| dc.description.abstract | 1.4 Conclusions
The core and valence level binding energies and vibrational stretching frequencies of nitrogen adsorbed on clean as well as modified transition?metal surfaces are summarized in Tables I.1–I.3. From Table I.1 it is evident that N? interacts weakly with the (100), (110) and (111) faces of clean Ni as well as with the polycrystalline Ni and Pd surfaces, giving rise to the characteristic two?peak N(1s) spectrum. On a polycrystalline Fe surface, however, both weakly chemisorbed and strongly chemisorbed molecular species are observed.
Coadsorption of molecular N? with atomic nitrogen on a Ni(110) surface gives strongly bound precursor nitrogen which subsequently dissociates at room temperature (Table 1.2). On carbon?covered Ni(110) surface, we observed atomic, weakly chemisorbed and strongly chemisorbed nitrogen species depending on the carbon coverage. N? interacts mainly with C(2p?) orbitals on the Ni(110)–C surface to form the precursor species. While N? is weakly adsorbed even on the polycrystalline Ni surface modified by Ba or Al, only the strongly chemisorbed precursor state is found on a Ba?covered polycrystalline Fe surface. Accordingly, we observe a single N–N vibrational band on the Ba?covered Fe surface with a reduced bond order of 1.9 compared to that on the clean surface (Table 1.3). The bond order of the weakly adsorbed nitrogen is also reduced on Al?covered Ni surface, indicating the increase in the charge transfer from the metal to the 1?g_gg? orbital of adsorbed N?.
We have shown that CO can be stabilized in the molecular form in the presence of electronegative chlorine on polycrystalline Mn, Fe and Ni surfaces. This becomes possible by virtue of the electron?withdrawing ability of chlorine which makes the adsorption sites more acidic. Electropositive elements such as Ba and Al, which donate charge to the metal surface, increase the basicity of the adsorption sites. Molecules (N? or CO) adsorbed on these sites tend to dissociate at higher temperatures. We believe that these results are of some relevance to catalysis and suggest the possibility of using a wider choice of transition metals as catalysts by suitably modifying them with electronegative or electropositive additives.
The following features in the N(1s) and He II spectra of adsorbed nitrogen are found to be useful in characterizing the various adsorbed species. Physical adsorption of N? is characterized by a single feature around 403 eV (e.g., on the TiO? surface). Two types of weakly chemisorbed N? species are identified, one showing only a single N(1s) feature corresponding to an unscreened final state at 405 eV (e.g., on Ti and Ni–Ti alloy surfaces), and the other showing two features at 401 and 406 eV corresponding to screened and unscreened final states respectively (e.g., on Ni, Ni/Al?O? and non?annealed Ni/TiO? surfaces). The feature at 397 eV corresponds to the atomic species (e.g., on Ti and annealed Ni/TiO? surfaces). Observation of a single feature at 405 eV due to chemisorption signifies much weaker adsorption than when two features at 401 and 405 eV are found. Physisorbed N? shows three features in the He II spectrum around 9, 11 and 14 eV due to 3?g_gg?, 1?u_uu? and 2?u_uu? orbitals respectively. Chemisorbed N? shows only two features around 8.5 and 13 eV due to (3?g_gg? + 1?u_uu?) and 2?u_uu? orbitals respectively. The atomic nitrogen shows a single feature around 5.6 eV in the He II spectrum. In Table III.1, we have summarized the N(1s) and valence orbital binding energies of N? adsorbed on the various surfaces studied by us.
The main conclusions with regard to the nature of the adsorption of N? on the different surfaces are as follows:
(i) Adsorption of N? is molecular on both Ni/Al and Ni/Al?O? surfaces at 80 K.
(ii) N? physisorbs on TiO? and reduced TiO? surfaces at 80 K; on the latter there is also dissociation.
(iii) N? is adsorbed partly dissociatively on clean Ti and Ti–Ni alloy surfaces. There is also very weak chemisorption with a single N(1s) feature around 405 eV.
(iv) On a Ni/TiO? surface, there is substantial molecular chemisorption (with two N(1s) features at 401 and 406 eV) accompanied by dissociation.
(v) Only molecular adsorption of N? is seen on the non?annealed Ni/TiO? surface just as on a Ni/Al?O? surface.
(vi) On annealed Ni/TiO?, however, we find both dissociative and molecular adsorption at 80 K (the latter desorbs at 125 K) similar to the Ni–Ti alloy surface, suggesting that the annealed Ni/TiO? surface may represent the SMSI state. | |
