Particle size effect surface structure facets

A key to achieving improvements in electrocatalytic activity in fuel cell electrodes lies in understanding how the structure of the catalyst determines the rates of relevant surface processes (Bayati et al., 2008 Eikerling et al., 2007a Maillard et al., 2004 Mulla et al., 2006 Vielstich et al., 2003). On nanoparticles, the proportions of the different surface sites, located at facets, edges, and corners, and the surface electronic structure, are closely related to the particle size (Bradley, 2007 Mayrhofer et al., 2005 Mukerjee, 1990 Mukerjee and McBreen, 1998 Somorjai, 1994). These particle size effects exhibit varying trends for different reactions (Ahmadi et al., 1996 ... 

In many catalytic systems, nanoscopic metallic particles are dispersed on ceramic supports and exhibit different stmctures and properties from bulk due to size effect and metal support interaction etc. For very small metal particles, particle size may influence both geometric and electronic structures. For example, gold particles may undergo a metal-semiconductor transition at the size of about 3.5 nm and become active in CO oxidation [10]. Lattice contractions have been observed in metals such as Pt and Pd, when the particle size is smaller than 2-3 nm [11, 12]. Metal support interaction may have drastic effects on the chemisorptive properties of the metal phase [13-15]. Therefore the stmctural features such as particles size and shape, surface stmcture and configuration of metal-substrate interface are of great importance since these features influence the electronic stmctures and hence the catalytic activities. Particle shapes and size distributions of supported metal catalysts were extensively studied by TEM [16-19]. Surface stmctures such as facets and steps were observed by high-resolution surface profile imaging [20-23]. Metal support interaction and other behaviours under various environments were discussed at atomic scale based on the relevant stmctural information accessible by means of TEM [24-29]. 

An ideal study of support effects requires model catalysts with metal particles that are identical in size and shape (so that only the support oxide varies). This is difficult to achieve for impregnated catalysts, but identical metal particles can be prepared via epitaxial model catalysts [36]. Well-faceted Rh nanocrystals were grown on a 100-cm area NaCl(OOl) thin film at 598 K. One half of a Rh/NaCl sample was covered with Al Oj, and the other half with TiO. The preparation of Rh particles for both Al Oj- and TiO -supported model catalysts in a single step prevents any differences in particle size, shape, and surface structure which could occur if the samples were prepared in separate experiments. Three model catalysts were prepared, with a mean Rh particle size of 7.8, 13.3, and 16.7 mn (the films were finally removed from the NaCl substrate by flotation in water). Activation was performed by O /H treatments, with the structural changes followed by TEM (Fig. 15.6). Oxidation was carried out in 1 bar O at 723 K prodncing an epitaxially grown rhodium oxide shell on a Rh core (cf Fig. 15.5e), whereas the hydrogen reduction temperature was varied. 

As to the mechanistic origin of the activity enhancement in dealloyed Pt-Cu catalyst, the authors believe geometric effects play a key role, because the low residual Cu near-surface concentrations make significant electronic interactions between Pt and Cu surface atoms unlikely. Therefore, they suspect that the dealloying creates favorable structural arrangements of Pt atoms at the particle surface, such as more active crystallographic facets or more favorable Pt-Pt interatomic distances for the electroreduction of oxygen, as predicted by DPT calculations [47]. A fourfold enhancement in Pt mass activity on monodispersed PtsCo nanoparticles with particle size of 4.5 nm was also reported recently [95]. 

All relevant electrochemical reactions in PEFCs exhibit peculiar sensitivities to the surface structure of the catalyst (Boudart, 1969). The abundances of the different surface sites, for example, edge sites, comer sites, or sites on crystalline facets, are related to the size of nanopartieles (Kinoshita, 1990). Support-particle interactions may alter the electronic stmeture of catalyst surface atoms at the rims with the support (Mukerjee, 2003). Moreover, the support may serve as a source or sink of reactants via the so-called spillover effect (Eikerling et al., 2003 Liu et al., 1999 Wang et al., 2010 Zhdanov and Kasemo, 2000). 

In this chapter, we will discuss a number of aspects of the effect of catalyst structure on catalytic activity, while the topic of poisoning and promotion will be discussed in Chapter 10. First, we will cover the variation in intrinsic catalytic activity of different facets and defects of a surface. Next, we will show how this leads to variations in activity with particle size and shape. This leads us to a close look at the nature of the active sites of the surface where most of the catalysis takes place. 

---