Mono-, Bi- and Multifunctional Single-Sites: Exploring the Interface Between Heterogeneous and Homogeneous Catalysis

This mini-review contrasts the characteristics of traditional heterogeneous (solid) catalysts with those of homogeneous ones: the nature of the active sites in each case is very different, a fact well illustrated in ammonia synthesis. It is recalled that certain chemical transformations can be effected only with heterogeneous catalysts. It is also demonstrated that the scope for introducing multifunctional sites is greater with open-structured inorganic heterogeneous catalysts than with homogeneous ones: for example, Ti^IV ions distributed in a spatially isolated and accessible manner at the large areas of a nanoporous support smoothly convert cyclohexene to adipic acid (with H₂O₂) in a cascade of six consecutive reactions. A sharp distinction is drawn between nanocluster and nanoparticle “metal” catalysts, both electronic and geometric arguments being utilized to explain this difference. In the extreme case, a few (or single) metal atoms (supported on oxides) have been shown (see refs. Fu et al. Science 301:935, 2003 and Rim et al J Phys Chem C 113:10198, 2009) to be more important determinants of catalytic activity than nanoparticle metals such as Au and Pd. Recent advances in high-resolution electron microscopy is a key technique in this facet of catalysis. The merits of immobilizing single-site homogeneous catalysts and of creating atomically well-defined single-site heterogeneous ones on high-area solids are illustrated both from a practical viewpoint and also as a strategy for the design of new catalysts.