Crystal-Phase and Surface-Structure Engineering of Bi<sub>2</sub>O<sub>3</sub> for Enhanced Electrochemical N<sub>2</sub> Fixation to NH<sub>3</sub>
Pengju Guo, Fengxiang Yin, Jie Zhang, Biaohua Chen, Ziyang Ni, Liuliu Shi, Mengyan Han, Zumai Wu, Guoru Li
Abstract
The nitrogen reduction reaction (NRR) for ammonia synthesis is hindered by weak N 2 adsorption/activation abilities and the hydrogen evolution reaction (HER). In this study, αBi 2 O 3 (monoclinic) and βBi 2 O 3 (tetragonal) were first synthesized by calcination at different temperatures. Experiments and calculations revealed the effects of Bi 2 O 3 with different crystal phases on N 2 adsorption/activation abilities and HER. Then, αBi 2 O 3 - x and βBi 2 O 3 - x series catalysts with surface oxygen vacancies (OVs) and Bi 0 active sites were synthesized through the partial in situ reduction method. The results demonstrate the following: (I) Tetragonal βBi 2 O 3 can better adsorb N 2 and cleave the N≡N bond, thereby obtaining a lower NRR rate-limiting energy barrier (*N≡N → *N≡N–H, 0.51 eV). Meanwhile, βBi 2 O 3 can effectively suppress HER by limiting proton adsorption (H + + e – → *H, 0.54 eV). Therefore, βBi 2 O 3 - x series catalysts exhibit higher NH 3 yield and FE than αBi 2 O 3 - x . Meanwhile, in situ FTIR further confirms that βBi 2 O 3 could better adsorb/activate N 2, and the NRR distal mechanism occurs on the Bi 2 O 3 surface. (II) The introduction of NaBH 4 promotes the conversion of part of Bi 3+ on the Bi 2 O 3 surface into Bi 0 and releases OVs. The additional active sites (OVs and Bi 0 ) enhance the overall catalyst’s adsorption/activation capacity for N 2, further increasing the NH 3 yield and FE. Meanwhile, semimetal Bi 0 can effectively limit electron accessibility, thereby inhibiting the combination of charges and adsorbed protons, reducing the HER reaction and improving the FE of NRR. Therefore, the introduction of NaBH 4 effectively improved the NH 3 yield and FE of the αBi 2 O 3 - x and βBi 2 O 3 - x series catalysts. After optimization, the βBi 2 O 3 -0.6 catalyst has the best NRR performance (NH 3 yield: 51.36 μg h –1 mg –1 cat.; FE: 38.67%), which is superior to the majority of bismuth-based NRR catalysts. This work not only studies the effects of Bi 2 O 3 with different crystal phases on N 2 and HER reaction but also effectively regulates the active components of Bi 2 O 3 surface, thereby realizing efficient NRR to NH 3 reaction, which provide valuable insights for the rational design of Bi-based NRR electrocatalysts.