Nuclear Spin Relaxation and Diffusion Studies of Adsorption and Dynamics at the Catalyst-Liquid Interface

The rational design of active materials for liquid-phase heterogeneous catalytic processes requires a detailed understanding of interactions occurring at the solid-liquid interface. The elucidation of such dynamics is of particular relevance to the study and development of solvated green chemical reaction processes, such as the production of chemicals and fuels from biomass. Nuclear spin relaxation time measurements have recently emerged as a novel tool for probing surface dynamics within such systems; herein, we detail the state-of-the-art of such measurements, and extend our current understanding of how such characteristics may be interpreted in terms of formal surface interaction phenomena. Initially, a simple protocol is developed to illustrate the sensitivity of longitudinal nuclear spin relaxation to hydrogen-bond-mediated adsorption interactions occurring between a prototypical polar liquid (methanol) and a range of common mesoporous catalyst support materials ($\gamma$-$Al_{2}O_{3}$, $\alpha$-$Al_{2}O_{3}$, anatase-$TiO_{2}$ and $SiO_{2}$) exhibiting hydroxylated pore surfaces. Proton longitudinal relaxation time constant ($T_{1}$) measurements are shown to demonstrate significant sensitivity to changes in adsorption mechanism within these systems. Specifically, the acquired $T_{1}$ data indicates that the dynamics of methanol within the adsorbed surface layer is notably enhanced upon passivation of surface hydroxyl groups with alkyl chains, and tends towards that of the unrestricted bulk liquid. A complex analysis in which we account for the influence of changing pore morphology and surface layer structure upon passivation is found to be in agreement with these observations, validating the widely applied assumption that the surface relaxivity of polar adsorbates is sensitive to interactions with hydroxyl groups at the pores surface. The ability of nuclear spin relaxation measurements to probe surface interaction strengths in a quantitative manner is then explored through the application of two-dimensional $T_{1}-T_{2}$ correlation experiments. The ratio of longitudinal-to-transverse relaxation time constants $T_{1}/T_{2}$ is readily obtained from such experiments, and is considered to provide a non-destructive indication of the surface affinities exhibited by species at the solid-liquid interface. We detail the application of such measurements to probe the surface interaction strengths of a homologous series of primary alcohols and cyclohexane within an industrial silica support material. The resulting $T_{1}/T_{2}$ values are shown to be in excellent agreement with the results of extensive density functional theory-based adsorption energy calculations, performed on single molecules interacting with an idealised silica surface. The observed correlation demonstrates the remarkable ability of this metric to provide a quantitative indication of adsorption energetics within liquid-saturated mesoporous media, and validates previous theoretical efforts to link adsorption energetics and the ratio $T_{1}/T_{2}$. Supplementary diffusion measurements illustrate that the effective self-diffusion coefficients obtained from these liquid/silica systems also exhibit sensitivity to interactions with the pore surface, leading to a reduction in alcohol mobility beyond that expected purely from the tortuosity of the porous material. For the first time, it is shown that a clear correlation between reduced diffusivity and $T_{1}/T_{2}$ ratio is evident.