Figure 4. H2-TPR profiles of MnOx catalysts (MnO2-H-200, MnO2, Mn2O3 and Mn3O4
The type and mobility of oxygen species were determined by O2-TPD and the results were shown in Figure 5. Generally, the O2-TPD profile of MnO2could be divided into three evolution peaks including low temperature(<350°C), medium temperature(350-600°C) and high temperature (>600°C) [30]. For the above MnOx, the major peak centered at 519°C was ascribed to the transformation of MnO2 to Mn2O3 due to the release of lattice oxygen. The high temperature peak (>800°C) was attributed to the successive transformation to Mn3O4. As shown in Figure 5, the oxygen desorption behaviors on manganese oxides at different valences were obviously different. Usually, the lower the desorption temperature of oxygen is, the looser it is bound to Mn within the MnOxlattice. Therefore, MnO2 and MnO2-H-200 had higher oxygen mobility. The peak at below 200°C was sourced from the desorption of physically adsorbed O2 [31]. As shown in the inset of Figure 5, the peak area of physically adsorbed O2 is remarkably larger than that of other samples, indicating that more active sites were provided on MnO2-H-200. For MnO2-H-200, a new peak at 270°C was assigned as the release of surface lattice oxygen and chemisorbed oxygen located at surface vacancies, indicating that hydrogen reduction enhance the mobility of oxygen species at low temperature, which was important for the ozone elimination activity.