Supplementary Information. Size-Dependent Electrocatalytic Reduction of CO 2 over Pd Nanoparticles

Transcription

1 Supplementary Information Size-Dependent Electrocatalytic Reduction of CO 2 over Pd Nanoparticles Dunfeng Gao,,, Hu Zhou, Jing Wang,, Shu Miao, Fan Yang, Guoxiong Wang,*, Jianguo Wang,*, and Xinhe Bao State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian , China. College of Chemical Engineering, Zhejiang University of Technology, Hangzhou , China. University of Chinese Academy of Sciences, Beijing , China. S1

2 Experimental section Catalysts preparation Carbon-supported Pd nanoparticles (NPs) were synthesized with sodium borohydride ([NaBH 4 ], Sinopharm Chemical Reagent Co. Ltd.) as a reductive agent and sodium citrate ([Na 3 C 6 H 5 O 7 2H 2 O], Sinopharm Chemical Reagent Co. Ltd.) as a stabilizing agent. The particle size of Pd NPs was controlled by varying the ratio of sodium citrate to palladium chloride ([PdCl 2 ], Sinopharm Chemical Reagent Co. Ltd.) and the reduction temperature. The samples were named with the number of particle size, for example, 3.7 nm Pd indicates that Pd NPs in the Pd/C catalyst have an average particle size of 3.7 nm by counting more than 200 particles from TEM images. The detailed preparation procedure for 3.7 nm Pd was as follows: 0.5 mmol PdCl 2 (dissolved in 0.1 M HCl solution) and 4 mmol sodium citrate were dissolved into 200 ml water, and then mg Vulcan XC72R carbon black (Carbot Corp.) was added, and sonicated for 30 min. 50 ml of 0.1 M NaBH 4 solution was added into the suspension dropwise under vigorous stirring at 25 o C. After the suspension was stirred for 8 h, the black precipitate was filtered, washed and dried overnight in a vacuum oven at 25 o C. For the preparation of 2.4 nm and 4.5 nm Pd, the ratio of sodium citrate to PdCl 2 was 8, and the reduction temperatures were 0 o C and 60 o C, respectively. For the preparation of 6.2 nm and 7.8 nm Pd, the ratio of sodium citrate to PdCl 2 was 2 and 0, respectively, and the reduction temperature was 25 o C nm Pd was obtained by bubbling 3.7 nm Pd for 12 h with H 2 in 0.1 M HCl solution at 25 o C. Physicochemical characterization X-ray diffraction (XRD) was performed on a Rigaku D/MAX 2500 diffractometer with Cu Kα radiation (λ= Å) at 40 kv and 200 ma. The scan speed was 2 o min -1 and the step size was 0.02 o. The actual loading of Pd in Pd/C catalyst was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES). Transmission electron microscopy (TEM) and high resolution TEM were carried out on a JEM-2100 microscope operated at an accelerating voltage of 200 kv. Gas diffusion electrode preparation Carbon black ink containing Vulcan XC72R carbon black and S2

3 polytetrafluoroethylene (PTFE, Sigma-Aldrich) was painted onto a piece of Toray carbon paper (Toray TGP-H-060, Toray Industries Inc.) to form a microporous layer. The carbon black loading was about 1 mg cm -2 and the PTFE content in the microporous layer was 15 wt%. To fabricate the catalyst layer, the as-prepared catalyst and Nafion ionomer solution (5 wt%, DuPont) were ultrasonically suspended in a water/alcohol mixture and then brushed onto the microporous layer. The loading of Pd/C catalyst was 2.0±0.1 mg cm -2, and the Nafion content in the catalyst layer was 10 wt%. Electrochemical measurements The electrochemical measurements were carried out in an H-cell (separated by Nafion 115) system as shown in Scheme S1. The Toray carbon fiber paper with the catalyst layer was cut into a size of 1 cm 2 cm acting as the working electrode. The Pt wire and Ag/AgCl electrode were used as the counter electrode and reference electrode, respectively. The potentials were controlled by an Autolab potentiostat/galvanostat (PGSTAT 302N). All potentials in this study were measured against the Ag/AgCl reference electrode and converted to the RHE reference scale by E (vs. RHE) = E (vs. Ag/AgCl) V ph. Electrocatalytic reduction of CO 2 was conducted in CO 2 -saturated 0.1 M KHCO 3 (Sinopharm Chemical Reagent Co. Ltd.) solution (ph 6.8) at room temperature and under atmospheric pressure. After CO 2 was purged into the KHCO 3 solution for at least 30 min to remove residual air in the reservoir, controlled potential electrolysis was performed at each potential for 30 min. The oxygen generated at the anode was vented out of the reservoir. The gas products of CO 2 electrocatalytic reduction were monitored by an online micro gas chromatography (GC) (Agilent 490) equipped with a TCD detector and Molsieve 5A column once every three minutes. The KHCO 3 solution after electrolysis was collected and analyzed on a Varian 400 MHz NMR spectrometer to quantify liquid products. Standard curve was made by using sodium formate (HCOONa 2H 2 O, Sinopharm Chemical Reagent Co. Ltd.) and the internal standard (1-Propanesulfonic acid 3-(trimethylsilyl) sodium salt, DSS, Sigma-Aldrich), as shown in Figure S4. A 0.5 ml sample of the KHCO 3 solution after electrolysis was S3

4 mixed with the addition of 0.1 ml D 2 O and 0.1 ml 6 mm DSS solution as an internal standard. The 1 H spectrum was measured with water suppression by a pre-saturation method. The area ratio of the formate peak to the DSS peak was compared to the standard curve to quantify the concentration of formate. The cyclic voltammetry measurements were conducted in a three-electrode cell equipped with an Ag/AgCl reference electrode and a Pt counter electrode at 25 o C. The electrolyte was 0.1 M HClO 4 (Sinopharm Chemical Reagent Co. Ltd.) solution. Six mg of the catalyst was dispersed in a mixture of 2 ml ethanol and 50 µl Nafion ionomer solution (5 wt%, DuPont) with ultrasonic stirring to form a homogenous ink. The catalyst layer was prepared by dropping 20 µl of the ink onto a glassy carbon (GC) disk electrode by a micropipette and drying at room temperature to form a catalyst layer with a loading of 0.3 mg cm -2. Cyclic voltammograms of Pd/C catalysts were conducted from 0 V to 1.2 V with a scan rate of 20 mv s -1 in Ar atmosphere to measure the reduction peak potential of Pd(OH) 2. The CO adsorption procedure in CO stripping voltammogram measurements was accomplished by polarizing the electrode at 0.4 V and bubbling the electrolyte solution with CO for 10 min and subsequently with Ar for another 30 min. [S1] Then cyclic voltammograms were conducted from 0.4 V to 1.2 V with a scan rate of 20 mv s -1. Based on DFT calculations, corner and edge sites on Pd NPs are considered to be active for CO 2 reduction, whereas corner, edge and terrace sites are all active for hydrogen evolution reaction. The calculation of active site numbers on Pd NPs of different sizes was quantified based on an ideal icosadeltahedral model of NPs. Therefore, TOF for CO and H 2 was calculated as follows: TOF: turnover frequency, h -1 ; I product nf TOF = 3600 m cat ω α M Pd I product : partial current for certain product, A; n: the number of electron transferred for product formation, which is 2 for both CO and H 2 production; F: Faradaic constant, C mol -1 ; S4

5 m cat : catalyst mass in the electrode, g; ω: Pd loading in the catalyst; α: the ratio of active atoms in Pd NPs (step and corner Pd atoms for CO production; corner, edge and terrace Pd atoms for H 2 production); M Pd : atomic mass of Pd, g mol -1. DFT and Gibbs free energy calculations Density functional theory calculations were performed using the Vienna Ab Initio Simulation package (VASP) within the generalized gradient approximation (GGA-PBE).[S2-4] A plane-wave basis set with a cutoff energy of 400 ev and ultrasoft Vanderbilt pseudopotentials (U.S.-PP) was employed. The Brillouin zone integration was carried out with k-point sampling for Pd(111) and Pd(211), and single k-point (gamma point) was used for Pd55 and Pd38, where the convergence criterion of the force and energy is 10 mev Å -1 and 0.1 mev. The Pd(111) and Pd(211) surfaces were chosen to represent the terraces and edges, while Pd55 and Pd38 clusters were chosen to represent the corner sites of Pd nanoparticles. For Pd model surfaces, four layers (3 3) Pd(111) and (1 4) Pd(211) were used, while the slab is separated from its periodic images in the vertical direction by a vacuum space of 20 Å. During the geometry optimizations, only two bottom layers were fixed while the upper two layers were fully relaxed. The Pd55 and Pd38 clusters were cut with cuboctahedral and truncated-octahedron shapes in (20.0Å 20.0Å 20.0Å) orthorhombic boxes. All atoms and adsorbates of Pd clusters were fully relaxed during geometry optimization. In free energies calculations, the entropic corrections and zero-point energy (ZPE) have been included. In this work, we consider both the CO 2 reduction and the hydrogen evolution reactions, the reaction mechanism we considered are: CO 2 (g) + + H + (aq) + e COOH (S1) COOH + e + H + (aq) CO + H 2 O(l) CO CO(g) + (S2) (S3) S5

6 + H + (aq) + e H + H + 0.5H 2 (g) (S4) (S5) The asterisk (*) of above means the substrate. The reaction free energies of (S1) and (S2) as function of the applied potentials are given by ΔG 1 = G COOH G μco 2 (g) μ(h + + e ) = G COOH G μco 2 (g) 1/2H Θ 2 + eu (S6) and ΔG 2 = G CO G COOH + μh 2 O(l) μ(h + + e ) = G CO G COOH + μh 2 O(l) 1/2H Θ 2 + eu (S7) based on the computational hydrogen electrode reference state according to the methodology proposed by Nørskov et al.,[s5, S6] where U is the potential relative to the reversible hydrogen electrode (RHE) at the same ph as the reactions. The free energy of a species is calculated according to the standard formula: G = E DFT + ZPE + δh 0 TS (S8) So equation S6 and S7 can be rewritten as: ΔG 1 = Δ 1 E + Δ 1 ZPE + Δ 1 δh 0 Δ 1 TS + eu (S6-1) here, Δ 1 is (COOH*-*- CO 2-1/2H 2 ) and ΔG 2 = Δ 2 E + Δ 2 ZPE + Δ 2 δh 0 Δ 2 TS + eu (S7-1) here, Δ 2 is (CO*- COOH*+H 2 O-1/2H 2 ) where E DFT is the DFT total energy, E ZPE is the zero point energy, δh 0 is the integrated heat capacity, T is the temperature of product, and S is the entropy. Zero-point energies (ZPE) are calculated with the vibrational frequencies of adsorbates and molecules as calculated within DFT. The integrated heat capacity (δh 0 ) and entropy (S) at K are obtained from the references.[s6, S7] The relevant thermodynamic data of gas-phase species are given in Table S3. The relevant data of gas phase species, adsorbates, substrates, and free energies S6

7 (ΔG) under the standard conditions and 0 V are given in Table S4-S7. Free energy diagrams for CO 2 reduction to CO are shown in Figure 3, while the energy diagrams for H + to H 2 are shown in Figure S10. S7

8 Table S1. The nominal and actual loadings of Pd and average particle size in different Catalysts Pd/C catalysts. Pd loading (wt%) Pd particle size (nm) nominal ICP-OES XRD TEM 2.4 nm Pd nm Pd nm Pd nm Pd nm Pd nm Pd Table S2. Faradaic efficiencies for formate over different Pd/C catalysts at V vs. RHE in CO 2 -saturated 0.1 M KHCO 3 solution. Catalysts Formate Faradaic efficiencies (%) 2.4 nm Pd nm Pd nm Pd nm Pd nm Pd nm Pd 1.2 S8

9 Table S3. Relevant thermodynamic data of gas-phase species. DFT total energies, zero point energy, the integrated heat capacity and entropy, the entropy of H 2 O (g=l) is calculated at atm, which corresponds to the vapor pressure of liquid water. Molecules E DFT ZPE δh 0 TS H CO CO H 2 O Total COOH * CO * Table S4. DFT total energies, zero point energies of H, CO and COOH adsorbed on Pd(111) Adsorbate E (ev) ZPE(eV) ΔG(eV) H * CO * COOH * Table S5. DFT total energies, zero point energies of H, CO and COOH adsorbed on Pd(211) Adsorbate E(eV) ZPE(eV) ΔG(eV) H * CO * COOH * S9

10 Table S6. DFT total energies, zero point energies of H, CO and COOH adsorbed on Pd(55) Adsorbate E(eV) ZPE(eV) ΔG(eV) H * CO * COOH * Table S7. DFT total energies, zero point energies of H, CO and COOH adsorbed on Pd(38) Adsorbate E(eV) ZPE(eV) ΔG(eV) H * CO * COOH * S10

11 Scheme S1. Schematic illustration of an H-cell (separated by Nafion 115) system. S11

12 Figure S1. TEM and HRTEM images of (a, b) 2.4 nm, (c, d) 4.5 nm and (e, f) 7.8 nm Pd. S12

13 Figure S2. Histograms of particle-size distributions of (a) 2.4 nm, (b) 3.7 nm, (c) 4.5 nm, (d) 6.2 nm, (e) 7.8 nm and (f) 10.3 nm Pd. In Figure S2, the percentage is based on the number ratio of differently-sized Pd NPs by counting more than 200 particles from the TEM images. The loading of Pd is similar in different Pd/C catalysts (18.3±0.5 wt%, Table S1). The weight percentage of differently-sized Pd NPs in Pd/C catalyst is calculated based on the number percentage and the third power of particle size in Pd/C catalyst. From Figure S11, the ratio of corner and edge atoms decreases drastically from 4-6 nm to 6-8 nm and remains nearly constant for the size larger than 9 nm. For 7.8 nm-pd, there is 6.2 wt% of Pd distributed in the size range from 6 to 8 nm and 0.4 wt% of Pd distributed in the size range from 4 to 5 nm (total Pd loading of 18.3 wt%, Table S1). In 10.3 nm-pd (total Pd loading of 18.0 wt%, Table S1), 16.7 wt% of Pd is distributed in the large size region ( 9 nm), and only 1.3 wt% of Pd is distributed in the size range from 6 to 8 nm. In addition, the increased ratio of terrace atoms to corner and edge atoms on large Pd NPs inhibits CO 2 reduction at corner and edge sites since hydrogen evolution is greatly facilitated at the drastically increased terrace sites. The low weight percent of small-sized Pd NPs should account for the much lower Faradaic efficiency of 10.3 nm-pd. S13

14 Figure S3. XRD patterns of Pd/C catalysts synthesized in different conditions. S14

15 Figure S4. (a) 1 H-NMR spectrum for formate determination; (b) Linear relationship between the formate concentration and relative area vs. DSS. The standard curve was made as follows: A 0.5 ml of formate sodium solution with different concentration (from 1 mm to 20 mm) was mixed with 0.1 ml D 2 O and 0.1 ml 6 mm DSS solution added as an internal standard. S15

16 50 j CO (A g -1 ) Pd nm 3.7 nm 4.5 nm 6.2 nm 7.8 nm 10.3 nm E (V vs. RHE) Figure S5. Mass activities for CO production over Pd NPs with different sizes at various potentials. S16

17 Figure S6. Adsorption of CO 2, (a) Pd(111), (b) Pd(211), (c) Pd55, (d) Pd38. S17

18 Figure S7. Adsorption of CO, (a) Pd(111), (b) Pd(211), (c) Pd55, (d) Pd38. S18

19 Figure S8. (a) Cyclic voltammograms of Pd NPs with different particle sizes in Ar-saturated 0.1 M HClO 4 solution with a scan rate of 20 mv s -1 at 25 o C. (b) The enlargement of the region from 0.9 to 0.5 V in the negative scan. (c) The linear relationship between reduction peak potential of Pd(OH) 2 and particle size. S19

20 Figure S9. CO stripping voltammograms of differently-sized Pd NPs in 0.1 M HClO 4 solution with a scan rate of 20 mv s -1 at 25 o C. S20

21 Figure S10. Free energy diagrams for H + electroreduction to H 2 on Pd(111), Pd(211), Pd55 and Pd38. S21

22 Figure S11. The ratio of step, corner, terrace and bulk atoms on ideal icosadeltahedral Pd NPs with different sizes. S22

23 Figure S12. Calculated turn of frequency (TOF) for CO (a) and H 2 (b) on Pd NPs with different sizes at various potentials. S23

24 Figure S13. Size dependence of TOF for H 2 production on Pd NPs at various potentials. S24

25 References: (S1) Xiao, L.; Zhuang, L.; Liu, Y.; Lu, J.; Abruna, H. D. J. Am. Chem. Soc. 2009, 131, 602. (S2) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 48, (S3) Kresse, G.; Furthmuller, J. Phys. Rev. B 1996, 54, (S4) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, (S5) Durand, W. J.; Peterson, A. A.; Studt, F.; Abild-Pedersen, F.; Nørskov, J. K. Surf. Sci. 2011, 605, (S6) Heine A. Hansen.; Joel B. Varley.; Andrew A. Peterson.; Nørskov, J. K. J. Phys. Chem. Lett. 2013, 4, 388. (S7) Peterson, A. A.; Abild-Pedersen, F.; Studt, F.; Rossmeisl, J.; Nørskov, J. K. Energy Environ. Sci. 2010, 3, S25