3.6 DFT calculations
The first-principle calculation was carried out using VASP program to further understand the formation of the oxygen vacancy and its effect on ozone decomposition. Based on the TEM images (Figure 3), the MnOx selectively exposed different favorable facets. On three types of facets, including MnO2 (110), Mn3O4 (200) and Mn2O3 (211), the deoxidation kinetics were studied. Top, bridge and hollow oxygen vacancy sites were all examined for each facet (Figure S3-S5). Formation energy (E f) of oxygen vacancy was theoretically studied to reveal the energy of removal of lattice oxygen, as shown in Table 3. The process of oxygen vacancy formation often played a key role in catalytic activity, as for it was generally accepted as the active sites for ozone adsorption [34]. And literatures [9, 17] reported that the oxygen vacancy easier to be formed led to more active sites, resulting in a higher performance for ozone removal. In comparison with the oxygen vacancy formation energy, it can be drawn that the oxygen vacancy is difficult to form on Mn2O3(211) surface, owing to the relatively higher formation energy. It could be explained by the different coordination number of oxygen to manganese atoms.
The process that the O2* transforms into oxygen molecule can be viewed as oxygen desorption, so oxygen adsorption energy (E o) on oxygen vacancy has been widely adopted as a descriptor for ozone elimination activity. The adsorption energy of O2 on different kinds of oxygen vacancies on MnO2 and Mn3O4 was also calculated and the results were shown in Table 3. Generally, a largeE o leads to the difficulty of the removal of O2*, thus catalytic activity was lowered. Compared to Mn3O4(200), MnO2(110) was favorable for adsorbed O2* to desorb from the oxygen vacancy sites. A three-step sequence mechanism was reasonable to describe ozone decomposition on manganese oxides, (i) dissociative adsorption of ozone to form an O2 molecule and an atomic O, with low energy barrier of 6 kJ/mol, (ii) reaction of the atomic O with gaseous ozone to form a peroxide species and an O2 molecule, and (iii) desorption of O2 to restore oxygen vacancy. The step (iii) is proved to be the rate-limiting step. Comparing with the oxygen adsorption energy, MnO2(110) was favorable for the regeneration of the oxygen vacancies, enhancing the O3elimination capability, as shown in Figure 8. It is in good agreement with the experimental results.
Table 3. Formation energy (E f) and oxygen adsorption energy (E o) of different kinds of oxygen vacancies