Ethylene Hydrogenation Molecular Mechanism on MoC_y Nanoparticles

Abstract

Ethylene hydrogenation catalyzed by MoC y nanoparticles has been studied by means of density functional theory methods and several models. These include MetCar (Mo 8 C 12 ), Nanocube (Mo 14 C 13 ), and Mo 12 C 12 nanoparticles as representatives of experimental MoC y nanostructures. The effect of hydrogen coverage has been studied in detail by considering low-, intermediate-, and highhydrogen regimes. The calculated enthalpy and energy barriers show that ethylene hydrogenation is feasible on the MetCar, Mo 12 C 12 , 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 C 2 H 4 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 (C 2 H 4 → C 2 H 5 ) 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 Mo 14 C 13 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.

C1 Chemistry on Metal Carbide Nanoparticles: Boosting the Conversion of CO2 and CH4

“…As we will see below, in spite of a large carbon/metal ratio, the metcars display a good ability to bind key molecules in C1 chemistry like H 2 , CO, H 2 O, and CO 2 . ,,, The M 8 C 12 compounds adopt a T d -like structure that includes low- and high-coordinated M atoms and six C 2 groups (Figure ). In principle, the four M atoms located in edge sites of the structure are expected to be the most active centers for binding but, depending on the nature of a reactant, the C atoms also can be active. ,,, Over the years, different types of M x C y molecular clusters have been synthesized and also studied in theoretical calculations. ,,− Figure displays the structures of several molecular clusters. The large clusters at the bottom are derived from cuts in the rock-salt lattice of bulk δ-MoC.…”

“…This type of site is very active in interacting with methane and other hydrocarbons. 28,49 structure and chemical changes of Mo species in MoO 3 / H-ZSM-5 during CH 4 reactions were monitored using in situ XAFS, temperature-programmed oxidation after the reaction, and isotopic of D 2 with OH groups in H-ZSM-5 before and after CH 4 reaction. 33 Exposure of MoO 3 /H-ZSM-5 to methane led to reduction and carburization of molybdenum with the C/Mo ratio reaching a value close (∼0.4) to that expected for Mo 2 C (Figure 10).…”

“…33 The last two were molecules that were also observed after exposing the MoC x /Au(111) systems seen in Figure 3 to methane. 28 Inside the H-ZSM-5 zeolite, catalytically inactive MoO x species reacted with CH 4 to form the two types of sites necessary for the transformation of methane to benzene: MoC x for C−H bond rupture and starting C−C bond formation, 28,33,49 and acid sites in the zeolite for addition and cyclization of C 2 + hydrocarbons to yield a highly stable aromatic ring. 33 Depending on the methodology followed to prepare the MoC x /ZSM-5 active phase, the selectivity in the methane to benzene conversion could be as high as 85%.…”

“…32−35 As mentioned above, methane coupling to ethane and ethylene has been seen on free and supported TMC nanoparticles. 28,33,49 This chemical transformation has been optimized on catalysts containing molybdenum and tungsten carbides. 77 Fast carbon diffusion on/in the carbide is a key factor.…”

“…Since the 1980s, methods have been reported in the literature for the synthesis of clusters, aggregates, and nanostructures of TMCs. − These methods involve direct reaction of a given metal with C-containing molecules in the gas phase, , metal deposition on layers of C-rich molecules, reduction and carburization of oxide precursors, ,, mechanochemical and ball milling approaches, and sol–gel chemistry . Molecular clusters, nanoparticles, and 2D systems frequently can adopt carbon/metal ratios and structures not seen in bulk TMCs. − They exhibit an “abnormal” behavior that can be quite useful for the activation of key molecules like H 2 , , CO 2 , ,,,, CH 4 or C 2 H 4 . Many interesting structural and electronic properties have been identified for clusters, aggregates, and nanostructures of TMCs, − but only a few of these systems have been actually tested in the binding or activation of CO 2 and CH 4 . ,,,,− In this section, we describe what is known about their structural and electronic properties.…”