Influence of Ni substitution on redox properties of In2(1−x) Ni2xTiO5−δ oxides

The thermal behavior of mixed metal oxides with nominal compositions of In2(1−x)Ni2xTiO5−δ where 0.0 ≤ x ≤ 0.2, were investigated by recording their temperature programmed reduction (TPR). The samples were synthesized by ceramic route and analyzed for phase composition using powder X-ray diffraction. The TPR profile of pristine In2TiO5 indicated the reduction of In3+, which takes place predominantly over the other species Ti4+ in In2TiO5 sample. Ni substitution at In3+ site induced ease in reducibility as indicated by lowering of onset reduction temperature and Tmax. Moreover, it also facilitated the reduction of otherwise non reducible cation, Ti4+. Ni metal initially formed by reduction, dissociates and activates hydrogen which then reacts and reduces the oxide (even Ti4+), a case of hydrogen spillover. The metallic Ni forms an alloy Ti2Ni with reduced Ti metal and δIn3Ni2 alloy with the reduced In metal as evident by XRD and XPS results of the reduced residue of 20% Ni doped (In1.6Ni0.4TiO5−δ) sample. Due to formation of new alloy phases of δIn3Ni2 and Ti2Ni, the rate of hydrogen adsorption diminishes which slows down further hydrogen activation and consequent further Ti4+ reduction.