Chalcogenide catalysts

Table 3 Non-noble metal ternary and pseudo-ternary chalcogenide catalysts studied... 

Trends in the electronic structure of the chalcogenide catalysts have proved to be helpful in the design and understanding of the catalyst clusters. During ORR, the molecular oxygen has been found to react with the cluster as a whole, rather than individual metal atoms.177 The overall number of electrons per cluster unit (NEC) in the valence bond has been shown to have a factor in the activity and stability of the cluster catalysts.177,181 The unsubstituted Chevrel phases have a NEC of 20.177,181 Substituting or intercalating other transition metals into the crystal lattice to make ternary or pseudo-binary Chevrel phases allows for the increase of NEC. It has been found that as the NEC approaches 24, the catalytic activity improves.181 Alonso-Vante compiled the results from his previous studies to show the effect of NEC in... 

Carbonyls. Related to the chalcogenide and metal cluster studies are the carbonyl studies for ORR.189 I92-198-201 The majority of the carbonyl work has been reported by several Mexican research groups. This body of work appears to have developed from studies on ruthenium-containing chalcogenide catalysts in the mid-1990 s. The precursors used for these catalysts involved Ru-carbonyls.202... 

Clearly, the way to understand the tailoring of nanomaterials is the follow-up of the chemical process. This has been done with the mthenium-chalcogenide materials. Thus using transition metal molecular cluster compounds paved to a certain extent, the bottom-up approach for chalcogenide catalysts in the nanoscale domain. We believe we have given a fairly comprehensive account of this chemical process. The use of the various clue techniques such as e g., NMR, XPS, XRD, not all mentioned in this chapter, provided information as to the nature of the rathenium-selenide material. Such information caimot be obtained if limited solely to py-rolsis, and to electrochemical techniques. 

Liu G, Zhang H, Hu JW (2007) Novel synthesis of a highly active carbon-supported Ru85Sel5 chalcogenide catalyst for the oxygen reduction reaction. Electrochem Commun 9 (ll) 2643-2648... 

Different methods have been reported for the synthesis of transition metal chalcogenide catalysts of the RhSx type, ineluding reaetion of metals [5] and caibonyl precursors [6] with sulfur or selenium in different solvents, deposition-precipitation from solution with H2S [7], high temperature treatment with gas H2S [8] and post treatment with selenic aeid [9], However, the preparation of RhSx with well-defined properties still remains as a ehallenge. Furthermore, Rh is one of the most expensive noble metals. Consequently, optimizing the dispersion and loading is of vital importance. 

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 cadmium chalcogenide semiconductors (qv) have found numerous appHcations ranging from rectifiers to photoconductive detectors in smoke alarms. Many Cd compounds, eg, sulfide, tungstate, selenide, teUuride, and oxide, are used as phosphors in luminescent screens and scintiUation counters. Glass colored with cadmium sulfoselenides is used as a color filter in spectroscopy and has recently attracted attention as a third-order, nonlinear optical switching material (see Nonlinear optical materials). DiaLkylcadmium compounds are polymerization catalysts for production of poly(vinyl chloride) (PVC), poly(vinyl acetate) (PVA), and poly(methyl methacrylate) (PMMA). Mixed with TiCl, they catalyze the polymerization of ethylene and propylene. 

Aqueous solutions have low conductivities resulting from extensive complex ion formation. The haUdes, along with the chalcogenides, are sometimes used in pyrotechnics to give blue flames and as catalysts for a number of organic reactions. 

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

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. 

With respect to non-noble and non-Ru catalysts, transition metal chalcogenides with spinel and pyrite structures have been investigated and shown that these can also be active to oxygen reduction processes. The motivation in the present case is that chalcogen addition might enhance the stability and activity toward the ORR... 

Ozenler SS, Kadirgan F (2006) The effect of the matrix on the electro-catalytic properties of methanol tolerant oxygen reduction catalysts based on ruthenium-chalcogenides. J Power Sources 154 364-369... 

Diphosphine mono-chalcogenides are able to bind to metal centres from either one of or both the phosphorus(III) and chalcogen donor centres. This has led to their study as hemi-labile ligands in catalyst systems, whereby one of the donor centres binds strongly to the metal centre and the other one is weakly bound and hence can be readily dissociated or reattached to reveal or protect the active site at a metal centre.28... 

The interest in noble metal free chalcogenides has recently been renewed. The Campbell group reported in 2006 on the use of Co-Se catalysts for ORR.197 The group also indicated they were involved in a more extensive study of non-precious metal chalcogenides as ORR catalysts. 

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