Article pubs.acs.org/JPCC
The Key Gold: Enhanced Platinum Catalysis for the Selective Hydrogenation of α,β-Unsaturated Ketone Yajie Xu, Lingli Liu, Hanbao Chong, Sha Yang, Ji Xiang, Xiangming Meng,* and Manzhou Zhu* Department of Chemistry and Center for Atomic Engineering of Advanced Materials, Anhui University, Hefei, Anhui 230601, P. R. China S Supporting Information *
ABSTRACT: AuPt alloy nanoparticles (NPs) were facilely synthesized with oleylamine as the stabilizing ligand and characterized by high-resolution transmission electron microscopy, powder X-ray diffraction, inductively coupled plasma-atomic emission spectrometer analysis, and so on. In addition, the AuPt alloys supported by the nano CeO2 exhibit high selectivity and efficiency in hydrogenation of benzylidene acetone under ambient temperature and pressure. By analyzing the catalytic performance over the NPs with different Au:Pt compositions, we found that the TONPt values (based on the number of Pt atoms) vary in the same trend with the change of conversion. Despite that gold itself shows no catalytic activity, the improved conversion and TONPt with the alloy catalysts clearly show the promotion effect of gold on the catalytic activity of the platinum. The inactive metal significantly improves the catalytic activity of active metal, which shows that the AuPt alloy exhibits an interesting synergistic effect.
1. INTRODUCTION In the past decades, metal nanoparticles (NPs)-catalyzed organic reactions have been widely explored.1−5 On the nanometer scale, the particles have high surface-to-volume ratios, unique electronic structure, and active centers.6,7 So far, NPs have been extensively used in catalyzing many reactions such as selective hydrogenation,8 oxidation,9−11 electrocatalysis,12,13 and coupling reactions.14−16 Compared with the homogeneous catalysts, the heterogeneous NP catalysts are easy to recycle and reuse. In addition, the recent studies indicate that the doping of different metal atoms might effectively modify the size and morphology of the target NPs and result in alloy NPs with enhanced properties.17−20 In this context, the alloy NPs have shown many advantages over the monometallic NPs, such as in the improved catalytic activity, enhanced conversion, and selectivity17−19 and milder reaction conditions.20,21 In our recent studies, a series of Pd-based alloys, such as Pd−Ni, Au−Pd, and so on, have been facilely prepared.22−24 These bimetallic alloys have uniform size and morphology and display significantly enhanced activity in catalyzing the coupling reactions. The improved properties of alloy NPs (relative to the monometallic NPs) have been frequently proposed to be caused by the synergistic effect.25 The synergistic effect means that the alloy (with two or more ingredients) shows better properties than either one of them or the summation of these individuals (i.e., 1 + 1 > 2). The physical origin of the synergistic effect might be related to the inherent interactions between the components. For example, Stamenkovic et al. recently utilized a series of Pt-based alloys NPs catalysts (e.g., PtNi, PtCo, etc.) in fuel cells and observed enhancement in the kinetics of the oxygen-reduction reaction. © XXXX American Chemical Society
In their study, the synergistic effect in the surface electronic structure is concluded as the main driving force.26,27 Elsewhere, Chen et al. reported NiRu alloy NP-catalyzed H2 generation via the ammonia borane hydrolysis, and the significantly improved catalytic activity was ascribed to the synergistic effect between Ni and Ru.28 As a traditional active catalyst, Pt has been widely used in hydrogenation reactions.28−31 Specifically, the Pt-based NPs are widely explored and applied in catalytic hydrogenation. In addition, the recent studies imply that the catalytic activity can be greatly improved by forming alloy Pt with other metals such as Pd.29,31 In this study, we facilely synthesized the size uniform AuPt alloy NPs (with different Au:Pt ratios) using oleylamine as the stabilizing ligand. These AuPt alloys show good stability, and they are uniformly dispersed with ultrasmall sizes of about 3 to 4 nm. In addition, the AuPt alloy NPs supported on CeO2 exhibit enhanced catalytic activity and selectivity for the hydrogenation of benzylidene acetone (compared with the monometallic Pt NPs). Interestingly, according to the experimental observations, despite the fact that gold in the alloys is ineffective in the catalytic hydrogenation reaction, its existence indeed improves the catalytic activity of Pt.
2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Chloroauric(III) acid (HAuCl4·3H2O, 99%), chloroplatinic(IV) acid (H2PtCl6· 6H2O, 99%), oleylamine (OAm, Aladdin, 70%), cerium nitrate Received: February 22, 2016 Revised: May 19, 2016
A
DOI: 10.1021/acs.jpcc.6b01808 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C hexahydrate (Ce(NO3)3·6H2O, Aladdin, 99.5%), borane tertbutylamine (C4H14BN, jkchemical, 97%), and benzylidene acetone (C10H10O, Energy Chemical, 98%) were used. Other chemicals, unless specified, were of reagent grade. Triply distilled water with a resistivity of >18.0 MΩ·cm was used in the preparation of aqueous solutions. 2.2. Preparation of Catalysts. 2.2.1. Preparation of CeO2 NPs. The CeO2 was synthesized as previously described.32 A mixture of 2.0 g Ce(NO3)3·6H2O, 2 mL of deionized water, 2 mL of propionic acid, and 60 mL of ethylene glycol were added in 100 mL of PTFE (polytetrafluoroethylene) liner and stirred for a few minutes. Then, the sealed PTFE liner was put in an autoclave and heated to 180 °C for 200 min. After it was cooled to room temperature, the product was separated from solution by centrifugation, washed with water three times, and then washed with ethanol for another three times. The final product was dried at 40 °C for 24 h. 2.2.2. Preparation of AuPt Alloy NPs and Pt NPs. Herein, the synthetic method for Au0.59Pt0.41 is used as an example. After pouring 10 mL of oleylamine in a 25 mL flask, 200 μL of HAuCl4 (0.2 g/mL) and 250 μL of H2PtCl6 (0.2 g/mL) were added and heated to 60 °C, then stirred for 10 min. Then, 200 mg borane-tert-butylamine complex and 3 mL of oleylamine mixture were added rapidly to the mixed solution until the light-yellow solution turned black. Meanwhile, the above mixture was heated to 90 °C and kept for 30 min. Finally, the mixture was cooled to room temperature and washed by methanol three times, then dried after centrifugation. A similar method has also been used to synthesize the other alloys with different Au:Pt compositions: (a) Au0.31Pt0.69, (b) Au0.48Pt0.52, (c) Au0.59Pt0.41, (d) Au0.76Pt0.24, and (e) Au0.87Pt0.13. (The numbers indicate the proportions of Au and Pt.). The preparation of the Pt NPs is as follows: 10 mL of oleylamine and 400 μL of H2PtCl6 (0.2 g/mL) were added to a 50 mL flask. The solution was stirred at 60 °C for 10 min. Then, 200 mg borane-tert-butylamine complex, which was dissolved in 3 mL of oleylamine, was added rapidly to the mixed solution. The solution became black slowly. The above mixture was heated to 90 °C and kept for 30 min. Finally, the mixture was cooled to room temperature and washed by methanol three times. The yield of Pt NPs was lower than that of alloy NPs. 2.2.3. Preparation of CeO2-Supported AuPt Bimetallic Catalysts. A specific amount of AuPt alloys (with the Pt wt % being consistent at 1.2 wt %) and CeO2 were dispersed in toluene and stirred for 24 h. Then, the above suspension was separated by centrifugation and dried at 40 °C in air. Finally, the catalysts were annealed by heating to 150 °C for 120 min. 2.3. Typical Procedure for the Catalytic Hydrogenation Reaction. In a Schlenk flask, benzylidene acetone (1 mmol) and the catalyst of AuPt/CeO2 (10 mg) were added to 3 mL of ethanol and the solution was stirred at room temperature (25 °C), with a hydrogen balloon to provide hydrogen for the catalytic reaction. After completion of the reaction, the used catalyst was recovered by centrifugation and washed with ethanol. The product was analyzed by gas chromatography (GC) and isolated by silica gel column chromatography, and the structure of the product was determined by NMR. Three kinds of products (A, B, C) were obtained in this reaction (Scheme 1). The product A (benzyl acetone) is the main product, which is generated via the CC hydrogenation. The yields of B (4-phenylbut-3-en-2-
Scheme 1. Reactions Involved in the Hydrogenation of Benzylidene Acetone
ol) and C (4-phenylbutan-2-ol) are much lower than A and thus are categorized as byproducts. 2.4. Characterization. Thin layer chromatography (TLC, Merck Silica Gel 60 F254, USA) analysis was performed before the separation of product by column chromatography. A mixed solution of petroleum ether and ethyl acetate was used as the developing solvent (10:1). Then, Merck Kieselgel 200-300 (USA) was used in a preparative column chromatography with the eluent of the mixed petroleum ether and ethyl acetate (20:1) to separate the crude products. 1H NMR spectra were acquired on a Bruker AM 400 (Switzerland) operating at 400 MHz (solvent: DMSO-d6 and CDCl3). The yield of the products was analyzed by GC 2010 Plus (Shimadzu, Japan) equipped with an rtx-1 capillary column and a flame ionization detector. The programmed temperature increased from 80 to 240 °C at a rate of 10 °C/min and finally remained at 240 °C for 5 min. The quantification of each component was integrated by an external standard method. Transmission electron microscopy (TEM) images were obtained by a JEM 2100 microscope (Japan). High-resolution transmission electron microscopy (HRTEM) was assessed by a JEOL-2010F instrument (Japan). The supported NPs were deposited on Cu grids after ultrasonic dispersion of the samples in ethanol, and the unsupported NPs were dissolved in dichloromethane without any treatment to disperse them. Powder X-ray diffraction (XRD) patterns were obtained by a MXP18AHF diffractometer (China) with Cu−K radiation (λ = 1.5418 Å) at 40 kV and 200 mA (scanning speed 4° min−1). For the unsupported NPs, the sample was coated on the glass slide. Inductively coupled plasma-atomic emission spectrometry (ICP-AES) was obtained by an Atomscan Advantage instrument (Thermo Jarrell Ash Corporation, USA) to quantify the Au loading on the composite catalysts, and the samples were digested before tests. To dissolve CeO2 and NPs in water, hydrogen peroxide and aqua regia were added in turn under heating.
3. RESULTS AND DISCUSSION A series of AuPt alloys and Pt NPs were synthesized as catalysts: (a) Au0.31Pt0.69, (b) Au0.48Pt0.52, (c) Au0.59Pt0.41, (d) Au0.76Pt0.24, and (e) Au0.87Pt0.13. To investigate the morphology of the catalysts, we carried out TEM. As shown in Figure 1, all of these AuPt bimetallic catalysts are uniform, with ultrasmall sizes of about 3 to 4 nm (Figures 1a−e). Furthermore, the TEM images of all of these AuPt NPs are similar. By contrast, B
DOI: 10.1021/acs.jpcc.6b01808 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Figure 1. TEM images and the size distributions of (a) Au0.31Pt0.69 (3.5 ± 0.5 nm), (b) Au0.48Pt0.52 (3.0 ± 0.5 nm), (c) Au0.59Pt0.41 (3.8 ± 0.5 nm), (d) Au0.76Pt0.24 (3.2 ± 0.5 nm), and (e) Au0.87Pt0.13 (3.0 ± 1.0 nm), (f) Pt NPs.
Figure 2. (A) TEM image of CeO2 nanosphere. (B−D) EDS maps of the Au0.59Pt0.41/CeO2 (B: Au-L; C: Pt-L; D: B+C); E: HRTEM image of Au0.59Pt0.41 NPs.
The XRD patterns of various samples are shown in Figure 3. The peak at 38.184° is the (111) crystal plane of Au (profile g), and the peak at 39.763° is the (111) crystal plane of Pt (profile a). Although these two peaks are close, the peaks of the alloys with different Au:Pt ratios are all located between 38.184 and 39.763°. These results provide strong evidence of the formation of the AuPt alloy NPs. Meanwhile, no obvious peaks for Au, Pt, or AuPt were found over the supported catalysts (Figure S1 in Supporting Information), indicating that the loading of the metal NPs is very low and the NPs are well-dispersed on the surface of CeO2.20 The exact proportion of the alloys in this studies is determined by ICP-AES. With the Pt NPs and alloy NPs, we then characterized their catalytic activity using the hydrogenation of benzylidene acetone as a probe reaction. According to the results in Table 1, no target product A was detected from the catalytic reaction over CeO2-supported Au NPs (Table 1, entry1), and only 0.9% of the benzylidene acetone was converted into the byproducts. By contrast, the CeO2-supported Pt NPs gave rise to A as the main product (in 62.4% yield), while the combined yield of the
Figure 1f revealed that several pure Pt NPs aggregated and formed the “silkworm”-shaped NPs. The results indicate that the presence of gold is beneficial to the shape and the size distribution in these AuPt bimetallic catalysts. In other words, gold plays an important role in the morphology control and size distribution of AuPt alloy NPs. The mesoporous CeO2 microsphere acts as an excellent support to stabilize the AuPt bimetallic catalysts. To confirm the structure of CeO2-supported AuPt alloy NPs, TEM and the energy-dispersive spectrometry (EDS) mapping of AuPt NPs was conducted (Figure 2A−D). Figure 2A shows that the size of CeO2 is ∼150 nm. In particular, the EDS mapping results imply that the Pt and Au are uniformly distributed. The elements of Au and Pt were observed in the EDS mapping in Figure 2B,C, respectively. In addition, the formation of the alloy is also validated from the lattice fringes in HRTEM (Figure 2E). The lattice spacing of the alloy is ∼0.230 nm, which is between the gold (111) value of 0.235 nm and the platinum (111) value of 0.226 nm. The result verifies the formation of bimetallic AuPt NPs. C
DOI: 10.1021/acs.jpcc.6b01808 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C Table 2. Benzylidene Acetone Hydrogenation under Different Conditionsa yields
Figure 3. XRD patterns of the as-prepared samples. (a) Pt, (b) Au0.31Pt0.69, (c) Au0.48Pt0.52, (d) Au0.59Pt0.41, (e) Au0.76Pt0.24, (f) Au0.87Pt0.13, and (g) Au.
A%
B+C %
TONPtb
1 2 3 4 5 6 7 8c 9d 10e 11f
Au/CeO2 Pt/CeO2 Au0.31Pt0.69/CeO2 Au0.48Pt0.52/CeO2 Au0.59Pt0.41/CeO2 Au0.76Pt0.24/CeO2 Au0.87Pt0.13/CeO2 Au/CeO2+Pt/CeO2 Au0.59Pt0.41/CeO2 Au0.59Pt0.41/CeO2 Pd/C
0.0 62.4 72.8 79.0 87.2 80.1 58.8 63.2 81.8 76.6 80.6
0.9 9.5 5.7 8.7 9.5 5.7 3.0 9.6 8.4 8.5 9.7
1171 1342 1428 1574 1397 1007 1187 1543 1539 1462
solvents
A%
B+C %
1 2 3 4 5 6 4 4 4 4 4 4 4
ethanol ethanol ethanol ethanol ethanol ethanol ethanol + TOL i-propanol THF n-hexane TOL FA ethanol + TOL
26.9 47.1 61.5 87.2 89.1 89.7 58.3 80.7 47.0 40.3 32.5 21.5 30.7
0 2.9 6.7 9.5 10.4 10.3 2.7 2.1 3.0 2.7 0.0 0.0 2.5
Reaction conditions: benzylidene acetone 1 mmol, catalyst Au0.59Pt0.41/CeO2 10 mg, solvents 3 mL, 25 °C, 1 atm H2. b Unsupported NPs as catalyst.
was ∼90%, and the yield of the byproducts was ∼10% under room temperature and 1 atm pressure. The solvent effect was then investigated. As shown in Table 2, the highest yield of A was achieved in alcoholic solvents (Table 2, entries 4 and 8). In ethanol and isopropanol solvents, the yields of A were 87.2 (Table 2, entry 4) and 80.7% (Table 2, entry 8), respectively. The yields of the byproducts (B+C) were 9.5 and 2.1%, respectively. The previously used mixed solution of ethanol and toluene in selective hydrogenation of CO (yielding B as the main product) was found to be inferior to the pure ethanol in our reaction system (Table 2, entries 4 vs 7).33 In tetrahydrofuran (THF), n-hexane, toluene (TOL), and formic acid (FA) solvents, the yield of the main product A was 1). This study is hoped to benefit the future improvement of heterogeneous catalysts, especially in the replacement of noble metals by cheap metals.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b01808. XRD images, table, and NMR spectra of products concerning supplementary results and discussion (PDF)
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