4. Conclusions
A series of MnOx catalysts (MnO2,
Mn2O3 and
Mn3O4) with different Mn valences were
synthesized by calcining γ-MnOOH through tuning the calcination
temperature and atmosphere. MnO2-H-200 was obtained by
reducing MnO2 using hydrogen. The catalytic activity of
the above MnOx towards ozone decomposition followed the
order of MnO2-H-200
>MnO2>
Mn2O3>
Mn3O4.MnO2 showed better
activity than Mn2O3 and
Mn3O4 due to its lower formation energy
of oxygen vacancy and desorption energy of peroxide species. Especially
among three catalysts, the least desorption energy of peroxide species
on MnO2 facilitated to the occurrence of rate-limiting
reaction step. The surface oxygen vacancy got enriched for
MnO2-H-200 by H2-reduction. The results
elucidated that both the nature and abundance of oxygen vacancies have a
decisive influence on the catalytic decomposition of ozone. Take an
example of MnO2-H-200, the insights on deactivation
mechanism further validated that the catalytic stability of
O3 decomposition was strongly associated with the
regeneration capacity of oxygen vacancy. The transformation from
adsorbed oxygen species to lattice oxygen led to an irreversible
generation of oxygen vacancy, which changed the property of the surface
of the catalysts and resulted in the difficult desorption of
O2*. Hence it caused the deactivation
of the catalyst.