Bifunctional/bimetallic catalysts

LCB-PE (long chain branched polyethylene) is not accessible through simple ethyl-ene/a-olefin copolymerization. Therefore, a bifunctional bimetallic catalyst was developed, in which one active center oligomerizes ethene to long-chain a-olefins, while the other copolymerizes them with ethene. 

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 ... 

When one component of a bimetallic alloy is leached out, a finely divided metal powder of high surface area results. One of the oldest of these so-called skeletal metal catalysts is Raney nickel10,11. Nickel boride is a more recently developed hydrogenation catalyst prepared by the reduction of nickel salts with sodium borohydride12-14. Bimetallic catalysts are often used to achieve selective saturation of a double bond in bifunctional unsaturated systems, e.g. in dienes. Amorphous metal alloys, a newly developed class of metal catalysts15,16, have also been applied in the hydrogenation of alkenes and dienes. 

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,... 

Hydrotreating. In order to successfully reform a naphtha with the conventional bifunctional reforming catalyst, heteroatoms must be reduced to a level such that they will not deactivate the reforming catalyst. Sulfur and nitrogen levels of well below 5 ppm seem to be required for conventional reforming catalysts while levels below 1 ppm appear necessary for the newer bimetallics. The assumption was made that, if sulfur and nitrogen levels could be reduced to specification, the chemical oxygen levels would follow. 

Dynamic Monte Carlo simulations have been employed to study the effect of the bimetallic catalyst structure and CO mobility in a simple model for the electrochemical oxidation of CO on Pt-Ru alloy electrodes. The Pt-Ru surface was modeled as a square lattice of surface sites, which can either be covered by CO or OH, or be empty. The important reactions taken into account in the model reflect the generally accepted bifunctional model, in which the OH with which CO is... 

As but one example of bimetallic catalysts, Lee and Rhee75 prepared a series of bifunctional bimetallic M-Pt/H-Beta (M = Cu, Ga, Ni, and Pd) catalysts and... 

Reviewing the work on tire Pt-Ru electrocatalysts is beyond the scope of this article. We will briefly comment on some key advances in this area. Although early discovery by Petrii, and Bockris and Wrob Io wa established the catalytic activity of Pt-Ru alloys for methanol oxidation, despite of active investigation that followed, even the optimum composition of Pt-Ru is yet to be firmly settled. An early explanation for the mechanism by which bimetallic catalysts improve upon the performance of pure Pt, that is, the bifunctional mechanism proposed by Watanabe and Motoo, was recently challenged. 

The possible reactions of n-pentane on the bifunctional catalysts were presented in Equations (2) through (5). The possible types of reactions of n-hexane are the same with the addition of aromatiza-tion leading to benzene. It was shown that on mono- as well as on bimetallic catalysts, naphthenes with rings of five carbon atoms... 

Bimetallic bifunctional catalysts containing different proportions of Ni and Pt supported in HUSY zeolite were prepared and characterized by TEM, punctual EDX analysis and -hexane isomerization. The EDX analysis of the Ni and Pt bimetallic catalysts shows that the metal particles contain both metals and from HRTEM it was observed that the bimetallic particles have crystallographic parameters of metallic nickel. The presence of small platinum amounts in the nickel catalysts produces more active catalysts for the -hexane isomerization, and presents also higher selectivity for the formation of dibranched hexane than the ones containing only platinum. 

At present, there is a general consensus that PtRu offers the most promising results, and has been the benchmark bimetallic catalyst since the mid-1960s. The reason for the enhanced rate of methanol oxidation on PtRu is often invoked by the bifunctional mechanism (see Section 9.3.1), where the first step of the reaction is adsorption of methanol ... 

At present, the Pt-Ru bimetallic system is recognized as the most promising CO-tolerant anode catalyst for the DMFCs. A large body of hterature exist demonstrating improvement of the CO oxidation on the Pt-Ru alloy and Ru-modified Pt catalysts. The superior CO tolerance of the Pt-Ru bimetallic catalysts compared with the monometallic Pt catalyst is frequently explained with concepts of bifunctional mechanism [17] and ligand effect [22, 23]. The former mechanism proposed by Watanabe and Motoo is widely accepted. They claimed that the Ru has higher reactivity with water than Pt and that formation of Ru-OH at a lower potential promotes the electrooxidation of the chemisorbed CO on the Pt (formulas (4) and (5)). 

The correlation of the MOR activity of the Pt-Ru bimetallic catalysts with their microstructures was described. On the basis of the bifunctional mechanism and ligand effect, the control of the microstructures in the catalysts is crucial for the enhancement of the CO tolerance, the MOR activity, and the durability. The co-reduction of the Pt and Ru precursors is a realistic process in the industrial production of the Pt-Ru bimetallic catalysts. In the synthesis, a key is decreasing the difference in the effective reduction potentials of the Pt and Ru cations. The decrease promotes the simultaneous reduction of the cations and realizes the well-mixed microstructure, which enhances the MOR activity and the durability of the Pt-Ru bimetallic catalysts. 

A new generation of bifunctional catalysts was introduced in 1967. The catalyst containing rhenium in addition to platinum provides greater stability.In 1975, the process using a catalyst containing platinum and iridium was commercialized. These catalysts are called bimetallic catalysts. The bimettillic catalysts are typically 3 to 4 times more active than the all-platinum catalyst. A bimetallic catalyst with rhenium typically contains about 0.3% platinum and 0.3% rhenium. The reasons for the effectiveness of these bimetallic catalysts are beyond the scope of this volume and the readers should refer to the appropriate monographs or reviews. ... 

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... 

P. A. (1997) Isomerization and hydrocracking of heptane over bimetallic bifunctional PtPd/H-beta and PtPd/USY zeolite catalysts. . Catal,... 

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