Effects of Adsorbent Properties on Nuclear Magnetic Resonance Line Widths of Adsorbates

Abstract

An analytical NMR spectrometer has been used to measure line widths of molecules adsorbed on silica so as to investigate the capabilities and limitations of such measurements. Comparisons of line widths with transverse relaxation times (T ₂) for the same samples showed that the observed resonance lines were inhomogeneously broadened. Consequently, line widths are not a reliable way of obtaining T ₂ values or activation energies. The porosity of silica appeared to be an important factor governing adsorbate line widths, the narrowest lines being observed with adsorbents having the largest pore diameters. The narrow resonance lines observed for molecules adsorbed at low coverages on pyrogenic silica have been attributed to the nonporous surface, small particle size, and high purity of that adsorbent. Broadening of lines at high surface coverages on pyrogenic silica appeared to be due to slow exchange of molecules between surface and bulk states having different chemical shifts. Potential applications of high-resolution NMR to the study of gas-solid adsorption are outlined.