Ethylene Hydrogenation Molecular Mechanism on MoC_y Nanoparticles

Ethylene hydrogenation catalyzed by MoC_y nanoparticles has been studied by means of density functional theory methods and several models. These include MetCar (Mo₈C₁₂), Nanocube (Mo₁₄C₁₃), and Mo₁₂C₁₂ nanoparticles as representatives of experimental MoC_y nanostructures. The effect of hydrogen coverage has been studied in detail by considering low-, intermediate-, and high-hydrogen regimes. The calculated enthalpy and energy barriers show that ethylene hydrogenation is feasible on the MetCar, Mo₁₂C₁₂, and Nanocube but at low, medium, and high hydrogen coverages, respectively. An additional step, related to the H* migration from a Mo to a C site in the nanoparticle, has been found to be the key to establishing the best hydrogenation system. In most cases, the reactions are exothermic, featuring low hydrogenation energy barriers, especially for the Nanocube at high hydrogen coverage. In addition, the calculated adsorption Gibbs free energy shows that, for this system, the C₂H₄ adsorption is feasible in the 300-400 K temperature range and pressures from 10^-10 to 2 atm. For the hydrogenation steps, calculated transition state theory rates show that the overall process is limited by the first hydrogenation step (C₂H₄ → C₂H₅) at temperatures of 330-400 K. However, at the lower temperatures of 300-320 K, the reaction rates are comparable for the two steps. The present results indicate that the Mo₁₄C₁₃ Nanocube models of MoC_y nanoparticles exhibit appropriate thermodynamic and kinetic features to catalyze ethylene hydrogenation at a high-hydrogen-coverage regime. The present findings provide a basis for understanding the chemistry of active MoC_y catalysts, suggest appropriate working conditions for the reaction to proceed, and provide a basis for future experimental studies.