Reactant adsorption sulfurization effects

Bimolecular surface reactions reactants adsorption, 29 111-112 with single reactant, 29 108-109 1,1 -Binaphthyl, dehydrocyclization, 28 318 Binary oxides, 32 119 Binding energy, 32 160-162 chemisorbed sulfur, 37 281 hydrogen, sulfur effect, 37 295-296 shift, Pd, 37 62-64 ZnO/SiOj, 37 21-22 Binor-S, see Norbomadiene Biological systems, hydrogen in, activation of, 11 301... 

In the reactions of the many component mixtures that make up most commercial feedstocks, the relationship between the two catalytic components may be further altered by preferential adsorption of certain hydrocarbon reactants on catalytic sites ( 8). For example, polycyclic aromatics have a highly variable effect dependent on the type and amount as well as catalyst and reaction conditions. Catalyst poisons such as sulfur, nitrogen, and oxygen may affect either or both catalyst components ( ). 

An effect due to lateral interactions between adsorbed sulfur and a coadsorbed molecule. Such interactions can enhance or reduce the adsorption free energy of the reactant under consideration. 

Because sulfur adsorbs very strongly on metals and prevents or modifies the further adsorption of reactant molecules, its presence on a catalyst surface usually effects substantial or complete loss of activity in many important reactions, particularly in hydrogenation reactions. Where the reaction network leads to two or more products, adsorbed sulfur can markedly affect the selectivity by reducing the rate of one of the reactions more than the other(s). In a few reaction systems these changes in selectivity are desirable however, in many others they are not. 

From previous studies and the qualitative nature of the rate data a likely combination appeared to be a controlling surface reaction between adsorbed atomic oxygen and unadsorbed sulfur dioxide. In order to determine ail the constants in the rate equation for this mechanism, it is necessary to vary each partial pressure independently in the experimental work. Thus measuring the rate of reaction at different total pressures but at constant composition is not sufficient to determine all the adsorption equilibrium constants. Similarly, if the data are obtained at constant composition of initial reactants but varying conversions, the partial pressures of the individual components do not vary independently. However, in these cases it is possible to verify the validity of the rate equation even though values of the separate adsorption equilibrium constants cannot be ascertained. Olson and Schuler studied the effect of conversion alone and obtained the data in Table 9-1 at 480°C. 

SO2 in absence of CO or Oj in the gas phase, the Pt surface is covered by sulfur little CO chemisorption is observed right after resuming the reactant s flow. As the temperature is increased, CO removes part of the sulfur from the Pt surface, and CO adsorption reaches levels similar to those previously observed on the equilibrated catalysts. This effect is very similar to the effect of Cl presented in Section 17.1, suggesting a common pathway to all these catalyst finishing effects. 

Figure 26. Cluster modeling results for the oxidation of galena by Fe suggesting the importance of proximity effects to surface reactivity on galena, modified from Becker et al. (2001). The adsorption of an Fe ion to one side of the cluster results in a coupled exchange of electron density from comer sulfur sites to the ferric ion and spin density (indicated by vertical arrows) from the ferric ion to the comer sulfur sites (a). This causes the equilibrium position of a hydroxyl on the opposite side of the cluster to migrate towards tlK nearest spin polarized comer site (b). The calculations demonstrate that delocalized orbitals typical of semiconducting minerals can couple reactants spatially separated by several bond lengths in the substrate.

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