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

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 tempera tures between 250 and 1100 K. Kinetic parameters (activation barrier and pr eexponential 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 cycloalka ne isotopomer pair, and also by comparison with other relevant alkane activ ation barriers. On the Ir(111) surface, the dissociative chemisorption of c yclobutane, cyclopentane, and cyclohexane occurs via two different reaction pathways: initial C-C bond cleavage dominates on Ir(111) at high temperatu re (T > similar to 600 K), while at low temperature (T < <similar to>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 th e entire temperature range studied, whereas dissociative chemisorption of b oth cyclohexane and cyclooctane occurs via initial C-H bond cleavage. Compa rison of the cycloalkane C-C bond activation barriers measured here with th ose reported previously in the literature qualitatively suggests that the d ifference in ring-strain energies between the initial state and the transit ion state for ring-opening C-C bond cleavage effectively lowers or raises t he activation barrier for dissociative chemisorption via C-C bond cleavage, depending on whether the transition state is less or more strained than th e 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.