Large enhancement of the catalytic activity for CO oxidation on hole doped (Ln,Sr)NiO3 (Ln = Pr, Sm, Eu) Perovskites

The effect of Sr substitution in LnNiO(3) (Ln = Pr, Sm, Eu) Perovskites on the structural and catalytic properties was studied by means of textural measurements, X-ray and neutron powder diffraction (XRD, NPD), thermogravimetric analysis, temperature-programmed reduction and catalytic activity in CO oxidation. All the samples were prepared using soft-chemistry procedures, decomposing citrate precursors at 1073 K, and subsequently oxygenating them at 1273 K under a very high oxygen pressure (200 bar). This treatment, necessary for the stabilization of Ni3+-Ni4+ mixed valences, resulted in very low specific areas (1-2 m(2) g(-1)). The X-ray diffraction profiles indicated pure Perovskite phases with orthorhombic distortion, which increased in the order Pr < Sm < Eu. The Rietveld refinement of NPD data for a selected Pr0.95Sr0.05NiO3 sample showed a significant reduction in the Ni-O distances with respect to the undoped compound, concomitant with the hole-doping of the d-bands of the solid. Temperature-programmed reduction profiles revealed that the reduction of Ni3+ occurs through two consecutive reduction steps (Ni (3+/4+) --> Ni2+ --> Ni-0). Both unsubstituted and Sr-substituted samples were tested in CO oxidation and it was found that the intrinsic activity is enhanced with Sr-substitution particularly for the Pr0.95Sr0.05NiO3 sample. This higher activity in CO oxidation appears to be related to the relative ease of oxygen removal from the hole-doped Perovskites and to the presence of oxygen vacancies. (C) 2000 Elsevier Science B.V. All rights reserved.