Elsevier

Applied Catalysis B: Environmental

Volume 250, 5 August 2019, Pages 408-418
Applied Catalysis B: Environmental

Insights into the role of singlet oxygen in the photocatalytic hydrogen peroxide production over polyoxometalates-derived metal oxides incorporated into graphitic carbon nitride framework

https://doi.org/10.1016/j.apcatb.2019.02.031Get rights and content

Highlights

  • Polyoxometalates-derived oxides have been incorporated into g-C3N4 framework.

  • g-C3N4-CoWO can catalyze the H2O2 production efficiently under visible light.

  • g-C3N4-CoWO can promote the singlet oxygen-involved H2O2 production.

Abstract

To develop a new strategy of enhancing the photoinduced holes (h+) consumption to promote the photoinduced electrons (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 graphitic carbon nitride (g-C3N4) precursor of 3-amino 1, 2, 4-triazole (3-AT) and the polyoxometalates (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 value of electrons transfer during the oxygen reduction reaction (ORR) process obtained from the Koutecky-Levich plot for g-C3N4-CoWO (n = 1.95) is higher than that for g-C3N4 (n = 1.18), suggesting that the CoWO incorporated into g-C3N4 framework can generate more e for O2 reduction. The superoxide radicals (radical dotO2-) quantitative and scavenger experiments combined with the electron spin resonance (ESR) results reveal that the negative shifts of the conduction band (CB) level from g-C3N4 to g-C3N4-CoWO can enhance the single-electron reduction of O2 to radical dotO2-. The h+ and 1O2 scavenger experiments results combined with the ESR results demonstrate that the CoWO incorporated into g-C3N4 framework can promote the oxidation of radical dotO2- to 1O2 by h+. The 1O2 quantitative experiments results indicate that the 1O2 can proceed two-electron reduction to H2O2. The enhanced h+ consumption and the 1O2 transferred from radical dotO2- 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.

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 → radical dotO2 (−0.13 V vs. NHE)radical dotO2 + 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 (radical dotOH) [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)radical dotO2 + h+ → 1O2 (0.34 V vs. NHE)

Since the oxidation of superoxide radicals (radical dotO2) to singlet oxygen (1O2) is thermodynamically favored (0.34 V vs NHE), the radical dotO2 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 radical dotO2 and further promote the photocatalytic H2O2 production [6b,6c]. In addition, g-C3N4 is chemical stable towards radical dotO2 [9]. Therefore, incorporating the electrons transfer materials into the g-C3N4 framework and investigating the 1O2 generation from radical dotO2 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 radical dotO2 and furthermore promote the radical dotO2 oxidation to 1O2 by h+. The enhanced h+ consumption and the 1O2 transferred from radical dotO2 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).

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