Dissociation reactions, oxide surface

Elementary steps. The reactivity of the anchored complexes has been studied in only a few cases. Evidence for a few elementary steps on surfaces has been obtained, including ligand association and dissociation reactions, oxidative addition, and insertion. Understanding of these will be a prerequisite to the fundamental understanding of the elementary steps of catalysis. At this point, these can at best be inferred by analogy to molecular chemistry in solution. Therefore, a systematic approach to the chemistry of these elementary steps on surfaces is recommended. 

Numerous quantum mechanic calculations have been carried out to better understand the bonding of nitrogen oxide on transition metal surfaces. For instance, the group of Sautet et al have reported a comparative density-functional theory (DFT) study of the chemisorption and dissociation of NO molecules on the close-packed (111), the more open (100), and the stepped (511) surfaces of palladium and rhodium to estimate both energetics and kinetics of the reaction pathways [75], The structure sensitivity of the adsorption was found to correlate well with catalytic activity, as estimated from the calculated dissociation rate constants at 300 K. The latter were found to agree with numerous experimental observations, with (111) facets rather inactive towards NO dissociation and stepped surfaces far more active, and to follow the sequence Rh(100) > terraces in Rh(511) > steps in Rh(511) > steps in Pd(511) > Rh(lll) > Pd(100) > terraces in Pd (511) > Pd (111). The effect of the steps on activity was found to be clearly favorable on the Pd(511) surface but unfavorable on the Rh(511) surface, perhaps explaining the difference in activity between the two metals. The influence of... 

For a precipitated iron catalyst, several authors propose that the WGS reaction occurs on an iron oxide (magnetite) surface,1213 and there are also some reports that the FT reaction occurs on a carbide surface.14 There seems to be a general consensus that the FT and WGS reactions occur on different active sites,13 and some strong evidence indicates that iron carbide is active for the FT reaction and that an iron oxide is active for the WGS reaction,15 and this is the process we propose in this report. The most widely accepted mechanism for the FT reaction is surface polymerization on a carbide surface by CH2 insertion.16 The most widely accepted mechanism for the WGS reaction is the direct oxidation of CO with surface 0 (from water dissociation).17 Analysis done on a precipitated iron catalyst using bulk characterization techniques always shows iron oxides and iron carbides, and the question of whether there can be a sensible correlation made between the bulk composition and activity or selectivity is still a contentious issue.18... 

In a typical inorganic oxide, the oxide surface acquires a charge by the dissociation or adsorption of potential determining ions at specific amphoteric surface groups or sites. As a consequence the equation of state of such surfaces will involve parameters that characterize surface reactions. In addition, one may also allow for the adsorption of anions and cations of the supporting electrolyte. However, in this paper we shall ignore this possibility to keep the discussion clear. Such embellishments of the model of the surface do not alter the key ideas presented here. 

Charge on the oxide surface is established by dissociation (ionization) of the surface hydroxyl groups. The situation corresponds to adsorption or desorption of protons depending on the pH of the solution (Fig. 10.4). These reactions can be trea-... 

Charging Hydroxylated Surfaces Definition of IEP(s). A hydroxyl-ated surface should be expected on all oxidic materials which have had a chance to come to equilibrium with an aqueous environment. Charge can develop on a hydroxylated surface through amphoteric dissociation of the surface hydroxide groups. Dissociation reactions can be written as follows, where underscored symbols refer to species forming part of the surface. Symbols not underscored refer to species assumed aqueous unless otherwise specified. 

Hydroxyl groups on the surface of oxide in aqueous solutions of the electrolyte have an amphoteric character and undergo addition and dissociation reactions of proton according to the following ... 

When oxidic surfaces become exposed to water, basic hydroxyls, as well as Bransted acidic protons, are generated upon dissociation of the water. This also occurs on the surfaces of basic oxides. There is an abundance of infrared spectroscopic information confirming the appearance of different OH groups by adsorption of HO on oxidic surfaces. Catalytic reactions induced by Bransted acidic sites take place on such surfaces. The acidic proton is located on bridging oxygen sites. 

Even if, except at defect sites[58, 59], the hydrogen does not easily dissociate on the surface oxides[l, 60], distributions of hydrogen atoms are involved in surface reactions (for instance the decomposition of the formic acid) and have motivated both experimental and theoretical works. The decomposition is made at high temperature on activated MgO powders[61, 62], On a non reducible oxides, the heterolytic... 

The oxide surface becomes reduced in this process if this were a step in a catalytic reaction, some other O-containing species would have to donate an O atom back to the substrate in order for the reaction to continue. Like the other types of reaction, 0-transfer adsorption can be either molecular or dissociative. 

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