Deactivation of Catalysts by Water and Dust

Catalyst activity loss is one of the most common operational challenges in industrial catalytic units. Among the various deactivation factors, physical coverage and chemical occupation of active sites by water vapor and dust represent the most direct, frequent, and easily misdiagnosed root cause of short-term failure, particularly in high‑dust, high‑humidity processes such as coal‑fired power plant SCR denitrification, cement kiln denitrification, and VOCs catalytic combustion.

Taking SCR denitrification catalysts as an example, the initial installation cost typically accounts for 30%–50% of the total investment in the denitrification system. Premature deactivation due to active‑site coverage not only incurs high replacement costs but may also lead to NOx emission exceedances. Unlike permanent deactivation mechanisms such as chemical poisoning and thermal sintering, the activity loss caused by water vapor and dust coverage is largely reversible or recoverable through technical measures. Accurately identifying the failure type, understanding the underlying mechanisms, and implementing targeted countermeasures are key to extending catalyst service life and reducing operation and maintenance costs.

carbon monoxide catalyst

1. Mechanisms of Water Vapor Action: From Reversible Adsorption to Synergistic Poisoning

The impact of water vapor on active sites spans a continuous spectrum from physical adsorption to chemical modification.

1.1 Competitive Adsorption (Reversible)

Water molecules compete with reactant molecules for the finite active sites. Studies show that the inhibition of the SCR reaction by water vapor is concentration‑dependent: when the water vapor content is below 8 vol%, competitive adsorption dominates. Temperature is the key controlling factor—at low temperatures, water molecules adsorb molecularly, occupying Bronsted acid sites and Lewis acid sites; as temperature increases, the surface residence time of water shortens, and the competitive effect weakens significantly. DFT calculations indicate that H₂O interacts strongly with titanium atoms on the V₂O₅/TiO₂ surface, and when water coverage on adsorption sites exceeds 15%, the NH₃ adsorption rate on VOx sites is markedly hindered.

1.2 Surface Hydroxylation and Active‑Phase Transformation (Irreversible)

Under prolonged water‑vapor exposure, the interaction evolves from physical adsorption to chemical transformation. For Pd‑based catalysts, activity loss in the initial stage is mainly due to competitive adsorption, but after long‑term operation, PdO gradually converts to Pd(OH)₂ species, causing irreversible damage. Different systems exhibit varying sensitivities: on MnCeTi catalysts, H₂O more readily dissociates to form hydroxyl groups, which more strongly suppress NH₃ adsorption; CuCeTi catalysts show relatively better water resistance at low temperatures.

1.3 Water Vapor–SO₂–Alkali Metal Ternary Synergy (Most Destructive)

Under low‑temperature SCR conditions, SO₂ is oxidized to SO₃, which then reacts with water vapor to form H₂SO₄, and subsequently with NH₃ to deposit ammonium sulfate or ammonium bisulfate on the catalyst surface. A more insidious pathway is “dissolution‑migration‑redeposition”: when the flue gas temperature drops below the dew point, condensed water films leach soluble alkali metals (K⁺, Na⁺) and alkaline‑earth metals from fly ash, which migrate into the internal pores via capillary condensation and, after drying, deposit as oxides or sulfates. This process combines both physical blockage and chemical poisoning, and is generally irreversible.

2. Dust Deposition and Water‑Dust Synergy

The impact of dust manifests as spatial occupation and mass‑transfer obstruction at the particle level.

2.1 Surface Deposition

Fly ash particles deposit on the catalyst surface through inertial impaction, interception, diffusion, and electrostatic adhesion, forming a covering layer that constitutes a physical barrier difficult for reactant molecules to overcome. Uneven flue‑gas flow distribution is a key factor—low‑velocity zones promote ash deposition and channel blocking, while high‑velocity zones cause erosion and insufficient contact time. According to GB/T21509‑2008, the relative standard deviation of flue‑gas velocity upstream of the catalyst layer should be controlled within 15%.

2.2 Pore Blockage

Fine particles (0.1–5 μm) deposit at pore mouths or inside micropores and mesopores. An evaluation of a 600 MW unit showed that pore blockage by calcium sulfate and other species from fly ash hindered the diffusion of NOx and NH₃ to the active particles, reducing the denitrification efficiency to 65%—the active sites were not chemically destroyed, but the blocked mass‑transfer channels rendered the catalyst “functionally deactivated”.

2.3 Water‑Dust Hard Scale Formation

When the catalyst surface temperature falls below the dew point, condensed liquid film mixes with fly ash and, after drying or thermal sintering, forms hard scale. In one coal‑fired power station, the as‑received coal ash content was as high as 41.3% (SiO₂ 64.1%, Al₂O₃ 27.1%); the high ash loading led to catalyst blockage and premature deactivation, with conventional sootblowing ineffective and offline treatment required.

3. On‑Site Diagnosis of Failure Types

The three failure types often overlap in practice, but their recoverability and countermeasures differ drastically. The following on‑site diagnostic framework is provided:

Diagnostic logic: If activity fluctuates rapidly with humidity and improves significantly upon heating → prioritize thermal desorption. If pressure drop continues to rise with obvious ash accumulation → enhance sootblowing or consider offline washing. If the above measures are ineffective → suspect chemical poisoning.

4. Tiered Countermeasure System

4.1 Source Prevention

Flue‑gas pretreatment and flow optimization. Install high‑efficiency dust removal upstream to control the inlet dust concentration below the design value. Use CFD numerical simulation to optimize flow distribution and keep the velocity relative standard deviation within 15%. In one northern coal‑fired power station, by modifying the deflector arrangement, the velocity deviations upstream of the first and second catalyst layers were reduced from 33.12% and 27.93% a 13.81% and 9.92%, respectively, effectively reducing the risk of physical deactivation.

Operating temperature and sootblowing management. Maintain the reaction temperature above the critical range where water‑vapor effects are pronounced (for SCR, preferably 320–380°C) and avoid frequent crossing of dew points. Establish a dynamic sootblowing strategy based on differential‑pressure signals. In one cement‑plant SCR unit, after cleaning and unblocking the catalyst and sootblowers, the pressure drop dropped significantly and the denitrification efficiency recovered to above 80%.

4.2 On‑Line In‑Situ Regeneration (Without Shutdown)

Thermal desorption. For competitive adsorption by water vapor, increase the temperature by 30–50°C and hold for 4–8 hours, or purge with hot dry air, achieving recovery rates of 80%–95%.

High‑temperature steam blowing. For ammonium‑salt or organic deposits, treat with 300–350°C steam to effectively remove some sulfates and restore NOx conversion to near‑fresh‑catalyst levels, without requiring shutdown or catalyst removal.

Combined sootblowing. Use acoustic sootblowing (low‑frequency, loosening) together with steam sootblowing (high‑frequency, impacting) in a staged approach.

4.3 Off‑Line Deep Cleaning and Regeneration

Applicable when on‑line measures are ineffective, or when hard scale or severe pore blockage is present.

Physical de‑ashing. Use compressed‑air blowing combined with negative‑pressure suction to restore permeability.

Chemical washing. For ash‑dominated deposits: wash with deionized water containing penetrating agents and surfactants. For sulfate or alkali‑metal deposits: dissolve with dilute acid solutions (oxalic or citric acid, 1%–5%) and rinse to neutral pH; studies indicate that 0.3 wt% sulfuric acid is relatively effective. For silica‑rich fly ash: dilute hydrofluoric acid can effectively remove SiO₂.

Active component replenishment. Impregnate with active species (e.g., V₂O₅, WO₃). After reloading with a VOSO₄ solution at 0.25 mol/L, the specific surface area and denitrification activity can be restored to fresh‑catalyst levels. A single regeneration method is often insufficient; combined regeneration (washing + active replenishment) is a more feasible approach.

Economic aspects of regeneration. Regeneration of intact catalysts can achieve about 90% of the fresh catalyst activity at a cost of approximately 30% of replacement. Water‑washing alone can increase denitrification activity by about 12%. For arsenic‑poisoned catalysts, specific regeneration techniques can remove more than 94% of arsenic.

4.4 Material Improvement Directions

Hydrophobic modification (e.g., using a Silicalite‑1 molecular‑sieve protective layer) can reduce water adsorption energy and improve stability in humid environments. Large‑pore design (pore size >50 nm) enhances anti‑blocking performance. Doping with Ce and Zr increases oxygen‑vacancy concentration and oxygen‑storage capacity, improving stability under combined water‑vapor and SO₂ conditions.

5. Economic Decision Points

Cost of misdiagnosis. For a 600 MW unit, the replacement cost for a single catalyst layer ranges from 8 a 12 million CNY. Mistaking a reversible coverage‑type failure for irreversible poisoning and replacing prematurely can result in direct losses of several million CNY.

Decision thresholds:

• Prioritize on‑line regeneration: activity decline >30% but <50%, pressure drop normal, diagnosed as water‑vapor coverage or loose ash.
• Consider off‑line regeneration: activity decline >50%, pressure drop significantly elevated, cleaning assessment shows recovery potential >60%.
• Consider replacement: activity decline >60%, pressure drop increase >50%, and cleaning recovery rate <60%.

Adopting a “preventive maintenance + on‑line regeneration + off‑line regeneration” combined strategy can significantly reduce overall costs compared to “replacement at end of life” and extend the effective service life by 1–2 operating cycles.

6. Conclusion

Coverage of catalyst active sites by water vapor and dust is the most common and most easily misdiagnosed root cause of short‑term failure in industrial catalysis. Water vapor “occupies” sites through competitive adsorption and hydroxylation, while dust “isolates” sites through deposition and pore blocking—and in practice, the two often act synergistically to accelerate degradation. The key to tackling this problem lies in systematic diagnosis and tiered intervention: prevention through flow optimization and operational control, on‑line repair through thermal desorption and sootblowing, deep regeneration through chemical washing and active‑species replenishment, and long‑term improvement through material modification. By combining technical measures with economic considerations, it is possible to minimize the total life‑cycle cost of catalysts while ensuring compliance.

author: Gloria
date:2026/6/24