Accepted Manuscript A simple way to prepare reduced graphene oxide nanosheets/Fe2O3-Pd/N-doped carbon nanosheets and their application in catalysis Tongjie Yao, Junshuai Zhang, Quan Zuo, Hao Wang, Jie Wu, Xiao Zhang, Tieyu Cui PII: DOI: Reference:

S0021-9797(16)30027-3 http://dx.doi.org/10.1016/j.jcis.2016.01.027 YJCIS 21010

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

7 November 2015 14 December 2015 14 January 2016

Please cite this article as: T. Yao, J. Zhang, Q. Zuo, H. Wang, J. Wu, X. Zhang, T. Cui, A simple way to prepare reduced graphene oxide nanosheets/Fe2O3-Pd/N-doped carbon nanosheets and their application in catalysis, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.01.027

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A simple way to prepare reduced graphene oxide nanosheets/Fe2O3-Pd/N-doped carbon nanosheets and their application in catalysis

Tongjie Yaoa, Junshuai Zhanga, Quan Zuoa, Hao Wanga, Jie Wub, Xiao Zhanga, and Tieyu Cuia,*

a

The Academy of Fundamental and Interdisciplinary Science, Harbin Institute of

Technology, Harbin, Heilongjiang 150080, PR China b

Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education,

School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, PR China.

*Corresponding authors. Tel.:+86-451-86403646.

E-mail address: [email protected] (T. Cui); 1

ABSTRCT The catalysts with Pd and γ-Fe2O3 nanoparticles embedded between reduced graphene oxide nanosheets (rGS) and N-doped carbon nanosheets (NCS) were prepared through a two-step method. Firstly, graphene oxide nanosheets (GS)/prussian blue (PB)-Pd/polypyrrole (PPy) composites were synthesized by using pyrrole monomer as reductant, K3Fe(CN)6 and PdCl2 as oxidants in the presence of GS via a redox reaction. Subsequently, the as-obtained GS/PB-Pd/PPy composites were calcinated in N2 atmosphere. During the heat-treatment, carbonization of PPy to NCS, conversion of nonmagnetic PB to magnetic γ-Fe2O3 nanoparticles, and reduction of GS to rGS were finished, simultaneously. rGS/Fe2O3-Pd/NCS composites exhibited good catalytic activity towards reduction of 4-nitrophenol. The rate constant k and turnover frequency were calculated and compared with recent reports. Owing to γ-Fe2O3 nanoparticles, the rGS/Fe2O3-Pd/NCS composites could be quickly separated by magnet and reused without obvious decrease in activity.

Keywords: Polypyrrole; γ-Fe2O3 nanoparticles; Pd nanoparticles; reduced graphene oxide; composites

2

1. Introduction Recently, metal nanoparticles based heterogeneous catalysts have attracted much attention because of the high catalytic efficiency in numerous catalytic reactions, including Heck C-C coupling, reduction of dyes and hydrogenation [1-3]. Their catalytic activity is strongly dependent on the active atoms on the surface. It has been reported that only the first or first few layers of metal species are responsible for catalytic reactions [4,5]. Therefore, it is necessary to synthesize highly dispersed metal nanoparticles with small size in order to increase the surface-to-volume ratio. However, two problems appear with reduction of diameter: first, the surface energy inevitably increases, which leads to the tendency of aggregation; second, the nanoparticles are difficult to be separated from reaction solution. Such disadvantages usually result in the decreases of both catalytic activity and reusability. To overcome these drawbacks, extensive efforts have been done. It has been confirmed that stabilizing nanoparticles on an appropriate support is a feasible way to solve these drawbacks. As the support for metal nanoparticles, it is required to provide high surface area, good stability, robust surface chemistry and excellent dispersion characteristic, which is important for optimizing the synergistic nanoparticle-support interaction and maximizing the reactive activity of metal nanoparticles [6,7]. According to the principle, reduced graphene oxide nanosheets (rGS) are suitable candidates. Besides large surface area and special electrical property, the electrostatic force generated

3

between positively charged metal ions and negatively charged graphene oxide nanosheets (GS), is beneficial for uniform assembly of metal nanoparticles [8,9]. Therefore, many groups have selected rGS as the support to load various metal nanoparticles [10]. Although these composites usually exhibit superior catalytic activity, a disadvantage still exists, especially after long-time use: the nanoparticles easily fall off from support surface, as they directly expose to the reaction media and no strong interaction exists between the support and nanoparticles. Compared with anchoring metal nanoparticles on single support surface, embedding them between two supports is an acceptable approach to solve above problem. Recently, N-doped carbon nanosheets (NCS) have drawn much attention in the field of catalysis, since the nitrogen atoms can serve as anchoring sites to stabilize metal nanoparticles, which leads to their high dispersity [11,12]. Once the metal nanoparticles are placed between rGS and NCS, not only the stability of metal nanoparticles is improved, but also the advantages of both rGS and NCS in uniformly dispersing nanoparticles are fully explored. Based on the aforementioned considerations, we have tried to prepare catalysts with functional nanoparticles embedding between rGS and NCS. In addition to Pd nanoparticles, γ-Fe2O3 nanoparticles were also incorporated due to their magnetic property. In traditional procedure, it usually needed multiple steps to prepare rGS/Fe2O3-Pd/NCS composites, including preparation of γ-Fe2O3 nanoparticles, synthesization of Pd nanoparticles, formation of NCS precursor on the GS surface and

4

following carbonization. To avoid the complex procedure, in this paper, a novel synthetic process with only two-step was designed by selecting polypyrrole (PPy) and prussian blue (PB) as precursors of NCS and γ-Fe2O3 nanoparticles, respectively. The catalytic reduction of 4-nitrophenol (4-NP) was chosen as a probe reaction to test the catalytic property of rGS/Fe2O3-Pd/NCS composites. The rate constant k and turnover frequency (TOF) were calculated and compared with recent reports. Owing to γ-Fe2O3 nanoparticles, the catalysts could be easily separated from reaction solution by magnet, and their reusability was also addressed in detail.

2. Experiment 2.1. Chemicals The pyrrole monomer was purchased from Sigma-Aldrich and it was distilled under reduced pressure and stored at -4 oC prior to use. Graphite, KNO3, KMnO4, H2SO4, H2O2 (30 wt%), PdCl2, K3Fe(CN)6, FeCl3 ·6H2O, NaBH4, 4-NP and absolute ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were analytical grade and used as received. The water used in the experiments was deionized with a resistivity of 18.2 MΩ·cm.

2.2. Preparation of rGS/Fe2O3-Pd/NCS composites Fig. 1 outlines the preparation procedure of rGS/Fe2O3-Pd/NCS composites, which begins with preparation of GS. When PdCl2 and K3 Fe(CN)6 were added into

5

the mixture of pyrrole monomer and GS solution, the polymerization of pyrrole monomer was initiated by these oxidants on GS surface. Meanwhile, PdCl2 was reduced to Pd nanoparticles and K3 Fe(CN)6 was converted to PB nanoparticles. Therefore, during the redox reaction, PPy layer, Pd and PB nanoparticles were prepared in one-step. Subsequently, the as-prepared GS/PB-Pd/PPy composites were sintered at 600 oC under N2 atmosphere. In heat-treatment, carbonization of PPy layer to NCS, conversion of nonmagnetic PB nanoparticles to magnetic γ-Fe2O3 nanoparticles, and reduction of GS to rGS were combined together, and the resulting rGS/Fe2O3-Pd/NCS composites with Pd and γ-Fe2O3 nanoparticles embedding between rGS and NCS were obtained. In typical experiment, GS was prepared by Hummer’s method and the concentration was adjusted to 1.7 mg/mL [13]. To prepare GS/PB-Pd/PPy composites, firstly, 10 mL aqueous solution containing 30 μL pyrrole monomer was added into 10 mL GS solution. After magnetic stirring for 30 min, 5 mL aqueous solution containing 2.0 mg PdCl2, 30 mg K3 Fe(CN)6 and 10 mg FeCl3·6H2O was added into above mixture. The reaction was allowed to proceed for 6.0 h. Finally, the products were centrifuged and washed by water for 3 times. The as-prepared GS/PB-Pd/PPy composites were heated at 600 oC under N2 atmosphere for 2.0 h, and the resulting rGS/Fe2O3-Pd/NCS composites were prepared.

2.3. Catalyzed reduction of 4-NP

6

The catalytic property of rGS/Fe2O3-Pd/NCS composites was explored by studying the change of the absorbance intensity at the maximum absorbance wavelength of the samples. Typically, 10 μg catalysts were dispersed into 2.95 mL deionized water. Then, 50 μL 4-NP solution (1.0 mg/mL) was gradually added. Finally, 20 μL NaBH4 solution (50 mg/mL) was rapidly injected into the mixture to start the reaction. During the reaction, the absorbance of solution was recorded by the UV-Vis spectrophotometer. In recycling experiment: firstly, the catalysts were separated from reaction solution by placing a magnet at the bottom of cuvette. After carefully removing the upper solution, 2.95 mL deionized water were added into the cuvette. Then, the catalysts were re-dispersed by ultrasound. Finally, 20 μL NaBH4 solution and 50 μL 4-NP solution were quickly injected into cuvette. The absorbance intensity of the reaction solution was immediately measured.

2.4. Characterization Scanning electron microscopy (SEM) was employed to examine the morphology of products. The structures of the catalysts were characterized by transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and scanning transmission electron microscopy (STEM) on a Tecnai G2 F30 TEM operating at an acceleration voltage of 300 KV. X-ray photoelectron spectrum (XPS) measurements were carried out on a VG ESCALAB MKII spectrometer with Mg Kα excitation

7

(1253.6 eV). X-ray diffraction (XRD) measurements were carried out on a Siemens D-5005 apparatus, using the Cu Kα radiation (λ = 1.5418 Å). Fourier transform infrared (FT-IR) spectra were measured over the wavenumber ranging from 400 to 4000

cm-1

using

a

Nicolet

Avatar

360

FT-IR

spectrophotometer.

The

field-magnetization dependence of the products was measured using a MPMS-7 superconducting quantum interference device magnetometer at magnetic fields up to 50 kOe. The UV-Vis absorption spectra were obtained from a Lambda 750 spectrophotometer.

3. Results and discussion

Fig. 1. Schematic diagram of the fabrication of rGS/Fe2O3-Pd/NCS composites.

In our experiment, after adding oxidants into the mixture of GS and pyrrole monomer, the solution color quickly changed from brown (the color of GS) to black-blue (the color of PPy homopolymer and PB nanoparticles is black and blue, respectively), suggesting PPy layer and PB nanoparticles were successfully prepared. Fig. 2a shows the XRD pattern of GS/PB-Pd/PPy composites. Four diffraction peaks at 2θ = 17.4, 24.6, 35.4 and 39.9o correspond to (200), (220), (400) and (420) Bragg’s

8

reflections of face-centered cubic lattice of PB [14], which is well indexed to the reported data (No. 73-0687). No diffraction peaks of Pd nanoparticles can be found mainly due to their small size and high dispersity (see below); After calcination, the XRD pattern of rGS/Fe2O3-Pd/NCS composites is shown in Fig. 2b. The diffraction peaks of PB nanoparticles completely vanish. Five new diffraction peaks at 2θ = 30.2, 35.5, 43.2, 57.1 and 62.6o are corresponding to (200), (311), (400), (511) and (220) planes in γ-Fe2O3 (No. 39-1346) [15]. In addition, another two peaks located at 40.8 and 47.3o are assigned to (111) and (200) lattices of Pd nanoparticles (No. 05-0681) [16]. According to Scherrer equation, the average diameter of γ-Fe2O3 and Pd nanoparticles is calculated to be 24.6 and 8.5 nm, respectively.

Fig. 2. XRD patterns of (a) GS/PB-Pd/PPy and (b) rGS/Fe2O3-Pd/NCS composites. ■ represents PB nanoparticles; ● represents γ-Fe2 O3 nanoparticles;  represents Pd nanoparticles.

The morphology of GS/PB-Pd/PPy composites is shown in Fig. S1. Fig. 3a and 3b show the SEM images of rGS/Fe2O3-Pd/NCS composites. The typical crinkled 9

structure of rGS can be observed. Numerous of nanoparticles disperse on their scrolled surface, which leads to a rather rough surface. The homogeneous distribution of nanoparticles in a large area can be verified from a low resolution SEM image (Fig. 3a). In magnified image (Fig. 3b), we can see these nanoparticles have spherical shape and their average size is measured to be about 30 nm. The corresponding TEM images are exhibited in Fig. 3c and 3d. Plenty of nanoparticles appeared as black-dots densely anchor on the whole surface of rGS. No nanoparticles can be found outside of the rGS, suggesting the rGS and NCS successfully protect them from falling off. Due to the difference in size calculated by XRD data, Pd nanoparticles can be easily distinguished from γ-Fe2O3 nanoparticles. In Fig. 3d, Pd and γ-Fe2O3 nanoparticles are pointed out by red and blue arrows, respectively. At the edge of rGS, we can see both of them are wrapped by NCS. In addition to the difference in diameter, in HRTEM images, their lattices are also different. As shown in Fig. 3e and 3f, the inter-planar distance of two kinds of nanoparticles is about 2.31 and 2.16 Å, which agrees well with the lattice spacing of the (311) and (111) lattice of γ-Fe2O3 and Pd nanoparticles, respectively [17,18].

10

Fig. 3. SEM images of (a) rGS/Fe2O3-Pd/NCS composites; (b) corresponding magnified image. TEM images of (c) rGS/Fe2O3-Pd/NCS composites; (d) corresponding magnified image; HRTEM images of (e) γ-Fe2O3 nanoparticles; (f) Pd nanoparticles; (g~k) STEM images and corresponding elemental mapping images of N, Fe and Pd, respectively.

To better observe the distribution of different components in composites, the STEM measurement and corresponding elemental mapping analysis are carried out. In Fig. 3g, many bright-dots densely appear on the rGS surface, confirming the Pd and γ-Fe2O3 nanoparticles uniformly deposit on the whole rGS surface. The elemental mappings in box section are shown in Fig. 3h~k. Fig. 3i shows the whole basal plane of composites contains a large amount of element N with a homogeneous distribution density, suggesting the surface of rGS is covered by NCS. XPS data indicated the rGS/Fe2O3-Pd/NCS composites had nitrogen content of 1.75 wt%. From Fig. 3j and 3k, we can infer both γ-Fe2O3 and Pd nanoparticles in composites well disperse 11

without aggregation. As commonly known, during the heat-treatment, metal and metal oxide nanoparticles tended to aggregate together. In our study, owing to the restriction of PPy chains covered on their surface, such tendency was hindered.

Fig. 4. FT-IR spectra of (a) GS; (b) GS/PB-Pd/PPy composites and (c) rGS/Fe2O3-Pd/NCS composites.

Fig. 4a shows the FT-IR spectrum of original GS [19]. A broad absorption at 3410 cm-1 is ascribed to the O-H stretching vibration. The peak at 1730 cm-1 is assigned to the C=O stretching vibration of the carboxylic acid and carbonyl moieties. The peak emerged at 1620 cm-1 is attributed to skeletal vibration of C=C group. Moreover, two peaks located at 1225 and 1050 cm-1 are corresponding to C-O band in epoxy and alkoxy groups, respectively. After polymerization (Fig. 4b), the characteristic peaks of GS are covered, since PPy homopolymer is a strong absorber in infrared region [20]. The feature peaks of PPy layer can be distinguished clearly: characteristic bands at 1570 and 1493 cm-1 are attributed to the stretching mode of the C-C and C-N in the pyrrole ring. The peaks at 1342 and 1196 cm-1 are related to 12

in-plane vibrations of C-H group. The ring deformation at 910 cm-1 is also observed [21]. Moreover, the band at 2083 cm-1 corresponds to the stretching vibration of the C-N group, and the absorption band at 491 cm-1 is due to vibration of Fe2+-CN-Fe3+, which indicates the formation of PB nanoparticles [16]. Fig. 4c shows the FT-IR spectrum of rGS/Fe2O3-Pd/NCS composites. Compared with Fig. 4b, the characteristic bands corresponding to PPy completely vanish and a new peak appeared at 1554 cm-1 is attributed to the skeletal vibration of rGS, suggesting carbonization occurs [22]. Additionally, the band at 491 cm-1 corresponding to Fe2+-CN-Fe3+ disappears; instead, a peak emerges at 571 cm-1, which is due to the vibration of Fe-O band [23], indicating the conversion of PB to γ-Fe2O3 nanoparticles during the heat-treatment.

Fig. 5. Core-level XPS spectra of (a) C 1s in GS; (b) C 1s in rGS/Fe2O3-Pd/NCS composites; (c) Fe 2p in rGS/Fe2O3-Pd/NCS composites; (d) Pd 3d in rGS/Fe2O3-Pd/NCS composites; (e) N 1s in pure PPy homopolymer; (f) N 1s in GS/PB-Pd/PPy composites. 13

Fig. 5a and 5b show the core-level C 1s XPS spectra before and after calcination. In original GS (Fig. 5a), the carbon species are divided into four peaks: C-C/C=C (284.6 eV), C-O (286.5 eV), C=O (287.2 eV) and O-C=O (288.2 eV) [24,25]. For comparison, the core-level C 1s XPS spectrum of rGS/Fe2O3-Pd/NCS composites can be fitted into five peaks, as an additional C-N peak (285.3 eV) originated from NCS appears [26]. Moreover, the content of oxidized carbon species dramatically decreases from original 62% to 22%, suggesting most of oxygen containing groups are removed due to the thermal reduction (Fig. 5b). As commonly known, the XRD patterns of γ-Fe2O3 and Fe3O4 nanoparticles were similar; therefore, they could not be simply distinguished via XRD measurements. Fig. 5c shows the core-level Fe 2p XPS spectrum of rGS/Fe2O3-Pd/NCS composites. Two peaks at 710.5 and 724.7 eV are observed, which is attributed to Fe 2p3/2 and Fe 2p1/2, respectively. Importantly, the characteristic satellite peak at 719.3 eV is corresponding to Fe 2p3/2 of Fe3+ in γ-Fe2O3, suggesting formation of γ-Fe2O3 nanoparticles instead of Fe3O4 nanoparticles [27,28]. The appearance of Pd nanoparticles can also be detected by XPS analysis. The high-resolution Pd 3d spectrum in Fig. 5d exhibits two individual peaks. Each peak can be further deconvoluted into two peaks yielding two doublets. The doublets with binding energy of 337.9 and 343.2 eV are corresponding to Pd 3d5/2 and 3d3/2 in Pd oxides; while another two peaks with binding energy of 335.6 and 340.8 eV are corresponding to Pd 3d5/2 and 3d3/2 in Pd0 nanoparticles. The interaction between Pd

14

and γ-Fe2O3 nanoparticles were investigated. Compared with reference rGS/Pd/NCS composites (Fig. S2), the peaks of Pd nanoparticles in rGS/Fe2O3-Pd/NCS composites shift to higher binding energy, indicating the electrons transfer from Pd nanoparticles to γ-Fe2O3 nanoparticles. The electronic structure was very important for catalysts, since it determined their catalytic property [29]. In our study, the small size and high dispersity of Pd nanoparticles were mainly caused by three reasons: first, highly negatively charged GS supplied strong driving force for positively charged Pd2+ ions, which was beneficial for uniform assembly of Pd nanoparticles [30]; second, during the reduction of Pd2+ ions, pyrrole monomer also polymerized around the metal nanoparticles to prevent their aggregations by PPy chains [31,32]; third, the synergistic interaction generated between Pd2+ ions and PPy homopolymer was favorable to further improve dispersity of Pd nanoparticles, which could be confirmed by XPS spectrum. For both pure PPy homopolymer and GS/PB-Pd/PPy composites (Fig. 5e and 5f), the core-level N 1s XPS spectra can be curve-fitted into three peaks with binding energy at 396.9, 399.4 and 401.2 eV. They are attributed to the amine (-NH-), imine (=N-), and positively charged nitrogen (N+) species, respectively [33]. Compared with pure PPy homopolymer (Fig. 5e), the content of =N- groups in GS/PB-Pd/PPy composites dramatically increases, which is caused by formation of PB nanoparticles that contain cyano groups [16]. Although a large number of cyano groups appear, the content of N+ species still increases from original 12% to 22%, suggesting the Pd2+ ions coordinate with amino groups through

15

sharing the electron pairs of amino groups. The synergistic effect was beneficial for improving dispersity of as-prepared Pd nanoparticles and reducing their diameter [34,35].

Fig. 6. (a) UV-Vis spectra of the 4-NP solution at different experimental procedure; (b) time-dependent UV-Vis spectra of the reaction mixture of catalytic reduction of 4-NP; (c) The dependence of ln(At/A0) on reaction time t for the reactions catalyzed by catalysts; (d) conversion of 4-NP using 10 μg catalysts with different PdCl2 usage. As commonly known, 4-NP had already been listed on the “Priority Pollutant List” by the U.S. Environmental Protection Agency [36]. It was highly toxic to human beings, since long-term exposure could cause damage of kidney, blood cells and liver. However, their reduction product, 4-aminophenol (4-AP), was a valuable intermediate and commonly applied for preparation of analgesic and antipyretic drug [37]. Reduction of 4-NP in the presence of metal catalysts had been accepted as an effective and friendly route to produce 4-AP in industry. Therefore, in this paper, to 16

monitor the catalytic activity of rGS/Fe2O3-Pd/NCS composites, we have examined the reduction reaction of 4-NP with NaBH4 as reductant. The conversion from 4-NP to 4-AP occurred via formation of intermediate 4-nitrophenolate ions. Fig. 6a shows the UV-Vis spectrum of original 4-NP solution, an absorption peak emerges at 317 nm (curve a). When NaBH4 solution is added, the solution color immediately changes from light-yellow to bright-yellow, and the wavelength of maximum absorption peak moves to 400 nm corresponding to the intermediate 4-nitrophenolate ions (curve b). The reduction of 4-NP to 4-AP in aqueous NaBH4 solution was thermodynamically favorable (E0 for 4-NP/4-AP = -0.76 V and H3BO3/BH4- = -1.33 V vs. NHE) [38]. However, kinetic barrier existed due to the large potential difference between the donor (BH4− ions) and the acceptor (4-NP). Although the above reaction proceeds for 24.0 h, the intensity of peak only reduces 3.7% (curve c), indicating the reaction proceeds very slowly. Metal nanoparticles were known to catalyze this reaction by facilitating electron transfer from the donor to acceptor; therefore, the reaction greatly accelerated [39]. When 0.2 mg catalysts were added into mixture of 4-NP and NaBH4 solution, the solution color quickly turned from bright-yellow to colorless within 3.0 min. In spectrum (curve d), the peak located at 400 nm vanishes; instead, a new peak assigned to 4-AP appears at 300 nm. We have monitored the reaction process by measuring the absorption spectra of 4-NP as a function of time. Fig. 6b shows the time-dependent UV-Vis spectra of 4-NP solution after adding only 10 μg rGS/Fe2O3-Pd/NCS composites (the usage of PdCl2

17

was 2.0 mg and the Pd nanoparticle loading was 4.3 wt%). The characteristic absorption peak of 4-nitrophenolate ions at 400 nm gradually disappears within 18 min; while an additional absorption peak at 300 nm becomes stronger and stronger. Since the amount of NaBH4 largely exceeded that of 4-NP (C4-NP:CNaBH4 = 1:74), the catalytic reaction should follow pseudo-first-order reaction [40,41]. The rate constant k was determined from the slope of the plots of ln(Ct/C0) vs. time t. (the ratio of Ct to C0, where Ct and C0 are the 4-NP concentrations at time t and 0, respectively, was measured from the relative intensity of the respective absorbance At/A0). Fig. 6c shows the linear relationship between ln(At/A0) and reaction time t in reduction reaction catalyzed by rGS/Fe2O3-Pd/NCS composites. As all these plots match the pseudo-first-order reaction kinetics very well, the rate constant k is calculated to be 0.163 min-1. It was well known that the rate constant was influenced by many factors, such as metal nanoparticle loading, the usage of NaBH4 and catalysts. Table 1 lists some k values and experimental parameters in recent studies. Take small usage of catalysts (10 μg) and high k/mPd value into account, the catalytic performance of rGS/Fe2O3-Pd/NCS composites is superior than other catalysts based on Pd nanoparticles [42-46]. Besides k value, the TOF was also calculated and their value was about 1.0 s-1. Compared with recent studies with catalysts based on Pd nanoparticles, the TOF value of rGS/Fe2O3-Pd/NCS composites was also higher than others [47-49].

Table 1. Comparison of k for the reduction of 4-NP in different catalytic systems. 18

Catalysts

Catalysts

dosage(mg)

Pd loading(wt%)

NaBH4 dosage

k value (min-1)

k /mPd value (min-1 mg-1)[a]

Ref.

Mesoporous Pd leaves

0.25 mg

nearly 100 wt%

0.3 mL×0.1 M

0.49

1.96

41

Pd/rGS

1.0 mg

0.5 wt%

0.1 mL×0.01 M

0.141

28.2

42

Pd/rGS

0.5 mg

7.5 wt%

1.0 mL×0.1 M

0.27

7.2

43

0.1 mg

1.9 wt%

0.5 mL×0.26 M

0.096

50.53

44

0.4 mg

16.6 wt%

0.5 mL×0.5 M

1.22

18.37

45

0.01 mg

4.3 wt%

0.02 mL×1.3 M

0.163

379.07

here

FexOy/Pd@mSiO2 Ni@Pd/KCC-1

[b]

rGS/Fe2O3-Pd/NCS

[a] The value of k/mPd is calculated based on the

k value and mass of Pd nanoparticles in corresponding catalysts.

[b] KCC-1 is fibrous nanosilica

Fig. 7. Magnetic hysteresis loops of rGS/Fe2O3-Pd/NCS composites at 295 K. Insets show the separation of rGS/Fe2O3-Pd/NCS composites in the presence of a magnet (bottom right) and the magnetic hysteresis loop at a lower field (top left).

The catalytic activity of rGS/Fe2O3-Pd/NCS composites could be easily manipulated by controlling the dosage of PdCl2. Fig. 6d shows the conversion of 4-NP as a function of PdCl2 usage. The conversion of 4-NP improves with the increase of PdCl2 usage. It was easy to understand that the more PdCl2 used, the more Pd nanoparticles loading on the composites. When the dosage of PdCl2 is 1.0, 4.0 and 19

6.0 mg, the rate constant k was calculated to be 0.097, 0.229 and 0.356 min-1, respectively. It was necessary to mention that nearly no catalytic activity existed in absence of PdCl2 during the preparation experiment, suggesting the catalytic property of rGS/Fe2O3-Pd/NCS composites originated from Pd nanoparticles.

Fig. 8. Conversion of 4-NP in 4 consecutive reaction cycles.

After calcination at 600 oC, nonmagnetic PB nanoparticles converted to magnetic γ-Fe2O3 nanoparticles, which makes the catalysts can be quickly recycled from reaction media by magnet (bottom right of Fig. 7). To determine the magnetic property of rGS/Fe2O3-Pd/NCS composites, the magnetic measurement was carried out at 295 K. The inset of Fig. 7 shows the magnetic hysteresis loop at a lower field (top left), both the remnant magnetization (2.65 emu/g) and coercivity (75 Oe) are very small, demonstrating the superparamagnetic property of catalysts. The saturation magnetization is 21.5 emu/g with the 29.8 wt% γ-Fe2O3 loading. This implied that the rGS/Fe2O3-Pd/NCS composites exhibited nearly no magnetic property in the absence of an magnetic field and they could be magnetic separation in presence of an magnetic

20

field, which was favorable for their application in the field of catalysis. The reusability of the catalysts was also investigated. As shown in Fig. 8, the reaction rate slightly reduces, which maybe caused by inevitable loss of catalysts (10 μg) during the separation and rinsing by water. However, the rGS/Fe2O3-Pd/NCS composites still can be reused at least 4 times without obvious significant loss of conversion, indicating the catalysts exhibit good reusability.

4. Conclusions A two-step method for preparation of rGS/Fe2O3-Pd/NCS composites with catalytic Pd and magnetic γ-Fe2O3 nanoparticles embedding between rGS and NCS via a redox reaction and following carbonization was introduced. This special structure not only prevented the aggregation and loss of supported nanoparticles, but also fully explored the advantages of rGS and NCS in uniformly dispersing nanoparticles. During the redox reaction, PPy layer, Pd and PB nanoparticles were prepared in one-step. In heat-treatment, carbonization of PPy layer to NCS, conversion of PB nanoparticles to γ-Fe2O3 nanoparticles and reduction of GS to rGS were combined together. Although the preparation procedure was simplified, the catalytic activity and reusability were not sacrificed. The rGS/Fe2O3-Pd/NCS composites showed a high catalytic activity in reduction of 4-NP. Even the usage of catalysts was only 10 μg, the rate constant k still reached as high as 0.163 min-1.

21

Furthermore, owing to magnetic γ-Fe2O3 nanoparticles, these catalysts were easily separated from the solution by magnet and reused at least 4 times.

Acknowledgements This work was supported by the National Nature Science Foundation of China (Grant no. 51273051, 21174033, 21204015 and 21404035). The Open Project of Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education. Natural Science Foundation of Heilongjiang Province of China (E2015005).

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Figure Captions: Fig. 1. Schematic diagram of the fabrication of rGS/Fe2O3-Pd/NCS composites.

Fig. 2. XRD patterns of (a) GS/PB-Pd/PPy and (b) rGS/Fe2O3-Pd/NCS composites. ■ represents PB nanoparticles; ● represents γ-Fe2 O3 nanoparticles;  represents Pd nanoparticles.

Fig. 3. SEM images of (a) rGS/Fe2O3-Pd/NCS composites; (b) corresponding magnified image. TEM images of (c) rGS/Fe2O3-Pd/NCS composites; (d) corresponding magnified image; HRTEM images of (e) γ-Fe2O3 nanoparticles; (f) Pd nanoparticles; (g~k) STEM images and corresponding elemental mapping images of N, Fe and Pd, respectively.

Fig. 4. FT-IR spectra of (a) GS; (b) GS/PB-Pd/PPy composites and (c) rGS/Fe2O3-Pd/NCS composites.

Fig. 5. Core-level XPS spectra of (a) C 1s in GS; (b) C 1s in rGS/Fe2O3-Pd/NCS composites; (c) Fe 2p in rGS/Fe2O3-Pd/NCS composites; (d) Pd 3d in rGS/Fe2O3-Pd/NCS composites; (e) N 1s in pure PPy homopolymer; (f) N 1s in GS/PB-Pd/PPy composites.

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Fig. 6. (a) UV-Vis spectra of the 4-NP solution at different experimental procedure; (b) time-dependent UV-Vis spectra of the reaction mixture of catalytic reduction of 4-NP; (c) The dependence of ln(At/A0) on reaction time t for the reactions catalyzed by catalysts; (d) conversion of 4-NP using 10 μg catalysts with different PdCl2 usage.

Fig. 7. Magnetic hysteresis loops of rGS/Fe2O3-Pd/NCS composites at 295 K. Insets show the separation of rGS/Fe2O3-Pd/NCS composites in the presence of a magnet (bottom right) and the magnetic hysteresis loop at a lower field (top left).

Fig. 8. Conversion of 4-NP in 4 consecutive reaction cycles.

Table 1. Comparison of k for the reduction of 4-NP in different catalytic systems.

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A simple way to prepare reduced graphene oxide nanosheets/Fe2O3-Pd/N-doped carbon nanosheets and their application in catalysis

Tongjie Yaoa, Junshuai Zhanga, Quan Zuoa, Hao Wanga, Jie Wub, Xiao Zhanga, Tieyu Cuia,*

Reduced graphene oxide nanosheets/Fe2O3-Pd/N-doped carbon nanosheets composites were prepared via a redox reaction and following calcination. They showed superior catalytic activity and reusability in reduction of 4-nitrophenol.

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