Synthesis, characterisation, TPR/TPO and activity studies on LaMnxV1-xO4-δ catalysts

LaMnxV1-xO4-delta samples (0 less than or equal tox less than or equal to1) were synthesised and characterised using XRD and FTIR techniques, while their oxygen content, reducibility, stability and ionic mobility were investigated by TPR, TPO and TPD methods. All mixed oxides were found to be of single phase; they were stable in air and exhibited reproducible TPR/TPO patterns over consecutive six to seven cycles. The doping of Mn resulted in lowering of reduction and oxidation temperatures. The substitution of B cations with Mn4+ also facilitated CO oxidation reaction in a lower temperature range of 200-500 degreesC, while LaVO4 or LaMnO4-delta showed very poor catalytic activity. The XRD powder patterns of all the Mn-doped samples revealed a redox behaviour, i.e. conversion of LaVO4 to LaVO3 and back to LaVO4, on subjecting a sample to H-2-reduction and re-oxidation treatments. Our results substantiate the important role of oxygen vacancies generated in the crystal lattice on introduction of Mn4+ such vacancies in turn lead to improved catalytic activity because of a greater oxygen mobility in the lattice. An ionic redox mechanism, where the re-oxidation step is a limiting process. is applicable to CO oxidation at temperatures above 200 degreesC. (C) 2001 Elsevier Science B.V. All rights reserved.