Enriching Locally the Reactive Intermediates of Ammonium Nitrate via Simple Pore-Size Modulation for Robust NH3–SCR Denitration

Porous materials are extensively utilized in the catalytic elimination of environmental pollutants. However, the intrinsic role of pore size in modulating catalytic performance remains poorly understood. Herein, we report that the reactivity of ammonium nitrate (AN), a critical meltable intermediate in the reaction of NH3 selective catalytic reduction (NH3–SCR), can be effectively manipulated by simply adjusting the mesopore size of silica supports, while preserving identical active sites. For the investigated catalysts of Fe2O3 confined in mesoporous silica (Fe2O3@meso-SiO2), we demonstrate that catalytic activity exhibits a strong positive correlation with mesopore size for both SBA-15 and KIT-6 confined catalysts in the medium- to low-temperature range, independent of the mesoporous architecture. AN-temperature-programmed desorption (AN-TPD) revealed that the behavior of AN can be systematically tuned by mesopore size. Larger pores raise the equilibrium vapor pressure of molten AN via the Kelvin effect, enriching the local gaseous AN concentration. Following Le Chatelier’s principle, this elevated concentration shifts the surface adsorption equilibrium of AN on active sites and accelerates the reaction between AN and NO to yield N2. Our work underscores the importance of pore size engineering for tuning the local concentration of meltable reactive intermediates, offering a new avenue for the rational design of high-performance NH3–SCR catalysts.