Transparent ZrO2 Aerogel Spheres Turn Sunlight Into Heat for CO2 Recycling

The chemical industry sits among the most stubborn sources of carbon dioxide on the planet, responsible for roughly one gigatonne of annual emissions from the heat it consumes and another gigatonne from the fossil-derived feedstocks it converts into products. Together, these two burdens account for about five percent of global greenhouse gas output, and both problems share an uncomfortable root: almost every industrial chemical transformation needs heat, and nearly all of that heat still comes from burning coal, oil, or gas. A team at ETH Zurich now reports a catalytic platform that attacks both problems at once, using concentrated light to heat a catalyst directly and deploying that self-heating catalyst to recycle carbon dioxide into carbon monoxide, a versatile building block for fuels and chemicals. The work, published in Advanced Science, describes translucent zirconia aerogel spheres studded with palladium nanoparticles that outperform conventional powder catalysts by nearly a factor of four under identical illumination.

The appeal of photothermal catalysis lies in its efficiency arithmetic. Most chemical processes demand either low-temperature heat below 150 degrees Celsius or medium-temperature heat between 150 and 400 degrees Celsius. Techno-economic studies identify heat pumps powered by renewable electricity as the cheapest option for the low-temperature range, but the medium-temperature band remains difficult to decarbonize. When sunlight is the primary renewable resource, the conventional route of converting photons to electricity and then electricity to heat suffers substantial losses; total light-to-heat efficiencies often fall below 20 percent for resistive heating and below 15 percent when electrolytic hydrogen is burned. Concentrated solar thermal systems, which convert sunlight directly into heat, can reach efficiencies of up to 60 percent. Photothermal catalysis pushes this logic one step further by eliminating the separate heat-transfer loop altogether: the catalyst itself absorbs the light and becomes the reactor’s heater.

Translating that concept from laboratory model systems to scalable technology has been the central obstacle. Many of the most celebrated photothermal catalysts in the literature are fabricated by colloidal lithography, self-assembly, or physical vapor deposition, producing highly regular nanostructures that are perfect for mechanistic studies but hold only vanishingly small catalyst volumes. Powder-supported alternatives scatter light so strongly that photons penetrate only tens of micrometers, leaving most of the catalyst bulk cold and idle. The Zurich team, led by researchers in the laboratory of Marcos Niederberger, reasoned that transparent aerogels could resolve this dilemma. Aerogels combine minimal light scattering with high surface area and open porosity, and they possess exceptionally low thermal conductivity, so heat generated deep inside the material stays there instead of leaking away. Previous work had incorporated metal nanoparticles into aerogels by co-gelation, but that route requires pre-synthesizing the nanoparticles and carefully controlling their dispersibility, which is difficult to scale. Standard wet impregnation, meanwhile, tends to destroy transparency or collapse the delicate pore network.

The new strategy hinges on a simple electrostatic trick performed in water. The team first synthesized zirconia nanocrystals of roughly three nanometers by a nonaqueous sol-gel route, then gelled them into uniform translucent spheres about a millimeter in diameter by extruding a partially gelled droplet through a syringe into heated silicone oil. Surface tension rounded each droplet into a glassy pearl before it solidified. Supercritical drying with carbon dioxide preserved the porosity, and calcination at 300 degrees Celsius removed residual organics. The resulting spheres retained about 92 percent porosity and a surface area of roughly 244 square meters per gram, far exceeding the 30 to 150 square meters per gram typical of conventional opaque zirconia powders. Because rewetting a dried gel collapses its pores, the metal had to be introduced before drying, and the researchers chose strong electrostatic adsorption using cheap, water-soluble palladium nitrate paired with ethylenediaminetetraacetic acid, or EDTA.

EDTA proved to be the linchpin of the method. At a pH of about 3.5, the zirconia surface carries a positive charge, while the Pd-EDTA complex is negatively charged, so the metal complex locks onto the gel surface electrostatically and survives the subsequent solvent exchange and supercritical drying. EDTA coordinates an unusually wide range of catalytically relevant metals and, crucially, prevents the hydroxide precipitation that plagues metals such as indium when ammonia-based complexation is attempted. Calcination cleanly burns the ligand away, and X-ray photoelectron spectroscopy confirmed that no nitrogen or significant carbon residues remained. Subsequent reduction in hydrogen generated metallic palladium nanoparticles with an average diameter of only about 1.5 nanometers, among the smallest values ever reported for supported palladium catalysts and well below the three to four nanometers typical of conventional impregnation or precipitation methods. The reduced spheres turned jet black, a visible sign that the palladium nanoparticles now absorbed light across the visible spectrum and could act as embedded nanoheaters.

Under concentrated white light from a high-power LED delivering 4.8 watts per square centimeter, roughly 48 suns, the catalyst converted a hydrogen and carbon dioxide feed into carbon monoxide through the reverse water-gas shift reaction. The researchers found that carbon monoxide production rose with palladium loading up to about 1.5 weight percent and then saturated, revealing a key design principle: once the nanoheaters absorb essentially all incident light, adding more metal yields no further heating and no further activity. The catalysts did deactivate, losing roughly 42 percent of their initial productivity within 16 hours on stream, a decline the team traced mainly to sintering of the palladium particles, which grew from about 1.5 to 3 nanometers, along with shrinkage and densification of the aerogel backbone. That densification is a deactivation pathway unique to photothermal operation, because a denser support conducts heat away faster and lowers the operating temperature by more than 10 degrees Celsius.

The most striking results came from adding indium. Co-depositing indium through the same EDTA route slowed deactivation dramatically, cutting it from about 42 percent to between 25 and 30 percent at indium loadings of one to two weight percent, and it pushed carbon monoxide selectivity above 99 percent, suppressing the parasitic formation of methane that would otherwise lock the carbon into a dead-end product. Electron microscopy suggested partial co-location of indium with palladium, and the authors attribute the improved selectivity to electronic and geometric effects: electron donation from indium weakens carbon monoxide adsorption on neighboring palladium atoms, while indium breaks up contiguous palladium ensembles that would otherwise bind carbon monoxide strongly enough to hydrogenate it all the way to methane. Over a 110-hour stability test, the optimized palladium-indium catalyst settled into a steady productivity of about 0.7 grams of carbon monoxide per gram of catalyst per hour, placing it at the upper end of reported photothermal reverse water-gas shift catalysts, which typically deliver between 0.1 and 0.6 grams per gram per hour.

Perhaps the most persuasive evidence for the aerogel concept came from a set of destruction experiments. The researchers took their intact spheres and ground them, gently in one case and thoroughly in ethanol in another, producing reference materials with identical chemistry but progressively destroyed architecture. The dry-ground aerogel reflected about 11 percent more light and ran roughly 10 degrees cooler, losing about 35 percent of its carbon monoxide productivity. The wet-ground sample, which resembled an ordinary powder catalyst, fared far worse: it ran about 44 degrees Celsius cooler than the intact spheres, a gap far too large to explain by reflectance alone, and produced only about 29 percent as much carbon monoxide. Cooling experiments confirmed that the collapsed materials dissipated heat faster, demonstrating that the aerogel’s twin gifts of transparency and ultralow thermal conductivity are what allow the entire catalyst bulk, not just its surface, to reach reaction temperature.

The broader implication is that photothermal catalysis need not remain a curiosity of precisely patterned model surfaces. By pairing a versatile aqueous impregnation chemistry with a support that lets light in and keeps heat trapped, the ETH Zurich team has outlined a scalable recipe for reactors in which sunlight both powers the process and supplies its carbon feedstock from recycled carbon dioxide. Challenges remain, particularly the gradual densification of the aerogel backbone and the residual sintering of the active metal, and the authors note that further compositional optimization is possible. But with a simple, inexpensive route to loading a wide range of metals into translucent, thermally insulating supports, and with productivity figures already competitive with the best photothermal systems reported, the path from glassy millimeter-sized pearls to industrial reactors in high-sunlight regions suddenly looks considerably shorter. For an industry searching for both clean heat and clean carbon, these self-heating spheres offer a rare two-for-one proposition.

Subject of Research: Development of translucent ZrO2 aerogel-supported Pd and PdIn nanoparticle catalysts for photothermal CO2 reduction via the reverse water-gas shift reaction

Article Title: ZrO2 Aerogel‐Supported Pd Nanoparticles for Photothermal CO2 Reduction

Article References: Kiwic, D., Räz, L., Tervoort, E., & Niederberger, M. (2026). ZrO 2 Aerogel‐Supported Pd Nanoparticles for Photothermal CO 2 Reduction. Advanced Science, 13(52), Article e76221. https://doi.org/10.1002/advs.76221

Image Credits: AI Generated

DOI: 10.1002/advs.76221

Keywords: photothermal catalysis, CO2 reduction, zirconia aerogel, palladium nanoparticles, reverse water-gas shift, strong electrostatic adsorption, solar-to-heat conversion, indium promotion, aerogel spheres, decarbonization, nanoparticle sintering, renewable process heat