Metal/overlayer and encaged carbonyl cluster catalysis Original Research Article

Two classes of metal catalyzed reactions are described to which the classical Langmuir–Hinshelwood or Eley–Rideal mechanisms do not apply. In one class the metal is partially covered with an overlayer, for instance a hydrocarbonaceous deposit. Hydrogen transfer from this layer to adsorbed intermediates is followed by migration of H atoms from the metal surface to the overlayer. When unsaturated molecules are fed with an excess of D2, this situation results in the initial formation of hydrogenation products with a much lower D/H ratio than the feed. If the overlayer contains asymmetric sites, molecules with prochiral C atoms are hydrogenated to a product with significant enantiomeric excess, i.e. the product carries the geometric imprint of overlayer groups. A second class of modified metal catalysts contains transition metal clusters in zeolite cavities. At high partial pressure of CO they are transformed to carbonyl clusters characterized by distinctive IR spectra. The transformation from metal-to-metal carbonyl clusters induces dramatic changes in activity and selectivity for catalytic reactions such as CO hydrogenation or olefin hydroformylation. With Rh/NaY the hydrogenation of CO can be directed towards preferential formation of acetic acid. Article Outline