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Catalytic behaviors of manganese oxides in electro-assisted catalytic air oxidation reaction: Influence of structural properties
Applied Surface Science 511 (2020) 145536
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Catalytic behaviors of manganese oxides in electro-assisted catalytic air oxidation reaction: Influence of structural properties Min Sun, Li-Ming Fang, Xiao-Hui Hong, Feng Zhang, Lin-Feng Zhai
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Anhui Province Key Laboratory of Advanced Catalytic Materials and Reaction Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Manganese oxide Catalytic wet air oxidation Bisphenol A Electric field Room condition
The electro-assisted catalytic wet air oxidation (ECWAO) process is effective for pollutants removal, by taking advantage of an anodic electric field to activate O2 on a catalyst surface. Herein, three different manganese oxides, including MnO2, Mn3O4 and MnO, are fabricated on graphite felt (GF) support to serve as the catalysts of ECWAO reaction. Structural properties of these manganese oxides are analyzed, and their catalytic behaviors in ECWAO reaction are investigated with bisphenol A as a probe pollutant. The manganese centers on the surface of manganese oxides are found to be present in multi-valence states, favoring the formation of chemisorbed oxygen species which are responsible for pollutants oxidation in the ECWAO process. Therefore, the manganese oxides demonstrate good and stable activity for catalytic oxidation of bisphenol A under room condition. The sequence of their catalytic activities is as follows: Mn3O4 > MnO > MnO2, depending on their low-temperature reducibility and oxygen mobility in them. These results illustrate a guideline to the design of highly active manganese oxide catalysts for the ECWAO reaction.
1. Introduction Wet air oxidation (WAO) serves as an effective aqueous oxidation technique for de-pollution of industrial wastewaters, especially those containing toxic and/or biorefractory organic pollutants [1]. In noncatalytic WAO processes, organic pollutants are oxidized at high temperatures and pressures to achieve their intensive mineralization within a short time. Application of a catalyst can significantly reduce the energy demand of WAO and alleviate the severity of operation condition [2]. In catalytic wet air oxidation (CWAO) processes, the catalysts accelerate the oxidation reaction by activating the reactant molecules or the oxygen (O2) molecules [3]. So far, great achievements have been obtained in the development of catalysts for reducing the temperature of CWAO. However, an operation temperature higher than room temperature is still required due to the high activation energy of the aqueous air oxidation reaction [4]. Recently, we have developed an electro-assisted catalytic wet air oxidation (ECWAO) process, which takes advantage of an anodic electric field to initiate the oxidation of organic pollutants by air under room condition [5–8]. In the process, O2 molecules are first adsorbed onto the metal oxide catalyst and undergo dissociation to form chemisorbed oxygen species. These chemisorbed oxygen species are then activated in the anode electric field so that they are able to oxidize ⁎
organic pollutants under room condition. Such an ECWAO process is eco-friendly and energy-saving as compared to conventional WAO/ CWAO processes needing higher operation temperature and/or pressure. Notably, the ECWAO process with MnO or MnO2 catalyst show extensive effectiveness in mineralizing dyes, pharmaceuticals and personal care products [5,8]. The date, manganese oxide catalysts have been widely employed in various heterogeneous catalytic reactions for environmental applications [9–12]. The catalytic activities of manganese oxides are dependent upon the valence states of manganese as well as the participation of active oxygen species, including adsorbed oxygen, oxygen vacancy and lattice oxygen, in the oxidation reactions [13,14]. For example, the Mn3+ species were found to be much more active than the Mn2+/Mn4+ species in photocatalytic water oxidation reaction, because Mn3+-rich structures with labile MneO bonds allow for facile formation of surface Mn-OH2 species and cleavage of MneO2 bonds [15]. Kim and Shim reported a series of manganese oxide catalysts for combustion of volatile organic compounds [16]. They found their catalytic activities were ordered as Mn3O4 > Mn2O3 > MnO2, which was correlated with oxygen mobility on the catalysts. These results address the precise control over the structural properties of manganese oxides to present highly active catalysts for the ECWAO reaction. Herein, a series of manganese oxides, including MnO2, Mn3O4 and
Corresponding author. E-mail address: [email protected] (L.-F. Zhai).
https://doi.org/10.1016/j.apsusc.2020.145536 Received 5 November 2019; Received in revised form 16 January 2020; Accepted 24 January 2020 Available online 25 January 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.
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2.3. Cyclic voltammetry (CV)
MnO, are fabricated on a graphite felt (GF), and the catalytic behaviors of MnOx catalysts in ECWAO process are systematically analyzed. Valence states of manganese centers and oxygen species in MnOx are characterized, and reducibility of MnOx as well as oxygen mobility in them is investigated. A correlation between structural property and catalytic activity of MnOx is constructed to explain their different catalytic behaviors in the ECWAO process.
The CV was conducted in a three-electrode system with platinum wire as counter electrode and saturated calomel electrode (SCE) as reference electrode. The working electrode was fabricated by loading the MnOx/GF composite powder on a glassy carbon electrode (5 mm in diameter). Briefly, 5 mg of MnOx/GF composite powder with a mean particles size of less than 20 μm was dispersed in 450 μL of anhydrous ethanol and 50 μL of Nafion solution (5 wt%) by ultrasonication. 10 μL of the resulting ink was dropped onto the glassy carbon electrode and dried at room temperature. The CV was carried out in 50 mM Na2SO4 solution (pH 7.0) from −0.5 to +1.5 V at 0.1 V s−1.
2. Material and methods 2.1. Preparation of MnOx/GF composites and structure characterization The MnOx/GF composites were prepared by a hydrothermal and pyrolysis method. Briefly, a piece of GF (4 × 2.5 cm2, 2 mm in thickness) was pretreated in a mixture of HNO3 and H2SO4 (1:1 in volume) for 1 h at 100 °C, and transferred to an autoclave filled with 30 mL of 10 g L-1 KMnO4. Here, the GF was chosen as the support of MnOx due to its good conductivity, favorable chemical stability and large surface area [17,18], as well as a reducing agent of KMnO4 during hydrothermal treatment [5]. The autoclave was sealed and maintained at 120 °C for 24 h. Then, the GF loaded with manganese compound was took out from the cooled autoclave, and calcined at 350 °C in air for 2 h to obtain the MnO2/GF composite. The Mn3O4/GF and MnO/GF composites were prepared in nitrogen atmosphere, at calcination temperature of 350 and 700 °C, respectively. Crystalline structures and morphologies of the MnOx/GF composites were characterized on a Bruker D8 advance X-ray diffractometer (XRD, Holland) and a JEM2100F transmission electron microscope (TEM, Japan). Manganese contents in the composites were determined by thermogravimetry (TG) on a TGA DT-50 apparatus (Japan) in air atmosphere. Valence states of manganese and oxygen species in the composites were identified using an ESCALAB 250 X-ray photoelectron spectrometer (XPS, USA). The composites were ground into powder for XRD, TEM, TG and XPS analyses.
2.4. Operation of the ECWAO process The ECWAO process was conducted in a 175 mL single-chamber glass-made reactor, with the prepared MnOx/GF composite as working electrode, a platinum wire (0.5 mm in diameter) as counter electrode and a SCE as the reference. The electrolyte was 130 mL of Na2SO4 solution (50 mM, pH 7.0) which contains 100 mg L-1 bisphenol A (BPA) as a probe pollutant. The MnOx/GF electrode was first electrochemically activated at +1.0 V (vs. SCE) for 2 h in the Na2SO4 electrolyte, and then BPA was added. The degradation of BPA was performed at a constant anodic current density of 10 A m−2, and air was bubbled at a rate of 1.5 L min−1 to supply sufficient dissolved O2. The concentration of BPA was determined on a Waters high performance liquid chromatography system (USA) equipped with a multiple wavelength UV detector and a SunFireTM C18 column (150 mm × 4.6 mm, 5 μm), and total organic carbon (TOC) was measured on a HTM-CT1000M TOC analyzer (Tailin, China). Intermediate products of BPA were identified by gas chromatography-mass spectrometry (GC–MS) on a Thermo Trace 1300 system equipped with a TR-V1 capillary column (30 m × 0.25 mm ID × 1.4 μm film, USA). The analytical procedure of GC–MS and extraction of intermediate products were described in our previous work [6]. Dissolved manganese was quantified by inductively coupled plasma and optical emission spectrometer (ICP-OES) on an Agilent 700 apparatus (USA). All experiments were conducted in triplicate, and average values with standard deviations were presented.
2.2. Temperature programmed reduction of hydrogen (H2-TPR) and temperature programmed desorption of O2 (O2-TPD)
3. Results and discussion
H2-TPR and O2-TPD profiles of the MnOx/GF composites were obtained on a Quantachrome ChemBET TPR/TPD apparatus (USA). The MnOx/GF composite powder was pretreated in helium gas at 100 °C for 30 min to remove adsorbed water. The flowing gas was 5 vol% H2/ helium mixed gas for H2-TPR and helium gas for O2-TPD, at a flow rate of 110 mL min−1. The sample was heated from room temperature to 800 °C at a rate of 10 °C min−1.
3.1. Crystalline structure and morphology of MnOx on the GF The XRD patterns of as-prepared MnOx/GF composites are shown in Fig. 1a. The broad peak located at 2θ value of 25.7° is ascribed to the (0 0 2) plane of graphic carbon of GF support. For the MnO2/GF
Fig. 1. XRD patterns (a) and TG curves (b) of the MnOx/GF composites prepared under different calcination conditions. 2
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Fig. 2. TEM images of MnO2/GF (a, b), Mn3O4/GF (c, d) and MnO/GF (e, f).
0.248 nm belong to the (1 1 2) and (2 1 1) planes of Mn3O4 (JCPDS No. 24-0734). The clear interplanar spacing on high resolution TEM of MnO/GF is 0.222 nm, in good agreement with the (2 0 0) plane of MnO (JCPDS No. 07-0230) (Fig. 2f).
composite, the diffraction peaks found at 37.1°, 41.5°, 49.8°, 56.0°, 64.8°, and 69.7° corresponds well to the tetragonal phase of α-MnO2 (JCPDS No. 44-0141). However, the intensity of these peaks is very weak, indicating poor crystallinity of MnO2. On the XRD pattern of Mn3O4/GF composite, diffraction peaks are clearly observed at 29.0°, 31.1°, 32.5°, 36.2°, 38.1°, 44.6°, 51.0°, 53.9°, 56.0°, 58.8°, 60.0°, and 64.7°, which are all assignable to the tetragonal phase of Mn3O4 (JCPDS No. 24-0734). The strong diffraction peaks appearing at 35.0°, 40.6° and 58.8° on the XRD pattern of MnO/GF composite confirm tetragonal phase of MnO (JCPDS No. 07-0230) with good crystallinity. As shown in Fig. 1b, the TG curves of MnOx/GF composites reveal a sharp weight loss over 450–750 °C, reflecting the combustion of GF as well as the conversion from MnOx to Mn2O3 in air. Manganese contents in the three different MnOx/GF composites are almost the same as 21.2 wt%, according to the weight percentage of the final product Mn2O3 in TG analysis (30.5%). The morphologies of as-prepared MnOx/GF composites are displayed by TEM images given in Fig. 2. As shown in Fig. 2a, the MnO2 is present as nanorods with diameter ranging from 10 to 30 nm, and the high resolution TEM image in Fig. 2b reveals a set of lattice fringes with interplanar distances of 0.490 and 0.240 nm attributed to the (2 0 0) and (2 1 1) planes of α-MnO2 (JCPDS No. 44-0141). The Mn3O4 and MnO are present as irregular particles with sizes no more than 100 nm (Fig. 2c and 2e). On the high resolution TEM image of Mn3O4/GF (Fig. 2d), the lattice fringes with interplanar distances of 0.308 and
3.2. Valence states of manganese and surface oxygen species The valence states of manganese as well as oxygen species on the surface of MnOx/GF composites are investigated by XPS analysis. As shown in Fig. 3a, the Mn 3 s level is split into two peaks with separation energy of 4.9 eV for MnO2, 5.4 eV for Mn3O4 and 5.9 eV for MnO, corresponding well with the oxidation state of manganese from high to low valence in these MnOx [19–21]. The Mn 2p XPS spectra of all MnOx/GF samples in Fig. 3b are split to Mn 2p3/2 and Mn 2p1/2 peaks located at ca. 642.2 and 653.9 eV, respectively. The Mn 2p3/2 peak can be divided into three components at 640.8 eV, 642.0 eV, and 643.0 eV, together with a satellite at 645.8 eV. The three components are assigned to the surface Mn2+ (640.8 eV), Mn3+ (642.0 eV), and Mn4+ species (643.0 eV), respectively [22,23]. Fig. 3c shows the asymmetrical O1s signal is also divided into three components with binding energies of 529.8, 531.4, and 533.2 eV, which are attributable to the surface lattice oxygen, chemisorbed oxygen, and physically adsorbed oxygen species, respectively [24,25]. The accurate contents of surface manganese and oxygen species, calculated by a quantitative analysis from their peak areas, are also given in Fig. 3. The coexistence of Mn2+, Mn3+ and 3
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Fig. 3. Mn 3s (a), Mn 2p (b) and O 1s (c) XPS spectra of the MnOx/GF composites. Fig. 4. H2-TPR profiles (a), O2-TPD profiles (b) and CV curves (c) of the MnOx/ GF composites.
Mn4+ on the surface of MnOx has an important impact on the formation of surface oxygen species, because mixed-valence state of metal can promote oxygen mobility in the oxides [26]. Notably, all the three manganese oxides display high amounts of chemisorbed oxygen species, i.e. O2- and O-, on their surfaces. As shown in Fig. 3b, the low valence Mn2+ and Mn3+ species are found on the surface of MnO2, which usually appear at oxygen vacancies to combine with chemisorbed oxygen species [27]. For MnO, the high valence Mn3+ and Mn4+ species on its surface are generated from the oxidation of Mn2+, accompanied by the transformation of adsorbed O2 into chemisorbed oxygen species [8,28]. As for Mn3O4, the coexistence of different
manganese valence states in its spinel structure endows high mobility of its structural lattice oxygen, which can convert to chemisorbed oxygen species via the cleavage of the labile Mn3+-O bond [15]. The results of previous work have demonstrated the involvement of chemisorbed oxygen species, as the direct oxidants of pollutants, in the ECWAO process [5,8]. Therefore, an effective ECWAO process desires abundant chemisorbed oxygen species enriched on the catalyst surface. The relative contents of chemisorbed oxygen species in as prepared MnOx are ordered as Mn3O4 > MnO > MnO2. 4
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Fig. 5. Pseudo-first-order kinetics for BPA oxidation in the ECWAO process with MnO2/GF (a), Mn3O4/GF (b) and MnO/GF (c) electrodes at different currents and the comparison of their rate constants (d).
Fig. 6. Proposed degradation pathway of PBA in the ECWAO process with MnOx/GF electrodes.
which displays no obvious H2-reduction peak, the H2-TPR profiles of MnOx/GF composites feature a series of distinct peaks arising from the reduction of MnOx by H2. The reduction of MnO2 can be divided into low-temperature reduction (ca. 300–350 °C) and high-temperature
3.3. Reducibility of MnOx and oxygen mobility The H2-TPR is carried out to investigate the reducibility of MnOx. As shown in Fig. 4a, in comparison to the H2-TPR profile of the GF support 5
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Fig. 8. TG curves (a), XRD patterns (b) and Mn 2p XPS spectra (c) of the MnOx/ GF electrodes underwent eight fed-batch runs.
reduction peak is related to the active oxygen species located on the surface of metal oxide [30,31]. It can be observed from Fig. 4a that the Mn3O4/GF and MnO/GF composites have better low-temperature reducibility than the MnO2/GF composite, suggesting higher activity of their surface oxygen species in oxidation reaction. Mobility of oxygen species in as-prepared MnOx is investigated by O2-TPD. As shown in Fig. 4b, the low-temperature peaks in the region of 150–300 °C result from evolution of chemisorbed oxygen species weakly bonded to the surface of MnOx, and the high-temperature ones from 300 to 800 °C are due to desorption of lattice oxygen in MnOx [32,33]. Notably, the liberation of chemisorbed oxygen species can be initiated below 200 °C, suggesting good oxygen mobility in the MnOx. The liberation of chemisorbed oxygen species in Mn3O4 is initiated at
Fig. 7. Mineralization of BPA in eight successive fed-batch runs of ECWAO process with MnO2/GF (a), Mn3O4/GF (b) and MnO/GF (c) electrodes.
reduction (ca. 350–400 °C), corresponding to the successive phase conversion from MnO2 to MnO via Mn3O4 [29]. As for Mn3O4, the weak peak at 200–350 °C is ascribed to the reduction of surface Mn4+, and the strong peak at 350–550 °C is to the reduction of Mn3O4 phase into MnO phase. While MnO itself cannot be reduced in the experimental temperature range, the reduction of Mn4+/Mn3+ on its surface gives a weak peak at ca. 250–350 °C. As is known, the low-temperature H26
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reduction of high-valence manganese. Fig. 8c shows the Mn 2p XPS spectra of used MnOx/GF electrodes. It is found that the proportion of Mn4+ in the MnOx is reduced after the ECWAO reaction. Meanwhile, the proportion of Mn2+ in MnO2 and Mn3O4 is greatly increased. For MnO, increment of Mn3+ proportion is observed. These results confirm the involvement of multiple valences of manganese, notably Mn2+/ Mn3+ redox couple, in the ECWAO reaction.
the lowest temperature, indicating relatively higher oxygen mobility. In comparison, chemisorbed oxygen species in MnO2 are relatively inert as evidenced by the highest temperature required for their liberation. The electro-oxidation of chemisorbed oxygen species in anodic electric field is an important O2 activation step in the ECWAO process. The CV is performed to evaluate reactivity of chemisorbed oxygen species in the ECWAO process. As shown in Fig. 4c, the anodic peaks representing the activation of chemisorbed oxygen species in the anodic electric field [5,8], are found between 0.0 and +0.5 V (vs. SCE) on the cyclic voltammograms of all the MnOx/GF composites. Intensities of these peaks are in the order Mn3O4/GF > MnO/GF > MnO2/GF, in good agreement with the relative contents of chemisorbed oxygen species as well as oxygen mobility in corresponding MnOx.
4. Conclusion MnOx on GF support demonstrate favorable and stable activities in catalyzing the air oxidation of BPA with assistance of anodic electric field. Mn2+, Mn3+ and Mn4+ are found to coexist on the surface of MnOx, which facilitates the formation of chemisorbed oxygen species as the direct oxidants of BPA in ECWAO reaction. The relative contents of chemisorbed oxygen species in as prepared MnOx are ordered as Mn3O4 > MnO > MnO2. Chemisorbed oxygen species of Mn3O4 possess the highest low-temperature reactivity. As a result, Mn3O4 demonstrates the highest catalytic activity towards ECWAO reaction. In comparison, chemisorbed oxygen species of MnO2 are relatively inert and it displays the lowest catalytic activity among the tested MnOx.
3.4. Catalytic activities of MnOx in the ECWAO process The degradation of 100 mg L-1 BPA in the ECWAO process is studied by monitoring the concentration of BPA ([BPA], mg L-1) along with the reaction time (t, min). The experimental results regarding to [BPA] and t are fitted to a pseudo-first-order kinetic equation:
d[BPA] = -k '[BPA] dt
(1)
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where k′ is a pseudo-first-order rate constant. As shown in Fig. 5a, a linear relationship between ln([BPA]0/[BPA]t) and t is observed at all applied currents, suggesting a pseudo-first-order kinetics for BPA oxidation. The degradation rate of BPA is increased with increase of applied current, as expected from the fact that the activation of chemisorbed oxygen species in MnOx is favored in a stronger electric field. Totally speaking, BPA can be completely removed within 120 min on the MnOx/GF composites, suggesting favorable catalytic activities of the prepared MnOx. At any applied current, the highest k′ is always obtained on the Mn3O4/GF electrode, and the lowest is on the MnO2/GF electrode. Therefore, Mn3O4 is catalytically the most active in the ECWAO process, and MnO2 is the most inert. Moreover, BPA can be deeply mineralized by the ECWAO process. The mineralization efficiency of BPA is higher than 97% after it is treated on the Mn3O4/GF or MnO/GF electrode for 180 min. Such a value is lower, i.e. approximately 90%, on the MnO2/GF electrode. Intermediate products generated from BPA degradation are identified by GC–MS. As shown in Fig. 6, p-isopropenylphenol (m/z = 134) and hydroquinone (m/z = 110) are detected as intermediate products of BPA. Based on the evolution profiles of these intermediate products, BPA is proposed to be first cleaved at the CeC bond between aromatic rings with p-isopropenylphenol and hydroquinone as products. The pisopropenylphenol and hydroquinone are subsequently oxidized into pbenzoquinone. Further oxidation of p-benzoquinone results in CO2, H2O and small molecular organic products, as occurs in CWAO process [34]. Fig. 7 demonstrates mineralization of BPA in eight successive fedbatch runs. No drop of the degradation rate or mineralization efficiency was observed during the eight runs, suggesting good reusability of the MnOx/GF electrodes. After each run, concentrations of manganese in the solution are determined to be lowered that 0.5 mg L-1, which are below the discharge limit of manganese in China (GB8978-1996). As a matter of fact, manganese contents in the MnOx/GF electrodes underwent eight runs are as high as 20.7% (MnO2), 20.1% (Mn3O4) and 21.2% (MnO), respectively, reflecting excellent stability of the MnOx/ GF electrodes (Fig. 8a). While negligible metal loss was observed during the eight cycles, the diffraction peaks of MnOx on XRD pattern of the used electrodes are obviously weaker as compared to the fresh ones (Fig. 8b), suggesting the participation of manganese in the ECWAO reaction. As is known, the chemisorption of O2 and consumption of chemisorbed oxygen species on MnOx surface is manipulated by the inter-conversion of different valence states of manganese [27,28]. The low-valence manganese is oxidized upon chemisorption of O2, and the consumption of chemisorbed oxygen species is accompanied by the
Min Sun: Conceptualization, Methodology, Supervision. Li-Ming Fang: Conceptualization, Data curation, Writing - original draft. XiaoHui Hong: Visualization, Investigation. Feng Zhang: Validation. LinFeng Zhai: Conceptualization, Writing - review & editing, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The authors wish to thank the National Natural Science Foundation of China (51478157, 21707133) and the Program for New Century Excellent Talents in University (NCET-13-0767) for financial support of this work. References [1] J. Levec, A. Pintar, Catalytic wet-air oxidation processes: a review, Catal. Today 124 (2007) 172–184. [2] K.M. Sushma, A.K. Saroha, A.K. Saroha, Performance of various catalysts on treatment of refractory pollutants in industrial wastewater by catalytic wet air oxidation: A review, J. Environ. Manage. 228 (2018) 169–188. [3] K.H. Kim, S.K. Ihm, Heterogeneous catalytic wet air oxidation of refractory organic pollutants in industrial wastewaters: A review, J. Hazard. Mater. 186 (2011) 16–34. [4] H. Debellefontaine, J.N. Foussard, Wet air oxidation for the treatment of industrial wastes. Chemical aspects, reactor design and industrial applications in Europe, Waste Manage. 20 (2000) 15–25. [5] M. Sun, L.M. Fang, J.Q. Liu, F. Zhang, L.F. Zhai, Electro-activation of O2 on MnO2/ graphite felt for efficient oxidation of water contaminants under room condition, Chemosphere 234 (2019) 269–276. [6] M. Sun, Y. Zhang, S.Y. Kong, L.F. Zhai, S. Wang, Excellent performance of electroassisted catalytic wet air oxidation of refractory organic pollutants, Water Res. 158 (2019) 313–321. [7] M. Sun, Y. Zhang, H.H. Liu, F. Zhang, L.F. Zhai, S. Wang, Room-temperature air oxidation of organic pollutants via electrocatalysis by nanoscaled Co-CoO on graphite felt anode, Environ. Int. 131 (2019) 104977. [8] L.F. Zhai, M.F. Duan, M.X. Qiao, M. Sun, S. Wang, Electro-assisted catalytic wet air oxidation of organic pollutants on a MnO@C/GF anode under room condition, Appl. Catal. B- Environ. 256 (2019) 117822. [9] C. Liu, J.W. Shi, C. Gao, C. Niu, Manganese oxide-based catalysts for low-temperature selective catalytic reduction of NOx with NH3: A review, Appl. Catal. A Gen. 522 (2016) 54–69. [10] L. Miao, J. Wang, P. Zhang, Review on manganese dioxide for catalytic oxidation of airborne formaldehyde, Appl Surf. Sci. 466 (2019) 441–453. [11] H. Xu, N. Yan, Z. Qu, W. Liu, J. Mei, W. Huang, S. Zhao, Gaseous heterogeneous
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Full Length Article
Catalytic behaviors of manganese oxides in electro-assisted catalytic air oxidation reaction: Influence of structural properties Min Sun, Li-Ming Fang, Xiao-Hui Hong, Feng Zhang, Lin-Feng Zhai
T
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Anhui Province Key Laboratory of Advanced Catalytic Materials and Reaction Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Manganese oxide Catalytic wet air oxidation Bisphenol A Electric field Room condition
The electro-assisted catalytic wet air oxidation (ECWAO) process is effective for pollutants removal, by taking advantage of an anodic electric field to activate O2 on a catalyst surface. Herein, three different manganese oxides, including MnO2, Mn3O4 and MnO, are fabricated on graphite felt (GF) support to serve as the catalysts of ECWAO reaction. Structural properties of these manganese oxides are analyzed, and their catalytic behaviors in ECWAO reaction are investigated with bisphenol A as a probe pollutant. The manganese centers on the surface of manganese oxides are found to be present in multi-valence states, favoring the formation of chemisorbed oxygen species which are responsible for pollutants oxidation in the ECWAO process. Therefore, the manganese oxides demonstrate good and stable activity for catalytic oxidation of bisphenol A under room condition. The sequence of their catalytic activities is as follows: Mn3O4 > MnO > MnO2, depending on their low-temperature reducibility and oxygen mobility in them. These results illustrate a guideline to the design of highly active manganese oxide catalysts for the ECWAO reaction.
1. Introduction Wet air oxidation (WAO) serves as an effective aqueous oxidation technique for de-pollution of industrial wastewaters, especially those containing toxic and/or biorefractory organic pollutants [1]. In noncatalytic WAO processes, organic pollutants are oxidized at high temperatures and pressures to achieve their intensive mineralization within a short time. Application of a catalyst can significantly reduce the energy demand of WAO and alleviate the severity of operation condition [2]. In catalytic wet air oxidation (CWAO) processes, the catalysts accelerate the oxidation reaction by activating the reactant molecules or the oxygen (O2) molecules [3]. So far, great achievements have been obtained in the development of catalysts for reducing the temperature of CWAO. However, an operation temperature higher than room temperature is still required due to the high activation energy of the aqueous air oxidation reaction [4]. Recently, we have developed an electro-assisted catalytic wet air oxidation (ECWAO) process, which takes advantage of an anodic electric field to initiate the oxidation of organic pollutants by air under room condition [5–8]. In the process, O2 molecules are first adsorbed onto the metal oxide catalyst and undergo dissociation to form chemisorbed oxygen species. These chemisorbed oxygen species are then activated in the anode electric field so that they are able to oxidize ⁎
organic pollutants under room condition. Such an ECWAO process is eco-friendly and energy-saving as compared to conventional WAO/ CWAO processes needing higher operation temperature and/or pressure. Notably, the ECWAO process with MnO or MnO2 catalyst show extensive effectiveness in mineralizing dyes, pharmaceuticals and personal care products [5,8]. The date, manganese oxide catalysts have been widely employed in various heterogeneous catalytic reactions for environmental applications [9–12]. The catalytic activities of manganese oxides are dependent upon the valence states of manganese as well as the participation of active oxygen species, including adsorbed oxygen, oxygen vacancy and lattice oxygen, in the oxidation reactions [13,14]. For example, the Mn3+ species were found to be much more active than the Mn2+/Mn4+ species in photocatalytic water oxidation reaction, because Mn3+-rich structures with labile MneO bonds allow for facile formation of surface Mn-OH2 species and cleavage of MneO2 bonds [15]. Kim and Shim reported a series of manganese oxide catalysts for combustion of volatile organic compounds [16]. They found their catalytic activities were ordered as Mn3O4 > Mn2O3 > MnO2, which was correlated with oxygen mobility on the catalysts. These results address the precise control over the structural properties of manganese oxides to present highly active catalysts for the ECWAO reaction. Herein, a series of manganese oxides, including MnO2, Mn3O4 and
Corresponding author. E-mail address: [email protected] (L.-F. Zhai).
https://doi.org/10.1016/j.apsusc.2020.145536 Received 5 November 2019; Received in revised form 16 January 2020; Accepted 24 January 2020 Available online 25 January 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.
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2.3. Cyclic voltammetry (CV)
MnO, are fabricated on a graphite felt (GF), and the catalytic behaviors of MnOx catalysts in ECWAO process are systematically analyzed. Valence states of manganese centers and oxygen species in MnOx are characterized, and reducibility of MnOx as well as oxygen mobility in them is investigated. A correlation between structural property and catalytic activity of MnOx is constructed to explain their different catalytic behaviors in the ECWAO process.
The CV was conducted in a three-electrode system with platinum wire as counter electrode and saturated calomel electrode (SCE) as reference electrode. The working electrode was fabricated by loading the MnOx/GF composite powder on a glassy carbon electrode (5 mm in diameter). Briefly, 5 mg of MnOx/GF composite powder with a mean particles size of less than 20 μm was dispersed in 450 μL of anhydrous ethanol and 50 μL of Nafion solution (5 wt%) by ultrasonication. 10 μL of the resulting ink was dropped onto the glassy carbon electrode and dried at room temperature. The CV was carried out in 50 mM Na2SO4 solution (pH 7.0) from −0.5 to +1.5 V at 0.1 V s−1.
2. Material and methods 2.1. Preparation of MnOx/GF composites and structure characterization The MnOx/GF composites were prepared by a hydrothermal and pyrolysis method. Briefly, a piece of GF (4 × 2.5 cm2, 2 mm in thickness) was pretreated in a mixture of HNO3 and H2SO4 (1:1 in volume) for 1 h at 100 °C, and transferred to an autoclave filled with 30 mL of 10 g L-1 KMnO4. Here, the GF was chosen as the support of MnOx due to its good conductivity, favorable chemical stability and large surface area [17,18], as well as a reducing agent of KMnO4 during hydrothermal treatment [5]. The autoclave was sealed and maintained at 120 °C for 24 h. Then, the GF loaded with manganese compound was took out from the cooled autoclave, and calcined at 350 °C in air for 2 h to obtain the MnO2/GF composite. The Mn3O4/GF and MnO/GF composites were prepared in nitrogen atmosphere, at calcination temperature of 350 and 700 °C, respectively. Crystalline structures and morphologies of the MnOx/GF composites were characterized on a Bruker D8 advance X-ray diffractometer (XRD, Holland) and a JEM2100F transmission electron microscope (TEM, Japan). Manganese contents in the composites were determined by thermogravimetry (TG) on a TGA DT-50 apparatus (Japan) in air atmosphere. Valence states of manganese and oxygen species in the composites were identified using an ESCALAB 250 X-ray photoelectron spectrometer (XPS, USA). The composites were ground into powder for XRD, TEM, TG and XPS analyses.
2.4. Operation of the ECWAO process The ECWAO process was conducted in a 175 mL single-chamber glass-made reactor, with the prepared MnOx/GF composite as working electrode, a platinum wire (0.5 mm in diameter) as counter electrode and a SCE as the reference. The electrolyte was 130 mL of Na2SO4 solution (50 mM, pH 7.0) which contains 100 mg L-1 bisphenol A (BPA) as a probe pollutant. The MnOx/GF electrode was first electrochemically activated at +1.0 V (vs. SCE) for 2 h in the Na2SO4 electrolyte, and then BPA was added. The degradation of BPA was performed at a constant anodic current density of 10 A m−2, and air was bubbled at a rate of 1.5 L min−1 to supply sufficient dissolved O2. The concentration of BPA was determined on a Waters high performance liquid chromatography system (USA) equipped with a multiple wavelength UV detector and a SunFireTM C18 column (150 mm × 4.6 mm, 5 μm), and total organic carbon (TOC) was measured on a HTM-CT1000M TOC analyzer (Tailin, China). Intermediate products of BPA were identified by gas chromatography-mass spectrometry (GC–MS) on a Thermo Trace 1300 system equipped with a TR-V1 capillary column (30 m × 0.25 mm ID × 1.4 μm film, USA). The analytical procedure of GC–MS and extraction of intermediate products were described in our previous work [6]. Dissolved manganese was quantified by inductively coupled plasma and optical emission spectrometer (ICP-OES) on an Agilent 700 apparatus (USA). All experiments were conducted in triplicate, and average values with standard deviations were presented.
2.2. Temperature programmed reduction of hydrogen (H2-TPR) and temperature programmed desorption of O2 (O2-TPD)
3. Results and discussion
H2-TPR and O2-TPD profiles of the MnOx/GF composites were obtained on a Quantachrome ChemBET TPR/TPD apparatus (USA). The MnOx/GF composite powder was pretreated in helium gas at 100 °C for 30 min to remove adsorbed water. The flowing gas was 5 vol% H2/ helium mixed gas for H2-TPR and helium gas for O2-TPD, at a flow rate of 110 mL min−1. The sample was heated from room temperature to 800 °C at a rate of 10 °C min−1.
3.1. Crystalline structure and morphology of MnOx on the GF The XRD patterns of as-prepared MnOx/GF composites are shown in Fig. 1a. The broad peak located at 2θ value of 25.7° is ascribed to the (0 0 2) plane of graphic carbon of GF support. For the MnO2/GF
Fig. 1. XRD patterns (a) and TG curves (b) of the MnOx/GF composites prepared under different calcination conditions. 2
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Fig. 2. TEM images of MnO2/GF (a, b), Mn3O4/GF (c, d) and MnO/GF (e, f).
0.248 nm belong to the (1 1 2) and (2 1 1) planes of Mn3O4 (JCPDS No. 24-0734). The clear interplanar spacing on high resolution TEM of MnO/GF is 0.222 nm, in good agreement with the (2 0 0) plane of MnO (JCPDS No. 07-0230) (Fig. 2f).
composite, the diffraction peaks found at 37.1°, 41.5°, 49.8°, 56.0°, 64.8°, and 69.7° corresponds well to the tetragonal phase of α-MnO2 (JCPDS No. 44-0141). However, the intensity of these peaks is very weak, indicating poor crystallinity of MnO2. On the XRD pattern of Mn3O4/GF composite, diffraction peaks are clearly observed at 29.0°, 31.1°, 32.5°, 36.2°, 38.1°, 44.6°, 51.0°, 53.9°, 56.0°, 58.8°, 60.0°, and 64.7°, which are all assignable to the tetragonal phase of Mn3O4 (JCPDS No. 24-0734). The strong diffraction peaks appearing at 35.0°, 40.6° and 58.8° on the XRD pattern of MnO/GF composite confirm tetragonal phase of MnO (JCPDS No. 07-0230) with good crystallinity. As shown in Fig. 1b, the TG curves of MnOx/GF composites reveal a sharp weight loss over 450–750 °C, reflecting the combustion of GF as well as the conversion from MnOx to Mn2O3 in air. Manganese contents in the three different MnOx/GF composites are almost the same as 21.2 wt%, according to the weight percentage of the final product Mn2O3 in TG analysis (30.5%). The morphologies of as-prepared MnOx/GF composites are displayed by TEM images given in Fig. 2. As shown in Fig. 2a, the MnO2 is present as nanorods with diameter ranging from 10 to 30 nm, and the high resolution TEM image in Fig. 2b reveals a set of lattice fringes with interplanar distances of 0.490 and 0.240 nm attributed to the (2 0 0) and (2 1 1) planes of α-MnO2 (JCPDS No. 44-0141). The Mn3O4 and MnO are present as irregular particles with sizes no more than 100 nm (Fig. 2c and 2e). On the high resolution TEM image of Mn3O4/GF (Fig. 2d), the lattice fringes with interplanar distances of 0.308 and
3.2. Valence states of manganese and surface oxygen species The valence states of manganese as well as oxygen species on the surface of MnOx/GF composites are investigated by XPS analysis. As shown in Fig. 3a, the Mn 3 s level is split into two peaks with separation energy of 4.9 eV for MnO2, 5.4 eV for Mn3O4 and 5.9 eV for MnO, corresponding well with the oxidation state of manganese from high to low valence in these MnOx [19–21]. The Mn 2p XPS spectra of all MnOx/GF samples in Fig. 3b are split to Mn 2p3/2 and Mn 2p1/2 peaks located at ca. 642.2 and 653.9 eV, respectively. The Mn 2p3/2 peak can be divided into three components at 640.8 eV, 642.0 eV, and 643.0 eV, together with a satellite at 645.8 eV. The three components are assigned to the surface Mn2+ (640.8 eV), Mn3+ (642.0 eV), and Mn4+ species (643.0 eV), respectively [22,23]. Fig. 3c shows the asymmetrical O1s signal is also divided into three components with binding energies of 529.8, 531.4, and 533.2 eV, which are attributable to the surface lattice oxygen, chemisorbed oxygen, and physically adsorbed oxygen species, respectively [24,25]. The accurate contents of surface manganese and oxygen species, calculated by a quantitative analysis from their peak areas, are also given in Fig. 3. The coexistence of Mn2+, Mn3+ and 3
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Fig. 3. Mn 3s (a), Mn 2p (b) and O 1s (c) XPS spectra of the MnOx/GF composites. Fig. 4. H2-TPR profiles (a), O2-TPD profiles (b) and CV curves (c) of the MnOx/ GF composites.
Mn4+ on the surface of MnOx has an important impact on the formation of surface oxygen species, because mixed-valence state of metal can promote oxygen mobility in the oxides [26]. Notably, all the three manganese oxides display high amounts of chemisorbed oxygen species, i.e. O2- and O-, on their surfaces. As shown in Fig. 3b, the low valence Mn2+ and Mn3+ species are found on the surface of MnO2, which usually appear at oxygen vacancies to combine with chemisorbed oxygen species [27]. For MnO, the high valence Mn3+ and Mn4+ species on its surface are generated from the oxidation of Mn2+, accompanied by the transformation of adsorbed O2 into chemisorbed oxygen species [8,28]. As for Mn3O4, the coexistence of different
manganese valence states in its spinel structure endows high mobility of its structural lattice oxygen, which can convert to chemisorbed oxygen species via the cleavage of the labile Mn3+-O bond [15]. The results of previous work have demonstrated the involvement of chemisorbed oxygen species, as the direct oxidants of pollutants, in the ECWAO process [5,8]. Therefore, an effective ECWAO process desires abundant chemisorbed oxygen species enriched on the catalyst surface. The relative contents of chemisorbed oxygen species in as prepared MnOx are ordered as Mn3O4 > MnO > MnO2. 4
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Fig. 5. Pseudo-first-order kinetics for BPA oxidation in the ECWAO process with MnO2/GF (a), Mn3O4/GF (b) and MnO/GF (c) electrodes at different currents and the comparison of their rate constants (d).
Fig. 6. Proposed degradation pathway of PBA in the ECWAO process with MnOx/GF electrodes.
which displays no obvious H2-reduction peak, the H2-TPR profiles of MnOx/GF composites feature a series of distinct peaks arising from the reduction of MnOx by H2. The reduction of MnO2 can be divided into low-temperature reduction (ca. 300–350 °C) and high-temperature
3.3. Reducibility of MnOx and oxygen mobility The H2-TPR is carried out to investigate the reducibility of MnOx. As shown in Fig. 4a, in comparison to the H2-TPR profile of the GF support 5
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Fig. 8. TG curves (a), XRD patterns (b) and Mn 2p XPS spectra (c) of the MnOx/ GF electrodes underwent eight fed-batch runs.
reduction peak is related to the active oxygen species located on the surface of metal oxide [30,31]. It can be observed from Fig. 4a that the Mn3O4/GF and MnO/GF composites have better low-temperature reducibility than the MnO2/GF composite, suggesting higher activity of their surface oxygen species in oxidation reaction. Mobility of oxygen species in as-prepared MnOx is investigated by O2-TPD. As shown in Fig. 4b, the low-temperature peaks in the region of 150–300 °C result from evolution of chemisorbed oxygen species weakly bonded to the surface of MnOx, and the high-temperature ones from 300 to 800 °C are due to desorption of lattice oxygen in MnOx [32,33]. Notably, the liberation of chemisorbed oxygen species can be initiated below 200 °C, suggesting good oxygen mobility in the MnOx. The liberation of chemisorbed oxygen species in Mn3O4 is initiated at
Fig. 7. Mineralization of BPA in eight successive fed-batch runs of ECWAO process with MnO2/GF (a), Mn3O4/GF (b) and MnO/GF (c) electrodes.
reduction (ca. 350–400 °C), corresponding to the successive phase conversion from MnO2 to MnO via Mn3O4 [29]. As for Mn3O4, the weak peak at 200–350 °C is ascribed to the reduction of surface Mn4+, and the strong peak at 350–550 °C is to the reduction of Mn3O4 phase into MnO phase. While MnO itself cannot be reduced in the experimental temperature range, the reduction of Mn4+/Mn3+ on its surface gives a weak peak at ca. 250–350 °C. As is known, the low-temperature H26
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reduction of high-valence manganese. Fig. 8c shows the Mn 2p XPS spectra of used MnOx/GF electrodes. It is found that the proportion of Mn4+ in the MnOx is reduced after the ECWAO reaction. Meanwhile, the proportion of Mn2+ in MnO2 and Mn3O4 is greatly increased. For MnO, increment of Mn3+ proportion is observed. These results confirm the involvement of multiple valences of manganese, notably Mn2+/ Mn3+ redox couple, in the ECWAO reaction.
the lowest temperature, indicating relatively higher oxygen mobility. In comparison, chemisorbed oxygen species in MnO2 are relatively inert as evidenced by the highest temperature required for their liberation. The electro-oxidation of chemisorbed oxygen species in anodic electric field is an important O2 activation step in the ECWAO process. The CV is performed to evaluate reactivity of chemisorbed oxygen species in the ECWAO process. As shown in Fig. 4c, the anodic peaks representing the activation of chemisorbed oxygen species in the anodic electric field [5,8], are found between 0.0 and +0.5 V (vs. SCE) on the cyclic voltammograms of all the MnOx/GF composites. Intensities of these peaks are in the order Mn3O4/GF > MnO/GF > MnO2/GF, in good agreement with the relative contents of chemisorbed oxygen species as well as oxygen mobility in corresponding MnOx.
4. Conclusion MnOx on GF support demonstrate favorable and stable activities in catalyzing the air oxidation of BPA with assistance of anodic electric field. Mn2+, Mn3+ and Mn4+ are found to coexist on the surface of MnOx, which facilitates the formation of chemisorbed oxygen species as the direct oxidants of BPA in ECWAO reaction. The relative contents of chemisorbed oxygen species in as prepared MnOx are ordered as Mn3O4 > MnO > MnO2. Chemisorbed oxygen species of Mn3O4 possess the highest low-temperature reactivity. As a result, Mn3O4 demonstrates the highest catalytic activity towards ECWAO reaction. In comparison, chemisorbed oxygen species of MnO2 are relatively inert and it displays the lowest catalytic activity among the tested MnOx.
3.4. Catalytic activities of MnOx in the ECWAO process The degradation of 100 mg L-1 BPA in the ECWAO process is studied by monitoring the concentration of BPA ([BPA], mg L-1) along with the reaction time (t, min). The experimental results regarding to [BPA] and t are fitted to a pseudo-first-order kinetic equation:
d[BPA] = -k '[BPA] dt
(1)
CRediT authorship contribution statement
where k′ is a pseudo-first-order rate constant. As shown in Fig. 5a, a linear relationship between ln([BPA]0/[BPA]t) and t is observed at all applied currents, suggesting a pseudo-first-order kinetics for BPA oxidation. The degradation rate of BPA is increased with increase of applied current, as expected from the fact that the activation of chemisorbed oxygen species in MnOx is favored in a stronger electric field. Totally speaking, BPA can be completely removed within 120 min on the MnOx/GF composites, suggesting favorable catalytic activities of the prepared MnOx. At any applied current, the highest k′ is always obtained on the Mn3O4/GF electrode, and the lowest is on the MnO2/GF electrode. Therefore, Mn3O4 is catalytically the most active in the ECWAO process, and MnO2 is the most inert. Moreover, BPA can be deeply mineralized by the ECWAO process. The mineralization efficiency of BPA is higher than 97% after it is treated on the Mn3O4/GF or MnO/GF electrode for 180 min. Such a value is lower, i.e. approximately 90%, on the MnO2/GF electrode. Intermediate products generated from BPA degradation are identified by GC–MS. As shown in Fig. 6, p-isopropenylphenol (m/z = 134) and hydroquinone (m/z = 110) are detected as intermediate products of BPA. Based on the evolution profiles of these intermediate products, BPA is proposed to be first cleaved at the CeC bond between aromatic rings with p-isopropenylphenol and hydroquinone as products. The pisopropenylphenol and hydroquinone are subsequently oxidized into pbenzoquinone. Further oxidation of p-benzoquinone results in CO2, H2O and small molecular organic products, as occurs in CWAO process [34]. Fig. 7 demonstrates mineralization of BPA in eight successive fedbatch runs. No drop of the degradation rate or mineralization efficiency was observed during the eight runs, suggesting good reusability of the MnOx/GF electrodes. After each run, concentrations of manganese in the solution are determined to be lowered that 0.5 mg L-1, which are below the discharge limit of manganese in China (GB8978-1996). As a matter of fact, manganese contents in the MnOx/GF electrodes underwent eight runs are as high as 20.7% (MnO2), 20.1% (Mn3O4) and 21.2% (MnO), respectively, reflecting excellent stability of the MnOx/ GF electrodes (Fig. 8a). While negligible metal loss was observed during the eight cycles, the diffraction peaks of MnOx on XRD pattern of the used electrodes are obviously weaker as compared to the fresh ones (Fig. 8b), suggesting the participation of manganese in the ECWAO reaction. As is known, the chemisorption of O2 and consumption of chemisorbed oxygen species on MnOx surface is manipulated by the inter-conversion of different valence states of manganese [27,28]. The low-valence manganese is oxidized upon chemisorption of O2, and the consumption of chemisorbed oxygen species is accompanied by the
Min Sun: Conceptualization, Methodology, Supervision. Li-Ming Fang: Conceptualization, Data curation, Writing - original draft. XiaoHui Hong: Visualization, Investigation. Feng Zhang: Validation. LinFeng Zhai: Conceptualization, Writing - review & editing, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The authors wish to thank the National Natural Science Foundation of China (51478157, 21707133) and the Program for New Century Excellent Talents in University (NCET-13-0767) for financial support of this work. References [1] J. Levec, A. Pintar, Catalytic wet-air oxidation processes: a review, Catal. Today 124 (2007) 172–184. [2] K.M. Sushma, A.K. Saroha, A.K. Saroha, Performance of various catalysts on treatment of refractory pollutants in industrial wastewater by catalytic wet air oxidation: A review, J. Environ. Manage. 228 (2018) 169–188. [3] K.H. Kim, S.K. Ihm, Heterogeneous catalytic wet air oxidation of refractory organic pollutants in industrial wastewaters: A review, J. Hazard. Mater. 186 (2011) 16–34. [4] H. Debellefontaine, J.N. Foussard, Wet air oxidation for the treatment of industrial wastes. Chemical aspects, reactor design and industrial applications in Europe, Waste Manage. 20 (2000) 15–25. [5] M. Sun, L.M. Fang, J.Q. Liu, F. Zhang, L.F. Zhai, Electro-activation of O2 on MnO2/ graphite felt for efficient oxidation of water contaminants under room condition, Chemosphere 234 (2019) 269–276. [6] M. Sun, Y. Zhang, S.Y. Kong, L.F. Zhai, S. Wang, Excellent performance of electroassisted catalytic wet air oxidation of refractory organic pollutants, Water Res. 158 (2019) 313–321. [7] M. Sun, Y. Zhang, H.H. Liu, F. Zhang, L.F. Zhai, S. Wang, Room-temperature air oxidation of organic pollutants via electrocatalysis by nanoscaled Co-CoO on graphite felt anode, Environ. Int. 131 (2019) 104977. [8] L.F. Zhai, M.F. Duan, M.X. Qiao, M. Sun, S. Wang, Electro-assisted catalytic wet air oxidation of organic pollutants on a MnO@C/GF anode under room condition, Appl. Catal. B- Environ. 256 (2019) 117822. [9] C. Liu, J.W. Shi, C. Gao, C. Niu, Manganese oxide-based catalysts for low-temperature selective catalytic reduction of NOx with NH3: A review, Appl. Catal. A Gen. 522 (2016) 54–69. [10] L. Miao, J. Wang, P. Zhang, Review on manganese dioxide for catalytic oxidation of airborne formaldehyde, Appl Surf. Sci. 466 (2019) 441–453. [11] H. Xu, N. Yan, Z. Qu, W. Liu, J. Mei, W. Huang, S. Zhao, Gaseous heterogeneous
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