Interface structures, product catalysis

Examples of the diversity of possible interface structures are as follows. Topotactic interfaces. Primary valence forces may link closely juxtaposed, or perhaps coherent, reactant and product phases so that the crystalline product retains the orientation and some structural features of the reactant [29,30]. The interface, with thickness of molecular dimensions, is defined by the discontinuity of structure and bonding within which reactivity is locally enhanced by the strain field. Product catalysis. If the residual solid is a catalyst for breakdown of a reactant constituent, decomposition may occur within chemisorbed material at the product... 

It is important to distinguish clearly between the surface area of a decomposing solid [i.e. aggregate external boundaries of both reactant and product(s)] measured by adsorption methods and the effective area of the active reaction interface which, in most systems, is an internal structure. The area of the contact zone is of fundamental significance in kinetic studies since its determination would allow the Arrhenius pre-exponential term to be expressed in dimensions of area"1 (as in catalysis). This parameter is, however, inaccessible to direct measurement. Estimates from microscopy cannot identify all those regions which participate in reaction or ascertain the effective roughness factor of observed interfaces. Preferential dissolution of either reactant or product in a suitable solvent prior to area measurement may result in sintering [286]. The problems of identify-... 

Surfaces and interfaces chemistry is the study of the structure and reactivity of liquid and solid surfaces. The surfaces may be extended or may be limited to the nanometer scale. The surface, often a transition metal, may be a catalyst for a chemical reaction. Such studies provide the fundamental principles of the commercially important area of heterogeneous catalysis, which is essential to fuel and metal production, food processing, and commodity chemical manufacturing. The surface may also be consumed as a reactant, such as in semiconductor etching. These studies provide the basic chemistry of the manufacturing of electronic components and devices. 

Both surface and bulk properties are relevant to catalytic reactivity. Although heterogeneous reactions by definition occur at the interface between a catalyst and reactant/product phase, the process of catalysis actually includes activation of an as-synthesized catalyst, catalytic reaction, and adverse processes leading to the deactivation of a working catalyst. Activation may involve chemical transformations of both the catalyst surface and bulk. For example, the iron oxide Fe Oj is chemically transformed into the active iron carbide during activation for the Fischer-Tropsch synthesis (FTS) from CO and [32, 33]. There are numerous other examples of reduction of a metal oxide to an active metal or oxidation of a metal to an active oxide, carbide, sulfide, or similar. Characterization of chemistry and structure of the surface and bulk of a catalyst nanoparticle using representative techniques are presented in Chapter 4. 

The cyclization mechanism of type 11 terpene cyclases is exemplified by the reaction of the SHC (Scheme 87.19). Important insights into the reaction mechanisms have been obtained from structural data [199, 208]. One of the inner helices of the p-domain of SHC contains a conserved DxD(D,E) motif that is located in the central cavity at the interface between the p- and y-domains. Its central aspartate residue D376 is polarized via hydrogen bonding to an adjacent histidine residue and protonates the double bond of the squalene substrate to initiate the reaction cascade. Conserved aromatic residues stabilize the intermediate cations by cation-tt-interactions. A water molecule is a candidate to act as catalytic base, and this water may also account for the formation of the by-product hopanol. Another bridging water molecule connects D376 to a tyrosine residue and can restore the active site after catalysis by reprotonation of D376. 

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