Chemisorption olefins

The active site on the surface of selective propylene ammoxidation catalyst contains three critical functionalities associated with the specific metal components of the catalyst (37—39) an a-H abstraction component such as Sb ", or Te" " an olefin chemisorption and oxygen or nitrogen insertion component such as Mo " or and a redox couple such as Fe " /Fe " or Ce " /Ce" " to enhance transfer of lattice oxygen between the bulk and surface... 

The use of silver fluoroborate as a catalyst or reagent often depends on the precipitation of a silver haUde. Thus the silver ion abstracts a CU from a rhodium chloride complex, ((CgH )2As)2(CO)RhCl, yielding the cationic rhodium fluoroborate [30935-54-7] hydrogenation catalyst (99). The complexing tendency of olefins for AgBF has led to the development of chemisorption methods for ethylene separation (100,101). Copper(I) fluoroborate [14708-11-3] also forms complexes with olefins hydrocarbon separations are effected by similar means (102). 

Cobalt, nickel and copper naked metal clusters and olefin chemisorption models. G. A. Ozin, Coord. Chem. Rev., 1979, 28, 117-146 (45). 

Demuth JE (1977) Chemisorption of C2H2 on Pd(l 11) and Pt(l 11) formation of a thermally activated olefinic surface complex. Chem Phys Lett 45 12-17... 

Active centers, nature of, 10 96 Active site, 27 210-221 in catalysts, 17 103-104, 34 1 for olefin chemisorption, 17 108-113 dual-site concept, 27 210 electrical conductivity, 27 216, 217 ESCA, 27 218, 219 ESR, 27 214-216 infrared spectroscopy, 27 213, 214 model, 27 219-221 molybdena catalyst, 27 304-306 Mdssbauer spectroscopy, 27 217, 218 nonuniform distribution, transport-limited pellets, 39 288-291... 

The use of CO as a chemical probe of the nature of the molecular interactions with the surface sites of metallic catalysts [6] was the first clear experimental example of the transposition to surface science and in particular to chemisorption of the concepts of coordination chemistry [1, 2, 5], In fact the Chatt-Duncanson model [7] of coordination of CO, olefins, etc. to transition metals appeared to be valid also for the interactions of such probes on metal surfaces. It could not fit with the physical approach to the surface states based on solid state band gap theory [8], which was popular at the end of 1950, but at least it was a simple model for the evidence of a localized process of chemical adsorption of molecules such as olefins, CO, H, olefins, dienes, aromatics, and so on to single metal atoms on the surfaces of metals or metal oxides [5]. 

The duality of cracking mechanisms is summarized in Fig. 5, where RH paraffin feed, R -C=C = olefinic product, Kq = equilibrium constant of olefin chemisorption. Free Bronsted acid sites HZ interact directly with the paraffin feed by protonation, producing monomolecular cracking. When the acid sites are covered with adsorbed olefins to form... 

Early in the nineties Ruiz et al. reported enhanced catalyst activities and increased selectivities to alkenes and higher hydrocarbons upon addition of V, Mg, and Ce oxides to Co-based F-T catalysts.These variations were attributed to electronic effects induced by the transition metal oxide. Similar results were obtained by Bessel et al. using a Cr promoter in Co/ZSM-5 catalysts.This group observed that the addition of Cr improved the catalyst activity, and shifted the selectivity from methane to higher, generally more olefinic, hydrocarbons. Based on H2 and CO chemisorption, as well as TPR and TPD results, they suggested that the promotion was caused by an interaction between the transition metal oxide and the cobalt oxide, which inhibits... 

Two Carbon Atom Complexes Formed upon Olefin Chemisorption... 

The answer to the first question is undoubtedly a positive one. The classical papers by Beeck et al., Rideal et al., and others have shown that ethylene disproportionates upon chemisorption into ethane and carbonaceous (adsorbed) residues (see 162). This disproportionation takes place at relatively low temperatures at room temperature and lower (see 162 for review). Moreover, the intensity analysis of LEED data has shown that upon chemisorption of ethylene, ethylidyne structures are formed. Similar structures are also formed by dissociative adsorption of higher olefins (181,182). There is thus no doubt with regard the first question. 

To minimize overpotential effects, cathodes axe usually made of finely divided platinum on a porous support, for aqueous electrolytes. The catalytic surfaces of the anodes are particularly susceptible to poisoning by CO, olefins, sulfur compounds, and other impurities in the fuel. These lie above H2 in the chemisorption series (Eq. 6.3). 

It is now generally accepted that the chemisorption of an olefin precedes its hydrogenation. Since any mechanism will depend upon the nature of the adsorbed state of the olefin, it is pertinent to examine the... 

Evidence for the Fe2+/Fe3+ redox cycle was provided later by ESR measurements [205], while recent experiments with deuterium-labelled butene indicate that C—H cleavage is involved in the rate-controlling step [138]. In agreement with the views of Schuit [281], chemisorption of the olefin on an anion vacancy is assumed, but O- is postulated as the active oxygen species rather than O2-. An argument in favour of O" is that otherwise much more, and rather complicated, elementary reaction steps are required to account for the transfer of charge. 

The divalent catalyst is highly coordinatively unsaturated and therefore exhibits some unusual chemistry (33-42). It has a light green color but quickly truns blue when exposed to N2, indicating a weak chemisorption. Carbon monoxide is adsorbed to yield a violet color, and of course it poisons the polymerization. Up to two molecules can be adsorbed. Olefins also adsorb in a 2 1 ratio, and acetylene is converted to benzene. Polar compounds like alcohols, ethers, or amines are strongly held. Nitric oxide (NO) attaches in a 2 1 ratio. 

Chemisorption [9] is an adsorptive interaction between a molecule and a surface in which electron density is shared by the adsorbed molecule and the surface. Electrochemical investigations of molecules that are chemisorbed to electrode surfaces have been conducted for at least three decades. Why is it, then, that the papers that are credited with starting the chemically modified electrode field (in 1973) describe chemisorption of olefinic substances on platinum electrodes [10,11] What is it about these papers that is different from the earlier work The answer to this question lies in the quote by Lane and Hubbard at the start of this chapter. Lane and Hubbard raised the possibility of using carefully designed adsorbate molecules to probe the fundamentals of electron-transfer reactions at electrode surfaces. It is this concept of specifically tailoring an electrode surface to achieve a particularly desired goal that distinguishes this work from the prior literature on chemisorption, and it is this concept that launched the chemically modified electrode field. 

In chemisorption, the electrochemically reactive material is strongly (and to a large extent irreversibly) adsorbed onto the electrode surface. Lane and Hubbard [4] were among the first to use this approach when they chemisorbed quinone-bearing olefins on platinum electrodes and demonstrated a pronounced effect of the adsorbed molecules on electrochemical reactions at the metal surface. 

This increasing adsorption energy of olefinic compounds by sulfurization of metals can induce a partial desorption of sulfur or else a surface reaction giving thiols, as was pointed out by Oudar et al. (71) in the case of butadiene chemisorption at 100°C on Pt(110) and Pt(lll) faces. 

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