Catalytic Oxidation of Toluene over Fe-Rich Palygorskite Supported Manganese Oxide: Characterization and Performance

: A series of Fe–rich palygorskite supported manganese oxide (X%Mn–Pal) catalysts were prepared by co-precipitation method and used as catalysts for toluene oxidation. The components and structure of the as-prepared catalysts were characterized by XRD, Raman, TEM, XPS, and in situ DRIFTS. The results showed that the 15%Mn–Pal catalyst exhibited the highest catalytic activity (T 90 = 227 ◦ C) and excellent cycling stability for the oxidation of toluene compared with other catalysts. The characterization results indicated that remarkable activity of the 15%Mn–Pal catalyst for toluene oxidation should be ascribed to the abundant surface oxygen vacancies. In situ DRIFTS results elucidated that benzoate was the main intermediate, which can be further oxidized into H 2 O and CO 2 . The objectives of this study are to (i) investigate the synergistic effect between Fe and Mn for toluene oxidation, (ii) develop an efﬁcient catalyst for toluene abatement with high activity and low–cost, and (iii) promote the application of natural Fe–rich palygorskite in the control of VOCs. effect of relative humidity ( a ) and GHSV ( b ) on the catalytic activity of 15%Mn–Pal.


Introduction
Volatile organic compounds (VOCs) are the significant precursors of photochemical smog, ozone, and secondary organic aerosols. Many countries and organizations have identified hundreds of chemicals as VOCs, which are seriously harmful to both the ecological environment and human health [1][2][3]. In recent years, numerous post-treatment techniques have been developed to eliminate VOCs emission, including adsorption, photocatalytic oxidation, plasma, absorption, and catalytic oxidation [4]. Among these techniques, catalytic oxidation was considered to be one of the most promising methods with the advantages of high removal efficiency, low reaction temperature, and less secondary pollutants [5].
Up to date, researchers have developed a variety of catalysts for the removal of VOCs, mainly including precious metal materials [6,7] and transition-metal oxides [8,9]. Although the noble metal catalysts display excellent properties, the problems of high cost, easy sintering, and poisoning have not been effectively solved, which limit their further practical applications [10]. Compared to noble-metal materials, transition metal oxides are considered as the candidate catalysts for VOCs oxidation due to their low cost, high thermal stability, and considerable reducibility [11][12][13][14]. Recently, the mixed metal oxide catalysts have attracted increasing attention due to their synergistic effect with many beneficial properties [15]. Of the mixed metal oxide-based catalysts, Fe-Mn oxide catalysts have  [12]. The peak intensity of Pal on X%Mn-Pal catalysts were decreased compared to that of Pal-Raw, which is likely due to the poor crystallinity and the high distribution of Mn oxides on Pal. The XRF result shows that roughly 7.65% of Fe 2 O 3 was detected in Pal, but no XRD reflections of α-Fe 2 O 3 can be observed, which should be ascribed to the low crystallinity of α-Fe 2 O 3 and the detective restriction of XRD instrument.
The manganese oxides on the surface of Pal were further characterized by Raman spectroscopy. As presented in Figure 1b, all X%Mn-Pal catalysts display a strong band at 650 cm −1 corresponding to a characteristic symmetric stretch of Mn-O-Mn of Mn 2 O 3 , while a weak peak at 311 cm −1 is consistent with the asymmetric stretch of Mn-O-Mn. These results are in good agreement with the previous reports [28,29]. The bands relating to α-Fe 2 O 3 can be obviously observed in Raman spectrum, which locates at 398 cm −1 and 1317 cm −1 [30]. Therefore, based on the XRD and Raman results, the Fe-rich palygorskite supported manganese oxide catalysts were successfully prepared. The manganese oxides on the surface of Pal were further characterized by Raman spectroscopy. As presented in Figure 1b, all X%Mn-Pal catalysts display a strong band at ~650 cm −1 corresponding to a characteristic symmetric stretch of Mn-O-Mn of Mn2O3, while a weak peak at 311 cm −1 is consistent with the asymmetric stretch of Mn-O-Mn. These results are in good agreement with the previous reports [28,29]. The bands relating to α-Fe2O3 can be obviously observed in Raman spectrum, which locates at 398 cm −1 and 1317 cm −1 [30]. Therefore, based on the XRD and Raman results, the Fe-rich palygorskite supported manganese oxide catalysts were successfully prepared.

TEM Analysis
The TEM and EDS-mapping images of 15%Mn-Pal are shown in Figure 2. Palygorskite displays the typical rod-like structure with a diameter of 20 nm in width (Figure 2a, b). Moreover, Figure. 2c-e shows that the distributions of Al, Mg, and Si further confirm the rod-like structure of Pal. The EDS-mapping images show that Fe highly disperses on the surface of Pal ( Figure 2f); meanwhile, Mn has the similar distribution to that of Fe ( Figure 2g). This result indicated that Mn might have intimate interaction with Fe on the surface of Pal. Furthermore, the lattice spacing of the 15%Mn-Pal catalyst was clearly calculated, and according to the previous reports, the obvious lattice fringes with the spacing of 0.27  The manganese oxides on the surface of Pal were further characterized by Raman spectroscopy. As presented in Figure 1b, all X%Mn-Pal catalysts display a strong band at ~650 cm −1 corresponding to a characteristic symmetric stretch of Mn-O-Mn of Mn2O3, while a weak peak at 311 cm −1 is consistent with the asymmetric stretch of Mn-O-Mn. These results are in good agreement with the previous reports [28,29]. The bands relating to α-Fe2O3 can be obviously observed in Raman spectrum, which locates at 398 cm −1 and 1317 cm −1 [30]. Therefore, based on the XRD and Raman results, the Fe-rich palygorskite supported manganese oxide catalysts were successfully prepared.

TEM Analysis
The TEM and EDS-mapping images of 15%Mn-Pal are shown in Figure 2. Palygorskite displays the typical rod-like structure with a diameter of 20 nm in width (Figure 2a, b). Moreover, Figure. 2c-e shows that the distributions of Al, Mg, and Si further confirm the rod-like structure of Pal. The EDS-mapping images show that Fe highly disperses on the surface of Pal ( Figure 2f); meanwhile, Mn has the similar distribution to that of Fe ( Figure 2g). This result indicated that Mn might have intimate interaction with Fe on the surface of Pal. Furthermore, the lattice spacing of the 15%Mn-Pal catalyst was clearly calculated, and according to the previous reports, the obvious lattice fringes with the spacing of 0.27 Furthermore, the lattice spacing of the 15%Mn-Pal catalyst was clearly calculated, and according to the previous reports, the obvious lattice fringes with the spacing of 0.27 and 0.254 nm corresponding to the (222) plane of Mn 2 O 3 [31] and the (110) plane of Fe 2 O 3 [32], respectively.

XPS Analysis
XPS experiments were performed to understand the surface elemental chemical state of the as-prepared catalysts. The results were shown in Figure 3 and the ratios of various elements for the as-prepared samples were summarized in Table 1. The survey spectrum demonstrated that O, Mn, and Fe species were found in the samples. In the Figure 3b, the Mn 2p 3/2 spectra could be divided into three components corresponding to the surface Mn 4+ (~643.4 eV), Mn 3+ (~641.9 eV), Mn 2+ (~640.8 eV) species, respectively [17,33,34]. Meanwhile, the 56.1% of Mn 3+ contents for 15%Mn-Pal is higher than that of 20%Mn-Pal (49.8%) (as shown in Table 1). According to the previous reports [35,36], Mn 3+ plays an important role in the oxidation of VOCs, and the content of Mn 3+ is used to evaluate the concentration of oxygen vacancy, while more oxygen vacancies favored the VOCs oxidation at low temperature.
Mn 4+ (~643.4 eV), Mn 3+ (~641.9 eV), Mn 2+ (~640.8 eV) species, respectively [17,33,34]. Meanwhile, the 56.1% of Mn 3+ contents for 15%Mn-Pal is higher than that of 20%Mn-Pal (49.8%) (as shown in Table 1). According to the previous reports [35,36], Mn 3+ plays an important role in the oxidation of VOCs, and the content of Mn 3+ is used to evaluate the concentration of oxygen vacancy, while more oxygen vacancies favored the VOCs oxidation at low temperature.  Three kinds of surface oxygen species were clearly observed from the O1s spectra in Figure 3c. The one peak located at ~530.1 eV was is assigned to the lattice oxygen species (Olatt), and the peaks located at around 531.6 eV and 532.5 eV are ascribed to the surface adsorbed oxygen (Oads) and surface hydroxyl oxygen (OOH) species [22,37], respectively. Literature demonstrates that the surface adsorbed oxygen species play an important role in the deep oxidation of VOCs [12,38]. The higher content of surface adsorbed oxygen indicates more oxygen vacancies exist on the surface of the catalysts [39]. As shown in Table 1, 15%Mn-Pal (57.4%) shows a relatively higher surface adsorbed oxygen, which is consistent with the content of Mn 3+ .
The Fe 2p3/2 region is composed of two peaks, the peak at around 711.2 eV is indicative of the presence of Fe 2+ , whereas the other peak at 712.9 eV should be assigned to the surface Fe 3+ [17,31]. The Fe 3+ content of 15%Mn-Pal (66.7%) is higher than that of 20%Mn-Pal (54.1%). The large amount of Fe 3+ requires the generation of free electron in the crystal  Three kinds of surface oxygen species were clearly observed from the O1s spectra in Figure 3c. The one peak located at~530.1 eV was is assigned to the lattice oxygen species (O latt ), and the peaks located at around 531.6 eV and 532.5 eV are ascribed to the surface adsorbed oxygen (O ads ) and surface hydroxyl oxygen (O OH ) species [22,37], respectively. Literature demonstrates that the surface adsorbed oxygen species play an important role in the deep oxidation of VOCs [12,38]. The higher content of surface adsorbed oxygen indicates more oxygen vacancies exist on the surface of the catalysts [39]. As shown in Table 1, 15%Mn-Pal (57.4%) shows a relatively higher surface adsorbed oxygen, which is consistent with the content of Mn 3+ .
The Fe 2p 3/2 region is composed of two peaks, the peak at around 711.2 eV is indicative of the presence of Fe 2+ , whereas the other peak at 712.9 eV should be assigned to the surface Fe 3+ [17,31]. The Fe 3+ content of 15%Mn-Pal (66.7%) is higher than that of 20%Mn-Pal (54.1%). The large amount of Fe 3+ requires the generation of free electron in the crystal matrix to achieve charge balance via the reaction of Fe 2+ + Mn 4+ → Fe 3+ + Mn 3+ , which favors the total oxidation of toluene [40].

Electrochemical Property
The electrochemical properties of the as-prepared samples were also investigated. As shown in Figure 4a, the 15%Mn-Pal catalyst has higher anodic and catholic currents, and the cyclic voltammetry (CV) curve area is larger than that of the other catalysts. The results demonstrated that 15%Mn-Pal has higher electrical activity which is beneficial to electrochemical reactions [41,42]. The Tafel curve in Figure 4b shows the similar trend with that of the CV results, in which 15%Mn-Pal shows a lower negative potential than those of the other samples. This result demonstrated the easier electron transfer occurring in 15%Mn-Pal, which is conducive to the redox reactions, and thus, enhancing its catalytic activity for toluene oxidation [43].
The electrochemical properties of the as-prepared samples were also investigated. As shown in Figure 4a, the 15%Mn-Pal catalyst has higher anodic and catholic currents, and the cyclic voltammetry (CV) curve area is larger than that of the other catalysts. The results demonstrated that 15%Mn-Pal has higher electrical activity which is beneficial to electrochemical reactions [41,42]. The Tafel curve in Figure 4b shows the similar trend with that of the CV results, in which 15%Mn-Pal shows a lower negative potential than those of the other samples. This result demonstrated the easier electron transfer occurring in 15%Mn-Pal, which is conducive to the redox reactions, and thus, enhancing its catalytic activity for toluene oxidation [43].

BET Analysis
It is generally believed that the catalytic activity of catalysts is closely related to their surface texture. The N2 adsorption-desorption isotherms and pore size distributions of all samples were displayed in Figure 5. It can be seen from Figure 5a that these catalysts all showed typical type IV adsorption-desorption isotherms with H3 hysteresis loop, and indicated that mesopores were formed in all the catalysts. Figure 5b showed the pore size distributions of each sample, which further confirmed the existence of mesoporous structure in the catalysts. Specifically, the textural parameters of the different catalysts were listed in Table 2. It was observed that the 15%Mn-Pal catalyst showed the maximum surface area (82.7 m 2 /g) and biggest pore volume (0.17 cm 3 /g), which may promote the adsorption of toluene on the surface of catalyst, and contribute to the improved catalytic performance.

BET Analysis
It is generally believed that the catalytic activity of catalysts is closely related to their surface texture. The N 2 adsorption-desorption isotherms and pore size distributions of all samples were displayed in Figure 5. It can be seen from Figure 5a that these catalysts all showed typical type IV adsorption-desorption isotherms with H 3 hysteresis loop, and indicated that mesopores were formed in all the catalysts. Figure 5b showed the pore size distributions of each sample, which further confirmed the existence of mesoporous structure in the catalysts. Specifically, the textural parameters of the different catalysts were listed in Table 2. It was observed that the 15%Mn-Pal catalyst showed the maximum surface area (82.7 m 2 /g) and biggest pore volume (0.17 cm 3 /g), which may promote the adsorption of toluene on the surface of catalyst, and contribute to the improved catalytic performance.

Mechanism Analysis
In situ DRIFTS experiments were conducted to understand the reaction mechanism of toluene oxidation over 15%Mn-Pal. Figure 6a shows a typical set of temperature dependent DRIFTS spectra during the toluene oxidation process over the 15%Mn-Pal catalyst under 25-275 °C. When toluene was introduced into the reaction system at 25 °C for 30 min, a series of peaks can be observed. The peak at 3029 cm −1 is the signal of C-H stretching vibration on aromatic ring, while two peaks at 2927 cm −1 and 2875 cm −1 represent the methyl and methylene species, respectively [44]. Strong broad band at 1635 cm −1 (δ(H-O-H)) is ascribed to the isolated -OH species on the catalyst surface [45], and it gradually disappeared as the temperature increased. In addition, the peak at 1497 cm −1 (ν(C=C)) is ascribed to the aromatic ring in-plane vibration [46]. These absorption bands indicated that the toluene was adsorbed on the surface of the 15%Mn-Pal. The adsorption and activation of toluene on the catalyst surface changed significantly as the temperature increased. The intensities of these bands at 1497 cm −1 and 3029 cm −1 decreased and gradually disappeared with the increase in the reaction temperature. Meanwhile, a weak and  In situ DRIFTS experiments were conducted to understand the reaction mechanism of toluene oxidation over 15%Mn-Pal. Figure 6a shows a typical set of temperature dependent DRIFTS spectra during the toluene oxidation process over the 15%Mn-Pal catalyst under 25-275 • C. When toluene was introduced into the reaction system at 25 • C for 30 min, a series of peaks can be observed. The peak at 3029 cm −1 is the signal of C-H stretching vibration on aromatic ring, while two peaks at 2927 cm −1 and 2875 cm −1 represent the methyl and methylene species, respectively [44]. Strong broad band at 1635 cm −1 (δ(H-O-H)) is ascribed to the isolated -OH species on the catalyst surface [45], and it gradually disappeared as the temperature increased. In addition, the peak at 1497 cm −1 (ν(C=C)) is ascribed to the aromatic ring in-plane vibration [46]. These absorption bands indicated that the toluene was adsorbed on the surface of the 15%Mn-Pal. The adsorption and activation of toluene on the catalyst surface changed significantly as the temperature increased. The intensities of these bands at 1497 cm −1 and 3029 cm −1 decreased and gradually disappeared with the increase in the reaction temperature. Meanwhile, a weak and new band at 1176 cm −1 was found, which can be assigned to the C-O stretching vibration of benzyl alcohol (C 6 H 5 -CH 2 O-). This result indicates that the toluene adsorbed on the surface of catalyst can be converted into benzyl alcohol [12]. In the high wavenumber range, the peak appears at 3068 cm −1 is attributed to the υ(CH-) stretching vibration of methyl group [12]. Additionally, two peaks appear at 1228 cm −1 and 1257 cm −1 may belong to the phenolate (C 6 H 5 -OH) species [44], and the peak located at 1535 cm −1 can correspond to the asymmetric υ(C=O) stretching vibration of a typical carboxylate group [12]. Furthermore, the peak at 1597 cm −1 can be ascribed to the skeleton υ(C=C) vibration of the aromatic ring [47], and the 1405 cm −1 is a C-O symmetric stretching vibration, indicating that the formation of benzoate. The band located at 1357 cm −1 is the characteristic of acetate species [15]. With the temperature increased from 150 to 275 • C, the intensities of 1597 and 1535 cm −1 increased first and then decreased; meanwhile, the peak of surface maleic anhydride species at 1305 cm −1 and 1357 cm −1 gradually increased at 200 • C, indicating that these intermediates were gradually accumulated on the catalyst. This phenomenon can be explained by the further conversion of benzyl alcohol and benzoate to anhydride and acetate. Afterwards, these species can be deeply oxidized to CO 2  The characteristic IR peak of the intermediates (1176 cm −1 benzyl alcohol, 1535 cm −1 benzoate, 1305 cm −1 anhydride, and 1357 cm −1 acetate) is calculated by integral area, and the results were shown in Figure 6b. The amount of benzyl alcohol species initially increased when the temperature was lower than 225 °C, and then decreased. The formation  The characteristic IR peak of the intermediates (1176 cm −1 benzyl alcohol, 1535 cm −1 benzoate, 1305 cm −1 anhydride, and 1357 cm −1 acetate) is calculated by integral area, and the results were shown in Figure 6b. The amount of benzyl alcohol species initially increased when the temperature was lower than 225 • C, and then decreased. The formation of the four intermediates on the surface of the 15%Mn-Pal was accumulated to the maximum at the reaction temperature of 225 • C. Among them, the benzoate species has the largest amount on the surface of the 15%Mn-Pal, indicating that the aromatic ring of benzoate species is difficult to break.
Based on the temperature-dependent DRIFTS results, it suggests that the toluene oxidation reaction occurs via the benzyl alcohol → benzoate → anhydride →acetate pathway, and as shown in Figure 7. The toluene adsorbed on the catalyst surface first undergoes a dehydrogenation with oxygen to form alcohols, and then oxidized into benzoate, anhydride and CO 2 and H 2 O. Furthermore, the deep oxidation of benzoate species or the C=C breakage of the aromatic ring could be the rate-controlling step in the toluene oxidation.
acteristic IR peak area of four intermediates versus temperature.
The characteristic IR peak of the intermediates (1176 cm −1 benzyl alcohol, 1 benzoate, 1305 cm −1 anhydride, and 1357 cm −1 acetate) is calculated by integral a the results were shown in Figure 6b. The amount of benzyl alcohol species ini creased when the temperature was lower than 225 °C, and then decreased. The fo of the four intermediates on the surface of the 15%Mn-Pal was accumulated to t mum at the reaction temperature of 225 °C. Among them, the benzoate specie largest amount on the surface of the 15%Mn-Pal, indicating that the aromatic rin zoate species is difficult to break.
Based on the temperature-dependent DRIFTS results, it suggests that the to idation reaction occurs via the benzyl alcohol → benzoate → anhydride →acet way, and as shown in Figure 7. The toluene adsorbed on the catalyst surface fir goes a dehydrogenation with oxygen to form alcohols, and then oxidized into b anhydride and CO2 and H2O. Furthermore, the deep oxidation of benzoate spec C=C breakage of the aromatic ring could be the rate-controlling step in the toluen tion.

Catalytic Performance
The catalytic activities of as-prepared catalysts for toluene oxidation were ev The component of the reaction gas mixture contains 1000 ppm toluene, the total 100 mL/min, and the gas hourly space velocity (GHSV) was 20,000 mL/(g⋅h). The ship between the reaction temperature and toluene conversion is shown in Figu   Figure 7. Proposed mechanism for toluene oxidation.

Catalytic Performance
The catalytic activities of as-prepared catalysts for toluene oxidation were evaluated. The component of the reaction gas mixture contains 1000 ppm toluene, the total flow was 100 mL/min, and the gas hourly space velocity (GHSV) was 20,000 mL/(g·h). The relationship between the reaction temperature and toluene conversion is shown in Figure 8, and the T 50 and T 90 (the temperatures for 50% and 90% conversion of toluene, respectively) are used to evaluate the catalytic performance of the catalysts. As shown in Figure 7, Pal-Raw has no obvious effect on toluene oxidation. The conversion of toluene was less than 6% in the entire temperature range, which should be due to the adsorption of toluene on the surface of Pal-Raw. Compared with Pal-Raw, the catalytic activity of 5%Mn-Pal was significantly improved, and the T 50 and T 90 were 223 • C and 248 • C, respectively. This result indicates that Mn plays an important role in the catalytic oxidation of toluene. With the Mn loading increased from 5% to 20%, the catalyst activity increased first and then decreased, the order of catalytic activity is 15%Mn-Pal > 20%Mn-Pal > 10%Mn-Pal > 5%Mn-Pal. Compared to 15%Mn-Pal, the lower catalytic activity of 20%Mn-Pal is likely due to the aggregation of excessive Mn on the surface of the catalyst. Based on the above results, Pal is considered as an acceptable support to synthesis the supported catalysts for toluene elimination.
Mn loading increased from 5% to 20%, the catalyst activity increased first and then decreased, the order of catalytic activity is 15%Mn-Pal > 20%Mn-Pal > 10%Mn-Pal > 5%Mn-Pal. Compared to 15%Mn-Pal, the lower catalytic activity of 20%Mn-Pal is likely due to the aggregation of excessive Mn on the surface of the catalyst. Based on the above results, Pal is considered as an acceptable support to synthesis the supported catalysts for toluene elimination.

Stability Test
It is well known that the stability of the catalyst determines its potential application to some extent. Therefore, three consecutive catalytic cycling experiments were carried out on the 15%Mn-Pal catalyst to investigate its catalytic stability, and the results are shown in Figure 9. It is found that the toluene conversion of 15%Mn-Pal catalyst keeps almost unchanged during the four-cycle tests, indicating its good stability for toluene oxidation. In addition, to further examine the possible application of catalyst in real exhaust conditions, the effect of water vapor on the catalytic performance of 15%Mn-Pal was explored at the GHSV of 20,000 mL/(g⋅h). As shown in Figure 10a, the toluene conversion decreased but remained above 90% after 5 vol% of water vapor was introduced into the stream at 250 °C at least for 5 h. After turning off the addition of water vapor, the toluene conversion was perfectly recovered. This result demonstrated that water vapor has little influence on the catalytic activity of 15%Mn-Pal for toluene conversion. The effect of different GHSV on the catalytic activity of the 15%Mn-Pal catalyst has also been investigated, as shown in Figure 10b. Obviously, the catalytic activity of 15%Mn-Pal is influenced by the GHSV values. With the GHSV increased from 20,000 to 60,000 mL/(g⋅h), the curves of toluene conversion versus reaction temperature shift to a higher temperature range. Nevertheless, the T50 and T90 of toluene conversion only increased by approximately 10 °C. The results indicated that the GHSV has a moderate effect on the catalytic activity of the catalyst. According to the above results, the prepared catalyst has persistent catalytic performance under the simulated real exhaust conditions.

Stability Test
It is well known that the stability of the catalyst determines its potential application to some extent. Therefore, three consecutive catalytic cycling experiments were carried out on the 15%Mn-Pal catalyst to investigate its catalytic stability, and the results are shown in Figure 9. It is found that the toluene conversion of 15%Mn-Pal catalyst keeps almost unchanged during the four-cycle tests, indicating its good stability for toluene oxidation. In addition, to further examine the possible application of catalyst in real exhaust conditions, the effect of water vapor on the catalytic performance of 15%Mn-Pal was explored at the GHSV of 20,000 mL/(g·h). As shown in Figure 10a, the toluene conversion decreased but remained above 90% after 5 vol% of water vapor was introduced into the stream at 250 • C at least for 5 h. After turning off the addition of water vapor, the toluene conversion was perfectly recovered. This result demonstrated that water vapor has little influence on the catalytic activity of 15%Mn-Pal for toluene conversion. The effect of different GHSV on the catalytic activity of the 15%Mn-Pal catalyst has also been investigated, as shown in Figure 10b. Obviously, the catalytic activity of 15%Mn-Pal is influenced by the GHSV values. With the GHSV increased from 20,000 to 60,000 mL/(g·h), the curves of toluene conversion versus reaction temperature shift to a higher temperature range. Nevertheless, the T 50 and T 90 of toluene conversion only increased by approximately 10 • C. The results indicated that the GHSV has a moderate effect on the catalytic activity of the catalyst. According to the above results, the prepared catalyst has persistent catalytic performance under the simulated real exhaust conditions.

Kinetic Study
The kinetics study of x%Mn-Pal catalysts for toluene oxidation were investigated in the absence of internal and external diffusion limitations. To obtain the reliable kinetic data, the toluene conversion should be controlled below 20% [37]. The apparent activation energy (Ea, kJ/mol) for toluene oxidation can be calculated according to the following equations: where rtoluene denotes the reaction rate (rtoluene, molC7H8/g/h), Cin is the concentration of toluene in the gas mixture (ppm), η is the toluene conversion, F is the total flow rate (mL/min), m is the catalyst weight (g), T is the reaction temperature, R is the gas constant, and Ea (kJ/mol) is the apparent activation energy. As can be seen from Figure 11a, the 15%Mn-Pal catalyst showed the highest toluene conversion than that of 10%Mn-Pal and 20%Mn-Pal at the same reaction temperatures. According to the toluene conversion, the Ea was evaluated and shown in Figure 11b. It is evidence that the activation energy of 15%Mn-Pal (125.7 kJ/mol) was lower than those of 10%Mn-Pal (157.9 kJ/mol) and 20%Mn-Pal (142.4 kJ/mol), which indicates the excellent catalytic activity of 15%Mn-Pal for toluene oxidation.

Kinetic Study
The kinetics study of x%Mn-Pal catalysts for toluene oxidation were investigated in the absence of internal and external diffusion limitations. To obtain the reliable kinetic data, the toluene conversion should be controlled below 20% [37]. The apparent activation energy (E a , kJ/mol) for toluene oxidation can be calculated according to the following equations: where r toluene denotes the reaction rate (r toluene , molC 7 H 8 /g/h), C in is the concentration of toluene in the gas mixture (ppm), η is the toluene conversion, F is the total flow rate (mL/min), m is the catalyst weight (g), T is the reaction temperature, R is the gas constant, and Ea (kJ/mol) is the apparent activation energy. As can be seen from Figure 11a, the 15%Mn-Pal catalyst showed the highest toluene conversion than that of 10%Mn-Pal and 20%Mn-Pal at the same reaction temperatures. According to the toluene conversion, the Ea was evaluated and shown in Figure 11b. It is evidence that the activation energy of 15%Mn-Pal (125.7 kJ/mol) was lower than those of 10%Mn-Pal (157.9 kJ/mol) and 20%Mn-Pal (142.4 kJ/mol), which indicates the excellent catalytic activity of 15%Mn-Pal for toluene oxidation.
Catalysts 2022, 12, x FOR PEER REVIEW 10 of 14 Figure 11. The Arrhenius fitting curves (a) and the toluene conversion with different reaction temperatures (b) over as-prepared catalysts. Reaction condition: catalyst weight 0.1 g, total flow rate 100 mL/min; GHSV 60,000 mL/(g⋅h), toluene 1000 ppm.

Discussion
It is well known that the catalytic activity of the Mn oxide catalysts is affected by their physical and chemical properties, such as the active component, specific surface area, the valence state of Mn, and the surface adsorbed oxygen or lattice oxygen concentration [7]. In this work, the x%Mn-Pal catalysts were successfully obtained. It was found that Mn oxides are the main active components and the content of Mn affected the catalytic performance in toluene oxidation. The higher Mn content favors the catalytic activity of Figure 11. The Arrhenius fitting curves (a) and the toluene conversion with different reaction temperatures (b) over as-prepared catalysts. Reaction condition: catalyst weight 0.1 g, total flow rate 100 mL/min; GHSV 60,000 mL/(g·h), toluene 1000 ppm.

Discussion
It is well known that the catalytic activity of the Mn oxide catalysts is affected by their physical and chemical properties, such as the active component, specific surface area, the valence state of Mn, and the surface adsorbed oxygen or lattice oxygen concentration [7]. In this work, the x%Mn-Pal catalysts were successfully obtained. It was found that Mn oxides are the main active components and the content of Mn affected the catalytic performance in toluene oxidation. The higher Mn content favors the catalytic activity of x%Mn-Pal catalyst, but excessive Mn loading inhibited the catalytic activity for toluene conversion due to the aggregation of Mn oxides [48]. The 15%Mn-Pal had the highest catalytic activity for toluene oxidation among these samples, which obtained 90% conversion of toluene at 227 • C and had excellent catalytic stability and water resistance.
In addition, the concentrations of Mn 3+ and O ads species of catalyst were also crucial for the deep oxidation of VOCs [49,50]. It has proved that higher Mn 3+ and O ads are conducive to promote the formation of oxygen vacancies, and then produce more reactive oxygen species, which is beneficial to improve the catalytic activity of samples [51]. As shown in Table 1, the 15%Mn-Pal showed a higher Mn 3+ and O ads species than those of the other samples, which was consistent with the results of catalytic activity in Figure 7. Generally, the catalytic mechanism of toluene over Mn-based catalysts belongs to the Marsvan Krevelen mechanism, in which the adsorbed toluene molecules can react with the surface mobile O latt species to form CO 2 and H 2 O. Meanwhile, abundant surface adsorbed oxygen species provide the stronger oxygen mobility, which is conducive to supplementing gaseous oxygen through oxygen vacancies, thus, efficiently accelerating the replenishment of the O latt species in the reaction process and improving the catalytic performance [52,53]. Based on the characterization results, 15%Mn-Pal has more oxygen vacancies and stronger oxygen mobility than those of the other catalysts, exhibiting the best catalytic performance for toluene oxidation in this study.  2 ) solution 50 wt% were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals used in this work were analytical reagent (AR).

Catalyst Preparation
The Pal-supported catalysts were synthesized via a co-precipitation method. In a typical synthesis process, 10 g of the Pal was placed into a clean 500 mL beaker, and different amounts of NH 4 HCO 3 and 50 wt% Mn(NO 3 ) 2 solution (5, 10, 15, 20 wt% in weight percent) were dissolved into 200 mL ethanol and 100 mL deionized water to obtained clear I and II solutions, respectively. Then, the solution of I and II were added quickly to the beaker containing 10 g Pal under vigorous stirring, and the above-mixed solution was further aged at room temperature for 12 h. The formed precipitates were collected by the centrifugation method and washed several times with deionized water and ethanol. After that, the collected samples were dried at 105 • C overnight and then ground to a particle size of 40~60 mesh. The samples were calcinated at 400 • C in the air for 4 h with a heating rate of 5 • C/min. The as-prepared catalysts were labeled as X%Mn-Pal, where X denotes as the loading content of Mn in weight percent, and Pal-Raw represents the natural palygorskite. As shown in Table 2, the actual Mn loadings of the as-prepared catalysts measured by the flame atomic absorption spectrometer, which was in good agreement with the theoretical calculated value, indicating that the X%Mn-Pal catalyst was successfully prepared.

Catalyst Characterization
The X-ray diffraction (XRD) patterns of catalysts were recorded on an X-Ray diffractometer (Dandong Haoyuan-DX-2700, Dandong, China) using Cu-Kα radiation generated at 50 kV and 100 mA in the 5 and 70 • range. Raman spectra of catalysts were obtained using a LabRAM HR Evolution with a 532 nm line. Transmission electron microscopy (TEM) measurements were obtained on a JEM-2100 microscope with an energy dispersive spectrometer. The X-ray photoelectron spectroscopy (XPS) signals of catalysts were measured on a Thermo Scientific K-Alpha instrument equipped with an Al Kα source. The specific surface area was measured by N 2 adsorption-desorption at liquid nitrogen temperature using a Quantachrome NOVA 3000e analyzer. The electrochemical properties of the catalysts were obtained using an electrochemical workstation (CHI-660, Chenhua, Shanghai, China). Real Mn loading of the catalysts determined by the flame atomic absorption spectrometer (WYS2200, WAYEE, Hefei, China). In situ DRIFTS was carried on a spectrometer (VERTEX 70, Bruker, Karlsruhe, Germany) in the range of 400~4000 cm −1 with a resolution of 4 cm −1 and an accumulation of 64 scans.

Catalytic Activity Measurement
The dynamic activities of the as-prepared catalysts for toluene oxidation were carried out in a homemade fixed-bed quartz reactor. For each test, 0.4 g catalyst (40-60 mesh) was placed in the middle of the reactor (inner diameter 8 mm) and supported by quartz wool. The required concentration of toluene (1000 ppm) was injected by a syringe pump and balanced by pure air with a total flow of 100 mL/min. To avoid the condensation of toluene vapor, a heating tape with 60 • C was attached to the gas pipeline. A gas chromatograph (GC-9790II, FULI) equipped with an FID detector was used to monitor the trend of toluene concentration online. The toluene conversion (η, %) was calculated according to the following equation: where C in and C out are the influent and effluent concentration of toluene.

Conclusions
In this paper, x%Mn-Pal catalysts with different Mn contents were successfully prepared via co-precipitation method and their catalytic performance for toluene combustion was evaluated. It was found that 15%Mn-Pal catalyst exhibited the highest catalytic activity and 90% conversion of toluene was obtained at 227 • C, and exhibited practical cycle stability. Moreover, the relationship between the catalytic performance and structure was deeply investigated by various characterizations, including XRD, Raman, TEM, BET, XPS, and in situ DRIFTS. The results showed that the addition of Mn promotes the catalytic oxidation of toluene. The 15%Mn-Pal catalyst has the maximum surface area of 82.7 m 2 /g, which is conducive to the adsorption of toluene on the catalyst surface. In addition, 15%Mn-Pal catalyst possessed more oxygen vacancies, which were related to the higher contents of Mn 3+ and O ads species. The large number of oxygen vacancies lead to an increase in oxygen mobility and beneficial to the catalytic oxidation of toluene. This study implied that Fe-rich palygorskite can be used as the low cost and environmentally friendly catalyst support for catalytic oxidation of VOCs.