Synthesis and electrochemical properties of layered Li–Ni–Mn–O compounds

An attempt to prepare solid solutions in the system of LiNiO₂, LiMnO₂ and Li₂MnO₃ was performed by heating metal acetates. The solid solutions between end members LiNiO₂ and Li₂MnO₃ can be successfully prepared in the overall compositional ranges. Both the structure and capacity were compared based on Rietveld analysis and electrochemical investigation on solid solutions between LiNiO₂ and Li₂MnO₃. The result showed that the cationic disorder as well as capacity was closely related to the ratio of Li, Mn and Ni in formula. The investigation of chronopotentiogram and ex situ XRD on the solid solutions indicated that the complex phase transitions in LiNiO₂ during delithiation were strongly suppressed with low Mn content (Mn/(Mn+Ni) ratio was 0.1 or 0.2) and completely suppressed with the ratio more than 0.5.     

Effects of Fe substitutions by Ni in La–Ni–O perovskite-type oxides in reforming of methane with CO2 and O2

LaNiO₃ and LaNi_1−xFe_xO₃ (x = 0.2, 0.4, 0.6, 0.8 and 1) perovskites were prepared by the citrate sol–gel method. The prepared compounds were characterized by using thermogravimetric analysis (TGA) and X-ray diffraction (XRD), temperature programmed reduction (TPR), and inductively coupled plasma (ICP) techniques. Specific surface area of the samples was measured by BET method. Morphology study of the prepared catalysts was performed using scanning and transmission electron microscopy (SEM and TEM, respectively). The XRD patterns of fresh catalysts indicated the formation of well-crystallized perovskite structure as the main phase present in the prepared samples. The results showed that the highly homogeneous and pure solids with particle sizes in the range of nanometers were obtained through this synthesis method. TPR analysis revealed that by increasing the degree of substitution (x) the reduction of the prepared samples became difficult. The effects of the partial substitution of Ni by Fe and reaction temperatures at atmospheric pressure were investigated in the combined reforming of methane with CO₂ and O₂ (CRM), after reduction of the samples under hydrogen. LaNiO₃ exhibited high activity and selectivity without coke formation between all of the studied perovskites. Among Fe-substituted catalysts, the following order of activity was observed: LaNiO₃>LaNi_0.4Fe_0.6O₃>LaNi_0.6Fe_0.4O₃ > LaNi_0.8Fe_0.2O₃ > LaNi_0.2Fe_0.8O₃ > LaFeO₃.     

The chemistry involved in the third explosion limit of H2–O2 mixtures

The third explosion limit of hydrogen oxidation in closed vessels has always been thought to be the result of the competition between homogeneous gas-phase reactions and diffusion of hydroperoxyl radicals to the walls, where they are destroyed. It has recently been observed that this species actually follows a chemical-kinetic steady state in this regime, with the consequence that its diffusive rate toward the catalytic walls becomes irrelevant. Here we show that the critical explosion conditions are determined instead by the fate of hydrogen peroxide, which emerges as the controlling reactant for the resulting gas-phase chemistry. A simple, accurate analytic expression for the third explosion limit follows from identification of the critical conditions for existence of weakly reactive, diffusion–reaction solutions, thereby providing the answer to a long-standing problem that in early work was characterized as being hopelessly difficult.     

Revision of the NaBO2–H2O phase diagram for optimized yield in the H2 generation through NaBH4 hydrolysis

The binary phase diagram NaBO₂–H₂O at ambient pressure, which defines the different phase equilibria that could be formed between borates, end-products of NaBH₄ hydrolysis, has been reviewed. Five different solid borates phases have been identified: NaBO₂·4H₂O (Na[B(OH)₄]·2H₂O), NaBO₂·2H₂O (Na[B(OH)₄]), NaBO₂·2/3H₂O (Na₃[B₃O₄(OH)₄]), NaBO₂·1/3H₂O (Na₃[B₃O₅(OH)₂]) and NaBO₂ (Na₃[B₃O₆]), and their thermal stabilities have been studied. The boundaries of the different Liquid + Solid equilibria for the temperature range from −10 to 80 °C have been determined, confirming literature data at low temperature (20–50 °C). Moreover the following eutectic transformation, Liq. → Ice + NaBO₂·4H₂O, occurring at −7 °C, has been determined by DSC. The Liquid–Vapour domain has been studied by ebullioscopy. The invariant transformation Liq. → Vap. + NaBO₂·2/3H₂O has been estimated at 131.6 °C. This knowledge is paramount in the field of hydrogen storage through NaBH₄ hydrolysis, in which borate compounds were obtained as hydrolysis reaction products. As a consequence, the authors propose a comparison with previous NaBO₂–H₂O binary phase diagrams and its consequence related to hydrogen storage through NaBH₄ hydrolysis.     

Hydrogen storage properties of destabilized MgH2–Li3AlH6 system

To improve the dehydrogenation properties of MgH₂, a novel hydrogen storage system, MgH₂–Li₃AlH₆, is prepared by mechanochemical milling. Three physical mixtures containing different mole ratios (1:4, 1:1 and 4:1) of MgH₂ and Li₃AlH₆ are studied and there exists a mutual destabilization effect between the components. The last mixture shows a capacity of 6.5 wt% H₂ with the lowest starting temperature of dehydrogenation (170 °C). First, Li₃AlH₆ decomposes into Al, LiH and H₂, and then the as-formed Al can easily destabilize MgH₂ to form the intermetallic compound Mg₁₇Al₁₂ at a temperature of 235 °C, which is about 180 °C lower than the decomposition temperature of pristine MgH₂. Finally, the residual MgH₂ undergoes a self-decomposition whose apparent activation energy has been reduced by about 22 kJ mol⁻¹ compared with pristine MgH₂. At a constant temperature of 250 °C, the mixture can dehydrogenate completely under an initial vacuum and rehydrogenate to form MgH₂ under 2 MPa H₂, showing good cycle stability after the first cycle with a capacity of 4.5 wt% H₂. The comparison between 4 MgH₂ + Li₃AlH₆ and 4 MgH₂ + LiAlH₄ mixtures is also investigated.     

Laminar burning velocities and flame characteristics of CO–H2–CO2–O2 mixtures

Laminar burning velocities of CO–H₂–CO₂–O₂ flames were measured by using the outwardly spherical propagating flame method. The effect of large fraction of hydrogen and CO₂ on flame radiation, chemical reaction, and intrinsic flame instability were investigated. Results show that the laminar burning velocities of CO–H₂–CO₂–O₂ mixtures increase with the increase of hydrogen fraction and decrease with the increase of CO₂ fraction. The effect of hydrogen fraction on laminar burning velocity is weakened with the increase of CO₂ fraction. The Davis et al. syngas mechanism can be used to calculate the syngas oxyfuel combustion at low hydrogen and CO₂ fraction but needs to be revised and validated by additional experimental data for the high hydrogen and CO₂ fraction. The radiation of syngas oxyfuel flame is much stronger than that of syngas–air and hydrocarbons–air flame due to the existence of large amount of CO₂ in the flame. The CO₂ acts as an inhibitor in the reaction process of syngas oxyfuel combustion due to the competition of the reactions of H + O₂ = O + OH, CO + OH = CO₂ + H and H + O₂(+M) = HO₂(+M) on H radical. Flame cellular structure is promoted with the increase of hydrogen fraction and is suppressed with the increase of CO₂ fraction due to the combination effect of hydrodynamic and thermal-diffusive instability.     

Another view of the upper and intermediate explosion limits of a H2–O2 system

In the present note we consider the two simultaneous mechanisms that are dominated by gas-phase reactions, namely the upper and intermediate explosion limits, and show that by employing the Le-Chatelier rule, a unified explosion limit is obtained in the form of a single analytical expression.     

Theoretical predictions vs. experimental measurements of the electrical conductivity of molten Li2CO3–K2CO3 modified by additives

The control of the physicochemical properties of the molten carbonate electrolyte is a key issue for optimising the performance and lifetime of the Molten Carbonate Fuel Cell (MCFC). In particular, the Li_xNi_1−xO cathode dissolution should be kept low by increasing the oxobasicity of the electrolyte, which can be done by adding small amounts of alkali or alkali-earth carbonates. Nevertheless, these additives may present drawbacks with respect to other features such as the electrical conductivity. This work aims at determining the evolution of this last property in the classical eutectic used in MCFC, Li₂CO₃–K₂CO₃ (62–38 mol%), containing small amounts of alkaline or alkaline-earth carbonates, such as Ca²⁺, Rb⁺, Cs⁺, or La³⁺. Experimental conductivity measurements carried out by impedance spectroscopy were correlated to theoretical predictions for each carbonate melt as a function of the amount of additive (1–10 mol%) at different temperatures within the range of (550 °C < T < 700 °C). In both experimental and theoretical cases, the conductivity slightly decreases with the quantity of additives except for the addition of lithium carbonate. The prediction gives general tendency but the calculated values are always lower than experimental ones because the chosen model is too simple.     

Ni/La2O3 and Ni/MgO–La2O3 catalysts for the decomposition of NH3 into hydrogen

The decomposition of NH₃ for hydrogen production was studied using Ni/La₂O₃ catalysts at varying compositions and temperatures prepared via surfactant-templated synthesis to elucidate the influence of catalyst active metal content, support composition and calcination temperature on the catalytic activity. The catalytic performance of all samples was studied between 300 and 600 °C under atmospheric pressure. The catalytic activity of the sample were as follows: 10Ni/La₂O₃-450 > 10Ni/La₂O₃-550 > 10Ni/La₂O₃-650 ≈ 10Ni/La₂O₃-750 ≈ 10Ni/La₂O₃-850. The excellent activity (100%) of 10Ni/La₂O₃-450 could be due to the high surface area, basicity strength and concentration of surface oxygen species of the catalyst as evidenced by BET, CO₂-TPD and XPS. In addition, to adjust the activity of the catalyst support, the molar ratios of Mg and La were varied (1:1, 3:1, 5:1, 7:1 and 9:1). The 5Ni/5MgLa (5:1 M ratio) was found to be the most active (100%) relative to other Ni/MgLa formulations. Furthermore, the Ni content in the Ni/5MgLa sample was adjusted between 10 and 40 wt%. Increasing the Ni content of the catalysts increased NH₃ conversion with the 40 wt% Ni formulation demonstrating complete NH₃ conversion at 600 °C and a high gas hourly space velocities (GHSV) (30,000 mL∙h⁻¹∙g_cat⁻¹).     

Adiabatic burning velocity of H2–O2 mixtures diluted with CO2/N2/Ar

Global warming due to CO₂ emissions has led to the projection of hydrogen as an important fuel for future. A lot of research has been going on to design combustion appliances for hydrogen as fuel. This has necessitated fundamental research on combustion characteristics of hydrogen fuel. In this work, a combination of experiments and computational simulations was employed to study the effects of diluents (CO₂, N₂, and Ar) on the laminar burning velocity of premixed hydrogen/oxygen flames using the heat flux method. The experiments were conducted to measure laminar burning velocity for a range of equivalence ratios at atmospheric pressure and temperature (300 K) with reactant mixtures containing varying concentrations of CO₂, N₂, and Ar as diluents. Measured burning velocities were compared with computed results obtained from one-dimensional laminar premixed flame code PREMIX with detailed chemical kinetics and good agreement was obtained. The effectiveness of diluents in reduction of laminar burning velocity for a given diluent concentration is in the increasing order of argon, nitrogen, carbon dioxide. This may be due to increased capabilities either to quench the reaction zone by increased specific heat or due to reduced transport rates. The lean and stoichiometric H₂/O₂/CO₂ flames with 65% CO₂ dilution exhibited cellular flame structures. Detailed three-dimensional simulation was performed to understand lean H₂/O₂/CO₂ cellular flame structure and cell count from computed flame matched well with the experimental cellular flame.     

Plasma-reduced Ni/γ–Al2O3 and CeO2–Ni/γ–Al2O3 catalysts for improving dry reforming of propane

Pristine Ni/γ–Al₂O₃ and CeO₂–Ni/γ–Al₂O₃ catalysts were prepared by co-impregnation technique for dry reforming of propane. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were used to examine the structure and morphology of the catalysts before and after the reforming reactions. The excellent interaction between catalyst active phases was observed in both CeO₂–Ni/γ–Al₂O₃ and Ni/γ–Al₂O₃ stabilized with polyethelene glycol (Ni/γ–Al₂O₃–PEG). Towards C₃H₈ and CO₂ conversion, the CeO₂–Ni/γ–Al₂O₃ and Ni/γ–Al₂O₃–PEG showed improved catalytic activity when compared to the pristine Ni/γ–Al₂O₃ catalyst. Interestingly, high H₂ concentration was achieved with the CeO₂–Ni/γ–Al₂O₃ and high CO concentration with the Ni/γ–Al₂O₃–PEG, which is due to the nanoconfinement of nickel particles within the support and favorable metal-support interaction as a result of plasma reduction. The CeO₂–Ni/γ–Al₂O₃ catalyst exhibited better stability for anti-sintering and coke resistance, thus exhibiting high reactivity and durability in the dry reforming.     

Ni/MgO–Al2O3 and Ni–Mg–O catalyzed SiC foam absorbers for high temperature solar reforming of methane

Ni catalyst supported on MgO–Al₂O₃ (Ni/MgO–Al₂O₃) prepared from hydrotalcite, and Ni–Mg–O catalyst are studied in regard to their activity in the CO₂ reforming of methane at high temperatures in order to develop a catalytically activated foam receiver–absorber for use in solar reforming. First, the activity of their powder catalysts is examined. Ni/MgO–Al₂O₃ powder catalyst exhibits a remarkable degree of high activity and thermal stability as compared with Ni–Mg–O powder catalyst. Secondly, a new type of catalytically activated ceramic foam absorber – Ni/MgO–Al₂O₃/SiC – and Ni–Mg–O catalyzed SiC foam absorber are prepared and their activity is evaluated using a laboratory-scale receiver–reactor with a transparent quartz window and a sun-simulator. The present Ni-based catalytic absorbers are more cost effective than conventional Rh/γ-Al₂O₃ catalyzed alumina and SiC foam absorbers and the alternative Ru/γ-Al₂O₃ catalyzed SiC foam absorbers. Ni/MgO–Al₂O₃ catalyzed SiC foam absorber, in particular, exhibits superior reforming performance that provides results comparable to that of Rh/γ-Al₂O₃ catalyzed alumina foam absorber under a high flux condition or at high temperatures above 1000 °C. Ni/MgO–Al₂O₃ catalyzed SiC foam absorber will be desirable for use in solar receiver–reactor systems to convert concentrated high solar fluxes to chemical fuels via endothermic natural-gas reforming at high temperatures.     

Production of syngas via autothermal reforming of methane in a fluidized-bed reactor over the combined CeO2–ZrO2/SiO2 supported Ni catalysts

Production of syngas via autothermal reforming of methane (MATR) in a fluidized bed reactor was investigated over a series of combined CeO₂–ZrO₂/SiO₂ supported Ni catalysts. These combined CeO₂–ZrO₂/SiO₂ supports and supported Ni catalysts were characterized by nitrogen adsorption, XRD, NH₃-TPD, CO₂-TPD and H₂-TPR. It was found that the combined supports integrated the advantages of SiO₂ and CeO₂, ZrO₂. That is, they have bigger surface area (about 300 m²/g) than pure CeO₂ and ZrO₂, stronger acidity and alkalescence than that of pure SiO₂, and enhanced the mobility of H adatoms. Ni species dispersed highly on these combined CeO₂–ZrO₂/SiO₂ supports, and became more reducible. Ni catalysts on the combined supports possess higher CO₂ adsorption ability, higher methane activation ability and exhibited higher activity for MATR. H₂/CO ratio in product gas could be controlled successfully in the range of 0.99–2.21 by manipulating the relative concentrations of CO₂ and O₂ in feed.     

Syngas production from CH4 dry reforming over Co–Ni/Al2O3 catalyst: Coupled reaction-deactivation kinetic analysis and the effect of O2 co-feeding on H2:CO ratio

The dry and oxidative dry reforming of CH₄ over alumina-supported Co–Ni catalysts were investigated over 72-h longevity experiments. The deactivation behaviour at low CO₂:CH₄ ratio (≤2) suggests that carbon deposition proceeds via a rapid dehydropolymerisation step resulting in the blockage of active sites and loss in CO₂ consumption. In particular, at high temperatures of 923 K and 973 K, a ‘breakthrough’ point was observed in which deactivation that was previously slow suddenly accelerated, indicating rapid polymerisation of deposited carbon. Only with feed CO₂:CH₄ > 2 or with O₂ co-feeding was coke-induced deactivation eliminated. In particular, O₂ co-feeding gave improved carbon removal, product H₂:CO ratios more suitable for downstream GTL processing and stable catalytic performance. Conversion-time data were adequately fitted to the generalised Levenspiel reaction-deactivation model. Activation energy estimate (66–129 kJ mol⁻¹) was dependent on the CO₂:CH₄ ratio but representative of other hydrocarbon reforming reactions on Ni-based catalysts.     

Oxidative steam reforming of ethanol over Ni/Al2O3 catalysts promoted by CeO2, ZrO2 and CeO2–ZrO2

Oxidative steam reforming of ethanol at low oxygen to ethanol ratios was investigated over nickel catalysts on Al₂O₃ supports that were either unpromoted or promoted with CeO₂, ZrO₂ and CeO₂–ZrO₂. The promoted catalysts showed greater activity and a higher hydrogen yield than the unpromoted catalyst. The characterization of the Ni-based catalysts promoted with CeO₂ and/or ZrO₂ showed that the variations induced in the Al₂O₃ by the addition of CeO₂ and/or ZrO₂ alter the catalyst's properties by enhancing Ni dispersion and reducing Ni particle size. The promoters, especially CeO₂–ZrO₂, improved catalytic activity by increasing the H₂ yield and the CO₂/CO and the H₂/CO values while decreasing coke formation. This results from the addition of ZrO₂ into CeO₂. This promoter highlights the advantages of oxygen storage capacity and of mobile oxygen vacancies that increase the number of surface oxygen species. The addition of oxygen facilitates the reaction by regenerating the surface oxygenation of the promoters and by oxidizing surface carbon species and carbon-containing products.     

Effects of N2O addition on the ignition of H2–O2 mixtures: Experimental and detailed kinetic modeling study

Ignition delay times of H₂/O₂ mixtures highly diluted with Ar and doped with various amounts of N₂O (100, 400, 1600, 3200 ppm) were measured in a shock tube behind reflected shock waves over a wide range of temperatures (940–1675 K). The pressure range investigated during this work (around 1.6, 13 and 32 atm) allows studying the effect of N₂O on hydrogen ignition at pressure conditions that have never been heretofore investigated. Ignition delay times were decreased when N₂O was added to the mixture only for the higher nitrous oxide concentrations, and some changes in the activation energy were also observed at 1.5 and 32 atm. When it occurred, the decrease in the ignition delay time was proportional to the amount of N₂O added and depended on pressure and temperature conditions. A detailed chemical kinetics model was developed using kinetic mechanisms from the literature. This model predicts well the experimental data obtained during this study and from the literature. The chemical analysis using this model showed that the decrease in the ignition delay time was mainly due to the reaction N₂O + M ? N₂ + O + M which provides O atoms to strengthen the channel O + H₂ ? OH + H.     

Synthesis and characterization of bimetallic Cu–Ni/ZrO2 nanocatalysts: H2 production by oxidative steam reforming of methanol

Cu/ZrO₂, Ni/ZrO₂ and bimetallic Cu–Ni/ZrO₂ catalysts were prepared by deposition–precipitation method to produce hydrogen by oxidative steam reforming of methanol (OSRM) reaction in the range of 250–360 °C. TPR analysis of the Cu–Ni/ZrO₂ catalyst showed that the presence of Cu facilitates the reduction of the Ni at lower temperatures. In addition, this sample showed two reduction peaks, the former peak was attributed to the reduction of the adjacent Cu and Ni atoms which could be forming a bimetallic Cu-rich phase, and the second was assigned to the remaining Ni atoms forming bimetallic Ni-rich nanoparticles. Transmission Electron Microscopy revealed Cu or Ni nanoparticles on the monometallic samples, while bimetallic nanoparticles were identified on the Cu–Ni/ZrO₂ catalyst. On the other hand, Cu–Ni/ZrO₂ catalyst exhibited better catalytic activity than the monometallic samples. The difference between them was related to the Cu–Ni nanoparticles present on the former catalyst, as well as the bifunctional role of the bimetallic phase and the support that improve the catalytic activity. All the catalysts showed the same selectivity toward H₂ at the maximum reaction temperature and it was ∼60%. The high selectivity toward CO is associated to the presence of the bimetallic Ni-rich nanoparticles, as evidenced by TEM–EDX analysis, since this behavior is similar to the one showed by the monometallic Ni-catalyst.     

Hydrogen production from oxidative steam-reforming of n-propanol over Ni/Y2O3–ZrO2 catalysts

Oxidative steam reforming (OSR) of n-propanol was studied over new Ni catalysts (ca. 7% Ni wt/wt) supported on Y₂O₃–ZrO₂ oxides with different yttrium content (2–41 % Y₂O₃ wt/wt). Materials were characterized by X-ray diffraction, temperature-programmed reduction, X-ray photoelectron and Raman spectroscopy, scanning electron microscopy with energy dispersive X-ray analysis and high resolution transmission electron microscopy. Samples were used in calcined form and tested in the temperature range 673–773 K using a reactant feed of n-propanol/water/O₂ at a molar ratio 1/9/0.5. Hydrogen production is related with the support composition and Ni dispersion.