Unraveling the Structure and Role of Mn and Ce for Selective Catalytic Reduction of NOx in Application-Relevant Catalysts and Conditions

Mn-based oxides are promising catalysts 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 role of Ce as Mn promoter on the SCR reaction. To solve this, here we have systematically studied the effect Ce by preparing, characterizing and testing around 30 catalysts aiming for a well-controlled homogeneous dispersion of the metal oxides. Our results show that, at low-temperature SCR conditions, the intrinsic activity of the Mn active sites is not positively affected by Ce species in intimate contact. In fact, the results suggest that Ce is electronically interacting with Mn and decreasing the active-site average activity. 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, increasing catalyst surface area and therefore the total number of active sites. Besides, addition of Ce is increasing N2 selectivity as it suppresses second-step oxidation reactions and thus N2O formation by structurally diluting the MnOx active sites. Therefore, the textural promoting effect still makes Ce an interesting additive for Mn catalysts.