Topotactic Nitrogen Transfer: Structural Transformation in Cobalt Molybdenum Nitrides

The temperature-programmed reaction of Co 3 Mo 3 N under H 2 /Ar results in a new nitride with composition Co 6 Mo 6 N. Powder neutron diffraction (PND) studies have confirmed unequivocally that the compound possesses the η-12 carbide structure, in which N atoms are exclusively located at 8a crystallographic sites, as opposed to the 16c sites exclusively occupied in the Co 3 Mo 3 N phase. On this basis, it possible to rationalize the observation that 50% nitrogen loss occurs under the high-temperature reduction conditions employed. Reaction of the reduced η-12 phase under N 2 /H 2 results in the regeneration of the η-6 Co 3 Mo 3 N nitride and return of nitrogen to the 16c sites (only). Although established for corresponding ternary carbide structures, the η-12 carbide structure is unprecedented in nitrides and a topotactic cycling between η-carbide structures is hitherto unknown. The ammonia synthesis activity of the η-6 nitride at ambient pressure and 400 °C is 167 μmol g −1 h −1, whereas the η-12 structure is unstable and reverts back to the η-6 structure under reaction conditions. Studies of the magnetic properties of the cobalt molybdenum nitrides demonstrate that both the η-6 and η-12 nitrides exhibit no long-range magnetic ordering and are superparamagnetic, although the coercive field ( H c ) observed in magnetization measurements is diminished in the reduced nitride.