The chemical nature of surface point defects on MoO3(010): Adsorption of hydrogen and methyl

We report density functional theory calculations using the Adaptive Coordin ate Real-space Electronic structure (ACRES) method of the terminal oxygen v acancy on the (010) surface of MoO3, within a (2 x 2) ordered array of vaca ncies on the surface. Analysis of the electronic structure of this surface shows that there are unoccupied dangling d(xz) and d(z)(2) orbitals perpend icular to the surface that are created by the removal of terminal oxygen. T he Mo-oxygen bonds surrounding the vacancy contract; however, the overall m orphology of the surface is not drastically distorted. The vacancies alter the chemical character of the surface, as shown by studies of hydrogen and methyl binding. On both the "perfect" and vacancy surfaces, hydrogen was mo st strongly adsorbed over the terminal oxygen and most weakly bound over th e symmetric bridging oxygen. Hydrogen is bound over the Mo atom, with a sli ghtly smaller binding energy than hydrogen over the asymmetric bridging oxy gen. The most favorable binding site for methyl on the vacancy surface is o ver the Mo atom exposed by removal of a terminal oxygen, whereas methyl bou nd to terminal oxygen is most stable on the perfect surface. There is no lo cal minimum for adsorption over the symmetric bridging oxygen; instead, a m ethyl placed over this site moves toward the terminal oxygen vacancy. Anayl ysis of the bonding shows that methyl is bound more strongly than hydrogen over the Mo atom because the C 2p orbital has better overlap with the Mo d( z2) orbital than the hydrogen Is. In addition, the steric repulsion observe d for methyl over the perfect MoO3(010) surface is more easily relieved wit h the presence of the terminal oxygen vacancy.