Low-temperature steam reforming of methanol to produce hydrogen over various metal-doped molybdenum carbide catalysts

Various transition metals (M = Pt, Fe, Co, and Ni) were selected to support on molybdenum carbides by in-situ carburization metal-doped molybdenum oxide (M-MoO_x) via temperature-programmed reaction (TPR) with a final temperature of 700 °C in a reaction gas mixture of 20% CH₄/H₂. XRD analysis results indicated that β-Mo₂C phase was formed in the case of Fe, Co, or Ni doping while α-Mo₂C phase was appeared with the β-MoC_1−x phase in the case of Pt doping. With the increase in Pt doping amount, more α-MoC_1−x phase was produced. As-prepared metal doped molybdenum carbides were investigated as alternative catalysts for the steam reforming of methanol. Comparing with the undoped molybdenum carbide such as β-Mo₂C, metal-doped one showed higher methanol conversion and hydrogen yield. It is found that Pt doped molybdenum carbide had the highest catalytic activity and selectivity among the prepared catalysts and methanol conversion reached 100% even at a temperature as low as 200 °C, and remained a long-time stability with a stable methanol conversion.     

Thermally stable Pt/Ti mesh catalyst for catalytic hydrogen combustion

In this study, platinum (Pt) supported on titanium (Ti) mesh catalysts for catalytic hydrogen combustion were prepared by depositing Pt as a thin-layer on metallic or calcined Ti mesh. The Pt thin-layer could be stabilized as uniformly distributed, near nano-sized particles on the surface of calcined Ti mesh by exposing the freshly sputtered Pt to hydrogen. Temperatures between 478 and 525 °C were reached during hydrogen combustion and could be maintained at a hydrogen flow rate of 0.4 normal liter (Nl)/min for several hrs. It was determined that Ti mesh calcination at ≥900 °C formed an oxide layer on the surface of Ti wires, which prevented significant Pt aggregation. X-ray photoelectron spectroscopy revealed that the surface of Ti mesh was fully converted to TiO₂ at ≥900 °C. Raman spectroscopy showed that the majority of TiO₂ was present in the rutile phase, with some minor contribution from anatase-TiO₂. The calcined Ti support was stable through all investigations and did not indicate any signs of degradation.     

Ni/β-Mo2C as noble-metal-free anodic electrocatalyst of microbial fuel cell based on Klebsiella pneumoniae

A composite of nickel and β-molybdenum carbide (Ni/β-Mo₂C) was prepared from solution derived precursor and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and Brunauer-Emmett-Teller (BET). The activity of Ni/β-Mo₂C as noble-metal-free anodic electrocatalyst of microbial fuel cell (MFC) based on Klebsiella pneumoniae (K. pneumoniae) was investigated by electrochemical measurements. The results from voltammetric measurements show that Ni/β-Mo₂C has high electrocatalytic activity towards the oxidation of formate, lactate, ethanol, and 2,6-di-tert-butyl-p-benzoquinon (2,6-DTBBQ), which are the main metabolites of K. pneumoniae. The results from polarization curve measurement indicate that the MFC using Ni/β-Mo₂C as anodic electrocatalyst delivers a higher power density than the MFC using β-Mo₂C as anodic electrocatalyst. Ni/β-Mo₂C provides the MFC based on K. pneumoniae with a novel noble-metal-free anodic electrocatalyst of high activity.     

Porous carbon incorporated β-Mo2C hollow sphere: An efficient electrocatalyst for hydrogen evolution reaction

Hydrogen is undoubtedly considered as the cleanest energy source and hence the urgency for production of H₂ is a concern to the scientific community. The major bottleneck for electrocatalytic hydrogen production is still exists as the reported catalysts, mainly noble metal based, are extremely costly. Thus the fabrication of cheap and noble metal free electrocatalyst for Hydrogen Evolution Reaction (HER) is still seeking. Here, a simplistic method is developed for the synthesis of carbon assimilated molybdenum carbide (β-Mo₂C) hollow sphere which showed effective electrocatalysis for HER. The synthesis of desired β-Mo₂CC hollow sphere is based on high temperature carbothermal reduction (800 °C in 10%-H₂/N₂ atmosphere) of carbon incorporated MoO₂ spheres. The synthesized β-Mo₂C with an optimal carbon loading reaches 10 mA/cm^?2 current density at a low overpotential of 146 mV with a small tafel slope of 90 mV dec^?1 for the electrochemical hydrogen evolution (HER) in 0.5 M H₂SO₄. The improved catalytic performance, long-term stability make β-Mo₂CC hollow sphere an auspicious HER electrocatalyst.     

Self-assembled ionic liquid synthesis of nitrogen-doped mesoporous TiO2 for visible-light-responsive hydrogen production

Nitrogen-doped mesoporous TiO₂ has been synthesized by a simple solvent evaporation-induced self-assembly method using a nitrogen-containing ionic liquid concurrently as a nitrogen source and mesoporous template. After being evaporated and subsequently calcined at various temperatures (300–900 °C), the synthesized samples were thoroughly characterized by X-ray diffraction (XRD), Raman, small-angle X-ray scattering patterns (SAXS), N₂ adsorption-desorption isotherms, X-ray photoelectron (XPS) and UV–Vis diffuse reflectance (UV–Vis DR) spectroscopies. The obtained results suggest that the calcination temperature greatly influences the crystallization of TiO₂, formation of mesoporous structure, specific surface area and N-doping amounts. Among the fabricated photocatalysts, the samples calcined at 600 °C exhibit superior photocatalytic performance for hydrogen production in water/methanol solution under visible light illumination if compared to other synthesized samples and commercial TiO₂ (Degussa P25). The finding is possibly due to the synergy of more N-doping amounts on the well-defined mesoporous TiO₂ with highly anatase crystal phase and moderate surface area in the catalysts.     

Halide-free Grignard reagents for the synthesis of superior MgH2 nanostructures

Grignard reagents can provide a simple path to generate, through their thermal decomposition, magnesium (Mg) and/or its hydride (MgH₂). However, existing compounds lack the ability to lead to “adequate” MgH₂ structures to enable effective hydrogen storage. Herein, we report on the possibility to tune Grignard reagent's chemical structure, i.e. number of β-hydrogen, and the activation energy for their thermal decomposition to lead to Mg/MgH₂. For example, di-tert-butylmagnesium with nine β-hydrogen was able to decompose at a very low temperature of 167 °C to generate MgH₂/Mg, which is 100 °C lower than the temperature required to generate MgH₂ from di-n-butylmagnesium, i.e. the only compound known to date. More remarkably, the MgH₂ synthesized from the di-tert-butylmagnesium precursor was able to release hydrogen from 100 °C. These promising hydrogen storage properties are credited to the formation of the metastable γ-MgH₂ phase, which is believed to result from the structural defects generated upon the thermal decomposition of di-tert-butylmagnesium.     

γ-MgH2 induced by high pressure for low temperature dehydrogenation

The formation of metastable γ-MgH₂ upon application of ultra-high pressure and its dehydrogenation properties were studied. Magnesium-nickel alloy (14 wt.% Ni) was hydrogenated and compressed at ultra-high pressures of 2.5 and 4 GPa. The phase composition and desorption properties of the products were investigated. Powder X-ray diffraction indicated that some α-MgH₂ converted to γ-MgH₂ during compression. This resulted in the onset of hydrogen desorption at 60 °C under vacuum. Our findings thus show that application of ultra-high pressure can facilitate the formation of γ-MgH₂, which has a lower dehydrogenation temperature (≤200 °C) than α-MgH₂, which desorbs at temperatures above 300 °C. The metastable phase possessed a high hydrogen storage capacity of at least 4.5 wt.%. These properties revealed the potential of γ-MgH₂ as a future hydrogen storage material.     

Synthesis of Ni/TiO2 catalyst by sol-gel method for hydrogen production from sodium borohydride

Hydrogen is a sustainable, renewable and clean energy carrier that meets the increasing energy demand. Pure hydrogen is produced by the hydrolysis of sodium borohydride (NaBH₄) using a catalyst. In this study, Ni/TiO₂ catalysts were synthesized by the sol-gel technique and characterized by X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) methods. The effects of Ni loading ratio (20–40%), catalyst amount (75–200 mg), the concentration of sodium hydroxide (NaOH, 0.25–1 M), initial amount of NaBH₄ (75–125 mg) and the reaction temperature (20–60 °C) on hydrogen production performance were examined. The hydrogen yield (100%) and hydrogen production rate (110.87 mL/g_cat.min) were determined at the reaction conditions of 5 mL of 0.25 M NaOH, 100 mg NaBH₄, 100 mg Ni/TiO₂, 60 °C. Reaction order and activation energy were calculated as 0.08 and 25.11 kJ/mol, respectively.     

Kinetics and microstructural changes during fluidized reduction of magnetite with hydrogen at low temperatures

Hydrogen reduction has received much attention in reducing carbon dioxide emissions and becoming carbon neutral in the iron and steel industry. In this study, the low-temperature hydrogen reduction (H₂: N₂ = 80%: 20%) process of natural magnetite in a fluidized bed was investigated. The reduction kinetics, phase transformation, and microstructure changes were characterized using chemical analyses, X-ray diffraction, Brunauer–Emmett–Teller method, and scanning electron microscopy. The results revealed that the magnetite was reduced to metallic iron in one step in the temperature range of 495 °C to 570 °C, and the reaction rate increased with increasing temperature. The one-step reduction of magnetite was controlled by the phase boundary reaction, and the apparent activation energy was 29.76 kJ/mol. Microstructural analysis indicated that as the reduction reaction progressed, micropores and cracks on the surface of the solid particles gradually developed. The dense magnetite core was wrapped by the porous metallic iron, which promoted the penetration of hydrogen into the particles and continued the reaction.     

β-Mo2C/MoO2 heterostructures for hydrogen evolution reaction: Morphology and catalytic activity regulated by heteroatom doping

Improving the activity of non-noble metallic electrocatalyst for hydrogen evolution reaction (HER) is an important issue for hydrogen energy utilization. In this work, we reported a new strategy for morphology regulation of β-Mo₂C/MoO₂ heterostructure via heteroatom doping to enhance the electrocatalytic HER activity. Electron microscopy observations found that N and S doping resulted in nanosphere and nanorod morphology, respectively. Amongst the S doping catalyst (denoted as S@β-Mo₂C/MoO₂) exhibited remarkably improved electrocatalytic HER activity and stability as compared to the pristine β-Mo₂C/MoO₂ catalyst, whereas the N doping induced significant degradation of catalytic activity and stability. The mechanism investigations reveal that the nanorod morphology of S@β-Mo₂C/MoO₂ endows it lower charge transfer resistance, higher electrochemical active surface area and lower valence state of Mo species, which contributes positively and importantly to its better electrocatalytic HER activity.     

Reduced graphene oxide-supported Ni-MoxC electrocatalyst for hydrogen evolution reaction prepared by ultrasonication and lyophilization

A Ni and Mo_xC hybrid (Ni-Mo_xC) supported on N-doped reduced graphene oxide (N-rGO) electrocatalyst with high hydrogen evolution reaction (HER) activity was prepared by ultrasonication and lyophilization. Notably, benefiting from the synergistic effect between Ni and Mo_xC nanoparticles, the optimized electrocatalyst displayed excellent catalytic activity with low overpotentials of 183 mV and 216 mV for the HER at the current density 10 mA cm⁻² in 1.0 M KOH and 0.5 M H₂SO₄ solution. The stability of the electrocatalyst could be well maintained for 24 h. These results indicate that the method to prepare hybrid (Ni-Mo_xC) is a simple way to produce cost-effective and high-efficient molybdenum carbide for hydrogen evolution.     

One-step synthesis of Ni/yttrium-doped barium zirconates catalyst for on-site hydrogen production from NH3 decomposition

On-site produced hydrogen from ammonia decomposition can directly fuel solid oxide fuel cells (SOFCs) for power generation. The key issue in ammonia decomposition is to improve the activity and stability of the reaction at low temperatures. In this study, proton-conducting oxides, Ba(Zr,Y) O_3-δ (BZY), were investigated as potential support materials to load Ni metal by a one-step impregnation method. The influence of Ni loading, Ba loading, and synthesis temperature, of Ni/BZY catalysts on the catalytic activity for ammonia decomposition were investigated. The Ni/BZY catalyst with Ba loading of 20 wt%, Ni loading of 30 wt%, and synthesized at 900 °C attained the highest ammonia conversion of 100% at 600 °C. The kinetics analysis revealed that for Ni/BZY catalyst, the hydrogen poisoning effect for ammonia decomposition was significantly suppressed. The reaction order of hydrogen for the optimized Ni/BZY catalyst was estimated as low as ?0.07, which is the lowest to the best of our knowledge, resulting in the improvement in the activity. H₂ temperature programmed reduction and desorption analysis results suggested that a strong interaction between Ni and BZY support as well as the hydrogen storage capability of the proton-conducting support might be responsible for the promotion of ammonia decomposition on Ni/BZY. Based on the experimental data, a mechanism of hydrogen spillover from Ni to BZY support is proposed.     

Nano-sized RuO2 electrocatalyst improves the electrochemical performance for hydrogen oxidation reaction

Hydrogen oxidation reaction (HOR) can be applied to proton exchange membrane fuel cells to generate electrical energy and anode discharge. Due to its special properties, RuO₂ has been applied to supercapacitors, phenolic wastewater, textile industry wastewater, and degrading organic substances. However, there is few reports on the application of the RuO₂ catalyst to hydrogen oxidation reaction (HOR). In this study, we successfully obtained RuO₂ NPs using a simple and eco-friendly hydrothermal method. Furthermore, the electrochemical activity of RuO₂ NPs prepared at different concentration (0.15 M, 0.20 M) and different hydrothermal temperature (150 °C, 160 °C, and 170 °C) was evaluated by the hydrogen oxidation reaction. The particle size, composition, dispersion and morphology of the obtained RuO₂ catalysts were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). In addition, cyclic voltammograms (CV) were utilized to investigate the electrochemical activity of the RuO₂ catalysts. The results showed that the obtained catalyst at a hydrothermal temperature of 160 °C and a concentration of 0.15 M displayed a Brunauer-Emmett-Teller (BET) surface area of 26.74 m² g⁻¹. Meanwhile, the catalyst had a uniform distribution. The hydrogen oxidation current density of the obtained RuO₂ catalysts is upto 6 mA cm⁻², showing a good electrochemical activity for hydrogen oxidation reaction.     

Catalytic decomposition of tar derived from biomass pyrolysis using Ni-loaded chicken dropping catalysts

The Ni-loaded chicken droppings (Ni/CD) and chicken dropping ash (Ni/CDA) were prepared by the impregnation method and applied as catalysts for biomass tar decomposition at low temperature (450 °C) under N₂ and steam/N₂ conditions. The prepared samples and the supports were characterized by N₂ adsorption measurements, X-ray diffraction, H₂ temperature-programmed reduction, and X-ray photoelectron spectroscopy. The results reveal that Ni/CD and Ni/CDA showed higher catalytic activity for tar decomposition, in terms of producing hydrogen-rich gas, relative to commercial Ni/Al₂O₃ under N₂ conditions. This higher activity was caused by lower interactions of Ni with the support and the presence of additional reduced Ni. In the case of steam reforming, Ni/CDA also showed higher hydrogen yield and a lower amount of carbon deposition than Ni/Al₂O₃. This result indicates that a hydrophilic hydroxyapatite in the CDA support promoted the water–gas shift reaction to suppress carbon deposition and increase hydrogen yield.     

Low temperature synthesis of Cu1−xAl2.5 spinel solid solution as sustained release catalyst for methanol to hydrogen

In order to find a method to synthesize spinel solid solution at low temperature, in this paper, pseudo-boehmite and copper nitrate were used as raw materials, and citric acid was used as accelerator for the first time, the Cu_1?xAl_2.5 spinel solid solutions were synthesized by ball milling treatment of the solid mixture and then calcined at elevated temperatures. The obtained materials were characterized by X-ray diffraction (XRD), Brunner?Emmett?Teller (BET) measurements, H₂-temperature programmed reduction (H₂-TPR), infrared (IR) and thermogravimetric methods (TG), and the mechanism of citric acid promoting the synthesis of the spinel solid solution was suggested. The results show that an optimal amount of citric acid can significantly promote the formation of the spinel structure, thus substantially decreasing the synthetic temperature from the above 900 °C–700 °C. At 700 °C, the preparation without the accelerator resulted in small amount of the spinel solid solution, while the use of accelerator gave rise to 82.96% spinel solid solution. Importantly, the Cu_1?xAl_2.5 spinel solid solution prepared at 700 °C demonstrates the best sustained release catalytic performance, and the catalytic activity reached more than 90% and remained stable within 50 h. The findings of this report might be provide guidance for the solid phase preparation of catalysts at relatively lower temperatures, thus facilitating the commercialization of the sustained release catalyst system.     

Evolution of PM2.5 from biomass high-temperature pyrolysis in an entrained flow reactor

In this study, wheat straw pyrolysis was conducted in an entrained flow reactor at 900–1300 °C, and PM_2.5 were sampled from the flue gas through a heated sampling system. Multi-phase PM_2.5 including carbonaceous matter, potassium-containing particles, and ash particles, was separated and quantified using a thermogravimetric analyzer (TGA). The micro-morphologies and chemical compositions of these three phases were characterized by scanning electron microscopy (SEM), scanning transmission electron microscope (STEM), energy dispersive X-ray spectrometry (EDS), and X-ray diffraction (XRD). Results show that PM_2.5 yields during biomass pyrolysis are in the range of 7–34 g/kg (dry-basis biomass) and increase with the increase of pyrolysis temperature. At low pyrolysis temperatures (900–1000 °C), the carbonaceous matter is dominated by char-carbon. When the pyrolysis temperature increase from 1000 °C to 1100 °C, the production of soot is greatly enhanced and soot becomes dominant in PM_2.5, and the amorphous morphologies of soot are replaced by the concentric graphitic layers. With the further increasing in pyrolysis temperature, soot particles become more spherical and onion-like. Above 1100 °C, the KCl content in PM_2.5 declines, which is because of the capture of KCl and the formation of low-melting potassium aluminosilicates in large char particles. At 1300 °C, the fragmentation of char particles is significantly strengthened, resulting in more ash in PM_2.5.     

Cobalt-molybdenum carbide@graphitic carbon nanocomposites: Metallic cobalt promotes the electrochemical hydrogen evolution reaction

In this work, a series of metallic cobalt-molybdenum carbide@graphitic carbon (CoMo(x:y)-T@GC) nanocomposites for the electrochemical hydrogen evolution reaction (HER) were synthesized by a sol-gel method. In the as-prepared nanocomposites, β-Mo₂C and metallic Co coexisted and were encapsulated by graphitic carbon. The presence of metallic Co effectively enhanced the crystallinity of β-Mo₂C, charge transfer efficiency and electrochemical active surface area (ECSA), thus resulting in the improved HER catalytic activities of the CoMo(x:y)-T@GC nanocomposites. The optimized electrocatalyst CoMo(0.5:0.5)-800@GC required the lowest overpotential of ～165 mV to deliver a current density of 10 mA cm^?2 in 0.1 M KOH, which was at the forefront compared with recently reported Mo₂C-based electrocatalysts.     

Synthesis,characterization and corrosion performance evaluation of DDR membrane for H2 separation from HI decomposition reaction

An all silica DDR (deca dodecasil rhombohedral) zeolite membrane with dense, interlocked structure has been developed for separation of H₂ from HI/I₂ mixture of HI decomposition reaction. In this work, the DDR zeolite membrane was synthesized on the seeded clay-alumina substrate within 5 days. The seeds were synthesized by sonication mediated hydrothermal process within short crystallization time which enhanced the nucleation for the membrane growth. The synthesized membranes along with seed crystals were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), Field emission scanning electron microscope (FESEM) and energy dispersive X-ray spectroscopy (EDAX). The selectivity of hydrogen with respect to CO₂ and Ar was evaluated by single gas permeation studies at room temperature. The tests for corrosion resistance were carried out upto 120 h with both support and DDR membrane at 130 °C which confirmed the stability of membrane under the harsh HI/I₂ environment.     

Remarkably improved hydrogen storage properties of carbon layers covered nanocrystalline Mg with certain air stability

Different nanocrystalline magnesium with carbon layers were successfully synthesized via a facile wet-chemical ball milling method for 20, 30 and 40 h, respectively. Based on Scherrer formula and X-ray diffraction results, the average crystallite size of all the three samples was below 30 nm. TEM observations showed that the hydrogenated Mg particles were covered with carbon layers. Moreover, the 40 h ball milled Mg sample showed outstanding hydrogen storage performance especially in the aspect of hydrogen absorption. The as-prepared sample started to take up hydrogen at nearly room temperature and eventually absorbed 6.8 wt% hydrogen at 200 °C. The apparent activation energy (E_a) of hydrogen absorption for the sample was decreased to 26.7 kJ/mol, much lower than that of other reported systems. For the dehydrogenation experiments, the hydrogenated sample could start to release hydrogen at about 275 °C and 6.5 wt% hydrogen was desorbed in 20 min at 325 °C. Interestingly, the prepared samples showed noteworthy air stability. Been placed in the air for 60 min, the dehydrogenation kinetics and hydrogen capacity of the three samples were basically unchanged, making it possible to be used in future commercial applications.     

Ternary transition metal alloy FeCoNi nanoparticles on graphene as new catalyst for hydrogen sorption in MgH2

The present investigation deals with the synthesis of ternary transition metal alloy nanoparticles of FeCoNi and graphene templated FeCoNi (FeCoNi@GS) by one-pot reflux method and there use as a catalyst for hydrogen sorption in MgH₂. It has been found that the MgH₂ catalyzed by FeCoNi@GS (MgH₂: FeCoNi@GS) has the onset desorption temperature of ~255 °C which is 25 °C and 100 °C lower than MgH₂ catalyzed by FeCoNi (MgH₂: FeCoNi) (onset desorption temperature 280 °C) and the ball-milled (B.M) MgH₂ (onset desorption temperature 355 °C) respectively. Also MgH₂: FeCoNi@GS shows enhanced kinetics by absorbing 6.01 wt% within just 1.65 min at 290 °C under 15 atm of hydrogen pressure. This is much-improved sorption as compared to MgH₂: FeCoNi and B.M MgH₂ for which hydrogen absorption is 4.41 wt% and 1.45 wt% respectively, under the similar condition of temperature, pressure and time. More importantly, the formation enthalpy of MgH₂: FeCoNi@GS is 58.86 kJ/mol which is 19.26 kJ/mol lower than B.M: MgH₂ (78.12 kJ/mol). Excellent cyclic stability has also been found for MgH₂: FeCoNi@GS even up to 24 cycles where it shows only negligible change from 6.26 wt% to 6.24 wt%. A feasible catalytic mechanism of FeCoNi@GS on MgH₂ has been put forward based on X-ray diffraction (XRD), Raman spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Photoelectron Spectroscopy (XPS), and microstructural (electron microscopic) studies.