Topochemical Synthesis of Ca3 CrN3 H Involving a Rotational Structural Transformation for Catalytic Ammonia Synthesis

Abstract

Topochemical reactions have led to great progress in the discovery of new metastable compounds with novel chemical and physical properties. With these reactions, the overall crystal structure of the host material is generally maintained. Here we report a topochemical synthesis of a hexagonal nitride hydride, h-Ca₃ CrN₃ H, by heating an orthorhombic nitride, o-Ca₃ CrN₃ , under hydrogen at 673 K, accompanied by a rotational structural transformation. The hydrogen intercalation modifies the Ca-N rock-salt-like atomic packing in o-Ca₃ CrN₃ to a face-sharing octahedral chain in h-Ca₃ CrN₃ H, mimicking a "hinged tessellation" movement. In addition, the h-Ca₃ CrN₃ H exhibited stable ammonia synthesis activity when used as a catalyst.

Keywords: Ammonia Synthesis Catalysis; Hydrides; Mixed Anions; Nitrides; Topochemistry.