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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

本项目的主要目的是改进我们已经发展起来的能量色散NEXAFS方法的能量分辨率和测量时间，并将其应用于表面化学、表面磁性和表面动力学等方面的研究，具体包括：（1）Pt（111）表面上CO的氧化反应。采用分散NEXAFS方法追踪了O预覆盖Pt（111）上CO暴露过程中O和CO覆盖度的变化。详细的反应速率分析和蒙特卡罗模拟给出了两个新的发现。第二是CO吸附对氧的扩散有控制作用，整个反应不能用简单的Langmuir-Hirishelwood反应来解释。（2）Pt（111）上的水生成反应。用能量色散NEXAFS研究了催化水生成反应。在Pt（111）表面制备了氧覆盖的（2 × 2）Pt（111）表面， ...更多信息在恒定的表面温度（120-140 K）下将结构暴露于气态氢（5.0 × 10 × 9托）。在反应期间以35 s的时间间隔测量K边NEXAFS谱。光谱的定量分析提供了吸附物种（O，OH和H_2O）的覆盖度变化。该反应由三个步骤组成，其特征是：有一个诱导期（I），随着O的消耗，OH和H_2O的覆盖度迅速增加（II），当O完全消耗后，OH缓慢转化为H_2O（III）。研究还发现，在较高的温度下，最大OH覆盖率变得较小。动力学Monte Carlo模拟再现了反应的三个特征步骤：第一步是通过H_2O的二维聚集产生OH畴（I），成核过程后的第二步是OH畴通过自催化循环传播，直到它们彼此接触（II），最后合并的OH畴转化为H_2O（III）。反应扩散方法也被应用到这个系统。它解释了在宽表面积的反应行为。(3)NO Ph（111）上的+N反应：用色散NEXAFS方法成功地观察到反应中间体N_2O。我们还发现了一个有趣的反应行为：在100-370 K范围内，降低衬底温度，反应速率增加。（4）用扫描能和扫描角S2 p光电衍射方法，明确地确定了甲硫醇（CH_3S）在Au（111）表面自组装的吸附结构。甲硫醇分子占据了顶部位置，S-Au距离为2.42 °A。由于前向散射产生的S 2 p光电子的角分布表明，S-C键从表面法线向[^-211]和[^-12^-1]（最近邻硫醇盐）方向倾斜约50±。(5)A本文提出了X射线磁性圆二色性（XMCD）中的深度分辨技术，并将其应用于Fe/Cu（100）和Fe/Ni/Cu（100）中，以直接观察磁性深度分布。结果表明，在130 K时，7 ML Fe/Cu（100）薄膜的表层两层为铁磁耦合，内层为自旋密度波（SDW）态。该技术能够分别从表面铁磁（FM）和内部SDW区域提取XMCD谱，表明FM/SDW界面具有反平行磁耦合，并且SDW区域具有块状特征。对于Fe/Ni/Cu（100），我们观察到了磁性活性表面层以及Fe表面和Ni膜之间的一些厚度依赖的磁耦合。少