A semi-empirical molecular orbital method was used to investigate the molecular and dissociative adsorption of water at rutile (110) and anatase (001) surfaces. These surfaces were simulated by model clusters, and the effect of long-range interactions was accounted for by an increase in the clusters. It was shown that qualitatively and semi-quantitatively correct descriptions of adsorption were possible only with the use of sufficiently large model clusters, and under conditions of local relaxation. It was found that, in agreement with published results, dissociative adsorption was energetically favored at both surfaces. In the case of rutile, water adsorption was also studied in the presence of O defects. The adsorption energies at defect positions were found to be higher, by a factor of 2 to 3, than those for the ideal surfaces. This was consistent with experimental investigations which indicated an increased activity at surfaces with defects. Local relaxation at the defect site played an important role in determining the relative stabilities of various adsorption sites.

Theoretical Investigation of Water Adsorption at Rutile and Anatase Surfaces. Bredow, T., Jug, K.: Surface Science, 1995, 327[3], 398-408