Catalysts intermetallic compounds

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... 

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],... 

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). 

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],... 

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. 

Metals frequently used as catalysts are Fe, Ru, Pt, Pd, Ni, Ag, Cu, W, Mn, and Cr and some of their alloys and intermetallic compounds, such as Pt-Ir, Pt-Re, and Pt-Sn [5], These metals are applied as catalysts because of their ability to chemisorb atoms, given an important function of these metals is to atomize molecules, such as H2, 02, N2, and CO, and supply the produced atoms to other reactants and reaction intermediates [3], The heat of chemisorption in transition metals increases from right to left in the periodic table. Consequently, since the catalytic activity of metallic catalysts is connected with their ability to chemisorb atoms, the catalytic activity should increase from right to left [4], A Balandin volcano plot (see Figure 2.7) [3] indicates apeak of maximum catalytic activity for metals located in the middle of the periodic table. This effect occurs because of the action of two competing effects. On the one hand, the increase of the catalytic activity with the heat of chemisorption, and on the other the increase of the time of residence of a molecule on the surface because of the increase of the adsorption energy, decrease the catalytic activity since the desorption of these molecules is necessary to liberate the active sites and continue the catalytic process. As a result of the action of both effects, the catalytic activity has a peak (see Figure 2.7). 

For metals promoting other metals, an interesting case was studied by Hurst and Rideal.2 In the combustion of mixtures of hydrogen and carbon monoxide, using copper as the basic catalyst the ratio of the gases burnt depends on the temperature, and also on the amount of small additions of palladium made to the copper. The proportion of carbon monoxide burnt is increased by addition of palladium, a maximum proportion of carbon monoxide being burnt when 0-2 per cent, of palladium is. present. With further amounts of palladium, the ratio CO H2 burnt falls off slowly until, with 5 per cent, palladium, it is nearly the same as with pure copper. This effect of palladium is ascribed to the introduction of a new type of surface, the line of contact between palladium and copper, though the proof that this is the cause of promotion is perhaps not complete. Mit-tasch and others,3 in elaborate studies of the promotion of various metal catalysts, particularly molybdenum, for the synthesis or decomposition of ammonia, concluded that the formation of intermetallic compounds... 

These intermetallic compounds react with Hj at RT, provided that the pressure is high enough , but there is often an induction period ranging from seconds to days depending on previous treatment of the alloy and time of exposure to air . Freshly prepared samples not exposed to air usually react in seconds because of the catalytic action of Ni and Co on Fe. The intermetallic compound is oxidized at its surface to form the rare-earth oxide (e.g., LajOj) and free metallic Ni (or Co or Fe), which acts as a catalyst to dissociate H. Impurity gases, such as CO, and HjO, decrease the rates of hydride formation and can poison the alloy for reaction with Hj. 

High surface area metal catalysts can be prepared by selectively removing one component of a bimetallic or polymetallic alloy or intermetallic compound. The remaining material has a microscopic spongy network of pores and is referred to as a skeletal metal catalyst. 

Other specialized alloys have also been used to prepare skeletal metal catalysts. Raney ruthenium has been prepared from the ruthenium aluminum alloy. 20 A colloidal platinum has been prepared by the action of acetic acid on a platinum lithium alloy. l Skeletal nickel catalysts have been made from a number of intermetallic compounds of nickel with the rare earth elements, lanthanum and samarium. The rare earth element is removed from the alloy by reaction with diiodoethane or dibromoethane which convert the rare earths to the soluble halide salts. 22 Several multicomponent catalysts have also been prepared from the corresponding aluminum alloys. 23-126... 

This paper discusses transition metal intermetallic compounds, in the context of the reactivity and physical properties expected for materials produced via solid-solid reactions at the metal catalyst oxide-support interface. It is shown that several observable and proposed features of the so called Strong Metal-Support Interaction (SMSI) — chemisorption activity, phase segregation, and encapsulation — follow naturally from the chemistry of these materials. Both literature precedent and experimental data are presented to support the close relationship suggested above. 

What, then, need pertain in order for intermetallic compounds to help explain what has been called SMSI First, and most importantly, they must form as a result of the high temperature reduction of the metal catalyst (typically Group VIII) on an... 

The TPR characterization of RhCoMOR catalyst showed a signal that could be attributed to intermetallic compounds, and another signal at high temperature, attributed to Co" which would be responsible for the catalytic activity. 

Intermetallic compounds and Co at exchange positions would be responsible for the improved performance of the catalysts. 

Figure 3 also shows conversion and selectivity to C2H4 with Sn added to the Pt catalyst[15]. Both the alkane conversion and the selectivity to olefins increase significantly with added Sn. X-ray diffraction and XPS of the Pt-Sn catalyst indicate intermetallic compound formation rather than fee metal, and this surface evidently increases the alkane conversion and reduces the decomposition of olefins. 

In previous works [8,9], bimetallic Bi-Pd catalysts supported on activated carbon and characterized by various Bi/Pd molar ratios ((Pd+Bi)=10 wt.%) were prepared from the thermal degradation of Bi and Pd acetate-type precursors under nitrogen at 773 K. Because several binary Bi-Pd alloys were heavily suspected in the supported catalysts, three intermetallic compounds, Bi2Pd, BiPd and BiPda were also prepared from the same precursors, according to the same... 

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