Effect of interfacial interactions on modification of cerium oxide and its reduction in composite materials under hydrogen containing atmosphere

Ce-based materials have attracted much attention because of their applications as electronic ceramics, membranes, electrodes in fuel cells, and catalysts. The redox activity of cerium cations, which is the basis of its application as the catalysts, was usually observed in undoped cerium oxide at elevated temperatures and low oxygen partial pressure. One of the questions remained to be answered is whether cerium oxide in a composite material would reduce during long-term exposure to hydrogen containing atmosphere. If it is, how wide extend would be. To explore this aspect, the redox activity of the composite systems La_0.6Sr_0.4CoO₃ -CeO₂ (LSCCe), La_0.95Ni_0.6Fe_0.4O₃ -CeO₂ (LNFCe), and La_0.8Sr_0.2MnO₃ -CeO₂ (LSMCe) fabricated through the reactive sintering in air at 1350 °C was studied under hydrogen containing atmosphere. During the reactive sintering the cross-dissolution of cations at the phase interfaces occurs and results in the modification of the initial phases, forming cerium oxides with different degree of oxygen deficiency. The degree of the cross-dissolution at the phase interfaces increases in the following sequence: LSMCe → LSCCe → LNFCe. The results obtained in the present study demonstrate that reduction of modified cerium oxides in composite materials with highly reducible transition metal cations occurs under hydrogen atmosphere at the initial stage of reduction. During long-term exposure to H₂-Ar atmosphere the involvement of cerium oxide in the reduction process seems to depend on its lattice parameter that determines the length of the pathway for oxygen ion transport through the lattice. Kinetics of the reduction of composite materials at 800 °pC has been discussed in more details.