【Haichuan Frontier Technology Sharing】Wang Shuangyin’s research team at Hunan University develops a new method for producing urea under normal temperature and pressure conditions
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The use of nitrogen fertilizers has enabled the feeding of 27% of the world’s population, with urea, as a widely used nitrogen fertilizer, playing an important role in human life. However, traditional urea synthesis methods typically require two reaction steps: ① N2 + H2 → NH3 and ② NH3 + CO2 → CO(NH2)2. These two steps demand high temperatures (150 °C–200 °C) and high pressures (15 Mpa–25 Mpa), consume a large amount of energy, and require complex equipment as well as multiple cycles to improve conversion efficiency. Faced with the increasingly severe energy problems, developing green and efficient synthetic methods meets the requirements of the new concepts for societal development. Electrocatalytic nitrogen fixation offers advantages such as mild reaction conditions, the use of clean energy, and the direct utilization of protons in water. However, in practice, separating NH3 from the aqueous electrolyte and further purifying it to obtain high-purity gaseous NH3 makes subsequent urea synthesis complex and impractical. So, is it possible to synthesize urea using simple and energy-efficient methods? To address the aforementioned challenges, the research team led by Wang Shuangyin from Hunan University, in collaboration with other teams, has for the first time developed a new method for producing urea under normal temperature and pressure conditions. By loading palladium-copper alloy nanoparticles on the surface of titanium dioxide nanosheets, researchers utilized electrocatalytic reactions to couple nitrogen and carbon dioxide in water to form urea. Isotopically labeled synchrotron radiation-FTIR spectroscopy and theoretical calculations confirmed that this coupling reaction occurs through the spontaneous formation of C-N bonds by adsorbed *N=N* molecules and carbon dioxide reduction products (CO), and it holds promise for further application in the urea synthesis process. This achievement was published in the journal Nature Chemistry under the title “Coupling N2 and CO2 in H2O to synthesize urea under ambient conditions”.
Preparation and characterization of Pd1Cu1/TiO2-400 electrocatalyst: The researchers modified PdCu alloy nanoparticles on the surface of oxygen-vacancy-rich titanium dioxide nanosheets using a co-reduction method with metal precursors. Their structure was confirmed by TEM and XPS, and the optimal material composition (titanium dioxide treated at 400 °C and Pd1Cu1 alloy) was identified; under these conditions, the nanosystem exhibited the highest urea formation rate.
Electrocatalytic performance of Pd1Cu1/TiO2-400 in a flow cell: Experimental results show that the introduction of oxygen vacancies promotes the activation of nitrogen and carbon dioxide by the nanomaterial. Under the condition that the onset voltage for the hydrogen evolution reaction was -0.4 V, a relatively satisfactory urea yield of 3.36 mmol g–1 h–1 was achieved, with a corresponding Faradaic efficiency of 8.92%.
Adsorption of gas molecules by catalytic materials: The chemical adsorption of nitrogen is the first step in the electrocatalytic production of urea. Through temperature-programmed desorption, researchers found that the nanocomposites exhibited enhanced capacity for adsorbing gas molecules after being modified with palladium-copper alloy particles (the peaks shifted to higher temperatures of 270 °C and 380 °C); they also showed an enhanced adsorption capacity for carbon dioxide molecules (with enhanced peaks appearing at 261 °C and 365 °C). More importantly, in the competitive adsorption reaction between carbon dioxide and nitrogen, the shift of the corresponding peaks is almost negligible; this healthy competition between the two leads to an efficient urea synthesis rate.
Investigation into the electrocatalytic mechanism of Pd1Cu1/TiO2-400: Researchers used advanced SR-FTIR technology to determine the intermediate products formed by Pd1Cu1/TiO2-400 during the electrocatalytic reduction of carbon dioxide and nitrogen to produce urea. At a voltage of -0.2 V, nitrogen begins to be activated to produce NH and NH2; when the voltage reaches -0.25 V, carbon dioxide starts to be activated to produce C=O and C-O. At a potential of -0.3 V, an infrared vibration peak for C-N appears, indicating that a coupling reaction has taken place to form urea.
Theoretical calculations for Pd1Cu1/TiO2-400 electrocatalysis: Results based on density functional theory (DFT) show that the presence of *N2 facilitates the reduction of CO2; the resulting CO then reacts with *N2 to form urea with high activity and selectivity. The activation of nitrogen molecules to produce the intermediate *NCON* is thermodynamically and kinetically feasible, and it plays an important role in the formation of urea.
Theoretical calculations for Pd1Cu1/TiO2-400 electrocatalysis: Results based on density functional theory (DFT) show that the presence of *N2 facilitates the reduction of CO2; the resulting CO then reacts with *N2 to form urea with high activity and selectivity. The activation of nitrogen molecules to produce the intermediate *NCON* is thermodynamically and kinetically feasible, and it plays an important role in the formation of urea.
【Summary】 In summary, titanium dioxide nanosheets modified with palladium-copper alloy nanoparticles are used to catalytically couple nitrogen and carbon dioxide into urea under normal temperature and pressure conditions. This method overcomes a series of drawbacks associated with traditional synthesis processes, such as high temperature and pressure, low conversion efficiency, and high energy consumption. It provides new approaches for the fixation of inert gas molecules like nitrogen as well as for urea production processes, and holds promise for further application as a green synthesis method. Original link: https://www.nature.com/articles/s41557-020-0481-9