Photocatalytic reduction of carbon dioxide to methanol by Cu2O/SiC nanocrystallite under visible lig

Available online at www.sciencedirect.com| Joumal ofScienceDirect7I Natural GasChemistryEL SEVIERJournal of Natural Gas Chemistry 20(201 1)145-150www.elsevier.com/locate/jngcPhotocatalytic reduction of carbon dioxide to methanol by Cu2O/SiCnanocrystallite under visible light irradiationHuiling Lil，Yonggen Leil，Ying Huang'，Yueping Fang'，YuehuaXu'， Li Zhu'，XinLil2*I. Institute of Biomaterial, College of Science, South China Agricutural University, Guangzhou 510642, Guangdong, China;2. The Guangdong Provincial Laboratory of Green Chemical Teclnology, School of Chemistry and Chemical Engineering,South China University of Technology, Guangzhou 51064 I, Guangdong, Chin[ Manuscript reeved November 11, 2010; revised December 17. 2010 ]JomdAbstractThe Cu2O/SiC photocatalyst was obtained from SiC nanoparticles (NPs) modified by Cu2O. Their photocatalytic activities for reducing CO2to CH3OH under visible light irradiation have been int stigated. Thepresults indicated that besides a small quantity of 6H-SiC, SiC NPs mainlyconsisted of 3C-SiC. The band gaps of SiC and Cu2O were estpnated to be alht 1.95 and 2.23 eV from UV-Vis spectra, respectively. TheCu2O modification can enhance the photocatalytic pl rformanceof SiC NPs, and the largest yields of methanol on SiC, Cu2O and Cu2O/SiCphotocatalysts under visible light iradiation were 153, 104 anddiationwere 152 10andlImtively.Key wordsphotocatalytic reduction; carbon dioxide; heterogeneo s catalysts1. IntroductionV| tbttlingnttaliation. At the same time, Cu2O is a nrrowbandgap semiconductor (about 2.0 eV) [19,20] and has betterNowadays, the release of carbon dioxide (CO2) into theselectivity for the photoreduction of CO2 to methanol [6].In this study, SiC nanoparticles (NPs) were modified byenvironment is one of the most serious problems due to green-Cu2O and the obtained Cu2O/SiC was utilized as photocathouse effect. The photocatalytic reduction of CO2 with H2Olysts for photocatalytic reduction of CO2 with water under vis-utilizing solar energy was found to be a promising way byible light iradiation. The SiC, Cu2O and Cu2O/SiC photocapwhich can solve the problem of the greenhouse gas and alsoalysts were characterized by X-ray diffraction (XRD), scaconvert CO2 to reusable hydrocarbon resources [1- -9].In recent years, TiO2 has been widely studied for itsning electron microscope (SEM), ultraviolet-visible (UV-Viapplication as photocatalysts because of its excellent stabil-spectroscope and X-ray photoelectron spectroscopeity, strong oxidizing property and innocuity. However, TiO2The photocatalytic activities of Cu2O/SiC photocatalyst foris a wide bandgap semiconductor (3.03eV for rutile andreducing CO2 to CH3OH under visible light irradiation have3.18eV for anatase) and can only absorb about 5% of sun-been investigated in detail.light in the ultraviolet region, which limits its photocatalytic2. Experimentalactivity [10-12].is more negative than those of other photocatalysts [1] and the2.1. Preparation of photocatalystsphotoelectrons in SiC have much stronger reduction perfor-mance. Therefore, SiC has been widely used to reduce CO2All of the chemical reagents used in this experiment were[1,13-15], split water [16,17] or degrade organic pollutantsanalytical grade and used without further purification. Com-. [18] under UV light irradiation. However, SiC has not beenmercial SiC NPs with the size of 40- -50 nm were obtainedused for photocatalytic reduction of CO2 with water underfrom Aladdin.* Corresponding author. Tel: +86-20-85280323; E-mail: xinliscau @ yahoo.comThe work was supported by the National Natural Science Foundation of China (Grant No. 209060中国煤化工the 3rd Phase“211.Poject" of South China Agricultural University (Grant No. 2009B010100001) and China Postdoctoral ScYHCNMHG430820).CopyrightO201 1, Dalian Institute of Chemical P, Chinese Academy of Sciences.All rights reserved.doi: 10.1016/S 10039953( 10)60166-1146Huiling Li et al./ Jourmal of Natural Gas Chemistry Vol. 20 No.220110The synthesis of Cu2O was performed according to themethod reported by Wang et al. [21]. In a typical prepara-continuously bubbled through the above solution in the reac-gn process, CuSO4.5H2O (3.3 g) was dissolved in 400 mLtor during the whole iradiation. The radiation time was5h. Adeionized water, and 20 mL NaOH aqueous solution (1.5 M)needle-type probe was inserted into the solution in the reactorwas added under vigorous stirring. After stirring for 15 min,to withdraw a small amount of liquid sampleat 1 h, 2h, 2.5h,1.5 mL N2H4:H2O aqueous solution (13.7 M) was added3h, 3.5h, 4h, 4.5 h and 5 h, respectively. The concentration6opwise into the blue Cu(OH)2 colloidal solution with con-of methanol in the samples was analyzed using a GC9560 gasstant stiring. After the blue precipitates were completely re-chromatograph equipped with a flame ionization detector andduced by N2H4H2O, they turned into red. Then the red pre-a stainless steel packed column (Porapak-Q, 2 mmx3 m).cipitates were filtered, washed with distilled water for severalAnes, and finally dried in a vacum oven at 60 °C for 3 h. .3. Results and discussionThe preparation of Cu2O/SiC photocatalysts was similarto that of Cu2O. SiC NPs (3 g) and CuSO4.5H2O (0.52g)3.1. Surface area and pore structure analyseswere dispersed in 400 mL deionized water, then precipitated好20 mL 1.5 M NaOH solution and reduced by N2H4:H2O.I he nitrqgen. adsorption-desorption isotherms of threeThe obtained samples世tehin vduoven afor3h.8-m. m06儆的妒弋卞却一0^下婶=to the Bet lclasAfilcation |24]. VAnd each Isothelm'showedisohern saccordingCharacterizationa distinct hysteresis loop, which is associated with the capil-lary condensation taking place in mesopores, indicating thatThe specific surface area, pore volume, and average poreall photocatalysts are mesoporous [25,26]. .Gameter of the photocatalysts were measured by nitrogen ad-sorption at the liquid nitrogen temperature of 77 K with thehelp of an ASAP 2010 volumetric adsorption analyzer Mi-Cu.0/SiCcyomeritics Instrument Corp. USA) [22,23]. The morphology5 the photocatalysts was observed on scanning electron mi-croscope (SEM, LEO 1530VP Field Emission Scanning Elec-tron Microscope, LEO Electron Microscopy Inc., Germany).2The XRD patterns were obtained at room temperature usingaMSAL-XD2 dfractometer with Cu Ka radiation (operatedat 36 kV and 30 mA,入= 0.15406 nm). The UV-Vis spectra insiCrange of the 200- -800 nm were measured using a Daojin UV-2550PC Diffuse Reflectance Spectroscope. The XPS spectrawere collected by a Kratos AXis Ultra (DLD) with pass en-ergy of 20 eV, and the excitation of the spectra was performedby means of monochromatized Mg Ka radiation. Correction①he energy shifts due to static charging of the samples was悟胭CuCaccomplished by referring to the C 1s line from the residualpump-line oil contamination taken at 284.6 eV.83. Photocatalytic reaction test0.81.gotochemical reactor (Nanjing Xujiang Machine-electronicRelative pressure (p/po)PIant), equipped with magnetic stirrer, quartz cool trap andFigure 1. N2 adsorption desorption isotherms on the three photocatalysts atcondensation tube. A 500 W Xe lamp was located in the77 Kquartz cool trap as iluminant. The wavelength of Xe lampFigure 2 shows the DFT pore size distributions of three4nged from 200 to 700 nm. The UV light under 400 nm wasphotocatalysts. It is noticed that there was an obvious poreremoved by a 2.0 M sodium nitrite solution (NaNO2) [20]. size distribution in the mesopore region for each photocata-Firstly, NaOH (0.8 g) and absolute sodium sulfite (Na2SO3,lyst, and a distinct peak could be seen in the DFT pore size dis-2.52 g) were dissolved in 200 mL disilledl water. This solu-tribution of Cu2O and Cu2O/SiC photocatalyst, respectively.a2n was then put into a photochemical reaction instrument.It means that the mesopores are dominant for the three photo-Before irradiation, ultrapure CO2 was bubbled through the so-catalysts.中国煤化工lution in the reactor for at least 30 min to ensure that all dis-The specificM出CNMHG pore volumesolved oxygen was eliminated. Then, 200 mg of catalyst pow-(Vlota), and averagliwi (uaverage/ uf the three pho-Q was added into above solution, and the irradiation lamptocatalysts are summarized in Table 1. It can be seen from0.00.20.40.60.8][.0Rel ative pressure p/ po)Journal of Natural Gas Chemistry Vol. 20 No.2 20111470.001 0Table 1 that all the photocatalysts examinedstudy were mesoporous materials according to IUPAC clas-spectrum of the Cu2O were in good agreement with the re-sification [24]. It was also noticed that Cu2O/SiC photocata-ported results [21]. There were five peaks with 20 values oflyst had larger total pore volume and average pore diameter,29.60, 36.5°, 42.40, 61 .5, and 73.7°, corresponding to thebut smaller specific surface area, compared with the other two .crystal planes of (110), (111), (200), (220) and (311) of crys-photocatalysts. This suggests that surface modification withtalline Cu2O, respectively. And in other studies, it has beenCu2O increases the average pore diameter and decreases theconcluded that Cu2O exposing (111) facets can be used as aspecific surface area. It can be attributed to the formation ofstable photocatalyst [30].02900 0osurface of sic which probably resuted in thecomplete iling of the small pores, and enlarging of the aver-10000age pore diameter [27].8000 Cu,O/SiC●Cu,0。SiC6000上0.0010 .2000上u,0/SiC0.00058000 E。sic3GSSC6000 E4000 E0.0000(c2oC 3SSiC三2000ic15000Cu,0!●Cu,O百主0.000102003!Cu,0/0-2(3(40 50( 0000 t20/(* )Figure 3. XRD patterns of various photocatalysts0.00000It was also seen from Figure 3 that peaks of Cu2O/SiC001000were distinctly similar to those for pure SiC, indicating thatPore diameter(A)SiC has no obvious change in the modification. Three smallFigure 2. DFT pore size distributions of three photocatalystspeaks of Cu2O were also found in the pattern of Cu2O/SiC,which was caused by the small loading amount of Cu2O, sothat the peaks of Cu2O in Cu2O/SiC were very weak [31].Table 1. Surface areas and pore structure of photocatalystsThis also shows that Cu2O may be compounded with the SiC,0P0O00 SBET (m2/g) Votal (cm/g) daverge (nm)which can be further confirmed by XPS analysis.Cu2O34.50.1818.334.90.2021.5_Cu2O/SiC25.10.2829.83.3. UV-Vis spectroscopy analysisThe UV-Vis diffuse reflectance spectra of various photo-catalysts are shown in Figure 4. As shown in Figure 4, the SiC3.2. XRD analysisNPs had photoabsorption from UV light to visible light, andthere were two marked absorption edges, which were aboutThe XRD patterns of the three photocatalysts are shown410 and 560 nm, respectively. The optical band gaps can beiO FiQQ05 It was observed from Figure 3 that theredetermined by the following equation using the optical ab-were three sharp peaks at 35.70, 60.00 and 71.50, and twosorption data near the band edge [32]:small peaks at 34. 1and 41 .50, respectively, in the patternsof SiC and Cu2O/SiC [28.291. According toJCt ps caahv= A(hu- Eg(1No.29- - 1129 and No.29- 1131, three sharp peal are conswhere, a, v, A, and Eg are the absorp on ' oefficient, lighttent with the peak positions of 3C-SiC (11),(22 )frequency, proportionality constant al 1 ba d gap, respec-and 3C- SiC (311), while two small peaks are cively. Followings of band gapthe peak positions of 6H- SiC (110) and 6H-SiC( , ), resec- 101 ur wete estir中国煤化工which are intively. The XRD patterns show that the main phas OSiCNPsgood agreement WMYHCNMHGPeV (for2Ci0cul0OOOBC-SiC), although there exists a feov haagnalSiC) and 3.0eV (Jpopuluvcly This fu，siC (6H-SiC).indicates th富piC NPs contain both 6H SiC and 3C41 001 000Pore di anet er( A)20000148Huiling Li et al./ Jourmal of Natural Gas Chsristry Vol. 20No.220113.4. XPS analysis1.1.2 !Figor 5 shows the C1s and Si 2p patterns of XPS for15000 .1.0SiC NPs, and Cu 2p3/2 and Cu 2p1/2 patterns of XPS for .Cu2O/SiC and Cu2O. It can be seen from Figure 5(a) thatC 1s(20.8main peak of XPS spectra for SiC NPs exhibited an asymmet-(3ric line shape, indicating that there were different types of car-0.6bon eisting on the surface of SiC NPs. The C 1s main peak:一SiCo年Cu,Oof XPS spectra could be split into three sub- peaks at about282.8, 283.5 and 284.4 eV, which were ascribed to Si- C [34],10000.2Si1-xCx alloy (0.5