Migration, noble metals

The double bond migration in steroid hydrocarbons catalyzed by acids or noble metals (see, for example, ref. 185) will not be discussed here. A general review of nonsteroid olefin isomerization has recently been published. Iron carbonyl has been used to isomerize steroidal dienes. 

This study, in conjunction with that discussed in 12.2.1.2, show that when using aqueous electrolytes or Nafion saturated with H20, the induction of NEMCA on finely dispersed noble metal catalysts is rather straightforward. The role of the electronically conducting porous C support is only to conduct electrons and to support the finely dispersed catalyst. The promoting species can reach the active catalyst via the electrolyte or via the aqueous film without having to migrate on the surface of the support, as is the case when using ceramic solid electrolytes. 

The adhesion of noble metals to the insulating substrates is generally poor. In order to overcome this problem, a thin layer (10-50nm) of adhesive metal (Cr, Ti, W) is deposited under the noble metal. Often this adhesive metal migrates through the noble metal and forms an oxide layer that significantly alters its surface... 

X-Ray diffraction has been used to identify the locations of noble metal ions in faujasites after subjecting the amminated precursors to a variety of calcination conditions (141-148). These ions remain in the supercages after partial oxidation of the ammine ligands at low temperature, but they migrate into sodalite cages after calcination at high temperature. This has been shown in detail for Pt and Pd ions in zeolite Y. 

Recently titania appeared as a non-conventional support for noble metal catalysts, since it was found to induce perturbations in their H2 or CO adsorption capacities as well as in their catalytic activities, This phenomenon, discovered by the EXXON group, was denoted "Strong Metal-Support Interactions" (SMSI effect) (1) and later extended to other reducible oxide supports (2). Two symposia were devoted to SMSI, one in Lyon-Ecully (1982) (3) and the present one in Miami (1985) (4) and presently, two main explanations are generally proposed to account for SMSI (i) either the occurence of an electronic effect (2,5-13) or (ii) the migration of suboxide species on the metal particles (14-20). The second hypothesis was essentially illustrated on model catalysts with spectroscopic techniques.lt can be noted that both possibilities do not necessarily exclude each other and can be considered simultaneously (21). 

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