Revealing the role of dual-type oxygen vacancies on SmMn2O5 mullite for catalytic oxidation reactions

Abstract

Oxygen vacancies (V_O) can modulate the electronic structure of transition metal oxides (TMOs) and act as active sites in catalytic oxidation reactions. However, the respective roles and joint mechanisms of different types of V_O remain ambiguous. Here, we have engineered the content of dual-type V_O (i.e., O_α vacancy (V_Oα), O_β vacancy (V_Oβ)) in SmMn₂O₅ mullite to reveal their roles and joint catalytic mechanisms at the molecular level. X-ray photoelectron spectroscopy, electron paramagnetic resonance, and thermogravimetry characterization all demonstrated that different concentrations of V_O (i.e., V_Oα, V_Oβ) were made to provide highly effective active sites for promoting NO oxidation by precisely displacing Mg²⁺ at the Sm³⁺ site. Higher Fermi energy level V_Oα/V_Oβ enhance the adsorption activation of O₂ (outermost layer (π* 2px, π* 2py))/NO (outermost layer (π* 2px)), respectively, which jointly results in the fast pathway to generate more nitrate and eventual desorption to NO₂ as evidenced by O₂-temperature programmed desorption (O₂-TPD), NO-TPD, and in situ diffuse reflectance infrared Fourier transform spectra. Besides, the activity results showed that the temperature of 50 % NO conversion of the optimal sample was reduced to 265 °C, which was much lower than that of 320 °C for pristine SmMn₂O₅ mullite. And the reaction rate of the optimal catalyst was 9.68∙10⁻⁶ mol g⁻¹ s⁻¹ at 200 °C. Furthermore, the constructed catalysts exhibited excellent stability and water resistance. This study provides a profound insight the role of different types of V_O in TMOs for catalytic oxidation reactions.