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Ru catalysts promoted with alkali and alkaline earth have shown superior ammonia (NH₃) synthesis activities under mild conditions. Although these promoters play a vital role in enhancing catalytic activity, their function has not been clearly understood. Here, we synthesize a series of Ba-Ru/MgO catalysts with an optimal Ru particle size (∼2.3 nm) and tailored BaO–Ru interfacial structures. We discover that the promoting effect is created through the separate storage of H⁺/e^– pairs at the BaO–Ru interface. Chemisorbed H atoms on Ru dissociate into H⁺/e^– pairs at the BaO–Ru interface, where strongly basic, nonreducible BaO selectively captures H⁺ while leaving e^– on Ru. The resulting electron accumulation in Ru facilitates N₂ activation via enhanced π-backdonation and inhibits hydrogen poisoning during NH₃ synthesis. Consequently, the formation of intimate BaO–Ru interface without an excessive loss of …