Surface structure conversion activity

Correlation Between Surface Structure and CO Conversion Activity... 

Before any attempt to establish a correlation between the surface structure of the oxidized alloys and their CO conversion activity one must stress that the surface composition of the samples under reaction conditions may not necessarily be Identical to that determined from ESCA data. Moreover, surface nickel content estimates from ESCA relative Intensity measurements are at best seml-quantlta-tlve. This can be readily rationalized If one takes Into consideration ESCA finite escape depth, the dependence of ESCA Intensity ratio... 

Quantum-chemical cluster models, 34 131-202 computer programs, 34 134 methods, 34 135-138 for chemisorption, 34 135 the local approach, 34 132 molecular orbital methods, 34 135 for surface structures, 34 135 valence bond method, 34 135 Quantum chemistry, heat of chemisorption determination, 37 151-154 Quantum conversion, in chloroplasts, 14 1 Quantum mechanical simulations bond activation, 42 2, 84—107 Quasi-elastic neutron scattering benzene... 

Regardless of the exact mechanism at work, HCl catalyst pretreatment have been demonstrated to enhance the photocatalytic oxidation of toluene at low concentrations [68,69]. The apparent deactivation of the photocatalyst is noticeably delayed over HCl-pretreated catalyst samples in a manner similar to that seen with cofed toluene and TCE (Fig. 13). However, the pseudo-steady-state level of conversion appears to be nearly identical on both untreated and HCl-pretreated catalysts. Because the batch HCl pretreatment process incorporates a limited quantity of HCl into the catalyst surface structure, this similarity in longterm activity may be the result of surface chlorine depletion. 

In these case studies, in addition to a brief discussion of the catalytic applications, representative reactions are discussed with the aim of illustrating in detail the relationships between surface structures (as inferred from investigations with probe molecules) and catalytic activity. The following topics are discussed in detail (i) MgO as a model catalyst for base-catalyzed reactions (ii) the mechanism of ethene hydrogenation on ZnO (iii) Cu20 as an oxidation catalyst for the conversion of methanol to formaldehyde, with... 

The comprehensive investigation of the interactions of simple alcohols (methanol, ethanol, propan-l-ol, and butan-l-ol) with Fe203 powders (514) by a combination of surface analytical techniques and conversion measurements under high vacuum and at atmospheric pressure is an example of the attempts to establish correlations between surface structure and catalytic activity. IR and XPS data showed that methanol is chemisorbed mainly disso-ciatively, giving formate species, whereas molecular chemisorption prevails for higher alcohols, which form hydrocarbons as the major products. 

Another method to improve the structural order of CMs is the conversion of the precursors to fibers prior to the pyrolysis step [377]. The precursor polymer may be stretched in addition. Carbon fibers are manufactured in large quantities as reinforcements in composite materials, after Bowen [403] and Fitzer [404]. Surface and bulk activation can be accomplished by anodic oxidation in dilute aqueous electrolytes (cf. Besenhard et al. [405, 406]). But carbon fibers with various degrees of graphitization have also been employed recently in rechargeable batteries [407-411] and in electrochemical double layer capacitors [18, 412-416]. This takes advantage of two fiber specific effects, namely... 

When water is present in the gas stream, it reacts with the SO, and O2 to produce sulfuric acid on the carbon surface, and can subsequently desorb. The overall SO adsorption capacity is enhanced due to its solubility in the water film that forms on the carbon surface. Conversely, active sites for SO2 capture are simultaneously reduced by water coverage. In general, the SO2 adsorption characteristics of an activated carbon are dependent upon its physical form, the pore structure, the surface area, and the surface chemistry. Similarly, both temperature and contact time also affect the efficiency of the process. The temperature for practical application is usually between ambient and 200°C, with ambient to 50°C being favored due to the decreasing solubility of SO2 in water at higher temperatures. 

Bimetallic Pt-Sn catalysts are useful commercially, e.g., for hydrocarbon conversion reactions. In many catalysts, Pt-Sn alloys are formed and play an important role in the catalysis. This is particularly true in recent reports of highly selective oxidative dehydrogenation of alkanes [37]. In addition, Pt-Sn alloys have been investigated as electrocatalysts for fuel cells and may have applications as gas sensors. Characterization of the composition and geometric structure of single-crystal Pt-Sn alloy surfaces is important for developing improved correlations of structure with activity and/or selectivity of Pt-Sn catalysts and electrocatalysts. 

A number of fundamental studies explore catalyst activity at an atomistic scale. DFT calculations can reveal how the rates of surface processes depend on the local electronic structure of surface atoms [233-238]. Monte Carlo simulations and mean-field approaches can incorporate this information in order to rationalize the effects of nanoparticle sizes and surface structure on the overall rates of current conversion [233, 239], Thereby the nontrivial dependence of reactivity on particle size could be explained. 

For the characteristic differences between the low and high activity states we suggest the following explanations. When the reaction starts in the low activity state, palladium is in metallic form and hence, the catalyst must be trained to develop a surface structure characteristic of high activity state in the total conversion of m-xylene. In this state the combustion occurs in a typical heterogeneous oxidation, so the conversion increases with temperature in an exponential manner. 

Structure Sensitivity of Hydrocarbon Conversion Reactions on Platinum Surfaces How does the reaction rate depend on the atomic structure of the platinum catalyst surface To answer this question, reaction rate studies using flat, stepped, and kinked single-crystal surfaces with variable surface structure were very useful indeed. For the important aromatization reactions of n-hexane to benzene and Ai-heptane to toluene, it was discovered that the hexagonal platinum surface where each surface atom is surrounded by six nearest neighbors is three to seven times more active than the platinum surface with the square unit cell [155, 156]. Aromatization reaction rates increase further on stepped and kinked platinum surfaces. Maximum aromatization activity is achieved on stepped surfaces with terraces about five atoms wide with hexagonal orientation, as indicated by reaction rate studies over more than 10 different crystal surfaces with varied terrace orientation and step and kink concentrations (Figure 7.38). 

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