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Dec 7, 2009 - and the half-peak breadth is also broadened. Ag/C60 NCC exhibits excellent activity and selectivity in the hydrogenation...
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J. Phys. Chem. C 2009, 113, 21526–21530

Experimental Evidence for the Interface Interaction in Ag/C60 Nanocomposite Catalyst and Its Crucial Influence on Catalytic Performance Baojun Li, Hongbian Li, and Zheng Xu* State Key Laboratory of Coordination Chemistry and Nanjing National Laboratory of Microstructure, Department of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed: July 19, 2009; ReVised Manuscript ReceiVed: NoVember 14, 2009

New model catalysts are important for studying the interface interaction and their influence on the catalytic performance. In this paper, a novel Ag/C60 nanocomposite catalyst (Ag/C60 NCC) has been designed, prepared, and applied for the first time in the hydrogenation of aromatic nitro compounds. A new band at 1100 cm-1 appears in the Fourier transform infrared (FTIR) spectra of the Ag/C60 NCC, and the peak of the Ag (2) mode of C60 shifts clearly from 1468 to 1464 cm-1 in the Raman spectrum. X-ray photoelectron (XPS) spectrum of Ag 3d electron can be curve fitted into two pairs of peaks, corresponding to neutral Ag and Ag+, and the half-peak breadth is also broadened. Ag/C60 NCC exhibits excellent activity and selectivity in the hydrogenation of chloronitrobenzenes and nitrobenzaldehydes, whereas Ag nanocrystals (NCs), the mixture of Ag NCs, and C60 NCs do worse. These results evidence the presence of the charge transfer from Ag to C60 and clearly indicate the catalytic performance’s dependence on the interface interaction between Ag NCs and support C60 NCs. 1. Introduction Due to extraordinarily high catalytic activities and excellent selectivity for different types of reaction, oxide-supported metal nanoparticles have attracted intensive attention in the past several decades.1–4 Extensive research efforts have been paid on seeking to elucidate the role of supports played in such catalysts, but the fundamental understanding in this respect remains far from complete; moreover, some results reported contradict each other. By general consensus, the supports are able to assist the metal atoms or clusters to be dispersed well, therefore inhibiting the agglomeration of the metal clusters. Recently, some researchers proposed that the active sites are the lattice defects on the oxide support, possibly modified by metal clusters, and function together with sites on the metal nanoparticles.5–8 Others attributed the catalytic activity to the interaction between metal nanoparticles and oxide support that causes partial electron donation to the metal cluster,9 but no direct experimental evidence was obtained. In most reports, the active sites are placed at the metal oxide-support interface,10–12 also without direct experimental evidence except transmission electron microscopy (TEM) images. It is better to exploit a new model for searching some direct experimental evidence for the interface interaction and to study their influence on the catalytic performance. Ag is able to donate electrons more easily than gold; meanwhile, Ag nanocatalysis is still an undeveloped area.13–15 C60, having a definite molecule structure and novel redox properties,16 is of benefit to the electron transfer from Ag to C60 and to obtain the experimental evidence of interface interaction. Ag is selected as an active component, and C60 nanocrystals (NCs) are adopted as the support for fabrication of Ag/C60 nanocomposite catalyst (Ag/C60 NCC). If the interface interaction is a key factor for the catalytic performance of C60 NCs-supported Ag NCs catalyst, such metal-support combination will exhibit excellent catalytic performance. * Corresponding author. E-mail: [email protected].

The hydrogenation of chloronitrobenzenes (CNBs) and nitrobenzaldehydes (NBAs) was selected as the probe reaction to evaluate the activity and selectivity of the model catalyst fabricated here. The hydrogenation products chloroanilines (CANs) and aminobenzaldehydes (ABAs) are important intermediates for production of many fine chemicals. Although most catalysts used at present have high catalytic activity in hydrogenation reactions,17 a common problem remaining is the side reaction of catalytic hydrodechlorization of CNBs and parallel hydrogenation of aldehyde.18 Several strategies have been developed to solve this problem,19 but the side reaction of hydrodechlorization still cannot be avoided completely for Pd, Pt, and Ni catalysts. For Ru-based catalyst, high selectivity can be obtained, but the conversion of substrate into the target product is not high enough except when loaded on suitable supports. Another problem in the hydrogenation of nitro compounds is the accumulation of hydroxylamine intermediates. Due to its exothermic decomposition, toxicity, and ability to form colored products,20 eliminating the accumulation of hydroxylamine intermediates from the hydrogenation reaction system is a topic of industrial importance. In this paper, the Ag/C60 NCC was prepared from Ag+ and C60 monoanion and its catalytic performance was examined by the selective hydrogenation of aromatic nitro compounds. As expected, the Ag/C60 NCC exhibits excellent catalytic performance with 100% conversion and 100% selectivity for the hydrogenation of CNBs to CNAs and 100% selectivity for the NBAs to ABAs. The experimental results clearly reveal the presence of the charge transfer at the interface of Ag NCs and C60 NCs support compared with the mixture of Ag NCs and C60 NCs (Ag/C60 M). The excellent catalytic performances of Ag/C60 NCC depend on the strong interface interaction between Ag NCs and C60 NCs.

10.1021/jp906821h  2009 American Chemical Society Published on Web 12/07/2009

Ag/C60 Nanocomposite Catalyst for Hydrogenation

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SCHEME 1: Illustration of the Preparation of Ag/C60 NCC rt

growth

AgNO3 + C60- 98 Ag + C60 98 Ag/C60 nanocomposite catalyst THF, H2O

2. Experimental Section Preparation of Ag/C60 NCC. Preparation of C60 monoanion follows our previous work:21 C60 (34 mg), Ni-Al alloy (130 mg, excess), and NaOH (400 mg) were put in a bottle, which was evacuated to exclude oxygen and filled with N2, and then tetrahydrofuran (THF, 20 mL) was added to it with stirring. After adding oxygen-free water (5 mL), the reduction reaction took place rapidly. After 10 min, THF solution was separated from the colorless aqueous solution. The freshly prepared C60 monoanion was added into aqueous AgNO3 solution quickly at a mole ratio of 1:1 with stirring. After stirring for 20 min, the precipitates were separated by centrifugation at 4000 rpm, rinsed with distilled water twice to remove inorganic salts, and dried in vacuum at 50 °C for 4 h, and Ag/C60 NCC was obtained. Preparation of Ag NCs. In a typical synthesis, AgNO3 (0.1-3.0 g), dodecylamine (10 mL), and 1-octadecene (10 mL) were added into a three-neck flask. Subsequently, the reactor was heated from room temperature to 300-320 °C at a rate of 8 °C · min-1 and maintained at 300-320 °C for 10 min. After cooling to room temperature, the sample was rinsed with n-heptane and dried in vacuum at 50 °C for 24 h, and Ag NCs (8-10 nm) were obtained. Preparation of Ag/C60 M. Because it is difficult to load Ag NCs onto C60 NCs by a traditional impregnation method, the mixture of Ag NCs and C60 NCs is used. THF solution of C60 monoanion was stirred in air for 2 h, and some precipitates (C60 NCs) were deposited. After centrifugation at 4000 rpm for 3 min, the brown precipitate was obtained. C60 NCs and Ag NCs were dispersed in THF and ultrasonicated for 1 h. The sample was centrifugated at 4000 rpm for 3 min and dried in vacuum at 50 °C for 24 h. The brown precipitate was obtained and used as catalyst. Hydrogenation of Nitro Compounds. The hydrogenation of nitro compounds was carried out in a 100 mL stainless steel autoclave. For each reaction, substrate (150 mg), catalyst (15 mg), and ethanol (25 mL) were added into the autoclave. The autoclave was flushed with N2 three times to get rid of air. The hydrogenation reaction was performed under 3.0 MPa of hydrogen atmosphere at 140 °C and stirring at 900 rpm for 3 h. Then, the autoclave was cooled to room temperature and flushed twice with N2. The catalyst was separated by centrifugation at 12 000 rpm and washed with ethanol (3 × 5 mL) for reuse. The organic phase was dried over anhydrous Na2SO4 and evaporated under reduced pressure. The products and intermediates were identified by gas chromatography-mass spectroscopy (GCMS). The conversions of substrate were determined with 4,4′-methylenedianiline as the internal standard by GCMS. Characterization. TEM images were taken with a JEOL 1010 transmission electron microscope operated at an accelerating voltage of 100 kV and JEM-4000EX TEM using an accelerating voltage of 400 kV. Fourier transform infrared (FTIR) spectra were recorded on a Bruker VECTORTM 22 FTIR spectrometer. The X-ray photoelectron (XPS) spectrum measurements were made with an Axis Ultra photoelectron spectrometer using monochromatic Mg K X-ray (1253.6 eV), and binding energies were referred to C1s (284.2 eV). The X-ray powder diffraction (XRD) pattern was taken on Japan Rigaku D/Max R-A diffractometer with Cu KR irradiation (λ ) 1.5418

Å) in the 2θ range from 5° to 80°. GCMS was performed on a GCMS-QP2010 (SHIMADZU) spectrometer equipped with a DB-ms capillary column comparing with authentic samples. The analysis of the immediate products was carried out by highperformance liquid chromatography (HPLC) with a 250 mm × 4.6 mm VP-ODS tracer Lc-10A reverse phase column from Shimadzu, and the eluent is a solvent mixture of methanol and H2O with a volume ratio of 8:2 and flow rate of 1 mL · min-1. 3. Results and Discussion The preparation of Ag/C60 NCC is one-pot reaction by reducing AgNO3 with C60 monoanion (Scheme 1). The electrostatic interaction between Ag+ and C60 monoanion and the strong reducing behavior of C60 monoanion cause Ag+ to be reduced to form Ag NCs; meanwhile, C60 monoanion was oxidized to product C60 molecule to form C60 NCs instantaneously. Both products of oxidation-reduction reaction integrate with each other and then give a novel nanostructure in one step. From the TEM image of Ag/C60 NCC, we can see that Ag NCs are highly dispersed on C60 NCs (Figure 1a). The diameters of most Ag NCs are less than 10 nm with a mean diameter of 5.5 nm (the bottom inset in Figure 1a). But Ag NCs were not quite uniform; several are in the range of 10-20 nm or even bigger. In the high-resolution TEM (HRTEM) image of the Ag/ C60 NCC (Figure 1b), the clear lattice fringe evidence that Ag NCs are partially embedded in the surface layer of C60 NCs support and partly exposed to the ambient; meanwhile, the envelope can obscure the lattice fringe of the Ag NCs. The 0.32 nm lattice spacing is assigned to the (111) plane of the facecentered cubic (fcc) phase Ag. Those Ag NCs partially embedded in the surface layer of C60 NCs enhance the interface interaction between Ag NCs and C60 NCs support. The lattice spacing calculated from the bright diffraction spots in selective area electron difraction (SAED, the top inset in Figure 1a) is in accordance with the HRTEM image. The ringlike pattern can be attributed to the diffraction of polycrystalline C60. The XRD pattern (Figure 2a) of the Ag/C60 NCC showed no additional new peak, except the fcc C60 (JCPDS 82-0505) and fcc Ag (JCPDS 04-0783), indicating that no Ag-C60 alloy was formed in the oxidation-reduction process. XPS spectrum of the Ag 3d electron shows that the half-peak breadth is broadened from 1.1 to 1.45 (Figure 2b). The peak can be curve fitted into two pairs of peaks with binding energies of 368.7 and 374.7 eV attributed to Ag+ and 368 and 374 eV attributed to neutral Ag atom.22 In order to get an insight into the nature of the interface interaction in Ag/C60 NCC, Raman and FTIR spectra are conducted. Raman scattering spectra of pristine C60 (curve I), as prepared Ag/C60 NCC (curve II), and Ag/C60 M (curve III) were collected with the laser power of 0.06 mW in order to minimize the effect of photopolymerization (Figure 3a). The vibration frequencies of Hg (7), Ag (2), and Hg (8) modes for pristine C60 in curve I are 1421, 1468, and 1570 cm-1, respectively, in agreement with the previous report.23 Compared with pristine C60, the spectrum of the Ag/C60 NCC changes considerably (curve II). The peak of Ag (2) mode is broadened and the center of the peak shifts to a lower frequency from 1468 to 1464 cm-1. It is evidence of the presence of the electron

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Figure 1. (a) TEM and (b) HRTEM images of Ag/C60 NCC. The top inset is the SAED image of Ag/C60 NCC, and the bottom inset is the diameter distribution from 465 Ag NCs.

Figure 2. (a) XRD pattern and (b) XPS spectrum of the Ag/C60 NCC.

Figure 3. (a) Room temperature Raman spectra using a 514.5 nm laser as the light source and (b) FTIR spectra of pristine C60 (curve I), Ag/C60 NCC (curve II), and Ag/C60 M (curve III), respectively.

transfer from Ag to C60, namely, a strong interface interaction.24 Although the peaks of the Hg (7) and Hg (8) modes are almost the same with the pristine C60, the width and the relative intensity increase significantly in Ag/C60 NCC, which is similar to the results in Ag/C60 film25 and is difficult to interpret at present, so further studies are needed. As a comparison, the Raman spectrum of Ag/C60 M (curve III) is the same as pristine C60, except for a broadening of the peak of the Hg (8) mode. This proves the presence of the strong interface interaction in the Ag/C60 NCC. FTIR spectra of the pristine C60 (curve I), Ag/

C60 NCC (curve II), and Ag/C60 M (curve III) are shown in Figure 3b. Compared with the absorption of pristine C60 in curve I, in addition to four characteristic bands of C60 cage at 525, 575, 1182, and 1428 cm-1,26 three new peaks appear in the absorption spectrum of Ag/C60 NCC (curve II) at 1100, 1047, and 1635 cm-1. The former one is similar to the absorption of C60 monoanion.21 This is further evidence of the presence of electron transfers from Ag to C60 in the Ag/C60 NCC. The latter two correspond to the bonds of the minor oxide of C60 formed during the oxidation-reduction process of C60 monoanion.27

Ag/C60 Nanocomposite Catalyst for Hydrogenation

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TABLE 1: Catalytic Selective Hydrogenation of CNBs and NBAs with Ag/C60 NCCa

a

Reaction conditions: catalyst, 15 mg, 13% Ag; substrate, 150 mg, 0.95 mmol for entries 1-3 and 0.99 mmol for entries 4-6 and ethanol (25 mL); H2 pressure, 3.0 MPa; reaction temperature, 140 °C; reaction time, 3 h.

However, they are absent in curve I, the reason for this being that the pristine C60 was prepared by arc discharge and not by oxidation of C60 monoanion. The lack of distinguishable bands at 1100 cm-1 in curve III shows that there is no obvious electron transfers in Ag/C60 M. The reason for this is clearly because of the absence of a tight contact or obvious interface between Ag and C60 NCs in the mixture; i.e., the interface interaction between Ag and C60 NCs was weakened. The XPS, Raman, and FTIR spectra directly demonstrate the presence of the interface interaction between Ag NCs and C60 NCs, which strongly induces the formation of the charge bilayer at the interface. In order to get an insight into the relation between the interface interaction and the catalytic performance of the Ag/ C60 NCC, the catalytic hydrogenation experiments of CNBs and NBAs were conducted, and the results are listed in Table 1. Despite the size of Ag NCs being bigger, the catalytic performance is still excellent. Three CNB compounds including o-CNB, m-CNB, and p-CNB are all converted into their corresponding CANs with 100% conversion and 100% selectivity (entries 1-3). The 100% selectivity for CANs indicates that hydrodechlorization is suppressed completely. The Ag/C60 NCC also exhibits an excellent chemoselective reduction of nitro group in the compound containing aldehyde group (Table 1, entries 4-6). The conversion of the o-NBA is lower than the other two, indicating the capability, to some extent, of the regioselective reduction of the nitro group in the presence of other reducible functional groups.20 To trace the formation of hydroxylamine in the hydrogenation process, the reaction is quenched after 1 h. The HPLC result shows that there is no hydroxylamine intermediate in the sample except for the product p-CAN and unreacted p-CNB (see Supporting Information, Figure S3). In addition, the reaction solution is colorless throughout. These results further prove that accumulation of hydroxylamine is avoided efficiently. Because it is difficult to load Ag NCs on the C60 NCs and to control the size of Ag NCs by a traditional impregnation method,

TABLE 2: Catalytic Performance of Various Catalysts for Selective Hydrogenation of p-CNBa

entry

catalyst

amount of Ag (mol %)

conv (%)

1 2 3 4

Ag/C60 NCC Ag/C60 M Ag NCsb C60 NCs

1.89 1.89 3.78 0

100 35 27 0

a Reaction conditions: catalyst, 15 mg, 13% Ag; substrate, 150 mg, 0.95 mmol and ethanol (25 mL); H2 pressure, 3.0 MPa; reaction temperature, 140 °C; reaction time, 3 h. b 3.9 mg of Ag.

Ag/C60 M was used. As a comparison, a mixture of C60 NCs ∼50 nm in diameter and Ag NCs ∼6 nm in diameter is used as the catalyst. The conversion of p-CNB is 35% under the same experimental conditions, whereas Ag NCs ∼6 nm in diameter alone as the catalyst exhibited even lower conversion (27%), and no activity for hydrogenation of p-CNB was observed with the C60 NCs alone as the catalyst (Table 2, entries 2-4). The results from XPS, Raman, and FTIR spectra and catalytic hydrogenation experiments clearly show that there is a strong interface interaction in the Ag/C60 NCC, which has a significant influence on the catalytic performance of the Ag/C60 NCC.28 4. Conclusions In conclusion, the Ag/C60 NCC was designed, prepared, and applied as a novel model to study the role of interface interaction in metal NCs suppported catalyst. The preparation method for the catalyst is novel, which integrates the formation of Ag NCs and C60 NCs support and Ag loading in one step. The catalyst exhibited high activity and selectivity for the hydrogenation of CNBs and NBAs to the corresponding CANs and ABAs. The charge transfer at the interface between the metal NCs and the support was firmly evidenced by XPS, Raman, and FTIR spectra, and it played a crucial role for excellent catalytic

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performance. This work are significantly valuable for understanding the relation between catalytic performance and structure of the metal NCs supported catalysts. Acknowledgment. Financial support from the National Natural Science Foundation of China under the Major Research Project (No. 90606005) and the Jiangsu Province Foundation of Natural Science (No. BK2006717) is acknowledged. Supporting Information Available: Scanning electron microscopy image, energy dispersive X-ray spectrum pattern, and TEM images of the fresh and used Ag/C60 NCC; HPLC result of the quenched reaction mixture. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Bell, A. T. Science 2003, 299, 1688. (2) Zhang, X.; Shi, H.; Xu, B. Q. Angew. Chem., Int. Ed. 2005, 44, 7132. (3) Liu, Z. P.; Wang, C. M.; Fan, K. N. Angew. Chem., Int. Ed. 2006, 45, 6865. (4) Fu, Q.; Saltsburg, H.; Stephanopoulos, M. F. Science 2003, 301, 935. (5) Grunwaldt, J. D.; Baiker, A. J. Phys. Chem. B 1999, 103, 1002. (6) Schubert, M. M.; Hackenberg, S.; van Veen, A. C.; Muhler, M.; Plzak, V.; Behm, R. J. J. Catal. 2001, 197, 113. (7) Molina, L. M.; Hammer, B. Phys. ReV. Lett. 2003, 90, 20610214. (8) Abad, A.; Concepcion, P.; Corma, A.; Garcia, H. Angew. Chem., Int. Ed. 2005, 44, 4066. (9) Sanchez, A.; Abbet, S.; Heiz, U.; Schneider, W. D.; Hzˇkkinen, H.; Barnett, R. N.; Landman, U. J. Phys. Chem. A 1999, 103, 9573. (10) Iwasaws, T.; Tokunaga, M.; Obora, Y.; Tsuji, Y. J. Am. Chem. Soc. 2004, 126, 6554. (11) Zong, X.; Yan, H. J.; Wu, G. P.; Ma, G. J.; Wen, F. Y.; Wang, L.; Li, C. J. Am. Chem. Soc. 2008, 130, 7176. (12) Zhang, J.; Xu, Q.; Feng, Z.; Li, M. Angew. Chem., Int. Ed. 2008, 47, 1766. (13) (a) Gru1nert, W.; Bru1ckner, A.; Hofmeister, H.; Claus, P. J. Phys. Chem. B 2004, 108, 5709. (b) Bron, M.; Teschner, D.; Knop-Gericke, A.; Steinhauer, B.; Scheybal, A.; Ha¨vecker, M.; Wang, D.; Fo¨disch, R.; Ho¨nicke, D.; Wootsch, A.; Schlo¨gl, R.; Claus, P. J. Catal. 2005, 234, 37. (c) Bron, M.; Teschner, D.; Knop-Gericke, A.; Scheybal, A.; Steinhauer, B.; Ha¨vecker, M.; Fo¨disch, R.; Ho¨nicke, D.; Schlo¨gl, R.; Claus, P. Catal. Commun. 2005,

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