Ethylene Hydrogenation Molecular Mechanism on MoC<i>_y</i> Nanoparticles

Ethylene hydrogenation catalyzed by MoCy 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 MoCy 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 MoCy 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 MoCy catalysts, suggest appropriate working conditions for the reaction to proceed, and provide a basis for future experimental studies

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

Mo₂C catalysts are widely used in hydrogenation reactions; however, the role of the C and Mo terminations in these catalysts is not clear. Understanding the binding of adsorbates is key for explaining the activity of Mo₂C. The adsorption of acetylene and ethylene, probe molecules representing alkynes and olefins, respectively, was studied on a β-Mo₂C­(100) surface with C and 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-Mo₂C) surfaces. The geometry and electronic properties of the clean δ-MoC(001) and β-Mo₂C­(100) surfaces have a 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-Mo₂C­(100) than on δ-MoC(001). The C termination in β-Mo₂C­(100) actively participates in both acetylene and ethylene adsorption and is not merely a spectator. The results of this work suggest that the β-Mo₂C­(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_<i>x</i> fragments

Systematic Theoretical Study of Ethylene Adsorption on δ‑MoC(001), TiC(001), and ZrC(001) Surfaces

A systematic study of ethylene adsorption over δ-MoC(001), TiC(001), and ZrC(001) surfaces was conducted by means of calculations based on periodic density functional theory. The structure and electronic properties of each carbide pristine surface had a strong influence in the bonding of ethylene. It was found that the metal and carbon sites of the carbide could participate in the adsorption process. As a consequence of this, very different bonding mechanisms were seen on δ-MoC(001) and TiC(001). The bonding of the molecule on the TMC­(001) systems showed only minor similarities to the type of bonding found on a typical metal like Pt(111). In general, the ethylene binding energy follow the trend in stability: ZrC(001) < TiC(001) < δ-MoC(001) < Pt(111). The van der Waals correction to the energy produces large binding energy values, modifies the stability orders and drives the ethylene closer to the surface but the adsorbate geometry parameters remain unchanged. Ethylene was activated on clearly defined binding geometries, changing its hybridization from sp² to sp³ with an elongation (0.16–0.31 Å) of the CC bond. On the basis of this theoretical study, δ-MoC(001) is proposed as a potential catalyst for the hydrogenation of olefins, whereas TiC(001) could be useful for their hydrogenolysis

Bandgap- and Local Field-Dependent Photoactivity of Ag/Black Phosphorus Nanohybrids

Black phosphorus (BP) is the most exciting post-graphene layered nanomaterial that serendipitously bridges the 2D materials gap between semimetallic graphene and large bandgap transition-metal dichalcogenides in terms of high charge-carrier mobility and tunable direct bandgap, yet research into BP-based solar to chemical energy conversion is still in its infancy. Herein, a novel hybrid photocatalyst with Ag nanoparticles supported on BP nanosheets is prepared using a chemical reduction approach. Spin-polarized density functional theory (DFT) calculations show that Ag nanoparticles are stabilized on BP by covalent bonds at the Ag/BP interface and Ag−Ag interactions. In the visible-light photocatalysis of rhodamine B by Ag/BP plasmonic nanohybrids, a significant rise in photoactivity compared with pristine BP nanosheets is observed either by decreasing BP layer thickness or increasing Ag particle size, with the greatest enhancement being up to ∼20-fold. By virtue of finite-difference time domain (FDTD) simulations and photocurrent measurements, we give insights into the enhanced photocatalytic performance of Ag/BP nanohybrids, including the effects of BP layer thickness and Ag particle size. In comparison with BP, Ag/BP nanohybrids present intense local field amplification at the perimeter of Ag NPs, which is increased by either decreasing the BP layer thickness from multiple to few layers or increasing the Ag particle size from 20 to 40 nm. Additionally, when the BP layer thickness is decreased from multiple to few layers, the bandgap becomes favorable to generate more strongly oxidative holes in the proximity of the Ag/BP interface to enhance photoactivity. Our findings illustrate a synergy between locally enhanced electric fields and BP bandgap, in which BP layer thickness and Ag particle size can be independently tuned to enhance photoactivity. This study may open a new avenue for further exploiting BP-based plasmonic nanostructures in photocatalysis, photodetectors, and photovoltaics

Low-Temperature Conversion of Methane to Methanol on CeO_<i>x</i>/Cu₂O Catalysts: Water Controlled Activation of the C–H Bond

An inverse CeO₂/Cu₂O/Cu­(111) catalyst is able to activate methane at room temperature producing C, CH_<i>x</i> fragments and CO_<i>x</i> species on the oxide surface. The addition of water to the system leads to a drastic change in the selectivity of methane activation yielding only adsorbed CH_<i>x</i> fragments. At a temperature of 450 K, in the presence of water, a CH₄ → CH₃OH catalytic transformation occurs with a high selectivity. OH groups formed by the dissociation of water saturate the catalyst surface, removing sites that could decompose CH_<i>x</i> fragments, and generating centers on which methane can directly interact to yield methanol

High Activity of Au/K/TiO₂(110) for CO Oxidation: Alkali-Metal-Enhanced Dispersion of Au and Bonding of CO

Images from scanning tunneling microscopy show high mobility for potassium (K) on an oxidized TiO₂(110) surface. At low coverages, the alkali metal occupies mainly terrace sites of the o-TiO₂(110) system. The results of X-ray photoelectron spectroscopy indicate that K is fully ionized. The electron transferred from K to the titania affects the reactivity of this oxide, favoring the dispersion of Au particles on the terraces of the o-TiO₂(110) surface. When small coverages of K and Au are present on the o-TiO₂(110) system, only a few K–Au pairs are formed and the alkali metal affects Au chemisorption mainly through the oxide interactions. Addition of K to Au/o-TiO₂(110) enhances the reactivity of the system, opening new reaction paths for the adsorption and oxidation of carbon monoxide. CO can undergo disproportionation (2CO → C_ads + CO_2,ads) on K/o-TiO₂(110) and Au/K/o-TiO₂(110) surfaces. The Au–KO_<i>x</i> interface binds CO much better than plain Au–TiO₂, increasing the surface coverage of CO and facilitating its oxidation. Kinetic tests show that K promotes CO oxidation on Au/TiO₂. Turnover frequencies of 2.1 and 10.8 molecules (Au site)⁻¹ s^–1 were calculated for oxidation of CO on Au/o-TiO₂(110) and Au/K/o-TiO₂(110) catalysts, respectively

CO Oxidation on Gold-Supported Iron Oxides: New Insights into Strong Oxide–Metal Interactions

Very active FeO_<i>x</i>–Au catalysts for CO oxidation are obtained after depositing nanoparticles of FeO, Fe₃O₄, and Fe₂O₃ on a Au(111) substrate. Neither FeO nor Fe₂O₃ is stable under the reaction conditions. Under an environment of CO/O₂, they undergo oxidation (FeO) or reduction (Fe₂O₃) to yield nanoparticles of Fe₃O₄ that are not formed in a bulk phase. Using a combined experimental and theoretical approach, we show a strong oxide–metal interaction (SOMI) between Fe₃O₄ nanostructures and Au(111), which gives the oxide special properties, allows the formation of an active phase, and provides a unique interface to facilitate a catalytic reaction. Our work highlights the important role that the SOMI can play in enhancing the catalytic performance of the oxide component in metal–oxide catalysts

Theoretical Study of the Interaction of CO on TiC(001) and Au Nanoparticles Supported on TiC(001): Probing the Nature of the Au/TiC Interface

The interaction of CO with the bare TiC(001) surface and with Au_<i>n</i> (<i>n</i> = 4, 9, 13) nanoparticles supported on the same TiC(001) surface has been studied by means of periodic density functional theory (DFT) based calculations with large supercell slab models. CO adsorption on the bare TiC(001) surface involves the direct interaction with a C surface atom and leads to a significant deformation of the underlying substrate. Because of this feature the calculated adsorption energy significantly varies with coverage. A comparison with available experimental data shows that this system is more complex than expected. The interaction of CO with the Au nanoparticles involves preferential bonding to low coordinated Au atoms. However, although the supported Au nanoparticles bind CO well, the adsorption energy of the molecule on the admetal is somewhat smaller than the one corresponding to the naked carbide surface and decreases with increasing the particle size, which is also consistent with a rather small red shift of the vibrational frequency of the adsorbed CO molecule that also decreased with increasing particle size. Implications for the use of Au/TiC systems in catalytic reactions involving CO are also discussed

Importance of Low Dimensional CeO_<i>x</i> Nanostructures in Pt/CeO_<i>x</i>–TiO₂ Catalysts for the Water–Gas Shift Reaction

CO₂ and H₂ production from the water–gas shift (WGS) reaction was studied over Pt/CeO_<i>x</i>–TiO₂ catalysts with incremental loadings of CeO_<i>x</i>, which adopts variations in the local morphology. The lowest loading of CeO_<i>x</i> (1 wt % to 0.5 at. %) that is configured in its smallest dimensions exhibited the best WGS activity over larger dimensional structures. We attribute this to several factors including the ultrafine dispersed one-dimensional nanocluster geometry, a large concentration of Ce³⁺ and enhanced reducibility of the low loadings. We utilized several in situ experiments to monitor the active state of the catalyst during the WGS reaction. X-ray diffraction (XRD) results showed lattice expansion that indicated reduced ceria was prevalent during the WGS reaction. On the surface, Ce³⁺ related hydroxyl groups were identified by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The enhanced reducibility of the catalyst with the introduction of ceria was further revealed by H₂-temperature programed reduction (H₂-TPR) and good thermal stability was confirmed by <i>in situ</i> environmental transmission electron microscopy (ETEM). We also investigated the formation of the low dimensional structures during catalyst preparation, through a two-stage crystal growth of ceria crystallite on TiO₂ nanoparticle: fine crystallites ∼1D formed at ∼250 °C, followed by crystal growth into 2D chain and 3D particle from 250–400 °C

Room-Temperature Activation of Methane and Dry Re-forming with CO₂ on Ni-CeO₂(111) Surfaces: Effect of Ce³⁺ Sites and Metal–Support Interactions on C–H Bond Cleavage

The results of core-level photoemission indicate that Ni-CeO₂(111) surfaces with small or medium coverages of nickel are able to activate methane at 300 K, producing adsorbed CH_<i>x</i> and CO_<i>x</i> (<i>x</i> = 2, 3) groups. Calculations based on density functional theory predict a relatively low activation energy of 0.6–0.7 eV for the cleavage of the first C–H bond in the adsorbed methane molecule. Ni and O centers of ceria work in a cooperative way in the dissociation of the C–H bond at room temperature, where a low Ni loading is crucial for the catalyst activity and stability. The strong electronic perturbations in the Ni nanoparticles produced by the ceria supports of varying natures, such as stoichiometric and reduced, result in a drastic change in their chemical properties toward methane adsorption and dissociation as well as the dry reforming of methane reaction. The coverage of Ni has a drastic effect on the ability of the system to dissociate methane and catalyze the dry re-forming process