Surface Composition and Diffusion in Catalysts Probed by PFG NMR Techniques

Abstract

We have recently developed an experimental protocol to study surface self-diffusion using Pulsed Field Gradient (PFG) Nuclear Magnetic Resonance (NMR) techniques. While the study of diffusion phenomena with PFG NMR techniques evolved in the 1960s and is nowadays part of the standard NMR toolbox, our novel approach allows us the simultaneous quantification of diffusion coefficients of molecules in a surface layer and the bulk pore liquid of a porous medium. This is of relevance when attempting to explain solvent effects in catalysis. Solvent effects are known to have a significant impact on catalyst performance. For example, water has been reported to influence the reaction rates during carbonyl hydrogenations. In this work we have assessed the effect of two solvents – water and isopropyl alcohol (IPA) - on the bulk pore and surface diffusivity of methyl ethyl ketone (MEK) in porous Ru/SiO2 catalyst pellets. We find that when MEK is dissolved in water, it appears to interact more closely with the surface than when it is dissolved in IPA.