The site densities of solid catalysts

Abstract

The number of catalytically-active sites per unit area, the site density, is often several orders of magnitude less than usually supposed. From the results obtained for representative systems, it is estimated that in 20–40% of modern catalytic systems the site densities are 10 11 cm −2 (i.e., 0.01% coverage) or less, with many site densities 10 9 cm −2 or less. Several unrelated methods of obtaining site densities lead to the same general conclusion. The catalysts of the representative systems are amorphous and crystalline acidic oxides, transition metal and rare earth oxides, transition metals, and salts. The reactions discussed are hydrocarbon isomerization, cracking, and polymerization; decomposition of NH 3, HCOOH, and alcohols; oxidation, hydrogenation, and dehydrogenation; O transfer, p- o H 2 conversion, and H 2D 2 equilibration. It is shown that in spite of the frequency of occurrence of low site densities, that high site densities are often uncritically assumed. Therefore, attempts to correlate surface properties with catalytic activity are open to question.