Heterogeneous Catalysis 4 Nanoparticle-Based Catalysts

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. 

Because carbon black is the preferred support material for electrocatalysts, the methods of preparation of (bi)metallic nanoparticles are somewhat more restricted than with the oxide supports widely used in gas-phase heterogeneous catalysis. A further requirement imposed by the reduced mass-transport rates of the reactant molecules in the liquid phase versus the gas phase is that the metal loadings on the carbon support must be very high, e.g., at least lOwt.% versus 0.1-1 wt.% typically used in gas-phase catalysts. The relatively inert character of the carbon black surface plus the high metal loading means that widely practiced methods such as ion exchange [9] are not effective. The preferred methods are based on preparation of colloidal precursors, which are adsorbed onto the carbon black surface and then thermally decomposed or hydrogen-reduced to the (bi)metallic state. This method was pioneered by Petrow and Allen [10], and in the period from about 1970-1995 various colloidal methods are described essentially only in the patent literature. A useful survey of methods described in this literature can be found in the review by Stonehart [11]. Since about 1995, there has been more disclosure of colloidal methods in research journals, such as the papers by Boennemann and co-workers [12]. 

This section describes in detail three topics in heterogeneous catalysis to which DFT calculations have recently been applied with great effect, the prediction of CO oxidation rates over RuO2(110), the prediction of ammonia synthesis rates by supported nanoparticles of Ru, and the DFT-based design of new selective catalysts for ethylene epoxidation. All three examples involve the careful application of DFT calculations and other appropriate theoretical methods to make quantitative predictions about the performance of heterogeneous catalysts under realistic operating conditions. 

Molybdenum sulfide-based catalysts are widely used in heterogeneous catalysis, especially in hydrotreating of petroleum cuts. The control of the morphology and size of MoSx nanoparticles is thus of great interest to obtain highly active and selective catalysts [1], Various methods have been used to synthesize nanoparticles with controlled morphology and structure, and the use of microemulsions seems especially suited for tailoring particle size at the nanolevel [2]. 

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