MVMoO7 substituted phases (M=Fe, Cr): Structural stability, reducibility and catalytic properties in methanol oxidation

Abstract

Structural, spectroscopic and thermal properties of MVMoO7 (M= Fe, Cr) phases, of potential interest in the catalysis field, are studied by means of X-ray Powder Diffraction XRD, Fourier Transformed Infrared Spectroscopy FTIR, Electronic Scanning Microscopy and X Ray Energy Dispersive Scattering Micro-Analysis SEM-EDS and Temperature Programmed Reduction TPR techniques. Thus, the formation of a triclinic solid solution in a complete range of composition is supported by a little M(III) size difference (about 5 %). In relation to the thermal reduction behaviour, iron is the unique reducible trivalent species. The reduction products of the Fe-Cr solid solution seem to be governed by the structural arrangement and particularly by the presence of M2O10 dimmers. The results are analyzed and compared with TPR data of MoO3, V2O5, M2O3,M2(MoO4)3 and MVO4 (M= Fe, Cr) binary and ternary systems. For the Fe-end member of the isomorphous series, a part of iron remains unreduced as FeV2O4–spinel phase at 1000oC whereas (V,Cr)2O3 is the final oxidizing product for the Cr-end member. However, the (Fe0.5Cr0.5)VMoO7 reduction occurs through the formation of FeMoO4 intermediate, affecting the Mo(VI)-Mo(IV)-Mo stability field and the V(V)-V(III) reduction steps. Finally, MVMoO7 (M= Fe, Cr) phases are chemically proved in the methanol oxidation reaction by Transients Study and the relationships among structural and chemical features and the catalytic behaviour are analyzed and discussed.