Surface Reconstruction of Decalcified CaMnO3 Perovskites for Low-Temperature NO Oxidation

Abstract

Effective oxide-based catalysts with the potential to replace noble metal catalysts remain elusive for NO-to-NO₂ oxidation, which is essential for low-temperature NO_x abatement. Herein, we report a time-controlled selective decalcification strategy to reconstruct dense CaMnO₃ perovskite into Mn-rich porous oxides for NO oxidation. Acid etching progressively removed Ca from the CaOMn framework, forming MnO₂-like porous structures with well-defined [MnO₆] units and MnOMn bridges. The optimized catalyst, denoted as CMO-24 h, decreased the temperature required for 50% NO conversion from 378 to 181 °C and increased the maximum NO conversion from 54% to 92%. The promoted activity arose from an optimized MnOMn lattice-oxygen microenvironment within the reconstructed porous framework. Mn enrichment and pore formation increased the accessibility of Mn-rich surface domains, while the connected MnOMn network facilitated Mn redox cycling among Mn²⁺/Mn³⁺/Mn⁴⁺ at low temperatures. Meanwhile, enriched surface-accessible lattice oxygen and an expanded surface/subsurface lattice-oxygen reservoir provided reactive oxygen for the oxidation of adsorbed NO-derived species. Temperature-programed desorption and in situ diffuse reflectance infrared Fourier transform spectroscopy (with NO or NO + O₂ as probes) further revealed that CMO-24 h enhanced NO_x adsorption/desorption and accelerated the evolution of nitrosyl/nitrite-type species toward nitrate- and NO₂-related species.