AVS 47th International Symposium
    Surface Science Wednesday Sessions
       Session SS1-WeM

Paper SS1-WeM8
Trapping-mediated Dissociative Chemisorption of Cycloalkanes on Ru(001) and Ir(111): The Influence of Ring Strain on the Activation of C-C and C-H Bonds

Wednesday, October 4, 2000, 10:40 am, Room 208

Session: Chemical Interactions and Surface Reactivity
Presenter: T.-W. Kim, University of California, Santa Barbara
Authors: T.-W. Kim, University of California, Santa Barbara
C.J. Hagedorn, University of California, Santa Barbara
M.J. Weiss, University of California, Santa Barbara
W.H. Weinberg, University of California, Santa Barbara
Correspondent: Click to Email
We have measured the initial probabilities of dissociative chemisorption of perhydrido and perdeutero cycloalkane isotopomers on the hexagonally close-packed (hcp) 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 initial reaction pathways: initial C-C bond cleavage dominates on Ir(111) at high temperatures (T > 600 K), while at low temperature (T < ~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, suggests that the difference in ring strain energies between the initial state and 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 invoked to explain the observed trend of decreasing C-H bond activation barrier with decreasing cycloalkane ring size.