Tanaka Ken-Ichi, Xie Zhao-Xiong
Advanced Science Research Laboratory, Saitama Institute of Technology, 1690 Fusaiji, Okabe, Saitama, Japan.
J Chem Phys. 2005 Feb 1;122(5):54706. doi: 10.1063/1.1825376.
CH(3)OH undergoes dissociation on a Si(111)-7 x 7 surface via a two dimensionally free precursor. The sticking probability attained by the STM (scanning tunneling microscopy) was entirely coverage independent, where the observed image represented the final state of the adsorption. CH(3)OH dissociates equally on the faulted and unfaulted halves at room temperature. However, the dissociation at the center adatom-rest atom site is four times preferential to that at the corner adatom-rest atom site in each half unit cell. Such site selectivity, center/corner, changes with the occupation of adatoms in corresponding half unit cell, that is, center/corner=4 for the half unit cell with one reacted adatom, but 2.6 and 1.8 for the half unit cells with two and three reacted adatoms, respectively. Such site selectivity is well rationalized by the dissociation depending on the local conformation of the site instead of the local density of states (LDOS). The site selectivity of center/corner is well reproduced by considering the occurrence probability of the whole dissociation pattern. As the STM image represents the final state of the adsorption, if the final step of adsorption involves dissociation of molecule or precursor, the STM image reflects the dissociation probability depending on the local structure. On the other hand, if no dissociation of molecule or precursor is involved at the final step, the adsorption probability might depend on the LDOS. The adsorption of H(2)S, H(2)O, and NH(3) is also discussed from this general viewpoint of adsorption. The concept of a two dimensionally free precursor will be important to understand the kinetics of heterogeneous catalysis.
CH₃OH通过二维自由前驱体在Si(111)-7×7表面发生解离。扫描隧道显微镜（STM）测得的 sticking概率完全与覆盖度无关，其中观察到的图像代表吸附的最终状态。CH₃OH在室温下在有缺陷和无缺陷的两半上均等解离。然而，在每个半晶胞中，中心吸附原子 - 静止原子位点的解离比角部吸附原子 - 静止原子位点的解离优先四倍。这种中心/角部的位点选择性随相应半晶胞中吸附原子的占据情况而变化，即对于有一个反应吸附原子的半晶胞，中心/角部 = 4，但对于有两个和三个反应吸附原子的半晶胞，分别为2.6和1.8。这种位点选择性通过取决于位点的局部构象而非局部态密度（LDOS）的解离得到了很好的解释。通过考虑整个解离模式的发生概率，很好地再现了中心/角部的位点选择性。由于STM图像代表吸附的最终状态，如果吸附的最后一步涉及分子或前驱体的解离，STM图像反映取决于局部结构的解离概率。另一方面，如果在最后一步不涉及分子或前驱体的解离，吸附概率可能取决于LDOS。还从这个一般的吸附观点讨论了H₂S、H₂O和NH₃的吸附。二维自由前驱体的概念对于理解多相催化的动力学将很重要。
