Date of Award
Doctor of Philosophy in Chemical Engineering - (Ph.D.)
Chemical and Materials Engineering
Ammonia (NH3) is one of the most important chemicals to the whole human society. The invention of the Haber-Bosch process enabled the industrial production of NH3. However, owing to the high capital costs of the centralized plant and the equipment and the negative environmental impact, it is no longer suitable for today's needs of human development. As a result, there is an urgent need to investigate sustainable approaches for ammonia production. Among those reported studies, nitrogen reduction (NRR) and nitrate reduction reaction (NO3RR) are considered applicable in the future. However, after decades of studying them for years, the catalysts are still suffering from limited activity.
The main objective of this study is to develop a series of promising transition metal based single atom catalysts for NRR and NO3RR. For the stabilization of metal atoms as the single atomic sites, porous boron nitride (BNx) is used as the substrate. Catalysts with tuned metal loading and substrate porosity are synthesized by controlling the content of precursors in the synthesis process. First, different metals are screened for the optimized NH3 productivity and obtain the highest NH3 productivity of 118.3µg /h/mgcat in NRR is obtained on the CuBNx catalysts. Then the CuBNx catalysts are studied in NO3RR to further improve the NH3 yield. The Lewis acidity and interaction between boron atoms and Cu atoms are the origin of the promising activity. Meanwhile, the catalysts are investigated using X-ray Absorption Spectroscopy (XAS), the XAS results confirm the formation of Cu single atomic sites by bonding with BNx. With XAS and H2H temperature programmed reduction, we prove the preference of higher oxidation states of Cu for NO3RR. Thus, O2 plasma is used to further improve the NH3 production rate by increasing the oxidation state of Cu atoms of the single atomic sites. The highest NH3 productivity in this work obtained is 4029 µg /h/mgcat which is currently the highest productivity ever reported. A systemic study with different plasma treatment further confirms the interaction between Cu, BN and O species plays a key role for NRR and NO3RR . However, a thorough understanding of the Cu-BNx-O system, and NRR and NO3RR reaction mechanisms are required for the future work in order to further improve NH3 productivity.
Huo, Siming, "Cu/BNx catalysts for electrocatalytic reduction of nitrogen and nitrate waste" (2023). Dissertations. 1663.