Heterogeneities in Structure and Diffusion within Porous Catalyst Support Pellets Observed by NMR Imaging

Abstract

The pore structure of porous oxide pellets commonly used as catalyst supports or as adsorbents in separation processes continues to attract much attention. This paper presents the results of a nuclear magnetic resonance (NMR) imaging study of various alumina and silica pellets. Both spin-echo and gradient-reversal techniques have been employed to produce two-dimensional maps of the spin density, spin-lattice relaxation time, and self-diffusion coefficient in the samples. NMR images show that the porous pellets contain significant structural heterogeneity, and this is reflected in the spatially resolved liquid-phase diffusion coefficients. The work suggests that NMR imaging techniques provide additional information which may be used to complement existing porosimetry and adsorption methods in characterising pore structures. These data will aid in the development of more realistic models of porous solids, which may then be used to predict transport phenomena. The advantage of NMR imaging methods is that they allow the characterisation of pore structure and transport processes within the same experimental environment. The NMR images presented in this paper represent the first application of this technique to visualize the internal structure of and diffusion within porous catalyst support pellets.