Surface Chemistry and Adsorption

The three steps postulated for the catalytic hydrogenation of ethylene indicate that the rate may be influenced by both adsorption and desorption (steps 1 and 3) and the surface reaction (step 2). Two extreme cases can be imagined that step 1 or step 3 is slow with respect to step 2 or that step 2 is relatively slow. In the first situation rates of adsorption or desorption are of interest, while in the second the surface concentration of the adsorbed species corresponding to equilibrium with respect to steps 1 and 3 is needed. In any case we should lilce to Tnow the n of sites on the catalyst surface, or at least the surface area of the catalyst. These questions require a study of adsorption. More is known about adsorption of gases, and this will be emphasized in the sections that follow. 

At these locations the surface atoms of the solid may attract other, atoms cTr molectiles in the surrounding gas or liquid phase. Similarly, the surfaces of pure crystals have nonunifonn force fields because of the atomic structure in the crystal. Such surfaces also have sites or active centers where adsorption is enhanced. Two types of adsorption may occur. 

Physical Adsorption The first type of adsorption is nonspecific and somewhat similar to the process of condensation. The forces attracting the fluid molecules to the solid surface are relatively wc, and the l at e lved fluring the adsorptrdh procesQs of Jhe jame of magnitude 

The amount of physical adsorption decreases rapidly as the temperature is raised and is generally very small above the critical temperatures of the adsorbed component. This is further evidence that physical adsorption is not responsible for catalysis. For example, the rate of oxidation of sulfur dioxide on a platinum catalyst becomes appreciable only above 300°C yet this is considerably above the critical temperature of sulfur dioxide (157°C) or of oxygen ( — 119°C). Physical adsorption is not highly dependent on the irregularities in the nature of the surface, but is usually directly proportional to the amount of surface. However, the extent of adsorption is not limited to a monomolecular layer on the solid surface, especially near the condensation temperature. As the layers of molecules build up on the solid surface, the process becomes progressively more like one of condensation. 

Physical-adsorption studies are valuable in determining the physical properties of solid catalysts. Thus the questions of surface area and pore-size distribution in porous catalysts can be answered from physical-adsorption measurements. These aspects of physical adsorption are considered in Secs. 8-5 and 8-7. 

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J.S. Mattson and H.B. Mark, Activated Carbon Surface Chemistry and Adsorption from Solution, Marcel Dekker, 1971. 

Alumina, silica and many other metal oxides are insulators. However, recent experiments indicate that the surfaces of these insulators are mainly ionic (Masel, 1996). The pristine or freshly cleaved surfaces of single crystals of these oxides (cleaved under ultrahigh vacuum) are fairly inert and do not have significant adsorption capacities for even polar molecules such as CO and S02 (Masel, 1996 Henrich and Cox, 1994). However, the surface chemistry and adsorption properties are dominated by defects on real surfaces. For example, oxide vacancies on alumina expose the unsaturated aluminum atoms, which are electron acceptors, or Lewis acid sites. 

Surface chemistry and adsorption properties of AI13 colloids... 

Mattson, J.S. and Mark, H.B., Jr. Activated Carbon — Surface Chemistry and Adsorption from Solution. M. Dekker, New York 1971. 

P, Matson and H.B. Mark, Activated carbon surface chemistry and adsorption from solution, Dekker, New York, 1971. 

Marsh et al. (1997, 2(K)0) have published texts on carbon technology and science. Mattson and Mark (1971) discuss activated carbons in terms of surface chemistry and adsorption from solution. 

Activated Carbon Surface Chemistry and Adsorption from Solution... 

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