INFRARED SPECTRA OF NITROGEN ADSORBED ON NICKEL-ON-AEROSIL CATALYSTS - EFFECTS OF INTERMOLECULAR INTERACTION AND ISOTOPIC SUBSTITUTION

Previous work1-3) on the adsorption of nitrogen on nickel is extended by a study of the spectra of adsorbed nitrogen isotopes. Detailed analysis of the spectra shows that at low surface coverage the nitrogen molecules are adsorbed at remote positions (singletons). At high surface coverage, distinct changes occur in the spectra as a result of intermolecular interaction. The degree of coverage proves to have an effect on frequency and line width and, when mixtures of nitrogen isotopes are adsorbed, also on the ratio of the intensities of the various absorption bands. A quantitative description of all effects occurring in the spectra is given with the aid of a perturbation treatment, it being assumed that the interaction has a dipole-dipole nature. From the results it is concluded that nitrogen cannot be adsorbed on all, but only on alternate B5-sites in a ridge. At high surface coverage deviations from Beer's law occur. The non-linear relation between the extinction and the surface coverage is caused by the dependence of the linewidth of the absorption band on the surface coverage. After correction for linewidth variations, the extinction coefficient is ε{lunate}({Δσ 1 2 = 17 cm-1}) = 2.0 × 10-18 cm2 molecule. © 1969.