Ethylene Conversion to Ethylidyne over Pd(111): Revisiting the Mechanism with First-Principles Calculations

Abstract

On Pd(111), thermal activation of ethylene has been reported to yield ethylidyne. Using more approximate models, a plausible three-step mechanism, ethylene f vinyl f ethylidene f ethylidyne, was recently proposed for this process on the basis of DFT calculations. We employed more elaborate computational models and characterized the thermodynamics and kinetics of the mechanism of ethylene conversion to ethylidyne on Pd(111). We carried out density functional slab-model studies for three coverages of the adsorbate, 1/3, 1/4, and 1/9. The resulting refined potential energy landscape turned out to differ notably from that reported previously: our calculated barriers for the various elementary steps are significantly lower than those of previous studies, and we determined the overall process to be exothermic, in contrast to earlier computational results. We show that the three-step mechanism is thermodynamically and kinetically feasible on Pd(111), with the dehydrogenation of ethylene to vinyl being the rate-limiting step at all coverages considered. Direct conversion of ethylene to ethylidene is unlikely due to a very high barrier. Coverage effects have been found important. At high coverage, the rate-limiting first reaction barrier is ∼50 kJ mol -1 above the desorption energy of ethylene, whereas at low coverages the two energies become comparable.

From Static to Reacting Systems on Transition-Metal Surfaces

“…Although the present chapter is focused on the reactivity issues, it is worth mentioning that ethylene and other C 2 H x (x ¼ 0À5) species usually prefer adsorption configurations on TMs with both C atoms sp 3 -hybridized, as in the C 2 H 6 molecule, so that the missing bonds with H atoms are replaced by interactions with the underlying surface metal atoms. Later, Moskaleva et al 65 revisited this mechanism using more elaborate models and more precise computational strategy (thicker slab with larger structural flexibility, denser k-point grid, and stricter convergence criteria). The exact mechanism of this reaction could not be unequivocally determined experimentally and several theoretical studies briefly surveyed below used the tools of modern DFT and kinetic Monte Carlo (kMC) simulations to identify accessible reaction pathways and to determine the preferred reaction route at given experimental conditions.…”

“…Zhao et al found that the barrier of ethylidene conversion to ethylidyne is remarkably low, only $ 20-30 kJ mol À1 , which implies that ethylidene species will be rapidly converted to ethylidyne and should not be detected experimentally. Finally, it should be mentioned that direct isomerization processes of ethylene to ethylidene and vinyl to ethylidyne via 1,2 H-shifts on Pd(111), 65,80 Pt(111), 82,84 Pt(110), 86 and Rh (111) 85 surfaces were theoretically found to have very high reaction barriers and thus unlikely to occur under relevant experimental conditions. 74 85 modeled ethylene conversion to ethylidyne on Rh (111) using periodic DFT approach.…”

Hierarchical approach to conformational search and selection of computational method in modeling the mechanism of ester ammonolysis

“…The data included in the other three clusters differ more significantly from the experimental data. Cluster {2} consists mainly of methods which take into account the electron correlation, combined with basis sets without polarization functions or with basis sets including polarization functions but with a few primitives (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21). The other two clusters contain the HF method with or without polarization function in the basis sets (clusters {1} and {3}, respectively).…”

“…The computational modeling of reaction mechanisms and energetics has become an important step in the investigation of different processes, occurring in biological systems [1][2][3][4], catalytic systems [5][6][7][8], organic synthesis [9][10][11], etc. The selection of the computational method [12] and construction of the initial structures of the reactants, transition states, intermediates and products are often carried out after comparison with previous knowledge and chemical intuition.…”

Spectroscopic characterization of C2Hx intermediates in the dissociation of vinyl iodide on Pt(111)

“…A mechanism in which ethylidene is formed not directly from ethylene, but via a vinyl intermediate, has therefore been proposed for ethylidyne formation from acetylene on Pd(111) [23][24][25]. In the case of ethylidyne formation from ethylene on Pt(111), kinetic Monte Carlo (kMC) simulations were carried out to gain information on how the coverages of various surface species evolve with time at a specific temperature.…”