as an Efficient Inorganic Material for CO2 Reduction

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A Softoxometalate [{K6.5Cu(OH)8.5(H2O)7.5}0.5 @{K3PW12O40}]n (n= 1348-2024) as an Efficient Inorganic Material for CO2 Reduction with Concomitant Water Oxidation Santu Das, Saurabh Kumar, Somenath Garai, Ramudu Pochamoni, Shounik Paul, and Soumyajit Roy ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13507 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 18, 2017

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ACS Applied Materials & Interfaces

A Softoxometalate [{K6.5Cu(OH)8.5(H2O)7.5}0.5 @{K3PW12O40}]n (n= 1348-2024) as an Efficient Inorganic Material for CO2 Reduction with Concomitant Water Oxidation Santu Das, Saurabh Kumar, Somenath Garai, Ramudu Pochamoni, Shounik Paul, Soumyajit Roy* Eco-Friendly Applied Materials Laboratory (EFAML), Materials Science Centre, Department of Chemical. Sciences, Mohanpur Campus, Indian Institute of Science Education & Research, Kolkata, 741246 West Bengal, India. *Email: [email protected], [email protected] KEY WORD Softoxometalate. CO2 reduction. Polyoxometalate. Photochemistry. Water Oxidation. Copper Tungstate based Cluster.

ABSTRACT An immediate challenge for chemists is to devise different methods to trap chemical energy using light by reduction of carbon dioxide to a transportable fuel. To reach this goal the major obstacle lies in finding a suitable material that is abundant and possesses catalytic power to effect such reduction reaction and perform this reduction reaction without using any external photosensitizer. Here we report first time a softoxometalate, based on {[K6.5Cu(OH)8.5(H2O)7.5]0.5[K3PW12O40]} metal oxide framework which is stable in reaction condition that effectively performs photochemical CO2 reduction reaction in water with a very high turnover number of 613 and TOF of 47.15 h-1. We observe that during this reaction water gets oxidized to oxygen, while the electrons released directly goes to CO2 reducing it to formic acid. A detailed account of characterization of the catalyst along with that of products of this reaction is reported.

ACS Paragon Plus Environment


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INTRODUCTION An alternative clean energy resource which can be obtained from a renewable material source like greenhouse gas, or from water is always of great interest in our daily life

1-2

. Production of fuel

via artificial photosynthesis where solar energy is converted to chemical energy can be extremely effective in our energy crisis ridden world3-4. Carbon dioxide initially undergoes two-electron reduction to produce formic acid which may be directly used in oxidized fuel cells5-6. It is also an important precursor in organic industry for synthesis of different organic molecules 7. However, activation of carbon dioxide requires a large amount of energy8 which is a drawback. Many effective catalysts have been developed during the last few decades which can activate carbon dioxide to different small molecules like formic acid, formaldehyde, methanol, methane, carbon monoxide etc. Some of these catalysts activate CO2 chemically

9-13

by direct binding with CO2 molecule, while others utilize electrochemical

14-19

, as

well as photochemical means for CO2 reduction and have attracted enormous interest in recent times20-26. Photo electrochemical methods

27-28

also offer one of the promising routes for carbon

dioxide activation.Lehn29 and Sauvage30 pioneered the field of photoelectrochemical CO2 reduction using macrocyclic complex of Ni2+, Co2+.30-31, Re+, and Ru+.Metal free CO2 reduction carried out using organo-catalyst in ionic liquids has also been reported.32 Molybdenum based compound can also act as a potential catalyst for CO2 reduction.3334 Optical semi-conductors coupled with a metal complex can also act as an effective catalyst for CO2 reduction.3539

Possibility of achieving CO2 reduction in water has also been proposed.3,

15, 40-44

. We have

recently shown that CO2 reduction and water oxidation reaction depends on one another when we performCO2 reduction in water. For that purpose we have used giant mixed valent molybdenum based softoxometalate and manganese tungsten based softoxometalate as catalysts.45 Designing of the catalyst materials using 3d-transition based metal ions makes the process inexpensive and with this end in view in our present work we have synthesized a copper tungsten


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oxide {[K6.5Cu(OH)8.5(H2O)7.5]0.5[K3PW12O40]}based softoxometalate46-49. The material is chosen such that the tungsten cores especially PW12 cores are photo-active and these tungsten cores serve as very active catalyst for photochemical water oxidation reaction.50-51Tungsten cores also make the overall metal oxygen frame work robust and stable in a photo chemical environment. It is also known that Cu-based material especially Cu based inorganic complexes52-53 as well as copper oxide based nano materials54-57 are also very effective for carbon dioxide reduction reaction. Especially Cu based electrodes are very efficient to reduce CO2 to different hydrocarbons.58 So with those functional attributes of copper in mind we have designed our catalyst material which is composed of both tungstate and copper units. To render the catalysts reusable under the photo catalytic conditions and to enhance the rate of the reaction we have prepared SOM of {CuPW12}unit. We have previously shown the photo activity of related softoxometalates.47, 59-60 We have

characterized

{[K6.5Cu(OH)8.5(H2O)7.5]0.5[K3PW12O40]}by

Fourier

transform

infrared

spectroscopy, Raman spectroscopy, and single crystal X-ray diffraction technique. Further morphology of the material is obtained from scanning electron microscopy and size distribution of the particles has been obtained from dynamic light scattering method. We now use SOMs of {[K6.5Cu(OH)8.5(H2O)7.5]0.5[K3PW12O40]}as a catalyst in photochemical carbon dioxide reduction reaction in water.

RESULT AND DISCUSSION Synthesis of active catalyst material We

have

synthesized

metal

oxide

framework

[{K6.5Cu(OH)8.5(H2O)7.5}0.5@{K3PW12O40}]≡

{{K6.5Cu(OH)8.5(H2O)7.5}0.5@1a}(1) by following conventional procedure. Further we have synthesized SOM of {Cu-PW12}n based on 1 in water by sonication. The morphology of the 1a is obtained from scanning electron microscopy. 1a undergoes self-assembly in water to form nano cubes in dispersion (Figure 1a). We can also control the size of nano cube by varying the loading of solid 1 in fixed amount of water. Hydrodynamic radius (Rh) of 1a increases with increasing loading of solid 1 in water, minimum



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Rh of 1a is 111 nm for loading of 0.2 µmol solid 1, and maximum Rh is 136 nm for corresponding loading of 4.8 µmol of solid 1 (Figure 1b). As 1 forms superstructure in dispersion therefore active catalytic species which participates in reaction is 1a. Thus the effective concentration of active catalytic material i.e. concentration of 1a in dispersion is different from that of concentration of solid catalyst 1 (Figure 1c). To calculate the concentration of 1a in dispersion we need to know the number of metal oxide frameworks present in each superstructure 1a.59 Number of metal oxide framework =

.

= 1348 to 2024 (For different value of R.) (1)

[R= hydrodynamic radius of 1a, and σ= length of a side of the constituent POM] Concentration of 1a can be calculated by following equation 1. Weight percentage of 1a is 0.01% to 0.2% in dispersion.

Figure 1. a) SEM image of 1a shows nano cube type morphology. b) A plot of hydrodynamic radius of 1a vs catalyst loading,. c) Concentration of SOM i.e. 1a in water with loading of solid catalyst. Detailed characterization of the catalyst material Now we go through detailed molecular level characterization of 1a. We have characterized it using FT-IR (Figure 2a) and Raman spectroscopy (Figure 2b). FT-IR spectrum shows peaks at 1628, 1412, 808, and 501 cm-1 respectively. A peak around 501 cm-1 is due to Cu-O stretching,61 Finger print region FTIR spectrum shows the presence of W12 unit in metal oxide framework. Presence of W-O bond can be confirmed from FTIR spectrum corresponding to peaks at 1412 cm-1 and 804 cm-1.61 In Raman spectrum


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we obtain peaks at 101, 160, 219, 504, 878 and 967 cm-1 respectively, where peak at 504 cm-1 is due to anti symmetric stretching of Cu-O bond.61 The peaks corresponding to W-O bond are obtained at 967, 878, and 219 cm-1 respectively, whereas peak at 967 cm-1 is due to W=Ot stretching and that of 878 cm-1 corresponds to stretching of W-O-W bridge bond.61 Now we describe detailed molecular structure which was obtained from single crystal X-ray diffraction of 1 (Figure 2c & 2d). [PW12O40]3- form α-Keggin type {PW12}

based

metal

oxide

framework

in

solid

state

where

copper

is

present

in

{K6.5Cu(OH)8.5(H2O)7.5}0.5-moiety coordinated to the Oh hole of the Keggin ion based 3D-contnuous structure. In this structure the tungstate core shows the photo activity and plays a crucial role in overall photo catalytic reaction. The disposition of the penta-coordinated [Cu(H2O)5]2+ in the octahedral holes between the 3D Keggin ion POMOF based SOM is the likely site for CO2 reduction as will be shown later. Note: (Cu-O(equatorial) bond distance is 2.40 Å while the Cu-O(axial) bond distance is 2.15 Å, is consistent with the reported related literature values) is the noteworthy feature (Figure S5). For detailed description of the structure, see ESI and also CCDC 1564166).



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Figure 2. a) Single crystal XRD structure of 1; b) Pictorial depiction of the [Cu(H2O)5]2+-moiety coordinated to the Keggin ion via. the intermediacy of the K+ ions. Colour code: W, blue; P, yellow; O, red; K, violet; Cu green. c) FT-IR spectroscopy of 1. d) Raman spectrum of 1a using excitation wavelength of 633 nm. Photochemical CO2 reduction in water Photochemical reactions are performed under nitrogen atmosphere. We have performed photochemical CO2 reduction in water using 1a as catalyst without any photo sensitizer. CO2 is reduced to formic acid selectively which is characterized by HPLC with respect to external standard formic acid (Figure 3). No other liquid and gaseous reduced product is obtained from the reaction mixture. We have also observed a peak around -0.6 V from cyclic voltammetry Vs Ag/AgCl electrode which corresponds to the reduction of CO2 to HCOOH which further confirmed that formic acid is formed in our reaction (Figure 4a). To show HCOOH is not formed from any other source like acetate buffer and other carbon impurity present in reaction mixture we have performed a controlled cyclic voltammetry experiment and followed by HPLC experiment (Figure 3b, 3c & S8).To further prove formic acid is formed from CO2 and not from any other carbon source present in the reaction medium we have performed the photochemical CO2 reduction reaction with

13

CO2. In this case we have obtained H13COOH (H13COOH is characterized by Raman

spectrum. We observe a shift in C-O stretching frequency upon use of 13CO2 by few wavenumbers) after the completion of the reaction which indicates HCOOH is formed from CO2 and not from any other carbon impurity (Figure 3d).The selectivity of the reduced produced can be obtained from the band position of the 1. The position of valance band and conduction band is obtained experimentally using cyclic voltammetry.62 the position of valance band and conduction band Vs NHE (pH-0) is 2.413 V and 1.008 V respectively (Figure 3a). Thus the electronic band gap of 1 is 3.42 eV. Further we have considered the optical band gap of 1 which is obtained from the electronic absorption spectrum. We determine the band gap of 1to be 3.40 eV from the Tauc plot63 (Figure S7). The position of conduction band is closest to the band position of CO2/HCOOH redox couple than others. Thus we get selectively


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formation of formic acid from CO2. In our present case during photo catalytic reduction of CO2 water gets oxidized to oxygen, which is detected by YSI dissolved oxygen meter. The formation of oxygen from water can also be explained from the position of valance band. The position of valance band is 2.413 V (Vs NHE, pH-0) which is lower in energy compared to the position of H2O/O2 band (1.23 V vs NHE, pH0). Therefore 1 can oxidize water to produce oxygen simultaneously.64 Therefore 1 upon photo excitation generates hole and electron pair. The hole oxidizes water to liberate oxygen and electrons reduce CO2 to formic acid. The ratio of the amount of final product obtained i.e. HCOOH: O2 is not 2:1 although theoretically the product ratio should have been 2:1. In our present case the electrons released during photochemical water oxidation are not completely transferred to the CO2, and we believe to a certain extent the electrons are solvated and quenched by the reaction medium which leads to a decrease in the formic acid yield with respect to the theoretical possibility of formic acid formed in the reaction. Hence the ratio of formation of formic acid to oxygen is not 2:1 but lesser. Further we have also characterized oxygen from Cyclic Voltammetry by increasing current at 1.2 V vs Ag/AgCl electrode with respect to blank sample (without illuminating light to the reaction mixture) (Figure 4a).To prove further oxygen is formed from water not from any other source, we have performed photochemical CO2 reduction reaction with a mixture of H218O and H216O (1:9). In that case we have obtained 16O16O, as well as 18

18

O16O, and

O18O from the mass spectrum of gaseous reaction mixture (Figure S9). These results conclude that here

simultaneously water is getting oxidized to liberate oxygen and CO2 is photochemically getting reduced to formic acid.



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Figure 3. a) Band position of 1 with respect to NHE at pH-0, and schematic representation of light driven CO2 reduction. b) Formation of formic acid from CO2. c) HPLC profile of formic acid and the reaction mixture: In blue, with CO2 – photo chemical CO2 reduction reaction purging with CO2; in red, photo chemical CO2 reduction reaction without purging CO2 keeping other reaction conditions unaltered. d) Raman spectrum of 12CO2 purged (Black line) and 13CO2 purged (violet line) post reaction solution. Now we discuss in detail CO2 reduction reaction. We observe that for a maximum loading of 4.8 µmol of 1 ([1a] = 2.37 nmol) of catalyst we get a maximum yield of 0.64mmol formic acid at pH 6 with maximum turnover number of 613 with respect to 1 (TON= no. of moles of product formed/ no. of moles of catalyst taken)(detailed Calculation is given in SI).The maximum turnover number with respect to 1a is 8.3X105 (TON= no. of moles of formic acid formed/concentration of 1 in moles) which is high as compared to recent developments in the field of CO2 reduction catalysis. We have also varied the loading of 1 within the range of 0.24 µmol to 4.8 µmol ([1a]= 0.17nmol to 2.37nmol) and it increases with increasing loading of the catalyst but after a certain limit of catalyst loading the formation of formic acid becomes constant (Figure 4c). It leads to a decrease in turnover number of the reaction (Figure 4e and 4f). We have also


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performed the time dependent study of the reaction at pH 6 using 4.8 µmol of 1 ([1a] = 2.37 nmol) (Figure 4b). Completion of reaction takes 13h with a TOF of 47.15 h-1 with respect to 1 (63,621 h-1 with respect to 1a). We have also performed pH dependent experiment in the stability window of 1 for different pH and this shows an interesting result (Figure 4d). As CO2 reduction reaction is a proton coupled electron transfer process hence it increases with decreasing pH (Eq. 3). But we have observed a breakdown in the general trend, where we got maximum formic acid at pH-6 instead of more acidic pH-5. Furthermore it gradually decreases with increasing pH from neutral pH to basic pH (Figure 3d). We believe water is getting oxidized and it releases electron which is transferred to the CO2 reduction reaction. As we know that water oxidation increases ongoing from acidic to basic pH (Eq. 2), hence the total number of available electrons in the reaction mixture is increased as we move from acidic to basic pH.

2H2O CO2 + 2H+ + 2e-

O2 + 4H+ + 4e-

(2)

HCOOH

(3)



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Figure 4. a) CV diagram, blank represents CV diagram of irradiated sample without addition of catalyst. 1-CO2 red. represents CO2 reduction at pH 7, using 1 with loading of 1.42 nmol 1. b) Time dependent formic acid formation at pH 6. c) Variation of formic acid formation with different loading of 1 at pH 6. d) pH dependent formic acid formation using 2.37 nmol of 1. e) Effect of loading of 1a on turnover number of reaction at pH 6. f) Variation of turnover number of photochemical CO2 reduction with respect to concentration of 1. Dependency of CO2 reduction on water and light We have already mentioned that pH dependent study already shows CO2 reduction depends on solvent water present in the reaction. To prove our point we have performed a set of experiments where we prepared different reaction sets with varying DMF and water ratio. We have observed that with increasing amount of water in DMF, CO2 reduction is initiated and it increases with increasing amount of water in reaction mixture (Figure 5). The increment of formation of formic acid is obtained from HPLC (Figure 5f) and cyclic voltammetry (Figure 5c), parallel oxygen evolution also increases with increasing water contain in reaction medium. This proves that CO2 reduction depends on the water present in the reaction system.Now the yield of the formic acid is observed to be maximum at pH 6. This result can be explained if we consider simultaneous water oxidation reaction in the system. Water oxidation increases with increasing pH of the solution, therefore the amount of electron release in the reaction medium increases with the increasing pH of the solution. As the reducing equivalents i.e. electrons of CO2 reduction reaction mainly come from water oxidation reaction therefore the CO2 reduction increases from pH 5 to 6. The yield of formic acid decreases when we move from pH 6 to 7 although water oxidation enhances due to the decrease of proton concentration in the reaction medium. At pH 5 and 6 excess protons are present in the reaction medium which enhance the forward reaction of the equilibrium. Thus all the above results indicate that CO2 reduction depends on water oxidation reaction. We have also performed the same reaction in the absence of light to show the effect of light in CO2 reduction reaction and in that case we do not get any reduced CO2 product from the reaction mixture


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(Figure 5a & 5d)which implies that our reaction depends on light. To further know which one is the effective catalyst, we have performed the same reaction using CuCl2 and Na2WO4, separately as catalysts and in that case also no reduced product was observed with those reagents (Figure 5b and 5e). This proves that 1a present in the core of 1 is responsible for CO2 reduction reaction and not the constituent reagents from which we prepare 1 or 1a.

Figure 5. CV diagram of CO2 reduction: a) in presence & absence of light; b) With 1, and precursor constituents of 1 and without catalyst at pH 6; c) at different ratios of water: DMF mixture, water amount decreases from 1 to 6. Quantitative estimation of formic acid d) in presence & absence of light; e) 1, precursor constituents of 1 and without catalyst at pH 6; f) at different ratios of water: DMF mixture, where water amount decreases from 1 to 6; Now, it is possible to draw the possible reaction pathway of this photochemical CO2 reduction reaction (Figure 6). Upon CO2 purging on the dispersion of the 1 we observe an increase in current as well as we also get two new peaks in cyclic voltammogram of 1 (Figure 6a). This result indicates that the CO2 is adsorbed on the surface of the 1. And those new peaks obtained in CV corresponds to CO2 adsorbed catalyst species. Now upon photo excitation of the 1 it goes to excited state 1*. The photo activity of 1 is further proven by dye degradation experiment using methylene blue as a dye (Figure 6b).65Here the



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methylene blue degrades gradually (with gradual decrease of absorbance of methylene blue dye) with time under light in presence of 1which supports further the photoactivity of the 1.Possibly{PW12O40} core of the 1 is the photo active species of the catalyst material. Now on photo-excitation {PWVI12O40} goes to the excited state to form {PWVI12O40}*. This species oxidizes water to liberate oxygen and the {W12O40} core gets reduced by the electrons which are released during the water oxidation process66-67 (Figure 6). Thus few WVI centres get reduced to WV. The reduced species shows inter-valance charge transfer (IVCT) from WV to WVI which gives rise to absorbance maxima at VIS-NIR region. The formation of reduced softoxometalate during the reaction is observed from time dependent electronic adsorption spectrum (Figure 6c).68-69 Now probably CO2 reduction takes place at the Cu centre. First CuII get reduced to Cu0 by the reduced tungsten, and the {PW12O40} core again comes back to its initial + VI oxidation state. Possibly the Cu0 is the active catalyst centre for the CO2 reduction reaction,70 and it reduces CO2 to formic acid via proton coupled electron transfer process and Cu0 goes to its initial oxidation state i.e. CuII again (The oxidation state of the post reaction catalyst material is obtain from EPR spectrum (Figure S11)), and the catalytic cycle continues further.The stability of the 1 is obtained from the XRD pattern of the post reaction catalyst material which is similar to that of the XRD pattern of the starting catalyst material (Figure 6d). Further the stability studies of 1are mentioned in detail in the next section. Thus here photochemical CO2 reduction takes place in water, and water acts as electron as well as proton donor in the reaction.


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Figure 6. Possible reaction pathway of photochemical CO2 reduction in water using 1 as a catalyst. A) Cyclic Voltammogram of 1 in Ar atmosphere and CO2 atmosphere. B) EAS spectrum of methylene blue dye during dye degradation experiment with 1. C) EAS spectrum of 1 at different time during photochemical CO2 reduction reaction. D) XRD pattern of 1 before and after reaction. Determination of Stability of catalyst material 1 The stability of the catalyst in such photo catalytic reaction is always of extreme importance. To check the stability of the catalyst we have also performed FT-IR (Figure 7a) and Raman spectroscopy (Figure 7b) of the catalyst 1 after reaction, and compared it with the spectra of the starting catalyst material. It is observed that the catalyst remains stable throughout the reaction time. Further the composition of the post reaction catalyst material is obtained from ICP-MS analysis. The analysis indicates 7.82% (Atomic percentage) of Cu and 92.18% (Atomic percentage) of W is present in the catalyst material. It represents an atomic ration of Cu: W is 1:11.8 which indicates that the catalyst remains intact after the completion of



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reaction. Now as our catalyst is stable in our reaction condition we can reuse our catalyst further. We have used our catalyst upto ten catalytic cycles. It is observed that the catalyst remains intact after all catalytic cycles and yield of formic acid is similar in all catalytic cycles (Figure 7c).Thus we have developed a catalyst material which is stable under photo catalytic condition and reusable for further catalytic cycles.

Figure 7. a) FT-IR spectrum, b) Raman spectrum of 1 before and after reaction indicates catalyst is stable throughout reaction condition. c) Recyclability of 1 in the matter of photochemical CO2 reduction reaction.

CONCLUSION From

single

crystal

X-ray

diffraction

[{K6.5Cu(OH)8.5(H2O)7.5}0.5@{K3PW12O40}].

analysis 1.

It

we

forms

have

explained

soft-oxometalate

the

structure

superstructure

of {Cu-

PW12}n=1348-2024 in water via self-assembly which participates in photochemical CO2 reduction coupled with water oxidation reaction, with a high turnover number of 8.3 x 105. In the course of reaction we do not use any photosensitizer as well as sacrificial electron donor e.g; amine which also makes the process cheap and eco-friendly. The SOM is stable throughout the irradiation time therefore the catalyst is reusable for photochemical CO2 reduction reaction. Our system can enable the operation of two different redox processes in one reaction medium. Thus in short we have developed a model carbon dioxide reduction system where water is oxidized to oxygen and transfers and donates electron to reduce carbon dioxide to formic acid. We have developed the molecular polyoxometalate as a model system, converted


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it into softoxometalate the efficient colloidal system and have shown the process of photochemical CO2 reduction with concomitant water oxidation. The work opens up avenues for understanding and designing of colloidal systems for carrying out such redox reactions where the molecular insight on the process can be obtained precisely down to the last atomic detail.

EXPERIMENTAL SECTION Preparation of [{K6.5Cu(OH)8.5(H2O)7.5}0.5@{K3PW12O40}] (1) Na2WO4·2H2O (1.68g, 0.108 mol), CuCl2.4H2O (160mg, 0.77 mmol) were mixed in 30 ml of water. Na2HPO4·7H2O (78.5 mg, 0.012 mol) was added to maintain the pH of solution at 7-8. A green suspension was formed, this green suspension was then refluxed at 100°C for two hours. After reflux, 300 mg of CH3COOK was added and the resulting mixture was allowed to cool to room temperature. Then the reaction mixture was filtered through glass frit. A green solution was obtained which was stored for crystallization. The resulting green crystals appeared after a week. The crystals were collected and dried under vacuum overnight (yield 32% based on Cu). Preparation of [{K6.5Cu(OH)8.5(H2O)7.5}0.5@{K3PW12O40}] based soft-oxometalate (1a) SOM based on 1a is prepared by dispersing it in 10 ml water. 0.24 µmol of 1 is added to 10 ml deionized water. The reaction mixture is sonicated upto 1hr at room temperature. After 1hr SOM 1a is formed in dispersion. General reaction procedure for photo catalytic CO2 reduction Photo catalytic carbon dioxide reduction reactions are carried out as follows. Desired amount of catalyst (1a) is taken in 10 ml of degassed double distilled water. The reaction mixture is sealed in a reaction vial and CO2 gas is purged for 2 hours. Then the reaction mixture is kept in the photo reactor under uv-light (150W lamp, λ= 280-400nm) for various intervals of time. 20 µL reaction mixture is taken out and further diluted with 10 ml double distilled water. We have performed HPLC measurement by injecting the above diluted reaction mixture in carbohydrate column with an external standard 0.1M formic acid solution.



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Here also observe that formic acid is present in our reaction mixture. We all the quantitative measurements of formic acid is performed by HPLC experiment. Next we performed CV with the reaction mixture using 0.1 M KCl as an electrolyte, in a potential range of +1.3V to -1.2 V with respect to Ag/AgCl reference electrode in standard 3-electrode system. Reference electrode is filled with saturated KCl solution. We get a peak around -0.6V in cyclic voltammogram which further confirms the formation of formic acid from carbon dioxide in our reaction mixture. During the reaction oxygen also evolve from the reaction mixture which is confirmed from cyclic voltammetry where we observe a rise in current at 1.2V. SUPPORTING ONLINE MATERIAL X-ray crystallographic file (CIF), detailed experimental procedure, characterization techniques are given in supporting online material. CORRESPONDING AUTHOR [email protected], [email protected] (SR) ACKNOWLEDGEMENT SR gratefully acknowledges the FIRE and start-up grants from IISER-Kolkata, India, DST-fast track, BRNS-DAE grant. Thanks are due to Dr. A. Dey of IACS, Kolkata for the help with 13CO2 experiments.

REFERENCE

1. Jacobson, M. Z.; Colella, W. G.; Golden, D. M. Cleaning the Air and Improving Health with Hydrogen Fuel-Cell Vehicles. Science 2005, 308, 1901-1905. 2. Aresta, M., Ed. Carbon Dioxide as Chemical Feedstock. John Wiley & Sons: 2010. 3. Nocera, D. G. The Artificial Leaf. Acc. Chem. Res. 2012, 45, 767-776. 4. Zhang, T.; Lin, W. Metal–Organic Frameworks for Artificial Photosynthesis and Photocatalysis. Chem. Soc. Rev. 2014, 43, 5982-5993. 5. Chang, J.; Feng, L.; Liu, C.; Xing, W.; Hu, X. An Effective Pd–Ni2P/C Anode Catalyst for Direct Formic Acid Fuel Cells. Angew. Chem. Int. Ed. 2014, 53, 122-126. 6. Wang, R.; Liu, J.; Liu, P.; Bi, X.; Yan, X.; Wang, W.; Ge, X.; Chen, M.; Ding, Y. Dispersing Pt Atoms onto Nanoporous Gold for High Performance Direct Formic Acid Fuel Cells. Chem. Sci. 2014, 5, 403-409.


Page 17 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


7. Miller S. L.; Urey, H. C. Organic Compound Synthes on the Primitive Earth. Science 1959, 130, 245251. 8. Graetz, J. New Approaches to Hydrogen Storage. Chem. Soc. Rev. 2009, 38, 73-82. 9. Castro-Rodriguez, I.; Meyer, K. Carbon Dioxide Reduction and Carbon Monoxide Activation Employing a Reactive Uranium (III) Complex. J. Am. Chem. Soc. 2005, 127, 11242-11243. 10. Sakakura, T.; Choi, J.-C.; Yasuda, H. Transformation of Carbon Dioxide. Chem. Rev. 2007, 107, 23652387. 11. Mandal, S. K.; Roesky, H. W. Group 14 Hydrides with Low Valent Elements for Activation of Small Molecules. Acc. Chem. Res. 2011, 4, 298-307. 12. Laitar, D. S.; Müller, P.; Sadighi, J. P. Efficient Homogeneous Catalysis in the Reduction of CO2 to CO. J. Am. Chem. Soc. 2005, 127, 17196-17197. 13. Langer, R.; Diskin-Posner, Y.; Leitus, G.; Shimon, L. J.; Ben-David, Y.; Milstein, D. Low-Pressure Hydrogenation of Carbon Dioxide Catalyzed by an Iron Pincer Complex Exhibiting Noble Metal Activity. Angew. Chem. Int. Ed. 2011, 50, 9948-9952. 14. Amatore, C.; Saveant, J. M. Mechanism and Kinetic Characteristics of the Electrochemical Reduction of Carbon Dioxide in Media of Low Proton Availability. J. Am. Chem. Soc. 1981, 103, 5021-5023. 15. Costentin, C.; Robert, M.; Savéant, J.-M. Catalysis of the Electrochemical Reduction of Carbon Dioxide. Chem. Soc. Rev. 2013, 42, 2423-2436. 16. Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F. New Insights into the Electrochemical Reduction of Carbon Dioxide on Metallic Copper Surfaces. Energy Environ. Sci. 2012, 5, 7050-7059. 17. Zhang, S.; Kang, P.; Meyer, T. J. Nanostructured Tin Catalysts for Selective Electrochemical Reduction of Carbon Dioxide to Formate. J. Am. Chem. Soc. 2014, 136, 1734-1737. 18. Kornienko, N.; Zhao, Y.; Kley, C. S.; Zhu, C.; Kim, D.; Lin, S.; Chang, C. J.; Yaghi, O. M.; Yang, P. MetalOrganic Frameworks for Electrocatalytic Reduction of Carbon Dioxide. J. Am. Chem. Soc. 2015, 137, 14129-14135. 19. Lin, S.; Diercks, C. S.; Zhang, Y.-B.; Kornienko, N.; Nichols, E. M.; Zhao, Y.; Paris, A. R.; Kim, D.; Yang, P.; Yaghi, O. M. Covalent Organic Frameworks Comprising Cobalt Porphyrins for Catalytic CO2 Reduction in Water. Science 2015, 349, 1208-1213. 20. Matsuoka, S.; Yamamoto, K.; Ogata, T.; Kusaba, M.; Nakashima, N.; Fujita, E.; Yanagida, S. Efficient and Selective Electron Mediation of Cobalt Complexes with Cyclam and Related Macrocycles in the pTerphenyl-Catalyzed Photoreduction of Carbon Dioxide. J. Am. Chem. Soc. 1993, 115, 601-609. 21. Morris, A. J.; Meyer, G. J.; Fujita, E., Molecular Approaches to the Photocatalytic Reduction of Carbon Dioxide for Solar Fuels. Acc. Chem. Res. 2009, 42, 1983-1994. 22. Takeda, H.; Koike, K.; Inoue, H.; Ishitani, O. Development of an Efficient Photocatalytic System for CO2 Reduction Using Rhenium (I) Complexes Based on Mechanistic Studies. J. Am. Chem. Soc. 2008, 130, 2023-2031. 23. Sato, S.; Morikawa, T.; Kajino, T.; Ishitani, O. A highly Efficient Mononuclear Iridium Complex Photocatalyst for CO2 Reduction under Visible Light. Angew. Chem. Int. Ed. 2013, 52, 988-992. 24. Wang, Y.; Bai, X.; Qin, H.; Wang, F.; Li, Y.; Li, X.; Kang, S.; Zuo, Y.; Cui, L. Facile One Step Synthesis of Hybrid Graphitic Carbon Nitride and Carbon Composites as High Performance Catalysts for CO2 Photocatalytic Conversion. ACS Appl. Mater. Interfaces 2016, 8,17212-17219. 25. Wang, S.; Hou, Y.; Wang, X. Development of a Stable MnCo2O4 Cocatalyst for Photocatalytic CO2 Reduction with Visible Light. ACS Appl. Mater. Interfaces 2015, 7, 4327-4335. 26. Wang, S.; Yao, W.; Lin, J.; Ding, Z.; Wang, X. Cobalt Imidazolate Metal–Organic Frameworks Photosplit CO2 under Mild Reaction Conditions. Angew. Chem. Int. Ed. 2014, 53, 1034-1038. 27. Barton, E. E.; Rampulla, D. M.; Bocarsly, A. B. Selective Solar-Driven Reduction of CO2 to Methanol Using a Catalyzed P-Gap Based Photoelectrochemical Cell. J. Am. Chem. Soc. 2008, 130, 6342-6344.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 21

28. Halmann, M. Photoelectrochemical Reduction of Aqueous Carbon Dioxide on p-Type Gallium Phosphide in Liquid Junction Solar Cells. Nature 1978, 275, 115-116. 29. Hawecker, J.; Lehn, J.-M.; Ziessel, R. Electrocatalytic Reduction of Carbon Dioxide Mediated by Re (bipy)(CO)3Cl (bipy= 2, 2′-bipyridine). Chem. Commun. 1984, 6, 328-330. 30. Beley, M.; Collin, J.-P.; Ruppert, R.; Sauvage, J.-P. Nickel (II)-Cyclam: an Extremely Selective Electrocatalyst for Reduction of CO2 in Water. Chem. Commun. 1984, 19, 1315-1316. 31. Fisher, B. J.; Eisenberg, R. Electrocatalytic Reduction of Carbon Dioxide by Using Macrocycles of Nickel and Cobalt. J. Am. Chem. Soc. 1980, 102, 7361-7363. 32. Wang, S.; Wang, X. Imidazolium Ionic Liquids, Imidazolylidene Heterocyclic Carbenes, and Zeolitic Imidazolate Frameworks for CO2 Capture and Photochemical Reduction. Angew. Chem. Int. Ed. 2016, 55, 2308-2320. 33. Porosoff, M. D.; Yang, X.; Boscoboinik, J. A.; Chen, J. G. Molybdenum Carbide as Alternative Catalysts to Precious Metals for Highly Selective Reduction of CO2 to CO. Angew. Chem. Int. Ed. 2014, 53, 67056709. 34. Chen, W. F.; Sasaki, K.; Ma, C.; Frenkel, A. I.; Marinkovic, N.; Muckerman, J. T.; Zhu, Y.; Adzic, R. R. Hydrogen-Evolution Catalysts Based on Non-Noble Metal Nickel–Molybdenum Nitride Nanosheets. Angew. Chem. Int. Ed. 2012, 51, 6131-6135. 35. Zhao, J.; Wang, X.; Xu, Z.; Loo, J. S. Hybrid Catalysts for Photoelectrochemical Reduction of Carbon Dioxide: A Prospective Review on Semiconductor/Metal Complex Co-Catalyst Systems. J. Mater. Chem. A 2014, 2, 15228-15233. 36. Kuriki, R.; Matsunaga, H.; Nakashima, T.; Wada, K.; Yamakata, A.; Ishitani, O.; Maeda, K. NatureInspired, Highly Durable CO2 Reduction System Consisting of a Binuclear Ruthenium (II) Complex and an Organic Semiconductor Using Visible Light. J. Am. Chem. Soc. 2016, 138, 5159-5170. 37. Yoshitomi, F.; Sekizawa, K.; Maeda, K.; Ishitani, O. Selective Formic Acid Production via CO2 Reduction with Visible Light Using a Hybrid of a Perovskite Tantalum Oxynitride and a Binuclear Ruthenium (II) Complex. ACS Appl. Mater. Interfaces 2015, 7, 13092-13097. 38. Kuriki, R.; Sekizawa, K.; Ishitani, O.; Maeda, K. Visible-Light-Driven CO2 Reduction with Carbon Nitride: Enhancing the Activity of Ruthenium Catalysts. Angew. Chem. Int. Ed. 2015, 54, 2406-2409. 39. Wang, S.; Wang, X., Photocatalytic CO2 reduction by CdS Promoted with a Zeolitic Imidazolate Framework. Appl. Catal., B 2015, 162, 494-500. 40. Iizuka, K.; Wato, T.; Miseki, Y.; Saito, K.; Kudo, A. Photocatalytic Reduction of Carbon Dioxide Over Ag Cocatalyst-Loaded ALa4Ti4O15 (A= Ca, Sr, and Ba) Using Water as a Reducing Reagent. J. Am. Chem. Soc. 2011, 133, 20863-20868. 41. Xie, S.; Wang, Y.; Zhang, Q.; Deng, W.; Wang, Y. MgO-and Pt-promoted TiO2 as an Efficient Photocatalyst for the Preferential Reduction Of Carbon Dioxide in the Presence of Water. ACS Catal. 2014, 4, 3644-3653. 42. Arai, T.; Sato, S.; Kajino, T.; Morikawa, T. Solar CO2 reduction using H2O by a Semiconductor/MetalComplex Hybrid Photocatalyst: Enhanced Efficiency and Demonstration of a Wireless System Using SrTiO3 Photoanodes. Energy Environ. Sci. 2013, 6, 1274-1282. 43. Arai, T.; Sato, S.; Morikawa, T. A Monolithic Device for CO2 Photoreduction to Generate Liquid Organic Substances in a Single-Compartment Reactor. Energy Environ. Sci. 2015, 8, 1998-2002. 44. Schreier, M.; Curvat, L.; Giordano, F.; Steier, L.; Abate, A.; Zakeeruddin, S. M.; Luo, J.; Mayer, M. T.; Grätzel, M. Efficient Photosynthesis of Carbon Monoxide from CO2 Using Perovskite Photovoltaics. Nat. commun. 2015, 6, 7326(1-6). 45. Das, S.; Biswas, S.; Balaraju, T.; Barman, S.; Pochamoni, R.; Roy, S. Photochemical Reduction of Carbon Dioxide Coupled with Water Oxidation Using Various Soft-Oxometalate (SOM) Based Catalytic Systems. J. Mater. Chem. A 2016, 4, 8875-8887.


Page 19 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


46. Roy, S. Soft-Oxometalates Beyond Crystalline Polyoxometalates: Formation, Structure and Properties. CrystEngComm 2014, 16, 4667-4676. 47. Das, S.; Thomas, P.; Roy, S. Photoinduced Topological Transformation in Mesoscopic Inorganic Nanoparticles: Application as a UV Sensor. Eur. J. Inorg. Chem. 2014, 2014, 4551-4557. 48. Das, S.; Kumar, S.; Mallick, A.; Roy, S. Competitive Self-Assembly Manifests Supramolecular Darwinism in Soft-Oxometalates. J. Mol. Eng. Mater. 2015, 3, 1540008(1-7). 49. Roy, S. “Soft Oxometalates”(SOMs): A Very Short Introduction. Comments Inorg. Chem. 2011, 32, 113-126. 50. Lv, H.; Geletii, Y. V.; Zhao, C.; Vickers, J. W.; Zhu, G.; Luo, Z.; Song, J.; Lian, T.; Musaev, D. G.; Hill, C. L. Polyoxometalate Water Oxidation Catalysts and the Production Of Green Fuel. Chem. Soc. Rev. 2012, 41, 7572-7589. 51. Al-Oweini, R.; Sartorel, A.; Bassil, B. S.; Natali, M.; Berardi, S.; Scandola, F.; Kortz, U.; Bonchio, M. Photocatalytic Water Oxidation by a Mixed-Valent MnIII3MnIVO3 Manganese Oxo Core that Mimics the Natural Oxygen-Evolving Center. Angew. Chem. Int. Ed. 2014, 53, 11182-11185. 52. Angamuthu, R.; Byers, P.; Lutz, M.; Spek, A. L.; Bouwman, E. Electrocatalytic CO2 Conversion to Oxalate by a Copper Complex. Science 2010, 327, 313-315. 53. Haines, R. J.; Wittrig, R. E.; Kubiak, C. P. Electrocatalytic Reduction of Carbon Dioxide by the Binuclear Copper Complex [Cu2 (6-(diphenylphosphino-2, 2'-bipyridyl) 2 (MeCN)2][PF6]2. Inorg. Chem. 1994, 33, 4723-4728. 54. Li, C. W.; Kanan, M. W. CO2 Reduction at Low Overpotential on Cu Electrodes Resulting from the Reduction of Thick Cu2O Films. J. Am. Chem. Soc. 2012, 134, 7231-7234. 55. Kim, D.; Resasco, J.; Yu, Y.; Asiri, A. M.; Yang, P. Synergistic Geometric and Electronic Effects for Electrochemical Reduction of Carbon Dioxide Using Gold–Copper Bimetallic Nanoparticles. Nat. commun. 2014, 5, 4948(1-8). 56. Fang, B.; Xing, Y.; Bonakdarpour, A.; Zhang, S.; Wilkinson, D. P. Hierarchical CuO–TiO2 Hollow Microspheres for Highly Efficient Photodriven Reduction of CO2 to CH4. ACS Sus. Chem. Eng. 2015, 3, 2381-2388. 57. Back, S.; Kim, J.-H.; Kim, Y.-T.; Jung, Y. Bi-Functional Interface of Au and Cu for Improved CO2 Electroreduction. ACS Appl. Mater. Interfaces 2016, 8, 23022-23027. 58. Xie, M. S.; Xia, B. Y.; Li, Y.; Yan, Y.; Yang, Y.; Sun, Q.; Chan, S. H.; Fisher, A.; Wang, X. Amino Acid Modified Copper Electrodes for the Enhanced Selective Electroreduction of Carbon Dioxide Towards Hydrocarbons. Energy Environ. Sci. 2016, 9, 1687-1695. 59. Das, K.; Roy, S. Direct Synthesis of Controlled-Size Nanospheres inside Nanocavities of Self-Organized Photopolymerizing Soft Oxometalates [PW12O40]n (n= 1100–7500). Chem. Asian J. 2015, 10, 1884-1891. 60. Das, S.; Misra, A.; Roy, S. Enhancement of Photochemical Heterogeneous Water Oxidation by a Manganese Based Soft Oxometalate Immobilized on a Graphene Oxide Matrix. New J. Chem. 2016, 40, 994-1003. 61. Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds. (Fourth Eds.) Wiley Online Library: 1986. 62. Inamdar, S. N.; Ingole, P. P.; Haram, S. K. Determination of Band Structure Parameters and the QuasiParticle Gap of CdSe Quantum Dots by Cyclic Voltammetry. ChemPhysChem 2008, 9, 2574-2579. 63. Tauc, J. Optical Properties and Electronic Structure of Amorphous Ge and Si. Mater. Res. Bull. 1968, 3, 37-46. 64. Chang, X.; Wang, T.; Gong, J. CO2 Photo-Reduction: Insights into CO2 Activation and Reaction on Surfaces of Photocatalysts. Energy Environ. Sci. 2016, 9, 2177-2196. 65. Whang, T.-J.; Huang, H.-Y.; Hsieh, M.-T.; Chen, J.-J. Laser-Induced Silver Nanoparticles on Titanium Oxide for Photocatalytic Degradation of Methylene Blue. Int. J. Mol. Sci. 2009, 10, 4707-4718.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 21

66. Tu, W.; Zhou, Y.; Feng, S.; Xu, Q.; Li, P.; Wang, X.; Xiao, M.; Zou, Z. Hollow Spheres Consisting of Ti 0.91O2/CdS Nanohybrids for CO2 Photofixation. Chem. Commun. 2015, 51, 13354-13357. 67. Wang, Y.; Li, F.; Li, H.; Bai, L.; Sun, L. Photocatalytic Water Oxidation Via Combination of BiVO4–RGO and Molecular Cobalt Catalysts. Chem. Commun. 2016, 52, 3050-3053. 68. Feng, W.; Zhang, T. R.; Liu, Y.; Lu, R.; Zhao, Y. Y.; Yao, J. N. Photochromic Behavior of Nanocomposite Hybrid Films of Finely Dispersed Phosphotungstic Acid Particles into Polyacrylamide. J. Mater. Sci. 2003, 38, 1045-1048. 69. Chen, D.; Sahasrabudhe, A.; Wang, P.; Dasgupta, A.; Yuan, R.; Roy, S. Synthesis and Properties of a Novel Quarternerized Imidazolium [α-PW12O40]3- Salt as a Recoverable Photo-Polymerization Catalyst. Dalton Trans. 2013, 42, 10587-10596. 70. Kim, W.; Edri, E.; Frei, H. Hierarchical Inorganic Assemblies for Artificial Photosynthesis. Acc. Chem. Res. 2016, 49, 1634-1645.


Page 21 of 21

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Graphic for manuscript

A Softoxometalate [{K6.5Cu(OH)8.5(H2O)7.5}0.5 @{K3PW12O40}]n (n= 1348-2024) as an Efficient Inorganic Material for CO2 Reduction with Concomitant Water Oxidation

Santu Das, Saurabh Kumar, Somenath Garai, Ramudu Pochamoni, Shounik Paul, Soumyajit Roy* __________ ACS Appl. Mater. Interfaces, 2017, Vol, Page – Page A softoxometalate, based on {[K6.5Cu(OH)8.5 (H2O)7.5]0.5[K3PW12O40]} metal oxide 3D-framework which is stable in reaction condition that effectively performs photochemical CO2 reduction reaction in water with a very high turnover number of 613 and TOF of 47.15 h-1 is reported. We observe that during this reaction water gets oxidized to oxygen, while the electrons released directly reduce CO2 to formic acid. A detailed account of characterization of the catalyst along with that of products of this reaction is reported.


as an Efficient Inorganic Material for CO2 Reduction

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