Atomic chemisorption, cluster requirements

Theoretical calculations, for example, for the adsorption of H2 on transition metals also show that much smaller clusters are required for chemisorption than for bulk properties.229 230 Also, for CO adsorption, extended Hiickel calculations on chains of Ni atoms showed only minor changes when the chain length exceeded 3 Ni atoms.231 This suggests a very small cluster as compared with the adsorption of halogen (see above). If the calculations are reliable this difference may reflect differences in the mode of bonding of CO and halogens.165... 

Some topics have been omitted from this review. This holds for the structure and function of metal sites in metalloproteins and metalloenzymes in relation to enzymatic catalysis, for which the reader is referred to Cramer and Hodgson (68) and Doniach et al. (80). Also, chemisorption studies and the structure of adsorbate-covered surfaces are not considered in this review, which deals with XAES in transmission, thus characterizing bulk material. It is noted that even in the case of chemisorbed atoms XANES data analysis requires physically the definition of clusters of considerable size. On the other hand, the analysis is simplified for adsorbed molecules. Very pronounced near-edge effects (usually obtained by electron-stimulated Auger measurements) are observed for low-Z-atom(C, N, O, F>containing chemisorbed... 

Table 1 summarizes the information required for a detailed characterization of a supported metal catalyst for supported bimetallics there are additional questions, e.g., the distribution of atoms in bimetallic clusters and the surface composition of larger alloy crystallites. For the support and the prepared catalyst, the total surface area, pore size distribution, and surface acidity are routinely measured, if required, while other characteristics, e.g., thermal and chemical stability, will have been assessed when selecting the support. The surface structure of alumina, silica, charcoal, and other adsorbents used as catalyst supports has been reviewed. Undoubtedly, the most commonly measured property is the metal dispersion, often expressed in terms of the specific metal area and determined by selective chemisorption or titration but, as discussed (Section 2), there is the recurring problem of deciding the correct adsorption stoicheiometry. 

The lack of activity of Co/MgO is due to the non availability of cobalt metal due to the formation of a CoO/MgO solid solution during activation. In contrast, the relatively high dispersion of Co/zeolite suggests that cobalt metal is available on the catalyst surface. The sample is inactive possibly because the number of Co° atoms in each cluster is less than a lcwer limit required for dissociative CO chemisorption. Chemisorption with retention of integrity, as is measured in the dispersion experiment, is not subject to such a constraint. 

If the degree of coverage of the ruthenium by the copper is very high, as can be inferred from the chemisorption and catalysis studies already discussed, the copper atoms should be coordinated extensively to ruthenium atoms. It is emphasized that the ruthenium-copper clusters are of such a size (average diameter of 32 A by electron microscopy) that the surface metal atoms constitute almost half of the total. Hence for a Cu/Ru atomic ratio of one, the number of copper atoms would correspond roughly to that required to form a monolayer on the ruthenium. 

Size and shape requirements of metal cluster models for reaching convergence of the chemisorption energy constituted the main topic of our study [255]. To reach convergence, we proposed to use symmetric models that can be accurately treated for clusters comprising of up to more than hundred atoms. A series of octahedral and cuboctahedral palladium clusters was considered, from Pdss to... 

Interest in the synthesis of transition metal carbonyl clusters incorporating interstitial main-group atoms is because they provide a conceptual bridge between or-ganometallic chemistry and the areas of inorganic solid-state and surface chemistry. In addition to serving as useful models for either solid-state binary alloys or for the chemisorption of heteroatoms on the step-site of a metal surface, these discrete molecular clusters are often markedly more stable, especially toward the temperatures and pressures required for catalytic reactions. 

Thus, it appears that high temperature chemisorption of N2 counts correctly, not the iron surface atoms, but the sites required for chemisorption of N2, the rate of which is the rate determining step in armnonia synthesis reaction. It would be wrong to conclude that the reaction is structure-insensitive because the turnover rate is independent of particle size when the sites are counted with chemisorption of N2. On the contrary, all the combined results confirm the view that a site for ammonia synthesis comprises a certain number of atoms, including C7 atoms, forming a multiple, or ensemble, or cluster. Perhaps these consist of the six atoms as shown in Fig. 2.30. ... 

Clusters involving several hydrocarbon fragments located at contiguous metal atoms are of interest to provide models for surface adsorption sites known as ensembles or multiplets. In addition, their dynamic properties and intramolecular transformations could provide valuable information concerning the niimerous steps of the catalytic conversion or synthesis of hydrocarbons. Finally, improved model compounds for the dissociative chemisorption of CO and possibly N2 are required. 

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