Nanoparticles in Heterogeneous Catalysis

Nano-sized materials offer a large variety of potential applications, especially in the field of catalysis due to their characteristic high-surface areas and distinctive properties. Chemical reactions are known to take place on the surface of catalysts, which become more active towards a specific reaction as their surface areas increase. Hence, catalysts at the nanoscale (nanocatalysts) efficiently provide the way to make many chemical processes. Understanding their electronic and dynamic properties has become one of the most active areas in modern physical chemistry, and tremendous advances have been achieved due to the permanent interaction between theory and experiments for the past several decades.

In the field of heterogeneous catalysis metal nanoparticles have achieved good selectivity towards specific reactions due to their unusual chemical and physical properties (Aiken-III et al. 1999). The observed catalytic enhancement in metal alloys is determined by several variables such as their overall composition, preparation of the catalyst, and external conditions (Campbell 1990; Toshima et al. 1998; Aiken-III et al. 1999; Mainardi et al. 2001). Changes in these macroscopic variables and processes are responsible for changes at the micro and nanoscopic levels, which in turn define the reactivity behavior of the material. Proper control of the surface chemistry, geometry, and composition at the nano-and macroscales are needed when designing heterogeneous metal catalysts (Bazin et al. 2000). This effect is most efficiently accomplished by alloying two or three metals, a process that causes variations in shape, structure, and surface atomic distribution in the nanoclusters depending on their size, shape, and overall composition (Mainardi et al. 2001).

In many cases, the active components of dispersed metal catalysts are small nanoparticles with well-defined crystallographic surfaces, and therefore nanoparticle properties, rather than the bulk properties are responsible for the observed catalytic characteristics. In an attempt to reduce the amount of platinum used in fuel cells, small atomic clusters of platinum deposited on carbon supports were proposed (Bockris et al. 1993; Huang et al. 2003). For such supported nanocatalysts, metal-metal and metal-adsorbate interactions were observed to be key factors for controlling their activity and reactivity (Combe et al. 2000; Huang et al. 2002). Those interactions are responsible for the specific crystallographic surfaces that are formed on the overall nanoparticle surface where catalytic reactions take place. Hence, a major component in the formulation of a molecular basis to surface-chemical reactivity is the study of single crystal surfaces of catalytically active nanoparticles.

The interaction of oxygen with transition metal surfaces has been a topic of high interest in surface science due to its technological applications. In particular, the importance of platinum as a catalyst in many oxidation and reduction reactions has lead to numerous studies on the O2/Pt (111) system (Puglia et al. 1995; Eichler et al. 1997; Adzic et al. 1998; Nolan et al. 1999; Mainardi et al. 2003). A significant amount of studies has been oriented towards the understanding of adsorption and reactions of molecular oxygen on Pt (111) surfaces (Mainardi et al.

2003) and clusters (Li et al. 2001). The Pt (111) surface consist of a two-dimensional hexagonal lattice and contains six adsorption sites per unit cell, three independent bridge, two hollow sites, and one top site (Fig. 18.2).

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