Trapping-mediated dissociative chemisorption of cycloalkanes on Ru(001) and Ir(111): influence of ring strain and molecular geometry on the activation of C-C and C-H bonds.
Trapping-mediated dissociative chemisorption of cycloalkanes on Ru(001) and Ir(111): influence of ring strain and molecular geometry on the activation of C-C and C-H bonds.
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DOI：
10.1021/ja002459z
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发表时间：
2001-02
影响因子：
15
通讯作者：
C. Hagedorn;M. Weiss;T. W. Kim;W. Weinberg
中科院分区：
文献类型：
--
作者：
C. Hagedorn;M. Weiss;T. W. Kim;W. Weinberg
我们测量了六方密排Ru（001）和Ir（111）单晶表面在250 ~ 1100k温度范围内，过氢氧和过氘环烷烃同位素体离解化学吸附的初始概率。描述每个表面上每个环烷烃同位素体的初始、限速C-H或C-C键裂解反应的动力学参数（激活势垒和指数前因子）被量化。通过每个环烷烃同位素体对的激活势垒之间是否存在动力学同位素效应，并与其他相关的烷烃激活势垒进行比较，确定了主要的初始反应机制是初始C-C键裂解还是C-H键裂解。在Ir（111）表面，环丁烷、环戊烷和环己烷的离解化学吸附通过两种不同的反应途径发生：在Ir（111）表面，高温（T <约600 K）下以初始C-C键裂解为主，而在低温（T <约400 K）下以初始C-H键裂解为主。在Ru（001）表面，环戊烷的离解化学吸附在整个研究温度范围内通过初始C-C键裂解发生，而环己烷和环辛烷的离解化学吸附通过初始C-H键裂解发生。本文测量的环烷烃C-C键激活势垒与先前文献报道的定性比较表明，开环C-C键裂解的初始态和过渡态之间的环应变能差异有效地降低或提高了通过C-C键裂解进行解离化学吸附的激活势垒，这取决于过渡态比初始态的应变更小还是更大。此外，还提出了立体参数和金属-碳键强度参数来解释所观察到的C-H键激活势垒随着环烷烃环尺寸的减小而减小的趋势。
We have measured the initial probabilities of dissociative chemisorption of perhydrido and perdeutero cycloalkane isotopomers on the hexagonally close-packed Ru(001) and Ir(111) single-crystalline surfaces for surface temperatures between 250 and 1100 K. Kinetic parameters (activation barrier and preexponential factor) describing the initial, rate-limiting C-H or C-C bond cleavage reactions were quantified for each cycloalkane isotopomer on each surface. Determination of the dominant initial reaction mechanism as either initial C-C or C-H bond cleavage was judged by the presence or absence of a kinetic isotope effect between the activation barriers for each cycloalkane isotopomer pair, and also by comparison with other relevant alkane activation barriers. On the Ir(111) surface, the dissociative chemisorption of cyclobutane, cyclopentane, and cyclohexane occurs via two different reaction pathways: initial C-C bond cleavage dominates on Ir(111) at high temperature (T > approximately 600 K), while at low temperature (T < approximately 400 K), initial C-H bond cleavage dominates. On the Ru(001) surface, dissociative chemisorption of cyclopentane occurs via initial C-C bond cleavage over the entire temperature range studied, whereas dissociative chemisorption of both cyclohexane and cyclooctane occurs via initial C-H bond cleavage. Comparison of the cycloalkane C-C bond activation barriers measured here with those reported previously in the literature qualitatively suggests that the difference in ring-strain energies between the initial state and the transition state for ring-opening C-C bond cleavage effectively lowers or raises the activation barrier for dissociative chemisorption via C-C bond cleavage, depending on whether the transition state is less or more strained than the initial state. Moreover, steric arguments and metal-carbon bond strength arguments have been evoked to explain the observed trend of decreasing C-H bond activation barrier with decreasing cycloalkane ring size.