Figure 9 . (a) Mn 3s, (b) Mn 2p3/2 and (c) O1s XPS spectra of MnO2-H-200 at different ozone conversion (1, 0.6 and 0.2)
To further verify the irreversible O2*desorption on MnO2-H-200, O2-TPD was conducted and was shown in Figure 10. The O2-TPD profile in lower temperature range was deconvoluted into several peaks (Figure 10b). According to previous studies, the desorption peaks of O2* and O* located at less than 200oC and in the range of 200-350oC, respectively. The O2* was regarded as the intermediate of ozone decomposition. The desorption temperature of the peaks attributed to O2* followed the order: MnO2-H-200-0.2 >MnO2-H-200-0.6 >MnO2-H-200, suggesting that O2* species were relatively stable on the spent catalysts. As the desorption of O2* is the rate-limiting step for ozone decomposition, the difficult desorption of O2* on the catalyst hindered the recovery of the oxygen vacancies. As a result, it caused a decrease of active sites and a low ozone conversion. The results indicated though the O2* desorption process was relatively reversible for MnO2-H-200, the residual O2* difficult to desorb from the surface of the catalyst might hinder the subsequent cycles, hence decreasing its active sites with time.