有色金属科学与工程  2019, Vol. 10 Issue (2): 68-76
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CeO2/Bi2MoO6纳米复合材料的制备及其增强光催化降解性能研究[PDF全文]
王书红 , 刘新 , 孔斌 , 卢家伟 , 李鸿 , 刘烈泉 , 王津津 , 陈娟 , 黄微雅     
江西理工大学冶金与化学工程学院,江西 赣州 341000
摘要:采用水热法制备了一系列不同的CeO2/Bi2MoO6纳米复合材料,分别考察了pH值(2~9)和Ce/Bi摩尔比(3 %~10 %)等因素对其光催化性能的影响.利用X射线粉末衍射、扫描电镜、红外光谱、紫外-可见漫反射光谱、光致发光光谱、瞬态光电流-时间响应等一系列分析测试手段对催化剂的组成、结构、光电性能等进行表征.结果表明,当pH=6时,所制备的Bi2MoO6(BMO)晶体形貌为细针状,复合CeO2后形貌为厚片状,比表面积减小、晶体颗粒增大.在实验室模拟太阳光条件下(300 W氙灯)进行光催化活性测试,分别以罗丹明B(RhB)、亚甲基蓝(MB)和苯酚为模拟污染物,对复合光催化剂的光催化活性进行考察,结果表明,当CeO2的含量为5 %时呈现出最高的光催化降解活性. 5 % CeO2/BMO对RhB、MB和苯酚的光催化反应的速率常数分别为0.037 min-1、0.016 min-1和0.007 min-1,相对于纯的BMO分别提高了3.19倍、1.70倍和4.58倍.其光催化性能增强主要原因是CeO2与Bi2MoO6复合后形成异质结有利于光生电子-空穴的有效分离,从而提高了活性自由基的含量.在自由基捕获实验中,超氧根离子自由基(·O2-)、羟基自由基(·OH)和空穴(h+)参与了光催化降解,其影响顺序为:·O2->·OH>h+.
关键词CeO2/Bi2MoO6复合    光催化    降解    
Synthesis of CeO2/Bi2MoO6 nanocomposites and their enhanced photocatalytic degradation performance
WANG Shuhong , LIU Xin , KONG Bin , LU Jiawei , LI Hong , LIU Liequan , WANG Jinjin , CHEN Juan , HUANG Weiya     
School of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, China
Abstract: A series of CeO2/Bi2MoO6 nanocomposites were prepared by hydrothermal method. The effect of pH value (2~9) and Ce/Bi molar ratio (3 %~10 %) on the photocatalytic performance of the prepared composites were investigated. The composition, structure and photoelectric properties of the catalyst were characterized by X-ray powder diffraction, scanning electron microscopy, infrared spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy, photoluminescence spectroscopy and transient photocurrent-time response spectrum, etc. The results showed that the morphology of Bi2MoO6 (BMO) crystals was needle-like at pH 6, and it became thick after recombination with CeO2. At the same time, the specific surface area reduced and the crystal particles enlarged. Photocatalytic activity of the catalyst was tested by photocatalytic degradation of Rhodamine B (RhB), methylene blue (MB) and phenol, respectively, under simulated sunlight irradiation (a 300 W Xenon lamp). The results showed that the optimal composite containing 5 % CeO2 could achieve the maximum photocatalytic degradation rate. Under the same experimental conditions, reaction rate constants of photocatalytic degradation of RhB, MB and phenol by 5 % CeO2/BMO were 0.037, 0.016 and 0.007 min-1, respectively, which were 3.19, 1.70 and 4.58 times higher than pure BMO, respectively. The enhanced photocatalytic performance was attributed to the formation of heterojunctions between CeO2 and Bi2MoO6, which were beneficial to the efficient separation of photogenerated electrons and holes. This, as a result, increased the content of active free radicals. Free radical trapping experiments showed that superoxide ion radical (O2-)、hydroxyl radical (OH) and holes (h+) all participated in the photocatalytic degradation, and their influence was in the order of O2->OH>h+.
Keywords: CeO2 /Bi2MoO6 composites    photocatalysis    degradation    

近年来,随着工业的快速发展,工业废水的处理成为突出问题[1].工业废水成分复杂,往往涉及多种有机、无机污染物,例如:偶氮染料、酚类(包括苯酚[2]、4-氯苯酚[3]、二硝基酚[4]、双酚A[5]等)以及重金属污染物等.其中,有机污染物往往浓度高且难降解[6],直接排放将对水环境生态系统造成严重影响,甚至威胁人类健康[7, 8].因此,如何有效处理工业废水中的有机污染物是当前环境领域的热点问题.

目前,工业废水处理方法包括吸附、膜处理技术、电化学处理、活性污泥法、离子交换和光催化等[9-16].其中,光催化技术能够充分利用太阳能将废水中的有机污染物降解为CO2和H2O等无机小分子,具有效率高、无二次污染等优点,被认为是一种绿色的技术,在化学、材料、能源、环境方面备受关注[17-25].在半导体光催化材料中,铋(Ⅲ)基半导体受到了研究者的极大关注,如Bi2O3[26],BiOX(X=Cl, Br, I)[27],BiVO4[28],BiPO4[29],Bi2WO6[30]和Bi2MoO6[31].其中,Bi2MoO6(BMO)的带隙能[32](2.6~2.8 eV)相比传统的半导体材料如:TiO2更窄,因而,能够利用太阳光中的可见光而备受关注.但是,纯BMO光催化剂存在光生电子-空穴极易复合的缺点,导致其光催化活性不高,从而限制了其在光催化降解废水中污染物方面的应用.将BMO与其它半导体复合构建异质结是降低其电子-空穴对复合率的有效手段.根据近期的研究报道,CeO2是一种良好的可见光催化剂,具有优异的光催化活性、无毒性和高耐久性[33-37].尤其值得一提的是,CeO2可以作为一种高效的助催化剂来提升复合光催化剂的活性[38].因此,通过构建CeO2和BMO异质结获得CeO2/BMO纳米复合材料,有望提高其对污水中有机污染物的光催化降解效率. Li等[39]采用溶剂热法合成了具有花状微结构的CeO2/BMO复合材料,并研究了其在可见光下的光催化降解活性.研究发现,具有最优化组成的CeO2/BMO复合材料在光催化反应75 min后对RhB的降解效率可达100 %,远超过相同实验条件下CeO2(26.8 %)和BMO(80.3 %)的光催化降解效率.然而,水热法合成CeO2/BMO复合材料及其光催化活性的研究尚未见报道.

文中通过水热法合成了一系列不同CeO2/BMO纳米复合材料,考察了pH值以及Ce/Bi摩尔比对所制备的复合材料光催化活性的影响.在实验室模拟太阳光条件下(300 W氙灯),分别以RhB、MB和苯酚为污染物模型,分别研究和比较所制备CeO2/BMO纳米复合材料的增强光催化降解活性,并探讨其光催化降解机理.

1 实验部分 1.1 化学试剂

Ce(NO3)2·6H2O(AR,西陇化工股份有限公司),尿素(AR,上海展云化工有限公司),乙醇、乙二醇、丙三醇、浓氨水、Bi(NO3)3·5 H2O(AR,阿拉丁),浓硝酸、柠檬酸、钼酸铵(AR,上海国药集团).

1.2 催化剂的制备

CeO2纳米颗粒的制备参考文献[40]方法,具体的实验过程如下:称量1.00 g Ce(NO3)2·6 H2O与0.50 g尿素,溶解于20.00 mL乙醇-丙三醇混合水溶液中(乙醇:丙三醇:水的体积比为5:5:10).搅拌30 min后置于高压反应釜中,170 ℃保温6 h.产物冷却至室温后取出抽滤,并用蒸馏水清洗直至滤液的pH=7.收集固体颗粒,90 ℃干燥2 h,350 ℃煅烧4 h,所得到的产物即为CeO2纳米颗粒.

CeO2/BMO复合材料合成采用改进文献[41]方法.具体实验步骤如下:称量1.07 g Bi(NO3)3·5H2O溶解在62.50 mL HNO3(浓硝酸与水体积比1:10)溶液中,然后加入0.59 g钼酸铵,将混合物在40 ℃下加热并剧烈搅拌.加入一定量的CeO2纳米颗粒(Ce/Bi摩尔比3 %、5 %和10 %)和2.03 g柠檬酸,用浓氨水调节溶液pH至6.然后,加入1.75 mL乙二醇,80 ℃继续磁力搅拌2 h,随后将溶液转移到100 mL聚四氟乙烯反应釜内,在150 ℃下反应8 h.待产物自然冷却至室温后抽滤,分别用蒸馏水和无水乙醇对样品洗涤至中性,60 ℃干燥过夜. 400 ℃煅烧4 h(升温速度为5 ℃/min)后获得复合材料,样品记为x CeO2/BMO,其中x为Ce/Bi摩尔比,分别对应为3 %、5 %和10 %.

作为对照,纯钼酸铋的制备方法同上,只是没有加入CeO2纳米颗粒.并且,分别在pH=2~9的条件下,考察了pH值对纯钼酸铋晶型的影响.

1.3 催化剂的表征

采用FEI公司FEG-250型扫描电子显微镜(SEM)观察样品的形貌;用Rigaku公司D/max-2500型X射线衍射仪(XRD)分析样品的晶相结构,其中,辐射源为Cu阳极靶,Kα射线,波长为λ=0.154 18 nm,测试电压为40 kV,测试电流为40 mA,扫描速率为5°/min.用美国麦克仪器公司ASAP2020型物理吸附仪,测定样品的比表面积.用日本日立F-4500型荧光光谱仪测定样品的光致发光光谱,扫描范围为200~700 nm.用Nicolet-470型红外光谱仪(FTIR)测样品的红外光谱,样品以KBr为底物做成压片.用日本岛津UV-2550型紫外可见分光光度计进行紫外可见漫反射(UV-Vis DRS)测试,以BaSO4作为参比,扫描范围200~800 nm.光电流的测试用三电极电化学工作站(CHI-660E,China),0.10 mol /L Na2SO4饱和溶液作为电解质,Ag/AgCl电极和Pt电极分别作为参比电极和辅助电极.

1.4 光催化性能评价

光催化实验以300 W的氙灯作为光源模拟太阳光.具体实验过程如下:称量0.08 g催化剂分散于80 mL,10 mg /L的RhB、MB和苯酚水溶液中.光催化反应前,悬浮液在黑暗中磁力搅拌30 min以达到催化剂对染料的吸附-脱附平衡.实验过程中使用循环冷却以保持光催化反应温度维持在室温.每间隔10 min,取3 ~ 4 mL混合液,在高速离心机中离心20 min(9 000 r/min)后取上层清液测定染料的吸光度.降解率η计算公式为:η=(C0-Ct)/C0 ×100 %,其中C0Ct分别为染料的初始浓度与不同时间时的浓度.

2 结果与讨论 2.1 X-射线衍射(XRD)、比表面积(BET)和SEM分析

图 1所示为所制备样品的XRD图.从图 1(a)中可见,不同pH条件下所制备的BMO样品在衍射角2θ为10.88°、28.44°、33.20°、36.26°、46.91°、56.4°和58.79°处出现衍射峰,分别对应四方晶系Bi2MoO6(JCPDF No.21-102)的(020)、(131)、(002)、(202)、(133)和(262)晶面.当pH=2 ~ 6时,随着pH的升高,BMO的特征衍射峰(131)晶面的衍射峰强度逐渐增强,而pH值进一步增大时,其衍射峰的强度又有所降低.其中pH=6时,(131)晶面的衍射峰的强度最高,半峰宽最小,且峰最尖锐,呈现出最好的结晶度[42].从图 1(b)中,除钼酸铋的衍射峰外,还可以观察到在33.2°、56.4°处分别出现CeO2的衍射峰,对应CeO2的(200)和(311)晶面.表明CeO2/Bi2MoO6复合材料被成功合成.

图 1 样品的XRD谱 Fig. 1 XRD patterns of the samples

图 2所示为BMO和5 % CeO2/BMO复合材料的SEM图.从图 2中可见,纯BMO的晶体形貌为细针状,复合CeO2后,5 % CeO2/BMO复合材料的形貌为厚片状.

图 2 样品的SEM像 Fig. 2 SEM images of the samples

此外,根据比表面积测试结果,纯BMO的比表面积为4.16 m2/g,复合CeO2后形貌为厚片状,比表面积减小、晶体颗粒增大.复合材料3 % CeO2/BMO、5 % CeO2/BMO和10 % CeO2/BMO的比表面积分别为3.36 m2/g、2.21 m2/g和3.58 m2/g.

2.2 傅里叶红外光谱分析(FTIR)

图 3所示为3 % CeO2/BMO、5 % CeO2/BMO、10 % CeO2/BMO复合材料和BMO样品的FTIR谱图.从图 3中可见,纯BMO样品在840 cm-1和797 cm-1处出现吸收峰,对应MoO6八面体中的Mo-O键的伸缩振动. 449 cm-1处的吸收峰对应为Bi-O的伸缩振动和形变振动模式,而586 cm-1附近的峰对应Mo-O-Bi的振动[43-45]. xCeO2/BMO复合材料的FTIR图谱中分别出现BMO的上述吸收峰,说明样品中存在Bi2MoO6,进一步证实了XRD分析结果.

图 3 xCeO2/BMO和BMO样品的FTIR谱 Fig. 3 FTIR spectra of xCeO2 /BMO composites and BMO samples

2.3 紫外-可见漫反射吸收光谱分析(UV-Vis DRS)

图 4所示为CeO2、BMO和5 % CeO2/BMO复合材料UV-Vis DRS谱图.从图 4中可见,纯BMO的吸收边缘在475.09 nm附近,对应带隙能为2.61 eV表明BMO在可见光区具有光吸收.当复合5 % CeO2后,所制备5 % CeO2/BMO复合材料的吸收边呈现红移,说明复合材料对可见光具有增强光吸收性能.根据半导体的光吸收阀值与带隙能公式:Eg = 1240 /λg,所制备样品的带隙能如表 1所列.由表 1可见,BMO、CeO2带隙能分别为2.61 eV和2.00 eV,3 % CeO2/BMO、5 % CeO2/BMO和10 % CeO2/BMO复合材料的带隙能先从2.39 eV降低到1.50 eV,再增加到2.44 eV.可见,通过调节Ce/Bi摩尔比,复合材料的带隙能在一定范围内具有可调控性.其中,5 % CeO2/BMO复合材料的带隙能最低.

图 4 样品的UV-Vis DRS谱 Fig. 4 UV-Vis DRS spectra of Samples

表 1 样品的带隙能 Table 1 Band-gaps of the samples
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2.4 光致发光谱(PL)

图 5所示为样品BMO、3 % CeO2/BMO、5 % CeO2/BMO、10 % CeO2/BMO在激发波长为320 nm下得到的光致发光(PL)谱图. PL发射峰的强弱可以反映光生载流子的复合几率,强度越弱表明光生电子-空穴复合的几率越低[46].从图 5可见,所测样品的发射峰在480 nm左右,强度次序:BMO>10 % CeO2/BMO > 3 % CeO2/BMO > 5 % CeO2/BMO.可见样品5 % CeO2/BMO具有最低的光生电子-空穴复合几率,有利于其光催化活性的提高.

图 5 样品的PL图 Fig. 5 PL spectra of samples

2.5 光电流分析

为了进一步揭示光生电子-空穴的分离与转移情况,对样品进行光电流测试,如图 6所示.一般而言,光电流响应强度越强,意味着半导体催化剂的光生电子转移效率越高[47].从图 6可见,复合CeO2之后,复合材料的光电流强度增强,其增强顺序为:BMO < 10 % CeO2/BMO < 3 % CeO2/BMO < 5 % CeO2/BMO.可见,当CeO2的复合量为5 %时,复合材料5 % CeO2/BMO的光电流最强,因而,其光电子转移效率最高,有利于光催化活性的提高.其原因可能是,CeO2可以承担电子俘获中心的作用,BMO在光激发后产生的电子可以有效转移到CeO2上,从而极大提高光生电子转移效率,降低电子-空穴的复合.光电流的结果与PL的结果一致.

图 6 BMO、3 %CeO2/BMO、5 % CeO2/BMO、10 % CeO2/BMO样品的光电流 Fig. 6 Photocurrent measurements of BMO、3 % CeO2/BMO、5 % CeO2/BMO、10 % CeO2/BMO

2.6 光催化性能测试

图 7所示为不同pH条件下所合成的BMO样品对RhB的光催化降解性能对比.从图 7中可见,pH=6时,所制备的样品BMO在光照60 min后,对RhB的降解率为49.06 %,高于其他pH条件下制备的BMO(5.91 % ~ 44.46 %).

图 7 不同pH条件下所合成BMO对RhB的光催化降解性能对比 Fig. 7 Photocatalytic degradation of RhB B by the samples of different pH BMO

图 8所示为催化剂对不同染料光催化降解图及其动力学拟合图.从图 8中可见,5 % CeO2/BMO复合材料在光催化降解RhB、MB和苯酚中具有最高的催化活性(图 8(a)~图 8(c)).当光照60 min时,5 % CeO2/BMO对RhB、MB和苯酚的降解率分别为90.01 %、62.80 %和35.20 %,高于3 % CeO2/BMO(69.19 %、45.93 %和26.79 %)和10 % CeO2/BMO(16.32 %、40.38 %和22.61 %)复合材料,并且远高于纯BMO(49.06 %、43.75 %和8.71 %).此外,5 % CeO2/BMO对RhB、MB和苯酚的降解率相应的光催化反应的速率常数分别为0.037 0 min-1、0.016 4 min-1和0.007 2 min-1图 8(d)~图 8(f)),相对于纯的BMO分别提高了3.19倍、1.70倍和4.58倍(具体数据见表 2).可见,当在BMO表面复合适当量的CeO2后,所形成的复合材料的光催化活性能够明显提升.

图 8 样品的光催化活性比较 Fig. 8 Photocatalytic activity comparison of samples Photocatalytic degradation of(a) RhB, (b) MB, (c) phenolby the differert samples; Degradation kinetics of (d)RhB, (e)MB, (f) Phenol

表 2 催化剂在降解RhB、MB和苯酚的准一级速率常数 Table 2 Pseudo-first order rate constants of catalyst samples in the photocatalytic degradation of RhB、MB and Phenol
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图 9所示为5 % CeO2/BMO纳米复合材料的自由基捕获实验.采用乙二胺四乙酸(EDTA)、对苯醌(BZQ)和叔丁醇(TBA)作为空穴(h+)、超氧自由基(·O2-)和羟基自由基(·OH)的捕获剂[48-50].从图 9可见,当不加捕获剂时,5 % CeO2/BMO对RhB的降解率为90.01 %,而加入EDTA、TBA和BZQ后其对RhB的降解率分别为62.13 %、23.16 %和7.62 %.可见,所对应活性物质对光催化的影响程度的大小顺序为:·O2->·OH>h+.其中,加入BZQ后,对其降解的抑制作用最明显,说明·O2-是RhB光催化氧化降解的主要活性物种,跟文献报道的其他钼酸铋复合材料一致[51-52].

图 9 捕获剂EDTA、TBA和BZQ对5 % CeO2/BMO光催化降解罗丹明B的性能影响 Fig. 9 Effect of quencher (EDTA, TBA and BZQ) on the photocatalytic degradation of rhodamine B by 5 % CeO2/BMO

5 % CeO2/BMO复合材料的增强光催化降解机理如图 10所示.在模拟太阳光照射下,CeO2/BMO复合材料光催化剂中的CeO2和BMO都能够被激发产生光生电子.根据文献[39],BMO的导带、价带位置均略高于CeO2的导带和价带位置.因而,当CeO2和BMO形成异质结时,BMO导带上的光生电子容易转移到CeO2导带上,而CeO2价带上的h+能够转移到BMO的价带上,从而使得电子-空穴有效分离,从而提升其光催化降解活性.复合材料表面的光生电子能够与溶液中的溶解氧反应生成·O2-,而表面的h+可能与OH-反应生成·OH.所生成的·O2-、·OH以及h+作为活性物质参与RhB的光催化降解.

图 10 5 % CeO2/BMO复合材料光催化剂对RhB的降解机理 Fig. 10 Mechanism of degradation of RhB by 5 %CeO2/BMO composite photocatalyst

3 结论

通过水热法合成不同pH条件的钼酸铋光催化剂, 并在其基础上复合CeO2,制备了一系列不同比例的CeO2/BMO复合材料(摩尔比为3 % ~ 10 %).以可见光为光源,RhB、MB和苯酚为降解对象进行光催化活性测试,具体实验结论如下:

1)当pH=6时制得的钼酸铋光催化活性最高,在其基础上复合CeO2后其光催化活性显著提高.

2)CeO2复合量为5 %时,模拟太阳光照射60 min后,其对RhB的光催化降解率可达90.01 %,相同条件下,纯BMO对RhB降解率仅为49.23 %.

3)5 % CeO2/BMO复合材料对可见光吸收能力明显增强,CeO2复合后与BMO形成异质结有效促进了光生电子-空穴的分离,从而提高光催化活性.

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