Tungsten adsorption rate

A tungsten sulfide catalyst which had been inactivated by extended operation had, as expected, a considerably lower adsorption capacity than new catalyst. This is shown in Fig. 3 for the adsorption of ethane at 100°C. The adsorption rate of gases on used, inactive WS2 catalyst seems to be decreased to an even greater extent. 

Mention has been made variously that the rate of adsorption of hydrogen on nickel and iron is very fast at room temperature. In fact, the rate is so fast that the limiting factor appears to be the rate at which gases enter the reaction chamber through the stopcock from the reservoir. This essentially instantaneous adsorption has been observed for many other metals, including platinum, rhodium, palladium, tungsten, tanta-... 

This reaction is actually slightly endothermic (A// = 88 kJ/mol), but the large net increase in entropy and the nonequilibrium nature of most CVD processes lead to significant tungsten deposition. As with the Ge example, the deposition mechanism involves adsorption steps and surface reactions. At low pressures and under conditions of excess hydrogen gas, the deposition rate follows the general form ... 

From experiments with the field emission microscope it is learned that for a system like oxygen on tungsten (a) the crystallographic plane of the tungsten has a marked influence on the adsorption properties (6) the heat of adsorption increases with the number of W atoms a particular 0 atom can contact (c) the heat of adsorption for the first layer, in which 0 atoms make first valence bonds with W atoms, is about 4 ev., for the second layer, in which 0 atoms make second valence bonds with W atoms, only about 2 ev. (d) at a constant pressure the rate of adsorption is constant until the first layer is complete, and for the second layer it is slower by a factor of 100 or more (e) beyond the second layer oxygen is adsorbed as admoles of O2, O4, Os. 

The study of the adsorption of cesium on tungsten is particularly fruitful because the rate of adsorption and the amount adsorbed can be measured easily and also because it is possible to measure the rate of evaporation of electrons, atoms, and positive ions from the same surface. From these data also can be deduced the nature of the adsorbed species and the forces between the adsorbed species and the tungsten atoms (3,4). 

Many theories of adsorption, following Langmuir, have assumed that the rate of adsorption is proportional to (1 — 0), i.e., to the fraction of the surface which is bare or not yet covered. Langmuir first proved the (1 — 0) law by measuring experimentally how the thermionic work function

changed with time as thorium reached the surface of a tungsten filament at a constant rate (10). He then assumed that tp decreased linearly with 0 and thus deduced that dd/di was proportional to (1 — 0). But this assumption has been shown to be incorrect for such cases as Cs on W, Ba on W, SrO on W, and other systems. Hence it follows that the (1 — 0) law is not valid. The experiments described above for N2 on W not only show that dQ/dt is not proportional to (1 — 0), but they show by a direct experiment that dd/dt for a constant arrival rate is independent of 0 between 0 = 0 and 1.0. 

That such a contaminant was present in Davis s experiments for N2 on tungsten powders can be deduced by comparing his results with our results for N2 on a tungsten ribbon. He found that the rate of adsorption at temperatures between 300° and 700°K. was extremely slow and that the rate increased with temperature. We find that the rate of N2 adsorption for clean tungsten is not extremely slow and that it decreases as the temperature increases. Furthermore Davis finds that at a pressure of 10 mm. and a temperature of 900°K. the equilibrium amount adsorbed is only about 2 X 10 N atoms/cm. under the same conditions we find about 50 X 10 N atoms/cm. . 

In Fig. 4.52 the activity of H2 oxidation for different metals is plotted as a function of heat of oxygen adsorption. A volcano-type plot is formed. A maximum in rate is found for the metal that can dissociate 02, but does not bind CO or oxygen too strongly. Gold cannot dissociate 02, tungsten is inactive because it... 

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