Study of the Interaction of Ammonia with Tungsten Surfaces by Thermal Desorption Spectrometry
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abstract
The interaction of ammonia gas, at pressures between 10−7 and 10−2 torr, with a polycrystalline tungsten filament, at temperatures between 200 and 700°K, has been investigated by thermal desorption mass spectrometry. Several procedures have been adopted to overcome the problems caused by the persistence of ammonia gas in ultrahigh vacuum systems. The adsorbed phase obtained by interaction at 200°K produces a desorption spectrum with a single low-temperature hydrogen peak (peak maximum 450°K) and a single high-temperature β-nitrogen peak (peak maximum 1450°K) in agreement with earlier field emission observations. Increasing the adsorption temperature in the range 200–700°K causes the single hydrogen desorption peak to shift to higher temperatures and the nitrogen desorption to increase and shift to lower temperatures, eventually forming two well-resolved desorption peaks. The nitrogen desorption features resemble those obtained by adsorption of nitric oxide (ω), electron bombardment of γ nitrogen (λ), and by repeated ammonia adsorption at 300°K with intervening flashing to 800°K (δ). In these experiments, with the reaction vessel cooled to 77°K, little hydrogen desorption accompanies the nitrogen desorption. However, experiments carried out with the reaction vessel at room temperature (or coated with an ammonia layer at 77°K) show that for adsorption at the higher temperatures the low-temperature nitrogen desorption peak is accompanied by the simultaneous desorption of hydrogen. These clearly resolved desorption features are designated η nitrogen and η hydrogen; desorption occurs by first-order kinetics with peak maxima at 970 and 985°K, respectively. Characteristic behavior of the η-hydrogen peak indicates that it desorbs as hydrogen atoms. Surface coverage estimates show that ammonia interacts with tungsten at 700°K to form, successively, surfaces of stoichiometry W2N(β), WN(δ), and W2N3H (η). The initial sticking probability in the formation of the η species is 10−6 at 700°K. It is concluded that, except at extremely low reactant pressure, the catalytic decomposition of ammonia on tungsten has as its limiting step the desorption of the η species W2N3H(η)→W2N(β)+N2(g)+H(g) and not desorption of nitrogen alone as has been sometimes suggested. Moreover, since the η-desorption reaction is expected to involve the breaking of N–H bonds, the observed hydrogen isotope effect in this zero-order reaction can be readily understood.
