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Catalysis Looks to the Future
evated temperature and pressure. Additional information on the structure of adsorbed hydrocarbons has been obtained from LEED studies. More recently, molecular beam investigations have proved useful in determining the dynamics of hydrocarbon adsorption and decomposition on metal surfaces. Because the thermal energy of the hydrocarbon can be controlled in such experiments, it is possible to determine the kinetics of hydrocarbon reactions in great detail. Studies carried out with well-defined single-crystal surfaces of platinum have demonstrated the sensitivity of reaction dynamics to the crystallographic planes exposed and to the presence of defects such as steps and kinks.
Ethylene oxide and propylene oxide are critical intermediates in chemical manufacture. Ethylene can be catalytically oxidized by molecular oxygen over a silver catalyst to give an excellent yield of ethylene oxide. The synthesis of propylene oxide requires the catalytic transfer of oxygen from an alkyl peroxide to propylene. Transition metal catalysts, usually based on molybdenum, are essential for the reaction. These two reactions have a major impact on the commodity chemical industry. Recently, the synthesis of epoxides has been modified to yield enantiomerically pure products, an advance that has had a major impact on the manufacture and synthesis of bioactive molecules. Barry Sharpless' group, at MIT, has shown that titanium complexed to tartrate esters (obtained from nature as one optical isomer) will transfer oxygen from tertiary-butyl hydroperoxide to allyl alcohols to give enantiomerically pure epoxides. Such studies have provided the paradigm for the role of structure, mechanism, and dynamics in the discovery of selective catalysts for the production of enantiomerically pure products.
The conversion of methanol to gasoline over ZSM-5, a medium-pore zeolite, has also been investigated extensively as part of the effort to develop alternatives to petroleum as a source of transportation fuels. Both 1H and 13C NMR have been useful techniques for characterizing the intermediates formed between olefins and protic sites inside the zeolite and for characterizing the structure of coke, which builds up slowly with time and contributes to catalyst deactivation. This type of information has proved useful in understanding how zeolite acidity and pore size influence the rate at which deactivation occurs.
The selective catalytic reduction of nitric oxide by ammonia over titania-supported vanadia catalysts provides an effective means for reducing NOx emissions from stationary sources. Isotopic tracers have revealed that this reaction is initiated by the formation of an adduct between nitric oxide and ammonia, whereas Raman and NMR studies of adsorbed ammonia have shown how ammonia complexes with vanadium ions present on the catalyst surface. Such investigations suggest that oxygen atoms bound to vanadium ions play an important role in activating the adsorbed ammonia for reaction. The role of additive oxides that are known to enhance catalyst activity and