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

A comprehensive new review published in Discover Green Chemistry argues that a quiet revolution is underway. Researchers led by Xinli Tang, Huayue Sun, and Jiankui Sun of North China University of Science and Technology systematically survey two decades of progress in chemically catalyzing the conversion of biomass, especially non-edible lignocellulosic material such as straw, wood, and agricultural waste, into lactic acid. Their analysis organizes the field into three competing routes: alkaline catalysis, acid catalysis, and an emerging family of photocatalytic and photothermal approaches that harness sunlight to drive the reaction at room temperature. Each route, the authors conclude, shares a common reaction network but differs in which step limits the overall rate, a unifying insight that could accelerate catalyst design across the entire field.

That shared network begins with sugars. Glucose, a six-carbon aldose, must first be isomerized into fructose, a transformation that Lewis acid sites catalyze through an intramolecular hydride shift known as the Lobry de Bruyn–van Ekenstein rearrangement. Fructose then undergoes retro-aldol cleavage, splitting into two three-carbon fragments, dihydroxyacetone and glyceraldehyde. These trioses dehydrate to pyruvaldehyde, which finally rearranges into lactic acid via a 1,2-hydride shift. Because fructose skips the isomerization step, it consistently outperforms glucose under identical conditions, while xylose, a five-carbon sugar, inevitably sacrifices part of its carbon skeleton to glycolic or formic acid, capping its lactic acid yield. The review reports yields exceeding 70 percent for glucose and up to 97 percent under optimized acid catalysis, but warns that raw lignocellulose typically delivers less than 50 percent because the recalcitrant lignin matrix blocks catalyst access and poisons active sites.

Alkaline catalysis, the oldest chemical route, exploits strong bases such as sodium and potassium hydroxide to cleave carbon-carbon bonds under hydrothermal conditions at or above 473 kelvin. Early work by Yan and colleagues showed that calcium and barium hydroxides form transition complexes with sugar intermediates, promoting selective C3–C4 bond cleavage, and that cellulose and starch could be converted directly to lactic acid in yields near 19 percent. More strikingly, Li’s group later achieved a 95.4 percent lactic acid yield from glucose at room temperature under anaerobic conditions, using barium hydroxide both as catalyst and as a reactant that traps the product as barium lactate. Yet the route carries a heavy price: high alkali concentrations generate salt waste, corrosion, and costly neutralization steps, and homogeneous bases cannot be recycled at all, making the economics unattractive for large-scale production.

Heterogeneous base catalysts attempt to resolve these problems. Layered double hydroxides of magnesium and aluminum, for example, enabled Albuquerque and colleagues to convert hydroxyacetone to lactic acid with 100 percent selectivity at just 40 degrees Celsius, using a recyclable solid base that eliminates neutralization entirely. Copper-based systems have proven particularly versatile: CuO supported on zirconia achieved complete glycerol conversion with 94.6 percent lactic acid selectivity, while copper oxide loaded on magnesia delivered a 70 percent yield from glucose at a relatively mild 393 kelvin. Glycerol itself, a cheap byproduct of biodiesel production, has emerged as a star feedstock, with noble-metal and copper catalysts converting it to lactic acid at yields of 80 to 96 percent, its simple C3 structure sidestepping the isomerization bottleneck that plagues six-carbon sugars.

Acid catalysis, however, is where the review places its strongest bet. Lewis acid zeolites, metal oxides, and rare-earth catalysts convert carbohydrates directly in water without the neutralization burden of alkaline chemistry. Tin-substituted beta zeolite, whose isolated tetrahedral Sn4+ sites act as water-tolerant Lewis acids, achieved a 67.1 percent lactic acid yield from glucose, while hierarchical zirconium zeolites reached 67.9 percent from xylose. Dealuminated ZSM-5 supported with erbium pushed yields to 69.1 percent by suppressing the formation of humins, the insoluble carbonaceous byproducts that plague sugar conversion. Rare-earth metals proved even more striking: erbium chloride delivered lactic acid from cellulose at yields up to 91 percent, and ytterbium chloride converted sugarcane bagasse to lactic acid within 15 minutes. Computational studies showed that heavier lanthanide ions lower the energy barrier for the critical C3–C4 bond cleavage, explaining their exceptional activity.

The most eye-catching numbers, though, come from the newest branch of the field: photocatalysis and its hybrid cousin, photothermal catalysis. Cao and colleagues developed a nitrogen-doped titanium dioxide catalyst that produced lactic acid from sugars with a 98.9 percent yield at just 60 degrees Celsius within 30 minutes under visible light. Huang’s team engineered a highly crystalline carbon nitride with structural oxygen that converted glucose at room temperature in 50 minutes, while Liu’s triazole-modified carbon nitride delivered yields of 85.5 to 98.3 percent from various sugars with 98.6 percent selectivity. Life cycle assessments attached to these systems are remarkable: the fluorine-doped carbon nitride route was calculated to generate only 0.7 kilograms of carbon dioxide equivalent per kilogram of lactic acid, roughly one-sixth of the petrochemical route, with an 87.8 percent reduction in fossil resource depletion.

Photocatalysis has historically been hobbled by poor selectivity. Mechanistic work by Zhang and colleagues revealed why: on pristine titanium dioxide, pyruvaldehyde preferentially follows low-barrier proton-coupled electron transfer pathways, producing unwanted C3 oxygenates, while the selective hydride shift to lactic acid faces a barrier of 1.22 electron volts. The solution proved elegant. By introducing oxygen vacancies that create Lewis acid sites and adding plasmonic gold nanoparticles that convert absorbed light into localized heat, the researchers steered the reaction toward the desired Cannizzaro-type pathway, achieving more than 90 percent lactic acid selectivity, a 3.4-fold improvement. Similar atomic-level heterojunctions, such as copper–sulfur moieties embedded in a cadmium zinc sulfide host, boosted glycerol conversion tenfold with selectivity above 95 percent, demonstrating that rational catalyst architecture can overcome the intrinsic kinetic limitations of light-driven chemistry.

Economics and durability remain the field’s stubborn obstacles. A landmark techno-economic assessment based on a 50,000-ton-per-annum plant suggested that erbium chloride-catalyzed glucose conversion could deliver an internal rate of return above 20 percent, but only if the expensive rare-earth catalyst is efficiently recovered and reused. Metal leaching from zeolites in hot water, carbonaceous fouling of oxide surfaces, and photocorrosion of semiconductors all erode catalyst lifetimes, and the review proposes a stability ranking that places zirconia and niobia at the top, followed by tin zeolites, carbon nitride photocatalysts, and layered double hydroxides. The authors argue that acid catalysis currently offers the best near-term balance of yield, feedstock flexibility, and practicality, while photocatalysis represents the most sustainable long-term option, pending breakthroughs in quantum efficiency and compatibility with real, untreated biomass.

What emerges from this sweeping analysis is a field in transition, moving from model sugars toward genuine waste streams, from precious metals toward abundant copper, zinc, and aluminum, and from brute-force heating toward sunlight-driven, carbon-negative chemistry. If researchers can marry the anti-leaching catalyst designs and standardized regeneration protocols the review calls for with the ambient-condition promise of photothermal systems, lactic acid could shift from a fermentation commodity to a cornerstone of the sustainable bioeconomy, and the biodegradable plastics built from it may finally compete with, and replace, the petrochemical polymers that now choke the planet.

Subject of Research: Chemo-catalytic conversion of biomass into lactic acid using alkaline, acid, and photocatalytic processes

Article Title: Research progress in the preparation of lactic acid from biomass by chemical catalytic process

Article References: Tang, X., Sun, H., Shi, Q., Zheng, D., Xie, J., & Sun, J. (2026). Research progress in the preparation of lactic acid from biomass by chemical catalytic process. Discover Green Chemistry, 1(1), Article 36. https://doi.org/10.1007/s44509-026-00038-8

Image Credits: AI Generated

DOI: 10.1007/s44509-026-00038-8

Keywords: lactic acid, biomass, catalysis, Lewis acid, photocatalysis, photothermal catalysis, polylactic acid, lignocellulose, zeolites, rare-earth catalysts, green chemistry, bioplastics