Reaction rate oscillations during co oxidation over Pt/γ-Al 2O 3; The issue of propagating waves

Abstract

In this paper we discuss the issue of temporal and spatio-temporal oscillations observed during catalytic oxidation of CO. It has recently been reported in the literature that the oscillations observed with this catalytic reaction, for both single crystal (Pt(100)) and supported catalysts (Pt, Pd/SiO 2), are exclusively of spatio-temporal nature, that is a temperature and concentration wave is propagating on the catalyst surface. This observation has also been reported as direct evidence that the same mechanism is responsible for the oscillations observed with supported and unsupported catalysts, i.e., a hex ↔ (1 × 1) phase transition. We report here isothermal oscillations for the CO oxidation reaction system over Pt/γ-Al 2O 3 catalysts under intermediate pressure conditions (1–10 Torr). These oscillations are only of temporal nature, that is the temperature and concentration of the catalyst are spatially uniform, and there was no evidence of waves propagating on the catalytic surface. With this catalytic system we have been unable to observe any spatio-temporal oscillations (propagating waves) under all conditions we have investigated. This would seem to indicate that the claim the same mechanism, i.e. a hex ↔ (1 × 1) transition, is responsible for the oscillatory behavior of both single crystal and supported catalysts is somewhat premature.