Kinetics and Mechanism of NH 3 Formation by the Hydrogenation of Atomic Nitrogen on Rh(111)

The reaction between atomic nitrogen and H 2 has been studied in order to elucidate the mechanism of NH 3 formation on Rh(111). Atomic nitrogen layers of 0.10 monolayer (ML) coverage were obtained by adsorbing NO at 120 K and selectively removing the atomic oxygen from dissociated NO by reaction with H 2 at 375 K. The rate of NH 3 formation is first order in the atomic nitrogen coverage and linearly proportional to the H 2 pressure below 5 × 10 -7 mbar. Static secondary ion mass spectrometry (SSIMS) indicates that N and NH 2 are the predominant reaction intermediates, while small amounts of NH 3 are also detected. The NH 2 surface coverage increases with increasing H 2 pressure. The presence of NH 2 is also indicated by the appearance of a reaction-limited H 2 desorption state in temperature-programmed desorption (TPD) spectra. The hydrogenation of NH 2 to NH 3 is expected to be the rate-determining step in the NH 3 formation. From the temperature dependence of the NH 3 formation rate an effective activation energy of 40 kJ/mol was determined, which could be translated into an activation energy of 76 kJ/mol for the hydrogenation from NH 2 to NH 3 .