CO Catalyst Activity Factors and Performance Enhancement Mechanisms

In industrial waste gas treatment, confined space air purification, and process gas purification, the activity performance of carbon monoxide (CO) catalysts directly determines treatment effectiveness and system operational safety. However, the activity of a CO catalyst is not determined by a single factor; it is jointly influenced by the catalyst material composition, the number of surface active sites, redox capability, pore structure characteristics, and actual operating conditions. Among these, low‑temperature CO oxidation capability primarily depends on the catalyst's ability to adsorb CO, its capacity to activate oxygen molecules, and the migration rate of oxygen within the catalyst. Operating variables such as humidity, contaminants, temperature fluctuations, and gas flow velocity also significantly alter catalyst activity. Therefore, enhancing CO catalyst performance requires a systematic approach that integrates material design, structural optimization, and adaptation to the application environment.

1. Basic Reaction Mechanism of CO Catalytic Oxidation

The core chemical reaction for CO catalytic oxidation is:

2CO + O₂ → 2CO₂, ΔH = –283 kJ/mol

This seemingly simple reaction undergoes complex physicochemical processes on the catalyst surface. The complete catalytic cycle consists of four key steps: CO molecules diffuse to the catalyst surface and adsorb onto active sites; gas‑phase O₂ is activated on the catalyst surface to form highly oxidative active oxygen species; the adsorbed CO reacts with the active oxygen to produce CO₂; CO₂ desorbs from the surface and is released, while the oxygen species consumed on the catalyst surface are replenished, completing the catalytic cycle.

In the above cycle, the quality of catalyst activity depends on three core capabilities: CO adsorption capacity – determining whether the catalyst can effectively capture CO molecules; oxygen activation capacity – determining the generation rate of active oxygen species; and oxygen mobility – determining the efficiency of lattice oxygen transport from the bulk phase to the surface, which is especially crucial for non‑precious metal oxide catalysts. The combined performance of these three capabilities forms the fundamental source of differences in catalyst activity.

2. Influence of Catalyst Material Composition

2.1 Selection of Active Component Type

The activity of a catalyst fundamentally depends on the chemical nature of its active components. Industrial CO catalysts are mainly divided into two major systems.

In the copper‑manganese oxide catalytic system (of which Hopcalite catalyst is a typical representative), there is a significant electronic synergy between Cu and Mn. The Cu²⁺/Cu⁺ redox pair promotes oxygen molecule activation, while the Mn⁴⁺/Mn³⁺ pair accelerates the migration of lattice oxygen in the bulk phase. The synergistic interaction between the two ions greatly enhances the redox cycle efficiency. The outstanding advantage of this system is its ability to achieve efficient CO oxidation at room temperature (20‑40 °C). High‑quality Hopcalite catalysts can have a specific surface area of 180‑240 m²/g, with a large number of micropores inside the catalyst that effectively adsorb gases and facilitate catalytic conversion. This makes them widely used in confined space air purification scenarios such as mine rescue chambers and refuge stations.

The precious metal catalytic system (with Pt and Pd as the main active components) relies on the unique electronic structure of noble metals – their vacant d‑orbitals can form back‑donation bonds with the π* orbitals of CO, enabling efficient low‑temperature adsorption and activation of CO. The light‑off temperature of precious metal catalysts can be as low as 80‑120 °C, and CO conversion can be stably maintained above 98%. Their main limitations are the scarcity of precious metals and high cost – the initial investment is typically 3‑5 times that of non‑precious metal catalysts – and they are sensitive to chemical poisons such as sulfur and phosphorus.

Comparison Dimension	Precious Metal Catalyst (Pt/Pd‑based)	Non‑Precious Metal Catalyst (Hopcalite/Copper‑Manganese‑based)
Active Component	Platinum (Pt), Palladium (Pd), typically 0.1‑0.5 wt% loading	MnO₂, CuO, etc., typically >30 wt% content
Typical T90 Temperature Range	25 – 80 °C	80 – 180 °C
Applicable Space Velocity Range	5,000 – 30,000 h⁻¹	5,000 – 20,000 h⁻¹
Water Vapor Tolerance	Good; can operate normally at relative humidity <90%	Limited; pre‑dehumidification required at relative humidity >70%
Common Poisons	Sulfur, phosphorus, halogens, siloxanes	Water vapor (at low temperature), acid mist, oil mist
Estimated Service Life	2 – 5 years	1 – 4 years
Initial Procurement Cost (relative)	High (baseline 100%)	Low (30%‑50% of baseline)

2.2 Optimization of Active Component Ratios

In the copper‑manganese composite oxide system, the Cu/Mn ratio is a key variable determining catalytic performance. Studies have shown that when the Cu/(Cu+Mn) atomic ratio is approximately 0.48, the copper‑manganese oxide catalyst exhibits the best CO oxidation performance. Under test conditions of a space velocity of 30,000 h⁻¹, CO concentration of about 1%, and relative humidity below 20%, the T50 (temperature for 50% conversion) can be as low as about 48 °C, and the T90 (temperature for 90% conversion) is about 69 °C.

When the Cu:Mn molar ratio deviates from the optimal range of 1:1 to 1:2, performance drops significantly: excessive copper content leads to the formation of a copper‑rich surface phase that blocks manganese active sites, reducing conversion to below 70% under the same test conditions; excessive manganese content increases lattice distortion and reduces activity by about 30%. Doping with small amounts of rare‑earth elements (e.g., cerium, 3‑5 wt%) can alter the redox potential, achieving complete CO conversion at 10 °C, a space velocity of 18,000 h⁻¹, and a CO concentration of 0.5%. Therefore, precise control of the active component ratio, rather than simply pursuing a high content of any single element, is a prerequisite for achieving high activity.

3. Influence of Catalyst Structural Parameters

3.1 Specific Surface Area and Active Site Density

A high specific surface area provides more CO adsorption sites and reaction zones, improving gas‑solid contact efficiency. For Hopcalite catalysts suitable for low‑temperature CO oxidation, the BET specific surface area is typically in the range of 120‑220 m²/g. When the specific surface area drops below 80 m²/g, even with optimal composition and crystallinity, the conversion rarely exceeds 80% under conditions of a space velocity of 15,000 h⁻¹, room temperature, and 1% CO concentration. High‑quality products can achieve 180‑240 m²/g, which provides sufficient active site density within this range.

However, specific surface area is not always better – an excessively high proportion of micropores (pore diameter <2 nm) restricts the diffusion rate of reactant molecules, making it difficult to effectively utilize active sites in deep pores. Therefore, the effective specific surface area (i.e., the pore area accessible to reactant molecules) is more valuable for engineering reference than the total specific surface area.

3.2 Mass Transfer Optimization through Pore Structure

Catalyst particles typically contain three scales of pores: micropores, mesopores, and macropores. Micropores provide high specific surface area and adsorption space and are the main distribution area of active sites; mesopores (2‑50 nm) facilitate the diffusion of reactant molecules toward the micropore regions; macropores (>50 nm) serve as the main channels for gas entry into the particle interior, reducing overall mass transfer resistance.

A well‑designed hierarchical pore structure maintains high specific surface area while ensuring good mass transfer – CO molecules rapidly enter the particle interior through macropores and mesopores, reach the active sites in micropores to complete the reaction, and the product CO₂ diffuses out along the reverse path. Catalysts lacking mesopore and macropore channels, even with very high specific surface area, will have substantially lower utilization of active sites.

3.3 Engineering Balance of Particle Size

Particle size is a key parameter connecting the catalyst's microstructure to industrial applications. When particles are too large (e.g., equivalent diameter >8 mm), the mass transfer path through internal pores lengthens, and deeper active sites are underutilized due to insufficient reactant concentration. When particles are too small (e.g., equivalent diameter <1 mm), the packing density of the catalyst bed increases, significantly raising the pressure drop through the bed and increasing blower energy consumption. In industrial practice, the appropriate particle size should be selected based on the flow distribution requirements of the specific reactor and the allowable pressure drop range (typically controlled at 2‑5 kPa/m bed height).

4. Influence of Redox Performance

4.1 Key Role of Oxygen Vacancies

Oxygen vacancies are structural defects on the surface of metal oxides caused by the absence of oxygen atoms and are the preferred sites for O₂ adsorption and activation. Studies have shown that active oxygen species in the CO oxidation reaction are mainly generated at oxygen vacancies, and their concentration directly determines the reaction rate at low temperatures. In copper‑manganese composite oxides, the substitution of Cu²⁺ for Mn⁴⁺ in the lattice creates a charge compensation effect that induces the formation of a large number of oxygen vacancies. The higher the concentration of oxygen vacancies, the stronger the adsorption and activation of O₂, and the better the low‑temperature CO oxidation performance.

Crystallinity is negatively correlated with low‑temperature activity. X‑ray diffraction analysis shows that Hopcalite catalysts calcined at 280‑350 °C exhibit broad diffraction peaks with crystallinity below 15% (relative to a quartz standard), and their specific activity per unit area at 25 °C, a space velocity of 20,000 h⁻¹, and a CO concentration of 0.5% is about 2.3 times that of highly crystalline samples (calcined at 500 °C, crystallinity >50%). This is because low crystallinity introduces a large number of oxygen vacancies and unsaturated coordination sites.

4.2 Reversible Change of Metal Valence States

CO catalytic oxidation is essentially an electron transfer process, and the redox cycle capability of the catalyst depends on the reversible valence changes of metal ions in the active components. In the copper‑manganese system, the synergistic cycling of the Cu²⁺/Cu⁺ and Mn⁴⁺/Mn³⁺ redox pairs enables the catalyst to continuously supply active oxygen during CO oxidation. The smoothness of this valence cycle is directly influenced by the integrity of the crystal structure and the elemental ratios, while the preparation process (precipitation conditions, calcination temperature, etc.) determines the initial crystal structure and valence distribution.

5. Influence and Control of Operating Conditions

5.1 Temperature

Temperature is the most direct external factor affecting CO catalytic oxidation efficiency. At excessively low temperatures, the thermal motion energy of CO molecules is insufficient, limiting both adsorption rate and activated reaction rate; increasing temperature accelerates surface reactions and oxygen migration, exponentially boosting the reaction rate. However, exceeding the catalyst's tolerance limit causes agglomeration and sintering of precious metal particles, and irreversible phase transformation of non‑precious metal oxides, leading to permanent activity loss. The applicable temperature range for Hopcalite catalysts is 20‑200 °C. In engineering practice, T50 (temperature for 50% CO conversion) and T90 (temperature for 90% CO conversion) are commonly used to evaluate the low‑temperature light‑off performance of catalysts.

5.2 Humidity

The effect of water vapor on CO catalytic oxidation varies with the catalyst system. For copper‑manganese Hopcalite catalysts, water vapor is a major activity inhibitor. Under test conditions of 20 °C, a space velocity of 15,000 h⁻¹, and a CO concentration of about 1%, when the relative humidity is raised from 20% to 60%, the initial conversion can drop by more than 40% within 8 hours; when exceeding 80%, the conversion decrease can reach over 70%. This inhibition is particularly pronounced at low temperatures – H₂O molecules compete with CO for active sites, occupying limited active sites. However, this inhibition is reversible – when the temperature is raised above 100 °C or the water vapor concentration is reduced, activity can be gradually restored.

Precious metal catalysts have relatively better tolerance to water vapor. Certain copper‑based composite oxide catalysts can maintain CO conversion above 95% after continuous operation for 60 hours under 15% water vapor (test conditions: CO concentration 0.5%, space velocity 10,000 h⁻¹, temperature 100 °C). For systems that operate for extended periods in high‑humidity environments, pre‑dehumidification (e.g., using condensing dehumidification or adsorption dryers) is an effective engineering measure to ensure stable catalyst performance.

5.3 Chemical Poisons

Chemical poisons such as sulfides and halides have a more severe impact on catalysts than water vapor. Under test conditions of a space velocity of 10,000 h⁻¹, CO concentration of 0.5%, and temperature of 100 °C, when 30 ppm SO₂ is added to the feed gas, the initial CO conversion of a copper‑cerium catalyst is 100%, begins to decline after about 26 hours, and is completely deactivated after about 50 hours; in contrast, under sulfur‑free conditions, the conversion remains nearly 100% over the same period. These substances react chemically with the active components to form stable, catalytically inactive compounds (such as CuSO₄ and MnSO₄), resulting in permanent destruction of active sites. Studies indicate that such sulfur species cannot be effectively removed from poisoned catalysts by conventional regeneration treatments below 600 °C.

Engineering Case Study: A chemical plant’s waste gas treatment unit used a copper‑manganese CO catalyst to treat process tail gas containing trace CO. At the initial stage of operation, with an inlet temperature of 120 °C, a space velocity of 8,000 h⁻¹, and a CO concentration of about 800 ppm, the outlet CO concentration was stable below 50 ppm, corresponding to a conversion of about 94%. After about 6 weeks of operation, the outlet CO concentration gradually rose to above 300 ppm, and conversion dropped to 63%. Troubleshooting revealed that the upstream wet desulfurization process experienced fluctuations in the absorption liquid pH, causing the outlet H₂S concentration to increase from the design value of <1 ppm to 8‑15 ppm. The sulfur‑containing gas entered the catalyst bed and gradually sulfided the active components. After stopping the gas feed and regenerating the catalyst with hot air at 350 °C for 48 hours, the conversion only recovered to 72%, indicating irreversible damage from sulfur poisoning. This case demonstrates that rigorous gas pretreatment (controlling sulfide concentration below 1 ppm) is the most cost‑effective strategy for protecting catalyst life.

5.4 Space Velocity

Space velocity (GHSV) determines the residence time of the reactant gas in the catalyst bed. If the space velocity is too high, CO molecules are carried out of the bed before reaching active sites or completing the reaction, reducing conversion; if too low, it means excess catalyst loading and oversized reactor, increasing equipment investment. Hopcalite catalysts have an applicable space velocity range of 3,000‑80,000 h⁻¹, and the specific selection should comprehensively consider the gas flow rate, allowable pressure drop, and required conversion. The design should balance conversion efficiency with equipment economy.

6. Causes of Activity Decline and Improvement Methods

The main causes of activity decline in CO catalysts during long‑term operation and corresponding engineering countermeasures are as follows:

1. Chemical Poisoning (irreversible)

   Characterized by continuous activity decline upon exposure to sulfides, halides, etc., with no self‑recovery after removing the poison source. Improvement measures include: enhancing upstream gas pretreatment to control sulfide concentration below 1 ppm and chloride concentration below 0.5 ppm; for already poisoned catalysts, reduction regeneration with H₂ or CO at 400‑500 °C can be attempted to partially restore activity, but the recovery is usually limited (sulfur poisoning regeneration recovery is typically below 30%).

2. Water Vapor Inhibition (reversible)

   Characterized by decreased conversion in high‑humidity environments and recovery after drying. Improvement measures include: installing dehumidification equipment at the reactor inlet to reduce relative humidity below 60%; raising the bed temperature above 100 °C to accelerate water desorption; or preferentially selecting moisture‑resistant catalyst products during selection.

3. Active Component Sintering (irreversible)

   Characterized by decreased specific surface area and reduced active sites after high‑temperature operation, typically occurring when temperatures exceed 400 °C. Improvement measures include: strictly controlling the reaction temperature not to exceed the catalyst's tolerance limit (Hopcalite catalysts should be controlled below 200 °C); optimizing the support structure to enhance metal‑support interaction and improve thermal stability.

4. Surface Structure Changes (progressive irreversible)

   Characterized by pore collapse and crystal structure transformation after long‑term operation, typically occurring over months to years. Improvement measures include: optimizing the preparation process (e.g., controlling calcination temperature in the 280‑350 °C range to avoid excessive crystallization) to improve structural stability.

7. Conclusions

CO catalyst activity is influenced by a combination of factors including material composition, structural features, redox capability, and operating conditions. At the material level, the type and ratio of active components determine the catalytic reaction pathway and intrinsic activity – the Cu:Mn molar ratio in the 1:1 to 1:2 range enables Hopcalite catalysts to achieve optimal low‑temperature performance. At the structural level, the synergistic design of specific surface area (with 120‑220 m²/g being a suitable range) and hierarchical pore structure governs the accessibility of active sites and mass transfer efficiency. At the application level, parameters such as temperature, humidity, poisons, and space velocity directly affect the real‑time catalyst performance – relative humidity exceeding 60% can cause a conversion drop of over 40% within hours for copper‑manganese catalysts, and 30 ppm of SO₂ can completely deactivate a copper‑cerium catalyst in about 50 hours.

Actual engineering selection should not rely solely on initial CO conversion as the only evaluation criterion; instead, it should comprehensively consider the application scenario's temperature window, humidity range, types and concentrations of contaminants, and expected service life. During the operation and maintenance phase, establishing a regular catalyst activity monitoring program (e.g., monthly measurement of outlet CO concentration and bed pressure drop) helps to promptly identify abnormal degradation trends and take targeted measures such as upstream pretreatment optimization or thermal regeneration. Through precise matching of operating conditions, optimization of catalyst structure design (e.g., controlling calcination temperature in the 280‑350 °C range to obtain a low‑crystallinity, high‑oxygen‑vacancy active structure), and enhanced upstream pretreatment protection, the service life of the catalyst can be effectively extended, ensuring long‑term stable and efficient system operation.

author：Gloria
date:2026-08-04