【作者】 严彦刚；

【导师】 蔡文斌；

【作者基本信息】 复旦大学 ， 物理化学， 2007， 博士

【摘要】 电化学是研究电和化学反应相互关系的科学，它主要研究电能与化学能相互转化的过程。探明电极／电解液界面的基本物理化学性质是认识、控制和利用电化学过程的基础。电催化是表面电化学研究的重要方面，其研究热点包括燃料电池相关的CO、甲醇等催化氧化反应，以及环保相关的NO催化还原反应等。由此，对具有高催化活性的Pt族金属电极表面上的吸附结构与反应过程进行多角度和多层次的详尽分析研究，将具有重要的科学意义和应用前景。近三十年来，为了从分子水平上深化对电化学界面的认识，各类谱学技术和扫描探针技术被引入到原位（in-situ）电化学研究中，使电化学研究从宏观向微观、由统计平均向分子水平不断发展；以此同时，时间分辨的动力学研究也进入亚微秒的水平。在这些现场方法中，表面增强振动光谱尤其是表面增强拉曼光谱（SERS）的贡献巨大，特别是近十年由于高性能拉曼谱仪的逐渐普及以及特殊纳米结构金属表面的涌现，电化学SERS被广泛应用于电极／电解质界面结构和性质的研究。以此相对比，比它晚出现的表面增强红外光谱（SEIRAS）由于其表面增强因子远小于SERS，一直没有受到广泛的关注而发展较慢，无论是其技术本身的进步及在表面电化学中的应用都远未达到与SERS可比的程度。实际上，红外吸收截面数量级为10^-20 cm² molecule^-1，而拉曼散射截面数量级为10^-29cm² molecule^-1，因而SEIRAS的检测灵敏度与SERS相当，某些时候甚至更高。特别是SEIRAS几乎可以在所有过渡金属表面均能获得与银、金、铜表面相近的增强效应，同时其表面选率简单，因而应具有更大的发展空间和新的应用前景。电化学衰减全反射表面增强红外光谱（ATR-SEIRAS），可以克服外反射模式红外光谱（IR-RAS）测量中的薄层电解质结构的诸多限制，有利于检测电极表面的吸附和反应，尤其适合于电极表面极性较高的小分子不可逆反应过程的时间分辨研究。将ATR-SEIRAS用于研究Pt族金属的电催化吸附与反应，可提供其它原位谱学技术难于提供的诸如参与界面反应的共吸附水等重要信息。如何制备具有SEIRA效应的金属薄膜是SEIRAS发展的瓶颈，对此我们提出了简易的“两步湿法镀膜”策略，使电化学ATR-SEIRAS技术全面拓展到Pt族金属表面。同时，由于该方法制备的金属膜为纳米颗粒金属薄膜，其表面结构比本体电极更接近实际催化体系。使用ATR-SEIRAS方法研究Pt族金属纳米薄膜电极表面的CO和甲醇氧化、NO还原以及电极界面共吸附水结构等，应有特别的参考价值。本论文主要围绕以下工作进行研究：自行设计了电化学原位-衰减全反射-表面增强红外光谱（ATR-SEIRAS）附件系统；将SEIRAS较全面地拓展到Pt族金属表面，并将其应用于CO和甲醇的吸附与氧化反应、NO的吸附及还原反应、共吸附水分子的可能界面结构，以及芳香小分子的吸附构型等研究。另外，初步研究了质子交换膜燃料电池（PEMFC）膜电极（MEA）的制备对单体电池性能的影响。具体主要内容和结果如下：1．提出“两步湿法镀膜”作为通用策略，制备具有SEIRA效应的Pt族纳米金属薄膜，为在Pt族金属电极开展电化学ATR-SEIRAS研究奠定了基础采用了简便易行的“两步湿法镀膜”，即在红外窗口Si半圆柱的底面上先通过化学镀镀上一层Au膜，然后采用合理电沉积方式在其上覆盖Pt族金属外层（Pt、Pd、Rh或Ru），成功地制备了具有SEIRA效应的Pt族金属纳米薄膜电极。使用电化学方法对上述Pt族金属（Pt、Pd和Rh）纳米薄膜进行测试，结果表明，镀层几乎无针孔，而且所得的各Pt族金属（Pt、Pd、Rh和Ru）纳米薄膜的电化学特征行为与本体金属一致，从而为与本体纯金属有可比性的电化学性质研究奠定了基础。另一方面，应用CO作为探针分子，对上述Pt族金属纳米薄膜的表面增强红外效应以及它在金膜基底上的覆盖程度进行了测试。结果显示该方法制备的Pt族金属纳米薄膜不仅几无“针孔”而且呈现极强SEIRA效应，增强效果超过其它方式制得的金属薄膜。同时吸附物种的吸收峰方向与定义的一致，不发生倒转，峰形单一，不出现所谓的“类-Fano”非对称双极峰扭曲现象，便于光谱的分析和解释，提高了结果的准确性。这些特性对现场ATR-SEIRAS的拓展应用至关重要。最后需指出的是，该“两步湿法镀膜”，作为一种通用的方法应用于在其它更多的过渡金属电极的ATR-SEIRAS研究，相关工作已由本实验室的其他同学相继展开。2．应用表面增强红外光谱研究燃料电池相关电催化基础反应（1）CO在Pt电极表面的吸附及氧化（a）UPD氢区0.1V下Pt电极表面CO的吸附及其氧化行为0.1 M HClO₄溶液中，Pt电极上CO吸附开始时较分散，随着CO总覆盖度增加，CO的局部浓度开始升高，当COB和COL峰的红外总吸收达饱和吸附时的红外总吸收的19％左右(简称θ_CO-IR)，发生界面自由水的共吸附（简称H₂O_共）。之后各分子之间发生相互作用，CO_B和CO_L发生强烈偶合，待CO吸附饱和时，形成了一个CO和H₂O共的稳定共吸附层。该条件下形成的CO吸附层在氧化过程中，其H₂O共均在CO氧化完成之前消失，对应的θ_IR要比吸附过程的高得多。若采用扫描电位方式氧化，H₂O共消失时对应的θ_CO-IP约为40％；如采用电位阶跃氧化，相应的θ_CO-IR约为60％。以上CO生成和氧化过程的θ_CO-IR的差异，意味着这些H₂O共优先参与了反应。动电位扫描过程，在0.54 V之前不发生CO氧化，而在0.45 V左右开始出现微弱的阳极电流，伴随着H₂O共的ν_OH的同步缓慢减弱，由此可推测此阳极电流弱峰可能是H₂O共在活性位上的电活化所致，这种预活化的水将直接参与CO的氧化。在0.1 V预吸附CO层的氧化过程是一个整体结构崩溃的过程，即各共吸附组分（CO_L、CO_B和H₂O共）的局部覆盖度从氧化反应被激活之后就开始快速降低。反应机制更适合用“平均场近似模型”（Mean field approximation model）来描述。（b）双层区0.45 V下Pt电极表面CO的吸附及其氧化行为与前述吸附过程不同的是，该电位下CO在Pt电极上的吸附一开始时就表现为聚集吸附的过程，即虽然CO总体覆盖度不高，但是局部的CO浓度较高，几乎一开始吸附就检测到H₂O共。当CO吸附达到饱和时，CO的局部浓度也没有发生太大的改变，吸附过程类似“成核生长”，形成CO和水的共吸附层结构较稳定。氧化过程中各吸附物种几乎同步减小和消失，各谱峰形状、频率及半峰宽几乎不变。由此推测这一过程更适合于用“成核生长模型”（Nucleation and growth model）来描述。总的来看，Pt电极表面CO氧化过程依赖于CO的吸附电位区（UPD氢区或双层区），并且CO的氧化过程动力学特征与其对应的吸附过程的动力学特征具有相似性。（2）CO在Rh或Pd电极及CH₃OH在Pt电极表面氧化初探在0.0 V预吸附的CO在Rh电极氧化过程类似于0.45 V预吸附Pt电极上CO的氧化过程；而在Pd电极上CO氧化开始时各谱峰的频率、半峰宽变化不大，但后期可发生CO_B向CO_T转移；结合电化学循环伏安扫描和ATR-SEIRAS光谱同步测量，获得了甲醇在Pt电极表面按双路径机理氧化及不同电位区间的表面吸附物种的信息。3．利用ATR-SEIRAS研究NO在Pt族电极表面的吸附和还原行为（1）NO在Ru电极表面的吸附和还原研究Ru电极预吸附NO在0.1 M HClO₄中只呈现1874-1840 cm^-1的谱峰，归属为与氧共吸附的线性NO或吸附在Ru氧化物表面的NO（表示为v₂（O）-NO物种）。在负向电位扫描过程中，并未发现NO吸附在还原态Ru表面的红外吸收峰（v₂-NO），这说明包括NO分子和共吸附氧物种在内的v₂（O）-NO物种作为整体被还原的，即并非分开独立进行还原的。光谱结果还表明，v₂（O）-NO物种在还原过程中可能存在快速扩散的行为；或者在整个电极表面的任何区域的v₂（O）-NO物种同时发生还原反应，导致v₂（O）-NO物种的覆盖度在还原过程中表现为快速均匀的降低过程。Ru在溶液中含有HNO₂的0.1 M HClO₄条件下，SEIRAS光谱中可以观察到1886-1850 cm^-1左右的v₂（O）-NO物种的红外吸收峰，以及1520-1578 cm^-1左右的与氧共吸附的三重吸附NO（v₁（O）-NO）的红外吸收峰。在负电位区观测到的1740-1790 cm^-1的谱带可归属为NO线性吸附在还原态Ru上v₂-NO物种。在阳极扫描的0.0到0.3 V之间v₂（O）-NO峰的积分强度为零，这说明在该条件下Ru表面氧化物在0.0到0.3 V之间几乎可以完全被还原。当电位高于0.35 V，电极表面开始生成氧化物种，v₂-NO物种随即开始转换为v₂（O）-NO物种。（2）预吸附单层NO在Pt电极表面的吸附和还原研究SEIRAS光谱研究发现了1760-1737 cm^-1和1609-1524 cm^-1的两个谱带。前者归属为线性吸附的NO分子（NOL）的红外吸收峰；后者可归属为双桥式吸附的NO分子（NO_B）的红外吸收峰。试验结果表明，在电化学条件下，预吸附NO的还原并不是一个结构敏感反应，而是一个吸附位敏感的还原反应。即NO_L和NO_B的还原是完全分步的过程，并且在还原过程中并没有发生吸附位的转移现象。（3）预吸附单层NO在Pd电极表面的吸附和还原研究在Pd电极表面可获得三个NO的红外吸收峰：1770-1724 cm^-1可归属为线性吸附的NO分子（NO_L）的红外吸收峰；1690-1572 cm^-1可归属为双桥式吸附的NO分子（NO_B）的红外吸收峰。1548-1487 cm^-1为三重吸附的NO分子（NO_M）的红外吸收峰。实验结果还表明，在电化学条件下，Pd电极表面预吸附NO还原也是一个吸附位敏感的还原反应，即NO_L、NO_B和NO_M还原是分步过程，但是在NO_B的还原过程中，明显地发生从NO_B到NO_M吸附位的转变。4．芳香小分子在Pt电极表面的吸附行为的增强红外光谱研究研究了HClO₄溶液中对硝基苯甲酸（PNBA）和KClO₄溶液中吡啶（Py）在Pt电极表面的吸脱附。研究结果表明在较高电位下（约0.3～0.7 V）PNBA是通过-COOH脱去质子之后的-COO^-吸附在Pt电极表面，并且两个氧原子几乎是等位地吸附在Pt电极表面，分子平面几乎垂直或以某小角度倾斜于电极表面。PNBA从约0.35 V以负电位开始脱附，脱附中间电位为0.2V。在0.1～-0.1 V区间，吸附PNBA分子中羧基的一个氧原子可能脱离电极表面，另一个氧原子可能以某种方式与Pt-H表面相结合。至于吡啶在Pt电极上的吸附，在相当宽的电位区间内（0.4～-0.4 V），吡啶在Pt电极上的吸附主要是通过N原子的孤对电子及脱氢后的α碳原子共同与Pt电极表面成键，并且分子平面与电极表面几乎是垂直或以某一小角度倾斜于电极表面，即采取edge-on而非end-on吸附构型。5．Pt族电极界面H₂O共分子结构的SEIRAS初探（1）Pt族电极界面与CO共吸附的H₂O共的ATR-SEIRAS特征我们在Pt、Pd、Rh和Ru电极表面都获得了IRAS难于检测到的H₂O共振动谱峰。3631-3658 cm^-1谱带被指认为共吸附单体水（水分子摆脱了通常的氢键作用，也称为自由水）的伸缩振动ν_OH，而1626-1633 cm^-1为剪式振动峰δ_（HOH）。当电位正移至使CO氧化脱附的电位时，此单体水的特征峰也消失。这其中，电极材料较明显地影响ν_（OH）的振动频率，如ν_（OH）分别为3643 cm^-1（Ru）、3631 cm^-1（Rh）、3648cm^-1（Pd）和3658 cm^-1（Pt）；而δ_（HOH）频率受电极材料影响小，均在1630 cm^-1附近。δ_（HOH）谱峰易受到近电极表面其它水分子的干扰。（2）溶液中的卤素离子对Pt电极上CO和H₂O共吸附结构的影响CO氧化之前，加入Cl^-对CO和H₂O共峰影响较小。只有在发生CO氧化后才观察到与Cl^-共吸附水的吸收峰，其ν_（OH）和δ_（HOH）分别位于3568和1622cm^-1。Cl^-的竞争吸附一定程度上抑制了CO的氧化过程。Ⅰ^-的加入在红外谱图上出现了3652 cm^-1、3610 cm^-1、3485 cm^-1的ν_（OH）及1637cm^-1左右的δ_（HOH）吸收峰。3652 cm^-1的谱峰可归为与CO共吸附的‘Ⅰ’类自由水的ν_O-H；3610 cm^-1为与Ⅰ^-离子共吸附的‘Ⅱ’类自由水ν_（O-H）；3485 cm^-1为在‘Ⅰ’和‘Ⅱ’类水外围更多的还有氢键作用的‘Ⅲ’类水分子。而1637 cm^-1应更多的为‘Ⅲ’水的δ_（HOH）贡献。Ⅰ^-的强吸附导致部分CO非氧化性脱落。（3）极端析氢电位条件下Pt电极表面的CO和水共吸附结构初探0.1 V或0.45 V下预吸附的CO，在经历强烈析氢后氧化的动力学过程分别与预吸附的CO直接进行氧化的动力学过程一致，说明CO与水的共吸附层结构稳定，以及氢析出主要发生在CO吸附层的缺陷空位Pt原子。在强析氢的条件下，CO_B和H₂O共的谱峰明显改变。对H₂O共而言，δ_HOH的同时随CO_B谱峰增加，而ν_OH的强度几乎不变。在更负的电位条件下，大量的气泡甚至可导致界面水部分脱离电极表面的现象，并造成了少部分CO_L和少部分H₂O共脱落。电位正扫，H₂O共依然存在，直到CO彻底氧化为止。δ_HOH的振动峰提前消失，表明其对外层环境的影响更敏感。在更负电位条件下，高频段(ν_OH)的谱峰拟合得到3640 cm^-1、3498 cm^-1和3373 cm^-1的三个谱峰。这三个谱峰分别对应于自由水H₂O共、紧邻外层水（氢键部分解离）及外层类冰结构的界面水。6．低Pt载量的质子交换膜燃料电池的研制在新的还原体系中合成了高分散、Pt粒径2-5 nm、高催化活性的Pt／C催化剂。开发了低载铂量(0.15 mg cm^-2)、超薄MEA（催化层厚10μm左右）制备技术，从而使电池性能有较大改进(自制催化剂在：室温；H₂：0.03 MPa；O₂：0.05 MPa；载铂0.15mg cm^-2的条件下，电池单体功率密度达0.17 W cm^-2。更多还原

【Abstract】 Electrochemistry is the branch of chemistry concerned with the interrelation of electrical and chemical effects. A large part of this field deals with the study of chemical changes caused by the passage of an electric current and the production of electrical energy by chemical reactions. Special attention has been focusing on the physical and chemical properties of the interface of electrode/electrolyte. The electrocatalysis is an important interrelating area of surface electrochemistry and surface catalysis. Its main topics include the electrocatalytic oxidation of CO & methanol which is related to fuel cell fundamentals and applications, and the reduction of NO which is of environmental concerns. Thus, the studies of adsorptions and reaction processes of CO, NO and methanol on Pt group metal electrodes will be of great significance in both scientific and application aspects.In the past 30 years, the optical spectroscopy and scanning probe technology （SPM） have been applied in electrochemical study in order to get the interface information at the molecular level. The improvement in instrumental performances and analytical methodology promotes electrochemical research from macroscopic to microscopic, from statistic average to molecular level and into the dynamics atμs time-resolution. In particular, pushed by the advent of new-generation high throughput confocal Raman microprobe and the delicate construction of nano-templates in last decade, surface-enhanced Raman spectroscopy （SERS） has been extensively and intensively applied in probing electrode/electrolyte interfaces. By contrast, as its counterpart and complementary technique, surface-enhance IR absorption spectroscopy （SEIRAS） received far less attention and development. A possible misunderstanding causing this kind of imbalance can be attributed to low enhancement factor for SEIRAS (ca. 10-10³ as compared to that of ca. 10⁵ to 10¹² for SERS). In fact, under most circumstances, the detection sensitivity of SEIRAS is essentially comparable to, and even higher than that of SERS, because the infrared absorption cross section and Raman scattering cross section are at the orders of 10^-20 and 10^-29 (cm² molecule^-1 ), respectively. Strong SERS enhancement is limited to silver, gold, copper and alkali metals, but similar SEIRAS enhancement can be available for nearly all metals. Consequently, SEIRAS should be promising for further development and broader application. Free from problems caused by the thin-layer structure of conventional IRRAS, SEIRAS with Kretschmann ATR configuration （ATR-SEIRAS） is a powerful tool for the studies of adsorptions and reactions at electrodes, especially in real-time monitoring irreversible reactions involving highly polar small molecules. Application of ATR-SEIRAS to Pt group metal electrodes will provide special insights to the interfacial structures and reaction mechanisms of interested electrocatalytic systems. The prerequisite of applying ATR-SEIRAS to surface electrocatalysis is the appropriate construction of the SEIRA-active Pt group metal electrodes consisting of conductive nanoparticle films. The structure of as-prepared nano-particles is closer to that of actual catalysts, as compared to previous bulk materials, and the results yielded for CO & methanol oxidation and NO reduction should have more specific significance.This thesis focuses on the following aspects: Building up a set of custom-made system for in-situ ATR-SEIRAS; Presenting a versatile two-step wet process to construct Pt, Pd, Rh, and Ru nanoparticle films on Si prism for electrochemical ATR-SEIRAS study; In-situ ATR-SEIRAS investigation of CO and methanol adsorption and oxidation, NO adsorption and reduction, adsorption configuration of small aromatic molecules, and interfacial structure of coadsorbed free water; Preparing low-Pt loading membrane electrode assembly （MEA） and testing the PEMFC performance. The main results and conclusions are summarized as follows:1．A versatile two-step wet process to fabricate Pt, Pd, Rh, and Ru nanofilm electrodes for in-situ ATR-SEIRAS study:A versatile two-step wet process to fabricate Pt, Pd, Rh, and Ru nanoparticlefilms for electrochemical ATR-SEIRAS study is presented, which incorporates aninitial chemical deposition of a Au underlayer on the basal plane of a silicon prismwith the subsequent electrodepostion of desired platinum group metal overlayers.Galvanostatic electrodeposition of Pt, Rh, and Pd from phosphate or perchloric acidelectrolytes, or potentiostatic electrodeposition of Ru from a sulfuric acid electrolyte,yields sufficiently "pinhole-free" overlayers as evidenced by electrochemical andspectroscopic characterizations. The Pt group metal nanofilms thus obtained exhibitstrongly enhanced IR absorption. In contrast to the corresponding metal filmselectrochemically deposited directly on glassy carbon and bulk metal electrodes, theobserved enhanced absorption for the probe molecule CO exhibits normal unipolarband shapes. This ubiquitous strategy is expected to open a wide avenue for extending ATR SEIRAS to explore molecular adsorption and reactions on technologically important transition metals.2．A study of oxidation of CO and methanol at Pt group nanofilm electrodes by in-situ ATR-SEIRAS（1） Adsorption and oxidation of CO at Pt electrode in 0．1 M HClO₄In the case of CO adsorption at 0．1 V （vs. SCE）, the increase of local surface concentration of CO adlayer is relatively slow. The coadsorbed water molecules (H₂O_free ) were detected when the total integrated intensity of CO_L and CO_B bands (denoted asθ_CO-IR) is 19 % that of saturated coverage, and the strong interaction and dipole coupling occurred between coadsorbates. In the potentiodynamic oxidation process, bands for H₂O_free disappeared at aθ_CO-IR of 40%, whereas in the potential step （to 0．65 V） oxidation process, the H₂O_free bands disappeared at theθ_CO-IR 60%. These facts indicate that H₂O_free molecules are actively involved in the oxidation of CO adlayer on Pt electrode. Moreover, theυOH band gradually weakens its intensity in response to the anodic prewave ranging from 0．45 to 0．54 V, which may be caused by the electro-activation of H₂O_free at some active sites before it reacts with CO to formed CO₂．The oxidation process of CO adlayer preformed at 0．1 V results in collapse of the entire structure, which may be better described with the so-called ’mean field approximation’ model.In the case of CO adsorption at 0．45V, the CO molecules packed locally at the beginning of absorption process, as demonstrated by the fact that H₂O_free bands show up at a lowθ_CO-IR of 4%, and until saturation, the local structure of CO and H₂O_free remains rather stable as judged by the insignificant changes of band positions. The oxidation process of CO adlayer preformed at 0．45 V may proceed via "nucleation and growth" mechanism.（2） Oxidation of CO on Rh and Pd electrodes & methanol oxidation on Pt electrodeThe results showed that the oxidation process of CO adlayer on Rh electrode was similar to that of CO predosed at 0．45 V on Pt electrode in 0．1 M HClO₄．In the potentiodyanmic scan of CO predosed Pd electrode, CO oxidation proceeds in the potential region 0．9～0．96 V does not change significantly the band frequencies and widths of adsorbates, and the transformation of CO_B to CO_M occurred in the late oxidation process. The in-situ time-resolved spectra indicate that methanol was oxidized through two-pathway mechanism on Pt electrode.3．A study of adsorption and reduction of NO at Pt group electrode surface:（1） Adsorption and reduction of NO at Ru electrodeFor a NO-predosed Ru electrode, only one band located at 1840-1874 cm^-1 was detected in 0．1 M HClO₄, attributable to atop NO coadsorbed with oxygen-containing species （denoted asυ₂（O）-NO species）. For a Ru electrode in 0．1 M HClO₄ containing 20 mM NaNO₂, two IR bands located at 1850-1886 cm^-1 and 1740-1790 cm^-1 were observed. The former, predominant at relatively high potentials, is ascribable to theυ₂（O）-NO species, whereas the latter to atop NO adsorbed on nominal Ru sites at relatively low potentials （denoted asυ₂-NO species）. In addition, a very weak band at 1520-1578 cm^-1 may be assigned to multi-coordinated NO coadsorbed with oxygen-containing species.The real-time spectral results suggest that the reduction of NO molecules and the coadsorbed oxygen-containing species proceed simultaneously rather than separately. No evidence was found for the conversion ofυ₂（O）-NO toυ₂-NO species during its reduction. Rather, the reverse process may occur at higher potentials. The net accumulation of theυ₂-NO species in CaseⅡresulted from the re-adsorption of NO on the nominally reduced Ru sites at lower potentials. In both cases, theυ₂（O）-NO species at Ru electrode can start to be reduced without the need of thecomplete removal of surface oxides.（2） Adsorption and reduction of NO adlayer at Pt electrodeTwo NO bands at 1760-1737 and 1609-1524 cm^-1 were obtained at NO-predosed Pt electrode. The former is assigned to linear adsorbed NO molecules on Pt atoms （NO_L）, and the latter can be attributed to bridge-adsorbed NO molecules on Pt atoms （NO_B）. The in-situ time-resolved spectral results indicated that the electroreduction of NO was not a structure-sensitive reaction but a site-sensitive reaction at Pt electrode. NO_L and NO_B were reduced separately, and the adsorptionsites transfer did not occurred in the reduction.（3） Adsorption and reduction of NO adlayer at Pd electrodeThree IR bands of NO at 1770-1724 cm^-1 , 1690-1572 cm^-1 and 1548-1487 cm^-1 were detected at NO-predosed Pd electrode which can be attributed to NO_L, NO_B and multi-coordinated NO_M, respectively. The real-time spectral results also indicated that the reduction of NO adlayer is not structure sensitive, but site sensitive. The molecules of NO_L, NO_B and NO_M were reduced separately, with sites transfer from NO_B to CO_M occurring.4．A study of electroadsorption of aromatic molecules on Pt electrodes with ATR-SEIRAS:In situ ATR surface-enhanced IR absorption spectroscopy （ATR-SEIRAS） hasbeen applied to study the adsorption of p-nitrobenzoic acid （PNBA） in 0．1 M HClO₄or pyridine （Py） in 0．1 M KClO₄ on Pt electrodes. The results indicate that adsorptionof PNBA at positive potential of 0．3 V vs. SCE yielded p-nitrobenzoate species boundto the surface through the carboxylate oxygen atoms with a bridging coordination.The PNBA was desorbed gradually as the potential shifted negatively, and mightadopt single oxygen-atom coordination to H-bonded Pt surface. The transitionpotential for electroadsorption of PNBA on Pt electrodes was centered ca 0．2 V vs.SCE. As for the adsorption of Py on Pt electrodes, spectroscopic evidence pointed tothe formation ofα-pyridyl species nearly perpendicular to the Pt surface. Over thepotential range between-0．4 and 0．4 V vs. SCE, the configuration and orientation ofadsorbed Py remained virtually unchanged.5．Interfacial structure of H₂O_free at CO-predosed Pt group electrodes（1） The SEIRAS features of H₂O_free at CO-predosed Pt group electrodesH₂O_free coadsorbed with CO was detected on Pt, Pd, Rh and Ru electrodes byATR-SEIRAS. The band at 3631-3658 cm^-1 is assigned toυOH of H₂O_free, and theband at 1626-1633 cm^-1 is assigned to the correspondingδHOH of H₂O_free. TheυOHfrequency is sensitive to electrode material, with 3643 cm^-1 for Ru, 3631 cm^-1 for Rh,3648 cm^-1 for Pd and 3658 cm^-1 for Pt at saturated CO coverage. But theδHOHfrequency is nearly independent of electrode materials, rather it is affected by otherwater molecules.（2） The effect of halide anions on the interfacial H₂O_free at CO-predosed PtelectrodeIn the presence of Cl in solution, spectral results showed only when CO adlayerwas stripped or partially stripped did the Clˉadsorb on Pt electrode. Two bands,υOH(3568 cm^-1 ) andδHOH (1622 cm^-1 ), emerged with the adsorption of Clˉ. The adsorbedClˉinhibited CO oxidation to some extent. In the presence of Iˉin solution, IR bandsat 3652 cm^-1 , 3610 cm^-1 and 3485 cm^-1 （υOH） and at 1637 cm^-1 （δHOH） were detectedat CO-predosed Pt electrode. The first three bands are assigned to H₂O coadsorbed with CO （typeⅠ）, H₂O coadsorbed with I （typeⅡ）, and outer-layer H₂O （typeⅢ）,respectively. Stronger adsorption of I at Pt electrode may expel partially CO adlayer.（3） CO-predosed Pt electrode at very negative potentialsAfter being subjected to very negative potential excursion, CO-predosed ateither 0．1 V or 0．45 V exhibits same oxidation kinetics. The hydrogen evolutionreaction （HER） occurs probably at defects sites of the adlayer. The entire spectrum ofthe infrared spectra of interface water can be detected under HER condition with theATR configuration. At strong HER potentials, CO_B band increases. The broaderυOHpeak can be separated into three peaks at 3640, 3498 and 3373 cm^-1 by fitting. Thebands 3640 cm^-1 and 3498 cm^-1 are assigned to H₂O_free with CO and outer layer H₂Owith partially dissociated hydrogen bonds, respectively. The 3373 cm^-1 band iscontributed from the bulk water of ice-like structure.6．Preparation of the membrane electrode assembly （MEA）The highly dispersed Pt/C catalyst with Pt particle sizes of 2-5 nm was synthesized in the new solution system. A special process of preparing MEA with a low loading of Pt (ca. 0．15 mg cm^-2 ) and ultrathin catalytic layer （ca. 10μm thickness） was developed, and the battery performance was measured at room temperature (with the conditions: H₂ 0．03 MPa, O₂ 0．05 MPa, Pt 0．15 mg cm^-2 ) to yield a power density of 0．17 W cm^-2 .更多还原