Establishment of the soft X-ray energy dispersive XAFS method and its applications to surface chemistry

软X射线能量色散XAFS方法的建立及其在表面化学中的应用

基本信息

批准号：
14204069
负责人：
OHTA Toshiaki
金额：
$ 35.61万
依托单位：
The University of Tokyo
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (A)
财政年份：
2002
资助国家：
日本
起止时间：
2002 至 2004
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-14204069/
关键词：
XAFS surface reaction CO oxidation reaction water formation reaction surface magnetism alkanethioalte SAM depth-profiling XMCD エネルギー分散型XAFS 時間分解分子吸着 XMCD XPD

项目摘要

The purpose of this project is to improve the energy-dispersive NEXAFS method which we have developed both in energy resolution and measuring time and to apply this method to surface chemistry, surface magnetism and surface dynamics.The activities during the project are as follows ;(1)CO oxidation reaction on Pt(111). The dispersive NEXAFS method was used to trace the change of the O and CO coverages during the CO exposure on O pre-covered Pt(111). Detailed reaction rate analysis and Monte Carlo simulations have given two new findings. One is that there is another reaction path of CO oxidation as well as that proposed with the previous STM measurement Second is that CO adsorption has an effect to control the oxygen diffusion and the total reaction cannot be explained by a simple LangmuirHirishelwood reaction.(2)Water formation reaction on Pt(111). The catalytic water formation reaction was investigated by the energy dispersive NEXAFS. An oxygen covered Pt(111) surface with the (2x2) st … More ructure was exposed to gaseous hydrogen (5.0x10x9 Torr) at constant surface temperatures (120-140 K). O K-edge NEXAFS spectra were measured during the reaction with a time interval of 35 s. Quantitative analyses of the spectra provided the coverage changes of the adsorbed species (O,OH, and H_2O). The reaction is composed of three steps, which are characterized by an induction period (I), fast increase in coverage of OH and H_2O with consuming O (II), and slow conversion of OH to H_2O after the complete consumption of O (III). It was also found that the maximum OH coverage becomes smaller at a higher temperature. The kinetic Monte Carlo simulation has reproduced the three characteristic reaction steps ; in the first step OH domains are created through 2D aggregation of H_2O (I), after the nucleation process the second step sets in where the OH domains propagate by the autocatalytic cycle until they contact with each other (II), and finally the merged OH domains convert to H_2O (III). The Reaction Diffusion method was also applied to this system. It explained the reaction behavior in a wide surface area.(3)NO+N reaction on Ph(111) : By applying the dispersive NEXAFS, method, we succeeded in observing a reaction intermediate, N_2O. We also found an interesting reaction behavior : the reaction rate increases by decreasing the substrate temperature in the region of 100-370 K.(4)The adsorption structure of methylthiolate (CH_3S) self-assembled on Au(111), a long-standing controversial issue, has been unambiguously determined by scanned-energy and scanned-angle S 2p photoelectiMn difrraction. The methylthiolate molecules are found to occupy atop sites with a S-Au distance of 2.42 °A. The angular distribution of the S 2p photoelectrons due to forward scattering reveals that the S-C bond is inclined by approximately 50± from the surface normal towards both the [^-211] and [^-12^-1] (nearest-neighbor thiolate) directions.(5)A depth-resolved technique in the x-ray magnetic circular dichroism (XMCD) has been devebped and applied to Fe/Cu(100) and Fe/Ni/Cu(100) in order to observe the magnetic depth profile directly. It was confirmed that the surface two layers of the 7 ML Fe/Cu(100) are ferromagnetically coupled, while the inner layers are in the spin density wave (SDW) state at 130 K. The technique enables to extract the XMCD spectra from the surface ferromagnetic (FM) and inner SDW regfons separately, indicating that the FM/SDW interface has an antiparallel magnetic coupling, and that the SDW region has the bulk-like feature. For Fe/Ni/Cu(100), we have observed magnetically live surface layers and some thickness-dependent magnetic coupling between the Fe surface and Ni film. Less

该项目的目的是改善我们在能量分辨率和测量时间中开发的能量分散性Nexafs方法，并将这种方法应用于表面化学，表面磁和表面动力学。项目期间的活性如下；（1）PT上的CO氧化反应（111）。使用色散Nexafs方法在O预覆盖的PT上的CO暴露期间（111）追踪O和CO覆盖率的变化。详细的反应速率分析和蒙特卡洛模拟给出了两个新发现。一个是，还有另一种反应路径的CO氧化路径，以及先前的STM测量结果第二个反应路径第二是，CO添加吸附具有控制氧扩散的作用，并且总反应无法通过简单的Langmuirhirishelwood反应来解释。（2）PT（111）。通过能量分散性Nexafs研究了催化水的形成反应。在恒定表面温度（120-140 K）下，将氧覆盖的氧覆盖的Pt（111）表面与（2x2）ST…更大的规则暴露于气态氢（5.0x10x9 torr）。在反应期间以35 s的时间间隔测量了o k边缘Nexafs光谱。对光谱的定量分析提供了吸附物种的覆盖率变化（O，OH和H_2O）。该反应由三个步骤组成，其特征在于诱导周期（i），OH和H_2O的覆盖率快速增加，而O（ii）的覆盖范围以及在O（iii）完全消耗后，OH转换为H_2O的缓慢转化为H_2O。还发现，在较高温度下，最大OH覆盖率变小。动力学蒙特卡洛模拟已重现了三个特征反应步骤。在第一步中，OH域是通过h_2o（i）的2D聚集创建的，在成核过程之后，第二步设置在OH域通过自动催化循环传播，直到它们相互接触（II），最后是合并的OH域转换为H_2O（III）。反应扩散方法也应用于该系统。（3）pH（111）上的无+n反应：通过应用分散性nexafs，方法，我们成功地观察了一个反应中间体N_2O。 We also found an interesting reaction behavior : the reaction rate increases by decreasing the substrate temperature in the region of 100-370 K.(4)The addsorption structure of methylthiolate (CH_3S) self-assembled on Au(111), a long-standing controversial issue, has been unambiguously determined by scanned-energy and scanned-angle S 2p photoelectiMn diflation.发现甲基硫醇分子在S-AU距离为2.42°A的位置上占据。由于向前散射，S 2P光电子的角度分布表明，S-C键从正常的表面倾向于[^-211]和[^-12^-1]和[^-12^-1]（最近的邻苯甲酸酯）方向。和Fe/Ni/Cu（100），以直接观察磁深度曲线。已经证实，在130 K处的7 ml Fe/Cu（100）的表面两层是铁磁耦合的，而内层则处于130 K处的自旋密度波（SDW）状态。该技术可以从表面铁磁性（FM）和内部的SDW统治中提取XMCD光谱，以使XMCD频谱从表面上磁磁进行了。耦合，并且SDW区域具有散装的特征。对于Fe/Ni/Cu（100），我们已经观察到磁性活的表面层以及Fe表面和Ni膜之间的厚度依赖性磁耦合。较少的