Spillover hydrogen

For each class, one or more of the following questions needs to be asked, and if possible answered. 

This framework should be helpful in the following survey of the five classes of behaviour. 

When the support contains cations that are not easily reducible (i.e. those of Al, Si, Mg, Zr), hydrogen spillover occurs above 573 K without observable chemical reaction (Class A). However if it contains ferric ions as impurities, as is often the case with alumina, reduction to ferrous ion is detectable by EPR and if it contains sulfate ion, as may be the case with titania, reduction of the precursor with hydrogen automatically generates hydrogen sulfide which poisons the metal (Class B). If deuterium is used in place of hydrogen, support hydroxyls 

In a recent and perhaps more significant development, the classical mechanism for the bifunctional cracking of alkanes on acidic zeolites has had to be revised to provide a role for the involvement of spiltover hydrogen at the acidic centres of a Pt/erionite catalyst.  

Perhaps the most fascinating exemplar of hydrogen spillover, and certainly the most visually convincing is the process whereby it reacts with certain oxides to be incorporated in their structures without the elimination of water. The products, which are known collectively as hydrogen bronzes, are formed especially by the oxides of tungsten, molybdenum, vanadium and rhenium (see Further Reading section). The highly coloured products (W blue Mo violet) contain ions of lower oxidation state (e.g. W , Mo , V ) formed for example as 

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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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