Role of Cryptomelane in Surface-Adsorbed Oxygen and Mn Chemical

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C 2019, 123, 17255−17264

Role of Cryptomelane in Surface-Adsorbed Oxygen and Mn Chemical Valence in MnOx during the Catalytic Oxidation of Toluene Xuejun Zhang,† Zi’ang Ma,† Zhongxian Song,*,‡ Heng Zhao,† Wei Liu,*,† Min Zhao,† and Jinggang Zhao† †

Shenyang University of Chemical Technology, Shenyang 110142, People’s Republic of China Faculty of Environmental and Municipal Engineering, Henan Key Laboratory of Water Pollution Control and Rehabilitation Technology, Henan University of Urban Construction, Pingdingshan 467036, People’s Republic of China

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‡

ABSTRACT: A series of manganese oxide catalysts were prepared via a coprecipitation method by adjusting the molar ratio of Mn(NO3)2/KMnO4 and used to catalyze the oxidation of toluene. The MnOx catalyst prepared by the Mn(NO3)2/KMnO4 ratio of 3:7 showed the best catalytic activity, and more than 90% conversion was obtained at 238 °C. The superior catalytic activity of MnOx could be attributed to its slightly higher specific surface area and better pore size distribution. Furthermore, more surface-adsorbed oxygen and Mn3+ species provided more oxygen vacancies, which could increase the ability of oxygen migration, resulting in the improvement of the redox performance and catalytic activity, which depended on the Mn(NO3)2/KMnO4 ratios. Importantly, the formation of cryptomelane (KMn8O16) contributed greatly to the toluene catalytic oxidation ability and CO2 selectivity.

1. INTRODUCTION Volatile organic compounds (VOCs) are the main air pollutants in today’s society, which are harmful to the human health and environment.1,2 There are many ways to remove VOCs, such as adsorption, thermal combustion, catalytic combustion, and using biofilms.3−6 Among these removal techniques, catalytic combustion is one of the promising methods for removing VOCs because of its high efficiency, low temperature, and low cost.7 Although precious metal catalysts showed excellent catalytic performance, high price and easier sintering limited its further applications.8,9 In contrast, transition metal catalysts have attracted widespread attention because of their lower cost and splendid stability.10,11 The manganese-based catalysts were usually considered to be a powerful alternative to precious metal catalysts for lowtemperature combustion of VOCs due to the higher catalytic performance.12−15 Generally, the oxidative capacity and variable valence of Mn species were the main factors influencing its catalytic activity.16,17 Piumetti et al.18 reported that the Mn3O4 catalyst exhibited excellent catalytic performance in the catalytic oxidation of toluene. Hou et al.19 found that the doping of MnOx into Ce−Zr composite oxide could enhance the oxygen mobility and the redox performance, resulting in an increase in catalytic performance. Likewise, Qu et al.20 revealed that the doping of Co into MnOx could improve the concentration of reducible oxygen species, which was in favor of the catalytic activity. It was obvious that the addition of other metal oxides into MnOx could increase the catalytic performance. However, Santos et al.21 demonstrated that the manganese-based catalyst prepared by KMnO4 could improve the oxidation ability. Li et al.22 prepared CeO2−MnO2 by the mixtures of KMnO4 and Ce species, which could © 2019 American Chemical Society

improve the generation of more oxygen vacancies and the migration of lattice oxygen, improving the catalytic performance. It was reported in the previous literature23 that the cryptomelane-type manganese oxides were prepared by a solvent-free method, which presented much higher activity for ethyl acetate oxidation than the platinum-based catalyst. Ma et al.24 proved that the doping of cerium, cobalt, and iron into cryptomelane-type manganese oxide (M-OMS-2) catalysts could improve the surface defects and the content of Mn3+, causing the improvement in the decomposition of ozone. Silver-modified cryptomelane-type manganese oxide could lead to a partial decomposition of the ordered structure of manganese oxide, which could increase the reducibility and activity of the catalyst.25 Hernández et al.26 prepared Cumodified cryptomelane by a solid-state reaction, which promoted the lattice oxygen mobility and availability and then showed high catalytic activity for CO oxidation. Based on the above discussion, it was obvious that the Mn species, structure and oxygen species over catalysts with cryptomelane were responsible for the outstanding catalytic performance. In brief, the MnOx catalysts prepared by the introduction of KMnO4 could improve the catalytic oxidation ability and then promoted the catalytic activity. Furthermore, the oxidation capacity over catalysts played an important role in the catalytic oxidation of toluene.20 In the present work, a series of manganese oxide catalysts were prepared by the coprecipitation of Mn(NO3)2 and KMnO4 and used to the catalytic oxidation of toluene. The aim Received: March 16, 2019 Revised: June 25, 2019 Published: June 25, 2019 17255

DOI: 10.1021/acs.jpcc.9b02499 J. Phys. Chem. C 2019, 123, 17255−17264

Article

The Journal of Physical Chemistry C

quartz tube and pretreated in a stream of pure N2. The temperature was raised to 400 °C at a rate of 20 °C/min for 40 min and then cooled to 100 °C. Thereafter, the sample was reduced with a 5% H2 and Ar gas mixture at a total flow rate of 30 mL/min. At the same time, the H2 consumption was recorded by a TCD while the temperature was gradually increased to 900 °C. To demonstrate the ability of the sample to bind oxygen, O2 temperature-programmed desorption (O2-TPD) was performed by a chemical adsorption instrument (Beijing Builder Electronic Technology Company, PCA-1200). The sample (200 mg) was used and pretreated with pure He gas at 200 °C for 60 min. The sample was cooled to 50 °C, and a He gas mixture with 20 vol % O2 was passed over the sample at a rate of 30 mL/min for 60 min. Finally, the O2 consumption was recorded by a TCD while the temperature was gradually increased to 900 °C. 2.4. Catalytic Activity. Catalytic evaluation of toluene was carried out on a fixed-bed reactor (i.d. = 8 mm, o.d. = 10 mm) by loading 0.1 mL of catalyst (40−60 mesh). The catalytic activity was measured at a temperature of 180−300 °C, and the temperature was obtained by a K-type thermocouple in a quartz tube. In the test, the feed was set to a toluene concentration of 500 ppm, the flowing N2 mixture contained 20% by volume of O2, and the total flow rate was 100 mL/min. The concentration of toluene in the inlet and outlet gases was measured by a FULI 9790 II online gas chromatograph equipped with FID detectors. To avoid deviations caused by gas adsorption, all tests were started after the reaction was stable for 30 min. The toluene conversion rate was calculated by the following formula

of this article was to study the effect of surface oxygen species and Mn chemical valence on complete oxidation of toluene over the MnOx catalysts. Therewith, the catalysts were characterized by X-ray diffraction (XRD), temperatureprogrammed studies (H2-TPR, O2-TPD), Raman spectroscopy, transmission electron microscope (TEM), N2 adsorption− desorption isotherm tests, and X-ray photoelectron spectroscopy (XPS).

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. All reagents were of AR grade and used directly without purification. Manganese(II) nitrate (Mn(NO3)2), concentrated ammonia (NH3·H2O), and KMnO4 were purchased from Shenyang-Laboratory Science and Trading Co., Ltd. (Shenyang, China). 2.2. Catalyst Preparation. A series of MnOx catalysts were composited by a coprecipitation method using different molar ratios of Mn(NO3)2/KMnO4. The appropriate amount of Mn(NO3)2 (23 mmol) was dissolved in 100 mL of H2O and stirred constantly. Subsequently, KMnO4 was added into the above solution to prepare the MnO2 catalyst with Mn(NO3)2/ KMnO4 molar ratios of 1:9, 3:7, 5:5, 7:3, and 9:1 and the samples were denoted Cat-1, Cat-2, Cat-3, Cat-4, and Cat-5, respectively. The mixture was vigorously stirred, and NH3·H2O was slowly added into it till the pH value reached 10. Then, it was transferred to a magnetic stirrer at 80 °C for 5 h. After water bath aging, the sample were cooled down to room temperature and stewed overnight. The precipitate was separated by extraction filtration, washed several times with distilled water until the pH reached 7, and then dried overnight at 105 °C. Finally, the samples were calcined at 550 °C for 5 h. For the control experiment, MnOx was prepared using manganese nitrate, expressed as Cat-0. For comparative experiments, 1.62 g of KNO3 was dissolved in 3 mL of deionized water to be sufficiently dissolved, followed by the addition of 1.38 g of Cat-0. It was stirred at room temperature for 2 h, then in a water bath at 60 °C for 4 h, and finally at 70 °C for 1 h and then dried overnight at 105 °C. Finally, the samples were calcined at 550 °C for 5 h. The sample was denoted as IM-2. 2.3. Catalyst Characterization. XRD of the powder was carried out on a Bruker D8 ADVANCE with a Cu Kα X-ray source (2θ range = 10−70°; step = 0.05°; step time = 3 s). Raman spectroscopy was performed on a Renishaw InVia 2000 Raman microscope (Renishaw plc, U.K.) using a 532 nm wavelength Ar-ion laser. The specific surface area (SBET) and the total pore volume (Vp) were determined out by a nitrogen adsorption− desorption isotherm at −196 °C using a physical adsorption instrument (Beijing Builder Electronic Technology Company, SSA-6000). The morphology and crystal structure of six samples were studied using a JEM-2010 transmission electron microscope (TEM) operating at 200 kV. XPS measurements were performed using an ESCALAB 250 photoelectron spectrometer (Thermo Fisher Scientific) with a monochromatic microfocus Al Kα X-ray source. The charge effect of the sample was corrected by setting the binding energy of the indefinite carbon (C 1s) at 284.6 eV. The H2 temperature-programmed reduction (H2-TPR) was measured on a chemical adsorption instrument (Beijing Builder Electronic Technology Company, PCA-1200). The required amount of sample (20 mg) was placed in a U-shaped

toluene conversion rate =

* − C toluene C toluene × 100% * C toluene

In this equation, Ctoluene * refers to the inlet concentration of toluene, while Ctoluene is the outlet concentration at different temperatures. Toluene oxidation was also characterized by the CO2 generation rate CO2 generation rate =

CCO2 × 100% * CCO 2

In the above equation, CCO2* is the concentration of CO2 in the effluent when toluene is oxidized completely and CCO2 is the one at different temperatures.

3. RESULTS AND DISCUSSION 3.1. Material Textural Properties. Figure 1 depicts the XRD patterns of the catalysts. Peaks with 2θ at 23.1, 32.9, 38.2, 45.1, 49.4, 55.2, 60.6, 64.1, and 65.8° were observed for Cat-0 and Cat-5, which were assigned to Mn2O3 (PDF #89-2809).27 Besides, the peaks with 2θ at 23.1, 32.9, 49.4, and 64.1° corresponding to Mn2O3 were also found for Cat-3 and Cat-4. With the addition of KMnO4, cryptomelane (KMn8O16) appeared in Cat-1, Cat-2, Cat-3, and Cat-4 (PDF #771796).28 Bastos et al.29 researched that KMn8O16 was powerfully active in VOC oxidation and improved the CO2 selectivity. Santos et al.30 proved that the formation of cryptomelane could improve the structural stability of the catalyst and then improved the mobility of lattice oxygen, resulting in the enhancement of the catalytic performance. 17256


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the Mn−O bond of trivalent manganese ions in MnO6 octahedra.31,32 It was worth nothing that the peaks of Cat-2, Cat-3, and Cat-4 samples became weaker and broader, indicating the existence of more oxygen vacancy defects. The increase in the concentration of oxygen vacancy defects significantly enhanced the reactivity of lattice oxygen, which greatly promoted the catalytic activity of VOCs. Besides, the increase of oxygen vacancy defects would lead to a weakening of the Mn−O bond and then improved the activity of lattice oxygen species.33 The peaks at 350 cm−1 for Cat-0, Cat-1, and Cat-2 were due to the asymmetric stretching of bridge oxygen species (Mn−O−Mn) and out-of plane bending modes of Mn2O3.28,34 In addition, Mn2O3 was not observed on Cat-1 and Cat-2 in XRD, which meant that Mn 2 O 3 was predominantly amorphous or was present as a microcrystal, which was not detected by the XRD technique. The N2 adsorption/desorption isotherms and BJH pore size distribution curves of the catalysts were presented in Figure 3. According to Figure 3, the isotherms of all of the samples belonged to the type IV isotherm, which were representatives of mesoporous materials.35 Besides, the H3-type hysteresis loop could be examined in the P/P0 scope of 0.8−1.0, meaning the presence of irregular porosity. Compared to those of the Cat-2 samples, the slopes of the adsorption−desorption isotherms on other samples minished at higher pressures and the closed dot shifted to a higher value, which indicated that the micropores and mesopores disappeared and the macropores formed, which led to a reduction of the surface area.36 As shown in Figure 3, the pore size distribution of all catalysts was located in the range of 6−80 nm. It is worth noting that the main pore size of Cat-2 was 20 nm, which was smaller than that of other samples. The Brunauer−Emmett−Teller (BET) surface area results are summarized in Table 1. The specific

Figure 1. XRD patterns of the Cat-0, Cat-1, Cat-2, Cat-3, Cat-4, and Cat-5 catalysts.

Thus, the change in the molar ratio of Mn(NO3)2/KMnO4 could affect the phase composition of Mn species and then affected the catalytic performance. To obtain more structural information, Raman spectroscopy was characterized, and the results are shown in Figure 2. It was

Table 1. Specific Surface Area, Total Pore Volume, and Average Pore Diameter of the Cat-0, Cat-1, Cat-2, Cat-3, Cat-4, and Cat-5 Catalysts

Figure 2. Raman spectra of the Cat-0, Cat-1, Cat-2, Cat-3, Cat-4, and Cat-5 catalysts.

obvious that the peak at 638 cm−1 was detected in all of the catalysts, which was attributed to the symmetric stretching of

samples

BET (m2/g)

total pore volume (cm3/g)

average pore diameter (nm)

Cat-0 Cat-1 Cat-2 Cat-3 Cat-4 Cat-5

11 17 21 20 20 11

0.119 0.136 0.152 0.190 0.227 0.115

44.8 32.06 28.9 37.9 45.7 43.2

Figure 3. N2 adsorption/desorption isotherms (A) and pore size distributions (B) of the Cat-0, Cat-1, Cat-2, Cat-3, Cat-4, and Cat-5 catalysts. 17257


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Figure 4. TEM micrographs of (A) Cat-0, (B) Cat-1, (C) Cat-2, (D) Cat-3, (E) Cat-4, and (F) Cat-5 catalysts.

Figure 5. Mn 2p3/2 (A) and O 1s (B) XPS spectra of the Cat-0, Cat-1, Cat-2, Cat-3, Cat-4, and Cat-5 catalysts.

more inner surface area and pore volume and then displayed the excellent catalytic performance.4 Thus, the microstructure of the catalyst could be influenced by different Mn(NO3)2/

surface areas of Cat-0, Cat-1, Cat-2, Cat-3, Cat-4, and Cat-5 were 11, 17, 21, 20, 20, and 11 m2/g, respectively. It was claimed that micropores and mesopores could offer much 17258


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The Journal of Physical Chemistry C KMnO4 molar ratios and Cat-2 showed abundant mesopores and the largest BET surface area, which could promote the formation of the favorable pore structure, resulting in the increase of mesopores and surface area. The TEM images of the catalysts prepared by the coprecipitation method are shown in Figure 4. It could be seen that Cat-0 displayed a nanoparticle morphology. Compared to that of Cat-0, the TEM images of other catalysts showed a nanorod morphology with the introduction of KMnO4. It was reported37 that the cryptomelane-type manganese showed a nanorod morphology, which exhibited excellent activity for the catalytic oxidation of toluene. It is well known that the catalytic activity of MnOx was associated with several factors, such as defect nature and density, reducibility, surface area, and morphology. Generally, the nanorod structure reveal a higher structural defect density and a stronger reducibility, which was beneficial for the activation of oxygen molecules to active oxygen adspecies and showed better catalytic performance.38 Hence, the formation of cryptomelane over the MnOx catalyst was advantageous for the improvement of the catalytic activity. 3.2. XPS Analysis. To further determine the form of the active oxygen species present and the oxidation state of metal elements on the surface of catalysts, XPS analysis was carried out, and the results are shown in Figure 5. In Figure 5A, the peak value of 642.2−643.5 eV was ascribed to Mn4+, while the peak value of 640.7−642.3 eV was attributed to Mn3+.23 The quantitative analysis with respect to the surface Mn3+/Mn4+ ratios of all of the samples is listed in Table 2. From Table 2, it

provided more active sites, which was beneficial to the improvement of catalytic oxidation activity. Similarly, Yu et al.40 reported that the increase of Mn3+ would improve the catalytic activity due to the improvement of the redox performance. However, Deng et al.41 found that the concentration of surface oxygen vacancy in the catalysts could be affected by the total amount of Mn4+ ions, which resulted in the difference in catalytic activity of the catalyst. Wang et al.42 reported that the mutual conversion among Mn4+, Mn3+, and Mn2+ valences could facilitate the electrons and radical species in the gas to be adsorbed on the MnO2 surface, which could improve the catalytic performance. Combined with the catalytic activity results, Cat-2 with the most amounts of Mn3+ showed the superior catalytic performance and excellent CO2 selectivity among the samples. Therefore, it was inferred that the formation of Mn3+ over MnOx could improve the catalytic oxidation of VOCs. Figure 5B displays the O 1s spectra of the Cat-0, Cat-1, Cat2, Cat-3, Cat-4, and Cat-5 samples. The two bands at around 531.3 and 529.6 eV could be assigned to adsorbed oxygen (Oads) and lattice oxygen (Olatt), respectively.27 As reported in Table 2, the surface Oads/Olatt atomic ratios of Cat-0, Cat-1, Cat-2, Cat-3, Cat-4, and Cat-5 were 20, 26, 38, 22, 27, and 23%, respectively. The abundant surface-adsorbed oxygen species were easily evaporated at low temperatures, which was beneficial for the stronger reducibility and then improved the oxidation performance.43 Li et al.22 reported that the absorption of surface oxygen could strengthen the migration of lattice oxygen, which might increase the catalytic activity. Qu et al.20 pointed that the large amount of surface adsorption oxygen was conducive to gas-phase oxygen circulation and promoted the oxidation of toluene. Clearly, Cat-2 exhibited the most abundant chemically adsorbed oxygen species, which increased its redox capacity and reduced the temperature of the catalytic oxidation of toluene. 3.3. Temperature-Programmed Studies. The reducibility of catalysts was investigated by means of H2-TPR analysis, and its profiles of all catalysts are revealed in Figure 6A. The reduction peaks of all of the samples could be divided into three parts: MnO2 → Mn2O3 → Mn3O4 → MnO.44 Compared to that of Cat-0, the reduction temperature of other catalysts shifted to a lower value with the introduction of KMnO4, indicating the improvement of the redox ability. It was obvious that Cat-2 possessed the lowest initial reduction temperature at 313 °C. Based on a previous research,30 the lower initial reduction temperature meant a large amount of

Table 2. Surface Atomic Ratios from XPS catalysts

Mn3+/Mn4+

Oads/Olatt


0.675 1.072 1.201 1.135 1.070 0.606

0.198 0.262 0.384 0.230 0.267 0.233

was obvious that the surface Mn3+/Mn4+ molar ratios of Cat-0, Cat-1, Cat-2, Cat-3, Cat-4, and Cat-5 were 0.68, 1.07, 1.20, 1.13, 1.07, and 0.61, respectively. Generally, more active sites and abundant surface-active oxygen species were beneficial to the catalytic activity of the catalyst. The mixed valence state of manganese oxide catalysts played an important role in redox catalysis. Genuino et al.39 proposed that the presence of Mn3+

Figure 6. H2-TPR pattern (A) and initial H2 consumption (B) of the Cat-0, Cat-1, Cat-2, Cat-3, Cat-4, and Cat-5 catalysts. 17259


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the results of XPS. Generally, the peak at around 600 °C could be assigned to the oxygen mobility of the catalyst. It meant that Cat-2 exhibited stronger oxygen mobility. According to Liu et al.,46 the oxygen mobility of the catalyst played a crucial role in the catalytic oxidation of VOCs and the catalytic performance of the catalyst would be improved by the enhancement of the oxygen mobility. In summary, the difference of Mn(NO3)2/ KMnO4 molar ratios led to the change in the oxygen mobility of the catalyst, which could influence the catalytic oxidation activity and the CO2 selectivity of the catalyst.33 3.4. Catalytic Activity Tests. The catalytic activity of all samples in the catalytic oxidation of toluene was measured, and the results are shown in Figure 8. The effect of the Mn(NO3)2/KMnO4 ratio on the catalytic activity over MnOx was investigated by comparing the T50 and T90 reaction temperatures (corresponding to 50 and 90% toluene conversion, respectively). As shown in Figure 8A and Table 3, the T50 order of toluene conversion was as follows: Cat-2 (224 °C) > Cat-3 (235 °C) >

surface-reactive species, which was beneficial to the catalytic performance of the catalyst. To further investigate the difference in the redox properties of the six catalysts, the initial H2 consumption rate is presented in Figure 6B. Obviously, Cat-2 showed the highest initial H2 consumption rate. As reported by Deng et al.,45 a higher initial H2 consumption rate indicated a stronger redox ability and an improved catalytic performance. Hence, the reduction performance of the MnOx catalysts was obviously increased with the addition of KMnO4 and the appearance of cryptomelane, which depended on the different Mn(NO3)2/ KMnO4 molar ratios. Figure 7 shows the O2-TPD profiles of the catalysts. As shown in Figure 7, the samples showed three types of

Table 3. Catalytic Activities of the Cat-0, Cat-1, Cat-2, Cat3, Cat-4, and Cat-5 Catalystsa,b toluene

CO2

catalysts

T50 (°C)

T90 (°C)

T50* (°C)

T90* (°C)


260 297 225 235 243 257

268

262

270

238 249 255 269

230 247 244 260

246 262 257 274

a

T50 means the temperature of conversion of toluene reached 50%, and T50* means the temperature of yield of CO2 reached 50%. bT90 means the temperature of conversion of toluene reached 90%, and T90* means the temperature of yield of CO2 reached 90%.

Figure 7. O2-TPD patterns of the Cat-0, Cat-1, Cat-2, Cat-3, Cat-4, and Cat-5 catalysts.

desorption peaks in the ranges of 100−400, 400−700, and 700−900 °C, which could be ascribed to the desorption of surface-adsorbed oxygen (α), the oxygen atoms in the framework bound to Mn3+ (β), and the oxygen atoms bound to Mn4+ (γ), respectively.30 The difference in the intensity of the peak at around 580−600 °C could be explained by the structural changes caused by the change of the Mn(NO3)2/ KMnO4 molar ratios. It could be seen that larger amounts of β species were released in Cat-2. This phenomenon could be attributed to the abundant Mn3+ in Cat-2, in agreement with

Cat-4 (242 °C) > Cat-5 (257 °C) > Cat-0 (260 °C) > Cat-1 (296 °C). Furthermore, the T90 over the catalysts showed the same trend as the sequence of T50. With the addition of KMnO4, the catalytic performance was improved. The MnO2 prepared by KMnO4 and Mn(NO3)2 showed the best catalytic oxidation of toluene when the Mn(NO3)2/KMnO4 ratio was 3:7. Nevertheless, the Mn(NO3)2/KMnO4 molar ratio reached to 1:9, and the catalytic activity decreased significantly.

Figure 8. Toluene conversion (A) and CO2 yield (B) as a function of reaction temperature over the Cat-0, Cat-1, Cat-2, Cat-3, Cat-4, and Cat-5 catalysts under the conditions of toluene concentration of 500 ppm, 20 vol % O2, and gas hourly space velocity (GHSV) of 60 000 h−1. 17260


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reaction stability for oxidation within 100 h. The removal rate of toluene remained above 90% during the whole period of detection. Consequently, the Cat-2 sample possessed the excellent stability and durability.

Although Cat-1 and Cat-2 both have cryptomelane-type structures, similar specific surface areas, and similar reducibilities, the catalytic properties of MnO2 catalysts were affected by the surface oxygen and Mn3+ contents. It could be seen from XPS that Cat-2 exhibited the most abundant chemisorbed oxygen species and Mn3+. As reported by Qu et al.,20 Mn3+ facilitated the supplement of oxygen, which could increase the oxygen transfer in the catalysts and then promoted the catalytic oxidation activity. The previous research47 showed that the surface oxygen played a critical role in the oxidation of toluene and the active surface oxygen in catalysts facilitated the oxidation reaction at low temperatures. Therefore, Cat-2 possessed better catalytic activity than Cat-1. It is worth noting that Cat-1 and Cat-4 samples exhibited similar BET surface areas, Mn3+/Mn4+ ratios, and Oads/Olatt ratios but revealed large differences in the catalytic activity. According to the reported studies,48,49 the activity of MnOx catalysts in the catalytic oxidation of toluene was associated with the reducibility of the species. Cat-4 revealed a lower reducible temperature. Moreover, the highest initial H2 consumption rate was found for Cat-4, which meant that the Cat-4 catalyst possessed stronger reducibility, enhancing the catalytic performance. As shown in Raman spectroscopy, Cat-1 and Cat-4 exhibited different intensities for the peak at 638 cm−1. The broadening and weakening of the peak in the Cat-4 sample was mainly due to the lattice defects and oxygen vacancies, which would contribute to larger specific surface area and then improved catalytic activity. Thus, the catalytic activities for VOCs over Cat-4 were higher than those over Cat-1.45 As shown in Figure 8B, in terms of CO2 selectivity, the following performance trend was observed: Cat-2 > Cat-4 > Cat-3 > Cat-0 ≈ Cat-5 > Cat-1. Santos et al.21 reported that the catalytic activity was related to the manganese species and the structure on the surface of the catalyst. The literature30 pointed out that cryptomelane could improve the catalytic oxidation performance and CO2 conversion capacity. Corresponding to XRD, XPS, H2-TPR, and O2-TPD characterization results, Cat-2 possessed more Mn3+, more surface-adsorbed oxygen, and the strongest oxidation capacity. Thus, Cat-2 showed better catalytic oxidation performance. 3.5. Catalytic Stability in Toluene Oxidation. In this study, the stability of Cat-2 sample was tested at 260 °C, and the results are shown in Figure 9. The Cat-2 showed excellent

4. DISCUSSION Different valence states of metals, redox properties, and active oxygen species were key factors to influence the catalytic performance of manganese oxide catalysts.50,51 4.1. Role of K+ and Cryptomelane in the Catalytic Oxidation of Toluene. It was reported that the presence of potassium ions had a significant effect on the catalytic activity of MnOx catalysts. According to the findings of Santos,21,33 cryptomelane-type oxide catalyst showed the excellent catalytic activity. Wang et al.52 researched that the effect of interlayered K+ with different structures (separation and localized) was used in manganese oxide to enhance the oxidation of formaldehyde. Hou et al.28 demonstrated that the doping of K+ could improve the catalytic activity over an OMS-2 molecular sieve due to an increase in the lattice oxygen activity. To further evaluate the role of cryptomelane and potassium ions in the MnOx catalyst, the MnOx catalyst with the same theoretical potassium ion content as that of the Cat-2 catalyst was prepared by the impregnation method. The catalytic activity of IM-2 in the catalytic oxidation of toluene was measured, and the results are shown in Figure 10.

Figure 10. Toluene conversion as a function of reaction temperature over the IM-2 catalyst under the conditions of toluene concentration of 500 ppm, 20 vol % O2, and GHSV of 60 000 h−1.

The catalytic oxidation ability of toluene was poor over the IM2 catalyst, and the maximum conversion rate of toluene was 5%. Hence, the presence of K+ over MnOx inhibited the catalytic oxidation ability of toluene. Corresponding to the XRD results, it was inferred that the formation of cryptomelane (KMn8O16) were responsible for the excellent catalytic performance over Cat-2. Santos et al.21 found that the cryptomelane catalyst possessed stronger oxidation performance and higher catalytic activity. Sun et al.37 proved that the OMS-2 material exhibited the typical cryptomelane structure and could release a large number of lattice oxygen species, which improved the oxidation performance of the catalyst. Genuino et al.39 reported that the OMS-2 catalyst showed the excellent catalytic oxidation of VOCs because of the presence of a cryptomelane-type structure. Hence, the addition of KMnO4 into MnOx promoted the formation of cryptomelane,

Figure 9. Stability test of the Cat-2 catalyst under the conditions for toluene oxidation at 260 °C. 17261



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which was responsible for the superior catalytic oxidation performance and CO2 selectivity over Cat-2. 4.2. Role of Mn3+/Mn4+ and Surface Oxygen Species in the Catalytic Performance over MnOx. Generally, more active sites and abundant surface-active oxygen species were beneficial to the catalytic activity of the catalyst. The mixed valence state of manganese oxide catalysts played an important role in redox catalysis. Genuino et al.39 proposed that the presence of Mn3+ provided more active sites, which was beneficial to the improvement of the catalytic oxidation activity. Similarly, Yu et al.40 reported that the increase of Mn3+ would improve the catalytic activity due to the improvement of redox performance. It was reported in the previous literature33 that the cryptomelane-type octahedral molecular sieve (OMS-2) nanorod samples possess both Mn3+ and Mn4+ species. The experimental results proved that Mn3+ provided more oxygen vacancy defects and was conducive to increasing the catalytic oxidation activity. Thus, the catalytic performance was closely related to the Mn3+/Mn4+ ratios and a large amount of Mn3+ could improve the catalytic performance over MnOx catalysts. It is well known that the better low-temperature reduction was more beneficial for catalytic oxidation. According to a previous study,39 the release of oxygen led to the formation of skeletal oxygen vacancies, which could act as catalytic active sites for oxidation reactions. However, the change of Mn−O lattice in the cryptomelane structure led to an increase in defects, which promoted the exchange of oxygen and increased the oxidation activity of VOCs. Hou et al.33 reported that the improvement of oxygen vacancy defects enhanced the lattice oxygen reactivity, which could tremendously promote the catalytic activity for VOC oxidation. The lattice oxygen mainly provided the oxidizing ability to the catalyst. When the amount of lattice oxygen participating in the reaction was sufficient, the superior oxygen mobility promoted the progress of the reaction. However, the increase of oxygen adsorption could improve the oxygen fluidity of the catalyst and speed up the lattice oxygen replenishment in the reaction process, thus improving the catalytic performance. From the O2-TPD, XPS, and XRD results, it was found that the Cat-2 catalyst possessed more surface-adsorbed oxygen, abundant Mn3+/Mn4+, and stronger redox ability, resulting in the enhancement of the catalytic activity. Therefore, the increase of Mn3+/Mn4+ and Oads/Olatt ratios over MnOx contributed to the enhancement of the catalytic activity, which relied on the different Mn(NO3)2/ KMnO4 molar ratios.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: (+86)-375-2089031 (Z.S.). *E-mail: [email protected]. Tel: (+86)-024-89384363 (W.L.). ORCID

Zhongxian Song: 0000-0002-4733-3676 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (No. 21872096), the Doctoral Research Start-up Project of Henan University of Urban Construction (No. 990/Q2017011) and Liaoning Province Doctorla Startup Fund (No.20170520402).

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5. CONCLUSIONS Manganese oxide catalysts were prepared via a coprecipitation method by adjusting the molar ratio of Mn(NO3)2/KMnO4. The results of TPR revealed that the Cat-2 sample (Mn(NO3)2/KMnO4 of 3:7) possessed excellent redox performance, which could be related to the existence of cryptomelane. Besides, O2-TPD indicated that the Cat-2 catalyst contained more reactive oxygen species, which were more conducive to its activity in the catalytic oxidation of toluene. Moreover, higher Mn3+/Mn4+ and Oads/Olatt ratios of Cat-2 were the important factors influencing its catalytic performance. Hence, the Cat-2 catalyst with a cryptomelane-type structure and a larger specific surface area exhibited higher catalytic activity and CO2 selectivity at low temperatures, which was closely related to the abundant Mn3+ and Oads and outstanding redox properties. 17262


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Role of Cryptomelane in Surface-Adsorbed Oxygen and Mn Chemical

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