Adsorption assisted processes and active sites on catalyst surfaces

It is generally accepted that the rate of desorption is a function of the amount of adsorbed species. However, it has been demonstrated in recent years that the desorption rate is a function of not only the coverage, but also the pressure of ambient gas. Similar examples were found with respect to the reactivity of adsorbed species. Hydrogen is heterolytically absorbed on ZnO and ZrO2 to form oxygen-H bond and metal-H bond, which are easily observed by IR technique. When hydrogenation of olefins takes place on these oxides, however, these heterolytically chemisorbed hydrogen does not participate in the hydrogenation. Active sites are seemingly formed by the chemisorption of olefins. Very few efforts have been made so far to estimate the number of active sites on the catalyst surface. The experimental methods to estimate the number of active sites will be discussed in this paper