Recent advances in volatile organic compounds abatement by catalysis and catalytic hybrid processes: A critical review

Journal Pre-proof Recent advances in volatile organic compounds abatement by catalysis and catalytic hybrid processes: A critical review

Jung Eun Lee, Yong Sik Ok, Daniel C.W. Tsang, JiHyeon Song, Sang-Chul Jung, Young-Kwon Park PII:

S0048-9697(20)30915-3

DOI:

https://doi.org/10.1016/j.scitotenv.2020.137405

Reference:

STOTEN 137405

To appear in:

Science of the Total Environment

Received date:

17 November 2019

Revised date:

4 February 2020

Accepted date:

16 February 2020

Please cite this article as: J.E. Lee, Y.S. Ok, D.C.W. Tsang, et al., Recent advances in volatile organic compounds abatement by catalysis and catalytic hybrid processes: A critical review, Science of the Total Environment (2020), https://doi.org/10.1016/ j.scitotenv.2020.137405

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© 2020 Published by Elsevier.

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Recent Advances in Volatile Organic Compounds Abatement by Catalysis and Catalytic Hybrid Processes: A Critical Review

Jung Eun Leea, Yong Sik Okb, Daniel C.W. Tsangc, JiHyeon Songd,

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Sang-Chul Junge, Young-Kwon Parka,*

School of Environmental Engineering, University of Seoul, Seoul 02504, Korea

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Division of Environmental Science and Ecological Engineering, Korea University, Seoul 02841,

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a

Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University,

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c

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Korea

Hung Hom, Kowloon, Hong Kong, China

Department of Civil and Environmental Engineering, Sejong University, Seoul 05006, Korea

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Department of Environmental Engineering, Sunchon National University, Suncheon 57922,

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Korea

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d

*Corresponding author: [email protected]

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Abstract Air pollution, particularly for toxic and harmful compounds to humans and the environment, has aroused increasing public concerns. Among air pollutants, volatile organic compounds (VOCs) are the main sources of air pollution. Many attempts have been made to control VOCs using catalysts, plasma, photolysis, and adsorption. Among them, oxidative catalysis by noble metals

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or transition metal oxides is considered one of the most feasible and effective methods to control

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VOCs. This paper reviews the experimental achievements on the abatement of VOCs using

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noble metals, transition metals and modified metal oxide catalysts. Although the catalytic degradation of VOCs appears to be feasible, there are unavoidable problems when only catalysis

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treatments are applied to the field. Therefore, catalysts including hybrid processes are developed

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to improve the removal efficiency of VOCs. This review addresses new hybrid treatments to

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remove VOCs using catalysts, including hybrid treatment combined with plasma, photolysis, and adsorption. The mechanism of the oxidation of VOCs by catalysts is explained by adsorption-

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desorption principles, such as the Langmuir-Hinshelwood, Eley-Rideal, and Mars-van-Krevelen

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mechanisms. A π-backbonding interaction between unsaturated compounds and transition metals is introduced to better understand the mechanism of VOC removals. Finally, several factors affecting the catalytic activities, such as support, component ratio, preparation method, metal loading, and deactivation factor, are discussed. Keywords: VOCs, catalysis, plasma, hybrid treatment, indoor air quality

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1. Introduction As air pollution is becoming increasingly serious, tremendous efforts have been made to investigate the abatement of harmful pollutants (Baird and Cann, 2005; EPA US 2018). Hundreds of thousands of chemicals are produced from various sources and some of them have been restricted under legal control. Among the harmful chemical pollutants that deteriorate the air

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quality, volatile organic compounds (VOCs), which have boiling points in the range of 50 ~

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260C, should be treated (EPA US 2018; Yang et al., 2019). In particular, harmful volatile

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chemicals need to be removed from the sources before they are distributed in the air; otherwise,

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they are difficult to treat after spreading outside (Lee et al., 2018). Alkyl or halogenated substituted benzene compounds are notorious and carcinogenic to people

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(Harb et al., 2020). Among them, benzene, toluene, and chlorobenzene are representative organic

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compounds for aromatic VOCs. Simple unsaturated alkenes or alcohols, and ketones have also been treated to improve the catalyst efficiency. In particular, alkyl benzenes with low molecular

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weights, such as toluene, xylene, and phenolic compounds, have been treated widely by various

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research groups with the aim of removing air pollutants (Jung and Suh, 2018; Khan et al., 2019; Ryu et al., 2019) because they are byproducts of many chemical processes using reformed or natural petroleum. Therefore, many treatments have attempted to remove VOCs using catalysts with noble metals (Fu et al., 2016; Zhang et al., 2018), transition metals or modified metal oxides catalysts, advanced adsorbents (Huang et al., 2015a) and noble metals-modified catalysts as well (Liu et al., 2018; Zhang et al., 2019) due to feasibility and relatively simple facilities needed. The photolytic degradation of indoor VOCs (Jeon et al., 2018; Harb et al., 2020; Saoud et al., 2020), 3

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such as formaldehyde, was performed using UV or a VUV lamp because of being performed safely at room temperature without heating system. Plasma (Ding et al., 2005; Sivachandiran et al., 2015; Zhao et al., 2011) and plasma combined hybrid treatments (Jiang et al., 2020; Feng et al., 2020) were performed for the removal of VOCs due to a high efficiency for the degradation of VOCs in a short reaction time. Biofiltration methods have been also employed for the removal

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of VOCs using bacterial biofilm, fungal biocatalysts, and modified biofilters with mixed microorganisms, because of their eco-friendly nature and bio-resource utilization (Cheng et al.,

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2016). Yang et al. (2018a) performed the biodegradation of multicomponent VOCs using

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biofilters and investigated the interactions such as antagonistic, neutral, and synergetic effects

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between VOCs. The interactive effects make the biodegradation rates of VOCs even lower,

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resulting in difficulties in field applications. Compared to other methods, the catalytic oxidation of VOCs using modified noble metal or transition metal oxides is very compatible, and it

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requires relatively low cost with high efficiency.

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There are several kinds of VOCs sources such as industrial processes, stacks, exhaust gas from

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automobiles and various vehicles, food industry, incineration of waste and combustion systems, fuels, chemical manufacturing processes and natural sources, such as fires and volcanoes. In addition, according to the assessment about indoor air, the concentrations for some chemical compounds of indoor air were found to contain two to five times higher VOCs than outdoor air. VOCs can be divided into several types according to the chemical structures or substituted functional groups. Aliphatic VOCs contain alkenes, ketones, alcohols, acids, aldehydes, ester, and chlorinated substituted compounds. Aromatic VOCs include benzene and benzene derivatives with various functional groups, such as toluene, xylene, phenols, chlorobenzene, and 4

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halogenated substituted compounds. Indoor VOC gases, such as formaldehyde, toluene, acetaldehyde, chlorobenzene, ethylbenzene, chloroethylene, esters, and ketones are generated mostly from organic solvents used in paints, cleaning solutions, cosmetics, adhesives, coating material, perfumes, dry-cleaned clothes, combustion sources, degreasing, disinfectants, and fuels, which are used in daily life. Some specific VOCs, halogenated or incompletely oxidized

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chemicals, may cause dermal or respiratory diseases on humans as well as environmental problems. Unfortunately, it is very rare to treat with the decomposition pathways for various

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removals of VOCs and influencing factors related with the conversion of VOCs. Therefore, this

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paper outlines the reaction mechanisms of decomposition of organic pollutants over

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heterogeneous catalysts and several key factors affecting the conversion efficiencies to help

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researchers develop novel techniques for the removal of VOCs.

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Among the various treatments for the abatement of VOCs, the catalytic oxidation of VOCs using transition metal oxides or modified transition metal oxide catalysts (Machocki et al., 2004;

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Park et al., 2000; Wu et al., 2013) is one of the most feasible and compatible treatments

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compared to noble metal-containing catalysts, despite noble metal-containing catalysts showing the best efficiency. The conversion of VOCs differs according to the catalyst activity, which would be dependent on the preparation methods, such as precursors, pH, and morphology. Although tremendous achievements have been made using various modified catalysts, they are effective toward only a few chemical compounds and catalyst deactivation problems occur when they are applied to a real field. Moreover, the majority of their results have been accomplished on a laboratory scale at high VOC concentrations and low flow rates compared to the real field conditions, which raise questions about the transferability of the bench-scale results to scale-up 5

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applications. To understand better the chemical activity and reaction mechanisms of transition metals for VOCs degradation, this paper provides the valence shell electrons configurations of manganese according to the oxidation states as one of the representative low temperature catalysts after introducing various techniques of VOCs removal. This paper reviews the literature on the catalytic oxidation of representative VOCs using

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improved metal oxides (Yang et al., 2018b; Wei et al., 2019), photolysis (Li et al., 2018; Wang et

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al., 2020), plasma, and integrated treatments of catalysis and photolysis (Shu et al., 2018; Shu et

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al., 2019; Wu et al., 2019), catalysis and plasma (Feng et al., 2020; Jiang et al., 2019; Zhu et al., 2020), and catalysis/adsorbent combined with plasma (Veerapandian et al., 2019; Yao et al, 2018).

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In this paper, based on the published results, the catalytic oxidation of a few of VOCs, including

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toluene, small ketones, and simple alcohols over various metal oxide catalysts are reviewed to

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better understand the chemical properties of VOCs to be removed using various modified catalysts. Photolysis and plasma discharge treatments are then introduced. In addition, although

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the catalytic activity for controlling a few specified pollutants shows remarkably improved

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efficiency, there has been a limitation in removing VOCs and by-products produced from a bench scale or real field because of being unable to completely control experimental parameters. Recently, several kinds of catalysts, including the hybrid system with plasma or photolysis, were investigated. In this paper, several hybrid processes (Huang et al., 2015b; Jiang et al., 2017; Shu et al., 2018; Yao et al, 2018) are introduced for the removal of VOCs because multifunctional treatments, such as catalyst-combined plasma processes, shows high efficiency for the removal of VOCs compared to mono-treatments. After introducing the catalytic degradation of VOCs, catalyst-loaded hybrid systems for the removal of VOCs are then reviewed. 6

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To determine the reaction mechanisms of the removal of VOCs by a catalyst, the removal process for several organic pollutants with regard to the adsorption over metal oxides catalysts will be provided after mentioning a range of treatments for the removal of VOCs. In general, the reaction mechanisms on the catalysts have been explained at the active sites on the catalyst surface (Huang et al., 2016b; Liotta, 2010). Three theories have been proposed to describe the

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reaction mechanism on the catalyst surface: Mars-van-Krevelen (MvK) mechanisms, EleyRideal (E-R), and Langmuir-Hinshelwood (L-H). To explain the adsorption and degradation

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process of organic pollutants, these theories are provided to understand the reaction of molecules

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on the catalyst surface (Huang et al., 2016b; Liotta, 2010). This section discusses the catalytic

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oxidation of several organic molecules regarding these theories. In addition, the catalytic activity

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can be influenced by a range of effects, such as support, metal loading, component ratio, preparation method, and precursor of metal oxides (Wei et al., 2019). These effects will be

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introduced in the last section of this paper. Based on the understanding of the reaction

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mechanism of VOCs and the critical effects to influence catalysts, this paper reviews the recently

field.

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published literature on low temperature catalysts to determine if they can be employed in the real

2. Removal of VOCs by catalytic oxidation To remove VOCs, many research groups have employed transition metal oxides and precious metals, as well as their mixtures. The catalytic oxidation of VOCs involves bond dissociation or the rearrangement of molecules, leading to the generation of intermediates and products. In 7

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contrast, physical processes for VOCs removal, such as adsorption and condensation, can be temporarily employed because of their limited capacities, while chemical reactions are applied for the permanent removals of VOCs. Among the removal of VOCs, catalytic oxidation of VOCs appears to be more feasible and compatible in terms of the cost and removal efficiency. Noble metals and transition metals were applied to remove VOCs. Many experimental studies have

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been conducted on the oxidation of VOCs over metal oxides MOx (M = V, Ce, Mn, Cr, Cu, Co, and Ni) for the removal of VOCs (Huang et al., 2015b; Li et al., 2009; Liotta, 2010; Rezaei et al.,

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2013; Wu et al., 2013). Moreover, the addition of noble metals into transition metal oxides

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results in increased catalytic activity. Even though the oxidative degradation by the combination

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of noble and transition metals are more effective for the removal of VOCs due to enhanced

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electrons transfer capability, the degradation efficiency for each of contaminant molecules might be different according to the support and target compounds. The addition of Au, Pt, Pd, Ru, and

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Ag into transition metals leads to the enhanced catalytic activity for the degradation of VOCs,

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but it depends on the composition and the ratio of precious metals and transition metals (Huang et al., 2015b; Huang et al., 2016b; Liotta, 2010). In addition, the catalytic activity for the

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degradation of VOCs is dependent on the preparation methods, support, acidity, relative humidity, and physical properties. Therefore, the catalytic activity related to the preparation method, characteristics of the support, and physical structure, such as the surface area, pore size, pore volume, morphology, and oxidation state, has attracted tremendous attention. According to the literature on the oxidation of VOCs by metal oxides, cerium oxide was reported to be very effective in the removal of 1000 ppm trichloroethylene (TCE) but a high temperature was required to achieve high efficiency (Li et al., 2009). Mn-containing metal 8

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oxides showed the improved efficiency for the removal of VOCs as listed in Table 1. Manganese oxides showed 100% conversion for 125 ppm n-hexane at 180℃. In addition, Mn/Zr bimetal oxide, which was composed at a relative ratio of Mn : Zr = 2:3 was tested for the removal of 1000 ppm 1,2-dichloroethane (1,2-DCE) and 1000 ppm TCE. They reported 100% conversion for 1,2-DCE and TCE at 450℃ and 550℃, respectively. Mn-containing bimetal catalysts, Mn/Zr

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and Mn/Cu, have been applied to remove toluene. One hundred percent conversion of (0.35%)

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toluene was estimated at 260℃ and 220℃ for Mn/Zr at Mn:Zr = 1:1, and Mn/Cu at Mn:Cu =

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2:1, respectively (Li et al., 2004). Delimaris and Ionnides (Delimaris and Ioannides, 2008) performed the degradation of 1600 ppm ethanol using a Mn-Ce catalyst and obtained 100%

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complete conversion of ethanol at 170℃. In addition, a Mn-Cu bi-metal component catalyst was

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applied to remove (1%) ethanol with 100% conversion achieved at 210℃ (Morales et al., 2008).

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The removal efficiency of VOCs differed significantly according to the ratio of mixed metals and

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the synergetic effects of mixed components. The addition of noble metals to metal oxides can increase the catalytic activity for VOCs

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removal compared to a noble metal unloaded catalyst (Huang et al., 2015b; Liotta, 2010; Scire et al., 2003). The oxidation of toluene was performed using Au-added CeO2 with 100% toluene conversion achieved at 360℃, whereas the conversion of toluene at the same concentration showed only 50% by Au unloaded bare CeO2 even at 600℃ (Scire et al., 2003). The addition of gold to cerium oxides enhanced the catalytic efficiency of cerium oxide. They explained the enhanced catalytic efficiency of Au/CeO2 for increased oxygen mobility. Table 2 lists the published results for the conversion of toluene through catalytic oxidation by noble metals, 9

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transition metals, and their mixed catalysts (Benard et al., 2009; Scire et al., 2003; Tahir and Koh, 1999). As shown in Table 2, the catalytic activity for the degradation of toluene differed according to the Pt loading. The optimal amount of noble metal depends on the molecular characteristics of the VOCs. For toluene removal, the Pt-loaded catalyst showed improved catalytic efficiency

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(Ordonez et al., 2002; Santos et al., 2010).

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As shown in Table 2, the Pt loading and composition of mixed metal oxides affected the

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catalytic efficiency for the degradation of toluene. With Pt(0.9 wt. %)/Al 2O3, 1000 ppm toluene

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was removed completely at 227℃, whereas with Pt(0.56 wt. %)/Al2O3, 100% conversion of

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1000 ppm toluene was achieved at 250℃ (Benard et al., 2009). In addition, the removal of 600 ppm toluene was performed with 50% conversion of toluene obtained at 180℃ over Pt(0.4

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wt.%)/Al2O3. In contrast, 50% conversion of 600 ppm of toluene was obtained at 263℃ over

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Pt(0.05 wt.%)/Al2O3 (Tahir and Koh, 1999). The amount of noble metal can also affect the

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catalytic activity for the removal of toluene. The oxidative conversions of hexane, methanol, butylamine, toluene, propane, and propene were testified over the Pt/Al2O3 catalyst (Benard et al., 2009; Gluhoi et al., 2006; Kim et al., 2004; Radic et al., 2004; Tahir and Koh, 1999). The oxidation of benzene, toluene, and 1-hexene was performed using Pt, Pd, and Rh cordierite monoliths over alumina. The Pt/alumina and Pd/alumina catalyst were more effective in removing aromatic compounds, such as benzene and toluene, whereas Rh/alumina showed better efficiency in removing aliphatic compounds than 10

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aromatic VOCs (Patterson et al., 2000).

3. Catalysis including the hybrid for the removal of VOCs Combined systems for the abatement of VOCs have been investigated to improve the removal

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efficiency, particularly for low concentrations of VOCs from a large flow gas (Jiang et al., 2017; Yao et al, 2018). Compared to the system using a mono-functional process, such as catalysis or

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plasma systems, hybrid processes, such as catalysis and plasma (Fan et al., 2011; Jiang et al.,

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2017) or catalysis and photolysis (Shu et al., 2018; Shu et al., 2019; Wu et al., 2019), have been

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proven to be efficient in the removal of VOCs. We described the hybrid multi-functional

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Saoud et al., 2020; Zhu et al., 2020).

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treatments of catalysis, photolysis, and plasma for the removal of VOCs (Feng et al., 2020;

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3.1. Removal of VOCs using photolysis and catalysts

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Photocatalysis is one of the most feasible treatments for the removal of VOCs in terms of cost and efficiency. Various modified TiO2 photocatalysts (Li et al., 2018; Yang et al., 2018b), such as transition metal-added TiO2 or N-, C-doped and electrochemically self-doped TiO2, have been used to control the indoor VOCs (Huang et al., 2016b; Wang et al., 2020). Various improved photocatalysts were prepared for the removal of indoor VOCs (Wang et al., 2020; Jafari et al., 2019). The photocatalytic degradation of benzene, toluene, acetone, styrene, xylene was performed using modified photocatalysts under the irradiation of a specific range of wavelengths. 11

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In addition, the photocatalytic degradation of formaldehyde was carried out using UV or visible light. Pt-doped TiO2 was quite effective in removing benzene, even under black light irradiation of 300 ~ 420 nm and the complete degradation of benzene was achieved by the photo-deposited Pt/TiO2 (Einaga et al., 2001). In addition, Ce addition to TiO2 resulted in increasing catalytic efficiency for the conversion of toluene and 90% conversion of toluene was achieved on Ce-TiO2

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made from a sol-gel method under visible light irradiation (Sidheswaran and Tavlarides, 2008). In contrast, only 52% toluene removal was achieved by TiO2 prepared using a sol-gel method

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under black light irradiation (Cao et al., 2000). Photolysis under vacuum ultraviolet (VUV)

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irradiation is associated with the direct cleavage of chemical bonds in organic molecules. The

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initiation step of the photolysis of toluene under VUV irradiation (185 nm) was considered to

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detach hydrogen from the methyl group of toluene. By further oxidation of hydrogen-detached toluene by active oxygen species, incomplete carbonaceous intermediates were generated, such

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as peroxides, benzoic acid, and anhydrides. The problem of using VUV for the removal of VOCs

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is the generation of O3 and byproducts produced by the reaction of VOCs and O3, which can produce secondary pollutants (Huang et al, 2015; Ochiai et al., 2013; Suzuki et al, 2015). With

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the photocatalysis only, it is difficult to completely oxidize toxic secondary pollutants and byproducts as residues of VOCs. On the other hand, the photocatalysis combined with oxidative catalysts prepared with transition metals or mixed transition metal oxides showed substantial improvement in the removal of not only thermodynamically-stable aromatic compounds including benzene, toluene, and their by-products, but also ozone and incompletely oxidized harmful intermediates (Shu et al., 2018; Shu et al., 2019; Wu et al., 2019). Shu et al. performed the photolysis of toluene using VUV irradiation over Mn/TiO2/activated carbon (Shu et al., 2018). 12

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To overcome the catalyst deactivation caused by adsorption from the degradation of carbonaceous pollutants and ozone generation, they utilized the combined multifunctional removal process of catalytic oxidation by a transition metal, Mn, including the TiO2 catalyst with an activated carbon support and photolysis using 185 nm VUV. The following reactions could occur in the presence of water vapor or oxygen molecules by the irradiation of 185 nm VUV

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(Albuquerque et al., 2015; Zhang et al, 2003).

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The activated carbon employing TiO2/AC and Mn/TiO2/AC catalysts showed remarkably

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improved ozone removal compared to that under VUV irradiation only. In the presence of ozone, although toluene may not be degraded by the reaction of ozone, the O3 adsorbed on the

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Mn/TiO2/AC catalyst was reported to affect the degradation of toluene by assisting the active

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oxygen sites generated from Mn (Suzuki et al, 2015).

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They performed the photocatalytic removal of toluene by Mn/TiO2/AC according to the amount of TiO2 to examine the effects of the amount of TiO2 on the conversion efficiency of

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toluene. They prepared five different samples, 0.1%Mn/10%TiO2/AC, 0.1%Mn/20%TiO2/AC,

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0.1%Mn/40%TiO2/AC, 0.1%Mn/60%TiO2/AC, and 0.1%Mn/80%TiO2/AC. Among them, 0.1%Mn/40%TiO2/AC showed the highest efficiency for the removal of toluene. They also examined the effects of the Mn loading on the conversion of toluene and performed the photocatalytic conversion of toluene using Mn/40%TiO2/AC according to the Mn loading of 0.1% to 10%. They obtained the highest conversion of toluene at 0.1%Mn/40%TiO2/AC. The order of toluene conversion was 0.1%Mn/40%TiO2/AC > 1%Mn/40%TiO2/AC > 5%Mn/40%TiO2/AC > 10%Mn/40%TiO2/AC. The highest catalytic conversion of toluene (87%) and complete ozone 13

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decomposition were achieved with 0.1%Mn/40%TiO2/AC (Shu et al., 2018). Based on previous studies, VUV (185 nm) light combined with catalysts was used to remove the VOCs, but catalyst deactivation occurred by intermediates deposited on the catalyst surface. Manganese oxides were reported to be effective in reducing catalyst deactivation by the decomposition of O3 (Ma et al., 2017; Xu et al., 2017). ZSM-5-supported manganese oxides

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were effective in removing O3 because of the large surface area of ZSM-5 and the well-dispersed

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manganese oxide on the catalyst surface (Huang et al., 2016a) Ce-added manganese oxides

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showed remarkably higher activity for the removal of toluene (Weng et al., 2017). Shu et al. performed VUV photolysis for toluene over Ce-MnOx/ZSM-5 because ZSM-5 was reported to

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be an effective support for the removal of toluene (Shu et al., 2019). They adjusted the Ce

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loading over manganese at Mn : Ce = 1:1, 1:3, and 1:5 to obtain the optimal composite of Mn

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and Ce in the Ce-MnOx/ZSM-5 catalyst for the conversion of toluene under VUV irradiation at 185 nm. Among them, Mn-3Ce/ZSM-5 showed the best activity for toluene removal in the VUV

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photolysis-combined system. Based on an analysis of toluene removal and the output CO2

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concentration from the conversion of toluene, Mn-3Ce/ZSM-5 showed the highest efficiency for toluene removal, more than 90%, even after 150 min. In the absence of a catalyst, only 70% toluene removal was achieved. The removal of toluene over ZSM-5 was very high, initially by adsorption, but after 80 min, similar results (70%) were obtained by photolysis. ZSM-5 appears to act as a support not a catalyst. When Mn/ZSM-5 was applied to the removal of toluene, 80% remained after 150 min. On the other hand, the ozone concentration by VUV irradiation was 83 ppm and 78 ppm by photolysis and with ZSM-5 under photolysis, respectively. On the other hand, Ce/MnOx catalysts reduced the ozone concentration of the outlet remarkably to 6 ppm, 1.8 14

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ppm, 0.9 ppm, and 2 ppm using Mn/ZSM-5, Mn-Ce/ZSM-5, Mn-3Ce/ZSM-5, and Mn5Ce/ZSM-5, respectively. They explained the improved activity of Mn-3Ce/ZSM-5 for the removal of toluene and ozone by the increase in active oxygen anionic species and the more available adsorption sites for toluene degradation.

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3.2. Catalyst and plasma hybrid system for the removal of VOCs

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Plasma combined with an adsorbent using various zeolites has been applied to the removal of

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toluene (Lee et al., 2019; Li et al., 2019). In general, the plasma discharge process can make

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various active oxidant species, OH radicals, and super oxide anion radicals (•O2-) by the reaction

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with water and oxygen. There are two types of plasma/catalyst hybrid systems for the removal of VOCs; a post-plasma catalyst (PPC) system and in-plasma catalyst (IPC) system. The active

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oxidant species produced from the plasma discharge can react with VOCs on the catalyst surface

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or in the gas phase. In the post-plasma catalyst system, the residual products generated after the reaction with radical species are decomposed by the catalytic oxidation. Comparatively, in the in-

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plasma catalyst system, the destructive oxidation of VOCs occurs both on the catalyst and by the active oxidant species produced from the plasma discharge. According to the characteristics and properties of zeolite, the removal efficiency of toluene showed different levels of conversion, as shown in Table 3. To increase the removal efficiency of VOCs, an integrated system of a plasma system and catalyst/adsorbent was used for the abatement of low concentrations of pollutants (Lee et al., 2020a; Lee et al., 2020b; Song et al., 2018; Sultana et al., 2015; Veerapandian et al., 2019). Based on the plasma efficiency of VOCs, the catalyst placement should be carefully 15

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adjusted. Jiang et al. performed the oxidative catalytic degradation of benzene using a plasmacombined system (Jiang et al., 2017). Compared to the conversion of benzene using either a plasma system only or catalysis, the removal of VOCs was improved significantly with the combined process of plasma and catalyst. They performed the degradation of benzene nonthermal plasma instead of conventional plasma because of the high cost in cleaning up the

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unwanted by-product adsorbed on the active sites. They performed the degradation of benzene using improved plasma, which was called surface/packed-bed hybrid discharge (SPBHD) plasma

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combined with several metal oxides. Alumina-supported metal oxides, such as AgOx, MnOx,

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CuOx, and FeOx, were placed at downstream of the plasma-like post-plasma catalysis (PPC)

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process to obtain the synergetic effect for the removal of pollutant. Among the applied metal

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oxides, AgOx showed the highest removal efficient for the degradation of benzene according to their results. In addition, they examined the effects of the Ag loading and optimized the GHSV

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for the degradation of benzene. Regarding the influence of the Ag loading, benzene removal

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showed the highest efficiency with the 15 wt. % of the Ag catalyst while the catalysts prepared with more than 15% Ag or less than 15% Ag were inferior to the 15% Ag loading catalyst in the

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degradation of benzene. At the optimized flow rate, the removal efficiency of benzene was highest at 22,856 h-1 GHSV. They obtained 98% conversion of benzene over 15 wt. % AgO x/γAl2O3 with 22,856 h-1 GHSV. The increased removal efficiency of benzene by the combined treatment of plasma and catalyst was due to the active species generated by ozone decomposition (Jiang et al., 2017). On the other hand, for the in-plasma catalyst (IPC) system, the catalyst bed is placed inside the plasma so that the polluted gas flows over the catalyst/adsorbent to store VOCs adsorbed on the 16

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surface of the catalyst/adsorbent followed by the oxidation of VOCs and by-products by the active species generated by the plasma system. For the process of VOCs removal, a catalystintegrated plasma discharge could be ignited continuously during the process in both IPC and PPC (Ding et al., 2005; Sivachandiran et al., 2015; Zhao et al., 2011). In contrast, to reduce the energy consumed for the removal of VOCs, the discharge can be adjusted in the sequential order

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of cyclic VOCs storage or adsorption and oxidation of VOCs by the catalyst and active species generated by the plasma instead of a continuous discharge. Compared to a continuous plasma

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process, the sequential process can reduce the energy cost by more than 10 fold. For the removal

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of isopropyl alcohol, the energy cost was reduced 14.5 fold using the manganese oxides

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combined with the NTP reactor following the sequential process compared to the continuous

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process (Sivachandiran et al., 2015). To purify indoor air, the removal of formaldehyde (HCHO) was performed using a cycled storage-discharge (CSD) sequential discharge method over

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AgCu/HZSM-5 (Zhao et al., 2011). The catalyst-combined sequential discharge system had a

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low energy cost and showed high tolerance toward moisturized air. Their experiments showed that lengthy storage resulted in low energy cost for the degradation of HCHO (6.3 ppm). In

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addition, through the sequential operation of the catalyst bed of Ag/HZSM-5 and dielectric barrier discharge (DBD), complete degradation of 4.7 ppm benzene was achieved with 99.8% CO2 selectivity (Ding et al., 2005). In addition, several metal catalysts, such as Ce, Co, Ag, Mn, Fe, Ni, Cu, and Zn, supported on HZSM-5 were employed for the degradation of benzene using the same process (Fan et al., 2011). Among several metal catalysts, 0.8 wt. % Ag/HZSM-5 showed the highest catalytic efficiency for the removal of benzene and resulted in 100% CO2 selectivity with the strong suppression of unwanted by-products. Compared to the PPC system 17

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for the removal of benzene, IPC could approach the complete degradation of benzene, even within 10 min, whereas PPC showed less than 60% removal efficiency for longer than 30 minutes. DC plasma placed upstream of the zeolite support metal catalyst, such as Ag, Mn, Ce, and Ag-Mn, was used to examine the catalytic efficiency for the removal of toluene at low concentrations. The Ag-loaded catalyst, Ag/HZSM-5 and Ag-Mn/HZSM-5, showed the higher

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removal of toluene because of the interaction of Ag and unsaturated hydrocarbon. Based on their results, the conversion of toluene was in the following order: Ag-Mn/HZSM-5 > Mn/HZSM-5 >

ro

Ag/HZSM-5 > Ce-Mn/HZSM-5 > Ce/HZSM-5. They explained the improved catalytic activity

-p

of Ag-Mn/HZSM-5 for the removal of toluene because of the more available lattice oxygen due

re

to the presence of Mn on the Ag catalyst surface (Qu et al., 2013). The increased ratios of lattice

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oxygen to surface oxygen on the Mn enhanced both CO2 conversion (99.9%) and the suppression

na

of ozone generation as well.

For the removal of toluene, trimetallic (Mn-Co-Ni) oxide catalyst combined with non-thermal

ur

plasma was employed to improve the efficiency (Feng et al., 2020). In addition, the multistage

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plasma coupled with the CoOx-CeO2 catalyst was used to remove toluene (Jiang et al., 2019). The toluene removal was performed using the plasma-catalyst hybrid system including Au nanocatalysts leading to suppression of NOx and O3 generation (Zhu et al., 2020). In addition Jiang et al. employed the modified multistage sliding plasma system combined with a series of CoOx-CeO2 catalysts to remove toluene and obtained higher efficiency for the removal of toluene due to the increased surface oxygen species (Jiang et al., 2019).

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3.3. Ozone effects for the removal of VOCs The effects of the oxygen species generated from plasma operations toward the removal of VOCs were proposed based on the experimental performance for the degradation of VOCs. Among them, the effects of the ozone produced from active oxygen species in the plasma system were also studied and the degradation mechanisms of VOCs were suggested (Jiang et al., 2017).

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The ozone concentration measured at the outlet of the plasma reactor decreased in the presence

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of benzene compared to that in the absence of benzene. Although there was no benzene

-p

degradation by the reaction of ozone, the ozone generation going through the plasma-catalyst combined system was reduced in the presence of benzene. This could be the result of

re

competition between ozone generation and VOCs degradation by active oxygen species. The

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active oxygen species appeared to react favorably with VOCs rather than produce ozone. The ozone concentration measured from the post plasma catalyst (PPC) was reduced in the presence

na

of benzene in the following order, AgOx/γ-Al2O3 < MnOx/γ-Al2O3 < CuOx/γ-Al2O3 < FeOx/γ-

ur

Al2O3 < γ-Al2O3, with a concomitant increase in SIE (J/L). This order was the same order as the

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catalytic efficiency for benzene degradation. The ozone amounts at the outlet of the PPC were measured 450, 231, and 133 mg/m3 from the plasma system, MnOx/γ-Al2O3 of the PPC and AgOx/γ-Al2O3 of PPC, respectively, under the pure air condition without benzene. In the presence of benzene (370 mg/m3), the ozone concentrations were 197, 111, and 73 mg/m3 at the outlet of the plasma only, MnOx/γ-Al2O3 of PPC, and AgOx/γ-Al2O3 of PPC, respectively. The plasma combined with the AgOx/γ-Al2O3 catalyst was the most effective for benzene removal and the suppression of ozone. The more efficient catalyst for ozone suppression, the higher conversion of benzene. The efficiencies of the plasma-combined catalyst for the removal of 19

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benzene and ozone suppression were in the following order according to the increase of SIE (J/L): AgOx/γ-Al2O3 > MnOx/γ-Al2O3 > CuOx/γ-Al2O3 > FeOx/γ-Al2O3 > γ-Al2O3 (Jiang et al., 2017). Ozone generation was suppressed by the competition of active oxygen species toward benzene. To investigate the effects of ozone on the degradation of benzene, they performed the removal of benzene under the addition of ozone, but no benzene degradation occurred in the

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presence of ozone. On the other hand, the amount of ozone evaluated at the outlet decreased remarkably in the presence of benzene. The ozone suppression by the presence of benzene was

ro

explained by the selectivity for the active oxygen species toward VOCs rather than oxygen

re

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molecule, leading to the formation of ozone molecules.

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4. Oxidative catalytic degradation mechanisms (adsorption/desorption)

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For various kinds of VOCs oxidation processes, the appropriate mechanism could be

ur

described depending on the molecular properties and characteristics of the catalyst, such as acidity or hydrophobicity. The catalytic oxidation of VOCs using metal oxides occurs by the

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adsorption of oxidants and/or reactants (VOCs) on the catalyst surface, and the degradation mechanisms are quite difference depending on the reactants and catalysts. In addition, after the reaction between VOCs and oxidants, products or intermediates should leave from the surface effectively. Otherwise, the active sites would be occupied and deactivated. In order to investigate the kinetics and mechanisms for the catalytic degradation of VOCs and to develop more efficient catalysts or hybrid catalytic processes, it is important to understand the adsorption/desorption of VOCs and oxidants on the catalysts. To describe catalytic oxidation by metal oxides, three 20

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models were applied to explain the reaction mechanism of VOCs degradation on the catalytic surface: Mars-van-Krevelen (MvK), Eley-Rideal (E-R), and Langmuir-Hinshelwood (L-H) mechanisms (Huang et al., 2015b; Liotta, 2010).

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4.1. The Langmuir-Hinshelwood (LH) mechanism and the π-backbonding interaction For the reaction described by the LH mechanism, the adsorption of reactant and oxygen on the

ro

catalyst surface should occur before the reaction between oxygen and reactant begins. After the

-p

generation of products from the reaction of the reactant and oxygen, the product desorbs from the

re

surface of catalyst. This mechanism is used frequently to describe the catalytic oxidation of

lP

VOCs. The reaction rate of the LH model can be determined depending on both the number of active catalyst sites and concentration of reactants (Liotta, 2010; Rezaei et al., 2013). The

na

catalyst surface provides active sites to undergo degradation of reactants. In the oxidation of

ur

VOCs in the presence of ozone, the reaction rate depends on the adsorption of ozone on the catalyst. After the active sites on the catalyst surface are prepared by oxygen or ozone, the

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adsorbed VOCs can be initiated by a reaction with active oxidant species. Qiu et al. (2019) investigate the catalytic degradation of chlorobenzene in the concentration range of 500 – 2500 ppm using a CrCeOx (Cr:Ce = 5:1) catalyst with Al/Fe support, and showed the complete degradation of chlorobenzene at the combustion temperature less than 300C over 1000 hours with high stability. They also studied the reaction mechanism for the decomposition of chlorobenzene by the CrCeOx/Al-Fe, which was estimated to follow the LH mechanism. As one of the theories used to help better understand the interaction of metal oxides and 21

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aromatic compounds or olefins, the π-backbonding interaction could be used to explain the interaction of metal and VOCs (Brill, 1973). In the reaction described by the L-H mechanism, the adsorption of reactant on the catalyst surface should be considered important. Regarding the oxidation of aromatic compounds, such as benzene or toluene, the orbital interaction of the reactants and surface could contribute to adsorption between the molecules and active sites of the

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noble metal or transition metal catalyst. According to the literature, π-backbonding is one of the interactions between manganese and toluene to help better understand the catalytic activity of

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transition metals or noble metals with the available d orbitals involving π molecular orbital

-p

interactions (Brill, 1973; Streitwieser et al., 1992). π-Backbonding is used to describe the

re

interaction between a transition metal and olefins or aromatic compounds. Transition metals can

lP

accept an electron into an empty or available d orbital from the π molecular orbital of olefins or aromatic compounds. In addition, if there are available electrons in the d-orbital of the metal, and

na

the d orbital electrons of a transition metal can be transferred to π* molecular orbital of the

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reactant, the interaction of π-backbonding with unsaturated organic pollutants, such as olefins or

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aromatic compounds, can occur among their molecular orbitals. With Mn, there are five available 3d orbitals (dxy, dyz, dzx, dz2, and dx2-y2) on manganese that can interact with toluene. Among the five d orbitals, the three t2g d orbitals (dxy, dyz, and dzx) can interact with the π* orbital of toluene, whereas the two eg d orbitals (dz2, dx2-y2) interact with the π-orbital of toluene (Greenwood and Earnshaw, 1997). Because the orbital interactions between the dz2 or dx2-y2 orbital of Mn and the π-bonding molecular orbital of toluene occur according to the molecular axis, x, y, and z, this interaction is called a σ-interaction. On the other hand, another interaction exists between one of the t2g orbitals (dxy, dyz, dzx) of Mn and π* antibonding 22

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molecular orbital of toluene, which is called a π-backbonding interaction because these orbitals overlap in the perpendicular direction with respect to the σ-framework of the atomic bonding axis (Atkins and Paula, 2006). There are two types of interaction between manganese and toluene resulting in diminishing the bonding character of toluene. This is because of both the addition of an electron into the

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antibonding molecular orbital of toluene and the transfer of electrons from the bonding

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molecular orbital of toluene to the d orbitals of Mn (Krchar and Janik, 2013). Through the

-p

increased antibonding character, a toluene molecule can be activated to react with oxygen species or ozone. In the presence of ozone, toluene may undergo electrophilic addition with ozone to

re

form an ozone adduct, molozonide (Kochi, 1973; Streitwieser et al. 1992). Through a rapid

lP

intramolecular transformation, the molozonide structure can be transformed to ozonide, which

na

produces two carbonyl groups, a ketone or aldehyde, with the concomitant bond breaking of the corresponding carbon atoms. Further oxidation can occur by active oxygen reagents, leading to

ur

various incomplete oxidized intermediates or by-products of carboxylic acids, anhydrides, ester,

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and epoxides at low temperatures (Aghbolaghy et al., 2018; Yao et al, 2018).

4.2. The Eley-Rideal (ER) mechanism Some oxidation process can occur through a slightly different mechanism called the EleyRideal (ER) mechanism (Burgos et al., 2002; Huang et al., 2015b). In this case, the oxidative process occurs between the oxygen adsorbed on the catalyst surface and the reactants in the gas phase without chemisorption on the catalyst surface. After the generation of products by the 23

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active oxygen species and reactants, the products desorb from the surface of catalyst. Burgos et al. performed the oxidative degradation of 2-propanol, toluene, methylethylketone (MEK), and acetone as well as mixtures of them using Pt-impregnated metallic monoliths made of aluminum foil and obtained the complete conversion of these molecules except for 2-propanol (Burgos et al., 2002). They also carried out experiments of TPD (temperature programmed

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desorption) and TPSD (temperature programmed surface oxidation) to investigate the reaction

ro

mechanism for VOCs degradation on Al2O3/Al and Pt/Al2O3/Al. Among 2-propanol, MEK,

-p

toluene, and acetone, only 2-propanol appeared to undergo the oxidation reaction with the adsorbed oxygen atoms on Pt after adsorbing on the Al2O3 surface, which is a two-center

re

reaction called the LH mechanism. On the other hand, the oxidation of toluene and ketones were

lP

considered to undergo oxidative degradation with the oxygen adsorbed on the catalyst without

na

adsorption in the gas phase, which could be ascribed as the ER mechanism (Burgos et al., 2002). They also performed the oxidation of mixtures of VOCs and reported no significant difference in

ur

the conversion efficiency of toluene and MEK, and acetone. In contrast, the oxidation of 2-

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propanol was inhibited significantly in the presence of toluene. The oxidation of 2-propanol was inhibited by the coexistence of toluene or MEK based on the catalytic oxidation of mixtures of toluene, 2-propanol, acetone, and MEK. With increasing toluene concentration, the oxidation of 2-propanol was inhibited remarkably. Interestingly, although toluene and MEK adsorbed on the alumina support, they did not appear to participate in oxidation rather than just remaining like spectators (Busca, 1996; Paulis et al., 2000). This suggests the oxidative degradation of toluene and MEK by the E-R mechanism and the oxidative degradation of 2-propanol via the L-H mechanism (Burgos et al., 2002). Although there is some controversy regarding the reaction 24

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mechanism for the catalytic oxidation of VOC alone and VOCs mixtures, aromatic compounds and olefins were unaffected by alkane or saturated hydrocarbons because aromatic compounds and alkenes undergo a dissociative reaction with oxygen adsorbed on the catalyst without adsorption on the catalyst surface. On the other hand, dissociative oxidation occurred with ethylbenzene or aliphatic hydrocarbon after adsorption on the active sites provided by the

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4.3. The Mars-van-Krevelen (MvK) mechanism

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catalyst or support with adsorbed oxygen.

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With the exception of the previous L-H and E-R mechanism, the Mars-van-Krevelen (MvK)

lP

model (Burgos et al., 2002; Huang et al., 2015b; Liotta, 2010; Radic et al., 2004) was developed to provide an appropriate explanation for oxidation on transition metal oxides, particularly for

na

the reaction involving cyclic reduction and oxidation between gaseous oxygen and active sites on

ur

the catalyst surface. In this model, the oxidation of VOCs takes place in two steps. The first step is the reaction of adsorbed oxygen on the catalyst surface and molecule in the gas phase, which is

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followed by the generation of a product on the surface, and the desorption of that product from the surface, which can be described as the reduction of the catalyst. The second step reaction involved the occupation of an oxygen vacancy from the bulk to the surface, which can be described as the re-oxidation of the catalyst. The MvKrevenlen mechanism was developed to explain the partial oxidation of organic compounds over the metal oxide catalysts. Based on the published results, the oxidation of VOCs over the Pt catalyst was explained using the MvK mechanism (Gangwal et al., 1988; Garetto and Apestequia, 2000; Ordonez et al., 2002). Radic et 25

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al. (Radic et al., 2004) performed the oxidation of toluene and hexane over a Pt/Al2O3 catalyst and examined the reaction rates for toluene and hexane by Pt/Al2O3 using the catalysts synthesized with two different Pt crystalline sizes, 1.0 and 15.5 nm. They reported that the catalytic efficiency was independent of the compound. On the other hand, the rate of oxidation of toluene and hexane on the Pt/Al2O3 catalyst increased with increasing Pt crystalline size. They

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explained the decomposition of toluene and hexane by the MVK mechanism. In addition, benzene, toluene, and hexane were oxidized using a commercial Pt/γ-Al2O3 catalyst. The

ro

reaction mechanism was described using the MVK model (Ordonez et al., 2002). The catalytic

-p

oxidation of cyclopentane and methane over Pt/Al2O3 were also described using the MVK

re

mechanism (Garetto and Apestequia, 2000). Although the components ratio of mixed metal

lP

catalysts was employed for the removal of the same compounds, the degradation efficiency will differ according to the preparation method or catalyst precursor. In general, the catalytic

na

efficiency for the removal of VOCs is dependent on the surface area and adsorption capability of

ur

the catalyst/adsorbent, which are determined using the methods for preparing the catalyst. For the decomposition of toluene using a Pt alumina support catalyst according to the size of the

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active sites of catalyst, the rate showed a dependency on the crystallite size of the active sites on the catalyst surface (Radic et al., 2004). With the catalyst active site with a mean value of 1.0 nm, the rate of oxygen adsorption on the catalyst surface was estimated to be approximately zero order and the rate with respect to the concentration of toluene was estimated to be first order. On the other hand, in the case of larger active sites (15.5 nm), the rate with respect to oxygen adsorption on the catalyst surface was first order and the rate with respect to the toluene concentration was approximately zero order. This is because the larger Pt crystallites, which play 26

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a role of a reductant, result in a weaker bond between the Pt and oxygen, which makes oxygen more active toward toluene. On the other hand, for the catalytic conversion of aromatic compounds using Mn/Ce mixed oxides, higher catalytic activity was obtained from the catalyst with larger lattice oxygen molecules, e.g., 3Mn1Ce metal oxides (Chen et al., 2018). This means that toluene removal

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would follow the Mars-van-Krevelen mechanism because the lattice oxygen concentration was

-p

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crucial for the removal of VOCs (Ma et al., 2018; Ren et al., 2016).

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5. Impacts on the catalytic activity

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5.1. Precursor effects on the activity of the catalysts

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According to the preparation method, degree of dispersion of active sites on metal oxides showed a difference toward VOCs oxidation. In particular, the conversion of VOCs using metal

ur

oxide catalysts differs according to the preparation methods. In addition, residual chlorine from

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the catalyst precursor was reported to affect the activity of the catalyst (Paulis et al., 2001). To examine the effects of chlorine generated from Pt precursors, the conversion of toluene was performed using a Pt/Al2O3 catalyst prepared from Pt(NH3)4(OH)2 and H2PtCl6 (Paulis et al., 2001). According to the results, the residual chlorine would suppress the catalytic activity of Pt because it causes the complete oxidation of metal and deactivates the active sites on the Pt surface. After eliminating the residual chlorine by a pretreatment, they achieved a higher level of toluene conversion over Pt/Al2O3 (Liotta, 2010). 27

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Various morphologies of manganese oxides, MnOx, have multivalent oxidation states, 2+ to 7+ manganese; the chemical composite of MnO, Mn2O3, and MnO2 correspond to the oxidation states of Mn2+, Mn3+, and Mn4+, respectively. The catalytic activity of MnOx showed different conversion of VOCs depending on the oxidation state of manganese due to the different nucleophilicity affecting active oxygen. Two types of oxygen atoms may exist in manganese

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oxide involving the redox cyclic oxidation of VOCs: lattice oxygen and surface oxygen. Wu et al. performed the oxidative conversion of o-xylene using manganese oxides prepared by a

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precipitation method (α-MnO2) and conventional method (MnOx) and compared the

-p

characteristics of these catalysts based on H2-TPR, BET, XRD, XPS, and SEM/EDS

re

measurements (Wu et al., 2013). They obtained the complete conversion of o-xylene to CO2 at

lP

220 ℃ using α-MnO2 made from redox-precipitation, which is 50 ℃ lower than the conventionally synthesized MnOx catalyst. They concluded that the different efficiency was

na

caused by the Mn4+ content on the catalyst surface. The higher the manganese ion concentration

ur

with a higher oxidation state on the surface, the higher catalytic efficiency in the removal of VOCs. Based on their results, α-MnO2 was covered by 100% Mn4+ on the surface, whereas

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MnO2 and Mn2O3 were found in MnOx, which was covered with only 31% Mn4+ on the catalyst surface. They explained the reason for the improved catalyst made from the redox-precipitation method due to the higher porous hierarchical structure and high BET surface area, which enhanced the oxygen mobility and adsorption of reactants. Compared to previous results, CO conversion showed higher conversion on PdOx/MnO2 than PdOx/Mn2O3 (Park et al., 2000). In addition, the oxidation of methane was performed on LaMnO3 and a higher conversion of methane was achieved on LaMnO3 with a higher Mn4+/Mn3+ ratio (Machocki et al., 2004). They 28

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also mentioned that pH and potassium ions in manganese oxide affected the catalytic activity of o-xylene (Wu et al., 2013). In addition, different morphologies of manganese oxides can be synthesized according to the manganese oxide precursor. MnO2 and Mn2O3 particles were observed in MnOx prepared from the nitrate precursor, whereas the Mn3O4 phase with better dispersion was observed in MnOx

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prepared from the acetate precursor (Rezaei and Soltan, 2012; Rezaei et al., 2009). For the

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decomposition of benzene in the presence of ozone, MnOx synthesized from the acetate

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precursor was highly dispersed and showed better catalytic efficiency than that prepared from the

lP

re

nitrate precursor (Park et al., 2012a; Park et al., 2012b).

5.2. Effect according to preparation methods

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Chen et al. performed the catalytic oxidation of benzene, toluene, chlorobenzene, and xylene

ur

using manganese oxides and Ce-added manganese oxides (Chen et al., 2018) because hybrid

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transition metal oxides show improved catalytic activity for the removal of VOCs (Castano et al, 2015; Si et al., 2016). Compared to MnOx or CoOx, Mn-Co mixed oxides were more effective in the removal of toluene and propanol (Castano et al, 2015). In addition, Mn-Fe binary oxides were superior to MnOx in the removal of toluene (Si et al., 2016). Ce- added manganese oxides were reported to increase the catalytic activity for the removal of VOCs, even chlorine containing molecules, owing to the abundant oxygen vacancies provided by Ce (He et al., 2015; Huang et al., 2014; Wang et al., 2008; Xingyi et al., 2009). To increase the degree of dispersion of active sites on the catalyst surface, Arena et al. examined the redox co-precipitation and 29

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synthesized well-dispersed Mn-Ce mixed oxides (Arena et al., 2007; Arena et al., 2008), which resulted in an increased removal efficiency for the oxidation of o-xylene (Wu et al., 2010). As more developed synthesis methods, hydrolysis-driving redox co-precipitation methods were applied to compare the catalytic efficiencies for the removal of aromatic compounds and improved removal was achieved (Chen et al., 2016; Si et al., 2016). Chen et al. prepared Mn-Ce

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mixed oxides and performed the oxidation of aromatic compounds (Chen et al., 2018). They obtained the improved conversion of aromatic compounds, toluene, benzene, xylene, and

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chlorobenzene over 3Mn-Ce oxides, which had been prepared by a hydrolysis-driving co-

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precipitation reaction. To examine the relationship between the structure and catalytic activity,

re

they synthesized several metal oxides, such as MnO2 and CeO2, as well as three different

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3MnOx/CeOy catalysts prepared by simple physical mixing, co-precipitation, and hydrolysis driving redox co-precipitation. According to the literature, MnO2 was prepared by potassium

na

permanganate (KMnO4) and hydrogen peroxide (H2O2) and CeO2 was prepared by the addition

ur

of a NH3H2O solution to a cerium (III) nitrate hexahydrate (Ce(NO3) 36H2O) solution. Coprecipitated 3MnOx-CeOy (Cop-3Mn1Ce) was prepared by the co-precipitation of a mixture of

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manganese (II) nitrate (Mn(NO3)2) and Ce(NO3) 36H2O by the addition of a dilute NH3H2O solution. For the mixed catalyst, the mixed 3MnOx-CeOy (Mixed-3Mn1Ce) was prepared by mixing MnO2 and CeO2 at a ratio of Mn:Ce = 3:1. The catalyst prepared by hydrolysis-driving co-precipitation, 3MnOx-CeOy (3Mn1Ce) at a molar ratio of Mn:Ce= 3:1, was synthesized by dissolving 38 mmol KMnO4 and 12.7 mmol Ce(NO3) 36H2O in deionized water and adding a 19.5 mmol H2O2 solution. The experimental results of O2-TPD revealed four types of oxygen species in the manganese oxides catalyst, which were physically adsorbed molecular oxygen 30

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(Ophy), chemically bonded oxygen on the surface of catalyst (Osurf), surface lattice oxygen bonded on the surface (Olatt), and lattice oxygen bonded chemically to the inside surface lattice oxygen (O’latt) (Liotta et al., 2008). According to the preparation methods, the mixing patterns of Mn and Ce of Mn/Ce metal oxides were significantly different and the catalytic activity for the removal of toluene showed a difference. Among the catalysts synthesized, 3Mn1Ce showed the

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highest activity for the conversion of toluene and the CO2 conversion yield. The temperature for

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the 90% conversion of toluene, T90%, over 3Mn1Ce was 239 ℃, whereas T90% of toluene over

-p

MnO2, CeO2, Mixed-3Mn1Ce, and Cop-3Mn1Ce were 267 ℃, 280 ℃, 256 ℃, and 256 ℃, respectively. They attributed the improved efficiency of 3Mn1Ce to the evenly well-mixed Ce

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and Mn structures. In addition, based on the experimental results, the amount of Olatt desorption

lP

on 3Mn1Ce was highest, 242 μmol/g, compared to the values of MnO2, CeO2, and Cop-3Mn1Ce, which were 188, 75, and 134 μmol/g, respectively (Chen et al., 2018). Among the molecular

na

dispersion structure of Mn/Ce catalysts, only 3Mn1Ce was formed without the aggregation of Ce.

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In addition, the oxidative removal of a few aromatic compounds was performed using the

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3Mn1Ce catalyst, and T90% values 268 ℃, 210 ℃, and 355 ℃ were obtained for benzene, oxylene, and chlorobenzene, respectively.

5.3. Influence of the support As for support materials, TiO2 is used most widely as a noble metal support for the removal of VOCs. Pt/TiO2, Pd/TiO2, and Rh/TiO2 were prepared by liquid phase reduction deposition and by incipient wetness impregnation for the abatement of ethanol, CO, and toluene (Liotta, 2010). The 31

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catalytic activities differed considerably according to the preparation method. Pt/TiO2 showed better catalytic efficiency for the degradation of CO, ethanol, and toluene than Pd/TiO 2 or Rh/TiO2 (Santos et al., 2010). Among the various kinds of transition metal oxides employed for the oxidation of VOCs, manganese oxides showed high catalytic efficiency for the degradation of toluene at relatively

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low temperatures (Alvim-Ferraz and Gaspar, 2005; Lahousse et al., 1998; Li et al., 2011a; Li et

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al., 2011b; Saqer et al., 2011). The catalytic activity of manganese oxides showed a significant

-p

difference according to the zeolites as a support.

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To examine the efficiency of various zeolites, the oxidation of toluene was carried out using the plasma catalytic process equipped with several zeolites, MCM-41, ZSM-5, and 13X, and the

lP

conversion of toluene was compared (Yao et al, 2018). Among the several zeolites, they reported

na

that the MnOx/MCM-41 showed higher efficiency than MnOx/ZSM-5 or MnOx/13X because of the interaction of MnOx. They suggested that the zeolite appeared to have a large effect on the

ur

catalytic ability (Yao et al, 2018). They also explained that the morphology, surface area, and

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porous size of the zeolite were related to the oxidation state of Mn. They obtained 99.4% and 73% toluene and CO2 conversion, respectively, by the plasma-combined manganese oxide with the MCM-41 support. They employed a dielectric barrier discharge (DBD) reactor with zeolite supported MnOx for toluene degradation. Both Mn2+ and Mn4+ were observed in MnOx/ZSM-5, MnOx/13X, and MCM-41, but the Mn3+ state species were observed only in MnOx/MCM-41. The Mn oxidation states must be related to the specific morphologies of zeolites for toluene degradation. They suggested that the plasma-catalyst with a hybrid zeolite-support system was 32

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quite effective in the removal of VOCs.

5.4. Effect of the amount of metal loading Rezaei et al. examined the effects of the Mn loading on an alumina support below 100℃

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(Rezaei et al., 2013). The effects of the Mn loading on the alumina supported Pt catalyst were also found to depend on the Mn loading; the catalytic activity for the conversion of toluene was

ro

different. They performed the catalytic degradation of toluene by MnOx/Pt/Al2O3 and examined

-p

the dependency on the Mn concentration in the MnOx/Pt/Al2O3 catalyst for the decomposition of

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toluene in the presence of ozone. According to their results, at Mn loadings less than 10%,

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Mn2O3 was the major species in MnOx, whereas MnO2 was observed to increase with increasing Mn loading, and Mn2O3 and MnO2 coexisted in MnOx. In addition, they found that a lower

na

oxidation state of Mn showed higher catalytic efficiency for the removal of toluene in the

ur

presence of ozone. Compared to the decomposition of benzene in the presence of ozone, Einaga et al. examined effect of the Mn loading on γ-alumina for the removal of benzene in the presence

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of ozone at room temperature. Based on their results, a lower content (1~7.5%) of Mn catalyst resulted in higher conversion of benzene rather than a higher content of Mn catalyst. On the other hand, in the conversion of acetone on the silica supported Mn catalyst (Reed et al., 2006), a higher concentration of Mn catalyst resulted in higher catalytic activity. They proposed that the rate of acetone conversion was controlled by ozonation by Mn and the conversion of acetone would proceed by active oxygen species generated after ozone decomposition through interactions with Mn rather than the decomposition of acetone by the Mn catalyst. 33

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5.5. Impacts of CO and oxygen concentration on the activity of catalysts The Pt catalytic activity can be reduced in the presence of CO. The oxidation of benzene, toluene, and hexene over the Pt/alumina catalyst was performed and 100% conversion was obtained at temperatures < 200℃ in the absence of CO (Liotta, 2010). In contrast, in the

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presence of CO, complete conversion of benzene, toluene, and hexene occurred over Pt/alumina

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at temperatures > 300℃. In addition, they examined the effects on the oxygen concentration on

-p

the Pt catalytic activity and reported that the oxidation of VOCs adjusted to the concentration of

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the oxygen (0.6%) environment was superior to the conversion at the low oxygen concentrations

lP

(0.1%) (Liotta, 2010; Patterson et al., 2000). The complete oxidation of benzene, toluene, and hexene in a 0.1% oxygen environment was obtained when the temperatures were increased by

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40~50℃ compared to the conversion in a 0.6% oxygen environment.

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5.6. Deactivation of MnOx

At low temperatures, the conversion of VOCs in the presence of ozone resulted in catalyst deactivation and incompletely oxidized intermediates, including peroxides and active oxygen species generated from the decomposition of ozone by metal oxides. Feeding water vapor can prevent catalyst deactivation by incompletely oxidized carbonaceous intermediates at low temperatures (Einaga and Futamura, 2004; Einaga and Futamura, 2005; Einaga and Futamura, 2006; Eingaga et al., 2011; Rezaei and Soltan, 2012; Zhao et al., 2012). Among various metal 34

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oxides to be utilized for the degradation of VOCs, manganese oxides are more active at low temperature and more effective for the ozone removal due to multivalent manganese oxidation states. Zhao et al. obtained the complete conversion of formaldehyde by the addition of water vapor over the MnOx catalyst at room temperature (Zhao et al., 2012). In the decomposition of benzene, water vapor suppressed the deactivation of MnOx/γ-alumina catalyst and MnOx/Y at

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low temperatures (Einaga and Futamura, 2006; Einaga et al., 2011).

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5.7. Synergistic effects in VOCs mixtures

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The physical and chemical properties of the support, such as hydrophobicity, acidity, pore size

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or volume, surface area, and preparation method, affect the catalytic activity remarkably. Aghbolaghy et al. (Aghbolaghy et al., 2018) performed the catalytic degradation of toluene,

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acetone, and a mixture of the two compounds in the presence of ozone to examine the mixture

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effects of VOCs on the MnOx/γ-Al2O3 catalyst at 25℃, 60℃, and 90℃, respectively. The

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activation energies for the catalytic degradation of toluene, acetone, and mixture of toluene and acetone in the presence of ozone were also estimated. The activation energy, ∆Eact, of toluene and acetone was estimated to be 31 kJ/mol and 40 kJ/mol, respectively. The catalysis of a mixture of toluene and acetone, toluene degradation by MnOx/γ-Al2O3 was superior to acetone. On the other hand, the rate of acetone decomposition in a toluene-coexisting mixture was inferior to that of pure acetone because of the lower activation energy for toluene decomposition.

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6. Perspectives and Conclusion The use of only one functional treatment may not be effective for the complete removal of low concentrations of VOCs and their mixtures from large volumes. Therefore, hybrid processes are required for the abatement of VOCs in a real field. In general, most air pollutants exhausted from various sources are organic or inorganic chemicals with relatively low molecular weights, but

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their chemical activity and characteristics are dependent on the molecular structure or electron

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configuration of the compounds. Among several types of hybrid processes, the sequential type of

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plasma and catalyst/adsorbent hybrid system should be a remarkable treatment for the control of VOCs compared to other treatments in terms of energy cost. According to the catalyst and

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adsorbent, the conversion efficiency differs greatly for the same component of VOCs. Among the

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adsorbents, zeolite can be used as a catalyst support. Depending on the composition, pore size,

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and morphology, the ability of a zeolite to adsorb VOCs or charged species shows great difference. Although the cationic metal loading catalyst/adsorbent tends to decrease the surface

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area of the catalyst compared to bare zeolites, if there is an available orbital in the loaded metal,

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the adsorption of VOCs on the catalyst/adsorbent can be elongated, resulting in higher removal of VOCs by plasma discharge. In addition, the synergetic effects of a mixture of VOCs and secondary pollutants by the oxidative removal of VOCs using plasma or catalysts should be taken into account for the cleaner and more eco-friendly treatment of VOCs. The complete conversion of acetone, cyclohexane, benzene, and toluene was achieved in the presence of ozone over manganese oxides at low temperatures, < 100℃. One of the great benefits of a manganese catalyst is its ability to decompose ozone at room temperature. On the 36

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other hand, deactivation of the catalyst surface by the adsorption of incomplete oxidized intermediates is a serious concern. Water vapor suppressed the deactivation by peroxides and oxygen species. Manganese oxides are used widely in the control of VOCs because of their ability to decompose ozone. The catalytic activity increases with decreasing oxidation state of Mn. Based on the results of the published literature, among the transition metal oxides, MnO2

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showed the highest activity in the decomposition of ozone to active oxygen species. Consequently, a lower Mn loading results in higher dispersion and a majority of lower oxidation

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states of Mn, leading to the faster formation of active oxygen sites on the catalyst surface. This

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means that improved catalytic activity for the conversion of toluene can be achieved by

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manganese oxide with higher reducibility and higher dispersion in the presence of ozone at low

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temperatures. Considering the components of VOCs and exhaust flow rate, the control system composed of catalyst/adsorbent inside a plasma discharge is expected to be more plausible

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provided that water vapor and by-products can be resolved. Despite the tremendous

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achievements in techniques for the abatement of VOCs, more efforts and powerful treatments

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will be needed to remove air pollution at low temperatures.

Acknowledgement This research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT (2017M1A2A2086839).

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Table 1. Temperatures of 100% conversion of VOCs by the multi-component catalysts Compound Benzene

Catalyst 3Mn1Ce

Conc. 1000 ppm

Chlorobenzene

3Mn1Ce

1000 ppm

355℃

1,2-dichloroethane

Mn/ZrOx

1000 ppm

450℃

(Li et al., 2009)

Ethanol

Mn/CeOx

1600 ppm

170℃

(Delimaris and Ioannides, 2008)

Ethanol

Mn/CuOx

1%

210℃

(Morales et al., 2008)

n-Hexane

MnOx

125 ppm

180℃

O-xylene

3Mn1Ce

1000 ppm

268℃

Trichloroethylene

Mn/ZrOx

1000 ppm

550℃

(Li et al., 2009)

Toluene

Mn/ZrOx

0.35%

260℃

(Li et al., 2004)

Toluene

Mn/CuOx

0.35%

220℃

(Li et al., 2004)

Toluene

3Mn1Ce

1000 ppm

239℃

Toluene

CuO-MnOx

600 ppm

240℃

Toluene

LaCoO3

1000 ppm

References (Chen et al., 2018)

a

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(Chen et al., 2018)

(Li et al., 2009)

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.

T90 (temperature of 90% toluene conversion)

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a

Temperature 210℃ a

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223℃

a

(Chen et al., 2018)

(Chen et al., 2018) (Wei et al., 2019)

a

(Yang et al., 2018b)

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Table 2. Conversion of toluene by metal oxides, noble metal and the multi-component catalysts Conc.

Conversion

References

Au/CeO2

0.7 vol%

100% at 360℃

(Scire et al., 2003)

CeO2

0.7 vol%

50% at 600℃

(Scire et al., 2003)

MnOx-CeO2

600 ppm

90% less than 250℃

(Delimaris and Ioannides, 2008)

MnOx/ZrO2

0.35%

90% less than 250℃

(Li et al., 2004)

Pd/FAU zeolites

1000 ppm

90% at 180℃

(Tidahy et al., 2007)

Pd/ZrO2

1000 ppm

90% at 280℃

(Tidahy et al., 2006)

Pt (0.9 wt%) /Al2O3

1000 ppm

100% at 227℃

Pt (0.56 wt%) /Al2O3

1000 ppm

100% at 250℃

Pt (0.4 wt%) /Al2O3

600 ppm

50% at 180℃

Pt (0.05 wt%) /Al2O3

600 ppm

50% at 263℃

(Tahir and Koh, 1999)

Pt/ZrBiO/γ-Al2O3

900 ppm

90% at 120℃

(Masui et al., 2010)

Pt/HPMOR

1000 ppm

90% at 190℃

(Zhang et al., 2018)

Pt/MCM-41

1000 ppm

90% at 192

(Yan et al., 2009)

Pt/MCM-41

4340-45000 ppm

90% at 150℃

(Tidahy et al., 2008)

Pt/ZSM-5

1000 ppm

90% at 175℃

(Chen et al., 2015)

Pt-Au/ZnO/Al2O3

1.8 mol%

90% less than 200℃

(Kim and Ahn, 2009)

90% at 160

(Liu et al., 2018)

1000 ppm

90% at 179℃

(Zhang et al., 2019)

Pt-Pd/HMS

1000 ppm

90% at 197℃

(Zhang et al., 2019)

Pd-Pt/SiO2

1000 ppm

98% at 160℃

(Wang et al., 2017)

90% at 175

(Fu et al., 2016)

Pt-Pd-HMS

Pt-Pd/MCM-41

950 ppm

500 ppm

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(Benard et al., 2009) (Benard et al., 2009)

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Pt-Pd-Al2O3

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Catalyst

(Tahir and Koh, 1999)

Table 3. Toluene Removal using plasma-adsorption or adsorption-plasma catalyst (APC) hybrid process 49

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Zeolite

Discharge Type

Conc. (ppm)

Removal (%)

References

0.334 a

90

(Lee et al., 2019)

DDBD b

1632 c

100

(Li et al., 2019)

ZSM-5/BaTiO3

PBDBD

1632 c

100

(Qin et al., 2018)

AgMn-ZSM-5 /BaTiO3

PBDBD

1632 c

100

(Qin et al., 2018)

Ag-Mn/HZSM-5

DBD (periodic operation)

3

Honeycomb zeolite

DBD (plasma desorption)

30

Honeycomb zeolite

DBD (plasma desorption)

30

Honeycomb zeolite

DBD

25

Y

DBD with the electrodes

γ-Al2O3(inner tube)//ZSM-5

segmented

of

(outer tube)

(Wang et al., 2015)

83

(Kuroki et al., 2007)

78~82

(Kuroki et al., 2009)

93

(Yamagata et al., 2006)

79.8

(Huang et al., 2015c)

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100

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(APC-closed discharge) PBDBD (continuous)

HZSM-5

PBDBD (continuous)

100

80.9

(Huang et al., 2015c)

Hβ (6.6Å pore size)

PBDBD (continuous)

100

98.4

(Huang et al., 2015c)

H-Y (7.4Å pore size)

PBDBD (continuous)

100

85.2

(Huang et al., 2015c)

Ag/H-Y

PBDBD (continuous)

100

97

(Huang et al., 2015c)

13X

DBD (cyclic and ventilated discharge)

150

95

(Yi et al., 2018)

DBD (open discharge)

150

94

(Yi et al., 2017)

DBD (cyclic)

200

52

(Teramoto et al., 2015)

DBD (cyclic)

200

55

(Teramoto et al., 2015)

DBD (cyclic)

200

50

(Teramoto et al., 2015)

Surface discharge plasma (cyclic)

200

50

(Oh et al., 2006)

(26.6Å pore size) Na-Y d

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MS-13X

100

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5A (5Å pore size)

(7.4Å pore size) H-Y e (7.4Å pore size) H-Y f (7.4Å pore size) NaY

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unit in mmole, b double dielectric barrier discharge, c unit in mg/m3, d SBET (750 m2/g), e SBET (650 m2/g),f SBET (520 m2/g)

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Declaration of interests

☒ 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.

☐The authors declare the following financial interests/personal relationships which may be

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considered as potential competing interests:

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Highlights

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Catalytic degradation of VOCs were introduced.

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New hybrid treatments to remove VOCs using catalysts was addressed.

- The mechanism of the oxidation of VOCs by a catalyst was suggested.

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- Several factors affecting the catalytic activities were discussed.

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