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Article

Structure-Activity Relationship of Manganese Oxide Catalysts for the Catalytic Oxidation of (chloro)-VOCs

by
Jian Wang
1,†,
Hainan Zhao
1,2,†,
Jianfei Song
2,
Tingyu Zhu
1,3 and
Wenqing Xu
1,3,*
1
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
2
State Key Laboratory of Heavy Oil Processing, College of Mechanical and Transportation Engineering, China University of Petroleum (Beijing), Beijing 102249, China
3
Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work and should be considered co-first authors.
Catalysts 2019, 9(9), 726; https://doi.org/10.3390/catal9090726
Submission received: 23 July 2019 / Revised: 23 August 2019 / Accepted: 26 August 2019 / Published: 28 August 2019
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Abstract

:
Manganese oxide catalysts, including γ-MnO2, Mn2O3 and Mn3O4, 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 Mn3O4 were converted to Mn2O3. 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 Mn2O3 and Mn3O4. 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.

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 [2,3,4], 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 [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]. In particular, these catalysts often show better resistance to the chlorine poisoning [26]. Among these catalysts, the manganese oxide catalysts are highly active [16,17,18,19,20,21,22,23,24,25] and have an ability to 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 [29,30,31]. Manganese oxides (Mn3O4, Mn2O3 and MnO2, etc.) are known to exhibit a good activity in the catalytic oxidation of the VOCs [18,19,20,21,22,24,25]. 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 [16,17,18,19,20,21,22,23,24,25,32], 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, Mn2O3 and Mn3O4, 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), Mn2O3 (JCPDS PDF 78-0390), Mn3O4 (JCPDS PDF 24-0734), MnCO3 (JCPDS PDF 44-1472) and MnC2O4 (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 Mn2O3. As shown in Figure 1b, the crystal structure of the MnC2O4 precipitate also remains unchanged after drying. With the increase in the calcination temperature, MnC2O4 is first converted into a mixture of the Mn3O4 and Mn2O3 (mainly Mn2O3), and then Mn2O3 becomes the sole crystal phase. As shown in Figure 1c, the Mn(OH)2 precipitate has already been converted into Mn3O4 through the dehydration process. With the increase in the calcination temperature, Mn3O4 is also converted into Mn2O3. 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 Mn3O4 are converted into Mn2O3 with the increase in the calcination temperature, suggesting that Mn2O3 has the best thermal stability. However, a peak for MnCO3 is observed in the XRD patterns of the MnCO3-350 and MnCO3-425 catalysts, indicating that the catalysts contain the MnCO3 species.
The catalytic evaluation of the manganese oxide catalysts for the benzene oxidation were performed, and the results are shown in Figure 2. In addition, the 50% conversion temperature (T50) and 90% conversion temperature (T90) of these catalysts for benzene are summarized in Table 1.
As shown in Figure 2a–b, the T50 and T90 for the MnCO3-350 (182 °C and 202 °C) and MnCO3-425 (191 °C and 218 °C) are lower than that for the MnC2O4-425 (202 °C and 229 °C), which shows the best activity among the manganese oxide catalysts with the crystal structure of the Mn2O3 (the MnC2O4-350 catalyst is a mixture of the Mn3O4 and Mn2O3). Therefore, it is clear that the γ-MnO2 is more active than the Mn2O3 in benzene oxidation. As shown in Figure 2c, the activity of the Mn(OH)2-500 catalyst with the crystal structure of Mn2O3 is better than that of the Mn(OH)2-425 catalyst with the crystal structure of Mn3O4, suggesting that the transformation of the Mn3O4 to the Mn2O3 is beneficial to the catalytic activity. Thus, the γ-MnO2 has the best activity in the benzene oxidation, followed by the Mn2O3 and Mn3O4. Furthermore, the activity of the manganese oxide catalysts with the same crystal structures decrease significantly with the increase in the calcination temperature. Overall, the activity of the manganese oxide catalysts for the benzene oxidation is very high, and benzene can not only be completely converted at 200 °C by the γ-MnO2 catalyst but also can be completely oxidized below 275 °C by the Mn3O4 catalyst. Figure 2d shows the benzene conversion and the CO2 selectivity of the MnCO3-425, MnC2O4-500 and Mn(OH)2-425 catalysts as a function of the temperature. As shown, CO2 is the only oxidation product.
The porous structures of the manganese oxide catalysts are shown in Table 2. Overall, it was found that both the specific area and the pore volume of the catalysts decrease with the increase in the calcination temperature, and the pore size of the catalyst changes inversely. For instance, the specific 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 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 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.
The MnCO3-425, MnC2O4-500 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 MnC2O4-500 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 Mn2O3 (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 MnC2O4-500 catalyst (Figure 3h), and the lattice spacing is 0.25 nm, corresponding to the (211) crystal plane of the Mn3O4 (Figure 3i).
The H2-TPR analyses were conducted to investigate the reducibility of the manganese oxide catalysts, and the results are shown in Figure 4. The reduction of the manganese oxides can be described as follows: MnO2 → Mn2O3 → Mn3O4 → MnO [34]. As shown in Figure 4, each H2-TPR profile of the manganese oxide catalyst shows two or three main peaks that can be attributed to the reduction of the manganese species in different valence states. In general, the peaks in the temperature range from 270 °C to 370 °C can be assigned to the reduction of MnO2 to Mn2O3 or Mn2O3 to Mn3O4, and the peak in the temperature range of 370 °C to 460 °C can be assigned to the reduction of Mn3O4 to MnO. Specifically, the MnCO3-350 and MnCO3-425 catalysts with the crystal structure of the γ-MnO2 show 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 Mn3O4 to MnO (Figure 4a). For the Mn(OH)2-350 and Mn(OH)2-425 catalysts with the crystal structure of the Mn3O4, the characteristic peaks at 450 °C can be assigned to the reduction of Mn3O4 to MnO, but the peaks in the temperature range from 270 °C to 370 °C are quite weak, indicating that the reducibility of the Mn3O4 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 structure of the Mn2O3 are similar, showing two strong peaks assigned to the reduction of the Mn2O3 and Mn3O4 at approximately 340 °C and 450 °C, respectively.
In addition to the abovementioned peaks, characteristic peaks in the temperature range from 210 °C to 270 °C can be assigned to the reactive oxygen species (mainly the surface oxygen species and highly reactive lattice oxygen species). It can be seen that the peak intensity of these species decreases with the increase in the calcination temperature for all the catalysts, and these peaks are 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 increased as well. All the above phenomena indicate that the crystal structures of the manganese oxides gradually become more intact, and the oxygen mobility and thus the reducibility of the catalyst decrease with the increase in the calcination temperature. Similar phenomena are observed in many transition metal oxide catalysts [15,35], in which these processes decrease the availability of the reactive oxygen species in the oxidation reactions. It is well known that the Mars-van-Krevelen mechanism is operational in the transition metal oxide catalysts during the oxidation reactions, which involves the process of releasing and replenishing lattice oxygen. Therefore, the reducibility of the catalyst will be closely related to the catalytic activity. Overall, the reducibility of the manganese oxides with different crystal structures is γ-MnO2 > Mn2O3 > Mn3O4, which shows a strong correlation with the main valence states of the manganese oxides.
The surface manganese species and oxygen species of the manganese oxide catalysts were investigated by XPS. Figure 5 shows the XPS spectra of the Mn 2p for these catalysts. The determination of the different oxidation states of manganese by XPS is not trivial because of the relative importance of the intra and inter-atomic effects [36], so Mn 2p has not been treated as a peak separation. However, 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 shows the XPS spectra of the O 1s for these catalysts, and the relative content of the different surface species was calculated and shown in Table 2. As presented 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 can be assigned to lattice oxygen species (O2−) in a fully coordinated environment [23,25]. The peak at 531.3 ± 0.2 eV can be attributed to the surface oxygen species (O−, O2− and O22−) in the vicinity of the surface defects [37], and the peak at 533.4 ± 0.2 eV can be described as the OH groups [23,25], together known as the adsorbed oxygen species. From the calculated data in Table 2, it can be concluded that the catalyst calcined at high temperatures has a higher ratio of the lattice oxygen species, and a lower ration of the surface oxygen species, indicating that the crystal structure of the catalyst gradually becomes more intact, and the surface defects are reduced. Therefore, increasing 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 [38,39,40] because they are prone to produce active oxygen species, such as the surface oxygen species and highly reactive lattice oxygen species. In addition, these species are easily involved in the catalytic reaction. Thus, increasing the calcination temperature will have adverse effects on the catalyst activity.
Figure 7 summarizes the catalytic properties for the manganese oxide catalysts. The results show a complex structure-activity relationship of the manganese oxidation catalysts. The high activity can be attributed to the large specific surface area and more abundant surface oxygen species. These catalyst characteristics are related to the oxygen mobility and the reducibility of the catalyst, which is significantly influenced by the manganese oxide crystal structure and the calcination temperature.
The in-situ FTIR spectra of the surface species during the adsorption (with O2) and desorption of benzene over the MnCO3-425, MnC2O4-500 and Mn(OH)2-425 catalysts are shown in Figure 8. As shown in Figure 8, the bands at 1572 cm−1 (C=C stretching vibration) and 1236 cm−1 (C–O stretching vibration) can be assigned to a surface phenolate species [41], and the bands at 1425 and 1313 cm−1 can be assigned to the maleate species (Olefin rocking vibration), consistent with the former reports [42,43,44]. In addition, the bands at 1561 cm−1 (asymmetric stretching vibration of COO−), 1377 cm−1 (CH2 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 MnC2O4-500 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 MnC2O4-500 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 < 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 converted to the intermediate or final products.
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, MnC2O4-500 and Mn(OH)2-425 catalysts. The band at 1478 cm−1 that is assigned to the C=C vibration of the adsorbed benzene over the MnC2O4-500 and Mn(OH)2-425 catalysts cannot be detected, indicating that there is no adsorbed benzene at these temperatures. In comparison to Figure 8, similar bands at 1561, 1425, 1377, 1368 and 1313 cm−1 due to different organic species are found in Figure 9 [46]. For the MnC2O4-500 catalyst (Figure 9b), the bands at 1478 cm−1 and 1368 cm−1 disappeared, and the bands at 1377 cm−1 (CH2 stretching vibration) and 1330 cm−1 (C–O stretching vibration) emerged [45,46]. For the Mn(OH)2-425 catalyst, further analysis of the band changes is required. The bands at 1727 cm−1 (C=O stretching vibration) and 1187 cm−1 (C–C stretching vibration) can be assigned to a surface aldehyde species [46], and the bands at 1561 cm−1 (asymmetric stretching vibration of COO−) and 1368 cm−1 (C–C stretching vibration) can be assigned to the acetate species [43,45,46]. Additionally, the bands at 1425 cm−1 and 1307 cm−1 can be assigned to the maleate species (Olefin rocking vibration), and the bands at 1596 cm−1 (C=C stretching vibration) and 1320 cm−1 (C–O stretching vibration) can be assigned to a surface phenolate species [41,46]. Hence, it could be proposed that benzene was oxidized into the phenolate, aldehyde, maleate and acetate species over the manganese oxide catalysts at 200 °C in the presence of an O2/N2 mixture [46]. Overall, the strongest of the intermediate species strength at 200 °C. With the increase in the reaction temperature, the 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 ability for benzene, which can cause the benzene ring pyrolysis at low temperatures.
To evaluate the catalytic properties of the manganese oxide catalysts in the catalytic oxidation of CVOCs, MnCO3-425, MnC2O4-500 and Mn(OH)2-425 catalysts with the crystal structures (γ-MnO2, Mn2O3 and Mn3O4) were chosen as representatives, and the catalytic oxidations of 1,2-DCE by these catalysts were studied, as shown in Figure 10. Due to the reason that the catalyst deactivation was observed in the oxidation process, three identical activity tests were performed. Figure 10a–c shows the 1,2-DCE conversion and reaction temperature as a function of time. As shown, the activity of the catalysts decreases continuously when the temperature is below 300 °C in the first experiment. After increasing the catalytic temperature above 300 °C, all the catalysts can reach stable states, and 1,2-DCE can be completely converted at 400 °C. In the second and third experiments, the activity of the catalysts are very similar, although inevitably lower than that in the first experiment, indicating that these manganese oxide catalysts are able to resist the chlorine poisoning. However, the porous structure of the catalysts appears to have been damaged (Table 2). For example, the specific surface area of the MnCO3-425 catalyst decreases from 105.8 m2/g to 37.6 m2/g. This decrease may be related to the effects of the chlorine poisoning. During the catalytic oxidation of the CVOCs, the catalyst is inevitably chlorinated, even causing changes in the specific surface area or the crystal structure of the catalyst [47]. Hence, we speculated that the catalyst channel was blocked by the newly formed structure in the 1,2-DCE oxidation process. However, the manganese oxide catalyst ultimately reaches a stable catalytic state, indicating that it has some ability to resist the chlorine poisoning by achieving an equilibrium state of chlorination and dechlorination.
It could be deduced that the toxicity of the chlorinated VOCs on the manganese oxide catalysts was mainly reflected in two aspects. Firstly, it destroyed the structure of the catalyst, resulting in a significant reduction of the specific surface area and reduction of the catalytic reaction interface. Secondly, it was difficult to remove the chlorine species, resulting in the chlorination of the catalyst. According to the experimental results, it was found that the activity of the manganese oxide catalysts decreased continuously during the 1,2-DCE oxidation under 350 °C, and only when the reaction temperature exceeds 350 °C could the activity of the catalyst be stable. It suggested that a higher reaction temperature could facilitate the removal of the chlorine species. However, the reduction of the catalyst interface was irreversible.
Figure 10d shows the 1,2-DCE conversion and CO2 selectivity as a function of the temperature in the third experiment. As shown, the MnCO3-425 catalyst demonstrates the best activity in the 1,2-DCE oxidation, followed by the MnC2O4-500 and Mn(OH)2-425 catalysts. This result is similar to that for the benzene oxidation. However, the selectivity of CO2 for these catalysts is not 100%. This observation may also be related to the effects of the chlorine poisoning, which can directly affect the capacity of the catalyst to provide the reactive oxygen species [48]. Therefore, CO is released as an incomplete oxidation product. Notably, the selectivity of CO2 can gradually increase to 100% for all the catalysts after the total conversion of 1,2-DCE. Moreover, almost no by-products were 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 H2O) was used as a manganese precursor, and NH4HCO3 (>99.9%), (NH4)2C2O4·H2O (>99.8%) and NaOH (>96%) were used as precipitants. First, 0.8 mol NH4HCO3, 0.096 mol (NH4)2C2O4·H2O and 0.192 mol NaOH were dissolved in 400 mL of deionized H2O respectively, and a 85.89 g Mn(NO3)2 solution (0.08 mol Mn(NO3)2) was diluted in the 1.2 L deionized H2O and then divided into three equal parts. Typically, the NH4HCO3 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)2C2O4 and NaOH as precipitants are denoted as MnC2O4-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 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
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
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 Mn2O3 and Mn3O4, and Mn2O3 shows the best thermal stability. Additionally, there is no essential difference in the benzene oxidation processes for γ-MnO2, Mn2O3 and Mn3O4. 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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Figure 1. XRD patterns of (a) MnCO3 (b) MnC2O4 (c) Mn(OH)2 calcined at different temperatures, and (d) standard XRD patterns of γ-MnO2, Mn2O3, Mn3O4 and MnCO3.
Figure 1. XRD patterns of (a) MnCO3 (b) MnC2O4 (c) Mn(OH)2 calcined at different temperatures, and (d) standard XRD patterns of γ-MnO2, Mn2O3, Mn3O4 and MnCO3.
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Figure 2. Catalytic evaluation of manganese oxide catalysts obtained by calcining (a) MnCO3, (b) MnC2O4 and (c) Mn(OH)2 for the benzene oxidation. (d) Benzene conversion and CO2 selectivity of the MnCO3-425, MnC2O4-500 and Mn(OH)2-425 catalysts as a function of the temperature. (1000 ppm benzene/20 vol.% O2/N2; 200 mg catalyst; WHSV 30,000 mL·g−1·h−1).
Figure 2. Catalytic evaluation of manganese oxide catalysts obtained by calcining (a) MnCO3, (b) MnC2O4 and (c) Mn(OH)2 for the benzene oxidation. (d) Benzene conversion and CO2 selectivity of the MnCO3-425, MnC2O4-500 and Mn(OH)2-425 catalysts as a function of the temperature. (1000 ppm benzene/20 vol.% O2/N2; 200 mg catalyst; WHSV 30,000 mL·g−1·h−1).
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Catalysts 09 00726 g003 550
Figure 3. Representative SEM, TEM micrographs of the (a–c) MnCO3–425, (d–f) MnC2O4-500 and (g–i) Mn(OH)2-425 catalysts. The high resolution TEM micrograph shows the yellow dotted area in the corresponding TEM micrograph of the catalyst.
Figure 3. Representative SEM, TEM micrographs of the (a–c) MnCO3–425, (d–f) MnC2O4-500 and (g–i) Mn(OH)2-425 catalysts. The high resolution TEM micrograph shows the yellow dotted area in the corresponding TEM micrograph of the catalyst.
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Figure 4. H2-TPR profiles of the manganese oxide catalysts obtained by calcining (a) MnCO3, (b) MnC2O4 and (c) Mn(OH)2.
Figure 4. H2-TPR profiles of the manganese oxide catalysts obtained by calcining (a) MnCO3, (b) MnC2O4 and (c) Mn(OH)2.
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Figure 5. XPS spectra of Mn 2p for the manganese oxide catalysts obtained by calcining (a) MnCO3, (b) MnC2O4 and (c) Mn(OH)2.
Figure 5. XPS spectra of Mn 2p for the manganese oxide catalysts obtained by calcining (a) MnCO3, (b) MnC2O4 and (c) Mn(OH)2.
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Figure 6. XPS spectra of O 1s for the manganese oxide catalysts obtained by calcining (a) MnCO3, (b) MnC2O4 and (c) Mn(OH)2.
Figure 6. XPS spectra of O 1s for the manganese oxide catalysts obtained by calcining (a) MnCO3, (b) MnC2O4 and (c) Mn(OH)2.
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Figure 7. Summary of catalytic properties for the manganese oxide catalysts.
Figure 7. Summary of catalytic properties for the manganese oxide catalysts.
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Figure 8. In-situ FTIR spectra of the adsorption and desorption process of the (a) MnCO3-425 (b) MnC2O4-500 (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.
Figure 8. In-situ FTIR spectra of the adsorption and desorption process of the (a) MnCO3-425 (b) MnC2O4-500 (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.
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Figure 9. In-situ FTIR spectra of the (a) MnCO3-425 (b) MnC2O4-500 (c) Mn(OH)2-425 catalysts collected at 200 °C during oxidation at 30 min in a 1000 ppm benzene/20 vol% O2/N2 stream followed by the temperature programmed to 250 °C, 300 °C and 350 °C.
Figure 9. In-situ FTIR spectra of the (a) MnCO3-425 (b) MnC2O4-500 (c) Mn(OH)2-425 catalysts collected at 200 °C during oxidation at 30 min in a 1000 ppm benzene/20 vol% O2/N2 stream followed by the temperature programmed to 250 °C, 300 °C and 350 °C.
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Figure 10. Catalytic evaluation of the (a) MnCO3-425, (b) MnC2O4-500 and (c) Mn(OH)2-425 catalysts for the 1,2-DCE oxidation. (d) 1,2-DCE conversion and CO2 selectivity of the MnCO3-425, MnC2O4-500 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).
Figure 10. Catalytic evaluation of the (a) MnCO3-425, (b) MnC2O4-500 and (c) Mn(OH)2-425 catalysts for the 1,2-DCE oxidation. (d) 1,2-DCE conversion and CO2 selectivity of the MnCO3-425, MnC2O4-500 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).
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Table
Table 1. Catalytic evaluation results for the manganese oxide catalysts.
Table 1. Catalytic evaluation results for the manganese oxide catalysts.
CatalystBenzene
T50 (°C)T90 (°C)
MnCO3-350182202
MnCO3-425191218
MnCO3-500205230
MnCO3-575213241
MnC2O4-350195223
MnC2O4-425202229
MnC2O4-500213240
MnC2O4-575223252
Mn(OH)2-350225252
Mn(OH)2-425225252
Mn(OH)2-500223250
Mn(OH)2-575227255
Table
Table 2. Characterization data for the manganese oxide catalysts.
Table 2. Characterization data for the manganese oxide catalysts.
CatalystSpecific Surface Area
(m2/g)		Pore Size
(nm)		Pore Volume
(cm3/g)		O
Oads cOlatt d
MnCO3-350153.25.60.290.6980.302
MnCO3-425105.86.50.240.5870.413
MnCO3-425 (B) a104.16.60.250.2950.705
MnCO3-425 (DCE) b37.617.50.220.2710.729
MnCO3-50045.117.50.240.3820.618
MnCO3-57543.917.40.290.2820.718
MnC2O4-35058.69.60.240.2850.715
MnC2O4-42547.79.60.240.2990.701
MnC2O4-50033.817.70.220.3750.625
MnC2O4-500 (B) a33.317.40.200.2720.728
MnC2O4-500 (DCE) b24.231.10.320.2610.739
MnC2O4-57525.231.50.190.2740.726
Mn(OH)2-35030.331.00.230.4330.567
Mn(OH)2-42530.130.50.210.3040.696
Mn(OH)2-425 (B) a37.130.90.230.2120.788
Mn(OH)2-425 (DCE) b17.246.40.240.2640.736
Mn(OH)2-50027.031.20.360.2750.725
Mn(OH)2-57521.646.30.850.2960.704
a 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 O22− species, and OH groups. d Lattice oxygen species (O2−).

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Wang, J.; Zhao, H.; Song, J.; Zhu, T.; Xu, W. Structure-Activity Relationship of Manganese Oxide Catalysts for the Catalytic Oxidation of (chloro)-VOCs. Catalysts 2019, 9, 726. https://doi.org/10.3390/catal9090726

AMA Style

Wang J, Zhao H, Song J, Zhu T, Xu W. Structure-Activity Relationship of Manganese Oxide Catalysts for the Catalytic Oxidation of (chloro)-VOCs. Catalysts. 2019; 9(9):726. https://doi.org/10.3390/catal9090726

Chicago/Turabian Style

Wang, Jian, Hainan Zhao, Jianfei Song, Tingyu Zhu, and Wenqing Xu. 2019. "Structure-Activity Relationship of Manganese Oxide Catalysts for the Catalytic Oxidation of (chloro)-VOCs" Catalysts 9, no. 9: 726. https://doi.org/10.3390/catal9090726

APA Style

Wang, J., Zhao, H., Song, J., Zhu, T., & Xu, W. (2019). Structure-Activity Relationship of Manganese Oxide Catalysts for the Catalytic Oxidation of (chloro)-VOCs. Catalysts, 9(9), 726. https://doi.org/10.3390/catal9090726

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