Fabrication of efficient nanostructured Co3O4-Graphene bifunctional catalysts: Oxygen evolution,hydrogen evolution,and H2O2 sensing

Nanostructured Co₃O₄-graphene hybrid catalysts are fabricated by a one-step vacuum kinetic spray technique from microparticles of Co₃O₄ and graphite powders. The Co₃O₄-graphene hybrid catalysts with various Co₃O₄ contents are studied concerning the oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) in 1.0 M KOH, as well as, H₂O₂ sensing in 0.1 M NaOH. We find that increasing graphene content in the hybrid catalysts results in an overall improvement of the OER electrocatalytic activity due to the enhancement in the charge transfer kinetics. The hybrid catalyst with 25 wt% Co₃O₄ reveals the optimum electrocatalytic activity toward the OER with the lowest overpotential (η) of 283 mV@ 10 mA cm⁻² and superior reaction kinetics with a low Tafel slope of 25 mV dec⁻¹. Besides, the OER stability at 50 mA cm⁻² for 50 h in 1.0 M KOH was verified. The hybrid catalyst with 50 wt% Co₃O₄ revealed the highest activity toward the HER with η of 108 mV@ 10 mA cm⁻², Tafel slope of 90 mV.dec⁻¹, and stability at 50 mA cm⁻² for nearly 30 h. Moreover, it reveals ultrahigh H₂O₂ amperometric detection with superior sensitivity of 18,110 μA mM⁻¹ cm⁻², linear detection range from 20 μM to 1 mM, and a limit of detection of 0.14 μM.     

Highly efficient hybrid electrocatalyst Fe4.5Ni4.5S8/Fe7S8 extracted from nickel ore for hydrogen evolution reaction

Nanoarchitecture materials are widely investigated to enhance hydrogen evolution reaction (HER) performance. However, it is beset with difficulties to obtain nanomaterials at industrial-scale in comparison with bulk materials. Herein, bulk electrocatalyst Fe_4.5Ni_4.5S₈/Fe₇S₈ is prepared using flotation method and alkali cleaning from natural nickel ore. Sulfur vacancies and electron interaction between Fe₇S₈ and Fe_4.5Ni_4.5S₈ may efficiently reduce hydrogen adsorption energy's absolute value, thus significantly improving hydrogen production efficiency. Hybrid catalyst requires ultralow overpotential of 48 mV at 10 mA cm^?2 for HER in 0.5 M H₂SO₄ and exhibits high stability (maintaining an almost constant overpotential after 20,000 cyclic voltammetry sweeps). Therefore, it can be affirmed that it demonstrates promising industrial application as an electrocatalyst towards HER.     

Enhanced hydrogen evolution reaction catalyzed by carbon-rich Mo4.8Si3C0.6/C/SiC nanocomposites via a PDC approach

In this study, mesoporous carbon-rich Mo_4.8Si₃C_0.6/C/SiC ceramic nanocomposites were successfully prepared via a single-source precursor route, starting from allylhydridopolycarbosilane (AHPCS, SMP-10), bis(acetylacetonato) dioxomolybdenum (VI) [MoO₂(acac)₂], and divinylbenzene (DVB). Besides, polystyrene (PS) was used as a pore former. The obtained carbon-rich single-source precursor/PS mixtures were pyrolyzed at 1100°C, and then annealed at 1350°C-1600°C to fabricate a series of carbon-rich Mo_4.8Si₃C_0.6/C/SiC ceramics comprised of high carbon content above 50 wt%. In comparison to the carbon-poor materials, the carbon-rich samples retain the higher specific surface area up to 214.6-304 m²/g at higher annealing temperatures (1350°C-1600°C) due to the enhancement of carbothermal reaction. The carbon-rich samples synthesized at 1500°C, denoted as SM/Mo/PS/DVB 2-1-4-2 1500 exhibit enhanced electrocatalytic performance with ultra-low overpotentials of 119 mV vs reversible hydrogen electrode at a current density of 10 mA cm⁻² in acidic media, which is superior to that of the Mo_4.8Si₃C_0.6/C/SiC ceramic (138 mV) with lower carbon content reported in our previous study. Therefore, our porous materials comprised of high carbon content and Nowotny phase (Mo_4.8Si₃C_0.6, NP) are considered as promising catalysts for the hydrogen evolution reaction (HER).     

Catalytic performance of multicomponent bismuth molybdates (NiXFe3Bi1Mo12O42+X) in the oxidative dehydrogenation of C4 raffinate-3 to 1,3-butadiene: Effect of nickel content and acid property

Multicomponent bismuth molybdate (Ni_XFe₃Bi₁Mo₁₂O_42+X) catalysts were prepared by a co-precipitation method with a variation of nickel content, and were applied to the oxidative dehydrogenation of C₄ raffinate-3 to 1,3-butadiene. Conversion of n-butene and selectivity for 1,3-butadiene over Ni_XFe₃Bi₁Mo₁₂O_42+X catalysts showed volcano-shaped curves with respect to nickel content. As a consequence, yield for 1,3-butadiene also showed a volcano-shaped curve with respect to nickel content. Among the catalysts tested, Ni₉Fe₃Bi₁Mo₁₂O₅₁ showed the best catalytic performance. Acid properties of Ni_XFe₃Bi₁Mo₁₂O_42+X catalysts were measured by NH₃-TPD experiments, with the aim of correlating the catalytic performance with the acid property of the catalysts. It was observed that either total acidity or acid strength was not directly correlated with the catalytic performance of Ni_XFe₃Bi₁Mo₁₂O_42+X catalysts. However, the conversion of n-butene was increased with increasing surface acidity of the catalyst. The largest surface acidity of the Ni₉Fe₃Bi₁Mo₁₂O₅₁ catalyst was responsible for its enhanced catalytic performance in the oxidative dehydrogenation of C₄ raffinate-3. The facile oxygen mobility of the γ-Bi₂MoO₆ phase in the Ni₉Fe₃Bi₁Mo₁₂O₅₁ catalyst also played an important role in enhancing the catalytic performance of the Ni₉Fe₃Bi₁Mo₁₂O₅₁ catalyst.     

Renewable hydrogen production from steam reforming of glycerol (SRG) over ceria-modified γ-alumina supported Ni catalyst

Excess crude glycerol derived as a by-product from biodiesel industry prompts the need to valorise glycerol to value-added chemicals. In this context, catalytic steam reforming of glycerol (SRG) was proposed as a promising and sustainable alternative for producing renewable hydrogen (H₂). Herein, the development of nickel (Ni) supported on ceria-modified mesoporous γ-alumina (γ-Al₂O₃) catalysts and their applications in catalytic SRG (at 550–750 °C, atmospheric pressure and weight hourly space velocity, WHSV, of 44,122 ml·g⁻¹·h⁻¹ (STP)) is presented. Properties of the developed catalysts were characterised using many techniques. The findings show that ceria modification improved Ni dispersion on γ-Al₂O₃ catalyst support with highly active small Ni particles, which led to a remarkable catalytic performance with the total glycerol conversion (ca. 99%), glycerol conversion into gaseous products (ca. 77%) and H₂ yield (ca. 62%). The formation rate for H₂ production (14.4 × 10⁻⁵ mol·s⁻¹·g⁻¹, TOF (H₂) = 3412 s⁻¹) was significantly improved with the Ni@12Ce-Al₂O₃ catalyst, representing nearly a 2-fold increase compared with that of the conventional Ni@Al₂O₃ catalyst. In addition, the developed catalyst also exhibited comparatively high stability (for 12 h) and coke resistance ability.     

A novel Ni2Mo3N/MCM41 catalyst for the hydrogenation of aromatics

Bulk and MCM41-supported Ni₂Mo₃N catalysts were prepared using a temperature-programmed reduction (TPR) method and characterized using XRD, TEM, ICP-AES, and N₂ adsorption analysis techniques. Their catalytic properties were measured for the simultaneous hydrogenation of p-xylene and naphthalene and compared with Ni/MCM41 and Mo₂N/MCM41 catalysts having similar metal loadings. The results indicate that the Ni₂Mo₃N/MCM41 catalyst exhibits excellent deep hydrogenation activity under mild condition (T = 483 K, P = 3.0 MPa), and that it is more active than either Ni/MCM41 or Mo₂N/MCM41 catalyst.     

P-doped MoS2/Ni2P/Ti3C2Tx heterostructures for efficient hydrogen evolution reaction in alkaline media

By combining the advantages of doping to change the electronic structure of molybdenum disulfide (MoS₂), transition metal phosphides, and MXene, we proposed the idea of designing and preparing a new type of composite material, P-doped MoS₂/Ni₂P/Ti₃C₂T_x heterostructures (denoted as P@MNTC), to serve as the hydrogen evolution reaction (HER) catalyst of electrochemical water splitting. The as-prepared P@MNTC heterostructures show a significant HER activity with an overpotential of 120 mV at 10 mA cm^–2 in alkaline electrolyte, with decreasing 105 and 125 mV compared with those of MoS₂ and MXene, respectively. The density functional theory indicates that the P doping and synergy effect of Ti₃C₂T_x can enhance the activation of MoS₂ and thus promote dissociation and absorption of H₂O during HER process. This strategy provides a promising way to develop high-efficiency MoS₂- and Ti₃C₂T_x-based composite catalysts for alkaline HER.     

Crystalline/amorphous Ni/NixSy supported on hierarchical porous nickel foam for high-current-density hydrogen evolution

Manufacturing efficient and durable electrodes for alkaline water electrolysis under high-current-density conditions is a great challenge. Herein, Ni/Ni_xS_y heterostructure was grown on designed porous nickel foam (PNF) to construct Ni/Ni_xS_y-PNF electrode by a facile and scalable method for alkaline hydrogen evolution reaction (HER). The hierarchical porous structure and super-hydrophilic surface of Ni/Ni_xS_y-PNF accelerate the mass transfer and bubble detachment at high current densities. Theoretical calculations reveals that the enhanced HER activity of Ni/Ni_xS_y heterostructure is attributed to the stronger H₂O adsorption. The resulting Ni/Ni_xS_y-PNF electrode achieves current densities of 100 and 500 mA cm⁻² at low overpotentials of 61 and 121 mV, respectively. When applied for overall water splitting, the Ni/Ni_xS_y-PNF||Fe-Ni₃S₂-PNF electrolyser reaches a current density of 10 mA cm⁻² at an extremely low voltage of 1.32 V. This work provides an effective strategy for fabricating high-performance electrodes for practical water electrolysis.     

Effect of Ni promoter in the oxide precursors of MoS2/MgO–Al2O3 catalysts tested in dibenzothiophene hydrodesulphurization

NiMoS catalysts supported on MgO–Al₂O₃ oxides, with 95 and 80 mol% of MgO, were synthesized by sol–gel method. In order to study the Ni promoter effect, MgO–Al₂O₃ supports were impregnated with a pH = 9 solution of Mo and Ni–Mo, respectively; the catalysts were dried (D) and calcinated (C). Catalytic tests showed a Ni promoter effect of 4.5 on the NiMoMg95Al5-D catalyst and 8.5 on the calcinated one. The latter catalyst is more active than a commercial NiMo/Al₂O₃ catalyst. On the other side, the catalyst supported on Mg80Al20 solid did not show any Ni promoter effect. Raman and UV–vis diffuse reflectance spectroscopy showed that during the impregnation step, a strong support interaction with the ion MoO₄^2? takes place on the Mo/MgO–Al₂O₃ solids. After calcination, MoO₄^2? ion remained on the catalyst surface, but increased its interaction with the support. The presence of Ni²⁺_Th, Ni²⁺_Oh and MoO₄^2? ions on dried NiMo/Mg95Al5 catalysts was confirmed, as well as the presence of Ni²⁺_Th, Ni²⁺_Oh, MoO₄^2? and Mo₇O₂₄^6? ions on the calcinated catalyst. This suggests that Ni²⁺ ion allows polymerization of MoO₄^2? to Mo₇O₂₄^6?, produced by Ni²⁺_Oh–MoO₄^2? and Ni²⁺_Oh–Mo₇O₂₄^6? close interactions. The NiMo/Mg80Al20 solids also showed MoO₃ species and a high Ni²⁺_Th concentration. Thus, the Ni promoter effect and therefore, catalytic activity decreased, due to the formation of Ni²⁺_Th–MgO and Ni²⁺_Th–Al₂O₃ spinels.     

Comparison of the catalytic performance of YIG garnets and Fe-containing oxides catalysts for oxidation of ethylbenzene

Iron-containing garnets (YIG) were used as catalysts for selective oxidation of ethylbenzene (EB) in the presence of H₂O₂ as oxidant. The catalysts comprising of two series of garnets e.g., Y₃(Fe_1–xZn_x)₅O₁₂ and Y₃(Fe_1–xNi_x)₅O₁₂ had distinct Zn and Ni contents (x = 0.00 0.01, 0.03, and 0.05). XRD, Raman and FTIR spectroscopies revealed that the cubic structure of Y₃Fe₅O₁₂ garnet was present for x = 0.00 and 0.01. For higher contents, the garnets had the Y₃Fe₅O₁₂ phase besides hematite (α-Fe₂O₃). The catalytic activity was dependent on the contents of metals in the garnets with Y₃Fe_4·97Ni_0·03O_12-γ and Y₃Fe_4·95Zn_0·05O_12-γ catalysts achieving better results. The influence of the reaction conditions such as reaction time, reaction temperature and effect of the solvents as well as the substrates to H₂O₂ molar ratios were studied. SEM-EDS, XPS and EPR results demonstrated the affinity of the Fe²⁺/Fe³⁺ pairs with Ni²⁺ species for the ethylbenzene molecule, which gave an EB conversion of 77% with a good production of acetophenone over the Y₃Fe_4·97Ni_0·03O_12-γ catalyst compared to other binary and ternary solids.     

Selective Isopropylation of Biphenyl to 4,4′-DIPB over Mordenite (MOR) Type Zeolite Obtained from a Layered Sodium Silicate Magadiite

Molybdenum carbide catalysts for water–gas shift (WGS) reaction were investigated to develop an alternate commercial LTS (Cu-Zn/Al₂O₃) catalyst for an onboard gasoline fuel processor. The catalysts were prepared by a temperature-programmed method and were characterized by N₂ physisorption, CO chemisorption, XRD and XPS. It was found that the Mo₂C catalyst showed higher activity and stability than the commercial LTS catalyst, even though both catalysts were deactivated during the thermal cycling runs. The optimum carburization temperature for preparing Mo₂C was in the range of 640–650 °C. It was found that the deactivation of the Mo₂C catalyst was caused by the transition of Mo^δ+ (IV < δ⁺ < VI, MoO_xC_y), Mo^IV and Mo₂C on the surface of the Mo₂C catalyst to Mo^VI (MoO₃) with the reaction of H₂O in the reactant. It was identified that molybdenum carbide catalyst is an attractive candidate for the alternate Cu-Zn/Al₂O₃ catalyst for automotive applications.     

Synthesis and characterization of ternary layered i-MAX (Mo2/3Y1/3)2AlC ceramics fabricated by spark plasma sintering

In this paper, the i-MAX phase (Mo_2/3Y_1/3)₂AlC ceramic with high purity of 98.29 wt% (1.13 wt% Y₂O₃ and 0.58 wt% Mo₂C) and high relative density of 98.59% was successfully synthesized by spark plasma sintering (SPS) at 1500°C with the molar ratio of n(Mo):n(Y):n(Al):n(C) = 4:2:3.3:2.7. The positions of C atoms in the crystal of (Mo_2/3Y_1/3)₂AlC were determined. Microstructure and physical and mechanical properties of (Mo_2/3Y_1/3)₂AlC ceramic were systematically investigated. It was found that the obtained (Mo_2/3Y_1/3)₂AlC ceramic had an average grain size of 32.1 ± 3.1 μm in length and 14.2 ± 1.7 μm in width. In terms of physical properties, the measured thermal expansion coefficient (TEC) of (Mo_2/3Y_1/3)₂AlC was 8.99 × 10⁻⁶ K⁻¹, and the thermal capacity and thermal conductivity at room temperature were 0.43 J·g⁻¹·K⁻¹ and 13.75 W·m⁻¹·K⁻¹, respectively. The room temperature electrical conductivity of (Mo_2/3Y_1/3)₂AlC ceramic was measured to be 1.25 × 10⁶ Ω⁻¹·m⁻¹. In terms of mechanical properties, Vickers hardness under 10 N load was measured as 10.54 ± 0.29 GPa, while flexural strength, fracture toughness, and compressive strength were determined as 260.08 ± 14.18 MPa, 4.51 ± 0.70 MPa·m^1/2, and 855 ± 62 MPa, respectively, indicating the promising structural applications.     

AuPd bimetal immobilized on amine-functionalized SBA-15 for hydrogen generation from formic acid: The effect of the ratio of toluene to DMF

The volume ratio of toluene to N,N-dimethylformamide (DMF) was adjusted during the amine functionalization of SBA-15 to change the amine content of SBA-15. XPS, FTIR, and TGA analyses indicated that under the experimental synthetic conditions the number of amine groups varies with the ratio of toluene/DMF, and the highest content of amine could be obtained with a volume ratio of toluene/DMF = 3:2. The hydrogen production experiment of formic acid decomposition showed that the hydrogen production efficiency over the Au-Pd-SBA-15-NH₂ catalysts increased with the increase in the surface amine content of the Au-Pd-SBA-15-NH₂. The optimal Au-Pd-SBA-15-NH₂-TD (toluene/DMF = 3:2) catalyst proved to have the smallest Au-Pd bimetal nanoparticle size and exhibited a turnover frequency (TOF) = 631 hours⁻¹ and an activation energy (Ea) = 20.8 kJ · mol⁻¹ at 25°C. The catalytic performance of hydrogen generation from the formic acid was improved due to the synergistic effect between the amine-functionalized SBA-15 and the Au-Pd bimetal (metal-support interactions).     

Liquid-phase oxidation of alcohols with oxygen catalysed by modified palladium(II) oxide

Benzyl and trans-cinnamyl alcohols are heterogeneously oxidised to the corresponding aldehydes by O₂ in liquid phase at 100 °C and ambient pressure using hydrous binary Pd^II–M oxides (M=Co^III, Fe^III, Mn^III and Cu^II) as catalysts. Modification of Pd^II oxide with transition metal cations greatly improves the catalytic activity and selectivity to aldehydes, Co^III and Fe^III being the most effective promoters. In benzyl alcohol oxidation in toluene solution, the Pd–Co system gives 85–100% selectivity to aldehydes at 53–95% alcohol conversion in 15–60 min reaction time. The catalyst can be re-used without loss of its activity and selectivity. The presence of a certain amount of water in the catalysts is essential for their performance. From TGA, the composition of the optimal Pd–Co catalyst can be approximated as PdO·(0.13–1.0)CoO(OH)·(2–3)H₂O. The oxidation of alcohols on Pd–M oxide catalysts is accompanied by transfer hydrogenation and decarbonylation side reactions, which is similar to the oxidation on the palladium metal. This indicates that the oxidation of alcohols on Pd–M oxide catalysts occurs via a dehydrogenation mechanism, with hydrogen being present on the catalyst surface.     

Decomposition of metal carbides as an elementary step of carbon nanotube synthesis

The role of catalyst components in catalysts containing molybdenum, Mo/M/MgO (MNi, Co, and Fe), as well as Mo-free catalysts, M/MgO (MNi, Co, and Fe), for carbon nanotube (CNT) synthesis have been investigated by TEM, XRD, and Raman spectroscopy. CNT synthesis by the catalytic decomposition of CH₄ over M/MgO catalysts can proceed at reaction temperatures higher than the decomposition temperature of the metal carbides (Ni₃C, Co₂C, and Fe₃C), which indicates that carbon in the CNT originates from the graphitic carbon formed on the catalyst surface by the decomposition of metal carbides. For all catalysts containing Mo, thin CNT formation starts at an identical temperature of 923 K, corresponding to the decomposition temperature of MoC_1−x into Mo₂C. The significant effect of the addition of Mo is concerned with the formation of Mo₂C in a catalyst particle during CNT synthesis at high reaction temperatures. The presence of a stable Mo₂C phase leads to the formation of thin CNT with better crystallinity at high reaction temperatures. The role of Ni, Co, and Fe in the Mo/M/MgO catalysts is ascribed to the dissociation of CH₄.     

Influence of phosphorous addition on Bi<Subscript>3</Subscript>Mo<Subscript>2</Subscript>Fe<Subscript>1</Subscript> oxide catalysts for the oxidative dehydrogenation of 1-butene

Bi₃Mo₂Fe₁P _x oxide catalysts were prepared by a co-precipitation method and the influence of phosphorous content on the catalytic performance in the oxidative dehydrogenation of 1-butene was investigated. The addition of phosphorous up to 0.4mole ratio to Bi₃Mo₂Fe₁ oxide catalyst led to an increase in the catalytic performance; however, a higher phosphorous content (above P=0.4) led to a decrease of conversion. Of the tested catalysts, Bi₃Mo₂Fe₁P_0.4 oxide catalyst exhibited the highest catalytic performance. Characterization results showed that the catalytic performance was related to the quantity of a π-allylic intermediate, facile desorption behavior of adsorbed intermediates and ability for re-oxidation of catalysts.     

Oxidative dehydrogenation of n-butene to 1,3-butadiene over multicomponent bismuth molybdate (M9Fe3Bi1Mo12O51) catalysts: Effect of divalent metal (M)

Multicomponent bismuth molybdate (M^II₉Fe₃Bi₁Mo₁₂O₅₁) catalysts with different divalent metal (M^II = Mg, Mn, Co, Ni, and Zn) were prepared by a co-precipitation method, and were applied to the oxidative dehydrogenation of n-butene to 1,3-butadiene. Effect of divalent metal (M^II) on the catalytic performance of M^II₉Fe₃Bi₁Mo₁₂O₅₁ catalysts was investigated. X-ray photoelectron spectroscopy (XPS) measurements were conducted to determine the oxygen mobility of M^II₉Fe₃Bi₁Mo₁₂O₅₁ catalysts. It was found that the catalytic performance of M^II₉Fe₃Bi₁Mo₁₂O₅₁ catalysts was closely related to the oxygen mobility of the catalysts. The yield for 1,3-butadiene was monotonically increased with increasing oxygen mobility of the catalysts. Among the catalysts tested, the Co₉Fe₃Bi₁Mo₁₂O₅₁ catalyst with the highest oxygen mobility showed the best catalytic performance in the oxidative dehydrogenation of n-butene.     

Introducing trace Co to molybdenum carbide catalyst for efficient ammonia synthesis

Development of the cost-effective catalysts with excellent catalytic performance is highly demanded for ammonia synthesis, and the strong adsorption of ammonia greatly hinders the design of cost-effective and high-performance ammonia synthesis catalysts. Herein, we report that the addition of a small amount of Co species (0.1 wt%) into Mo₂C catalyst, which can provide electrons to Mo₂C, not only leads to improvement of the adsorption and migration of hydrogen, but also facilitates the adsorption and desorption of ammonia. Consequently, Mo₂C catalyst with 0.1 wt%Co offers a 40% higher ammonia synthesis activity and a lower negative reaction order with respect to NH₃ in comparison to Mo₂C. This work stresses the importance of the minor components in improving the ammonia synthesis activity by accelerating the migration of reactants over the catalyst surface and the escape of products from the catalyst.     

Dual-phase membrane reactor for hydrogen separation with high tolerance to CO2 and H2S impurities

Catalytic membrane reactors based on oxygen-permeable membranes are recently studied for hydrogen separation because their hydrogen separation rates and separation factors are comparable to those of Pd-based membranes. New membrane materials with high performance and good tolerance to CO₂ and H₂S impurities are highly desired. In this work, a new membrane material Ce_0.85Sm_0.15O_1.925–Sr₂Fe_1.5Mo_0.5O_6-δ (SDC–SFM) was prepared for hydrogen separation. It exhibits high conductivities at low oxygen partial pressures, which is benefit to electron transfer and ion diffusion. A high hydrogen separation rate of 6.6 mL cm⁻² min⁻¹ was obtained on a 0.5-mm-thick membrane coated with Ni/SDC catalyst at 900°C. The membrane reactor was operated steadily for 532 h under atmospheres containing CO₂ and H₂S impurities. Various characterizations reveal that SDC–SFM has good stability in the membrane reactor for hydrogen separation. All facts confirm that SDC–SFM is promising for hydrogen separation in practical applications. © 2018 American Institute of Chemical Engineers AIChE J, 65: 1088–1096, 2019     

Effect of cobalt (nickel) content on the catalytic performance of molybdenum carbides in dry-methane reforming

The dry-methane reforming (DMR) behavior of Co-Mo and Ni-Mo carbide catalysts has been studied in order to establish the effect of the cobalt or nickel content of molybdenum carbide DMR catalysts. The results indicate that incorporating cobalt into the Mo₂C structure at a Co/Mo ratio of 0.4, i.e. a Co_0.4Mo₁C_x catalyst, gives a DMR activity and stability that are markedly higher than those of Mo₂C catalysts. With respect to the Ni-Mo carbide catalysts, a Ni/Mo atom ratio of 0.2 (i.e. an Ni_0.2Mo₁C_x catalyst), gives the maximum synergistic interaction between Ni and Mo. However, higher molar ratios decrease the promoting effect and facilitate the phase separation of the promoter. These results are proved by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy and N₂-adsorption studies, and are also reflected in the poor catalytic stability of both the Co-Mo and the Ni-Mo carbide catalysts.