Oxygen chemisorption on platinum: II. Model calculations

Abstract

There exist two conflicting sets of data in the literature concerning the sticking probability of oxygen on platinum. One set suggests a large value of the order of 0.1, while other results indicate that the sticking probability is much smaller. Recent Auger electron spectroscopy data of Weinberg et al. yield a value of ca. 7 × 10 −7 on a clean platinum (111) surface. Reasons are given for believing that the large sticking probabilities are measured when the platinum surfaces are contaminated with carbon, and the smaller values are obtained when the platinum is clean. Absolute rate theory of gas phase kinetics together with the Crystal Field Surface Orbital-Bond Energy Bond Order (CFSO-BEBO) Model of Weinberg and Merrill is applied to obtain a theoretical sticking probability of 7 × 10 −6 ⩽ S ⩽ 7 × 10 −5 in reasonable agreement with experiment. The activation energy to chemisorption is predicted to be ca. 2.1 kcal mole . A model is also developed for the case of carbon stimulated adsorption of oxygen on a contaminated surface, and its properties investigated by the same theoretical approach. In the latter case the adsorption is found to be non-activated, but the principal reason for the great increase in the adsorption rate lies in a change in the entropy of activation. Substitution of reasonable numerical values for the parameters in the rate expression leads to a calculated sticking probability of the order of 0.1 for the contaminated surface, i.e. close to the high values reported by several workers.