Studies on reduction of chromium doped iron oxide catalyst using hydrogen and various concentration of carbon monoxide

Chemical reduction behaviour of 3% chromium doped (Cr–Fe₂O₃) and undoped iron oxides (Fe₂O₃) were investigated by using temperature programmed reduction (TPR). The reduced phases were characterized by X-ray diffraction spectroscopy (XRD). The reduction processes were achieved with 10% H₂ in nitrogen (%, v/v), 10% and 20% of carbon monoxide (CO) in nitrogen (%, v/v). In hydrogen atmosphere, the TPR results indicate that the reduction of Cr–Fe₂O₃ and Fe₂O₃ proceed in three steps (Fe₂O₃ → Fe₃O₄ → FeO → Fe) with Fe₃O₄ and FeO as intermediate states. A complete reduction to metallic iron for both samples occurred at 900 °C. As for CO reductant, the profiles show the reduction of Fe₂O₃ also proceeded in three steps with a complete reduction occurs at 900 °C in 10% CO with no sign of carbide formation. Nevertheless, a 20% CO was able to reduce the completely at lower temperature at 700 °C and there is a formation of iron carbide at 500 °C but the carbide disappeared as the reduction temperature increase. Meanwhile in 10% CO atmosphere, Cr–Fe₂O₃ shows a better reducibility compared to Fe₂O₃ with a complete reduction at 700 °C, which is 200 °C lower than Fe₂O₃. A Cr dopant in the Fe₂O₃ can lead to formation of various forms of iron carbides such as F₂C, Fe₅C₂, Fe₂₃C₆ and Fe₃C as the CO concentration was increased to 20%. The transformation profile of iron phases during carburization follows the following forms, Fe₂O₃ → Fe₃O₄ → iron carbides (Fe_xC). The XRD pattern shows the diffraction peaks of Cr–Fe₂O₃ are more intense with improved crystallinity for the characteristic peaks of Fe₂O₃ compare to undoped Fe₂O₃. No visible sign of chromium particles peaks in the XRD spectrum that indicates the Cr particles loaded onto the iron oxide are well dispersed. The uniform dispersion with no sign of sintering effects of the Cr dopant on the Fe₂O₃ was confirmed by FESEM. The study shows that Cr dopant gives a better reducibility of iron oxide but promotes the formation of carbides in an excess CO concentration.     

Influence of hydrogen and various carbon monoxide concentrations on reduction behavior of iron oxide at low temperature

The aims of this study are to produce Fe₃O₄ from Fe₂O₃ using hydrogen (H₂) and carbon monoxide (CO) gases by focusing on the influence of these gases on reduction of Fe₂O₃ to Fe₃O₄ at low temperature (below 500 °C). Low reduction temperature behavior was investigated by using temperature programmed reduction (TPR) with the presence of 20% H₂/N₂, 10% CO/N₂, 20% CO/N₂ and 40% CO/N₂. The TPR results indicated that the first reduction peak of Fe₂O₃ at low temperature appeared faster in CO atmosphere compared to H₂. Furthermore, reducibility of first stage reduction could be improved when increasing CO concentration and reduction rate were followed the sequence as: 40% CO > 20% CO > 10% CO > 10% H₂. All reduction peaks were shifted to higher temperature when the CO concentration was reduced. Although, initial reduction by H₂ occurred slower (first peak appeared at higher temperature, 465 °C) compared to CO, however, it showed better reduction with Fe₂O₃ fully reduced to Fe at temperature below 800 °C. Meanwhile, complete reduction happened at temperature above 800 °C in 10% and 20% CO/N₂. Thermodynamic calculation revealed that CO acts as a better reducer than H₂ as the enthalpy change of reaction (ΔH_r) is more exothermic than H₂ and the change in Gibbs free energy (ΔG) at 500 °C is directed to more spontaneous reaction in converting Fe₂O₃ to Fe₃O₄. Therefore, formation of magnetite at low temperature was thermodynamically more favorable in CO compared to H₂ atmosphere. XRD analysis explained the formation of smaller crystallite size of magnetite by H₂ whereas reduction of CO concentration from 40, 20 to 10% enhanced the growth of highly crystalline magnetite (31.3, 35.5 and 39.9 nm respectively). All reductants were successfully transformed Fe₂O₃ → Fe₃O₄ at the first reduction peak except for 10% CO/N₂ as there was a weak crystalline peak due to remaining unreduced Fe₂O₃. Overall, less energy consumption needed in reducing Fe₂O₃ to Fe₃O₄ by CO. This proved that CO was enhanced the formation of magnetite, encouraged formation of highly crystalline magnetite in more concentrated CO, considered better reducing agent than H₂ and these are valid at lower temperature.     

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).     

Metal oxide hydrogen,oxygen, and carbon monoxide sensors for hydrogen setups and cells

Use of hydrogen, oxygen, and carbon oxide semiconductor sensors made of metal oxides allows controlling electronically the content of these gases in operation of many hydrogen setups, cells and devices. Present review-paper gives a general idea of achievements in this field.     

Thermally reduced graphite oxide/carbon nanotubes supported molybdenum disulfide as catalysts for hydrogen evolution reaction

We report an efficient molybdenum disulfide (MoS₂) supported by thermally reduced graphite oxide and carbon nanotubes (TRGO-CNT) for hydrogen evolution reaction. The TRGO-CNT-MoS₂ composite is successfully prepared by a simple sonication process, exhibiting excellent catalytic activity of the hydrogen evolution reaction (HER) with a low overpotential of −0.14 V, which is much lower compared to that of MoS₂, CNT-MoS₂ and TRGO-MoS₂, respectively. TRGO-CNT-MoS₂ also exhibits high stability even after 1000 cycles and strong durability after 48 h. The high HER performance of TRGO-CNT-MoS₂ attributes to a synergic effect of thermal reduced GO and CNT that support MoS₂ due to significant decrease of electrochemical impedance and reliable supporting material for the efficient HER.     

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).     

Synthesis of Pt nano catalyst in the presence of carbon monoxide: Superior activity towards hydrogen evolution reaction

The effect of carbon monoxide (CO) on the reduction of Pt ion to metallic Pt is studied. The modified GC electrode with platinum metal synthesized in the presence of CO shows excellent activity for hydrogen evolution reaction (HER). Despite the decrease in the loading of platinum (4.5 × 10⁻⁴ mg cm⁻²) a substantial increase in its electrocatalytic activity towards HER is observed in a sulfuric acid environment. The observed electrocatalytic activity is comparable to available commercial catalysts like Pt/C. Tafel slope was obtained to be 34 mV.dec^−1, and the overpotential was acquired to be 31 mV at the mass activity of 10 mA mg⁻¹ were observed which was very close to kinetic parameters of Pt/C catalyst.     

A new approach to the manufacturing of elemental boron from boron oxide by carbon monoxide

Elemental boron is one of the most valuable high-tech boron products and it has highest energy density 14 kcal/g in the world for this type of product. With the rapid advancements in technology in recent years, a demand has grown for a light materials with functionality and excellent properties such as high hardness, high melting point, high strength, high chemical resistance and nuclear characteristics that can be used in the fields of aerospace, aviation, automotive and solar cells. In this study boron oxide was reduced using carbon monoxide via a batch system to produce elemental boron. To determine the most suitable conditions for the reduction reaction different temperatures and different CO/B₂O₃ mol ratio parameters were studied. As a result of thermodynamic calculations for the most efficient parameters for reaction temperature was 140–210 °C and the CO/B₂O₃ mol ratio being studied was 3/1 and 2/1 for the batch system. Boron oxide reduction was performed by carbon monoxide gas with the pressure set at 10 bar. Characterization of the product was carried out by using X-Ray Diffractometer (XRD), Fourier Transform Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) at optimum temperature and mol ratio (140 °C and 3/1). Boron phase was seen in both XRD and FT-IR analysis. Also, SEM analysis was performed in order to observe morphological structure of elemental boron.     

Carbon-coated molybdenum carbide nanosheets derived from molybdenum disulfide for hydrogen evolution reaction

The design and synthesis of efficient non-noble metal catalyst is important for the practical application of hydrogen evolution reaction (HER) from water electrolysis. In this study, molybdenum carbides were prepared by a novel synthetic method, which involved the first exfoliation of 2D MoS₂ based on the principle of cold expansion of water below 4 °C and then carburization of exfoliated MoS₂. In this method, MoS₂ played roles of morphology template and molybdenum source simultaneously. Carbon-coated molybdenum carbide nanosheets were obtained and confirmed by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. The stripping degree of MoS₂ was found to have an important influence on physical properties and catalytic performance of molybdenum carbides. Interestingly, Mo₂C nanosheets encapsulated in carbon nanotubes were observed when the MoS₂ with a high peeling degree was used in the preparation. It showed high activity and good durability towards HER in acid solution.     

Coupling molybdenum carbide nanoparticles with N-doped carbon nanosheets as a high-efficiency electrocatalyst for hydrogen evolution reaction

Searching for earth-abundant and high-efficiency electrocatalysts for the hydrogen evolution reaction (HER) is of critical importance for future energy conversion devices. To facilitate the HER on a nonprecious metal-based catalyst, integration of catalytically active nanoparticles with highly conductive carbon supports represents a promising strategy since the formed nanohybrid can offer available active sites and improved electron transfer capability. Herein, we demonstrate a feasible and scalable approach to fabricate well-dispersed Mo₂C nanoparticles firmly anchored on 2D ultrathin N-doped carbon nanosheets (denoted as Mo₂C@NC nanosheets) using inexpensive NaCl as recyclable templates. The adoption of NaCl template provides a 2D space for the one-step concurrent growth of Mo₂C nanoparticles and N-doped carbon nanosheets. Benefiting from the synergy between fine Mo₂C nanoparticles with high dispersity and N-doped C nanosheets, the resultant Mo₂C@NC nanosheets exhibit an outstanding HER performance with a low overpotential, a small Tafel slope and excellent stability under acidic medium, making them a promising noble-metal-free HER catalyst.     

Steam reforming of methanol for hydrogen production over nanostructured wire-like molybdenum carbide catalyst

Steam reforming of methanol over nanostructured wire-like molybdenum carbides, which were synthesized by direct carburization of aniline-intercalated molybdenum based organic-inorganic hybrid composite at different temperatures, were investigated in details. SEM and TEM images revealed that the obtained wire-like molybdenum carbide was composed of nano-particles (10–20 nm) and had microporous structure. These wire-like molybdenum carbides showed higher catalytic activity than those synthesized via the conventional temperature programmed reaction (TPRe) method. It is found that the wire-like molybdenum carbide synthesized at 675 °C had higher surface area (93.2 m²/g) and showed more excellent catalytic activity and longer term stability. XRD results indicated that more α-MoC_1−x phase existed in this kind of molybdenum carbide. It indicated that an alternative high-performance molybdenum carbide catalyst can be easily obtained by adjusting the synthesis condition.     

Kinetic interplay between hydrogen and carbon monoxide in syngas-fueled catalytic micro-combustors

The combined oxidation of hydrogen and carbon monoxide over platinum in micro-combustors at catalytic surface temperatures below 600 K was studied numerically, using a two-dimensional computational fluid dynamics (CFD) model with detailed heterogeneous and homogeneous reaction mechanisms and multicomponent transport. Simulations were performed at different surface temperatures and feed compositions to study the kinetic interplay between hydrogen and carbon monoxide. A sensitivity analysis of the heterogeneous reaction mechanism was performed to identify the rate-controlling steps. Finally, possible mechanisms for the observed behavior were discussed. It was shown that there is significant kinetic interplay between hydrogen and carbon monoxide. Carbon monoxide significantly inhibits the catalytic oxidation of hydrogen. In contrast, the presence of hydrogen was found to promote the catalytic oxidation of carbon monoxide, with the largest effect shown for the small addition of hydrogen, then this effect progressively decreases with the further increase of hydrogen concentration. Accordingly, the apparent reaction order with respect to hydrogen changes from positive to negative, then to zero. The promoting effect of hydrogen can be attributed to the carboxyl pathway, which is crucial to describe the process.     

Highly graphitic carbon shell on molybdenum carbide nanosheets by iron doping for stable hydrogen evolution

The graphitic degree of carbon layer on molybdenum carbide (MC) surface might affect its catalytic performance toward the hydrogen evolution reaction (HER). In this study, an iron (Fe)-doping method was investigated to adjust the graphitic degree of the carbon layer, and subsequently explore the catalytic activity of the generated MCs. A series of Fe-doped MCs was synthesized, and the optimization of their doping level led to distinctly higher HER activity compared to the undoped MC, in both acidic and alkaline solutions, due to the electronic effect. Transmission electron microscopy was applied to determine carbon-coated structure of the obtained Fe-doped MCs while Raman spectra demonstrated the successful adjustment of the carbon layer's graphitic degree with the change of the doping level. As a result, the well-preserved catalytic activity after a 10,000-circles stability test indicated the stable performance of HER on the Fe-doped MC, whereas the enhanced durability was supported by the high graphitic degree of carbon coating layer.     

Alumina supported nano-platinum on copper nanoparticles prepared via galvanic displacement reaction for preferential carbon monoxide oxidation in presence of hydrogen

Improved catalytic centres with a minimum mass-loading of expensive platinum (Pt) have been anticipated for various catalytic applications, for instance preferential oxidation (PROX) of carbon monoxide (CO) in the presence of Hydrogen. Here, we report the synthesis of nano-Pt on the surface of copper (Cu) nanoparticles (NPs) supported on γ-Al₂O₃ (Pt_n(Cu)/γ-Al₂O₃) via galvanic displacement reaction (GDR) for the catalytic CO-PROX reaction. Pt_n(Cu)/γ-Al₂O₃ showed much improved CO-PROX performance compared to that of the as-synthesized Pt_l(Cu)/γ-Al₂O₃ catalyst. Importantly, no significant conversion of hydrogen at a lower temperature range (<200 °C) is observed during the CO-PROX reaction which is one of the essential prerequisites for the CO-PROX reaction. Moreover, Pt_n(Cu)/γ-Al₂O₃ showed the durable, long-term catalytic CO-PROX performance for 120 h. These results infer that realization of nano-Pt on the surface of the Cu NPs holds the promise as the catalytic centres with the minimum mass-loading of Pt for the CO-PROX reaction.     

Zeolite imidazolate framework-8 derived molybdenum carbide/nitrogen-doped carbon for highly-efficient hydrogen evolution reaction

A metal-organic framework-derived method was developed to synthesize highly efficient non-noble metal electrocatalyst for alkaline hydrogen evolution reaction (HER). Zn²⁺, phosphomolybdic acid were coordinated with 2-methylimidazole, and zinc (Zn) and phosphorus (P) species were removed by annealing at 850 °C in N₂ atmosphere, resulting in micro/mesoporous molybdenum carbide (Mo₂C) composited with nitrogen-doped carbon (denoted as ZIF8-xMo-850). The optimized sample ZIF8-12Mo-850 displayed a low overpotential of ~85.7 mV to deliver a current density of 10 mA cm⁻², with a corresponding Tafel slope of ~69.7 mV dec⁻¹ in 1 M KOH. This HER catalytic activity was competitive with the most recently developed Mo₂C-based HER electrocatalysts. From further investigation, the high HER catalytic activity of ZIF8-12Mo-850 is owing to three aspects: (i) The appropriate Mo feeding amount of ZIF8-12Mo-850 resulted in the highest surface content of Mo²⁺ active site; (ii) The evaporation of Zn and P in the ZIF8-12Mo precursor formed its largest average pore diameter of 32.3 nm, which leaded to the highest electrochemically active surface area (ECSA) of 64.29 cm² (iii) The 2-methylimidazole in the precursor resulted in the highest surface content of pyridinic N in ZIF8-12Mo-850 (14.63%), which efficiently improved its conductivity and charge transfer efficiency.     

Microbial fuel cell operation on carbon monoxide: Cathode catalyst selection

Electricity production from carbon monoxide (CO) in a microbial fuel cell (MFC) has recently been demonstrated. Efficient operation of this MFC requires a CO-tolerant and preferably inexpensive cathode. Pyrolised CoTMPP, FeTMPP, and Co/FeTMPP gas diffusion cathodes were tested in MFCs operated on acetate or CO. When the MFC was fed with acetate the best cathode performance was obtained when using a Co/FeTMPP (3:1) cathode with a Me catalyst load of 0.5 mg cm⁻², although this performance was slightly lower than that obtained with a cathode containing 0.5 mg-Pt cm⁻². Tests using a MFC operated on CO showed a higher power output when using the Co/FeTMPP cathode when compared both with CoTMPP and Pt cathodes.     

Synthesis and characterization of Ni-Mo2C particles supported over hydroxyapatite for potential application as a catalyst for hydrogen production

Ni-Mo₂C particles supported over hydroxyapatite were synthesized as potential catalysts to hydrogen production applications due their physiochemical properties observed in characterization results, this favorable for biomass gasification. Mo₂C particles doped with Ni were synthesized by temperature programmed reaction method at 900 °C under hydrogen reducing atmosphere. Hydroxyapatite support was obtained from thermal extraction of bovine bones, in a temperature range from 700 to 900 °C. Ni-Mo₂C impregnation over hydroxyapatite support was made by incipient humidity method. X-ray diffraction analysis determined crystallographic phases of β-Mo₂C, NiC and hydroxyapatite. Though, bone organic matter degradation was observed by X-ray diffraction and confirmed by Infrared Spectroscopy with Fourier Transform (FTIR) the final structure of hydroxyapatite was maintained. Finally, textural properties analysis of support showed an increase in porosity structure with the increment of temperature. β-Mo₂C and NiC were obtained with similar catalytic activity than noble metals, also nickel improves hydrocarbons bonds rupture. Hydroxyapatite showed high stability and porosity at elevated temperatures attributable to synthesis conditions.     

Transient analysis of carbon monoxide poisoning and oxygen bleeding in a PEM fuel cell anode catalyst layer

The presence of carbon monoxide in the fuel stream hinders the performance of a polymer electrolyte membrane (PEM) fuel cell, known as carbon monoxide (CO) poisoning. Introducing oxygen in the fuel stream lessens CO poisoning. Since CO poisoning is a phenomenon that occurs over a substantial period of time, a transient model has been developed in this study, taking into account the effect of CO concentration, operating pressure and temperature, as well as oxygen bleeding on the performance of the cell. It is found that at a lower CO concentration the poisoning effect takes a much longer time to reach the steady state, even though for a better steady state anode performance. A higher operating temperature results in a better steady state performance, but the performance drops faster toward the steady state value at higher temperature. A higher operating pressure leads to an enhanced performance over the entire transient history, although the benefit diminishes as pressure is increased. Even with a small amount of oxygen (0.5%) introduced into the fuel stream, the anode performance can be improved significantly. Finally, it is observed that the use of pure hydrogen interspersed in carbon monoxide containing fuel improves the anode performance. However, performance recovery when operating on pure hydrogen is much slower than the performance degradation due to the CO poisoning.     

Periodically regenerating diesel particulate filter with a hydrogen/carbon monoxide mixture addition

This investigation analyses the effect of introducing a H₂/CO mixture, upstream of a diesel particulate filter (DPF), in an attempt to support the regeneration process. The introduction of the mixture was achieved via various periodic strategies in an attempt to reduce the volume of mixture required while still maintaining proficient regeneration qualities. In addition to this, the effect of space velocity and engine load on the regeneration process was also investigated. The experimental data showed that the mixture addition supported the regeneration process by increasing the filter temperature via an exothermic reaction. The most beneficial spraying strategy introduced the mixture to the DPF every 20 s, with each injection event lasting for a period of 10 s. This strategy required 50% less mixture volume than the constant spray strategy but still induced similar regeneration capabilities. In addition to this, it was noted that a decrease in space velocity reduced the rate of temperature increase but improved the peak filter temperature resulting in increased regeneration proficiency. It was also noted that a reduction in engine load reduced the mean filter temperature but overall had minimal effect on the regeneration process.     

New insights into high-valence state Mo in molybdenum carbide nanobelts for hydrogen evolution reaction

Hydrogen evolution reaction (HER) is considered to be one of the most promising strategies to create hydrogen. Recently, searching high-efficient, stable, and earth-abundant electrocatalysts to replace precious metals for practical utilizations of HER is attracting more and more attentions. Herein, novel molybdenum carbide nanobelts containing Mo of high-valence state derived from MoO₃-ethylenediamine inorganic/organic hybrid precursors are successfully synthesized via a facile one-pot pyrolysis method. The molybdenum carbide nanobelts are characterized using XRD, SEM, TEM and XPS. Moreover, the high-valence state Mo and their relative content in the molybdenum carbide nanobelts can be identified by XPS. The high-resolution XPS spectra of Mo 3d indicates in the molybdenum carbide nanobelts the proportion of high-valence state Mo in active Mo components is 51.3%. More importantly, the as-synthesized products exhibit excellent electrocatalytic activity for HER with a low onset overpotential of 50 mV and a small Tafel slope of 49.6 mV dec^?1 in acidic medium (0.5 M H₂SO₄). Besides, the catalysts require only overpotentials of 143 and 234 mV to achieve current densities of 10 and 220 mA cm^?2, respectively. Furthermore, they also exhibit good durability after 2000 cycles and constant current density test. Such excellent electrocatalytic HER performance can be ascribed to the high intrinsic activity of high-valence state Mo in Molybdenum Carbide. Synthesizing molybdenum carbide with high-valence state Mo electrocatalysts for HER will open up an exciting alternative avenue to acquire outstanding HER electrocatalytic activity.