TY - JOUR
T1 - Density functional theory studies of acetylene hydrogenation on clean, vinylidene- and ethylidyne-covered Pt(1 1 1) surfaces
AU - Podkolzin, Simon G.
AU - Alcalá, Rafael
AU - Dumesic, James A.
PY - 2004/8/24
Y1 - 2004/8/24
N2 - Density functional theory (DFT) calculations for acetylene hydrogenation on clean, vinylidene CCH2-covered (0.25 ML) and ethylidyne CCH 3-covered (0.25 ML) Pt(1 1 1) surfaces were performed to probe the reaction mechanism and evaluate energetic changes due to high hydrocarbon coverage. A comparison between the reaction energetics on the clean and pre-covered surfaces shows that high coverage trends are similar for vinylidene and ethylidyne species: surface hydrocarbon species and hydrogen are destabilized by up to 150 and 30 kJ/mol, respectively. Unsaturated, multiply-bonded species are destabilized more than species forming fewer bonds with the surface. Activation energies are not affected, unless the spatial formation of a transition state is hindered or a reactant is significantly distorted. In these cases, activation barriers can be different by up to 50 kJ/mol and the relative significance of parallel steps may change. For example, CH2CH2 formation is hindered at high coverage and the relative propensity of CHCH2 for forming either CH2CH 2 or CHCH3 is reversed. The calculations confirm that vinylidene CCH2 and ethylidyne CCH3 are spectator species in the overall reactions of ethylene and ethane formation. However, at the evaluated surface coverage of 0.25 ML, these spectator species may undergo hydrogen disproportionation with other hydrocarbon fragments, serving as a hydrogen reservoir and providing lower-energy pathways. As a result, the predicted energetics for acetylene hydrogenation at high coverage are affected by not only the extent of destabilization of active species and their transition states, but also by the relative stability of spectator species and their possible participation in disproportionation reactions.
AB - Density functional theory (DFT) calculations for acetylene hydrogenation on clean, vinylidene CCH2-covered (0.25 ML) and ethylidyne CCH 3-covered (0.25 ML) Pt(1 1 1) surfaces were performed to probe the reaction mechanism and evaluate energetic changes due to high hydrocarbon coverage. A comparison between the reaction energetics on the clean and pre-covered surfaces shows that high coverage trends are similar for vinylidene and ethylidyne species: surface hydrocarbon species and hydrogen are destabilized by up to 150 and 30 kJ/mol, respectively. Unsaturated, multiply-bonded species are destabilized more than species forming fewer bonds with the surface. Activation energies are not affected, unless the spatial formation of a transition state is hindered or a reactant is significantly distorted. In these cases, activation barriers can be different by up to 50 kJ/mol and the relative significance of parallel steps may change. For example, CH2CH2 formation is hindered at high coverage and the relative propensity of CHCH2 for forming either CH2CH 2 or CHCH3 is reversed. The calculations confirm that vinylidene CCH2 and ethylidyne CCH3 are spectator species in the overall reactions of ethylene and ethane formation. However, at the evaluated surface coverage of 0.25 ML, these spectator species may undergo hydrogen disproportionation with other hydrocarbon fragments, serving as a hydrogen reservoir and providing lower-energy pathways. As a result, the predicted energetics for acetylene hydrogenation at high coverage are affected by not only the extent of destabilization of active species and their transition states, but also by the relative stability of spectator species and their possible participation in disproportionation reactions.
KW - Acetylene
KW - Adsorption
KW - DFT
KW - Ethane
KW - Ethylene
KW - Ethylidyne
KW - Hydrogen
KW - Hydrogenation
KW - Lateral interactions
KW - Platinum
KW - Spectator species
KW - Vinylidene
UR - http://www.scopus.com/inward/record.url?scp=2942650704&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=2942650704&partnerID=8YFLogxK
U2 - 10.1016/j.molcata.2004.04.015
DO - 10.1016/j.molcata.2004.04.015
M3 - Article
AN - SCOPUS:2942650704
SN - 1381-1169
VL - 218
SP - 217
EP - 227
JO - Journal of Molecular Catalysis A: Chemical
JF - Journal of Molecular Catalysis A: Chemical
IS - 2
ER -