REGULAR ARTICLE Charge state of metal atoms on oxide supports: a systematic study based on simulated infrared spectroscopy and density functional theory Antonio M. Ma ´rquez • Jesu ´s Graciani • Javier Fdez Sanz Received: 27 July 2009 / Accepted: 18 November 2009 / Published online: 11 December 2009 Ó Springer-Verlag 2009 Abstract A long standing question in the study of sup- ported 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 [a-Al 2 O 3 (0001) and rutile TiO 2 (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 spe- cies 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 Al 2 O 3 surface, this is not the case for Au adatoms, that are basi- cally neutral on the most stable adsorption site. Pd and Pt adatoms also show a significative amount of charge transfer to this surface. On the TiO 2 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 d? – CO complexes. The other two coinage metal atoms are found to significatively reduce the TiO 2 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. Keywords Copper Á Silver Á Gold Á Palladium Á Platinum Á Aluminum oxide Á Titanium oxide Á Adsorption Á Density functional calculations Á Charge state 1 Introduction Deposition of metal atoms and clusters on the surface of oxide materials is a topic that has been extensively studied for a long time. The several technological applications of metal–oxide interfaces, ranging from microelectronics to catalysis, make the field one of the most active in surface science and technology research [1–16]. In recent years, very accurate ultrahigh vacuum (UHV) experimental techniques have been developed to gain insight into the properties and structure of small metal aggregates on oxide surfaces. Samples may be characterized by, e.g. scanning Dedicated to the memory of Professor Jean-Pierre Daudey and published as part of the Daudey Memorial Issue. A. M. Ma ´rquez (&) Á J. Graciani Á J. F. Sanz Departamento de Quı ´mica Fı ´sica, Facultad de Quı ´mica, Universidad de Sevilla, CL/Prof. Garcı ´a Gonza ´lez, 1, 41012 Sevilla, Spain e-mail: marquez@us.es 123 Theor Chem Acc (2010) 126:265–273 DOI 10.1007/s00214-009-0703-0