NANOPOROUS MATERIALS FOR THE PHOTONIC FIXATION OF NITROGEN TO AMMONIA: A COMPREHENSIVE SIMULATION, EXPERIMENTATION, AND COMPARISON STUDY

Abstract

Ammonia is one of the most significant precursors to fertilizers, making it one of the most produced chemicals in the world. Nitrogen is naturally converted to ammonia under benign conditions catalyzed by the nitrogenase enzyme. The natural nitrogen fixation process motivated researchers to develop materials (photocatalysis) to produce ammonia under ambient conditions. To this day, catalysts suffer from low ammonia yield and an incomprehensive understanding of the mechanism. A technical-economic study of the newly developed process's performance target for the photofixation process was set to explore the feasibility of the solar-based nitrogen fixation. Considering five years of break-even point, the current ammonia yield must be increased to nine-folds of the current experimental laboratory conversion. The proposed process is not only compared to the Haber-Bosch process but also to other sustainable routes to minimize greenhouse gases emissions. Two novel processes are introduced and scrutinized from thermodynamics and environmental perspectives, which are (1) methane solar cracking integrated system and (2) ammonia production via coupling of thermochemical cycles. Furthermore, this research aims to develop a systematic and comprehensive approach to designing durable and efficient catalysts that can drive the photofixation of nitrogen. By utilizing Quantum mechanics, molecular simulation techniques, and structural and mechanical properties, metal-organic frameworks (MOFs) libraries are screened to identify a set of promising catalysts for the photofixation of nitrogen. Besides technical assessment, the methodology considers all sustainability aspects; energy, environmental impact, and logistics. As a result of a study on an experimental-based dataset, four MOFs, Fe2Cl2(BBTA), Fe2(mDOBDC), Zn2(mDOBDC), and Ni-BTP, have been selected based on their band edges, while only Fe2Cl2(BBTA) MOF exhibited a bandgap less than 3 eV. Moreover, a set of MOFs’ combinations are proposed for heterojunction application to enhance charge carriers’ separation. Intriguingly, the predictability of MOF’s bandgap and edges from was demonstrated MOF’s organic linker bandgap and metal node type and correlations were built for some MOF families. Environmental impact of producing a gram of Fe2Cl2(BBTA), Zn2(mDOBDC), Fe2(mDOBDC), and Ni-BTP results in 2.19, 1.49, 1.33, and 0.35 kg CO2 (eq.) of total global warming potential emissions. A selected number of MOFs are synthesized using the solvothermal method and characterized using various techniques. These MOFs were tested for activity towards nitrogen fixation under visible-light irradiation. Due to enhanced charge separation, MOF/semiconductor showed four times higher ammonia conversion than pristine MOF.

Date of Award	2022
Original language	American English
Awarding Institution	HBKU College of Science and Engineering