# Visible-light Homogeneous Photocatalytic Conversion of CO₂ into CO in Aqueous Solutions with an Fe catalyst

Heng Rao,[a] Julien Bonin*[a] and Marc Robert*[a]

Laboratoire d'Electrochimie Moléculaire, UMR CNRS 7591, Université Paris Diderot, Sorbonne Paris Cité, 15 Rue Jean-Antoine de Baïf, F-75205 Paris (France)

E-mail: julien.bonin@univ-paris-diderot, robert@univ-paris-diderot.fr

**Abstract:** We used an iron substituted tetraphenyl porphyrin bearing positively charged trimethylammonio groups at the para position of each phenyl ring for the photoinduced conversion of CO₂. This complex is water soluble and acts as a molecular catalyst to selectively reduce CO₂ into CO under visible light irradiation in aqueous solutions (acetonitrile:water 1:9 v:v) with the assistance of purpurin, a simple organic photosensitizer. CO is produced with a catalytic selectivity of 95% and turnover up to 120, illustrating the possibility of photo-catalysing the reduction of CO₂ in aqueous solution using visible light, a simple organic sensitizer coupled to an amine as sacrificial electron donor and an earth abundant metal based molecular catalyst.

## Introduction

Converting solar light into storable, re-usable forms of energy has recently attracted a lot of attention.<sup>1-2</sup> It notably includes the generation of solar fuels through water splitting and carbon dioxide reduction but it remains a great challenge for chemists.<sup>3-6</sup> Regarding the latter reaction, a number of molecular catalysts based on earth abundant metals have been proposed, the two main reaction products being carbon monoxide or formate.<sup>7</sup> Ni, Co, Mn and Fe based complexes including polypyridines, carbonyl and porphyrin ligands have been shown as the most efficient compounds.<sup>8-13</sup> Recently, we demonstrated that an Fe porphyrin was able to catalyze the eight electrons reduction of CO₂ to methane.<sup>14</sup>

All these catalytic reactions have been performed in organic, aprotic solvent, such as acetonitrile (ACN) or N,N-dimethylformamide (DMF). Aqueous CO₂ reduction remains rare and challenging due to H₂ evolution competition and relatively low CO₂ concentration. Photochemical CO₂ conversion to CO in water with inexpensive molecular catalysts has thus been reported only in very few cases. It includes the use of Co(cyclam) at pH 4 with a maximum turnover number (TON) of 22,<sup>15</sup> and the use of Ni(cyclam) at pH between 4 and 6 with a maximum TON of 4.4,<sup>16-20</sup> with [Ru(bpy)₃]²⁺ as dye. Recently a Ni complex bearing a 2,2':6,2"-terpyridine ligand and anchored at the surface of CdS quantum dots has been shown to reach a maximum TON of 20 at pH 6.5, with a catalytic selectivity of 90% after a few hours irradiation, progressively decreasing to produce mainly dihydrogen after 24 h.<sup>21</sup> Herein we report that a tetraphenyl Fe porphyrin bearing trimethylammonio substituents at all para positions of the four phenyl rings (Fe-p-TMA, Scheme 1) is a very selective and stable catalyst for the photochemical conversion of CO₂-to-CO in aqueous solution (pH 6.7) under visible light irradiation (> 420 nm) using a cheap organic photosensitizer (purpurin PP, Scheme 1).

<figure><img src="image_1.png" /><figcaption>Fe-p-TMA</figcaption></figure>

<figure><img src="image_2.png" /><figcaption>Purpurin</figcaption></figure>

**Scheme 1.** Molecular structures of the catalyst (Fe-p-TMA) and of the photosensitizer (purpurin, PP).

## Results and Discussion

Photocatalytic CO₂ reduction by Fe-p-TMA (Scheme 1) was studied in pH 6.7 aqueous solutions containing 0.1 M NaHCO₃ with the minimum amount of CH₃CN (ACN, 10%) so as to solubilize purpurin (PP, Scheme 1) employed as sensitizer. At this pH, PP is in its monoanionic form (pKs 4.7, 9.5)<sup>22</sup> and thus will be indicated as PP in the following. Several amines were used as sacrificial electron donors (SD). The best results were obtained with triethylamine (TEA). As illustrated in Figure 1 and in Table 1 (entry 1), CO is produced upon visible light irradiation (λ > 420 nm). The amount of CO keeps increasing for about two days and then levels off, with a turnover
number (TON) of 60. Small amount of H₂ is also produced with a TON of 5. Hence the selectivity for CO production is 95%. In the absence of catalyst no product was detected, while photocatalysis using ¹³C-labelled CO₂ lead to ¹³CO production as detected by GC-MS, confirming that carbon monoxide was issued from CO₂ reduction.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** CO (black symbols) and H₂ (red symbols) production upon visible light irradiation (> 420 nm) of a CO₂-saturated ACN:H₂O (1:9, v:v) solution containing 2 µM **Fe-p-TMA**, 0.2 mM **PP**, 0.1 M NaHCO₃ and 0.05 M **SD**: TEA (●, ■), TEOA (▲, ◆) or EDTA (▼, ●).</figcaption></figure>

Interestingly, with no **PP**\*, a TON of 6 was obtained for CO after 47h irradiation, showing that the Fe catalyst is also able to slowly convert CO₂ into CO without any sensitizer. When the amine was omitted, TONs of 8.4 for CO and 1.6 for H₂ were measured, indicating that water itself could act as a sacrificial electron donor. Figure 1 illustrates the effect of the nature of the amine used. TEA gave slightly better results than triethanolamine (TEOA) and ethylenediamine tetraacetic acid (EDTA), although TON are not very different (Table 1, entries 3-4), falling in line with the fact all three donors have similar redox potentials.²³ It is remarkable that the selectivity for CO formation remained very high even in these aqueous conditions. As discussed elsewhere,⁸,²⁴ the active species reacting with CO₂ is the reduced Fe⁰ porphyrin, implying that the later compound does not lead to hydride formation followed by hydrogen evolution. It also indicates that the porphyrin ring itself is not reductively hydrogenated since it would have led to rapid degradation of the catalyst, as previously seen with similar derivatives.²⁵ These results are also in accordance with recent electrochemical studies which showed that **Fe-p-TMA** is one of the few molecular catalysts keeping its high activity towards CO₂ in aqueous solutions.²⁶ One major reason for the catalytic activity leveling off is due to the photosensitizer instability, as illustrated in Figure 2 where the main absorption peak of **PP**\* (at 510 nm) starts decreasing after a couple of hours, reaching a 60% lessening after 47h. **PP**\* photobleaching has otherwise been described.²⁷ Re-adding **PP**\* to the solution mixture after two days of irradiation almost fully restored the catalytic activity (Figure 3), with a TON of 120 for CO and still 95% catalytic selectivity at the end of a four days experiment (Table 1, entry 6). When initially using a more concentrated solution of the photosensitizer (0.4 mM **PP**\*), more CO was also produced (TON 71, see Table 1, entry 2) again with maintaining the same excellent catalytic selectivity towards carbon dioxide reduction. Upon adding more catalyst to the solution after 47h of irradiation, only a slight increase of the TON for CO was observed (Figure 3 left and Table 1, entry 3), confirming that sensitizer stability over time is the main factor explaining the leveling off formation of the reduction product. It also falls in line with recent studies showing that **Fe-p-TMA** remains stable even after extensive visible light irradiation.¹⁴,²⁴

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Spectral evolution with time upon visible irradiation (> 420 nm) of a CO₂-saturated ACN:H₂O (1:9, v:v) solution containing 2 µM **Fe-p-TMA**, 0.2 mM **PP**, 0.1 M NaHCO₃ and 0.05 M TEA.</figcaption></figure>

Quenching experiments of a **PP**\* solution excited at 420 nm showed that **PP**\* reacts with TEA extremely fast, close to the diffusion limit since the quenching rate using the Stern-Volmer equation was measured to equal 1.2×10¹⁰ L mol⁻¹ s⁻¹ (Figure 4).
In contrast, PP\* is only poorly quenched by Fe-p-TMA, as shown in the Supporting Information (see Figure S3 and ensuing explanations for details). The reduced purpurin species PP\* has a standard redox potential (E\* = -0.66 V vs. SCE, Figure S2) negative enough to reduce the Fe\* porphyrin to Fe\*, since E\* (Fe\*\*/Fe\*) \approx -0.3 V vs. SCE.²⁶ It can however not reduce Fe\* (E\* (Fe\*\*/Fe\*)) \approx -1 V vs. SCE) and Fe\* (E\* (Fe\*\*/Fe\*)) \approx -1.3 V vs. SCE) species,²⁶ the driving forces being then largely negative. PP\* being a stable species, it may be excited to PP\*\* and further quenched by TEA to the doubly reduced species PP\*\*. The redox standard potential of the former species was determined by cyclic voltammetry and is equal to -0.83 V vs. SCE (Figure S2). PP\*\* could then react by electron transfer with both Fe\* porphyrin and further reduce Fe\* to the catalytically active Fe\* species. The series of the following electron transfer steps (i)-(iii) (the global charge is also indicated for all porphyrins) could thus be proposed:

$$ [\text{Fe}^{\text{III}}\text{-p-TMA} - \text{Cl}^{-}]^{4+} + \text{PP}^{2-} \rightarrow [\text{Fe}^{\text{II}}\text{-p-TMA}]^{4+} + \text{PP}^{-} + \text{Cl}^{-} \quad (i) $$

<figure><img src="image_5.png" /><figcaption>**Figure 3.** CO (black squares) and H₂ (red circles) production upon visible light irradiation (> 420 nm) of a CO₂-saturated ACN:H₂O (1:9, v:v) solution containing 2 µM Fe-p-TMA, 0.2 mM PP, 0.1 M NaHCO₃ and 0.05 M TEA. After 47 hours irradiation, 1 equivalent of Fe-p-TMA catalyst (left) or of PP photosensitizer (right) was added to the solution.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Emission spectra of a 0.2 mM PP in ACN:H₂O (1:9, v:v) solution with increasing concentration of TEA. Inset: linear plot (R² = 0.996) of the PP emission ratio at 605 nm versus TEA concentration according to the Stern-Volmer equation.</figcaption></figure>

$$ [\text{Fe}^{\text{II}}\text{-p-TMA}]^{4+} + \text{PP}^{3-} \rightarrow [\text{Fe}^{\text{I}}\text{-p-TMA}]^{3+} + \text{PP}^{2-} \quad (ii) $$

$$ [\text{Fe}^{\text{I}}\text{-p-TMA}]^{3+} + \text{PP}^{3-} \rightarrow [\text{Fe}^{\text{0}}\text{-p-TMA}]^{2+} + \text{PP}^{2-} \quad (iii) $$

Thermodynamics for reactions (ii) and (iii) are still uphill by +0.17 eV and +0.47 eV respectively. However, when taking into account work terms, these reactions are in fact favorable in the case of Fe\*\* reduction and only slightly endergonic for Fe\*\* reduction. Indeed, the driving force for electron transfer is given by the following equation (1):

$$ \Delta G^{\circ} = E^{\circ}_{\text{PP}^{3-}/\text{PP}^{2-}} - E^{\circ}_{\text{Fe}^{\text{II}}}/\text{Fe}^{\text{II-1}} + w_{\text{R}} + w_{\text{P}} \quad (1) $$

where w_R and w_P are the work terms required to bring the reactants and products, respectively, from infinite to reacting distance. These terms are assumed to be only from electrostatic origin and are significant since both reactants and products are charge. They can be estimated as follows: w_R + w_P = (-3 x 4 - 2 x 3) x w_0 = -18 x w_0 = -0.47 eV for reaction (ii) and w_R + w_P = (-3 x 3 - 2 x 2) x w_0 = -13 x w_0 = -0.338 eV for reaction (iii); w_0 = e₀²/(4π x ε₀ x ε_S x d) = 0.026 eV with d, the distance between the two reactants (it has been estimated
in similar cases to be 7 Å),²⁸ and εS the solvent dielectric constant (78.8 for water). Using equation (1), ΔG⁰ = -0.3 eV for (ii) and 0.13 eV for (iii), showing that the two electron transfer reactions are indeed feasible. In the case of reaction (iii), the reaction is slightly unfavorable but it is followed by the fast, favorable binding to CO₂.

After reduction to the Fe⁰ catalytically active species, the porphyrin binds to CO₂. The Fe-CO₂ adduct [Fe¹CO₂⁻ ↔ Fe¹¹CO₂⁻²] is likely stabilized by through space coulombic interaction between the positive charges borne by the phenyl rings and the partial negative charges of the oxygen atoms of the carbon dioxide, as already suggested.²⁹ C-O bond cleavage may then occur from protonation of the Fe-CO₂ adduct, leading to the release of a water molecule and a Fe-CO adduct. The later species furnishes CO either upon photo-irradiation or by a one-electron homogeneous reduction by the Fe⁰ species.²⁴

<table><caption>**Table 1.** Catalytic performance (TON) and selectivity (CS) of a 2 µM **Fe-p-TMA** CO₂-saturated ACN:H₂O (1:9, v:v) solution after 47 h of visible-light (> 420 nm) irradiation in the presence of 0.1 M NaHCO₃ 50 mM of different **SD** and 0.2 mM **PP**.</caption><thead><tr><th rowspan="2">entry</th><th rowspan="2">SD</th><th colspan="2">TON</th><th colspan="2">CS (%)</th></tr><tr><th>H₂</th><th>CO</th><th>H₂</th><th>CO</th></tr></thead><tbody><tr><td>1</td><td>TEA</td><td>3</td><td>60</td><td>5</td><td>95</td></tr><tr><td>2<sup>[a]</sup></td><td>TEA</td><td>4</td><td>71</td><td>5</td><td>95</td></tr><tr><td>3</td><td>TEOA</td><td>4</td><td>42</td><td>9</td><td>91</td></tr><tr><td>4</td><td>EDTA</td><td>4</td><td>46</td><td>8</td><td>92</td></tr><tr><td>5<sup>[b]</sup></td><td>TEA</td><td>3 / 5</td><td>58 / 78</td><td>5 / 6</td><td>95 / 94</td></tr><tr><td>6<sup>[c]</sup></td><td>TEA</td><td>3 / 6</td><td>63 / 120</td><td>5 / 5</td><td>95 / 95</td></tr></tbody></table>

[a] 0.4 mM **PP**. [b] After 47h / 47h irradiation and addition of one equivalent of **Fe-p-TMA** after the first 47h. [c] After 47h / 47h irradiation and addition of one equivalent of **PP** after the first 47h.

## Conclusions

Introducing trimethylammonio groups on the para position of the phenyl rings of an iron porphyrin provides a water soluble molecular catalyst able to perform photochemical CO₂-to-CO conversion in aqueous solutions, at close to neutral pH. By using visible light and a cheap organic sensitizer, selective production of CO was maintained over two days of irradiation. Catalysis is ultimately limited by progressive sensitizer degradation but it could be started again by adding fresh sensitizer. This work opens new perspectives for developing efficient CO₂ photocatalysis in water. Further studies aiming at this goal will include the development of new sensitizers combining efficient light absorption properties as well as increased stability over long time irradiation.

## Experimental Section

**Chemicals:** The synthesis of chloro iron(III) 5,10,15,20-tetra(40-N,N,N-trimethylanilinium)porphyrin (Fe-p-TMA 1) has been previously described.[7] 1,2,4-trihydroxyanthraquinone (or purpurin, PP, Sigma-Aldrich, 90%), triethylamine (TEA, Acros Organics, 99%), triethanolamine (TEOA, Sigma, > 99%), ethylenediamine tetraacetic acid disodium salt dihydrate (EDTA, Fluka, > 99%), sodium hydrogen carbonate (NaHCO₃, Alfa Aesar, 99%) and acetonitrile (ACN, Acros Organics, 99.9 %) were used as received. Ultrapure water was obtained from a TKA MicroPure system. Argon (> 99.998%) and ¹²CO₂ (> 99.7%) were from Air Liquide whereas ¹³CO₂ (99% content in atom ¹³C) was from Aldrich.

**Photocatalysis:** Irradiations of ACN:H₂O 1:9 (v:v) solutions containing 0.1 M NaHCO₃, the catalyst (Fe-p-TMA), the photosensitizer (PP) and a sacrificial electron donor (SD = TEA, TEOA or EDTA) were conducted at right angle in a closed 1x1 cm, 3.5 mL quartz suprasil cuvette (Hellma 117.100F-QS) equipped with a home-designed headspace glassware. Solutions were saturated with gas for 20 minutes before irradiation. The pH of all CO₂-saturated solutions was ca. 6.7. The light source was a Newport LCS-100 solar simulator, delivering 1 Sun irradiance thanks to an AM1.5 G standard filter, associated with a Schott GG420 longpass filter and 2 cm long glass OS cell filled with deionized water to limit catalyst absorbance and to cut off IR and low UV.

**Physical measurements:** UV-Visible absorption spectra were measured with an Analytik Jena Specord 600 UV/Vis spectrophotometer. Emission UV-Visible absorption spectra were measured with an Analytik Jena Specord 600 UV/Vis spectrophotometer. Emission quenching measurements were conducted with a Cary Eclipse fluorescence spectrophotometer (Agilent Technologies), with the excitation wavelength set at 420 nm. Emission intensities used for the Stern-Volmer analysis were taken at 605 nm, i.e. the emission maximum of PP in the solvent mixture. The lifetime of the emissive excited state of PP (in its mono-anion from) was taken as 3 ns as a reasonable value based on reported studies in different solvent.[22, 27] Cyclic voltammograms were recorded with a Metrohm Autolab (PGSTAT 128N) potentiostat interfaced with the Nova software using a typical three-electrode cell with a 3-mm diameter glassy carbon electrode as work electrode, a platinum wire as counter electrode and a saturated calomel electrode (SCE) as reference electrode.
*Products analysis:* Gaseous products analysis was performed with an Agilent Technology 7820A GC system equipped with a capillary column (CarboPLOT P7, 25 m length, 25 mm inner diameter) and a thermal conductivity detector. Manual injections were made using a graduated gastight syringe (Hamilton). Calibration curves for H₂ and CO were established individually. Control experiments, with no catalyst, no CO₂ or no light were conducted in the same conditions than the full system. Ionic chromatography measurements were performed with a Thermo Scientific Dionex ICS-1100 system. Mass spectra were obtained by a ThermoFisher Scientific TRACE Ultra gas chromatograph equipped with a CP 7514 column (Agilent Technologies) and coupled to a DSQ II mass spectrometer in positive ionisation mode, using a TriPlus headspace autosampler.

**TON calculation:** TON is the number of catalytic cycles per catalyst amount. Mol number of H₂ and CO were obtained from the conversion of GC peak areas into moles in the sample headspace thanks to calibration curves.

## Acknowledgements

H. R. thanks the China Scholarship Council for a PhD fellowship (CSC grant 201507040033).

**Keywords:** CO₂ reduction • photocatalysis • porphyrin • solar fuels • renewable energy

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