The ternary Fe-Co-Mn composition was chosen deliberately. Iron and cobalt are the two most active Fischer-Tropsch metals, and previous work has shown that combining them produces product streams richer in olefins and alcohols than either metal alone. Manganese, a well-known iron promoter, enhances selectivity toward valuable light olefins while suppressing the formation of unwanted methane. Yet even the best metal formulation can be undermined by the support. Conventional oxide supports such as silica, alumina, titania, and zeolites can react with iron and cobalt nanoparticles during high-temperature treatment, forming hard-to-reduce silicates and other mixed compounds that remain catalytically inactive. Carbonaceous supports, being comparatively inert, sidestep this problem while offering high surface areas, good thermal conductivity, and chemical stability.

To isolate the effect of the support, the team prepared all three catalysts by the same impregnation route, using nitrate precursor salts at an equimolar Fe:Co:Mn ratio of 1:1:1, with a total metal loading of 30 weight percent. The supports were first treated with nitric acid under reflux to clean and functionalize their surfaces, then dried, impregnated, and calcined at 500 degrees Celsius under nitrogen. Each catalyst was subsequently tested in a fixed-bed reactor at 220 degrees Celsius, 20 bar, and a hydrogen-to-carbon monoxide ratio of 2, conditions representative of industrial Fischer-Tropsch operation.

The performance gap between the supports was striking. The graphene-supported catalyst converted 68.7 percent of the carbon monoxide feed, while the carbon nanotube-supported version fell in between and the activated-carbon-supported catalyst managed only 37.9 percent. Methane selectivity, an undesirable byproduct, rose from 11.2 percent on graphene to 17.7 percent on activated carbon, and selectivity toward the valuable C5-plus liquid hydrocarbons dropped from 76.3 percent to 67.9 percent across the same comparison. Apparent reaction rates followed the same ordering: the graphene-supported catalyst achieved the highest carbon monoxide consumption rate at 1.48 times ten to the minus four millimoles per gram of catalyst per minute, along with the highest hydrocarbon formation rate. When the best performer was subjected to a 122-hour stability test, it maintained its activity and selectivity with only minor variations, a promising sign for continuous operation.

Why does graphene win? The answer, assembled from an extensive characterization campaign involving X-ray diffraction, nitrogen physisorption, temperature-programmed reduction, X-ray photoelectron spectroscopy, electron microscopy, and infrared spectroscopy, is that the support shapes nearly every property that matters. Nitrogen adsorption measurements showed that graphene conferred by far the largest surface area, 567.88 square meters per gram after metal loading, compared with 275.93 for the carbon nanotube catalyst and 181.51 for the activated carbon version. Pore volume followed the same trend, reaching 1.38 cubic centimeters per gram on graphene. Although metal incorporation reduced surface area and pore volume on all three supports by partially occupying pore spaces, graphene’s layered structure, with its interlayer and interparticle voids, preserved the most accessible architecture, facilitating reactant transport and maximizing the catalyst-reactant interfacial area.

Electron microscopy added a second piece of the puzzle. Transmission electron images revealed thin, wrinkled graphene sheets with nanoparticles anchored across their surfaces and edges, whereas the nanotube and activated carbon samples showed progressively more particle aggregation. Measuring one hundred nanoparticles per sample, the team found the most frequent particle sizes were approximately 40 nanometers on graphene, 60 on carbon nanotubes, and 70 on activated carbon. Smaller particles expose more metal surface per unit mass, increasing the number of available active sites. Selected area electron diffraction patterns and lattice fringe imaging further confirmed the multicrystalline nature of the metal phases on all three supports.

The reduction behavior, critical because the oxidized catalyst precursors must be converted to active metallic species before the reaction, also depended strongly on the support. Temperature-programmed reduction showed that the activated-carbon catalyst was the hardest to activate: its reduction began only around 350 degrees Celsius and stretched to about 750 degrees in one broad, overlapping peak. The carbon nanotube catalyst displayed four distinct reduction steps starting near 190 degrees, corresponding to the stepwise reduction of cobalt oxide to metallic cobalt and iron oxide through magnetite and wustite to metallic iron. The graphene catalyst reduced most favorably, beginning near 170 degrees with clearly resolved peaks and a total hydrogen consumption of 5.36 millimoles per gram, intermediate between the carbon nanotube catalyst’s 3.12 and the activated carbon catalyst’s 6.97, but achieved over lower and more accessible temperature windows.

Surface chemistry provided a third differentiator. X-ray photoelectron spectroscopy of the freshly calcined catalysts revealed that the graphene-supported sample carried the highest proportions of lower-valence metal species at its surface, with a Co2+/Co3+ ratio of 1.50 and an Fe2+/Fe3+ ratio of 0.78, compared with 0.82 and 0.43 respectively for the activated carbon catalyst. Manganese existed mainly in the trivalent state on all three samples. While these measurements reflect the state after calcination rather than during the reaction itself, they indicate that the local chemical environment created by graphene alters the oxidation-state distribution of the active species, a factor closely tied to metal-support interactions and, ultimately, to catalytic behavior.

Structural analysis filled in the remaining details. X-ray diffraction confirmed that the graphitic structure of all three supports survived catalyst preparation, with the characteristic carbon reflection near 26.5 degrees two-theta retained in every pattern. Reflections consistent with iron, cobalt, and manganese oxide and spinel-type phases were observed, but heavy overlap in the 30 to 36 degree region prevented unambiguous phase assignment, a common challenge in multicomponent oxide systems. Notably, no distinct crystalline metal-support compound was detected, supporting the idea that carbon supports avoid the inactive compound formation that plagues oxide supports. Infrared spectroscopy corroborated this picture, detecting metal-oxygen vibrations in the 400 to 800 per centimeter region while confirming that the sp2-bonded carbon lattice, signaled by the unchanged C=C band near 1600 per centimeter, remained intact after metal incorporation.

Taken together, the study demonstrates that no single property explains graphene’s advantage; rather, its high surface area, generous pore volume, smaller metal particles, easier reducibility, and favorable surface oxidation states act in concert. The authors caution that these correlations should not be read as evidence of one dominant factor, since the support simultaneously influences multiple characteristics, and they suggest that future work should probe the reaction mechanism over each support to fully unravel the origins of the performance differences. For a field seeking cleaner routes to liquid fuels from syngas, the message is clear: when it comes to carbon supports for Fe-Co-Mn Fischer-Tropsch catalysts, the flat, two-dimensional architecture of graphene offers a measurable edge over its tubular and porous rivals.

Subject of Research: Effect of carbonaceous support materials on Fe-Co-Mn nanocatalyst performance in Fischer-Tropsch CO hydrogenation

Article Title: Effect of carbonaceous supports on the performance and characteristics of Fe-Co-Mn nanocatalysts for CO hydrogenation

Article References: Arsalanfar, M., Ostovar, S., Rezazadeh, E., Abdouss, M., & Milad Jourshabani (2026). Effect of carbonaceous supports on the performance and characteristics of Fe-Co-Mn nanocatalysts for CO hydrogenation. Results in Engineering, 32, Article 113159. https://doi.org/10.1016/j.rineng.2026.113159

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113159

Keywords: Fischer-Tropsch synthesis, graphene support, Fe-Co-Mn catalyst, CO hydrogenation, syngas, nanocatalysts, activated carbon, carbon nanotubes, temperature-programmed reduction, XPS, BET surface area, liquid fuels

To understand why this matters, it helps to recall how a dye-sensitized solar cell actually works. In these devices, light is absorbed by dye molecules attached to a nanostructured titanium dioxide photoanode. Excited electrons are injected into the titanium dioxide and travel through an external circuit to the counter electrode. Meanwhile, the oxidized dye is regenerated by an electrolyte, usually containing the iodide-triiodide redox couple, which itself must be regenerated at the counter electrode. The counter electrode therefore has to catalyze the reduction of triiodide back to iodide efficiently; if it does not, the whole cycle stalls and the cell’s fill factor and efficiency suffer. Platinum has been the benchmark catalyst for this reaction because of its exceptional activity and stability, but it is scarce, expensive and increasingly unattractive for mass-market deployment.

The search for platinum alternatives has become one of the most active corners of dye-sensitized solar cell research. Over the past decade, researchers have explored transition metal sulfides, selenides, nitrides, carbides and oxides as catalytic counter electrode materials, often pairing them with conductive carbon scaffolds such as graphene, carbon nanotubes or activated carbon. The logic is straightforward: the catalyst provides the electrochemical activity, while the carbon provides the electrical conductivity and surface area needed to shuttle electrons and expose active sites to the electrolyte. Ferrites—mixed metal oxides with the general formula MFe2O4—have emerged as promising candidates because they combine reasonable electrocatalytic activity with chemical stability and low cost.

The new study, led by D. Sengeni of C.K. College of Engineering and Technology in Cuddalore, Tamil Nadu, with colleagues from several Chennai institutions, takes this approach a step further by sourcing the carbon scaffold from renewable agro-food waste rather than from commercially produced activated carbon. The team converted Bean Curd Stick waste into porous biocarbon and then combined it with nickel ferrite, NiFe2O4, a spinel oxide in which nickel and iron ions occupy the crystal lattice in a way that supports electrocatalytic activity. The novel aspect of the study, according to the authors, is precisely this utilization of renewable agro-food waste as a conductive carbon matrix to fabricate a high-performance ferrite-carbon hybrid electrode.

Structural and compositional analyses confirmed the successful formation of the NFO@BC composite, while microscopic studies revealed the uniform anchoring of NiFe2O4 nanoparticles on the porous biocarbon surface. That uniformity is not merely cosmetic. In hybrid counter electrodes, the distribution of catalyst particles across the conductive scaffold determines how effectively electrons can reach active sites. If the ferrite particles cluster together, large portions of the carbon surface remain catalytically inert and the electrolyte cannot access much of the catalyst. Uniform anchoring, by contrast, means that nearly every ferrite nanoparticle is electrically connected to the carbon network and simultaneously exposed to the iodide-triiodide electrolyte, maximizing the density of accessible electrocatalytic sites.

One of the most striking quantitative results from the study comes from Brunauer-Emmett-Teller, or BET, surface area analysis, which revealed a specific surface area of 286.7 square meters per gram for the NFO@BC composite. For an electrode material, that is a substantial figure, and it has direct electrochemical consequences. A high surface area provides abundant accessible electrocatalytic sites and improved contact with the electrolyte, allowing the triiodide reduction reaction to proceed rapidly at the electrode-electrolyte interface. Porosity also matters for ion transport: the pores within the biocarbon matrix create pathways through which the electrolyte can penetrate the electrode, shortening diffusion distances and reducing the charge-transfer resistance that would otherwise limit current output.

The authors attribute the enhanced photovoltaic performance of their cells to the synergistic effect of the conductive biocarbon and the electrocatalytically active NiFe2O4. This division of labor is the central design principle of the hybrid. The biocarbon, derived from biomass, acts as a three-dimensional conductive network that collects electrons arriving from the external circuit and distributes them across the electrode. The nickel ferrite nanoparticles, dispersed across that network, serve as the catalytic workhorses where the triiodide ions are actually reduced. Neither component alone would perform as well: bare biocarbon lacks the catalytic activity of the ferrite, while bare nickel ferrite suffers from the modest electrical conductivity typical of spinel oxides. Together, they address each other’s weaknesses.

The broader context of this work is the push to commercialize dye-sensitized solar cells, which the authors identify as the motivation for developing inexpensive, effective and sustainable platinum-free counter electrodes. Dye-sensitized cells are attractive for applications where conventional panels struggle—indoor lighting, building-integrated photovoltaics, and devices that benefit from semi-transparency and flexibility. But every cost component counts. Platinum loading on a counter electrode may seem like a small fraction of a cell’s material budget, yet at the scale of gigawatt production the price of platinum becomes prohibitive, and its supply chain carries significant environmental and geopolitical burdens. Replacing it with a material synthesized from food waste and abundant iron and nickel compounds changes that calculus fundamentally.

The choice of Bean Curd Stick waste also fits into a growing body of research on biomass-derived carbons for energy devices. Biomass precursors are attractive because they are renewable, widely available and often rich in the heteroatoms that can enhance the catalytic properties of the resulting carbon. Previous work by some of the same research community has examined heteroatom doping in Bean Curd Stick-derived porous carbon for platinum-free counter electrodes, and other groups have used bamboo-derived activated carbon in similar ferrite composites. The present study extends this line of inquiry by demonstrating that the biocarbon matrix can host nickel ferrite nanoparticles uniformly, creating a composite whose texture and chemistry are both favorable for triiodide reduction.

The implications reach beyond a single laboratory result. If hybrid electrodes of this kind can be produced reliably from waste streams, dye-sensitized solar cells could move closer to the cost profile that made them famous after their invention by Brian O’Regan and Michael Grätzel in 1991. The study also illustrates a broader trend in materials science: the convergence of waste valorization and clean energy technology, where one environmental problem—food waste—is enlisted to solve another, the cost and scarcity of critical materials in renewable energy hardware. The authors conclude that NFO@BC stands out as a low-cost, sustainable and efficient platinum-free counter electrode for dye-sensitized solar cell applications, and their results suggest that the humble tofu stick, of all things, may have a role to play in the solar panels of the future.

Subject of Research: Pt-free dye-sensitized solar cell counter electrodes based on NiFe2O4 and biomass-derived biocarbon

Article Title: Development of NiFe2O4@Biocarbon hybrid counter electrode for efficient Pt-free dye-sensitized solar cells

Article References: Sengeni, D., Vinayagam, P., Karuppiah, S., Velmurugan, V., Bindhu, M., & Sevvanthi, S. (2026). Development of NiFe2O4@Biocarbon hybrid counter electrode for efficient Pt-free dye-sensitized solar cells. Ionics. https://doi.org/10.1007/s11581-026-07557-1

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07557-1

Keywords: dye-sensitized solar cells, counter electrode, NiFe2O4, biocarbon, platinum-free, Bean Curd Stick waste, nickel ferrite, biomass-derived carbon, triiodide reduction, photovoltaics, BET surface area, agro-food waste

The research team, led by Rim Ismail, Sabah Mariyam, Mohammad Alherbawi, and Tareq Al-Ansari, framed their work as a waste-to-waste valorization strategy. Rather than treating sludge, farm residues, and food waste as disposal problems, they converted each into functional materials and then used one to upgrade another. Three feedstocks were compared: dried sewage sludge pellets supplied by Qatar’s Public Works Authority, mixed dried agricultural residues from a local farm, and fresh banana peels from a single purchase batch. Each material was ground, homogenized, and sieved to particles below 120 micrometers to eliminate heat and mass transfer limitations, then characterized through proximate and ultimate analysis, calorimetry, electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy.

The compositional differences among the feedstocks proved decisive. Banana peel stood out with the highest volatile matter content at 87.7 percent and the highest calorific value at 17.7 megajoules per kilogram, alongside a remarkably low ash fraction of just 7.52 percent. Sewage sludge, by contrast, contained 28.79 percent ash and only 31.10 percent carbon, yielding a modest heating value of 15.77 megajoules per kilogram. X-ray diffraction revealed that sludge was dominated by crystalline quartz, calcite, and calcium phosphate phases, while banana peel showed a broad amorphous hump typical of cellulose-rich biomass. Energy-dispersive spectroscopy also detected a notable potassium signal of 3.06 percent on the banana peel surface, a detail that would later prove central to the catalytic story.

Biochars were produced in a fixed-bed quartz reactor at 600 degrees Celsius under two atmospheres: inert nitrogen and reactive carbon dioxide. The carbon dioxide atmosphere enables in-situ activation through the Boudouard reaction, in which carbon reacts with carbon dioxide to form carbon monoxide, enlarging micropores and introducing oxygenated surface groups. Sewage sludge produced the highest biochar yields, reflecting its inorganic content, while agricultural residues produced the lowest, consistent with their readily decomposed cellulose and hemicellulose fractions. Thermogravimetric analysis showed that under carbon dioxide, the lignocellulosic feedstocks suffered sharp additional mass loss above roughly 700 degrees Celsius, whereas the mineral-rich sludge char resisted high-temperature conversion. All single-step biochars, however, exhibited disappointingly low surface areas below 4.2 square meters per gram, underscoring the limits of pyrolysis alone.

To unlock real porosity, the team applied a two-step chemical activation. Each biochar was impregnated with a one molar potassium carbonate solution at a one-to-three biochar-to-activator weight ratio, dried, and then heated to 700 degrees Celsius for two hours under nitrogen. Critically, the researchers skipped the acid-washing step commonly used to demineralize activated carbons, deliberately retaining potassium species within the carbon matrix. The result was a family of K-modified activated biochars rather than fully demineralized carbons. Scanning electron microscopy revealed dramatic transformations: the activated banana peel biochar developed a honeycomb-like network of thin walls and interconnected pores, while sludge-derived samples remained fragmented but mineral-dominated.

The headline result came from gas adsorption measurements. Potassium carbonate activation catapulted the surface area of carbon-dioxide-derived banana peel biochar from 3.17 to 311.9 square meters per gram, a hundredfold increase, with t-plot analysis confirming that micropores accounted for roughly 90.8 percent of that area. X-ray diffraction showed near-complete amorphization of the material, and X-ray photoelectron spectroscopy revealed an enhanced surface potassium signal. Yet the other feedstocks barely responded; sludge and agricultural residue biochars gained only a few square meters per gram, because their mineral contents hindered activator penetration and promoted side reactions. The banana peel material, BPBK-CO2, was selected for catalytic testing alongside its non-activated counterpart, BPB-CO2.

The proof-of-concept application targeted hydrothermal liquefaction of sewage sludge, a process that converts wet biomass into biocrude at 250 to 370 degrees Celsius and elevated pressures without the energy penalty of drying. In a 500-milliliter batch reactor, 30 grams of sludge were processed with 170 milliliters of water at 310 degrees Celsius for 45 minutes, reaching autogenous pressures near 100 bar in subcritical water. Biochar catalysts were added at 5 weight percent of the dry feedstock. The catalyst-free baseline produced a biocrude yield of 25.75 percent. Adding the non-activated banana peel biochar raised this to 37.88 percent, while the activated, high-surface-area version yielded 31.35 percent, a striking inversion that challenged the assumption that porosity governs catalytic performance.

The explanation lay in potassium. Inductively coupled plasma analysis showed the non-activated biochar contained 189.3 milligrams of potassium per gram, more than double the 79.6 milligrams per gram in the activated material. During liquefaction, aqueous-phase potassium rose 3.56-fold over baseline with the non-activated char and 2.03-fold with the activated one, and biocrude yield tracked the aqueous potassium concentration almost perfectly. The researchers propose that dissolved alkali species promote hydrolysis, depolymerization, and decarboxylation of sludge macromolecules while suppressing the repolymerization of reactive intermediates into solids, with surface-associated potassium providing complementary basic sites. Consistent with this mechanism, carbon dioxide in the aqueous phase increased dramatically, from 17.47 percent of the headspace profile at baseline to over 56 percent with either biochar, signaling enhanced decarboxylation.

Biocrude quality improved across the board. The higher heating value rose from 30.15 megajoules per kilogram at baseline to 31.13 with the non-activated char and 31.86 with the activated char, both within the typical range for liquefaction biocrudes. Oxygenated compounds in the oil decreased from 43.18 percent to roughly 36 percent with both catalysts, a shift that could reduce hydrogen demand during downstream hydrodeoxygenation and improve storage stability. Gas chromatography-mass spectrometry also revealed a strong shift toward lighter compounds: the fraction below C8 grew from 41.75 percent at baseline to 57.75 to 64.15 percent with the biochars. Notably, the activated biochar retained a larger share of the biocrude in the C8 to C16 range, the hydrocarbon window most relevant to jet fuel, suggesting activation moderated over-cracking by limiting potassium release.

The study’s authors are careful to frame these results as a paired proof of concept rather than a comprehensive screening, and they acknowledge open questions, including the need for soluble-potassium and potassium-free controls, quantification of potassium leaching, catalyst reuse trials, and full elemental mass balances. Nevertheless, the implications are compelling. The best catalyst for maximizing biocrude yield was not the most porous one but the one richest in a naturally occurring alkali metal, meaning that the cheapest, least-processed waste char may outperform its engineered counterpart depending on the process objective. For a world drowning in sludge and food waste, the idea that banana peels discarded after breakfast could help transform a city’s sewage into liquid fuel is a vivid demonstration of circular bioenergy, where the byproducts of one waste stream become the key that unlocks the value of another.

Subject of Research: Waste-derived potassium-modified activated biochars as catalysts for sewage-sludge hydrothermal liquefaction

Article Title: Comparative preparation of waste-derived K-modified activated biochars and proof-of-concept application in sewage-sludge hydrothermal liquefaction

Article References: Ismail, R., Mariyam, S., Alherbawi, M., & Al-Ansari, T. (2026). Comparative preparation of waste-derived K-modified activated biochars and proof-of-concept application in sewage-sludge hydrothermal liquefaction. Case Studies in Chemical and Environmental Engineering, 14, Article 101493. https://doi.org/10.1016/j.cscee.2026.101493

Image Credits: AI Generated

DOI: 10.1016/j.cscee.2026.101493

Keywords: biochar, hydrothermal liquefaction, sewage sludge, banana peel, potassium carbonate activation, biocrude, pyrolysis, waste valorization, catalysis, renewable energy, BET surface area, circular economy

The researchers, led by Bernadetta Kaźmierczak, began with dried used coffee grounds sieved to a particle size of 600 to 400 micrometers. They pyrolyzed 120-gram batches in a chamber furnace at 600 degrees Celsius under a carbon dioxide protective gas flowing at 5 liters per minute, following a carefully staged temperature program with holding periods at 200, 550 and 600 degrees Celsius. This produced an unactivated reference biochar, designated BFK_600, with a yield of about 20 percent of the starting mass. For the activated variants, the team cut off the carbon dioxide supply once 600 degrees Celsius was reached and introduced water vapor instead, at flow rates of either 2 or 4 milliliters per minute, for activation times of 15, 30 or 45 minutes. Six activated biochars resulted, each named for its exact activation recipe.

Chemical characterization began with Fourier-transform infrared spectroscopy, which tracks the vibrational fingerprints of functional groups on the biochar surface. The spectra revealed a dramatic transformation during pyrolysis itself. The raw coffee grounds displayed broad hydroxyl bands between 3530 and 3300 inverse centimeters, aliphatic C-H bands at 2923 and 2856 inverse centimeters, and a suite of carbonyl, amide and ether signals. After pyrolysis at 600 degrees Celsius, most of these bands vanished, reflecting the dehydration, decarboxylation and decarbonylation reactions that strip oxygen from the carbon matrix at high temperature. In other words, the very process that creates the carbon skeleton also destroys many of the oxygen-containing groups that make a sorbent chemically active.

The striking finding was that water vapor activation partially reversed this loss. Activated biochars developed new hydroxyl bands in the 3800 to 3000 inverse centimeter region that were absent from the unactivated material, along with shifted C-O, C-N and C-O-C signals near 1144 to 1146 inverse centimeters and new bands attributed to C=C, N-H and N-COO structures. The effect depended on the steam dose. At a flow of 4 milliliters per minute, the biochars gained additional hydroxyl bands at 3619, 3608, 3361 and 3331 inverse centimeters, plus a band at 1240 inverse centimeters corresponding to C-O, C-OH and N-COO groups that appeared only in the 15-minute sample. Notably, the longest and most intense activation, 4 milliliters per minute for 45 minutes, produced no hydroxyl bands at all, suggesting that excessive steam exposure can push the surface chemistry past a useful point.

Surface area measurements told an equally nuanced story. The unactivated biochar had a BET specific surface area of just 7.4 square meters per gram, typical of a poorly developed pyrolysis char. Steam activation increased this figure in every case, but not linearly. At the 2 milliliter per minute flow, the best result was 37.9 square meters per gram after 15 minutes, falling to 17.4 after 45 minutes. At 4 milliliters per minute, the optimum shifted to 30 minutes, yielding 62.6 square meters per gram, roughly a ninefold expansion over the unactivated char, while the 15-minute sample at that flow reached only 12.9. The team concluded that steam first carves open new pores, but prolonged or overly intense treatment begins to destroy the porous framework it created. Nitrogen adsorption isotherms classified all the materials as mesoporous, with dominant pore diameters between 2 and 2.5 nanometers.

Scanning electron microscopy coupled with energy-dispersive X-ray analysis added morphological and elemental detail. Raw coffee grounds showed a flat, fibrous structure with no visible porosity. After pyrolysis, the material fragmented into a heterogeneous network resembling broken fibers, with recesses hinting at pores. Activated samples retained this rough, folded texture, with cavities confirming porosity. The elemental analysis was particularly revealing: pyrolysis raised the relative carbon content from 49.7 to 65.7 percent while oxygen fell from 32.7 to 9.7 percent, as volatile oxygen compounds escaped. Steam activation then pushed oxygen back up, by factors of 1.3 to 1.7 depending on the recipe, and nitrogen also rose relative to the unactivated char, likely because the water vapor blanket on the particle surface limits the volatilization of nitrogen species while oxidation-reduction reactions rebuild organooxygen groups.

With the surface chemistry mapped, the researchers turned to the practical question: could these biochars remove chromium(III) from water? They selected the biochar with the richest oxygen functionality, BFK_600_4ml_15min, and the one with the largest surface area, BFK_600_4ml_30min, and compared them against the unactivated reference. Adsorption tests used a chromium(III) solution of 120 milligrams per liter at pH 4 and 20 degrees Celsius, conditions chosen to keep the metal dissolved while mimicking the mildly acidic character of real tannery wastewater streams. Chromium concentrations were tracked by inductively coupled plasma mass spectrometry.

The results showed that surface chemistry mattered more than raw surface area. The biochar activated at 4 milliliters per minute for 15 minutes, despite its modest BET area, achieved the best performance: 51 percent chromium removal with a 0.5-gram sorbent dose and 56 percent with 1.0 gram, compared with 44 and 47 percent for the unactivated char. The high-surface-area 30-minute biochar reached 47 and 55 percent respectively. The team attributes the advantage of the 15-minute material to its hydroxyl bands and the distinctive 1240 inverse centimeter C-OH signal, which provide active binding sites for trivalent chromium ions. Kinetics followed a familiar pattern: adsorption was fastest in the first ten minutes and approached equilibrium by 210 minutes for activated samples, versus 150 minutes for the unactivated char. Statistical analysis confirmed a significant effect of contact time, with Pearson correlation coefficients between 0.87 and 0.99 linking contact time to removal efficiency.

The broader significance lies in the process itself. Physical activation with steam avoids the corrosive reagents such as potassium hydroxide or phosphoric acid used in chemical activation, which can generate surface areas above 3000 square meters per gram but leave behind liquid waste streams requiring careful disposal. Steam activation produces no such by-products, and the new study demonstrates that at 600 degrees Celsius it does something chemical activation often cannot: it regenerates the oxygen-bearing functional groups that high-temperature pyrolysis destroys, even if the resulting surface areas remain modest. Earlier work by the same group showed that at 700 degrees Celsius steam activation instead wipes out organooxygen groups and boosts surface area far more dramatically, so the activation temperature emerges as a dial for choosing between a chemically rich surface and a geometrically extensive one.

For a waste stream generated at a scale of millions of tons annually, the prospect of turning espresso dregs into a green sorbent for heavy-metal-laden wastewater is compelling, and the study’s careful dissection of steam flow and activation time gives future work a clear parameter map. The removal percentages reported here, in the 44 to 56 percent range under conservative single-pass conditions, are lower than the 90 to 99 percent figures reported in the literature for lead, cadmium and manganese on coffee biochars, and the authors are explicit that their experiments were comparative rather than optimized. Even so, the message is clear: the humble used coffee ground, activated with nothing but water vapor, can be engineered into a functional water-cleaning material, and the recipe that best restores its chemical appetite for chromium is a short, gentle blast of steam rather than a long, aggressive one.

Subject of Research: Water vapor activation of biochars produced from used coffee grounds for chromium(III) adsorption from water

Article Title: Water vapor activated biochars made from used coffee grounds

Article References: Kaźmierczak, B., Radulski, P., Osuch-Słomka, E., & Zięba, E. (2026). Water vapor activated biochars made from used coffee grounds. Results in Chemistry, 31, Article 103908. https://doi.org/10.1016/j.rechem.2026.103908

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103908

Keywords: used coffee grounds, biochar, water vapor activation, pyrolysis, chromium(III), adsorption, FTIR spectroscopy, BET surface area, SEM/EDS, wastewater treatment, circular economy, functional groups

The research team, led by Tim-Patrick Fellinger with contributions from Simon W. J. Dietzmann, Asad Mehmood, Jian Liang Low and colleagues, set out to understand a puzzling observation: commercial activated carbons and advanced nitrogen-doped carbons frequently show extremely sharp peaks in their calculated ultramicropore distributions, with full widths at half maximum far below what physical considerations should allow. For pores differing in size by even a single carbon layer, theory suggests a peak width of at least 0.125 nanometres, yet some measurements report peaks as narrow as 0.04 nanometres. Such precision would imply pores defined to a fraction of an atom’s diameter, which is physically implausible in the disordered, amorphous world of non-graphitic carbon.

To resolve the mystery, the researchers prepared a pair of elegantly matched model materials. Starting from a zinc-containing zeolitic imidazolate framework, they synthesised a zinc-nitrogen-carbon material containing tetrapyrrolic ZnN4 sites, atomically dispersed metal centres coordinated by four nitrogen atoms embedded in the carbon plane. Acid extraction of the zinc then produced a second material in which the metal ions were removed but the carbon framework remained essentially unchanged, leaving behind tetrapyrrolic H2N4 cavities with nitrogen-nitrogen distances of roughly 0.4 nanometres. Spectroscopic analysis using extended X-ray absorption fine structure confirmed the tetrapyrrolic coordination in both materials, while X-ray photoelectron spectroscopy tracked the transformation from ZnN4 to H2N4 sites. Powder X-ray diffraction confirmed that no change in graphitisation occurred, meaning only the zinc was removed and the carbon skeleton was preserved.

With these precisely defined materials in hand, the team measured gas sorption using three different probe gases: nitrogen at 77.4 kelvin, carbon dioxide at 273 kelvin, and argon at 87.3 kelvin. Both materials displayed type I isotherms characteristic of microporous solids, and zinc extraction increased the apparent specific surface area by about 23 percent. But then came a surprise. At the lowest accessible relative pressures, around ten to the minus seven, the zinc-containing material actually adsorbed more nitrogen and more carbon dioxide than the supposedly more porous metal-free version. This trend reversal was counterintuitive: if the removal of zinc created new ultramicropores, the leached material should have taken up more gas, not less.

The key clue came from argon. Unlike nitrogen and carbon dioxide, argon is essentially non-polar and possesses no quadrupole moment, the asymmetry in its charge distribution that allows it to interact with electrically polarised surfaces. Argon adsorption followed the expected trend, with the metal-free material consistently taking up more gas across the entire pressure range, and no reversal appeared at low pressures. Because the anomalous behaviour depended on the quadrupole moment of the gas, with carbon dioxide showing the strongest effect, followed by nitrogen and effectively none for argon, the researchers concluded that the origin was microscopic and chemical rather than macroscopic and structural. The specific interactions between polar gas molecules and the embedded nitrogen-metal sites were distorting the measurements.

The pore size distributions told the same story. Analysis of nitrogen and carbon dioxide isotherms produced razor-thin apparent ultramicropore peaks, with full widths at half maximum of roughly 0.09 and 0.04 nanometres respectively, at apparent pore diameters between 0.53 and 0.82 nanometres depending on the gas used. Argon, in contrast, revealed only broader supermicropores centred around 1.3 nanometres with a full width of about 0.6 nanometres, features the team assigns to genuine physical porosity. The apparent surface areas themselves also diverged depending on the gas: for the zinc-containing material, nitrogen analysis suggested 750 square metres per gram while argon gave 566, with nitrogen consistently overestimating the accessible surface.

To explain these observations at the molecular level, the team turned to electronic density functional theory calculations of adsorption energies for each gas on three sites: plain graphitic carbon, the tetrapyrrolic H2N4 cavity, and the ZnN4 centre. For all three gases, adsorption at ZnN4 was stronger than on graphitic carbon, and the deviation grew with increasing quadrupole moment of the adsorbate. The zinc atom carries a substantial partial charge of about plus 1.6 electrons due to its ionic bonding character, creating a strongly polarised site that attracts quadrupolar molecules like carbon dioxide and nitrogen through quadrupole-ion interactions. Binding energies at these sites fall squarely in the range normally associated with ultramicropore filling, so the porosity analysis software interprets them as pores smaller than 0.7 nanometres, even though no pore physically exists there. Carbon dioxide also binds preferentially to the H2N4 cavity through quadrupole-dipole interactions aligned with the nitrogen-hydrogen bonds, though more weakly than at ZnN4.

The most striking validation came from a new analytical approach. Because standard pore size distribution fitting cannot directly yield adsorption energies, the researchers applied a multi-Langmuir analysis to the carbon dioxide isotherms, deconvoluting the data into contributions from three classes of binding sites: the carbon support, real micropores, and the strongest specific sites. For the strongest site class, the fitted adsorption free energies differed by 0.036 electron volts between the zinc-containing and metal-free materials, a value that matches almost exactly the 0.035 electron volt difference predicted by the density functional theory calculations for carbon dioxide binding at ZnN4 versus H2N4 sites. The internal consistency between experiment and simulation is compelling evidence that the phantom pores are indeed the chemical fingerprints of atomically dispersed metal-nitrogen sites. Encouragingly, the extracted maximum capacities at these sites, between 0.08 and 0.13 millimoles per gram, are comparable to active site densities independently reported for iron-nitrogen-carbon catalysts, hinting that simple gas sorption could one day serve as a quantitative probe of catalytically active sites, a quantity that has remained notoriously difficult to measure.

The practical implications for the field are substantial. The authors recommend that narrow ultramicropore peaks arising from in-plane functionalities be flagged as artefacts, for example with an asterisk, and that corresponding pore volumes be reported as apparent rather than true micropore volumes. For atomically dispersed metal-nitrogen-carbon materials and related nitrogen-doped carbons, surface areas and total pore volumes are best determined from argon sorption, which avoids the overestimation caused by specific adsorption. Beyond correcting current practice, the findings open a genuine opportunity: gas sorption, one of the most widely accessible characterisation techniques in materials chemistry, could be transformed into a tool for detecting and quantifying specific surface sites and in-plane functionalities, provided new analysis kernels are developed. That would help researchers disentangle morphological properties such as porosity and surface area from chemical properties such as active site geometry and concentration, both crucial for optimising carbon-based materials in energy storage, conversion and catalysis. What looked like a flaw in the measurement may turn out to be one of its most useful features.

Subject of Research: Pore size distribution analysis artefacts in non-graphitic carbons and atomically dispersed M-N-C materials caused by specific gas adsorption

Article Title: A Pore or not a Pore? Understanding Pore Size Distributions of Non‐Graphitic Carbon and Atomically‐Dispersed M‐N‐C Materials

Article References: Dietzmann, S. W. J., Mehmood, A., Low, J. L., Wu, S.-H., Prinz, C., Buzanich, A. G., Radnik, J., Appel, P. A., Emmerling, F., & Fellinger, T.-P. (2026). A Pore or not a Pore? Understanding Pore Size Distributions of Non‐Graphitic Carbon and Atomically‐Dispersed M‐N‐C Materials. Advanced Science, 13(50), Article e76048. https://doi.org/10.1002/advs.76048

Image Credits: AI Generated

DOI: 10.1002/advs.76048

Keywords: pore size distribution, gas sorption, M-N-C materials, nitrogen-doped carbon, ultramicropores, ZnN4 sites, density functional theory, activated carbon, BET surface area, argon sorption, Langmuir analysis, electrocatalysis