N2 chemisorption on Ni(111). An infrared investigation under steady-state conditions

Abstract

The adsorption ofN 2 on Ni( 111) at 89-115 K under steady-state (quasiequilibrium) conditions has been studied using Fourier-transform infrared reflection absorption spectroscopy (FT-lRAS) and low-energy electron diffraction (LEED). At very low coverage, a single N==N absorption band is observed at 2218 cm -I which is assigned to a singleton N2 adspecies. With increasing N z pressure, two other bands develop at 2212-2208 and 2204-2203 cm -I, and a faint (v1xyj)R 30· LEED pattern is observed. At even higher pressure, a sharp (YJxV3)R 30· LElED pattern develops together with a relatively broad single band at 2194 cm-I • The enthalpy of adsorption ofN2 on Ni(111) is estimated to be -8.4 kcaVmol at zero coverage from isothermal IRAS measurements. Slight attractive interactions between N2 adspecies are detected. The origins of the vibrational features which are related to island formation will be discussed in detail.J. Chern. Phys. 95 (12). 15

Local electronic states in the topmost surface layer probed by metastable atom electron spectroscopy: N2adsorbed and condensed onNi(111Suzuki1, Taoka2, Aoki3 et al. 2001Phys. Rev. B5040View full textAdd to dashboardCite

“…LEED measurements have been also performed, but the results seem controversial. Quick et al observed a p(2ϫ2) pattern after 1 L exposure at 83 K, 10 while Yoshinobu et al reported a ()ϫ))R30°structure under steady-state conditions at 89 K. 11 The expected coverages in the two ordered structures are 0.25 and 0.33, both of which are somewhat higher than the saturation coverage estimated by TDS. This indicates that the N 2 overlayer consists of various size ordered domains.…”

“…This indicates that the N 2 overlayer consists of various size ordered domains. 11 Figure 3 shows a schematic view of the collision geometry between He*(2 3 S) and N 2 on Ni͑111͒, if the p(2ϫ2) overlayer is formed in the domains. As is clearly seen in the figure, the spatial region of N 2 exposed to He*(2 3 S) is very limited owing to the presence of neighboring N 2 molecules.…”

“…9 They found that ͑i͒ the chemisorbed species only appears below 70 K where the N 2 molecules stand up on the surface, ͑ii͒ at sufficiently low temperature ͑e.g., 20 K͒ the chemisorbed and physisorbed states grow simultaneously leading to the formation of a monolayer, and ͑iii͒ the N 2 molecules in the multilayers are regarded as physisorbed and condensed species. LEED measurements have been carried out also, but the conclusions seem controversial; Quick et al observed a p(2ϫ2) pattern after 1 L (1 Lϭ1ϫ10 Ϫ6 Torr s) exposure at 83 K, 10 while Yoshinobu et al reported a ()ϫ))R30°structure under steady-state conditions at 89 K. 11 Höfer et al measured vibration-resolved photoemission spectra of N 2 physisorbed on Ni͑111͒ and indicated that the line shape of the 1 u -derived band is reproduced well using a hopping model, where a photohole in the ionized molecule transfers to neighbors. 12 In contrast to the case of Ni͑111͒, the coexistence of the chemisorbed and physisorbed species has not been observed in the case of Ni͑100͒ or Ni͑110͒ in the submonolayer region.…”

On the encapsulation of nickel clusters by molecular nitrogen

“…For the nickel-nickel potential, the method to obtain the analytic first derivatives is discussed in Ref. 5. We then follow the paths generated by the eigenvectors of the Hessian in the usual fashion until we either locate a transition state or execute so many iterations in the procedure that we are convinced that for that eigenvector, the procedure is not convergent.…”

“…In separate work Parks et al 4 have found that the binding energy of molecular nitrogen on nickel clusters for most cluster sizes is about 0.75 eV, near the 0.3-0.5 eV adsorption energy experimentally determined for the binding of molecular nitrogen on bulk nickel surfaces. 5 Using an extended Hückel model parameterized to ab initio and experimental information, Curotto et al 6 have examined the isomer distributions of bare nickel clusters and have found that the differences between adjacent isomers are as small as 0.05 eV. The origin of the small energy differences appears to be Jahn-Teller distortions, and the high density of low energy minima may be correct even though the details of the potential model itself is not quantitatively accurate.…”