What is SCR Catalyst?
SCR (Selective Catalytic Reduction) is an emission control technology that converts nitrogen oxides (NOx) in exhaust gas into nitrogen (N₂) and water using a catalyst and a reducing agent — typically ammonia (NH₃) or urea-derived ammonia. SCR catalysts are the core component of modern diesel emission systems (Euro VI / EPA / China 6) and of stationary NOx abatement for power plants, industrial boilers, and gas turbines.
How SCR Works
In the SCR reaction, ammonia reacts selectively with NO and NO₂ over the catalyst surface to form nitrogen and water:
- 4 NO + 4 NH₃ + O₂ → 4 N₂ + 6 H₂O
- NO + NO₂ + 2 NH₃ → 2 N₂ + 3 H₂O
“Selective” means the catalyst directs ammonia toward NOx reduction rather than oxidation by the excess oxygen always present in exhaust gas. Without a catalyst, these reactions require temperatures well above typical exhaust conditions; with a zeolite catalyst, >90% NOx conversion is achieved from 200 °C onward.
Zeolite vs Vanadia SCR Catalysts
Commercial SCR catalysts fall into two families:
| Catalyst Type | Active Phase | Temperature Window | Key Limitation |
|---|---|---|---|
| Vanadium-based | V₂O₅-WO₃ on TiO₂ | 300-450 °C | Thermal deactivation above ~550 °C; vanadia volatility; disposal cost |
| Zeolite-based | Cu- or Fe-exchanged zeolite | 150-600 °C | Higher material cost; sensitive to sulfur (Cu forms) |
Zeolite SCR catalysts replaced vanadia in mobile diesel applications because they combine a wider active temperature window with the hydrothermal stability to survive diesel particulate filter (DPF) regeneration at 800 °C. In stationary applications, zeolite catalysts are increasingly preferred where tighter NOx limits or higher operating temperatures push vanadia beyond its thermal limit.
Common SCR Zeolite Catalysts
- Cu-SSZ-13 — the industry benchmark for mobile diesel SCR: best low-temperature activity, CHA framework stabilises copper against high-temperature migration. See SSZ-13 for SCR.
- Fe-SSZ-13 — preferred for high-sulfur exhaust and high-temperature operation (marine, stationary engines).
- Cu-Beta / Fe-Beta — cost-effective for stationary SCR with stable temperatures below 500 °C.
For a full selection comparison, see best zeolite for SCR. When requesting a sample, specify your application (mobile or stationary), exhaust temperature range, fuel sulfur content, and target emission standard.
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