Supporting Information Electrochemical CO2 Reduction to ...
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Supporting Information Electrochemical CO2 Reduction to Hydrocarbons on a Heterogeneous Molecular Cu Catalyst in Aqueous Solution Zhe Weng,a,b Jianbing Jiang,a,b Yueshen Wu,a,b Zishan Wu,a,b Xiaoting Guo,a,b,c Kelly L. Materna,a,b Wen Liu,a,b Victor S. Batista,a,b Gary W Brudvig,a,b Hailiang Wanga,b a
Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States
b
Energy Sciences Institute, Yale University, West Haven, Connecticut 06516, United States
c
Department of Chemistry, Nankai University, Tianjin 300071, The People's Republic of China
Material Synthesis and Characterization Materials. All chemicals and solvents were commercially available and used as obtained without further purification. The water used throughout all experiments was deionized with 8.2 MΩ from a Millipore system. Characterizations. 1H NMR spectra were recorded at an Agilent 400 MHz NMR instrument. Chemical shifts are reported as ppm from the internal reference tetramethylsilane (TMS). Absorption spectra were recorded on a Varian Cary 50 Bio UV-visible spectrophotometer. Fluorescence spectra were recorded on a Shimadzu RF-5301pc spectrofluorophotometer. SEM and EDS were taken on a Hitach SU70 field emission SEM. TEM measurements were performed on an FEI Tecnai Osiris S/TEM (200 kV). XPS measurements were performed on a PHI Versa Probe II system using monochromatic 1486.7 eV Al Kα X-ray source. 5,10,15,20-tetrakis(2,6-dimethoxyphenyl)porphyrin (PorHCH3).
Following
a
reported
procedure,1 a mixture of pyrrole (278 µL, 4.0 mmol) and 2,6-dimethoxybenzaldehyde (665 mg, 4.0 mmol) in CHCl3 (400 mL) and C2H5OH (3.0 mL) was degassed by bubbling with nitrogen for 15 min, followed by addition of BF3·OEt2 (160 µL, 1.3 mmol). The solution was stirred for 1.5 h. 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ, 681 mg, 3.0 mmol) was added in one portion and stirred for 90 min at 60 °C. The solution was cooled to room temperature, and triethylamine (1.0 mL) was added to the solution. The solvents were removed by rotatory evaporation. The dark solid was passed through a silica pad (CH2Cl2), and then column chromatographed (silica, pure CH2Cl2 to CH2Cl2/ethyl acetate (9:1)) to afford a purple solid.
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(128 mg, 15%). 1H NMR (CDCl3) δ –2.50 (s, 2H), 3.49 (s, 24), 6.98 (d, J = 12 Hz, 8H), 7.68 (t, J = 12Hz, 4H), 8.67 (s, 8H). 13C NMR (CDCl3) δ 160.6, 129.8, 120.5, 104.3, 56.1; MALDI-MS M+ = 854.23, calculated 854.33 (M = C52H46N4O8); λmax (CH2Cl2) = 417, 512, 544, 587, 643 nm. 5,10,15,20-tetrakis(2,6-dihydroxyphenyl)porphyrin (PorH).
Following
a
reported
procedure,1 a sample of PorHCH3 (177 mg, 0.207 mmol) in anhydrous CH2Cl2 (3.54 mL) was added to BBr3 (403 mL, 4.25 mmol). The solution color turned from red to green.
The
reaction mixture was stirred gently under nitrogen at room temperature for 4 h. The reaction was then quenched with water (0.53 mL) and stirred for another 40 min. The crude mixture was then neutralized with a saturated solution of sodium bicarbonate and extracted with ethyl acetate. The extract was then washed with citric acid. The organic solution was dried (Na2SO4) and concentrated to afford a purple solid (138 mg, 90%). 1H NMR (CD3OD) δ 6.81 (d, J = 12Hz, 8H), 7.47 (t, J = 12Hz, 4H), 8.89 (s, 8H).
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C NMR (CD3OD) δ 158.1, 129.8, 116.8, 111.0, 106.7;
MALDI-MS M+ = 742.30, calculated 742.21 (M = C44H30N4O8); λmax (CH3OH) = 414, 512, 544, 586, 641 nm. Copper(II)-5,10,15,20-tetrakis(2,6-dihydroxyphenyl)porphyrin (PorCu). A solution of PorH (33 mg, 0.039 mmol) in CHCl3 (15 mL) was added to a solution of copper(II) acetate (21 mg, 0.12 mmol) in CH3OH (5.0 mL). The solution was refluxed for 19 h. After cooling down to room temperature, the solution was washed with brine, and extracted with ethyl acetate. The organic extract was dried (Na2SO4) and concentrated to afford the title compound (35 mg, 89%). MALDI-MS (M + H)+ = 804.28, calculated 804.12 (M = C44H28CuN4O8); λmax (CH3OH) = 413, 539 nm. Copper(II)-5,10,15,20-tetrakis(2,6-dimethoxyphenyl)porphyrin (PorCH3Cu). A solution of PorHCH3 (37 mg, 0.05 mmol) in CHCl3 (20 mL) was added to a solution of copper(II) acetate (27 mg, 0.15 mmol) in CH3OH (4.0 mL). The solution was refluxed for 11 h. After cooling down to room temperature, the solution was washed with brine, and extracted with ethyl acetate. The organic extract was dried (Na2SO4) and concentrated to afford the title compound (30 mg, 85%). MALDI-MS (M + H)+ = 916.16, calculated 916.24 (M = C52H45CuN4O8); λmax (CH2Cl2) = 415, 538 nm.
Electrochemical measurements Electrochemical experiments were performed on a Bio-Logic VMP3 Multi Potentiostat using a
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home-made gas-tight two-compartment electrochemical cell. The Pt wire counter electrode and the Ag/AgCl reference electrode were purchased from Pine Research Instrumentation. Carbon fiber paper (AvCarb GDS3250) was purchased from Fuel Cell Store. 250 µL of 2 mg mL1
catalyst of methanol solution was drop dried onto the carbon fiber paper to form a 1×2 cm 2
catalyst area (corresponding to 0.25 mg cm-2) to form a working electrode. The working electrode compartment and the counter electrode compartment were separated by an anion exchange membrane (Selemion DSV). Each compartment contained 15 mL of electrolyte and ~15 mL of gas headspace. For all experiments, 0.5 M KHCO3 aqueous solution (99.7% metals basis, Sigma-Aldrich) was used as the electrolyte after electrochemical purification for more than 16 h to remove trace metal ions.2 Before measurements, the electrolyte was presaturated with CO2 by bubbling the gas for 15 min. During measurements, CO2 was continuously bubbled into the electrolyte at a flow rate of 10 sccm. Current densities were calculated based on the geometric area of the working electrode. All potentials were referred to the reversible hydrogen electrode (RHE) and were recorded with iR compensation.
Product quantification Gas products of electrocatalysis were analyzed by a GC-MS (Agilent 6890N gas chromatograph / 5973 mass selective detector) equipped with a 60 m long Agilent CarbonPlot column. A low gauss magnet and careful tuning were applied to the MS detector to improve hydrogen sensitivity. During electrocatalytic measurements, 200 µL of gas sample was injected into the split inlet of the GC-MS system using a Hamilton 1725SL SampleLock syringe. The split ratio was set to 20:1. Helium (1 mL min-1) was used as the carrier gas. During electrocatalysis, the gas phase was first sampled 8 min after the start, and then every 18 min. A typical GC-MS trace is shown in Figure S4. The peak areas were converted to gas volumes using calibration curves. Liquid products were quantified after electrocatalysis by a 400 MHz NMR spectrometer (Agilent). 700 µL of electrolyte was mixed with 35 µL of 10 mM dimethyl sulfoxide (DMSO) and 50 mM phenol used as internal standards in D2O for 1H NMR analysis. The 1H NMR spectrum was recorded with water suppression by a presaturation method. The relaxation time was 5 s.
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TOF Calculation CV measurements were conducted at various scan rates in 0.5 M KHCO3 solution saturated with CO2 within the potential window from 0.025 to 0.225 V vs. RHE where no Faradaic reaction took place. The electrochemical double-layer (EDL) capacitance can be calculated by the equation below: 𝐶=
𝑖 𝑣
Where i is the current (mA) and v is the scan rate (mV s-1). We derived the EDL capacitance from the slope of the linear regression in the current-scan rate plot. Since the EDL capacitance is proportional to the actual number of PorCu molecules exposed in the electrolyte, TOF can be calculated by the equation below: 𝑇𝑂𝐹 =
𝑗 𝑗 𝑘𝑗 = = 𝑛𝑒𝑚 𝑛𝑒(𝐶𝑉/𝑘𝑒) 𝑛𝐶𝑉
Where j is the partial current (mA) of one product, n is the number of electrons transferred to produce one molecule of the product, e is the elementary charge (1.602×10-19 C), m is the actual number of exposed PorCu molecules, k is the number of elementary charges adsorbed on each molecule, C is the EDL capacitance (mF) and V is the potential window (V) of the CV measurements. Assuming one exposed PorCu molecule contributes to the EDL capacitance by adsorbing one elementary charge (e.g. a K+ ion), we have k = 1 and the TOFs can be calculated for all the CO2 reduction products.
Molecular integrity tests After 1 h of CO2 reduction electrocatalysis at –0.976 V vs. RHE, the PorCu catalyst was recovered from the working electrode by dissolving with methanol. Both absorption and fluorescence spectra of PorH, PorCu and the recovered PorCu were collected in methanol (Figure S12). The excitation wavelengths were 410, 413 and 413 nm for PorH, PorCu and the recovered PorCu, respectively. The weak fluorescence of the recovered PorCu (blue trace, bottom panel) indicates the presence of only a tiny amount of demetallated porphyrin, which is quantitated to be 1% by the equation below, assuming the fluorescence quantum yield of PorH is 1 (100%): ø𝑎 =
𝐴𝑏𝑠𝑟 𝐸𝑚𝑠𝑎 × × ø𝑟 𝐴𝑏𝑠𝑎 𝐸𝑚𝑠𝑟
Where øa is the fluorescence quantum yield of the recovered PorCu, Absr is the absorption 4
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intensity of PorH at the excitation wavelength, Absa is the absorption intensity of the recovered PorCu at the excitation wavelength, Emsa is the integrated emission area of the recovered PorCu, Emsr is the integrated emission area of PorH, and ør is the fluorescence quantum yield of the recovered PorCu. TEM and XPS measurements were also performed for samples prepared by drop casting the methanol solution containing the recovered PorCu on TEM grids and Si chips.
Computational Method All geometries were fully optimized along with the PCM aqueous continuum solvation model3 at the B3LYP level of density functional theory as implemented in Gaussian 094 using the LANL2DZ pseudopotential basis set on transition metal atoms5 and the 6-31G(d,p) basis set on H, C, N and O atoms.6 Analytic vibrational frequencies were computed to verify the nature of all stationary points. The structures were then used to compute the electronic energy using LANL2DZ pseudopotential for transition metal atoms5 and the 6-311+G(d,p) basis set on H, C, N and O atoms.7 To reduce systematic errors, the reduction potentials of PorCu0/- and PorCu-/2- vs. FeCp+/0 (ferrocinium/ferrocene redox pair) were calculated as Ered/ox(V vs. FeCp+/0) = -[Esolv(red, eV) + Esolv(FeCp+, eV) - Esolv(ox, eV) - Esolv(FeCp0, eV)] where Esolv refers to the solvated single-point energy and “red” and “ox” refer to the reduced and oxidized species in the redox pair, respectively.8 The calculated potentials were then converted to the RHE scale according to the following formula: E(V vs. RHE) = E(V vs. FeCp+/0) + 0.64 V + 0.0592 V*pH.
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Figure S1. (A) 1H NMR and (B) 13C NMR spectra of PorHCH3 in CDCl3.
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Figure S2. (A) 1H NMR and (B) 13C NMR spectra of PorH in CD3OD.
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Figure S3. Electronic absorption spectra of (A) PorCu (CH3OH), (B) PorHCH3 (CH2Cl2) and (C) PorH (CH3OH).
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Figure S4. A representative 1H NMR spectrum of liquid products generated from CO2 reduction catalyzed by PorCu. Phenol and DMSO were used as internal references.
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S10 Figure S5. A representative GC-MS trace of gas products generated from CO2 reduction catalyzed by PorCu.
Figure S6. Partial current densities and Faradaic efficiencies of the main liquid product (formic acid) generated from CO2 reduction over the PorCu catalyst at various electrode potentials, measured after 1 h of electrocatalysis.
Figure S7. Faradaic efficiencies of gas products generated from CO2 reduction over the PorCu catalyst at various electrode potentials, measured 8 min after electrolysis was started.
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Figure S8. Time-dependent total current densities for the PorH, PorCH3Cu and PorCu electrodes at –0.976 V vs. RHE.
Figure S9. CV measurements and analysis of the PorCu electrode over time during CO2 reduction electrocatalysis at –0.976 V vs. RHE. (A) Nyquist plots (200 kHz ~ 1 Hz, inset showing a zoom in view of the high frequency region). (B) CV curves at various scan rates at 56 min after electrocatalysis was started. (C) The linear regression of the current-scan rate plots. (D) EDL capacitance.
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Figure S10. CV curves of the PorCu electrode taken at the scan rate of 100 mV s-1 before and after electrocatalysis at -0.976 V vs. RHE for ~1 h. No obvious redox peaks were observed immediately after the electrocatalysis was stopped, which we attribute to slow recovery to equilibrium. After one hour of rest, the CV curve shows redox peaks with larger areas than the ones observed before electrocatalysis, which is consistent with our finding that the electrochemically active surface area of the catalyst has increased during the catalysis.
Figure S11. SEM image of the PorCu electrode after 1 h of electrocatalysis at –0.976 V vs. RHE.
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Figure S12. Partial current densities and Faradaic efficiencies of gas products generated from CO2 reduction over PorCu electrodes over time at various electrode potentials.
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Figure S13. A flow chart showing a set of control experiments carried out to exclude the in situ formation of Cu(0) nanoparticles.
Figure S14. UV-Vis absorption and fluorescence spectra for PorH, PorCu and recovered PorCu in methanol.
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Figure S15. TEM images of PorCu after ~1 h of electrolysis at -0.976 V vs. RHE.
Figure S16. Cu 2p core level spectra for PorCu before and after electrolysis at –0.976 V vs. RHE for ~1 h.9, 10
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Figure S17. Partial current and Faradaic efficiencies of gas products generated from CO2 reduction on a methanol-washed PorCu electrode over time at –0.976 V vs. RHE.
Figure S18. Faradaic efficiencies of gas products generated from CO2 reduction on the PorCu electrode with the electrode potential cycled between –0.676 and –0.976 V over time.
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Figure S19. Partial current densities and Faradaic efficiencies of gas products generated from CO2 reduction catalyzed by the PorCH3Cu over time at –0.976 V vs. RHE.
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S18 Table 1. Comparison with other metal-porphyrin and Cu based electrocatalyst materials for CO2 electroreduction Potential
j Total1
FE2
Main
/V vs RHE
/mA cm-2
/%
Products (FE)
Catalyst
CH4 (27%), PorCu
-0.976
49
59
C2H4 (17%), CO (10%)
Cu phthalocyanine
Fe porphyrin
-1.6
-1.16 (V
N/A
~50
CO, HCOOH
KHCO3 0.4M EtNCO2CH3 + 0.1M NBu4PF6 in DMF + 2M H2O
94
CO
-0.67
~3.2
91
CO
-0.8
1.32
9
CH4 (2.3%)3
Co porphyrin
protoporphyrin
KHCO3 0.5M
COF comprising
Co
0.5M
CH4 (30%),
0.31
vs NHE)
Electrolyte
0.5M KHCO3
Reference
This work
11
12
2
0.1M HClO4
13
0.1M KCl
14
CH4 (7%), Cu2O/Cu
-1
3
~62
C2H4 (12%), CO, HCOOH CH4 (18%),
Cu
-1.01
~4
~64
C2H4 (18%), CO, HCOOH
Cu nanoparticles Reduced Cu2O film
-1.35
12
76
CH4 CO (40%),
-0.55
2.6
1
Total current.
2
Faradaic efficiency for CO2 reduction.
75
HCOOH (33%)
18
0.1M KHCO3 0.1M NaHCO3 0.5M NaHCO3
15
16
17
S19 3
P(CO2) = 10 atm.
References: (1) Ellis, A.; Twyman, L. J. Macromolecules 2013, 46, 7055. (2) Lin, S.; Diercks, C. S.; Zhang, Y. B.; Kornienko, N.; Nichols, E. M.; Zhao, Y. B.; Paris, A. R.; Kim, D.; Yang, P.; Yaghi, O. M.; Chang, C. J. Science 2015, 349, 1208. (3) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999. (4) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M. J.; Heyd, J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.; Gaussian, Inc.: Wallingford, CT, USA, 2009. (5) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (6) Hariharan, P. C.; Pople, J. A. Theor. Chem. Acc. 1973, 28, 213. (7) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. (8) Konezny, S. J.; Doherty, M. D.; Luca, O. R.; Crabtree, R. H.; Soloveichik, G. L.; Batista, V. S. J. Phys. Chem. C 2012, 116, 6349. (9) Bird, R. J.; Swift, P. J Electron Spectrosc 1980, 21, 227. (10) Drolet, D. P.; Manuta, D. M.; Lees, A. J.; Katnani, A. D.; Coyle, G. J. Inorg Chim Acta 1988, 146, 173. (11) Furuya, N.; Matsui, K. J. Electroanal. Chem. 1989, 271, 181. (12) Costentin, C.; Drouet, S.; Robert, M.; Saveant, J. M. Science 2012, 338, 90. (13) Shen, J.; Kortlever, R.; Kas, R.; Birdja, Y. Y.; Diaz-Morales, O.; Kwon, Y.; Ledezma-Yanez, I.; Schouten, K. J. P.; Mul, G.; Koper, M. T. M. Nat. Commun. 2015, 6, 8177. (14) Lee, S.; Kim, D.; Lee, J. Angew. Chem. Int. Ed. 2015, 54, 14701. 19
S20 (15) Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F. Energ. Environ. Sci. 2012, 5, 7050. (16) Manthiram, K.; Beberwyck, B. J.; Aivisatos, A. P. J. Am. Chem. Soc. 2014, 136, 13319. (17) Li, C. W.; Kanan, M. W. J. Am. Chem. Soc. 2012, 134, 7231.
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Supporting Information Electrochemical CO2 Reduction to Hydrocarbons on a Heterogeneous Molecular Cu Catalyst in Aqueous Solution Zhe Weng,a,b Jianbing Jiang,a,b Yueshen Wu,a,b Zishan Wu,a,b Xiaoting Guo,a,b,c Kelly L. Materna,a,b Wen Liu,a,b Victor S. Batista,a,b Gary W Brudvig,a,b Hailiang Wanga,b a
Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States
b
Energy Sciences Institute, Yale University, West Haven, Connecticut 06516, United States
c
Department of Chemistry, Nankai University, Tianjin 300071, The People's Republic of China
Material Synthesis and Characterization Materials. All chemicals and solvents were commercially available and used as obtained without further purification. The water used throughout all experiments was deionized with 8.2 MΩ from a Millipore system. Characterizations. 1H NMR spectra were recorded at an Agilent 400 MHz NMR instrument. Chemical shifts are reported as ppm from the internal reference tetramethylsilane (TMS). Absorption spectra were recorded on a Varian Cary 50 Bio UV-visible spectrophotometer. Fluorescence spectra were recorded on a Shimadzu RF-5301pc spectrofluorophotometer. SEM and EDS were taken on a Hitach SU70 field emission SEM. TEM measurements were performed on an FEI Tecnai Osiris S/TEM (200 kV). XPS measurements were performed on a PHI Versa Probe II system using monochromatic 1486.7 eV Al Kα X-ray source. 5,10,15,20-tetrakis(2,6-dimethoxyphenyl)porphyrin (PorHCH3).
Following
a
reported
procedure,1 a mixture of pyrrole (278 µL, 4.0 mmol) and 2,6-dimethoxybenzaldehyde (665 mg, 4.0 mmol) in CHCl3 (400 mL) and C2H5OH (3.0 mL) was degassed by bubbling with nitrogen for 15 min, followed by addition of BF3·OEt2 (160 µL, 1.3 mmol). The solution was stirred for 1.5 h. 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ, 681 mg, 3.0 mmol) was added in one portion and stirred for 90 min at 60 °C. The solution was cooled to room temperature, and triethylamine (1.0 mL) was added to the solution. The solvents were removed by rotatory evaporation. The dark solid was passed through a silica pad (CH2Cl2), and then column chromatographed (silica, pure CH2Cl2 to CH2Cl2/ethyl acetate (9:1)) to afford a purple solid.
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(128 mg, 15%). 1H NMR (CDCl3) δ –2.50 (s, 2H), 3.49 (s, 24), 6.98 (d, J = 12 Hz, 8H), 7.68 (t, J = 12Hz, 4H), 8.67 (s, 8H). 13C NMR (CDCl3) δ 160.6, 129.8, 120.5, 104.3, 56.1; MALDI-MS M+ = 854.23, calculated 854.33 (M = C52H46N4O8); λmax (CH2Cl2) = 417, 512, 544, 587, 643 nm. 5,10,15,20-tetrakis(2,6-dihydroxyphenyl)porphyrin (PorH).
Following
a
reported
procedure,1 a sample of PorHCH3 (177 mg, 0.207 mmol) in anhydrous CH2Cl2 (3.54 mL) was added to BBr3 (403 mL, 4.25 mmol). The solution color turned from red to green.
The
reaction mixture was stirred gently under nitrogen at room temperature for 4 h. The reaction was then quenched with water (0.53 mL) and stirred for another 40 min. The crude mixture was then neutralized with a saturated solution of sodium bicarbonate and extracted with ethyl acetate. The extract was then washed with citric acid. The organic solution was dried (Na2SO4) and concentrated to afford a purple solid (138 mg, 90%). 1H NMR (CD3OD) δ 6.81 (d, J = 12Hz, 8H), 7.47 (t, J = 12Hz, 4H), 8.89 (s, 8H).
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C NMR (CD3OD) δ 158.1, 129.8, 116.8, 111.0, 106.7;
MALDI-MS M+ = 742.30, calculated 742.21 (M = C44H30N4O8); λmax (CH3OH) = 414, 512, 544, 586, 641 nm. Copper(II)-5,10,15,20-tetrakis(2,6-dihydroxyphenyl)porphyrin (PorCu). A solution of PorH (33 mg, 0.039 mmol) in CHCl3 (15 mL) was added to a solution of copper(II) acetate (21 mg, 0.12 mmol) in CH3OH (5.0 mL). The solution was refluxed for 19 h. After cooling down to room temperature, the solution was washed with brine, and extracted with ethyl acetate. The organic extract was dried (Na2SO4) and concentrated to afford the title compound (35 mg, 89%). MALDI-MS (M + H)+ = 804.28, calculated 804.12 (M = C44H28CuN4O8); λmax (CH3OH) = 413, 539 nm. Copper(II)-5,10,15,20-tetrakis(2,6-dimethoxyphenyl)porphyrin (PorCH3Cu). A solution of PorHCH3 (37 mg, 0.05 mmol) in CHCl3 (20 mL) was added to a solution of copper(II) acetate (27 mg, 0.15 mmol) in CH3OH (4.0 mL). The solution was refluxed for 11 h. After cooling down to room temperature, the solution was washed with brine, and extracted with ethyl acetate. The organic extract was dried (Na2SO4) and concentrated to afford the title compound (30 mg, 85%). MALDI-MS (M + H)+ = 916.16, calculated 916.24 (M = C52H45CuN4O8); λmax (CH2Cl2) = 415, 538 nm.
Electrochemical measurements Electrochemical experiments were performed on a Bio-Logic VMP3 Multi Potentiostat using a
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S3
home-made gas-tight two-compartment electrochemical cell. The Pt wire counter electrode and the Ag/AgCl reference electrode were purchased from Pine Research Instrumentation. Carbon fiber paper (AvCarb GDS3250) was purchased from Fuel Cell Store. 250 µL of 2 mg mL1
catalyst of methanol solution was drop dried onto the carbon fiber paper to form a 1×2 cm 2
catalyst area (corresponding to 0.25 mg cm-2) to form a working electrode. The working electrode compartment and the counter electrode compartment were separated by an anion exchange membrane (Selemion DSV). Each compartment contained 15 mL of electrolyte and ~15 mL of gas headspace. For all experiments, 0.5 M KHCO3 aqueous solution (99.7% metals basis, Sigma-Aldrich) was used as the electrolyte after electrochemical purification for more than 16 h to remove trace metal ions.2 Before measurements, the electrolyte was presaturated with CO2 by bubbling the gas for 15 min. During measurements, CO2 was continuously bubbled into the electrolyte at a flow rate of 10 sccm. Current densities were calculated based on the geometric area of the working electrode. All potentials were referred to the reversible hydrogen electrode (RHE) and were recorded with iR compensation.
Product quantification Gas products of electrocatalysis were analyzed by a GC-MS (Agilent 6890N gas chromatograph / 5973 mass selective detector) equipped with a 60 m long Agilent CarbonPlot column. A low gauss magnet and careful tuning were applied to the MS detector to improve hydrogen sensitivity. During electrocatalytic measurements, 200 µL of gas sample was injected into the split inlet of the GC-MS system using a Hamilton 1725SL SampleLock syringe. The split ratio was set to 20:1. Helium (1 mL min-1) was used as the carrier gas. During electrocatalysis, the gas phase was first sampled 8 min after the start, and then every 18 min. A typical GC-MS trace is shown in Figure S4. The peak areas were converted to gas volumes using calibration curves. Liquid products were quantified after electrocatalysis by a 400 MHz NMR spectrometer (Agilent). 700 µL of electrolyte was mixed with 35 µL of 10 mM dimethyl sulfoxide (DMSO) and 50 mM phenol used as internal standards in D2O for 1H NMR analysis. The 1H NMR spectrum was recorded with water suppression by a presaturation method. The relaxation time was 5 s.
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S4
TOF Calculation CV measurements were conducted at various scan rates in 0.5 M KHCO3 solution saturated with CO2 within the potential window from 0.025 to 0.225 V vs. RHE where no Faradaic reaction took place. The electrochemical double-layer (EDL) capacitance can be calculated by the equation below: 𝐶=
𝑖 𝑣
Where i is the current (mA) and v is the scan rate (mV s-1). We derived the EDL capacitance from the slope of the linear regression in the current-scan rate plot. Since the EDL capacitance is proportional to the actual number of PorCu molecules exposed in the electrolyte, TOF can be calculated by the equation below: 𝑇𝑂𝐹 =
𝑗 𝑗 𝑘𝑗 = = 𝑛𝑒𝑚 𝑛𝑒(𝐶𝑉/𝑘𝑒) 𝑛𝐶𝑉
Where j is the partial current (mA) of one product, n is the number of electrons transferred to produce one molecule of the product, e is the elementary charge (1.602×10-19 C), m is the actual number of exposed PorCu molecules, k is the number of elementary charges adsorbed on each molecule, C is the EDL capacitance (mF) and V is the potential window (V) of the CV measurements. Assuming one exposed PorCu molecule contributes to the EDL capacitance by adsorbing one elementary charge (e.g. a K+ ion), we have k = 1 and the TOFs can be calculated for all the CO2 reduction products.
Molecular integrity tests After 1 h of CO2 reduction electrocatalysis at –0.976 V vs. RHE, the PorCu catalyst was recovered from the working electrode by dissolving with methanol. Both absorption and fluorescence spectra of PorH, PorCu and the recovered PorCu were collected in methanol (Figure S12). The excitation wavelengths were 410, 413 and 413 nm for PorH, PorCu and the recovered PorCu, respectively. The weak fluorescence of the recovered PorCu (blue trace, bottom panel) indicates the presence of only a tiny amount of demetallated porphyrin, which is quantitated to be 1% by the equation below, assuming the fluorescence quantum yield of PorH is 1 (100%): ø𝑎 =
𝐴𝑏𝑠𝑟 𝐸𝑚𝑠𝑎 × × ø𝑟 𝐴𝑏𝑠𝑎 𝐸𝑚𝑠𝑟
Where øa is the fluorescence quantum yield of the recovered PorCu, Absr is the absorption 4
S5
intensity of PorH at the excitation wavelength, Absa is the absorption intensity of the recovered PorCu at the excitation wavelength, Emsa is the integrated emission area of the recovered PorCu, Emsr is the integrated emission area of PorH, and ør is the fluorescence quantum yield of the recovered PorCu. TEM and XPS measurements were also performed for samples prepared by drop casting the methanol solution containing the recovered PorCu on TEM grids and Si chips.
Computational Method All geometries were fully optimized along with the PCM aqueous continuum solvation model3 at the B3LYP level of density functional theory as implemented in Gaussian 094 using the LANL2DZ pseudopotential basis set on transition metal atoms5 and the 6-31G(d,p) basis set on H, C, N and O atoms.6 Analytic vibrational frequencies were computed to verify the nature of all stationary points. The structures were then used to compute the electronic energy using LANL2DZ pseudopotential for transition metal atoms5 and the 6-311+G(d,p) basis set on H, C, N and O atoms.7 To reduce systematic errors, the reduction potentials of PorCu0/- and PorCu-/2- vs. FeCp+/0 (ferrocinium/ferrocene redox pair) were calculated as Ered/ox(V vs. FeCp+/0) = -[Esolv(red, eV) + Esolv(FeCp+, eV) - Esolv(ox, eV) - Esolv(FeCp0, eV)] where Esolv refers to the solvated single-point energy and “red” and “ox” refer to the reduced and oxidized species in the redox pair, respectively.8 The calculated potentials were then converted to the RHE scale according to the following formula: E(V vs. RHE) = E(V vs. FeCp+/0) + 0.64 V + 0.0592 V*pH.
5
S6
Figure S1. (A) 1H NMR and (B) 13C NMR spectra of PorHCH3 in CDCl3.
6
S7
Figure S2. (A) 1H NMR and (B) 13C NMR spectra of PorH in CD3OD.
7
S8
Figure S3. Electronic absorption spectra of (A) PorCu (CH3OH), (B) PorHCH3 (CH2Cl2) and (C) PorH (CH3OH).
8
S9
Figure S4. A representative 1H NMR spectrum of liquid products generated from CO2 reduction catalyzed by PorCu. Phenol and DMSO were used as internal references.
9
S10 Figure S5. A representative GC-MS trace of gas products generated from CO2 reduction catalyzed by PorCu.
Figure S6. Partial current densities and Faradaic efficiencies of the main liquid product (formic acid) generated from CO2 reduction over the PorCu catalyst at various electrode potentials, measured after 1 h of electrocatalysis.
Figure S7. Faradaic efficiencies of gas products generated from CO2 reduction over the PorCu catalyst at various electrode potentials, measured 8 min after electrolysis was started.
10
S11
Figure S8. Time-dependent total current densities for the PorH, PorCH3Cu and PorCu electrodes at –0.976 V vs. RHE.
Figure S9. CV measurements and analysis of the PorCu electrode over time during CO2 reduction electrocatalysis at –0.976 V vs. RHE. (A) Nyquist plots (200 kHz ~ 1 Hz, inset showing a zoom in view of the high frequency region). (B) CV curves at various scan rates at 56 min after electrocatalysis was started. (C) The linear regression of the current-scan rate plots. (D) EDL capacitance.
11
S12
Figure S10. CV curves of the PorCu electrode taken at the scan rate of 100 mV s-1 before and after electrocatalysis at -0.976 V vs. RHE for ~1 h. No obvious redox peaks were observed immediately after the electrocatalysis was stopped, which we attribute to slow recovery to equilibrium. After one hour of rest, the CV curve shows redox peaks with larger areas than the ones observed before electrocatalysis, which is consistent with our finding that the electrochemically active surface area of the catalyst has increased during the catalysis.
Figure S11. SEM image of the PorCu electrode after 1 h of electrocatalysis at –0.976 V vs. RHE.
12
S13
Figure S12. Partial current densities and Faradaic efficiencies of gas products generated from CO2 reduction over PorCu electrodes over time at various electrode potentials.
13
S14
Figure S13. A flow chart showing a set of control experiments carried out to exclude the in situ formation of Cu(0) nanoparticles.
Figure S14. UV-Vis absorption and fluorescence spectra for PorH, PorCu and recovered PorCu in methanol.
14
S15
Figure S15. TEM images of PorCu after ~1 h of electrolysis at -0.976 V vs. RHE.
Figure S16. Cu 2p core level spectra for PorCu before and after electrolysis at –0.976 V vs. RHE for ~1 h.9, 10
15
S16
Figure S17. Partial current and Faradaic efficiencies of gas products generated from CO2 reduction on a methanol-washed PorCu electrode over time at –0.976 V vs. RHE.
Figure S18. Faradaic efficiencies of gas products generated from CO2 reduction on the PorCu electrode with the electrode potential cycled between –0.676 and –0.976 V over time.
16
S17
Figure S19. Partial current densities and Faradaic efficiencies of gas products generated from CO2 reduction catalyzed by the PorCH3Cu over time at –0.976 V vs. RHE.
17
S18 Table 1. Comparison with other metal-porphyrin and Cu based electrocatalyst materials for CO2 electroreduction Potential
j Total1
FE2
Main
/V vs RHE
/mA cm-2
/%
Products (FE)
Catalyst
CH4 (27%), PorCu
-0.976
49
59
C2H4 (17%), CO (10%)
Cu phthalocyanine
Fe porphyrin
-1.6
-1.16 (V
N/A
~50
CO, HCOOH
KHCO3 0.4M EtNCO2CH3 + 0.1M NBu4PF6 in DMF + 2M H2O
94
CO
-0.67
~3.2
91
CO
-0.8
1.32
9
CH4 (2.3%)3
Co porphyrin
protoporphyrin
KHCO3 0.5M
COF comprising
Co
0.5M
CH4 (30%),
0.31
vs NHE)
Electrolyte
0.5M KHCO3
Reference
This work
11
12
2
0.1M HClO4
13
0.1M KCl
14
CH4 (7%), Cu2O/Cu
-1
3
~62
C2H4 (12%), CO, HCOOH CH4 (18%),
Cu
-1.01
~4
~64
C2H4 (18%), CO, HCOOH
Cu nanoparticles Reduced Cu2O film
-1.35
12
76
CH4 CO (40%),
-0.55
2.6
1
Total current.
2
Faradaic efficiency for CO2 reduction.
75
HCOOH (33%)
18
0.1M KHCO3 0.1M NaHCO3 0.5M NaHCO3
15
16
17
S19 3
P(CO2) = 10 atm.
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