Metal Carbide Catalysts Research Articles

Atomic-level insight into the carburization process of iron-based catalysts: A ReaxFF molecular dynamics study

Transition metal carbide catalysts have garnered widespread attention due to their outstanding catalytic performance. The carbide catalysts, such as FexC and MoxC are typically obtained from their oxide carburization. The reduction and carburization processes are key steps during the activation of oxide precursors, which determine the activity and stability. An understanding of the carburization process mechanism is very important for the optimization of carbide catalysts. Iron-based catalysts are widely used in the large-scale coal-to-liquid industry because of their low price and high activity. In this paper, the carburization process of iron oxide nanoparticles was simulated by the Reactive force field (ReaxFF) molecular dynamics method. The results illustrate that the competition between CO dissociation and reduction of oxides occurs at the 100 ps time scale, which can be tuned by particle size and oxygen vacancy. We have explained the counter-intuitive finding that small oxide particles are more difficult to achieve fully carburization than larger ones; this is because rapid carbon accumulation on the surface blocks the oxygen diffusion channels from bulk to surface. The atomic distribution heat map was carried out to demonstrate the carbon and oxygen penetration processes. Meanwhile, the volumetric strain analysis reveals the driving force comes from the strong tendency to form Fe-C bonds. The dependence on surface orientation was further systematically investigated. This work provides microscopic insights into the carburization process of iron-based materials and the fundamental knowledge for the optimization of catalyst synthesis procedures.

Nitrogen Reduction Reaction to Ammonia on Transition Metal Carbide Catalysts.

The development of a low-cost, energy-efficient, and environmentally friendly alternative to the currently utilized Haber-Bosch process to produce ammonia is of great importance. Ammonia is an essential chemical used in fertilizers and a promising high-density fuel source. The nitrogen reduction reaction (NRR) has been explored intensively as a potential avenue for ammonia production using water as proton source, but to this day a catalyst capable of producing this chemical at high Faradaic efficiency (FE) and commercial yield and rates has not been reported. Here, we investigate the activity of transition metal carbide (TMC) surfaces in the (100) facets of the rocksalt (RS) structure as potential catalysts for the NRR. In this study, we use density functional theory (DFT) to model reaction pathways, estimate stability, assess kinetic barriers, and compare adsorbate energies to determine the overall performance of each TMC surface. For pristine TMC surfaces (with no defects) we find that none of the studied TMCs possess both exergonic adsorption of nitrogen and the capability to selectively protonate nitrogen to form ammonia in the desired aqueous solution. ZrC, however, is shown to be a potential catalyst if used in a non-aqueous electrolyte. To circumvent the endergonic adsorption of nitrogen onto the surface, a carbon vacancy was introduced. This provides a well-defined high coordination active site on the surface. In the presence of a vacancy VC, NbC, and WC showed efficient nitrogen adsorption, selectivity towards ammonia, and a low overpotential (OP). NbC did, however, display an unfeasible kinetic barrier to nitrogen dissociation for ambient-condition purposes, and thus it is suggested for high tempearture/pressure ammonia synthesis. Both WC and VC in their RS (100) structure are promising materials for experimental investigations in aqueous electrolytes, and ZrC could potentially be interesting for non-aqueous electrolytic systems.

Transition metal carbide catalysts for Upgrading lignocellulosic biomass-derived oxygenates: A review of the experimental and computational investigations into structure-property relationships

Transition metal carbide (TMC) catalysts, due to their noble metal like characteristics,have emerged as potential candidates for the hydrodeoxygenation (HDO) of biomass derived species, including the pyrolysis product bio-oil. However, the selectivity for deoxygenation, excessive hydrogenation and stability under operating conditions,remain as key challenges. Hence, this paper presents a comprehensive review of the state of the art with regard to (i) the preparation and structure of TMCs, (ii) characterization of active sites on TMCs and (iii) fundamental knowledge of the reaction mechanisms/pathways and associated energetics of HDO of model compounds which are present in biomass derived bio-oil. We believe that this fundamental knowledge will greatly assist researchers to systematically modify the composition and structure of TMCs to optimize their HDO performance.

Co3C-stabilized-CoFe heterostructure wrapped by bamboo-like N-doped carbon nanotubes for highly-efficient oxygen electrocatalysis

Bimetallic alloys have been showing unexpectedly high activity for oxygen reduction reaction (ORR) in alkaline electrolytes. However, many fundamental questions regarding the ORR stability and poor oxygen evolution reaction (OER) performance of bimetallic alloys remain unresolved. Here, we prepare a cobalt carbide-stabilized CoFe alloy wrapped by hollow bamboo-like nitrogen-doped carbon nanotubes (CoFe-Co3C@NCNTs) using bimetallic Prussian-blue-analogue and dicyandiamide as precursors. Interestingly, the spherical CoFe-Co3C heterojunctions are always encapsulated at one end of the tube. The as-marked CoFe-Co3C@NCNTs-20 catalyst exhibits an ultra-high ORR activity (half-wave potential of 0.934 V) and a relatively good OER activity (overpotential of 0.320 V), which are superior to Pt/C and RuO2, respectively. CoFe-Co3C@NCNTs-20 also shows excellent ORR/OER stabilities and potential difference between ORR and OER (0.616 V). Excellent activity and stability benefit from the synergies between CoFe-Co3C and NCNTs, while the hollow structure/conductive skeleton enhances the mass/charge transfers, respectively. The NCNTs coating greatly enhances the stability of the CoFe-Co3C heterojunctions to avoid the corrosion of active sites in alkaline media. Moreover, rechargeable zinc-air battery assembled with CoFe-Co3C@NCNTs-20 cathode shows high power-density (209.15 mW cm−2) and open-circuit voltage (1.505 V). It provides a new reference for promoting the development of bimetallic alloy-coupled transition metal carbide catalysts.

Synthesis of sulfur-free Co-Mo nitride catalysts for the hydrotreating of atmospheric gasoil and co-processing of rapeseed oil

Sulfur-free transition metal carbide and nitride catalysts have been shown to be reasonable substitutes for sulfidic catalysts for the hydrotreating middle distillates and vegetable oils. However, their catalyst activity is still lower than commercial catalysts. In this paper, we build on our previous research by studying the synthesis and testing of nine sulfur-free Al2O3-supported Co-Mo nitride catalysts for the hydrotreating of atmospheric gasoil and its blends with rapeseed oil (25 wt.%) under industrial conditions (330–350 °C, 5.5 MPa, WHSV 1–2 h−1). Nitration temperatures play the most significant role in the synthesis and activity of the catalyst, obtaining a CoMoNx catalyst whose catalytic behaviour competes with the current commercial sulfidic Ni-Mo or Co-Mo catalysts (reduction of sulfur and nitrogen content up to 99.0 and 84.0%, respectively). Thus, our results allow us to evaluate and define the most suitable synthesis conditions for preparing high activity sulfur-free Al2O3-supported Co-Mo nitride catalysts.

Supported transition metal (Mo, W) carbide and nitride catalysts for lignin hydrodeoxygenation: interplay of supports, structure, and catalysis

In-depth understanding of the structure–performance relationship for transition metal (Mo, W) carbide- and nitride-catalyzed lignin hydrodeoxygenation was summarized.

Coupling multifunctional catalytic active sites into metal carbide catalysts to promote the multipath bidirectional conversion of Li-S redox

Li-S batteries are hampered by sluggish liquidsolid redox and shuttle effects. Herein, a strategy of coupling multifunctional catalytic sites is proposed to fabricate bidirectional catalysts to promote multipath bidirectional Li-S redox conversion. By engineering Co or Zr dopants into Ni3C/C, the obtained Co0.30Ni2.70C/C and Zr0.30Ni2.70C/C electrocatalysts play different roles in polysulfide adsorption and multipath bidirectional conversion. During the discharging/charging process, due to limited electron transfer between the Co3+/Co2+ pair and polysulfides, Co sites themselves are catalytically inactive for the conversion of polysulfides and only act as promoters to improve the catalytic activity of Ni sites. In contrast, in addition to being promoters, Zr sites serve as electron acceptors and donors to catalyze bidirectional Li-S redox due to the flexible electron transfer between the Zr4+/Zr2+ pair and polysulfides. Such bidirectional electron transfer is accompanied by thiosulfate-mediated redox. Therefore, Zr doping not only accelerates the inherent solid (S8)-liquid (polysulfide)-solid (Li2S) conversion pathway but also propels the reversible thiosulfate-mediated conversion pathway. As a result, when the Zr0.30Ni2.70C/C nanosheets were used to modify the commercial PP separator, the Li-S batteries exhibit improved electrochemical performance (a decay of 0.0756 % over 200 cycles with a sulfur loading of 5.0 mgs cm−2).

Activating molybdenum carbide via a surface sulfur modification to enhance hydrogen evolution activity

Molybdenum carbide (Mo2C) was regarded as one of the most promising catalysts for hydrogen evolution reaction (HER) due to its Pt-like electronic structure. In practice, however, the catalytic performance of Mo2C is always hampered by the inevitable oxidation on its surface. Herein, we developed a simple and efficient strategy to improve the HER activity of Mo2C using sulfur as a "modifier", in which sulfur atoms were used to replace a portion of the oxygen atoms in the surface oxide layer to activate the surface. The experimental results demonstrated that a moderate sulfur modification could significantly improve the HER performance. Theoretical studies further revealed that the strong hydrogen bonding of Mo atoms in the oxide layer is the primary reason that impairs HER performance, whereas the S modification can shift down the d-band center of Mo, effectively weakening the Mo-H bond. As expected, the S-modified Mo2C exhibited superior performance, affording a low overpotential of 117 mV at 10 mA cm−2 for catalyzing the HER in 0.5 M H2SO4. This work provides a convenient and effective way for the development of transition metal carbide catalysts.

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.

Selective upgrading of biomass-derived benzylic ketones by (formic acid)–Pd/HPC–NH2 system with high efficiency under ambient conditions

Selective upgrading of biomass-derived benzylic ketones by (formic acid)–Pd/HPC–NH2 system with high efficiency under ambient conditions

Synthesis of Transition Metal Carbide Catalysts for Vanadium Redox Flow Batteries

Vanadium redox flow batteries (VRFBs) proposed by Skyllas-Kazacos’ group in the 1980s [1] have recently attracted considerable attention for large-scale energy storage due to their desirable features such as flexible and safe design, high efficiency, and long cycle life. Graphite or carbon materials have been the most common catalysts for VRFBs [2]. However, they often show limited activity and reversibility. This issue may be circumvented with the development of new and efficient catalysts. Transition metal carbides are regarded as attractive candidates because they possess good electronic conductivity, low cost, high abundance, and outstanding thermal and chemical stabilities [3]. However, transition metal carbides fabricated by traditional carbothermal reaction methods exhibit a small specific surface area and inevitable aggregation at high reaction temperatures.A transition metal carbide fabrication process is under development at Hawaii Natural Energy Institute, which favors the formation of nano-sized particles that are expected to possess a large specific surface area. Figure 1a shows X-ray diffraction patterns for graphite as carbon source and the resulting vanadium carbide. The dominant phase is V8C7 and the particle size is approximately 50 nm. As shown in Figure 1b, cathodic currents for both electrodes in 3 M H2SO4 are due to the hydrogen evolution reaction. Although the cathodic current remarkably increases for graphite in 1 M (V2++V3+) + 3 M H2SO4 in Figure 1c, a significant anodic current is not detected indicating that the reversibility of the graphite toward the V2+/V3+ redox reactions is poor. In contrast, anodic and cathodic currents substantially increase on vanadium carbide, signifying an improvement in catalytic activity and reversibility toward the negative electrode reactions in comparison to graphite. The undesired hydrogen evolution reaction appears to have an important influence on the V3+ to V2+ reduction reaction. This influence was removed to more clearly show the vanadium V2+/V3+ redox reactions partial currents. Figure 1d shows the catalytic activity of graphite and vanadium carbide after background current subtraction. Vanadium carbide reveals more pronounced redox peaks than graphite signaling that vanadium carbide has a higher catalytic activity and enhanced reversibility toward V2+/V3+ redox reactions than graphite. The vanadium carbide composition, structure, morphology, and particle size will be tuned to improve activity.Figure 1. (a) X-ray diffraction patterns and (b, c, d) cyclic voltammograms (positive scan) for vanadium carbide and graphite. Acknowledgments Authors are grateful to the Office of Naval Research (award N00014-18-1-2127) and the Hawaiian Electric Company.

Enhanced alcohol production over binary Mo/Co carbide catalysts in syngas conversion

Recently metal carbide catalysts have attracted attention as alternatives to pure metals for the conversion of syngas to higher oxygenates, a process which could enable the sustainable production of fuels, polymers, and chemicals. Although Mo and Co carbides have both shown promise for higher oxygenate production, they have not achieved the requisite activity and selectivity for practical implementation. In this work we synthesize and characterize a binary Mo and Co carbide catalyst that exhibits improved activity and oxygenate selectivity relative to either pure metal carbide. We apply a combination of advanced electron microscopy and X-ray diffraction to show that the binary Mo/Co carbide catalyst forms uniformly mixed amorphous nanoparticles. Through in situ X-ray absorption spectroscopy studies, we determine that the structure of the mixed metal carbide catalyst under reaction conditions consists of both carbidic and bimetallic components. By testing the catalytic properties of a series of Mo/Co carbide catalysts prepared by different synthesis methods, we find that the Mo and Co sites must be in close contact to achieve improved syngas conversion to higher alcohols. Through in situ DRIFTS measurements, both Mo and Co atoms at the surface of the catalyst are identified as adsorption sites for reactive species.

Understand the Fe3C nanocrystalline grown on rGO and its performance for oxygen reduction reaction

Transition metal carbide catalysts are the alternative products of traditional noble metal Pt/C in oxygen reduction reaction (ORR). Oleic acid-coated iron oxide nanoparticles (F3O4@OA) was prepared by pyrolysis, assembled with reduced graphene oxide (rGO), and carbonized at 800 °C to obtain N doped iron carbide nanoparticles supported rGO (N–Fe3C/rGO). The particle size of nanocrystals increases significantly with the graphene loading risen can be seen in the transmission electron microscope (TEM) image which roughly distributed around 40 nm. The linear scanning voltammetry (LSV) curve shows the material has an ORR performance equivalent to Pt/C in alkaline media.

Biomass derived metal carbide catalysts formed using a salt flux synthesis

Metal carbide compounds have recently become highly studied materials as non-precious metal catalysts for several energy related reactions. These materials often require high temperatures and long annealing time to produce the desired phase. Using a low-temperature salt flux synthesis method, molybdenum metal and biomass samples were reacted to form porous nanoparticle Mo2C. Several different biomass and biochar sources were used to demonstrate the versatility and generality of this process. The biomass derived carbides were also tested and shown to be active for the Hydrogen Evolution Reaction (HER). In addition to abundant and inexpensive carbon sources, the overall reaction takes place at reduced temperatures compared to traditional carbide synthesis methods and utilizes very simple and environmentally friendly techniques. This process provides a clear path toward greener carbide synthesis using economical and renewable resources and could be implemented using nearly any biomass available.

Janus Segregation at the Carbon Nanotube-Catalyst Interface.

The contact between a carbon nanotube (CNT) edge and a catalyst is a curvilinear interface of fundamental and practical importance. Here, the first-principles evidence shows that on a rigid/solid catalyst the faceted CNT edge is significantly lower in energy compared to the minimal-length circle, with the interface energy difference decreasing on more compliant surfaces. This universal trend, found for typical monometallic (Ni, Co), bimetallic (Co7W6), and metal carbide (WC) catalysts, results in a peculiar edge segregation into one-dimensional Janus (armchair-zigzag) interface. Its lowered energy greatly enhances the nucleation probability of chiral tubes, dramatically affecting their growth kinetics. This offers a richer basis for understanding, modeling, and control of catalytic CNT synthesis.

Impact of Morphological Effects on the Activity and Stability of Tungsten Carbide Catalysts for Dry Methane Reforming

Fundamental understanding of the effects of phase and exposed facets of transition metal carbide (TMC) catalysts on reactivity is a route to preparing more active and stable materials for targeted ...

Hydrothermal Conversion of Cd-Enriched Rice Straw and Cu-Enriched Elsholtzia splendens with the Aims of Harmless Treatment and Resource Reuse

In the present work, we report a greener and efficient process of hydrothermal conversion of rice straw (Cd low accumulated crop) and Elsholtzia splendens (Cu accumulated plant) to recover the metals and simultaneously produce biofuel for harmless treatment and resource reuse. The impacts of different types of additives, raw materials/solvent ratios, reaction times, and temperatures are investigated, and the composition of solid phase left is also studied for future efficient use. As the results suggest, almost all enriched metals (95 ± 2% of Cu in E. splendens and almost all Cd in rice straw) are concentrated in solid phase after hydrothermal conversion with a considerable amount of carbohydrates contained, making the solid residue an excellent material for producing metal carbide catalysts. During the hydrothermal conversion, the additives have little impact on the metals recovery, and higher raw material/solvent ratios have higher yields of Cu/Cd. Finally, 22.0–25.0% yields of bio-oils without any meta...

Fe5C2 nanoparticles as low-cost HER electrocatalyst: the importance of Co substitution

Constructing and understanding the doping effect of secondary metal in transition metal carbide (TMC) catalysts is pivotal for the design of low-cost hydrogen evolution reaction (HER) electrocatalysts. In this work, we developed a wet-chemistry strategy for synthesizing Co-modified Fe5C2 nanoparticles ((Fe1−xCox)5C2 NPs) as highly active HER electrocatalysts in basic solution. The structure of (Fe1−xCox)5C2 NPs was characterized by X-ray diffraction (XRD), extended X-ray absorption fine structure spectra (EXAFS) and scanning/transmission electron microscopy (S/TEM), indicating that the isomorphous substitution of cobalt in the lattice of Fe5C2. (Fe0.75Co0.25)5C2 exhibited the best HER activity (174 mV for j = −10 mA/cm2). Computational calculation results indicate that Co provides the most active site for HER. X-ray adsorption spectra (XAS) studies further suggested that the electron transfer in Fe–C bonds are enhanced by the substitution of Co, which modulates the hydrogen adsorption on the adjacent electronic-enriched carbon, and therefore promotes HER activity. Our results affirm the design of low-cost bimetallic TMCs based HER catalysts.

Highly Dispersed Metal Carbide on ZIF-Derived Pyridinic-N-Doped Carbon for CO2 Enrichment and Selective Hydrogenation.

Catalytic conversion of CO2 into chemicals is a critical issue for energy and environmental research. Among such reactions, converting CO2 into CO has been regarded as a significant foundation to generate a liquid fuels and chemicals on a large scale. In this work, zeolitic imidazolate framework-derived N-doped carbon-supported metal carbide catalysts (M/ZIF-8-C; M=Ni, Fe, Co and Cu) with highly dispersed metal carbide were prepared for selective CO2 hydrogenation. Under the same metal loadings, catalytic activity for CO2 hydrogenation to CO follows the order: Ni/ZIF-8-C≈Fe/ZIF-8-C>Co/ZIF-8-C>Cu/ZIF-8-C. These catalysts are composed of carbide or metal supported on pyridinic N sites within the N-doped carbon structure. ZIF-8-derived pyridinic nitrogen and carbide effect CO2 adsorption, whereas dispersed Ni or Fe carbide and metal species serve as an active site for CO2 hydrogenation. The supported Ni catalyst exhibits extraordinary catalytic performance, which results from high dispersion of the metal and exposure of the carbide. Based on high-sensitivity low-energy ion scattering (HS-LEIS) and line scan results, density functional theory (DFT) was used to understand reaction mechanism of selective CO2 hydrogenation over Ni/ZIF-8-C. The product CO is derived mainly from the direct cleavage of C-O bonds in CO2 * rather than decomposition of COOH*. The CO* desorption energy on Ni/ZIF-8-C is lower than that for further hydrogenation and dissociation. Comparison of Ni/ZIF-8-C with ZIF-8-C indicates that the combined effects of the highly dispersed metal or carbide and weak CO adsorption result in high CO selectivity for CO2 hydrogenation.

Metal Carbides for Biomass Valorization

Transition metal carbides have been utilized as an alternative catalyst to expensive noble metals for the conversion of biomass. Tungsten and molybdenum carbides have been shown to be effective catalysts for hydrogenation, hydrodeoxygenation and isomerization reactions. The satisfactory activities of these metal carbides and their low costs, compared with noble metals, make them appealing alternatives and worthy of further investigation. In this review, we succinctly describe common synthesis techniques, including temperature-programmed reaction and carbothermal hydrogen reduction, utilized to prepare metal carbides used for biomass transformation. Attention will be focused, successively, on the application of transition metal carbide catalysts in the transformation of first-generation (oils) and second-generation (lignocellulose) biomass to biofuels and fine chemicals.