Mechanisms of the catalytic carbon monoxide oxidation on Pt (110)

Abstract

The kinetics of the carbon monoxide oxidation on a clean Pt (110) crystal were investigated in an ultra-high vacuum system by utilizing Auger electron spectroscopy, low-energy electron diffraction and residual gas analysis. Two different catalytic reaction mechanisms were found to prevail for the experimental conditions chosen. In the temperature range, 100 < T ⩽ 220°C, where essentially CO was preadsorbed on the Pt surface the subsequent adsorption of O 2 was competitive and the reaction exhibited the characteristics of a Langmuir-Hinshelwood mechanism. In this case the onset of the CO 2 formation was delayed by a characteristic time which depended strongly on temperature (“induction period”). A simple model for the Langmuir-Hinshelwood reaction was developed which permitted a more detailed evaluation of the kinetic curves yielding an activation energy for the catalytic reaction of 2.9 kcal/mole. On the other hand, when oxygen was preadsorbed on the Pt surface ( T > 90°C) the subsequent reaction with CO occurred immediately and was temperature independent. This behavior was interpreted in terms of an Eley-Rideal mechanism. Both reactions were used for titration of the adsorbed species. From area measurements under the titration curves it was concluded that the saturation coverage for CO and oxygen on Pt(110) is approximately the same.