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.