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.