Design and development of Non-Noble metal catalysts in N-ethylcarbazole hydrogenation for Liquid Organic Hydrogen Carrier (LOHC) systems

Abstract

Hydrogen can play a significant role as a clean energy vector due to its abundance, high gravimetric energy density, and environmental friendliness. However, efficient hydrogen storage remains a bottleneck that poses economic and technological challenges to the current system. Liquid organic hydrogen storage (LOHSs) have drawn intensive attention for their excellent compatibility with the existing fuel infrastructure. Among the Liquid Organic Hydrogen Carrier's (LOHC) under development, N-ethylcarbazole (NEC) is an attractive option mainly due to its ability to undergo hydrogenation cycle below 473K. Recent research have investigated noble metal catalysts for hydrogenation of N-ethylcarbazole and very limited efforts have been conducted for noble metal free catalysts. It is noted that comprehensive analysis involving both catalyst and carrier (N-ethylcarbazole) change during hydrogenation process is lacking in literature hindering the development of this technology. In our first work, we have comprehensively investigated noble-metal-free catalyst, Cu-doped NiO as well as the carrier (N-ethylcarbazole) during the hydrogenation process. Insights into the role of Cu doping during hydrogenation of N-ethylcarbazole was obtained through in-depth experimental and theoretical analysis of carrier and catalyst. Our findings indicate that optimal Cu doping enhances the hydrogenation performance of NiO, with a hydrogen storage capacity of up to 5.43 wt% in the first cycle and achieving hydrogenation efficiency greater than 90% during three cycles. Our study explored the structural formation of 12H-NEC diastereomers using advanced spectral techniques, revealing the complex nature of the hydrogenation process. The structural evolution as well as recyclability of Cu4-NiO catalyst during the hydrogenation process was investigated and it was found that reducing environment of hydrogen converted the Cu4-NiO catalyst into an alloy during the first cycle. Our results revealed that the catalysts acts as a reservoir during the hydrogenation process confirmed by the growth of nickel hydroxide during the hydrogenation process. In the following work, Cu-doped NiFe2O4 catalyst for NEC hydrogenation and the effect of Cu-doping on the catalytic performance was investigated. Our results show that optimum doping improved the hydrogenation performance of NiFe2O4, and 4 mol% Cu doped catalyst showed the best performance among all the doped catalysts with hydrogen storage capacity of 5.41 wt% and 93.4% hydrogenation efficiency. Enhanced surface charge density was identified as key contributor to the catalytic performance. The catalyst was stable in reducing hydrogen environment confirmed through comprehensive analytical techniques. To understand the structure of the fully hydrogenated 12H-NEC, investigation of the hydrogenated product was carried out using Nuclear Magnetic Resonance (NMR, including Heteronuclear Single Quantum Coherence (HSQC) and Heteronuclear Multiple Bond Correlation (HMBC)). Also, Diffusion Resolved Encoded Acquisition Method for Transverse Relaxation and Mixing Time Evaluation (DREAMTIME) Nuclear Magnetic Resonance (NMR) spectroscopy was employed for the first-time in the field of hydrogen storage to understand the composition of the hydrogenated product. Through our analysis, we confirmed that the structure obtained from 12H-NEC was (4aR, 4bS, 8aS, 9aS) -9-ethyl-4a, 4b, 8a, 9a-tetramethyldodecahydro-1H-carbazole and our analysis paves way for understanding the relationship between catalyst design and hydrogenation process. To the best of my knowledge, this is among the first PhD thesis in Australia dedicated to the investigation of catalysts for hydrogenation of liquid organic hydrogen carriers (N-ethylcarbazole). Our work makes a pioneering contribution to the national research landscape in the field of hydrogen storage specifically in relation to liquid organic hydrogen carriers.