Molybdenum nitride as a metallic photoelectrocatalyst for hydrogen evolution reaction via introduction of electron traps to improve the separation efficiency of photogenerated carriers

Metallic photoelectrocatalysts possess a wide light absorption range and the fast hydrogen evolution reaction (HER) kinetics, which can be used as the next generation of catalysts towards photoelectrocataytic HER. In this work, molybdenum nitride has been fabricated via an in-suit growth method on metal molybdenum substance (Mo₃N₂/Mo foil). The metallic and optical property of Mo₃N₂ was confirmed by the DFT calculations and experimental results from UV–visible absorption spectrum and valence X-ray photoelectron spectroscopy spectrum. Photocatalytic HER rate of Mo₃N₂ reached to 158.78 μmol h^?1 g^?1. Furthermore, Mo₃N₂–MoS₂/Mo foil was prepared to improve photoelectrocatalytic performance. Herein, a suitable energy band alignment for Mo₃N₂–MoS₂/Mo foil was proposed based on experiments and DFT calculations, and the formation of a heterojunction (Mo₃N₂–MoS₂) effectively suppressed the recombination of photo-generated carriers. The results of photoelectrocatalytic experiments suggested that the photocurrent density of Mo₃N₂–MoS₂/foil was effectively enhanced about 1.5 times than that of simplex Mo₃N₂/Mo foil. The electrochemical experiments (LSV and EIS) indicated that the metallic nature of Mo₃N₂ was also beneficial to electrocatalytic HER, and the overpotential of Mo₃N₂–MoS₂/Mo foil at 10 mA cm^?2 was ?173 mV. This work provides a potential candidate for photoelectrocatalytic electrodes.     

Influence of hydrogen and carbon monoxide on reduction behavior of iron oxide at high temperature: Effect on reduction gas concentrations

The purposes of this study are to reduce Fe₂O₃ using hydrogen (H₂) and carbon monoxide (CO) gases at a high temperature zone (500 °C–900 °C) by focusing on the influence of reduction gas concentrations. Reduction behavior of hematite (Fe₂O₃) at high temperature was examined using temperature programmed reduction (TPR) under non-isothermal conditions with the presence of 10% H₂/N₂, 20% H₂/N₂, 10% CO/N₂, 20% CO/N₂ and 40% CO/N₂. The TPR_CO results indicated that the first and second reduction peaks of Fe₂O₃ at a temperature below 660 °C appeared rapidly when compared to TPR_H2. However, TPR_H2 exhibited a better reduction in which Fe₂O₃ entirely reduced to Fe at temperature 800 °C (20% H₂) without any remaining of wustite (FeO) whereas a temperature above 900 °C is needed for a complete reduction in 10% H₂/N₂, 10% and 20% CO/N₂. Furthermore, the reduction of hematite could be improved when increasing CO and H₂ concentrations since reduction profiles were shifted to a lower temperature. Thermodynamic calculation has shown that enthalpy change of reaction (ΔHr) for all phases transformation in CO atmosphere is significantly lower than in H₂. This disclosed that CO is the best reductant as it is a more exothermic, more spontaneous reaction and able to initiate the reduction at a much lower temperature than H₂ atmosphere. Nevertheless, the reduction of hematite using CO completed at a temperature slightly higher compared to H₂. It is due to the presence of an additional carburization reaction which is a phase transformation of wustite to iron carbide (FeO → Fe₃C). Carburization started at the end of the second stage reduction at 600 °C and 630 °C under 20% and 40% CO, respectively. Therefore, reduction by CO encouraged the formation of carbide, slower the reduction and completed at high temperature. XRD analysis disclosed the formation of austenite during the final stage of a reduction under further exposure with high CO concentration. Overall, less energy consumption needed during the first and second stages of reduction by CO, the formation of iron carbide and austenite were enhanced with the presence of higher CO concentration. Meanwhile, H₂ has stimulated the formation of pure metallic iron (Fe), completed the reduction faster, considered as the strongest reducing agent than CO and these are effective at a higher temperature. Proposed iron phase transformation under different reducing agent concentrations are as followed: (a) 10% H₂, 20% H₂ and 10% C; Fe₂O₃ → Fe₃O₄ → FeO → Fe, (b) 20% CO; Fe₂O₃ → Fe₃O₄ → FeO → Fe₃C → Fe and (c) 40% CO; Fe₂O₃ → Fe₃O₄ → FeO → Fe₃C → Fe → F,C (austenite).     

Physical and chemical behaviour of tungsten oxide in the presence of nickel additive under hydrogen and carbon monoxide atmospheres

The physical and chemical behaviour of bulk tungsten oxide (WO₃) and Ni doped tungsten oxide (15% Ni/WO₃) were examined by performing a temperature-programmed reduction (TPR) technique. The chemical composition, morphology, and surface composition of both samples before and after reduced were analysed by X-ray diffraction (XRD), scanning electron microscopy (FESEM), and X-ray photoelectron spectroscopy (XPS) analysis. The XRD pattern of calcined Ni doped tungsten oxide powder comprised of WO₃ and nickel tungstate (NiWO₄) phases. The reduction behaviour was investigated by a non-isothermal reduction up to 900 °C achieved under (10 and 20% v/v) hydrogen in nitrogen (H₂ in N₂) and (20 and 40% v/v) carbon monoxide in nitrogen (CO in N₂) atmospheres. The H₂-TPR were indicated the reduction of bulk WO₃ and 15% NiWO₃ proceed in three steps (WO₃ → WO₂ → WO₂ + W) and (WO₃ → WO₂ → W + Ni₄W) respectively under 20% H₂. Whereas, the reduction of 15% WO₃ under 40% CO involves of two following stages: (i) low temperature (<800 °C) transformation of WO₃ → WO_2.72 → WO₂ and, (ii) high temperature (>800 °C) transformation of WO₂ → W → WC. Furthermore, NiWO₄ alloy phase was transformed according to the sequence NiWO₄ → Ni + WO_2.72 → Ni + WO₂ → Ni + W → Ni₄W + W at temperature >700 °C and >800 °C in H₂ and CO atmospheres, respectively. It can be concluded that the reduction behaviour of WO₃ is matched with the thermodynamic data. In addition, the reduction under H₂ is more favourable and have better reducibility compared to the CO gas. It is due to the small molecule size and molecule mass of H₂ that encourages the diffusion of H₂ molecule into the internal surface of the catalyst compared to CO. Moreover, Ni additive had improved the WO₃ reducibility and enhancing the CO adsorption and promotes the formation of tungsten carbide (WC) by carburisation reaction. Besides, the formation of Ni during the reduction of 15% Ni/WO₃ under CO reductant catalysed the Boudouard reaction to occur, which disproportionated the carbon monoxide to carbon dioxide and carbon (CO → CO₂ + C).     

Effect of MoO3 on Pd nanoparticles for efficient formic acid electrooxidation

Palladium is a promising formic acid electro-oxidation (FAO) catalyst due to its higher initial activity than platinum. However, suffering from the adsorption of hydrogen and CO-like species, the activity and stability of Pd are still unsatisfied. Herein, palladium nanoparticles deposited on carbon supported molybdenum trioxide (Pd–MoO₃/C) is prepared with MoO₃ as the promoter for FAO. X-ray photoelectron spectroscopy analysis proves the close contact between Pd and MoO₃, which generates the hydrogen spillover effect and forms the Pd–Mo structure. The hydrogen spillover effect enhances the desorption of hydrogen from Pd and facilitates the FAO activity. Both the spillover effect and Pd–Mo structure contribute to the removal of CO_ad and facilitate the durability and anti-CO poisoning ability of the catalyst. With the optimized ratio of MoO₃ to carbon black, the Pd–MoO₃/C-20 catalyst owns the best specific activity of 5.86 mA cm⁻², which is 1.86 times of Pd/C.     

Hydrogen production by catalytic decomposition of methane over Fe based bi-metallic catalysts supported on CeO2–ZrO2

Methane decomposition to produce hydrogen was studied over iron based bimetallic catalysts supported on cerium-zirconium oxide in a continuous flow fixed bed reactor at 700 °C. 15 wt% Fe/CeZrO₂ was prepared by wetness impregnation and the promoted Fe catalysts (15 Fe-5 Co/CeZrO₂ and 15 Fe-5 Mo/CeZrO₂) were prepared by co-impregnation technique. Mo promoted Fe catalyst exhibited the maximum surface area of 24.08 m²/g. X-ray diffraction studies revealed that Fe₂O₃, Co₃O₄ and MoO₃ were the phases present in freshly calcined catalysts, while the reduced catalysts consisted of phases including elemental Fe, Mo and Fe–Co alloys. Both X-ray diffraction and temperature programmed reduction studies confirmed the complete reduction of metal oxide species under H₂ at 700 °C. The catalytic activity of Fe/CeZrO₂ was enhanced upon addition of Co and Mo as promoters. The initial hydrogen yield on 15 Fe-5 Mo/CeZrO₂ was ~90% and it decreased with increase in time on stream (TOS), and finally stabilized around ~50% after 125 min of TOS. The Co promoted catalyst exhibited similar activity while the initial hydrogen yield on 15 Fe/CeZrO₂ was ~83% and dropped to ~33% after 125 min of TOS. Graphitic carbon, Fe₃C and Mo₂C phases were observed in the XRD patterns of spent catalysts along with elemental Fe and Fe–Co alloy. It was evident from temperature programmed oxidation results that coke formation which deactivates the catalyst was dominant in 15 Fe/CeZrO₂ when compared to the promoted (Co and Mo) Fe catalysts where carbon nanostructures were dominant. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) confirmed the formation of carbon nanostructures on the surface of spent catalysts. The Fe based catalysts supported both tip and base-growth mechanisms for the growth of carbon nanostructures.     

nPt (Hx−2nMoO3) as a promising catalyst for the oxidation of methanol. Synthesis and electrocatalytic properties

A new method for the synthesis of the catalyst systems Pt–Mo was suggested. nPt⁰(H_x−2nMoO₃)/GC electrodes were prepared by a redox reaction between the hydrogen-containing molybdenum bronzes and potassium tetrachloroplatinate (II) in acid solutions at open circuit potential. The electrodes were characterized by CVA, SEM, X-ray microanalysis, XRD, XPS and ICP-AES. Pt⁰conglomerates formation with nonuniform distribution over the molybdenum bronzes surface has been revealed. nPt⁰(H_x−2nMoO₃)/GC electrodes showed high catalytic activity (not inferior to Pt–Ru-catalyst) in the oxidation of carbon monoxide and methanol as compared with Pt/GC-electrodes. Catalytic effect is apparently achieved by effective oxidation of strongly chemisorbed species (CO_ads, HCO_ads), which occurs at boundaries platinum – molybdenum oxide. Therefore nPt⁰(H_x−2nMoO₃) can be considered as one of perspective catalysts for DMFC.     

CO tolerance of nano-architectured Pt–Mo anode electrocatalysts for PEM fuel cells

PEM fuel cell membrane electrode assemblies with Nafion electrolytes and commercial Pt-based cathodes were tested with Pt_0.8Mo_0.2 alloy and MoO_x@Pt core–shell anode electrocatalysts for CO tolerance and short-term stability to corroborate earlier thin-film RDE results. Polarization curves at 70 °C for the Pt_0.8Mo_0.2 alloy in H₂ with 25–1000 ppm CO showed a significant increase in CO tolerance based on peak power densities in comparison to PtRu electrocatalysts. MoO_x@Pt core–shell electrocatalysts, which showed extremely high activity for H₂ in 1000 ppm CO during RDE studies, performed relatively poorly in comparison to the Pt_0.8Mo_0.2 and PtRu alloys for the same total catalyst loading on a per area basis in MEA testing. The discrepancy is attributed to residual stabilizer from the core–shell synthesis impacting catalyst-ionomer interfaces. Nonetheless, the MoO_x@Pt electrochemical performance is superior on a per-gram-of-precious-metal basis to the Pt_0.8Mo_0.2 electrocatalyst for CO concentrations below 100 ppm. Due to cross-membrane Mo migration, the stability of the Mo-containing anode electrocatalysts remains a challenge for developing stable enhanced CO tolerance for low-temperature PEM fuel cells.     

Preliminary studies of the electrochemical performance of Pt/X@MoO3/C (X = Mo2C,MoO2, Mo) catalysts for the anode of a DMFC: Influence of the Pt loading and Mo-phase

The present study is focused on the influence of Pt loading on the reactivity of catalysts prepared supporting the metal on novel core–shell molybdenum substrates. The electrocatalytic activity and stability of nine Pt/X@MoO3/C catalysts (where X denotes the nature of Mo-phases in the core of the core–shell Mo-particle: Mo₂C, MoO₂ and/or Mo⁰) with three Pt loading (5, 20 and 30 wt% Pt) were tested for carbon monoxide and methanol electro–oxidation reactions.     

Effect of Mo content on hydrogen evolution reaction activity of Mo2C/C electrocatalysts

Recent research suggests that molybdenum carbide (β-Mo₂C) has the potential to be a cheap and active substitute for Pt-based electrocatalyst for hydrogen evolution reaction. In this article molybdenum carbide (Mo₂C) electrocatalysts immobilized on carbon support were synthesized and evaluated for hydrogen evolution reaction (HER). The quantity of Mo in the samples was varied to understand the effect of Mo content in Mo₂C/C electrocatalyst on the structure, morphology, electrochemical properties and HER. The Mo weight percentages determined by ICP-OES technique in four Mo₂C/C samples prepared were found as ~9.3, 15.8, 20.4 and 28.0. SAXS studies revealed that the pore size of the carbon increased with an increase in Mo content, most probably to accommodate the Mo₂C motifs. X-ray photoelectron spectra showed that the amount of low valent Mo increased as we increased the Mo content up to 20 wt % but decreased in the 28 wt % sample. All the samples were active for electrochemical HER with the sample having ~20 wt % Mo showing the highest activity and exhibited a Tafel slope of 69 mVdec⁻¹. Among all samples the 20 wt% Mo sample exhibited the highest electrochemical surface area (ECSA) of ~2.92 mFcm⁻² and minimum charge transfer resistance for the HER. Thus, it is concluded that 20 wt% Mo in Mo₂C/C electrocatalyst evolves with ideal pore size, highest ECSA, smooth charge transfer and thus exhibits the best electrochemical properties for HER.     

Structural engineering of hierarchically hetestructured Mo2C/Co conformally embedded in carbon for efficient water splitting

Developing low-cost and high efficient electrocatalysts for both oxygen and hydrogen evolution reaction in an alkaline electrolyte toward overall water splitting is still a significant challenge. Here, a novel hierarchically heterostructured catalyst composed of ultrasmall Mo₂C and metallic Co nanoparticles confined within a carbon layer is produced by a facile phase separation strategy. During thermal reduction of CoMoO₄ nanosheets in CO ambient, in-situ generated nanoscale Co and ultrafine Mo₂C conformally encapsulated in a conductive carbon layer. In addition, some carbon nanotubes catalyzed by Co nanoparticles vertically grew on its surface, creating 3D interconnected electron channels. More importantly, the integrated C@Mo₂C/Co nanosheets assembled into the hierarchical architecture, providing abundant active surface and retaining the structural integrity. Benefiting from such unique structure, the constructed hierarchical heterostructure shows low overpotentials of 280 mV and 145 mV to reach a current density of 10 mA cm⁻² for OER and HER in an alkaline electrolyte. Furthermore, the symmetrical electrolyzer assembled with catalyst exhibits a small cell voltage of 1.67 V at 10 mA cm⁻² in addition to outstanding durability, demonstrating the great potential as a high efficient bifunctional electrocatalyst for overall water splitting.     

Efficient overall water splitting over a Mo(IV)-doped Co3O4/NC electrocatalyst

To meet the demand of producing hydrogen at low cost, a molybdenum (Mo)-doped cobalt oxide (Co₃O₄) supported on nitrogen (N)-doped carbon (x%Mo–Co₃O₄/NC, where x% represents Mo/Co molar ratio) is developed as an efficient bifunctional electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). This defect engineering strategy is realized by a facile urea oxidation method in nitrogen atmosphere. Through X-ray diffraction (XRD) refinement and other detailed characterizations, molybdenum ion (Mo⁴⁺) is found to be doped into Co₃O₄ by substituting cobalt ion (Co²⁺) at tetrahedron site, while N is doped into carbon matrix simultaneously. 4%Mo–Co₃O₄/NC is the optimized sample to show the lowest overpotentials of 91 and 276 mV to deliver 10 mA cm^?2 for HER and OER in 1 M potassium hydroxide solution (KOH), respectively. The overall water splitting cell 4%Mo–Co₃O₄/NC||4%Mo–Co₃O₄/NC displays a voltage of 1.62 V to deliver 10 mA cm^?2 in 1 M KOH. The Mo⁴⁺ dopant modulates the electronic structure of active cobalt ion (Co³⁺) and boosts the water dissociation process during HER, while the increased amount of lattice oxygen and formation of pyridinic nitrogen due to Mo doping benefits the OER activity. Besides, the smaller grain size owing to Mo doping leads to higher electrochemically active surface area (ECSA) on 4%Mo–Co₃O₄/NC, resulting in its superior bifunctional catalytic activity.     

A highly active and stable Pt modified molybdenum carbide catalyst for steam reforming of dimethyl ether and the reaction pathway

To improve the stability of molybdenum carbide catalysts in dimethyl ether steam reforming (DSR), the inactivation mechanism and the performance of Pt modified catalyst has been investigated. The Mo₂C oxidation induced by H₂O is verified to be the main reason of catalytic deactivation. After modified with Pt, the H₂ production rate and selectivity are greatly enhanced, reaches 1605 μmol min⁻¹·g_cat⁻¹ at 350 °C, in comparison to that of the Mo₂C/Al₂O₃ catalyst. Moreover, the 2%Pt–Mo₂C/Al₂O₃ catalyst is more stable with only 20% activity loss after 50 h on stream compares to the 73% activity loss in 12 h with Mo₂C/Al₂O₃ catalyst. By means of in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), the enhancement brought by Pt is ascribed to the consumption acceleration of intermediate oxygen species on catalyst surface and the decline of onset temperature of DSR reaction. It is expected that these findings can lead us to more practical molybdenum carbide catalysts in DSR.     

High-performance bifunctional Fe-doped molybdenum oxide-based electrocatalysts with in situ grown epitaxial heterojunctions for overall water splitting

The design and development of low-cost, abundant reserves, high catalytic activity and durability bifunctional electrocatalysts for water splitting are of great significance. Here, simple hydrothermal and hydrogen reduction methods were used to fabricate a uniform distribution of Fe-doped MoO₂/MoO₃ sheets with abundant oxygen vacancies and heterojunctions on etched nickel foam (ENF). The Fe– MoO₂/MoO₃/ENF exhibited a small overpotential of 36 mV at 10 mA cm⁻² for hydrogen evolution reaction (HER), an excellent oxygen evolution reaction (OER) overpotential of 310 mV at 100 mA cm⁻² and outstanding stabilities of 95 h and 120 h for the HER and OER, respectively. As both cathode and anode catalysts, the heterogeneously structured Fe– MoO₂/MoO₃/ENF required a low cell voltage of 1.57 V at 10 mA cm⁻². Density functional theory (DFT) calculations show that Fe doping and MoO₂/MoO₃ heterojunctions can significantly reduce the band gap of the electrode, accelerate electron transport and reduce the potential barrier for water splitting. This work provides a new approach for designing metal ion doping and heterostructure formation that may be adapted to transition metal oxides for water splitting.     

Ethanol oxidation on PtRuMo/C catalysts: In situ FTIR spectroscopy and DEMS studies

The electrooxidation of ethanol on carbon supported PtRuMo nanoparticles of different Mo compositions has been studied in the temperature range of 30–70 °C. Current–time curves have shown an increase of the current density with the Mo introduction during the ethanol oxidation at 0.5 V in a whole temperature range. The incorporation of different amount of MoO_x (∼Mo⁵⁺) like species over PtRu systems produces ternary catalyst with similar structural characteristics as particle size or crystal phases, but the catalytic behavior depended on both the surface amount of Mo and on the applied potential. In situ spectroelectrochemical studies have been used to identity adsorbed reaction intermediates and products (in situ Fourier transform infrared spectroscopy, FTIR) and volatile reaction products (differential electrochemical mass spectrometry, DEMS). For all catalysts, incomplete ethanol oxidation to C2 products (acetaldehyde and acetic acid) prevails under the conditions selected in this study. The higher CO tolerance of PtRuMo/C catalysts at very low potentials (<0.3 V) results to minimum or no CO poisoning of the Pt and Ru surfaces, in contrast to the PtRu/C catalyst, which are rapidly blocked by CO. Therefore, catalyst with higher amount of Mo allows a fast “replenishment” of the active sites leading to the formation of acetaldehyde and, especially, acetic acid at potentials above 0.3 V.     

Biomass-derived carbon nanosheets coupled with MoO2/Mo2C electrocatalyst for hydrogen evolution reaction

Transition metal carbide such as molybdenum carbide has been widely used in electrolytic water for hydrogen production due to its potential catalytic property. The synthesis of molybdenum carbide-based high-efficient catalysts by simple process remains great challenges. Herein, Mo oxide/carbide material with hybrid morphology was synthesized by carbonizing mixture of lotus roots and Mo salt. The as-obtained material consists of MoO₂/Mo₂C (MOMC) anchored on biomass-derived nitrogen-doped carbon (NC) matrix. The results show that as-prepared material displays leaf-like and belt-like nanosheets, and the MOMC/NC catalyst with optimal Mo contents exhibits an excellent activity with a low overpotential of 138 mV to drive 10 mA cm^?2 and Tafel slope is 56.7 mV dec^?1 in alkaline medium, indicating that as-prepared catalyst will have promising application in the field of catalysis.     

Nano composite composed of MoOx-La2O3Ni on SiO2 for storing hydrogen into CH4 via CO2 methanation

The crucial problems for the catalysts of CO₂ methanation are the low activity at low temperature and deactivation caused by metal sintering. In order to overcome the problems or to improve the shortages, a new scheme has been put forward by loading LaNi_1-xMo_xO₃ with perovskite-type structure on SiO₂. After reduction, Ni nanoparticles, MoO_x and La₂O₃ would be all stay together and highly dispersed on SiO₂ (Ni/MoO_x-La₂O₃/SiO₂). The techniques of BET, XRD, H₂-TPR, HRTEM, ICP, H₂ chemisorption and XPS were used to characterize the prepared samples. Through effectively combining MoO_x which is active for the reaction of reverse water gas shift and Ni which can catalyze CO methanation, the resultant Ni/MoO_x-La₂O₃/SiO₂ catalyst exhibited pretty good performance for CO₂ methanation, especially showing very good resistance to metal sintering. NiLa₂O₃/SiO₂ catalyst without adding Mo was investigated for comparison. Since many metallic ions can enter into the lattice of a perovskite-type oxide, therefore, many combined catalysts for sequential reactions may be designed via this scheme.     

One-pot synthesis of pure phase molybdenum carbide (Mo2C and MoC) nanoparticles for hydrogen evolution reaction

Herein, we report the one-step synthesis of pure phase molybdenum carbide (Mo₂C and MoC) nanoparticles via the in-situ carburization reduction route without using any reducing agent. The X-ray diffraction (XRD) results confirm the formation of pure phase Mo₂C and MoC at 800 °C for 8 h and 15 h respectively. The as-synthesized powders have been investigated for hydrogen production and energy storage applications. The pure phase Mo₂C shows high performance towards the hydrogen evolution reaction (HER) with a Tafel slope of 129.7 mV dec⁻¹ however, MoC exhibits a low activity towards HER with a Tafel slope of 266 mV dec⁻¹. Both the phases show high stability up to 5000 cyclic voltammetry (CV) cycles in the potential range of 0–0.4 V. In the case of MoC, the specific capacitance increases during the initial 2000 CV cycles which may be attributed to the electrode activation during the CV test. The Mo₂C powder shows a double layer capacitance (C_dl) value of 2.47 mF cm⁻² and a specific capacitance of 2.24 mF g⁻¹. The MoC phase shows a higher C_dl value of 8.99 mF cm⁻² and a specific capacitance of 8.17 mF g⁻¹.     

Hydrogen absorption and desorption kinetics of MgH2 catalyzed by MoS2 and MoO2

The catalytic effect of MoS₂ and MoO₂ on the hydrogen absorption/desorption kinetics of MgH₂ has been investigated. It is shown that MoS₂ has a superior catalytic effect over MoO₂ on improving the hydrogen kinetic properties of MgH₂. DTA results indicated that the desorption temperature decreased from 662.10 K of the pure MgH₂ to 650.07 K of the MgH₂ with MoO₂ and 640.34 K of that with MoS₂. Based on the Kissinger plot, the activation energy of the hydrogen desorption process is estimated to be 101.34 ± 4.32 kJ mol⁻¹ of the MgH₂ with MoO₂ and 87.19 ± 4.48 kJ mol⁻¹ of that with MoS₂, indicating that the dehydriding process energy barrier of MgH₂ can be reduced. The enhancement of the hydriding/dehydriding kinetics of MgH₂ is attributed to the presence of MgS and Mo or MgO and Mo which catalyze the hydrogen absorption/desorption behavior of MgH₂. The detailed comparisons between MoS₂ and MoO₂ suggest that S anion has superior properties than O anion on catalyzing the hydriding/dehydriding kinetics of MgH₂.     

Effect of preoxidation on the reduction of ilmenite concentrate powder by hydrogen

In this study, hydrogen reduction of raw and preoxidized ilmenite concentrates powder, with particle size less than 74 μm, is investigated using a thermal analyzer at 1123, 1173, and 1223 K in a 30% H₂ + 70% Ar atmosphere. The reduction rate by hydrogen was found to be accelerated due to preoxidation treatment of the raw ilmenite concentrate. The reduction of both raw and preoxidized ilmenite concentrates can be divided into two stages: Fe³⁺→Fe²⁺ and Fe²⁺→Fe. The Fe₃O₄→FeO stage overlaps with the Fe₂O₃→Fe₃O₄ stage during reduction process of the preoxidized ilmenite concentrate. Moreover, the preoxidation treatment can effectively decrease the reduction activation energy. The scanning electron microscopy (SEM) and the X-ray powder diffraction (XRD) techniques were used to characterize the micromorphology and phase of the products. The promotion mechanism of reduction of ilmenite by hydrogen through preoxidation treatments is also discussed.     

Study of Mo-based sepiolite catalyst on depolymerization of lignin under supercritical ethanol

A unique Mo/SEP catalyst using low-cost and available sepiolite as support was prepared by wet impregnation method. All catalyst performances of the Mo/SEP catalysts were studied in process of lignin catalytic depolymerization (LCD) under supercritical ethanol with nitrogen pressure, and the effects of reaction temperatures and reaction time on LCD process were also investigated. X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy techniques were used to characterize the structural characteristics of the fresh and spent catalysts, and gas chromatography-mass spectrometry (GC-MS) was employed to analyze the compositions of the obtained liquid product. The results indicated that Mo/SEP catalyst had unique performance for LCD, and the highest soluble fraction yield of petroleum ether of 47.6% and yield of liquid product of 63.5% were obtained with constantly reacting for 4 hours at 290°C and 6.5 MPa N₂. In addition, relevant characterizations demonstrated that the reaction temperature could cause the phase transfer of catalyst and change of Mo⁶⁺ to Mo⁵⁺ species. The conversion degree of Mo⁶⁺ to Mo⁵⁺ was the major reason responding for the catalytic performance of Mo/SEP catalyst during LCD process.