Chemisorption process

Sellers H 1991 On modeling chemisorption processes with metal cluster systems. II. Model atomic potentials and site specificity of N atom chemisorption on Pd(111) Chem. Phys. Lett. 178 351-7... 

The incorporation of the new material without any increase in the overall length of the book has been achieved in part by extensive re-writing, with the compression of earlier material, and in part by restricting the scope to the physical adsorption of gases (apart from a section on mercury porosimetry). The topics of chemisorption and adsorption from solution, both of which were dealt with in some detail in the first edition, have been omitted chemisorption processes are obviously dependent on the chemical nature of the surface and therefore cannot be relied upon for the determination of the total surface area and methods based on adsorption from solution have not been developed, as was once hoped, into routine procedures for surface area determination. Likewise omitted, on grounds of... 

Anodic polarization also may occur. Typically, this begins with the formation of a thin, impervious oxide film, chemisorbed at the anode (as on the surface of stainless steels). However, for most metals used in boiler plant systems this chemisorption process must be aided by anodic corrosion inhibitors to reduce corrosion rates to tolerable levels. An example is the application of nitrite-based inhibitors, widely used in HW heating systems. 

In the case of action of specific chemical forces between sorbent and solute a chemisorption process may result. This process could be regarded as a complex formation reaction and would be specific and selective, concentration dependent, possibly slow and partly irreversible. 

The hydrogen molecule does not chemisorb onto clean sintered gold surfaces at or above 78 K [147] but on unsintered films, a small amount of H2 is chemisorbed if gold surface atoms of low coordination number are present [148]. Stobinski [149] found that H2 can also chemisorb on thin sintered Au films if the surface is covered at low temperatures with a small amount of gold equivalent to 1-3 Au monolayers prior to H2 exposure. This suggests a fundamental role of surface Au atoms of low coordination number in the chemisorption process. Deuterium molecules also chemisorb in a similar fashion on gold films at 78 K and isotope effects were... 

The adsorption action of activated carbon may be explained in terms of the surface tension (or energy per unit surface area) exhibited by the activated particles whose specific surface area is very large. The molecules on the surface of the particles are subjected to unbalanced forces due to unsatisfied bonds and this is responsible for the attachment of other molecules to the surface. The attractive forces are, however, relatively weak and short range, and are called Van der Waals forces, and the adsorption process under these conditions is termed as a physical adsorption (physisorption) process. In this case, the adsorbed molecules are readily desorbed from the surface. Adsorption resulting from chemical interaction with surface molecules is termed as chemisorption. In contrast to the physical process described for the adsorption on carbon, the chemisorption process is characterized by stronger forces and irreversibility. It may, however, be mentioned that many adsorption phenomena involve both physical and chemical processes. They are, therefore, not easily classified, and the general term, sorption, is used to designate the mechanism of the process. 

Kineti cs. To date only addition reactions have been reported. These reactions produce products or adducts that are the result of complete addition, or addition and subsequent elimination. An example of the later reaction is dehydrogenation of hydrocarbons on platinum clusters. These addition reactions are in many ways analogs to the chemisorption process on metal surfaces. 

It has also to be remembered that the band model is a theory of the bulk properties of the metal (magnetism, electrical conductivity, specific heat, etc.), whereas chemisorption and catalysis depend upon the formation of bonds between surface metal atoms and the adsorbed species. Hence, modern theories of chemisorption have tended to concentrate on the formation of bonds with localized orbitals on surface metal atoms. Recently, the directional properties of the orbitals emerging at the surface, as discussed by Dowden (102) and Bond (103) on the basis of the Good-enough model, have been used to interpret the chemisorption behavior of different crystal faces (104, 105). A more elaborate theoretical treatment of the chemisorption process by Grimley (106) envisages the formation of a surface compound with localized metal orbitals, and in this case a weak interaction is allowed with the electrons in the metal. 

Heats of chemisorption may vary from a few kilocalories per mole of adsorbed gas to 100 kcal mole-1 (or more). It is certainly not necessary to employ a very sensitive calorimeter for their determination and, actually, many chemisorption processes have been studied in calorimeters with a low sensitivity. However, it is necessary to introduce a large quantity of... 

In this paper we have utilized the changes in the SHG from metal surfaces to monitor chemisorption processes at electrode surfaces. In particular, we have seen that ... 

Based upon analogies between surface and molecular coordination chemistry outlined in Table 1, we have recently set forth to investigate the interaction of surface-active and reversibly electroactive moieties with the noble-metal electrocatalysts Ru, Rh, Pd, Ir, Pt and Au. Our interest in this class of compounds is based on the fact that chemisorption-induced changes in their redox properties yield important information concerning the coordination/organometallic chemistry of the electrode surface. For example, alteration of the reversible redox potential brought about by the chemisorption process is a measure of the surface-complex formation constant of the oxidized state relative to the reduced form such behavior is expected to be dependent upon the electrode material. In this paper, we describe results obtained when iodide, hydroquinone (HQ), 2,5-dihydroxythiophenol (DHT), and 3,6-dihydroxypyridazine (DHPz), all reversibly electroactive... 

As we have seen, the chemisorption properties of the substrate depend on its electronic structure, so that changes in the latter are reflected in the former. In the case of electrified substrates, the strength of the applied electric field governs the substrate modification and, thereby, regulates the chemisorption process in a controllable manner. 

In conclusion, we have seen that an applied held has the ability to strongly affect the chemisorption process. One trend, clearly observable in both Fig. 7.6(b) and Fig. 7.7(b), is that the sign of F determines whether or not Aq is enhanced by the presence of the held, i.e., Aq is increased (decreased) when F is positive (negative). More variable is the dependence of AE on F, due to the variability in the existence (or not) of Absolutions, which, when they do occur, represent the more stable interaction. Consequently, the presence of the held may either enhance or suppress the chemisorption process. 

---