Novel reactivity and physical properties on transition metal carbide surfaces by controlling atomic structures and surface electronic states

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Since the finding in 1970's that early transition metal carbides behave like noble metals rather than the native transition metals in surface catalysis, this class of materials has accepted great interests as potential substitutes for the precious noble metals in catalytic applications. The materials also show high activities for hydrodesulfurization and hydrodenitrogenation of hydrocarbons, which are relevant to environmental catalysis. Other than the catalysis, transition metal carbides of Group 4-6 have unique properties combining those of both metal and ceramics ; they are good electrical and thermal conductors typical of metals, while they possess ultra-hardness, very high melting points, and stable sub-stoichiometry typical of ceramics. Up to date, however, few groups have presented their investigations on single crystal surfaces of the chemically active transition metal carbides at atomic scale resolution which would address key issues for unique catalytic performances of the ma … More terials. We have successfully prepared well-ordered C-terminated surfaces on Mo_2C(0001) and obtained their images by scanning tunneling scanning tunneling microscopy (STM) for the first time.The structure of an α-Mo_2C(0001) surface varies depending on surface coverage of C atoms which was regulated by the annealing temperature and its period. By careful control of the preparation procedure, well-ordered C-terminated surfaces with (ν3xν3)R30-honeycomb structure and c(2x4)-zigzag row structure were successfully prepared and imaged by STM. It was revealed that each C atom inside the structures became visible as a shallow sombrero protrusion at low tunneling resistance less than 1 MΩ.Novel selective etching reaction of the surface carbon atoms by oxygen gas was found to occur even at room temperature. On a carbon terminated Mo_2C(0001) surface with both (ν3xν3)R30-honeycomb and c(2x4)-zigzag row structures, the etching reaction selectively occurred on the c(2x4)-zigzag row structure, leading to exposure of underlying (1x1) Mo layer. Less