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catalysts Article

Structure-Activity Relationship of Manganese Oxide Catalysts for the Catalytic Oxidation of (chloro)-VOCs Jian Wang 1,† , Hainan Zhao 1,2,† , Jianfei Song 2 , Tingyu Zhu 1,3 and Wenqing Xu 1,3, * 1

2 3

* †

Beijing Engineering Research Center of Process Pollution Control, National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China State Key Laboratory of Heavy Oil Processing, College of Mechanical and Transportation Engineering, China University of Petroleum (Beijing), Beijing 102249, China Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China Correspondence: [email protected]; Tel./Fax.: +86-10-8254-4823 These authors contributed equally to this work and should be considered co-first authors.





Received: 23 July 2019; Accepted: 26 August 2019; Published: 28 August 2019

Abstract: Manganese oxide catalysts, including γ-MnO2 , Mn2 O3 and Mn3 O4 , were synthesized by a precipitation method using different precipitants and calcination temperatures. The catalytic oxidations of benzene and 1,2-dichloroethane (1,2-DCE) were then carried out. The effects of the calcination temperature on the catalyst morphology and activity were investigated. It was found that the specific surface area and reducibility of the catalysts decreased with the increase in the calcination temperature, and both the γ-MnO2 and Mn3 O4 were converted to Mn2 O3 . These catalysts showed good activity and selectivity for the benzene and 1,2-DCE oxidation. The γ-MnO2 exhibited the highest activity, followed by the Mn2 O3 and Mn3 O4 . The high activity could be associated with the large specific surface area, abundant surface oxygen species and excellent low-temperature reducibility. Additionally, the catalysts were inevitably chlorinated during the 1,2-DCE oxidation, and a decrease in the catalytic activity was observed. It suggested that a higher reaction temperature could facilitate the removal of the chlorine species. However, the reduction of the catalytic reaction interface was irreversible. Keywords: benzene; 1,2-Dichloroethane; catalytic oxidation; manganese oxide catalysts

1. Introduction Volatile organic compounds (VOCs) are not only toxic to the human body [1] but also act as the precursors of the PM2.5 and near-surface ozone [2–4], causing severe environmental problems. In recent years, many countries have enacted laws and regulations to limit the emissions of VOCs. Generally, catalytic oxidation methods can achieve a complete decomposition of the VOCs at low temperatures (i.e., 250–500 ◦ C) [5,6], which is suitable for the abatement of the VOCs that have no recycling value, such as waste gases from printing or spraying industries, which mainly consist of the BTX (benzene, toluene and xylene) and oxygenated VOCs (OVOCs), as well as partially chlorinated VOCs (CVOCs) [7,8]. Noble metal catalysts have been successfully utilized for the abatement of the VOCs [9,10]. However, the cost of these catalysts is very high. Recent investigations have shown that the transition metal oxide catalysts (Mn, Co and Cu, etc.) show an activity that is comparable with the noble metal catalysts [11–25]. In particular, these catalysts often show better resistance to the chlorine poisoning [26]. Among these catalysts, the manganese oxide catalysts are highly active [16–25] and have an ability to

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resist the chlorine poisoning in the catalytic oxidation of the CVOCs [27]. Additionally, these catalysts are considered to be environmentally friendly. Manganese has many valence states, including Mn2+ , Mn3+ , Mn4+ , Mn6+ and Mn7+ , allowing the electron transfer to readily occur [28]; thus, the manganese materials are widely used as catalysts [29–31]. Manganese oxides (Mn3 O4 , Mn2 O3 and MnO2 , etc.) are known to exhibit a good activity in the catalytic oxidation of the VOCs [18–22,24,25]. The morphology of the manganese oxide catalysts can vary with different precursors and preparation conditions [23], which clearly affects their catalytic behaviors. It is generally agreed that the specific surface area, the valence states of the surface manganese species, the active oxygen species and the surface defects of the catalyst are the most important factors for the catalytic activity [20,22,25,32]. The reduction of the calcination temperature of the catalyst seems beneficial as well [23]. According to previous reports [16–25,32], most studies focus on approaches to increase the catalytic activity of the manganese oxide catalysts. However, in real applications, controlling a stable reaction temperature may not be very practical because the concentration of the VOCs is continually changing, and the catalytic oxidation process is an exothermic reaction. Therefore, the effects of the thermal stability and crystal structure change on the catalytic activity warrant an in-depth study. Manganese oxide catalysts undergo a catalyst deactivation during the catalytic oxidation of the CVOCs [27,33]. It is thus necessary to evaluate the chlorine resistance of the catalysts, since the CVOCs are prevalent in the practical waste gases [7,8]. Notably, previous studies often studied only a few catalyst samples, so the results of the structure-activity relationship were not comprehensive enough [21,24], and the results of different studies are often inconsistent. In addition, only a few reports include the simultaneous study of the CVOCs. Consequently, lack of a clear understanding regarding these issues has obviously hindered further improvements in the manganese oxide catalysts and their applications. In this study, we synthesized a series of manganese oxide catalysts by the traditional precipitation method, and typical manganese oxide structures, such as MnO2 , Mn2 O3 and Mn3 O4 , were obtained by changing the precipitants and calcination temperatures. In addition to benzene, 1,2-dichloroethane (1,2-DCE) was also used as a model reactant to investigate the chlorine resistance of the catalysts. The manganese catalysts exhibited a good activity in the benzene and 1,2-DCE oxidation. The morphology of the catalysts was characterized thoroughly by X-ray diffraction (XRD), N2 adsorption/desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), H2 -temperature programmed reduction (H2 -TPR) and in-situ Fourier transform infrared spectroscopy (FTIR). Various properties of the catalysts, including the crystal structure, were found to be related to the catalytic activity, and thus the structure-activity relationship was established. 2. Results and Discussion Figure 1 shows the XRD patterns of the manganese oxide catalysts (Figure 1a–c), and the standard XRD patterns including the γ-MnO2 (JCPDS PDF 30-0820), Mn2 O3 (JCPDS PDF 78-0390), Mn3 O4 (JCPDS PDF 24-0734), MnCO3 (JCPDS PDF 44-1472) and MnC2 O4 (JCPDS PDF 01-0160) (Figure 1d). As shown in Figure 1a, the MnCO3 precipitate retains its original crystal structure after drying at 100 ◦ C. With the increase in the calcination temperature, MnCO3 is first converted to the γ-MnO2 and then converted to a pure Mn2 O3 . As shown in Figure 1b, the crystal structure of the MnC2 O4 precipitate also remains unchanged after drying. With the increase in the calcination temperature, MnC2 O4 is first converted into a mixture of the Mn3 O4 and Mn2 O3 (mainly Mn2 O3 ), and then Mn2 O3 becomes the sole crystal phase. As shown in Figure 1c, the Mn(OH)2 precipitate has already been converted into Mn3 O4 through the dehydration process. With the increase in the calcination temperature, Mn3 O4 is also converted into Mn2 O3 . These results show that, although the structures of the manganese oxide catalysts are quite different as a result of different precipitated precursors at low calcination temperatures, both the γ-MnO2 and Mn3 O4 are converted into Mn2 O3 with the increase in the calcination temperature, suggesting that Mn2 O3 has the best thermal stability. However, a peak for

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2.MnCO Results discussion observed in the XRD patterns of the MnCO3 -350 and MnCO3 -425 catalysts, indicating that 3 isand the catalysts contain the MnCO3 species. (b) MnCO3-575

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Figure 1. XRD patterns of (a) MnCO (b) MnC2 O4 (c) Mn(OH)2 calcined at different temperatures, Figure 1. XRD patterns of (a) MnCO33(b) MnC2O 4 (c) Mn(OH)2 calcined at different temperatures, and (d) standard XRD patterns of γ-MnO , Mn O3 , Mn3 O4 and MnCO3 . and (d) standard XRD patterns of γ-MnO2, 2Mn2O23, Mn 3O4 and MnCO3.

The catalytic evaluation of the manganese oxide catalysts for the benzene oxidation were performed, Figure 1 shows the XRD patterns of the manganese oxide catalysts (Figure 1a–c), and the and the results are shown in Figure 2. In addition, the 50% conversion temperature (T50 ) and 90% standard XRD patterns including the γ-MnO2 (JCPDS PDF 30-0820), Mn2O3 (JCPDS PDF 78-0390), conversion temperature (T90 ) of these catalysts for benzene are summarized in Table 1. Mn3O4 (JCPDS PDF 24-0734), MnCO3 (JCPDS PDF 44-1472) and MnC2O4 (JCPDS PDF 01-0160) (Figure 1d). As shown in1.Figure 1a,evaluation the MnCO 3 precipitate retains its original crystal structure after Table Catalytic results for the manganese oxide catalysts. drying at 100 °C. With the increase in the calcination temperature, MnCO3 is first converted to the Benzene γ-MnO2 and then converted to a pure Mn2O3. As shown in Figure 1b, the crystal structure of the Catalyst ◦ MnC2O4 precipitate also remains unchanged after drying. With T50 ( C) T90 (◦ C) the increase in the calcination temperature, MnC2O4 is first converted a mixture 3O4 and Mn2O3 (mainly Mn2O3), and MnCOinto 182of the Mn 202 3 -350 then Mn2O3 becomes the sole crystal phase. As shown in Figure 1c, the Mn(OH)2 precipitate has MnCO -425 191 218 3 MnCO -500 205 230 already been converted into Mn3O4 through the dehydration process. With the increase in the 3 213Mn2O3. These 241 3 -575 calcination temperature, Mn3O4 isMnCO also converted into results show that, although the MnC2 O4 -350 195 223 structures of the manganese oxide catalysts are quite different as a result of different precipitated MnC2 O4 -425 202 229 precursors at low calcination temperatures, both the γ-MnO2 240 and Mn3O4 are converted into Mn2O3 MnC2 O4 -500 213 with the increase in the calcination suggesting MnC2temperature, O4 -575 223 252that Mn2O3 has the best thermal Mn(OH) 225in the XRD 252 patterns of the MnCO3-350 and stability. However, a peak for MnCO 3 is observed 2 -350 Mn(OH) -425 225 252 2 MnCO3-425 catalysts, indicating that the catalysts contain the MnCO 3 species. Mn(OH)2 -500 Mn(OH)2 -575

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Figure 2. Catalytic evaluation of manganese oxide catalysts obtained by calcining (a) MnCO , Figure 2. Catalytic evaluation of manganese oxide catalysts obtained by calcining (a) MnCO3, (b)3 (b) MnC2 O4 and (c) Mn(OH)2 for the benzene oxidation. (d) Benzene conversion and CO2 selectivity of MnC2O4 and (c) Mn(OH)2 for the benzene oxidation. (d) Benzene conversion and CO2 selectivity of the MnCO -425, MnC O -500 and Mn(OH) -425 catalysts as a function of the temperature. (1000 ppm a function of the temperature. (1000 the MnCO33-425, MnC22O44-500 and Mn(OH)22-425 catalysts as −1 benzene/20 vol.% O2 /N2 ; 200 mg catalyst; WHSV 30,000 mL·g ·h−1−1 ).−1 ppm benzene/20 vol.% O2/N2; 200 mg catalyst; WHSV 30000 mL·g ·h ).

As shown in Figure 2a–b, the T50 and T90 for the MnCO3 -350 (182 ◦ C and 202 ◦ C) and MnCO3 -425 The catalytic evaluation of the manganese oxide catalysts for the benzene oxidation were ◦ (191 C and 218 ◦ C) are lower than that for the MnC2 O4 -425 (202 ◦ C and 229 ◦ C), which shows the best performed, and the results are shown in Figure 2. In addition, the 50% conversion temperature (T50) activity among the manganese oxide catalysts with the crystal structure of the Mn2 O3 (the MnC2 O4 -350 and 90% conversion temperature (T90) of these catalysts for benzene are summarized in Table 1. catalyst is a mixture of the Mn3 O4 and Mn2 O3 ). Therefore, it is clear that the γ-MnO2 is more active As shown in Figure 2a–b, the T50 and T90 for the MnCO3-350 (182 °C and 202 °C) and than the Mn2 O3 in benzene oxidation. As shown in Figure 2c, the activity of the Mn(OH)2 -500 catalyst MnCO3-425 (191 °C and 218 °C) are lower than that for the MnC2O4-425 (202 °C and 229 °C), which with the crystal structure of Mn2 O3 is better than that of the Mn(OH)2 -425 catalyst with the crystal shows the best activity among the manganese oxide catalysts with the crystal structure of the structure of Mn3 O4 , suggesting that the transformation of the Mn3 O4 to the Mn2 O3 is beneficial to the Mn2O3 (the MnC2O4-350 catalyst is a mixture of the Mn3O4 and Mn2O3). Therefore, it is clear that the catalytic activity. Thus, the γ-MnO2 has the best activity in the benzene oxidation, followed by the γ-MnO2 is more active than the Mn2O3 in benzene oxidation. As shown in Figure 2c, the activity of Mn2 O3 and Mn3 O4 . Furthermore, the activity of the manganese oxide catalysts with the same crystal the Mn(OH)2-500 catalyst with the crystal structure of Mn2O3 is better than that of the Mn(OH)2-425 structures decrease significantly with the increase in the calcination temperature. Overall, the activity catalyst with the crystal structure of Mn3O4, suggesting that the transformation of the Mn3O4 to the of the manganese oxide catalysts for the benzene oxidation is very high, and benzene can not only be Mn2O3 is beneficial to the catalytic activity. Thus, the γ-MnO2 has the best activity in the benzene completely converted at 200 ◦ C by the γ-MnO2 catalyst but also can be completely oxidized below oxidation, followed by the Mn 2 O 3 and Mn3O4. Furthermore, the activity of the manganese oxide 275 ◦ C by the Mn3 O4 catalyst. Figure 2d shows the benzene conversion and the CO2 selectivity of the catalysts with the same crystal structures decrease significantly with the increase in the calcination MnCO3 -425, MnC2 O4 -500 and Mn(OH)2 -425 catalysts as a function of the temperature. As shown, temperature. Overall, the activity of the manganese oxide catalysts for the benzene oxidation is very CO2 is the only oxidation product. high, and benzene can not only be completely converted at 200 °C by the γ-MnO2 catalyst but also The porous structures of the manganese oxide catalysts are shown in Table 2. Overall, it was can be completely oxidized below 275 °C by the Mn3O4 catalyst. Figure 2d shows the benzene found that both the specific area and the pore volume of the catalysts decrease with the increase in the conversion and the CO2 selectivity of the MnCO3-425, MnC2O4-500 and Mn(OH)2-425 catalysts as a calcination temperature, and the pore size of the catalyst changes inversely. For instance, the specific function of the temperature. As shown, CO2 is the only oxidation product. surface areas of the MnCO3 -425 and MnCO3 -500 catalysts are 105.8 m2 /g and 45.1 m2 /g, respectively, and the pore sizes of these two catalysts are 6.5 nm and 17.5 nm, respectively. This is a typical sintering phenomenon, suggesting that the manganese oxide crystalline grains aggregate during the calcination process. In addition, the specific surface areas of the catalysts with the crystal structures of the Mn2 O3

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and Mn3 O4 are similar, but the specific surface areas of the MnCO3 -350 and MnCO3 -425 catalysts with the crystal structure of the γ-MnO2 are very large. The above results indicate that the precipitated precursor has a strong influence on the morphology of the catalyst. Moreover, almost no changes can be observed in the microstructure of the catalysts after the benzene oxidation. Table 2. Characterization data for the manganese oxide catalysts. Catalyst MnCO3 -350 MnCO3 -425 MnCO3 -425 (B) a MnCO3 -425 (DCE) b MnCO3 -500 MnCO3 -575 MnC2 O4 -350 MnC2 O4 -425 MnC2 O4 -500 MnC2 O4 -500 (B) a MnC2 O4 -500 (DCE) b MnC2 O4 -575 Mn(OH)2 -350 Mn(OH)2 -425 Mn(OH)2 -425 (B) a Mn(OH)2 -425 (DCE) b Mn(OH)2 -500 Mn(OH)2 -575

Specific Surface Area (m2 /g)

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Pore Volume (cm3 /g)

153.2 105.8 104.1 37.6 45.1 43.9 58.6 47.7 33.8 33.3 24.2 25.2 30.3 30.1 37.1 17.2 27.0 21.6

5.6 6.5 6.6 17.5 17.5 17.4 9.6 9.6 17.7 17.4 31.1 31.5 31.0 30.5 30.9 46.4 31.2 46.3

0.29 0.24 0.25 0.22 0.24 0.29 0.24 0.24 0.22 0.20 0.32 0.19 0.23 0.21 0.23 0.24 0.36 0.85

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After evaluation as a catalyst for the catalytic oxidation of benzene. b After evaluation as a catalyst for the catalytic oxidation of 1,2-DCE. c O− , O2 − and O2 2− species, and OH groups. d Lattice oxygen species (O2− ).

The MnCO3 -425, MnC2 O4 -500 and Mn(OH)2 -425 catalysts were characterized by SEM and TEM, and Figure 3 shows the representative micrographs. As shown in Figure 3a, the MnCO3 -425 catalyst consists of spherical particles approximately 0.3–1 µm in diameter. The manganese oxide crystalline grain size is approximately 5–7 nm (Figure 3b), and the lattice spacing is 0.21 nm, corresponding to the (101) crystal plane of the γ-MnO2 (Figure 3c). As shown in Figure 3d, the MnC2 O4 -500 catalyst consists of spherical particles similar to those observed in the MnCO3 -425 catalyst. The manganese oxide crystalline grain size, which is much larger, is approximately 20–50 nm (Figure 3e), and the lattice spacing is 0.27 nm, corresponding to the (222) crystal plane of the Mn2 O3 (Figure 3f). As shown in Figure 3g, the Mn(OH)2 -425 catalyst consists of large cubic particles approximately 10–20 µm in diameter. The manganese oxide crystalline grain size is slightly smaller than that of the MnC2 O4 -500 catalyst (Figure 3h), and the lattice spacing is 0.25 nm, corresponding to the (211) crystal plane of the Mn3 O4 (Figure 3i).

typical sintering phenomenon, suggesting that the manganese oxide crystalline grains aggregate during the calcination process. In addition, the specific surface areas of the catalysts with the crystal structures of the Mn2O3 and Mn3O4 are similar, but the specific surface areas of the MnCO3-350 and MnCO3-425 catalysts with the crystal structure of the γ-MnO2 are very large. The above results indicate2019, that9, 726 the precipitated precursor has a strong influence on the morphology of the catalyst. Catalysts 6 of 17 Moreover, almost no changes can be observed in the microstructure of the catalysts after the benzene oxidation.

Figure 3. micrographs of the (a–c)(a–c) MnCO (d–f)(d–f) MnCMnC and (g–i) 3. Representative RepresentativeSEM, SEM,TEM TEM micrographs of the MnCO 3–425, 2O4-500 and 3 –425, 2 O4 -500 Mn(OH) The The highhigh resolution TEMTEM micrograph shows the yellow dotted areaarea in the (g–i) Mn(OH) -425 catalysts. resolution micrograph shows the yellow dotted in 2 -4252catalysts. corresponding TEMTEM micrograph of the the corresponding micrograph of catalyst. the catalyst.

The analyses conducted to investigate the reducibility of the manganese 2 -TPR3-425, The H MnCO MnCwere 2O4-500 and Mn(OH) 2-425 catalysts were characterized by SEMoxide and catalysts, and the results are shown in Figure 4. The reduction of the manganese oxides can described TEM, and Figure 3 shows the representative micrographs. As shown in Figure 3a, the be MnCO 3-425 as follows: MnO2of →spherical Mn2 O3 →particles Mn3 O4 → MnO [34]. As0.3–1 shown in in Figure 4, eachThe H2 -TPR profile of the catalyst consists approximately μm diameter. manganese oxide manganese oxide catalyst shows two or three main peaks that can be attributed to the reduction of the crystalline grain size is approximately 5–7 nm (Figure 3b), and the lattice spacing is 0.21 nm, manganese species in different valence states. In γ-MnO general, the peaks theshown temperature range corresponding to the (101) crystal plane of the 2 (Figure 3c).inAs in Figure 3d,from the ◦ ◦ 270 370 catalyst C can be assigned the reduction of MnO Mn2 O3 in to Mn the 2 totoMn 2 O3 or 4 , and MnCC 2Oto 4-500 consists of to spherical particles similar those observed the3 O MnCO 3-425 ◦ can be assigned to the reduction of Mn O to MnO. peak in the temperature 370 ◦ C to 460 3 4 catalyst. The manganeserange oxideofcrystalline grainCsize, which is much larger, is approximately 20–50 Specifically, andspacing MnCO3 -425 catalysts with the crystal the γ-MnO show 3 -350 nm (Figure the 3e), MnCO and the lattice is 0.27 nm, corresponding to structure the (222) of crystal plane2of the strong reduction peaks at approximately 280 ◦ C and 310 ◦ C, which can be attributed to the Mn4+ and Mn3+ reduction, respectively, and the peaks at approximately 420 ◦ C can be attributed to the reduction of Mn3 O4 to MnO (Figure 4a). For the Mn(OH)2 -350 and Mn(OH)2 -425 catalysts with the crystal structure of the Mn3 O4 , the characteristic peaks at 450 ◦ C can be assigned to the reduction of Mn3 O4 to MnO, but the peaks in the temperature range from 270 ◦ C to 370 ◦ C are quite weak, indicating that the reducibility of the Mn3 O4 is poor due to the lack of the high valance manganese species (Figure 4c). The reduction profiles of the other manganese oxide catalysts with the crystal

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Catalysts 2019, 9, x FOR PEER REVIEW 7 of 17 structure of the Mn2 O3 are similar, showing two strong peaks assigned to the reduction of the Mn 2 O3 ◦ ◦ and Mn3 O4 at approximately 340 C and 450 C, respectively.

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In addition to the abovementioned peaks, characteristic peaks in the temperature range from

2-TPR analyses were conducted to investigate the reducibility the manganese oxide ◦The ◦ C can C to H 270 be assigned to the reactive oxygen species (mainly theofsurface oxygen species

210 catalysts, the results shown in Figure 4. The reduction ofpeak the manganese can be and highlyand reactive latticeare oxygen species). It can be seen that the intensity ofoxides these species described as follows: MnO 2 → Mn2O3 → Mn3O4 → MnO [34]. As shown in Figure 4, each H2-TPR decreases with the increase in the calcination temperature for all the catalysts, and these peaks are profileobservable of the manganese oxide catalyst showsattwo or three main peaks that be attributed to the ◦ C. The barely when the catalyst is calcined a temperature exceeding 500 can main reduction reduction of the manganese species in different valence states. In general, the peaks in the peaks of the manganese species also shift to higher temperatures. For instance, the main reduction temperature range from 270 °C to 370 °C can be assigned to the reduction of MnO 2 to Mn 2 O 3 or peaks at 312 ◦ C and 417 ◦ C for the MnCO3 -350 catalyst gradually shift to 350 ◦ C and 441 ◦ C for the Mn2O3 to Mn3O4, and the peak in the temperature range of 370 °C to 460 °C can be assigned to the MnCO 3 -575 catalyst. It should also be noted that the initial reduction temperature has increased as reduction of above Mn3O4phenomena to MnO. Specifically, thethe MnCO 3-350 and MnCO3-425 catalysts with the crystal well. All the indicate that crystal structures of the manganese oxides gradually structure of the γ-MnO 2 show strong reduction peaks at approximately 280 °C and 310 °C, which become more intact, and the oxygen mobility and thus the reducibility of the catalyst decrease with the 4+ and Mn3+ reduction, respectively, and the peaks at approximately 420 can be attributed to the Mntemperature. increase in the calcination Similar phenomena are observed in many transition metal °C can be attributed to the reduction of Mn3O4 to MnO (Figure 4a). For the Mn(OH)2-350 and oxide catalysts [15,35], in which these processes decrease the availability of the reactive oxygen species Mn(OH)2-425 catalysts with the crystal structure of the Mn3O4, the characteristic peaks at 450 °C can in the oxidation reactions. It is well known that the Mars-van-Krevelen mechanism is operational be assigned to the reduction of Mn3O4 to MnO, but the peaks in the temperature range from 270 °C in the transition metal oxide catalysts during the oxidation reactions, which involves the process of to 370 °C are quite weak, indicating that the reducibility of the Mn3O4 is poor due to the lack of the releasing and replenishing lattice oxygen. Therefore, the reducibility of the catalyst will be closely high valance manganese species (Figure 4c). The reduction profiles of the other manganese oxide related to the catalytic activity. Overall, the reducibility of the manganese oxides with different crystal catalysts with the crystal structure of the Mn2O3 are similar, showing two strong peaks assigned to structures is γ-MnO2 > Mn2 O3 > Mn3 O4 , which shows a strong correlation with the main valence the reduction of the Mn2O3 and Mn3O4 at approximately 340 °C and 450 °C, respectively. states of the manganese oxides. In addition to the abovementioned peaks, characteristic peaks in the temperature range from The surface manganese species and oxygen species of the manganese oxide catalysts were 210 °C to 270 °C can be assigned to the reactive oxygen species (mainly the surface oxygen species investigated by XPS. Figure 5 shows the XPS spectra of the Mn 2p for these catalysts. The determination and highly reactive lattice oxygen species). It can be seen that the peak intensity of these species of the different oxidation states of manganese by XPS is not trivial because of the relative importance decreases with the increase in the calcination temperature for all the catalysts, and these peaks are of the intra and inter-atomic effects [36], so Mn 2p has not been treated as a peak separation. However, barely observable when the catalyst is calcined at a temperature exceeding 500 °C. The main reduction peaks of the manganese species also shift to higher temperatures. For instance, the main reduction peaks at 312 °C and 417 °C for the MnCO3-350 catalyst gradually shift to 350 °C and 441 °C for the MnCO3-575 catalyst. It should also be noted that the initial reduction temperature has

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it can be seen that the peak shifts to the low binding energy region with the increase in the calcination temperature, indicating that the amount of the high valent manganese species decreases. Figure 6 Catalysts 2019, 9, x FOR PEER REVIEW 8 of 17 shows the XPS spectra of the O 1s for these catalysts, and the relative content of the different surface species was and in Table 2. As presented in Figureexceeding 6, the peak500 of O°C. 1s The can also be barely observable whenREVIEW theshown catalyst is calcined at a temperature Catalysts 2019, 9, xcalculated FOR PEER 8main of 17 divided into three peaks [23,25]. The peak at 529.4 ± 0.2 eV can be assigned to lattice oxygen species reduction peaks of the manganese species also shift to higher temperatures. For instance, the main (O2− ) in aas fully coordinated [23,25]. The peak at 531.3 ± structures 0.2 eVshift can be attributed the increased well. phenomena indicate that the crystal of reduction peaks at All 312the °C above andenvironment 417 °C for the MnCO 3-350 catalyst gradually to the 350 manganese °C andto441 − , O − and O 2− ) in the vicinity of the surface defects [37], and the peak at surface oxygen species (O oxides gradually more2 It intact, and thebeoxygen and thus the reducibility of has the 2 also °C for the MnCO3become -575 catalyst. should noted mobility that the initial reduction temperature 533.4 ±decrease 0.2 can be the described asinthe groups [23,25], together known as theofadsorbed oxygen catalyst with increase theOH calcination temperature. Similar phenomena observed increased as eV well. All above phenomena indicate that the crystal structures theare manganese species. From the calculated in Table 2, can be concluded thatthus the catalyst at high in manygradually transition metal oxide catalysts [15,35], which these processes decrease thecalcined availability of oxides become moredata intact, and theitinoxygen mobility and the reducibility of the temperatures has a higher ratio of the lattice oxygen species, and a lower ration of the surface oxygen the reactive oxygen species in the oxidation reactions.temperature. It is well known that the Mars-van-Krevelen catalyst decrease with the increase in the calcination Similar phenomena are observed indicating that the structure of in thewhich catalyst gradually becomes more intact, and the mechanism is operational incrystal the transition metal oxide catalysts during the oxidation reactions, inspecies, many transition metal oxide catalysts [15,35], these processes decrease the availability of surface defects are reduced. Therefore, increasing the calcination temperature causes a reduction in the which involves the process of releasing and replenishing lattice oxygen. Therefore, the reducibility the reactive oxygen species in the oxidation reactions. It is well known that the Mars-van-Krevelen catalyst specific surface area, leading to a decrease in the catalytic reaction interface. It has been widely of the catalyst will be closely to the catalytic the oxidation reducibility of the mechanism is operational in therelated transition metal oxide activity. catalysts Overall, during the reactions, reported that the defect sites exhibit areplenishing high oxidation activity [38–40] because they areshows prone ato manganese oxides with different crystal structures is γ-MnO 2 > Mn 2O3 Therefore, > Mn 3O4, which which involves thesurface process of releasing and lattice oxygen. the reducibility active will oxygen species, as the species and highly lattice of oxygen strong with main such valence states theoxygen manganese oxides. ofproduce the correlation catalyst bethe closely related to surface theofcatalytic activity. Overall, thereactive reducibility the species. In oxides addition, these speciescrystal are easily involved the catalytic reaction. Thus, increasing manganese with different structures is in γ-MnO 2 > Mn2O 3 > Mn3O 4, which showsthe a calcination temperature will have adverse effects on the catalyst activity. strong correlation with the main valence states of the manganese oxides.

Figure 5. XPS spectra of Mn 2p for the manganese oxide catalysts obtained by calcining (a) MnCO3, andspectra (c) Mn(OH) 2. 2p for the manganese oxide catalysts obtained by calcining (a) MnCO , (b) MnC25.O4XPS Figure of Mn 3 Figure 5. XPS spectra of Mn 2p for the manganese oxide catalysts obtained by calcining (a) MnCO3, (b) MnC2 O4 and (c) Mn(OH)2 . (b) MnC2O4 and (c) Mn(OH)2.

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Figure 6. XPSenergy spectra oxide calcining (a) MnCO3 , Binding (eV)of O 1s for the manganese Binding energy (eV)catalysts obtained by Binding energy (eV) Figure 6. XPS spectra of O 1s for the manganese oxide catalysts obtained by calcining (a) MnCO3, (b) (b) MnC2 O4 and (c) Mn(OH)2 . MnC2O4 and (c) Mn(OH)2. Figure 6. XPS spectra of O 1s for the manganese oxide catalysts obtained by calcining (a) MnCO3, (b) Figure 7 summarizes the catalytic properties for the manganese oxide catalysts. The results show MnC2surface O4 and (c) Mn(OH)2. species and oxygen species of the manganese oxide catalysts were The manganese

a complex structure-activity relationship of the manganese oxidation catalysts. The high activity can be investigated Figure 5surface showsarea theand XPS spectra of the Mn oxygen 2p for species. these catalysts. The attributed toby theXPS. large specificspecies more abundant surface These catalyst The surface manganese and oxygen species of the manganese oxide catalysts were determination of the different states and of manganese by XPS iscatalyst, not trivial because of the characteristics related to the5oxidation oxygen the reducibility the2p which is significantly investigated byareXPS. Figure showsmobility the XPS spectra of the of Mn for these catalysts. The relative importance of the intra and inter-atomic effects [36], so Mn 2p has not been treated as a influenced by of thethe manganese crystal structure and the calcination determination different oxide oxidation states of manganese by XPS istemperature. not trivial because of the peak separation. However, it can be seen that the peak shifts to the low binding energy region with relative importance of the intra and inter-atomic effects [36], so Mn 2p has not been treated as a the increase in the calcination temperature, indicating that the amount of the high valent peak separation. However, it can be seen that the peak shifts to the low binding energy region with manganese species decreases. Figure 6 shows the XPS spectra of the O 1s for these catalysts, and the the increase in the calcination temperature, indicating that the amount of the high valent relative content of the different surface species was calculated and shown in Table 2. As presented manganese species decreases. Figure 6 shows the XPS spectra of the O 1s for these catalysts, and the in Figure 6, the peak of O 1s can also be divided into three peaks [23,25]. The peak at 529.4 ± 0.2 eV relative content of the different surface species was calculated and shown in Table 2. As presented 2−

the calcination temperature causes a reduction in the catalyst specific surface area, leading to a decrease in the catalytic reaction interface. It has been widely reported that the surface defect sites exhibit a high oxidation activity [38–40] because they are prone to produce active oxygen species, such as the surface oxygen species and highly reactive lattice oxygen species. In addition, these Catalysts 2019, 9, 726 involved in the catalytic reaction. Thus, increasing the calcination temperature 9 of 17 species are easily will have adverse effects on the catalyst activity.

Figure7.7.Summary Summaryofofcatalytic catalyticproperties propertiesfor forthe themanganese manganeseoxide oxidecatalysts. catalysts. Figure

The in-situ FTIR spectra the surface species for during adsorption (with O2 ) andThe desorption Figure 7 summarizes theofcatalytic properties the the manganese oxide catalysts. results of benzene over the MnCO3 -425, MnC Mn(OH) arecatalysts. shown inThe Figure 2 O4 -500 and 2 -425 catalysts show a complex structure-activity relationship of the manganese oxidation high8. −1 (C=C stretching vibration) and 1236 cm−1 (C–O stretching As shown in Figure 8, the bands at 1572 cm activity can be attributed to the large specific surface area and more abundant surface oxygen −1 can vibration) can be assigned to a surface are phenolate andmobility the bands at 1425 and 1313 cmof species. These catalyst characteristics relatedspecies to the [41], oxygen and the reducibility the be assigned to theismaleate species influenced (Olefin rocking consistent with the former reports [42–44]. catalyst, which significantly by vibration), the manganese oxide crystal structure and the In addition, the bands at 1561 cm−1 (asymmetric stretching vibration of COO− ), 1377 cm−1 (CH2 calcination temperature. stretching vibration), 1368 cm−1 (C–C stretching vibration) and 1354 cm−1 (CH3 stretching vibration) can be assigned to the acetate species [43,45,46], and the band at 1478 cm−1 can be assigned to the adsorbed benzene (C=C stretching vibration) [46]. In the adsorption process of benzene over MnCO3 -425, only the band at 1561 cm−1 can be observed, while other bands that are assigned to the adsorbed benzene or other intermediate products cannot be observed (Figure 8a). For the MnC2 O4 -500 and Mn(OH)2 -425 catalysts, not only benzene but also the phenolate, maleate and acetate species can be observed (Figure 8b,c). Notably, in the adsorption process of benzene over the MnCO3 -425 catalyst, there is an inverse peak in the 1300–1500 cm−1 region. This peak may arise because the MnCO3 -425 catalyst has a residual MnCO3 , which absorbs the infrared light in the range of 1300–1500 cm−1 , and these species are covered by intermediates. This phenomenon is confirmed the XRD hypothesis. The band at 1478 cm−1 that is assigned to the C=C vibration of the adsorbed benzene over the MnC2 O4 -500 and the Mn(OH)2 -425 catalysts decreases somewhat during the exposure of the adsorbed species to a flowing 20 vol% O2 /N2 stream (Figure 8a,c), indicating that benzene is gradually removed or converted to the intermediate products. In contrast, the bands assigned to the intermediate products do not decrease but increase somewhat, indicating that the catalytic oxidation performance of the catalysts is weak at a low temperature, hindering the conversion of the intermediate products to the final products. Nevertheless, the adsorbed benzene can be oxidized under the action of the active oxygen or lattice oxygen on the catalyst surface, resulting in the accumulation of the intermediate products. Clearly, the peak strength of the adsorbed benzene and intermediate products shows the rule of MnCO3 -425 < MnC2 O4 -500 < Mn(OH)2 -425, which is related to the catalytic oxidation performance of the catalysts. When benzene is adsorbed on the surface of the manganese oxide catalysts with a stronger oxidation ability, it can be rapidly converted to the intermediate or final products.

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Figure 8. In-situ FTIR spectra of the adsorption and desorption process of the (a) MnCO3 -425 Figure FTIR spectra of the adsorption and desorption process of the (a) MnCO3-425 (b) ◦ (b) MnC8.2 OIn-situ 4 -500 (c) Mn(OH)2 -425 catalysts collected at 100 C after 30 min on a 1000 ppm benzene/20 MnC 2O4-500 (c) Mn(OH)2-425 catalysts collected at 100 °C after 30 min on a 1000 ppm benzene/20 vol.% O2 /N2 stream followed by exposure of the adsorbed species to a flowing 20 vol% O2 /N2 stream. vol.% O2/N2 stream followed by exposure of the adsorbed species to a flowing 20 vol% O2/N2 stream. Figure 9 shows the in-situ FTIR spectra collected at different temperatures in a 1000 ppm benzene/20

vol% O2 /N2 stream over the MnCO3 -425, MnC2 O4 -500 and Mn(OH)2 -425 catalysts. The band at spectra of the surface species during the adsorption (with O2MnC ) and2 O desorption 1478 The cm−1in-situ that isFTIR assigned to the C=C vibration of the adsorbed benzene over the 4 -500 and of benzene over the MnCO 3-425, MnC 2O4-500indicating and Mn(OH) 2-425 catalysts are shown in Figure 8. As Mn(OH) -425 catalysts cannot be detected, that there is no adsorbed benzene at these 2 −1 −1 (C–O stretching −1 shown in Figure 8, the bands at 1572 cm (C=C stretching vibration) and 1236 cm temperatures. In comparison to Figure 8, similar bands at 1561, 1425, 1377, 1368 and 1313 cm due to vibration) can bespecies assigned a surface phenolate species [41], 2and the catalyst bands at 1425 and cm−1 different organic areto found in Figure 9 [46]. For the MnC O4 -500 (Figure 9b), 1313 the bands −1 disappeared, −1 (CH stretching can be assigned the cm maleate species (Olefin rocking vibration), consistent with the former reports at 1478 cm−1 andto1368 and the bands at 1377 cm vibration) and 2 −1 (asymmetric stretching vibration of COO−), 1377 cm−1 −1 [42–44]. In addition, the bands at 1561 cm 1330 cm (C–O stretching vibration) emerged [45,46]. For the Mn(OH)2 -425 catalyst, further analysis of −1 −1 (C–C (CH 2 stretching vibration), 1368 cm−1 (C–C stretching vibration) and 1354 cm stretching the band changes is required. The bands at 1727 cm−1 (C=O stretching vibration) and (CH 11873 cm −1 −1 vibration) can be assigned to the acetate speciesaldehyde [43, 45, species 46], and[46], the and band 1478 at cm1561 can stretching vibration) can be assigned to a surface theatbands cmbe − −1 assigned to the adsorbed benzene (C=C stretching vibration) [46]. In the adsorption process of (asymmetric stretching vibration of COO ) and 1368 cm (C–C stretching vibration) can be assigned − −1 canbands benzene overspecies MnCO[43,45,46]. 3-425, only the band at 1561 cm 1 can be observed, while that are to the acetate Additionally, the bands at 1425 cm−1 and 1307 cmother be assigned to −1 assigned to the adsorbed benzene or other intermediate products cannot be observed (Figure 8a). the maleate species (Olefin rocking vibration), and the bands at 1596 cm (C=C stretching vibration) For MnC O4(C–O -500 and Mn(OH)vibration) 2-425 catalysts, not only benzene but also the phenolate, maleate and the 1320 cm2−1 stretching can be assigned to a surface phenolate species [41,46]. and acetate species can be observed (Figure and 8c). in the aldehyde, adsorptionmaleate processand of Hence, it could be proposed that benzene was 8b oxidized intoNotably, the phenolate, −1 region. This ◦ benzene over the MnCO 3-425 catalyst, there is an inverse peak in the 1300–1500 cm acetate species over the manganese oxide catalysts at 200 C in the presence of an O2 /N2 mixture [46]. ◦ C. With peak may arise because the intermediate MnCO3-425 catalyst a residual 3, which absorbs the infrared Overall, the strongest of the species has strength at 200 MnCO the increase in the reaction −1 light in the range of 1300–1500 cm , and these species are covered by intermediates. temperature, the peak strengths of the intermediate products gradually decreased, indicating that This they phenomenon is confirmed the XRD hypothesis. are gradually oxidized to the final products. By comparison of Figures 8 and 9, it can be concluded Thecatalytic band atoxidation 1478 cm−1mechanism that is assigned to theover C=Cdifferent vibration of the adsorbed benzene the that the of benzene crystalline manganese oxide over catalysts MnC 2O4-500 and the Mn(OH)2-425 catalysts decreases somewhat during the exposure of the are essentially the same, and the intermediate products are mainly the phenolate, aldehyde, maleate, adsorbed species to aAdditionally, flowing 20 vol% O2/N2 stream (Figure 8a and 8c),a indicating that benzene is and acetate species. the manganese oxide catalysts have strong catalytic oxidation gradually or converted to the the bands assigned to the ability for removed benzene, which can cause theintermediate benzene ringproducts. pyrolysisIn at contrast, low temperatures. intermediate products do not decrease but increase somewhat, indicating that the catalytic oxidation performance of the catalysts is weak at a low temperature, hindering the conversion of

the accumulation of the intermediate products. Clearly, the peak strength of the adsorbed benzene and intermediate products shows the rule of MnCO3-425 < MnC2O4-500 < Mn(OH)2-425, which is related to the catalytic oxidation performance of the catalysts. When benzene is adsorbed on the surface of the manganese oxide catalysts with a stronger oxidation ability, it can be rapidly Catalysts 2019,to9,the 726 intermediate or final products. 11 of 17 converted

Figure 9. In-situ FTIR spectra of the (a) MnCO3 -425 (b) MnC2 O4 -500 (c) Mn(OH)2 -425 catalysts Figure 9.atIn-situ spectra of the (a)min MnCO 3-425 (b) MnC2O4-500 (c) Mn(OH)2-425 catalysts collected 200 ◦ CFTIR during oxidation at 30 in a 1000 ppm benzene/20 vol% O2 /N2 stream followed ◦ ◦ ◦ C.ppm benzene/20 vol% O2/N2 stream collected at 200 °C during oxidation at 30 min in a350 1000 by the temperature programmed to 250 C, 300 C and followed by the temperature programmed to 250 °C, 300 °C and 350 °C.

To evaluate the catalytic properties of the manganese oxide catalysts in the catalytic oxidation of Figure 9 shows in-situ FTIR collected at different temperatures in a 1000 ppm2 , CVOCs, MnCO MnC andspectra Mn(OH) with the crystal structures (γ-MnO 3 -425, the 2 O4 -500 2 -425 catalysts benzene/20 2/N 2 stream over the MnCO 3-425, MnC 2O 4-500 and Mn(OH) 2-425 catalysts. The Mn2 O3 and vol% Mn3 OO ) were chosen as representatives, and the catalytic oxidations of 1,2-DCE by these 4 −1 that is assigned to the C=C vibration of the adsorbed benzene over the band at 1478 cm catalysts were studied, as shown in Figure 10. Due to the reason that the catalyst deactivation was MnC 2O4-500 andoxidation Mn(OH)process, 2-425 catalysts cannot be detected, that there is no adsorbed observed in the three identical activity tests indicating were performed. Figure 10a–c shows benzene at these temperatures. In comparison to Figure 8, similar bands at 1561, 1425, 1377, 1368 the 1,2-DCE conversion and reaction temperature as a function of time. As shown, the activity of −1 due to different organic species are found in Figure 9 [46]. ◦ and 1313 cm For the MnC 2 O 4 -500 the catalysts decreases continuously when the temperature is below 300 C in the first experiment. −1 disappeared, and the bands at 1377 cm−1 ◦ C, catalyst (Figure 9b), bands temperature at 1478 cm−1 and 1368 After increasing thethe catalytic above 300cm all the catalysts can reach stable states, −1 (C–O ◦ (CH 2 stretching vibration) and 1330 cm stretching vibration) emerged [45,46]. the and 1,2-DCE can be completely converted at 400 C. In the second and third experiments, theFor activity Mn(OH) 2-425 catalyst, the band changes required. The bands at 1727 cm−1 of the catalysts are very further similar, analysis althoughof inevitably lower than is that in the first experiment, indicating −1 (C=O stretching vibration) 1187 cm (C–C stretching vibration) can be assigned a surface that these manganese oxideand catalysts are able to resist the chlorine poisoning. However,tothe porous −1 (asymmetric stretching vibration of COO−) and aldehyde species [46], and the bands at 1561 cm structure of the catalysts appears to have been damaged (Table 2). For example, the specific surface −1 (C–C stretching vibration) can be assigned to 1368 cmthe Additionally, area of MnCO3 -425 catalyst decreases from 105.8 m2 /gthe to acetate 37.6 m2species /g. This [43,45,46]. decrease may be related −1 −1 the bands at 1425 cm and 1307 cm can be assigned to the maleate species (Olefin rocking to the effects of the chlorine poisoning. During the catalytic oxidation of the CVOCs, the catalyst is −1 (C=C stretching vibration) and 1320 cm−1 (C–O stretching vibration), and the bands at 1596 cm inevitably chlorinated, even causing changes in the specific surface area or the crystal structure of vibration) be Hence, assigned a surface phenolate specieschannel [41,46].was Hence, it could be proposed that the catalystcan [47]. wetospeculated that the catalyst blocked by the newly formed benzene was oxidized into the phenolate, aldehyde, maleate and acetate species over the structure in the 1,2-DCE oxidation process. However, the manganese oxide catalyst ultimately reaches manganese oxidestate, catalysts at 200 that °C init the of an 2/N2 the mixture [46].poisoning Overall, the strongest a stable catalytic indicating has presence some ability to O resist chlorine by achieving an equilibrium state of chlorination and dechlorination.

peak strengths of the intermediate products gradually decreased, indicating that they are gradually oxidized to the final products. By comparison of Figure 8 and Figure 9, it can be concluded that the catalytic oxidation mechanism of benzene over different crystalline manganese oxide catalysts are essentially the same, and the intermediate products are mainly the phenolate, aldehyde, maleate, and acetate species. Additionally, the manganese oxide catalysts have a strong catalytic oxidation Catalysts 2019, 9, 726 12 of 17 ability for benzene, which can cause the benzene ring pyrolysis at low temperatures. 500 400

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Figure 10. Catalytic evaluation of the (a) MnCO3 -425, (b) MnC2 O4 -500 and (c) Mn(OH)2 -425 catalysts Figure 10. Catalytic evaluation of the MnCO3-425, MnC2O4-500 and Mn(OH)2-425 catalysts for the for the 1,2-DCE oxidation. (d) 1,2-DCE conversion and CO selectivity of the MnCO3 -425, MnC O -500 1,2-DCE oxidation. (100 ppm 1,2-DCE/20 vol% O2/N2; 200 2mg catalyst; WHSV 30000 mL·g−1·h−12). 4 and Mn(OH)2 -425 catalysts as a function of the temperature. (100 ppm 1,2-DCE/20 vol% O2 /N2 ; 200 mg catalyst; WHSV 30,000 mL·g−1 ·h−1 ).

To evaluate the catalytic properties of the manganese oxide catalysts in the catalytic oxidation of CVOCs, 3-425, MnC2O4-500 and Mn(OH)2-425 catalysts with the crystal structures It couldMnCO be deduced that the toxicity of the chlorinated VOCs on the manganese oxide catalysts (γ-MnO 2, Mn2O3 and Mn3O4) were chosen as representatives, and the catalytic oxidations of was mainly reflected in two aspects. Firstly, it destroyed the structure of the catalyst, resulting in 1,2-DCE by these catalysts were studied, as shown Figure 10. Due to the reasonreaction that theinterface. catalyst a significant reduction of the specific surface area in and reduction of the catalytic deactivation was observed in the oxidation process, three identical activity tests were performed. Secondly, it was difficult to remove the chlorine species, resulting in the chlorination of the catalyst. Figure 10a-cto shows the 1,2-DCE conversion and reaction temperature a functionoxide of time. As According the experimental results, it was found that the activity of theas manganese catalysts shown, the activity of the during catalysts decreases when350 the◦temperature below °C in decreased continuously the 1,2-DCEcontinuously oxidation under C, and only is when the300 reaction the first experiment. After increasing the catalytic temperature above 300 °C, all the catalysts can ◦ temperature exceeds 350 C could the activity of the catalyst be stable. It suggested that a higher reach stable states, and 1,2-DCE can be completely converted at 400 °C. In the second and third reaction temperature could facilitate the removal of the chlorine species. However, the reduction of the experiments, the activity of the catalysts are very similar, although inevitably lower than that in the catalyst interface was irreversible. first experiment, indicating that these manganese oxide catalysts are able to resist the chlorine Figure 10d shows the 1,2-DCE conversion and CO 2 selectivity as a function of the temperature in poisoning. However, the porous the structure the catalysts appears to have been damaged (Table 2). the third experiment. As shown, MnCOof 3 -425 catalyst demonstrates the best activity in the 1,2-DCE For example, the specific surface area of the MnCO 3-425 catalyst decreases from 105.8 m2/g to 37.6 oxidation, followed by the MnC2 O4 -500 and Mn(OH) 2 -425 catalysts. This result is similar to that for 2/g. This decrease may be related to the effects of the chlorine poisoning. During the catalytic m the benzene oxidation. However, the selectivity of CO2 for these catalysts is not 100%. This observation oxidation CVOCs, catalyst is inevitably chlorinated, evencan causing changes in the specific may also of be the related to thethe effects of the chlorine poisoning, which directly affect the capacity of surface area or the crystal structure of the catalyst [47]. Hence, we speculated that the catalyst the catalyst to provide the reactive oxygen species [48]. Therefore, CO is released as an incomplete channel was blockedNotably, by the newly formed structure in gradually the 1,2-DCE oxidation process. However, the oxidation product. the selectivity of CO2 can increase to 100% for all the catalysts manganese oxide catalystofultimately reaches a almost stable no catalytic state, were indicating that it has some after the total conversion 1,2-DCE. Moreover, by-products observed. 3. Experimental 3.1. Catalysts Preparation All the chemicals used in this study were purchased from Aladdin (Shanghai, China) without further purification. Mn(NO3 )2 (50 wt% in H2 O) was used as a manganese precursor, and NH4 HCO3

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(>99.9%), (NH4 )2 C2 O4 ·H2 O (>99.8%) and NaOH (>96%) were used as precipitants. First, 0.8 mol NH4 HCO3 , 0.096 mol (NH4 )2 C2 O4 ·H2 O and 0.192 mol NaOH were dissolved in 400 mL of deionized H2 O respectively, and a 85.89 g Mn(NO3 )2 solution (0.08 mol Mn(NO3 )2 ) was diluted in the 1.2 L deionized H2 O and then divided into three equal parts. Typically, the NH4 HCO3 solution was added dropwise into the Mn(NO3 )2 solution under stirring at room temperature (R.T.), resulting in the formation of a precipitate. After stirring for 0.5 h, the precipitate was aged for 4 h under static conditions and then filtered and washed with the deionized water and dried at 100 ◦ C overnight. Subsequently, the precipitate was calcined at 350–575 ◦ C in air, generating the manganese oxide catalyst MnCO3 -T, where T is the calcination temperature. Similarly, the manganese oxide catalysts obtained using (NH4 )2 C2 O4 and NaOH as precipitants are denoted as MnC2 O4 -T and Mn(OH)2 -T, respectively. 3.2. Catalyst Characterizations 3.2.1. X-ray Diffraction (XRD) XRD patterns of the samples were recorded on a powder diffractometer (Rigaku D/Max-RA, Shimadzu, Kyoto, Japan) using the Cu Kα radiation (40 kV and 120 mA). The diffractograms were recorded from 10◦ to 80◦ with a step size of 0.02◦ and a step time of 8 s. 3.2.2. N2 Adsorption/Desorption The porous structures of the catalysts were characterized using the N2 adsorption at 77 K in an automatic surface area and porosity analyzer (Autosorb iQ, Quantachrome, Boynton, FL, USA). The specific surface areas of the catalysts were calculated from the N2 adsorption isotherms using the Brunauer–Emmett–Teller (BET) equation, and the pore sizes and pore volumes of the catalysts were determined from the N2 desorption isotherms using the Barrett–Joyner–Halenda (BJH) method. 3.2.3. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) The SEM analyses were performed with a JEOL JSM-7500F Field Emission scanning electron microscope (JEOL, Tokyo, Japan) at 5 kV. The TEM analyses were collected on an FEI Tecnai G2 F20 field emission electron microscope (FEI, Hillsboro, OR, USA) at 200 kV. 3.2.4. X-ray Photoelectron Spectroscopy (XPS) The XPS measurements were collected on a photoelectron spectrometer (ESCALAB 250, Thermo Fisher Scientific, Waltham, MA, USA) using the Al Kα (1486.8 eV) radiation as the excitation source (powered at 10 mA and 15 kV). Charging of the samples was corrected by setting the binding energy of the adventitious carbon (C1s) at 284.6 eV. 3.2.5. H2 -Temperature-Programmed Reduction (H2 -TPR) The H2 -TPR measurements were conducted on an automated chemisorption analyzer (ChemBET Pulsar TPR/TPD, Quantachrome, Boynton, FL, USA). The sample (30 mg) was pretreated in He at 300 ◦ C for 1 h. After being cooled to R.T., the TPR experiments were carried out in a flow of 10% H2 /Ar from R.T. to 600 ◦ C at a ramp of 10 ◦ C·min−1 . 3.3. Catalytic Oxidation of VOCs Catalytic evaluations were carried out in a quartz tube, single-pass fixed-bed microreactor (4 mm i.d.) with a 200 mg catalyst (40–60 mesh). The reactor was heated by an electric furnace, and the temperature was monitored through a K-type thermocouple next to the sieve plate. Benzene and 1,2-DCE were introduced into the reaction flow directly from the gas cylinders. The total flow rate of the mixed stream was 100 mL·min−1 with a gas composition of 1000 ppm benzene/20 vol% O2 /N2 or 100 ppm 1,2-DCE/20 vol% O2 /N2 (corresponding weight hourly space velocity (WHSV) was 30,000 mL·g−1 ·h−1 ). The reactants and products (CO2 and CO) were analyzed on-line with a gas

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chromatograph (GC 2010 Plus, Shimadzu, Kyoto, Japan), which was equipped with a methanizer (MTN, Shimadzu, Kyoto, Japan) and two flame ionization detectors. Additionally, the relative error was less than ±1%. The conversion of benzene and 1,2-DCE was calculated by Equation (1): x (%) = [C(in) − C(out)]/C(in) × 100

(1)

where x is the conversion, and C(in) and C(out) are the inlet and outlet concentrations, respectively. The CO2 selectivity was calculated using Equation (2): S(CO2 ) (%) = C(CO2 )/[C(CO2 ) + C(CO)] × 100

(2)

where S(CO2 ) is the CO2 selectivity, and C(CO2 ) and C(CO) are the outlet concentrations of the CO2 and CO, respectively. 3.4. In-Situ Fourier Transform Infrared Spectroscopy (FTIR) The FTIR spectra were collected with a FTIR spectrometer (Nicolet 6700, Thermo Fisher Scientific, Waltham, MA, USA). The FTIR spectrometer was equipped with an MCT (mercury-cadmium-telluride) detector and a stainless-steel IR cell. The powder sample was pressed into a self-supported disk, and the sample was pretreated at 400 ◦ C for 1 h in a 20 vol% O2 /N2 stream before the experiment. After being cooled to a desired temperature, the spectra of the clean sample surfaces were collected as the background. In addition, the spectra of the in-situ reactions of the sample in specific mixed streams at given temperatures were collected. 4. Conclusions A series of manganese oxides were synthesized and used as catalysts for the benzene and 1,2-DCE oxidation. The complete conversion temperatures for the benzene and 1,2-DCE are lower than 300 ◦ C and 400 ◦ C, respectively. Generally, the γ-MnO2 exhibits the highest activity, followed by Mn2 O3 and Mn3 O4 , and Mn2 O3 shows the best thermal stability. Additionally, there is no essential difference in the benzene oxidation processes for γ-MnO2 , Mn2 O3 and Mn3 O4 . The high activity of the manganese oxides is associated with the large specific surface area, abundant surface oxygen species and excellent low-temperature reducibility, and increasing the calcination temperature has obvious adverse effects. During the catalytic oxidation of 1,2-DCE, the catalyst structure is irreversibly damaged, which leads to the decrease of the reaction interface and activity. A higher reaction temperature could facilitate the removal of the chlorine species to maintain the catalyst activity. These results indicate that, in addition to increasing the catalytic activity, further improvements in the thermal stability and chlorine resistance of the manganese oxide catalysts are essential. Author Contributions: Conceptualization, J.W. and W.X.; data curation, J.W. and H.Z.; investigation, H.Z.; methodology, J.S.; validation, T.Z.; writing—original draft, H.Z.; writing—review and editing, J.W. Funding: This work was financially supported by the Key Research Program of the Chinese Academy of Sciences (Grant NO. ZDRW-ZS-2017-6-2) and the National Natural Science Foundation of China (Grant NO. 51708540). Conflicts of Interest: The authors declare no conflict of interest.

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