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 × 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.