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