Hiromitsu Tanaka & Tsutomu Kajino

Transcription

1 Selective CO 2 conversion to formate conjugated with H 2 O oxidation utilizing semiconductor/complex hybrid photocatalysts Shunsuke Sato,* Takeo Arai,* Takeshi Morikawa, Keiko Uemura, Tomiko M. Suzuki, Hiromitsu Tanaka & Tsutomu Kajino Toyota Central Research and Development Laboratories, Inc., Nagakute, Aichi , Japan * To whom correspondence should be addressed. ssato@mosk.tytlabs.co.jp ; takeo-arai@mosk.tytlabs.co.jp S1

2 Supporting Information Method Materials The complexes, [Ru{4,4 -di(1h-pyrrolyl-3-propyl carbonate)-2,2 -bipyridine}(co) 2 Cl 2 ] (MCE1), [Ru(4,4 -diphosphate ethyl-2,2 -bipyridine)(co) 2 Cl 2 ] (MCE2-A) and [Ru(4,4 -dicarboxylic acid-2,2 -bipyridine)(co) 2 Cl 2 ] (MCE3-A) were synthesized according to previously reported method. 1,2 Several type semiconductors such as zinc doped gallium phosphide (GaP), zinc doped indium phosphide (InP) and nitrogen doped tantalum pentoxide (N-Ta 2 O 5 ) were selected as photocathode materials. InP wafer and GaP wafer were obtained from Sumitomo Electric Industries, Ltd. and Sumitomo Metal Mining Co. Ltd., respectively. Those wafers were cut to oblong plate (20 8 mm 2 or mm 2 ) and the copper wire was soldered on the top of plate by indium. The plates were reinforced by attaching grass substrate in their reverse side and their edges were covered with silicone rubber. N-Ta 2 O 5 electrode was synthesized as previously reported and the copper wire was soldered on the top of plate by indium and its edges were covered with silicone rubber. 3 S2

3 The position of the valence band maximum (E VBM ) of each p-type semiconductors was estimated by the results of liner sweep voltammetry comparing with the data measured by photoelectron spectroscopy in air (PESA). 4 The position of the conduction band minimum (E CBM ) was calculated by subtracting the bandgap value of p-type semiconductor from E VBM. As the results, E CBM s versus Ag/AgCl at ph 4 was estimated approximately to be -2.2 V, V and -1.6 V for GaP, InP and N-Ta 2 O 5, respectively. Those E CBM s were more negative than the potential required for CO 2 reduction over a ruthenium complex catalyst (E red = -0.8 V). Synthesis of [Ru{4,4 -di(1h-pyrrolyl-3-propyl carbonate)-2,2 -bipyridine}(co)(mecn)cl 2 ] (MCE4) As a typical run, a MeCN solution (500 ml) containing mg (0.2 mmol) of MCE1 was irradiated using a white fluorescent light for 24 h at room temperature. After the solvent was evaporated under reduced pressure, the residual red solid was recrystallized with MeCN/ether. Yield: 89 %. 1 H NMR (δ, 400MHz, (acetone-d 6 ): (d, 1H, J = 5.6 Hz, bpy-6 ), 9.39 (d, 1H, J = 5.6 Hz, bpy-6), 9.06 (s, 1H, bpy-3 ), 8.94 (s, 1H, bpy-3), 8.41 (d, 1H, J = 5.6 Hz, bpy-5 ), 8.02 (d, 1H, J = 5.6 Hz, bpy-5), 6.78 (m, 2H, pyrrol), 6.02 (m, S3

4 2H, pyrrol), 4.45 (m, 2H, -CH 2 ), 4.21 (m, 2H, -CH 2 ), 2.32 (m, 2H, -CH 2 ), 2.25 (s, 3H, CH 3 -CN). FT-IR (MeCN) ν CO / cm -1 = m/z (ESI-MS): [M +Na] + ; [M MeCN+MeOH+Na] +. Preparation of Semiconductor/Metal-complex-electrocatalyst (SC/[MCE]) hybrid photocatalyst As a typical run, MCE2-A(0.648 μmol) and MCE4(0.648 μmmol) dissolving in 1 ml of MeCN were polymerized using chemical polymerization initiators {pyrrol(2.86 nmol) and FeCl 3 (4μmol) in 25μl EtOH solution}. The polymer solution (0.05 ml) was dropped on the surface of semiconductor and dried out at 333K for five minutes. These coating procedures were repeated five times. The resulting SC/[MCE] hybrid photocathode put in the dark at room temperature over night. Finally, SC/[MCE] hybrid photocatalyst was rinsed with pure water. Total quantity of complex catalysts was μmol on each semiconductor. S4

5 Preparation of InP/[MCE2-A+MCE4] photocatalyst for Z-scheme system InP wafer (Sumitomo Electric Industries, Ltd., ) was cut to oblong plate (20 15 mm 2 ) and the copper wire was soldered on the top of plate by indium. The plates were reinforced by attaching grass substrate in their reverse side and their edges were covered with silicone rubber. Then, the electrode was combined with [MCE2-A+MCE4] by chemical polymerization process mentioned above. Preparation of Pt-loaded TiO 2 (TiO 2 /Pt) photocatalyst for Z-scheme system A paste of anatase TiO 2 was prepared by mixing 200 mg of TiO 2 powder (Degussa, P25), 30µl of acetylacetone, 400 µl of water, and 1 drop of Triton X-100. The paste was painted on conducting glass (FTO; Fluorine doped tin oxide, mm 2, AGC Fabritech Co., LTD.) by the squeegee method, and then calcined at 823 K for 1hour in air. The electrode was subsequently treated with TiCl 4 for particle-necking. 10mM TiCl 4 solution was applied on the electrode by spin-coating method at 1000 rpm for 10 seconds, and then calcined at 823 K for 30 minutes. After these processes, platinum was loaded on TiO 2 photoanode by impregnation method. TiO 2 photoanode was immersed in 1mM H 2 PtCl 6 S5

6 solution for a few minutes, and then calcined at 573 K for 30 minutes. Finally, we obtained translucent TiO 2 /Pt photocatalyst. S6

7 Photoelectrochemical reduction of CO 2 with a three-electrode configuration Photoelectrochemical measurements were performed using an electrochemical analyzer (ALS600B, ALS Co., Ltd.). Photocathode, silver-silver chloride electrode (Ag/AgCl) and glassy carbon electrode (GCE) were used as working, reference and counter electrode, respectively. A Pyrex glass cell was used as a reactor, and xenon light source (MAX-302, Asahi Spectra Co., Ltd., about 70 sun) equipped with an optical filter (LUX422, Asahi Spectra Co., Ltd., λ > 400 nm) and a cold mirror was used to irradiate visible light. Pure water and electrolyte aqueous solutions such as 10mM of sodium hydrogen carbonate (NaHCO 3 ), sodium phosphate (Na 3 PO 4 ) and sodium sulfate (Na 2 SO 4 ), were used as the reaction solution. 5 ml of electrolyte solution was introduced in the reactor and bubbled with Ar gas for 20 minutes to remove dissolved air, and then it was bubbled with CO 2 gas for 10 minutes before experiment. CO 2 gas was continuously flowed into the reactor during the experiment. S7

8 Photochemical reduction of CO 2 with Z-scheme system Photochemical measurements were performed using an electrochemical analyzer (ALS2323, ALS Co., Ltd.). CO 2 reduction was performed with Z-scheme system in a two-electrode configuration. InP/[MCE2-A+MCE4] photocatalyst (20x15 mm 2 ) and TiO 2 /Pt photocatalyst (20x15 mm 2 ) were used as working and counter electrode, respectively. Two-compartment Pyrex cell separated with proton exchange membrane (Nafion117, Dupont) was used as a reactor. 4ml of 10mM NaHCO 3 aqueous solution was introduced as electrolyte solution in working and counter electrode side of Z-scheme system, respectively. A solar simulator (HAL-320, Asahi Spectra Co., Ltd.) was used as a light source. The intensity was adjusted at 1SUN (Air Mass 1.5 (AM1.5)) by 1 SUN checker (CS-20, Asahi Spectra Co., Ltd.). The irradiation area was limited to 10mm x 10mm by a slit. The light was irradiated from TiO 2 /Pt side, and InP/[ MCE2-A+MCE4] was irradiated with the transmitted light through TiO 2 /Pt. No external electric bias was applied between two photoelectrodes. The electrolyte solution in working electrode side was bubbled with Ar gas for 20 minutes to remove dissolved air, and then it was bubbled with CO 2 gas for 10 minutes before experiment. During the experiment, CO 2 gas and Ar S8

9 gas were continuously flowed into the working and counter electrode side of Z-scheme system, respectively. MCE2-A MCE1 N N O O O O N N Cl Ru MeCN MCE3-A OC Cl MCE4 Scheme S1. Structure of metal complex electrocatalyst(mce). S9

10 Table S1 The results of photoelectrochemical CO 2 reduction over InP alone and InP/[MCE] in several aqueous solution. Hybrid photocatalyst SC MCE Solvent HCOO - / μmol cm -2 EFF / % InP MCE2-A+MCE4 pure water InP MCE2-A+MCE4 10 mm NaHCO InP MCE2-A+MCE4 10 mm Na 3 PO InP MCE2-A+MCE4 10 mm Na 2 SO InP - 10 mm NaHCO The photoelectrochemical CO 2 reduction was performed in three-electrode configuration. Photocathode, glassy carbon and Ag/AgCl were used as working, counter and reference electrode, respectively. A Pyrex glass cell was used as a reactor, and xenon light source (ca. 70 SUN) equipped with an optical filter (λ > 400 nm) and a cold mirror was used to irradiate visible light for 1h in 5ml aqueous solution. Applied potential was -0.4 V(vs Ag/AgCl). The current efficiency for formate formation (EFF) was calculated by dividing the total number of electrons stored in formate by the amount of consumed charge. Total quantity of complex catalysts was μmol on each semiconductor. S10

11 e - (a) Electrochemical analyzer (b) e - HCOO - e - HCOO - e - CO 2 Light e - CO 2 Light h + Red Ox e - h + H 2 O O 2 H + h + SC/[MCE] Ag/AgCl GCE TiO 2 /Pt InP/[MCE] Proton exchange membrane Figure S1. Schematic illustration of the photoelectrochemical reduction of CO 2 (a) with a three-electrode configuration and (b) with a two-electrode configuration in Z-scheme system with no electrical bias comprising of translucent Pt-loaded TiO 2 (TiO 2 /Pt) and [MCE2-A+MCE4]-modified InP (InP/[MCE2-A+MCE4]) S11

12 Determination of products The amount of HCOO- was determined using an ion chromatograph (ICS-2000, Dionex Corporation) with IonPacAS15 and IonPacAG15 columns. The column temperature was maintained at 308 K. A 3 mm KOH solution was used as the first eluent for 10 minutes, and then the eluent was gradually changed to a 10 mm KOH solution for the next 5 minutes, after which the eluent was gradually changed to a 30 mm KOH solution for the next 5 minutes. Isotope analysis To detect the formation of H 13 COO - and DCOO -, an ion chromatograph, interfaced with a time-of-flight mass spectroscopy system (IC-TOFMS, JEOL JMS-T100LP), was used with MeOH added as the mobile phase. 18 O 2 generation was detected by gas chromatography-mass spectrometry (GC-MS, 6890 and 5973, Agilent Technologies, Inc.) in the isotope analysis using H 2 18 O. S12

13 Results Rupolymer(MCE4) InP 0h Rupolymer(MCE4) InP 24h Figure S2. Cross-sectional SEM image of InP/MCE4 hybrid photocatalyst. (a) Irradiation time for 0h. (b) Irradiation time for 24h. Because the thickness of MCE4 is distributed in-plane, it is different depending on observation area. S13

14 Rupolymer(MCE2-A+MCE4) InP 1h Rupolymer(MCE2-A+MCE4) InP 19h Figure S3. Cross-sectional SEM image of InP/[MCE2-A+MCE4] hybrid photocatalyst. (a) Irradiation time for 1h. (b) Irradiation time for 19h. Because the thickness of catalyst is distributed in-plane, it is different depending on observation area S14

15 Isotope tracer analysis utilizing 13 CO 2 in Z-scheme system. 13 CO 2 isotope tracer analysis was conducted to verify the carbon source of formate generated in the Z-scheme system. The electrolyte solution in working electrode side was bubbled with Ar gas for 20 minutes to remove dissolved air. Then, the working electrode side of Z-scheme system was purged with 13 CO 2 gas by bubbling for 10 minutes and sealed with septum. A clear peak due to H 13 COO - (m/z = 46) was observed by LC-TOFMS only when the reactor was purged with 13 CO 2, while no peak was found for 12 CO 2. This result indicates that the carbon source for formate is CO 2. S15

16 Intensity (a) m/z=45(h 12 COO - ) under 13 CO 2 Intensity (b) m/z=46(h COO 10 - ) Retention time [min] Retention time [min] Intensity (c) m/z=45(h 12 COO - ) under 12 CO 2 Intensity (d) m/z=46(h COO 10 - ) Retention time [min] Retention time [min] Figure S4. IC-TOFMS spectra from a tracer analysis utilizing 13 CO 2 (a,b) and 12 CO 2 (c,d). The photoelectrochemical reaction utilizing Z-scheme system was conducted in a closed system, purged with 13 CO 2. S16

17 Isotope tracer analysis utilizing D 2 O in Z-scheme system. D 2 O isotope tracer analysis was conducted to verify the proton source of formate generated in the Z-scheme system. 10mM NaHCO 3 solution was prepared as electrolyte solution using D 2 O as solvent. 4ml of the solution was introduced in working and counter electrode sides of Z-scheme system, respectively. After photoelectrochemical reduction of CO 2, a clear peak due to DCOO - (m/z = 46) was observed only in the case that D 2 O was used as reactant. This result indicates that the proton source for formate is H 2 O. Intensity (e) m/z=45(hcoo - ) under D 2 O Intensity (f) m/z=46(dcoo ) Retention time [min] Retention time [min] Figure S5. IC-TOFMS spectra from a tracer analysis utilizing D 2 O (e,f). The photoelectrochemical reaction utilizing Z-scheme system was conducted by using 10mM NaHCO 3 solution containing D 2 O as electrolyte solution. S17

18 Isotope tracer analysis utilizing H 2 18 O in Z-scheme system. H 2 18 O isotope tracer analysis was conducted to verify the oxygen generation from water in the Z-scheme system reaction. 8mk of 10 mm NaHCO 3 aqueous solution containing 25% H 2 18 O solution was used as electrolyte solution for working and counter electrode sides. The electrolyte solution in working side was bubbled with Ar gas for 20 minutes to remove dissolved air, and then it was bubbled with CO 2 gas for 10 minutes before experiment. During the experiment, CO 2 gas was continuously flowed into working side of Z-scheme system. Counter side of Z-scheme system was used in closed condition. After photocatalytic reduction of CO 2, a clear peak due to 18 O 2 (m/z = 36) and 16 O 18 O (m/z = 34) was observed in the gas phase of counter side. This result indicates that H 2 O was oxidized to O 2 over TiO 2 /Pt. In other words, H 2 O was used as electron donor to reduce CO 2. S18

19 Intensity Intensity m/z = 34 ( 16 O 18 O) Retention time (minute) 4000 m/z = 36 ( O 2 ) Retention time (minute) Figure S6. GC-MS spectra from a tracer analysis utilizing 10mM NaHCO 3 aqueous solution containing 25% of H 2 18 O. The spectra were calculated by subtracting the signal of air from the signal of sample which corrected after photocatalytic reaction in Z-scheme system. S19

20 Reference (1) Anderson, P. A. et al., Inorg. Chem. 1995, 34, (2) Chardon-Noblat, S.; Deronzier, A.; Ziessel, R.; Zsoldos, D. J. Electrochem. Soc. 1998, 444, 253. (3) Morikawa, T.; Saeki, S.; Suzuki, T.; Kajino, T.; Motohiro, T. Appl. Phys. Lett. 2010, 96, (4) (a) Uchida, T.; Mimura, T.; Ohtsuka, M.; Otomo, T.; Ide, M.; Shida, A.; Sawada, Y. Thin Solid Films, 2006, 496, 75 (b) Nakano, Y.; Saeki, S.; Morikawa, T. Appl. Phys. Lett. 2009, 94, S20