Ryder, Christopher P. C.
(2024)
High Pressure and Standard Photoelectron Spectroscopy of Materials for Hydrogen Storage and Production.
PhD thesis, University of Nottingham.
Abstract
It is widely agreed that hydrogen, a clean, renewable fuel which produces only water as a by-product when reacted with oxygen in air, can and must be used to replace fossil fuels for applications across all sectors if global targets for greenhouse gas emission reductions are to be met, and the harmful effects of the climate crisis are to be mitigated and reversed. However, many of the technologies associated with the lifecycle of truly green hydrogen are still in their infancy. Production still relies heavily on fossil fuels, and electrolysis alone cannot be expected to cover the entire capacity required for their replacement, while the storage of hydrogen commonly uses very high-pressure compression (up to 700 bar) with large associated costs and safety concerns. This thesis therefore looks to investigate materials which may provide alternative methods in each case, namely solar water splitting photocatalysed by titanium dioxide for production, and solid-state storage of hydrogen in intermetallic compounds.
These investigations are performed with core-level, X-ray-based spectroscopies, which can be used to effectively probe the surfaces of materials, providing detailed information about the constituent elements and the chemical state of the system. However, these analytical techniques generally require ultra-high vacuum conditions, while reactions of materials for solid-state hydrogen storage often occur at ambient pressures and above. A solution to overcome this so called “pressure gap” is therefore required for their effective analysis.
It is for this reason that, in this thesis, a novel differentially pumped multi-stage transfer device was designed to enable the rapid transfer of a sample from reaction to analysis conditions, such that the measured results of analysis techniques including XPS could more accurately represent the actual behaviour of a sample under those reaction conditions. A ‘proof of concept’ study was performed to display the unique capabilities of the system, in which argon ion sputtering was used to create defective states in the surface and sub-surface layers of a titanium dioxide crystal. The defective crystal was then moved to the reaction chamber of the device, where it was exposed to the atmosphere in an attempt to heal the defects. It could then be transferred back into position for XPS analysis within just 5 minutes for observation of the healed titanium states.
The surface of a high entropy alloy, (Ti0.65Zr0.35)1.05MnCr0.8Fe0.2, with potential for applications in hydrogen storage, has been investigated with X-ray photoelectron spectroscopy and Near-edge X-ray absorption Fine Structure spectroscopy before and after exposure to conditions associated with hydrogen activation. A surface oxide layer seen on the as-received alloy was reduced when heated to 650oC under vacuum for 30 minutes. After another sample of the alloy was heated to the same temperature in 1 bar of hydrogen, only chromium and iron appeared to reduce to metallic states, while manganese was seen on the surface in significant amounts in the form of Mn3O4 and MnO. It is suggested that this sacrificial oxidation of manganese may allow chromium and iron to remain in reduced metallic states to provide a pathway for the dissociation of the hydrogen molecules and diffusion of hydrogen atoms into the bulk for activation.
Finally, a combination of X-ray Photoelectron spectroscopy, Near-edge X-ray Absorption Fine Structure spectroscopy and Resonant Photoemission spectroscopy was used to investigate the interactions between titanium dioxide, in the form of nanoparticles, and a gold crystal surface. The two materials are commonly combined for use in solar water splitting for the production of green hydrogen, so an improved understanding of the interactions and charge transfer dynamics may aid in the further development of the technology. Through analysis of the measured spectra and the core-hole clock method, no evidence of charge transfer between the two materials was observed on the timescale of the core-hole lifetime. Argon ion sputtering of the deposited nanoparticles did however appear to show a reduction in their size, providing a potentially unexplored method of introducing quantum confinement effects into the semiconductor for possible improvements in efficiencies of titanium dioxide photocatalysed devices for solar water splitting.
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