Topochemical Synthesis of Ca₃CrN₃H 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-Ca 3 CrN 3 H, by heating an orthorhombic nitride, o-Ca 3 CrN 3 , under hydrogen at 673 K, accompanied by a rotational structural transformation. The hydrogen intercalation modifies the CaÀ N rock-salt-like atomic packing in o-Ca 3 CrN 3 to a face-sharing octahedral chain in h-Ca 3 CrN 3 H, mimicking a "hinged tessellation" movement. In addition, the h-Ca 3 CrN 3 H exhibited stable ammonia synthesis activity when used as a catalyst.

Revisiting group 4–7 transition metals for heterogeneous ammonia synthesis

“…20,36,37 The intermetallic compounds containing group 4-7 metals, such as ZrCr 2 , ZrMn 2 , and ZrV 2 , can absorb both H 2 and N 2 , and they have been tested for ammonia synthesis recently and form the intermetallic nitride hydride phase, although their catalytic activities are not high. 19 Wang et al reported that a CoMoFeNiCu high-entropy alloy is active for ammonia decomposition, the reverse reaction of ammonia synthesis. 38 There has been so far little work on high-entropy alloys or high-entropy metal hydrides and nitrides for ammonia synthesis catalysis, which is worthy of future study.…”