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 110 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 O-2 and H2O for the five-coordinated Cr site is quantified. The Cr(VI) compound is subject to hydrolysis to form free Cr(VI)02(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)O-2(OH)(2)(g) is in qualitative agreement with the experimentally obtained 60 kJ/mol. (C) 2003 Elsevier B.V. All rights reserved.