Combined EPR and TG techniques : comparison of the thermal reduction of chromium (VI) ions in some zinc chromates and chromate-oxalate mixtures

Abstract

Lower oxydation state chromium species which are successively formed and stabilized upon thermal reduction of chromate (VI) ions, are studied by combined EPR and TG-techniques. The spontaneous oxygen release occuring during the vacuum thermal treatment of some zinc chromates leads to the formation of both Cr(V) and Cr(III) species, the latter being either dispersed in ZnO, clustered in a Cr 2O 3-like phase or ordered in a ZnCr 2O 4-spinel phase. The Cr/Zn ratio in the initial compound as well as the presence of some reducing OH − ions are critical for the stability of such intermediate Cr species. When solid mixtures of ZnCrO 4 and ZnC 2O 4 are calcined, the CrO 4 = reduction is enhanced by the in-situ genesis of reducing gaseous or adsorbed carbon monoxyde stemming from the oxalate. Surprisingly such a reducing system favours the high temperature formation of some other very stable Cr(V) species.