Catalysis thin oxide film supports

In conclusion, the doping of oxide materials opens promising new routes to change the morphology and electronic properties of supported metal particles as used in heterogeneous catalysis. Thin oxide films are ideally suited to elucidate such doping effects, as they can be explored by means of conventional surface science techniques at a fundamental level. The identified mechanisms can be transferred to real catalysts later, as the doping approach is not based on specific thin-film effects. 

In comparison to most other methods in surface science, STM offers two important advantages (1) it provides local information on the atomic scale and (2) it does so in situ [50]. As STM operates best on flat surfaces, applications of the technique in catalysis relate to models for catalysts, with the emphasis on metal single crystals. Several reviews have provided excellent overviews of the possibilities [51-54], and many studies of particles on model supports have been reported, such as graphite-supported Pt [55] and Pd [56] model catalysts. In the latter case, Humbert et al. [56] were able to recognize surface facets with (111) structure on palladium particles of 1.5 nm diameter, on an STM image taken in air. The use of ultra-thin oxide films, such as AI2O3 on a NiAl alloy, has enabled STM studies of oxide-supported metal particles to be performed, as reviewed by Freund [57]. 

Catalysis by Noble Metal Nanoparticles Supported on Thin-Oxide Films... 

With the advent of synthetic methods to produce more advanced model systems (cluster- or nanoparticle-based systems either in the gas phase or on planar surfaces), we come to the modern age of surface chemistry and heterogeneous catalysis. Castleman and coworkers demonstrate the large influence that charge, size, and composition of metal oxide clusters generated in the gas phase can have on the mechanism of a catalytic reaction. Rupprechter (Chap. 15) reports on the stmctural and catalytic properties of planar noble metal nanocrystals on thin oxide support films in vacuum and under high-pressure conditions. The theme of model systems of nanoparticles supported on planar metal oxide substrates is continued with a chapter on the formation of planar catalyst based on size-selected cluster deposition methods. In a second contribution from Rupprecther (Chap. 17), the complexities of surface chemistry and heterogeneous catalysis on metal oxide films and nanostructures, where the extension of the bulk structure to the surface often does not occur and the surface chemistry is often dominated by surface defects, are discussed. 

Another possible way to separate they catalyst from the fatty products was found by Davis [52-54] and further investigated by Fell [55]. This new method is supported aqueous-phase catalysis (SAPC cf. Section 4.7). On a hydrophilic support, e.g., silicon oxide with a high surface area, a thin aqueous film is applied which contains the water-soluble rhodium catalyst, for instance HRh(CO)L3 with sodium TPPTS ligands. Oleyl alcohol and syngas react at the organic/aqueous interface and form the formylstearyl alcohol in a yield of 97%. The catalyst can be separated from the product by simple filtration without loss of activity. 

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. 

The research field of metal-oxide interfaces is very active, partly because of their important technological applications. For example, in heterogeneous catalysis, oxide powders or porous compounds, such as zeolites, are used as supports for transition metal clusters, because they provide a large - external or internal - specific area of contact with the metal. In many cases, it is also recognized that they modify the cluster reactivity (Dufour and Perdereau, 1988). Oxide surfaces, such as those of MgO or SrTiOs, whose quality and planarity are well controlled, have been used as substrates for the deposition of thin superconductor films. This has been particularly important since the discovery that some copper oxide based compounds remain superconductors above liquid nitrogen temperature. Thin metallic films are also deposited on various oxides in the fabrication of optical devices, or on glass in the fabrication of mirrors. 

Later work by Stevenson [72] supported this hypothesis. The preparation of PET catalysed by antimony trioxide was studied in thin films on metal surfaces that were carefully selected to avoid catalysis by surface effects or by dissolved metal as mentioned earlier, a large number of metals and their oxides, salts or other derivatives catalyse the polyesterification reaction. On inactive surfaces like silver or rhodium the catalysed polycondensation rate increased with decrease in film thickness. In the absence of added catalyst there was no tendency for the rate to increase with decreasing film thickness. Stevenson proposed that in thin films the catalyst-deactivating component was more readily lost, thereby increasing the reaction rate. 

Model catalysts, consisting of metal particles supported on thin-film or single-crystal metal oxide surfaces, have been utilized successfully for more than a decade in an effort to understand particle size and support effects in catalysis [1 ]. However, a... 

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