A molecular beam study of the catalytic oxidation of CO on a Pt(111) surface

Abstract

The oxidation of carbon monoxide catalyzed by Pt(111) was studied in ultrahigh vacuum using reactive molecular beam–surface scattering. Under all conditions studied, the reaction follows a Langmuir–Hinshelwood mechanism: the combination of a chemisorbed CO molecule and an oxygen adatom. When both reactants are at low coverage, the reaction proceeds with an activation energy E^*_LH =24.1 kcal/mole and a pre‐exponential υ₄ =0.11 cm² particles⁻¹ sec⁻¹. At very high oxygen coverage, E^*_LH decreases to about 11.7 kcal/mole and υ₄ to about 2×10⁻⁶ cm² particles⁻¹ sec⁻¹. This is largely attributed to the corresponding increase in the energy of the adsorbed reactants. When a CO molecule incident from the gas phase strikes the surface presaturated with oxygen, it enters a weakly held precursor state to chemisorption. Desorption from this state causes a decrease in chemisorption probability with temperature. Once chemisorbed, the CO molecule then has almost unit probability of reacting to produce CO₂ below 540 K. The CO₂ product angular distribution varies from cosγ to cos⁴γ depending sensitively upon the adsorbed reactant concentrations.