Unraveling the structure and role of Mn and Ce for NOx reduction in application-relevant catalysts

Abstract

Mn-based oxides are promising for the selective catalytic reduction (SCR) of NOx with NH₃ at temperatures below 200 °C. There is a general agreement that combining Mn with another metal oxide, such as CeOx improves catalytic activity. However, to date, there is an unsettling debate on the effect of Ce. To solve this, here we have systematically investigated a large number of catalysts. Our results show that, at low-temperature, the intrinsic SCR activity of the Mn active sites is not positively affected by Ce species in intimate contact. To confirm our findings, activities reported in literature were surface-area normalized and the analysis do not support an increase in activity by Ce addition. Therefore, we can unequivocally conclude that the beneficial effect of Ce is textural. Besides, addition of Ce suppresses second-step oxidation reactions and thus N₂O formation by structurally diluting MnOx. Therefore, Ce is still an interesting catalyst additive.

The role of Ce one Mn-based catalysts for the selective catalytic reduction of NOx at low temperature is an ongoing debate. Here the authors demonstrate that Ce has a structural promoting effect but impacts negatively on the intrinsic catalytic activity of Mn.