单晶金属氧化物表面分子吸附与反应的超高真空红外光谱研究

发布时间：2018-05-19 00:10

  本文选题：光催化 + 表面科学　； 参考：《山东大学》2017年博士论文

【摘要】：1972年Fujishima和Honda教授发现光照下TiO_2电极可以把水裂解为氢气和氧气,之后迅速掀起了光催化领域的研究热潮。光催化材料在催化分解水制氢、降解各种有机和无机污染物、太阳能电池以及光还原二氧化碳等领域具有重要的应用前景。分子尺度上对光催化反应基本过程的理解,对提高催化剂的催化效率和选择性具有重要意义。催化剂表面是多相催化发生的直接场所,研究表面与反应物分子的相互作用行为,对深入理解光催化机理是非常重要的。氧化钛(TiO_2)和氧化锌(ZnO)是目前研究和应用最为广泛的光催化材料,把这两种材料表面作为模型表面来研究光催化机理具有很强的代表性。人们利用多种超高真空表面科学分析技术,在单晶TiO_2与ZnO表面分子吸附与反应过程的研究中取得了丰富成果。但是,很多分子尺度上的基础问题依然存在争议,利用更多的实验手段从不同角度来研究表面化学过程对于澄清这些问题是非常有必要的。表面红外反射吸收光谱(Infrared Reflection Absorption Spectroscopy,IRRAS)对吸附分子具有很强的化学分辨能力,并对分子周围环境比较敏感,非常适合研究表面分子的吸附位置、吸附构型、反应路径和分子-分子相互作用等问题,已经成功地应用于金属单晶表面化学和催化反应机理研究中。但是由于单晶氧化物表面吸附分子振动的红外吸收信号极弱,其上表面化学过程的红外光谱研究非常缺乏。我们设计搭建了新型的超高真空-真空红外光谱(Ultrahigh Vacuum-Vacuum Fourier Transform Infrared Spectrometer,UHV-FTIRS)系统,该系统经过优化的光路设计,具有极高的稳定性与灵敏度,可以在超高真空环境中原位监测氧化物单晶表面的分子过程。利用这套装置,我们系统地研究了金红石型单晶TiO_2(110)表面和纤锌矿型单晶ZnO(1010)表面等典型氧化物表面的分子吸附位置与吸附构型、分子间相互作用以及分子-表面电荷转移等问题。主要研究内容与结果如下:1.分别利用CO和NO分子研究了 TiO_2(110)表面氧空位(Vo)引入极化子参与的分子吸附过程。对于CO吸附,位于次表层六配位Ti位(Ti6c)的极化子转移到CO下方的五配位Ti位(Ti5c),分子周围静电场环境的改变引起了分子振动频率的微弱红移;对于NO吸附,次表层的Ti6c极化子直接转移到Ti:3d-NO:2p杂化轨道,引起了 N-O伸缩频率的巨大红移。不同的极化子参与的分子吸附图像取决于极化子态与分子最低未占据分子轨道(Lowest Unoccupied Molecular Orbital,LUMO)的能级排布情况。2.利用偏振分辨和方向分辨的IRRAS,我们系统研究了 TiO_2(110)理想配比表面和还原性表面NO吸附与反应路径。在理想配比TiO_2(110)表面,确定了吸附于Ti5c的双配位顺式(NO)_2二聚体构型(cis-(NO)_2/TiTi),并给出了 NO→cis-(NO)_2/TiTi → N_2O + Oa的反应路径。而在还原性TiO_2(110)表面,Vo的存在明显地改变了 NO的吸附与反应路径。实验发现三种与Vo相关的NO吸附构型,它们都和Vo引入的极化子有着强的相互作用,引起了分子振动频率的极大红移。NO通入量足够高时,所有的NO中间态最终全部转化为N_2O分子。3.我们研究了 TiO_2(110)表面Au团簇的生长、电荷态表征及其调控。以CO为探针分子,利用其振动频率的改变来表征Au团簇的电荷态,我们发现TiO_2(110)表面原位生长的Au团簇以电中性形式存在。通过TiO_2(110)表面2NO →N_2O+ Oa反应,成功实现了对Au/TiO_2界面处Au原子从电中性到正电性的调控,反映到CO频率上有20-26 cm-1的较大蓝移。频率蓝移大小,即Au正电性强弱,取决于表面Oa原子数目及Au团簇尺寸。4.我们研究了 CO_2在TiO_2(110)表面的吸附位置、吸附构型和分子-分子相互作用。随着(CO_2覆盖度的增加,90K时,CO_2依次吸附于Vo位、Vo近邻的Ti5c位、远离Vo的Ti5c位和桥位氧(Obr)位。红外光沿[001]和[1(?)0]晶向入射的IRRAS谱均发现了(CO_2的v3(O(CO)反对称伸缩振动分裂为两个吸收峰,意味着近邻CO_2之间发生了振动耦合。结合密度泛函(DFT)计算,我们提出了两种(CO_2的耦合转子结构来解释近邻CO_2在两个方向的吸收峰分裂。CO_2覆盖度为1.5分子单层(monolayer,ML)时,通过方向分辨和偏振分辨的IRRAS,确定了吸附于Obr位的CO_2沿[1(?)0]方向的水平吸附构型。5.我们发现了ZnO(10(?)0)表面CO_2一维分子链的形成并研究了其演化过程。在9OK时ZnO(10(?)0)表面CO_2的吸附过程中,随着覆盖度增加,IRRAS谱中出现了一系列CO_2吸收峰的精细结构,结合DFT计算,依次把它们归属为沿[0001]方向形成的CO_2单体、二聚体、三聚体以及更长的分子聚合体。这种新奇的链式生长过程是由于界面电荷再分布引起的增强的CO_2分子与表面三配位Zn原子(Zn3c)之间的库伦吸引作用导致的。低(CO_2覆盖度时,表面退火可以使较短的分子链变的更长。高覆盖度时,退火至150K局域的(2×1)结构会演化为一种局域的(1×1)结构中间态,进一步升高温度最终演化为规范的(2×1)结构。我们提出了一个动力学机制解释了这种相演化过程。
[Abstract]:In 1972, Fujishima and Honda found that the TiO_2 electrode can break water into hydrogen and oxygen under light, and then quickly set off the research boom in the field of photocatalysis. Photocatalytic materials have important applications in the field of catalytic decomposition of water for hydrogen production, degradation of various organic and inorganic pollutants, solar cells and light and carbon dioxide. The understanding of the basic process of photocatalytic reaction on the molecular scale is of great significance for improving the catalytic efficiency and selectivity of the catalyst. The surface of the catalyst is a direct place for the occurrence of multiphase catalysis. It is very important to study the interaction between the surface and the reactant molecules. It is very important to understand the mechanism of photooxidation. Titanium oxide (TiO_2) and oxygen are very important. Zinc (ZnO) is the most widely used photocatalytic material for research and application. It is very representative to study the photocatalytic mechanism of the surface of these two materials as a model surface. A variety of ultra high vacuum surface scientific analysis techniques have been used in the study of the adsorption and reaction of TiO_2 and ZnO on the surface of single crystal. However, many basic problems on the molecular scale are still controversial. It is necessary to use more experimental methods to study the surface chemical processes from different angles to clarify these problems. The Infrared Reflection Absorption Spectroscopy (IRRAS) has a strong chemical fraction for the adsorbate molecules. It is very suitable for the study of the adsorption location of surface molecules, the adsorption configuration, the reaction path and the interaction of molecules and molecules, which have been successfully applied to the study of the surface chemistry and catalytic reaction mechanism of the single crystal. The signal is very weak, and the infrared spectrum of the upper surface chemical process is very short. We designed and built a new ultra high vacuum vacuum infrared spectroscopy (Ultrahigh Vacuum-Vacuum Fourier Transform Infrared Spectrometer, UHV-FTIRS) system. The system has been designed with optimized optical path, with high stability and sensitivity. The molecular process of in-situ monitoring of the surface of a single oxide single crystal in an ultra high vacuum environment is used. Using this set of devices, we have systematically studied the molecular adsorption position and adsorption configuration, intermolecular interaction and molecular surface charge transfer on the surface of the TiO_2 (110) and the ZnO (1010) surface of the monocrystalline monocrystalline monocrystalline rutile The main contents and results are as follows: 1. using CO and NO molecules, the molecular adsorption process of TiO_2 (110) surface oxygen vacancy (Vo) is introduced into the polaron. For CO adsorption, the polaron at the subsurface six coordination Ti bit (Ti6c) is transferred to the five coordination Ti bit (Ti5c) below CO, and the environment of the electrostatic field around the molecule is changed. The weak red shift of the molecular vibration frequency; for NO adsorption, the Ti6c Polaron in the subsurface is transferred directly to the Ti:3d-NO:2p hybrid orbit, causing a huge red shift of the N-O expansion frequency. The molecular adsorption images of different polarons are dependent on the polaron and the molecular lowest unoccupied sub orbits (Lowest Unoccupied Molecular Orbital, LUM). O's energy level arrangement.2. uses polarization resolution and directional resolution IRRAS, we systematically study the NO adsorption and reaction path of TiO_2 (110) ideal ratio surface and reductive surface. In the ideal ratio TiO_2 (110) surface, the dual coordination CIS (NO) _2 two polymer (cis- (NO) _2/TiTi) adsorbed on Ti5c is determined. The reaction path of /TiTi to N_2O + Oa, while on the reduced TiO_2 (110) surface, the existence of Vo obviously changes the adsorption and reaction path of NO. The experiment found that the NO adsorption configurations related to Vo have strong interaction with the polaron introduced by Vo, which caused the maximum red shift.NO of the molecular vibrational frequency to be high enough. Some NO intermediate states were eventually converted to N_2O molecule.3.. We studied the growth of Au clusters on the TiO_2 (110) surface, the characterization of the charge state and its regulation. CO was used as a probe molecule to characterize the charge state of the Au cluster by the change of its vibrational frequency. We found that the Au clusters of the original growth of the TiO_2 (110) surface exist in the form of electrical neutrality. Through TiO_2 (1) 10) the surface 2NO to N_2O+ Oa reaction, successfully realized the regulation of Au atom from electrical neutral to positive on the Au/TiO_2 interface, reflecting a large blue shift of 20-26 cm-1 on the frequency of CO. The frequency blue shift, that is, the magnitude of the Au positive power, depends on the number of Oa atoms on the surface and the Au cluster size.4.. With the addition of the adsorption configuration and molecular molecular interaction, with the increase of CO_2 coverage, 90K, CO_2 adsorbed on Vo bit, Vo near Ti5c bit, far away from Vo Ti5c and bridge position oxygen (Obr). In conjunction with the density functional (DFT) calculation, two kinds of coupling rotor structure (CO_2) are proposed to explain the IRRAS that is resolved and polarization resolved by the directional resolution and polarization resolution of the adjacent CO_2 in two directions with the absorption peak division.CO_2 coverage of 1.5 molecular monolayers (monolayer, ML), and the CO_2 adsorbed on the Obr bit is along [1 (?) 0. ] the horizontal adsorption configuration.5. in the direction of the ZnO (10 (?) 0) surface CO_2 one-dimensional molecular chain formation and its evolution process. In the 9OK ZnO (10 (?) 0) surface CO_2 adsorption process, with the increase of the coverage, IRRAS spectrum appears a series of CO_2 absorption peak of the fine structure, combined with DFT calculation, they belong to [0001] in turn along [0001]. The CO_2 monomer, the two polymer, the trimer, and the longer molecular polymer. This novel chain growth process is caused by the Kulun attraction between the enhanced CO_2 molecule and the surface three coordination Zn atom (Zn3c) caused by the redistribution of the interface charge. Low (CO_2 coverage, the surface annealing can make the shorter molecular chain change. " When the high coverage is high, the (2 * 1) structure annealed to the local area of 150K will evolve into a local (1 * 1) structure intermediate state, which will further increase the temperature to the standard (2 * 1) structure. We have proposed a kinetic mechanism to explain the phase evolution process.
【学位授予单位】：山东大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O644.1;O647.3
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本文编号：1907826