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Synthesis, characterization, and application of PdPt and PdRh bimetallic nanoparticles encapsulated within amine-terminated poly(amidoamine) dendrimers
Catalysis Communications 11 (2009) 62–66
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Synthesis, characterization, and application of PdPt and PdRh bimetallic nanoparticles encapsulated within amine-terminated poly(amidoamine) dendrimers Xiaohong Peng a,*, Qinmin Pan b,*, Garry L. Rempel b, Sheng Wu a a b
Department of Polymer Science and Engineering, South China University of Technology, Guangzhou 510640, PR China Department of Chemical Engineering, University of Waterloo, Ontario, Waterloo, Canada N2L 3G1
a r t i c l e
i n f o
Article history: Received 25 April 2009 Received in revised form 17 August 2009 Accepted 22 August 2009 Available online 27 August 2009 Keywords: Poly(amidoamine) dendrimer PdPt and PdRh bimetallic nanoparticles 1-Hexene Catalytic hydrogenation
a b s t r a c t PdPt and PdRh bimetallic dendrimer-encapsulated nanoparticles (DENs) were first prepared within fourth-generation amine-terminated poly(amidoamine) dendrimers (G4-NH2) by a co-complexation route, which included both co-complexation of bimetallic ions with dendrimers in HCl aqueous solution at pH 3 and subsequent reduction of the bimetallic ions in composite. The dendrimer-encapsulated bimetallic ions and reduction courses were analyzed by UV–vis spectroscopy and X-ray photoelectron spectroscopy. High-resolution transmission electron microscopy was used to characterize the two types of bimetallic nanoparticle size, size distribution, and particle morphology. The resulting PdPt and PdRh DENs show promising catalytic activities for the hydrogenation of 1-hexene. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The emergence of nanotechnology for making metal nanoparticles is providing new opportunities for a potential revolution in the field of catalysis. Dendrimers are particularly attractive hosts for catalytically active metal nanoparticles [1]. The dendrimertemplated formation of nanoparticles can be realized via two methods: the encapsulation of particles in the internal cavity of a dendrimer and the formation of particles surrounded by dendrimer branches [2,3]. Dendrimers with noncomplexing functional groups, such as hydroxyl groups [4,5] or partially quaternized groups [6] are generally favored for forming dendrimer-encapsulated nanoparticles (DENs). However, in the case of amine-terminated poly(amidoamine) (PAMAM), the stronger basic nature of the surface primary amine (pKa 9.23) compared to that of the interior tertiary amine (pKa 6.30) may lead to the formation of nanoparticles surrounded by complexation of metal ions with dendrimeric surface amine groups [7]. The control of dendrimeric solution pH could prevent the metal complexes from attaching to the primary amine groups on the PAMAM periphery, which may
* Corresponding authors. Tel./fax: +86 20 87114799 (X. Peng); tel./fax: +1 519 746 4979 (Q. Pan). E-mail addresses: [email protected] (X. Peng), [email protected] (Q. Pan). 1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2009.08.011
also be available to obtain either only platinum or only palladium DENs [8]. Although most of the previous studies have been confined to the ‘‘monometallic” nanoparticles, bimetallic nanoparticles composed of two different metal elements have drawn greater interest in the field of catalysis than monometallic ones. This is because bimetallization would make it possible not only to obtain catalysts with improved catalytic activity but also to create new types of catalysts, which may not be achieved by monometallic catalysts [9]. In some cases, hydrogenation rates can provide information about the atomic composition of the noble metal catalyst [5,10]. Accordingly, Crooks and co-workers [5] measured turnover frequencies (TOFs) for the hydrogenation of allyl alcohol in water using G4-OH[(Pd)x(Pt)40 x] DENs at various atomic ratios of Pd and Pt. The results showed that the TOFs of PdPt DENs were significantly higher than the physical mixtures of the single-metal analogues. The similar experimental results also appeared in the hydrogenation of allyl alcohol when the PdAu bimetallic DENs were used as catalysts [6], the maximal enhancement in the catalytic activity was observed for PdAu alloys at 70 mol% Pd. Here, PdPt and PdRh bimetallic DENs were first prepared by a co-complexation route with fourth-generation amine-terminated PAMAM dendrimer. For the hydrogenation of 1-hexene, the bimetallic DENs showed enhanced catalytic activities compared to physical mixtures of corresponding monometallic DENs.
X. Peng et al. / Catalysis Communications 11 (2009) 62–66
2. Experimental sections
63
The TOFs were determined on the basis of hydrogen uptake vs time.
2.1. Materials 3. Results and discussion The fourth-generation amine-terminated PAMAM dendrimer (G4-NH2) having an ethylenediamine core was purchased in a methanol solution with 10–25 wt.% from Dendritech Inc. (Midland, MI, USA). Prior to use, the methanol was removed under vacuum at room temperature (22 °C). K2PdCl4 (98 wt.%), K2PtCl4 (98 wt.%), RhCl3 (98 wt.%), NaBH4 (99 wt.%) (Aldrich Chemical Co., Milwaukee, WI, USA) were used as received. 2.2. Syntheses of bimetallic PdPt and PdRh DENs G4-NH2(PdPt) and G4-NH2(PdRh) DENs were synthesized by simultaneous co-complexation of two different metal ions, followed by a single reduction step. Firstly, dry G4-NH2 was dissolved in sufficient deionized water to yield a dendrimeric solution of 50 lM, the solution pH was adjusted to 3 by addition of 0.1 M HCl aqueous solution. Secondly, co-complexation of metal ions with dendrimers was carried out by adding a total volume of 1.0 mL of both 0.10 M K2PdCl4 and 0.10 M K2PtCl4 solutions, or of both 0.10 M K2PdCl4 and 0.10 M RhCl3 solutions to 50 mL or 85 mL, respectively, of a 50 lM aqueous solution of G4-NH2 dendrimers in a 100-mL shake-flask. After stirring at 200 rpm for 76 h (PdPt system) or 24 h (PdRh system), 2.0 mL of a 0.5 M NaBH4 solution was slowly added with vigorous stirring using a magnetic stirrer and then such formed mixture was stirred at 200 rpm for 1 h. Finally, the pH value of the solution was adjusted to 7–8 with 0.1 M HCl aqueous solution. 2.3. Characterization and analyses UV–vis absorption spectra can be used for characterizing the co-complexation of G4-NH2 with the two different metal ions followed by reduction, which were recorded using a Shimadzu UVmini-1240 UV–vis spectrophotometer. High-resolution transmission electron microscopy (HRTEM) micrographs having a point-to-point resolution of 0.23 nm were obtained using a JEM2010HR transmission electron microscope equipped with an ISIS300 X-ray energy dispersive spectrometer (EDS). For the HRTEM characterization, samples of bimetallic DENs were prepared by depositing a drop of the DEN solution on a carbon-coated Cu grid and dried naturally. X-ray photoelectron spectroscopy (XPS) studies were carried out by an ES-2402 device using Mo Ka radiation, samples were prepared as thin films from aqueous solutions of dendrimers containing metal nanoparticles on soaking silicon wafers.
PdPt DENs were prepared by simultaneous co-complexation of K2PdCl4 and K2PtCl4 with G4-NH2. Firstly, the peripheral amine groups of G4-NH2 can be selectively protonated by the control of solution pH 3 and then bimetallic ions can be exclusively trapped within the interior of dendrimers by simultaneous co-complexation with internal functional groups. Each dendrimer molecule coordinates with essentially the same number of bimetallic ions. Subsequent reduction produces zero-valent bimetallic nanoparticles, uniform in size, with one encapsulated in the interior of each dendrimer molecule. The intradendrimer templating mechanism is thought to predominate when metal ions (like Cu2+) or ionic pre2 cursors (like PtCl4 ) preferentially coordinate with the interior 2 groups of G4-NH2 [8,12]. Although PdCl4 was very quickly com2 plexed with the G4-NH2 (within 15 min) at pH 3, PtCl4 required several days (within 76 h) to completely complex at pH 5, so the lengthy stirring time (within 76 h) for the co-complexation of K2PdCl4 and K2PtCl4 with G4-NH2 was necessary during the synthesis procedure of PdPt bimetallic DENs [8]. Finally, an aqueous solution of 0.5 M NaBH4 was slowly added and the light yellow solution of dendrimer/metal ions immediately turned into dark brown, indicating the formation of bimetallic nanoparticles. We also found that the resulting solutions of nanoparticles were very stable for at least one month after adjusting the final solution pH to 7–8. Fig. 1a and Table 1 show UV–vis absorbance spectra of Pd and Pt metals during the course of co-complexation and the subsequent reduction. For the maximum sensitivity wavelength (kmax) of the monometal ions, the characteristic peaks at 207 and 237 nm arise 2 from the absorbance of PdCl4 , and the characteristic peak at 2 194 nm from PtCl4 . In the absence of dendrimer, the solution mix2 2 ture of PdCl4 and PtCl4 also exhibits the mixed absorption peaks of two metal ions, which are at 204 and 236 nm. When the addi-
2.4. Hydrogenation using gas uptake apparatus The hydrogenation of 1-hexene was carried out in the presence of bimetallic DENs in a solvent mixture of tetrahydrofuran/water (v/v = 6/1). The hydrogenation reactions were run at 50 psi and 25 °C. Ten milliliters of the bimetallic DEN solution and 120 mL of solvent were placed into a 300 mL hydrogenation gas uptake autoclave developed by Mohammadi and Rempel [11]. The system was degassed by bubbling with H2 for 10 min. When the temperature and H2 pressure of the autoclave were stabilized, 4.8 mL (37.0 mmol) of 1-hexene was pressurized into the solution by a 5 mL substrate addition device with agitation at 600 rpm, followed by measurement of the uptake of H2. There was no evidence for DEN aggregation during or after the hydrogenation reaction, indicating that the catalysts were stable within the reaction periods.
Fig. 1. (a) Variations in the UV–vis spectra of Pd and Pt metals during the course of co-complexation and the subsequent reduction. (b) UV–vis spectra of G4NH2(PdxPty) with various Pd/Pt ratios. In all cases, the final concentrations of total metal and G4-NH2 were 0.1 mM and 2.5 lM, respectively.
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X. Peng et al. / Catalysis Communications 11 (2009) 62–66
Table 1 The maximum spectral sensitivity wavelength (kmax) for the UV–vis spectra of Pd and Pt metals during the course of co-complexation and the subsequent reduction. Substrate
Pd2+
Pt2+
(Pd2+7) + (Pt2+3)
G4-NH2 (Pd2+7Pt2+3)
G4-NH2 (Pd7Pt3)
G4-NH2
kmax (nm)
207,237
194
204,236
212
212
196
2
2
tion of G4-NH2 to the solution mixture of PdCl4 and PtCl4 , however, a new strong absorption peak at 212 nm appears (see Fig. 1a and Table 1). This indicates that two metal ions are complexed with the internal functional groups of the dendrimer and are encapsulated in the dendrimer host. After reduction of the composite, the absorption peak at 212 nm is weakened. Fig. 1b presents a series of UV–vis spectra of the PdPt bimetallic nanoparticles with various Pd/Pt ratios (G4-NH2(PdxPty), where x/y = Pd/Pt molar ratio). Each spectrum in Fig. 1b exhibits a single peak and the peak height decreases as the Pt loading increases, the absorption bands are of nearly exponential shape and this indicates the complete reduction of metal ions [4]. Moreover, it is noted that the spectra of the bimetallic nanoparticles are different from those of the monometallic Pd and Pt nanoparticles. The change can be primarily attributed to the change in dielectric function caused by mixing the two different metal atoms [13]. Therefore, the characteristic monotonic absorption spectra strongly suggest that bimetallic nanoparticles are formed in the cavity of the dendrimers. The representative HRTEM micrograph of G4-NH2(Pd7Pt3) is shown in Fig. 2a. The nanoparticles are nearly uniform and their shape is almost spherical. The microscopy demonstrates that the nanoparticles are a Gaussian-like size distribution. Average particle diameter (D) and standard deviation (S) of PdPt bimetallic DENs are 2.5 and 0.5 nm, respectively (Fig. 2b).
Fig. 2. HRTEM micrograph of G4-NH2(Pd7Pt3) nanoparticles with a Pd/Pt ratio of 70/30 (a), and the corresponding size distribution histogram (b).
EDS analyses of both Pd and Pt elements were carried out and only their average compositions were obtained because it was rather difficult to acquire the elements of one nanoparticle. The result of XPS analysis showed that the metal ions were completely reduced irrespective of the Pd/Pt ratios. The Pd 3d5/2 peak shifts from Pd2+ 338.1 eV to Pd0 335.7 eV after reduction, and a shift from 72.5 to 71.3 eV was found for the Pt 4f7/2 peak. These XPS results are very similar to those found for monometallic DENs. After adjusting the pH of G4-NH2 solution to 3 using HCl, the preparation of PdRh bimetallic nanoparticles within G4-NH2 dendrimers was carried out via the co-complexation method. By preloading a dendrimer ‘‘nanoreactor” with stoichiometric bimetallic ions of Pd2+ and Rh3+ and then the dendrimer-encapsulated PdRh nanoparticles were prepared via slowly adding NaBH4 solution to the composite in situ. The light red dendrimer-encapsulated metal ion solution immediately turned to a homogeneous brownishblack solution, indicating the formation of PdRh bimetallic nanoparticles. The resulting nanoparticles were stable for one month after adjusting the solution pH to 7–8. The UV–vis spectrum of G4-NH2(Pd5Rh5) DENs in Fig. 3a reveals an absorbance peak at 212 nm. Formation of the particles was confirmed by the decrease of the absorption peak intensity after chemically reducing G4-NH2(Pd2+5Rh3+5). It demonstrated that the spectra of the obtained nanoparticles were different not only from those of the monometallic Pd and Rh nanoparticles but also from those of their physical mixtures. Fig. 3b presents a series of UV–vis spectra of PdRh bimetallic nanoparticles with various Pd/ Rh ratios. As the Pd/Rh ratio decreases, the kmax shifts to a longer wavelength. The change in the absorption spectra of the bimetallic nanoparticles from those of monometallic ones can be primarily
Fig. 3. (a) Variations in the UV–vis spectra of G4-NH2(Pd5Rh5) and G4NH2(Pd2+5Rh3+5) compared to their monometallic and physical mixture DENs. (b) UV–vis spectra of G4-NH2(PdxRhy) with various Pd/Rh ratios. In all cases, the final concentrations of total metal and G4-NH2 were 0.07 mM and 3.0 lM, respectively.
X. Peng et al. / Catalysis Communications 11 (2009) 62–66
Fig. 4. HRTEM image of G4-NH2(Pd5Rh5) nanoparticles with a Pd/Rh ratio of 50/50 (a), and the corresponding size distribution histogram (b).
attributed to the change in dielectric function caused by mixing the two different metal atoms. Thus, the characteristic absorption spectra also suggest the formation of PdRh DENs. The representative HRTEM image of G4-NH2(Pd5Rh5) is shown in Fig. 4a. The average particle diameter and dispersion of the nanoparticles are 3.0 ± 1.1 nm (Fig. 4b). The micrograph demonstrates that the particle size is nearly uniform and the shape is roughly spherical. Although the particle size distribution of PdRh bimetallic nanoparticles is somewhat broader compared to those of monometallic and bimetallic noble metal nanoparticles [14,15], the size distribution of the resulting nanoparticles is nearly monodispersed. In addition, only average compositions of Pd and Rh nanoparticles were obtained via the EDS characterization like PdPt system. XPS analysis following reduction indicates that the Pd and Rh salts are all completely reduced: The Pd 3d5/2 peak shifts from 337.5 to 335.2 eV upon reduction, and the Rh 3d5/2 peak is shifted from 311.1 to 308.8 eV. In order to investigate the catalytic activity, the PdPt bimetallic DEN catalysts were applied in hydrogenation of 1-hexene at 50 psi and 25 °C. Fig. 5a shows that the PdPt bimetallic DENs present high catalytic activities in the hydrogenation reaction. Monometallic Pt DENs yielded the lowest TOF (100 mol H2(mol Pt) 1 h 1), while the TOFs of physical mixtures of Pd-only and Pt-only DENs were located on the straight line. The PdPt DENs (Pd/Pt ratios of 3/7, 5/5, and 7/3) gave larger TOFs than those of the physical mixtures of monometallic DENs with equal composition. The maximum TOF (1150 mol H2(mol Pd + Pt) 1 h 1) was at Pd/Pt ratio of 7/3, which was even larger than that of Pd-only DENs (TOF = 800 mol H2(mol Pd) 1 h 1). The enhanced catalytic activity of the bimetallic DENs could be due to a synergistic electronic effect [5,10]. As was discussed above, the catalytic activity of PdRh system (see Fig. 5b) is similar with the PdPt system. However, TOFs of
65
Fig. 5. Turnover frequencies (TOFs) for the hydrogenation of 1-hexene as functions of mole percentages of palladium and rhodium. (a) PdPt bimetallic DENs and physical mixtures containing Pd-only and Pt-only DENs. (b) PdRh bimetallic DENs and physical mixtures containing Pd-only and Rh-only DENs. The (j) represents the bimetallic DENs, while the (.) represents the data obtained from their monometallic physical mixtures. Conditions: (a) 25 °C, 50 psi, the substrate/metal = 1850/1 and [Pd + Pt] = 150 lM; (b) 25 °C, 50 psi, the substrate/metal = 3150/1, and [Pd + Rh] = 90 lM.
the PdRh bimetallic DENs were higher than those of PdPt bimetallic DENs in the hydrogenation of 1-hexene, and the highest TOF of PdRh system was achieved at a Pd/Rh ratio of 5/5 while PdPt system was at a Pd/Pt ratio of 7/3. These results indicate that the catalytic activities are dependent upon the metal composition of bimetallic nanoparticles. According to the view of the synergistic electronic effect of Toshima [10], the electronic interaction between two different metals may provide an uneven electron distribution, which could make the electron density of one metal become lower. This effect of one metal upon another could cause the bimetallic cluster to be more active than monometallic ones because the substrate containing a double bond favors the electron-deficient surface. These results also provided additional evidence for the formation of PdPt and PdRh DENs.
4. Conclusions We have shown that the PdPt and PdRh bimetallic nanoparticles can be prepared within amine-terminated G4-NH2 dendrimers by controlling protonation of the terminal amine groups and selective co-complexation of bimetallic ions with interior amine groups. The UV–vis absorbance spectra and catalytic hydrogenation activities of the PdPt and PdRh bimetallic nanoparticles are different not only from those of their monometallic nanoparticles but also from those of their physical mixtures. For 1-hexene hydrogenation, the highest TOFs of PdPt and PdRh systems were different and the catalytic activity of PdRh bimetallic DENs were found to be more effective than those of PdPt bimetallic DENs.
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Acknowledgments The authors gratefully acknowledge the supports of Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation (CFI), the China Scholarship Fund and the Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars (State Education Ministry). References [1] X.-H. Peng, Q.-M. Pan, G.L. Rempel, Chem. Soc. Rev. 37 (2008) 1619–1628. [2] Y.-M. Chung, H.-K. Rhee, Catal. Surv. Asia 8 (2004) 211–223.
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
W. Zhang, L. Li, Y. Du, X. Wang, P. Yang, Catal. Lett. 127 (2009) 429–436. Y.-M. Chung, H.-K. Rhee, J. Mol. Catal. A: Chem. 206 (2003) 291–298. R.W.J. Scott, A.K. Datye, R.M. Crooks, J. Am. Chem. Soc. 125 (2003) 3708–3709. R.W.J. Scott, O.M. Wilson, S.-K. Oh, E.A. Kenik, R.M. Crooks, J. Am. Chem. Soc. 126 (2004) 15583–15591. Y. Niu, L. Sun, R.M. Crooks, Macromolecules 36 (2003) 5725–5731. H. Ye, R.W.J. Scott, R.M. Crooks, Langmuir 20 (2004) 2915–2920. J.H. Sinfelt, Acc. Chem. Res. 20 (1987) 134–139. N. Toshima, T. Yonezawa, New J. Chem. 22 (1998) 1179–1201. N.A. Mohammadi, G.L. Rempel, Comput. Chem. Eng. 11 (1987) 27–35. Y. Gu, H. Xie, J. Gao, D. Liu, C.T. Williams, C.J. Murphy, H.J. Ploehn, Langmuir 21 (2005) 3122–3131. K. Torigoe, Y. Nakajima, K. Esumi, J. Phys. Chem. 97 (1993) 8304–8309. K. Esumi, A. Suzuki, A. Yamahira, K. Torigoe, Langmuir 16 (2000) 2604–2608. O.M. Wilson, R.W.J. Scott, J.C. Garcia-Martinez, R.M. Crooks, J. Am. Chem. Soc. 127 (2005) 1015–1024.
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Synthesis, characterization, and application of PdPt and PdRh bimetallic nanoparticles encapsulated within amine-terminated poly(amidoamine) dendrimers Xiaohong Peng a,*, Qinmin Pan b,*, Garry L. Rempel b, Sheng Wu a a b
Department of Polymer Science and Engineering, South China University of Technology, Guangzhou 510640, PR China Department of Chemical Engineering, University of Waterloo, Ontario, Waterloo, Canada N2L 3G1
a r t i c l e
i n f o
Article history: Received 25 April 2009 Received in revised form 17 August 2009 Accepted 22 August 2009 Available online 27 August 2009 Keywords: Poly(amidoamine) dendrimer PdPt and PdRh bimetallic nanoparticles 1-Hexene Catalytic hydrogenation
a b s t r a c t PdPt and PdRh bimetallic dendrimer-encapsulated nanoparticles (DENs) were first prepared within fourth-generation amine-terminated poly(amidoamine) dendrimers (G4-NH2) by a co-complexation route, which included both co-complexation of bimetallic ions with dendrimers in HCl aqueous solution at pH 3 and subsequent reduction of the bimetallic ions in composite. The dendrimer-encapsulated bimetallic ions and reduction courses were analyzed by UV–vis spectroscopy and X-ray photoelectron spectroscopy. High-resolution transmission electron microscopy was used to characterize the two types of bimetallic nanoparticle size, size distribution, and particle morphology. The resulting PdPt and PdRh DENs show promising catalytic activities for the hydrogenation of 1-hexene. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The emergence of nanotechnology for making metal nanoparticles is providing new opportunities for a potential revolution in the field of catalysis. Dendrimers are particularly attractive hosts for catalytically active metal nanoparticles [1]. The dendrimertemplated formation of nanoparticles can be realized via two methods: the encapsulation of particles in the internal cavity of a dendrimer and the formation of particles surrounded by dendrimer branches [2,3]. Dendrimers with noncomplexing functional groups, such as hydroxyl groups [4,5] or partially quaternized groups [6] are generally favored for forming dendrimer-encapsulated nanoparticles (DENs). However, in the case of amine-terminated poly(amidoamine) (PAMAM), the stronger basic nature of the surface primary amine (pKa 9.23) compared to that of the interior tertiary amine (pKa 6.30) may lead to the formation of nanoparticles surrounded by complexation of metal ions with dendrimeric surface amine groups [7]. The control of dendrimeric solution pH could prevent the metal complexes from attaching to the primary amine groups on the PAMAM periphery, which may
* Corresponding authors. Tel./fax: +86 20 87114799 (X. Peng); tel./fax: +1 519 746 4979 (Q. Pan). E-mail addresses: [email protected] (X. Peng), [email protected] (Q. Pan). 1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2009.08.011
also be available to obtain either only platinum or only palladium DENs [8]. Although most of the previous studies have been confined to the ‘‘monometallic” nanoparticles, bimetallic nanoparticles composed of two different metal elements have drawn greater interest in the field of catalysis than monometallic ones. This is because bimetallization would make it possible not only to obtain catalysts with improved catalytic activity but also to create new types of catalysts, which may not be achieved by monometallic catalysts [9]. In some cases, hydrogenation rates can provide information about the atomic composition of the noble metal catalyst [5,10]. Accordingly, Crooks and co-workers [5] measured turnover frequencies (TOFs) for the hydrogenation of allyl alcohol in water using G4-OH[(Pd)x(Pt)40 x] DENs at various atomic ratios of Pd and Pt. The results showed that the TOFs of PdPt DENs were significantly higher than the physical mixtures of the single-metal analogues. The similar experimental results also appeared in the hydrogenation of allyl alcohol when the PdAu bimetallic DENs were used as catalysts [6], the maximal enhancement in the catalytic activity was observed for PdAu alloys at 70 mol% Pd. Here, PdPt and PdRh bimetallic DENs were first prepared by a co-complexation route with fourth-generation amine-terminated PAMAM dendrimer. For the hydrogenation of 1-hexene, the bimetallic DENs showed enhanced catalytic activities compared to physical mixtures of corresponding monometallic DENs.
X. Peng et al. / Catalysis Communications 11 (2009) 62–66
2. Experimental sections
63
The TOFs were determined on the basis of hydrogen uptake vs time.
2.1. Materials 3. Results and discussion The fourth-generation amine-terminated PAMAM dendrimer (G4-NH2) having an ethylenediamine core was purchased in a methanol solution with 10–25 wt.% from Dendritech Inc. (Midland, MI, USA). Prior to use, the methanol was removed under vacuum at room temperature (22 °C). K2PdCl4 (98 wt.%), K2PtCl4 (98 wt.%), RhCl3 (98 wt.%), NaBH4 (99 wt.%) (Aldrich Chemical Co., Milwaukee, WI, USA) were used as received. 2.2. Syntheses of bimetallic PdPt and PdRh DENs G4-NH2(PdPt) and G4-NH2(PdRh) DENs were synthesized by simultaneous co-complexation of two different metal ions, followed by a single reduction step. Firstly, dry G4-NH2 was dissolved in sufficient deionized water to yield a dendrimeric solution of 50 lM, the solution pH was adjusted to 3 by addition of 0.1 M HCl aqueous solution. Secondly, co-complexation of metal ions with dendrimers was carried out by adding a total volume of 1.0 mL of both 0.10 M K2PdCl4 and 0.10 M K2PtCl4 solutions, or of both 0.10 M K2PdCl4 and 0.10 M RhCl3 solutions to 50 mL or 85 mL, respectively, of a 50 lM aqueous solution of G4-NH2 dendrimers in a 100-mL shake-flask. After stirring at 200 rpm for 76 h (PdPt system) or 24 h (PdRh system), 2.0 mL of a 0.5 M NaBH4 solution was slowly added with vigorous stirring using a magnetic stirrer and then such formed mixture was stirred at 200 rpm for 1 h. Finally, the pH value of the solution was adjusted to 7–8 with 0.1 M HCl aqueous solution. 2.3. Characterization and analyses UV–vis absorption spectra can be used for characterizing the co-complexation of G4-NH2 with the two different metal ions followed by reduction, which were recorded using a Shimadzu UVmini-1240 UV–vis spectrophotometer. High-resolution transmission electron microscopy (HRTEM) micrographs having a point-to-point resolution of 0.23 nm were obtained using a JEM2010HR transmission electron microscope equipped with an ISIS300 X-ray energy dispersive spectrometer (EDS). For the HRTEM characterization, samples of bimetallic DENs were prepared by depositing a drop of the DEN solution on a carbon-coated Cu grid and dried naturally. X-ray photoelectron spectroscopy (XPS) studies were carried out by an ES-2402 device using Mo Ka radiation, samples were prepared as thin films from aqueous solutions of dendrimers containing metal nanoparticles on soaking silicon wafers.
PdPt DENs were prepared by simultaneous co-complexation of K2PdCl4 and K2PtCl4 with G4-NH2. Firstly, the peripheral amine groups of G4-NH2 can be selectively protonated by the control of solution pH 3 and then bimetallic ions can be exclusively trapped within the interior of dendrimers by simultaneous co-complexation with internal functional groups. Each dendrimer molecule coordinates with essentially the same number of bimetallic ions. Subsequent reduction produces zero-valent bimetallic nanoparticles, uniform in size, with one encapsulated in the interior of each dendrimer molecule. The intradendrimer templating mechanism is thought to predominate when metal ions (like Cu2+) or ionic pre2 cursors (like PtCl4 ) preferentially coordinate with the interior 2 groups of G4-NH2 [8,12]. Although PdCl4 was very quickly com2 plexed with the G4-NH2 (within 15 min) at pH 3, PtCl4 required several days (within 76 h) to completely complex at pH 5, so the lengthy stirring time (within 76 h) for the co-complexation of K2PdCl4 and K2PtCl4 with G4-NH2 was necessary during the synthesis procedure of PdPt bimetallic DENs [8]. Finally, an aqueous solution of 0.5 M NaBH4 was slowly added and the light yellow solution of dendrimer/metal ions immediately turned into dark brown, indicating the formation of bimetallic nanoparticles. We also found that the resulting solutions of nanoparticles were very stable for at least one month after adjusting the final solution pH to 7–8. Fig. 1a and Table 1 show UV–vis absorbance spectra of Pd and Pt metals during the course of co-complexation and the subsequent reduction. For the maximum sensitivity wavelength (kmax) of the monometal ions, the characteristic peaks at 207 and 237 nm arise 2 from the absorbance of PdCl4 , and the characteristic peak at 2 194 nm from PtCl4 . In the absence of dendrimer, the solution mix2 2 ture of PdCl4 and PtCl4 also exhibits the mixed absorption peaks of two metal ions, which are at 204 and 236 nm. When the addi-
2.4. Hydrogenation using gas uptake apparatus The hydrogenation of 1-hexene was carried out in the presence of bimetallic DENs in a solvent mixture of tetrahydrofuran/water (v/v = 6/1). The hydrogenation reactions were run at 50 psi and 25 °C. Ten milliliters of the bimetallic DEN solution and 120 mL of solvent were placed into a 300 mL hydrogenation gas uptake autoclave developed by Mohammadi and Rempel [11]. The system was degassed by bubbling with H2 for 10 min. When the temperature and H2 pressure of the autoclave were stabilized, 4.8 mL (37.0 mmol) of 1-hexene was pressurized into the solution by a 5 mL substrate addition device with agitation at 600 rpm, followed by measurement of the uptake of H2. There was no evidence for DEN aggregation during or after the hydrogenation reaction, indicating that the catalysts were stable within the reaction periods.
Fig. 1. (a) Variations in the UV–vis spectra of Pd and Pt metals during the course of co-complexation and the subsequent reduction. (b) UV–vis spectra of G4NH2(PdxPty) with various Pd/Pt ratios. In all cases, the final concentrations of total metal and G4-NH2 were 0.1 mM and 2.5 lM, respectively.
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X. Peng et al. / Catalysis Communications 11 (2009) 62–66
Table 1 The maximum spectral sensitivity wavelength (kmax) for the UV–vis spectra of Pd and Pt metals during the course of co-complexation and the subsequent reduction. Substrate
Pd2+
Pt2+
(Pd2+7) + (Pt2+3)
G4-NH2 (Pd2+7Pt2+3)
G4-NH2 (Pd7Pt3)
G4-NH2
kmax (nm)
207,237
194
204,236
212
212
196
2
2
tion of G4-NH2 to the solution mixture of PdCl4 and PtCl4 , however, a new strong absorption peak at 212 nm appears (see Fig. 1a and Table 1). This indicates that two metal ions are complexed with the internal functional groups of the dendrimer and are encapsulated in the dendrimer host. After reduction of the composite, the absorption peak at 212 nm is weakened. Fig. 1b presents a series of UV–vis spectra of the PdPt bimetallic nanoparticles with various Pd/Pt ratios (G4-NH2(PdxPty), where x/y = Pd/Pt molar ratio). Each spectrum in Fig. 1b exhibits a single peak and the peak height decreases as the Pt loading increases, the absorption bands are of nearly exponential shape and this indicates the complete reduction of metal ions [4]. Moreover, it is noted that the spectra of the bimetallic nanoparticles are different from those of the monometallic Pd and Pt nanoparticles. The change can be primarily attributed to the change in dielectric function caused by mixing the two different metal atoms [13]. Therefore, the characteristic monotonic absorption spectra strongly suggest that bimetallic nanoparticles are formed in the cavity of the dendrimers. The representative HRTEM micrograph of G4-NH2(Pd7Pt3) is shown in Fig. 2a. The nanoparticles are nearly uniform and their shape is almost spherical. The microscopy demonstrates that the nanoparticles are a Gaussian-like size distribution. Average particle diameter (D) and standard deviation (S) of PdPt bimetallic DENs are 2.5 and 0.5 nm, respectively (Fig. 2b).
Fig. 2. HRTEM micrograph of G4-NH2(Pd7Pt3) nanoparticles with a Pd/Pt ratio of 70/30 (a), and the corresponding size distribution histogram (b).
EDS analyses of both Pd and Pt elements were carried out and only their average compositions were obtained because it was rather difficult to acquire the elements of one nanoparticle. The result of XPS analysis showed that the metal ions were completely reduced irrespective of the Pd/Pt ratios. The Pd 3d5/2 peak shifts from Pd2+ 338.1 eV to Pd0 335.7 eV after reduction, and a shift from 72.5 to 71.3 eV was found for the Pt 4f7/2 peak. These XPS results are very similar to those found for monometallic DENs. After adjusting the pH of G4-NH2 solution to 3 using HCl, the preparation of PdRh bimetallic nanoparticles within G4-NH2 dendrimers was carried out via the co-complexation method. By preloading a dendrimer ‘‘nanoreactor” with stoichiometric bimetallic ions of Pd2+ and Rh3+ and then the dendrimer-encapsulated PdRh nanoparticles were prepared via slowly adding NaBH4 solution to the composite in situ. The light red dendrimer-encapsulated metal ion solution immediately turned to a homogeneous brownishblack solution, indicating the formation of PdRh bimetallic nanoparticles. The resulting nanoparticles were stable for one month after adjusting the solution pH to 7–8. The UV–vis spectrum of G4-NH2(Pd5Rh5) DENs in Fig. 3a reveals an absorbance peak at 212 nm. Formation of the particles was confirmed by the decrease of the absorption peak intensity after chemically reducing G4-NH2(Pd2+5Rh3+5). It demonstrated that the spectra of the obtained nanoparticles were different not only from those of the monometallic Pd and Rh nanoparticles but also from those of their physical mixtures. Fig. 3b presents a series of UV–vis spectra of PdRh bimetallic nanoparticles with various Pd/ Rh ratios. As the Pd/Rh ratio decreases, the kmax shifts to a longer wavelength. The change in the absorption spectra of the bimetallic nanoparticles from those of monometallic ones can be primarily
Fig. 3. (a) Variations in the UV–vis spectra of G4-NH2(Pd5Rh5) and G4NH2(Pd2+5Rh3+5) compared to their monometallic and physical mixture DENs. (b) UV–vis spectra of G4-NH2(PdxRhy) with various Pd/Rh ratios. In all cases, the final concentrations of total metal and G4-NH2 were 0.07 mM and 3.0 lM, respectively.
X. Peng et al. / Catalysis Communications 11 (2009) 62–66
Fig. 4. HRTEM image of G4-NH2(Pd5Rh5) nanoparticles with a Pd/Rh ratio of 50/50 (a), and the corresponding size distribution histogram (b).
attributed to the change in dielectric function caused by mixing the two different metal atoms. Thus, the characteristic absorption spectra also suggest the formation of PdRh DENs. The representative HRTEM image of G4-NH2(Pd5Rh5) is shown in Fig. 4a. The average particle diameter and dispersion of the nanoparticles are 3.0 ± 1.1 nm (Fig. 4b). The micrograph demonstrates that the particle size is nearly uniform and the shape is roughly spherical. Although the particle size distribution of PdRh bimetallic nanoparticles is somewhat broader compared to those of monometallic and bimetallic noble metal nanoparticles [14,15], the size distribution of the resulting nanoparticles is nearly monodispersed. In addition, only average compositions of Pd and Rh nanoparticles were obtained via the EDS characterization like PdPt system. XPS analysis following reduction indicates that the Pd and Rh salts are all completely reduced: The Pd 3d5/2 peak shifts from 337.5 to 335.2 eV upon reduction, and the Rh 3d5/2 peak is shifted from 311.1 to 308.8 eV. In order to investigate the catalytic activity, the PdPt bimetallic DEN catalysts were applied in hydrogenation of 1-hexene at 50 psi and 25 °C. Fig. 5a shows that the PdPt bimetallic DENs present high catalytic activities in the hydrogenation reaction. Monometallic Pt DENs yielded the lowest TOF (100 mol H2(mol Pt) 1 h 1), while the TOFs of physical mixtures of Pd-only and Pt-only DENs were located on the straight line. The PdPt DENs (Pd/Pt ratios of 3/7, 5/5, and 7/3) gave larger TOFs than those of the physical mixtures of monometallic DENs with equal composition. The maximum TOF (1150 mol H2(mol Pd + Pt) 1 h 1) was at Pd/Pt ratio of 7/3, which was even larger than that of Pd-only DENs (TOF = 800 mol H2(mol Pd) 1 h 1). The enhanced catalytic activity of the bimetallic DENs could be due to a synergistic electronic effect [5,10]. As was discussed above, the catalytic activity of PdRh system (see Fig. 5b) is similar with the PdPt system. However, TOFs of
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Fig. 5. Turnover frequencies (TOFs) for the hydrogenation of 1-hexene as functions of mole percentages of palladium and rhodium. (a) PdPt bimetallic DENs and physical mixtures containing Pd-only and Pt-only DENs. (b) PdRh bimetallic DENs and physical mixtures containing Pd-only and Rh-only DENs. The (j) represents the bimetallic DENs, while the (.) represents the data obtained from their monometallic physical mixtures. Conditions: (a) 25 °C, 50 psi, the substrate/metal = 1850/1 and [Pd + Pt] = 150 lM; (b) 25 °C, 50 psi, the substrate/metal = 3150/1, and [Pd + Rh] = 90 lM.
the PdRh bimetallic DENs were higher than those of PdPt bimetallic DENs in the hydrogenation of 1-hexene, and the highest TOF of PdRh system was achieved at a Pd/Rh ratio of 5/5 while PdPt system was at a Pd/Pt ratio of 7/3. These results indicate that the catalytic activities are dependent upon the metal composition of bimetallic nanoparticles. According to the view of the synergistic electronic effect of Toshima [10], the electronic interaction between two different metals may provide an uneven electron distribution, which could make the electron density of one metal become lower. This effect of one metal upon another could cause the bimetallic cluster to be more active than monometallic ones because the substrate containing a double bond favors the electron-deficient surface. These results also provided additional evidence for the formation of PdPt and PdRh DENs.
4. Conclusions We have shown that the PdPt and PdRh bimetallic nanoparticles can be prepared within amine-terminated G4-NH2 dendrimers by controlling protonation of the terminal amine groups and selective co-complexation of bimetallic ions with interior amine groups. The UV–vis absorbance spectra and catalytic hydrogenation activities of the PdPt and PdRh bimetallic nanoparticles are different not only from those of their monometallic nanoparticles but also from those of their physical mixtures. For 1-hexene hydrogenation, the highest TOFs of PdPt and PdRh systems were different and the catalytic activity of PdRh bimetallic DENs were found to be more effective than those of PdPt bimetallic DENs.
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Acknowledgments The authors gratefully acknowledge the supports of Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation (CFI), the China Scholarship Fund and the Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars (State Education Ministry). References [1] X.-H. Peng, Q.-M. Pan, G.L. Rempel, Chem. Soc. Rev. 37 (2008) 1619–1628. [2] Y.-M. Chung, H.-K. Rhee, Catal. Surv. Asia 8 (2004) 211–223.
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