A method for estimating surface reaction energetics: Application to the mechanism of ethylene decomposition on Pt(111)

We present a scheme for calculating the heats of formation of adsorbed species on surfaces. Using this approach, we can estimate the relative stabilities of surface intermediates and adsorbate-surface bond strengths. We report the application of this method to obtaining a better understanding of the mechanism and energetics of ethylene decomposition on clean and potassium-promoted Pt(111). We estimate heats of formation, heats of reaction, and Pt-carbon bond strengths for adsorbed ethylene, ethylidene, ethylidyne, ethyl, vinyl, acetylene, vinylidene, ethynyl, methyl, methylene, methyne, and carbide. These equilibrium thermodynamic estimates clearly rule out several previously proposed mechanisms, favoring instead a reaction sequence in which ethylene isomerizes to ethylidene, followed by dehydrogenation to form ethylidyne, which then dehydrogenates further to form ethynyl (via vinyl and acetylene) and C_2(a). Carbon-carbon bond cleavage is also thermodynamically accessible throughout the decomposition. Potassium alters the metal-carbon bond character in these adsorbed hydrocarbon fragments such that hydrogen-rich adsorbates are favored over the more fully dehydrogenated species, an effect attributable to the relative stabilities of the isolated hydrocarbanions. As a result, although adsorbed hydrogen is stabilized by potassium, the activation energies for C-H and C-C bond cleavage increase dramatically.