Bimetallic catalytic data

The results of the EXAFS studies on supported bimetallic catalysts have provided excellent confirmation of earlier conclusions (21-24) regarding the existence of bimetallic clusters in these catalysts. Moreover, major structural features of bimetallic clusters deduced from chemisorption and catalytic data (21-24), or anticipated from considerations of the miscibility or surface energies of the components (13-15), received additional support from the EXAFS data. From another point of view, it can also be said that the bimetallic catalyst systems provided a critical test of the EXAFS method for investigations of catalyst structure (17). The application of EXAFS in conjunction with studies employing ( mical probes and other types of physical probes was an important feature of the work (25). 

The spectroscopic and kinetic data from this reaction indicated the existence of a long sought catalytic reaction topology, bimetallic catalytic binuclear elimination. The kinetic data provided a linear-bilinear form in organometallics [95]. One term represented the classic unicyclic rhodium catalyzed hydroformylation and the other represented the attack of manganese hydride carbonyl on an acyl rhodium tetracarbonyl species. A representation of the interconnected topology is shown in Figure 4.12. 

It is particularly interesting that selectivity effects have been observed even for bimetallic systems that do not form solid solutions in the bulk. For example, ruthenium and copper are virtually completely immiscible in the bulk (12), yet the catalytic data provide evidence of marked interaction between these two components in ruthenium-copper catalysts. This phenomenon is discussed in greater detail in Chapters 3 and 4. 

Colloids embedded in a silica sol-gel matrix were prepared by using fully alloyed Pd-Au particles. The Mossbauer data have yielded evidence that alloying Pd with Au in bimetallic colloids leads to enhanced catalytic hydrogenation and also to improved selectivity [426]. 

Catalytic asymmetric cyanide addition to imines constitutes an important C—C bondforming reaction, as the product amino nitriles may be converted to non-proteogenic a-amino acids. Kobayashi and co-workers have developed two different versions of the Zr-catalyzed amino nitrile synthesis [73]. The first variant is summarized in Scheme 6.22. The bimetallic complex 65, formed from two molecules of 6-Br-binol and one molecule of 2-Br-binol in the presence of two molecules of Zr(OtBu)4 and N-methylimidazole, was proposed as the active catalytic species. This hypothesis was based on various NMR studies more rigorous kinetic data are not as yet available. Nonetheless, as depicted in Scheme 6.22, reaction of o-hydroxyl imine 66 with 5 mol% 65 and 1—1.5 equiv. Bu3SnCN (CH2C12, —45 °C) leads to the formation of amino nitrile 67 with 91 % ee and in 92 % isolated yield. As is also shown in Scheme 6.22, electron-withdrawing (— 68) and electron-rich (—> 69), as well as more sterically hindered aryl substituents (— 70) readily undergo asymmetric cyanide addition. 

The electrocatalytic activities data indicate that there is a major enhancement in comparison with that of Au/C catalysts in terms of the peak potential (by -500 mV) and the peak current (by 20 x). The presence of a small fraction of Pt in the Au-based bimetallic nanoparticles significantly modified the catalytic properties. While a detailed characterization is needed, it is clear that the bimetallic AuPt composition played a significant role in the observed modification of the catalytic properties. 

Compensation trends are found in the kinetic data reported for the catalytic hydrogenation of several hydrocarbons on bimetallic systems, including the reactions of ethylene on Ni-Cu and on Ni-Au alloys (252), buta-1,3-diene on Ni- Cu alloys (183), benzene on Ni-Cu (but the behavior of benzene on Cu-Pd was less obviously compensatory) (253), and the liquid phase hydrogenation of nitrobenzene on Pd Ag alloys (56). The kinetic characteristics of but-2-yne on Pd-Au alloys (28) was different in that E underwent little variation, while the value of log A systematically diminished as the proportion of palladium in the alloys was reduced. It was concluded that palladium was the active constituent and gold was an inactive diluent. 

Just as DENs particle sizes have some distribution (albeit relatively narrow), there is surely some distribution in particle compositions for bimetallic DENs. This is a fundamentally important aspect of DENs, particularly with regard to their catalytic properties however, there are presently no reliable characterization methods for evaluating particle composition distributions. One method that has been applied to PdAu [21] and PtPd [19] DENs, as well as dendrimer-templated PtAu [24] is to collect single particle EDS spectra from several (15-20) nanoparticles. These experiments indicate that individual particle composition distributions may vary widely, but the difficulty in obtaining data from the smallest particles may skew the results somewhat. EDS spectra collected over large areas, which sample tens or hundreds of particles, generally agree well with the bulk composition measurements [24] and with stoichiometries set in nanoparticle synthesis [19,21,24]. 

In the course of IR pyrolysis, according to mass spectrometry and gas chromatography data, various gas products of destruction of PAN polymeric chain are present in the reaction chamber, including hydrocarbons such as ethylene and propylene [17, 18], These hydrocarbons provide the carbon source. Catalytic decompositions of hydrocarbons at high intensity IR-radiation in the presence of metallic Gd leads to the formation of carbon nanostructures such as observed bamboo-like CNT. It is well known that Ni, Co Fe have conventionally been used widely as metallic catalysts for high temperature pyrolysis of hydrocarbons. Recently bimetallic components was shown to be more effective than single metals as catalysts. Especially transition metals with addition of rare-earth metals such as Y, Ce, Tb, La and Ho [19]. In this work catalytic activity of single metallic Gd in the IR-pyrolysis of hydrocarbons are found by us for the first time. 

The catalytic activity of similar polymer-protected bimetallics has been found to vary strongly with composition (182). Because of the lack of NMR data for the palladium sites, we cannot quantitatively relate this variation to the surface LDOS, but a qualitative discussion can be given. The Pt NMR has shown that the interior of the alloy particles is bulk-like. In the bulk alloys the Ef LDOS on both Pt and Pd sites varies strongly with composition around x = 0.8 (184). It is supposed, but not proven, that on the surfaces of the alloy particles the E[ LDOS changes strongly with composition as well and that this explains the variation in catalytic activity. 

Among ordered bimetallic systems, the Pt-Sn one can be considered at present as the most in-depth studied not only for its surface structural properties, but also for its reactivity and catalytic properties. A comparable detailed knowledge exists only for a few other cases, among platinum alloys we can cite the Ni-Pt and Co-Pt systems, examined for their catalytic properties and the Pt-Ti system studied for their electrocatalytic properties [5]. Sparse data relative to the surface properties of several other Pt alloys exist (e.g. FeaPt and CuaPt -[3] and PtaMn [51]. All these data available pertain to fee phases either random substitutional or ordered compounds. Data exist also for other cubic ordered alloys which are isostructural with the PtaSn compound, e.g. NiaAl [52, 53] and AuaPd [28] and finally the Au-Cu system, which has been object of interest as the prototypical LI2 or Pm3m ordered system in the CuaAu composition [54, 55]. 

In contrast to the behavior of CO, the decomposition of ethylene is a facile process when performed on a nickel catalyst, but does not occur when the hydrocarbon is passed over iron. Based on these data we can rationalize the observed deactivation behavior observed in the present investigations according to the notion that at 725°C, the surface of the bimetallic particles become enriched in nickel, a condition that favors decomposition of adsorbed ethylene molecules, but is inert with regard to catalyzed disproportionation of CO. Subsequent lowering of the temperature to 600°C results in the restoration of the original surface composition and the concomitant attainment of the initial catalytic reactivity pattern. 

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