Single-Site Molybdenum on Solid Support Materials for Catalytic Hydrogenation of N2 -into-NH3

Abstract

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 g_Mo ^-1 using a well-defined Mo-hydride grafted on silica (SiO_2-700 ). 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 g_Mo ^-1 under similar operating conditions and maximum yield of 29×10³ μmol h^-1 g_Mo ^-1 when the pressure is increased to 30 atm.

Keywords: N2 fixation; NH3 synthesis; heterogeneous catalysis; in silico design; molybdenum.