Oscillatory oxidation of CO over Pt, Pd and Ir catalysts: Theory

A simple physical model is presented to explain the oscillatory oxidation of CO over Pt, Pd and Ir catalysts. Oscillations are believed to occur between two branches of a Langmuir-Hinshelwood reaction mechanism. The slow oxidation and reduction of the metal surface layer induces transitions between these two branches. The kinetics of the oscillatory oxidation of CO are simulated using three coupled differential equations which describe the time evolution of: the fractional coverage of chemisorbed oxygen on the catalyst surface, θ 1 , the fractional coverage of chemisorbed carbon monoxide, θ 2 , and the fraction of surface sites blocked by oxide formation, θ 3 . A detailed comparison is presented between the model calculations and our experimental results for Pt. Almost all the oscillatory characteristics observed over Pt are quantitatively reproduced by the model.