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

通过控制原子结构和表面电子态来实现过渡金属碳化物表面的新颖反应性和物理性质

基本信息

批准号：
12450019
负责人：
IWASAWA Yasuhiro
金额：
$ 9.02万
依托单位：
The University of Tokyo
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (B)
财政年份：
2000
资助国家：
日本
起止时间：
2000 至 2001
项目状态：
已结题

项目摘要

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

自20世纪70年代发现早期过渡金属碳化物在表面催化中表现得像贵金属而不是天然过渡金属以来，这类材料作为贵重贵金属的潜在替代品在催化应用中受到了极大的关注。该材料还具有较高的烃类加氢脱硫和加氢脱氮活性，这与环境催化有关。除了催化作用外，4-6族过渡金属碳化物还具有结合金属和陶瓷的独特性能；它们是典型金属的良好导电体和导热体，同时具有超硬度、高熔点和稳定的陶瓷亚化学计量。然而，到目前为止，很少有研究小组在原子尺度上对化学活性过渡金属碳化物的单晶表面进行了研究，这将解决材料独特催化性能的关键问题。我们成功地在Mo_2C（0001）上制备了有序的c端表面，并首次通过扫描隧道显微镜（STM）获得了它们的图像。α-Mo_2C（0001）表面的结构受退火温度和退火周期的影响，并与C原子的表面覆盖程度有关。通过精心控制制备过程，成功制备了具有（ν3xν3） r30 -蜂窝结构和c(2x4)-之字形结构的有序c端表面，并通过STM成像。结果表明，在隧道阻力小于1 MΩ的情况下，结构内部的每个C原子都以浅草帽状突起的形式出现。发现即使在室温下，氧也能对表面碳原子产生新的选择性蚀刻反应。在具有（ν3xν3） r30 -蜂窝结构和c(2x4)-之字形结构的碳端Mo_2C（0001）表面上，蚀刻反应选择性地发生在c(2x4)-之字形结构上，导致下伏的（1x1） Mo层暴露。少