 36, 645 (1994).