Low temperature catalytic ozonation of toluene in flue gas over Mn-based catalysts: Effect of support property and SO2/water vapor addition
Graphical abstract
Introduction
In the past several years, the traditional air pollutants, i.e. NOx, SO2 and particle matter (PM), from coal-fired utilities have been greatly reduced via the “Ultra-Low Emission” retrofit due to very stringent emission standards in China (NOx< 50 mg/Nm3, SO2< 35 mg/Nm3 and PM < 5 mg/Nm3). The volatile organic compounds (VOCs) from combustion and many different industry processes draw much more attention for its environmental potential hazards and further atmospheric chemical reactions [1,2]. It has been proved that VOCs are the crucial precursors for particular matter and photochemical ozone formation via atmospheric chemical reactions [3,4]. National governments are all trying to formulate more complete regulations for VOCs reduction. Especially, Chinese government announced that the total VOCs emissions need to be cut by more than 10 % by 2020 compared with 2017. Thus, developing new technologies for VOCs “green” treated is greatly demanded.
Many VOCs removal techniques are presented to improve air quality, including carbon adsorption [5], photocatalytic oxidation (PCO) [6], non-thermal plasma (NTP) [7], biological treatment [8], catalytic oxidation [9] and catalytic ozonation [10]. PCO and NTP method would consume great amount of other energies (i.e., light and electrical energy), while the byproduct treatment is a big problem for carbon adsorption and biological treatment. Catalytic method stands out among these techniques, which could directly convert VOCs to COx with less energy consumption and additional equipment installation. Besides, catalytic ozonation is more efficient for low temperature waste gas treatment comparing with catalytic oxidation. As to the dropping metal selection, noble metal (Pd and Pt) could achieve satisfied catalytic activity under low temperature, but its high price pushes researchers to find a cheap substitute. Luckily, transition metal (Mn, Co, Fe and Cu), which has reasonable price and variable valence, could be good alternative candidates for oxidation catalyst development. Among them, MnOx performed well in VOCs degradation, and its reaction mechanisms for catalytic ozonation have been investigated via kinetic and spectroscopic studies [[11], [12], [13]].
Besides active metals doping on the catalyst, the different types of support also greatly influence the final performance including surface area, metal interaction and dispersion. Hisahiro Einaga et al. [14] carried out experiments on benzene catalytic ozonation over MnOx supported on SiO2-, Al2O3-, TiO2- and ZrO2-support. They illustrated that the higher specific area was beneficial for catalytic activity, thereby SiO2-supported manganese oxides obtained the best benzene oxidation rate. Ebrahim Rezaei et al. [15] investigated MCM-41, γ-Al2O3 (Nit) and γ-Al2O3 (Ace) supporting MnOx on low temperature toluene oxidation, and found that higher manganese dispersion leaded to less active during catalytic ozonation. Peng Liu et al. [16] modified MnOx on diatomite, which showed strong stability and high selectivity to CO2 for toluene oxidation. Overall, SiO2, TiO2 and γ-Al2O3 are the commonly used supports, and they were investigated extensively in VOCs removal [[17], [18], [19]]. Therefore, they are selected to investigate the effect of support on toluene catalytic degradation by ozone in this paper.
There are a wide range of VOCs in the industrial waste gas. Among them, toluene is one of the typical organic compounds that is commonly generated from different emission sources [6,20,21]. Thus, it was selected as the target pollutant. Industrial and residential coal firing is one of the major VOCs emitted sources, so that investigating catalytic oxidation of VOCs under the related industrial condition is crucial [22]. Especially, because toluene is used as cleaning agent for electron device, the combustion of these waste electron device in waste incineration plant would release tens of ppm toluene. In addition, SO2 and water vapor are the essential ingredients in flue gas, however, they could lead to catalyst poisoning and lower the performance in some case. Until now, water vapor effect in catalytic reaction has been evaluated [[23], [24], [25], [26]], but the investigation on SO2 effect is insufficient in real case application. In this paper, the effect of support, reaction temperature, initial O3 and C7H8 concentration, SO2 and water vapor were systematically evaluated under the typical industrial conditions.
Section snippets
Catalyst preparation
Manganese oxides were dispersed on γ-Al2O3 (Alfa Aesar), SiO2 (Alfa Aesar) and TiO2 (Degussa) using impregnation method. Firstly, γ-Al2O3 and SiO2 were pretreated in calcination at 550 °C for 2 h under 1 L/min air flow to remove impurities, while TiO2 was calcined at 450 °C. After pretreatment, 1.5 g three supports were individually added into 30 mL acetone containing 0.4885 g 50 % manganese nitrate solution (Sinopharm Chemical Reagent Co., Ltd.). The impregnated samples were stirred at room
Effect of different supports
Three MnOx supported catalysts were investigated to compare different toluene ozonation performance with different supports. The testing results are shown in Fig. 2, and the values are listed in Table 1. Clearly, the toluene oxidation efficiency at 120 °C decreased in the following order: MnOx/γ-Al2O3> MnOx/SiO2>MnOx/TiO2. Nearly 100 % of the toluene was degraded over MnOx/γ-Al2O3 catalyst. CO2 and H2O should be the final target products after toluene oxidation that can achieve green
Conclusion
In summary, three Mn-based catalysts with γ-Al2O3, SiO2 and TiO2 as support were prepared via impregnation method. The obtained catalysts were used to oxidize toluene to CO2 and H2O. Results demonstrate that the catalytic activity at 120 °C for toluene conversion follows MnOx/γ-Al2O3 > MnOx/SiO2 > MnOx/TiO2, which is basically corresponding to the ozone decomposition efficiency. XPS analysis suggests that Mn3+ and surface oxygen species played a positive role on toluene conversion. Both Lewis
CRediT authorship contribution statement
Jiaming Shao: Writing - original draft, Data curation, Investigation. Fawei Lin: Writing - review & editing, Funding acquisition. Zhihua Wang: Conceptualization, Writing - review & editing, Funding acquisition. Peixi Liu: Data curation, Investigation. Hairong Tang: Resources, Investigation. Yong He: Writing - review & editing, Validation, Methodology. Kefa Cen: Supervision, Resources.
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
National Key Research and Development Program of China (2018YFB0605200), Fundamental Research Funds for the Central Universities (2019XZZX005-1-01) and National Natural Science Foundation of China (51906175).
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