Characterization, kinetics and redox properties of lanthanum chromium nickel oxide perovskites: Implications for combustion and solid oxide fuel cells.

摘要

LaCr_1−xNi_xO₃ perovskite oxides (x = 0, 0.25, 0.50, 0.75, 1.00) were investigated as possible materials for combustion of exhaust gases and as a potential solid oxide fuel cell (Sore) anode. This involved the determination of surface cation composition, kinetics of methane oxidation, catalyst stability under reducing conditions (P(O ₂) 1.10−6 atm), and redox properties.; XPS investigation to obtain surface cation compositions showed that surface and bulk compositions are indistinguishable. The methane oxidation rate depended on catalyst composition, and the highest activity was measured with LaNiO ₃. Ni-O-Ni ensembles in the nickel-containing materials are good candidates for the reactive species since a local defect can be formed without cation migration. A random population of B sites by Ni ions would produce a surface density of Ni-O-Ni ensembles proportional to x2 (x in LaCr _1−xNi_xO₃), in agreement with the dependence found in this study. During transient methane reduction (0.21 kPa ≤ P(CH ₄) ≤ 1.4 kPa and no gas phase oxygen), initial catalyst activities were comparable to rates in the presence of gas phase oxygen, indicating the same initial reaction pathways. The materials with high Cr (x ≥ 0.5) content are capable of preserving the perovskite lattice even after complete reduction of Ni3+ $\to$ Ni2+ in methane or hydrogen and subsequent reoxidation in the oxygen/argon mixture. Oxygen mobility studies showed that at least three distinct oxygen pools exist in these catalysts at lower temperatures (50°–300°C): adsorbed CO₂; easily accessed sites associated with Ni; less accessible sites that become involved at higher temperatures (≥300°C). LaCr_1−xNi_xO₃ materials have a better oxygen ion mobility than La(Sr)MnO_3−δ perovskites and are comparable to (LaCa)(CoFe)O_3−δ materials. LaCr_0.75Ni_0.25O₃ oxygen ion diffusion coefficient at 300°C is 4.1010 cm 2/s. In summary: LaCr_0.5Ni_0.5O₃ is a potential combustion catalyst due to its high hydrocarbon oxidation rate, good performance during upset conditions (oxygen lean atmosphere), and material stability to reduction/reoxidation cycles. LACr_1−xNi _xO₃ materials are possible anodes for solid oxide fuel cells, because they provide a substantial increase in the number of active surface sites over standard anode materials.