Hydrogen spillover mechanism

A comparison of the 1H chemical shift for H2 adsorbed on supported Pt metals revealed substantially different shift-versus-coverage behaviour between, for example, Ru and Rh.163 The interpretation of these observations is, however, not yet clear, but discussion is raised on a hydrogen spillover mechanism. 

Recently, it has also been proposed (36) that desorption of the branched carbenium ion can also occur by H species formed by dissociation of the hydrogen reactant on the Pt centers and its diffusion to the acid sites by a hydrogen spillover mechanism ... 

In addition, if we accept the conventional mechanism of Brpnsted acid site generation by hydrogen spillover from Pt sites, as proposed for Pt-S04=/Zr02, we must recognize that this would be in fact a reduction process (zerovalent hydrogen yielding -OH groups). 

Hence the 30 % that are lost could be on the support and slowly react with 1-hexanol to form HA. However when aniline is reacted there is no significant loss of material, which suggests that aniline cannot interact directly with the surface hydroxyls. This suggests that the interaction between aniline and the support hydroxyls is not as simple as shown above, rather it is more likely that the reaction operates via a spillover mechanism involving an intermediate in the nitrobenzene hydrogenation sequence rather than aniline. The alkylation reaction between aniline and 1-hexanol takes place on the metal function, therefore the reaction with the missing aniline associated with the support will be slow as it requires a reverse spillover and a diffusion across the support surface. 

Recent experiments confirmed the role of tin added to platinum on alumina (present mainly as Sn " ) as a regulator for surface hydrogen 75a). A very specific spillover mechanism is assumed between platinum and tin atoms. The hydrogen accessibility of Pt increases proportionally to the amount of added Sn. To this end, it is sufiicient that 3-6% of the tin atoms... 

Sachtler proposes a "dual site" mechanism where the hydrogen is dissociated on the Ni surface and then migrates to the substrate which is coordinated to the adsorbed nickel-tartrate complex. In this context it is of interest that the well known Sharpless epoxidation probably takes place on a dimeric tartrate complex of Ti. Sachtler suggests that both the anion and the cation have a function which varies according to the conditions used. It is not clear whether the spillover mechanism is also proposed for the reaction in solution [55]. 

Scheme 9.1 A mechanism for metal-catalysed hydrogen spillover, shown by the exchange of support hydroxyls with deuterium. The process can extend to the whole surface (A), but HD is formed by reverse spillover (B), followed by desorption.

Several studies have attempted to generalize hydrogen spillover as one of the three types of species or to reject spillover based on studies disproving one of the assumed species types. We firmly believe that spillover is more general and may occur by a variety of mechanisms involving, under certain circumstances, any of the species. 

Associated with the presence of the dispersed platinum phase, a through>the-metal, back-spillover, mechanism, much faster than the direct recombination of the hydrogen species chemisorbed on the support, would mainly govern the desorption reaction. Also remarkable is the close analogy between trace B3 in Figure 4.7, and diagram reported in ref (204) for a Rh/Ce02 catalyst reduced at 773 K and fiirther treated with H2 in the same way as indicated above... 

Donors and Donees in Permanent Contact.—A spillover mechanism has been put forward to account for the catalysis of the hydrogenation of high area carbon to methane by impregnated metals.The activity series found (Rh >... 

On the basis of these results the following mechanisms are deduced (i) Partial reduction of the support adjacent to the platinum via hydrogen spillover, resulting in a special deactivation for MCP hydrogenolysis and in a change of product distribution. This... 

First and the most obvious conclusion, which can be drawn from the experiments described above, is that the SMSI state of Pt and other noble metals dispersed on a Ti02 support can be induced, probably more effectively, by means of the reduction with Ti metal. Consequently, the hydrogen spillover is not the only mechanism leading to SMSI. 

Further carbonaceous material is then distributed mainly to the support, whose surface area is two orders of magnitude greater than that of the platinum crystallites, by a spillover mechanism.27,47,61-65 Spillover is the transport of active species, dissociated or molecular, sorbed on a first surface, onto a second surface that does not sorb under the same conditions. Hydrogen spillover is the most rapid,63 and has been demonstrated over distances of the order of at least millimetres.66... 

A kinetic study of -hexadecane hydroconversion has been carried out over bifunctional catalysts prepared from EMT and FAU zeolites, MCM-41-type molecular sieves and amorphous siliea alumina. A unique influence of hydrogen partial pressure characterized by a sharp optimum of activity at intermediate pressures has been revealed. A reaction model including the conventional bifunctional mechanism and the generation of active sites by hydrogen spillover is proposed to describe the experimental results. 

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