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.