Adsorption and dissociation of NH₃ on clean and hydroxylated TiO₂ rutile (110) surfaces: A computational study

The adsorption and dissociation of NH₃ on the clean and hydroxylated TiO₂ rutile (110) surfaces have been investigated by the first-principles calculations. The monodentate adsorbates such as H₃NTi(a), H₂NTi(a), NTi(a), H₂NO(a), HNO(a), NO(a) and HO(a), as well as the bidentate adsorbate, TiNTi(a) can be formed on the clean surface. It is found that the hydroxyl group enhances the adsorption of certain adsorbates on the five-fold-coordinated Ti atoms (5c-Ti), namely H₂NTi(a), HNTi(a), NTi(a) and TiNTi(a). In addition, the adsorption energy increases as the number of hydroxyl groups increases. On the contrary, the opposite effect is found for those on the two-fold-coordinated O atoms (2c-O). The enhanced adsorption of NHx (x = 1 − 2) on the 5c-Ti is due to the large electronegativity of the OH group, increasing the acidity of the Ti center. This also contributes to diminish the adsorption of NHx (x = 1 − 2) on the two-fold-coordinated O atoms (2c-O) decreasing its basicity. According to potential energy profile, the NH₃ dissociation on the TiO₂ surface is endothermic and the hydroxyl group is found to lower the energetics of H₂NTi(a)+HO(a) and HNTi(a)+2{HO(a)}, but slightly raise the energetic of TiNTi(a)+3{HO(a)} compare to those on the clean surface. However, the dissociation of NH₃ is found to occur on the hydroxylated surface with an overall endothermic by 31.8 kcal/mol and requires a barrier of 37.5 kcal/mol. A comparison of NH₃ on anatase surface has been discussed. The detailed electronic analysis is also carried out to gain insights into the interaction nature between adsorbate and surface.