Tungsten in Catalysis

Filaments are usually refractory metals such as tungsten or iridium, which can sustain high temperatures for a long time (T > 3000 K). The lifetime of filaments for electron sources can be prolonged substantially if an adsorbate can be introduced that lowers the work function on the surface so that it may be operated at lower temperature. Thorium fulfills this function by being partly ionized, donating electrons to the filament, which results in a dipole layer that reduces the work function of the tungsten. In catalysis, alkali metals are used to modify the effect of the work function of metals, as we will see later. 

Langmuir s research on how oxygen gas deteriorated the tungsten filaments of light bulbs led to a theory of adsorption that relates the surface concentration of a gas to its pressure above the surface (1915). This, together with Taylor s concept of active sites on the surface of a catalyst, enabled Hinshelwood in around 1927 to formulate the Langmuir-Hinshelwood kinetics that we still use today to describe catalytic reactions. Indeed, research in catalysis was synonymous with kinetic analysis... 

There has been to some degree the belief that Mossbauer spectroscopy, although in principle an ideal technique for catalyst studies, for practical purposes can only be applied to problems in catalysis if the catalyst contains either iron or tin. Therefore, one of the main purposes of this review is to show how Mossbauer spectroscopy can be directly extended to many additional Mossbauer atoms or isotopes (such as antimony, europium, nickel, ruthenium, gold, and tungsten) and, perhaps more importantly, how the technique can be extended to obtain information about systems that do not contain a Mossbauer atom. ... 

In the last 20 years, a number of researchers have shown that some transition metal carbides, such as those of molybdenum7 and tungsten,8,9 have catalytic behaviour similar to the noble metals. Since the starting materials for the production of the group 6 transition metal carbides are abundant and cheap it has been suggested that they can replace the scarce and expensive noble metals in catalysis. One of the major problems with the metal carbide catalysts has been the difficulty in producing them with... 

Bell, A., Dicyclopentadiene Polymerisation using Well-characterized Tungsten Phenoxide Complexes , in Catalysis in Polymer Synthesis, ACS Symp. Ser. 496, Washington, DC, 1992, pp. 121-133. 

Surface Vanadate, Molybdate and Tungstate Species The pure and mixed oxides and the salts of vanadium, molybdenum and tungsten in their higher oxidation states are used widely as heterogeneous catalysts, for selective oxidation as well as for acid catalysis. Similarly, supported chromia and rhenium oxides find wide application in different catalytic processes. 

The methods for the preparation of the triply bonded dimers, (C5Hj)2M2(CO)4, of molybdenum and tungsten involve either the photolysis or thermolysis of (C5H5)2M2(C0)6. The photolysis route is appropriate only for the preparation of very small amounts of (CjHj)2Mo2(CO)4. An improved version of the thermolysis route is presented that shortens the reaction time and produces (C5H,)2M2(CO)4 in high yield and purity. These compounds are of interest in catalysis, as precursors to sulfided clusters, and in preparative chemistry due to the broad reactivity of the metal-metal triple bond. ... 

Below 2798 K, the stable tvmgsten carbide phases are a-WC with a simple hex structure and W2C with a hep structure of metal atoms (6). At higher temperatures, several other carbide phases exist, but mostly nonstoichiometrically. Among these various tungsten carbide phases, hep a-W2C and fee p-WCi-x (x 0.5) attracted the most attention in catalysis (13,14). The hep a-W2C has an L3 structure, and the fee /3-WCi has a B1 rock salt structure. For nonstoichiomet-ric carbide phases such as a-MoCi- and fi-WCi-x, the lattice dimensions were found to increase along with an increase in the carbon/metal ratio. 

Rhenium exhibits a greater resistance than tungsten to the water cycle effect, in which lamps and electron tubes become blackened by deposition of metal. This phenomenon involves catalysis by small quantities of water that react with the metal in a hot filament to produce a volatile metal oxide and hydrogen. The oxide condenses on the surface of the bulb and is reduced back to the metal by hydrogen. 

The chemical uses of tungsten have increased substantially in more recent years. Catalysis (qv) of photochemical reactions and newer types of soluble organometaUic complexes for industrially important organic reactions are among the areas of these new applications. 

Reactivity studies of organic ligands with mixed-metal clusters have been utilized in an attempt to shed light on the fundamental steps that occur in heterogeneous catalysis (Table VIII), although the correspondence between cluster chemistry and surface-adsorbate interactions is often poor. While some of these studies have been mentioned in Section ll.D., it is useful to revisit them in the context of the catalytic process for which they are models. Shapley and co-workers have examined the solution chemistry of tungsten-iridium clusters in an effort to understand hydrogenolysis of butane. The reaction of excess diphenylacetylene with... 

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