g-C_3N_4修饰β-ZnMoO_4光催化降解磺胺二甲嘧啶:合成方法的影响、降解动力学及机理(英文)

在过去几十年中,钼酸盐在功能材料领域的应用备受关注.例如,半导体材料二价金属钼酸盐MMoO_4(M=Ca,Mg,Zn)在发光、催化、电容器、闪烁探测器等方面已有良好的应用.研究表明,钼酸锌在紫外或可见光照射下能够有效降解甲基橙、维多利亚蓝、苯酚等污染物.中国拥有丰富的钼资源,目前钼主要用于生产高强度钢.制备钼基高效除污除材料可作为钼资源的另一种高附加值利用模式.氮化碳(g-C_3N_4)作为一种低成本的光活性改性剂,可提高半导体材料的光催化性能.迄今为止,基于氮化碳复合材料的制备方法包括:原位水热合成、超声波复合、一步升温合成和沉淀法等.然而,很少讨论合成方法对复合材料性能的影响.本文以β-ZnMoO_4为主体材料,g-C_3N_4为修饰材料,首次制备了两者复合的新型光催化剂.采用不同的方法和条件制备了β-ZnMoO_4和β-ZnMoO_4/C_3N_4复合材料,探讨了合成方法对复合材料光催化性能的影响,并进一步研究了材料光催化降解磺胺二甲嘧啶的动力学和降解途径.以钼酸钠和硝酸锌为原料,在不同温度和时间条件下,采用水热法合成得到了两种不同形貌的β-ZnMoO_4材料.光催化降解实验结果显示,水热合成条件对催化剂的光催化活性影响很大,280℃水热条件下维持24 h,得到表面光滑的不规则微米颗粒(β-ZnMoO_4-280),其光催化活性高于180℃条件下获得的片状形貌的钼酸锌材料(β-ZnMoO_4-180).β-ZnMoO_4/C_3N_4复合材料通过原位水热法和超声法合成,结果显示,原位水热合成条件下获得的β-ZnMoO_4-180/C_3N_4光催化剂对磺胺二甲嘧啶表现出显著增强的降解能力.相比之下,在280℃水热条件下,C_3N_4颗粒发生逐步分解,且反应开始时C_3N_4颗粒会扰乱β-ZnMoO_4-280晶体生长的连续性,使复合材料性能下降.对于超声法合成的β-ZnMoO_4/C_3N_4材料,两种β-ZnMoO_4/C_3N_4复合材料的光催化活性均提高,但提高程度不及水热法180℃条件下制备的材料.结果表明,对于光催化复合材料的制备,要选择适当的合成方法,才能得到高性能复合光催化材料,本文采用180℃的水热合成条件,添加3%g-C_3N_4,可得到性能最佳的β-ZnMoO_4-180/C_3N_4复合光催化剂.添加自由基抑制剂的光催化降解实验结果表明,超氧负离子(·O_2~–)和空穴(h~+)在降解中起主导作用.β-ZnMoO_4/C_3N_4复合材料光催化活性的增强归因于C_3N_4与β-ZnMoO_4之间形成异质结,该异质结提高了光生电子-空穴对的分离效率.通过液相-质谱联用手段,测定了磺胺二甲嘧啶降解的中间产物,结果表明,污染物的光催化降解途径主要包括脱氨基和脱甲基过程.

Photocatalytic degradation of sulfamethazine by graphitic carbon nitride-modified zinc molybdate: Effects of synthesis method on performance,degradation kinetics,and mechanism

In the present study, zinc molybdate (β-ZnMoO4) and graphitic carbon nitride (g-C3N4)-modifiedβ-ZnMoO4 (β-ZnMoO4/g-C3N4) were prepared to decontaminate aqueous solutions from the antibi-otic sulfamethazine (SMZ). Our results revealed that the hydrothermal synthesis method greatly influenced the photocatalytic activity of the resultant catalysts. The pristine β-ZnMoO4 samples obtained under more intensive synthesis conditions (24 h at 280 °C) showed higher photocatalytic activity than that prepared for 12 h at 180 °C (denoted β-ZnMoO4-180). In the case of in situ hydro-thermal synthesis of β-ZnMoO4/g-C3N4, a surface-modified sample was only obtained under the reaction conditions of 180 °C for 12 h. Compared with the sheet-like β-ZnMoO4-180 sample, theβ-ZnMoO4-180/g-C3N4 composite showed enhanced photocatalytic activity for the degradation of SMZ. By contrast, the hydrothermal reaction at 280 °C caused the gradual decomposition of g-C3N4. It is believed that the structural incorporation of g-C3N4 into β-ZnMoO4 at 280 °C mightdisrupt the crystal growth, thereby deteriorating the performance of the composite catalysts formed at this temperature. For the composite catalysts prepared by the ultrasonic method, a remarkable increase in the degradation rate of SMZ was only observed at a high g-C3N4 content of 8 mol%. The photo-catalytic degradation of SMZ by β-ZnMoO4-180/g-C3N4 composite catalysts followed pseu-do-first-order kinetics. Further study of the photocatalytic mechanism revealed that holes and su-peroxide radicals were the dominant oxidative species in the photodegradation process. The en-hanced photocatalytic performance of the composites was attributed to the higher separation effi-ciency of the photogenerated electron-hole pairs at heterogeneous junctions. The degradation in-termediates of SMZ were detected by liquid chromatography-mass spectrometry, from which plau-sible reaction pathways for the photodegradation of SMZ were proposed. Our results indicated that the synthesis method for g-C3N4 composites should be carefully selected to achieve superior photo-catalytic performance.