Mechanism of the catalytic oxidation of carbon monoxide on zinc oxide

Abstract

Pure zinc oxide is active at 200°C in the catalysis of the oxidation of carbon monoxide. Oxygen appears to be adsorbed in an ionic as well as a non-ionic form, while carbon monoxide and carbon dioxide are adsorbed essentially in non-ionic forms. Two surface sites, probably Zn⁺ ions, are required to fix O₂ and also CO₂. Carbon dioxide is not an inhibitor in the reaction but mutual displacement of CO₂ and O₂ is observed.

The kinetic study of the reaction shows that the rate is proportional to the square root of the oxygen pressure and is independent of CO pressure. These results may be explained by assuming that O₂ is adsorbed weakly and CO strongly on two different types of sites. As, however, an inhibiting effect is shown by non-stoichiometric mixtures with an excess of CO and an accelerating effect for mixtures with an excess of O₂, it is necessary to consider a competitive adsorption of CO and O₂ on one type of site and the reaction with either CO from the gas phase or with CO weakly adsorbed on the sites of a different type. However, since the concentration of the ionic species O^–(ads) practically does not vary on the surface of ZnO, the order ½ with respect to oxygen is interpreted as involving the participation in the reaction of non-ionic oxygen O (ads). As with NiO, the slowest step of the oxidation of CO on ZnO does not involve the transfer of electrons. Thus no effect of doping ZnO by Li⁺ or Ga³⁺ on catalytic activity is expected and none is observed.