A First Principles Analysis of C−H Bond Formation in Ethylene Hydrogenation

Abstract

The Horiuti−Polanyi mechanism for ethylene hydrogenation over Pd(111) is examined using first-principle density functional quantum chemical calculations. Cluster and periodic slab DFT-GGA calculations were carried out to determine the modes and energies of chemisorption for a sequence of proposed intermediates, along with overall reaction energies and activation barriers for each of the speculated elementary steps. The DFT-calculated binding energies for ethylene (π), ethylene (di-σ), ethyl, vinyl, ethylidyne, atomic oxygen, and atomic carbon on the Pd19 cluster (and the Pd(111) slab) were found to be −30 (−27), −60 (−62), −130 (−140), −237 (−254), −620 (−636), −375 (−400), and −610 (−635) kJ/mol. The slab results were found to be within 20 kJ/mol of the cluster results. Frequency calculations along with predicted chemisorption energies indicate that ethylene adsorbs in both π- and di-σ-configurations. At moderate temperatures, the binding energies for π- and di-σ-bound ethylene are comparable. At low surface coverages, the predicted intrinsic activation barriers are +72 kJ/mol for the hydrogenation of ethlyene to surface ethyl and +71 kJ/mol for ethyl to ethane. The corresponding overall reaction energies for these two steps are +3 and −5 (without lateral interactions) kJ/mol, respectively. At low surface coverages, the di-σ-intermediate appears to be the precursor to reaction. At low coverage the π-bound intermediate is first converted to the di-σ-species before it will react with hydrogen. The apparent activation barrier for ethylene hydrogenation to surface ethyl is 26 kJ/mol which is significantly lower than the intrinsic activation barrier. The apparent barrier is measured with respect to the gas-phase rather than the adsorbed ethylene state. Higher surface coverages alter the favored reaction state. At higher coverages, the activation barriers for ethylene hydrogenation to surface ethyl were calculated to be +82 and +36 kJ/mol for the di-σ- and π-bound intermediates, respectively. Higher surface coverages weaken both the metal−hydrogen and metal−carbon bonds. This promotes hydrogenation from the π-bound state. The calculated barrier of +36 kJ/mol (from the π-bound state) at higher surface coverages is consistent with experimentally reported ethylene hydrogenation barriers.

Direct Observation of Surface Ethyl to Ethane Interconversion upon C2H4 Hydrogenation over Pt/Al2O3 Catalyst by Time-Resolved FT-IR Spectroscopy

“…26 The most striking observations upon decrease of the reaction temperature are 23 We conclude that surface ethyl to ethane interconversion is the rate determining step of ethylene hydrogenation under reaction conditions. The fact that the The very large temperature dependence of the intensity ratio of the 2 surface species CH 3 CH 2 Pt and CH 3 CPt 3 is worth noting.…”

“…9,20,21 These experimental findings on Pt or Pd surfaces are supported by recent DFT computational work. [22][23][24][25] The papers predict that the barrier to hydrogenation is substantially lower for π-C 2 H 4 than for di-σ ethylene (at the high surface coverage relevant for reaction conditions), and that the barrier for ethyl to ethane conversion exceeds that of step (1) by a factor between 2 and 5.…”

A machine learning approach to graph-theoretical cluster expansions of the energy of adsorbate layers

“…60 The hydrogenation of ethylene over Pd is a widely studied model system. [61][62][63] Previous theoretical studies have identified two single-molecule adsorption modes: The most stable mode is the di-s mode in which the molecule is bonded to two palladium atoms, distorting its geometry quite strongly according to the Dewar-Chatt-Duncanson model 64,65 . The latter model accounts for hydrocarbon-metal interactions in which a fraction of metal electron occupies the anti-bonding orbitals of the molecule.…”

Studies on the reaction pathway of arsenate adsorption at water–TiO2 interfaces using density functional theory

“…Yet, measuring it precisely through experiments is challenging. First-principle DFT calculations have been used to study the surface reaction processes of gas, water, or organic small molecules [9][10][11][12]. To our knowledge, the reaction pathway calculation for the adsorption of oxyanions on mineral surfaces is very limited.…”