Dissociation reactions of CO Gas on Fe and Fe3O4 surfaces observed by Raman-ellipsometry spectroscopy

A combined system of Raman spectroscopy and ellipsometry is developed for the study of catalyst surfaces on which a chemical reaction is taking place. The dielectric function and the thickness of a surface layer or a surface compound produced in the reaction are analyzed by ellipsometry; vibrational modes of the compound and surface species are analyzed by Raman spectroscopy, and the products in the gas phase by mass spectrometry. The system is applied to the study of the Boudouard and dissociation reactions of CO molecules on Fe and Fe₃O₄ catalysts. A surface layer produced by diffused C and O atoms, a thin oxide layer produced on the Fe by the CO dissociation reaction, and graphite layers produced by the Boudouard reaction are analyzed. Raman spectra reveal the existence of several kinds of carbon species such as defective graphite and an Fe carbide formed on the Fe and Fe₃O₄ surfaces. Specific Raman bands at 1121 and 1147 cm^-1 are tentatively concluded to be produced from microscopic clusters of surface graphite. The reaction of the defective graphite with H₂ gas is found to be stepwise on a time scale of seconds.