Sites on tungsten

Huang X, Zhai HJ, Waters T, Li J, Wang LS (2006) Experimental and theoretical characterization of superoxide complexes [W O iO " )] and [WjO,(O W)] Models for the interaction of O with reduced W sites on tungsten oxide surfaces. Angew Chem Int Ed 45 657... 

Different types of chemisorption sites may be observed, each with a characteristic A value. Several adsorbed states appear to exist for CO chemisorbed on tungsten, as noted. These states of chemisorption probably have to do with different types of chemisorption bonding, maybe involving different types of surface sites. Much of the evidence has come initially from desorption studies, discussed immediately following. 

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... 

Lewis Bases. A variety of other ligands have been studied, but with only a few of the transition metals. There is still a lot of room for scoping work in this direction. Other reactant systems reported are ammoni a(2e), methanol (3h), and hydrogen sulfide(3b) with iron, and benzene with tungsten (Tf) and plati num(3a). In a qualitative sense all of these reactions appear to occur at, or near gas kinetic rates without distinct size selectivity. The ammonia chemisorbs on each collision with no size selective behavior. These complexes have lower ionization potential indicative of the donor type ligands. Saturation studies have indicated a variety of absorption sites on a single size cluster(51). 

In view of this discussion, why compare the experimentally determined value of cesium adsorbed per square centimeter at the optimum emission with the number of available sites per square centimeter on the 110 plane rather than on any other plane There are two reasons (7) the 110 plane is the densest packed plane and is more likely to form on a tungsten surface than any other plane and 2) Martin (5) has shown that in the case of cesium on tungsten near the optimum activity, the 110 plane contributes more current than any other plane. Hence in the experiments described above, fc.pt is observed when the 110 plane reaches its optimum. The fact that the electron emission changes its trend abruptly when all the available sites on the 110 plane are filled fits in with the concept of a monolayer. 

Returning now to the observed effect of particle size on catalytic activity, van Hardeveld and Hartog 219) have calculated that the relative concentration of C7 sites on octahedral iron crystallites decreases with decreasing particle size and that, in general, the C7 site is not a small-particle surface site. The above correlation of increased catalytic activity with increased C7 site surface concentration thus also explains the observed structure sensitivity (particle size dependence) for this reaction. Finally, this correlation is consistent with results obtained from field electron microscopy of iron (220), single crystal reaction studies on tungsten (also a bee metal) (227), and symmetry considerations (222). 

Moffat, Johnson, and Clark86)found the propylene disproportionation reaction on tungsten oxide-silica catalyst to be limited by interphase diffusional effects in spite of calculations which predict that no diffusional limitation should occur. They postulate that widely separated and very active sites could have their inherent activity limited by localized film diffusional effects which are functions of Reynolds and Schmidt numbers. Activity of cobalt molybdate-alumina was not limited by interphase or prediffusional effects. 

Transport properties (continued.) semiclassical model, 192-193 temperature-dependent resistivity of nanowires, 193-198 Triplet sites on supports, 63-64 Tungsten species, SiC>2-supported, 63 Turnover numbers (TON), nanostructured materials, 6... 

There are a number of lattice-gas chemisorption systems, where atoms or molecules are adsorbed in well-defined sites on the surface, as determined by vibrational spectroscopy, but where there is no long range order. The adsorption sites and bond lengths for such systems can be determined by diffuse LEED calculations, as has been done for oxygen on the tungsten (100) surface./13/... 

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