Porous materials such as zeolites, activated carbons and metal-organic frameworks (MOFs) have been shown to be potentially useful in a wide variety of different applications, including gas separation, energy storage and catalysis. In these applications, pores of different sizes and geometries play a crucial role in the capture and diffusion of guest molecules. Researcher Huan Doan (who is completing his PhD in the group of Dr Valeska Ting at the University of Bristol) is currently interested in exploring the possibility of enhancing the mass transfer and the gas storage capacity of existing nanoporous materials, creating materials for improved catalytic performance and selective gas storage.
Previous work in the Ting group has used neutron scattering to show how the shape and size of these pores can greatly influence the interaction of gases with the porous material host. Thanks to the combination of neutron spectroscopy and experimental gas sorption techniques, Ting and co-workers were able to study the behaviour of gases in porous materials with a diversity of pore characteristics. Neutron spectroscopy proved especially useful for the investigation of porous hydrogen storage materials, where information on the behaviour of light gases such as hydrogen within the pores of a solid material can be very challenging to extract using other techniques.
The group initially took inelastic neutron scattering (INS) measurements using TOSCA at the ISIS Neutron and Muon Source. These experiments resulted in surprising spectroscopic evidence for the hydrogen adsorbed into the pores of a carbon nanomaterial at 77 K having characteristics typically associated with solid hydrogen1. This confirmed the key role of optimal pore dimensions in compressing and densifying the adsorbed hydrogen.
They did further investigations of hydrogen confined in different nanoporous carbons with identical pore size, but with varying pore geometries (slit-shaped, cylindrical and disordered pores), using INS with in-situ hydrogen dosing on TOSCA. These experiments, to study the effects of pore geometry on the confined H2 at pressures up to 10 MPa, demonstrated that the geometry of the pores may also have an effect on how densely gases can be packed into a pore.
Such fundamental investigations aim to improve the understanding of how the nanoporous structure of different storage materials can influence gas storage capacities, leading to more accurate evaluation methods for current porous materials as well as potentially informing the design of new hydrogen storage materials.
Inspired by these findings, Huan Doan, a postgraduate researcher from the University of Bristol, has been investigating MOF materials containing pore dimensions varying from micropores (less than 2 nm in diameter) to macropores (more than 50 nm) and containing different pore geometries (non-geometrical or hexagonal) obtained from novel synthetic methods.2,3
Image: HKUST-1 MOF with different pore dimensions and geometries obtained through use of supercritical CO2 (scCO2) and defect formation
Improvements in catalytic activity in both liquid phase and gas phase reactions have been attributed to the enhanced molecular diffusion in the larger pores of the MOFs. The on-going INS measurements complemented by other characterisation techniques such as XRD, SEM and gas sorption will produce a complete picture of the material, and will be used to evaluate the results from the catalytic tests. Huan Doan will present this work at the European Conference on Neutron Scattering (ECNS2019) in St Petersburg thanks to a student travel grant from the ISIS Neutron and Muon Source.
Dr Ting, who led the research group working on these experiments says, “Our research goal is in developing functional nanoporous materials to address challenges in sustainable technologies. Neutron spectroscopy has allowed us to obtain information on the molecules contained within these tiny pores- information we cannot get from other techniques. This has allowed us to better understand how the structures of our materials govern their interactions with gases. I am very happy that Huan will have a chance to present his research into these porous systems at the European Conference on Neutron Scattering".