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Supporting Information

Ultra-small Pd Nanoparticles Derived from

Supramolecular Assembly for Enhanced

Electrochemical Reduction of CO2 to CO

Ruru Chen,ab Minna Cao,*a Weiguang Yang,ab Huimin Wang,a

Suyuan Zhang,a Hongfang Li*a and Rong Cao*a

a. State Key Laboratory of Structural Chemistry, Fujian Institute

of Research on the Structure of Matter, Chinese Academy of

Sciences, Fuzhou 350002, China.

b. College of Chemistry, Fuzhou University, Fuzhou 350108,

China.

Corresponding Authors

*Minna Cao (mncao/fjirsm.ac.cn)

*Hongfang Li (hongfangli/fjirsm.ac.cn)

*Rong Cao (rcao/fjirsm.ac.cn)

Electronic Supplementary Material (ESI) for Chemical Communications.This journal is © The Royal Society of Chemistry 2019

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1. Experimental1.1. MaterialsAll reagents and chemicals were obtained commercially and used without further purification. Palladium chloride (PdCl2), hydrochloric acid (HCl), potassium chloride (KCl) and potassium bicarbonate (KHCO3) were bought from Sinopharm Chemical Reagent Co., Ltd (China). KOH was purchased from Aladdin. Vulcan XC-72R carbon black was purchased from Cabot Corp. Commercial Pd/C (10 wt%) was purchased from Alfa. Me10CB[5] was synthesized according to the process reported in the literature.1 Ultrapure water (18 MΩ) used in the experiment was supplied by a Millipore System.1.2. InstrumentsThe X-ray diffraction data was collected on Super Nova CCD diffractometer equipped with graphite-monochromated Cu Kα radiation (λ = 1.5418 Å). Empirical absorption correction was applied to the data using the spherical harmonics. Structure was solved by direct methods and refined on F2 by full-matrix least-squares with the SHELX-2015 program package.2 All non-hydrogen atoms, except some water oxygen atoms, were refined anisotropically. The hydrogen atoms of organic molecules were generated geometrically. Crystal data and structure refinement parameters are given in Table S1. CCDC-1893955 contains supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif

Power X-ray diffraction (PXPD) was performed with a Rigaku Mini Flex 600 diffractometer with a Cu Kα radiation source (λ = 1.5418 Å) at a low scan rate of 1 min-1. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) measurements were performed using a FEI Tecnai G2 F20 electron microscope. X-ray photoelectron spectroscopy (XPS) was characterized by an ESCALAB 250 Xi XPS system. Elemental analysis (EA) was carried on an Elementar Vario EL III analyzer. Thermal gravimetric analysis (TGA) was performed using a TA SDT-Q600 instrument, with a heating rate of 10C min-1 under a flow of nitrogen atmosphere. The loading amount of Pd was determined with a Jobin Yvon Ultima 2 inductively coupled plasma emission spectrometer (ICP). All the electrochemical experiments were performed on CHI600E electrochemical workstation. The gas-phase products were detected using a gas chromatograph (GC) (Agilent 7820A), while liquid-phase products were detected by ion-exchange chromatography (IC) (Metrohm China Limited).1.3. Preparation catalyst1.3.1. Preparation of PdCl4-Me10CB[5]0.17 mmol of PdCl2 and 1 mL HCl (6 M) was dissolved in 20 mL deionized water to obtain a clear aqueous solution of H2PdCl4 (solution I). 0.1 mmol Me10CB[5] was dissolved into the deionized water (20 mL) under ultrasonic (solution II). Solution I and II were carefully transferred to each side of the H-

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tube, respectively. Brown crystals were obtained by slow diffusion after three days in the H-tube. Anal. Calcd for C40H64Cl4N20O16Pd: C, 36.11%; H, 4.81%; N, 21.1%. Found: C, 36.39%; H, 4.55%; N, 22.45%.1.3.2. Preparation of Pd-Me10CB[5] catalystPd-Me10CB[5] was prepared by thermal reduction of supramolecular assembly at 300C for 3 h under H2 atmosphere (5% H2 + 95% N2) in a triangulation furnace with the heating rate of 5 C min-1. After cooling down to room temperature, 9.8 mg of Pd-Me10CB[5] and half quality of Vulcan XC-72R carbon black were dispersed in the mixed solution of 0.5 mL water, 0.5 mL isopropyl alcohol and 40 μL Nafion (5 wt %) solution, ultrasonic for 2 h. Following this, 120 µL ink was loaded onto a carbon fiber paper (N1S1007) (1*1 cm2) uniformly. The loading of Pd was 120 µg cm-2. The detailed information can be seen in Table S2.1.3.3. Preparation of Pd/Me10CB[5] catalystThe Me10CB[5] and PdCl2 were ground fully with the same amount with PdCl4-Me10CB[5]. Then the physical mixture was reduced following the same procedure as Pd-Me10CB[5]. The catalyst was prepared using the same method as Pd-Me10CB[5]. Meanwhile, the amount of Pd loaded onto the carbon fiber paper is identical.1.3.4. Preparation of bulk Pd catalystBulk Pd was synthesized using the same method as Pd/Me10CB[5], but without adding Me10CB[5]. Moreover, the amount of Pd loaded onto the carbon fiber paper is identical with Pd-Me10CB[5].1.4. Electrochemical measurement1.4.1. Electrocatalytic activityCO2RR measurements were carried out in a H-cell (separated by Nafion 117) system. The carbon fiber paper loaded with the catalyst, Ag/AgCl with saturated KCl solution electrode and Pt network acted as the working electrode, reference electrode and counter electrode, respectively. 70 mL 0.5 M KHCO3

solution was fed into the anodic and cathodic compartments. A magnetic stirrer was used to accelerate the mass transport. All the potentials were converted to the reversible hydrogen electrode (RHE) by adding a value of (0.197 + 0.0591 × pH) V. The working electrode was cycled for 15 scans between 0 V and -1.20 V to clean the Pd surface. Then Linear sweep voltammetry (LSV) was performed under N2-saturated (pH = 8.5) and CO2-saturated (pH = 7.2) 0.5 M KHCO3 solution, with the scan rate of 10 mV s-1, respectively. Then the electrode was used to potentiostatic electrolysis with the cathode electrolyte purged CO2 (> 99.99%) at 30 mL min-1. The gas was taken to detect after at least 15 minutes electrolysis. The cathodic electrolyte was collected after electrolysis half an hour and analyzed using IC. Durability test was performed in an electrolytic cell with 0.5 M KHCO3 solution for 12 h with CO2 bubbled into the solution continuously.1.4.2. Electrochemically active surface area (ECSA) measurements

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The electrochemically active surface area (ECSA) of the catalyst is estimated from the reduction peak of the PdO formed in the positive scan of cyclic voltammetry (CV) in N2-saturated 2 M KOH solution.3 The CV measurement was conducted in a three-electrode cell equipped with an Ag/AgCl with saturated KCl solution reference electrode and a Pt network counter electrode with the scan rate of 100 mV s-1. The catalyst with the same amount of Pd (5 µg) was loaded onto a glassy carbon (GC) (0.197 cm2) electrode and dried at room temperature to form a catalyst layer.1.5. Products analysisThe gas products of CO2RR were detected by an offline micro GC equipped with FID and TCD detector. After electrolysis at least 15 minutes, 1 mL of the gas above the electrolyte was taken and injected into the GC for gas-phase product detection. The collected KHCO3 electrolyte was used IC to quantify liquid-phase products, in which 1 mL of the electrolyte was diluted by 9 mL of H2O (deionized water). Two major gas products CO and H2 were generated, and only a minor formate was detected as the sole liquid-phase product.

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Fig. S1. The PXRD of PdCl4-Me10CB[5].

Fig. S2. PXRD of the samples reduction from the supramolecular assembly at

different temperature (200C, 300C and 400C) for 3 h. There appears no obvious

peak of Pd NPs owing to the inadequate reduction when the supramolecular assembly

reduction at 200C for 3 h. But the peak of Me10CB[5] disappears when the

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supramolecular assembly reduction at 400C for 3 h, indicating the collapse of

Me10CB[5].

Fig. S3. PXRD of the samples reduction from the supramolecular assembly at 300C

for different time (1 h, 3 h, 6 h). There appears a small peak of Pd (111) owing to the

inadequate reduction at 300 C for 1 h. But the peak of Me10CB[5] almost disappears

at 300C for 6 h.

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Fig. S4. TGA of Me10CB[5] and PdCl4-Me10CB[5].

Fig. S5. The three-dimensional structure of PdCl4-Me10CB[5].

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Fig. S6. HAADF-STEM image of Pd-Me10CB[5] and the corresponding elemental

mapping images of C (red), N (orange), O (yellow), Pd (green).

Fig. S7. (a, b). TEM images of Pd/Me10CB[5], the inset is corresponding sizes

distribution. (c, d) The TEM images of the bulk Pd.

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Fig. S8. PXRD of the Pd/Me10CB[5] and bulk Pd.

Fig. S9 The XPS survey spectrum of Pd-Me10CB[5].

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Fig. S10 The EDS of Pd-Me10CB[5].

Fig. S11. XPS survey spectrum of Pd/Me10CB[5] and bulk Pd.

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Fig. S12. Pd 3d high resolution spectrum of Pd/Me10CB[5]

Fig. S13. Pd 3d high resolution spectrum of bulk Pd

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Electrochemical Characterization

Cyclic voltammetries (CVs) in 2 M KOH have been recorded for the

Electrochemically active surface area measurements (ECSA). Fig. S14a shows the CV

in a different potential range of the Pd-Me10CB[5]. The CV upper potential value for

the measurement of the area was 1.3 V. Such a value was chosen as Czerwiński

4unequivocally demonstrated that this value corresponds to the condition for the

formation of the first complete PdO monolayer in alkaline conditions. Fig. S14b

reports the CVs for the Pd-Me10CB[5], Pd/Me10CB[5] and bulk Pd. Such CVs were

used for the determination of the ECSA values reported in the main text of the paper.

Fig. S14. (a) CVs for different potential range of the Pd-Me10CB[5] in 2 M

KOH solution with a sweep rate of 100 mV/s. (b) The CV of the Pd-Me10CB[5],

Pd/Me10CB[5] and bulk Pd in the 2 M KOH solution with a sweep rate of 100

mV/s.

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Fig. S15. Electrochemical active surface area (ECSA) of all the samples.

Fig. S16. Partial current density of CO normalized to the ECSA.

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Fig. S17. HRTEM image of Pd-Me10CB[5] after CO2RR for 12 h at the potential of -

0.6 V in 0.5 M KHCO3 solution

Fig. S18. TEM image of Pd-Me10CB[5] after CO2RR for 12 h at the potential of -0.6

V in 0.5 M KHCO3 solution and no obvious accumulation observed.

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Fig. S19. FT-IR spectra of the samples. The carbonyl stretch peak (~1750 cm-1) of the

Me10CB[5] portals is marked with dashed line. There is no obvious shift of the C=O

bond, indicating no covalent interaction formed. Moreover, the intact structure of

Me10CB[5] still keeps in the samples of Pd-Me10CB[5] and Pd/Me10CB[5].

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Table S1: Crystal data and structure refinement of PdCl4-Me10CB[5]

Sample 1

Empirical formula C40H64Cl4N20O16Pd

Formula weight 1329.31

Crystal system monoclinic

Space group I2/a

a (Å) 13.1597(2)

b (Å) 17.5930(2)

c (Å) 22.3163(3)

α (°) 90

β (°) 95.1030(10)

γ (°) 90

V (Å3) 5146.16(12)

Z 4

F(000) 2744.0

T (K) 295

Rint 0.0163

GOOF 1.087

R1 [I > 2σ(I)]a 0.0477

wR2 [I > 2σ(I)]b 0.1478

aR1 = ║Fo│–│Fc║/∑│Fo│. bwR2 = [∑w(Fo2Fc

2)2/∑w(Fo2)2]1/2.

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Table S2: ICP of all the samples.

Samples Pd (wt %) Mass loading(mg cm-2)

Pd-Me10CB[5] 10.28 0.12

Pd/Me10CB[5] 11.22 0.12

Bulk Pd 91.86 0.12

Commercial Pd/C 10 0.12

PdCl4-Me10CB[5] 9.49

Table S3: Different valence state distributions of Pd species in all the samples.

Binding Energy (eV)

Samples Pd species

Pd 3d5/2 Pd 3d3/2

Relative area

peak (%)

Pd(0) 335.4 340.7 0.76

Pd-Me10CB[5]Pd(II) 337.1 342.4 0.24

Pd(0) 335.8 341.1 0.81

Pd/Me10CB[5]Pd(II) 337.3 342.4 0.19

Pd(0) 335.6 340.9 0.65Bulk Pd

Pd(II) 336.5 341.7 0.35

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Table S4: The element distribution of Pd-Me10CB[5] in the EDS.

Element C N O Cl Cu Pd

Weight% 28.63 17.55 0.76 0.26 34.43 16.37

Table S5: Comparisons of Faradaic Efficiency (FE) and partial current density of CO

based electrocatalyst reported in the literatures.

Catalysts ElectrolytePotential

(V vs. RHE)

FECO

(%)

jCO

(mA cm−2)

Ref.

Pd-Me10CB[5] 0.5 M KHCO3 -0.60 92.5 3.98This

work

Pd/Me10CB[5] 0.5 M KHCO3 -0.70 91.8 2.71This

work

3.7 nm Pd 0.1 M KHCO3 -0.79 91 6 5

Pd0.25

Au0.75

film 0.1 M KHCO3 -0.60 87 0.8 6

Pd icosahedra 0.1 M KHCO3 -0.80 91.1 2 7

Mesoporous

Pd7Cu

3

0.1 M KHCO3 -0.80 80 1 8

3.3 nm

Pd85

Cu15

0.1 M KHCO3 -0.89 86 6.9 9

Commercial Pd/C 0.5 M NaHCO3 -0.60 50 0.3 10

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3.7 nm Pd 1 M KHCO3 -0.70 93.4 22.9 11

Twisted Pd-Au

NW0.5 M KHCO3 -0.60 94.3 / 12

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