【作者】 李玮琨；

【导师】 龚学庆；

【作者基本信息】 华东理工大学 ， 工业催化， 2014， 博士

【摘要】 二氧化钛金属氧化物材料广泛应用于催化化工、环境保护、新能源开发、生物医疗、食品科学等诸多领域。尤其在多相催化和光催化应用领域,二氧化钛一直都是一个热门催化剂,这主要是由于二氧化钛不仅制备技术和工艺十分成熟,而且适用于许多实验分析方法。因此,许多研究人员都在致力于二氧化钛催化剂的性能提高和应用领域拓展工作。另一方面,多相催化剂在催化过程中的性能主要由其表面结构决定,因此,为了能够对二氧化钛的性能形成全面完整的认识,就必须从二氧化钛的表面结构入手,分析其性能及潜在应用。在本文中我们采用了密度泛函理论(Density Functional Theory, DFT)方法对二氧化钛的结构和性能进行了以下研究工作：金红石二氧化钛(110)面台阶结构研究：金红石二氧化钛(110)表面是金红石晶体表面中热力学上最稳定的表面。具有缺陷的金红石(110)表面通常比没有缺陷的表面呈现更高的催化活性,尤其是台阶缺陷结构。在本论文中,我们采用DFT方法和扫描隧道显微镜(STM)模拟研究了金红石(110)面上几种不同取向台阶缺陷结构。通过对比不同取向台阶结构的形成能,我们发现取向为<110>的台阶结构经过2×1重构后呈现出最稳定的构型。同时,此种台阶结构经过STM模拟的结果与实验中的STM表征结果完全一致。金红石二氧化钛(110)表面上新的台阶模型的确定为进一步研究其结构和性能提供了坚实的理论基础。金红石二氧化钛(011)表面对H20分子的吸附性能研究：对金红石二氧化钛(011)表面的研究现在越来越多,主要由于它在很多催化反应中都呈现出极高的反应活性。最近的一些研究表明经过2×1重构的‘’brookite(001)-like"(011)表面具有最低表面形成能。在本论文中,我们采用DFT方法和STM模拟技术研究了此重构表面对H20分子的吸附行为。研究发现,在完整的2×1重构表面上,H20分子以较弱的分子吸附形式吸附；在具有氧缺陷的表面上,H20分子以较强的解离形式吸附,并在表面形成OH基团。随着表面上吸附的H20分子数量增加时,H20分子则以分子-解离吸附形式聚集形成H20团簇。而且,根据STM模拟结果与实验中的STM表征结果,我们发现H20团簇在表面上的生长均为一维水链结构,主要由于‘’brookite(001)-like"重构表面的特殊“波纹型”结构。经过2×1重构‘’brookite(001)-like"(011)表面上存在的一维纳米通道拓展了二氧化钛在纳米流体、酶催化等领域中的应用。锐钛矿二氧化钛(101)和板钛矿二氧化钛(210)表面结构及催化性能对比研究：锐钛矿二氧化钛(101)表面和板钛矿二氧化钛(210)表面分别为锐钛矿和板钛矿晶体暴露的主要表面。同时,它们在结构上具有一定相似性,但在活性上具有很大区别。这两种表面具有相同的结构单元,但是,两种表面上的结构单元内部原子距离和排列方式均不同。在板钛矿(210)表面上,两行结构单元之间的连接处具有极高的活性,能够有效地推进分子吸附和催化反应在表面上发生。在本论文中,我们采用DFT方法对比研究了它们吸附探针分子(H2O和HCOOH)和吸附金团簇的结构和性能。研究表明：(1)对于H20分子,无论是分子吸附还是解离吸附,在板钛矿(210)表面上都比在锐钛矿(101)表面上强。对于HCOOH分子,HCOOH在锐钛矿(101)表面上只表现为分子吸附,而在板钛矿(210)表面上却只表现为解离吸附,并且存在一种最稳定的双齿吸附结构。(2)对于金团簇,Au纳米颗粒倾向于吸附在两种表面的氧缺陷处。在(210)表面上小尺寸的纳米金团簇倾向于分散,形成3D结构,而(101)表面上小尺寸的纳米金团簇则倾向于聚集而形成2D的金膜结构。同时,通过研究Au6-8/brookite催化剂上CO低温氧化的反应机理,我们发现,金团簇中金原子的流动性和改变构象的能力能够明显促进CO氧化过程中的分子吸附和反应进程。板钛矿(210)表面的特殊结构预示着板钛矿材料在催化和光催化中具有更为广阔的应用前景。硼氮共掺杂锐钛矿二氧化钛催化剂中协同作用研究：硼氮共掺杂二氧化钛体系在催化领域中一直备受关注。然而,共掺杂体系中非金属间的协同作用一直备受争议。在本论文中,我们采用DFT方法对硼氮共掺杂锐钛矿二氧化钛(101)和(001)表面的几何和电子性质进行了研究。与实验结果相结合,根据制备过程中硼源和氮源的掺入顺序不同,在表面上生成的结构也不相同。B-N协同作用在锐钛矿表面上主要体现为Ti-N-B-O和Ti-B-N-Ti两种结构,这两种结构都能够有效减小共掺杂二氧化钛的禁带宽度。其中,硼氮共掺杂对(101)表面的带宽影响较大,而对(001)表面影响较小。二氧化钛混晶材料的异质结构及电子性质研究：混晶二氧化钛材料比单晶二氧化钛催化剂具有更高的光催化活性。在本论文中,我们采用DFT方法对金红石、锐钛矿、板钛矿二氧化钛两两结合形成的混晶材料进行了全面系统的理论研究。结果表明,影响混晶材料的电子性质有几个因素：构成混晶材料的两种成分本身的HOMO和]HUMO态的相对位置；两个板层之间的晶格匹配程度和两个板层间界面结构的形变程度。特别是当其中一种成分的HOMO和LUMO态比另一种都高时,每种成分将会维持本身的电子结构,则在混晶材料中,电子和空穴在空间上实现分离。这种电子性质规律能够对高活性混晶半导体材料的设计和制备起到有效的指导作用。更多还原

【Abstract】 As a versatile metal oxide, titanium dioxide (TiO2) has been extensively studied and widely used in many application fields, such as materials science, catalysis, environmental protection, energy development, biological science and medical treatment, etc. In particular, TiO2is universally applied in heterogeneous catalysis and photocatalysis, mainly because of its mature manufacturing process and suitability for almost all the experimental characterization and analysis methods. A great deal of effort has been devoted to the catalytic performance of TiO2over the past few decades. However, the surface structure of catalysts largely determines their properties in promoting catalytic process and therefore a comprehensive understanding of relationship between structure and the related reactivity is definitely necessary. In this dissertation, density functional theory (DFT) studies of structure and activity of T1O2catalyst are performed as below:Step-edge structures of rutile TiO2(101):rutile TiO2(110) surface is considered as the most thermodynamically stable surface of rutile. Defective rutile TiO2(110) surface always presents higher activity than the defect-free surface, particullay for step-edge structure. In this dissertation, we performed a DFT study and scanning tunneling microscope (STM) simulations of several low index directions of step edge on rutile TiO2(110) surface. According to comparing formation energy of different step-edge structures on rutile TiO2(110) surface, we determined the2X1reconstructed step edge along<110> orientation is the most stable. Simultaneously, the simulated STM image of this step-edge structure is completely consistent with the experimental STM result. It should be mentioned that the new step-edge model on rutile TiO2(110) surface is extremely important for a deeper understanding of structures and reactivities of stepped structures.Adsorption of H2O molecule on rutile TiO2(011) surface:rutile TiO2(011) surface is studied with more interests due to its considerably high performance in many catalytic reactions. Some resent studies proposed a new model of2×1reconstructed rutile TiO2(011) surface as "brookite(001)-like" with a lower surface energy. In this dissertation, we performed a DFT study and STM simulation of H2O adsorption on the reconstructed surface. It is suggested that weak molecular adsorption is predicted on perfect2×1reconstructed surface, however, stronger dissociated adsorption on O-defected surface foming hydroxyl group. Water molecules are aggregated into clusters as molecular-dissociated adsorption as the number of adsorbed water increases. On the "brookite(001)-like" surface, water cluster growth is observed along one dimension in simulated STM images and experimental STM data. This is mainly because of the corrugated geometry on the surface. The ID water clusters in nanosized channels on2×1reconstructed rutile TiO2(011) surface is important in different fields, such as, nanofluidics, enzyme catalysis, and biosensors.Comparative study of structures and reactivities of anatase and brookite TiC>2surfaces: Anatase TiO2(101) and brookite TiO2(210) surfaces are the two major surfaces exposed at anatase and brookite crystals, which have the similar structures but obviously different reactivities. Both two surfaces have the same structural building blocks, but the interatomic distances are slightly shorter and the blocks are arranged in a different way. Highly active junction sites are generated between rows of blocks on brookite TiO2(210) surface, which can greatly promote molecular adsorption and catalytic reaction on the surface. In this disertation, we performed a comparative DFT study of probe molecules (H2O and HCOOH) and gold clusters adsorption at two surfaces. The results shown as follows:(1) For H2O molecule, stronger molecular and dissociated adsorption are both confirmed at brookite TiO2(210) than anatase TiO2(101). For HCOOH molecule, only molecular adsorption exists at anatase TiO2(101), while HCOOH dissociates at brookite TiO2(210). Moreover, a rather stable dissociated structure (bidentate configuration) is found to occur at junction site.(2) For Au nanoparticles, they prefer to nucleate and grow at O vacancies of defected surfaces. Extended2D Au-film structures are favored at anatase TiO2(101), while dispersion of3D Au clusters is favored at brookite TiO2(210). Furthermore, CO oxidation reaction over Au6-8/brookite catalyst is also studied. The mobility of Au atoms in Au clusters and their capacity to tune conformations can significantly facilitate the adsorption and reaction processes during CO oxidation. The unique surface structure makes brookite TiO2(210) a promising candidate in catalysis and photocatalysis.B-N synergistic effect in co-doped anatase TiO2:boron and nitrogen co-doped TiO2has attracted much attention and been widely studied. However, the synergistic effect between the two dopants is still controversial. In this disertation, we performed a DFT study of geometric and electric properties of boron and nitrogen co-doped TiO2(anatase TiO2(101) and (001) surfaces). Combining with experimental results, newly forming structures are different according to the doping order of B and N dopants. The B-N synergistic effect is shown as Ti-B-N-Ti and Ti-B-N-O structures at anatase surfaces, which can both improved photocatalytic activity of TiO2. Furthermore, the B-N synergistic effect can largely decrease the bandgap of (101) surface, which is not obvious at (001) surface. Heterostructures and electronic properties of mixed-phase TiO2:mixed-phase TiO2has been reported to exhibit improved photocatalytic activities than single-phase TiO2. In this disertation, we performed a systematic DFT study of geometric and electronic properties of two-phase TiO2by joinig two single-phase TiO2slabs. Our results indicated that electronic properties of mixed-phase TiO2are affected by several factors:the relative positions of HOMO and LUMO states of the two composites; the lattice match between the two slabs of the compositon and how well the two slabs interact with each other in the interfacial region. In particular, when the HOMO and LUMO levels of one of the two single-phase TiO2slabs are higher than the corresponding ones of the other, the composite may have native electronic structures with phase separated HOMO-LUMO states. Such electronic properties may provide useful information to guide the design and preparation of highly active semiconductor materials with mixed phases.更多还原