Quantification of the selective activation of C--H bonds in short chain alkanes: The reactivity of ethane, propane, isobutane, {ital n}-butane, and neopentane on Ir(111)
- Department of Chemical Engineering, University of California, Santa Barbara, California 93106 (United States)
The initial probabilities of precursor-mediated, dissociative chemisorption of the saturated hydrocarbons {sup 13}C-labeled ethane, propane, isobutane, {ital n}-butane, and neopentane on the close-packed Ir(111) surface have been measured. The selective activation of primary (1{degree}), secondary (2{degree}), and tertiary (3{degree}) C--H bonds has been quantified by examining the reactivities of the selectively deuterated isotopomers of propane, C{sub 3}H{sub 8}, CH{sub 3}CD{sub 2}CH{sub 3}, and C{sub 3}D{sub 8}, and of isobutane, (CH{sub 3}){sub 3}CH, (CH{sub 3}){sub 3}CD, and (CD{sub 3}){sub 3}CH. With respect to the bottom of the physically adsorbed well for each hydrocarbon, the apparent C--H bond activation energies have been found to be 10.4{plus_minus}0.3 kcal/mol (ethane), 11.4{plus_minus}0.3 kcal/mol (propane), 11.5{plus_minus}0.3 kcal/mol ({ital n}-butane), 11.3{plus_minus}0.3 kcal/mol ({ital i}-butane), and 11.3{plus_minus}0.3 kcal/mol (neopentane). For all the alkanes examined, the ratios of the preexponential factors of the rate coefficients of reaction and desorption are 1{times}10{sup {minus}2}. The C--D bond activation energies are higher than the corresponding C--H bond activation energies by 480 cal/mol (ethane), 630 cal/mol (propane), and 660 cal/mol ({ital i}-butane). By analyzing the primary kinetic isotope effects for the selectively deuterated isotopomers of propane and isobutane, the 2{degree} C--H bond activation energy is found to be 310{plus_minus}160 cal/mol less than the 1{degree} C--H bond activation energy on this surface, and similarly, 3{degree} C--H bond cleavage is less by 80{plus_minus}70 cal/mol. The quantification of the branching ratios within the C--H bond activation channel for propane and isobutane on this surface shows that the formation of 1{degree}-alkyl intermediates is, in general, favored over the formation of either 2{degree}- or 3{degree}-alkyl intermediates. (Abstract Truncated)
- DOE Contract Number:
- FG03-89ER14048
- OSTI ID:
- 249405
- Journal Information:
- Journal of Chemical Physics, Vol. 103, Issue 13; Other Information: PBD: 1 Oct 1995
- Country of Publication:
- United States
- Language:
- English
Similar Records
Direct dissociative chemisorption of alkanes on Pt(111): Influence of molecular complexity
Isotope effects in trapping-mediated chemisorption of ethane and propane on Ir(110)