No abstract

Intellectually, the advantages that flow from the availability of single-site heterogeneous catalysts (SSHC) are many. They facilitate the determination of the kinetics and mechanism of catalytic turnover-both experimentally and computationally-and make accessible the energetics of various intermediates (including short-lived transition states). These facts in turn offer a rational strategic principle for the design of new catalysts and the improvement of existing ones. It is generally possible to prepare soluble molecular fragments that circumscribe the single-site, thus enabling a direct comparison to be made, experimentally, between the catalytic performance of the same active site when functioning as a heterogeneous (continuous solid) as well as a homogeneous (dispersed molecular) catalyst. This approach also makes it possible to modify the immediate atomic environment as well as the central atomic structure of the active site. From the practical standpoint, SSHC exhibit very high selectivities leading to the production of sharply defined molecular products, just as do their homogeneous analogues. Given that mesoporous silicas with very large internal surface areas are ideal supports for SSHC, and that more than a quarter of the elements of the Periodic Table may be grafted as active sites onto such silicas, there is abundant scope for creating new catalytic opportunities.

Metal oxides, in the form of dispersed powders, have been tested as potential catalysts for the four-electron oxidation of water to 0, under photochemical conditions. The most efficient catalysts were found to be IrO,, Co,O,, RuO,, NiCo,O,, Rh,O, and Mn,O, and, in particular, high activity was observed with IrO,. Comparison of the oxide structure with its observed rate of 0, generation under standard conditions has allowed formulation of a few general requisites for an effective catalyst. Samples of iridium oxide deposited onto the surface of a second (inert) oxide were tested for their 0,-evolving capability. The efficiency of the system depended markedly upon the nature of the support. Materials that favour formation of small deposits of iridium oxide (e.g. ZnO, MgO, TiO,) are the best supports, whilst 0, production is almost completely inhibited with acidic supports. Many metal oxides can be prepared in the form of hydrates of variable composition. These materials also function as 0,-evolving catalysts, the efficiency of the process depending upon any thermal pretreatment. This finding is explained in terms of changes in structure and composition of the oxide that occur upon heating.As part of a protracted research programme, concerned with building a photosystem capable of storing solar energy via the photodissociation of water, a range of 0,-evolving catalysts has been developed. 1-6 Mostly, these catalysts have contained ruthenium dioxide in some form, and all have been prone to corrosion under high anodic bias. homogeneous,'^ colloidal5* and heterogeneou~l.~ Ru0,-based catalysts have been tested for their 0,-evolving capacities under photochemical conditions. In certain cases, quite high efficiencies have been reported. Despite such detailed studies, the poor performance of the catalyst remains as the most serious obstacle in developing a suitable anodic branch of a solar-energy storage device and, if a satisfactory catalyst is to be identified, attention should be given to materials other than RuO,. Indeed, it has been shown that Ir0,,lo Pt0210 and MnO," powders will function as heterogeneous catalysts for water oxidation in related model systems.Using a well established photochemical test system,6 we have evaluated the 0,-evolving capabilities of a range of metal oxides. These materials have been used in the form of powders suspended in aqueous solution. Most of the materials were found to be inferior to RuO, powders, but a few oxides, notably IrO,, were effective catalysts for water oxidation and demonstrated much higher levels of corrosion resistance than the catalysts used previously. Several of these oxides exist in the form of hydrates for which the 0,-evolving capability is increased upon dehydration. In order to extend the scope of the work, a series of catalysts was prepared in which IrO, was deposited onto the surface of a second (inert) oxide. By this means, the effect of the supporting agent on the 2795

Bimetallic nanoparticles (Ru(6)Pd(6), Ru(6)Sn, Ru(10)Pt(2), Ru(5)Pt, Ru(12)Cu(4), and Ru(12)Ag(4)) anchored within silica nanopores exhibit high activities and frequently high selectivities, depending upon the composition of the nanocatalyst, in a number of single-step (and often solvent-free) hydrogenations at low temperatures (333-373 K). The selective hydrogenations of polyenes (such as 1,5,9-cyclododecatriene and 2,5-norbornadiene) are especially efficient. Good performance is found with these nanoparticle catalysts in the hydrogenation of dimethyl terephthalate to 1,4 cyclohexanedimethanol and of benzoic acid to cyclohexanecarboxylic acid or to cyclohexene-1-carboxylic acid, and also in the conversion of benzene to cyclohexene (or cyclohexane), the latter being an increasingly important reaction in the context of the production of Nylon. Isolated atoms of noble metals (Pd, Rh, and Pt) in low oxidation states, appropriately complexed and tethered to the inner walls of nanoporous (ca. 3 nm diameter) silica, are very promising enantioselective hydrogenation catalysts. Nanoporous carbons, as well as other nanoporous oxides, may also be used to anchor and tether the kind of catalysts described here.