Kinetics and Mechanism of NH₃ Formation by the Hydrogenation of Atomic Nitrogen on Rh(111)

Abstract

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 .

C−N Coupling in Reactions between Atomic Nitrogen and Ethylene on Rh(111)

“…Assuming a value of 0.68 ML for the NO-saturated surface, calibration toward the NO uptake curve indicated that an atomic nitrogen coverage of 0.10 ML was obtained starting from an NO exposure of 0.25 langmuir. The procedure has been described in detail in a previous publication on the hydrogenation of atomic nitrogen …”

“…The procedure has been described in detail in a previous publication on the hydrogenation of atomic nitrogen. 18 Figure 1 shows the TPRS spectra of H 2 (2 amu), HCN (27 amu), N 2 (28 amu), and C 2 N 2 (52 amu) obtained from a Rh-(111) surface covered with 0.10 ML of N ads and various amounts of ethylene coadsorbed at 120 K. We also monitored CH 3 CN (51 amu) and NH 3 (17 amu), but these products were not detected. For comparison, we have added the N 2 desorption spectrum obtained from a surface solely covered with 0.10 ML of atomic nitrogen.…”

A fast x-ray photoelectron spectroscopy study of the NO-H2 reaction over Rh(533): Identifying surface species

“…The existence of such species has been suggested in the first oscillatory work over a Rh field emitter, 25 as a possible cause of the observed high emission, because NH x is an electropositive species. 30 Under the conditions of a large excess of H 2 , formation of NH x intermediates is not impossible. 9 In photoemission electron microscopy ͑PEEM͒ work on Rh͑111͒, NH x was also suggested as a possible species for creating low work function areas as an alternative to the present sub-surface oxygen.…”

Formation of NH3 and N2 from atomic nitrogen and hydrogen on rhodium (111)

“…The order of ammonia formation in the coverage of N atoms equals nϭ1, as follows from the straight line plot of ln N versus time, measured at 375 K and a constant pressure of 2ϫ10 Ϫ7 mbar of H 2 . 6 The order in hydrogen was determined at 375 K and H 2 pressures between 5ϫ10 Ϫ8 and 10 Ϫ6 mbar, for a fixed reaction time of 160 s, ͑see Fig. 3͒.…”