Metal tolerance manipulation

Catalyst surface activity may be manipulated to alter the ratio of HDM activity to metal compound diffusivity with a predictable impact on optimum pore size (Howell etal., 1985). Lowering the intrinsic surface activity by varying the quantity, chemical composition, or distribution of active catalytic metals will increase the Ni and V penetration into the catalyst. The lower surface activity catalysts may be able to tolerate a smaller pore size (higher total surface area) and still maintain an acceptable performance for the HDM reactions. 

There will undoubtedly be many new catalysts described in the near future. In particular, we anticipate that later transition-metal complexes will play a much larger role in this type of chemistry. Although bis-Cp complexes of Groups 3 and 4, the lanthanides, and the actinides have shown exceptional activity and thus far have dominated reported investigations on catalytic homodehydrocoupling, it is most unlikely that they are unique in this property. It is to be expected that other classes of complexes, such as mono-Cp and -rj -alkyl complexes of these elements, will also be active. Other directions for future evolution are the development of catalysts that are air stable, that are tolerant of more functionalities on substrates, and that are more easily manipulated. 

The utility of carbide and nitride catalysts has prompted numerous studies of their reactivity that use carbide and nitride overlayers as the catalyst rather than bulk carbides or nitrides. This approach permits careful manipulation of the surface metal/nonmetal stoichiometry, which is crucial to probing reactivity. These studies consistently reveal the catalytic activity of carbide and nitride overlayers and, in several cases, the similarities between their behavior and that of noble metal catalysts. For example, the same benzene yield and reaction pathway for the dehydrogenation of cyclohexane was observed for both p(4x4)-C/Mo(110) and Pt(l 11) surfaces. Furthermore, carbon-modified tungsten may be a more desirable catalyst for direct methanol fuel cells than Pt or Ru surfaces because the transition metal carbide exhibits higher activity toward methanol and water dissociation and is more CO-tolerant. ... 

Electrochemical deposition of metals in the pores of templating membranes requires that one side of the membrane be in direct contact with a metallic layer. This can be produced by plasma or vacuum deposition of a metal layer on one side of the membrane (25) and requires that the membrane film be robust enough to tolerate this kind of manipulation. The thickness of the conductive layer is typically 100-1000 nm (45-47). The metal which produces the conductive layer can be the same or different from the one that will provide the final template structure. In electrochanical template deposition, the coated film is placed in an electrochemical ceU where the template membrane acts as the cathode and a counter electrode is the anode. The deposition can be carried out under galvanostatic or... 

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