Ozone decomposition catalysts are widely used in industrial off-gas treatment, semiconductor cleanrooms, ozone oxidation in water treatment, and medical disinfection. However, rapid deactivation in high-humidity environments remains a core bottleneck limiting long-term stable operation. The root cause lies in competitive adsorption between water molecules and ozone molecules at the catalyst active sites, along with multiple deactivation mechanisms including surface hydroxylation, pore blockage, and accumulation of intermediate oxygen species. In recent years, researchers have achieved significant breakthroughs at the material level through elemental doping, crystalline phase regulation, hydrophobic modification, and heterojunction construction — Ce-doped NiO achieves >98% conversion of 200 ppm ozone at 90% relative humidity (RH) for 60 hours, Mn-doped hexagonal cobaltite perovskite reaches 100% decomposition at 90% RH with stability over 60 hours, and mesoporous carbon-coated MnO runs stably for 100 hours at 65% RH. These advances indicate that the water-resistance bottleneck is moving from a materials-science challenge toward an engineering-solvable problem. However, translating laboratory data into reliable industrial operation still requires a systematic engineering methodology in catalyst selection and system design. The following discussion addresses deactivation mechanisms, material modification strategies, and engineering supporting measures.
Water Attack Pathways: Three Major Mechanisms of High-Humidity Deactivation for Ozone Decomposition Catalysts
Taking the most widely applied manganese-based oxides (MnOx) as an example, their ozone decomposition activity mainly relies on surface oxygen vacancies — the active sites for O₃ adsorption and cleavage. The catalytic cycle proceeds as follows: O₃ molecules first adsorb onto surface oxygen vacancies, then dissociate into O₂ and reactive oxygen species; the reactive oxygen species further react with another O₃ molecule to produce oxygen, completing the catalytic loop. In high-humidity environments, however, water molecules attack the catalyst active centers through at least three pathways.
Pathway 1: Competitive Adsorption
Water molecules directly compete with ozone molecules for active sites. Because water is polar, its adsorption energy on oxide surfaces is often lower than that of ozone, so water preferentially occupies oxygen vacancies, directly reducing the number of sites available for ozone decomposition. Studies have shown that competitive adsorption of water on MnTi₂.₂/γ-Al₂O₃/Al catalyst active sites is a primary cause of deactivation.
Pathway 2: Surface Hydroxylation
Water not only physically adsorbs on the catalyst surface but also chemisorbs to form hydroxyl groups (–OH). These hydroxyl groups combine with reactive oxygen species, blocking the catalytic cycle and converting regenerable active sites into irreversible deactivated sites. Moreover, residual surface acid ions aggravate this process under high humidity — it was found that at 65% RH, the activity of insufficiently washed Ce-modified γ-MnO₂ dropped significantly, indicating that acid ions severely damage catalyst activity.
Pathway 3: Pore Blockage and Mass-Transfer Limitation
Water molecules condense on the catalyst surface, forming a water film that covers active surfaces and fills micropores and mesopores. This drastically reduces the effective specific surface area and impedes diffusion of ozone molecules to active sites, manifesting as a continuous decline in catalytic efficiency. Conventional manganese-based catalysts begin to show obvious activity loss above 50% RH. Accelerated deactivation tests over 3000 hours further confirm that these multiple mechanisms act synergistically under high-humidity conditions, posing severe challenges to long-term stability.
From Elemental Doping to Crystalline Phase Engineering: Four Technical Routes to Enhance Intrinsic Water Resistance
Once the deactivation mechanisms were understood, researchers explored various materials-science approaches to improve intrinsic water resistance.
Route 1: Elemental Doping – Dual Modulation of Electronic Structure and Surface Properties
Elemental doping is one of the most thoroughly investigated modification methods. Introducing dopants such as Ce, Ti, Ag, or Bi into manganese- or nickel-based catalysts can stabilise the crystal structure, increase oxygen vacancy concentration, and tune surface hydrophilicity/hydrophobicity.
A typical example is the CeO₂-doped NiO catalyst developed by a team at East China University of Science and Technology. Using a simple co-precipitation method, the Ce-NiO catalyst maintained >98% conversion of 200 ppm ozone at 30°C, 90% RH, and a gas hourly space velocity (GHSV) of 1.2 million mL·g⁻¹·h⁻¹ for 60 hours. Characterisation revealed that CeO₂ doping not only increased defect sites and expanded reaction sites but also reduced water desorption energy. More importantly, the study found that water is not simply a "poison" on this catalyst — water molecules can dissociate on the surface to form hydroxyl groups, which then react with ozone to regenerate water, opening an additional reaction pathway that avoids occupation of active sites by intermediate oxygen species. This finding partly explains why certain doped catalysts actually perform better under high humidity.
Ti doping also shows excellent water resistance. Researchers prepared MnTi₂.₂/γ-Al₂O₃/Al structured catalysts by in-situ growth of γ-Al₂O₃ layers on Al mesh; these achieved nearly 100% ozone degradation at 50% RH and maintained ~95% performance at 70% RH. Ag-doped hollandite (Ag-Hollandite) catalysts maintained 98% ozone conversion for 8 hours at 70% RH.
Route 2: Crystalline Phase and Morphology Engineering – Exposing Active Sites
The crystalline phase and morphology of a catalyst directly affect its water resistance. Researchers prepared different MnO₂ phases (α-, amorphous, and δ-) at room temperature by simple pH adjustment. Among them, amorphous MnO₂, having the lowest average oxidation state and highest oxygen vacancy concentration, achieved 98% ozone removal at 90% RH. In-situ diffuse reflectance infrared spectroscopy confirmed that this catalyst accumulated only minimal hydroxyl groups under humid conditions, verifying its excellent water resistance.
More significantly, the amorphous MnO₂ preparation method was successfully applied to cordierite honeycomb supports. The resulting monolithic catalyst modules (100 mm × 100 mm × 20 mm) maintained 60% stable ozone removal for 60 hours in a duct (O₃ concentration 400±30 ppb, temperature 25±5°C, air velocity 1 m/s). This demonstrates that crystalline-phase control is not only effective in powder catalysts but also transferable to industrial-scale monoliths.
Route 3: Defect Engineering – Constructing Active Centres with Lewis Acid–Base Pairs
In recent years, defect-based Lewis acid–base pair strategies have opened a new direction for water-resistant catalyst design. Researchers developed hydrophobic carbon-coated mesocrystalline MnO (Meso-MnO@C), rich in manganese vacancy (V_Mn)-based Lewis acid–base pairs. Manganese vacancies induce electronic reconstruction in MnO, forming V_Mn-Mn acid sites and adjacent lattice oxygen base sites. These Lewis acid–base pairs act as electron donors and acceptors, facilitating fast electron transfer and lowering the energy barrier for O₃-to-O₂ conversion. This catalyst achieved nearly 100% ozone decomposition at a high GHSV of 1500 L·g⁻¹·h⁻¹ and ran stably for 100 hours at 65% RH. The hydrophobic carbon layer effectively prevented water accumulation on the catalyst surface, ensuring long-term performance under humid conditions.
Route 4: MOF-Derived Materials – Building Novel Water-Resistant Catalytic Structures
Metal-organic frameworks (MOFs), with their high surface area and abundant active ionic sites, have emerged as a frontier in catalytic materials. Researchers employed MOF-to-MOF conversion to prepare Co-based MOF-derived porous carbons with different N-doping levels via high-temperature pyrolysis. The Co-N structure exhibited strong ozone adsorption and weak water adsorption, effectively suppressing water uptake. Moreover, the abundant micropores enhanced exposure of active sites and contact between ozone and active centres. This strategy offers a new material option for ozone decomposition under extreme conditions of high humidity and high space velocity.
Surface Hydrophobicity and Heterojunction Construction: Emerging Directions in Water-Resistant Catalyst Design
Hydrophobic Modification – Giving the Catalyst a "Waterproof Coat"
If elemental doping and crystalline-phase engineering modify the catalyst from the inside, hydrophobic modification builds a "water barrier" from the outside. The team of Professor Zhu Yongfa at Tsinghua University developed a hierarchical structure of graphene-encapsulated α-MnO₂ nanofibers; the hydrophobic graphene shell effectively prevented chemisorption of water vapour and avoided water enrichment on the catalyst surface. The optimised catalyst achieved 70% ozone conversion at 50% RH while maintaining good water resistance.
Fluorine doping is another effective surface-regulation strategy. Researchers found that substituting fluorine atoms for lattice oxygen in α-MnO₂ nanowires simultaneously achieved both surface hydrophobicity and oxygen-vacancy regulation — promoting oxygen-vacancy formation (increasing active sites) and enhancing surface hydrophobicity (suppressing water competition). The optimised catalyst decomposed 96% of ozone within 72 hours at 30°C, 65% RH, and a GHSV of 840 L·g⁻¹·h⁻¹. Density functional theory calculations and mechanistic studies indicated that fluorine doping not only lowered the reaction energy barrier but also altered the reaction pathway under high humidity by preventing water activation.
Researchers also modified Mn/ZSM-5 with butyltriethoxysilane (C4); the C4-Mn/ZSM-5 catalyst maintained over 85% ozone conversion for 6 hours at 80% RH. Composite filters of reinforced MnOx catalyst with hydrophobic polymer particles achieved 47.5% efficiency at 80% RH, 1.6 times that of other catalysts.
Heterojunction Construction – Repelling Water Molecules via Interfacial Electric Fields
Heterojunction construction is another emerging route. Researchers discovered that Mn-doped hexagonal cobaltite perovskite Sr₆Co₅O₁₅ undergoes in-situ phase transformation under humid ozone to form the active hexagonal SrCoO₃ phase, raising ozone conversion from 31.8% to 54.6%. Further Mn doping produced a SrMnO₃/Mn-Sr₆Co₅O₁₅ heterojunction, generating a built-in electric field at the interface that triggers charge transfer from SrMnO₃ to Mn-Sr₆Co₅O₁₅, accumulating negative charge on the Sr₆Co₅O₁₅ surface. This negative charge layer weakens water adsorption on active sites by repelling the oxygen-terminated ends of water molecules — a "electric-field repulsion" mechanism completely different from conventional hydrophobic modification. This catalyst achieved 100% ozone decomposition at 25°C and 90% RH, with stability exceeding 60 hours.
Engineering Supporting Strategies: Catalyst Selection and System Design Recommendations for High-Humidity Conditions
Material-level breakthroughs provide more options for ozone catalytic decomposition under high humidity, but in real industrial scenarios, relying solely on intrinsic catalyst performance is often insufficient. The following engineering strategies and selection recommendations are derived from practical experience.
Assess Operating Conditions First – Humidity Is the Primary Selection Parameter
Before selecting a catalyst, systematically evaluate the actual operating conditions. Humidity is the primary external factor affecting catalyst life. When the process gas RH is below 50%, conventional manganese-based catalysts generally meet requirements; between 50% and 70%, Ti- or Ag-doped catalysts are recommended; above 70% or up to 90%, specialised water-resistant catalysts such as Ce-NiO, F-doped MnO₂, or heterojunction types should be used. Additionally, if the gas contains poisons like sulfides or chlorides, the catalyst's poisoning resistance must be assessed — residual acid ions on the surface significantly accelerate deactivation under high humidity.
Upstream Pre-treatment – The Most Direct and Mature Engineering Approach
Installing a dehumidification device upstream of the catalyst bed is the most mature and cost-effective solution in current industrial practice. For extremely humid streams (e.g., wastewater off-gas, typically >80% RH), a mist eliminator or condensation dewatering unit must be installed before the catalyst bed. By placing desiccants such as silica gel or molecular sieves upstream, or using cooling/condensation to remove most moisture before the catalyst stage, catalyst life can be greatly extended. Although this does not fundamentally solve the intrinsic water-resistance issue, it is the easiest and fastest approach in actual projects.
Case Study 1: Ozone Off-Gas Treatment System at a Wastewater Treatment Plant
An off-gas treatment system for the ozone oxidation process at a municipal wastewater plant had a design flow of 5000 m³/h, ozone concentration ~80 ppm, and off-gas RH constantly above 85%. The initial system used conventional manganese-based pellet catalysts; after about 400 hours of operation, ozone decomposition efficiency dropped from 98% to 62%. Investigation attributed deactivation to water film formation on the catalyst surface, blocking micropores and covering active sites. The retrofit included installing a cooling-condensation dewatering device ahead of the catalyst bed to reduce inlet RH below 55%, and replacing the catalyst with a Ti-doped Mn-based monolithic catalyst. After the upgrade, the system ran continuously for over 2000 hours with ozone decomposition efficiency stably above 92%.
Support Engineering and Structured Design
Loading catalysts onto structured supports such as honeycomb ceramics or metal meshes improves mass transfer, reduces bed pressure drop, and minimises water aggregation on the catalyst surface. Monolithic modules are also more convenient for installation and replacement. For high-space-velocity applications, structured design is particularly critical — mesoporous carbon-coated MnO maintained nearly 100% decomposition at a GHSV of 1500 L·g⁻¹·h⁻¹.
Case Study 2: Ozone Removal Unit in a Semiconductor Fab Cleanroom HVAC System
An ozone removal unit in a semiconductor fab's cleanroom HVAC system handled 20,000 m³/h and required outlet ozone <10 ppb. Located in a humid southern region, the ambient air had high moisture content year-round. The initial system combined activated carbon adsorption with conventional catalysts; after about 3 months, efficiency dropped to 70% of design. Analysis showed the cooling coil dehumidification was insufficient, delivering air at 65-70% RH to the catalyst section. The solution was to add a dedicated desiccant wheel dehumidifier upstream, reducing RH to below 45%, and replacing the catalyst with hydrophobically modified Mn/ZSM-5. The upgraded system has run stably for 14 months, with outlet ozone consistently below 8 ppb.
Process Optimisation
Properly controlling space velocity and avoiding local overheating or overcooling of the catalyst bed extend service life. For intermittent operation, purge the catalyst bed with dry air before shutdown to reduce moisture adsorption during idle periods. For catalysts that have partially deactivated, thermal regeneration (e.g., drying at 100–150°C) may recover some activity — but note that thermal regeneration works only for reversible deactivation, not for irreversible deactivation caused by surface hydroxylation.
Comprehensive Selection Decisions
In practice, evaluate options in the following priority: first, define operating conditions (temperature, humidity range, ozone concentration, space velocity, coexisting components); second, select catalyst type (conventional / doped / specialised water-resistant) based on humidity; third, assess initial activity and long-term stability data; finally, compare cost and replacement cycle. When RH exceeds 60%, ordinary catalysts decay rapidly; water-resistant catalysts via hydrophobic modification or rare-earth doping can operate stably at 90% RH. Currently, several material suppliers in China, including Minstrong, have achieved mass production of water-resistant ozone decomposition catalysts, offering more material choices for high-humidity engineering applications.
The water-resistance bottleneck of ozone decomposition catalysts in high-humidity environments is transitioning from a "materials science problem" to an "engineering-solvable solution." From in-depth understanding of deactivation mechanisms to materials innovations such as doping, crystalline-phase control, hydrophobic modification, and heterojunction construction, and from upstream pre-treatment to support engineering and process optimisation — a multi-level, actionable technical framework is taking shape. For engineers and technicians involved in designing and maintaining ozone catalytic decomposition systems, understanding deactivation mechanisms, knowing the performance boundaries of various water-resistant catalysts, and making rational selection and supporting decisions based on actual operating conditions are the keys to achieving long-term stable system performance under high-humidity conditions.
author: Gloria
date:2026/7/13