Development of High-Performance, Low-Environmental-Impact Ammonia Synthesis Catalysts

A research group led by Professor Masaaki Kitano of the MDX Research Center for Element Strategy (MDXES), Institute for Integrated Research (IIR), Institute of Science Tokyo, has developed a novel ammonia synthesis catalyst that does not rely on transition metals, which have traditionally been considered essential. The catalyst was created using silicate compounds that are abundant on Earth as starting materials. A catalyst is a substance that accelerates chemical reactions without being consumed.

The research group synthesized a catalyst by partially substituting oxygen atoms in barium silicate (Ba_₃SiO_₅) with hydride ions (H^⁻)^*1 or nitride ions (N^³⁻). During the reaction, the substituted ions leave the crystal lattice, creating anion vacancies^*2. Electrons trapped in these vacancies efficiently activate nitrogen molecules. As a result, the catalyst exhibited an ammonia synthesis rate that substantially exceeds that of conventional ruthenium-based catalysts.

*1 Hydride ion
A hydrogen atom that has gained one electron and carries a negative charge

*2 Anion vacancy
A defect in which an anion is missing from the crystal lattice. When an anion leaves, electrons tend to remain localized in the surrounding region.

Challenges of Conventional Ammonia Synthesis Methods Toward a Decarbonized Society

Conventional methods that emit massive amounts of carbon dioxide

Ammonia (NH₃) is an essential raw material for fertilizers and a wide range of chemical products. In recent years, its high hydrogen (H₂) density has also made it a promising hydrogen carrier. A hydrogen carrier is a substance that safely stores and transports hydrogen by chemically binding it into another form.

Industrial ammonia is currently produced via the Haber-Bosch process, which requires high temperatures and pressures. This method uses hydrogen produced from fossil fuels, resulting in large quantities of carbon dioxide as a byproduct. To realize “green ammonia” synthesis using hydrogen derived from renewable energy sources, there is a need to develop catalysts capable of efficiently synthesizing ammonia under low-temperature and low-pressure conditions.

Transition-metal-containing catalysts as a conventional approach

Historically, ammonia synthesis catalysts have largely relied on transition-metal-containing materials^*3. This is because nitrogen molecules—the raw material for ammonia—form very strong interactions with transition metals, enabling efficient activation of nitrogen molecules. Conversely, it was long believed that materials lacking transition metals could not effectively activate nitrogen.

*3 Transition metals
Metals that facilitate chemical reactions. Because they can readily donate or accept electrons, they can convert otherwise unreactive molecules into more reactive states.

A New Ammonia Synthesis Method in Which Electrons Activate Nitrogen

A compound exhibiting high ammonia synthesis rates

The research group synthesized a mixed-anion compound “Ba₃SiO_5-xN_yH_z” by introducing high concentrations of hydride ions (H^⁻) and nitride ions (N^³⁻) into the oxygen sites of barium silicate (Ba₃SiO₅) (Fig. 1). This compound not only maintains stable ammonia synthesis for over 100 hours but also exhibits an ammonia synthesis rate more than ten times higher at low temperatures than conventional transition-metal catalysts, such as ruthenium supported on magnesium oxide.

Electron-trapping catalyst

At temperatures used for ammonia synthesis, this compound readily releases hydride ions (H^⁻) and nitride ions (N^³⁻), generating ammonia while forming anion vacancies (Fig. 2, right). Electrons become trapped in these anion vacancies, resulting in the compound becoming overall electron-rich. Upon heating in the presence of nitrogen and hydrogen, these electrons activate N_₂ and H_₂ molecules, allowing hydrogen and nitrogen to be reincorporated into the compound and restoring the original structure (Fig. 2, left).

In this way, the compound was found to function as a catalyst that continuously drives ammonia synthesis while undergoing structural changes.

Ammonia synthesis achieved at lower temperatures and pressures than conventional methods

The research group increased the ammonia synthesis rates by more than 100 times at maximum by depositing small ruthenium particles onto the surface of Ba3SiO5₋xNyHz. In this system, ruthenium does not function as an active site. Instead, it enhances reactivity by promoting the formation of anion vacancies. As a result, an ammonia synthesis rate at one of the highest levels reported to date was achieved even under the relatively low-temperature and low-pressure conditions of 300°C and 1.0 MPa.

Toward Foundational Technology for Next-Generation Catalyst Design

This research challenges the conventional view that transition metals are indispensable for ammonia synthesis catalysts. The study presents a new catalyst design based on silicate compounds abundant on Earth, utilizing electrons trapped in anion vacancies for ammonia synthesis. This suggests that combinations of elements not previously considered catalyst candidates may become targets for future materials exploration. In the future, this technology is expected to serve as a foundational approach that accelerates the practical realization of green ammonia synthesis.