Stability,reactivity and decomposition kinetics of surface passivated α-AlH3 crystals

As a novel high energy fuel, aluminum hydride (AlH₃) has great potential in the field of solid propellants because of its high hydrogen capacity, which can significantly improve the specific impulse of solid propellants. In order to improve the stability of α-AlH₃, hydrochloric acid has been used to stabilize AlH₃ and the stabilization mechanism has been investigated. Various characterization techniques including scanning electron microscopy, X-ray electron spectrometer, X-ray diffraction, thermal analysis, and vacuum stability test have been employed to investigate the morphology, crystal structure, thermal stability, and decomposition kinetics of raw and passivated α-AlH₃. The results showed that the honeycomb-like structures could be formed on the surface of α-AlH₃ after passivation. First of all, the initial decomposition temperatures of the passivated samples were slightly increased. In particular, for the optimized sample with 105 min passivation time (AlH₃-105min), the initial decomposition temperature (173.4 °C) is increased by 5.6 °C. Moreover, the total decomposition time (1652 min) is improved by about 50% than that of the raw sample (1098 min). Besides, the decomposition activation energies (E_a) of passivated samples are much higher than that of the raw sample (84.8 kJ mol^?1), in which the optimized sample (AlH₃-105min) reaches 107.1 kJ. mol^?1. The decomposition kinetics model may change from 3-D nucleation and nucleus growth model to 2-D nucleation and nucleus growth model. It demonstrates the passivated samples have a lower decomposition rate and higher thermal stability. The stabilization mechanism is as follows: removing the impurities on the surface and accelerating the hydrolysis reaction of AlH₃ to generate complete and dense oxide layers.     

II — kinetics of the catalytic decomposition of hydrogen peroxide solution by manganese dioxide samples

The catalytic decomposition rates of hydrogen peroxide solution by twelve MnO₂ samples have been shown to follow a similar order to those of the reThe difference in the catalytic decomposition rates of H₂O₂ over the various crystalline manganese dioxides may, in view of the proton diffusio     

Solid particle decomposition and hydrolysis reaction kinetics in Cu–Cl thermochemical hydrogen production

This paper examines cupric chloride solid conversion during hydrolysis in the thermochemical copper-chlorine (Cu–Cl) cycle of hydrogen production. The hydrolysis reaction is a challenging step, due to the excess steam requirement and decomposition of cupric chloride (CuCl₂) into cuprous chloride (CuCl) and chlorine (Cl₂). In this paper, the hydrolysis and decomposition reactions are analyzed with respect to chemical equilibrium conversion and the reaction kinetics. The effects of operating parameters are examined, including the temperature, pressure and excess steam, on equilibrium conversion. It is shown that the reaction kinetics expression that represents a reversible reaction reflects the equilibrium limitation on the solid conversion, rather than first-order kinetics.     

Stability and kinetics of powder-type and pellet-type iron (III) oxide catalysts for sulfuric acid decomposition in practical Iodine–Sulfur cycle

The efficiency of sulfuric trioxide decomposition strongly affects to the overall efficiency of the Iodine–Sulfur (IS) cycle, because the decomposition process is highly endothermic and most energy-consuming process. Therefore, we carefully selected iron (III) oxide (Fe₂O₃) for high temperature decomposition on the assumption of commercialization considering its proper activity, stability and price. This study starts with collecting the very basic empirical data as well as considering practical implications for actual design. Experimental results of stability test with Fe₂O₃ powder showed that over 65% of SO₃ conversion was secured during 110 h operation. We experimentally investigated the effects related to the weight hourly space velocity (WHSV) and gas inlet composition to SO₃ conversion with pellet-type catalyst in the temperature range of 730–910 °C. Reaction order of SO₃ decomposition was empirically verified as first-order reaction. The activation energy was also obtained as 141.4 kJ/mol under four different WHSV values.     

Pyrolysis kinetics and reactivity of algae–coal blends

This paper presents results from a thermogravimetric analysis and modelling based study using a fresh water alga, Chlorococcum humicola, and a Victorian Brown Coal and their blends at different proportions. Pyrolysis was studied using the pure coal and pure algae as well as their blends to a final temperature of 1000 °C at different heating rates to understand the kinetics. The kinetic data of pure algae and pure coal were used to predict the pyrolysis characteristics of coal–algae blends at various heating rates using a modified distributed activation energy model which closely matched the experimental data. The experimental results also indicate that there is no chemical interaction between the algae and coal during pyrolysis.     

Pyrolysis characteristics and kinetics of upgraded Yi’an gas coal

This article studied pyrolysis characteristics and kinetics of upgraded Yi’an gas coal by low temperature pyrolysis in N₂ atmosphere. The quality of coal before and after modification was examined by thermogravimetric analysis. The pyrolysis of the coal becomes violent in the middle temperature range and is calculated to be a third-order reaction. After upgrading, total weight losses and coal caking properties of different temperatures increase and 250°C is the best upgrading temperature.     

I — kinetics of reduction of manganese dioxide by ascorbic acid and hydrazine solutions

The reduction rates of 7 commercial (4 γ-MnO₂ and 3 ε-MnO₂) and 5 prepared MnO₂ samples by ascorbic acid solution at 25 °C have been     

Hydrogen absorption and desorption kinetics of Ag–Mg–Ni alloys

The hydrogen absorption/desorption (A/D) kinetics of hydrogen storage alloys Mg_2−xAg_xNi (x=0.05, 0.1) prepared by hydriding combustion synthesis in two-phase (α–β) region in the temperature range of 523–$\text{573}\text{K}$ have been investigated. The hydriding/dehydriding (H/D) reaction rate constants were extracted from the time-dependent A/D curves. The obtained hydrogen A/D kinetic curves were fitted using various rate equations to reveal the mechanism of the H/D processes. The relationships of rate constant with temperature were established. It was found that the three-dimensional diffusion process dominates the hydrogen A/D. The apparent activation energies of 63±5 and $\text{61±7}\text{kJ/mol}\text{H}{}_2$ in Mg_1.95Ag_0.05Ni alloy and $\text{52±2}\text{kJ/mol}\text{H}{}_2$ and of $\text{50±2}\text{kJ/mol}\text{H}{}_2$ in Mg_1.9Ag_0.1Ni alloy were found for the H/D processes in two-phase (α–β) coexistence region from 523 to $\text{573}\text{K}$, respectively. With the increasing content of Ag in Ag–Mg–Ni alloys, the apparent energy was decreased and the reaction rate was faster. It is reasonable to explain that the hydriding kinetics of Mg₂Ni was improved by adding Ag.     

Methane decomposition kinetics and reaction rate over Ni/SiO2 nanocatalyst produced through co-precipitation cum modified Stöber method

Co-precipitation cum modified Stöber method was adopted to produce nano-Ni/SiO₂ (n-Ni/SiO₂) catalyst and conducted a series of methane decomposition kinetic experiments in a fixed bed pilot plant. Methane decomposition activity of n-Ni/SiO₂ catalyst was quantified by considering thermodynamic deposition of carbon at a temperature range of 550–650 °C and methane partial pressure from 0.2 to 0.8 atm. The utmost methane conversion of 18.87 mmol/g_cat min was obtained at 650 °C and methane partial pressure of 0.8 atm. The findings concluded that the enhancement occurred with carbon formation rate when increasing the methane partial pressure is very much evident at higher temperature such as 650 °C. However, the intensity in methane decomposition descending tendency was declined at lower reaction temperature. The effects of methane partial pressure and reaction temperature on the specific molar carbon formation rate were also examined. The calculated reaction order and activation energy were 1.40 and 61.1 kJ mol^?1, respectively. The kinetic experiments showed the existence of an optimum reaction condition to achieve the highest performance of n-Ni/SiO₂ catalyst in terms of methane decomposition rate. However, carbon accumulation ceases once complete catalyst deactivation occurred at certain reaction conditions such as high temperature and lower methane partial pressure. Virgin nanocatalyst and as-produced nanocarbons were studied with BET, XRD, and TEM.     

Characterization and catalytic behavior of hydrotalcite-derived Ni–Al catalysts for methane decomposition

A series of Ni catalysts were prepared from Ni–Al hydrotalcite-like compounds (HTlcs) by varying the Ni/Al molar ratio (1–4) and calcination temperature (773–1173 K) of HTlcs. The catalysts were reduced with H₂ at 1073 K and tested for CH₄ decomposition at 773–923 K on a thermal gravimeter. Various techniques including N₂ physical adsorption, XRD, H₂-TPR, XPS, HAADF-STEM, TEM, and Raman were applied to characterize the catalysts and the as-produced carbon. The characterizations show that calcination of Ni–Al HTlcs leads to Ni(Al)O solid solution and minor NiO and/or NiAl₂O₄ spinel may be formed depending on the Ni/Al ratio and calcination temperature; upon reduction at 1073 K, most nickel species are reduced to metallic Ni. In CH₄ decomposition, carbon yield shows a volcano-type dependence on the Ni content with the optimum Ni/Al ratio equal to 3. On the other hand, carbon yield is affected by the calcination temperature of the Ni₃Al HTlcs to a small extent. Carbon yield is also significantly affected by the reaction temperature, which decreases remarkably with a rise of temperature to 923 K. TEM and Raman indicate that fish-bone carbon nanofibers are formed at 773–823 K, whereas multi-walled carbon nanotubes are formed at 873–923 K.     

Performance and stability of SOFC anode fabricated from NiO–YSZ composite particles

Ni–YSZ cermet anodes for solid oxide fuel cells (SOFCs) were fabricated at various sintering temperatures from NiO–YSZ composite particles made by spray pyrolysis (SP) technique. NiO particles covered with fine YSZ (Y₂O₃ stabilized ZrO₂) particles were used as the composite particles, and the initial ratio of Ni and YSZ was set at 75:25 (mol%). As a result, the cermet anode sintered at 1350 °C showed the morphology in which fine YSZ grains were uniformly dispersed on the surface of Ni grain network. Electrical performance such as electrochemical activity and internal resistance of a Ni–YSZ cermet anode changed with sintering temperature. The anode fabricated at 1350 °C showed the highest electrical performance. Especially, a single cell voltage with the Ni–YSZ cermet anode kept very stable for 8000 h at 1000 °C in the SOFC operation condition of H₂—3% H₂O and air. The cermet anode after a long-term test had its initial morphology. It indicates that the Ni–YSZ cermet anode fabricated from NiO–YSZ composite particles is a very promising material for its practical use as SOFCs.     

Hydrogen and multiwall carbon nanotubes production by catalytic decomposition of methane: Thermogravimetric analysis and scaling-up of Fe–Mo catalysts

Fe-based catalysts doped with Mo were prepared and tested in the catalytic decomposition of methane (CDM), which aims for the co-production of CO₂-free hydrogen and carbon filaments (CFs). Catalysts performance were tested in a thermobalance operating either at isothermal or temperature programmed mode by monitoring the weight changes with time or temperature, respectively, as a result of CF growth on the metal particles. Maximum performance of Fe–Mo catalysts was found at the temperature range of 700–900 °C. The addition of Mo as dopant resulted in an increase in the rate and amount of deposited carbon, reaching an optimum in the range 1.7–5.1% (mol) of Mo for Fe–Mo/Al₂O₃ catalysts, whereas for Fe–Mo/MgO catalyst an optimum at 5.1% Mo loading was obtained. XRD study revealed the effect of the Mo addition on the Fe₂O₃/Fe crystal domain size in the fresh and reduced catalysts. Tubular carbon nanostructures with high structural order were obtained using Fe–Mo catalysts, mainly as multiwall carbon nanotubes (MWCNTs) and bamboo carbon nanotubes. Fe–Mo catalysts showing best results in thermobalance were tested in a rotary bed reactor leading to high conversions of methane (70%) and formation of MWCNTs (5.3 g/h).     

Improved hydrogen storage kinetics and thermodynamics of RE-Mg-based alloy by co-doping Ce–Y

Aiming at improving hydrogen storage performance of Mg-base alloy, the Mg₉₀Ce₅Y₅ alloy, which has high capacity and high stability, was prepared by vacuum induction melting. The XRD, SEM, TEM, PCI, and DSC were used to characterize the microstructure and phase transformation of alloy as well as hydrogen storage property. The results indicate that the Mg₉₀Ce₅Y₅ alloy consists of multiphase structure, including the CeMg₁₂, Y₅Mg₂₄, Ce₂Mg₁₇ as well as residual Mg phase, besides, part Y dissolved in both Mg and CeMg₁₂/Ce₂Mg₁₇ phase to form solid solutions. After hydrogen absorption, these phases transform into the MgH₂, CeH_2.73 and YH₂ phase, while after hydrogen desorption, the MgH₂ transforms into the Mg phase, but the rare earth hydride phase was not changed. There is another reversible transformation between the CeH_2.73 and CeO₂ phase, which is beneficial for the cyclic stability of the alloy. The alloy has the reversible hydrogen capacity of about 6.0 wt% H₂ as well as the activation energy of 114.3 kJ/mol, and also exhibits enhanced kinetics compared with the pure MgH₂ sample, as a result of the synergistic effect of rare earth hydride phase. Meanwhile, it is also noted that the Mg₉₀Ce₅Y₅ alloy begins to release hydrogen below 250 °C and the rate of hydrogen desorption is mainly dominated by surface controlled.     

Formation kinetics and microstructure of Mg–Ti hydrides made by reactive ball milling

In this work we report the formation kinetics and microstructural properties of Mg_1−xTi_x powder metal hydride systems (x = 0.1, 0.2, 0.3, 0.4, 0.5), obtained by reactive ball milling (RBM) via three different ways. The resulting MgH₂–TiH₂ nanocomposites are characterized and an optimal composition is found. Differences among the preparation routes are analyzed.     

The mechanism and kinetics study of Fischer‒Tropsch reaction over iron-nickel-cerium nano-structure catalyst

The kinetic of Fischer‒Tropsch reaction was investigated using the iron‒nickel‒cerium nano-structure catalyst synthesized by the hydrothermal method in the fixed-bed micro reactor for the first time. The kinetic tests carried out under operating conditions including the pressure of 2–10 bar, temperature of 230–250 °C, molar ratio H₂/CO of 1, and the gas space velocity of 3000 h⁻¹. Twenty-two set of reaction mechanisms were proposed on the basis of the adsorption nature of carbon monoxide, and hydrogen using the Langmuir‒Hinshelwood‒Hougen‒Watson, and Eley Rideal adsorption theories for the FT reaction. The rate equations of CO consumption were obtained based on the proposed reaction mechanisms. The best kinetic model was chosen by non-linear regression analysis and its kinetic parameters including activation energy, adsorption enthalpies of H₂, and CO were estimated 60.4, −3.24, and −65.7 kJ/mol respectively. The nanocatalyst was characterized by various techniques such as XRD, FESEM, and Brunauer–Emmett–Teller surface area measurements.     

Towards integration of hydrolysis,decomposition and electrolysis processes of the Cu–Cl thermochemical water splitting cycle

The thermochemical copper–chlorine (Cu–Cl) cycle is a promising technology that can utilize various energy sources such as nuclear and solar energy to produce hydrogen with minimal or no emissions of greenhouse gases. Past investigations have primarily focused on the design and testing of individual unit operations of the Cu–Cl cycle. This paper investigates the chemical streams flowing through each individual process from the aspect of system integration. The interactions between each of the two immediate upstream and downstream processes are examined. Considering the integration of electrolytic hydrogen production and cupric chloride hydrolysis steps, it is evident that an intermediate step to concentrate CuCl₂ and reduce HCl composition is required. Spray drying and crystallization, serving as the intermediate steps, are examined from the aspects of energy requirements and viability of engineering. Regarding the integration of the hydrolysis and oxygen production steps, thermodynamic and XRD analysis results are presented to study the mutual impacts of these two steps on each other. Within the hydrolysis reactor, high conversion of CuCl₂ to Cu₂OCl₂ is preferable for the integration because it reduces the release of chlorine gas during the oxygen production. Considering the integration of the oxygen production step and electrolysis of CuCl, pulverization is needed for the solidified CuCl. The recovery of CuCl vapour entrained in oxygen gas requires further research. Residual CuCl₂ introduced from the hydrolysis step into the oxygen production step may be further entrained by CuCl into the electrolytic hydrogen production cell. Additionally, thermal energy integration patterns are briefly discussed while integrating the various chemical streams of the Cu–Cl cycle. Steam generated from the heat recovery of cuprous chloride can be introduced into the hydrolysis reactor to serve as a reactant.     

Preparation of Ru–Cs catalyst and its application on hydrogen production by ammonia decomposition

Ammonia can be a hydrogen source for many applications including fuel cells. Using Ru or Cs–Ru as the catalyst, hydrogen is generated from ammonia by decomposition reaction. These catalysts are deposited on carbon powder by either chemical reduction or precipitation method in this study. Different carbon powder pre-treatment solutions and catalyst deposition conditions are evaluated. Nitric acid pre-treatment followed by precipitation at pH of 6 produces the highest catalyst loading from solution with given concentration of catalyst precursor. Hydrogen generation rate is measured at different catalyst compositions, ammonia inlet flow rates, decomposition temperatures, amount of catalyst packing, and ratio of Cs/Ru. The optimal condition for the ammonia decomposition reaction is Cs/Ru weight ratio at 3, ammonia inlet flow at 6 ml min⁻¹, reaction temperature at 400 °C. At this condition, the ammonia conversion rate reaches 90% and hydrogen generation rate reaches 29.8 mmol/min-g_cat.     

Enhanced kinetics for the LiBH4:MgH2 multi-component hydrogen storage system – The effects of stoichiometry and decomposition environment on cycling behaviour

In situ neutron diffraction was undertaken on stoichiometric 2LiBD₄ : MgD₂ and non-stoichiometric 0.3LiBD₄:MgD₂ with both ratios decomposed under 1 bar deuterium and under dynamic vacuum. The subsequent cycling behaviour under 100 bar D₂ at 400 °C was investigated in situ. Analysis of the uptake through formation of deuterided products showed fast kinetics for the magnesium rich system, 0.3:1, with 90% deuteriding occurring within 10 min. This compares to only 60% deuteriding for the 2:1 sample after 4 h under similar conditions. These results demonstrate the strong influence of stoichiometry in the cycling kinetics compared to decomposition conditions, although the later determines the phase progression.     

Influence of adding graphene on the hydrogen storage thermodynamics and kinetics of as-milled CeMg12–Ni alloy

Graphene can been used to improve the hydrogen storage performance of magnesium based materials because of its outstanding electrical conductivity, high specific surface area, and good mechanical qualities. In this paper, mechanical ball milling method was utilized to prepare the CeMg₁₂/Ni alloy combined with graphene in order to investigate the impact of graphene on microstructure, kinetics, and thermodynamic properties of alloy. The results reveal that the addition of graphene can refine the grains of alloy, increase the surface defects of alloy particles and promote Ni to be dissolved in the main phase of the alloy. The absolute value of enthalpy drops from 65.27 kJ/mol to 60.3 kJ/mol when Graphene is added at a higher concentration. The addition of graphene improves the dehydrogenation performance of alloy hydride by lowering its thermal stability. Because of a minimum dehydrogenation activation energy of 85.23 kJ/mol and dehydrogenation characteristic time of 1090s, the alloy with 6 wt% Graphene exhibits the best dehydrogenation kinetic performances.