Gianfranco Pacchioni

Oxide ultra-thin films on metals: new materials for the design of supported metal catalysts

Ultrathin oxide films on metals offer new opportunities for the design of supported nanoclusters with potential use in catalysis. This requires a characterization at the atomistic level of the structure and composition of the thin film, of its morphology and defect structure. A proper selection of metal/oxide interface, film thickness, lattice mismatch, etc. makes it possible to prepare collections of supported metal particles with novel properties. This critical review describes some illustrative examples, emphasizes the role of the interplay between theory and experiment, and relates some recent findings related to the possibility to control the charge state of a supported nanoparticle on an ultrathin oxide film (211 references).

Cluster Models for Surface and Bulk Phenomena

It is widely recognized that an understanding of the physical and chemical properties of clusters will give a great deal of important information relevant to surface and bulk properties of condensed matter. This relevance of clusters for condensed matter is one of the major motivations for the study of atomic and molecular clusters. The changes of properties with cluster size, from small clusters containing only a few atoms to large clusters containing tens of thousands of atoms, provides a unique way to understand and to control the development of bulk properties as separated units are brought together to form an extended system. Another important use of clusters is as theoretical models of surfaces and bulk materials. The electronic wavefunctions for these cluster models have special advantages for understanding, in particular, the local properties of condensed matter. The cluster wavefunctions, obtained with molecular orbital theory, make it possible to relate chemical concepts developed to describe chemical bonds in molecules to the very closely related chemical bonding at the surface and in the bulk of condensed matter. The applications of clusters to phenomena in condensed matter is a cross-disciplinary activity which requires the interaction and collaboration of researchers in traditionally separate areas. For example, it is necessary to bring together workers whose background and expertise is molecular chemistry with those whose background is solid state physics. It is also necessary to bring together experimentalists and theoreticians.

Chemisorption and Reactivity on Supported Clusters and Thin Films

Preface. Introduction to Heterogeneous Catalysis R.M. Lambert. Thin Films as Model Catalysts D.R. Rainer, D.W. Goodman. Metal Deposits on Thin Well Ordered Oxide Films: Morphology, Adsorption and Reactivity M. Baumer, et al. The Growth and Stability of Ultrathin Films on Metal and Oxide Surfaces T.E. Madey. Size Effects in Heterogeneous Catalysis: A Surface Science Approach C. Henry, et al. Supported Clusters, Structure, Reactivity and Microscopic Processes in Catalysis M. Ichikawa. Quantitative Determination of Molecular Adsorbate Structures D.P. Woodruff. The Structure and Reactivity of TiO2(110) Supported Palladium and Rhodium B.E. Hayden. Angle-Scanned Photoelectron Diffraction: A Structural Probe for Near-Surface: Atomic Layers G. Granozzi, M. Sambi. Co-Adsorption on Metal-Oxide Crystal Surfaces: Cases of CO/Cu/ZnO(0001) and CO2/Na/TiO2(110) P.J. Moller. Theory of Adsorption and Surface Reactions B. Hammer, J.K. Norskov. Density Functional Cluster Calculations on Metal Deposition at Oxide Surfaces N. Rosch, G. Pacchioni. Theory of Heterogeneous Catalytic Reactivity Using the Cluster Approximation R.A. van Santen. Cluster Modelling of Oxide Surfaces: Structure, Adsorption and Reactivity G. Pacchioni. Theoretical Modelling of Chemisorption and Reactions on Metal-Oxide Surfaces L.G.M. Pettersson, et al. Stability of Polar Oxide Surfaces: Oxygen Vacancies and Non-Stoichiometric Reconstructions C. Noguera, et al. Computer Simulation of Structural, Defect and Surface Properties of Solids C.R.A. Catlow, et al. Index.

Mechanisms responsible for chemical shifts of core-level binding energies and their relationship to chemical bonding

Abstract A comprehensive review of different mechanisms which contribute to the chemical shifts of core-level binding energies, BEs, is made. A principle focus is on showing how the mechanisms can be used to relate the BE shifts to features of the chemical bonding and chemical interactions in the studied system. Several initial state mechanisms are identified; while some are well known, the importance of others has been only recognized fairly recently. A theoretical framework is presented which places the analysis and interpretation of these BE shifts on a firm foundation. A rigorous definition and distinction of initial and final state effects is presented. This definition is applied to show that initial state effects are often the dominant factors for the chemical BE shifts. It is also shown that, in many cases, theoretical approaches involving the use of constrained variations can permit a clear and definitive separation of the contributions of the different mechanisms. Several representative applications to the analysis and interpretation of core-level BE shifts are described which show how the theoretical methods of analysis can be used to identify the mechanisms important for the BE shifts. Often more than one mechanism makes an important contribution to the shifts and it is common that the contributions will be canceling. When all of the relevant mechanisms are taken into account in the analysis of the BE shifts, these shifts do provide valuable information about the chemical bonding and electronic structure of the materials being studied. The mechanisms presented and the theoretical frameworks described provide a unified view of BE chemical shifts.

Modeling doped and defective oxides in catalysis with density functional theory methods: Room for improvements

Due to the well-known problem of the self-interaction, standard density functional theory (DFT) methods tend to produce delocalized holes and electrons in defective oxide materials even when there is ample experimental evidence of a strong localization. For late transition metal compounds or rare earth oxides, this results in the incorrect description of the electronic structure of the system (e.g., magnetic insulators are predicted to be metallic). Practical ways to correct this deficiency are based on the use of hybrid functionals or of the DFT+U approach. In this way, most of the limitations related to the self-interaction are removed, and the electronic structure is properly described. What is less clear is to what extent hybrid functionals, DFT+U approaches, or standard DFT functionals can properly describe the strength of the chemical bonds at the surface of an oxide. This is a crucial question if one is interested in the catalytic properties of oxide surfaces. Oxidation reactions often involve oxygen detachment from the surface and incorporation into an organic substrate. Oxides are doped with heteroatoms to create defects and facilitate oxygen removal from the surface, with formation of oxygen vacancies. Do standard DFT calculations provide a good binding energy of the missing oxygen despite the failure in giving the right electronic structure? Can hybrid functionals or the DFT+U approach provide a simple yet reliable way to get accurate reaction enthalpies and energy barriers? In this essay, we discuss these problems by analyzing some case histories and the relatively scarce data existing in the literature. The conclusion is that while modern electronic structure methods accurately reproduce and predict a wide range of electronic, optical, and magnetic properties of oxides, the description of the strength of chemical bonds still needs considerable improvements.

Silicon and germanium clusters. A theoretical study of their electronic structures and properties

Silicon and germanium clusters containing three to seven atoms have been studied with the pseudopotential MO‐LCAO method followed by configuration interaction procedure. Si and Ge clusters have very similar electronic structures and consequently analogous physico‐chemical properties but differ substantially from small carbon clusters. Linear structures are clearly less favorable than more compact structures. On the other hand, some planar geometries possess considerable stability. The Si and Ge clusters which are sections of the diamond‐type crystal lattice are less stable than clusters which can be considered as segments of closed‐packed lattices or as steps in pentagonal crystal growth. The reason is that the majority of atoms in small clusters are surface atoms which cannot assume the tetrahedral coordination characteristic of Si and Ge bulk atoms. The appearance of typical bulk properties is expected only for very large Si and Ge clusters with small surface atoms/bulk atoms ratio.

Characterization of oxide surfaces by infrared spectroscopy of adsorbed carbon monoxide: a theoretical investigation of the frequency shift of CO on MgO and NiO

Abstract We report the results of ab initio cluster model calculations of the interaction of the CO molecule with a finite cluster representing Mg 2+ and Ni 2+ chemisorption sites on MgO- and NiO(100) surfaces. We compare the CO adsorption at these two oxide surfaces which have the same NaCl-like crystal structure and very similar bulk moduli but which have different d-orbital occupations, Mg 2+ d 0 and Ni 2+ d 8 . We found that NiO behaves very similarly as the non-transition metal oxide MgO. In particular, the bonding with CO is almost entirely electrostatic in nature; it does not involve any significant σ donation or, in the NiO case, any π back-donation. Moreover, we found that the CO vibrational shift toward higher frequencies arises essentially from the repulsion originating when the CO molecule stretches in the presence of the rigid surface (wall effect). Contributions to the frequency shift from chemical bonding mechanisms are negligible.

Supported nickel and copper clusters on MgO(100): A first‐principles calculation on the metal/oxide interface

The interaction of Ni and Cu atoms as well as Ni4 and Cu4 clusters with cationic and anionic sites of the MgO(100) surface has been studied by means of gradient‐corrected density functional calculations using cluster models. We found that the cationic surface atoms and the fourfold hollow sites are essentially inert while Ni and Cu atoms as well as their clusters are weakly oxidized by the surface oxygens. The adhesion energy is 0.62 eV/atom for Ni4 and 0.36 eV/atom for Cu4. This reflects the stronger bonding of a surface oxygen with a Ni atom, 1.24 eV, compared to a Cu atom, 0.28 eV. The reason for the stronger bonding of Ni is the presence of the uncomplete 3d shell. In fact, the mixing of the 3d orbitals with the O 2p band leads to the formation of a covalent polar bond of moderate strength. Cu binds mainly via the 4s electrons and the interaction is weaker. An important conclusion is that the metal–metal bonds in the cluster are stronger than the metal–substrate bonds. The adsorbed clusters feature so...

Adhesion energy of Cu atoms on the MgO(001) surface

We have studied theoretically the interaction of an isolated Cu atom adsorbed on the oxygen sites of the regular MgO (001) surface with the aim of providing an accurate estimate of the adhesion energy. We performed cluster model calculations using a variety of first principles quantum-chemical approaches; local (spin) density approximation [L(S)DA], density functionals that include density gradient corrections (GC-DF), hybrid density functional (B3LYP), and explicitly correlated wave functions. Various combinations of exchange-correlation functionals and different methods to introduce electron correlation, including MP2 and CCSD(T), have been considered. The dependence of the results on cluster and basis set size has been carefully checked. We found that the hybrid DF method, B3LYP, and explicitly correlated wave functions, CCSD(T), give similar results with an adhesion energy of about 0.40±0.05 eV; GC-DF methods suggest a higher binding energy of 0.6 eV. Therefore, Cu atoms can be considered to bind to o...

Physisorbed and chemisorbed CO2 at surface and step sites of the MgO(100) surface

Abstract The interaction of CO 2 with regular and defect sites of the MgO(100) surface has been investigated by means of ab initio cluster model SCF and correlated calculations. Full geometry optimization of the surface complexes has been carried out at the SCF level. CO 2 adsorbs molecularly at the Mg 2+ surface and step sites, forming an end-on linear complex with the molecular axis normal to the surface; a “horizontal” orientation where the molecule lies flat on the surface and interacts with two Mg 2+ cations, is only slightly less favorable. The interaction of CO 2 with surface oxide anions, O 2− , and the consequent formation of surface carbonates has also been considered. While five-coordinated surface O 2− ions are rather unreactive, carbonates form at low-coordinated defect sites. The carbonate formation at defect sites is a non-activated process. The local electrostatic interactions between the surface ions and the chemisorbed CO 2 play an important role in determining the orientation of the surface carbonate. The analysis of the vibrational frequencies and of the core level binding energies of physisorbed and chemisorbed CO 2 is fully consistent with the experimental observation that two distinct species coexist at the surface of MgO at low temperatures.