Hydrogen Breathes New Life Into Exhausted Ozone-Destroying Catalysts

Ozone is a paradox in atmospheric chemistry. High above our heads, the stratospheric ozone layer shields life on Earth from lethal ultraviolet radiation, but at street level the very same molecule is a corrosive pollutant that damages lungs, degrades materials, and complicates industrial air-cleaning systems. As cities and factories increasingly deploy ozone-based oxidation to destroy volatile organic compounds, or VOCs, a stubborn engineering problem has emerged: what to do with the unreacted ozone that slips through these treatment units, and how to keep the catalysts that scrub it working over months and years of continuous operation. A new open-access study from researchers in South Korea, published in the journal Advances in Industrial and Engineering Chemistry, now offers a carefully quantified answer to the second half of that problem, demonstrating that a deactivated manganese catalyst can be brought back to life either by heating it to a few hundred degrees Celsius or, remarkably, by exposing it to hydrogen at barely more than body temperature.

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