Role of hydroxyl groups in the NH_x (x = 1-3) Adsorption on the TiO₂ anatase (101) surface determined by a first-principles study

A spin-polarized density functional theory calculation was carried out to study the adsorption of NH_x species (x = 1-3) on a TiO₂ anatase (101) surface with and without hydroxyl groups by using first-principles calculations. It was found that the present hydroxyl group has the effect of significantly enhancing the adsorption of monodentate adsorbates H ₂N-Ti(a) compared to that on a bare surface. The nature of the interaction between the adsorbate (NH_x) and the hydroxylated or bare surface was analyzed by the Mulliken charge and density of states (DOS) calculations. This facilitation of NH₂ is caused by the donation of coadsorbed H filling the nonbonding orbital of NH₂, resulting in an electron gain in NH₂ from the bonding. In addition, the upper valence band, which originally consisted of the mixing of O 2p and Ti 3d orbitals, has been broadened by the two adjacent H 1s and NH₂ -y_b orbitals joined to the bottom of the original TiO₂ valence band. The results are important to understand the OH effect in heterogeneous catalysis.