Strong metal-support interactions explanations

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). 

Different catalysts may form coke at different rates. There has been a report [16] of studies conducted on the effect of coke formation on platforming catalysts with different metal dispersions. In the report, it was found that small crystallites had a stronger resistance to coke deposition than larger ones. The explanation for this fact was that cyclopentadiene (cyclopentane in nitrogen was used as feed) adsorbed more strongly on the big crystallites and formed coke. The effect was due to less electronic interaction with the support than for smaller crystallites. Another study [7] on promotion of reforming catalysts with lithium found out that the promoted catalysts became more stable against coke formation. 

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