Taming NO oxidation efficiency by γ-MnO2 morphology regulation

Nitric oxide (NO) emitted from the combustion of fossil fuels has drawn global concern, and the oxidation of NO contributes greatly to the DeNOx process. Herein, single-crystal γ-MnO2 catalysts with well-defined hollow-sphere-, sea-urchin-, and flower-like morphologies (γ-MnO2-HS, γ-MnO2-SU, and γ-MnO2-F) were rationally designed and synthesized via an environmental-friendly template-free hydrothermal strategy. The physicochemical properties of the prepared materials were characterized by XRD, FE-SEM, TEM, BET, XPS, H2-TPR, O2/NO-TPD and in situ DRIFTS, and their catalytic activities in NO oxidation were evaluated. The results showed the γ-MnO2-HS sample possessed the highest activity and could oxidize 91.1% of NO at 275 °C, which was obviously higher than those of the γ-MnO2-SU and γ-MnO2-F catalysts. It was found that the presence of abundant surface-adsorbed oxygen and the largest quantity of Mn3+ over the γ-MnO2-HS material respectively accelerated the processes of NO adsorption and O2 activation, which greatly promoted the NO oxidation process. This work provides significant insights into NO oxidation over γ-MnO2 catalysts.