Catalytic activity of oxides and halides on hydrogen storage of MgH2

Hydrogen sorption kinetics of ball milled MgH₂ with and without chemical additives were studied. We observed kinetics and capacity improvement with increasing the number of sorption cycles that contributed to the micro/nano cracking of MgH₂ particles, shown by XRD and SEM studies. In addition, to investigate the proposed specific role of O²⁻-based additives on the sorption kinetics of MgH₂, we have undertaken a comparative study evaluating the performances of MgH₂ containing the NbCl₅, CaF₂ or Nb₂O₅ additives. At 300 °C, addition of NbCl₅ and CaF₂ improved the sorption capacity to 5.2 and 5.6 wt% within 50 min, respectively, in comparison to the required 80 min in the case of Nb₂O₅. This suggests the importance of the chemical nature of the catalyst for hydrogen sorption in MgH₂. In addition, the catalyst specific surface area was shown to be very critical. High surface area Nb₂O₅ (200 m² g⁻¹), prepared by novel precipitation method, exhibits an excellent catalytic activity and helped to desorb 4.5 wt% of hydrogen from MgH₂ within 80 min at a temperature as low as 200 °C.     

Mercury adsorption from fuel ethanol onto phosphonated silica gel prepared by heterogenous method

Ethanol is one of the most important renewable biofuels contributing to the reduction of negative environmental impacts generated by the worldwide utilization of fossil fuels, and the presence of metal ions in fuel ethanol has significant effect on the performance and the quality of fuel. In the present work, silica gel functionalized by poly(triethylenetetraminomethylenephosphonic acid) SG-Cl-T-P was successfully developed by heterogenous synthesis method, and the adsorption capacity of Hg(II) from fuel ethanol via SG-Cl-T-P was examined. The adsorption isotherms were fitted by the Langmuir model, the Freundlich model and the Dubinin–Radushkevich (D–R) model. Furthermore, the adsorption study was analyzed kinetically. The thermodynamic parameters, including the Gibbs free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS) were calculated, they were −3.24 kJ mol⁻¹ (35 °C) , 29.25 kJ mol⁻¹, and 106.20 J K⁻¹ mol⁻¹, respectively.     

The kinetics of oxidation of Diesel soots by NO2

The kinetics of oxidation of three soots, from a Diesel engine fuelled by either Ultra-Low Sulphur Diesel (ULSD) or biodiesel, by NO₂ have been measured in a packed bed at various temperatures (300–550 °C) and [NO₂] (20–880 ppm) relevant to regenerating a Diesel Particulate Filter. Adsorbed hydrocarbons and oxygen accounted for a significant fraction (～20% by mass) of the otherwise carbonaceous material. After pre-treatment (heating up to 550 °C in a flow of pure Ar and holding the temperature at 550 °C for 1 h) to ensure consistency between samples, they were subsequently burned at a fixed temperature in a flow of NO₂ + Ar. For this, a balance on oxygen atoms entering and leaving the packed bed showed that during oxidation in NO₂ any oxygen remaining in a soot after pre-treatment was not rapidly liberated as CO or CO₂. A mass balance on the element nitrogen demonstrated that no N₂ or N₂O was formed below 550 °C; mass balances on carbon and oxygen demonstrated that all the carbon ended up as CO or CO₂ and below 550 °C the nitrogen yielded only NO. The oxidation of soot in NO₂ was found to be first-order with respect to NO₂. Also, the soot derived from biodiesel was more reactive than soot from ULSD; nevertheless, the apparent activation energies for oxidation by NO₂ were the same (70 ± 18 kJ mol^?1) for each carbon. When the distribution of diameters of the individual spherules of soot was taken into account, it was not possible to tell whether there was internal burning of porous spherules or, on the other hand, non-porous, solid spherules were burning on their exteriors.     

Formulating and optimizing a combustion pathways for oil shale and its semi-coke

Enhanced technologies from oil recovery to unconventional fuels - oil shale, oil sands and extra-heavy oil – have in common complex chemical reactions processes. This paper is about the formulation and optimization of the chemical mechanism especially in oil shale and semi-coke combustion. The Levenberg–Marquardt algorithm was used to minimize the error between estimated values and the thermogravimetric data for combustion mechanisms of 4-steps and 3-steps proposed for the oil shale and its semi-coke respectively. The kinetic parameters such as reaction order, pre-exponential factor, activation energy and stoichiometric coefficients that affect drying, pyrolysis, oxidation and decarbonation reactions were estimated with success. The values of activation energies were 54–67 kJ mol^?1 for oil shale drying, 62–65 kJ mol^?1 for pyrolysis reaction, up to 100 kJ mol^?1 for Fixed Carbon (FC) oxidation reaction, and 162–418 kJ mol^?1 for decarbonation reaction. Regarding to the semi-coke combustion, the activation energies were 33 kJ mol^?1 for drying reaction, 211 kJ mol^?1 for oxidation reaction and 291 kJ mol^?1 for decarbonation reaction. The chemical reactions suggest reaction order superior to one, except to the decarbonation reaction at 3 K min^?1. Considering the estimated parameters, as well as a heating rate at 3 K min^?1, an oil shale containing about 20 wt.% of organic matter and 34.6 wt.% of CaCO₃, the species mass fractions formed during combustion process were 3.4 wt.% of FC, 10.6 wt.% of Oil, 3.3 wt.% of HC and 1.8 wt.% of CO. The fraction of CO₂ formed accounts a total of 21.6 wt.%. For a semi-coke containing 3.4 wt.% of FC and 40.6 wt.% of CaCO₃, its combustion formed 2.1 wt.% of CO. The CO₂ fraction from oxidation and decarbonation reactions accounts 10.2 wt.%, considering that the stoichiometric mass coefficient γ = 0.75 in decarbonation reaction.     

Biodiesel production from palm oil via heterogeneous transesterification

This paper presents the study of the transesterification of palm oil via heterogeneous process using montmorillonite KSF as heterogeneous catalyst. This study was carried out using a design of experiment (DOE), specifically response surface methodology (RSM) based on four-variable central composite design (CCD) with α (alpha) = 2. The transesterification process variables were reaction temperature, x₁ (50–190 °C), reaction period, x₂ (60–300 min), methanol/oil ratio, x₃ (4–12 mol mol^?1) and amount of catalyst, x₄ (1–5 wt%). It was found that the yield of palm oil fatty acid methyl esters (FAME) could reach up to 79.6% using the following reaction conditions: reaction temperature of 190 °C, reaction period at 180 min, ratio of methanol/oil at 8:1 mol mol^?1 and amount of catalyst at 3%.     

Prediction of mineral scale formation in geothermal and oilfield operations using the extended UNIQUAC model: Part I. Sulfate scaling minerals

Pressure parameters are added to the Extended UNIQUAC model presented by Thomsen and Rasmussen (Modeling of vapor–liquid–solid equilibrium in gas–aqueous electrolyte systems. Chemical Engineering Science 54, 1787–1802, 1999). The improved model has been used for correlation and prediction of solid–liquid equilibrium (SLE) of scaling minerals (CaSO₄, CaSO₄·2H₂O, BaSO₄ and SrSO₄) at temperatures up to 300 °C and pressures up to 1000 bar. The results show that the Extended UNIQUAC model, with the proposed pressure parameters, is able to represent binary, ternary and quaternary solubility data within the experimental accuracy in the temperature range from −20 to 300 °C, and the pressure range from 1 to 1000 bar.     

Synthesis and electrochemical properties of lithium cobalt oxides prepared by molten-salt synthesis using the eutectic mixture of LiCl–Li2CO3

Lithium cobalt oxide powders have been successfully prepared by a molten-salt synthesis (MSS) method using a eutectic mixture of LiCl and Li₂CO₃ salts. The physico-chemical properties of the lithium cobalt oxide powders are investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), particle-size analysis and charge–discharge cycling. A lower temperature and a shorter time (∼700°C and 1 h) in the Li:Co=7 system are sufficient to prepare single-phase HT-LiCoO₂ powders by the MSS method, compared with the solid-state reaction method. Charge–discharge tests show that the lithium cobalt oxide prepared at 800°C has an initial discharge capacity as high as 140 mA h g⁻¹, and 100 mA h g⁻¹ after 40 cycles. The dependence of the synthetic conditions of HT-LiCoO₂ on the reaction temperature, time and amount of flux with respect to starting oxides is extensively investigated.     

Effects of KNbO3 catalyst on hydrogen sorption kinetics of MgH2

The influences of Nb-containing oxides and ternary compound in hydrogen sorption performance were investigated. As faster desorption kinetic and lower activation energy were reported by addition of a ternary compound catalyst such as K₂NiF₆, the influence of KNbO₃ on hydrogen storage properties of MgH₂ has been investigated for the first time. The MgH₂ - KNbO₃ composite desorbed 3.9 wt% of hydrogen within 10 min, while MgH₂ and MgH₂-Nb₂O₅ composites desorbed 0.66 wt% and 3.2 wt% respectively under similar condition. For MgH₂ with other Nb-contained catalysts such as Nb, NbO and Nb₂O₃, the desorption rate was almost the same as the one registered for as-milled MgH₂. The analysis of differential scanning calorimetry (DSC) showed that MgH₂-KNbO₃ composite started to release hydrogen at ∼335 °C which is 50 °C lower compared to as-milled MgH₂ without any additives. The activation energy for the hydrogen desorption was estimated to be about 104 ± 6.8 kJ mol⁻¹ for this material, while for the as-milled MgH₂ was about 165 ± 2.0 kJ mol⁻¹. It is believed that Nb-ternary oxide catalyst (KNbO₃) showed a good catalytic effect and enhance the sorption kinetics of MgH₂.     

Proton conduction in ceria-doped Ba2In2O5 nanocrystalline ceramic at low temperature

Sintered pellets of Ce-doped Ba₂In₂O₅ (BIC) were prepared from nanopowders. The electrical conductivities were measured using ac impedance spectroscopy under different atmospheres and temperatures. The electrical conductivity of sintered BIC was found sensitive to environmental humidity when the temperature was below 300 °C. However, in the presence of hydrogen, the electrical conductivities were independent of water content in the range of 0–30 vol%. The electrical conductivities of BIC were significantly affected by the presence of hydrogen in a temperature range of 100–300 °C. The estimated protonic transference number and the measured open circuit voltage suggested the existence of electronic conduction. The coefficient of thermal expansion of BIC is 11.2 × 10⁻⁶ K⁻¹ from 25 to 1250 °C.     

High voltage stability of nanostructured thin film catalysts for PEM fuel cells

This paper provides a comparative evaluation of electrocatalyst surface area stability in PEM fuel cells under accelerated durability testing. The two basic electrocatalyst types are conventional carbon-supported dispersed Pt catalysts (Pt/C), and nanostructured thin film (NSTF) catalysts. Both types of fuel cell electrocatalysts were exposed to continuous cycling between 0.6 and 1.2 V, at various temperatures between 65 and 95 °C, with H₂/N₂ on the anode and cathode, while periodic measurements of electrochemical surface area were recorded as a function of the number of cycles. The NSTF electrocatalyst surface areas were observed to be significantly more stable than the Pt/C electrocatalysts. A first order rate kinetic model was applied to the normalized surface area changes as a function of number of cycles and temperature, and two parameters extracted, viz. the minimum stable surface area, S_min, and the activation energy, E_a, for surface area loss in this voltage range. S_min was found to be 10% versus 66%, and E_a 23 kJ mole⁻¹ versus 52 kJ mole⁻¹, for Pt/C versus NSTF-Pt, respectively. The loss of surface area in both cases is primarily the result of Pt grain size increases, but the Pt/C XRD grain sizes increase significantly more than the NSTF grain sizes. In addition, substantial peak shifts occur in the Pt/C CVs, which ultimately end up aligning with the NSTF peak positions, which do not change substantially due to the voltage cycling. NSTF catalysts should be more robust against shut down/start-up, operation near OCV and local H₂ starvation effects.     

Sulfonic-functionalized heteropolyacid–silica nanoparticles for high temperature operation of a direct methanol fuel cell

Sulfonic-functionalized heteropolyacid–SiO₂ nanoparticles were synthesized by grafting and oxidizing of a thiol-silane compound onto the heteropolyacid–SiO₂ nanoparticle surface. The surface functionalization was confirmed by solid-state NMR spectroscopy. The composite membrane containing the sulfonic-functionalized heteropolyacid–SiO₂ nanoparticles was prepared by blending with Nafion^® ionomer. TG–DTA analysis showed that the composite membrane was thermally stable up to 290 °C. The DMFC performance of the composite membrane increased the operating temperature from 80 to 200 °C. The function of the sulfonic-functionalized heteropolyacid–SiO₂ nanoparticles was to provide a proton carrier and act as a water reservoir in the composite membrane at elevated temperature. The power density was 33 mW cm⁻² at 80 °C, 39 mW cm⁻² at 160 °C and 44 mW cm⁻² at 200 °C, respectively.     

Hydrogen storage properties of nanocrystalline Mg–Ce/Ni composite

Mg–Ce alloy was prepared by induction melting under vacuum, hydrided firstly and then Mg–Ce/Ni composite was obtained by mechanical milling Mg–Ce hydrides under Ar for 50 h with addition of nano-sized Ni powder. XRD results showed CeMg₁₂ formed in melted alloy. CeMg₁₂ disappeared and CeH_2.53 emerged during subsequent hydriding. The phase composition was not changed during ball milling process. Compared with Mg and Mg/Ni, Mg–Ce/Ni composite showed significant hydriding/dehydriding performance without any prior activation. The enthalpy of hydride formation for Mg–10.9 wt.% Ce/10 wt.% Ni composite was −70.58 kJ mol⁻¹ H₂. Improved hydrogen storage properties were attributed to the catalytic effect of addition of nano-sized Ni particles and existence of CeH_2.53, as well as the grain refinement, defects, etc. in the material introduced by ball milling process.     

Comparative studies of nickel oxide films on different substrates for electrochemical supercapacitors

Thin nickel oxide (NiO) films were obtained by post-heating of the corresponding precursor films of nickel hydroxide (Ni(OH)₂) cathodically deposited onto different substrates, i.e., nickel foils, and graphite at 25 °C from a bath containing 1.5 mol L⁻¹ Ni(NO₃)₂ and 0.1 mol L⁻¹ NaNO₃ in a solvent of 50% (v/v) ethanol. The surface morphology of the obtained films was observed by scanning electron microscope (SEM). Electrochemical characterization was performed using cyclic voltammetrty (CV), chronopotentiometry (CP) and electrochemical impedance analysis (EIS). When heated at 300 °C for 2 h in air, the specific capacitance of the prepared NiO films on nickel foils and graphite, with a deposition charge of 250 mC cm⁻², were 135, 195 F g⁻¹, respectively. When the deposition charge is less than 280 mC cm⁻², the capacitance of both appears to keep the linear relationship with the deposition charge. The specific capacitance, cyclic stability of the NiO/graphite hybrid electrodes in 1 mol L⁻¹ KOH solution were superior to those on nickel foils mainly due to the favorable adhesion, the good interface behavior between graphite and the NiO films, and the extra pseudo-capacitance of the heated graphite substrates.     

Investigation of the processes for reversible hydrogen storage in the Li–Mg–N–H system

The hydrogen storage performances of the Li–Mg–N–H system are investigated starting either from 1:2 Mg(NH₂)₂–LiH or 1:2 MgH₂–LiNH₂ ball-milled mixtures. It is shown that, for 1:2 MgH₂–LiNH₂, an ammonia release occurs if the first heating is conducted under a dynamic vacuum, leading to a fast degradation of the material. The positive role of LiH, if initially present in the mixture, is therefore emphasized as LiH rapidly reacts with ammonia and avoids the contamination of the hydrogen desorbing flow. The desorption kinetics of the ball-milled 1:2 Mg(NH₂)₂–LiH mixture are fast: a total amount of 5.0 wt.% of hydrogen is desorbed in 25 min at 220 °C. This material exhibits a nice reversibility at 200 °C with an experimental capacity around 4.8 wt.%. Preliminary results are given on the structure of Li₂Mg(NH)₂, formed upon desorption: this phase crystallizes in a cubic unit cell with a lattice parameter of 10.06(1) Å. In addition, by plotting an absorption isotherm of the Li₂Mg(NH)₂ phase at 200 °C, two pressure plateaus are observed revealing the existence of an intermediary phase between Li₂Mg(NH)₂ and the rehydrided material, which is the 1:2 Mg(NH₂)₂–LiH mixture.     

Exploration of high capacity LiNi0.5Mn1.5O4 synthesized by solid-state reaction

LiNi_0.5Mn_1.5O₄ was prepared by an improved solid-state reaction at high heating and cooling rates, the mixed precursors were initially heated up to 900 °C, then directly cooled down to 600 °C and heated for 24 h in air. X-ray diffraction (XRD) pattern shows that LiNi_0.5Mn_1.5O₄ has cubic spinel structure; scanning electron microscopic (SEM) image shows that the particle size is about 0.2 μm together with homogenous distribution. Electrochemical measurements show that LiNi_0.5Mn_1.5O₄ powders delivered up to 143 mAh g⁻¹ with superior cycling performance at the rate of 5/7C.     

Thermogravimetric kinetic analysis of the combustion of biowastes

Thermogravimetric (TG) analysis was used to study and compare the combustion of sewage sludge (SS), animal manure (AM) and the organic fraction of municipal solid waste (OFMSW). TG curves are in correspondence with the volatiles and carbon content of the materials studied. Non-isothermal thermogravimetric data were used to assess the kinetics of the combustion of these carbonaceous materials. The paper reports on the application of a model-free isoconversional method for the evaluation of the activation energy corresponding to the combustion of these biowastes. The activation energy related to AM combustion (E ～ 140 kJ mol^?1) was similar to that corresponding to SS (E ～ 143 kJ mol^?1) while the OFMSW showed to have a higher value (E ～ 173 kJ mol^?1).     

Wide-optical bandgap with improved conductivity p-μc-Si:Ox:H films prepared by Cat-CVD

Boron-doped hydrogenated microcrystalline silicon oxide (p-μc-Si:O_x:H) films have been deposited using catalytic chemical vapor deposition (Cat-CVD). The single-coiled tungsten catalyst temperature (T_fil) was varied from 1850 to 2100 °C and films were deposited on glass substrates at the temperatures (T_sub) of 100–300 °C. Different catalyst-to-substrate distances of 3–5 cm and deposition pressures from 0.1 to 0.6 Torr were considered.Optical and electrical characterizations have been made for the deposited samples. The sample transmittance measurement shows an optical-bandgap (Eg_opt) variation from 1.74 to 2.10 eV as a function of the catalyst and substrate temperatures. One of the best window materials was obtained at T_sub=100 °C and T_fil=2050 °C, with Eg_opt=2.10 eV, dark conductivity of 3.0×10^?3 S cm^?1 and 0.3 nm s^?1 deposition rate.     

Fabrication and characterization of micro tubular SOFCs for operation in the intermediate temperature

Tubular SOFC systems appear to be well-suited to accommodate repeated cycling under rapid changes in electrical load and in cell operating temperatures. Our goal is to develop innovative processing method to fabricate new micro tubular SOFCs with sub-millimeter diameter and its stack module which enable to generate high volumetric power density. In this study, micro tubular SOFCs under 1 mm diameter have been successfully fabricated and tested in the intermediate temperature region (550 °C or under). The cell consists of NiO–Gd doped ceria (GDC) as an anode (support tube), GDC as an electrolyte and (La, Sr)(Fe, Co)O₃ (LSCF)–GDC as a cathode. The single tubular cell with 0.8 mm diameter and 12 mm length generated over 70 mW at 550 °C with H₂ fuel, which indicates that the cell generated over 0.3 W cm⁻² at 550 °C.     

Dynamic behaviour of SOFC short stacks

Electrical output behaviour obtained on solid oxide fuel cell stacks, based on planar anode supported cells (50 or 100 cm² active area) and metallic interconnects, is reported. Stacks (1–12 cells) have been operated with cathode air and anode hydrogen flows between 750 and 800 °C operating temperature. At first polarisation, an activation phase (increase in power density) is typically observed, ascribed to the cathode but not clarified. Activation may extend over days or weeks. The materials are fairly resistant to thermal cycling. A 1-cell stack cycled five times in 4 days at heating/cooling rates of 100–300 K h⁻¹, showed no accelerated degradation. In a 5-cell stack, open circuit voltage (OCV) of all cells remained constant after three full cycles (800–25 °C). Power output is little affected by air flow but markedly influenced by small fuel flow variation. Fuel utilisation reached 88% in one 5-cell stack test. Performance homogeneity between cells lay at ±4–8% for three different 5- or 6-cell stacks, but was poor for a 12-cell stack with respect to the border cells. Degradation of a 1-cell stack operated for 5500 h showed clear dependence on operating conditions (cell voltage, fuel conversion), believed to be related to anode reoxidation (Ni). A 6-cell stack (50 cm² cells) delivering 100 W_el at 790 °C (1 kW_el L⁻¹ or 0.34 W cm⁻²) went through a fuel supply interruption and a thermal cycle, with one out of the six cells slightly underperforming after these events. This cell was eventually responsible (hot spot) for stack failure.