This account provides an overview of our development of catalytic ammonia synthesis under ambient reaction conditions with well-designed molecular molybdenum complexes as catalysts. It also reviews recent progress in our research, particularly the application of catalytic ammonia synthesis driven by electrical energy and visible light.

Green ammonia synthesis, which utilizes H₂ produced from renewable energies such as wind and solar light, is now becoming a hot topic over the world. To realize this new process, the development of new catalysts that can function under lower temperatures and pressures than Haber-Bosch process is strongly desired. We have recently developed new solid materials with functional heteroanion sites such as H⁻ and N³⁻ ions as catalysts for ammonia synthesis under mild reaction conditions. Here, we mainly focus on the role of heteroanion sites of oxyhydrideand oxynitride-supported transition metal catalysts on the catalytic performance in ammonia synthesis.

Ammonia is expected to be used on a much larger scale as hydrogen carrier and fuel that does not emit carbon dioxide at the time of combustion in the next 30 years. Industrially, ammonia is produced under severe reaction conditions (350–550 °C and 15–35 MPa) using the Haber-Bosch process. The development of new catalysts for ammonia synthesis under milder conditions is urgently necessary. We have developed a novel supported ultra-small molybdenum metal cluster catalyst with a size of less than 1 nm on average. This metal cluster has been prepared from molybdenum chloride cluster supported on HY zeolite, followed by removal of all the chloride ligands by hydrogen heat treatment. The resulting metal cluster effectively cleaves N≡N bond in dinitrogen (N₂) by taking advantage of the multiple molybdenum sites, and constantly produces ammonia even at 200 °C under 5 MPa. The stability of the chloride cluster precursor in air and the easy availability of the metal cluster catalyst by the simple hydrogen treatment are advantageous for industrial use. This report describes the development and future view of the ultra-small molybdenum cluster catalyst for ammonia synthesis.

For ammonia synthesis, the dissociation of nitrogen molecules with stable triple bond is an important issue. Since alkali metals such as Li and Na possess high nitrogen dissociation properties, their compounds are useful for ammonia synthesis. In this work, synthesis techniques by using alkali metal compounds were proposed and investigated. The chemical looping process of lithium hydride is composed of two step reactions, and ammonia is synthesized under 0.1 MPa below 400 °C. The nitrogen dissociation properties of Li compounds were correlated with the chemical state of Li. The more metallic Li can dissociate nitrogen at lower temperatures. Considering the nitrogen dissociation properties, the chemical looping process by alloys composed of Li and group 14 elements was proposed. The chemical looping of Li alloys consists of three step reactions and can generate ammonia under 0.1 MPa as well. For alloys composed of Na and group 14 elements, catalytic ammonia synthesis properties were investigated since Na does not form stable nitrides and amides like Li. All the Na alloys showed catalytic activity for ammonia synthesis, and then the ammonia generation temperature and ammonia formation rate depended on thermal stability and chemical state of the alloys, respectively.

The rising demand for renewable hydrogen is driving up water requirements globally so seawater is considered as a potential water source for electrolysis. There are two main approaches to seawater electrolysis: (1) direct seawater electrolysis, where seawater is fed directly into the stack, and (2) indirect seawater electrolysis, where desalinated seawater is used. For direct seawater electrolysis, it is essential to develop advanced anode catalysts that favor the oxygen evolution reaction (OER) over the chlorine oxidation reaction (COR). The current density and system costs are also key considerations. The indirect seawater electrolysis resembles the conventional electrolysis but includes a seawater desalination step. The combination of offshore wind and PEM electrolysis is gaining interest as a preferred method for renewable hydrogen production especially in Europe.

The direct one-step hydrogenation of toluene to methylcyclohexane facilitated by a protonexchange membrane water electrolyzer driven by renewable energy has garnered considerable attention for stable hydrogen storage and safe hydrogen transportation. However, a persistent challenge lies in the crossover of toluene from the cathode to the anode chamber, which deteriorates the anode and decreases its energy efficiency and lifetime. To address this challenge, the catalyst-poisoning mechanism is systematically investigated using IrO₂ and high-entropic non-noble-metal alloys as anodes in acidic electrolytes saturated with toluene and toluene-oxidized derivatives, such as benzaldehyde, benzyl alcohol, and benzoic acid. Benzoic acid plays an important role in polymer-like carbon-film formation by blocking the catalytically active sites on the anode surface. Moreover, Nb and the highly entropic state on the surface of the multi-element alloy lower the adsorbing ability of toluene and prevent polymer-like carbon film formation. This study contributes to the design of catalyst-poisoning-resistant anodes for organic hydride technology, advanced fuel cells, and batteries.