Room-temperature coupling of methane with singlet oxygen

Abstract

Owing to emission of methane (CH₄) causing global warming and waste of resources, conversion of CH₄ to value-added chemicals can mitigate environmental sustainability and energy concerns. Direct room-temperature coupling of CH₄ to form ethane (CH₃CH₃) challenges chemists owing to the strong C–H bonds requiring high temperature (>700 °C) for dehydrogenation of CH₄. Oxidative coupling is a promising approach for CH₄ conversion to C₂H₆ using solar energy at room temperature. To achieve high efficiency of C₂H₆ formation, using an appropriate oxidant is a potential strategy to avoid overoxidation during the CH₄ coupling process. Singlet oxygen (¹O₂) has typically manifested a mild redox capacity with a high selectivity to attack organic substrate CH₄. Here, we report a synergistic photocatalytic-oxidative route for direct CH₄ coupling. Under solar light irradiation, a high CH₃CH₃ generation rate of 647 μmol g⁻¹ h⁻¹ is achieved at 25 °C. Our work demonstrates that the solar-oxidative route can result in new and useful C1-based catalytic behaviors.

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Environmental significance

Owing to emission of methane (CH₄) causing global warming and waste of resources, conversion of CH₄ to value-added chemicals can mitigate environmental and energy concerns. Direct room-temperature coupling of CH₄ to form ethane (CH₃CH₃) challenges chemists owing to the strong C–H bonds requiring high temperature (>700 °C) for dehydrogenation of CH₄. Oxidative coupling is a promising approach for CH₄ conversion to C₂H₆ using solar energy at room temperature. To achieve high efficiency of C₂H₆ formation, using an appropriate oxidant is a potential strategy to avoid overoxidation during the CH₄ coupling process. Singlet oxygen (¹O₂) has typically manifested a mild redox capacity with a high selectivity to attack organic substrate CH₄. Here, we report a synergistic photocatalytic-oxidative route for direct CH₄ coupling. Under solar light irradiation, a high CH₃CH₃ generation rate of 647 μmol g⁻¹ h⁻¹ is achieved at 25 °C. Our work demonstrates that the solar-oxidative route can result in new and useful C1-based catalytic behaviors.

Taking into account the environmental pollution and global warming caused by the use of traditional fossil energy and the shortage of its reserves, increasing the use of natural gas mainly composed of methane (CH₄) is an inevitable trend,¹ since methane has the advantage of being abundant and relatively inexpensive and clean. However, methane itself is also a greenhouse gas whose greenhouse effect is about 25 times that of carbon dioxide of the same mass.² Thus, methane emissions contribute to global warming. On the other hand, the direct use of natural gas as a fuel will also cause environmental pollution and waste of resources, since methane storage and transportation are difficult and it is prone to leakage. These factors have made scientists invest a lot of energy in the research and development of simple and feasible technologies for converting methane into value-added chemical raw materials.

However, a high temperature (>700 °C) is required for thermodynamic dehydrogenation of CH₄ due to the strong C–H bonds (434 kJ mol⁻¹), leading to energy consumption and low selectivity of CH₄ conversion.³ Photocatalytic methane conversion is a safe, low-energy and environmentally friendly strategy for the direct conversion of methane, since the dissociation of methane at room temperature can be achieved by means of photocatalytic methods using the light energy of sunlight and a suitable photocatalyst. Photocatalytic oxidation is also a promising approach for coupling of CH₄ to form C₂H₆ using solar energy at room temperature.⁴ Nevertheless, the major challenge of CH₄ coupling via the photocatalytic route is insufficient yield of target CH₃CH₃ and large production of by-products, e.g., HCOOH, CO, and CO₂. Furthermore, noble metal co-catalysts, such as Au, Pd, and Pt, were generally used for promoting the efficiency of coupling of CH₄.⁵ Developing oxidative-coupling and noble-metal-free catalyst systems, thus, is highly desirable for photocatalytic CH₄ coupling at room temperature.

Traditionally, chemical oxidants, including O₂, H₂O₂, and CO₂, have been proven to be important in oxidative activation of CH₄ to hydrocarbons, such as methanol.⁶ Actually, these oxidants potentially generate reactive oxygen species (ROS), such as the superoxide radical anion (O₂˙⁻), hydroxyl radical (˙OH), sulfate radical (SO₄˙⁻) and singlet oxygen (¹O₂), which are crucial in activation of CH₄.⁷ In particular, the ¹O₂-based system typically manifested a mild redox capacity (2.2 V) with a high selectivity to attack organic substrates, compared to other free radicals such as SO₄˙⁻ (2.5–3.1 V) and ˙OH (2.7 V) (see Fig. 1).⁸

Fig. 1 Band structure of TiO₂ (Degussa P25) and redox potentials of reactive oxygen species.

Peroxymonosulfate (P) as an excellent alternative oxidant has been confirmed to be the main source of HSO₅⁻ which can produce ¹O₂, ˙OH, and SO₄˙⁻ radicals.⁹ In particular, P can be utilized for selective oxidation of organic substances during which ¹O₂ is generated and serves as a mild oxidant with distinct reactivity towards different substrates.¹⁰ Importantly, the HSO₅⁻ molecule has a higher oxidizing potential (1.82 V) than H₂O₂ (1.76 V), and is thus more promising for activation of CH₄.⁷ Therefore, P is often applied as an electron acceptor in photocatalytic degradation of organic pollutants.¹¹ Nevertheless, P has never been studied for selective activation of CH₄.

Herein, we develop a TiO₂–¹O₂ system for the photocatalytic-oxidative route for CH₄ coupling to form CH₃CH₃ with solar light at room temperature. Other oxidants, including O₂, H₂O₂, and CO₂, have been investigated to illustrate the important role of HSO₅⁻ in selectively controlling the coupling of CH₄ to form C₂H₆. Further, an ¹O₂ involving radical-mediated pathway is proposed to explain the high activity of C₂H₆ formation from CH₄. This work provides an alternative new approach for effective coupling of CH₄ to form C₂H₆ at room temperature.

The XRD patterns of TiO₂ in Fig. S1a† show the typical anatase and rutile diffraction peaks. The particle morphology with the size range of 10–30 nm and crystalline structure have been clearly indicated by the TEM and HRTEM images of TiO₂, respectively (Fig. S1b and c†). Fig. 1 shows the band structure of TiO₂ and the redox potentials of H₂O₂/˙OH, H₂O/˙OH, O₂/O₂˙⁻, ¹O₂/O₂˙⁻, SO₅˙⁻/HSO₅⁻, and HSO₅⁻/SO₄˙⁻.^12,13 Based on these band and redox positions, the TiO₂ material is expected to present enhanced performance for radical generation and activation of CH₄.

In order to reveal the photocatalytic performance of the TiO₂–¹O₂ system, we first made a comparison of control experiments based on different reaction conditions, including light, catalyst, and HSO₅⁻ (see Fig. 2a). Under solar light irradiation, TiO₂ with HSO₅⁻ as oxidant gave rise to an excellent performance for selective generation of C₂H₆, with a rate of 647 μmol g⁻¹ h⁻¹, much higher than the 180 and 89 μmol g⁻¹ h⁻¹ for the two by-products CH₃OH and HCOOH, respectively, leading to a calculated C₂H₆ selectivity up to 75%. Notably, HSO₅⁻ can be independently activated by solar light with the corresponding reaction: HSO₅⁻ → SO₄˙⁻ + ˙OH.¹⁴ The ˙OH radical enables activation of CH₄ to produce ˙CH₃ species which are essential for C₂H₆ and CH₃OH generation. Under these conditions only a little C₂H₆, CH₃OH, HCOOH, and CO were detected, as displayed in Fig. 2a. For TiO₂ as catalyst, the photo-generated carriers reacting with HSO₅⁻ generate more ¹O₂ which activates CH₄ to generate ˙CH₃ species, thus accelerating the coupling of ˙CH₃ to form C₂H₆.

Fig. 2 Photocatalytic performance of the TiO₂–¹O₂ system under solar light irradiation: (a) comparison of this work (Exp.) and control experiments by varying conditions (no light, no P, and no cat.); (b) products obtained with different oxidants (¹O₂, O₂, H₂O₂, and CO₂).

To reveal the crucial role of ¹O₂ in selective conversion of CH₄ to C₂H₆, a control experiment was conducted using different oxidants for the conversion of CH₄. Fig. 2b summarizes the results of CH₄ oxidation with various oxidants (¹O₂, H₂O₂, O₂, and CO₂) under solar light irradiation. Apart from a little bit of CH₃OH, trace amounts of C₂H₆ were found for H₂O₂, O₂, and CO₂ as oxidants, as shown in Fig. 2b. In contrast, the reaction with ¹O₂ as oxidant remarkably promotes the conversion of CH₄ and selective generation of C₂H₆. Therefore, we conclude that ¹O₂ possesses superiority in view of the photocatalytic activity and selectivity for C₂H₆ generation. This is probably attributed to the specific band structure of TiO₂ and more positive redox potential of HSO₅⁻/SO₄˙⁻, thus favouring generation of ¹O₂, as shown in Fig. 1. Additionally, to further understand the ability of ¹O₂, we controlled the amount of P which is the source of ¹O₂ (Fig. S2a†). As the amount of P was increased from 0 to 0.10 mmol, more C₂H₆ was selectively produced in addition to two other products CH₃OH and HCOOH. Meanwhile, much more over-oxidation by-products (HCOOH, CO, and CO₂) were generated as it increased to 0.4 mmol, as displayed in Fig. 3a and b. This is presumably owing to the over-oxidation of CH₄. It is possible that excessive P may undergo a photoreaction (HSO₅⁻ → SO₄˙⁻ + ˙OH) and produce ˙OH, leading to the formation of CH₃OH and subsequent over-oxidization to HCOOH.

Fig. 3 (a) ESR spectra of ˙CH₃, ˙OH, SO₄˙⁻, and ˙O²⁻ radicals produced after photocatalytic reaction for 10 min. (b) Products of CH₄ conversion after adding scavengers para-quinone, K₂Cr₂O₇, Na₂C₂O₄, and salicylic acid in the reaction system for trapping ˙O²⁻, e⁻, h⁺, and ˙OH, respectively.

Therefore, excessive P normally results in formation of other by-products, leading to less C₂H₆. This result further suggests that an appropriate amount of P contributes to selective conversion of CH₄ to C₂H₆. Importantly, such a noble-metal free catalyst system presents remarkable coupling of CH₄ to form CH₃CH₃ compared to the various reported noble metal-based catalysts (see Table 1).

Table 1 Comparison of photocatalytic conversion of CH₄ to CH₃CH₃ over reported noble-metal-based catalysts

Catalysts	Light source	Temperature (°C)	C2H6 (μmol g^−1 h^−1)	Ref.
6.0 wt% Ag–HPW/TiO2	Xe lamp 400 W (200 < λ < 1000 nm)	30	20.3	15
1.0 wt% Pt/HGTS	Xe lamp 300 W	60	0.63	16
11.7 wt% Au/m-ZnO	Xe lamp 300 W (solar light)	30	11.3	17
AuPd/ZnO (Pd, 1.0 wt%)	Xe lamp 300 W (solar light)	30	17.7	18
0.5 wt% Pd/Ga2O3	Xe lamp 300 W (λ = 254 nm)	45	0.28	19
WO3, H2O2 (2 mM)	Mercury-vapor lamp (UVC-visible light)	55	3.40	20
HBEA	Hg lamp 450 W	70	14.3	21
Cu0.1Pt0.5/PC-50	LED 40 W (λ = 365 nm)	40	68.0	22
Au–ZnO/TiO2	Xenon lamp 300 W (300 < λ < 500 nm)	26	188	23
TiO2, ^1O2	Xe lamp 300 W (solar light)	25	647	This work

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To gain a better understanding of the mechanism of the photocatalytic process for selective conversion of CH₄ to C₂H₆, we used electron spin resonance (ESR) characterization and performed trapping experiments of active species, where 5-dimethyl-1-pyrroline-N-oxide (DMPO) was used as a trapping reagent to detect ˙CH₃, SO₄˙⁻, ˙OH, and O₂˙⁻ species. 2,2,6,6-Tetramethylpiperidine (TEMP) was used for detection of ¹O₂. As shown in Fig. 3a, the typical ˙CH₃, SO₄˙⁻, ˙OH, O₂˙⁻, and ¹O₂ radical species were obviously formed during the photocatalytic process. To verify the distinctive roles of these species in selective conversion of CH₄, we carried out trapping experiments using the corresponding scavengers, as displayed in Fig. 2a. The detailed reactions between scavenger reagents and active species are described in the ESI.† In Fig. 3b, the generation of C₂H₆ was significantly suppressed after trapping the ¹O₂ in Fig. 2b, while CH₃OH production was slightly promoted during this process. This result suggests that ¹O₂ remarkably facilitates the formation of C₂H₆ and the ˙OH radical prefers to activate CH₄ for generation of CH₃OH. However, a fraction of C₂H₆ was still detectable even after trapping ˙OH in the reactive system, which is probably attributed to the remaining ¹O₂ radicals. Apart from a certain amount of CH₃OH, C₂H₆ was never found in the absence of O₂˙⁻ active species (see Fig. 3b). This indicates that O₂˙⁻ also remarkably determined the selective formation of C₂H₆ which is related to the formation of ¹O₂. The e⁻ was also essential for the selective conversion as it initiated the ¹O₂ generation through chain reactions, which was proven by the absence of C₂H₆ in products after elimination of photogenerated electrons. On the other hand, h⁺ only partially controlled the formation of C₂H₆ based on an h⁺ trapping experiment.

Based on the above experimental analysis, we proposed a plausible mechanism. As displayed in Scheme 1, photo-induced electrons reacted with HSO₅⁻ and generated SO₄˙⁻ and ˙OH radicals (eqn (1) and (2)).²⁴ Meanwhile, direct light irradiation accelerated the generation of SO₄˙⁻ and ˙OH radicals (eqn (3)).⁷ O₂ was formed based on the reactions described by eqn (4)–(6),⁷ which agrees well with the trace amount of O₂ when ¹O₂ is used as oxidant in Fig. 2b. This O₂ further generated the O₂˙⁻ radical according to the reactions described by eqn (7). Consequently, ¹O₂ was finally produced as a result of the presence of the O₂˙⁻ radical (see eqn (8) and (9)).²⁵ The synthesized ¹O₂ was able to selectively dehydrogenize CH₄ and generate the ˙CH₃ radical which further underwent coupling, hence producing CH₃CH₃ (eqn (10)–(13)). It is noted that an increasing amount of ˙CH₃ prefers to form C₂H₆,²⁶ which is competitive with the CH₃OH generation (˙CH₃ + ˙OH → CH₃OH).^27,28 Therefore, when more ˙OH or O₂˙⁻ was present, CH₃OH could be generally produced. This well indicates that the dominant product was CH₃OH when radicals H₂O₂ and O₂ were selected as oxidants in Fig. 2b. Taken together, ¹O₂ favoured selective production of CH₃CH₃, in comparison with H₂O₂ or O₂-based systems. Apart from the products CH₃CH₃ and CH₃OH, over-oxidation by-products such as HCOOH, CO, and even CO₂ could also be formed (see eqn (14) and (15)) in the presence of the O₂˙⁻ radical.²⁹

HSO₅⁻ + e⁻ → OH⁻ + SO₄˙⁻ (1)

HSO₅⁻ + e⁻ → ˙OH + SO₄²⁻ (2)

HSO₅⁻ → ˙OH + SO₄˙⁻ (3)

2HSO₅⁻ + 2˙OH → 2SO₄˙⁻ + 2H₂O + O₂ (4)

HSO₅⁻ + h⁺ → H⁺ + SO₅˙⁻ (5)

SO₅˙⁻ + SO₅˙⁻ → 2SO₄˙⁻ + O₂ (6)

O₂ + e⁻ → O₂˙⁻ (7)

O₂˙⁻ + h⁺ → ¹O₂ (8)

˙OH + O₂˙⁻ → ¹O₂ + OH⁻ (9)

¹O₂ + CH₄ → ˙CH₃ (10)

CH₄ + ˙OH → ˙CH₃ + H₂O (11)

˙CH₃ + ˙OH → CH₃OH (12)

˙CH₃ + ˙CH₃ → C₂H₆ (13)

CH₃OH + O₂˙⁻ → HCOOH (14)

HCOOH + O₂˙⁻ → CO + CO₂ (15)

Scheme 1 Singlet oxygen involving radical-pathway mechanism for conversion of methane to ethane with the TiO₂–¹O₂ system.

Conclusions

Solar-light driven selective conversion of methane to ethane has been achieved through a photocatalytic reaction at room temperature. By introducing HSO₅⁻ into a TiO₂-based photocatalytic system, enhanced yields and selectivity of CH₃CH₃ are obtained largely due to the presence of ¹O₂ provided by HSO₅⁻. Solar light stimulates the TiO₂ catalyst to produce charge carriers (excited electrons and holes) which further activate HSO₅⁻ to generate ¹O₂. Detection and trapping experiments of active species further prove that the photocatalytic TiO₂–¹O₂ system involves the ¹O₂ radical pathway mechanism. This report opens up a new possibility for efficient conversion of methane to ethane with solar energy at ambient temperature.

Author contributions

Anhua Huang and Jingsheng Wang prepared the samples, carried out the experiments, analysed the data and prepared the paper; Xingyang Wu and Hangchen Liu assisted with the characterization and photocatalytic tests; Jun Cai and Guo Qin Xu reviewed and edited the manuscript; Song Ling Wang supervised this work and reviewed/edited the manuscript; all authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was sponsored by the Shanghai Pujiang Talent Program (No. 19PJ1405200) and the Startup Fund for Youngman Research at SJTU (SFYR at SJTU, No. WF220516003).

Notes and references

1. H. Herzog, B. Eliasson and O. Kaarstad, Capturing greenhouse gases, Sci. Am., 2000, 282, 72–79.
2. H. Song, X. Meng, Z.-j. Wang, H. Liu and J. Ye, Solar-energy-mediated methane conversion, Joule, 2019, 3, 1606–1636.
3. Y. Zeng, H. C. Liu, J. S. Wang, X. Y. Wu and S. L. Wang, Synergistic photocatalysis–Fenton reaction for selective conversion of methane to methanol at room temperature, Catal. Sci. Technol., 2020, 10, 2329–2332.
4. L. Yuliati and H. Yoshida, Photocatalytic conversion of methane, Chem. Soc. Rev., 2008, 37, 1592–1602.
5. L. Yu, Y. Shao and D. Li, Direct combination of hydrogen evolution from water and methane conversion in a photocatalytic system over Pt/TiO₂, Appl. Catal., B, 2017, 204, 216–223.
6. Y. Kang, Z. Li, X. Lv, W. Song, Y. Wei, X. Zhang, J. Liu and Z. Zhao, Active oxygen promoted electrochemical conversion of methane on two-dimensional carbide (MXenes): from stability, reactivity and selectivity, J. Catal., 2020, 393, 20–29.
7. F. Ghanbari and M. Moradi, Application of peroxymonosulfate and its activation methods for degradation of environmental organic pollutants, Chem. Eng. J., 2017, 310, 41–62.
8. S. Zhu, X. Li, J. Kang, X. Duan and S. Wang, Nonradical oxidation in persulfate activation by graphene-like nanosheets (GNS): differentiating the contributions of singlet oxygen (¹O₂) and sorption-dependent electron transfer, Environ. Sci. Technol., 2019, 53, 307–315.
9. D. Dai, Z. Yang, Y. Yao, L. Chen, G. Jia and L. Luo, Persulfate activation on crystallographic manganese oxides: mechanism of singlet oxygen evolution for nonradical selective degradation of aqueous contaminants, Catal. Sci. Technol., 2017, 7, 934–942.
10. Y. Zhou, J. Jiang, Y. Gao, J. Ma, S.-Y. Pang, J. Li, X.-T. Lu and L.-P. Yuan, Activation of peroxymonosulfate by benzoquinone: a novel nonradical oxidation process, Environ. Sci. Technol., 2015, 49, 12941–12950.
11. M. Ahmadi, F. Ghanbari and M. Moradi, Photocatalysis assisted by peroxymonosulfate and persulfate for benzotriazole degradation: effect of pH on sulfate and hydroxyl radicals, Water Sci. Technol., 2015, 72, 2095–2102.
12. Y. Shiraishi, Y. Ueda, A. Soramoto, S. Hinokuma and T. Hirai, Photocatalytic hydrogen peroxide splitting on metal-free powders assisted by phosphoric acid as a stabilizer, Nat. Commun., 2020, 11, 3386.
13. J. Lim, Y. Yang and M. Hoffmann, Activation of Peroxymonosulfate by Oxygen Vacancies-Enriched Cobalt-Doped Black TiO₂ Nanotubes for the Removal of Organic Pollutants, Environ. Sci. Technol., 2019, 53, 6972–6980.
14. M. G. Antoniou, A. A. de la Cruz and D. D. Dionysiou, Degradation of microcystin-LR using sulfate radicals generated through photolysis, thermolysis and e-transfer mechanisms, Appl. Catal., B, 2010, 96, 290–298.
15. X. Yu, V. L. Zholobenko, S. Moldovan, D. Hu, D. Wu, V. V. Ordomsky and A. Y. Khodakov, Stoichiometric methane conversion to ethane using photochemical looping at ambient temperature, Nat. Energy, 2020, 5, 511–519.
16. S. Wu, X. Tan, J. Lei, H. Chen, L. Wang and J. Zhang, Ga-doped and Pt-loaded porous TiO₂–SiO₂ for photocatalytic nonoxidative coupling of methane, J. Am. Chem. Soc., 2019, 141, 6592–6600.
17. L. Meng, Z. Chen, Z. Ma, S. He, Y. Hou, H.-H. Li, R. Yuan, X.-H. Huang, X. Wang and X. Wang, Gold plasmon-induced photocatalytic dehydrogenative coupling of methane to ethane on polar oxide surfaces, Energy Environ. Sci., 2018, 11, 294–298.
18. W. Jiang, J. Low, K. Mao, D. Duan, S. Chen, W. Liu, C.-W. Pao, J. Ma, S. Sang and C. Shu, Pd-Modified ZnO–Au Enabling Alkoxy Intermediates Formation and Dehydrogenation for Photocatalytic Conversion of Methane to Ethylene, J. Am. Chem. Soc., 2020, 143, 269–278.
19. S. P. Singh, A. Anzai, S. Kawaharasaki, A. Yamamoto and H. Yoshida, Non-oxidative coupling of methane over Pd-loaded gallium oxide photocatalysts in a flow reactor, Catal. Today, 2020, 375, 264–272.
20. K. Villa, S. Murcia-López, T. Andreu and J. R. Morante, Mesoporous WO₃ photocatalyst for the partial oxidation of methane to methanol using electron scavengers, Appl. Catal., B, 2015, 163, 150–155.
21. S. Murcia-López, M. C. Bacariza, K. Villa, J. M. Lopes, C. Henriques, J. R. Morante and T. Andreu, Controlled photocatalytic oxidation of methane to methanol through surface modification of beta zeolites, ACS Catal., 2017, 7, 2878–2885.
22. X. Li, J. Xie, H. Rao, C. Wang and J. Tang, Platinum-and CuO_x-Decorated TiO₂ Photocatalyst for Oxidative Coupling of Methane to C₂ Hydrocarbons in a Flow Reactor, Angew. Chem., Int. Ed., 2020, 59, 19702–19707.
23. S. Song, H. Song, L. Li, S. Wang, W. Chu, K. Peng, X. Meng, Q. Wang, B. Deng, Q. Liu, Z. Wang, Y. Weng, H. Hu, H. Lin, T. Kako and J. Ye, A selective Au-ZnO/TiO₂ hybrid photocatalyst for oxidative coupling of methane to ethane with dioxygen, Nat. Catal., 2021, 4, 1032–1042.
24. X. Chen, W. Wang, H. Xiao, C. Hong, F. Zhu, Y. Yao and Z. Xue, Accelerated TiO₂ photocatalytic degradation of Acid Orange 7 under visible light mediated by peroxymonosulfate, Chem. Eng. J., 2012, 193, 290–295.
25. X. Li, J. Liu, A. I. Rykov, H. Han, C. Jin, X. Liu and J. Wang, Excellent photo-Fenton catalysts of Fe–Co Prussian blue analogues and their reaction mechanism study, Appl. Catal., B, 2015, 179, 196–205.
26. S. Murcia-López, K. Villa, T. Andreu and J. R. Morante, Partial oxidation of methane to methanol using bismuth-based photocatalysts, ACS Catal., 2014, 4, 3013–3019.
27. K. Sahel, L. Elsellami, I. Mirali, F. Dappozze, M. Bouhent and C. Guillard, Hydrogen peroxide and photocatalysis, Appl. Catal., B, 2016, 188, 106–112.
28. M. Hayyan, M. A. Hashim and I. M. AlNashef, Superoxide ion: generation and chemical implications, Chem. Rev., 2016, 116, 3029–3085.
29. J. Xie, R. Jin, A. Li, Y. Bi, Q. Ruan, Y. Deng, Y. Zhang, S. Yao, G. Sankar and D. Ma, J. Highly selective oxidation of methane to methanol at ambient conditions by titanium dioxide-supported iron species, Tang, Nat. Catal., 2018, 1, 889–896.

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Footnotes

† Electronic supplementary information (ESI) available. See https://doi.org/10.1039/d2va00034b

‡ These authors contributed equally to this work.

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