Vacuum microbalance investigations on the pressure and temperature dependence of solid/gas reactions

The pressure and temperature dependdence of the evaporation-analogous heterogeneous reaction ZnO(s) + CO(v), H2(v) → Zn(v) + CO2(v), H2O(v) was studied with a vacuum microbalance apparatus. The reaction rate as a function of pressure, temperature and surface structure of the solid was determined. It was found that the order n and the activation energy EA depend on temperature and pressure. The results can be explained by the assumption of different reaction mechanisms. At high temperatures and small pressures the activation of the Zn–O bond on the surface of the solid is rate determining. For high temperatures and hogh pressures the reflection of the reaction products Zn(v) and Co2(v) from the gas phase on the surface auses the change in the order n and hast to be considered. At low temperature the desorption of the products become rate determining. Experiments with Co/argon mixtures confirm the proposed interpretation for higher pressures