Surfaces facets

How does a support affect the morphology of a particle on top of it Which surface planes does the metal single crystal expose The thermodynamically most stable configuration of such small crystallites is determined by the free energy of the surface facets and the interface with the support, and can be derived by the so-called Wulff construction, which we demonstrate for a cross section through a particle-support assembly in two dimensions (Fig. 5.13). 

Surface faceting may be particularly significant in chiral heterogeneous catalysis, particularly in the N i/P-ketoester system. The adsorption of tartaric add and glutamic acid onto Ni is known to be corrosive and it is also established that modifiers are leached into solution during both the modification and the catalytic reaction [28]. The preferential formation of chiral step-kink arrangements by corrosive adsorption could lead to catalytically active and enantioselective sites at step-kinks with no requirement for the chiral modifier to be present on the surface. 

In comparison to most other methods in surface science, STM offers two important advantages (1) it provides local information on the atomic scale and (2) it does so in situ [50]. As STM operates best on flat surfaces, applications of the technique in catalysis relate to models for catalysts, with the emphasis on metal single crystals. Several reviews have provided excellent overviews of the possibilities [51-54], and many studies of particles on model supports have been reported, such as graphite-supported Pt [55] and Pd [56] model catalysts. In the latter case, Humbert et al. [56] were able to recognize surface facets with (111) structure on palladium particles of 1.5 nm diameter, on an STM image taken in air. The use of ultra-thin oxide films, such as AI2O3 on a NiAl alloy, has enabled STM studies of oxide-supported metal particles to be performed, as reviewed by Freund [57]. 

The beauty of a gem is measured in terms of its clarity, brilliance, and color. Its natural beauty can be enhanced by the way it is cut. There are two basic kinds of gem cuts faceted and cabochon. The faceted cut has many flat cut surfaces (facets) with an overall shape that might be round, oval, square, rectangular, or pear-shaped. Faceted cuts are preferred for brilliant transparent stones such as diamond. The cabochon cut has a smooth rounded top, usually with a flat base, and it is mainly used for opaque or translucent stones. 

Figure 5.3.8 HRTEM images of a C11/Z11O/AI2O3 methanol synthesis catalyst consisting of porous aggregates (A) of metallic Cu and ZnO nanoparticles (B [57]) showing details of the surface faceting, decoration, and defect structure (C [58]), which is discussed in detail in the text.

Recently, experimental and theoretical evidence for a model of the active site of industrial methanol synthesis that combines the role of ZnO and defects in Cu has been presented [58]. Planar defects have been shown to lead to changes in surface faceting of the Cu nanoparticles (Figure 5.3.8C) associated with formation of steps and kinks that were assumed to represent high-energy surface sites of special catalytic activity. For a series of Cu/ZnO-based catalysts, a linear correlation of the defect concentration with the intrinsic activity of the exposed Cu surface was observed. In addition, (partial) surface decoration of Cu with ZnOx by SMSI has been... 

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. 

Second, apart from single crystals, nanoparticle model catalysts should be employed to better mimic the complex properties of supported metals. Nevertheless, the metal nanoparticles should still exhibit well-defined surface facets to allow more reliable data interpretation and a comparison with single-crystal results. 

Fig. 21. Schematic models of truncated cuboctahedra of various sizes and aspect ratios, exhibiting a (111) top facet and (1 1 1) and (100) side facets. According to HRTEM images of palladium nanoparticles, the terraces may be incomplete, leading to surface facets with steps. For structural characteristics, see Table II.

Chen Q, Richardson NV (2003) Surface facetting induced by adsorbates. Prog Surf Sci 73 59... 

Fig. 15.5 Oxidation and reduction of epitaxially grown polyhedral Rh nanoparticles (mean size 5 nm) on alumina, monitored ex situ by HRTEM. In the as-prepared state, most of the Rh particles were half-octahedra with 111 and 100 surface facets, as revealed by combining results from HRTEM and WBDF (a, d), and SAED (b). Upon oxidation in 1 bar at 723 K, an epitaxial Rh-oxide shell developed on top of a Rh core (c, e). Reduction in 1 bar H at 523 and 723 K led to polycrystalline (f) and rounded crystalline (g) nanoparticles, respectively. The microstructural changes were correlated with changes in catalytic hydrogenolysis activity (see text for details) adapted in part from [20] with permission. Copyright (1998) Elsevier...

Surface reconstruction usually leads to the formation of stable overlayers. This cannot occur without some mobility of catalyst substrate atoms. The reconstructed phase often has a different surface atom density from the non-reconstructed surface. For this reason the macroscopic consequence may be facetting of the catalyst along particular preferred crystallographic directions. This explains the often observed phenomenon that a heterogeneous catalyst often shows only stable performance after some initiation period. In a reactive system the surface composition of the adlayer may strongly vary with conditions and hence the details of surface facetting and surface reactivity. 

The phenomenon of surface reconstruction is quite general and is one of the causes of surface facetting observed in working catalysts. 

Hanrath, T. Korgel, B.A. Crystallography and surface faceting of germanium nanowires. Small 2005, 1, 717-721. 

One can set limits of validity to the SAPS model. The cluster cannot be too small, because small clusters with open electronic shells deform away from spherical symmetry. On the other hand, very large clusters have the tendency to form planar surface facets. The intermediate cluster range between those two limits is well adapted to SAPS. 

Figure 13.5 Reprinted from Heffelhnger, J.R., Bench, M.W., and Carter, C.B. (1995) On the faceting of ceramic surfaces, Surf. Sci. 343, L1161. Copyright 1995, with permission from Elsevier. See also Heffelhnger, J.R. and Carter, C.B. (1997) Mechanisms of surface faceting and coarsening. Surf. Sci. 389, 188.

---