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Article

Photocatalytic Reduction of CO2 to Methanol by Cu2O/TiO2 Heterojunctions

by
S.-P. Cheng
,
L.-W. Wei
and
H.-Paul Wang
*
Department of Environmental Engineering, National Cheng Kung University, Tainan 70101, Taiwan
*
Author to whom correspondence should be addressed.
Sustainability 2022, 14(1), 374; https://doi.org/10.3390/su14010374
Submission received: 5 November 2021 / Revised: 4 December 2021 / Accepted: 15 December 2021 / Published: 30 December 2021
(This article belongs to the Special Issue Selected Papers from International Conference on Innovations in Energy Engineering & Cleaner Production (IEECP'21&22))
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Abstract

:
The conversion of CO2 to low-carbon fuels using solar energy is considered an economically attractive and environmentally friendly route. The development of novel catalysts and the use of solar energy via photocatalysis are key to achieving the goal of chemically reducing CO2 under mild conditions. TiO2 is not very effective for the photocatalytic reduction of CO2 to low-carbon chemicals such as methanol (CH3OH). Thus, in this work, novel Cu2O/TiO2 heterojunctions that can effectively separate photogenerated electrons and holes were prepared for photocatalytic CO2-to-CH3OH. More visible light-active Cu2O in the Cu2O/TiO2 heterojunctions favors the formation of methanol under visible light irradiation. On the other hand, under UV-Vis irradiation for 6 h, the CH3OH yielded from the photocatalytic CO2-to-CH3OH by the Cu2O/TiO2 heterojunctions is 21.0–70.6 µmol/g-catalyst. In contrast, the yield of CH3OH decreases with an increase in the Cu2O fraction in the Cu2O/TiO2 heterojunctions. It seems that excess Cu2O in Cu2O/TiO2 heterojunctions may lead to less UV light exposure for the photocatalysts, and may decrease the conversion efficiency of CO2 to CH3OH.

1. Introduction

In recent years, the significant rise of greenhouse gas CO2 concentrations on the earth causing serious problems has received much attention. There are major challenges in recycling high-thermal stability CO2, which may involve severe reaction conditions (high pressures or high temperatures) with extra energy consumption that may lead to the additional formation of CO2. Thus, the recycling of CO2 into low-carbon chemicals or fuels using solar energy is considered an economically attractive and environmentally friendly route. The desired photocatalysts for the photocatalytic reduction of CO2 to chemicals or fuels under mild conditions are being developed to achieve the goal of CO2 recycling [1,2,3].
Titanium dioxide (TiO2), an n-type semiconductor, has been used for the photocatalytic reduction of CO2 to chemicals such as formic acid, formaldehyde, methane and methanol [4,5]. TiO2 has shown great potential for various photocatalytic reactions, mainly due to its chemical stability, nontoxicity, high oxidation efficiency and environmentally friendly nature [6]. However, because of its fairly wide bandgap (3.2 and 3.0 eV for anatase and rutile phases, respectively), TiO2 can only be activated by ultraviolet (UV) light, equivalent to about 5% of natural solar light. A variety of strategies, such as metal ion doping, cation or anion doping, and coupling with narrow-bandgap semiconductors, have been developed to extend absorption into the visible light region [7,8,9,10]. The doping of anions (e.g., N, F, S, and C) onto TiO2 could shift the absorption edge to a relatively low energy, and its photo-response into the visible spectrum [11,12,13,14]. Cation-, anion- or metal ion-doped TiO2 could lead to better solar energy harvesting in the visible light region; however, this still suffers from relatively high photogenerated electron and hole recombination rates, causing difficulties in engineering applications [15,16,17].
CO2 may be activated by a one-electron transfer step and form a ·CO2− radical ion. The ·CO2− may be reduced to yield a hydroxyformyl radical (·COOH), which recombines a hydrogen radical (H+) and an electron (e−) to form formic acid [18]. In the following step, formic acid accepts H+ and e− to form formaldehyde. Formic acid and formaldehyde seem to be formed prior to methanol generation. Thus, the key points that control the photocatalytic CO2-to-CH3OH reaction may include reaction conditions, photocatalyst activity, bandgap energy, light source and process parameters. To effectively suppress the rapid recombination of photoexcited electrons and holes, a heterojunction structure could facilitate electron migration [19,20]. Cuprous oxide (Cu2O), a typical p-type semiconductor, has wide application prospects in solar cells, photocatalysis, and hydrogen evolution reactions (HER) [21]. Cu2O, with a bandgap energy of 2.0–2.2 eV, could effectively harvest visible light for photocatalysis. However, while the photocatalytic CO2-to-CH3OH reaction facilitated by Cu2O is thermodynamically feasible, its CH3OH yield suffers from the low solar conversion efficiency [22]. By the heterojunction between the p-type Cu2O and n-type TiO2, the recombination of photo-excited charges could be effectively retarded and facilitate photocatalytic reactions [23,24]. In this work, novel Cu2O/TiO2 heterojunctions were thus prepared by a simple soft chemical method as the visible light photocatalysts used for the enhanced photocatalytic reduction of CO2 to methanol.

2. Materials and Methods

Cu2O was prepared by the facile soft chemical method (Figure 1). Briefly, CuCl2 (97%, Merck, Kenilworth, NJ, USA) (10.1 mmol) was dispersed in a NaCl solution (5 M) (100 mL) with a dispersant (polyethylene glycol 20,000 (Sigma-Aldrich, Burlington, MA, USA) (0.025 mmol)), which was stirred at 298 K for 1 h. Na3PO4 (96%, Sigma-Aldrich, USA) (9.76 mmol) was added to the solution and stirred for 1 h. The Cu2O was centrifuged and cleaned with distilled water and ethanol three times. Titanium butoxide (Ti(OBu)4) (97%, Sigma-Aldrich, USA) and Cu2O at the XCu2O mole fractions (Cu2O/(Cu2O + TiO2)) of 0.1, 0.2 and 0.5 were mixed in deionized water, and were then centrifuged, dried at 378 K for 4 h, and heated at 723 K under N2 flow (99.99%) (20 mL/min) for 2 h to obtain the Cu2O/TiO2 heterojunctions used for photocatalysis experiments.
The crystalline structures of the Cu2O, TiO2 and Cu2O/TiO2 heterojunctions were determined by X-ray diffraction (D8, Discover with Gadds, Bruker AXS Gmbh). The crystalline sizes of the Cu2O, TiO2 and Cu2O/TiO2 heterojunction photocatlysts were calculated by the Scherrer equation (t = kλ/Bcosθ) using the Jade software. The images of the Cu2O/TiO2 heterojunctions were investigated by scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS) (AURIGA) and scanning transmission electron microscopy (JEOL JEM-2100F CS STEM). The room temperature photoluminescence spectra of the photocatalysts were determined on the LabRAM HR (Horiba Jobin Yvon, Palaiseau, France) using the 325 nm excitation wavelength. The diffuse reflection absorption spectra of the photocatalysts at 200–800 nm were studied on a UV–visible spectrophotometer (Varian, Cary 100, Palo Alto, CA, USA). BaSO4 was used as the standard in the absorption spectroscopic experiments. The bandgap energy was studied via the Kubelka–Munk equation (αhν = A(hν-Eg)n). The specific surface area, pore size and pore volume distribution of the photocatalysts were measured on a nitrogen adsorption–desorption analyzer (Micromeritics, ASAP 2020) using the Brunauer–Emmett–Teller (BET) and Barret–Joyner–Halenda (BJH) model. The zeta potential data were determined from the specific surface areas and active sites of the photocatalysts.
The photocatalytic experiments were carried out in a closed cylindrical quartz reactor to prevent oxygen/air access. The photocatalyst (0.1 g) was dispersed in a sodium hydroxide (0.025 M) aqueous solution (100 mL). Before the photocatalysis experiments, high-purity CO2 was bubbled through the solution until the pH reached 7.00 at 298 K. A 300 W Xenon arc lamp (Burgeon Instrument Co., Ltd., Taoyuan City, Taiwan) with the light cut off (λ > 400 nm) by a filter (FSQ-CG400, Newport, Taipei, Taiwan) was used for the experiments on the photocatalytic reduction of CO2 to methanol. The concentrations of the photocatalytic product methanol were measured by GC-MS (JEOL JMS-700 and Shimadzu, QP2010).

3. Results and Discussion

The XRD patterns of the photocatalysts are shown in Figure 2. The diffraction peaks at 29.6°, 36.5°, 42.4°, 61.4°, 73.6° and 77.5° correspond to the (110), (111), (200), (220), (311) and (222) phases of the crystalline Cu2O (JCPDS card No. 78-2076), respectively [25]. A high-intensity diffraction peak at 36.4° confirms the existence of Cu2O in the Cu2O/TiO2 heterojunctions. Other diffraction peaks at 25.3°, 37.8°, 48.0°, 53.9°, 55.0°, 62.7°, 68.8°, 70.3° and 75.0° can be indexed to the (101), (004), (200), (105), (211), (204), (116), (220) and (215) planes of TiO2 (JCPDS card No. 71-1167), associated with the anatase phase [26,27], indicating that the Cu2O/TiO2 heterojunctions consist of anatase predominantly. Note that a peak at 38.7° related to the CuO(111) plane is also observed, suggesting the existence of a small amount of CuO. The crystalline sizes of the TiO2, Cu2O, and Cu2O/TiO2 heterojunctions derived by Scherrer’s equation are 50–100, 50–70, and 40–100 nm, respectively.
The TEM images of the TiO2 and Cu2O/TiO2 heterojunctions are shown in Figure 3. It is clear that the Cu2O (in the Cu2O/TiO2 heterojunction) and TiO2 have nanoparticle diameters of ~5 and 70–130 nm, respectively. The presence of Cu, Ti, and O in the Cu2O/TiO2 heterojunction could be revealed by energy-dispersive X-ray (EDX) spectroscopy (see Figure 3c). The HRTEM image of the sample in Figure 3d shows lattice fringes spacing of 0.212 and 0.237 nm, corresponding to the (200) and (111) planes of Cu2O, respectively [28]. The TiO2 with high crystallinity has the d-spacings of 0.352 and 0.246 nm, related to the (100) and (004) planes of anatase TiO2 [28].
The nitrogen adsorption–desorption isothermals of the pristine TiO2 and Cu2O/TiO2 heterojunctions are shown in Figure 4A. The absorption isothermals of the Cu2O/TiO2 heterojunctions can be classified as type IV with H1 hysteresis loops, suggesting that they have a mesoporous structure. In Figure 4B, the Cu2O/TiO2 heterojunctions have greater pore volumes, with pore diameters between 10 and 40 nm, than the pristine TiO2, possessing relatively high pore diameters of 30–70 nm. It seems that the smaller Cu2O nanoparticles may, to some extent, be incorporated into the pores of the TiO2, which creates more internal surfaces in the interfaces of the Cu2O and TiO2 nanoparticles. In Table 1, as expected, the Cu2O/TiO2 heterojunctions have relatively high specific surface areas (94–120 m2/g) and small average pore diameters, which may benefit the photocatalytic reduction of CO2 to CH3OH.
According to the linear regression analysis, the relationship between the zeta potentials (see in Table 1) and BET surfaces of the Cu2O/TiO2 heterojunctions and TiO2 nanoparticles was well fitted (R2 > 0.9). The zeta potential data were determined by the specific surface areas and active sites of the photocatalysts. The Cu2O/TiO2 heterojunctions with negative potential can provide more active sites for CO2 reduction, suggesting that the Cu2O/TiO2 heterojunctions are feasible for photocatalytic CO2-to-CH3OH reactions.
The diffuse reflectance ultraviolet–visible spectra of the photocatalysts were also determined. In Figure 5, the absorbance of the Cu2O/TiO2 heterojunctions in the range of 200–800 nm can be observed. Compared with TiO2, the fundamental absorbance cuts at 400 nm, and the TiO2 mixed with Cu2O reveals a significantly enhanced absorption in the visible light region. It is clear that TiO2 with Cu2O causes a red-shift to 400–800 nm in the visible light range, possibly due to the forming of the Cu2O/TiO2 heterojunctions [28].
In Figure 6, the bandgap energies of the photocatalysts were determined by the Kubelka–Munk transforms [29]. The direct bandgaps of the TiO2, Cu2O and Cu2O/TiO2 heterojunctions with the XCu2O fractions of 0.1, 0.2 and 0.5 were estimated to be 3.20, 2.09, 3.03, 3.0 and 2.94 eV, respectively. It seems that the coupling of Cu2O with TiO2 can effectively decrease the bandgap energies of both. The Cu2O/TiO2 heterojunctions turn out to be more photoactive than TiO2 under visible light irradiation.
The charge separation efficiency of photoinduced electrons and holes is also one of the important factors in photocatalysis. The photoluminescence spectra can provide information on charge carrier trapping, migration and transfer [30]. The photoluminescence spectra of the Cu2O/TiO2 heterojunctions are shown in Figure 7. The photoluminescence intensity of the Cu2O/TiO2 heterojunctions is less than that of TiO2. A clear quenching of the photoluminescence emission of the Cu2O/TiO2 heterojunctions is observed, especially for the Cu2O/TiO2 heterojunctions with the XCu2O fraction of 0.2, which showed maximum quenching. Such quenching of the photoluminescence suggests that the separation of photogenerated electron and hole pairs in the Cu2O/TiO2 heterojunctions has been effectively improved. Cu2O can transfer the photogenerated holes from TiO2 to inhibit the recombination of photogenerated electrons and holes significantly, and this may consequently lead to enhanced photocatalysis.
Figure 8A shows the yield of CH3OH from the photocatalytic reduction of CO2 by the Cu2O/TiO2 heterojunctions under visible light irradiation. After 6 h of visible light irradiation, the yields of CH3OH photocatalyzed by the Cu2O/TiO2 heterojunctions with the XCu2O fractions of 0.1, 0.2, and 0.5 are 9.29, 11.44, and 13.06 µmol/g-catalyst, respectively. Note that, as expected, TiO2 is not very effective for the photocatalytic CO2-to-CH3OH reaction. Additionally, more visible light-active Cu2O in the Cu2O/TiO2 heterojunctions favors the formation of methanol. On the other hand, under UV-Vis irradiation for 6 h, the CH3OH yielded from the photocatalytic CO2-to-CH3OH reaction by the Cu2O/TiO2 heterojunctions is 21.0–70.6 µmol/g-catalyst (see Figure 8B). It is clear that the yields of CH3OH under UV-Vis irradiation are greater than those under visible irradiation. In contrast, the yield of CH3OH decreases with an increase in the Cu2O fraction in the Cu2O/TiO2 heterojunctions. It seems that excess Cu2O in the Cu2O/TiO2 heterojunctions may lead to less UV light exposure for the photocatalysts, and may reduce the conversion efficiency of CO2 to CH3OH.
The schematic diagram of the charge separation in the Cu2O/TiO2 heterojunction structure is depicted in Scheme 1. When the Cu2O/TiO2 heterojunctions are irradiated by visible light, only the electrons of Cu2O can be excited to the conduction band, and then move to the TiO2, leading to the better separation of electron and hole pairs. However, photoexcited electrons in Cu2O and TiO2 are excited to the conduction band when irradiated under UV–visible light, whereas the holes of TiO2 may quickly transfer to the Cu2O, which may reduce the recombination of photogenerated holes and electrons and promote the photocatalytic activity.
CO2 can be activated by a one-electron transfer step to form·CO2−, which may be reduced to yield the hydroxyformyl radical (COOH), which recombines with a hydrogen radical (H+) and an electron (e−) to form formic acid and formaldehyde (which seem to be formed prior to the CH3OH generation) [18]. Thus, the key points that control the photocatalytic CO2-to-CH3OH reaction may include reaction conditions, photocatalyst activity, bandgap energy, light source and process parameters, and the comparison between different methods is shown in Table 2 [31,32,33,34,35,36,37,38]. It is clear that the Cu2O/TiO2 heterojunctions prepared in this work offer much better CH3OH yields under visible light irradiation.

4. Conclusions

The novel Cu2O/TiO2 heterojunction photocatalysts prepared by a simple soft chemical method have relatively high specific surface areas and small average pore diameters, which may benefit the photocatalytic reduction of CO2 to CH3OH. The Cu2O in conjunction with TiO2 decreases its bandgap energy, and extends the absorption to the visible light region. The p-n-type heterojunction can effectively suppress charge carrier recombination. After the 6 h photocatalytic reduction, 9–13 and 21–76 µmol/g-catalyst of methanol can be yielded under visible and UV-Vis irradiation, respectively. The comparison between different methods suggests that the Cu2O/TiO2 heterojunctions prepared in this work offer much better CH3OH yields under visible and UV light irradiation.

Author Contributions

S.-P.C. designed the concept and drafted the manuscript. L.-W.W. provided support of the literature search and manuscript revision. H.-P.W. supervised the research work and revised the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Taiwan Ministry Science and Technology and EPA.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The financial supports of the Taiwan Ministry Science and Technology and EPA for sponsoring this project (MoST 108-2221-E-006-165-MY3, MoST 109-2221-E-006 -042 -MY3, MoST 110-2221-E-006-107-MY2, and EPA 110GA00008001047) are gratefully acknowledged.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Preparation procedure for the Cu2O/TiO2 heterojunction photocatalysts.
Figure 1. Preparation procedure for the Cu2O/TiO2 heterojunction photocatalysts.
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Figure 2. X-ray diffraction patterns of the (a) Cu2O and Cu2O/TiO2 heterojunctions with the XCu2O fractions of (b) 0.5, (c) 0.2, (d) 0.1, and (e) TiO2 nanoparticles.
Figure 2. X-ray diffraction patterns of the (a) Cu2O and Cu2O/TiO2 heterojunctions with the XCu2O fractions of (b) 0.5, (c) 0.2, (d) 0.1, and (e) TiO2 nanoparticles.
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Figure 3. The TEM images of the (a) pristine TiO2 and (b) Cu2O/TiO2 heterojunctions (XCu2O = 0.5) with (c) EDS mapping, and (d) the HRTEM image.
Figure 3. The TEM images of the (a) pristine TiO2 and (b) Cu2O/TiO2 heterojunctions (XCu2O = 0.5) with (c) EDS mapping, and (d) the HRTEM image.
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Figure 4. (A) N2 absorption–desorption isothermals and (B) pore size distributions of the Cu2O/TiO2 heterojunctions with the XCu2O fractions of 0.1, 0.2, 0.5, and pristine TiO2 nanoparticles.
Figure 4. (A) N2 absorption–desorption isothermals and (B) pore size distributions of the Cu2O/TiO2 heterojunctions with the XCu2O fractions of 0.1, 0.2, 0.5, and pristine TiO2 nanoparticles.
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Figure 5. UV-Vis DR spectra of the Cu2O/TiO2 heterojunctions with the XCu2O fractions of 0.1–0.5.
Figure 5. UV-Vis DR spectra of the Cu2O/TiO2 heterojunctions with the XCu2O fractions of 0.1–0.5.
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Figure 6. The Tauc plot of the Cu2O/TiO2 heterojunctions with the XCu2O fractions of 0.1–0.5.
Figure 6. The Tauc plot of the Cu2O/TiO2 heterojunctions with the XCu2O fractions of 0.1–0.5.
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Figure 7. The photoluminescence spectra of the Cu2O/TiO2 heterojunctions with the XCu2O fractions of (a) 0.1, (b) 0.2 and (c) 0.5, and (d) pristine TiO2 nanoparticles.
Figure 7. The photoluminescence spectra of the Cu2O/TiO2 heterojunctions with the XCu2O fractions of (a) 0.1, (b) 0.2 and (c) 0.5, and (d) pristine TiO2 nanoparticles.
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Figure 8. Photocatalytic reduction of CO2 to methanol under (A) visible and (B) UV–visible irradiation by the Cu2O/TiO2 heterojunctions with the XCu2O fractions of (a) 0.1, (b) 0.2, (c) and 0.5, and (d) TiO2 nanoparticles.
Figure 8. Photocatalytic reduction of CO2 to methanol under (A) visible and (B) UV–visible irradiation by the Cu2O/TiO2 heterojunctions with the XCu2O fractions of (a) 0.1, (b) 0.2, (c) and 0.5, and (d) TiO2 nanoparticles.
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Scheme 1. The charge separation with the Cu2O/TiO2 heterojunctions under (a) ultraviolet–visible and (b) visible light irradiation.
Scheme 1. The charge separation with the Cu2O/TiO2 heterojunctions under (a) ultraviolet–visible and (b) visible light irradiation.
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Table
Table 1. The BET surface areas, average pore diameters, and zeta potentials of the pristine TiO2 and Cu2O/TiO2 heterojunctions with the XCu2O fractions of 0.1–0.5.
Table 1. The BET surface areas, average pore diameters, and zeta potentials of the pristine TiO2 and Cu2O/TiO2 heterojunctions with the XCu2O fractions of 0.1–0.5.
PhotocatalystsXCu2O
Fractions		BET Surface
Area (m2/g)		BJH Average Pore
Diameter (nm)		Zeta Potential (mV)
Pristine TiO20485052.7
Cu2O/TiO20.19423−21.1
0.210515−26.6
0.512015−39.1
Table
Table 2. Method comparison for photocatalytic CO2-to-CH3OH reaction.
Table 2. Method comparison for photocatalytic CO2-to-CH3OH reaction.
PhotocatalystLight SourceBandgap
Energy (eV)		ReactionsCH3OH Yield
(µmol/g/h)		Ref.
Cu2O/TiO2UV
Vis		2.9–3.0100 mg photocatalysts,
CO2 in deionized water		9–13
12–70		This work
Co/TiO2UV-100 mg photocatalysts,
CO2 in NaHCO3 (1 M)		0.05[31]
Anatase TiO2 Vis2.9–3.2500 mg photocatalysts,
CO2 in deionized water		2.74[32]
SnO2/g-C3N4UV-20 mg photocatalysts,
CO2 in water vapor		0.02[33]
rGO/ZnOVis2.8100 mg photocatalysts,
CO2 in water vapor		0.42[34]
rGO/Cu2OUV2.7–2.8100 mg photocatalysts,
CO2 in NaOH (1 M)		8.77[35]
CQDs/Cu2OVis2.4–2.6150 mg photocatalysts,
CO2 in deionized water		1.96[36]
ZnTe/SrTiO3UV2.8–3.420 mg photocatalysts,
CO2 in deionized water		0.75[37]
ZnO/g-C3N4UV2.6–3.010 mg photocatalysts,
CO2 in deionized water		0.06[38]
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Cheng, S.-P.; Wei, L.-W.; Wang, H.-P. Photocatalytic Reduction of CO2 to Methanol by Cu2O/TiO2 Heterojunctions. Sustainability 2022, 14, 374. https://doi.org/10.3390/su14010374

AMA Style

Cheng S-P, Wei L-W, Wang H-P. Photocatalytic Reduction of CO2 to Methanol by Cu2O/TiO2 Heterojunctions. Sustainability. 2022; 14(1):374. https://doi.org/10.3390/su14010374

Chicago/Turabian Style

Cheng, S.-P., L.-W. Wei, and H.-Paul Wang. 2022. "Photocatalytic Reduction of CO2 to Methanol by Cu2O/TiO2 Heterojunctions" Sustainability 14, no. 1: 374. https://doi.org/10.3390/su14010374

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