Accepted Manuscript Regular article A simple way to prepare Pd/Fe3O4/polypyrrole hollow capsules and their applications in catalysis Tongjie Yao, Hao Wang, Quan Zuo, Jie Wu, Baifu Xin, Fang Cui, Tieyu Cui PII: DOI: Reference:
S0021-9797(15)00271-4 http://dx.doi.org/10.1016/j.jcis.2015.03.012 YJCIS 20322
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
8 February 2015 6 March 2015
Please cite this article as: T. Yao, H. Wang, Q. Zuo, J. Wu, B. Xin, F. Cui, T. Cui, A simple way to prepare Pd/ Fe3O4/polypyrrole hollow capsules and their applications in catalysis, Journal of Colloid and Interface Science (2015), doi: http://dx.doi.org/10.1016/j.jcis.2015.03.012
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A simple way to prepare Pd/Fe3O4/polypyrrole hollow capsules and their applications in catalysis
Tongjie Yaoa, Hao Wanga,Quan Zuoa, Jie Wub, Baifu Xinb, Fang Cuia, Tieyu Cuia,*
a
The Academy of Fundamental and Interdisciplinary Science, Harbin Institute of
Technology, Harbin 150080,People’s Republic of China b
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education,
School of Chemistry and Materials Science, Heilongjiang University, Harbin, 150080, People’s Republic of China
*
Corresponding authors. Tel.:+86-451-86403646. E-mail address: [email protected] (T. Cui); 1
ABSTRACT Preparation of catalysts with good catalytic activity and high stability, together with magnetic separation property, in a simple way is highly desirable. In this paper, we reported a novel strategy to construct magnetic recyclable hollow capsules with Pd and Fe3O4 nanoparticles embedded in polypyrrole (PPy) shell via only two steps: first, synthesization of Pd nanoparticles, preparation of Fe 3O4 nanoparticles, and formation of PPy shell were finished in one-step on the surface of polystyrene (PS) nanospheres; then, the PS core was selectively removed by tetrahydrofuran. The Pd/Fe3O4/PPy hollow capsules exhibited good catalytic property in reduction of 4-nitrophenol with NaBH4 as reducing agent, and the reaction rate constants were calculated through pseudo-first-order reaction equation. Due to incorporation of Fe3O4 nanoparticles, the catalysts could be quickly separated from the reaction solution by magnet and reused without obvious catalytic loss. Besides catalytic property and reusability, their stability was also examined by HNO3 etching experiment. Compared with bare Pd and Fe3O4 nanoparticles, the stability of both Pd and Fe3O4 nanoparticles in hollow capsules was largely improved owing to the protection of PPy shell. The good catalytic performance, ease of separation, high stability and especially a simple preparation procedure, made Pd/Fe3O4/PPy hollow capsules highly promising candidates for diverse applications.
Keywords: polypyrrole (PPy); Fe3O4 nanoparticles; Pd nanoparticles; hollow capusles; catalysis
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1. Introduction Metal nanoparticles have attracted enormous interest because of their novel properties and corresponding applications in a broad range of areas, such as surface enhanced spectroscopy, biological imaging, optoelectronics, thermal therapy and especially in catalysis [1-3]. Over the last two decades, metal nanoparticles have been demonstrated to be efficient catalysts in many different reactions by taking the advantages of their large surface-to-volume ratios and shaped-dependent specific crystallographic surfaces [4,5]. However, when the particle size decreases to nanoscale, the aggregation caused by high surface energy and difficult separation caused by small size are practical obstacles, which hinders the application of nanoparticles in industrial use, even though they have high catalytic activity. To overcome these drawbacks, extensive efforts have been done. It has been proven through investigations that anchoring metal nanoparticles on supports is an effective approach to solve the above two drawbacks. Therefore, many materials with active sites for anchoring metal nanoparticles have been developed as candidates for catalyst supports [6-8]. Of wide range of supports, magnetic hollow capsules have drawn much attentions. Compared with typical solid support, they usually possess lower density, higher surface area and larger void space [9-11]. Moreover, incorporation of magnetic nanoparticles endows such support with magnetic property; therefore, they can be quickly separated from reaction solution by external magnetic field, thus avoiding the drawbacks of traditional centrifugation and filtration technique, such as inefficiency and time-wasting [12]. Because of these advantages, some researchers are
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working on preparing magnetic hollow capsules as supports to load metal nanoparticles [13-15]. However, at present, the most related studies were focused on either the catalytic activity or reusability of catalysts, while the simplicity of preparation procedure was largely neglected. Take most frequently used strategy for constructing hollow capsules, hard-template method, as an example, to prepare magnetic hollow capsule, the typical preparation procedure usually involved four steps: preparation of magnetic nanoparticles, synthesization of metal nanoparticles, construction of shell around the hard-template, and removal of hard-template [16,17]. This traditional preparation procedure was too complex and time-wasting, which was not beneficial for industrial production. In our previous study [18], preparation of magnetic nanoparticles and construction of PPy shell were successfully finished in one-step; however, it still acquired extra procedures to synthesize metal nanoparticles and assemble them on the surface of hard-template. Therefore, To further simplify the experimental process, and prepare magnetic hollow capsules with good catalytic activity and high stability by a simpler approach still remained a grand challenge. Polypyrrole (PPy), as a conducting polymer, has been widely applied in the field of sensors, supercapacitors and biomedicine due to their high conductivity, environmental friendly and excellent stability [19-22]. Recently, many works reported their applications in catalysis [23-25], and the related results showed that PPy was an ideal catalyst support: on one hand, numerous of amino groups on their chains could supply enough active sites to increase the loading of metal nanoparticles and promote
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the intimate contact between the metal nanoparticles and PPy; on the other hand, synergism between the PPy and the metal ions could further make the catalyst more stable and enhance their catalytic performance. In addition, by taking advantage of good reducibility of pyrrole monomer, once metal salts acted as oxidants and pyrrole monomer served as reductants, the pyrrole monomer would be oxidized to the PPy, while the metal salts were reduced to the corresponding metal nanoparticles [26]. During the process, if magnetic nanoparticles could be synthesized at the same time, the formation of metal nanoparticles, preparation of magnetic nanoparticles, and construction of PPy support were combined into one-step; therefore, the preparation procedure was largely simplified. On the basis of the above statements, herein, we have designed a novel strategy to synthesize Pd/Fe3O4/PPy hollow capsules through only two-steps. Fig. 1 illustrates the synthetic procedure for the Pd/Fe3O4/PPy hollow capsules. First, the polystyrene (PS)@Pd/Fe3O4/PPy composites were synthesized in one-step since the oxidation of pyrrole monomer, reduction of PdCl2 and hydrolysis of Fe3+ and Fe2+ ions took place on PS nanosphere surface at the same time. The as-prepared PS@Pd/Fe3O4/PPy composites were then converted to Pd/Fe3O4/PPy hollow capsules through a consequent tetrahydrofuran (THF) etching. To the best of our knowledge, to date, this was the simplest procedure to prepare magnetic recyclable hollow capsules. Their catalytic property was investigated by reducing the 4-nitrophenol (4-NP) with NaBH4 as the reducing agent, and rate constant was calculated through pseudo-first-order reaction equation. Owing to the incorporation of Fe3O4 nanoparticles, the catalysts 5
could be easily separated from reaction solution by magnet and reused. More significantly, the stability of Pd/Fe3O4/PPy hollow capsules was 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. Styrene, acrylic acid, methylacrylic acid, absolute ethanol, THF, 4-NP, NH3·H2O (28 wt%), K2S2O8, FeCl3·6H2O, FeCl2·4H2O, PdCl2 and NaBH4 were purchased from Sinopharm Chemical Reagent Co., Ltd. All of chemicals were analytical grade and used as received. The water used in the experiments was deionized with a resistivity of 18.2 MΩ·cm-1.
2.2 Preparation of Pd/Fe3O4/PPy hollow capsules The carboxylic-capped PS nanospheres were synthesized according to the previous work [27]. Firstly, 50 μL pyrrole monomer was injected into the 20 mL PS nanosphere (55 mg) solution under mechanical stirring at ambient temperature. 0.5 h later, 10.0 mL mixture containing 50 mg FeCl3·6H2O and 10 mg PdCl2 was added into the above solution slowly. The oxidation polymerization was maintained for 5.0 h before nitrogen was purged into solution to remove oxygen. Then, 15 mg FeCl2·4H2O and 1.5 mL NH3·H2O were added into system successively. The reaction was allowed to proceed for another 2.0 h under nitrogen protection, and the PS@Pd/Fe 3O4/PPy 6
composites were prepared. Finally, to obtain Pd/Fe3O4/PPy hollow capsules, the as-prepared PS@Pd/Fe3O4/PPy composites were suspended into 30 mL THF solution. After stirring for a day, the PS nanospheres were removed, and the resulting Pd/Fe3O4/PPy hollow capsules were separated by magnet and washed by ethanol.
2.3. Catalyzed reduction of 4-NP The catalytic property of Pd/Fe3O4/PPy hollow capsules was explored by studying the change of the absorbance intensity at the maximum absorbance wavelength (λmax) of the 4-NP. In a typical procedure, 1.0 mg Pd/Fe3O4/PPy hollow capsules were homogeneously dispersed into the 2.0 mL 4-NP solution (25 mg·L-1), followed by a rapid injection of 0.5 mL of NaBH4 solution (10 mg·mL-1) under stirring. The color of the mixture gradually changed from intense yellow to colorless, indicating that the Pd/Fe3O4/PPy hollow capsules catalyzed the reduction of 4-NP. In the recycling study, the catalysts were separated from the solution by magnet when the reduction reaction completely finished. After washed by water twice, they were reused in the next reaction run. The procedure was repeated for 5 times. After reaction with 1.0 mg catalysts for 3.0 min in each cycle, the absorbance intensity of reaction solution was immediately measured and the conversation of 4-NP was calculated.
2.4. Characterization A JEOL JSM-6700F scanning electron microscope (SEM) with primary electron energy of 3 kV was employed to examine the surface morphologies of products. The
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structure and shell thickness of the materials were determined by a Tecnai G 2 F30 transmission electron microscope (TEM) operating at 300 kV. 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. A Lambda 750 ultraviolet and visible (UV-Vis) spectrophotometer was employed for analysis of 4-NP reduction. X-ray diffraction (XRD) data were collected on a Siemens D-5005 X-ray diffractometer with Cu Karadiation (λ = 1.5418 Å). 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. X-ray photoelectron spectroscopy (XPS) was collected by using a VG ESCALAB MKII spectrometer with Mg Ka excitation (1253.6 eV). Binding energy calibration was based on C 1s at 284.6 eV.
3. Results and discussion The morphology of PS nanospheres is presented in Fig. S1. They are very uniform with an average diameter of 450 nm. From a magnified image (inset of Fig. S1), we could see their surface is smooth, suggesting the as-prepared PS nanospheres were ideal hard-template for constructing resulting hollow capsules. The FT-IR spectrum of PS nanospheres is shown in Fig. 2a, the PS main peaks locate at 3027, 2921, 1499, 1454 and 696 cm−1. In addition, a peak appears at 1732 cm−1, suggesting carboxylic group is successfully modified on their surface, which is beneficial for deposition of PPy shell in next-step [28].
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Fig. 1. Formation process of Pd/Fe3O4/PPy hollow capsules.
As the oxidation potential of Pd2+/Pd0 (0.987 V) is much higher than that of Fe3+/Fe2+ (0.771 V), PdCl2 accelerates the oxidation polymerization of pyrrole monomer. An obvious phenomenon was that the color of solution quickly turned black within 3.0 min with PdCl2 and FeCl3·6H2 O as oxidant; in contrast, if only FeCl3·6H2O was used as oxidant, the color of solution was not changed to black until 20 min later. When the mixture of PdCl2 andFeCl3·6H2O was added into the system, Pd2+ ions were quickly reduced to Pd0 and Fe3+ ions were gradually reduced to Fe2+ ions by pyrrole monomer. Once NH3·H2O was introduced into the reaction system, the alkalinity of mixture increased, which resulted in the hydrolysis of both Fe 3+ and Fe2+ ions. Therefore, Fe(OH)3 and Fe(OH)2 were produced under alkaline condition, and finally Fe3O4 nanoparticles formed through dehydration [18]. The as-prepared Pd and Fe3O4 nanoparticles were deposited on the PS nanosphere surface and wrapped by PPy shell. Based on above analysis, In our system, PdCl2, FeCl3·6H2O and pyrrole monomer all played two roles in preparation procedure: for PdCl2, it acted as precursor of Pd nanoparticles and oxidant of pyrrole monomer; for FeCl3·6H2O, it acted as source of Fe3O4 nanoparticles and oxidant of pyrrole monomers; for pyrrole monomer, it acted as precursor of resulting PPy shell and reductants of both PdCl2 and 9
FeCl3·6H2O. Because of multifunctions of these raw materials, formation of Pd nanoparticles, preparation of Fe3O4 nanoparticles, and construction of PPy shell were finished in one-step; therefore, the preparation procedure was greatly reduced.
Fig. 2. FT-IR spectra of (a) PS nanospheres; (b) PS@Pd/Fe3O4/PPy composites; (c) Pd/Fe3O4/PPy hollow capsules.
Fig. 3a shows the SEM image of PS@Pd/Fe3O4/PPy composites. They exhibit uniform spherical shape. A magnified image reveals that the surface of PS@Pd/Fe3O4/PPy composites is considerable rough (inset of Fig. 3a), which looks like cauliflower, and this is the feature of PPy homopolymer [29]. Compared with original PS nanospheres, their diameter dramatically increases to 530 nm, suggesting the successful coverage of Pd/Fe3O4/PPy shell and their thickness is about 40 nm. Fig. 3b shows the corresponding TEM image of PS@Pd/Fe3O4/PPy composites. It is difficult to resolve the interface between PPy shell and PS core due to their similar electron contrast, and only solid composites with spherical shape can be seen. In magnified TEM image (inset of Fig. 3b), it is easy to distinguish that many inorganic
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nanoparticles randomly decorate in the PPy shell. The FT-IR spectrum of PS@Pd/Fe3O4/PPy composites is shown in Fig. 2b, the PS feature peaks appear at 3027, 2923, 1495, 1454 and 697 cm−1. In addition, the bands at 1551 and 1454 cm-1 are assigned to the stretching vibration of the C-C and C-N in pyrrole ring, and the ring deformation at 928 cm-1 is also observed [30]. Moreover, a peak at 573 cm-1 is due to the stretching mode of Fe-O bond, indicating Fe3O4 nanoparticles are synthesized in the PPy shell [31]. The FT-IR spectrum, together with SEM and TEM images, suggested the PS@Pd/Fe3O4/PPy composites had been successfully prepared. After PS@Pd/Fe3O4/PPy composites were added into THF solution and stirred for a day, the PS nanospheres were removed and Pd/Fe3O4/PPy hollow capsules were obtained. Fig. 3c shows the morphology of resulting capsules. Some of the capsules are collapse with many random folds due to high-vacuum condition under the SEM measurements (inset of Fig. 3c), suggesting the removal of PS core and survival of PPy shell during the etching process. In corresponding TEM image (Fig. 3d), the appearance of a large void further confirms the Pd/Fe3O4/PPy capsules possess a well-defined hollow structure, and the thickness of Pd/Fe3O4/PPy shell is measured to be 44 nm, which is in a good agreement with the value calculated from the SEM image. Although, PS nanospheres completely disappear in the view of TEM image, the FT-IR spectrum of Pd/Fe3O4/PPy hollow capsules reveals that the trace amounts of PS chains still remain in Pd/Fe3O4/PPy hollow capsules. In Fig. 2c, the peaks located at 1540, 1196, 1034 and 928 cm-1 belong to the PPy chain, and a weak peak appeared at 697 cm-1 is corresponding to residual PS chains. This might be because a
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number of pores in PPy shell were blocked by Pd and Fe3O4 nanoparticles during the preparation, which greatly increased the difficulty for dissolved PS chains completely diffusing out of the capsule shell.
Fig. 3. (a) SEM image of PS@Pd/Fe3O4/PPy composites; (b) TEM image of PS@Pd/Fe3O4/PPy composites; (c) SEM image of Pd/Fe3O4/PPy hollow capsules; (d) TEM image of Pd/Fe3O4/PPy hollow capsules; (e) magnified TEM image of Pd/Fe3O4/PPy hollow capsules; (f) HRTEM image of Fe3O4 and Pd nanoparticles; (g~i) TEM image and corresponding element mappings. The insets show corresponding magnified image.
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Fig. 4. XRD pattern of Pd/Fe3O4/PPy hollow capsules. ■represents Fe3O4 nanospheres; ▲represents Pd nanoparticles.
A magnified image shows that Pd and Fe3O4 nanoparticles randomly decorate in the hollow capsules (Fig. 3e). Because of similar electron contrast and spherical shape, it is difficult to identify them via different shade. To solve the problem, XRD measurement was done to determine their diameters. Fig. 4 shows the XRD pattern of resulting Pd/Fe3O4/PPy hollow capsules. Six diffraction peaks appear at 2θ= 30.2, 35.5, 43.2, 53.6, 57.1 and 62.6°, which are corresponding to (220), (311), (400), (422), (511) and (440) Bragg diffractions of cubic lattice of Fe3O4 nanospheres (JCPDS No. 19-0629), respectively. A diffraction peaks located at 39.7° appear due to the (111) Bragg diffractions of face centered Pd nanoparticles (JCPDS No. 05-0681). The size of Fe3O4 and Pd nanoparticles calculated from the (311) and (111) peak based on Scherrer formula is 22.4 and 7.8 nm, respectively, indicating the diameter of Fe3O4 nanoparticles is nearly three times larger than that of Pd nanoparticles. Based on this difference, in Fig. 3e and Fig. S2, we point Pd nanoparticles by red circles and Fe 3O4 13
nanoparticles by blue boxes. No obvious aggregation of Pd nanoparticles can be observed in TEM view. When PdCl2 was reduced to Pd nanoparticles by pyrrole monomer, the pyrrole monomer was also oxidized to PPy. During the redox reaction, Pd2+ ions were tend to concentrated around the PPy chains due to the coordination interaction between amino groups and Pd2+ ions (see below), which led PPy shell cover around the Pd nanoparticles and prevent them from further aggregation (indicated by arrows in Fig. 3e). Small size and uniform distribution of Pd nanoparticles could largely increase the active sites and promote intimate contact between them and PPy support, which was important for improving their catalytic activity [32]. In addition, when Fe3O4 nanoparticles started to form under alkaline condition, the polymerization of pyrrole monomer still proceeded; therefore, Fe 3O4 nanoparticles were also coated by a thin PPy shell (indicated by arrows in Fig. 3e), this was very important for their stability and we would discuss it later. In the high resolution TEM (HRTEM) image (Fig. 3f), the inter-planar distance of two kinds of nanoparticles is approximately 2.204 and 2.538 Å, which agrees well with the lattice spacing of the (111) and (311) plane of Pd and Fe3O4 nanoparticles, respectively. To further investigate the distribution of Pd and Fe3 O4 nanoparticles in hollow capsules, the element mappings of Fe and Pd were shown in Fig. 3g~i. The element mappings exhibit a homogenous distribution of Pd and Fe elements in the hollow capsules, confirming that Pd and Fe3O4 nanoparticles distribute homogenously rather than form respective aggregates. Meaningfully, the content of the Pd and Fe elements in the middle of the hollow capsules is less than that on the shell, further confirming the
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hollow structure of Pd/Fe3O4/PPy catalysts (Fig. 3h, i).
Fig. 5. XPS spectra of Pd/Fe3O4/PPy hollow capsules: (a) survey spectrum; (b) high-resolution Pd 3d XPS spectrum; (c) core-level spectrum of N 1s in Pd/Fe3O4/PPy hollow capsules; (d) core-level spectrum of N 1s in Fe3O4/PPy hollow capsules;.
XPS measurements were employed to characterize the surface chemical compositions and valence states of Pd/Fe3O4/PPy hollow capsules (Fig. 5). The survey spectrum of Pd/Fe3O4/PPy hollow capsules in Fig. 5a reveals the presence of C, N, O, Pd and Fe elements. In high-resolution Pd 3d XPS spectrum (Fig. 5b), two relatively strong satellite features are corresponding to 3d3/2 (341.25 eV) and 3d5/2 (336.20 eV), which confirms the oxidation state of Pd species in hollow capsules is Pd0 [33]. The signal of element N in survey spectrum mainly originates from amino groups in PPy chains. To further confirm the coordination interaction between the 15
amino groups and Pd2+ ions, the core-level spectrum of N 1s is investigated. For comparison, a control experiment to prepare Fe3O4/PPy hollow capsules was designed by a similar method to that for preparing Pd/Fe3 O4/PPy hollow capsule except that PdCl2 was not used as oxidant. In Fig. 5c and 5d, the binding energy of N 1s in Fe3O4/PPy hollow capsules appears at 399.75 eV, while it locates at 400.05 eV in Pd/Fe3O4/PPy hollow capsules. The N 1s core-level spectrum of these samples can be curve-fitted into three peak components with binding energy at 398.6, 399.8 and 400.6 eV, attributable to the amine (-NH-), imine (=N-), and positively charged nitrogen (N+) species, respectively. The content of N+ species increases from 19.7% (Fe3O4/PPy hollow capsules) to 27.5% (Pd/Fe3O4/PPy hollow capsules), which results in the red-shift of N 1s binding energy in absence or presence of Pd nanoparticles. The red-shift of N 1s binding energy confirms the electron donating effect of amino groups in PPy backbone, suggesting the Pd2+ ions coordinates with amino groups through sharing the electron pairs of amino groups [34]. Based on the previous study, such interaction was beneficial for increasing the loading and preserving the dispersion of metal nanoparticles [35]. It has been experimentally demonstrated that Pd nanoparticles have high catalytic activity in carbon–carbon coupling reaction, hydrogenation and reduction of various dyes [36,37]. Here, the catalytic reduction of 4-NP by NaBH4 to produce 4-aminophenol (4-AP) is chosen as a probe reaction to test the catalytic properties of Pd/Fe3O4/PPy hollow capsules. The preliminary catalytic testing was carried out by reducing 4-NP in water with NaBH4 as the reducing agent. Fig. 6a illustrates the 16
UV–Vis spectra of the 4-NP during the reaction in the presence or absence of the Pd/Fe3O4/PPy hollow capsules. Curve (a) is the UV–Vis spectrum of the initial 4-NP solution and the λmax appears at 317 nm. As commonly known, PPy homopolymer is good adsorbents for various dyes [38]. Therefore, before studying catalytic activity of Pd/Fe3O4/PPy hollow capsules, we had to investigate the adsorption of 4-NP in the presence of Pd/Fe3O4/PPy catalysts. Curve (b) shows the UV-Vis spectrum of 4-NP solution after adding Pd/Fe3O4/PPy hollow capsules for 1.0 h, as a nearly 9.7 % concentration decrease is observed, indicating that the adsorption has small influence on reduction of initial 4-NP solution concentration. Curve (c) is corresponding to the UV–Vis spectrum of the 4-NP solution after adding NaBH4 solution immediately. Addition of NaBH4 changed the pH of mixture from acidic to highly basic and resulted in formation of 4-nitrophenolate ions. Therefore, λmax remarkably red-shifts from original 317 to 400 nm, which is corresponding to a color change from light yellow to intense yellow. The reduction reaction was allowed to proceed for 24 h in absence of catalysts. In curve (d), the intensity of λmax reduce only 8.1%, indicating it is difficult for the reduction to proceed without the catalysts. In contrast, after the addition of 1.0 mg catalysts, the absorptions of 4-NP at 400 nm quickly disappears within 3.0 min, with the concomitant increase of the 300 nm peak of 4-AP (curve e). The solution color turns from intense yellow to colorless. Based on above UV–Vis analysis, it could be concluded that the Pd/Fe3O4/PPy hollow capsules had excellent catalytic activity. To study the reduction rate of 4-NP, the evolution of the catalytic reduction
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reaction was monitored by recording the absorbance of 4-NP. Fig. 6b illustrates the reduction reaction of 4-NP observed at different time intervals using 0.02 mg Pd/Fe3O4/PPy hollow capsule as the catalyst. The intensity of λ max gradually decrease, while the peak of 4-AP located at 300 nm appears and increases in intensity. Considering the much higher concentration of NaBH4 compared to that of 4-NP in overall reduction process (CNaBH4:C4-NP = 348:1), the pseudo-first-order kinetics can be applied with respect to 4-NP only [39,40]. The rate constant k is determined by a linear plot of ln(Ct/C0) and reaction time t in minutes ( the ratio of Ctto C0, where Ctand C0were the 4-NP concentrations at time t and 0, respectively, is measured from the relative intensity of the respective absorbance At/A0). Inset of Fig. 6b shows the linear relationship between ln(At/A0) and reaction time t in reduction reaction catalyzed by Pd/Fe3O4/PPy hollow capsules. As all these plots match the pseudo-first-order reaction equations very well, the rate constant k can be calculated from the rate equation and the value is 0.122 min-1 (in this case, the dosage of PdCl2 is 10.0 mg, and the loading of Pd nanoparticles is about 11.8 wt%). Table 1 lists the experimental parameters and k value of recent work, the catalytic performance of Pd/Fe3O4/PPy hollow capsule could be comparable with other studies [41-45]. The dosage of PdCl2 had a direct influence on the catalytic activity of Pd/Fe3O4/PPy hollow capsules since it determined the weight content of Pd nanoparticles. To investigate the relationship between the loading of Pd nanoparticles and the catalytic activity of the Pd/Fe3O4/PPy hollow capsules, four samples with the PdCl2 dosage of 0.0, 3.0, 6.0 and 15.0 mg were prepared. ICP measurements showed 18
the loading of Pd nanoparticles was 0.0, 5.56, 7.60 and 18.2 wt%, respectively. These samples with the same mass (0.02 mg) were added into the identical solution of 4-NP and NaBH4. The mixture was measured by a UV–Vis spectrometer at given time. As shown in Fig. 6c, although no PdCl2 used in sample preparation, the catalysts still shows the catalytic property due to the existence of Fe3O4 nanoparticles. Some studies have indicated that iron oxide nanoparticles could be used in catalytic reaction [46]. However, their catalytic activity was largely determined by diameter. In our study, the size of as-prepared Fe3O4 nanoparticles was large (22.4 nm), which resulted in a very poor catalytic activity. Therefore, the catalytic property of Pd/Fe3O4/PPy hollow capsules mainly originated from Pd nanoparticles. The rate constants of other three samples were also calculated according to the pseudo-first-order equation, and their value was 0.051, 0.085 and 0.160 min-1, respectively. Table 2 exhibits the relationship between the dosage of PdCl2, loading of Pd nanoparticles and k value. It is easy to appreciate that the more the PdCl2 used, the more Pd nanoparticles embedded in the PPy shell, and hence the better the catalytic property of Pd/Fe3O4/PPy hollow capsules.
Table 1. Comparison of catalytic property of different catalysts in recent studies.
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Fig. 6. (a) UV-Vis spectra of the 4-NP solution in different experimental procedure; (b) UV-Vis spectra of the 4-NP and NaBH4 mixture in the presence of 0.02 mg Pd/Fe3O4/PPy catalysts at different times, inset shows the rate constant k estimated by the slopes of straight lines of ln(At/A0) vs. reduction time; (c) conversion of 4-NP using 0.02 mg Pd/Fe3O4/PPy catalysts with different PdCl2usage for various times; (d) UV-Vis spectra of 4-NP reduction with catalysts in different cycles.
According to above discussion, one of the obstacles to hinder application of metal nanoparticles is that they are difficult to be separated from reaction solution. Therefore, the reusability of Pd/Fe3O4/PPy hollow capsules was needed to be investigated. In our experiments, Fe3O4 nanoparticles were synthesized during the preparation procedure, which endowed the catalysts with magnetic property. Fig. 7 shows the magnetization curve of the Pd/Fe3O4/PPy hollow capsules. The nearly zero 20
coercivity and the reversible hysteresis behavior indicate their super-paramagnetic nature at room-temperature. At 295 K, the saturation magnetization value of the Pd/Fe3O4/PPy hollow capsules is 26.0 emu/g (the content of Fe3O4 is about 44.4 wt%), which permits the catalysts can be quickly recycled from reaction solution by external magnetic field (inset of Fig. 7). In reduction of 4-NP, when the reduction reaction finished, the catalysts were directly magnetic separated from the reaction solution and rinsed with water for twice. Then, they were reused for the next-run under the same conditions. After reaction with 1.0 mg catalysts for 3.0 min in each cycle, the absorbance intensity of reaction solution was immediately measured. The results demonstrated no significant loss of activity for the reduction of 4-NP over Pd/Fe3O4/PPy hollow capsules in five successive catalytic cycles and the conversion in each cycle was nearly 100%, indicating the Pd/Fe3O4/PPy catalysts have good reusability (Fig. 6d). This was mainly because Pd nanoparticles in hollow capsules were separated by PPy chains, they could hardly contact with each other during the reused cycles, therefore, aggregation could not occur and their catalytic activity was well preserved during the repeated cycles.
Table 2. The relationship between the dosage of PdCl2, loading of Pd nanoparticles and k value.
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Fig. 7 Magnetization curves at 295 K of Pd/Fe3O4/PPy hollow capsules. Inset shows the digital image of magnetic separation of catalysts after reaction.
Besides high catalytic activity and reusability, another important property for catalysts is stability. As the catalytic reaction took place on the surface of metal nanoparticles, direct contact between metal nanoparticles and reaction solution was beneficial for enhancing catalytic activity. However, solution erosion usually resulted in their deactivation, especially after long-time contact. Therefore, compared with anchoring metal nanoparticles on the support surface, embedding metal nanoparticles in the porous shell was an alternative way to improve the service-life of catalysts, which not only make metal nanoparticles contact with reaction solution through porous channel, but also reduced the erosion caused by solution. In our study, both Pd and Fe3O4 nanoparticles were wrapped by PPy chains during the preparation. Due to the coverage of PPy shell, the direct contact between reaction solution and nanoparticles was avoided, and hence, the stability of catalysts was enhanced. Here,
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we have compared the stability between Pd/Fe3O4/PPy hollow capsules, bare Pd and Fe3O4 nanoparticles through HNO3 etching test. Pd/Fe3O4/PPy hollow capsules, bare Pd and Fe3O4 nanoparticles with the same charge were added into the identical HNO 3 solution with the concentration of 1.2 M. After 1.0 h, the Pd and Fe 3O4 nanoparticles were nearly completely dissolved. In contrast, the weight percentage of the dissolved Fe3O4 and Pd nanoparticles in hollow capsules was only 7.0 and 20 wt%, respectively. The catalysts after etching was also applied in reduction of 4-NP, and the reaction rate constant k reduced to 0.098 min-1 (Fig. S3). Based on above investigation, in our system, PPy shell not only played roles as traditional support to load Pd and Fe 3O4 nanoparticles, but also supplied coordination interaction with Pd nanoparticles, separated them from aggregation and protected them from being corroded by reaction solution.
4. Conclusions In summary, we have introduced a simple way to construct Pd/Fe3O4/PPy hollow capsules. During the preparation, PdCl2, FeCl3·6H2O and pyrrole monomer all played two roles, which led synthesization of Pd nanoparticles, preparation of Fe3O4 nanoparticles, and formation of PPy support can be finished in one-step, and hence reducing the preparation steps. In hollow capsules, PPy shell not only served as traditional support to load Pd and Fe3O4 nanoparticles, but also supplied coordination interaction with Pd nanoparticles, separated them from aggregation and protected them from being corroded. Therefore, catalytic activity, reusability and stability of
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catalysts were improved. In reduction of 4-NP by using NaBH4 as reductants, the Pd/Fe3O4/PPy hollow capsules exhibited excellent catalytic property. The rate constant was calculated through pseudo-first-order reaction equation and their value depended on the usage of PdCl2. In HNO3 etching test, the stability of nanoparticles in hollow capsules was superior to that of corresponding bare Pd and Fe3O4 nanoparticles owing to the coverage of PPy shell. In addition to catalytic activity and stability, reusability was also an important property for catalysts. The Pd/Fe3O4/PPy hollow capsules could be quickly recycled from the reaction solution due to their room-temperature super-paramagnetic property endowed by Fe3O4 nanoparticles. In recycling test, no significant decrease in catalytic activity was detected, even after the catalytic experiments were repeated 5 times, suggesting the catalysts showed good reusability. Since Pd/Fe3O4/PPy hollow capsules presented excellent catalytic activity, good reusability and high stability, together with a very simple preparation procedure, they were potentially applicable in industrial catalysis.
Acknowledgements This work was supported by the National Nature Science Foundation of China (Grant no. 51273051, 21174033, 21204015 and 21404035).
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Figure captions: Fig. 1. Formation process of Pd/Fe3O4/PPy hollow capsules.
Fig. 2. FT-IR spectra of (a) PS nanospheres; (b) PS@Pd/Fe3O4/PPy composites; (c) Pd/Fe3O4/PPy hollow capsules.
Fig. 3. (a) SEM image of PS@Pd/Fe3O4/PPy composites; (b) TEM image of PS@Pd/Fe3O4/PPy composites; (c) SEM image of Pd/Fe3O4/PPy hollow capsules; (d) TEM image of Pd/Fe3O4/PPy hollow capsules; (e) magnified TEM image of Pd/Fe3O4/PPy hollow capsules; (f) high-resolution TEM image of Fe3O4 and Pd nanoparticles; (g~i) TEM image and corresponding element mappings. The insets show corresponding magnified image.
Fig. 4. XRD pattern of Pd/Fe3O4/PPy hollow capsules. ■represents Fe3O4 nanospheres; ▲represents Pd nanoparticles.
Fig. 5. XPS spectra of Pd/Fe3O4/PPy hollow capsules: (a) survey spectrum; (b) high-resolution Pd 3d XPS spectrum; (c) core-level spectrum of N 1s in Pd/Fe3O4/PPy hollow capsules; (d) core-level spectrum of N 1s in Fe3O4/PPy hollow capsules.
Fig. 6. (a) UV-Vis spectra of the 4-NP solution in different experimental procedure; (b) UV-Vis spectra of the 4-NP and NaBH4 mixture in the presence of 0.02 mg 32
Pd/Fe3O4/PPy catalysts at different times, inset shows the rate constant k estimated by the slopes of straight lines of ln(At/A0) vs. reduction time; (c) conversion of 4-NP using 0.02 mg Pd/Fe3O4/PPy catalysts with different PdCl2usage for various times; (d) UV-Vis spectra of 4-NP reduction with catalysts in different cycles.
Fig. 7 Magnetization curves at 295 K of Pd/Fe3O4/PPy hollow capsules. Inset shows the digital image of magnetic separation of catalysts after reaction.
Table 1. Comparison of catalytic property of different catalysts in recent studies.
Table 2. The relationship between the dosage of PdCl2, loading of Pd nanoparticles and k value.
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Graphical Abstract
A simple way to prepare Pd/Fe3O4/polypyrrole hollow capsules and their applications in catalysis
Tongjie Yaoa, Hao Wanga, Quan Zuoa, Jie Wub, Baifu Xinb, Fang Cuia, Tieyu Cuia ,*
*
Corresponding authors. Tel.:+86-451-86403646. E-mail address: [email protected] (T. Cui) 34
S0021-9797(15)00271-4 http://dx.doi.org/10.1016/j.jcis.2015.03.012 YJCIS 20322
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
8 February 2015 6 March 2015
Please cite this article as: T. Yao, H. Wang, Q. Zuo, J. Wu, B. Xin, F. Cui, T. Cui, A simple way to prepare Pd/ Fe3O4/polypyrrole hollow capsules and their applications in catalysis, Journal of Colloid and Interface Science (2015), doi: http://dx.doi.org/10.1016/j.jcis.2015.03.012
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A simple way to prepare Pd/Fe3O4/polypyrrole hollow capsules and their applications in catalysis
Tongjie Yaoa, Hao Wanga,Quan Zuoa, Jie Wub, Baifu Xinb, Fang Cuia, Tieyu Cuia,*
a
The Academy of Fundamental and Interdisciplinary Science, Harbin Institute of
Technology, Harbin 150080,People’s Republic of China b
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education,
School of Chemistry and Materials Science, Heilongjiang University, Harbin, 150080, People’s Republic of China
*
Corresponding authors. Tel.:+86-451-86403646. E-mail address: [email protected] (T. Cui); 1
ABSTRACT Preparation of catalysts with good catalytic activity and high stability, together with magnetic separation property, in a simple way is highly desirable. In this paper, we reported a novel strategy to construct magnetic recyclable hollow capsules with Pd and Fe3O4 nanoparticles embedded in polypyrrole (PPy) shell via only two steps: first, synthesization of Pd nanoparticles, preparation of Fe 3O4 nanoparticles, and formation of PPy shell were finished in one-step on the surface of polystyrene (PS) nanospheres; then, the PS core was selectively removed by tetrahydrofuran. The Pd/Fe3O4/PPy hollow capsules exhibited good catalytic property in reduction of 4-nitrophenol with NaBH4 as reducing agent, and the reaction rate constants were calculated through pseudo-first-order reaction equation. Due to incorporation of Fe3O4 nanoparticles, the catalysts could be quickly separated from the reaction solution by magnet and reused without obvious catalytic loss. Besides catalytic property and reusability, their stability was also examined by HNO3 etching experiment. Compared with bare Pd and Fe3O4 nanoparticles, the stability of both Pd and Fe3O4 nanoparticles in hollow capsules was largely improved owing to the protection of PPy shell. The good catalytic performance, ease of separation, high stability and especially a simple preparation procedure, made Pd/Fe3O4/PPy hollow capsules highly promising candidates for diverse applications.
Keywords: polypyrrole (PPy); Fe3O4 nanoparticles; Pd nanoparticles; hollow capusles; catalysis
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1. Introduction Metal nanoparticles have attracted enormous interest because of their novel properties and corresponding applications in a broad range of areas, such as surface enhanced spectroscopy, biological imaging, optoelectronics, thermal therapy and especially in catalysis [1-3]. Over the last two decades, metal nanoparticles have been demonstrated to be efficient catalysts in many different reactions by taking the advantages of their large surface-to-volume ratios and shaped-dependent specific crystallographic surfaces [4,5]. However, when the particle size decreases to nanoscale, the aggregation caused by high surface energy and difficult separation caused by small size are practical obstacles, which hinders the application of nanoparticles in industrial use, even though they have high catalytic activity. To overcome these drawbacks, extensive efforts have been done. It has been proven through investigations that anchoring metal nanoparticles on supports is an effective approach to solve the above two drawbacks. Therefore, many materials with active sites for anchoring metal nanoparticles have been developed as candidates for catalyst supports [6-8]. Of wide range of supports, magnetic hollow capsules have drawn much attentions. Compared with typical solid support, they usually possess lower density, higher surface area and larger void space [9-11]. Moreover, incorporation of magnetic nanoparticles endows such support with magnetic property; therefore, they can be quickly separated from reaction solution by external magnetic field, thus avoiding the drawbacks of traditional centrifugation and filtration technique, such as inefficiency and time-wasting [12]. Because of these advantages, some researchers are
3
working on preparing magnetic hollow capsules as supports to load metal nanoparticles [13-15]. However, at present, the most related studies were focused on either the catalytic activity or reusability of catalysts, while the simplicity of preparation procedure was largely neglected. Take most frequently used strategy for constructing hollow capsules, hard-template method, as an example, to prepare magnetic hollow capsule, the typical preparation procedure usually involved four steps: preparation of magnetic nanoparticles, synthesization of metal nanoparticles, construction of shell around the hard-template, and removal of hard-template [16,17]. This traditional preparation procedure was too complex and time-wasting, which was not beneficial for industrial production. In our previous study [18], preparation of magnetic nanoparticles and construction of PPy shell were successfully finished in one-step; however, it still acquired extra procedures to synthesize metal nanoparticles and assemble them on the surface of hard-template. Therefore, To further simplify the experimental process, and prepare magnetic hollow capsules with good catalytic activity and high stability by a simpler approach still remained a grand challenge. Polypyrrole (PPy), as a conducting polymer, has been widely applied in the field of sensors, supercapacitors and biomedicine due to their high conductivity, environmental friendly and excellent stability [19-22]. Recently, many works reported their applications in catalysis [23-25], and the related results showed that PPy was an ideal catalyst support: on one hand, numerous of amino groups on their chains could supply enough active sites to increase the loading of metal nanoparticles and promote
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the intimate contact between the metal nanoparticles and PPy; on the other hand, synergism between the PPy and the metal ions could further make the catalyst more stable and enhance their catalytic performance. In addition, by taking advantage of good reducibility of pyrrole monomer, once metal salts acted as oxidants and pyrrole monomer served as reductants, the pyrrole monomer would be oxidized to the PPy, while the metal salts were reduced to the corresponding metal nanoparticles [26]. During the process, if magnetic nanoparticles could be synthesized at the same time, the formation of metal nanoparticles, preparation of magnetic nanoparticles, and construction of PPy support were combined into one-step; therefore, the preparation procedure was largely simplified. On the basis of the above statements, herein, we have designed a novel strategy to synthesize Pd/Fe3O4/PPy hollow capsules through only two-steps. Fig. 1 illustrates the synthetic procedure for the Pd/Fe3O4/PPy hollow capsules. First, the polystyrene (PS)@Pd/Fe3O4/PPy composites were synthesized in one-step since the oxidation of pyrrole monomer, reduction of PdCl2 and hydrolysis of Fe3+ and Fe2+ ions took place on PS nanosphere surface at the same time. The as-prepared PS@Pd/Fe3O4/PPy composites were then converted to Pd/Fe3O4/PPy hollow capsules through a consequent tetrahydrofuran (THF) etching. To the best of our knowledge, to date, this was the simplest procedure to prepare magnetic recyclable hollow capsules. Their catalytic property was investigated by reducing the 4-nitrophenol (4-NP) with NaBH4 as the reducing agent, and rate constant was calculated through pseudo-first-order reaction equation. Owing to the incorporation of Fe3O4 nanoparticles, the catalysts 5
could be easily separated from reaction solution by magnet and reused. More significantly, the stability of Pd/Fe3O4/PPy hollow capsules was 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. Styrene, acrylic acid, methylacrylic acid, absolute ethanol, THF, 4-NP, NH3·H2O (28 wt%), K2S2O8, FeCl3·6H2O, FeCl2·4H2O, PdCl2 and NaBH4 were purchased from Sinopharm Chemical Reagent Co., Ltd. All of chemicals were analytical grade and used as received. The water used in the experiments was deionized with a resistivity of 18.2 MΩ·cm-1.
2.2 Preparation of Pd/Fe3O4/PPy hollow capsules The carboxylic-capped PS nanospheres were synthesized according to the previous work [27]. Firstly, 50 μL pyrrole monomer was injected into the 20 mL PS nanosphere (55 mg) solution under mechanical stirring at ambient temperature. 0.5 h later, 10.0 mL mixture containing 50 mg FeCl3·6H2O and 10 mg PdCl2 was added into the above solution slowly. The oxidation polymerization was maintained for 5.0 h before nitrogen was purged into solution to remove oxygen. Then, 15 mg FeCl2·4H2O and 1.5 mL NH3·H2O were added into system successively. The reaction was allowed to proceed for another 2.0 h under nitrogen protection, and the PS@Pd/Fe 3O4/PPy 6
composites were prepared. Finally, to obtain Pd/Fe3O4/PPy hollow capsules, the as-prepared PS@Pd/Fe3O4/PPy composites were suspended into 30 mL THF solution. After stirring for a day, the PS nanospheres were removed, and the resulting Pd/Fe3O4/PPy hollow capsules were separated by magnet and washed by ethanol.
2.3. Catalyzed reduction of 4-NP The catalytic property of Pd/Fe3O4/PPy hollow capsules was explored by studying the change of the absorbance intensity at the maximum absorbance wavelength (λmax) of the 4-NP. In a typical procedure, 1.0 mg Pd/Fe3O4/PPy hollow capsules were homogeneously dispersed into the 2.0 mL 4-NP solution (25 mg·L-1), followed by a rapid injection of 0.5 mL of NaBH4 solution (10 mg·mL-1) under stirring. The color of the mixture gradually changed from intense yellow to colorless, indicating that the Pd/Fe3O4/PPy hollow capsules catalyzed the reduction of 4-NP. In the recycling study, the catalysts were separated from the solution by magnet when the reduction reaction completely finished. After washed by water twice, they were reused in the next reaction run. The procedure was repeated for 5 times. After reaction with 1.0 mg catalysts for 3.0 min in each cycle, the absorbance intensity of reaction solution was immediately measured and the conversation of 4-NP was calculated.
2.4. Characterization A JEOL JSM-6700F scanning electron microscope (SEM) with primary electron energy of 3 kV was employed to examine the surface morphologies of products. The
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structure and shell thickness of the materials were determined by a Tecnai G 2 F30 transmission electron microscope (TEM) operating at 300 kV. 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. A Lambda 750 ultraviolet and visible (UV-Vis) spectrophotometer was employed for analysis of 4-NP reduction. X-ray diffraction (XRD) data were collected on a Siemens D-5005 X-ray diffractometer with Cu Karadiation (λ = 1.5418 Å). 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. X-ray photoelectron spectroscopy (XPS) was collected by using a VG ESCALAB MKII spectrometer with Mg Ka excitation (1253.6 eV). Binding energy calibration was based on C 1s at 284.6 eV.
3. Results and discussion The morphology of PS nanospheres is presented in Fig. S1. They are very uniform with an average diameter of 450 nm. From a magnified image (inset of Fig. S1), we could see their surface is smooth, suggesting the as-prepared PS nanospheres were ideal hard-template for constructing resulting hollow capsules. The FT-IR spectrum of PS nanospheres is shown in Fig. 2a, the PS main peaks locate at 3027, 2921, 1499, 1454 and 696 cm−1. In addition, a peak appears at 1732 cm−1, suggesting carboxylic group is successfully modified on their surface, which is beneficial for deposition of PPy shell in next-step [28].
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Fig. 1. Formation process of Pd/Fe3O4/PPy hollow capsules.
As the oxidation potential of Pd2+/Pd0 (0.987 V) is much higher than that of Fe3+/Fe2+ (0.771 V), PdCl2 accelerates the oxidation polymerization of pyrrole monomer. An obvious phenomenon was that the color of solution quickly turned black within 3.0 min with PdCl2 and FeCl3·6H2 O as oxidant; in contrast, if only FeCl3·6H2O was used as oxidant, the color of solution was not changed to black until 20 min later. When the mixture of PdCl2 andFeCl3·6H2O was added into the system, Pd2+ ions were quickly reduced to Pd0 and Fe3+ ions were gradually reduced to Fe2+ ions by pyrrole monomer. Once NH3·H2O was introduced into the reaction system, the alkalinity of mixture increased, which resulted in the hydrolysis of both Fe 3+ and Fe2+ ions. Therefore, Fe(OH)3 and Fe(OH)2 were produced under alkaline condition, and finally Fe3O4 nanoparticles formed through dehydration [18]. The as-prepared Pd and Fe3O4 nanoparticles were deposited on the PS nanosphere surface and wrapped by PPy shell. Based on above analysis, In our system, PdCl2, FeCl3·6H2O and pyrrole monomer all played two roles in preparation procedure: for PdCl2, it acted as precursor of Pd nanoparticles and oxidant of pyrrole monomer; for FeCl3·6H2O, it acted as source of Fe3O4 nanoparticles and oxidant of pyrrole monomers; for pyrrole monomer, it acted as precursor of resulting PPy shell and reductants of both PdCl2 and 9
FeCl3·6H2O. Because of multifunctions of these raw materials, formation of Pd nanoparticles, preparation of Fe3O4 nanoparticles, and construction of PPy shell were finished in one-step; therefore, the preparation procedure was greatly reduced.
Fig. 2. FT-IR spectra of (a) PS nanospheres; (b) PS@Pd/Fe3O4/PPy composites; (c) Pd/Fe3O4/PPy hollow capsules.
Fig. 3a shows the SEM image of PS@Pd/Fe3O4/PPy composites. They exhibit uniform spherical shape. A magnified image reveals that the surface of PS@Pd/Fe3O4/PPy composites is considerable rough (inset of Fig. 3a), which looks like cauliflower, and this is the feature of PPy homopolymer [29]. Compared with original PS nanospheres, their diameter dramatically increases to 530 nm, suggesting the successful coverage of Pd/Fe3O4/PPy shell and their thickness is about 40 nm. Fig. 3b shows the corresponding TEM image of PS@Pd/Fe3O4/PPy composites. It is difficult to resolve the interface between PPy shell and PS core due to their similar electron contrast, and only solid composites with spherical shape can be seen. In magnified TEM image (inset of Fig. 3b), it is easy to distinguish that many inorganic
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nanoparticles randomly decorate in the PPy shell. The FT-IR spectrum of PS@Pd/Fe3O4/PPy composites is shown in Fig. 2b, the PS feature peaks appear at 3027, 2923, 1495, 1454 and 697 cm−1. In addition, the bands at 1551 and 1454 cm-1 are assigned to the stretching vibration of the C-C and C-N in pyrrole ring, and the ring deformation at 928 cm-1 is also observed [30]. Moreover, a peak at 573 cm-1 is due to the stretching mode of Fe-O bond, indicating Fe3O4 nanoparticles are synthesized in the PPy shell [31]. The FT-IR spectrum, together with SEM and TEM images, suggested the PS@Pd/Fe3O4/PPy composites had been successfully prepared. After PS@Pd/Fe3O4/PPy composites were added into THF solution and stirred for a day, the PS nanospheres were removed and Pd/Fe3O4/PPy hollow capsules were obtained. Fig. 3c shows the morphology of resulting capsules. Some of the capsules are collapse with many random folds due to high-vacuum condition under the SEM measurements (inset of Fig. 3c), suggesting the removal of PS core and survival of PPy shell during the etching process. In corresponding TEM image (Fig. 3d), the appearance of a large void further confirms the Pd/Fe3O4/PPy capsules possess a well-defined hollow structure, and the thickness of Pd/Fe3O4/PPy shell is measured to be 44 nm, which is in a good agreement with the value calculated from the SEM image. Although, PS nanospheres completely disappear in the view of TEM image, the FT-IR spectrum of Pd/Fe3O4/PPy hollow capsules reveals that the trace amounts of PS chains still remain in Pd/Fe3O4/PPy hollow capsules. In Fig. 2c, the peaks located at 1540, 1196, 1034 and 928 cm-1 belong to the PPy chain, and a weak peak appeared at 697 cm-1 is corresponding to residual PS chains. This might be because a
11
number of pores in PPy shell were blocked by Pd and Fe3O4 nanoparticles during the preparation, which greatly increased the difficulty for dissolved PS chains completely diffusing out of the capsule shell.
Fig. 3. (a) SEM image of PS@Pd/Fe3O4/PPy composites; (b) TEM image of PS@Pd/Fe3O4/PPy composites; (c) SEM image of Pd/Fe3O4/PPy hollow capsules; (d) TEM image of Pd/Fe3O4/PPy hollow capsules; (e) magnified TEM image of Pd/Fe3O4/PPy hollow capsules; (f) HRTEM image of Fe3O4 and Pd nanoparticles; (g~i) TEM image and corresponding element mappings. The insets show corresponding magnified image.
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Fig. 4. XRD pattern of Pd/Fe3O4/PPy hollow capsules. ■represents Fe3O4 nanospheres; ▲represents Pd nanoparticles.
A magnified image shows that Pd and Fe3O4 nanoparticles randomly decorate in the hollow capsules (Fig. 3e). Because of similar electron contrast and spherical shape, it is difficult to identify them via different shade. To solve the problem, XRD measurement was done to determine their diameters. Fig. 4 shows the XRD pattern of resulting Pd/Fe3O4/PPy hollow capsules. Six diffraction peaks appear at 2θ= 30.2, 35.5, 43.2, 53.6, 57.1 and 62.6°, which are corresponding to (220), (311), (400), (422), (511) and (440) Bragg diffractions of cubic lattice of Fe3O4 nanospheres (JCPDS No. 19-0629), respectively. A diffraction peaks located at 39.7° appear due to the (111) Bragg diffractions of face centered Pd nanoparticles (JCPDS No. 05-0681). The size of Fe3O4 and Pd nanoparticles calculated from the (311) and (111) peak based on Scherrer formula is 22.4 and 7.8 nm, respectively, indicating the diameter of Fe3O4 nanoparticles is nearly three times larger than that of Pd nanoparticles. Based on this difference, in Fig. 3e and Fig. S2, we point Pd nanoparticles by red circles and Fe 3O4 13
nanoparticles by blue boxes. No obvious aggregation of Pd nanoparticles can be observed in TEM view. When PdCl2 was reduced to Pd nanoparticles by pyrrole monomer, the pyrrole monomer was also oxidized to PPy. During the redox reaction, Pd2+ ions were tend to concentrated around the PPy chains due to the coordination interaction between amino groups and Pd2+ ions (see below), which led PPy shell cover around the Pd nanoparticles and prevent them from further aggregation (indicated by arrows in Fig. 3e). Small size and uniform distribution of Pd nanoparticles could largely increase the active sites and promote intimate contact between them and PPy support, which was important for improving their catalytic activity [32]. In addition, when Fe3O4 nanoparticles started to form under alkaline condition, the polymerization of pyrrole monomer still proceeded; therefore, Fe 3O4 nanoparticles were also coated by a thin PPy shell (indicated by arrows in Fig. 3e), this was very important for their stability and we would discuss it later. In the high resolution TEM (HRTEM) image (Fig. 3f), the inter-planar distance of two kinds of nanoparticles is approximately 2.204 and 2.538 Å, which agrees well with the lattice spacing of the (111) and (311) plane of Pd and Fe3O4 nanoparticles, respectively. To further investigate the distribution of Pd and Fe3 O4 nanoparticles in hollow capsules, the element mappings of Fe and Pd were shown in Fig. 3g~i. The element mappings exhibit a homogenous distribution of Pd and Fe elements in the hollow capsules, confirming that Pd and Fe3O4 nanoparticles distribute homogenously rather than form respective aggregates. Meaningfully, the content of the Pd and Fe elements in the middle of the hollow capsules is less than that on the shell, further confirming the
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hollow structure of Pd/Fe3O4/PPy catalysts (Fig. 3h, i).
Fig. 5. XPS spectra of Pd/Fe3O4/PPy hollow capsules: (a) survey spectrum; (b) high-resolution Pd 3d XPS spectrum; (c) core-level spectrum of N 1s in Pd/Fe3O4/PPy hollow capsules; (d) core-level spectrum of N 1s in Fe3O4/PPy hollow capsules;.
XPS measurements were employed to characterize the surface chemical compositions and valence states of Pd/Fe3O4/PPy hollow capsules (Fig. 5). The survey spectrum of Pd/Fe3O4/PPy hollow capsules in Fig. 5a reveals the presence of C, N, O, Pd and Fe elements. In high-resolution Pd 3d XPS spectrum (Fig. 5b), two relatively strong satellite features are corresponding to 3d3/2 (341.25 eV) and 3d5/2 (336.20 eV), which confirms the oxidation state of Pd species in hollow capsules is Pd0 [33]. The signal of element N in survey spectrum mainly originates from amino groups in PPy chains. To further confirm the coordination interaction between the 15
amino groups and Pd2+ ions, the core-level spectrum of N 1s is investigated. For comparison, a control experiment to prepare Fe3O4/PPy hollow capsules was designed by a similar method to that for preparing Pd/Fe3 O4/PPy hollow capsule except that PdCl2 was not used as oxidant. In Fig. 5c and 5d, the binding energy of N 1s in Fe3O4/PPy hollow capsules appears at 399.75 eV, while it locates at 400.05 eV in Pd/Fe3O4/PPy hollow capsules. The N 1s core-level spectrum of these samples can be curve-fitted into three peak components with binding energy at 398.6, 399.8 and 400.6 eV, attributable to the amine (-NH-), imine (=N-), and positively charged nitrogen (N+) species, respectively. The content of N+ species increases from 19.7% (Fe3O4/PPy hollow capsules) to 27.5% (Pd/Fe3O4/PPy hollow capsules), which results in the red-shift of N 1s binding energy in absence or presence of Pd nanoparticles. The red-shift of N 1s binding energy confirms the electron donating effect of amino groups in PPy backbone, suggesting the Pd2+ ions coordinates with amino groups through sharing the electron pairs of amino groups [34]. Based on the previous study, such interaction was beneficial for increasing the loading and preserving the dispersion of metal nanoparticles [35]. It has been experimentally demonstrated that Pd nanoparticles have high catalytic activity in carbon–carbon coupling reaction, hydrogenation and reduction of various dyes [36,37]. Here, the catalytic reduction of 4-NP by NaBH4 to produce 4-aminophenol (4-AP) is chosen as a probe reaction to test the catalytic properties of Pd/Fe3O4/PPy hollow capsules. The preliminary catalytic testing was carried out by reducing 4-NP in water with NaBH4 as the reducing agent. Fig. 6a illustrates the 16
UV–Vis spectra of the 4-NP during the reaction in the presence or absence of the Pd/Fe3O4/PPy hollow capsules. Curve (a) is the UV–Vis spectrum of the initial 4-NP solution and the λmax appears at 317 nm. As commonly known, PPy homopolymer is good adsorbents for various dyes [38]. Therefore, before studying catalytic activity of Pd/Fe3O4/PPy hollow capsules, we had to investigate the adsorption of 4-NP in the presence of Pd/Fe3O4/PPy catalysts. Curve (b) shows the UV-Vis spectrum of 4-NP solution after adding Pd/Fe3O4/PPy hollow capsules for 1.0 h, as a nearly 9.7 % concentration decrease is observed, indicating that the adsorption has small influence on reduction of initial 4-NP solution concentration. Curve (c) is corresponding to the UV–Vis spectrum of the 4-NP solution after adding NaBH4 solution immediately. Addition of NaBH4 changed the pH of mixture from acidic to highly basic and resulted in formation of 4-nitrophenolate ions. Therefore, λmax remarkably red-shifts from original 317 to 400 nm, which is corresponding to a color change from light yellow to intense yellow. The reduction reaction was allowed to proceed for 24 h in absence of catalysts. In curve (d), the intensity of λmax reduce only 8.1%, indicating it is difficult for the reduction to proceed without the catalysts. In contrast, after the addition of 1.0 mg catalysts, the absorptions of 4-NP at 400 nm quickly disappears within 3.0 min, with the concomitant increase of the 300 nm peak of 4-AP (curve e). The solution color turns from intense yellow to colorless. Based on above UV–Vis analysis, it could be concluded that the Pd/Fe3O4/PPy hollow capsules had excellent catalytic activity. To study the reduction rate of 4-NP, the evolution of the catalytic reduction
17
reaction was monitored by recording the absorbance of 4-NP. Fig. 6b illustrates the reduction reaction of 4-NP observed at different time intervals using 0.02 mg Pd/Fe3O4/PPy hollow capsule as the catalyst. The intensity of λ max gradually decrease, while the peak of 4-AP located at 300 nm appears and increases in intensity. Considering the much higher concentration of NaBH4 compared to that of 4-NP in overall reduction process (CNaBH4:C4-NP = 348:1), the pseudo-first-order kinetics can be applied with respect to 4-NP only [39,40]. The rate constant k is determined by a linear plot of ln(Ct/C0) and reaction time t in minutes ( the ratio of Ctto C0, where Ctand C0were the 4-NP concentrations at time t and 0, respectively, is measured from the relative intensity of the respective absorbance At/A0). Inset of Fig. 6b shows the linear relationship between ln(At/A0) and reaction time t in reduction reaction catalyzed by Pd/Fe3O4/PPy hollow capsules. As all these plots match the pseudo-first-order reaction equations very well, the rate constant k can be calculated from the rate equation and the value is 0.122 min-1 (in this case, the dosage of PdCl2 is 10.0 mg, and the loading of Pd nanoparticles is about 11.8 wt%). Table 1 lists the experimental parameters and k value of recent work, the catalytic performance of Pd/Fe3O4/PPy hollow capsule could be comparable with other studies [41-45]. The dosage of PdCl2 had a direct influence on the catalytic activity of Pd/Fe3O4/PPy hollow capsules since it determined the weight content of Pd nanoparticles. To investigate the relationship between the loading of Pd nanoparticles and the catalytic activity of the Pd/Fe3O4/PPy hollow capsules, four samples with the PdCl2 dosage of 0.0, 3.0, 6.0 and 15.0 mg were prepared. ICP measurements showed 18
the loading of Pd nanoparticles was 0.0, 5.56, 7.60 and 18.2 wt%, respectively. These samples with the same mass (0.02 mg) were added into the identical solution of 4-NP and NaBH4. The mixture was measured by a UV–Vis spectrometer at given time. As shown in Fig. 6c, although no PdCl2 used in sample preparation, the catalysts still shows the catalytic property due to the existence of Fe3O4 nanoparticles. Some studies have indicated that iron oxide nanoparticles could be used in catalytic reaction [46]. However, their catalytic activity was largely determined by diameter. In our study, the size of as-prepared Fe3O4 nanoparticles was large (22.4 nm), which resulted in a very poor catalytic activity. Therefore, the catalytic property of Pd/Fe3O4/PPy hollow capsules mainly originated from Pd nanoparticles. The rate constants of other three samples were also calculated according to the pseudo-first-order equation, and their value was 0.051, 0.085 and 0.160 min-1, respectively. Table 2 exhibits the relationship between the dosage of PdCl2, loading of Pd nanoparticles and k value. It is easy to appreciate that the more the PdCl2 used, the more Pd nanoparticles embedded in the PPy shell, and hence the better the catalytic property of Pd/Fe3O4/PPy hollow capsules.
Table 1. Comparison of catalytic property of different catalysts in recent studies.
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Fig. 6. (a) UV-Vis spectra of the 4-NP solution in different experimental procedure; (b) UV-Vis spectra of the 4-NP and NaBH4 mixture in the presence of 0.02 mg Pd/Fe3O4/PPy catalysts at different times, inset shows the rate constant k estimated by the slopes of straight lines of ln(At/A0) vs. reduction time; (c) conversion of 4-NP using 0.02 mg Pd/Fe3O4/PPy catalysts with different PdCl2usage for various times; (d) UV-Vis spectra of 4-NP reduction with catalysts in different cycles.
According to above discussion, one of the obstacles to hinder application of metal nanoparticles is that they are difficult to be separated from reaction solution. Therefore, the reusability of Pd/Fe3O4/PPy hollow capsules was needed to be investigated. In our experiments, Fe3O4 nanoparticles were synthesized during the preparation procedure, which endowed the catalysts with magnetic property. Fig. 7 shows the magnetization curve of the Pd/Fe3O4/PPy hollow capsules. The nearly zero 20
coercivity and the reversible hysteresis behavior indicate their super-paramagnetic nature at room-temperature. At 295 K, the saturation magnetization value of the Pd/Fe3O4/PPy hollow capsules is 26.0 emu/g (the content of Fe3O4 is about 44.4 wt%), which permits the catalysts can be quickly recycled from reaction solution by external magnetic field (inset of Fig. 7). In reduction of 4-NP, when the reduction reaction finished, the catalysts were directly magnetic separated from the reaction solution and rinsed with water for twice. Then, they were reused for the next-run under the same conditions. After reaction with 1.0 mg catalysts for 3.0 min in each cycle, the absorbance intensity of reaction solution was immediately measured. The results demonstrated no significant loss of activity for the reduction of 4-NP over Pd/Fe3O4/PPy hollow capsules in five successive catalytic cycles and the conversion in each cycle was nearly 100%, indicating the Pd/Fe3O4/PPy catalysts have good reusability (Fig. 6d). This was mainly because Pd nanoparticles in hollow capsules were separated by PPy chains, they could hardly contact with each other during the reused cycles, therefore, aggregation could not occur and their catalytic activity was well preserved during the repeated cycles.
Table 2. The relationship between the dosage of PdCl2, loading of Pd nanoparticles and k value.
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Fig. 7 Magnetization curves at 295 K of Pd/Fe3O4/PPy hollow capsules. Inset shows the digital image of magnetic separation of catalysts after reaction.
Besides high catalytic activity and reusability, another important property for catalysts is stability. As the catalytic reaction took place on the surface of metal nanoparticles, direct contact between metal nanoparticles and reaction solution was beneficial for enhancing catalytic activity. However, solution erosion usually resulted in their deactivation, especially after long-time contact. Therefore, compared with anchoring metal nanoparticles on the support surface, embedding metal nanoparticles in the porous shell was an alternative way to improve the service-life of catalysts, which not only make metal nanoparticles contact with reaction solution through porous channel, but also reduced the erosion caused by solution. In our study, both Pd and Fe3O4 nanoparticles were wrapped by PPy chains during the preparation. Due to the coverage of PPy shell, the direct contact between reaction solution and nanoparticles was avoided, and hence, the stability of catalysts was enhanced. Here,
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we have compared the stability between Pd/Fe3O4/PPy hollow capsules, bare Pd and Fe3O4 nanoparticles through HNO3 etching test. Pd/Fe3O4/PPy hollow capsules, bare Pd and Fe3O4 nanoparticles with the same charge were added into the identical HNO 3 solution with the concentration of 1.2 M. After 1.0 h, the Pd and Fe 3O4 nanoparticles were nearly completely dissolved. In contrast, the weight percentage of the dissolved Fe3O4 and Pd nanoparticles in hollow capsules was only 7.0 and 20 wt%, respectively. The catalysts after etching was also applied in reduction of 4-NP, and the reaction rate constant k reduced to 0.098 min-1 (Fig. S3). Based on above investigation, in our system, PPy shell not only played roles as traditional support to load Pd and Fe 3O4 nanoparticles, but also supplied coordination interaction with Pd nanoparticles, separated them from aggregation and protected them from being corroded by reaction solution.
4. Conclusions In summary, we have introduced a simple way to construct Pd/Fe3O4/PPy hollow capsules. During the preparation, PdCl2, FeCl3·6H2O and pyrrole monomer all played two roles, which led synthesization of Pd nanoparticles, preparation of Fe3O4 nanoparticles, and formation of PPy support can be finished in one-step, and hence reducing the preparation steps. In hollow capsules, PPy shell not only served as traditional support to load Pd and Fe3O4 nanoparticles, but also supplied coordination interaction with Pd nanoparticles, separated them from aggregation and protected them from being corroded. Therefore, catalytic activity, reusability and stability of
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catalysts were improved. In reduction of 4-NP by using NaBH4 as reductants, the Pd/Fe3O4/PPy hollow capsules exhibited excellent catalytic property. The rate constant was calculated through pseudo-first-order reaction equation and their value depended on the usage of PdCl2. In HNO3 etching test, the stability of nanoparticles in hollow capsules was superior to that of corresponding bare Pd and Fe3O4 nanoparticles owing to the coverage of PPy shell. In addition to catalytic activity and stability, reusability was also an important property for catalysts. The Pd/Fe3O4/PPy hollow capsules could be quickly recycled from the reaction solution due to their room-temperature super-paramagnetic property endowed by Fe3O4 nanoparticles. In recycling test, no significant decrease in catalytic activity was detected, even after the catalytic experiments were repeated 5 times, suggesting the catalysts showed good reusability. Since Pd/Fe3O4/PPy hollow capsules presented excellent catalytic activity, good reusability and high stability, together with a very simple preparation procedure, they were potentially applicable in industrial catalysis.
Acknowledgements This work was supported by the National Nature Science Foundation of China (Grant no. 51273051, 21174033, 21204015 and 21404035).
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Figure captions: Fig. 1. Formation process of Pd/Fe3O4/PPy hollow capsules.
Fig. 2. FT-IR spectra of (a) PS nanospheres; (b) PS@Pd/Fe3O4/PPy composites; (c) Pd/Fe3O4/PPy hollow capsules.
Fig. 3. (a) SEM image of PS@Pd/Fe3O4/PPy composites; (b) TEM image of PS@Pd/Fe3O4/PPy composites; (c) SEM image of Pd/Fe3O4/PPy hollow capsules; (d) TEM image of Pd/Fe3O4/PPy hollow capsules; (e) magnified TEM image of Pd/Fe3O4/PPy hollow capsules; (f) high-resolution TEM image of Fe3O4 and Pd nanoparticles; (g~i) TEM image and corresponding element mappings. The insets show corresponding magnified image.
Fig. 4. XRD pattern of Pd/Fe3O4/PPy hollow capsules. ■represents Fe3O4 nanospheres; ▲represents Pd nanoparticles.
Fig. 5. XPS spectra of Pd/Fe3O4/PPy hollow capsules: (a) survey spectrum; (b) high-resolution Pd 3d XPS spectrum; (c) core-level spectrum of N 1s in Pd/Fe3O4/PPy hollow capsules; (d) core-level spectrum of N 1s in Fe3O4/PPy hollow capsules.
Fig. 6. (a) UV-Vis spectra of the 4-NP solution in different experimental procedure; (b) UV-Vis spectra of the 4-NP and NaBH4 mixture in the presence of 0.02 mg 32
Pd/Fe3O4/PPy catalysts at different times, inset shows the rate constant k estimated by the slopes of straight lines of ln(At/A0) vs. reduction time; (c) conversion of 4-NP using 0.02 mg Pd/Fe3O4/PPy catalysts with different PdCl2usage for various times; (d) UV-Vis spectra of 4-NP reduction with catalysts in different cycles.
Fig. 7 Magnetization curves at 295 K of Pd/Fe3O4/PPy hollow capsules. Inset shows the digital image of magnetic separation of catalysts after reaction.
Table 1. Comparison of catalytic property of different catalysts in recent studies.
Table 2. The relationship between the dosage of PdCl2, loading of Pd nanoparticles and k value.
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Graphical Abstract
A simple way to prepare Pd/Fe3O4/polypyrrole hollow capsules and their applications in catalysis
Tongjie Yaoa, Hao Wanga, Quan Zuoa, Jie Wub, Baifu Xinb, Fang Cuia, Tieyu Cuia ,*
*
Corresponding authors. Tel.:+86-451-86403646. E-mail address: [email protected] (T. Cui) 34
