Evidence for support effects in metal oxide supported cobalt catalysts

Abstract

Abstract Cobalt supported on a number of different metal oxides are often used in the Fischer–Tropsch (FT) reaction. No obvious rationale for the composition of the metal oxide used exists. In this paper we examine the possibility that some form of interaction between the metal and the metal oxide support exists which enhances the activity of the Co metal. To this effect, we have supported Co metal on a series of metal oxides of different degrees of reducibility: (i) Al2O3 which is unreducible under FT conditions, (ii) ZrO2 which can be reduced to a small extent under FT conditions, and (iii) ZnO which can be reduced to a measurable degree under FT conditions. The postulate was that the more reducible support will have the greater number of trapped electrons at anion vacancies, and so will have the greater possibility of transferring these electrons from the support to the metal, giving rise to a sequentially greater metal/metal oxide interaction. We have previously shown that temperature programmed desorption/decomposition of formate species adsorbed on Cu/ZnO/Al2O3 (methanol synthesis catalysts) has been able to identify adsorption sites on: (i) the Cu metal, (ii) the ZnO, (iii) Al2O3 and (iv) at the Cu/ZnO interface. Here we have used the same probe reaction, namely the adsorption and temperature programmed desorption/decomposition of a formate species, to identify and predict the extent of metal/metal oxide interaction. The whole system, the kinetics of the decomposition of the formate used, and the different forms of metal oxide support used constituted a predictive possibility of what would constitute an active metal oxide supported Co catalyst on (i) Co supported on Al2O3, on (ii) Co supported on ZrO2 and (iii) on Co supported on ZnO. The ZnO support is shown to provide the greatest extent of charge transfer from the support to the Co, evidenced by the lowest temperature for desorption/decomposition of the formate of all the supports used, and the largest amount of CO in the product spectrum. The in situ N2O reactive frontal chromatography measurement of the Co metal area showed the formate species to be closely packed on the Co. Schematic diagram of closely packed formate species on Co, showing the overlap of the electron charge clouds, demonstrating the possibility of reaction between neighbouring formate strands (red balls: O atoms, grey balls: C atoms, white balls: H atoms and blue balls: Co atoms)