Site density indirect method determination

There have been attempts to count active centers ever since their existence was postulated in 1925. Normally site densities, where the site density is the number of active centers per unit rea< are thought to be near the maximum value, 10 cm , but in some cases values which are several orders of magnitude smaller have been suggested. A direct method of determining site density is one which depends on results of kinetic studies. Several direct methods, including one using transition state theory, are described results are presented. Many indirect methods, along with results, are also discussed. 

There are both direct and indirect ways of determining site density. All the direct ways are kinetic. Some of the kinetic methods use trasition state theory (TST) while others do not. We shall look first at the direct, non-TST methods. 

An indirect method is one in which it is assumed that the group of active centers is that group of surface sites which has a certain characteristic, say, the ability to adsorb a certain kind of molecule. Thus, counting those sites is the same as determining the site density. But as long ago as 1929 Taylor ( ) stated that sites on the same surface could qualitatively or quantitatively differ with respect to catalytic activity. Sometimes one is justified in assuming that a certain set of sites, such as all metal atoms or all sites which adsorb a certain poison molecule, is the same as the set of sites which catalyzes a reaction. But unfortunately this assumption is not always justified. 

Probably the most popular indirect methods of determining site density are the poison method and the method in which the amount of chemisorption of reactant is measured. The number of either the poison or the reactant molecules which chemisorb per unit area is then taken to be the site density. Some examples will be discussed. 

Other indirect methods, those not involving poisoning or reactant chemisorption, have been used to determine site densities. Perhaps the most common of these methods is to count the surface metal atoms, with either pure metal crystals or metal supported on oxides, with the counting carried out by determination of the amount of O2 or CO2 which chemisorbs ( ) the assumption has then sometimes been made that all such atoms are catalytically active. 

Other methods—indirect, but not utilizing chemisorption— have been used to determine site densities. In 1958 the magnetic moment of surface atoms was taken to be a measure of the number of catalytically active atoms ( ). The number of surface free valencies was used for the same purpose in 1964 ( ). In 1965 Mellor and coworkers ( ) oxidized KI over silica-alumina by chemical analysis the number of surface atoms capable of oxidation was determined and taken to be the site density. The number of hydrogen vibrations on the ZnO surface which catalyzed the dehydrogenation of isopropanol was used to calculate the site density (91). 

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