Metallic clusters microscopic catalysis

The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

The two versions of the Miilheim electrochemical process provide colloidal solutions (e.g., in THF) of a variety of transition metal or bimetallic nanoparticles, and constitute a simple, clean and reliable alternative to chemical processes such as reduction by borohydrides in which the excess reducing agent and/or the oxidized form thereof have to be removed from the product (in fact, boron originating from boron hydrides is sometimes incorporated in the nanopaiticles) [26], But are these methods of any use in catalysis. One possibiUty is immobUization on soUd carriers, deUvering materials having islands of metal clusters of a predefined size. Moreover, they allow for the design of heterogeneous catalysts with well-defined compositional and structural features on a macroscopic and microscopic level. 

M. Baumer, J. Libuda, and H. Freund, Metal deposits on thin well ordered oxide films Morphology, adsorption and reactivity. In Lambert, R. M., Pacchioni, G. Chemisorption and Reactivity on Supported Clusters and Thin Films Towards an Understanding of Microscopic Processes in Catalysis. Boston Kluwer Academic Publishers, NATO ASI Series E Applied Sciences-Advanced Study Institute, vol. 331, pp. 61-104, 1997. 

---