Evolution of oxygen vacancies in MnOx-CeO2 mixed oxides for soot oxidation

Oxygen vacancy (O-vacancy) is essential in catalytic oxidation but little is known about its insight. Herein, a series of MnOx-CeO2 catalysts with various Mn/(Mn + Ce) molar ratios were synthesized with citric acid complex method for O-vacancy study in soot catalytic combustion. The samples were characterized by X-ray powder diffraction (XRD), N2 adsorption/desorption, O2-temperature programmed desorption (O2-TPD), H2-temperature programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS) and in situ Raman spectroscopy. It has been shown that MnOx(0.4)-CeO2 catalyst presented more O-vacancies, thus exhibiting the highest catalytic activities and redox properties. With the utilization of in situ Raman, two types of O-vacancies, including Frenkel-type oxygen vacancy (FOV) and intrinsic oxygen vacancy (IOV), were clarified. Furthermore, the transform relation between FOV and IOV was found. Those two types of O-vacancies favored to the migration and transformation of active species, enhancing further the oxidation-reduction cycle and the catalytic activity for soot oxidation. In addition, Mn4+/Mn3+(Mn2+), Olatt, Osur and Ce4+/Ce3+ were believed to play important roles in soot oxidation. Finally, evolution of O-vacancies was proposed, which is of significance for soot catalytic oxidation.