China team reports ammonia synthesis through electroreduction of nitrogen on black phosphorus nanosheets
More than 100 years after the introduction of the Haber–Bosch process, scientists continue to search for alternative ammonia production routes that are less energy-demanding. A team from the South China University of Technology in Guangzhou has now discovered that black phosphorus is an excellent catalyst for the electroreduction of nitrogen to ammonia. The catalyst can achieve a high ammonia yield of 31.37 μg h-1 mg-1cat under ambient conditions.
According to their study published in the journal Angewandte Chemie, layered black phosphorus nanosheets are a highly selective and efficient catalyst in this process.
Ammonia (NH3) is a basic raw chemical material used in modern industry and agriculture. At present, the energy-intensive Haber-Bosch process is the main artificial synthesis route for ammonia, and this process uses more than 1% of global annual energy consumption and produces carbon dioxide emissions. In contrast, electrochemical reduction of nitrogen into ammonia under ambient conditions is a potential strategy for sustainable ammonia production. However, due to the strong dipole moment of the N≡N triple bond and the vigorous hydrogen evolution reaction (HER) competing, the development of highly effective catalysts with sufficient activity and selectivity is essential.
… Here, for the first time, we focus our attention on a nonmetallic simple substance and demonstrate that orthorhombic black phosphorus in the form of few-layer nanosheets is a superior metal-free single element electrocatalyst for nitrogen reduction under ambient conditions.—Zhang et al.
In the Haber–Bosch process, nitrogen from air is reduced with hydrogen or synthesis gas under high pressure and temperature over a transition-metal catalyst. The energy demand of this process is so high that one to two percent of the global energy supply is devoted to industrial production of ammonia.
Black phosphorus is a rising star in electronic applications because of its metallic-like appearance and unusual electronic properties. Moreover, its puckered two-dimensional sheet-like structure may provide the necessary edges and sites for adsorption and molecular activation.
With this idea in mind, researcher Haihui Wang and colleagues, prepared thin layers of bulk black phosphorus, by a facile liquid exfoliation method. The catalyst nanosheets were included in a carbon fiber electrode for electrolysis. To provide a nitrogen supply, a hydrochloride electrolyte solution was saturated with nitrogen.
On application of a voltage, the electrode readily and selectively produced ammonia from nitrogen, and the layered black phosphorus even outperformed most nonmetallic and metal-base catalysts reported at present. The extraordinary activity and selectivity of this material are explained by the structure and energetics of the phosphorus sheets.
Using theoretical calculations, the authors found that the zigzag arrangement in the phosphorus layers, in contrast to other layered or flat materials, provided ideal sites for nitrogen adsorption and the electronic structure at the edges was best suited for binding, activating, and reducing nitrogen by a low-energy pathway.
Despite the generally good stability of black phosphorus under ambient conditions, its performance declined in the long term because of oxidation. The authors said that further improvements in preventing black phosphorus degradation in the electrolyte would be beneficial.
This work opens up a novel and attractive application for black phosphorus. In electrocatalytic nitrogen reduction, the performance of black phosphorus is superior to other nonmetallic and even metallic catalysts, suggesting that this material may soon play a bigger role in electrocatalysis.
Zhang, L., Ding, L., Chen, G., Yang, X. and Wang, H. (2019), “Ammonia Synthesis Under Ambient Conditions: Selective Electroreduction of Dinitrogen to Ammonia on Black Phosphorus Nanosheets.” Angew. Chem. Int. Ed. doi: 10.1002/anie.201813174