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Articles you may be interested inInteraction of hydrogen with chemical vapor deposition diamond surfaces: A thermal desorption study J. Vac. Sci. Technol. A 12, 3033 (1994); 10.1116/1.578932 Studies of interactions between NO and CO on a Pd surface by thermal programmed desorption and molecular beam relaxation spectrometry Interaction of CO with a Pd(110) surface, studied by low energy electron diffraction, thermal desorption spectroscopy, and Δφ Summary Abstract: Surface reactions studied by laserinduced thermal desorption with Fourier transform mass spectrometry detectionThe interaction of ammonia gas, at pressures between 10-7 and 10-2 torr, with a polycrystalline tungsten filament, at temperatures between 200 and 700 o 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 0 K produces a desorption spectrum with a single low-temperature hydrogen peak (peak maximum 450 0 K) and a single high-temperature /1-nitrogen peak (peak maximum 1450 0 K) in agreement with earlier field emission observations. Increasing the adsorption temperature in the range 200-700 o 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 (w), electron bombardment of l' nitrogen (A), and by repeated ammonia adsorption at 300 0 K with intervening flashing to 800 0 K (Il). In these experiments, with the reaction vessel cooled to nOK, 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 nOK) 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 1] nitrogen and 1] hydrogen; desorption occurs by first-order kinetics with peak maxima at 970 and 985°K, respectively. Characteristic behavior of the 1]-hydrogen peak indicates that it desorbs as hydrogen atoms. Surface coverage estimates show that ammonia interacts with tungsten at 700 0 K to form, successively, surfaces of stoichiometry W2N (J3), WN (Il), and W2NaH (1]). The initial sticking probability in the formation of the 1] species is 10-6 at 700oK. 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 1] species and not desorption of nitrogen alone as has been sometimes suggested. Moreover, since the 1]-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.

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The catalytic decomposition of ammonia on tungsten has been investigated by field electron-emission microscopy. Physisorbed ammonia causes a large lowering of work function (∼ − 2.8 eV). Ammonia chemisorbs at low temperatures by the formation of a coordinate bond and the heat of adsorption and activation energy for decomposition appear to be highest on planes of highest clean work function. Decomposition to nitrogen and hydrogen occurs in the interval 200°–400°K. Interaction of ammonia with a clean tungsten surface at higher temperatures causes the successive appearance of three species, presently interpreted as N, NNH2, and NNH3+. The interaction of ammonia with nitrogen adatoms to produce the additional species will proceed at lower temperatures and therefore requires little activation energy. The rate-limiting step in the decomposition of ammonia on tungsten is the decomposition on the surface of NNH2. The proposed scheme for decomposition satisfies the existing kinetic data for tungsten. The implication of these results on the structure of nitrogen adatoms is discussed.