Reduction of aromatic nitro compounds over Ni nanoparticles confined in CNTs

Journal Pre-proof Reduction of aromatic nitro compounds over Ni nanoparticles confined in CNTs Yingmin Qu, Guodong Xu, Jiahao Yang, Zhongshen Zhang

PII:

S0926-860X(19)30466-1

DOI:

https://doi.org/10.1016/j.apcata.2019.117311

Reference:

APCATA 117311

To appear in:

Applied Catalysis A, General

Received Date:

26 June 2019

Revised Date:

7 October 2019

Accepted Date:

20 October 2019

Please cite this article as: Qu Y, Xu G, Yang J, Zhang Z, Reduction of aromatic nitro compounds over Ni nanoparticles confined in CNTs, Applied Catalysis A, General (2019), doi: https://doi.org/10.1016/j.apcata.2019.117311

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Reduction

of

aromatic

nitro

compounds

over

Ni

nanoparticles confined in CNTs

Yingmin Qu, a * Guodong Xu, b Jiahao Yang, b Zhongshen Zhang a*

National Engineering Laboratory for VOCs Pollution Control Material & Technology Research

of

a

Academy of Sciences, Huairou District, Beijing, 101408, China

School of Chemistry and Environmental Engineering, Yancheng Teachers University, Yancheng,

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b

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Center for Environmental Material and Pollution Control Technology, University of Chinese

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224002, China.

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*Corresponding author. Tel.: +86 13914640967.

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E–mail: [email protected]; [email protected]

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Graphical Abstract

1

Highlights 

The synergetic confinement effect of Ni/CNTs(c) made Ni reduction easier and retarded the oxidation.



Ni/CNTs(c) exhibited excellent catalytic performance with k of 1.475 min-1.



The conversion efficiency over Ni/CNTs(c) maintained over 95.5% after 10 cycles.



The catalytic mechanism for 4-NP reduction was provided.

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Abstract

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Ni within carbon nanotubes (CNTs) channels (Ni/CNTs(c)) and located on the outer CNTs surface (Ni/CNTs(d)) were prepared to investigate the effect of the

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interaction between Ni and CNTs on the catalytic performance of Ni/CNTs for

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aromatic nitro compounds reduction. The characterizations illustrated that the reduction of NiO within the CNTs nanotubes was easier than that of those particles

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located on the outer CNTs surface. Ni/CNTs(c) exhibited superior catalytic performance to Ni/CNTs(d). For Ni/CNTs(c), 1.475 min-1 of the rate constant k was

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obtained, and the conversion efficiency still maintained over 95.5% after 10 cycles. The high catalytic activity and the excellent stability of Ni/CNTs(c) were attributed to the synergetic confinement effect of Ni particles inside CNTs, which made oxidized

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Ni easy reduction and prevent Ni nanoparticles from agglomeration.

Keywords: synergetic confinement effect, CNTs, Ni, reduction, aromatic nitro compounds

1. Introduction 2

4-nitrophenonol (4-NP), a kind of aromatic nitro compounds, has been considered as one of the toxic organic pollutants in industrial and agricultural waste water [1, 2]. An environment friendly and efficient conversion route of 4-NP is the catalytic reduction of 4-NP with sodium borohydride (NaBH4) to 4-aminophenol (4-AP) which is an important intermediate for the manufacture of aniline, antipyretic

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drugs and analgesic [3, 4]. This catalytic reduction of 4-NP is also one of the most widely used model reactions due to the easy measurement of both 4-NP and 4-AP by

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UV-Vis spectroscopy and no by-products are formed [5, 6]. Noble metals and

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non-noble metals have been employed to catalyze this reaction [7-12]. Although the noble metals are effective, the high cost and the loss of the noble metals during

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recycling process limits their applications. Among non-noble metals, magnetic

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nanoparticles like Fe, Co and Ni have attracted significant attention for their high catalytic activity, magnetic separability and low cost [13-15]. However, magnetic

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nanoparticles are easy agglomeration, which results in a remarkable decrease in their catalytic activity. An effective approach to avoid aggregation is to immobilize the nanoparticles on supports with high surface area. Compared to most of the supported Ni nanoparticles (Ni NPs), Ni supported on silica hollow microspheres (Ni/SiO2

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MHMs) exhibited superior catalytic performance for the reduction of 4-NP, attributed to the high specific surface area of silica support, the small size and high dispersion of Ni NPs [16]. However, the catalytic performance of the Ni-based catalysts was still low, which needed long reaction time [17, 18]. Therefore, it is of great significance to develop an efficient, environmental-friendly and stable catalyst. It suggested that 3

supports with high surface area and high electron conductivity could improve the catalytic activities of Ni-based catalysts [19]. Carbon especially carbon nanomaterials, such as carbon nanotubes (CNTs) have high surface area, excellent electron conductivity, chemical inertness and mechanical strength, which may be superior to other supports for the reduction of 4-NP. In

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addition, the well-defined hollow interior of CNTs leads to the π electron density shifting from the interior to the exterior surface. Therefore, the properties and

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behavior of substances and materials introduced into the channels of CNTs are

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different from those on the exterior walls of CNTs and in the bulk. The confinement effects and electronic effect of CNTs could affect the catalytic activity, particularly for

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reactions involving reduction [20-25]. Furthermore, it suggested that the channels of

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CNTs could use as nanoreactors, which also displayed special catalytic performance [21]. The Pd nanocatalyst inside the channels of CNTs exhibited superior activity and

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enantioselectivity to that of Pd nanocatalyst outside the channels, due to the enrichment of reactant, chiral modifier, and additive in the channels of CNTs [21]. Although many studies have reported different activities of CNTs-based catalysts for hydrogenation reactions, few studies have discussed the impact of the interaction of

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Ni with carbon nanotubes on the activity of Ni/CNTs for the reduction of aromatic nitro compounds. In the present, Ni encapsulated into CNTs (denoted as Ni/CNTs(c)) and deposited on the outer walls of CNTs (denoted as Ni/CNTs(d)) were prepared to investigate the effect of the interaction of Ni with carbon nanotubes on the catalytic 4

performance of Ni/CNTs for the 4-NP reduction. The stability of the catalysts was also studied.

2. Experimental 2.1. Materials CNTs (diameter, 20-30 nm; length, 0.5-2μm) were purchased from Chengdu

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Organic Chemicals. All chemicals and solvents employed were of analytical grade

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and used without further purification. 2.2. Synthesis of Ni-CNT catalysts

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The preparation of Ni/CNTs was referred to methods reported by Pan et al [22]. Raw CNTs were refluxed in 68 wt% nitric acid for 14 h at 140℃, then filtered and

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washed with deionized water, followed by drying at 60℃ for 12 h, the obtained

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CNTs were opened ends and denoted as CNTs-c. The CNTs-c was then immersed into the solution of Ni(NO3)2 under stirring to prepare Ni/CNTs(c) with 5% of Ni loading.

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The mixture was ultrasonic treatment for 0.5 h, and stirred for 12 h, followed by heating in air at 120℃ for 10 h. The resulting was reduced by N2H4·H2O. Generally, the resulting was dissolved in deionized water, and the mixture was heated at 80℃, an 1.0 M NaOH solution was added to adjust the pH of the solution to 12. Then

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N2H4·H2O ( Ni/N2H4·H2O molar ratio of 1:3 ) was added dropwise to the mixture and

heated at 80℃ for 4 h under stirring. The resultant was filtered and washed with

water and absolute ethanol successively, and kept in absolute ethanol. For the preparation of Ni/CNTs(d), CNTs with closed ends which were obtained by refluxing raw CNTs in 37 wt% nitric acid at 110℃ for 5 h were used. Then an 5

impregnation procedure was conducted similar to the above to disperse the active components on the outer surface of CNTs. 2.3. Catalyst Characterization Powder X-ray diffraction (XRD) pattern was performed on Philip X’Pert PRO MPD diffractometer using a radiation source of Cu Kα (λ=1.54056 Å). The

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morphologies of the samples were characterizaed by transmission electron microscopy (TEM, JEM-2100F). N2 adsorption/desorption were measured on a

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calculated by the Brunauer-Emmett-Teller (BET) method.

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Micromeritics ASAP 2020 PLUS HD88 instrument. The specific surface area was

The X-ray photoelectron spectroscopy (XPS) measurements were recorded on a

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XSAM 800 apparatus with an Al Ka=1486.6 eV exciting source (12 kV, 12 mA). The

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binding energy was corrected using the C1s peak (284.0 eV) as the reference to reduce the charge effect of samples.

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H2-temperature programmed reduction (TPR) was performed in an in-house constructed system equipped with a TCD (thermal conductivity detector) to measure H2 consumption. 100 mg of catalyst dried at 100℃ was pretreated by calcining in Ar at 400℃ for 1 h and then was cooled down to ambient temperature in Ar. After that, it

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was reduced with a 10 vol.% H2/Ar mixture ( 30 mL·min-1) by heating up to 600℃ at a ramp rate of 5℃·min-1. 2.4. Catalytic reduction of nitrophenols Catalytic reduction reactions of nitrophenols with excess NaBH4 were conducted in aqueous solutions at the atmospheric pressure and the designed temperature. In a 6

typical experiment, a 50 mL 4 mM 4-NP solution was freshly prepared and 5 mg of catalyst were dispersed in the solution under 30℃, and the mixture was continuous stirred for 5 min to reach the adsorption-desorption equilibrium. After that, 50 mL freshly prepared NaBH4 solution (0.4 M) was put into the mixture under stirring. Monitoring the reaction progress was conducted by taking 1 mL of the reaction

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mixture (diluted 50 times with deionized water) at regular intervals and measuring the UV–Vis spectra (Perkin Elmer Lambda 35 UV-Vis) of 4-NP at 400 nm. After the

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reaction, the catalyst was removed from the solution, washed with water and reused.

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3 Results and discussion 3.1 Characterization

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Fig. 1 presented the XRD patterns of Ni/CNTs. For the three samples, the

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diffraction peaks at 25.9o, 42.9o, 53.7o, 77.9o could be assigned to the diffraction peaks of CNTs. No peaks assigned to Ni or oxidized Ni appeared, illustrating that Ni

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species in fresh and used catalysts all highly dispersed or were amorphous. Fig. 2A and B displayed the TEM image of Ni/CNTs(c). It could be seen that the

Ni nanoparticles located inside the pores of CNTs evenly. The average diameter of particles was ∼12 nm. Fig. 2B showed clear lattice fringes of 0.203 nm which was

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corresponding to Ni (111) planes. It proved that metallic Ni existed in the interior of CNTs. Fig. 2C and D provided the TEM image of Ni/CNTs(d). It was found that the characteristic d-spacings of 0.219 nm (shown in Fig. 2D) corresponding to the Ni(OH)2 (103) planes, and Ni(OH)2 was agglomeration on the external of CNTs. This

illustrated that Ni species on the outer surface was not reduced by N2H4·H2O. These 7

results implied that the reduction of the inner Ni was easier than Ni on the outer surface of CNTs, which was consistent with results reported by Pan et al [26]. This was ascribed to the interaction between the electron-deficient inner CNTs surface and the anionic oxygen in oxidized Ni, which weakened bonding strength of Ni-O and reduced the activation energy of Ni reduction, resulting in the reduction of the inner

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Ni easy. The TEM image of used Ni/CNTs(c) was shown in Fig. 2(E) and (F). 0.203 nm of d-spacings for used Ni/CNTs(c) was corresponded to the Ni(111) planes. It

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illustrated that Ni almost not oxidized during reaction, which implied that the

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interaction of inner Ni nanoparticles with CNTs could enhance the oxidation resistance of Ni nanoparticles. This result was consistent with that of reported by

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Chen et al. [27]. Compared to the fresh, the inner Ni particles (Fig. 2 (E) and (F))

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were larger in length due to agglomeration along the axis. The specific surface area of samples was summarized in Table 1. CNTs-c and

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CNTs-d both exhibited higher specific surface area than that of CNTs. Moreover, the specific surface area of samples loading Ni was almost unchanged compared with that of their supports, which suggested that the channel of CNTs weren’t blocked

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completely with Ni NPs.

8

Intensity (a. u.)

CNTs

Ni/CNTs(d)

Ni/CNTs(c) used Ni/CNTs(c)

10

20

30

40

50

60

70

80

2 (°)

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Fig. 1 XRD patterns of Ni/CNTs

Fig. 2 TEM image of (A) and (B) of Ni/CNTs(c), (C) and (D) of Ni/CNTs(d), （E） and (F) of used Ni/CNTs(c) 9

Table 1 BET surface area of samples SBET (m2·g-1)

CNTs

118.340

CNTs-c

185.562

CNTs-d

133.258

Ni/CNTs(c)

180.126

Ni/CNTs(d)

149.838

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sample

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H2-TPR profiles of Ni/CNTs were shown in Fig. 3. H2-TPR profile of Ni/CNTs(c)

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showed three peaks with maximums at about 101, 275, and 439℃ which could be attributed to the reduction of NiO originated from oxidation of Ni, pure NiO crystal

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and methanation of CNTs, respectively. H2-TPR profile of Ni/CNTs(d) also appeared three peaks at 161, 286 and 458℃, which was simlar with that of Ni/CNTs(c). The

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lower reduction temperature of NiO in Ni/CNTs(c) illustrated that confinement inside CNTs facilitated reduction of NiO. This results was consistent with the TEM results

Hydrogen consumption (a.u.)

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and the results reported [26, 27].

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Ni/CNTs(d)

100

Ni/CNTs(c)

200

300

400

500

Temperature (oC)

Fig. 3 TPR profiles of Ni/CNTs 10

600

XPS was conducted to acqure further insight into the valence information of Ni and the results were shown in Fig. 4. The Ni2p spectra of fresh and used Ni/CNTs(c) were deconvoluted into three contributions at 853.6, 855.3 and 859.7 eV in the Ni2p3/2 region, assigned to the metallic Ni, the oxidized Ni and the satellite, respectively. The oxidized Ni might be ascribed to the incomplete reduction of Ni by N2H4·H2O. On the

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contrary, there were two peaks at 855.0 and 859.4 eV in the Ni2p3/2 region for Ni/CNTs(d), indicating only oxidized Ni were exstence for the outer Ni deposited on

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CNTs. Compared to Ni/CNTs(d), the binding energy of oxidized Ni (Ni2p3/2) in

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Ni/CNTs(c) had a shift of 0.3 eV towards a higher value. It was a hint that the

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electronic structures of Ni inside and outside the CNTs were different.

Ni 2p

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Ni/CNTs(d)

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Intensity (a.u.)

Ni/CNTs(c)

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850

860

used Ni/CNTs(c)

870

880

Binding Energy (eV)

Fig. 4 XPS spectra of Ni2p of catalysts

3.2 Catalytic reduction of 4-NP The catalytic activity of Ni/CNTs for 4-NP reduction was investigated and the 11

results were shown in Fig. 5. From Fig. 5(A), it suggested that the degradation of peak at 400 nm corresponding to 4-NP was accompanied by the increase of peak at 300 nm assigned to 4-AP, implying that the formation of 4-AP from the reduction of 4-NP. Due to the concentration of NaBH4 greatly exceeded that of 4-NP. The kinetics of this reaction can be considered as a pseudo-first-order reaction [28]. The kinetic equation

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of this reduction can be written as: ln(C/C0)=ln(A/A0)=-kt

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where C and A are the concentration of 4-NP and the absorbance at 400 nm at time t,

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ur na

lP

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at 400 nm, respectively; k is the rate constant.

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respectively; C0 and A0 are the initial concentration of 4-NP and the initial absorbance

Fig. 5 (A) UV-vis absorption spectra of 4-NP catalyzed by Ni/CNTs(c), (B) C/C0 versus time for 4-NP reduction; (C) pseudo-first-order plot of ln(C/C0) against reaction time for 4-NP reduction 12

A plot of C/C0 versus reaction time and pseudo-first-order plot of ln(C/C0) against reaction time for the 4-NP reduction were portrayed in Fig. 5(B) and (C), respectively. In the absence of catalyst, C/C0 was 97.3% and the conversion of 4-NP was 2.7% within 8 min. It could be observed that the catalytic performance of Ni/CNTs(c) was higher than that of Ni/CNTs(d). The enhancement of the activity of

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Ni/CNTs(c) compared with Ni/CNTs(d) could be ascribed to the confinement of Ni particles within CNTs and the peculiar interaction of CNTs with the inner Ni species,

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making Ni easy reduction and preventing Ni nanoparticles from agglomeration.

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Furthermore, the confinement within CNTs channels could increase the density of reactants resulting in a locally higher density of reactants and H2 pressure, which

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accelerated the reduction of 4-NP [20]. It was observed that the 4-NP conversion

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reached about 100% within 6 min over Ni/CNTs(c). The rate constant (k) calculated by a linear plot of ln(C/C0) vs time for different catalysts was shown in Table 2. The k

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reached 1.475 min-1 for Ni/CNTs(c) which was impressively higher than those of other catalysts shown in Table 2. The excellent catalytic performance of Ni/CNTs(c) was attributed to the synergistic confinement effect of Ni nanoparticles inside CNTs described above.

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3.3 Cycling performance

It is of great importance to study the reusability of Ni/CNTs(c). The catalyst was

used to catalyze the 4-NP reduction at 30℃. The catalyst was reused after being separated and washed with water. As shown in Fig. 6, the 4-NP conversion still maintained over 95.5%, which was better than that of other nickel-based catalyst [17, 13

29]. The Ni nanoparticles of the used became larger in length compared with that of the fresh Ni/CNTs(c), which was responsible for the decreased efficiency. The excellent stability of Ni/CNTs(c) was due to the synergetic confinement effect of Ni nanoparticles inside CNTs retarded the oxidation and agglomeration of Ni

Table 2 The catalytic performance of catalysts k (min-1)

Ni/CNT(c) (this work)

1.475

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Catalyst

Ni/CNT(d) (this work)

0.345

0.1632

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Ni/CC-CH [28]

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Ni/SiO2 hollow spheres [16]

0.27

Ni-Ca-Al2O3 [29]

0.171

Ni-RGO hybrids [30]

0.108

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nanoparticles.

Ni/C composites [31]

0.168

Ni/CNT [17]

0.102

Fe/CC-CH [28]

0.2937

P(AMPS)-Co [32]

0.12

Ca-doped Co3O4 [33]

0.2256

3.4 Catalytic activity of Ni/CNT(c) for other aromatic nitro compound reduction Catalytic reduction of o-nitrophenol (2-NP), p-nitroaniline (4-NA) and 14

o-nitroaniline (2-NA) over Ni/CNT(c) was also performance under identical reaction conditions and the results were shown in Table 3. It could be found that Ni/CNTs(c) exhibited outstanding catalytic performance for the reduction of these aromatic nitro compounds. The reactivity order was 4-NP＞2-NP＞4-NA＞2-NA. The difference in reactivity of aromatic nitro compounds may be explained by the steric effect,

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conjugation effect and inductive effect of the substituent group [34]. In these cases, the steric effect played a predominant role, which reduced the reactivity of 2-NP and

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2-NA compared with the corresponding aromatic nitro compound 4-NP and 4-NA.

Fig. 6 Cycling performance of Ni/CNTs(c) for 4-NP reduction

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3.5 Catalytic mechanism for 4-NP reduction According to the experimental results and the literatures [34], it can be concluded that the reduction reaction involves the following processes: (1) the π-π stacking

interaction between the inside surface of CNTs and aromatic nitrophenol results in higher aromatic nitrophenol concentration around the inner Ni nanoparticles, which made substrates easier access to Ni nanoparticles. (2) NaBH4 dissociates Na+ and 15

BH4- in water, and produced BH4- is easy to diffuse toward the electron-deficient internal surface and react with water to produce more H2, leading to high H2 pressure. (3) H2 adsorbed on Ni nanoparticles cleaves and forms Ni-H bonds. (4) The positively charged N in –NO2 of nitrophenols can be attacked by negative charged H to form nitroso group, and then reduces to hydroxylamine which is finally reduced to the

Time (min)

4-NP

10

2-NP

13

4-NA

20

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＞99.9

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2-NA

Conversion (%)

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substrate

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Table 3 aromatic nitro compound reduction over Ni/CNTs(c)

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aminophenol by the reductive addition of two hydrogen atoms (Schem 1 and Fig. 7).

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＞99.9 ＞99.9 ＞99.9

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Reaction conditions: aromatic nitro compound (4 mM, 50 mL), Catalyst amount 5 mg, NaBH4

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(0.4 M, 50 mL), 30℃.

Scheme 1. Reduction of nitrophenols over Ni/CNTs(c)

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Fig. 7 Mechanisms of catalyst of nitrophenols reduction

The outstanding activity of Ni/CNTs(c) could be due to the synergistic effect of

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the inner Ni nanoparticles and the inside surface of CNTs. First of all, the synergetic confinement effect of Ni particles inside CNTs promoted the reduction of Ni and retarded the oxidation and agglomeration of Ni nanoparticles, which resulted in more active sites available and excellent stability. Secondly, the interior surface of CNTs

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had strong adsorption ability for the aromatic nitrophenol and BH4-, which provided higher 4-NP concentration and H2 concentration in the channel of CNTs, leading to

highly efficient contact between the active sites and the reactants.

4 Conclusions Ni/CNTs(c) and Ni/CNTs(d) were successfully prepared and employed to 17

catalyze the nitrophenols reduction. Ni/CNTs(c) exhibited excellent catalytic performance and high stability, which was superior to Ni/CNTs(d) and those reported catalysts. The results of TEM, H2-TPR and XPS illustrated that the reduction of NiO was easier within the CNTs nanotubes than that located on the outer CNTs surface. The k reached 1.475 min-1, and the conversion efficiency still maintained over 95.5%

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after 10 cycles. The high catalytic activity and outstanding stability of Ni/CNTs(c) was attributed to the synergetic confinement effect of Ni nanoparticles inside CNTs

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which made Ni reduction easy and retarded the oxidation and agglomeration of Ni

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nanoparticles. Ni/CNTs(c) also exhibited outstanding catalytic activity for other

Acknowledgements acknowledge

the

University

of

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We

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aromatic nitro compound reduction.

Chinese

Academy

of

Sciences

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(Y8540XX1N2), and National Natural Science Foundation of China (21707152).

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