Effects of carbon on the stability and chemical performance of transition metal carbides: A density functional study

Surface Termination (-O, -F or -OH) and Metal Doping on the HER Activity of Mo2CTx MXene

Two-dimensional MXenes have recently garnered significant attention as electrocatalytic materials for hydrogen evolution reaction (HER). However, previous theoretical studies mainly focused on the effect of pure functional groups while neglecting hybrid functional groups that are commonly observed in experiments. Herein, we investigated the hybrid functionalized Mo2CTx MXene (T=-O, -F or -OH) to probe the HER properties. In binary O/F co-functionalization, the presence of F groups would attenuate the H adsorption and lead to the enhanced HER activity than the fully O-terminated Mo2CO2. However, the surface HER activity of ternary O/F/OH functionalized Mo2CTx is not satisfactory owing to the relatively weak H adsorption capacity. To further enhance the catalytic activity, modification was performed by introducing another metal element into its lattice structure. The doped metal (Fe, Co, Ni, Cu) exhibits reduced charge transfer to O compared to Mo atoms, leading to enhanced H adsorption and improved overall activity. The synergistic effect of hybrid functionalization and TM modification provides useful guidance for achieving feasible Mo2CTx candidates with high HER performance, which can be applied to the electrocatalytic applications of other MXenes.

Tuning the degree of CO2 activation by carbon doping Cun- (n = 3-10) clusters: an IR spectroscopic study

Copper clusters on carbide surfaces have shown a high catalytic activity towards methanol formation. To understand the interaction between CO₂ and the catalytically active sites during this process and the role that carbon atoms could play in this, they are modeled by copper clusters, with carbon atoms incorporated. The formed clusters Cu_nC_m^- (n = 3-10, m = 1-2) are reacted with CO₂ and investigated by IR multiple-photon dissociation (IR-MPD) spectroscopy to probe the degree of CO₂ activation. IR spectra for the reaction products [Cu_nC·CO₂]^-, (n = 6-10), and [Cu_nC₂·CO₂]^-, (n = 3-8) are compared to reference spectra recorded for products formed when reacting the same cluster sizes with CO, and with density functional theory (DFT) calculated spectra. The results reveal a size- and carbon load-dependent activation and dissociation of CO₂. The complexes [Cu_nC·CO₂]^- with n = 6 and 10 show predominantly molecular activation of CO₂, while those with n = 7-9 show only dissociative adsorption. The addition of the second carbon to the cluster leads to the exclusive molecular activation of the CO₂ on all measured cluster sizes, except for Cu₅C₂^- where CO₂ dissociates. Combining these findings with DFT calculations leads us to speculate that at lower carbon-to-metal ratios (CMRs), the C can act as an oxygen anchor facilitating the OCO bond rupture, whereas at higher CMRs the carbon atoms increasingly attract negative charge, reducing the Cu cluster's ability to donate electron density to CO₂, and consequently its ability to activate CO₂.

Carbide coating on nickel to enhance the stability of supported metal nanoclusters

The influence on the growth of cobalt (Co)-based nanostructures of a surface carbide (Ni₂C) layer formed at the Ni(100) surface is revealed via complementary scanning tunneling microscopy (STM) measurements and first-principles calculations. On clean Ni(100) below 200 °C in the sub-monolayer regime, Co forms randomly distributed two-dimensional (2D) islands, while on Ni₂C it grows in the direction perpendicular to the surface as well, thus forming two-atomic-layers high islands. We present a simple yet powerful model that explains the different Co growth modes for the two surfaces. A jagged step decoration, not visible on stepped Ni(100), is present on Ni₂C. This contrasting behavior on Ni₂C is explained by the sharp differences in the mobility of Co atoms for the two cases. By increasing the temperature, Co dissolution is activated with almost no remaining Co at 250 °C on Ni(100) and Co islands still visible on the Ni₂C surface up to 300 °C. The higher thermal stability of Co above the Ni₂C surface is rationalized by ab initio calculations, which also suggest the existence of a vacancy-assisted mechanism for Co dissolution in Ni(100). The methodology presented in this paper, combining systematically STM measurements with first-principles calculations and computational modelling, opens the way to controlled engineering of bimetallic surfaces with tailored properties.

Tuning product selectivity in CO2 hydrogenation over metal-based catalysts

Conversion of CO2 into chemicals is a promising strategy for CO2 utilization, but its intricate transformation pathways and insufficient product selectivity still pose challenges. Exploiting new catalysts for tuning product selectivity in CO2 hydrogenation is important to improve the viability of this technology, where reverse water-gas shift (RWGS) and methanation as competitive reactions play key roles in controlling product selectivity in CO2 hydrogenation. So far, a series of metal-based catalysts with adjustable strong metal-support interactions, metal surface structure, and local environment of active sites have been developed, significantly tuning the product selectivity in CO2 hydrogenation. Herein, we describe the recent advances in the fundamental understanding of the two reactions in CO2 hydrogenation, in terms of emerging new catalysts which regulate the catalytic structure and switch reaction pathways, where the strong metal-support interactions, metal surface structure, and local environment of the active sites are particularly discussed. They are expected to enable efficient catalyst design for minimizing the deep hydrogenation and controlling the reaction towards the RWGS reaction. Finally, the potential utilization of these strategies for improving the performance of industrial catalysts is examined.

Activation of Carbon Dioxide by CoCDn- (n = 0-4) Anions

Laser ablation generated CoCD_n^- (n = 0-4) anions were mass selected and then reacted with CO₂ in an ion trap reactor. The reactions were characterized by mass spectrometry and quantum chemical calculations. The experimental results demonstrated that the CoC^- anion can convert CO₂ into CO. In contrast, the bare Co^- anion is inert toward CO₂. Coordinated D ligands can modify the reactivity of CoCD_1-4^- in which CoCD_1-3^- can reduce CO₂ into CO selectively and CoCD₄^- can only adsorb CO₂. The crucial roles of the coordinated C and D ligands to tune the reactivity of CoCD_n^- (n = 0-4) toward CO₂ were rationalized by theoretical calculations. Note that the hydrogenation process that is usually observed in the reactions of gas-phase metal hydrides with CO₂ is completely suppressed for the reactions CoCD_n^- + CO₂. This study provides insights into the molecular-level origin for the observations that CO can be selectively generated from CO₂ catalyzed by cobalt-containing carbides in heterogeneous catalysis.

Stable and Antisintering Tungsten Carbides with Controllable Active Phase for Selective Cleavage of Aryl Ether C-O Bonds

Transition-metal carbides are important materials in heterogeneous catalysis. It remains challenging yet attractive in nanoscience to construct the active phase of carbide catalysts in a controllable manner and keep a sintering-resistant property in redox reactions, especially hydroprocessing. In this work, an integrated strategy was presented to synthesize stable and well-defined tungsten carbide nanoparticles (NPs) by assembling the metal precursor onto carbon nanotubes (CNTs), wrapping a thin polymeric layer, and following a controlled carburization. The polymer served as a soft carbon source to modulate the metal/carbon ratio in the carbides and introduced amorphous carbons around the carbides to prevent the NPs from sintering. The as-built p-W_xC/CNT displayed high stability in the hydrogenolysis of aryl ether C-O bond in guaiacol for more than 150 h. Its activity was more than two and six times higher than those prepared via typical temperature-programmed reduction with gaseous carbon (W_xC/CNT-TPR) and carbothermal reduction with intrinsic carbon support (W_xC/CNT-CTR), respectively. Our p-W_xC/CNT catalyst also achieved high efficiency for selective cleavage of the aryl ether C-O bonds in lignin-derived aromatic ethers, including anisole, dimethoxylphenol, and diphenyl ether, with a robust lifespan.

Role of Transition Metals on TM/Mo2C Composites: Hydrogen Evolution Activity in Mildly Acidic and Alkaline Media

Modification of electronic and chemical properties of a material by the introduction of another element into its lattice is one of the most common methods for designing new catalysts for different applications. In this work the effect of modifying molybdenum carbide with transition metals (Fe, Co, Ni, Cu), TM-Mo₂C composites, upon the catalytic activity toward hydrogen evolution reaction (HER) in mild acidic and alkaline media has been studied. Catalysts were prepared by carbothermal reduction of molybdenum and TM oxides precursors and were characterized by different physicochemical techniques. Results evidenced a strong pH effect on the catalytic performance of TM-Mo₂C, while, at pH = 5, inclusion of TM into the Mo₂C lattice has a deleterious effect on the HER activity and, at pH = 9, a promoting effect was observed, highlighting the importance of considering specific operation conditions during the catalyst design process. Analysis of in situ near-edge X-ray adsorption data reveals a decrease on the oxidation state and average bond ionicity of dopant metal upon a pH increase, shedding light of the different effects of TMs on the resulting HER activity in acidic and alkaline media. Finally, stability tests demonstrated no deterioration on catalysts' performance after 8 h of continuous cycling within the HER working range, confirming the suitability of Mo₂C materials as promising HER catalysts.

Sustainable hydrogen production by molybdenum carbide-based efficient photocatalysts: From properties to mechanism

Hydrogen is considered to be a promising energy carrier to solve the issue of energy crisis. Molybdenum carbide (Mo_xC) is the typical material, which has similar properties of Pt and thought to be an attractive alternative to noble metals for H₂ evolution. The study of Mo_xC as alternative catalyst for H₂ production is almost focused on electrocatalytic field, while the application of Mo_xC as a co-catalyst in photocatalytic H₂ evolution has received in-depth research in recent years. Particularly, Mo_xC exhibits significant enhancement in the H₂ production performance of semiconductors under visible light irradiation. However, a review discussing Mo_xC serving as a co-catalysts in the photocatalytic H₂ evolution is still absent. Herein, the recent progress of Mo_xC on photocatalytic H₂ evolution is reviewed. Firstly, the preparation methods including chemical vapor deposition, temperature programming, and organic-inorganic hybridization are detailly summarized. Then, the fundamental structure, electronic properties, and specific conductance of Mo_xC are illustrated to illuminate the advantages of Mo_xC as a co-catalyst for H₂ evolution. Furthermore, the different heterojunctions formed between Mo_xC and other semiconductors for enhancing the photocatalytic performance are emphasized. Finally, perspectives regarding the current challenges and the future research directions on the improvement of catalytic performance of Mo_xC-based photocatalysts are also presented.

Activation of Gold on Metal Carbides: Novel Catalysts for C1 Chemistry

This article presents a review of recent uses of Au-carbide interfaces as catalysts for C1 Chemistry (CO oxidation, low-temperature water-gas shift, and CO₂ hydrogenation). The results of density-functional calculations and photoemission point to important electronic perturbations when small two-dimensional clusters of gold are bounded to the (001) surface of various transition metal carbides (TiC, ZrC, VC, Ta C, and δ-MoC). On these surfaces, the C sites exhibited strong interactions with the gold clusters. On the carbide surfaces, the Au interacts stronger than on oxides opening the door for strong metal-support interactions. So far, most of the experimental studies with well-defined systems have been focused on the Au/TiC, Au/δ-MoC, and Au/β-Mo₂C interfaces. Au/TiC and Au/δ-MoC are active and stable catalysts for the low-temperature water-gas shift reaction and for the hydrogenation of CO₂ to methanol or CO. Variations in the behavior of the Au/δ-MoC and Au/β-Mo₂C systems clearly show the strong effect of the metal/carbon ratio on the performance of the carbide catalysts. This parameter substantially impacts the chemical behavior of the carbide and its interaction with supported metals, up to the point of modifying the reaction rate and mechanism of C1 processes.

The shape-dependent surface oxidation of 2D ultrathin Mo2C crystals

2D atomic crystals have been widely explored, usually owing to their numerous shapes, of which the typical hexagon has drawn the most interest. However, the relationship between shape and properties has not been fully probed, owing to the lack of a proper system. Here, we demonstrate for the first time the shape-dependent surface oxidation of 2D Mo₂C crystals, where the elongated flakes are preferentially oxidized under ambient conditions when compared with regular ones, showing higher chemical activity. The gradual surface oxidation of elongated Mo₂C crystals as a function of time is clearly observable. Structural determinations reveal that a discrepancy in the arrangement of Mo and C atoms between elongated and regular crystals accounts for the selective oxidation behavior. The identification of the shape-dependent surface oxidization of Mo₂C crystals provides significant possibilities for tuning the properties of 2D materials via shape-control.

Enhanced Synergetic Catalytic Effect of Mo2C/NCNTs@Co Heterostructures in Dye-Sensitized Solar Cells: Fine-Tuned Energy Level Alignment and Efficient Charge Transfer Behavior

A highly efficient and stable electrocatalyst with the novel heterostructure of Co-embedded and N-doped carbon nanotubes supported Mo₂C nanoparticles (Mo₂C/NCNTs@Co) is creatively constructed by adopting the one-step metal catalyzed carbonization-nitridation strategy. Systematic characterizations and density functional theory (DFT) calculations reveal the advanced structural and electronic properties of Mo₂C/NCNTs@Co heterostructure, in which the Co-embedded and N-doped CNTs with tunable diameters present electron-donating effect and the work function is correspondingly regulated from 4.91 to 4.52 eV, and the size-controlled Mo₂C nanoparticles exhibit Pt-like 4d electronic structure and the well matched work function (4.85 eV) with I^-/I₃^- redox couples (4.90 eV). As a result, the conductive NCNTs@Co substrate with fine-tuned energy level alignment accelerates the electron transportation and the electron migration from NCNTs@Co to Mo₂C, and the active Mo₂C shows high affinity for I₃^- adsorption and high charge transfer ability for I₃^- reduction, which reach a decent synergetic catalytic effect in Mo₂C/NCNTs@Co heterostructure. The DSSC with Mo₂C/NCNTs@Co CE achieves a high photoelectric conversion efficiency of 8.82% and exceptional electrochemical stability with a residual efficiency of 7.95% after continuous illumination of 200 h, better than Pt-based cell. Moreover, the synergistic catalytic mechanism toward I₃^- reduction is comprehensively studied on the basis of structure-activity correlation and DFT calculations. The advanced heterostructure engineering and electronic modulation provide a new design principle to develop the efficient, stable, and economic hybrid catalysts in relevant electrocatalytic fields.

Molybdenum carbide catalyst for the reduction of CO2 to CO: surface science aspects by NAPPES and catalysis studies

Carbon dioxide is a greenhouse gas, and needs to be converted into one of the useful feedstocks, such as carbon monoxide and methanol. We demonstrate the reduction of CO₂ with H₂ as a reducing agent, via a reverse water gas shift (RWGS) reaction, by using a potential and low cost Mo₂C catalyst. Mo₂C was evaluated for CO₂ hydrogenation at ambient pressure as a function of temperature, and CO₂ : H₂ ratio at a gas hourly space velocity (GHSV) of 20 000 h^-1. It is demonstrated that the Mo₂C catalyst with 1 : 3 ratio of CO₂ : H₂ is highly active (58% CO₂ conversion) and selective (62%) towards CO at 723 K at ambient pressure. Both properties (basicity and redox properties) and high catalytic activity observed with Mo₂C around 700 K correlate well and indicate a strong synergy among them towards CO₂ activation. X-ray diffraction and Raman analysis show that the Mo₂C catalyst remains in the β-Mo₂C form before and after the reaction. The mechanistic aspects of the RWGS reaction were determined by near-ambient pressure X-ray photoelectron spectroscopy (NAPXPS) with in situ generated Mo₂C from carburization of Mo-metal foil. NAPXPS measurements were carried out at near ambient pressure (0.1 mbar) and various temperatures. Throughout the reaction, no significant changes in the Mo²⁺ oxidation state (of Mo₂C) were observed indicating that the catalyst is highly stable; C and O 1s spectral results indicate the oxycarbide species as an active intermediate for RWGS. A good correlation is observed between catalytic activity from atmospheric pressure reactors and the electronic structure details derived from NAPXPS results, which establishes the structure-activity correlation.

An electronic perturbation in TiC supported platinum monolayer catalyst for enhancing water-gas shift performance: DFT study

The water-gas shift (WGS) reaction behaviors over the TiC(0 0 1) supported Pt monolayer catalyst (Pt_ML/TiC(0 0 1)) are investigated by using the spin-unrestricted density functional theory calculations. Importantly, we find that the Pt_ML/TiC(0 0 1) system exhibits a much lower density of Pt-5d states nearby the Fermi level compared with that for Pt(1 1 1), and the monolayer Pt atoms undergo an electronic perturbation when in contact with TiC(0 0 1) support that would strongly improve the WGS activity of supported Pt atoms. Our calculations clearly indicate that the dominant reaction path follows a carboxyl mechanism involving a key COOH intermediate, rather than the common redox mechanism. Furthermore, through the detailed comparisons, the results demonstrate that the strong interactions between the monolayer Pt atoms and TiC(0 0 1) support make Pt_ML/TiC(0 0 1) a highly active catalyst for the low-temperature WGS reaction. Following the route presented by Bruix et al (2012 J. Am. Chem. Soc. 134 8968-74), the positive effect derived from strong metal-support interaction in the metal/carbide system is revealed.

Molybdenum carbide as an efficient and durable catalyst for aqueous Knoevenagel condensation

Molybdenum carbide showed an efficient performance for the Knoevenagel condensation in aqueous media at room temperature, affording the corresponding products in high yields within a short reaction time.

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.

W2C nanorods with various amounts of vacancy defects: determination of catalytic active sites in the hydrodeoxygenation of benzofuran

Phase-pure W₂C nanorods were prepared by pyrolysis of a metatungstate and melamine hybrid, and showed excellent performance in the hydrodeoxygenation of benzofuran.

A novel route towards well-dispersed short nanofibers and nanoparticles via electrospinning

A novel, simple, and widely applicable electrospinning–calcination–grinding route capable of preparing well-dispersed inorganic nanoparticles and short nanofibers is reported.

Molybdenum carbide as an efficient catalyst for low-temperature hydrogenation of dimethyl oxalate

Silica-supported molybdenum carbide (Mo₂C/SiO₂) is found to be a highly active, selective and stable catalyst for the hydrogenation of dimethyl oxalate to ethanol at low temperatures (473 K).

Identifying trends and descriptors for selective CO2 conversion to CO over transition metal carbides

Transition metal carbides are promising catalysts for CO₂ reduction and their activity is correlated with oxygen binding energy and reducibility.

Adsorption and desulfurization reaction mechanism of thiophene and its hydrogenated derivatives over NbC(001) and NbN(001): an ab initio DFT study

Herein, we present periodic DFT-based calculations on the thiophene and its H-derivatives adsorption and reaction pathways over niobium carbide and nitride cubic face-centered (001) surfaces.

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.