Photo-induced reduction of biomass-derived 5-hydroxymethylfurfural using graphitic carbon nitride supported metal catalysts

Abstract

Photo-catalytic reduction of biomass-derived 5-hydroxymethylfurfural (HMF) into 2,5-dihydroxymethylfuran (DHMF) under visible light irradiation is achieved by using a platinum catalyst supported on graphitic carbon nitride (Pt/g-C₃N₄). Pt/g-C₃N₄ acts as a multifunctional and tandem catalyst to successively promote the photo-induced water splitting to form hydrogen and the successive activation of the produced hydrogen for HMF reduction, yielding DHMF yield of 6.5% with TOF of 0.457 h⁻¹. This research provides a sustainable and green pathway for biomass conversion using solar radiation as a driving force.

---

The production of fuels and chemicals sustainably obtained from plentiful and renewable biomass has recently attracted global attention.^1–6 Biomass is a carbon-neutral resource and the only renewable source of organic carbon; moreover, the use of biomass can substantially minimize the emission of net carbon dioxide. From this point of view, 5-hydroxymethylfurfural (HMF), produced from sugar dehydration, has been deemed as one of key precursors for the production of biomass-derived chemicals and biofuels.^7,8 Notably, hydrogenation of HMF is one of the typically important biomass-related chemical transformations to obtain 2,5-dihydroxymethylfuran (DHMF) and 2,5-dihydroxymethyl-tetrahydrofuran (DHMTHF, Fig. 1).^9–16 Typically, DHMF has been used as a six-carbon monomer not only in the synthesis of drugs but also in applications such as artificial fibers, polymers and resins.^7,8,17

Fig. 1 Reaction scheme of photo-induced reduction of HMF over Pt/g-C₃N₄.

Several researches regarding DHMF and DHMTHF synthesis have been reported.^18–24 Previously, Tomishige et al. demonstrated Ir-ReO_x/SiO₂ promoted HMF hydrogenation to DHMF and Pd–Ir alloy catalyzed total HMF reduction to DHMTHF.¹⁸ Tomishige and Schiavo investigated selective reduction HMF and furfural into DHMF, DHMTHF and 1,5-pentanediol by using nickel, copper, platinum, palladium, ruthenium, Pd–Ir alloy, Rh–Ir alloy, and Ni–Pd bimetallic catalysts.^{10,11,21–24}

Since biomass is organic matter that contains sunlight with the form of chemical energy through the process of photosynthesis, the use of sunlight again as a driving force to promote biomass-related transformations with various photo-catalysts should promise a sustainable and green pathway for HMF conversion. Recently, graphitic carbon nitride (g-C₃N₄)-based photo-catalysts attracted increasing interests for visible light-induced hydrogen production via water splitting due to their unique electronic properties and high chemical and thermal stability.^25–32 Inspired by these workers, a catalyst of Pt supported on g-C₃N₄ (Pt/g-C₃N₄) was investigated as photo-catalysis for HMF reduction to DHMF under a visible light irradiation in this research (Fig. 1). The Pt/g-C₃N₄ acts as tandem catalyst to promote the photo-induced water splitting to form hydrogen and the subsequent activation of the produced hydrogen for HMF reduction. Under an optimal reaction condition, a 6.5% DHMF yield with TOF of 0.457 h⁻¹ was obtained. It should be pointed out that, as far as we known, this is the first time for a direct photo-catalytic conversion of HMF, and such photo-induced process provides a pathway for biomass conversion with solar as a driving force.

As shown from N₂ adsorption/desorption isotherm (Fig. 2), the Brunauer–Emmett–Teller (BET) surface area of g-C₃N₄ is 11.7 m² g⁻¹ and slightly increases to 12.8 m² g⁻¹ for Pt/g-C₃N₄ after Pt loading, which is in accordance with the reported ref. 33.

Fig. 2 Typical nitrogen adsorption–desorption isotherm of the g-C₃N₄ and Pt/g-C₃N₄ (Pt 2.0 wt%) samples.

X-ray powder diffraction (XRD) of g-C₃N₄ and Pt/g-C₃N₄ are compared in Fig. 3a; for g-C₃N₄, the peak at 27.4° was a characteristic peak (002) corresponding to the stacking of conjugated inter-layers; whereas, the peak at 13.1° was indexed as (100) showing in-plane ordering of tri-s-triazine units (JCPDS 87-1526).^34–36 Notably, the diffraction peaks of platinum species in both Pt/g-C₃N₄ (Pt, 2.0 wt%) and Pt/g-C₃N₄ (Pt, 5.0 wt%) are unobserved, which is presumably due to the small nanoparticles size and high dispersion of noble nanoparticles on the support surface. In addition, the diffraction patterns of Pt/g-C₃N₄ remained nearly unchanged when compared with their corresponding supports, which confirmed that the crystal structures of g-C₃N₄ were not altered by the addition of noble metals.

Fig. 3 Characterization of g-C₃N₄ and Pt/g-C₃N₄ with various Pt loading levels by (a) XRD and (b) FTIR spectra.

The bonding of carbon and nitrogen in the g-C₃N₄ and Pt/g-C₃N₄ samples was analyzed through Fourier-transform infrared (FT-IR) spectra. As shown in Fig. 3b, several major bands centered at about 3180, 2358, 1200–1650, and 806 cm⁻¹. The broad 3180 cm⁻¹ band was assigned to the stretching vibration of N–H groups.³⁷ The peak at 2358 cm⁻¹ was attributed to C≡N stretching modes.³⁸ All samples showed stretching modes in the 1200–1650 cm⁻¹ range, which were typical for aromatic C–N heterocycle.³⁹ The sharp peak at 806 cm⁻¹ was considered to be the characteristic breathing mode of the triazine units.^38,40

The high resolution transmission electron microscopy (HRTEM) analysis of Pt/g-C₃N₄ revealed that highly dispersed Pt nanoparticles were successfully generated onto g-C₃N₄ via photo-deposition method and the average nanoparticle size is about 2.5 nm (Fig. 4).

Fig. 4 (a, b, c) TEM images and (d) particle size distribution of Pt/g-C₃N₄ (Pt 2.0 wt%).

The compositions and chemical states of Pt/g-C₃N₄ (Pt 2.0 wt%) were tested by X-ray photoelectron spectroscopy (XPS). As shown in Fig. 5a, peaks corresponding to oxygen, nitrogen, carbon and platinum clearly observed in the survey scans of Pt/g-C₃N₄. The high-resolution spectra of carbon, nitrogen and platinum elements were further investigated. Fig. 5b shows three distinct carbon species presented in the C 1s spectra. The main C 1s peak at 288.1 eV corresponds to sp²-hybridized carbon (N–C=N), which is considered as the major carbon species in the polymeric g-C₃N₄.³⁸ The peak at 284.6 eV is regarded as sp² C–C.^41,42 Another peak with binding energy of around 286.0 eV is attributed to the C–NH₂ species on the g-C₃N₄.⁴²

Fig. 5 XPS scan survey (a), C 1s XPS spectra (b), N 1s XPS spectra (c) and Pt 4f XPS spectra (d) of Pt/g-C₃N₄ (Pt 2.0 wt%).

For the N 1s XPS of Pt/g-C₃N₄, the N 1s line can be deconvoluted into three superimposed peaks at 398.5 eV, 399.7 eV and 401.0 eV as shown in Fig. 5c. The predominant peak in N 1s spectra of Pt/g-C₃N₄ at 398.5 eV was assigned to the sp²-hybridized nitrogen atoms in C–N=C, demonstrating the presence of triazine rings. The peak at about 399.7 eV was indexed as the sp³-hybridized tertiary nitrogen N–(C)₃; whereas, the peak at 401.0 eV was attributed to the amino group functionalized with a hydrogen (C–N–H), suggesting the existence of the defects.^38,43–45

Fig. 5d shows XPS spectra of Pt 4f of Pt/g-C₃N₄. The larger peaks at 71.2 eV and 74.5 eV were assigned to metallic platinum.⁴⁶ There is a 3.3 eV difference between the binding energy (B.E.) of Pt 4f_7/2 and Pt 4f_5/2 peaks, which is the characteristic of metallic Pt 4f states.⁴⁷ The two small peaks at 72.2 eV and 75.8 eV were indexed as Pt(II) oxidation state.^48,49 Based on the relative areas of these peaks, the percentage of metallic Pt(0) and Pt(II) species are 67% and 33%, respectively. Notably, the Pt⁰ is predominant on the surface of Pt/g-C₃N₄. In the case of many hydrogen-involving catalytic reactions, metallic Pt can serve as active component and can also provide much more active sites than PtO and PtO₂.⁵⁰

The optical properties of the samples were accessed by UV-visible absorption spectra (Fig. 6). Both g-C₃N₄ and mesoporous g-C₃N₄ (mpg-C₃N₄) had fundamental absorption edges at 460 nm, showing an intrinsic semiconductor-like absorption in the blue region of the visible spectrum.³⁴ The Pt/mpg-C₃N₄ and Pt/g-C₃N₄ showed the absorption edge at 470 nm and 568 nm, respectively. The corresponding band gap values of g-C₃N₄ (2.70 eV), mpg-C₃N₄ (2.70 eV), Pt/mpg-C₃N₄ (2.64 eV) and Pt/g-C₃N₄ (2.18 eV) were obtained according to the relationship E_g = 1240/λ, where E_g is the band gap energy and λ is the cutoff wavelength of the photocatalyst.³⁴ The increased absorption in the visible region of light and narrower band gap energies can contribute a positive impact to Pt/g-C₃N₄ as the photocatalysis. In addition, the UV-visible absorption spectra further indicated that all samples can be excited by visible light. Therefore, all experiments were performed under visible light irradiation (λ > 420 nm) in this research.

Fig. 6 UV-Vis absorbance spectrum of the g-C₃N₄, Pt/g-C₃N₄, mpg-C₃N₄ and Pt/mpg-C₃N₄ samples.

In this research, photo-induced reduction of HMF to DHMF was investigated over various g-C₃N₄-related photo-catalysts with trimethylamine (TEA) as a sacrificial electron donor in aqueous medium under an irradiation of a visible light source using 210 W Xenon lamp equipped with a 420 nm cut-off filter. For the preparation of photo-catalysts Pt/g-C₃N₄ and Pd/g-C₃N₄, in situ photo-deposition of noble nanoparticles over g-C₃N₄ was applied.^51,52 In the cases of the syntheses of various metal-doped g-C₃N₄ (M/g-C₃N₄, M = Zn, Ni, Cu, Co, La, Ce), dicyandiamide was copolymerized in the presence of the corresponding metal halide at 550 °C under flowing nitrogen atmosphere.^40,53

Initially, Pt/g-C₃N₄ was selected as a potential photo-catalyst because light-illuminated Pt/g-C₃N₄ has been proven to promote hydrogen evolution from water.⁵⁴ In addition, it has been proven that g-C₃N₄ with a band gap of 2.7 eV is a kind of visible-light photocatalyst to split water, but the quantum yield of the system is low.⁵⁴ Pt deposition can successfully enhance the photocatalytic activities of Pt/g-C₃N₄, indicating that Pt species efficiently acts as a cocatalyst.^55,56 As expected, the photo-catalytic reduction of HMF proceeded with a DHMF yield of 4.5% with turnover frequency (TOF) of 0.32 h⁻¹ in water containing TEA at room temperature under an illumination of a visible light source (Table 1, Run 5). Moreover, improving reaction temperature enhanced catalytic/photo-catalytic activity of the Pt/g-C₃N₄ system with DHMF yield and TOF of 6.5% and 0.457 h⁻¹, respectively (Table 1, Run 6). In addition, DHMF was always the predominant product with the selectivity higher than 90% for photo-induced reduction of HMF under the investigated conditions. However, full hydrogenation product DHMTHF was unobserved in the photo-catalytic reduction of HMF, indicating that the Pt/g-C₃N₄ is not effective for the further transformation of DHMF to DHMTHF under the visible light irradiation.

Table 1 Photo-induced HMF reduction over various catalysts^a

Run	Catalyst	DHMF yield (%)	TOF^b × 100 (h^−1)
a Reaction conditions: HMF (13 mg, 0.1 mmol), catalyst (30 mg), triethylamine (0.5 mL), water (4 mL), reaction time (4 hours) at room temperature under an irradiation of a visible light source using 210 W xenon lamp equipped with a 420 nm cut-off filter.b Turnover frequency was donated as the amount of produced DHMF per amount of total metal per hour for the investigated catalyst.c The photo-induced reaction was performed at 80 °C under the visible light irradiation.d Without triethylamine.e In the dark.
1	—	0	—
2	g-C3N4	0	—
3	mpg-C3N4	0.5	—
4	Pt/mpg-C3N4 (Pt 2.0 wt%)	4.1	28.7
5	Pt/g-C3N4 (Pt 2.0 wt%)	4.5	32.0
6^c	Pt/g-C3N4 (Pt 2.0 wt%)	6.5	45.7
7^d	Pt/g-C3N4 (Pt 2.0 wt%)	0	—
8^e	Pt/g-C3N4 (Pt 2.0 wt%)	0.3	3.4
9	Pd/g-C3N4 (Pd 2.0 wt%)	3.2	18.3
10	Zn/g-C3N4 (Zn 10 wt%)	2.7	1.3
11	Ni/g-C3N4 (Ni 10 wt%)	1.8	0.8
12	Cu/g-C3N4 (Cu 10 wt%)	0.7	0.4
13	Co/g-C3N4 (Co 10 wt%)	0.6	0.3
14	La/g-C3N4 (La 10 wt%)	1.9	1.8
15	Ce/g-C3N4 (Ce 10 wt%)	0.6	0.5

---

Both g-C₃N₄ and mpg-C₃N₄ are medium-bandgap semiconductor.²⁷ Here, mpg-C₃N₄ is obtained by generating nanopore structure into the polymeric matrix to improve the structural and electronic functions of g-C₃N₄. Compared with g-C₃N₄, mpg-C₃N₄ feathers unique semiconductor properties along with an open crystalline pore wall and a large surface area, which facilitates mass transfer.⁵⁴ Pt/mpg-C₃N₄ demonstrated a slightly decreased catalytic activity if compared with Pt/g-C₃N₄ under the employed conditions (Table 1, Runs 4 and 5). As shown in Fig. 6, Pt/g-C₃N₄ showed longer wavelength region in comparison with that of Pt/mpg-C₃N₄. The enhanced light adsorption for Pt/g-C₃N₄ can presumably result in the higher photocatalytic activity. A reference experiment in the absence of visible light irradiation, but in the presence of Pt/g-C₃N₄ and TEA demonstrated a negligible TOF of 0.034 h⁻¹ for DHMF formation (Table 1, Run 8), which suggested a significant promotion effect of the light irradiation. Our control experiments further showed that no reaction took place in the absence of Pt/g-C₃N₄ or TEA base on HPLC and HPLC-MS analysis (Table 1, Runs 1 and 7), reflecting the tandem catalysis reaction by coupling water reduction for hydrogen evolution with subsequent HMF hydrogenation to afford DHMF. In addition, g-C₃N₄ was inactive for the photo-catalytic synthesis of DHMF (Table 1, Run 2). While, DHMF was obtained with a low yield of 0.5% by using mpg-C₃N₄, which was still in sharp contrast to Pt/mpg-C₃N₄ (Table 1, Runs 3 and 4), indicating the multifunctional roles of Pt nanoparticles to promote electron transfer (for water reduction) and for the catalytic hydrogenation of HMF.

In addition to Pt/g-C₃N₄, Pd/g-C₃N₄ and various metal-doped g-C₃N₄ (M/g-C₃N₄, M = Zn, Ni, Cu, Co, La, Ce) were also active in the photo-catalytic synthesis of DHMF with, however, rather lower activity (Table 1, Runs 9–15). It is therefore evident that Pt shows the best promotional effect for photo-induced HMF reduction among the examined metals. Previously, Pt/graphene and Pt/ionic polymer were investigated as catalysts and Pt-support interactions were examined.^57,58 To probe the influence of g-C₃N₄ support, the photo-induced reactions were also performed with commercial Pt/C and Pt/Al₂O₃; however, both of them were inactive to the photo-induced reactions, indicating the key role of semiconductor g-C₃N₄.

It was reported that Pt can capture electrons from the conductive band of g-C₃N₄; therefore, a suitable Pt loading level caused its well and homogeneous dispersion on the g-C₃N₄ surface, which favored the effective transfer and separation of photo-generated electron–hole pairs.^59,60 The influence of Pt content on the photo-catalytic performance of Pt/g-C₃N₄ for HMF reduction revealed that both DHMF yield and TOF improved with increasing Pt loading to 2.0 wt%, indicating a key role of Pt metal as an active component in the Pt/g-C₃N₄ (Fig. 7a). Further increase of the Pt loading to 5.0 wt% in Pt/g-C₃N₄ led to a decrease in both DHMF yield and TOF. These results suggested that photocatalytic activity of Pt/g-C₃N₄ may be related to the metal dispersion on the support. As proved by Wang et al., after optimization of preparation parameters, 10 wt% Fe-amount of the carbon nitride polymer leads to a direct synthesis of phenol under visible light irradiation.⁵³

Fig. 7 Effects of (a) Pt loading level in Pt/g-C₃N₄, (b) catalyst Pt/g-C₃N₄ (Pt 2.0 wt%) amount and (c) catalyst cycle times on photo-induced HMF reduction to DHMF. Reaction conditions: HMF (13 mg, 0.1 mmol), catalyst [Pt/g-C₃N₄ (30 mg) with various Pt loading level for (a), Pt/g-C₃N₄ (Pt 2.0 wt%) with various amount for (b) and Pt/g-C₃N₄ (30 mg, Pt 2.0 wt%) for (c)], triethylamine (0.5 mL), water (4 mL), reaction time (4 hours) at ambient temperature under an irradiation of a visible light source using 210 W xenon lamp equipped with a 420 nm cut-off filter.

Fig. 7b shows the effect of Pt/g-C₃N₄ loading amount on the HMF reduction in the investigated photo-catalytic system. DHMF yields smoothly increased to a maximum of 4.5% with an optimal Pt/g-C₃N₄ loading amount of 30 mg. The above results suggested that the reaction rate was mainly limited by the number and availability of catalytically active sites of the catalyst in the regime 1 (Fig. 7b, catalyst <30 mg); while, the reaction rate was generally determined by electron-transfer kinetics in the regime 2 (Fig. 7b, catalyst >30 mg).⁶¹ Therefore, the formation of DHMF stayed almost unaltered in the regime 2 even with an excessive amount of Pt/g-C₃N₄. The reusability of Pt/g-C₃N₄ was further investigated as shown in Fig. 7c. The recovered catalyst was washed, dried at 60 °C under vacuum, and then reused for the next run. There was no obvious loss of its catalytic activity.

In summary, Pt/g-C₃N₄ is active for photo-catalytic reduction of biomass-based HMF to DHMF under a visible light irradiation. Pt/g-C₃N₄ promotes a tandem catalysis reaction by coupling water reduction for hydrogen evolution with subsequent HMF hydrogenation to afford DHMF. In addition, it should be pointed out that this is the first time for the direct photocatalytic hydrogenation of HMF to DHMF, and such photocatalytic hydrogenation process provides a path to convert solar into chemical energy.

Acknowledgements

The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (21472189), Science and Technology Planning Project of Guangdong Province, China (2015A010106010), Natural Science Foundation of Guangdong Province, China (2015A030312007), and Guangdong Key Laboratory of New and Renewable Energy Research and Development (Y607jl1001).

Notes and references

1. C. O. Tuck, E. Pérez, I. T. Horváth, R. A. Sheldon and M. Poliakoff, Science, 2012, 337, 695–699.
2. P. Gallezot, Chem. Soc. Rev., 2012, 41, 1538–1558.
3. J. N. Chheda, G. W. Huber and J. A. Dumesic, Angew. Chem., Int. Ed., 2007, 46, 7164–7183.
4. Y. Roman-Leshkow, J. N. Chheda and J. A. Dumesic, Science, 2006, 312, 1933–1937.
5. G. W. Huber and A. Corma, Angew. Chem., Int. Ed., 2007, 46, 7184–7201.
6. G. W. Huber, S. Iborra and A. Corma, Chem. Rev., 2006, 106, 4044–4098.
7. R. J. van Putten, J. C. van der Waal, E. de Jong, C. B. Rasrendra, H. J. Heeres and J. G. de Vries, Chem. Rev., 2013, 113, 1499–1597.
8. A. A. Rosatella, S. P. Simeonov, R. F. M. Frade and C. A. M. Afonso, Green Chem., 2011, 13, 754–793.
9. Y. Nakagawa, M. Tamura and K. Tomishige, ACS Catal., 2013, 3, 2655–2668.
10. V. Schiavo, G. Descotes and J. Mentech, Bull. Soc. Chim. Fr., 1991, 128, 704–711.
11. Y. Nakagawa and K. Tomishige, Catal. Commun., 2010, 12, 154–156.
12. R. Alamillo, M. Tucker, M. Chia, Y. Pagán-Torres and J. Dumesic, Green Chem., 2012, 14, 1413–1419.
13. Y. L. Yang, Z. T. Du, J. P. Ma, F. Lu, J. J. Zhang and J. Xu, ChemSusChem, 2014, 7, 1352–1356.
14. J. Z. Chen, F. Lu, J. J. Zhang, W. Q. Yu, F. Wang, J. Gao and J. Xu, ChemCatChem, 2013, 5, 2822–2826.
15. J. Ohyama, A. Esaki, Y. Yamamoto, S. Arai and A. Satsuma, RSC Adv., 2013, 3, 1033–1036.
16. T. Pasini, G. Solinas, V. Zanotti, S. Albonetti, F. Cavani, A. Vaccari, A. Mazzanti, S. Ranieri and R. Mazzoni, Dalton Trans., 2014, 10224–10234.
17. T. Stahlberg, W. J. Fu, J. M. Woodley and A. Riisager, ChemSusChem, 2011, 4, 451–458.
18. M. Tamura, K. Tokonami, Y. Nakagawa and K. Tomishige, Chem. Commun., 2013, 49, 7034–7036.
19. Y. Kwon, E. de Jong, S. Raoufmoghaddam and M. T. M. Koper, ChemSusChem, 2013, 6, 1659–1667.
20. J. Z. Chen, R. L. Liu, Y. Y. Guo, L. M. Chen and H. Gao, ACS Catal., 2015, 5, 722–733.
21. Y. Nakagawa, K. Takada, M. Tamura and K. Tomishige, ACS Catal., 2014, 4, 2718–2726.
22. S. Liu, Y. Amada, M. Tamura, Y. Nakagawa and K. Tomishige, Green Chem., 2014, 16, 617–626.
23. Y. Nakagawa, H. Nakazawa, H. Watanabe and K. Tomishige, ChemCatChem, 2012, 4, 1791–1797.
24. S. Liu, Y. Amada, M. Tamura, Y. Nakagawa and K. Tomishige, Catal. Sci. Technol., 2014, 4, 2535–2549.
25. X. H. Li and M. Antonietti, Chem. Soc. Rev., 2013, 42, 6593–6604.
26. G. P. Dong, Y. H. Zhang, Q. W. Pan and J. R. Qiu, J. Photochem. Photobiol., C, 2014, 20, 33–50.
27. Y. Wang, X. C. Wang and M. Antonietti, Angew. Chem., Int. Ed., 2012, 51, 68–89.
28. J. J. Zhu, P. Xiao, H. L. Li and S. A. C. Carabineiro, ACS Appl. Mater. Interfaces, 2014, 6, 16449–16465.
29. S. W. Cao and J. G. Yu, J. Phys. Chem. Lett., 2014, 5, 2101–2107.
30. S. W. Cao, J. X. Low, J. G. Yu and M. Jaroniec, Adv. Mater., 2015, 27, 2150–2176.
31. Y. T. Gong, M. M. Li, H. R. Li and Y. Wang, Green Chem., 2015, 17, 715–736.
32. Z. W. Zhao, Y. J. Sun and F. Dong, Nanoscale, 2015, 7, 15–37.
33. O. Fontelles-Carceller, M. J. Muñoz-Batista, M. Fernández-García and A. Kubacka, ACS Appl. Mater. Interfaces, 2016, 8, 2617–2627.
34. X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen and M. Antonietti, Nat. Mater., 2009, 8, 76–80.
35. H. F. Shi, G. Q. Chen, C. L. Zhang and Z. G. Zou, ACS Catal., 2014, 4, 3637–3643.
36. F. Goettmann, A. Fischer, M. Antonietti and A. Thomas, Angew. Chem., Int. Ed., 2006, 45, 4467–4471.
37. X. C. Wang, X. F. Chen, A. Thomas, X. Z. Fu and M. Antonietti, Adv. Mater., 2009, 21, 1609–1612.
38. A. Thomas, A. Fischer, F. Goettmann, M. Antonietti, J.-O. Müller, R. Schlögl and J. M. Carlsson, J. Mater. Chem., 2008, 18, 4893–4908.
39. S. C. Yan, Z. S. Li and Z. G. Zou, Langmuir, 2009, 25, 10397–10401.
40. H. Q. Sun, G. L. Zhou, Y. X. Wang, A. Suvorova and S. B. Wang, ACS Appl. Mater. Interfaces, 2014, 6, 16745–16754.
41. J. S. Zhang, M. W. Zhang, G. G. Zhang and X. C. Wang, ACS Catal., 2012, 2, 940–948.
42. J. H. Li, B. Shen, Z. H. Hong, B. Z. Lin, B. F. Gao and Y. L. Chen, Chem. Commun., 2012, 48, 12017–12019.
43. E. Raymundo-Piñero, D. Cazorla-Amorós, A. Linares-Solano, J. Find, U. Wild and R. Schlögl, Carbon, 2002, 40, 597–608.
44. C. Chang, Y. Fu, M. Hu, C. Y. Wang, G. Q. Shan and L. Y. Zhu, Appl. Catal., B, 2013, 142–143, 553–560.
45. F. He, G. Chen, Y. G. Yu, S. Hao, Y. S. Zhou and Y. Zheng, ACS Appl. Mater. Interfaces, 2014, 6, 7171–7179.
46. L. Wei, Y. J. Fan, J. H. Ma, L. H. Tao, R. X. Wang, J. P. Zhong and H. Wang, J. Power Sources, 2013, 238, 157–164.
47. J. G. Yu, L. F. Qi and M. Jaroniec, J. Phys. Chem. C, 2010, 114, 13118–13125.
48. Y. Q. Huang, H. L. Huang, Y. J. Liu, Y. Xie, Z. R. Liang and C. H. Liu, J. Power Sources, 2012, 201, 81–87.
49. J. Xu, X. C. Xu, L. K. Ouyang, X. J. Yang, W. Mao, J. J. Su and Y. F. Han, J. Catal., 2012, 287, 114–123.
50. F. X. Zhang, J. X. Chen, X. Zhang, W. L. Gao, R. C. Jin, N. J. Guan and Y. Z. Li, Langmuir, 2004, 20, 9329–9334.
51. X. Xu, H. R. Li and Y. Wang, ChemCatChem, 2014, 6, 3328–3332.
52. L.-H. Gong, Y.-Y. Cai, X.-H. Li, Y.-N. Zhang, J. Su and J.-S. Chen, Green Chem., 2014, 16, 3746–3751.
53. X. F. Chen, J. S. Zhang, X. Z. Fu, M. Antonietti and X. C. Wang, J. Am. Chem. Soc., 2009, 131, 11658–11659.
54. X. C. Wang, K. Maeda, X. F. Chen, K. Takanabe, K. Domen, Y. D. Hou, X. Z. Fu and M. Antonietti, J. Am. Chem. Soc., 2009, 131, 1680–1681.
55. T. Toyao, M. Saito, Y. Horiuchi, K. Mochizuki, M. Iwata, H. Higashimura and M. Matsuoka, Catal. Sci. Technol., 2013, 3, 2092–2097.
56. X.-H. Li, X. C. Wang and M. Antonietti, Chem. Sci., 2012, 3, 2170–2174.
57. Y. J. Gao, X. Chen, J. G. Zhang, H. Asakura, T. Tanaka, K. Teramura, D. Ma and N. Yan, Adv. Mater., 2015, 27, 4688–4694.
58. S. Siankevich, G. Savoglidis, Z. F. Fei, G. Laurenczy, D. T. L. Alexander, N. Yan and P. J. Dyson, J. Catal., 2014, 315, 67–74.
59. M. Y. Liu, W. S. You, Z. B. Lei, G. H. Zhou, J. J. Yang, G. P. Wu, G. J. Ma, G. Y. Luan, T. Takata, M. Hara, K. Domen and C. Li, Chem. Commun., 2004, 2192–2193.
60. K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y. Inoue and K. Domen, Nature, 2006, 440, 295.
61. T. Maschmeyer and M. Che, Angew. Chem., Int. Ed., 2010, 49, 1536–1539.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19153c

---