The crystallization and the hydrogen absorption properties of a Ni32Nb28Zr30Fe10 melt spun ribbon were investigated. X-ray diffraction measurements reveal that a small fraction of the ribbon is in a crystalline state, whereas the main component is amorphous. The bulk crystallization process of the alloy ribbon occurs in two steps above 770 K, as measured by differential scanning calorimetry. At each step of the crystallization process, an unusually low activation energy of the order of 180 kJ/mol, was observed. Hydrogen absorption pressure-composition isotherms measured between 598 K and 673 K showed that the enthalpy of hydrogenation is quite high (similar to 85 kJ/mol), as compared to that of analogous ribbons. The isotherms of these ribbons do not exhibit any plateau, similarly to other amorphous materials, but they exhibited extremely slow kinetics for hydrogen absorption. To simulate the local atomic structure involving cluster formation in amorphous Ni32Nb28Zr30Fe10 alloy, DFT-MD approach was used to construct an amorphous supercell of this alloy with 108 atoms. Calculations predicted that a fully amorphous structure of Ni32Nb28Zr30Fe10 can form. The low activation energy of crystallization observed before hydrogenation is due to the presence of only 3 full icosahedra without any Ni-centered icosahedra, that could provide resistance against crystallization. Moreover, a cluster analysis of the Ni32Nb28Zr30Fe10 alloy after hydrogenation showed interaction of hydrogen atoms with only two icosahedra out of four found in this case, and this could be the probable reason for the extremely slow kinetics of hydrogen absorption. (C) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Vanadium-based alloy membranes with body-centred-cubic (BCC) structure are considered as one of the leading alternatives to Pd-based alloys for hydrogen separation applications due to their lower cost and higher permeability. As permeability and mechanical properties depend on what microstructure can be produced mainly by alloy composition under same processing conditions, the effect of alloy composition on microstructure, mechanical properties and hydrogen permeability has been investigated for the V(85)Ni(15)and V85Ni10Ti5(at%) alloys prepared by a same process route. All Ni atoms dissolve into the V-matrix to form a single highly supersaturated solid solution with dendritic segregation of Ni-solute atoms in the binary alloy. A part of Ni replacement with 5 at% Ti leads to the formation of small interdendritic phases NiTi and NiTi(2)in addition to major phase of V-based solid solution. The mechanical property testing shows that the ultimate strength of the ternary alloy is higher than that of the binary alloy, but the elongation and rollability are lower due to a combination of solid solution hardening and particle strengthening effect. The addition of Ti can greatly increase permeability about 4 times greater than the binary alloy at a permeation testing of 400 degrees C. But the presence of small amounts of interdendritic compounds provides a barrier to hydrogen migration, resulting in a relatively lower hydrogen diffusion coefficient. In theory, the diffusivity and solubility of hydrogen atom in the presence of alloying element Ti is higher than that in the presence of alloying element Ni in vanadium. This is demonstrated using first principles calculation which further explains the mechanism of hydrogen permeation.

A LiOH-promoted Ru-based catalyst was recently reported to have a high TOF of 17.7 s(-1) at 623 K, compared to 2.7 s(-1) for an un-promoted Ru-based catalyst, and has been reproduced for this study to develop further understanding of the catalyst activity under a range of conditions. The kinetic values were calculated using a Temkin-Pyzhev-like power law rate expression model. Reaction orders, pre-exponential factors (A) and activation energies (E) were calculated for two temperature ranges, 623-748 K, and 748-873 K. The TOF of this catalyst at 623 K is not similar to that previously reported, being only 1.6 s(-1) in this study. A follow-up CFD analysis supports the fact that the kinetic model effectively describes performance of the catalyst at a range of temperatures and pressures, and can be used in the future on similar catalysts. H-2 partial pressure has an inhibitory effect on the rate of decomposition of NH3 at all temperatures, not just near or below 673 K as previously proposed in the literature, however equilibrium decomposition is still possible with sufficient catalyst loading. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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 degrees 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. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Ammonia is a prospective hydrogen transport intermediate due to its high volumetric and gravimetric H-2 densities, and existing production and distribution infrastructure. However, its ultimate use in mobile proton exchange membrane (PEM) fuel cells necessitates decomposition and purification at or near the point of use. In this study, the production of high purity H-2 from NH3 using a two-stage process has been demonstrated by coupling separate decomposition (150 g of 1 wt % Ru on Al2O3 catalyst) and purification (a single 150 cm(2), Pd-coated tubular vanadium membrane) stages. Equilibrium NH3 decomposition and >90% H-2 recovery was demonstrated with a catalyst temperature of 450 degrees C and membrane temperature of 340 degrees C, with an overall H-2 production rate of 0.75 kg/day. Mass spectrometry showed that levels of N-2 and NH3 impurities were below detection limits. This configuration is readily scalable by increasing the catalyst loading and membrane area (through use of multiple tubes in parallel), and could enable a pathway for distributed use of H-2 from NH3 in mobile and stationary power generation.

It is widely accepted that vanadium is unsuitable as a membrane material due to an extreme susceptibility to hydrogen embrittlement. Consequently, the focus of R&D effort towards hydrogen-selective, vanadium-based metal membranes in recent years has been the development of robust alloys with improved embrittlement resistance. What the literature hasn't really addressed, however, is whether vanadium's shortcomings can be overcome through the implementation of suitable controls. This work attempts to address this question by closely examining V-H phase equilibrium and undertaking practical demonstrations of Pd-coated vanadium membranes in a tubular geometry. Membranes were prepared from a 500 mm-long tubular 99.9% V substrate, coated on each surface with Pd catalyst layers. This single tube was sectioned for several permeation and hydrogen absorption tests. An examination of the V-H phase diagram show that hydride phase transitions (alpha ->beta, alpha ->gamma) and corresponding miscibility gaps can readily be avoided using appropriate operating procedures. Hydrogen permeating testing showed these membranes exhibit very high permeability (initially exceeding 3.0 x 10(-7) mol m(-1) s(-1) Pa-0.5 at 320 degrees C and above) which allows the use of thick-walled (similar to 0.25 mm), self-supporting, pinhole-free vanadium tubes as the membrane substrate. These membranes also exhibited robustness, with mechanical integrity being maintained through multiple thermal and hydride cycles and over several hundred hours of testing. This work shows that the main natural advantages of vanadium (low cost relative to Pd and very high permeability which affords the use of self-supporting, defect-free substrates) can still be exploited if used in conjunction with appropriate geometry and operating procedures.

Despite the growing use of hydrogen (H2) as a transport fuel, one of the major barriers still remaining is efficient and inexpensive fuel distribution and storage. Current approaches, such as compression, liquefaction or metal hydride formation, incur a significant energy penalty. Ammonia (NH3) has long been considered a prospective H2 medium, exhibiting a higher volumetric H2 density than liquid H2, through liquid-phase storage at mild pressure. Decomposition of NH3 into H2 and N2 can be achieved via use of catalytic reactors and fuel-cell-grade H2 can be produced using metal membranes at H2 distribution sites.In this study, a 3-Dimensional (3D) Computational Fluid Dynamics (CFD) model has been developed to understand the performance of the H2 separation process in gas mixtures derived from an NH3-cracking reaction. The reactor consists of 19 tubular membrane tubes, each 470 mm long, inside a tubular shell with an inner diameter of 130 mm. Standard transport and energy equations governing a 3D, pressure-based, steady-state model were derived from the laws of conservation of mass, momentum and energy. The governing equations were solved using commercial CFD software ANSYS Fluent 18.0. Gas flow and mixing were modelled by the two-equation standard k-epsilon model for closure. Coupled solver was used for pressure-velocity coupling, enabling a pseudo-transient option with pseudo time steps of 0.01 s. To estimate H2 permeation through the metal membrane, a constant H2 permeability of 3.0E–07 mol.m−1 s−1 Pa−0.5 derived from series of experiments tested under a range of industrial conditions, was used. Model simulations were conducted for an adiabatic temperature of 300 °C, a feed-side pressure of 7.8 bara and a permeate side pressure of 0.1 bara. A parametric analysis was carried out to explore the effects of variation in total feed-gas flow and effects of changes in NH3-cracking efficiency on H2 production rates and H2 yields. The model estimated that 4.6–11.6 kg H2/day can be produced from a 30-70 L min−1 NH3 inlet flow with 80-90% NH3-cracking efficiency. At lower NH3 inlet flow rates, higher H2 yields can be obtained within a shorter distance of the membrane tubes due to relatively slower velocities and longer residence times. At high inlet flow rates, H2 yields were significantly lower due to their faster velocities and shorter resident times, but high yields (>95%) were still observed at the membrane reactor outlet. A sensitivity analysis of the model showed that even if metal membranes functioned at only 50% of the maximum permeability, a high H2 yield similar to that estimated using 100% permeability can still be achieved at the H2 outlets.

Hydrogen isotope selectivity of palladium membranes has long been known and studied, but the emergence of vanadium-based membranes as a low-cost alternative naturally inspires curiosity as to whether these membranes exhibit similar properties. Accordingly, experiments to calculate the permeability of hydrogen and deuterium through a palladium-coated vanadium membrane at 300 degrees C were undertaken, and they revealed that hydrogen permeates at a rate 1.5 x faster than deuterium. With hydrogen absorption experiments at the same temperature showing very little difference in the amount of each isotope absorbed over a wide pressure range, it can be concluded that atomic hydrogen diffuses through vanadium 1.5 x faster than atomic deuterium. In practice, this gives rise to a significant separation factor, with deuterium being depleted in the permeate stream, but enriched in the retentate stream. Creating a cascading series of membranes, with successive retentate streams combined, will allow the deuterium concentration to be enriched far beyond the natural value of 0.015%. This work suggests that further work is warranted to explore whether this separation factor can be enhanced (e.g., through alloying), and to demonstrate a cascading membrane system to deliver high purity deuterium from a natural hydrogen source. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Amorphous metallic membranes display promising properties for hydrogen purification up to an ultrapure grade (purity > 99.999%). The hydrogen permeability through amorphous membranes has been widely studied in the literature. In this work we focus on two additional properties, which should be considered before possible application of such materials: the propensity to crystallize at high temperatures should be avoided, as the crystallized membranes can become brittle; the hydrogen solubility should be high, as solubility and permeability are proportional. We investigate the crystallization process and the hydrogen solubility of some membranes based on Ni, Nb, and Zr metals, as a function of Zr content, and with the addition of Ta or B. The boron doping does not significantly affect the crystallization temperature and the thermal stability of the membrane. However, the hydrogen solubility for p ~7 bar is as high as H/M ~0.31 at T = 440 °C and H/M ~0.27 at T = 485 °C. Moreover, the membrane does not pulverize even after repeated thermal cycles and hydrogenation processes up to 485 °C and 7 bar, and it retains its initial shape.

Ni-Nb-Zr amorphous membranes, prepared by melt-spinning, show great potential for replacing crystalline Pd-based materials in the field of hydrogen purification to an ultrapure grade (>99.999%). In this study, we investigate the temperature evolution of the structure of an amorphous ribbon with the composition Ni32Nb28Zr30Cu10 (expressed in atom %) by means of XRD and DTA measurements. An abrupt structural expansion is induced between 240 and 300 °C by hydrogenation. This structural modification deeply modifies the hydrogen sorption properties of the membrane, which indeed shows a strong reduction of the hydrogen capacity above 270 °C.

Amorphous (Ni0.6Nb0.4)1−xZrx membranes were investigated by means of X-ray diffraction, thermogravimetry, differential thermal analysis and tensile modulus measurements. Crystallization occurs only above 673 K, and even after hydrogenation the membranes retain their mainly amorphous nature. However, after exposure to gaseous hydrogen, the temperature dependence of the tensile modulus, M, displays large variations. The modulus of the hydrogen reacted membrane is higher with respect to the pristine samples in the temperature range between 298 K and 423 K. Moreover, a sharp drop in M is observed upon heating to approximately 473 K, well below the glass transition temperature of these glasses. We propose that the changes in the moduli as a function of temperature on the hydrogenated samples are due to the formation of nanocrystalline phases of Zr hydrides in (Ni0.6Nb0.4)1−xZrx-H membanes.

A 2-dimensional, axis-symmetric CFD model of a tubular CMR has been developed using a commercial software package FLUENT for the purposes of guiding the design and operation of a HTWGS-CMR for the processing of coal-derived syngas. Development of the model has been approached in a stepwise manner through the successive incorporation of sub-models for the CMR processes. For each step, performance of model was checked and validated against measurements using a prototype CMR with same set-up applied in simulation.The optimum catalyst loading, which yield the maximum CO conversion within targeted operating reactor temperatures (350-450 degrees C), was found to be 11.6 kg/(COmol/s) for the inlet syngas temperature of 350 degrees C with a reactor having a 1 '' shell diameter. The CMR model was validated experimentally with a simulated coal-derived syngas (64.5% of CO, 33.0% of H-2 and 2.5% of CO2 with a 3:1 steam to carbon (S:C) ratio) at a total dry gas flow of 4 L-N/min and a feed pressure of 15 bar(g). These tests were performed using a prototype reactor which incorporated with a tubular (0.1 mm thick, 150 cm(2), 3/8 '' OD) Pd/Ag23 wt% membrane. The CMR model was simulated using a wider range of operating parameters (namely permeation rate, inlet temperature, catalyst loading, pressure at permeate side and S:C ratios) to examine its sensitivity to these variables. Outcomes of these parametric analyses have enhanced our understanding of CMR operation in order to optimise its performance. (C) 2015 Elsevier B.V. All rights reserved.

Layered alloy membranes, with sub-micron Pd catalyst layers over a highly permeable vanadium alloy core, provide a low-cost alternative to supported Pd-alloy membranes. Despite the minimal Pd consumption, Pd still comprises a significant fraction of the overall membrane costs, and further cost reductions can be achieved by replacing Pd with suitable alternative catalytic layers. Ni is an obvious candidate, exhibiting high catalytic activity for a range of reactions while being relatively inexpensive, but its instability in syngas renders it unsuitable as a teed-side catalyst. The permeate surface of an alloy membrane is exposed to H-2 only during operation, making Ni of interest as a permeate-side catalyst.Asymmetric Pd/V/Ni alloy membranes have been fabricated with varying Ni thickness, and hydrogen permeance has been examined over a wide range of pressures and temperatures. Hydrogen permeance increases with decreasing Ni thickness, down to a limiting thickness of 150 nm, beyond which permeance degraded due to incomplete Ni coverage. The permeance was 65% that of a symmetrical Pd/V/Pd membrane with 500 nm Pd layers. The cost-effectiveness is dependent on raw Pd price and manufacturing costs. (C) 2015 Published by Elsevier B.V.

The microstructure, hardness, and precipitate free zones (PFZ) of V55Ti30Ni15 alloys during heat treatment have been investigated in this study. The microstructure resulting from different heat treatment conditions has a great influence on hardness. The microstructure resulting from different heat treatment conditions has a great influence on hardness. Fine NiTi particles precipitate from the supersaturated V-matrix solid solution at 750 °C, increase in quantity until 800 °C, and then dissolve back into the V-matrix at 850 °C. The resultant hardness decreases with temperature until 800 °C, and then increases from 800 to 850 °C. The microstructure containing small NiTi precipitates resulting from the treatment of 18 h at 800 °C has a good soft condition for workability. PFZ formed at the grain boundary of V-matrix during heat treatment was observed. Vacancies depletion in V-matrix maybe led to the formation of PFZ.

Vanadium is highly permeable to hydrogen which makes it one of the leading alternatives to Pd alloys for hydrogen-selective alloy membrane applications, but it is prone to brittle failure through excessive hydrogen absorption and transitions between the BCC alpha and BCT beta phases. V-Ti-Ni alloys are a prospective class of alloy for hydrogen-selective membrane applications, comprising a highly-permeable vanadium solid solution and several interdendritic Ni-Ti compounds. These Ni-Ti compounds are thought to stabilise the alloy against brittle failure. This hypothesis was investigated through a systematic study of V70Ti15Ni15 by hydrogen absorption and X-ray diffraction under conditions relevant to membrane operation. Dissolved hydrogen concentration in the bulk alloy and component phases, phase identification, thermal and hydrogen-induced expansion, phase quantification and hydride phase transitions under a range of pressures and temperatures have been determined. The vanadium phase passes through three different phase fields (BCC, BCC + BCT, BCT + BCT) during cooling under H-2 from 400 to 30 degrees C. Dissolution of Ni and Ti into the vanadium phase increases the critical temperature for beta-hydride formation from <200 to >400 degrees C. Furthermore, the Ni-Ti phases also exhibit several phase transitions meaning their ability to stabilise the alloy is questionable. We conclude that this alloy is significantly inferior to V with respect to its stability when used as a hydrogen-selective membrane, but the hydride phase transitions suggest potential application for high-temperature hydrogen and thermal energy storage. (C) 2014 Published by Elsevier B. V.

Current research into desulfurization sorbents for coal gasification is aimed at maximizing sorption capacity, kinetics and attrition resistance, while minimizing cost and volatility. Isolating the chemical aspects of desulfurization sorbent performance, however, is difficult because sorption performance is highly dependent on the physical form of the sorbent. To guide sorbent development, thermodynamic calculations offer a consistent basis from which to compare various aspects of sorbent performance, which includes conversion, but also speciation, volatility and regeneration. This approach has been used to compare 24 potential sorbents for the removal of sulfur from syngas, including oxides of transition metals, alkaline earths, alkalis and borates. Oxides of copper, manganese and zinc are most favourable for high-temperature sulfur sorption and these oxides are chemically regenerable. The thermodynamic calculations are confirmed by the available literature, with the ZnO, MnO and CuO receiving the most attention in recent years. Each oxide has characteristic disadvantages, such as the tendency of copper oxide to reduce, the high volatility of zinc oxide and the high regeneration temperature of manganese oxide. This is driving the development of fabrication techniques which maximize sorbent dispersion, stability and tolerance to repeated sulfidation/regeneration cycles. Copyright (C) 2010 Curtin University of Technology and John Wiley & Sons, Ltd.

ABSTRACTPre‐oxidation treatment of Fe3Al alloy, a material used in high temperature gas filtration, is common practice to improve corrosion resistance. This work evaluates the various pre‐oxidation procedures in terms of the quality of the protective layers formed, using microstructural analysis and corrosion tests. The effect of atmosphere on the oxidation kinetics at 950 °C and the oxide layer morphology was determined. The corrosion resistance of Fe3Al filters was investigated in simulated coal‐derived synthesis gas in the temperature range 600–800 °C. It was found that the corrosion resistance of Fe3Al was significantly improved by the development of the surface oxide layer, and that the quality of oxide layer is dependent on the oxidation treatment conditions, such partial oxygen pressure, pO2. The best corrosion resistance was achieved with a two‐stage oxidation treatment, involving initial pre‐oxidation at low pO2 and final pre‐oxidation at high pO2. © 2011 Curtin University of Technology and John Wiley & Sons, Ltd.