Supported size-dependent properties

Abstract Immobilized metallic and bimetallic complexes and clusters on oxide or zeolite supports made from well-defined molecular organometaUic precursors have drawn wide attention because of their novel size-dependent properties and their potential applications for catalysis. It is speculated that nearly molecular supported catalysts may combine the high activity and selectivity of homogenous catalysts with the ease of separation and robustness of operation of heterogeneous catalysts. This chapter is a review of the synthesis and physical characterization of metaUic and bimetallic complexes and clusters supported on metal oxides and zeohtes prepared from organometaUic precursors of well-defined molecularity and stoichiometry. 

Free clusters are ideal model systems to probe the influence of their intrinsic, size-dependent properties on the catalytic activity due to the lack of any support interactions. Free clusters are prepared from cluster sources [20] and only very low densities are obtained. They are highly unstable under normal conditions and, even under UHV conditions, exothermal catalytic reactions may lead to fragmentation without the presence of a buffer gas. Thus, free clusters may not become relevant for industrial applications. Nevertheless, they are important vehicles to gain a fundamental understanding of nanocatalysis. 

In Section 2 the general features of the electronic structure of supported metal nanoparticles are reviewed from both experimental and theoretical point of view. Section 3 gives an introduction to sample preparation. In Section 4 the size-dependent electronic properties of silver nanoparticles are presented as an illustrative example, while in Section 5 correlation is sought between the electronic structure and the catalytic properties of gold nanoparticles, with special emphasis on substrate-related issues. 

A.K. Santra and D.W. Goodman, Size-dependent electronic, structural, and catalytic properties of metal clusters supported on ultrathin oxide films, in Catalysis and Electrocatalysis at Nanoparticle Surfaces, eds. A. Wieckowski, E.R. Savinova, and C.G. Vayenas. Marcel Dekker, New York, 2003, pp. 281-309. 

H. Topsbe, N. Topsbe, H. Bohlbro, and J. A. Dumesic, Supported iron catalysts Particle size dependence of catalytic and chemisorptive properties, Proc. 7th Int. Congress Catalysis, edited by T. Seyama, K. Tanabe (Kondansha, Tokyo), p. 247 (1981). 

HREM methods are powerful in the study of nanometre-sized metal particles dispersed on ceramic oxides or any other suitable substrate. In many catalytic processes employing supported metallic catalysts, it has been established that the catalytic properties of some structure-sensitive catalysts are enhanced with a decrease in particle size. For example, the rate of CO decomposition on Pd/mica is shown to increase five-fold when the Pd particle sizes are reduced from 5 to 2 nm. A similar size dependence has been observed for Ni/mica. It is, therefore, necessary to observe the particles at very high resolution, coupled with a small-probe high-precision micro- or nanocomposition analysis and micro- or nanodiffraction where possible. Advanced FE-(S)TEM instruments are particularly effective for composition analysis and diffraction on the nanoscale. ED patterns from particles of diameter of 1 nm or less are now possible. 

To understand heterogeneous catalysis it is necessary to characterize the surface of the catalyst, where reactants bond and chemical transformations subsequently take place. The activity of a solid catalyst scales directly with the number of exposed active sites on the surface, and the activity is optimized by dispersing the active material as nanometer-sized particles onto highly porous supports with surface areas often in excess of 500m /g. When the dimensions of the catalytic material become sufficiently small, the properties become size-dependent, and it is often insufficient to model a catalytically active material from its macroscopic properties. The structural complexity of the materials, combined with the high temperatures and pressures of catalysis, may limit the possibilities for detailed structural characterization of real catalysts. 

This combined theoretical-experimental approach shows the potentiality of generating defective surfaces to modify the chemical activity of supported nanoaggregates for catalytic applications. It also shows the importance to obtain an atomistic control on the species deposited on the smface for the proper characterization of their activity. Through a specifically designed experimental apparatus which makes it possible to deposit size-selected metal clusters it has become possible to study the properties of a supported cluster as a function of its size. This opens new, unprecedented perspectives for the understanding of size-dependent activity and selectivity of a supported metal catalyst. To reach this ambitious goal theory plays a role which cannot be overlooked. 

STM measurements of the shape of gold clusters supported on flat gold substrates and FEM measurements of the melting temperature of gold clusters supported on tungsten tips are presented. The size dependence of these cluster properties is obtained for cluster diameters ranging from 12 nm to 2 nm. 

Chapter 4, by Batzill and his coworkers, describes modern surface characterization techniques that include photoelectron diffraction and ion scattering as well as scanning probe microscopies. The chapter by Hayden discusses model hydrogen fuel cell electrocatalysts, and the chapter by Ertl and Schuster addresses the electrochemical nano structuring of surfaces. Henry discusses adsorption and reactions on supported model catalysts, and Goodman and Santra describe size-dependent electronic structure and catalytic properties of metal clusters supported on ultra-thin oxide films. In Chapter 9, Markovic and his coworkers discuss modern physical and electrochemical characterization of bimetallic nanoparticle electrocatalysts. 

Size-Dependent Electronic, Structural, and Catalytic Properties of Metal Clusters Supported on Ultrathin Oxide Films... 

Both examples revealed unique and size-dependent catalytic properties of very small metal clusters. As mentioned in the introduction, gas phase clusters will probably never be important in real catalysis, however, such small clusters can be stabilized on support materials so they may become relevant for future applications. 

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