Spiltover hydrogen

For metal-on-carbon systems two aspects should be noted (i) metals and sulfided metals can act as sources of spiltover hydrogen species, and (ii) as active carbon adsorbs hydrogen by itself at high temperatures [T > 300°C (75)], the effect of spillover is to increase the rate of uptake but not the extent. With the inorganic oxides, spillover most often results in an increase in the amount of adsorption. 

The kinetics of the chemisorption of spiltover hydrogen species have been fully detailed from a mathematical and physical point of view for carbon as well as for alumina by Robell et al. (17) and Kramer and Andre (18), respectively. Both groups of authors concluded that surface diffusion is the rate-determining step in the overall process of hydrogen spillover. This is shown in Fig. 3 for platinized carbon and Fig. 4 for platinized alumina. 

Spiltover hydrogen was found by Van Meerbeck et al. (26) to reduce the surface of silica gel containing only traces of metals such as Ni, Pd, Pt, W, and Ta. When their silica samples were heated to about 1000°C under an H2 atmosphere, they detected an IR bond at 2300 cm"1 that was attributed to a... 

The intermediate states in this two-hydrogen atom reaction are not known, although Teichner et al, basing their view on the overall kinetics, have suggested that H3 is involved in the reaction H3 has been observed spectroscopically in other studies (125). Alternatively, because of the low concentration of spiltover hydrogen on the surface, a two-dimensional-three-center (2Hsp + OMe) reaction seems improbable. It may be that the reaction occurs stepwise by the association of spiltover hydrogen with a methoxyl as a first step, i.e. ... 

The extension of this concept to other surfaces would mean that spiltover hydrogen may be able to associate reversibly with surface hydroxyls. 

The host oxide lattice, moreover, is able to be reduced by spiltover hydrogen, producing water. Spillover induces lower temperatures of reduction for vanadium, uranium, chromium, cobalt, cadmium, and tin oxides (127), among others. The reduction may involve bulk transformation or it may be confined to the surface. The most studied example of this phenomena involves Ti02 and the resultant changes in sorption capabilities of the surface (SMSI), as discussed above. SMSI seems to be an extreme example of the change in chemisorptive properties with reduction and subsequent occulta-tion of the supported metal. 

A variety of surfaces have been shown to react with spiltover hydrogen. The above discussion focused on oxide surfaces where polar association is possible. Boudart et al. found that spillover occurs from Pt to carbon at modest temperatures (114). Indeed, it has been suggested that a hydrocarbon bridge assists in spillover from Pt onto the supports (46,85). The spiltover... 

In Section V below we show that spillover can induce catalytic activity on the support. The nature of the active site created on the support may result from the surface reduction, or the adsorbed hydrogen may be a center and site for reaction (123). On the other extreme, spiltover hydrogen has been shown to inhibit ortho-para conversion over sapphire and ruby surfaces... 

This brings the discussion of the changes in the solid full circle. Spiltover hydrogen can exchange with the surface. It may react with and replace methoxyls with hydroxyls. It may be incorporated into the bulk with a change in the bulk crystal structure. Bulk reduction may occur. The species spilling over may react only with the surface, with coke, or with other sorbed species. In addition, spillover may promote or inhibit reaction on the surface. 

When first proposed, the concept of spillover was not easily accepted. It has been presumed that spiltover hydrogen is present as a single species. Many recent studies have discussed the unique nature of the interface between the activating metal and the support. It is not clear that this is or is not a distinct energetic state. There is very recent evidence that there may be more than one form of adsorbed-spiltover hydrogen. These states are distinguishable from hydroxyls. 

Further evidence of the multiple nature of spiltover hydrogen comes from the pioneering studies of Beck and White 44,109,138). Hydrogen and deuterium were sorbed separately on Pt/Ti02 at 227 and 27°C, respectively the sorptions and evacuation between the sorptions were performed in rapid sequences and the sample was cooled to — 133°C and subsequently programmed with an increasing temperature to over 480°C. Separate D2 (at 77°C) and H2 (at 327°C) peaks were observed (Fig. 12). An insignificant... 

We find this explanation not totally satisfactory. Unless subsequent spillover displaces previous spillover, diffusion will involve a monotonic gradient from the source. Subsequent spillover should not displace but intermix with previously sorbed species. Since it is generally accepted that the spiltover species is atomic, it is difficult to accept that little HD is formed and the desorption peaks occur with a 250°C difference in temperature.These studies seem to give credence to the hypothesis that multiple states of spiltover hydrogen (or deuterium) exist on the surface. Further studies are needed to clarify the nature of the sorbed states, their energetics, and their number on the support surface. 

The capacity for spiltover hydrogen varies from surface to surface, although there is less discrepancy on similar supports. For oxides, in general, the coverage is less than 1 % of the surface and is, therefore, far less than the number of surface hydroxyls (25). 

Our interpretation of these diverse studies is that for most systems the spillover from metal to support is the rate-controlling step, although this may not always be the case. Our intuition is that spiltover hydrogen has only a weak bond with the support compared to its bond with the metal. The... 

Further cautions should be discussed. Whereas transport of hydrogen may occur at temperatures well below 400°C, the induction of catalytic activity on the support by spiltover hydrogen is an activated process and requires considerable time (up to 12 h of treatment at 430°C in hydrogen). Comparison of catalytically active surfaces must be done with similar temperatures and times of spillover pretreatment. To further complicate the analysis, there is evidence that an activated support (e.g., Al2Oa) may be able to dissociate hydrogen. The process may, therefore, be autocatalytic that is, the support first activated by spillover may be able to adsorb, dissociate, spill over, and consequently activate more support surface (137). 

A. Interaction between Spiltover Hydrogen, the Support, and the Reactants... 

A new explanation for catalyst synergy between two solid phases of a catalyst has recently been advanced by Delmon (174) in studies of hydrodesulfurization (HDS) with mixtures of MoS2 and Co9S8. Both the activity and the selectivity of the HDS reaction increased if the contact between the admixed phases was improved. Spiltover hydrogen from the Co9S8 partially reduces the MoS2 modest reduction creates hydrogenation sites and further reduction creates HDS sites. 

The activated silica is unable to reform the spiltover hydrogen when heated in H2 at 430°C (12 h) without the Pt/Al203 catalyst. Indeed, the hydrogenation of ethylene at 200°C shows that the activity pattern is very close to that of Fig. 11, curve B (134). 

The reaction is slow (lower abscissa) due to the presence of spiltover hydrogen. This is no longer the case for the second dose (curves B, upper... 

The spiltover hydrogen can simply be added to benzene (forming cyclohexane and cyclohexene) in a noncatalytic reaction which exhausts entirely this hydrogen species, as shown below. Also, in order to have a clearcut picture, the reaction of benzene is carried out after evacuation of silica, which has been activated. The evacuation desorbs the spiltover hydrogen. Figure 17 shows the conversion at 170°C of benzene (8 cm3) with hydrogen (1000 cm3) into ethane and initially into acetylene (182). 

Fig. 17. Hydrogenolysis of benzene at 170°C and hydrogenation of ethylene at 200°C on SiOz (A) cracking and hydrogenolysis of benzene after evacuation of the spiltover hydrogen (B) hydrogenolysis of benzene after run (A) (C) hydrogenolysis of benzene after interaction between Si02 and 02 at 430°C (D) hydrogenation of ethylene at 200°C. , Acetylene A, ethane.

It may be hydrogenated by the spiltover hydrogen in a noncatalytic reaction (see below). 

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