Quantification of Primary Versus Secondary C-H Bond Cleavage in Alkane Activation: Propane on Pt

The trapping-mediated dissociative chemisorption of three isotopes of propane (C(3)H(8), CH(3),CD(2)CH(3), and C(3)D(8)) has been investigated on the Pt(110)-(1 x 2) surface, and both the apparent activation energies and the preexponential factors of the surface reaction rate coefficients have been measured. In addition, the probabilities of primary and secondary C-H bond cleavage for alkane activation on a surface were evaluated. The activation energy for primary C-H bond cleavage was 425 calories per mole greater than that of secondary C-H bond cleavage, and the two true activation energies that embody the single measured activation energy were determined for each of the three isotopes. Secondary C-H bond cleavage is also preferred on entropic grounds, and the magnitude of the effect was quantified.