Bio-template-assisted synthesis of hierarchically hollow SiO2 microtubes and their enhanced formaldehyde adsorption performance

Applied Surface Science 274 (2013) 110–116

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Bio-template-assisted synthesis of hierarchically hollow SiO2 microtubes and their enhanced formaldehyde adsorption performance Yao Le, Daipeng Guo, Bei Cheng, Jiaguo Yu ∗ State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Luoshi Road 122#, Wuhan 430070, PR China

a r t i c l e

i n f o

Article history: Received 3 February 2013 Received in revised form 27 February 2013 Accepted 28 February 2013 Available online 15 March 2013 Keywords: Silica tubes Hollow TEPA Adsorption Formaldehyde

a b s t r a c t The indoor air quality is crucial for human health, taking into account that people often spend more than 80% of their time in houses, offices and cars. Formaldehyde (HCHO) is a major pollutant and longterm exposure to HCHO may cause health problems such as nasal tumors and skin irritation. In this work, for the first time, hierarchically hollow silica microtubes (HHSM) were synthesized by a simple sol–gel and calcination method using cetyltrimethyl ammonium bromide (CTAB) and bio-template poplar catkin (PC) as co-templates and the PC/SiO2 weight ratio R was varied from 0, 0.1, 0.3 and 1. The prepared samples were further modified with tetraethylenepentamine (TEPA) and characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), differential thermal analysis (DTA), thermal gravimetric analysis (TGA), and N2 physisorption techniques. This was followed by formaldehyde adsorption tests at ambient temperature. The results showed that all the prepared HHSM samples contained small mesopores with peak pore size at ca. 2.5 nm and large several tens of nanometer-sized pores on the tube wall. The R exhibited an obvious influence on specific surface areas and the sample prepared at R = 0.3 exhibited highest specific surface area (896 m2 /g). All the TEPA-modified samples exhibited enhanced formaldehyde adsorption performance. The maximum HCHO adsorption capacity (20.65 mg/g adsorbent) was achieved on the sample prepared at R = 0.3 and modified by 50 wt.% TEPA. The present study will provide new insight for the utilization of bio-template used for the fabrication of inorganic hollow tubes with high HCHO adsorption performance for indoor air purification. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The indoor air quality plays an important role in human health, because people often spend more than 80% of their time in houses, offices and cars, which contribute a higher risk from inhalation of indoor pollutants than outdoors [1–5]. In particular, the increased air-tightness required for energy saving results in the accumulation of pollutants in less-ventilated indoor air. In recent years, indoor air pollution has become more and more serious. Formaldehyde, as a representative indoor pollutant, mainly comes from indoor furniture paint, decorated materials, polymerizing plate, binders and chemical fiber carpets. Formaldehyde is also one of the main contents of cigarette smoke. A high concentration of formaldehyde can cause a series of symptoms including nausea, headache, pharyngitis, coryza, emphysema, lung cancer, and even death [6–9].

∗ Corresponding author. Tel.: +86 27 87871029; fax: +86 27 87879468. E-mail address: [email protected] (J. Yu). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.02.123

So it is necessary to take some measures to remove formaldehyde from indoor air. The purification technology for removing indoor formaldehyde is of great importance because indoor air pollution has become an important social issue with increasing desire to improve the quality of life. Adsorption, scrubbing, and advanced oxidation have been applied to remove volatile organic compounds (VOCs) in air, such as formaldehyde and toluene [9]. Among the above methods, adsorption is a simpler and more effective method to remove gaseous formaldehyde. Various adsorbents, including potassium permanganate, aluminum oxide and ceramic materials, have been prepared and utilized to remove gaseous HCHO from a test chamber and homes [10]. However, the adsorption capacity of these conventional adsorbents is rather limited. Although the adsorption performance of activated carbon is excellent, it is not very efficient for the adsorption of polar species, such as formaldehyde due to the difference of both properties [8,11–13]. Recently, surface amine-modification of activated carbon and mesoporous silica with amine-containing compounds has become an effective approach

Y. Le et al. / Applied Surface Science 274 (2013) 110–116

to enhance their formaldehyde adsorption performance because Schiff bases and amino groups can effectively interact with HCHO [12–14]. Moreover, it was reported that the adsorption capacity of mesoporous silica material is around three times higher than that of activated carbon [12,15]. Therefore, it is necessary to further develop more efficient and environmentally friendly SiO2 -based adsorbents for efficient removal of formaldehyde. The fabrication of versatile micro-nanostructured materials, including nanoparticles (NPs), nanowires, nanorods, and nanotubes and their assembly have attracted extensive attentions in fundamental science and industrial research, since they have fascinating geometries and novel physicochemical properties as well as various potential applications [16]. Among them, inorganic nanotubes with a unique hollow structure allow the independent modification of their inner and outer surface with desired functional molecules to develop smart multifunctional nanotubes and provide straight nanochannels for the nanofluidic sensors [17–20]. As a tubular nanomaterial, silica nanotubes display empty inner space which can be filled by functional loads. In addition, the surface is hydrophilic and biocompatible which results in the applications of the material in bioseparation, biocatalysis, biosensoring, and as drug/gene delivery carriers [21–23]. In addition to the mere properties of pristine silica tubes, adding amine-modification will be beneficial for applications in adsorption. Amine groups, which can react with CO2 and formaldehyde, are expected to enhance adsorption efficiency if the adsorbent surface is amine-modified [24]. For example, Srisuda et al. used co-condensation method to introduce the organic amine groups into the silica network for enhancing the adsorption of formaldehyde [14]. Further, Zhu et al. reported the synthesis of amine-functionalized SBA-15 as highly sensitive chemosensors to detect formaldehyde vapor [7]. However, to the best of our knowledge, the amine-modified hierarchical hollow silica microtubes (HHSM) assembled by NPs have not been reported as formaldehyde adsorbents. In this work, for the first time, we present a simple and effective strategy to fabricate HHSM using cetyltrimethyl ammonium bromide (CTAB) and bio-template poplar catkin (PC) from poplar tree as co-templates with varying the PC/SiO2 weight ratio (R), followed by their surface amine modification. Experimental studies of their performance as adsorbents for formaldehyde removal under ambient conditions showed that they are very promising materials for formaldehyde removal from indoor air. 2. Experimental 2.1. Preparation 2.1.1. Materials All reagents used in the experiments were analytical grade. And all the reagents were from Shanghai Chemical Reagent Co, China and used without further purification. Poplar catkin (plants flower) from Poplar tree was used as biological templates. Distilled water was used for all synthesis and treatment processes. 2.1.2. Preparation of HHSM The HHSM samples are prepared by a sol–gel method using tetraethylorthosilicate as precursor and silicon source, CTAB and PC were used as co-templates and the weight ratio of PC/SiO2 (R) was varied. Typically, 0.2 g of CTAB was dissolved in 96 mL of distilled water, with stirring at room temperature. NaOH aqueous solution (0.7 mL, 2 M) and a given amount of PC (0.1 g) were then added to the solution (R = 0.3). The temperature of the solution was raised and kept at 80 ◦ C. Then, tetraethylorthosilicate (TEOS, 1.34 mL) was added to the above solution. The mixture was continuously stirred for an additional 2 h. The resulting products

111

Table 1 Preparation conditions and textural properties of the mesoporous SiO2 tubes. Samples

R

PC(g)

CTAB(g)

SBET (m2 /g)

APS (nm)

PV (cm3 /g)

P0 P0.1 P0.3 P1.0

0 0.1 0.3 1.0

0 0.033 0.1 0.33

0.2 0.2 0.2 0.2

611 667 896 829

2.9 3.1 3.1 2.2

0.69 0.52 0.69 0.46

were collected by filtration and dried at room temperature. The CTAB and PC templates were removed by calcination at 600 ◦ C for 5 h. The samples were also prepared at R = 0 (without PC), 0.1 and 1.0. The samples prepared at R = 0, 0.1, 0.3 and 1.0 were labeled as P0, P0.1, P0.3 and P1.0, respectively (see Table 1). 2.1.3. Surface amine modification of HHSM samples The amine-modified HHSM samples were prepared by a wet impregnation method. In a typical surface amine modification experiment, 0.1 g of tetraethylenepentamine (TEPA) was dissolved in 10 g of ethanol solution under stirring for 0.5 h, and then 0.2 g of the P0, P0.1, P0.3 or P1.0 samples were separately added into the TEPA ethanol solution, and the solution was kept at 80 ◦ C. After stirring for 2 h, the samples were centrifuged and dried at 80 ◦ C for 6 h. the nominal weight% (wt.%) of TEPA/SiO2 in the sample is 50%. The above amine-modified samples were designated as P0-50, P0.1-50, P0.3-50 and P1.0-50. 2.2. Characterization Scanning electron microscopy (SEM) observation was performed by an S-4800 Field Emission SEM (FESEM, Hitachi, Japan) at an accelerating voltage of 10 kV. The thermal chemical and physical properties of the amine-modified samples were characterized by differential thermal analysis (DTA) and thermal gravimetric analysis (TGA). The DTA-TGA curves was obtained by a DTG-60H analyzer (Shimadzu Corp., Tokyo, Japan). About 10 mg of the sample was heated at 10 ◦ C/min to 600 ◦ C in air. Transmission electron microscopy (TEM) observation was conducted by a JEM-2100F electron microscope (JEOL, Japan) at an accelerating voltage of 200 kV. The Brunauer–Emmett–Teller (BET) specific surface areas (SBET ) and porous structures of the samples were characterized using a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). All of the samples were degassed at 180 ◦ C before nitrogen adsorption measurements. The SBET was determined by a multipoint BET method using the adsorption data in the relative pressure (P/P0 ) range of 0.05–0.3. The nitrogen adsorption volume at the relative pressure (P/P0 ) of 0.99 was used to determine the pore volume (PV). The desorption isotherm was used to determine the pore size distribution via the Barret-Joyner-Halender (BJH) method, assuming a cylindrical pore modal [25]. The results were shown in Table 1. Fourier-transform infrared (FTIR) spectra were collected by a Shimadzu IRAffinity-1 FTIR spectrometer in the frequency range of 4000–500 cm−1 . X-ray photoelectron spectroscopy (XPS) measurement was performed in an ultrahigh vacuum VG ESCALAB 210 electron spectrometer equipped with a multichannel detector. The spectra were excited using Mg K␣ (1253.6 eV) radiation (operated at 200 W) of a twin anode in the constant analyzer energy mode with a pass energy of 30 eV. 2.3. Formaldehyde adsorption experiments Formaldehyde adsorption was performed in an organic glass box covered by a layer of aluminum foil paper on its inner wall at ambient temperature. The experimental setup is shown in Fig. 1. 0.1 g of adsorbent was dispersed on the bottom of glass petri dish with a diameter of 14 cm. After placing the sample-contained dishes in the

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NPs aggregate into several hundreds of nano-sized particles, which form the wall of micrometer-sized silica microtubes. Further observation from Fig. 2d indicates that the wall of silica tubes possess a large amount of pores and the pore size is about several tens of nanometers, which is beneficial to the diffusion and transfer of gas molecules. It is generally accepted that the resulting H2 O and alcohol during the hydrolysis and condensation of TEOS remain in the pore of the Si O Si network [26]. Therefore, H2 O and alcohol are easily removed at high temperature leaving lots of holes, which provide the channels for the entrance of O2 and the release of CO or CO2 . Furthermore, the composition of the sample was determined with energy-dispersive X-ray spectroscopy (EDS) (Fig. 2e), indicating that the peaks of Si and O are very strong, and the peaks of C and Al are also observed, which come from carbon conductive adhesive pads and circular aluminum stubs, respectively. 3.2. Nitrogen adsorption isotherms

3. Results and discussion

The BET surface areas and pore parameters of the prepared HHSM samples were characterized by nitrogen sorption measurements. The effects of bio-template PC adding amount (niz. R) on nitrogen adsorption isotherms and corresponding pore size distributions of HHSM samples are presented in Fig. 3. According to the Brunauer–Deming–Deming–Teller (BDDT) classification, all prepared four samples have Type IV isotherms, shown in Fig. 3a, indicating the presence of mesopores within these samples [25,27,28]. Three well-distinguished regions of the adsorption isotherm can be noticed: a linear increase of the adsorbed volume at low pressure due to monolayer-multilayer adsorption on the pore walls; a sharp increase in the adsorbed volume at intermediate pressure due to capillary condensation; and a subsequent linear increase in the adsorbed volume vs pressure at high pressure due to multilayer adsorption on the outer surface [29]. The pore size distributions of the samples are shown in Fig. 3b. The P0, P0.1 and P0.3 samples show quite narrow pore size distributions centered at around 2.8, 2.6 and 2.8 nm, respectively. The P1.0 sample, however, shows a significantly broader distribution with peak pore diameter at 2.1 nm. The R presents an obvious influence on the pore structure and BET surface areas of the above four samples (see Table 1). With increasing R, the isotherms shift upwards till R = 0.3, implying an increase in the specific surface area. Then, the isotherms shift downwards at R = 1. All structural parameters derived from these isotherms are summarized in Table 1.

3.1. SEM and TEM images

3.3. DTA-TGA analysis

The morphologies and microstructures of the prepared samples were observed by SEM and TEM. Fig. 2a and b show SEM images of bio-template PC and the prepared P0.3 sample, respectively. It can be observed from Fig. 2a that a large amount of one-dimensional wires with diameters of about 6 ␮m and lengths of several hundred ␮m can be easily observed for bio-template PC. Further observation from inset of Fig. 2a indicates that the surface of PC is very smooth. On the contrary, the prepared silica microtube P0.3 sample displays very rough surface morphologies (see Fig. 2b), which are composed of a lot of small NPs. To get more information about the microstructure of silica microtube, it was further investigated by TEM. Fig. 2c shows a typical TEM image of silica microtubes, confirming the prepared P0.3 sample with hollow tubular structure and morphology. Typically, the out diameter of the tubes is about 5 ␮m, the inner diameter is ca. 4 ␮m and the thickness is about 500 nm. High-magnification TEM image shown in Fig. 2d indicates that the tube surface contains 300–400 nm sized particles, which are composed of smaller NPs (about 50–100 nm in size). Therefore, SEM and TEM results indicate that the prepared HHSM sample have a hierarchical structure, namely, several tens of nano-sized

The amount of TEPA loaded into the HHSM samples was characterized by differential thermal and thermal gravimetric analysis. Fig. 4a shows comparison of DTA curves of un-modified P0.3 and TEPA-modified P0.3-50 samples. It can be seen from the DTA curves that for un-modified P0.3 sample, only a great endothermic peak at about 50 ◦ C is observed. This is attributed to desorption of physically adsorbed water molecules. The corresponding weight loss in the TGA curves (Fig. 4b) is about 19%. This high adsorption to moisture in air is due to the P0.3 sample with high specific surface areas and pore volume. On the contrary, for the TEPA-modified P0.3-50 sample, three main exothermic peaks at ca. 179, 273 and 500 ◦ C and two small endothermic peaks at ca. 59 and 110 ◦ C could be clearly observed. Two small endothermic peaks at 59 and 110 ◦ C are attributed to desorption of physically adsorbed water, ethanol and TEPA molecules. In the temperature range of 110–600 ◦ C, three exothermic peaks at about 179, 273 and 500 ◦ C are due to the combustion of the organic matter. The total weight loss (see Fig. 4b) for the TEPA-modified P0.3-50 sample is about 49% in the temperature range of 25–600 ◦ C. If the adsorbed water and solvent ethanol are excluded from the total weight loss, the actual loading amount

Fig. 1. Schematic diagram of experimental set-up for room-temperature formaldehyde adsorption: (1) organic glass box, (2) glass slide cover, (3) glass petri dish with adsorbent, (4) sampling port, (5) injection port, (6) fine wire, (7) sample recovering port, (8) 1412 Photoacoustic IR Multigas Monitor, (9) computer, (10) fan and (11) door.

bottom of reactor with a glass slide cover, 25 ␮L of HCHO (38%) solution was injected into the reactor and a 5 W fan was placed in the bottom of reactor during the whole adsorption process. After 2–3 h, the formaldehyde solution was volatilized completely and the concentration of formaldehyde was stabilized. HCHO, CO2 , CO and water vapor was on-line analyzed with a Photoacoustic IR Multigas Monitor (INNOVA air Tech Instruments Model 1412) [3]. The HCHO vapor was allowed to reach adsorption/desorption equilibrium with wall of the reactor prior to the adsorption experiment. The initial concentration of HCHO after adsorption/desorption equilibrium was controlled at ca. 300 ppm, which remained constant until the glass slide cover on the petri dish was removed to start HCHO adsorption. In the recycle experiments, the adsorbents were heated at 80 ◦ C for 2 h before next run of HCHO adsorption.

Y. Le et al. / Applied Surface Science 274 (2013) 110–116

113

Fig. 2. SEM (a,b) and TEM (c,d) images of bio-template PC (a) and the P0.3 sample (b–d), and EDS spectrum (e) of the P0.3 sample.

of TEPA on the P0.3-50 sample was estimated to be about 33%. The TGA result implies that the actual loading TEPA amount in the sample is lower than nominal adding TEPA amount in the experiment. This is easy to understand because the P0.3 sample has a limit of adsorption capacity toward TEPA. 3.4. FTIR analysis FTIR was used to characterize surface amine modification of the prepared HHSM samples. The FTIR spectra of all unmodified and TEPA-modified samples were similar, thus only the FTIR spectra of representative P0.3 and P0.3-50 samples are shown in Fig. 5. It can be seen from Fig. 5a that the un-modified P0.3 sample exhibits a rather strong and broad FTIR adsorption peak at around 3000–3800 cm−1 wavenumber range due to the stretching modes

of O H bonds related to free water (capillary pore water and surface adsorbed water). The peak at 1626 cm−1 is due to the bending vibration of O H bonds, which is assigned to the chemisorbed water. The FTIR results suggest that the calcined HHSM sample contains a large amount of adsorbed water due to its high BET surface areas and rich hydroxyl groups, and strong affinity to water molecules. This is consistent with the previous DTA and TGA results. For the P0.3 and P0.3-50 samples, the peaks at 1000–1250 cm−1 wavenumber range are clearly visible, which is due to the asymmetric stretching vibrations of Si O Si band. The absorption peaks at 802 and 468 cm−1 are due to the symmetric stretching vibrations of Si O Si band. Another adsorption peak at about 960 cm−1 is related to SiOH group. Additionally, a small absorption peak at 2380 cm−1 is observed in the Fig. 5a, which is ascribed to a stretching vibration of gaseous carbon dioxide [30]. After TEPA surface modification (the

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Y. Le et al. / Applied Surface Science 274 (2013) 110–116

Volume adsorbed (cm STP/g)

500

(a)

450

b

3

Transmittance (%)

400 350 300 250 200

P0 P0.1 P0.3 P1.0

150 100 50 0.0

0.2

0.4

0.6

0.8

1.0

2930 C-H

1542 1466 -NH2-NH

2850 C-H

2

2380 CO2

a

1626 H2O

Si-O-Si 3425 H2O

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

Relative pressure (P/P0)

Fig. 5. FTIR spectra of the P0.3 (a) and TEPA-modified P0.3-50 (b) samples.

dV/dlog(W) (cm3 STP/g)

9

(b)

8

P0.3-50 sample), the peak at 3000–3800 cm−1 region corresponding to O-H stretching vibration is still observed in the FTIR spectra, but its intensity obviously decreases (Fig. 5b), implying that the strong interaction occurs between the surface hydroxyl groups of the sample and the amine groups in TEPA, possibly forming new Si O− N+ H3 R and/or Si O− N+ H2 R2 groups [31–33]. However, some new adsorption peaks at 1466, 1542, 2850 and 2930 cm−1 can be clearly observed in the FTIR spectrum of TEPA-modified P0.3-50 sample (Fig. 5b) comparing with that of un-modified P0.3 sample. Two small absorption peaks at 2930 and 2850 cm−1 are due to the C H stretching vibrations. Other two absorption peaks at 1542 and 1466 cm−1 are characteristics of the asymmetric and symmetric bending of primary amines (NH2 ), respectively.

P0 P0.1 P0.3 P1.0

7 6 5 4 3 2 1 0

1

10

100

Pore diameter (nm) Fig. 3. Nitrogen adsorption–desorption isotherms (a) and corresponding pore-size distribution curves (b) of the P0, P0.1, P0.3 and P1.0 samples. o

179 C

DTA (µV)

150

(a) P0.3-50

100 50

o

273 C

P0.3

0

o

500 C o

-50

110 C o

59 C 100

200

300

400

500

o

Temperature ( C) 110

(b)

Weight (%)

100 90

P0.3

80 70

P0.3-50

60 50 40

3.5. XPS analysis XPS characterization was performed to further investigate the interaction between TEPA and the HHSM surface. XPS survey spectrum (Fig. 6a) of TEPA-modified P0.3-50 sample indicates that the sample contains Si, C, O and N elements, with sharp XPS peaks centered at binding energies of 102.8 (Si 2p), 153.9 (Si 2s), 285.0 (C 1s), 532.3 (O 1s) and 399.2 (N 1s), implying the presence of amine groups in the sample due to the surface amine modification. Fig. 6b shows high-resolution C1s XPS spectrum of the P0.3-50 sample. The C 1s peak can be fitted into three smaller peaks located at 284.7, 285.0 and 286.2 eV, respectively. The lower binding energy peak at 284.7 eV can be attributed to carbons of C H configuration of the TEPA, while the higher binding energy peak at 286.2 eV can be attributed to carbon in C N configuration. And the binding energy peak at 285.0 eV can be ascribed to C C configuration of TEPA or the adventitious hydrocarbon from the XPS instrument itself [34]. To further study the chemical nature and status of N element, the high-resolution XPS spectrum of the N 1s XPS region is shown in Fig. 6c. The asymmetrical and broad features of N 1s XPS peak suggest N element with different chemical status. A signal deconvolution after Gaussian curve fitting is shown in Fig. 6c, pointing out chemically different N species in P0.3-50, with their N 1s binding energies at about 398.9 eV, 400.2 eV and 401.4 eV. The main N 1s peak at binding energy of 398.9 eV can be assigned to C NH C. The two weak peaks at 400.2 eV and 401.4 eV can be respectively attributed to NH2 and NH3 + which come from amino groups [35,36].

30 20

3.6. Adsorption performances of formaldehyde 100

200

300

400

500

600

o

Temperature ( C) Fig. 4. DTA (a) and TGA (b) profiles of the P0.3 and TEPA-modified P0.3-50 samples.

Fig. 7 presents the comparison of formaldehyde uptake on the P0.3, P0.3-50, TEPA (the same amount contained in P0.3-50) and commercial zeolite. The HCHO adsorption on the above adsorbents

Y. Le et al. / Applied Surface Science 274 (2013) 110–116

C1S

20

HCHO adorption (mg/g)

O1S

N1S

R0.3-50

Si2P Si2S

100

200

300

400

500

600

P1.0-50

15

P0.1-50 10

0

10

20

Biding energy (eV)

30

40

50

60

t (min) 1000

SBET (m /g) 2

P1

0 -5 .0

-5 P0

.0

0 .3

0 0

200

P1

290

5

0

285

Biding energy (eV)

400

-5

280

10

P0

286.2

600

0 P0 .1

284.7

15

-5

C-N

800

HCHO adsorption S

.1

C-H

(b)

P0

Relative intensity (a.u.)

285.0

20

P0

C1s

C-C

H C H O a d s o r p ti o n (m g / g )

(b)

N1S

(c) Relative intensity (a.u.)

Fig. 8. (a) Effects of R on the formaldehyde adsorption capacity of the P0.3, P0-50, P0.1-50, P0.3-50 and P1-50 samples and (b) comparison of specific surface areas and formaldehyde adsorption capacity of the samples. C-NH-C -NH 398.9 400.2 -NH 401.4

392

394

396

398

400

402

404

406

408

Biding energy (eV) Fig. 6. XPS survey spectrum (a) and high-resolution XPS spectra of C 1s (b) and N 1s (c) for the TEPA-modified P0.3-50 sample.

H C H O a d s o r p ti o n (m g / g )

P0.3

P0-50

5 0

700

P0.3-50

.3

0

(a)

P0

Relative intensity (a.u.)

(a)

115

20

P0.3-50

15 10 P0.3 5 TEPA 0

Zeolite 0

10

20

30

40

50

60

t (min) Fig. 7. HCHO uptake as a function of time on the P0.3, P0.3-50, pure TEPA and commercial zeolite samples.

is very rapid within the first 20 min and then gradually slows down, approaching the equilibrium within ca. 30 min. Pure TEPA and commercial zeolite have a relative low HCHO adsorption capacity of 5.25 and 2.02 mg/g, respectively. In contrast, the TEPA-modified P0.3-50 sample shows the largest HCHO adsorption capacity with 20.65 mg/g. The P0.3-50 sample exhibits a higher HCHO adsorption capacity than pure TEPA. This can be ascribed to the synergistic effects of TEPA and P0.3. First, the high specific surface area and porous volume of the P0.3 sample lead to a good dispersion of TEPA, and thus more HCHO affinity sites are exposed to the adsorbate. The inherent affinity of TEPA for HCHO along with the good dispersion of TEPA in the P0.3 sample synergistically increases the HCHO adsorption capacity of the P0.3-50 sample. A possible mechanism for the formaldehyde adsorption is because formaldehyde can reacts with the amine group to produce new imine group (R NH2 + HCHO → R N = CH2 + H2 O) and is trapped in the form of an imine compound [37]. Consequently, it is not surprising that the formaldehyde adsorption capacity increases with increasing the amount of amine groups functionalized to the adsorbent surface [24]. Fig. 8a shows the comparison of formaldehyde adsorption capacities of the P0.3, P0-50, P0.1-50, P0.3-50 and P1.0-50 samples. As can be seen from Fig. 8a, the adsorption capacity of the P0.3-50 sample is the highest. Fig. 8b shows the comparison of specific surface areas for the P0, P0.1, P0.3 and P1.0 samples and HCHO adsorption capacities of the P0-50, P0.150, P0.3-50 and P1.0-50 samples. For P0.3-50 sample it exhibits the greatest formaldehyde adsorption capacity (20.65 mg/g). This is because the P0.3 sample has highest specific surface area and pore volume (Table 1). Obviously, the HCHO adsorption capacity

Y. Le et al. / Applied Surface Science 274 (2013) 110–116

Percentage of adsorption amount(%)

116

100

21177100 and 51272199), Fundamental Research Funds for the Central Universities and Self-determined and Innovative Research Funds of SKLWUT.

80

References

60 40 20 0

1

2 3 4 5 Number of adsorption cycles

6

Fig. 9. Comparison of formaldehyde adsorption and recycle times over the P0.3-50 sample.

has positive relationship with the specific surface area of the sample [38,39]. To further test repeatability of HCHO adsorption on the P0.3-50 sample, the adsorption experiments were repeatedly performed by 6 times. In the multiple cycle adsorption–desorption tests, HCHO adsorption was performed at ambient temperature and atmosphere with the initial HCHO concentration of ca. 300 ppm for 60 min. After HCHO adsorption equilibrium, then, the HCHO was desorbed at 80 ◦ C for 2 h. As shown in Fig. 9, the P0.3-50 sample exhibits relative good thermal stability and repeatability for HCHO adsorption. This is crucial for its practical application. 4. Conclusion In conclusion,hierarchical hollow silica microtubes (HHSM) are successfully prepared by a sol–gel method using TEOS as precursor and CTAB and bio-template PC as co-templates, which are calcined at 600 ◦ C and modified with TEPA. All the prepared samples contained small mesopores with peak pores at ca. 2.8 nm. Several tens of nanometer-sized mesopores are also observed in the wall of hollow tubes from TEM images. The PC/SiO2 weight ratio R exhibits a significant influence on specific surface areas, and the P0.3 sample has highest specific surface area (896 m2 /g). All the TEPA-modified HHSM samples present good formaldehyde adsorption capacity, which is related to the surface amine modification and specific surface areas of the samples. With increasing the specific surface areas, the formaldehyde adsorption capacity increases. The maximum formaldehyde adsorption capacity (20.65 mg/g adsorbent) is obtained on the P0.3-50 sample due to the synergistic effects of amine groups of TEPA and the high specific surface area and porous volume of the P0.3 sample. This study will provide new insight into the design and fabrication of high performance adsorbents for removing indoor formaldehyde. Acknowledgements This work was partially supported by the 863 Program (2012AA062701), 973 program (2013CB632402), NSFC (51072154,

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