Adsorbate bonding-selectivity

OYAMA ET AL. Adsorbate Bonding Selectivity in Partial Oxidation... 

Linic and co-workers provided two additional examples of modifying selectivity for the ethylene epoxidation reaction. They demonstrated the promotional effect of Cs on EO formation using DFT calculations Cs atoms increase AEa by up to 0.2 eV vs. Ag only, via an induced electric field that interacts with the different dipoles of the two TS s." More recently Christopher et al. synthesized and tested (100) facet-dominated Ag nanowire catalysts, based on the DFT results that AEa is ca. 0.1 eV larger on Ag(lOO) than on Ag(lll), because of the extra elongation of the Ag-adsorbate bonds required to form the TS to AC on Ag(lOO). The Ag nanowire catalysts were indeed more selective than conventional Ag catalysts, in which Ag particles mainly exposes the (111) facet." Incidentally,... 

As mentioned above, it is reasonable to assume that this tetrahedral V species forms at defect sites (hydroxyl nests) in the zeolite framework, but is stabilized by this interaction in a well defined environment through V-O-Si bonds. As indicated by the characterization data, the local coordination of vanadium must be different from that found for well dispersed vanadium sites on silica. This stabilization probably limits the unselective metal-bonded propane or propylene adsorption, in agreement with the role of adsorbate bonding on the selection of partial and total oxidation pathways of ethane on vanadium supported on silica (76) and in agreement with IR evidence (Fig. 

Furthermore, the organic functionalization studies have indicated that multiple reaction products can form even for simple systems. Kinetic and thermodynamic influences must be considered in any analysis of the product distribution. Moreover, the studies have revealed differences in the dominance of kinetic vs. thermodynamic control between the silicon and germanium surfaces. The dissimilarity primarily stems from the fact that adsorbate bonds are usually weaker on Ge than on Si. This difference in energetics leads to observable differences in the degree of selectivity that can be achieved on the two surfaces. Another important motif is the significance of interdimer bonding in the products. Many molecules, even as small as ethylene, have been observed to form products that bridge across two dimers. Consequently, each analysis of adsorption products should include consideration of interdimer as well as intradimer species. 

It is obvious that chemisorptive titration is most ideally applied under such conditions of pressure and temperature that only adsorbates bonded to one surface atom exist. If, in addition, there are surface species bonded to several surface atoms, selective chemisorption applies if... 

Olsen C, Rowntree PA. (1998) Bond-selective dissociation of alkanethiol based self-assembled monolayers adsorbed on gold substrates, using low-energy electron beams. / Chem Phys 108 3750-3764. 

Similar results have been obtained for Ni(100)c(2 x 2)-CO and Ni(100)c(2 X 2)-0. For CO best agreement between theory and experiment is found for the adsorbate bonded in the atop or terminal position, in agreement with LEED observations. It has been emphasized by Plummer and Gustafsson that at present the interpretation of experimental data depends heavily on comparison with theoretical calculations for each individual adsorbate-surface combination. General symmetry arguments (selection rules), however, are now being formulated which should allow identification of the symmetry of the initial-state orbital or adsorption site, without complicated calculations. 

Charge transfer is not unique to electrocatalysis as even a cursory survey of the catalytic literature can show. Indeed, oxidation (18-21), desulfurization (21), and reduction (22) mechanisms have been proposed, involving electron transfer between catalyst and reactant, to explain activity and selectivity effects. Electronic interactions between adsorbate bonds and d-band electrons of the catalyst are also used commonly to explain strength of adsorption (21,23,24). This electron exchange or transfer in conventional catalysis and electrocatalysis, and steps such as adsorption, surface reaction, and desorption, point toward expected similarities between the two catalytic... 

Several chapters in this volume describe new experimental evidence of adsorbate bonding with different in situ spectroscopic techniques, the effect of the interfacial structure on kinetics and mechanisms of selected electrode reactions with bond breaking and surface bonding Kke oxygen... 

Oyama, S. (1991). Adsorbate Bonding and the Selection of Partial and Total Oxidation Pathways, J. Catal., 128, pp. 210-217. 

Syntheses via both surface and homogeneous chemistry are important in electrochemical technology. When the reactions occur on the surface, the most important factor will be the strengths of the electrode-adsorbate bonds and, thus, the electrode material. Examples of such reactions would include CI2, O2 and H2 evolution as well as the Kolbe reaction and electrocatalytic hydrogenation. There are many examples of syntheses where the chemistry occurs in solution and a selection will be discussed in Chapter 6. 

Formation of the oxide tetrahedron and its kinetics is common while the adsorbate-site selectivity, the order of the ionic bonds formation, and the orientation of the oxide tetrahedron vary with the bonding environment. Except for the initial stage of oxidation, the oxygen adsorbate prefers a position inside a tetrahedron. However, the scale and geometry of the lattice, and the electronegativity of the host determine the bond formation order and the site-and-orientation specificity of the tetrahedron. 

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

The most common hydrophobic adsorbents are activated carbon and siUcahte. The latter is of particular interest since the affinity for water is very low indeed the heat of adsorption is even smaller than the latent heat of vaporization (3). It seems clear that the channel stmcture of siUcahte must inhibit the hydrogen bonding between occluded water molecules, thus enhancing the hydrophobic nature of the adsorbent. As a result, siUcahte has some potential as a selective adsorbent for the separation of alcohols and other organics from dilute aqueous solutions (4). 

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