Acidic chalcogenides catalysts

Several chalcogenide catalyzed MBH-type reactions are reported [146], Instead of the common ferf-amines or phosphanes, also higher organochalcogenides can act as nucleophilic activator. Such Morita-Baylis-Hillman reactions catalyzed by sub-stoichiometric amounts of sulfides and selenides in the presence of Lewis acid to activate the carbonyl group were described by Kataoka and co-workers [147, 148]. The reaction of p-nitrobenzaldehyde and 2-cyclohexenone has been used for screening a series of chalcogenide catalysts in dichloromethane at room temperature. The best result was found when 10 mol% of chalcogenide where employed with a stoichiometric amount of TiCl in the presence of excess 2-cyclohexenone as Michael acceptor (3 equiv., Table 7.10). 

The different classes of Ru-based catalysts, including crystalline Chevrel-phase chalcogenides, nanostructured Ru, and Ru-Se clusters, and also Ru-N chelate compounds (RuNj), have been reviewed recently by Lee and Popov [29] in terms of the activity and selectivity toward the four-electron oxygen reduction to water. The conclusion was drawn that selenium is a critical element controlling the catalytic properties of Ru clusters as it directly modifies the electronic structure of the catalytic reaction center and increases the resistance to electrochemical oxidation of interfacial Ru atoms in acidic environments. 

The novel cluster-like chalcogenide material RuxSey deposited in thin [5, 26, 31, 36] and ultra-thin layers [9, 11] or in powder form embedded in a polymer matrix [30] was found to be an efficient catalyst for the molecular oxygen reduction in acid medium. Fig. 5.10 summarizes the current-potential (j-E) characteristics as a function of the substrate s nature. First of all, one can appreciate that similar activities are obtained from materials synthesized in powder or in colloidal form when deposited onto GC (Fig. 5.10, compare curves (1) and (2)). For the sake of comparison, the j-E characteristic generated on the naked GC substrate for the electrochemical process is contrasted in curve (5). 

The oxidation of monosulfonated triphenylphosphine with nitric oxide and nitrous acid was investigated in aqueous solution. The possible interconversion of phosphine chalcogenides was studied in the presence of Pd(0) catalyst. The reaction of a phosphine selenide with sulfur takes place via the phosphine that was separated as the Pd(II)complex. ... 

Recently it has been reported that Ru addition to PtCo/C results in catalysts with improved tolerance to methanol under typical ORR reaction conditions i.e. potentials more positive than 0.7 V NHE and O2 saturated acid electrolyte. This is because under these reaction conditions, upper oxide Ru species are stable and hinder methanol adsorption. Another interesting alternative is the Ru-based chalcogenides. In particular Ru Sey-based electrocatalysts have received a great deal of attention because of their high tolerance to methanol, even if their performance as electrocatalysts for the ORR is inferior to Pt/C by =40%. ... 

Gago AS, Morales-Acosta D, Arriaga LG, Alonso-Vante N (2011) Carbon supported ruthenium chalcogenide as cathode catalyst in a microlluidic fOTuiic acid fuel cell. J Power Sources 196(3) 1324-1328... 

Other materials, including carbides, nitrides, oxynitrides, carbonitrides, and chalcogenides, have also been explored as new non-noble metal catalysts. Some of them demonstrate promising catalytic ORR activity and superior stability in acidic media. 

Electrocatalysis of the ORR of various Chevrel type chalcogenide compounds was first studied by Alonso-Vante and Tributsch [19] in an acid medium and an enhancement in the ORR activity was observed when Mo was partially substituted by Ru. The compounds containing non-substituted octahedral Mo show metallic behavior while Ru substituted Mo octahedral (Moe-rM Xg, where X = S, Se, Te) systems show semiconducting behavior. The compound Mo4.2Rui,8Se8 displayed catalytic behavior comparable to that of Pt. The ORR overpotentials on catalysts of the series MogXg followed the trend Te[Pg.468]

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