| Methyne, methylene, and methyl intermediates have been directly observed by SIMS to be adsorbed on a Ni(111) surface after reaction of CO + H(,2) at 50-100 torr. Deuterium and ('13)CO labeling studies confirm that these intermediates are formed from the reactant gases. The variation in ion intensity with temperature indicate that the CH(,2) desorbes first at 138(DEGREES)C followed by CH(,3) at 170(DEGREES)C and CH at 210-230(DEGREES)C. This indicates all three species exist as separate entities on the surface. After correcting for ionization effects, their concentrations are estimated to be within an order of magnitude of each other. The activation energy and turnover numbers for the Ni single crystal are similar to those of more conventional catalysts, indicating that their kinetic behavior is similar. These data are most consistent with a mechanism involving sequential hydrogenation of surface carbon. ESCA analysis indicates a linear increase of surface carbon with increasing reaction temperature. A series of FeMn and KFeMn carbonyl clusters were supported on a high surface area carbon so that a variety of stoichiometric metal compositions could be obtained. There is a maximum in activity and selectivity towards C(,2)-C(,4) olefins with the K Fe(,2)Mn(CO)(,12) and Et(,4)NFe(,2)Mn(CO)(,12) clusters where Et(,4)N is a tetraethylammonium cation. The metal ratio is optimized for the formation of a manganospinel (MnFe(,2))O(,4). Potassium incorporation into these FeMn catalysts helps retain their selectivity toward olefins after high temperature pretreatments. Potassium also increases the olefin/paraffin ratio such that, except for methane, only C(,2)-C(,4) olefins are produced. FeCO and KFeCo carbonyl clusters were deposited on carbon supports. High activities and selectivities toward olefin production for the K promoted catalysts was observed. The mixed-metal catalysts are more active than most other reported FeCo catalysts. With increasing Co content the activity steadily increases and the olefin/paraffin ratio decreases. Activation energies increase with increasing K/metal ratios. A similar series of FeRu carbonyl clusters supported on carbon show how the catalytic nature of each metal at the surface is additive in terms of activity and selectivity. Surface enrichment of Fe for the FeRu catalysts is seen by CO and H(,2) uptakes and activity data. High resolution STEM photographs of the used FeRu(,2)(CO)(,12)/carbon catalyst indicate raft-like structure of the metal particles. |
