Regular ArticleEnhanced catalytic oxidation of VOCs over porous Mn-based mullite synthesized by in-situ dismutation
Graphical abstract
Introduction
With the rapid development of urbanization and industrialization, more and more volatile organic compounds (VOCs) are emitted artificially into the surrounding environment [1], [2]. These VOCs have serious impacts on human health and environment safety [3]. Especially aromatic VOCs, such as benzene, chlorobenenze, etc. not only severely destroy the ozone layer and produce photochemical smog, but also cause physical weakness, nausea, memory loss, dizziness, and death [4], [5]. Catalytic oxidation technology is one of the effective methods to remove VOCs pollutants [6]. The development of catalysts for the removal of VOCs with both low-temperature activity and stability is the key but remains challenging. The current catalysts are divided into supported precious metals and non-noble metal oxides [7]. It has been reported that oxide catalysts such as perovskite, spinel, hydrotalcite and other oxides have insufficient catalytic activity, while precious metals are expensive, resource limited, low water resistance, and short durability [8], [9], [10], [11], [12]. Hence, the development of oxide catalysts with high activity comparable to precious metals is still on the way.
Recently, Mn-based mullite-type oxides, e.g. SmMn2O5, GdMn2O5, etc. have been considered as one of potential substitute for noble metal catalysts due to its low cost, great thermal stability, and high efficiency [8]. Mn-based mullites have been widespread applied as catalysts for NO oxidation, oxygen reduction reaction, and combustion of methane and soot [13], [14], [15], [16], [17]. Wang et al. synthesized SmMn2O5 mullite through treating the mixed precipitates at 800 °C for NO oxidation in diesel exhaust and achieved excellent catalytic activity compared to the commercial Pt/Al2O3 catalyst [13]. Jin et al. reported the SmMn2O5 mullite prepared via sol–gel method for efficiently removal of soot [17]. Dong et al. studied GdMn2O5 mullite developed by hydrothermal method to remove acetone with a high efficiency and stability in total oxidation [8]. However, it is found that the mullite catalysts prepared by these methods have insufficient activity in the catalytic oxidation of aromatic VOCs. The mullite (SmMn2O5) possess an orthorhombic structure, wherein Mn shows two oxidation states of Mn4+ and Mn3+, and the Mn4+ is coordinated with oxygen atoms to form Mn4+O6 octahedral chain shared by the side parallel to the c-axis as well as the adjacent chains are connected by a twisted square Mn3+O5 pyramid in a-b plane [17], [18]. The Mn4+–Mn4+ dimers of Mn-based mullite have been identified as the active sites and the electron hybridization between Mn-O promotes the participation of active oxygen species in the catalytic reaction [8], [13]. However, a large amount of rare earth elements often covers the manganese sites on the surface of Mn-based mullite, restraining the number of active oxygen species. In addition, the researches on the catalytic oxidation of VOCs often focus on the oxidation of single-component organic compounds, and there is little study on the simultaneous removal of multi-component VOCs.
Taking into account the above challenges, we herein first provided a sample and scalable strategy to prepare rich-mesoporous Mn-based mullite with high catalytic activity for VOC oxidation. In this method, the rich-mesoporous SmMn2O5 mullite catalyst was directly formed by the in-situ dismutation rection of solid state Mn3+ in bulk SmMnO3 perovskite at room temperature. The as-prepared mullite showed the advanced number and mobility of surface oxygen species, and the increase exposure of active manganese sites as well as hoisted specific surface area, which promoted the catalytic oxidation of benzene. In order to study the catalytic mechanism, the SmMn2O5 mullite was also synthesized by the molten polymerization, co-precipitation, hydrothermal, sol–gel methods, resepctively. Combined with the characterization (XPS, H2-TPR, O2-TPD) and performance, the relationship of structures and properties were revealed, and the reactant intermediate and catalytic mechanism of benzene oxidation were explored. Ultimately, the capability of the removal of mixed VOCs with different concentration of benzene and chlorobenzene and the stability also was researched in deep.
Section snippets
Synthesis of catalysts
The novel porous SmMn2O5 mullite was prepared by the in situ dismutation of Mn3+ ions in bulk SmMnO3 perovskite and as illustrated in Fig. 1. The bulk SmMnO3 was synthesized according to the molten polymerization strategy our reported earlier [19]. 1 g of SmMnO3 powder was immersed in 40 mL of absolute ethanol and stirred for 5 min. Afterwards, 2.4 mL of concentrated nitric acid was added to the above mixture and agitated for 1 h at room temperature. The as-obtained precipitate was washed and
Crystal phase structure
Fig. 2 displays the XRD patterns of SmMn2O5-ID, SmMn2O5-MP and SmMnO3. The XRD diffraction peaks of SmMnO3 and SmMn2O5-MP are corresponding to the standard cards of PDF# 25-0747 and PDF# 52-1095, respectively, indicating the formation of perovskite structure and mullite structure [19], [20]. Meanwhile, the XRD diffraction peaks of SmMn2O5-MP and SmMn2O5-ID are in the same position, showing that the SmMnO3 perovskite was completely converted into SmMn2O5 mullite after being treated with acid. In
Conclusion
Highly active SmMn2O5 mullite was successfully synthesized at room temperature for the first time by partially removing Sm3+ and dismutation reaction of Mn3+ from SmMnO3 perovskite. Compared with that of SmMnO3 and bullk SmMn2O5, SmMn2O5 synthesized by partially removal of Sm and Mn has larger specific surface area (35.8 m2·g−1), higher molar ratios of Mn4+/Mn3+ (1.21) and Olatt/Oads (3.01), and better active oxygen desorption capacity, reducibility and manganese exposure (Sm/Mn = 0.34). These
CRediT authorship contribution statement
Ruoyu Liu: Investigation, Formal analysis, Writing - original draft. Bing Zhou: Investigation, Formal analysis, Writing - original draft. Lizhong Liu: Conceptualization, Supervision, Writing - review & editing. Yan Zhang: Writing - review & editing. Yu Chen: Writing - review & editing. Qiaoling Zhang: Writing - review & editing. Mingliang Yang: Writing - review & editing. Lanping Hu: Writing - review & editing. Miao Wang: Conceptualization, Supervision, Writing - review & editing. Yanfeng Tang:
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The financial supports from the National Natural Science Foundation of China (Grant No.: 21776140) and the Large Instruments Open Foundation of Nantong University (Grant No.: KFJN2027).
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These authors contributed equally to this work.