NGR study of iron molybdates and iron-cobalt molybdates catalysts

Abstract

Iron molybdates FM and cobalt-iron molybdates CF _x M (which means Co_1−y Fe _y MoO₄ withy=x/100 6≤x≤67), used as matrix of catalysts for propene mild oxidation, were studied by Mössbauer spectroscopy at 295 K and 4 K. All the spectra exhibit three Fe²⁺ doublets, two of them correspond to β-FeMoO₄ and the third one to α-FeMoO₄. Enhancement of quadrupole splittings of Fe²⁺ in the spectra of the CF _x M catalysts is ascribed to the occurrence of a solid solution of either iron in CoMoO₄ or cobalt in FeMoO₄. In each spectrum at 295 K one or several Fe³⁺ doublets were present, which low temperature spectra allow to be assigned to Fe₂(MoO₄)₃, small particles of Fe₂O₃, solid solution of Fe³⁺ in ferrous molybdates and even Fe₂MoO₄ in some cases. The behaviour of all the iron species of these solids during reduction by hydrogen is described.