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The adsorption properties of Ni, Pd, Pt, Cu, Ag, and Au atoms on MgO ultrathin films deposited on Mo(100) or Ag(100) substrates are determined from first principle DFT calculations and compared with the corresponding adsorption characteristics on the bare MgO(100) surface. The three supports exhibit different behaviors. On MgO/Mo(100) thin films there is an electron transfer from the metal/oxide interface to all considered adatoms, In some cases, like for Pd, the charge transfer is small and the species remains atomic-like, while for other atoms like Cu, Ag, Au or Pt there is a net transfer of one electron with formation of full anions. This has dramatic consequences on the preferred adsorption sites, on the bond strength, and on the magnetic state of the adatoms: all these quantities are totally different from what is found for the same atoms on the bare MgO(100) surface. MgO/Ag(100) films have an intermediate behavior between that of MgO/Mo(100) thin films and of the MgO(100) surface: the charge transfer is found only for Au and, to a smaller extent, for Pt atoms. All other atoms considered keep essentially the same electronic structure and preferred adsorption site shown on MgO(100). The reasons for the different behavior are rationalized in terms of global properties of the metal/oxide interface, and in particular of its work function.

The adsorption properties of CO molecules adsorbed on free and MgO supported Au clusters have been investigated by means of a first principles density functional theory (DFT) approach. We have focused our attention on the vibrational properties of CO adsorbed on gas-phase Au3-7, Au 12, and Au22 cluster anions, and on Au3-5, Au12, and Au22 clusters deposited on MgO/Ag(001) films or on the bare MgO(001) surface. We also considered a full gold monolayer deposited on MgO/Ag(001). CO does not bind to the high-coordinated Au atoms of one- or two-dimensional (1D or 2D) gold clusters or islands but only to the low-coordinated atoms at the periphery of these structures. A red-shift of about 50-60 cm-1 is found in ω (CO) for gold clusters deposited on MgO/Ag(001) thin films compared to the same clusters deposited on bare MgO(001) or to the neutral gas-phase counterparts. This shift, due to the occurrence of a charge transfer from the metal support to the deposited gold cluster, is found only for small 1D or 2D clusters or, for 3D clusters, when the CO molecule is bound at the MgO/Au interface. On 3D gold clusters, CO adsorbs on the top layers with binding energies and vibrational frequencies typical of neutral supported gold particles. This shows that the charging effect observed on ultrathin MgO films is largely restricted to the gold layer at the interface with the oxide support. © 2009 American Chemical Society.

On the basis of experimental evidence and DFT calculations, we propose a simple yet viable way to stabilize and chemically activate gold nanoclusters on MgO. First the MgO surface is functionalized by creation of trapped electrons, (H+)(e-) centers (exposure to atomic H or to H2 under UV light, deposition of low amounts of alkali metals on partially hydroxylated surfaces, etc.); the second step consists in the self-aggregation of gold clusters deposited from the gas phase. The calculations show that the (H+)(e-) centers act both as nucleation and activation sites. The process can lead to thermally stable gold cluster anions whose catalytic activity is enhanced by the presence of an excess electron. © 2008 American Chemical Society.

The formation of gold atoms and clusters from the reduction by hydrogen of mononuclear [Au(CH3)(2)(acac)] organometallic complexes deposited on the MgO surface has been studied by first principle density functional theory calculations. The comparison of measured and computed infrared data suggests the occurrence of a dissociative adsorption of the complex on the surface of the oxide, although a given fraction of undissociated molecules can cohexist. The reduction can follow various mechanisms, leading to the formation of neutral or positively charged mononuclear gold intermediates. We show that some of these intermediates can easily diffuse on the surface at room temperature and lead to nucleation of stable partially oxidized gold clusters

In the 80’s Masatake Haruta pointed out the striking catalytic activity of supported gold clusters toward carbon monoxide oxidation at the temperature of 200 K (Haruta’s catalysts). Several experimental and theoretical efforts have been dedicated to clarify the properties that actually enhance the chemical activity of gold. In particular, the charge state of the particles is still now at the center of a debate all over the scientific community. Several studies have shown that negatively charged clusters are chemically more active than neutral or positively charged clusters: the presence of an extra negative charge results in the activation of adsorbed molecules and thus in the enhanced catalytic activity. Charge transfer phenomena are due to the interaction between clusters and the support. A simple yet viable way to stabilize and chemically activate metals nanoclusters on MgO supports consists in functionalizing the MgO surface with creation of trapped electrons by exposure to atomic hydrogen, molecular hydrogen and UV light or deposition of small amounts of alkali metals on partially hydroxylated surfaces. The calculations show that electron traps act as nucleation and charging sites for clusters. A catalytic cycle for CO oxidation has been proposed in order to verify the catalytic activity of gold clusters. Charging of supported metal atoms can also be obtained with the use of ultra-thin oxide films grown on a metal substrate. It has been found that electrons can actually flow by direct tunneling from the metal substrate through the oxide thin film to adsorbed molecules and clusters. CO is largely used as a probe molecule: according to the Blyholder model, the vibrational frequency of the C-O stretching mode is very sensitive to changes in the electron density of the atoms where the molecule is bound. In this scenario, CO adsorption can represent a powerful tool to distinguish between neutral and anionic clusters formed on MgO/Ag.

We report density functional theory calculations on the interaction of Pd atoms and small I'd clusters with an electron-rich MgO surface. This surface can be generated by forming a specific kind of defects, named (H+)(e(-)) centers, using well known chemical recipes. By deposition of gas-phase I'd atoms on the properly functionalized MgO surface, one can generate collections of small Pd cluster anions with peculiar chemical properties. The (H+)(e(-)) centers act as nucleation sites for diffusing Pd atoms and favor the formation of small, thermally stable Pd. clusters. The presence of an extra charge on the metal cluster results in a large vibrational red-shift of adsorbed CO molecules. The present results intend to stimulate experimental work to produce stable metal cluster anions on the surface of an ionic oxide. (C) 2008 Elsevier B.V. All rights reserved.

Charging of metal atoms or clusters on oxide surfaces has important consequences on their chemical and physical properties. Recently it is has been shown that negatively charged gold atoms and clusters form spontaneously from neutral Au atoms deposited on ultrathin MgO films. The formation of anions on the surface remains difficult to prove experimentally. Also theoretically, the discrimination between neutral and charged adsorbed species is not straightforward. In this paper we perform an accurate analysis of the observable consequences of the formation of Au anions on an oxide surface. To this end we consider the following properties: spin distribution, density of states, Bader charges, substrate relaxation, simulated scanning tunneling microscopy images, work function changes, CO vibrational frequency, electric field effects, and core level shifts. Most of these properties are accessible experimentally, at least in principle. Taken individually, these properties do not necessarily provide conclusive evidence about the charged nature of the adsorbate. Taken together, they offer a complete and unambiguous characterization of the formation of Au anions. (C) 2007 American Institute of Physics.

The electronic structure and spectral properties of self-trapped holes (STH's) in $\mathrm{Si}{\mathrm{O}}_{2}$ are revisited using pure and hybrid density functional theory (DFT) approaches and cluster models. The structure and electron spin resonance (ESR) parameters [hyperfine coupling constants (hfcc's) and $g$ factors] have been determined for the two classical variants of STH's: ${\mathrm{STH}}_{1}$, consisting of a hole trapped at the $2p$ nonbonding orbital of an O atom bridging two Si atoms, and ${\mathrm{STH}}_{2}$, a metastable defect where the hole is delocalized over the $2p$ orbitals of two bridging O atoms. The computed ESR parameters fully support the experimental assignments based on the analysis of the ESR spectra. However, a nearly quantitative agreement between measured and computed hfcc's and $g$ factors is found only with a hybrid functional where 50% of the Hartree-Fock exchange is mixed in with the DFT exchange. Standard hybrid functionals, such as B3LYP, or pure DFT functionals, such as BLYP, fail completely in describing the two defects. According to these results, ${\mathrm{STH}}_{2}$ can form only in correspondence of specific O-Si-O angles of about 80\ifmmode^\circ\else\textdegree\fi{}--90\ifmmode^\circ\else\textdegree\fi{}, which classifies the defect center as a molecular polaron.

We have performed density functional theory calculations on the properties of small PtAun (n = 1-4) bimetallic clusters adsorbed on ultrathin MgO films deposited over the Ag(100) substrate. The PtAu bimetallic clusters show the occurrence of a charge transfer from the MgO/Ag interface to the supported nanoparticle with the formation of cluster anions. The Pt atom, which exhibits a stronger bonding with the oxide film and a higher diffusion barrier compared to Au, acts as a nucleation site for the bimetallic cluster and limits its diffusion on the surface. © the Owner Societies.