Hydrogen spillover activated process

The above studies are not necessarily in conflict with more conventional methods of spillover activation of an oxide in the presence of a metal. Each of these studies involved attempts to activate oxides at low temperatures. However, the results of these studies and more conventional spillover experiments confirm that the process is activated and may be enhanced by treatment at higher temperatures. Yet each study suggests that atomic hydrogen (as a radical or an ion) is involved in the creation of active sites on various oxides. 

Recent studies by Maret et al. have investigated the process of spillover activation of amorphous alumina (135,136). If the sample was cooled in H2 from 430 to 180°C with the source of spillover present (supported Pt), no induction period was found for ethylene hydrogenation. This implies that... 

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). 

The conclusion must be that the support is the cause of the slow uptake of H2 by the catalyst and this suggests that, in addition to H2 being bonded to the metal, H2 must be bonded to the support. It is of course well known that supports like Ti(>2 and WO3 can be reduced by hydrogen atoms which spill over from metal particles to the support and it is not unlogic to presume that spillover is an activated process, and thus is slow at room... 

Note that this method enables one to observe variation of electric conductivity of a sample due to adsorption of hydrogen atoms appearing as a result of the spillover effect, no more. In a S3rstem based on this effect it is rather difficult to estimate the flux intensity of active particles between the two phases (an activator and a carrier). The intensity value obtained from such an experiment is always somewhat lower due to the interference of two opposite processes in such a sample, namely, birth of active particles on an activator and their recombination. When using such a complicated system as a semiconductor sensor of molecular hydrogen (in the case under consideration), one should properly choose both the carrier and the activator, and take care of optimal coverage of the carrier surface with metal globules and effect of their size [36]. 

Fig. 21. Possible energy coordinate for the processes associated with spillover from a metal (M) onto an oxide support. The circled numbers refer to the equations in the introduction (1 and 2), activated adsorption (4), spillover (5), surface diffusion on the oxide (6), activation of the oxide support. The active sites are created by spiltover hydrogen at high temperatures. See text for more details.

Another influence may arise from the oxide support (Si02, AI2O3, Ti02, La203, MgO, ZnO, etc.). Although the discussion of the role of the support in CO hydrogenation is still controversial (249), strong support effects on activity and selectivity are well established (368,370,380,381). Spillover processes were observed (382), and possible intermediates (e.g., formate) were found to be bound to the oxide (379). 

Spillover hydrogen can also be demonstrated through its participation in catalytic processes. A recent report (22) described HTR Pt/titania as more active for acetone hydrogenation than Pt/Si02- This reaction is believed to occur via coordination of acetone to the oxide surface followed by attack of spillover hydrogen. 

The Eqs. (6.41), (6.42) and (6.43) are all the hydrogenation processes of the edge carbon atom with unsaturated bond in activated carbon. Edge carbon atoms have unsaturated dangling bond, which is easily covalently bonded with spillover hydrogen atom. 

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