C. Goyhenex

Size effect in the CO chemisorption on palladium clusters supported on magnesium oxide

Abstract In a previous paper [C. Duriez et al., Surf. Sci. 253 (1991) 190], molecular beam measurements were made of the adsorption-desorption kinetics of CO on Pd clusters, epitaxially grown on MgO(100). A large increase (∼8 kcal/mol) in the adsorption energy for clusters smaller than 3 nm grown on clean air-cleaved MgO was observed relative to clusters larger than 5 nm grown on UHV-cleaved MgO. New experimental measuremnts have shown that clusters smaller than 5 nm, grown on UHV-cleaved MgO or mica, give the same enhancement of reactivity as on a clean air-cleaved MgO surface, ruling out a possible substrate effect. The origin of this size effect is explained by the presence of a strong adsorption energy on some sites, the proportion of which increasing when the size of the clusters is decreased. Checks for carbon contamination by CO dissociation or CO disproportionation were negative.

Limitation of Auger electron spectroscopy in the determination of the metal-on-oxide growth mode: Pd on MgO(100)

A discussion on the use of Auger electron spectroscopy as a quantitative tool to determine the growth mode of metals on single crystal oxide surfaces is presented. In the case of Pd grown epitaxially on MgO(100), the three-dimensional character of the growth is easily seen at coverage above one monolayer. However, in the submonolayer regime, and mainly at low substrate temperatures, the AES results are ambiguous. The combination of AES with the more sensitive helium-atom diffraction method allows us to demonstrate that the growth is three-dimensional from the early stages, the particles becoming flatter when the substrate temperature decreases. We compare our results with other growth studies on different metal/oxide systems. At low temperature, the ideal growth modes are not always observed, the final morphology of the films being determined mainly by kinetic effects. Thus a pseudo-Stranski-Krastanov growth mode is often obtained with formation of 2D islands followed by 3D clustering from a critical submonolayer coverage.

FTIR studies of the adsorption of CO on supported metallic clusters PdMgO(100)

Recently, we have evidenced a strong size effect in the adsorption energy of CO on small palladium clusters supported on single crystalline magnesium oxide surfaces. One possible interpretation is the presence of several types of adsorption sites on the metallic clusters. In order to investigate more precisely the adsorption of CO, a new experiment has been developed. The main technique is the fast Fourier transform infrared spectroscopy (FTIR) used to measure the stretching vibration frequencies of the adsorbed CO molecules. New results demonstrating the availability of FTIR to determine adsorption sites of CO on Pd/MgO(100) model catalysts are presented. Actually, two distinct absorption bands are observed, tentatively assigned one to the facets of the cluster and the other to the edges.

Size Effects in Heterogeneous Catalysis

Often the activity and the selectivity of heterogeneous catalysts depend on the size of the metal particles. The origin of these size effects is not yet understood. A new way to investigate the origin of the size effects is to use supported model catalysts. They are prepared by vapour growth, under UHV, on oxide single crystals. By epitaxy at high temperature, homogeneous collections of clusters are obtained. The various techniques available to characterize the nucleation and growth, the structure and morphology and the electron properties of the metal clusters are described on practical examples. Two examples of size effects, for Pd clusters supported on oxide single crystals, are described: the CO chemisorption and the CO oxidation. In these two particular cases, the size effects are interpreted by a stronger binding energy of CO at low coordinated Pd atoms (i.e. edge atoms). More generally chemical and electronic properties of small metal clusters depend not only on their size but also on their morphology and surface structure.

Magnetism of CoPd self-organized alloy clusters on Au(111)

Magnetic properties of gold-encapsulated CoxPd1−x self-organized nano-clusters on Au(111) are analyzed by x-ray magnetic circular dichroism for x = 0.5, 0.7, and 1.0. The clusters are superparamagnetic with a blocking temperature decreasing with increasing Pd concentration, due to a reduction of the out-of-plane anisotropy strength. No magnetic moment is detected on Pd in these clusters, within the detection limit, contrary to thick CoPd films. Both reduction of anisotropy and vanishing Pd moment are attributed to strain.

Advances in the Atomic-Scale Modeling of Nanosystems and Nanostructured Materials

Collective Electron Dynamics in Metallic and Semiconductor Nanostructures.- Weak Chemical Interaction and van der Waals Forces: A Combined Density Functional and Intermolecular Perturbation Theory #x2013 Application to Graphite and Graphitic Systems.- Reactive Simulations for Biochemical Processes.- Molecular Dynamics Simulations of Liquid-Crystalline Dendritic Architectures.- Surface Diffusion on Inhomogeneous Surfaces.- Electronic, Magnetic and Spectroscopic Properties of Vanadium, Chromium and Manganese Nanostructures.- Electronic Structure and Magnetism of Double Perovskite Systems.- Effect of Spin-Orbit Coupling on the Magnetic Properties of Materials: Theory.- Effect of Spin-Orbit Coupling on the Magnetic Properties of Materials: Results.- Nanostructural Units in Disordered Network-Forming Materials and the Origin of Intermediate Range Order.

Rules for tight-binding calculations in bi-metallic compounds based on density functional theory: the case of Co?Au

Even though recent developments in electronic structure calculations based on density functional theory (DFT) allow us to use them for more and more realistic systems, they still remain unsuited for comprehensive studies of complex transition metal compounds involving intricate structural and chemical effects. In that case, the tight-binding approximation (TBA) is a good compromise to get reliable results with only a minimal set of parameters, provided that clear rules enable a proper self-consistent treatment of charge transfers between inequivalent sites. Thus, in the case of the Co-Au system, DFT calculations demonstrate that a local neutrality rule is obeyed per orbital and per chemical species. Shifting the atomic levels accordingly in TBA calculations is then sufficient to accurately determine the local densities of states whatever the chemical configuration. In addition, this also allows us to justify the derivation from TBA of pairwise ordering pair interactions and to determine them self-consistently.

IrPd nanoalloys: simulations, from surface segregation to local electronic properties

Using semi-empirical modeling, namely tight-binding at different levels of accuracy, the chemical, crystallographic, and electronic structures of bimetallic IrPd nanoparticles are characterized. For the purpose, model cuboctahedral particles containing 561 atoms are considered. Atomistic simulations show that core–shell nanoparticles are highly stable, with a strong surface segregation of Pd, at least for one atomic shell thickness. Within self-consistent tight-binding calculations founded on the density functional theory, an accurate insight is given into the electronic structure of these materials which have a high potential as catalysts.

Atomic Migration Phenomena in Intermetallics with High Superstructure Stability

Most of the contemporary materials based on intermetallic phases are either multiple bulk phases, or nanostructured layers deposited on appropriate substrates. In each case, the desired properties of the materials are due to chemical order and the preparation technology consists of a generation of specific processes mediated by atomic migration. It is shown how a nanoscopic (atomistic) image of the atomic migration phenomena results from an indirect experimental technique in combination with Monte Carlo (MC) and Molecular Dynamics (MD) simulations. “Order-order” relaxations were observed in phases representing three typical cubic superstructures of high stability: L12 (Ni3Al), B2 (NiAl), and L10 (FePt).

Electronic structure of CoPt based systems: from bulk to nanoalloys.

An accurate description of the local electronic structure is necessary for guiding the design of materials with targeted properties in a controlled way. For complex materials like nanoalloys, self-consistent tight-binding calculations should be a good alternative to ab initio methods, for handling the most complex and large systems (hundreds to thousands of atoms), provided that these parameterized method is well founded from ab initio ones that they intend to replace. Ab initio calculations (density functional theory) enabled us to derive rules for charge distribution as a function of structural change and alloying effects in Co and Pt based systems, from bulk to nanoalloys. A general local neutrality rule per site, orbital and species was found. Based on it, self-consistent tight-binding calculations could be implemented and applied to CoPt nanoalloys. A very good agreement is obtained between tight-binding and DFT calculations in terms of local electronic structure.