Adsorption surface intermediates

In connection with the adsorption of organic molecules at the surface of an electrode it is possible to distinguish two types (a) adsorption of undissociated molecules and (b) adsorption of intermediates formed by dissociation of the original molecule. The variation of coverage of the surface of a... 

It has been often stressed that low eoordinated atoms (defeets, steps, and kink sites) play an important role in surfaee ehemistry. The existenee of dangling bonds makes steps and kinks espeeially reaetive, favoring the adsorption of intermediate species on these sites. Moreover, smdies of single-crystal surfaces with a eomplex geometry have been demonstrated very valuable to link the gap between fundamental studies of the basal planes [Pt( 111), Pt( 100), and Pt(l 10)] and applied studies of nanoparticle eatalysts and polycrystalline materials. In this context, it is relevant to mention results obtained with adatom-modified Pt stepped surfaces, prior to discussing the effect of adatom modification on electrocatalysis. 

Weakly adsorbed molecules (A in Figure 8. lb), which can exist only under catalytic reaction conditions, play an important role in surface catalytic reactions even if the adsorption of the promoter is very weak or is undetectable at the surface. Surface intermediates (Cat-X in Figure 8. lb) under the ambient gas molecules behave in a different way from those under vacuum, showing rate enhancement and selectivity change of the surface reaction in the presence of ambient gas. 

The components of the starting mixture are in rapid adsorption-desorption interaction with the surface. For example, a part of adsorbed -hexane desorbs as -hexane another part reacts to give benzene. If benzene formation involves an n-hexene surface intermediate, this hexene—the concentration of which may be eventually so small that it does not appear in the gas phase—interacts with the inactive hexene in the starting material and increases its specific radioactivity. 

The competition for adsorption sites is very important for the kinetics of a heterogeneous catalytic reaction. For this reason sites,, are included as a reactant in the kinetic model. As a site must be either free or occupied by one of the surface intermediates, there is a conservation law for the coverages... 

The concept of electrocatalysis and its relation to chemical surface bonding of reactive intermediates is closely related to that of heterogeneous catalysis. Following the previous section, simple Gibbs energy curves can illustrate the essential ideas of how adsorption of intermediates and their associated Gibbs energy affect the rate of an inner-sphere reaction. 

V / RHE. Figure 6.32 provides a schematic illustration of the series pathway with CO as intermediate. Methanol adsorbs on Pt forming some H-containing surface intermediate H atoms are abstracted in a sequence of steps, until absorbed CO is formed. Removal of CO requires adsorption of water to yield water-based oxygenated surface species. 

Theoretical investigations indicated641 642 that two main pathways are possible for the first, dehydration step. One proceeds via a methoxy surface intermediate (10) formed as a result of the adsorption of methanol on acidic site of the zeolite (HO-Z) to give dimethyl ether [Eqs (3.63) and (3.64)]. The other, direct route is a reaction between two methanol molecules adsorbed simultaneously on the same active site [Eqs (3.65) and (3.66)] ... 

Adsorption modes and hydrogenolysis were also correlated with other metal-catalyzed reactions. Gault noticed striking similarities in product distributions of isomerization and ring opening of cycloalkanes. Kinetic and tracer studies provided useful data252 268 to arrive at the conclusion that a common surface intermediate is... 

Large pores are desirable but not always possible when high surface area is also demanded. Therefore, a compromise is necessary. This compromise is evident in the choice of XAD-8, with its lower surface area, over XAD-7 when humic acids are accumulated from environmental waters (143, 167, 319-321). The same, but more subtle, considerations apply to XAD-2 and XAD-4. The 780 m2/g for XAD-4 will be available for the adsorption of intermediate weight organic solutes only if the water is relatively free of humic material that can block many of the pores. Therefore, a situation can exist for environmental waters for which the lower surface area XAD-2 is more effective than the XAD-4 polymer. 

The scheme involves a number of unknown surface intermediates, while the appearance of product molecules in the gas phase depends on the relative rates of adsorption, desorption, formation and conversion. The calcu-... 

At present we have evidence for the complexity of higher temperature adsorption/desorption phenomena while, in general, the kinetic characteristics observed for many catalytic reactions are perhaps deceptively simple. The estimations of the concentrations of the participating surface intermediates are, in contrast, experimentally very difficult. Mechanistic investigations of many heterogeneous catalytic processes yield insufficient information to allow clear distinctions to be drawn between alternative reaction modelsf 125). 

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