Topotactic Nitrogen Transfer: Structural Transformation in Cobalt Molybdenum Nitrides

Abstract

The temperature-programmed reaction of Co3Mo3N under H2/Ar results in a new nitride with composition Co6Mo6N. 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 Co3Mo3N 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 N2/H2 results in the regeneration of the η-6 Co3Mo3N 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 (Hc) observed in magnetization measurements is diminished in the reduced nitride.