Cleavage of N-H bonds by active oxygen on Ag(110): I. Ammonia

Ammonia adsorbs without dissociation on clean Ag(110) with a binding energy of 11 kcal/mol. Coadsorption of ammonia and atomic oxygen at 105 K produces adsorbed hydroxyl groups and NH_x species. Coadsorption of ammonia and molecular oxygen leads to the stabilization of molecular oxygen, as is shown by the increase in the desorption peak temperature of dioxygen from 180 to 210 K. The reaction of ammonia with both forms of adsorbed oxygen produces the same products at the same temperatures. Water desorbs in a series of peaks at 310, 340, and 400 K resulting from hydroxyl recombination and hydrogen transfer from NH_x species to adsorbed oxygen atoms. NO and N₂ desorb together at 530 K. Oxygen recombination at 590 K only occurs following small ammonia doses such that excess oxygen persists on the surface. No hydrogen was seen to desorb under any reaction conditions. Vibrational spectroscopy shows that NH groups persist on the surface at temperatures well into the water desorption peak at 310 K and possibly to significantly higher temperatures, indicative of the difficulty of N-H bond cleavage by metallic silver.