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In situ synthesis of MnO2@SiO2–TiO2 nanofibrous membranes for room temperature degradation of formaldehyde
Composites Communications 16 (2019) 61–66
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Composites Communications journal homepage: www.elsevier.com/locate/coco
In situ synthesis of MnO2@SiO2–TiO2 nanofibrous membranes for room temperature degradation of formaldehyde
T
Fuhai Cuia, Weidong Hana, Yang Sib, Wenkun Chenb, Meng Zhangb, Hak Yong Kima,c,∗∗, Bin Dingb,∗ a
Department of BIN Convergence Technology, Chonbuk National University, Jeonju, 561-756, South Korea Innovation Center for Textile Science and Technology, Donghua University, Shanghai, 200051, China c Department of Organic Materials and Fiber Engineering, Chonbuk National University, Jeonju, 561-756, South Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrospun nanofibers Titanium dioxide Manganese dioxide Formaldehyde Catalytic degradation
Catalytic oxidation is an efficient and cost-effective technology to eliminate HCHO, and MnO2 shows excellent performance towards this kind of reactions. In this work, a birnessite type manganese dioxide nanoparticles modified silica-doped titanium dioxide (MnO2@SiO2–TiO2) nanofibrous membranes were successfully synthesized by using an electrospinning technique and liquid phase synthesis. It is found that MnO2@SiO2–TiO2-4 nanofibrous membrane showed the best catalytic activity for HCHO removal. The as-fabricated nanofibrous membranes exhibited good and reversible catalytic oxidation activity towards formaldehyde, and more than 90% of the catalytic performance could remain after 5 testing cycles. The successfully constructed MnO2 nanoparticles decorated soft SiO2–TiO2 nanofibrous membranes, which can be a great promise for effectively solving the problem of environmental remediation.
1. Introduction Formaldehyde (HCHO), as the most typical and common indoor air pollution, may cause a range of diseases, including nausea, eye irritation, headaches, allergic dermatitis, and bronchial asthma [1–3]. HCHO can be emitted from the widespread use of building materials, furnishings, and decoration materials, which has become one of the top risks to human health and a global problem with more and more widespread attention [4–8]. In 2010, the World Health Organization established an indoor air standard, which limited the content of HCHO below 0.08 mg m−3. Globally, in most of the developing country, especially in new buildings or remodeled houses, HCHO content often exceeded the standard value [9–11]. To solve the above-mentioned problem, various air purification technologies have been widely utilized for HCHO decomposition, for example, active carbon adsorption, photocatalytic degradation, and catalytic oxidation degradation [12–15]. Compared with other methods, catalytic oxidation degradation has been considered as the most hopeful to remove HCHO because of its no secondary pollution and light source assistance. Thus, developing a fast and efficient catalytic material to remove HCHO is of great significance. Among the catalytic oxidation catalysts, transition metal-based and noble metal-based catalysts, with the merit of decomposing HCHO into ∗
CO2 under mild condition, have been studied for years [16–19]. However, high cost greatly limits the noble metal-based catalysts in the practical application. Therefore, transition metal oxides, particularly, MnO2 has been widely studied for formaldehyde decomposition due to its abundant amount and low cost [20–22]. However, to our knowledge, no matter what noble-metal-based or transition metal oxide catalysts are usually used as powder. Generally, these powder catalyst should be blended with polymer binder and then coated on the nonwoven fabrics to realize their practical application, which leads to the decrease of the exposed active sites and catalytic activity. Fortunately, electrospun nanofibers have been deemed as an ideal candidate carrier, due to their attractive characteristics (e.g. structural controllability, high porosity, composition tenability, and interspatial connectivity, and large specific surface area), which has attracted great attention from researchers [23–26]. However, the nanostructure of the electrospun nanofibrous membrane will be ineffective during the above-mentioned surface coating process [27–29]. Thus, it is of great importance to develope a suitable method to construct granular and fibrous composite materials with enough accessible active sites and good catalytic activity. In this work, we demonstrated a simple and versatile method for fabricating soft MnO2@SiO2–TiO2 nanofibrous membranes, where the
Corresponding author. Corresponding author. Department of BIN Convergence Technology, Chonbuk National University, Jeonju, 561-756, South Korea. E-mail addresses: [email protected] (H.Y. Kim), [email protected] (B. Ding).
∗∗
https://doi.org/10.1016/j.coco.2019.08.002 Received 26 June 2019; Received in revised form 26 July 2019; Accepted 5 August 2019 Available online 05 September 2019 2452-2139/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic for the fabrication of the as-prepared MnO2@SiO2–TiO2 nanofibrous membrane via electrospinning and liquid phase synthesis.
25 ± 2 °C and 50 ± 5%, respectively. Finally, the as-prepared electrospun nanofibrous membrane was calcined in air at 700 °C by using a muffle resistance furnace (SX-G12133, Tianjin Zhonghuan Furnace Co., Ltd., China), the calcination temperature maintained for 2 h to sufficiently burn off the polymer and obtain the soft SiO2–TiO2 nanofibrous membranes.
nanostructured MnO2 nanoparticles were uniformly formed on the surface of SiO2–TiO2 nanofibers. The design of MnO2@SiO2–TiO2 nanofibrous membrane is primarily based on three rules: (1) the SiO2–TiO2 nanofibrous membrane should possess high porosity for HCHO transportation and adsorption. (2) The nanostructured MnO2 nanoparticles must be stably immobilized on the surface of SiO2–TiO2 nanofibers via a facile and versatile method. (3) The as-prepared membranes should possess stable structure and strong mechanical performance to provide prolonged utilization. Fig. 1 illustrates the synthetic process of MnO2@SiO2–TiO2 nanofibrous membrane. First of all, soft SiO2–TiO2 nanofibrous membranes were obtained by doping SiO2 through a versatile electrospinning and subsequently calcination. Then, MnO2 nanoparticles were in situ synthesized on SiO2–TiO2 nanofibers via a simple liquid phase synthesis method under a mild condition. Significantly, the loading amount and size of MnO2 nanoparticles could be controlled by altering synthesis cycles. Additionally, the as-prepared MnO2@SiO2–TiO2 nanofibrous membrane showed excellent catalytic oxidation activity towards HCHO degradation and could be easily reused without any complicated treatment process. We prospect that such materials could potentially serve as highly efficient and cost-effective catalysts for indoor air purification.
2.3. Synthesis of MnO2@SiO2–TiO2 nanofibrous membranes The hierarchical structured MnO2@SiO2–TiO2 nanofibrous membranes were obtained by seeding MnO2 nanoparticles on the SiO2–TiO2 nanofiber through liquid phase synthesis. Typically, 1.8 g KMnO4 and 0.12 g Glc were separately dissolved in 200 mL deionized water. After that, the soft SiO2–TiO2 nanofibrous membrane was first immersed into the KMnO4 aqueous solution for 5 min, and then the Glc solution was transferred to the KMnO4 solution, and the glass beaker was placed in a thermostatic oscillator at 60 °C for 1 h. Subsequently, the purple KMnO4 was reduced into brown MnO2 and deposited onto the SiO2–TiO2 nanofibers due to the reduction properties of Glc, soon afterward, the obtained sample was rinsed by deionized water to remove the residual ions. After different synthesis cycles, MnO2@SiO2–TiO2 nanofibrous membrane was dried in oven at 80 °C for 10 h. The as-prepared samples with n synthesis cycles were denoted as MnO2@SiO2–TiO2-n nanofibrous membranes (n = 1, 2, 3, 4, and 5).
2. Experimental 2.1. Materials
2.4. Characterization Polyvinylpyrrolidone (PVP, Mw = 1 300 000), tetrabutyl titanate (TBT), glucose (Glc), acetylacetone (Acac), N,N-dimethylformamide (DMF), absolute ethyl alcohol (EtOH), and ethanoic acid (HAc) were purchased from Aladdin (Shanghai, China). Potassium permanganate (KMnO4) and tetraethyl orthosilicate (TEOS) were obtained from Lingfeng Chemical Co., Ltd., China. Deionized water was supplied by a Heal-Force System.
The morphologies and microstructures of the resultant materials were observed by scanning electron microscopy (SEM). The nanostructures and element distribution of the obtained MnO2@SiO2–TiO2 nanofibrous membrane were performed by high-resolution transmission electron microscopy (HR-TEM) and The scanning transmission electron microscopy (STEM). X-ray diffraction (XRD) was used to characterize the crystal structure of the synthesized materials. The specific surface area (SSA) of the fabricated samples was analyzed by BET instrument. X-ray photoelectron spectroscopy (XPS) was used to describe the surface chemical states of the obtained materials.
2.2. Synthesis of SiO2 doped TiO2 electrospun nanofibrous membranes The SiO2–TiO2 electrospun nanofibrous membrane was fabricated via electrospinning and subsequently calcination methods. Firstly, EtOH was used as solvent to dissolve PVP powder at 25 °C through vigorously stirring more than 1 h, and transparent PVP solution with a concentration of 5 wt% was obtained. The Si sol was prepared by blending TEOS, EtOH, HAc and deionized water with stirring for at least 4 h. Afterward, the TBT and Acac (molar ratio of Ti:Si was 7:3) were mixed with the obtained Si sol and stirring for more than 1 h. Then, the spinning solution was prepared by mixing the Ti–Si sol and PVP solution. Subsequently, the electrospinning process was performed on an EXES-4 electrospinning device. During the spinning process, the room temperature and relative humidity (RH) were maintained at
2.5. Catalytic test The HCHO degradation activity of MnO2@SiO2–TiO2 nanofibrous membranes was evaluated under static mode, which was operated at 25 °C and RH of 40 ± 5%. Typically, the as-synthesized samples (size of 10 × 10 cm2) with different loading cycles were put in the 5 L organic glass reactor to test their activity of HCHO removal, respectively. Before each catalytic reaction test, the reactor was adequately cleaned to eliminate the influence of residual HCHO. The original concentration of HCHO was controlled at approximately 200 ppm. A BSQ-GCH20 62
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Fig. 3. (a) Low magnification TEM and (b) HR-TEM images of MnO2@ SiO2–TiO2-4. (c) STEM images of MnO2@SiO2–TiO2-4 with elemental mapping images of (d) Ti, (e) Mn, (f) Si, and (g) O, respectively.
Fig. 2. SEM images of (a) SiO2–TiO2, (b) MnO2@SiO2–TiO2-1, (c) MnO2@ SiO2–TiO2-2, (d) MnO2@SiO2–TiO2-3, (e) MnO2@SiO2–TiO2-4, (f) MnO2@ SiO2–TiO2-5 nanofibrous membrane, respectively.
study the synthesized MnO2@SiO2–TiO2 nanofibrous membrane, by taking MnO2@SiO2–TiO2-4 as an example. Fig. 3a showed the typical TEM image of the heterostructure of MnO2@SiO2–TiO2, it can be observed that the MnO2 nanoparticles with an average size of 40 nm were tightly immobilized onto the SiO2–TiO2 nanofiber after ultrasonic dispersion, revealing a strong adhesion force between MnO2 nanoparticles and SiO2–TiO2 nanofiber surface. As shown in Fig. 3b, the lattice distance of 0.24 nm can be observed, which is in accordance with the lattice distance of δ-MnO2 [30]. Furthermore, HR-TEM result confirmed the formation of intimate MnO2@SiO2–TiO2-4 heterostructures. The STEM was performed to further study the structure of the as-synthesized MnO2@SiO2–TiO2-4. The distribution of the elements of Ti, Si, Mn, and O was demonstrated in Fig. 3c–g. During the synthetic reaction, KMnO4 was reduced into MnO2 by Glc, which was irregularly grown on the surface of SiO2–TiO2 nanofibers [31]. Furthermore, the above result could be proved in Fig. 3f, the Mn element was dispersed onto the fiber. In addition, the corresponding results further verified the formation of the secondary nanostructure. The XRD patterns of the as-synthesized SiO2–TiO2, and MnO2@ SiO2–TiO2 nanofibrous membrane were shown in Fig. 4a. For the SiO2–TiO2 nanofibers, the weak diffraction peaks implying their poor crystallinity, which can be ascribed to the doped SiO2. However, the crystalline without any transition after inducing the component of SiO2. In addition, the peaks of MnO2@SiO2–TiO2-4 nanofibrous membrane were in accordance with the δ-MnO2 (JCPDS No. 80–1098). These two broad and weak peaks of the as-prepared sample revealed the poor crystallinity. Furthermore, compared with the previously reported MnO2 nanoparticles, the (002) plane showed a stronger intensity, implying that the MnO2 with highly exposed (002) facet was inclined to form onto the surface of the SiO2–TiO2 nanofiber [32]. XPS was applied to demonstrate the chemical compositions on the surface of MnO2@SiO2–TiO2-4 nanofibrous membrane. XPS survey spectra of MnO2@SiO2–TiO2-4 were shown in Fig. 4b. The chemical states of Mn element on the surface of the as-prepared MnO2@ SiO2–TiO2-4 nanofibrous membrane were demonstrated in Fig. 4c, Mn 2p3/2 spectra displayed two characteristical peaks were corresponding to Mn3+ and Mn4+, respectively, which was on the surface of the asprepared MnO2@SiO2–TiO2 nanofibrous membrane. In addition, as
formaldehyde online detector was utilized to monitor the change of HCHO concentration. For comparison, neat SiO2–TiO2 nanofibrous membrane without loading of MnO2 was also employed as a control sample for the catalytic degradation of HCHO under the same conditions. The reacted samples were sufficiently flushed with air and rinsed with deionized water to remove the adsorbent species. Subsequently, MnO2@SiO2–TiO2 nanofibrous membranes were reused for another catalytic reaction cycle, which was operated under the same experimental condition as demonstrated above. The removal efficiency of HCHO was evaluated as follow: C −C HCHO removal efficiency (%) = 0C t × 100%Where C0 is the in0 itial concentration of HCHO, and Ct is the concentration of formaldehyde at an interval time, respectively. 3. Results and discussion 3.1. Morphology and structure analysis Fig. 2a displayed the morphology of SiO2–TiO2 nanofibrous membrane, showing that the fibers were randomly distributed and the average diameter is ~250 nm. Compared with the brittle pure TiO2 nanofibrous membrane with many breakages, the SiO2–TiO2 nanofibrous membrane, as the substrate, exhibited ultra-large aspect ratios, smooth surface, and robust softness. This result revealed that SiO2 doping could effectively decrease cracks on the surface of the nanofiber, which could ensure good mechanical properties of TiO2 nanofibrous membrane during the synthesis process of MnO2 nanoparticles. As shown in Fig. 2b, after 1 cycle of synthesis, granular MnO2 nanostructure with a size of ~20 nm was grown on the SiO2–TiO2 nanofiber surface. The coverage rate and size of MnO2 nanostructures can be controlled by the loading amount through changing the synthesis cycles. Obviously, as shown in Fig. 2c–e, the loading amount of granular MnO2 gradually increased with the increase of synthesis cycles. After 5 synthesized cycles, the integrality of SiO2–TiO2 nanofibrous membrane was destroyed by the formation of dense MnO2 aggregation (Fig. 2f). TEM and elemental mapping analysis were performed to further 63
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Fig. 4. (a) XRD patterns of SiO2–TiO2 and MnO2@Si–TiO2-4 nanofibrous membrane. XPS spectra of MnO2@SiO2–TiO2-4 nanofibrous membrane: (b) survey spectra, (c) Mn 2p3/2, and (d) O 1s.
structure. The pore size distribution of the as-synthesized samples were in the range of 10–70 nm (Fig. 5b). Additionally, the BJH pore volume of MnO2@SiO2–TiO2-4 nanofibrous membrane was much larger than the initial SiO2–TiO2 nanofibrous membrane. Generally, for catalytic materials, hierarchical structures on the fiber surface could enhance their SSA of the as-prepared samples, which would contribute to improving their performance. The significantly increased surface area would supply much more active sites for the oxidation reaction of the as-prepared samples. The catalytic activity of the as-prepared samples were carried out under static test mode at room temperature. As shown in Fig. 6a, SiO2–TiO2 nanofibrous membrane almost has no activity for HCHO removal, implying that no chemical reaction or physical adsorption were occurred on the SiO2–TiO2 nanofibers. The sharp decline of the HCHO concentration in the first 5 min was ascribed to the gradually
shown in Fig. 4d, the O 1s spectra showed one characteristical peak at 529.6 eV attributing to lattice oxygen (Olatt) on the as-synthesized MnO2 nanoparticles [32]. The porous structure was measured through aperture analyzer to investigate the effect of the mass loading of MnO2 nanoparticles on the SiO2–TiO2 nanofibrous membranes. As shown in Fig. 5a, a careful observation demonstrated that the XRD curves of modified samples displayed the formation of mesopores. This phenomenon could be assigned to the voids among the MnO2 nanoparticles on the SiO2–TiO2 nanofibrous membrane. Obviously, the gradually increasing BET surface area from 4.95 to 48.59 m2 g−1 was ascribed to the increased loading amount of MnO2 with the increase of synthesis cycles from 0 to 5, revealing that nanostructured MnO2 played an important role in enlarging the effective surface area [33]. The MnO2@SiO2–TiO2 nanofibrous membranes also showed a typically polydisperse pore
Fig. 5. (a) BET isotherms of these as-synthesized materials. (b) Pore size distribution of relevant samples by employing BJH analysis. 64
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Fig. 6. (a) Catalytic oxidation degradation curves of HCHO over different samples. (b) Catalytic oxidation activity of MnO2@SiO2–TiO2-4 nanofibrous membrane towards HCHO with 5 cycles.
~8.5%. In theory, according to the calculated results, the catalyst would be deactivation after 60 catalytic cycles. However, to our knowledge, during the degradation process, the catalyst could only change the rate of the reaction without entering into it. The decreased catalytic oxidation property could be assigned to the reduction of active sites. Thus, the service life of the as-prepared catalytic materials would be much longer than the calculated results in practical application. One of the unique advantages of electrospinning technology is in scaling up the fabrication. For example, as shown in Fig. S1, a large scale (40 × 40 cm2) membrane was successfully fabricated. As the above-mentioned, we proposed a possible mechanism of HCHO removal on MnO2 at room temperature (Fig. 6d). First of all, hydrogen bondings which were formed between HCHO and surface hydroxyl groups of MnO2. Secondly, the adsorbed HCHO could be oxidized into formate and carbonate by structural hydroxyl groups. Thirdly, the regeneration of surface hydroxyl groups could be taken place during the reaction between active oxygen (O2−, O−, etc.) and gas phase water through this reaction (O2−, O− + H2O → 2-OH). Finally, the desorption of carbonate from the surface of MnO2 could be stimulated by competitive adsorption of water in air [34].
increased loading amount of MnO2 nanoparticles, the HCHO removal efficiency of the as-synthesized samples of MnO2@SiO2–TiO2 nanofibrous membranes also gradually increased with the increase of synthesis cycles, respectively. In addition, the HCHO removal efficiency of MnO2@SiO2–TiO2-4 and MnO2@SiO2–TiO2-5 nearly reached 100% within 20 min. This results could reveal that the HCHO could be easily oxidized by the as-prepared samples, and the HCHO removal efficiency depended on the loading amount of MnO2 nanoparticles. To further investigate the catalytic activity of the as-prepared samples, the oxidation kinetics of HCHO were analyzed by utilizing the equation as follows: ln(C0/Ct) = kt Where C0 is the concentration of HCHO at the initial time (min), and Ct is the concentration of formaldehyde at an interval time (min), respectively, and k (min−1) is the HCHO removal rate constant. The fitting curves of the as-synthesized samples obeyed the pseudo-first-order reaction dynamic model, which were demonstrated in Fig. 6b. The constant k for the samples of MnO2@SiO2–TiO2-4 and MnO2@ SiO2–TiO2-5 nanofibrous membranes have a similar calculated value of 0.21 and 0.22 min−1, respectively. This result revealed that MnO2@ SiO2–TiO2-4 and MnO2@SiO2–TiO2-5 exhibited the highest HCHO removal efficiency among the as-synthesized materials. However, due to the excess mass loading of MnO2, SiO2–TiO2 nanofibrous membrane could not maintain integrity after 5 synthesis cycles. The recoverability plays a very important role in evaluating the practical application of a catalyst. However, it is a challenge to keep well catalytic properties in a long-term use process. As shown in Fig. 6c, the sample exhibited good reversible catalytic oxidation properties towards HCHO and 91.57% of the catalytic activity still could be preserved after 5 utilization cycles, implying the good reusability of MnO2@SiO2–TiO2-4. As the above-mentioned testing results, the theoretical service life of the as-prepared catalytic materials could be calculated, after 5 cycles, the catalytic performance was reduced by
4. Conclusions In summary, soft MnO2@SiO2–TiO2 nanofibrous membranes were constructed by the combination of electrospinning method and subsequent liquid phase synthesis. The resultant nanostructured MnO2 nanoparticles endowed the samples with large SSA, which supplied more catalytic reaction active sites. More interestingly, profiting from the increased SSA and high porosity, the as-prepared nanostructured MnO2 nanoparticles decorated on the surface of SiO2–TiO2 exhibited high HCHO removal efficiency of 100%. After 5 times of catalytic testing cycles, The sample still preserved 91.57% of the initial catalytic activity towards HCHO. Considering various advantages of the as-synthesized MnO2 modified SiO2–TiO2 nanofibrous membranes, this work 65
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will afford a facial and efficient method for developing efficient noble metal-free catalysts to degrade VOCs at room temperature.
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Author contributions Fuhai Cui and Weidong Han contributed equally to this work. Notes The authors declare no competing financial interest. Acknowledgements This work was supported by the National Science Foundation of China (No. 51873029), the Innovation Program of Shanghai Municipal Education Commission (No. 2017-01-07-00-03-E00024) and the Program of Shanghai Academic Research Leader (No. 18XD1400200). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.coco.2019.08.002. References [1] W. Kanchongkittiphon, M.J. Mendell, J.M. Gaffin, G. Wang, W. Phipatanakul, Indoor environmental exposures and exacerbation of asthma: an update to the 2000 review by the Institute of Medicine, Environ. Health Perspect. 123 (2015) 6–20. [2] T. Salthammer, S. Mentese, R. Marutzky, Formaldehyde in the indoor environment, Chem. Rev. 110 (2010) 2536–2572. [3] L. Cao, Q. Fu, Y. Si, B. Ding, J. Yu, Porous materials for sound absorption, Compos. Commun. 10 (2018) 25–35. [4] J. Quiroz Torres, S. Royer, J.P. Bellat, J.M. Giraudon, J.F. Lamonier, Formaldehyde: catalytic oxidation as a promising soft way of elimination, Chem. Sus. Chem. 6 (2013) 578–592. [5] C. Jiang, D. Li, P. Zhang, J. Li, J. Wang, J. Yu, Formaldehyde and volatile organic compound (VOC) emissions from particleboard: identification of odorous compounds and effects of heat treatment, Built. Environ. 117 (2017) 118–126. [6] Q. Fu, Y. Si, C. Duan, Z. Yan, L. Liu, J. Yu, B. Ding, Highly carboxylated, cellular structured, and underwater superelastic nanofibrous aerogels for efficient protein separation, Adv. Funct. Mater. 29 (2019) 1808234. [7] J. Wang, P. Zhang, J. Li, C. Jiang, R. Yunus, J. Kim, Room-temperature oxidation of formaldehyde by layered manganese oxide: effect of water, Environ. Sci. Technol. 49 (2015) 12372–12379. [8] E. Uhde, T. Salthammer, Impact of reaction products from building materials and furnishings on indoor air quality-a review of recent advances in indoor chemistry, Atmos. Environ. 41 (2007) 3111–3128. [9] R. Averlant, S. Royer, J.-M. Giraudon, J.-P. Bellat, I. Bezverkhyy, G. Weber, J.F. Lamonier, Mesoporous silica-confined manganese oxide nanoparticles as highly efficient catalysts for the low-temperature elimination of formaldehyde, ChemCatChem 6 (2014) 152–161. [10] X. Tang, Y. Bai, A. Duong, M.T. Smith, L. Li, L. Zhang, Formaldehyde in China: production, consumption, exposure levels, and health effects, Environ. Int. 35 (2009) 1210–1224. [11] F. Cui, W. Han, J. Ge, X. Wu, H. Kim, B. Ding, Electrospinning: a versatile strategy for mimicking natural creatures, Compos. Commun. 10 (2018) 175–185. [12] S. Rong, P. Zhang, Y. Yang, L. Zhu, J. Wang, F. Liu, MnO2 framework for instantaneous mineralization of carcinogenic airborne formaldehyde at room
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In situ synthesis of MnO2@SiO2–TiO2 nanofibrous membranes for room temperature degradation of formaldehyde
T
Fuhai Cuia, Weidong Hana, Yang Sib, Wenkun Chenb, Meng Zhangb, Hak Yong Kima,c,∗∗, Bin Dingb,∗ a
Department of BIN Convergence Technology, Chonbuk National University, Jeonju, 561-756, South Korea Innovation Center for Textile Science and Technology, Donghua University, Shanghai, 200051, China c Department of Organic Materials and Fiber Engineering, Chonbuk National University, Jeonju, 561-756, South Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrospun nanofibers Titanium dioxide Manganese dioxide Formaldehyde Catalytic degradation
Catalytic oxidation is an efficient and cost-effective technology to eliminate HCHO, and MnO2 shows excellent performance towards this kind of reactions. In this work, a birnessite type manganese dioxide nanoparticles modified silica-doped titanium dioxide (MnO2@SiO2–TiO2) nanofibrous membranes were successfully synthesized by using an electrospinning technique and liquid phase synthesis. It is found that MnO2@SiO2–TiO2-4 nanofibrous membrane showed the best catalytic activity for HCHO removal. The as-fabricated nanofibrous membranes exhibited good and reversible catalytic oxidation activity towards formaldehyde, and more than 90% of the catalytic performance could remain after 5 testing cycles. The successfully constructed MnO2 nanoparticles decorated soft SiO2–TiO2 nanofibrous membranes, which can be a great promise for effectively solving the problem of environmental remediation.
1. Introduction Formaldehyde (HCHO), as the most typical and common indoor air pollution, may cause a range of diseases, including nausea, eye irritation, headaches, allergic dermatitis, and bronchial asthma [1–3]. HCHO can be emitted from the widespread use of building materials, furnishings, and decoration materials, which has become one of the top risks to human health and a global problem with more and more widespread attention [4–8]. In 2010, the World Health Organization established an indoor air standard, which limited the content of HCHO below 0.08 mg m−3. Globally, in most of the developing country, especially in new buildings or remodeled houses, HCHO content often exceeded the standard value [9–11]. To solve the above-mentioned problem, various air purification technologies have been widely utilized for HCHO decomposition, for example, active carbon adsorption, photocatalytic degradation, and catalytic oxidation degradation [12–15]. Compared with other methods, catalytic oxidation degradation has been considered as the most hopeful to remove HCHO because of its no secondary pollution and light source assistance. Thus, developing a fast and efficient catalytic material to remove HCHO is of great significance. Among the catalytic oxidation catalysts, transition metal-based and noble metal-based catalysts, with the merit of decomposing HCHO into ∗
CO2 under mild condition, have been studied for years [16–19]. However, high cost greatly limits the noble metal-based catalysts in the practical application. Therefore, transition metal oxides, particularly, MnO2 has been widely studied for formaldehyde decomposition due to its abundant amount and low cost [20–22]. However, to our knowledge, no matter what noble-metal-based or transition metal oxide catalysts are usually used as powder. Generally, these powder catalyst should be blended with polymer binder and then coated on the nonwoven fabrics to realize their practical application, which leads to the decrease of the exposed active sites and catalytic activity. Fortunately, electrospun nanofibers have been deemed as an ideal candidate carrier, due to their attractive characteristics (e.g. structural controllability, high porosity, composition tenability, and interspatial connectivity, and large specific surface area), which has attracted great attention from researchers [23–26]. However, the nanostructure of the electrospun nanofibrous membrane will be ineffective during the above-mentioned surface coating process [27–29]. Thus, it is of great importance to develope a suitable method to construct granular and fibrous composite materials with enough accessible active sites and good catalytic activity. In this work, we demonstrated a simple and versatile method for fabricating soft MnO2@SiO2–TiO2 nanofibrous membranes, where the
Corresponding author. Corresponding author. Department of BIN Convergence Technology, Chonbuk National University, Jeonju, 561-756, South Korea. E-mail addresses: [email protected] (H.Y. Kim), [email protected] (B. Ding).
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https://doi.org/10.1016/j.coco.2019.08.002 Received 26 June 2019; Received in revised form 26 July 2019; Accepted 5 August 2019 Available online 05 September 2019 2452-2139/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic for the fabrication of the as-prepared MnO2@SiO2–TiO2 nanofibrous membrane via electrospinning and liquid phase synthesis.
25 ± 2 °C and 50 ± 5%, respectively. Finally, the as-prepared electrospun nanofibrous membrane was calcined in air at 700 °C by using a muffle resistance furnace (SX-G12133, Tianjin Zhonghuan Furnace Co., Ltd., China), the calcination temperature maintained for 2 h to sufficiently burn off the polymer and obtain the soft SiO2–TiO2 nanofibrous membranes.
nanostructured MnO2 nanoparticles were uniformly formed on the surface of SiO2–TiO2 nanofibers. The design of MnO2@SiO2–TiO2 nanofibrous membrane is primarily based on three rules: (1) the SiO2–TiO2 nanofibrous membrane should possess high porosity for HCHO transportation and adsorption. (2) The nanostructured MnO2 nanoparticles must be stably immobilized on the surface of SiO2–TiO2 nanofibers via a facile and versatile method. (3) The as-prepared membranes should possess stable structure and strong mechanical performance to provide prolonged utilization. Fig. 1 illustrates the synthetic process of MnO2@SiO2–TiO2 nanofibrous membrane. First of all, soft SiO2–TiO2 nanofibrous membranes were obtained by doping SiO2 through a versatile electrospinning and subsequently calcination. Then, MnO2 nanoparticles were in situ synthesized on SiO2–TiO2 nanofibers via a simple liquid phase synthesis method under a mild condition. Significantly, the loading amount and size of MnO2 nanoparticles could be controlled by altering synthesis cycles. Additionally, the as-prepared MnO2@SiO2–TiO2 nanofibrous membrane showed excellent catalytic oxidation activity towards HCHO degradation and could be easily reused without any complicated treatment process. We prospect that such materials could potentially serve as highly efficient and cost-effective catalysts for indoor air purification.
2.3. Synthesis of MnO2@SiO2–TiO2 nanofibrous membranes The hierarchical structured MnO2@SiO2–TiO2 nanofibrous membranes were obtained by seeding MnO2 nanoparticles on the SiO2–TiO2 nanofiber through liquid phase synthesis. Typically, 1.8 g KMnO4 and 0.12 g Glc were separately dissolved in 200 mL deionized water. After that, the soft SiO2–TiO2 nanofibrous membrane was first immersed into the KMnO4 aqueous solution for 5 min, and then the Glc solution was transferred to the KMnO4 solution, and the glass beaker was placed in a thermostatic oscillator at 60 °C for 1 h. Subsequently, the purple KMnO4 was reduced into brown MnO2 and deposited onto the SiO2–TiO2 nanofibers due to the reduction properties of Glc, soon afterward, the obtained sample was rinsed by deionized water to remove the residual ions. After different synthesis cycles, MnO2@SiO2–TiO2 nanofibrous membrane was dried in oven at 80 °C for 10 h. The as-prepared samples with n synthesis cycles were denoted as MnO2@SiO2–TiO2-n nanofibrous membranes (n = 1, 2, 3, 4, and 5).
2. Experimental 2.1. Materials
2.4. Characterization Polyvinylpyrrolidone (PVP, Mw = 1 300 000), tetrabutyl titanate (TBT), glucose (Glc), acetylacetone (Acac), N,N-dimethylformamide (DMF), absolute ethyl alcohol (EtOH), and ethanoic acid (HAc) were purchased from Aladdin (Shanghai, China). Potassium permanganate (KMnO4) and tetraethyl orthosilicate (TEOS) were obtained from Lingfeng Chemical Co., Ltd., China. Deionized water was supplied by a Heal-Force System.
The morphologies and microstructures of the resultant materials were observed by scanning electron microscopy (SEM). The nanostructures and element distribution of the obtained MnO2@SiO2–TiO2 nanofibrous membrane were performed by high-resolution transmission electron microscopy (HR-TEM) and The scanning transmission electron microscopy (STEM). X-ray diffraction (XRD) was used to characterize the crystal structure of the synthesized materials. The specific surface area (SSA) of the fabricated samples was analyzed by BET instrument. X-ray photoelectron spectroscopy (XPS) was used to describe the surface chemical states of the obtained materials.
2.2. Synthesis of SiO2 doped TiO2 electrospun nanofibrous membranes The SiO2–TiO2 electrospun nanofibrous membrane was fabricated via electrospinning and subsequently calcination methods. Firstly, EtOH was used as solvent to dissolve PVP powder at 25 °C through vigorously stirring more than 1 h, and transparent PVP solution with a concentration of 5 wt% was obtained. The Si sol was prepared by blending TEOS, EtOH, HAc and deionized water with stirring for at least 4 h. Afterward, the TBT and Acac (molar ratio of Ti:Si was 7:3) were mixed with the obtained Si sol and stirring for more than 1 h. Then, the spinning solution was prepared by mixing the Ti–Si sol and PVP solution. Subsequently, the electrospinning process was performed on an EXES-4 electrospinning device. During the spinning process, the room temperature and relative humidity (RH) were maintained at
2.5. Catalytic test The HCHO degradation activity of MnO2@SiO2–TiO2 nanofibrous membranes was evaluated under static mode, which was operated at 25 °C and RH of 40 ± 5%. Typically, the as-synthesized samples (size of 10 × 10 cm2) with different loading cycles were put in the 5 L organic glass reactor to test their activity of HCHO removal, respectively. Before each catalytic reaction test, the reactor was adequately cleaned to eliminate the influence of residual HCHO. The original concentration of HCHO was controlled at approximately 200 ppm. A BSQ-GCH20 62
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Fig. 3. (a) Low magnification TEM and (b) HR-TEM images of MnO2@ SiO2–TiO2-4. (c) STEM images of MnO2@SiO2–TiO2-4 with elemental mapping images of (d) Ti, (e) Mn, (f) Si, and (g) O, respectively.
Fig. 2. SEM images of (a) SiO2–TiO2, (b) MnO2@SiO2–TiO2-1, (c) MnO2@ SiO2–TiO2-2, (d) MnO2@SiO2–TiO2-3, (e) MnO2@SiO2–TiO2-4, (f) MnO2@ SiO2–TiO2-5 nanofibrous membrane, respectively.
study the synthesized MnO2@SiO2–TiO2 nanofibrous membrane, by taking MnO2@SiO2–TiO2-4 as an example. Fig. 3a showed the typical TEM image of the heterostructure of MnO2@SiO2–TiO2, it can be observed that the MnO2 nanoparticles with an average size of 40 nm were tightly immobilized onto the SiO2–TiO2 nanofiber after ultrasonic dispersion, revealing a strong adhesion force between MnO2 nanoparticles and SiO2–TiO2 nanofiber surface. As shown in Fig. 3b, the lattice distance of 0.24 nm can be observed, which is in accordance with the lattice distance of δ-MnO2 [30]. Furthermore, HR-TEM result confirmed the formation of intimate MnO2@SiO2–TiO2-4 heterostructures. The STEM was performed to further study the structure of the as-synthesized MnO2@SiO2–TiO2-4. The distribution of the elements of Ti, Si, Mn, and O was demonstrated in Fig. 3c–g. During the synthetic reaction, KMnO4 was reduced into MnO2 by Glc, which was irregularly grown on the surface of SiO2–TiO2 nanofibers [31]. Furthermore, the above result could be proved in Fig. 3f, the Mn element was dispersed onto the fiber. In addition, the corresponding results further verified the formation of the secondary nanostructure. The XRD patterns of the as-synthesized SiO2–TiO2, and MnO2@ SiO2–TiO2 nanofibrous membrane were shown in Fig. 4a. For the SiO2–TiO2 nanofibers, the weak diffraction peaks implying their poor crystallinity, which can be ascribed to the doped SiO2. However, the crystalline without any transition after inducing the component of SiO2. In addition, the peaks of MnO2@SiO2–TiO2-4 nanofibrous membrane were in accordance with the δ-MnO2 (JCPDS No. 80–1098). These two broad and weak peaks of the as-prepared sample revealed the poor crystallinity. Furthermore, compared with the previously reported MnO2 nanoparticles, the (002) plane showed a stronger intensity, implying that the MnO2 with highly exposed (002) facet was inclined to form onto the surface of the SiO2–TiO2 nanofiber [32]. XPS was applied to demonstrate the chemical compositions on the surface of MnO2@SiO2–TiO2-4 nanofibrous membrane. XPS survey spectra of MnO2@SiO2–TiO2-4 were shown in Fig. 4b. The chemical states of Mn element on the surface of the as-prepared MnO2@ SiO2–TiO2-4 nanofibrous membrane were demonstrated in Fig. 4c, Mn 2p3/2 spectra displayed two characteristical peaks were corresponding to Mn3+ and Mn4+, respectively, which was on the surface of the asprepared MnO2@SiO2–TiO2 nanofibrous membrane. In addition, as
formaldehyde online detector was utilized to monitor the change of HCHO concentration. For comparison, neat SiO2–TiO2 nanofibrous membrane without loading of MnO2 was also employed as a control sample for the catalytic degradation of HCHO under the same conditions. The reacted samples were sufficiently flushed with air and rinsed with deionized water to remove the adsorbent species. Subsequently, MnO2@SiO2–TiO2 nanofibrous membranes were reused for another catalytic reaction cycle, which was operated under the same experimental condition as demonstrated above. The removal efficiency of HCHO was evaluated as follow: C −C HCHO removal efficiency (%) = 0C t × 100%Where C0 is the in0 itial concentration of HCHO, and Ct is the concentration of formaldehyde at an interval time, respectively. 3. Results and discussion 3.1. Morphology and structure analysis Fig. 2a displayed the morphology of SiO2–TiO2 nanofibrous membrane, showing that the fibers were randomly distributed and the average diameter is ~250 nm. Compared with the brittle pure TiO2 nanofibrous membrane with many breakages, the SiO2–TiO2 nanofibrous membrane, as the substrate, exhibited ultra-large aspect ratios, smooth surface, and robust softness. This result revealed that SiO2 doping could effectively decrease cracks on the surface of the nanofiber, which could ensure good mechanical properties of TiO2 nanofibrous membrane during the synthesis process of MnO2 nanoparticles. As shown in Fig. 2b, after 1 cycle of synthesis, granular MnO2 nanostructure with a size of ~20 nm was grown on the SiO2–TiO2 nanofiber surface. The coverage rate and size of MnO2 nanostructures can be controlled by the loading amount through changing the synthesis cycles. Obviously, as shown in Fig. 2c–e, the loading amount of granular MnO2 gradually increased with the increase of synthesis cycles. After 5 synthesized cycles, the integrality of SiO2–TiO2 nanofibrous membrane was destroyed by the formation of dense MnO2 aggregation (Fig. 2f). TEM and elemental mapping analysis were performed to further 63
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Fig. 4. (a) XRD patterns of SiO2–TiO2 and MnO2@Si–TiO2-4 nanofibrous membrane. XPS spectra of MnO2@SiO2–TiO2-4 nanofibrous membrane: (b) survey spectra, (c) Mn 2p3/2, and (d) O 1s.
structure. The pore size distribution of the as-synthesized samples were in the range of 10–70 nm (Fig. 5b). Additionally, the BJH pore volume of MnO2@SiO2–TiO2-4 nanofibrous membrane was much larger than the initial SiO2–TiO2 nanofibrous membrane. Generally, for catalytic materials, hierarchical structures on the fiber surface could enhance their SSA of the as-prepared samples, which would contribute to improving their performance. The significantly increased surface area would supply much more active sites for the oxidation reaction of the as-prepared samples. The catalytic activity of the as-prepared samples were carried out under static test mode at room temperature. As shown in Fig. 6a, SiO2–TiO2 nanofibrous membrane almost has no activity for HCHO removal, implying that no chemical reaction or physical adsorption were occurred on the SiO2–TiO2 nanofibers. The sharp decline of the HCHO concentration in the first 5 min was ascribed to the gradually
shown in Fig. 4d, the O 1s spectra showed one characteristical peak at 529.6 eV attributing to lattice oxygen (Olatt) on the as-synthesized MnO2 nanoparticles [32]. The porous structure was measured through aperture analyzer to investigate the effect of the mass loading of MnO2 nanoparticles on the SiO2–TiO2 nanofibrous membranes. As shown in Fig. 5a, a careful observation demonstrated that the XRD curves of modified samples displayed the formation of mesopores. This phenomenon could be assigned to the voids among the MnO2 nanoparticles on the SiO2–TiO2 nanofibrous membrane. Obviously, the gradually increasing BET surface area from 4.95 to 48.59 m2 g−1 was ascribed to the increased loading amount of MnO2 with the increase of synthesis cycles from 0 to 5, revealing that nanostructured MnO2 played an important role in enlarging the effective surface area [33]. The MnO2@SiO2–TiO2 nanofibrous membranes also showed a typically polydisperse pore
Fig. 5. (a) BET isotherms of these as-synthesized materials. (b) Pore size distribution of relevant samples by employing BJH analysis. 64
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Fig. 6. (a) Catalytic oxidation degradation curves of HCHO over different samples. (b) Catalytic oxidation activity of MnO2@SiO2–TiO2-4 nanofibrous membrane towards HCHO with 5 cycles.
~8.5%. In theory, according to the calculated results, the catalyst would be deactivation after 60 catalytic cycles. However, to our knowledge, during the degradation process, the catalyst could only change the rate of the reaction without entering into it. The decreased catalytic oxidation property could be assigned to the reduction of active sites. Thus, the service life of the as-prepared catalytic materials would be much longer than the calculated results in practical application. One of the unique advantages of electrospinning technology is in scaling up the fabrication. For example, as shown in Fig. S1, a large scale (40 × 40 cm2) membrane was successfully fabricated. As the above-mentioned, we proposed a possible mechanism of HCHO removal on MnO2 at room temperature (Fig. 6d). First of all, hydrogen bondings which were formed between HCHO and surface hydroxyl groups of MnO2. Secondly, the adsorbed HCHO could be oxidized into formate and carbonate by structural hydroxyl groups. Thirdly, the regeneration of surface hydroxyl groups could be taken place during the reaction between active oxygen (O2−, O−, etc.) and gas phase water through this reaction (O2−, O− + H2O → 2-OH). Finally, the desorption of carbonate from the surface of MnO2 could be stimulated by competitive adsorption of water in air [34].
increased loading amount of MnO2 nanoparticles, the HCHO removal efficiency of the as-synthesized samples of MnO2@SiO2–TiO2 nanofibrous membranes also gradually increased with the increase of synthesis cycles, respectively. In addition, the HCHO removal efficiency of MnO2@SiO2–TiO2-4 and MnO2@SiO2–TiO2-5 nearly reached 100% within 20 min. This results could reveal that the HCHO could be easily oxidized by the as-prepared samples, and the HCHO removal efficiency depended on the loading amount of MnO2 nanoparticles. To further investigate the catalytic activity of the as-prepared samples, the oxidation kinetics of HCHO were analyzed by utilizing the equation as follows: ln(C0/Ct) = kt Where C0 is the concentration of HCHO at the initial time (min), and Ct is the concentration of formaldehyde at an interval time (min), respectively, and k (min−1) is the HCHO removal rate constant. The fitting curves of the as-synthesized samples obeyed the pseudo-first-order reaction dynamic model, which were demonstrated in Fig. 6b. The constant k for the samples of MnO2@SiO2–TiO2-4 and MnO2@ SiO2–TiO2-5 nanofibrous membranes have a similar calculated value of 0.21 and 0.22 min−1, respectively. This result revealed that MnO2@ SiO2–TiO2-4 and MnO2@SiO2–TiO2-5 exhibited the highest HCHO removal efficiency among the as-synthesized materials. However, due to the excess mass loading of MnO2, SiO2–TiO2 nanofibrous membrane could not maintain integrity after 5 synthesis cycles. The recoverability plays a very important role in evaluating the practical application of a catalyst. However, it is a challenge to keep well catalytic properties in a long-term use process. As shown in Fig. 6c, the sample exhibited good reversible catalytic oxidation properties towards HCHO and 91.57% of the catalytic activity still could be preserved after 5 utilization cycles, implying the good reusability of MnO2@SiO2–TiO2-4. As the above-mentioned testing results, the theoretical service life of the as-prepared catalytic materials could be calculated, after 5 cycles, the catalytic performance was reduced by
4. Conclusions In summary, soft MnO2@SiO2–TiO2 nanofibrous membranes were constructed by the combination of electrospinning method and subsequent liquid phase synthesis. The resultant nanostructured MnO2 nanoparticles endowed the samples with large SSA, which supplied more catalytic reaction active sites. More interestingly, profiting from the increased SSA and high porosity, the as-prepared nanostructured MnO2 nanoparticles decorated on the surface of SiO2–TiO2 exhibited high HCHO removal efficiency of 100%. After 5 times of catalytic testing cycles, The sample still preserved 91.57% of the initial catalytic activity towards HCHO. Considering various advantages of the as-synthesized MnO2 modified SiO2–TiO2 nanofibrous membranes, this work 65
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will afford a facial and efficient method for developing efficient noble metal-free catalysts to degrade VOCs at room temperature.
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Author contributions Fuhai Cui and Weidong Han contributed equally to this work. Notes The authors declare no competing financial interest. Acknowledgements This work was supported by the National Science Foundation of China (No. 51873029), the Innovation Program of Shanghai Municipal Education Commission (No. 2017-01-07-00-03-E00024) and the Program of Shanghai Academic Research Leader (No. 18XD1400200). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.coco.2019.08.002. References [1] W. Kanchongkittiphon, M.J. Mendell, J.M. Gaffin, G. Wang, W. Phipatanakul, Indoor environmental exposures and exacerbation of asthma: an update to the 2000 review by the Institute of Medicine, Environ. Health Perspect. 123 (2015) 6–20. [2] T. Salthammer, S. Mentese, R. Marutzky, Formaldehyde in the indoor environment, Chem. Rev. 110 (2010) 2536–2572. [3] L. Cao, Q. Fu, Y. Si, B. Ding, J. Yu, Porous materials for sound absorption, Compos. Commun. 10 (2018) 25–35. [4] J. Quiroz Torres, S. Royer, J.P. Bellat, J.M. Giraudon, J.F. Lamonier, Formaldehyde: catalytic oxidation as a promising soft way of elimination, Chem. Sus. Chem. 6 (2013) 578–592. [5] C. Jiang, D. Li, P. Zhang, J. Li, J. Wang, J. Yu, Formaldehyde and volatile organic compound (VOC) emissions from particleboard: identification of odorous compounds and effects of heat treatment, Built. Environ. 117 (2017) 118–126. [6] Q. Fu, Y. Si, C. Duan, Z. Yan, L. Liu, J. Yu, B. Ding, Highly carboxylated, cellular structured, and underwater superelastic nanofibrous aerogels for efficient protein separation, Adv. Funct. Mater. 29 (2019) 1808234. [7] J. Wang, P. Zhang, J. Li, C. Jiang, R. Yunus, J. Kim, Room-temperature oxidation of formaldehyde by layered manganese oxide: effect of water, Environ. Sci. Technol. 49 (2015) 12372–12379. [8] E. Uhde, T. Salthammer, Impact of reaction products from building materials and furnishings on indoor air quality-a review of recent advances in indoor chemistry, Atmos. Environ. 41 (2007) 3111–3128. [9] R. Averlant, S. Royer, J.-M. Giraudon, J.-P. Bellat, I. Bezverkhyy, G. Weber, J.F. Lamonier, Mesoporous silica-confined manganese oxide nanoparticles as highly efficient catalysts for the low-temperature elimination of formaldehyde, ChemCatChem 6 (2014) 152–161. [10] X. Tang, Y. Bai, A. Duong, M.T. Smith, L. Li, L. Zhang, Formaldehyde in China: production, consumption, exposure levels, and health effects, Environ. Int. 35 (2009) 1210–1224. [11] F. Cui, W. Han, J. Ge, X. Wu, H. Kim, B. Ding, Electrospinning: a versatile strategy for mimicking natural creatures, Compos. Commun. 10 (2018) 175–185. [12] S. Rong, P. Zhang, Y. Yang, L. Zhu, J. Wang, F. Liu, MnO2 framework for instantaneous mineralization of carcinogenic airborne formaldehyde at room
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