Japanese Team Maps the Full Catalytic Cycle of a Molybdenum Route to Ammonia

Nitrogen makes up most of the air around us, yet its N≡N triple bond is so stable that plants and people cannot directly use atmospheric N₂ as a nutrient. Breaking that bond and converting nitrogen into ammonia is one of the foundational chemical transformations of modern civilization. A Japanese research team has now mapped, step by step, the complete catalytic cycle of a molecular molybdenum system that turns dinitrogen into ammonia using samarium diiodide and water under ambient conditions.^[1][2]

The work was led by Kazunari Yoshizawa of Kyoto University’s Fukui Institute for Fundamental Chemistry, Yoshiaki Nishibayashi of the University of Tokyo, Hiromasa Tanaka of Daido University and Taiji Nakamura of Kyoto Institute of Technology, with colleagues across the four institutions. By combining mechanistic experiments with quantum-chemical calculations, the team connected previously isolated pieces of the chemistry into a full cycle—from nitrogen activation to ammonia release and catalyst regeneration. The paper appeared in Nature Communications on September 24.^[1]

The century-old benchmark: Haber–Bosch

Industrial ammonia is still dominated by the Haber–Bosch process. Fritz Haber demonstrated that nitrogen and hydrogen could be converted into ammonia at high temperature and pressure in the early twentieth century. Carl Bosch then transformed the chemistry into a workable high-pressure industrial process, with commercial-scale plants operating by the 1910s. Haber received the 1918 Nobel Prize in Chemistry; Bosch shared the 1931 prize for the development of high-pressure chemical methods.^[3][4]

The achievement reshaped agriculture. Synthetic ammonia made mineral nitrogen fertilizer available on an unprecedented scale. But the industrial system remains energy- and carbon-intensive. The International Energy Agency estimates that ammonia production accounts for about 2% of global final energy consumption. Direct emissions are roughly 450 million tonnes of CO₂ per year, and about 70% of ammonia is used to make fertilizer.^[5]

High temperature and pressure are part of the energy burden, but the largest carbon issue is often hydrogen. Most ammonia is still made with hydrogen derived from natural gas or coal. “Green ammonia,” in which renewable electricity powers water electrolysis to produce hydrogen for Haber–Bosch plants, is one leading decarbonization route, but it requires large amounts of clean power, electrolyzer capacity and new infrastructure.

Biology proves nitrogen can be fixed without a high-pressure reactor

Chemists have long been fascinated by nitrogenase, the enzyme complex used by certain microorganisms to reduce atmospheric nitrogen under mild conditions. Nitrogenase contains sophisticated metal clusters, including molybdenum in its most studied form, and uses electrons, protons and ATP to make ammonia. Biology is not performing the reaction for free—it spends chemical energy—but it demonstrates that the nitrogen triple bond can be overcome without a 500°C industrial reactor.

Nishibayashi’s group reported a major step toward an artificial analogue in Nature in 2019. A molybdenum catalyst, paired with samarium(II) diiodide, or SmI₂, as a reductant and water or alcohol as a proton source, produced ammonia from nitrogen under ambient conditions. The system reached as many as 4,350 ammonia equivalents per molybdenum catalyst, and the formation rate approached that of nitrogenase enzymes. The paper also stated clearly that the reaction, in its then-current form, was not suitable for industrial use.^[6]

In 2023 the team improved the catalyst using PCP-type pincer ligands. At 25°C and one atmosphere of nitrogen, one optimized molybdenum complex generated 60,000 equivalents of ammonia over 72 hours when supplied with large excesses of SmI₂ and water—a dramatic increase over the 2019 system.^[7]

A four-part catalytic cycle

The Nature Communications study resolves the cycle into four major processes. First, a molybdenum nitride complex is hydrogenated stepwise from Mo≡N to Mo–NH, then Mo–NH₂ and finally Mo–NH₃. Second, two molybdenum complexes form a dimer bridged by N₂. Third, ammonia ligands are released. Fourth, the bridging N≡N bond is cleaved to regenerate the molybdenum nitride and close the catalytic cycle.^[2]

That last point matters. A reaction is catalytic only if the active species is regenerated. Earlier studies had identified several key pieces, but not every transition and intermediate could be observed directly. The new study combines chemical trapping and computation to make the full loop coherent.

Capturing fleeting intermediates by building stable analogues

Some of the crucial species—especially molybdenum imide and amide intermediates—are too reactive to isolate easily in their exact catalytic forms. The researchers therefore synthesized stable methyl-substituted analogues: molybdenum methylimide and methylamide complexes. By stepping those compounds through methylation, protonation and reduction, they produced methylamine and regenerated the molybdenum nitride, experimentally supporting the proposed sequence.^[2]

The methyl compounds are models, not the actual Mo–NH and Mo–NH₂ intermediates. Density-functional-theory calculations were therefore used to compare their Mo–N bond structures and electronic properties with the species expected during catalysis. The close correspondence gave the team a bridge between experimentally isolable compounds and the fleeting species of the real reaction.

SmI₂ does more than donate electrons

The SmI₂–water combination is central to the unusual reactivity. Water is stable, and its O–H bond is difficult to break in a controlled way. The team proposed, through theoretical analysis, a solution structure in tetrahydrofuran in which samarium binds a water molecule: [SmI(H₂O)(thf)₅]I.^[2]

The calculated bond dissociation free energy of an O–H bond in free water is 107.2 kcal/mol. Once coordinated to samarium in the proposed complex, it falls to 36.1 kcal/mol. In effect, SmI₂ is not simply an electron reservoir; coordination makes the water molecule much more willing to transfer hydrogen to the molybdenum-bound nitrogen.^[2]

Proton-coupled electron transfer is the key choreography

The N–H bonds form through proton-coupled electron transfer, or PCET. Instead of an electron arriving in one independent step and a proton in another, proton transfer and electron transfer are coupled. The calculations trace electron motion from the SmI₂–H₂O complex as the proton moves toward the nitrogen atom bound to molybdenum.^[2]

That means the chemistry depends on balance. A stronger reductant is not automatically better, and a stronger proton donor is not automatically better. The electron-donating ability and proton-donating ability of the pair have to be matched so that PCET proceeds efficiently. The new mechanism explains why SmI₂ and water, an initially counterintuitive combination, accelerated ammonia formation so strongly in the group’s earlier experiments.

The hardest step is making the first Mo–NH bond

The full free-energy profile points to one clear bottleneck: conversion of the molybdenum nitride to the molybdenum imide. At 298 K, the imide-forming step is endergonic by 3.9 kcal/mol. The calculated activation free energy for conversion of the Sm(II)I₂(H₂O)/Mo–nitride complex to the Sm(III)I₂(OH)/Mo–imide complex is 5.1 kcal/mol.^[2]

Once past that point, the overall ammonia-formation sequence is exergonic by 12.0 kcal/mol, while the later dimerization, ammonia release and N≡N cleavage steps are also calculated to be strongly thermodynamically favorable. The practical implication is unusually specific for a mechanistic paper: improving the chemistry of imide formation should be a prime target for designing a faster, more efficient catalyst.^[2]

Ambient conditions do not automatically mean an energy-efficient commercial process

This is the most important caveat. The reaction itself operates at room temperature and pressure, but SmI₂ is a powerful stoichiometric reductant. Any industrial comparison has to account for the energy, cost and materials required to manufacture and regenerate that reductant, as well as solvent recovery, catalyst lifetime, ammonia separation and continuous operation. The 2019 Nature paper explicitly warned that the system was not then suited to industrial use.^[6]

That makes it misleading to compare the system with Haber–Bosch solely on reactor temperature and pressure. Haber–Bosch has more than a century of optimization behind it and runs continuously at enormous scale. Molecular nitrogen-fixation chemistry remains a research platform. Its value lies in opening different mechanistic options, not in having already won an industrial efficiency contest.

Why understanding the mechanism changes what chemists can design

Catalyst discovery and catalyst engineering are different stages. Once a reaction is known to work, mechanistic understanding tells researchers what to change. Should the ligand make molybdenum more electron-rich? Should the proton donor be altered? Can a cheaper, recyclable electron source replace SmI₂? Which intermediate needs stabilization, and which transition state should be lowered?

The new answer is that imide formation is the most demanding point and that successful PCET requires a finely tuned combination of reducing power and proton-donating strength. That gives catalyst designers a much more concrete target than “make the catalyst faster.”

There will not be a single route to low-carbon ammonia

Decarbonizing ammonia is likely to involve several technologies. Renewable hydrogen can feed conventional Haber–Bosch plants. Fossil-derived hydrogen can be paired with carbon capture and storage. Researchers are also developing electrochemical, photochemical and plasma routes, alongside molecular catalytic nitrogen fixation.

The IEA’s net-zero scenarios require a rapid expansion of near-zero-emission ammonia production, including electrolysis-based hydrogen and carbon capture, while also depending on continued innovation in technologies that are not yet commercially mature.^[5]

Ammonia is becoming an energy molecule as well as a fertilizer feedstock

The stakes are rising because ammonia is being considered not just as the basis of nitrogen fertilizer but also as an energy carrier. NH₃ can transport hydrogen in a form that is easier to liquefy than pure hydrogen, and Japan and other countries have explored ammonia for power generation and marine fuel. Those applications remain contested because combustion can create nitrogen oxides and because climate performance depends heavily on how the ammonia itself is produced.

If new energy uses materially increase demand, low-carbon production becomes even more important. Expanding conventional fossil-based ammonia output to serve energy markets would undermine much of the climate rationale.

From high-pressure engineering to molecular control

Haber and Bosch solved nitrogen fixation industrially by mastering catalysts, heat, pressure and materials engineering. More than a century later, chemists are trying to solve the same molecular problem by controlling electrons and protons one step at a time.

The 2026 work does not deliver a commercial ammonia plant. It delivers something more fundamental: a mechanistic map. It identifies how the catalyst takes nitrogen apart, how hydrogen is delivered from water, where the highest energetic hurdle lies and how the catalyst regenerates.

The next challenge is to turn that understanding into chemistry that uses cheaper and recyclable electron sources, longer-lived catalysts and processes that remain efficient when energy and materials are counted end to end. But every industrial technology begins with understanding what the molecules are actually doing. For this molybdenum system, the black box is now considerably more transparent.