Low-coordinated atoms

It is noteworthy that surface carbon did not come from those CO molecules responsible for the HT peak but from sites that are able to disproportionate CO and correspond to the LT peak. Because the latter sites are important only on quite small particles, it is tempting to associate them with low coordination number surface metal atoms, the relative concentration of which increases rapidly as the particle size decreases below 2 nm (8). Thus, these atoms may be the sites responsible for the relatively weakly adsorbed state of CO. Results similar to our work were found on other Group VIII metals. In the case of a Ru/Si02 sample, Yamasaki et al. (9) have shown by infrared spectroscopy that the deposition of carbon occurs rapidly by CO disproportionation on the sites for weakly held CO. The disproportionation also occurred on a Rh/Al20 sample with 66% metal exposed so that appreciable concentrations of low coordination atoms are expected (10). 

For atomic chemisorption, similar structural effects are found (see the middle panel of Figure 4.10). As for molecular chemisorption, low-coordinated atoms at steps bind adsorbates stronger and have lower barriers for dissociation than surfaces with high coordination numbers and lower d band centers. The d band model thus explains the many observations that steps form stronger chemisorption bonds than flat surfaces [1,20,24-28]. The finding that the correlation with the d band center is independent of the adsorbate illustrates the generality of the d band model. 

Lin X, Nilius N, Sterrer M, Koskinen P, Haekkinen H, Freund H-J. Characterizing low-coordinated atoms at the periphery of MgO-supported Au islands using scanning tunneling microscopy and electronic structure calculations. Phys Rev B. 2010 81 153406 (1-4). 

We may note that if the above ideas are generalized to small particles, which expose mostly low-coordinated atoms, the TOF would logically decrease as FE increases, as is usually observed. However, the electronic environment of an atom in a cluster is quite different from that of an atom at a step or kink on a large crystal. 

The nature of the bonding at the interface between metal species and oxide support is known only very approximately. Ideally, one would like to be able to prepare monodispersed catalyst particles, all with the same and, if possible, very small number of atoms. In fact, the active part of the catalyst is its surface where low-coordinated atoms are responsible for most of the specific chemistry [20]. However, surface atoms form only a small fraction of the total number of atoms of the particle. Because heterogeneous catalysis often deals with precious metals, reducing the size of the particle immediately results in reduced cost due to a better surface-to-volume ratio with a given quantity of material, it is... 

For low dispersion catalysts, the addition of Au also induces an increase in the carvotanacetone formation. However the variation in selectivity is lower to the one obtained on a catalyst with higher dispersion. The magnitude of this effect is justified by the small number of low coordination atoms that exist in large particles ( 140 A) in comparison with the large number of low coordination atoms that exist on the small particles ( 20 A). The previous results can be explained if... 

The scaling of the metal-metal bond length of Pd species with cluster size was recently studied in detail [168]. There is considerable evidence that the metal-metal bonds contract for smaller clusters [174-177]. In a liquid droplet model, this phenomenon is easily rationalized by the increasing pressure with decreasing particle radius a more chemical argument refers to the fact that the fraction of low-coordinated atoms increases with decreasing cluster size. 

Low coordinated atoms on metallic NPs have been proposed to be catalytically active sites. How many atoms with a coordination of six or lower are there on icosahedral, octahedral, and truncated octahedral Au NPs. 

The shape of the gold particles is an important factor for catalytic activity since it determines the relative amount of corners and edges, i.e., of low-coordinated atoms. It depends on the support through the interfacial energy this is well known from works performed on model samples (Table 15.1). The higher the energy, the flatter the gold particles. 

Bradley et al. employed P VP-stabilized palladium colloids, which were pretreated with the base and the olefin at the reaction temperature prior to addition of the aryl halide (Table 1) [19]. The colloids were prepared by stirring [Pd(dba)2] in the presence of PVP at variable hydrogen pressures, affording particles of different average particle size (diameter 1.7 0.5 nm to 3.7 0.3 nm). The initial activities of Heck reactions catalyzed by these colloids of different average particle size were correlated with the number of low-coordinated atoms, i.e., comer and edge atoms in the palladium particles rather than all surface atoms. A general difficulty in such considerations is that the particles are not monodisperse, and also are not ideal cuboctahedra. 

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