Novel Ordered Mesoporous γ-MnO2 Catalyst for High-Performance

Jul 3, 2019 - Institutions, School of Chemical Engineering and Light Industry, Guangdong University of. Technology, Guangzhou, 510006, P. R. China. b...
0 downloads 0 Views 6MB Size
Download PDF
Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

pubs.acs.org/IECR

Novel Ordered Mesoporous γ‑MnO2 Catalyst for High-Performance Catalytic Oxidation of Toluene and o‑Xylene Xiaohong Zeng,†,§ Gao Cheng,†,§ Qi Liu,† Weixiong Yu,† Runnong Yang,† Huajie Wu,† Yongfeng Li,† Ming Sun,† Canyang Zhang,‡ and Lin Yu*,† †

Downloaded via BUFFALO STATE on July 29, 2019 at 02:36:51 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Key Laboratory of Clean Chemistry Technology of Guangdong Regular Higher Education Institutions, School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, P. R. China ‡ Guangdong ZhongTou Environmental Protection Co., Ltd, Guangzhou 510700, P. R. China S Supporting Information *

ABSTRACT: The main bottleneck in the catalytic removal of volatile organic compounds (VOCs) is the desire to search for a cheap, highly efficient, and durable catalyst. Herein, we developed a facile synthesis of an ordered mesoporous γMnO2 nanostructure (meso-γ-MnO2) by treating an ordered mesoporous Mn2O3 precursor (meso-Mn2O3) with a dilute HNO3 solution, during which the ordered mesoporous framework was well retained. The resultant meso-γ-MnO2 was born with high specific surface area and rich surface oxygen vacancies, which could greatly improve its number of surface active sites and reactivity of surface oxygen species. Surprisingly, the meso-γ-MnO2 exhibited significantly enhanced catalytic activities compared to nonporous α-MnO2 nanorods and γ-MnO2 microurchins, achieving T90 (the temperatures for 90% conversion of toluene or o-xylene) at 219 and 237 °C for removing toluene and o-xylene under a WHSV of 40,000 mL·g−1· h−1, respectively. Our results provide an effective and facile route to synthesize a highly efficient γ-MnO2 catalyst for VOC removal. porosity,12,13 doping with metal cations,14 and inducing oxygen vacancies,15 in addition to discovering new catalysts.16 Among the various forms of non-noble metal oxides, MnO2 has received extensive interest as the heterogeneous catalyst for catalytic oxidation reaction, mainly because of its innate superiority including multivalence (Mn2+, Mn3+, and Mn4+) and changeable structure (δ-, α-, γ-, R-, and β- MnO2).17,18 γMnO2, a disordered structure composed of the intergrowth of β-MnO2 and R-MnO2, is one of the most intensively studied manganese dioxides.19 Previously, the literature showed that the γ-MnO2-based catalysts were highly active for heterogeneous catalytic oxidation reactions. For instance, Fu et al. found that the hollow γ-MnO2 spheres could oxidize benzyl alcohol to benzaldehyde with preferable catalytic performance and selectivity compared to hollow β-MnO2 spheres, conventional MnO2, and Mn2O3 particles.20 Pal et al. reported that the lotus-shaped porous γ-MnO2 nanostructure achieved much higher catalytic activity in oxidizing α-pinene to verbenone than that of other porous γ-MnO2 particles with different shapes (rods, spheres and nanoaggregates), bulk MnO2, and other reported catalysts.21 Gong et al. probed into their relationship between different MnO2 crystalline phases and

1. INTRODUCTION Volatile organic compounds (VOCs), such as alkanes, alcohols, ketones, and aromatic compounds, are considered as the main components of air pollutants from the urban and industrial processes.1 Since VOCs present negative influence on human health and environment, efficient approaches for their abatement are of great necessity and significance.2,3 Catalytic oxidation is regarded to be a complete and promising solution for VOCs abatement due to the low energy consumption, high efficiency, and less amount of byproducts. Thus, highperformance and durable catalysts are urgently needed to achieve excellent catalytic performances. Supported noble metals (e.g., Ru/HZSM-5,4 Pd-ceramic fiber,5 Pd/Al2O3,6 and Ag/SBA-157) are recognized as the best catalytic materials for their remarkable low-temperature performances. However, high noble metal loading is in general necessary, which extremely restricts the commercial mass production of supported noble metals owing to their expensive nature and easy poisoning and sintering.8 To this end, more attention is directed to new earth-abundant non-noble metal oxides (e.g., MnO2, Fe2O3, Co3O4, NiO, and CeO2) with low cost and good thermostability,9 which have been developed as versatile alternative materials to supported noble metals. Nevertheless, the catalytic performances of non-noble metal oxide catalysts are still unacceptable; thus, there is considerable room for enhancement by, for example, controlling shape,10 fabricating heterogeneous structure,11 establishing mesoporosity/macro© XXXX American Chemical Society

Received: Revised: Accepted: Published: A

April 16, 2019 June 23, 2019 July 3, 2019 July 3, 2019 DOI: 10.1021/acs.iecr.9b02087 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

M) was used for removing the KIT-6 template. The mesoMn2O3 precursor was obtained after filtering, washing, and drying at 60 °C. The mesoporosity of the meso-Mn2O3 precursor was determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques, as displayed in Figure S1. A facile synthetic method for preparing the meso-γ-MnO2 sample is described as follows: (1) 1 g of meso-Mn2O3 precursor was added into dilute HNO3 solution with stirring for 2 h at 80 °C; (2) the acquired precipitate was filtered and dried at 60 °C for 12 h. 2.1.2. Synthesis of γ-MnO2 Microurchins and α-MnO2 Nanorods. In a reported preparation of γ-MnO2 microurchins,27 MnCl2·H2O and (NH4)2S2O8 were dissolved with 30 mL of water, and then the solution was kept warm at 90 °C for 24 h. For α-MnO2 nanorods,28 a mixture of KMnO4 and MnSO4·H2O was hydrothermally heated at 140 °C for 12 h. Finally, the black participates were filtered and dried at 60 °C. 2.2. Materials Characterizations. The crystalline phases of products were characterized with a Bruker D8 advance X-ray diffractometer under Cu Kα radiation (40 kV, 40 mA); the data were collected from 10° to 70° at a scan rate of 3°/min. The Raman spectra were acquired from a dispersive Renishaw inVia Reflex Raman spectrometer equipped with a 50 mW diode pumped solid state laser of 532 nm. Field Emission transmission electron microscopy (FETEM) images were taken on a FEI Talos F200S apparatus operating at 200 kV to investigate the microstructures of samples. The morphology observation was done via field emission scanning electron microscopy (FESEM, Hitach, SU8220) under an operation voltage of 15 kV. The N2 adsorption−desorption tests were conducted on an ASAP 2020 adsorption analyzer at −196 °C. The samples were pre-degassed at 120 °C for 10 h under vacuum conditions prior to measurement. The specific surface area (SSA) and pore size (Dp) were measured according to Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) methods. X-ray photoelectron spectroscopy (XPS) was carried out on an Escalab 250Xi device equipped with a Al Kα source, and 284.8 eV of the C1 BE value was severed as the corrected value to determine the binding energy of samples. Hydrogen temperature-programmed reduction (H2-TPR) was performed on a Micromeritics AutoChem II 2920 instrument. The sample (30 mg) was pretreated in Ar at 250 °C for 1 h and then reduced in a feed of 5 vol % H2−Ar with a heating rate of 10 °C·min−1. The H2 consumption was calibrated by reducing a standard AgO sample. 2.3. Catalytic Activity Evaluation. The catalytic activity measurements of toluene or o-xylene complete oxidation were taken in a static bed quartz reactor (i.d. = 8 mm). A mixture of catalyst (50 mg, 40−60 mesh) and quartz sand (50 mg, 40−60 mesh) supported by the glass wool was placed in the middle of the quartz reactor. The gas intake flow rate (1000 ppm toluene or o-xylene + 20 vol % O2 + N2 (balance)) is 33.4 mL·min−1, which achieves a weight hourly space velocity (WHSV) of 40,000 mL·g−1·h−1 and a toluene/O2 molar ratio of 0.005. Toluene or o-xylene vapor was released from an ice-bath bubbler, and an online gas chromatograph (Agilent 6820) with flame ionization detector (FID) was used for monitoring the exhausts. Notably, the final oxidation products contain only CO2 and H2O. Each catalytic activity measurement was repeated five times, and the average toluene conversion with error bars (defined as the standard error) was calculated accordingly. Furthermore, the apparent activation energy (Ea,

their NO catalytic reduction activities, and demonstrated that the wirelike γ-MnO2 performed better than the other three MnO2 catalysts (δ-, α-, and β-MnO2).22 Whereas, when applied in VOC complete oxidation, the biggest issue of recently reported γ-MnO2 catalysts is that their catalytic performances still underperformed the other MnO2 crystalline phases. For example, Sun et al. investigated the crystalline structure-dependent activity of MnO2 catalysts in toluene combustion, following the sequence of α-MnO2 ultralong nanowires > γ-MnO2 urchins > β-MnO2 prisms.23 Zhang et al. tested the activities over the MnO2 catalysts for HCHO removal, and they revealed that the activities varied in the order of dendritic α-MnO2 ≈ spherical δ-MnO2 > nanoneedlelike γ-MnO2 > dendritic β-MnO2.24 Consequently, developing advanced γ-MnO2 catalysts with superior activities in VOCs oxidation is still of great challenge and necessity. Due to the highly efficient oxidation reactions requiring sufficient surface active sites and high reactivity of the surface, the texture properties (e.g., porosity and specific surface area) and surface chemistry structures (e.g., oxygen vacancies and reactivity of oxygen species) of catalytic materials are the primary determinants of their catalytic performances.25 Based on these considerations, we rationally proposed a facile method to prepare meso-γ-MnO2 via treating meso-Mn2O3 with dilute HNO3 solution, as illustrated in Scheme 1. After Scheme 1. Schematic Illustration of Transformation Process from meso-Mn2O3 into meso-γ-MnO2

the acid treatment, the as-prepared meso-γ-MnO2 was endowed with ordered mesoporous framework, high specific surface area and rich surface oxygen vacancies. Furthermore, it could be found that the amount of surface-absorbed oxygen species and reducibility were well improved in meso-γ-MnO2. Benefiting from the above merits, the novel meso-γ-MnO2 showed substantially superior catalytic performance for toluene and o-xylene oxidation compared to the nonporous γ-MnO2 microurchins and α-MnO2 nanorods. To our best knowledge, the successful synthesis of meso-γ-MnO2 with ordered mesopores has not yet been reported for the γ-MnO2 materials. Our results open up an efficient avenue to fabricate mesoporous γ-MnO2 material as a high-performance catalyst in VOCs complete oxidation.

2. EXPERIMENTAL SECTION 2.1. Preparation of MnO2 Catalysts. 2.1.1. Synthesis of Ordered Mesoporous γ-MnO2. Typically,26 1 g of KIT-6 was impregnated into Mn(NO3)2·4H2O ethanol solution (0.3 M) under continuous stirring, and then it was dried at 60 °C. The as-obtained powder was repeatedly impregnated and further calcined at 500 °C for 3 h. A hot NaOH aqueous solution (2 B

DOI: 10.1021/acs.iecr.9b02087 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. (a) Wide-angle XRD patterns of the MnO2 products; (b) low-angle XRD pattern of the meso-γ-MnO2; (c) Raman spectra of the MnO2 products.

Figure 2. FETEM and HR-FETEM images of (a, b) meso-γ-MnO2 (the inset of (b) is a magnification of the yellow dashed box), (c, d) γ-MnO2 microurchins, and (e, f) α-MnO2 nanorods.

kJ·mol−1) and reaction rate (r, μmol·g−1·s−1) were calculated as below equations: r=

X tolueneCin mcat

ln r = −

Ea 1 + ln A R T

Furthermore, as seen from Figure 1b, the low-angle XRD pattern of the meso-γ-MnO2 shows an obvious diffraction peak (2θ = ∼0.9°) indexed as the (211) plane, which is a feature for cubic ordered mesoporous structure.26 This result suggests that the ordered mesoporous framework can be well retained in the meso-γ-MnO2 after the acid treatment of meso-Mn2O3 precursor. The structural information of the three MnO2 samples was further studied by Raman spectroscopy, which provides a complete and credible illustration of nanostructured materials, such as crystalline disorders and lattice vibrational properties.30 As displayed in Figure 1c, the Raman bands can be scoped to the wavenumber shift of 180−750 cm−1. According to the reported assignments, the Raman bands of 550−700 cm−1 and 450−550 cm−1 are ascribed to the Mn−O stretching mode of [MnO6], and the metal−oxygen chains of Mn−O−Mn in the MnO2 framework, respectively.31 The other Raman bands within 200−450 cm−1 are assigned to the frame bending vibration in the [MnO6] octahedral lattice.32 Obviously, the meso-γ-MnO2 clearly shows five bands at 271, 381, 514, 579, 645 cm−1, while the γ-MnO2 microurchins exhibit six bands at 275, 383, 493, 524, 578, and 654 cm−1. These are in conformity with the reported studies on γ-MnO2 materials.33 Particularly, the Raman bands of 577 and 578 cm−1 for the meso-γ-MnO2 and γ-MnO2 microurchins mark the presence of

(1)

(2)

Where Cin and mcat in eq 1 are the inlet toluene concentration and catalyst mass, respectively.

3. RESULTS AND DISCUSSION 3.1. Structure Characterizations. To confirm the crystalline phases of the as-obtained MnO2 products, wideangle XRD analysis was performed. As displayed in Figure 1a, the characteristic peaks of meso-γ-MnO 2 and γ-MnO 2 microurchins are both well indexed with the pure orthorhombic γ-MnO2 phase (JCPDS No. 14-0644), where the 2θ values of 22.4°, 37.1°, 42.6°, 56.1°, and 66.7° correspond to the (120), (131), (300), (160), and (003) planes of γ-MnO2, respectively.29 Compared to the γ-MnO2 microurchins, the meso-γ-MnO2 exhibits broader and weaker diffraction peaks, revealing its slightly low crystallinity. With regard to the αMnO2 nanorods, the diffraction peaks are readily identified to the tetragonal α-MnO2 phase (JCPDS No. 44-0141).28 C

DOI: 10.1021/acs.iecr.9b02087 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research (1× 2) channels in an orthorhombic structure.30 These Raman bands of meso-γ-MnO2 appear much broader and less intense than that of γ-MnO2 microurchins, which verified the XRD analysis that meso-γ-MnO2 possesses poor crystallinity. Moreover, the Raman band of meso-γ-MnO2 located at 645 cm−1 is lower than that of γ-MnO2 microurchins (654 cm−1). The red band-shift phenomenon suggests that there is a more defective structure in the meso-γ-MnO2.34,35 For α-MnO2 nanorods, the Raman spectrum shows seven bands at 184, 329, 389, 513, 581, 637, and 751 cm−1 in all. Note that the two strong Raman bands at 581 and 637 cm−1 demonstrate the feature of a tetragonal framework with (2 × 2) tunnels.36 3.2. Morphology and Microstructure. The ordered mesoporosity of the meso-γ-MnO2 is further investigated by FETEM. As seen from Figure 2a, the meso-γ-MnO2 possesses ordered mesoporous framework composed of uniform mesopores, where the average diameter of mesopores is around ∼9.5 nm. This result also supported the low-angle XRD measurement, confirming that the acid treatment process can preserve the ordered mesoporous framework originated from meso-Mn2O3. The high-resolution FETEM (HRFETEM) image of meso-γ-MnO2 (Figure 2b) displays clear interplanar spacing of 0.24 nm, corresponding to the (131) plane of γ-MnO2 structure. For the nonporous γ-MnO2 microurchins and α-MnO2 nanorods, FESEM was first carried out for investigating their morphologies. As revealed in Figure S2, the γ-MnO2 microurchins show homogeneous urchin-like morphology with an average diameter of 2−4 μm, and each microurchin consists of numerous nanorods (Figure S2a and b). For α-MnO2 nanorods, uniform nanorods with a length of 1−3 μm and a diameter of 30−50 nm are observed (Figure S2c and d). The FETEM images further confirm the urchinlike and rod-like shapes for the γ-MnO2 microurchins and αMnO2 nanorods, respectively (Figure 2c and e). From Figure 2d and f, the exposed facet of γ-MnO2 microurchins is ascribed to the (131) crystal plane according to the interplanar distance of 0.40 nm, while the exposed facet of α-MnO2 nanorods is closed to the (200) crystal plane with the interplanar distance of 0.49 nm. Moreover, a deeper HR-FETEM observation (inset of Figure 2b, marked by the yellow boxes) reveals that the meso-γ-MnO2 has poorer surface structure than γ-MnO2 microurchins and α-MnO2 nanorods, indicative of more surface defects in the meso-γ-MnO2.37,38 3.3. The Process of Phase Transformation. In order to understand the phase transformation process from mesoMn2O3 precursor to meso-γ-MnO2, we performed the timedependent experiments by XRD measurement (Figure 3). After 10 min of acid treatment, a new crystalline structure of γMnO2 is produced as indicated by the appearance of the (131) and (300) diffraction peaks, while the intensity of all the diffraction peaks ascribed to Mn2O3 begins to decrease simultaneously. This implies that the Mn2O3 phase started to transform into the γ-MnO2 phase. When the reaction proceeds for 30 min, the (120) diffraction peak of γ-MnO2 appears, accompanied by the increasing peak intensity of γ-MnO2. When comes to 60 min, all the diffraction peaks assigned to γMnO2 can be detected, and the Mn2O3 phase still exists. With a longer acid treatment to 120 min, no diffraction peaks corresponding to Mn2O3 are observed and only γ-MnO2 phase presents, indicating a complete transformation from Mn2O3 phase to γ-MnO2 phase. The above XRD results reveal that the acid treatment is favorable to form the γ-MnO2 phase. Most agree that the acid treatment can promote the disproportio-

Figure 3. Phase transformation from Mn2O3 to γ-MnO2 through acid treatment at different reaction times (10, 30, 60, and 120 min).

nation of Mn2O3, which is converted into soluble Mn2+ and MnO2 via a disproportionation effect.39 A similar synthetic route could be found in the fabrication of γ-MnO2 solids, where the Mn2O3 precursor was immersed in H2SO4 solution via a hydrothermal method.40 Therefore, treating the mesoMn2O3 precursor under HNO3 solution could lead to the complete transformation into meso-γ-MnO2. 3.4. Catalytic Performance. The toluene oxidation tests over meso-γ-MnO2, α-MnO2 nanorods, and γ-MnO2 microurchins were carried out at WHSV = 40,000 mL·g−1·h−1 (Figure 4a). Obviously, the toluene conversion of all the MnO2 catalysts increases monotonically as the reaction temperature rises from 100 to 260 °C. Herein T10 and T90 (the temperatures for 10% and 90% conversion of toluene) were used to compare the catalytic activities for convenience, which are provided in Table 1. Clearly, the T10 and T90 values follow the same order: γ-MnO2 microurchins (205 and 239 °C) > α-MnO2 nanorods (200 and 236 °C) > meso-γ-MnO2 (179 and 219 °C), implying that the meso-γ-MnO2 presents the best catalytic activity. This result proves that the presence of mesopores in the meso-γ-MnO2 can facilitate the toluene complete combustion dramatically. Furthermore, the apparent activation energy (Ea) of the three MnO2 samples was also determined to further compare their catalytic activities, which was computed from the typical linearity between ln k and 1000/T. As tabulated in Table 1, the meso-γ-MnO2 exhibits the lower Ea value (68.0 kJ·mol−1) than γ-MnO2 urchins (91.5 kJ·mol−1) and α-MnO2 nanorods (81.4 kJ·mol−1), demonstrating that toluene is more efficiently oxidized over the meso-γMnO2. The catalytic activities (T10 and T90) of previously reported catalysts for toluene removal under similar catalytic test conditions are depicted in Figure 5 and also listed in Table S1. Significantly, it can be seen that our meso-γ-MnO2 sample exhibits better catalytic performance than those of supported noble metals and non-noble metal oxides even at a higher WHSV, such as 0.12 Ag/Mn2O3-redn,41 5.8 wt % Au/3DOM Mn2O3,42 flower-like ε-MnO2,43 and 10% MnOx/HZSM-525.44 Such a result further confirms that the meso-γ-MnO2 has a remarkably enhanced oxidation ability in total combustion of toluene. The catalytic performance of mese-γ-MnO2 affected by distinguishing WHSV was explored (Figure 6a). When the WHSV value rises from 20,000 to 60,000 mL·g−1·h−1, the toluene conversion follows a decreasing trend. This is mainly due to the insufficient contact time between the gaseous reactants (O2 and toluene) and the surface of mese-γ-MnO2.45 For purpose of examining the stability, we performed an D

DOI: 10.1021/acs.iecr.9b02087 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 4. (a) Toluene conversion over MnO2 products; (b) Arrhenius plots of MnO2 products at WHSV of 40,000 mL·g−1·h−1.

Table 1. Textural Properties, Activity Test, and Kinetic Parameters of As-Obtained MnO2 Products catalytic activities

kinetic parameters

sample

SSA (m /g)

Dp (nm)

T10 (°C)

T90 (°C)

Ea (kJ·mol )

Rs (μmol·h−1·m−2)b

meso-γ-MnO2 γ-MnO2 microurchins α-MnO2 nanorods

123.2 68.3 75.4

9.5 −c −c

179 205 200

219 239 236

68 91.5 81.4

8.54 4.68 6.79

2

−1 a

a Ea was computed by the slope of Arrhenius plots under WHSV of 40,000 mL·g−1·h−1, when toluene conversion is less than 20%. bRs was computed by the formula (Rs = (XtolueneCin)/(mcat × SSA)) at temperature of 220 °C. cThe − symbol represents the sample is a nonporous material.

spent meso-γ-MnO2 remains the same. Moreover, the FETEM image in Figure S3b indicates that the mesopores of the spent meso-γ-MnO2 are well preserved, as further verified by the textural results in terms of specific surface area (122.5 m2/g) and pore size (∼ 11.2 nm) (Figure S3c and d). The above results suggest that the meso-γ-MnO2 is catalytically stable after ∼ 72 h stability test. In addition, the complete oxidation of o-xylene was also carried out for surveying the catalytic activities of the asobtained three MnO2 catalysts (Figure S4). As expected, the catalytic activity trend of o-xylene oxidation is in perfect agreement with that of toluene oxidation. As shown in Table S2, meso-γ-MnO2 (T10 = 178 °C and T90 = 237 °C) is more active than the α-MnO2 nanorods (T10 = 194 °C and T90 = 246 °C) and γ-MnO2 microurchins (T10 = 210 °C and T90 = 260 °C). Moreover, the catalytic activities (T10 and T90) of some recently reported catalysts for o-xylene removal are displayed in Figure S5 and also given in Table S2. The T10 and T90 values of meso-γ-MnO2 are comparable to those of catalysts reported in recent literatures. Based on the above catalytic test results, the resultant meso-γ-MnO2 is found to display enhanced performance and excellent stability for

Figure 5. Comparation of T10 and T90 values with reported catalysts for toluene oxidation.

uninterrupted reaction experiment over meso-γ-MnO2 catalyst for a whole running time of ∼72 h (Figure 6b). After two runs of catalytic reaction, a 40 h long test at 220 °C was carried out, and no significant decrease in toluene conversion is observed. Then two runs of catalytic reaction were conducted again, both of which exhibit the same results as the first two runs. After that, the spent meso-γ-MnO2 was characterized via XRD, FETEM, and N2 absorption−desorption techniques (Figure S3). As illustrated in Figure S3a, the crystalline phase of the

Figure 6. (a) Different WHSV for toluene oxidation over meso-γ-MnO2 product; (b) catalytic stability test of toluene oxidation over meso-γ-MnO2 at 220 °C and WHSV of 40,000 mL·g−1·h−1. E

DOI: 10.1021/acs.iecr.9b02087 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 7. (a) N2 adsorption−desorption isotherms and (b) BJH pore size distributions of MnO2 products.

toluene and o-xylene complete combustion, suggesting meso-γMnO2 as an ideal catalyst applied in VOCs removal. 3.5. N2 Adsorption−Desorption Analysis. Figure 7 illustrates the N2 adsorption−desorption isotherms and BJH pore size distribution curves of the three MnO2 samples, and their textural data are summarized in Table 1. As for meso-γMnO2, its isotherm is reasonably ascribed to a type IV isotherm with a H2-type hysteresis ring, reconfirming the mesopores exist in the meso-γ-MnO2.45 According to the BJH pore size distribution, the meso-γ-MnO2 shows ∼9.5 nm average pore size. These textural results are fully aligned with the low-angle XRD and FETEM observations of meso-γMnO2. For the α-MnO2 nanorods and γ-MnO2 microurchins, their shapes of isotherms are defined as type II with a welldeveloped H3-type hysteresis ring, suggesting they are nonporous characteristics, which can be further proved by the BJH pore size distributions.46,47 As seen from Table 1, the specific surface area reduces sequentially as follow: meso-γMnO2 > α-MnO2 nanorods > γ-MnO2 microurchins, which is in fair agreement with the catalytic activity tendency. To further elucidate whether the superior catalytic property of meso-γ-MnO2 is closely associated with its higher specific surface area, the Rs values of the as-synthesized MnO2 samples at 220 °C were computed as tabulated in Table 1. The meso-γMnO2, which exhibits the greatly enhanced catalytic activity, displays the highest Rs value (8.54 μmol·h−1·m−2), followed by that of α-MnO2 nanorods (6.79 μmol·h−1·m−2) and that of γMnO2 microurchins (4.68 μmol·h−1·m−2). This analysis reveals that the specific surface area is a main factor influencing the catalytic activities of all the MnO2 samples. Generally, mesopores and a high specific surface area in the nanostructured catalyst could provide rich pore structures and abundant surface active sites, which are particularly favorable for heterogeneous catalytic oxidation processes, such as the adsorption, diffusion and activation of gaseous reactants.48,49 Therefore, the meso-γ-MnO2 with uniform mesopores (∼9.5 nm) and high specific surface area (123.2 m2/g) can exhibit outstanding catalytic performance for VOCs complete oxidation. 3.6. XPS Analysis. XPS technique was used to investigate the surface elemental states and adsorbed species of meso-γMnO2, α-MnO2 nanorods, and γ-MnO2 microurchins, and the corresponding results are listed in Table 2. Figure 8a reveals that the binding energies (BEs) of Mn 2p1/2 and Mn 2p3/2 are concentrated approximately at 653.7 and 641.9 eV, respectively, thus the spin orbit splitting is 11.8 eV, which approaches to those of reported MnO2 materials.50 On comparing the Mn

Table 2. Surface Components, H2-TPR Profiles, of AsObtained MnO2 Products H2-TPR reducibility product meso-γ-MnO2 γ-MnO2 microurchins α-MnO2 nanorods a

AOS

O A/ OL

peak 1

peak 2

H2 consumption (mmol·g−1)

3.64 3.87

0.57 0.27

230 282

360 388

6.74 10.41

3.72

0.31

280

−a

10.12

The − symbol represents that there is no H2-TPR peak.

2p spectrum, the Mn 3s spectrum was proved to be more accurate for identifying the oxidation state of Mn. Hence, the average oxidation state (AOS) of Mn can be achieved on the basis of the equation (AOS = 8.95−1.13ΔE3s).50 Figure 8b presents the Mn 3s spectrum of all the MnO2 samples. Obviously, the difference in BE (ΔE3s) between the two Mn 3s peak for meso-γ-MnO2, γ-MnO2 microurchins, and α-MnO2 nanorods is 4.7, 4.5, and 4.6 eV, respectively. Accordingly, the AOS of Mn for meso-γ-MnO2 is evaluated to be 3.64, which is lower than that of the γ-MnO2 microurchins (3.87) and αMnO2 nanorods (3.72). On the basis of the electroneutrality principle, the lowest AOS of Mn for meso-γ-MnO2 indicates the existence of richest surface oxygen vacancies,51 further confirming the above structural results revealed by Raman and HRTEM characterizations. Figure 8c shows the O 1s spectra of three MnO2 samples. Here, each spectrum is deconvoluted into two regions at ∼529.1−530.0 and ∼531.1−532.0 eV, which correspond to the surface-lattice oxygen species (OL) and surface-absorbed oxygen species (OA), respectively.43,52 It can be clearly seen from Table 2 that the OA/OL ratio of the three MnO2 samples decreases in the sequence of meso-γ-MnO2 (0.57) > α-MnO2 nanorods (0.31) > γ-MnO2 microurchins (0.27), following the order of their catalytic performances. This implies that the largest amount of OA exists on the meso-γ-MnO2 surface. It could be explained that surface oxygen vacancy in metal oxides plays a vital role as the active site for adsorbing and activating the gaseous O2 due to its coordinatively unsaturated environment,38,53 thus leading to the formation of OA locating on the surface.42 Therefore, the meso-γ-MnO2 containing the richest surface oxygen vacancies possesses the largest amount of OA on its surface. As indicated in the Mars−van Krevelen (MVK) mechanism, the OL species of the nanostructured catalyst were directly F

DOI: 10.1021/acs.iecr.9b02087 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 8. XPS spectra of (a) Mn 2p, (b) Mn 3s, and (c) O 1s for as-prepared MnO2 samples.

catalytic performances for toluene and o-xylene oxidation. Furthermore, the total H2 consumption amount of meso-γMnO2 is the lowest among the three MnO2 samples, further revealing that the meso-γ-MnO2 possesses the lowest AOS of Mn confirmed by XPS analysis. The better reducibility of meso-γ-MnO2 means that its OL species has higher mobility, which could benefit the redox process and enhance the catalytic performance for VOCs complete oxidation.39

involved in VOCs complete oxidation and then the reduced surface could be refilled by the OA species.46,54 During the oxidation process, the presence of the surface oxygen vacancies increases the mobility and reactivity of the OL species adjacent to the surface oxygen vacancies and thus the catalytic activity can be boosted.38 Meanwhile, the OA species timely replenish the consumed OL species and the reduced catalyst surface can be readily recovered. Therefore, richer surface oxygen vacancies and more OA species of meso-γ-MnO2 can synergistically contribute to its superior catalytic oxidation ability for VOCs removal. 3.7. H2-TPR Analysis. H2-TPR measurement was applied to probe into the reducibility of meso-γ-MnO2, α-MnO2 nanorods and γ-MnO2 microurchins, as displayed in Figure 9. As we can see, the meso-γ-MnO2 exhibits two reduction

4. CONCLUSIONS To sum up, a facile route was developed to prepare meso-γMnO2 via a disproportionation effect that involved treating meso-Mn2O3 with a dilute HNO3 solution, and the phase transformation process was thoroughly studied. As a result, the resultant meso-γ-MnO2 presented remarkably boosted performance for toluene (T10 = 179 °C and T90 = 219 °C) and oxylene catalytic oxidation (T10 = 178 °C and T90 = 237 °C) at a WHSV of 40, 000 mL·g−1·h−1, as compared with the nonporous counterparts, including γ-MnO2 microurchins and α-MnO2 nanorods. A series of characterizations indicated that the superior catalytic combustion activity of meso-γ-MnO2 was closely attributed to its ordered mesoporous framework, higher specific surface area, and richer surface oxygen vacancies. Moreover, the meso-γ-MnO2 exhibited excellent stability after ∼72 h of uninterrupted reaction, suggesting its possibility for the practical application of VOC removal. Our present work not only demonstrates a facile pathway to fabricate a novel mesoporous γ-MnO2 catalyst, but also offers a new insight into designing a high-efficiency γ-MnO2 catalyst toward VOC oxidation.

Figure 9. H2-TPR profiles of the three MnO2 samples.

■

peaks located at 230 and 360 °C, respectively. Similar reduction property is observed for γ-MnO2 microurchins, which also exhibits two reduction peaks, but both of them shift to higher temperatures at 282 and 388 °C, respectively. Generally, the following two reduction processes of MnO2 take place with the reduction temperature increasing: (1) Mn4+ → Mn3+ and (2) Mn3+ → Mn2+.38,55 Accordingly, for the two γMnO2 samples, the first peak is ascribed to the reduction of Mn4+ to Mn3+, and the second peak is due to the reduction of Mn3+ to Mn2+. However, only a single reduction peak (280 °C) is observed for α-MnO2 nanorods, which probably corresponds to the reductions of Mn4+ and Mn3+ simultaneously.56 The reduction temperature of meso-γ-MnO2 is significantly lower than that of the two nonporous MnO2 samples, indicating the reducibility decreases in the sequence of meso-γ-MnO2 > α-MnO2 nanorods > γ-MnO2 microurchins. This order corresponds well with the variation in their

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b02087. XRD pattern and FETEM images of meso-Mn2O3 precursor; FESEM images of γ-MnO2 microurchins and α-MnO2 nanorods; XRD pattern, FETEM images and N2 absorption−desorption results of the spent meso-γ-MnO2 after the stability test; o-xylene conversion over the three MnO2 samples at a WHSV of 40,000 mL· g−1·h−1; comparison of T10 and T90 values with the reported catalysts for o-xylene oxidation; catalytic activities of the reported catalysts for toluene complete combustion; catalytic activities of the reported catalysts for o-xylene complete combustion (PDF) G

DOI: 10.1021/acs.iecr.9b02087 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

■

applications. Chem. Commun. (Cambridge, U. K.) 2018, 54 (50), 6484−6502. (14) Wang, Y.; Arandiyan, H.; Scott, J.; Dai, H.; Amal, R. Hierarchically porous network-like Ni/Co3O4: noble metal-free catalysts for carbon dioxide methanation. Adv. Sustain. Syst. 2018, 2, 1700119. (15) Li, X.; Ma, J.; Yang, L.; He, G.; Zhang, C.; Zhang, R.; He, H. Oxygen vacancies induced by transition metal doping in gammaMnO2 for highly efficient ozone decomposition. Environ. Sci. Technol. 2018, 52 (51), 12685−12696. (16) Yang, P.; Fan, S.; Chen, Z.; Bao, G.; Zuo, S.; Qi, C. Synthesis of Nb2O5 based solid superacid materials for catalytic combustion of chlorinated VOCs. Appl. Catal., B 2018, 239, 114−124. (17) Kitchaev, D. A.; Dacek, S. T.; Sun, W.; Ceder, G. Thermodynamics of phase selection in MnO2 framework structures through alkali intercalation and hydration. J. Am. Chem. Soc. 2017, 139 (7), 2672−2681. (18) Zhang, Z.; Tian, Y.; Zhao, W.; Wu, P.; Zhang, J.; Zheng, L.; Ding, T.; Li, X. Hydroxyl promoted preferential and total oxidation of CO over ε-MnO2 catalyst. Catal. Today 2019, DOI: 10.1016/ j.cattod.2019.04.064. (19) Hunter, J. C. Preparation of a new crystal form of manganese dioxide: λ-MnO2. J. Solid State Chem. 1981, 39, 142−147. (20) Fu, X.; Feng, J.; Wang, H.; Ng, K. M. Room temperature synthesis of a novel gamma-MnO2 hollow structure for aerobic oxidation of benzyl alcohol. Nanotechnology 2009, 20 (37), 375601. (21) Pal, P.; Pahari, S. K.; Giri, A. K.; Pal, S.; Bajaj, H. C.; Panda, A. B. Hierarchically order porous lotus shaped nano-structured MnO2 through MnCO3: chelate mediated growth and shape dependent improved catalytic activity. J. Mater. Chem. A 2013, 1 (35), 10251. (22) Gong, P.; Xie, J.; Fang, D.; Han, D.; He, F.; Li, F.; Qi, K. Effects of surface physicochemical properties on NH3-SCR activity of MnO2 catalysts with different crystal structures. Chin. J. Catal. 2017, 38 (11), 1925−1934. (23) Sun, M.; Lan, B.; Lin, T.; Cheng, G.; Ye, F.; Yu, L.; Cheng, X.; Zheng, X. Controlled synthesis of nanostructured manganese oxide: crystalline evolution and catalytic activities. CrystEngComm 2013, 15 (35), 7010. (24) Zhang, J.; Li, Y.; Wang, L.; Zhang, C.; He, H. Catalytic oxidation of formaldehyde over manganese oxides with different crystal structures. Catal. Sci. Technol. 2015, 5, 2305−2313. (25) Xu, H.; Yan, N.; Qu, Z.; Liu, W.; Mei, J.; Huang, W.; Zhao, S. Gaseous heterogeneous catalytic reactions over Mn-based oxides for environmental applications: a critical review. Environ. Sci. Technol. 2017, 51 (16), 8879−8892. (26) Jiao, F.; Harrison, A.; Hill, A. H.; Bruce, P. G. Mesoporous Mn2O3 and Mn3O4 with Crystalline Walls. Adv. Mater. 2007, 19 (22), 4063−4066. (27) Zhou, M.; Zhang, X.; Wang, L.; Wei, J.; Wang, L.; Zhu, K.; Feng, B. Growth process and microwave absorption properties of nanostructured γ-MnO2 urchins. Mater. Chem. Phys. 2011, 130 (3), 1191−1194. (28) Wang, X.; Li, Y. Rational synthesis of α-MnO2 single-crystal nanorods. Chem. Commun. 2002, No. 7, 764−765. (29) Zeng, J. H.; Wang, Y. F.; Yang, Y.; Zhang, J. Synthesis of seaurchin shaped γ-MnO2 nanostructures and their application in lithium batteries. J. Mater. Chem. 2010, 20 (48), 10915. (30) Gao, T.; Fjellvag, H.; Norby, P. Structural and morphological evolution of beta-MnO2 nanorods during hydrothermal synthesis. Nanotechnology 2009, 20 (5), No. 055610. (31) Chen, D.; Ding, D.; Li, X.; Waller, G. H.; Xiong, X.; El-Sayed, M. A.; Liu, M. Probing the charge storage mechanism of a pseudocapacitive MnO2 electrode using in operando Raman spectroscopy. Chem. Mater. 2015, 27 (19), 6608−6619. (32) Dong, Y.; Li, K.; Jiang, P.; Wang, G.; Miao, H.; Zhang, J.; Zhang, C. Simple hydrothermal preparation of α-, β-, and γ-MnO2 and phase sensitivity in catalytic ozonation. RSC Adv. 2014, 4 (74), 39167.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lin Yu: 0000-0001-6187-6514 Author Contributions §

X.Z. and G.C. contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work has been provided the financial support from the National Natural Science Foundation of China (21576054, 51678160), Natural Science Foundation of Guangdong Province (2018A030310563), the Scientific Program of Guangdong Province (2016B020241003, 2016A020221033), the Foundation of Higher Education of Guangdong Province (2015KTSCX027), and the Scientific Program of Guangzhou (201704020202).

■

REFERENCES

(1) Guo, Y.; Yang, D.-P.; Liu, M.; Zhang, X.; Chen, Y.; Huang, J.; Li, Q.; Luque, R. Enhanced catalytic benzene oxidation over a novel waste-derived Ag/eggshell catalyst. J. Mater. Chem. A 2019, 7, 8832− 8844. (2) Li, Q.; Odoom-Wubah, T.; Zhou, Y.; Mulka, R.; Zheng, Y.; Huang, J.; Sun, D.; Li, Q. Coral-like CoMnOx as a highly active catalyst for benzene catalytic oxidation. Ind. Eng. Chem. Res. 2019, 58 (8), 2882−2890. (3) Yang, K.; Liu, Y.; Deng, J.; Zhao, X.; Yang, J.; Han, Z.; Hou, Z.; Dai, H. Three-dimensionally ordered mesoporous iron oxidesupported single-atom platinum: Highly active catalysts for benzene combustion. Appl. Catal., B 2019, 244, 650−659. (4) Wang, Y.; Yang, D.; Li, S.; Chen, M.; Guo, L.; Zhou, J. Ru/ hierarchical HZSM-5 zeolite as efficient bi-functional adsorbent/ catalyst for bulky aromatic VOCs elimination. Microporous Mesoporous Mater. 2018, 258, 17−25. (5) Deng, H.; Kang, S.; Wang, C.; He, H.; Zhang, C. Palladium supported on low-surface-area fiber-based materials for catalytic oxidation of volatile organic compounds. Chem. Eng. J. 2018, 348, 361−369. (6) Huang, S.; Zhang, C.; He, H. Effect of pretreatment on Pd/ Al2O3 catalyst for catalytic oxidation of o-xylene at low temperature. J. Environ. Sci. 2013, 25 (6), 1206−1212. (7) Qu, Z.; Bu, Y.; Qin, Y.; Wang, Y.; Fu, Q. The improved reactivity of manganese catalysts by Ag in catalytic oxidation of toluene. Appl. Catal., B 2013, 132−133, 353−362. (8) Wang, Y.; Arandiyan, H.; Liu, Y.; Liang, Y.; Peng, Y.; Bartlett, S.; Dai, H.; Rostamnia, S.; Li, J. Template-free scalable synthesis of flower-like Co3‑xMnxO4 spinel catalysts for toluene oxidation. ChemCatChem 2018, 10 (16), 3429−3434. (9) Kamal, M. S.; Razzak, S. A.; Hossain, M. M. Catalytic oxidation of volatile organic compounds (VOCs)-A review. Atmos. Environ. 2016, 140, 117−134. (10) Ren, Q.; Mo, S.; Peng, R.; Feng, Z.; Zhang, M.; Chen, L.; Fu, M.; Wu, J.; Ye, D. Controllable synthesis of 3D hierarchical Co3O4 nanocatalysts with various morphologies for the catalytic oxidation of toluene. J. Mater. Chem. A 2018, 6 (2), 498−509. (11) Deng, Y.; Tang, W.; Li, W.; Chen, Y. MnO2-nanowire@NiOnanosheet core-shell hybrid nanostructure derived interfacial effect for promoting catalytic oxidation activity. Catal. Today 2018, 308, 58−63. (12) Si, W.; Wang, Y.; Peng, Y.; Li, J. Selective dissolution of A-site cations in ABO3 perovskites: A new path to high-performance catalysts. Angew. Chem., Int. Ed. 2015, 54 (27), 7954−7. (13) Arandiyan, H.; Wang, Y.; Sun, H.; Rezaei, M.; Dai, H. Ordered meso- and macroporous perovskite oxide catalysts for emerging H

DOI: 10.1021/acs.iecr.9b02087 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research (33) Julien, C.; Massot, M.; Rangan, S.; Lemal, M.; Guyomard, D. Study of structural defects in γ-MnO2 by Raman spectroscopy. J. Raman Spectrosc. 2002, 33 (4), 223−228. (34) González-Prior, J.; López-Fonseca, R.; Gutiérrez-Ortiz, J. I.; de Rivas, B. Oxidation of 1,2-dichloroethane over nanocube-shaped Co3O4 catalysts. Appl. Catal., B 2016, 199, 384−393. (35) Gao, R.; Liu, L.; Hu, Z.; Zhang, P.; Cao, X.; Wang, B.; Liu, X. The role of oxygen vacancies in improving the performance of CoO as a bifunctional cathode catalyst for rechargeable Li-O2 batteries. J. Mater. Chem. A 2015, 3 (34), 17598−17605. (36) Gao, T.; Fjellvag, H.; Norby, P. A comparison study on Raman scattering properties of alpha- and beta-MnO2. Anal. Chim. Acta 2009, 648 (2), 235−9. (37) Zhu, G.; Zhu, J.; Jiang, W.; Zhang, Z.; Wang, J.; Zhu, Y.; Zhang, Q. Surface oxygen vacancy induced α-MnO2 nanofiber for highly efficient ozone elimination. Appl. Catal., B 2017, 209, 729−737. (38) Huang, N.; Qu, Z.; Dong, C.; Qin, Y.; Duan, X. Superior performance of α@β-MnO2 for the toluene oxidation: Active interface and oxygen vacancy. Appl. Catal., A 2018, 560, 195−205. (39) Si, W.; Wang, Y.; Peng, Y.; Li, X.; Li, K.; Li, J. A high-efficiency gamma-MnO2-like catalyst in toluene combustion. Chem. Commun. (Cambridge, U. K.) 2015, 51 (81), 14977−80. (40) Walanda, D. K.; Lawrance, G. A.; Donne, S. W. Hydrothermal MnO2: synthesis, structure, morphology and discharge performance. J. Power Sources 2005, 139 (1−2), 325−341. (41) Deng, J.; He, S.; Xie, S.; Yang, H.; Liu, Y.; Guo, G.; Dai, H. Ultralow loading of silver nanoparticles on Mn2O3 nanowires derived with molten salts: a high-efficiency catalyst for the oxidative removal of toluene. Environ. Sci. Technol. 2015, 49 (18), 11089−95. (42) Xie, S.; Dai, H.; Deng, J.; Yang, H.; Han, W.; Arandiyan, H.; Guo, G. Preparation and high catalytic performance of Au/3DOM Mn2O3 for the oxidation of carbon monoxide and toluene. J. Hazard. Mater. 2014, 279, 392−401. (43) Shi, F.; Wang, F.; Dai, H.; Dai, J.; Deng, J.; Liu, Y.; Bai, G.; Ji, K.; Au, C. Rod-, flower-, and dumbbell-like MnO2: Highly active catalysts for the combustion of toluene. Appl. Catal., A 2012, 433− 434, 206−213. (44) Zhang, C.; Huang, H.; Li, G.; Wang, L.; Song, L.; Li, X. Zeolitic acidity as a promoter for the catalytic oxidation of toluene over MnOx/HZSM-5 catalysts. Catal. Today 2019, 327, 374−381. (45) Du, Y.; Meng, Q.; Wang, J.; Yan, J.; Fan, H.; Liu, Y.; Dai, H. Three-dimensional mesoporous manganese oxides and cobalt oxides: High-efficiency catalysts for the removal of toluene and carbon monoxide. Microporous Mesoporous Mater. 2012, 162, 199−206. (46) Cheng, G.; Yu, L.; He, B.; Sun, M.; Zhang, B.; Ye, W.; Lan, B. Catalytic combustion of dimethyl ether over α-MnO2 nanostructures with different morphologies. Appl. Surf. Sci. 2017, 409, 223−231. (47) Thommes, M.; Kaneko, K.; Neimark, A. V.; Olivier, J. P.; Rodriguez-Reinoso, F.; Rouquerol, J.; Sing, K. S. W. Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report). Pure Appl. Chem. 2015, 87 (9−10), 1051−1069. (48) Ren, Q.; Feng, Z.; Mo, S.; Huang, C.; Li, S.; Zhang, W.; Chen, L.; Fu, M.; Wu, J.; Ye, D. 1D-Co3O4, 2D-Co3O4, 3D-Co3O4 for catalytic oxidation of toluene. Catal. Today 2019, 332, 160−167. (49) Xie, S.; Deng, J.; Liu, Y.; Zhang, Z.; Yang, H.; Jiang, Y.; Arandiyan, H.; Dai, H.; Au, C. T. Excellent catalytic performance, thermal stability, and water resistance of 3DOM Mn2O3-supported Au−Pd alloy nanoparticles for the complete oxidation of toluene. Appl. Catal., A 2015, 507, 82−90. (50) Tang, X.; Li, Y.; Chen, J.; Xu, Y.; Shen, W. Synthesis, characterization, and catalytic application of titanium-cryptomelane nanorods/fibers. Microporous Mesoporous Mater. 2007, 103 (1−3), 250−256. (51) Jia, J.; Zhang, P.; Chen, L. Catalytic decomposition of gaseous ozone over manganese dioxides with different crystal structures. Appl. Catal., B 2016, 189, 210−218. (52) Genuino, H. C.; Meng, Y.; Horvath, D. T.; Kuo, C.-H.; Seraji, M. S.; Morey, A. M.; Joesten, R. L.; Suib, S. L. Enhancement of

catalytic activities of octahedral molecular sieve manganese oxide for total and preferential CO oxidation through vanadium ion framework substitution. ChemCatChem 2013, 5 (8), 2306−2317. (53) Sun, L.; Cao, Q.; Hu, B.; Li, J.; Hao, J.; Jing, G.; Tang, X. Synthesis, characterization and catalytic activities of vanadiumcryptomelane manganese oxides in low-temperature NO reduction with NH3. Appl. Catal., A 2011, 393, 323−330. (54) Liu, Y.; Liu, Y.; Guo, Y.; Xu, J.; Xu, X.; Fang, X.; Liu, J.; Chen, W.; Arandiyan, H.; Wang, X. Tuning SnO2 surface area for catalytic toluene deep oxidation: on the inherent factors determining the reactivity. Ind. Eng. Chem. Res. 2018, 57 (42), 14052−14063. (55) Xie, Y.; Yu, Y.; Gong, X.; Guo, Y.; Guo, Y.; Wang, Y.; Lu, G. Effect of the crystal plane figure on the catalytic performance of MnO2 for the total oxidation of propane. CrystEngComm 2015, 17 (15), 3005−3014. (56) Kim, S. C.; Shim, W. G. Catalytic combustion of VOCs over a series of manganese oxide catalysts. Appl. Catal., B 2010, 98 (3−4), 180−185.

I

DOI: 10.1021/acs.iecr.9b02087 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX