Photocatalytic reduction of CO2 in methanol to methyl formate over CuO–TiO2 composite catalysts

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Abstract

Photocatalytic reduction of CO2 on CuO–TiO2 composite catalysts in the presence of methanol to prepare methyl formate had been investigated. Methanol was used as sacrificial reagent to react with the photo-generated holes in the valence band, and CO2 was reduced by the electrons in the conduction band. CuO–TiO2 was optimized for CuO loading, preparation method and calcination temperature. The catalyst of 1.0CuO–TiO2, calcined at 450 °C and CTAB as a dispersant showed the highest overall activity. The heterojunction between CuO and TiO2 demonstrated with HRTEM played an important role in enhancing the photocatalytic activity.

Graphical abstract

The surface-phase junction of the CuO–TiO2 composite catalyst, band positions of CuO, TiO2 and the potentials of several redox couples.

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Research highlights

► Methanol was utilized as reductant for the photoreduction of CO2 because the reducibility of methanol is stronger than water and methanol can dissolve much more CO2 than water. ► CuO–TiO2 composite catalysts were used and the catalysts were prepared through a sol–gel method using cetyltrimethylammonium bromide (CTAB) as an auxiliary reagent. ► The surface-phase junction between CuO and TiO2 was investigated by HRTEM and it could enhance the photocatalytic activity.

Introduction

The rapid increase in atmospheric CO2 leads to climate change, which is one of the greatest threats of times. It is urgent to reduce the accumulation of CO2 in the atmosphere. There are three ways to reduce CO2 emissions: reducing the amount of the produced CO2, using CO2 and storing CO2, where transformation of CO2 into chemicals is an attractive option and fulfills the recycle use of CO2.

Photoreduction of CO2 is a possible avenue to convert CO2. In 1979 Honda et al. first reported photoreduction of CO2 to organic compound such as HCOOH, HCHO, CH3OH and CH4 in aqueous suspension of semiconductor powders [1]. A number of researches focused on the photocatalytic reduction of CO2 with H2O [2], [3], [4], [5], [6], [7], [8]. Because of the weak reducibility of water and the low solubility of CO2 in water, many other reductants were used for the photoreduction of CO2. Teramura et al. used Ga2O3 as a photocatalyst for the photocatalytic reduction of CO2 in the presence of H2 which worked as a reductant and CO gas was selectively produced [9]. Lo et al. studied photocatalytic reduction of CO2 in a self-designed circulated photocatalytic reaction system under TiO2 and ZrO2 photocatalysts and the highest yield of the photoreduction of CO2 was obtained by using TiO2 with H2 + H2O and ZrO2 with H2 [10]. Teramura et al. used MgO for photoreducing CO2 to CO in the presence of H2 or CH4 and clarified the mechanism of the CO2 photocatalytic reduction [11]. Al-Jubori described the photoelectrocatalytic reduction of carbon dioxide in aqueous solution to C1 organic compounds (formic acid and formaldehyde) in the presence of n-Bi2S3 and n-CdS semiconductor powders and hydrogen sulfide could enhance the rate of the photoreduction processes [12].

Due to the low rate of photocatalytic CO2 conversion, following strategies had been employed to enhance photocatalytic carbon dioxide disappearance rate in this paper: (i) use cetyltrimethylammonium bromide (CTAB) as a dispersant to synthesized nanocatalysts with suitable particle size; (ii) CuO and TiO2 formed composite or heterojunction to reduce the recombination of the electrons and holes; (iii) methanol was used as reductant for the photoreduction of CO2 because of the stronger reducibility of methanol and solubility of CO2 in methanol.

Section snippets

Preparation of catalysts

All the reagents were of analytical purity and purchased from Tianjin Benchmark Chemical Reagent Company. TBOT (tetrabutyl titanate) and CTAB were dissolved in absolute ethanol by sonicating for 15 min and followed by stirring for 15 min respectively. The two solutions were mixed under stirring for 30 min to form a transparent solution. Cu(NO3)2 5H2O was dissolved under stirring in absolute ethanol, then the solution of Cu(NO3)2 was added dropwise to the TBOT + CTAB solution. After stirring for 30 

Catalysts characterization with XRD, UV–vis and TEM

As shown in Fig. 2, both position and shape of the diffractive peaks of 1.0CuO–TiO2 were quite similar to pure anatase TiO2, which indicated that loading small amount of CuO almost did not change the crystalline phase of TiO2. The diffraction peak of crystal plane [1 0 1] was selected to estimate the crystallite size of the sample by the Debye–Scherrer equation. The mean particle size was nearly 13.8 nm. CuO was not found due to a bit of CuO loading and perfect dispersion.

The 1.0CuO–TiO2 catalyst

Conclusion

The photocatalytic reduction of CO2 could be enhanced by CuO–TiO2 composite catalysts by using methanol as sacrificial reagent, and methyl formate was produced. The optimal catalyst was 1.0 wt% CuO loading on TiO2, CTAB as dispersant, and calcined at 450 °C. The heterojunction between CuO and TiO2 was found for improving photocatalytic reduction rate of CO2. The reaction mechanism was also explained by the conventional band theory of charge transfer at the semiconductor materials.

Acknowledgments

We gratefully acknowledge financial support by the National Natural Science Foundation of China (NSFC) 20876109 and 21046006.

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