Few-Layered Graphene-like Boron Nitride: A Highly Efficient

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Few-layered Graphene-like Boron Nitride: A Highly Efficient Adsorbent for Indoor Formaldehyde Removal Jiawei Ye, Xiaofeng Zhu, Bei Cheng, Jiaguo Yu, and Chuanjia Jiang Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.6b00426 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 7, 2016

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Environmental Science & Technology Letters

Few-layered Graphene-like Boron Nitride: A Highly Efficient Adsorbent for Indoor Formaldehyde Removal §

§

Jiawei Ye , Xiaofeng Zhu , Bei Cheng, Jiaguo Yu* and Chuanjia Jiang*

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, P. R. China §

These authors contributed equally to this work.

*Corresponding authors. Tel.: 0086-27-87871029, Fax: 0086-27-87879468, E-mail: [email protected]; [email protected]

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ABSTRACT:

2

Highly porous boron nitride (BN) composed of flexible network of hexagonal BN

3

nanosheets was synthesized via thermal treatment of a boric acid and urea mixture.

4

The as-prepared sponge-like BN displayed fast adsorption rates and ultra-high

5

adsorption capacities for gaseous formaldehyde (HCHO), e.g. 19 mg/g in equilibrium

6

with approximately 20 ppm HCHO in air, which is an order of magnitude higher than

7

other tested materials, including commercial hexagonal BN and various metal oxides.

8

The superb HCHO adsorption performance of the porous BN is mainly due to its high

9

specific surface area (627 m2/g), as well as the abundant surface hydroxyl and amine

10

groups. Moreover, chemisorption can occur on the BN layers and contribute to the

11

high HCHO uptake via Cannizzaro-type disproportionation reactions, during which

12

HCHO is transformed into less toxic formic acid and methanol. This porous BN is a

13

promising adsorbent for indoor HCHO removal, and may serve as the support for

14

highly efficient HCHO decomposition catalysts.


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INTRODUCTION

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Formaldehyde (HCHO) is a major indoor air pollutant, which is mainly emitted

17

from building and furnishing materials.1,2 Long-term exposure to HCHO may cause

18

serious health problems, including respiratory disease, skin irritation and cancer.1 To

19

reduce indoor HCHO concentration, various techniques have been developed for

20

HCHO removal, such as adsorption (physical adsorption,3-7 chemisorption8,9), plasma

21

oxidation,10-12 photocatalytic degradation13-15 and thermal catalytic oxidation

22

decomposition.16-27 Catalytic oxidation is potentially capable of continuous and

23

complete removal of gaseous HCHO, but the catalysts with high room-temperature

24

HCHO decomposition activity typically contain noble metals28 and are prone to

25

deactivation.29 On the other hand, adsorption offers a feasible strategy for indoor

26

HCHO removal, due to its low cost and easy operation. Various adsorbents such as

27

activated carbon,3,30,31 AlOOH,6 and CeO232 have been studied for removing gaseous

28

HCHO, but their performances are still unsatisfactory even after surface modification

29

or combination as composite materials. Considering the polarity of HCHO molecules,

30

it is hypothesized that materials with hydrophilic surface and a high specific surface

31

area may serve as an effective adsorbent for HCHO.

32

As an analogue of graphene, hexagonal boron nitride (BN) has recently attracted

33

wide research interests, due to its unique two-dimensional layered structure and

34

physicochemical properties, such as ultra-high specific surface area, outstanding

35

electrical-insulating properties, high thermal conductivity and chemical stability.33

36

These characteristics make BN a promising material in broad applications, especially 3 ACS Paragon Plus Environment


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in hydrogen storage,34,35 electronics,36 and adsorptive removal of organic water

38

pollutants.37 In addition to its large specific surface area, BN has high hydrophilicity,

39

thus porous BN may exhibit good adsorption performance toward gaseous HCHO.

40

Although the adsorption of HCHO molecules on BN has been theoretically studied,38

41

porous BN has rarely been evaluated for the adsorptive removal of indoor HCHO

42

under ambient condition.

43

Herein, highly porous sponge-like BN (p-BN) composed of interconnected

44

network of curled nanosheets was prepared by thermal treatment of a boric acid and

45

urea mixture. The as-prepared material exhibited outstanding performance for the

46

adsorption of gaseous HCHO, as compared with commercial BN (c-BN), activated

47

carbon (AC) and various metal oxides. Furthermore, we reported for the first time that

48

this few-layered porous BN can trigger the Cannizzaro-type reaction, and thus HCHO

49

can be transformed into less toxic species (i.e., formic acid and methanol). This work

50

may shed light on the design of environmentally benign adsorbent for efficient indoor

51

HCHO removal.

52 53

MATERIALS AND METHODS

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Preparation of p-BN. All chemicals used were reagent-grade without further

55

treatment. Typically, 1.24 g of boric acid (Sinopharm Chemical Reagent Co., Ltd) and

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14.5 g of urea (Sinopharm) were dissolved in 40 mL of distilled water to form a clear,

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colorless solution, which was heated at 65 °C under magnetic stirring until water

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evaporated completely. The dried mixtures were collected into a quartz boat and 4 ACS Paragon Plus Environment

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heated to 900 °C at a rate of 10 °C/min for 5 h under nitrogen (N2) atmosphere,

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yielding white and fluffy products. Sources of commercial materials for comparison

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are provided in Table S1 in Supporting Information (SI).

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Characterization. The phase structures of p-BN and c-BN were analysed by

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X-ray diffraction (XRD) (Rigaku D/Max-RB, Japan), while the morphology of p-BN

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was observed by field-emission scanning electron microscopy (FESEM) (JEOL 7500F,

65

Japan), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM)

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(JEM-2100F). X-ray photoelectron spectra (XPS) measurements were performed on

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VG ESCALAB210, and Fourier transform infrared (FTIR) spectra were collected

68

using a Shimadzu IRAffinity-1 FTIR spectrometer. The Brunauer-Emmett-Teller

69

(BET) specific surface area (SBET), pore size distribution, pore volume and average

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pore size of the samples were determined by N2 adsorption measurement

71

(Micromeritics ASAP 2020). In situ diffuse reflectance infrared Fourier transform

72

spectroscopy (DRIFTS) was performed for p-BN exposed to a gas mixture of HCHO

73

and O2 at room temperature (Thermo Fisher Nicolet iS50). More details of the

74

characterization methods are provided in the SI.

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HCHO adsorption test. HCHO adsorption experiments of the tested materials

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were performed in the dark and at 25 °C in an organic glass box reactor, with the

77

real-time concentrations of HCHO, carbon dioxide (CO2), carbon monoxide (CO) and

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water vapor online detected by a Photoacoustic Field Gas Monitor (INNOVA AirTech

79

Instruments, Model 1412).9 For measuring the adsorption isotherms of p-BN and

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c-BN, initial HCHO concentrations ranged from 20 to 646 ppm. The adsorption 5 ACS Paragon Plus Environment


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kinetics and isotherms were modeled according to Xu et al.39 Detailed procedures are

82

provided in the SI.

83 84

RESULTS AND DISCUSSION

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Phase Structure and Morphology. The XRD pattern of c-BN (Figure 1a) can be

86

assigned to hexagonal BN (JCPDS 45-0893), while that of p-BN exhibits two

87

broadened peaks corresponding to the (002) and (100) planes of hexagonal BN, which

88

is characteristic of partially disordered BN phases with low crystallinity.37 The shifted

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(002) peak corresponds to an interplanar spacing of 0.37 nm, which is larger than the

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standard (002) distance of 0.33 nm in c-BN.35

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In the FTIR spectra of both p-BN and c-BN (Figure 1b), two intense absorption

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peaks were observed at 1388 and 800 cm–1, which are attributed to in-plane B-N

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transverse optical mode of the sp2-bonded BN and the out-of-plane B-N-B bending

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mode, respectively.40 The FTIR spectrum of p-BN exhibits two additional weak

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absorption bands at 3410 and 3200 cm–1, which are attributed to the O-H stretching

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bands from hydroxyl groups and N-H stretching bands from amine groups,

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respectively.41

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The FESEM image (Figure 1c) shows that p-BN has a porous sponge-like

99

morphology, which is constructed by interconnected network of twisted nanosheets,

100

while TEM characterization (Figure 1d) also confirmed the porous structure of p-BN.

101

The HRTEM image (Figure 1d, inset) shows parallel fringes at the edge of the BN

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nanosheets, suggesting that these nanosheets are composed of a few (e.g. six) stacked 6 ACS Paragon Plus Environment

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BN layers. The average spacing between adjacent fringes was 0.37 nm, consistent

104

with that calculated from the (002) peak in the XRD spectrum.

105

106 107 108 109

Figure 1. XRD patterns (a) and FTIR spectra (b) of the as-synthesized porous BN (p-BN) and the commercial BN (c-BN). FESEM (c), TEM (d) and HRTEM (inset of part d) images of p-BN.

110 111

XPS Surface Chemistry Analysis. The XPS survey spectra (Figure S1a) of

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p-BN and c-BN indicate the presence of B, N, O and C on both samples, with

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corresponding binding energies of approximately 191 (B 1s), 398 (N 1s), 532 (O 1s),

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and 285 eV (C 1s).42 The C 1s peak is due to the adventitious hydrocarbon.

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High-resolution B 1s spectra of c-BN and p-BN (Figure S1b) show main peaks at

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190.3–190.9 eV and shoulder peaks at 191.9–192.3 eV. The former peaks correspond

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to BN3 trigonal units in layered hexagonal BN, while the latter correspond to B-O 7 ACS Paragon Plus Environment


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bonds due to surface hydroxyl or the exposure to air. Notably, the B-O bond peak

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intensity of p-BN is higher than that of the c-BN due to extra hydroxyl groups

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originating from the boric acid precursor.37 As compared to c-BN, the positive shift of

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the B 1s binding energy in p-BN is caused by surface hydroxyl or amine groups

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bonded to B. The binding energy of N 1s (Figure S1c) from NB3 trigonal units in

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c-BN and p-BN are located at 397.9 and 398.2 eV, respectively, consistent with

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reported values.43 A shoulder peak at 399.0 eV in the N 1s spectrum of p-BN can be

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assigned to amine groups.42 These results, in combination with the FTIR analysis,

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further indicate the presence of hydroxyl and amine groups in p-BN.

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Textural Properties. Nitrogen adsorption-desorption isotherms of p-BN and

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c-BN (Figure S2) are both type IV isotherms with type H3 hysteresis loops,

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suggesting the presence of slit-shaped mesopores formed between hexagonal BN

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nanosheets.44 The drastic increases of N2 adsorption at the low relative pressure (P/P0)

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range for p-BN confirms the presence of micropores,35 as compared to c-BN with

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minimal N2 adsorption. On the other hand, the hysteresis loops at a P/P0 range

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between 0.4–1.0 indicate the presence of large mesopores and macropores in c-BN

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and p-BN.

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The textural properties of p-BN and the commercial samples are summarized in

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Table 1. The p-BN sample exhibits significantly larger SBET (627 m2/g) than the other

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materials. Based on the above results, p-BN is supposed to have high adsorption

138

performance for removal of HCHO from air.

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Table 1. Textural properties and HCHO adsorption capacity of the samples. Sample p-BN c-BN AC f MS g SiO2 γ-Al2O3 α-Fe2O3 Co2O3 TiO2 (P25) CeO2

141 142 143 144

SBET a (m2/g) 627 25 106 389 229 124 109 61 47 14

dp b (nm) 2.7 10.6 5.1 2.5 7.7 6.9 14.6 9.9 9.2 11.0

Vp c (cm3/g) 0.42 0.07 0.14 0.25 0.44 0.21 0.40 0.15 0.11 0.04

qd (mg/g) 19.0 1.93 0.90 1.68 1.61 1.75 1.51 h 1.65 1.08 2.19

q’ e (mg/m2) 0.030 0.077 0.008 0.004 0.007 0.014 0.014 0.027 0.023 0.156

Notes: a BET specific surface area; b average pore diameter; c total pore volume; d HCHO adsorption capacity, measured after 60 min. e HCHO adsorption capacity normalized to SBET; f activated carbon, g molecular sieve (13X). h Estimated from HCHO uptake minus CO2 generation.

145 146

HCHO Adsorption Activity. After the adsorption started, HCHO concentration

147

quickly decreased in the presence of p-BN (Figure S3a), while CO2 concentration did

148

not change significantly (Figure S3b), indicating that HCHO was removed by

149

adsorption rather than catalytic decomposition. The HCHO adsorption kinetics of

150

p-BN

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pseudo-first-order model (Figure S4), with a calculated pseudo-second-order rate

152

constant of 0.023 g/mg/min (Table S2). In contrast, HCHO concentration only slightly

153

decreased in the presence of c-BN and other commercial materials (Figure S3a).

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Correspondingly, the amount of HCHO adsorption on p-BN rapidly increased within

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10 min and reached 19.0 mg/g after 1 h, which was much higher than the other

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materials (< 2.3 mg/g) (Figure 2 and Table 1). Moreover, after 60 min of adsorption,

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p-BN was in equilibrium with a much lower HCHO concentration (~20 ppm) than the

were

better

fitted

with

the

pseudo-second-order


model

than

the


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other materials (≥120 ppm). Given the same equilibrium HCHO concentration,

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HCHO adsorption by p-BN is expected to be even higher than by the other materials.

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For example, with an equilibrium HCHO concentration of 133 ppm (i.e., 0.169 mg/L),

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the adsorption capacity of p-BN was 26.8 mg/g (Figure S5). The adsorption isotherm

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of c-BN (Figure S6) can be described by the Freundlich model (R2 = 0.91) but not the

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Langmuir model (R2 < 0.01), while that of p-BN cannot be well described by either of

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these models (Figure S7). These results were due to the fact that partial oxidation of

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adsorbed HCHO occurred (shown in later sections), which were not considered in the

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classical adsorption models. The HCHO adsorption of the commercial materials were

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also fast (Figure 2), despite the low adsorption capacities; due to the low time

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resolution of the measured data compared to the adsorption rates, the adsorption

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kinetics could not be accurately modeled.

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It should be noted that α-Fe2O3 was able to decompose HCHO at room

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temperature,45 with HCHO concentration decreased from 149 ppm to 119 ppm and

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CO2 concentration increased by approximately 20 ppm within 60 min (Figure S3).

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This suggests that approximately two thirds of the HCHO removed by α-Fe2O3 was

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decomposed into CO2 and H2O, and approximately one third remained adsorbed on

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α-Fe2O3, either as HCHO or as transformed species. Thus, while HCHO removal by

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α-Fe2O3 was 4.5 mg/g after 60 min of reaction, the HCHO adsorption capacity of

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α-Fe2O3 was estimated to be approximately 1.5 mg/g.


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Figure 2. HCHO uptake over time on p-BN and commercial materials.

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The as-prepared p-BN had much higher HCHO adsorption capacity than c-BN,

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mainly due to its larger SBET, while both the BN samples were more effective in

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adsorbing HCHO than most of the other tested materials (except CeO2) when SBET is

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taken into account (Table 1). Therefore, in additional to the large SBET, surface

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properties of boron nitride also have great contributions to the high HCHO adsorption.

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Hydrogen bonding between HCHO and hydroxyl or amine groups plays an important

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role in the adsorption of HCHO,6,30 and this can explain why boron nitride has higher

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HCHO adsorption capacity than activated carbon, which is less hydrophilic than BN.

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However, boron nitride is also more effective in adsorbing HCHO than materials with

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high surface hydroxyl content, e.g. γ-Al2O3, SiO2 and molecular sieve (13X). Hence, 11 ACS Paragon Plus Environment


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another reason for the high adsorption capacity of boron nitride is that HCHO

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molecules and BN layers are both planar configuration, resulting in lower adsorption

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resistance. Furthermore, HCHO molecules have π-conjugation between C and O

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atoms, while BN also exhibits a large 2D delocalized π-conjugated structure. Thus,

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π−π conjugation interaction can occur between BN and HCHO, and this strong

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interaction can significantly enhance the adsorption of HCHO. As a result of the

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above-mentioned advantages, p-BN exhibited outstanding performance for HCHO

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adsorption.

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The HCHO adsorption performance stability of p-BN was measured by testing

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used p-BN after heating under an infrared lamp (Figure S8). The adsorption capacity

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decreased with recycling times, probably due to partial blocking of micropores.

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However, the performance became stable after the fourth run, and HCHO uptake

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remained at 8.5 mg/g, which is still high compared to other materials and previously

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reported value.4

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Adsorption Mechanisms Study by in situ DRIFTS. When p-BN was exposed

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to the HCHO/O2 mixture at ambient temperature, the DRIFTS spectra exhibited

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multiple bands (Figure S9), which could be assigned to formic acid (HCOOH),

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methanol (CH3OH) and HCHO (Table S3). Specifically, the bands at 2986, 2863,

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2750, 1610, 1566 and 1371 cm–1 are attributed to adsorbed formate,17,22,25,27,46-50 those

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at 2912, 2804 and 1467 cm–1 to methoxy group,48 and those at 1771, 1700 and 1413

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cm–1 to molecularly adsorbed HCHO.47,51 Moreover, the broad bands at 3200–3600

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cm–1 are attributed to the stretching vibration of hydroxyl groups.27 These results 12 ACS Paragon Plus Environment

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indicate that HCOOH and CH3OH are the products of HCHO adsorption on p-BN,

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likely via a Cannizzaro-type disproportionation reaction pathway. Notably, an

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intensive negative peak was observed at 1640 cm–1, corresponding to adsorbed

216

water,17,21,23

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disproportionation reaction of HCHO.

which

suggests

that adsorbed

water

was

consumed

in

the

218

Thus, a possible mechanism for the Cannizzaro-type disproportionation reaction

219

of HCHO on p-BN surface is proposed (Figure S10). Briefly, HCHO molecules are

220

adsorbed on the surface of p-BN. Then the N atoms of BN layer, which act as Lewis

221

base, trigger the nucleophilic addition reaction, and a H atom of HCHO transfers to

222

another HCHO molecule (step I). As a result, some adsorbed HCHO molecules

223

transform into formamide while others form methoxy salts (step II). Finally, the

224

formamide and methoxy salts react with surface adsorbed water molecules and

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transform into less toxic HCOOH and CH3OH (step III). Thus, while water vapor can

226

compete with HCHO for adsorption,31 water molecules are necessary in the

227

chemisorption of HCHO onto p-BN surface. The complex role of air moisture in the

228

overall HCHO uptake performance needs further investigation.

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Outlook. The porous BN exhibited superb HCHO adsorption performance at

230

room temperature, which was mainly due to its high specific surface area, as well as

231

the abundant surface hydroxyl and amine groups. This material can be used as a

232

support for synthesizing highly efficient HCHO decomposition catalysts, e.g.

233

supported Pt catalysts. Moreover, chemisorption occurred on the BN layers and

234

contributed to the high HCHO uptake via Cannizzaro-type disproportionation 13 ACS Paragon Plus Environment


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reactions. Thus, the adsorption isotherms could not be well described by the classical

236

adsorption models; alternatively, models are needed for describing such reaction

237

systems, which should consider both partitioning between phases and chemical

238

reactions occurring at the heterogeneous surfaces.

239

This study highlighted the complex role of water vapor in affecting the HCHO

240

uptake performance by the porous BN, which needs to be more systematically

241

investigated. In addition, another important influencing factor is temperature, since

242

exothermic and endothermic adsorptions are affected by temperature in different

243

ways.39

244

temperature-dependent adsorption isotherms,39,52 but a prerequisite for such

245

approaches is the definition of standard reference state for the adsorbed species in

246

order to calculated the chemical activity of these species at different interfaces.

247

Alternatively, the adsorption energy of the materials can be calculated by density

248

functional theory (DFT) computation,38 but it is necessary to quantitatively

249

characterize the atomic structure of the materials, including the presence and quantity

250

of vacancies and other types of defects, as well as the precise contents of hydroxyl,

251

amino and other functional groups involved in the adsorption of HCHO.

The

adsorption

thermodynamics

can

be

investigated

from

252 253

Acknowledgments

254

This work was supported by the NSFC (51320105001, 51372190, 21573170,

255

51272199 and 21433007), 973 program (2013CB632402), the Natural Science

256

Foundation of Hubei Province (2015CFA001), the Fundamental Research Funds for 14 ACS Paragon Plus Environment

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the Central Universities (WUT: 2015-III-034) and Innovative Research Funds of

258

SKLWUT (2015-ZD-1).

259 260

Supporting

Information

Available:

Additional

information

related

to

the

261

characterization and HCHO adsorption test procedures, sources of commercial

262

materials (Table S1), kinetics parameters (Table S2), In situ DRIFTS band assignment

263

(Table S3), XPS (Figure S1), N2 adsorption-desorption (Figure S2), concentration

264

change of HCHO and CO2 over time (Figure S3), HCHO adsorption kinetics

265

modeling (Figure S4), HCHO adsorption isotherms (Figure S5), data fitting of HCHO

266

adsorption isotherms (Figure S6 and S7), HCHO uptake in recycle tests (Figure S8),

267

in situ DRIFTS (Figure S9) and proposed disproportionation reaction mechanism

268

(Figure S10). This material is available free of charge via the Internet at

269

http://pubs.acs.org.

270 271

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arsenosilicate in H+, Na+ and Ca2+ forms. Colloids Surf. 1987, 24, 33-42.



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Few-Layered Graphene-like Boron Nitride: A Highly Efficient

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