THEORETICAL INVESTIGATION OF WATER-ADSORPTION AT RUTILE AND ANATASE SURFACES

The semiempirical MO method SINDO1 was used for the investigation of m olecular and dissociative adsorption of water at titanium dioxide surf aces rutile (110) and anatase (001). The surfaces were simulated with model clusters. The influence of long-range interactions was accounted for by increase of the clusters. It was shown that a qualitatively an d semiquantitatively correct description of adsorption is possible onl y with sufficiently large model clusters and under consideration of lo cal relaxations. In agreement with the literature, the dissociative ad sorption is energetically favored at both surfaces. For the rutile (11 0) surface the water adsorption was studied also in the case of oxygen defects. The adsorption energies at defect positions were found to be higher by a factor of two to three than for the ideal surface. This c onfirms experimental investigations which find increased activity at s urfaces with defects. The local relaxation at the defect site plays an important role for the determination of relative stabilities of diffe rent adsorption sites. A potential curve was calculated for the dissoc iation of a water molecule adsorbed at a rutile (110) surface, and the influence of hydrogen bonding and local relaxation on the reaction ba rrier was determined.