Application of infrared spectroscopy and surface potential measurements in a study of the oxidation of carbon monoxide on platinum

Abstract

The kinetics of the heterogeneous oxidation of carbon monoxide by oxygen over an Aerosilsupported platinum catalyst and over evaporated platinum films have been investigated, between 250 and 370 °K, both with infrared spectrophotometry and by using the change of work function of the catalyst brought about by the presence of adsorbed species. Thus, the concentration of chemisorbed carbon monoxide has been continuously monitored during the oxidation process by measuring the height of the absorption b and maximum at ca. 2070 cm -1, and the concentration of chemisorbed oxygen atoms by recording their surface potential. Since reactions involving chem isorbed species are probably rate determining in hetero ­ geneous catalytic reactions, this direct measurement leads to a considerable simplification of the kinetics and the rate of reaction is well represented by the expression the kinetics and the rate of reaction is well represented by the expression rate = κ[CO(a)]² [O₂(a)] exp (—E₁/RT); the α represents an adsorbed species. Rates of oxidation on films an d supported catalyst are substantially the same w hen reproducible activity of the catalysts is obtained by alternate treatment of their surfaces with gaseous carbon monoxide an d oxygen. The possible primary steps are: 2CO(a) + O2(a) → 2CO₂(g), (1) CO(a) + O(a) → CO₂(g), (2) CO(g) + O(a) → CO₂(g), (3) The activation energy of (1) is ca. 10 kcal/mole and that of (3) is ca. 0 kcal/mole, while (2) is thermodynamically improbable. Under the conditions used in the present investigation, reaction (1) is overwhelmingly the most important, whereas reaction (3) is rate determining when a mixture of the two reactants are in contact with the catalyst at 500 °K (Langmuir 1922); the kinetics are then described by rate = {κ[O₂ (g))]/[CO(g)]} exp ( - q₁/RT ). Not only is this expression qualitatively in accord with the predictions based on reactions (1) and (3), but the results of both investigations are shown to be incomplete quantitative agreement.