NH3 adsorption on Ni(110) and the production of the NH2 species by electron irradiation

The adsorption of NH₃ on Ni(110) has been examined using electron stimulated desorption ion angular distribution (ESDIAD), low energy electron diffraction (LEED) and thermal desorption spectrometry (TDS). At ～ 85 K the NH₃ molecule enters into a series of chemisorption and physisorption states whose structures have been partially characterized by means of ESDIAD and LEED. Upon heating, these NH₃ states desorb without dissociation; for adsorption below 300 K there is essentially no thermal decomposition. The ammonia adiayer was found to be extremely sensitive to electron irradiation effects. Evidence was found to support the irradiation induced conversion of NH₃(ads) to an amido intermediate, nh_2(ads). The NH₂ adsorbs with its C_2v axis normal to the surface and its NH bonds aligned along the 001] and 001?] directions. In the absence of further electron irradiation the nh_2(ads) species is stable to 375 K whereupon it dissociates to N_(ads)and H_2(g). The remaining N_(ads) desorbs near 750 K with significant attractive N…N interaction. No evidence is found for an imido intermediate, nh_(ads). nh_2(ads) also undergoes a disproportionation/recombination reaction upon heating to produce an additional NH₃ desorption state. A significant isotope effect for NH versus ND scission, sensitive to the adsorption state of the ammonia, is found to occur upon electron irradiation.