Activities of bimetallic catalysts

Activities and selectivities of bimetallic catalysts for hydrogenation of carvone are reported in Table 3. The results show a decrease of the specific activity of bimetallic catalysts compared to that of monometallic ones. Gold addition also modifies the selectivity patterns. The partial hydrogenation of the exo double bond is increased on both large and small particles on these catalysts carvotanacetone is the main product. 

A similar dependence of the activity of bimetallic catalysts on their composition is observed in the synthesis of hydrocarbons with increasing content of palladiiun, the activity drops by 1-2 orders of magnitude. Supporting of iron, cobalt, rutheniiim, or rhodium carbonyls on Cu/Si02 also suppressed the activity of these metals for synthesis of hydrocarbons from CO and H2 by 1-2 orders of magnitude. 

A sharp fall in activity of bimetallic catalysts for ethane hydrogenolysis with increasing palladium content seems to be due to dilution of ruthenium atoms by less-active palladiiim atoms. As the palladium content grows, the surface concentration of multiple active centers for ethane hydrogenolysis (consisting of several ruthenium atoms) decreases. But the nature of these centers appears to be unchanged, since the activation energy for the reaction remains constant. 

The activity of all catalysts were evaluated for the CO hydrogenation reaction. The histogram shown in Fig. 8 reveals that the bimetallic Co-Mo nitride system has appreciable hydrogenation activity with exception of samples 2 and 4. This apparent anomaly was probably due to the relatively high heat of adsorption for these two catalysts, which offered strong CO chemisorption but with imfavourable product release. 

It is known that the addition of Pt errhances the activity of hyam catalysts while sacrificing selectivity [4-5]. Bimetallic catalysts were prepared and tested to see if one could improve the activity of these catalysts while maintaining the good selectivity. As expected, Catalyst A with an 8% Pd + 2% Pt loading showed an increased activity of 57.3 compared to 32.7 for Catalyst A with a 10% Pd loading (Table 10.2). On the other hand, the selectivity of the bimetallic catalyst was... 

Ffirai and Toshima have published several reports on the synthesis of transition-metal nanoparticles by alcoholic reduction of metal salts in the presence of a polymer such as polyvinylalcohol (PVA) or polyvinylpyrrolidone (PVP). This simple and reproducible process can be applied for the preparation of monometallic [32, 33] or bimetallic [34—39] nanoparticles. In this series of articles, the nanoparticles are characterized by different techniques such as transmission electronic microscopy (TEM), UV-visible spectroscopy, electron diffraction (EDX), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) or extended X-ray absorption fine structure (EXAFS, bimetallic systems). The great majority of the particles have a uniform size between 1 and 3 nm. These nanomaterials are efficient catalysts for olefin or diene hydrogenation under mild conditions (30°C, Ph2 = 1 bar)- In the case of bimetallic catalysts, the catalytic activity was seen to depend on their metal composition, and this may also have an influence on the selectivity of the partial hydrogenation of dienes. 

A major improvement in the selectivity towards crotyl alcohol by the hydrogenation of crotonaldehyde has been attained by Margitfalvi et al. [91] through the modificahon of Pt/Si02 by Sn addition via SnEfi, which was then reduced at 573 K. For Sn/Pb = 1.2, both the overall activity of the catalyst and its selectivity towards the formahon of crotyl alcohol were strongly increased. On this bimetallic catalyst, the selechvity of the formation of crotyl alcohol was over 70%. 

In heterogeneous catalysis by metal, the activity and product-selectivity depend on the nature of metal particles (e.g., their size and morphology). Besides monometallic catalysts, the nanoscale preparation of bimetallic materials with controlled composition is attractive and crucial in industrial applications, since such materials show advanced performance in catalytic processes. Many reports suggest that the variation in the catalyst preparation method can yield highly dispersed metal/ alloy clusters and particles by the surface-mediated reactions [7-11]. The problem associated with conventional catalyst preparation is of reproducibility in the preparative process and activity of the catalyst materials. Moreover, the catalytic performances also depend on the chemical and spatial nature of the support due to the metal-support interaction and geometrical constraint at the interface of support and metal particles [7-9]. 

Alternative catalyst formulations for methane ATR based on bimetallic catalysts have been studied, aiming at increasing the activity of nickel catalysts by the addition of low contents of noble metals. 

Our design of bimetallic catalysts based on crown-complexed alkaline-earth metal ions, for use in reactions of ester and activated amides endowed with a distal carboxylate anchoring group, is based on the mechanistic hypothesis outlined in Scheme 5.3. Such hypothesis critically rests on the finding that in EtOH solution... 

A brief overview of bimetallic catalysts is presented. Electronic vs. ensemble effects are discussed, and literature is reviewed on single crystal bimetallics, and supported bimetallic clusters. Bimetallic cluster compounds are considered as models. Structural considerations, effects of potential poisons, particles from bimetallic cluster compounds, and catalytic activity/selectivity studies are briefly reviewed and discussed. 

In summary, the technique of catalytic reduction for the preparation of bimetallic catalysts can be extensively used with a variety of parent metals and re-ductants. However, some structure sensitivities of the reduction reactions become apparent and the modifying metal can be selectively deposited on specific sites of the parent-supported metal. Furthermore, such structure sensitivity depends on the nature of the re-ductant, and a given modifier can be deposited, according to the reductant used, selectively onto different parts of the metallic surface. In fact, a bimetallic catalyst can be tailored to provide the optimum activity, selectivity and lifetime for a given reaction. 

In the preparation and activation of a catalyst, it is often the case that the chemical form of the active element used in the synthesis differs from the final active form. For example, in the preparation of supported metal nanoclusters, a solution of a metal salt is often used to impregnate the oxide support. The catalyst is then typically dried, calcined, and finally reduced in H2 to generate the active phase highly dispersed metal clusters on the oxide support. If the catalyst contains two or more metals, then bimetallic clusters may form. The activity of the catalyst may depend on the metal loading, the calcination temperature, and the reduction temperature, among others. 

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