MnO2 Framework for Instantaneous Mineralization of Carcinogenic

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MnO Framework for Instantaneous Mineralization of Carcinogenic Airborne Formaldehyde at Room Temperature Shaopeng Rong, Pengyi Zhang, Yajie Yang, Lin Zhu, Jinlong Wang, and Fang Liu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02833 • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 21, 2016

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MnO2 Framework for Instantaneous Mineralization of Carcinogenic Airborne Formaldehyde at Room Temperature

Shaopeng Rong, Pengyi Zhang*, Yajie Yang, Lin Zhu, Jinlong Wang, Fang Liu State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, P. R. China.

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ABSTRACT: Formaldehyde (HCHO) causes increasing concerns due to its ubiquitously found in the indoor environment and being irritative and carcinogenic to humans. The fast abatement of HCHO is of significant practical interest at room temperature. In this paper, we fabricated a three-dimensional manganese dioxide framework (3D-MnO2), which owns interconnected network structures, low mass density (~7.3 mg cm-3), and high absorption capacity for organic liquids. In particular, the 3D-MnO2 showed excellent activity and stability for HCHO oxidation at room temperature, achieving 45% of 100 ppm HCHO mineralized into CO2 under high gas hourly space velocity of 180 L gcat-1 h-1. The excellent performance of 3D-MnO2 catalysts in decomposing HCHO can be ascribed to their quickly reversible and high water content for replenishing the consumed surface hydroxyl groups during HCHO decomposition, and fully exposed active reaction sites. This is a valuable finding that inexpensive metal oxides such as MnO2 can transform ppm-level HCHO into harmless CO2 in as short as sub-second at room temperature.

KEYWORDS: three-dimensional network, manganese dioxide, formaldehyde, catalytic oxidation, indoor air


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1. INTRODUCTION Nowadays, people typically spend over 80% of their time indoors, the indoor air quality will be closely related to human health. As one of the priority indoor air pollutants emitted from furnishing and building materials 1, formaldehyde (HCHO) has attracted a great deal of attention. With the high consumption and widespread use of building materials and furnishings, the problem of indoor HCHO pollution can be almost inevitable, especially in newly built or remodeled houses. Long-term exposure to HCHO may result in adverse human health effects and even cancer 2. Both in China and worldwide, the public concern on the health effects of HCHO exposure continues to grow today. The World Health Organization has classified HCHO as group 1 carcinogen for humans, and set an indoor guideline value of 0.1 mg m-3 for HCHO in 2010 3, which is also adopted as the Chinese national standard. The surveys reveal that indoor HCHO in US commercial and residential buildings were as high as 42 ppb, often exceeding recommended exposure limits 4. As the most populous developing country in the world, the HCHO level in 70% of Chinese newly built or remodeled houses have exceeded the safety standards 2. Therefore, it is of great significance to develop the cost-effective and practical HCHO removal technologies. The development of new catalytic materials, which could quickly transform HCHO into CO2 at room temperature, will be an alternative method. Many efforts have been devoted to develop efficient materials for HCHO removal. Among noble metals, supported Pt catalysts, such as 1 wt% Pt/TiO2 5, 0.1 wt% Pt/TiO2 6, 1 wt% Pt/Fe2O3 7

and 3 wt% Pt/MnOx-CeO2

8

have been reported to show the ability to completely transform

ppm-level HCHO into CO2 at room temperature. Considering the high cost of noble metals, transition metal oxides especially manganese dioxides (MnO2) have been investigated for HCHO oxidation. Sekine et al.

9

first found MnO2 had the best activity among 13 commercial


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transitional metal oxides. Our group 10 have found that the layer structured MnO2, i.e. birnessite (δ-MnO2) could oxidize HCHO into CO2 at room temperature. Zhang et al.

11

also reported that

δ-MnO2 exhibited the best catalytic activity among four phase structures MnO2 catalysts, achieving almost 100% conversion at 80 °C with a gas hourly space velocity of 100 L gcat-1 h-1. Sidheswaran et al

12

coated MnO2 consisting of mixed crystal phases on the ventilation filter

media and used to treat 150-200 ppb HCHO, achieving a stable 80% conversion within 35 days at room temperature at low air velocities (0.2 cm s-1). However, up to now, for non-noble catalysts, it is very difficult to completely oxidize HCHO to CO2 and H2O at room temperature. Examination of the literatures reveals that the HCHO completely conversion requires the temperature is much higher than room temperature for most of the non-noble catalysts. With the decrease of temperature, the conversion of HCHO decreased sharply. Most of the non-noble catalysts exhibit very low HCHO conversion at room temperature at high face velocity required for indoor air purification. On the other hand, whether noble catalysts or non-noble catalysts are usually in the form of powder, which is not convenient for practical applications. Thus, if the catalyst can maintain a certain controllable macroscopic shape, it will greatly facilitate its practical application. However, both noble catalysts and non-noble catalysts may encounter the problem of losing activity when loaded on the carriers or assembled into particles. Therefore, how to make the catalyst expose active sites to remain its high activity and stability at room temperature is not only of great significance but also a huge challenge. Three-dimensional (3D) network structure has been attracting much attention, either as inherent catalysts or supports for other catalysts. Since the first aerogels were prepared by Kistler in 1930s

13

, 3D-structured materials such as sponges, foams, and aerogels have been attracting

much attention, and their fabrication methods and application technology have been significantly


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developed

14-17

. These 3D-structured materials appear continuously interconnected network

structures, high porosity, ultralow mass density, large surface area, and fast mass transport kinetics, which offer great promise for wide applications. For the catalytic process, the construction of catalytic systems based on 3D interconnected network may favor mass transfer, reduce transport limitation and effectively reduce the agglomeration of the nanomaterials to expose more active sites. Zhang’ group prepared a 3D-structured TiO2/GA composites by onestep hydrothermal method, which exhibited highly recyclable photocatalytic activity past few years, two kinds of preparation methods, i.e. self-assembly approaches

24-30

19-23

18

. In the

and template-guided

, have been developed to synthesize various 3D-structured materials. Though

composite 3D materials containing MnO2 have been reported in recent years, the fabrication of 3D pure MnO2 network is rarely reported. He et al. 31 reported a composites MnO2/carbon foam by growing a layer MnO2 nanosheets on the surface of carbon foam, which showed excellent capacitance performance. Wu et al. 32 papered interconnected porous 3D MnO2/rGO hydrogel by a reduction and self-assembly process, which exhibited remarkable energy density and stable cycling performance. Zhu et al. 33 assembled a hierarchical rGO/β-MnO2 hydrogel by dispersion of pre-synthesized ultrathin β-MnO2 nanobelts in graphene oxide solution by a hydrothermal reaction. The construction of above 3D composite MnO2 structures is usually based on carbon materials (GO, rGO, carbon nanotubes), which are inert components and may mask the active sites for catalytic reactions. In this work, we reported an easy approach to fabricate three-dimensional manganese dioxide (3D-MnO2) without the assistance of organic components and template. The resulting 3D-MnO2 not only can combine a number of excellent structural properties such as interconnected network, low mass density (~7.3 mg cm-3), rich porosity and highly exposed active sites, but also showed


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the high absorption capacity for organic solvent. Particularly, as-synthesized 3D-MnO2 showed the excellent activity for the removal of HCHO, which could quickly transform ppm-level HCHO into CO2 at room temperature. 2. EXPERIMENTAL SECTION 2.1 Chemicals All chemicals were of analytical grade and used with received. Methanol, MnCl2▪4H2O, MnSO4▪H2O, KClO3, CH3COOK, CH3COOH and H2O2 (30 wt% in H2O) were all purchased from Beijing Chemical Reagent Co., Ltd. Tetramethylammonium hydroxide (TMA▪OH, 25 wt% in H2O) was supplied by Aladdin Reagent Co., Ltd. Ultrapure water was used in all experiments, which was obtained from an ultrapure water system (Thermo Co., USA). 2.2 Catalyst Preparation Synthesis of MnO2 nanosheets: Ultrathin MnO2 nanosheets were synthesized through a onestep procedure reported in the literature

34

. Firstly, TMA▪OH (4.4 mL), H2O2 (30 wt%, 2 mL)

and 15 mL ultrapure water were mixed together. Then the mixed solution was poured into MnCl2▪4H2O aqueous solution (10 mL, 0.3 M) with stirring, and kept stirring for 12 h in the open air at room temperature. Finally, MnO2 nanosheets colloid suspension was obtained. Synthesis of MnO2 nanowires: 0.6761 g MnSO4·H2O, 0.8576 g of KClO3 and 0.6870 g of CH3COOK were mixed with 60 mL ultrapure water. After the above chemicals were fully dissolved, 3.2 mL CH3COOH was poured. Then the clarifying mixed solution was transferred into a 100 mL Teflon-lined stainless steel autoclave. The autoclave was heated in electric oven and kept at 160 °C for 8 h. After the autoclave cooled naturally to room temperature, the


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precipitates were collected by filtration and washed several times with ultrapure water. Finally, the as-obtained powder was dried in air. Synthesis of 3D-MnO2: Firstly, MnO2 nanosheets precipitates was collected from as-papered colloid suspension by centrifugation at 14000 rpm for 30 min. Then the obtained MnO2 nanosheets was dispersed in 30 ml water with ultrasonic treatment. Then, equal volume of MnO2 nanowires aqueous dispersion (2.0 mg mL-1) and MnO2 nanosheets aqueous dispersion were mixed together. After stirred for 5 h in the open air at room temperature, the mixture suspension was poured into the desired container, then it was immersed in a liquid nitrogen bath to freeze. Subsequently, the obtained frozen samples were directly transferred to a freeze-drier and freezedried at -50 °C for 2-3 days to prepare 3D-MnO2. 2.3 Characterization The density (ρ) of the 3D-MnO2 was calculated by dividing the mass weight (m) by the volume (V) (ρ = m/V). The volume of 3D-MnO2 was calculated based on the geometric equation (V = π (d/2)2 h) for a relatively good cylindrical shape sample. The weight of the 3D-MnO2 including the weight of entrapped air was measured by an electronic balance (Mettler Toledo, Switzerland). Field emission scanning electron microscopy (FESEM) were conducted with a Hitachi S5500 field emission scanning electron microscope (Hitachi Co., Ltd, Japan). Transmission electron microscopy (TEM) images were obtained with a JEM-2011 electron microscope (JEOL, Japan). X-ray diffraction (XRD) patterns were carried out with a Bruker D8Advance X-ray diffractometer (Bruker, Germany) using Cu Kα radiation at a scanning rate of 8° min-1 in a 2θ range of 5-90°.

X-ray photoelectron spectroscopy (XPS) was measured by

ESCALAB 250Xi (Thermo Fisher, USA) at a pass energy of 50 eV, equipped with Al Kα as an


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exciting X-ray source. Binding energies were corrected for surface charging by referring to the energy of the C 1s peak at 284.8 eV. The XPSPEAK41 peak fitting program was applied to fit the Mn 2p3/2, Mn 3s and O 1s spectrum, respectively. In the spectra, the background was approximated by a Shirley-type profile. The ratio of elements with different valence states was calculated based on the peak areas processed by the XPSPEAK41. The Brunauer-Emmett-Teller (BET) specific surface area measurements were determined on an Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). The multipoint BET method and BJH (Barrett-JoynerHalenda) analyses were used to determine the total specific surface area and the pore volume of the samples, respectively. In order to determine the intermediate species during HCHO oxidation process and to learn the HCHO oxidation mechanism, an in-situ diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) was collected on Nicolet 6700 FTIR spectrometer (Thermo Fisher, USA). The spectra were measured with a resolution of 4 cm-1 and an accumulation of 32 scans. During HCHO oxidation process, the reactant gas mixture (∼80 ppm of HCHO + O2) was injected into the DRIFT cell at a flow rate of 30 mL min-1 at room temperature. Thermogravimetric (TG) analysis was performed using TGA/DSC 1 STARe (Mettler Toledo, Switzerland) with the heating rate of 5 °C min-1 from 25 °C to 800 °C in N2 (20 mL min-1). 2.4 Catalytic activity tests The samples of 50 mg were carefully pressed into a flow-through quartz with the diameter of 8 mm. The reactor was placed into a pipe furnace with the air heating. The temperature was controlled with the accuracy ± 0.5 °C using a temperature controller. In order to achieve the stable conversion, the temperature was maintained at every certain temperature for 40 min.


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HCHO gas was generated by flowing synthetic air (N2/O2 = 79%/21%) over the paraformaldehyde heated at 27 °C in a water bath. The HCHO gas was then diluted by synthetic air in the mixed gas cylinder to generate mixed gas. The total flow rate of mixed feed gas was 150 mL min-1, and the inlet HCHO concentration was ~100 ppm, corresponding to a gas hourly space velocity (GHSV) of 180 L gcat-1 h-1. When the water stream resistance experiment was performed, the synthetic air would pass through the water solution and then mixed with the HCHO gas. The relative humidity (RH) of outlet gas was ~65%. The concentration of HCHO in the outlet gas was determined by using MBTH method

35

. The concentration of CO2 generated

from the oxidation of HCHO was determined by a gas chromatograph (GC-2014, Shimadzu, Japan) equipped with amethanizer. The HCHO conversion ratio was then calculated as following Equation (1): [ ]

HCHO conversion = [] × 100% (1)

where [HCHO]in is the inlet concentration of HCHO and [CO2]out is the outlet concentration of CO2 in the effluent through the samples. For kinetics measurement, the HCHO conversion was controlled below 15%. The reaction rate (ν) was calculated using the following equation (2): #$ ( ! " #$ %&'( )=

*+*, -./0 1 23/

(2)

Where η denotes the stable HCHO conversion based on CO2 formation; CHCHO is the concentration of HCHO in gas mixture; Vgas is the total flow rate during HCHO decomposition; mcat is the mass of the catalyst in reactor bed.


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Supposing that the reaction is a first-order reaction for HCHO oxidation 36, the obtained kinetic model for catalytic decomposition of HCHO in the presence of excess air can be described as following kinetic models:

ln = −

6/ 78

− ln 9: + ln 3D-MnO2 > MnO2 nanowires. Correspondingly, the AOS of Mn is also in the same order, which was calculated according to the binding energy difference of Mn3s. Besides, the O1s spectra were also deconvoluted as shown in Figure 5d. Two recognizable peaks located at around 529.7 and 531.3 eV are ascribed to lattice oxygen (Olatt) and surface adsorbed oxygen species (Oads), respectively

46

. From Figure 5d and Table 1, we can clearly find that the percentage of Oads

follows the order: 3D-MnO2 > MnO2 nanosheets > MnO2 nanowires, and 3D-MnO2 exhibit the highest Oads/Ototal ratio. The surface adsorbed oxygen usually exists in the form of surface hydroxyl, indicating that 3D-MnO2 may have higher surface hydroxyl and/or adsorbed water content. It has been reported that the surface hydroxyl could promote the oxidation of HCHO at room temperature. Therefore, we assume that 3D-MnO2 will exhibit a higher HCHO catalytic activity than its precursors, which was confirmed by the following experiments. Besides, resulting from the hydrogen bonding between surface hydroxyl and water, higher surface hydroxyl content usually means more surface water content, which is supported by the following results of TG. It means that 3D-MnO2 not only has a high content of surface hydroxyl, but also surface water.


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Figure 5. XPS spectra of MnO2 nanosheets, MnO2 nanowires and 3D-MnO2. (a) Survey spectra, (b) Mn2p, (c) Mn3s, (d) O1s.

Table 1. Surface element compositions and BET surface areas Samples MnO2 nanosheets MnO2 nanowires 3D-MnO2

Mn4+/Mn3+ 5.16 3.91 4.31

Surface element compositions Oads/Ototal AOS a Mn/O b 0.38 3.75 0.40 0.36 3.68 0.48 0.54 3.73 0.35

K/Mn b 0 0.12 0.06

BET (m2 g-1) 202.9 16.7 99.8

Pore Volume (cm3 g-1) 0.245 0.038 0.203

a

The average oxidation state (AOS) of Mn was calculated according to the binding energy difference (△E) of Mn3s in XPS through an empirical formula, i.e. AOS = 8.956–1.126 ×△E b The element molar ratio is calculated from the XPS results.

The specific surface area of MnO2 nanosheets, nanowires and 3D-MnO2 were measured by fitting the isotherm to the BET model, which were 202.9 m2 g-1, 16.7 m2 g-1 and 99.8 m2 g-1, respectively (Table 1). Due to its hierarchical porous structure, the 3D-MnO2 exhibited excellent absorption capacity and absorption rate for organic solvents. The fast absorption progress is


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shown in a series of photos in Figure 6a, a small amount of 3D-MnO2 can completely adsorb 575 mg toluene within 5 s. The average absorption rate of 3D-MnO2 is ~2.3 g g-1 s-1, which is much faster than the graphene aerogel (∼0.57 g g-1 s-1) reported before 47. The fast absorption rate and high absorption capacity indicate the great potential use of the 3D-MnO2 for the facile removal of oil spillage and chemical leakage. Economic performance and environmental protection efforts require that functional materials can not only absorb pollutants, but also can be repeatedly use. Because the present 3D-MnO2 did not show a good compressibility, it is not feasible to remove absorbed organic solvents through squeezing. However, combustion may be an attractive alternative method. As shown in Figure 6b, the toluene can be directly combusted after being absorbed by 3D-MnO2, which maintained its shape, size, and 3D network structure after a short time combustion.

Figure 6. (a) Absorption process of toluene (stained with Sudan Black B) by the 3D-MnO2 within 6 seconds. (b) Digital photos showing the process of recycling 3D-MnO2 via combustion.


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3.2 Catalytic activity for HCHO oxidation Among the transition metal oxides, Mn-based catalysts have been widely investigated due to its best catalytic activity for the oxidation of HCHO. Figure 7a shows HCHO conversion under various reaction temperature over the different types of MnO2 samples during the purge of dry air and humid air. Whatever under the conditions of dry or humid air, the catalytic activities followed the sequence: 3D-MnO2 > MnO2 nanosheets > MnO2 nanowires. And all samples showed better performance in humid air than in dry air. As discussed above, the crystal phases of MnO2 nanosheets and nanowires were δ- and α- MnO2 respectively, whose results of the oxidation of HCHO are consistent with Zhang’s

11

. In the case of humid air, complete HCHO

conversion was achieved at ~ 90 °C. Interestingly, even at room temperatures under high gas hourly space velocity of 180 L gcat-1 h-1 (the corresponding retention time ~ 0.4 s), 3D-MnO2 also achieved HCHO conversion 25% in dry air and 45% in humid air, respectively, which are greatly higher than those over MnO2 nanosheets, nanowires, or any other metal oxides reported in literature. The reasons can be attributed to the 3D network structure, which favors mass transfer and fully expose the reaction sites. Comparing the HCHO complete oxidation temperature over three samples in the case of dry air or humid air, we can clearly find that the catalytic activity for the oxidation of HCHO is strongly dependent on the relative humidity. The promoting effect of water vapor on HCHO oxidation has also been reported for several noble metal catalysts (Pt/Fe3O4, Au/CeO2, and AgMnOx-CeO2) and transition metal oxide (CeO2 and MnOx)

48-50

. Huang et al., reported that the

water vapor and hydroxyl groups are essential for the oxygen supply and transfer during the catalytic oxidation of HCHO 51. Chen et al., suggested that water can react with surface oxygen to form surface hydroxyl group

52, 53

, which plays important role in HCHO oxidation. Previous


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literature suggests that HCHO could directly react with hydroxyl groups to generate formate species 54. In addition, the formate species could be further oxidized by surface hydroxyl groups 52, 54, 55

. Moreover, the reaction between water vapor and surface active oxygen (O2-, O- + H2O →

2-OH) could regenerate the surface hydroxyl groups consumed in the oxidation of HCHO. Our previous study

10

also found that the presence of moisture did not inhibit but rather enhanced

HCHO oxidation, and found that water can play two roles in HCHO removal, i.e. one is to compensate the consumed hydroxyl group on the catalyst surface and promote the further oxidation of HCHO intermediates into CO2; the other is to promote the absorption of HCHO via hydrogen bond. Thus, the presence of water can improve the HCHO conversion activity in humid air.


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Figure 7. (a) HCHO conversion as a function of reaction temperature over the different MnO2 catalysts (HCHO concentration= 100 ppm, 21% O2, N2 as balance gas, RH= 0 or 65%, GHSV= 180 L gcat-1 h-1). (b) Arrhenius plots for HCHO oxidation over the different MnO2 catalysts. (c) Stability test of the 3D-MnO2 under different reaction conditions (HCHO concentration = 100 ppm, 21% O2, N2 as balance gas, GHSV= 180 L gcat-1 h-1). Figure 7b shows the Arrhenius plots of the catalysts for HCHO oxidation, and the calculated apparent activation energy (Ea) was simultaneously given for every catalyst. Whatever in humid air or in dry air, the 3D-MnO2 material shows the lowest Ea values, which is much lower than those owned by MnO2 nanowires and nanosheets. With the introduction of moisture in feed gas, all three kinds of MnO2 catalysts exhibit lower Ea values than that in case of dry air, thus providing further evidence that the presence of moisture can enhance the oxidation of HCHO. The catalytic stability of 3D-MnO2 for HCHO oxidation was tested in a series of relatively longtime experiments. As shown in Figure 7c, complete conversion of HCHO was observed at 120 °C in dry air, while at 90 °C in humid air. And during the 10 h test periods, no deactivation was observed in both cases, implying that 3D-MnO2 catalyst is quite stable under these conditions. The stability of 3D-MnO2 for the oxidation of HCHO was also tested. In humid air, 45% HCHO conversion was achieved, which kept stable during the period of 10 h test. While in dry air, the HCHO conversion was 26%, and it gradually decreased to 21% within 10 h. Above results further indicate that the presence of water in air is good for HCHO oxidation by 3DMnO2, and 3D-MnO2 material can maintain stable activity even at room temperature in a long time. Considering the ubiquitous presence of water vapor in air, the present finding suggests water vapor in real environment is good for 3D-MnO2. 3.3 In-situ DRIFTS and TG study of reaction mechanism


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To elucidate why as-synthesized 3D-MnO2 material shows excellent activity for HCHO oxidation, in-situ observation of DRIFTS spectra of three samples in a flow of ~80 ppm HCHO/O2 at room temperature was performed. As shown in Figure 8, upon exposure to HCHO, the absorption bands at 1343, 1394, 1580, 1605, 1658, 2338 and 3580 cm-1 were observed on the surface of catalysts. The bands at 1343 (νs (COO-)), 1394 (δ (CH)), and 1605 (νas (COO-)) can be attributed to formate species, and the bands at 1580 cm-1 (νas (COO-)) and 1312 cm-1 (νs (COO-)) can be ascribed to monodentate carbonate species 5, 56, 57. It can be clearly found that the location of absorption peaks attributed to formate were detected for all three samples, and all of them first increased with exposure time and then nearly reached a constant value. Comparing the intensities of the carbonate at 1580 cm-1, it accumulated on the surface of the MnO2 nanosheets, no observable change on the MnO2 nanowires, while it decreased on the 3D-MnO2 sample. The above result suggests that on the 3D-MnO2 materials HCHO can be quickly transformed into formate and carbonate, and carbonate species on 3D-MnO2 desorbed faster than on other two MnO2 samples. It is well known that accumulation of intermediates on the surface of catalysts may lead to the deactivation of catalysts. The rapid desorption of intermediates will contribute to high activity and stability of catalytic materials.


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Figure 8. In-situ DRIFTS spectra of (a) MnO2 nanosheets, (b) MnO2 nanowires and (c) 3DMnO2 exposed to a flow of ~80 ppm HCHO at room temperature. It is interesting to know why carbonate intermediates desorb fast from 3D-MnO2 materials. As shown in Figure 8c, when the 3D-MnO2 sample was exposed to HCHO, the intensities of the bands at 3850 and 1658 cm-1 ascribed to structural hydroxyl groups (ν(OH)) and (δ(H-O-H)) increased with exposure time. In contrast, these bands decreased in the cases of MnO2 nanosheets and MnO2 nanowires. However, the intensities of carbonate species accumulated on MnO2 nanosheets and did not change on MnO2 nanowires, while it decreased on the 3D-MnO2 sample. When we linked the changes between hydroxyl groups and carbonate species, we could find that they were negatively correlated. In the case of 3D-MnO2, due to more content of hydroxyl groups, the carbonate species become easier to desorb, resulting in less accumulation of


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surface carbonate species on its surface. Previous study

36

also reported that there was less

accumulation of surface carbonate species with more content of hydroxyl groups and/or water during the oxidation of HCHO and that the carbonate was more easily desorbed. As mentioned above, the oxidation of HCHO generally is accompanied with consumption of surface hydroxyl groups. The above observation indicates that in the case of 3D-MnO2, consumed surface hydroxyl group for HCHO oxidation can be quickly compensated. As we reported earlier

10

,

water in air may be the source to compensate the consumed surface hydroxyl group. Thus, the fast compensation of surface hydroxyl groups on 3D-MnO2 should be associated with its water adsorption. In order to verify the above assumptions, the weight loss and evolution of water molecules from the samples were determined with TG. As shown in Figure 9a, the weight loss of three samples dropped dramatically with the increased of temperature. The weight loss below 100 °C is caused by the loss of adsorbed water. 3D-MnO2 showed the largest water loss, 14.85%, while it was only 3.05% for MnO2 nanosheet and 2.15% for MnO2 nanowires, respectively. According to our previous study 10, the activity of MnO2 for HCHO oxidation is strongly associated with its water content, the higher water content, the better activity it has. Therefore, 3D-MnO2 with the highest water content exhibited the best activity at room temperature. In addition, we found the 3D-MnO2 material also showed faster adsorption ability of water. As shown in Figure 9b, after the sample was quickly dehydrated at 120 °C in dry N2, the 3D-MnO2 soon recovered most of desorbed water at room temperature when exposed to humid air. The above results indicate that 3D-MnO2 not only has the high water content, but also can reversibly recover most of desorbed water soon. As we discussed above, surface hydroxyl groups play important role and consumed in the HCHO oxidation process, and the consumed surface hydroxyl group can be recovered by


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the reaction between surface active oxygen and absorbed water. Thus, due to its high content of water and good water adsorption ability, 3D-MnO2 showed excellent activity for HCHO oxidation.

Figure 9. (a) TG curves of the different MnO2. (b) the recovery of water content over 3D-MnO2. 4. CONCLUSION We have developed a simple organic and template-free approach to fabricate 3D-MnO2 by freeze-drying the MnO2 nanosheets and nanowires aqueous solutions. With cell walls of MnO2 nanosheets and ribs of MnO2 nanowires, the high-purity manganese dioxide with a monolith network

was

constructed.

As-synthesized

3D-MnO2

materials

exhibited

continuous


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interconnected structures, low mass density, and high absorption capacity for organic liquids. Owing to its high water adsorption ability and 3D network structure, which favors mass transfer and fully expose reaction active sites, the 3D-MnO2 materials exhibit excellent catalytic activity and stability for HCHO oxidation at room temperature, making 45% of 100 ppm HCHO continuously converted into CO2 under high gas hourly space velocity of 180 L gcat-1 h-1. It is expected that this 3D-MnO2 material not only has great potential for air purification, but also it may have potential application in other fields such as water treatment, supercapacitors and energy storage. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (P.Y.Z.). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was ﬁnancially supported by Natural Science Foundation of China (Nos. 21677083, 21521064, 21411140032) and Tsinghua University Initiative Scientiﬁc Research Program (20131089251). REFERENCES (1) Yu, C.; Crump, D. Build. Environ. 1998, 33, 357-374. (2) Tang, X. J.; Bai, Y.; Duong, A.; Smith M. T.; Li, L. Y.; Zhang, L. P. Environ. Int. 2009, 35, 1210-1224.


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(57) Davydov, D. D.; Rochester, C. H. Infrared spectroscopy of adsorbed species on the surface of transition metal oxides, John Wiley& Sons Press: Chichester, New York, Brisbane, Toronto, Singapore, 1990.


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Graphical table of contents 38x20mm (300 x 300 DPI)


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Figure 1. (a) Scheme of the procedure to fabricate 3D-MnO2 with MnO2 nanosheets and MnO2 nanowires as starting materials. (b) the weight measurement of the 3D-MnO2 showing a density estimated as 7.3 mg cm-3 (i.e. 13.120 mg in 1.8 cm3). (c) Digital photos of two shapes of 3D-MnO2 standing on the dandelion. 299x142mm (300 x 300 DPI)


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Figure 2. SEM images of MnO2 nanosheets (a, b), MnO2 nanowires (c, d) and 3D-MnO2 (e-h). 433x141mm (300 x 300 DPI)


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Figure 3. TEM and HRTEM images of MnO2 nanosheets (a, b), MnO2 nanowires (c, d) and 3D-MnO2 (e, f). 144x216mm (300 x 300 DPI)


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Figure 4. XRD patterns of MnO2 nanosheets, MnO2 nanowires and 3D-MnO2. 297x210mm (300 x 300 DPI)


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Figure 5. XPS spectra of MnO2 nanosheets, MnO2 nanowires and 3D-MnO2. (a) Survey spectra, (b) Mn2p, (c) Mn3s, (d) O1s. 305x210mm (300 x 300 DPI)


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Figure 6. (a) Absorption process of toluene (stained with Sudan Black B) by the 3D-MnO2 within 6 seconds. (b) Digital photos showing the process of recycling 3D-MnO2 via combustion. 115x66mm (300 x 300 DPI)


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Figure 7. (a) HCHO conversion as a function of reaction temperature over the different MnO2 catalysts (HCHO concentration= 100 ppm, 21% O2, N2 as balance gas, RH= 0 or 65%, GHSV= 180 L gcat-1 h-1). (b) Arrhenius plots for HCHO oxidation over the different MnO2 catalysts. (c) Stability test of the 3D-MnO2 under different reaction conditions (HCHO concentration = 100 ppm, 21% O2, N2 as balance gas, GHSV= 180 L gcat-1 h-1). 297x210mm (300 x 300 DPI)


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Figure 8. In-situ DRIFTS spectra of (a) MnO2 nanosheets, (b) MnO2 nanowires and (c) 3D-MnO2 exposed to a flow of ~80 ppm HCHO at room temperature. 297x210mm (300 x 300 DPI)


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Figure 9. (a) TG curves of the different MnO2. (b) the recovery of water content over 3D-MnO2. 130x184mm (300 x 300 DPI)


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MnO2 Framework for Instantaneous Mineralization of Carcinogenic

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