Oscillatory Oxidation of CO Over Pt at Pressures from 10 to 760 Torr

Oscillations in the rate of CO oxidation over polycrystalline Pt have been measured at total pressures of 760 and 10 Torr. Despite the pressure change of two orders of magnitude, oscillations occurred between similar values of high and low reactivity, and over roughly the same experimental parameters. The lack of dependence of the reaction kinetics on the pressure is not predicted by the Langmuir-Hinshelwood equations that provide the rate multiplicities underlying the oscillations in a number of recent models. This lack of agreement suggests that the current understanding of CO oxidation and oscillatory reaction kinetics remains incomplete. The primary difference between the oscillations at the two pressures occurred in the waveforms, which were highly structured and fractal-like at P = 760 Torr, and became simple square waves on the evolved catalyst surface at P = 10 Torr. Differences in the catalyst surface structure and in coupling between parts of the catalyst through concentration gradients in the gas may account for these changes in the complexity of the waveforms.