The chemisorption and decomposition of pyridine and ammonia at clean and oxidised Al(1 1 1) surfaces

The nature of the oxygen state at Al(1 1 1) surfaces has been probed by the adsorption of ammonia and pyridine using photoelectron spectroscopy to study reaction products as the sample was warmed from 130 to 540 K. Ammonia only physisorbs at the clean Al(1 1 1) surface but the presence of surface oxygen catalyses a decomposition reaction which leads to the formation of an amide and, on warming to 295 K and above, a nitride. There is no evidence in the XP spectra for the presence of an hydroxyl species or for the desorption of water. Higher oxygen concentrations result in a decrease in the extent of decomposition, with ammonia being stable to 295 K on the pre-oxidised surface. Pyridine partially decomposes at the clean Al(1 1 1) surface at 130 K, giving rise to a well defined intermediate tentatively assigned to an α-pyridyl on the basis of the C:N ratio. Further decomposition occurs on warming with the formation of carbide and nitride by 540 K. Pre-adsorbed oxygen reduces the extent of pyridine decomposition at low temperatures but the species assigned as an α-pyridyl is still formed by room temperature.