A Molecular CO2 Reduction Catalyst Based on Giant Polyoxometalate {Mo368}

Photocatalytic CO₂ reduction in water is one of the most attractive research pursuits of our time. In this article we report a giant polyoxometalate {Mo₃₆₈} based homogeneous catalytic system, which efficiently reduces CO₂ to formic acid with a maximum turnover number (TON) of 27,666, turnover frequency (TOF) of 4,611 h⁻¹ and external quantum efficiency of the reaction is 0.6%. The catalytic system oxidizes water and releases electrons, and these electrons are further utilized for the reduction of CO₂ to formic acid. A maximum of 8.3 mmol of formic acid was observed with the loading of 0.3 μmol of the catalyst. Our catalyst material is also stable throughout the reaction. The starting materials for this experiment are CO₂ and H₂O and the end products are HCOOH and O₂. The formic acid formed in this reaction is an important H₂ gas carrier and thus significant in renewable energy research.

Introduction

The CO₂ concentration in environment is ever increasing. Thus, to find out a suitable pathway to recycle CO₂ to an energy rich material is a crucial challenge nowadays (Hoffert et al., 2002; Crabtree and Lewis, 2007; Meinshausen et al., 2009; Mikkelsen et al., 2010; Garai et al., 2012; Bandeira et al., 2015; Twidell and Weir, 2015). The depletion of fossil fuel during production of energy increases the CO₂ level in the environment (Hoel and Kverndokk, 1996; Höök and Tang, 2013). It is also known that abundance of fossil fuel is limited. Thus, it is necessary to find out a new pathway which can produce energy without hampering the environment and burning fossil fuels. With this end in view if we convert a greenhouse gas like CO₂ to energy rich material it would be very interesting and helpful for a sustainable environment and economy (Khenkin et al., 2010; Rankin and Cummins, 2010; Wang W. et al., 2011; Bontemps et al., 2012; Kuhl et al., 2012; Ohtsu and Tanaka, 2012; Wesselbaum et al., 2012; Xi et al., 2012; Arai et al., 2013; Costentin et al., 2013; Asadi et al., 2014; Blondiaux et al., 2014; Herrero et al., 2014; Kim et al., 2014; Kou et al., 2014; Lu et al., 2014; Studt et al., 2014; Zhang et al., 2014; Gao et al., 2015; Kornienko et al., 2015; Liu et al., 2015; Marszewski et al., 2015; Matlachowski and Schwalbe, 2015; Roberts et al., 2015; Sypaseuth et al., 2015; Iwase et al., 2016; Kuriki et al., 2017). However, the challenge is as CO₂ is a very stable oxide of carbon at its stable oxidation state, a large amount of energy is required to activate CO₂. Activation of CO₂ is a challenge. Here this challenge is addressed via photochemical reduction of CO₂ to HCOOH using an unusually large giant POM cluster, first synthesized by Müller group in Bielefeld {Mo₃₆₈} (Müller et al., 2002, 2004; Müller and Roy, 2003). The TON (27,666) and TOF (4,611 h⁻¹) of this conversion is quite high as compared to other reported molecular catalysts (Tamaki et al., 2015). In photosynthesis, nature continuously activates CO₂ with ease where CO₂ from environment is converted into carbohydrates through photosensitization using sun as the source of light. The process happens in nature such that first water gets oxidized and releases electrons which further reduce CO₂ to carbohydrate in a long catalytic cycle (Hatch, 1976). Drawing inspiration from this process, use of a proper catalytic system can lead to the conversion of CO₂ to different high energy carbon material under light. Many potential catalytic systems have been developed over the last few decades for the synthesis of different fuel and organic materials from CO₂. Some of the catalysts initially bind with CO₂ and further reduce it to different reduced materials (Castro-Rodriguez and Meyer, 2005; Laitar et al., 2005; Sakakura et al., 2007; Sadique et al., 2008; Cokoja et al., 2011; Langer et al., 2011; Mandal and Roesky, 2011; Sato et al., 2011; Schmeier et al., 2011). Some catalysts convert CO₂ following electrochemical methods (Amatore and Saveant, 1981; Hori et al., 1989; Whipple and Kenis, 2010; Agarwal et al., 2011; Finn et al., 2012; Kuhl et al., 2012; Sullivan et al., 2012; Costentin et al., 2013; Zhang et al., 2014; Kornienko et al., 2015; Lin et al., 2015). Electrochemically CO₂ can be reduced to different alkane, ethylene, CO, and HCOOH. The major problem associated with this process is the selectivity in the reduction of CO₂ to different reduced products. Recently Yaghi group showed that electrochemically CO₂ can be reduced to CO selectively using Co-porphyrin based COF and MOF catalysts in water (Kornienko et al., 2015; Lin et al., 2015). Imminent challenge lies in the photochemical reduction of CO₂ (Matsuoka et al., 1993; Daniel and Astruc, 2004; Schwartzberg and Zhang, 2008; Takeda et al., 2008; Li and Zhang, 2009; Morris et al., 2009; Dhakshinamoorthy et al., 2012; Tornow et al., 2012; Sato et al., 2013; Sekizawa et al., 2013; Zhu et al., 2013; Wang et al., 2014; Kim et al., 2015; Li et al., 2015; Low et al., 2015). Among various proposed technologies, photocatalytic CO₂ reduction has been known as one of the most important strategies for solving both global energy and environmental problems due to its low cost, cleanliness, and environmental friendliness (Maginn, 2010; Iizuka et al., 2011; Sato et al., 2011; Yu et al., 2014). Lehn (Hawecker et al., 1984) and Sauvage (Beley et al., 1984) started photochemical and photoelectrochemical CO₂ reduction in 1980 using different rhenium, ruthenium, nickel, and cobalt complexes of different macrocycles as catalysts (Fisher and Eisenberg, 1980; Beley et al., 1984). Many hybrid nano materials are potential catalysts for this purpose. Also, some ruthenium and rhenium-based metal complexes can reduce CO₂ in presence of light. Photo-electrochemical method is another important tool in this regard (Halmann, 1978; Barton et al., 2008). The major problem associated with photochemical CO₂ reduction is the use of a sacrificial electron donor, an organic amine, which cannot be recovered from the reaction (Takeda et al., 2008; Morris et al., 2009). One of the interesting solution of this problem is using water as a sacrificial electron donor (Wang C. et al., 2011; Kim et al., 2014). However, all such avenues for CO₂ reduction suffers from low yield of the reduced product. Moreover, the catalyst materials are also expensive. Thus, it is necessary to develop a catalyst which is inexpensive, easy to synthesize and can reduce CO₂ in water with promising yield. Till date the photo catalyst materials used for CO₂ reduction in water are majorly heterogeneous (Barton et al., 2008; Xi et al., 2012; Kuriki et al., 2016). Homogeneous photochemical CO₂ reduction in water is also reported (Nakada et al., 2016).

A wide variety of catalysts both homogenous and heterogenous have been reported for the reduction of CO₂ to formic acid ranging from macrocycles (Chen et al., 2015; Ikeyama and Amao, 2018), hybrid materials (Yadav et al., 2012; Sekizawa et al., 2013; Yoshitomi et al., 2015), ionic liquids (Lu et al., 2017), nanoparticles (Kortlever et al., 2015), B doped nanodiamonds (Ikemiya et al., 2018) to alloys (Bai et al., 2017) with some of them reaching selectivity as high as 100%. On the other hand, reports on photochemical CO₂ reduction to HCOOH using molecular catalysts mainly employs Ru based complexes as the active catalyst (Boston et al., 2013). Use of bipyridine based Ru(II) complexes with an external light sensitizer have been reported for the selective reduction of CO₂ to formic acid (80%) with a TON = 526 (Rosas-Hernández et al., 2015). By employing Ru based supramolecular photocatalysts which acts as both light sensitizer and catalyst, Ishitani et al. have showed selective reduction of CO₂ to formic acid in the presence of an external reductant (Tamaki et al., 2012). Further they have tuned the catalytic activity of Ru(II)-Ru(II) supramolecular photocatalyst [Ru₂-Ru(CO)] by employing a suitable reductant to increase formic acid selectivity (87%) and TON_HCOOH = 2,766 (Tamaki et al., 2015). Here a synergistic interaction between the reductant employed and the photocatalyst is dictating the outcome of the reaction. These catalysts have an advantage of visible light absorption but require a need for an external electron donor, which was either added externally or added as a part of the solvent. Also, a similar RuReCl photocatalyst was investigated in aqueous solution but the efficiency of formic acid production was low due to the degradation of photocatalyst owing to the back-electron transfer from one electron reduced species to the photosensitizer unit (Nakada et al., 2015). In contrast, in this work a giant {Mo₃₆₈} POM based homogenous photocatalyst system is used to achieve higher selectivity and TON toward formic acid production by employing water solvent as the electron donor.

Recently our group has reported molybdenum based heterogeneous Soft-Oxometalate materials as efficient catalysts for CO₂ reduction reaction (Das et al., 2016). To achieve the same in homogenous realm, an oxo-molybdate based catalyst material which is completely soluble in water is chosen. In this work a molecular catalyst based on giant molybdenum polyoxometalate {Mo₃₆₈} is synthesized, which can reduce CO₂ to formic acid in water with a good yield. The mixed valent molybdenum based giant polyoxometalate {Mo₃₆₈}is synthesized following the literature procedure (Müller et al., 2002). Due to the presence of Mo^V and Mo^VI centers in the cluster, intra-valance charge transfer bands are observed in the polyoxometalate. This band imparts deep blue color to the solution. The cluster, a member of molybdenum blue family, is also photoactive and thus there is no need of an addition of any photosensitizer (Das et al., 2016a). Photoactivity of polyoxometalates is known and has been used by us as a catalyst in photo-polymerization (Chen et al., 2013; Das and Roy, 2015, 2016), as well as in photochemical water oxidation reaction (Roy et al., 2007a,b, 2008; Das et al., 2016b; Barman et al., 2018) without addition of any sensitizer and other reactions (Roy et al., 2013). The cluster {Mo₃₆₈} is extremely efficient as a catalyst and 8.3 mmol of formic acid is obtained with a loading of 0.3 μmol of catalyst. Here the catalyst acts with a maximum turnover number (TON) of 27,666 and turnover frequency (TOF) of 4,611 h⁻¹.

Materials and Methods

All the materials and reagents were purchased from commercially available source and used without further purification. Only water is used as a solvent which was distilled twice before starting any reaction. Before use, all the glass apparatus were first boiled in acid bath then cleaned first with tap water then with double distilled water and finally rinsed with acetone and dried in hot air oven overnight, the temperature of which was set at 90°C. A Luzchem UV photoreactor operating at a power of 64 W (8 × 8 W) with UVA lamp is used for photochemical CO₂ reduction reaction in water.

Synthesis of Na₄₈[H_XMo₃₆₈O₁₀₃₂(H₂O)₂₄₀(So₄)₄₈]. ca. 1,000H₂O

{Mo₃₆₈} is synthesized by following literature procedure. To a solution of Na₂MoO₄·2H₂O (3 g, 12.4 mmol) in water Na₂S₂O₄ (0.15 g, 0.86 mmol) is added as a reducing agent. The reaction mixture is acidified with 0.5 M H₂SO₄ (35 mL; immediate color change to blue). The solution is then stored in a closed flask for 2 weeks and after 2 weeks the precipitated deep-blue crystals of {Mo₃₆₈} are obtained by filtration, Yield: 135 mg.

General Reaction Procedure for Photo Catalytic CO₂ Reduction

Photo catalytic carbon dioxide reduction reactions are carried out as follows. Desired amount of {Mo₃₆₈} is taken in 10 ml of oxygen free double distilled deionized water. The reaction mixture is closed in a two neck round bottom flask and CO₂ gas is purged for 2.5 h. Then the reaction mixture is kept in the photo reactor under UV-light (eight 8 W lamps, λ = 373 nm) for different intervals of time. 20 μL of reaction mixture is taken out and further diluted with 10 ml HPLC grade water, which is used for carrying out MALDI-MS experiment. For MALDI-MS experiments the reaction mixture is co-crystallized with HCCA matrix and then the mass spectrum was recorded. Mass spectrum gives a molecular ion peak of formic acid (m/z = 46; Figure S1). Further to detect formic acid, HPLC measurement was performed by injecting the diluted reaction mixture in carbohydrate column with an external standard, 0.1 M formic acid solution. and all the quantitative measurements of formic acid is done by using HPLC experiments. CV measurements with the reaction mixture are done using 0.1 M KCl as an electrolyte, in a potential range of +0 to −2 V with respect to saturated Ag/AgCl reference electrode in a standard 3-electrode system and a peak around −0.59 V in cyclic voltammogram further confirms the formation of formic acid from carbon dioxide in our reaction mixture. To further prove formic acid is formed in our reaction mixture, a coupling reaction is performed with our reaction solution using the following method. A solution of aniline (100 μL) in 2 ml acetonitrile along with 20 mg HATU is added to the reaction mixture as a coupling reagent. The reaction mixture is stirred for 2 h at room temperature. The N-phenylformamide is formed in the reaction mixture. Organic components are extracted with EtOAc (3 × 15 ml) and the EtOAc is evaporated in vacuum. The extracted organic component is dissolved in acetonitrile to perform GC-MS and MALDIMS analysis by following above mentioned procedures. Also we have taken the proton NMR spectrum of amide in CDCl₃ solution to further confirm the formation of amide (Figure S1).

Determination of Oxygen Using YSI Dissolve Oxygen Meter

Oxygen formed in the reaction is detected by YSI dissolved oxygen meter. YSI dissolved oxygen meter is first calibrated by degassed water. For that purpose, Nitrogen gas is first bubbled through double distilled water for an hour and then put YSI dissolved oxygen meter into this water and recorded the amount of oxygen present. Next, dissolved oxygen meter is dipped into the reaction system i.e., photo illuminated sample and amount of oxygen present in the reaction system is recorded. Now from the difference of oxygen reading in YSI dissolved oxygen meter the total oxygen formed in the reaction is calculated.

pH Dependent Reaction

These experiments are performed by following the previous procedure using different buffer solutions in the range of pH 5 to 9. Measurement of the formic acid is carried out by similar methods as mentioned above.

Characterization Techniques

Fourier Transform Infra-Red Spectroscopy (FT-IR)

FTIR spectrum of {Mo₃₆₈} is performed by KBr pallet technique. Initially a pellet is prepared from the mixture of KBr and {Mo₃₆₈}. FTIR spectrum is recorded by using Perkin Elmer Spectrum RX1 spectrophotometer with FTTR facility in the range 2,000–450 cm⁻¹.

Electronic Absorption Spectroscopy (EAS)

A stable solution of {Mo₃₆₈} is taken in a quartz cuvette and the electronic absorption spectrum is recorded on U-4100 Spectrophotometer (Liquid).

Cyclic Voltammetry

PAR model 273 potentiostat is used for CV experiment. A platinum wire auxiliary electrode, a glassy carbon working electrode with surface area of 0.002826 cm² and an aqueous Ag/Ag⁺ reference electrode which is filled with saturated KCl solution, is used in a three-electrode configuration. All the measurements were performed at 298 K in an inert atmosphere.

Raman Spectroscopy

A LABRAM HR800 Raman spectrometer is employed using a He–Ne ion laser (λ = 1,024 nm) as the excitation source to analyse the sample.

HPLC

All reaction samples were monitored by HITACHI- HPLC system equipped with binary 2,130 pumps, a manual sampler, and 2,490 refractive index detector, maintained at 50°C. The products were separated in sugar ion-exclusion column (250 × 4.8 mm), maintained at 60°C using water as mobile phase with 0.8 mL/min flow rate. The HPLC system is controlled and processed by Inkarp software. Standard Formic acid and Formaldehyde solution were prepared and calibrated. Each time the product obtained is diluted with a known volume of milliq water before analysis to prevent the overloading of the column. All experiments were done in triplicates and the average values were reported within the standard deviations of <2.0%.

Gas Chromatography-Mass Spectrometry (GC-MS)

The products were identified and analyzed using a GCMS-QP-2010 Ultra (M/s. Shimadzu Instruments, Japan) with a HB-5 capillary column (20 m × 0.18 mm) supplied by M/s. J&W Scientific, USA and Trace 1300 GC and ISQ qd single quadruple Mass spectrometer with a TG-5MS capillary column (30 m × 0.32 mm × 0.25 μm) supplied by Thermo Fisher Scientific, India. Gas samples were detected by Molecular sieve 5Å packed column. Thermal Conductivity Detector (TCD) was used for gas samples and Mass Detector (MSD) was used for formic acid measurement.

NMR

¹H NMR and ¹³C NMR spectrum is recorded on 500 MHz Bruker and 400 MHz Jeol NMR machine. For detection of formic acid by ¹H NMR, solvent suppression (water) method is used.

Matrix Assisted Laser Desorption Ionization Mass Spectroscopy (MALDI-MS)

All samples were prepared in HPLC grade acetonitrile, by dissolving very minute amount of sample on 2 ml acetonitrile, and then filtering the sample using 0.2 micron syringe filter. Then the sample is cocrystalized with HCCA matrix.

Result and Discussion

Deep blue crystals of {Mo₃₆₈} with I4mm space group are isolated from the mother liquor in a week by following the literature procedure (Müller et al., 2002). The structure was first determined by A. Müller group using single crystal X-ray diffraction. In the structure, the unit cell contains two hedgehogs like anions which possess D₄ symmetry (Figure 1A). The complete cluster contains a central ball shaped unit {Mo₂₈₈O₇₈₄(H₂O)₁₉₂(SO₄)₃₂} and two other capping units {Mo₄₀O₁₂₄(H₂O)₂₄(SO₄)₈}. These two units also possess local symmetry, D_8d, and C_4V, respectively. The molecule has large cavity in which 400 water molecules can be encapsulated. Further this structure can be considered as a hybrid of ring shaped {Mo₁₇₆} structure and ball shaped {Mo₁₀₂} structure. The cluster shows typical peaks in the FTIR spectrum which are as follows (Figure 1B), 1,635 (δ_H2O), 1,123 (ν_as${\text{SO}}_4^{2-}$), 976 (νMo = Ot), 912, 618 cm⁻¹, respectively. Raman spectrum (λ = 1,024 nm) shows peak at 989, 908 (broad peak), 837, and 703 cm⁻¹, respectively (Figure 1C). Electronic absorption spectrum shows maximum absorbance at 747 nm which is observed due to the IVCT transition between Mo^V to Mo^VI which is characteristic for molybdenum blue species (Figure 1D). From the above all characterization details, it confirms that {Mo₃₆₈}is formed in our reaction. Further cyclic voltammogram of {Mo₃₆₈} under Ar atmosphere is recorded at different scan rates (Figure 1E), which gave three peaks at −0.7, 1.01, and 2.663 V, respectively vs. NHE. From the cyclic voltammogram of the material the position of HOMO and LUMO of the catalyst material was calculated. From the data, the position of HOMO is 2.63 V vs. NHE and the position of LUMO is −0.7 V vs. NHE (Figure 1F). The band gap of the catalyst material is 3.33 eV. Thus, from the band position it can be proposed that {Mo₃₆₈} is able to reduce CO₂ as well as oxidize water simultaneously under UV-light source of 373 nm (Figure S3). The external quantum efficiency (EQE) of the system is measured by irradiating monochromatic LED light source of wavelength 365 nm (I = 67 mW/cm²) and 745 nm (I = 32 mW/cm²) where we find the EQE of 0.6 and 1.54 × 10⁻³%, respectively (SI).

FIGURE 1

Figure 1. (A) Single crystal structure of {Mo₃₆₈} where Red polyhedra denotes Oxygen, yellow denotes {Mo₁} and blue with blue-turquoise pentagonal bipyramids denotes {Mo(Mo₅)}, (B) FT-IR spectrum, (C) Raman Spectrum, (D) EAS spectrum, (E) Cyclic Voltammogram (Na₂SO₄ was used as electrolyte at a scan rate 100 mV/s), (F) HOMO and LUMO band position of {Mo₃₆₈}.

To describe the detailed catalytic process of CO₂ reduction in water, CO₂ reduction under UV-light without addition of any photosensitizer as well as any organic sacrificial electron donor is performed. The reaction is carried out in an air tight quartz tube which is first purged with nitrogen to remove trace amount of oxygen from the reaction system. Later CO₂ is purged for 3 h. The quartz tube is sealed and kept in a photo reactor for different intervals of time. CO₂ undergoes reduction under aforementioned reaction condition, the reduced product is characterized first by GC-MS and further confirmed from ¹H-NMR by solvent suppression method (water peak suppression; Figure 2A). These above results confirm that formic acid is formed from CO₂ in our reaction condition. Further formic acid is characterized and quantified by HPLC against a standard 0.1 M formic acid solution. A peak at −0.6 V in cyclic voltammogram is also observed using saturated Ag/AgCl as a reference electrode (Figure 3A), which further confirms the formation of formic acid in the reaction mixture. Besides formic acid, trace amount of formaldehyde which is characterized by HPLC against 0.1 M formaldehyde solution is also found. To further check whether any gaseous reduced product is formed in the reaction, GC-MS is performed by injecting the gases from the reaction chamber and no CO, CH₄, or any other reduced gaseous product is observed from the reaction mixture. Thus, in our present reaction we get formic acid selectively over other reduced product obtained from the reduction of CO₂. Formic acid is formed from CO₂ only and not from any other carbon impurity. To further prove the source of carbon in formic acid, same reaction is performed using ¹³CO₂ as a reactant yielding H¹³COOH. The product is characterized by ¹³C-NMR (Figure 2B) taken before and after reaction and Raman spectroscopy (Figure S4). Prior to UV-light irradiation ¹³C-NMR gave strong signal at 125.5 ppm corresponding to ¹³CO₂. After the completion of 6 h of photoreduction, ¹³CO₂ signal is found to decrease and a new peak at 166.6 ppm corresponding to H¹³COOH is obtained (Tamaki et al., 2015). We also performed ¹H-NMR after reducing ¹³CO₂ enriched solution where a doublet (¹J_CH = 216 Hz) is found at 7.9 ppm which coupled with ¹³C atom (Figure 2D). These results indicate that formic acid is formed from CO₂ and not from any other carbon source. This can be proved in a control experiment by recording HPLC of the light illuminated sample without purging of CO₂. In that case, no trace of formic acid or formaldehyde is found in the reaction mixture. This indicates that formic acid and formaldehyde is obtained from CO₂ only. During the photo catalytic CO₂ reduction water also gets oxidized to oxygen simultaneously. Formation of oxygen is characterized by the YSI dissolved oxygen meter and GC-MS. The experiment is further repeated using mixture of ${\text{H}}_2^{18}$O (isotopic purity = 97%) and ${\text{H}}_2^{16}$O (1:8) where we found the corresponding masses at m/z = 32.01, 34.04 and 35.98. The mass ca. 32 corresponds to ¹⁶O¹⁶O whereas m/z = 34 and 36 resemble to ¹⁶O¹⁸O and ¹⁸O¹⁸O, respectively (Figure 2C) which confirms that oxygen is produced from oxidation of solvent water molecule. Besides, deuterium-labeling experiment was conducted using D₂O (isotopic purity = 99.9%) and H₂O as a solvent (3:2). We found that the deuterium was incorporated into formate moiety (m/z = 75), which was confirmed from mass spectrometry (Figures 2E,F). As exchangeable D is found in the reaction medium, it got incorporated during the formation of formic acid and later deuterium incorporated ethylformate is obtained after reaction with ethanol.

FIGURE 2

Figure 2. (A) Representative ¹H-NMR of the product from photochemical CO₂ reduction. (B) ¹³C-NMR spectrum of the reaction mixture before (red) and after (black) the reaction. (C) Full mass spectrum representing all the isotopes of the gaseous products (O₂, ¹⁶O¹⁸O, ¹⁸O¹⁸O and N₂). (D) ¹H-NMR spectrum of the reaction mixture using ¹³CO₂. (E) Mass of ethylformate (molecular peak) after reaction with H₂O and (F) with D₂Oand H₂O mixture.

FIGURE 3

Figure 3. (A) Cyclic voltammogram of the reaction mixture, blue line corresponds to blank where there is neither CO₂ nor catalyst, red line corresponds to CO₂ addition without UV light illumination and black line corresponds to photo CO₂ reduction in water using {Mo₃₆₈} as a catalyst. (B) Time dependent formic acid and oxygen formation by photochemical carbon dioxide reduction using {Mo₃₆₈} as a catalyst. (C) Formation of different reduced product using {Mo₃₆₈}, and their respective percentage yield in total CO₂ reduced product. (D) Effect of variation of {Mo₃₆₈} loading on formation of formic acid. (E) pH dependent formic acid formation using {Mo₃₆₈} as a catalyst. (F) TON of the formic acid and formaldehyde [Amount of product formed (mol)/amount of catalyst taken (mol)].

Further time dependent study reveals, 6 h is needed to complete the reduction of CO₂ to formic acid. The TOF of the reaction is quite high 4,611 h⁻¹ in water. The yield of the formic acid initially increases rapidly with time (Figure 3B). It increases almost linearly with time. After certain time of reaction, the rate of the increment of the formic acid formation with time decreases and reaction yield does not increase further after 6 h of the reaction. This indicates that the CO₂ reduction reaction is complete within 6 h and same trend can be observed for oxygen evolution too (Figure 3B). Both the processes i.e., CO₂ reduction and water oxidation are coupled which can be confirmed from Figure 3A where current increase occurs at 0.9 V vs. Ag/AgCl only after photoreduction. Furthermore, loading of the catalyst is varied in photochemical CO₂ reduction in a controlled fashion (Figure 3D). Here, initially the yield of the formic acid increases almost linearly with the increasing loading of the catalyst and further after certain range of the loading of the catalyst the reaction yield becomes independent of the catalyst loading (Figure 3D). We have observed a maximum yield of 8.3 mmol of formic acid with a loading of 0.3 μmol of the catalyst with maximum turnover number of 27,666 (Figure 3F). This clearly indicates high reactivity of the {Mo₃₆₈} unit. Other than formic acid, 37 μmol of formaldehyde is also obtained at the same loading of the catalyst under the same reaction condition. The selectivity of the formation of the formic acid with respect to the total CO₂ reduced product is around 95.73% for formic acid and 4.27% for formaldehyde (Figure 3C). To see the effect of the proton concentration in the photo catalytic CO₂ reduction reaction, pH is varied in the range from 5 to 9 in the course of the reaction. To do so, different buffer solutions are used (Acetate buffer was used to regulate the pH). pH dependent study reveals that the yield of formic acid is maximum at pH 7 (Figure 3E). The yield of the reaction is almost identical on moving toward acidic pH. But when we move toward the basic pH the yield of the reaction decreases drastically. This is due to the dissociation of {Mo₃₆₈} in basic medium. Also, this is may be due to the fact that CO₂ reduction is a proton coupled electron transfer (PCET) reaction, the reaction in basic pH, protons are quenched from the reaction medium therefore the yield of the reaction also decreases.

For carbon dioxide reduction reaction two major constituents, i.e., electrons and protons are required. As no sacrificial proton donor or electron donor is used in this case, there is a possibility that water acts as a source of both protons as well as electron in the reaction. To prove this observation, different sets of experiment were performed. Initially, when the same reaction was performed in dry DMF, due to lack of availability of protons, no trace of formic acid was detected indicating that water is playing a crucial role in carbon dioxide reduction reaction under the prevalent reaction conditions (Figure 4A). Further to prove the role of the water in the photo catalytic condition, different set of reactions with varying ratio of water and DMF in the reaction mixture was carried out keeping the total volume of the reaction mixture constant (Figure 4A). From the experimental result it can be concluded that with increasing loading of water in the reaction the yield of formic acid increases linearly, which supports the role of water as proton donor.

$\begin{array}{rc}C{O}_2+H_{2}O(aq)=H_{2}C{O}_3& (1)\end{array}$

$\begin{array}{rc}{H}_{2}C{O}_3(aq)={HC{O}_3}^{-}(aq)+H^{+}(aq)& (2)\end{array}$

$\begin{array}{rc}{C{O}_3^{2-}+4H}^{+}+2e^{-}=HCOOH(aq)+H_{2}O& (3)\end{array}$

The dependency of water on CO₂ reduction reaction is further obtained from pH dependent study. As, carbon dioxide reduction is proton dependent process (Equation 3), the yield of the reaction should increase on increasing the proton concentration in the reaction medium. But in our present case we observe a decrease in reduced product concentration upon going from neutral pH to acidic pH. This result indicates that another component of CO₂ reduction reaction equilibrium i.e., electron concentration may vary with the change in the pH of the reaction. There are generally two electron sources present in reaction system: one is the catalyst material itself and another is the water. Now if the cluster were to act as electron donor in the reaction then it would degrade in the solution. If it were the situation then a decrease in the absorbance of the catalyst with increasing time of illumination may be observed. But the absorption spectrum of cluster remains unchanged throughout the reaction (Figure 4B). Moreover, in all pH variation reactions the same amount of catalyst was used resulting in a constant electronic concentration in the reaction. On the other hand, if the reaction equilibrium totally depended on proton concentration of the reaction system then the yield should have increased at lower pH. However, this does not happen in our reaction system. Hence from the above two observations it can be concluded that the catalyst material does not act as a source of electrons in the photo chemical CO₂ reduction reaction and water must act as an electron source in the reaction.

FIGURE 4

Figure 4. (A) Variation of DMF and water content in reaction mixture (D-DMF, W-water, all values given in ml unit). (B) Plot of change of absorbance of {Mo₃₆₈} with time vs. reaction time.

As already discussed, oxygen formation was detected in the system during CO₂ reduction which proves that the oxidation of water releases electrons into the system. As it is known that the water oxidation reaction depends on the pH of the medium and it increases with increasing pH of the reaction i.e., ongoing from the acidic to basic pH water oxidation increases. As water oxidation increases at higher pH electron concentration also increases which will facilitate the reduction process. Thus, CO₂ reduction should increase at higher pH. It is also observed in our present reaction system that CO₂ reduction increases with increasing pH (Up to pH 7, as at basic pH the catalyst dissociates). Thus, from the above two observations we can conclude that water only acts as a sacrificial electron donor in the reaction and photo chemical CO₂ reduction depends on photo-chemical water oxidation reaction which also takes place parallelly in the reaction system. Thus, a maximum yield of formic acid is obtained in neutral pH as compared to acidic pH.

To investigate the active catalyst species in the reaction, the reaction is repeated with the precursor of {Mo₃₆₈} i.e., with sodium molybdate and no reduced product is found in the reaction. Thus, from this result, it can be concluded that the giant cluster {Mo₃₆₈} is only responsible for CO₂ reduction reaction and not a single molybdenum unit. Cluster cage thus plays a crucial role for the reaction. It is already mentioned that {Mo₃₆₈} is a photoactive materialand it absorbs UV-light of 373 nm wave length to generate holes (h⁺) and electrons (e⁻) in the system. These holes can oxidize water to generate electrons, protons and oxygen in the medium. The electrons and protons so generated reduce CO₂ to formic acid and formaldehyde. This also explains the high photo catalytic activity of {Mo₃₆₈}. {Mo₃₆₈} comprises of different small molybdenum based units i.e., {Mo^VI(${\text{Mo}}_5^{\text{VI}}$)}, {${\text{Mo}}_2^{\text{V}}$}, {${\text{Mo}}_1^{\text{V}}$}, respectively. We believe that upon photo excitation, {Mo^VI(${\text{Mo}}_5^{\text{VI}}$)} goes to excited state and forms {Mo^VI(${\text{Mo}}_5^{\text{VI}}$)}*. This species has potential to oxidize water to liberate oxygen and release protons and electrons. On the other hand, the {Mo₂} unit and {Mo₁} unit plays a crucial role for photochemical CO₂ reduction. It has been already shown by Müller group that CO₂ can coordinate with {Mo₂} units of the giant molybdenum based polyoxometalates (Garai et al., 2012; Bandeira et al., 2015). Thus, in our present case it is also reasonable to postulate that CO₂ can co-ordinate with {Mo₂} unit. It is also observed that there is change in cyclic voltammogram of {Mo₃₆₈} upon purging of CO₂ in {Mo₃₆₈} solution (Figure S2). Further this coordinated CO₂ unit can be reduced by electrons and protons present in the reaction medium. Possibly CO₂ also can coordinate with {Mo₁} unit and reduction of coordinated CO₂ takes place in a synergistic fashion. We believe that adsorbed CO₂ converted to C$O_2^{-·}$. The activation barrier of this reduction is lowered by the cluster. Later this species accept proton to generate carboxyhydroxyl intermediate species which participates in PCET process to form formic acid. Due to direct attachment, a large number of CO₂ molecules are reduced for every molecule of {Mo₃₆₈}, and hence the cluster shows such high photocatalytic activity.

The stability of the catalyst is discussed under prevalent photo catalytic conditions. After reaction the catalyst is recovered by slow evaporation of solvent. In this present case it is shown that the {Mo₃₆₈} is stable under the photocatalytic condition using different techniques. FT-IR spectrum of {Mo₃₆₈} after completion of the reaction shows peaks at 1632, 1124, 985, 916, 615 cm⁻¹, respectively which are almost similar to that of the solid catalyst. This indicates that the cluster is stable under the reaction conditions (Figure 5A). Moreover, the IVCT bands responsible for the Mo(V)->Mo(VI) transitions do not change with the reaction and it further indicates that there is no change in the catalyst composition and that the catalyst is stable under the reaction conditions. Similar result is also observed from Raman spectrum of catalyst which also shows the catalyst remains intact after the photochemical CO₂ reduction (Figure 5B). Also note that no presence of particulate matter is detected from DLS in this system during catalysis which indicates that the catalyst is homogeneous. Hence our catalyst is a stable homogeneous catalyst which reduces CO₂ to formic acid in water.

FIGURE 5

Figure 5. (A) FT-IR spectrum of {Mo₃₆₈}, black line corresponds to FT-IR spectrum of before reaction and brown line corresponds to FT-IR spectrum of after reaction. (B) Raman spectrum of {Mo₃₆₈}, black line corresponds to Raman spectrum of before reaction and brown line corresponds to Raman spectrum of after reaction.

Conclusion

Photochemical reduction of CO₂ to formic acid in water using molybdenum based giant polyoxometalate {Mo₃₆₈} molecular catalyst is reported. The photocatalytic system shows excellent selectivity for formic acid production (95.73%) with high TON (27,666) and TOF (4,419 h⁻¹); which is quite high in its class. The presence of different Mo based sub units in the cluster is responsible for the exceptional activity of the catalyst. As the system is photoactive, no external photosensitizer is added and water solvent acts as electron donor making the whole process environmentally benign and self-sustained. We believe that this work can lead to the development of a class of highly efficient homogeneous CO₂ reduction catalysts based on water soluble polyoxometalates.

Author Contributions

SD, TB and SB contributed equally. SD, TB and SB performed the experiments. SS assisted in those experiments. RP carried out GC-MS and HPLC experiments. SR designed the project, analyzed the results and wrote the paper with inputs from all other co-authors.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The authors thank Prof. Dr. Abhishek Dey, of IACS Kolkata, for his help with ¹³CO₂ experiments. SR gratefully acknowledges the start-up grant and FIRE and PRIS grant from IISER Kolkata, India, and grants from CCNU, and NSFC, P. R. China. TB acknowledges IISER Kolkata for his fellowship. SS thanks SERB for financial assistance (PDF/2017/000676) and SB acknowledges UGC for fellowship.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2018.00514/full#supplementary-material

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