Computational study of perovskite redox materials for solar thermal ammonia synthesis

Abstract

Ammonia is one of the most important precursors used in chemical engineering to manufacture products that we use every day. However, to help tackle the deepening global energy crisis, it is important to find new, alternative methods for ammonia synthesis, particularly those that utilise renewable energies, to replace the century-old, energy-intensive Haber–Bosch ammonia synthesis process. Among the new approaches that have been studied, solar thermal ammonia synthesis (STAS), which uses solar energy, water, and redox materials to produce ammonia in a chemical-looping process, has attracted the attention of researchers as an economical and environmentally friendly process for the production of ammonia. However, finding the right redox materials that can efficiently catalyse the STAS reactions through trial-and-error experiments are extremely difficult and time-consuming. Recently, with the help of Density Functional Theory-based calculations and high-performance computers, high-throughput computational screening of promising redox materials for STAS has provided the community with an important tool to expedite the development of STAS technology. According to the literature, previous research has focused on binary redox materials – yet these compounds only occupy 10 % of the larger material space. As such, more complex materials are worthy of further study. In this thesis, we theoretically analysed the thermodynamics of perovskite oxide/oxynitride pairs and their potential as active materials for STAS. First, we investigated the metal-alloyed perovskite redox materials for STAS. It is believed that metal intermixing alters the bonding states of the thermodynamically stable perovskite oxides. This can potentially activate a highly stable material for use as a thermal catalyst. Combining random structural sampling, high-throughput DFT calculations, and a machine-learning Gibbs free energy descriptor, we found that, compared to the pristine perovskite SrTiO3/YTiO3, A-site-alloyed perovskite Sr0.875Y0.125TiO3/Sr0.875Y0.125TiO2.875N0.125 is mostly activated via cooperative enhancement; i.e., its limiting reaction energy is more negative than that of the pristine perovskite redox pairs. This insight may aid the future design of redox materials via compositional engineering. Consequently, we found that perovskite oxynitrides are compounds that possess intriguing properties that are yet to be investigated, and we further proceeded to assess the thermodynamics of pristine perovskite oxide/oxynitride. The results show that lattice strains affect the formation enthalpies of perovskite oxides/oxynitrides. Complete relaxation, which has a greater influence on lattice strains, will make some of the perovskite oxynitrides reach the true local minimum. The formation enthalpies of some perovskite oxynitrides, mostly structures that contain transition metals as the A-site cation, will change accordingly. The electronic interaction between the nitrogen dopant and the B-site cation is one of the most important factors affecting the difference in the formation energies between the perovskite oxynitrides and their oxide counterparts. This work establishes a thermodynamic database of 430 perovskite oxide/oxynitride pairs, which paves the way for further research on their potential as redox pairs for STAS. In the final part of this study, we calculated the Gibbs formation energies of these candidates at different reaction temperatures. This enabled us to perform further reaction thermodynamic analysis. We found that BeTiO3/BeTiO2.875N0.125 is the most promising redox pair for use in STAS. In addition, using this set of cubic perovskites, we benchmarked the accuracies of the Gibbs free energies calculated using the machine-learning descriptor against those obtained from quasiharmonic phonon calculations. Although many perovskite oxides are known to exhibit strong vibrational anharmonicity, our benchmark shows that, within the temperature range for thermal catalytic reactions, the free energies calculated using these two approaches agree well with each other. The results from this research suggest that STAS using suitable redox materials has potential as a process for small-scale ammonia production. As such, the identification of promising active materials for STAS will be the focus of future research. The methodology proposed in this work can also be used in the design of novel functional materials for energy storage and solid-state solar thermal chemical processes.