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二氧化钛纳米带负载双金属催化剂的制备及催化和光催化性能研究
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摘要
二氧化钛(TiO2)是一种常见的多功能无机非金属材料,其广泛应用在工业和人们日常生活的各领域,其中纳米TiO2在光催化、太阳能电池、催化剂载体以及生物传感器等方面均具有重要应用。作为催化剂载体,近年来TiO2在纳米金催化领域被广泛研究。而在半导体光催化技术方面,对其进行异质化修饰改性的研究也是目前该领域的研究热点。通常纳米TiO2直接作为粉体或制备为薄膜材料使用,由此导致了其在某些领域的实际工业应用难以实现或降低了其性能,如作为半导体光催化材料在环境治理技术中的应用。近来,一维纳米材料的发展为纳米TiO2的应用和研究开辟了一条新途径。TiO2一维纳米材料所具有的类纤维单晶结构使其具有可集成性等优点使它可以自组装为宏观整体纸状材料。
     本论文主要以二氧化钛粉末(P25)为前驱体,水热法制备二氧化钛纳米带(TiO2-NBs)一维结构。采用光沉积法在二氧化钛表面沉积银纳米颗粒,然后将所制备样品(Ag/TiO2-NBs)在氯金酸溶液中进行原位置换反应,合成二氧化钛纳米带负载金银双金属异质结构(Au-Ag/TiO2-NBs),得到一种二氧化钛纳米带负载金银双金属纳米颗粒催化剂。透射电子显微镜观察(TEM)发现,所合成的金属颗粒尺寸均在3nm以下,并且高度均匀地分布在二氧化钛纳米带表面。X射线电子能谱(XPS)结合紫外可见吸光光谱(UV-vis)测试表明,双金属纳米颗粒中的银多数以氧化银的形式存在,而Au以金属态存在。相对于单一金属催化剂(Au/TiO2-NBs或Ag/TiO2-NBs),二氧化钛纳米带负载金银双金属结构具有的协同作用赋予其对苯甲醇选择性氧化高催化活性及稳定性。更为重要的是,基于这种纳米带负载双金属的异质结构,可以采用改良的造纸工艺技术,组装形成一种新颖的高性能的多孔结构的整体纳米纸催化剂。将该整体式催化剂应用于苯甲醇的气相选择性催化氧化,在较温和的反应条件下,即常压下反应温度为240℃,空气组成(O2/N2比为1:4)进气,Au-Ag/TiO2-NBs纳米纸催化剂表现出了高催化活性及对苯甲醛的高选择性(>99%),其空间转化率(TOF)为2570h-1。30h稳定性运转试验表明,该催化剂在较高的反应温度下(240℃)一直保持了高的反应速率,表明其具有很好的稳定性。TEM观察发现,二氧化钛纳米带上负载的金银双金属颗粒粒径在反应前后变化很小,经过反应30h后,粒径尺寸由1.48m左右增加至1.92nm左右。扫描电子显微镜(SEM)观察表明,反应后二氧化钛纳米带之间没有发生烧结。综上所述,所制备的Au-Ag/TiO2-NBs纳米纸催化剂具有很高的催化稳定性。该催化材料的优良稳定性来自于两方面:1、二氧化钛纳米带作为金属纳米颗粒的载体所表现出来的高抗烧结性能;2纳米双金属颗粒在二氧化钛纳米带表面所具有的高抗团聚烧结性能。由于光沉积形成的金属颗粒以半球状附着于二氧化钛纳米带的表面,二者之间的强相互作用使得二氧化钛纳米带对金属颗粒起到了有效的固着作用;另一方面双金属结构中存在的银大部分以氧化银的形式存在,氧化银的存在对催化剂的稳定性也起到了一定的作用。
     采用光沉积-原位置换法,我们还制备了TiO2纳米带负载其它贵金属的双金属/TiO2-NBs异质结构,并且将其应用于光催化体系中,通过对有机染料的光催化降解实验评价了所制备样品的光催化性能。研究表明,二氧化钛纳米带负载银钯(Ag-Pd/TiO2-NBs)、银-铂(Ag-Pt/TiO2-NBs)和金-铜(Au-Cu/TiO2-NBs)等双金属异质结构,均具有一定的光催化性能。对有机染料的降解性能较好,在污水处理方面有一定的应用前景。
Titanium dioxide is a common multifunctional semiconductor. It was widely used in different industries, including photocatalyst, solar cells and biosensors. It was more and more important for TiO2used as catalyst support, especially in the catalytic and photocatalytic fields. The development and application of TiO2were limited because of its existence of powder. Recently, the development of one dimensional nanomaterials provide a new way for TiO2material. Meanwhile, the research of TiO2one dimensional material used as catalyst support increased quickly. So TiO2one dimensional material has a bright future in industrial application.
     In this paper, we synthesized TiO2nanobelts through the hydrothermal method used titanium dioxide (P25) as a precursor. Ag/TiO2-NBs was obtained by the photodeposited method, Au-Ag/TiO2-NBs was fabricated by a galvanic replacement in HAuCl4solution. The bimetallic structure of Au-Ag/TiO2-NBs was detected by transmission electronic microscope (TEM), the metal particles were all sub-3nm, and finely dispersed on the surface of TiO2nanobelts. X-ray photoelectron spectroscopy (XPS) and UV-vis spectra were conducted to make sure the composition of this structure. The test suggested that silver existed in the form of silver oxide, and gold was metallic state.
     Compared with monometallic structure (Ag/TiO2-NBs or Au/TiO2-NBs), Au-Ag/TiO2-NBs has a synergistic effect, so it has a higher catalytic performance. More importantly, we synthesized a monolithic catalyst through the modified paper-making process based on the bimetallic Au-Ag/TiO2-NBs. And this catalyst exhibited good catalytic performance in the gas phase oxidation of benzyl alcohol at240℃and atmospheric pressure (The composition of carrier gas is O2/N2=1:4). Its TOF was2570h"1, and selectivity of benzaldehyde was over99%. The metal particle size of this catalyst had no obvious change after the gas phase oxidation. The size of Au-Ag nanoparticle became1.92nm from1.48nm. Furthermore, TiO2nanobelts had no sintering during the oxidation process showed by the scanning electron microscope (SEM). There are two reasons for the high stability of this catalyst, on one hand, the sintering resistance of TiO2nanobelts act as the noble metal catalyst support plays an important role. On the other hand, the metal particles had no agglomeration during the process because of the fix effect of metal particles on TiO2nanobelts. The existence of silver oxide in the metal particle also plays a role for the stability.
     We synthesized other noble metal bimetallic catalysts supported on TiO2-NBs through this method. And we conducted the photocatalysis experiment of organic dye degragation to evaluate the photocatalytic activity of those samples. As a result, Ag-Pd/TiO2-NBs, Ag-Pt/TiO2-NBs and Au-Cu/TiO2-NBs have certain photocatalytic activity, and they have widely use in the treatment of waste water in the future.
引文
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