Crystal-Phase and Surface-Structure Engineering of Bi₂O₃ for Enhanced Electrochemical N₂ Fixation to NH₃

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.

Review on synthetic approaches and PEC activity performance of bismuth binary and mixed-anion compounds for potential applications in marine engineering

“…β-Bi 2 O 3 nanocrystals were synthesized from bismuth nitrate and bismuth oxalate by various solution processes. 113–116 The synthesis of β-Bi 2 O 3 is facing some challenges due to its specific crystal structure and properties. β-Bi 2 O 3 is a metastable phase and tends to transform into the thermodynamically more stable α-Bi 2 O 3 at high temperatures.…”