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Sulfur-containing gases can reduce the activity of manganese-based catalysts and, in severe cases, cause irreversible deactivation. The extent of deactivation depends on several factors, including the type and concentration of sulfur compounds, catalyst composition, reaction temperature, gas humidity, oxygen concentration, and exposure time. For manganese oxide catalysts, sulfur dioxide (SO₂) may interfere with catalytic reactions through competitive adsorption, surface sulfate formation, and active-site blockage. Hydrogen sulfide (H₂S), on the other hand, may undergo different surface reactions that alter the chemical state of manganese species or produce other sulfur-containing compounds. Determining whether a catalyst has been poisoned by sulfur requires more than monitoring an increase in outlet pollutant concentrations. Feed gas composition, operating parameters, and material characterization should also be evaluated to distinguish sulfur poisoning from temperature fluctuations, dust accumulation, uneven gas distribution, and other operational issues.
Manganese-based catalysts are commonly used in catalytic reactions involving oxidation-reduction processes. In some catalytic oxidation reactions involving manganese oxides, for example, target reactants first adsorb onto the catalyst surface and become activated. They then react with adsorbed oxygen species or lattice oxygen. During this process, different oxidation states of manganese and surface oxygen species participate in electron transfer and redox cycles.
When SO₂, H₂S, or organic sulfur compounds in industrial exhaust gas enter a catalytic reactor, they may adsorb onto the catalyst surface or undergo chemical reactions with it. These processes can alter the accessibility of active sites, the state of surface oxygen species, and mass transfer through the catalyst pores.
Sulfur poisoning is not a single chemical reaction but the result of several surface processes occurring together. Different manganese oxides may have significantly different sulfur adsorption characteristics and resistance levels because of variations in crystal structure, manganese oxidation states, support materials, and preparation methods. Therefore, the fact that a material contains manganese dioxide does not, by itself, establish its sulfur resistance.
SO₂ is a common sulfur-containing component in industrial flue gas from combustion processes and in certain chemical industry exhaust streams. Its effects on manganese-based catalysts mainly involve competitive adsorption, surface sulfate formation, and interference with active sites and redox processes.
Catalytic reactions require target reactants to interact with active sites on the catalyst surface. If SO₂ adsorbs at these sites, it may compete with the target reactants, reducing the number of sites available for the desired reaction.
Under certain conditions, competitive adsorption can reduce the opportunities for target reactants to adsorb and become activated, thereby lowering the reaction rate. The extent of this effect depends on SO₂ concentration, exposure time, target reactant concentration, and catalyst surface properties.
If SO₂ is present only in a relatively weakly adsorbed state, some of its effects may diminish after the sulfur-containing gas is removed. However, if stable sulfur-containing species form on the surface, the resulting deactivation may be more difficult to reverse.
In oxygen-containing atmospheres, SO₂ may undergo oxidation on the catalyst surface, forming sulfur-containing species such as sulfites and sulfates. The specific products and their stability depend on catalyst composition, reaction temperature, and gas-phase conditions.
Some sulfates may interact with manganese active components or the catalyst support and accumulate on the surface. Their effects may include:
A study on VOC oxidation over MnO₂ found that adsorbed sulfates can interfere with the function of surface oxygen species and active sites, thereby reducing catalytic performance.
This indicates that SO₂-induced deactivation is not simply a matter of physical surface coverage. It may also involve changes in the catalyst's surface chemistry.
Oxygen vacancies are defect sites in the lattice or on the surface of metal oxides. In certain catalytic reactions, they participate in oxygen adsorption and activation, as well as oxygen-species migration.
For manganese oxide catalytic systems that rely on surface oxygen or lattice oxygen, sulfate formation may occupy some oxygen vacancies or hinder oxygen migration between the surface and the lattice. This can interfere with the catalyst's redox cycle.
A 2024 study on manganese-cerium composite oxides for ozone decomposition found that SO₂ could form stable metal sulfates in the presence of ozone, affecting oxygen vacancies, electron transfer, and the catalyst's redox properties. The study also observed that water vapor could partially mitigate SO₂ poisoning. However, this finding applies to a specific material and ozone decomposition system and should not be generalized to all manganese-based catalytic reactions.
If sulfur-containing species continue to deposit on the catalyst, or accumulate together with dust, salts, and other reaction byproducts, they may cover some pore openings or reduce the effective pore diameter, limiting gas diffusion into the catalyst.
However, pore blockage is not necessarily the primary mechanism in every SO₂ poisoning process. For some materials, the loss of accessible active sites or changes in redox properties may have a more direct impact on catalytic activity than restricted pore diffusion.
Although both SO₂ and H₂S contain sulfur, their chemical properties and surface reaction pathways differ. Their deactivation mechanisms should not be treated as identical.
H₂S has reducing properties. Under certain atmospheric and temperature conditions, it may react with manganese oxide surfaces, causing surface sulfidation and changes in the chemical state of manganese species.
The catalytic performance of manganese oxides is closely related to their different oxidation states and the interconversion between them. If H₂S changes the redox state of surface manganese species or produces sulfur-containing compounds that block active sites, the original catalytic cycle may be disrupted.
When H₂S enters an oxygen-containing exhaust stream, its surface reactions may differ from those occurring under oxygen-deficient conditions. In the presence of oxygen, H₂S may undergo further oxidation to form elemental sulfur, oxygen-containing sulfur species, or other sulfur-containing products. Sulfides may also form under certain conditions.
The specific products and their stability depend on oxygen concentration, temperature, catalyst composition, and other coexisting gases. Therefore, the presence of H₂S in the feed gas alone is not sufficient to establish that a specific sulfidation reaction has occurred on the catalyst.
| Comparison | SO₂ | H₂S |
|---|---|---|
| Main chemical characteristics | Oxygen-containing sulfur compound | Sulfur compound with reducing properties |
| Possible surface processes | Adsorption, oxidation, and sulfate formation | Adsorption, surface sulfidation, and oxidation reactions |
| Potential effects | Active-site blockage and changes in surface oxygen species | Changes in manganese species and formation of various sulfur-containing compounds |
| Key influencing conditions | Oxygen, temperature, water vapor, and catalyst composition | Oxygen, temperature, gas composition, and catalyst composition |
| Key evaluation criteria | Surface sulfates and their effects on catalytic reactions | Sulfur species and changes in surface manganese chemistry |
For exhaust streams containing both SO₂ and H₂S, interactions between the two components should also be considered. Results from tests involving individual sulfur compounds cannot be directly applied to mixed-gas conditions.
Sulfur concentration affects the amount of sulfur species that come into contact with the catalyst surface. Higher concentrations generally increase the opportunities for adsorption and chemical reactions.
However, long-term exposure to low sulfur concentrations also deserves attention. Even when the sulfur concentration is relatively low, continuous exposure may gradually lead to the accumulation of sulfur-containing species on the catalyst surface. Therefore, sulfur resistance evaluations should consider both sulfur concentration and cumulative exposure time.
Temperature affects the adsorption, oxidation, conversion, and decomposition of sulfur species. It also influences the redox properties of the catalyst itself.
Different sulfur species have different levels of stability, and the same sulfur species may behave differently on different catalyst surfaces. Therefore, it is not safe to assume that increasing the temperature will eliminate sulfur poisoning.
Actual testing should cover the catalyst's intended operating temperature range and monitor performance changes both during sulfur exposure and after the sulfur-containing gas is removed.
Industrial exhaust gas usually contains some water vapor. Water vapor may change the surface adsorption environment and influence the formation and accumulation of sulfur species.
It is important to note that water vapor does not have the same effect in every catalytic system. In some manganese-based low-temperature SCR systems, water vapor may aggravate SO₂-related deactivation. In a specific manganese-cerium composite oxide system for ozone decomposition, however, water vapor was observed to partially mitigate SO₂ poisoning.
These findings indicate that the effect of water vapor depends on catalyst composition and reaction pathways. It should not be universally classified as either promoting or suppressing sulfur poisoning. Sulfur resistance tests should reproduce the humidity conditions of the actual exhaust stream as closely as possible.
Different manganese oxides vary in crystal structure, manganese oxidation-state distribution, specific surface area, pore size, and surface oxygen properties. These characteristics influence sulfur adsorption and subsequent surface reactions.
Supports and other active components may also change the surface properties of a catalyst. For example, certain metal components in composite oxides may react preferentially with SO₂, potentially changing the extent to which active manganese species are affected. However, this protective effect must be verified for each specific material and should not be assumed to be a general property of all composite catalysts.
A decline in catalytic performance does not necessarily indicate sulfur poisoning. Deviations from the operating temperature range, changes in gas flow, dust buildup, and uneven gas distribution can produce similar symptoms. The recommended approach is to review operating data first and then use material characterization to identify the cause.
First, record the inlet and outlet concentrations of the target pollutant and calculate the corresponding removal or conversion efficiency. Comparisons should be made under the same or comparable conditions, including temperature, flow rate, and space velocity.
If inlet conditions remain relatively stable but the outlet pollutant concentration continues to increase, the system's treatment capacity may be declining. Further investigation of the catalyst and equipment is then necessary.
When inlet concentrations fluctuate significantly, outlet concentrations alone are not sufficient for evaluation. Conversion rates should also be calculated based on inlet concentrations, while accounting for potential errors caused by sampling, analytical instruments, and gas dilution.
A decline in catalytic activity may be accompanied by changes in outlet pollutant concentrations, but not all forms of catalyst deactivation cause an increase in pressure drop.
If the pressure drop increases significantly, investigate dust accumulation, bed blockage, catalyst attrition, and gas distribution. If the pressure drop remains relatively stable while catalytic performance declines, other possibilities should be considered, including active-site poisoning, changes in material structure, and unsuitable operating temperatures.
Therefore, pressure drop is useful for evaluating catalyst bed operating conditions, but it cannot serve as the sole indicator of sulfur poisoning.
X-ray photoelectron spectroscopy (XPS) can analyze the elemental composition and chemical states of species on the catalyst surface.
By comparing the S 2p and manganese-related spectral regions of fresh and spent catalyst samples, it is possible to determine whether sulfur has accumulated on the surface and to investigate the chemical forms of sulfur and changes in manganese species.
However, XPS primarily provides information about the near-surface region and does not represent the sulfur distribution throughout the entire catalyst particle. A single spectral peak is also insufficient to establish the complete deactivation mechanism.
X-ray diffraction (XRD) can be used to analyze the crystal structure and phase composition of a catalyst. If operation produces a new crystalline phase at a sufficiently high concentration and with adequate crystallinity, XRD may detect the corresponding changes.
However, low-concentration, amorphous, or predominantly surface-bound sulfur species may not produce distinct diffraction signals. Therefore, the absence of new phases in an XRD pattern does not prove that sulfur poisoning has not occurred.
Scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM-EDS) can be used to examine catalyst surface morphology and the local distribution of sulfur. However, this technique generally cannot determine the precise chemical form of sulfur on its own.
Specific surface area and pore size measurements can help determine whether the catalyst's pore structure has changed. Combining these results with XPS, XRD, and catalytic performance tests provides a more comprehensive assessment of sulfur accumulation and its relationship to catalyst deactivation.
The appropriate treatment depends on the sulfur compounds present, the extent of deactivation, the condition of the catalyst, and the reaction process. Desulfurization, catalyst regeneration, and catalyst replacement should not be treated as universally applicable solutions.
First, analyze the exhaust gas for SO₂, H₂S, and other sulfur-containing compounds to determine whether the sulfur load exceeds the original process design conditions.
If the inlet sulfur concentration remains excessively high, upstream desulfurization may be considered based on the gas composition and treatment requirements. The specific desulfurization process should be selected according to temperature, humidity, treatment capacity, and other pollutants. It is also important to evaluate whether the pretreatment process could introduce additional impurities.
Under controlled conditions, stop feeding the sulfur-containing gas and monitor whether catalytic performance recovers. If performance improves, some of the deactivation may have resulted from reversible adsorption or temporary surface coverage. If performance does not recover significantly, the formation of stable sulfur species or changes in material structure should be investigated further.
However, performance recovery alone cannot establish the specific deactivation mechanism. Surface characterization and material testing are still necessary.
Catalyst regeneration must account for the stability of sulfur-containing species, the thermal stability of the catalyst itself, the properties of the support, and potential structural changes caused by the regeneration process.
A 2026 study on chlorobenzene oxidation over Mn₂O₃ used acid washing to treat sulfur-poisoned catalyst samples. The study found that acid washing reduced the sulfur content and partially restored catalytic activity. However, some sulfur species remained, and the catalyst did not fully recover its original performance. This demonstrates that acid washing may be effective under certain conditions, but it does not mean that all manganese-based catalysts are suitable for this treatment.
High-temperature calcination, gas purging, or water washing should not be applied without prior testing. Inappropriate regeneration conditions may cause active-component sintering, changes in support structure, or other performance losses. A regeneration procedure should first be validated through small-scale testing before its suitability for larger-scale application is assessed.
If catalytic activity cannot be restored and testing confirms significant sulfur accumulation or structural changes, the decision to replace the catalyst should consider its remaining activity, equipment operating requirements, regeneration costs, and replacement costs.
The sulfur load at the inlet must also be addressed after replacement. Otherwise, the new catalyst may experience the same type of deactivation.
For projects involving sulfur-containing exhaust gas, catalyst selection should not be based solely on initial activity under sulfur-free conditions. The following factors should also be verified.
| Evaluation Item | Information to Verify | Engineering Significance |
|---|---|---|
| Sulfur components | Types and concentrations of SO₂, H₂S, and organic sulfur compounds | Identifies potential sulfur poisoning species |
| Gas conditions | Temperature, humidity, oxygen concentration, and flow rate | Defines the actual reaction environment |
| Reaction requirements | Target pollutants, inlet concentrations, and treatment requirements | Establishes catalyst evaluation criteria |
| Material performance | Activity changes and long-term stability under sulfur exposure | Determines suitability for sulfur-containing conditions |
| Catalyst bed operation | Pressure drop, attrition, dust accumulation, and gas distribution | Evaluates operational risks |
| Maintenance requirements | Desulfurization methods, regeneration conditions, and replacement costs | Supports long-term operating plans |
For example, consider an industrial exhaust gas treatment system that initially meets its treatment requirements, but gradually experiences an increase in outlet pollutant concentrations after a period of operation. The first step is to check whether the inlet sulfur concentration has changed. Next, review the operating temperature, flow rate, pressure drop, and performance of the upstream desulfurization unit. If obvious process abnormalities have been ruled out, spent catalyst samples can be collected and compared with fresh samples to examine sulfur content and surface chemical states. These findings should then be evaluated alongside catalytic activity test results to determine whether sulfur poisoning has occurred.
This troubleshooting approach can be used for engineering investigations. However, specific corrective actions must be determined based on the gas composition and material test results. An increase in outlet pollutant concentration alone is not sufficient evidence that the catalyst has been deactivated by sulfur.
For projects involving sulfur-containing exhaust gas, sulfur resistance evaluation should include both short-term exposure tests and long-term stability tests.
Short-term exposure tests primarily examine whether catalytic performance changes immediately after the catalyst comes into contact with sulfur-containing gases. During testing, operating conditions such as temperature, space velocity, target pollutant concentration, and oxygen concentration should be controlled. A sulfur-free control test should also be included.
Long-term stability tests focus on whether sulfur species continue to accumulate and whether catalytic performance gradually declines with exposure time. During testing, outlet concentrations, pressure drop, and operating temperature should be recorded periodically. Catalyst samples should also be collected when necessary to analyze surface sulfur content and chemical states.
At a minimum, the following variables should be considered:
It is important to emphasize that short-term sulfur resistance tests cannot replace long-term operating evaluations. A catalyst that remains stable during brief exposure may still experience sulfur accumulation during extended continuous operation.
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