Effect of multi-wall carbon nanotubes supported palladium addition on hydrogen storage properties of magnesium hydride

To improve the hydrogen storage performance of magnesium hydride, multi-wall carbon nanotubes supported palladium (Pd/MWCNTs) was introduced to the magnesium-based materials. Pd/MWCNTs catalysts with different amounts of Pd (20 wt.%, 40 wt.%, 60 wt.%, 80 wt.%) were synthesized by a solution chemical reduction method. Afterwards, Mg₉₅–Pd_m/MWCNTs_5−m (m = 0, 1, 2, 3, 4, 5) were prepared for the first time by hydriding combustion synthesis (HCS) and mechanical milling (MM). It is determined by X-ray diffraction (XRD) analysis that Pd/MWCNTs can significantly increase the hydrogenation degree of magnesium during the HCS process. The microstructures of the composites obtained by transmission electron microscope (TEM) and field emission scanning electronic microscopy (FESEM) analyses show that Pd nanoparticles are well supported on the surface of carbon nanotubes and the Pd/MWCNTs are dispersed uniformly on the surface of MgH₂ particles. Moreover, it is revealed that there is a synergistic effect of MWCNTs and Pd on the hydrogen storage properties of the composites. The Mg₉₅–Pd₃/MWCNTs₂ shows the optimal hydriding/dehydriding properties, requiring only 100 s to reach its saturated hydrogen absorption capacity of 6.67 wt.% at 473 K, and desorbing 6.66 wt.% hydrogen within 1200 s at 573 K. Additionally, the dehydrogenation activation energy of MgH₂ in this system is decreased to 78.6 kJ/mol H₂, much lower than that of as-received MgH₂.     

Improved hydrogen permeation through thin Pd/Al2O3 composite membranes with graphene oxide as intermediate layer

Here we proposed the decreasing in the roughness of asymmetric alumina (Al₂O₃) hollow fibers by the deposition of a thin graphene oxide (GO) layer. GO coated substrates were then used for palladium (Pd) depositions and the composite membranes were evaluated for hydrogen permeation and hydrogen/nitrogen selectivity. Dip coating of alumina substrates for 45, 75 and 120 s under vacuum reduced the surface mean roughness from 112.6 to 94.0, 87.1 and 62.9 nm, respectively. However, the thicker GO layer (deposited for 120 s) caused membrane peel off from the substrate after Pd deposition. A single Pd layer was properly deposited on the GO coated substrates for 45 s with superior hydrogen permeance of 24 × 10⁻⁷ mol s⁻¹m⁻² Pa⁻¹ at 450 °C and infinite hydrogen/nitrogen selectivity. Activation energy for hydrogen permeation through the Al₂O₃/GO/Pd composite membrane was of 43 kJ mol⁻¹, evidencing predominance of surface rate-limiting mechanisms in hydrogen transport through the submicron-thick Pd membrane.     

Three-dimensional layered porous graphene aerogel hydrogen getters

Organic getters are often introduced into sealed systems to remove the excessive reactive hydrogen gas. In this work, the formable graphene aerogel hydrogen getters are prepared by integrating the alkyne-containing molecules (e.g., DEB) into the palladium-loaded three-dimensional layered porous graphene aerogel (Pd-GA). The performance of Pd-GA/DEB composite materials in reducing reactive hydrogen gas is examined at pressures of 0.1–1 bar at a temperature of 25 °C, and the hydrogen consumption is measured as a function of time. Results suggest that the hydrogen uptake capability of Pd-GA/DEB getters increases with the loading of Pd particles on the GA and the content of Pd⁰. The highest hydrogen absorption capacity is up to 215.5 cm³/g, and the hydrogenation rate of DEB molecules is 89.3%. This study promotes the fundamental understanding of solid-phase catalytic hydrogenation and the applications of 3D layered porous graphene aerogel in hydrogen absorption.     

Fabrication of highly dispersed palladium/graphene oxide nanocomposites and their catalytic properties for efficient hydrogenation of p-nitrophenol and hydrogen generation

In this report, graphene oxide (GO) nanosheets decorated with ultrafine Pd nanoparticles (Pd NPs) have been successfully fabricated through a reaction between [Pd₂(μ-CO)₂Cl₄]²⁻ and water in the presence of GO nanosheets without any surfactant or other reductant. The as-synthesized small Pd NPs with average diameter of about 4.4 nm were well-dispersed on the surface of GO nanosheets. The Pd/GO nanocomposites show remarkable catalytic activity toward the hydrogenation of p-nitrophenol at room temperature. The kinetic apparent rate constant (k_app) could reach about 34.3 × 10⁻³ s⁻¹. Furthermore, the as-prepared Pd/GO nanocomposites could also be used as an efficient and stable catalyst for hydrogen production from hydrolytic dehydrogenation of ammonia borane (AB). The catalytic activity is much higher than the conventional Pd/C catalysts.     

Substitutional V–Pd alloys for the membranes permeable to hydrogen: Hydrogen solubility at 150–400 °C

The development of non-palladium membrane for separation of hydrogen from gas mixtures is one of critical challenges of hydrogen energy. Vanadium based materials are most promising for such membranes. The alloying of pure vanadium is crucially important for reduction of hydrogen solubility to an optimal value. Solution of hydrogen in substitutional V-xPd alloys (x = 5, 7.3, 9.7, 12.3, 18.8 at%) was investigated. The pressure–composition-isotherms were obtained in the range of pressure (10–10⁶) Pa, temperature (150–400) °С and concentration of hydrogen, H/M, from 4·10⁻⁴ to 0.6. The alloying of vanadium with palladium was found to reduce the hydrogen solubility substantially greater than the alloying with other elements, e.g. by Ni and Cr. The hydrogen absorption in the V–Pd alloys obeyed Siverts' law including the range of undiluted solution with hydrogen concentration H/M > 0.1. The reduction in the hydrogen solubility due to the alloying of V with Pd was caused mainly by increase in the enthalpy of solution at nearly constant entropy factor. Changes in the gross electronic structure of metal are most probably responsible for the effects of alloying on the hydrogen solubility in the substitutional V–Pd alloys.     

Room-temperature hydrogen storage performance of metal-organic framework/graphene oxide composites by molecular simulations

Metal-organic framework/graphene oxide (MOF/GO) composites have been regarded as potential room-temperature hydrogen storage materials recently. In this work, the influence of MOF structural properties, GO functional group contents and different amounts of doped lithium (Li⁺) on hydrogen storage performance of different MOF/GO composites were investigated by grand canonical Monte Carlo (GCMC) simulations. It is found that MOF/GO composites based on small-pore MOFs exhibit enhanced hydrogen storage capacity, whereas MOF/GO based on large-pore MOFs show decreased hydrogen storage capacity, which can be ascribed to the novel pores at MOF/GO interface that favors the enhanced hydrogen storage performance due to the increased pore volume/surface area. By integrating the small-pore MOF-1 with GO, the hydrogen storage capacity was enhanced from 9.88 mg/go up to 11.48 mg/g. However, the interfacial pores are smaller compared with those in large-pore MOFs, resulting in significantly reduced pore volume/surface area as well as hydrogen storage capacities of large-pore MOF/GO composite. Moreover, with the increased contents of hydroxyl, epoxy groups as well as carboxyl group modification, the pore volumes and specific surface areas of MOF/GO are decreased, resulting in reduced hydrogen storage performance. Furthermore, the room-temperature hydrogen storage capacities of Li⁺ doped MOF/GO was improved with increased Li⁺ at low loading and decrease with the increased Li⁺ amounts at high loading. This is due to that the introduced Li⁺ effectively increases the accessible hydrogen adsorption sites at low Li⁺ loading, which eventually favors the hydrogen adsorption capacity. However, high Li⁺ loading causes ion aggregation that reduces the accessible hydrogen adsorption sites, leading to decreased hydrogen storage capacities. MOF-5/GO composites with moderate Li⁺ doping achieved the optimum hydrogen storage capacities of approximately 29 mg/g.     

Hydroxyapatite-supported palladium(0) nanoclusters as effective and reusable catalyst for hydrogen generation from the hydrolysis of ammonia-borane

Herein we report the preparation, characterization and catalytic use of hydroxyapatite-supported palladium(0) nanoclusters in the hydrolysis of ammonia-borane. Palladium(0) nanoclusters were formed in situ from the reduction of palladium(II) ion exchanged hydroxyapatite during the hydrolysis of ammonia-borane and supported on hydroxyapatite. The hydroxyapatite-supported palladium(0) nanoclusters are stable enough to be isolated as solid materials and characterized by using a combination of advanced analytical techniques. They are isolable, redispersible and reusable as an active catalyst in the hydrolysis of ammonia-borane even at low concentration and temperature. They provide a maximum hydrogen generation rate of ∼1425 mL H₂ min⁻¹ (g Pd)⁻¹ and 12300 turnovers in the hydrolysis of ammonia-borane at 25 ± 0.1 °C before deactivation. The work reported here also includes the full experimental details for the collection of a wealth of kinetic data to determine the activation energy (E_a = 54.8 ± 2.2 kJ/mol) and the effect of catalyst concentration on the rate for the catalytic hydrolysis of ammonia-borane.     

Graphene oxide supported Pd-Fe nanohybrid as an efficient electrocatalyst for proton exchange membrane fuel cells

The experimental realization and computational validation for graphene oxide (GO) supported palladium (Pd)-iron (Fe) nanohybrids as a new generation electrocatalyst for proton-exchange membrane fuel cells (PEMFCs) has been reported. The experimental apprehension of the present catalyst system has been initiated with the graphene oxide, followed by the doping of Pd and Fe via thermal inter calation of palladium chloride and iron chloride with the in-situ downstream reduction to get nanohybrids of the GO-Pd-Fe. These nanohybrids are subsequently characterized by RAMAN, FT-IR, UV–Vis, XRD, SEM, EDS, TEM and HRTEM analysis. Furthermore, the first principle calculations based on Density Functional Theory (DFT) with semi-empirical Grimme DFT-D₂ correction has been performed to support the experimental findings. Computational results revealed the alteration of graphene electronic nature from zero-band gaped to metallic/semi-metallic on adsorption of transition metal clusters. Moreover, the defect sites of the graphene surface are more favorable than the pristine sites for transition metal adsorption owing to the strong binding energies of the former. Electrochemical studies show that GO-Pd-Fe nanohybrids catalyst (Pd: Fe = 2:1) demonstrates excellent catalytic activity as well as the higher electrochemical surface area of (58.08 m²/g Pd–Fe)⁻¹ which is higher than the commercially available Pt/C catalyst with electrochemical surface area 37.87 m²/(g Pt)⁻¹.     

Palladium nanoparticles supported on chemically derived graphene: An efficient and reusable catalyst for the dehydrogenation of ammonia borane

Chemically derived graphene (CDG) was prepared by hydrazine hydrate reduction of graphene oxide and used as support for palladium nanoparticles (Pd NPs) generated ex situ with controllable particle size and dispersion. The Pd NPs supported on CDG were well characterized by using a combination of advance analytical techniques and employed as catalyst in the dehydrogenation and hydrolysis of ammonia borane (AB) in organic solvents and aqueous solutions, respectively. Monodisperse Pd NPs of 4.5 nm were prepared from the reduction of palladium(II) acetylacetonate by tert-butylamine borane in the presence of oleylamine. They were readily impregnated on CDG which has BET surface area of 500 m² g⁻¹. Pd NPs retain their particle size dispersion and stability when supported on chemically derived graphene. The resulting materials are highly active and stable catalyst for the dehydrogenation and hydrolysis of AB. In addition to their high activity and stability, these Pd NPs are also reusable catalyst in both dehydrogenation and hydrolysis of AB preserving 85% and 95% of initial activity after 5th and 10th runs, respectively.     

Theoretical study of hydrogen adsorption on nitrogen doped graphene decorated with palladium clusters

Density functional theory was used to study the adsorption of hydrogen on small palladium clusters (Pd_n, n = 1–4) supported on pyridine-like nitrogen doped graphene. Charge transfer and strong binding (up to four times higher than binding energy of Pd cluster supported on graphene) between graphene–nitrogen and palladium clusters prevent detachment of clusters and leads to three types of adsorption states of hydrogen. The first type is a molecular hydrogen physisorbed, the second one is an activated state of H₂ without adsorption barriers where H–H bond is relaxed and the third type is dissociated state. In dissociated cases, we found barriers below 0.56 eV. This means the process might occur spontaneously at room temperature. We also show that metal–N–vacancy complexes are very stable and involve nitrogen sp² and p_z, carbon p_z and palladium d orbitals. Besides, these systems exhibit an interesting magnetic behavior.     

Polymer-immobilized palladium supported on TiO2 (Pd-PVB-TiO2) as highly active and reusable catalyst for hydrogen generation from the hydrolysis of unstirred ammonia-borane solution

Herein we report the preparation, characterization and the catalytic use of the polymer-immobilized palladium catalyst supported on TiO₂ (Pd-PVB-TiO₂) in the hydrolysis of unstirred ammonia-borane solution. The polymer-immobilized palladium catalyst is stable enough to be isolated as solid materials and characterized by XRD, SEM, and EDX. The immobilized palladium catalyst supported on TiO₂ is found highly active, isolable, and reusable in the hydrolysis of unstirred ammonia-borane even at low concentrations and temperature. The work reported here also includes the full experimental details for the collection of a wealth of kinetic data to determine the activation energy (E_a = 55.9 kJ/mol) and the effects of catalyst and substrate concentration on the rate for the hydrolysis of unstirred ammonia-borane solution. Maximum H₂ generation rate of ∼642 mL H₂ min⁻¹ (g Pd)⁻¹ and ∼4367 mL H₂ min⁻¹ (g Pd)⁻¹ was measured by the hydrolysis of AB at 25 °C and 55 ± 0.5 °C, respectively.     

Characteristics of resistivity-type hydrogen sensing based on palladium-graphene nanocomposites

We describe the characteristics of resistivity-type hydrogen (H₂) sensors made of palladium (Pd)-graphene nanocomposites. The Pd-graphene composite was synthesized by a simple chemical route capable of large production. Synthesis of Pd nanoparticles (PdNPs) of various sizes decorated on graphene flakes were easily controlled by varying the concentration of Pd precursors. Resistivity H₂ sensors were fabricated from these Pd-graphene composites and evaluated with various concentrations of H₂ and interfering gases at different temperatures. Characteristics for sensitivity, selectivity, response time and operating life were studied. The results from testing the Pd-graphene indicated a potential for hydrogen sensing materials at low temperature with good sensitivity and selectivity. Specifically H₂ was measurable with concentrations ranging from 1 to 1000 ppm in laboratory air, with a very low detection limit of 0.2 ppm. The response of the sensors is almost linear. The resistivity of sensors changed approximately 7% in its resistance with 1000 ppm H₂ even at room temperature. The robust mechanical properties of graphene, which supported these PdNPs, exhibit structural stability and durability in H₂ sensors for at least six months. Moreover, the advantages in this work are experimental reproducibility in synthesis Pd-graphene composite and sensor fabrication process.     

Platinum deposition onto g-C3N4 with using of labile nitratocomplex for generation of the highly active hydrogen evolution photocatalysts

The chemisorption of the labile dimeric platinum nitratocomplex [Pt₂(μ-OH)₂(NO₃)₈]^2- onto graphene oxide doped graphitic C₃N₄ (g–C₃N₄–GO) was performed for the first time to prepare PtO_x/g–C₃N₄–GO composites with ionic platinum species (Pt(II)) bonded with N-donor groups of the g-C₃N₄. The thermal treatment of the obtained composites in the hydrogen atmosphere results in a gradual reduction of Pt(II) species with the formation of Pt/g–C₃N₄–GO photocatalysts. The Pt/g–C₃N₄–GO catalysts paired with TEOA sacrificing agent in an aqueous solution were tested in a photoinduced hydrogen evolution reaction under visible light (λ = 425 nm). The photocatalytic activity of prepared materials strongly influenced by the temperature of the reduction stage so that the maximal rate of H₂ evolution was revealed for the catalyst (0.5 wt% of Pt) reduced at 400 °C with a quantum efficiency of 3.0% and rate of HER of 5.1 mmol h^?1 per 1 g of photocatalysts. The photocatalytic activity of this sample was much higher than the activity of 0.5% Pt/g–C₃N₄–GO photocatalysts prepared by conventional photoreduction of H₂PtCl₆ or reduction of this precursor with NaBH₄. The Pt/g–C₃N₄–GO photocatalyst with Pt concentration of 0.1 wt% was prepared using the described protocol and shown specific activity of about 1.4 mol h^?1 per 1 g of Pt outperforming analogous material reported to date.     

Facile synthesis of hybrid porous composites and its porous carbon for enhanced H2 and CH4 storage

The anticipated energy crisis due to the extensive use of limited stock fossil fuels forces the scientific society for find prompt solution for commercialization of hydrogen as a clean source of energy. Hence, convenient and efficient solid-state hydrogen storage adsorbents are required. Additionally, the safe commercialization of huge reservoir natural gas (CH₄) as an on-board vehicle fuel and alternative to gasoline due to its environmentally friendly combustion is also a vital issue. To this end, in this study we report facile synthesis of polymer-based composites for H₂ and CH₄ uptake. The cross-linked polymer and its porous composites with activated carbon were developed through in-situ synthesis method. The mass loadings of activated carbon were varied from 7 to 20 wt%. The developed hybrid porous composites achieved high specific surface area (SSA) of 1420 m²/g and total pore volume (TPV) of 0.932 cm³/g as compared to 695 m²/g and 0.857 cm³/g for pristine porous polymer. Additionally, the porous composite was activated converted to a highly porous carbon material achieving SSA and TPV of 2679 m²/g and 1.335 cm³/g, respectively. The H₂ adsorption for all developed porous materials was studied at 77 and 298 K and 20 bar achieving excess uptake of 4.4 wt% and 0.17 wt% respectively, which is comparable to the highest reported value for porous carbon. Furthermore, the developed porous materials achieved CH₄ uptake of 8.15 mmol/g at 298 K and 20 bar which is also among the top reported values for porous carbon.     

Palladium nanoparticles “breathe” hydrogen; a surgical view with X-ray diffraction

High resolution in situ XRD was used to study the hydriding behavior of palladium electrocatalysts, prepared from two precursors, (NH₄)₂PdCl₆ (4.3 nm), and palladium acetylacetonate [Pd(acac)₂] (6.2 nm), and supported on porous carbon XC-72R. X-ray line profile analysis revealed a defective fcc lattice with internal strains and a high stacking fault probability of 12%. Importantly, no change, neither of the size, nor the state of defects was observed during the phase transition (α ↔ β). Based on this finding, a two-phase model adopted from bulk palladium hydride was proposed to describe the transition trough the miscibility gap (MG). Apparently, Pd nanoparticles can “breathe” hydrogen, without modifying their intrinsic crystal structure: A 3-parameter algorithm perfectly reproduces the anomalous line profiles observed inside the MG by a simple rescaling of the intensity profiles measured outside the MG. The algorithm delivers accurate values for the phase boundaries α-max, β-min. They depend sensitively on the particles size and surface state, but also surprisingly on the branch of the hysteresis loop. Time dependent studies verify a hindered kinetics of hydride formation in the presence of surface oxide species.     

Preparation of a novel recyclable cocatalyst wool–Pd for enhancement of photocatalytic H2 evolution on CdS

In recent years, the research of photocatalyst splitting of water to hydrogen by using semiconductor has been developed rapidly. CdS are attractive photocatalytic materials for the conversion of solar energy into chemical energy under visible-light irradiation. In this paper, a kind of recyclable cocatalyst that the wool supported palladium cocatalyst was synthetized and characterized by X-ray diffraction, diffuse reflectance UV–vis spectroscopy, energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopic (XPS) studies and transmission electron microscopy (TEM). The optimal weight percentage of wool–Pd was found to be 3.0 wt %, which resulted in a high visible-light photocatalytic average H₂-production rate of 1555 μ mol/h. It showed that the recycled cocatalyst wool–Pd could improve the efficiency of photocatalytic hydrogen production because of introducing oxidation cocatalyst PdS and reduction cocatalyst Pd. This study indicated that the prepared recyclable cocatalyst wool–Pd not only could improve the efficiency of photocatalytic hydrogen production, but also eco-friendly and recyclable.     

Stability of a hydrogen molecule in a vacancy of palladium hydrides

We report our ab-initio calculations of energy states of equilibrium H–H separation in a vacancy of palladium and palladium hydrides at a variety of H/Pd loading ratios. In a vacancy of pure palladium, the H₂ molecule has a shallow local energy minimum only in the [001] direction at a separation of 0.96 Å and it dissociates into positions near interstitial sites due to its high energy state. Increasing the H/Pd ratio to the beta phase deepens the energy well of the H₂ molecule and results in a shorter H–H separation. At a loading ratio around 1, the H₂ molecule is mostly affected by surrounding hydrogen neighbors and the H–H separation reaches 0.77 Å. The H₂ molecule is then fairly stable and its energy state is comparable to that of nearby interstitial sites. Our calculations suggest that the loading ratio of hydrogen in palladium has a significant effect on the stability of the H₂ molecule in the vacancy.     

Fabrication and electrochemical hydrogen storage performance of Ti49Zr26Ni25 alloy covered with Cd/Pd core/shell particles

A facile two-step reduction method is employed to obtain the Cd/Pd core/shell particles. Mechanical alloying and subsequent annealing are used to fabricate the Ti₄₉Zr₂₆Ni₂₅ quasicrystal. Composite materials of Ti₄₉Zr₂₆Ni₂₅ mixed with different contents of Cd/Pd particles are obtained via ball-milling. The electrochemical performance and kinetics properties of the alloy electrodes for Ni/MH secondary batteries are studied. Ultimately, a maximum discharge capacity of 272.9 mA h/g is achieved for 7% additive content of Cd/Pd. Ti₄₉Zr₂₆Ni₂₅ + Cd/Pd shows higher capacity than Ti₄₉Zr₂₆Ni₂₅ + Pd (246.8 mA h/g) and original Ti₄₉Zr₂₆Ni₂₅ (212.5 mA h/g). Moreover, the composites also exhibit improved cyclic stability and high-rate dischargeability. The Cd/Pd particles with special core/shell microstructure can enhance the electro-catalytic activity of Pd. The Cd/Pd material covered on the surface of alloy can further decrease the charge-transfer resistance and accelerate the hydrogen transmission, thus improving the electrochemical properties and reaction kinetics of the electrode.     

Hydrogen sorption studies of palladium decorated graphene nanoplatelets and carbon samples

With the increasing population of the world, the need for energy resources is increasing rapidly due to the development of the industry. 88% of the world's energy needs are met from fossil fuels. Since there is a decrease in fossil fuel reserves and the fact that these fuels cause environmental pollution, there is an increase in the number of studies aimed to develop alternative energy sources nowadays. Hydrogen is considered to be a very important alternative energy source due to its some specific properties such as being abundant in nature, high calorific value and producing only water as waste when burned. An important problem with the use of hydrogen as an energy source is its safe storage. Therefore, method development is extremely important for efficient and safe storage of hydrogen. Surface area, surface characteristics and pore size distribution are important parameters in determining the adsorption capacity, and it is needed to develop new adsorbents with optimum parameters providing high hydrogen adsorption capacity. Until recently, several porous adsorbents have been investigated extensively for hydrogen storage. In this study, it was aimed to develop and compare novel Pd/carbon, Pd/multiwalled carbon nanotube, and Pd/graphene composites for hydrogen sorption. All the palladium/carbon composites were characterized by t-plot, BJH desorption pore size distributions, N₂ adsorption/desorption isotherms, and SEM techniques. The maximum hydrogen storage of 2.25 wt.% at −196 °C was achieved for Pd/KAC composite sample. It has been observed that the spillover effect of palladium increases the hydrogen sorption capacity.     

Graphitic carbon nitride‐chitosan composites–anchored palladium nanoparticles as high‐performance catalyst for ammonia borane hydrolysis

The novel composites consisting of graphitic carbon nitride and chitosan (denoted as g‐C₃N₄‐CS) is synthesized for anchoring palladium nanoparticles. The results reveal that the resultant catalysts possess superior catalytic activity for ammonia borane (AB) hydrolysis. The corresponding turnover frequency reaches up to 27.7 at 30.0°C, and the activation energy is as low as 35.3 kJ mol^?1. Kinetics study reveals that the hydrolysis reaction is 0.50 and 0.68 orders with AB concentration and palladium concentration, respectively. In addition, the catalytic activity of the resultant Pd(0)/g‐C₃N₄‐CS catalysts is stable even after 10 runs. The result will be helpful for the development of hydrogen generation and functional materials.