CNRS n 7591, Université Paris Diderot, Bâtiment Lavoisier, 15 rue Jean de Baïf, Paris Cedex 13, France. Wavenumber (cm -1 )

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1 Supporting Information Catalysis and Inhibition in the Electrochemical Reduction of CO 2 on Platinum in the Presence of Protonated Pyridine. New Insights into Mechanisms and Products. Hachem Dridi a, Clément Comminges b, Claudia Morais b, Jean-Claude Meledje a, Kouakou Boniface Kokoh b, Cyrille Costentin a *, and Jean-Michel Savéant a * a Sorbonne Paris Cité, Laboratoire d'electrochimie Moléculaire, Unité Mixte de Recherche Université - CNRS n 7591, Université Paris Diderot, Bâtiment Lavoisier, 15 rue Jean de Baïf, Paris Cedex 13, France. b IC2MP UMR-CNRS 7285, Université de Poitiers, 4 rue Michel Brunet, TSA 51106, Poitiers Cedex 9, France. cyrille.costentin@univ-paris-diderot.fr, saveant@univ-paris-diderot.fr Additional figures a) 1 % b) 2 % R/R 0 c) 2 % d) 2 % e) 10 % f) 0.5 % g) 0.5 % Wavenumber (cm -1 ) Fig S1. Reference FTIR spectra for (a) AcOH, (b) pyridine, (c) formic acid, (d) methanol, (e) HCO 3 -. FTIR spectra obtained after 30 min electrolysis in 10 mm acetic acid/acetate solution at ph 5.05 in the presence of 0.4 M K 2 SO 4 and CO 2 at (f) V vs. SHE and (g) V vs. SHE. S1

3 in the presence of 0.4 M K 2 SO 4 at 0.05 V s -1 after a 10 min electrolysis at - 0.21 V vs. SHE. Scan is started at -0.

(b) Cyclic voltammogram under CO on a Pt disk electrode (5 mm diameter) of a 10 mm acetic acid/acetate solution at ph 5.

36 V vs. SHE in the anodic direction. Fig S3.

16 in the presence of 0.4 M K 2 SO 4 at 0.05 V s -1 after a 10 min electrolysis at -0.36 V vs. SHE. Scan is started at -0.

2 Fig S2. (a) Cyclic voltammogram under CO on a Pt disk electrode (5 mm diameter) of a 10 mm acetic acid/acetate solution at ph 5.3 in the presence of 0.4 M K 2 SO 4 at 0.05 V s -1 after a 10 min electrolysis at V vs. SHE. Scan is started at V vs. SHE in the anodic direction. (b) Cyclic voltammogram under CO on a Pt disk electrode (5 mm diameter) of a 10 mm acetic acid/acetate solution at ph 5.3 in the presence of 0.4 M K 2 SO 4 at 0.05 V s -1 after a 10 min electrolysis at V vs. SHE. Scan is started at V vs. SHE in the anodic direction. Fig S3. Cyclic voltammogram under CO on a Pt disk electrode (5 mm diameter) of a 10 mm pyridinium/pyridine solution at ph 5.16 in the presence of 0.4 M K 2 SO 4 at 0.05 V s -1 after a 10 min electrolysis at V vs. SHE. Scan is started at V vs. SHE in the anodic direction. Fig S4. Current recorded during anodic scan corresponding to an anodic scan (1 mv s -1 ) after a 30 min electrolysis under CO 2 at (blue) V vs. SHE in a 10 mm pyridinium/pyridine solution at ph 5.35 with 0.4 M K 2 SO 4 under CO 2 and (red) V vs. SHE in a 10 mm acetic acid/acetate solution at ph 5.05 with 0.4 M K 2 SO 4 under CO 2. Black arrow indicates initial potential and scan direction. S2

3 R/R 0 (%) R/R 0 (%) R/R 0 (%) R/R 0 (%) a) H 2 O CO L CO 2 HSO % 0.2 % Wavenumber (cm -1 ) Wavenumber (cm -1 ) b) CO 2 2.5% HCO Wavenumber (cm -1 ) c) HCO % CO Wavenumber (cm -1 ) Fig S5. (a) FTIR spectra at various potentials at a Pt electrode recorded during anodic scan (at 1 mv s -1 ) after CO adsorption at vs. SHE in a 0.4 M K 2 SO 4 solution (ph 6.64). From top to bottom, spectra correspond to potentials changing by 50 mv interval from the starting potential (b) FTIR spectra at various times at a Pt electrode recorded during electrolysis at V vs. SHE in a 0.4 M K 2 SO 4 solution under CO 2 (the final ph value was 4.13) (c) FTIR spectra at various potentials at a Pt electrode during anodic scan (1 mv s -1 ) after a 20 min electrolysis at -0.41V vs. SHE in a 0.4 M K 2 SO 4 solution with under CO 2. From top to bottom, spectra correspond to potentials changing by 50 mv interval from the starting potential at V vs. SHE. S3

4 Materials and Methods Chemicals Electrolyte solutions were prepared from potassium sulfate (Merck, > 99.0 %) or potassium chloride (Merck, > 99.5 %) diluted with ultra-pure water (TKA MicroPure, 0.06 μs/cm at room temperature). Pyridine (Fluka, > %) or acetic acid (Sigma-Aldrich, 99.99%) were added to reach the desired concentration of acid. ph was adjusted using a concentrated solution of sulfuric acid (Normapur, Prolabo, 99 %) or hydrochloric acid (Normapur, Prolabo, 37%) and potassium hydroxide (Rectapur, Prolabo, > 85 %,). Methanol (VWR, 99.8 %) and deuterium oxide (Euriso-Top, %) were used in NMR experiments. All the aforementioned chemicals were used as received. Argon, nitrogen, carbon monoxide and carbon dioxide gases were purchased from Air Liquide. In situ Fourier transform infrared (FTIR) spectroscopy measurements coupled with electrochemical experiments In situ FTIR measurements were carried out in a Bruker IFS 66v spectrometer which was modified for beam reflection on the electrode surface at a 65 C incidence angle. A 10-6 bar vacuum was used in order to remove interferences from atmospheric water and CO 2. The detector was a MCT (HgCdTe) type, beforehand cooled by liquid nitrogen. The spectral resolution was of 4 cm -1 and the FTIR spectra were recorded in the wavenumber 1000 and 4000 cm -1 IR region. A homemade three-electrode spectroelectrochemical cell, fitted on the bottom with an MIR transparent window (CaF 2 ) was used. A slab of glassy carbon served as a counter electrode and the reference electrode was a SCE. The working electrode was a 7 mm diameter Pt disk. To minimize the absorption of the infrared beam by the solution, the working electrode was pressed against the window and a thin layer of electrolytic solution was obtained. Two methods were used: SPAIRS (Single Potential Alteration IR Spectroscopy) and Chronoamperometry/FTIRS coupling. In the SPAIRS method the electrode reflectivity R Ei was recorded at different potentials E i, each separated by 0.05 V during the anodic scan at a sweep rate of 1 mv s -1. In the second spectro-electrochemical method the potential was maintained at a fixed value and spectra were recorded every 3 min. IR spectra were calculated for each potential value (or time) as changes in the reflectivity (R) relative to a reference single-beam spectrum (R ref ) as R = R i /R ref. Upward and downward going absorption bands represent, respectively, the decrease and increase of corresponding species. Cyclic voltammetry Cyclic voltammetry experiments were carried out using a Metrohm AUTOLAB potentiostat (model PGSTAT12) in a standard three-compartment electrochemical cell with a platinum wire counter electrode and a saturated calomel reference electrode (SCE, Radiometer Analytical XR110). The working electrode was either a 1, 5 or a 7 mm diameter polycrystalline platinum disk carefully polished on a polishing cloth with an aqueous suspension of alumina (1 and 0.05 μm) for 1 min at each size. The residual alumina S4

5 particles were removed by sonication in distilled water for 30 s. Then, the electrodes were air-dried before use. Ohmic drop was compensated using the positive feedback compensation implemented in the instrument. Experiments were made at room temperature (at 22 ± 2 C) either under saturated Ar, N 2, CO or CO 2 solutions, obtained after about 30 min of gas bubbling. Controlled potential electrolysis Controlled potential electrolysis were carried out with a Parstat 2273 potentiostat using the experimental setup shown on Figure S6. The working electrode was a polycrystalline platinum basket (Ögussa). The area was determined by electrochemical calibration and found to be cm 2. The basket was pre-cleaned in concentrated nitric acid then soaked for 5 min in ultrasonicated acetone and finally rinsed thoroughly in boiling ultra-pure water. The counter electrode was a platinum mesh isolated from the main cell compartment by immersion in a glass tube terminated by a porous glass frit to avoid oxygen creation around the cathode. The anolyte simply contained a 0.1 M solution of KCl as supporting electrolyte. A saturated calomel electrode (SCE, Radiometer Analytical XR110) was used as reference electrode without saline bridge to avoid an increase of the ohmic drop. The volume of the electrolyzed solution was 55 ml. Before each experiment, CO 2 (or Ar) was continuously bubbled through the catholyte for 30 min to reach saturation. Fig. S6. Schematic of the device used in controlled potential electrolysis experiments. Product Analysis Gas Chromatography Gas chromatography analyses of gas evolved in the headspace during the electrolysis were performed with an Agilent Technologies 7820A GC system equipped with a thermal conductivity detector. H 2 production was quantitatively detected using a CP-CarboPlot P7 capillary column (27.46 m in length and S5

6 25 μm internal diameter). Temperature was held at 150 C for the detector and 34 C for the oven. The carrier gas was argon flowing at 9.5 ml/min at constant pressure of 0.5 bars. Injection was performed via a 250-μL gas-tight (Hamilton) syringe previously degassed with CO 2. Conditions allowed detection of H 2, O 2, N 2, CO, and CO 2. However, analysis of the head-space shows only H 2 and trace of air but no CO. The total amount of molecular H 2 generated by electrolysis was calculated using Q nnf with Q the charge measured during catalysis (in C), n the number of electrons involved in the catalytic reaction (which is 2, as 2 electrons are required to form one molecule of dihydrogen), N the amount of H 2 produced (in mol), and F the Faraday constant (in C/mol). N was then calculated by gas chromatography analysis of the gas phase using the calibration curve method and by measurement of the volume collected in the graduated cylinder (Fig S6), then compared with the value obtained from the equation above in order to calculate the Faradaic yield. Calibration curves for H 2 were determined separately by injecting known quantities of pure gas. Ion chromatography Ion chromatography was used in this study to detect other potential products of the CO 2 reduction, particularly formate ions (HCOO - ). Electrolyzed solutions were diluted 20-fold with ultra-pure water, then analyzed using a Dionex DX100 ionic chromatograph equipped with a CD20 conductivity detector, an IonPac AG10 guard column (4 x 50 mm), and an IonPac AS10 analytical column (4 x 250 mm). A 50 mm NaOH solution was used as eluent. Formate ions are detected at 3.2 min for an eluent flow rate of 1 ml/min. Before each analysis, the sample loop was rinsed with distilled water and then by the solution to be injected. The absence of formate formed during electrolysis is shown by comparison of figures S7 and S8. 1 H NMR 1 H NMR spectroscopy was used to detect the presence of methanol in the electrolyzed solutions. Spectra were recorded at room temperature with a 400 MHz Bruker (Advance III) spectrometer. The solutions analyzed consist of a sample (0.5 ml) taken directly from the electrolyzed solution and mixed with 0.3 ml of D 2 O. The water signal (~ 4.79 ppm) was reduced by means of a solvent pre-saturation sequence. A solution of methanol (30 mm) prepared in the presence of 10 mm pyridinium/pyridine and 0.1 M KCl was used to determine the methanol signal position. Under these conditions, 1 H NMR spectroscopy shows one singlet at 3.34 ppm corresponding to the three protons in the methyl group. However, this signal has not been observed in any of the electrolyzed solutions (see figures S9 to S14). Knowing that S6

7 Signal (V) the detection limit of NMR for aqueous solutions of methanol was determined to be below 1 μm, S1 this indicates the absence of any significant amount of methanol resulting from CO 2 reduction Experiment a b c d e f HCOO Retention Time (min) Fig S7. Ionic chromatograms recorded after the electrolysis of 10 mm PyrH + solutions carried out in the conditions depicted in Table S1 (partial reproduction of Table 1). No significant peak corresponding to the production of formate is detected. Table S1. Production of bulk products upon electrolysis at a Pt electrode i Experiment Applied Potential (V vs. SHE) gas ph Charge (C) Faradaic yields (%) H 2 a Ar b CO c CO d CO e CO f CO i: 10 mm PyH + /Py solutions in the presence of 0.1 M of supporting electrolyte (KCl) under 1 atm of either argon or CO 2. S7

8 Peak Area (V x min x 10 3 ) Signal (V) (a) [NaHCOO] ( M) HCOO Retention Time (min) 25 (b) y = x R 2 = NaHCOO concentration ( M) Fig S8. (a) Chromatograms corresponding to sodium formate solutions used for calibration. (b) IC calibration curve of peak area vs. sodium formate concentration (in ultra-pure water). S8

9 Fig S9. 1 H NMR of a sample taken directly from the electrolyzed solution (experiment a) in the conditions depicted in Table S1. Fig S10. 1 H NMR of a sample taken directly from the electrolyzed solution (experiment b) in the conditions depicted in Table S1 (red: before electrolysis; black: after electrolysis). Fig S11. 1 H NMR of a sample taken directly from the electrolyzed solution (experiment c) in the conditions depicted in Table S1 (red: before electrolysis; black: after electrolysis). S9

10 Fig S12. 1 H NMR of a sample taken directly from the electrolyzed solution (experiment d) in the conditions depicted in Table S1 (red: before electrolysis; black: after electrolysis). Fig S13. 1 H NMR of a sample taken directly from the electrolyzed solution (experiment e) in the conditions depicted in Table S1. Fig S14. 1 H NMR of a sample taken directly from the electrolyzed solution (experiment f) in the conditions depicted in Table S1 (red: before electrolysis; black: after electrolysis). S10

11 References S1. Costentin, C.; Canales, J. C.; Haddou, B; Savéant, J-M.. J. Am. Chem. Soc. 2013, 135, S11

Single Catalyst Electrocatalytic Reduction of CO 2 in Water to H 2 :CO Syngas Mixtures with Water Oxidation to O 2

Electronic Supplementary Material (ESI) for Energy & Environmental Science. This journal is The Royal Society of Chemistry 2014 Supporting Information Single Catalyst Electrocatalytic Reduction of CO 2