Catalytic ignition in the CO[sbnd]O2 reaction on platinum: experiment and simulations

Abstract

Catalytic ignition temperatures for the CO+O2 reaction on a Pt wire, and the corresponding transient ignition curves, have been measured and analyzed theoretically for nonflammable reactant mixtures in Ar and He at atmospheric pressure. At very low relative CO concentrations (< 0.02%), the experimental ignition temperature rises rapidly with increasing concentration, and then more slowly at higher concentrations. These data are explained by employing a kinetic model based on the standard three-step mechanism for CO oxidation on Pt(111), combined with time-dependent mass and heat transport. The simulations reproduce both the dependence of ignition temperatures on the reactant concentrations and the transient behavior after ignition without any kinetic fitting parameters. The model predicts that ignition at intermediate CO concentrations is entirely governed by the interplay between the reaction kinetics on the CO-covered surface and the heat transport. At very low CO/O2 ratios, CO concentration gradients are established in the pre-ignition regime, which facilitates ignition. After ignition, but before the mass-transport controlled steady-state is established, a kinetic phase transition occurs from a CO-covered to an oxygen-covered surface.