title: Interpretation of NMR relaxation as a tool for characterising the adsorption strength of liquids inside porous materials.
abstract: Nuclear magnetic resonance (NMR) relaxation times are shown to provide a unique probe of adsorbate-adsorbent interactions in liquid-saturated porous materials. A short theoretical analysis is presented, which shows that the ratio of the longitudinal to transverse relaxation times (T1/T2) is related to an adsorbate-adsorbent interaction energy, and we introduce a quantitative metric esurf (based on the relaxation time ratio) characterising the strength of this surface interaction. We then consider the interaction of water with a range of oxide surfaces (TiO2 anatase, TiO2 rutile, γ-Al2O3, SiO2, θ-Al2O3 and ZrO2) and show that esurf correlates with the strongest adsorption sites present, as determined by temperature programmed desorption (TPD). Thus we demonstrate that NMR relaxation measurements have a direct physical interpretation in terms of the characterisation of activation energy of desorption from the surface. Further, for a series of chemically similar solid materials, in this case a range of oxide materials, for which at least two calibration values are obtainable by TPD, the esurf parameter yields a direct estimate of the maximum activation energy of desorption from the surface. The results suggest that T1/T2 measurements may become a useful addition to the methods available to characterise liquid-phase adsorption in porous materials. The particular motivation for this work is to characterise adsorbate-surface interactions in liquid-phase catalysis.
authors.affiliation: Department of Chemical Engineering & Biotechnology, University of Cambridge, Pembroke Street, Cambridge CB2 3RA (UK).
authors.forename: Carmine
authors.initials: C
authors.lastname: D'Agostino
authors.forename: Jonathan
authors.initials: J
authors.lastname: Mitchell
authors.forename: Michael D
authors.initials: MD
authors.lastname: Mantle
authors.forename: Lynn F
authors.initials: LF
authors.lastname: Gladden
keywords: NMR spectroscopy; catalysis; mesoporous materials; porous media; relaxation times; surfaces and interfaces