Ab Initio Study of the Interaction of Dimethyl Methylphosphonate with Rutile (110) and Anatase (101) TiO2 Surfaces

摘要

Density functional theory has been applied to an ab initio study of the molecular and the dissociative adsorption of dimethyl methylphosphonate (DMMP) on TiO2. Rutile (110)-(1x 1) and anatase (101)-(1x 1) surfaces, free of defects and of OH, have been modeled using two-dimensionally periodic slabs (2-DPS) and also clusters with unsaturated edge atoms terminated with pseudohydrogen. Molecular adsorption occurs via the formation of a Ti_(5c)···O=P dative bond at unsaturated, 5-fold-coordinated Ti surface sites. C-H···O bonds between CH3 groups and surface O atoms also contribute, but dative bonding between methoxy O atoms and Ti_(5c) sites has no significant effect. In 2-DPS calculations, all TiO2 surfaces considered give about the same adsorption energy (ΔE_(ads)) for molecular adsorption. There does, however, appear to be a small variation among the different substrates whereby ΔE_(ads) increases with surface energy. Two spectroscopic signatures of molecular adsorption have been computed. One is a shift of -70 cm~(-1), relative to the gas-phase value, in the v(P=O) stretching frequency, and the other is a shift of 0.5 eV to higher binding energy, relative to the gas-phase value, of the molecular HOMO. Several hypothetically possible products have been considered for the first step in dissociative adsorption. The most favorable is a (CH3)(CH3O)P(-O-)2 bridge between two Ti_(5c) sites together with a CH3 bonded to the O atom of a (Ti-)2O_(2c) site. This model is consistent with experimental results for DMMP adsorption on OH-free rutile (110) surfaces. On all TiO2 surfaces considered, dissociative adsorption is more exothermic than molecular adsorption. In 2-DPS calculations, ΔE_(ads) for dissociative adsorption is essentially the same for rutile (110) and anatase (101), but is smaller than that for anatase (010). The cluster models give results that are generally consistent with those of 2-DPS models; however,ΔE_(ads) is somewhat overestimated in the cluster calculations.