Bifunctional mechanisms

Hence, the rate depends only on the ratio of the partial pressures of hydrogen and n-pentane. Support for the mechanism is provided by the fact that the rate of n-pentene isomerization on a platinum-free catalyst is very similar to that of the above reaction. The essence of the bifunctional mechanism is that the metal converts alkanes into alkenes and vice versa, enabling isomerization via the carbenium ion mechanism which allows a lower temperature than reactions involving a carbo-nium-ion formation step from an alkane. 

It is shown that these new supports induce an enhancement of the NO reduction which cannot be explained by a modification of the Pd electronic properties. A bifunctional mechanism is proposed and discussed in addition to the intrinsic roles of the metal and of the... 

In conclusion, a "specific role" of these dual "Metal-Support" sites at the metal-support interface has to be considered in addition of the "intrinsic roles" of the metal and the support and a bifunctional mechanism can be reasonably proposed. 

In conclusion, Cu on Ti02 or Zr02 show a unique and interesting behaviour since their deNOx activity is promoted and not inhibited by the presence of sulfur in the feed. This effect can hardly be attributed to a selective inhibition of the oxidation of decane, and is better explained by the promotion of a bifunctional mechanism involving the acid sites created on the support by the reaction of SO2. 

The synergistic elfect seen in Pt-Rn alloys has aronsed great interest, since it opened perspectives for their nse in efficient methanol fnel cells. Many studies were performed to elucidate the origins of this effect. Some workers believe that it is due to changes in the electron strnctnre of platinnm npon alloy formation with ruthenium. A popular interpretation is the bifunctional mechanism, according to which the organic species are preferentially chemisorbed on platinnm sites while the ruthenium sites facilitate the adsorption of the species needed for oxidation of the orgaiuc species. 

The most essential question is why the CO-free sites are secured for H2 adsorption and oxidation. Watanabe and Motoo proposed a so-called bifunctional mechanism originally found at Pt electrodes with various oxygen-adsorbing adatoms (e.g., Ru, Sn, and As), which facilitate the oxidation of adsorbed COad at Pt sites [Watanabe and Motoo, 1975a Watanabe et al., 1985]. This mechanism has been adopted for the explanation of CO-tolerant HOR on Pt-Ru, Pt-Sn, and Pt-Mo alloys [Gasteiger et al., 1994, 1995], and recently confirmed by in sim FTIR spectroscopy [Yajima et al., 2004]. To investigate the role of such surface sites, we examined the details of the alloy surface states by various methods. 

Finally, we want to compare the main mechanistic findings of our study with the classic bifunctional mechanism, which is generally used to explain the improved CO oxidation reactivity of PtRu surfaces and catalyst particles [Watanabe and Motoo, 1975]. According to that mechanism, Ru acts as a promotor for the formation of oxygenated adspecies on bimetallic PtRu surfaces, which can then react with CO... 

Thus, under HCK conditions, and under hydrodewaxing (HDW) as well, paraffins are hydroisomerized and hydrocracked by a bifunctional mechanism involving the metallic and the acid sites. This classical mechanism involves ... 

As was previously mentioned, PtRu alloys exhibit improved performance over pure Pt alloys.117,118 This is primarily a result of the ability of Ru to dissociate H20 for reaction with CO adsorbed on Pt sites.115,116 That CO oxidation on pure Ru is unfavorable indicates that on the bimetallic surface, CO is oxidized only on the Pt sites.119 Thus, CO is oxidized on Pt sites adjacent to Ru sites, where water is activated.120,121 This is known as a bifunctional mechanism. In addition, the presence of Ru atoms reduces the adsorption energy of CO on neighboring Pt atoms, lowering the activation energy of CO oxidation.122 This effect is purely electronic and is less significant than the bifunctional effect of Ru.123 One significant limitation of PtRu is the weak adsorption of methanol on Ru, particularly at room temperature.117,124 The weak adsorption severely hinders methanol decomposition, which is evident in Fig. 7 by the drop in current density for PtRu electrodes with high Ru composition.125... 

Prominent co-catalysts for the Pt-on-carbon anode catalyst in the oxidation of polyhydric alcohols are Ru or Ce02 [54, 60]. Their increased resistance to poisoning with mainly CO during operation is associated with the existence of a bifunctional mechanism (Scheme 11.6). 

Q dehydrocyclization, 29 311 ring enlargement, 29 311-316 Bifunctional Fisher-Tropsch/hydroformylation catalysts, 39 282 Bifunctional mechanism, 30 4 Bifurcation diagram, oscillatory CO/O, 37 233-234... 

On the basis of the combined weight of the above results, we believe that bifunctional electrocatalytic properties may be operative for both MOR and ORR on the AuPt bimetallic nanoparticle catalysts depending on the nature of the electrolyte. For ORR in acidic electrolyte, the approaching of both the reduction potential and the electron transfer number for the bimetallic catalyst with less than 25%Pt to those for pure Pt catalyst is indicative of a synergistic effect of Au and Pt in the catalyst. For MOR in alkaline electrol)he, the similarity of both the oxidation potential and the current density for the bimetallic catalyst with less than 25%Pt to those for pure Pt catalyst is suggestive of the operation of bifunctional mechanism. Such a bifunctional mechanism may involve the following reactions ... 

Fig. 21. General synthesis of p-CD-linked ruthenium complexes asymmetric transfer hydrogenation is described as a metal-ligand bifunctional mechanism according to 31).

It is likely that both mechanisms are active and dependent on potential. At low potentials (<200 mV) on PtRu, the bifunctional mechanism is not active because Ru is unable to dissociate adsorbed H2O to produce OH. However, above 250 mV, this does occur and CO oxidation by adsorbed OH becomes the dominant reaction in achieving CO tolerance. This is strongly related to the use of PtRu as a MeOH oxidation catalyst because CO oxidation is also the rate-determining step for this reaction. 

The general mechanism of MeOH on Pt and PtRu is well established. First, MeOH is adsorbed and subjected to multiple dehydrogenation steps to give adsorbed CO. This dehydration step is known to occur at low potentials. The adsorbed CO is then oxidized by active OH species produced by the dissociation of H2O. This is fhe pofenfial-driven rate-determining step because OH formation does not occur on Pt until higher potentials. The addition of Ru pro-mofes fhe reaction because it is able to produce OH species at lower potentials. This promotional effect is known as the "bifunctional" mechanism ... 

Figure 13.48 Bifunctional mechanism for isomerizationof ethylbenzene to xylenes via methylethylyclopentene intermediates.

Ru provides sites for water activation as well as having an electronic effect on the Pt atoms, such that CO is less strongly adsorbed. In situ XAS measurements have been used to determine the structure of PtRu catalysts, to assess the magnitude of any electronic effect that alloy formation may have on the Pt component of the catalyst, and to provide evidence in support of the bifunctional mechanism. 

The excellent catalytic activity is rationalized by a nonclassical metal-ligand bifunctional mechanism using an NH effect. As shown in Figure 1.18, frani-RuH(ri -BH4)(tolbinap)(dpen) (18A) (TolBINAP see Figure 1.2), a precata-... 

Figure 1.24. Metal-ligand bifunctional mechanism in asymmetric transfer hydrogenation of...

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