Thermogravimetric and EPR study of the solid-state reactions between zinc chromate and zinc oxalate

Abstract

The formation of some catalytically active Cr(V) and Cr(III) species is studied when the progressive thermal reduction of zinc chromate is enhanced by the addition of solid zinc oxalate, the reducing agent CO being formed in situ, during thermal analysis.When compared with the decomposition of pure ZnCrO4, Cr(VI) reduction, as followed by EPR, is similar concerning the nature of the intermediate Cr species formed: Cr(V), Cr(III) in a Cr2O3-like bulk phase and Cr(III) dispersed in ZnO. However, the temperature range of “Cr2O3” formation, as well as the magnitude of the Cr(III)-Cr(III) interaction in such a phase is found to be strongly dependent on the amount of ZnO formed. On the other hand, after the Cr(V) species resulting from progressive oxygen release have disappeared, some other stable Cr(V) species are formed again at higher temperatures. Their formation may be related to a further reduction of the remaining Cr(VI) species by CO stemming from the oxalate.