A sea cucumber-like nanoporous TiO2 modified by bimetal PtAu through the dealloying for water splitting

Bimetal PtAu modifying nanoporous TiO₂ composites are prepared by dealloying. X-ray diffraction is used to determine the phase constitution. The specific surface area and mesoporous pores are measured by Brunauer-Emmett-Teller. X-ray photoelectron spectroscopy, photoluminescence spectroscopy, UV–Vis diffuse reflectance spectroscopy and Raman spectroscopy are employed to analyse the mutual synergy between Pt and Au. The hydrogen generation rate is measured to determine the photocatalytic performance. The results reveal that TiO₂ is the anatase structure and possess a sea cucumber-like morphology with a large specific surface area. The hydrogen generation rate is 1.745 mmol h^?1 g^?1 for the dealloyed Al₉₂Ti_7.86Pt_0.04Au_0.1 ribbons under the full spectrum irradiation. This value is 29.6 times, 4.4 times and 1.8 times those of the dealloyed Al₉₂Ti₈, Al₉₂Ti_7.9Au_0.1, and Al₉₂Ti_7.96Pt_0.04 ribbons, respectively. The above results indicate that the addition of a small amount of Au and Pt significantly enhances the photocatalytic activity for H₂ production. The Au promotes the absorption of visible light, and Pt serve as an electron sink for effective electron-hole pairs separation during the photocatalytic process. It is a feasible and an effective strategy to take full advantages of respective virtues of noble metals and their strong interactions to enhance photocatalytic activity.     

Hydrogen gettering of titaniumpalladium/palladium nanocomposite films synthesized by cosputtering and vacuum-annealing

Nano-engineered composite film, prepared by the combination of titanium (Ti) nanoparticles with surrounding layers of palladium (Pd), has been suggested as a high performance hydrogen (H₂) getter. Uniform TiPd film covered by a 35-nm-thick Pd layer was deposited on a silicon wafer via cosputtering and post-vacuum-annealing. As the annealing temperature increased from 200 to 400 °C, amorphous alloy and nano-aggregates were observed, and efficient structural modulation occurred at 400 °C, where dewetting of Pd cover layer from the getter surface was observed. This led to the enhancement of the chemisorption capacity of the 400^oC-annealed sample, two-times higher than that of the 300^oC-annealed sample. Abrupt change in residual gases, which typically come from a bonding process, can be mitigated by minimizing the gas transfer distance through the dewetting of the cover layer; since Ti nanoparticles surrounded by Pd exist independently of each other in the gettering layer, external H₂ gas molecules can be continuously adsorbed onto still-unreacted Ti particles by passing through the dewetted channels in the Pd cover layer. This concept demonstrates a pathway towards a useful synthetic approach for high-performance thin-film getters with high adsorption capacity, fast gettering rate and good device compatibility.     

Fatigue limit of carbon and CrMo steels as a small fatigue crack threshold in high-pressure hydrogen gas

The fatigue limit properties of a carbon steel and a low-alloy CrMo steel were investigated via fully-reversed tension-compression tests, using smooth specimens in air and in 115-MPa hydrogen gas. With respect to the CrMo steel, specimens with sharp notches were also tested in order to investigate the threshold behavior of small cracks. The obtained SN data inferred that the fatigue limit was not negatively affected by hydrogen in either of the steels. Observation of fatigue cracks in the unbroken specimens revealed that non-propagating cracks can exist even in 115-MPa hydrogen gas, and that the crack growth threshold is not degraded by hydrogen. The experimental results provide justification for the fatigue limit design of components that are to be exposed to high-pressure hydrogen gas.     

Hydrogen storage properties of nanocrystalline Mg2Ni prepared from compressed 2MgH2Ni powder

In the present work, nanocrystalline Mg₂Ni with an average size of 20–50 nm was prepared via ball milling of a 2MgH₂Ni powder followed by compression under a pressure of 280 MPa. The phase component, microstructure, and hydrogen sorption properties were characterized by using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), pressure-composition-temperature (PCT) and synchronous thermal analyses (DSC/TG). Compared to the non-compressed 2MgH₂Ni powder, the compressed 2MgH₂Ni pellet shows lower dehydrogenation temperature (290 °C) and a single-phase Mg₂Ni is obtained after hydrogen desorption. PCT measurements show that the nanocrystalline Mg₂Ni obtained from dehydrogenated 2MgH₂Ni pellet has a single step hydrogen absorption and desorption with fairly low absorption (?57.47 kJ/mol H₂) and desorption (61.26 kJ/mol H₂) enthalpies. It has very fast hydrogen absorption kinetics at 375 °C with about 3.44 wt% hydrogen absorbed in less than 5 min. The results gathered in this study show that ball milling followed by compression is an efficient method to produce Mg-based ternary hydrides.     

Nanostructured CeCu mixed oxide synthesized by solid state reaction for medium temperature shift reaction: Optimization using response surface method

Solid state reaction was applied as a simple and cheap process for synthesizing Ce_0.9Cu_0.1O_1.9 mixed oxide by milling of CuO and CeO₂. Response surface methodology (RSM) technique was used to design a three-levels-two-factors (milling times of 20, 95 and 170 min and milling speeds of 100, 150 and 200 rpm) to investigate and optimize CO conversion in medium temperature shift (MTS) reaction (300–390 °C). According to contour plots, the interaction between parameters (time and speed of) is so important. Determined optimal values of milling time and speed which lead to conversions of 34.9, 47.2, 61.4 and 71.7% at 300, 330, 360 and 390 °C, are 120 min and 162 rpm, respectively. X-ray diffraction (XRD) analysis showed that the trend of crystalline size versus milling speed is different for each milling time: constant for low times, with a maximum for medium times and incremental for high times. Increasing milling time and speed leads to BET surface area decrease and CuCe spices formation which causes better reducibility (TPR) and larger particles (SEM). It was determined that mild values of time and speed (95 min and 150 rpm) lead to the best catalytic performance with good catalytic stability.     

Mechanism of Ni-catalyzed selective CO cleavage of lignin model compound benzyl phenyl ether under mild conditions

Selective cleavage of CO bonds in benzyl phenyl ether (BPE) as a typical lignin α-O-4 ether to produce aromatics is a challenging and attractive topic. Herein, the earth-abundant first-row transition metals, such as Co, Ni and Cu were supported on activated carbon (Co/AC, Ni/AC and Cu/AC) to identify their ability for cleaving CO bond of BPE. Among these catalysts, Ni/AC exhibit highest activity for cleavage of CO bond. The reaction with BPE was carried out at pretty mild condition of 140 °C and 2 MPa H₂, which is highly selective afforded toluene and phenol as the major products with the optimum yields of 88.5 and 86.5%, respectively. Based on the test, the reaction pathways were proposed. A abundant of dissociated H· atoms on Ni(0) sites forms surface active NiH species; Ni(0) activates and facilitates cleavage of the CO bond in BPE to form benzyl (C₆H₅C·) and phenoxy radicals (C₆H₅O·); H· atoms spill from active species NiH recombined together with C₆H₅C· and C₆H₅O· forming the products of toluene and phenol, respectively.     

Synthesis of nanocrystalline mesoporous Ni/Al2O3SiO2 catalysts for CO2 methanation reaction

A series of nanocrystalline mesoporous Ni/Al₂O₃SiO₂ catalysts with various SiO₂/Al₂O₃ molar ratios were prepared by the sol-gel method for the carbon dioxide methanation reaction. The synthesized catalysts were evaluated in terms of catalytic performance and stability. The catalysts were studied using XRD, BET, TPR and SEM. The BET results indicated that the specific surface area of the samples with composite oxide support changed from 254 to 163.3 m²/g, and an increase in the nickel crystallite size from 3.53 to 5.14 nm with an increment of Si/Al molar ratio was visible. The TPR results showed a shift towards lower temperatures, indicating a better reducibility and easier reduction of the nickel oxide phase into the nickel metallic phase. Furthermore, the catalyst with SiO₂/Al₂O₃ molar ratio of 0.5 was selected as the optimal catalyst, which showed 82.38% CO₂ conversion and 98.19% CH₄ selectivity at 350 °C, high stability, and resistivity toward sintering. Eventually, the optimal operation conditions were specified by investigating the effect of H₂/CO₂ molar ratio and gas hourly space velocity (GHSV) on the catalytic behavior of the denoted catalyst.     

Preferential CO oxidation over CuOCeO2 catalyst synthesized from MOF with nitrogen-containing ligand as precursor

A set of highly dispersed copper ceria catalysts were synthesized by using the CeMOF precursor featured rich nitrogen-containing ligand. Owing to the existence of coordination interactions between metal ions and nitrogen atoms, the copper ions could be adsorbed into the pore of Ce-MOF and stabilized by the ordered nitrogen atom on the pore wall. After calcinations, the generated CuO/CeO₂ catalyst featured more well-dispersed active sites, which was evidenced by varieties of characterizations such as FT-IR, UV-vis spectroscopy, PXRD, TEM, H₂-TPR, Raman spectroscopy and XPS. The as-synthesized CuO/CeO₂ catalysts displayed outstanding catalytic activities and stabilities for preferential carbon monoxide oxidation in H₂-rich stream.     

Extraordinary room-temperature hydrogen sensing capabilities with high humidity tolerance of PtSnO2 composite nanoceramics prepared using SnO2 agglomerate powder

Two kinds of PtSnO₂ composite nanoceramics have been prepared using SnO₂ nanoparticles and SnO₂ agglomerate powder separately. One is of a relatively uniform and porous microstructure with a specific surface area of 8.1 m²/g, and the other is of a rather non-uniform microstructure with large SnO₂ agglomerates and crack-like pores and a specific surface area of 6.4 m²/g. While the samples of uniform microstructure typically show a sensitivity of 150 to 1% H₂ – 20% O₂ – N₂ in air of 50% relative humidity (RH) at room temperature, those of non-uniform microstructure surprisingly show much higher sensitivities of 850 and 450 in air of 50% and 70% RH, respectively, to the same concentration of hydrogen. The influence of humidity on the samples has been further studied and a much higher humidity tolerance has been revealed for those samples of non-uniform microstructure. All these results demonstrate a clear and unexpected advantage of a non-uniform microstructure over a uniform one in humidity tolerance for room-temperature hydrogen-sensitive PtSnO₂ composite nanoceramics.     

Study on steam reforming of coal tar over NiCo/ceramic foam catalyst for hydrogen production: Effect of Ni/Co ratio

The effect of Ni/Co ratio on the catalytic performance of NiCo/ceramic foam catalyst for hydrogen production by steam reforming of real coal tar was studied. The NiCo/ceramic foam catalyst was synthesized by deposition-precipitation (DP) method and characterized with different methods. The experiments were conducted in a two-stage fixed-bed reactor. The results showed that the reducibility of the metallic oxides in bimetallic NiCo/ceramic foam catalysts was influenced obviously by the Ni/Co ratio.Both gas and hydrogen yield increased first and then decreased with the decline of Ni/Co ratio, and the highest hydrogen yield of 31.46 mmol g^?1 was obtained when the Ni/Co ratio was 5/5. The lowest coke deposition of 0.34 wt% was generated at the same Ni/Co ratio. The lifetime test showed the catalyst maintained catalytic activity after 14 cycles (28 h), indicating the coal tar steam reforming on NiCo/ceramic foam catalyst is a promising method for hydrogen production.     

Noble metal-free FeN-CNFs as an efficient electrocatalyst for oxygen reduction reaction

Carbonaceous materials containing non-precious metal atoms and doped with nitrogen have enthralled stunning attention in the field of electrochemical energy conversion systems. Herein, we demonstrated a facile method to fabricate iron and nitrogen doped carbon nanofiber (FeN-CNFs) catalyst material from ferric chloride and interfacial synthesized polyaniline (PANI) nanofibers, by carbonization process in an inert atmosphere at 800 °C. Further, synthesized material was characterized by elemental analysis and X-ray photoelectron spectroscopy (XPS) that confirms the presence of FeN bonds. The structural and morphological features are studied using various microscopy and spectroscopy techniques. The oxygen reduction reaction (ORR) activity of synthesized catalyst materials was examined by rotating disk electrode experiments in 0.1 M KOH. Among all these synthesized materials FeN-CNFs material showed enhanced ORR activity regarding current density and onset potential. Also, FeN-CNFs catalyst exhibited tolerance to methanol and durability in comparison to commercial Pt/C catalyst. The superior performance of FeN-CNFs may be attributed due to the introduction of Fe and formation of FeN bond in catalyst material.     

Numerical analysis of H2 formation during partial oxidation of H2SH2O upon activation of oxidizer by an electric discharge

The numerical analysis of H₂ production during partial oxidation of H₂SH₂O in a plug-flow reactor at atmospheric pressure and a rather low temperature (T₀ = 500 K) was conducted, when the oxidizer (oxygen or air) was preliminarily activated by an electrical discharge with different values of reduced electric field and input energy. It was shown that a significant hydrogen yield in flow reactor can be obtained only after ignition of the mixture. The ignition delay length depends on the reduced electric field E/N and input energy E_s in the discharge and is minimal at E/N～8–10 Td for the discharge in oxygen and at E/N～4–10 and 120–150 Td in air discharge, when O₂(a¹Δ_g) mole fraction in the discharge products is maximal. If the H₂SH₂OO₂(air) mixture ignites inside the flow reactor, the mole fraction of hydrogen and its relative yield do not depend on E/N. The relative hydrogen yield increases monotonically with an addition of water to H₂S. It was found, that the approach based on the partial oxidation of the H₂SH₂O mixture upon activation of oxygen by an electric discharge can ensure very low energy cost for H₂ production. The minimum specific energy requirement, obtained for the H₂SO₂ mixture, was found to be 0.83 eV/(molecule H₂) and 0.18 eV/(molecule H₂S) at atmospheric pressure and can be further decreased if the energy released during partial oxidation of H₂S is spent on heating the reagents. The use of air as an oxidizer requires higher energy costs and seems to be less promising.     

In situ fabrication of porous graphene electrodes for high-performance lithiumoxygen batteries

These years, LiO₂ batteries attract wide interest because of its high theoretical energy density. However, the catalytic activity and porous structure of cathode remains a great challenge. In this work, we developed a hierarchical porous graphene foam to serve as a battery cathode, which has much richer active sites for cathodic reaction and channels for Li⁺ transfer and O₂ diffusion. The cathode exhibits a superior specific capacity as high as 9559 mAh g^?1 at 57 mA g^?1 and remains a high-rate capability of 3988 mAh g^?1 at an increased current density of 285 mA g^?1. Benefiting from the well-designed cathode structure, the battery can be stably operated for 150 cycles with a stable voltage profile and voltage efficiency up to 65%. The well-designed graphene has a potential to be a superior free-standing cathode to other carbon-based materials due to its good combination of its hierarchical and porous structure, large surface area, abundant defects and excellent mechanical stability.     

One-pot synthesis of ultrasmall PtAg nanoparticles decorated on graphene as a high-performance catalyst toward methanol oxidation

A one-pot synthesis method is utilized for the fabrication of ultrasmall platinum-silver nanoparticles decorated on graphene (PtAg/G) catalyst. This method has several advantages such as inexpensiveness, simplicity, low temperature, surfactant free, reductant free, being environmentally friendly and greenness. In this work, graphene and silver formate were dispersed in ultrapure water in an ultrasonic bath at 25 °C followed by through a galvanic displacement reaction; to prepare PtAg/G, PtCl₂ was added to the suspension under mild stirring condition. The morphology, crystal structure and chemical compositions of the as-fabricated PtAg/G and Pt/C catalysts were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Energy dispersive X-ray spectroscopy (EDS) techniques. Electrochemical techniques, including cyclic voltammetry (CV) and chronoamperometry (CA) measurements were used to analyze the electrochemical activity of the PtAg/G and Pt/C catalysts. The TEM images illustrate the uniform distribution of ultrasmall PtAg nanoparticles with the average size of 2–3 nm on the graphene nanosheets. The PtAg/G promoted the current density 2.46 times as much as Pt/C with a negative shift in onset oxidation potential and peak potential for oxidation reaction of methanol. Besides, the novel PtAg/G catalyst shows large electrochemically active surface area, lower apparent activation energy, and higher levels of durability in comparison to the Pt/C catalyst for the oxidation of methanol. The PtAg/G catalyst depicts extraordinary catalytic performance and stability to those of the Pt/C catalyst toward methanol oxidation in alkaline media.     

Temperature-hydrogen pressure phase boundaries and corresponding thermodynamics for ZrCoH system

The thermodynamically and kinetically stable regions of the temperature–H₂ pressure phase boundaries for the ZrCoH system were established using the Temperature-Concentration-Isobar (TCI) method. Based on this, the enthalpy change and entropy change values of dehydrogenation and disproportionation reactions were successfully obtained. The average enthalpy change (ΔH) and entropy change (ΔS) estimated from the phase boundaries for dehydrogenation of ZrCoH₃ to ZrCo are respectively 103.07 kJ mol^?1H₂ and 148.85 J mol^?1 H₂ K^?1, which are well agreement with the data reported in literature. The average ΔH and ΔS were estimated to be ?120.91 kJ mol^?1H₂ and -149.32 J mol^?1 H₂ K^?1 for the disproportionation of ZrCoH₃, whereas the ΔH and ΔS were calculated to be ?84.6 kJ mol^?1H₂ and -92.29 J mol^?1 H₂ K^?1 for disproportionation of ZrCo. In addition, it was found from the established phase boundaries that the anti-disproportionation property of ZrCo alloy can be enhanced if the phase boundaries of hydrogenation/dehydrogenation are far away from the phase boundaries of disproportionation by adjusting the thermodynamics. Meanwhile, it is possible to keep ZrCo away from disproportionation even at high temperature of 650 °C under hydrogen atmosphere, if the temperature-H₂ pressure trajectory is carefully controlled without crossing the phase boundaries of disproportionation. Therefore, the established phase boundaries can be used as a guide to the eye avoiding disproportionation and improving the anti-disproportionation property of ZrCo alloy.     

AuNi alloy nanoparticles supported on reduced graphene oxide as highly efficient electrocatalysts for hydrogen evolution and oxygen reduction reactions

Bimetallic nanoparticles of Au and Ni in the form of alloy nanostructures with varying Ni content are synthesized on reduced graphene oxide (rGO) sheets via a simple solution chemistry route and tested as electrocatalysts towards the hydrogen evolution (HE) and oxygen reduction (OR) reactions using polarization and impedance studies. The AuNi alloy NPs/rGO nanocomposites display excellent electrocatalytic activity which is found to improve with increasing Ni content in the AuNi/rGO alloy nanocomposites. For HER, the best AuNi alloy NPs/rGO electrocatalyst, the one with the highest Ni content, exhibits high activity with an onset overpotential approaching zero versus the reversible hydrogen electrode and an overpotential of only 37 mV at 10 mA cm^?2. Additionally, a low Tafel slope of 33 mV dec^?1 and a high exchange current density of 0.6 mA cm^?2 are measured which are very close to those of commercial Pt/C catalyst. Also, in the ORR tests, this electrocatalyst displays comparable activity to Pt/C. The Koutecky–Levich plots referred to a 4-electron mechanism for the reduction of dissolved O₂ on the AuNi alloy NPs/rGO catalyst. The electrocatalyst thus demonstrates excellent activity towards HER and ORR. Additionally, it exhibits outstanding operational durability and activation after 10,000^th cycles assuring its practical applicability.     

PtPd nanoparticles supported on sulfonated nitrogen sulfur co-doped graphene for methanol electro-oxidation

In this paper, sulfonated nitrogen sulfur co-doped graphene (S-NS-GR) nanocomposite, i.e., nitrogen sulfur co-doped graphene functionalized with SO₃H group as a novel catalyst support material was prepared. PtPd nanoparticles (PtPd NPs) were deposited on the surface of S-NS-GR by a facile electrochemical approach. The morphology and structure of Pd-PtNPs/S-NS-GR were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and electrochemical impedance spectroscopy (EIS), respectively. In addition, the electrocatalytic performance of catalyst for methanol oxidation reaction (MOR) was systematically studied by cyclic voltammetry and chronoamperometry in alkaline media. Compared with PtPd NPs supported on nitrogen sulfur co-doped graphene (Pt-PdNPs/NS-GR), the excellent performance of Pd-PtNPs/S-NS-GR is mainly ascribed to the embedding of abundant functional groups (SO₃H) into the NS-GR layers, which not only facilitate the homogeneous distribution of metal NPs, but also strengthen the interaction between metals and support material, thus improve the stability of catalyst in MOR.     

The catalytic effect of an inert additive (SrTiO3) on the hydrogen storage properties of 4MgH2Na3AlH6

Investigations on the catalytic effects of a non-reactive and stable additive, SrTiO₃, on the hydrogen storage properties of the 4MgH₂Na₃AlH₆ destabilized system were carried out for the first time. The Na₃AlH₆ compound and the destabilized systems used in the investigations are prepared using ball milling method. The doped system, 4MgH₂Na₃AlH₆SrTiO₃, had an initial dehydrogenation temperature of 145 °C, which 25 °C lower as compared to the un-doped system. The isothermal absorption and desorption capacity at 320 °C has increased by 1.2 wt% and 1.6 wt% with the addition of SrTiO₃ as compared to the 4MgH₂Na₃AlH₆ destabilized system. The decomposition activation energy of the doped system is estimated to be 117.1 kJ/mol. As for the XRD analyses at different decomposition stages, SrTiO₃ is found to be stable and inert. In addition to SrTiO₃, similar phases are found in the doped and the un-doped system during the decomposition and dehydrogenation processes. Therefore, the catalytic effect of the SrTiO₃ is speculated owing to its ability to modify the physical structure of the 4MgH₂Na₃AlH₆ particles through pulverization effect.