THE REDUCTION OF ZINC TITANATE AND ZINC-OXIDE SOLIDS

The reduction of bulk mixed oxides of zinc and titanium of various compositions and Zn-Ti-O crystalline phases was studied in a thermogravimetric apparatus in H2-H2O-N2 gas mixtures at 550-1050-degrees-C. Comparative reduction experiments with ZnO were also performed. In the absence of water vapor, activation energies of 24 and 37 kcal mol-1 were obtained for ZnO and Zn-Ti-O, respectively. The addition of water vapor inhibited reduction and resulted in a change in activation energy to 44 kcal mol-1 for both ZnO and Zn-Ti-O solids. Similar to H2O, H2S inhibited the initial reduction rate of ZnO and Zn-Ti-O materials. Based on kinetic experiments, a two-site model is proposed for ZnO reduction. One type of sites is characterized by a rapid reduction rate but is poisoned by water vapor as well as by the presence of titanium atoms in the solid. The other type of sites has a lower reduction rate, is not poisoned by H2O, and is slowly eliminated by the presence of titanium.