Toward Visible-Light Photochemical CO2-to-CH4 Conversion in

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Towards Visible-Light Photochemical CO-to-CH Conversion in Aqueous Solutions Using Sensitized Molecular Catalysis Heng Rao, Julien Bonin, and Marc Robert J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00950 • Publication Date (Web): 28 Apr 2018 Downloaded from http://pubs.acs.org on April 29, 2018

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The Journal of Physical Chemistry

Towards Visible-light Photochemical CO2-to-CH4 Conversion in Aqueous Solutions using Sensitized Molecular Catalysis Heng Rao, Julien Bonin* and Marc Robert* Laboratoire d’Electrochimie Moléculaire, UMR 7591, Université Paris Diderot, Sorbonne Paris Cité, CNRS, 15 rue Jean-Antoine de Baïf, F-75205 Paris, France.

ABSTRACT: Solar fuels may be generated upon visible light induced catalytic reduction of carbon dioxide. This appealing approach remains highly challenging, especially when using earth abundant catalysts, mild conditions and water as a solvent. Upon employing an iron tetraphenyl porphyrin complex substituted by positively charged trimethylammonio groups at the para position of each phenyl ring, and after reduction with three electrons by the excited state of an iridium sensitizer (λ > 420 nm), CO2 could be reduced to CO and to CH4 in both acetonitrile and aqueous solutions (acetonitrile/water 3:7 v:v) with good selectivity. Stability of the catalytic system remains a weakness and the reasons were analyzed.

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1. INTRODUCTION Storing solar light energy into chemical bonds has recently attracted a lot of attention.1 It includes the generation of solar fuels through water splitting and carbon dioxide reduction. High challenges are still to design efficient, selective, stable and scalable catalytic systems. Regarding carbon dioxide, it could be reduced into various fuels such as methanol and methane, although catalytic systems leading to such highly reduced compounds remain rare, as well as commodity chemicals or precursors to fuels, like e.g. CO that can further be used in Fischer-Tropsch chemistry.2-5 For activating CO2 reduction using light, catalytic materials have been employed. Alternatively, molecular catalysts based on earth abundant metals have also been proposed, the two main reaction products being carbon monoxide or formate.6-8 Among these abundant metals, Ni, Co, Mn and Fe-based complexes including polypyridines, carbonyl and porphyrin ligands have been shown to be the most efficient compounds.9-15 Copper containing complexes have been rarely employed, with two recent, notable exceptions leading to highly efficient CO production.16,17 In all these cases, reactions have been done in organic solvent, mainly acetonitrile (ACN) and N,N-dimethylformamide (DMF). In contrast, the photochemical CO2-toCO conversion in water has only been achieved in few cases, with modest turnover number and limited stability over time.18-21 We have recently demonstrated the remarkable performance of Fe-p-TMA catalyst (Scheme 1) under visible light irradiation to both achieve the two electrons CO2-to-CO catalytic reduction in aqueous solutions (acetonitrile/water 1:9 v/v)22 and the eight electrons CO2-to-CH4 conversion in ACN when combined with an iridium-based photosensitizer,23 belonging to a well-known class of compounds for photoredox catalysis.24 We now show that CH4 generation can be realized in aqueous solutions (ACN/water 3:7 v:v) when using a water-compatible analog of the iridium complex sensitizer, [Ir(ppy)2(bpy)]+ (Scheme 1).


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This sensitizer presents relevant photophysical properties such as a long excited state lifetime (several hundreds of nanoseconds)25-28 and a highly negative redox potential (ca. -1.45 V vs SCE)29-31 in its reduced form. Although the performances remain limited by the instability of the sensitizer upon prolonged irradiation, it opens the door for the use of water as solvent in the generation of solar fuels from CO2 catalysis in homogeneous systems. Scheme 1. Structure of the Fe-p-TMA catalyst (left) and of the [Ir(ppy)2(bpy)]+ photosensitizer (right).

2. METHODS 2.1.

Chemicals.

The

preparation

of

chloro

iron(III)

5,10,15,20-tetra(4’-N,N,N-

trimethylanilinium)porphyrin (Fe-p-TMA 1) has been previously described.32 fac-(tris-(2phenylpyridine))iridium(III)

(Ir(ppy)3,

Aldrich,

99%),

(2,2'-bipyridine)bis(2-

phenylpyridinato)iridium(III) hexafluorophosphate (Ir(ppy)2(bpy),PF6 TCI, >90%), triethylamine (TEA,

Acros

Organics,

99%),

triethanolamine

(TEOA,

Sigma,

>

99

%),

N,N-

diisopropylethylamine (DIPEA, Fisher Scientific, > 99%), 2,2,2-trifluoroethanol (TFE, Romil Ltd, > 99.5 %,) and acetonitrile (ACN, Acros Organics, 99.9 %) were used without further purification. Tetrabutylammonium hexafluorophosphate (TBAPF6, Sigma–Aldrich, 98%) was recrystallized twice from ethanol and dried before use. Ultrapure water was from a TKA MicroPure system. Argon (> 99.998%), CO2 (> 99.7%) and CO (> 99.997%) were from Air


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Liquide, whereas CH4 (> 99 %) was from Fluka. H2 was obtained from a hydrogen generator (FDBS NMH2 500). 2.2. Photochemical catalysis. Irradiation of either CO2- or argon-saturated 3.5 mL solution containing the catalyst, the photosensitizer and the sacrificial electron donor were conducted in a closed 1 cm × 1 cm quartz suprasil cuvette (Hellma 117.100F-QS) equipped with home-designed headspace glassware for further gaseous product quantification. A Newport LCS-100 solar simulator, equipped with an AM1.5 G standard filter allowing 1 Sun irradiance and combined with a Schott GG420 longpass filter and a 2-cm-long glass (OS) cell filled with deionized water to prevent catalyst absorbance and to cut off infrared and low ultraviolet, was used as the light source and was placed at right angle of the sample. The iron ferrioxalate (K3Fe(C2O4)3) chemical actinometer was used to estimate the number of incident photons to the sample to ca. (2.2 ± 0.2) × 1019 photons per hour. 2.3. Absorption, emission and infrared spectroscopy. UV-Visible absorption spectra were recorded with an Analytik Jena Specord 600 spectrophotometer. Emission quenching measurements were conducted with an Agilent Technologies Cary Eclipse fluorescence spectrophotometer with the excitation wavelength set at 420 nm. Emission intensities used for the Stern-Volmer analysis of were taken at 592 nm, the emission maximum of *[Ir(ppy)2(bpy)]+. Infrared spectra were recorded on a Perkin Elmer Spectrum BX FTIR spectrometer equipped with a Specac Omni Cell P/N 800 measurement cell. 2.4. Time-resolved absorption spectroscopy. Transient absorption measurements were conducted with an Edinburgh Instruments LP920-KS laser flash photolysis spectrometer. The solutions were excited at 420 nm (5 ns pulse, 5 to 8 mJ cm-2) via a Continuum SLOPO Plus OPO


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pumped by a frequency-tripled Continuum Surelite II-10 Nd:YAG laser. Right-angle probe light was provided by an Osram XBO 450 W ozone free pulsed xenon lamp and then collected into a spectrograph. Kinetics at 592 nm for *[Ir(ppy)2(bpy)]+ was measured thanks to a Hamamatsu R928 photomultiplier tube linked to a Tektronix TDS 3012C 100 MHz oscilloscope. Temperature of the sample port was set at 20°C by a Quantum Northwest TC125 Peltier effect controller. The Edinburgh Instruments L900 software ensured the control and the synchronization of the whole setup. 2.5. Gas chromatography and turnover calculation. Gaseous products analysis was performed with an Agilent Technology 7820A gas chromatography (GC) system set with a CarboPLOT P7 capillary column (25 m length and 25 mm inner diameter) and a thermal conductivity detector. Calibration curves for H2, CO and CH4 were established separately. Control experiments, with no catalyst, no sensitizer, no CO2 or no light were conducted under the same conditions otherwise as the full system and revealed no product formation. Turnover number (TON) was defined as the number of catalytic cycles per catalyst amount. The number of moles of H2, CO and CH4 was determined by converting peak integrations from GC data into moles in the sample headspace by using individual calibration curves and taking into account the irradiated sample volume. Typical uncertainty on TON, based on repeated experiments, is about 5%, thus corresponding to the size of the data points in Figures 1, 2 and 3. 2.6. Electrochemical measurements. Cyclic voltammetry experiments were performed using a Metrohm Autolab 302N potentiostat. 0.5 mM ACN solution of [Ir(ppy)2(bpy)]+ was prepared with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte and then degassed with argon or carbon monoxide for 20 minutes. Voltammograms were recorded at 0.1 V/s in a typical three electrodes cell with a 3 mm diameter glassy carbon working


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electrode, a platinum wire as counter electrode and a saturated calomel electrode as reference electrode. 3. RESULTS AND DISCUSSION Before testing the catalytic system in aqueous solutions, we conducted a series of experiments in ACN (Fig. 1) varying the nature of the tertiary amine used as sacrificial electron donor (SD) and with CO2 as reactant. Similarly to what has been observed with Ir(ppy)3 sensitizer, CO, H2 and CH4 were produced with CO being the major product (ca. 60% catalytic selectivity with a maximum TON of 178, Table 1), while methane was produced with ca. 10% selectivity (TON of 32 with TEA as sacrificial donor). Table 1. Turnover number (TON, ± 5%) and catalytic selectivity (CS) of a 2 µM Fe-p-TMA CO2-saturated solution in the presence of 0.2 mM photosensitizer (PS) and 0.05 M sacrificial electron donor (SD) after 47 h of visible (> 420 nm) light irradiation.

PS

Solvent

SD

Ir(ppy)2(bpy)

ACN

Ir(ppy)2(bpy)

Ir(ppy)3

TON

CS (%)

H2

CO

CH4

H2

CO

CH4

TEA

103

178

32

33

57

10

TEOA

67

134

23

30

60

10

DIPEA

77

151

24

31

60

9

TEA

5

24

3

16

75

9

TEOA

4

19

3

15

73

12

DIPEA

4

20

3

15

74

11

ACNa

TEA

24

198

31

10

78

12

ACNb

TEA

18

-

89

17

-

83

ACNc

TEA

19

-

100

16

-

84

ACN/H2O


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ACNd

TEA

45

-

195

19

-

81

a

Under CO2 atmosphere Ref 8, Table 1, entry 2. b Under CO atmosphere, Ref 8, Table 1, entry 10. c Under CO atmosphere + 0.5 M TFE, this study. d Under CO atmosphere + 0.5 M TFE, this study (102 h irradiation).

Using TEA as SD led to the highest turnover number for the reduction products (CO, CH4) among the three amines tested (TEA, DIPEA and TEOA), but no major difference could be observed in terms of both efficiency and catalytic selectivity (Table 1). We then conducted a series of experiments using a mix of 30% ACN and 70% water (v/v) as the reaction solvent, chosen to maximize the water proportion while allowing a suitable solubility for [Ir(ppy)2(bpy)]+ to ensure an efficient light absorption. Keeping all other parameters (including concentration) identical, we observed (Fig. 2) a lower amount of all reduction products, which may be ascribed to a significantly lower solubility of the reactant (CO2) in the reaction mixture. The sacrificial electron donor giving the best efficiency is TEA, once again with no major difference in terms of efficiency and catalytic selectivity for the three SDs that have been employed (Table 1). Regarding methane formation, catalytic selectivity was close to 10%, with a maximum TON of 3 obtained with all SDs. Overall, a ca. 75% catalytic selectivity for CO was measured, corresponding to a TON in the 20-25 range. At first look, these results are not surprising since all SDs have relatively close oxidation potentials (0.5 - 0.8 V vs SCE range)33,34, however TEOA has a lower pKa (7.9)34 than TEA (10.7) and DIPEA (10.5)35 so one could have expected a higher generation of H2 in the former case, which was not observed. To help understanding the reaction mechanism in aqueous solutions, we conducted emission quenching experiments upon exciting the [Ir(ppy)2(bpy)]+ sensitizer at 420 nm in the presence of increasing concentration of TEA, TEOA, DIPEA or Fe-p-TMA (Fig. S1 to S5). Analysis of emission quenching using the Stern-Volmer analysis and the excited-state lifetime that was


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measured by time-resolved laser kinetics (145 ns, see Fig. S6), give quenching rate constants of 5.8×106, 4.6×106, 1.4×108 M-1 s-1 for TEA, TEOA and DIPEA, respectively, while no quenching was detected with Fe-p-TMA (Table S1).

Figure 1. CO (black squares), H2 (red circles) and CH4 (blue diamonds) formation (turnover numbers) upon visible-light irradiation (λ > 420 nm) of a CO2-saturated ACN solution containing 2 µM Fe-p-TMA catalyst, 0.2 mM [Ir(ppy)2(bpy)]+ sensitizer and 0.05 M TEA (top), TEOA (middle) or DIPEA (bottom) sacrificial electron donor.


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These results first revealed that the excited sensitizer is quenched via a reductive pathway from the sacrificial amine and is thus reduced into the anionic form [Ir(ppy)2(bpy•‒)] (see below), as reported before.36-38 Second, the quenching rate constants are well below the diffusion-limit, as

Figure 2. CO (black squares), H2 (red circles) and CH4 (blue diamonds) formation (turnover numbers) upon visible-light irradiation (λ > 420 nm) of a CO2-saturated ACN/H2O (3:7 v/v) solution containing 2 µM Fe-p-TMA catalyst, 0.2 mM [Ir(ppy)2(bpy)]+ sensitizer and 0.05 M TEA (top), TEOA (middle) or DIPEA (bottom) sacrificial electron donor.


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already reported39-41 so that the initial electron transfer steps leading to the catalytic active species (Fe0) are of low efficiency. It is also important to note that these characteristics are in contrast with what we observed before with Ir(ppy)3 in organic solvent. In that later case, the quenching process goes through an oxidative electron transfer with the catalyst at a rate constant close to the diffusion limit.23 In addition, even if the quenching rate constant is ca. 25 times faster for DIPEA than for TEA, TON with DIPEA remains lower than for TEA, thus highlighting, as already noticed, the absence of simple rationalization in the choice of a particular amine as SD.34 When employing the neutral Ir(ppy)3 as sensitizer in organic media, we previously determined that CO was a key intermediate product in the CO2 reduction process, and that the FeIICO adduct species was further reduced in a multi-electron multi-proton process to afford CH4.23 Without any addition of acid, residual water and/or SD protonated form could act as the proton source. Addition of a weak acid such as trifluoroethanol (TFE) accelerates the reaction and as shown in Fig. 3, optimized conditions consisting in the addition of 0.5 M TFE allow the system to reach, after 102 h irradiation, 195 TON in CH4, with a selectivity of 81% (see also Table 1). Using larger acid concentration or a stronger acid may lead to selectivity decrease and larger amount of hydrogen.


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Figure 3. Gaseous products formation (turnover numbers) upon visible-light irradiation (λ > 420 nm) of a CO-saturated ACN solution containing 2 µM Fe-p-TMA catalyst, 0.2 mM Ir(ppy)3 sensitizer, 0.05 M TEA as sacrificial electron donor and 0.5 M TFE as proton source. We further transposed this approach to the aqueous system containing [Ir(ppy)2(bpy)]+ as photosensitizer and performed irradiation in CO saturated solutions. We observed in this case a different behavior. After few tens of minutes of visible-light irradiation, the color of the solution drastically faded and a precipitate finally appeared, as showed in Fig. 4, whereas no change at all was observed during similar experiment conducted under CO2 atmosphere. The observed precipitation could be ascribed to a structural modification of the iridium sensitizer giving an insoluble form. Otherwise, it has been reported42 that cationic [Ir(ppy)2L2]+ complexes (L being ancillary ligand such as pyridine, acetonitrile) can bind to CO and/or to acetonitrile and optical signatures of these complexes give peaks close to 450 and 485 nm respectively, the latter being observable in our experiment (Fig. 4).


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Figure 4. Spectral evolution upon visible-light irradiation (λ > 420 nm) for a CO-saturated ACN/H2O (3:7 v/v) solution containing 2 µM Fe-p-TMA as catalyst, 0.2 mM [Ir(ppy)2(bpy)]+ as sensitizer and 0.05 M TEA as electron donor We thus hypothesize that electron transfer from the sacrificial electron donor to the excited sensitizer induces the loss of the bpy ligand43 and its replacement by CO and/or ACN ligands, as illustrated in Scheme 2. To confirm this hypothesis, we conducted infrared spectroscopy measurement (Fig. S7) which revealed two vibrations at ca. 2255 and 2295 cm-1 which closely match with CO vibration in Ir(CO)x complexes.44,45 Cyclic voltammetry (Fig. S8) also revealed the apparition of an irreversible reduction wave at ca. -2 V vs. SCE under CO, as previously observed.42 Scheme 2. Possible mechanism leading to the decomposition of [Ir(ppy)2(bpy)]+ in the presence of CO as substrate.


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Finally, upon electron transfer to the *[Ir(ppy)2(bpy)]+ sensitizer from a sacrificial reductant such as TEA, the spin density in the reduced sensitizer is predominantly located on the bpy ligand, generating a bpy•‒ radical anion.46 Such electron localization in the antibonding LUMO contributes to the poor observed photostability and favors the bpy ligand loss. Such lower stability of cationic IrIII complexes compared to neutral cyclometalated IrIII complexes (such as Ir(ppy)3) could be assigned to the stronger ligand field of the C^N ligands compared to the N^N coordinating ligand.38 4. CONCLUSIONS Photochemical reduction of CO2 has been achieved using an iron porphyrin as catalyst, an iridium complex [Ir(ppy)2bpy]+ as sensitizer and various amines as sacrificial electron donor. Upon visible light irradiation, CO was the main product (178 TON in optimized conditions) and a substantial amount of CH4 was also produced (32 TON, 10% catalytic selectivity) while H2 was formed as a minor by-product when acetonitrile was used as a solvent. Among the various amine employed, TEA appeared as the best choice to maximize CO2 product formation. Remarkably, the CO2 catalytic reduction could also be realized in aqueous conditions (acetonitrile/water 3:7 v:v). In that case, the main product was again carbon monoxide while methane was produced with selectivity up to 10% (3 TON). Efficiency of the catalytic system is hampered by the limited CO2 solubility and most importantly by instability of the sensitizer that leads to CO ligandation and further loss of activity. Despite these limitations, this work opens the


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door to further studies in pure water once more stable sensitizers have been designed. Studies along these lines will be reported in due time. ASSOCIATED CONTENT The Supporting Information (PDF file), containing emission quenching experiments, timeresolved absorption kinetics, infrared and voltammetry data, is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.xxxxxx AUTHOR INFORMATION Corresponding Author *E-mail [email protected], Tel +33 1 57 27 57 93 (J.B.); [email protected], Tel +33 1 57 27 87 90 (M.R.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ORCID Heng Rao: 0000-0001-7270-3301 Julien Bonin: 0000-0001-9943-0219 Marc Robert: 0000-0001-7042-4106 Notes


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The authors declare no competing financial interest.

ACKNOWLEDGMENTS H.R. thanks the China Scholarship Council for his PhD fellowship (CSC student number 201507040033). ABBREVIATIONS ACN,

acetonitrile;

DMF,

N,N-dimethylformamide;

GC,

gas

chromatography;

TFE,

trifluoroethanol; TEA, triethylamine, TEOA, triethanolamine; DIPEA, diisopropylethylamine, SD, sacrificial donor; PS, photosensitizer. REFERENCES (1) Kamat, P. V. Semiconductor Surface Chemistry as Holy Grail in Photocatalysis and Photovoltaics. Acc. Chem. Res. 2017, 50, 527-531. (2) Jhong, H.-R. M.; Ma, S.; Kenis, P. J. A. Electrochemical Conversion of CO2 to Useful Chemicals: Current Status, Remaining Challenges, and Future Opportunities. Curr. Opin. Chem. Eng. 2013, 2, 191-199. (3) Aresta, M.; Dibenedetto, A.; Angelini, A. Catalysis for the Valorization of Exhaust Carbon: from CO2 to Chemicals, Materials, and Fuels. Technological Use of CO2. Chem. Rev. 2014, 114, 1709-1742. (4) Kim, W.; McClure, B. A.; Edri, E.; Frei, H. Coupling Carbon Dioxide Reduction with Water Oxidation in Nanoscale Photocatalytic Assemblies. Chem. Soc. Rev. 2016, 45, 3221-3243. (5) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355, eaad4998. (6) Schneider, J.; Jia, H.; Muckerman, J. T.; Fujita, E. Thermodynamics and Kinetics of CO2, CO, and H+ Binding to the Metal Centre of CO2 Reduction Catalysts. Chem. Soc. Rev. 2012, 41, 2036-2051. (7) Sahara, G.; Ishitani, O. Efficient Photocatalysts for CO2 Reduction. Inorg. Chem. 2015, 54, 5096-5104. (8) Takeda, H.; Cometto, C.; Ishitani, O.; Robert, M. Electrons, Photons, Protons and EarthAbundant Metal Complexes for Molecular Catalysis of CO2 Reduction. ACS Catal. 2017, 7, 7088.


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(9) Bonin, J.; Robert, M.; Routier, M. Selective and Efficient Photocatalytic CO2 Reduction to CO Using Visible Light and an Iron-Based Homogeneous Catalyst. J. Am. Chem. Soc. 2014, 136, 16768-16771. (10) Rosas-Hernandez, A.; Alsabeh, P. G.; Barsch, E.; Junge, H.; Ludwig, R.; Beller, M. Highly Active and Selective Photochemical Reduction of CO2 to CO using Molecular-Defined Cyclopentadienone Iron Complexes. Chem. Commun. 2016, 52, 8393-8396. (11) Takeda, H.; Ohashi, K.; Sekine, A.; Ishitani, O. Photocatalytic CO2 Reduction Using Cu(I) Photosensitizers with a Fe(II) Catalyst. J. Am. Chem. Soc. 2016, 138, 4354-4357. (12) Alsabeh, P. G.; Rosas-Hernandez, A.; Barsch, E.; Junge, H.; Ludwig, R.; Beller, M. IronCatalyzed Photoreduction of Carbon Dioxide to Synthesis Gas. Catal. Sci. Technol. 2016, 6, 3623-3630. (13) Guo, Z.; Cheng, S.; Cometto, C.; Anxolabéhère-Mallart, E.; Ng, S.-M.; Ko, C.-C.; Liu, G.; Chen, L.; Robert, M.; Lau, T.-C. Highly Efficient and Selective Photocatalytic CO2 Reduction by Iron and Cobalt Quaterpyridine Complexes. J. Am. Chem. Soc. 2016, 138, 9413-9416. (14) Ouyang, T.; Hou, C.; Wang, J.-W.; Liu, W.-J.; Zhong, D.-C.; Ke, Z.-F.; Lu, T.-B. A Highly Selective and Robust Co(II)-Based Homogeneous Catalyst for Reduction of CO2 to CO in CH3CN/H2O Solution Driven by Visible Light. Inorg. Chem. 2017, 56, 7307-7311. (15) Cao, L.-M.; Huang, H.-H.; Wang, J.-W.; Zhong, D.-C.; Lu, T.-B. The Synergistic Catalysis Effect within a Dinuclear Nickel Complex for Efficient and Selective Electrocatalytic Reduction of CO2 to CO. Green Chem. 2018, 20, 798-803. (16) Guo, Z.; Yu, F.; Yang, Y.; Leung, C.-F.; Ng, S.-M.; Ko, C.-C.; Cometto, C.; Lau, T.-C.; Robert, M. Photocatalytic Conversion of CO2 to CO by a Copper(II) Quaterpyridine Complex. ChemSusChem 2017, 10, 4009-4013. (17) Zhong, D.-C.; Liu, W.-J.; Huang, H.-H.; Ouyang, T.; Jiang, L.; Zhang, W.; Lu, T.-B. A Copper(II) Molecular Catalyst for Efficiently and Selectively Photochemical Reduction of CO2 to CO in a Water-containing System. Chem.-Eur. J. 2018, Early View, doi: 10.1002/chem.201705566. (18) Tinnemans, A. H. A.; Koster, T. P. M.; Thewissen, D. H. M. W.; Mackor, A. TetraazaMacrocyclic Cobalt(II) and Nickel(II) Complexes as Electron-Transfer Agents in the Photo(electro)chemical and Electrochemical Reduction of Carbon Dioxide. Recl. Trav. Chim. Pays-Bas 1984, 103, 288-295. (19) Mochizuki, K.; Manaka, S.; Takeda, I.; Kondo, T. Synthesis and Structure of [6,6‘-Bi(5,7dimethyl-1,4,8,11-tetraazacyclotetradecane)]dinickel(II) Triflate and Its Catalytic Activity for Photochemical CO2 Reduction. Inorg. Chem. 1996, 35, 5132-5136. (20) Neri, G.; Forster, M.; Walsh, J. J.; Robertson, C. M.; Whittles, T. J.; Farras, P.; Cowan, A. J. Photochemical CO2 Reduction in Water using a Co-Immobilised Nickel Catalyst and a Visible Light Sensitiser. Chem. Commun. 2016, 52, 14200-14203. (21) Kuehnel, M. F.; Orchard, K. L.; Dalle, K. E.; Reisner, E. Selective Photocatalytic CO2 Reduction in Water through Anchoring of a Molecular Ni Catalyst on CdS Nanocrystals. J. Am. Chem. Soc. 2017, 139, 7217-7223. (22) Rao, H.; Bonin, J.; Robert, M. Visible-light Homogeneous Photocatalytic Conversion of CO2 into CO in Aqueous Solutions with an Iron Catalyst. ChemSusChem 2017, 10, 4447-4450. (23) Rao, H.; Schmidt, L. C.; Bonin, J.; Robert, M. Visible-light-driven Methane Formation from CO2 with a Molecular Iron Catalyst. Nature 2017, 548, 74-77. (24) Koike, T.; Akita, M. Visible-light Radical Reaction Designed by Ru- and Ir-based Photoredox Catalysis. Inorg. Chem. Front. 2014, 1, 562-576.


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TOC Graphic (updated)


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Toward Visible-Light Photochemical CO2-to-CH4 Conversion in

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