摘要
光催化氧化技术是一种新型的废水处理技术,具有处理效率高、工艺设备简单、操作条件易控制、无选择性地降解有机污染物等突出优点,已在废水处理领域显示出巨大的应用潜力。
有关CeO_2作为三效催化剂、化学机械抛光(CMP)材料、氧气传感器、固体氧化物燃料电池(SOFC)、紫外吸收材料等的研究较多。在光催化应用方面,CeO_2均被作为一种助剂掺杂到其它氧化物中来增强本体氧化物的光催化活性,鲜见对以CeO_2为基质的光催化剂的研究。
因此,本论文以制具具有高降解能力的铈基光催化剂为目标,探讨用实验室常规方法制备二氧化铈基光催化剂的可能性,为其工业化应用提供理论依据。
主要研究内容和获得的认识如下:
(1)以铈盐为原料,分别采用四种常规湿化学方法,即高分子网络凝胶法、直接沉淀法、两步沉淀法、溶胶聚沉法制备出纳米CeO_2。以亚甲基兰溶液为降解模型,考察了纯纳米CeO_2的紫外光催化性能。实验结果表明,采用实验室常规方法制备的纯CeO_2纳米粉体的光催化性能极其微弱。
(2)为了改善CeO_2纳米粉体的光催化性能,分别以两步沉淀法、溶胶聚沉法制备出的CeO_2为起始,采用浸渍一沉淀法在其表面复合微量Ag,经过低温焙烧,得到Ag/CeO_2复合物并考察了其紫外光光催化性能。光催化结果表明,与纯CeO_2纳米粉体相比,Ag/CeO_2-1(溶胶聚沉法所得粉体复合Ag所得)的紫外光催化性能显著改善,表现出良好的紫外光催化性能;而Ag/CeO_2-2(两步沉淀法所得粉体复合Ag所得)的紫外光催化性能未见明显提高。催化效果的差异体现了溶胶聚沉法制备出的CeO_2性质的特殊性。
(3)对高降解能力铈基光催化剂Ag/CeO_2-1的制备工艺和光催化降解条件进行了优化研究。优化的制备工艺为:用0.1mol/L AgNO_3浸渍CeO_2后,加入2%Na_2CO_3溶液与之反应,将所得沉淀依次水洗、醇洗,350℃焙烧2h。优化的粉体投加量为2g/L;初始pH值对催化剂的降解性能有明显的影响,强酸或强碱性条件有利于亚甲基兰溶液的降解;前30min亚甲基兰溶液的紫外光催化降解速率方程为ln(C_0/C)=—0.0222+0.02056t,表观降解速率常数为0.02056 min~(-1),显示出一级动力学特征。
(4)初步研究了Ag/CeO_2-1催化剂在阳光下的降解性能。阳光下,Ag/CeO_2-1催化剂对可见光具有良好的光响应性,表明其是一种广谱的光催化剂。
(5)针对回流胶溶法制备溶胶耗时的缺点,改进了纳米CeO_2水溶胶的制备工艺,即利用铈(Ⅳ)与非铈稀土的氢氧化物的溶解pH值的差异,选择性地溶解非铈稀土氢氧化物,快速制备出含有La~(3+)的纳米CeO_2水溶胶。该改进方法不仅使制备溶胶的流程大为简化,获得的溶胶稳定性高,而且最终获得的目标产物的光催化能力与回流胶溶法所得溶胶衍生产物的催化能力相近。
本论文的研究的价值在于,首次采用实验室常规方法制备出了具有高降解能力的、对紫外光和可见光均具有良好光响应的广谱铈基催化剂。该研究拓宽了光催化剂的开发思路,也为新型催化剂的开发提供了理论基础,这对于充分利用我国得天独厚的铈稀土资源必将产生深远的影响。
Photocatalytic oxidation technology has a great potential for the removal of organic pollutants from wastewater by virtue of its high treatment efficiency, simple processing equipment, easy operation conditions and decomposing organic pollutants non-selectively.
Many researches have been focused on the performances of cerium oxide as a excellent material in the fields of three-way catalysts, chemical-mechanical polishing (CMP), oxygen gas sensor, solid oxide fuel cells (SOFC), UV absorbent and so on. It is well known that the photocatalytic activity of MO_x photocatalysts can be reinforced by doping limited cerium. However, study on photocatalytic activity of pure CeO_2 has rarely been reported.
In the present work, the probability of preparing photocatalyst of ceria matrix with high activity will be studied.
The main research contents and conclusions are as followed:
(1)Nanometer ceria was prepared by four different methods (polyacrylamide method; direct precipitation method; two-stage precipitation method and sol-aggregation method) using cerium salts as starting materials. The tests on photocatalytic degrading of methylene blue (MB) using as-prepared pure CeO_2 were investigated. The photocatalytic activity of pure CeO_2 was very weak according to the experimental results.
(2) Ag/CeO_2 composite powders were obtained by loading little Ag on the surface of pure CeO_2 through immersion-precipitation method. Then photocatalytic experiments were carried out under ultra-violet light. Ag/CeO_2-1 sample (derived from sol-aggregation) showed an excellent photocatalytic performance compared to non-modified ceria powders. In contrast, there was no obvious enhancement in the photocatalytic activity for Ag/CeO_2-2(derived from two-stage precipitation). The significant different photocatalytic performance for degrading MB between the two samples indicated the specialty of CeO_2 powders from sol-aggregation method.
(3) In order to obtain Ag/CeO_2 photocatalyst with good photocatalytic behavior, the preparation treatments were optimized. In addition, the suitable conditions for degrading organic pollutants were systematically studied. The optimum preparation conditions for the catalyst were as followed: nanometer CeO_2 powders from sol-aggregation method were immersed with 0.1 mol/L AgNO_3, then 2% Na_2CO_3 was added to react. Subsequently, the precipitation was washed with distilled water and absolute alcohol, successively. Finally, the intermediate was calcinated at 350℃for 2h. Effects of dosage of powders and reaction pH on degradation ratio were determined, respectively. The results indicated that the photocatalytic activity reached the maximum, when the amount of catalyst was 2.0 g/L, the initial pH value was distributed in the range of strong acidity or alkalinity. The rate of photodegradation in aqueous solution was followed first-order kinetics before the first 30 minutes. It can be stated as the equation: In (C_0/C)=-0.0222+0.02056t.
(4) The photocatalytic activity under solar light was studied also. The photocatalyst of Ag/CeO_2 exhibited a good visible-light response, namely, had a broad-spectrum activity for degrading organic pollutants.
(5) An improved method for preparing CeO_2 sol was proposed. Namely, CeO_2 sol accompanied with La~(3+) could be obtained rapidly based on the different pH values when hydroxide mixture composed of cerium (Ⅳ) and other non-cerium rare earth were solubilised by nitric acid. It overcame the time-consuming disadvantages compared with the original method. Not only was the process treatment via the new improved method shortened dramatically, but the mixed sol was very stable. What's the most important was the property of the objected-product wasn't impaired.
The value of this paper was that the photocatalyst of ceria matrix with good activity was firstly obtained by laboratory conventional method. The present work will enlarge the research idea and offer a theoretical basis for developing new photocatalyst, which has far reaching influence on benefiting a unique advantage of cerium resources in our country.
引文
[1] Gogate PR, Pandit AB. A review of imperative technologies for wastewater treatment Ⅰ: Oxidation technologies at ambient conditions[J]. Advances in environmental research. 2004, 8: 501-551.
[2] 孟楠,王柯.环境污染的国际考察[J].复旦学报(社会科学版),1998,13(3):46-49.
[3] Fujishima A, Honda K. Electrochemical Photolysis of Water at a Semiconductor Electrode[J]. Nature, 1972, 238(5358): 37-46.
[4] John. H. Carey, et al. Photodechlorination of PCB's in the presence of Titanium dioxide in aqueous suspensions[J]. Bull. Envirom. Contam. Toxic. 1976, 16(6): 697-701.
[5] Frank S N, Bard A J. Heterogeneous photocatalytic oxidation of Cyanide and Sulfite in aqueous solution at semiconductor powder[J]. J. Phys. Chem. 1977, 81: 1484-1495.
[6] Pruden, A. L. Ollis, D. F. J. Catal, 1983, 82: 404.
[7] Mills A., Le Hunte. S.. An overview of semiconductor photocatalysis[J]. J. Photochem. Photobiol A: Chem, 1997, 108: 1-35.
[8] 包南,孙剑,张锋,等.N 掺杂TiO_2纳米晶:水热—热分解法制备及光催化活性[J].无机化学学报,2007,23(1):101-108.
[9] 李立清,刘宗耀,唐新村,等.B/Fe_2O_3共掺杂纳米TiO_2可见光下的催化性能[J].中国有色金属学报,2006,16(12):2098-2103.
[10] 樊彩梅,唐琦,孙彦平.掺杂钕离子纳米TiO_2的制备及光催化降解水中苯酚的研究[J].稀有金属,2006,30(6):774-778.
[11] 袁慧,赵高凌,韩高荣.高纯金属酞菁的制备及其对TiO_2光催化性能的影响研究[J].功能材料,2005,36(4):580-582.
[12] 李敏,王振岭,石恒真,等.敏化的TiO_2纳米晶表面形貌、晶相、光谱及光催化灭菌研究[J].无机材料学报,2003,18(6):1261-1266.
[13] 郑凤英,李顺兴,钱沙华,等.水杨酸和5—磺基—水杨酸表面修饰纳米TiO_2吸附去除对硝基苯酚[J].环境科学学报,2007,27(1):64-68.
[14] 井立强,周强,王百齐,等.表面修饰Ru的TiO_2纳米粒子的表征及其光催化活性[J].哈尔滨工业大学学报,2005,37(11):1543-1545,1559.
[15] 井立强,蔡伟民,孙晓君.Pd/ZnO和Ag/ZnO复合纳米粒子的制备、表征及光催化活性[J].催化学报,2002,23(4):336-340.
[16] 陈孝云,陈曦,刘守新.电场协助对Ag/TiO_2光催化降解效率的影响[J].科学技术与工程,2006,6(5):523-528.
[17] 邵忠宝,王成燕,陈雪冰,等.纳米ZnO/Ag的制备及其光催化性能[J].材料研究学报, 2005,19(1):59-63.
[18] 鄂磊,徐明霞.贵金属修饰型TiO_2的光催化活性及Fermi能级对其光催化性能的影响[J].硅酸盐学报,2004,32,(12):1538-1541.
[19] 章福祥,张秀,陈继新,等.Ag/TiO_2复合纳米催化剂的制备和表征及其光催化活性[J].催化学报,2003,24(11):877-880.
[20] 井立强,孙晓君,蔡伟民,等.Pd/ZnO和Ag/ZnO复合纳米粒子的SPS和XPS研究[J].物理化学学报,2002,18(8):754-758.
[21] 刘永正,徐自力,杨秋景,等.复合氧化物Co_3O_4/Bi_2O_3对甲苯的光催化研究[J].吉林大学学报(理学版),2006,44(2):279-283.
[22] 刘兴芝,李春梅,房大维,等.纳米Re_2O_7的萃取法制备及光催化特性研究[J].无机化学学报,2006,22(4):612-616.
[23] 李小忠,王连军,赵铭,等.纳米CeO_2晶体的制备及其光催化性能的研究[J].环境化学,2006,25(2):149-153.
[24] 刘自力,秦祖赠,韦江慧.焙烧温度对钨酸铋光催化剂的影响[J].广西大学学报(自然科学版),2006,31(1):82-85.
[25] 娄向东,刘双枝,王天喜,等.氧化铁纳米棒的制备及其在光催化降解水溶性活性染料中的应用[J].工业水处理,2006,26(1):40-42.
[26] 陈盟,韩相春,尤铁学.TiO_2光催化氧化机动车排放污染物的影响因素[J].东北林业大学学报,2006,34(6):70-71,79.
[27] 王晓钰,祈巧艳,孙剑辉.纳米TiO_2在废水处理中的光催化活性[J].工业水处理,2005,25(1):6-9.
[28] 朱振中,任建荣,陈坚.悬浮态TiO_2光催化降解偶氮染料中性深黄GL[J].江南大学学报(自然科学版),2003,2(2):179-182.
[29] 王赟,阎永胜,沈湘黔,等.ZnO包覆Al_2O_3的制备表征及光催化性能研究[J].环境科学与技术,2006,29(12):33-35.
[30] 彭祺,汪玉庭,张伟安,等.纳米TiO_2—CS微球光催化降解亚甲基兰的研究[J].环境科学与技术,2006,29(1):23-25.
[31] 钱湛,铁柏清,孙建,等.UV/Fe~(3+)/H_2O_2体系降解活性艳橙X-GN废水[J].印染,2006,(2):12—16.
[32] 李俊杰,左永福,阎智英,等.不同酸介质对TiO_2光催化还原Cr(Ⅵ)体系的影响[J].云南大学学报(自然科学版),2005,27(5):434-436.
[33] 王艳荣.纳米二氧化铈的资源及应用[J].广州化工,2005.33(5):24-26.
[34] 周益明,忻新泉.低热固相合成化学[J].无机化学学报,1999,15(3):273-292.
[35] 周益明,忻新泉.我国固体无机化学的研究进展[J].化学通报,1999,(11):1-6.
[36] 庄稼,朱达川,迟燕华,等.固相反应制备纳米CeO_2粉体[J].化学研究与应用,2004,16(6):839-840.
[37] 庄稼,迟燕华,吴修贤,等.固相反应两步法制备纳米CeO_2及其机制研究[J].中国稀土学报,2004,22(5):641-646.
[38] Yong Xiu Li, Xue Zhen Zhou, Yong Wang, et al. Preparation of nano-sized CeO_2 by mechanochemical reaction of cerium carbonate with sodium hydroxide[J]. Materials Letters, 2003, 58: 245-249.
[39] 胡盛东,朱达川,涂铭旌,等.机械力固相化学反应法制备纳米氧化铈粉末[J].稀土,2004,25(5):20-23.
[40] Boro Djuriclic, Stephen Pickering. Nanostructured cerium oxide: preparation and properties of weakly-agglomerated powders[J]. Journal of the European Ceramic Society, 1999, (19): 1925-1934.
[41] 吴君毅,高玮,张凡.小颗粒氧化铈制备过程中的团聚控制[J].稀土,2001,22(1):5-8.
[42] 董相廷,李铭,张伟,等.沉淀法制备CeO_2纳米晶与表征[J].中国稀土学报,2001,19(1):24-26.
[43] Matijevic E, HSU W P. Preparation and properties of monodispersed colloidal particles of lanthanide compounds[J]. J. Collid & Interface Sci., 1987, 118(2): 506.
[44] Chu X, Chung W, Schmidt L D. Sintering of sol-gel prepared submicroneter particles studied by transimission electronmicroscopy[J]. J. Am. Ceram. Soc., 1993, 76(8): 215.
[45] 杜玉成,孙立柏,张久兴.纳米颗粒氧化铈的制备研究[J].矿冶,2003,12(4):51-53,64.
[46] 景晓燕,洪广言,李有谟.醇盐法制备稀土氧化物超微粉末[J].应用化学,1990,7(12):92-94.
[47] 董相廷,刘桂霞,孙晶,等.透明纳米CeO_2的合成与表征[J].中国稀土学报,2002,20(2):123-125.
[48] 董相廷,麦世坚,张伟,等.硬脂酸凝胶法制备CeO_2纳米粉体[J].材料科学与工程,2001,19(1):99-101.
[49] 王春秀,胡克良,李福利.纳米CeO_2的制备及其红外吸收性能研究[J].激光与红外,2006,36(5):399-402.
[50] 董相廷,洪广言.溶胶—凝胶法合成CeO_2纳米晶[J].长春理工大学学报,2002,25(2):43-46.
[51] 王成云,苏庆德,钱逸泰.非水溶剂水热法制备CeO_2纳米粉[J].化学研究与应用,2001,13(4):402-405.
[52] 梅燕,韩业斌,李彦,等.水/油微乳液技术制备单分散纳米CeO_2[J].硅酸盐学报,2006,34(3):340-344.
[53] 张凤林,潘湛昌,张环华,等.电化学法和沉淀法制备的纳米结构CeO_2的微观结构比较[J].稀有金属,2003,27(3):332-334.
[54] 李小忠,王连军,赵铭,等.离子交换—双氧水氧化法制备纳米CeO_2晶体[J].无机化学学报,2006,22(1):79-82.
[55] 郭广生,李铎,王志华,等.激光蒸凝法制备氧化铈纳米粒子[J].稀有金属材料与工程,2005,34(6):928-931.
[56] 王健,邱新平,武增华,等.沉淀—熔盐法制备纳米CeO_2粉体[J].中国稀土学报,2005,23(增刊):42-45.
[57] Jacques B, Laetitia Oliviero, Benoist Renard, et al. Catalytia wet air oxidation of ammonia over M/CeO_2 catalysts in the treatment of nitrogen-containing pollutants[J]. Catalysis Today, 2002, 75: 29-34.
[58] Tschiipe A, Birringer R. Oxyreduction studies on nanostructured cerium oxide[J]. Nanostructeed Materials, 1997, 9: 591-594.
[59] Steele B C H. Appraisal of Ce_(1-y)Gd_yO_(2-y/2) electrolytes for IT-SOFC operation at 500℃[J]. Solid State Ionics, 2000, 129: 95-110.
[60] Masatoshi Katsuki, Shaorong Wang, Kenji Yasumoto, et al. The oxygen transport in Gd-doped ceria[J]. Solid State Ionics, 2002, 154-155: 589-595.
[61] 董相廷,曲晓刚,洪广言,等.CeO_2纳米晶的制备及其在电化学上的应用[J].科学通报,1996,41(9):847-850.
[62] Tetsuya Hoshino, Yasushi Kurata, Yuuki Terasaki, et al. Mechanism of polishing of SiO_2 films by CeO_2 particles[J]. Journal of Non-Crystalline Solids, 2001, 83: 129-136.
[63] 马燕合.我国稀土应用开发现状及其展望[J].材料导报,2000,14(1):3-5.
[64] 宋晓岚,王海波,吴雪兰,等.纳米CeO_2的制备技术及应用[J].稀土,2004,25(3):55-61,70.
[65] 尚静,谢绍东,刘建国.SnO_2/TiO_2复合半导体纳米薄膜的研究进展[J].化学进展,2005,17(6):1012-1018
[66] Juan M Coronado, Javier A Maira, Arturo Mart'ynez-Arias et al., EPR Study of the Radicals Formed upon UV Irradiation of Ceria-Based Photocatalysts[J]. Journal of Photochemistry and Photobiology A; Chemistry, 2002, 150(2): 213-221.
[67] Fornasiero P, Fonda E, Monte R D et al, UV-Enhanced Exchange of O_2 with H_2O Adsorbed on TiO_2[J]. J. Catal., 1999, 18(1): 155-159.
[68] 米纳切夫(MNHaueb O M).稀土在催化中的应用[M].北京:科学出版社,1987.
[69] 庞秀江,龚茂初,王敏,等,沉淀方法对CeO_2-ZrO_2系储氧材料性能的影响[J].物理化学学报,2004,20(9):1155-1158.
[70] Fan Zhang, Shi-ping Yang, Hong-mei Chen, et al. Preparation of discrete nanosize ceria powder[J]. Ceramics International, 2004, (30): 997-1002.
[71] 张立德,牟季美.纳米材料与纳米结构[M].北京:科学出版社,2001:84.
[72] 康俊勇,S.Tsunekawa,A.Kasuya.超细SnO_2纳米晶粒带边光吸收的限度效应[J].物理学报.2001,50(11):2198-2202.
[73] T. Masui, K. Machida, T. Sakata, H. Mori, G. Adachi. Preparation and characterization of cerium oxide ultrafine particles dispersed in polymer thin films[J]. Journal of Alloys and Compounds, 1997, (256): 97-101.
[74] Shinryo Yabe, Mika Yamashita, Shigeyoshi Momose Synthesis and UV-shielding properties of metal oxide doped ceria via soft solution chemical processes[J]. International Journal of Inorganic Materials, 2001(3): 1003-1008.
[75] 侯林瑞,原长洲,彭秧.磁载光催化剂Ag-TiO_2/ZnO/γ—Fe_2O_3的制备及其光催化性能[J].应用化学,2006,23(11):1278-1281.
[76] 周新文,李东亮,郑红霞.光化学还原法制备Ag—TiO_2抗菌材料的制备及性能研究[J].化工新型材料,2006,34(11):68-70.
[77] 俞洁,鲍小恒,堵永国,等.Yb_2O_3粉体化学镀银研究[J].电工材料,2006,(4):35-39.
[78] 尹晓敏,程永清,徐杰.掺Ag~+二氧化钛粉体抗菌性能的影响因素[J].钢铁钒钛,2006,27(1):50-54.
[79] 石雷,霍晓敏,刘春艳,等.预处理条件对一步法合成吲哚用Ag/SiO_2催化剂活性的影响[J].催化学报,2005,26(5):403-406.
[80] 张旋,姜洪雷,于学龄.均匀沉淀法制备载银TiO_2薄膜及其光催化性能的研究[J].山东轻工业学院学报,2006,20(1):67-71.
[81] Linsebigler A L, Lu G Q, Yates J T. Photocatalysis on TiO_2 surfaces: principle, mechanisms, and selected results[J]. Chem Rev, 1995, 95(3): 735-758.
[82] 宋晓岚,邱冠周,杨振华,等.水相介质中纳米CeO_2的分散行为[J].稀有金属,2005,29(2):167-172.
[83] 宋晓岚,邱冠周,史训达,等.混合表面活性剂分散纳米CeO_2颗粒的协同效应[J].湖南大学学报(自然科学版),2005,32(5):95-99.
[84] 漆新华,王中华,庄益源,等.改性TiO_2对X-3B的光催化性能及其降解动力学研究[J].环境污染与防治,2004,26(2):84-100.
[85] 漆新华,王中华,庄益源,等.二氧化钛对活性艳红X-3B的光催化降解研究[J].化工环保,2004,24(1):1-4.
[86] 王中华.掺铁TiO_2光催化降解活性艳红X-3B的研究[J].天津化工,2004,18(5):4-6,62.
[87] 董相廷,刘桂霞,孙晶,等.CeO_2纳米水溶胶的制备[J].稀有金属材料与工程,2002,31(3):229-231.
[88] 刘桂霞,孙多先,董相廷.一步法合成纳米CeO_2溶胶及其分散稳定机理[J].天津大学学报,2004,37(3):203-206.
[89] 张其春.制取纯氢氧化铈(Ⅳ)的工艺方法[P]ZL99114762.6.
[90] 奚旦立主编.环境工程手册(环境监测卷)[M].北京:高等教育出版社,1998年(第一版).