Amount Of H2 Research Articles

Novel mesoporous Pt@n-AgI/n-α-MoO3 ternary nanocomposite for enhanced hydrogen production from glycerol under visible illumination

A novel mesoporous n-AgI/n-α-MoO3 nanocomposites with large surface area were constructed by hydrothermal approach utilizing hexamethylenetetramine (HMT) as an organic building block (cage-like). Pt nanoparticles (NPs) were highly distributed over AgI/α-MoO3 nanocomposites in situ during the photocatalysis reaction for photocatalytic H2 evolution under visible exposure. The H2 evolution results indicated that 9%AgI/α-MoO3 nanocomposite exhibited the best H2 evolution ability, and the total H2 amount was determined at about 24100 μmolg−1 within 9 h illumination using 1 g/L the photocatalyst dosage, which increased to 28920 μmolg−1 at 2.5 g/L. The H2 amount of AgI/α-MoO3 photocatalyst was fostered ∼40 and 10 times larger than that of α-MoO3 and AgI NPs. The optimal 9%AgI/α-MoO3 (2521.8 μmolg−1h−1) presented 34.4 and 8.61 times higher H2 evolution rate compared to α-MoO3 (73.23 μmolg−1h−1) AgI (292.91μmolg−1h−1). The resultant S-scheme AgI/α-MoO3 heterojunctions enhanced the separation efficiency of photocarriers with a remarkable enhancement of H2 evolution. The n-n heterojunction AgI/α-MoO3 nanocomposite inhibited the recombination of electrons and holes, decreased photoluminescence intensity, enhanced the current density, and promoted photocatalytic ability. The obtained AgI/α-MoO3 nanocomposite can maintain stability for 45 h of illumination continuously in the photocatalysis reactions for five consecutive cycles. These results provide a simple approach to designing S-scheme photocatalysts with a high separation rate of carriers to promote solar energy conversion efficiency.

Confinement of LiAlH4 in a Mesoporous Carbon Black for Improved Near-Ambient Release of H2

LiAlH4 is a potential solid-state H2 storage material, where safe and efficient H2 storage is of critical importance for the transition towards a sustainable emission-free economy. To improve the H2 release and storage properties of LiAlH4, confinement in porous media decreases the temperature of H2 release and improves the kinetics, where considerably improved H2 release properties are accompanied by a loss in the total amount of H2 released. The capability of mesoporous carbon black to improve the H2 storage properties of confined LiAlH4 is investigated with temperature-programmed desorption and time-stability measurements using X-ray diffraction and N2 gas adsorption measurements to characterize the composite materials’ composition and structure. Here, we present the capability of commercial carbon black to effectively lower the onset temperature of H2 release to that of near-ambient, ≥295 K. In addition, the confinement in mesoporous carbon black destabilized LiAlH4 to a degree that during ≤14 days in storage, under Ar atmosphere and at ambient temperature, 40% of the theoretically contained H2 was lost due to decomposition. Thus, we present the possibility of destabilizing LiAlH4 to a very high degree and, thus, avoiding the melting step before H2 release at around 440 K using scaffold materials with fine-tuned porosities.

The Pt-interfacial-reaction-based quantitative detection of trace H2 released from palladium hydride in aqueous solutions

Hydrogen gas holds tremendous potential across various domains, encompassing industrial applications, medical usage, and scientific research, among others. The precise and reliable detection of hydrogen gas is of paramount importance to ensure its safe utilization and accurate control of associated reactions, among other considerations. However, conventional approaches for quantitatively detecting H2, such as gas chromatography and hydrogen sensors, primarily focus on gas-phase detection, neglecting the detection of trace amounts of H2 in the liquid phase. This limitation becomes particularly pertinent when employing palladium hydride (PdHx) as a slow-releasing agent of hydrogen for medical purposes, necessitating the quantification of H2 released from PdHx in aqueous solutions, which is not straightforward to accomplish. This study presents a straightforward and convenient electrochemical method for quantifying H2 in an aqueous solution released from PdHx. The proposed method establishes a linear relationship between the theoretical concentration of H2 in the solution and the amount of H2 that reacted on the working electrode. Consequently, by measuring the amount of H2 that reacted on the working electrode in a PdHx dispersion, the actual concentration of H2 can be determined using the established linear relation. Specifically, it was found that the concentration of H2 released by 200 μg/mL PdHx at room temperature was determined to be 0.23 mM. To validate the findings, a simulation of the hydrogen detection reaction was conducted, which exhibited good agreement with the experimental results. The simulation results suggest that the presence of a peak near the primary hydrogen oxidation peak could be attributed to the adsorption of H2 and H+ near the electrode surface, and this effect diminishes with increasing temperature due to the weakening of species adsorption at elevated temperatures. Moreover, this study also demonstrated the quantitative verification of O2 in an aqueous solution using a similar method. The detection of trace amounts of H2 or O2 in aqueous solutions can find applications in determining the actual production of H2 in hydrogen evolution reactions facilitated by molecular catalysts and evaluating the trace amounts of H2 in photocatalysis as a complementary technique to gas chromatography. This methodology holds significant potential for advancing various areas of research.

CO2 capture and conversion to methane with Ni-substituted hydrotalcite dual function extrudates

Nickel-substituted hydrotalcites promoted with cesium were synthesized, and their dual-function ability for acting as a high-temperature CO2 adsorbent and methanation catalyst was assessed. The Ni-substituted hydrotalcites were synthesized by the co-precipitation method with (Ni + Mg)/Al and Ni/(Ni + Mg) molar ratios of 3 and 0.4, respectively. Extrudates of these materials were prepared and tested for CO2 capture from a synthetic flue gas mixture (with 15 vol% CO2) and subsequent methanation. Two wetness impregnation routes with cesium were considered: i) powder impregnation before the extrudates shaping (PI) or ii) the direct impregnation of the extrudates (EI). EI materials were able to sorb more CO2 due to their higher number of basic sites, especially those with strong strength. However, while the amount of CH4 produced by both types of samples is similar, EI materials are less selective to methane (i.e., they present a slightly higher formation of CO). For this reason, Cs-doped PI extrudates were selected and used in the parametric study. Under cyclic steady-state and operation at 350 °C and 1 bar, the reactor packed with this material allowed capturing ca. 0.24 molCO2 kgDFM-1 and the complete conversion of the captured CO2 into CH4. However, a substantial amount of H2 is released from the reactor without being converted, hindering the CH4 purity level. This indicator can be increased at the expense of some product loss by performing a short-duration purge with CH4 between the capture and methanation stages. Remarkedly, this material showed outstanding stability during the continuous cyclic operation, without CO formation.

Core-shell structure of reduced graphene oxide@Ag–TiO2 for photocatalytic H2O splitting and CH3OH dehydrogenation under UV light irradiation

The H2 production from water-splitting and dehydrogenation of waste alcohols is widely investigated in current years to find out greener, and more efficient photocatalyst materials. Reduced graphene oxide (rGO; 1-10 wt%) coated TiO2 and Ag(1 wt%)-TiO2 core-shell nanocomposites are fabricated for enhanced photocatalytic H2 production from H2O-splitting and CH3OH-dehydrogenation under UV irradiation. These rGO@TiO2 and rGO@Ag–TiO2 catalysts are characterized by XRD, Raman, FE-SEM, HR-TEM, XPS, BET, CV, EDS-mapping, PL, and UV–vis DRS for morphological, structural, and optical properties. After rGO(1-3 wt%) coating over TiO2, the H2 amount is increased from 5 to 14 mmol for H2O-splitting and 13–83 mmol via CH3OH-dehydrogenation during 5 h UV irradiation, respectively. Due to the synergetic effect of rGO and Ag, core-shell rGO(2-4 wt%) coated Ag–TiO2 produced the highest amount of 108–133 mmol of H2 from water-splitting and 440–450 mmol from CH3OH-dehydrogenation during 7 and 5 h UV irradiation, respectively. The work illustrates the potential of Ag-deposition and rGO-coating over TiO2 for efficient H2 production.

Photocatalytic activity of non-oxides materials TiB2, TiC, and TiN

This study aims to explore, for the first time, the effect of the anion species on the photocatalytic activity of the compounds TiB2, TiC, and TiN for H2 production. Therefore, non-oxide titanium compounds have been investigated as alternative semiconductor materials to replace TiO2. In this case, these non-oxide materials were evaluated as photocatalysts for the production of hydrogen due to their negative conduction band energies, which facilitate the accumulation of electrons on the surface and promote the water-splitting reaction. Specifically, TiN exhibits the lowest electron-hole recombination, favoring the generation of hydrogen because more electrons are transferred in this material compared to TiB2 and TiC (2- and 3-fold higher amounts of H2, respectively). Photoluminescence (PL) analysis revealed that TiN spectra showed the lowest emission, indicating a lower concentration of recombination sites, attributed to the presence of Ti-3d orbitals, resulting in improved electrical conductivity and enhanced charge transfer during the reaction. However, in the case of TiC, some of the Ti-3d orbitals are not occupied by free electrons, resulting in partial oxidation, this leads to a higher resistance to charge transfer, promoting electron-hole recombination, which in turn affects the activity of hydrogen generation. Therefore, commercial TiN can be considered a viable candidate for use in photocatalytic systems.

Biohydrogen production of a halophytic cyanobacteria Phormidium keutzingium and activated sludge co-culture using different carbon substrates and saline concentrations

Hydrogen (H2) production using a biological pathway such as anaerobic digestion suffers limitations in the survival of microorganisms. Usually, microalgae are used as feedstock but the acidic pH of digestate ruptures the cell wall of microalgae resulting in dead cellular debris. Whereas some cyanobacteria (blue-green microalgae) are very resilient and tolerant to stressful environments such as hypersaline conditions. Therefore, in this study, a microbial consortium (co-culture) of cyanobacteria Phormidium keutzingianum and activated sludge bacteria (ASB) was established to generate H2 production from waste resources. Different co-culturing ratios 2:1, 1:1, and 1:2 v/v (P. keutzingianum: ASB) were analyzed for H2 production with glucose, sorbitol, and mannitol as carbon substrates. In addition, the effect of different concentrations (0, 10, and 20 g/L) of salt (NaCl) on H2 production was also tested. Results indicated that the amount of cumulative H2 produced changed significantly by varying the carbon substrates. Glucose-supplemented co-culture produced the lowest amount of H2 at 278 mL/L as compared to 980 and 562 mL H2/L in sorbitol and mannitol-supplemented co-cultures, respectively. The addition of NaCl negatively affected H2 production by increasing the salinity level from 0 to 20 g/L, and the cumulative H2 reduced from 980 to 176.8 mL/L and 562 to 333 mL/L in sorbitol and mannitol-supplemented co-culture, respectively. Sorbitol showed cyanobacterial growth in terms of chlorophyll from 600 to 1200 μg/L and optical density from 1 to 12 at λ 620 nm. Total organic carbon (TOC) in sorbitol supplemented co-cultures also increased to the peak 5.7, 6.7, and 7.7 g/L in 2:1, 1:2, and 1:1 v/v ratios, respectively, indicating anaerobic growth. Hence, this study signifies that sorbitol appeared to be the significant carbon substrate that helped cyanobacteria grow under anaerobic conditions resulting in higher H2 production. Moreover, cyanobacteria are more tolerant to stressful environments such as hypersaline and acidic conditions compared to microalgae.

Simulation And Optimization Of Hydrogen Production By Steam Reforming Of Natural Gas

Hydrogen (H2) production through natural gas steam reforming is widely adopted due to its cost-effectiveness and energy efficiency. A simulation and optimization study was performed on an industrial natural gas steam reforming system using Aspen Hysys V12 software to optimize this process. The study focused on optimizing various parameters, including the Reformer Reactor, Water Gas Shift Reactor, and purification units such as the Separator and Pressure Swing Adsorption (PSA). The Reformer and Water Gas Shift reactors were set at 900 °C and 300 °C, respectively, to maximize hydrogen production. Under specific conditions of 5 atm pressure and a steam-to-carbon ratio (S/C) of 2.5, the process achieved a hydrogen production rate of 402.2 kg/h. The treatment zone effectively eliminated ~ 100% of undesirable CO2 and CO gases, with only trace amounts of CH4 and H2 remaining in the waste gases. Additionally, the PSA unit efficiently removed ~ 100% of the water from the separator, ensuring water-free dry gases were sent to the PSA unit. The integration of heating and cooling heat exchange units reduced energy consumption by approximately 51.6%. After the removal of undesired gases in our PSA unit, the production yield for the final product (H2, based on dry gas inlet to PSA) is 77.83%, resulting in 100% pure dry H2. In the waste gas outlet (tail gas) of PSA a composition (22.17%), includes CO, CO, H2O, and CH4. Resulting high-quality hydrogen is well-suited for a wide range of applications, including fuel cells, petroleum refining, natural gas refineries, and petrochemical processes.

Quantifying Carbon Active Sites Chemisorbing Hydrogen on Oxygen Containing Activated Carbons during Heat Treatment in Hydrogen Atmosphere.

Carbon edge sites have been widely studied because of their importance in surface reactivity and electronic properties. The surface chemistry of the carbon edge sites is relevant to various reactions, and carbon active sites are key topics in many applications. Temperature-programmed desorption (TPD) and temperature-programmed reaction (TPR) techniques are used to clarify the fate of oxygen atoms present as CO-yielding functional groups on the activated carbon during heat treatment in hydrogen with an argon balance atmosphere. It has been elucidated that CO is decomposed, H2O is released by a reduction reaction with atmospheric H2, and CO2 is evolved by secondary reactions from the CO-yielding functional groups during TPR. Atmospheric H2 consumption during TPR is observed and its rate is characterized. The amounts of carbon active sites are quantified by determining the amount of H2 chemisorbed onto the carbon surfaces. Finally, it is quantitatively determined that the active sites that chemisorb hydrogen are generated after the decomposition of CO and CO2 caused by secondary reactions between ca. 700 and 1100 K from the CO-yielding functional groups. The origin of these CO-yielding functional groups is generally attributed to phenol/ether groups. In addition to these oxygen-containing functional group decompositions, some free sites on the edge sites are activated for H2 chemisorption by heat treatment between ca. 700 and 1100 K.

Quantifying H2 emissions under different nutritional mitigation strategies and its impact on improving the prediction of enteric methane emissions of ruminants

Stoichiometric models that predict methane (CH4) production of ruminants assume that the amount of hydrogen gas (H2) produced during fermentation in the rumen equals the amount of H2 consumed by electron sinks in the rumen and thus that H2 should not be emitted into the environment. However, some studies have demonstrated that H2 emissions occur under practical conditions. In addition, the H2 emissions increase when nutritional mitigation strategies are used. Hence, this study hypothesized that considering H2 emission would improve the prediction of enteric CH4 emission, especially when using mitigation strategies that induce high variation in the H2 emission. The objective of this study was to develop and evaluate the performance of CH4 emission prediction models using H2 emission as an explanatory variable. A database of CH4 and H2 emission (mean treatment data) from ruminants (dairy cattle, growing cattle and sheeps) was built to develop mixed-effects models at three levels of complexity: level 1 - H2 production/yield, DMI, or both; level 2 - level 1 explanatory variables plus chemical composition of the diet (i.e. CP, EE, NDF, OM, or PCO); and level 3 - level 2 explanatory variables plus animal metabolic weight (BW0.75, kg). When all animal categories were grouped, including H2 production improved the performance of CH4 production prediction only in level 1 models for the electron-receptor mitigation strategy, reducing the root mean square of prediction error (RMSPE) by 18 %. For dairy cattle and sheep, dry matter intake was not a significant explanatory variable in level 1 or 2 models. For growing beef cattle, for all mitigation strategies together and the inhibitor-mitigation strategy alone, including H2 production reduced RMSPE by 13 % and 27 %, respectively. Overall, H2 production was included in 60 % of level 1 models and 100 % of level 2 and 3 models, which best predicted CH4 production. Thus, including H2 emission does improve prediction of enteric CH4 emission, especially when mitigation strategies that induce variation in the H2 are used.

Kinetic investigations of coal gasification for high-purity H2 production with carbon capture and storage potential

A novel coal conversion pathway for clean hydrogen production with carbon capture and storage (CCS) is proposed. The alkaline coal gasification for high-purity hydrogen production with CCS is conducted in a designed reactor at moderate conditions (1 atm, 873 K). Hydroxides and a Ni catalyst are used to produce 51.12 mmol H2 with the purity of 99.03% from 1 g of coal. The NaOH can accelerate the coal decomposition and dehydrogenation while both Group I and Group II hydroxides can effectively capture CO2 in situ. The Ni catalyst can greatly promote the methane recombination reaction, water gas shift and macromolecular condensate recombination to generate large amounts of H2. The carbon analysis in the solid and gaseous products shows the carbon capture and storage potential of the alkaline gasification.

Investigation on the pre-reduction of ilmenite pellet using H2/CO atmospheres

ABSTRACT In this study, ilmenite and bentonite were uniformly mixed, pelletised, and baked at temperatures up to 1200°C. The reduction of ilmenite pellet by H2/CO mixtures and H2 pure gas was studied in the temperature range of 760–890°C. Reduced pellet microstructures and phases analysed by, Optical Microscopy (OM), Scanning Electron Microscopy (SEM), and Energy Dispersive X-Ray Analysis (EDX) Mapping. Experimental works shown that between 760°C, 880°C, and 890°C, the H2 amount and reduction temperature have a considerable influence on the degree of metallisation. The obtain results demonstrate the degree of metallisation increased with increase of reduction temperature and reaction time. The degree of metallisation could reach 93.64% when the sample was reduced to 890°C for 360 min in pure H2 atmosphere. In microstructural analysis, metallic iron particles became smaller and more uniformly distributed as hydrogen content of the reducing gas increased.

Silver vanadate-coupled yttrium orthovanadate nanocomposite: An improved photocatalyst for amended visible-light-driven hydrogen evolution

The synthesis of yttrium orthovanadate (YVO4) by surfactant-aided sol-gel method is presented in this work. To enhance the visible-light activity of YVO4, 4.0–16.0 wt % of silver vanadate (AVO) nanoparticles were coupled by simple impregnation. The formed nanocomposites were applied for hydrogen (H2) production by photocatalytic reaction in a glycerol/water mixture with traces of platinum as a cocatalyst. The incorporation of 12.0 wt% AVO has boosted the H2 evolution by > 130 times (3.000 mmol g−1h−1) to the pure YVO4 (0.023 mmol g−1h−1). These outcomes were attributed to the efficient separation of photogenerated charges as well as bandgap reduction to 2.35 eV compared to 3.54 eV in pristine YVO4. The produced H2 could be furtherly enhanced to 3.750 mmol g−1h−1 by optimizing the photocatalyst amount at 2.0 g L−1. This innovative nanocomposite could also maintain its ability to produce 96% of its original H2 amount after the fifth regeneration cycle as well.

The potential of green hydrogen fuel as an alternative in Cameroon's road transport sector

The objective of this paper is to evaluate and assess the potential of green hydrogen (H2) as a substitute fuel for Cameroon's road transportation system. To achieve this objective, we start by providing a status quo of Cameroon's road transport. Then, we conduct an empirical analysis on the potential of H2 as a fuel for vehicles. Based on photovoltaic electrolysis, we estimate the amount of H2 needed to fuel vehicles with the associated costs as well as the quantity of carbon emissions that could be reduced in the event of an energy switch over the period 2020–2035. Finally, techno-economic indicators are used to assess the cost-effectiveness of an H2 vehicle in comparison to a petrol car. Results reveal that if a transition were to take place before 2035 the Cameroonian road transport sector would need between 1.75 MT and 2.5 MT of H2 fuel per year. With this transition, Cameroon could reduce its emissions by around 26% (or 429 megatons of CO2 equivalent). Hydrogen-fuelled vehicles offer the greatest potential for initiating this transition with the current level of technology. Indeed, starting from an investment cost for PV of 0.42 USD/Wp, H2 in a hydrogen-fuelled vehicle can be cost-effective compared with petrol in an internal combustion engine with a PV efficiency of 40%. Therefore, a hydrogen fuel-based transport sector could emerge through technological and production advances. This study contributes to solving the problems related to fossil fuel availability and greenhouse gas emissions arising from the expansion of the road transport sector in Cameroon, envisaging H2 as an alternative fuel.

H2, CH4 and CO2 adsorption on Cameo coal: Insights into the role of cushion gas in hydrogen geological storage

Large-scale hydrogen underground storage is widely considered as an important solution for energy transition to mitigate global warming, however, our understanding on the various chemical and physical processes leading to H2 loss during geo-storage remains largely unknown. We conducted adsorption isotherms measurements of H2, CH4 and CO2 on Cretaceous Cameo coal at 35, 50 and 65 °C, to explore adsorption mechanisms of H2 and deciphering the effect of cushion gas (e.g., CH4, CO2) on H2 loss in the subsurface. The adsorbed amounts of H2 are just 12% and 6% of CH4 and CO2 at temperature of 35 °C and pressure of 2.3 MPa, respectively, and H2 adsorption behavior on Cameo coal can be described well by Langmuir monolayer theory. The extremely high Langmuir pressures (35–49 MPa) for H2 indicate very weak affinity with coal, compared to CH4 and CO2. Differences of Langmuir adsorption capacities suggest that the gas adsorption of H2 and CH4 are mainly by physical interactions while both physical and chemical bonding with coal structures occurs for CO2. The affinity with solid materials for the three gases can be quantitatively evaluated by the heat of adsorption, which was determined to be ∼9.2 kJ/mol for H2. Extrapolation of our experimental results to natural coal seams indicates that H2 presents much smaller adsorption amounts at depth up to 2000 m, corresponding to temperatures lower than 65 °C and pressure less than 30 MPa, geological conditions that are suitable for hydrogen underground storage. This study confirms that the injection of CH4 or CO2 as cushion gas can largely reduce the H2 loss by adsorption in the subsurface. Empirical calculation suggests H2 adsorption will be insignificant with chemical composition of CH4 above 8% or CO2 above 2% at the storage sites (e.g., abandoned mines, depleted coal seams). Our study lays a foundation on the understanding of the risk of H2 loss in large-scale hydrogen underground storage.

Vine shoots pre-treatment strategies for improved hydrogen production and metabolites redistribution in Clostridium butyricum

This work deals with the use of vine shoots, a renewable, largely available, lacking of alternatives lignocellulosic material as a feedstock for hydrogen production. Physical pre-treatments by steam explosion (SE), chemical by organosolv (OS) and biological by laccase (LAC) were carried out in vine shoots to disrupt the cell fiber and increase the biomass hydrolysis and fermentation into hydrogen (H2). After SE, there was a slight decrease in cellulose and hemicellulose contents in biomass fibers, while a decrease in lignin content occurred after OS pre-treatment. There were no quantifiable changes after laccase pre-treatment, however the enzyme-substrate oxidative reactions were favorable for hydrolysis and fermentation since an increase in soluble sugars and H2 production was observed with LAC vine shoots as substrate. 300.1 mL H2/L were obtained from raw material vine shoots, while 649.4, 399.8 and 749.7 mL H2/L were obtained from biomass pre-treated by SE, OS and LAC, respectively. Furthermore, the hydrolysis of pre-treated biomass by addition of cellulase was evaluated to improve H2 production. Higher amounts of H2 were obtained from hydrolyzed biomass in relation to non-hydrolyzed ones (154.2%, 602.0% and 167.1% more with SE, OS and LAC hydrolyzed, respectively). In all cases, the mixed acid pathway was carried out by Clostridium butyricum, since acetic and butyric acids were produced.

Facile synthesis of ultrahigh-surface-area and hierarchically porous carbon for efficient capture and separation of CO2 and enhanced CH4 and H2 storage applications

Ultrahigh surface area and hierarchically microporous carbons functionalized with heteroatoms are critical for many applications, particularly gas storage and separation. However, the facile synthesis of such carbons remains challenging. Herein, we present a single-step activation strategy for producing porous polymer-based ultrahigh surface area carbons through melamine-mediated potassium oxalate activation. As prepared carbon possesses a uniform distribution of heteroatoms and hierarchical micropores, and exhibits a maximum specific surface area of 3463 m2/g, which is among the best reported for carbon materials. The presence of heteroatoms (N and O), along with its ultra-high surface area and hierarchical microporous structure, plays a significant role in storing large amounts of CO2 (22.8 mmol/g at 298 K/25 bar), CH4 (10.1 mmol/g at 298 K/25 bar) and H2 (5.2 wt% at 77 K/50 bar). Furthermore, it demonstrates a selective capture of CO2 over N2, CH4 and H2. The developed carbon is highly suitable for gas storage and the separation of CO2 from flue gas and syngas.

Studies on the Reduction Behavior of Iron Oxide Pellet Fines with Hydrogen Gas: Mechanism and Kinetic Analysis

The present study contributes to the current worldwide activities aiming to replace fossil carbon in steel making processes with hydrogen causing considerable reduction of greenhouse gas emissions. Compacts prepared from iron oxide pellets fines were isothermally reduced in pure hydrogen gas and a mixture of hydrogen and argon in the temperatures range from 700 to 1100 °C. The total weight loss produced during the reduction process was continuously recorded using thermogravimetric analysis (TG) technique. The findings demonstrated that the temperature has a considerable impact on the conversion and reduction rates. At a given temperature, the reduction rate was accelerated as the amount of H2 increased in the reducing gas. The results indicated that H2 content does not have an effect on reduction behavior, when it is higher than 80%. The reduction reaction of samples was shown to takes place in a step wise manner from hematite to metallic iron. The reduction kinetic and mechanism were deduced from the application of mathematical models and the morphological structure of the reduced samples and correlated with the apparent activation energy (Ea) values. The Ea values at the early, intermediate and final stages were 16.36, 29.24 and 49.35 kJ/mole, respectively. The early stage of the reduction process was controlled by chemical reaction, whereas the gaseous diffusion was controlled the latter stage. At the intermediate stage, the reduction process was controlled by mixed mechanism of gaseous diffusion and chemical reaction.Graphical

Chemical insights into the two-stage ignition behavior of NH3/H2 mixtures in an RCM

This study investigated the two-stage ignition behavior of NH3/H2 mixtures using a rapid compression machine (RCM). The ignition delay times and first-stage ignition delay times of NH3/H2 blends were measured at temperatures ranging from 881 to 1127 K, pressures of 15 and 25 bar, and equivalence ratios (ϕ) of 1.0 and 1.5. Experimental results showed that H2 had a significant promotional effect on the ignition of NH3, and two-stage ignition behavior was observed in some test mixtures. Time-resolved species concentrations were recorded using the gas chromatography (GC) method during the single- and two-stage ignition process. Species evolution suggested that the consumption of NH3 and H2 separated during the two-stage ignition process. The oxidation of H2 primarily occurred at the first-stage ignition point, while NH3 oxidation occurred at the total ignition point. A kinetic model was developed to predict the two-stage ignition behavior of NH3/H2 and the species profiles. Model analysis showed that H2 played a key role in initiating the oxidation process and contributed to the early heat release. When H2 content was high (e.g. 50%), its oxidation led to the simultaneous total oxidation of NH3, resulting in single-stage ignition characteristics. However, when a small amount of H2 was present (e.g. 10%), its oxidation only partially consumed NH3, leading to the two-stage ignition behavior. Sensitivity analyses indicated that the co-oxidation of fuels during the first-stage ignition was primarily dominated by H2 oxidation chemistry, while NH3 oxidation became dominant during the following total ignition stage as H2 was completely consumed. Additional model simulations revealed that the ignition behavior of NH3/H2 mixtures is strongly influenced by temperature and pressure. A distinct threshold for temperature and pressure was identified, demarcating the transition between single-stage and two-stage ignition phenomena. Moreover, the fraction of H2 in the mixture had a significant impact on the ignition behavior, with higher fractions leading to increased intensity of the first-stage ignition and closer proximity to the total ignition point. However, when the H2 fraction exceeds a certain threshold, two-stage ignition behavior disappears.

Low- and intermediate-temperature ammonia/hydrogen oxidation in a flow reactor: Experiments and a wide-range kinetic modeling

Understanding the chemistry behind the oxidation of ammonia/hydrogen mixtures is crucial for ensuring the flexible use of such mixtures in several applications, related to propulsion systems and power generation. In this work, the oxidation of ammonia/hydrogen blends was investigated through an experimental and kinetic-modeling study, where the low- and intermediate-temperature conditions were considered. An experimental campaign was performed in a flow reactor, at stoichiometric conditions and near-atmospheric pressure (126.7 kPa). The mole fraction of fuels, oxidizer and final products was measured. At the same time, a comprehensive kinetic model was set up, following a modular and hierarchical approach, and implementing the recently-available elementary rates. Such a model was used to interpret the experimental results, and to extend the analysis to literature data, covering several oxidation features. The reactivity boost provided by H2 addition was found to be approximately linear with its mole fraction in both flow- and jet-stirred-reactor conditions (except for the smallest H2 amounts in the flow reactor), in contrast with the more-than-linear increase in the laminar flame speed. The key role of HO2 in regulating fuel conversion and autoignition at low temperature was confirmed for binary mixtures, with H2NO being the bottleneck to the low-temperature oxidation of NH3-rich blends. On the other hand, the nitrogen fate was found to be mostly regulated by NHx + NO propagation and termination channels.