Photochemical reduction of CO₂ into CO coupling with triethanolamine decomposition

Abstract

In this work, the impacts of triethanolamine (TEOA) on the performance of photochemical CO₂ reduction were investigated in a simple homogeneous system. We demonstrates that CO₂ can be converted into CO coupling with the decomposition of triethanolamine in TEOA aqueous solution without other additives under light irradiation. About 7.5 μmol CO product is achieved within 7 h with a maximum apparent quantum yield (AQY) of 0.086% at 254 nm. The isotope labelling experiment confirms that CO product originates from the reduction of CO₂ rather than the decomposition of TEOA. In addition, the photochemical system exhibits excellent stability, no obvious inactivation is observed during long-term photochemical CO₂ reduction reaction. This work provides a deep understanding of the effects of TEOA on the performance of photocatalytic CO₂ reduction. Upon utilizing TEOA as a sacrificial electron donor in photocatalytic system, the contribution of TEOA must be considered once evaluating the catalytic activity of catalysts.

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Introduction

Artificial photochemical conversion of CO₂ into fuels or value-added chemicals using solar energy is considered as a promising strategy for addressing both energy crisis and environmental issues associated with the consumption of fossil fuels.^1–5 However, the CO₂ reduction reaction faces significant challenges due to the thermodynamic stability and low reactivity of CO₂ molecule. To overcome these challenges, researchers have been focusing on the development of photochemical systems, including homogeneous and heterogeneous systems. The homogeneous system, which involves molecular catalysts, has gained attention due to its advantages in high activity, selectivity, tunable structure, and trackable chemical reactivity.^6,7

Generally, a typical homogeneous photochemical system for CO₂ reduction includes a photosensitizer, a sacrificial reductant, and a molecular catalyst. Designing effective molecular catalysts has been a key focus in achieving high activity and selectivity for CO₂ reduction. In the past decades, various types of catalysts, such as metal pyridyl complexes,^8,9 metal porphyrin catalysts,¹⁰ metal salen complexes,^11,12 imidazolium salts,¹³ and ionic liquids,^14,15 have been developed and shown excellent catalytic activity in homogeneous systems. In these photochemical systems, molecular complexes usually exhibit dual function. First, molecular catalyst can function as a photosensitizer, which converts solar light into chemical energy for CO₂ reduction. While the metal center of the complex acts as an active site, transferring the photoinduced electron into the reduction reaction of CO₂. In addition, it is crucial to ensure the rapid acceptance of an additional electron from sacrificial electron donors to avoid the accumulation of charge carriers, which can induce catalyst decomposition and diminish the durability of the photocatalytic system.^16,17 TEOA, acting as a sacrificial reductive quencher for chromophore excited states, is commonly used as an electron donor in photochemical CO₂ reduction reaction. However, it should be noted that previous reports have overlooked the potential secondary roles of TEOA in photochemical CO₂ reduction performance. In other words, TEOA might directly reduce CO₂ under light irradiation. Thereby, further research is needed to fully understand the potential applications and contributions of TEOA in photochemical CO₂ reduction mechanisms.

Herein, the effects of TEOA on the performance of homogeneous photochemical CO₂ reduction were studied in detail. The gaseous and liquid products were analyzed by ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy, mass spectroscopy (MS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and gas chromatography (GC). These studies collectively provide a comprehensive understanding of the reaction mechanism during the photochemical CO₂ reduction in the presence of TEOA aqueous solution without other additives.

Results and discussion

We first optimized the performance of photochemical CO₂ reduction in a 250 mL custom-built cell with different volume ratio of TEOA and H₂O (Fig. S1–S3†). A 300 W Xe lamp was used as the irradiation source, and the irradiation area was 12.5 cm². It can be observed that the generation rate of CO exhibits similar trend in three photochemical systems, which increases upon increasing irradiation time. About 1.29, 1.58 and 1.65 μmol h⁻¹ of CO generation rates are achieved at 7 h irradiation, corresponding to 3, 5 and 10 vol% TEOA aqueous solution respectively. As a result, 5 vol% TEOA aqueous solution system was applied for the following comparison. The UV-vis absorption spectrum was performed to monitor reaction intermediate. As depicted in Fig. S4,† the original TEOA aqueous solution mainly absorb ultraviolet light below 275 nm. Upon irradiation, a new absorption band emerges, possibly attributed to the formation of carbamate species.^18,19 The peak intensity gradually increases with prolonged irradiation time and reaches a maximum at 7 h irradiation. Such evolution is consistent with that of CO₂ generation rate, suggesting the presence of carbamate intermediate.

Fig. 1a shows the time-dependent CO generation amount during photochemical CO₂ reduction reaction in 5 vol% TEOA aqueous solution under light irradiation. About 7.5 μmol CO product is achieved within the testing time frame. Note that the initial rate for CO generation is low at the first 3 h of photochemical reaction, which may be associated to an induced period.²⁰ The quasi-linear plot of CO generation amount with irradiation time implies CO gas comes from photochemical reduction.²¹ Table S1† summarizes the photochemical CO₂ reduction under different sets of conditions. The result suggests that the TEOA aqueous solution systems exhibit significant performance in the conversion of CO₂ into CO under light irradiation. However, no CO product is detected (entry 4) after equipping with a cut-off filter (λ > 420 nm), implying that this photochemical reaction needs to be initiated by ultraviolet light, which is supported by the test of UV-vis absorption spectrum. Once CO₂ is replaced with N₂ (entry 5), no detectable CO is observed, suggesting that CO₂ serves as the carbon source in our photochemical reaction, in other words, the CO product originates from CO₂ rather than the decomposition of TEOA. In order to confirm that CO is generated exclusively from CO₂, the isotope labeling experiment of ¹³CO₂ gas was conducted as shown in Fig. S5.† The mass spectra presents one peak at m/z = 29 assigned to ¹³CO, verifying that the CO product comes from the reduction of CO₂ molecule. The apparent quantum yield (AQY) was performed with a 300 W Xe lamp irradiation equipped with various band-pass filters (254, 275, 295, 313, 350, 365 and 380 nm) as depicted in Table S2.† The result suggests that this photochemical system exhibits low AQY value, and the maximum AQY of CO generation is calculated to be 0.086% at 254 nm. In addition, the stability of the photochemical system was performed as shown in Fig. 1b, there is no obvious inactivation during photochemical CO₂ reduction process after 100 h irradiation, suggesting the robustness of this photochemical system.

Fig. 1 (a) Time-dependent performance of photochemical CO₂ reduction in 5 vol% TEOA aqueous solution under light irradiation. The error bars represent the standard deviation of three separate measurements. (b) The stability investigation during photochemical CO₂ reduction in 5 vol% TEOA aqueous solution under light irradiation.

Note that the sources of protons and electrons are required during CO₂ reduction reaction because they play a vital role in enabling the deoxygenation process of CO₂ molecule. The primary source of the protons can be attributed to H₂O or TEOA in our photochemical system. However, the electron can only come from TEOA molecule rather than H₂O since no oxygen has been detected during CO generation. In particular, TEOA can donate two protons and two electrons during its decomposition (see Scheme 1).^22–25 Therefore, we speculate that the electrons from TEOA decomposition participate in CO₂ reduction reaction.

Scheme 1 The decomposition pathway of TEOA upon monoelectronic oxidation.

In order to confirm this point, ¹H and ¹³C NMR spectra of pure TEOA, TEOA after 12 h irradiation and TEOA after 100 h irradiation were recorded on a Bruker 600 MHz spectrometer as shown in Fig. 2. Prior to test, the solution after irradiation was freeze-dried for 48 h to remove residual water. In Fig. 2a, three ¹H NMR signals are observed for pure TEOA, the peaks centred at 4.73, 3.55 and 2.60 ppm are attributed to OH, O–CH₂ and N–CH₂ groups according to previous reports.^26–29 After 12 h irradiation under CO₂ bubbling condition, two new signals are monitored, one peak at 3.58 ppm is assigned to O–CH₂ group, while the other at 2.68 ppm is assigned to N–CH₂ groups in diethanolamine (DEOA) molecules. However, the OH signal of DEOA disappears due to its overlaps with TEOA, as supported by the area integration analysis and the inserted figure in Fig. 2a. After 100 h irradiation under CO₂ bubbling condition, the peak intensity of DEOA is significantly enhanced. The results indicate the presence of DEOA in the solution after irradiation. Furthermore, the content of DEOA increases once prolonging the irradiation time. This observation suggests that the photochemical CO₂ reduction reaction leads to the decomposition of TEOA. It is also confirmed by ¹³C NMR spectra shown in Fig. 2b, two ¹³C NMR signals are detected for pure TEOA, the peaks centred at 58.82 and 55.62 ppm are attributed to N–CH₂ and O–CH₂ groups of TEOA molecules, respectively.^30–32 After 12 h irradiation under CO₂ bubbling condition, two new signals are observed, one peak at 59.49 ppm is assigned to O–CH₂ group, while the other at 49.62 ppm is assigned to N–CH₂ groups in DEOA molecules. Similarly, the peak intensity of DEOA is significantly enhanced after 100 h irradiation under CO₂ bubbling condition.

Fig. 2 (a) ¹H and (b) ¹³C NMR spectra of pure TEOA, TEOA after 12 or 100 h irradiation under CO₂ bubbling condition. The solution consists of 100 μL TEOA and 500 μL D₂O. The TEOA after photochemical reaction in 5 vol% TEOA aqueous solution is achieved by freeze-drying for 48 h to remove residual water.

The ATR-FTIR spectra were also performed to study the decomposition of TEOA as shown in Fig. S6.† For pure TEOA, it shows the typical vibration modes of TEOA molecules, consistent with previous reports.³³ After 12 h irradiation under CO₂ bubbling condition, two new peaks centred at 1647 and 1544 cm⁻¹ are observed, which derives from the vibration modes of N–H deformation in DEOA molecules and C=O stretching in NCOO⁻ group^34–36 that generated from the adsorption of CO₂ by amine species.²⁹ While their intensity increases as irradiation proceeds, proving that the formation process of DEOA is caused by photochemical reaction. In addition, the ESI-MS spectra were applied to further confirm the formation of DEOA from the decomposition of TEOA as shown in Fig. S7–S9.† For pure TEOA, the signals at m/z = 150 and 172 correspond to TEOA, while the new signal at m/z = 106 was attributed to DEOA after light irradiation.

To identify the photochemical process is stoichiometric in TEOA aqueous solution, the amount of TEOA was monitored during photochemical CO₂ reduction by ¹H NMR technique. As a result, the standard curves for different amount of TEOA was quantified by the peak area as shown in Fig. S10.† Fig. 3 presented the decomposition amount of TEOA, which is determined by subtracting the remaining TEOA quantity from the initial total TEOA content within the system. As we can see, about 63.7 μmol TEOA is consumed within the testing time frame. This value is much higher than that of CO generation amount as shown in Fig. 1, suggesting the photochemical CO₂ reduction reaction is nonstoichiometric in TEOA aqueous solution. This is possibly caused by the self-decomposition of excited TEOA molecules.^37–39 Nonetheless, it exhibited similar trend with the CO generation amount, implying the photochemical reduction of CO₂ into CO coupled with TEOA decomposition.

Fig. 3 The time-depended consumption during photochemical CO₂ reduction in 5 vol% TEOA aqueous solution under light irradiation.

According to the previous reports^40–43 and our experimental results, a possible mechanism for photochemical CO₂ reduction in our system is proposed in Scheme 2. TEOA firstly adsorbs CO₂ molecules to form carbamate, which subsequently converts into a positively charged aminyl radical (TEOA˙⁺) and carbon dioxide radical anions (CO₂˙⁻) by a monoelectronic transfer under UV light irradiation condition. Then the deprotonation of the TEOA˙⁺ transfers into a carbon centered radical via rearrangement,⁴⁴ which will contribute to the deoxygenation of CO₂˙⁻ to generate CO product. Subsequently, the iminium species further degrades into glycolaldehyde and DEOA in the present of H₂O.

Scheme 2 Proposed mechanism for CO₂ reduction coupling with TEOA decomposition.

Conclusion

In summary, our work demonstrates that CO₂ can be converted into CO in a TEOA aqueous solution under light irradiation without other additives. The isotope labelling experiment confirms the CO product originates from CO₂ rather than the decomposition of TEOA. This photochemical system exhibits excellent stability during long-term photochemical CO₂ reduction reaction. In addition, the decomposition of TEOA was also analyzed by ¹H and ¹³C NMR, MS and ATR-FTIR. At last, a possible mechanism is proposed for CO₂ reduction coupling with TEOA decomposition. We believe that this work will provide a deep understanding of the effects of TEOA on the performance of photocatalytic CO₂ reduction. When TEOA is served as a sacrificial electron donor in photocatalytic system, the investigation of intrinsic activity for catalyst needs to take TEOA contributions into consideration.

Author contributions

C. L. Yang: investigation, validation, data analysis, writing. Y. S. Su, Y. H. Cheng, Y. J. Cui: investigation. S. Y. Liu, Y. W. Fang: supervision. Z. Li: review, editing, supervision, project administration, funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Natural Science Foundation of Guangdong Province (2022A1515012359), National Natural Science Foundation of China (21902121), STU Scientific Research Foundation for Talents (NTF21020), the 2020 Li Ka Shing Foundation Cross-Disciplinary Research Grant (2020LKSFG09A) and the Department of Education of Guangdong Province (2021KCXTD032).

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Footnote

† Electronic supplementary information (ESI) available: Photochemical CO₂ reduction performance, UV-vis absorption spectra, photochemical reaction under different sets of conditions, mass spectra, AQY measurements, ATR-FTIR observation, standard curve of TEOA for ¹H NMR analysis. See DOI: https://doi.org/10.1039/d3ra06585e

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