N-Formylation of Amines with CO2 and H2 Using PdâAu Bimetallic

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N-Formylation of Amines with CO2 and H2 Using Pd-Au Bimetallic Catalysts Supported on Polyaniline-Functionalized Carbon Nanotubes Pengpeng Ju, Jinzhu Chen, Aibing Chen, Limin Chen, and Yifeng Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02865 • Publication Date (Web): 07 Feb 2017 Downloaded from http://pubs.acs.org on February 13, 2017

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ACS Sustainable Chemistry & Engineering

N-Formylation of Amines with CO2 and H2 Using Pd-Au Bimetallic Catalysts Supported on Polyaniline-Functionalized Carbon Nanotubes

Pengpeng Ju,†,#,§ Jinzhu Chen,*,†,§,‡ Aibing Chen,*,# Limin Chen,‡ and Yifeng Yu#

†

College of Chemistry and Materials Science, Jinan University. No. 601 Huangpu Avenue West,

Tianhe District, Guangzhou 510632, P.R. China #

College of Chemical and Pharmaceutical Engineering, Hebei University of Science and

Technology. No. 70 Yuhua Road, Shijiazhuang 050018, P.R. China §

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences. No. 2 Nengyuan Rd,

Wushan, Tianhe District, Guangzhou 510640, P.R. China ‡

Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control. School of

Environment and Energy, South China University of Technology. 382 Zhonghuan Road East, Guangzhou Higher Education Mega Centre, Panyu District, 510006, P.R. China * Corresponding author, Tel.: (+86)-20-8522-2191, Fax: (+86)-20-8522-0223, E-mail address: [email protected] (J. Chen), [email protected] (A. Chen)

1

ACS Paragon Plus Environment


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Abstract: Bimetallic Pd-Au catalyst was prepared by depositing the Pd-Au alloy nanoparticles on polyaniline-functionalized carbon nanotubes (PANI-CNT) and the resulting Pd-Au/PANI-CNT catalyst exhibited excellent catalytic activity for the N-formylation of pyrrolidine using CO2/H2. The structural and electronic properties of the Pd-Au/PANI-CNT was characterized by X-ray powder diffraction (XRD), nitrogen adsorption-desorption, transmission electron microscopy (TEM), high angle annular dark-field scanning transmission electron microscopy (HAADF–STEM) and X-ray photoelectron spectroscopy (XPS). Under optimized conditions, an N-formylpyrrolidine yield of 98.3% was obtained from pyrrolidine and CO2/H2 at 125 °C by using Pd-Au/PANI-CNT with the Pd/Au molar ratio of 1:1. Our research further reveals that Pd atoms should be the true active sites for the hydrogenation reaction and the N-formylation reaction might occur mainly over Pd atoms or over the interface between Pd atoms and Au atoms for the bimetallic Pd-Au/PANI-CNT catalyst. The enhanced catalytic performance of bimetallic Pd-Au/PANI-CNT is mainly related to beneficial interactions between Pd atoms and Au atoms, leading to the changes of the electronic properties of the formed bimetallic Pd-Au nanoparticles.

Keywords: bimetallic catalyst, carbon dioxide, electronic property, formylation, hydrogenation

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1. INTRODUCTION As a renewable, abundant, low-cost and nontoxic C1 resource, the transformation of carbon dioxide (CO2) into value-added compounds has attracted wide attentions in the both fields of green and synthetic chemistry.1-8 Moreover, the utilization of CO2 as carbon source for fine chemical synthesis is one of primary methods to reduce the amount of CO2 in the atmosphere.9-13 Recent research reported catalytic N-formylation of amines to formamides with CO2 and hydrogen (H2) as carbonylation

reagent.14-16

In

addition

to

H2,

hydrosilane,

hydrosiloxane

and

polymethylhydrosiloxane (PMHS) were investigated as reducing agents for N-formylation of amines with CO2.17-31 Generally, CO2/hydrosilane and CO2/hydrosiloxane systems allow diverse amine scope and mild reaction conditions for formamide synthesis when compared with CO2/H2 system.32-39 Formamides are typically important intermediates with widespread applications in organic synthesis and pharmaceutical compound synthesis.40-47 For example, formamides are useful reagents in Vilsmeier formylation reaction,48 important precursors in the preparation of formamidines, isocyanides and heterocycles, intermediates for the productions of methylated amines from amines, and serve as Lewis base catalysts in hydrosilylation and alkylation of carbonyl compounds.49,50 Moreover, N,N-dimethylformamide served as important industrial solvent as well as versatile reagent for synthetic transformation.51,52 The N-formylation reactions with CO2/H2 as carbonylation reagent was reported by mainly using homogeneous ruthenium-complex catalysts such as Ru-pincer-type complexes RuCl2(PMe3)4 [PMe3 = trimethylphosphine],17,18 RuCl2(dppe)2 [dppe = 1,2-bis(diphenylphosphino)ethane],19-21 RuCl2(dppbz)2 [dppbz = 1,2-bis(diphenylphosphino)benzene],22 [HRu3(CO)11]−,23 PdCl2/KHCO3,24 Pt2(µ-dppm)3 [dppm, bis(diphenylphosphino)methane],25,26 (PPh3)2(CO)IrCl,27 Molybdenum-silyl 3



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complex,28 (PPh3)3CuCl,27 Fe(BF4)2·6H2O-phosphine ligand,29,30 and Co(BF4)2·6H2O-phosphine ligand.31 In the case of CO2/hydrosilane- and CO2/hydrosiloxane-based homogeneous carbonylation systems, the investigated catalytic systems involved both transition metal complexes such as Cu-diphosphine complexes-PMHS,32,33 chelating bis(tzNHC) [tz = 1,2,3-triazol-5-ylidene, NHC = N-heterocyclic carbene)] rhodium complexes-Ph2SiH2,34 Ir(H)(CF3SO3)(NSiN)(coe) [NSiN = fac-coordinated bis(pyridine-2-yloxy)methylsilyl, coe = cis-cyclooctene]-HSiMe(OSiMe3)2,35 and organocatalytic

systems

including

imidazolium-based

ionic

liquids-PhSiH3,36

1,3,2-diazaphospholene-Ph2SiH2,37 NHC-PMHS,38 and thiazolium carbine-PMHS.39 Homogeneous ruthenium complex catalysts evidently show excellent catalytic performance for the N-formylation of amines with CO2. Although a modification of heterogeneous catalyst Ru/Al2O3

with

phosphine

ligand

dppe

was

recently

reported

for

N-formylation

of

3-methoxypropylamine with supercritical CO2 and H2, the formation of a Ru-dppe species was suggested as the “true” homogeneous catalyst for this transformation. Notably, as far as we known, very limited heterogeneous catalysts including Pd/Al2O3-NR-RD (NR, nano-rod; RD, reductive deposition),53 Ir/HSA-TiO2 (HSA, high surface area) and Cu/ZnO were reported for N-formylation of amines with CO2.54,55 In addition to N-formylation of amines with CO2, recent research reported reductive methylation of amines to give N-methyl amines or N,N-dimethyl amines by using CO2 and a reducing agent. The investigated catalytic systems included [Pt] Karstedt’s catalyst-HCOOH,56 Pt-MoOx/TiO2-H2,57

Au/Al2O3-VS-H2,58

Pd/CuZrOx-H2,59

[Cu(OtBu)-(IMes)]-PhSiH3,60

ZnCl2-IPr-PhSiH3,61 N-heterocyclic carbenes-PhSiH3,62 B(C6F5)3-PhSiH3.63 The reported catalytic systems

for

N-formylation

(Table

S1,

Supporting

Information),

4


N-methylation

and

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N,N-dimethylation (Table S2, Supporting Information) of various amines with CO2 were systematically compared in the Supporting Information. Besides CO2/H2, CO was also investigated as the carbonylation reagent of amines for the synthesis of amides. For example, Dyson and co-workers reported Pd-promoted oxidative carbonylation of C(sp3)–H bonds with CO and amines to give substituted phenyl amides.64 In industry, N,N-dimethylformamide (DMF) is obtained by dimethylamine carbonylation with CO using CH3ONa catalyst in methanol.65 Compared with the toxic CO, CO2 is abundant, cheap and safe. However, due to kinetic inertness and thermodynamic stability, reductive amination of CO2 to give amide remains a great challenge although highly attractive. Pd and Au were reported respectively as heterogeneous catalysts for CO2 reduction.66-68 Notably, Pd-Au bimetallic catalysts have recently attracted widespread attention and are the most widely investigated bimetallic catalyst systems.69-73 Generally, Pd-Au bimetallic catalyst shows easy accessibility as well as broad catalytic scope in oxidation, coupling, hydrogenation, hydrogenolysis and so on.74-80 Typically, Pd-Au bimetallic catalysts were commercially used in the industrial production of vinyl acetate and extensively investigated in various reactions such as oxidation of alcohols to aldehydes, low-temperature CO oxidation, Suzuki coupling reaction, direct H2O2 synthesis from H2 and O2, acetylene trimerization, hydrodechlorination of chlorine-containing pollutants, hydrodesulfurization of sulphur-containing compounds, hydrogenation of hydrocarbon, catalytic reduction of 4-nitrophenol with an excess amount of NaBH4, electrochemical hydrogen generation,

electrooxidation

of

formic

acid,

and

electrocatalytic

oxidation

of

5-hydroxymethylfurfural to 2,5-furandicarboxylic acid.71,81 Among these reactions, Pd-Au bimetallic catalyst often exhibits remarkably enhanced catalytic performances when compared with 5



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either of the constituent metals. The enhancement of the catalytic performance for Pd-Au bimetallic catalyst is sometimes caused by synergetic effect, alloy effect, ensemble effect and ligand effect, which is often closely correlated with its alloy composition and controllable morphology.82-90 Previously, the polyaniline (PANI)-immobilized bimetallic Pd-Au catalyst was reported for benzyl alcohol oxidation.91 Vanadium dioxide (VO2) supported on PANI-functionalized carbon nanotubes (PANI-CNT) was examined for 5-hydroxymethylfurfural transformation into 2,5-diformylfuran.92 Palladium nanoparticles deposited on PANI-CNT were investigated in selective hydrogenation of phenol to cyclohexanone.93-95 Moreover, Pt-Ru bimetallic catalysts supported on PANI-CNT were reported for electrochemical oxidation of methanol.96,97 Therefore, in this research, we demonstrated the preparation of bimetallic Pd-Au catalysts deposited on composite PANI-CNT owing to stabilization effect of PANI towards nanoparticles and reversible acid/base chemistry, redox property as well as non-solubility of PANI in most organic solvents. The bimetallic Pd-Au/PANI-CNT exhibits a narrow size distribution of Pd-Au nanoparticles with an average nanoparticle size of 3.0 nm over PANI-CNT. Moreover, heterogeneous catalyst Pd-Au/PANI-CNT shows outstanding catalytic activity towards the N-formylation of pyrrolidine with the corresponding N-formylpyrrolidine yield of 98.3% at 125 °C using CO2/H2 (Scheme 1). The Pd-Au bimetallic catalyst, containing Pd/Au in a molar ratio of 1:1, is significantly more active than pure Pd/PANI-CNT and pure Au/PANI-CNT catalysts, indicating a strong alloy effect. Our research further reveals that Pd atoms should be the true active sites for the hydrogenation reaction and the N-formylation reaction might occur mainly over Pd atoms or over the interface between Pd and Au in the phase of alloyed bimetallic nanoparticles in Pd-Au/PANI-CNT catalyst. The enhanced catalytic performance of alloyed bimetallic Pd-Au/PANI-CNT is mainly related to 6


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beneficial interactions between Pd and Au, leading to the changes of the electronic properties of the bimetallic Pd-Au nanoparticles.

Scheme 1. N-formylation of pyrrolidine to N-formylpyrrolidine over Pd-Au/PANI-CNT, Pd/PANI-CNT and Au/PANI-CNT catalysts

2. EXPERIMENTAL SECTION 2.1 Materials and catalyst preparation The information of chemical materials and the preparations of PANI, PANI/CNT, acid-treated CNTs, reduced graphene oxide (RGO) were provided in the Supporting Information. Synthesis of various Pd-Au bimetallic catalysts In a typical synthesis of Pd-Au/PANI-CNT, an aqueous solution containing HAuCl4 (13.6 mg, 1×10-3 M), H2PdCl4 (PdCl2 5.8 mg in 2~3 drops of hydrochloric acid, 1×10-3 M) and polyvinyl alcohol (PVA 30 mg, molar ratio of PVA monomer/(Au+Pd) = 10:1) was prepared. The mixture was stirred vigorously at 0 °C (ice-water bath) for 0.5 h. A freshly prepared aqueous solution of NaBH4 (12.4 mg, 0.1 M, 3.3 mL, molar ratio of NaBH4/(Au+Pd) = 5:1) was quickly added to give a dark brown solution. The PANI-CNT (200 mg) was then added to the above colloidal solution under vigorous stirring for 4 h. The resulting Pd-Au/PANI-CNT was filtered and washed thoroughly with 7



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deionized water, further dried under vacuum for 12 h at 80 °C, and then reactivated at 200 °C in H2 (3 MPa) for 2 h. Pd-Au/PANI, Pd-Au/CNT, Pd-Au/RGO were obtained with the same procedure used for Pd-Au/PANI-CNT except that the support PANI-CNT (200 mg) was replaced by PANI (200 mg), acid-treated CNTs (200 mg), and RGO (200 mg), respectively. Synthesis of Au/PANI-CNT and Pd/PANI-CNT In a typical procedure for Au/PANI-CNT preparation, an aqueous solution of HAuCl4 (1×10-3 M, 300 mL) with polyvinyl alcohol (30 mg, molar ratio PVA monomer/Au = 10:1) was prepared. The mixture was stirred vigorously at 0 °C for 0.5 h. A freshly prepared aqueous solution of NaBH4 (12.4 mg, 0.1 M, 3.3 mL, molar ratio NaBH4/(Au+Pd) = 5:1) was quickly added to give a dark brown solution. The PANI-CNT (200 mg) was then added to the above colloidal solution under vigorous stirring for 4 h. The resulting Au/PANI-CNT was filtered and washed thoroughly with deionized water, further dried under vacuum for 12 h at 80 °C, and then reactivated at 200 °C in H2 (3 MPa) for 2 h.98 Pd/PANI-CNT was obtained with the same procedure used for Au/PANI-CNT except that HAuCl4 (1×10-3 M, 300 mL) was replaced by H2PdCl4 (1×10-3 M, 300 mL). 2.2 Experimental Section In a typical N-formylation experiment, a 25 mL-stainless steel autoclave reactor was continuously loaded with a magnetic stir bar, Au-Pd/PANI-CNT catalyst (50 mg, Pd 1.6 wt.%, Au 3.0 wt.%, 0.008 mol% Pd relative to pyrrolidine, Pd/Au molar ratio = 1), 1,4-dioxane solvent (5.0 mL), and pyrrolidine (71 mg, 1.0 mmol). After flushing the reactor three times with CO2, the CO2 pressure was maintained at 3.5 MPa at ambient temperature in the reactor. H2 was then introduced 8


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into the reactor up to the desired partial pressure of 3.5 MPa. The stainless steel was stirred at 125

°C for 48 h. After the reaction was halted, the autoclave reactor was quickly cooled down to ambient temperature. The reaction mixtures were analyzed by Gas Chromatography (GC Fuli 9790II) equipped with a capillary column (KB-5, 0.32 mm × 30 m) and a FID detector with nitrogen as carrier gas.

N-formylpyrrolidine. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.08 (s, 1H), 3.33 (s, 2H), 3.23 (s, 2H), 1.74 (s, 4H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 160.57, 45.73, 42.80, 24.59, 23.92.

N-formylmorpholine. 1H NMR (400 MHz, 25 °C, CD3CN) δ 7.98 (s, 1H), 3.65–3.58 (m, 2H), 3.58–3.53 (m, 2H), 3.46–3.41 (m, 2H), 3.37–3.32 (m, 2H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 160.43, 66.78, 65.93, 45.32, 40.10. 1-formyl-4-ethylpiperazine. 1H NMR (400 MHz, 25 °C, CDCl3) δ 7.99 (s, 1H), 3.74–3.60 (m, 2H), 3.58–3.46 (m, 2H), 2.71 (ddd, J = 14.5, 8.8, 4.5 Hz, 6H), 1.15 (t, J = 7.3 Hz, 3H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 160.72, 51.99, 51.95, 50.90, 43.83, 38.21, 10.21. 1-formyl-4-methylpiperazine. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.09 (s, 1H), 3.62–3.50 (m, 2H), 3.44–3.32 (m, 2H), 2.42–2.33 (m, 4H), 2.30 (s, 3H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 160.76, 55.30, 54.07, 46.03, 45.52, 39.76. 3,4-dihydroisoquinoline-2(1H)-carbaldehyde. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.23 (s, 1H), 7.36–6.95 (m, 4H), 4.62 (s, 2H), 3.72 (t, J = 5.9 Hz, 2H), 3.00–2.71 (m, 2H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 162.14, 161.59, 134.43, 133.55, 132.17, 131.67, 129.26, 129.00, 127.27, 126.88, 126.80, 126.72, 126.64, 125.98, 47.56, 43.53, 42.47, 38.30, 29.77, 27.84.

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1,4-diformylpiperazine. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.02 (s, 1H), 3.50 (dd, J = 15.9, 10.4 Hz, 2H), 3.32 (dd, J = 15.1, 9.9 Hz, 2H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 160.90, 160.78, 45.98, 44.86, 40.40, 39.35.

N-n-butylformamide. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.04 (s, 1H), 6.74 (s, 1H), 3.18 (dd, J = 13.2, 6.6 Hz, 2H), 1.53–1.35 (m, 2H), 1.26 (dq, J = 14.5, 7.2 Hz, 2H), 0.83 (t, J = 7.3 Hz, 3H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 165.38, 161.89, 41.79, 37.91, 32.99, 31.31, 19.90, 19.42, 13.50, 13.45.

N-n-octylformamide. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.14 (s, 1H), 5.85 (s, 1H), 3.53–3.04 (m, 2H), 1.50 (d, J = 6.6 Hz, 2H), 1.27 (d, J = 10.8 Hz, 10H), 0.99–0.75 (m, 3H).

13

C

NMR (101 MHz, 25 °C, CDCl3) δ 164.89, 161.40, 41.96, 38.34, 31.85, 31.31, 29.61, 29.25, 26.95, 26.49, 22.72, 14.16.

N-n-propylformamide. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.15 (s, 1H), 5.92 (s, 1H), 3.21 (dd, J = 13.5, 6.7 Hz, 2H), 1.55 (dt, J = 14.5, 7.3 Hz, 2H), 0.92 (td, J = 7.4, 2.6 Hz, 3H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 165.20, 161.66, 43.86, 40.05, 24.42, 22.81, 11.37, 10.97.

N-benzylformamide. 1H NMR (400 MHz, 25 °C, CD3CN) δ 8.17 (s, 1H), 7.44–7.21 (m, 5H), 4.37 (d, J = 6.3 Hz, 2H).

13

C NMR (101 MHz, 25 °C, CD3CN) δ 165.7, 162.22, 140.27, 140.03,

129.49, 128.36, 128.12, 128.07, 45.83, 42.12.

N,N-diethylformamide. 1H NMR (400 MHz, 25 °C, CDCl3) δ 7.95 (s, 1H), 3.26 (q, J = 7.2 Hz, 2H), 3.18 (q, J = 7.2 Hz, 2H), 1.09 (t, J = 7.2 Hz, 3H), 1.03 (t, J = 7.2 Hz, 3H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 162.06, 41.73, 36.46, 14.79, 12.66.

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N,N-di-n-butylformamide. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.02 (s, 1H), 3.38–2.97 (m, 4H), 1.50 (m, 4H), 1.38–1.19 (m, 4H), 0.92 (dd, J = 9.5, 4.6 Hz, 6H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 162.01, 46.55, 41.25, 30.23, 28.78, 19.68, 19.17, 13.28, 13.13.

N-cyclohexylformamide. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.05 (s, 1H), 6.17 (s, 1H), 3.97–2.99 (m, 1H), 1.97–1.42 (m, 5H), 1.41–1.01 (m, 5H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 163.94, 160.13, 50.65, 46.22, 34.04, 32.31, 24.97, 24.56, 24.34.

N-tert-butylformamide. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.33–7.82 (m, 1H), 1.30 (d, J = 15.5 Hz, 10H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 163.22, 160.58, 51.26, 50.33, 30.84, 28.91.

N-phenylformamide. 1H NMR (400 MHz, 25 °C, CDCl3) δ 8.68 (s, 1H), 8.36 (s, 1H), 7.60–7.48 (m, 1H), 7.43–7.28 (m, 2H), 7.24–7.05 (m, 2H). 13C NMR (101 MHz, 25 °C, CDCl3) δ 162.95, 159.30, 137.03, 136.95, 129.86, 129.21, 125.39, 124.92, 120.15, 118.92.

3. RESULTS AND DISCUSSION In this research, bimetallic Pd-Au/PANI-CNT catalyst was prepared and systematically characterized by X-ray powder diffraction (XRD), nitrogen adsorption-desorption, transmission electron microscopy (TEM), high angle annular dark-field scanning transmission electron microscopy (HAADF–STEM), and X-ray photoelectron spectroscopy (XPS). For comparison purpose, bimetallic Pd-Au/PANI-CNT, pure Pd/PANI-CNT, and pure Au/PANI-CNT catalysts were investigated for the N-formylation of pyrrolidine to N-formylpyrrolidine using CO2/H2 as the carbonylation reagent. In addition, to probe the effect of support on the N-formylation reaction, bimetallic Pd-Au catalysts loaded on various supports including PANI, CNT, PANI-CNT and reduced graphene oxide (RGO) were prepared, the resulting bimetallic catalysts were examined for 11



the N-formylation of pyrrolidine. In order to further understand the relationship between the structure and catalytic activity of bimetallic catalyst, Pd-Au/PANI-CNT with various Pd/Au molar ratio were synthesize, analyzed with XPS, and applied for the N-formylation reaction.

CNT PANI-CNT Au (200)

Au/PANI-CNT

Pd (200)

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Pd/PANI-CNT

Au-Pd (200)

Au-Pd/PANI-CNT

Au-Pd (200)

Au-Pd/PANI

Au-Pd (200)

Au-Pd/CNT RGO

Au-Pd (200) 10

20

30

40

Au-Pd/RGO

50

60

70

80

Two theta / degree

Figure 1. XRD patterns of CNT, RGO, PANI-CNT and various supported Pd, Au, Pd-Au catalysts.

The XRD patterns of CNT, PANI-CNT, RGO and supported Pd, Au, Pd-Au catalysts are shown in Figure 1. Both CNT and RGO exhibit two characteristic peaks. The diffraction peak with high intensity at 26.1° corresponds to the characteristic carbon (002) plane; while, the other peak with low intensity at 43.1° matches the characteristic peak of the (100) packing of graphitic structure.99-101 After functionalization CNT with PANI, the XRD patterns of the obtained PANI/CNT were almost the same with CNT; however, decreased intensities of the both two 12


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characteristic diffraction peaks were observed.102 For Au/PANI-CNT, the characteristic reflection peaks at 38.4° and 43.9° were attributed to the (110) and (200) planes, respectively, for the metallic Au, indicating the presence of metallic Au nanoparticles.103 In the case of Pd/PANI-CNT, the diffraction peaks in the XRD patterns at 2θ values of 39.9°, 43.7° and 68.3° corresponded to the (111), (200) and (220) planes, respectively, for the face-centered cubic (fcc) crystalline Pd.104,105 The XRD patterns of bimetallic samples in Figure 1, including Pd-Au/PANI-CNT, Pd-Au/PANI, Pd-Au/CNT and Pd-Au/RGO, exhibit the characteristic peaks corresponding to both crystalline Pd and Au, which confirms the presence of bimetallic nanoparticles in the samples. In addition, a comparison of characteristic peaks for (110) planes of metallic Au among Au/PANI-CNT and Pd-Au/PANI-CNT shows a slight shift toward a lower diffraction angle, indicating the formation of Pd–Au alloyed structure.106,107 Moreover, the diffraction peak for the (200) plane of Pd-Au/PANI-CNT appears between 43.7° and 43.9°, further indicating the presence of Pd–Au alloyed structure.108 Our HAADF–STEM analysis also proves the existence of alloyed bimetallic nanoparticles.

1800 CNT PANI-CNT Pd/PANI-CNT Au/PANI-CNT Pd-Au/PANI-CNT Pd-Au/CNT Pd-Au/RGO RGO

1600 1400

Vads / cm3g-1 (STP)

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Figure 2. Nitrogen adsorption-desorption isotherms and pore-size distribution curves of CNT, RGO, PANI-CNT and various supported Pd, Au, Pd-Au catalysts. 13



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Figure 2 shows nitrogen adsorption–desorption isotherms of CNT, RGO, PANI-CNT and various supported Pd, Au, Pd-Au catalysts with inset image of pore size distribution patterns. The resulting surface area, pore diameter and pore volume of these materials are shown in Table 1. CNT endows a high Brunauer–Emmett–Teller (BET) surface area up to 148.62 m2 g−1 and a large pore volume of 2.70 cm3 g−1. Moreover, the hysteresis loop at 0.9

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