Ab initio reaction path analysis of benzene hydrogenation to cyclohexane on Pt(111).
First-principles density functional theory calculations were performed to obtain detailed insight into the mechanism of benzene hydrogenation over Pt(111). The results indicate that benzene hydrogenation follows a Horiuti-Polanyi scheme which involves the consecutive addition of hydrogen adatoms. A first-principles-based reaction path analysis indicates the presence of a dominant reaction path. Hydrogenation occurs preferentially in the meta position of a methylene group. Cyclohexadiene and cyclohexene are expected to be at best minor products, since they are not formed along the dominant reaction path. The only product that can desorb is cyclohexane. Along the dominant reaction path, two categories of activation energies are found: lower barriers at approximately 75 kJ/mol for the first three hydrogenation steps, and higher barriers of approximately 88 kJ/mol for steps four and six, where hydrogen can only add in the ortho position of two methylene groups. The highest barrier at 104 kJ/mol is calculated for the fifth hydrogenation step, which may potentially be the rate-determining step. The high barrier for this step is likely the result of a rather strong C-H...Pt interaction in the adsorbed reactant state (1,2,3,5-tetrahydrobenzene) which increases the barrier by approximately 15 kJ/mol. Benzene and hydrogen are thought to be the most-abundant reaction intermediates.
Saeys
Mark
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Laboratorium voor Petrochemische Techniek, Ghent University, Krijgslaan 281(S5), B-9000 Gent, Belgium.
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