Abstract
Very stable in operando and low-loaded atomic molybdenum on solid-support materials have been prepared and tested to be catalytically active for N-2-into-NH3 hydrogenation. Ammonia synthesis is reported at atmospheric pressure and 400 degrees C with NH3 rates of approximately 1.3 x 10(3 )mu molh(-1) g(Mo) (-1) using a well-defined Mo-hydride grafted on silica (SiO2-700 ). DFT modelling on the reaction mechanism suggests that N-2 spontaneously binds on monopodal [(equivalent to Si-O-)MoH3]. Based on calculations, the fourth hydro-genation 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 x .10(3) mu molh(-1) g(Mo)(-1) under similar operating conditions and maximum yield of 29 x 10(3) mu molh(-1) gMo(-1) when the pressure is increased to 30 atm.