Enhanced hydrogen adsorption in ordered mesoporous carbon through clathrate formation

Ordered mesoporous carbons were synthesized with a soft-template approach and modified with a water and tetrahydrofuran mixture having a H₂O/THF molar ratio of 17:1 as potential adsorbent media for hydrogen storage. Hydrogen adsorption equilibrium on the carbon adsorbents was measured gravimetrically at 270 K and hydrogen pressures up to 163 bar. Enhanced hydrogen adsorption was observed on the carbon adsorbents doped with 0.5 wt.% and 0.75 wt.% of H₂O/THF due to the combined effects of hydrogen adsorption on the carbon surface and formation of a binary H₂–H₂O–THF clathrate. Hydrogen adsorption capacities on the carbon adsorbents doped with 0.5 wt.%, 0.75 wt.% of H₂O/THF, and the pure carbon at 270 K and 163 bar are 0.747 wt.%, 0.646 wt.% and 0.585 wt.%, respectively. The hydrogen adsorption isotherms on all the doped carbon adsorbents are of typical Type III and can be well correlated by the Freundlich equation. A desorption hysteresis loop was observed on the carbon adsorbents doped with 0.5 wt.% and 0.75 wt.% of H₂O/THF, which was probably caused by the pore size difference during the adsorption and desorption steps.     

Recyclable hydrogen storage system composed of ammonia and alkali metal hydride

Ammonia (NH₃) reacts with alkali metal hydrides MH (M = Li, Na, and K) in an exothermic reaction to release hydrogen (H₂) at room temperature, resulting that alkali metal amides (MNH₂) which are formed as by-products. In this work, hydrogen desorption properties of these systems and the condition for the recycle from MNH₂ back to MH were investigated systematically. For the hydrogen desorption reaction, the reactivities of MH with NH₃ were better following the atomic number of M on the periodic table, Li < Na < K. It was confirmed that the hydrogen absorption reaction of all the systems proceeded under 0.5 MPa of H₂ flow condition below 300 °C.     

Kinetic interactions between hydrogen and carbon monoxide oxidation over platinum

The heterogeneous chemistry coupling of H₂ and CO over platinum was investigated experimentally and numerically for H₂/CO/O₂/N₂ mixtures with overall lean equivalence ratios φ = 0.13–0.26, H₂:CO molar ratios 1:5–3:1, and a pressure of 5 bar. Experiments were performed in an optically accessible channel-flow reactor at surface temperatures 510–827 K and involved in situ Raman measurements of major gas-phase species concentrations and thermocouple measurements of surface temperatures. Emphasis was placed on the low temperature range 510–600 K, whereby H₂ inhibited the CO oxidation, and which was of particular relevance to gas turbine idling and part-load operation. Comparisons of measurements with 2-D simulations attested the aptness of the employed kinetic scheme, not only for H₂/CO fuel mixtures but also for pure CO. Measured and predicted transition temperatures below which H₂ inhibited CO oxidation agreed well with each other, showing a main dependence on the overall equivalence ratio (550 ± 5 K at φ = 0.13 and 600 ± 5 K at φ = 0.26) and a weaker dependence on the H₂:CO ratio. Furthermore, this inhibition was non-monotonically dependent on the H₂:CO ratio, becoming higher at a value of 1:1. The inhibiting kinetic effect of H₂ was an outcome of the competition between H₂ and CO/O₂ for surface adsorption and, most importantly, of the competition between the adsorbed H(s) and CO(s) for surface-deficient O(s). Finally, transient simulations in practical catalytic channels revealed the interplay between kinetic and thermal effects. While at φ = 0.13 the H₂/CO reactive mixture exothermicity was insufficient to overtake the kinetic inhibition, at φ = 0.26 catalytic ignition could still be achieved at temperatures well-below the transition temperature. The effect of H₂:CO molar ratio on the light-off times was quite strong, suggesting care when designing syngas catalytic rectors with varying compositions.     

The role of differently distributed vanadium nanocatalyst in the hydrogen storage of magnesium nanostructures

Using a glancing angle (co)deposition technique, ∼4.6 at.% V has been coated on the surface of individual Mg nanoblades and doped into Mg nanostructures fabricated at different deposition angles. The hydrogen storage properties of the formed V-decorated and V-doped Mg nanostructures depend strongly on how the nanocatalyst V is surrounded by the host Mg. The V-doped Mg sample has lower activation energies for hydrogen absorption (E_a^a = 35.3 ± 0.9 kJ/mol H₂) and hydrogen desorption (E_a^d = 38.9 ± 0.3 kJ/mol H₂) than the V-decorated Mg sample when deposited at the same deposition angle of θ = 70°. The activation energies of the doped samples increase gradually with the decrease of the θ angle. We also find that the porosity of the Mg nanostructures plays a secondary role. A phenomenological model based on a heterogeneous reaction is proposed to explain the different hydrogen desorption activation energies obtained for different V–Mg nanostructured samples.     

High density hydrogen storage in nanocavities: Role of the electrostatic interaction

High pressure H₂ adsorption isotherms at N₂ liquid temperature were recorded for the series of cubic nitroprussides, Ni_1−xCo_x[Fe(CN)₅NO] with x = 0, 0.5, 0.7, 1. The obtained data were interpreted according to the effective polarizing power for the metal found at the surface of the cavity. The cavity volume where the hydrogen molecules are accumulated was estimated from the amount of water molecules that are occupying that available space in the as-synthesized solids considering a water density of 1 g/cm³. The calculated cavity volume was then used to obtain the density of H₂ storage in the cavity. For the Ni-containing material the highest storage density was obtained, in a cavity volume of 448.5 Å³ up to 10.4 hydrogen molecules are accumulated, for a local density of 77.6 g/L, above the density value corresponding to liquid hydrogen (71 g/L). Such high value of local density was interpreted as related to the electrostatic contribution to the adsorption potential for the hydrogen molecule within the cavity.     

Interrelationships between bioreactor volume,effluent recycle rate,temperature, pH, %H2, hydrogen productivity and hydrogen yield with undefined bacterial cultures

Simultaneous achievement of high volumetric productivities (HP = 231.3 mmol H₂/L/h) and high hydrogen yields (HY = 3.55 mol H₂/mol glucose) was obtained by increasing the temperature to 70 °C and by reducing the total bioreactor system volume (V) to 5.74 L and increasing the degassed effluent recycle rate (F_er) to 3.2 L/min, giving a V/F_er value of 1.8 min. The bioprocess involved the recycling of degassed effluent at a high flow rate through a quasi-stationary fluidized granular bed. In this process the rate of physical removal of H₂ trapped in the bulk liquid phase surrounding the fluidized granules reduced the thermodynamic constraints preventing the simultaneous achievement of high HPs and high HYs in the anaerobic fluidized granular bed bioreactor. Energy balance analysis showed that with heat recycling the bioreactor system could achieve a net positive volumetric energy output of 11.76 W/L at an energy efficiency of 49.3%.     

Nanocarbon boosts energy-efficient hydrogen production in carbon-assisted water electrolysis

To improve upon our previously reported slow hydrogen evolution rate R_H at the energy-efficient lower voltages in CAWE (carbon-assisted water electrolysis) at room temperature, new results using different carbons and catalysts to improve R_H are reported here. Compared to earlier results with carbon GX203, about a ten-fold increase in R_H is reported using high surface area carbon BP2000 at the operating voltage E^o = 1.12 V. With added FeSO₄ catalyst, E^o is lowered to 0.72 V without lowering R_H, representing about 30% decrease in the energy barrier of the process. For comparison, in water electrolysis without carbon, measurable R_H is observed only for E^o ≥ 2 V. This large improvement in R_H at the energy efficient E^o = 0.72 V is suggested to result from nanoscale particle size of carbon BP2000 as well as from electrons provided by the catalyst through the reaction Fe²⁺ ? Fe³⁺ + e⁻. By measuring the amounts of H₂ evolved at the cathode and CO₂ evolved at the anode using gas chromatography, the mechanism for CAWE is established to be the reaction: C (s) + 2H₂O (?) → CO₂ (g) + 2H₂ (g). The reaction slows down with time as carbon is depleted by oxidation.     

Correlation study between hydrogen absorption property and lattice structure of Mg-based BCC alloys

Nanostructured Mg₆₀Ni₅Co_mX_35 − m (X = Co, B, Al, Cr, V, Pd and Cu) body centered cubic (BCC) alloys were synthesized by mechanical alloying method. These Mg-based alloys with different lattice parameters can show significantly different hydrogen absorption properties. The BCC alloys with lattice parameter in the range of 0.300∼0.308 nm absorb large amount of hydrogen at 373 K and the BCC alloys with the parameter larger than 0.313 nm have difficulty to absorb hydrogen at this temperature. Geometric effect is thought to be one of the dominant factors to affect the hydrogen absorption property of interstitial alloys. Nanostructure, fresh surface area and defects produced during mechanical alloying process are also important facts that make Mg-based alloys absorb hydrogen at 373 K.     

Commercializable power source from forming new states of hydrogen

The data from a broad spectrum of investigational techniques strongly and consistently indicates that hydrogen can exist in lower-energy states than previously thought possible. The predicted reaction involves a resonant, nonradiative energy transfer from otherwise stable atomic hydrogen to a catalyst capable of accepting the energy. The product is H(1/p  ), fractional Rydberg states of atomic hydrogen called “hydrino atoms” wherein n=(1/2,1/3,1/4,…1/p)$n=(1/2,1/3,1/4,\dots 1/p)$ (p ≤ 137 is an integer) replaces the well-known parameter n = integer in the Rydberg equation for hydrogen excited states. Atomic lithium and molecular NaH served as catalysts since they meet the catalyst criterion—a chemical or physical process with an enthalpy change equal to an integer multiple m of the potential energy of atomic hydrogen, 27.2 eV (e.g. m = 3 for Li and m = 2 for NaH). Specific predictions based on closed-form equations for energy levels of the corresponding hydrino hydride ions H⁻(1/4) of novel alkali halido hydrino hydride compounds (MH*X; M = Li or Na, X = halide) and molecular hydrino H₂(1/4) were tested using chemically generated catalysis reactants.     

Deoxidation of Ti–Al intermetallics via hydrogen treatment

Alloys of Ti–47Al were deoxidized via a simple method called hydrogen treatment (HT), which involves deoxidation with hydrogen in a melting process. Because of the increase in the partial pressure of hydrogen and the melting duration, the oxygen content of the alloys greatly decreased after HT. Activated hydrogen atoms dissociated at high temperatures, and the hydrogen molecules in the melting chamber seemed to affect the deoxidation reactions, which are represented by the following equations: O + 2H = H₂O and O + H₂ = H₂O. Based on a comparison of the changes in Gibbs free energy, the hydrogen atoms were found to play a major role in deoxidation.     

Influence of molecular hydrogen on acetylene pyrolysis: Experiment and modeling

The effect of molecular hydrogen on the formation of molecular carbonaceous species important for soot formation is studied through a combination of shock-tube experiments with high-repetition-rate time-of-flight mass spectrometry and detailed chemistry modeling. The experiment allows to simultaneously measure the concentration–time profiles for various species with a time resolution of 10 μs. Concentration histories of reactants and polyacetylene intermediates (C_2xH₂, x = 1–4) are measured during the pyrolysis of acetylene with and without H₂ added to the gas mixture for a wide range of conditions. In the 1760–2565 K temperature range, reasonable agreement between the experiment and the model predictions for C₂H₂, C₄H₂, C₆H₂, and C₈H₂ is achieved. H₂ addition leads to the depletion of important building blocks for particle formation, namely of polyacetylenes due to an enhanced consumption of important radicals by H₂, which are required for the fast build-up of carbonaceous material.     

Liquid hydrogen production via hydrogen sulfide methane reformation

Hydrogen sulfide (H₂S) methane (CH₄) reformation (H₂SMR) (2H₂S + CH₄ = CS₂ + 4H₂) is a potentially viable process for the removal of H₂S from sour natural gas resources or other methane containing gases. Unlike steam methane reformation that generates carbon dioxide as a by-product, H₂SMR produces carbon disulfide (CS₂), a liquid under ambient temperature and pressure—a commodity chemical that is also a feedstock for the synthesis of sulfuric acid. Pinch point analyses for H₂SMR were conducted to determine the reaction conditions necessary for no carbon lay down to occur. Calculations showed that to prevent solid carbon formation, low inlet CH₄ to H₂S ratios are needed. In this paper, we analyze H₂SMR with either a cryogenic process or a membrane separation operation for production of either liquid or gaseous hydrogen. Of the three H₂SMR hydrogen production flowsheets analyzed, direct liquid hydrogen generation has higher first and second law efficiencies of exceeding 80% and 50%, respectively.     

Physical and photo-electrochemical characterizations of α-Fe2O3. Application for hydrogen production

The optical, electrical and photo-electrochemical properties of dense hematite α-Fe₂O₃ have been studied for the photo-catalytic hydrogen production. The band gap was evaluated at 1.96 eV from the diffuse reflectance spectrum and the transition is directly allowed; further indirect transition occurs at 2.04 eV. The oxygen deficiency permits the altering of the transport properties and the oxide exhibits n type behavior with activation energy of 0.11 eV. α-Fe₂O₃ is found to be photo-electrochemically active. The flat band potential V_fb (−0.51 V_SCE) and the density N_D (19.12 × 10¹⁹ cm⁻³) were obtained respectively by extrapolating the linear part to C⁻² = 0 and the slope of the Mott–Schottky plot. The complex impedance pattern is circular in the high frequency region followed by a straight line in the low frequency one, a behavior attributed to the Warburg ionic diffusion. The conduction band edge (−0.62 V_SCE) lies below the H₂O/H₂ level (−0.50 V_SCE) and Fe₂O₃ offers the possibility to be used as hydrogen photocathode. The best activity was obtained in SO₃²⁻ (0.5 M, pH 13.8) solution with a rate evolution of 6 ml (g catalyst)⁻¹ min⁻¹.     

Ionic conduction in Sn1−xScxP2O7 for intermediate temperature fuel cells

A novel series of mixed ion conductors, Sn_1−xSc_xP₂O₇ (x = 0.03, 0.06, 0.09, 0.12), were synthesized by a solid-state reaction method. The conduction behaviors of the ion conductors in wet hydrogen atmosphere were investigated by some electrochemical methods including AC impedance spectroscopy, gas concentration cells in the temperature range of 323-523 K. It was found that the doping limit of Sc³⁺ in SnP₂O₇ was between 9 mol% and 12 mol%. The highest conductivity was observed to be 2.76 × 10⁻² S cm⁻¹ for the sample of x = 0.06 under wet H₂ atmosphere at 473 K. The ionic conduction was contributed mainly to proton and partially to oxide ion in wet hydrogen atmosphere from 373 K to 523 K. The H₂/air fuel cells using Sn_1−xSc_xP₂O₇ (x = 0.03, 0.06, 0.09) as electrolytes (1.7 mm in thickness) generated the maximum power densities of 11.16 mW cm⁻² for x = 0.03, 25.02 mW cm⁻² for x = 0.06 and 14.34 mW cm⁻² for x = 0.09 at 423 K, respectively. The results indicated that Sn_1−xSc_xP₂O₇ is a promising solid electrolyte system for intermediate temperature fuel cells.     

Optimization of media composition for hydrogen gas production from hydrolyzed wheat starch by dark fermentation

Effects of N/C, P/C and Fe(II)/C ratios in fermentation medium on biohydrogen production by dark fermentation of acid-hydrolyzed wheat starch was investigated. The powdered wheat was autoclaved at pH = 3 and 90 °C for 15 min and the resulting sugar solution was fermented after external addition of N, P and Fe(II) to overcome nutrient limitations. Box–Wilson statistical experiment design was used by considering the N/C (0–0.05, w w⁻¹), P/C (0–0.02) and Fe(II)/C (0–0.03) ratios as the independent variables while the hydrogen yield and specific hydrogen production rate (SHPR) were the objective functions to be optimized. A quadratic response function was used to correlate the response functions with the independent variables. Low levels of the variables (N/C < 0.02, P/C < 0.01, Fe(II)/C < 0.01) resulted in low hydrogen yield and SHPR due to nutrient limitations and high levels of nutrients caused inhibitions. The optimum conditions yielding the maximum hydrogen yield (Y = 2.84 mol H₂ mol⁻¹ glucose) were N/C = 0.02, P/C = 0.008 and Fe(II)/C = 0.015. The maximum SHPR (96 mL H₂ g⁻¹ biomass h⁻¹) was obtained at N/C = 0.025, P/C = 0.008 and Fe(II)/C = 0.015 (w w⁻¹).     

Methane steam reforming for producing hydrogen in an atmospheric-pressure microwave plasma reactor

A methane steam reforming process for producing mainly hydrogen in an atmospheric-pressure microwave plasma reactor is demonstrated. Nano carbon powders, CO_x, C₂H₂, C₂H₄, and HCN were also formed. Intermediates such as OH, NH, CH, and active N₂ were identified using optical emission spectroscopy. The selectivity of H₂ was greater than 92.7% at inlet H₂O/CH₄ molar ratio (R) ≧ 0.5, and was higher than that obtained using methane plasmalysis because steam inhibited the formation of C₂H₂. The highest methane conversion was obtained at R = 1, reaching 91.6%, with the lowest specific energy consumption of H₂ formation at [CH₄]_in = 5%, 1.0 kW, and 12 slpm. The plasma-assisted catalysis process, which packed Ni/Al₂O₃ catalysts in the discharge zone and supplied heat using hot effluents, was used to elevate the methane conversion and hydrogen selectivity. However, large amounts of 40–70 nm carbon powder, which is electrically conductive, were produced, resulting in rapid catalyst deactivation due to carbon being deposited on the surface and in the pores of catalysts.     

Experimental and kinetic study on ignition delay times of DME/H2/O2/Ar mixtures

Ignition delay times of dimethyl ether (DME)/hydrogen/oxygen/argon mixtures (hydrogen blending ratio ranging from 0% to 100%) were measured behind reflected shock waves at pressures of 1.2–10 atm, temperature range of 900–1700 K, and for the lean (? = 0.5), stoichiometric (? = 1.0) and rich (? = 2.0) mixtures. For more understanding the effect of initial parameters, correlations of ignition delay times for the lean mixtures were obtained on the basis of the measured data (X_H2 ? 95%) through multiple linear regression. Ignition delay times of the DME/H₂ mixtures demonstrate three ignition regimes. For X_H2 ? 80%, the ignition is dominated by the DME chemistry and ignition delay times show a typical Arrhenius dependence on temperature and pressure. For 80% ? X_H2 ? 98%, the ignition is dominated by the combined chemistries of DME and hydrogen, and ignition delay times at higher pressures give higher ignition activation energy. However, for X_H2 ? 98%, the transition in activation energy for the mixture was found as decreasing the temperature, indicating that the ignition is dominated by the hydrogen chemistry. Simulations were made using two available models and different results were presented. Thus, sensitivity analysis was performed to illustrate the causes of different simulation results of the two models. Subsequently, chemically interpreting on the effect of hydrogen blending ratio on ignition delay times was made using small radical mole fraction and reaction pathway analysis. Finally, high-pressure simulations were performed, serving as a starting point for the future work.     

Study of the H + O + M reaction forming OH: Kinetics of OH chemiluminescence in hydrogen combustion systems

The temporal variation of OH^∗ (A²Σ⁺) chemiluminescence in hydrogen oxidation chemistry has been studied in a shock tube behind reflected shock waves at temperatures of 1400-3300 K and at a pressure of 1 bar. The aim of the present work is to obtain a validated reaction scheme to describe OH^∗ formation in the H₂/O₂ system. Temporal OH^∗ emission profiles and ignition delay times for lean and stoichiometric H₂/O₂ mixtures diluted in 97-98% argon were obtained from the shock-tube experiments. Based on a literature review for the hydrogen combustion system, the key reaction considered was H + O + M = OH^∗ + M (R1). The temperature dependence of the measured peak OH^∗ emission from the shock tube and the peak OH^∗ concentration from a homogeneous closed reactor model are compared. Based on these results a reaction rate coefficient of k₁ = (1.5 ± 0.4) × 10¹³ exp(−25 kJ mol⁻¹/RT) cm⁶ mol⁻² s⁻¹ was found for the forward reaction (R1) which is slightly higher than the rate coefficient suggested by Hidaka et al. (1982). The comparison of measured and simulated absolute concentrations shows good agreement. Additionally, a one-dimensional laminar premixed low-pressure flame calculation was performed for where absolute OH^∗ concentration measurements have been reported by Smith et al. (2005). The absolute peak OH^∗ concentration is fairly well reproduced if the above mentioned rate coefficient is used in the simulation.     

Effect of organic loading on a novel hydrogen bioreactor

This study investigated the impact of six organic loading rates (OLR) ranging from 6.5 gCOD/L-d to 206 gCOD/L-d on the performance of a novel integrated biohydrogen reactor clarifier systems (IBRCSs) comprised a continuously stirred reactor (CSTR) for biological hydrogen production, followed by an uncovered gravity settler for decoupling of solids retention time (SRT) from hydraulic retention time (HRT). The system was able to maintain a high molar hydrogen yield of 2.8 mol H₂/mol glucose at OLR ranging from 6.5 to 103 gCOD/L-d, but dropped precipitously to approximately 1.2 and 1.1 mol H₂/mol glucose for the OLRs of 154 and 206 gCOD/L-d, respectively. The optimum OLR at HRT of 8 h for maximizing both hydrogen molar yield and volumetric hydrogen production was 103 gCOD/L-d. A positive statistical correlation was observed between the molar hydrogen production and the molar acetate-to-butyrate ratio. Biomass yield correlated negatively with hydrogen yield, although not linearly. Analyzing the food-to-microorganisms (F/M) data in this study and others revealed that, both molar hydrogen yields and biomass specific hydrogen rates peaked at 2.8 mol H₂/mol glucose and 2.3 L/gVSS-d at F/M ratios ranging from 4.4 to 6.4 gCOD/gVSS-d. Microbial community analysis for OLRs of 6.5 and 25.7 gCOD/L-d showed the predominance of hydrogen producers such as Clostridium acetobutyricum, Klebsiella pneumonia, Clostridium butyricum, Clostridium pasteurianum. While at extremely high OLRs of 154 and 206 gCOD/L-d, a microbial shift was clearly evident due to the coexistence of the non-hydrogen producers such as Lactococcus sp. and Pseudomonas sp.     

Exergy analysis of hydrogen production from biomass gasification

For a given set of operating conditions, the hydrogen production from biomass gasification can be improved through optimization of the operating parameters and efficiencies. The present approach can predict hydrogen production via biomass gasification in a range of 10–32 kg/s from biomass (sawdust wood). The biomass is introduced to a gasifier at an operating temperature range of 1000–1500 K. Also, 4.5 kg/s of steam at 500 K is used as gasification medium. Results indicate that improvement in hydrogen production from biomass steam gasification depending on the amount of steam and quantity of biomass feeding to the gasifier as well the operating temperature. Over the range of feeding biomass, the hydrogen yield reaches 80–130 g H₂/kg biomass while in the operating temperature examined, the hydrogen yield reaches 80 g H₂/kg biomass. On mole basis it is found that, in the first range of H₂ varies from 51 to 63% in the studied range of feeding biomass in existing 4.5 kg/s from steam while H₂ gets to 51–53% in existing of 6.3 kg/s from steam.