STUDY OF MULTIPHASIC MOLYBDATE-BASED CATALYSTS .1. ELECTRICAL-CONDUCTIVITY STUDY OF VALENCE STATES AND SOLUBILITY LIMITS IN MIXED IRON AND COBALT MOLYBDATES

The electrical conductivity of a series of iron-cobalt molybdates has been measured at 370°C, as a function of the iron content. An important increase of the electrical conductivity occurred when small amounts of iron were substituted for cobalt. This takes place because iron is partially in the 3+ oxidation state. This species, which is stabilized into the CoMoO4 structure, creates a valence induction effect which is responsible for the drastic variation of the electrical conductivity with compounds whose iron content ranges from 0 to 20%. UV spectroscopy allows us to propose that the ferric species involved in this valence induction phenomenon are those giving rise to the intervalence charge transfer between iron cations (Fe2+-O-Fe3+). By contrast, ESR spectroscopy is silent for such a phenomenon (too short relaxation time) and shows two typical Fe3+ (d5 ion) spectra for Fe3+ engaged either in a different manner in the solid solution (strongly distorted octahedral site) or a given compound such as Fe2(MoO4)3 observed at high iron content. © 1993 Academic Press, Inc.