Bismuth oxybromide-based photocatalysts for photocatalytic nitrogen fixation to ammonia
Creators: name: Kheradmand, Amanj
Publisher: name: Macquarie University
Year: 2023
Subjects: classificationCode: 400402
subject: Chemical and thermal processes in energy and combustion

description: Ammonia (NH<sub>3</sub>) is vital for producing chemicals and fertilisers. The Haber Bosch process, which is responsible for 2 % of world energy usage and global annual CO<sub>2</sub> emissions, is an industrial process to synthesize ammonia. Thus, the need for low-cost, environmentally benign methods of artificial ammonia generation under room conditions is critical. One of the promising alternatives is photocatalytic nitrogen (N<sub>2</sub>) fixation. In this thesis, various nanomaterials were fabricated through a bismuth-rich, solid solution, bimetallic loading, and heterojunction for photocatalytic NH<sub>3</sub> synthesis under ambient conditions. Firstly, a range of bismuth oxybromide (BiOBr) with different ratios of Bi/Br were fabricated via a facile method. The photoactivity results show that the higher the Bi/Br ratio, the higher the photocatalytic activity. Thorough characterisation results reveals that the enhanced photocatalytic activity of Bi<sub>3</sub>O<sub>4</sub>Br may be credited to the higher light absorption capacity, more negative conduction band, effective charge carrier separation and migration, and longer charge carrier lifespan. More importantly, the oxygen vacancy of Bi<sub>3</sub>O<sub>4</sub>Br can enhance the cleavage of N<sub>2</sub> molecules as the rate-determining step of N<sub>2</sub> fixation. A novel bismuth-rich Bi<sub>3</sub>O<sub>4</sub>Br<sub>x</sub>I<sub>1-x</sub> solid solution containing oxygen vacancies (OVs) was fabricated via solvothermal reaction by altering the Br/I molar ratio which Bi<sub>3</sub>O<sub>4</sub>Br<sub>0.5</sub>I<sub>0.5</sub> shows the highest photocatalytic NH<sub>3</sub> generation. This improved photocatalytic activity contributes to the more effective separation of photogenerated electron-hole pairs and faster charge transfer. By changing x from 1 to 0.25 in Bi<sub>3</sub>O<sub>4</sub>Br<sub>x</sub>I<sub>1-x</sub> samples, the bandgap value and morphology of samples would be altered, narrowing the bandgap, and shifting the nanosheet structure to nanorods one. The spectroelectrochemical measurements reveal that Bi<sub>3</sub>O<sub>4</sub>Br<sub>0.5</sub>I<sub>0.5</sub> has the highest concentration of accessible states to the reactions and shifts the conduction band edge toward a lower potential. To further extend the photocatalyst activity of Bi<sub>3</sub>O<sub>4</sub>Br, Co and Ni nanoparticles were co-deposited on the Bi<sub>3</sub>O<sub>4</sub>Br surface for photocatalytic nitrogen reduction by a one-step hydrothermal technique. The bimetallic Co-Ni/Bi<sub>3</sub>O<sub>4</sub>Br photocatalyst presents a higher photocatalytic activity in comparison with the monometallic Ni/Bi<sub>3</sub>O<sub>4</sub>Br and Co/Bi<sub>3</sub>O<sub>4</sub>Br. This finding could be ascribed to the beneficial synergistic effect between the two metals leading to remarkable photocatalytic properties. Photoelectrochemical analysis indicates that Co-Ni/Bi<sub>3</sub>O<sub>4</sub>Br possesses the highest charge separation and inhibited charge recombination, leading to a high N<sub>2</sub> photofixation performance. At the last, LaFeO<sub>3</sub> and Bi<sub>3</sub>O<sub>4</sub>Br heterojunctions were synthesis for N<sub>2</sub> photofixation. The highest enhancement was observed for 10 wt% LaFeO<sub>3</sub>/Bi<sub>3</sub>O<sub>4</sub>Br, whose activity is 2.3 times higher than that of Bi<sub>3</sub>O<sub>4</sub>Br. This improvement would be credited to the synergistic effect between Bi<sub>3</sub>O<sub>4</sub>Br and LaFeO<sub>3</sub> in the photocatalysis composite, which resulted in advanced visible light harvesting, successful charge separation and the longer lifetime of excited electrons based on comprehensive characterisation results. Reactive species trapping experiments were proposed the Z-scheme mechanism over LaFeO<sub>3</sub>/Bi<sub>3</sub>O<sub>4</sub>Br. In summary, this thesis explores the synthesis of various nanomaterials for photocatalytic NH<sub>3</sub> synthesis under ambient conditions. Bismuth-rich solid solution, bimetallic loading, and heterojunction were used to fabricate Bi<sub>3</sub>O<sub>4</sub>Br, Bi<sub>3</sub>O<sub>4</sub>Br<sub>x</sub>I<sub>1-x</sub>, Co-Ni/Bi<sub>3</sub>O<sub>4</sub>Br, and LaFeO<sub>3</sub>/Bi<sub>3</sub>O<sub>4</sub>Br photocatalysts, which demonstrated enhanced photocatalytic activity due to effective charge carrier separation and migration, longer charge carrier lifespan, and advanced visible light harvesting.