Exploration of potassium X-ray absorption spectroscopy in Prussian blue and catalytic materials

The alkali metal potassium is often utilised as a promoter in heterogeneous catalysis and more recently as a charge carrier in sustainable battery materials, as an alternative to relatively scarce and highly in demand Li⁺. As many green energy solutions are often intermittent new sustainable storage systems are needed, such as storage in chemical bonds, ie synthetic fuels, or in large grid scale batteries. In the former, K⁺ is an essential promotor in catalysts for ammonia synthesis, CO₂ reduction and Fischer-Tropsch chemistry. In the latter, K⁺ ion is considered as one of the alternatives to Li⁺. In both scenarios the importance of potassium in these essential green energy storage solutions is evident, however, a fundamental understanding of the speciation, structure and evolution of potassium within these processes is less well-known. One possible technique for characterisation of potassium is through XAS.

Herein potassium K-edge XAFS will be employed to investigate the nature of potassium within several different potassium compounds and molecules, in an operando electrochemical cell of Prussian blue materials, and a potassium that is used as a promoter in Fischer-Tropsch synthesis. Furthermore, potassium K-edge XANES simulations are employed to assess the performance of different simulation methods and to gain a greater understanding of the transitions that occur in a Potassium K-edge spectrum.

Chapter 3 investigates potassium within two different Prussian blue materials in operando conditions, analysing the charge carrier as the battery is cycled. Potassium was shown to be the species that balances the structural charge during the redox reactions in the battery contrary to theories where the charge is balanced by protons (H₃O⁺). A high crystalline PB sample was found to be reproducible during continuous cycles, whereas a low crystalline PB sample showed high degradation and became electrochemically inactive as the material became stuck in the PB phase, due to the formation of Fe-O species ferrihydrite.

Chapter 4 systematically investigated the potassium K-edge XANES of different potassium compounds experimentally and computationally using three different methods, multiple scattering theory (MST), Finite difference Method (FDM), and Density Functional theory (DFT). Potassium K-edge XANES simulations were performed successfully and then utilised to understand the transitions in the excitation of the 1s electron in potassium. The DFT calculated projected DOS showed that transitions in potassium are a result of 1s to 4p excitation. Transitions to peaks before the edge are attributed to p orbital overlap between potassium and its nearby neighbouring atoms. In PB, it was found that the K⁺ resides within the cavities of the structure but located away from the centre of the primitive cubic cavities (8c site) and towards the corners of the cavities (32f’ position), due to structural defects or interstitial water. The ‘pre-edge’ in the XANES is attributed to overlap of potassium p orbitals with iron, and the shoulder on the white line, is influenced by the CN ligand anti bonding π* orbital and this overlap is promoted by the inclusion of a core-hole.

Chapter 5 demonstrates the ability to investigate and validate new synthesised perovskite materials containing substituted potassium within structure, used as supports for Fe Fischer-Tropsch synthesis. The materials were investigated by potassium K-edge XAS. It was successfully proven that potassium sits in the A site of a perovskite material through a combination of potassium K-edge fingerprint XANES analysis, EXAFS fitting to perovskite scattering paths and visual confirmation K-A site and K-B site of a perovskite scattering paths through use of wavelet transformations.