Density functional theory studies of acetylene hydrogenation on clean, vinylidene- and ethylidyne-covered Pt(111) surfaces.

Simon G. Podkolzin;^a Rafael Alcala;  James A Dumesic

Journal of Molecular Catalysis A: Chemical 218(2), 217-227 (2004)

Link to publication details on publisher's server

Abstract

    DFT calculations for acetylene hydrogenation on clean, vinylidene CCH₂- (0.25 ML) and ethylidyne CCH₃-covered (0.25 ML) Pt(111) surfaces were performed to elucidate 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 CH₂CH₂ 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.

Address:

Department of Chemical Engineering, University of Wisconsin, Madison, WI 53706, USA.

[a] Present address: The Dow Chemical Company, Core Research and Development, Midland, MI 48674, USA.

Publisher:

Elsevier B.V.

CODEN: JMCCF2, ISSN: 1381-1169, CAN 141:224889, AN 2004:479635

Animation of DFT calculations and Monte Carlo simulations