What are the common causes of deactivation for carbon monoxide catalysts in semiconductor applications? How can this be prevented?

I. Introduction
Carbon monoxide catalysts—primarily copper-manganese oxide systems (such as Hopcalite) in the semiconductor sector—play a pivotal role in high-purity gas production, cleanroom air purification, and process exhaust gas treatment. However, during operation, these catalysts gradually lose their activity due to a variety of factors. Engineering data indicates that approximately 65% ​​of cases involving the premature replacement of catalysts are directly attributable to deactivation. Understanding the mechanisms of deactivation and implementing targeted countermeasures are therefore central to ensuring the long-term, stable operation of purification systems.

Carbon Monoxide Catalysts

II. Major Types and Mechanisms of Deactivation
2.1 Moisture Poisoning: The Most Common, Yet Partially Reversible
Water molecules compete with CO for adsorption sites on the catalyst surface, thereby hindering reactants from accessing the active copper-manganese centers. In environments with a relative humidity exceeding 60%, the activity of standard catalysts can decline by more than 50% within just 24 hours; if humidity rises to 80% and persists for 72 hours, the CO conversion rate may plummet from 98% to below 40%. While activity can be partially restored once a dry atmosphere is re-established, repeated exposure to moisture shocks can result in irreversible damage to the catalyst’s crystal lattice structure.

Case Study: In a semiconductor air separation system, the catalyst’s conversion rate dropped precipitously during the high-humidity summer season; with an inlet CO concentration of 2.5 ppm, the outlet concentration surged to 1.8 ppm (whereas the design specification required it to be ≤ 0.5 ppm). An inspection revealed that the dew point at the dryer outlet had risen from -45°C to -28°C, resulting in a relative humidity of 65% at the catalyst bed inlet. Following the replacement of the desiccant, the catalyst’s activity gradually recovered over a period of 48 hours.

2.2 Chemical Poisoning: Irreversible Loss of Activity
Sulfur Poisoning: H₂S or SO₂ reacts with the CuO component of the catalyst to form CuS—a product of extreme stability. When the H₂S concentration reaches 0.5 ppm and persists for 24 hours, the CO conversion rate drops by approximately 40%; after 72 hours, it falls below 20%, rendering the catalyst incapable of recovery through standard regeneration procedures.

Chlorine Poisoning: Cl₂ or HCl reacts to form CuCl₂ or MnCl₂. This form of poisoning is even more potent than sulfur poisoning—exposure to just 0.2 ppm of Cl₂ for 48 hours is sufficient to cause the catalyst to lose more than 80% of its activity. Case Study: In a specific CVD exhaust gas treatment system, the conversion rate of the catalyst dropped from 95% to 58% after six months of operation. Surface analysis revealed a chlorine content of 3.2% (compared to <0.05% for fresh catalyst). Following the installation of an upstream alkaline scrubbing tower, the service life of the same batch of catalyst was extended to 14 months.

2.3 Thermal Sintering: Structural Failure at High Temperatures
The oxidation of CO is a strongly exothermic reaction (ΔH = -283 kJ/mol). When there is a sudden surge in inlet CO concentration or inadequate heat dissipation, the local temperature within the catalyst bed can exceed 100°C—far surpassing the catalyst’s optimal operating temperature range (0–50°C). A fresh catalyst with a specific surface area of ​​220 m²/g, after operating continuously at 80°C for 500 hours, saw its specific surface area decrease to 95 m²/g, resulting in a reduction in activity of approximately 60%. For every 20°C increase in temperature, the sintering rate increases by approximately one order of magnitude.

Case Study: In a high-purity gas production facility, an adsorption breakthrough in the upstream section caused the inlet CO concentration to rise from 2 ppm to 15 ppm. The ensuing vigorous reaction caused the temperature at the center of the catalyst bed to reach 95°C, with a radial temperature difference of 25°C. Subsequently, the conversion rate of this specific batch of catalyst never recovered to levels above 90%.

III. Systematic Prevention Strategies
3.1 Prevention of Moisture Poisoning
Upstream Drying: For air separation and high-purity gas production applications, the dew point at the dryer outlet should be ≤ -40°C; for exhaust gas treatment scenarios, the relative humidity should be ≤ 40%. It is recommended to install an online dew point monitoring and alarm system.

Selection of Moisture-Resistant Catalysts: For scenarios where the presence of moisture cannot be completely avoided (e.g., the fresh air intake sections of cleanrooms), catalysts modified with rare-earth doping (Ce, La) or hydrophobic carriers should be selected. These catalysts can maintain over 80% of their initial activity for more than six months in environments with a relative humidity of ≤ 90%.

Periodic Regeneration: Catalysts that have already suffered from moisture poisoning can be regenerated by purging with hot nitrogen gas at 80–100°C for 12–24 hours, thereby restoring 70%–90% of their initial activity. 3.2 Prevention of Chemical Poisoning
Upstream Removal: In scenarios involving sulfur- or chlorine-containing waste gas, configure upstream alkaline scrubbing towers (using 5%–10% NaOH solution, capable of removing over 95% of HCl and Cl₂) and activated carbon/molecular sieve adsorption beds. Install an inline pH meter to monitor the concentration of the alkaline solution.

Early Warning Monitoring:Install inline H₂S and Cl₂ detectors at the catalyst inlet. Set the alarm thresholds at H₂S > 0.1 ppm and Cl₂ > 0.05 ppm. If these limits are exceeded, activate backup pretreatment systems or shut down the unit to investigate the issue.

Poisoning-Resistant Catalysts: Select catalysts containing sacrificial components (e.g., ZnO) that preferentially react with sulfur or chlorine, thereby protecting the primary active components.

3.3 Prevention of Thermal Sintering
Activity Matching: For operating conditions characterized by significant fluctuations in inlet CO concentration, select catalysts with moderate activity and superior thermal stability. The guiding principle is to ensure that “outlet emission standards are met,” rather than blindly pursuing the absolute highest conversion rate.

Temperature Monitoring: Establish at least three temperature monitoring points across the radial cross-section of the catalyst bed. If the radial temperature difference exceeds 10°C, inspect the gas flow distribution; if the bed temperature exceeds 70°C, reduce the inlet CO load or enhance heat dissipation.

Space Velocity Optimization: For waste gas streams with CO concentrations ranging from 500 to 1,000 ppm, design the space velocity to be approximately 15,000 h⁻¹. This allows for an optimal balance between conversion efficiency and temperature rise.

4. Diagnosis and Recovery from Deactivation
Moisture Poisoning: Manifests as a decline in conversion efficiency while the pressure drop across the catalyst bed remains essentially normal. Furthermore, activity can be gradually restored when the catalyst is exposed to a dry atmosphere. Regeneration can be achieved via purging with hot nitrogen gas under conditions of 80–100°C for a duration of 12–24 hours; this process typically restores 70%–90% of the catalyst’s initial activity. Note that prolonged and repeated exposure to moisture may result in irreversible structural damage, in which case the effectiveness of recovery efforts will be significantly diminished.

Sulfur and Chlorine Poisoning: These constitute forms of irreversible deactivation. Diagnosis is based on a persistent decline in conversion efficiency that remains unresponsive to standard regeneration procedures. Furthermore, elemental analysis of the catalyst surface will reveal sulfur or chlorine concentrations significantly higher than those found in fresh catalyst samples (e.g., a chlorine content exceeding 0.1%). This type of deactivation cannot be reversed through regeneration methods; the entire catalyst charge must be replaced.

Thermal Sintering: This constitutes an irreversible and permanent form of deactivation. Its characteristics include a reduction in specific surface area exceeding 40% compared to the fresh state, and X-ray diffraction patterns revealing significant growth in the crystallite size of the active components. Once thermal sintering occurs, the catalyst cannot be repaired and requires immediate replacement. The key to prevention lies in strictly controlling the catalyst bed temperature to avoid localized overheating.

Dust Fouling: This is a form of physical deactivation, typically manifested by an increase in the catalyst bed pressure drop exceeding 30% of the initial value. For mild fouling, fine particulates can be removed via back-flushing or mechanical screening, with an expected recovery rate of 50%–70%. If the fouling is severe or accompanied by chemical poisoning, the entire catalyst charge must be replaced.

Maintenance Recommendations: It is recommended to establish an operational log for each catalyst unit, recording key parameters—such as inlet and outlet CO concentrations, bed pressure drop, and radial temperature distribution—on a weekly basis. If the conversion rate drops by more than 15% and the underlying cause remains unclear, operations should be halted to collect samples for analysis; targeted remedial measures can then be implemented once the specific type of deactivation has been identified.

V. Conclusion
Deactivation of carbon monoxide catalysts in semiconductor applications primarily stems from three mechanisms: competitive adsorption of moisture, chemical poisoning (by sulfur or chlorine), and thermal sintering. Reversible deactivation caused by moisture can be prevented through upstream drying (maintaining a dew point of ≤ -40°C) or by selecting moisture-resistant modified catalysts. Irreversible poisoning caused by sulfur or chlorine requires upstream alkaline scrubbing or adsorption-based removal, coupled with the implementation of online monitoring and early warning systems. Thermal sintering necessitates a proper balance between catalyst activity and operational load conditions, as well as enhanced control over the catalyst bed temperature. Through systematic preventive measures and periodic diagnostics, the service life of the catalyst can be extended by 30%–50%, thereby significantly reducing the operation and maintenance costs of the purification system.

author：Gloria
date:2026-05-27