Insights into the role of singlet oxygen in the photocatalytic hydrogen peroxide production over polyoxometalates-derived metal oxides incorporated into graphitic carbon nitride framework
Graphical abstract
Introduction
As a clean environment and green oxidant, hydrogen peroxide (H2O2) is convenient and safe storage and transportation in liquid form and producing water (H2O) as the sole by-product. Therefore, it can be widely utilized in organic synthesis, environmental remediation, disinfection and one-compartment fuel cells alternative to hydrogen (H2) [1]. However, the anthraquinone method (the Riedl-Pfleiderer process) utilized in industry catalyzed by Pd-based catalysts requires the regeneration of anthrahydroquinone by H2 [2a], and the direct synthesis of H2O2 with H2 and O2 catalyzed by Pd or Au-Pd catalysts should pay more attention to the potentially explosive nature of H2/O2 mixed gases [2b]. A noble metal-free approach capable of producing H2O2 without H2 is therefore desired.H2O + 1/2O2 → H2O2 (ΔGo = 117 kJ∙mol−1)O2 + 2H+ + 2e− → H2O2 (0.68 V vs. NHE)O2 + e− → O2− (−0.13 V vs. NHE)O2− + 2H+ + e− → H2O2 (1.44 V vs. NHE)
The photocatalytic H2O2 production method through proton-coupled electron transfer (PCET) process can meet the above requirement because it only needs H2O, dioxygen (O2) and light (Eq. (1)) [3]. Since the formed 1,4-endoperoxide species on the graphitic carbon nitride (g-C3N4) surface actually gets transformed as H2O2 molecule, it exhibit efficient performance for photocatalytic H2O2 production [[4]]. Generally, the photocatalytic H2O2 production over g-C3N4-based catalysts can process either a direct two-electron O2 reduction (Eq. (2)) [[5]] or a sequential two-step single-electron O2 reduction (Eqs. (3) and (4)) [6]. However, the photoinduced holes (h+) of g-C3N4 are less active for water oxidation because of the relative low valence band (VB) potential (Eq. (5)) [7]. The relative low h+ consumption leads to electron-hole recombination, thus limiting the utilization of e− for O2 reduction to H2O2. To solve the above problem, two approaches have been adopted: 1) quenching the h+ with the addition of organic electron donor to utilize more e− for O2 reduction to H2O2 [8]; and 2) positively shifting the VB potential of g-C3N4-based catalysts to promote the water oxidation [[5]]. However, the above approaches possess disadvantages: 1) using organic electron donor results in the loss of chemical energy, the introduction of side products, and the increase in cost due to the use of chemicals other than H2O [5b]; and 2) the positive shifting of the VB potential may generate hydroxyl radical (OH) [4d,5e], which can tear the heptazine unit directly from g-C3N4 to form cyameluric acid and further release nitrates into the aqueous environment [9]. Therefore, it is highly desired to develop a new strategy of enhancing the h+ consumption to promote the e− utilization for O2 reduction to H2O2 and maintaining the chemical stability of g-C3N4-based catalysts.H2O + 2 h+ → 1/2O2 + 2H+ (1.23 V vs. NHE)O2− + h+ → 1O2 (0.34 V vs. NHE)
Since the oxidation of superoxide radicals (O2−) to singlet oxygen (1O2) is thermodynamically favored (0.34 V vs NHE), the O2− can be oxidized by h+ to 1O2 (Eq. (6)) [10]. Furthermore, it has been proved that the 1O2 can promote the H2O2 production in the presence of dissolved organic matter (DOM) as organic electron donors [11]. Negatively shifting the CB potential of g-C3N4-based catalysts by incorporating the electrons transfer materials into the g-C3N4 framework can enhance the single-electron reduction of O2 to O2− and further promote the photocatalytic H2O2 production [6b,6c]. In addition, g-C3N4 is chemical stable towards O2− [9]. Therefore, incorporating the electrons transfer materials into the g-C3N4 framework and investigating the 1O2 generation from O2− during the photocatalytic H2O2 production over g-C3N4-based catalysts is essential for enhancing the h+ consumption and maintaining the chemical stability of g-C3N4-based catalysts. As efficient electrocatalysts for hydrogen evolution reaction (HER) and overall water splitting, the polyoxometalates (POMs)-derived metal oxides are capable of accepting, transporting and storing electrons [12]. Therefore, incorporating the POMs-derived metal oxides into g-C3N4 framework can generate more e- for O2 reduction. Herein, the hybrid catalyst of g-C3N4-CoWO has been prepared through the calcination of the g-C3N4 precursor of 3-amino 1, 2, 4-triazole (3-AT) and the POMs precursor of (NH4)8Co2W12O42 (NH4-Co2W12). The hybrid catalyst of g-C3N4-CoWO with well-defined and stable structure exhibits efficient catalytic performance (9.7 μmol h-1) for photocatalytic H2O2 production in the absence of organic electron donor under visible light. The catalytic and characterization results reveal that the incorporation of CoWO into g-C3N4 framework can enhance the single-electron reduction of O2 to O2− and furthermore promote the O2− oxidation to 1O2 by h+. The enhanced h+ consumption and the 1O2 transferred from O2− can promote the photocatalytic H2O2 production over g-C3N4-CoWO. In addition, the recycle experiment results reveal that the heterogeneous g-C3N4-CoWO is catalytic stable.
Section snippets
Chemicals
3-amino 1, 2, 4-triazole (3-AT), melamine, urea, cobalt(II) acetate (Co(Ac)2), ammonium metatungstate ((NH4)6H2W12O40), nitro blue tetrazolium (NBT), 1,3-diphenylisobenzofuran (DPBF), p-benzoquinone (PBQ), l-Histidine (L-His), ammonium oxalate ((NH4)2C2O4), Rose Bengal (RB), and tris(2,2′-bipyridine)ruthenium dichloride ([Ru(bpy)3]Cl2·6H2O) were purchased from Alfa Asear company and used as received without further purification. MWCNT (purity > 95%) were purchased from Chengdu Organic Chemicals
Preparation of g-C3N4-CoWO
As shown in Fig. 1, the hybrid catalyst of g-C3N4-CoWO has been prepared through the calcination of the g-C3N4 precursor of 3-AT and the POMs precursor of NH4-Co2W12. The XRD patterns of NH4-Co2W12, CoWO, g-C3N4, g-C3N4-Co, g-C3N4-WO and g-C3N4-CoWO are shown in Fig. S1A. The NH4-Co2W12 possesses the typical peaks of [Co2W12O40]8− cluster [13]. The NH4-Co2W12 has been completely transformed to the CoWO4 (JCPDS 15-0867) phase after calcination [16]. The g-C3N4 has two distinct diffraction peaks
Conclusions
In summary, to develop a new strategy of enhancing the h+ consumption to promote the e− utilization for O2 reduction to H2O2 and maintaining the chemical stability of g-C3N4-based catalysts, the hybrid catalyst of g-C3N4-CoWO has been prepared through the calcination of the g-C3N4 precursor of 3-AT and the POMs precursor of NH4-Co2W12. The hybrid catalyst of g-C3N4-CoWO with well-defined and stable structure exhibits efficient catalytic performance (9.7 μmol h-1) for photocatalytic H2O2
Acknowledgments
This work is supported by the National Natural Science Foundations of China (Grant Nos. 21777176, 21707154, 51578532) and the Chinese Academy of Sciences (QYZDB-SSW-DQC018).
References (44)
- et al.
Environ. Sci. Technol.
(2017) - et al.
J. Am. Chem. Soc.
(1956) - et al.
Angew. Chem.
(2016) - et al.
J. Mater. Chem.
(2008) - et al.
Ind. Eng. Chem. Res.
(2015)et al.Ind. Eng. Chem. Res.
(2017) - et al.
Environ. Sci. Technol.
(2011) - et al.
Chem. Comm.
(2014) - et al.
J. Mater. Chem. A
(2014)et al.Energy Environ. Sci.
(2016) - et al.
Appl. Catal. B: Environ.
(2016)et al.ACS Appl. Mater. Interfaces
(2017) - et al.
Angew. Chem.
(2006)et al.Angew. Chem.
(2006)et al.Energy Environ. Sci.
(2012)
Ind. Eng. Chem. Res.
Angew. Chem.
Energy Environ. Sci.
Nat. Comm.
Adv. Funct. Mater.
J. Catal.
ChemSusChem
Chem. Eur. J.
J. Am. Chem. Soc.
Appl. Catal. B: Environ.
Angew. Chem.
J. Catal.
ACS Catal.
ChemSusChem
Nano Energy
Energy Environ. Sci.
Appl. Catal. B: Environ.
J. Am. Chem. Soc.
Appl. Catal. B: Environ.
J. Catal.
Chem. Commun.
Angew. Chem.
ACS Catal.
ACS Catal.
Adv. Funct. Mater.
Small
J. Catal.
Acc. Chem. Res.
Appl. Catal. B: Environ.
J. Phys. Chem. C
Environ. Sci. Technol.
Environ. Sci. Technol.
Adv. Energy Mater.
ACS Catal.
Angew. Chem.
J. Mater. Chem. A
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