The acid-base concept in heterogeneous catalysis

It is recognized that the use of solid catalysts presents several advantages over their liquid homologues. They frequently have a high activity and selectivity. They do not corrode the vessels or the reactors, and induce no environmental pollution. It is easy to separate them from the reaction products and, thus, to make repeated use of them. Due to their industrial interest, they have been the subject of many studies over the last thirty years. 

A solid with a high positive has strong basic sites and 

The thermo-desorption of molecules in the gas phase also yields information on the surface acidity. Since desorption of a base from a strongly acidic site is more difficult than from a weakly acidic one, the number of desorbed molecules as a function of temperature gives an estimate of the strength of the reactive sites. The test molecules which are commonly used include ammonia, pyridin and amines to quantify the surface acidity, and CO2, nitric acid and phenol to quantify their basicity. 

The measurement of the activity in a specific catalytic reaction, such as isopropanol decomposition or butene isomerization is also often used in the field. Catalysts may, for example, be characterized by their ability to dehydrate isopropanol molecules into propene, or to dehydrogenate them in acetone. The former reaction tests their acidity and the latter their basicity (Ai, 1977 Cunningham et al., 1981 Nollery and Ritter, 1984 Tanabe et al., 1989 Gervasini and Auroux, 1991). 

Values in brackets give the measured oxygen Is-level binding energies. A correlation has been established between this energy and the I EPS in several oxides (Mullins and Averbach, 1988 Delamar, 1990 Casamassima et al, 1991). However, some care must be taken, because the binding energies are also dependent upon electron relaxation around the Is hole, not present in electrochemical measurements. 

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