Carbide Catalysts Research Articles

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.

Carbon nanofiber supported Mo2C catalysts for hydrodeoxygenation of guaiacol: The importance of the carburization process

Molybdenum carbide catalysts supported on carbon nanofibers (β-Mo2C/CNF) were synthetized employing different carburization parameters: five temperatures (550–750 °C) and four heating rates (1–10 °C/min) were tested. The carburization process of the Mo precursor in the catalysts was studied by thermogravimetric analysis, X-ray diffraction, X-ray photoelectron spectroscopy, N2 physisorption, inductively coupled plasma optical emission spectrometry and transmission electron microscopy. The formation of the carbide phase was confirmed by the presence of the oxycarbide and carbide phases which were observed on the surface of all catalysts. Higher carburization temperatures resulted in an increase of the carbide phase content and crystal size at the expenses of the oxycarbide phase disappearance. High carburization temperatures and low heating rates were needed in order to obtain well-defined β-Mo2C crystals over the catalysts, which involved a substantial improvement of their catalytic activity in the hydrodeoxygenation of guaiacol.

Hydrocracking of vacuum residue using nano-dispersed tungsten carbide catalyst

In this study, nano-sized tungsten carbide (2.8 nm) particles with enhanced properties were successfully synthesized using removable ceramic coating method and then applied as dispersed catalysts to hydrocracking of vacuum residue, which was carried out at 673 K for 4 h with initial hydrogen pressure of 70 bar. The nano-sized particles were then characterized with XRD, TEM, and BET analysis. The temperature programmed desorption analysis results showed that nano-sized tungsten particles are capable of superior hydrogen adsorption (compared to bulk tungsten carbide) as well as high catalytic performance not only in commercial liquid yield (Naphtha, middle distillate, and gas oil; 46.7 wt%) but also in coke formation (5.9 wt%). Finally, the characterization and performance correlation of the nano-sized tungsten carbide catalyst were analyzed.

Molybdenum Carbide Catalysts for Chemoselective Transfer Hydrogenation of Nitroarenes

AbstractThe catalytic transfer hydrogenation of nitroarenes to corresponding amines is an important organic synthesis method in the process of industrial production. Molybdenum carbide catalysts were prepared by temperature‐programmed reduction of the as‐prepared molybdenum trioxide in the flowing 20% CH4/H2 and exhibited excellent catalytic performance for the reduction of nitroarenes under mild conditions. In spite of the presence of halogens and reducible substituents, the catalysts also deliver high chemoselectivity (>99%) for transformation nitroarenes to corresponding amines. The Cycling tests of molybdenum carbide catalysts demonstrated that it is highly stable and could easily be recovered by simple filtration for at least eight recycling reactions without decrease of catalytic activity and chemoselectivity.

(Invite) Prospective Investigations of Tungsten and Molybdenum Carbide-Containing Materials As Catalysts for Electrochemical Energy Conversion Processes: Proton, Oxygen, and CO/CO2 Reduction Reaction

In the last decade, transition metal carbides, particularly based of molybdenum and tungsten, have emerged as novel materials with promising catalytic properties toward several important practical electrochemical reactions. In this presentation, the activity of such materials will be discussed for the following electrochemical approaches/reactions: i) the electrocatalysis of oxygen reduction reaction (ORR) in acid media on Pt- and Pt2Ni-nanoparticles supported on carbon forming composites with tungsten and molybdenum carbides with cubic (α) and hexagonal (β) structures; ii) the hydrogen evolution reaction (HER) in acid and alkaline media on TM-modified molybdenum carbide (TM-Mo2C) catalysts, where TM is a transition metal, such as, Fe, Co, Ni and Cu; iii) the CO2 and CO electro-reduction reactions in 0.1 M NaClO4 and 0.5 and 1 M KHCO3 solutions on carbon supported tungsten and molybdenum carbides. Syntheses of the carbide-based catalysts have been conducted employing different procedures, after which the produced material are characterized by X-Ray diffraction (XRD), energy dispersive X-ray spectroscopy, (EDS), X-ray photoelectron spectroscopy (XPS), in situ X-ray absorption near edge structure (XANES), and transmission electron microscopy (TEM). Cyclic voltammetry and polarization measurements on stationary and rotation disk electrodes have been employed for the electrochemical investigations. For the CO/CO2 reduction, reaction products were analyzed by differential electrochemical mass spectrometry (DEMS) and high performance liquid chromatography (HPLC) analyses. Enhanced mass and specific ORR activities are seen for Pt–molybdenum carbide-derived composites compared to Pt-α-WC/C, which in turn presents specific activity for the ORR similar to that of a standard carbon supported Pt catalyst (Pt/C). For Pt-β-WC1-x/C composites, the specific activity is also higher than that of Pt/C when a carbide-to carbon load of 40 wt.% is used. In the case of the HER, high activities are observed for all TM-Mo2C catalysts in acid and alkaline media. Nonetheless, while in acid there is a decrease in the activity upon metal doping, with an activity trend of α-Mo2C > Fe-Mo2C > Co-Mo2C > Ni-Mo2C > Cu-Mo2C, the opposite tendency is found in alkaline solutions. In situ near-edge X-ray adsorption analysis reveals different oxidation states of the TM inside the carbide in both solutions. In acid, a positive charge of the TM in the materials in the electrochemical environment is possibly at the origin of the deleterious effect of Fe, Co, Ni and Cu. In addition, catalysts stability against dissolution in acid media is increased by doping of molybdenum carbide with TM. Regarding of CO and CO2 electro-reductions in 0.1 M NaClO4 and 0.5 and 1 M KHCO3 solutions, results suggest that neither molybdenum nor tungsten carbides can be considered as active catalysts, in contrast to reported results in gas phase. It appears that both the aqueous environment and their activity toward the HER surpass the reactivity toward these latter reactions. Acknowledgements: Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP - Procs. 2013/16930-7 and 2014/23486-9) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) (Procs. 1423454), Brazil, for financial supports.

Cobalt-modified molybdenum carbide as a selective catalyst for hydrodeoxygenation of furfural

The hydrodeoxygenation (HDO) reaction is crucial to the upgrading of biomass-derived furfural to produce a promising fuel additive, 2-methylfuran. In order to enhance the stability of the molybdenum carbide (Mo2C) catalyst, this work utilizes cobalt (Co) modification to tune the oxygen and furfural binding energies on Mo2C. Density functional theory (DFT) calculations of the adsorption configuration of furfural on Mo2C(0001) and cobalt-modified molybdenum carbide (Co/Mo2C(0001)) reveal that the CO bond of furfural is elongated, facilitating the selective CO scission to produce 2-methylfuran. The reduced oxygen and furfural binding energies on Co/Mo2C(0001) allow the facile removal of surface oxygen and furfural to improve the stability of the catalyst. Temperature-programmed desorption (TPD) experiments on model surfaces confirm the enhanced stability and overall HDO performance of Co/Mo2C/Mo(110). Based on the results from high-resolution electron energy loss spectroscopy (HREELS), 2-methylfuran-like intermediates are observed on both Mo2C/Mo(110) and Co/Mo2C/Mo(110). Parallel reactor evaluations over the corresponding powder catalysts further demonstrate the enhanced stability of Co/Mo2C over Mo2C at ambient pressure. This work illustrates the important roles of oxygen and furfural binding energies in the furfural HDO reaction on both model surfaces and powder catalysts, which in turn provides insights into designing selective and stable carbide-based catalysts for HDO reactions.

Impact of the presence of Mo carbide species prepared ex situ in Mo/HZSM-5 on the catalytic properties in methane aromatization

The catalytic activity of HZSM-5 supported Mo-oxide (MoOx) and Mo-carbide (MoCy) for methane aromatization was studied using a packed-bed microreactor. MoOx/HZSM-5 catalysts with 3, 6, 10, and 12 wt.% Mo loading were prepared by incipient wetness impregnation method followed by calcination at 500 °C. The MoCy/HZSM-5 catalysts were prepared ex situ by treating the oxide catalysts by temperature-programmed reduction and carburization. The MoCy/HZSM-5 catalysts show significantly higher activities and stability compared to those of the MoOx/HZSM-5 catalysts. Unlike the oxide catalysts, not only methane conversion and benzene yield improve with higher Mo loading but also the deactivation rate becomes much slower for the carbide catalysts. The optimum carbide catalyst has a Mo loading of 10 wt% The catalysts were characterized by XRD, N2 adsorption, TPR, NH3-TPD, TPO,TGA and 27Al-NMR. The results show that the oxide catalysts at a high loading face pore blockage after few hours of reaction, which makes the active sites inaccessible to CH4. Carbide catalysts, on the other hand face no pore blockage even after a long period of reaction, making them much better catalysts than the oxides for nonoxidative methane aromatization reaction.

Selective production of aromatic hydrocarbons from lignocellulosic biomass via catalytic fast-hydropyrolysis using W2C/γ-Al2O3

This study is focused on fast hydropyrolysis of biomass followed by upgradation of pyrolysates over γ-alumina-supported tungsten carbide catalyst (W2C/γ-Al2O3). The catalyst was prepared by wet-impregnation followed by carburization, and was characterized by various techniques. Catalytic fast-hydropyrolysis experiments were performed at 500 °C in an analytical pyrolyzer coupled with gas chromatograph-mass spectrometer using pine-wood, cellulose and lignin as the feedstocks. W2C/γ-Al2O3 is proved to be an efficient hydrodeoxygenation catalyst producing aromatic and aliphatic hydrocarbons at 82% and 15% selectivities, respectively. Further, the activity of W2C/γ-Al2O3 catalyst is better than γ-Al2O3 and WO3/γ-Al2O3 in terms of both hydrocarbon selectivity and biomass conversion.

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.

Nitrogen-doped mesoporous SiC materials with catalytically active cobalt nanoparticles for the efficient and selective hydrogenation of nitroarenes

Mesoporous nitrogen-doped silicon carbide catalysts with integrated cobalt nanoparticles (Co@N-SiC) were synthesized by the thermal decomposition of a microphase-separated block copolymer of polycarbosilane and polyethylene. The catalysts are highly active, reusable and offer selective hydrogenation of the nitro group in the presence of hydrogenation-sensitive functional groups.

Ni-Doping Effects on Oxygen Removal from an Orthorhombic Mo2C (001) Surface: A Density Functional Theory Study

Density functional theory (DFT) calculations were used to investigate the effect of Ni dopants on the removal of chemisorbed oxygen (O*) from the Mo-terminated (TMo) and C-terminated (TC) Mo2C(001) surfaces. The removal of adsorbed oxygen from the catalytic site is essential to maintain the long-term activity and selectivity of the carbide catalysts in the deoxygenation process related to bio-oil stabilization and upgrading. In this contribution, the computed reaction energetics and reaction barriers of O* removal were compared among undoped and Ni-doped Mo2C(001) surfaces. The DFT calculations indicate that selected Ni-doped surfaces such as Ni adsorbed on TMo and TC Mo2C(001) surfaces enable weaker binding of important reactive intermediates (O*, OH*) compared to the undoped counterparts, which is beneficial for the O* removal from the catalyst surface. This study thus confirms the promoting effect of the Ni dopant on O* removal reaction on the TMo Mo2C(001) and TC Mo2C(001) surfaces. This computational...

Oxygen reduction electrocatalysis on transition metal-nitrogen modified tungsten carbide nanomaterials

Considering the technological importance of the oxygen reduction reaction (ORR) and the cost constraints of highly catalytically active precious metals, recent research efforts have been focused on designing and synthesizing earth abundant non-precious metal catalysts for this reaction. Among recent advances in this area, transition metal-nitrogen modified tungsten carbides can be pointed as prominent candidates as ORR electrocatalysts. Nevertheless, mechanistically understandings of which active sites are responsible for the ORR electrocatalysis on these materials are required for the rational design of suitable materials. In the present work, various tungsten carbides and iron-nitrogen modified tungsten carbides catalysts are synthesized and in-depth characterized through various physical and electrochemical techniques towards gaining insights on the ORR process on these materials in both acidic and alkaline media. High performance materials are developed, with the most active presenting only a ca. 0.060 V increase in the ORR overpotential, compared to a standard platinum catalyst in an alkaline medium. The in-depth analyses allowed for suggestions on reaction pathways for the oxygen reduction on the hybrid Fe/N/WC/C nanomaterials in terms of active sites. These finds might direct further developments in the research on transition metal-nitrogen modified tungsten carbide materials.

Hydrotreatment of vegetable oil for green diesel over activated carbon supported molybdenum carbide catalyst

Catalysts were prepared and utilized in the hydrotreatment of vegetable oil and FAME to produce diesel-like hydrocarbons. Mo2C/AC catalyst displayed the higher catalytic activity with 100.00% conversion and 21.01% cracking ratio, compared to MoO/Al2O3 (85.64%, 25.79%), MoS2/Al2O3 (83.46%, 11.88%), Mo/Al2O3 (67.99%, 33.19%), NiP/Al2O3 (48.72%, 3.49%), Ni/Al2O3 (18.12%, 0.00%), and MoO/AC (56.05%, 18.55%). The reaction condition was also optimized, and the bio-diesel achieved 100.00% conversion, 9.67% cracking ratio and 0.73 HDO/DC ratio over Mo2C/AC under the optimal condition. The conversion over Mo2C/AC dropped from 100.00% to 87.08% and 71.06% with cycles. The results of XRD, XPS and N2 adsorption–desorption demonstrated that the coke deposited on the surface of catalyst and the formation of MoO2 and MoO3 led to the deactivation. After regeneration, the catalyst recovered and achieved 99.36% conversion.

Effects of graphitization of carbon nanospheres on hydrodeoxygenation activity of molybdenum carbide

The hydrodeoxygenation catalytic activity and stability of carbon supported molybdenum carbide catalysts could be improved by graphitizing their carbon supports.

Acetic acid hydrodeoxygenation on molybdenum carbide catalysts

Kinetics and site requirements of acetic acid hydrodeoxygenation on molybdenum carbide – a stable and selective catalyst under atmospheric hydrogen pressure.

Organic−Inorganic‐Hybrid‐Derived Molybdenum Carbide Nanoladders: Impacts of Surface Oxidation for Hydrogen Evolution Reaction

AbstractMolybdenum carbides are widely adopted as efficient noble‐metal‐free electrocatalysts for hydrogen evolution reaction (HER). Their surface oxidation is inevitable under air but is often ignored, which can be detrimental for the accurate understanding and precise design of stable carbide catalysts. Herein, molybdenum carbide nanoladders are derived via pyrolyzing MoOx/amine hybrid precursors, and two types of impact of surface oxidation on their HER performance are obvious: (1) the downshift of d‐band center in Mo, and (2) the introduced steric hindrance from surface oxygen. Consequently, the negative hydrogen‐binding free energy (ΔGH*, −0.52 eV) of pristine MoC gradually increases to positive values with surface oxidation, indicating the effectively weakened strength of Mo−H. With moderate surface oxygen incorporation, a ΔGH* of almost 0 eV is in accordance with the efficiently promoted HER performance via a good balance of Volmer and Heyrovsky/Tafel steps in kinetics. The optimal catalyst supported on a Ti plate (1 cm2) gives a high hydrogen production rate of 0.35 mL min−1 at 300 mV with high stability over the self‐made water‐splitting configuration. This work does not only make us reacquaint the surface oxidation of carbides on electrocatalysis but also paves the way for developing efficient and stable catalysts via surface engineering.

Hydrogen production from formic acid over morphology-controllable molybdenum carbide catalysts

The morphologies of molybdenum carbide catalysts were designed by elaborately selecting organic carbon sources. Based on the scanning electron microscopy (SEM) images and Brunauer-Emmett-Teller (BET) surface area data, it is found that the structure and surface area of MoxCy catalysts can be adjusted by selecting different organic carbon sources. Wire-like MoxCy, nanosheet-like MoxCy and rod-like MoxCy catalysts were obtained by using aniline and p-phenylenediamin (pPd), 4-chloro-o-phenylenediamin (4CloPd), and o-phenylenediamin (oPd) as the carbon sources, respectively, and the nanosheet-like MoxCy showed a relatively high surface area of 30.3 m2 g−1. Furthermore, the crystal structure of MoxCy was found to be controlled by the organic carbon source. Notably, α-MoxCy nanosheets prepared by using 4CloPd showed the best catalytic performance, in which the formic acid conversion approached 100% with a low CO selectivity when the reaction temperature was higher than 250 °C.

Comparison of oxide, sulfide, carbide and nitride Ni-W catalysts supported USY-Al2O3 for ring opening of decalin

The bimetallic Ni-W oxide catalyst supported on USY- Al2O3 has been prepared and converted into Ni-W sulfide, carbide and nitride catalysts. These four types of catalysts have been characterized by surface area measurements, pulsed CO chemisorption, thermogravimetric analysis, X-ray diffraction, temperature-programmed reduction and pyridine infrared spectroscopy. The initial decalin reaction activity of the catalysts decreases in the order of carbide catalyst>oxide catalyst>sulfide catalyst>>nitride catalysts. The catalysts deactivation decreases in the order of: sulfide catalyst>carbide catalyst>>nitride catalyst>oxide catalyst. The initial ring-opening activity decreases in the order of carbide catalyst>oxide catalyst ∼sulfide catalyst>>nitride catalyst. The catalyst activity has relation with the metal sites, the amount of the strong Brönsted acid sites and the surface area on the catalyst. Besides these, the catalyst deactivation also has the close relation with the coke produced on the catalyst during the reaction.

Ni-Assisted Low Temperature Synthesis of MoCx with Enhanced HER Activity.

Here we report a novel one-pot approach to synthesize MoCx catalysts at much lowered temperature for hydrogen evolution reaction (HER) in acidic solution. A room-temperature co-precipitation reaction among Ni2+ , MoO42- and 2-methylimidazole in an aqueous solution produced a Mo-Ni-imidazole complex, which was used as the precursor to fabricate MoCx . The morphological and chemical properties of the obtained MoCx affected by calcination and other additive metal sources (like Fe, Co, Cu) were investigated in detail. We demonstrated that Ni-Mo alloying took place prior to MoCx formation during heating, which catalyzed the crystallization of MoCx at relatively low temperature. The MoCx /Ni-650 catalyst exhibited a quite low overpotential of 172 mV at 20 mA cm-2 . The high performance originates from the synergistic effect between MoCx and Ni-Mo alloy, high ratio of Mo3+ /Mo2+ , as well as high electrochemically active surface area and low charge-transfer resistance. This work would open a new way to the design and fabrication of other transition-metal carbide catalysts.