Acetylene and Ethylene Adsorption on a β-Mo₂C(100) Surface: A Periodic DFT Study on the Role of C- and Mo-Terminations for Bonding and Hydrogenation Reactions

Abstract

Mo 2 C catalysts are widely used in hydrogenation reactions, however, the role of the C-and Moterminations in these catalysts is not clear. Understanding the binding of adsorbates is key for explaining the activity of Mo 2 C. The adsorption of acetylene and ethylene, probe molecules representing alkynes and olefins, respectively, was studied on a β-Mo 2 C(100) surface with Cand Mo-terminations using calculations based on periodic density functional theory. Moreover, the role of the C/Mo molar ratio was investigated to compare the catalytic potential of cubic (δ-MoC) and orthorhombic (β-Mo 2 C) surfaces. The geometry and electronic properties of the clean δ-MoC(001) and β-Mo 2 C(100) surfaces have strong influence on the binding of unsaturated hydrocarbons. The adsorption of ethylene is weaker than that of acetylene on the surfaces of the cubic and orthorhombic systems; adsorption of the hydrocarbons was stronger on β-Mo 2 C(100) than on δ-MoC(001). The C-termination in β-Mo 2 C(100) actively participates in both acetylene and ethylene adsorption and is not merely a spectator. The results of this work suggest that the β-Mo 2 C(100)-C surface could be the one responsible for the catalytic activity during the hydrogenation of unsaturated C≡C and C=C bonds, while the Mo-terminated surface could be poisoned or transformed by the strong adsorption of C and CH x fragments.

Size and Stoichiometry Effects on the Reactivity of MoCy Nanoparticles toward Ethylene

“…Reference data on most stable δ-MoC(001) and β-Mo 2 C­(001) surfaces, either C- or Mo-terminated, as provided. See complete Table S4. b References and . c Reference . d Reoptimized by using the present computational setup. …”

“…All conceivable binding sites and connectivities have been explicitly considered. The notation for the adsorption modes is as earlier reported. ,, Briefly, π-M relates to ethylene binding atop a metal site; di-σ-MM, di-σ-CM, and di-σ-CC correspond to the binding bridging metal–metal, carbon–metal, and carbon–carbon bond, respectively; and finally, σ-M, μ-M stand for a situation in which one has one C atom of C 2 H 4 atop a metal atom (σ-M) and one C 2 H 4 C atom located perpendicularly atop a M–M bridge (μ-M). For every NP, several initial adsorption geometries were evaluated to cover all binding possibilities, so that a complete scanning of the potential energy surface for C 2 H 4 binding in every NP can be assured.…”

“…Several studies have shown that transition metal carbides have excellent catalytic properties and appear as a good alternative to replace Pt-group-based catalysts, especially on several hydrogenation reactions. Among them, molybdenum carbides are highlighted as promising materials because of their remarkable interaction with hydrocarbons and with several successful applications in hydrogenation reactions. − Not surprisingly, several studies have been reported on the interaction of C 2 H 4 with molybdenum carbides surfaces, precisely to explore the capability of these materials as a potential alternative catalyst to Pt-group metals. − The binding and hydrogenation of the ethylidyne species on δ-MoC(001) have also been considered because these species have been found to be responsible of poisoning in Pt-based catalysts. Interestingly, there is evidence that this material could avoid surface poisoning, in contrast to Pt and Pd extended surfaces …”

Determination of Molybdenum Species Evolution during Non‐Oxidative Dehydroaromatization of Methane and its Implications for Catalytic Performance

“…As proposed by some authors, the role of Mo x C y would be to promote the C−H bond dissociation in CH 4 , leading to the formation of CH x species that undergo dehydrogenation and C−C coupling resulting in C 2 intermediates. Previous Density Functional Theory (DFT) studies have suggested that the catalytic reactivity of Mo−C species can vary with the Mo : C ratio ,. Since XAS is a bulk characterization technique and Mo/H‐ZSM‐5 is continuously evolving it is difficult to unambiguously differentiate between MoC species with varied Mo/C ratios which will likely present different activity.…”

“…Previous Density Functional Theory (DFT) studies have suggested that the catalytic reactivity of MoÀC species can vary with the Mo : C ratio. [42,43] Since XAS is a bulk characterization technique and Mo/H-ZSM-5 is continuously evolving it is difficult to unambiguously differentiate between MoC species with varied Mo/C ratios which will likely present different activity. What can be clearly observed however, is that with further time on stream, a continuous intensity increase of the FT 3.3 Å peak occurs during MDA (bottom spectra in Figure 2).…”