1、available at 催 journal homepage: (Special Issue on Electrocatalysis Transformation)Electrochemical CO2 reduction to formic acid on crystalline SnO2 nanosphere catalyst with high selectivity and stabilityYishu Fu a, Yanan Li a, Xia Zhang a, Yuyu liu b,c,#, Xiaodong Zhou d, Jinli Qiao a,*abCollege of
2、 Environmental Science and Engineering, Donghua University, Shanghai 201620, ChinaCollege of Environmental Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China c Graduate School of Environmental Studies, Tohoku University, Sendai 980-8579, Japand Department of Che
3、mical Engineering, University of South Carolina, Columbia, SC 29208, USAArticle history:Received 18 December 2015 Accepted 27 January 2016 Keywords:Carbon dioxide reduction Tin dioxide FormateFaradaic efficiency1. IntroductionA novel catalyst for CO2 electroreduction based on nanostructured SnO2 was
4、 synthesized using a facile hydrothermal self-assembly method. The electrochemical activity showed that the catalyst gave outstanding catalytic activity and selectivity in CO2 electroreduction. The catalytic activity and formate selectivity depended strongly on the electrolyte conditions. A high far
5、adaic efficiency, i.e., 56%, was achieved for formate formation in KHCO3 (0.5 mol/L). This is attributed to control of formate production by mass and charge transfer processes. Electrolysis experiments using SnO2-50/GDE (an SnO2-based gas-diffusion electrode, where 50 indicates the 50% ethanol conte
6、nt of the electrolyte) as the catalyst, showed that the electrolyte pH also affected CO2 reduction. The optimum electrolyte pH for obtaining a high faradaic efficiency for formate production was 8.3. This is mainly because a neutral or mildly alkaline environment maintains the oxide stability. The f
7、ara-daic efficiency for formate production declined with time. X-ray photoelectron spectroscopy showed that this is the result of deposition of trace amounts of fluoride ions on the SnO2-50/GDE surface, which hinders reduction of CO2 to formate.? 2016, Dalian Institute of Chemical Physics, Chinese A
8、cademy of Sciences.The increased amount of CO2 in the atmosphere is claimed to be one of the major contributors to the greenhouse effect, and will result in serious global warming issues 1. Among various conversion methods, the electrochemical synthesis of high-value chemicals from CO2 offers severa
9、l advantages such as process simplicity and flexibility, and production of various organic chemicals, depending on the type of catalyst used 2,3.CO2 is a stable molecule and generally produced by fossil fuel combustion and respiration. Converting CO2 to useful chemi-cals at the same rate as its pres
10、ent production is beyond our current scientific and technological abilities 4. The reduction of CO2 involves the use of specific metal catalysts, to achieve product selectivity, and because of the sluggish kinetics of CO2 electroreduction 5.The study of CO2 electroreduction in aqueous solutions at a
11、mbient temperature has focused on metal electrodes 3,6.* Corresponding author. Tel: +86-21-67792379; Fax: +86-21-67792159; E-mail: # Corresponding author. Tel: +81-90-60089342; Fax: +81-22-7953859; E-mail: liumail.kankyo.tohoku.ac.jpThis work was supported by the Innovation Program of the Shanghai M
12、unicipal Education Commission (14ZZ074), the International Academic Coop-eration and Exchange Program of Shanghai Science and Technology Committee (14520721900), Graduate Innovation Fund of Donghua University (15D311304) and the College of Environmental Science and Engineering, State Environmental P
13、rotection Engineering Center for Pollution Treatment and Control in Textile Industry, Donghua University. All the financial supports are gratefully acknowledged.DOI: 10.1016/S1872-2067(15)61048-8 | http:/ | Chin. J. Catal., Vol. 37, No. 7, July 2016The catalytic reduction of CO2 to methanol was achi
14、eved over Cu under hydrothermal conditions with a methanol yield of 11.4% 7. The product distributions and faradaic efficiencies obtained in the electrochemical reduction of CO2 with Cu foams differ significantly from those obtained at smooth electropo-lished Cu electrodes. This is attributed to the
15、 high surface roughness, hierarchical porosity, and confinement of reactive species in the case of Cu foams. The faradaic efficiency for for-mic acid production at Cu foam electrodes was higher at all tested potentials, with a maximum efficiency of 37% at ?1.5 V, which is the highest value obtained
16、for the electroreduction of CO2 to formic acid at a Cu electrode under ambient pressure 8. The use of gas-diffusion electrodes (GDEs) for electro-chemical reduction of CO2 on Pb, In, and Sn under acidic condi-tions gave high efficiencies for formic acid (pH 2) production 9. An Sn-based GDE (SGDE) sh
17、owed good stability during CO2 reduction; the faradaic efficiency for conversion of CO2 to for-mate reached 18% during the initial 5 min and remained at about 12% until the end of the reduction time, i.e., 1 h 10. Recently, SGDEs have attracted much attention for CO2 reduc-tion. Wang et al. 11,12 re
18、ported that an SGDE with polyte-trafluoroethylene as an additive gave a good electrochemical performance, because of the increased active catalyst surface area and CO2 diffusion, and high catalyst loading, i.e., 5 mg/cm2. The deactivation of Sn-metal-based electrodes during CO2 reduction is fast, an
19、d the reduction reaction on these electrodes requires an overpotential of at least 860 mV at a current den-sity of 4?5 mA/cm2 in an aqueous solution saturated with CO2 at 0.1 kPa 13. It is vital to explore the use of metal oxides in CO2 reduction to overcome this problem, but there have been few rep
20、orts of such studies. The role of metal oxides, whether as catalysts for the formation of formic acid or as precursors for the fabrication of well-structured catalysts, remains unclear. Kanans group 14 published several reports on the metal oxide effect in CO2 reduction. The faradaic efficiency for
21、CO2 reduction depended greatly on the presence of SnOx; Sn/SnOx thin-film electrodes catalyzed the formation of CO and formic acid as the main reaction products. The faradaic efficiency for formic acid reached 30% at ?0.7 V vs the normal hydrogen electrode. An important result of this study is the o
22、bservation that controlling the size of tin oxide nanoparticles (NPs) on carbon supports enables overpotentials as low as 340 mV to be achieved for CO2 reduction to formate, with significant en-hancements in current density to over 10 mA/cm2 on high-surface-area graphene supports. Reduced nanoscale
23、tin oxide catalysts are highly stable during controlled-potential electro-lysis 15.The most important and valuable products of CO2 reduction are formate and formic acid. SnO2 shows good catalytic activity in formate production, but the electrolyte conditions greatly affect the formation of formic ac
24、id 1619. The pH value of the electrolyte significantly affects the electrode potentials for the reduction of H2O and CO2 20:H+ + 2e? ? H2CO2 + H+ + 2e? ? HCOO?An environment that is too acidic promotes hydrogen for-mation, and one that is too alkaline does not favor formation of1082 Yishu Fu et al.
25、/ Chinese Journal of Catalysis 37 (2016) 10811088formic acid. CO2 electrolysis in a neutral or mildly alkaline en-vironment stabilizes the oxide. The electrolyte concentration also greatly influences the formation of formic acid 21. The faradaic efficiency for formic acid production in KHCO3 (0.5 mo
26、l/L) was greater than that in K2CO3 (0.1 mol/L) with an Sn granule electrode in a fixed-bed reactor 16. The highest achieved faradaic efficiency for formate production was 88.4% in 0.1 mol/L KHCO3 at ?1.72 V vs the saturated calomel elec-trode (SCE) 17, and the faradaic efficiency was between 65.0%
27、and 79.9% in KHCO3 (0.5 mol/L) 22.In this work, we developed a novel SnO2 NP catalyst with a high catalytic efficiency for CO2 electroreduction, based on a GDE. Unlike that used in Wangs group 11, the catalyst was a nanostructured tin oxide consisting of SnO2 NPs with highly porous structures, and w
28、as synthesized using a facile hydro-thermal self-assembly process. SnO2-50/GDE (an SnO2-based gas-diffusion electrode, where 50 indicates the 50% ethanol content of the electrolyte) was prepared by coating SnO2 cata-lyst ink on a gas-diffusion carbon paper sheet. The SnO2 cata-lyst ink was prepared
29、by homogeneously mixing SnO2 catalyst particles, 5 wt% Nafion solution, and isopropyl alcohol. The electrolyte conditions, i.e., the pH and concentration, were con-trolled, to enable a better understanding of the mechanisms of the effects of the electrolyte on formic acid formation and the faradaic
30、efficiency. The SnO2 NP catalyst morphology was ex-amined using scanning electron microscopy (SEM). The elec-trochemical properties of the modified electrode, i.e., SnO2-50/GDE, in CO2 reduction were investigated thoroughly using cyc-lic voltammetry (CV), linear sweep voltammetry (LSV), CO2 electrol
31、ysis, and ion chromatography. The production rate and faradaic efficiency for formate, which can be used as a liquid fuel during CO2 reduction, were also investigated.2. Experimental2.1. Catalyst synthesisAn SnO2 NP catalyst was synthesized from SnCl4 and D-glucose monohydrate using a facile hydroth
32、ermal self-as-sembly process. SnCl4 (4 mmol) was mixed with D-glucose monohydrate (10 mmol) and the mixture was dissolved in dis-tilled water and ethanol (totally 35 mL) with stirring until a transparent solution was obtained. The mixture solution was transferred to a 100 mL Teflon-lined stainless-s
33、teel autoclave, which was sealed and kept at 180 C for 24 h. The formed black powder was collected, washed several times with etha-nol/water, and dried in a vacuum oven at 60 C for 5 h. The obtained powder was calcined in air at 550 C for 5 h, during which the black sediment gradually turned white,
34、indicating the successful removal of carbon by oxidation in air, to give the SnO2 NP catalyst. The catalyst is denoted by SnO2-50, where 50 indicates that the percentage of ethanol content in the mixture solution is 50%.2.2. Electrode preparation and electrochemical testsFor all electrochemical meas
35、urements, the SnO2-50 NP cat-Yishu Fu et al. / Chinese Journal of Catalysis 37 (2016) 10811088 1083alyst was coated on a gas-diffusion carbon paper sheet (Toray, TGP-H-090) to form a working electrode. The catalyst ink was prepared by suspending the SnO2-50 NP catalyst (15 mg) in a mixture of 5 wt%
36、Nafion solution (100 mg) and 99.7 wt% iso-propyl alcohol (1.4 mL; Sinopharm Chemical Reagent Co.). A catalyst-coated gas-diffusion layer on 4 cm2 Toray carbon pa-per (TGP-H-090), denoted by SnO2-50/GDE, was used as the working electrode. It was tested using a conventional three-electrode electrochem
37、ical H-type cell.Electrochemical characterization was performed using a standard H-type cell (Aldrich Nafion?117) equipped with a gas inlet and outlet, which allowed the passage of either N2 (99.99%) or CO2 (99.99%) through the solution, to investigate the catalyst properties and CO2 reduction perfo
38、rmance. A standard H-type cell, with a piece of Nafion?117 cation-ex-change membrane (H+ form) as a separator, SnO2-50/GDE as the working electrode, a Pt foil electrode as the counter elec-trode, and an SCE as the reference electrode were used. All electrochemical measurements were performed using a
39、 CHI 660E instrument. For the CO2 reduction measurements, aqueous electrolytes of KHCO3 concentration from 0.1 to 1.0 mol/L were used as the functional electrolyte to obtain the desired solution concentration; CO2 gas (99.99%) at 1 atm was bubbled through the solution for 30 min before all measure-m
40、ents.The electrocatalytic activity of the SnO2-50/GDE working electrode was tested using CV and LSV at potential scan rates of 50 and 5 mV/s, respectively, in the potential range 1.0 to ?1.6 V vs the SCE. Controlled-potential electrolysis was performed using a CHI 660E electrochemical analyzer, with
41、 the same standard H-type cell. An aqueous KHCO3 (0.5 mol/L) electrolyte was used for the CO2 reduction measurements; CO2 gas (99.99%) at 0.1 kPa was bubble through the solution for at least 30 min before each measurement. For long-term electro-lysis measurements, a constant potential of ?1.70 V vs
42、the standard hydrogen electrode (SHE) was imposed for 28 h. For soluble product measurements, a constant potential of ?1.70 V vs the SHE was imposed for 1 h, and the electrolysis currents were recorded continuously. All tests were performed at am-bient temperature and pressure.(which is convenient f
43、or formate detection in the mobile phase) before determination of the soluble reduction products. The product solution was filtered using a filtration membrane (0.22 m) and the formate concentrations in the electrolyte were determined directly by ion chromatography (ICS-90, Dionex, USA), using an AS
44、144 mm 250 mm separation column and a flow rate of 1 mL/min. The mobile phase used in instrument preparation was a mixed aqueous solution of Na2CO3 (3.5 mmol/L) and NaHCO3 (1.0 mmol/L), and aqueous H2SO4 (20 mmol/L) was used as the regenerating liquid. 3. Results and discussion3.1. Physical characte
45、rization of SnO2 NP catalystThe morphology and crystal structure (phase composition) of SnO2 were examined using SEM and XRD, respectively. Fig. 1 shows SEM images at two magnifications of the SnO2-50 NP catalyst, which was prepared using a solvent of ethanol : dis-tilled water = 1 : 1. Fig. 1(a) sh
46、ows that the sample consisted of a mixture of NP and nanosphere aggregates of diameter 500 nm to 1 m with a highly porous structure. The high-resolution SEM image (Fig. 1(b) shows that these large NPs and nanos-pheres have a clear three-dimensioned hierarchical structure, consisting entirely of seco
47、ndary structures composed of ag-gregated small primary SnO2 NPs of diameter 2025 nm. This special morphology provides catalysts with large surface areas, and greatly affects the catalytic activity in CO2 electroreduction 23,24.The crystalline structure of as-prepared SnO2 was con-firmed using XRD. T
48、he XRD pattern in Fig. 2 shows that the hierarchical structure consisted of small SnO2 NPs with good(a)2.3. Physical characterization and reduction product determinationThe crystal-phase X-ray diffraction (XRD) pattern of the SnO2-50 NP catalyst synthesized at 180 C for 24 h was ob-tained using a Ph
49、ilips PW3830 X-ray diffractometer equipped with a Cu K radiation ( = 0.15406 nm) source. The intensity data were collected at 25 C in the 2 range from 0 to 90, at a scanning rate of 1.20/min. The morphology of the SnO2-50 catalyst was examined using SEM (Ultra plus thermal field-emission instrument, Carl Zeiss SMT AG, Germany). The chem-ical composition of SnO2-50/GDE after long-term electrolysis was determ
