Simultaneous measurement of CO oxidation rate and surface coverage on [formula omitted] using infrared spectroscopy: Rate hysteresis and CO island formation

Abstract

The catalytic oxidation of CO on a commercial Pt Al 2 O 3 catalyst has been studied at a total pressure of one atmosphere using Fourier transform infrared spectroscopy. Simultaneous measurements of steady-state reaction rates and CO surface coverages were obtained at temperatures between 460 and 520 K. During oxidation, the frequency of the infrared absorption peak of linearbonded CO is invariant at 2081 cm −1 at 520 K over nearly a 100-fold change in CO surface coverage. This peak position is within 1 cm −1 of that found for saturation coverage in the presence of pure CO at one atmosphere. In contrast, under nonreactive desorption, large shifts in frequency are found as the CO surface coverage changes. The ratio of the intensities of the absorption bands due to both linear and bridge-bonded CO was constant during oxidation. These observations are interpreted as resulting from the formation of islands of CO during the oxidation reaction. Hysteresis in both CO reaction probability and CO surface coverage are found which are inversely related. Thus CO blocks active sites on the Pt for the reaction. The width and shape of the hysteresis loops as a function of temperature are qualitatively understood in terms of the rate of CO desorption from the Pt surface. At very high surface coverages, changes in the reaction probability are not accompanied by changes in adsorbed CO. This may be due either to the formation of surface species, unobserved in ir, which block the reaction or to the presence of a small number of very reactive Pt sites which are blocked with only a very small change in CO surface coverage.