CHEMICAL SCIENCE

Research progress in nitrogen cycle electrochemistry driven by nano-heterogeneous catalysts


On May 30, 2022, Nano Research Energy, an energy journal hosted by Tsinghua Universityhttps://www.sciopen.com/journal/2790-8119Associate editor, Professor Sun Xuping of the University of Electronic Science and Technology of China published a title“Recent advances in nanostructured heterogeneous catalysts for N-cycle electrocatalysis”The latest roundup.

Nitrogen (N) is one of the most abundant elements on Earth and plays an irreplaceable role in life. It exists primarily as an inert molecular structure of a non-polar diazo (N2) gas (~78% of atmospheric volume) with a high bonding energy of 941 kJ·mol–1, which cannot be used directly by most flora and all living things. Interestingly and thankfully, N2 can be converted to reactive nitrogen by lightning or in a specific biological way. In nature, there are also inorganic nitrogen-containing compounds such as nitrate (NO3)), nitrite (NO2″),nitric oxide (NO), nitrogen dioxide (N2O), hydrazine (N2H4), ammonia (NH3), and most of them can be converted into each other. These mutual transformation processes (such as N2 fixation of NH3, NH3 nitrification to no2–/NO3–, NO3–denitrification back to N2, etc.) can form a biogeochemical nitrogen cycle (N-cycle) that is as important as the carbon cycle, which has sufficient research significance.

However, N-cycles mediated by a range of natural systems are heavily influenced by human activities, and the resulting imbalances in N-cycles are accompanied by serious environmental problems (N-cycle imbalances caused by human activities have serious negative impacts on terrestrial, marine and atmospheric ecosystems), which in turn threaten the survival of human beings. For example, NO and N2O are both atmospheric pollutants formed by the burning of fossil fuels in factories and vehicles, and the rapid emission and accumulation of NO has led to problems such as acid rain, ozone depletion and smog. In addition, the overuse of artificial nitrogen fertilizers leads to higher concentrations of active “fixed” nitrogen (such as NO3 – and NO2 – in groundwater), with significant side effects on terrestrial and aquatic ecosystems, and the excessive absorption of this nitrogen oxygen anion is harmful to both humans and animals. Therefore, no/N2O emission control and NO3–/NO2– groundwater remediation are important areas of nitrogen chemistry research.

Over the past few decades, human activities have had a serious impact on the circulation and balance of the nitrogen cycle. One of the greatest inventions of Fritz Haber and Carl Bosch in the 20th century (H-B process, N2+H2→ NH3) made it possible to industrially produce NH3. However, not only is the huge capital cost of concentrating plants and equipment, but also the negative impact of the H-B process on the environment makes it no longer suitable for the needs of human development today. As a result, we are faced with the great need to explore sustainable/distributed approaches to controlling the nitrogen cycle and achieving a circular nitrogen economy. GALOx nitrogen provides a sustainable way for the electrification industry not only to store intermittent electrical energy into useful chemicals such as NH3, but also to directly offset global CO2 emissions from traditional H-B processes. Unfortunately, N2 is one of the most thermodynamically stable species, making the conversion of N2 to NH3 a highly endothermic process. In fact, the electrochemical nitrogen reduction reaction (NRR) is so tricky that it even drives the development of nitrate reduction reactions (NO3RR) and nitric oxide reduction reactions (NORR) to produce NH3. The use of more reactive but harmful NO3–, NO3–, NO, etc. as precursors will not only help improve the conversion efficiency, but also be expected to alleviate the related environmental pollution problems. Further, as shown in Figure 1, the electrochemical conversion of inorganic nitrides also includes oxidation reactions with N2 (or NO3–) as the main product, such as ammonia oxidation reaction (AOR), hydrazine oxidation reaction (HzOR), nitrogen oxidation reaction (NOR) and the like.

The development of sustainable programmes to mitigate this anthropogenic imbalance is essential to address serious environmental problems. Flexible, sustainable and compatible artificial nitrogen cycle (N-cycle) electrocatalytic techniques are considered a viable option for shaping the future of the Earth’s nitrogen cycle. Electrocatalysis refers to the acceleration of the conversion process using electricity, electrolytes (reactions usually use water as a proton source), and catalysts that are efficient enough. On the surface of the catalyst, N2, gaseous nitrogen oxides, and nitrogen and oxygen anions can be used as nitrogen sources for the electrosynthesis of NH3. NH3 and N2H4 can be converted to N2 in fuel cells.

The latest review of nitrogen cycling by Professor Sun Xuping’s team highlights the important advances of multiphase nanocatalysts in various semi-reactions (e.g., NOR, AOR, HzOR, NOOR, NRR, NORR, NO3RR, NO2RR) and energy devices (e.g., metal-N2 batteries) (mainly in the past three years), including their preparation details and electrochemical properties. The effectiveness of circulating nitrogen species in closed-loop electrochemistry for the simultaneous generation of useful chemicals (e.g., NH3, NH2OH and N2H4) and the reduction of pollutants (e.g., NO3–, NO2−, NO) was highlighted. In addition, many examples are given to briefly illustrate the flexibility of catalytic system design, for example, modifying COFs on a typical carbon surface. More importantly, there are still problems and challenges in the development of nitrogen cycle electrocatalysts, and the review summarizes some possible future trends in the field, hoping that a recent overview of N-cycle electrocatalysis will inspire further research.

Figure 1: Typical electrochemical transformation of small molecules containing nitrogen. The left side of the figure includes common redox reactions and also shows some reactions coupled with CO2 reduction reactions (CO2RR) to produce higher value-added products such as urea. On the right are common inorganic nitrogen-containing substances and their oxidation states.

Figure 2: The main mechanisms and pathways of nitrogen cycle electrocatalysis.

Figure 3: Challenges and objectives of electrocatalysis of the nitrogen cycle. From left to right: (i) the key transformation steps in the nitrogen cycle; (ii)the current challenges and targets for the electrocatalytic conversion of nitrogen species; (iii) Higher objectives.

Figure 4: Some electrochemical devices, such as fuel cells, electrolyzers, and batteries, convert nitrogen-containing substances to form a cyclic triangle.

Related paper information:

Liang, J.; Liu, Q.; Alshehri, A. A.; Sun, X. P. Recent advances in nanostructured heterogeneous catalysts for N-cycle electrocatalysis. Nano Res. Energy 2022, DOI: 10.26599/NRE.2022.9120010. https://doi.org/10.26599/NRE.2022.9120010

As a sister journal of Nano Research, Nano Research Energy (ISSN: 2791-0091; e-ISSN: 2790-8119; Official website: https://www.sciopen.com/journal/2790-8119It was launched in March 2022 and is co-edited by Professor Qu Liangti of Tsinghua University and Professor Chunyi Zhi of the City University of Hong Kong. Nano Research Energy is an international multidisciplinary, all-English open access journal, focusing on the cutting-edge research and application of nanomaterials and nanoscience technology in new energy-related fields, benchmarking against the top international energy journals, and committed to publishing high-level original research and review papers. Before 2023No APC feesTeachers are welcome to submit articles. Please contact: NanoResearchEnergy@tup.tsinghua.edu.cn.

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