Formation of NH3 and N-2 from Atomic Nitrogen and Hydrogen on Rhodium(111)

Reactions of adsorbed N atoms on Rh(111) to N2 and NH3 were studied with temp. programmed desorption, temp. programmed reaction spectroscopy, and static secondary ion mass spectrometry. For N-atom coverages below ~0.15 monolayers, desorption of N2 follows simple second-order kinetics, but at higher coverages the desorption traces broaden to higher temps. Hydrogenation to NH3 can be described by a stepwise addn. of H atoms to Nads in which the reaction from NH2,ads+Hads to NH3,ads dets. the rate. The activation energy for the rate detg. step is 76 kJ/mol. The desorption of NH3 from Rb(111) was studied sep. The kinetic parameters for desorption at low NH3 coverage are 81 kJ/mol and 1013 s-1, but the rate of desorption increases rapidly with increasing NH3 coverage. It is argued that the remarkable coverage dependence of the desorption rate is unlikely to be caused by lateral repulsive interactions but may be due to a coverage of the pre-exponential facto