Hydrogenation Reactions on Au/TiC(001): Effects of AuC Interactions on the Dissociation of H2

Transition Metal Carbides as Supports for Catalytic Metal Particles: Recent Progress and Opportunities

Transition metal carbides (TMCs) constitute excellent alternatives to traditional oxide-based supports for small metal particles, leading to strong metal-support interactions, which drastically modify the catalytic properties of the supported metal atoms. Moreover, they possess extremely high melting points and good resistance to carbon deposition and sulfur poisoning, and the catalytic activities of some TMCs per se have been shown to be similar to those of Pt-group metals for a considerable number of reactions. Therefore, the use of TMCs as supports can give rise to bifunctional catalysts with multiple active sites. However, at present, only TiC and MoxC have been tested experimentally as supports for metal particles, and it is largely unclear which combinations may best catalyze which chemical reactions. In this Perspective, we review the most significant works on the use of TMCs as supports for catalytic applications, assess the current status of the field, and identify key advances being made and challenges, with an eye to the future.

Reconstruction and catalytic activity of hybrid Pd(100)/(111) monolayer on γ-Al2O3(100) in CH4, H2O, and O2 dissociation

The importance of the "heterogeneity" of a Pd monolayer induced by interaction with a semi-ionic support in catalysis was evaluated. The geometry of the Pd monolayer was optimized on the (100) plane of γ-Al2O3 at fixed unit cell parameters defined by the oxide. Simulation of the deposition of a whole Pd monolayer in the flat Pd(100) form cut from the bulk led to the formation of a slightly distorted Pd(111) monolayer. The subsequent chemisorption or dissociation of CH4 or H2O on the Pd(111) layer resulted in a new hybrid Pd(100)/(111) structure containing alternating elements of (100) and (111) planes (the parallel bands of squares and triangles), which are similar for both CH4 and H2O reactions, and two isolated Pd mono-vacancies, respectively. The hybrid Pd(100)/(111) layer without chemisorbed species was found to be more stable than the initial distorted Pd(111) layer. The catalytic capabilities of these monolayer structures were investigated for the dissociation of methane and the water-gas shift reaction (WGSR) due to the lower predicted activation barriers for CH4, H2O, and O2 dissociation on the hybrid Pd(100)/(111) layer compared to that on the pure (bulk) Pd(100) surface. Moreover, the exothermic heats of these reactions were calculated to be moderate instead of endothermic heats on the Pd(100) or Pd(111) surfaces. The heats of H2O and NH3 adsorption on various monolayers were tested, revealing their dependence on Pd atomic charges. The relevance of the model of the heterogeneous Pd monolayer for explaining the maximum reaction rate experimentally observed at different Pd coverages was discussed. The transferability of the geometry and the extent of charge inhomogeneity of the hybrid monolayer without vacancies were also tested on the same γ-Al2O3(100) support for Pt, Rh, and Ag.

Structural, electronic, and magnetic properties of Ni nanoparticles supported on the TiC(001) surface

Metals supported on transition metal carbides are known to exhibit good catalytic activity and selectivity, which is interpreted in terms of electron polarization induced by the support. In the present work we go one step further and investigate the effect that a titanium carbide (TiC) support has on the structural, electronic, and magnetic properties of a series of Ni nanoparticles of increasing size exhibiting a two- or three-dimensional morphology. The obtained results show that three-dimensional nanoparticles are more stable and easier to form than their homologous two-dimensional counterparts. Also, comparison to previous results indicates that, when used as the support, transition metal carbides have a marked different chemical activity with respect to oxides. The analysis of the magnetic moments of the supported nanoparticles evidences a considerable quenching of the magnetic moment that affects mainly the Ni atoms in close contact with the TiC substrate indicating that these atoms are likely to be responsible for the catalytic activity reported for these systems. The analysis of the electronic structure reveals the existence of chemical interactions between the Ni nanoparticles and the TiC support, even if the net charge transfer between both systems is negligible.

Structural, thermodynamic and kinetic factors in the desorption/absorption of a hydrogen molecule in the M3AlH10−xNa (M = Be or Mg; x = 0 or 2) hydrides

In the context of the design and study of new materials for hydrogen storage, the thermodynamic and kinetic-mechanistic factors for the desorption/absorption of a hydrogen molecule in the M₃AlH_10−xNa (M = Be or Mg; x = 0 or 2) hydrides are evaluated.

Assessing the usefulness of transition metal carbides for hydrogenation reactions

Transition Metal Carbides (TMCs) are proposed as viable replacements for scarce and expensive late Transition Metals (TMs) for heterogeneous catalysis involving hydrogenation reactions or steps.

MoS2-supported gold nanoparticle for CO hydrogenation

Employing dispersion-corrected density functional theory, we examine the geometry, electronic structure, and reactivity of 13-atom Au nanoparticle supported on defect-laden single-layer MoS₂. The planar structure of Au₁₃ favored in isolated phase, transforms into the three-dimensional structure when supported on MoS₂. We find that charge is transferred from MoS₂ to Au₁₃, and that the electron density is also distributed away from the Au₁₃/MoS₂ interfacial region-making Au sites away from the interface catalytically active. Owing to effect of the support, the Au d states become narrower, and the frontier states appear close to the Fermi level. Consequently, in contrast to the reactivity of Au₁₃/TiO₂ toward methanol decomposition, Au₁₃/MoS₂ offers excellent activity toward methanol synthesis, as demonstrated here, via CO hydrogenation.

First-principles investigation of H2S adsorption and dissociation on titanium carbide surfaces

The adsorption and dissociation reactions of H₂S on TiC(001) are investigated using first-principles density functional theory calculations. The geometric and electronic structures of the adsorbed S-based species (including H₂S, SH and S) on TiC(001) are analyzed in detail. It is found that the H₂S is bound weakly, while SH and atomic S are bound strongly on the TiC(001) surface. The transition state calculations show that the formation of SH from H₂S (H₂S → SH + H) is very easy, while the presence of a co-adsorbed H will inhibit the further dissociation of SH (SH + H → S + H + H). In contrast, the hydrogenation of the adsorbed SH is rather easy (SH + H → H₂S). Therefore, the dissociative SH can be removed via the hydrogenation reaction. It is concluded that it is difficult for H₂S to dissociate completely to form atomic S and poison the TiC surface. The results will further provide understanding of the mechanism of the sulfur tolerance of the TiC anode of proton exchange membrane fuel cells (PEMFCs).

The bending machine: CO2 activation and hydrogenation on δ-MoC(001) and β-Mo2C(001) surfaces

The adsorption and activation of a CO₂ molecule on cubic δ-MoC(001) and orthorhombic β-Mo₂C(001) surfaces have been investigated by means of periodic density functional theory based calculations using the Perdew–Burke–Ernzerhof exchange–correlation functional and explicitly accounting for (or neglecting) the dispersive force term description as proposed by Grimme.

CO2 Activation and Methanol Synthesis on Novel Au/TiC and Cu/TiC Catalysts

Small Cu and Au particles in contact with a TiC(001) surface undergo a charge polarization that makes them very active for CO2 activation and the catalytic synthesis of methanol. The binding energy of CO2 on these systems is in the range of 0.6 to 1.1 eV, much larger than those observed on surfaces or nanoparticles of Cu and Au. Thus, in spite of the poor CO2 hydrogenation performance of Cu(111) and Au(111), the Cu/TiC(001) and Au/TiC(001) systems display a catalytic activity for methanol synthesis substantially higher than that of conventional Cu/ZnO catalysts. The turnover frequencies for methanol production on Cu/TiC(001) are 170-500 times much larger than on Cu(111). The present study moves away from the typical approach of using metal/oxide catalysts for the synthesis of methanol via CO2 hydrogenation. This work shows that metal carbides can be excellent supports for enhancing the ability of noble metals to bond and activate CO2.