Chemisorption, electron transfer

In the foregoing examples the spectral data indicated a Lewis acid-base reaction on the surface where the alkali and alkaline earth cations acted as the electron acceptors while the adsorbates were the electron donors. It is quite natural that the reverse situation might be possible that is, the adsorbent be basic while the adsorbate show acidic properties so that in the chemisorption electron transfer will occur in the reverse direction. Several examples of such adsorption have already been discussed in this chapter. Kortiim (22) found another example in the adsorption of symmetrical trinitrobenzene on magnesia and on alumina. Whereas trinitrobenzene adsorbed on calcium fluoride or silica was colorless, on magnesia it was red with an absorption maximum at 4650 A (Fig. 26) and the spectrum of the adsorbed species was very... 

Forces of Adsorption. Adsorption may be classified as chemisorption or physical adsorption, depending on the nature of the surface forces. In physical adsorption the forces are relatively weak, involving mainly van der Waals (induced dipole—induced dipole) interactions, supplemented in many cases by electrostatic contributions from field gradient—dipole or —quadmpole interactions. By contrast, in chemisorption there is significant electron transfer, equivalent to the formation of a chemical bond between the sorbate and the soHd surface. Such interactions are both stronger and more specific than the forces of physical adsorption and are obviously limited to monolayer coverage. The differences in the general features of physical and chemisorption systems (Table 1) can be understood on the basis of this difference in the nature of the surface forces. 

Most adsorption processes are exothermic (AH is negative). Adsorption processes involving nonspecific interactions are referred to as physical adsorption, a relatively weak, reversible interaction. Processes with stronger interactions (electron transfer) are termed chemisorption. Chemisorption is often irreversible and has higher heat of adsorption than physical adsorption. Most dispersants function by chemisorption, in contrast to surfactants, which... 

Metal oxides possess multiple functional properties, such as acid-base, redox, electron transfer and transport, chemisorption by a and 71-bonding of hydrocarbons, O-insertion and H-abstract, etc. which make them very suitable in heterogeneous catalysis, particularly in allowing multistep transformations of hydrocarbons1-8 and other catalytic applications (NO, conversion, for example9,10). They are also widely used as supports for other active components (metal particles or other metal oxides), but it is known that they do not act often as a simple supports. Rather, they participate as co-catalysts in the reaction mechanism (in bifunctional catalysts, for example).11,12... 

A key aspect of metal oxides is that they possess multiple functional properties acid-base, electron transfer and transport, chemisorption by a and 7i-bonding of hydrocarbons, O-insertion and H-abstraction, etc. This multi-functionality allows them to catalyze complex selective multistep transformations of hydrocarbons, as well as other catalytic reactions (NO,c conversion, for example). The control of the catalyst multi-functionality requires the ability to control not only the nanostructure, e.g. the nano-scale environment around the active site, " but also the nano-architecture, e.g. the 3D spatial organization of nano-entities. The active site is not the only relevant aspect for catalysis. The local area around the active site orients or assists the coordination of the reactants, and may induce sterical constrains on the transition state, and influences short-range transport (nano-scale level). Therefore, it plays a critical role in determining the reactivity and selectivity in multiple pathways of transformation. In addition, there are indications pointing out that the dynamics of adsorbed species, e.g. their mobility during the catalytic processes which is also an important factor determining the catalytic performances in complex surface reaction, " is influenced by the nanoarchitecture. 

Adsorption/chemisorption at the surface (adsorption of gas molecules occurs on the solid surface because of attractive forces between them). Gas molecules approaching the surface may lose some of their momentum (in the component normal to the surface) and become trapped in the potential well. The energy required to overcome the attractive potential barrier of the surface and the attraction of neighbouring molecules is the heat of adsorption (Van der Wall forces) and several monolayers may be adsorbed. However, if there is some interaction or electron transfer between the gas molecule and the surface (forming, e.g., a surface compound), it is defined as chemisorption. The heat of chemisorption is usually greater than the heat of adsorption. The extent of chemisorption depends upon the specific nature of the solids and gases. 

Vickerman and Ertl (1983) have studied H2 and CO chemisorption on model Cu-on-Ru systems, where the Cu is deposited on single-crystal (0001) Ru, monitoring the process using LEED/Auger methods. However, the applicability of these studies carried out on idealized systems to real catalyst systems has not been established. Significant variations in the electronic structure near the Eermi level of Cu are thought to occur when the Cu monolayer is deposited on Ru. This implies electron transfer from Ru to Cu. Chemical thermodynamics can be used to predict the nature of surface segregation in real bimetallic catalyst systems. 

There have been many attempts to relate bulk electronic properties of semiconductor oxides with their catalytic activity. The electronic theory of catalysis of metal oxides developed by Hauffe (1966), Wolkenstein (1960) and others (Krylov, 1970) is base d on the idea that chemisorption of gases like CO and N2O on semiconductor oxides is associated with electron-transfer, which results in a change in the electron transport properties of the solid oxide. For example, during CO oxidation on ZnO a correlation between change in charge-carrier concentration and reaction rate has been found (Cohn Prater, 1966). 

When a gas is adsorbed at a metal surface, the observed change in work function is brought about by electronic interaction between the metal and the adsorbate. Most chemisorptions involve electron transfer, the nature of which is related to the electronic structure and the surface properties of the metal. At the outset, therefore, it is desirable to consider the adsorption process and the formation of chemical bonds at metal surfaces in general terms. 

The excess negative charge located in the interior of metallic silver colloids could also be transferred to other electron acceptors, including methylviologen, nitrobenzene, nitropyridinium oxide, anthracene quinone sulfonic add, and potassium cyanohexaferrate(III)[506, 531], The efficiency and, indeed, the direction of electron transfer were found to depend on the position of the Fermi level of the surface-modified silver particles. For example, chemisorption of AgN to a silver particle is shown to result in a shift of the Fermi level to a more positive potential, as shown in the lower line in Fig. 84. 

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. 

It has often been pointed out that the electrical conductivity of sintered samples of ZnO and of other n-conducting oxides is frequently caused by the conductivity of thin layers near the surface, and not by the conductivity of the bulk (25-28). According to our present knowledge, these thin layers near the surface of oxides are caused by electron transfer from the layers to the chemisorbate during the chemisorption, and the amount of chemisorption may be related to the electronic properties of the gas molecules and of the solids. The dependence of the electrical conductivity of some semiconductors on the pressure of CO, COj, and on the vapor pressure of ethanol, methanol, acetone, and water, as observed by Ljaschenko and Stepko (29), can be explained by the same mechanism. The dependence of conductivity of some mixed oxides at high temperatures can be explained in a similar way (30). 

If we first consider only those chemisorption processes in which an electron transfer takes place from the semiconductor to the chemisorbing gas, we can summarize the result of these calculations as follows The value of the work function must increase if a chemisorption takes place with the consumption of electrons by the chemisorbed gas. The increase of the work function can be expressed in the case of an w-conducting adsorbent by a quadratic, and in the case of a p-conducting adsorbent by a combined linear-logarithmic dependence on the surface concentration of the... 

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