Study on catalytic effect and mechanism of MOF (MOF = ZIF-8, ZIF-67, MOF-74) on hydrogen storage properties of magnesium

Using a deposition-reduction method, Mg/MOF nanocomposites were prepared as composites of Mg and metal-organic framework materials (MOFs = ZIF-8, ZIF-67 and MOF-74). The addition of MOFs can enhance the hydrogen storage properties of Mg. For example, within 5000 s, 0.6 wt%, 1.2 wt%, 2.7 wt%, 3.7 wt% of hydrogen were released from Mg, Mg/MOF-74, Mg/ZIF-8, Mg/ZIF-67, respectively. Activation energy values of 198.9 kJ mol⁻¹ H₂, 161.7 kJ mol⁻¹ H₂, 192.1 kJ mol⁻¹ H₂ were determined for the Mg/ZIF-8, Mg/ZIF-67, Mg/MOF-74 hydrides, which are 6 kJ mol⁻¹ H₂, 43.2 kJ mol⁻¹ H₂, and 12.8 kJ mol⁻¹ H₂ lower than that of Mg hydride, respectively. Moreover, the cyclic stability characterizing Mg hydride was significantly improved when adding ZIF-67. The hydrogen storage capacity of the Mg/ZIF-67 nanocomposite remained unchanged, even after 100 cycles of hydrogenation/dehydrogenation. This excellent cyclic stability may have resulted from the core-shell structure of the Mg/ZIF-67 nanocomposite.     

Synergistic catalytic effects of ZIF-67 and transition metals (Ni,Cu, Pd,and Nb) on hydrogen storage properties of magnesium

MgTM/ZIF-67 nanocomposites were prepared by the deposition-reduction method using ZIF-67, MgCl₂, and TMCl_x (TM = Ni, Cu, Pd, Nb) as raw materials. The dehydrogenation activation energies of MgTM/ZIF-67 (TM = Ni, Cu, Pd, Nb) nanocomposites were calculated to be 115.4 kJ mol⁻¹ H₂, 115.7 kJ mol⁻¹ H₂, 113.6 kJ mol⁻¹ H₂, and 75.8 kJ mol⁻¹ H₂, respectively; hence, the MgNb/ZIF-67 nanocomposite manifested the best comprehensive hydrogen storage performance. The hydrogen storage capacity of the MgNb/ZIF-67 nanocomposite was hardly attenuated after the 100th hydrogen absorption-desorption cycle. The dehydrogenated enthalpies of MgH₂ and CoMg₂H₅ in MgNb/ZIF-67 hydride were calculated to be 72.4 kJ mol⁻¹ H₂ and 81.0 kJ mol⁻¹ H₂, respectively, which were lower than those of additive-free MgH₂ and Mg/ZIF-67. The improved hydrogen storage properties of MgNb/ZIF-67 can be ascribed to the CoMg₂–Mg(Nb) core-shell structure and the catalytic effects of NbH and niobium oxide nanocrystals.     

Effect of in-situ formed Mg2Ni/Mg2NiH4 compounds on hydrogen storage performance of MgH2

Magnesium-based hydrogen storage materials (MgH₂) are promising hydrogen carrier due to the high gravimetric hydrogen density; however, the undesirable thermodynamic stability and slow kinetics restrict its utilization. In this work, we assist the de/hydrogenation of MgH₂ via in situ formed additives from the conversion of an MgNi₂ alloy upon de/hydrogenation. The MgH₂–16.7 wt%MgNi₂ composite was synthesized by ball milling of Mg powder and MgNi₂ alloy followed by a hydrogen combustion synthesis method, where most of the Mg converted to MgH₂, and the others reacted with the MgNi₂ generating Mg₂NiH₄, which produced in situ Mg₂Ni during dehydrogenation. Results showed that the Mg₂Ni and Mg₂NiH₄ could induce hydrogen absorption and desorption of the MgH₂, that it absorbed 2.5 wt% H₂ at 473 K, much higher than that of pure Mg, and the dehydrogenation capacity increased by 2.6 wt% at 573 K. Besides, the initial dehydrogenation temperature of the composite under the promotion of Mg₂NiH₄ decreased greatly by 100 K, whereas it is 623 K for MgH₂. Furthermore, benefiting from the catalyst effect of Mg₂NiH₄ during dehydrogenation, the apparent activation energy of the composite reduced to 73.2 kJ mol⁻¹ H₂ from 129.5 kJ mol⁻¹ H₂.     

Synthetical catalysis of nickel and graphene on enhanced hydrogen storage properties of magnesium

Reduced graphene-oxide-supported nickel (Ni@rGO) nanocomposite catalysts were synthesized, and incorporated into magnesium (Mg) hydrogen storage materials with the aim of improving the hydrogen storage properties of these materials. The experimental results revealed that the catalytic effect of the Ni@rGO nanocomposite on Mg was more effective than that of single nickel (Ni) nanoparticles or graphene. When heated at 100 °C, the Mg–Ni and Mg–Ni@rGO composites absorbed 4.70 wt% and 5.48 wt% of H₂, respectively, whereas the pure Mg and Mg@rGO composite absorbed almost no hydrogen. The addition of the Ni@rGO composite as a catalyst yielded significant improvement in the hydrogen storage property of the Mg hydrogen storage materials. The apparent activation energy of the pure Mg sample (i.e., 163.9 kJ mol⁻¹) decreased to 139.7 kJ mol⁻¹ and 123.4 kJ mol⁻¹, respectively, when the sample was modified with single rGO or Ni nanoparticles. Under the catalytic action of the Ni@rGO nanocomposites, the value decreased further to 103.5 kJ mol⁻¹. The excellent hydrogen storage properties of the Mg–Ni@rGO composite were attributed to the catalytic effects of the highly surface-active Ni nanoparticles and the unique structure of the composite nanosheets.     

Effects of trimesic acid-Ni based metal organic framework on the hydrogen sorption performances of MgH2

A metal-organic framework based on Ni (II) as metal ion and trimasic acid (TMA) as organic linker was synthesized and introduced into MgH₂ to prepare a Mg-(TMA-Ni MOF)-H composite through ball-milling. The microstructures, phase changes and hydrogen storage behaviors of the composite were systematically studied. It can be found that Ni ion in TMA-Ni MOF is attracted by Mg to form nano-sized Mg₂Ni/Mg₂NiH₄ after de/rehydrogenation. The hydriding and dehydriding enthalpies of the Mg-MOF-H composite are evaluated to be −74.3 and 78.7 kJ mol⁻¹ H₂, respectively, which means that the thermodynamics of Mg remains unchanged. The absorption kinetics of the Mg-MOF-H composite is improved by showing an activation energy of 51.2 kJ mol⁻¹ H₂. The onset desorption temperature of the composite is 167.8 K lower than that of the pure MgH₂ at the heating rate of 10 K/min. Such a significant enhancement on the sorption kinetic properties of the composite is attributed to the catalytic effects of the nanoscale Mg₂Ni/Mg₂NiH₄ derived from TMA-Ni MOF by providing gateways for hydrogen diffusion during re/dehydrogenation processes.     

Microstructural evolution and hydrogen storage proprieties of melt-spun eutectic Mg76.87Ni12.78Y10.35 alloy with low hydrides formation/decomposition enthalpy

Ternary eutectic Mg_76.87Ni_12.78Y_10.35 (at. %) ribbons with mixed amorphous and nanocrystalline phases were prepared by melt spinning. The microstructures of the melt-spun, hydrogenated and dehydrogenated samples were examined and compared by X-ray diffraction and transmission electron microscopy. The amorphous structure transforms into a thermally stable nanocrystalline structure with a grain size of about 5 nm during hydrogen ab/desorption cycles. The Mg, Mg₂Ni and phases with Y in the melt-spun state transform into MgH₂, Mg₂NiH₄, Mg₂NiH_0.3, YH₂ and YH₃ after hydrogenation, and transform back to Mg, Mg₂Ni and YH₂ upon subsequent dehydrogenation. The reaction enthalpy (ΔH) and entropy (ΔS) of the higher plateau pressure corresponding to Mg₂Ni hydride formation are −53.25 kJ mol⁻¹ and −107.74 J K⁻¹ mol⁻¹, respectively. The amorphous/nanocrystalline structure effectively reduces the enthalpy and entropy of Mg₂Ni hydride formation, but has little effect on Mg. The activation energy for dehydrogenation of the hydrogenated ribbons is 69 kJ mol⁻¹. This suggests that Mg–Ni–Y with ternary eutectic composition can form an amorphous/nanocrystalline structure by melt spinning, and this nanostructure efficiently improves the thermodynamics and kinetics for hydrogen storage.     

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.     

New mechanism and improved kinetics of hydrogen absorption and desorption of Mg(In) solid solution alloy milling with CeF3

This paper presents improving the hydrogen absorption and desorption of Mg(In) solid solution alloy through doped with CeF₃. A nanocomposite of Mg_0.95In_0.05-5 wt% CeF₃ was prepared by mechanical ball milling. The microstructures were systematically investigated by X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy. And the hydrogen storage properties were evaluated by isothermal hydrogen absorption and desorption, and pressure-composition-isothermal measurements in a temperature range of 230 °C–320 °C. The mechanism of hydrogen absorption and desorption of Mg_0.95In_0.05 solid solution is changed by the addition of CeF₃. Mg_0.95In_0.05-5 wt% CeF₃ nanocomposite transforms to MgH₂, MgF₂ and intermetallic compounds of MgIn and CeIn₃ by hydrogenation. Upon dehydrogenation, MgH₂ reacts with the intermetallic compounds of MgIn and CeIn₃ forming a pseudo-ternary Mg(In, Ce) solid solution, which is a fully reversible reaction with a reversible hydrogen capacity～4.0 wt%. The symbiotic nanostructured CeIn₃ impedes the agglomeration of MgIn compound, thus improving the dispersibility of element In, and finally improving the reversibility of hydrogen absorption and desorption of Mg(In) solution alloy. For Mg_0.95In_0.05-5 wt% CeF₃ nanocomposite, the dehydriding enthalpy is reduced to about 66.1 ± 3.2 kJ⋅mol⁻¹⋅H₂, and the apparent activation energy of dehydrogenation is significantly lowered to 71.9 ± 10.0 kJ⋅mol⁻¹⋅H₂, a reduction of ～73 kJ⋅mol⁻¹⋅H₂ relative to that for Mg_0.95In_0.05 solid solution. As a result, Mg_0.95In_0.05-5 wt% CeF₃ nanocomposite can release ～57% H₂ in 10 min at 260 °C. The improvements of hydrogen absorption and desorption properties are mainly attributed to the reversible phase transition of Mg(In, Ce) solid solution combing with the multiphase nanostructure.     

Enthalpy change (ΔH) and entropy change (ΔS) measurement of CeMn1−xAl1−xNi2x (x=0.00, 0.25, 0.50 and 0.75) hydrides by electrochemical P–C–T curve

The thermodynamic properties of CeMn_1−xAl_1−xNi_2x (x=0.00, 0.25, 0.50 and 0.75) hydrides have been investigated in this paper. With increasing Ni substitution content, the hydrogen concentration (H/M) in CeMn_1−xAl_1−xNi_2x (x=0.00, 0.25, 0.50 and 0.75) hydride increases from 0.129 wt% for x=0.00 to 0.421 wt% for x=0.75 at 293 K. The pressure–concentration isotherm (P–C–T) curves show that no hydrogen equilibrium pressure plateau has been observed for CeMnAl hydride while the slope of the plateau become flatter and longer with increasing Ni content. Meanwhile, the enthalpy change (ΔH⁰) and the entropy change (ΔS⁰) of the hydrides for dehydrogenization shift from −67.44 kJ mol⁻¹ (x=0.00) to 21.16 kJ mol⁻¹ (x=0.75) and from −0.24 kJ mol⁻¹ K⁻¹ (x=0) to −0.03 kJ mol⁻¹ K⁻¹ (x=0.75), respectively. With increasing Ni content, both ΔH⁰ and ΔS⁰ for dehydrogenization shift to the positive direction and make alloy hydrides more stable and hydrogen desorption much easier.     

Theoretical prediction and experimental study on catalytic mechanism of incorporated Ni for hydrogen absorption of Mg

Transition metals, including Ni, show good catalytic activity in the hydrogen storage reaction of Mg. In the present paper, first-principles calculation is performed to predict and analyze the hydriding reaction of Ni-incorporated Mg and experimental study is used to verify the accuracy of the forecast. Theoretical studies show that the hydriding reaction of Ni-incorporated Mg is a diffusion-controlled process. With Ni incorporation, the energy barrier of H₂ dissociation is significantly decreased and the diffusion becomes the limiting step. Experimental studies confirm the results of theoretical studies. Besides, the material with Ni incorporation shows excellent activation performance and rapid absorption rates, leading to a high hydrogen content of 4.1 wt% in 60 s under 240 °C 3.0 MPa H₂ and a low activation energy of 56.1 kJ mol⁻¹ versus 0.4 wt% and 73.5 kJ mol⁻¹ for the material without Ni incorporation. Atomic Ni only plays a role of catalyst.     

High-value utilization of intermetallic Cu9Al4 as an additive to improve the hydrogen storage performance of magnesium

As a product of in the preparation of Cu–Al clad composites, intermetallic Cu₉Al₄ greatly affects the mechanical properties of the composites due to its high hardness and poor plasticity, which limits its application. Here, Cu₉Al₄ is used as an additive to improve the hydrogen storage performance of Mg. Thereby, the Cu₉Al₄ was introduced into Mg to prepare the Mg‒x wt.% Cu₉Al₄ composites (x = 0, 5, 10, 15, 20 and 25) by high-energy ball milling. The microstructure and phase composition of the composites under different states were analyzed. The hydrogen absorption and desorption kinetics and thermodynamics of the composites at different temperatures are investigated. Compared with pure Mg, the addition of Cu₉Al₄ can significantly improve the dehydrogenation kinetics of Mg. With the increase of Cu₉Al₄ content, the hydrogen desorption rate gradually increased. When the Cu₉Al₄ content was 20 wt.%, the dehydrogenation activation energy calculated according to JMAK kinetic model and Arrhenius equation was the lowest (96.84 kJ mol⁻¹), and it had the best hydrogen desorption kinetics. Especially, the phase transition of the first hydrogen absorption process for the Mg‒20 wt.% Cu₉Al₄ was studied in detail. The results show that the Cu₉Al₄ phase first transforms into (Cu_1.3Al_0.7)Mg during the first hydrogenation, and then Mg reacts with H₂ to generate MgH₂. In the subsequent dehydrogenation and re-hydrogenation cycles, the (Cu_1.3Al_0.7)Mg is stable and does not change.     

Thermodynamic and kinetic properties of calcium hydride

Calcium hydride has shown great potential as a hydrogen storage material and as a thermochemical energy storage material. To date, its high operating temperature (above 800 °C) has not only hindered its opportunity for technological application but also prevented detailed determination of its thermodynamics of hydrogen sorption. In addition, calcium metal suffers from high volatility, high corrosivity from Ca (and CaH₂), slow kinetics of hydrogen sorption, and the solubility of Ca in CaH₂. In this work, a literature review of the wide-ranging thermodynamic properties of CaH₂ is provided along with a detailed experimental investigation into the thermodynamic properties of molten and solid CaH₂. The thermodynamic values of hydrogen release from both molten and solid CaH₂ were determined as ΔH_des (molten CaH₂) = 216 ± 10 kJ mol⁻¹.H₂, ΔS_des (molten CaH₂) = 177 ± 9 J K⁻¹ mol⁻¹.H_2, which equates to a 1 bar hydrogen equilibrium temperature for molten CaH₂ of 947 ± 65 °C. Similarly, in the solid-state: ΔH_des (solid CaH₂) = 172 ± 12 kJ mol⁻¹.H₂, ΔS_des (solid CaH₂) = 144 ± 10 J K⁻¹ mol⁻¹.H₂. Moreover, the activation energy of hydrogen release from CaH₂ was also calculated using DSC analysis as E_a = 203 ± 12 kJ mol⁻¹. This study provides the first thermodynamics for the Ca–H system in over 60 years, providing more accurate data on this emerging energy storage material.     

Destabilization effects of Mg(AlH4)2 on MgH2: Improved desorption performances and its reaction mechanism

Both kinetics and thermodynamics properties of MgH₂ are significantly improved by the addition of Mg(AlH₄)₂. The as-prepared MgH₂–Mg(AlH₄)₂ composite displays superior hydrogen desorption performances, which starts to desorb hydrogen at 90 °C, and a total amount of 7.76 wt% hydrogen is released during its decomposition. The enthalpy of MgH₂-relevant desorption is 32.3 kJ mol⁻¹ H₂ in the MgH₂–Mg(AlH₄)₂ composite, obviously decreased than that of pure MgH₂. The dehydriding mechanism of MgH₂–Mg(AlH₄)₂ composite is systematically investigated by using x-ray diffraction and differential scanning calorimetry. Firstly, Mg(AlH₄)₂ decomposes and produces active Al^∗. Subsequently, the in-situ formed Al^∗ reacts with MgH₂ and forms Mg–Al alloys. For its reversibility, the products are fully re-hydrogenated into MgH₂ and Al^∗, under 3 MPa H₂ pressure at 300 °C for 5 h.     

Remarkable isosteric heat of hydrogen adsorption on Cu(I)-exchanged SSZ-39

Hydrogen storage capacity on Cu(I)-exchanged SSZ-39 (AEI), -SSZ-13 (CHA) and Ultra stable-Y (US–Y, FAU) at temperatures between 279 K and 304 K are investigated. The gravimetric hydrogen storage capacity values reaching 83 μmol H₂ g⁻¹ (at 279 K and 1 bar) are found to be comparable with the highest adsorption capacity values reported on metal-organic frameworks. The volumetric hydrogen storage capacity values; on the other hand, are found to be more than three times of those reported on metal-organic frameworks (0.57 g/L on Cu(I)-SSZ-39 at 1 bar and 296 K vs. ca. 0.18 g/L on Co₂(m-dobdc) at 1 bar and 298 K (Kapelewski MT, Runčevski T, Tarver JD, Jiang HZH, Hurst KE, Parilla PA et al. Record High Hydrogen Storage Capacity in the Metal-Organic Framework Ni₂(m-dobdc) at Near-Ambient Temperatures. Chem Mater 2018; 30:8179–89)). The isosteric heat of adsorption values are calculated to be between 80 kJ mol⁻¹ and 49 kJ mol⁻¹ on Cu(I)-SSZ-39 and between 22 kJ mol⁻¹ and 15 kJ mol⁻¹ on Cu(I)-US-Y indicating H₂ adsorption mainly at Cu(I) cations located at the eight-membered rings on Cu(I)-SSZ-39 and at six-membered rings on Cu(I)-US-Y. Hydrogen adsorption experiments performed at 77 K showed higher adsorption capacity values for Cu(I)-SSZ-39 at 1 bar, but Cu(I)-US-Y showed potential for hydrogen storage at higher pressure values.     

Hydrogen adsorption properties of in-situ synthesized Pt-decorated porous carbons templated from zeolite EMC-2

To increase the interaction between the adsorbed hydrogen and the adsorbent surface to improve the hydrogen storage capacity at ambient temperature, decorating the sorbents with metal nanoparticles, such as Pd, Ni, and Pt has attracted the most attention. In this work, Pt-decorated porous carbons were in-situ synthesized via CVD method using Pt-impregnated zeolite EMC-2 as template and their hydrogen uptake performance up to 20 bar at 77, 87, 298 and 308 K has been investigated with focus on the interaction between the adsorbed H₂ and the carbon matrix. It is found that the in-situ generated Pt-decorated porous carbons exhibit Pt nanoparticles with size of 2–4 nm homogenously dispersed in the porous carbon, accompanied with observable carbon nanowires on the surface. The calculated H₂ adsorption heats at/near 77 K are similar for both the plain carbon (7.8 kJ mol⁻¹) and the Pt-decorated carbon (8.3 kJ mol⁻¹) at H₂ coverage of 0.08 wt.%, suggesting physisorption is dominated in both cases. However, the calculated H₂ adsorption heat at/near 298 K of Pt-decorated carbon is 72 kJ mol⁻¹ at initial H₂ coverage (close to 0), which decreases dramatically to 20.8 kJ mol⁻¹ at H₂ coverage of 0.014 wt.%, levels to 17.9 at 0.073 wt.%, then gradually decreases to 2.6 kJ mol⁻¹ at 0.13 wt.% and closes to that of the plain carbon at H₂ coverage above 0.13 wt.%. These results suggest that the introduction of Pt particles significantly enhances the interaction between the adsorbed H₂ and the Pt-decorated carbon matrix at lower H₂ coverage, resulting in an adsorption process consisting of chemisorption stage, mixed nature of chemisorption and physisorption stage along with the increase of H₂ coverage (up to 0.13 wt.%). However, this enhancement in the interaction is outperformed by the added weight of the Pt and the blockage and/or occupation of some pores by the Pt nanoparticles, which results in lower H₂ uptake than that of the plain carbon.     

Production of an Mg/Mg2Ni lamellar composite for generating H2 and the recycling of the post-H2 generation residue to nickel powder

The magnesium hydrolyzing reaction was catalyzed in situ using a layered Mg₂Ni compound, rapidly producing hydrogen in NaCl solution. The post-H₂ generation residue (mixture of Mg(OH)₂ and Mg₂Ni catalyst) was recycled to recover pure Ni powder from the waste mixture. Pure Mg (153 g) and pure Ni (47 g) in a eutectic composition were easily melted to form a molten alloy by a super-high-frequency (35,000 Hz) induction furnace. The lamellar material had an Mg/Mg₂Ni/Mg/Mg₂Ni… layered structure, in which each layer was ∼0.8 μm thick; Mg was an anodic phase and Mg₂Ni was a cathodic phase (the catalyst). Bulk Mg/Mg₂Ni composite alloy contains many microgalvanic cells. Owing to the lamellar microstructure, no dense hydrated oxide film that might have caused surface passivation was found, allowing continuous H₂ generation until no magnesium remained to participate in the hydrolysis. The activation energy of the hydrolysis reaction in simulated sea water was ∼36.35 kJ mol⁻¹.     

Enhanced Hydrogen Ab/De-sorption of Mg(Zn) solid solution alloy catalyzed by YH2/Y2O3 nanocomposite

YH₂/Y₂O₃ nanocomposite was prepared and introduced to Mg_0.97Zn_0.03 solid solution alloy forming a nanocomposite of Mg_0.97Zn_0.03-10 wt%YH₂/Y₂O₃ by mechanical milling. The phase components and microstructure were systematically investigated by XRD, SEM and STEM. Hydrogenation of Mg_0.97Zn_0.03 solid solution resulted in phase segregation into MgH₂ and intermetallic compound MgZn₂. The in-situ formed ultra-fine MgZn₂ homogeneously dispersed in MgH₂ matrix, and returned to Mg(Zn) solid solution through dehydrogenation. This reversible phase transition of Mg(Zn) solid solution benefited to thermodynamic destabilization of MgH₂. The co-dopant of YH₂ and Y₂O₃ exhibited synergistic catalytic effects on the hydrogen absorption and desorption of Mg_0.97Zn_0.03 solid solution alloy. As a result, Mg_0.97Zn_0.03-10 wt%YH₂/Y₂O₃ nanocomposite showed significantly improved kinetics with obviously lowered hydriding and dehydriding activation energy of 45.8 kJ⋅mol⁻¹⋅H₂ and 74.7 kJ⋅mol⁻¹⋅H₂, respectively, and the enthalpy of hydrogen desorption was reduced to 72.2 kJ⋅mol⁻¹⋅H₂.     

Enhanced hydrogen generation behaviors and hydrolysis thermodynamics of as-cast Mg–Ni–Ce magnesium-rich alloys in simulate seawater

In this study, we developed as-cast (Mg10Ni)_1-xCe_x (x = 0, 5, 10, 15 wt%) ternary alloys by using a flux protection melting method and investigated their hydrolysis hydrogen generation behaviour in simulate seawater. The phase compositions and microstructures of as-cast (Mg10Ni)_1-xCe_x ternary alloys are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) equipped with electron energy dispersion spectrum (EDS) and transition electron microscope (TEM). Their kinetics, thermodynamics, rate-limiting steps and apparent activation energies are investigated by fitting the hydrogen generation curves at different temperatures. With increasing Ce content, the (Mg10Ni)_1-xCe_x ternary alloys show increased electrochemical activities and decreased eutectic. When 10 wt% and 15 wt% Ce added, the active intermediate phase of Mg₁₂Ce has been observed. The hydrogen generation capacity of (Mg10Ni)₉₅Ce₅ is as high as 887 mLg⁻¹ with a hydrolysis conversion yield of 92%, which is higher than that of Mg₁₀Ni alloys (678 mLg⁻¹) with a yield only 75% at 291 K. The initial hydrolysis reaction kinetics of Mg–Ni–Ce alloys is mainly controlled by the electrochemical activity and the mass transfer channels formed in the alloys. Such a structure-property relationship will provide a possible strategy to prepare Mg-based alloys with high hydrogen conversion yield and controlled hydrolysis kinetics/thermodynamics.     

Aqueous-phase reforming of n-BuOH over Ni/Al2O3 and Ni/CeO2 catalysts

The aqueous-phase reforming (APR) of n-butanol (n-BuOH) over Ni(20 wt%) loaded Al₂O₃ and CeO₂ catalysts has been studied in this paper. Over 100 h of run time, the Ni/Al₂O₃ catalyst showed significant deactivation compared to the Ni/CeO₂ catalyst, both in terms of production rates and the selectivity to H₂ and CO₂. The Ni/CeO₂ catalyst demonstrated higher selectivity for H₂ and CO₂, lower selectivity to alkanes, and a lower amount of C in the liquid phase compared to the Ni/Al₂O₃ sample. For the Ni/Al₂O₃ catalyst, the selectivity to CO increased with temperature, while the Ni/CeO₂ catalyst produced no CO. For the Ni/CeO₂ catalyst, the activation energies for H₂ and CO₂ production were 146 and 169 kJ mol⁻¹, while for the Ni/Al₂O₃ catalyst these activation energies were 158 and 175 kJ mol⁻¹, respectively. The difference of the active metal dispersion on Al₂O₃ and CeO₂ supports, as measured from H₂-pulse chemisorption was not significant. This indicates deposition of carbon on the catalyst as a likely cause of lower activity of the Ni/Al₂O₃ catalyst. It is unlikely that carbon would build up on the Ni/CeO₂ catalyst due to higher oxygen mobility in the Ni doped non-stoichiometric CeO₂ lattice. Based on the products formed, the proposed primary reaction pathway is the dehydrogenation of n-BuOH to butaldehyde followed by decarbonylation to propane. The propane then partially breaks down to hydrogen and carbon monoxide through steam reforming, while CO converts to CO₂ mostly through water gas shift. Ethane and methane are formed via Fischer-Tropsch reactions of CO/CO₂ with H₂.     

Syngas production from glycerol-dry(CO2) reforming over La-promoted Ni/Al2O3 catalyst

A 3 wt% La-promoted Ni/Al₂O₃ catalyst was prepared via wet co-impregnation technique and physicochemically-characterized. Lanthanum was responsible for better metal dispersion; hence higher BET specific surface area (96.0 m² g⁻¹) as compared to the unpromoted Ni/Al₂O₃ catalyst (85.0 m² g⁻¹). In addition, the La-promoted catalyst possessed finer crystallite size (9.1 nm) whilst the unpromoted catalyst measured 12.8 nm. Subsequently, glycerol dry reforming was performed at atmospheric pressure and temperatures ranging from 923 to 1123 K employing CO₂-to-glycerol ratio from zero to five. Significantly, the reaction results have yielded syngas as main gaseous products with H₂:CO ratios always below than 2.0 with concomitant maximum 96% glycerol conversion obtained at the CO₂-to-glycerol ratio of 1.67. In addition, the glycerol consumption rate can be adequately captured using power law modelling with the order of reactions equal 0.72 and 0.14 with respect to glycerol and CO₂ whilst the activation energy was 35.0 kJ mol⁻¹. A 72 h longevity run moreover revealed that the catalyst gave a stable catalytic performance.