The Importance of Transient States at Higher Coverages in Catalytic Reactions

Abstract

DFT-GGA periodic slab calculations were used to examine the adsorption and hydrogenation of ethylene to a surface ethyl intermediate on the Pd(111) surface. The reaction was examined for two different surface coverages, corresponding to (2×3) [low coverage] and ( 3× 3)R 30°[high coverage] unit cells. For the low coverage, the di-σ adsorption of ethylene (-62 kJ/mol) is 32 kJ/mol stronger than the π-adsorption mode. The intrinsic activation barrier for hydrogenation of di-σ bonded ethylene to ethyl, for a (2×3) unit cell, was found to be +88 kJ/mol with a reaction energy of +25 kJ/mol. There appeared to be no direct pathway for hydrogenation of π-bonded ethylene to ethyl, for low surface coverages. At higher coverages, however, lateral repulsive interactions between adsorbates destabilize the di-σ adsorption of ethylene to a binding energy of -23 kJ/mol. A favorable surface geometry for the ( 3× 3)R 30°coverage is achieved when ethylene is π-bound and hydrogen is bound to a neighboring bridge site. At high coverage, the hydrogenation of di-σ bound ethylene to ethyl has an intrinsic barrier of +82 kJ/mol and a reaction energy of -5 kJ/mol, which is only slightly reduced from the low coverage case. For a ( 3× 3)R 30°unit cell, however, the more favorable reaction pathway is via hydrogenation of π-bonded ethylene, with an intrinsic barrier of +36 kJ/mol and an energy of reaction of -18 kJ/mol. This pathway is inaccessible at low coverage. This paper illustrates the importance of weakly bound intermediates and surface coverage effects in reaction pathway analysis.

Controlling CH 2 dissociation on Ru(0001) through surface site blocking by adsorbed hydrogen

“…CH þ H reaction. Prior work in other catalytic systems has highlighted the role of carbon and hydrogen surface coverage in controlling similar surface chemistry [45]. As mentioned in the methods section, the dosing protocol we followed with our MBS creates coadsorbed hydrogen atoms in addition to those originating from CH 4 molecules that dissociatively adsorb.…”

“…More generally, while the effects of carbon coverage on the thermal stability of hydrocarbon fragments have received attention in prior work [45], the effect of non-equilibrium concentrations of surface hydrogen, has been much less appreciated. This study suggests that such effects can dramatically change the thermodynamics and kinetics of hydrocarbon chemistry at catalytic surfaces.…”

Kinetics of ethylene combustion in the synthesis of vinyl acetate over a Pd/SiO2 catalyst

“…Adsorbed ethylene, C 2 H 4(a) , or its dehydrogenated product may block the dissociative adsorption of O 2 . Previous experimental [26][27][28] and theoretical [29][30][31][32] studies of the adsorption of C 2 H 4 and O 2 on Pd single crystals and clusters show two kinds of adsorption sites on Pd surface: (i) atop adsorption site on a Pd atom, the so-called π -type; and (ii) bridge adsorption sites called di σ -type, related to adsorption of a rehybridized (sp 2 → sp 3 ) C 2 H 4 molecule. However, molecular C 2 H 4 adsorbed on Pd(111), (110), and (100) has only been detected at low adsorption temperatures (< 150 K) [26][27][28][33][34][35][36].…”

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

“…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.…”