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| Fig. 1: A photoelectrochemical solar cell of plasmon-enhanced black silicon. (Source: J. Li, after Ali et al. [4]) |
Nitrogen fixation is an essential reaction as a part of nitrogen cycling in nature. From this process, stable N2 molecules in atmosphere could be converted into compounds containing nitrogen atoms, including ammonia, nitrates, and urea, which then become useful for most living organisms. [1]
Human participating in nitrogen fixation by large scale artificial methods starts from the synthesis of ammonia using nitrogen via Haber-Bosch process. [2] This is a thermochemical catalytic conversion of N2 and H2 at elevated temperatures and pressures, typically over an iron catalyst. Firstly discovered in 1909, the Haber-Bosch process has been heralded as the most important invention of the twentieth century. Ammonia is an important chemical having vast need in agriculture, pharmaceutical production and solar energy storage. However, Haber-Bosch process is a highly energy intensive reaction, and people are trying to develop other chemical, photo or electrochemical and biological production methods for synthesizing ammonia from N2.
Photochemical or photoelectrochemical conversion provides a promising approach to convert nitrogen into ammonia by using solar energy. This is because the photochemical strategy employs inexpensive semiconductors as the catalysts, water as the solvent, and solar energy as the driving force, all of which are cheap and easily avaliable. Meanwhile, ammonia could be synthesized by much milder conditions compared with Haber- Bosch process.
Researchers from China reported a device fabricated using BiOBr nanosheets of oxygen vacancies (OVs) on the exposed {001} facets (Fig. 1). Along with the electron-donating nature of BiOBr under visible light, the designed catalytic centers of OVs could activate N2 and significantly promote the interfacial electron transfer from the excited BiOBr nanosheets to the adsorbed N2. The N2 fixation rate of BiOBr {001} OVs in this study became much higher than those previous semiconductors. [3]
A solar-driven nanostructured photoelectrochemical cell based on plasmon-enhanced black silicon was described to convert atmospheric N2 to ammonia effectively. And the yield could increase with N2 pressure: at 7 atm, the highest yield of 60 mg m-2 h-1 was observed. [4] In the presence of sulfite as a reactant, the process also offers a direct solar energy route to ammonium sulfate, a fertilizer of economic importance. Significant potential exists for further development of this approach to ammonia generation. [4]
© Jiachen Li. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] A. J. Medford and M. C. Hatzell, "Photon-Driven Nitrogen Fixation: Current Progress, Thermodynamic Considerations, and Future Outlook," ACS Catal. 7, 2624 (2017).
[2] J. W. Erisman et al., "How a Century of Ammonia Synthesis Changed the World," Nat. Geosci. 1, 636 (2008).
[3] H. Li et al., "Efficient Visible Light Nitrogen Fixation with BiOBr Nanosheets of Oxygen Vacancies on the Exposed {001} Facets," J. Am. Chem. Soc. 137, 6393 (2015).
[4] M. Ali et al., "Nanostructured Photoelectrochemical Solar Cell for Nitrogen Reduction Using Plasmon-Enhanced Black Silicon," Nat. Commun. 7, 11335 (2016).