Investigations of catalyst and energy storage materials using 57Fe Mössbauer spectroscopy

To improve the understanding of how functional materials operate is key to developing the next generation of materials in their respective fields. In complex, mixed-phase systems this can often be problematic due to the presence and interference from non-active phases such as support systems or matrices that complicate the data from spectroscopic techniques. The use of techniques that can observe single elements can be a powerful method by which to observe the phase of interest. Mössbauer spectroscopy is a powerful, isotope specific spectroscopic technique that uses gamma rays to probe the hyperfine structure of the nucleus. In this work 57Fe Mössbauer spectroscopy was employed in a collaborative effort with industrial scientists from Johnson Matthey to help develop shared understanding of a selection of industrially important materials. Lanthanum ferrites are used as heterogeneous three-way catalyst materials for petrol car emission control; iron carbides play important roles in Fischer-Tropsch synthesis; iron molybdates are catalysts for the oxidation of methanol to formaldehyde; and lithium iron phosphate is an energy storage material. These were all chosen to be a part of the studies contained in this thesis. The motivations behind the catalytic studies were of a similar theme – improve the understanding of how these materials operate, how they age, and how changes to the synthesis process and resulting material properties can influence the performance of the materials. The materials chosen, the methods of their synthesis and the differences between them, with performance data for their respective application, chosen for their industrial relevance, were related to their hyperfine structure through 57Fe Mössbauer spectroscopy. Debye temperatures of iron carbides and rare-earth orthoferrite perovskites were approximated through variable temperature Mössbauer spectroscopic studies and other techniques. The studies of lithium iron phosphate, initially of powdered samples, some of which were extracted from electrodes, culminated in in-operando Mössbauer measurements of full cells at the ESRF synchrotron facility. Commercial materials were studied along with the effect of fabricating electrodes, and the charge state of electrodes, yielding useful information on their hyperfine structure. The studies herein, driven by industrial questions, have shown the strengths of Mössbauer spectroscopy for these various research fields.