The Accommodation Coefficients of Carbon Monoxide, Oxygen, and Carbon Dioxide on Platinum

Abstract

For the purpose of investigating the energy transfer at the gas-solid interface in the catalytic oxidation of carbon monoxide on platinum, the accommodation coefficients (α) of carbon monoxide, oxygen, and carbon dioxide were determined by measuring the heat loss from the platinum ribbon at temperatures of 165∼ 544° C and under gas pressures of 1∼ 5× 10-2 Torr. For carbon monoxide, a is 0.652 at 165° C and decreases with increasing ribbon temperature to 0.615 at 291°C, but increases at higher temperatures. For oxygen, α is 0.471 at 165°C and decreases up to about 400° C, and then shows nearly constant value of 0.39. For carbon. dioxide, a is 0.482 at 165° C and decreases to 0.354 at 544° C monotonously. The temperature dependence of α can be explained as follows. Since carbon dioxide is not chemisorbed on platinum, the impinging gas molecules are adsorbed physically on a platinum surface and exchange their energywith the surface, and a decreases with increasing ribbon temperature. Carbon monoxide and oxygen are chemisorbed on platinum forming a layer at low temperatures, and most of the energy transfer takes place between the layer and the gas molecules physisorbed on it. At higher temperatures, the chemisorbed layer becomes unstable, and the energy transfer also takes place on the bear surface. Consequently, the temperature dependence of α deviates from that observed at low temperatures.