Dissociation of SO2on Au/TiC(001): Effects of Au-C Interactions and Charge Polarization

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.

DFT study the water-gas shift reaction over Cu/α-MoC surface

Cu-based catalysts have been widely used for water-gas shift reaction (WGS, CO + H₂O → CO₂ + H₂), and α-MoC support also shows the good performance for the reaction. Therefore, WGS reaction is systematically studied over Cu/α-MoC by using density functional theory (DFT). DFT result shows the strong metal-support interaction between Cu and α-MoC(111) support. As a result, an extensive tensile strain is introduced in the Cu lattice due to α-MoC support, and Cu 3d band center shifts to Fermi level. However, the strong metal-support interaction does not lead to significant polarization of the Cu/α-MoC surface due to the less charge transfer from Mo to Cu. For the WGS reaction, small Cu particles on α-MoC(111) are likely to facilitate the reaction. At the interface of Cu-α-MoC(111), oxygen stabilizes the dissociated *H, which is benefit of H₂O scission. Then, the activity increases compared with Cu(111) surface. In general, small Cu particles on α-MoC support also have good activity for WGS reaction compared with Au deposition on α-MoC. Graphical abstract.

Molybdenum carbide supported metal catalysts (Mn/MoxC; M = Co, Ni, Cu, Pd, Pt) – metal and surface dependent structure and stability

The surface and metal-dependent morphologies and energies of molybdenum carbide supported metal catalysts (M_n/Mo_xC; M = Co, Ni, Cu, Pd, Pt) have been systematically investigated on the basis of periodic density functional theory computations.

First-Principles Investigation of the Formation of Pt Nanorafts on a Mo2C Support and Their Catalytic Activity for Oxygen Reduction Reaction

We use first-principles calculations to study the formation of Pt nanorafts and their oxygen reduction reaction (ORR) catalytic activity on Mo₂C. Due to the high Pt binding energy on C atoms, Pt forms sheet-like structures on the Mo₂C surface instead of agglomerating into particles. We find that the disordered Mo₂C surface carbon arrangement limits the Pt sheet growth, leading to the formation of 4-6 atom Pt nanorafts. The O-O repulsion between the O atoms on the Mo₂C and O adsorbate enhances the ORR activity by weakening the O adsorption energy. We find a significant change from the usual scaling of the energies of the intermediates in the ORR pathway and a strong interaction between the nanoraft and water that lead to a high activity of the Pt nanorafts. Fundamentally, our work demonstrates that the activity of metal catalysts can be strongly affected by manipulation of the atomic arrangement of the supporting carbide surface.

Highly active Au/δ-MoC and Au/β-Mo2C catalysts for the low-temperature water gas shift reaction: effects of the carbide metal/carbon ratio on the catalyst performance

Water gas shift reaction catalyzed by Mo carbides surfaces and on Au supported thereon is studied by experiments and computational methods.

Hägg carbide surfaces induced Pt morphological changes: a theoretical insight

Comprehensive spin-polarized density functional theory (DFT) combined with ab initio molecular dynamic (AIMD) simulations have been performed to explore the structures, energies, and diffusion behavior of platinum on Fe₅C₂ surfaces with importance in Fischer–Tropsch (F–T) catalysis.

Methane capture at room temperature: adsorption on cubic δ-MoC and orthorhombic β-Mo2C molybdenum carbide (001) surfaces

By means of an surrealistic picture, one can see the most prominent result in this paper; the capacity of δ-MoC(001) surface to sequester methane molecule at room temperature.

The activation of gold and the water-gas shift reaction: insights from studies with model catalysts

The activation of gold in catalytic reactions has been the subject of intensive research that has led to the transformation of one of the least chemically reactive elements to a catalyst with excellent activity and selectivity. Scientists have performed numerous systematic experimental and theoretical studies using model systems, which have explained the role of Au in chemical reactions with progressively increasing degrees of structural and chemical complexity. We present an overview of recent studies of model Au(111), CeOx/Au(111), and Au/CeOx/TiO2(110) surfaces that use Au in different structural configurations specifically for the water-gas shift reaction (WGS, CO + H2O → CO2 + H2), an important industrial process for the purification of CO. We demonstrate the significance of key structural components of the Au-based supported catalysts such as the metal-oxide interface (Au-Ox) toward the WGS catalytic activity, a "structure-activity" relationship. In the WGS reaction, Au(111) or Au nanoparticles have poor catalytic performance due to their inability to activate one of the most important steps of the reaction, the breaking of O-H bonds in the dissociation of water (H2O → OH + H). The relatively large energetic barrier can be overcome by using O on Au(111) to facilitate the formation of OH at low temperatures, with eventual CO2 and H2 production upon reaction between CO and the adsorbed OH. However, the inability to replace the reacted O prevents a sustainable catalytic process from occurring on Au(111). The addition of a small concentration of CeOx nanoparticles on top of the Au(111) surface facilitates this rate-determining step and easily continues the catalytic cycle in the production of H2. We have discovered that CeOx nanoparticles in contact with Au(111) are rich in Ce(3+). They also have a distinct metal-oxide interface, which sustains excellent activity for the WGS reaction via the formation of a unique carboxylate intermediate, making CeOx/Au(111) more active than Cu/ZnO(0001̅), Cu(100), and Cu(111) which are the typical catalysts for this reaction. Taking this knowledge one step further, bringing these components (oxide and metal nanoparticles) together over a second oxide in Au/CeOx/TiO2 produces a system with unique morphological and electronic properties. The result is a superior catalyst for the WGS reaction, both as a model system (Au/CeOx/TiO2(110)) and as powder material (Au/CeOx/TiO2(anatase)) optimized directly in a series of systematic investigations.

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.

Heterogeneous mercury oxidation on au(111) from first principles

Density functional theory (DFT) studies of mercury oxidation on Au(111) are conducted to determine the potential Hg oxidation mechanisms taking place on catalytic gold surfaces by using the Perdew and Wang approximation (PW91) described by a generalized gradient approximation (GGA). The Hg oxidation was examined via a Langmuir-Hinshelwood mechanism where each Hg(0) and Cl2 (or HCl) species is separately adsorbed on the gold surface and the bimolecular reaction occurs through the formation of bound HgCl and HgCl2. For this, the Climbing Image-Nudged Elastic Band (CI-NEB) method has been employed to calculate the activation energies of HgCl and HgCl2 formation pathways. In the three-step Hg oxidation mechanism (Hg → HgCl → HgCl2), the second Cl attachment step is endothermic which is the reaction rate-limiting step, while the first Cl attachment step is exothermic. This observation implies that Hg oxidation prefers a pathway in which HgCl and HgCl2 are formed, rather than a pathway directly oxidizing Hg to HgCl2. In the presence of H atoms due to HCl dissociation on the Au surface, the H atoms lower the activation energy for Hg oxidation by consuming the electron charge of Au atoms, thereby weakening the strength of interaction between Cl and the Au surface and lowering an energy required to detach Cl from the Au surface. This mechanism is in the absence of site competition on the Au surface. In addition, details of the electronic properties of these systems are discussed.

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.

Investigation of adsorption behavior of mercury on Au(111) from first principles

The structural and electronic properties of Hg, SO(2), HgS, and HgO adsorption on Au(111) surfaces have been determined using density functional theory with the generalized gradient approximation. The adsorption strength of Hg on Au(111) increases by a factor of 1.3 (from -9.7 to -12.6 kcal/mol) when the number of surface vacancies increases from 0 to 3; however, the adsorption energy decreases with more than three vacancies. In the case of SO(2) adsorption on Au(111), the Au surface atoms are better able to stabilize the SO(2) molecule when they are highly undercoordinated. The SO(2) adsorption stability is enhanced from -0.8 to -9.3 kcal/mol by increasing the number of vacancies from 0 to 14, with the lowest adsorption energy of -10.2 kcal/mol at 8 Au vacancies. Atomic sulfur and oxygen precovered-Au(111) surfaces lower the Hg stability when Hg adsorbs on the top of S and O atoms. However, a cooperative effect between adjacent Hg atoms is observed as the number of S and Hg atoms increases on the perfect Au(111) surface, resulting in an increase in the magnitude of Hg adsorption. Details of the electronic structure properties of the Hg-Au systems are also discussed.

Sulfur multilayer formation on Au(111): new insights from the study of hexamethyldisilathiane

Hexamethyldisilathiane was successfully used as a new precursor for the formation of S layers on Au and to study their interaction. Characterization of the S modified gold surface was done by X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and scanning tunneling microscopy (STM). Key findings include the direct observation by STM of (i) coexistence of different phases, (ii) multiple sulfur layers formation, (ii) formation of rectangular structures not only on the adlayer but also on the top layer, and (iv) rectangular structure mobility on different layers. These results provide clear evidence regarding the adsorbate nature of the rectangular structures, solving a highly debated question.