Molecular beam study of CO oxidation on a (111) platinum surface

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Abstract
The heterogeneous oxidation of CO on epitaxially grown Pt(111) surfaces has been investigated using modulated molecular beam techniques. Within the range of pressures from 10−7 to 10−5 torr and temperatures from 450 to 950°K, the overall reaction is apparently first order. The observance of higher order kinetics in these experiments is argued to be an artifact of the beam technique used here and suggests a high surface diffusion rate for adsorbed CO. The measured angular dependence of the CO2 desorption rate from the Pt(111) surfaces does not follow the Knudsen law of random (cosine) desorption, but is much more sharply peaked about the surface normal. As with the desorption of hydrogen, the desorption law for CO2 is given by cosdθr where in this case d ≤ 6, depending on the condition of the surface. The adsorption of CO is also shown to have an angular dependence and is not activated on Pt(111). The results of this study strongly suggest that a Langmuir‐Hinshelwood mechanism dominates the reaction, with CO being the principal diffusing species.