The research team, led by Min Seok Kwon and corresponding author No-Kuk Park of Yeungnam University, together with colleagues from the Institute of Clean Technology and Inwoo Eco Corporation, set out to understand precisely why manganese-oxide catalysts lose their appetite for ozone and then to compare two fundamentally different regeneration strategies. Their choice of material was deliberate. Manganese dioxide sits among the most effective and inexpensive catalysts for splitting ozone into ordinary oxygen at ambient conditions, which makes it attractive for small-scale VOC treatment systems where energy budgets are tight and elaborate heating equipment is impractical. Yet manganese oxides carry a well-known weakness: they are highly susceptible to moisture, and their performance decays steadily during prolonged use.

To build their catalyst, the researchers employed a potassium permanganate-assisted precipitation route. Aqueous manganese nitrate was combined with potassium permanganate in a 3:2 molar ratio at 80 degrees Celsius, a slow addition that drove oxidation and precipitation of manganese oxide from solution. After roughly 24 hours of aging, the precipitate was filtered, washed three times with deionized water to strip away residual nitrate and unreacted permanganate, dried at 110 degrees Celsius overnight, and pulverized. The powder was then shaped by extrusion with methyl cellulose and colloidal silica binders into cylindrical pellets a few millimeters across, later ground and sieved to particles between 300 and 800 micrometers for testing. X-ray fluorescence confirmed a product dominated by manganese oxide, roughly 93.2 weight percent MnO with about 4.6 percent SiO2 carried in from the binder. Crucially, because the synthesis avoided high-temperature calcination, the resulting catalyst remained only partially crystalline, a structure rich in lattice defects and oxygen vacancies that previous work has linked to exceptional ozone-decomposition activity.

The deactivation experiments told a vivid story. When about one gram of the catalyst was loaded into a tubular reactor and fed roughly 15 ppmv of ozone in air at room temperature and atmospheric pressure, the outlet ozone concentration plummeted to zero. Complete decomposition held for approximately 40 minutes before ozone began to creep back into the effluent, ultimately stabilizing near 8 ppmv, equivalent to about 50 percent conversion. This classic breakthrough curve is the fingerprint of a catalyst whose active sites are being progressively occupied. The mechanism behind ozone destruction on manganese oxide proceeds in three steps: ozone adsorbs onto the surface and splits off an adsorbed atomic oxygen species, a second ozone molecule reacts with that atomic oxygen to form two adsorbed molecular oxygen species, and finally those molecular species desorb as gaseous oxygen, freeing the site for another cycle. At low ozone concentrations, that third step, desorption, becomes the bottleneck. When desorption lags, oxygen intermediates pile up on the surface like cars in a parking garage with no exit, sealing off the very sites the reaction depends on.

Temperature-dependence experiments reinforced this interpretation. When the team gradually heated a deactivated catalyst, ozone removal climbed from partial conversion at 30 degrees Celsius to essentially complete destruction at 40 degrees Celsius, with no ozone detectable at the outlet above that point. Thermal energy, in other words, helps the trapped oxygen intermediates escape, temporarily re-exposing active sites. But definitive evidence came from X-ray photoelectron spectroscopy, or XPS, which probes the chemical state of the outermost atomic layers. On the fresh catalyst, the oxygen 1s spectrum was dominated by a lattice oxygen peak near 529.9 electronvolts. On the deactivated sample, a feature near 532 electronvolts, corresponding to surface-adsorbed oxygen species, swelled to encompass roughly 72 percent of the total spectral envelope. The poison was not some exotic contaminant; it was the reaction’s own intermediate, hoarded on the surface until the catalyst choked on its product.

With the deactivation mechanism established, the team tested the conventional remedy: thermal treatment. Deactivated catalyst samples were heated in an electric furnace at 100 to 600 degrees Celsius in 100-degree intervals for four hours each, at a controlled ramp of 5 degrees per minute, and then re-examined by X-ray diffraction and re-run in the ozone reactor. Between 100 and 500 degrees Celsius, the XRD patterns remained broad and weak, the signature of amorphous or partially crystalline phases studded with defects and vacancies, including traces of tunnel-structured epsilon-MnO2 that readily hosts oxygen vacancies. But above 600 degrees Celsius the picture changed dramatically: sharp diffraction peaks of alpha-Mn2O3 appeared, marking crystallization into a stable cubic phase with few vacancies and a high desorption energy for oxygen. In effect, the intense heat welded the catalyst into a form that no longer wanted to work.

The activity measurements tracked this structural narrative precisely. The fresh catalyst had a breakthrough time of about 40 minutes; samples treated at 100, 200, 300, 400, 500, and 600 degrees Celsius held out for approximately 8, 15, 23, 26, 22, and 3 minutes respectively. Expressed as recovery of initial activity, treatments at 100 and 200 degrees Celsius managed only about 20 and 37.5 percent, while the 300-to-500-degree-Celsius window consistently delivered around 57.5 percent. Beyond that, performance collapsed: catalysts treated at 500 and 600 degrees Celsius allowed steady-state outlet ozone concentrations of 9.5 and 11 ppmv, far above the roughly 8 ppmv seen after milder treatments. XPS confirmed that thermal treatment did strip away much of the adsorbed oxygen responsible for poisoning, but the accompanying loss of lattice defects at high temperatures meant that even a clean surface had fewer places to work. Thermal regeneration, the study showed, is a compromise: hot enough to free the surface, but not so hot as to crystallize the active phase.

The more surprising result came from the alternative strategy. Instead of heating the catalyst to hundreds of degrees, the researchers flowed a dilute mixture of 10 percent hydrogen in nitrogen over the spent catalyst at just 40 and 60 degrees Celsius. Embedded thermocouples revealed the chemistry happening in real time: the catalyst bed temperature fluctuated by up to 0.15 degrees Celsius at 40 degrees and within a 0.65-degree band at 60 degrees, transient swings produced when adsorbed oxygen species oxidized the incoming hydrogen to water in small local exotherms. Those gentle temperature excursions are direct evidence that the adsorbed oxygen, the poison itself, was being selectively consumed. After hydrogen exposure at 40 degrees Celsius, the catalyst regained about 25 percent of its initial activity; at 60 degrees Celsius, recovery reached approximately 55 percent, achieved with a fraction of the energy demanded by thermal treatment.

Spectroscopy sealed the case for selectivity. After hydrogen reduction, the 531.6-electronvolt XPS peak assigned to adsorbed oxygen dropped sharply, falling to an area fraction of 26.31 percent at 60 degrees Celsius and 29.61 percent at 40 degrees Celsius, while the lattice oxygen and oxygen vacancy signals remained essentially untouched. The hydrogen had removed the poison without disturbing the underlying active structure, avoiding the crystallization and defect loss that plague high-temperature regeneration. Intriguingly, catalysts regenerated by hydrogen outperformed some heat-treated samples even when their residual adsorbed-oxygen levels were comparable, because the hydrogen-treated surfaces retained their defect-rich, vacancy-laden architecture. The authors also noted that the reaction rate in ozone decomposition is governed by the interplay between adsorbed atomic oxygen and ozone forming adsorbed molecular oxygen, and that hydrogen regeneration short-circuits the poisoning cycle by converting the trapped species directly to water. For an industry searching for ways to extend catalyst lifetimes in compact, low-energy air-purification systems, the message is clear: sometimes the gentlest treatment, a warm whisper of hydrogen rather than a blast of furnace heat, is precisely what a poisoned catalyst needs to breathe again.

Subject of Research: Regeneration of deactivated manganese-based ozone decomposition catalysts by thermal treatment and hydrogen reduction

Article Title: Optimization of regeneration conditions for ozone decomposition catalysts via thermal treatment and hydrogen reduction

Article References: Kwon, M. S., Jang, J. G., Kim, M., Kwon, B. C., Park, N.-K., Kong, J. S., & Kong, S. W. (2025). Optimization of regeneration conditions for ozone decomposition catalysts via thermal treatment and hydrogen reduction. Advances in Industrial and Engineering Chemistry, 1(1), Article 29. https://doi.org/10.1007/s44405-025-00030-z

Image Credits: AI Generated

DOI: 10.1007/s44405-025-00030-z

Keywords: ozone decomposition, manganese catalyst, catalyst regeneration, hydrogen reduction, thermal treatment, XPS, adsorbed oxygen, oxygen vacancies, VOC treatment, air purification, MnO2, deactivation

The study, published in the Journal of Saudi Chemical Society, describes ZIF-8/CA composite porous carbons: materials built by combining zeolitic imidazolate framework-8, a metal-organic framework prized for its ultrahigh surface area, with cellulose acetate, the biocompatible and biodegradable polymer that already dominates commercial cigarette filter manufacturing. The research was led by Wei-li Xu, Pei-jian Sun and Cong Nie of the Key Laboratory of Tobacco Chemistry at the Zhengzhou Tobacco Research Institute of CNTC, working with colleagues from China Tobacco Shaanxi Industrial and China Tobacco Shandong Industrial. Their central insight is deceptively simple: the ratio in which the two ingredients are mixed before carbonization decides everything about how well the final carbon can grab benzene molecules from a moving gas stream.

To build the composites, the team turned to a dual emulsion-solvent evaporation method, a technique borrowed from soft-matter chemistry that is rarely applied to metal-organic framework composites. First they synthesized ZIF-8 powder by mixing zinc nitrate hexahydrate with 2-methylimidazole in water at a precisely controlled molar ratio and stirring the milky suspension for 24 hours. They then created a primary emulsion by injecting an aqueous internal phase containing ammonium bicarbonate into a solution of cellulose acetate dissolved in dichloromethane, adjusting the ZIF-8 to cellulose acetate mass ratios to 3/7, 4/6, 6/4 and 7/3. Droplets of this emulsion were dispersed into a dilute polyvinyl alcohol solution to form a secondary emulsion. As the solvent evaporated over six hours of stirring, uniform composite microspheres formed. The final and most transformative step was pyrolysis: heating the microspheres to 950 degrees Celsius at 5 degrees per minute under nitrogen and holding them there for two hours, converting the polymer-framework hybrid into porous carbon.

Why go to such lengths? The answer lies in the complementary weaknesses of existing adsorbents. Conventional activated carbon, the workhorse of volatile organic compound control, suffers from a moderate specific surface area, irregular pore structures and weak affinity for benzene at low concentrations. ZIF-8, by contrast, offers exceptional microporosity, strong pi-pi interactions with aromatic molecules and a gate-opening flexibility that accommodates bulky guests like benzene and toluene — but the raw powder is notoriously difficult to process, tends to agglomerate, and cannot easily be shaped into practical devices such as filter cartridges. Cellulose acetate solves the processability problem: its hydroxyl and acetyl groups form hydrogen bonds with ZIF-8, promoting even dispersion and mechanical stability, while its carbonization generates additional pores. What remained unknown, and what this study set out to map systematically, was how the ZIF-to-polymer mass ratio shapes the full hierarchy of pore sizes and, in turn, the benzene uptake.

The characterization data tell a striking story of structural optimization. Scanning electron microscopy revealed spherical particles between 100 and 500 micrometers in diameter, but their internal architecture varied dramatically with composition. At the 3/7 ratio, excess cellulose acetate caused the polymer matrix to aggregate, producing a dense, nearly pore-free cross-section. Pushing ZIF-8 content too high, at 6/4 and 7/3, triggered the opposite failure: agglomerated carbon particles and larger but poorly distributed pores. The sweet spot arrived at 4/6, where the surface roughened into uniform microscale protrusions and the cross-section displayed a beautifully interconnected porous network. Nitrogen physisorption measurements confirmed the visual evidence: the 4/6 composite achieved a Brunauer-Emmett-Teller specific surface area of 1380 square meters per gram and a mesopore volume of 0.77 cubic centimeters per gram, far exceeding the 577, 657 and 796 square meters per gram recorded for the other formulations. All samples showed Type IV isotherms with H4 hysteresis, a fingerprint of mesoporosity, with mesopores concentrated near 2.5 nanometers.

Mercury intrusion porosimetry added the final piece of the hierarchical puzzle, probing pores far too large for nitrogen adsorption to detect. The dominant macropore diameter increased progressively with ZIF-8 content, a trend the researchers attribute to gas evolution — carbon dioxide and nitrogen released as the framework decomposes — which inflates and expands the pores during carbonization. This revealed an elegant division of labor: cellulose acetate carbonization builds mesopores and surface area, while ZIF-8 decomposition carves macropores. The two effects compete, and the 4/6 composition strikes the optimal balance between abundant mesoporous adsorption domains and macroporous highways for rapid molecular transport. Many high-surface-area carbons perform poorly in dynamic adsorption because blocked channels or excessive micropores strangle diffusion; this composite avoids both traps by design.

Dynamic benzene vapor adsorption tests brought the structure-property relationship into sharp focus. In breakthrough experiments with a benzene stream flowing at 100 milliliters per minute through a thermostatted column at 25 degrees Celsius, the saturation adsorption capacity traced a volcano-shaped curve across the composition series, peaking at 235.0 milligrams per gram for the 4/6 sample — well above its siblings and competitive with previously reported adsorbents. To dissect the kinetics, the team fitted the breakthrough curves with two classical fixed-bed models. The Apiratikul-Chu model reproduced the entire breakthrough curves with correlation coefficients exceeding 0.99, capturing the asymmetric tailing that arises from internal diffusion through tortuous pore networks. The 4/6 composite also displayed the highest rate constant, at 65.2 per minute, thanks to plentiful active sites and macropores that slash mass-transfer resistance. The Adams-Bohart model, applied to the initial stage of adsorption, delivered correlation coefficients above 0.97, confirming that surface adsorption and external mass transfer govern the onset of uptake.

The chemistry behind the capture is as important as the physics of the pores. Both the graphitic carbon formed from cellulose acetate and the residual framework structure of ZIF-8 are rich in delocalized pi-electrons, which form strong pi-pi stacking interactions with the aromatic ring of benzene — an affinity that physical adsorption alone cannot provide. Meanwhile, the graded pore hierarchy produces a confinement effect: micropores and mesopores physically trap benzene molecules, while interconnected channels accelerate their diffusion toward those sites. Macropores lower external diffusion resistance, mesopores and micropores dominate intraparticle diffusion, and the result is a material whose adsorption rate is as impressive as its capacity.

The most headline-grabbing result came when the material faced its intended real-world challenge: actual cigarette smoke. Using a custom-built apparatus connected to an SM-450 smoking machine operating under the ISO standard protocol — 35-milliliter puffs drawn over two seconds with 60-second intervals — the researchers loaded 10 milligrams of adsorbent into each cigarette filter and analyzed trapped benzene by gas chromatography-mass spectrometry. The ZIF-8/CA 4/6 composite removed 49.0 percent of benzene from mainstream smoke, dramatically outperforming a ZIF-8/polylactic acid control composite prepared under identical conditions, which managed only 20.7 percent. Even in the chemical chaos of real smoke, where countless gaseous components compete for adsorption sites, the hierarchical pore structure and pi-pi selectivity for aromatic compounds allowed the composite to maintain high benzene capture. The authors note that only benzene was quantified in this study, with multi-component analysis and regeneration and long-term cycling tests planned for follow-up work, and that static adsorption isotherms will be examined in future research.

Beyond the laboratory numbers, the study delivers a genuinely versatile design principle. By simply tuning a mixing ratio before a single carbonization step, researchers can dial in a micro-meso-macroporous architecture tailored to a target pollutant — a rational, adaptable strategy that could extend well beyond cigarette filters to industrial off-gas treatment, indoor air purifiers and protective respirators. The rigid carbon skeleton also promises structural stability for cyclic use. For a compound as pervasive and as dangerous as benzene, a scalable material that more than doubles the removal efficiency of a comparable commercial polymer composite represents meaningful progress — and a reminder that sometimes the biggest advances in environmental health come not from exotic new chemistry, but from getting the architecture of familiar ingredients exactly right.

Subject of Research: Hierarchical pore engineering of ZIF-8/cellulose acetate composite porous carbon for benzene vapor adsorption and cigarette smoke purification.

Article Title: Hierarchical pore structure modulation of ZIF-8/CA composite porous carbon for efficient benzene vapor adsorption

Article References: Xu, W.-L., Sun, P.-J., Sun, X.-H., Wang, Y.-P., Li, J.-L., Ge, C., Liu, Q., Zhou, J., Yang, F., Song, X.-H., & Nie, C. (2026). Hierarchical pore structure modulation of ZIF-8/CA composite porous carbon for efficient benzene vapor adsorption. Journal of Saudi Chemical Society, 30(4), Article 56. https://doi.org/10.1007/s44442-026-00098-2

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00098-2

Keywords: ZIF-8, cellulose acetate, porous carbon, benzene adsorption, volatile organic compounds, hierarchical pores, metal-organic frameworks, cigarette smoke filtration, air purification, carbonization, adsorption kinetics, breakthrough curves