Metal surface stepped, kinked

Recenl work has defined more carefully ihe nature of active sites. Metal surfaces are thought to contain three main types of sites terraces, ledges (or steps) and kinks, which correspond to one, two. and three coordinatively unsaturated sites of organometallic chemistry. These sites display differing activities toward saturation, isomerization, and CKChiingQ 7 J0,68 JO 1.103,104,105). 

As the crystal surface exposed to the atmosphere is usually not ideal, specific sites exist with even much lower co-ordination numbers. This is shown schematically in Fig. 3.5, which gives a model comprising so-called step, kink and terrace sites (Morrison, 1982). This analysis suggests that even pure metal surfaces contain a wide variety of active sites, which indeed has been confirmed by surface science studies. Nevertheless, catalytic surfaces often behave rather homogeneously. Later it will be discussed why this is the case. In short, the most active sites deactivate easiest and the poorest active sites do not contribute much to the catalytic activity, leaving the average activity sites to play the major role. 

Since the most active catalytic sites are usually steps, kinks, and surface defects, atomically resolved structural information including atomic distribution and surface structure at low pressure, possible surface restructuring, and the mobility of adsorbate molecules and of the atoms of the catalyst surface at high temperature and high pressure is crucial to understanding catalytic mechanisms on transition metal surfaces. The importance of studying the structural evolution ofboth adsorbates... 

Fig. 2.18 Eleclrocrystallization on a metal surface. The upper layer of atoms grows by surface diffusion of adatoms (A) across plane jc-j until they encounter a step site of lower energy, such as B. The adatom may now move along the step until it is located at the even lower energy kink site at C...

Perhaps one of the important conclusions of these studies that points to the unique chemistry of surface irregularities, steps, and kinks, which appear to be active sites, is the controlling influence of the local atomic structure, local surface composition, and local bonding between adsorbates and surface sites. The microstructure of the metal surface controls bond scission and thus the rate and path of chemical reactions. Calculations taking into account this local bonding picture should help to unravel the elementary bond-breaking steps in catalytic surface reactions. 

Steps at metal surfaces were argued to be good candidates for an increased reactivity because of the lower coordination number of the atoms [8]. In a seminal work Davis and Somoijai [9] concluded, e.g. that the ability of Pt to break C-H bonds during hydrocarbon dehydrogenation is enhanced at kinked Pt surfaces pre-dosed with oxygen, while Gland and coworkers demonstrated that CO adsorbs preferentially on the atomically rough steps of Pt(3 21) [10] and that some 02 dissociation [11] may occur at the same surface. 

One problem that is amenable to this technique is transition metal-mediated chemical reactions. They range from industrial processes such as petroleum refinery to biological cycles such as nitrogen fixation and photosynthesis. These reactions have been thought to take place at an unsaturated metal site, for example, the kinks or step edges on a metal surface. Ideally, one would like to observe the chemical events that evolve... 

Surface Crystallography and Composition. Platinum (11) and nickel (8,9,12) have been the metal surfaces examined in our surface science studies to date. The surface coordination chemistry has been examined as a function of surface crystallography and surface composition. Surfaces specifically chosen for an assay of metal coordination number and of geometric effects were the three low Miller index planes (111), (110) and (100) as well as the stepped 9(lll)x(lll) and stepped-kinked 7(lll)x(310) surfaces (both platinum and nickel are face centered cubic). 

Thus, methyl hydrogen atoms cannot easily closely approach surface metal atoms even through vibrationally excited states as shown schematically at the left in the figure. However, acetonitrile molecules bound near step or kink sites can have methyl hydrogen atoms in positions from which there can be a facile close approach of these hydrogen atoms to the surface metal atoms. This geometric or stereochemical feature explains the reactivity (irreversible C—H bond-breaking processes) observed for acetonitrile on the stepped, stepped-kinked, and super-stepped (110) nickel surfaces. 

However, this assumption is not necessarily justified. Even for a well-faceted nanoparticle there are a number of nonequivalent adsorption sites. For example, in addition to the low-index facets, the palladium nanoparticle exhibits edges and interface sites as well as defects (steps, kinks) that are not present on a Pd(l 1 1) or Pd(lOO) surface. The overall catalytic performance will depend on the contributions of the various sites, and the activities of these sites may differ strongly from each other. Of course, one can argue that stepped/kinked high-index single-crystal surfaces (Fig. 2) would be better models (64,65), but this approach still does not mimic the complex situation on a metal nanoparticle. For example, the diffusion-coupled interplay of molecules adsorbed on different facets of a nanoparticle (66) or the size-dependent electronic structure of a metal nanoparticle cannot be represented by a single crystal with dimensions of centimeters (67). It is also shown below that some properties are merely determined by the finite size or volume of nanoparticles (68). Consequently, the properties of a metal nanoparticle are not simply a superposition of the properties of its individual surface facets. 

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