Water adsorption and hydrolysis on molecular transition metal oxides and oxyhydroxides

Addition of water to molecular transition metal oxides (TiO2(g) and CrO3(g)) and oxyhydroxides (ScO(OH)(g), VO2(OH)(g), and MnO3(OH)(g)) was studied by means of quantum chemistry. In the investigated reactions, each reaction step comprised the breaking of one M=O bond and the formation of two OH groups. Exothermicity was observed when the product had tetrahedral or lower oxygen coordination. The reactions were found to involve stable water complexes as intermediates. The stabilities of such complexes were accentuated in the addition reaction Sc(OH)(3)(g) + H2O(g), in which the formation of a tetrahedral complex was found exothermic. For VO(OH)(3)(g), CrO2(OH)(2)(g), and MnO3(OH)(g), water addition to the remaining M=O bonds was found endothermic, whereas the formation of water complexes, using hydrogen bonds and preserving the oxyhydroxide kernel, was preferred. Thus, the sequence of such kernels for water clustering in the investigated reactions was found to be Sc(OH)(3). H2O(g), Ti(OH)(4)(g), VO(OH)(3)(g), CrO2(OH)(2)(g), and MnO3(OH)(g). These stability considerations are important, as CrO2(OH)(2)(g) is believed to be the product of water-induced degradation of the protective chromium oxide scale on stainless steel at elevated temperatures.