An insight into geometries and catalytic applications of CeO2 from a DFT outlook

Rare earth metal oxides (REMOs) have gained considerable attention in recent years owing to their distinctive properties and potential applications in electronic devices and catalysts. Particularly, cerium dioxide (CeO2), also known as ceria, has emerged as an interesting material in a wide variety of industrial, technological, and medical applications. Ceria can be synthesized with various morphologies, including rods, cubes, wires, tubes, and spheres. This comprehensive review offers valuable perceptions into the crystal structure, fundamental properties, and reaction mechanisms that govern the well-established surface-assisted reactions over ceria. The activity, selectivity, and stability of ceria, either as a stand-alone catalyst or as supports for other metals, are frequently ascribed to its strong interactions with the adsorbates and its facile redox cycle. Doping of ceria with transition metals is a common strategy to modify the characteristics and to fine-tune its reactive properties. DFT-derived chemical mechanisms are surveyed and presented in light of pertinent experimental findings. Finally, the effect of surface termination on catalysis by ceria is also highlighted

Geometries, electronic properties and stability of molybdenum and tungsten nitrides low-index surfaces

Motivated by the vital role played by transition metal nitride (TMN) composites in various industrial applications, the current study reports electronic properties, thermodynamic stability phase diagram, and vacancy formation energies of the plausible surfaces of NiAs and WC-type structures of δ 3-MoN and δ-WN hexagonal phases, respectively. Low miller indices of various surface terminations of δ 3-MoN and δ-WN namely, (100), (110), (111), and (001) have been considered. Initial cleaving of δ 3-MoN bulk unit cell offers separate Mo and N terminations signified as δ 3-MoN (100): Mo, δ 3-MoN(100):N, δ 3-MoN(111):Mo, δ 3-MoN(111):Mo, and δ 3-MoN(001):Mo. However, the (110) plane reveals mix-truncated with both molybdenum and nitrogen atoms i.e. δ 3-MoN (110): MoN. Likewise, the δ-WN faces incur analogous surface terminations. Ab initio atomistic thermodynamic analyses predict that, N-terminated (111) and (100) slabs to be the most energetically favourable surface terminations amongst the explored surfaces of δ 3-MoN and δ-WN, respectively. Evidenced by plotted density of states (DOS), bulk and surfaces of δ 3-MoN and δ-WN display a metallic character. In terms of surface relaxation and reconstructions, most investigated surfaces experience mainly downward displacements of their topmost layers. Most notably, the relaxed Mo-termination in (111) and (100) surfaces of δ 3-MoN demonstrate significant reconstructions resulted in the first layer to be solely truncated with nitrogen atoms instead of molybdenum in the un-relaxed geometry. Nevertheless, no surface reconstruction has been noticed in most of considered δ-WN configurations. Calculated Bader's electronic charges reveal charge transfer from Mo/W atoms to N atoms, largely retaining the ionic bond nature in their bulk phases. Finally, vacancy formation energy (VFE) calculations showed that introducing nitrogen vacancies through the surface is an endothermic process. Furthermore, the energy required to create a vacant cite in the inner layers differ than that needed in the outer layers. Nitrogen-terminated slabs hold the highest concentrations. Results from this study should be useful when studying the activation of doubly and triply bonded molecules such as N2 at surface vacancies

Catalytic Hydrogenation of p-Chloronitrobenzene to p-Chloroaniline Mediated by γ-Mo2N

Promoting the production of industrially important aromatic chloroamines over transition-metal nitrides catalysts has emerged as a prominent theme in catalysis. This contribution provides an insight into the reduction mechanism of p-chloronitrobenzene (p-CNB) to p-chloroaniline (p-CAN) over the γ-Mo2N(111) surface by means of density functional theory calculations. The adsorption energies of various molecularly adsorbed modes of p-CNB were computed. Our findings display that, p-CNB prefers to be adsorbed over two distinct adsorption sites, namely, Mo-hollow face-centered cubic (fcc) and N-hollow hexagonal close-packed (hcp) sites with adsorption energies of −32.1 and −38.5 kcal/mol, respectively. We establish that the activation of nitro group proceeds through direct pathway along with formation of several reaction intermediates. Most of these intermediaries reside in a significant well-depth in reference to the entrance channel. Central to the constructed mechanism is H-transfer steps from fcc and hcp hollow sites to the NO/–NH groups through modest reaction barriers. Our computed rate constant for the conversion of p-CNB correlates very well with the experimental finding (0.018 versus 0.033 s–1 at ∼500 K). Plotted species profiles via a simplified kinetics model confirms the experimentally reported high selectivity toward the formation of p-CAN at relatively low temperatures. It is hoped that thermokinetics parameters and mechanistic pathways provided herein will afford a molecular level understanding for γ-Mo2N-mediated conversion of halogenated nitrobenzenes into their corresponding nitroanilines; a process that entails significant industrial applications

Selective hydrogenation of Acetylene over γ-Mo2N (100) catalytic surface

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An atomistic approach of thiophene hydrodesulfurization over γ-Mo2N catalyst

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Thermo-elastic and optical properties of molybdenum nitride

This contribution aims to investigate volume-dependent thermal and mechanical properties of two most studied phases of molybdenum nitride (c-MoN and h-MoN) by means of quasi harmonic approximation approach (QHA) via first-principles calculations up to their melting point and a pressure of 12 GPa. Lattice constants, band gaps and bulk modulus at 0 K match well corresponding experimental measurements. Calculated Bader’s charges indicate that Mo-N bonds exhibit more ionic nature in the cubic MoN phase. Based on estimated Gibbs free energies, the cubic phase presents thermodynamic stability higher than that detected for hexagonl, with no phase transition observed in the selected T-P conditions as detected experimentally. The elastic stiffness coefficients of MoN in hexagonal structure revealed that it is stable elastically; in contrast to the cubic structure. The temperature dependence on the bulk modulus is more profound on the dense cubic phase than on the hexagonal phase. Overall, the two considered structures of molybdenum nitride display very minimal harmonic effects, evidenced by the slight variation of thermal and mechanical properties with the increase of pressure and temperature. The optical conductivity of both phases near a zero photon energy coincides well with their metalic character inferred by their Correponding DOS curves. It is expected that the thermo-elastic properties of saturated molybdenum nitrides reported in this study will aid in the continuous pursuit to enhance their catalytic and mechanical utilizations

Mechanisms and kinetics of thiophene hydrodesulfurization over γ-Mo2N catalyst

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Molybdenum nitrides from structures to industrial applications

Owing to their remarkable characteristics, refractory molybdenum nitride (MoNx)-based compounds have been deployed in a wide range of strategic industrial applications. This review reports the electronic and structural properties that render MoNx materials as potent catalytic surfaces for numerous chemical reactions and surveys the syntheses, procedures, and catalytic applications in pertinent industries such as the petroleum industry. In particular, hydrogenation, hydrodesulfurization, and hydrodeoxygenation are essential processes in the refinement of oil segments and their conversions into commodity fuels and platform chemicals. N-vacant sites over a catalyst’s surface are a significant driver of diverse chemical phenomena. Studies on various reaction routes have emphasized that the transfer of adsorbed hydrogen atoms from the N-vacant sites reduces the activation barriers for bond breaking at key structural linkages. Density functional theory has recently provided an atomic-level understanding of Mo–N systems as active ingredients in hydrotreating processes. These Mo–N systems are potentially extendible to the hydrogenation of more complex molecules, most notably, oxygenated aromatic compounds

Structural, electronic and thermodynamic properties of bulk and surfaces of terbium dioxide (TbO2)

This contribution reports a comprehensive investigation into the structural, electronic and thermal properties of bulk and surface terbium dioxide (TbO2); a material that enjoys wide spectra of catalytic and optical applications. Our calculated lattice dimension of 5.36 Å agrees well with the corresponding experimental value at 5.22 Å. Density of states configuration of the bulk structure exhibits a semiconducting nature. Thermo-mechanical properties of bulk TbO2 were obtained based on the quasi-harmonic approximation formalism. Heat capacities, thermal expansions and bulk modulus of the bulk TbO2 were obtained under a wide range of temperatures and pressures. The dependency of these properties on operational pressure is very evident. Cleaving bulk terbium dioxide affords six distinct terminations. Bader's charge distribution analysis for the bulk and the surfaces portrays an ionic character for Tb-O bonds. In an analogy to the well-established finding pertinent to stoichiometric CeO2 surfaces, the (111):Tb surface appears to be the thermodynamically most stable configuration in the nearness of the lean-limit of the oxygen chemical potential. For the corresponding non-stoichiometric structures, we find that, the (111):O + 1VO surface is the most stable configuration across all values of accessible oxygen chemical potentials. The presence of an oxygen vacant site in this surface is expected to enable potent catalytic-assisted reactions, most notably production of hydrogen from water

Dissociation of chlorobenzene (C6H5CI), Chlorothane (C2H5CI) and ispropyl chloride (C3H7CI) on ceria (CeO2) 111 surfaces

Chlorinated volatile organic compounds (Cl-VOCs) are toxic materials that are emitted from any fuel whenever a chlorine source is present. These compounds are often of environmentally persistent nature and induce toxic effects to the wildlife as well as to human health and the best way to control these harmful gases being by destructing them into smaller fragments. At first glance, these materials have been decomposed via thermal oxidation at temperature ~ 1000°C; in processes that are very energy intensive and may lead to the formation of the notorious dioxins compounds in post-combustion chambers. CeOz or CeOzbased materials as alternative of the expensive noble metals based material are widely used as catalysts in prominent industrial applications. Very recently, mounting experimental evidences have established robust catalytic behaviour of Ce02 toward dechlorinating and degradation of Cl-VOCs [ 1 ,2]. Among the various plausible terminations, Ce02(111) is the most stable surface involving mixed Ce/0 terminations. In this work, we inspected the dissociation of representative Cl-VOCs, namely chlorobenzene (C6H5Cl), chloroethane (CzH5Cl) and ispropyl chloride (C3H7Cl), termed as CB, CE and IC respectively, over the Ce02 (111) surface using density functional theory (DFT). The on-site Coulomb interaction correction (DFT +U) method developed by Dudarev et al [3] was employed to produce more accurate energetic profiles of these systems. Nudged Elastic Band (NEB) method [4] was used to calculate the reaction paths. The DFT calculations revealed that Ce02(111) surface mediates fission of the carbon-chlorine bonds in CB, CE and IC via accessible energy barriers of 3.32, 2.82 and 1.56 eV, respectively. The dissociated chlorine atoms and CxHy moieties exists in 1.64, 1.36 and 1.70 eV below the entrance channel for CB, CE and IC molecules, respectively. Results from this study should be helpful to understand the role of Ce02-based catalysts in destruction of persistent halogenated pollutants