Catalyst Used in Internal Combustion Engines

Gasoline Vehicle Catalysts: Three-Way Catalyst (TWC) and Gasoline Particulate Filter (GPF) under China VI Standards
These two technologies are core aftertreatment solutions for modern gasoline vehicles to meet stringent emission regulations (especially China VI and Euro 6).

🔧 1. Three-Way Catalyst (TWC)

• Core Objective: Simultaneously reduce three major harmful pollutants in gasoline engine exhaust:
  • Carbon Monoxide (CO): Toxic byproduct of incomplete combustion.
  • Hydrocarbons (HC): Unburned fuel and lubricant components contributing to photochemical smog.
  • Nitrogen Oxides (NOₓ): Formed under high-temperature oxygen-rich conditions, causing acid rain and smog.
• Core Structure:
  • Substrate: Honeycomb ceramic (e.g., cordierite) or metal structure providing high surface area.
  • Washcoat: Porous material (e.g., alumina) coating the substrate to enhance surface area.
  • Active Catalysts: Precious metals embedded in the washcoat:
    • Platinum (Pt) & Palladium (Pd): Catalyze oxidation (CO → CO₂, HC → CO₂ + H₂O).
    • Rhodium (Rh): Catalyzes reduction (NOₓ → N₂ + O₂).
• Working Principle:
  1. Hot exhaust gas (400–800°C optimal) enters the catalyst.
  2. CO, HC, and NOₓ diffuse into the washcoat micropores.
  3. Synchronized redox reactions occur on the catalyst surface:
     • Oxidation:
       • 2CO + O₂ → 2CO₂
       • HC + O₂ → CO₂ + H₂O
     • Reduction:
       • 2CO + 2NO → 2CO₂ + N₂
       • HC + NO → CO₂ + H₂O + N₂
  4. Purified gas (N₂, CO₂, H₂O) is released.
• Key Points:
  • Requires stoichiometric air-fuel ratio (λ=1, ±0.01) for >90% efficiency, managed by ECU and oxygen sensors.
  • Light-off temperature: ~250–300°C (low efficiency during cold starts).

🌫 2. Gasoline Particulate Filter (GPF)

• Background: Mandated by China VIb/Euro 6d for particulate number (PN) and mass (PM) control.
• Core Objective: Trap and oxidize particles (soot, organics, inorganics).
• Core Structure:
  • Substrate: Wall-flow honeycomb ceramic (e.g., cordierite/silicon carbide) with alternating blocked channels.
  • Coating: Catalytic layer (Pt/Pd) for oxidation assistance (cGPF).
• Working Principle (Trapping):
  1. Exhaust flows into GPF; particles are physically trapped by porous walls.
  2. Cleaned gas exits.
• Regeneration (Soot Removal):
  • Passive: Uses NO₂ (from upstream TWC) to oxidize soot at >550°C:
    C + 2NO₂ → CO₂ + 2NO
  • Active: ECU-triggered (via ignition delay, extra fuel injection) to raise temperature >600°C:
    C + O₂ → CO₂
• Key Points:
  • Positioned downstream of TWC to utilize NO₂ for passive regeneration.
  • Differential pressure sensors monitor clogging.
  • Requires low-sulfur fuel/ashless oil to avoid irreversible damage.

Comparison: TWC vs. GPF

🛠 Summary

• TWC purifies gaseous pollutants via redox reactions but requires precise air-fuel control.
• GPF traps particles and regenerates via passive (NO₂) or active (high-temp O₂) oxidation, essential for China VI PN limits.
  Together, they enable compliance with stringent emissions standards.

Diesel Engine Catalysts

Diesel aftertreatment combines DOC, DPF, SCR, and ASC for near-zero emissions.

🧪 1. DOC (Diesel Oxidation Catalyst)

• Function: Oxidizes CO, HC, and soluble organics.
• Reaction:
  2CO + O₂ → 2CO₂, HC + O₂ → CO₂ + H₂O
• Key: Converts NO to NO₂ for DPF regeneration; operates at 200–400°C.

⚫ 2. DPF (Diesel Particulate Filter)

• Function: Traps PM (soot, ash).
• Regeneration:
  • Passive: C + 2NO₂ → CO₂ + 2NO (>250°C)
  • Active: C + O₂ → CO₂ (>600°C).
• Efficiency: >99% PM removal.

🔁 3. SCR (Selective Catalytic Reduction)

• Function: Reduces NOₓ using urea (AdBlue®).
• Reaction:
  4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O
• Key: Operates at 200–450°C; >85% NOₓ reduction.

🌀 4. ASC (Ammonia Slip Catalyst)

• Function: Eliminates excess NH₃ from SCR.
• Reaction:
  4NH₃ + 3O₂ → 2N₂ + 6H₂O
• Position: Downstream of SCR.

System Workflow:

图表

代码

⚠️ Challenges & Trends:

• Low-Temperature Activity: Advanced SCR catalysts (Cu/Fe-zeolite) for cold starts.
• Integration: SCRF (SCR-coated DPF) to save space.
• NH₃ Control: Optimizing ASC to minimize N₂O emissions.
• Sulfur Sensitivity: Requires ultra-low-sulfur fuel (<10 ppm).

💡 Applications: Commercial vehicles, non-road machinery (generators/excavators/ships), compliant with China VI/Euro VI.
Efficiency: >95% reduction in PM and NOₓ.