  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.
