Mechanism of Activity Decline, Diagnosis, and Recovery Strategies for CO Catalysts After Moisture Exposure

The decline in CO catalyst activity caused by moisture exposure is generally reversible, with the extent of recovery depending on the severity of the exposure and the specific type of catalyst. Mild moisture exposure can be remedied by heat treatment at 100–130°C, restoring over 90% of the initial activity, whereas moderate exposure requires thermal regeneration at 150–200°C; however, prolonged exposure to high-humidity environments may induce changes in the lattice structure, potentially leading to partial, irreversible deactivation. Understanding the interaction mechanism between moisture and active sites facilitates informed decision-making regarding diagnosis and activity recovery under various operating conditions.

I. Mechanisms of CO Catalyst Deactivation by Moisture
1.1 Competitive Adsorption and Active Site Occupation
The catalytic oxidation of CO relies on the adsorption of CO and O₂ onto active sites on the catalyst surface. When water vapor enters the reaction system, H₂O molecules compete with CO for these active sites, thereby inhibiting CO adsorption. In-situ DRIFT studies have revealed that, in the presence of 3.1 vol% water vapor, competitive adsorption between water molecules and CO—along with the formation of carboxylate and bicarbonate species—is the primary cause of the decline in catalyst performance.

The impact of moisture is particularly pronounced for copper-manganese-based catalysts. Fourier-transform infrared (FTIR) spectroscopy indicates that as water content increases, the CO adsorption peak associated with Cu⁺ gradually diminishes, signaling a reduction in CO adsorption capacity; this is the primary cause of catalyst deactivation in humid environments. Cu²⁺ ions form coordination hydrates with water, occupying active sites that would otherwise be available for CO adsorption.

1.2 Hydroxyl Blocking and Reaction Pathway Shifts
Water vapor dissociates on the catalyst surface to form hydroxyl groups (-OH). These hydroxyl species occupy active sites; however, surface hydroxyl groups are inert toward CO oxidation at low temperatures and cannot participate in the catalytic cycle. Studies indicate that the deactivation effect of water vapor can be attributed to H₂O blocking the active sites on the catalyst surface; hydroxyl groups formed by the reaction between surface lattice oxygen and water obstruct the generation pathways of active oxygen species.

For MCo₂O₄ (M = Mn, Ce, Cu) systems, water molecules adsorbed on oxygen vacancies inhibit the formation of active oxygen species, leading to catalyst deactivation in humid air. Additionally, if formate intermediates fail to convert into CO₂, the accumulation of these species on active sites can also result in reduced catalytic activity.

1.3 Pore Blockage and Increased Mass Transfer Resistance
When moisture enters the catalyst pores, water occupies some micropores and mesopores, increasing gas diffusion resistance; this slows the rate at which CO penetrates the catalyst interior and reduces the effective reaction area. In cases of severe moisture exposure, catalyst particles may adhere or agglomerate, preventing the gas from passing uniformly through the catalyst bed.

1.4 Determining the Reversibility of Deactivation
The reversibility of deactivation caused by moisture depends on the mode of interaction between water molecules and active sites. When water molecules temporarily occupy active sites through competitive adsorption, activity can usually be restored once the moisture source is removed. Studies show that catalyst activity recovers to varying degrees after the water vapor supply is cut off, indicating that water-induced deactivation is partially reversible. However, prolonged exposure to high-humidity environments can cause water molecules to induce surface hydroxylation of active components or alter the lattice structure, leading to an irreversible loss of some activity. Water-induced deactivation involves both reversible and irreversible pathways; irreversible deactivation is typically attributed to the decomposition of water on the catalyst surface into hydroxyl radicals, which trigger structural degradation.

II. Differences in Moisture Resistance Among Catalyst Types
2.1 Copper-Manganese-Based Catalysts
Copper-manganese-based catalysts are among the most widely used catalysts for low-temperature CO oxidation, yet they exhibit significant sensitivity to moisture. Water vapor poisoning rapidly degrades copper-manganese-based catalysts; when relative humidity exceeds 40%, catalytic activity can drop by more than 50% within a few hours. Upon exposure to moisture, the redox cycle between copper oxide and manganese dioxide is disrupted and oxygen migration efficiency declines, manifesting as an increase in the reaction light-off temperature or a decrease in CO removal efficiency. While CuMnO₂ maintains 100% CO conversion in dry air for 10 hours, its activity drops significantly in humid air, as H₂O competes with CO for adsorption sites on the catalyst surface.

2.2 Noble Metal Catalysts
The moisture resistance of noble metal catalysts varies. Platinum-based catalysts retain significant activity under humid conditions (10%–95% RH); studies have confirmed that water enhances CO oxidation on Pt/ceria and Pt/alumina catalysts, though the extent of this effect depends on water concentration. Water promotes the CO oxidation light-off process for Pd and Pt-Pd catalysts. However, on Pt-based catalysts, H₂O molecules cover Pt and Co³⁺ active sites, inhibiting the adsorption of CO and O₂. Commercial gold-based catalysts exhibit high activity under dry conditions, but their activity drops markedly when relative humidity exceeds 25%.

2.3 Modified Transition Metal Oxide Catalysts
Doping can significantly enhance the moisture resistance of catalysts. Taking copper-doped OMS-2 (octahedral molecular sieve) catalysts as an example, Cu doping not only boosts CO oxidation activity but also substantially improves water resistance. Research indicates that optimal performance is achieved at a Cu doping level of 1.91 wt%, with the Cu-doped OMS-2 catalyst demonstrating excellent activity retention under humid conditions. The NaOH-treated Cu-OMS-2 catalyst maintains 100% CO conversion for up to 50 hours in the presence of 15 vol% H₂O and retains approximately 80% of its activity even after undergoing three cycles of alkali impregnation and calcination.

Incorporating SnO₂ into the Cu-Mn catalyst system significantly inhibits H₂O adsorption on the catalyst surface while promoting synergistic effects between Cu and Mn. This facilitates the formation of bidentate carbonate intermediates that decompose more readily, thereby enhancing catalytic activity in humid environments. The CuMnSn8 catalyst achieves 100% CO conversion at 85% relative humidity and 55°C.

A carbon-coating strategy is equally effective. In MOF-derived Co₃O₄/C composite catalysts, the residual amorphous carbon matrix acts as a "water-resistant barrier"; its weak affinity for water molecules effectively prevents competitive H₂O adsorption.

III. Methods for Assessing Moisture Exposure
Visual inspection is the first step. Mild moisture exposure results in negligible changes to the catalyst's appearance, potentially manifesting only as a hygroscopic state; moderate to severe exposure is indicated by damp particle surfaces, inter-particle adhesion, and noticeable agglomeration.

Assessing operational data provides a more reliable diagnosis. If the outlet CO concentration rises, higher temperatures are required to achieve the design performance, or the catalyst service life is shortened—all while system operating parameters remain unchanged—moisture exposure should be investigated as a primary cause. Activity loss due to moisture is typically accompanied by an increase in the reaction light-off temperature, whereas sulfur poisoning is characterized more by the permanent destruction of active sites.

For critical industrial applications, it is recommended to verify the catalyst's actual condition through CO catalytic oxidation performance tests and moisture content analysis. Temperature-Programmed Desorption (H₂O-TPD) allows for the quantitative assessment of the catalyst's water adsorption capacity and desorption temperature range, providing a basis for formulating subsequent regeneration strategies. 4. Methods for Restoring Moisture-Affected Catalysts
4.1 Mild Moisture Exposure: Low-Temperature Heat Treatment
For slight moisture absorption caused by ambient humidity fluctuations, preheating at 80–150°C can be employed; the optimal parameters involve heating at 100–130°C for 4–10 minutes. Catalysts treated under these conditions can recover over 90% of their initial activity. The underlying principle is that heating causes the desorption of water molecules adsorbed on active sites, thereby re-exposing those sites. Air calcination is another effective restoration method; calcining the deactivated catalyst in air at 300°C for 2 hours can restore its activity.

4.2 Moderate Moisture Exposure: Thermal Regeneration
For copper-manganese-based catalysts, standard thermal regeneration involves heating at 150–200°C. For cobalt-doped manganese oxide catalysts, treatment at 200°C for 1 hour achieves significant activity recovery while maintaining the integrity of the crystal structure. After pretreatment at 150°C in an N₂ atmosphere, the CO oxidation activity of moisture-deactivated Co₃O₄ catalysts at 0°C can be fully restored. Air-purging regeneration has also been proven effective in restoring the activity of catalysts deactivated by water vapor poisoning.

4.3 Severe Moisture Exposure: Replacement or Deep Regeneration
If the catalyst has been immersed in water for an extended period, is severely agglomerated, or exhibits a significant drop in activity, complete recovery via simple heat treatment is usually difficult. For noble metal catalysts, deep regeneration can be achieved through alternating oxidation-reduction treatments: the oxidation stage oxidizes surface-adsorbed CO into CO₂ (which then desorbs), while the reduction stage converts oxidized active metals back into metallic active sites.

For Pt-CeO₂ catalysts, in-situ treatment with CO at 200°C can generate surface oxygen vacancies, thereby enhancing the adsorption and activation of O₂ at the interface. In industrial practice, the catalyst can be regenerated via a reduction reaction by contacting it with a hydrogen-nitrogen mixture; the regenerated catalyst retains approximately 90% of the stability of the fresh catalyst. Oxidative regeneration typically employs a gas mixture of oxygen and nitrogen with an oxygen content of 0.1%–21.0% by volume, at temperatures ranging from 300°C to 600°C (preferably 350°C–500°C). Vacuum activation/regeneration is also an option for noble metal catalysts; the optimal temperature range is 350°C–400°C, allowing the catalyst to gradually recover its initial CO conversion activity within a reaction mixture containing a small amount of hydrogen.

4.4 Precautions for Regeneration Operations
During oxidative regeneration, excessively high oxygen concentrations or rapid heating rates can trigger intense exothermic reactions, leading to localized high temperatures and sintering of the active components. Industrial practice typically employs low oxygen concentrations (1%–5% O₂/N₂) combined with programmed temperature control. The regeneration temperature must balance activity recovery with the structural stability of the catalyst: temperatures that are too low fail to effectively desorb moisture, while excessively high temperatures can cause grain growth in the active components or damage the support structure.

V. Engineering Measures to Prevent Catalyst Moisture Exposure
Controlling humidity during storage and loading is fundamental. Catalysts should remain in sealed packaging during storage and be loaded for use as soon as possible after opening. Outdoor loading operations should be avoided during rainy or high-humidity weather.

Pre-treatment and protection during the operational phase are even more critical. A drying pre-treatment unit (such as a desiccant wheel or adsorption dryer) should be installed upstream of the catalytic module to reduce the humidity of the gas stream entering the catalyst bed. In environments where humidity is difficult to maintain below 45%—such as life-saving capsules and refuge chambers—a desiccant should be installed at the catalyst inlet; moisture-laden air passes through the desiccant to remove water vapor before flowing through the CO catalyst layer.

The choice of desiccant and its configuration directly affect the effectiveness of the protection. In engineering practice, a combination of silica gel and molecular sieves is frequently employed: silica gel serves as the primary, coarse desiccant, while 5A molecular sieves act as the secondary, deep-drying agent; using them in series achieves superior dehydration performance. In a field test conducted in a high-humidity environment (average humidity of 85%), installing a drying canister containing 2 kg of molecular sieves in series within the intake line extended the catalyst's service life from 15 days to 90 days, with desiccant regeneration required only once a month to maintain protective efficacy.

Another effective strategy involves a layered arrangement of the desiccant and the CO catalyst. By placing a layer of desiccant upstream of the CO catalyst, the incoming air is dehumidified before reaching the catalyst bed, thereby preventing moisture-induced catalyst deactivation. This layered configuration allows for multiple alternating layers, with the specific ratio adjusted based on actual operating conditions.

For applications subject to significant humidity fluctuations, a dual-tower switching design is recommended: one tower operates while the other undergoes regeneration, with the roles periodically swapped. Established engineering practices in air purification systems already position the CO oxidation zone downstream of the drying zone; gas is first dehydrated in the drying zone before entering the CO oxidation zone for catalytic oxidation, fundamentally eliminating moisture interference with the catalyst at the process design level.

Moisture resistance must also be a key consideration during catalyst selection. For high-humidity environments, priority should be given to moisture-resistant catalyst systems, such as Cu-doped OMS-2 or SnO₂-modified CuMnOx catalysts. Existing catalyst systems can be post-treated using hydrophobic modification techniques, such as the application of PVA or PVP. By understanding the interaction mechanism between moisture and the catalyst, fully accounting for operating humidity during selection, and incorporating drying pre-treatment units during operation, the impact of water vapor on the catalyst can be minimized at the source, thereby extending its service life.

Author: Gloria
Date: October 12, 2026