Quantification of Active Sites and Elucidation of the Reaction Mechanism of the Electrochemical Nitrogen Reduction Reaction on Vanadium Nitride

• Published in: Angewandte Chemie (International ed.) 2019-09, Vol.58 (39), p.13768-13772
• Main Authors: Yang, Xuan, Kattel, Shyam, Nash, Jared, Chang, Xiaoxia, Lee, Ji Hoon, Yan, Yushan, Chen, Jingguang G., Xu, Bingjun
• Format: Article
• Language: English
• Subjects: ammonia synthesis; Catalysis; Catalysts; Chemical reduction; Density functional theory; electrochemical nitrogen reduction; Electrochemistry; isotopic labeling; Nitrogen; Reaction mechanisms; Vanadium; vanadium nitride
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201906449

Despite recent intense interest in the development of catalysts for the electrochemical nitrogen reduction reaction (ENRR), mechanistic understanding and catalyst design principles remain lacking. In this work, we develop a strategy to determine the density of initial and steady‐state active sites on ENRR catalysts that follow the Mars–van Krevelen mechanism via quantitative isotope‐exchange experiments. This method allows the comparison of intrinsic activities of active sites and facilitates the identification and improvement of active‐site structures for ENRR. Combined with detailed density functional theory calculations, we show that the rate‐limiting step in the ENRR is likely the initial N≡N bond activation via the addition of a proton and an electron to the adsorbed N2 on the N vacancies to form N2H. The methodology developed and mechanistic insights gained in this work could guide the rational catalyst design in the ENRR. A quantitative isotope‐exchange method was used to determine the density of initial and steady‐state active sites on an oxygen‐modified vanadium nitride (VNO) catalyst used for the electrochemical nitrogen reduction reaction. The activation of adsorbed N2 on the N vacancy of VNO to adsorbed N2H is likely the rate‐limiting step.