FORMATION OF NH3 AND N-2 FROM ATOMIC NITROGEN AND HYDROGEN ON RHODIUM(111)

Reactions of adsorbed N atoms on Rh(lll) to N-2 and NH3 were studied w ith temperature programmed desorption, temperature programmed reaction spectroscopy, and static secondary ion mass spectrometry. For N-atom coverages below approximate to 10.15 monolayers, desorption of N-2 fol lows simple second-order kinetics, but at higher coverages the desorpt ion traces broaden to higher temperatures. Hydrogenation to NH3 can be described by a stepwise addition of H atoms to N-ads in which the rea ction from NH2,ads+H-ads to NH3,ads determines the rate. The activatio n energy for the rate determining step is 76 kJ/mol. The desorption of NH3 from Rh(111) was studied separately. The kinetic parameters for d esorption at low NH3 coverage are 81 kJ/mol and 10(13) s(-1), but the rate of desorption increases rapidly with increasing NH3 coverage. It is argued that the remarkable coverage dependence of the desorption ra te is unlikely to be caused by lateral repulsive interactions but may be due to a coverage dependence of the pre-exponential factor. (C) 199 7 American Vacuum Society.