Transition Atoms Pathway on Stoichiometric and Reduced TiO$_{2}$ Rutile (110) Surface: Interfacial Charge Transfer and Distribution of Ti$^{3+}$ States

Charge transfer between metal nanoparticles and the supported TiO2 surface is primarily important for catalytic applications as it greatly affects the catalytic activity and the thermal stability of the deposited nanoparticles on the surface. Herein, a systematic spin-polarized density functional calculation is performed to evaluate the adsorption, diffusion, and charge state of several transition metal monomers on both stoichiometric and reduced rutile TiO2 (110) surface. The role of oxygen vacancy (Ov) with its accompanying excess electrons in influencing the activation of the monomers is examined. For pristine reduced surface, our hybrid functional calculation shows that only a small portion (around 5%) of the excess electrons occupy the topmost surface, which are mainly delocalized at the second nearest and third nearest fivefold coordinated Ti (Ti5c) atoms. The small amounts of excess electrons populating at the Ti5c atoms can be transferred to strongly electronegative adsorbates like Au and Pt thus enabling a moderate adsorption, whereas no stable adsorption at the Ti5c site is found for other less electronegative TM adatoms(Ag, Cu, Fe, Co, Ni and Pd) on the reduced surface and for all the adatoms on stoichiometric surface. This finding clarify the origin of the experimental observation of the adsorption of O2 and CO molecules at Ti5c sites in connection with charge transfer. In addition, the spatial redistribution of the excess electrons at Ti5c sites around the Ov upon the adsorption of the monomers is thoroughly examined. Our finding of an accumulation of excess electrons at the Ti5c sites around the monomers explains the critical role of the perimeter interface of the deposited nanoparticles in promoting the adsorption and activation of reactants observed in experiments.