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 (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.

First-principles study on the effects of N and Al doping on the mechanical properties and electronic structures of TiC

“…Titanium carbide, a typical transition metal carbide, has been applied in cutting tools, 1 hard coatings, 2,3 grain inhibitors 4 as well as a catalyst [5][6][7] for years. It is not only due to its desirable physical properties such as high hardness, high melting points, low density and corrosion resistance but also the chemical stability and low thermal expansion coefficient draw huge attention in manufacturing industries.…”

Heterogeneous Catalysis and Parahydrogen‐Induced Polarization

“…One of the modern trends in the field of catalysis is the search for a replacement for noble metal catalysts, and metal carbides, especially molybdenum carbides, are extensively studied in this context. [78,79] Recently, carbides have been studied for the first time in hydrogenation with p-H 2 as well -it was shown that Mo 2 C catalysts can provide selectivities to the hydrogen pairwise addition route comparable to those of supported metal catalysts. [80] After a comprehensive characterization and performance comparison of two differently prepared Mo 2 C catalysts, the authors came to the conclusion that the presence of hexagonal-close-packed phase of Mo 2 C in these catalysts is crucial for the enhanced degree of the pairwise addition.…”

“…One of the modern trends in the field of catalysis is the search for a replacement for noble metal catalysts, and metal carbides, especially molybdenum carbides, are extensively studied in this context [78,79] . Recently, carbides have been studied for the first time in hydrogenation with p ‐H 2 as well – it was shown that Mo 2 C catalysts can provide selectivities to the hydrogen pairwise addition route comparable to those of supported metal catalysts [80] .…”

Concepts, models, and methods in computational heterogeneous catalysis illustrated throughCO2conversion

“…The use of slab models, given their possibilities, accessibility, and moderate computational expenses, has become the de facto workhorse in the last two decades; particularly when modeling reactivity at high coverage regimes of the clean surfaces, [68][69][70] although lately the increased computational resources allowed studying situations at a low coverage, [71][72][73] and including also surface defects such as vacancies, [74][75][76] subsurface species, [77][78][79] surface atoms, [80][81][82] and so on. Notice that slab models also permit to represent regular defects, such as surface steps, see Figure 2(a), by using vicinal surfaces slab models, [83][84][85] allowing also chemical resolution studies on chiral surfaces.…”