Catalytic activity crystallographic surface structure

Readsorption of water leads to a lifting of the reconstruction and the reoccurrence of the (1 x 1) structure [254], Note for completeness that the reconstructed surface exhibits a considerably higher chemical activity, for example in the DcNO reaction, than the hydroxyl-covered surface which is basically inactive towards further chemisorption [256]. In other words, water desorption and readsorption leads to a strong change in the chemical activity of certain crystallographic planes of oxide surfaces which may be relevant with respect to the catalytic activity of powders of real samples. 

Crystallographic analysis has provided us with a detailed structure of hCp on the other hand, essentially all of the structure-function analyses have been done on FetSp. Also, except for the copper site structural homology, the two proteins are quite different. hCp is composed of six plastocyanin-like domains (plastocyanin is a type 1 copper-containing protein) that are arranged in a trigonal array (Zaitseva et al., 1996). One result of this domain replication is a conformational fold that produces a distinct, negatively charged patch on the protein surface adjacent to the catalytically active type 1 Cu(II). This copper atom is in domain 6. (Domains 2 and 4 contain type 1-like copper sites that do not participate in the ferroxidase reaction.) Lindley et al. (1997) have proposed that this... 

There are, therefore, a number of distinct structural characteristics which must be identified in order to fuUy understand the action of oxide catalysts. Bulk properties of interest include the identification of distinct crystallographic phases present in the catalyst the local environment of the nuclei in either crystalline phases or amorphous materials and the redox properties of the catalyst. Surface properties impacting on catalytic activity include the local environment of nuclei at the surface acid-base behavior the number and concentration of acid sites, including hydroxyl groups and the nature of these acid sites. 

The catalytic activity of the Cu-Si surface species explained above was a logical, but only an indirect derivation from our former experiments. For some time, we were not able to present more direct evidence. The reason was the complicated surface topography of a working contact mass. According to all experiences, the Rochow reaction exclusively takes place in sharply demarcated pits on the silicon surface, the shape of which usually corresponds to the respective crystallographic planes. The inner surface of these pits mostly has an own structure, i.e, it is partly covered by particles of copper species, often doubtless by ri-CuaSi particles [15]. But, the characteristic dimensions of these structures are generally below the resolving power of appropriate surface methods like the combination SEM/EDX. 

The catalytic activity was also calculated on the basis of surface sites, calculated by titration method, to obtain turnover frequencies, TOE (expressed as mol isopropanol formed mof Rh-min ). TOE values for Rh/ZEDIP, Rh/ZESEP and Rh/ZESEX are nearly the same, although the dispersion values are different. This fact confirms that the acetone hydrogenation is insensitive to the structure of the metal, its reaction rate is proportional to the amount of exposed rhodium atoms, and is not affected by the metallic particle size or the crystallographic plane exposed. 

Researchers in the area of heterogeneous catalysis have recently focussed considerable attention to the relationships among catalytic activity, product selectivity and the size and shape of metal particles for reactions catalyzed by metals (15). Reactions that are influenced by the size and shape of metal particles or electronic interactions of the metal particles with the support are known as structure sensitive reactions. Theoretical calculations of various crystallographic structures (16) have shown that the number of specific type of surface atoms (face, corner, edge) change as a function of particle size. For example, for a face centered cubic system, the number of face atoms decreases as particle size decreases. If, therefore, a reaction is catalyzed on a face and there are a substantial number of face atoms necessary for catalysis to occur, then as particle size decreases catalytic activity will decrease. This idea often runs counter to principles discussed in general science texts (17). 

Other changes can occur to render the surface catalytically inactive for high surface area and/or supported metal oxides. Metal oxides can exist in various crystallographic structures, of which only a limited set (or only one) may be active. The transformation of one form to another can represent a deactivation of the catalyst. The formation of mixed metal oxides by reaction between the oxide and the support may represent a system with little or no catalytic activity as compared to the original catalyst. 

In the treatment presented above a special model of nonuniformity was presented which takes into account sophisticated surface structure and the existance of different crystallographical planes with different reactivity. An interesting and industrially relevant situation is when nm size metal crystallites on various supports act as catalytically active material. Metal nanoparticles supported on inorganic and organic matrices have shown promising features, like higher catalytic activity and/or selectivity than conventional catalysts in many catalytic reactions. The origin for... 

The deposition of gas phase species on the surface creates thin layers of well deflned crystallographic structure. Therefore, the combustion of a CH4/H2-mixture or of acetylene can lead to diamond deposition on a catalytically active surface [7-11]. 

The former phenomenon is usual referred to as particle-size effect and is pronounced for structure-sensitive reactions [1,2], i.e., catalytic reactions where the rate and/or selectivity is significantly different from one crystallographic plane to another. Structure-sensitive reactions (e.g., isomerizations) frequently occur on catalytic sites consisting of an ensemble of surface atoms with specific geometry. It is thus reasonable to expect that as the active-phase crystallite size decreases, there will be a different distribution of crystallographic planes on the catalyst surface, with the possible disappearance of ensemble sites, so that both the catalyst activity and... 

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