Modelling of Influence of Oxygen Bulk Diffusion in Nickel on Oscillatory Kinetics of Catalytic Oxidation of Methane
Oscillatory reaction kinetics is unusual and, nevertheless, well-known phenomenon in heterogeneous catalysis. Sinusoidal or relaxation-type oscillations, and chaotic behavior have been observed in approximately 70 catalytic reactions in a wide pressure range, from ultrahigh vacuum up to atmospheric pressure, over many types of catalysts, including single-crystals, polycrystalline foils, wires, and supported catalysts. Several mechanisms describing the rate oscillations for different reactions were proposed. However, the most of them is based on the Langmuir-Hinshelwood mechanism and do not take into account the diffusion in subsurface layers of catalysts. This study is devoted to theoretical analysis of the influence of the diffusion of oxygen atoms into nickel on the self-sustained rate oscillations in the catalytic oxidation of methane. The mathematical model of the reaction consists of a system of ordinary differential equations and takes into account the concentrations of surface intermediates, oxygen concentration in subsurface layers, and the heat balance. Owing to the oxygen diffusion, an induction period appeared before the self-sustained rate oscillations. The concentration of oxygen in the subsurface layers of nickel oscillates synchronously with the concentrations of reaction products in the gas phase.