Synthesis of heterogeneous catalysts with well shaped platinum particles to control reaction selectivity

Abstract

Colloidal and sol-gel procedures have been used to prepare heterogeneous catalysts consisting of platinum metal particles with narrow size distributions and well defined shapes dispersed on high-surface-area silica supports. The overall procedure was developed in three stages. First, tetrahedral and cubic colloidal metal particles were prepared in solution by using a procedure derived from that reported by El-Sayed and coworkers [Ahmadi TS, Wang ZL, Green TC, Henglein A, El-Sayed MA (1996) Science 272: 1924 -1926]. This method allowed size and shape to be controlled independently. Next, the colloidal particles were dispersed onto high-surface-area solids. Three approaches were attempted: (i) in situ reduction of the colloidal mixture in the presence of the support, (ii) in situ sol-gel synthesis of the support in the presence of the colloidal particles, and (iii) direct impregnation of the particles onto the support. Finally, the resulting catalysts were activated and tested for the promotion of carbon-carbon doublebond cis-trans isomerization reactions in olefins. Our results indicate that the selectivity of the reaction may be controlled by using supported catalysts with appropriate metal particle shapes. On the basis of their kinetic behavior, catalytic reactions are often classified as either mild or demanding (1-3). Demanding reactions-such as the oxidation of CO, NO, or hydrocarbons; the synthesis of ammonia; and most oil processing conversionsusually require high temperatures and pressures, and involve small concentrations of intermediates similar to those identified under vacuum. The performance of these reactions often depends strongly on the structure of the catalyst used (4, 5). In contrast, mild reactions-in particular, hydrogenations and isomerizations of unsaturated hydrocarbons-take place under less-demanding temperature and pressure conditions. Mild reactions have historically been considered structure-insensitive (6-8), but that conclusion has been drawn from studies on reactivity vs. metal dispersion that used ill-defined supported catalysts (9, 10) and has been questioned by more recent studies using better catalytic models (11). For instance, both experimental (12-14) and theoretical (15) studies on the selective catalytic hydrogenation of CAO bonds in unsaturated aldehydes have suggested that such reactions may be promoted by close-packed (111) surfaces. In another example, the dehydrogenation of cyclohexene was found to be faster on Pt (111) than on Pt (100) single-crystal surfaces (16). Our recent surface-science investigations on the isomerization of unsaturated olefins (17-19) strongly suggest that selectivity toward the formation of the cis isomer may be favored by Pt (111) facets. Additional surfacescience reports on the conversion of alkyl and alkene adsorbates under vacuum conditions (20-25), as well as studies with more realistic model systems (26, 27), point to a potential structure sensitivity in the conversion of other olefins and unsaturated hydrocarbons.These results not only s...

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