Kinetics of the NO + CO Reaction on Clean Pt: Steady-State Rates

Article abstract of DOI:10.1021/j100269a006

The kinetics of reaction between NO and CO on clean polycrystalline Pt are studied for temperatures between 300 and 1200 K, pressures between 1 and 10^-8 torr, compositions from X_CO = 0.0005 to 0.99, and rate variations over a factor of 10⁸.Surfaces are shown to be clean before reaction and are found to contain only monolayers of NO or CO following cooling and pumpdown after reaction at any pressure.No nonstoichiometric residues of reactant or contaminant species are observed on the surfaces under any conditions.In excess NO (X_CO < 0.01) the steady-state kinetics can be fit quantitatively assuming a Langmuir-Hinshelwood bimolecular rate expression with a heat of adsorption of NO of 17 kcal/mol.For X_CO > 0.05, the reaction can be fit by a bimolecular rate expression, r_R ca.P_NO/P_CO, at low temperatures, but at high temperatures the rate is proportional to P_NO and independent of P_CO.Near stoichiometric ratios the reaction appears to be limited by the adsorption rate of NO at high temperatures.For pressures between 10^-6 and 10^-8 torr the rate can be fit quantitatively by the same LH mechanism valid at high pressures with a heat of adsorption of CO of 30 kcal/mol.These results strongly suggest that this reaction on Pt is a true bimolecular reaction between adsorbed NO and CO rather than unimolecular NO decomposition with CO scavenging of adsorbed oxygen because the kinetics fit only the former mechanism and because rates are up to 10⁴ higher than those of NO decomposition.