Fast exchange approach in NMR studies of specific adsorption in oxides

Abstract

The averaging of NMR spectra by fast exchange between physisorbed and chemisorbed molecules was examined both theoretically and experimentally. The analysis of the theoretical consideration was applied to the interpretation of experimental data on complex formation of adsorbed molecules with active surface sites. It was shown from line broadening in the 1H-NMR spectra of ethylene adsorbed on silica—alumina that under conditions of fast exchange the heat and equilibrium constants of its physisorption could be obtained. Investigation of the dependences of the line shifts on coverage in 13C-NMR spectra of acetone adsorbed on silica gel allowed the determination of the real shift of the 13C line for molecules coordinates by OH groups, the equilibrium constants of complex formation and the surface concentration of hydroxyls. Particular attention was paid to analysis of the NMR spectra of adsorbed molecules when the conditions of fast exchange were violated. It was shown that temperature and surface coverage could affect the rate of exchange and hence the NMR spectra of adsorbates.