Bimetallic catalysts mechanism

TPR of supported bimetallic catalysts often reveals whether the two metals are in contact or not. The TPR pattern of the 1 1 FeRh/SiOi catalyst in Fig. 2.4 shows that the bimetallic combination reduces largely in the same temperature range as the rhodium catalyst does, indicating that rhodium catalyzes the reduction of the less noble iron. This forms evidence that rhodium and iron are well mixed in the fresh catalyst. The reduction mechanism is as follows. As soon as rhodium becomes metallic it causes hydrogen to dissociate atomic hydrogen migrates to iron oxide in contact with metallic rhodium and reduces the oxide instantaneously. 

To summarize, TPR is a highly useful technique, which provides a quick characterization of metallic catalysts. It gives information on the phases present after impregnation and on the eventual degree of reduction. For bimetallic catalysts, TPR patterns often indicate whether or not the two components are mixed. In favorable cases, where the catalyst particles are uniform, TPR yields activation energies for the reduction as well as information on the mechanism of reduction. 

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 ... 

Current views on the surface enrichment of one component over another in alloy systems are, surprisingly, more a consequence of gas titration and Auger electron spectroscopy than XPS and UPS. There is little doubt, however, that looking to the future XPS will provide important clues regarding the mechanism of bimetallic catalysts, the significance of promoters. 

Cg Dehydrocyclization. Arguments have been put forward that primary ring closure produces six-membered rings over three important catalyst types oxides, supported platinum, and bimetallic catalysts (107). The postulation of metal catalyzed Cg ring closure does not involve any definite suggestion whether its mechanism is direct or stepwise. ... 

In core- (and focal point-) functionalized dendrimers, the catalyst may benefit from the site isolation created by the environment of the dendritic structure. Site-isolation effects in dendrimers can also be beneficial for other functionalities (a review of this topic has appeared in Reference (10)). When reactions are deactivated by excess ligand and when a bimetallic deactivation mechanism is operative, core-functionalized dendrimers can minimize the deactivation. 

A stepwise reaction mechanism which involves adsorption of nitrate at a bimetallic site, reduction to nitrite, desorption in to the aqueous phase and re-adsorption at a monometallic e.g. Pd) site has been proposed and is supported by theoretical prediction. A reaction scheme based on the use of a bimetallic catalyst is illustrated in Fig. 2. 

In the presence of a large excess of EtO ion, the bimetallic catalyst is fully saturated with EtO as shown by structure I in Scheme 5.3. Incremental additions of a carboxylate substrate would cause the gradual conversion of I into the 1 1 productive complex II, but further additions would yield the unproductive complex III. As expected from this mechanism a bell-shaped profile is observed in a plot of initial rate versus substrate concentration related to the catalyzed ethanolysis of 16 (Figure 5.5). The fairly good quality of the fit supports the validity of Scheme 5.3. Further confirmation comes from the finding that benzoate anions behave as competitive inhibitors of the reaction. Since the reaction product of the ethanolysis of 16 is also a benzoate anion, product inhibition is expected. Indeed, only four to five turnovers are seen in the ethanolysis of 16 before product inhibition shuts down the reaction. The first two turnovers are shown graphically in Figure 5.6. 

Shubina and Koper also claimed that for the electrochemical oxidation of CO on bimetallic catalyst surfaces, the bifunctional effect is the most dominant mechanism. The more oxophylic element Ru or Sn seems to provide the oxygen donor, which is commonly believed to be adsorbed hydroxyl group, via activation of adsorbed water at a smaller electrode overpotential,... 

For a further discussion of the structure and properties of bimetallic systems, see Sections 2.6 and 3.2.3 for the preparation of bimetallic catalysts, see Section 4.6 and for the mechanisms by which they work in oxidations, see Section 8.2.2. Most textbooks of physical chemistry have sections on adsorption and catalysis, but they frequently focus on studies made under ultra-high vacuum conditions with single crystal surfaces. While this work produces beautiful pictures, it has limited relevance to the more mundane world of practical catalysis. Other introductory treatments of about the level of this chapter, or slightly more advanced, are available,5,7,11 as are deeper discussions of the kinetics of catalysed reactions.12 14 Industrial processes using catalysts have also been described in detail.15,16... 

In bimetallic catalysts prepared by catalytic reduction of copper by hydrogen, copper is deposited as three-dimensional agglomerates which are located, at low copper loadings, on the edges, corners, and rims of the parent metallic particles. The mechanism of deposition can be transferred from that proposed in corrosion and involving a local electrochemical cell ... 

We have reported on a tandem procedure for the synthesis of 3-allyl-N-(alkoxycarbonyl)indoles 115 via the reaction of 2-(alkynyl)phenylisocyanates 114 and allyl carbonates 5 in the presence of Pd(PPh3)4 (lmol%) and CuCl (4 mol%) bimetallic catalyst [80]. A proposed mechanism is shown in Scheme 35. Initially, the insertion of the isocyanates 114 into the complex 7, formed by the reaction of 5 with Pd(0), would form the 7r-allylpalladium intermediates 117. This intermediate, with Pd - N bonding, could be in equilibrium with the Pd - O bonded intermediates 118, which should more probably be represented as the bis-7r-allylpalladium analogue 119. Insertion of the alkyne then occurs to form the indoles 115 and the Pd(0) species is regenerated. It should be emphasized that no carboamination takes place at all in the absence of CuCl the product 116 was obtained. 

Since on pure platinum, methanol oxidation is strongly inhibited by poison formation, bimetallic catalysts such as PtRu or PtSn, which partially overcome this problem, have received renewed attention as interesting electrocatalysts for low-temperature fuel cell applications, and consequently much research into the structure, composition, and mechanism of their catalytic activity is now being undertaken at both a fundamental and applied level [62,77]. Presently, binary PtRu catalysts for methanol oxidation are researched in diverse forms PtRu alloys [55,63,95], Ru electrodeposits on Pt [96,97], PtRu codeposits [62,98], and Ru adsorbed on Pt [99]. The emphasis has recently been placed on producing high-activity surfaces made of platinum/ruthenium composites as a catalyst for methanol oxidation [100]. 

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