Passivation agents and conditions for Mo2C and W2C: Effect on catalytic activity for toluene hydrogenation

Anisotropic Growth of One-Dimensional Carbides in Single-Walled Carbon Nanotubes with Strong Interaction for Catalysis

Tungsten and molybdenum carbides have shown great potential in catalysis and superconductivity. However, the synthesis of ultrathin W/Mo carbides with a controlled dimension and unique structure is still difficult. Here, inspired by the host-guest assembly strategy with single-walled carbon nanotubes (SWCNTs) as a transparent template, we reported the synthesis of ultrathin (0.8-2.0 nm) W₂C and Mo₂C nanowires confined in SWCNTs deriving from the encapsulated W/Mo polyoxometalate clusters. The atom-resolved electron microscope combined with spectroscopy and theoretical calculations revealed that the strong interaction between the highly carbophilic W/Mo and SWCNT resulted in the anisotropic growth of carbide nanowires along a specific crystal direction, accompanied by lattice strain and electron donation to the SWCNTs. The SWCNT template endowed carbides with resistance to H₂O corrosion. Different from normal modification on the outer surface of SWCNTs, such M₂C@SWCNTs (M = W, Mo) provided a delocalized and electron-enriched SWCNT surface to uniformly construct the negatively charged Pd catalyst, which was demonstrated to inhibit the formation of active PdH_x hydride and thus achieve highly selective semihydrogenation of a series of alkynes. This work could provide a nondestructive way to design the electron-delocalized SWCNT surface and expand the methodology in synthesizing unusual 1D ultrathin carbophilic-metal nanowires (e.g., TaC, NbC, β-W) with precise control of the anisotropy in SWCNT arrays.

Synthesis and Characterization of Supported Mixed MoW Carbide Catalysts

For mixed MoW carbide catalysts, the relationship between synthesis conditions, evolution of (mixed) phases, extent of mixing, and catalytic performance of supported Mo/W carbides remains unclear. In this study, we prepared a series of carbon nanofiber-supported mixed Mo/W-carbide catalysts with varying Mo and W compositions using either temperature-programmed reduction (TPR) or carbothermal reduction (CR). Regardless of the synthesis method, all bimetallic catalysts (Mo:W bulk ratios of 1:3, 1:1, and 3:1) were mixed at the nanoscale, although the Mo/W ratio in individual nanoparticles varied from the expected bulk values. Moreover, the crystal structures of the produced phases and nanoparticle sizes differed depending on the synthesis method. When using the TPR method, a cubic carbide (MeC_1-x ) phase with 3-4 nm nanoparticles was obtained, while a hexagonal phase (Me₂C) with 4-5 nm nanoparticles was found when using the CR method. The TPR-synthesized carbides exhibited higher activity for the hydrodeoxygenation of fatty acids, tentatively attributed to a combination of crystal structure and particle size.

Establishing tungsten carbides as active catalysts for CO2 hydrogenation

Molybdenum carbides are promising catalysts for the reverse water-gas shift (RWGS) reaction, and we aim to understand if similar performance can be observed across the library of transition metal carbides. Although tungsten and molybdenum carbides exhibit similar catalytic properties for hydrogenation reactions, tungsten carbide has not been thoroughly evaluated for CO₂ hydrogenation. We hypothesize that the extreme synthesis conditions necessary for carburizing tungsten can cause sintering, agglomeration, and carbon deposition, leading to difficulty evaluating the intrinsic activity of tungsten carbides. In this work, tungsten is encapsulated in silica to preserve particle size and demonstrate correlations between the active phase composition and RWGS performance.

The structural evolution of Mo2C and Mo2C/SiO2 under dry reforming of methane conditions: morphology and support effects

The thermal carburization of MoO₃ nanobelts (nb) and SiO₂-supported MoO₃ nanosheets under a 1 : 4 mixture of CH₄ : H₂ yields Mo₂C-nb and Mo₂C/SiO₂. Following this process by in situ Mo K-edge X-ray absorption spectroscopy (XAS) reveals different carburization pathways for unsupported and supported MoO₃. In particular, the carburization of α-MoO₃-nb proceeds via MoO₂, and that of MoO₃/SiO₂via the formation of highly dispersed MoO_x species. Both Mo₂C-nb and Mo₂C/SiO₂ catalyze the dry reforming of methane (DRM, 800 °C, 8 bar) but their catalytic stability differs. Mo₂C-nb shows a stable performance when using a CH₄-rich feed (CH₄ : CO₂ = 4 : 2), however deactivation due to the formation of MoO₂ occurs for higher CO₂ concentrations (CH₄ : CO₂ = 4 : 3). In contrast, Mo₂C/SiO₂ is notably more stable than Mo₂C-nb under the CH₄ : CO₂ = 4 : 3 feed. The influence of the morphology of Mo₂C and its dispersion on silica on the structural evolution of the catalysts under DRM is further studied by in situ Mo K-edge XAS. It is found that Mo₂C/SiO₂ features a higher resistance to oxidation under DRM than the highly crystalline unsupported Mo₂C-nb and this correlates with an improved catalytic stability. Lastly, the oxidation of Mo in both Mo₂C-nb and Mo₂C/SiO₂ under DRM conditions in the in situ XAS experiments leads to an increased activity of the competing reverse water gas shift reaction.

In situ Mo K-edge X-ray absorption spectroscopy reveals different carburization pathways for unsupported and supported MoO₃ yielding Mo₂C and Mo₂C/SiO₂. Mo₂C/SiO₂ features higher resistance to oxidation under dry reforming of methane than Mo₂C.

Metal (Mo, W, Ti) Carbide Catalysts: Synthesis and Application as Alternative Catalysts for Dry Reforming of Hydrocarbons-A Review

Dry reforming of hydrocarbons (DRH) is a pro-environmental method for syngas production. It owes its pro-environmental character to the use of carbon dioxide, which is one of the main greenhouse gases. Currently used nickel catalysts on oxide supports suffer from rapid deactivation due to sintering of active metal particles or the deposition of carbon deposits blocking the flow of gases through the reaction tube. In this view, new alternative catalysts are highly sought after. Transition metal carbides (TMCs) can potentially replace traditional nickel catalysts due to their stability and activity in DR processes. The catalytic activity of carbides results from the synthesis-dependent structural properties of carbides. In this respect, this review presents the most important methods of titanium, molybdenum, and tungsten carbide synthesis and the influence of their properties on activity in catalyzing the reaction of methane with carbon dioxide.

Molybdenum-Tungsten Blue Nanoparticles as a Precursor for Ultrafine Binary Carbides

Herein, we demonstrate a promising method for the synthesis of ultrafine carbide particles using dispersions of molybdenum–tungsten nanoparticles. Dispersions of molybdenum–tungsten blue nanoparticles with different initial molar ratios of molybdenum/tungsten were synthesized through the reduction of molybdate and tungstate ions by ascorbic acid in an acidic medium (pH = 1.0–2.5). Molybdenum–tungsten blue nanoparticles were characterized by ultraviolet–visual (UV–VIS), infrared (FTIR), and X-ray photoelectron (XPS) spectroscopies; transmission electronic microscopy (TEM); and dynamic light scattering (DLS). We demonstrated that molybdenum–tungsten blue nanoparticles belong to toroidal polyoxometalate clusters (λ_max = 680–750 nm) with a predominant particle size of 4.0 nm. Molybdenum–tungsten blue dispersions were shown to be monodispersed systems with a small particle size and long-term stability (>30 days) and are suitable for further catalytic applications.

A Review of Preparation Strategies for α-MoC1-x Catalysts

Transition metal carbides are attracting growing attention as robust and affordable alternative heterogeneous catalysts to platinum group metals, for a host of contemporary and established hydrogenation, dehydrogenation, and isomerisation reactions. In particular, the metastable α-MoC1-x phase has been shown to exhibit interesting catalytic properties for low temperature processes reliant on O-H and C-H bond activation. While demonstrating exciting catalytic properties, a significant challenge exists in the application of metastable carbides, namely the challenging procedure for their preparation. In this review we will briefly discuss the properties and catalytic applications of α-MoC1-x, followed by a more detailed discussion on available synthesis methods and important parameters that influence carbide properties. Techniques are contrasted with properties of phase, surface area, morphology and Mo:C being considered. Further, we briefly relate these observations to experimental and theoretical studies of α-MoC1-x in catalytic applications. Synthetic strategies discussed are, the original temperature programmed ammonolysis followed by carburisation, alternative oxycarbide or hydrogen bronze precursor phases, heat treatment of moybdate-amide compounds and other low temperature synthetic routes. The importance of carbon removal and catalyst passivation in relation to surface and bulk properties are also discussed. Novel techniques that by-pass the apparent bottle neck of ammonolysis are reported, however a clear understanding of intermediate phases is required to be able to fully apply these techniques. Pragmatically, the scaled application of these techniques requires the pre-pyrolysis wet chemistry to be simple and scalable. Further, there is a clear opportunity to correlate observed morphologies/phases and catalytic properties with findings from computational theoretical studies. Detailed characterisation throughout the synthetic process is essential and will undoubtedly provide fundamental insights that can be used for the controllable and scalable synthesis of metastable α-MoC1-x.

Synthesis of Microporous Mo2C-W2C Binary Carbides by Thermal Decomposition of Molybdenum-Tungsten Blues

Molybdenum and tungsten carbides are perspective catalytic systems. Their activity in many reactions is comparable to the activity of platinum group metals. The development of the synthesis method for of highly dispersed binary molybdenum and tungsten carbides is an important task. Dispersions of molybdenum-tungsten blue were used as a precursor for synthesis of binary molybdenum and tungsten carbides. The synthesis of carbides was carried out by thermal decomposition of molybdenum-tungsten blue xerogels in an inert atmosphere. The binary carbides were characterized by XRD, TGA, SEM and nitrogen adsorption. The influence of the molar ratio reducing agent/Me [R]/[ΣMe], molar ratio molybdenum/tungsten [Mo]/[W] on phase composition, and morphology and porous structure of binary carbides was investigated. Samples of binary molybdenum and tungsten carbides with a highly developed porous structure and a specific surface area were synthesized.

On Catalytic Behavior of Bulk Mo2C in the Hydrodenitrogenation of Indole over a Wide Range of Conversion Thereof

The catalytic activity of bulk molybdenum carbide (Mo2C) in the hydrodenitrogenation (HDN) of indole was studied. The catalyst was synthesized using a temperature-programmed reaction of the respective oxide precursor (MoO3) with the carburizing gas mixture of 10 vol.\% CH4/H2. The resultant material was characterized using X-ray diffraction, CO chemisorption, and nitrogen adsorption. The catalytic activity was studied in the HDN of indole over a wide range of conversion thereof and in the presence of a low amount of sulfur (50 ppm), which was used to simulate the processing of real petroleum intermediates. The molybdenum carbide has shown high activity under the tested operating conditions. Apparently, the bulk molybdenum carbide turned out to be selective towards the formation of aromatic products such as ethylbenzene, toluene, and benzene. The main products of HDN were ethylbenzene and ethylcyclohexane. After 99% conversion of indole HDN was reached (i.e., lack of N-containing compounds in the products was observed), the hydrogenation of ethylbenzene to ethylcyclohexane took place. Thus, the catalytic behavior of bulk molybdenum carbide for the HDN of indole is completely different compared to previously studied sulfide-based systems.

Transition Metal Carbides (TMCs) Catalysts for Gas Phase CO2 Upgrading Reactions: A Comprehensive Overview

Increasing demand for CO2 utilization reactions and the stable character of CO2 have motivated interest in developing highly active, selective and stable catalysts. Precious metal catalysts have been studied extensively due to their high activities, but their implementation for industrial applications is hindered due to their elevated cost. Among the materials which have comparatively low prices, transition metal carbides (TMCs) are deemed to display catalytic properties similar to Pt-group metals (Ru, Rh, Pd, Ir, Pt) in several reactions such as hydrogenation and dehydrogenation processes. In addition, they are excellent substrates to disperse metallic particles. Hence, the unique properties of TMCs make them ideal substitutes for precious metals resulting in promising catalysts for CO2 utilization reactions. This work aims to provide a comprehensive overview of recent advances on TMCs catalysts towards gas phase CO2 utilization processes, such as CO2 methanation, reverse water gas shift (rWGS) and dry reforming of methane (DRM). We have carefully analyzed synthesis procedures, performances and limitations of different TMCs catalysts. Insights on material characteristics such as crystal structure and surface chemistry and their connection with the catalytic activity are also critically reviewed.

Hydrogenation of furfural by noble metal-free nickel modified tungsten carbide catalysts

Nickel–tungsten carbide catalysts convert furfural to high value products in a liquid phase catalytic reaction. The product distribution depends on the solvent and the Ni–W-ratio of the catalyst. In isopropyl alcohol a combination of Ni and W_xC enables the opening of the furan ring to yield 1,2-pentanediol. Nickel accelerates the tungsten oxide reduction in the tungsten carbide catalyst synthesis and facilitates the carbon insertion. Nickel modified tungsten carbide is a promising, noble metal-free catalyst system for the upgrading of furfural based renewable resources. Its preparation is facilitated compared to unmodified tungsten carbide catalysts.

Furfural is converted over a noble metal-free, nickel modified tungsten carbide catalyst to high value products, including 1,2-pentanediol.

Surface reconstruction and the effect of Ni-modification on the selective hydrogenation of 1,3-butadiene over Mo2C-based catalysts

In the current study, Mo₂C, NiMo₂C, H–Mo₂C and H–NiMo₂C were synthesized to understand the effects of Ni modification and surface reconstruction.

Tuning the reactivity of molybdenum (oxy)carbide catalysts by the carburization degree: CO2 reduction and anisole hydrodeoxygenation

Structure-performance relations for molybdenum (oxy)carbide catalysts evaluated for CO₂ hydrogenation and anisole hydrodeoxygenation.

Effect of Platinum, Gold, and Potassium Additives on the Surface Chemistry of CdI2-Antitype Mo2C

Transition metal carbides are versatile materials for diverse industrial applications including catalysis, where their relatively low cost is very attractive. In this work, we present a rather extensive density functional theory study on the energetics of adsorption of a selection of atomic and molecular species on two Mo terminations of the CdI₂ antitype phase of Mo₂C. Moreover, the coadsorption of CO in the presence of preadsorbed metal atoms and its dissociative adsorption in the absence and presence of preadsorbed Pt and K were investigated. By using CO as a probe to understand the structural/electronic effects of the preadsorption of the metal atoms on the Mo₂C(001) surface, we showed that K further enhances CO adsorption/activation on the surface, in contrast to the precious metals considered. Moreover, it was observed that the presence of both Pt and K stabilizes the transition state for the C-O bond dissociation, lowering the activation barrier for the dissociation of the C-O bond by about 0.3 and 0.4 eV, respectively.