Intermetallic compounds, as catalysts

Elattar, A., W.E. Wallace and R.S. Craig, 1978, Intermetallic compounds as catalysts for hydrogenation of carbon oxides, in Rare Earths in Modem Science and Technology, eds G.J. McCarthy and J.J. Rhyne (Plenum, New York) p. 87. 

Urazbaeva, K.A.(1990) Modified hydrides of intermetallic compounds as catalysts of asymmetric hydrogenation, Cand. Diss. Thesis, N.D. Zelinskii Institute of Organic Chemistry, Academy of Sciences of the USSR, Moscow (supervisors Klabunovskii, E.l. and Konenko, I.R.). 

The idea to use unsupported intermetallic compounds as catalysts is not new, and the first to explore the catalytic properties of unsupported... 

Despite the large number of publications, where intermetallic compounds in a pure and unsupported form have been used as catalysts, the stability under reaction conditions was not investigated or established in any of these studies. How the combination of unsupported intermetallic compounds as catalysts and thorough studies on their stability under reaction conditions enables a knowledge-based development is shown in the next section. 

Imamura et al. (1984, 1986a) described a new way of preparing Raney catalysts using intermetallic compounds as starting materials. They found that, when treated with 1,2-diiodoethane (or dibromoethane) the lanthanide could be etched from the alloy to form the iodide (bromide), leaving a spongy, high-surface-area skeleton of the other element (Ni or Co) ... 

Imamura et al. (1984, 1986a) described a new way of preparing Raney catalysts using intermetallic compounds as starting materials as reported in sect. 1. The catalytic results... 

Because of the hurdles in the synthesis and their structural complexity, intermetallic compovmds as catalysts may sound like an exotic application—perhaps with only academic interest. Intermetallic compounds do, however, offer a unique combination of valuable properties that outweighs by far the difficulties in their catalytic application—a picture which is corroborated by a careful literature search. Besides the well-known examples of the Raney-catalysts, there are about 1200 reports, where intermetallic compounds are involved in catalytic processes. On the one hand, they are used as precursors for high surface area catalysts, on the other hand they are commonly observed after catalytic processes where hydrogen is involved as a result of reactions between the supported noble metal and the partly reduced support. Also, very rarely, the catalytic properties of well-characterized intermetallic compounds have been investigated. 

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). 

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. 

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... 

Bi- and multicomponent metallic catalysts supported on metal oxides are complex objects for preparation and characterization. It is difficult to achieve a uniform particle composition within the catalyst [1] and exclude strong metal/support interactions [2]. Moreover, the question whether the supported particles of the two or more metals are coexisting as elements (next to each other, separated or as core/shell particles), or forming alloys (randomly distributed elements on the crystallographic sites) or intermetallic compounds... 

For other reactions like methane, ammonia or methanol synthesis the intermetallic compounds are used as precursors of new active catalysts. In this case a bulk decomposition of the starting compounds is required. This can be performed either under a synthesis gas reactant or by applying either an oxidation pretreatment under air, O2, CO2, N2O or by hydrogenation at high temperature. 

Imamura and Wallace (1980) used various intermetallic compounds (LaNis, CeNis, ThNis, TI1C05) as precursors. After decomposition under oxygen at 350°C they obtained catalysts which were clearly more active than the corresponding classical Ni catalysts. They suggested that geometrical effects were responsible for such a behavior. 

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