No abstract

No abstract

No abstract

No abstract

No abstract

Reactions of adsorbed N atoms on Rh͑111͒ to N 2 and NH 3 were studied with temperature programmed desorption, temperature programmed reaction spectroscopy, and static secondary ion mass spectrometry. For N-atom coverages below Ϸ0.15 monolayers, desorption of N 2 follows simple second-order kinetics, but at higher coverages the desorption traces broaden to higher temperatures. Hydrogenation to NH 3 can be described by a stepwise addition of H atoms to N ads in which the reaction from NH 2,ads ϩH ads to NH 3 ,ads determines the rate. The activation energy for the rate determining step is 76 kJ/mol. The desorption of NH 3 from Rh͑111͒ was studied separately. The kinetic parameters for desorption at low NH 3 coverage are 81 kJ/mol and 10 13 s Ϫ1, but the rate of desorption increases rapidly with increasing NH 3 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 dependence of the pre-exponential factor.

Temperature programming of NO and C2H4 coadsorbed on Rh(111) gives rise to the desorption of a number of gases. Where H2, H20, CO2 and N2 are the main products at low C2H4 coverages, significant amounts of HCN, CO and NO evolve at higher C2H4 coverages. Static SIMS indicates the formation of a large supply of adsorbed CN species, part of which desorbs as HCN, while the remainder decomposes and is responsible for delayed formation of N2. For the highest C2H4 coverages the majority of the initially adsorbed NO desorbs as HCN.

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 .