Quantification of the selective activation of C–H bonds in short chain alkanes: The reactivity of ethane, propane, isobutane, n-butane, and neopentane on Ir(111)

Abstract

The initial probabilities of precursor-mediated, dissociative chemisorption of the saturated hydrocarbons 13C-labeled ethane, propane, isobutane, n-butane, and neopentane on the close-packed Ir(111) surface have been measured. The selective activation of primary (1°), secondary (2°), and tertiary (3°) C–H bonds has been quantified by examining the reactivities of the selectively deuterated isotopomers of propane, C3H8, CH3CD2CH3, and C3D8, and of isobutane, (CH3)3CH, (CH3)3CD, and (CD3)3CH. 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±0.3 kcal/mol (ethane), 11.4±0.3 kcal/mol (propane), 11.5±0.3 kcal/mol (n-butane), 11.3±0.3 kcal/mol (i-butane), and 11.3±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 1×10−2. 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 (i-butane). By analyzing the primary kinetic isotope effects for the selectively deuterated isotopomers of propane and isobutane, the 2° C–H bond activation energy is found to be 310±160 cal/mol less than the 1° C–H bond activation energy on this surface, and similarly, 3° C–H bond cleavage is less by 80±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°-alkyl intermediates is, in general, favored over the formation of either 2°- or 3°-alkyl intermediates. This result is a direct consequence of the disproportionate number of 1° C–H bonds relative to the number of 2° and 3° C–H bonds in these alkanes. These results are compared to those for the reaction of these alkanes on the reconstructed Pt(110)-(1×2) surface, and the influence of surface structure on the selective activation of 1°, 2°, and 3° C–H bonds is discussed.