Reduction of CeO₂ in composites with transition metal complex oxides under hydrogen containing atmosphere and its correlation with catalytic activity

Reduction behavior of pure and doped CeO₂, the multi-phase La_0.6Sr_0.4CoO₃·xCeO₂, La_0.8Sr_0.2MnO₃·xCeO₂, and La_0.95Ni_0.6Fe_0.4O₃·xCeO₂ composites, was studied under hydrogen containing atmosphere to address issues related to the improvement of electrochemical and catalytic performance of electrodes in fuel cells. The enhanced reduction of cerium oxide was observed initially at 800°C in all composites in spite of the presence of highly reducible transition metal cations that could lead to the increase in surface concentration of oxygen vacancies and generation of the electron enriched surface. Due to continuous reduction of cerium oxide in La_0.6Sr_0.4CoO₃·x-CeO₂ and La_0.8Sr_0.2MnO₃·xCeO₂ (up to 10 h) composites the redox activity of the Ce⁴⁺/Ce³⁺ pair could be suppressed and additional measures are required for reversible spontaneous regeneration of Ce⁴⁺. After 3 h exposure to H₂-Ar at 800°C the reduction of cerium oxides and perovskite phases in La_0.95Ni_0.6Fe_0.4O₃·xCeO₂ composites was diminished. The extent of cerium oxide involvement in the reduction process varies with time, and depends on its initial deviation from oxygen stoichiometry (that results in the larger lattice parameter and the longer pathway for O_2- transport through the fluorite lattice), chemical origin of transition metal cations in the perovskite, and phase diversity in multi-phase composites.