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A systematic study of ethylene adsorption over δ-MoC(001), TiC(001), and ZrC(001) surfaces was conducted by means of calculations based on periodic density functional theory. The structure and electronic properties of each carbide pristine surface had a strong influence in the bonding of ethylene. It was found that the metal and carbon sites of the carbide could participate in the adsorption process. As a consequence of this, very different bonding mechanisms were seen on δ-MoC(001) and TiC(001). The bonding of the molecule on the TMC(001) systems showed only minor similarities to the type of bonding found on a typical metal like Pt(111). In general, the ethylene binding energy follow the trend in stability: ZrC(001) < TiC(001) < δ-MoC(001) < Pt(111). The van der Waals correction to the energy produces large binding energy values, modifies the stability orders and drives the ethylene closer to the surface but the adsorbate geometry parameters remain unchanged. Ethylene was activated on clearly defined binding geometries, changing its hybridization from sp 2 to sp 3 with an elongation (0.16−0.31 Å) of the CC bond. On the basis of this theoretical study, δ-MoC(001) is proposed as a potential catalyst for the hydrogenation of olefins, whereas TiC(001) could be useful for their hydrogenolysis.

The molecular mechanism of ethylene (C 2 H 4 ) 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, θ H , 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 H 2 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 and Pd (111). At a high H coverage of ~0.85 of a monolayer, the C 2 H 4 adsorbs at 1 atm and 300 K over a system having as-formed CH 3 moiety species, which critically favors the C 2 H 4 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 and Pd (111) catalyst surfaces. The ethane desorption rate is larger than the surface intermediate elementary reaction rates, pointing for its desorption upon formation, closing the catalytic cycle. The present results put δ-MoC under the spotlight as an economic and improved replacement catalyst to Pt and Pd, with significant improvements in enthalpy and activation energy barriers. Here, we provid a detailed study for the C 2 H 4 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.

Transition metal carbides are emerging alternative catalysts to Pt-group metals for the transformation of CO2 and methane greenhouse gases. Recently, the effect of nanostructuring of such carbides has started to...