Heterostructured nanocomposite tin phthalocyanine@mesoporous ceria (SnPc@CeO₂) for photoreduction of CO₂ in visible light

Abstract

Heterostructured tin phthalocyanine supported to mesoporous ceria was synthesized and used a photocatalyst for CO₂ reduction under visible light. The photoreduction CO₂ activities of the heterostructures were investigated in the presence of triethylamine as sacrificial agent. The developed photocatalyst exhibited high catalytic activity for photoreduction of CO₂ and after 24 hours of visible light irradiation 2342 μmol g⁻¹ cat of methanol (ϕ_MeOH = 0.0223 or 2.23%) and 840 μmol g⁻¹ cat of CO (ϕ_CO = 0.0026 or 0.26%) were obtained as the major reaction products. The methanol formation rate (R_MeOH) and CO formation rate (R_CO) was found to be 97.5 μmol h⁻¹ g⁻¹ cat and 35.0 μmol h⁻¹ g⁻¹ cat respectively. While under the identical experimental conditions mesoporous ceria (meso-CeO₂) gave only 316 μmol g⁻¹ cat of methanol (ϕ_MeOH = 0.003 or 0.30%) and 126 μmol g⁻¹ cat CO (ϕ_CO = 0.0004 or 0.04%) with product formation rate R_MeOH = 13.2 μmol h⁻¹ g⁻¹ cat and R_CO = 5.3 μmol h⁻¹ g⁻¹ cat. Furthermore, the recovered catalyst showed consistent catalytic activity for at least five runs without any significant loss in product yields.

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1 Introduction

Continuous rise in emission of anthropological carbon dioxide (CO₂) and the depletion of fossil resources have driven the research interest in developing processes for converting CO₂ to value added chemicals.¹ Among the different processes known, solar assisted photocatalytic conversion of CO₂ to renewable fuels is regarded as a long term beneficial approaches that not only tackles global warming but also partly fulfils energy demands.² In this regard, Inoue et al.³ in 1979 first reported photoreduction of CO₂ to organic compounds under UV light and after that increasing efforts are being focused on extending the light response of TiO₂ to visible region.⁴ However, only limited successes were achieved due to the wide band gap of the matrix TiO₂ (3.0–3.2 eV). Recently, several semiconductor based photocatalysts such as modified TiO₂,⁵ CdS,⁶ ZnS,⁷ mixed metal oxides⁸ for CO₂ reduction have been developed, but the efficiencies are still far from practical applications because of fast charge recombination rates. To overcome the limitations of the narrow visible-light absorption and fast charge recombination of semiconductor photocatalysts, several strategies for instance band-gap engineering by doping of various metals (Cu,⁹ Ag,¹⁰ Ru¹¹ etc.) and by attaching visible light absorbing molecular complexes¹² on photoactive supports to give heterostructured materials have recently been developed. Particularly, construction of heterostructured materials by heterogenization of homogeneous metal complexes is fascinating as it provides high efficiencies as compared to semiconductors as well as facile recovery and recycling ability of these catalysts make them more attractive from economical viewpoints. Among the various transition metal based complexes, polypyridyl complexes of ruthenium,¹³ iridium¹⁴ have been used as photocatalyst for photoreduction of CO₂ under visible light. However, expensive nature and limited accessibility of these precious metal catalysts make the developed processed less desirable from economical view points.

Phthalocyanines and porphyrins are important class of organic dyes which posses a number of unique properties such as excellent semi-conductivity, photoconductivity, chemical stability and optical absorption in the UV-Vis region.¹⁵ Particularly, metallophthalocyanines owing to their ease of synthesis, high thermal stability, excellent electrical and optical properties have attracted a great deal of attention and have been used as light-harvesting materials to broaden the absorption range of semiconductors.¹⁶ Anchoring of these dye complex units to the nanocrystalline semiconductor supports enable ultrafast injection of electrons from the excited state into the conduction band of semiconductor to give improved conversion efficiency. In this regard, recently we reported graphene oxide (GO)–cobalt phthalocyanine complex as an effective, recyclable visible light photocatalyst to photoreduce CO₂ to methanol.¹⁷ In continuation to our on-going research on visible light assisted photoreduction of CO₂,¹⁸ herein we report an efficient heterostructured composite constructed by the combination of tin phthalocyanine and mesoporous ceria for the photoreduction of carbon dioxide using triethylamine as sacrificial donor under visible light irradiation.

2 Results and discussion

2.1 Synthesis and characterization of catalyst

During the present study, meso-CeO₂ was prepared by combustion method using chitosan as template as suggested in the literature.¹⁹ The synthesized meso-CeO₂ was first treated by water to enhance the –OH groups on the surface. Tin(IV) phthalocyanine dichloride was attached to meso-CeO₂ through axial position by taking advantage of –OH functional groups located on the surface of meso-CeO₂ (Scheme 1).

Scheme 1 Synthesis of SnPc@CeO₂ catalyst.

Fig. 1 shows the SEM images of uncalcined ceria, meso-CeO₂ and supported SnPc@CeO₂. The SEM image of uncalcined CeO₂ samples show unorganised rough surface (Fig. 1a). The SEM image of meso-CeO₂ show erupted ridge type structures that may be appeared due to the burning of template in the combustion step (Fig. 1b). The supported catalyst i.e. SnPc@CeO₂ also showed similar crinkled morphology as of meso-CeO₂ Fig. 1c. Furthermore appearance of Sn, Ce, C, N and O elements in EDX pattern of SnPc@CeO₂ confirmed the presence of tin phthalocyanine in the synthesized material.

Fig. 1 SEM images of (a) uncalcined CeO₂ (b) meso-CeO₂ and (c) SnPc@CeO₂ and EDX pattern of (d) uncalcined CeO₂ (e) meso-CeO₂ and (f) SnPc@CeO₂.

The microstructure of meso-CeO₂ and SnPc@CeO₂ was determined with TEM as shown in Fig. 2a and b, respectively. TEM image clearly indicated that the size of particles in the synthesized meso-CeO₂ was in the range of 5–10 nm diameters (Fig. 2a). After the attachment of SnPc to meso-CeO₂ although the morphology remained almost same with 5–10 nm diameter, however some aggregation was observed due to cementing with SnPc (Fig. 2b). SAED pattern of SnPc@CeO₂ clearly depicted rings due to diffraction of 111, 220 and 311 plane with 1.65 Å, 1.05 Å and 0.82 Å d spacing of meso-CeO₂ which explained the crystalline morphology of material (Fig. 2c). Further absence of sharp spot defines polycrystalline structure of material because of rapid heating during combustion process prevents large crystal growth.

Fig. 2 TEM images of (a) meso-CeO₂ (b) SnPc@CeO₂ and (c) SAED pattern of SnPc@CeO₂.

FTIR spectra of SnPcCl₂ showed characteristics peaks due to phthalocyanine ring vibration at 723 cm⁻¹ and 781 cm⁻¹. The peaks at 1124 cm⁻¹, 1581 cm⁻¹ and 1656 cm⁻¹ were assumed due to the aromatic ring vibrations, C–H bending vibrations, C=N vibration and pyrrole ring vibrations of the phthalocyanine ring (Fig. 3a).²⁰ FTIR spectra of meso-CeO₂ showed its characteristic peaks at 1357 cm⁻¹ and 1544 cm⁻¹ (Fig. 3b). The peak at 1631 cm⁻¹ and 3428 cm⁻¹ were observed due to the adsorbed water on the surface.²¹ The presence of peaks of SnPc at 711 cm⁻¹, 782 cm⁻¹, 1118 cm⁻¹, 1344 cm⁻¹, 1463 cm⁻¹ and 1583 cm⁻¹ in heterostructured materials i.e. SnPc@CeO₂ confirmed the successful attachment of tin(IV) phthalocyanine to meso-ceria support (Fig. 3c).

Fig. 3 FT-IR Spectra of (a) SnPcCl₂ (b) meso-CeO₂ (c) SnPc@CeO₂.

Fig. 4 shows X-ray diffraction pattern of meso-CeO₂ and SnPc@CeO₂. The XRD diffraction pattern of meso-CeO₂ shows its characteristics peaks at the 2θ value 28.4° (111), 33.5° (200), 47.5° (220), 56.2° (311), 69.8° (400) and 76.4° (331) which can be indexed to fcc cubic space group Fm3m (225) structure and was in good agreement with JCPDS card no. 34-0394 (Fig. 4a).²² Immobilization of tin phthalocyanine to meso-CeO₂ in SnPc@CeO₂ catalyst did not change diffraction pattern of meso-CeO₂ which was most likely due to the lower loading (Fig. 4b).

Fig. 4 XRD diffraction pattern of (a) meso-CeO₂ (b) SnPc@CeO₂.

Nitrogen adsorption–desorption isotherm was used to determine surface properties such as BET surface area (S_BET), mean pore diameter (r_p) and total pore volume (V_p). For the meso-CeO₂ and SnPc@CeO₂ the type (IV) loop of N₂ adsorption–desorption isotherm was observed which suggested the mesoporous nature of materials according to IUPAC recommendation (Fig. S1†).²³ The BET surface area (S_BET) of meso-CeO₂ (75.92 ± 2% m² g⁻¹) was significantly decreased in SnPc@CeO₂ (20.52 ± 2% m² g⁻¹) which is due to the attachment of SnPc to the meso-CeO₂ surface.

UV-Vis spectrum of tin(IV) phthalocyanine dichloride (SnPcCl₂) collected in DMF, showed its characteristics absorption band at 298 nm (Soret band) and 688 nm (Q band) respectively due to macrocyclic π → π* ring transitions (Fig. 5a).²⁴ The broad Q band of tin(IV) phthalocyanine was observed because tin(IV) phthalocyanine exist in aggregated form in solution and thus, the absorption band was originated due to the combined absorption of monomeric and aggregated forms.²⁵ The UV-Vis spectrum of meso-CeO₂ exhibited a strong absorption band in the UV region due to O-2p to Ce-4f transition (Fig. 5b).²⁶ After the immobilization of SnPc to meso-CeO₂, a relatively sharp Q band of tin(IV) phthalocyanine was appeared. This is most likely due to the formation of monomeric form of SnPc during the immobilization to meso-CeO₂ support. The appearance of sharp Q band in UV-Vis spectra of SnPc@CeO₂ was a strong indication of successful attachment of SnPc to meso-ceria.²⁷ The increased visible light absorption profile indicates that the developed catalyst can work efficiently under visible light irradiation (Fig. 5c).

Fig. 5 UV/Vis absorption spectra of (a) SnPcCl₂ (b) meso-CeO₂ (c) SnPc@CeO₂.

Thermal stability of materials was determined by thermo gravimetric analysis as shown in Fig. 6. TGA thermogram of SnPcCl₂ shows a very small weight loss between 150–180 °C due to degradation of residual solvent molecules and 1,2-dicyanobenzene used for the synthesis of SnPc (Fig. 6a). The sharp weight loss at around 535 °C was observed due to the degradation of phthalocyanine ring structure.²⁸ For CeO₂, first weight loss around at 100 °C was due to the evaporation of water and then a steady weight loss was observed (Fig. 6b).²⁹ Heterostructured SnPc@CeO₂ catalyst showed the first a very small weight loss at 50–100 °C due to moisture and second major weight loss between 350–575 °C due to degradation of immobilized SnPc and oxy-functionalities presented on the support (Fig. 6c).

Fig. 6 DT-TGA thermogram of (a) SnPcCl₂ (b) meso-CeO₂ (c) SnPc@CeO₂.

2.2 Photo-catalytic activation of CO₂

The photocatalytic CO₂ activation experiments were carried out by using meso-CeO₂, SnPc@CeO₂, and homogeneous SnPcCl₂ in DMF and water and triethylamine mixture (3 : 1 : 1). The reaction mixture was irradiated with 20 watt white cold LED flood light. To monitor the progress of reaction, liquid samples were withdrawn during the reaction and 1 μL sample was injected in GC-FID and gaseous products were analyzed with GC-TCD and GC-FID (using RGA column). Further a calibration curve was plotted by injecting calculated amount of methanol in GC-FID. Methanol was observed as major liquid reaction product so its yield was used for evaluating the performance of catalyst (Fig. S2†). A plot of methanol yield during time by using meso-CeO₂, equimolar SnPcCl₂ to SnPc@CeO₂ catalyst and SnPc@CeO₂ catalyst and was plotted in Fig. 7. After 24 hours of visible light irradiation the yield of methanol by using meso-CeO₂, SnPcCl₂ in equimolar to SnPc@CeO₂ and SnPc@mesoCeO₂ was found to be 316 μmol g⁻¹-cat, 1402 μmol g⁻¹ cat and 2342 μmol g⁻¹ cat respectively. The corresponding methanol formation rate R_MeOH by using meso-CeO₂, homogeneous SnPcCl₂ in equimolar to SnPc@CeO₂ and SnPc@mesoCeO₂ was calculated 13.2 μmol h⁻¹ g⁻¹ cat, 58.4 μmol h⁻¹ g⁻¹ cat and 97.5 μmol h⁻¹ g⁻¹ cat with quantum yield for methanol (ϕ_MeOH) 0.003 (0.30%), 0.0133 (1.33%) and 0.0223 (2.23%) respectively. The quantum yield for product was determined as follows:

Quantum yield of product (ϕ_product) = [number of moles of product] × number of electrons required for reduction/[moles of incident photons]

Fig. 7 Photocatalytic methanol formation versus time by using (a) blank reaction (b) meso-CeO₂ and (c) SnPcCl₂ equimolar amount as in SnPc@CeO₂ and (d) SnPc@CeO₂.

For methanol and CO formation, number of electrons required is, respectively, 6 and 2.

The analysis of gaseous products after 24 h gave carbon monoxide as major reaction product along with the small amounts of hydrogen and oxygen. The yield of carbon monoxide by using meso-CeO₂, homogenous SnPcCl₂ and SnPc@CeO₂ was found to be 126 μmol g⁻¹ cat, 514 μmol g⁻¹-cat and 840 μmol g⁻¹ cat respectively. The carbon monoxide formation rate (R_CO) by using meso-CeO₂, homogenous SnPcCl₂ and SnPc@CeO₂ was found to be 5.3 μmol h⁻¹ g⁻¹ cat, 21.4 μmol h⁻¹ g⁻¹ cat, 35.0 μmol h⁻¹ g⁻¹ cat with the quantum yield 0.0004 (0.04%), 0.0016 (0.16%) and 0.0026 (0.26%) respectively. Hydrogen was observed only in the case of SnPc@CeO₂, whereas it was found to be absent in case of using other catalyst components like CeO₂ and SnPcCl₂. The yield of hydrogen with SnPc@CeO₂ catalyst was found to be 13.5 μmol g⁻¹ cat (R_H2 = 0.56 μmol h⁻¹ g⁻¹ cat) with 4.28 × 10⁻⁵ quantum yield. The catalytic selectivity for methanol by using meso-CeO₂, SnPcCl₂ and SnPc@CeO₂ was calculated as 71%, 73% and 73% respectively.

Production of methanol with time under blank condition as well as using different catalyst components like meso-CeO₂, homogeneous SnPcCl₂ and heterogeneous SnPc@CeO₂ is shown in Fig. 7. It is clear from the plot that SnPc@CeO₂ gave highest product yield in comparison to SnPcCl₂ and meso-CeO₂. Furthermore, the lower product yield with homogeneous SnPcCl₂ as compared to heterogeneous SnPc@CeO₂ suggested the promoting role of ceria support in enhancing the photoreduction reaction. As shown in Fig. 7, the shape of the highest curve (SnPc@CeO₂) typically looks like a saturation curve, i.e. is not linear. This is most likely due to the following reason: as we performed the photoreduction experiments in a closed vessel, so after the consumption of available CO₂ a saturation curve is reached (Fig. 7). In order to confirm the stability, after 24 h of illumination we again re-purged the reaction mixture with CO₂ and continued the reaction for additional 24 h by withdrawing the samples after every 4 h. After re-purging the solution with CO₂, the methanol yield was found to be increased as similar as in first run and then approaching to the saturation point with the consumption of available CO₂ (Fig. S4†).

The origin of methanol as a result of photoreduction product of carbon dioxide was ensured with three blank reactions. The first reaction was carried out in the absence of catalyst, second in the absence of light and the third by purging nitrogen in place of CO₂ under otherwise identical conditions. There were no product was observed after a long period of exposure.

Further we have checked recyclability of catalyst. After the reaction SnPc@CeO₂ catalyst was separated by centrifugation and washed with ethanol and dried. This catalyst was used for further reactions. After five recycling there was no significant loss in activity was observed (Fig. 8). The tin metal content in the recovered catalyst after five runs was found to be 0.34 wt% that was comparable to fresh catalyst 0.38 wt%. Furthermore to prove the heterogeneous nature of catalyst we added 100 mg catalyst in 10 mL water and then stirred it for 24 h. The catalyst was separated with centrifugation and the resulting filtrate was analysed by ICP-AES. The tin content in the filtrate was found to be 0.02 wt% which revealed that a marginal leaching had occurred during the reaction. These results suggested the higher stability as well as heterogeneous nature of the developed heterostructured photocatalyst without giving any significant leaching.

Fig. 8 Reuse experiments of photocatalytic CO₂ reduction by using SnPc@CeO₂ catalyst.

The better photocatalytic activity of SnPc@CeO₂ towards the CO₂ reduction was explained due to the better charge injection in the conduction band of CeO₂. As mentioned in literature meso-ceria can absorb only in the UV region due to its high band gap.³⁰ Tin(IV) phthalocyanine has good visible light absorption capacity with longer life time of its excited species as compared to ruthenium bipyridyl complexes and other phthalocyanine.³¹ So after absorption of the visible light SnPc excited to singlet (S₁) excited state and transfer electrons to the conduction band of meso-CeO₂.³² Due to continuous pumping of electrons by SnPc in the conduction band of CeO₂ multiple electron transfer necessary for methanol formation is possible.^17,18 These electrons in the conduction band of CeO₂ were used for the reduction of CO₂ adsorbed on the surface of CeO₂. Triethylamine was used as a sacrificial donor to provide necessary electrons for the photoreduction process. After providing electrons triethylamine was converted to its degradation products.^33,2f However, water provided necessary protons for the reaction as shown in the Scheme 2.³⁴

SnPc + hν → SnPc*(S₁) (excited singlet state)

SnPc*(S₁) + CeO₂ → SnPc^o+ + 6e⁻ (CeO₂ conduction band)

6e⁻ (CeO₂ conduction band) + CO₂ + 6H⁺ → CH₃OH + H₂O

SnPc^o+ + TEA → SnPc + TEA^o+

TEA^o+ → degradation products + e⁻ (transferred to SnPc)

3H₂O → 6H⁺ + 3/2O₂ + 6e⁻

Scheme 2 Plausible mechanism of CO₂ reduction over SnPc@CeO₂ catalyst.

3 Conclusions

In summary, heterostructured SnPc@CeO₂ photocatalyst was successfully achieved by the immobilization of tin(IV) phthalocyanine to mesoporous ceria through axial position by taking advantage of hydroxyl groups presented on the surface of meso-CeO₂. The synthesized photocatalyst exhibited good photocatalytic activity under visible light irradiation and afforded methanol and carbon monoxide as major products. The developed catalyst was found to be more efficient as compared to the meso-CeO₂ as well as homogeneous SnPcCl₂ with the added benefits of facile recovery and consistent activity for recycling runs. With the use of SnPc@CeO₂, the yield of methanol and CO after 24 h was attained as 2342 μmol g⁻¹ cat and 840 μmol g⁻¹ cat with quantum yields (ϕ_MeOH = 0.0223) and (ϕ_CO = 0.0026) respectively. The attained product formation rate for methanol (R_MeOH) and CO (R_CO) by using SnPc@CeO₂ was 97.5 μmol h⁻¹ g⁻¹ cat and 35.0 μmol h⁻¹ g⁻¹ cat. The superior photocatalytic activity of the heterostructured catalyst was defined on the basis of better charge injection in the conduction band of meso-CeO₂. We believe that our findings will be helpful for scientific community to develop low cost visible light active photocatalytic materials for conversion of CO₂ to value added chemicals.

4 Experimental section

4.1 Materials

Tin(II) chloride dihydrate (98%), phthalonitrile (≥96.0%), cerium nitrate (99.99%) and the template chitosan (85% deacylated) was purchased from Alfa Aesar. Chloro naphthalene (≥85%) was purchased from Sigma-Aldrich. DMF and water used were of HPLC grade and procured from Merck. All other chemicals were of analytical grade and used without any further purification.

4.2 Techniques used

The rough surface morphology of materials were determined with the help of Field Emission Scanning Electron Microscopy (FE-SEM) performed on FE-SEM (Jeol Model JSM-6340F). Transmission Electron Microscopy (TEM) was used for the determination of ultrafine structure of materials and performed on FEI-TecnaiG² TwinTEM operating at an acceleration voltage of 200 kV. TEM images were collected by depositing very dilute aqueous suspension of sample on carbon coated TEM grid. Vibrational spectra (FTIR) for identification of functionalities present in samples were collected on Perkin-Elmer spectrum RX-1 IR spectrophotometer using potassium bromide window. UV-Vis absorption spectra of SnPcCl₂ in DMF and solid UV-Vis of other samples were collected on Perkin Elmer lambda-19 UV-Vis-NIR spectrophotometer using a 10 mm quartz cell, using BaSO₄ as reference. To executing the phase structure and crystalline nature of material and powder X-ray diffractogram was recorded on Bruker D8 Advance diffractometer at 40 kV and 40 mA with Cu K_α radiation (λ = 0.15418 nm). Before analysis samples was prepared in glass slides and dried properly. Brunauer–Emmett–Teller (BET) surface area, Barret–Joiner–Halenda (BJH) porosity, pore volume etc. surface properties of samples were examined by N₂ adsorption–desorption isotherm at 77 K by using VP; Micromeritics ASAP 2010. To obtaining thermal stability data and rough idea of chemical moiety presents in materials Thermo Gravimetric Analyses (TGA) was performed by using a thermal analyzer TA-SDT Q-600. Analysis was carried out in the temperature range of 40 to 800 °C under nitrogen flow with heating rate 10 °C min⁻¹ metal content of SnPc@CeO₂ catalyst was determine by ICP-AES analysis that was carried out by using Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES, DRE, PS-3000UV, Leeman Labs Inc, USA). Elemental analysis was done for confirming the intact ring structure of phthalocyanine and no metal was leached out on catalyst. For illumination LED light (Model: HP-FL-20W-F, Hope LED Opto-Electric Co., Ltd) was used having following specification – Chip: KWE, Bridgelux, Epistar COB LED, 20 W, lum. output: 80–90l m W⁻¹, beam angle: 120 degree and wavelength λ > 400 (maximum at 515 nm). Fig. S5† shows spectral power distribution (spectrum profile) for different wavelength.

4.3 Synthesis of mesoporous-ceria (meso-CeO₂)¹⁹

For the synthesis of mesoporous CeO₂ literature procedure using a modified template method was followed. In a 100 mL of 5% acetic acid solution, 3 g chitosan powder was dissolved with stirring for about 1 h. An aqueous solution of cerium nitrate (1.5 g) was added to this solution and stirred for 2 h. The obtained precursor was precipitated by adding 50% aqueous ammonia solution. Precipitate was dried at 60 °C and calcined at 550 °C for 5 hours and grinded.

4.4 Immobilization of tin(IV) phthalocyanine dichloride (SnPcCl₂) to meso-CeO₂ (SnPc@CeO₂)

Tin(IV) phthalocyanine dichloride (SnPcCl₂) was synthesized by following the literature method.³⁵ Mesoporous ceria was first treated with water for 24 h to increase the –OH functionalities on the surface and then separated by centrifuge, dried at 60 °C. The immobilization of tin(IV) phthalocyanine dichloride to meso-CeO₂ was carried out by refluxing SnPcCl₂ (0.25 g) and meso-CeO₂ (2.0 g) in ethanol for 12 hours in the presence of triethylamine. After cooling SnPc@CeO₂ was separated by centrifuge and washed thoroughly with water, ethanol and dried at 70 °C under vacuum for 12 h. The Sn-metal content in the synthesized sample was determined by ICP-AES and was found to be 0.38 wt% (53 μmol SnPcCl₂ g⁻¹ cat).

4.5 Photocatalytic CO₂ activation experiment

All the synthesized materials i.e. meso-CeO₂, SnPc@CeO₂ and homogeneous SnPcCl₂ were tested for photocatalytic CO₂ reduction using triethylamine as sacrificial donor. In a borosil vessel (5 cm dia.) water (10 mL), triethylamine (10 mL), and DMF (30 mL) was added and nitrogen was purged through this solution for 15 min to replace other dissolved gaseous. After that carbon dioxide was purged through this solution for 30 minutes for saturating the solution with carbon dioxide. The vessel was charged with 100 mg of catalyst and sealed with a rubber septum and irradiated under visible light by using 20 watt LED (Model no. HP-FL-20W-F, Hope LED Opto-Electric Co., Ltd) with stirring. To preventing from outer interference whole reaction setup was placed in a box. The intensity of light on vessel's surface was found 85 W m⁻² measures with the help of intensity meter. Samples were withdrawn after every 2 hours interval with the help of a long needle and catalyst was removed by syringe filter (PTFE, 0.2 μm, 13 mm dia.). The obtained samples were injected in GC-FID (Varian CP-3800 by using 30 m long Stabilwax® w/Integra-Guard® column, flow rate: 0.5 mL min⁻¹, injector temperature: 250 °C, FID detector temperature: 275 °C) for determination of liquid products. A calibration curve was plotted by injecting various concentration of methanol in GC-FID for quantification and to checking linear response and sensitivity to various concentrations. Gaseous products were determined with the help of GC-TCD and GC-FID (Agilent 7890A GC system) using capillary column (RGA, refinery gas analyzer) at the flow rate (H₂: 35 mL min⁻¹, air: 350 mL min⁻¹, makeup flow: 27 mL min⁻¹, for TCD reference flow: 45 mL min⁻¹, helium flow: 2 mL min⁻¹), injector temperature: 220 °C, TCD detector temperature and FID detector temperature: 220 °C. For determining the gaseous product a separate reaction was carried out and product was analysed after 24 h of vis-irradiation by injecting 20 μL sample in GC-TCD and GC-FID.

Blank reactions were performed for ensuring that originated methanol was photoreduction product of CO₂ and not originated due to degradation of organic materials. First blank reaction was carried out in identical conditions except no catalyst was added. In the second blank reaction, catalyst and same solvent system was used except reaction was placed in dark. An additional blank experiment was also carried out in Vis-irradiation containing catalyst but purging N₂ rather than CO₂. No any product was detected in all the blank reactions. Further, to checking effect of solvent system we have carried out reaction in another aprotic solvent acetonitrile and water system in identical conditions. The yield of methanol in this system was found to be comparable to DMF (Fig. S2†).

Acknowledgements

We would like to thanks Director IIP for granting permission to publish these findings. PK and AK are also thankful to UGC and CSIR, New Delhi for providing fellowships. Analytical department is kindly acknowledged for analysis of samples.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: NMR spectra and surface characterization data of rGO-immobilized catalyst. See DOI: 10.1039/c5ra06449j

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