Ultrafine Ni–Pt Alloy Nanoparticles Grown on Graphene as Highly Efficient Catalyst for Complete Hydrogen Generation from Hydrazine Borane

Ultrafine Ni–Pt alloy NPs grown on graphene (NiPt/graphene) have been facilely prepared via a simple one-step coreduction synthetic route and characterized by transmission electron microscopy, energy-dispresive X-ray spectroscopy, X-ray diffraction, inductively coupled plasma atomic emission spectroscopy, X-ray photoelectron spectroscopy, Raman and Fourier transform infrared methods. The characterized results showed that ultrafine Ni–Pt NPs with a small size of around 2.3 nm were monodispersed on the graphene nanosheet. Compared to the pure Ni_0.9Pt_0.1 alloy NPs, graphene supported Ni_0.9Pt_0.1 alloy NPs exhibited much higher activity and hydrogen selectivity (100%) toward conversion of hydrazine borane (HB) to hydrogen. It is first found that the durability of the catalyst can be greatly enhanced by the addition of an excess amount of NaOH in this reaction, because of the neutralization of NaOH by the byproduct H₃BO₃ produced from the hydrolysis of HB. After six cycles of the catalytic reaction, no appreciable decrease in activity was observed, indicating that the Ni_0.9Pt_0.1/graphene catalysts have good durability/stability

Synergetic Catalysis of Non-noble Bimetallic Cu–Co Nanoparticles Embedded in SiO₂ Nanospheres in Hydrolytic Dehydrogenation of Ammonia Borane

Ultrafine non-noble bimetallic Cu–Co nanoparticles (∼2 nm) encapsulated within SiO₂ nanospheres (Cu–Co@SiO₂) have been successfully synthesized via a one-pot synthetic route in a reverse micelle system and characterized by SEM, TEM, EDS, XPS, PXRD, ICP, and N₂ adsorption–desorption methods. In each core–shell Cu–Co@SiO₂ nanosphere, several Cu–Co NPs are separately embedded in SiO₂. Compared with their monometallic counterparts, the bimetallic core–shell nanospheres Cu_<i>x</i>Co_1–<i>x</i>@SiO₂ with different metal compositions show a higher catalytic performance for hydrogen generation from the hydrolysis of ammonia borane (NH₃BH₃, AB) at room temperature, due to the strain and ligand effects on the modification of the surface electronic structure and chemical properties of Cu–Co NPs in the SiO₂ nanospheres. Especially, the Cu_0.5Co_0.5@SiO₂ nanospheres show the best catalytic performance among all the synthesized Cu_<i>x</i>Co_1–<i>x</i>@SiO₂ catalysts in the hydrolytic dehydrogenation of AB. In addition, the activation energy (<i>E</i>_a) of Cu_0.5Co_0.5@SiO₂ core–shell structured nanospheres for the hydrolysis of AB is estimated to be 24 ± 2 kJ mol^–1, relatively low values among the bimetallic catalysts reported for the same reaction. Furthermore, the multi-recycle test shows that the bimetallic Cu_0.5Co_0.5@SiO₂ core–shell nanospheres are still highly active for hydrolytic dehydrogenation of AB even after 10 runs, implying a good recycling stability in the catalytic reaction

Controlled Synthesis of MOF-Encapsulated NiPt Nanoparticles toward Efficient and Complete Hydrogen Evolution from Hydrazine Borane and Hydrazine

The catalytic dehydrogenation of hydrazine borane (N₂H₄BH₃) and hydrous hydrazine (N₂H₄·H₂O) for H₂ evolution is considered as two of the pivotal reactions for the implementation of the hydrogen-based economy. A reduction rate controlled strategy is successfully applied for the encapsulating of uniform tiny NiPt alloy nanoclusters within the opening porous channels of MOFs in this work. The resultant Ni_0.9Pt_0.1/MOF core–shell composite with a low Pt content exerted exceedingly high activity and durability for complete H₂ evolution (100% hydrogen selectivity) from alkaline N₂H₄BH₃ and N₂H₄·H₂O solution. The features of small NiPt alloy NPs, strong synergistic effect between NiPt alloy NPs and the MOF, and open pore structure for freely mass transfer made NiPt/MIL-101 an excellent catalyst for highly efficient H₂ evolution from N₂H₄BH₃ or N₂H₄·H₂O

Molecular Dynamics Simulations of Hydrogen Bond Dynamics and Far-Infrared Spectra of Hydration Water Molecules around the Mixed Monolayer-Protected Au Nanoparticle

Molecular dynamics simulations have been performed to systematically investigate the structure and dynamics properties, hydrogen bond (HB) dynamics, and far-infrared (far-IR) spectra of hydration water molecules around the mixed monolayer-protected Au nanoparticles (MPANs) with different ligand compositions and length. Our simulation results demonstrate that the translational and rotational motions of hydration water molecules in the proximity of charged terminal NH₃⁺ and COO^– groups are suppressed significantly with respect to the bulk water. Compared to the bulk water, meanwhile, longer structural relaxation times of hydration H₂O–H₂O HBs indicate enhanced strength of H₂O–H₂O HBs at the interface of mixed MPANs. Accordingly, these hydration water molecules around the charged terminal groups can exhibit a considerable blue-shift in far-IR spectra for all ligand compositions and length studied here. A series of detailed HB analyses manifest that above restricted behavior of hydration water molecules can be attributed to the stronger H₂O–NH₃⁺ and H₂O–COO^– HBs and the corresponding structural relaxation times are much greater than those of hydration H₂O–H₂O HBs. Furthermore, we find that increasing ligand length can affect much the morphology of self-assemble monolayers in water owing to enhanced hydrophobic interactions between alkane chains and water molecules and favor the translational mobility of hydration water molecules owing to weaken electrostatic interactions. Unlike the translational motions, our comparison results among different ligand lengths clearly confirm that the rotational relaxation of hydration water molecules should be dominated by the local and directional HBs at the interfaces, rather than the previous explanation of the ratio between hydrophobic/hydrophilic exposed regions. More importantly, our simulations reveal at a molecular level that the ligand composition has a little influence on the structure, dynamics, HBs, and far-IR spectra of hydration water molecules around the mixed MPANs mainly due to the comparable strength between H₂O–NH₃⁺ and H₂O–COO^– HBs

Defect-Patching of Zeolite Membranes by Surface Modification Using Siloxane Polymers for CO₂ Separation

Grain boundary defects are normally formed in zeolite membranes during membrane preparation and calcination processes. In this work, a siloxane polymer coating with an imidazole group was grafted on the surface of defective SSZ-13 membranes by chemical liquid deposition to seal the defects. The parameters, such as silanization time, polymerization time, monomer type, and concentration, were optimized. Characterizations including Fourier transform infrared spectroscopy, field-emission scanning electron microscopy, and energy-dispersive X-ray spectroscopy showed that siloxane polymers were coated on the surfaces of SSZ-13 crystals and membrane. Six modified membranes showed decreased CO₂ permeance by only 21 ± 5% [average CO₂ permeance of 1.9 × 10^–7 mol/(m² s Pa)] and increased CO₂/CH₄ selectivity by a factor of 9 ± 3 (average CO₂/CH₄ selectivity of 108) for an equimolar CO₂/CH₄ mixture at 298 K. CO₂/CH₄ and CO₂/N₂ selectivities of the modified membrane decreased with pressure and temperature. Membrane stability was investigated by a long-time test and exposures to water vapor at temperatures up to 378 K and to some organic solutions. This modification method is also effective in sealing the defects of other zeolite membranes, such as AlPO-18 membranes

Efficient Photocatalytic Hydrogen Evolution on Band Structure Tuned Polytriazine/Heptazine Based Carbon Nitride Heterojunctions with Ordered Needle-like Morphology Achieved by an In Situ Molten Salt Method

Polymeric carbon nitride (CN) is a fascinating metal-free photocatalyst for active solar energy conversion via water splitting. However, the photocatalytic activity of CN is significantly restricted by the intrinsic drawbacks of fast charge recombination because of incomplete polymerization. Herein, an in situ ionothermal molten salt strategy has been developed to construct polytriazine/heptazine based CN isotype heterojunctions from low cost and earth-abundant urea as the single-source precursor, with the purpose of greatly promoting the charge transfer and separation. The engineering of crystallinity and phase structure of CN has been attempted through facile tailoring of the condensation conditions in a molten salt medium. Increasing the synthetic temperature and eutectic salts/urea molar ratio leads to the formation of CN from bulk heptazine phase to crystalline polytriazine imide (PTI) phase, while CN isotype heterojunctions are in situ created at moderate synthetic temperature and salt amount. As evidenced by the measurements of UV–vis DRS and Mott–Schottky plots, the conduction band potentials can be tuned in a wide range from −1.51 to −0.96 V by controlling the synthetic temperature and salt amount, and the apparent band gap energies are reduced accordingly. The difference in band positions between PTI and heptazine phase CN enables the formation of CN heterojunctions, greatly promoting the separation of charge carriers. These metal-free CN heterojunctions demonstrate a well ordered needle-like morphology, and the optimal sample yields a remarkable hydrogen evolution rate (4813.2 μmol h^–1 g^–1), improved by a factor of 12 over that of bulk heptazine-based CN and a factor of 4 over that of PTI. The enhanced photocatalytic performance can be directly ascribed to the synergistic effect of the improved crystallinity with reduced structural defects, the decreased band gap energy with tunable band positions, and the efficient separation of charge carriers induced by the formation of heterostructures

Molecular-Level Understanding of Solvation Structures and Vibrational Spectra of an Ethylammonium Nitrate Ionic Liquid around Single-Walled Carbon Nanotubes

Molecular dynamics simulations have been performed to explore the solvation structures and vibrational spectra of an ethylammonium nitrate (EAN) ionic liquid (IL) around various single-walled carbon nanotubes (SWNTs). Our simulation results demonstrate that both cations and anions show a cylindrical double-shell solvation structure around the SWNTs regardless of the nanotube diameter. In the first solvation shell, the CH₃ groups of cations are found to be closer to the SWNT surface than the NH₃⁺ groups because of the solvophobic nature of the CH₃ groups, while the NO₃^– anions tend to lean on the nanotube surface, with three O atoms facing the bulk EAN. On the other hand, the intensities of both C–H (the CH₃ group of the cation) and N–O (anion) asymmetric stretching bands at the EAN/SWNT interface are found to be slightly higher than the corresponding bulk values owing to the accumulation and orientation of cations and anions in the first solvation shell. More interestingly, the N–O stretching band exhibits a red shift of around 10 cm^–1 with respect to the bulk value, which is quite contrary to the blue shift of the O–H stretching band of water molecules at the hydrophobic interfaces. Such a red shift of the N–O stretching mode can be attributed to the enhanced hydrogen bonds (HBs) of the NO₃^– anions in the first solvation shell. Our simulation results provide a molecular-level understanding of the interfacial vibrational spectra of an EAN IL on the SWNT surface and their connection with the relevant solvation structures and interfacial HBs

Concentration-Dependent Hydrogen Bond Behavior of Ethylammonium Nitrate Protic Ionic Liquid–Water Mixtures Explored by Molecular Dynamics Simulations

The detailed hydrogen bond (HB) behavior of ethylammonium nitrate (EAN) ionic liquid (IL)–water mixtures with different water concentrations has been investigated at a molecular level by using classical molecular dynamics simulations. The simulation results demonstrate that the increasing water concentration can weaken considerably all cation–anion, cation–water, anion–water, and water–water HBs in EAN–water mixtures, and the corresponding HB networks around cations, anions, and water molecules also change significantly with the addition of water. Meanwhile, both the translational and the rotational motions of anions, cations, and water molecules are found to be much faster as the water concentration increases. On the other hand, the order of their HB strength is found to be cation–anion > anion–water > cation–water > water–water at low water mole fractions (<38%), while the corresponding order is cation–anion > cation–water > anion–water > water–water at high water mole fractions (>38%). The opposite orders of anion–water and cation–water HBs at low and high water concentrations, as well as the different changes of HB networks around cations and anions, should be responsible for the increasing deviation in diffusion coefficient between cations and anions with the water concentration, which is favorable to the cation–anion dissociation. In addition, the competing effect between ionic mobility and ionic concentration leads to that the ionic conductivity of EAN–water mixtures initially increases with the water mole fraction and follows a sharp decrease beyond 90%. Our simulation results provide a molecular-level concentration-dependent HB networks and dynamics, as well as their relationship with unique structures and dynamics in protic IL–water mixtures

Molecular Dynamics Simulations for Loading-Dependent Diffusion of CO₂, SO₂, CH₄, and Their Binary Mixtures in ZIF-10: The Role of Hydrogen Bond

The loading-dependent diffusion behavior of CH₄, CO₂, SO₂, and their binary mixtures in ZIF-10 has been investigated in detail by using classical molecular dynamics simulations. Our simulation results demonstrate that the self-diffusion coefficient <i>D</i>_<i>i</i> of CH₄ molecules decreases sharply and monotonically with the loading while those of both CO₂ and SO₂ molecules initially display a slight increase at low uptakes and follow a slow decrease at high uptakes. Accordingly, the interaction energies between CH₄ molecules and ZIF-10 remain nearly constant regardless of the loading due to the absence of hydrogen bonds (HBs), while the interaction energies between CO₂ (or SO₂) and ZIF-10 decease rapidly with the loading, especially at small amounts of gas molecules. Such different loading-dependent diffusion and interaction mechanisms can be attributed to the relevant HB behavior between gas molecules and ZIF-10. At low loadings, both the number and strength of HBs between CO₂ (or SO₂) molecules and ZIF-10 decrease obviously as the loading increases, which is responsible for the slight increase of their diffusion coefficients. However, at high loadings, their HB strength increases with the loading. Similar loading-dependent phenomena of diffusion, interaction, and HB behavior can be observed for CH_4, CO₂, and SO₂ binary mixtures in ZIF-10, only associated with some HB competition between CO₂ and SO₂ molecules in the case of the CO₂/SO₂ mixture

Vertical Graphene-Supported High-Hydrogen Permeance ZIF‑8 Membranes

The deployment of green hydrogen energy plays a pivotal role in propelling sustainable development and achieving carbon neutrality. Separating H2 and CH4 is a crucial step in industrial hydrogen purification. Metal–organic framework (MOF) membranes offer vast prospects for applications in gas separation. Breaking the “trade-off” between permeance and selectivity has consistently remained a primary challenge in the realm of separation membranes. In this work, highly permeable H2 separation ZIF-8 membranes were fabricated on a vertical graphene (VG)-modified α-Al2O3 support (VG@α-Al2O3), and the VG layer can afford active sites for synthesizing ZIF-8 membranes and provide more gas transport path to reduce the mass transfer resistance. With the aid of O2 plasma in improving the hydrophilicity of the VG layer, nano-ZIF-8 crystals can be synthesized on the VG to act as seeds (ZIF-8@VG@α-Al2O3) for membrane synthesis, and green synthesis membranes were realized in aqueous solution. At 90 °C for 12 h, about 900 nm-thick membrane layers were synthesized, with a high H2 permeance of 1.8 × 10–6 mol·m–2·s–1·Pa–1 and H2/CH4 separation factor of 9.6. After the synthesis period was increased to 24 h, the denser ZIF-8 membrane resulted in a higher H2/CH4 selectivity (17.2). In contrast to the MOF membranes described in previous studies, satisfactory hydrogen permeance of ZIF-8 membranes can be achieved while maintaining high selectivity