Catalytic Mechanisms in Selective Catalytic Reduction

Selective catalytic reduction (SCR) constitutes a keystone technology for the abatement of nitrogen oxides (NOₓ) in industrial and vehicular emissions. At its core, SCR employs a catalyst—typically a metal‐exchanged zeolite or mixed metal oxide—to facilitate the reaction of ammonia (NH₃) with NOₓ, converting pollutants into benign N₂ and H₂O. Mechanistically, the process may proceed via Langmuir–Hinshelwood (LH) routes, in which both NH₃ and NOₓ adsorb onto neighbouring active sites before surface reaction, or through Eley–Rideal (ER) pathways, where gaseous NOₓ interacts directly with adsorbed NH₃. Central to high performance are the redox properties of transition metal centres (such as Cu, Fe or Mn), which shuttle between oxidation states to activate NOₓ and regenerate active sites. The interplay of acid sites for NH₃ adsorption, redox sites for NOₓ reduction and pore architecture that governs diffusion underpins catalyst design. Recent advances have focused on tuning the atomic dispersion of metal species, engineering pore environments to stabilise key intermediates and mitigating deactivation by water and sulphur oxides. These developments are paving the way to catalysts that combine low‐temperature activity, hydrothermal stability and resistance to poisoning, thereby broadening the global applicability of SCR in diverse emission control scenarios.

Research from Nature Portfolio

Recent studies have elucidated the nature of dinuclear active sites in SCR catalysts by anchoring single acidic metal ions on reducible oxide supports. For example, a model system demonstrated that individual Mo ions paired with neighbouring Fe ions form uniform dinuclear sites whose density directly correlates with catalytic rate while maintaining constant activation energy. This work highlighted how tuning the acid–redox synergy at atomic scale can optimise turnover frequency and guide the rational design of next‐generation SCR materials.

Catalytic Mechanisms in Selective Catalytic Reduction publication trend

The graph below shows the total number of articles in catalytic mechanisms in selective catalytic reduction across all publications each year (not limited to Nature Index journals).

Technical terms

Active site: Specific atom or ensemble of atoms on a catalyst surface where reactants adsorb and react.

Redox cycling: Alternating oxidation and reduction of metal centres that drives NOₓ activation and catalyst regeneration.

Eley–Rideal mechanism: Reaction pathway in which a gas‐phase molecule reacts directly with an adsorbed species.

Langmuir–Hinshelwood mechanism: Reaction route involving two adsorbed species that diffuse on the surface before reacting.

Zeolite: Crystalline microporous aluminosilicate framework used as a high‐surface‐area catalyst support and ion‐exchange medium.

References

1. Understanding deNO x mechanisms in transition metal exchanged zeolites. Chemical Society Reviews (2024).
2. Revisiting the nature of Cu sites in the activated Cu-SSZ-13 catalyst for SCR reaction. Chemical Science (2015).
3. Single-atom catalysts reveal the dinuclear characteristic of active sites in NO selective reduction with NH3. Nature Communications (2020).
4. A Review of Low Temperature NH3-SCR for Removal of NOx. Catalysts (2019).