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oxidized carbon nanotube nanocomposites for CO2 capture
Accepted Manuscript Title: Layered double hydroxides/oxidized carbon nanotube nanocomposites for CO2 capture Author: Junya Wang Liang Huang Qianwen Zheng Yaqian Qiao Qiang Wang PII: DOI: Reference:
S1226-086X(16)00072-1 http://dx.doi.org/doi:10.1016/j.jiec.2016.02.010 JIEC 2832
To appear in: Received date: Revised date: Accepted date:
13-7-2015 13-2-2016 15-2-2016
Please cite this article as: J. Wang, L. Huang, Q. Zheng, Y. Qiao, Q. Wang, Layered double hydroxides/oxidized carbon nanotube nanocomposites for CO2 capture, Journal of Industrial and Engineering Chemistry (2016), http://dx.doi.org/10.1016/j.jiec.2016.02.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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nanocomposites for CO2 capture
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Layered double hydroxides/oxidized carbon nanotube
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Junya Wang, Liang Huang, Qianwen Zheng, Yaqian Qiao, Qiang Wang*
College of Environmental Science and Engineering, Beijing Forestry University, 35
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Qinghua East Road, Haidian District, Beijing 100083, P. R. China
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*Corresponding author: Professor Qiang Wang
College of Environmental Science and Engineering, Beijing Forestry University, 35 Qinghua East Road, Haidian District, Beijing 100083, P. R. China E-mail: [email protected]; [email protected]
Tel: +86 13699130626
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Highlights
Layered double hydroxide/oxidized carbon nanotube hybrid was prepared for
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CO2 capture
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Electrostatic self-assembly and direct co-precipitaiton methods were compared
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The CO2 capture performance of LDH/OCNT nanocomposite was studied
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The introduction of OCNT improved the performance of LDH derived CO2
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Abstract
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adsorbent
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A systematic investigation on the influence of synthesis method and chemical composition of LDH/oxidized carbon nanotube (LDH/OCNT) nanocomposites on their CO2 capture performance were performed. Three LDH/OCNT nanocomposites
were prepared using two methods: “electrostatic self-assembly” and “direct co-precipitaiton”. All LDH/OCNT nanocomposites were thoroughly characterized. The isothermal CO2 adsorption at different temperatures (60–400
o
C) and
adsorption/desorption cycling tests were conducted. Both the CO2 adsorption capacity and the multi-cycle stability of LDH derived adsorbents were improved by the introduction of OCNT. In particular, the absolute CO2 capture capacity of Mg-Al-NO3 LDH was increased by more than twice by adding 9.1 wt% OCNT. Keywords: CO2 adsorbent; hydrotalcite; sorption enhanced water gas shift; synthesis
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1. Introduction
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method; stability
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CO2 is one of the major greenhouse gases (GHG) which continuous increases due to
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the large-scale burning of fossil fuels.[1, 2] It also has been predicted that fossil fuels will be the dominant energy source in the coming decades, and the amount of energy demand will increase further by 53% by 2030.[3] Therefore, the reduction of CO2
emission is highly demanded. For CO2 capture, store and utilize, several new
technologies have been developed,[4] and many kinds of solid CO2 adsorbents have
been synthesized.[5-9] One feasible step towards reducing CO2 emissions is to capture
the CO2 generated during combustion and store it in a suitable place.[10] Sorption enhanced water gas shift (SEWGS) is well known as a promising pre-combustion CO2 capture technology, which is a combination of WGS reaction and CO2 sorption, as shown in equation (1). Due to the existence of a solid CO2 adsorbent, the produced CO2 can be in-situ captured. And in the meantime, the conversion of CO and the
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production of H2 can be increased as well.[11-14] CO (g) + H2O (g) + adsorbent (s) ↔ adsorbent-CO2 (s) + H2 (g)
(1)
The key to ensure the success of this process is choosing a suitable CO2
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capturing material. Among various CO2 adsorbents for SEWGS, layered double hydroxides (LDHs) derived mixed oxides are one of the best solid CO2 adsorbent,
formula of LDH is [M1−x
2+
M
3+ x
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which showed good performance in the temperature range of 200–400 oC. The general (OH)2][An-]x/n·ZH2O, where M2+ and M3+ are
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divalent (Mg2+, Zn2+, Ni2+, etc.) and trivalent cations (Al3+, Ga3+, Fe3+, Mn3+, etc.),
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respectively. This compound consists of positively charged brucite-like layers with an interlayer region containing charge compensating anions and solvation molecules.[3]
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LDH as the CO2 adsorbents had been widely investigated in recent years. For instance, the effects of divalent cations,[15] trivalent cations,[16] charge compensating
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anions,[17-19] Mg/Al ratio,[20] synthesis method,[3] the presence of SO2 and H2O,[14,
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21] particle size,[22, 23] alkali metal (K, Cs) doping,[18, 23-29] the uploading of LDHs on supports,[30-32] and operational pressure[33] on CO2 adsorption have all
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been reported.
Recently, LDH based hybrid materials become promising materials for CO2
capture, for instance, LDH/carbon nanotube (LDH/CNT) and LDH/graphene oxide (LDH/GO) hybrids, etc. In general, the synthesis method can be divided into two general
strategies.
One
is
one-pot
co-precipitation
method,
by
which
Garcia-Gallastegui et al.[31] synthesized a LDH/GO hybrid for CO2 capture, with
improved CO2 adsorption capacity and recyclability. They also used multi-walled carbon nanotubes (MWCNTs) as support to synthesize LDH/MWCNTs hybrid as CO2 adsorbent, by which the absolute CO2 capture capacity and the stability of LDH were improved.[32] The other synthesis method is based on the electrostatically
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driven self-assembly between the delaminated positively charged LDH single sheets and the negatively charged monolayer. Wang et al.[34] synthesized Mg-Al-NO3 LDH-NS/GO nanocomposite via electrostatic self-assembly from exfoliated LDH and
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GO nanosheet dispersions as novel high-temperature CO2 adsorbent, which showed good CO2 capture capacity and good CO2 adsorption/desorption cycling performance.
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However, the LDH/CNT synthesized by the self-assembly method has not been studied for CO2 capture.
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In this paper, we performed a comprehensive and comparative study on the
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influence of synthesis method and the chemical composition of LDH/oxidized carbon nanotube (LDH/OCNT) nanocomposites on their CO2 capture performance. First, two
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synthesis methods including “exfoliation‒self-assembly” and “direct co-precipitation” were used for the preparation of nanocomposites. By varying the key synthesis
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parameters, a series of nanocomposites with different chemical compositions were
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synthesized. All samples were then thoroughly characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy
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(TEM), etc. Finally, the isothermal CO2 capture capacity and the CO2 adsorption/desorption cycling performance were evaluated in detail. We believe that the existence of CNTs can not only improve the dispersion of LDHs and accessibility of gas, but also make it easier to pelletize the LDH-based CO2 adsorbents, which are
crucial for its practical applications.
2. Experimental 2.1 Synthesis of samples 2.1.1 Synthesis of Mg-Al-NO3 LDH-NS A salt solution (100 ml) containing a mixture of 19.2 g Mg(NO3)2·6H2O and 9.4 g
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Al(NO3)3·9H2O was added drop-wise to a basic solution (100 ml) containing 4.3 g NaNO3 in a 500 ml round bottom flask. The pH of the mixture solution was kept constant at 10 by addition of a NaOH solution (3.4 M). The resulting mixture was
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separated into two portions and hydrothermally treated in 200 ml Teflon lined stainless autoclaves at 120 oC for overnight. After hydrothermal aging, the sample
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was filtered and washed with deionized water until pH = 7, then dried at 100 oC in an oven. The obtained sample is Mg-Al-NO3 LDHs. The delamination of Mg-Al-NO3
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LDHs into single nanosheets was performed as follows. 0.5 g Mg-Al-NO3 LDHs were
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put into 50 mL formamide in a 100 ml beaker, followed by magnetic stirring till no sediment was observed upon standing. The concentration of delaminated Mg-Al-NO3
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LDHs dispersion was determined to be 10 g L‒1, which is marked Mg-Al-NO3 LDH-NS.
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2.1.2 Synthesis of OCNT
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Oxidization of carbon nanotubes was achieved by a modified Hummers method. 6 g CNTs were first purified by calcination at 500 oC for 1 h and washed with 70 mL of
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10 wt% HCl to remove metal residues. The products were then filtered, washed and lyophilized. After that, it was put together with 1.5 g sodium nitrate into the 70 mL concentrated H2SO4 with stirring in a 1000 ml beaker. Then 9 g KMnO4 was added
gradually with stirring while keeping the temperature of the mixture below 20 oC with
ice-bath. The mixture was then stirred at 35 oC for 30 min, followed by the addition of some 140 mL distilled water. After another 30 min stirring, 420 mL distilled water was then added to terminate reaction. Subsequently, 30 wt% H2O2 was added. After 10 min, the mixture was centrifuged and washed with 10 wt% HCl solution to remove residual metal ions. The precipitate was then washed with Milli-Q water and centrifuged repeatedly until the solution became neutral, then dried at 100 oC in an
Page 6 of 32
oven. The obtained sample is oxidized carbon nanotube. 2.1.3 Synthesis of nanocomposites using “exfoliation‒self-assembly” method The nanocomposites synthesized using the “exfoliation‒self-assembly” method was
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denoted as LDH-NO3-NS/OCNT nanocomposites, which contain exfoliated Mg-Al-NO3 LDH nanosheets and OCNT. The synthesis procedure is described briefly
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as follows. The positively charged LDH-NO3-NS suspension (10 g L‒1) was obtained by the delamination of Mg-Al-NO3 LDH in formamide, and the negatively charged
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OCNT suspension (1 g L‒1) was prepared by dispersing OCNT powders in water by
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sonication treatment. Then the LDH-NO3-NS/OCNT nanocomposites were prepared by mixing the above two suspensions together. Due to the opposite charge these two
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suspensions, LDH-NO3-NS/OCNT nanocomposites can be formed by electrostatic self-assembly. Six different LDH-NO3-NS/OCNT nanocomposites were prepared by
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varying the LDH-NO3-NS:OCNT volume ratios (VR) of 1: 0.5, 1: 0.7, 1: 1, 1: 1.5 and
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1: 3. The weight ratios between LDH and OCNT were listed in Table 1. The precipitants were centrifuged under 8000 rpm for 10 min, followed by washing with
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Milli-Q water and anhydrous methanol repeatedly to remove formamide and water completely. The nanocomposites were finally dried at 100 oC for 24 h. 2.1.4 Synthesis of nanocomposites using “direct coprecipitation” method The nanocomposites synthesized using the “direct coprecipitation” method were denoted as Mg-Al-NO3/OCNT and Mg-Al-CO3/OCNT nanocomposites. The
synthesis procedure is described briefly as follows. In brief, a salt solution (100 ml) containing a mixture of 19.2 g Mg(NO3)2·6H2O and 9.4 g Al(NO3)3·9H2O was added
drop-wise to a basic solution (100 ml) containing NaNO3 (or Na2CO3) and different mass of highly dispersed OCNT (1 g L‒1) in a 500 ml round bottom flask (The mass of NaNO3 + OCNT = 4.3 g, Na2CO3 + OCNT = 2.7 g). The pH of the mixture solution
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was kept constant at 10 by addition of a NaOH solution (3.4 M). The resulting mixture was stirring overnight at room temperature. Then the sample was filtered and washed with deionized water until pH = 7, then dried at 100 oC in an oven. Depending
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on the anions used (NO3‒ and CO32‒), LDH-NO3/OCNT and LDH-CO3/OCNT nanocomposites can be obtained. And the mass ratio of OCNT in the LDH/OCNT
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nanocomposite was calculated by the formula of LDH. 2.2. Characterization of samples
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Powder XRD analyses were conducted on a Shimadzu XRD-7000 X-ray diffractometer with Cu Kα radiation. Diffraction patterns were recorded within the
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range of 2θ = 5–65o with a step size of 0.02o. The morphologies of samples were
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characterized by field emission scanning electron microscope (FE-SEM, SU-8020). High resolution transmission electron microscopy (HR-TEM) images were obtained
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on a JEOL 2010, operating at 200 kV. Samples were prepared by dispersing the
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sample in isopropanol using 0.01 mg of sample per mL of solvent, and allowing a drop to dry onto a holey carbon copper grid (300 mesh, Agar Scientific). The zeta
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potentials of delaminated LDHs and OCNTs were measured using zeta potential analyzer (ZPA, Nanosizer Nano ZS, Malvern Instruments). 2.3 Evaluation of CO2 capture capacity Thermogravimetric adsorptions of CO2 on the samples were measured using a Q50
TGA analyzer. Samples were first calcined at 400 oC for 5 h in N2 (60 ml min‒1)
before performing adsorption. To avoid the error caused by the memory effect, the test was carried out immediately after the first calcination. And the samples were further calcined in situ at 400 oC for 1 h in N2 (60 ml min‒1) before adsorption. CO2 adsorption experiments were carried out at 1 atm with a constant flow of CO2 (20 ml
min‒1) for 2 h. The regeneration and stability of the adsorbents were assessed by
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adsorption/desorption cycling tests, in which the adsorption step was carried out at 200 oC for 30 min with a constant flow of pure CO2 (20 ml min‒1), and the desorption
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was performed at 400 oC for 30 min with a constant flow of pure N2 (60 ml min‒1).
3. Results and discussion
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3.1 Synthesis and characterization of LDH-NS/OCNT nanocomposites
The Mg-Al-NO3 LDHs were synthesized using hydrothermal method. Powder XRD
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data confirmed that the synthesized Mg-Al-NO3 LDHs was phase pure, which had a
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hydrotalcite-type structure, as shown in Figure 1(a). However, after the Mg-Al-NO3 LDH powder was put in formamide, and stirred for several hours, a clear and
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transparent colloidal dispersion was obtained. The XRD pattern of delaminated LDH in Figure 1(b) shows that the characteristic diffractions of LDH structure disappeared,
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with only a halo at 2θ = 20–30o came from formamide. The absence of sharp basal
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peaks clearly suggested that the host sheets were not in parallel to induce interference of the X-rays, which confirmed that the LDH had been delaminated into individual
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nanosheets.[35]
Zeta potential is often used as an index of the magnitude of electrostatic
interaction between colloidal particles, which is thus a measure of the colloidal stability of the solution.[36] The zeta potential of exfoliated Mg-Al-NO3 LDH
nanosheet colloidal dispersion and the OCNT dispersion were measured, which were +27.1 and –45.8 mV, respectively. This data suggested that both of these two dispersions were stable and well-dispersed. Due to the opposite charge of the exfoliated LDH and the OCNT dispersions, the LDH-NO3-NS/OCNT nanocomposite was synthesized by electrostatic self-assembly of Mg-Al-NO3 LDH and OCNT nanosheets. It was found that the colloidal
Page 9 of 32
suspension of LDH was broken down, and the sediment was immediately formed once the LDH-NO3-NS dispersion was added to the OCNT dispersion. This behavior assures the strong interaction between LDH nanosheets and OCNT. The
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nanocomposite was easily obtained by separating with centrifugation. The negatively charged OCNT is complementary to the positive charge of the delaminated LDH
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nanosheets and is likely to contribute to the stabilization of the growing nanocomposite. The chemical interaction between the LDH and GO has been also
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observed in a similar hybrid LDH/GO.[34]
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The XRD patterns of pristine LDH-NO3, OCNT and LDH-NO3-NS/OCNT nanocomposites are shown in Figure 2. For the LDH-NO3-NS/OCNT nanocomposites,
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the characteristic reflections of Mg-Al-NO3 LDH were clearly observed, which can be indexed to (003), (006) and (110) planes.[37] Figure 2(g) shows the XRD of OCNT,
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in which the 2θ peak at 26o can be indexed to the (002) plane of graphitic carbon, with
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an interplanar distance of 0.337 nm (JCPDS No. 41-1487). However, when the VR is below 1:1.5, the typical diffraction peak of OCNTs can not be noticed in the
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nanocomposites, suggesting that the OCNT might be highly dispersed. And the small amount of OCNT imbedded into the LDH did not impede the growth of LDH crystals.[38] While, with the increase of the OCNT amount added in nanocomposites (VR of 1:1.5 and 1:3), the peak which is indexed to the (002) plane of graphitic carbon appeared, confirming the obvious existence of OCNT. Furthermore, with more OCNTs were added to the nanocomposites, the (00l) peaks of LDHs were weakened,
corresponding to a lower crystallinity of LDHs. And their obvious broadening of the diffraction peaks indicates smaller LDH particle size in LDH-NO3-NS/OCNT nanocomposites. This data suggests that the addition of OCNTs could improve the dispersion of LDH in the nanocomposites.
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The size, morphology and structure of nanocomposites were characterized using FE-SEM analysis. It can be clearly seen from Figure 3(a) that the OCNTs are
multi-walled nanotubes with outer diameter of about 15‒20 nm. And the LDH-NO3
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showed a typical morphology of layered structures, consisting of aggregates of flake-like particles. However, Figure 3(c) and 3(d) show the SEM images of
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LDH-NO3-NS/OCNT nanocomposite, which is composed of numerous LDH nanosheets and OCNT intercrossed with each other, forming a house-of-cards-type
(Figure
3(b))
with
the
flake-like
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stacking structure. Moreover, it is obviously different from the pristine LDH-NO3 morphology.
The
LDH-NO3-NS/OCNT
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nanocomposites are more porous comparing to neat LDHs, which is favorable for
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CO2 adsorption.[39]
In order to further explore the morphology and structure of the nanocomposite,
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the samples were also characterized using HR-TEM analysis. The TEM image in
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Figure 4(a) shows the pristine CNTs with an average diameter of 10‒20 nm. However, after surface oxidation, the surface of the CNTs became heterogeneous due to the
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attack by the oxidative acid. It can be clearly seen that the smooth surface of the pristine CNTs (Figure 4(a)) was changed to a much rougher surface (Figure 4(b)). It has been reported that the rough surface of OCNT was attributed to the acidic etching of CNT surface.[40] In Figure 4(c), it is clearly shows that the delaminated LDH nanosheets have a translucent plate-like morphology, which further proved that the LDH has been completely delaminated into individual nanosheets.[35] The HR-TEM image of LDH-NO3-NS/OCNT nanocomposite in Figure 4(d) obviously indicated that the OCNT was tightly absorbed and randomly distributed on the surface of LDH nanosheets or interactied with LDH nanosheets, which is agreement with the result of FE-SEM. These result further indicated the strong interfacial electrostatic interaction
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between negatively charged functional OCNT and positively charged layers of LDH. The reason is that carboxylic acid groups (COOH) on the surface of OCNT exist as carboxylate anions (COO‒) in the alkaline solution, thus yielding negatively charged
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OCNT, and the surface of LDH nanosheets are positively charged.[41] Therefore, the negatively charged 1D OCNT is easily attached to the positively charged surface of
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2D LDH nanosheet, which can form a stable 3D nanostructure under the dominance
of charge attractions,[37] similar to the LDH/GO nanocomposite that we previously
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reported.[34]
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3.2 Synthesis and characterization of LDH-NO3/OCNT and LDH-CO3/OCNT nanocomposites
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LDH-NO3/OCNT and LDH-CO3/OCNT were synthesized using a one-pot “direct co-precipitation” method. For all samples, the characteristic peaks of LDHs were
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clearly seen, suggesting the successful formation of LDHs. During the synthesis,
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certain amount of OCNT powders was directly dispersed in the mixed salt solution. Unlike the electrostatic driven self-assembly method, negatively-charged OCNT
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played an important role in the LDH growth process in co-precipitation method.[32] The XRD patterns of LDH-NO3, LDH-CO3, and their nanocomposites with OCNT
are shown in Figure 5. With the VR of OCNT increasing gradually in the nanocomposites (VR = 1: 1.5 and 1: 3), the characteristic peak OCNT started to be observed. Comparing to the peaks from LDH, the intensity of the peak from OCNT is very weak, which suggests that the OCNT is highly dispersed within the nanocomposites. The presence of OCNT during the LDH growth may introduce defects into the LDH structure, through modified nucleation conditions, or induced curvature, which is favorable for CO2 capture.[32] The
FE-SEM
images
of
LDH-NO3/OCNT
and
LDH-CO3/OCNT
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nanocomposites are shown in Figure 6. The morphology and structure of the nanocomposites synthesized from the “direct coprecipitation” method is similar to that from “exfoliation‒self-assembly” method. As it is well known that pure
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Mg-Al-CO3 LDH generally forms sand-rose like morphology, while Figure 6(b) indicates that the LDH-CO3/OCNT nanocomposite is also LDH nanosheets and
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OCNT intercrossed with each other. The HR-TEM images of LDH-NO3/OCNT and LDH-CO3/OCNT nanocomposites (Figure 7) also showed that they formed a stable
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3D nanostructure during the synthesis process, the OCNT can be seen clearly in the
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composites which interspersed among the LDHs. Therefore the XRD, SEM, and TEM analyses all indicated that the synthesis method has little influence on the morphology
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and structure of the formed nanocomposites. For the samples prepared from “direct
coprecipitation” method, the presence of OCNT also can improve the dispersion of
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LDH.
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3.3 Comparison of CO2 capture performance The CO2 capture capacity of the above mentioned three types of nanocomposites
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including LDHl-NO3-NS/OCNT, LDH-NO3/OCNT, and LDH-CO3/OCNT were
evaluated using isothermal CO2 adsorption tests. All the samples were first calcined at 400 oC for 5 h before each CO2 adsorption test. Then the thermo-gravimetric
adsorptions of CO2 on the samples were measured at 200 oC using a Q50 TGA analyzer.
In the present work, we are particularly interested in whether the two different synthesis methods would influence the CO2 adsorption capacity of nanocomposite or not. The CO2 adsorption capacities of pure LDH and its nanocomposites with OCNT are listed in Table 1. We can see that the LDH-NO3-NS/OCNT nanocomposite had better
adsorption
capacity
than
that
without
OCNT.
Moreover,
the
LDH-NO3-NS/OCNT nanocomposite with 9.1 wt% OCNT showed the maximum
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adsorption capacity, which is more than twice larger than that of pure Mg-Al-NO3 LDH. Similarly, the CO2 adsorption capacity of LDH-NO3/OCNT nanocomposite synthesized from “direct coprecipitation” method was also significantly improved by
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the addition of OCNT, and the optimal OCNT weight loading was also 9.1 wt%. The enhancement in adsorption capacity in the presence of OCNT might be attributed to
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the highly dispersion and stabilization of LDH nanosheets.[42] With 9.1 wt% of
OCNT, the geometric and electrostatic compatibility between LDH single sheets and
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OCNT appears to favor heterogeneous nucleation, dispersion, and stabilization,[31]
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which is agreement with the results reported before.[34] The other possible reason is the presence of OCNT during LDH growth, may also contribute positively to the
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adsorption capacity, by creating more active sites. Meis et al.[30] attributed the increase in CO2 adsorption capacity to a greater number of LDH edge sites when
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supported. For LDH-CO3/OCNT nanocomposite, its CO2 capture capacity was
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slightly decreased comparing to neat Mg-Al-CO3 LDH. Although no improvement was observed in the CO2 capture capacity, it is believed that the dispersion of the
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LDH on the OCNT support is likely to mitigate sintering problem during calcination leading to a more active and stable material.[32] In order to integrate the composite into certain processes, we choose
LDH-NO3-NS/OCNT nanocomposite as a sample to investigate its CO2 adsorption
capacity performance at different temperatures, as shown in Figure 8. The results showed that the maximum CO2 capture capacity took place at 60 oC (0.89 mmol g‒1). And the CO2 adsorption capacity decreased with the increase in adsorption temperature, which is in agreement with the result of the LDH/GO nanocomposite.[34] The LDH-NO3-NS/OCNT nanocomposite also showed good CO2 capture capacity in a wide temperature range, which makes it a promising CO2 adsorbent not only for the
Page 14 of 32
SEWGS process but also for the post-combustion CO2 capture from flue gases. 3.4 CO2 adsorption/desorption cycling test In addition to the CO2 capture capacity, the continuous CO2 adsorption/desorption
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cycling stabilities of LDH-NS/OCNT and LDH/OCNT nanocomposites (VR = 1: 1)
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were also evaluated. The CO2 adsorption/desorption cycling tests were performed in a
typical temperature swing adsorption (TSA) mode, as shown Figure 9. The adsorption
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was performed at 200 oC for 30 min with pure CO2, and the desorption was performed
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at 400 oC for 30 min with pure N2. For all these three adsorbents, the CO2 capture capacity showed a relatively obvious decrease in the second cycle, and a very slow
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decline in the following 22 cycles. For LDH-NO3-NS/OCNT nanocomposite, the CO2 capture capacity dropped from 0.38 to 0.32 mmol g‒1 from the first cycle to the
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second cycle, and kept almost constant from the 14th cycle, with a value of 0.3 mmol
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g‒1. For LDH-NO3 OCNT nanocomposite, the CO2 capture capacity dropped from
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0.37 to 0.34 mmol g‒1 from the first cycle to the second cycle, and kept almost constant from the 6th cycle, with a value of 0.32 mmol g‒1. For LDH-CO3/OCNT
nanocomposite, the CO2 capture capacity dropped from 0.4 to 0.38 mmol g‒1 from the
first cycle to the second cycle, and kept almost constant from the 14th cycle, with a value of 0.34 mmol g‒1. This trend is consistent with that from pure LDH-based CO2 adsorbent, which have attributed the first stage to a small amount of CO2 irreversibly chemisorbed on the LDO.[10, 32, 43, 44] From Figure 9(a) and (b), it can be seen that the CO2 adsorption capacities of the nanocomposites were still much higher than that of the pure Mg-Al-NO3 LDH after 22 cycles. And LDH-CO3/OCNT nanocomposite
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also showed good CO2 capture capacity even after 22 cycles. This result demonstrated that
these
LDH
based
nanocomposites
with
OCNT
have
good
CO2
adsorption/desoprtion cycling stability and are potential for practical applications.
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Another potential advantage of the LDH/OCNT nanocomposites type CO2 adsorbent
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is that they are much easier to be pelletized than pure LDH. And the mechanical and
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thermal stability of such obtained LDH/OCNT pellets will be much higher than that of pure LDH, which make it very promising for the practical SEWGS applications.
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Furthermore, we also compared our work with literature reports. The CO2 capture performances of neat LDH derived and LDH-based hybrids derived
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adsorbents are summarized in Table 2. It can be seen that the CO2 capture capacities were both promoted by introduced GO or OCNT, indicating that the structural
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disorder of the materials may enhance the adsorbent/gas contact. Table 2 shows that the CO2 capture capacity of LDH-CO3/OCNT nanocomposites was comparable to
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literature reports. What’s more, it is believed that the mechanical property of the LDH derived adsorbents can be significantly increased by introducing OCNT. This hybrid type CO2 adsorbents are easier to be pelletized and can prevent the “pasting” problem. Also CNTs are much cheaper than GO and have more defined morphology typically with small diameter, high mechanical strength and hence a higher accessible surface area to act as a support.[32, 38] In this contribution, we demonstrated LDH/OCNT nanocomposites will be a promising adsorbent for CO2 capture. Conclusions A comprehensive and comparative study on the influence of synthesis method and
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chemical
composition
of
LDH/oxidized
carbon
nanotube
(LDH/OCNT)
nanocomposites on their CO2 capture performance was conducted. Three types of LDH/OCNT nanocomposites including Mg-Al-NO3-NS/OCNT, Mg-Al-NO3/OCNT,
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and Mg-Al-CO3/OCNT were prepared using “electrostatic self-assembly” and “direct
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co-precipitaiton” methods. XRD, SEM, and TEM analyses revealed that the synthesis
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method has little influence on the morphology and structure of the formed nanocomposites. In all three nanocomposites, the OCNT was tightly absorbed and
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randomly distributed on the surface of LDH nanosheets or interactied with LDH nanosheets, which improves the dispersion of LDH in the nanocomposites. For
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LDH-NO3-NS/OCNT and LDH-NO3/OCNT, the optimal OCNT weight loading was also 9.1 wt% and the CO2 capture capacities were significantly improved, which is
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d
more than twice larger than the pure Mg-Al-NO3 LDH. For Mg-Al-CO3/OCNT nanocomposite, the addition of OCNT resulted in negligible influence on its CO2
Ac ce p
capture capacity, which is ca. 0.35–0.45 mmol g‒1. The CO2 adsorption capacity of
LDH-NO3-NS/OCNT nanocomposite increased with the decrease in adsorption temperature, and the maximum CO2 capture capacity of 0.89 mmol g‒1 was obtained
at 60 oC. The results also demonstrated that these LDH/OCNT nanocomposites have good CO2 adsorption/desoprtion cycling stability and are potential for practical
applications.
Acknowledgment This work was supported by the Program for New Century Excellent Talents in University (NCET–12–0787), the Beijing Nova Programme (Z131109000413013),
Page 17 of 32
the National Natural Science Foundation of China (51308045), the Fundamental Research Funds for the Central Universities (TD–JC–2013–3, BLYJ201509), and the Project Sponsored by the Scientific Research Foundation for the Returned Overseas
ip t
Chinese Scholars, State Education Ministry.
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References
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[28] Q. Wang, H.H. Tay, L.W. Chen, Y. Liu, J. Chang, Z.Y. Zhong, J.Z. Luo, A. Borgna, J. Nanoeng. Nanomanuf., 1 (2011) 298. [29] Y.J. Wu, P. Li, J.G. Yu, A.F. Cunha, A.E. Rodrigues, Chem. Eng. Technol., 36
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(2013) 567. [30] N.N.A.H. Meis, J.H. Bitter, K.P. de Jong, Ind. Eng. Chem. Res., 49 (2010) 1229.
cr
[31] A. Garcia-Gallastegui, D. .Iruretagoyena, M. Mokhtar, A.M. Asiri, S.N. Basahel,
S.A. Al-Thabaiti, A.O. Alyoubi, D. Chadwick, M.S.P. Shaffer, Chem. Mater., 24 (2012)
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Commun., 50 (2014) 10130.
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[39] J.L. Gunjakar, I.Y. Kim, J.M. Lee, N.-S. Lee, S.-J. Hwang, Energy Environ. Sci., 6 (2013) 1008. [40] Y. Xing, L. Li, C. Chusuei, V. Hull, Langmuir, 21 (2005) 4185. [41] H. Wang, X. Xiang, F. Li, J. Mater. Chem., 20 (2010) 3944.
Page 20 of 32
[42] Z. Huang, P. Wu, B. Gong, Y. Fang, N. Zhu, J. Mater. Chem., 2 (2014) 5534. [43] Y. Ding, E. Alpay, Chem. Eng. Sci., 55 (2000) 3929.
Table
1.
The
te
d
M
an
us
cr
ip t
[44] Y. Ding, E. Alpay, Chem. Eng. Sci., 55 (2000) 3461.
chemical
composition
and
CO2
capture
capacity
of
Ac ce p
LDH-NO3-NS/OCNT, LDH-NO3/OCNT, and LDH-CO3/OCNT nanocomposites.
LDH/OC NT [V/V]
OCNT amount (wt%)
‒
CO2 capture capacity (mmol g 1)
LDH-NO3-NS/OCNT
LDH-NO3/OCNT
LDH-CO3/OCNT
LDH
0
0.24
0.24
0.46
1: 3
23.1
0.35
0.37
0.42
1: 1.5
13.0
0.32
0.41
0.36
1: 1
9.1
0.43
0.43
0.45
1: 0.7
6.5
0.39
0.36
0.43
1: 0.5
4.8
0.37
0.32
0.40
Page 21 of 32
ip t cr us an M d te
Ac ce p
Table 2. Summary of the CO2 capture performance of LDH/OCNT nanocomposites and the similar CO2 adsorbents reported in literature. CO2 capture
Testing
Testing
capacity (mmol g 1)
Temperature (oC)
Time (min)
LDH-NO3
0.24
200
120
This work
LDH-CO3
0.46
200
120
This work
LDH-NS-NO3/OCNT
0.43
200
120
This work
LDH-NO3/OCNT
0.43
200
120
This work
LDH-CO3/OCNT
0.45
200
120
This work
LDH-CO3/OCNT
0.42
300
120
[32]
LDH-NS-NO3/GO
0.47
200
120
[34]
LDH-CO3/GO
0.45
300
120
[31]
LDH composite
‒
Ref.
Page 22 of 32
ip t cr us an M d te Ac ce p Figure 1. XRD diffraction patterns of (a) Mg-AL-NO3 LDH, and (b) Mg-AL-NO3-NS dispersion, (▼) sample holder.
Page 23 of 32
ip t cr us an M d te Ac ce p Figure 2. XRD patterns of (a) LDH-NO3 and LDH-NO3-NS/OCNT nanocomposite
Page 24 of 32
with different volume ratios, (b) VR = 1:0.5, (c) VR = 1:0.7, (d) VR = 1:1, (e) VR =
te
d
M
an
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cr
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1:1.5, (f) VR = 1:3 and (g) OCNT, (▼) sample holder.
Ac ce p
Figure 3. FE-SEM of (a) OCNT, (b) Mg-Al-NO3 LDH, (c) LDH-NO3-NS/OCNT
nanocomposite (VR = 1:1), and (d) high image of LDH-NO3-NS/OCNT nanocomposite (VR = 1:1).
Page 25 of 32
ip t cr us an M d te Ac ce p
Figure 4. HR-TEM images of (a) CNT, (b) OCNT, (c) LDH-NO3-NS, and (d)
LDH-NO3-NS/OCNT nanocomposite.
Page 26 of 32
ip t cr us an M d te Ac ce p Figure 5. XRD patterns of (a) LDH-NO3 and LDH-NO3/OCNT nanocomposites with different volume ratios, and (b) LDH-CO3 and LDH-CO3/OCNT nanocomposite with different volume ratios, (▼) sample holder.
Page 27 of 32
ip t cr us an M d te
Ac ce p
Figure 6. FE-SEM images of (a) LDH-NO3/OCNT nanocomposites, and (b)
LDH-CO3/OCNT nanocomposite.
Page 28 of 32
ip t cr us
an
Figure 7. HR-TEM images of (a) LDH-NO3/OCNT nanocomposites, and (b)
Ac ce p
te
d
M
LDH-CO3/OCNT nanocomposite.
Page 29 of 32
ip t cr us an M
Figure 8. The effect of adsorption temperature on the CO2 adsorption capacity of
Ac ce p
te
d
LDH-NO3-NS/OCNT nanocomposite.
Page 30 of 32
ip t cr us an M d te Ac ce p Figure 9. CO2 adsorption/desorption cycling tests of (a) LDH-NO3-NS/OCNT nanocomposite, (b) LDH-NO3/OCNT nanocomposite, and (c) LDH-CO3/OCNT nanocomposite.
Page 31 of 32
Graphical abstract
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nanocomposites for CO2 capture
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Layered double hydroxides/oxidized carbon nanotube
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Junya Wang, Liang Huang, Qianwen Zheng, Yaqian Qiao, Qiang Wang*
College of Environmental Science and Engineering, Beijing Forestry University, 35
Ac ce p
te
d
M
an
Qinghua East Road, Haidian District, Beijing 100083, P. R. China
Page 32 of 32
S1226-086X(16)00072-1 http://dx.doi.org/doi:10.1016/j.jiec.2016.02.010 JIEC 2832
To appear in: Received date: Revised date: Accepted date:
13-7-2015 13-2-2016 15-2-2016
Please cite this article as: J. Wang, L. Huang, Q. Zheng, Y. Qiao, Q. Wang, Layered double hydroxides/oxidized carbon nanotube nanocomposites for CO2 capture, Journal of Industrial and Engineering Chemistry (2016), http://dx.doi.org/10.1016/j.jiec.2016.02.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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nanocomposites for CO2 capture
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Layered double hydroxides/oxidized carbon nanotube
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Junya Wang, Liang Huang, Qianwen Zheng, Yaqian Qiao, Qiang Wang*
College of Environmental Science and Engineering, Beijing Forestry University, 35
te
d
M
an
Qinghua East Road, Haidian District, Beijing 100083, P. R. China
Ac ce p
*Corresponding author: Professor Qiang Wang
College of Environmental Science and Engineering, Beijing Forestry University, 35 Qinghua East Road, Haidian District, Beijing 100083, P. R. China E-mail: [email protected]; [email protected]
Tel: +86 13699130626
Page 1 of 32
Highlights
Layered double hydroxide/oxidized carbon nanotube hybrid was prepared for
cr
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CO2 capture
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Electrostatic self-assembly and direct co-precipitaiton methods were compared
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The CO2 capture performance of LDH/OCNT nanocomposite was studied
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The introduction of OCNT improved the performance of LDH derived CO2
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Abstract
d
adsorbent
Ac ce p
A systematic investigation on the influence of synthesis method and chemical composition of LDH/oxidized carbon nanotube (LDH/OCNT) nanocomposites on their CO2 capture performance were performed. Three LDH/OCNT nanocomposites
were prepared using two methods: “electrostatic self-assembly” and “direct co-precipitaiton”. All LDH/OCNT nanocomposites were thoroughly characterized. The isothermal CO2 adsorption at different temperatures (60–400
o
C) and
adsorption/desorption cycling tests were conducted. Both the CO2 adsorption capacity and the multi-cycle stability of LDH derived adsorbents were improved by the introduction of OCNT. In particular, the absolute CO2 capture capacity of Mg-Al-NO3 LDH was increased by more than twice by adding 9.1 wt% OCNT. Keywords: CO2 adsorbent; hydrotalcite; sorption enhanced water gas shift; synthesis
Page 2 of 32
1. Introduction
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M
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method; stability
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CO2 is one of the major greenhouse gases (GHG) which continuous increases due to
Ac ce p
the large-scale burning of fossil fuels.[1, 2] It also has been predicted that fossil fuels will be the dominant energy source in the coming decades, and the amount of energy demand will increase further by 53% by 2030.[3] Therefore, the reduction of CO2
emission is highly demanded. For CO2 capture, store and utilize, several new
technologies have been developed,[4] and many kinds of solid CO2 adsorbents have
been synthesized.[5-9] One feasible step towards reducing CO2 emissions is to capture
the CO2 generated during combustion and store it in a suitable place.[10] Sorption enhanced water gas shift (SEWGS) is well known as a promising pre-combustion CO2 capture technology, which is a combination of WGS reaction and CO2 sorption, as shown in equation (1). Due to the existence of a solid CO2 adsorbent, the produced CO2 can be in-situ captured. And in the meantime, the conversion of CO and the
Page 3 of 32
production of H2 can be increased as well.[11-14] CO (g) + H2O (g) + adsorbent (s) ↔ adsorbent-CO2 (s) + H2 (g)
(1)
The key to ensure the success of this process is choosing a suitable CO2
ip t
capturing material. Among various CO2 adsorbents for SEWGS, layered double hydroxides (LDHs) derived mixed oxides are one of the best solid CO2 adsorbent,
formula of LDH is [M1−x
2+
M
3+ x
cr
which showed good performance in the temperature range of 200–400 oC. The general (OH)2][An-]x/n·ZH2O, where M2+ and M3+ are
us
divalent (Mg2+, Zn2+, Ni2+, etc.) and trivalent cations (Al3+, Ga3+, Fe3+, Mn3+, etc.),
an
respectively. This compound consists of positively charged brucite-like layers with an interlayer region containing charge compensating anions and solvation molecules.[3]
M
LDH as the CO2 adsorbents had been widely investigated in recent years. For instance, the effects of divalent cations,[15] trivalent cations,[16] charge compensating
d
anions,[17-19] Mg/Al ratio,[20] synthesis method,[3] the presence of SO2 and H2O,[14,
te
21] particle size,[22, 23] alkali metal (K, Cs) doping,[18, 23-29] the uploading of LDHs on supports,[30-32] and operational pressure[33] on CO2 adsorption have all
Ac ce p
been reported.
Recently, LDH based hybrid materials become promising materials for CO2
capture, for instance, LDH/carbon nanotube (LDH/CNT) and LDH/graphene oxide (LDH/GO) hybrids, etc. In general, the synthesis method can be divided into two general
strategies.
One
is
one-pot
co-precipitation
method,
by
which
Garcia-Gallastegui et al.[31] synthesized a LDH/GO hybrid for CO2 capture, with
improved CO2 adsorption capacity and recyclability. They also used multi-walled carbon nanotubes (MWCNTs) as support to synthesize LDH/MWCNTs hybrid as CO2 adsorbent, by which the absolute CO2 capture capacity and the stability of LDH were improved.[32] The other synthesis method is based on the electrostatically
Page 4 of 32
driven self-assembly between the delaminated positively charged LDH single sheets and the negatively charged monolayer. Wang et al.[34] synthesized Mg-Al-NO3 LDH-NS/GO nanocomposite via electrostatic self-assembly from exfoliated LDH and
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GO nanosheet dispersions as novel high-temperature CO2 adsorbent, which showed good CO2 capture capacity and good CO2 adsorption/desorption cycling performance.
cr
However, the LDH/CNT synthesized by the self-assembly method has not been studied for CO2 capture.
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In this paper, we performed a comprehensive and comparative study on the
an
influence of synthesis method and the chemical composition of LDH/oxidized carbon nanotube (LDH/OCNT) nanocomposites on their CO2 capture performance. First, two
M
synthesis methods including “exfoliation‒self-assembly” and “direct co-precipitation” were used for the preparation of nanocomposites. By varying the key synthesis
d
parameters, a series of nanocomposites with different chemical compositions were
te
synthesized. All samples were then thoroughly characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy
Ac ce p
(TEM), etc. Finally, the isothermal CO2 capture capacity and the CO2 adsorption/desorption cycling performance were evaluated in detail. We believe that the existence of CNTs can not only improve the dispersion of LDHs and accessibility of gas, but also make it easier to pelletize the LDH-based CO2 adsorbents, which are
crucial for its practical applications.
2. Experimental 2.1 Synthesis of samples 2.1.1 Synthesis of Mg-Al-NO3 LDH-NS A salt solution (100 ml) containing a mixture of 19.2 g Mg(NO3)2·6H2O and 9.4 g
Page 5 of 32
Al(NO3)3·9H2O was added drop-wise to a basic solution (100 ml) containing 4.3 g NaNO3 in a 500 ml round bottom flask. The pH of the mixture solution was kept constant at 10 by addition of a NaOH solution (3.4 M). The resulting mixture was
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separated into two portions and hydrothermally treated in 200 ml Teflon lined stainless autoclaves at 120 oC for overnight. After hydrothermal aging, the sample
cr
was filtered and washed with deionized water until pH = 7, then dried at 100 oC in an oven. The obtained sample is Mg-Al-NO3 LDHs. The delamination of Mg-Al-NO3
us
LDHs into single nanosheets was performed as follows. 0.5 g Mg-Al-NO3 LDHs were
an
put into 50 mL formamide in a 100 ml beaker, followed by magnetic stirring till no sediment was observed upon standing. The concentration of delaminated Mg-Al-NO3
M
LDHs dispersion was determined to be 10 g L‒1, which is marked Mg-Al-NO3 LDH-NS.
d
2.1.2 Synthesis of OCNT
te
Oxidization of carbon nanotubes was achieved by a modified Hummers method. 6 g CNTs were first purified by calcination at 500 oC for 1 h and washed with 70 mL of
Ac ce p
10 wt% HCl to remove metal residues. The products were then filtered, washed and lyophilized. After that, it was put together with 1.5 g sodium nitrate into the 70 mL concentrated H2SO4 with stirring in a 1000 ml beaker. Then 9 g KMnO4 was added
gradually with stirring while keeping the temperature of the mixture below 20 oC with
ice-bath. The mixture was then stirred at 35 oC for 30 min, followed by the addition of some 140 mL distilled water. After another 30 min stirring, 420 mL distilled water was then added to terminate reaction. Subsequently, 30 wt% H2O2 was added. After 10 min, the mixture was centrifuged and washed with 10 wt% HCl solution to remove residual metal ions. The precipitate was then washed with Milli-Q water and centrifuged repeatedly until the solution became neutral, then dried at 100 oC in an
Page 6 of 32
oven. The obtained sample is oxidized carbon nanotube. 2.1.3 Synthesis of nanocomposites using “exfoliation‒self-assembly” method The nanocomposites synthesized using the “exfoliation‒self-assembly” method was
ip t
denoted as LDH-NO3-NS/OCNT nanocomposites, which contain exfoliated Mg-Al-NO3 LDH nanosheets and OCNT. The synthesis procedure is described briefly
cr
as follows. The positively charged LDH-NO3-NS suspension (10 g L‒1) was obtained by the delamination of Mg-Al-NO3 LDH in formamide, and the negatively charged
us
OCNT suspension (1 g L‒1) was prepared by dispersing OCNT powders in water by
an
sonication treatment. Then the LDH-NO3-NS/OCNT nanocomposites were prepared by mixing the above two suspensions together. Due to the opposite charge these two
M
suspensions, LDH-NO3-NS/OCNT nanocomposites can be formed by electrostatic self-assembly. Six different LDH-NO3-NS/OCNT nanocomposites were prepared by
d
varying the LDH-NO3-NS:OCNT volume ratios (VR) of 1: 0.5, 1: 0.7, 1: 1, 1: 1.5 and
te
1: 3. The weight ratios between LDH and OCNT were listed in Table 1. The precipitants were centrifuged under 8000 rpm for 10 min, followed by washing with
Ac ce p
Milli-Q water and anhydrous methanol repeatedly to remove formamide and water completely. The nanocomposites were finally dried at 100 oC for 24 h. 2.1.4 Synthesis of nanocomposites using “direct coprecipitation” method The nanocomposites synthesized using the “direct coprecipitation” method were denoted as Mg-Al-NO3/OCNT and Mg-Al-CO3/OCNT nanocomposites. The
synthesis procedure is described briefly as follows. In brief, a salt solution (100 ml) containing a mixture of 19.2 g Mg(NO3)2·6H2O and 9.4 g Al(NO3)3·9H2O was added
drop-wise to a basic solution (100 ml) containing NaNO3 (or Na2CO3) and different mass of highly dispersed OCNT (1 g L‒1) in a 500 ml round bottom flask (The mass of NaNO3 + OCNT = 4.3 g, Na2CO3 + OCNT = 2.7 g). The pH of the mixture solution
Page 7 of 32
was kept constant at 10 by addition of a NaOH solution (3.4 M). The resulting mixture was stirring overnight at room temperature. Then the sample was filtered and washed with deionized water until pH = 7, then dried at 100 oC in an oven. Depending
ip t
on the anions used (NO3‒ and CO32‒), LDH-NO3/OCNT and LDH-CO3/OCNT nanocomposites can be obtained. And the mass ratio of OCNT in the LDH/OCNT
cr
nanocomposite was calculated by the formula of LDH. 2.2. Characterization of samples
us
Powder XRD analyses were conducted on a Shimadzu XRD-7000 X-ray diffractometer with Cu Kα radiation. Diffraction patterns were recorded within the
an
range of 2θ = 5–65o with a step size of 0.02o. The morphologies of samples were
M
characterized by field emission scanning electron microscope (FE-SEM, SU-8020). High resolution transmission electron microscopy (HR-TEM) images were obtained
d
on a JEOL 2010, operating at 200 kV. Samples were prepared by dispersing the
te
sample in isopropanol using 0.01 mg of sample per mL of solvent, and allowing a drop to dry onto a holey carbon copper grid (300 mesh, Agar Scientific). The zeta
Ac ce p
potentials of delaminated LDHs and OCNTs were measured using zeta potential analyzer (ZPA, Nanosizer Nano ZS, Malvern Instruments). 2.3 Evaluation of CO2 capture capacity Thermogravimetric adsorptions of CO2 on the samples were measured using a Q50
TGA analyzer. Samples were first calcined at 400 oC for 5 h in N2 (60 ml min‒1)
before performing adsorption. To avoid the error caused by the memory effect, the test was carried out immediately after the first calcination. And the samples were further calcined in situ at 400 oC for 1 h in N2 (60 ml min‒1) before adsorption. CO2 adsorption experiments were carried out at 1 atm with a constant flow of CO2 (20 ml
min‒1) for 2 h. The regeneration and stability of the adsorbents were assessed by
Page 8 of 32
adsorption/desorption cycling tests, in which the adsorption step was carried out at 200 oC for 30 min with a constant flow of pure CO2 (20 ml min‒1), and the desorption
ip t
was performed at 400 oC for 30 min with a constant flow of pure N2 (60 ml min‒1).
3. Results and discussion
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3.1 Synthesis and characterization of LDH-NS/OCNT nanocomposites
The Mg-Al-NO3 LDHs were synthesized using hydrothermal method. Powder XRD
us
data confirmed that the synthesized Mg-Al-NO3 LDHs was phase pure, which had a
an
hydrotalcite-type structure, as shown in Figure 1(a). However, after the Mg-Al-NO3 LDH powder was put in formamide, and stirred for several hours, a clear and
M
transparent colloidal dispersion was obtained. The XRD pattern of delaminated LDH in Figure 1(b) shows that the characteristic diffractions of LDH structure disappeared,
d
with only a halo at 2θ = 20–30o came from formamide. The absence of sharp basal
te
peaks clearly suggested that the host sheets were not in parallel to induce interference of the X-rays, which confirmed that the LDH had been delaminated into individual
Ac ce p
nanosheets.[35]
Zeta potential is often used as an index of the magnitude of electrostatic
interaction between colloidal particles, which is thus a measure of the colloidal stability of the solution.[36] The zeta potential of exfoliated Mg-Al-NO3 LDH
nanosheet colloidal dispersion and the OCNT dispersion were measured, which were +27.1 and –45.8 mV, respectively. This data suggested that both of these two dispersions were stable and well-dispersed. Due to the opposite charge of the exfoliated LDH and the OCNT dispersions, the LDH-NO3-NS/OCNT nanocomposite was synthesized by electrostatic self-assembly of Mg-Al-NO3 LDH and OCNT nanosheets. It was found that the colloidal
Page 9 of 32
suspension of LDH was broken down, and the sediment was immediately formed once the LDH-NO3-NS dispersion was added to the OCNT dispersion. This behavior assures the strong interaction between LDH nanosheets and OCNT. The
ip t
nanocomposite was easily obtained by separating with centrifugation. The negatively charged OCNT is complementary to the positive charge of the delaminated LDH
cr
nanosheets and is likely to contribute to the stabilization of the growing nanocomposite. The chemical interaction between the LDH and GO has been also
us
observed in a similar hybrid LDH/GO.[34]
an
The XRD patterns of pristine LDH-NO3, OCNT and LDH-NO3-NS/OCNT nanocomposites are shown in Figure 2. For the LDH-NO3-NS/OCNT nanocomposites,
M
the characteristic reflections of Mg-Al-NO3 LDH were clearly observed, which can be indexed to (003), (006) and (110) planes.[37] Figure 2(g) shows the XRD of OCNT,
d
in which the 2θ peak at 26o can be indexed to the (002) plane of graphitic carbon, with
te
an interplanar distance of 0.337 nm (JCPDS No. 41-1487). However, when the VR is below 1:1.5, the typical diffraction peak of OCNTs can not be noticed in the
Ac ce p
nanocomposites, suggesting that the OCNT might be highly dispersed. And the small amount of OCNT imbedded into the LDH did not impede the growth of LDH crystals.[38] While, with the increase of the OCNT amount added in nanocomposites (VR of 1:1.5 and 1:3), the peak which is indexed to the (002) plane of graphitic carbon appeared, confirming the obvious existence of OCNT. Furthermore, with more OCNTs were added to the nanocomposites, the (00l) peaks of LDHs were weakened,
corresponding to a lower crystallinity of LDHs. And their obvious broadening of the diffraction peaks indicates smaller LDH particle size in LDH-NO3-NS/OCNT nanocomposites. This data suggests that the addition of OCNTs could improve the dispersion of LDH in the nanocomposites.
Page 10 of 32
The size, morphology and structure of nanocomposites were characterized using FE-SEM analysis. It can be clearly seen from Figure 3(a) that the OCNTs are
multi-walled nanotubes with outer diameter of about 15‒20 nm. And the LDH-NO3
ip t
showed a typical morphology of layered structures, consisting of aggregates of flake-like particles. However, Figure 3(c) and 3(d) show the SEM images of
cr
LDH-NO3-NS/OCNT nanocomposite, which is composed of numerous LDH nanosheets and OCNT intercrossed with each other, forming a house-of-cards-type
(Figure
3(b))
with
the
flake-like
us
stacking structure. Moreover, it is obviously different from the pristine LDH-NO3 morphology.
The
LDH-NO3-NS/OCNT
an
nanocomposites are more porous comparing to neat LDHs, which is favorable for
M
CO2 adsorption.[39]
In order to further explore the morphology and structure of the nanocomposite,
d
the samples were also characterized using HR-TEM analysis. The TEM image in
te
Figure 4(a) shows the pristine CNTs with an average diameter of 10‒20 nm. However, after surface oxidation, the surface of the CNTs became heterogeneous due to the
Ac ce p
attack by the oxidative acid. It can be clearly seen that the smooth surface of the pristine CNTs (Figure 4(a)) was changed to a much rougher surface (Figure 4(b)). It has been reported that the rough surface of OCNT was attributed to the acidic etching of CNT surface.[40] In Figure 4(c), it is clearly shows that the delaminated LDH nanosheets have a translucent plate-like morphology, which further proved that the LDH has been completely delaminated into individual nanosheets.[35] The HR-TEM image of LDH-NO3-NS/OCNT nanocomposite in Figure 4(d) obviously indicated that the OCNT was tightly absorbed and randomly distributed on the surface of LDH nanosheets or interactied with LDH nanosheets, which is agreement with the result of FE-SEM. These result further indicated the strong interfacial electrostatic interaction
Page 11 of 32
between negatively charged functional OCNT and positively charged layers of LDH. The reason is that carboxylic acid groups (COOH) on the surface of OCNT exist as carboxylate anions (COO‒) in the alkaline solution, thus yielding negatively charged
ip t
OCNT, and the surface of LDH nanosheets are positively charged.[41] Therefore, the negatively charged 1D OCNT is easily attached to the positively charged surface of
cr
2D LDH nanosheet, which can form a stable 3D nanostructure under the dominance
of charge attractions,[37] similar to the LDH/GO nanocomposite that we previously
us
reported.[34]
an
3.2 Synthesis and characterization of LDH-NO3/OCNT and LDH-CO3/OCNT nanocomposites
M
LDH-NO3/OCNT and LDH-CO3/OCNT were synthesized using a one-pot “direct co-precipitation” method. For all samples, the characteristic peaks of LDHs were
d
clearly seen, suggesting the successful formation of LDHs. During the synthesis,
te
certain amount of OCNT powders was directly dispersed in the mixed salt solution. Unlike the electrostatic driven self-assembly method, negatively-charged OCNT
Ac ce p
played an important role in the LDH growth process in co-precipitation method.[32] The XRD patterns of LDH-NO3, LDH-CO3, and their nanocomposites with OCNT
are shown in Figure 5. With the VR of OCNT increasing gradually in the nanocomposites (VR = 1: 1.5 and 1: 3), the characteristic peak OCNT started to be observed. Comparing to the peaks from LDH, the intensity of the peak from OCNT is very weak, which suggests that the OCNT is highly dispersed within the nanocomposites. The presence of OCNT during the LDH growth may introduce defects into the LDH structure, through modified nucleation conditions, or induced curvature, which is favorable for CO2 capture.[32] The
FE-SEM
images
of
LDH-NO3/OCNT
and
LDH-CO3/OCNT
Page 12 of 32
nanocomposites are shown in Figure 6. The morphology and structure of the nanocomposites synthesized from the “direct coprecipitation” method is similar to that from “exfoliation‒self-assembly” method. As it is well known that pure
ip t
Mg-Al-CO3 LDH generally forms sand-rose like morphology, while Figure 6(b) indicates that the LDH-CO3/OCNT nanocomposite is also LDH nanosheets and
cr
OCNT intercrossed with each other. The HR-TEM images of LDH-NO3/OCNT and LDH-CO3/OCNT nanocomposites (Figure 7) also showed that they formed a stable
us
3D nanostructure during the synthesis process, the OCNT can be seen clearly in the
an
composites which interspersed among the LDHs. Therefore the XRD, SEM, and TEM analyses all indicated that the synthesis method has little influence on the morphology
M
and structure of the formed nanocomposites. For the samples prepared from “direct
coprecipitation” method, the presence of OCNT also can improve the dispersion of
d
LDH.
te
3.3 Comparison of CO2 capture performance The CO2 capture capacity of the above mentioned three types of nanocomposites
Ac ce p
including LDHl-NO3-NS/OCNT, LDH-NO3/OCNT, and LDH-CO3/OCNT were
evaluated using isothermal CO2 adsorption tests. All the samples were first calcined at 400 oC for 5 h before each CO2 adsorption test. Then the thermo-gravimetric
adsorptions of CO2 on the samples were measured at 200 oC using a Q50 TGA analyzer.
In the present work, we are particularly interested in whether the two different synthesis methods would influence the CO2 adsorption capacity of nanocomposite or not. The CO2 adsorption capacities of pure LDH and its nanocomposites with OCNT are listed in Table 1. We can see that the LDH-NO3-NS/OCNT nanocomposite had better
adsorption
capacity
than
that
without
OCNT.
Moreover,
the
LDH-NO3-NS/OCNT nanocomposite with 9.1 wt% OCNT showed the maximum
Page 13 of 32
adsorption capacity, which is more than twice larger than that of pure Mg-Al-NO3 LDH. Similarly, the CO2 adsorption capacity of LDH-NO3/OCNT nanocomposite synthesized from “direct coprecipitation” method was also significantly improved by
ip t
the addition of OCNT, and the optimal OCNT weight loading was also 9.1 wt%. The enhancement in adsorption capacity in the presence of OCNT might be attributed to
cr
the highly dispersion and stabilization of LDH nanosheets.[42] With 9.1 wt% of
OCNT, the geometric and electrostatic compatibility between LDH single sheets and
us
OCNT appears to favor heterogeneous nucleation, dispersion, and stabilization,[31]
an
which is agreement with the results reported before.[34] The other possible reason is the presence of OCNT during LDH growth, may also contribute positively to the
M
adsorption capacity, by creating more active sites. Meis et al.[30] attributed the increase in CO2 adsorption capacity to a greater number of LDH edge sites when
d
supported. For LDH-CO3/OCNT nanocomposite, its CO2 capture capacity was
te
slightly decreased comparing to neat Mg-Al-CO3 LDH. Although no improvement was observed in the CO2 capture capacity, it is believed that the dispersion of the
Ac ce p
LDH on the OCNT support is likely to mitigate sintering problem during calcination leading to a more active and stable material.[32] In order to integrate the composite into certain processes, we choose
LDH-NO3-NS/OCNT nanocomposite as a sample to investigate its CO2 adsorption
capacity performance at different temperatures, as shown in Figure 8. The results showed that the maximum CO2 capture capacity took place at 60 oC (0.89 mmol g‒1). And the CO2 adsorption capacity decreased with the increase in adsorption temperature, which is in agreement with the result of the LDH/GO nanocomposite.[34] The LDH-NO3-NS/OCNT nanocomposite also showed good CO2 capture capacity in a wide temperature range, which makes it a promising CO2 adsorbent not only for the
Page 14 of 32
SEWGS process but also for the post-combustion CO2 capture from flue gases. 3.4 CO2 adsorption/desorption cycling test In addition to the CO2 capture capacity, the continuous CO2 adsorption/desorption
ip t
cycling stabilities of LDH-NS/OCNT and LDH/OCNT nanocomposites (VR = 1: 1)
cr
were also evaluated. The CO2 adsorption/desorption cycling tests were performed in a
typical temperature swing adsorption (TSA) mode, as shown Figure 9. The adsorption
us
was performed at 200 oC for 30 min with pure CO2, and the desorption was performed
an
at 400 oC for 30 min with pure N2. For all these three adsorbents, the CO2 capture capacity showed a relatively obvious decrease in the second cycle, and a very slow
M
decline in the following 22 cycles. For LDH-NO3-NS/OCNT nanocomposite, the CO2 capture capacity dropped from 0.38 to 0.32 mmol g‒1 from the first cycle to the
d
second cycle, and kept almost constant from the 14th cycle, with a value of 0.3 mmol
te
g‒1. For LDH-NO3 OCNT nanocomposite, the CO2 capture capacity dropped from
Ac ce p
0.37 to 0.34 mmol g‒1 from the first cycle to the second cycle, and kept almost constant from the 6th cycle, with a value of 0.32 mmol g‒1. For LDH-CO3/OCNT
nanocomposite, the CO2 capture capacity dropped from 0.4 to 0.38 mmol g‒1 from the
first cycle to the second cycle, and kept almost constant from the 14th cycle, with a value of 0.34 mmol g‒1. This trend is consistent with that from pure LDH-based CO2 adsorbent, which have attributed the first stage to a small amount of CO2 irreversibly chemisorbed on the LDO.[10, 32, 43, 44] From Figure 9(a) and (b), it can be seen that the CO2 adsorption capacities of the nanocomposites were still much higher than that of the pure Mg-Al-NO3 LDH after 22 cycles. And LDH-CO3/OCNT nanocomposite
Page 15 of 32
also showed good CO2 capture capacity even after 22 cycles. This result demonstrated that
these
LDH
based
nanocomposites
with
OCNT
have
good
CO2
adsorption/desoprtion cycling stability and are potential for practical applications.
ip t
Another potential advantage of the LDH/OCNT nanocomposites type CO2 adsorbent
cr
is that they are much easier to be pelletized than pure LDH. And the mechanical and
us
thermal stability of such obtained LDH/OCNT pellets will be much higher than that of pure LDH, which make it very promising for the practical SEWGS applications.
an
Furthermore, we also compared our work with literature reports. The CO2 capture performances of neat LDH derived and LDH-based hybrids derived
M
adsorbents are summarized in Table 2. It can be seen that the CO2 capture capacities were both promoted by introduced GO or OCNT, indicating that the structural
te
d
disorder of the materials may enhance the adsorbent/gas contact. Table 2 shows that the CO2 capture capacity of LDH-CO3/OCNT nanocomposites was comparable to
Ac ce p
literature reports. What’s more, it is believed that the mechanical property of the LDH derived adsorbents can be significantly increased by introducing OCNT. This hybrid type CO2 adsorbents are easier to be pelletized and can prevent the “pasting” problem. Also CNTs are much cheaper than GO and have more defined morphology typically with small diameter, high mechanical strength and hence a higher accessible surface area to act as a support.[32, 38] In this contribution, we demonstrated LDH/OCNT nanocomposites will be a promising adsorbent for CO2 capture. Conclusions A comprehensive and comparative study on the influence of synthesis method and
Page 16 of 32
chemical
composition
of
LDH/oxidized
carbon
nanotube
(LDH/OCNT)
nanocomposites on their CO2 capture performance was conducted. Three types of LDH/OCNT nanocomposites including Mg-Al-NO3-NS/OCNT, Mg-Al-NO3/OCNT,
ip t
and Mg-Al-CO3/OCNT were prepared using “electrostatic self-assembly” and “direct
cr
co-precipitaiton” methods. XRD, SEM, and TEM analyses revealed that the synthesis
us
method has little influence on the morphology and structure of the formed nanocomposites. In all three nanocomposites, the OCNT was tightly absorbed and
an
randomly distributed on the surface of LDH nanosheets or interactied with LDH nanosheets, which improves the dispersion of LDH in the nanocomposites. For
M
LDH-NO3-NS/OCNT and LDH-NO3/OCNT, the optimal OCNT weight loading was also 9.1 wt% and the CO2 capture capacities were significantly improved, which is
te
d
more than twice larger than the pure Mg-Al-NO3 LDH. For Mg-Al-CO3/OCNT nanocomposite, the addition of OCNT resulted in negligible influence on its CO2
Ac ce p
capture capacity, which is ca. 0.35–0.45 mmol g‒1. The CO2 adsorption capacity of
LDH-NO3-NS/OCNT nanocomposite increased with the decrease in adsorption temperature, and the maximum CO2 capture capacity of 0.89 mmol g‒1 was obtained
at 60 oC. The results also demonstrated that these LDH/OCNT nanocomposites have good CO2 adsorption/desoprtion cycling stability and are potential for practical
applications.
Acknowledgment This work was supported by the Program for New Century Excellent Talents in University (NCET–12–0787), the Beijing Nova Programme (Z131109000413013),
Page 17 of 32
the National Natural Science Foundation of China (51308045), the Fundamental Research Funds for the Central Universities (TD–JC–2013–3, BLYJ201509), and the Project Sponsored by the Scientific Research Foundation for the Returned Overseas
ip t
Chinese Scholars, State Education Ministry.
cr
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[42] Z. Huang, P. Wu, B. Gong, Y. Fang, N. Zhu, J. Mater. Chem., 2 (2014) 5534. [43] Y. Ding, E. Alpay, Chem. Eng. Sci., 55 (2000) 3929.
Table
1.
The
te
d
M
an
us
cr
ip t
[44] Y. Ding, E. Alpay, Chem. Eng. Sci., 55 (2000) 3461.
chemical
composition
and
CO2
capture
capacity
of
Ac ce p
LDH-NO3-NS/OCNT, LDH-NO3/OCNT, and LDH-CO3/OCNT nanocomposites.
LDH/OC NT [V/V]
OCNT amount (wt%)
‒
CO2 capture capacity (mmol g 1)
LDH-NO3-NS/OCNT
LDH-NO3/OCNT
LDH-CO3/OCNT
LDH
0
0.24
0.24
0.46
1: 3
23.1
0.35
0.37
0.42
1: 1.5
13.0
0.32
0.41
0.36
1: 1
9.1
0.43
0.43
0.45
1: 0.7
6.5
0.39
0.36
0.43
1: 0.5
4.8
0.37
0.32
0.40
Page 21 of 32
ip t cr us an M d te
Ac ce p
Table 2. Summary of the CO2 capture performance of LDH/OCNT nanocomposites and the similar CO2 adsorbents reported in literature. CO2 capture
Testing
Testing
capacity (mmol g 1)
Temperature (oC)
Time (min)
LDH-NO3
0.24
200
120
This work
LDH-CO3
0.46
200
120
This work
LDH-NS-NO3/OCNT
0.43
200
120
This work
LDH-NO3/OCNT
0.43
200
120
This work
LDH-CO3/OCNT
0.45
200
120
This work
LDH-CO3/OCNT
0.42
300
120
[32]
LDH-NS-NO3/GO
0.47
200
120
[34]
LDH-CO3/GO
0.45
300
120
[31]
LDH composite
‒
Ref.
Page 22 of 32
ip t cr us an M d te Ac ce p Figure 1. XRD diffraction patterns of (a) Mg-AL-NO3 LDH, and (b) Mg-AL-NO3-NS dispersion, (▼) sample holder.
Page 23 of 32
ip t cr us an M d te Ac ce p Figure 2. XRD patterns of (a) LDH-NO3 and LDH-NO3-NS/OCNT nanocomposite
Page 24 of 32
with different volume ratios, (b) VR = 1:0.5, (c) VR = 1:0.7, (d) VR = 1:1, (e) VR =
te
d
M
an
us
cr
ip t
1:1.5, (f) VR = 1:3 and (g) OCNT, (▼) sample holder.
Ac ce p
Figure 3. FE-SEM of (a) OCNT, (b) Mg-Al-NO3 LDH, (c) LDH-NO3-NS/OCNT
nanocomposite (VR = 1:1), and (d) high image of LDH-NO3-NS/OCNT nanocomposite (VR = 1:1).
Page 25 of 32
ip t cr us an M d te Ac ce p
Figure 4. HR-TEM images of (a) CNT, (b) OCNT, (c) LDH-NO3-NS, and (d)
LDH-NO3-NS/OCNT nanocomposite.
Page 26 of 32
ip t cr us an M d te Ac ce p Figure 5. XRD patterns of (a) LDH-NO3 and LDH-NO3/OCNT nanocomposites with different volume ratios, and (b) LDH-CO3 and LDH-CO3/OCNT nanocomposite with different volume ratios, (▼) sample holder.
Page 27 of 32
ip t cr us an M d te
Ac ce p
Figure 6. FE-SEM images of (a) LDH-NO3/OCNT nanocomposites, and (b)
LDH-CO3/OCNT nanocomposite.
Page 28 of 32
ip t cr us
an
Figure 7. HR-TEM images of (a) LDH-NO3/OCNT nanocomposites, and (b)
Ac ce p
te
d
M
LDH-CO3/OCNT nanocomposite.
Page 29 of 32
ip t cr us an M
Figure 8. The effect of adsorption temperature on the CO2 adsorption capacity of
Ac ce p
te
d
LDH-NO3-NS/OCNT nanocomposite.
Page 30 of 32
ip t cr us an M d te Ac ce p Figure 9. CO2 adsorption/desorption cycling tests of (a) LDH-NO3-NS/OCNT nanocomposite, (b) LDH-NO3/OCNT nanocomposite, and (c) LDH-CO3/OCNT nanocomposite.
Page 31 of 32
Graphical abstract
cr
nanocomposites for CO2 capture
ip t
Layered double hydroxides/oxidized carbon nanotube
us
Junya Wang, Liang Huang, Qianwen Zheng, Yaqian Qiao, Qiang Wang*
College of Environmental Science and Engineering, Beijing Forestry University, 35
Ac ce p
te
d
M
an
Qinghua East Road, Haidian District, Beijing 100083, P. R. China
Page 32 of 32