Adsorption and Decomposition of Ammonia on a Polycrystalline Tungsten Filament

Abstract

Adsorption and decomposition of ammonia on a polycrystalline tungsten filament with and without preadsorbed nitrogen doses were studied by flash‐filament desorption spectroscopy. The hydrogen‐desorption spectrum resulting from ammonia decomposition was qualitatively similar to that which would have resulted from a comparable amount of hydrogen dosed onto a clean surface, desorption occurring over much the same temperature range but with a somewhat different spectrum shape. Hydrogen desorption was substantially complete by 900°K under all circumstances. With increasing nitrogen predose, the amount of ammonia taken up in a fixed dosing condition decreased, while total nitrogen left on the surface following the dose increased. Partial decomposition occurred during ammonia adsorption at 300°K resulting in desorption of approximately 1.5 atoms of hydrogen per ammonia molecule absorbed. Prolonged dosage of ammonia on clean tungsten at 300°K left 0.65NH_1.5 units on the surface per surface tungsten atom. When total nitrogen on the surface exceeds one‐half an atom per surface tungsten atom, a second desorption peak appears in the nitrogen‐desorption spectrum beginning at about 870°K, with a peak maximum around 1100°K. The nitrogen in this peak is designated x‐nitrogen and can be almost completely resolved from β‐nitrogen. Desorption of x‐nitrogen is found to be second order in the excess of nitrogen over one‐half nitrogen atom per surface tungsten atom, with an activation energy of 46 kcal/mole. When the surface was predosed with one isotope of nitrogen and subsequently dosed with ammonia having a different isotope of nitrogen, extensive mixing of isomers was found in both the x‐nitrogen and β‐nitrogen peaks but with some bias in the x‐nitrogen peak toward the predosed nitrogen isotope. The (β + x)‐nitrogen surface structure has approximately one nitrogen adatom per surface tungsten atom. This structure forms and decomposes under equivalent conditions to those leading to the surface complex of Dawson and Hansen for which the structure W₂NNH₂ was proposed, and to the (1 × 1) structure of Estrup and Anderson for which the structure WNH₂ was proposed. It is concluded that both of these latter assignments were incorrect and that the properties observed were those of (β + x)‐nitrogen. Furthermore, the average rate of x‐nitrogen desorption agreed well with ammonia decomposition rates on tungsten wires available in the literature. The (β + x)‐nitrogen structure was synthesized, so far as can be judged from its flash‐desorption spectrum, by dosing with pure nitrogen activated by electron bombardment. These results strongly suggest that the catalytic decomposition of ammonia on tungsten has as its limiting step the desorption of x‐nitrogen; it is shown that kinetic orders, activation energy, and temperature range for effective catalysis are in good accord with this model.