THE INFLUENCE OF PLATINUM CONCENTRATION AND PARTICLE-SIZE ON THE KINETICS OF PROPANE OXIDATION OVER PT/GAMMA-ALUMINA

For the regulation of automotive emissions it is of interest to understand the parameters that control the oxidation rates of different hydrocarbon species. Propane oxidation was selected as a model reaction for this investigation. Catalysts were prepared by multiple impregnation of γ-alumina with chloroplatinic acid. Concentrations ranged from 0.03 to 30 wt% Pt. The reaction took place in a recirculation batch reactor with an initial mixture of 10 Torr C3H8, 100 Torr O2, and 650 Torr Ar. The oxidation rate increased sixfold with increasing Pt concentration. When calculated per Pt surface atom, based on CO chemisorption, the rate increased by two orders of magnitude in the same Pt concentration range. The apparent activation energy remained unchanged at 22.1 kcal/mol within a standard deviation of ±3.4 kcal/mol. The increase in specific reaction rate with particle size was confirmed by sintering experiments. The rate increase reflects an increase in the preexponential factor. The results suggest that propane oxidation is expedited by a favorable ensemble of active Pt sites, which are more likely to form on larger crystallites. Kinetic parameters of propane and methane oxidation are compared. Theoretical site densities, derived from fundamental kinetics for common reaction mechanisms, are compared with the number of Pt surface atoms as measured by chemisorption. © 1991.