Anionic defects of H⁻ (H^∙_O) and oxygen vacancies with trapped electrons (V^∙_O) have been built in the lattice of a hydrothermally derived perovskite (Ba₅Nb₄O_15, BNO) by CaH₂ reduction in Ar flow at 600 °C for 12 h. The formed anionic defects in the CaH₂-reduced BNO (CaBNO) have been found to improve drastically the ammonia synthesis activity of Ru/CaBNO by donating electrons from both H^∙_O and/or V^∙_O to Ru atoms. The stoichiometry of H^∙_O and V^∙_O has been determined by TPR and TPO method, respectively. The H^∙_O defects have been proved to react with N₂ under ammonia synthesis conditions via TPD, XPS measurements and isotope (D₂) reaction, which provides secondary pathways for ammonia synthesis. Kinetic studies revealed that oxygen vacancies accept spilledover H atoms from Ru particles to alleviate the H₂ poisoning effect. Cs-promoter has been demonstrated to accelerate the formation of anionic defects in CaBNO under ammonia synthesis reactions.

Graphical Abstract

Anionic defects (V^∙_O and H^∙_O) formed in the lattice of Ba₅Nb₄O₁₅ perovskite via CaH₂-reduction drastically promote the ammonia synthesis activity of Cs-Ru/CaBNO catalyst