Surface retexturing of Pt wires during the catalytic oxidation of CO

Abstract

The catalytic properties of Pt, which have been studied in a variety of forms of the metal (single crystals, wires or dispersed on a support), may be influenced by the method of preparing the metal surface. Pt surfaces undergo retexturing¹ during catalytic reactions at high temperatures (>1,000 K) and a relatively facile sintering process has also been described². Engel and Ertl³ have reviewed the heterogeneous oxidation of carbon monoxide on Pt (CO + 1/2 O₂ → CO₂), emphasizing studies concerned with the behaviour of the reaction at very low pressures and on clean surfaces (for example, on single crystal faces). Under these conditions the kinetic behaviour is determined by the ratio of the partial pressures of the reactants and is relatively insensitive to the total pressure. Here, we describe microscopic and kinetic observations of the oxidation on Pt wire surfaces below 600 K and at atmospheric pressure. The surface reactions are apparently more complicated here than at low pressure; extrapolations from the low to the high pressure regimes may not be applicable because of changes in the rate-controlling parameters. We present evidence of the mobility of Pt atoms (often considered immobile) on the surface and suggest that the catalyst functions through formation of oxy- and/or carboxyl-Pt complexes in a two-dimensional chemisorbed layer, envisaged as a monolayer possessing fluid properties and enhanced reactivity, containing both adsorbed reactant gases and mobile catalyst atoms.