Kinetics of zinc oxide reduction with carbon monoxide

Reduction of ZnO single particles with CO was investigated at atmospheric pressure from 1000° to 1500°C. Weight loss data up to about 90 pct reduction were easily reproducible for the dense photoconductive grade ZnO particles but not for the American grade samples, whose scatter was attributed to the 13 pct residual internal porosity and to impurities. The data agreed closely with a mixed regime model, which pictures external diffusion acting in series with an irreversible first order kinetic process at the surface. After the diffusional contribution was subtracted, activation energies of 37,900 (±2040) cal per mole and 20,600 (±10,200) cal per mole were obtained for the photoconductive and American grades, respectively. For the photoconductive grade the mixed regime model gave a good fit over the entire temperature range. Diffusional limitations were approached at 1500°C.