Understanding Mn-based oxides in the selective catalytic reduction of NOx

The Selective Catalytic Reduction of NOx with NH3 (NH3-SCR) is a key technology for controlling nitrogen oxide emissions. Mn-based oxides are promising for targeting emissions at temperatures below 200 °C, where the current commercial deNOx catalysts fall short. However, achieving high NOx conversion and N2 selectivity at low temperatures, while maintaining a high tolerance for poisons such as H2O and SO2, remains a significant challenge. This thesis investigates strategies to overcome these limitations by incorporating different metals into Mn-based mixed oxides to tune their structure, redox properties, and active site distribution. In particular, the effects of Ce, Fe, and lanthanides (La, Pr) were systematically studied with a wide range of characterization tools, which allowed us to correlate structural and chemical changes at the nanoscale with catalytic performance. With this investigation, we established that these metals have vastly different properties: while Ce is a structural promoter, resulting in amorphous, high-surface-area materials with homogeneous metal dispersions, Fe is mostly a redox promoter, increasing Mn3+ species on the surface of the catalyst. However, Ce content should be kept at a minimal amount, as it has a negative effect on the intrinsic activity of Mn. Therefore, Fe was incorporated to counteract this, resulting in the simultaneous enhancement of NOx conversion and N2 selectivity at low temperatures. Further studies revealed that improvements in N2 selectivity caused by Ce and other lanthanides result purely from structural effects, specifically the dilution of MnOx active sites, which suppresses second-step oxidation of NH3 in the SCR reaction. Moreover, the incorporation of a metal with limited reducibility such as La proved that these metals do not participate in redox cycles during the reaction, as was previously speculated for Ce. By using a combination of in-situ STEM-EELS and DRIFTS, we demonstrated that catalyst heterogeneity at the nanoscale plays a critical role in the reaction pathway and performance. By monitoring Mn valence changes in-situ under NH3-SCR, a direct correlation between local structure, redox behavior, and catalytic function was established. Highly dispersed MnOx sites resulted in higher NOx reduction mostly through the E-R mechanism, while agglomerated MnOx nanoparticles favored by-product formation through both mechanisms. Overall, this thesis provides fundamental insights into the structure-activity relationships governing Mn-based catalysts, which can be used as a roadmap for the design of future catalysts.