ArticleA novel process of ozone catalytic oxidation for low concentration formaldehyde removal
Graphical Abstract
A promising novel process of cycled storage-ozone catalytic oxidation (OZCO) was successfully employed to remove low concentration formaldehyde (HCHO) from air over the optimal MnOx/Al2O3 catalyst (manganese acetate precursor and 10 wt% Mn loading) at room temperature.
Introduction
Indoor air quality has become a serious issue owing to the increasing amount of time people spent indoors. Formaldehyde (HCHO) is a common volatile organic compound (VOC) presence in indoor air, which poses serious risks to human health [1, 2]. Thus, removal of indoor HCHO has attracted considerable attention in the field of environment protection. Various methods of air purification, including catalytic oxidation [3, 4, 5], photocatalytic oxidation [6, 7], and plasma oxidation [8, 9] have been investigated for elimination of HCHO from air. However, considering the low concentration and long-term release of HCHO in indoor environments, most air purification methods are unsuitable for practical applications to indoor HCHO removal owing to their high operating temperatures and requirements for expensive noble metals [10, 11, 12].
Recently, ozone catalytic oxidation (OZCO), which involves reactions between active species from ozone, such as active oxygen atoms and hydroxyl radicals, and adsorbed VOCs can readily proceed at room temperature without the use of noble metal catalysts. This method is regarded as a promising alternative technique for low temperature oxidation of VOCs [13, 14, 15]. Great efforts have been devoted to the elimination of benzene [16, 17, 18], toluene [19, 20], and formaldehyde [21] by the OZCO approach. In our previous work, we demonstrated that ozone can completely oxidize HCHO into CO2 and H2O over a manganese oxide (MnOx) catalyst at room temperature [22]. Supported MnOx is the most commonly used catalyst in OZCO of VOCs because of its superior ozone decomposition ability [23, 24, 25, 26]. Nevertheless, the utilization of ozone in a conventional OZCO reaction is greatly depressed because HCHO is normally present at low concentrations indoors. This might lead to energy wasted on ozone production and cause secondary pollution owing to a continuous exhaust of incompletely decomposed ozone into the air.
To improve ozone utilization, effectively reduce energy costs, and minimize secondary pollution caused by undecomposed ozone during the HCHO removal process, in this work we used a cycled storage-OZCO approach to eliminate HCHO. The cycled HCHO storage-OZCO process was conducted over a supported MnOx/Al2O3 catalyst at room temperature and included two stages, namely, a HCHO storage stage and an OZCO stage. In the storage stage, a low concentration of HCHO in the air was first adsorbed on the MnOx/Al2O3 surface. Subsequently, at the OZCO stage, stored species, including formate and dioxymethylene (DOM), from HCHO adsorption were converted into CO2 and H2O by ozone oxidization [27]. The MnOx/Al2O3 adsorption sites were regenerated to allow further storage in the next cycle. This storage-OZCO process offers two main advantages for overcoming the limitations encountered by conventional continuous OZCO methods. First, ozone is supplied only at the OZCO stage; thus, the consumption of ozone and associated energy costs are considerably reduced. Second, at the OZCO stage, a large amount of surface species, accumulated during the storage stage, can react with oxidative species from ozone decomposition effectively over a relatively short period. This effect not only facilitates an improvement of ozone utilization compared with the continuous OZCO reaction, but also considerably reduces pollution caused by undecomposed ozone. Therefore, we expect that cycled storage-OZCO processes could be used for indoor HCHO purification. To the best of our knowledge, the removal of low concentrations of HCHO using a cycled storage-OZCO process has not yet been reported.
Section snippets
Catalyst preparation
In this work, γ-Al2O3 supported MnOx catalysts were prepared by an incipient wetness impregnation method. To examine the effects of the manganese precursor, manganese(II) nitrate (mass fraction 50%, Mn(NO3)2) and manganese(II) acetate (Mn(CH3COO)2) were used as the precursors. The impregnated samples were first aged at room temperature for 15 h, and then dried at 110 °C for 6 h. The dried samples were then calcined at 500 °C for 4 h in air to obtain fresh MnOx/Al2O3 catalysts.
Catalyst characterization
The phase
Results and discussion
The cycled storage-OZCO process was performed over MnOx/Al2O3 catalysts in this work because the excellent ozone decomposition activity of manganese oxides catalysts is favorable for highly efficient oxidation of HCHO. The breakthrough capacity and OZCO performance of the MnOx/Al2O3 catalysts are two important criteria for the cycled storage-OZCO process. The type of manganese precursor and the loading content of the manganese play important roles in the OZCO reaction over MnOx/Al2O3 catalysts
Conclusions
A novel process of cycled storage-OZCO was applied to low concentration HCHO removal over MnOx/Al2O3 catalysts at room temperature. The effects of the manganese precursor and loading on the storage-OZCO process were investigated to screen out optimal MnOx/Al2O3 catalysts. Characterization by XRD and XPS indicated that the manganese oxide catalysts prepared from acetate precursors showed a smaller MnOx particle size and possessed a Mn3+ content of 75%. These factors contributed to their good
Acknowledgments
We thank Andrew Jackson, PhD, from Liwen Bianji, Edanz Group China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript.
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This work was supported by the National Natural Science Foundation of China (21673030) and the Higher Education Development Fund (for Collaborative Innovation Center) of Liaoning Province, China (20110217004).
Published 5 October 2017