Interaction of CO with Ni(100). Adsorption isotherms and thermal decomposition

Abstract

The reversible adsorption of CO on Ni(100) in the temperature range 100–150 °C has been investigated with ellipsometry and Auger electron spectroscopy, as has the thermal decomposition at higher temperatures. From thermodynamic data the (coverage-independent) heat of adsorption was found to be 134 ± 4 kJ mol^–1. An isotherm equation is given, valid for temperatures up to 225 °C, consistent with the hexagonal structure of the CO adlayer at saturation. Decomposition, which occurs measurably above 150 °C, leads to the deposition of carbon and is influenced by an operating ion gauge. Decomposition occurs according to CO(a)+ CO(g)→ CO₂+ C(a). The rate of disappearance of CO has a small pressure dependence that is explained by a mechanism in which the formation of a highly reactive CO(a) molecule from adsorbed CO is rate-determining. For this step the availability of a site not poisoned by carbon is necessary. It is shown that the ion gauge reduces the number of sites poisoned by one carbon atom. The reaction rate for thermal decomposition has an activation energy of 130 kJ mol^–1.