Hydrogen permeation through a Pd-based membrane and RWGS conversion in H2/CO2, H2/N2/CO2 and H2/H2O/CO2 mixtures

This study investigated the effect of gases such as CO₂, N₂, H₂O on hydrogen permeation through a Pd-based membrane −0.012 m² – in a bench-scale reactor. Different mixtures were chosen of H₂/CO₂, H₂/N₂/CO₂ and H₂/H₂O/CO₂ at temperatures of 593–723 K and a hydrogen partial pressure of 150 kPa. Operating conditions were determined to minimize H₂ loss due to the reverse water gas shift (RWGS) reaction. It was found that the feed flow rate had an important effect on hydrogen recovery (HR). Furthermore, an identification of the inhibition factors to permeability was determined. Additionally, under the selected conditions, the maximum hydrogen permeation was determined in pure H₂ and the H₂/CO₂ mixtures. The best operating conditions to separate hydrogen from the mixtures were identified.     

Steam reforming of methanol over Cu/ZnO/ZrO2/Al2O3 catalyst

Steam reforming of methanol was investigated over Cu–ZnO–ZrO₂–Al₂O₃ catalysts at 473 and 573 K. The Cu:Zn:(Al + Zr) molar ratio was 3:3:4; however, the Zr:Al molar ratio was varied and the catalysts were pretreated at different calcination and reduction temperatures. The synthesized catalysts were characterized by N₂ physisorption, temperature-programmed reduction with H₂ (H₂-TPR), X-ray diffraction, oxidized surface TPR, and infrared spectroscopy after carbon monoxide chemisorption. The crystalline size of Cu decreased on increasing the calcination temperatures from 573 to 623 K and increased on increasing the reduction temperatures from 523 to 573 K. Among the tested catalysts, the Cu–ZnO–ZrO₂ catalyst exhibited the highest and lowest hydrogen-formation rates at 473 and 573 K, respectively. After the reaction at 573 K, all the tested catalysts exhibited an increase in the Cu crystalline size, causing the catalyst deactivation. Among the tested catalysts, the Cu–ZnO–ZrO₂–Al₂O₃ catalyst, where the Cu:Zn:Al:Zr molar ratio was 3:3:2:2, showed the highest and most stable catalytic activity at 573 K. Cu dispersion and catalyst composition affected the catalytic performance for steam reforming of methanol.     

Improvement of hydrogen-storage properties of MgH2 by Ni,LiBH4, and Ti addition

In this work, differently from our previous work, MgH₂ instead of Mg was used as a starting material. Ni, Ti, and LiBH₄ with a high hydrogen-storage capacity of 18.4 wt% were added. A sample with a composition of MgH₂–10Ni–2LiBH₄–2Ti was prepared by reactive mechanical grinding. MgH₂–10Ni–2LiBH₄–2Ti after reactive mechanical grinding contained MgH₂, Mg, Ni, TiH_1.924, and MgO phases. The activation of MgH₂–10Ni–2LiBH₄–2Ti for hydriding and dehydriding reactions was not required. At the number of cycles, n = 2, MgH₂–10Ni–2LiBH₄–2Ti absorbed 4.09 wt% H for 5 min, 4.25 wt% H for 10 min, and 4.44 wt% H for 60 min at 573 K under 12 bar H₂. At n = 1, MgH₂–10Ni–2LiBH₄–2Ti desorbed 0.13 wt% H for 10 min, 0.54 wt% H for 20 min, 1.07 wt% H for 30 min, and 1.97 wt% H for 60 min at 573 K under 1.0 bar H₂. The PCT (Pressure–Composition–Temperature) curve at 593 K for MgH₂–10Ni–2LiBH₄–2Ti showed that its hydrogen-storage capacity was 5.10 wt%. The inverse dependence of the hydriding rate on temperature is partly due to a decrease in the pressure differential between the applied hydrogen pressure and the equilibrium plateau pressure with the increase in temperature. The rate-controlling step for the dehydriding reaction of the MgH₂–10Ni–2LiBH₄–2Ti at n = 1 was analyzed.     

Flowerlike microspheres catalyst NiO/La2O3 for ethanol-H2 production

Catalysts of nano-sized nickel oxide particles based on flowerlike lanthanum oxide microspheres with high disperse were prepared to achieve simultaneous dehydrogenation of ethanol and water molecules on multi-active sites. XRD, SEM, 77K N₂ adsorption were used to analyze and observe the catalysts’ structure, morphology and porosity. Catalytic parameters with respect to yield of H₂, activity, selectivity towards gaseous products and stability with time-on-stream and time-on-off-stream were all determined. This special morphology NiO/La₂O₃ catalyst represented more than 1000 h time-on-stream stability test and 500 h time-on-off-stream stability test for hydrogen fuel production from ethanol steam reforming at 300 °C without any deactivation. During the 1000 h time-on-stream stability test, ethanol–water mixtures could be converted into H₂, CO, and CH₄ with average selectivity values of 57.0, 20.1, 19.6 and little CO₂ of 3.2 mol%, respectively, and average ethanol conversion values of 96.7 mol%, with H₂ yield of 1.61 mol H₂/mol C₂H₅OH. During the 500 h time-on-off-stream stability test, ethanol–water mixtures could be converted into H₂, CO, CH₄ and CO₂ with average selectivity values of 65.1, 17.3, 15.1 and 2.5 mol%, respectively, and average ethanol conversion values of 80.0 mol%. For the ethanol-H₂ and petrolic hybrid vehicle (EH–HV), the combustion value is the most important factor. So, it was very suitable for the EH–HV application that the low temperature ethanol steam reforming products’ distribution was with high H₂, CO, CH₄ and very low CO₂ selectivity over the special NiO/La₂O₃ flowerlike microspheres.     

Bench-scale WGS membrane reactor for CO2 capture with co-production of H2

This study investigated the water-gas shift reaction in a bench-scale membrane reactor (M-WGS), where three supported Pd membranes of 44 cm in length and ca. 6 μm in thickness were used, reaching a total membrane surface area of 580.6 cm². The WGS reaction was studied with the syngas mixture: 4.0% CO, 19.2% CO₂, 15.4% H₂O, 1.2% CH₄ and 60.1% H₂, under high temperature/pressure conditions: T = 673 K, p_feed = 20–35 bar(a), p_perm = 15 bar(a), mimicking CO₂ capture with co-production of H₂ in a natural gas fired power plant. High reaction pressure and high permeation of Pd membranes allowed for near complete CO conversion and H₂ recovery. Both the membranes and the membrane reactor demonstrated a long-term stability under the investigated conditions, indicating the potential of M-WGS to substitute conventional systems.     

Thermodynamic analysis of the para-to-ortho hydrogen conversion in cryo-compressed hydrogen vessels for automotive applications

Cryo-compressed hydrogen storage has potential applications in fuel-cell vehicles due to its large storing density and thermal endurance. The dormancy of storage can be extended when considering the endothermic conversion of para-to-ortho hydrogen. In present study, a thermodynamic model is established to analyze the effect of the conversion in a cryogenic pressure vessel. The influence of the parameters such as the filling density, initial temperature and initial ortho hydrogen fraction is studied. It is demonstrated that different “transition pressures” for the vessels exist for different filling densities. The conversion can carry out sufficiently and the dormancy can be extended significantly when the designed release pressure of the vessel matches with the transition pressure. The heat of absorption increases with the initial o-H₂ fraction, whereas the peak of conversion rate occurs earlier for the vessel with a large initial o-H₂ fraction. The dormancy can be extended by 163% for the vessel with filling density of 70 kg/m³. The investigations on the effect of the para-to-ortho hydrogen conversion can provide useful guideline for the design of cryo-compressed hydrogen vessels.     

Destabilization effects of Mg(AlH4)2 on MgH2: Improved desorption performances and its reaction mechanism

Both kinetics and thermodynamics properties of MgH₂ are significantly improved by the addition of Mg(AlH₄)₂. The as-prepared MgH₂–Mg(AlH₄)₂ composite displays superior hydrogen desorption performances, which starts to desorb hydrogen at 90 °C, and a total amount of 7.76 wt% hydrogen is released during its decomposition. The enthalpy of MgH₂-relevant desorption is 32.3 kJ mol⁻¹ H₂ in the MgH₂–Mg(AlH₄)₂ composite, obviously decreased than that of pure MgH₂. The dehydriding mechanism of MgH₂–Mg(AlH₄)₂ composite is systematically investigated by using x-ray diffraction and differential scanning calorimetry. Firstly, Mg(AlH₄)₂ decomposes and produces active Al^∗. Subsequently, the in-situ formed Al^∗ reacts with MgH₂ and forms Mg–Al alloys. For its reversibility, the products are fully re-hydrogenated into MgH₂ and Al^∗, under 3 MPa H₂ pressure at 300 °C for 5 h.     

Thermodynamic analysis of glycerol-steam reforming in the presence of CO2 or H2 as carbon gasifying agent

This paper deals with the thermodynamic analysis of glycerol-steam reforming with H₂ or CO₂ co-fed as carbon gasifying agents in order to mitigate carbon deposition. Thermodynamic calculations were carried out at temperatures from 500 to 800 K and steam-to-glycerol ratios of 0:1–20:1 at atmospheric pressure. Carbon deposition was significant (1.0–1.7 mol C/mol C₃H₈O₃) at low steam-to-glycerol ratio (<4.0) within the reaction temperature range (500–800 K). Carbon-free regime can only be achieved at temperatures above 700 K at steam-to-glycerol ratio of 3:1. Beyond the steam-to-glycerol ratio of 4:1, carbon deposition is essentially zero. The addition of H₂ (as co-reactant) reduced the carbon deposition (down to 0.58 mol C/mol C₃H₈O₃ from 1.70 mol C/mol C₃H₈O₃) even at steam-to-glycerol ratio of 0:1 and reaction temperature of 500 K. Above 5 mol H₂/mol C₃H₈O₃, thermodynamic analysis showed undetectable carbon deposition. Significantly, this could be attributed to the H₂-gasification of carbon species to produce CH₄ and hence, the concomitant increase in the latter. The introduction of CO₂ into the glycerol-steam system, however, led to increased carbon deposition at all temperatures considered in this study due to the reaction between CO₂ and CH₄ in forming carbon deposits. Nevertheless, the carbon yield can be reduced through reforming at higher temperatures. It was further concluded from the current work that H₂ co-feeding linearly increased the exothermicity of reforming system.     

Influence of CO2, CH4, and initial temperature on H2/CO laminar flame speed

The objective of this study is to investigate the impact of syngas composition by varying the H₂/CO ratio (1:3, 1:1, and 3:1 by volume), the CO₂ dilution (0%–40%), and methane addition (0%–40%) on laminar flame speed. Thus, laminar flame speeds of premixed syngas–air mixtures were measured for different equivalence ratios (0.8–2.2) and inlet temperatures (295–450 K) using the Bunsen-burner method. It was found that laminar flame speed increases with increasing H₂/CO ratio, while CO₂ dilution or CH₄ addition decreased it. The location of the maximum flame speed shifts to richer mixtures with decreasing H₂/CO ratio, while it shifts to leaner mixtures with the addition of CH₄ due to its inherent slower flame speed. The location of the maximum flame speed is also shifted towards leaner mixtures with the addition of CO₂ due to the preponderance of the reduction of the adiabatic flame temperature with increasing dilution. Comparison between experimental and numerical results shows a better agreement using a modified mechanism derived from GRI-Mech 3.0. A correlation, based on the experimental results, is proposed to calculate the laminar flame speed over a wide range of equivalence ratios, inlet temperatures, and fuel content.     

CO2 and H2O reduction by solar thermochemical looping using SnO2/SnO redox reactions: Thermogravimetric analysis

The thermochemical dissociation of CO₂ and H₂O from reactive SnO nanopowders is studied via thermogravimetry analysis. SnO is first produced by solar thermal dissociation of SnO₂ using concentrated solar radiation as the high-temperature energy source. The process targets the production of CO and H₂ in separate reactions using SnO as the oxygen carrier and the syngas can be further processed to various synthetic liquid fuels. The global process thus converts and upgrades H₂O and captured CO₂ feedstock into solar chemical fuels from high-temperature solar heat only, since the intermediate oxide is not consumed but recycled in the overall process. The objective of the study was the kinetic characterization of the H₂O and CO₂ reduction reactions using reactive SnO nanopowders synthesized in a high-temperature solar chemical reactor. SnO conversion up to 88% was measured during H₂O reduction at 973 K and an activation energy of 51 ± 7 kJ/mol was identified in the temperature range of 798-923 K. Regarding CO₂ reduction, a higher temperature was required to reach similar SnO conversion (88% at 1073 K) and the activation energy was found to be 88 ± 7 kJ/mol in the range of 973-1173 K with a CO₂ reaction order of 0.96. The SnO conversion and the reaction rate were improved when increasing the temperature or the reacting gas mole fraction. Using active SnO nanopowders thus allowed for efficient and rapid fuel production kinetics from H₂O and CO₂.     

Tri-reforming of methane over Ni@SiO2 catalyst

A nickel-silica core@shell catalyst was applied for a methane tri-reforming process in a fixed-bed reactor. To determine the optimal condition of the tri-reforming process for production of syngas appropriate for methanol synthesis the effect of reaction temperature (550–750 °C), CH₄:H₂O molar ratio (1:0–3.0) and CH₄:O₂ molar ratio (1:0–0.5) in the feedstock was investigated. CH₄ conversion rate and H₂/CO ratio in the produced syngas were influenced by the feedstock composition. Increasing the amount of steam above the proportion of CH₄:H₂O 1:0.5 reduced the H₂:CO molar ratio in produced syngas to ∼1.5. Increasing oxygen partial pressure improved methane conversion to 90% at 750 °C. At low ∼550 °C reaction temperature the tri-reforming process was not effective with low hydrogen production (H₂ yield ∼20%) and very low <5% CO₂ conversion. Increasing reaction temperature increased hydrogen yield to ∼85% at 750 °C. From all the tested reaction conditions the optimal for tri-reforming over the 11%Ni@SiO₂ catalyst was: feed composition with molar ratio CH₄:CO₂:H₂O:O₂:He 1:0.5:0.5:0.1:0.4 at T = 750 °C. The results were explained in the context of characterisation of the catalysts used. The obtained results showed that the tri-reforming process can be applied for production of syngas with composition suitable for methanol synthesis.     

Enhanced dormancy due to para-to-ortho hydrogen conversion in insulated cryogenic pressure vessels for automotive applications

A dynamic model has been developed to characterize dormancy and hydrogen loss from an insulated cryogenic pressure vessel that is filled with 99.79%-para liquid hydrogen to reach supercritical conditions. The model considers the thermodynamics and kinetics of the endothermic para-to-ortho conversion that occurs when the stored H₂ heats after the vessel is exposed to ambient conditions for an extended time. The thermal, thermodynamic, and kinetic aspects of the model were validated against experimental data obtained on a 151-L tank designed for service at nominal pressures up to 350 bar. Depending on the initial pressure, temperature, amount of H₂, and the rate of heat gain from the ambient, the endothermic para-to-ortho conversion can extend the loss-free dormancy time by up to 85%. Under conditions in which the endothermic conversion does not materially affect dormancy, it can still significantly reduce the H₂ loss rate and it can even introduce a secondary dormancy period.     

Mesoporous CexMn1−xO2 composites as novel alternative carriers of supported Co3O4 catalysts for CO preferential oxidation in H2 stream

The mesoporous Co₃O₄ supported catalysts on Ce–M–O (M = Mn, Zr, Sn, Fe and Ti) composites were prepared by surfactant-assisted co-precipitation with subsequent incipient wetness impregnation (SACP–IWI) method. The catalysts were employed to eliminate trace CO from H₂-rich gases through CO preferential oxidation (CO PROX) reaction. Effects of M type in Ce–M–O support, atomic ratio of Ce/(Ce + Mn), Co₃O₄ loading and the presence of H₂O and CO₂ in feed were investigated. Among the studied Ce–M–O composites, the Ce–Mn–O is a superior carrier to the others for supported Co₃O₄ catalysts in CO PROX reaction. Co₃O₄/Ce_0.9Mn_0.1O₂ with 25 wt.% loading exhibits excellent catalytic properties and the 100% CO conversion can be achieved at 125–200 °C. Even with 10 vol.% H₂O and 10 vol.% CO₂ in feed, the complete CO transformation can still be maintained at a wide temperature range of 190–225 °C. Characterization techniques containing N₂ adsorption/desorption, X-ray diffraction (XRD), H₂ temperature-programmed reduction (H₂-TPR) and scanning electron microscopy (SEM) were employed to reveal the relationship between the nature and catalytic performance of the developed catalysts. Results show that the specific surface area doesn’t obviously affect the catalytic performance of the supported cobalt catalysts, but the right M type in carrier with appropriate amount effectively improves the Co₃O₄ dispersibility and the redox behavior of the catalysts. The large reducible Co³⁺ amount and the high tolerance to reduction atmosphere resulted from the interfacial interaction between Co₃O₄ and Ce–Mn support may significantly contribute to the high catalytic performance for CO PROX reaction, even in the simulated syngas.     

Enhancement of hydrogen-storage performance of MgH2 by Mg2Ni formation and hydride-forming Ti addition

MgH₂, rather than Mg, was used as a starting material in this work. A sample with a composition of MgH₂–10Ni–4Ti was prepared by reactive mechanical grinding. Activation of the sample was completed after the first hydriding cycle. At n = 1, the sample desorbed 2.53 wt% H for 10 min, 3.99 wt% H for 20 min, 4.58 wt% H for 30 min, and 4.68 wt% H for 60 min at 593 K under 1.0 bar H₂. At n = 2, the sample absorbed 3.59 wt% H for 5 min, 4.55 wt% H for 25 min, and 4.60 wt% H for 45 min at 593 K under 12 bar H₂. The inverse dependence of the hydriding rate on the temperature in the initial stage and the normal dependence of the hydriding rate on the temperature in the later stage were discussed. The rate-controlling step for the dehydriding reaction of activated MgH₂–10Ni–4Ti was analyzed as the chemical reaction at the hydride/α-solid solution interface.     

Enhancement of the H2 desorption properties of LiAlH4 doping with NiCo2O4 nanorods

Lithium aluminum hydride (LiAlH₄) is considered as an attractive candidate for hydrogen storage owing to its favorable thermodynamics and high hydrogen storage capacity. However, its reaction kinetics and thermodynamics have to be improved for the practical application. In our present work, we have systematically investigated the effect of NiCo₂O₄ (NCO) additive on the dehydrogenation properties and microstructure refinement in LiAlH₄. The dehydrogenation kinetics of LiAlH₄ can be significantly increased with the increase of NiCo₂O₄ content and dehydrogenation temperature. The 2 mol% NiCo₂O₄-doped LiAlH₄ (2% NCO–LiAlH₄) exhibits the superior dehydrogenation performances, which releases 4.95 wt% H₂ at 130 °C and 6.47 wt% H₂ at 150 °C within 150 min. In contrast, the undoped LiAlH₄ sample just releases <1 wt% H₂ after 150 min. About 3.7 wt.% of hydrogen can be released from 2% NCO–LiAlH₄ at 90 °C, where total 7.10 wt% of hydrogen is released at 150 °C. Moreover, 2% NCO–LiAlH₄ displayed remarkably reduced activation energy for the dehydrogenation of LiAlH₄.     

High catalytic kinetic performance of amorphous CoPt NPs induced on CeOx for H2 generation from hydrous hydrazine

CeO_x-induced amorphization of CoPt nanoparticles (NPs) is achieved by a facile co-reduction method using sodium borohydride (NaBH₄) as the reducing agent at room temperature (298 K) under ambient atmosphere. The investigation results indicate that CeO_x plays a critical role in transferring the crystalline CoPt nanoalloy into the amorphous one. To our surprise, the resultant Co_0.65Pt_0.30(CeO_x)_0.05 NPs exhibit high catalytic kinetic performance with 72.1% hydrogen (H₂) selectivity for the H₂ generation from hydrous hydrazine (N₂H₄) within only a few minute at 298 K. Although complete conversion is not achieved, but the initial turnover frequency value of 194.8 h⁻¹ for the present amorphous catalyst is much higher than that of crystalline one. Moreover, such a highly rapid catalyst may greatly encourage the practical application of hydrous N₂H₄ as a hydrogen storage material.     

Efficient catalytic abatement of greenhouse gases: Methane reforming with CO2 using a novel and thermally stable Rh–CeO2 catalyst

2% Rh–CeO₂ catalyst was synthesized using the hard template method and characterized by means of N₂ adsorption/desorption, XRD and H₂-TPR methods. The prepared powdered catalyst exhibited high thermal stability and high surface area with negligible sintering during 24-h exposure to 973 K in an inert atmosphere. During the temperature programmed methane dry reforming reaction between 473 and 1073 K, an increase in the molar H₂/CO ratio from 0.3 at 623 K to as high as 0.96 at 1073 K was observed. Besides H₂ and CO, H₂O was identified among reaction products, originating from the simultaneously occurring reverse water-gas shift reaction. During the isothermal test performed at 923 K, the 2% Rh–CeO₂ catalyst exhibited stable performance and produced syngas with the average H₂/CO ratio equal to 0.62. A relative drop of catalyst activity equal to 11% was observed within 70-h time on stream at 1023 K, with the average H₂/CO ratio at the reactor outlet equal to 0.71.     

Effect of Ce on 5 wt% Ni/ZSM-5 catalysts in the CO2 reforming of CH4 reaction

The aim of this study is to investigate the promotional effect of Ce on Ni/ZSM-5 catalysts in the CO₂ reforming of CH₄ reaction. The evaluation of the catalytic performances of the composite catalysts was conducted in a fixed-bed reactor at atmospheric pressure. The influencing factors, including temperature, Ni and Ce loadings, molar feed ratio of CO₂/CH₄, and time-on-stream (TOS), were investigated. The characteristics of the catalysts were checked with Brunauer-Emmett-Teller (BET) analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The reduction and the basic properties of the composite catalysts were elucidated by temperature-programmed reduction by H₂ (H₂-TPR) and temperature-programmed desorption of CO₂ (CO₂-TPD), respectively. The reactivity of deposited carbon was studied by sequential temperature-programmed surface reaction of CH₄ (CH₄-TPSR) and temperature-programmed oxidation using CO₂ and O₂ (CO₂-TPO and O₂-TPO). Results indicate that higher CH₄ conversion, H₂ selectivity, and desired H₂/CO ratio for 5 wt% Ni & 5 wt% Ce/ZSM-5 could be achieved with CO₂/CH₄ feed ratio close to unity over the temperature range of 500–900 °C. Moreover, the addition of Ce could not only promote CH₄ decomposition for H₂ production but also the gasification of deposited carbon with CO₂. The dispersion of Ni particles could be improved with Ce presence as well. A partial reduction of CeO₂ to CeAlO₃ was observed from XPS spectra over 5 wt% Ni & 5 wt% Ce/ZSM-5 after H₂ reduction and 24 h CO₂–CH₄ reforming reaction. Benefiting from the introduction of 5 wt% Ce, the calculated apparent activation energies of CH₄ and CO₂ over the temperature range of 700–900 °C could be reduced by 30% and 40%, respectively.     

Explosion behavior of CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures

In this work, the explosion behavior of stoichiometric CH₄/O₂/N₂/CO₂ and H₂/O₂/N₂/CO₂ mixtures has been studied both experimentally and theoretically at different CO₂ contents and oxygen air enrichment factors. Peak pressure, maximum rate of pressure rise and laminar burning velocity were measured from pressure time records of explosions occurring in a closed cylindrical vessel. The laminar burning velocity was also computed through CHEMKIN–PREMIX simulations.