Improved hydrogen storage properties of MgH2 with Ni-based compounds

The nanoscaled Ni-based compounds (Ni₃C, Ni₃N, NiO and Ni₂P) are synthesized by chemical methods. The MgH₂-X (X = Ni₃C, Ni₃N, NiO and Ni₂P) composites are prepared by mechanical ball-milling. The dehydrogenation properties of Mg-based composites are systematically studied using isothermal dehydrogenation apparatus, temperature-programmed desorption system and differential scanning calorimetry. It is experimentally confirmed that the dehydrogenation performance of the Mg-based materials ranks as following: MgH₂Ni₃C, MgH₂Ni₃N, MgH₂NiO and MgH₂Ni₂P. The onset dehydrogenation temperatures of MgH₂Ni₃C, MgH₂Ni₃N, MgH₂NiO and MgH₂Ni₂P are 160 °C, 180 °C, 205 °C and 248 °C, respectively. The four Mg-based composites respectively release 6.2, 4.9, 4.1 and 3.5 wt% H₂ within 20 min at 300 °C. The activation energies of MgH₂Ni₃C, MgH₂Ni₃N, MgH₂NiO and MgH₂Ni₂P are 97.8, 100.0, 119.7 and 132.5 kJ mol^?1, respectively. It' found that the MgH₂Ni₃C composites exhibit the best hydrogen storage properties. Moreover, the catalytic mechanism of the Ni-based compounds is also discussed. It is found that Ni binding with low electron-negativity element is favorable for the dehydrogenation of the Mg-based composites.     

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.     

Synergistic effects of des tabilization,catalysis and nanoconfinement on dehydrogenation of LiBH4

In this report, Ni and TiO₂ are successfully embedded into porous carbon aerogel (CA) (donated as NiTiO₂@CA). Meanwhile, the synergistic effect of Ni, TiO₂ and CA on the dehydrogenation properties of LiBH₄ is systematically studied. Ni@CA, TiO₂@CA and CA are also investigated for comparisons. Compared to other three materials, NiTiO₂@CA exhibits better performance when used as a carrier to support LiBH₄. More than 6.75 wt% H₂ is released from LiBH₄NiTiO₂@CA system in nearly 120 min at 350 °C, exhibiting a higher dehydrogenation capacity than that of LiBH₄Ni@CA (3.15 wt %), LiBH₄TiO₂@CA (5.15 wt%) and LiBH₄-CA (2.05 wt %), respectively. Furthermore, the apparent energy (Ea) calculated with Kissinger method is 118.8 kJ/mol, much lower than that of pure LiBH₄. Dehydrogenation performance of LiBH₄NiTiO₂@CA may be due to the synergetic effect of destabilization of TiO₂, catalysis of Ni, as well as the nanoconfinement of CA.     

The effects of duty cycles on pulsed current electrodeposition of ZnNiAl2O3 composite on steel substrate: Microstructures,hardness and corrosion resistance

In this study, we examine the effect of duty cycles (33%, 50% and 67%) under square-wave galvanostatic pulses on the electrodeposition of zinc-nickel-alumina (ZnNiAl₂O₃) composites from a sulfate bath. XRD results showed that the dominant phases of the ZnNiAl₂O₃ electrodeposits were mixtures of Zn₂₁Ni₅ and Zn₂₂Ni₃ phases together with as Al₂O₃. The Ni content measured in the electrodeposits using EDS varied from 9.73 to 13.47 wt%. SEM results showed that finer and smoother surface electrodeposits were obtained by pulsed current electrodeposition at a low (33%) duty cycle. In addition, the corrosion properties of the electrodeposits were characterized by Tafel plots and electrochemical impedance spectroscopy (EIS), while the microhardness of the electrodeposits was measured by a Vickers hardness tester. In summary, this study revealed that pulsed current electrodeposition at a 33% duty cycle led to a finer and smoother surface morphology, an enhanced strength, a greater corrosion resistance, and a higher Ni content in the ZnNiAl₂O₃ composite coatings compared to plating at higher duty cycles or plating through DC electrodeposition.     

Grain refinement and Al-water reactivity of AlGaInSn alloys

AlGaInSn alloys were prepared by using traditional casting metallurgy method with different additions of Al5Ti1B grain refiner. Their microstructures were investigated by means of X-ray diffraction (XRD) and scanning electron microscope (SEM) with energy dispersed X-ray (EDX). The Al grains of alloys are refined significantly from 129 μm to 57 μm with increasing Ti content from 0.03 wt% to 0.24 wt%. Many thin dendrites that are a few micrometers thick are observed within Al grains.Al-water reactivities were performed under different water temperatures. The alloy with Ti content of 0.12 wt% shows the maximum H₂ generation rate under different water temperatures, which is above 5 times of Ti-free alloy. The H₂ yields of alloys drop from 87% to 30% with rising Ti content from 0.03 wt% to 0.24 wt% at the water temperature of 30 °C, but they rise to about 90% when the water temperature is above 50 °C.The growth mechanism of alloys and the effect of grain refinement on Al-water reactivities are discussed.     

Effect of combining Al,Mg, Ce or La oxides to extracted rice husk nanosilica on the catalytic performance of NiO during COx-free hydrogen production via methane decomposition

Amorphous nanosilica powder was extracted from rice husk and used as a catalyst support as well as a starting material for the preparation of different binary oxides, i.e., SiO₂Al₂O₃, SiO₂MgO, SiO₂CeO₂ and SiO₂La₂O₃. A series of supported nickel catalysts with the metal loading of 50 wt % were prepared by wet impregnation method and evaluated in methane decomposition to “CO_x-free” hydrogen production. The fresh and spent catalysts were extensively characterized by different techniques. Among the evaluated catalysts, both Ni/SiO₂Al₂O₃ and Ni/SiO₂La₂O₃ catalysts were the most active with an over-all H₂ yield of ca. 80% at the initial period of the reaction. This distinguishable higher catalytic activity is mainly referred to the presence of free mobile surface NiO and/or that NiO fraction weakly interacted with the support easily reducible at low temperatures. The Ni/SiO₂CeO₂ catalyst has proven a great potential for application in the hydrogen production in terms of its catalytic stability. The formation of Mg_xNi_(1?x)O solid solution caused the Ni/SiO₂MgO catalyst to lose its activity and stability at a long reaction time. Various types of carbon materials were formed on the catalyst surface depending on the type of support used. TEM images of as-deposited carbon showed that multi-walled carbon nanotubes (MWCNTs) and graphene platelets were formed on Ni/SiO₂, while only MWCNTs were deposited on all binary oxide supported Ni catalysts.     

Promoted activity of porous silica coated Ni/CeO2ZrO2 catalyst for steam reforming of acetic acid

Porous silica coated Ni/CeO₂ZrO₂ catalysts were synthesized for steam reforming of acetic acid. The silica coated Ni/CeO₂ZrO₂ catalyst showed a significantly enhanced activity (95% acetic acid conversion) than the Ni/CeO₂ZrO₂ catalysts (62% acetic acid conversion) at a low temperature (550 °C). Interaction between Ni/CeO₂ZrO₂ and silica layer was proved to be a crucial role on enhancing of catalytic activities. Further characterization (XPS, H₂-TPR) indicates this interaction facilitates the steam reforming reaction and raises the selectivity of CO by modifying the surface Ni electronic structure. In addition, the coated catalyst also exhibited a good stability and no obvious deactivation was detected at 550 °C and 600 °C within 30 h.     

Dimensionally stable NiFe@Co/Ti nanoporous electrodes by reactive deposition for water electrolysis

The development of efficient and dimensionally stable electrode plates is of a significant challenge for the oxygen evolution reaction in the industrial water electrolysis process. In this work, structurally stable electrode plates are developed based on the nanostructured NiFe catalysts on highly porous and dimensionally stable Co reactive deposited on Ti substrates, NiFe@Co/Ti. SEM analysis shows the hierarchically structured micro- and nano-porous structure of the Co electrode on Ti substrates by reactive deposition route. The surface area of the reactive deposited Co is 3 times larger than that of the conventional electrodeposited Co electrode, providing highly porous and stable base for the subsequent deposition of NiFe electrocatalysts. The as-prepared NiFe@Co/Ti electrode exhibits high catalytic activity towards oxygen evolution in alkaline solutions, achieving an onset potential of as low as 1.44 V (η = 210 mV) and delivering a current of 10 mA cm^?2 at an overpotential of 0.26 V. Most importantly, the electrode shows excellent stability with negligible degradation under the discharge current density at 100 mA cm^?2 for 100 h, demonstrating the practical applicability of the NiFe@Co/Ti nanostructured electrodes for industrial scale water electrolysis.     

Hollow nanoporous NiPd catalysts with enhanced performance for ethanol electro-oxidation

In this work, bimetallic NiPd hollow nanoporous (HNiPd) catalysts are prepared by in-situ deposition of Pd nanoparticles (Pd NPs) on hollow Ni (HNi) microspheres. Scanning electron microscope (SEM) and transmission electron microscopy (TEM) reveal the hollow nanoporous essence of HNiPd catalysts. Meanwhile, using high-angle annular dark-field scanning TEM (HAADF-STEM) and elemental mapping, it is found that tiny dendritic-like NiPd nanocomposites attach on the exterior of microspheres. The content of Pd is easily tailored to constitute HNiPd catalysts with different Ni/Pd atomic ratios. Further electrochemical evaluation vindicates that the as-prepared HNiPd catalysts have a good catalytic activity and stability toward ethanol oxidation reaction (EOR) in alkaline medium. Notably, the peak current density of HNi_3.1Pd catalyst and the chronoamperometric current density of HNi_4.6Pd catalyst are 4 and 2 times of Pd/C (JM) catalyst, respectively, which show that HNiPd catalysts hold great potential in application of alkaline direct ethanol fuel cells (DEFCs).     

Trimetallic NiFeCo selenides nanoparticles supported on carbon fiber cloth as efficient electrocatalyst for oxygen evolution reaction

Trimetallic NiFeCo selenides (NiFeCoSe_x) anchored on carbon fiber cloth (CFC) as efficient electrocatalyst for oxygen evolution reaction (OER) in alkaline medium have been synthesized via a facile two-step method. Firstly, trimetallic NiFeCo (oxy) hydroxides have been electrodeposited on CFC support (NiFeCo/CFC). Secondly, a solvothermal selenization process has been used to convert NiFeCo/CFC into NiFeCoSe_x/CFC using N, N-dimethylformamide (DMF) as solvent. The composition and homogeneous distribution of NiFeCoSe_x/CFC nanoparticles are determined by XRD, XPS, SEM elemental mapping and EDX images. Furthermore, SEM images reveal that NiFeCoSe_x/CFC has volcano-shaped morphology with rough surface and homogenously distributed on the surface of CFC, which may provide more active sites for OER. The electrochemical measurements show that trimetallic NiFeCoSe_x/CFC possesses the better electrocatalytic activity with the lower overpotential (150 mV at 10 mA cm^?2), Tafel slope (85 mV dec^?1), larger double-layer capacitance (200 mF cm^?2) and long-term stability than unary or binary metal selenides. The enhanced activity of NiFeCoSe_x/CFC may be attributed to the trimetallic NiFeCo selenides and selenides-CFC synergistic interaction. It may offer a promising way to design transition multimetallic selenides supported on conductive support as electrocatalysts for OER.     

Synergetic effects of K,Ti and F on the hydrogen storage properties of the LiNH system

In this paper, the hydrogen storage properties of the LiNH₂LiH system doped with K₂TiF₆ were investigated and discussed. Interestingly, the hydrogen storage properties are significantly enhanced by introducing K₂TiF₆ into the LiNH₂LiH system. By doping 5 mol% K₂TiF₆ in the LiNH₂LiH system, we obtain the hydrogen desorption peak temperature (233 °C) at a heating rate of 10 °C min^?1, which is approximately 66 °C lower than that of the pristine LiNH₂LiH system. Moreover, the system begins to desorb H₂ at 75 °C, which is approximately 124 °C lower than in the pristine LiNH₂LiH system. The isothermal desorption kinetics at 250 °C and 300 °C clearly reflects the dramatically improved kinetic properties. Additionally, the reversibility of the LiNH₂LiH system can be drastically enhanced by adding K₂TiF₆. We propose that the dehydrogenation property of the K₂TiF₆-doped LiNH₂LiH sample is improved by the synergetic effects of K, Ti and F.     

Biogas reforming for hydrogen enrichment by ceria decorated over nickel catalyst supported on titania and alumina

Catalytic dry reforming of biogas for hydrogen enrichment was studied over cerium oxide promoted nickel catalysts supported on titanium dioxide and aluminium oxide. The catalysts were prepared by wet impregnation method and characterized by H₂-TPR, XRD, BET and FESEM techniques. Their catalytic performance in the biogas dry reforming reaction was studied at temperature ranges from 650 to 850 °C, with a CH₄/CO₂ ratio of 1.5:1. The H₂-TPR results revealed that 11 wt % Ni impregnation on TiO₂ support makes the catalyst with strong metal-support interaction which moderates the metal sintering. Also, the addition of CeO₂ effectively improved the CH₄ and CO₂ conversions as well as H₂ enrichment. At 850 °C, 11 wt % Ni/TiO₂ catalyst leads to 70.5% CH₄ conversion with 32.0% H₂ enrichment, whereas, Ni_0·11/Ce_0.20 (Al₂O₃TiO₂) yielded high CH₄ conversion (84.9%) with 40.6% of H₂ enrichment. No significant change in the activity of the catalyst was observed with 8.8 wt % of carbon deposited on the Ni_0·11/Ce_0.20 (Al₂O₃TiO₂) catalyst, after 7 h of continuous reforming. Moreover, under combined (dry and oxidative) reforming of biogas, the stoichiometric H₂/CO ratio (1.2) was observed at 0.47 O₂/CH₄ ratios with negligible carbon deposition. Thus, Ni_0·11/Ce_0.20 (Al₂O₃TiO₂) catalyst exhibited better activity and selectivity with high catalyst stability at 850 °C.     

Influence of MgO content as an additive on the performance of Ni/MgOSiO2 catalysts for the steam reforming of glycerol

Ni catalysts supported on MgOSiO₂ were assessed in the steam reforming reaction of glycerol for the study of the H₂ production and carbon deposition with different MgO contents as additives. The catalysts were prepared with commercial SiO₂ by the aqueous impregnation method and characterized by energy dispersive X-ray spectroscopy, specific surface area, X-ray diffraction, thermogravimetric analysis, X-ray diffraction in situ with O₂, temperature programmed reduction with H₂, X-ray diffraction in situ with H₂, temperature programmed desorption with H₂ and scanning electron microscopy. The glycerol steam reforming reaction took place at 600 °C for 5 h, with a water/glycerol molar ratio of 12/1 at 5 mLh^?1. N₂ was used as the carrier gas. The characterization of the samples showed the interaction of Ni with the support increases with the MgO addition, due to the formation of a NiMg silicate hydrate and MgNiO₂ solid solution; as a result, both metallic area and dispersion also increased. Catalytic results showed similar gaseous products yields (H₂, CH₄, CO and CO₂) for mixed-matrix catalysts, however, a lower carbon deposition on 10 wt%Ni catalyst supported on 30 wt%MgOSiO₂ was observed.     

One-step synthesis of CuCo alloy/Mn2O3Al2O3 composites and their application in higher alcohol synthesis from syngas

CuCo alloy/Mn₂O₃Al₂O₃ composites were synthesized by a facile one-step sol–gel method using citric acid as a chelating agent. The catalytic performance of the as-prepared catalysts was investigated in reaction of CO hydrogenation to higher alcohols. According to the characterization data obtained by TG-DSC, XRD, TPR, BET, ICP, SEM, TEM and XPS, a stronger interaction between the Cu and Co ions in the CuCo₂O₄ particles led to the formation of CuCo alloy in the reduced catalysts, and the Mn/Al molar ratio significantly influenced the performance of the catalysts. Mn₂O₃Al₂O₃ composites reduce the unwanted CoAl₂O₄ spinel phase, offer tunable pore sizes and surface areas, and also appear to act as barriers to hinder the CuCo alloy particles sintering. The results suggest that the metal nanoparticles of CuCo alloy together with Mn₂O₃Al₂O₃ contributed to the high selectivity of higher alcohols as well as the good stability. A Mn/Al molar ratio of 5/3 was found to be most suitable for the catalyst properties in terms of activity and product distribution.     

Effect of heat treatment on AlMgGaInSn alloy for hydrogen generation through hydrolysis reaction

The composition, microstructure and corrosion behavior of AlMgGaInSn alloy in cast and heat-treatment were investigated by XRD (X-ray diffraction), SEM (scanning electron microscope) and EDS (energy dispersive spectrometer). The hydrogen evolution parameters, and the electrochemical properties based on different heat-treatment parameters conditions and immersion temperature were also tested. As the heat-treated temperature increased, the second phases were found to be spheroidizing or ellipsoidal shape due to the diffusion and solid solution. The reaction can be divided into three stages: i) the amalgamation initial stage; ii) the micro-galvanic reaction for propagation corrosion; iii) the dissolution-precipitation reaction for uniform corrosion. The hydrolysis rate reached the maximum value when the sample was annealed at temperature of 500 °C for 9 h. The hydrogen generation rate and the open electrochemical potential of the activated aluminum under water are both depending on heat treatment time, heat treatment temperature, and reaction temperature. A corrosion mechanism was also proposed in which Mg₂Sn and eutectic phase acted as the induction reaction stage during hydrolysis reaction.     

Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A = Ca,Sr and Ba)

The crystal structure of a photocatalyst generally plays a pivotal role in its electronic structure and catalytic properties. In this work, we synthesized a series of La/Cr co-doped perovskite compounds ATiO₃ (M = Ca, Sr and Ba) via a hydrothermal method. Their optical properties and photocatalytic activities were systematically explored from the viewpoint of their dependence on structural variations, i.e. impact of bond length and bond angles. Our results show that although La/Cr co-doping helps to improve the visible light absorption and photocatalytic activity of these wide band gap semiconductors, their light absorbance and catalytic performance are strongly governed by the TiO bond length and TiOTi bond angle. A long TiO bond and deviation of TiOTi bond angle away from 180° deteriorate the visible light absorption and photocatalytic activity. The best photocatalytic activity belongs to Sr_0.9La_0.1Ti_0.9Cr_0.1O₃ with an average hydrogen production rate ～2.88 μmol/h under visible light illumination (λ ≥ 400 nm), corresponding to apparent quantum efficiency ～ 0.07%. This study highlights an effective way in tailoring the light absorption and photocatalytic properties of perovskite compounds by modifying cations in the A site.     

Dry reforming of biogas in fluidized bed: Process intensification

Biogas is a renewable resource obtained mainly from the anaerobic fermentation of agro-industrial and anthropogenic residues. The production of hydrogen by dry reforming of methane represents a potential application for this renewable energy carrier. This could play a positive contribution towards meeting the challenge of providing a global supply of energetically sustainable and environmentally friendly energy. This work combines a catalytic reaction, a separation and the catalyst regeneration in a single reactor. To this end, a two zone fluidized bed reactor (TZFBR) with hydrogen selective membranes has been employed (TZFBR + MB). The operating conditions for the process of dry reforming of biogas have been optimized experimentally, both in TZFBR and TZFBR + MB. Several catalysts were prepared (Ni/Al₂O₃, NiCe/Al₂O₃, NiCo/Al₂O₃), characterized and tested in reactions in both TZFBR and in TZFBR + MB. Finally, the influence of using oxygen or carbon dioxide as regenerating gases in the process has been studied. Experimental results show the feasibility of using CO₂ for in situ catalyst regeneration, avoiding the potential problems associated with the use of O₂.     

Base-metal catalysts based on porous layered double hydroxides for alkaline-free sodium borohydride hydrolysis

Catalyzed hydrolysis of sodium borohydride (SBH) has demonstrated promise for generation of a pure hydrogen stream for use with fuel cells. In designing an improved continuous hydrogen generator that uses the substantial heat released in the hydrolysis reaction to more effectively separate the sodium borate by-product, we sought a robust base-metal catalyst that could tolerate the exothermic reaction under flow conditions. Working under base-free conditions in ethanol solvent we identified reduced nickel and iron-containing particles supported on layered double hydroxides (LDHs) as robust catalysts. Catalytic activity was enhanced further using high surface area hierarchical supports prepared using the ‘inverse opal’ method. In particular, macroporous NiMgAl and FeMgAl LDHs produced 0.4 and 1.0 mol of hydrogen per minute per mole of active metal of the supported catalyst in aqueous ethanol solvent.     

Enhancement of hydrogen storage properties in 4MgH2 Na3AlH6 composite catalyzed by TiF3

In this work, we have investigated the hydrogen release and uptake pathways storage properties of the MgH₂Na₃AlH₆ with a molar ratio of 4:1 and doped with 10 wt% of TiF₃ using a mechanical alloying method. The doped composite was found to have a significant reduction on the hydrogen release temperature compared to the un-doped composite based on the temperature-programme-desorption result. The first stage of the onset desorption temperature of MgH₂Na₃AlH₆ was reduced from 170 °C to 140 °C with the addition of the TiF₃ additive. Three dehydrogenation steps with a total of 5.3 wt% of released hydrogen were observed for the 4MgH₂Na₃AlH₆-10 wt% TiF₃ composite. The re/dehydrogenation kinetics of 4MgH₂Na₃AlH₆ system were significantly improved with the addition of TiF₃. Kissinger analyses showed that the apparent activation energy, E_A, of the 4MgH₂Na₃AlH₆ doped composite was 124 kJ/mol, 16 kJ/mol and 34 kJ/mol lower for un-doped composite and the as-milled MgH₂, respectively. It was believed that the enhancements of the MgH₂Na₃AlH₆ hydrogen storage properties with the addition of TiF₃ were due to formation of the NaF, the AlF₃ and the Al₃Ti species. These species may played a synergetic catalytic role in improving the hydrogenation properties of the MgH₂Na₃AlH₆ system.     

CO2 methanation on hard-templated NiOCeO2 mixed oxides

Mesostructured NiOCeO₂ mixed oxides, with Ni loadings in the range 5–35 wt% (g_Ni/g_CeO2), were synthesized by the hard template procedure using SBA-15 as the template. A hard-templated CeO₂ was also prepared and used as the support for depositing Ni (5–35 wt%) by impregnation. Two NiOAl₂O₃ catalysts were synthesized for comparison, by impregnating nickel on a commercial γ-alumina. All the samples were characterized by different techniques as to their chemical composition, structure, morphology, texture, and redox features. The catalytic performance was investigated in the CO₂ methanation reaction after mild reduction pretreatment (H₂ at 400 °C for 1 h). Catalytic testing was performed under atmospheric pressure, 300 °C, 72,000 cm³ h^?1 g_cat^?1, and stoichiometric H₂/CO₂ molar ratio. High catalytic activity, with CH₄ selectivity values ≥ 93 mol%, was obtained with the NiOCeO₂ mixed oxides (CO₂ conversions up to 76 mol%). When γ-Al₂O₃ was used as the support, catalysts with low activity (CO₂ conversion ≤ 20 mol%) were obtained. On selected samples, additional catalytic runs were also performed for reaction times up to 30 h or with a higher space velocity value. The catalytic results were explained by taking into account the role of the nickel-ceria interactions both during the reduction of NiO and in the reactants activation.