Supporting Information (SI)

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1 Supporting Information (SI) Selective CO Production in Photoelectrochemical Reduction of CO 2 with a Cobalt Chlorin Complex Adsorbed on Multiwalled Carbon Nanotubes in Water Shoko Aoi, Kentaro Mase, Kei Ohkubo,*, Tomoyoshi Suenobu, and Shunichi Fukuzumi*,, Department of Material and Life Science, Graduate School of Engineering, Osaka University, SENTAN, Japan Science and Technology Agency (JST), Suita, Osaka , Japan Division of Innovative Research for Drug Development, Institute for Academic Initiatives, Osaka University, Suita, Osaka, , Japan Department of Chemistry and Nano Science, Ewha Womans University, Seoul 12-75, Korea Faculty of Science and Technology, Meijo University, SENTAN, Japan Science and Technology Agency (JST), Nagoya, Aichi , Japan Experimental Section Materials. Chemicals were purchased from commercial sources and used without further purification, unless otherwise noted. Acetonitrile (MeCN), ethanol (EtOH) and dimethyl sulfoxide (DMSO) were obtained from Nacalai Tesque. Nafion perfluorinated ion exchange resin solution, Nafion perfluorinated membrane (Nafion 117), and vanadyl acetylacetonate (VO(acac) 2 ) were received from Sigma-Aldrich Co. LLC. Bismuth nitrate pentahydrate (Bi(NO 3 ) 3 5H 2 O), potassium iodide (KI), p-benzoquinone, Na 2 SO 4, sodium hydroxide (NaOH) and iron(ii) sulfate heptahydrate (FeSO 4 7H 2 O) were obtained from Wako Pure Chemical Industries, Ltd. p-benzoquinone was recrystallized from ethanol. Multiwalled carbon nanotubes (MWCNTs) (diameter: 6-9 nm average length: 5 µm, > 95%) was obtained from Sigma-Aldrich Co. LLC. MWCNTs were heated at 45 C in air for 15 min to remove amorphous carbon. The obtained powder was then shaken in aq. HCl (2. M) at room temperature for 2 h. These procedures were repeated once more to obtain the purified MWCNTs. A cobalt chlorin complex (Co II (Ch)) was prepared by the published methods. S1 Glass slides coated with FTO (transmittance: 83.6%) were supplied by Aldrich Chemicals Co. and cut by Asahi Glass Co., Ltd. Purified water was provided by a Millipore Milli-Q water purification system (Millipore, Direct-Q 3 UV) with an electronic conductance of 18.2 MΩ cm. Preparation of BiVO 4 /FTO Electrode. FTO glasses were cleaned prior to use by immersing in MeOH/HCl [1/1 (v/v)] solution for 3 min and washed by purified water. The resulting FTO glasses were hydroxylated in H 2 SO 4 for 2 h and then boiled in purified water for 3 min with subsequent drying under N 2. S2 BiVO 4 /FTO electrode was prepared according to the literature procedures. S3 A 1

2 .4 M of Bi(NO 3 ) 3 solution prepared by dissolving Bi(NO 3 ) 3 5H 2 O (291 mg) and KI (996 mg) in 15 ml of water. After the ph of the resulting solution was adjusted to 1.7 by adding HNO 3, 2 ml of EtOH containing.23 M of p-benzoquinone (149 mg) was added and vigorously stirred for 1 min. After removal of insoluble residues by filtration, a three-electrode cell composed of a fluorine-doped tin oxide (FTO) working electrode, a platinum coil counter electrode, an SCE reference electrode was employed for electrodeposition. A FTO working electrode was soaked into as prepared solution with a cm 2 area and cathodic deposition was performed with an applied potential at.14 V (vs SCE) until total passing charge of.13 C cm 2 at 298 K. The resulting orange-colored precipitate on the electrode corresponds to crystalline bismuth oxyiodide (BiOI). After washing with water,.1 ml of a DMSO solution containing.2 M vanadyl acetylacetonate (VO(acac) 2 ) was dropcast on the BiOI/FTO electrode. The resulting electrode was annealed to form crystalline BiVO 4 on FTO at 45 with ramping rate of 2 /min for 2 h. Excess V 2 O 5 existing in the BiVO 4 /FTO electrodes was removed by soaking them in 1. M of NaOH for 1 h and resulting BiVO 4 /FTO electrodes were repeatedly washed with water. Preparation of FeO(OH)/BiVO 4 /FTO Electrode. The FeO(OH)/BiVO 4 /FTO electrode was prepared by photodeposition of FeO(OH) on the BiVO 4 /FTO electrode. Photodeposition was performed in a N 2 -saturated aqueous solution containing.1 M of FeSO 4 7H 2 O. S3 A conventional three electrode cell was used with a BiVO 4 /FTO working electrode, a platinum coil counter electrode, and an SCE reference. The BiVO 4 /FTO working electrode was illuminated from the back side of the FTO electrode with a solar simulator (HAL-32, Asahi Spectra Co., Ltd.), where the light intensity was adjusted at 1 mw cm 2 (AM 1.5 G) at the sample position by 1 SUN checker (CS-2, Asahi Spectra Co., Ltd.) at 298 K. An external bias of.21 V (vs SCE) was applied to facilitate photodeposition until total passing charge of 45 mc cm 2 at 298 K. To ensure the deposition of FeO(OH) on any bare BiVO 4 surface, electrodeposition of FeO(OH) was performed with applied potential at V (vs SCE) for 9 s at 298 K. X-ray Diffraction. Powder X-ray diffraction patterns were recorded on a Rigaku MinFlex 6. Incident X-ray radiation (Cu Kα; λ = 1.54 Å) was produced by a Cu X-ray tube, operating at 4 kv and 15 ma. The scan rate was 1 min 1 from 2θ = 1 7. Preparation of Co II (Ch)-Modified Electrode. A Co II (Ch)-modified electrode for electrochemical reduction of CO 2 was prepared by drop-cast method. A mother MeCN (1. ml) solution consists of Co II (Ch) (1. mm), MWCNTs (1.3 mg), and 5% Nafion (12 µl). The solution was sonicated for 2 min. Then a 1 µl drop was applied on the surface of a polished glassy carbon working electrode and allowed to evaporate to afford a thin film with MWCNTs loading of.18 mg cm 2 and a catalyst loading of 1 nmol. The glassy carbon working electrode (BAS) was routinely polished with BAS polishing alumina suspension and rinsed with acetone before use. Co II (Ch) could be attached on the MWCNTs-modified electrode because the complex might hardly be dissolved in water. Photoelectrochemical Measurements. Photoelectrochemical measurements were performed on an ALS 63B electrochemical analyzer. A conventional three-electrode cell was used with the 2

3 FeO(OH)/BiVO 4 /FTO electrode as a working electrode, a platinum coil as a counter electrode and an SCE reference electrode in an Ar-saturated aqueous solution containing 5. mm Na 2 SO 4 at 298 K. The FeO(OH)/BiVO 4 /FTO electrode was illuminated from the back side of FTO electrode with a solar simulator (HAL-32, Asahi Spectra Co., Ltd.), where the light intensity was adjusted at 1 mw cm 2 (AM1.5G) at the sample position by a 1 SUN checker (CS-2, Asahi Spectra Co., Ltd). Detection of Formic Acid. Detection and determination of quantity of formic acid were performed on the indicator kit purchased from J. K. International. In the presence of formic acid and formate dehydrogenase, NAD + is reduced to NADH as given by eq 1. HCOOH + NAD + + H 2 O HCO 3 + NADH + H + (1) The concentration of formic acid formed in an aqueous solution after photoassisted controlled potential electrolysis was calculated from an increase in absorbance at λ = 34 nm due to formation of NADH (NADH: λ max = 34 nm, ε = 63 M 1 cm 1 ). Calcium phosphate buffer (1. ml) was poured to a lithium salt of NAD + (19 mg)). The resulting solution (1. ml) is denoted as 1. Distilled water (2. ml) was added to 1 (1. ml) and UV-vis absorption spectra of the mixture (3. ml) before and after addition of formate dehydrogenase (5 µl) were measured. An increase in absorbance at λ = 34 nm before and after addition of formate dehydrogenase (5 µl) was defined as ΔA Blank. An aqueous solution (2. ml) after photoassisted controlled potential electrolysis was added to 1 (1. ml) and UV-vis absorption spectra of the mixture (3. ml) before and after addition of formate dehydrogenase (5 µl) were measured. An increase in absorbance at λ = 34 nm before and after addition of formate dehydrogenase (5 µl) was defined as ΔAbs Sample solution. The authentic ΔAbs was calculated by eq 2. Abs (sample solution) Abs (blank) = ΔAbs (2) The concentration of formic acid was calculated as following eq 3. The concentration of formic acid (g L 1 ) = (V MW ΔAbs) / (ε d ν 1) (3) V (Volume of solution in UV-cell): 3.5 ml MW (Molecular weight of formic acid): 46. d (Optical pass length): 1. cm ε: (Absorption coefficient): 63 M 1 cm 1 ν (Volume of solution after electrolysis): 2. ml Photoelectrochemical Measurements. Photoelectrochemical measurements in a 2E system were performed in a quartz anode cell (light path length = 1 cm) connected with a pyrex cathode cell through a Nafion membrane (Figure S1a). The anode cell consists of the as-prepared FeO(OH)/BiVO 4 /FTO photoanode for the water oxidation in an Ar-saturated aqueous solution (1 ml) containing 5. mm Na 2 SO 4. The cathode cell consists of Co II (Ch)-modified electrode for the 3

4 CO 2 reduction in a CO 2 -saturated aqueous solution (13 ml) containing 5. mm Na 2 SO 4. The photoanode was connected with alligator clips and wire of a counter electrode and a reference electrode. The Co II (Ch)-modified cathode was connected with an alligator clip and wire of a working electrode. The anode and cathode solution were saturated by continuous bubbling with Ar and CO 2 gas for 3 min, before the photocatalytic reaction. Cyclic voltammetry (CV) measurements and controlled potential electrolysis were performed on an ALS 63B electrochemical analyzer with a Co II (Ch)-modified cathode as a working electrode, a FeO(OH)/BiVO 4 /FTO anode as a counter electrode and as a reference electrode in deaerated water containing 5. mm Na 2 SO 4 as a supporting electrolyte at 298 K. The FeO(OH)/BiVO 4 /FTO anode was illuminated from the back side of FTO electrode with a solar simulator (HAL-32, Asahi Spectra Co., Ltd.), where the light intensity was adjusted at 1 mw cm 2 (AM 1.5G) at the sample position by a 1 SUN checker (CS-2, Asahi Spectra Co., Ltd). The detection of produced gas with gas chromatography and bubbling with Ar in anode cell and CO 2 in cathode cell were carried out every 2 hours of controlled potential electrolysis. Controlled potential electrolysis was also conducted in a 3E system with the Co II (Ch)-modified cathode as a working electrode, the FeO(OH)/BiVO 4 /FTO anode as a counter electrode, and SCE as a reference electrode (Figure S1b). The anode cell consists of the as-prepared FeO(OH)/BiVO 4 /FTO anode for the water oxidation in an Ar-saturated aqueous solution (1 ml) containing 5. mm Na 2 SO 4. The cathode cell consists of the Co II (Ch)-modified cathode for the CO 2 reduction and an SCE reference electrode in a CO 2 -saturated aqueous solution (13 ml) containing 5. mm Na 2 SO 4. The FeO(OH)/BiVO 4 /FTO anode was connected with an alligator clip and wire of a counter electrode. The Co II (Ch)-modified cathode was connected with an alligator clip and wire of a working electrode. The SCE reference electrode was connected with an alligator clip and wire of a reference electrode. The anode and cathode solution was saturated by continuous bubbling with Ar and CO 2 gas for 3 min, respectively, before the photocatalytic reaction. The ph (2., 2.8 and 3.6) of deaerated water containing 5. mm Na 2 SO 4 in photoelectrochemical cell was adjusted by addition of an aqueous solution of H 2 SO 4 (18 mm). When ph was 6.7, KHCO 3 (.5 M) was used as a supporting electrolyte. All the photoelectrochemical and electrochemical measurements were conducted using a saturated calomel reference electrode (SCE) and all results in this work are presented against the SCE. The conversion of potentials vs. SCE to vs. NHE was performed according to eq 4. E (vs SCE at measured ph) = E (vs NHE at ph ).241 V.59 V ph (4) UV-Visible Absorption Spectral Measurements. Absorption spectra were recorded on a Shimadzu UV-31PC UV-Vis-NIR scanning spectrophotometer at 298 K using quartz cells (light path length = 1. cm). Measurements of the Potential Difference. The potential difference between the FeO(OH)/BiVO 4 /FTO electrode and the SCE reference electrode was measured with Precision 4

5 Source/Measure Unit B2911A possessing 1 fa and 1 nv measurement resolution and 1 pa and 1 µv sourcing resolution under open circuit conditions during controlled potential electrolysis with the Co II (Ch)-modified cathode and FeO(OH)/BiVO 4 /FTO photoanode at various applied potentials. The image of photoelectrochemical cell connected with Precision Source/Measure Unit is shown in Figure S1b. Gas Chromatography Analysis. The gas phase of the reaction vessel was analyzed by Shimadzu GC-17A gas chromatograph [Ar carrier, a capillary column with molecular sieves (Agilent Technologies, 1995PMS, 3 m.53 mm) at 313 K] equipped with a thermal conductivity detector. Each 2 µl gas sampled from the headspace of the anode cell (6.5 ml) or the cathode cell (5.8 ml) were injected in the column through the rubber septum. The amount of gas was quantified with calibration curve. References [S1] Mase, K.; Ohkubo, K.; Fukuzumi, S. Efficient Two-Electron Reduction of Dioxygen to Hydrogen Peroxide with One-Electron Reductants with a Small Overpotential Catalyzed by a Cobalt Chlorin Complex. J. Am. Chem. Soc. 213, 135, [S2] Krasnoslobodtsev, A. V.; Smirnov, S. N. Effect of Water on Silanization of Silica by Trimethoxysilanes. Langmuir 22, 18, [S3] Kim, T. W.; Choi, K.-S. Nanoporous BiVO 4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting. Science 214, 343, RE (a) RE WE (b) V CE Figure S1. The images of a photoelectrochemical cell (a) in a 2E system composed of the FeO(OH)/BiVO 4 /FTO photoanode and the Co II (Ch)-modified cathode and (b) in a 3E system composed of the FeO(OH)/BiVO 4 /FTO photoanode and the Co II (Ch)-modified cathode and a SCE reference electrode connected with Precision Source/Measure Unit. WE (green), CE (red) and RE (black) represent working electrode, counter electrode and reference electrode, respectively. 5

6 (211) (2) (132) (24) (22) (161) (123) (121) (c) (b) (a) (44) (2) (42) (321) θ, degree Figure S2. Powder X-ray diffraction patterns of (a) as-prepared BiVO 4 /FTO electrode, (b) as-prepared FeO(OH)/BiVO 4 /FTO electrode and (c) FeO(OH)/BiVO 4 /FTO photoanode after photoassisted controlled potential electrolysis with the Co II (Ch)-modified cathode at an applied bias voltage of 1.3 V at the cathode vs the FeO(OH)/BiVO 4 /FTO photoanode in a CO 2 -saturated aqueous solution containing 5. mm Na 2 SO 4 (ph 4.6) under simulated 1 sun (AM 1.5G) illumination at 298 K for 51 h. Black circles indicate the peaks derived from FTO electrode. Current, µa CO 2, ph 4.6 Ar, ph Applied Bias Voltage at the Cathode vs the Photoanode, V Figure S3. Cyclic voltammograms of CO 2 - (red) and Ar- (black) saturated aqueous solution at ph 4.6 containing 5. mm Na 2 SO 4 recorded at the Co II (Ch)-modified cathode at applied bias voltage at the cathode vs the photoanode under simulated 1 sun (AM 1.5G) illumination. Sweep rate: 1 mv s 1. 6

7 .3 Absorbance Wavelength, nm Figure S4. UV-vis absorption spectral changes of solutions after photoassisted controlled potential electrolysis with the Co II (Ch)-modified cathode at an applied bias voltage of 1.3 V at the cathode vs the FeO(OH)/BiVO 4 /FTO photoanode in a CO 2 -saturated aqueous solution containing 5. mm Na 2 SO 4 (ph 4.6) under simulated 1 sun (AM 1.5G) illumination at 298 K for 51 h. Black and red lines show the spectra before and after addition of formate dehydrogenase. Before the reaction After the reaction Absorbance.1 λ max = 43 nm λ max = 42 nm λ max = 652 nm λ max = 64 nm Wavelength, nm Figure S5. UV-vis absorption spectra (black and red lines) of the supernatant (2. ml) containing [Co III Ch] + and Co II Ch loaded on the MWCNTs cathode after removal of detached MWCNTs by centrifugation before and after the photoassisted controlled potential electrolysis with the Co II (Ch)-modified MWCNTs cathode at an applied bias voltage of 1.3 V at the cathode vs the FeO(OH)/BiVO 4 /FTO photoanode in a CO 2 -saturated aqueous solution containing 5. mm Na 2 SO 4 (ph 4.6) under simulated 1 sun (AM 1.5G) illumination at 298 K for 51 h, respectively. The concentration of Co II Ch after the reaction was 7% of the initial concentration of [Co III Ch] + before the reaction. 7

8 Amount of products, µmol.2.1 CO H Figure S6. Time courses of evolution of CO (red) and H 2 (black) in the controlled potential electrolysis with the Co II (Ch)-modified cathode at an applied bias voltage of 1.3 V at the cathode vs the FeO(OH)/BiVO 4 /FTO photoanode in a CO 2 -saturated aqueous solution containing 5. mm Na 2 SO 4 (ph 4.6) under dark at 298 K Under light illumination Under dark Current, µa Figure S7. Time courses of currents for the controlled potential electrolysis with the Co II (Ch)-modified cathode at an applied bias voltage of 1.3 V at the cathode vs the FeO(OH)/BiVO 4 /FTO photoanode in a CO 2 -saturated aqueous solution containing 5. mm Na 2 SO 4 (ph 4.6) under dark (black) and simulated 1 sun (AM 1.5G) illumination (red) at 298 K. 8

9 (a) (b) Amount of products, µmol CO H Amount of products, µmol CO H (c) Amount of products, µmol CO H (d) Amount of products, µmol CO H (e) Amount of products, µmol Figure S8. Time courses of evolution of H 2 (black) and CO (red) in the photoassisted controlled potential electrolysis with the Co II (Ch)-modified cathode at an applied bias voltage of 1.3 V at the cathode vs the FeO(OH)/BiVO 4 /FTO photoanode in a CO 2 -saturated aqueous solution containing 5. mm Na 2 SO 4 under simulated 1 sun (AM 1.5G) illumination at 298 K at ph (a) 2., (b) 2.8, (c) 3.6, (d) 4.6 and (e) 6.7. CO H

10 (a) (b) Amount of CO, µmol [Co II (Ch)] = 2. mm [Co II (Ch)] = 1. mm [Co II (Ch)] =.5 mm Rate of CO production, µmol h Concentration of Co II (Ch), mm Figure S9. (a) Time courses of production of CO in the controlled potential electrolysis with the Co II (Ch)-modified MWCNTs cathode at 1.1 V vs NHE in a CO 2 -saturated aqueous solution containing 5. mm Na 2 SO 4 (ph 4.6) at 298 K. The amounts of Co II (Ch) loaded on MWCNTs were determined from the concentrations in a sonicated MeCN solution, which are.5, 1. and 2. mm. (b) Plot of the initial rate (< 2 h) of the photoelectrocatalytic CO production vs different concentrations of Co II (Ch) loaded on MWCNTs in a sonicated MeCN solution. 1

11 Table S1. The Amount of Produced Gas, Faradaic Efficiency for Produced Gas and Total Electric Charge (Q) in Photoelectrochemical Reduction of CO 2 with the Co II (Ch)-Modified Cathode at Various Applied Bias Voltages at the Cathode vs the FeO(OH)/BiVO 4 /FTO Photoanode in a CO 2 -Saturated Aqueous Solution Containing 5. mm Na 2 SO 4 (ph 4.6) under Simulated 1 Sun (AM 1.5G) Illumination at 298 K for 2 h applied bias voltage (vs the FeO(OH)/BiVO 4 /FTO amount of CO, [µmol] (f CO %) amount of H 2, [µmol] (f H2 %) amount of O 2, [µmol] (f O2 %) total Q, [µmol] photoanode) (19).332 (81).24 (1) (77).15 (23).227 (1) (83).17 (17).31 (1) 1.25 Table S2. The Amount of Produced Gas, Faradaic Efficiency for Produced Gas and Total Electric Charge (Q) in the Electrolysis with the Co II (Ch)-Modified Cathode at 1.1 V vs NHE in a CO 2 -Saturated Aqueous Solution Containing 5. mm Na 2 SO 4 (ph 4.6) with the FeO(OH)/BiVO 4 /FTO Photoanode under Dark and Simulated 1 Sun (AM 1.5G) Illumination at 298 K for 2 h amount of CO, amount of H 2, amount of O 2, total Q, [µmol] (f CO %) [µmol] (f H2 %) [µmol] (f O2 %) [µmol] under dark.965 (88).13 (12).547 (1) 2.19 under light illumination.955 (88).13 (12).545 (1)

12 (a) Current, µa Under light illumination Under dark (b) Current, µa Under light illumination Under dark Time, s Time, s (c) Current, µa Under light illumination Under dark (d) Current, µa Under light illumination Under dark Time, s Time, s Figure S1. Time courses of currents for the electrolysis with the Co II (Ch)-modified cathode at (a) 1.1 V, (b) 1. V, (c).9 V and (d).8 V vs NHE in a CO 2 -saturated aqueous solution containing 5. mm Na 2 SO 4 (ph 4.6) with the FeO(OH)/BiVO 4 /FTO photoanode under dark (black) and simulated 1 sun (AM 1.5G) illumination (red) at 298 K. 12

13 Table S3. Potentials of the FeO(OH)/BiVO 4 /FTO Photoanode vs NHE during the Controlled Potential Electrolysis with the Co II (Ch)-Modified Cathode and the FeO(OH)/BiVO 4 /FTO Photoanode in a CO 2 -Saturated Aqueous Solution Containing 5. mm Na 2 SO 4 (ph 4.6) under Dark and 1 Sun (AM 1.5G) Illumination at 298 K E red vs NHE (V) E ox vs NHE (V) applied potential for controlled potential electrolysis at cathode potential of FeO(OH)/BiVO 4 /FTO photoanode under dark potential of FeO(OH)/BiVO 4 /FTO photoanode under light illumination