TY - JOUR

T1 - Observing solvent dynamics in porous carbons by nuclear magnetic resonance: Elucidating molecular-level dynamics of in-pore and ex-pore species

AU - Cervini, L.

AU - Barrow, N.

AU - Griffin, J.

PY - 2020/4/1

Y1 - 2020/4/1

N2 - The adsorption and diffusion of species in activated carbons is fundamental to many processes in catalysis and energy storage. Nuclear magnetic resonance (NMR) gives an insight into the molecular-level mechanisms of these phenomena thanks to the unique magnetic shielding properties of the porous carbon structure, which allows adsorbed (in-pore) species to be distinguished from those in the bulk (ex-pore). In this work we investigate exchange dynamics between expore and in-pore solvent species in microporous carbons using a combination of one-dimensional (1D) and two-dimensional (2D) NMR experiments. We systematically compare the effects of four variables: particle size, porosity, solvent polarity and solvent viscosity to build up a picture of how these factors influence the exchange kinetics. We show that exchange rates are greater in smaller and more highly activated carbon particles, which is expected due to the shorter in-pore-ex-pore path length and faster diffusion in large pores. Our results also show that in-pore-ex-pore exchange of apolar solvents is slower than water, suggesting that the hydrophobic chemistry of the carbon surface plays a role in the diffusion kinetics, and that increased viscosity also reduces the exchange kinetics. Our results also suggest the importance of other parameters, such as molecular diameter and solvent packing in micropores.

AB - The adsorption and diffusion of species in activated carbons is fundamental to many processes in catalysis and energy storage. Nuclear magnetic resonance (NMR) gives an insight into the molecular-level mechanisms of these phenomena thanks to the unique magnetic shielding properties of the porous carbon structure, which allows adsorbed (in-pore) species to be distinguished from those in the bulk (ex-pore). In this work we investigate exchange dynamics between expore and in-pore solvent species in microporous carbons using a combination of one-dimensional (1D) and two-dimensional (2D) NMR experiments. We systematically compare the effects of four variables: particle size, porosity, solvent polarity and solvent viscosity to build up a picture of how these factors influence the exchange kinetics. We show that exchange rates are greater in smaller and more highly activated carbon particles, which is expected due to the shorter in-pore-ex-pore path length and faster diffusion in large pores. Our results also show that in-pore-ex-pore exchange of apolar solvents is slower than water, suggesting that the hydrophobic chemistry of the carbon surface plays a role in the diffusion kinetics, and that increased viscosity also reduces the exchange kinetics. Our results also suggest the importance of other parameters, such as molecular diameter and solvent packing in micropores.

KW - Diffusion

KW - Energy storage

KW - Kinetics

KW - Magnetic shielding

KW - Magnetism

KW - Microporosity

KW - Nuclear magnetic resonance spectroscopy

KW - Particle size

KW - Porous materials

KW - Solvents

KW - Viscosity

KW - Activated carbon particles

KW - Diffusion kinetics

KW - Micro-porous carbons

KW - Molecular diameter

KW - Molecular level mechanisms

KW - Nuclear magnetic resonance(NMR)

KW - Shielding properties

KW - Two Dimensional (2 D)

KW - Nuclear magnetic resonance

U2 - 10.1595/205651320X15747624015789

DO - 10.1595/205651320X15747624015789

M3 - Journal article

VL - 64

SP - 152

EP - 164

JO - Johnson Matthey Technology Review

JF - Johnson Matthey Technology Review

IS - 2

ER -