Ultra-pure hydrogen production via ammonia decomposition in a catalytic membrane reactor

In this work two alternatives are presented for increasing the purity of hydrogen produced in a membrane reactor for ammonia decomposition. It is experimentally demonstrated that either increasing the thickness of the membrane selective layer or using a small purification unit in the permeate of the membranes, ultra-pure hydrogen can be produced. Specifically, the results show that increasing the membrane thickness above 6 μm ultra-pure hydrogen can be obtained at pressures below 5 bar. A cheaper solution, however, consists in the use of an adsorption bed downstream the membrane reactor. In this way, ultra-pure hydrogen can be achieved with higher reactor pressures, lower temperatures and thinner membranes, which result in lower reactor costs. A possible process diagram is also reported showing that the regeneration of the adsorption bed can be done by exploiting the heat available in the system and thus introducing no additional heat sources.     

Tube-wall catalytic membrane reactor for hydrogen production by low-temperature ammonia decomposition

Ammonia has attracted great interest as a chemical hydrogen carrier. However, ammonia decomposition is limited kinetically rather than thermodynamically below 400 °C. We developed a tube-wall catalytic membrane reactor that could decompose ammonia with high conversion even at temperatures below 400 °C. The reactor had excellent heat transfer characteristics, and thus nearly 100% conversion for an NH3 feed of 10 mL/min at 375 °C was achieved with a 2-μm-thick palladium composite membrane, and hydrogen removal from the decomposition side resulted in a large kinetic acceleration.     

3D simulation of hydrogen production by ammonia decomposition in a catalytic membrane reactor

Ammonia decomposition in an integrated Catalytic Membrane Reactor for hydrogen production was studied by numerical simulation. The process is based on anhydrous NH₃ thermal dissociation inside a small size reactor (30 cm³), filled by a Ni/Al₂O₃ catalyst. The reaction is promoted by the presence of seven Pd coated tubular membranes about 203 mm long, with an outer diameter of 1.98 mm, which shift the NH₃ decomposition towards the products by removing hydrogen from the reaction area. The system fluid-dynamics was implemented into a 2D and 3D geometrical model. Ammonia cracking reaction over the Ni/Al₂O₃ catalyst was simulated using the Temkin-Pyzhev equation.Introductory 2D simulations were first carried out for a hypothetic system without membranes. Because of reactor axial symmetry, different operative pressures, temperatures and input flows were evaluated. These introductory results showed an excellent ammonia conversion at 550 °C and 0.2 MPa for an input flow of 1.1 mg/s, with a residual NH₃ of only a few ppm. 3D simulations were then carried out for the system with membranes. Hydrogen adsorption throughout the membranes has been modeled using the Sievert’s law for the dissociative hydrogen flux. Several runs have been carried out at 1 MPa changing the temperature between 500 °C and 600 °C to point out the conditions for which the permeated hydrogen flux is the highest. With temperatures higher than 550 °C we obtained an almost complete ammonia conversion already before the membrane area. The working temperature of 550 °C resulted to be the most suitable for the reactor geometry. A good matching between membrane permeation and ammonia decomposition was obtained for an NH₃ input flow rate of 2.8 mg/s. Ammonia reaction shift due to the presence of H₂ permeable membranes in the reactor significantly fostered the dissociation: for the 550 °C case we obtained a conversion rate improvement of almost 18%.     

Direct high-purity hydrogen production from ammonia by using a membrane reactor combining V-10mol%Fe hydrogen permeable alloy membrane with Ru/Cs2O/Pr6O11 ammonia decomposition catalyst

The hydrogen production capabilities of the membrane reactor combining V-10 mol%Fe hydrogen permeable alloy membrane with Ru/Cs₂O/Pr₆O₁₁ ammonia decomposition catalyst are studied. The ammonia conversion is improved by 1.7 times compared to the Ru/Cs₂O/Pr₆O₁₁ catalyst alone by removing the produced hydrogen through the V-10mol%Fe alloy membrane during the ammonia decomposition. 79% of the hydrogen atoms contained in the ammonia gas are extracted directly as high-purity hydrogen gas. Both the Ru/Cs₂O/Pr₆O₁₁ catalyst and the V-10 mol% Fe alloy membrane are highly durable, and the initial performance of the hydrogen separation rate lasts for more than 3000 h. The produced hydrogen gas conforms to ISO 14687–2:2019 Grade D for fuel cell vehicles because the ammonia and nitrogen concentrations are less than 0.1 ppm and 100 ppm, respectively.     

The impact of ammonia feed distribution on the performance of a fixed bed membrane reactor for ammonia decomposition to ultra-pure hydrogen

In this study, the influence of distribution of ammonia feed along the height of a fixed bed membrane reactor (FBMR) for ammonia decomposition to hydrogen is investigated to understand the leverage of this approach. A rigorous heterogeneous model with verified kinetics is implemented to simulate the reactor. The simulation results indicate that the application of a distributed ammonia feed with equal distribution of injection points resulted in a 17.45% improvement in hydrogen production rate at a low temperature of 800.0 K over a FBMR without feed distribution. In the parameter space of this study, it has been shown that the ammonia conversion is sensitive to the number of distribution points and has an optimal value. It is found that the implication of the optimum number of injection points can substantially reduce the length of the reactor by 75.0% to achieve 100.0% ammonia conversion. The hydrogen reversal permeation phenomenon is observed at a low pressure and the upper part of the reactor. A novel configuration of a FBR and a FBMR with feed distribution is proposed for efficient production of ultra-pure hydrogen at a relatively low pressure. The critical reactor length ratio has been provided for this configuration.     

Preparation of a novel bimodal catalytic membrane reactor and its application to ammonia decomposition for COx-free hydrogen production

A novel bimodal catalytic membrane reactor (BCMR) consisting of a Ru/γ-Al₂O₃/α-Al₂O₃ bimodal catalytic support and a silica separation layer was proposed. The catalytic activity of the support was successfully improved due to enhanced Ru dispersion by the increased specific surface area for the γ-Al₂O₃/α-Al₂O₃ bimodal structure. The silica separation layer was prepared via a sol–gel process, showing a H₂ permeance of 2.6 × 10⁻⁷ mol Pa⁻¹ m⁻² s⁻¹, with H₂/NH₃ and H₂/N₂ permeance ratios of 120 and 180 at 500 °C. The BCMR was applied to NH₃ decomposition for CO_x-free hydrogen production. When the reaction was carried out with a NH₃ feed flow rate of 40 ml min⁻¹ at 450 °C and the reaction pressure was increased from 0.1 to 0.3 MPa, NH₃ conversion decreased from 50.8 to 35.5% without H₂ extraction mainly due to the increased H₂ inhibition effect. With H₂ extraction, however, NH₃ conversion increased from 68.8 to 74.4% due to the enhanced driving force for H₂ permeation through the membrane.     

Kinetic and thermodynamic analyses of mid/low-temperature ammonia decomposition in solar-driven hydrogen permeation membrane reactor

It is a promising method for hydrogen generation without carbon emitting by ammonia decomposition in a catalytic palladium membrane reactor driven by solar energy, which could also store and convert solar energy into chemical energy. In this study, kinetic and thermodynamic analyses of mid/low-temperature solar thermochemical ammonia decomposition for hydrogen generation in membrane reactor are conducted. Hydrogen permeation membrane reactor can separate the product and shift the reaction equilibrium forward for high conversion rate in a single step. The variation of conversion rate and thermodynamic efficiency with different characteristic parameters, such as reaction temperature (100–300 °C), tube length, and separation pressure (0.01–0.25 bar), are studied and analyzed. A near-complete conversion of ammonia decomposition is theoretically researched. The first-law thermodynamic efficiency, net solar-to-fuel efficiency, and exergy efficiency can reach as high as 86.86%, 40.08%, and 72.07%, respectively. The results of this study show the feasibility of integrating ammonia decomposition for hydrogen generation with mid/low-temperature solar thermal technologies.     

Characterization of an ammonia decomposition process by means of a multifunctional catalytic membrane reactor

Ammonia decomposition was studied in a multifunctional catalytic membrane reactor filled with Ruthenium catalyst and equipped with palladium-coated membranes. To characterize the system we measured NH₃ conversion, H₂ yield and its partial pressure, the internal and external temperatures of the reactor shell and the electric consumption under several NH₃ flow and pressure conditions. Experimental results showed that the combined effect of Ruthenium catalyst and palladium membranes allowed the reaction to reach the equilibrium in all the conditions we tested. At 450 °C the ammonia conversion resulted the most stationary, while at 7 bar the hydrogen yield was almost independent of both the ammonia flow and temperature. In addition, the experimental system used in this work showed high values of NH₃ conversion and H₂ permeation also without heating the ammonia tank and therefore renouncing to control the feeding gas pressure. When ultra-pure hydrogen is needed at a distal site, a reactor like this can be considered for in situ hydrogen production.     

Analysis of ammonia decomposition reactor to generate hydrogen for fuel cell applications

In this paper, reaction engineering principles are utilized to analyze process conditions for producing sufficient hydrogen in an ammonia decomposition reactor for generating net power of 100 W in a fuel cell. It is shown that operating the reactor adiabatically results in a sharp decrease in temperature due to endothermic reaction, which results in low conversion of ammonia. For this reason, the reactor is heated electrically to provide heat for the endothermic reactions. It is observed that when the reactor is operated non-adiabatically, it is possible to get over 99.5% conversion of ammonia. The weight of absorbent to reduce ammonia to ppb levels is calculated. An energy balance on the reactor exit gas indicates that there is sufficient heat available to vaporize enough water to achieve 100% relative humidity in the fuel cell. A suitable fuel cell stack is designed and it is shown that this stack is able to provide the necessary power to electrically heat the reactor and produce net power of 100 W.     

Ammonia for hydrogen storage; A review of catalytic ammonia decomposition and hydrogen separation and purification

Ammonia is of interest as a hydrogen storage and transport medium because it enables liquid-phase hydrogen storage under mild conditions. Although ammonia can be used directly for energy applications, its use in conventional fuel cell electric vehicles necessitates decomposition into nitrogen and hydrogen, and the purification of the hydrogen to the composition required for commercial proton exchange membrane fuel cells. This article provides a review of the material and process considerations for catalytic ammonia decomposition and shows that Ru-based catalysts on conductive support materials are active at < 500 °C, but further understanding around lifetimes and deactivation conditions is required. This review then explores materials and technologies for hydrogen purification from decomposed ammonia gas streams, and our experiments show that defect-free dense-metal membranes are uninhibited by ammonia and can achieve the required product purity.     

One-step synthesis of Ni/yttrium-doped barium zirconates catalyst for on-site hydrogen production from NH3 decomposition

On-site produced hydrogen from ammonia decomposition can directly fuel solid oxide fuel cells (SOFCs) for power generation. The key issue in ammonia decomposition is to improve the activity and stability of the reaction at low temperatures. In this study, proton-conducting oxides, Ba(Zr,Y) O_3-δ (BZY), were investigated as potential support materials to load Ni metal by a one-step impregnation method. The influence of Ni loading, Ba loading, and synthesis temperature, of Ni/BZY catalysts on the catalytic activity for ammonia decomposition were investigated. The Ni/BZY catalyst with Ba loading of 20 wt%, Ni loading of 30 wt%, and synthesized at 900 °C attained the highest ammonia conversion of 100% at 600 °C. The kinetics analysis revealed that for Ni/BZY catalyst, the hydrogen poisoning effect for ammonia decomposition was significantly suppressed. The reaction order of hydrogen for the optimized Ni/BZY catalyst was estimated as low as ?0.07, which is the lowest to the best of our knowledge, resulting in the improvement in the activity. H₂ temperature programmed reduction and desorption analysis results suggested that a strong interaction between Ni and BZY support as well as the hydrogen storage capability of the proton-conducting support might be responsible for the promotion of ammonia decomposition on Ni/BZY. Based on the experimental data, a mechanism of hydrogen spillover from Ni to BZY support is proposed.     

Catalyst support effect on ammonia decomposition over Ni/MgAl2O4 towards hydrogen production

Ammonia is a prospective fuel for hydrogen storage and production, but its application is limited by the high cost of the catalysts (Ru, etc.) to decompose NH₃. Decomposing ammonia using non-precious Ni as catalysts can therefore improve its prospects to produce hydrogen. This work proposes several Ni/MgAl₂O₄ with the support properties tuned and investigates the support effect on the catalytic performance. Ni/MgAl₂O₄-LDH shows high NH₃ conversion (～88.7%) and H₂ production rate (～1782.6 mmol g^?1 h^?1) at 30,000 L. kg^?1 h^?1 and 600 °C, which is 1.68 times as large as that of Ni/MgAl₂O₄-MM. The performance remains stable over 30 h. The characterizations manifest that the high specific surface area of Ni/MgAl₂O₄-LDH can introduce highly dispersed Ni on the surface. Kinetics analysis implies promoted NH₃ decomposition reaction and alleviated H₂ poisoning for Ni/MgAl₂O₄-LDH. A roughly linear relationship is obtained by fitting the curves of dispersed Ni on the surface vs the reaction orders regarding H₂ and NH₃. This indicates that enhanced NH₃ decomposition performance can be ascribed to the strengthened NH₃ decomposition reaction and weakened H₂ poisoning by the highly dispersed Ni on the MgAl₂O₄-LDH surface. This work provides an opportunity to develop highly active and cost-effective catalysts to produce hydrogen via NH₃ decomposition.     

Module design of silica membrane reactor for hydrogen production via thermochemical IS process

The potential of the silica membrane reactors for use in the decomposition of hydrogen iodide (HI) was investigated by simulation with the aim of producing CO₂-free hydrogen via the thermochemical water-splitting iodine-sulfur process. Simulation model validation was done using the data derived from an experimental membrane reactor. The simulated results showed good agreement with the experimental findings. The important process parameters determining the performance of the membrane reactor used for HI decomposition, namely, reaction temperature, total pressures on both the feed side and the permeate side, and HI feed flow rate were investigated theoritically by means of a simulation. It was found that the conversion of HI decomposition can be improved by up to four times (80%) or greater than the equilibrium conversion (20%) at 400 °C by employing a membrane reactor equipped with a tubular silica membrane. The features to design the membrane reactor module for HI decomposition of the thermochemical iodine-sulfur process were discussed under a wide range of operation conditions by evaluating the relationship between HI conversion and number of membrane tubes.     

Highly efficient Co/NC catalyst derived from ZIF-67 for hydrogen generation through ammonia decomposition

Small-size cobalt nanoparticles (NPs) distributed on nitrogen doped carbon support (Co/NC-X) were prepared by pyrolysis of ZIF-67 at various temperatures (X = 500, 600,700 and 800 °C) in nitrogen atmosphere and utilized as catalysts for hydrogen production through ammonia decomposition. Characterizations of the catalysts including XRD, HRTEM, XPS, H₂-TPR, CO₂-TPD, etc., were conducted for structure analysis. The N–C plate obtained from pyrolysis was coated with Co NPs to hinder its aggregation, which made the Co NPs dispersed evenly and increased their dispersion. The calcination temperature and the strong base of the support can adjust the strength of Co–N bond. The activity of the Co/NC-X catalysts is attributed to the high content of Co⁰ and the moderate Co–N bond strength. The ammonia decomposition activity of Co/NC-X catalysts in this paper is higher than many reported Co-based catalysts. Co/NC-600 catalyst demonstrates an ammonia conversion of 80% at 500 °C with a space velocity of 30,000 ml g_cat^?1 h^?1, corresponding to a hydrogen production rate of 26.8 mmol H₂ g_cat^?1 min^?1. The work provides insight for the development of highly active cobalt-based catalysts for hydrogen production through ammonia decomposition.     

Enhanced hydrogen permeance through graphene oxide membrane deposited on asymmetric spinel hollow fiber substrate

Membranes of graphene oxide (GO) present suitable application for hydrogen (H₂) purification. The deposition of a selective and high permeable GO membrane on a proper substrate is still a challenge. Here we applied the vacuum-assisted method to deposit a GO layer on asymmetric spinel (MgAl₂O₄) hollow fibers. The synthetized GO showed a nanosheeted morphological structure and a relative high degree of oxidation. The hollow fibers were produced with dolomite and alumina as the ceramic starting material and showed the desired asymmetric pore size distribution, in addition to suitable bending strength, 54.88 ± 4.25 MPa, and average surface roughness, 180 ± 8.2 nm. A continuous GO layer of 1.7 ± 0.2 μm was deposited onto the fiber outer surface. The composite MgAl₂O₄/GO membrane presented H₂ permeance of 8.2 ± 0.0 × 10^?7 mol s^?1 m^?2 Pa^?1 at room temperature (approximately 25 °C) and 0.3 MPa of transmembrane pressure. Ideal hydrogen/nitrogen and hydrogen/carbon dioxide selectivity values were of 3.3 ± 0.0 and 11.4 ± 0.1, respectively.     

Influence of basic dopants on the activity of Ru/Pr6O11 for hydrogen production by ammonia decomposition

Basic oxides such as alkali metal oxides, alkaline earth metal oxides, and rare earth oxides were added to Ru/Pr₆O₁₁, and the activity of the catalysts with respect to hydrogen production by ammonia decomposition was investigated. Ru/Pr₆O₁₁ doped with alkali metal oxides, except for Li₂O, achieved higher NH₃ conversions than bare Ru/Pr₆O₁₁. Cs₂O, the most basic of the alkali metal oxides, was the most effective dopant. In contrast, other dopants with lower basicity than the alkali metal oxides achieved lower NH₃ conversions than bare Ru/Pr₆O₁₁. Changing the Cs/Ru molar ratio revealed that the best Cs/Ru ratio was 0.5–2; the reaction was effectively promoted without negative effects from coverage of the Ru surface by the Cs₂O. Varying the order of loading the Ru and Cs₂O onto Pr₆O₁₁ revealed that loading Ru onto Cs₂O/Pr₆O₁₁ was an effective way to enhance NH₃ conversion, and coverage of the Ru surface was reduced.     

Fe-based catalyst derived from MgFe-LDH: Very efficient yet simply obtainable for hydrogen production via ammonia decomposition

The current study provided the first example to develop the Fe-based catalyst for CO_x-free hydrogen production via ammonia decomposition through the unique MgFe-layered double hydroxides (MgFe-LDHs) of different stoichiometric Mg/Fe ratio. The so obtained Fe-based catalyst is low-cost, readily obtainable, and environmentally friendly. Structurally, the Fe(FeN_x) species are 3D-isolated by the nano-MgO entities, improving anti-sintering potential of Fe(FeN_x); and electronically, the Fe (FeN_x) species are promoted by the nano-MgO matrix, showing the strongest promoting effect of MgO on Fe(FeN_x). At a GHSV_NH3 of 150,000 mL g_cat⁻¹ h⁻¹, the current N–Mg_5.3FeO_m catalyst can give an outstanding H₂ formation rate of 9.83 mol g_cat⁻¹ h⁻¹ at 680 °C and a TOF_H2 = 2.19 s⁻¹ at 530 °C. The influence of Mg/Fe constitution on catalyst structure, surface property, and performance was systematically investigated. The in-situ ammonia treatment was superior to the usually adopted hydrogen pre-reduction for the Fe–Mg oxide precursor, leading to easy development of small sized FeN_x specimen and activity enhancement.     

Effect of preparation methods on the catalyst performance of Co/MgLa mixed oxide catalyst for COx-free hydrogen production by ammonia decomposition

This work deals with the effect of catalyst preparation method of the mixed Co, Mg and La oxide catalysts on their structure and catalytic properties for ammonia decomposition. Two methods are used for catalysts preparations impregnation and co-precipitation (in air and in pure O₂ atmosphere), The Mg/La = 2 molar ratio and 5 wt% of cobalt content was maintained same in all catalysts. The catalyst performance was evaluated in the temperature range 300–550 °C at atmospheric pressure. The prepared catalysts were characterized by BET, XRD, TPR, XPS, CO₂-TPD and SEM techniques. No pronounced differences were observed in BET among the catalysts. It was found that the 5CML-OXY (5 wt%Co over MgLa catalyst prepared by co-precipitation method in oxygen atmosphere) has superior activity among the other catalysts. This could be attributed to availability of easily reducible cobalt species determined by TPR studies and enhanced interaction between Mg and La determined by SEM and XPS. The moderate basic site density determined by CO₂-TPD results was also increased in 5CML–OXY catalysts compared with other catalysts. These consequences are might be one of the reasons for enhanced activity of 5CML–OXY catalyst compared to other catalysts. Hence catalyst preparation by co-precipitation in oxygen atmosphere is the best method which might be one of the parameters that influenced on catalytic properties of the cobalt on MgOLa₂O₃ system, for ammonia decomposition.     

Steam reforming of propane in a fluidized bed membrane reactor for hydrogen production

Steam reforming of propane was carried out in a fluidized bed membrane reactor to investigate a feedstock other than natural gas for production of pure hydrogen. Close to equilibrium conditions were achieved inside the reactor with fluidized catalyst due to the very fast steam reforming reactions. Use of hydrogen permselective Pd₇₇Ag₂₃ membrane panels to extract pure hydrogen shifted the reaction towards complete conversion of the hydrocarbons, including methane, the key intermediate product. Irreversible propane steam reforming is limited by the reversibility of the steam reforming of this methane. To assess the performance improvement due to pure hydrogen withdrawal, experiments were conducted with one and six membrane panels installed along the height of the reactor. The results indicate that a compact reformer can be achieved for pure hydrogen production for a light hydrocarbon feedstock like propane, at moderate operating temperatures of 475–550 °C, with increased hydrogen yield.     

Equilibrium shift of methylcyclohexane dehydrogenation in a thermally stable organosilica membrane reactor for high-purity hydrogen production

A high-performance organosilica membrane was prepared via sol–gel processing for use in methylcyclohexane (MCH) dehydrogenation to produce high-purity hydrogen. The membrane showed a high H₂ permeance of 1.29 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹, with extremely high H₂/C₃H₈ and H₂/SF₆ selectivities of 6680 and 48,900, respectively, at 200 °C. The extraction of hydrogen from the membrane reactor led to the MCH conversion higher than the thermodynamic equilibrium, with almost pure hydrogen obtained in the permeate stream without considering the effect of carrier gas and sweep gas in the membrane reactor, and the organosilica membrane reactor was very stable under the reaction conditions employed.