Promotional role of La addition in the NO oxidation performance of a SmMn2O5 mullite catalyst

Abstract

A series of La_xSm_1−xMn₂O_δ (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 (La_0.96MnO_3.05) phase. The main impact of different La/Sm molar ratios is the amount of surface adsorbed oxygen (O_ads) 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.