| 研究実績の概要 |
Early 3d transition metals, such as Ti, V, Nb or Cr are known to be inactive for the Haber-Bosch process, due to their strong M-N bonds. I expanded the study to hydrides of other early transition metals, i.e., V and Nb. These metals benefit from body-centered cubic (bcc) related structures which enhance hydride diffusion, in addition to having relatively lower M-N bond strengths. The activity of vanadium hydride, most likely with an active composition of VH0.44N0.16, is superior to the previously reported TiH2 and BaTiO2.5H0.5, and comparable to Cs-Ru/MgO. These results show that there is more potential for developing new single-phase hydride catalysts of previously overlooked elements without sacrificing activity.
Topochemical reactions have led to great progress in the discovery of new metastable compounds with novel chemical and physical properties, making it an effective method of synthesizing new hydride materials. I reported a topochemical synthesis of a new hexagonal nitride hydride, h-Ca3CrN3H, by heating an orthorhombic nitride, o-Ca3CrN3, 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-Ca3CrN3 to a face-sharing octahedral chain in h-Ca3CrN3H. Impressively, the h-Ca3CrN3H exhibited stable ammonia synthesis activity when used as a catalyst, even though the early transition metal Cr is not an active element for ammonia synthesis. DFT calculations reveal that nitrogen reduction could be effectively achieved through an associative mechanism.
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