Chemical reactivity metal clusters, oxide surface

As we mentioned, oxide surfaces are important in the field of nanocatalysis by supported metals. In practical applications, the support has the crucial role of stabilizing small metallic particles, which act as the actual catalysts in a chemical process. Once the oxide surface is sufficiently well characterized, one can deposit small metal clusters and study their reactivity as a function of the support, of the metal, of the size of the cluster, etc. In this way, complex catalytic processes can be divided into a series of substeps, which allow a more detailed microscopic characterization. Despite the fact that only recently well-defined metal clusters have been deposited under controlled conditions on oxide surfaces and thin films, great advances have been obtained in the understanding of the mechanisms of adhesion and growth of the metal particles to the oxide surface. In this process, the role of theory is quite substantial. 

We have seen that a great variety of defect centers can form at the surface of an oxide like MgO (Table 1). Each surface defect has a direct and characteristic effect on the properties of absorbed species. This becomes particularly important in the analysis of the chemical reactivity of supported metal atoms and clusters [1-3]. The defects not only act as nucleation centers in the growth of metal islands or clusters [4,209], but also can modify the catalytic activity of the deposited metal by affecting the... 

The latter point brings us to an important question in the field of catalysis by supported metal particles to which extent is the chemical reactivity of a (sub-) nanocluster affected by the interaction with the substrate Very few theoretical studies were dedicated to this problem, and most of them are related to the surface of MgO, an oxide which interacts weakly widi the supported particle, as shown above. Still, the knowledge accumulated in the course of the years on the structure of surface defects and morphology of the MgO surface allows one to analyze some of the mechanisms which can modify the chemical properties of a supported cluster as a function of the site where nucleation has occurred. 

The inclusion of impurity atoms in MgO is much more interesting from a chemical point of view when alkali metals are used to replace Mg ions. In fact, this results in trapped-hole centers. The MVO pairs have been extensively studied in the bulk of alkaline-earth oxides by optical studies, EPR and ENDOR measurements [185,186] as well as by embedded cluster calculations [187]. The LiVO ions create an effective dipole which polarizes the surrounding lattice, with the two ions moving toward each other. The presence of an O radical, however, is most interesting when one is dealing with surface properties. This center in fact is very reactive and is the subject of the next paragraph. 

A great variety of oxides can be used as supports. These materials are chemically stable, but in several cases, interactions between the metallic particles and the support may occur. This interaction is very likely due to the creation of a chemical bond between the particle and the support. A model of such chemical bonding can be found in molecular cluster chemistry when Ru3(CO)i2 reacts with a silica surface, the sUanol makes a so-called oxidative addition to the Ru-Ru bond of the cluster and there is formation of an (Tj -siloxy) (Ru-Ru bond) in which the surface oxygen behaves as a 3-electron ligand in the M. L. H. Green formalism. There is no obvious reason why such reactivity would not occur when a particle of a zerovalent metal is adsorbed (chemisorbed) on a partially hydroxylated surface. 

The complexity of the matter is huge since the oxidation of nAl particles depends on thermodynamical, physical, and chemical features of the reactants involved. In addition to metal-oxidizer combustion, the characteristic size of nAl powders (typically 100 nm or less) deserves further consideration. In the nanometric range, the particle can be composed by few thousands or even few hundreds of atoms. The cohesive energy of atomic clusters is expected to be inversely dependent by the particle radius while surface energy increases and may become non-negligible. Several particle properties such as melting temperature, reactivity, and surface tension may differ from the bulk features [4, 10, 11]. Regarding calorimetric... 

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