Photoelectrochemical degradation of organic pollutants using BiOBr

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Photoelectrochemical degradation of organic pollutants using BiOBr anode coupled with simultaneous CO2 reduction to liquid fuels via CuO cathode Shan-Shan Liu, Qiu-Ju Xing, Ying Chen, Meng Zhu, XunHeng Jiang, Shi-Hao Wu, Weili Dai, and Jian-Ping Zou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04917 • Publication Date (Web): 01 Dec 2018 Downloaded from http://pubs.acs.org on December 2, 2018

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ACS Sustainable Chemistry & Engineering

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Photoelectrochemical degradation of organic pollutants using

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BiOBr anode coupled with simultaneous CO2 reduction to liquid

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fuels via CuO cathode

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Shan-Shan Liu, Qiu-Ju Xing,* Ying Chen, Meng Zhu, Xun-Heng Jiang, Shi-Hao Wu, Weili Dai, Jian-Ping Zou*

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Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources

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Recycle, Nanchang Hangkong University, 696 Fenghe south road, Nanchang 330063, P. R.

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China

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Corresponding authors. E-mail: [email protected] (J.-P. Zou) or [email protected] (Q.-J.

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Xing); Tel: +86 791 83953373; Fax: +86 791 83953373.

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Dedicated to Prof. Jin-Shun Huang on the occasion of his 80th birthday

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ABSTRACT: This study develops a novel system consisting of BiOBr as a photoanode and

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CuO as photocathode for coupling photoelectrocatalytic (PEC) oxidation of tetracycline

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(TC) with photoelectrocatalytic reduction of CO2 to form useful chemicals, such as CH3OH

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and CH3CH2OH. Under illumination, the degradation efficiency of TC was compared using

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photoelectrocatalysis (PEC), photocatalysis (PC) and electrocatalysis (EC) after 2.5 h. About

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80% of TC was removed in the PEC process against 39%, 63% removed in PC and EC

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process, respectively, and. the kinetic constants of them are estimated to be 0.418 h-1, 0.211

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h-1 and 0.286 h-1, respectively. Apparently, the results show the system of PEC has a

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synergistic effect between PC and EC for the degradation of TC. Furthermore, the onset

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potential of the total current was approximate -0.6 V (vs. AgCl/Ag) and the yield of CH3OH

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and C2H5OH is the best at the bias potential of -0.7 V. And the yield of CH3OH and C2H5OH

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was 125.9 μmol/L and 26.5 μmol/L, respectively. Results of UV-vis absorption spectra, total

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organic carbon (TOC), Liquid chromatography-Mass spectra (LC-MS), nuclear magnetic

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resonance (NMR), and in-situ IR confirm the carbons change from organic pollutants to CO2

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and then to liquid fuels. Finally, we proposed a catalytic mechanism to explain the synergetic

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effect of PEC oxidation and PEC reduction and the synergetic effect of PC and EC. The

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present work provides a new avenue to achieve the one-pot conversion of organic pollutants

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to liquid fuels and puts forward a new idea to couple PEC oxidation and PEC reduction.

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Keywords: CO2 reduction; Degradation; Organic pollutants; Photoelectrocatalytic oxidation;

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Photoelectrocatalytic reduction

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Introduction

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In recent years, the increasing of water pollution and emissions of greenhouse gases have

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caused serious global energy crisis and environmental problems, which threaten the survival

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of humans and thousands of other living species.1-5 Photocatalysis was considered a

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promising technology to synchronously solve the problems of environmental contaminations

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and energy shortage.6-13 For example, Zhu et al. reported that the photodegradation of phenol

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was achieved over g-C3N4@TiO2 core-shell with excellent activity.14 Yu et al. reported that

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Co-doped BiOCl nanosheets could be employed as efficient and stable photocatalyst for

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bisphenol A degradation under visible light illuminations.15 Zhang et al. illuminated that the

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photocatalytic carbon dioxide reaction could be achieved over Pt NPs supported on TiO2-SiO2

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porous materials and the catalytic performance was strongly depended on the size of the

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photocatalyst.16 Ye et al. reported that Au-Cu alloy nanoparticles supported on SrTiO3/TiO2

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coaxial nanotube arrays can selectively convert carbon dioxide to CO and hydrocarbons in the

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presence of hydrous hydrazine17. Wang et al. reported that the photocatalytic activity of

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hydrogen evolution over carbon nitride can be significantly improved by a facile bottom-up

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strategy.18

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In addition, aiming at achieving the coupling of photocatalytic degradation of organic

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pollutants with simultaneous energy supply, our group has developed some hybrid materials,

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which can serve as photocatalysts for organic pollutant degradation combined with

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simultaneous alcohol production. For example, we designed and synthesized GQD/V-TiO2

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photocatalysts that show superior photocatalytic performance for one-pot transformation of


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organic pollutants to methanol and ethanol by merging the photocatalytic MB degradation and

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CO2 reduction.19 Besides, rGO modified SrTi0.95Fe0.05O3 photocatalysts were also synthesized

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for one-pot transformation of RhB to methanol and ethanol.20 However, the efficiency of

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these developed catalytic systems is still unsatisfactory. Moreover, it is difficult to effectively

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realize the separation and recovery of catalysts.21-25 Therefore, exploration of new technology

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to solve the above problems and provide new avenues for the application of photocatalysis

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technology in environmental treatment and energy supply is highly appealing.26-30

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Photoelectrocatalytic (PEC) technology is an excellent technology for organic pollutants

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degradation and reduction of CO2 due to the high catalytic efficiency and facile recovery and

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reuse of the catalyst.31-35 Therefore, the PEC technology is expected to solve the key problems

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in the photocatalytic process. However, the most critical part of PEC technology is

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preparation of photoelectrode, which should be capable of efficiently degrading organic

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pollutants and reducing CO2.36-39 Recently, the bismuth-based semiconductor catalysts (BiOX,

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X = Cl, Br, I) have been used for the removal of organic pollutants, especially BiOBr, which

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is a well-known photocatalyst due to its suitable band gap and stable photocatalytic

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properties.40-43 In addition, various kinds of catalysts have been investigated for CO2

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reduction, including metallic, non-metallic and molecular catalysts. Among these catalysts,

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Cu-based catalysts exhibit good catalytic activity for CO2 reduction with wide product

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distribution, including HCOOH,44 CH3OH,45 CH446 and so on. Especially, CuO, a p-type

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semiconductor, is demonstrated to be a promising material as photocathode due to its suitable

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band gap that responds to visible light.47

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Therefore, we developed a novel PEC system to achieve efficient mineralization of

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organic pollutants coupled with simultaneous CO2 reduction to useful chemicals over BiOBr

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(photoanode) and CuO (photocathode). This article mainly studied PEC oxidation of

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tetracycline (TC) and PEC reduction of CO2 to liquid fuels such as methanol and ethanol. In

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addition, a catalytic mechanism was proposed to explain the synergetic effect of PEC

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oxidation and PEC reduction and the synergetic effect of PC and EC. The present work

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provides new ideas for the treatment of organic wastewater and good approaches for the

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promotion of low-cost wastewater treatment processes.



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Experimental Section

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Chemicals and Materials

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All chemicals were commercially obtained and used without further purification. The

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chemicals used for the preparation are A.R. grade. Sodium hydroxide (NaOH), Potassium

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Bromide (KBr), Potassium bicarbonate (KHCO3) and Ethylene glycol were provided by

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Xilong

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(Bi(NO3)3·5H2O), sodium tungstate (Na2WO4·2H2O), ammonium sulfate ((NH4)2S2O8),

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polyvinylpyrrolidone (PVP, K-30), and sodium dodecyl sulfate (SDS) were obtained from

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Shanghai Chemical Reagent Co., Ltd. (China, Shanghai). FTO was gained by OPV-FTO22-7

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Tech Co., Ltd. (China, Liaoning), and Cu mesh (100 mesh, 0.11 mm as wire diameter) was

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supplied from Alfa Aesar Company (China, Shanghai).

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Characterizations and analytical methods

Chemical

Co.,

Ltd.

(China,

Guangdong).

Bismuth

nitrate

pentahydrate

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The as-synthesized samples were characterized by X-ray diffraction (Bruker D8

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ADVANCE) using graphite monochromatized Cu-Ka (λ = 1.5406 Å) radiation. The XRD

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data were collected over the angular range of 10° to 80° with a scanning speed of 2°/min.

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Ultraviolet-visible (UV-vis) diffuse reflection spectra of the samples were obtained using a

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UV-vis U-3900H spectrophotometer. The morphologies of the samples were investigated

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using a scanning electron microscopy (SEM, FEI, Holland), transmission electron microscopy

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(TEM, Tecnai F20, FEI, USA) and high-resolution TEM (HRTEM) images. X-ray

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photoelectron spectroscopy (XPS) measurements were carried out on a VG 250 Escalab

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spectrometer. Raman spectroscopic measurements were recorded on a Renishaw Invia Raman

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System with a 532 nm Nd:YAG excitation source. The electrochemical properties were

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performed by CHI 660E electrochemical workstation (Shanghai Chenhua, China) using a

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standard three-electrode cell as prepared CuO electrode as working electrode, BiOBr

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electrode as counter electrode and Ag/AgCl electrode as reference electrode, respectively.

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Na2SO4 (0.1 M) and KHCO3 (0.1 M) was used as the anode and cathode electrolyte solution,

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respectively. Electrochemical Impedance Spectroscopy (EIS) was carried out at the open

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circuit potential. The amplitude of the sine wave was 5 mV, and frequency range was from

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0.01 Hz to 100 kHz. The reduction products of CH3OH and C2H5OH were detected by gas


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chromatography (GC7890A, Agilent., America) with a DB-WAXetr column (125-7032,

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Agilent Co.). At the same time, the products were also detected by Bruker ARX 400 nuclear

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magnetic resonance (NMR) spectrometer, in which 0.9 ml cathode electrolyte after reaction

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was mixed with 0.1 ml D2O (deuterated water). The intermediates of organic pollutants were

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analyzed by liquid chromatography-mass spectrometry (LC-MS, Thermo, Finnigan,

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LCQ-Deca xp) equipped with an electrospray ionization (ESI) source.

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Preparation of electrodes

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Preparation of anode

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In a typical procedure, a certain amount of Bi(NO3)3∙5H2O (98.0%) were dissolved into

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10 mL ethylene glycol (EG) solution, and then slowly added into 10 mL EG solution

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containing stoichiometric amounts of KBr (99.5%) with the Bi/Br molar ratio of 1:1. And 10

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mg polyvinylpyrrolidone (PVP, K-30) was added to the above mixed solution, and then

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stirred for 30 min at room temperature to obtain a new mixed solution. One piece of FTO

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substrate (4 cm × 2 cm) was ultrasonically cleaned with acetone, ethanol and deionized water

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for 0.5 h, and then was dipped into the above mixed solution at a 50 mL Teflon-liner. The

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autoclave was sealed and maintained at 150 oC for 8 h and then cooled to room temperature.

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The as-synthesized BiOBr/FTO anode was washed with deionized water and then dried.

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Preparation of cathode

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CuO cathode was prepared by hydrothermal method. Typically, 6.4 g NaOH, 1.0954 g

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(NH4)2S2O8 and 0.211 g Na2WO4·2H2O were added to 32 mL of water, followed by 0.4614 g

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of sodium dodecyl sulfate (SDS) added to the aqueous solution with stirring to form a white

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aqueous solution. The Cu meshes were ultrasonically washed with hydrochloric acid, acetone

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and ethanol for 10 min, respectively. Then it was dipped into the above mixed solution at a 50

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mL Teflon-liner. The autoclave was sealed and maintained at 130 oC for 24 h and then cooled

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to room temperature. Then the as-synthesized CuO cathode was washed with distilled water

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and dried at 60 oC for 3 h.

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Catalytic degradation of TC and catalytic reduction of CO2

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Catalytic degradation of TC



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Degradation of TC by PEC was carried out under potentiostatic conditions in H-type cell,

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in which a piece of Nafion®117 membrane was used as a separator. Ag/AgCl was the

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reference electrode, while a Pt foil and the as-prepared BiOBr was used as the cathode and

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anode, respectively. Visible light was acquired by a 300 W Xe lamp equipped with a 420 nm

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cutoff filter. The electrolytes used in the anodic and cathodic compartments are 0.1 M Na2SO4,

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10 ppm TC (80 mL) and 0.1 M KHCO3 (80 mL), respectively. The degradation system was

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continuously stirred throughout the process. The potential was set at 0, 0.6, 0.7 and 0.8 V,

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respectively. The solution was stirred for 0.5 h to achieve adsorption-desorption equilibrium

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in the dark. Solution was sampled from the anodic compartment at prescribed intervals, and

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filtered by a cellulose acetate membrane (0.45 m). The removal efficiency of TC was

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analyzed using UV-Vis spectrophotometer (MAPADA V-1100D). Similarly, the steps of

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degradation of TC by photocatalysis (PC) are same to the above PEC steps except that applied

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voltage was not used, and the steps of degradation of TC by electrocatalysis (EC) are also

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same to the above PEC except for the absence of light radiation.

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Catalytic reduction of CO2

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Three-electrode electrochemical H-type cell system was used in the process of PEC

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reduction of CO2. Ag/AgCl was used as the reference electrode, while Pt foil and CuO were

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used as anode and cathode, respectively. A 300 W Xe lamp with a 420 nm cutoff filter was

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used as visible light source. The electrolytes used in the anodic and cathodic compartments

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were 0.1 M Na2SO4 (80 mL) and 0.1 M KHCO3 (80 mL), respectively. Prior to reduction, the

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cathodic electrolyte was purged with CO2 for 0.5 h to be saturated with CO2. The potential

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was set by 0, -0.6, -0.7 and -0.8 V, respectively. During the reaction, solution was sampled

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from the cathodic reaction cell at compartment at prescribed intervals, and filtered by a

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cellulose acetate membrane (0.22 m). Subsequently, CH3OH and C2H5OH concentrations

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were measured by Agilent 7890A gas chromatograph (FID, DB-WAX column). Similarly, the

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experimental steps of photocatalytic reduction of CO2 are same as the above steps except that

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applied voltage was not used, and electrocatalytic reduction of CO2 are same as the above

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steps except for the absence of light radiation.

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Catalytic degradation of TC coupled with catalytic reduction of CO2


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The experimental process of TC degradation by PEC coupled with simultaneous

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reduction of CO2 by PEC is similar to that of PEC degradation of TC. The experimental

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conditions are same to the above photoelectrocatalytic TC degradation except that CuO was

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used as cathode. As TC degradation by EC coupled with simultaneous reduction of CO2 by

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EC, the experimental steps are same as the above PEC processes except for the absence of

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light radiation. In addition, the experimental conditions of degradation of TC by PC coupled

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with reduction of CO2 by PC are same to those of TC degradation by PEC coupled with

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simultaneous reduction of CO2 by PEC except that applied voltage was not used.

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Results and discussion

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Electrode characterizations

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The morphologies of the as-prepared BiOBr and CuO electrodes were tested using SEM

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and TEM. As shown in Figure 1a, the as-prepared BiOBr shows flower-like microstructure

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that consists of numerous nanosheets, and the average size of microspheres is about 3 m

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(Figure 1b). Meanwhile, High-resolution TEM (HRTEM) image of the BiOBr electrode

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shows the lattice spacing of 0.270 nm, corresponding to the (1 1 0) crystal plane of the BiOBr

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(Figure 1c). As shown in Figure 1d, the as-prepared CuO electrode shows sheets’

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microstructure that consists of numerous nanosheets. TEM image shows the width of a single

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nanosheet is approximate 110 nm (Figure 1e) and HRTEM image shows a lattice spacing of

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0.23 nm, corresponding to the (1 1 1) crystal facet of CuO (Figure 1f).



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Figure 1. (a-c) SEM, TEM and HRTEM images of the BiOBr anode; (e-f) SEM, TEM and

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HRTEM images of the CuO cathode.

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The light absorption of the electrodes were studied by the UV-visible diffuse reflectance

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spectrum. As shown in Figure S1(a-b), the band gaps of BiOBr and CuO are calculated to be

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2.86 and 1.48 eV with absorption edges extending to around 430 and 900 nm, respectively,

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which is similar to the case reported in the literature.48According to the DRS spectra and the

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Mott-Schottky (MS) plots (Figure S1c), the conduction band (CB) and valence band (VB) of

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the BiOBr are calculated to be -0.41 V and 2.45 V vs. SHE, respectively. Figure S1d shows a

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p-type semiconductor of CuO film due to the negative slope (tangent intersect at a negative

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value). The flat band potential (Vfb) of CuO is 0.63 V νs. SCE (equivalent 0.87 V νs. SHE)

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(Figure S1d). It is believed that the flat band potential is approximately equal to the potential

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of CB.47 Therefore, the CB and VB of the CuO are calculated to be -0.61 V and 0.87 V νs.

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SHE, respectively, demonstrating that CuO has the ability to reduce CO2. The separation of

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photo-generated electron-hole pairs was further evaluated by measuring the transient

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photocurrent. Figure S2(a-b) shows the photocurrent responses of BiOBr and CuO

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photoelectrode under open circuit potential, and rapid and reversible photocurrent responses

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are obtained.

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Degradation of TC


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Figure 2a displays the degradation efficiency of TC by PC, as well as EC and PEC at the

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applied potential from 0.6 to 0.8 V vs. Ag/AgCl. 39% of TC was degraded by PC after 2.5 h,

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while 54% and 68% of TC were degraded by EC and PEC at 0.7 V of bias potential after 2.5

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h, respectively. From 0.6 V to 0.8 V, the degradation efficiency of TC increases with the

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increase of bias potential. Meanwhile, the degradation efficiencies of TC by PC, as well as by

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EC and PEC at 0.7 V of bias potential all fit a pseudo-first-order kinetic model (ln(Co/Ct) =

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kt). And the kinetic constants are calculated as 0.211 h-1, 0.286 h-1 and 0.418 h-1 for PC, EC

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and PEC, respectively (Figure 2b). The results indicate PEC has much more efficiency for TC

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degradation than PC and EC, which could be due to the synergistic effect between

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electrocatalysis and photocatalysis.

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When the bias voltage was applied, the movements of photogenerated electrons from the

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external circuit to cathode were improved, leading to the efficient separation of electrons and

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holes, and finally promoting the degradation efficiency of TC. And Figure S3 shows that the

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current density of the PEC is significantly greater than that of the single EC, indicating that

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the PEC has better degradation efficiency of TC than the EC. As shown in Figure 2c, the

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degradation efficiency of TC by the PEC maintains above 68% and only 2.0% decrease for

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the degradation efficiency after four cycles. As shown in Figure 2d, the current density of the

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BiOBr electrode becomes stable at different applied potentials during the PEC process. The

228

above results show that the BiOBr electrode possesses good stability.



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Figure 2. Degradation efficiency of TC by PC, EC and PEC (anode = BiOBr, cathode = Pt,

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applied potential from 0.6 to 0.8 V vs. Ag/AgCl, visible light, λ > 420 nm) (a); Kinetic

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analysis with the PC, EC and PEC degradation on the BiOBr film (anode = BiOBr, cathode =

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Pt) (b); Cycle experiment of TC degradation at a potential of 0.7 V in PEC process by the

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BiOBr film (c); Current density as a function of time at different voltages during the

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degradation of TC by the BiOBr film (d).

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Reduction of CO2

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To investigate the photoelectrochemical performance of the as-prepared CuO for CO2

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reduction, linear sweep voltammetry (LSV) test in N2 or CO2 saturated solution were

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conducted over photoanode and photocathode. As shown in Figure S4a, the current density in

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CO2 saturated solution is obviously higher than that of N2 in the BiOBr/CuO system, and the

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current density further increases under illumination of visible light. And the current density of

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Pt/CuO system is lower than that of BiOBr/CuO system, confirming that the BiOBr/CuO

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system was more active for CO2 reduction than the Pt/CuO system.49 The results reveal a

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superior photoelectrocatalytic performance of CO2 reduction in the BiOBr/CuO system and


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there should have a significant synergy between photoanode (BiOBr) and photocathode

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(CuO). Figure S4a shows the onset potential of CO2 reduction on the as-prepared CuO

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electrode is about -0.5 V. Thus the cathodic potential should be set at negative more than -0.5

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V so that the reaction of CO2 reduction can be proceeded. Accordingly, the anodic potential

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should also set at above 0.5 V in the degradation of TC. In the present work, we set the

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potential at 0.6, 0.7 and 0.8 V in the PEC degradation process of TC.

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As shown in Figure 3a, the formation yield of CH3OH and C2H5OH is up to 100.3

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μmol/L and 12.00 μmol/L, respectively, at -0.7 V after 2.5 h of PEC reduction. From -0.6 V to

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-0.8 V, the bias potential of -0.7 V showed the best yield of CH3OH and C2H5OH. The

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phenomenon could be explained that the reaction is reduction of CO2 at the potential of -0.7 V

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rather than hydrogen evolution reaction (HER). When the applied voltage is lower than -0.7 V,

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HER becomes dominant reaction.50 Furthermore, CO2 cannot be reduced to ethanol by PC but

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ethanol was detected under the reduction by EC. The formation yield of CH3OH and C2H5OH

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by PEC is much higher than that by EC and PC (Figure 3a). And Figure 3b shows that the

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current density of the PEC is significantly greater than that of the single EC, indicating that

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the PEC has higher ability to reduce CO2 than the EC. The main products of CH3OH and

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C2H5OH are demonstrated by NMR test (Figure S4b).

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Figure 3. Formation yield of methanol and ethanol from CO2 reduction by PC, EC and PEC

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(a); Chronoamperometry at a potential of -0.7 V in EC and PEC process on the CuO electrode

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(electrolyte: 0.1 M KHCO3, CuO as working electrode, Pt foil as the counter electrode) (b).

266 267

Degradation of TC coupled with reduction of CO2



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The voltage showed influence on the degradation of TC coupled with PEC reduction of

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CO2. As shown in Figure 4a, with the potential changing from −0.6 to −1.0 V, the degradation

270

efficiency of the TC firstly increases and an optimum degradation efficiency was found at the

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bias voltage of 0.7 V. The appropriate applied potential could effectively promote the photo

272

electrons transferring to the counter electrode via external curcuit, which inhibits the

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recombination of photoelectrons and holes.21 When the voltage increases, the ability of BiOBr

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anode to degrade TC has a little decrease, which is attributed to the evolution of oxygen at

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BiOBr anode surface. Similarly, the yield of CH3OH and C2H5OH first increase and then

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decrease with the increase of voltage and the maximum yield of methanol and ethanol is

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125.9 μmol/L and 26.5 μmol/L at -0.7 V after 2.5 h, respectively. As shown in Figure S5a, TC

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can be well decomposed (80%) at 0.7 V in the PEC system according to the results of UV–vis

279

absorption spectra. Furthermore, the total organic carbon (TOC) result also confirms that 51%

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of TC can be mineralized after 2.5 h reaction (Figure S5b).

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In order to investigate the role of the anode and cathode electrodes in the different

282

catalytic system, some control experiments were done. As shown in Table 1, for the BiOBr/Pt

283

system with bias potential of 0.7 V, the removal efficiency of TC in the PEC process (68%) is

284

more than that in the PC (39%) and EC (54%) processes, respectively. For the Pt/CuO system

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at bias potential of 0.7 V, the yield of CH3OH and CH3CH2OH in the PEC process also shows

286

higher than that in both PC and EC processes. Moreover, the degradation efficiency of TC

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and the yield of CO2 reduction products in the BiOBr/CuO system are all much larger than

288

those in the BiOBr/Pt system and Pt/CuO system, indicating that the photoanode BiOBr film

289

has good degradation rate of TC and the photocathode CuO has good potential for CO2

290

reduction. The above results manifest that there exist significant synergy between PEC

291

degradation of TC and PEC reduction of CO2 in the BiOBr/CuO system.

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According to the LSV test results, the current density of BiOBr/CuO system was larger

293

than that of Pt/CuO system, confirming that the BiOBr/CuO system was more active in CO2

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reduction. Thus, the coupling with BiOBr photoanode and CuO photocathode could

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significantly improve the catalytic activity of the PEC system, and further indicates that the

296

PEC system driven by BiOBr photoanode was more favorable for CO2 reduction. The

297

synergistic effect in BiOBr/CuO system was attributed to the use of photovoltage to


Page 12 of 30

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298

compensate cathode potential for CO2 reduction. Due to the photovoltage generated under

299

light radiation, the required electromotive force was significantly reduced using the BiOBr

300

photoanode. 51 Therefore, the cathode potential for CO2 reduction became more negative due

301

to compensation of the bias voltage.52 In addition, the generated photoelectrons at the anode

302

compartment are driven to cathode compartment via an external circuit to participate in CO2

303

reduction in the BiOBr/CuO system. Thus, the full utilization of photovoltage and generated

304

photoelectron result in that there are more favorable for CO2 reduction and the higher yields

305

of CH3OH and C2H5OH in the BiOBr/CuO system than in the system of Pt/CuO.

306

From Figure 4b, the Faradaic efficiency of the PEC CO2 reduction changes with the bias

307

potential from -0.6 to -1.0 V. And the catalyst presents a higher selectivity toward CH3OH

308

formation than CH3CH2OH formation. When the applied voltage is -0.7 V, the faradaic

309

efficiency is the highest with 9.4% and 4.0% for CH3OH and CH3CH2OH, respectively.

310

When the bias potential increases from -0.8 V to -1.0 V, the Faradaic efficiency gradually

311

decreases because the HER became dominant but CO2 reduction was suppressed. As shown in

312

Figure 4c, the current density of BiOBr/CuO system increases firstly and then decreases with

313

time. The significant decrease of the current density was due to reduction of the surface CuO

314

during the reaction, which was confirmed by the XPS results of the photocathode before and

315

after the reaction (Figure S8b). However, the current density of BiOBr is relatively stable with

316

time due to the stability of the BiOBr electrode. In general, the current density in the

317

BiOBr/CuO system is obviously higher than that in the BiOBr/Pt system, indicating that the

318

PEC performance of BiOBr/CuO system is better than that of Pt/CuO system during the

319

degradation of TC.

320

To investigate the stability of the BiOBr electrode and CuO electrode, four cycles

321

experiment for degradation of TC coupled with reduction of CO2 were performed at the

322

applied potential of -0.7 V. Figure 4d shows the degradation efficiency of TC maintains above

323

76% and only has a decrease of 4% after four consecutive cycles. Furthermore, the results of

324

XRD, XPS and Raman of the BiOBr electrode show that there are nearly no change after 2.5

325

h PEC reaction, indicating the BiOBr has good stability (Figures S6-S7). The details are

326

described in the Supporting Information (Text S1). Figure 4d shows the yield of methanol and

327

ethanol are obviously reduced after the four consecutive cycles, indicating that observable



328

deactivation occurred on the CuO electrode. And Figure S9 shows a gradual decrease in the

329

current density of BiOBr/CuO system after four cycles experiment, which is also attributed to

330

the deactivation of the CuO electrode. The deactivation of the CuO electrode is also observed

331

over other systems reported in the previous literatures.53-54 In addition, the XRD and XPS

332

results of the CuO electrode before and after the reaction (Figures S8(b-c)) also confirm the

333

poor stability of the CuO electrode during CO2 PEC reduction. The details are described in

334

the Supporting Information (Text S2).

335 336

Figure 4. Degradation of TC and yield of CH3OH and C2H5OH at the different bias potential

337

(a); Faradaic efficiency of CO2 reduction (b); Current density as a function of time during the

338

PEC degradation of TC in different systems at bias potential of -0.7 V vs. AgCl/Ag (c); Four

339

cycles of TC degradation coupled with reduction of CO2 at bias potential of -0.7 V vs.

340

AgCl/Ag (d) in the PEC system with BiOBr and CuO as anode and cathode, respectively.

341

Table 1. Effect of different electrodes on the degradation of TC and the yield of CH3OH and

342

CH3CH2OH for CO2 reduction at applied potential -0.7 V vs. AgCl/Ag (in absence of applied

343

potential at the PC process).


Page 14 of 30

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Electrodes

BiOBr/Pt

Pt/CuO

BiOBr/CuO

Technologies

Degradation

Yield of

Yield of

efficiency of TC

CH3OH

CH3CH2OH

(%)

(μmol/L)

(μmol/L)

PC

39

/

/

EC

54

/

/

PEC

68

/

/

PC

/

9.4

/

EC

/

70.7

8.3

PEC

/

100.3

12.1

PC

39

9.4

/

EC

63

94.7

11.3

PEC

80

125.9

26.5

344 345

Synergetic mechanism of TC degradation coupled with CO2 reduction

346 347

Figure 5. Removal efficiency of TC with the addition of scavengers in different reaction

348

systems at applied potential of 0.7 V vs. AgCl/Ag.

349

Figures S10a and S10b show the electrochemical impedance spectra (EIS) of the BiOBr

350

electrode and CuO electrode under dark and visible light irradiation. The resistance of

351

electrodes became lower after photoirradiation, indicating more efficient charge separation

352

under visible light irradiation.55 The result indicates a large number of carriers generated

353

under visible light irradiation, which can well explain the PEC process has better catalytic



354

performance than the EC.

355

Furthermore, the active species trapping experiments were conducted to explore the

356

mechanism of the degradation of TC in PEC, PC and EC systems by the as-prepared BiOBr.

357

Isopropanol (IPA) and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) are used as

358

the scavengers of hydroxyl radicals (·OH) and holes (h+), respectively. Since the generated

359

electrons in the anode chamber go through the external circuit to the cathode chamber, it is

360

impossible to trap O2 to form superoxide anions (·O2-) in the anode chamber. Therefore, ·O2-

361

are not considered in this degradation process in the anode chamber. As shown in Figure 5,

362

the removal efficiency of TC just has a little decrease when IPA was added in the PC, EC and

363

PEC systems, whereas the removal efficiency of TC markedly decreases when EDTA was

364

added in the PC and PEC systems. This phenomenon suggests that h+ is the main active

365

species for TC degradation in the PC and PEC systems, while electro-oxidation plays a major

366

role for degradation of TC in the EC process. Thus, on the basis of the above analyses, the

367

direct oxidation by holes and electro-oxidation play major role for the degradation of TC in

368

the PEC process.

369

In order to further discuss the possible mechanism of degradation of TC, the

370

intermediates were analyzed using LC-MS (Figure S11). The results of HPLC-MS show that

371

the characteristic peak of TC (m/z= 445) gradually decreased and some new peaks appeared,

372

indicating the BiOBr can degrade TC into small molecules. Based on the m/z values of the

373

intermediate products and the structure of TC shown in the Figure S11, we put forward the

374

pathways of photoelectriccatalytic degradation (Figure 6). As displayed in Fig. 6, the TC

375

(m/z= 445(A)) was splintered into B (m/z=405) through the deamidation reaction. And the B

376

changes into C (m/z= 362) via loss of dimethylamino group with increasing the illumination

377

time. As the reaction proceeded, the C was fragmented into the D (m/z= 318) via

378

dehydroxylation, opening of benzene rings and deethylation.56 Then the D was changed to E

379

(m/z= 274) via deacetylation reaction.57 Finally, all the above intermediates could be

380

degraded into CO2 and H2O.


Page 16 of 30

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381 382

Figure 6. Proposed degradation pathway of TC during the 2.5 h PEC process in the

383

BiOBr/CuO system with a bias potential of 0.7 V vs. (AgCl/Ag).

384 385

The mechanism of photoelectrocatalytic CO2 reduction was shown in Figure 7, and H+ and O2 were formed at the anode according to the below reaction (Eq. 1):58 H2O → 2H+ + 1/2O2 + 2e-

386

(1)

387

The generated H+ at the anode compartment was driven to cathode compartment via

388

Nafion 117 to participate in CO2 reduction reaction. The CO2 of cathode compartment accepts

389

an electron from the CuO electrode to form a CO2*- radical, and further accepts an electron to

390

form the carboxyl (*COOH) intermediate. The generated *COOH could be reduced to *CO,

391

and then the generated *CO is subsequent hydrogenation to *COH or *CHO.53 The *COH or

392

*CHO further accepts proton-electron to form CH3OH. On the other hand, *CO would further

393

participate in the reaction by obtaining electrons and protons from the anode to form

394

ethanol.59-60



395 396

Figure 7. Proposed reaction pathway of photoelectrocatalytic CO2 reduction on CuO

397

electrode.

398

Furthermore, the pathway for photoelectrocatalytic reduction of CO2 on CuO electrode

399

was also studied by in-situ FTIR spectroelectrochemical techniques. Figure 8 shows

400

time-dependent infrared spectra for photoelectrocatalytic reduction of CO2 on CuO electrode

401

at -0.7 V. The background is the signal of CO2 saturated KHCO3 solution and the upward

402

band at 2359 cm-1 is associated with CO2.61 The absorption bands at 1364 and 1650 cm-1 are

403

attributed to the peak of COO-,62 while the band around 3429 cm-1 corresponds to the O-H

404

stretching of alcohol.53 The intensity of these peaks increases as the increase of reaction time,

405

indicating the formation of alcohol products. The bands at 2841 cm-1 and 1446 cm-1

406

correspond to C-H stretching vibration of -CH2- and deformation vibration of -CH3,

407

respectively.63 In addition, the peaks related to C-H and C-O are observed at 1109 cm-1 and

408

1002 cm-1, respectively.53 Therefore, the above results indicate the generation of methanol and

409

ethanol.


Page 18 of 30

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410 411

Figure 8. In-situ infrared spectra of CO2 reduction at -0.7 V with different reaction time by

412

the PEC process in the BiOBr/CuO system.

413

Therefore, the synergetic mechanism of TC degradation coupled with PEC CO2

414

reduction can be proposed in Scheme 1. In the part of anode, TC was degraded by direct holes

415

oxidation and electro-oxidation. Meanwhile, water can also be oxidized by the BiOBr

416

electrode to produce H+ and O2. And then the generated H+ at the anode compartment was

417

driven to cathode compartment via Nafion 117 and the generated e- at the anode was driven to

418

the cathode via a potentiostat, respectively. Finally, CO2 adsorbs on the CuO electrode and

419

then reacts with H+ to produce CH3OH and C2H5OH when it gets some electrons from the

420

CuO electrode. The reaction processes can be shown in Eqs. (2-13):

421

BiOBr + hv → h+ + e-

(2)

422

CuO + hv → h+ + e-

(3)

423

2H2O → O2 + 4H+ + 4e-

(4)

424

CO2 + e- → CO2*-

(5)

425

CO2*- + H+ + e- → *COOH

(6)

426

*COOH + H+ + e- →*CO + H2O

(7)

427

*CO + H+ + e- → *CHO

(8)

*CHO + H+ + e- → CH2O

(9)

429

CH2O + H+ + e- → *CH3O

(10)

430

*CH3O + H+ + e- → CH3OH

(11)

431

2*CO + 4H+ + 4e- → 2*C2H2O

(12)

428

432

2*C2H2O + 4H+ + 4e- → CH3CH2OH+ H2O


(13)


433 434

Scheme 1. Possible reaction mechanism of the photoelectrocatalytic degradation of TC

435

coupled with photoelectrocatalytic reduction of CO2 to form methanol and ethanol.

436

Conclusions

437

In summary, a novel strategy through the merging of PEC oxidation and PEC reduction

438

was proposed for the realization of organic pollutants degradation with simultaneous CO2

439

reduction to useful chemicals over BiOBr photoanode and CuO photocathode under visible

440

light. Experimental results demonstrate that there is a synergistic effect between PEC

441

oxidation and PEC reduction and the mechanism of synergistic catalysis was put forward.

442

This work for the first time provides an efficient catalytic system for pollutant degradation

443

and CO2 reduction via the coupling with PEC oxidation and reduction. And the present work

444

highlights the great potential of using photoelectrocatalytic coupling strategy for the

445

conversion of organic pollutant to value-added chemicals.

446


Page 20 of 30

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447


Acknowledgements

448

We gratefully acknowledge the financial support of the NSF of China (51622806,

449

51878325, 51868050 and 51720105001), the NSF of Jiangxi Province (20162BCB22017,

450

20165BCB18008, 20171ACB20017, and 20171BAB206049).

451

Supplementary data

452

UV-vis diffuse reflectance spectrum, Band gap energy, Electrochemical Impedance

453

Spectroscopy (EIS), Mott-Schottky plot and Transient photocurrent responses of the

454

as-prepared BiOBr and CuO; Linear sweep voltammetry (LSV) of the as-prepared CuO; XRD

455

and XPS of the BiOBr and CuO before and after 2.5 h reaction; 1H NMR of products in the

456

PEC system at 0.7 V vs. (AgCl/Ag); Chronoampherometry in EC and PEC process at -0.7 V

457

on the BiOBr film; UV–vis absorption spectra for degradation of TC in the PEC system at 0.7

458

V; electrospray ionization (ESI) spectra for TC solution in the PEC system. These

459

supplementary materials can be found in the online version at http://pubs.acs.org.

460

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461

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686 687 688 689 690

For Table of Contents Use Only

691 692 693

Synopsis. A novel system consisting of BiOBr as a photoanode and CuO as photocathode was

694

designed to couple photoelectrocatalytic degradation of TC with simultaneous reduction of

695

CO2.

696 697



221x153mm (96 x 96 DPI)


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Photoelectrochemical degradation of organic pollutants using BiOBr

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