Comparison of co-current and counter-current flow in a bifunctional reactor containing ammonia synthesis and 2-butanol dehydrogenation to MEK

In this study, a multi-tubular thermally coupled packed bed reactor in which simultaneous production of ammonia and methyl ethyl ketone (MEK) takes place is simulated. The simulation results are presented in two co-current and counter-current flow modes. Based on this new configuration, the released heat from the ammonia synthesis reaction as an extremely exothermic reaction in the inner tube is employed to supply the required heat for the endothermic 2-butanol dehydrogenation reaction in the outer tube. On the other hand, MEK and hydrogen are produced by the dehydrogenation reaction of 2-butanol in the endothermic side, and the produced hydrogen is used to supply a part of the ammonia synthesis feed in the exothermic side. Thus, 30.72% and 31.88% of the required hydrogen for the ammonia synthesis are provided by the dehydrogenation reaction in the co-current and counter-current configurations, respectively. Also, according to the thermal coupling, the required cooler and furnace for the ammonia synthesis and 2-butanol dehydrogenation conventional plants are eliminated, respectively. As a result, operational costs, energy consumption and furnace emissions are considerably decreased. Finally, a sensitivity analysis and optimization are applied to study the effect of the main process parameters variation on the system performance and obtain the minimum hydrogen make-up flow rate, respectively.     

Comparative study of the microstructure of Ti-6Al-4V titanium alloy sheets under quasi-static and high-velocity bulging

Journal of Mechanical Science and Technology - In order to reveal the high-velocity deformation mechanisms of Ti-6Al-4V titanium alloy sheets, the dynamic deformation behavior and the...     

Tungsten phosphide nanosheets seamlessly grown on tungsten foils toward efficient hydrogen evolution reaction in basic and acidic media

Exploring earth-abundant electrocatalyst with active and stable hydrogen evolution reaction (HER) properties is desirable but still challengeable. Herein, WP₂ nanosheets are seamlessly grown on W foil (WP₂ NSs/W) through phosphorization of WO₃/W. This seamless WP₂/W structure is beneficial to reducing the resistance between WP₂ and W. Along with the exposed large density of active sites, WP₂ NSs/W displays outstanding HER activity with a lower onset potential of about 0 V, a smaller overpotential of 90 mV for the current density of 10 mA/cm² in basic media. Notably, WP₂ NSs/W electrode also catalyzes HER efficiently in acid. The synthesis of WP₂ NSs/W provides us a straightforward strategy to gain more cost-effective cathode for HER.     

Micromechanisms of fatigue crack growth in cast aluminium piston alloys

The fatigue crack growth behaviour in as-cast and hot isostatically pressed (HIP) model cast aluminium piston alloys with hypoeutectic Si compositions of 6.9 wt% and 0.67 wt% has been investigated. The HIP alloys showed slightly improved fatigue crack growth resistance. Analysis of the crack path profiles and fracture surfaces showed that the crack tends to avoid Si and intermetallic particles at low ΔK levels up to a mid-ΔK of ∼7 MPa√m. However, some particles do fail ahead of the crack tip to facilitate crack advance due to the interconnected microstructure of these alloys. At higher levels of ΔK, the crack increasingly seeks out Si and intermetallic particles up to a ΔK of ∼9 MPa√m after which the crack preferentially propagates through intermetallic particles in the 0.67 wt%Si alloy or Si and intermetallics in the 6.9 wt%Si alloys. It was also observed that crack interaction with intermetallics caused crack deflections that led to roughness-induced crack closure and possibly oxide-induced crack closure at low to mid-ΔK. However, crack closure appears unimportant at high ΔK due to the large crack openings and evidenced by the fast crack growth rates observed.     

Effect of the Sm content on the structure and electrochemical properties of La1.3 − xSmxCaMg0.7Ni9 (x = 0–0.3) hydrogen storage alloys

La_1.3 − xSm_xCaMg_0.7Ni₉ (x = 0–0.3) hydrogen storage alloys were prepared by inductive melting and the effect of the Sm content on the structure and electrochemical properties was investigated in the paper. The Sm substitution for La in La_1.3 − xSm_xCaMg_0.7Ni₉ (x = 0–0.3) alloys does not change the main phase structure (the rhombohedral PuNi₃-type structure), but leads to a shrinkage of unit cell and a decrease of hydrogen storage capacity. With the increase of the Sm content in the alloys, the maximum discharge capacity of electrode decreases from 400.2 (x = 0) to 346.6 mAh g⁻¹ (x = 0.3), but the high-rate dischargeability and cycling stability is improved. After 100 cycles, the capacity retention rate increases from 75 (x = 0) to 85% (x = 0.3).     

Improvement of the cycleability of nano-crystalline lithium manganate cathodes by cation co-doping

Lithium manganate spinel is extensively studied as a positive electrode in lithium ion rechargeable batteries. Growth of nano-size cathode particles is proposed to improve the rate capabilities of these cathode materials. It remains controversial if the particle size in the nano-range (as compared to the conventional micrometer size particles of these materials) has any appreciable influence on the discharge capacity, rate capabilities, and cycleability of these materials. In the 4 V range, especially at slightly elevated temperature, lithium manganate exhibits capacity fading though the underlying mechanism for such fading is not yet clear. In the present work, we have successfully prepared nano-crystalline lithium manganate spinel powder by an acetate base solution route. Though the discharge capacity of these nano-crystalline cathodes was equivalent to their microcrystalline counterpart, these exhibited capacity fading in the 4 V range. Through a combined X-ray diffraction, micro-Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) analyses, we correlated the observed capacity fading with the onset of Jahn–Teller (J–T) distortion toward the end of the discharge in the cut-off limit between 4.2 and 3.4 V. It was postulated that if J–T distortion is the dominant fading mechanism of these nano-crystalline cathodes then by increasing the average oxidation state of the Mn ions in a virgin lithium manganate cathode, the onset of such distortion towards the end of the discharge could be delayed, and therefore, the cycleability of these cathodes could be improved. By synthesizing lithium and aluminum ion co-doped lithium manganate particles, we could increase the average oxidation state of the Mn ions in the virgin electrodes. Indeed, the cycleability of these co-doped cathodes was dramatically improved which supports our postulation. The doping contents of lithium and aluminum, however, should be further optimized to further increase the discharge capacity of these modified cathodes.     

In-situ hydrogen-induced evolution and de-/hydrogenation behaviors of the Mg93Cu7-xYx alloys with equalized LPSO and eutectic structure

Nanosizing is efficient as the dual-tuning of thermodynamics and kinetics for Mg-based hydrogen storage materials. The in-situ synthesis of nanocomposites through hydrogen-induced decomposition from long-period stacking ordered phase is proved effective to achieve active nano-sized catalysts with uniform dispersion. In this study, the Mg₉₃Cu_7-xY_x (x = 0.67, 1.33, and 2) alloys with equalized Mg–Mg₂Cu eutectic and 14H long-period stacking ordered phase of Mg₉₂Cu_3.5Y_4.5 are prepared. Its solidification path is determined as α-Mg, 14H–Mg₂Cu pair and Mg–Mg₂Cu eutectic. The increased Y/Cu atomic ratio lowers the first-cycle hydrogenation rate of the alloys due to the increased 14H–Mg₂Cu structure and reduced Mg–Mg₂Cu eutectic interfaces. After the hydrogen-induced decomposition of the long-period stacking ordered phase, MgCu₂ and YH₃ nanoparticles are in-situ formed, and the following activation process significantly accelerates. The YH₃ nanoparticles partly decompose to YH₂ at 300 °C in vacuum and Mg–Mg₂Cu-YH_x nanocomposites are in-situ formed. The nano-sized YH₂ helps catalyze H₂ dissociation and the YH_x/Mg interfaces stimulate H diffusion and the nucleation of MgH₂. Therefore, the Mg₉₃Cu₅Y₂ composite shows the fastest absorption rates. However, due to the positive effect of YH_x/Mg interfaces on H diffusion and the negative effect of YH₃ nanophases on the hydride decomposition, the minimum activation energy of 115.43 kJ mol⁻¹ is obtained for the desorption of the Mg₉₃Cu_5.67Y_1.33 hydride.     

One-step synthesis of amorphous Ni–Fe–P alloy as bifunctional electrocatalyst for overall water splitting in alkaline medium

Design of inexpensive and highly efficient bifunctional electrocatalyst is paramount for overall water splitting. In this study, amorphous Ni–Fe–P alloy was successfully synthesized by one-step direct-current electrodeposition method. The performance of Ni–Fe–P alloy as a bifunctional electrocatalyst toward both hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) was evaluated in 30 wt% KOH solution. It was found that Ni–Fe–P alloy exhibits excellent HER and OER performances, which delivers a current density of 10 mA cm^?2 at overpotential of ～335 mV for HER and ～309 mV for OER with Tafel slopes of 63.7 and 79.4 mV dec^?1, respectively. Moreover, the electrolyzer only needs a cell voltage of ～1.62 V to achieve 10 mA cm^?2 for overall water splitting. The excellent electrocatalytic performance of Ni–Fe–P alloy is attributed to its electrochemically active constituents, amorphous structure, and the conductive Cu Foil.     

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.     

Oxygen vacancy in magnesium/cerium composite from ball milling for hydrogen storage improvement

The hydrogen sorption performance of Mg is constrained by the difficulties of hydrogen dissociation on particle surface and mass transfer in particle bulk. This work focuses on oxygen vacancy and its effect on the performance of Mg-xCeO₂ (x = 0.7, 1.5, 3, and 6 mol.%) from ball milling for hydrogen storage. The HRTEM observation shows that the crystal domains of Mg from ball milling are reduced to nanoscale by the addition of hard CeO₂ nanoparticles. The XRD and XPS characterization shows that during heating for hydrogenation, some O atoms in CeO₂ transfer to Mg and form MgO, and CeO₂ converts to Ce₆O₁₁ with oxygen vacancies. The isothermal absorption (p-c-T) analysis shows that the hydrogen capacity of the materials increases with the increase of CeO₂ additive, and the optimum addition is 3.0 mol.%. The DSC analysis shows that with the addition of 3.0 mol.% of CeO₂, the hydrogen desorption peak temperature is 35 °C lower than that of pure MgH₂, and the calculated activation energy deceases by 31.3 kJ/mol. The improvement of hydrogen sorption performance is mainly attributed to the formation of oxygen vacancies.