Why ammonia production is important - Ammonia feeds 50% of the world’s population
In recent years, a new wave of ammonia-manufacturing technology development has been rising in developed countries, including China, the United States, and Australia. Ammonia is utilized to manufacture diverse industrial substances. In particular, it is most widely used as the nitrogen source in fertilizers essential for food production.
Since the Haber-Bosch process of synthesizing ammonia at high temperature and pressure was developed in 1908, food production has explosively increased. As statistics report that ammonia produced by the Haber-Bosch process feeds 50% of the world’s population, ammonia has become an indispensable compound at present. For this reason, the Haber-Bosch process has been recognized as an important ammonia synthesis method, and the Nobel Prize in Chemistry was awarded to three relevant researchers: Fritz Haber in 1918, Carl Bosch in 1931, and Gerhard Ertl in 2007.
Nobel Laureates in Chemistry for the synthesis of ammonia: Fritz Haber, Carl Bosch, and Gerhard Ertl (from left to right), www.nobelprize.com.
Ammonia, the most practical hydrogen storage material to accelerate the spread of fuel cell vehicles
As fuel cell vehicles have been recently attracting more attention, methods for producing and transporting hydrogen, a raw material for fuel cells, are also emerging as a crucial issue. Hydrogen is difficult to transport as it is a gas at room temperature, thus occupying a substantial volume, and yields explosive mixtures. In addition, the costs of liquefying hydrogen and transporting liquid hydrogen are substantially high.
To resolve those problems, studies have been carried out to convert hydrogen into other chemicals for transportation, which is then reconverted into hydrogen in hydrogen plants. Above all, ammonia is considered to have the highest potential as a source in terms of hydrogen conversion efficiency and cost, so developed countries are speeding up their research on the compound.
Importance of the electrochemical nitrogen reduction reaction - Eco-friendly ammonia production technology NRR
The Haber-Bosch process, which is used to produce ammonia, is currently one of the largest greenhouse gas emitters, accounting for 1% of global CO2 emissions. This is because ammonia is produced from chemical reactions between nitrogen and hydrogen at high temperature and high pressure. Another critical issue is that the energy consumed in this process amounts to 1~2% of global energy consumption. To cope with this, research has been actively conducted in recent years to produce ammonia in an eco-friendly manner through the electrochemical nitrogen reduction reaction (NRR) at room temperature and atmospheric pressure.
A schematic diagram of eco-friendly hydrogen production for a fuel cell using ammonia. PNAS, 116(8), 2794–2795, (2019)
Challenges of the electrochemical NRR to be resolved
To economically produce ammonia from the NRR, the ammonia production rate, which is the amount of produced ammonia per unit electrode area, needs to reach the level of 0.36 mmol cm-2 h-1. In addition, the Faraday efficiency (F.E.), an index calculated by comparing the measured ammonia production against the theoretical ammonia production that can be inferred from the supplied electrical energy, should also reach at least 50%. In other words, ammonia must be produced more than the amount of catalysts used (activity), and a large amount of ammonia must be produced compared to the competing reactions (selectivity).
However, as of 2020, even NRR catalysts with the best catalytic performance published in the academic community show the ammonia activity not exceeding 1.2 x 10-5 mmol cm-2 h-1. The F.E. also stands at 10~30%. Therefore, there is still a long way to go for commercialization. Some studies have reported a higher F.E. of 60~70%, but in such cases, the activity is mostly lower than 5.0 x 10-6 mmol cm-2 h-1.
The NRR utilizes an electrolytic cell to reduce nitrogen in a liquid phase through an electrode that contains catalysts for nitrogen reduction reaction on the cathode. The reason for the low ammonia production activity is that nitrogen gas has low reactivity and low solubility in an aqueous solution, so it is difficult to facilitate the NRR. In addition, the reason for the low F.E. is that the competing hydrogen evolution reaction (HER) takes place instead of the NRR that requires six electrons. In this case, the HER, which only requires two electrons, is relatively much easier.
To address these challenges, research is being conducted to develop various catalysts for nitrogen reduction along with electrolytic cell systems (using ionic liquids instead of water as a solvent).
Methods for accurately measuring ammonia production
Because of the very low activity of the NRR, it is difficult to additionally detect the ammonia production and secure reliable results. As the amount of ammonia produced by the actual NRR catalysts is very small, even a trace of impurities (some studies have shown that the results can be changed even with the amount of ammonia contained in exhaled human breath) can significantly change the values of the NRR activity. Therefore, a number of verification procedures must be carried out to report the NRR activity values.
The simplest method of measuring ammonia production is an indicator method. This method is an analytic technique for measuring the ammonia concentration through a series of procedures: adding an indicator to the NRR-generated solution, measuring the absorption spectrum through UV spectrometry to determine the exact color of the indicator, and using the Beer–Lambert law.
The indicators used for the measurement include Nessler’s reagent, indophenol, enzymatic assay kits, etc. Although the indicator method is the most basic and relatively easy-to-use measurement method, the reliability of the ammonia production measured by the method alone cannot be ensured. Therefore, it is necessary to confirm whether the ammonia production is consistent with the amount from other different measurement methods.
One of those methods involves the 1H nuclear magnetic resonance (NMR) method in conjunction with 15N isotope-labeled NRR experiments. In general, nitrogen exists in the form of 14N in nature, but 15N labeled nitrogen gas (15N2) is used as a reactant instead of the general N2 gas during the NRR experiment to determine whether the resultant ammonia contains the 15N isotope. This method is intended to identify whether the measured ammonia has resulted from impurities. While 1H coupled with 14N shows a doublet peak, 1H coupled with 15N shows a triplet peak. The areas where both peaks appear are different, so they are easily identifiable.
In addition, the integration of the 15N peak shown in the NMR spectrum can be used to quantitatively determine the ammonia production through calibration using standard solutions with known ammonia concentrations. The reliability of the determined ammonia production, compared with the amount obtained through the indicator method, can be increased. Considering the high cost of 15N2, it is essential to develop experimental environments that can recycle the used 15N2.
Ion chromatography is also widely used to measure ammonia production. Besides, various in situ analytical techniques (methods for measuring the specific properties of matter during the experiment) for observing how the NRR actually occurs in atomic units on the electrode surface are also utilized in conjunction with computer-aided simulation techniques using density functional theory (DFT).
(a)An illustration of the simultaneous detection of 14NH4+ and 15NH4+ in the 1H NMR (b) Testing equipment of the NRR cycle. Nature, 570, 504–508 (2019), https://doi.org/10.1038/s41586-019-1260-x