| 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 (H2Ofree ) were detected when the total integrated intensity of COL and COB bands (denoted asθCO-IR) is 19 % that of saturated coverage, and the strong interaction and dipole coupling occurred between coadsorbates. In the potentiodynamic oxidation process, bands for H2Ofree disappeared at aθCO-IR of 40%, whereas in the potential step (to 0．65 V) oxidation process, the H2Ofree bands disappeared at theθCO-IR 60%. These facts indicate that H2Ofree molecules are actively involved in the oxidation of CO adlayer on Pt electrode. Moreover, theÏ…OH 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 H2Ofree 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 H2Ofree bands show up at a lowθCO-IR of 4%, and until saturation, the local structure of CO and H2Ofree 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 COB to COM 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 asÏ…2(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 theÏ…2(O)-NO species, whereas the latter to atop NO adsorbed on nominal Ru sites at relatively low potentials (denoted asÏ…2-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 ofÏ…2(O)-NO toÏ…2-NO species during its reduction. Rather, the reverse process may occur at higher potentials. The net accumulation of theÏ…2-NO species in Caseâ…¡resulted from the re-adsorption of NO on the nominally reduced Ru sites at lower potentials. In both cases, theÏ…2(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 (NOL), and the latter can be attributed to bridge-adsorbed NO molecules on Pt atoms (NOB). 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. NOL and NOB 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 NOL, NOB and multi-coordinated NOM, respectively. The real-time spectral results also indicated that the reduction of NO adlayer is not structure sensitive, but site sensitive. The molecules of NOL, NOB and NOM were reduced separately, with sites transfer from NOB to COM 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 H2Ofree at CO-predosed Pt group electrodes(1) The SEIRAS features of H2Ofree at CO-predosed Pt group electrodesH2Ofree coadsorbed with CO was detected on Pt, Pd, Rh and Ru electrodes byATR-SEIRAS. The band at 3631-3658 cm-1 is assigned toÏ…OH of H2Ofree, and theband at 1626-1633 cm-1 is assigned to the correspondingδHOH of H2Ofree. TheÏ…OHfrequency 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 H2Ofree 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,Ï…OH(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 (Ï…OH) and at 1637 cm-1 (δHOH) were detectedat CO-predosed Pt electrode. The first three bands are assigned to H2O coadsorbed with CO (typeâ… ), H2O coadsorbed with I (typeâ…¡), and outer-layer H2O (typeâ…¢),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, COB band increases. The broaderÏ…OHpeak 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 H2Ofree 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 .
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