Catalyst intermetallic

Several catalyst types have received recent attention, all of which show promise of producing high-activity catalysts for Haber-type systems under mild conditions. These can be classified as supported catalysts, intermetallic catalysts, and precipitated catalysts. These classifications are not rigid, neither are they necessarily absolutely distinct from the current range of promoted catalysts made by fusion, and the references cited below are only exemplary. 

Multimetallic catalysts, alloy catalysts, intermetallic compounds, fuel cell catalysts, colloidal intermediates, metal complexes, and metal clusters have received considerable attention [12-21] because the metal-metal cooperating bifunctional catalysts which can activate reactants simultaneously showed high catalytic activity and stereoselectivity under mild conditions [19]. In fact, there have been many bifunctional multimetallic catalysts in which multimetallic alloy- and electro-catalysts offer a way to fine-tune the catalytic properties of metals, atomic composition, and microstrucrnres [16-18, 20]. Cooperative multimetallic activation of oxidants via the multielectron transfer is also a common feature in biological oxidation catalysis [14]. Artificial multimetallic complexes with two or more metal atoms that contain... 

Surface Characterization and Methanation Activity of Catalysts Derived from Binary and Ternary Intermetallics... 

The surface properties of three types of methanation catalysts obtained by oxidation of selected Intermetallics were examined In relation to their CO conversion activity. The first type (Ni Si, N1 A1 ) which corresponds to active phase-supporl iX the coXventionally prepared catalyst Is little affected by the oxidation treatment. The surface Nl is oxidized and relatively more abundant In the active solids. The second type (active phase-promoter ex Ni Th ) is extensively decomposed on oxidation. The transformation of these alloys Is accompanied by a surface enrichment in Nl. 

Fe, and B SI, Tl, Th, and Ce). The transforaed materials exhibit high specific activity In methanation, ammonia synthesis, and ethylene hydrogenation reactions. The similarity between the industrial methanation catalysts and catalysts obtained by decomposition of various Intermetallics Is striking. Most catalysts obtained by decomposition of a binary alloy Involve an associative combination of... 

The first set of experiments was conducted in methanol. The substrate concentration was varied from 15 to 50 mM at a 200 pM concentration of 1 for the determination of kinetic parameters for the transformation of 8 into 9. The catalytic rate constant was determined to be 0.04 min and the Michael constant was determined to be 40 mM at 30°C. The rate constant is comparable to those reported for other dinuclear Cu(ll) complexes with a comparable Cu -Cu distance of 3.5 A, but about one magnitude lower than those observed for complexes with a shorter intermetallic distances (12-14), e.g. 2.9 A (kcat = 0.21 min ) (12) or 3.075 A (kcat = 0.32 min (13). The rate constant Aion for the spontaneous (imcatalyzed) oxidation of 8 into 9 was determined to be 6 x 10" min and corresponds to the oxidation without catalyst under otherwise identical conditions. The rate acceleration (Arca/Aion) deduced from these values is 60,000-fold. 

Skeletal copper is best made from the CuA12 intermetallic compound which has very close to 50 wt% aluminum in the alloy and gives an active and selective catalyst [27-29], Skeletal nickel is also best made from an alloy of about 50 wt% aluminum [25] however, in this case, the alloy consists of more than one intermetallic phase, the combination of which provides the best activity while maintaining adequate strength in the catalytic residue. The most active skeletal cobalt catalysts are made from an alloy of about 60-65 wt% aluminum, which consists of two intermetallic phases, Co2A19 + Co4A113 [30],... 

If the desired catalyst is to consist of two or more catalytic metals after leaching or if a promoter metal is to be included, the precursor alloy becomes even more complicated with respect to phase diagrams. The approximate proportion of reactive metal (aluminum) in these ternary and higher alloys usually remains the same as for the binary metal system for the best results, although the different catalytic activities, leaching behavior and strengths of the various intermetallic phases need to be considered for each alloy system. 

The precursor alloy is quenched to form small grains readily attacked by the caustic solution [31], Quenching can also enable specific intermetallic phases to be obtained, although this is less common. Yamauchi et al. [32-34] have employed a very fast quench to obtain a supersaturation of promoter species in the alloy. It is even possible to obtain an amorphous metal glass of an alloy, and Deng et al. [35] provide a review of this area, particularly with Ni, Ni-P, Ni-B, Ni-Co, and Ni-Co-B systems. The increased catalytic activity observed with these leached amorphous alloy systems can be attributed to either chemical promotion of the catalyzed reaction or an increased surface area of the leached catalyst, depending on the components present in the original alloy. Promotion with additives is considered in more detail later. 

Bursian et al. (66a) suggested metallic platinum sites for dehydrogenation and Pt " sites for ring closure. They studied the effect of several elements added to platinum-on-silica catalyst on the aromatizing activity of n-hexane. Benzene yield increased parallel to the amount of soluble platinum (66b) at the same time, the crystallinity of platinum decreased in the presence of additives promoting aromatization. These are elements (e.g., Ce, Sc, Zr) which do not form an intermetallic compound with platinum (66c). 

Reaction 5.45 is at least partly hypothetical. Evidence that the Cl does react with the Na component of the alanate to form NaCl was found by means of X-ray diffraction (XRD), but the final form of the Ti catalyst is not clear [68]. Ti is probably metallic in the form of an alloy or intermetallic compound (e.g. with Al) rather than elemental. Another possibility is that the transition metal dopant (e.g. Ti) actually does not act as a classic surface catalyst on NaAlH4, but rather enters the entire Na sublattice as a variable valence species to produce vacancies and lattice distortions, thus aiding the necessary short-range diffusion of Na and Al atoms [69]. Ti, derived from the decomposition of TiCU during ball-milling, seems to also promote the decomposition of LiAlH4 and the release of H2 [70]. In order to understand the role of the catalyst, Sandrock et al. performed detailed desorption kinetics studies (forward reactions, both steps, of the reaction) as a function of temperature and catalyst level [71] (Figure 5.39). 

Figure 5.19. ED image of model Cu-Pd intermetallic catalyst system on the carbon support of interest in methanol synthesis showing the presence of both fee and bcc phases.

Modifications of the chemical nature of the catalyst under cathodic load are also possible. Sulphides can be reductively dissolved with liberation of H2S [139]. Oxides can be progressively reduced with loss of the specific activity [140]. In the latter case, an additive can be used to diminish the rate of reduction. Intermetallic compounds or alloys may exhibit preferential dissolution of one of the components during cathodic performances in concentrated alkali [141],... 

Finally, new catalytic processes (e.g., multi-step treatments, new supports, and intermetallic systems) which avoid the metallic contamination (mainly by V, Ni, Fe, Ti and the alkalis) must be explored, just as non-catalytic processes should not be neglected in order to prevent shortage of catalyst from limiting progress. 

These catalysts are composed of one or several metallic active components, deposited on a high surface area support, whose purpose is the dispersion of the catalytically active component or components and their stabilization [23-27], The most important metallic catalysts are transition metals, since they possess a relatively high reactivity, exhibit different oxidation states, and have different crystalline structures. In this regard, highly dispersed transition clusters of metals, such as Fe, Ru, Pt, Pd, Ni, Ag, Cu, W, Mn, and Cr and some alloys, and intermetallic compounds, such as Pt-Ir, Pt-Re, and Pt-Sn, normally dispersed on high surface area supports are applied as catalysts. 

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