Diversity of the kinetics of carbon monoxide oxidation on differently prepared zinc oxides

The fine kinetics of carbon monoxide oxidation over two differently prepared zinc oxides, Kadox ZnO (K25-ZnO) and Kanto Chemical Co. ZnO (Kan-ZnO), have been investigated and compared in detail using the steady-state and the transient-response methods. The morphology based on scanning electron micrographs showed fine crystallites with high surface area, 20 m² g^-1 for K25-ZnO and 4.0 m² g^-1 for Kan-ZnO, even though they gave the same X-ray diffraction pattern of wurtzite-type crystalline structure. Extraordinary differences in the steady-state rate surfaces for the two catalysts were found, being first order with respect to P_CO and zero order with respect to P _O2 for Kan-ZnO, and having an anomalous tendency with respect to P_CO, indicating both a maximum and a minimum, for K25-ZnO. Based on the electrical conductivity measurements of the catalysts and the characteristics of the transientresponse curves obtained, two reaction path models for the two catalysts have been proposed relating to two different oxygen species. All kinetic parameters evaluated from experimental data consistently explain both the steady-state rate data and the characteristic modes of transient-response curves obtained.