Surface reactivity of supported gold: I. Oxygen transfer between CO and CO 2

Abstract

The rate of redistribution of isotopic carbon between CO and CO 2 has been studied on Au supported on MgO in the temperature range 300 to 400 °C, P co 2 P co ratios 0.1 to 1.2 and total pressure of 50 Torr. A few experiments were also carried out on supported Ru and Pt. The effect of Au concentration, temperature, and catalyst preparation method have been selected for investigation. In addition, determinations of the particle size of Au have been carried out by X-ray to illustrate the effect of the temperature of reduction and decomposition of the Au salt upon the particle size of the metal in the supported catalyst. Chemical reduction of the Au salt at low temperature (< 100 °C) produced Au particles with diameters ≤ 150 Å, while particles with diameters 20 times larger were obtained by thermal decomposition (≤ 350 °C) of the Au salt. Since particle growth may occur by direct addition of the decomposing salt and/or by sintering among metal particles, it is suggested that the latter process cannot readily occur at temperatures ⩽ 0.3 Tm. The implications of these findings for separate control of the degree of dispersion and of the support coverage by the metal are pointed out. Kinetic observations have been employed to study the thermodynamic and kinetic factors contributing to the activity of Au surfaces in the oxygen transfer step between gas and surface phases. Au activity was found to decrease with increasing P co 2 P co ratio, indicating that reduced surface species (metal atoms) play a dominant role in the reactivity of the surface. A similar trend was found for Ru and Pt at low ratios P co 2 P co . For Pt at higher P co 2 P co ratios, a reactivity inversion was found. Under similar conditions of gas composition, temperature and support, the affinity of the Au surface for oxygen increased with decreasing particle size. The degree of dispersion of Au was found to influence the rate of the catalytic reaction. The effect has been interpreted in terms of a relation between metal particle size and gas mean free path. The usefulness of these studies for developing criteria for control of oxidation depth and selectivity behavior in catalytic oxidations through optimization of size, size distribution of metal particles, and their morphological connection with the supporting agent is emphasized.