Coadsorbed molecules

The surface composition and the availability of certain adsorption sites are not the only factors that determine how CO binds to the surface. Interactions between CO and coadsorbed molecules play an important part as well. A RAIRS study by Raval... 

REGULATION OF CATALYSIS BY COADSORBED MOLECULES 8.2.1 Self-Assisted Dehydrogenation of Ethanol on an Nb/Si02 Catalyst... 

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. 

Sulfur is known to block the reactive sites or compete for them with a reactant the catalytic path may be further altered by chemical interaction with coadsorbed molecules of reactants. 

Interaction between sulfur and coadsorbed molecules was studied by Bonzel and Ku who showed that sulfur present on a palladium catalyst inhibits the oxyda-tion of CO to CO2 by combining with adsorbed oxygen with successive formation of SO and SO2. 

Furtak and Macomber, the composition of the solution affected the slope of the maximum frequency/electric potential curves. Higher chloride concentrations resulted in higher slopes (though, perhaps still within the experimental reproducibility) and fluoride exhibited smaller slopes (1.46 eVV ). Furtak and Roy have shown that for coadsorbed chloride and pyridine, or chloride and thiocyanate, each molecule shows its own excitation potential dependence. Another interesting point was that the excitation maximum of one of the molecules was reflected in the excitation profile of the other coadsorbed molecule. 

De Feyter, S. et al., Expression of chirality by achiral coadsorbed molecules in chiral monolayers observed by STM, Angew. Chem. Int. Ed. 37, 1223-1226, 1998. 

Higher coverages of CO lead to repulsive interactions between the coadsorbed molecules. These higher coverages (a) lower the average heat of adsorption (as shown in Figure 3.20) and (b) push the CO molecules into new adsorption sites (as shown in Figure 2.31) to maximize the distance between them. When CO is coadsorbed... 

Alkali metals are often used as additives during catalytic reactions. They are bonding modifiers that is, they influence the bonding and thus the reactivity of the coadsorbed molecules. Potassium is a promoter in CO hydrogenation reactions where CO dissociation is desired and is one of the elementary reaction steps. The alkali metal also reduces the hydrogen chemisorption capacity of the transition metal. Potassium is a promoter in ammonia synthesis for the opposite reason, because it weakens the NH3 product molecule bonding to the metal, thereby reducing its sur-... 

Figure 17 illustrates the use of this concept for the interpretation of the influence of co-adsorbed, less mobile molecules (benzene, ethylene) on the mobility of a highly mobile species (methane) in zeolite Na-Y. It turns out that the reduction in the methane mobility with an increasing amount of coadsorbed molecules [159] may be satisfactorily explained by the assumption that any co-adsorbed molecule reduces the transition rate through a particu-... 

Xe NMR of adsorbed xenon has proved to be a powerful tool to study the physicochemical properties of zeolites [1]. The highly polarisable Xe atom is very sensitive to its environment, i.e. to the interactions with other chemical species, in particular to the nature and the concentration of coadsorbed molecules. So this technique can be used for the determination of the diffusion coefficients of gases in solids [2]. We have also shown that H NMR imaging can be used for such types of studies [3]. 

Xe NMR spectroscopy of adsorbed xenon, largely used to investigate static properties of porous solids, appears very useful to study the diffusion of coadsorbed molecules when the local concentration of these molecules changes as for example during the adsorption process. Coefficient of intracrystalline molecular transport can be obtained from the simulation of the NMR spectra using the solutions (adsorbate concentration profiles) of the diffusion equations. 

Coadsorption suppresses surface reactions that require sites consisting of several metal atoms by blocking. The close proximity of coadsorbate molecules may lead to steric hindrance effects, reducing translational or rotational motion. Short-range lateral effects may enhance as well as decrease the adsorbate-surface interaction strength. Enantiomeric selectivity as well as selective oxidation are examples of reactions that are sensitive to the composition of the surface adlayer. In the former case the selectivity is enhanced due to steric interactions. In the latter case the chemical nature of adsorbed surface atoms changes with surface adatom concentration. 

When a ketone is hydrogenated by a transition-metal catalyst, covered by one of the optically active enantiomers of a coadsorbate molecule (an experimental example is the adsorption of one of the enantiomeric forms of tartaric acid to nickel), the interaction between the coadsorbed optically active enantiomer and keton results in the preferential formation of one of the possible enantiomers of the alcohol. The coadsorbate plays a very similar role as a bulky ligand of a homogeneous catalyst, which influences the selectivity of a reaction by direct steric interactions. Coadsorbates may also alter the interaction of adsorbates by changing the interaction energies. 

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