Reaction on Bimetallic Catalysts

There is extensive work to report on dehydrogenation of cycloalkanes on bimetallic catalysts, mainly in the supported form. Its motivation is quite clear to minimise parasitic reactions such as carbon deposition and hydrogenolysis, so as to have a catalytic system capable of working for long periods of time and at high selectivity. 

Experiments with PtPb/Al203 and with PtSb/Al203 have shown that these additives have similar effects to those produced by tin. They decrease activity at low temperature by occupying active centre, but their beneficial effect is shown at high temperatures, where they diminish carbon formation on the metal, and help to maintain activity. In trying to assess the role of the second component, attention must be given to the conditions under which the measurements are made. 

The inclusion of tin in an Ir/AlaOs catalyst (5wt.% of each) completely suppressed the hydrogenolysis of cyclohexane, which in its absence gave n-hexane as a major by-product the rate at 526 K was however decreased, but the activation energy fell from 208 to 125 kJ mol. This remarkable and important result 

The platinum-ruthenium couple is well known to exhibit synergism in a number of catalytic processes, especially those of an electrochemical character, but the activity of ruthenium for hydrogenolysis greatly exceeds that of platinum. With PtRu/Al203, this was a linear function of composition in the cyclohexane-hydrogen reaction, but a maximum rate of dehydrogenation at 570 K was found when the surface contained about 55% platinum.  

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To test the prediction that Cu would improve the selectivity of Ag as an ethylene epoxidation catalyst, Linic, Jankowiak, and Barteau tested several Cu/ Ag catalysts on porous a-Al203 monoliths.56 The Cu concentrations in the catalysts varied from 0 to 0.8 at.%, so only a single bimetallic phase was expected. The catalysts were tested under conditions where the temperature, the feed composition of ethylene and oxygen, and the conversion of ethylene were held constant. Under these conditions, the selectivity of the catalyst increased significantly as the Cu content was increased from 0 to 0.2 at.%. This is a dramatic success the DFT calculations of transition states for competing reactions on bimetallic catalysts lead directly to the experimental identification of an improved catalyst. 

The results of our early exploratory studies on bimetallic catalysts clearly demonstrated that selectivity effects are important in hydrocarbon reactions on bimetallic catalysts. To obtain insight into the factors involved in the selectivity effects, it is useful at this point to inquire into the nature of the hydrocarbon reactions under consideration. 

There are few reports of alkene-deuterium reactions on bimetallic catalysts, but those few contain some points of interest. On very dilute solutions of nickel in copper (as foil), the only product of the reaction with ethene was ethene-di it is not clear whether the scarcity of deuterium atoms close to the presumably isolated nickels inhibits ethane formation, so that alkyl reversal is the only option, or whether (as with nickel film, see above) the exchange occurs by dissociative adsorption of the ethene. Problems also arise in the use of bimetallic powders containing copper plus either nickel, palladium or platinum. Activation energies for the exchange of propene were similar to those for the pure metals (33-43 kJ mol ) and rates were faster than for copper, but the distribution of deuterium atoms in the propene-di clearly resembled that shown by copper. It was suggested that the active centre comprised atoms of both kinds. On Cu/ZnO, the reaction of ethene with deuterium gave only ethane-d2. as hydrogens in the hydroxylated zinc oxide surface did not participate by reverse spillover. ... 

In the work of the author and his associates on bimetallic catalysts comprising various combinations of Group VIII and Group IB metals, it was discovered that the activity of the Group VIII metal for hydrogenolysis reactions of hydrocarbons was decreased markedly by the presence of the Group IB metal (11-15). It was shown that the inhibition of hydrogenolysis leads to improved selectivity for alkane isomerization reactions (11) and for reactions in which saturated hydrocarbons are converted to aromatic hydrocarbons (12,14,15). Interest in bimetallic catalysts increased markedly with the discovery of this selectivity phenomenon. 

During our research on bimetallic catalysts, it was evident very early that the activities of a metal catalyst for different reactions could be altered to markedly different degrees by the incorporation of a second metallic element into the catalyst. The discussion begins with a brief review of our early exploratory studies of this selectivity phenomenon, with emphasis on hydrocarbon reac tions such as isomerization of alkanes, aromatization of alkylcydopentanes, dehydrogenation of cyclohexanes to aromatic hydrocarbons, and hydro-genolysis of alkanes and cycloalkanes. 

The consideration of rates and mechanistic aspects of reactions such as hydrogenolysis, dehydrogenation, and isomerization provides a basis for interpreting selectivity data on bimetallic catalysts. 

The use of bimetallic catalysts in hydrocarbon reactions have extensively been studied because increased activity, selectivity and stability of the catalyst can be attained with the addition of a second metal. The disadvantage of studying catalytic phenomena on bimetallic catalysts prepared by a conventional coimpregnation method is that the catalyst surfaces are often heterogeneous, which makes it difficult to the catalytic systems. The use of bimetallic clusters as precursors has great advantages for preparation of relatively uniform bimetallic reaction sites well dispersed on oxide surfaces. 

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... 

According to Viljava et al. (2000) and Furminsky (2000), the preferred route hydrogenation-hydrogenolysis occurs on bimetallic catalysts sulfated, while the route HDO occurs on the acid sites. Therefore, the sulfidation is necessary for the activity of the catalysts and to promote the reaction of HDO. 

Based on the research results of monometallic catalysts, scientists also studied on bimetallic catalysts for N2 activation. They realized that the adsorption energy of N2 determines the catalysts properties. Under specific reaction conditions, it can estimate adsorption energy of N2 on catalyst. The catalytic efficiency of the elements for the synthesis and decomposition of ammonia was correlated with the chemisorption energy of nitrogen. An inverted parabolic function (volcano curve) was obtained by Ozaki et in which iron, ruthenium, and osmium mark the top of the volcano. 

To proceed with the topic of this section. Refs. 250 and 251 provide oversights of the application of contemporary surface science and bonding theory to catalytic situations. The development of bimetallic catalysts is discussed in Ref. 252. Finally, Weisz [253] discusses windows on reality the acceptable range of rates for a given type of catalyzed reaction is relatively narrow. The reaction becomes impractical if it is too slow, and if it is too fast, mass and heat transport problems become limiting. 

OH adsorption on Ru is a key factor that makes this metal the major component of various bimetallic catalysts for anode reactions. Ru-OH causes a signihcant inhibition of the ORR [Inoue et al., 2002]. In situ SXS data for the oxidation of Ru(OOOl) in acid... 

A significant volume of literature relates to our work. Concerning choice of support, Montassier et al. have examined silica-supported catalysts with Pt, Co, Rh Ru and Ir catalysts.However, these systems are not stable to hydrothermal conditions. Carbon offers a stable support option. However, the prior art with respect to carbon-supported catalysts has generally focused on Ru and Pt as metals.Additionally, unsupported catalysts have also been reported effective including Raney metals (metal sponges).Although the bulk of the literature is based on mono-metallic systems, Maris et al. recently reported on bimetallic carbon-supported catalysts with Pt/Ru and Au/Ru. In contrast, our work focuses primarily on the development of a class of rhenium-based carbon supported catalysts that have demonstrated performance equal to or better than much of the prior art. A proposed reaction mechartism is shown in Figure 34.2 °l... 

Besides direct reduction, a one-pot reductive amination of aldehydes and ketones with a-picoline-borane in methanol, in water, and in neat conditions gives the corresponding amine products (Scheme 8.2).40 The synthesis of primary amines can be performed via the reductive amination of the corresponding carbonyl compounds with aqueous ammonia with soluble Rh-catalyst (Eq. 8.17).41 Up to an 86% yield and a 97% selectivity for benzylamines were obtained for the reaction of various benzaldehydes. The use of a bimetallic catalyst based on Rh/Ir is preferable for aliphatic aldehydes. 

Lanthanides in combination with transition metals have been shown to have a positive effect in promoting heterogeneous catalytic reactions. The bimetallic Yb—Pd catalyst obtained from the precursor (pMF)i0Yb2 Pd(CN)4]3 K on a titania surface offers improved performance over a palladium-only catalyst for the reduction of NO by CH4 in the presence of 02.99 100 The structure, shown in Figure 6, consists of two inverted parallel zigzag chains that are connected through the lanthanide atoms by trans-bridging [Pd(CN)4]2- anions.101... 

Methylcyclopentane is a powerful probe molecule for the study of metal surfaces. The product distribution on platinum depends on the following factors particle size 491 reaction conditions 492-494 carbonaceous residues,492,493,495 and the extent of the interface between the metal and the support.492,493,495 The hydrogenolysis rate of methylcyclopentane depends on the hydrogen pressure.496,497 The rate exhibits a maximal value as a function of hydrogen pressure on EuroPt catalysts.498 The hydrogenolysis of methylcyclopentane has also been studied over Pt-Ru bimetallic catalysts.499... 

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