Exploring Surface Interactions in Catalysts Using Low-Field Nuclear Magnetic Resonance

Abstract

Fast field cycling (FFC) nuclear magnetic resonance (NMR) is applied to probe the slow dynamics of liquid molecules imbibed in porous catalysts. The FFC measurements are used to determine surface diffusion correlation and residence times that provide information on the molecular dynamics of surface adsorbed species. The longitudinal relaxation time T 1 dispersion curves reveal biphasic diffusion of adsorbed water that we attribute to the presence of “strongly bound” and “weakly bound” molecules. FFC measurements of organic liquids (2-butanone, 2-propanol) do not show such behavior. These observations agree with molecular dynamics simulations. The frequency dependence of the relaxation time ratio T 1 / T 2 is also considered; it is demonstrated that T 1 / T 2 remains a valid indicator of adsorption energy regardless of the field strength at which the measurement is taken in the range B 0 = 0.1 mT to 0.23 T.