Catalysis Science & Technology PAPER Cite this: Catal. Sci. Technol., 2018, 8, 1686 Received 8th January 2018, Accepted 13th February 2018 DOI: 10.1039/c8cy00043c rsc.li/catalysis Polyoxometalates covalently combined with graphitic carbon nitride for photocatalytic hydrogen peroxide production† Shen Zhao, a Xu Zhao, * ab Shuxin Ouyang cde and Yongfa Zhu f The polyoxometalate (POM) cluster [SiW 11 O 39 ] 8- (SiW 11 ) with photoreductive ability has been successfully covalently combined with graphitic carbon nitride (g-C 3 N 4 ) through an organic linker strategy. The hybrid catalyst g-C 3 N 4 –SiW 11 exhibits efficient catalytic performance (15.2 μmol h -1 ) for photocatalytic H 2 O 2 pro- duction in the presence of methanol and can stabilize the formed H 2 O 2 under sunlight irradiation (AM 1.5 filter). The Koutecky–Levich plot obtained from electrochemical rotating disk electrode (RDE) analysis of the oxygen reduction reaction (ORR) for g-C 3 N 4 –SiW 11 reveals that the value of electron transfer during the ORR process is 2.76. Combining the electron spin resonance (ESR), Koutecky–Levich plots, O 2 temperature programmed desorption (O 2 -TPD) and density functional theory (DFT) calculation results, the enhanced O 2 adsorption of g-C 3 N 4 –SiW 11 can promote the two-electron reduction of O 2 to H 2 O 2 . Introduction With increasing demands for a clean environment, green oxi- dant hydrogen peroxide (H 2 O 2 ) is used as the terminal oxygen source in pulp bleaching, disinfection, and organic synthesis, producing H 2 O as the sole byproduct. 1 Nowadays, many methods including the anthraquinone method, 2a the direct synthesis from H 2 and O 2 , 2b the alcohol oxidation method 2c and electrochemical syntheses 2d have been adopted to pro- duce H 2 O 2 . However, the above methods have two shortcom- ings: 1) large amounts of energy and organic solvents have been consumed and 2) organic impurities may contaminate the formed H 2 O 2 and increase the difficulty of extraction. Therefore, an efficient, energy-saving and green method to produce H 2 O 2 is highly desired. R‐CH 2 OH + 2h + → R‐CHO + 2H + (1) O 2 + 2H + + 2e - → H 2 O 2 (0.68 V vs. NHE) (2) The photocatalytic H 2 O 2 production through proton- coupled electron transfer (PCET) can meet the above require- ment because it needs only water (H 2 O), oxygen (O 2 ) and light. 3 The reaction is carried out by light irradiation of O 2 - saturated water with a catalyst in the presence of electron do- nors, such as alcohols. 4 Photoexcitation of the catalyst pro- duces hole (h + ) and electron (e - ) pairs. The h + oxidizes alco- hol and produces aldehyde and H + (eqn (1)), while the e - promotes two-electron reduction of O 2 and produces H 2 O 2 (eqn (2)). Though graphitic carbon nitride (g-C 3 N 4 )-based cat- alytic systems exhibit good efficiency for photocatalytic H 2 O 2 production, 4b,c,e two problems exist in the above systems which restrict their further improvement: 1) the limited inhi- bition of the one-electron reduction of O 2 (eqn (3)) 4b and 2) the subsequent decomposition of formed H 2 O 2 by absorbing light. 4c To promote the two-electron reduction of O 2 to H 2 O 2 and stabilize the formed H 2 O 2 under light irradiation, intro- ducing guest molecules to the g-C 3 N 4 host is therefore desired. 5 O 2 +H + +e - → ˙OOH(-0.13 V vs. NHE) (3) O 2 + 4H + + 4e - → H 2 O(1.23 V vs. NHE) (4) Polyoxometalates (POMs) are composed of cations and polyanion clusters with structural diversity, in which the oxo- metal polyhedra of MO x (M = W, Mo, V, Nb, Ta; x = 5, 6) are the basic construction units. 6 In the presence of light with abundant energy, the excitation of POMs refers to the charge transfer from O 2- to M n+ (n = 5, 6), leading to the formation of a hole center (O - ) and trapped electron center (M (n-1)+ ) 1686 | Catal. Sci. Technol., 2018, 8, 1686–1695 This journal is © The Royal Society of Chemistry 2018 a Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China. E-mail: zhaoxu@rcees.ac.cn; Fax: +86 10 62849667; Tel: +86 10 62849667 b University of Chinese Academy of Sciences, Beijing, 100049, China c TU-NIMS Joint Research Center, School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, China d Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China e Key Lab of Advanced Ceramics and Machining Technology, Ministry of Education, Tianjin 300072, China f Department of Chemistry, Tsinghua University, Beijing, 100084, China † Electronic supplementary information (ESI) available: Catalytic results and characterization of the catalysts. See DOI: 10.1039/c8cy00043c Published on 13 February 2018. Downloaded by Tsinghua University on 12/16/2019 3:14:03 PM. View Article Online View Journal | View Issue