Charge state of metal atoms on oxide supports: a systematic study based on simulated infrared spectroscopy and density functional theory

A long standing question in the study of supported clusters of metal atoms in the properties of metal–oxide interfaces is the extent of metal–oxide charge transfer. However, the determination of this charge transfer is far from straight forward and a combination of different methods (both experimental and theoretical) is required. In this paper, we systematically study the charging of some adsorbed transition metal atoms on two widely used metal oxides surfaces [α-Al₂O₃ (0001) and rutile TiO₂ (110)]. Two procedures are combined to this end: the computed vibrational shift of the CO molecule, that is used as a probe, and the calculation of the atoms charges from a Bader analysis of the electron density of the systems under study. At difference from previous studies that directly compared the vibrational vawenumber of adsorbed CO with that of the gas phase molecule, we have validated the procedure by comparison of the computed CO stretching wavenumbers in isolated monocarbonyls (MCO) and their singly charged ions with experimental data for these species in rare gas matrices. It is found that the computational results correctly reproduce the experimental trend for the observed shift on the CO stretching mode but that care must be taken for negatively charged complexes as in this case there is a significative difference between the total charge of the MCO complex and the charge of the M atom. For the supported adatoms, our results show that while Cu and Ag atoms show a partial charge transfer to the Al₂O₃ surface, this is not the case for Au adatoms, that are basically neutral on the most stable adsorption site. Pd and Pt adatoms also show a significative amount of charge transfer to this surface. On the TiO₂ surface our results allow an interpretation of previous contradictory data by showing that the adsorption of the probe molecule may repolarize the Au adatoms, that are basically neutral when isolated, and show the presence of highly charged Au^δ+–CO complexes. The other two coinage metal atoms are found to significatively reduce the TiO₂ surface. The combined use of the shift on the vibrational frequency of the CO molecule and the computation of the Bader charges shows to be an useful tool for the study the charge state of adsorbed transition metal atoms and allow to rationalize the information coming from complementary tools.