| Summary: | <p>Soft X-ray spectroscopies are powerful tools to study the chemical state of surfaces with nanometre-scale surface sensitivity, and thus are of significant interest in the field of heterogeneous catalysis. Understanding how catalysts operate under reaction conditions is vital to improve their performance in areas such as energy storage and conversion, but accessing both surface chemical information and realistic reaction conditions remains an outstanding challenge.</p>
<p>In this thesis, environmental cells are implemented to perform operando X-ray photoelectron (XPS) and absorption spectroscopy (XAS) over catalysts relevant for the conversion of CO2 to methanol; a promising route for sustainable chemical production.</p>
<p>Initially, an environmental cell sealed with a silicon nitride (SiNx) window (100 nm) was used to determine the role of CO over a Cu catalyst in the hydrogenation of CO2. High pressure XAS alongside complementary near ambient pressure (NAP)-XPS and mass spectrometry measurements determined that CO acts as an oxygen scavenger to recover active metallic sites on the Cu surface following oxidation, which is not achieved with H2 alone at these temperatures.</p>
<p>To extend the environmental cell approach to enable high pressure XPS, a method to transfer graphene from its growth substrate to a perforated SiNx membrane was developed. This method involved depositing a thin Au support onto graphene with an additional polymer frame atop. The Au layer protected the graphene from contamination and the polymer frame ensured the graphene did not wrinkle or tear substantially. The reliable transfer of graphene using this method enabled measurements of 1 bar of Ar within a NAP-XPS system, and the removal of Au with a liquid etchant in lab-based XPS.</p>
<p>Finally, using graphene suspended over perforated SiNx membranes, Cu/Zn nanoparticles were studied under similar conditions to those used to study pure Cu films. Both high pressure XPS and XAS (at 1 bar) were performed to determine the role of Zn in increasing catalytic activity. Complementary scanning transmission electron microscopy revealed how the distribution of chemical species in the catalyst changes with pre-treatment conditions. The order of dosing H2 and CO2 gas has a significant influence on whether Cu0/Zn0 alloy dominates or the nanoparticles exsolve into Cu0 and ZnOx; the addition of CO recovers some of the alloy component.</p>
<p>Overall, this thesis successfully demonstrates and applies new approaches to enable high pressure soft X-ray spectroscopy of industrially-relevant catalysts under more representative reaction conditions than previously possible. The developments in environmental design and manufacture of graphene windows allowed X-ray spectroscopy to be reliably performed at 1 bar within this thesis. Since the experiments shown in this thesis, X-ray spectroscopy is now being performed at 4 bar by the Weatherup research group using the same techniques, with ambition to further increase the pressures possible for X-ray spectroscopy.</p>
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