Ammonia is a promising renewable energy carrier because of its high energy density . The reaction condition for the ammonia synthesis in the industry is severe, 400-500 °C and 10-30 MPa. It is difficult for time variability and thin density of renewable energy to achieve the harsh reaction condition . Thus, high catalytic performance under mild conditions is inevitable for an energy-efficient synthesis. Catalysts (e.g., Ru) for ammonia synthesis generally employ electron-donating metals such as Ca enhance their activities , while oxygen is not preferable because of its electron-withdrawing ability. However, recent reports unraveled that oxygen (electrophilic species) addition enhances the activity of catalysts . As plausible explanations, hydrogen-absorbing, electron-donating properties, and an increase in active sites have been proposed, but the reasons are unclear .