Very stable in-operando and low-loaded atomic molybdenum on solid support materials have been prepared and tested to be catalytically active for N₂-into-NH₃ hydrogenation. Ammonia synthesis is reported at atmospheric pressure and 400°C with NH₃ rates of ca. 1.3·10³ μmol h¯¹ gMo¯¹ using a well-defined Mo-hydride grafted on silica (SiO₂-700). DFT modelling on the reaction mechanism suggests that N₂ spontaneously binds on monopodal [(≡Si-O-)MoH₃]. Based on calculations, the fourth hydrogenation step involving the release of the first NH₃ molecule represents the rate-limiting step of the whole reaction. The inclusion of cobalt co-catalyst and an alkali caesium additive impregnated on mesoporous SBA-15 support increases the formation of NH₃ with rates of ca. 3.5·10³ μmol h¯¹ gMo¯¹ under similar operating conditions and maximum yield of 29·10³ μmol h¯¹ gMo¯¹ when pressure is increased to 30 atm.