Composition-Dependent Morphology of Bi- and Trimetallic

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Composition-Dependent Morphology of Bi- and Trimetallic Phosphides: Construction of Amorphous Pd-Cu-NiP Nanoparticles as a Selective and Versatile Catalyst Ming Zhao, Yuan Ji, Mengyue Wang, Ning Zhong, Zinan Kang, Naoki Asao, Wen-Jie Jiang, and Qiang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08082 • Publication Date (Web): 22 Sep 2017 Downloaded from http://pubs.acs.org on September 23, 2017

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ACS Applied Materials & Interfaces

Composition-Dependent Morphology of Bi- and Trimetallic Phosphides: Construction of Amorphous Pd-Cu-Ni-P Nanoparticles as a Selective and Versatile Catalyst Ming Zhao,1,* Yuan Ji,1 Mengyue Wang,2 Ning Zhong,1 Zinan Kang,1 Naoki Asao,3,4,* Wen-Jie Jiang,5 Qiang Chen2 1

School of Chemical Engineering, China University of Mining and Technology, No.1, Daxue

Road, Xuzhou 221116, P. R. China 2

School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi

710049, China 3

Division of Chemistry and Materials, Graduate School of Science and Technology, Shinshu

University, Ueda 386-8567, Japan 4

WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

5

Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry,

Chinese Academy of Science, Beijing 100190, P. R. China

ACS Paragon Plus Environment

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KEYWORDS: palladium, amorphous alloys, selective catalysis, semi-hydrogenation, alcohol oxidation, synergistic effect

ABSTRACT Amorphous materials have been widely researched in heterogeneous catalysis and for next-generation batteries. However, the well-defined production of high-quality (e.g., monodisperse and high surface area) amorphous alloy nanomaterials has rarely been reported. In this work, we investigated the correlations among the composition, morphology, and catalysis of various Pd-M-P NPs (M = Cu or Ni), which indicated that less Cu (≤20 at%) was necessary for the formation of an amorphous morphology. The amorphous Pd-Cu-Ni-P NPs were fabricated with a controllable size and characterized carefully, which show excellent selective catalysis in the semi-hydrogenation of alkynes, hydrogenation of quinoline, and oxidation of primary alcohols. The uniqueness of the catalytic performance was confirmed by control experiments with monometallic Pd, amorphous Pd-Ni-P NPs, crystalline Pd-Cu-P NPs, and a crystalline counterpart of Pd-Cu-Ni-P catalyst. The catalytic selectivity likely arose from improved Pd-M (M = Cu or Ni) synergistic effects in the amorphous phase and the electron deficiency of Pd. The model reactions proceeded under H2 or O2 gas without any additives, bases, or metal oxide supports, and the catalyst could be reused several times. This report is expected to enlighten the design of amorphous alloy nanomaterials as green and inexpensive catalysts for atom-economic and selective reactions.

1. Introduction


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Catalytic hydrogenation and oxidation reactions are fundamental ways to produce various chemicals and intermediates. Directing transformations in a selective manner plays a key role to reduce the amount of industrial waste to ensure sustainable chemistry. Compared with homogeneous catalysts, heterogeneous ones are preferred in the chemical industry due to the ease of separating the catalyst and reuse, which saves production costs. Among others, palladium has been extensively studied as a highly active and versatile heterogeneous catalyst.1-5 Much research has been done to improve catalytic selectivities through different methods, such as alloying Pd with a secondary metal6-10 or pairing with a metal oxide support6, 11 or an organic ligand12-14. Typically, Pd-alloy nanoclusters immobilized on polystyrene-based polymers have been used in selective aerobic oxidations.9 Pd- and Pt-based catalysts coated by organic-capping or self-assembled monolayers (SAM) have performed well in selective hydrogenations.15-20 Polymeric mesoporous graphitic carbon nitride (mpg-C3N4),a type of effective support for metal catalysts, has been applied mainly in selective hydrogenation reactions.21-24 In most examples, the use of organic modifiers (supports, ligands, or additives), including hazardous reagents, is inevitable. In addition, metal alloys, traditionally utilizing two different precious metals with the helped of metal oxide supports, are very good selective catalysts.6, 8, 9, 25 This combination has drawbacks, such as high cost, bad compatibility of the oxides in chemical media, low resistance to deactivation by coke deposition (e.g., Al2O3), and poor mechanical performance (e.g., MgO). Therefore, it is critical for environmental, industrial, and economic issues to use metal or alloy nanocatalysts alone to achieve high catalytic selectivities. On the other hand, amorphous metal phosphides have drawn much attention in heterogeneous catalysis and next generation batteries,26-28 and supplied worldwide industries with phosphide semiconductors.29 In contrast to crystal, the absence of terraces of metal atoms in amorphous


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alloy creates them a high degree of coordinative unsaturation.30-32 Besides, metal-P covalent bonds result in electropositive metals, which contribute to high catalytic efficiencies and stabilities in aerobic oxidations14,

33

and high energy barriers for subsurface chemistry,

segregation, and metal hydride formation.10, 34, 35 In contrast to common M-P binary alloys whose preparation has been widely researched, multi-compositional (e.g., Pd-M-P) materials are typically formed with improved local disorder and enhanced thermal stability.26 More importantly, bimetallic synergistic effects would be highly improved in amorphous Pd-M-P NPs duo to the entire uniform morphology and shorten Pd-M atomic distance.36 However, these materials have been rarely studied and their application has only been focused on fuel-cell catalysis.33, 37, 38 The catalytic properties of Pd-M-P have not yet to be revealed in liquid-phase organic reactions. Herein, we aimed to investigated how the compositions affected the morphologies of the PdM-P NPs (M = Cu or Ni) to achieve enhanced Pd-M synergistic effects in the amorphous Pd-MP NPs. Cheap Cu and Ni are promising candidates to be alloyed with a precious metal resulting in the synergistic (or ligand) effects for selective reactions.7, 8, 39-44 For example, Cu was known as a promoter to Pd in semi-hydrogenation of alkynes. The maximum ethylene selectivity of PdCu is comparable to that of Pd-Ag alloy.40 In aerobic oxidation of primary alcohols, Cu could accelerate the deprotonation rate of β-hydrogen to increase the activity of the catalyst.45 To evaluate the selective catalysis of the amorphous Pd-M-P NPs, several liquid reactions, including the semi-hydrogenation of alkynes, the hydrogenation of quinoline, and the oxidation of primary alcohols, were selected as models utilizing molecular H2 and O2.


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2. Experimental Section 2.1. Chemicals. Copper (II) acetylacetonate [Cu(acac)2, ≥99.99%] were purchased from Sigma-Aldrich Co. Palladium (II) acetate [Pd(OAc)2, 47.5% Pd] and copper (II) acetate monohydrate [Cu(OAc)2·H2O, >98%] were purchased from Alfa Aesar. Other chemicals were described in our previous literature.33 2.2. Characterizations. All the equipments and operation conditions for characterizations including X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), selected area diffraction (SAED), energydispersive X-ray (EDX), and inductively coupled plasma mass spectrometry (ICP-MS) were described in our previous literatures.33, 38 2.3. Fabrication of Pd-(M)-P NPs. M(acac)2 (M = Pd, Cu, or Ni, 0.8 mmol), PPh3 (1.76 mmol), Bu4NBr (2 mmol, for 10 nm amorphous Pd-Cu-Ni-P NPs) or borane tert-butylamine complex (2 mmol, for others), (Oct)3PO (6 mmol), and oleylamine (OLA, 12.5 mL) were used, and the fabrication was carried out with a one-pot solution-phase reduction procedure based on our previous reports.14, 33 After reaction, the NPs were collected by centrifugation (13000 rpm, 10 min), washed with ethanol (5 mL), and then stored in hexane. TEM images of PdPx NPs (Pd72P28 determined by ICP-MS, the same below), Pd1Cu2Px NPs (Pd26Cu47P27), Pd1Cu2Ni1Px NPs (Pd12Cu45Ni13P30), Pd0.2Cu2Px NPs (Pd2Cu83P15), and Ni0.2Cu2Px NPs (Ni6Cu70P24) are shown in Figure S2 in Supporting Information. 2.4. Fabrication of Pd-(M)-P/C (C represents Vulcan XC-72). The mixing procedure for Pd-M-P NPs and Vulcan XC-72 was described in our previous literature.33 After deposition, the


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material was annealed at 300 °C under nitrogen gas for 1 h to remove OLA and the organic impurities from the surface. 2.5. Fabrication of impregnated catalyst Pd75Cu25/C. This material was produced by reducing a mixture of Pd(OAc)2 (0.06 mmol) and Cu(OAc)2·H2O (0.02 mmol) in methanol using NaBH4 (0.16 mmol) in the presence of Vulcan XC-72 (20 mg). An methanol solution of NaBH4 (2 mL) was added slowly into the above mentioned methanol solution (10 mL) at ambient temperature and the mixture was stirred for 20 min. The synthesized catalyst was collected by filtering and washing with methanol six times and dried under vacuum. 2.6. Acetic acid treatment. A suspension of the as-prepared 2 nm Pd-Cu-Ni-P/C (100 mg) in acetic acid (50 mL) was heated under N2 at 60 °C for 12 h. The reaction was quenched by adding 50 mL of EtOH. The material was collected by centrifugation and underwent additional washing with EtOH (50 mL×2). Then, the material was annealed at 300 °C for 1 h, and named as Pd-CuNi-P/C-Ac. 2.7. Semi-hydrogenation of 4-ethynyl-biphenyl. The reaction was conducted by mixing alkyne (0.2 mmol), catalyst (0.1-0.5 mol% of Pd), and MeOH/H2O (25/1, v/v; 1.0 M) as the solvent under a hydrogen atmosphere provided by a balloon, and monitored by a gas chromatograph-mass spectrometer (GC-MS). After the reaction finished, the catalyst was separated by centrifugation, and the supernatant was analyzed by GC-MS and/or nuclear magnetic resonance (NMR) spectroscopy to calculate the conversion of 4-ethynyl-biphenyl and selectivity of the corresponding alkene. 2.8. Partial oxidation of primary alcohols. Typically, the oxidation reactions were conducted by mixing primary alcohol (0.4 mmol), catalyst (0.6-2 mol% of Pd), and solvent (4 mL) under an


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oxygen atmosphere provided by a balloon. After 12 h (27 h in the case of cinnamic alcohol), the catalyst was separated by centrifugation, and the supernatant was analyzed by GC-MS and 1HNMR to calculate the conversion of alcohol and selectivity of product aldehyde. 2.9. Hydrogenation reaction of quinoline. The reaction was performed in an autoclave (30 cc). Quinoline (1.9 mmol), catalyst (2 nm amorphous Pd18Cu21Ni19P42/C, 30 mg), and solvent (MeOH, 1.9 mL)were added and the reaction was proceeded in an oil bath (80 oC) under a hydrogen atmosphere (0.5 MPa). The reactions were monitored by GC-MS. After 15 h, the reaction was finished and the catalyst powder was removed by centrifugation. The selectivity and yield of product were analyzed by 1H-NMR.

3. Results and discussion 3.1. Materials and characterizations. The morphologies of Pd-(M)-P/C were analyzed by XRD (Figure 1, see TEM images in Figure S2). It was found that the mole ratio of Cu(acac)2 highly affected the morphologies of the resulting Pd-Cu-(Ni)-P and Ni-Cu-P NPs. When >40 at% of Cu was contained, micro- or fully crystalline structures were formed in all cases (Figure 1a, 1b, 1d, and 1e). The XRD patterns of Ni6Cu70P24 and Pd2Cu83P15 match well with the face-centered cubic (fcc) structure of Cu due to the substantial proportion of Cu (Figure 1a and 1b). In Figure 1d, the peaks are slightly shifted to lower angles due to the insertion of Pd atoms. In Figure 1e, in the case of Pd27Cu46P27, a B2-structured Pd-Cu alloy (Pd-Cu_bcc) likely formed, which was mixed with a Cu-P phase (49.3 o).46 Without the addition of Cu(acac)2 and Ni(acac)2, the binary Pd-P NPs (Pd72P28) were also obtained as crystals, a precursor of the Pd3P0.95 phase (Figure 1f).14 These observations indicate that Pd, Ni, and P are essential for amorphous particle formation.33,


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In contrast, Pd, Cu, and P are likely less miscible. In Figure 1c, the amorphous Pd-Cu-Ni-P

NPs (their composition is Pd33Cu11Ni29P27 determined by ICP-MS) formed when the mole ratios among the metal precursors, Pd(acac)2/Cu(acac)2/Ni(acac)2, was 3/2/3.

Figure 1. XRD patterns of different Pd-(M)-P NPs. The compositions were determined by inductively coupled plasma mass spectrometry (ICP-MS). Note that all the catalysts were deposited on activated carbon (Vulcan XC-72) and annealed at 300 °C for 1 h. Before deposition, transmission electron microscopy (TEM, Figure 2a) shows the amorphous Pd-Cu-Ni-P NPs formed with a uniform size of 10 nm in average (Figure 2a, inset). Selected area electron diffraction (SAED, Figure 2b) and high-resolution TEM (HRTEM, Figure 2c) indicated the absence of long ordered atomic chains within the NPs, corresponding to the observation of XRD pattern (Figure 1c). After the deposition and annealing process, annular dark-field scanning transmission electron microscopy (ADF-STEM, Figure 2d), TEM (Figure S1a), and bright-field STEM (BF-STEM, Figure S1b) images indicated a good distribution of NPs on carbon. Highresolution ADF-STEM (Figure 2e) and high-resolution BF-STEM (inset in Figure S1b) further indicated the dense amorphous structure without any lattice planes. Energy-dispersive X-ray


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spectroscopy (EDX) analysis in the STEM confirmed the existence and homogeneity of elemental Pd, Cu, Ni, and P (Figure 2f). As mentioned above, the composition was determined to be Pd33Cu11Ni29P27 by ICP-MS. Pd/Ni atomic ratio corresponds to the mole ratio of Pd(acac)2 to Ni(acac)2, while Cu(acac)2 was not completely reduced. In our previous report, we supposed that Pd-Ni-P NPs were probably formed through a cleavage of metal-P bond of metal-phosphine complex.33 Therefore, there may be two reasons for the incomplete reduction of Cu(acac)2. First, Cu is difficult to form complex with PPh3 compared with Pd and Ni; Second, Cu(II) is less efficiently to be reduced than Pd(II). Another report also shown the incomplete reduction of Cu(acac)2 in OLA during the preparation of Pd-Cu alloy.47 Notably, the supported NPs remain amorphous after calcination at 300 °C, which agrees with the results from Pd-Cu-Ni-P bulk metallic glasses.48 Their crystallization temperature is generally higher than 350 °C. This result clearly demonstrates that the thermal stability of the amorphous phase in the quaternary NPs is significantly higher than those in binary metal phosphides, including Pt-P49 and Ni-P NPs50.


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Figure 2. TEM (a), size distribution (a, inset), SAED (b), and HRTEM (c) images of the 10 nm amorphous Pd-Cu-Ni-P NPs coated by OLA. ADF-STEM (d), high-resolution ADF-STEM (e), and STEM-EDX mapping (f) images of the 10 nm amorphous Pd-Cu-Ni-P/C. 3.2. Semi-hydrogenation reaction of alkynes. Interestingly, the different morphologies of the various Pd-(M)-P NPs led to different catalytic properties. As the selective hydrogenation of carbon-carbon triple bonds is an important route to achieve olefins, especially for stereo specific products, the semi-hydrogenation of alkynes was selected as a model reaction to study the catalysis of Pd-(M)-P NPs (Table 1). Notably, several catalysts, including Pd72P28, Pd26Cu47P27,


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Pd33Cu11Ni29P27, and Pd2Cu83P15, with similar loadings of Pd (95% with high conversions of the alkyne (entries 1-2, 4-5). Notably, the amorphous Pd33Cu11Ni29P27/C was found to be the best catalyst, and the hydrogenation of 4-ethynyl-biphenyl gave 4-phenyl styrene in >99%selectivity using 0.4 mol% of Pd (Table 1, entry 4). When comprising larger content of Cu, the crystalline catalyst Pd12Cu45Ni13P30/C showed a lower selectivity (entry 3). The reaction did not proceed with Ni-Cu-P/C (entry 6). The commercial Pd/C was the least selective catalyst, showing a decreased alkene selectivity of 86% and an alkyne consumption of 84% (entry 7). To the best of our knowledge, the titled NPs show the best catalytic performance among previously reported amorphous alloys.51-53 The same selectivity was also obtained when we changed the catalyst carriers to TiO2 or SiO2 (entries 8-9, see TEM images in Figure S3a and S3b). These observations clearly indicate that the catalysis of the amorphous Pd-Cu-Ni-P NPs was inherent and was not influenced by the supports. In entry 10, no alkene was obtained when we used an impregnated catalyst Pd75Cu25/C (named Pd75Cu25/C-Im, Figure S3c). Lindlar catalyst (PdCaCO3-PbO/PbAc2), a conventional heterogeneous catalyst for semi-hydrogenation, showed a lower selectivity of 95% after 0.5 h and then lost its selective catalysis completely after 3.2 h (entry 11). Furthermore, the catalytic uniqueness of the amorphous alloy was confirmed by a controlled experiment of its crystalline counterpart, which was prepared by simply annealing the amorphous catalyst at 400 °C.38 The TEM image (Figure S4a) indicated that the resulting NPs did not aggregate after the calcination, while HRTEM (Figure S4b) and XRD (Figure S4c) detected the appearance of lattice faces. When the reaction was conducted with the obtained partially crystallized catalyst, the selectivity decreased to 97:3 with only 89% conversion (entry


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12), likely due to the segregation of Pd to larger active sites, which decreased the Pd-Cu synergistic effect (vide infra). Table 1. Semi-hydrogenation of 4-ethynyl-biphenyl. Conv. Entry

Catalyst

Pd mol%

Sel.

Conditions (%)

(%)

1

Pd72P28/C

0.4

25 °C, 2.5 h

99

97

2

Pd26Cu47P27/C

0.5

45 °C, 3.5 h

95

96

3

Pd12Cu45Ni13P30/C

0.5

25 °C, 13 h

95

90

4

Pd33Cu11Ni29P27/C

0.4

45 °C, 3.2 h

>99

>99

5

Pd2Cu83P15/C

0.1

45 °C, 3.5 h

95

96

6

Ni6Cu70P24/C

0.1 (Ni)

65 °C, 2 h

0

-

7a

Pd/C

0.1

25 °C, 0.5 h

84

86

8

Pd33Cu11Ni29P27/TiO2

0.4

45 °C, 3.2 h

>99

99

9

Pd33Cu11Ni29P27/SiO2

0.4

45 °C, 3.2 h

47

100

10b

Pd75Cu25/C-Im

0.4

45 °C, 3.2 h

100

0

11c

Lindlar catalyst

0.4

45 °C, 3.2 h

100

0

(0.5 h)

(>99)

(95)

89

97

12

Pd33Cu11Ni29P27/C

0.4

45 °C, 0.5 h

(partially crystalline)


12

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Reaction conditions: 4-ethynyl-biphenyl (0.2 mmol), MeOH/H2O (25/1, v/v; 1.0 M) as the solvent, H2 (1 atm). Note that the reaction conditions have been optimized in entry 1-5. Conv. and Sel. represent the conversion of 4-ethynyl-biphenyl and the selectivity of 4-phenyl styrene, respectively, which were determined by 1H-NMR. aThe solvent was 0.05 M MeOH/H2O. b

Alkane product was obtained in nearly 100% yield. cThe solvent was 1.0 M EtOH/H2O (25/1,

v/v). With the titled amorphous catalyst, three other types of alkynes, a terminal aliphatic alkyne (Table 2, entry 2), diphenylacetylene (entry 3), and 1-phenyl-1-butyne (entry4), were investigated. Excellent selectivity of the alkene was achieved in all cases. It is noted that terminal alkynes were preferred compared with internal alkynes, transforming to the corresponding alkenes in a relatively higher selectivity. In the case of internal alkynes, the Z isomers of the alkenes were obtained in high yields with a Z:E ratio of >10:1 (entries 3-4). Table 2. Semi-hydrogenation of alkynes catalyzed by the amorphous Pd-Cu-Ni-P/C. Entry

Alkynes

Conv. (%)

Sel. (%)

1

>99

>99

2

>99

95

3a

>99

91 (Z:E>10:1)


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4a

>99

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91 (Z:E >10:1)

Reaction conditions: alkyne (0.4 mmol), catalyst (0.4 mol%), H2 (1 atm), MeOH/H2O (7.7 mL/0.3 mL) as the solvent, 45 °C. aThe reaction was carried out at 60 °C 3.3. Partial oxidation of primary alcohols. Encouraged by the selective catalysis of the amorphous NPs in the hydrogenations, they were then utilized for the oxidation of primary alcohols. The selective oxidation of benzyl alcohol to benzylaldehyde was studied as a preliminary model reaction. Unfortunately, the reaction did proceed at all with the 10 nm amorphous Pd-Cu-Ni-P/C (Table 3, entry 1) perhaps due to the limited number of lowcoordinated Pd atoms and low Cu content (11 at%). It occurred to us that 99

3b

2 nm Pd-Cu-Ni-P/C (0.6)

80

100

>99


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4

2 nm Pd-Ni-P/C (2.0)

120

30

99

5b,c

Commercial Pd/C (0.6)

80

91

68

6b

2 nm Pd-Cu-Ni-P/C-Ac (0.6)

80

100

>99

Reaction conditions: benzyl alcohol (0.4 mmol), p-xylene (4.0 mL) as the solvent, O2 (1 atm), 12 h. aDetermined by GC-MS. bIn the absence of p-xylene. cThe reaction time is 3 h. To further display the outstanding selective catalysis of the amorphous NPs, the contents of benzyl alcohol, benzylaldehyde and the main by-product benzyl benzoate were plotted against the reaction time. In Figure 4a, the highest selectivity of aldehyde of 68% was observed after 3 h with Pd/C; however, the value decreased quickly to 52% when prolonging the reaction time to 6 h. In contrast, the selectivity was maintained at 99% upon extending the reaction past 3 h after the full conversion of alcohol using the Pd-Cu-Ni-P catalyst (Figure 4b). Next, we modified the catalyst by partial removal of Cu by a simple acetic acid treatment without changing the morphology or catalytic properties (Table 3, entry 6); the treated material was named as Pd-CuNi-P/C-Ac (TEM image in Figure S8 and XRD pattern in Figure 3d). After reaction, the STEM image (Figure S10a) showed no aggregation of the NPs. The obtained catalyst could be reused at least four times without any loss of catalytic selectivity (Figure S9) or change of the amorphous structure (Figure S10b). We also compared our catalyst with various previously reported catalytic systems, including Pd on metal oxide and Pd-based alloys, which demonstrated that the amorphous alloy NPs are the most selective (Table S2).


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Figure 4. The contents of benzyl alcohol, benzylaldehyde, and benzyl benzoate over the course of the reaction at 80 °C using different catalysts: (a) Pd/C and (b) 2 nm Pd-Cu-Ni-P/C. The selective oxidation of cinnamic alcohol to cinnamic aldehyde is much more challenging due to the existence of a C=C bond, especially in the absence of base.56 To our delight, the selectivity of cinnamic aldehyde reached up to 82% with 95% conversion with the 2 nm Pd-CuNi-P amorphous catalyst, while much lower selectivities were observed with Pd/C, Pd/SiO2, and Pd/Al2O3 (Table 4). Table 4. Selective oxidation of cinnamic alcohol with different Pd-based catalysts.

OH

Catalsyt

O

O2, 1atm

Entry

Catalyst

Conv. (%)

Sel. (%)

Reference

1

2 nm Pd-Cu-Ni-P/C

95

82

This work

2

Pd/C

90

68

[57]

3

Pd/SiO2

83

66

[57]

4

Pd/Al2O3

31

30

[57]


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Reaction conditions for entry 1: cinnamic alcohol (0.4 mmol), catalyst (0.4 mol%), toluene (4 mL), 100 oC, 27 h. 3.4. Selective hydrogenation of quinoline. Considering the high activity of the 2 nm Pd-CuNi-P NPs in the oxidation reaction, we again investigated their hydrogenation catalysis using a more challenging substrate, quinoline. Previous work using Pd NPs supported on metal oxides has required the use of harsh reaction conditions.58, 59 We found the hydrogenation proceeded smoothly under relatively mild conditions with the 2 nm amorphous Pd-Cu-Ni-P/C (Scheme 1). The corresponding 1,2,3,4-tetrahydroquinoline was obtained in >99% selectively.

Scheme 1. Amorphous Pd-Cu-Ni-P/C catalyzed selective hydrogenation of quinoline to 1,2,3,4-tetrahydroquinoline. 3.5. Discussion. Given that the dissociation of molecular hydrogen on the catalyst surface is often the rate-limiting step, the unique catalytic property of Pd-Cu-Ni-P/C in semi-hydrogenation may stem from four factors. First, the amorphous morphology, or in other words, the entire coarse-grained surface of the NPs could decrease the hydriding behavior of Pd.35 Second, Pd atoms dissociate H-H bond and the resulting dissociated H on Pd could transfer to Cu, which would attack the alkyne due to its weak binding ability on Cu, allowing the selective semihydrogenation.7 Among the several Pd-Cu-(Ni)-P NPs shown in Table 1, the amorphous NPs showed distinguished selective catalysis, which indicates that the synergistic effect between Pd and Cu is greatly improved in the amorphous phase. Pd75Cu25/C-Im shows no selectivity in the


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semi-hydrogenation reaction of alkyne (Table 1, entry 10), which may be ascribed to the acceleration of hydriding process on the crystalline Pd phase and abundant Pd active sites on the "clear" surface of Pd-Cu NPs. Therefore, it cannot be excluded that the extra carbonaceous residues from Pd-Cu-Ni-P NPs would affect the selectivity, which would be the third factor. Four, XPS showed that the BEs of Pd3d in amorphous Pd-Cu-Ni-P/C (Figure 5, see SI for other details on XPS) exhibited positive shifts by approximately 0.2 eV compared with the values in Pd (0), indicating the electron deficiency of Pd, which could lead to a suppression of Pd-hydride formation and a preferential absorb of alkyne molecules on Pd atoms.10, 60 As a consequence, the neighboring Cu-hydride would react with the stable alkynyl-Pd to give alkene as a product. For alcohol oxidation, Cu could accelerate the beta-hydride elimination of the metal alkoxide intermediate to afford the aldehyde (Scheme 2, Path A),45, 55 which has been proposed as the likely rate-determining step.56, 61 In comparison with the amorphous Pd-Ni-P/C (Cu: 0 at%) and 10 nm Pd33Cu11Ni29P27/C (Cu: 11 at%), the 2 nm Pd18Cu21Ni19P42/C (Cu: 21 at%) shows much higher activity further demonstrated the assumption. At the same time, Pd-Cu synergistic effect would inhibit the reaction of the alkoxide with the aldehyde to form a hemiacetal and the subsequent benzyl benzoate (Scheme 2, Path B). Moreover, Ni could reduce the amount of Pd-H species by the generation of stable Ni(OH)x,62 resulting in the suppression of the reduction of benzyl alcohol (Scheme 2, Path C). In Figure S11d, the electron state of phosphorus might be unitary metal-P-Ox with a BE of 134.2 eV. It indicates that C-P bond cleavage occurred because the BE of P2p in the Pd(0)-triphenylphosphine complex is approximately 130.6 eV. This result also excluded an effect of organic ligand (i.e. PPh3) on the selective catalysis.


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Figure 5. XPS spectra of Pd(3d) in Pd/C and 10 nm amorphous Pd-Cu-Ni-P/C. OH Ph

+ O2

Metallic Pd regenerated

H

OH

OH Ph O

H

O Cu Cu

H

Pd Pd

Ph

O

O Ni path A

path B

Ph

O

Cu Cu OH

H

O

Cu Cu

Pd Pd

Ni Ni

path C

Ph

Pd Pd Pd

Ni Ni

path A

OH

OH Ph

O

path B

Ph O2 O

Ph

O

Ph

Scheme 2. Proposed mechanism for the aerobic oxidation of the alcohol.

4. Conclusions In summary, the morphologies in relation to the compositions of various Pd-M-P were investigated, and the quaternary Pd33Cu11Ni29P27 NPs were fabricated with an amorphous structure. The thermal stability of an amorphous phase in the Pd-Cu-Ni-P NPs was observed during the annealing process. The NPs exhibited remarkable catalytic activities and selectivities


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in the semi-hydrogenation of alkynes, hydrogenation of quinoline, and aerobic oxidation of primary alcohols without contributions from an interfacial effect with the support. A comparison with the crystalline counterpart disclosed that the amorphous structure was essential for the unique catalytic performance. This work directs a clue to design alloy nanomaterials with stable amorphous morphology as selective and versatile heterogeneous catalysts. Further studies to elucidate the mechanism and to extend the scope of the synthetic utility of the amorphous NPs are in progress in our laboratory.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Additional TEM and STEM images, XRD patterns, EDX spectra, XPS analysis of the amorphous Pd-Cu-Ni-P/C, and the stability test of Pd-Cu-NiP/C catalyst in the oxidation of benzyl alcohol.

AUTHOR INFORMATION Corresponding Author *[email protected]; *[email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This work was partially supported by the Fundamental Research Funds for the Central Universities (no. 2015XKMS048), the Natural Science Foundation of Jiangsu Province (no. BK20160254).

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234x113mm (96 x 96 DPI)


Composition-Dependent Morphology of Bi- and Trimetallic

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