Adsorption sites, for hydrogen

The difference between TOFchem and TOFitk for both structure-insensitive and structure-sensitive reactions, without a doubt, reflects the difference in the concentration of adsorption sites (for hydrogen or CO) and that of reaction sites occupied by the most active intermediates. The larger difference in the TOF s for structure-sensitive reactions is an outcome of ensemble size and geometric arrangement required for active reaction sites. 

The second explanation considered that the existence of chlorines can promote the adsorption of H2 (one chlorine can form about six adsorption sites for hydrogen), thus inhibits the activation of N2. For example, Lu studied and compared the conditions of H2 active adsorption on two samples of Ru/ABOs catalysts with RuCls and Ru3(CO)i2 as the precursors, respectively. The results have shown that H2 chemisorption on the sample with RuCls as the precursor is very active, but is not so obviously on the sample without chlorination. This proved that the existence of the CR can accelerate the adsorption of H2. It was proposed in the succeeding reports that due to the electronegativity, the chlorines are adsorbed on the surface sites of the high valence state and prevent the near surface site of low valence state... 

Metal sites in MOFs can serve as adsorption sites for hydrogen. Exposure of unsaturated metal centers in MOFs will increase the affinity for H2 through strong metal-H2 interaction. Coordinatively unsaturated metals into MOFs can be induced by various approaches... 

The palladium clusters and islands, which we examined in Section 1.5, show another important property of a good catalyst They have various adsorption sites for hydrogen with different adsorption energies, so that the reaction can pass through those sites whose energy is optimal in the sense of Sabatier s principle. They are therefore more active than a monolayer of Pd on Ah(lll), which offers only two sites, neither of which has the optimum energy. 

Deviations from this simple expression have been attributed to mechanistic complexity For example, detailed kinetic studies have evaluated the relative importance of the Langmuir-Hinshelwood mechanism in which the reaction is proposed to occur entirely on the surface with adsorbed species and the Eley-Rideal route in which the reaction proceeds via collision of a dissolved reactant with surface-bound intermediates 5 . Such kinetic descriptions allow for the delineation of the nature of the adsorption sites. For example, trichloroethylene is thought to adsorb at Ti sites by a pi interaction, whereas dichloroacetaldehyde, an intermediate proposed in the photo-catalyzed decomposition of trichloroethylene, has been suggested to be dissociatively chemisorbed by attachment of the alpha-hydrogen to a surface site... 

A number of contradictory views have been published concerning the structure of adsorbed alcohols and the nature of adsorption sites (for review see ref. 69). Experimental evidence from IR investigations has shown that, on alumina, alcohols form several surface complexes of very different chemical natures (e.g. refs. 31, 32, 117, 133—137) (i) alcohol molecules weakly bonded to the surface, very probably by hydrogen bonds (I) (such complexes are sometimes denoted as physically sorbed alcohols) (ii) surface alkoxides (alcoholates) (II) (iii) surface carboxy-lates (III). Less certain is the existence of species with partial double bonds or of ketone-like species. The formation of the various surface complexes is dependent on the structure of the alcohol. For examples, weakly bonded species (I) have been found with all alcohols, alkoxides (II) mostly with primary alcohols, sometimes also with secondary alcohols, but have never been reported for tertiary alcohols. 

These silanols are preferential adsorption sites for different molecules. As a result, surface hydroxyls are, indeed, particularly reactive with H20 and other polar molecules, such as NH3, which is likely to be physically adsorbed to form a multiple hydrogen-bonded layer [16]. 

Semiconductors are considered to be catalytic particles that contributed to the development of primitive metabolism. According to Wahtershauser life could have developed on the surface of iron sulphide minerals (eg mackinawite or pyrrhotite [FeS] or pyrite [FeS2]) [8], The chemoautotrophy theory is based on the reaction between iron sulphide and hydrogen sulphide, which acts as a reducing agent, whereas iron sulphide provides adsorption sites for substrates and acts as a catalyst (equation 10.1) ... 

There should not be independent adsorption sites for each reactant they should adsorb competitively, carbon monoxide much more strongly than hydrogen. 

The question as to what is the active site of Cu-based catalysts in MSR is still unclear and debated in the literature. Similar to the methanol synthesis reaction, either metallic Cu° sites, oxidized Cu+ sites dispersed on the oxide component or at the Cu-oxide interface, or a combination of both kinds of sites are thought to contribute to the active ensembles at the Cu surface. Furthermore, the oxidic surface of the refractory component may take part in the catalytic reaction and provide adsorption sites for the oxygenate-bonded species [126], whereas hydrogen is probably adsorbed at the metallic Cu surface. Similar to methanol synthesis, factors intrinsic to the Cu phase also contribute to the MSR activity in addition to SACu- There are two major views discussed in the literature relating these intrinsic factors either to the variable oxidation state of Cu, in particular to the in situ adjustment of the Cu°/Cu+ ratio at the catalyst s surface [102, 107, 127 132], or to the defect structure and varying... 

After calculating stable adsorption sites for each adsorbate low energy reaction pathways were determined. First we consider the hydrogenation reactions on Pt(lll), following this the C+0 and C+N reactions on Pt(lll) and the C+H reaction on Cu(lll). 

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