Ru catalysts promoted with alkali and alkaline earthhave shownsuperior ammonia (NH3) synthesis activities under mildconditions. Although these promoters play a vital role in enhancingcatalytic activity, their function has not been clearly understood.Here, we synthesize a series of Ba-Ru/MgO catalysts with an optimalRu particle size (similar to 2.3 nm) and tailored BaO-Ru interfacialstructures. We discover that the promoting effect is created throughthe separate storage of H+/e(-) pairs atthe BaO-Ru interface. Chemisorbed H atoms on Ru dissociateinto H+/e(-) pairs at the BaO-Ruinterface, where strongly basic, nonreducible BaO selectively capturesH(+) while leaving e(-) on Ru. The resultingelectron accumulation in Ru facilitates N-2 activation viaenhanced pi-backdonation and inhibits hydrogen poisoning duringNH(3) synthesis. Consequently, the formation of intimateBaO-Ru interface without an excessive loss of accessible Rusites enables the synthesis of highly active catalysts for NH3 synthesis.