Reactions with Oxide Surfaces

The reaction between the surface OH groups and SnCl4 leads to similar surface species (8 in Fig. 7.8).227 

Silica is known to have covalent Si—O bonds in which silicon always assumes a tetracoordinated structure. The OH groups on the silica surface are believed to occur in different forms (9-11 in Fig. 7.9). 

Spectroscopic data show that Sn remains tetracoordinated on the surface and one H atom of the terminal methyl is bonded to a surface OH group by hydrogen bonding (12 in Fig. 7.10). 

The same surface species is obtained at ambient temperature by the reaction of Bu3SnH and the silanol groups, suggesting that the Sn-H bond is more reactive in this case than the Sn-C bond. The surface reaction depends upon the degree of dehydroxylation of the surface of silica. On silica dehydroxylated at 500°C the reaction leads to one well-defined surface complex. On the other hand, on silica dehydroxylated at 200°C, the evolution of alkane is continuous. The difference in the latter case is related to the presence of neighboring OH groups, because the number of the surface vicinal OH groups capable of 

Alumina is known to have more ionic character and its surface has a more complex structure than that of silica. Reaction of Bu3SnH with the surface of partially dehydroxylated aluminas was followed and it was found that the extreme sensitivity of tin chemical shifts to the molecular environment constitutes a method whereby surface organometallic complexes of tin can be used as molecular probes for determining surface structures of oxides.248 

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MECHANISMS OF ORGANIC COMPOUND REACTIONS WITH OXIDES SURFACE SITES... 

The emphasis in examining sorption with spectroscopic techniques has been mostly related to reaction with oxide surfaces, primarily Fe and Al. In addition to the simplicity of the systems, these are regarded as the primary adsorbent minerals for oxyanions in natural systems. 

SO2 0-0.5 ppmv (urban) 20-200 pptv (remote) Oxidation of fossil fuel S Oxidation of S gases, volcanoes Direct reaction with earth surface, oxidation to sulfate... 

The reaction of tetraalkyltin complexes with oxide surfaces was studied244,245 but no description at the molecular level has been reported. The low-temperature reactivity of tetraalkyltin (SnR4, where R=Me, Et, i-Pr, Bu) complexes toward the surface of silica was studied in detail.246 At room temperature, the complex is physisorbed. Above 100°C, the adsorbed molecules react with the OH groups and the evolution of alkanes is observed (Scheme 7.15). 

Nozaki, Matsukawa, and Mano (81) suggested for Example 31 that the rate-determining step is CO reaction with a surface that has been oxidized by NO thus. Step 1 or 6 for CO can provide a lower limit (since the reaction is 0.4 order in NO) for logL. But logL = 18 is rather large for a lower limit, and other possibilities may have to be considered. Dissociative adsorption of NO (Step 2 for NO) could be the rate-determining step. Example 32 is for the same reaction, but using a different catalyst and the log L calculations are similar. 

Photo-induced oxidation reactions on oxide surfaces have been discussed in reviews by Bickley (206, 405) and by Formenti and Teichner (406). It is characteristic of these reactions that the energy of the irradiating light needs to be not less than that of the absorption band of the oxide. Under these conditions both electrons and holes are produced, which can then react with molecules adsorbed on the surface. Since in some cases the surface lattice ions can absorb light at an energy less than the bulk oxide (7), photo-oxidation may be observed at lower wavelengths than expected from the bulk absorption band. 

All the above species have been detected in various quantities at oxide surfaces. The discussion of this example serves mainly to show that catalytic reactions at oxide surfaces are very complex. This is a mixed blessing from the sensing point of view. It provides a broad spectrum of reactions that could be used. On the other hand, it can lead to great variation in the results obtained with only slightly different sensors. Another drawback of such a complex and diverse mechanism is the relatively slow time response which, in most cases, is limited by the rates of the chemical reactions (Fig. 8.10). Naturally, one tendency of the current research in this field is to increase the selectivity of the surface reactions by introducing additional catalytic control, for example, by incorporation of catalytic metals, metal clusters, and other surface modifiers. 

Table 1(b) on the formation or removal in vacua of carbon monoxide by reaction of surface oxides with carbon in the metal shows the results of these calculations. The reactions are feasible thermodynamically in vacua of the order of 10-10 atm. at temperatures of 600°C. or higher for the metals tungsten, chromium, and iron. Thus, carbon monoxide will be formed by the diffusion of carbon to the surface and subsequent reaction with the surface oxides. This reaction has been discussed for the case of steels by Holm (11). The effect of carbon content on the reaction is not shown in the table. However, the effect can be seen from the expression for the equilibrium constant K for the reaction of ferrous oxide with carbon in the iron ... 

In this section the reactivity of the adsorbed O ion is discussed for some elementary reactions on oxide surfaces for which there is direct spectroscopic evidence of the participation of the ion. Evidence from the gas phase shows that free O" ion reacts with H2, the lower hydrocarbons, and also with oxygen both associative and hydrogen abstraction reactions are known to occur (99, 100). 

Owing to fast competitive adsorption/desorption equilibria, the oxidation products intervene with oxidative surface reactions and thus are further oxidized, ultimately leading to complete mineralization. The Langmuir-Hinshelwood kinetic... 

H2O2, and NH4OH. This suggests that a high copper removal rate requires not only the mechanical reaction but also the chemical reaction. Abrasives and chemicals in the slurry solutions continuously accelerate oxidation, etching, and abrasion of copper surface by chemical and mechanical effects. Although the lowest friction force was observed in the alumina based slurry with the addition of citric acid, the highest removal rate of copper was observed in this slurry due to the chemical reaction with copper surface. The smallest adhesion force resulted in the lowest friction force in the alumina-based slurry with the addition of citric acid. 

Surface Acidity/Basicity. - Van Veen and co-workers were one of the first groups to discuss the importance of surface acidity/basicity as it relates to the interactions that favor decorating metal oxide surfaces with certain metal complexes. We alluded to the acidity/basicity of the surface vide supra) in describing how a metal complex may become subject to a reaction with the surface. Van Veen, et al. were interested in decorating surfaces with a wide range of metal acetylacetonate complexes to include Pt and Pd precursors. They showed that metal complexes that were susceptible to attack by acids were likely to decorate the surface of an acidic surface oxide by loss of a ligand to form a firm attachment with the surface. 

In a review article on oscillatory reactions (294), Sheintuch discusses the effect of introducing a heat balance for the catalyst rather than a mass balance for the reactor into the differential equation system for a surface reaction with oxidation/reduction cycles. Although the coverage equations alone can yield oscillatory behavior, as was the case for the models discussed in the previous section, Sheintuch s model is discussed in this section because introduction of the heat balance adds qualitatively new features. In this extended system complex, multiple peak behavior and quasiperiodicity was observed as shown in Fig. 8. Sheintuch also investigated the interaction of two oscillators. This work, however, will be treated in detail in Section V, were synchronization and chaos are discussed. 

If the Earth s ocean surface were to be kept hot for a sustained period, things would be very different. A hot surface would probably mean that water vapour was more abundant in the high atmosphere the stratosphere would be moist. Without an efficient cold trap, the oceans would imperceptibly be lost as water reached the higher levels of the atmosphere, was broken by photolysis, and produced H. Any H at the top of the atmosphere is vulnerable to ejection to space, leaving matching oxidant that will eventually find its way into reaction with a surface rock. 

Alkali metals can occur in the atomic state in the vapour phase and they show a very high activity towards all electron acceptors. The introduction of alkali metals on to oxide surfaces involves their reaction with all surface electron acceptor centres. Such acceptor centres are anionic vacancies, the holes trapped on oxygen anions near the cationic vacancies, and surface hydroxyl groups. Oxide surfaces possessing these defects can react with alkali metal in accordance with equations (l)-(4). 

The reactions with the surface of oxide semiconductors encompass all the foregoing, except dissociation, together with the formation of adsorbed species such as C02, CO3 and HCOs ", and reduction of the lattice. 

These reactions generate anion and cation exchange capacity on an iron oxide in this example. X and symbolize the anion and cation of the acid and base, where MX is an indifferent electrolyte. The chemical identity of the acid and base is lost upon reaction with the surface, but the exact chemical nature of the surface-bound ions need not be known to describe thermodynamically the equilibrium state of the oxide-electrolyte system. 

The aim of this paper is to provide a correlation between the catalytic pattern of differently loaded silica supported M0O3 and V2O5 catalysts in MPO and POD reactions with their surface and redox features in order to highlight the nature of the active surface species in the selective oxidation of light alkanes. 

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