材料工程  2017, Vol. 45 Issue (7): 71-76   PDF    
http://dx.doi.org/10.11868/j.issn.1001-4381.2015.001422
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文章信息

曲家惠, 都玲, 赵方昕, 杨丽丽, 张文杰
QU Jia-hui, DU Ling, ZHAO Fang-xin, YANG Li-li, ZHANG Wen-jie
溶胶-凝胶法制备La2Ti2O7/HZSM-5及其光催化活性
Sol-gel Synthesis and Photocatalytic Activity of La2Ti2O7/HZSM-5
材料工程, 2017, 45(7): 71-76
Journal of Materials Engineering, 2017, 45(7): 71-76.
http://dx.doi.org/10.11868/j.issn.1001-4381.2015.001422

文章历史

收稿日期: 2015-11-21
修订日期: 2017-03-06
溶胶-凝胶法制备La2Ti2O7/HZSM-5及其光催化活性
曲家惠1, 都玲1, 赵方昕2, 杨丽丽1, 张文杰1    
1. 沈阳理工大学 环境与化学工程学院, 沈阳 110159;
2. 沈阳理工大学 信息科学与工程学院, 沈阳 110159
摘要: 采用溶胶-凝胶法制备La2Ti2O7/HZSM-5光催化剂,并对其光催化活性进行研究。结果表明:La2Ti2O7经HZSM-5分子筛负载后,并未改变原有晶相,仍为单斜晶系钙钛矿结构。负载后的La2Ti2O7分散在分子筛表面,催化剂的比表面积大幅增加并形成新的中孔结构。HZSM-5制约了La2Ti2O7的生长,导致晶粒尺寸减小。材料的光谱吸收边界随负载量的减小而发生蓝移,禁带宽度增大。负载样品中La3d和O1s的电子结合能增大。La2Ti2O7/HZSM-5比纯La2Ti2O7具有更高的光催化活性。经紫外光照120min后,活性艳红X-3B在70%La2Ti2O7/HZSM-5上的总脱色率为91.8%,而在La2Ti2O7上仅为31.7%。
关键词: 溶胶-凝胶    钛酸镧    HZSM-5    光催化   
Sol-gel Synthesis and Photocatalytic Activity of La2Ti2O7/HZSM-5
QU Jia-hui1, DU Ling1, ZHAO Fang-xin2, YANG Li-li1, ZHANG Wen-jie1    
1. School of Environmental and Chemical Engineering, Shenyang Ligong University, Shenyang 110159, China;
2. School of Information Science and Engineering, Shenyang Ligong University, Shenyang 110159, China
Abstract: La2Ti2O7/HZSM-5 photocatalysts were prepared by sol-gel method, and its photocatalytic activity was studied. The results show that the original crystal phase of La2Ti2O7 is not changed after loading La2Ti2O7 on HZSM-5, and is still perovskite structure. La2Ti2O7 is dispersed on the surface of HZSM-5 after loading to apparent increasing specific surface area and formation of new mesoporous structure. The growth of La2Ti2O7 is constrainted by HZSM-5, resulting in the reduction of grain size. The absorption boundary has a blue shift and the band gap increases with decreasing La2Ti2O7 loading content. Binding energies of La3d and O1s electrons move to higher energy after loading La2Ti2O7 on HZSM-5. La2Ti2O7/HZSM-5 has enhanced photocatalytic activity as compared to La2Ti2O7. 91.8% of the reactive brilliant red(RBR) X-3B is decolorized on 70%La2Ti2O7/HZSM-5 after 120min of UV irradiation, while only 31.7% of the dye is removed on La2Ti2O7.
Key words: sol-gel    lanthanum titanate    HZSM-5    photocatalysis   

光催化净化技术在解决环境问题、应对环境污染方面起到了重要作用[1]。在被广泛研究的光催化剂中,镧系钛酸盐优良的物理化学性质引起研究人员的兴趣[2, 3]。La2Ti2O7作为其中重要的代表物质,近年来受到较多关注[4, 5]。La2Ti2O7的合成大都采用传统的固相反应法。自Milanova等[6]采用简单的柠檬酸聚合络合法制备纯相La2Ti2O7以来,液相法合成La2Ti2O7的报道逐渐增多。对于纯相La2Ti2O7而言,由于其较大的晶粒尺寸、低比表面积及高的电子-空穴复合率,而使其光催化降解能力较低[7]。很多学者运用掺杂、贵金属沉积等手段对其进行改性[8-10]。ZSM-5分子筛是一种常见载体,具有独特的交叉孔道体系和较大的比表面积。研究人员以分子筛为载体负载氧化钛和SrTiO3,提高了光催化剂的活性[11, 12]。本工作以HZSM-5为载体,采用溶胶-凝胶法制备La2Ti2O7/HZSM-5光催化剂,进行X射线衍射(XRD)、中远红外光谱(FT-IR/FIR)、紫外-可见漫反射光谱(UV-Vis)、比表面(BET)及X光电子能谱(XPS)表征,考察负载量对La2Ti2O7的晶体结构、表面形貌等物化性质的影响,并以活性艳红X-3B为初始污染物,研究负载型La2Ti2O7的光催化降解性能。

1 实验材料与方法 1.1 材料制备

将0.85mL钛酸四丁酯逐滴加入8mL无水乙醇中,充分搅拌,形成A液。称取1.0825g(n(La):n(Ti)=1:1) 硝酸镧溶于8mL蒸馏水,再加入8mL冰醋酸,形成B液。在强力搅拌下将A液滴至B液内,并向混合液中缓慢加入2mL乙二醇。向溶液中投加一定量HZSM-5,随后迅速将混合液转入水浴锅内,在70℃下搅拌,直至形成淡黄色凝胶。将凝胶置于110℃烘箱内干燥15h后研磨,再于马弗炉中以5℃·min-1的升温速率将温度升至800℃,恒温煅烧3h。冷却至室温后将样品研磨成细粉,即可制得负载量分别为20%,50%,70%,90%的La2Ti2O7/HZSM-5。文中所指负载量均为La2Ti2O7在样品中的质量分数。制备La2Ti2O7时,混合液中不加入分子筛,其他过程相同。

1.2 催化剂表征

采用D8 Advance X射线衍射仪(CuKαλ=0.1541nm)分析催化剂的晶体结构;采用Frontier FT-IR/FIR中远红外光谱仪对催化剂进行红外光谱分析;采用LAMBDA 35紫外-可见光谱仪分析催化剂的紫外漫反射光谱;采用F-Sorb3400比表面及孔径分析仪测试催化剂的比表面积及孔结构;采用Thermo ESCALAB 250Xi X光电子能谱仪分析催化剂表面元素的化学环境。

1.3 光催化活性评价

光催化剂活性评价在自制的反应器中进行,在100mL石英烧杯上方放置主波长253.7nm的20W紫外灯。反应溶液体积为50mL,活性艳红X-3B的初始浓度为30mg·L-1。使用721E型可见分光光度计测定活性艳红X-3B最大吸收波长540nm处的吸光度。在本实验中La2Ti2O7的浓度均为500mg·L-1,与染料溶液混合后于暗处搅拌至吸附-脱附平衡。取5mL溶液,经微滤膜(孔径为0.45μm)过滤后测定其吸光度。剩余溶液经光催化反应后过滤,测其吸光度。根据朗伯-比尔定律[13]计算活性艳红X-3B的吸附率以及光催化降解率。活性艳红X-3B的光催化降解符合一级反应方程ln(C0/C)= k×t[14],其中,k为光催化反应速率常数,C0为染料溶液初始浓度,C为光照一定时间后的染料溶液浓度,t为光照时间。

2 结果与讨论 2.1 样品的表征

La2Ti2O7及不同负载量的La2Ti2O7/HZSM-5的XRD谱图如图 1所示。La2Ti2O7各晶面衍射峰均与JCPDS 81-1066中的衍射峰一一对应,所制备的催化剂为单斜晶系钙钛矿结构。未负载La2Ti2O7的晶胞参数:a=0.7784nm,b=0.5529nm,c=1.2971nm,α=γ=90°,β=98.804°。每个晶胞所含的分子数Z=4,晶胞体积V=0.5516nm3。负载在HZSM-5上的La2Ti2O7保持了原有特征衍射峰,晶相未发生改变,晶胞参数亦未产生明显变化。根据Scherrer公式[15],利用La2Ti2O7(212) 晶面半峰宽数据,计算出La2Ti2O7及负载量分别为90%,70%,50%,20%的La2Ti2O7/HZSM-5样品中La2Ti2O7的晶粒尺寸分别是23.0,22.7,22.3,20.6,19.2nm。负载后La2Ti2O7的晶粒尺寸略有下降。这是因为La2Ti2O7在分子筛表面得到较好分散,晶粒生长有所减缓[16]

图 1 La2Ti2O7和不同负载量的La2Ti2O7/HZSM-5的XRD谱图 Fig. 1 XRD patterns of La2Ti2O7 and La2Ti2O7/HZSM-5 with different loading content

图 2为La2Ti2O7及不同负载量的La2Ti2O7/HZSM-5的FT-IR和FT-FIR光谱图。可知,3440cm-1及1631cm-1附近出现的是羟基伸缩和弯曲振动吸收峰,这是由于样品表面吸附的水分子及分子筛本身的表面羟基所致。分子筛表面的羟基可以捕集更多的光生空穴,形成具有强氧化能力的羟基自由基[17]。1232cm-1处的峰归属于Al—O—Al或Si—O—Si的反对称伸缩振动峰[18],1096cm-1处宽而强的吸收带为Si—O—Si键的反对称伸缩振动峰,796cm-1处为SiO4的四面体环振动峰[19]

图 2 La2Ti2O7及不同负载量的La2Ti2O7/HZSM-5的FT-IR(a)和FT-FIR谱图(b) Fig. 2 FT-IR spectra(a) and FT-FIR spectra(b) of La2Ti2O7 and La2Ti2O7/HZSM-5 with different loading content

图 2(b)远红外谱图可以更清楚地观察金属氧化物的成键信息。647cm-1和353cm-1附近的吸收带归属于La—O吸收峰[20],554cm-1附近强而宽的吸收带为TiO6八面体中Ti—O的伸缩振动峰,554cm-1处的吸收带随负载量的减小而增强,为HZSM-5特征双五环的反对称伸缩振动吸收峰[21]与其叠加所致,462cm-1处的吸收带由Ti—O—La振动引起[22],452cm-1处伴随负载量下降而变强的吸收峰为四面体Si—O的弯曲振动峰。可以看出,负载后的样品保持了La2Ti2O7的所有特征吸收峰,且在1033cm-1处未出现新吸收带,表明La3+并未进入到分子筛骨架[21]

图 3为La2Ti2O7和不同负载量的La2Ti2O7/HZSM-5的紫外-可见漫反射光谱。催化剂在紫外区有很好的吸收。根据爱因斯坦关系式Eg=1240/λg[14],计算可得La2Ti2O7及负载量为90%,70%,50%,20%的La2Ti2O7/HZSM-5的禁带宽度分别为2.84,3.11,3.20,3.21,3.32eV。可以看出,随着负载量的减小,光谱吸收边界发生蓝移,禁带宽度有所增大。由于负载到HZSM-5上的La2Ti2O7晶粒生长受到抑制,晶粒细化使带隙宽化。

图 3 La2Ti2O7及不同负载量的La2Ti2O7/HZSM-5的紫外-可见漫反射光谱图 Fig. 3 UV-Vis diffuse reflectance spectra of La2Ti2O7 and La2Ti2O7/HZSM-5 with different loading content

材料的比表面积、孔结构和吸附量如表 1所示。未负载的La2Ti2O7几乎没有孔结构,累计孔容仅为0.0011cm3·g-1。负载后样品的孔容及平均孔径有所增加。值得注意的是,低负载量(20%~50%)样品的孔容随分子筛的减少而下降,在高负载量时却未继续减少。这是由于低负载量样品的孔容由HZSM-5本身的微孔所提供,分子筛减少后孔容随之下降。高负载量的La2Ti2O7在分子筛表面密集堆积而形成新的中孔结构,增加的孔容抵消了分子筛含量下降而减少的孔容。此时,平均孔径的增加也证实了样品孔结构的改变。HZSM-5对染料没有任何吸附能力,对活性艳红分子的吸附全部来自于La2Ti2O7。当负载量较大时,La2Ti2O7与分子筛间新的孔结构使其具有较大的孔容,有利于对染料的吸附。

表 1 HZSM-5,La2Ti2O7和不同负载量的La2Ti2O7/HZSM-5的比表面积、孔结构和吸附量 Table 1 Specific surface area, porous structure and adsorption capacity of HZSM-5, La2Ti2O7 and La2Ti2O7/HZSM-5 with different loading conten
Sample BET surface area/(m2·g-1) Average pore size/nm Pore volume/ (cm3·g-1) Adsorption of RBR X-3B/%
HZSM-5 223.8 1.91 0.1068 0.0
20%La2Ti2O7/HZSM-5 187.3 1.84 0.0864 7.0
50%La2Ti2O7/HZSM-5 128.2 1.78 0.0572 8.9
70%La2Ti2O7/HZSM-5 80.6 3.31 0.0666 11.4
90%La2Ti2O7/HZSM-5 36.2 6.08 0.0549 12.0
La2Ti2O7 8.0 0.56 0.0011 6.4

图 4为70%La2Ti2O7/HZSM-5及La2Ti2O7光催化剂中La3d,Ti2p,O1s的XPS谱图。由图 3(a)可知,La3d显示出较为复杂的多峰图谱,这是由于其自旋轨道相互作用导致La3d的芯级能谱分裂所致。负载样品中834.2,836.0,838.8,847.4eV对应于La3d5/2结合能的位置,851.0,852.8,855.6,862.7eV则对应于La3d3/2结合能位置。这与文献报道非常接近[23],表明La处于+3价态,并以氧化物形式存在[24, 25]。位于La3d3/2,La3d5/2左侧高能端的振起(shake-up)伴峰的强度变化可反映出O的2p电子转移至La的能力。负载后La3d3/2与La3d5/2伴峰所占比例由纯La2Ti2O7的71.8%,76.7%减小至70.0%和67.7%。表明负载样品内La2Ti2O7中O的外层价电子向La转移的能力有所下降,即La—O间共价性[26]和La周围的电子云密度减小,电子结合能相应增加。这与拟合图谱中结果相对应,即负载样品中La3d的电子结合能向高能端偏移0.1~0.5eV。

图 4 La2Ti2O7及70%La2Ti2O7/HZSM-5的XPS谱图 (a)La3d;(b)Ti2p;(c)O1s Fig. 4 XPS spectra of La2Ti2O7 and 70%La2Ti2O7/HZSM-5 (a)La3d;(b)Ti2p;(c)O1s

图 4(b)可知,70%La2Ti2O7/HZSM-5样品中458.3,464.0eV处两对称的谱峰对应于Ti2p3/2及Ti2p1/2结合能位置,峰距为5.7eV,为Ti4+的特征XPS谱峰[27]。负载前后峰距未有变化,表明Ti所处的化学环境没有发生明显改变。

图 4(c)可知,结合能为529.4,529.7,531.3eV对应于La2Ti2O7的晶格氧、表面吸附氧和羟基中的氧。70%La2Ti2O7/HZSM-5中O1s拟合图谱内新出现的532.7eV处结合能位置归属于分子筛骨架中的晶格氧。负载后的样品中,由于La—O间共价性的减小,O周围的电子云密度应有所增加,电子结合能也应有所降低。然而,负载样品中O1s的电子结合能反而向高能端移动0.2~0.4eV,这是由于分子筛对La2Ti2O7中O周围电子较强的吸引作用所致。

2.2 光催化活性

图 5为不同负载量的La2Ti2O7/HZSM-5对活性艳红X-3B的降解活性。本工作使用的HZSM-5对染料无光催化降解能力,光催化活性均来自于负载到HZSM-5表面的La2Ti2O7。未经负载的La2Ti2O7的光催化活性比较低,经紫外光照30min后活性艳红X-3B的降解率仅为6.4%。负载后样品的光催化活性随负载量不同而有较大变化。当负载量为70%时,70%La2Ti2O7/HZSM-5样品有最佳的光催化活性,经紫外光照30min后活性艳红X-3B的降解率为34.8%。

图 5 不同负载量的La2Ti2O7/HZSM-5对活性艳红X-3B的降解率 Fig. 5 Photocatalytic degradation efficiency of RBR X-3B on La2Ti2O7/HZSM-5 with different loading content

活性艳红X-3B在70%La2Ti2O7/HZSM-5和La2Ti2O7上的光催化反应速率常数k值分别为0.02122,0.00311min-1,负载样品为未负载La2Ti2O7的6.8倍。一方面,分子筛对La2Ti2O7有着较好的分散作用,La2Ti2O7拥有更小的晶粒尺寸、更大的比表面积和新的孔结构,对染料分子有更好的吸附作用,提高了光催化反应效率。另一方面,XPS结果显示出HZSM-5可使La2Ti2O7周围电子云密度降低,抑制了电子与空穴的复合,进而增强了氧化还原能力。这与O′Neill等[28]研究相吻合,电子转移使材料的光催化活性提高。但是,随着分子筛含量的继续增加,降解率呈现出明显的下降趋势。这主要是因为在低负载量时,为保持La2Ti2O7催化剂含量不变,反应体系中HZSM-5颗粒增多,阻碍了La2Ti2O7对入射光的吸收。同时,催化剂对染料分子的吸附能力有所下降,不利于光催化反应进行。

图 6为不同光照时间时La2Ti2O7及70%La2Ti2O7/HZSM-5对活性艳红X-3B降解时溶液的紫外-可见吸收光谱图。随反应时间的增加,位于540nm处活性艳红的偶氮共轭体系吸收峰强度降低,说明其结构受到破坏,造成染料脱色。光催化反应120min时,染料在70%La2Ti2O7/HZSM-5上的总脱色率可达91.8%,而在La2Ti2O7上仅为31.7%。位于230,280,310nm处的吸收峰归属于染料分子中苯环结构和萘环结构的特征峰。未负载La2Ti2O7几乎不能对其降解,70%La2Ti2O7/HZSM-5的降解效果却很显著。

图 6 不同光照时间时La2Ti2O7(a)及70%La2Ti2O7/HZSM-5(b)光催化降解活性艳红X-3B的紫外-可见吸收光谱图 Fig. 6 UV-Vis absorption spectra of RBR X-3B photocatalytic degradation on La2Ti2O7(a) and 70%La2Ti2O7/HZSM-5(b) with different irradiation time
3 结论

(1) 采用溶胶-凝胶法制备La2Ti2O7/HZSM-5光催化剂。La2Ti2O7分散在HZSM-5分子筛表面,La2Ti2O7/HZSM-5的比表面积远大于La2Ti2O7,并可形成新的中孔结构。

(2) La2Ti2O7经分子筛负载后晶型没有改变,仍为单斜晶系的钙钛矿结构。HZSM-5制约了La2Ti2O7的生长,晶粒尺寸有所减小。

(3) La2Ti2O7/HZSM-5比La2Ti2O7有更强的吸附及光催化降解能力。当负载量为70%时,70%La2Ti2O7/HZSM-5具有最佳的光催化活性。

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