Chromic acid evaporation upon exposure of Cr2O3(s) to H2O(g) and O2(g) – mechanism from first principles

Density functional theory is employed to address the mechanism for chromic acid desorption from a Cr2O3 surface. The reaction path involves the 1 1 0 Cr2O3 surface, which is subject to initial oxidation to produce Cr(IV) surface sites. Effects of subsequent further oxidation, hydration and hydrolysis are described. Molecular oxygen exposure has the formation of a monolayer of Cr(VI) groups with 506 kJ/mol exothermicity. An intermediate molecularly chemisorbed peroxide species on a reactive five-coordinated surface Cr site is characterized. Competition between O2 and H2O for the five-coordinated Cr site is quantified. The Cr(VI) compound is subject to hydrolysis to form free Cr(VI)O2(OH)2(g) and surface hydroxides whereby an additional 33 kJ/mol is released. A net 44 kJ/mol endothermicity for the formation Cr(VI)O2(OH)2(g) is in qualitative agreement with the experimentally obtained 60 kJ/mol.