Critical hydrogen coverage effect on the hydrogenation of ethylene catalyzed by δ-MoC(001): an ab initio thermodynamic and kinetic study

Abstract

The molecular mechanism of ethylene (C2H4) hydrogenation on a δ-MoC(001) surface has been studied by periodic density functional theory methods. Activation energy barriers and elementary reaction rates have been calculated as a function of the hydrogen surface coverage, θH, with relevant properties derived from ab initio thermodynamics and kinetic rate estimates. The hydrogen coverage has a very strong effect on the adsorption energy and the second hydrogenation step of ethylene. A relatively low energy barrier favors the dissociation of H2 on δ-MoC(001) leading to medium H coverages (>0.4 of a monolayer) where the energy barrier for the full hydrogenation of ethylene is already below the corresponding barriers seen on Pt(111) and Pd(111). At a high H coverage of ∼0.85 of a monolayer, the C2H4 adsorbs at 1 atm and 300 K over a system having as-formed CH3 moiety species, which critically favors the C2H4 second hydrogenation, typically a rate limiting step, by reducing its activation energy to a negligible value of 0.08 eV, significantly lower than the equivalent values of ∼0.5 eV reported for Pt(111) and Pd(111) catalyst surfaces. The ethane desorption rate is larger than the surface intermediate elementary reaction rates, pointing to its desorption upon formation, closing the catalytic cycle. The present results put δ-MoC under the spotlight as an economic and improved replacement catalyst for Pt and Pd, with significant improvements in enthalpy and activation energy barriers. Here, we provide a detailed study for the C2H4 hydrogenation reaction mechanism over a carbide showing characteristics or features not seen on metal catalysts. These can be exploited when dealing with technical or industrial applications.