Site-concept

Lahaye J., Dentzer J., Soulard P., Ehrburger P. Carbon gasification The active site concept. In Fundamental Issues of Control of Carbon Gasification Reactivity, Lahaye J, Ehrburger P. Ed., Academic Publishers, London, 1991, p. 143-158... 

Active centers, nature of, 10 96 Active site, 27 210-221 in catalysts, 17 103-104, 34 1 for olefin chemisorption, 17 108-113 dual-site concept, 27 210 electrical conductivity, 27 216, 217 ESCA, 27 218, 219 ESR, 27 214-216 infrared spectroscopy, 27 213, 214 model, 27 219-221 molybdena catalyst, 27 304-306 Mdssbauer spectroscopy, 27 217, 218 nonuniform distribution, transport-limited pellets, 39 288-291... 

The initial collective electronic theory of the fifties, in its simplest form, implied that the electrons and holes-controlled by the bulk structure—of the catalytic solid are available for reactants anywhere on the surface. It largely ignored the geometric factor inherent in Taylor s active site concept or in Balandin s Multiplet Theory. 

The "Hidden Site" Concept in Faujasite. The infrared active hydroxyl vibrations in H-faujasite absorb at two distinct frequency regions, commonly denoted as high-frequency (HF) and low-frequency (LF) hydroxyl bands (17.). In the structure of faujasite, there are four crystallographically distinct oxygen atoms. The HF band vibrating in the 3680-3625 cm--1- range is generally... 

The selection on an empirical basis of collective-electron factors or active-site concepts or some combination thereof in order to account for the activity of surfaces in catalysing various processes has obvious disadvantages. Possibilities for a more systematic approach to the integration of collective-electron and localised-state aspects of surface structure have developed from theoretical treatments of intrinsic and extrinsic surface states, respectively. Models based on such developments, by reason of their relative novelty, have not yet been as widely applied as collective electron or active-site models to interpret catalytic activity of various surfaces and still less to considerations of sensitivity to irradiation. However, an abbreviated consideration of such surface state models is deemed essential here both as a basis for assessing their possible relevance in the explanation of radiation-induced effects and as an illustration of the integration of electronic and localised state aspects into a common framework. 

The model captures the decreased but sustained activity over platinum promoted sulfated zirconia in the presence of hydrogen by eliminating all contributions to the activity by the S sites and decreasing the activity over the S2 sites by 70%. The deactivation constant for the S2 sites was set to zero to match the experimental data. These results from the model are consistent with a two active site concept where hydrogen inhibits that activity of one type of active sites more than the other type of active site. 

Scheme 18.17 Summary of the single site concept applied to selective dehydrogenation of isobutene.

Up to this point only speculations have been presented. They were based on the assiomption that the peripheral process in olfaction is mediated by specific receptor sites of a group of membrane bound adenyl cyclases and that the Multiple Profile - Multiple Receptor Site concept is viable. If these assumptions are correct the following extrapolations could be made ... 

In the following, we present a scenario for CO electrooxidation on Pt nanoparticles to elucidate the role of particle size and surface structure. The model employs the active site concept and highlights the role of finite surface mobility of adsorbed CO. 

Active site concepts for Mn- and Ce-based systems are not yet well developed, which is partly due to problems in the reliable differentiation between oxidation states in case of Mn. 

Particularly in the case of natural events, some scenarios are treated as exclusion criteria for the site itself (e.g. local volcanism and local active fault) and so they are not discussed here [14]. Other scenarios are dealt with preferably through site protection features (e.g. the dry site concept, site drainage, protecting dams and levees) rather than by plant design measures and therefore they are discussed in the relevant publication on site evaluation. 

FIGURE 3.8 Mapping of a 3D nanoparticle model onto a 2D surface model with active and inactive sites. The active site concept is the integral part of the heterogeneous surface model of nanoparticle activity, explored here for CO d electro-oxidation. Immobile adsorbs, preferentially, on specific sites called the active sites. The mobile CO d is found on the remaining inactive catalyst sites. (Reprinted with permission from Andreaus, B.et al. 2006. Kinetic modeling of COad monolayer oxidation on carbon-supported platinum nanoparticles. J. Phys. Chem. B, 110, 21028-21040, Figure 2,7,8,9, American Institute of Physics.)... 

Figure 6.9. The bismetallo active site concept of Buckingham (253). Reprinted with permission. Copyright 1978 by the American Chemical Society.

Enantiomorphic sites concept The stereospecific polymerization of racemic methyloxirane produces generally a polymer with some cristallinity which can be fractionated by selective solubility (using acetone for example) in a crystalline fraction and an amorphous one (an intermediate "semi-crystalline fraction can be also isolated). 

---