Prospective catalyst based on the exsolution from lanthanum aluminate perovskite : from chemistry to electrochemistry

Abstract

Using renewable energy will play an essential role in energy conversion and storage on demand of energy consumption and environmental protection. With the advantages of high energy density, huge reserves, and zero emissions, therefore, hydrogen can enrich renewables and diversify the future energy territory. In this context, this study aims to establish a strategy related to chemistry and electrochemistry topics within a hydrogen centred theme. Here, the chemistry topic of the C1 chemistry-based catalyses is steam reforming for producing hydrogen and CO oxidation for environmental protection. The electrochemistry topic corresponds to the energy conversion in SOFC. Processing in these technologies, using a decent energy material is a challenging and crucial part. Regarding the efforts toward the energy materials family, perovskites have attracted much attention over the past decades in diverse application fields.
This work selects and pursues one of the ABO₃ type perovskite oxide, the LaAlO₃- based perovskite. as it is an excellent substrate and possesses cheap, stable, and other promising properties. After selecting the material system, a strategy in this work will be employed, which is a recent advanced approach referring to redox exsolution altering the traditional supported metallic nanoparticles system, especially in terms of durability. Therefore, the study undertakes a "materials + process" research skeleton to seek manifold catalysis and electrochemistry application based on LaAlO₃- based perovskites' exsolution.

In terms of the “material” strategy, by utilising the techniques of XRD, SEM, TEM, EDX, EELS, XPS, TGA, electrical conductivity, dilatometry and B.E.T. adsorption analysis, this thesis explores a series of transition metal-doped (B-site, Fe, Co, Ni, Mn, Cr) and alkali/alkaline metals doped (A-site, Na, K, Ca, Sr) in LaAlO₃ perovskites systematically, centred on exsolution. As a result, fundamental properties of various inherent compositions are developed from different cations and various doping levels, which indicate that high symmetric and stable perovskite structures have been achieved in LaAlO₃ systems after substitution. The study considers several factors for evaluating the obtained perovskites crystal structure variation, including the dopants' size, defects, doping levels, and unit cell parameters. Afterwards, a high-temperature reduction of these materials was conducted. Three series exsolution systems were investigated on three LaAlO₃-based materials with different dopants on the B-site, Fe/Co, Ni, and Mn-based systems. Results suggest that the perovskites are decorated with metallic nanoparticles of diverse compositions, the nature of the various size and populations, and heterostructural interfaces. Furthermore, lower temperatures favour the growth of particles, while higher temperatures sustain the nucleation of particles. There are termination surfaces for determining exsolution. Strikingly, enhanced exsolution in dual doping systems (Ni- based and Mn-based) was observed and discussed. Some differences between these three series of materials in the exsolution study were discussed and analysed.

On the other hand, in terms of the “process” strategy, the catalytical topics regarding CO oxidation and steam reforming and the electrochemical device regarding SOFC were launched. The Fe/Co series exsolution system is set for the CO oxidation test, demonstrating that the Co-based exsolution materials are an excellent candidate. Moreover, the Ni/Ni-Co series exsolution system is designed for steam reforming (MSR and MoSR), suggesting the requirement of further modified studies owing to the not very good catalytic performance. Mn-based materials were used for electrochemistry studies. The electrical conductivity and compatibility were studied before assembling a SOFC single cell. The related Mn-based exsolution seems to affect the final output performance.

Throughout the entire work, exsolution takes the primary part. Many of the principles and topical chemistry, including crystal chemistry, defect chemistry and coordination chemistry, are discussed and exemplified in this study on lanthanum aluminate-based perovskite newly. Further controlling of the chemistry topic can work adequately for chemistry (catalysis) and electrochemistry application.

Date of Award	15 Jun 2022
Original language	English
Awarding Institution	University of St Andrews
Supervisor	John Thomas Sirr Irvine (Supervisor)