Photoelectrochemical Nitrate Reduction to Ammonia Using Metal Oxide Based Photosensitisers

Abstract

Ammonia is one of the most important chemicals synthesised presently. It supports the lives of billions around the world through its utilisation as a precursor for fertilisers, enhancing agricultural yield. It has also been proposed that ammonia could be employed as a carbon free hydrogen storage molecule due to its high hydrogen content (40% higher than methanol).Currently, ammonia primarily is synthesised through the energy-intensive Haber Bosch process. This entails combining dinitrogen and hydrogen under high temperatures (300-500 °C) and high pressures (150-200 atm). These strenuous conditions are due to the extreme difficulty in cleaving the dinitrogen triple bond. This process, in addition to using steam reforming to produce hydrogen, results in large carbon dioxide output.Therefore, it is necessary to devise alternative methods to synthesise green ammonia. Photoelectrochemical synthesis of ammonia is one such route that couples electrochemical and photochemical processes to reduce the overpotential needed. Additionally, from the economic and environmental perspective, nitrate reduction to produce ammonia is a better option. Nitrates are present in the environment naturally or contaminated from run off fertilisers. These present a threat that if consumed in large quantities can lead to cancers and if left to accumulate in waters, lead to environmental issues such as eutrophication.This work thus focuses on the synthesis of materials to reduce nitrates to ammonia photoelectrochemically. P-type semiconductors with a conduction band position that encompass the reduction potential of nitrate to ammonia are employed. Utilising different p-type metal oxide photosensitisers such as copper oxide, nickel oxide and cobalt oxide, the surfaces are modified with metal catalysts to improve the conversion efficiency of nitrate to ammonia. The concentration of nitrite (the first reductive product) and ammonia formed is evaluated using ion chromatography. Additionally, the concentration of hydrogen and nitrogen formed will be evaluated using gas chromatography. To further evaluate the efficacy of these materials, the faradaic efficiency and yield rates for nitrate reduction are evaluated. The quality of the materials chosen are be judged on their selectivity for adsorption and reduction, and their stability.