KINETICS OF THE CO OXIDATION BY O2 AND N2O OVER CU-CR/AL2O3

The oxidation of CO by O2 and N2O over an oxidized 10 wt. % Cu-Cr/Al2O3 catalyst (Cu:Cr = 1:1) has been studied by temperature-programmed reactivity measurements (400-550 K) over a wide range of partial reactant pressures, including inhibition by CO2. The CO oxidation rate is zeroth-order in oxygen and has orders between 0-1 in CO and N2O, depending on the gas-phase composition. Mechanistic information from literature combined with the kinetic data resulted in the selection of an Eley-Rideal-type of kinetic model without a priori assumptions on rate-determining processes. The model consists of the oxidation of reduced sites by O2 and/or N2O, followed by a reaction with CO, yielding a surface intermediate that releases CO2 in a consecutive step. CO2 inhibits both by reversible adsorption on oxidized and reduces sites, the latter under formation of the surface reaction intermediate. Apart from the surface oxidation by O2, the reaction rates of all assumed elementary processes are of the same order of magnitude and, therefore, determine the overall rate. The surface oxidation by oxygen is about four orders of magnitude larger, which explains the zeroth-order in oxygen and the observation that oxygen first reacts with CO before N2O is able to oxidize CO. The obtained activation energies of the elementary processes agree with values in the literature for corresponding systems.