Kinetics and mechanism of NH3 formation by the hydrogenation of atomic nitrogen on Rh(111)

The reaction between at. N and H was studied in order to elucidate the mechanism of NH3 formation on Rh(111). At. N layers of 0.10 monolayer coverage were obtained by adsorbing NO at 120 K and selectively removing the at. O from dissocd. NO by reaction with H at 375 K. The rate of NH3 formation is 1st order in the at. N coverage and linearly proportional to the H pressure below 5 * 10-7 mbar. Static secondary ion mass spectrometry indicates that N and NH2 are the predominant reaction intermediates, while small amts. of NH3 are also detected. The NH2 surface coverage increases with increasing H pressure. The presence of NH2 is also indicated by the appearance of a reaction-limited H desorption state in temp.-programmed desorption spectra. The hydrogenation of NH2 to NH3 is expected to be the rate-detg. step in the NH3 formation. From the temp. dependence of the NH3 formation rate an effective activation energy of 40 kJ/mol was detd., which could be translated into an activation energy of 76 kJ/mol for the hydrogenation from NH2 to NH