Oxygen vacancy defect engineering in Mn-doped CeO2 nanostructures for nitrogen oxides emission abatement

Abstract

The presence of sufficient oxygen vacancy defects (OVDs) in metal oxides can be a good predictor of the selective catalytic reduction with NH3 (NH3-SCR) of nitrogen oxides. Here, Mn-doped CeO2 nanoparticles with considerable OVDs (OVDs-Mn-CeO2) were successfully synthesized by KMnO4via a facile strategy. The content of OVDs and Mn4+ significantly increased, as evidenced by characterizations. The amount of absorbed oxygen species trapped in oxygen vacancies of the OVDs-Mn-CeO2 catalyst was 31.8% higher than that in conventional Mn-CeO2, as evidenced by X-ray photoelectron spectroscopy (XPS). Furthermore, density functional theory plus U (DFT + U) calculations were performed to prepare a Mn-CeO2 material with sufficient oxygen vacancies. The catalytic NH3-SCR activity was improved by introducing the OVDs into the typical Mn-CeO2 catalyst. The OVDs-Mn-CeO2 catalyst afforded high NO conversion (＞90%) at 140–240 °C, better than that of the Pure-MnOx and traditional Mn-CeO2 catalysts. It has been suggested that surface enrichment of Mn4+ and Mn/Ce redox couples could accelerate the catalytic NH3-SCR of NOx. In situ DRIFTS revealed that bidentate nitrate and NH3 adsorbed on Lewis acid sites intermediates were produced and then consumed during the NH3-SCR reaction cycle according to a Langmuir-Hinshelwood mechanism. In particular, the intermediate of combined NH3-NO3− species over the OVDs-Mn-CeO2 catalyst with abundant oxygen vacancies might be much more activated in catalytic NH3-SCR than the intermediate of NH2- species over the Mn-CeO2 catalyst.