Kyushu University researchers watched nickel nanoparticles during microwave irradiation and found them running far hotter than the material around them, which explains why microwave reactors outpace conventional furnaces.
Microwave heating has a reputation in chemistry for producing results that conventional furnaces cannot match, and a long-running argument about why. Sceptics have maintained that a microwave is just a heater, and that any apparent advantage reflects poor temperature measurement. Advocates have suspected that microwaves deposit energy unevenly, creating hot spots that drive reactions faster than the measured bulk temperature suggests.
Researchers at Kyushu University have now watched it happen. Writing in the Chemical Engineering Journal, a team led by Professor Shuntaro Tsubaki of the Faculty of Engineering reports that nickel catalyst particles under microwave irradiation reach temperatures up to 80 °C higher than the material surrounding them, and that converting biomass to hydrogen proceeded more than six times faster than with conventional heating.
“Our findings provide rare, experimentally grounded insight into how microwaves interact with catalytic nanoparticles during reactions,” Tsubaki said.
Bamboo, cellulose and a commercial catalyst
The feedstocks were cellulose and bamboo powder, mixed with a commercial nickel catalyst supported on silica-alumina. The process is catalytic pyrolysis: heating organic material in the absence of oxygen so that it breaks down, with a catalyst steering the products toward hydrogen and other light gases rather than tars and char.
Bamboo is a sensible test material. It is a lignocellulosic biomass, built from cellulose, hemicellulose and lignin like wood and agricultural residues, and it grows fast enough to be harvested repeatedly. Converting such material to fuel gases has long been attractive and persistently difficult, because the reactions need high temperatures and the equipment to supply them is large and slow.
Rather than a conventional magnetron, the kind in a kitchen microwave, the team used a solid-state generator with precisely controlled frequency. Tuning the frequency matters because how strongly a material absorbs microwaves depends on it, and a controllable source allows the energy to be directed into the catalyst rather than scattered through the reactor.
Watching the catalyst while it works
The methodological contribution is the measurement. The team used in situ X-ray absorption fine structure, or XAFS, and in situ X-ray diffraction to monitor the catalyst during irradiation, rather than examining it before and after.
XAFS probes the local environment of a particular element’s atoms, revealing oxidation state and the arrangement of neighbours, and the signal is temperature-sensitive, which is how the team inferred that the nickel was hotter than its surroundings. X-ray diffraction reports on crystal structure and particle size. Doing both while microwaves were running required a synchrotron, a facility producing X-rays intense enough to collect data in the time a reaction takes.
This is why the dispute about microwave effects persisted for so long. A thermocouple or infrared camera reports the temperature of the bulk material or the reactor wall, not of a nanoparticle. If the particle is much hotter, a comparison between microwave and furnace heating at the same measured temperature is not comparing equivalent conditions.
The argument has a name in the literature: whether there are “non-thermal microwave effects” distinct from simple heating. Much of the dispute has turned out to be about measurement rather than physics, and work like this reframes the question, since a particle genuinely hotter than its surroundings is a thermal effect, just not the one the thermometer reports.
Sintering at 350 °C instead of 1,000
The clinching evidence came from an unwelcome phenomenon. Under microwave irradiation, the nickel particles sintered, fusing together, at bulk temperatures of 350 to 400 °C. Conventional heating normally requires 1,000 °C or higher to make nickel do that.
Sintering is a physical process that depends on the temperature of the particles themselves, not on what an instrument elsewhere in the reactor records. Particles fusing at a bulk temperature 600 °C below the usual threshold is difficult to explain except by concluding that the particles were far hotter than the measurement showed. The researchers took it as confirmation that the actual nickel surface temperature was substantially higher than external measurements indicated.
It is also a practical problem. Sintered catalyst particles have less surface area, and a catalyst with less surface area is less active. Any process built on selective microwave heating will have to manage the degradation that the same mechanism causes, which is a trade-off the paper identifies rather than solves.
Why selective heating helps
The advantage of concentrating heat in the catalyst is that chemistry happens at the catalyst surface. In a conventional furnace, the entire reactor and its contents must be brought to temperature, which consumes energy, takes time, and exposes the biomass to prolonged heat that can push reactions toward unwanted products.
Heating the catalyst selectively means the reactive sites are hot while the bulk stays cooler. That is where the sixfold acceleration comes from, and it also means less energy spent warming material that does not need to be warm.
Small, electric, local
The application Tsubaki emphasises is decentralised.
“One promising application is the rapid, electricity-driven production of hydrogen and light gases from agricultural and forestry residues, enabling compact, local energy systems that do not rely on large external furnaces,” he said.
That addresses a persistent economic difficulty with biomass. Crop and forestry residues are bulky and widely dispersed, and hauling them to a central plant can consume much of the energy they contain. A process that works in small, electrically driven units could be sited where the material is.
Being electrically driven matters for emissions too. Industrial furnaces generally burn fuel; a microwave reactor runs on electricity, which can come from renewable sources, which is why the release frames the work as a step toward electrified chemical manufacturing.
The researchers also suggest extending the approach to breaking down mixed plastic waste and producing functional carbon materials. “These applications illustrate how MW-driven catalysis can contribute simultaneously to sustainable energy production and advanced materials development,” Tsubaki said.
Mixed plastic waste is a plausible target for the same reason biomass is: it is a heterogeneous organic feedstock that current recycling handles poorly, and processes that tolerate variable input are what the problem calls for.
The work involved collaborators at the University of Osaka, Institute of Science Tokyo, the High Energy Accelerator Research Organization and International Christian University, and was supported by the Japan Science and Technology Agency and the Japan Society for the Promotion of Science.
Notes
- “Microwave-Triggered Local High-Temperature Formation on Ni Nanoparticles that Enables Ultra-Fast Catalytic Pyrolysis of Lignocellulose.” Chemical Engineering Journal, 2026. DOI: 10.1016/j.cej.2026.179202.
- Kyushu University. “Selective microwave heating speeds bamboo-to-hydrogen fuel conversion.” EurekAlert!, 3 August 2026. https://www.eurekalert.org/news-releases/1138692.