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 approximately 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 a mesoporous SBA-15 support increases the formation of NH3 with rates of circa 3.5×103 μmol h−1 gMo−1 under similar operating conditions and maximum yield of 29×103 μmol h−1 gMo−1 when the pressure is increased to 30 atm.