Unveiling the enhancement mechanism of NH3-SCR performance in Mn-modified α-Fe2O3 on Fe2O3-TiO2 catalysts with varied crystal facets

This study explored the impact of Mn modification and diverse crystal facets on iron-based catalysts, focusing on Mn-modified Fe2O3-TiO2 catalysts with various alpha-Fe2O3 crystal facets ({012}, {014}, and {113}). The modification of alpha-Fe2O3 with manganese significantly enhanced the NH3-SCR performance of the Mn/Fe2O3-TiO2 catalysts exhibiting varying effects based on the different exposed crystal facets. Notably, the Mn/Fe2O3{113}TiO2 catalyst demonstrated the highest efficacy, maintaining NOx conversion rates exceeding 90.0 % in the temperature range of 200-350 degrees C, peaking near 100.0 % between 250 and 300 degrees C, and N2 selectivity above 70.0 % within 150-300 degrees C. The {113} crystal facet of alpha-Fe2O3, characterized by its higher surface energy, enhanced the presence of surface defect species, thereby promoting the adsorption and activation of reactants while facilitating interactions among Fe, Mn, and Ti. These enhanced interactions facilitated electron migration, increasing the release of Mn4+ and O alpha species, which in turn increased the redox capacity. This intrinsic mechanism contributed to the superior activity of the Mn/Fe2O3{113}-TiO2 catalyst. Both the Mn/Fe2O3{014}TiO2 and Mn/Fe2O3{113}-TiO2 catalysts exhibited NH3 adsorption on both Br & oslash;nsted acid sites and Lewis acid sites. The adsorbed NH3 species reacted with gaseous NO species and adsorbed nitrate species through LangmuirHinshelwood (L-H) reaction and Eley-Rideal (E-R) mechanisms. Additionally, Mn4+ promoted a "Fast SCR" reaction, significantly boosting the catalysts' activity at low temperatures. The Mn/Fe2O3{113}-TiO2 catalyst, with elevated Mn4+ and O alpha levels, exhibited increased efficiency in both "Standard SCR" and "Fast SCR" pathways. This study provides valuable insights for developing high-performance alpha-Fe2O3-based NH3-SCR catalysts with extended operating temperature ranges.