A comparative study of the oxidation of CO and CH4 over Au/MOx/Al2O3 catalysts

The activity of Au/Al2O3 and Au/MOx/AlO3 (M = Cr, Mn. Fe, Co. Ni. Cu, and Zn) in low temperature CO oxidation and the oxidation of CH4 was studied. Generally, addition of MOx to Au/Al2O3 stabilizes small Au particles initially present on the support in heat treatments up to 700 degreesC. All multi-component catalysts show a remarkable enhancement in low temperature CO oxidation compared to the mono-component catalysts. The observed activities are directly related to the average Au particle size. whereas the identity of MOx is less important. The CH4 oxidation activity of Au/Al2O3 is improved upon addition of MnOx, FeOx, CoOx, and to a lesser. extent NiOx. Measured activities in CH4 oxidation over Au/MOx/Al2O3 decrease in the following order: CuOx > MnOx > CrOx > FeOx > CoOx > NiOx > ZnOx. The high activity observed for CuOx and CrOx containing catalysts is assigned to the intrinsically high CH4 oxidation capability of these oxides themselves. For Au/MnOx/Al2O3 a lower apparent activation energy was found than for Au/Al2O3 and MnOx/Al2O3 which might point to a promoting effect of MnOx on Au in the oxidation of CH4. The results presented support a similar model for both CO and CH4 oxidation. In this model the reaction takes place at the Au/MOx perimeter. which is defined as the boundary between Au, MOx and the gas phase. The reductant CO or CH4 is adsorbed on Au, and MOx is the supplier of O. (C) 2001 Elsevier Science B.V. All rights reserved.