Catalysis support-related clusters

Supported metal clusters play an important role in nanoscience and nanotechnology for a variety of reasons [1-6]. Yet, the most immediate applications are related to catalysis. The heterogeneous catalyst, installed in automobiles to reduce the amount of harmful car exhaust, is quite typical it consists of a monolithic backbone covered internally with a porous ceramic material like alumina. Small particles of noble metals such as palladium, platinum, and rhodium are deposited on the surface of the ceramic. Other pertinent examples are transition metal clusters and atomic species in zeolites which may react even with such inert compounds as saturated hydrocarbons activating their catalytic transformations [7-9]. Dehydrogenation of alkanes to the alkenes is an important initial step in the transformation of ethane or propane to aromatics [8-11]. This conversion via nonoxidative routes augments the type of feedstocks available for the synthesis of these valuable products. 

Supported nanoparticles are related to the idea of starting with polynuclear cluster precursors. While there is no clear line that divides polynuclear clusters from nanoparticles, clusters are generally small, low nuclearity (MnL n = 3-20), structurally well-characterized species approximately 1-2 nm in size. Nanoparticles are larger (>2 nm) and frequently defined by a size distribution rather than a discrete number of atoms and ligands [41]. In the area of catalysis, gold nanopar-... 

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. 

Related to the Surface Science approach to catalysis is the use of molecular beam techniques . With Mass Spectroscopy and other detection techniques the reactivity of small metal particles effusing into a vacuum is studied. Again reaction conditions are well defined, but the temperature is close to zero Kelvin. This approach enables the study of chemical reactivity 2ls a function of particle size. Knowledge on the reactivity of isolated particles is relevant especially for theoretical studies since these data refer to well-defined particles without interaction with a support. Theoretical studies can be conveniently done with isolated small clusters. As with surface science studies, molecular beam experiments are fundamental to the study of elementary reactions as a function of surface or particle structure. We will frequently refer to data collected by these approaches. 

Recently there has been considerable interest in the reactions of mixed-metal cluster complexes with oxide supports. To a large extent this emanates from the established phenomena within the field of heterogeneous catalysis which show that unusual activities and selectivities can be observed for supported "alloy catalysts [70]. The development of the solution chemistry of heterometallic cluster complexes has reached the stage of providing several series of related complexes which can be used as the basis of systematic studies. Indeed, the complexes Co2Rh2(CO)j 2 Co3Rh(CO)j 2 were reacted with silica and alumina some years ago with the aim of generating new alloy catalysts... 

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