Factors Influencing Reaction Temperature of CO Catalysts

I. Intrinsic Catalyst Properties Determine the Baseline Activity Temperature

Active Components and Chemical Composition

The active components and chemical composition are the primary factors. The core advantage of Hopcalite catalysts (copper‑manganese mixed oxides) is their ability to initiate CO oxidation at ambient temperature (20–40°C). The Cu:Mn molar ratio directly affects low‑temperature activity: when the ratio is between 1:1 and 1:2, the CO conversion at room temperature can exceed 90% (at a space velocity of 15,000 h⁻¹ and 30% relative humidity). Deviations from this range cause a marked decline in activity.

Crystal Phase Structure and Crystallinity

Crystal phase structure and crystallinity are equally critical. X‑ray diffraction analysis shows that Hopcalite samples calcined at 280–350°C exhibit low crystallinity, and their room‑temperature specific activity is more than twice that of highly crystalline samples calcined above 500°C. The reason is that the low‑crystallinity samples contain abundant oxygen vacancies and unsaturated coordination sites, which serve as the active centers for CO catalytic oxidation. Calcination below 300°C results in incomplete decomposition of the precursor; above 450°C, it causes particle sintering and a sharp drop in specific surface area.

Specific Surface Area and Pore Structure

Specific surface area and pore structure affect the number of active sites and the efficiency of gas diffusion. For Hopcalite catalysts suitable for low‑temperature CO oxidation, the BET specific surface area generally ranges from 120 to 220 m²/g, and a mesopore (2–10 nm) fraction exceeding 60% is more favorable for gas diffusion. When the specific surface area falls below 80 m²/g, even an optimal chemical composition cannot achieve practical conversion at room temperature.

II. Interactive Effects of Reactant Concentrations on Temperature

CO Concentration

An increase in CO concentration raises the reaction exotherm, which can cause a significant temperature rise in the catalyst bed. For example, with a CO concentration of 0.5% in the exhaust gas, the theoretical adiabatic temperature rise upon complete conversion is about 60–70°C (the exact value depends on the heat capacity of the flue gas). This temperature rise has both beneficial aspects—helping the system sustain a self‑sustaining reaction at a lower inlet temperature—and potential risks, such as thermal runaway when heat dissipation is inadequate.

Hopcalite catalysts begin to undergo irreversible structural changes above 100°C, and prolonged high‑temperature operation accelerates deactivation. Under high‑CO‑concentration conditions, the core task of temperature management is to strike a balance between utilising the reaction heat to maintain self‑sustaining operation and preventing overheating‑induced deactivation.

Oxygen Content

Oxygen content is equally important. Oxygen is a direct participant in CO oxidation. When the O₂ concentration is below 1%, temperature variations have a more pronounced effect on conversion efficiency; when the O₂ concentration reaches 10%, the reaction becomes less sensitive to temperature, and system stability improves. Insufficient oxygen cannot be compensated for by raising the temperature—the reaction is then limited by stoichiometry rather than kinetics.

III. Space Velocity: The Engineering Variable That Determines Contact Time

Space velocity (GHSV) defines the volume of gas processed per unit volume of catalyst per hour and is a core parameter in engineering selection. The higher the space velocity, the shorter the residence time of the gas in the catalyst bed, and the lower the single‑pass conversion. For a given catalyst and inlet conditions, doubling the space velocity typically requires a 10–30°C increase in temperature to achieve the same conversion.

Recommended space velocity ranges vary significantly across applications: for continuous industrial exhaust treatment, it is generally 8,000–15,000 h⁻¹; for intermittent respiratory protection equipment, it can reach 15,000–25,000 h⁻¹. For granular Hopcalite products under dry air conditions, the tolerable space velocity range is 3,000–80,000 h⁻¹.

An important corollary of this rule is that temperature data for different catalysts are comparable only when obtained under similar space velocities. Relying solely on the minimum light‑off temperature is not sufficient to judge the engineering applicability of a catalyst.

IV. Suppression of Low‑Temperature Activity by Humidity

Water vapor is one of the most critical external factors affecting the low‑temperature performance of CO catalysts. Hopcalite catalysts are sensitive to humidity—activity begins to decline noticeably when the relative humidity exceeds 45%.

The mechanism involves three aspects: competitive adsorption of water molecules with CO on active sites; formation of a water film on the surface at high humidity, which blocks access to active sites; and long‑term exposure to high humidity, which may cause structural changes in the active components.

Quantitative data show that at 25°C and a space velocity of 20,000 h⁻¹, a typical Hopcalite catalyst achieves a CO conversion above 95% at 30% relative humidity; when the relative humidity rises to 80%, the conversion can drop below 50% within 2 hours. To achieve the same conversion under wet conditions as under dry conditions, the reaction temperature typically needs to be increased by 20–40°C, or the catalyst loading needs to be increased by 30–50%. For conditions with humidity exceeding 70%, simply increasing the loading is of limited effectiveness; a drying pretreatment unit is usually required upstream of the catalyst bed.

V. Impurity Gases and Catalyst Poisoning

SO₂ and other sulfur‑containing species can react with active components to form stable sulfates, leading to permanent loss of active sites. When the inlet SO₂ concentration exceeds 30 ppm, the catalyst lifetime may be shortened to less than one‑third of its original value. Chlorine‑containing gases are also poisonous.

Once chemical poisoning occurs, the previously effective reaction temperature is no longer applicable, and a higher temperature is required to maintain the same conversion. However, increasing the temperature does not eliminate the poisoning effect—it only partially compensates for the loss of activity. For sulfur‑ or chlorine‑containing streams, the preferred strategy is to install desulfurization and dechlorination pretreatment upstream of the catalyst bed, or to select a poison‑resistant formulation.

VI. Positive Feedback from Reaction Heat

CO catalytic oxidation is a strongly exothermic reaction (ΔH = 283 kJ/mol). The heat released raises the bed temperature, and the elevated temperature in turn accelerates the reaction rate—creating a positive feedback loop.

A direct manifestation of this mechanism is the hysteresis effect between light‑off and extinction: once the reaction ignites, the bed temperature rises due to the exotherm, allowing operation at a lower inlet temperature; however, if the inlet temperature or CO concentration drops further, the reaction may extinguish, requiring reheating to recover.

In engineering practice, temperature monitoring should not be limited to the reactor inlet. It is recommended to install temperature measurement points at the bed inlet, at the bed midpoint (hot‑spot location), and at the outlet. A sudden decrease in the temperature difference (ΔT) across the bed may indicate a drop in conversion (less heat release); when the hot‑spot temperature persistently exceeds 100°C, the adequacy of heat dissipation and the risk of thermal runaway should be assessed.

VII. The Temperature Gap Between Laboratory and Industrial Sites

It is common for a catalyst to perform well in laboratory tests but require a higher temperature when applied in the field—this is a typical phenomenon in scale‑up.

The reasons are: laboratory tests usually use well‑controlled gas mixtures (stable CO concentration, humidity, and flow rate), whereas industrial exhaust often exhibits fluctuations in CO concentration, humidity variations, changes in space velocity, and interference from sulfur‑ and chlorine‑containing impurities. In addition, industrial reactors differ from laboratory setups in terms of gas distribution uniformity, bed aspect ratio, and heat‑transfer conditions. Engineering factors such as catalyst packing, flow distribution, and insulation can raise the actual required temperature by 20–50°C compared to laboratory data.

Therefore, laboratory temperature data can serve as a performance reference but should not be directly equated with the optimal operating temperature for the actual unit. Engineering selection should primarily rely on test data obtained under conditions that closely simulate the real‑world application.

VIII. Practical Methods for Determining Operating Temperature

Determining a reasonable operating temperature requires a comprehensive consideration of the following factors:

• Inlet CO concentration and O₂ content
• Gas flow rate and space velocity
• Gas humidity and impurity composition
• Target CO conversion
• Catalyst loading and bed design
• Allowable energy consumption of the system

On this basis, it is advisable to establish a temperature‑conversion curve by testing conversion at different temperatures, and further evaluate the effects of humidity, impurities, and long‑term operation on activity.

Two common pitfalls in engineering practice deserve attention:

Pitfall 1: Directly using the supplier’s “minimum light‑off temperature”

The light‑off temperature is usually defined as T₅₀ (the temperature at which 50% conversion is achieved) or T₉₀ (the temperature for 90% conversion). In actual industrial applications, the target conversion is often 95% or higher, so the corresponding operating temperature will necessarily be higher than the light‑off temperature. The light‑off temperature should not be used as a substitute for the normal working temperature.

Pitfall 2: Blindly raising the inlet temperature when conversion drops

Raising the temperature can only partially compensate for activity loss. If the performance decline is due to catalyst poisoning, increased humidity, or bed plugging, simply raising the temperature is not only of limited benefit but may also accelerate thermal deactivation. The root cause should be identified first, and then appropriate measures—such as dehumidification, removal of impurities, regeneration, or replacement—should be taken.

Conclusion: The reaction temperature of a CO catalyst is not a fixed number on a product data sheet; it is the result of coupled effects from intrinsic catalyst properties and multiple factors including CO concentration, oxygen content, space velocity, humidity, impurities, reaction heat, and scale‑up effects. In engineering practice, the reasonable operating temperature should be determined based on test data obtained under conditions as close as possible to the actual application, and a monitoring system covering the bed temperature profile should be established. Only by fully understanding the mechanisms of each factor can one achieve the optimal balance among purification efficiency, energy consumption, and catalyst lifetime.

author：Gloria
date:2026-08-31