Miller index facets

These difficulties have stimulated the development of defined model catalysts better suited for fundamental studies (Fig. 15.2). Single crystals are the most well-defined model systems, and studies of their structure and interaction with gas molecules have explained the elementary steps of catalytic reactions, including surface relaxation/reconstruction, adsorbate bonding, structure sensitivity, defect reactivity, surface dynamics, etc. [2, 5-7]. Single crystals were also modified by overlayers of oxides ( inverse catalysts ) [8], metals, alkali, and carbon (Fig. 15.2). However, macroscopic (cm size) single crystals cannot mimic catalyst properties that are related to nanosized metal particles. The structural difference between a single-crystal surface and supported metal nanoparticles ( 1-10 nm in diameter) is typically referred to as a materials gap. Provided that nanoparticles exhibit only low Miller index facets (such as the cuboctahedral particles in Fig. 15.1 and 15.2), and assuming that the support material is inert, one could assume that the catalytic properties of a... 

Several strategies were appUed to produce samples for TEM and kinetic studies [8, 21], but only one route is presented here (Fig. 15.3). Noble metal nanoparticles were grown via metal evaporation on a crystalline soluble substrate (e.g., NaCl(OOl)), leading to an epitaxial growth of particles with regular shape and well-developed low-Miller index facets (Fig. 15.3). Thereafter, the metal particles were embedded in a thin (25 nm) amorphous oxide fdm, before the metal-oxide system was lifted off the substrate via flotation in water [8, 18, 20, 31]. 

In summary, a Pd(l 1 1) single-crystal surface is not sufficient to model the complex adsorption behavior of palladium nanoparticles, even for nanoparticles which mostly exhibit (111) facets. High Miller index stepped or kinked single-crystal surfaces may provide better models of nanoparticles. However, one should remember that CO adsorbed on defects of defect-rich Pd(l 11) became invisible at high coverages Furthermore, it will be demonstrated in a following section that the... 

Chemisorption-induced restructuring can be very well seen using a small metal tip and field ion microscopy. In Figure 6.1 lb the field ion microscope picture of a rhodium tip is shown when clean and after exposure to carbon monoxide at 420 K at low pressures ( 10 Pa) [8]. The metal tip has been completely reshaped as a result of CO chemisorption. The tip becomes faceted and rougher, the step density is reduced, and extended low-Miller-index terraees are formed. 

The facets of a well-formed crystal or internal planes through a crystal structure are specified in terms of Miller Indices. These indices, h, k and I, written in round brackets (hkt), represent not just one plane but the set of all parallel planes (hkl). The values of h, k and I are the fractions of a unit cell edge, ao, bo and cq, respectively, intersected by this set of planes. A plane that lies parallel to a cell edge, and so never cuts it, is given the index 0 (zero). Some examples of the Miller indices of important crystallographic planes follow. 

Densely packed surfaces correspond to low Miller indices, as shown in Figure 3.1 for an fee crystal structure. The drawing shows that the [111] surface plane is most densely packed. The minimization of surface tension, discussed at the beginning of this chapter, generally favors formation of densely packed surfaces. On arbitrary oriented crystals this behavior produces faceting when the metal is heated in absence of surface contamination. Similarly, in the absence of adsorption effects the electrochemical dissolution or deposition of metals tends to expose low index surfaces. 

---