Acceptors adsorption

If acceptor adsorption particles with concentrations Nta possessing a value of affinity to electron Eta appear on the surface of adsorbent characterized by pre-adsorption band bending (1.49), then the condition of their transition into the charged form is provided by inequality... 

Fig. 1.9. The dependence of barrier height caused by complete occupation of ASS as a function of the initial height, a - acceptor adsorption, a = a(Eg -S)/kT b donor adsorption, 0 - 0(,Eg -S)/kT. Curve / does...

It was shown in [98] that during acceptor adsorption in a broad band semiconductor of -type characterized by availability of an a priori surface-adjacent depletion zone developing depletion of BSS levels slows... 

As for equilibrium values of as and P they are mainly dependent on relations between such parameters of the systems as initial electric conductivity of adsorbent, concentration of chemisorbed particles, reciprocal position of the energy levels of absorbate and adsorbent. Thus, during acceptor adsorption in case of small concentration of adsorption particles one can use (1.82) and (1.84) to arrive to expressions for equilibrium values of ohmic electric conductivity and the tangent of inclination angle of VAC ... 

In case of plausible BSS recharging the equilibrium values of a and P caused by acceptor adsorption can be described by the following expressions... 

The energetics of the reaction (reaction 21) for the case of the acceptor adsorption (see Table 2) are not favorable (i.e., AE > 0). Because this adsorption is precisely the type of adsorption onto siloxane bonds (see Fig. 15, below), the unfavorable AE confirms the hydrophobic nature of the dehydroxylated dis-iloxane-rich surface of silica. 

Figure 12. (a) f ully optimized 3-21G acceptor adsorption complex geometry, (b) Fully optimized M P2/6-31G acceptor adsorption complex geometry. 

Figures 14 and 15 show the analogous results for a 6-31G full optimization calculation on disiloxane and the acceptor adsorption of water onto the disiloxane bridge. In agreement with our localization hypothesis, the SiO and OH bond distances in Figure 15 agree closely with the corresponding distance in Figure 126.

Figure 15. Fully optimized 6-31G acceptor adsorption of water onto disiloxane.

Many solids have foreign atoms or molecular groupings on their surfaces that are so tightly held that they do not really enter into adsorption-desorption equilibrium and so can be regarded as part of the surface structure. The partial surface oxidation of carbon blacks has been mentioned as having an important influence on their adsorptive behavior (Section X-3A) depending on conditions, the oxidized surface may be acidic or basic (see Ref. 61), and the surface pattern of the carbon rings may be affected [62]. As one other example, the chemical nature of the acidic sites of silica-alumina catalysts has been a subject of much discussion. The main question has been whether the sites represented Brpnsted (proton donor) or Lewis (electron-acceptor) acids. Hall... 

Silica gel and aluminium oxide layers are highly active stationary phases with large surface areas which can, for example, — on heating — directly dehydrate, degrade and, in the presence of oxygen, oxidize substances in the layer This effect is brought about by acidic silanol groups [93] or is based on the adsorption forces (proton acceptor or donor effects, dipole interactions etc) The traces of iron in the adsorbent can also catalyze some reactions In the case of testosterone and other d -3-ketosteroids stable and quantifiable fluorescent products are formed on layers of basic aluminium oxide [176,195]... 

Increasing catalyst surface work function causes an increase in the heat of adsorption (thus chemisorptive bond strength) of electropositive (electron donor) adsorbates and a decrease in the heat of adsorption (thus chemisorptive bond strength) of electronegative (electron acceptor) adsorbates. 

Alkali promoters are often used for altering the catalytic activity and selectivity in Fischer-Tropsch synthesis and the water-gas shift reaction, where C02 adsorption plays a significant role. Numerous studies have investigated the effect of alkalis on C02 adsorption and dissociation on Cu, Fe, Rh, Pd, A1 and Ag6,52 As expected, C02 always behaves as an electron acceptor. 

The effect of alkali addition on the adsorption of NO on metal surfaces is of great importance due to the need of development of efficient catalysts for NO reduction in stationary and automotive exhaust systems. Similar to CO, NO always behaves as an electron acceptor in presence of alkalis. 

On K modified Ni(100) and Ni(lll)62,63 and Pt(lll)64 the dissociative adsorption of hydrogen is almost completely inhibited for potassium coverages above 0.1. This would imply that H behaves as an electron donor. On the other hand the peaks of the hydrogen TPD spectra shift to higher temperatures with increasing alkali coverage, as shown in Fig. 2.22a for K/Ni(lll), which would imply an electron acceptor behaviour for the chemisorbed H. Furthermore, as deduced from analysis of the TPD spectra, both the pre-exponential factor and the activation energy for desorption... 

It is worth remembering the complementary rule already demonstrated in Figure 2.19 regarding NO adsorption on Pt(lll) at high NO coverages where, on a clean surface NO behaves as a weak electron donor but shifts to a strong electron acceptor in presence of K ... 

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