Figure 10 . O2-TPD profile of MnO2-H-200 at different ozone conversion (1, 0.6 and 0.2): (a) the temperature range of 50-600°C and (b) the peak-peak fitting at the temperature range of 50-400°C
For MnO2-H-200, two desorption peaks occurring at 468°C and 491°C were attributed to the gradual escapes of the lattice oxygen close to the surface and the bulk oxygen species, respectively. They both shifted to higher temperature for the spent catalysts, which indicated that oxygen mobility got weakened and the strong Mn-O bond was formed in the octahedral MnO6 framework. The consistent results were obtained in H2-TPR (Figure 11). Generally, when oxygen vacancies were introduced to MnO2, the formation of Mn3+ could result in the Jahn-Teller distortion and thus facilitate to weaken Mn-O bond. So the lower oxygen mobility of the used catalysts might be caused by the unrecovered oxygen vacancies.