Ab Initio Reaction Path Analysis of Benzene Hydrogenation to Cyclohexane on Pt(111)

Abstract

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.

Heat of Adsorption of Naphthalene on Pt(111) Measured by Adsorption Calorimetry

“…We note that, for benzene adsorption on Pt(111), other adsorption heats, which are closer to the experimental value (but still much lower), have been computed (in kJ/mol): 132, 28 117, 29 104, 30 and 102. 31 The fact that all these studies underestimate the heat of adsorption suggests fundamental problems connected with DFT calculations for aromatic hydrocarbons adsorbed on metal surfaces. Similar problems seem to arise in the calculation of adsorption geometries and bond properties, as indicated by a controversy about DFT calculations on PTCDA/Ag(111).…”

“…The horizontal bars show further computed benzene adsorption energies. [28][29][30][31] terraces on Cu(111), 36 we attribute this to step edges. We conclude that the adsorption heat on steps (g331 kJ/mol) is >10% higher than the initial adsorption heat on terrace sites (∼300 kJ/mol).…”

Insights into the mechanism of nitrobenzene reduction to aniline over Pt catalyst and the significance of the adsorption of phenyl group on kinetics

“…To model the mechanism of nitrobenzene reduction, there are two significant problems to overcome. The first challenge is associated with the large size of nitrobenzene molecule, because a larger molecule would considerably increase the number of possible adsorption configurations [21][22][23][24][25][26][27][28][29][30][31][32], therefore, a significantly more adsorption configurations should be carefully tested in the calculations of nitrobenzene, in contrast to the much fewer adsorption configurations of other smaller organic molecules such as formic acid or methanol. In addition, an increase in adsorption configurations would also lead to an increase in the numbers of possible transition states, which again makes the calculations more complicated and time-consuming [21,22].…”

Extension of the Single-Event Microkinetic Model to Alkyl Substituted Monoaromatics Hydrogenation on a Pt Catalyst

“…A LHHW type of kinetic model for the same data set was also developed based on a detailed first-principles density functional theory calculations and corresponding reaction path analysis. 17,18 The Single-Event MicroKinetic (SEMK) methodology goes beyond the above-mentioned types of kinetic models by explicitly accounting for the rates of all elementary steps that are considered in the reaction mechanism and not assuming a rate-determining step. The adequate classification of elementary steps into reaction families ensures the feedstock independence of the corresponding kinetic parameters.…”