Photoreduction of CO2 to methanol over Bi2S3/CdS photocatalyst under visible light irradiation

Available online at www.sciencedirect.comJOURMLOFScienceDirectNATURAL GASCHEMISTRYEL SEVIERJournal of Natural Gas Chemistry 20(201 1)413- 417www.clscvicr.com/locatc/jngcPhotoreduction of CO2 to methanol over Bi2S3/CdS photocatalystunder visible light irradiationXin Li1,2*，Juntao Chen2，Huiling Li2，Jingtian Li2,Yitao Xu2，Yingju Liu2，Jiarong Zhou21. The Guangdong Provincial Laboratory of Green Chemical Technology, School of Chemistry and Chemical Engineering,South China University of Technology, Guangzhou 510641, Guangdong, China; 2. Instinute of Biomaterial, College of Science,South China Agriculural University, Cuangzhou 510642, GCuangdong, China[Manuscript reeived January 13, 2011; revised May 9, 2011 ]AbstractThe Bi2S3, Cas and Bi2S3/CdS photocatalysts were prepared by direct reactions between their corresponding salt and thiourea in a hy-drothermal autoclave. The photocatalytic activities of these photocatalysts for reducing CO2 to CH3OH under visible light irradiation havebeen investigated. The results show that the photocatalytic activity and visible light response of Bi2S3 are higher than those of Cds. The Bi2S3modification can enhance the photocatalytic activity and visible light response of Cas. The photocatalytic activity of BizS3/CdS hetero-junctionphotocatalyst was the highest and the highest yields of methanol was 613 umolg when the weight proportion of Bi2S3 to CdS was 15%, whichwas about three times as large as that of CdS or two times of that of Bi2S3.Key wordsphotocatalytic reduction; carbon dioxide; heterogeneous catalysts; CdS; Bi2S3; CO2 reduction1. Introduction[18] carried out the photoreduction of CO2 in the presenceof H2S over typical sulfides such as bismuth sulfide (Bi2S3)The photocatalytic reduction of CO2 to methanol has at-and cadmium sulfide (CdS). HCHO and HCOOH were pro-tracted much research attention in the past decades becauseduced in this reaction. Fujiwara and his co-workers [21] in-it transforms greenhouse gas CO2 to useful hydrocarbon re-vestigated the photocatalytic reduction of CO2 over hexago-sources [1-12].nal CaS nanocrystallites prepared in N, N-dimethylformamideAmong various photocatalysts, TiO2 has been widely(DMF), and they found that sulfur vacancies on the surfaceused for the photocatalytic reduction of CO2 to methanol.of nancrytallites can be formed by the adsorption of excessHowever, TiO2 is a wide bandgap semiconductor (3.03 eV forCd2+ to the surface, which resulted in a remarkable increaserutile and 3.18 eV for anatase) and can only absorb about 5%of photocatalytic activity. Eggins and his co-workers [22] per-sunlight in the ultraviolet region, which grealy limits its prac- formed the photocatalytice reduction of CO2 to dimeric andtical applications [13- 17].tetrameric products, namely oxalate, glyoxylate, glycolate andIt was well known that the bandgaps of CdS and Bi2S3tartrate using aqueous CdS or ZnS colloids containing tetram-were narrower and their potentials of conduction bands wereethylammonium chloride. However, these photocatalytic re-more negative than those of other photocatalysts [18- -20],ductions of CO2 over CdS and Bi2S3 had been carried out un-therefore, CdS and Biz2S3 have been widely applied to the pho-der UV light irradiation or by other reductants, and they havetocatalytic reduction of CO2. Inoue et al. [1] first reportednot been used for photocatalytic reduction of CO2 to methanolin 1979 that CO2 bubbled in water was reduced to HCHO,with water as reductant under visible light irradiation.HCOOH and CH3OH over various semiconductor photocata-In this study, CdS was modified by Bi2S3 and the obtainedlysts, such as CdS, TiO2, ZnO, GaP and SiC under photoir-Bi2S3/CdS was used as a photocatalyst for photocatalytic re-radiation of their aqueous suspension. Aliwi and AI-Juboriduction of CO2 with water under visible light irradiation. The* Corresponding author. Tel: +86-2085280323; E-mai: xinliscau@ yahoo.comThe work was supported by the National Natural Science Foundation of China (No. 20906034), the Key Academic Program of the 3rd Phase“211 Project"of South China Agricultural University (No.2009B010100001) and China Postdoctoral Science Foundation中国煤化工CopyrightO2011. Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.doi: 10.1016/S 1003-9953(10)60212-5TYHCNMHG414Xin Li et al./ Journal of Natural Gas Chemistry VoL 20 No. 42011Bi2S3, CdS and BizS3/CdS photocatalysts were characterizedual pump -line oil contamination taken at 284.6 eV. An Oxfordby X-ray diffraction (XRD), scanning electron microscopyInca 350 energy dispersive X-ray spectroscope (EDS) was(SEM), transmission electron microscopy (TEM), ultraviolet-used to identfy and analyze the element contents of surfacevisible (UV-Vis) spectroscopy, X-ray photoelectron spec-composite photocatalysts.troscopy (XPS) and energy dispersive X-Ray spectrometer(EDS). The photocatalytic activities of the BizS3, CdS and2.3. Photocatalytic reaction testingBi2S3/CdS photocatalysts for reducing CO2 to CH3OH undervisible light irradiation have been investigated in detail.The photocatalytic activites were performed in a XPA-IIphotochemical reactor (Nanjing Xujiang Machine- electronic2. ExperimentalPlant), equipped with a magnetic stirrer, a quartz cool trap,and a condensation tube. A 500 W Xe lamp was located2.1. Preparation of photocatalystsin the quartz cool trap as iluminant. The wavelength ofXe lamp ranged from 200 to 700 nm. The UV light underAll of the chemical reagents used in this work were ana-400 nm was removed by a 2.0 M sodium nitrite solution [26].lytical grade and used as received. CdS and Bi2S3 were syn-Firstly, sodium hydroxide (0.80 g) and absolute sodium sulfitethesized according to the reported method [19,23]. In a typical(2.52 g) were dissolved in 200 mL distilled water. This so-preparation process, pure CdS and BizS3 were prepared by di-lution was then put into a photochemical reaction instrument.rect hydrothermal reactions between their corresponding saltBefore iradiation, ultrapure CO2 was bubbled through the so-and thiourea (6.02 g Cd(NO3)2.4H2O and 1 .55 g thiourea forlution in the reactor for at least 30 min to ensure that all dis-CdS, 3.05 g Bi(NO3)3:.5H2O and 0.71 g thiourea for BiS3)solved oxygen was eliminated. Then, 200 mg catalyst powderin a hydrothermal autoclave at 150 °C for 24 h. When cooledwas added into the above solution, and the iradiation lampdown to room termperature, the precipitate was filtered off, andwas turned on to start the photoreaction. Ultrapure CO2 wassubsequently washed with distilled water and dried in vacuocontinuously bubbled through the above solution in the reac-at 50 °C overnight. At last, CdS was dried at 400°C for 1 h,tor during the whole iradiation. The radiation time was5h. Aand Bi2S3 was heat treated at 240°C for 1h.needle-type probe was inserted into the solution in the reactorSimilarly, the Bi2Sz/CdS photocatalysts were preparedto withdraw a small amount of liquid samples at 1 h,2 h, 2.5 h,by the same method, the weight proportions of Bi2S3 to3h, 3.5 h, 4h, 4.5 h and 5 h, respectively. The concentrationsCdS were 15% (Bi2S3/CdS(1)), 30% (Bi2S3/CdS(2)) andof methanol in the samples were analyzed using a GC9560 gas45% (Bi2S3/CadS(3)).The mixture of Cd(NO3)2.4H2O,chromatograph equipped with a flame ionization detector andBi(NO3)3-5H2O and thiourea was maintained in a hydrother-a stainless steel packed column (Porapak-Q, 2 mmx3 m).mal autoclave at 150 °C for 24 h, then the following processeswere as those of CdS or BizS3. At last, all samples were heat3. Results and discussiontreated at 240°C for 1 h in an oven.3.1. Surface areas and pore structure analyses2.2. CharacterizationThe specific surface area (SBEr), pore volume (Vota),photocatalysts are summarized in Table 1. It can be seen fromand average pore diameter (davernge) of the photocatalystsTable 1 that the Bi2S3, CdS and Bi2S3/CdS photocatalysts arewere measured by nitrogen adsorption at the liquid nitro-all mesoporous materials according to IUPAC clasificationgen temperature of 77K with the help of an ASAP 2010[27]. It was also noticed that Bi2S3/CdS(3) photocatalyst hasvolumetric adsorption analyzer (Micromeritics InstrumentCorp. USA) [24,25]. The morphologies of the photocata-age pore diameter in comparison with other photocatalysts,lysts were observed on scanning electron microscope (SEM,which suggests that surface modification with Bi2S3 greatlyLEO 1530VP Field Emission Scanning Electron Microscope,increases the average pore diameter and the specific surfaceLEO Electron Microscopy Inc., Germany) and transmissionarea of CdS. .electron microscope (TEM, FEI-Tecnai 12). The XRD pat-terns were obtained at room temperature using a MSAL-XD2Table 1. Element analyses, surface areas and porediffractometer with Cu Ka radiation (operated at 36 kV andstructures of photocatalysts30 mA, λ= 0.15406 nm). The UV-Vis spectra in the rangeMolar ratio of Bi/Cd SBETVrotal. daverageof 200- -800 nm were measured with a Daojin UV-2550PCPhotocatalystsEDSPSbn2.g-l) (cm'g少(nm)diffuse reflectance spectroscope. The XPS patterms were col-Cds0.02010.9lected by a Kratos AXis Ultra (DLD) with pass energy ofBizS30.05811.920eV, and the excitation of the spectra was performed byBi2Ss/CdS(1) 0.26:1 0.27:1240.06012.0means of monochromatized Mg Ka radiation. Correction ofBi2S3/CdS(2) 0.659.0633.0the energy shifts due to static charging of the samples wasBi2S/CdS(3) 0.92中国煤化工.0643.3accomplished by referencing to the C 1s line from the resid-a Determined by IMYHCNMHGsalyses416Xin Li et al./Jourmal of Natural Gas Chemnistry Vol. 20 No.4 2011(bc)200 rim200n馆Figure 4. TEM images of曲) BizSs/CdS(1), (b) Bi2S/CdS(2) and (C) BizSs/CdS(3)Bi2S3 was belt-shaped, and Cas with particle size from 100 tofrom Figure 6 that the generation rates of CH3OH over the400 nm was granulated. It can also be seen obviously that thethree Bi2S3/CdS photocatalysts were significantly higher thanspread of belt -shaped Bi2S3 on the surface of Cds particlesthat over Bi2S3 or CdS under visible light irradiation. Thedecreased with increasing the percentage of BizS3.highest yield of methanol over Bi2S3/CdS was 613 μmolgwhen the weight proportion of BizS3 to CdS was 15%, which3.5. Photocatalytic activitywas about three times of that of CdS or two times of that ofBi2S3. This may be because that doping BizS3 to CdS couldThe mechanism of photocatalytic reduction of CO2 withform hetero junction structures, which improves the separa-H2O to CH3OH are shown in Figure 5. It can be seen fromtion of electrons and holds, prevents the charge-carrier re-Figure 5 that both Bi2S3 and CdS can be used as the photocat-combination and prolongs the lifetime of photo-carriers inalysts for reducing CO2 to CH3OH under visible light. ThisBi2S3/CdS photocatalysts [34- 36]. In addition, the spread ofis because the potentials of conduction bands of Bi2S3 andbelt-shaped BiS3 on the surface of CdS particles was betterCds are more negative than all those of methanol yielded forthan those on other two and the effective compound areas inCO2 (g), H2CO3, Co5 etc. in water [8,29- -32]. The detailedhetero-junction structures were larger than those in other twowhen the weight proportion of Bi2S3 to Cds was 15%.band structures are also pictured in Figure 5 [1,26,33].70-1.5 EV vs NHE-1.0E BC一BiS/CdS(1)- BisS/CdS(2)500F -* Bi$/CdS(3)0.0--------.- H,CO,/CH,OHCO7CH,OH400 EO/H2O0E三3001.5E BV200 |2.5E CO,+6H'*+6e→CH,OH+HO-0.38 eV100H,CO,+6H* + 6e→CH,OH+2H,O 0.044 eVCO- +8H°+6e°→CH,0H+2H2O 0.209eVFigure 5. The mechanism of photocatalytic reduction of CO2 with H2O to1CH3OHTime (h)Figure 6. The yields of CH3OH in the photocatalytic reduction of CO2 withThe yields of CH3OH in the photocatalytic reduction ofH2O over various photocatalysts under visible light irradiationCO2 with H2O on various photocatalysts under visible light ir-radiation are shown in Figure 6. From Figure 6, it can be seenthat the yield of methanol over Bi2S3 was higher than that4. Conclusionsover CdS, which can be attributed to the better photoabsorp-tion performance, higher surface area and larger average poreThe Bi2S3, C中国煤化工lysts were pre-diameter of Bi2S3 compared with CdS. It also can be noticedpared by direct hy;YHC N M H Gen their corre-Journal of Natural Gas Chemistry Vol. 20 No.42011417sponding salt and thiourea in an ammonia bath autoclave. The[12] LiHL,Lei Y G, Huang Y P, Fang Y P, XuY H, ZhuL,LiX. JBi2S3, CdS and Bi2S3/CdS photocatalysts were characterizedNat Gas Chem, 2011, 20(2): 145by XRD and UV-Vis spectroscopy. It is found out that the13] Hoffmann M R, Martin S T, Choi W Y, Bahnemann D W. Chemphotocatalytic activity and visible light response of Bi2S3 areRev, 1995, 95(1): 69higher than those of CdS. Moreover, the modification of CdS[14] Linsebigler A L, Lu G Q, Yates JT. Chem Rev, 1995, 95(3): 735with Bi2S3 can enhance its photocatalytic activity and visi-[15] Mills A, Le Hunte s. J Photochem Photobiol A, 1997, 108(1): 1ble light response. 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