TY - JOUR
T1 - A method for estimating surface reaction energetics
T2 - Application to the mechanism of ethylene decomposition on Pt(111)
AU - Carter, Emily A.
AU - Koel, Bruce E.
N1 - Funding Information:
E.A.C. is grateful for support from the National Science Foundation and the Camille and Henry Dreyfus Foundation in the form of a Presidential Young Investigator Award (198881993) and a Distinguished New Faculty Award, respectively. B.E.K. acknowledges the US Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division for partial support of this research.
PY - 1990/2/2
Y1 - 1990/2/2
N2 - 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 C2(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.
AB - 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 C2(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.
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U2 - 10.1016/0039-6028(90)90498-W
DO - 10.1016/0039-6028(90)90498-W
M3 - Article
AN - SCOPUS:0011598913
VL - 226
SP - 339
EP - 357
JO - Surface Science
JF - Surface Science
SN - 0039-6028
IS - 3
ER -