Endothermic Chemisorption

It is important that these forms differ in the strength of the chemisorption bonding, i.e., in the heat of adsorption. The charged form is always stronger than the neutral form. Indeed, in the first case, unlike the second, desorption must be accompanied by the delocalization of an electron or hole this is always an endothermic process. 

Processes such as those represented by Equations (9) and (15) may be likened to the examples of endothermic chemisorption discussed by de Boer (7. ). 

Dissociative chemisorption energies calculated by density functional theory for various molecules on a number of stepped transition metal surfaces. All values are given in eV per molecule. Positive and negative values signify endothermal and exothermal chemisorption reactions, respectively. 

Thus the complete removal of an electron will undoubtedly require a prohibitively endothermic energy, I — <, as pointed out by Emmett and Teller (20). Such a view uses the concept of the removal of the electron to an infinite distance. If, however, the electron is moved to a finite distance in the solid (that is, partial ionization), the energy required, I, is less than I, and the small dipole moment of the chemisorption bond can be explained. Dowden takes care of this partial ionization by introducing the term without actually specifying the physical mechanism of the electron transfer. 

Fig. 17. An endothermic chemisorption with low activation energies for adsorption and desorption.

The potential curves of the adsorption of cesium on a CaF2 surface are given in Fig. 21, which shows that the curve for the ion represents an endothermic chemisorption. By the absorption of light of suitable wave length the system is transferred from minimum B to a point P of the upper curve and an electron is freed and may be drawn off as a photoelectron. The phenomenon of the selective photoelectric effect could be fully explained by this photoionization process (174). By thermal excitation the transfer can be effected at point electron emission of oxide cathodes. Point S is reached by taking up an amount of energy, which may be called the work function of the oxide cathode in this case but which is completely comparable with the energy of activation in chemisorption discussed in Sec. V,9 and subsequently. We shall not discuss these phenomena in this article but refer to a book of the author where these subjects are dealt with in detail (174) ... 

When a contamination is present which produces a dipole layer, as sulfur does, the dissociative chemisorption of molecular hydrogen is given by (Fig. 40) ABC D ) there is an activation energy (difference between levels C and A) the heat of adsorption is severely reduced it is even negative (endothermic chemisorption). The dissolution of molecular hydrogen proceeds less easily than in the case of a pure-iron surface. The kinetic energy has to be sufficient to overcome the difference between C ... 

The heat of chemisorption, which must be low in order to enable catalysis to take place, may even be negative. In various sections we have seen that endothermic chemisorption may play an important role (Secs. V,9, VI,3,4,5, and X,4). Figure 40 shows that surface contaminations can promote endothermic chemisorption. In nickel, as in iron, hydrogen atoms can be dissolved endothermically. It is highly probable that dissolved hydrogen atoms react from the metal phase with chemisorbed hydrocarbons. 

For chemisorption of 02 on Ag, two most recent ab initio calculations by McKee (147) and Upton et al. (148) found 02 to be endothermically chemisorbed, the conclusion being independent of the cluster size, for both Ag2 (147) and Ag24 (148). Only when special corrections had been made [for the electron affinity of 02 and the ionization energy of Ag2 (147) or by assuming bonding in an excited state of Ag24 (148)], the observed exothermicity of 02 chemisorption on Ag surfaces has been reproduced. 

Enthalpy changes on adsorption and desorption of probe molecules on catalyst surfaces may also be followed by differential thermal analysis (DTA) (67) although this method has been used only sporadically in the past. The experimental techniques have been described by Landau and Molyneux (67) very recently. As an example, Bremer and Steinberg (68) observed three endothermic peaks during the desorption of pyridine from a MgO-Si02 catalyst these peaks were assigned as three different chemisorption states of pyridine. 

In this section, the temperature is never allowed to rise so high that direct desorption of adatoms plays a significant role. Because their recombination behaviours are so different, we consider separately the cases of (a) moderately strong molecular chemisorption, (b) weak or endothermic molecular adsorption and (c) exceptionally strong chemisorption. 

Adsorption is normally exothermic thus a decrease in temperature will increase the extent of adsorption whereas an increase in temperature will increase the adsorption capacity for chemisorption (normally endothermic). The heat of adsorption, AHads, is defined as the total amount of heat evolved per a specific amount of adsorbate adsorbed on an adsorbent. Because adsorption from an aqueous solution occurs, AHads is small, and thus small changes in temperature do not alter the adsorption process much. The effect of temperature on adsorption can be expressed by ... 

An important detail is that an individual rate-limiting step may be endothermic whereas the overall reaction is exothermic as in this case. This is illustrated in Fig. 7.3. The chemisorption of N2 is exothermic and its dissociation is endothermic (1A). However, the overall reaction of N2 + H2 to NH3 is exothermic (1B). The overall activation energy and kinetics are dictated by the slow step. The reaction heat liberated (A H25°C) = — 11 kcal/mole is the thermodynamic value associated with the overall reaction. 

For example, one criterion is based on heats of adsorption chemisorption has high heats while physical adsorption has low heats. We know of chemisorption phenomena in which the heats of adsorption are probably zero and some that are endothermic. Another criterion is based on rates of adsorption, fast for physical, slow for chemical. There are chemisorption phenomena that are too fast to be measured, and also some physical adsorption phenomena are too slow to be measured. If 1 understand Dr. Hauffe, his criterion would be that physical adsorption does not perturb the electronic economy of the solid while chemisorption does.. Mignolet [Rec. tray, chlm., 74, 685 (1955)] found that noble gases adsorbed on evaporated metal films changed the work function by approximately one volt. 

Comparison of the two elementary processes (2) and (3) shows the latter to be the most endothermic one. Indeed, decomposition of N2O yielding NO and N entails the rupture of the N=N bond, which requires 85 kcal. mole (3.6 e.v. molecule ) whereas for N2O decomposition into Na and 0, the breaking of the NO bond requires only 38 kcal. mole (1.6 e.v. molecule 0-This explains that thermally, in the absence of radiation, reaction (2) is always considerably favored, compared to reaction (3). Under irradiation, however, a great number of excess free carriers are produced in the conduction and valency bands these carriers tend to recombine. In this respect the adsorbed N2O molecule may behave like a recombination center. This phenomenon can be accounted for by considering the adsorbed N2O molecule to be an acceptor level. Under this hypothesis, the N2O chemisorption results from the capture, by the weakly adsorbed molecule, of an electron from the conduction band. At the moment of recombination with a positive hole from the valency band, a variable amount of energy can be recovered, depending on the position of the level constituted by the adsorbed N2O molecule. For the silica and alumina we have utilized, the width of the forbidden region is about 10 e.v. process (3) which only requires 3.6 e.v. may thus become possible. 

Garcia de la Banda (67) has measured the chemisorption of hydrogen and oxygen on chromia at temperatures of about 100-150°. The amounts adsorbed increase with temperature. He considers that active sites are formed endothermically and increase in number with temperature. There are temperature ranges and adsorption conditions in which one could observe such behavior on our catalysts, but we believe that such phenomena are better explained by the treatment which we have given. 

Chemisorption differs from physisorption in that it involves the formation of chemical bonds and electron transfer between adsorbate and adsorbent. It is of primary concern with regard to catalysis, and it is only useful as an extreme measure for separation purposes. Removal of chemical warfare agents from breathing air is a prime example. One way to discem between physisorption and chemisorption is by heat effects and the effect of temperature. For example, physisorption is always exothermic, i.e., A// < 0. Furthermore, the adsorbed state is more ordered than the fluid state, so AA < 0. To be thermodynamically feasible, < 0, so AH < TAS. In contrast, endothermic adsorption is possible for dissociative chemisorption, even if the entropy of the adsorbate decreases, because the entropy of the adsorbent may increase to more than offset that, e.g., by expanding. 

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