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J. Phys. Chem. C 2008, 112, 472-475
Artificial Construction of the Magnetically Separable Nanocatalyst by Anchoring Pt Nanoparticles on Functionalized Carbon-Encapsulated Nickel Nanoparticles Yanwen Ma, Bing Yue, Leshu Yu, Xizhang Wang, Zheng Hu,* Yining Fan, and Yi Chen Key Laboratory of Mesoscopic Chemistry of MOE and Jiangsu ProVincial Lab for NanoTechnology, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China
Weiwei Lin National Laboratory of Solid State Microstructures, Nanjing UniVersity, Nanjing 210093, China
Yinong Lu College of Materials Science and Engineering, Nanjing UniVersity of Technology, Nanjing 210009, China
Junhui Hu Shenzhen Junye Nano Material Co., Ltd., Shenzhen, 518118, China ReceiVed: June 9, 2007; In Final Form: October 20, 2007
A magnetically separable nanocatalyst consisting of 2-5-nm Pt particles dispersed on carbon-encapsulated nickel nanoparticles has been artificially constructed. The magnetically separable function of the nanocatalyst is well demonstrated by recycled hydrogenation of nitrobenzene to aniline. The construction process has been clearly illuminated, which is composed of the nondestructive functionalization of magnetic Ni(C) nanocapsules with carboxylic acid end groups and then the immobilization of Pt nanoparticles. This strategy should be applicable to the construction of some other magnetically separable nanocatalysts.
1. Introduction In recent years, magnetically separable nanocatalysts as a new kind of catalysts have attracted increasing attention due to their scientific and technological importance.1 Such kinds of catalysts are expected to have many advantages, e.g., they could overcome the transport limitations in the liquid phase, and they could easily be separated from the products by an external magnetic field for recycling. This could greatly enhance the catalytic performance and decrease the related cost; hence they have great potential applications.1d,e Generally, the construction of an ideal magnetically separable nanocatalyst should simultaneously meet the following high standards: (1) good dispersion of the active species (usually noble metal nanoparticles) on the magnetic support; (2) enough binding strength between the active species and support for recycle demand; (3) good chemical inertness of the magnetic support; (4) high saturation magnetization for magnetic separation; and (5) soft ferromagnetism for redispersion. Hence the construction of the advanced magnetically separable nanocatalysts has become a challenging topic for the development of this field. It is known that carbonencapsulated magnetic nanoparticles integrate the high chemical stability, good biocompatibility, and low toxicity of the outer graphite shells with the ferromagnetism of the inner nanoparticles.2 Moreover, the carbon shells can be modified with appropriate organic functional groups for anchoring the desired catalytically active species; hereby, the magnetically separable nanocatalysts could be constructed. However, little has been done on this aspect probably due to the difficulty to obtain * To whom correspondence should be addressed. Phone: 0086-2583686015. Fax: 0086-25-83686251. E-mail:
[email protected].
SCHEME 1: Free Radical Addition Route to Functionalize Ni(C) Nanocapsules with Carboxylic Acid End Groups
enough quantity of carbon-encapsulated magnetic nanoparticles. By laser-induction complex heating evaporation synthesis, we have been able to produce carbon-encapsulated nickel nanoparticles [Ni(C) nanocapsules] at the kilogram level for further exploration.3 Presented in this paper is the artificial construction of a magnetically separable nanocatalyst through nondestructively functionalizing magnetic Ni(C) nanocapsules with carboxylic acid end groups followed by binding Pt nanoparticles. The magnetically separable function of this nanocatalyst has been well demonstrated by recycled hydrogenation of nitrobenzene. The magnetic Ni(C) nanocapsules thus prepared are composed of carbon shells of a few nanometers in thickness and nickel cores with diameters ranging from 5 to 100 nm as described in our previous paper.4 It is well-known that carboxylic acid groups could be effectively generated on the surface of carbon nanostructures by oxidation at the expense of their partial skeleton;5 such a treatment would inevitably destruct part of the structure of the Ni(C) nanocapsules with the formation of carbon nanocages as the carbon shells of the nanocapsules are rather thin.4 To overcome the destruction of the nanocapsules and to get a high yield of carboxylic-acid-functionalized Ni(C)
10.1021/jp074477+ CCC: $40.75 © 2008 American Chemical Society Published on Web 12/19/2007
Construction of the Magnetically Separable Nanocatalyst
J. Phys. Chem. C, Vol. 112, No. 2, 2008 473
Figure 2. C1s XPS spectra of the Ni(C) (a), Ni(C)-CE (b), and Ni(C)-CP (c) samples.
Figure 1. ATR-FTIR spectra of the Ni(C) (a), Ni(C)-CE (b), and Ni(C)-CP (c) samples.
nanocapsules, a nondestructive free radical addition route has been employed.6 The functionalization process is briefly shown in Scheme 1. The organic acyl peroxides of dicarboxylic acids, HOOC(CH2)nC(O)OO(O)C(CH2)nCOOH [succinic (n ) 2) or glutaric (n ) 3) acid peroxide], were used as precursors of the corresponding “functional” radicals. Thermal decomposition of succinic or glutaric acid peroxides results in the generation of 2-carboxyethyl or 3-carboxypropyl radicals, respectively. The radicals thus formed are very reactive and ready to functionalize the Ni(C) nanocapsules. The corresponding products are noted as Ni(C)-CE (n ) 2) and Ni(C)-CP (n ) 3), respectively. 2. Experimental Section The product produced by laser-induction complex evaporation synthesis was pretreated with concentrated HCl solution (12 mol/ L) for removing the un-encapsulated Ni nanoparticles.3,4 The Ni(C) nanocapsules obtained were thoroughly washed with distilled water and then dried at 70 °C in air for further study. The detailed synthesis of organic acyl peroxides of dicarboxylic acids, i.e., succinic (or glutaric) acid peroxides, can be found elsewhere.6 In short, 10 g of fine powder of succinic (or glutaric) anhydride (>98%, Alfa) was added to 20 mL of 8% hydrogen peroxide and stirred for 30 min in an ice bath. The solution was filtered to leave the deposit, which was washed with a little (ca. 10 mL) water. The white peroxide product was vacuum-dried at room temperature for 24 h. About 6.5 g of succinic (or glutaric) acid peroxide was obtained. The acid-functionalized Ni(C) nanocapsules were prepared as follows: 0.5 g of Ni(C) nanocapsules were added to 50 mL of o-dichlorobenzene and sonicated (70 W, 40 kHz) for 10 min to obtain a liquid suspension, which was kept at about 90 °C and mechanically stirred for 10 days. During this period of time, 0.5 g of succinic (or glutaric) acid peroxide was added to the suspension every day. After cooling to room temperature, the suspension was added into 200 mL of tetrahydrofuran, which was stirred for 24 h. The product was separated by centrifuging (3000 rpm) and was fully washed with ethanol; the washingcentrifuging process was repeated for several times in order to remove the unreacted peroxide and byproducts. Finally, the functionalized Ni(C) nanocapsules, i.e., Ni(C)-CE and Ni(C)CP with the precursors of succinic and glutaric acid peroxides, respectively, were vacuum dried at 70 °C overnight. Pt nanoparticles were immobilized onto Ni(C)-CE particles by the ethylene glycol (EG) method.7 Specifically, 0.2 g of Ni(C)-CE was placed in 50 mL of EG and sonicated for 10 min to obtain a suspension. Four milliliters of EG solution of hexachloroplatinic acid (7.5 mg Pt/mL EG) was added dropwise
to the solution and mechanically stirred for 4 h, and then 5 mL NaOH (10 mol/L) was added. The mixture was refluxed at 140 °C for 3 h to ensure the complete reduction of Pt species. The whole preparation process was protected under flowing argon. Finally, the solid sample was collected by centrifuging (3000 rpm), fully washed with ethanol, and vacuum dried at room temperature. After EG reduction, the first-run filtrate was tested with 50 mL of NaBH4 solution (1 mol/L), and no precipitate was observed indicating no Pt cation containing species can be detected in the solution, i.e., all the Pt species were reduced to Pt nanoparticles giving about a 15% Pt loading. This product was denoted as Pt/Ni(C). The magnetically separable function of the Pt/Ni(C) nanocatalyst was tested by its recycle use for the hydrogenation of nitrobenzene to aniline. In a typical run, 100 mg of Pt/Ni(C) catalyst was added into a 125-mL isopropanol solution containing 6.5 mL of nitrobenzene in a 250-mL flask, which was placed in an autoclave under 2 × 106 Pa hydrogen. The reaction was performed at 80 °C under vigorous stirring (600 rpm) for 3 h. After reaction, the Pt/Ni(C) catalyst was collected by an external magnet, thoroughly washed with ethanol, and then vacuum dried for reuse. The catalyst-free product was used for characterization. Characterization. The various functional groups on the samples were detected by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR, NEXUS870) and X-ray photoelectron spectroscopy (XPS, ESCALab MK-II). Thermogravimetry-differential scanning calorimetry (TG-DSC) profiles were recorded by a STA-499C thermal analyzer (NETZSCH) at a 10 °C/min heating rate in Ar from about 30 to 1000 °C, the effluent was analyzed by an online mass analyzer (QUADSTAR-422, PFEIFFER) when needed. X-ray diffraction (XRD) experiments were carried out on a Philips X’pert Pro X-ray diffractometer with Cu KR radiation of 1.5418 Å. The morphology and structure of the product were analyzed by transmission electron microscopy (TEM, JEOL-JEM-1005 at 100 kV) and high-resolution transmission electron microscopy (HRTEM, JEM2010 at 200 kV) equipped with an energy dispersive X-ray spectrometer (EDX, ThermoNORAN). Magnetic hysteresis loops were measured by a vibrating sample magnetometer in the range of -12 000 to 12 000 Oe at room temperature. The products obtained by the hydrogenation of nitrobenzene were analyzed by gas chromatography. 3. Results and Discussion The ATR-FTIR spectra of the pristine and functionalized Ni(C) nanocapsules are shown in Figure 1. Compared with the featureless spectrum for the pristine Ni(C) nanocapsules, the spectrum for Ni(C)-CE clearly displays absorption peaks around 1715, 1563, and 1407 cm-1. The peaks around 1715 and 1407 cm-1 could be assigned to >CdO and -OH functional groups in carboxylic acid moieties (-COOH),
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Figure 3. (a) Thermal analysis on the Ni(C), Ni(C)-CE, and Ni(C)-CP samples and (b) mass analysis of the Ni(C)-CE sample. In (a), curves 1, 2, and 3 are TG profiles of Ni(C), Ni(C)-CE, and Ni(C)-CP, and curves 2′ and 3′ are DSC of Ni(C)-CE and Ni(C)-CP, respectively. In (b), lines 1-5 denote the ion current vs time plot for m/z 28 (CH2CH2), 44 (COO), 45 (COOH), 72 (CH2CH2COO), and 73 (CH2CH2COOH), respectively.
respectively.8 The peak around 1563 cm-1 should come from the CdC double bonds located near the functionalities.8,9 A similar infrared spectrum is observed for Ni(C)-CP. These results clearly indicate the attachment of carboxylic acid groups to nanocapsules. XPS analysis could provide additional information about the oxygen-containing surface groups. Figure 2 displays the C1s XPS spectra for Ni(C), Ni(C)-CE, and Ni(C)-CP samples. The spectrum for Ni(C) only shows a singlet at 284.6 eV, corresponding to the graphitic carbon shells of Ni(C) nanocapsules. For the spectra of Ni(C)-CE or Ni(C)-CP, the main change is the appearance of a new satellite peak around 289.0 eV, which could be attributed to the -COO- functional groups,9 indicating the grafting of the oxygen containing groups to Ni(C) nanocapsules from the functionalization. This is in agreement with the ATR-FTIR results. Figure 3a presents the TG-DSC curves of the Ni(C), Ni(C)CE, and Ni(C)-CP samples. It is seen that the TG curve of the Ni(C) nanocapsules displays a continuously slight weight loss after 450 °C (curve 1), which is tentatively attributed to the slow oxidation of the carbon shells to gaseous carbon oxide due to the trace O2 in Ar carrier gas. The slight weight loss (
