Research on the Mechanism of Sulfur and Chlorine Poisoning of Hopcalite Catalysts and Poison-Resistant Design

I. Introduction

Hopcalite is a non‑noble metal catalytic material whose main active components are manganese dioxide (MnO₂) and copper oxide (CuO). Owing to its excellent catalytic oxidation activity toward carbon monoxide (CO) at ambient temperature, it is widely used in mine rescue, refuge chambers, confined‑space air purification, respiratory protective equipment, and similar applications. Its catalytic mechanism is based on the synergistic effect between copper and manganese oxides—manganese oxide acts as the oxygen donor, copper oxide as the oxygen acceptor, and lattice oxygen migrates between them. CO captures lattice oxygen on the catalyst surface to form CO₂ and generates oxygen vacancies, and gas‑phase O₂ subsequently fills these vacancies to complete the catalytic cycle.

In practical operation, however, the feed gas often contains impurities such as hydrogen sulfide (H₂S), sulfur dioxide (SO₂), chlorine (Cl₂), and hydrogen chloride (HCl). These substances react chemically with the active components of the catalyst, causing poisoning and deactivation. Conventional Hopcalite catalysts show a significant decline in activity when the relative humidity exceeds 45%, and they are sensitive to sulfur and chlorine compounds. Sulfur and chlorine poisoning are among the most important deactivation causes that Hopcalite catalysts face in real‑world applications. Understanding the poisoning mechanisms and exploring effective anti‑poisoning design strategies are of great significance for extending catalyst service life and reducing replacement costs.

II. Mechanism of Sulfur Poisoning

2.1 Chemical Nature of Sulfur Poisoning

When the reaction gas contains sulfur‑containing compounds such as hydrogen sulfide and sulfur dioxide, sulfur reacts chemically with copper oxide and manganese oxide in the catalyst. The main reaction products include copper sulfide (CuS), manganese sulfide (MnS), and sulfate species. XPS analysis shows that sulfur forms CuS with copper and MnS with manganese, which disrupts the Cu–O–Mn bonding network and directly weakens the electronic synergistic effect between copper and manganese.

In industrial flue‑gas scenarios, sulfur dioxide is one of the main sulfur sources. Studies have shown that SO₂ can react with active sites on Cu–Mn catalysts to generate sulfite and sulfate species. These reaction products gradually accumulate on the catalyst surface, leading to progressive coverage of the active sites.

2.2 Deactivation Mechanism of Sulfur Poisoning

Sulfur poisoning affects Hopcalite catalysts mainly in three aspects:

• Coverage of active sites. Sulfides and sulfates form a covering layer on the catalyst surface, physically blocking CO molecules from approaching the active sites. Because the active sites are occupied, CO cannot make effective contact with the catalyst, and the catalytic reaction is forced to terminate.
• Destruction of crystalline structure. When the sulfur concentration exceeds a certain threshold or the exposure time is too long, manganese oxide can be reduced to MnS or MnSO₄ crystalline phases, causing irreversible structural collapse. Such structural damage cannot be restored by simple heat treatment.
• Change in electronic structure. The reaction of sulfur with the active components alters the electronic synergy between copper and manganese and weakens the oxygen spillover capability. The lattice‑oxygen migration channels between copper and manganese oxides are blocked, making it difficult to sustain the catalytic cycle.

2.3 Quantitative Characteristics of Sulfur Poisoning

Sulfur poisoning has a cumulative nature—the amount of poison adsorbed on the catalyst surface increases gradually with exposure time, and the degree of deactivation is positively correlated with the cumulative poison exposure. Experimental data show that when the H₂S concentration in the atmosphere exceeds 5 ppm, the activity of Hopcalite catalyst drops by more than 90% within 40 hours. At low temperatures (e.g., below 80°C), sulfide adsorption on the catalyst surface is enhanced, and the poisoning rate is relatively faster.

The reversibility of sulfur poisoning depends on the poison concentration and exposure time. In cases of mild sulfur poisoning, partial regeneration can be achieved by inert‑gas thermal purging (150–300°C) to remove some physically adsorbed H₂S or sulfite species. However, when the sulfur concentration exceeds 100 ppm or continuous exposure lasts more than 100 hours, the catalyst undergoes irreversible structural destruction—once sulfates or crystalline sulfides are formed, recovery is essentially impossible.

III. Mechanism of Chlorine Poisoning

3.1 Chemical Nature of Chlorine Poisoning

Halogen compounds such as chlorine and hydrogen chloride react with the metal oxides in the catalyst, altering the catalyst structure and performance and leading to poisoning deactivation. Hydrogen chloride reacts with metal oxides to form metal chlorides, destroying the original oxide lattice structure.

Studies have also found that when the atmosphere contains carbon‑containing organic compounds, chlorides (such as CH₂Cl₂ and other organochlorines) or coexisting hydrocarbons can undergo cracking reactions under chlorine catalysis, generating soot deposits that further aggravate deactivation. The deactivation period for chlorine poisoning is relatively short, with literature reporting activity loss on the order of about ten minutes.

3.2 Deactivation Characteristics of Chlorine Poisoning

The deactivation rate of chlorine poisoning is generally higher than that of sulfur poisoning. Similar to sulfur poisoning, chlorine poisoning also exhibits some degree of reversibility when the poison concentration is reduced or the atmosphere is switched. However, unlike sulfur poisoning, the soot formation that accompanies chlorine poisoning further exacerbates catalyst performance degradation—carbonaceous deposits not only cover active sites but also block catalyst pores, preventing reactants from fully contacting the active centers.

When the gas contains moderate to high concentrations of chlorine, sulfur, phosphorus, arsenic, etc., there is a risk of catalyst poisoning.

3.3 Comparison of Sulfur Poisoning and Chlorine Poisoning

IV. Theoretical Basis for Multi‑Component Synergistic Anti‑Poisoning Design

4.1 Performance Limitations of Conventional Hopcalite

Conventional Hopcalite catalyst (Cu–Mn binary system) has inherent weaknesses in resisting sulfur and chlorine: Cu and Mn have high affinity for poisons, making them prone to react with sulfur‑ and chlorine‑containing species. Single‑element doping often addresses only one aspect of performance and cannot simultaneously improve multiple anti‑poisoning capabilities. The core idea of multi‑component synergistic design is to introduce a third component (X) to systematically enhance anti‑poisoning performance through electronic structure regulation, oxygen vacancy construction, and surface property modification.

4.2 Rare‑Earth Element Doping – Oxygen Vacancy Construction Strategy

Adding rare‑earth elements such as Ce and La can create more oxygen vacancies. Studies have shown that the addition of CeO₂ improves the dispersion of active components. The increase in oxygen vacancies helps enhance the lattice‑oxygen migration rate and redox capability of the catalyst, allowing it to maintain relatively high CO oxidation activity even in the presence of poisons.

In practice, doping with a small amount of rare‑earth element (e.g., cerium at 2–5 wt%) can alter the redox potential of the catalyst. After rare‑earth modification, the space velocity required for complete CO conversion at 10°C can be increased from 12,000 h⁻¹ to 18,000 h⁻¹. Under the same space velocity, Ce doping can increase the room‑temperature CO conversion by about 50 percentage points (e.g., from 40% to 90%); under the same conversion requirement, the allowable space velocity can be increased by about 8 times, significantly expanding the operating window. In addition, rare‑earth elements themselves can serve as "sacrificial sulfur capture sites," effectively preventing sulfation of copper active sites.

4.3 Incorporation of Transition Metals: Strategies for Constructing Multi-component Systems
Incorporating Co₃O₄ to form a multi-component Cu-Mn-Co (or Cu-Mn-Co-Ag) system can enhance resistance to poisoning. Research indicates that Hopcalite-type catalysts composed of Cu-Co-Mn spinel particles exhibit significantly higher CO oxidation activity than Cu-Mn systems—reportedly more than two to three times higher.

The incorporation of Co modulates the catalyst's electronic structure and alters the adsorption energy of active components toward poisons, shifting the adsorption mode from irreversible chemisorption to reversible physisorption. Different dopants operate via distinct mechanisms; for instance, low-valence Cu doping can markedly improve the low-temperature activity and SO₂ tolerance of Mn-based catalysts. Consequently, the choice of dopant must be tailored to the specific type of poison targeted.

4.4 Other Technical Routes for Anti‑Poisoning Modification

Hydrophobic surface treatment. Silanization treatment can enhance the surface hydrophobicity of the catalyst, reducing competitive adsorption of water molecules on active sites. Since water itself is also an important factor in Hopcalite deactivation, hydrophobic treatment has positive effects on both moisture resistance and poison resistance.

Support optimization. Optimizing the support ratio and particle size can improve sulfur and water resistance. Loading active components on a high‑surface‑area support can optimize catalytic performance without changing the main composition.

Cu/Mn ratio adjustment. Studies have shown that when the Cu:Mn molar ratio is between 1:1 and 1:2, the catalyst achieves over 90% CO conversion at 25°C and a space velocity of 30,000 h⁻¹. Outside this range, performance drops significantly—excess copper leads to a copper‑rich phase on the surface that blocks manganese active sites, reducing conversion to below 70%; excess manganese increases lattice distortion and reduces activity by about 30%.

V. Engineering Considerations for Multi‑Component Synergistic Anti‑Poisoning Design

5.1 Balancing Anti‑Poisoning Performance and Catalytic Activity

While multi‑component synergistic design improves anti‑poisoning performance, care must be taken to avoid compromising the intrinsic CO oxidation activity of the catalyst. The type, proportion, and introduction method of dopants must be systematically optimized. For example, the amount of promoter must be precisely controlled—adding 1–3 wt% of alkali metal oxides (such as K₂O, Na₂O) can improve active‑site dispersion, but exceeding 5% may cover active centers. The optimal amount of CeO₂ is generally in the range of 2–5 wt%, with the exact optimum depending on the preparation process and target service conditions; the literature commonly recommends 2–4%.

In practice, the modification scheme should be matched to the specific poison type and concentration of the operating conditions. For instance, in sulfur‑containing atmospheres, Ce‑modified Cu–Mn/Al₂O₃ catalysts can enhance sulfur resistance by improving the dispersion of active components through CeO₂.

5.2 Preparation Considerations for Modified Catalysts

The preparation process of multi‑component catalysts has a decisive influence on their final performance. Studies indicate that the activity of Hopcalite catalysts is inversely proportional to their crystallinity—samples calcined at 280–350°C have a crystallinity below 15% (relative to quartz standard), and their specific activity (conversion per unit area) at 25°C is about 2.3 times that of high‑crystallinity samples (calcined at 500°C, crystallinity >50%). Low crystallinity introduces a large number of oxygen vacancies and unsaturated coordination sites, which are key sources of activity.

Parameters such as precipitation pH (7–9) and calcination temperature (280–350°C) must also be strictly controlled during preparation.

5.3 Practical Challenges in Engineering Applications

The increased complexity of preparing multi‑component catalysts needs to be evaluated in terms of production costs. The synergistic poisoning effects of different poisons coexisting (e.g., accelerated deactivation when sulfur and chlorine are both present) require further investigation. In addition, the long‑term stability of modified catalysts under real operating conditions needs sufficient validation data.

It should be noted that "irreplaceable" does not mean "universal"—in extremely high poison concentrations or high‑temperature sulfur‑containing tail gases, Hopcalite catalysts (whether modified or not) have their applicable limits. Professional selection should be based on a scientific assessment of the specific operating conditions.

VI. Conclusion

Sulfur poisoning and chlorine poisoning are the two most important chemical deactivation modes of Hopcalite catalysts in practical applications. They differ in reaction products, deactivation rates, reversibility, etc.—chlorine poisoning is faster and accompanied by soot (when carbon‑containing species coexist), but both lead to deactivation through coverage of active sites or destruction of crystalline structure.

The quantitative boundaries of sulfur poisoning can be summarized as follows: when H₂S exceeds 5 ppm, activity drops by more than 90% in 40 hours; when the sulfur concentration exceeds 100 ppm or continuous exposure exceeds 100 hours, irreversible structural destruction occurs. Chlorine poisoning deactivation can occur within minutes.

The core logic of multi‑component synergistic anti‑poisoning design lies in introducing a third component to regulate the electronic structure, construct oxygen vacancies, and optimize surface properties, thereby fundamentally reducing the reactivity of active components toward poisons. Rare‑earth doping (Ce, La, recommended 2–5 wt%) and transition‑metal introduction (Co, Ag, etc.) are currently the two most promising anti‑poisoning modification routes. In addition, precise control of preparation parameters such as Cu/Mn ratio optimization (Cu:Mn = 1:1 to 1:2) and low crystallinity (calcination at 280–350°C) are also key factors in improving overall catalyst performance.

Future research directions include: evaluation of long‑term service performance of multi‑component catalysts under real conditions, in‑depth analysis of sulfur‑chlorine synergistic poisoning mechanisms, development of customized anti‑poisoning formulations for specific applications, and quantitative structure‑activity relationship modeling between anti‑poisoning performance and catalytic activity.

This article is based on general technical principles and publicly available research findings in the field of catalytic materials, and is intended to provide technical reference for the selection and performance optimization of Hopcalite catalysts under sulfur‑ and chlorine‑containing operating conditions.