High pressure palladium membrane reactor for the high temperature water-gas shift reaction

The water-gas shift (WGS) catalytic membrane reactor (CMR) incorporating a composite Pd-membrane and operating at elevated temperatures and pressures can greatly contribute to the efficiency enhancement of several methods of H₂ production and green power generation. To this end, mixed gas permeation experiments and WGS CMR experiments have been conducted with a porous Inconel supported, electroless plated Pd-membrane to better understand the functioning and capabilities of those processes. Binary mixtures of H₂/He, H₂/CO₂, and a ternary mixture of H₂, CO₂ and CO were separated by the composite membrane at 350, 400, and 450 °C, 14.4 bar (P_tube = 1 bar), and space velocities up to 45,000 h⁻¹. H₂ permeation inhibition caused by reversible surface binding was observed due to the presence of both CO and CO₂ in the mixtures and membrane inhibition coefficients were estimated. Furthermore, WGS CMR experiments were conducted with a CO and steam feed at 14.4 bar (P_tube = 1 bar), H₂O/CO ratios of 1.1-2.6, and GHSVs of up to 2900 h⁻¹, considering the effect of the H₂O/CO ratio as well as temperature on the reactor performance. Experiments were also conducted with a simulated syngas feed at 14.0 bar (P_tube  = 1 bar), and 400-450 °C, assessing the effect of the space velocity on the reactor performance. A maximum CO conversion of 98.2% was achieved with a H₂ recovery of 81.2% at 450 °C. An optimal operating temperature for high CO conversion was identified at approximately 450 °C, and high CO conversion and H₂ recovery were achieved at 450 °C with high throughput, made possible by the 14.4 bar reaction pressure.     

Enhanced dehydrogenation and rehydrogenation properties of LiBH4 catalyzed by graphene

The investigation of thermally induced dehydrogenation of LiBH₄ reveals that LiBH₄ doped with the graphene catalysts shows superior dehydrogenation and rehydrogenation performance to that of Vulcan XC-72, carbon nanotube and BP2000 doped LiBH₄. For doping with 20 wt.% graphene, thermal dehydrogenation of LiBH₄ is found to start at ca. 230 °C and a total weight loss of 11.4 wt.% can be obtained below 700 °C. With increased loading of graphene within a LiBH₄ sample, the onset dehydrogenation temperature and the two main desorption peaks from LiBH₄ are found to decrease while the hydrogen release amount is found to increase. Moreover, variation of the equilibrium pressure obtained from isotherms measured at 350–450 °C indicate the dehydrogenation enthalpy is reduced from 74 kJ mol⁻¹ H₂ for pure LiBH₄ to ca. 40 kJ mol⁻¹ H₂ for 20 wt.% graphene doped LiBH₄. Importantly, the reversible dehydrogenation/rehydrogenation process was achieved under 3 MPa H₂ at 400 °C for 10 h, with a capacity of ca. 4.0 wt.% in the tenth cycle. Especially, LiBH₄ is reformed and new species, Li₂B₁₀H₁₀, is detected after the rehydrogenation process.     

Reversible hydrogen storage in a 3NaBH4/YF3 composite

A 3NaBH₄/YF₃ hydrogen storage composite was prepared through ball milling and its hydrogen sorption properties were investigated. It is shown that NaBH₄ does not react with YF₃ during ball milling. The dehydrogenation of the composite starts at 423 °C, which is about 100 °C lower than the dehydrogenation temperature of pure NaBH₄, with a mass loss of 4.12 wt%. Pressure–Composition–Temperature tests reveal that the composite has reversible hydrogen sorption performance in the temperature range from 350 °C to 413 °C and under quite low hydrogenation plateau pressures (<1 MPa). Its maximum hydrogen storage capacity can reach up to 3.52 wt%. The dehydrogenated composite can absorb 3.2 wt% of hydrogen within 5 min at 400 °C. Based on the Pressure–Composition–Temperature analyses, the hydrogenation enthalpy of the composite is determined to be −46.05 kJ/mol H₂, while the dehydrogenation enthalpy is 176.76 kJ/mol H₂. The mechanism of reversible hydrogen sorption in the composite involves the decomposition and regeneration of NaBH₄ through the reaction with YF₃. Therefore, the addition of the YF₃ to NaBH₄ as a reagent forms a reversible hydrogen storage composite.     

Hydrogen storage in carbon fibers activated with supercritical CO2: Models and the importance of porosity

In this work we studied the adsorption of H₂ at 77 K and 0.0–0.12 MPa onto carbon fibers activated with supercritical CO₂ (ACFs) and with different burn-offs (10–53%). The highest amount of H₂ stored was 2.45 wt% in an ACF with a burn-off of 51% at 0.12 MPa. The measured isotherms were analyzed using an equilibrium model derived by analogy with a multiple-site Langmuir-type adsorption model. The different equilibria correspond to adsorption in pores of different sizes. The experimental results fitted a model with two different adsorption sites satisfactorily, allowing such sites to be related to the microporous structure of the ACFs. Thus, a high-energy adsorbent–adsorbate interaction site, associated with very small micropores, accessible only to very small molecules such as H₂, and another lower-energy site associated with larger pores can be proposed. The model also predicts the adsorption behavior under equilibrium conditions at higher pressures, allowing the maximum adsorption capacity of the ACFs to be determined. The results show that the ACFs adsorb most of the H₂ molecules at low equilibrium pressures, and that they become almost saturated at pressures around 1.0 MPa. The maximum H₂ storage capacity in these ACFs lies between 1.50 and 3.15 wt%.     

Improvement in the hydrogenation-dehydrogenation performance of a Mg–Al alloy by graphene supported Ni

Mg-based materials are very promising candidates for hydrogen storage. In this paper, the graphene supported Ni was introduced to the Mg₉₀Al₁₀ system by hydrogenation synthesis (HS) and mechanical milling (MM). The 80 wt%Ni@Gn catalyst was synthesized by a facile chemical reduction method. The microstructures of the catalyst and composite show that Ni nanoparticles are well supported on the surface of graphene and they are dispersed uniformly on the surface of MgH₂ particles. After heating to 450 °C and holding at 340 °C for 2 h subsequently under 2.0 MPa hydrogen pressure, all the samples are almost completely hydrogenated. According to the temperature programmed desorption test, the Mg₉₀Al₁₀-8(80 wt%Ni@Gn) composite could desorb 5.85 wt% H₂ which comes up to 96% of the theoretical hydrogen storage capacity. Moreover, it shows the optimal hydriding/dehydriding performance, absorbing 5.11 wt% hydrogen within 400 s at 523 K, and desorbing 5.81 wt% hydrogen within 1800 s at 573 K.     

Nickel on a macro-mesoporous Al2O3@ZrO2 core/shell nanocomposite as a novel catalyst for CO methanation

A novel nickel catalyst supported on Al₂O₃@ZrO₂ core/shell nanocomposites was prepared by the impregnation method. The core/shell nanocomposites were synthesized by depositing zirconium species on boehmite nanofibres. This contribution aims to study the effects of the pore structure of supports and the zirconia dispersed on the surface of the alumina nanofibres on the CO methanation. The catalysts and supports were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), H₂ temperature-programmed reduction (H₂-TPR), nitrogen adsorption–desorption, and thermogravimetry and differential thermal analysis (TG-DTA). The catalytic performance of the catalysts for CO methanation was investigated at a temperature range from 300 °C to 500 °C. The results of the characterization indicate that the metastable tetragonal zirconia could be stably and evenly dispersed on the surface of alumina nanofibres. The interlaced nanorods of the Al₂O₃@ZrO₂ core/shell nanocomposites resulted in a macropore structure and the spaces between the zirconia nanoparticles dispersed on the alumina nanofibres formed most of the mesopores. Zirconia on the surface of the support promoted the dispersion and influenced the reduction states of the nickel species on the support, so it prevented the nickel species from sintering as well as from forming a spinel phase with alumina at high temperatures, and thus reduced the carbon deposition during the reaction. With the increase of the zirconia content in the catalyst, the catalytic performance for the CO methanation was enhanced. The Ni/Al₂O₃@ZrO₂-15 exhibited the highest CO conversion and methane selectivity at 400 °C, but they decreased dramatically above or below 400 °C due to the temperature sensitivity of the catalyst. Ni/Al₂O₃@ZrO₂-30 exhibited a high and constant rate of methane formation between 350 °C and 450 °C. The excellent catalytic performance of this catalyst is attributed to its reasonable pore structure and good dispersion of zirconia on the support. This catalyst has great potential to be further studied for the future industrial use.     

Characterizations of composite NaNH2-NaBH4 hydrogen storage materials synthesized via ball milling

Composite NaNH₂-NaBH₄ (molar ratio of 2/1) hydrogen storage materials are prepared by a ball milling method with various ball milling times. The compositions and hydrogen generation characteristics are investigated by means of X-ray diffraction (XRD) and thermo gravimetric-differential thermal analysis (TG-DTA). The structural characteristics imply that ball milling produces a new phase of Na₃(NH₂)₂BH₄, and mechanical energy accumulated in the ball milling process may be responsible for the phase change of Na₃(NH₂)₂BH₄. TG-DTA demonstrates that the phase change temperature of the composite NaNH₂-NaBH₄ (2/1) ball milled for 16 h is 141.8 °C, and the melting point is 197.3 °C; below 400 °C, composite hydrogen storage material is mainly decomposed to give hydrogen and Na₃BN₂; while above 400 °C, the previous by-product Na₃BN₂ continues to decompose so as to give metal Na gradually.     

Rehydrogenation and cycle studies of LiBH4–CaH2 composite

Rehydrogenation behavior of 6LiBH₄ + CaH₂ composite with NbF₅ has been studied between 350 and 500 °C after dehydrogenation at 450 °C. The composite exhibits the best rehydrogenation feature at 450 °C in terms of the overall rehydrogenation rate and the amount of absorbed hydrogen. It is found that about 9 wt% hydrogen is absorbed at 450 °C for 12 h. Up to 10 dehydrogenation–hydrogenation cycles have been carried out for the composite. It is demonstrated that 6LiBH₄ + CaH₂ with 15 wt% NbF₅ maintains a reversible hydrogen storage capacity of about 6 wt% at 450 °C after a slight degradation between the 1st and 5th cycles. The addition of NbF₅ seems to improve the cycle properties by retarding microstructural coarsening during cycles.     

Thermochemical recycling of hydrolyzed NaBH4. Part II: Systematical study of parameters dependencies

This paper focuses on the yields of both main product NaBH₄ and byproduct MgH₂ of the thermochemical process. The influence of parameters such as i) the isothermal reaction temperature in the range 480 °C–660 °C, ii) the stoichiometric ratio of solid reactants NaBO₂:Mg prepared from 1:2 to 1:8, iii) H₂ pressure supplied from 2 to 31 bars and iv) the reaction time kept at isotherm from 0 to 16 h have been systematically investigated. The yields are estimated by in-situ and ex-situ evaluations. Two temperature regimes for MgH₂ and NaBH₄ formation are recognized from 370 °C to 450 °C and above 500 °C respectively. With regard to NaBH₄ regeneration, temperature is the most important factor that positively accelerates the apparent reaction rate between 500 °C and 650 °C providing a sufficient H₂ pressure. To efficiently obtain high NaBH₄ yield mixtures with molar stoichiometric ratio between solid reactants not less than 1:4 is suggested. Experimental results also reveal that at 12 bars of H₂ pressure high NaBH₄ yield is obtained. Hence, more efficient way to improve mass transfer of solid reactants (e.g. advance reactor enhances mobility of reactants) rather than increasing H₂ pressures is advised. Under optimized condition, 100% conversion of NaBO₂ can be achieved within 1.5 h.     

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.     

Pseudo catalytic ammonia synthesis by lithium–tin alloy

In this work, nitrogenation, ammonia generation, regeneration reactions of lithium-tin alloy is investigated as pseudo catalytic process of ammonia synthesis. Li₁₇Sn₄ synthesized by thermochemical method at 500 °C can react with 0.1 MPa of N₂ below 400 °C. Nano or amorphous lithium nitride would be formed by the nitrogenation. By reaction of the nitrogenated samples and H₂, ammonia is generated at 300 °C under 0.1 MPa. The initial alloy phase Li₁₇Sn₄ is regenerated below 350 °C from the products after the ammonia generation process. Based on the above three step process, ammonia can be pseudo-catalytically synthesized from N₂ and H₂ below 400 °C under ambient pressure. Furthermore, the reactivity for the ammonia synthesis using Li–Sn alloy is preserved during the NH₃ synthesis cycles due to the characteristic reaction process based on the Li extraction and insertion.     

Inhibition effect of CO on hydrogen permeability of Pd–Ag membrane applied in a microchannel module configuration

The surface adsorption effect of CO on the hydrogen permeability of a 12.5 micron-thick Pd₇₇Ag₂₃ membrane has been evaluated quantitatively under experimental conditions close to the operating conditions of the highly-efficient membrane reformer (MRF) system developed by Tokyo Gas. The permeability of the membrane was measured in the conditions of CO concentration between 1 and 5 vol.% at a temperature and pressure of up to 500 °C and 0.6 MPa, respectively. High feed flow rates and a microchannel module configuration were applied in the flux measurements to ensure that the results are obtained with limited influence of concentration polarization adjacent to the membrane surface and hydrogen depletion along the microchannel length. While the CO inhibition effect was close to negligible at 500 °C, it was significant at lower temperatures. At a feed pressure of 0.2 MPa, the CO inhibition effect was only 0.2% at a CO concentration of 1 vol.% and the effect was 3.6% at a CO concentration of 5 vol.% at 500 °C. The CO inhibition effect were 3.4% for 1 vol.% CO and 14.1% for 5 vol.% CO at 400 °C. Measurements were also carried out at a high feed pressure of 0.6 MPa to evaluate the pressure dependence of the CO inhibition effect. The CO inhibition effect decreased to 0.7% at a CO feed concentration of 5 vol.% at 500 °C. Lower CO inhibition effect were also observed at 450 and 400 °C compared to the data obtained with the feed pressure of 0.2 MPa, while the inhibition levels were almost the same at 350 °C. Though the CO inhibition effect is larger at a lower feed pressure of 0.2 MPa, the effect was only 0.2% at 1 vol.% CO at 500 °C, which is close to the operating conditions of the MRF system. This study quantitatively revealed that the CO inhibition effect on hydrogen flux is extremely small when the membrane is operated at temperatures equal to or higher than 500 °C, even for state-of-the-art thin membranes. The performance of the Tokyo Gas MRF seems thus mainly limited by concentration polarization effects.     

Energy storage of thermally reduced graphene oxide

The energy-storage capacity of reduced graphene oxide (rGO) is investigated in this study. The rGO used here was prepared by thermal annealing under a nitrogen atmosphere at various temperatures (300, 400, 500 and 600 °C). We measured high-pressure H₂ isotherms at 77 K and the electrochemical performance of four rGO samples as anode materials in Li-ion batteries (LIBs). A maximum H₂ storage capacity of ∼5.0 wt% and a reversible charge/discharge capacity of 1220 mAh/g at a current density of 30 mA/g were achieved with rGO annealed at 400 °C with a pore size of approximately 6.7 Å. Thus, an optimal pore size exists for hydrogen and lithium storage, which is similar to the optimum interlayer distance (6.5 Å) of graphene oxide for hydrogen storage applications.     

Co-electrospun Pd-coated porous carbon nanofibers for hydrogen storage applications

Electrospinning produces sub-micron sized continuous fibers from polymer solutions or melt by electric force. Due to its versatility and cost-effectiveness, this method has been recently adopted for the fabrication of one-dimensional materials. Here, we fabricated polyacrylonitrile (PAN) polymer fibers from which uniform nanoporous carbon fibers with diameters of 100-200 nm were obtained after carbonization at 800 °C in Ar + H₂O. Water vapor was injected during carbonization to be utilized as a nanoscale pore former. Additionally, a direct coating method using palladium nanoparticles on the carbon fibers was developed. Palladium salt solution was electrosprayed during the electrospinning of the polymer fibers. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were used to confirm surface chemical composition and degree of carbonization. The specific surface area of the palladium coated carbon fibers was 815.6 m²/g. Reversible hydrogen adsorption capacity was determined to be 0.35 wt% at 298 K, 0.1 MPa.     

A novel catalyst–sorbent system for an efficient H2 production with in-situ CO2 capture

This paper presents an experimental investigation for an improved process of sorption-enhanced steam reforming of methane in an admixture fixed bed reactor. A highly active Rh/Ce_αZr_1−αO₂ catalyst and K₂CO₃-promoted hydrotalcite are utilized as novel catalyst/sorbent materials for an efficient H₂ production with in situ CO₂ capture at low temperature (450–500 °C). The process performance is demonstrated in response to temperature (400–500 °C), pressure (1.5–6.0 bar), and steam/carbon ratio (3–6). Thus, direct production of high H₂ purity and fuel conversion >99% is achieved with low level of carbon oxides impurities (<100 ppm). A maximum enhancement of 162% in CH₄ conversion is obtained at a temperature of 450 °C and a pressure of 6 bar using a steam/carbon molar ratio of 4. The high catalyst activity of Rh yields an enhanced CH₄ conversion using much lower catalyst/sorbent bed composition and much smaller reactor size than Ni-based sorption enhanced processes at low temperature. The cyclic stability of the process is demonstrated over a series of 30 sorption/desorption cycles. The sorbent exhibited a stable performance in terms of the CO₂ working sorption capacity and the corresponding CH₄ conversion obtained in the sorption enhanced process. The process showed a good thermal stability in the temperature range of 400–500 °C. The effects of the sorbent regeneration time and the purge stream humidity on the achieved CH₄ conversion are also studied. Using steam purge is beneficial for high degree of CO₂ recovery from the sorbent.     

Pd–Ag thin film membrane for H2 separation. Part 2. Carbon and oxygen diffusion in the presence of CO/CO2 in the feed and effect on the H2 permeability

The effect of CO and CO₂ on the performance and stability of Pd–Ag thin film membranes prepared by electroless plating deposition (EPD) was investigated, observing the presence of dissociation to carbon and oxygen which slowly diffuse in the membrane influencing also H₂ permeability. The effect of the two carbon oxides was investigated both separately and combined in the 400–450 °C temperature range over long-term cumulative experiments (up to over 350 h) on a membrane that already worked for over 350 h in H₂ or H₂–N₂ mixtures. An increase of the H₂ permeation flux was observed feeding only CO₂ in the range 10–20%. This effect was interpreted as deriving from the facilitated H₂ flux caused from oxygen diffusion (deriving from CO₂ dissociation) in the membrane. CO induces instead a partial inhibition on the H₂ flux deriving from the negative effect of CO competitive chemisorption as well as C diffusion in the membrane, which overcome the positive effect associated to oxygen diffusion in the membrane. Carbon and oxygen diffuse through the membrane with a rate two order of magnitude lower than hydrogen, and recombinate at the permeate side forming CO, CO₂ and CH₄ which amount increases with time-on-stream. The effect is reversible and not associated with the creation of cracks or defects in the membrane, as supported by leak tests.     

Different effects of temperature on supramolecular protein and non-protein materials in hydrogen storage

The storage of large quantities of hydrogen at ambient temperature is a key factor in establishing a hydrogen-based economy. One strategy for hydrogen storage is to exploit the interaction between H₂ and a solid material by physisorption of hydrogen on porous materials. However, physisorption materials containing MOF, porous carbons, zeolites, clathrates, and synthesized organic polymers physisorb only about 1 wt% of H₂ at ambient temperature. One approach to solving this problem is to prepare new classes of physisorption materials which exhibits a mechanism different from the reported materials in hydrogen storage. Here we report the synthesis of apo cross-linked ferritin supramolecules by disulfide bonds, and their holo form. Unlike non-protein adsorbents, the hydrogen storage capacity of these protein materials increases as a function of temperature over the range of 20–40 °C. The holo supramolecules enable the adsorption of hydrogen up to 3.51 wt% at 40 °C and 40 bar H₂. In contrast, non-protein physisorption materials such as activated carbon and nano Fe₂O₃ marginally adsorb hydrogen, and, as reported, their ability to adsorb hydrogen decreases with increasing temperature under the same experimental condition. These results demonstrate that protein materials have a unique hydrogen storage mechanism which offers new opportunities in exploration of physisorption materials at ambient temperature.     

Light and stable LinB14(n=1–5) clusters for high capacity hydrogen storage at room temperature: A DFT study

In this article, we have explored the hydrogen (H₂) storage capacity of the Li doped B clusters Li_nB₁₄(n = 1–5) using density functional theory (DFT). The geometrical and Bader's topological parameters indicate that the clusters adsorb H₂ in the molecular form. The Li atom polarises the H₂ molecules for their effective adsorption on the clusters. The Li_nB₁₄ (n = 1–5) clusters are found to be stable even after H₂ adsorption at room temperature. The average adsorption energy is found to be in the range of 0.12–0.14 eV/H₂. Among the various clusters, the Li₅B₁₄ shows maximum H₂ storage capacity (13.89 wt%) at room temperature. The ADMP simulation reveals that within few femtoseconds (fs), the H₂ molecules begin to move away from the clusters and within 400 fs most of the H₂ molecules moved away from the clusters.     

High capacity potassium-promoted hydrotalcite for CO2 capture in H2 production

This paper presents an experimental study for a newly modified K₂CO₃-promoted hydrotalcite material as a novel high capacity sorbent for in-situ CO₂ capture. The sorbent is employed in the sorption enhanced steam reforming process for an efficient H₂ production at low temperature (400–500 ^°C). A new set of adsorption data is reported for CO₂ adsorption over K-hydrotalcite at 400 °C. The equilibrium sorption data obtained from a column apparatus can be adequately described by a Freundlich isotherm. The sorbent shows fast adsorption rates and attains a relatively high sorption capacity of 0.95 mol/kg on the fresh sorbent. CO₂ desorption experiments are conducted to examine the effect of humidity content in the gas purge and the regeneration time on CO₂ desorption rates. A large portion of CO₂ is easily recovered in the first few minutes of a desorption cycle due to a fast desorption step, which is associated with a physi/chemisorption step on the monolayer surface of the fresh sorbent. The complete recovery of CO₂ was then achieved in a slower desorption step associated with a reversible chemisorption in a multi-layer surface of the sorbent. The sorbent shows a loss of 8% of its fresh capacity due to an irreversible chemisorption, however, it preserves a stable working capacity of about 0.89 mol/kg, suggesting a reversible chemisorption process. The sorbent also presents a good cyclic thermal stability in the temperature range of 400–500 °C.     

Hydrogen storage properties of nanostructured MgH2/TiH2 composite prepared by ball milling under high hydrogen pressure

Nanostructured MgH₂/0.1TiH₂ composite was synthesized directly from Mg and Ti metal by ball milling under an initial hydrogen pressure of 30 MPa. The synthesized composite shows interesting hydrogen storage properties. The desorption temperature is more than 100 °C lower compared to commercial MgH₂ from TG-DSC measurements. After desorption, the composite sample absorbs hydrogen at 100 °C to a capacity of 4 mass% in 4 h and may even absorb hydrogen at 40 °C. The improved properties are due to the catalyst and nanostructure introduced during high pressure ball milling. From the PCI results at 269, 280, 289 and 301 °C, the enthalpy change and entropy change during the desorption can be determined according to the van’t Hoff equation. The values for the MgH₂/0.1TiH₂ nano-composite system are 77.4 kJ mol⁻¹ H₂ and 137.5 J K⁻¹ mol⁻¹ H₂, respectively. These values are in agreement with those obtained for a commercial MgH₂ system measured under the same conditions. Nanostructure and catalyst may greatly improve the kinetics, but do not change the thermodynamics of the materials.