Combining easily reducible iron with hard-to-reduce tantalum — Iron-based oxyhydride realized by Kyoto University and partners

Oxyhydrides, materials in which negative hydrogen ions (hydrides) are introduced into oxides, are expected to find applications as next-generation battery materials and high-performance catalysts for ammonia synthesis. In particular, the transition metal species to be combined is a crucial element determining functionality, and iron is viewed as promising due to its low cost and low toxicity. However, when attempts are made to introduce hydrogen by reducing iron-based oxides with the reducing agent CaH₂, the oxygen is merely extracted without being replaced by hydrogen, causing the structural framework to collapse.

A research group consisting of Program-Specific Researcher Yuki Sasahara (at the time of the research, currently Assistant Professor at the Graduate School of Engineering, Hokkaido University) and Professor Hiroshi Kageyama of the Graduate School of Engineering at Kyoto University, Associate Professor Susumu Fujii of the Department of Materials, Faculty of Engineering at Kyushu University, Professor Ko Mibu of the Department of Physical Science and Engineering, Nagoya Institute of Technology, and Professor Masatomo Yashima of the Department of Chemistry, School of Science, Institute of Science Tokyo, has successfully synthesized BaFe_0.5Ta_0.5O_2.7H_0.3, the first perovskite-type oxyhydride containing iron-hydrogen bonds. This was achieved by combining iron, which is easily reduced, with tantalum, which is difficult to reduce. This achievement is expected to serve as a new material design guideline for controlling redox reactions in solids, and the results were published in the Journal of the American Chemical Society.

The research group focused on tantalum, niobium, and hafnium, which had not been used in oxyhydrides because, in contrast to iron, they are difficult to reduce. By heating BaFe_0.5Ta_0.5O₃, an ABO₃ perovskite-type oxide containing both iron and tantalum at the same B-site, together with CaH₂, they successfully synthesized the first oxyhydride possessing iron-hydrogen bonds, BaFe_0.5Ta_0.5O_2.7H_0.3.

When examining the oxidation states of iron and tantalum, it was found that while tantalum remained pentavalent even after the reaction, only iron had changed to a lower oxidation state.

When investigating the reaction mechanism via first-principles calculations, it was revealed that if an oxygen vacancy occurs near tantalum, the surrounding bonding balance is significantly disrupted, causing local strain.

On the other hand, if the oxygen is replaced by a hydride instead of simply being extracted, this imbalance in the bonding distribution is alleviated, yielding a more stable local structure.

Tantalum does not function merely as a diluent or a structural stabilizer. It also plays a role in controlling the reduction reaction pathway, shifting it from oxygen deficiency toward hydride introduction.

The obtained substance was stable in both air and water. Despite being synthesized under strong reducing conditions, the iron was not completely reduced to a divalent state but was stabilized in a mixed-valence state where divalent and trivalent iron coexist.

Tantalum controls the progression of the reduction of iron. Furthermore, it was found that oxyhydrides can be formed even when using niobium or hafnium in addition to tantalum.

The group demonstrated that redox asymmetry, a state arising from combining an easily reducible element with a hard-to-reduce element, serves as a general guideline for designing iron-based oxyhydrides, significantly expanding the design space into material groups containing elements for which oxyhydride formation was previously difficult.

Sasahara stated: "While carefully looking over the coordination environment lists for each element, I came up with this combination of elements that happened to be unused together. Although the experiments progressed more smoothly than I had anticipated, we struggled with measurements to confirm that it was indeed successfully formed. It was synthesized using a ball mill, and because its stability is extremely high, we believe that a bit of ingenuity will be required to obtain high activity for use as a catalyst."

Journal Information
Publication: Journal of the American Chemical Society
Title: Redox Asymmetry Enables Fe-H Bonds in Perovskite Oxyhydrides
DOI: 10.1021/jacs.6c06588