Showing papers on "Carbide published in 2018"
TL;DR: In this paper, the influence of the morphology of MOF-derived nanostructures on their performance is elucidated, and the opportunities in this field are discussed, as well as the optimization strategies and optimized methods that enable control over the size, morphology, composition and structure of the derived nanomaterials.
Abstract: The thermal transformation of metal–organic frameworks (MOFs) generates a variety of nanostructured materials, including carbon-based materials, metal oxides, metal chalcogenides, metal phosphides and metal carbides. These derivatives of MOFs have characteristics such as high surface areas, permanent porosities and controllable functionalities that enable their good performance in sensing, gas storage, catalysis and energy-related applications. Although progress has been made to tune the morphologies of MOF-derived structures at the nanometre scale, it remains crucial to further our knowledge of the relationship between morphology and performance. In this Review, we summarize the synthetic strategies and optimized methods that enable control over the size, morphology, composition and structure of the derived nanomaterials. In addition, we compare the performance of materials prepared by the MOF-templated strategy and other synthetic methods. Our aim is to reveal the relationship between the morphology and the physico-chemical properties of MOF-derived nanostructures to optimize their performance for applications such as sensing, catalysis, and energy storage and conversion. Nanomaterials derived from metal–organic frameworks (MOFs) show good performance in sensing, gas storage, catalysis and energy-related applications. In this Review, the influence of the morphology of MOF-derived nanostructures on their performance is elucidated, and the opportunities in this field are discussed.
871 citations
TL;DR: It was found that the lattice parameter mismatch of the component monocarbides is a key factor for predicting single phase solid solution formation, revealing a vast new compositional space for the exploration of new UHTCs.
Abstract: Bulk equiatomic (Hf-Ta-Zr-Ti)C and (Hf-Ta-Zr-Nb)C high entropy Ultra-High Temperature Ceramic (UHTC) carbide compositions were fabricated by ball milling and Spark Plasma Sintering (SPS). It was found that the lattice parameter mismatch of the component monocarbides is a key factor for predicting single phase solid solution formation. The processing route was further optimised for the (Hf-Ta-Zr-Nb)C composition to produce a high purity, single phase, homogeneous, bulk high entropy material (99% density); revealing a vast new compositional space for the exploration of new UHTCs. One sample was observed to chemically decompose; indicating the presence of a miscibility gap. While this suggests the system is not thermodynamically stable to room temperature, it does reveal further potential for the development of new in situ formed UHTC nanocomposites. The optimised material was subjected to nanoindentation testing and directly compared to the constituent mono/binary carbides, revealing a significantly enhanced hardness (36.1 ± 1.6 GPa,) compared to the hardest monocarbide (HfC, 31.5 ± 1.3 GPa) and the binary (Hf-Ta)C (32.9 ± 1.8 GPa).
440 citations
TL;DR: This paper demonstrates an efficient fluoride-free etching method based on the anodic corrosion of titanium aluminium carbide in a binary aqueous electrolyte that paves a safe way to the scalable synthesis and application of MXene materials.
Abstract: Two-dimensional (2D) titanium carbide (Ti3 C2 ) is emerging as an important member of the MXene family. However, fluoride-based synthetic procedures remain an impediment to the practical applications of this promising class of materials. Here we demonstrate an efficient fluoride-free etching method based on the anodic corrosion of titanium aluminium carbide (Ti3 AlC2 ) in a binary aqueous electrolyte. The dissolution of aluminium followed by in situ intercalation of ammonium hydroxide results in the extraction of carbide flakes (Ti3 C2 Tx , T=O, OH) with sizes up to 18.6 μm and high yield (over 90 %) of mono- and bilayers. All-solid-state supercapacitor based on exfoliated sheets exhibits high areal and volumetric capacitances of 220 mF cm-2 and 439 F cm-3 , respectively, at a scan rate of 10 mV s-1 , superior to those of LiF/HCl-etched MXenes. Our strategy paves a safe way to the scalable synthesis and application of MXene materials.
362 citations
TL;DR: In this article, a novel high-entropy carbide ceramic, (Hf0.2Zr 0.2Ta 0.3Nb0.5Ti 0.4Nb1.2Ti0.4Ti 0.2Nb 0.5Nb 1.2C, with a single phase rock salt structure was synthesized by spark plasma sintering.
Abstract: A novel high‐entropy carbide ceramic, (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C, with a single‐phase rock salt structure, was synthesized by spark plasma sintering. X‐ray diffraction confirmed the formation of a single‐phase rock salt structure at 26‐1140°C in Argon atmosphere, in which the 5 metal elements may share a cation position while the C element occupies the anion position. (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C exhibits a much lower thermal diffusivity and conductivity than the binary carbides HfC, ZrC, TaC, and TiC, which may result from the significant phonon scattering at its distorted anion sublattice. (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C inherits the high elastic modulus and hardness of the binary carbide ceramics.
338 citations
TL;DR: The results presented herein show that transition-metal carbide are promising co-catalysts for photocatalytic hydrogen production.
Abstract: Hydrogen production through facile photocatalytic water splitting is regarded as a promising strategy to solve global energy problems. Transition-metal carbides (MXenes) have recently drawn attention as potential co-catalyst candidates for photocatalysts. Here, we report niobium pentoxide/carbon/niobium carbide (MXene) hybrid materials (Nb2 O5 /C/Nb2 C) as photocatalysts for hydrogen evolution from water splitting. The Nb2 O5 /C/Nb2 C composites were synthesized by one-step CO2 oxidation of Nb2 CTx . Nb2 O5 grew homogeneously on Nb2 C after mild oxidation, during which some amorphous carbon was also formed. With an optimized oxidation time of 1.0 h, Nb2 O5 /C/Nb2 C showed the highest hydrogen generation rate (7.81 μmol h-1 gcat-1 ), a value that was four times higher than that of pure Nb2 O5 . The enhanced performance of Nb2 O5 /C/Nb2 C was attributed to intimate contact between Nb2 O5 and conductive Nb2 C and the separation of photogenerated charge carriers at the Nb2 O5 /Nb2 C interface; the results presented herein show that transition-metal carbide are promising co-catalysts for photocatalytic hydrogen production.
279 citations
TL;DR: In this article, an equi-atomic high-entropy carbide powder was synthesized using spark plasma sintering (SPS), and its phase evolution, microstructure, composition and oxidation behaviour were investigated in detail.
248 citations
01 May 2018
TL;DR: In this article, the metal-support interaction of platinum with MXenes at moderate temperature is reported, using the water-gas shift reaction as an example to showcase the properties of a representative catalyst.
Abstract: The reactive metal–support interaction (RMSI) offers electronic, geometric and compositional effects that can be used to tune catalytic active sites. Generally, supports other than oxides are disregarded as candidates for RMSI. Here, we report an example of non-oxide-based RMSI between platinum and Nb2CT
x
MXene—a recently developed, two-dimensional metal carbide. The surface functional groups of the two-dimensional carbide can be reduced, and a Pt–Nb surface alloy is formed at a moderate temperature (350 °C). Such an alloy exhibits weaker CO adsorption than monometallic platinum. Water-gas shift reaction kinetics reveals that the RMSI stabilizes the nanoparticles and creates alloy–MXene interfaces with higher H2O activation ability compared with a non-reducible support or a bulk niobium carbide. This RMSI between platinum and the niobium MXene support can be extended to other members of the MXene family and opens new avenues for the facile design and manipulation of functional bimetallic catalysts. Reactive metal–support interactions can tune the activity of heterogeneous catalysts, but have mainly been reported for oxide supports. Now, the metal–support interaction of platinum with MXenes at moderate temperature is reported, using the water-gas shift reaction as an example to showcase the properties of a representative catalyst.
211 citations
TL;DR: In this paper, a self-assembled MXene-gold nanoparticles (MXene@AuNPs) nanocomposites with tunable sizes is reported, which are obtained via the self-reduction reactions of MXene material in a HAuCl4 solution at room temperature.
Abstract: MXene, a new type of two-dimensional layered transition metal carbide material differing from graphene, demonstrates intriguing chemical/physical properties and wide applications in recent years. Here, the preparation of the self-assembled MXene-gold nanoparticles (MXene@AuNPs) nanocomposites with tunable sizes is reported. The nanocomposites are obtained via the self-reduction reactions of MXene material in a HAuCl4 solution at room temperature. The sizes of the Au particles can be well-controlled by regulating the self-reduction reaction time. They can greatly influence the catalytic behaviors of the MXene@AuNPs composites. MXene@AuNPs composites with optimized reduction time show high catalytic performances and good cycle stability for model catalytic reactions of nitro-compounds, such as 2-nitrophenol and 4-nitrophenol. This work demonstrates a new approach for the preparation of tunable MXene-based self-assembled composites.
188 citations
184 citations
TL;DR: Fe3C‐Fe,N/C hollow spheres exhibit an ORR performance comparable to that of commercially available 20 wt% Pt/C in alkaline electrolyte, with a similar half‐wave potential, an electron transfer number close to 4, and lower H2O2 yield of less than 5%.
Abstract: Sub-50 nm iron-nitrogen-doped hollow carbon sphere-encapsulated iron carbide nanoparticles (Fe3C-Fe,N/C) are synthesized by using a triblock copolymer of poly(styrene-b-2-vinylpyridine-b-ethylene oxide) as a soft template. Their typical features, including a large surface area (879.5 m2 g-1), small hollow size (≈16 nm), and nitrogen-doped mesoporous carbon shell, and encapsulated Fe3C nanoparticles generate a highly active oxygen reduction reaction (ORR) performance. Fe3C-Fe,N/C hollow spheres exhibit an ORR performance comparable to that of commercially available 20 wt% Pt/C in alkaline electrolyte, with a similar half-wave potential, an electron transfer number close to 4, and lower H2O2 yield of less than 5%. It also shows noticeable ORR catalytic activity under acidic conditions, with a high half-wave potential of 0.714 V, which is only 59 mV lower than that of 20 wt% Pt/C. Moreover, Fe3C-Fe,N/C has remarkable long-term durability and tolerance to methanol poisoning, exceeding Pt/C regardless of the electrolyte.
184 citations
TL;DR: In this paper, the influence of different iron carbides on the activity and selectivity of iron-based Fischer-Tropsch catalysts has been studied, and the intrinsic FTS activity is assigned to each corresponding iron carbide based on the phase composition and the particle size.
Abstract: The influence of different iron carbides on the activity and selectivity of iron-based Fischer–Tropsch catalysts has been studied. Different iron carbide phases are obtained by the pretreatment of a binary Fe/SiO2 model catalyst (prepared by coprecipitation method) to different gas atmospheres (syngas, CO, or H2). The phase structures, compositions, and particle sizes of the catalysts are characterized systematically by XRD, XAFS, MES, and TEM. It is found that in the syngas-treated catalyst only χ-Fe5C2 carbide is formed. In the CO-treated catalyst, Fe7C3 and χ-Fe5C2 with a bimodal particle size distribution are formed, while the H2-treated catalyst exhibits the bimodal size distributed e-Fe2C and χ-Fe5C2 after a Fischer–Tropsch synthesis (FTS) reaction. The intrinsic FTS activity is calculated and assigned to each corresponding iron carbide based on the phase composition and the particle size. It is identified that Fe7C3 has the highest intrinsic activity (TOF = 4.59 × 10–2 s–1) among the three candidat...
TL;DR: In this paper, an interface design strategy by matrix-alloying with Ti for in-situ interfacial carbide formation in reduced graphene oxide (RGO)/CuTi composites was reported.
TL;DR: Mo1.33C-MXene was used for removing cations and anions and demonstrated the desalination performance in brackish water and seawater concentrations.
Abstract: Ion intercalation materials are emerging as a highly attractive class of electrodes for efficient energy water desalination. Most materials and concepts so far have focused on the removal of cations (especially sodium). Anion intercalation, however, remains poorly investigated in water desalination. We present a study on the capability of Mo1.33C-MXene for removing cations and anions and demonstrate the desalination performance in brackish water and seawater concentrations. Mo1.33C-MXene was prepared via acid treatment of the transition metal carbide MAX phase (Mo2/3Sc1/3)2AlC. Binder-free electrodes were obtained by entangling MXene with carbon nanotubes and tested without the use of any ion exchange membrane at low (5 mM NaCl) and high (600 mM NaCl) salt concentrations. Such electrodes showed a promising desalination performance of 15 mg/g in 600 mM NaCl with high charge efficiency up to 95%. By employing chemical online monitoring of the effluent stream, we separated the cation and anion intercalation ...
TL;DR: Support from the projects APVVV-15-0469, the authors 15-0621, VGA 2/0163/16, and VEGA 2/0082/17 is acknowledged.
Abstract: Support from the projects APVV-15-0469, APVV-15-0621, VEGA 2/0163/16, and VEGA 2/0082/17 is acknowledged. MJR and EGC acknowledge the support of EPSRC grant XMAT (EP/K008749/2).
TL;DR: In this article, a deuterium charge cell was used to visualize hydrogen atoms at trapping sites associated with nano-sized precipitates of titanium carbide and vanadium carbide in ferritic steels.
13 Jul 2018
TL;DR: The structural, elastic and electronic properties of two-dimensional (2D) titanium carbide/nitride based pristine and functionalized MXenes (Tin+1Cn/Tin + 1Nn) are investigated by density functional theory calculations as discussed by the authors.
Abstract: The structural, elastic and electronic properties of two-dimensional (2D) titanium carbide/nitride based pristine (Tin+1Cn/Tin+1Nn) and functionalized MXenes (Tin+1CnT2/Tin+1NnT2, T stands for the terminal groups: –F, –O and –OH, n = 1, 2, 3) are investigated by density functional theory calculations. Carbide-based MXenes possess larger lattice constants and monolayer thicknesses than nitride-based MXenes. The in-plane Young's moduli of Tin+1Nn are larger than those of Tin+1Cn, whereas in both systems they decrease with the increase of the monolayer thickness. Cohesive energy calculations indicate that MXenes with a larger monolayer thickness have a better structural stability. Adsorption energy calculations imply that Tin+1Nn have stronger preference to adhere to the terminal groups, which suggests more active surfaces for nitride-based MXenes. More importantly, nearly free electron states are observed to exist outside the surfaces of –OH functionalized carbide/nitride based MXenes, especially in Tin+1Nn(OH)2, which provide almost perfect transmission channels without nuclear scattering for electron transport. The overall electrical conductivity of nitride-based MXenes is determined to be higher than that of carbide-based MXenes. The exceptional properties of titanium nitride-based MXenes, including strong surface adsorption, high elastic constant and Young's modulus, and good metallic conductivity, make them promising materials for catalysis and energy storage applications.
TL;DR: The stability, hydrophilic nature, available adsorptive surfaces, and easy separation after reaction make the MGMX nanocomposite an efficient sorbent for the removal of toxic Hg(II) for water purification.
TL;DR: This work provides the area-controllable synthesis of a manufacturable MXene from a transition metal dichalcogenide material and the formation of a metal/semiconductor junction structure.
Abstract: The epitaxial synthesis of molybdenum carbide (Mo2C, a 2D MXene material) via chemical conversion of molybdenum disulfide (MoS2) with thermal annealing under CH4 and H2 is reported. The experimental results show that adjusting the thermal annealing period provides a fully converted metallic Mo2C from MoS2 and an atomically sharp metallic/semiconducting hybrid structure via partial conversion of the semiconducting 2D material. Mo2C/MoS2 hybrid junctions display a low contact resistance (1.2 kΩ·μm) and low Schottky barrier height (26 meV), indicating the material’s potential utility as a critical hybrid structural building block in future device applications. Density functional theory calculations are used to model the mechanisms by which Mo2C grows and forms a Mo2C/MoS2 hybrid structure. The results show that Mo2C conversion is initiated at the MoS2 edge and undergoes sequential hydrodesulfurization and carbide conversion steps, and an atomically sharp interface with MoS2 forms through epitaxial growth of ...
TL;DR: Two-dimensional transition metal carbides with a formula of M2C (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W) have been recently synthesized and isolated, and are presented as very promising candidates for carbon dioxide (CO2) capture, storage, and activation.
Abstract: Two-dimensional transition metal carbides with a formula of M2C (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W) have been recently synthesized and isolated, and are here presented as very promising candidates for carbon dioxide (CO2) capture, storage, and activation. By means of density functional theory investigations including dispersion we show the strong CO2 uptake and activation on M2C compounds, where the estimates of adsorption and desorption rates indicate their CO2 adsorption capacity even at low CO2 partial pressures and high temperatures. The M2C compounds feature noteworthy CO2 loading capacities ranging from 2.34 to 8.25 mol CO2 kg−1, making them practical materials for CO2 abatement.
TL;DR: This work reveals the active SMSI effect in metal/carbide catalysts induced by carbonization, which opens a new territory for this important concept in heterogeneous catalysis.
Abstract: Strong metal–support interaction (SMSI) has been regarded as one of the most important concepts in heterogeneous catalysis, which has been almost exclusively discussed in metal/oxide catalysts. Here, we show that gold/molybdenum carbide (Au/MoCx) catalysts feature highly dispersed Au overlayers, strong interfacial charge transfer between metal and support, and excellent activity in the low-temperature water–gas shift reaction (LT-WGSR), demonstrating the active SMSI state. Subsequent oxidation treatment results in strong aggregation of Au nanoparticles, weak interfacial electronic interaction, and poor LT-WGSR activity. The two interface states can be transformed into each other by alternative carbonization and oxidation treatments. This work reveals the active SMSI effect in metal/carbide catalysts induced by carbonization, which opens a new territory for this important concept.
TL;DR: In this article, the authors compared the hydrogen trapping ability of 15 Fe-C-X alloys with five types of carbides, i.e., Ti, Cr, Mo, W and V, and their effect on the hydrogen embrittlement susceptibility is considered while three carbon contents are prepared for each carbide former.
TL;DR: In this paper, a two-dimensional transition-metal carbide (MXenes) anode was used for high-rate pseudocapacitive energy storage, achieving an energy density of 39 Wh/kg.
Abstract: Two-dimensional transition-metal carbides (MXenes) have shown great promise as electrode materials for high-rate pseudocapacitive energy storage. In this study, we report on the fabrication of bistacked two-dimensional titanium carbide electrodes which are free of binder, conductive additives, and current-collector. This MXene electrode is capable of reversible electrochemical storage of sodium ions with good cycling stability and rate capability. A prototype hybrid Na-ion capacitor was assembled by combining the bistacked MXene anode with an activated carbon cathode, which showed an energy density of 39 Wh/kg (including the total weight of bistacked MXene and activated carbon, 6 mg/cm2) at 1C rate and maintained up to 60% of its performance at a 60C rate, in the operating voltage window of 3.4 V. This study opens new avenues for developing self-standing binder and additive-free MXene electrodes for metal-ion batteries and capacitors.
TL;DR: The carbides were extracted from the austenitic matrix and characterized by combining different techniques, showing the formation of various types of Mo-rich carbides, classified as M6C, M12C and MnCm type.
Abstract: Hastelloy X (HX) is a Ni-based superalloy which suffers from high crack susceptibility during the laser powder bed fusion (LPBF) process. In this work, the microstructure of as-built HX samples was rigorously investigated to understand the main mechanisms leading to crack formation. The microstructural features of as-built HX samples consisted of very fine dendrite architectures with dimensions typically less than 1 µm, coupled with the formation of sub-micrometric carbides, the largest ones were mainly distributed along the interdendritic regions and grain boundaries. From the microstructural analyses, it appeared that the formation of intergranular carbides provided weaker zones, which combined with high thermal residual stresses resulted in hot cracks formation along the grain boundaries. The carbides were extracted from the austenitic matrix and characterized by combining different techniques, showing the formation of various types of Mo-rich carbides, classified as M₆C, M12C and MnCm type. The first two types of carbides are typically found in HX alloy, whereas the last one is a metastable carbide probably generated by the very high cooling rates of the process.
TL;DR: In this paper, a strategy to turn non-electrocatalytic Ti2CTx into an active electrocatalyst by the nitridation of two-dimensional (2D) titanium carbide MXene nanosheets using sodium amide (NaNH2) was reported.
Abstract: Developing active and stable electrocatalysts from Earth-abundant elements is the key to water splitting for hydrogen production through electrolysis. Here, we report a strategy to turn non-electrocatalytic Ti2CTx into an active electrocatalyst by the nitridation of two-dimensional (2D) titanium carbide MXene (Ti2CTx) nanosheets using sodium amide (NaNH2). The addition of NaNH2 results in the chemical bonding of Ti–Nx at 500 °C on the surface of Ti2CTx, which was designed as an efficient electrocatalytic material for the hydrogen evolution reaction (HER). When used as an electrocatalytic material for the HER, the nitrided-Ti2CTx (N-Ti2CTx) exhibited high activity with an overpotential of −215 mV vs. NHE for the hydrogen evolution reaction (HER) at 10 mA cm−2. These values are over three times smaller than those for pristine-Ti2CTx (−645 mV vs. NHE for the HER). The as-synthesized sample showed excellent durability under acidic (0.5 M H2SO4) conditions, indicating its robust catalytic activity towards the HER. The nitridation strategy implemented here could be extended to other 2D transition metal carbide electrocatalysts to improve their catalytic performance.
TL;DR: In this article, the formation mechanism of Ti3AlC2 MAX phase, synthesized using elemental titanium, aluminum, and carbon (graphite) powders via wave propagation and thermal explosion techniques, was investigated.
TL;DR: In this article, the authors reported that the Pitting resistance for tempered condition was lower than the austenitized condition with least resistance at 550 °C. The observation was attributed to the presence of a Fe-rich surface film and massive carbide precipitation with a Cr depletion zone of 7-9 nm at carbide interface.
TL;DR: In this paper, the interfacial structure and mechanical properties of multi-layer graphene (MLG)/pure Ti composites produced by spark plasma sintering and subsequent hot-rolling (HR) have been investigated.
TL;DR: In this article, an extensive in situ Mossbauer spectroscopy study on highly dispersed Fe on carbon catalysts (Fe@C) produced through pyrolysis of a Metal Organic Framework was coupled to their FTS performance testing.
TL;DR: The experimental results indicate that the judicious choice of pyrolysis temperature is essential for creating abundant Mo2C/MoC interfaces and regulating the N,P-doping level in both Mo carbides and carbon matrixes, which leads to optimized electronic properties for accelerating HER kinetics.
Abstract: Molybdenum (Mo) carbide-based electrocatalysts are considered promising candidates to replace Pt-based materials toward the hydrogen evolution reaction (HER). Among different crystal phases of Mo carbides, although Mo2C exhibits the highest catalytic performance, the activity is still restricted by the strong Mo-H bonding. To weaken the strong Mo-H bonding, creating abundant Mo2C/MoC interfaces and/or doping a proper amount of electron-rich (such as N and P) dopants into the Mo2C crystal lattice are effective because of the electron transfer from Mo to surrounding C in carbides and/or N/P dopants. In addition, Mo carbides with well-defined nanostructures, such as one-dimensional nanostructure, are desirable to achieve abundant catalytic active sites. Herein, well-defined N,P-codoped Mo2C/MoC nanofibers (N,P-Mo xC NF) were prepared by pyrolysis of phosphomolybdic ([PMo12O40]3-, PMo12) acid-doped polyaniline nanofibers at 900 °C under an Ar atmosphere, in which the hybrid polymeric precursor was synthesized via a facile interfacial polymerization method. The experimental results indicate that the judicious choice of pyrolysis temperature is essential for creating abundant Mo2C/MoC interfaces and regulating the N,P-doping level in both Mo carbides and carbon matrixes, which leads to optimized electronic properties for accelerating HER kinetics. As a result, N,P-Mo xC NF exhibits excellent HER catalytic activity in both acidic and alkaline media. It requires an overpotential of only 107 and 135 mV to reach a current density of 10 mA cm-2 in 0.5 M H2SO4 and 1 M KOH, respectively, which is comparable and even superior to the best of Mo carbide-based electrocatalysts and other noble metal-free electrocatalysts.
TL;DR: The effect of carbon addition up to 1.84% on the mechanical performance and microstructure of a CoCrFeMnNi high-entropy alloy was studied in this article.