The Confocal Microprobe Raman Studies Of The Electrochemical Behavior Of Benzene On Pt And Rh Electrodes

The interaction between organic compounds with transition metals has been one of the hot fields in surface sciences. With the development of modern society and the pursuit for a better life, the contamination of organic compounds, especially aromatic compounds, to the environment has been the central issue to be solved. Benzene, as the simplest aromatic molecule, has been used as a model molecule by scientists to study the interaction between aromatic compounds and transition metals. As two of the most important transition metals, Pt and Rh have very high catalytic activity for the reduction or oxidation of organic compounds. Up to date, there are abundant of reports about the oxidation, reduction and adsorption of benzene on smooth or crystal Pt and Rh surfaces investigated by electrochemical and spectroscopic methods. However, some spectroscopic methods can only work on smooth or single surfaces, which make the application the spectroscopic techniques to the study of the reaction and adsorption on rough surfaces scarcely.Since the discovery of surface-enhance Raman scattering by Fleischmann, it has provided plenty of information about the behavior of molecules at the metal/solution interface at the molecular scale, due to its high sensitivity and the ease to obtain the whole vibrational spectra. But till now, there are few report on the application of SERS to the study of benzene adsorption on Pt and Rh metal due to the low SERS activity of these two metals. The fast development of SERS in the recent years, especially the obtaining of SERS signal from many transition metals by Tian et al. with the help of the unique procedures for roughening the electrodes and confocal microprobe system, has opened bright future for the study of catalytic process occurring on transition metals.Now, it is possible to extend SERS study to molecules of very small Raman crosssection, such as benzene, on Pt, Rh electrodes. Furthermore, the confocal feature of the Raman system enables the observation of the Raman signal to be limited to a very small volume. With the TC monitor equipped with the Raman system, we are able to monitor in-situ the whole reaction process of benzene on electrode surfaces. In this paper, the reduction, substitution and adsorption behaviors of benzene on Pt, Rh surfaces has been studied by confocal microprobe Raman to understand the electrochemical behavior of benzene on platinum group metals at the molecular scale.The main results and conclusions of this thesis work are summarized as follows:1). By using the confocal microprobe Raman system capable of performing three-dimensional analysis, the reduction process of benzene into cyclohexane, which was insoluable in water and formed drops adhered to the electrode surface, was monitored. The Raman signal of benzene from solution could be detected throughout the potential region studied, indicating that the reduction of benzene on these two metals is not thorough. The catalytic activity of Rh for the electrochemical reduction of benzene is higher than that of Pt, and the roughened electrode is higher than the smooth one. Furthermore, it was found that the initial reduction potential can also be influenced by the roughness of the electrode and the acidity of the solution. The reduction of benzene can only be observed in the potential region of hydrogen evolution with the cooperative effect of the hydrogen radical generated during the hydrogen evolution process on a smooth electrode; however, benzene can be reduced with the help of the adsorbed hydrogen.2). At very positive potentials, benzene can react with chloride (bromide) ions or chloride (bromide) formed to form insoluble species and adhere onto the electrode surface as droplets. The higher relative concentration of chloride (bromide) ions to benzene would make this substitution reaction complicated. The laser has severe effect oh the electrochemical bromination, which makes this reaction irreversible. The catalytic effect of Pt for the electrochemical substitution of benzene'is higher t...