NMR studies of adsorption and diffusion in porous carbonaceous materials

Abstract

Porous carbonaceous materials have many important industrial applications including energy storage, water purification, and adsorption of volatile organic compounds. Most of their applications rely upon the adsorption of molecules or ions within the interior pore volume of the carbon particles.Understanding the behaviour and properties of adsorbate species on the molecular level is therefore key for optimising porous carbon materials, but this is very challenging owing to the complexity of the disordered carbon structure and the presence of multiple phases in the system. In recent years, NMR spectroscopy has emerged as one of the few experimental techniques that can resolve adsorbed species from those outside the pore network. Adsorbed, or "in-pore" species are shielded with respect to their free (or "ex-pore") counterparts. This shielding effect arises primarily due to ring currents in the carbon structure in the presence of a magnetic field, such that the observed chemical shift differences upon adsorption are independent of the observed nucleus to a first approximation.Theoretical modelling has played an important role in rationalising and explaining these experimental observations. Together, experiments and simulations have enabled a large amount of information to be gained on the adsorption and diffusion of adsorbed species, as well as on the structural and magnetic properties of the porous carbon adsorbent. Here, we review the methodological developments and applications of NMR spectroscopy and related modelling in this field, and provide perspectives on possible future applications and research directions.

Understanding the Chemical Shifts of Aqueous Electrolyte Species Adsorbed in Carbon Nanopores

“…Thanks to this feature, it was possible to demonstrate that, for organic electrolytes, the variations of the magnetic shieldings, as a consequence of the changes in electronic structure with charging (negatively or positively), have a predominant influence on the overall chemical shift compared to ion organization . This result is consistent with experiments conducted on a range of electrolytes showing similar variations of the chemical shift with applied potential. ,, …”

“…While the ring currents are predominant in determining the chemical shifts for a range of electrolytes, , other parameters can lead to significant modifications of the chemical shifts. NMR spectra are known to be influenced by the hydration number of the ions under consideration; the dehydration of ions upon entering the micropores can thus lead to large chemical shift changes. , Due to exchange between the environments inside and outside of the pores and motional averaging, the carbon particle size can also lead to large variations .…”

“…The local structure, and especially the size of the aromatic domains, was also shown to have a large effect, which could be used to get insights into the structures of a range of porous carbons. 27 While the ring currents are predominant in determining the chemical shifts for a range of electrolytes, 28,29 other parameters can lead to significant modifications of the chemical shifts. NMR spectra are known to be influenced by the hydration number of the ions under consideration; the dehydration of ions upon entering the micropores can thus lead to large chemical shift changes.…”

Mesoscopic simulations of the in situ NMR spectra of porous carbon based supercapacitors: Electronic structure and adsorbent reorganisation effects

“…This effect has actually been seen experimentally on a larger range of electrolytes than the one explored here. 44 According to the results obtained here, the coarse-grained or all-atom representation of the ions and molecules in the molecular simulations do not lead to significant changes. The ion adsorption can still have an effect on the actual chemical shift as can be seen in particular at 0 V.…”

Engineering metal–organic frameworks for adsorption-based gas separations: from process to atomic scale

“…Looking at sorption kinetics, unlike volumetric and gravimetric sorption instruments, there is no commercial unit that allows direct measurement of sorption kinetics. Instead, one has to rely on other tools to study macropore and/or micropore diffusion, such as zero length column (ZLC), 149 frequency response measurement (FRM), pulsefield gradient (PFG) NMR, 150 interference microscopy (IFM) and infra-red microscopy (IRM). 151 A recent review by Ruthven et al describes these various techniques and their application in analysing transport properties.…”