Elucidating the facet-dependent reactivity of Cr[sbnd]Mn catalyst for selective catalytic reduction of NOx with NH3

Abstract

The facet-dependent reactivity of CrMn catalysts was still unclear, hindering the further enhancement of their low-temperature SCR performance. Herein, the facet-dependent reactivity of CrMn1.5O4 catalyst for NH3-SCR of NOx was innovatively illustrated by numerous characterizations and density functional theory (DFT) calculations. Exposed (100) facet of CrMn1.5O4 catalyst exhibited best low-temperature SCR activity with ≥90 % NO conversion within 148–296 °C and 2.86 × 10−3 mol/(g·s) reaction rate within 160–240 °C. The characterizations revealed that (100) facet could induce the increase of BET specific area, electron transfer, concentration of Mn4+ and Oα, surface acidity, redox ability, NH3 and NOx adsorption/activation capacity. Subsequently, DFT calculations demonstrated that (100) facet exhibited the strongest affinity for NH3 and NO due to its unique 3O3c-Mn5c-2O4c bond and abundant charges transfer near the active adsorption sites, and Brønsted acid and oxygen vacancies were most easily formed on (100) facet. Furthermore, H2O formation as the rate determining step easily occurred on (100) facet. Eventually, we successfully improved the low-temperature SCR activity of CrMn1.5O4 catalyst by further tailoring highly active (100) facet from 0.754 to 0.865. This work provides the deeper understanding of facet-dependent reactivity and a good strategy to improve the catalytic activity of the catalysts.