Birnessite-Type Manganese Oxide on Granular Activated Carbon for

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Birnessite-Type Manganese Oxide on Granular Activated Carbon for Formaldehyde Removal at Room Temperature Jinge Li, Pengyi Zhang, Jinlong Wang, and Mingxiao Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07217 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 13, 2016

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The Journal of Physical Chemistry

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Birnessite-Type Manganese Oxide on Granular Activated

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Carbon for Formaldehyde Removal at Room Temperature

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Jinge Li, Pengyi Zhang*, Jinlong Wang, Mingxiao Wang,

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State Key Joint Laboratory of Environmental Simulation and Pollution Control,

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School of Environment, Tsinghua University, Beijing 100084, China

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*

Corresponding author.

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Pengyi Zhang

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Tel: +86 10-62773720;

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Fax: +86-10-62797760;

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E-mail: [email protected]

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ACS Paragon Plus Environment


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ABSTRACT：Formaldehyde (HCHO) is a priority indoor air pollutant due to its

2

adverse impact to human health. A layer of birnessite-type manganese oxide (MnOx)

3

was facilely coated on the granular activated carbon (AC) via in-situ reduction of

4

permanganate with AC at room temperature. The properties and growing process of

5

MnOx were characterized by SEM, EDS, HRTEM, XRD and Raman. As-prepared

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MnOx/AC exhibited high activity to remove HCHO at room temperature. GC analysis

7

indicates that removed HCHO can be finally transformed into CO2 in a much longer

8

time. Accumulation of carbonate on the surface of MnOx and partial hydroxylation of

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MnOx lead to the deactivation of MnOx/AC. However, the deactivated sample can be

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regenerated at 60 °C for 2 h. These results indicate that as-prepared MnOx/AC can be

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applied for indoor air purification due to its easy-preparation and regeneration, and

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high activity for HCHO removal.

13 14 15

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1. INTRODUCTION

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People spend most of their time indoors, and exposure to indoor air pollutants poses

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risks to human health. Carbonyl compounds (aldehydes and ketones) especially

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formaldehyde (HCHO) are common indoor air pollutants and receive increasing

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concerns. HCHO has been classified as a human carcinogen that causes

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nasopharyngeal cancer and probably leukemia.1,2 An overview of indoor HCHO and

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its guideline values has been recently published.3 The recommended values for indoor

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HCHO range from 0.01 ppm to 0.1 ppm. In recently years, a number of investigations

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for the elimination of HCHO have been conducted focusing on physical adsorption

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and catalytic oxidation.3-7 Adsorption using porous carbonaceous materials has been

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believed to be a simple and economic way for indoor HCHO removal at room

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temperature, but the adsorption is only effective for a short time because of their

13

limited adsorption capacity.8-10 Photo- and thermo-catalysis technologies are also

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widely studied for HCHO oxidation.11-15 However, these processes often have the

15

disadvantage of producing incomplete intermediates and require input of heat and

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light energy. Catalytic oxidation at low temperature is regarded as a promising way

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for elimination of HCHO and has been extensively studied.16 Recently, supported

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noble metal (Au, Pd, and Pt) catalysts exhibit excellent activity for HCHO

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degradation even at room temperature.17-20 However, the high cost of noble metals

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still limits their wide application. Thus, to eliminate HCHO pollution and improve the

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indoor air quality, it is interesting to find an eco-effective material for the complete

22

oxidation of low level HCHO at room temperature. 3



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Manganese oxide (MnOx), an environment-friendly material, has been investigated

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to degrade many gaseous pollutants in the environment21-24 and attracts wide attention

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in recent years for indoor air purification, including as catalysts for CO25 and

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benzene26,27 oxidation, transformation of alcohols to aldehydes or ketones,28 and also

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for HCHO removal from indoor air. Chen et al.29 investigated the effect of square

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tunnel size of several MnOx materials on their activity for complete oxidation of

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HCHO. Chen and his co-workers30 prepared monodisperse MnOx honeycomb and

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hollow nanospheres for oxidation of high concentration HCHO at high temperature.

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Sidheswaran et al.31 synthesized a novel MnOx powder, which can effectively

10

eliminate low-level HCHO at room temperature. They also integrated MnOx with

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activated carbon fibers together to remove particles, volatile organic compounds and

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ozone from gas.32 In our previous work, HCHO would be continuously oxidized by

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the layered MnOx at room temperature, if the consumed hydroxyl groups on the

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surface of MnOx is compensated timely in air.33 To overcome the drawbacks of

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powder MnOx in the practical application, it is attractive to coat MnOx on solid

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substrates. Miyawaki et al.34 reported a hybrid catalyst for HCHO removal via the

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deposition of MnOx on a polyacrylonitrile-based activated carbon (AC) nanofiber,

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which was prepared by heating the polyacrylonitrile nanofiber at 600 °C for

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carbonation. Zhou et al.35 developed an in-situ synthesis method to deposit MnOx

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nanosheets on cellulose fibers. However, this composite material is only effective for

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HCHO removal at temperature over 60 °C.

4


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In this study, we developed a composite material MnOx/AC via a facile in-situ

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reduction method. The growth of MnOx on the AC surface to form a core-shell

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structure was illustrated. As-prepared MnOx/AC material showed a high activity for

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HCHO removal at room temperature and thermal regenerability at temperature as low

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as 60 °C.

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2. EXPERIMENTAL SECTION

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2.1 Preparation of MnOx/AC. Coal-based granular AC was purchased from the

9

Shanxi Xinhua Chemical Plant of China and used with received. The AC granules are

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cylindrical rods with a diameter of 0.9 mm and lengths of 2-3 mm. The MnOx/AC

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catalysts were prepared via in-situ reduction of KMnO4 by the granular AC. First, 2.0

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g of AC was immersed in 100 mL of a 240 mmol/L KMnO4 solution, where

13

permanganate reacted with the carbon. Then, MnOx formed in the reaction and

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deposited in-situ onto the AC surface. After 15 h, the dark-purple color of KMnO4

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solution disappeared, and the brownish supernatant was decanted. Finally, the

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remaining AC granules were dried at 105 °C for 1 h, without further high-temperature

17

treatment. In addition, to learn the morphology evolution of MnOx layer grown on the

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surface of AC, samples were taken at different preparation time for SEM observation.

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2.2 Characterization of MnOx/AC. The morphology of as-prepared MnOx/AC

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was observed by SEM (S-5500, Hitachi, Japan; Sirion200, FEI, USA). Surface

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elemental analysis was performed by EDS attached to the Sirion200 microscope. For

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the high-resolution TEM observation, MnOx powder suspended in reaction solution 5



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were deposited onto carbon-coated copper grids and observed on a TECNAIG2 20

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TEM at an acceleration voltage of 200 kV. The oxidation state (OS) of Mn was

3

determined by XPS (PHI Quantera SXM, ULVAC-PHI, Japan). The C 1s binding

4

energy of 284.8 eV was used for calibration. XRD analysis of the AC, MnOx/AC and

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MnOx powder collected from the solution were performed with a Rigaku D/max-RB

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using Cu Kα radiation (λ=0.15418 nm), operated at 40 kV and 100 mA. Raman

7

scattering spectra of AC and MnOx/AC were measured with a Renishaw inVia

8

microspectrometer using an excitation wavelength of 532 nm. Before each data

9

acquisition, the intensity of the Raman peak at 520 cm–1 from silicon was normalized,

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and the Raman spectra were then recorded between 100 and 1200 cm−1. The

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elemental contents of manganese (Mn) and potassium (K) in the MnOx/AC sample

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were determined using ICP-AES (IRIS Intrepid II, Thermo, USA). First a certain

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amount of desiccated MnOx/AC was digested with concentrated HCl and HNO3

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(volume ratio=1:3), and then the elemental concentrations in the digested solution

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were measured with ICP-AES. Thermogravimetric analysis (TGA) of MnOx/AC was

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performed on a TGA/DSC 1 STARe system (Mettler-Toledo, Swiss). The temperature

17

ramp was 10 °C/min.

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The in-situ IR observation was conducted on a Thermo Nicolet NEXUS 870 FTIR

19

spectrometer equipped with an in-situ cell. Its temperature can be adjusted. 0.05 g of

20

MnOx/AC was placed in the in-situ cell. The HCHO gas with the concentration of

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∼20 mg/m3 flowed through the cell at a flow rate of 150 mL/min with the synthetic

22

air as the balance gas. Spectra were collected at a resolution of 4 cm-1. 6


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2.3 Test of activity for HCHO removal. The activity for HCHO removal was

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performed in a fixed-bed reactor at room temperature. The experimental apparatus is

4

illustrated in Figure S1. The granular MnOx/AC or AC (0.5 g) was filled in a Teflon

5

tube with inner diameter of 9 mm. Gaseous HCHO was generated by passing cleaned

6

air through a glass bottle containing HCHO solution in a thermostat maintained at

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10 °C. The humidified air stream was generated by bubbling air from a glass bottle

8

containing deionized water. Then, the HCHO gas was mixed with humid and dry air

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in another glass bottle. The HCHO concentration was set at 0.5 mg/m3 or 5 mg/m3,

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and the relative humidity (RH) was controlled at 45±5%. The total flow rate was 1.0

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L/min, and the corresponding residence time was 0.06 s with the face velocity of 26

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cm/s, which is close to that typically used in the filtration unit. The inlet and outlet

13

HCHO concentrations were measured by the MBTH method.36,37 The HCHO

14

conversion (η) was calculated with Eq. (1), where [C]in and [C]out are the inlet and

15

outlet HCHO concentration, respectively (both expressed in mg/m3).

16

(1) The adsorption capacity was calculated using the following equation：

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(2)

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where q is the amount of HCHO removal (mg/g); C0 and Ct are HCHO

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concentrations (mg/m3) in the gas flow before and after the adsorbent/catalyst samples,

21

respectively; Q is flow rate (L/min); m is the mass of AC or MnOx/AC (g); t is the

22

reaction time (min) and T is the breakthrough time for AC or the running time for 7



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MnOx/AC.

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To analyze the products generated from the HCHO removal reaction, static

3

experiments were conducted in two 3.5-L glass vessels. 0.2 g AC and MnOx/AC were

4

separately placed at the bottom of the two glass vessels. The vessels were then filled

5

with high purity air (99.999% N2 + 99.995% O2) to remove CO2 and sealed quickly.

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A quantity of formalin solution was used as the source of HCHO and separately

7

injected into the two glass vessels. The gas samples were taken out at different time

8

intervals for analysis of HCHO, CO and CO2. The concentrations of CO and CO2

9

were determined by a GC-FID (Shimadzu GC 2014) equipped with a methane

10

convertor.

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3. RESULTS AND DISCUSSION

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3.1 Growth of MnOx on the AC surface. The morphology evolution of MnOx on

14

the AC surface at different stages is shown by the SEM images in Figure 1. Initially,

15

the MnOx layer seems like flower consisting of curled nanosheets (Figure 1a) as

16

described in literature.38 The thickness of the nanosheet is estimated to be ~ 3 nm. As

17

the reaction proceeded, these nanosheets became denser and assembled together

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(Figure 1b). When the reaction time lasted for 0.5 h, a small number of sphere-like

19

particles with a diameter of 10~30 nm appeared (Figure 1c). These changes are

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ascribed to the aggregation of individual nanosheet.39 Subsequently, these particles

21

continued to grow (Figure 1d) and assembled into microspheres (Figure 1e). On the

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surface of these microspheres, loose lichen-like nanosheets continuously grew and 8


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became denser (Figure 1f). Finally, a compact layer of MnOx was coated on the

2

surface of activated carbon. Li et al.40 and Zhou et al.41 reported that the nanosheets

3

can assemble into flower-like microspheres. In our experiments, the multilayered

4

structure

5

dissolution–recrystallization growing process (Figure S3). And finally these

6

microsphere with diameters of ~ 0.5 µm aggregated into a layer of MnOx on the AC

7

surface.

also

rolls

up

and

scrolls

into

microspheres

during

the

8

3.2 Structure and composition of MnOx/AC. Figure 2a shows the cross sectional

9

SEM image of MnOx/AC, as-prepared material features a core-shell structure, i.e.

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MnOx stacked closely on the AC surface and formed a thin shell with the thickness of

11

0.9 ~ 1.2 µm. The EDS results (Figure 2b) confirmed the presence of the elements Mn,

12

O, and K on the AC surface. The surface layer that was scraped from the sample was

13

ultrasonically dispersed in water and subjected to SEM and HRTEM analysis. After

14

dispersion, the irregular nanosheets randomly cross-linked to each other (Figure 2c).

15

HRTEM image (Figure 2d) confirmed the layered structure of the nanosheets. The

16

lattice fringes of 0.74 nm is consistent with interlayer distance of birnessite-type

17

MnOx 42.

18

Raman spectroscopy is a useful method for analyzing the local structure of MnOx,

19

especially for samples with poor crystallinity. Figure 3 shows the Raman spectra of

20

AC and MnOx/AC. Three absorption peaks at ~ 500, 575 and 639 cm-1 correspond to

21

the characteristic peaks of birnessite-type MnOx. Among them, the Mn–O symmetric

22

stretching vibration of the MnO6 groups at 639 cm-1 is in the range 635 - 645 cm-1, 9



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1

which indicates the presence of large cations such as K+ in its structure42,43. The peak

2

at 575 cm-1 with weak intensity is the specific fingerprint of the Mn-O vibration along

3

the chains in the MnOx framework44.

4

The XRD patterns of the AC, MnOx/AC and MnOx powder collected from the

5

reaction solution are shown in Figure 4. The weak and broad peaks indicate that both

6

MnOx deposited on the surface of AC and suspended in the solution are

7

poorly-crystallized due to the low preparation temperature. Compared with Figure 4a

8

and c, the peaks of MnOx/AC in the diffractogram (Figure 4b) are noticeable at 12°,

9

24°, 36° and 64° (2θ). The structure can be indexed to layered birnessite with a

10

turbostratic structure. The layered birnessite consists of two-dimensional edge-sharing

11

MnO6 octahedral layers with K+ cations and water molecules in the interlayer

12

space.30,45 The angular value of 2θ at 12° corresponds to the (001) basal reflection of a

13

layered birnessite phase with the interlay spacing of 7.4 Å. And the peak at 24° (3.4 Å)

14

is assigned to the (002) reflections, which is consistent with the HRTEM data shown

15

in Figure 4d.

16

The oxidation state of manganese (Mn OS) in MnOx/AC was measured with XPS.

17

The Mn 3s, Mn 2p, and O 1s core level spectra are used to determine the Mn OS.

18

Chigane et al.46 investigated the spectra of some commercial MnOx samples, such as

19

MnO, Mn3O4, Mn2O3 and MnO2. Galakhov et al.47 also studied the Mn 3s core level

20

spectra of MnOx to assess the Mn OS. A series of MnOx samples such as MnO,

21

Li2MnO3, LiMnO2.4, La0.9MnO3, and La1.2Sr1.8Mn2O7 were synthesized for XPS

22

analysis. The Mn 3s splitting of manganites is plotted as a function of the valence of 10


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the Mn ions. However, the samples prepared by Galakhov et al.47 contained many

2

cations which interferes the accurate determination of Mn OS. Therefore, the data

3

provided by Chigane et al was usually used as a reference to calculate the Mn OS.

4

According to Chigane et al.,46 the energy differences (∆E3s) between the two Mn 3s

5

peaks were 5.79, 5.50, 5.41, and 4.78 eV for MnO, Mn3O4, Mn2O3, and MnO2,

6

respectively. The ∆E3s is related to the Mn OS through the formula Eq. (2) with the

7

correlation coefficient of 0.99, indicating the feasibility and validity of this method to

8

estimate the Mn OS. The O 1s spectrum can be divided into three constituents

9

corresponding to different oxygen-containing chemical bonds, i.e. the Mn-O-Mn bond

10

at 529.3-530.0 eV, the Mn-O-H bond at 530.5-531.5 eV and the H-O-H bond at

11

531.8-532.8 eV.46 Toupin et al.48 estimated the Mn OS according to the signal area of

12

O 1s spectra as follows. OS = -1.91 ∆E3s + 13.2

13

(2) (3)

14

15

The XP spectra for the Mn 2p, Mn 3s and O 1s regions of MnOx/AC are shown in

16

Figure 5. The results revealed that the ∆E3s was approximately 4.95 eV (Figure 5a)

17

and the Mn OS value was 3.75 according to Eq. (2). The O 1s spectrum of MnOx/AC

18

(Figure 6b) was divided into three peaks and the Mn OS calculated with Eq. (3)48 was

19

3.72, which is consistent with the value calculated from the Mn 3s spectra. Kim and

20

Shim23 investigated the Mn 2p spectra of a series of MnOx compounds, and the

21

energy difference (∆E2p) between the Mn 2p3/2 and Mn 2p1/2 peaks was used to

22

estimate the Mn OS. The ∆E2p of several MnOx are shown in Table 1. Figure 5c is 11



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the Mn 2p spectrum of MnOx/AC, and the corresponding ∆E2p was 11.87 eV.

2

Assuming that the Mn OS between 3 and 4 is linear with the ∆E2p, the Mn OS value

3

was calculated as 3.73. Thus, the average Mn OS of MnOx on the surface of AC was

4

3.73 according to the results from Mn 3s, Mn 2p and O 1s.

5

The atomic ratio of K to Mn and the average Mn content in MnOx/AC determined

6

by ICP-AES were 0.24 and 20.28%, respectively. The water loss determined by TGA

7

in the range of 40 to 400 °C49 was ~ 11.5% (Figure S2). According to these data and

8

the Mn oxidation state, a mean chemical formula of K0.24MnO1.99·1.76 H2O can be

9

given to the as-prepared MnOx coated on the activated carbon.

10

3.3 Activity for HCHO removal and its mechanism. The activity of MnOx/AC

11

for HCHO removal was tested at room temperature. Figure 6 shows the performance

12

of MnOx/AC in the cases of two different HCHO concentrations. When the inlet

13

concentration of HCHO was 0.5 mg/m3 (which is 5 times higher than the guideline

14

value in China), the HCHO conversion was always higher than 70% in 80 h though it

15

showed the gradual decrease trend.(Figure 6a) However, the same amount of AC

16

quickly lost the ability to remove HCHO within 1.5 h. According to formaldehyde

17

breakthrough curve (Figure S4a), the estimated adsorption capacity is only ~ 31.5

18

µg/g AC, which is consistent with the results reported in literatures.50,51 While the

19

mass of HCHO removed by MnOx/AC within 80 h is over 3360 µg/g MnOx/AC

20

(Figure S4b), which is over 100 times higher than that observed for AC at a

21

concentration of 0.5 mg/m3 HCHO. Under the condition of 5 mg/m3 HCHO, such a

22

small amount (0.5 g) of MnOx/AC also lost its activity within 32 h (Figure 6b), the 12


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corresponding capacity for removing HCHO is ~ 6720 µg/g MnOx/AC. Furthermore,

2

as shown in Figure 6b, the ability of MnOx/AC to remove HCHO could be completely

3

recovered after the sample was heated at 60 oC in air for 2 h, which means an in-situ

4

thermal regeneration unit can be conveniently designed for its long-time use in an air

5

purifying device.

6

To illustrate the mechanism that MnOx/AC removes HCHO, the reaction products

7

of HCHO were analyzed. As shown in Figure 7, the HCHO concentration in the blank

8

vessel was approximately maintained at 400 ppm, and in the presence of 0.2 g AC it

9

gradually decreased ~ 67% within 5 h, and no significant amount of CO or CO2 were

10

detected. While in the presence of 0.2 g MnOx/AC, HCHO concentration quickly

11

decreased. Simultaneously, significant amount of CO2 was detected and no CO was

12

detected. Furthermore, nearly 100% conversion of HCHO to CO2 was achieved

13

within 5 h. In our previous study,52 we investigated the activity of birnessite-type

14

MnOx with different interlayer cations (K+, Mg2+, Ca2+, Fe3+). K-birnessite has the

15

strong ability to further oxidize formate to CO2 at room temperature. The above

16

results also demonstrate that as-prepared MnOx/AC material is efficient to completely

17

transform HCHO to CO2 at room temperature. Thus, it has great potential application

18

in indoor air purification due to its low cost, high capacity and safety without

19

producing secondary pollutants.

20

To verify the reason that the MnOx/AC deactivates, in-situ FTIR studies were

21

performed on am FTIR spectrometer. Figure 8 shows the IR spectra collected at

22

different times after the HCHO flowed through the in-situ reaction cell. The broad 13



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1

band at 3000~3600 cm-1 can be assigned to the interlayer hydrates and structural

2

hydroxyl groups of birnessite.53 The significant increase of absorption at 3000~3600

3

cm-1 with reaction time indicates that water molecules and/or hydroxyl groups

4

gradually accumulate on the surface of the MnOx/AC. In addition, the peaks of COO-

5

at 1567 and 1347 cm-1 decreased, and the peaks corresponding to bidentate carbonate

6

(b-CO32-) at 1484 and 1421 cm-1 as well as the OH band at 1648 cm-1 increased with

7

reaction time. The results indicate that exposure of the MnOx/AC sample to HCHO

8

led to rapid formation of formate and the formate was then oxidized to CO2, a small

9

amount of which adsorbed on the surface of MnOx/AC in the form of b-CO32-.54-56

10

The occurrence and accumulation of b-CO32- gradually led to the deactivation of

11

MnOx/AC samples. Figure 8c and d showed that these changes were recovered during

12

the thermal regeneration at 60 oC in air for 2 h. The accumulated b-CO32- and

13

hydroxyl groups could release from surface after the regeneration. In our previous

14

study,57 the adsorbed b-CO32- desorbed from surface in form of CO2.

15

The change of surface components of MnOx/AC during reaction was also

16

characterized with XPS. The O 1s spectra of MnOx/AC before reaction, after reaction

17

and after regeneration are shown in Figure 9. All the O 1s peaks are asymmetrical

18

with a hump on the higher binding energy (BE) side, which can be fitted to several

19

components. The fitting results and the Mn OS calculated with Eq. (3) are

20

summarized in Table 2.47 The Mn OS slightly decreased from 3.72 to 3.54 after the

21

reaction, indicating that the MnOx during reaction was partially reduced. Additionally,

22

a new splitting peak appeared in the O 1s spectra of the used MnOx/AC, which is 14


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1

assigned to O=C-O58 and further confirmed the transformation of HCHO into CO2.

2

Furthermore, the relative ratio of Mn-OH noticeably increased and correspondingly

3

the ratio of Mn-O-Mn decreased, indicating the partial hydroxylation of MnOx. The

4

accumulation of b-CO32- and surface hydroxylation of MnOx may impede the

5

competitive adsorption of HCHO and its subsequent degradation on the surface of

6

MnOx/AC, leading to its gradual deactivation. However, after the used sample was

7

regenerated at 60 oC in air for 2 h, the Mn OS was recovered to its previous level

8

(shown in Table 2), which indicates the adsorbed b-CO32- released from surface of

9

MnOx, and the MnOx/AC recovered its activity for HCHO removal. The result is

10

consistent with the in-situ FTIR data shown in Figure 8c and d.

11 12

According to the above-mentioned results and discussion, we proposed the

13

mechanism that formaldehyde decomposes on MnOx/AC. As shown in Figure 10,

14

HCHO first adsorbs on the MnOx surface, and then transforms into –OH and HCOO-

15

at room temperature. HCOO- would be further decomposed into CO2. As a result, the

16

content of Mn-OH noticeably increase, while a small amount of CO2 accumulates on

17

the surface of MnOx in the form of bidentate carbonate (b-CO32-). The accumulation

18

of b-CO32- and surface hydroxylation of MnOx lead to the deactivation of MnOx. After

19

the deactivated MnOx is heated at higher temperature in air, the Mn-OH is

20

dehydroxylated and b-CO32- desorbs from the surface. Accordingly the MnOx/AC

21

material is regenerated.

22 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

4. CONCLUSIONS

2

A layer of MnOx with thickness of ~ 0.9-1.2 µm was facilely coated on the surface of

3

granular AC via in-situ reduction of aqueous permanganate with AC at room

4

temperature. TEM, Raman, XRD and XPS analysis indicate that the coated MnOx is a

5

birnessite-type manganese oxide with poor crystallinity. As-prepared MnOx/AC

6

composite material showed high activity to remove HCHO at room temperature. Its

7

capacity for removing HCHO was as high as 3360 µg/g MnOx/AC, which is more

8

than 100 times higher than the adsorption capacity of the AC for low concentration

9

HCHO (0.5 mg/m3). Furthermore, the removed HCHO was completely transformed

10

into CO2 without producing secondary pollutants. The deactivation of MnOx/AC is

11

ascribed to the accumulation of carbon dioxide and surface hydroxylation. However,

12

it can be easily regenerated in air at 60 °C. Thus, as-prepared MnOx/AC has great

13

potential application for removing indoor HCHO due to its easy-preparation and

14

regeneration, low cost, safety and high activity at room temperature.

15 16

ASSOCIATED CONTENT

17

Sopporting Information.

18

Schematic diagram of the experimental setup, TGA plot for MnOx/AC and AC in a

19

N2 atmosphere, Growth of MnOx on the AC surface, and time-dependence of HCHO

20

by AC and MnOx/AC.

21 22 16


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1

ACKNOWLEDGMENTS

2

This work was financially supported by the National High Technology Research and

3

Development Program of China (2012AA062701), Natural Science Foundation

4

(21221004, 21411140032) and Tsinghua University Initiative Scientific Research

5

Program.

6 7

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3

25



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1 2 3 4 5

Page 26 of 38

Table 1. Binding energies of the Mn 2p peaks for a series of MnOx. Name Mn2p3/2 (ev) Mn2p1/2 (ev) ∆E2p (eV) Mn OS

Mn3O4

Mn2O3

MnO2

This work

641.01 653.61 12.60 2.67

642.19 654.22 12.03 3

642.61 654.42 11.81 4

642.72 654.59 11.87 3.73

6 7

26


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Table 2. Data obtained from the O1s spectra of different MnOx/AC samples

1 2

Sample

O1s

Calculated Mn OS

Component

BE/eV

Area/%

Fresh MnOx/AC

Mn-O-Mn Mn-OH H-O-H

529.8 531.0 532.4

69.8 19.5 10.7

3.72

Deactivated MnOx/AC

Mn-O-Mn Mn-OH O=C-O H-O-H

529.5 530.7 531.8 533.2

48.2 22.3 21.2 8.2

3.54

Regenerated MnOx/AC

Mn-O-Mn Mn-OH H-O-H

529.5 531.0 532.3

73.5 20.4 6.1

3.72

3

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

3 4 5 6 7 8

Figure 1. SEM images of MnOx/AC samples taken at different preparation times: (a) 2 min; (b) 10 min; (c) 0.5 h; (d) 2 h; (e) 8 h; and (f) 15 h.

28


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1 2

3 4 5 6 7

Figure 2. (a) Cross sectional SEM image and (b) EDS spectrum of MnOx/AC; (c) SEM image and (d) HRTEM image of MnOx scraped from MnOx/AC.

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2 3

4 5 6 7

Figure 3. Raman spectra of AC and MnOx/AC.

30


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1

2 3 4 5

Figure 4. XRD patterns of the samples: (a) AC powder, (b) MnOx/AC powder, and (c) MnOx powder collected from the mother solution.

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

2

Figure 5. XPS spectra of (a) Mn 3s, (b) O 1s and (c) Mn 2p of fresh MnOx.

3 4

32


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1 2 3

4 5 6 7 8 9

Figure 6. Comparison of AC and MnOx/AC for removing HCHO. a, HCHO concentration: 0.5 mg/m3; b, HCHO concentration: 5 mg/m3 .

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2 3 4

5 6 7 8 9

Figure 7. Formation of CO2 during HCHO removal by AC or MnOx/AC at room temperature.

34


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1 2

3 4 5 6 7 8 9

Figure 8. (a)Time dependence of in-situ FTIR spectra of MnOx/AC exposed to gaseous HCHO flow at room temperature, (b) magnified spectra of red region in (a), (c) in-situ FTIR spectra of MnOx/AC before, after reaction and regeneration, (d) magnified spectra of blue region in (c).

35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

2

Figure 9. XPS spectra of O1s of different MnOx/AC samples

3 4 5 6

36


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1

2

Figure 10. The reaction mechanism of HCHO on the surface of MnOx/AC.

3 4

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

TOC Graphic

3

4 5 6 7

A layer of birnessite-type manganese oxide was facilely coated on activated carbon for HCHO removal.

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Birnessite-Type Manganese Oxide on Granular Activated Carbon for

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