High H2 Selectivity Research Articles

Waste to resource: Developing red mud as low-cost catalysts to enhance catalytic co-pyrolysis of tobacco waste and low-density polyethylene

Solid waste co-utilization technology is an effective way to improve the resourceful use of solid waste. In this study, Ni- CRM600 catalysts were prepared from industrial solid waste red mud (RM) and used to catalyze the co-pyrolysis of tobacco waste (TW) and low-density polyethylene (LDPE) for H2-rich combustible gas (H2, CO and CH4) production. The results showed that the Ni-CRM600 catalyst was able to significantly promote the reforming reaction of volatile matter, compared to the original RM, the modified Ni-CRM600 catalyst improved the H2 yield by more than 8 times and the CO yield by more than 6 times in the co-pyrolysis of TW with LDPE. The highest yield of H2 reached 572.02 mL/g, and the low heating value (LHV) of the combustible gas was 143.54 J/g. In the regeneration experiments of the catalyst, the regenerated catalyst showed excellent stability and remained catalytic activity after 10 cycles. The characterization of the Ni-CRM600 catalyst showed that the large number of oxygen vacancies (Ov) in RM provided an active site for NiO, resulting in the excellent catalytic activity of Ni-CRM600. The investigation of the main active components of Ni-CRM600 showed that Fe2O3 significantly promotes the yield of CO, CO2 and light hydrocarbons (CH4, C2H4, C2H6) in the gas, while NiO has a high selectivity for H2. This study provided a novel solution for co-disposal of multi-source bulk solid waste with implications on high-quality combustible gas production and synergistic resource utilization.

Non‐selective Defect Minimization towards Highly Efficient Metal‐Organic Framework Membranes for Gas Separation

The persistence of defects in polycrystalline membranes poses a substantial obstacle to reaching the theoretical molecular sieving separation and scaling up production. The low membrane selectivity in most reported literature is largely due to the unavoidable non‐selective defects during synthesis, leading to a mismatch between the well‐defined pore structure of polycrystalline molecular sieve materials. This paper presents a novel approach for minimizing non‐selective defects in metal‐organic framework (MOF) membranes by a constricted crystal growth strategy in a confined environment. The in‐situ ZIF formation using the densely packed seeding array between the substrate and the pre‐grown top ZIF layer yields a confined membrane interlayer, which is highly uniform with a tightly packed crystalline structure. Unlike uncontrolled crystal growth, we purposely regulate the interlayer membrane growth in the direction parallel to the substrate. A notable 99% decrease in defects in the confined interlayer was achieved compared to the random‐grown top layer, leading to a ~353% increment in H2/N2 selectivity over the non‐confined reference MOF membrane. The performance of this new membrane sits in the optimal range above the Robeson upper bound. The membrane boasts a balanced high H2 permeability (>5000 Barrer) and selectivity (>50), significantly surpassing peer ZIF membranes.

Propane steam reforming over La0.8Sr0.2Ni1-yMyO3 (M = Cr, Mn, Fe, Co) perovskite-type oxides

Perovskite-type oxides have attracted significant attention in recent years as potential catalysts for hydrocarbon reforming reactions. Herein, a series of oxides with the formula La0.8Sr0.2Ni1-yMyO3 (LSNi1-yMy; M=Cr, Mn, Fe, Co) have been synthesized and tested for the propane steam reforming (PSR) reaction. Materials were characterized employing BET, XRD, H2-TPR, SEM/EDX, TEM/SAED and TPO techniques. Optimal results were obtained for LSNi0.5Cr0.5, which exhibited high propane conversion (78 %), H2-selectivity (98 %), and excellent stability with time-on-stream (40 hours) at 600 °C. This has been attributed to the exceptional ability of LSNi0.5Cr0.5 to maintain its perovskite structure under reaction conditions, the in situ formation of finely dispersed Ni-Cr alloy crystallites, and the suppression of reactions that lead to graphitic carbon deposition. This study provides deeper insight into the effects of structural properties and reducibility of perovskite-type oxides on their catalytic properties and can be used to design catalysts with improved performance.

Hydrogen purification in nitrogen-doped two-dimensional conjugated microporous polymers

Conjugated microporous polymers (CMPs) have emerged as advanced membranes with unique pore structures for the separation of small kinetic diameter gases, including H2. In this work, we present a comprehensive investigation into the potential of N-doped pyridinyl functionalized conjugated microporous polymers, denoted as CHNX (X = 1, 2, 6), in H2 purification by using molecular dynamic simulations. Our findings demonstrate that CHNX membranes exhibit exceptional H2 purification performance with both high H2 selectivity and permeance. Notably, the CHN6 membrane shows the highest H2 permeance of 2.61 × 10−5 mol m−2 s−1 Pa−1, owing to its larger pore area relative to the other membranes. Moreover, both CHN and CHN2 membranes achieve 100 % gas selectivity based on the molar sieving mechanism. To gain deeper insights into the gas separation process, we analyze the gas-membrane interaction, 2D density distribution, center of mass, mean square displacement, etc. The elucidation of these dynamics enhances our understanding of the underlying gas separation mechanisms. Ultimately, this work provides crucial theoretical guidance for the future development of gas separation membranes in hydrogen purification applications, thereby advancing the field of sustainable energy production.

Atomic level transition of graphene layer to carbon nanotubes over cobalt during ethanol decomposition reaction

Herein we report the catalytic ethanol decomposition over cobalt catalyst synthesized using conventional solution combustion synthesis (SCS). The reaction pathway proposed in-situ DRIFT analysis shows that the decomposition reaction proceeds through the formation of surface ethoxy intermediates that transformed into acetates and aldehydes which further decompose to releases CH4, H2 and CO2 with some deposition of carbon on the catalyst surface. The catalytic reaction shows 100 % ethanol conversion at 420 °C, with high selectivity of H2 in the output. The structural transformation of the catalyst and deposited carbon was analyzed using high-resolution transmission electron microscope (HRTEM). Presence of small Co nanoparticles (size <30 nm) surrounded with graphene and larger metal nanoparticles trapped inside MWCNTs was visible during a stability run over 50 h that motivates in proposing the reaction mechanism of the fluctuating metal nanoparticle along with carbon on the catalyst surface. Moreover, this work opens up the possibility of synthesizing highly dispersed Co nanoparticles in an efficient way without any complex experimental procedures and equipment that can further be used as effective catalysts in many industrial reactions.

Efficient Low‐temperature Hydrogen Production by Electrochemical‐assisted Methanol Steam Reforming

AbstractMethanol steam reforming (MSR) provides an alternative way for efficient production and safe transportation of hydrogen but requires harsh conditions and complicated purification processes. In this work, an efficient electrochemical‐assisted MSR reaction for pure H2 production at lower temperature (~140 °C) is developed by coupling the electrocatalysis reaction into the MSR in a polymer electrolyte membrane electrolysis reactor. By electrochemically assisted, the two critical steps including the methanol dehydrogenation and water‐gas shift reaction are accelerated, which is attributed to decreasing the methanol dehydrogenation energy and promoting the dissociation of H2O to OH* by the applied potential. Furthermore, the reduced H2 partial pressure by the hydrogen oxidation and reduction process further promotes MSR. The combination of these advantages not only efficiently decreases the MSR temperature but also achieves the high rate of hydrogen production of 505 mmol H2 g Pt−1 h−1 with exceptionally high H2 selectivity (99 %) at 180 °C and a low voltage (0.4 V), and the productivity is about 30‐fold than that of traditional MSR. This study opens up a new avenue to design novel electrolysis cells for hydrogen production.

Efficient Low-temperature Hydrogen Production by Electrochemical-assisted Methanol Steam Reforming.

Methanol steam reforming (MSR) provides an alternative way for efficient production and safe transportation of hydrogen but requires harsh conditions and complicated purification processes. In this work, an efficient electrochemical-assisted MSR reaction for pure H2 production at lower temperature (~140 °C) is developed by coupling the electrocatalysis reaction into the MSR in a polymer electrolyte membrane electrolysis reactor. By electrochemically assisted, the two critical steps including the methanol dehydrogenation and water-gas shift reaction are accelerated, which is attributed to decreasing the methanol dehydrogenation energy and promoting the dissociation of H2 O to OH* by the applied potential. Furthermore, the reduced H2 partial pressure by the hydrogen oxidation and reduction process further promotes MSR. The combination of these advantages not only efficiently decreases the MSR temperature but also achieves the high rate of hydrogen production of 505 mmol H2 g Pt -1 h-1 with exceptionally high H2 selectivity (99 %) at 180 °C and a low voltage (0.4 V), and the productivity is about 30-fold than that of traditional MSR. This study opens up a new avenue to design novel electrolysis cells for hydrogen production.

Highly stable and reversible hydrogen sensors using Pd-coated SnO2 nanorods and an electrode–substrate interface as a parallel conduction channel

We demonstrated highly stable and reversible H2 sensors using a dual electron-conduction channel comprising Pd-coated SnO2 nanorods (NRs) and a parallel conduction channel containing Cr/native oxide/p-type Si interface. The dual-channel sensor exhibited highly reversible resistance changes during H2 in-and-out cycles at operating temperatures of 25 °C− 100 °C, while typical single-channel sensors with SnO2 NRs fabricated on SiO2 (300 nm)/Si substrates showed irreversible response because of insufficient recovery from Pd-Hx to Pd-Ox. In particular, the sensor showed significantly high repeatability, long-term stability, and selectivity for H2 sensing at 80 °C. Additionally, the time taken to detect H2 above 1% was less than 2 s and the response to H2 above 500 ppm was unaffected by high humidity. These results indicate that the dual-channel Pd-coated SnO2 NR sensor is suitable for applications such as hydrogen leak detectors that require high speed and precision. Moreover, it was found that the sensor could detect H2 below 0.5 ppm even at low temperatures. This work provides a pathway for improving gas-sensing performance by interface engineering, enabling various practical hydrogen sensor applications.

Investigating the promotion of Fe–Co catalyst by alkali and alkaline earth metals of inherent metal minerals for biomass pyrolysis

The alkali and alkaline earth metals (AAEMs) in the inherent metallic minerals of biomass have a crucial effect on the pyrolysis process. In our previous work, the experiments mainly investigated the effects of Fe–Co catalysts on pine needle (PN) pyrolysis. This work further explored the synergistic effect of AAEMs (K, Ca, Na, and Mg) with Fe–Co-catalyzed biomass pyrolysis. The results showed that AAEMs in biomass could improve the gas yield by promoting decarbonization, decarboxylation, and hydrocarbon conversion. Adding Co to Fe enhanced the stability and anti-coking properties of the catalyst, which facilitated the breakage of C–H/C–C bonds and the conversion of oxygenated compounds to hydrocarbons, increasing the H2 yield (13.99 mL/g → 92.50 mL/g). AAEMs enhanced the catalytic activity of Fe–Co catalysts with high selectivity for H2 (92.50 mL/g→104.81 mL/g) and CO (65.00 mL/g→73.65 mL/g). The characterization results showed that AAEMs promoted the reduction of Fe–Co catalyst and the dispersion of metal active sites, which facilitated the transition from disordered to ordered carbon and the removal of oxygen functional groups such as O–H, thus increasing the CO yield, while the effective cleavage of CH4 and C2 compounds and the dehydrogenation of organic components promoted the increase of H2 yield, and the activity followed Na > Ca > Mg > K.

Poly-triphenylene membrane: A multifunctional two-dimensional porous carbon framework with ultra-high carrier mobilities and potassium ion storage capacity

The development of multifunctional two-dimensional (2D) carbon-based materials with novel, excellent properties and high performance is a challenge. By using density functional theory methods, a novel 2D planar and porous carbon material 2D-C18H6, which is a poly-triphenylene membrane composed of triphenylene molecules, is systematically investigated. The computational results confirm that the 2D-C18H6 structure has good lattice dynamics stability and high thermal stability. 2D-C18H6 displays great gas separation ability with high selectivity and high diffusive flux of He and H2 from other gas molecules, due to its dense pores of proper size in the structure. It is a semiconductor with a direct band gap of 2.36 eV and ultra-high carrier mobilities along both a and b directions, up to 7.6 × 104 cm2V−1s−1. The adsorption and migration of K atoms on 2D-C18H6 monolayer indicate it exhibits ultra-high theoretical specific capacity of 1449 mAh·g−1, low voltage of 0.285–0.024 V and low migration barrier of 0.125 eV. Possessing various excellent properties, 2D-C18H6 monolayer has potential applications in electronic device semiconductors, potassium ion battery anode materials, photocatalytic hydrogen evolution materials and gas separation membranes.

Feed Effects on Water–Gas Shift Activity of M/Co3O4-ZrO2 (M = Pt, Pd, and Ru) and Potassium Role in Methane Suppression

Water–gas shift (WGS) is an industrial process to tackle CO abatement and H2 upgradation. The syngas (CO and H2 mixture) obtained from steam or dry reformers often has unreacted (from dry reforming) or undesired (from steam reforming) CO2, which is subsequently sent to downstream WGS reactor for H2 upgradation. Thus, industrial processes must deal with CO2 and H2 in the reformate feed. Achieving high CO2 or H2 selectivities become challenging due to possible CO and CO2 methanation reactions, which further increases the separation costs to produce pure H2. In this study, M/Co3O4-ZrO2 (M = Ru, Pd and Pt) catalysts were prepared using sonochemical synthesis. The synthesized catalysts were tested for WGS activity under three feed conditions, namely, Feed A (CO and steam), Feed B (CO, H2 and steam) and Feed C (CO, H2, CO2 and steam). All the catalysts gave zero methane selectivity under Feed A conditions, whereas the methane selectivity was significant under Feed B and C conditions. Among all catalysts, PtCZ was found to be the best performing catalyst in terms of CO conversion and CO2 selectivity. However, it still suffered with low but significant methane selectivity. This best performing catalyst was further modified with an alkali component, potassium to suppress undesirable methane selectivity. All the catalysts were well characterized with BET, SEM, TEM to confirm the structural properties and effective doping of the noble metals. Additionally, the apparent activation energies were obtained to showcase the best catalyst.

Novel spinel oxide catalysts CuFexAl2−xO4 with high H2 selectivity with low CO generation in methanol steam reforming

A series of CuFexAl2−xO4 (0 <x < 2) spinel oxide catalysts are prepared via the sol-gel method for methanol steam reforming (MSR). The CuFexAl2−xO4 catalysts are studied by characterization of XRD, SEM, EDS, XPS, Raman techniques. The influence of Fe doping on the catalytic performance of CuFexAl2−xO4 is studied in-depth. The results show that the catalytic performance of the fresh CuFexAl2−xO4 is better than the reduced one. In addition, proper Fe doping could improve the activity of the catalysts. In the study, CuFe1.2Al0.8O4 showed the best catalytic performance among the CuFexAl2−xO4 (0 <x < 2). With Fe doping, the spinel Cu2+ was move to catalyst surface while partial Fe2+ cations incorporated into the spinel structure. Then the surface structure and micro-Cu environment are changed, which is conducive to the Cu releasing from spinel structure. The selectivity of hydrogen for the developed CuFe1.2Al0.8O4 is more than 99%, and the selectivity of CO is less than 1%.

Hydrogen-permeable DDR zeolite membrane packed with Zn/HZSM-5 catalyst for methane co-aromatization with ethylene

Co-aromatization of methane (CH4) and ethylene (C2H4) over a membrane reactor (MR) using hydrogen-permeable DDR zeolite membrane packed with Zn/HZSM-5 catalyst was studied for the first time. The adopted DDR zeolite membrane showed high H2 selectivity for separations of binary H2/CH4 and trinary H2/CH4/C2H4 mixtures. Typically, the selectivities for H2/CH4 and H2/C2H4 were 40 and 29 for trinary mixture separation at room temperature, respectively. The effects of reaction temperature, methane-ethylene feed ratio, space velocity and H2 removal on the membrane reaction performance were investigated systematically. The membrane reactor can achieve 8.43% methane conversion with BTX selectivity of 80.68% within 1 h under the optimal reaction conditions (reaction temperature 450 °C, methane-ethylene feed ratio 3:2, weight hourly space velocity (WHSV) 800 mL/(gcat h), sweep helium flow rate 15 mL/min). Compared to a fixed-bed reactor (FBR) with similar reaction condition, the achieved methane conversion and BTX yield were improved by ∼42% and ∼10%, respectively, due to the timely removal of H2 from the reaction system. The membrane also showed good stability, and the methane conversion maintained at about 7% for about 30 h. After burning the coke formed on the catalyst, the initial reaction performance can be restored, showing a good regeneration ability.

Ultrasound and Microwave-Assisted Synthesis of Hexagonally Ordered Ce-Promoted Mesoporous Silica as Ni Supports for Ethanol Steam Reforming

Solvothermal synthesis of mesoporous materials based on amphiphilic molecules as structure-directing agents can be enhanced using non-conventional technologies for stirring and thermal activation. Here, we disclose a green synthesis approach for the preparation of cerium-modified hexagonally ordered silica sieves. Ultrasound micromixing enabled us to obtain well-dispersed Ce in the self-assembled silica network and yielded ordered materials with high cerium content (Ce/Si molar ratio = 0.08). Microwave dielectric heating, applied by an innovative open-end coaxial antenna, was used to reduce the overall hydrothermal synthesis time and to improve the surface area and textural properties. These mesoporous materials were used as a Ni catalyst support (10 wt.% metal loading) for the ethanol steam reforming reaction. The new catalysts featured complete ethanol conversion, high H2 selectivity (65%) and better stability, compared to the same catalyst prepared with magnetic stirring and conventional heating. The Ce-promoted silica sieves offered a suitable support for the controlled growth of nanocarbon that does not result in catalyst deactivation or poisoning after 6 h on stream.

Hydrothermal Stability of Hydrogen-Selective Carbon-Ceramic Membranes Derived from Polybenzoxazine-Modified Silica-Zirconia.

This work investigated the long-term hydrothermal performance of composite carbon-SiO2-ZrO2 membranes. A carbon-SiO2-ZrO2 composite was formed from the inert pyrolysis of SiO2-ZrO2-polybenzoxazine resin. The carbon-SiO2-ZrO2 composites prepared at 550 and 750 °C had different surface and microstructural properties. A carbon-SiO2-ZrO2 membrane fabricated at 750 °C exhibited H2 selectivity over CO2, N2, and CH4 of 27, 139, and 1026, respectively, that were higher than those of a membrane fabricated at 550 °C (5, 12, and 11, respectively). In addition to maintaining high H2 permeance and selectivity, the carbon-SiO2-ZrO2 membrane fabricated at 750 °C also showed better stability under hydrothermal conditions at steam partial pressures of 90 (30 mol%) and 150 kPa (50 mol%) compared with the membrane fabricated at 500 °C. This was attributed to the complete pyrolytic and ceramic transformation of the microstructure after pyrolysis at 750 °C. This work thus demonstrates the promise of carbon-SiO2-ZrO2 membranes for H2 separation under severe hydrothermal conditions.

Atomic layer deposited aluminium oxide membranes for selective hydrogen separation through molecular sieving

Ultra-thin membranes have significant potential for the gas separation. However, the fabrication of ultra-thin membrane with high selectivity and permeance remains a challenge. Herein, we develop a high-performance hydrogen separation membrane using atomic layer deposition (ALD) method to deposit ultra-thin Al2O3 layer (ALD-Al2O3) on a porous anodic aluminum oxide (AAO) support. The pore openings of the AAO substrate were fully covered, but not sealed, by the Al2O3 overlayer after 260 ALD cycles. Due to the low deposition temperature (50 °C), angstrom-scale pores were formed within the Al2O3 overlayer. Single gas permeance results revealed that the size of the pores to be smaller than 0.364 nm, but larger than the kinetic diameter of molecular H2 (0.289 nm). This membrane at room temperature, exhibits high H2 permeance of ∼3.03 × 10−7 mol m−2 s−1 Pa−1 and high H2 selectivity for H2/CO2 (5148.2), H2/O2 (120.7), H2/N2 (161.5) and H2/CH4 (219.5), showing great potential for industrial H2 purification and recovery.

Engineering Supramolecular Binding Sites in a Chemically Stable Metal-Organic Framework for Simultaneous High C2 H2 Storage and Separation.

Developing porous materials to overcome the trade-off between adsorption capacity and selectivity for C2 H2 /CO2 separation remains a challenge. Herein, we report a stable HKUST-1-like MOF (ZJU-50a), featuring large cages decorated with high density of supramolecular binding sites to achieve both high C2 H2 storage and selectivity. ZJU-50a exhibits one of the highest C2 H2 storage capacity (192 cm3 g-1 ) and concurrently high C2 H2 /CO2 selectivity (12) at 298 K and 1 bar. Single-crystal X-ray diffraction studies on gas-loaded ZJU-50a crystal unveil that the incorporated supramolecular binding sites can selectively take up C2 H2 molecule but not CO2 to result in both high C2 H2 storage and selectivity. Breakthrough experiments validated its separation performance for C2 H2 /CO2 mixtures, providing a high C2 H2 recovery capacity of 84.2 L kg-1 with 99.5 % purity. This study suggests a novel strategy of engineering supramolecular binding sites into MOFs to overcome the trade-off for this separation.

Enhanced hydrogen production using a tandem biomass pyrolysis and plasma reforming process

Converting biomass into energy and fuels is considered a promising strategy for replacing the exhaustible fossil fuels. In this study, we report on a tandem process that combines cellulose pyrolysis and plasma-assisted reforming for H2 production. The hybrid pyrolysis/plasma reforming process was carried out in a two-stage reaction system incorporating a coaxial dielectric barrier discharge (DBD) plasma reactor. The effects of discharge power, steam, reforming temperature, and catalyst on the reaction performance were investigated. The results show that low temperatures are preferred in the non-catalytic plasma reforming process, whereas high temperatures are desired to achieve a high H2 yield and a high H2 selectivity in the plasma-catalytic reforming system. The synergistic effect of plasma catalysis was dominant in the plasma-catalytic reforming process at 250 °C. In contrast, the catalyst, rather than the plasma, played a dominant role in the plasma-catalytic reforming at higher temperatures (550 °C). Using Ni-Co/Al2O3 at a reforming temperature of 550 °C, a high H2 yield of 26.6 mmol/g was attainted, which was more than 8 times and about 100% greater than that obtained using plasma alone and catalyst alone, respectively. This work highlights the potential of non-thermal plasmas in low-temperature biomass conversion.

Evaluation of Different Oxygen Carriers for Chemical Looping Reforming of Toluene as Tar Model Compound in Biomass Gasification Gas: A Thermodynamic Analysis

A thermodynamic study on a toluene chemical looping reforming process with six metal oxides was conducted to evaluate the product distribution for selecting an appropriate oxygen carrier with thermodynamic favorability towards high syngas yield. The results show that a suitable operation temperature for most oxygen carriers is 900 °C considering syngas selectivity and solid C formation whether the toluene is fed alone or together with fuel gas. The syngas selectivity of all oxygen carriers decreases with the increasing equivalence ratio, but the decrease degrees are quite different due to their different thermodynamic natures. With the increasing amounts of H2 and CO, the syngas selectivity for various oxygen carriers correspondingly decreases. The addition of CO2 and H2O(g) benefits reducing the solid C formation, whereas the addition of CH4 leads to more solid C being produced. Under the simulated gasification gas atmosphere, a synergetic elimination of solid C and water–gas shift reactions are observed. In terms of syngas selectivity, Mn2O3 possesses the best performance, followed by CaFe2O4 and Fe2O3, but NiO and CuO exhibit the lowest performance. BaFe2O4 presents a high H2 selectivity but a very poor CO selectivity due to the formation of BaCO3, which has a high thermodynamic stability below 1200 °C. Nevertheless, Mn2O3 is more likely to form solid C than feeding toluene alone and has a lower melting point. Considering syngas selectivity, carbon deposit and melting point, CaFe2O4 exhibits the highest performance concerning the tar chemical looping.

Synergistic effect of Promoter addition and Nickel Loading for Catalysed Dry Reforming of Methane

This paper investigates the synergistic influence of both catalyst active sites Lanthanum promoter and Nickel loading on supported Ni catalysts for Dry Reforming of Methane (DRM) reaction. Alumina-supported nanosized Ni-based catalysts were synthesized using the impregnation method with varying percentages of Lanthanum promoter. Structural analysis via XRD, SEM and PSD show Nickel and lanthanum were homogeneously dispersed on Al2O3 supported catalyst with a range of 40-90 nm were obtained. Lanthanum addition favours equilibrated reaction equivalent to DRM mechanism. Increasing the La percentage and temperature however shifts the equilibrium to a higher conversion of CH4 (72%) hence higher H2 selectivity (70%).