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Bimetallic Pd–Cu nanocrystals and their tunable catalytic properties† Junjie Mao, Yuxi Liu, Zheng Chen, Dingsheng Wang* and Yadong Li

Received 10th February 2014, Accepted 14th March 2014 DOI: 10.1039/c4cc01051e

Monodisperse Pd–Cu bimetallic nanocrystals (NCs) with tunable compositions





Pd0.8Cu0.2) and controlled sizes (5.2 nm, 6.8 nm, 8.1 nm, 16.4 nm, 19.9 nm) were easily obtained in an octadecylamine (ODA) synthetic system, which exhibited tunable catalytic properties for styrene epoxidation and ethanol electro-oxidation.

Bimetallic Pd-based nanomaterials have received great attention because of their wide applications in hydrogenation reactions, C–C coupling reactions and proton-exchange membrane fuel cells.1–3 Recent research studies indicate that, upon appropriate modification of their nanoscale surface, composition, and structure, Pd-based nanomaterials can become promising catalysts by decreasing material cost and enhancing performance.4–6 Therefore, it is significant to obtain well-defined Pd-based bimetallic nanomaterials. Various methods have been developed for the preparation of Pd-based NCs.7–12 Recently, a facile and general strategy for bimetallic NCs based on a noble-metal-induced reduction (NMIR) mechanism has been designed, which provided a platform for the investigation of the unique properties of bimetallic NCs.13–16 Herein, we take Pd–Cu NCs as an example to explore their tunable catalytic properties for styrene epoxidation and ethanol electro-oxidation. Fig. 1 shows transmission electron microscopy (TEM) images of the as-obtained Pd–Cu NCs which have narrow size distribution and an average diameter of 5.2 nm. The high-resolution TEM (HRTEM) image shows the lattice fringes with an inter-fringe distance of 0.217 nm, which is between the characteristics of face centered cubic Pd (0.224 nm) and a Cu (0.209 nm) crystal phase in the (111) plane, indicating the formation of Pd–Cu alloys. The energy-dispersive X-ray (EDX) spectroscopy mapping profile and the EDX line scanning profile obviously manifested the homogeneous distribution of Pd and Cu in bimetallic Pd–Cu NCs. The Pd : Cu atomic ratio determined by EDX analysis and inductively coupled plasma-mass

Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental details, XRD patterns, TEM images. See DOI: 10.1039/c4cc01051e

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Fig. 1 (A) TEM image of Pd–Cu NCs. (B) HRTEM image of Pd–Cu NCs. (C) High-angle annular dark-field (HAADF) STEM image of Pd–Cu NCs. (D) Line-scanning profile across four Pd–Cu NCs. (E) EDX elemental maps for Cu (red) and Pd (blue).

spectrometry (ICP-MS) analysis is almost consistent with the ratio of precursors. The reaction process is exhibited in Fig. 2. When dissolving the reaction precursors in ODA for 10 min, the solution color became blue (Fig. 2a). With the elevation of the temperature from 110 1C to 185 1C, they displayed gradual evolution from blue to black (Fig. 2b–e), which indicated the gradual nucleation process. After the solution turns black at 185 1C, the growth process can be monitored by quenching the reaction at different temperatures. We collected different samples at 185 1C, 220 1C, 230 1C, and 240 1C during the reaction. The content of products was confirmed by EDX (Fig. S1, ESI†). At the beginning, Pd was found to be the dominant species (Pd : Cu = 7 : 3, Fig. S1a, ESI†) while substantial amount of Cu was present when the reaction temperature was raised to 240 1C (Pd : Cu = 1 : 1, Fig. S1d, ESI†). Moreover, the powder X-ray diffraction (XRD) patterns (Fig. S2, ESI†) show that the peaks shift to high angles when elevating the reaction temperature, which indicates that more

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Fig. 3 TEM images of Pd–Cu NCs with different sizes: (a) 5.2 nm, (b) 6.8 nm, (c) 8.1 nm, (d) 16.4 nm, (e) 19.9 nm. Insets: as-obtained Pd–Cu NCs can self-assemble into highly ordered patterns. (f) XRD patterns of Pd–Cu NCs with different sizes. Fig. 2 (A) Color evolution during the reaction process at different temperatures: (a) 110 1C, (b) 140 1C, (c) 160 1C, (d) 175 1C, (e) 185 1C. (B) TEM results of samples obtained at different temperatures: (f) 185 1C, (g) 220 1C, (h) 230 1C, (i) 240 1C.

and more Cu2+ was reduced in this system. Fig. 2f–i shows the size evolution during the particle growth process. The particle size increases with more and more Cu incorporation into Pd nanoparticles (Fig. S3, ESI†). These results indicate that, in our synthetic system, the Pd precursor is firstly reduced to zero-value Pd. Then, the zero-value Pd can induce the stepwise reduction of Cu2+ with the assistance of the ODA solvent until Cu2+ is reduced to zero-value Cu completely. The composition of Pd–Cu NCs was controlled by precursor molar ratios. By keeping the amount of the Pd precursor at 0.066 mmol and changing the amount of the Cu precursor, we could control the Pd–Cu NCs’ composition from a Cu content of 20% to 80% (Table S1, ESI†). Fig. S4a–e (ESI†) shows the TEM images of Pd–Cu NCs with different compositions. The corresponding XRD patterns of the as-obtained Pd–Cu NCs are given in Fig. S4f (ESI†). The diffraction peaks positioned between the standard peaks of Pd (JCPDS-65-6174) and Cu (JCPDS-04-0836) indicate the formation of Pd–Cu alloys. It can be seen that, with an increase of Cu content, the peaks position slightly shifted to higher 2y values compared to those of pure Pd, which could be attributed to the decreased lattice spacing resulting from the replacement of Pd atoms by smaller Cu atoms. By controlling the experimental conditions, Pd–Cu NCs with different sizes were successfully synthesized. For example, in a typical synthesis of Pd–Cu NCs with an average size of 8.1 nm, 20 mg Pd(acac)2 and 15.9 mg Cu(NO3)23H2O were added to a solution of 10 mL ODA (7.5 g) at 110 1C. After the mixture changed into a clear solution, it was then transferred to a 50 mL autoclave at 180 1C for 6 h. The products were collected at the bottom of the flask by decanting the supernatant and further washed with hexane and ethanol several times. As a result, high-quality Pd–Cu NCs with a

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well-distributed size were obtained. Fig. 3c displays the as-obtained nanocrystals with an average diameter of 8.1 nm. The Pd–Cu NCs with an average size of 6.8 nm (Fig. 3b) were synthesized by mixing Pd(acac)2 with Cu(NO3)23H2O in ODA (12 g) solvent at 80 1C. After all solid dissolved to form a homogeneous solution, the temperature of this system was elevated to 240 1C and maintained at 240 1C for 10 min. When we reduced the amount of ODA to 7.5 g, the average size of products decreased to 5.2 nm (Fig. 3a). XRD measurements (Fig. 3f) confirmed the successful synthesis of Pd–Cu NCs. The reason for size change is that the decreased ODA solvent results in the increase of the precursor concentration, leading to a large amount of nucleation in the initial stage of nanocrystal growth. It is also noteworthy that the metal precursor also plays a key role in controlling the particle size in an ODA synthetic system. For example, Pd–Cu NCs with an average size of 16.4 nm (Fig. 3d) were synthesized following a similar procedure. 11.6 mg PdCl2 and 15.9 mg Cu(NO3)2 3H2O were mixed in 3 g ODA and then heated to 260 1C. Likewise, if the amount of ODA was increased to 7.5 g, Pd–Cu NCs with an average size of 19.9 nm (Fig. 3e) could be synthesized. All the resulting highly monodisperse nanocrystals with a narrow size distribution can be spontaneously self-assembled into superstructures without any additional post processing steps (insets of Fig. 3a–e). Direct ethanol fuel cells (DEFC) have gained a surge of attention due to the distinct advantages of their high power density output, low pollutant emissions, and safety for storage and transportation.17 We chose Pd0.5Cu0.5, Pd0.7Cu0.3, and Pd black as electrocatalysts to study the composition effects on the electrochemical properties. Electrooxidation of ethanol on these catalysts was run in an alkaline media. Before each test, Pd–Cu NCs were washed by ethanol a further three times and irradiated by UV light (wavelength at 185 and 254 nm) for 24 h in air to remove the organic coating around the NCs. Fig. 4 plots the cyclic voltammogram (CV) curves for the electro-oxidation of ethanol on the Pd–Cu NCs and all of the catalysts showed the characteristic peaks for pure Pd in the forward and backward scans.

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Fig. 4 Cyclic voltammetric curves for (A) Pd0.5Cu0.5, (B) Pd0.7Cu0.3, (C) Pd black catalyst on a GC electrode, in 0.5 M KOH + 1 M ethanol solution at a scan rate of 50 mV s 1. Current densities were normalized with reference to the geometric area of a working electrode (0.07 cm2). Insets: CV curve in the absence of ethanol, scan rate: 100 mV s 1, reference electrode: Ag/AgCl with saturated KCl. (D) Mass activities of Pd0.5Cu0.5, Pd0.7Cu0.3 and Pd black, the mass activity of Pd based catalysts is evaluated by forward peak potential.

For electro-oxidation of ethanol, in particular, bimetallic Pd–Cu catalysts with a composition of Pd0.5Cu0.5 exhibited the highest mass activity toward electrochemical oxidation of ethanol almost 4 times higher than that of Pd black and 1.7 times that of Pd0.7Cu0.3, thus suggesting that bimetallic Pd–Cu catalysts with appropriate copper content have considerable potential as non-Pt electrocatalysts for DEFCs. The mechanism still needs a detailed analysis, which is part of our ongoing research. Styrene oxide is an industrially important organic intermediate widely used in the synthesis of fine chemicals and pharmaceuticals. Over the past few decades, many homogeneous and heterogeneous catalysts have been developed and used to catalyze the epoxidation of styrene.18 Here, the epoxidation of styrene was used to evaluate the catalytic performance of the as-prepared catalysts. Generally, benzaldehyde is a major by-product which is produced from the breaking of a CQC bond followed by direct oxidation or from further oxidation of styrene oxide. Therefore, a good catalyst with both fast conversion and high selectivity is always desired. The results showing the influence of composition of the as-prepared catalysts on styrene conversion and product selectivity are presented in Table 1. The Pd Table 1 Catalytic performance of the as-prepared catalysts for styrene epoxidationa

Selectivity (%) Catalyst

Styrene Styrene Benzoic conversion (%) oxide Benzaldehyde acid Others

Pd Pd0.5Cu0.5 Pd0.3Cu0.7 Pd0.2Cu0.8 Pd0.7Cu0.3

35 83 75 52 39

28 40 70 38 48

30 31 17 44 29


36 22 9 14 18

6 7 4 4 5

Reagents and conditions: 0.1 g catalyst, 10 mmol styrene, 20 mL acetonitrile, 15 mmol TBHP, 80 1C, 8 h.

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supported catalyst showed appreciable styrene conversion activity (styrene conversion of 35%) but with poor selectivity for styrene oxide (28%); in this case, benzoic acid (36%) was found to be the major product. The low selectivity for the benzaldehyde indicates that the oxidation of the aldehydes to corresponding acids by TBHP is faster than the aldehyde formation reactions. The Pd–Cu resulted in an increase in both substrate conversion and product selectivity of the catalyst, depending on the possible synergetic effects of alloys. This indicates the important role played by the alloy of the catalyst in styrene epoxidation. The Pd0.3Cu0.7 catalyst showed the best performance with a significant improvement in both styrene conversion (75%) and styrene oxide selectivity (70%) after a reaction for 8 h, which is higher than that reported in previous work.19,20 Fig. S5 (ESI†) shows the strong influence of the reaction time on the epoxidation of styrene. The conversion of styrene increased continuously as the reaction time and could reach as high as 93% for 16 h. The selectivity to styrene oxide slightly increased from 51 to 70%. Further prolonging the time led to a decrease in selectivity, which is mainly caused by the isomerization of styrene oxide or over-oxidation at high temperature reaction. In summary, we have successfully synthesized a series of Pd–Cu nanoalloys with tunable compositions (Pd0.2Cu0.8, Pd0.3Cu0.7, Pd0.5Cu0.5, Pd0.7Cu0.3, Pd0.8Cu0.2) and controlled sizes (5.2 nm, 6.8 nm, 8.1 nm, 16.4 nm, 19.9 nm). The as-prepared Pd–Cu catalysts showed much better performances toward both ethanol electrooxidation and styrene epoxidation. For ethanol electro-oxidation, Pd0.5Cu0.5 exhibited the highest mass activity which is about 4 times higher than that of Pd black and 1.7 times higher than that of Pd0.7Cu0.3. For styrene epoxidation, the as-prepared Pd0.3Cu0.7 catalyst displayed much better catalytic efficiency than Pd–Cu NCs with other Cu/Pd molar ratios. These preliminary results indicate that highperformance Pd–Cu alloyed nanocatalysts can be developed by tuning the microstructures of the NCs, which are expected to have promising applications in fuel cells and organic reactions. Financial support of this work by the State Key Project of Fundamental Research for Nanoscience and Nanotechnology (2011CB932401, 2011CBA00500, 2012CB224802), the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (201321), Specialized Research Fund for the Doctoral Program of Higher Education (20130002120013), and the National Natural Science Foundation of China (21221062, 21131004, 21390393, 21322107) is gratefully acknowledged.

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Chem. Commun., 2014, 50, 4588--4591 | 4591

Bimetallic Pd-Cu nanocrystals and their tunable catalytic properties.

Monodisperse Pd-Cu bimetallic nanocrystals (NCs) with tunable compositions (Pd0.2Cu0.8, Pd0.3Cu0.7, Pd0.5Cu0.5, Pd0.7Cu0.3, Pd0.8Cu0.2) and controlled...
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