Chromic acid evaporation upon exposure of Cr 2O 3(s) to H 2O(g) and O 2(g) – mechanism from first principles

• Published in: Chemical physics letters Vol. 383; no. 5; pp. 549 - 554
• Main Authors: Panas, Itai, Svensson, Jan-Erik, Asteman, Henrik, Johnson, Tobias J.R., Johansson, Lars-Gunnar
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 2004
• ISSN: 0009-2614; 1873-4448
• DOI: 10.1016/j.cplett.2003.11.079

Density functional theory is employed to address the mechanism for chromic acid desorption from a Cr 2O 3 surface. The reaction path involves the 1 1 0 Cr 2O 3 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 H 2O for the five-coordinated Cr site is quantified. The Cr(VI) compound is subject to hydrolysis to form free Cr(VI)O 2(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.