First-Principles Study of Hydrogenation of Ethylene on a HxMoO3(010) Surface

Hydrogenation of ethylene on a molybdenum oxide hydrogen bronze surface with the (010) orientation was studied using periodic density functional. theory (DFT). Significant surface relaxation was found to occur upon increase of hydrogen content in the bronze surface. Various ethylene adsorption configurations and the minimum energy reaction pathways that lead to the formation of an ethane molecule were systematically investigated. Ethylene adsorption with the tilted configuration. was found to be energetically most favorable due to the interaction between the pi electron of ethylene and the protonic H on the bronze surface. The weak physisorption of ethylene on the surface results in little bond activation on the molecule, leading to a one-step reaction of hydrogenation of ethylene on the HxMoO3(010) surface, in contrast to the strong chemisorption of the molecule on surfaces of transition metal catalysts. The process was found to be exothermic regardless of hydrogen contents in the bronze compounds. The calculated hydrogenation barriers on the HxMoO3(010) surface are comparable to or smaller than the activation energies on the Pt(111) surface, on which ethylene hydrogenation was reportedly a two-step process. An increase of hydrogen concentration can lead to a substantially exothermic reaction process with significantly enhanced kinetics, consistent with experimental observations. The calculated partial pressure of ethylene required for adsorption leading to its hydrogenation is in excellent agreement with the reported experimental results.