The oxidation of carbon monoxide over the (110) surface of iridium

Abstract

The heterogeneously catalyzed reaction of gaseous CO and O 2 to form CO 2 over Ir(110) has been studied, through measurements of the steady-state rate of CO 2 production, by mass spectrometry for surface temperatures between 300 and 1000 K, and for partial pressures of the reactants between 10 −8 and 3 × 10 −6 Torr. The rate expressions developed from a previous analysis of transient reactions were combined into a model that both qualitatively and quantitatively predicts trends in the steady-state measurements. However, the following four phenomena limit the applicability of this model: (1) The activation energies for CO desorption and CO oxidation depend markedly upon the composition of the adlayer on the surface. (2) Diffusion in the adlayer may limit the rates of CO desorption and CO oxidation, but this effect can be included empirically in the rate expression. (3) The catalyst surface is oxidized at temperatures where the steady-state rate of CO oxidation is rapid, i.e., a second species of atomically adsorbed oxygen forms irreversibly near 700 K. (4) Hysteresis in the rate of production of CO 2 with temperature occurs in the steady-state reaction.