Electrochemical nitrogen reduction reaction to ammonia using perovskite-type oxynitrides BaMO2N (M = Nb, Ta) and enhancement of the activity by acid treatment

The electrochemical nitrogen reduction reaction (ENRR) to NH3 is one of the potential alternatives to the Harber-Bosch (HB) process. Metal nitrides and oxynitrides have been proposed as suitable catalysts. However, an ENRR catalyst that is stable during the ENRR and demonstrates a high Faradaic efficiency (FE) has never been reported. Perovskite-type oxynitrides, BaMO2N (M = Nb, Ta) are promising catalysts for the ENRR because of the coexisting of N vacancies that function as N2 mole-cule adsorption sites and a robust crystal lattice resulting from the metal-oxygen bonds. Herein, we demonstrate electro-chemical NH3 production using the perovskite oxynitride catalysts with high FE values over 60%. The Ta and Nb oxynitrides showed NH3 production rates of 6.8 and 9.2 μgNH3/h·mgcat, with the FE values of 73% and 39%, respectively. Acid treatment using 6 M nitric acid enhanced their activity to 8.0 and 15 μgNH3/h·mgcat with 63% and 67% of FE values for the Ta and Nb oxynitrides, respectively. The amount of NH3 produced was approximately ten times larger than the nitrogen content of the oxynitride catalysts, indicating that the NH3 was dominantly produced from N2, not from decomposition of the catalysts.