Metal oxide based catalysts for photo/electrochemical ammonia synthesis
Creators: name: Saji, Sandra
Publisher: name: The Australian National University
Year: 2023

description: Ammonia is undoubtedly a chemical of paramount importance in the agriculture world as a fertilizer and in the chemical industry as a starting reagent for the production of other chemicals. It is also a reservoir of energy as a hydrogen carrier (17.6 weight % of H2). Owing to its large hydrogen capacity and high energy density, ammonia has therefore attracted great interest in the emerging carbon-free hydrogen economy. The Haber-Bosch process for the synthesis of ammonia represents a gigantic scientific breakthrough that has profoundly changed the whole world. However, the high temperature and pressure conditions, large energy and hydrogen consumption, and substantial energy-related CO2 emissions have also brought growing challenges. Aiming at replacing the traditional Haber-Bosch process, it is of great significance that a green and sustainable process can be developed for ammonia synthesis through nitrogen fixation. Considering the high bond dissociation enthalpy of 941 kJ/ mole for the N-N triple bond, sustainable external energy sources are being intensively pursued. This includes light, electromotive force, solar thermal energy, or a combination of light and electromotive force, which have been respectively utilized as the driving force in photocatalysis, electro-catalysis, photo-thermo-catalysis and photoelectrocatalysis for nitrogen fixation, which are promising ways to mitigate the energy crisis and green house effects. Another attractive ammonia synthesis method is the conversion of nitrate to ammonia which occurs under milder operating conditions. Compared with the synthesis of ammonia from nitrogen, the NO3- to NH3 conversion process is thermodynamically more favorable since N=O has a lower dissociation energy (204 kJ/mole) than the N-N triple bond (941 kJ/mole). Moreover, NO3- has nearly 40,000 fold higher solubility in water than nitrogen. Chapter 1 encompasses a brief review on why ammonia is a chemical of paramount importance. It also puts forward the significance of catalytic reduction of nitrogen/nitrate to ammonia, the mechanism of the reduction reaction, the methods to accurately estimate ammonia etc. Chapter 2 is a study on an economical, reliable, safe and quantitative protocol developed for isotopic labelling using liquid chromatography-mass spectroscopy in dinitrogen fixation. A circumstantial insight into various parameters, the experimental setup and instrument details guided to the reproducibility of experiments. Chapter 3 portrays a low cost, scalable manufacturing of gas-phase photoreactors for nitrogen reduction, based on molybdenum oxy-sulfides. They showed excellent yield and efficiency, along with remarkable performance. Chapter 4 focusses on a 2D assembly of cobalt oxide composites on reduced graphene oxide; the photocatalyst showed excellent efficiency, with good stability. The room temperature processes offer a facile way towards upscaling and hold great promise for zero emission nitrogen fixation. Chapter 5 depicts a study on 2D mixed-phase iron oxide as an effective catalyst for Electrochemical nitrate reduction, with high ammonia yield and efficiency. Chapter 6 includes the summary of the thesis, future recommendations and perspectives towards photo/electro catalytic ammonia synthesis.