Cycling stability and adsorption mechanism at room temperature of the upscaled Ni-doped hierarchical carbon scaffold

Hydrogen adsorption performance and mechanism upon cycling of the upscaled Ni-doped hierarchical carbon scaffold (HCS) are investigated. Upon 22 hydrogen ad/desorption cycles (T = 25–50 °C and p (H₂) = 1–50 bar), the upscaled Ni-doped HCS shows excellent cycling stability with gravimetric capacity of up to 1.51 wt % H₂. This is due to mechanical stability of HCS and good distribution of Ni nanoparticles. Hydrogen adsorption mechanism of Ni-doped HCS upon cycling is experimentally and theoretically characterized. Besides dissociative adsorption onto the surface, hydrogen diffusion into the lattice structure of Ni is observed. The latter enhances with the number of ad/desorption cycles and alters the electron sharing mechanisms between Ni and H during adsorption.     

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.     

In situ deposition of Pd nanoparticles with controllable diameters in hollow carbon spheres for hydrogen storage

A novel in situ synthesis of Pd nanoparticles supported in hollow carbon spheres (HCS) is reported. The size of the nanoparticles can be tuned via application of different Pd precursors. The hydrogen storage properties of Pd supported in HCS under room temperature were examined at partial pressures. We observed significant difference between the storage capacities of two samples containing Pd nanoparticles with different diameter distributions. The results showed that the sample with suitable diameters of Pd nanoparticles was more favorable for the H₂ storage, even lower mass of Pd was used. The maximum hydrogen storage of 0.36 wt % exhibited the sample with Pd nanoparticles with the diameter of 11 nm (measured at 298 K and 24 bar) and it was enhanced by the factor of two in respect to the pristine HCS. The enhanced storage capacity is due to cumulative hydrogen adsorption by HCS and Pd nanoparticles. We also propose the mechanism of hydrogen storage in our material.     

Computational insights of alkali metal (Li / Na / K) atom decorated buckled bismuthene for hydrogen storage

The mechanism of hydrogen molecule adsorption on 2D buckled bismuthene (b-Bi) monolayer decorated with alkali metal atoms was studied using density functional theory based first principles calculations. The decorated atoms Li, Na and K exhibited distribution on surface of b-Bi monolayer with increasing binding energy of 2.6 eV, 2.9 eV and 3.6 eV respectively. The adsorption of H₂ molecule on the slabs appeared stable which was further improved upon inclusion of van der Waals interactions. The adsorption behaviour of H₂ molecules on the decorated slabs is physisorption whereas the slabs were able to bind up to five H₂ molecules. The average adsorption energy per H₂ molecules are in range of 0.1–0.2 eV which is good for practical applications. The molecular dynamics simulation also confirmed the thermodynamic stabilities of five H₂ molecules adsorbed on the decorated slabs. The storage capacity values are found 2.24 wt %, 2.1 wt %, and 2 wt %, for respective cases of Li, Na and K atoms decorated b-Bi. The analysis of the adsorbed cases pointed to electrostatic interaction of Li and H₂ molecule. The adsorption energies, binding energies, charge analysis, structural stability, density of states, and hydrogen adsorption percentage specifies that the decorated b-Bi may serve as an efficient hydrogen storage material and could be an effective medium to interact with hydrogen molecules at room temperature.     

Renewable hydrogen production by steam reforming of butanol over multiwalled carbon nanotube-supported catalysts

The conversion of bio-oxygenates into hydrogen (H₂) via catalytic steam reforming is a green approach for H₂ generation. In the present work, butanol was chosen as renewable feedstock for producing H₂. Two catalysts supported on multiwalled carbon nanotubes, Ni/CNT and Co/CNT, were synthesized by the wetness impregnation method and used for butanol reforming. Trials were performed in a fixed-bed reactor in the 623–773 K range using S/C ratio equal to 33.3 mol/mol (here, S/C denotes steam to carbon ratio). The Ni/CNT catalyst exhibited higher reforming activity. The best catalytic performance for Ni/CNT was observed at T = 773 K. At this temperature, high values of butanol conversion (87.3%) and H₂ yield (0.75 mol/mol) were observed at W/F_A0 = 16.7 g h/mol (here, W is the catalyst mass and F_A0 is the molar flow rate of butanol at the inlet). The performance of Ni/CNT catalyst for steam reforming of synthetic bio-butanol was also investigated at T = 773 K and H₂ yield of 0.65 mol/mol was achieved.     

Oxygen-promoted hydrogen adsorption on activated and hybrid carbon materials

The effect of heteroatoms on hydrogen adsorption properties of activated and hybrid carbon materials is critically described. For that purpose, olive stones were activated chemically with KOH, and subsequently washed or not, and oxidised with ozone or not. Olive stones were also activated physically with CO₂. A series of activated carbons prepared by chemical activation of sucrose was also investigated for comparison. As a result, many activated carbons with different pore-size distributions, surface areas, average micropore widths, oxygen contents and amounts of mineral matter could be compared. All were thoroughly characterised by adsorption of N₂, CO₂ and H₂O, elemental analysis, XPS, thermogravimetry, and adsorption of H₂ at different pressures. Many correlations between textural parameters, composition and adsorption properties could be evidenced, and were critically discussed. We show that the hydrogen uptake at 77 K is controlled by the following parameters, listed by decreasing order of importance: specific surface area, average micropore size, surface chemistry and shape of the pore size distribution. At room temperature (i.e., at 298 K), the adsorbed hydrogen uptake was in the range of 0.19–0.42 wt %; the presence of large amounts of alkali metals can further improve the hydrogen adsorption properties, but surface chemistry still has a major influence, especially through the acidic surface functions.     

The synthesized nickel-doped multi-walled carbon nanotubes for hydrogen storage under moderate pressures

Hydrogen adsorption capacity of Multiwalled carbon nanotubes (MWCNTs) decorated with Nickel (Ni) nanoparticles has been presented at room temperature and under moderate pressures of 4–20 bar. The functionalization of carbon nanotubes was carried by H₂SO₄-HNO₃ reducing agents and the Ni supported MWCNTs (Ni-MWCNTs) were prepared by wet chemical method. The structure and morphology characterization of samples were performed by XRD, TEM, EDX and SEM analyses. These nanotubes then subjected to hydrogenation step by using Sievert's-like apparatus. The hydrogenation of the Ni-MWCNTs was performed at 298 K and moderate hydrogen pressures of 4–20 bar. The obtained results show that there is a correlation between hydrogen storage capacity and hydrogen pressure that; as the pressure was increased, hydrogen uptake capacity enhanced due to physisorption. In addition, maximum hydrogen storage capacity of Ni-MWCNTs was found to be 0.298 wt % at room temperature and under pressure of 20 bar.     

Sorption-enhanced reaction process for glycerol-to-hydrogen conversion over cobalt catalyst supported on promoted hydrotalcites

New bi-functional materials comprising the reforming catalyst, cobalt, and the CO₂-sorbent, hydrotalcite were used to produce pure hydrogen (H₂) from sorption-enhanced steam glycerol reforming (SESGR). Three promoters, calcium, copper and zinc, were used for modifying the properties of hydrotalcites. All materials were characterized using X-ray diffraction, nitrogen physisorption and electron microscopy techniques. They were found to be very proficient for glycerol-to-H₂ conversion in a fixed-bed reactor, even at low temperature (623–823 K). Copper-promoted materials were especially promising, due to longest duration of the pre-breakthrough stage (40 min) and highest H₂ content of the reformed gas (93.1%) at T = 823 K. Besides, their sorption capacity was the highest (1.1 mol CO₂/kg sorbent) at T = 823 K. The effects of temperature, steam-to-carbon ratio in feed (S/C ratio) and gas hourly space velocity (GHSV) on the SESGR process were investigated. Durability tests over 20 cycles of adsorption and regeneration showed that materials promoted with calcium, copper and zinc were stable up to 8 (at 773 K), 11 and 5 cycles (at 823 K) correspondingly. The role of cobalt metal and cationic hydrotalcite promoters in the reforming pathway was elucidated. This insightful study will assist in improved H₂ production from renewably producible glycerol.     

A porous 3d-4f heterometallic metal–organic framework for hydrogen storage

A heterometallic metal–organic framework, {[Ce(oda)₃Zn_1.5(H₂O)₃]·0.75H₂O}_n (1, H₂oda = oxydiacetic acid), has been synthesized under hydrothermal condition. The single-crystal X-ray diffraction analysis reveals that compound 1 belongs to hexagonal crystal system with space group P6/mcc and exhibits 3D porous framework. The hydrogen adsorption experiments suggest that 1 possesses reversible hydrogen storage capacity, up to 1.34 wt.% at 77 K and 0.86 wt.% at 298 K, respectively.     

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.     

Activated carbons doped with Pd nanoparticles for hydrogen storage

Three activated carbons (ACs) having apparent surface areas ranging from 2450 to 3200 m²/g were doped with Pd nanoparticles at different levels within the range 1.3–10.0 wt.%. Excess hydrogen storage capacities were measured at 77 and 298 K at pressures up to 8 MPa. We show that hydrogen storage at 298 K depends on Pd content at pressures up to 2–3 MPa, below which the stored amount is low (<0.2 wt.%). At higher pressures, the micropore volume controls H₂ storage capacity. At 77 K, Pd doping has a negative effect on hydrogen storage whatever the pressure considered. From N₂ adsorption at 77 K, TPR, XRD, TEM, and H₂ chemisorption studies, we concluded that: (i) Pd particles remained mainly decorating the outer surface of the ACs; (ii) increasing Pd content produced an increase of the metal particle size; (iii) ACs with higher surface area produced smaller metallic nanoparticles at a given Pd content.     

Melamine assisted preparation of nitrogen doped activated carbon from sustainable biomass for H2 and CO2 storage

Activated carbon (AC), as an effective solid adsorbent, is extensively employed in H₂ and CO₂ storage. To enhance its adsorption capability and selectivity, it is necessary to increase its surface area and dope heteroatoms by a simple and environment-friendly method. In this work, nitrogen doped activated carbon (NAC) has been synthesized from sustainable biomass by direct activation with the assistance of melamine. The obtained NAC with 2.1 wt% N dopants possesses a high surface area (2477.27 m²/g) and pore volume (1.93 cm³/g). The NAC displayed enhanced H₂ uptake capacity (2.29 wt% at 77 K, 1 bar and 0.83 wt% at 298 K, 100 bar) and adequate CO₂ uptake capacity (2.85 mmol/g at 298 K, 1 bar and 4.49 mmol/g at 273 K, 1 bar). Activation mechanism with the assistance of melamine was proposed in accordance with the experimental data. The facile method of preparing NAC is potential for large-scaled production.     

Hydrogen storage in magnesium based-composite hydride through hydriding combustion synthesis

Mg and Zr-based AB₂ hydride composite was prepared by hydriding combustion synthesis (HCS) and the hydriding–dehydriding properties of HCS Mg–(20, 40 wt%)AB₂ products were extensively examined. The dehydriding onset temperatures of the HCS Mg–20AB₂ and Mg–40AB₂ composites were 533 K and 493 K, respectively, which were lower than that of the MgH₂. It is suggested that the well-dispersed Zr-based AB₂ phase in a Mg composite prepared by HCS plays a crucial role in significantly improving its kinetic properties. Especially, the HCS Mg–20AB₂ composite showed fully activated hydrogenation within the 8th cycle and reached a saturated H₂ absorption capacity of 5.7 wt.% at 573 K in 10 min. In addition, the hydrogen capacity did not show any significant decrease even after 86 cycles. These results display a potential excellence of HCS processing in preparing Mg-based hydrogen storage materials.     

Enhanced hydrogen adsorption in ordered mesoporous carbon through clathrate formation

Ordered mesoporous carbons were synthesized with a soft-template approach and modified with a water and tetrahydrofuran mixture having a H₂O/THF molar ratio of 17:1 as potential adsorbent media for hydrogen storage. Hydrogen adsorption equilibrium on the carbon adsorbents was measured gravimetrically at 270 K and hydrogen pressures up to 163 bar. Enhanced hydrogen adsorption was observed on the carbon adsorbents doped with 0.5 wt.% and 0.75 wt.% of H₂O/THF due to the combined effects of hydrogen adsorption on the carbon surface and formation of a binary H₂–H₂O–THF clathrate. Hydrogen adsorption capacities on the carbon adsorbents doped with 0.5 wt.%, 0.75 wt.% of H₂O/THF, and the pure carbon at 270 K and 163 bar are 0.747 wt.%, 0.646 wt.% and 0.585 wt.%, respectively. The hydrogen adsorption isotherms on all the doped carbon adsorbents are of typical Type III and can be well correlated by the Freundlich equation. A desorption hysteresis loop was observed on the carbon adsorbents doped with 0.5 wt.% and 0.75 wt.% of H₂O/THF, which was probably caused by the pore size difference during the adsorption and desorption steps.     

Synthesis, characterization and catalytic performances of Ce-SBA-15 supported nickel catalysts for methane dry reforming to hydrogen and syngas

A series of Ce-incorporated SBA-15 mesoporous materials were synthesized through direct hydrothermal synthesis method and further impregnated with 12 wt.% Ni. The samples were characterized by ICP-AES, XRD, N₂ physisorption, XPS, TPR, H₂ chemisorption, TGA, temperature-programmed hydrogenation (TPH) and TEM measurements. The low-angle XRD and N₂ physisorption results showed the Ce successfully incorporated into the framework of SBA-15. The catalytic properties of these catalysts were investigated in methane reforming with CO₂. The Ce/Si molar ratio had a significant influence on the catalytic performance. The highest catalytic activity and long-term stability were obtained over the Ni/Ce-SBA-15 (Ce/Si = 0.04) sample. The improved catalytic behavior could be attributed to the cerium impact in the framework of SBA-15, where cerium promoted the dispersion of nano-sized Ni species and inhibited the carbon formation. In comparison with the effect of CeO₂ crystallites in SBA-15, cerium in the framework of SBA-15 promoted the formation of the nickel metallic particles with smaller size. The XRD and TGA results exhibited that carbon deposition was responsible for activity loss of Ni/SBA-15 and Ni/Ce-SBA-15 (Ce/Si = 0.06) catalysts. TEM results showed that the hexagonal mesopores of SBA-15 were still kept intact after reaction and the pore walls of SBA-15 prevented the aggregation of nickel.     

The mechanism and sorption kinetic analysis of hydrogen storage at room temperature using acid functionalized carbon nanotubes

The present work investigates the effect of acid functionalization of multiwalled carbon nanotubes (MWCNTs) on the physisorption based mechanism of hydrogen storage at room temperature. For this purpose, a suite of functionalized CNT samples is synthesized and subjected to a comprehensive range of material characterization techniques and hydrogen storage measurements. Nitric acid (HNO₃) and the mixture of sulphuric acid and nitric acid (H₂SO₄:HNO₃) are used for the synthesis at oxidation temperatures of 80 °C and 100 °C. Electron microscopy and X-ray photoelectron spectroscopy results reveal that acid functionalization causes major alternation in the physicochemical properties of the CNTs due to the varied concentration of oxygen functional groups. Particularly, the H₂SO₄:HNO₃ functionalized sample at 100 °C is found to have the highest interlayer spacing, oxygen to carbon ratio (26.09 at. %), defect content, and specific surface area (215.3 m²/g). These features collectively contribute to substantially improved hydrogen storage properties, including a ～150% increase in the hydrogen storage capacity at 298 K and 50 bar. Furthermore, kinetic analysis shows that the desorption follows a multiple diffusion process which is sensitive to the oxygen functional groups and structural defects, hence reducing the rate of desorption; whereas the adsorption is controlled by a more rapid, three-dimensional diffusion process.     

First-principles study of H2 adsorption on the pristine and oxidized (8,0) carbon nanotube

The effect of oxygen, hydrogen, and (oxygen + hydrogen) molecules adsorption on the structural and electrical properties of (8,0) carbon nanotube (CNT) are investigated through density functional theory. The obtained results indicate endothermical chemisorption of O₂ on the nanotube surface with a large binding energy of about 598 meV and a significant charge transfer of about 0.43 e per molecule. It is discussed that the O₂ chemisorption creates hole carries in the (8,0) carbon nanotube and thus increases the work function of the system. In the case of hydrogen molecule, a weak physisorption on the surface of CNT (∼−5 meV) is identified. The adsorption of H₂ on CNT is also accompanied by hole doping and increment of the work function of the CNT, while the charge transfer between CNT and H₂ is negligible. The band offsets in the H₂-CNT junction are calculated to examine and describe the observed hole doping in this system. The effect of oxygenation of CNT on hydrogen adsorption is also investigated and the most favorable adsorption configuration is found and the related adsorption energy is calculated. It is argued that the oxygenation of CNT enhances the physisorption of hydrogen molecules. It is shown that hydrogen molecule adsorption on the oxidized CNT cancels hole doping and hence decreases the work function of the system.     

Activated carbons with appropriate micropore size distribution for hydrogen adsorption

We measured hydrogen storage on five well-known commercial carbon materials (CCMs) and we compared their performances to those obtained on our lab-made activated carbons (ACs). H₂ uptake of our lab-made ACs was always higher than that of CCMs of similar S_BET, our best AC reached 6 wt.% H₂ excess adsorption at 77 K and 4 MPa. We calculated the contribution of four ranges of pores (<0.5 nm; 0.5-0.7 nm, 0.7-2 nm and >2 nm) to the H₂ excess adsorption for the 14 carbon materials considered in this study. We clearly demonstrated that: (i) the superiority in H₂ excess adsorption of lab-made ACs over the CCMs is related to their pore size distribution; (ii) H₂ uptakes higher than 3 wt.% are due to pores with diameter wider than 0.7 nm.     

Biomass to hydrogen-rich syngas via catalytic steam reforming of bio-oil

Hydrogen-rich syngas production from the catalytic steam reforming of bio-oil from fast pyrolysis of pinewood sawdust was investigated by using La_1−xK_xMnO₃ perovskite-type catalysts. The effects of the K substitution, temperature, water to carbon molar ratio (WCMR) and bio-oil weight hourly space velocity (W_bHSV) on H₂ yield, carbon conversion and the product distribution were studied in a fixed-bed reactor. The results showed that La_1−xK_xMnO₃ perovskite-type catalysts with a K substitution of 0.2 gave the best performance and had a higher catalytic activity than the commercial Ni/ZrO₂. Both high temperature and low W_bHSV led to higher H₂ yield. However, excessive steam reduced hydrogen yield. For the La_0.8K_0.2MnO₃ catalyst, a hydrogen yield of 72.5% was obtained under the optimum operating condition (T = 800 °C, WCMR = 3 and W_bHSV = 12 h⁻¹). The deactivation of the catalysts mainly was caused by coke deposition.     

Sustainable and selective hydrogen production by steam reforming of bio-based ethylene glycol: Design and development of Ni–Cu/mixed metal oxides using M (CeO2, La2O3, ZrO2)–MgO mixed oxides

Hydrogen (H₂) production in a clean and green manner via renewable sources is at present of great interest. Ethylene glycol, a bio-based feedstock, offers a sustainable route for high purity H₂ production. In the current investigation, MgO based mixed metal oxides containing CeO₂, La₂O₃ and ZrO₂ were synthesized and used to support 20 wt% Ni–Cu (1:1). The impacts of altering support characteristics on catalytic behavior have been studied and compared in H₂ synthesis via ethylene glycol steam reforming (SR), employing various characterization techniques such as XRD, SEM, EDX, TEM, H₂-TPR, H₂-TPD, TG-DSC and BET. Further, high resolution XPS studies were performed to explore the valence states and effectiveness of surface engineering of the catalysts. Assessment of the efficacy of catalysts was done via several parameters such as reactant conversion, H₂ concentration and long-term stability. All the synthesized materials produced encouraging results with high H₂ yield and conversion under the said operating conditions [T- 623 to 773 K; GHSV - 3120 to 6240 h^?1; P - 0.1 MPa; S/C - 3 to 7.5 mol/mol]. Amongst the three catalysts, Ni–Cu/La₂O₃–MgO and Ni–Cu/CeO₂–MgO exhibited superior behavior for high H₂ production. Ni–Cu/La₂O₃–MgO was better in comparison to Ni–Cu/CeO₂–MgO in terms of reactant conversion whereas Ni–Cu/CeO₂–MgO showed highest H₂ concentration (98 mol %) and improved stability along with absence of carbon deposition owing to its high mobile oxygen vacancies in its lattice. The highly active cubic CeO₂ species and its long-term durability (up to 8 cycles) owing to its exceptional redox property further justified its efficacy. The optimized process showed that at T = 773 K, GHSV = 3120 h^?1, S/C = 4.5 mol/mol for Ni–Cu/La₂O₃–MgO and Ni–Cu/CeO₂–MgO and at T = 773 K, GHSV = 3120 h^?1, S/C = 6 mol/mol and for Ni–Cu/ZrO₂–MgO, maximum H₂ concentration was obtained. At the end, reaction pathway followed by the catalysts was proposed.