Hydrogenation spillover

The effect of spillover was observed for different species such as H,68 O69 n,70 NO64 or CO.69 Most of the research has been carried out with hydrogen spillover. 

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). 

It is evident that the supported clusters have a strong affinity for hydride ligands provided by the support. The process by which the support delivers these ligands is referred to in the catalysis literature as reverse hydrogen spillover. The opposite process (spillover), well known for supported metals [36], is shown by the theoretical results to be a redox process in reverse spillover, the support hydroxyl groups oxidize the cluster. 

It was initially believed that the promoter and Mo sulfides were individual crystallites in intimate contact (touching) and that the promoter aided hydrogen activation. Then it was proposed that the crystallites may not need be in direct contact as hydrogen spillover to the support could accomplish the same objective (see Fig. 16b). However, slowly it became apparent that the promoter was not effective as a separate sulfide crystallite but was actually only effective if it was present in some form on the surface of the MoS, crystallites (1-3). An early proposal suggested that the promoter is bonded to the support, which would lead to higher stability of a deposited MoS2 monolayer (50), as illustrated in Fig. 16c. However, the chemistry was subsequently found to be more subtle. 

Ti02 with high surface area would hardly take place. The dependence of the water-photolysis yield on electrolyte, which was described before, may also be ascribed to the difference in the extent of hydrogen spillover. 

Diffusion of atoms from the point at which they dissociate on a metal surface to the edge of the metal crystallite is one of the component steps of hydrogen spillover. Quasielastic neutron scattering experiments have produced direct evidence for the diffusion coefficients of hydrogen on the surface of catalysts. The mean time between diffusional jumps for hydrogen on a Raney Ni surface has been found to be 2.7 0.5 x 10 9s at 150°C.72 For H on the surface of Pt crystals dispersed within a Y type zeolite the mean time between surface jumps was found73 to lie between 3.0 and 8 x 10-9s at 100 °C. 

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. 

L. Chen et al., Mechanistic study on hydrogen spillover onto graphitic carbon materials. J. Phys. Chem. C 111, 18995 (2007)... 

Keywords hydrogen absorption, composite alloys, metal hydride electrodes, hydrogen spillover... 

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.

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