Promotional role of La addition in the NO oxidation performance of a SmMn2O5 mullite catalyst†
A series of LaxSm1−xMn2Oδ (x = 0, 0.1, 0.3, 0.5) catalysts were synthesized through a co-precipitation method. The catalytic activity for NO oxidation was enhanced with La substitution, and the maximum activity was achieved at x = 0.3. XRD and HRTEM results revealed the formation of a multiphase oxide as well as the interface structure between the mullite (SmMn2O5) phase and Mn-rich perovskite (La0.96MnO3.05) phase. The main impact of different La/Sm molar ratios is the amount of surface adsorbed oxygen (Oads) and surface Mn4+ ions as revealed by XPS results. The NO oxidation performance was enhanced through La addition by promoting the decomposition of nitrate/nitrite species and desorption of NO2, improving the reducibility of surface adsorbed oxygen, as determined by H2-TPR, NO + O2-TPRD and in situ DRIFTS studies. Mono-, bi-dentate and bridged nitrates formed on the surface were determined to be the primary reaction intermediates.