Rhenium 10 surface reactions

To model the photochemical reaction of Re2(CO)io with silica in the presence of THE, silanolates other than silsesquioxanes [Et3SiOH, PhsSiOH, Ph2Si(OH)2 and (OH)Ph2SiOSiPh2(OH)j with [Re2(CO)8(THE)2] have also been synthesized [148], pointing to the formation of the surface species [((r-=SiO)((r-H)Re2(CO)8j in the above-mentioned reaction with silica. Rhenium carbonyl of the two diol derivatives listed above where both the SiOH have reacted are proposed as models for surface reaction with a vicinal hydroxyl residue, although no direct surface species is reported [148]. 

A recent research by McGeer on the exchange reaction between light and heavy nitrogen on iron synthetic ammonia catalysis and on rhenium surfaces at 450° C provides a clue to such an attempt. The velocity of... 

Another reaction model involves the compression of the ethylidyne overlayer at high pressure of ethylene. Because of repulsive adsorbate-adsorbate (ethylene-ethy-lidyne) interaction, and the expected small activation energy of ethylidyne surface diffusion, ethylene could adsorb on the metal in the small hole created near the compressed ethylidyne. Compression of this type has been detected by STM upon the adsorption of hydrocarbons on platinum and the coadsorption of CO and sulfur on both platinum and rhenium surfaces [218]. 

The authors determined the adsorption and the surface reaction kinetic parameters of methanol reaction over monolayer supported vanadium, molybdenum, chromium, rhenium catalysts, and bulk iron molybdate. 

Another facet of surface organometallic chemistry involves modelling of the mechanisms of surface reactions on the basis of the reactivity of molecular models. For example, the reactivity of metal-imine complexes of molybdenum is considered by CHAN, who proposes elementary steps constituting the catalytic cycle of the surface-catalyzed alkene ammoxidation reaction, which is of great industrial importance. HERRMANN provides some very fine examples of molecular models of the rhenium oxide catalysts used commercially in the alkene metathesis reaction. 

Solid catalysts for the metathesis reaction are mainly transition metal oxides, carbonyls, or sulfides deposited on high surface area supports (oxides and phosphates). After activation, a wide variety of solid catalysts is effective, for the metathesis of alkenes. Table I (1, 34 38) gives a survey of the more efficient catalysts which have been reported to convert propene into ethene and linear butenes. The most active ones contain rhenium, molybdenum, or tungsten. An outstanding catalyst is rhenium oxide on alumina, which is active under very mild conditions, viz. room temperature and atmospheric pressure, yielding exclusively the primary metathesis products. 

During the preparation of the hexamethoxides of rhenium, molybdenum and tungsten by co-condensation with excess tetramethoxysilane on a cold surface, simultaneous co-condensation is necessary to avoid the danger of explosion present when sequential condensation of the reactants is employed. In the latter case, the high concentrations of hexafluoride at the interface leads to violent reaction with the silane. 

Stable rhenium tricarbonyls bonded to the surface of MgO have been prepared and characterized by EXAFS. Heating under He, O2 or vacuum of a sample obtained by impregnation of Re2(CO)io produced the oxidative fragmentation of the initial surface organometallic species [39-41]. These types of supported well-characterized species can be used as models in the study of reaction mechanisms [42]. 

As most laboratories use commercially obtained rhenium metal rather than a material prepared in situ, the following modifications are used. The rhenium metal is heated in a stream of hydrogen at 1000°C. for an hour to remove surface oxides, and the reaction of rhenium metal and chlorine is carried out at 650°C. Because of these temperatures, a Vycor combustion tube is necessary. 

Metals that are known to be highly catalytic for hydrogenation and dehydrogenation reactions are similar to platinum with respect to the h.e.r. These include palladium, iridium, rhenium, nickel, and cobalt. In the case of palladium the situation is further complicated by the tendency of atomic hydrogen, which is formed on the surface as an intermediate, to diffuse into the metal and dissolve in it. This effect... 

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