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.
Highlights
Surface interactions of liquids in porous media are of great importance, in the field of heterogeneous catalysis,[1,2,3,4,5] and the ability to understand surface interactions is essential for efficient and rational catalyst design.[6]
In this work we have presented a theoretical analysis to show that the ratio of nuclear magnetic resonance (NMR) relaxation times, T1/T2, can be related directly to the adsorbate–adsorbent interaction energy characterising adsorption in a porous material, and we introduce a quantitative metric esurf characterising the strength of this surface interaction
Given that the relaxation time characteristics of the nuclear spin system will be dominated by the strongest adsorption sites, the hypothesis that ÀT2/T1 should correlate directly with the maximum activation energy of desorption observed in a temperature-programmed desorption experiment was tested; the data were found to support this hypothesis
Summary
Surface interactions of liquids in porous media are of great importance, in the field of heterogeneous catalysis,[1,2,3,4,5] and the ability to understand surface interactions is essential for efficient and rational catalyst design.[6]. Established techniques for probing the interaction of molecules at solid surfaces include isosteric heat of adsorption, temperature-programmed desorption (TPD), infra-red (IR) spectroscopy, and nuclear magnetic resonance (NMR) chemical shift (d). All these measurements have limitations, and none are able to probe, non-destructively, the behaviour of molecules on catalyst surfaces at realistic reaction conditions. Whilst isosteric heat of adsorption measurements are non-invasive,[7] they require the determination of adsorption isotherms at different temperatures, and are extremely time-consuming; characterisation of co-adsorbed systems may take in excess of well over ten hours.[8] TPD is known to cause in situ reactions of some organic molecules leading to dehydration, isomerisation and decomposition, and is limited in its ability to probe co-ad-
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