π-BondedN2on Fe(111): The Precursor for Dissociation

Abstract

N 2 chemisorbed on Fe(lll) is identified by photoelectron spectroscopy and highresolution electron energy-loss spectroscopy as a n-bonded surface complex in which both nitrogen atoms interact with the metal. This species forms the immediate precursor to dissociation.PACS numbers: 82.65.Jv, 31.20.Tz, 82.80.Di, 82.80.Pv Molecular nitrogen adsorption on transition metals has been studied in detail on various substrates by photoemission and vibrational spectroscopies, 1 and in all cases it has so far been concluded that the molecule bonds mainly via the 3o-^ and 2cr g molecular orbitals in an "end-on" configuration to the surface, i.e., M-N-N. Atomic nitrogen adsorption has also been studied widely, 2 but no experimental data have been reported for the actual surface intermediate in which the breaking of the dinitrogen bond occurs. There is a great practical interest in a better understanding of the dissociation mechanism on a molecular basis, since dissociative nitrogen adsorption has been identified as the rate-determining step in the ammonia synthesis reaction on iron. 3 The kinetics of dissociative nitrogen adsorption on Fe(l 11) have been studied in detail previously 4 ; it was concluded that this process proceeds through a molecularly adsorbed species with a rather low activation energy. Later it was found 5 that this species (a state) is preceded at low temperatures by a more weakly held, physisorbed y state, which desorbs at about 85 K. In the present work, the a state has been identified as a rr -bonded molecule with both N atoms interacting with the surface and thus forming the immediate precursor to dissociation. This evidence is based on experimental x-ray photoemission spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (not discussed here) supplemented by calculations for the N 1 s core-hole spectra and high-resolution electron-energy-loss spectroscopy (HREELS) data. N 15 core-level photoelectron experiments using Al Ka radiation were performed in a Leybold Heraeus photoelectron spectometer equipped with a rotable cold finger. 6 The vibrational data were obtained in a different vacuum system with a high-resolution electronenergy-loss spectrometer. The Fe(lll) crystals were cleaned and annealed following previously described procedures. 7 In Fig. 1, a sequence of N 15 spectra for the three different nitrogen bonding states on Fe(lll), denoted by 7, a (molecular), and ft (atomic) is displayed. The y state spectrum (curve a) was recorded at 85 K in an ambient of P N2 = 5xlO~7 mbar and includes emission from some a-N 2 , 5 giving rise to the shoulder at ~ 399-eV binding energy (E B ). The intensities of the two peaks at E B = 405.9 eV and E B -401 eV depend on the ambient N 2 pressure at T^ 85 K, since y-N 2 is only weakly adsorbed (£^^24 kJ/mol). 5 A similar N ls-doublet structure has been observed by Kishi and Roberts 8 for N 2 on polycrystalline iron but was interpreted incorrectly as arising from two different adsorption states of N 2 . After heating the predosed surface in vacuum to 97 K, ...

The 308 nm laser photodissociation of HN3 adsorbed on Si(111)-7 × 7

“…Fe(1 11) [30]. This peak is quite different from those for "end-on" physisorbed N 2 on metal surfaces, where two peaks at -406 and -401 eV were observed and attributed to the up and down N atoms, respectively [30].…”

“…When N 2 adsorbed "end-on" on Ni(110) [31], W(100) [32] and Pt(111) [33], this mode was observed to be 265 meV or higher frequencies, while it was adsorbed "side-on" on Fe(1 11) [30], its vibrational frequency was shifted down to 185 meV. The observed 206 meV peak here, thus suggests a "side-on" adsorption of N 2 on Si(1 11).…”

“…For N 2 adsorbed on metal surfaces, the NN stretching mode was found to range from 180 to 277 meV, depending upon the N 2 adsorption geometry [30][31][32][33]. When N 2 adsorbed "end-on" on Ni(110) [31], W(100) [32] and Pt(111) [33], this mode was observed to be 265 meV or higher frequencies, while it was adsorbed "side-on" on Fe(1 11) [30], its vibrational frequency was shifted down to 185 meV.…”

Ab initiocalculation of the solution enthalpies of substitutional and interstitial impurities in paramagnetic fcc Fe

“…(5) and (6), we used paramagnetic fcc Fe simulated within the magnetic SQS method and bcc crystals of V and Nb with theoretically calculated lattice parameters 3.03 and 3.30Å, respectively, which turned out to coincide with experimental ones (a V exp = 3.02Å [53] and a Nb exp = 3.30Å [53]). An isolated dimer with the experimental bond length 1.1Å was used as a reference state for N [54]. Because the van der Waals forces between graphite layers are not reproduced properly in the DFT-GGA calculations, for the reference state of C in Eq.…”

Dissociative and non-dissociative adsorption dynamics of N2 on Fe(110)

“…Early investigations aimed to characterize the interaction of N 2 with Fe(111) identified two distinct molecular species on this face [7][8][9][10]: a weakly bound state with the molecular axis normal to the surface and a more strongly bound state with the molecule lying parallel to the surface. Using photoelectron spectroscopy and high-resolution electron energy loss spectroscopy, the latter was identified as the immediate precursor state to dissociation [7]. Molecular beam experiments directed to probe the dynamics of the N 2 /Fe(111) system showed that the N 2 translational kinetic energy can be efficiently used to promote reactivity on this surface [11].…”