Raman - Ellipsometry Combined System for Solid Surface Analysis

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A combined system of Raman spectroscopy and ellipsometry is developed for the study of catalyst surfaces on which a chemical reaction is taking place. Surface layers or surface species under the reaction is analyzed by ellipsometry and Raman spectroscopy, and the products in gas phase are by mass spectroscopy. A special reaction cell was designed for the optical measurement under high temperature-high pressure conditions. By an automatized rotating analyzer ellipsometer, rapid changes in surface properties of catalysts are measured on the time scale of milliseconds.The reaction, 2CO --> <CO_2> + <C_(ad)> , on Fe and Co catalysts produce several kinds of surface carbon species, whose properties were studied in situ. The Raman observation reveals that an amorphous and a glassy graphites are produced at 400 C on pre-formed Fe or Co carbide of 5 to 30 A in thickness. The state of reactive carbons in the carbides is concluded to be <Fe_3C> and <Co_2C> in the Fe and Co carbides, respectively. It is found that the amorphous graphite does not contribute to the production of hydrocarbons in the hydrogenation reaction. The characterization of states of surface hydrogen on Fe and Co catalysts is very required to understand the mechanism of the hydrogenation reaction. It is found that states of surface hydrogens around 400゜C and at about 1 atm. pressure of <H_2> gas are very different from their states at room temperature and under low pressures. They are absorbed into the catalysts to be in protonic states and surface hydrides are formed afterwards on the absorbed layer. In consequence, we arrived at a conclusion as a new hydrogenation mechanism that the surface carbon atoms are attacked from the metal side by the dissolved hydrogen to produce hydrocarbons.