Adsorption centers

A detailed description is given to the role of point defects available in the volume and on the surface of oxide adsorbents on adsorption-induced change of electrophysical characteristics. We try to deduce the impact of the proper nature of adsorbent as well as the nature of adsorption centers. 

The effect of the nature of adsorbent on adsorption-caused change in its electrophysical characteristics. The nature of adsorption centers and their effect on the process of charging of the surface... 

Adopting the current standpoint that the process of chemisorption can be treated as chemical reaction of adsorption particle with adsorption center accounting for effect of both the reaction on the whole adsorbent and adsorbent on the reaction proper, i.e. accounting for the... 

The examples provided indicate the importance of accounting for local mechanism of chemisorption, i.e. accounting for local binding of adsorbent on certain adsorption centers during which the effect of the band bending in solid state can be consider as a weak perturbation. 

In conjunction with latest progress in quantum chemistry the availability of vast experimental data makes it possible to anal)rze the character of possible centers of adsorption of particles of various gases as well as type, chemical and electron properties of surface compounds formed during interaction of adsorption particles with adsorption centers. 

Considering chemisorption as chemical interaction, in our case interaction of oxygen with adsorption centers which are modeled by surface-adjacent superstoichiometric metal atoms we can write down... 

We can conclude now that one electron returns to the conductivity band during each act of formation of the vacant site to adsorb sensitizer. Because adsorption centers Zr(. ) are not accounted for by (2.81) the energetics of the process does not depend on the manner in which R is closing in A, i.e. on the fact which recombination mechanism (either Langmuire-Hinshelwood or Ili-Ridil) takes place. 

We shall assume that the molecules H2 and D2 dissociate into ions upon adsorption. Let us also assume that the adsorption centers for deuterium atoms are the same as for hydrogen atoms. The question as to the nature of these centers is here of no interest. We shall denote the surface concentration of adsorption centers by N. The surface concentrations of the chemisorbed atoms of hydrogen and deuterium are denoted, respectively, by JVh and Nd. Let us further assume that the surface is saturated by hydrogen and deuterium atoms so that... 

The pressure P is contained in formula (62) not only in an explicit form but also in terms of the parameters e8 and e8+, as seen from (5), because s and e8+ are, generally speaking, functions of pressure. In our model, however, 8- and e8+ may be regarded as independent of P since the surface is supposed to be saturated with hydrogen and deuterium atoms (all the adsorption centers are assumed to be occupied). Thus, the hydrogen-deuterium exchange proves, in accordance with (63), to be a reaction of the first order with respect to hydrogen (deuterium), which is consistent with numerous experimental data (see Section III.A). 

We shall assume that the surface of the catalyst contains chemisorbed atomic oxygen and that it is these chemisorbed oxygen atoms that act, when in the ion-radical state, as adsorption centers for CO molecules. In this case, during the adsorption of CO molecules, surface ion radicals C02-are formed as intermediate compounds, which, after being preliminarily neutralized, are desorbed in the form of C02 molecules. 

Fig. 10.9 Ammonia adsorption centers in M0G5-G0 [35] composite and FIKUST-l/GO composite [42].

The surface of silica turns hydrophobic on treatment with organo-silicon chlorides. Water vapor adsorption isotherms measured by Stober (219) showed a very marked decrease in reversible adsorption. Less than 0.3 primary adsorption centers per 100 A remained in the surface after covering with the organosiloxane layer. Similar effects were observed in the adsorption of ammonia. About 2.2 silanol groups per 100 A had not reacted with the trimethylsilyl chloride. Nevertheless, the greater part of these had become unaccessible for water vapor. Apparently, they were hidden underneath a trimethylsilyl umbrella. ... 

Weak chemisorption, in which the chemisorbed particle C (considered together with its adsorption center) remains electrically neutral and in which the free electrons or boles of the lattice do not contribute to the bond between the lattice and the particle. We shall denote such a bond by the symbol CL, where L is the mbol of the lattice. 

By its nature, the acceptor bond, like the donor bond, may be purely ionic or purely homopolar or, in the general case, a mixed one. As we shall see below, this depends on how the electron or the hole captured by the particle and participating in the bond is distributed between the adsorbed particle and the adsorption center. In other words, this depends on the type of localization of the electron or the hole, which in turn, is determined by the nature of the adsorbate and the adsorbent. 

In the case of weak chemisorption of a Cl atom depicted in Fig. 2a, the bonding electron is that drawn from the Cl ion of the lattice, the latter ion serving as the adsorption center. In other words, the bonding is due to a hole being drawn from the Cl atom into the lattice. We have here a bond of the same type as in the molecular ion Ch". The dipole moment which arises in this case is opposite in direction to that of the preceding case. 

Figure 2b depicts a strong acceptor bond for a Na atom. It is formed from the weak bond depicted in Fig. 2a, for example, as a result of the capture and localization of a free electron, that is, as a result of the transformation of a Na+ ion of the lattice serving as an adsorption center, into a neutral Na atom. We obtain a bond of the same type as in the molecules H2 or Na2. This is a typically homopolar two-electron bond formed by a valence electron of the adsorbed Na atom and an electron of the crystal lattice borrowed from the free electron population. The quantum-mechanical treatment of the problem 2, 8) shows that these two electrons are bound by exchange forces which in the given case are the forces keeping the adsorbed Na atom at the surface and at the same time holding the free electron of the lattice near the adsorbed atom. 

Figures 3a and 3a depict the weak bond of an O2 molecule with the lattice. It is formed by an electron being drawn from an ion of the lattice to an O2 molecule. Owing to the greater electron aflSnity of the O2 molecule, the electron may be considered completely transferred from the lattice to the molecule as a result, a molecular ion 02 is formed and a localized hole appears in the lattice attached to the ion Oi, The entire system (the adsorbed O2 molecule + adsorption center) acquires a noticeable dipole moment with negative pole directed outward, but remains electrically neutral as a whole. The bond is effected without the participation of a free lattice electron. The transition to a strong acceptor bond entails the localization of an electron, or, what amounts to the same thing, the delocalization of a hole. Such a strong acceptor bond is depicted in Figs. 3b and 3b. ...

Secondly, the imperfections may directly participate in the act of adsorption, inasmuch as they may play the part of adsorption centers. This is the second mechanism. 

Let us now consider another mechanism by which the imperfections affect the adsorptive and catalytic properties of the surface. This is based on their participation in the adsorption process as adsorption centers. The problem of chemisorption on an atomic imperfection has been treated quantum-mechanically by Bonch-Bruevich 98) it was discussed from the viewpoint of the boundary-layer theory by Hauffe 99) and has been investigated recently by Kogan and Sandomirsky 95). 

F centers may act as adsorption centers not only in the alkali halides, but in any other crystals as well. Take, for example, a crystal of ZnO, in which the F center is an oxygen valency with two (not one ) electrons localized near it, as depicted in Fig. 30. From the chemical point of view such a center represents two adjacent localized free valencies of like sign which on an ideal surface could never meet because of Coulomb repulsion between them. (This should be especially stressed.) As a result of this property, such an F center may play a specific role in catalysis acting as an active center for a number of reactions. 

Not only F centers, but also other surface imperfections may act as adsorption centers. For example, the foreign gas molecules adsorbed on the surface may act as adsorption centers for the molecules of another gas (e.g., chemisorbed 0 atoms may act as adsorption center for CO molecules, as in Sec. V,B). 

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