Behavior of chromium oxide on MgO or MgF2

Abstract

The molecular structure and oxidation state of the chromium oxide on MgO or MgF2 as a function of Cr loading were determined by XRD, Raman spectroscopy, and XPS. The effects of preparation method and the acidity of support were investigated. The Raman study on a series of chromium oxide supported on MgO prepared by an impregnation method showed that the chromium oxide was present as polychromate independent of Cr loading due to high acidity of the preparation solution. In samples prepared by a precipitation method, the chromium oxide was present as monochromate at 5% Cr loading, as dichromate at 10% Cr loading, and as mono- and polychromate at 20% Cr loading due to a larger surface area of the support prepared by this method. The chromium in CrO3/MgF2 was present as polychromate and monochromate for impregnated and precipitated samples, respectively. The oxidation state of Cr(VI) on MgO is maintained independent of Cr loadings. When the 10 wt % Cr/MgO was calcined at 800 degrees C, MgCr2O4 and Cr2O3 were formed. Preferred formation of MgCr2O4 was observed for the precipitated sample due to its larger surface area compared to that of the impregnated sample.