Field-Emission Study of the Adsorption and Decomposition of Ammonia on Tungsten

Dawson, P. T.; Hansen, Robert S.

doi:10.1063/1.1668693

Cited by 30 publications

(9 citation statements)

References 22 publications

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“…During thermal desorption the cell is at room temperature, thus no correction is required for delayed pressure bursts and the spectrum can be interpreted kinetically. 2 The hydrogen desorption spectra obtained from ammonia interaction at higher temperatures are much less satisfying in view of the predictions of the proposed mode1. This has been achieved by immediately repeating the experiment for a 300 0 K adsorption under the same conditions and preflashing to 800 0 K (Fig.…”

Section: Tungsten Produces Surface Species Which Decomposementioning

confidence: 92%

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Study of the Interaction of Ammonia with Tungsten Surfaces by Thermal Desorption Spectrometry

Dawson

1971

The Journal of Chemical Physics

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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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Section: Tungsten Produces Surface Species Which Decomposementioning

confidence: 92%

“…However, decomposition of the gas-phase ammonia on the filament becomes appreciable above 1000 o K, and the desorption spectra must be corrected for this effect. 2 No new peak is observed in the vicinity of 900°K. 7).…”

Section: Tungsten Produces Surface Species Which Decomposementioning

confidence: 95%

Study of the Interaction of Ammonia with Tungsten Surfaces by Thermal Desorption Spectrometry

Dawson

1971

The Journal of Chemical Physics

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“…The PFCs are saturated with hydrogen atoms, either deuterium from the plasma or protium. At the surface ammonia can be formed in a multi-step process [5]. This process is also active if ammonia reaches the surfaces as shown for the ammonia puff experiment mentioned above.…”

Section: Ammonia Behaviourmentioning

confidence: 97%

Ammonia production in nitrogen seeded plasma discharges in ASDEX Upgrade

Rohde

Oberkofler

2015

Journal of Nuclear Materials

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“…Ammonia will adsorb to the equivalent of a half-monolayer of nitrogen, WS,NH,, at 200 O K (6)(7)(8). Field electron emission microscope observations show that surface dissociation of hydrogen from chemisorbed ammonia occurs over the temperature interval 200-400 O K , being more rapid on crystallographic planes of low electron affinity (6 (8) and is thus indistinguishable from the desorption of pure P-hydrogen from a polycrystalline tungsten surface.…”

Section: Desorption Of Hydrogen Following Interaction Ofmentioning

confidence: 99%

“…Field electron emission microscope observations show that surface dissociation of hydrogen from chemisorbed ammonia occurs over the temperature interval 200-400 O K , being more rapid on crystallographic planes of low electron affinity (6 (8) and is thus indistinguishable from the desorption of pure P-hydrogen from a polycrystalline tungsten surface. Desorption of the P-nitrogen left behind does not commence until 1150 OK.…”

Section: Desorption Of Hydrogen Following Interaction Ofmentioning

confidence: 99%

The State of Hydrogen Desorbing from Intermediates Formed by Ammonia Interaction with Tungsten Surfaces

Yi¹,

Dawson²

1974

Can. J. Chem.

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Ammonia interaction with a tungsten surface can generate dense adlayers containing nitrogen and hydrogen, i.e. an η-species of surface stoichiometry Ws2N3H. In thermal desorption mass spectrometry experiments, hydrogen desorbing from the η-species interacts with the glass wall in a manner similar to that previously observed for atomic hydrogen. This paper describes two mass spectrometric techniques designed to confirm this conclusion directly. The first method uses a line-of-sight geometry between the tungsten filament and the ionization source of the mass spectrometer and the results indicate that, at least, part of the hydrogen desorbing from the η-species does so atomically. In the second method a multiple wall collision geometry is used but prior saturation of the wall with D atoms will result in an HD+ ion current for desorbing H atoms. The results suggest that 26% of the hydrogen desorbs atomically. Hydrogen atom desorption from the η-species occurs at tungsten filament temperatures below those required for hydrogen atom evaporation from a pure hydrogen adlayer. It is proposed that a reduced binding energy for adsorbed hydrogen atoms and a reduced mobility of these adatoms arises from the presence of a large surface concentration of nitrogen. This will result in the rates of atomic hydrogen desorption and bimolecular recombination becoming comparable at temperatures lower than is the case for pure hydrogen interaction with tungsten. The implications of these results for the ammonia synthesis reaction are discussed.

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Field-Emission Study of the Adsorption and Decomposition of Ammonia on Tungsten

Cited by 30 publications

References 22 publications

Study of the Interaction of Ammonia with Tungsten Surfaces by Thermal Desorption Spectrometry

Study of the Interaction of Ammonia with Tungsten Surfaces by Thermal Desorption Spectrometry

Ammonia production in nitrogen seeded plasma discharges in ASDEX Upgrade

The State of Hydrogen Desorbing from Intermediates Formed by Ammonia Interaction with Tungsten Surfaces

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