Abstract
Manganese oxides with varied Mn valance states but identical
morphology were synthesized via a
facile thermal treatment of γ-MnOOH.
And their behaviors of ozone
decomposition were investigated following the order of
Mn3O4 <
Mn2O3 < MnO2< MnO2-H-200. In combination with XRD, SEM,
BET, TEM, H2-TPR, O2-TPD and XPS
characterization, it was deduced that the superior O3decomposition capacity for MnO2-H-200 was strongly
associated with abundant oxygen vacancies on its surface. Among
Mn3O4,
Mn2O3 and MnO2, the
difference on O3 decomposition efficiency was dependent
on divergent nature of oxygen vacancy. DFT calculation revealed that
Mn3O4 and MnO2 possessed
lower formation energy of oxygen vacancy, while MnO2 had
the minimum desorption energy of peroxide species
(O2*).
It was deduced that the promotion of the O3decomposition capability was attributed to the easier
O2* desorption.
The insights on the deactivation
mechanism for MnO2-H-200 further validated the
assumptions. As the reaction proceeded, adsorbed oxygen species
accumulated on the catalyst surface, and a portion of them were
transformed to lattice oxygen. An irreversible generation of oxygen
vacancy led to the deactivation of the catalyst.
Introduction
Ozone (O3) as a
typical secondary pollution is considered to be detrimental to human
health and plant growth because of its strong oxidation capacity [1,
2]. It is generally recognized that the ground-level ozone is sourced
from the photochemical reaction between volatile organic compounds
(VOCs) and nitrogen oxides (NOx) in the presence of heat
and sunlight. Since the concentration of VOCs and NOxhas been increasing with the increasing of the population density of the
world, ozone pollution becomes more and serious [3]. Especially in
some special circumstance, ozone hazards are more prominent such as in
indoor environment and aircraft cabin [4]. Thus, the U.S.
Environmental Protection Agency updated the National Ambient Air Quality
Standards for ground-level ozone from 75 ppb to 70 ppb [5]. Among
the various routes known to eliminate ozone contamination, ozone
catalytic decomposition has shown significant promise as an alternative
way due to its efficiency and safety. Nevertheless there are a number of
desirable characteristics for an ozone decomposition catalyst, among
which superior activity and stability are crucial. Of course low cost is
also a pursuit in view of application. Though precious metals own higher
performance for ozone decomposition, transition metal oxides still are
the optimum option in consideration of the scarcity of precious metals.
Among transition metal oxides such as
Co3O4, CeO2, CuO and
MnO2 etc, MnO2 is the most active oxide
owing to multiple valence states [6]. As a result, numerous
manganese oxides (MnOx) with different Mn valences or
particle morphologies have been reported with satisfactory catalytic
performance for ozone elimination over the recent years [7, 8].
However, MnOx is easily deactivated in the presence or
absence of water vapor, so the reason why the catalyst deactivated is
important for the rational design of the effective catalyst [9, 10].
Possible ozone decomposition mechanism over MnOxcatalyst was investigated by detecting the intermediate species. The
peroxide species were identified by in situ Raman spectroscopy with
isotopic labeling experiments by Oyama et al, and the reaction mechanism
was elucidated as below [11, 12]:
O3 + □*→ O* +O2 (1)
O3 + O*→ O2* + O2 (2)
O2* → O2 + □* (3)
in which □* represents the active sites, and
O2* stands for peroxide species. The
primary reactions above are generally assumed to take place at oxygen
vacancies with cations on the MnOx surface, since oxygen
vacancies can influence the O3 and/or oxygen
intermediates adsorption/desorption behaviors [10, 13-17]. Zhu et al
showed that the adsorption energy of O3 was increased
when oxygen vacancies were generated into the α-MnO2lattice, indicating oxygen vacancies were more favorable for
O3 adsorption [15]. Gong et al [18] found that
cubic Cu2O exposed plane owned higher efficiency of
O3 decomposition and resistance to water vapor, which
was ascribed to weakly adsorbed O2*intermediate on the cubic Cu2O. According to defect
engineering, the adsorption strength of binding of adsorbed oxygen
intermediates to the MnOx surface depends on the
property of oxygen vacancies, which is related to the elemental
composition and structure of MnOx [19, 20]. As the
most efficient catalyst for O3 decomposition,
MnOx was widely studied owing to its multiple oxidation
states. Higher Mn3+ ratio on Mn/TiO2owned superior O3 decomposition activity [21].
Similar result was obtained that manganese with lower oxidation states
was favorable in decomposition of ozone [22, 23]. As indicated that
the valence of Mn can significantly influence the ozone decomposition
performance. Moreover, when Mn was coated on the support, the
introduction of support will add the complexity between Mn valence and
activity [24, 25]{Rakesh Radhakrishnan, 2001 #725}. Therefore, to
avoid the effects of support, it is necessary that pure
MnOx should be synthesized to elucidate the intrinsic
mechanism on O3 decomposition. Even for the unsupported
α-MnO2, the α-MnO2 nanofibers exhibited
the best activity, which is ascribed to abundant oxygen vacancy on its
preferentially
exposed (211) facet [26]. So the effects of morphology should be
further avoided.
In the study, MnO2,
Mn2O3 and
Mn3O4 nanorods with identical
morphologies were synthesized and oxygen vacancies were introduced to
the surface of MnO2-H-200 via hydrogen reduction. Their
behaviors of ozone decomposition were studied. The dependence of their
catalytic activity on the surface Mn valence was investigated. In
combination with DFT calculation, the intrinsic mechanisms were also
analyzed on O3 catalytic decomposition.