Hydrogen spillover catalytic activity

According to long-lasting experimental efforts, the use of alloy catalysts that contain a less noble metal whose oxide exhibits low solubilities in acid electrolytes—in particular Sn and Bi are effective in this respect—enhance the catalytic activity of platinum. The rationale of this effect has been that the oxide of the nonnoble component at close atomic distance from the Pt surface atoms supplies by spillover the oxygen that is necessary to oxidize the adsorbed CO species. Today research and development turn more to Ru and lr or Rh, the more easily oxidizable platinum metals as alloying metals that seem to be at least as efficient as Bi and Sn and are certainly more stable than those in acidic environments—in particular if the anode potential becomes more anodic in cases of poor supply of fuel (158). The Pt-Ru anode exhibits a sizeable higher oxidation current for methanol and for adsorbed hydrogen than the Pt electrode, indication that a smaller part of the Pt electrode surface is blocked by CO adsorption. Still the catalytic activity is too low because the onset of the anodic peak of methanol oxidation is at a... 

Burch and Flambard (113) have recently studied the H2 chemisorption capacities and CO/H2 activities of Ni on titania catalysts. They attributed the enhancement of the catalytic activities for the CO/H2 reaction (after activation in H2 at 450°C) to an interfacial metal-support interaction (IFMSI). This interaction is between large particles of Ni and reduced titanium ions the Ti3+ is promoted by hydrogen spillover from Ni to the support, as pictured in Fig. 8. The IFMSI state differs from the SMSI state since hydrogen still chemisorbs in a normal way however, if the activation temperature is raised to 650°C, both the CO/H2 activity and the hydrogen chemisorption are suppressed. They define this condition as a total SMSI state. Between the temperature limits, they assumed a progressive transition from IFMSI to SMSI. Such an intermediate continuous sequence had been... 

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... 

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). 

It has been shown in the previous sections that the addition of small amounts of a transition metal to various metal oxides lowers the temperature required for their reduction by H2. This phenomenon has been attributed to hydrogen spillover. It follows that a partial reduction of the host oxide can induce or modify the catalytic activity of the host material. 

The behavior of 1,3-cyclohexadiene in the presence of hydrogen at 170°C is very similar to that of benzene (Fig. 17), with a transient formation of acetylene. Similarly, in the presence of He, 1,3-cyclohexadiene is cracked into acetylene, and this reaction can be repeated for many successive doses of the reactant. Now, the behavior of the isomer, 1,4-cyclohexadiene, is different because in the presence of hydrogen as well as He this reactant is only cracked into acetylene (182,183). The explanation of this different evolution has been provided before 1,4-cyclohexadiene is indeed a poison for the hydrogenation of acetylene and the stepwise reaction of hydrogenolysis of 1,4-cyclohexadiene (in the presence of H2) stops with the production of acetylene. Finally, it should be mentioned that cyclohexene, either in H2 or in He, is not catalytically transformed on silica activated by hydrogen spillover. 

To sum up this section, it appears that some new and unforeseen catalytic properties may be induced on refractory oxides by hydrogen spillover, and to a lesser extent by oxygen spillover. If the activator, like Pt/Al203 catalyst, is not separated after the spillover activation from the activated oxide, these new properties could be masked by the catalytic activity of Pt/Al203. 

The above discussion demonstrates the multifaceted nature of spillover. The interphase transport of an activated species onto a surface (and sometimes into the bulk) where it is unable to be formed without the activator can induce a variety of changes on, and reactions with, the surface. All the reactions of atomic hydrogen are found to be induced by spillover exchange, bronze formation, reduction, demethoxylation, and catalytic activation. An activated species is able to gain indirect access to the nonsorbing surface. 

The influence of spillover species on an acceptor phase can be in the extreme either subtle or profound. Many of the phenomena associated with hydrogen spillover are as subtle as the influences of type-2 hydrogen on the activity of ZnO (189) or as significant as bulk reduction, bronze formation, or catalytic activation. The effects may be similar to the exposure of a surface to a hydrogen plasma. 

Pt/HZSM-5 showed high and stable catalytic activity for the hydrodesulfurization of thiophene at 400 C and its catalytic activity was higher than that of commercial C0M0/AI2O3 catalyst. It is concluded that the Brdnsted acid site and spillover hydrogen formed on Pt particle in Pt/HZSM-5 catalyst play an important role for the hydrodesulfurization of thiophene. 

Participation of spillover hydrogen in the hydrodesulfurization of thiophene over Pt/HZSM-5 catalyst was assumed and examined. It was found that catalytic activity of PWSi02(quartz) mixed mechanically with HZSM-5 was higher than that obtained by simple addition of the data for the pure components(Figure 5). This implies that spillover hydrogen on Pt/HZSM-5 catalyst participates in the hydrodesulfurization of thiophene. The mechanism proposed is shown in Scheme 1. 

Promotional effects of Co or Ni are also construed in terms of a "remote control" theory proposed by Delmon and co-workers [3]. On the basis of the findings that catalytic synergies are generated even for physical mixtures of supported component sulfides, it is claimed in this theory that the catalytic, activity of Mo sulfides is enhanced by spillover hydrogen originally generated on highly dispersed Co or Ni promoter sulfides in the proximity. 

The high and stable catalytic activity of Pd-hybrid catalyst in hydrogen atmosphere should be attributed the proton (H+so) formed in the spillover phenomenon lioni Pd to H-ZSM-5. 

Furthermore, it was assumed the existence of spillover hydrogen in the hydrodesulfurization of thiophene over RhAJSY catalyst. Thus, we tried to confirm the existence of spillover hydrogen in the hydrodesulfurization of thiophene over RH/USY catalyst. The catalytic activity of Rh/Si02(quartz) mixed mechanically with USY in the hydrodesulfurization of thiophene was examined. It was found that the activity of mixed catalyst obtained experimentally was higher than that calculated theoretically as shown in Figure 5. 

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