The kinetics of the heterogeneous oxidation of zinc vapor by carbon dioxide

The heterogeneous oxidation of Zn(g) is a promising reaction pathway for the conversion of CO₂ into CO in the two-step Zn/ZnO solar thermochemical cycle as it eliminates the solid-state diffusion limitation that plagues the oxidation of Zn(l,s). The rate of the heterogeneous oxidation of Zn(g) is measured gravimetrically in a quartz tubular flow reactor operated at atmospheric pressure for temperatures between 800 and 1150K, Zn(g) concentrations up to 36mol%, and CO₂ concentrations up to 45mol%. The surface kinetics are extracted from the global reaction rate using a numerical reacting flow model that accounts for the transport of reacting species in the gas phase. The oxidation of Zn(g) by CO₂ is rapid, on the order of 10^-8-10^-5molcm^-2s^-1, and the rate is proportional to the product of the Zn(g) and CO₂ partial pressures at the reaction surface. The activation energy for the Arrhenius reaction rate parameter is 44±3kJmol^-1 and the pre-exponential factor is (92±6)×10^-3molcm^-2s^-1atm^-2. As a result of the rapid rate of oxidation of Zn(g), less than 1s is required to convert more than 85% of Zn to ZnO.