Combined Modulation Excitation, Neutron, and X-rays Methods to Understand Catalytic Systems

Abstract

The development and growth of heterogeneous catalysis are directly connected to the knowledge of the structure and associated changes that arise from reactions it has, under specific environmental conditions. In liquid phase catalysed reactions, which was the focus of this thesis, information associated with the reaction, e.g. the active site, is often difficult to obtain due to the solvent being present at higher quantities in comparison to the much smaller quantity of active species. Additionally, difficulties associated with the characterisation of such systems arise from the frequently short lifetime of active species, and the tendency of catalytic events to occur on the surface of the catalyst, with the bulk structure barely participating in any reactions. The purpose of this thesis was to conduct a research study, integrating modulation excitation (ME) approach with total neutron scattering (TNS) and X-ray absorption spectroscopy (XAS) techniques. The combination of periodic modulation excitation with phase-sensitive detection (PSD) analysis, and their integration within TNS and XAS, allowed us to probe surface structural changes. This approach demonstrated an enhanced signal-to-noise ratio of the experimental data and significantly improved the sensitivity of the respective instruments to weak component contributions. Periodic electrical potential switches were employed as external stimulations to perturb the investigated systems reversibly and measure the active species contributions. In contrast to XAS, where ME methodology has been extensively implemented to the study of gas-phase catalytic reactions and most recently to liquid-phase catalytic reactions; combined ME-TNS studies is a novel approach that was successfully developed and demonstrated for the first time in this thesis. Ultimately, the essential instrumentation and innovative analysis procedures to extract useful structural information from the newly acquired ME-TNS data are demonstrated in the results chapters of this thesis. Finally, the ME technique was implemented at the Energy Dispersive EXAFS (EDE) branch of the I20 X-ray absorption spectroscopy beamline at Diamond Light Source, while the NIMROD instrument at ISIS neutron and muon source was developed to enable it to obtain ME-neutron scattering data.

Date of Award	31 Dec 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Xiaolei Fan (Supervisor) & Chris Hardacre (Supervisor)