Experimental and DFT studies on Sr-doped LaMnO3 catalysts for NOx storage and reduction

Abstract

The idea of rational design of perovskite catalysts for NO_x storage and reduction (NSR) starts from DFT studies on Sr-doped LaMnO₃ (001) plane models: a Mn-terminated plane (Mn-ter) has a higher activity than La- or Sr-ter planes; the number of A-site defects increases when Sr is doped on the surface; O-vacancy formation energies for both La- and Mn-ter gradually decrease with increased Sr loading, and the values for La-ter are always larger than for Mn-ter with the same Sr loading, indicating that the O-vacancy formation is facile on Mn-ter with Sr-doping. This model yields the highest reactivity for oxidation and the lowest energy barrier for O-vacancy formation. A surface tuning method for La_0.5Sr_0.5MnO₃ is introduced. With the exception of the promotion of surface area and pore volume, the ratio of Sr/La and the number of surface-active oxygen atoms and Mn⁴⁺ cations exposed on the outermost layers are improved with an increased contact time between the raw materials and dilute HNO₃. However, overtreatment leads to a less stable phase of MnO₂ with a high reducibility but significantly restrained NO_x adsorption. NSR performances under lean-burn/fuel-rich cycles are measured, and the results correspond well with the NO adsorption or oxidation behaviors and DFT calculations.