Site blocking effect

The rules are simple as long as we also keep site-blocking effects in mind and as long as we can identify the electron acceptor or electron donor character of the various adsorbates. 

Figure 2. The site blocking effects of the Na promoter on the Ru/Si02 catalysts...

The site blocking effects of alkali promoters as monitored via microcalorimetry indicated that K promoters significantly reduced the adsorbed hydrogen amounts and eliminated the intermediate (50 kJ/mol) and weakly (10 kJ/mol) bound states [13]. The authors concluded that K promoter selectively populated the defect like sites where the weakly bound hydrogen states are present. 

The site blocking effect of tin is slightly higher in (O) type catalysts ... 

There is an additional effect of the potassium promoter to be discussed. It was found that its effect is more pronounced at higher reaction rates, that is, higher stationary concentrations of adsorbed ammonia. Adsorbed NH3 blocks sites for the adsorption of the reactants. Its dipole moment has, however, the opposite sign of that of a-N2, so the presence of K will lower its adsorption energy and hence reduce its site-blocking effect [27], as illustrated in Fig. 6.6b. 

Diffusional constants may depend strongly on micropore filling. This, in essence, is due to site blocking effects. It explains the often observed relationship between overall experimentally measured diffusional rate constant activation energies and heats of adsorption. 

Sulfur adsorbed (S ds) on metal surfaces blocks or delays the formation of the passive film by blocking the adsorption sites for OH, which are the precursors of passive oxide film formation. This site blocking effect is represented in Fig. 3-22. 

Fig. 27. The site-blocking effect of sulfur on deuterium adsorption on Mo(lOO) as determined by deuterium thermal desorption. (0) = sulfur layer disordered, (x) = sulfur layer ordered. The broken line is a theoretical prediction of the site-blocking effect assuming that one sulfur atom blocks one deuterium atom adsorption site and that deuterium molecules chemisorb dissociatively in adjacent, unoccupied sites...

A large molecule, e.g., a hydrocarbon, may attach to a metal surface with several bonds. This is schematically illustrated in Figure 5.21. In this figure dissociative adsorption of pentane occurs by attachment of two of its carbon atoms to the metal surface. This mode of adsorption becomes suppressed when sulfur or carbon atoms are coadsorbed. The S or C atoms decrease the probability for a surface to have neighboring-reactive sites. As a result, the probability for pentane to form multiple bonds with the metal surface becomes suppressed. It will only adsorb with one of its carbon atoms attached to the surface. As a result of this site blocking effect, the heat of adsorption of pentane decreases and the probability for dissociative adsorption decreases as well. 

---