Catalyst selenium-based

Finally, it should be mentioned that two years later, an enantioselective oxidative (boron) Heck-type reaction was reported by Jung and coworkers [32] for a dinuclear NHC-derived CNO-based pincer complex for which for the first time a Pd /Pd" mechanism was suggested to be operative with palladium pincer-type crosscoupling catalysts. Reaction mechanisms without a change of the palladium s oxidation state have never been proposed to be operative for Heck cross-couphng reactions, but have been shown to be operative for xylene-derived selenium-based pincer complexes and related systems in the cross-couphng of vinyl epoxides (and aziridines) with organoboronic acids [24d, 33]. 

The permeation of methanol through the membrane from the anode to the cathode (cross-over) represents a source of severe performance loss. The combined reaction of methanol and oxygen on the cathodic platinum catalysts (or platinum-based alloys) leads to a mixed potential formation that reduces the maximum achievable potential considerably by up to 200 mV. Two strategies are followed to circumvent this phenomenon. On the one hand the general avoidance of methanol permeation through the membrane allows the use of standard catalysts. The other strategy is the use of methanol-tolerant catalysts for the cathode of DMFCs these materials are characterized by a complete inactivity towards methanol, which does not react on these catalyst surfaces. For the development of methanol-tolerant catalysts, several prerequisites have to be fulfilled to obtain competitive products the new material not only has to be as active as a comparable catalyst (platinum-based) but also the stability and cost aspect have to be considered. Recently, selenium-modified rathenium particles (RuSe ) were found to be a suitable alternative to platinum-based alloys where the addition of selenium increases the activity of pure rathenium particles to shift these catalysts in a competitive regime. ... 

Attempts have been made to develop methods for the production of aromatic isocyanates without the use of phosgene. None of these processes is currently in commercial use. Processes based on the reaction of carbon monoxide with aromatic nitro compounds have been examined extensively (23,27,76). The reductive carbonylation of 2,4-dinitrotoluene [121 -14-2] to toluene 2,4-diaLkylcarbamates is reported to occur in high yield at reaction temperatures of 140—180°C under 6900 kPa (1000 psi) of carbon monoxide. The resultant carbamate product distribution is noted to be a strong function of the alcohol used. Mitsui-Toatsu and Arco have disclosed a two-step reductive carbonylation process based on a cost effective selenium catalyst (22,23). 

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. 

There is increased interest in the use of Ru-based systems as catalysts for oxygen reduction in acidic media, because these systems have potential applications in practicable direct methanol fuel cell systems. The thermolysis of Ru3(CO)i2 has been studied to tailor the preparation of such materials [123-125]. The decarbon-ylation of carbon-supported catalysts prepared from Ru3(CO)i2 and W(CO)6, Mo(CO)is or Rh(CO)is in the presence of selenium has allowed the preparation of catalysts with enhanced activity towards oxygen reduction, when compared with the monometallic ruthenium-based catalyst [126],... 

The chloroform with base/phase-transfer catalyst or potassium rcrt-butoxide methods have been used for the preparation of selenium-substituted cyclopropanes 1-3. ... 

Indeed, one can easily conclude that selective allylic oxidation of olefins, in the context of fine chemicals, is a largely underdeveloped area of catalysis. Selenium dioxide catalyzes the allylic oxidation of a variety of olefins with TBHP, affording the corresponding allylic alcohols, but the system is homogeneous [3] and, hence, falls outside the scope of this book. The only heterogeneous catalysts for allylic oxidation which seem to have synthetic utility are palladium-based [4]. 

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