DFT investigation of high temperature water gas shift reaction on chromium–iron mixed oxide catalyst

As part of high temperature water gas shift reaction mechanism, CO adsorption and H₂O adsorption on Fe₃O₄ (111) and chromium atom substituted Fe₃O₄ (111) slab surfaces are investigated by means of periodic DFT approach using VASP. Fe₃O₄ bulk structure has been computed including the Hubbard (U) parameter. One oxygen site (Ooct1) is studied as a probable site among the six Fe₃O₄ (111) terminations. Cr atom substitution on this surface is also examined. Cr atoms prefer being on the surface rather than in the bulk structure and Cr atoms substitute on the octahedral iron atom layer (Ooct2Cr). Adsorption energies of CO on Ooct1 and Ooct2Cr are found as −96 kcal/mol and −47 kcal/mol. Water adsorption on Ooct1 surface is molecular with −54.88 kcal/mol adsorption energy. On the other hand, water adsorption on Ooct2Cr surface is dissociative with nearly same adsorption energy, −55.12 kcal/mol, indicating the catalytic effect of chromium atom.     

Evaluation of redox activity of brownmillerite-structured Ca2Fe2O5 oxygen carrier for chemical looping applications

The brownmillerite-structured Ca₂Fe₂O₅ oxygen carrier has shown its great potential in chemical looping processes, due to it can be completed regeneration in a H₂O or CO₂ atmosphere without the air reactor. However, the low reaction reactivity of Ca₂Fe₂O₅ restricts its application. In this study, Ca₂Fe₂O₅ oxygen carrier was prepared by sol-gel method. The reduction and oxidation kinetics of Ca₂Fe₂O₅ were evaluated in H₂ and CO₂ atmospheres, respectively. The reduction of Ca₂Fe₂O₅ in H₂ atmosphere in good agreement with the random nucleation and growth model with E_a and A of 53.82 kJ/mol and 2.65 s⁻¹, respectively. The dimension of the model increases with conversion (A1.5 → A2 → A3 → A4). The oxidation of reduced Ca₂Fe₂O₅ in CO₂ atmosphere can be described by the zero-order contraction model with E_a and A of 11.66 kJ/mol and 0.05 s⁻¹, respectively. The kinetics analysis showed that both the reduction of H₂ and the oxidation of CO₂ are one-step reactions, as evidenced by the fact that only Fe⁰ and Fe³⁺ phases were detected in semi-in situ XRD analysis. It was inferred that the release and recovery of lattice oxygen is from inside to outside for Ca₂Fe₂O₅ oxygen carrier in the redox process. By reducing the migration energy barrier of lattice oxygen between bulk and surface would be an effective means to improve the reactivity of Ca₂Fe₂O₅ oxygen carriers.     

Fundamental study of heterogeneous KCl sulfation under oxy‐fuel combustion conditions

Under oxy‐fuel combustion condition, SO₂ in the flue gas would be accumulated by recirculation, which is conducive to the heterogeneous sulfation reaction of alkali metals. In the present study, experiments were conducted in a fixed bed to investigate the effects of operating parameters and mineral additives (SiO₂, CaO, and Fe₂O₃) on the heterogeneous sulfation of potassium chloride under oxy‐fuel combustion atmosphere. According to the results here, the heterogeneous sulfation reaction was a kinetically controlled process, with the activation energy of 93.6 kJ/mol. The reaction orders with respect to SO₂, O₂, and H₂O were determined as 1, 0.6 and 0 (H₂O involved in the reaction). While the reaction would be promoted obviously in the absence of H₂O. The rate law of heterogeneous sulfation of potassium chloride was derived based on the experimental data. Compared with air combustion, the heterogeneous sulfation rate was lower under oxy‐fuel combustion. All the mineral additives employed would affect the sulfation reaction. The sulfation reaction can be catalyzed by Fe₂O₃. While CaO would suppress the reaction by competing for SO₂ with KCl. The reaction between CaO and SO₂ could also be catalyzed by Fe₂O₃. Besides, SO₂ was more reactive towards CaO than KCl.     

Preparation and application of binary acid–base CaO–La2O3 catalyst for biodiesel production

A simple method was developed for biodiesel production from non-edible Jatropha oil which contains high free fatty acid using a bifunctional acid–base catalyst. The acid–base catalyst comprising CaO and La₂O₃ mixed metal oxides with various Ca/La atomic ratios were synthesized via co-precipitation method. The effects of Ca/La compositions on the surface area, acidity–basicity and transesterification activity were investigated. Integrated metal–metal oxide between Ca and La enhanced the catalytic activity due to well dispersion of CaO on composite surface and thus, increased the surface acidic and basic sites as compared to that of bulk CaO and La₂O₃ metal oxide. Furthermore, the transesterification reactions resulted that the catalytic activity of CaO–La₂O₃ series were increased with Ca/La atomic ratio to 8.0, but the stability of binary system decreased by highly saturated of CaO on the catalyst surface at Ca/La atomic ratio of 10.0. The highest biodiesel yield (98.76%) was achieved under transesterification condition of 160 °C, 3 h, 25 methanol/oil molar ratio and 3 wt.%. In addition, the stability of CaO–La₂O₃ binary system was studied. In this study, Ca–La binary system is stable even after four cycles with negligible leaching of Ca²⁺ ion in the reaction medium.     

Chromium as reactant for solar thermochemical synthesis of ammonia from steam, nitrogen, and biomass at atmospheric pressure

Ammonia for fertilization plays a crucial role in agriculture. It is an important commodity chemical, and it can serve as a fuel for combustion engines or as a carrier molecule for hydrogen. Global NH₃ production of over 100 million metric tons per year relies almost entirely on natural gas for energy and hydrogen. About 2% of the world’s energy budget is spent to produce NH₃. Experiments towards a solar thermochemical cycle for NH₃ synthesis at near atmospheric pressure using a transition metal reactant and a Fresnel-lens solar furnace are reported here: reacting Cr metal powder with gaseous N₂ to Cr nitride, hydrolyzing Cr nitride powder with steam to NH₃ and Cr₂O₃, and finally reducing Cr₂O₃ powder back to Cr with mixtures of H₂, CO, and N₂. At about 1000 °C it was found that Cr readily fixes N₂ from the gas phase as Cr nitride (4.13 × 10⁻² mol N₂/mol Cr/min, 85 ± 4 mol% of hexagonal Cr₂N after 5.6 min). Cr₂N converts over time to a cubic CrN phase. Corrosion of Cr nitride with steam at 1000 °C and about 1 bar forms Cr₂O₃ and CrO while liberating 53 ± 11 mol% of the nitrogen contained in the solid Cr nitride in 60 min. Of the N liberated, 0.28 ± 0.07 mol% forms the desired NH₃. This results in a yield of 0.15 ± 0.02 mol% NH₃ relative to the N in the nitride (1.07 × 10⁻⁴ mol NH₃/mol Cr/min). Addition of CaO/Ca(OH)₂ powder or quartz wool to provide more reactive sites and promote protonation of N increased the yield of NH₃ only slightly (0.24 ± 0.01 or 0.39 ± 0.03 mol% NH₃ relative to the N in the nitride respectively). The thermochemical cycle is closed by heating Cr₂O₃ to 1200–1600 °C with a reduction yield near the surface of the particles of approximately 82.85 mol% (40 min at 1600 °C) in a gas stream of H₂ and CO (2.7 × 10⁻³ mol Cr/mol Cr₂O₃/min). An unreacted core model was applied to estimate the activation energy of Cr₂O₃ reduction with 128 ± 4 kJ/mol. Cr appears promising to promote nitridation and oxide reduction as a basis for a future custom-designed reactant with high specific surface area enabling sustainable and more scalable NH₃ production from N₂ and H₂O at ambient pressure without natural gas consumption.     

Improving hydrogen storage properties of covalent organic frameworks by substitutional doping

We proposed a possible way of promoting the binding of H₂ molecules on covalent organic frameworks crystals via substituting the bridge C₂O₂B rings with different metal-participated rings, which can naturally avoid the clustering of metal atoms. First-principles calculations on both crystalline phase and molecular fragments show that the H₂ binding energy can be enhanced by a factor of four with regard to the undoped crystal, i.e. reaching about 10 kJ/mol. Grand canonical Monte Carlo simulations further confirm that such substitutional doping would improve the room temperature hydrogen storage capacity by a factor of two to three.     

Reaction mechanism for the free-edge oxidation of soot by O2

The reaction pathways for the oxidation by O₂ of polycyclic aromatic hydrocarbons present in soot particles are investigated using density functional theory at B3LYP/6-311++G(d,p) level of theory. For this, pyrene radical (4-pyrenyl) is chosen as the model molecule, as most soot models present in the literature employ the reactions involving the conversion of 4-pyrenyl to 4-phenanthryl by O₂ and OH to account for soot oxidation. Several routes for the formation of CO and CO₂ are proposed. The addition of O₂ on a radical site to form a peroxyl radical is found to be barrierless and exothermic with reaction energy of 188 kJ/mol. For the oxidation reaction to proceed further, three pathways are suggested, each of which involve the activation energies of 104, 167 and 115 kJ/mol relative to the peroxyl radical. The effect of the presence of H atom on a carbon atom neighboring the radical site on the energetics of carbon oxidation is assessed. Those intermediate species formed during oxidation with seven-membered rings or with a phenolic group are found to be highly stable. The rate constants evaluated using transition state theory in the temperature range of 300–3000 K for the reactions involved in the mechanism are provided.     

Improved stability of nano-sized La0.6Sr0.4Co0.2Fe0.8O3-δ-Ce0.8Sm0.2O1.9 composite cathodes by substituting Sr2+ with Ca2+

The nano-sized composite cathodes prepared by infiltrating La_0.6Sr_{0. 4}Co_0.2Fe_0.8O_3-δ (LSCF) or La_0.6Ca_0.4Co_0.2Fe_0.8O_3-δ (LCCF) into the Ce_0.8Sm_0.2O_1.9 (SDC) scaffolds exhibit different electro-catalytic activity and microstructure evolution. Compared with LSCF-SDC nano-sized composite cathode, the LCCF-SDC composite cathode shows the higher microstructure stability. There is no observable coarsening or sintering and no diffraction peaks of other impurity phase are detected for both LSCF and LCCF after being aged at 600 °C for 500 h, but the lattice distortion is less if La³⁺ ions are substituted by Ca²⁺ ions instead of by Sr²⁺ in LaCo_0.2Fe_0.8O_3-δ (LCFO) lattice. The oxygen vacancy concentration is also less in LCCF than in LSCF. The less lattice distortion and oxygen vacancy concentration prohibit the Ca²⁺ ions segregation on the LCCF cathode surface because of less strain in the LCCF lattice and less electrostatic interactions between the negatively charge A-site dopants (Ca′_La) and the positively charged oxygen vacancies (V_o^··) on LCCF surface. The greater binding energy of Ca–O maybe also hinder the enrichment of Ca²⁺ ions on the cathode surface. After being aged in air at 600 °C for 500 h, more Sr²⁺ ions gather on the LSCF cathode surface to form a Sr-rich inert phase, which is detrimental to the oxygen reduction reaction on the cathode surface.     

A novel hybrid iron-calcium catalyst/absorbent for enhanced hydrogen production via catalytic tar reforming with in-situ CO2 capture

An iron-calcium hybrid catalyst/absorbent (Ca–Al–Fe) is developed by a two-step sol-gel method to enhance tar conversion, cyclic CO₂ capture and mechanical strength of absorbent for hydrogen production in calcium looping gasification. The developed catalyst/absorbent consists of CaO and brownmillerite (Ca₂Fe₂O₅) with mayenite (Ca₁₂Al₁₄O₃₃) as inert support. Comparing with three candidate absorbents without Ca₂Fe₂O₅ or Ca₁₂Al₁₄O₃₃, cyclic carbonation reactivity and mechanical strength of Ca–Al–Fe are largely promoted. Meanwhile, Ca–Al–Fe approaches the maximum conversion rate of 1-methyl naphthalene (1-MN) with enhanced hydrogen yield around 0.15 mol/(h·g) under reforming conditions of present study. Ca–Al–Fe also shows the largest CO₂ absorption and lowest coke deposition. Influences of operation variables on 1-MN reforming are evaluated and recommended conditions can be iron to CaO mass ratio of 10%, reaction temperature of 800 °C and steam to carbon in 1-MN mole ratio of 2.0. Ca–Al–Fe hybrid catalyst/absorbent presents good potential to be applied in future.     

Application of a micro-channel reactor for process intensification in high purity syngas production via H2O/CO2 co-splitting

A stainless steel micro-channel reactor was tailor-made to an in house-design for process intensification propose. The reactor was used for a two-step thermochemical cycles of H₂O and CO₂ co-splitting reaction, in the presence of La_0.3Sr_0.7Co_0.7Fe_0.3O₃ (LSCF). LSCF was coated inside the reactor using wash-coat technique. Oxygen storage capacity of LSCF was determined at 4465 μmol/g, using H₂-TPR technique. H₂O-TPSR and CO₂-TPSR results suggested that a formation of surface hydroxyl group was the cause of H₂O splitting favorable behavior of LSCF. Optimal operating reduction/oxidation temperature was found at 700 °C, giving 2266 μmol/g of H₂, 705 μmol/g of CO, and 67% of solid conversion, when the H₂O and CO₂ ratio was 1 to 1, and WSHV was 186,000 mL/g.h. Activation energy of H₂O spitting and CO₂ splitting was estimated at 87.33 kJ/mol, and 102.85 kJ/mol The pre-exponential factor of H2O splitting and CO2 splitting was 595.24 s^?1 and 698.79 s^?1, respectively.     

Ca–Al layered double hydroxides-derived Ni-based catalysts for hydrogen production via auto-thermal reforming of acetic acid

Biomass-derived acetic acid (HAc) is an alternative resource for hydrogen production, and heat self-sustained auto-thermal reforming (ATR) of HAc shows potential for practical application, while robust and stable catalysts remain as a key factor. Ca–Al layered double hydroxides (LDHs)-derived Ni-based catalysts were synthesized by co-precipitation, and tested in ATR of HAc for hydrogen production. Over the LDHs-derived Ca_2.55Ni_0.45AlO_4.5 catalyst, active sites of Ni–CaO–Ca₁₂Al₁₄O₃₃ were formed, and interaction among Ni, CaO and Ca₁₂Al₁₄O₃₃ was proved to be a pivotal role for 1) thermal stability, 2) resistance to oxidation of Ni⁰ species, and 3) inhibiting coke deposition through catalytic cycle, CaO + CO₂↔CaCO₃ and CaCO₃+*C↔CaO+2CO, for gasification of coking precursors. Hence, the Ca_2.55Ni_0.45AlO_4.5 catalyst showed enhanced activity with no obvious deactivation in ATR: the acetic acid conversion rate was 100%, and the hydrogen yield remained stable near 2.75 mol-H₂/mol-HAc at a rate of 40.34 mmol-H₂/s/g-catalyst.     

The nanometer magnetic solid base catalyst for production of biodiesel

Nanometer magnetic solid base catalysts were prepared by loading CaO on Fe₃O₄ with Na₂CO₃ and NaOH as precipitator, respectively. The optimum conditions for preparation of this catalyst were investigated. The influence of the proportion of Ca²⁺ to Fe₃O₄ on the catalytic performance has been studied. The catalyst with highest catalytic activity has been obtained when the proportion of Ca²⁺ to Fe₃O₄ is 7:1; the catalytic activity of the catalyst calcined from Ca(OH)₂ to Fe₃O₄ is better than that calcined from CaCO₃ to Fe₃O₄; under the conditions of methanol/oil molar ratio of 15:1, catalyst dosage of 2 wt% and temperature of 70 °C, the biodiesel yield reaches to 95% in 80 min, even to 99% finally. The catalytic activity and recovery rate of the nanometer magnetic solid base catalysts are much better than those of CaO. Calcination temperature was determined by differential thermogravimetric analysis. Ca₂Fe₂O₅, a kind of new metal multiple oxide, was found in the catalyst through X-ray diffraction. At the end, these catalysts were characterized by scanning electronic microscope (SEM), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM).     

Thermodynamics of hydrogen production from urea by steam reforming with and without in situ carbon dioxide sorption

The thermodynamic effects of molar steam to carbon ratio (S:C), of pressure, and of having CaO present on the H₂ yield and enthalpy balance of urea steam reforming were investigated. At a S:C of 3 the presence of CaO increased the H₂ yield from 2.6 mol H₂/mol urea feed at 940 K to 2.9 at 890 K, and decreased the enthalpy of bringing the system to equilibrium. A minimum enthalpy of 180.4 kJ was required to produce 1 mol of H₂ at 880 K. This decreased to 94.0 kJ at 660 K with CaO-based CO₂ sorption and, when including a regeneration step of the CaCO₃ at 1170 K, to 173 kJ at 720 K. The presence of CaO allowed widening the range of viable operation at lower temperature and significantly inhibited carbon formation. The feasibility of producing H₂ from renewable urea in a low carbon future is discussed.     

Hydrogen production by biomass gasification in a fluidized-bed reactor promoted by an Fe/CaO catalyst

Biomass gasification for hydrogen production was performed in a continuous-feeding fluidized-bed with the use of Fe/CaO catalysts. The relationship between catalyst properties and biomass gasification efficiencies was studied. The findings indicated that only CaO was involved in the enhancement of char gasification, resulting in an increased hydrogen production. However, CaO was also easily deactivated by biomass tar. The characterization results indicated that when CaO was impregnated with Fe, Ca₂Fe₂O₅ formed on the surface of the support. Ca₂Fe₂O₅ decomposed polyaromatic tar but was not effective in char gasification. The synergistic effects between Fe and CaO that effectively enhanced biomass gasification mainly involved combustion and pyrolysis, and the biomass gasification products, i.e., char and tar, were further gasified, indicating that tailor-made Fe/CaO catalysts prevented CaO deactivation by tar, thus promoting biomass gasification and hydrogen production.     

Hydrogen physisorption on palygorskite dehydrated channels: A van der Waals density functional study

The interaction between H₂ molecules within the structure of microporous clays is an interesting topic, with applications including gas storage, nuclear waste containment, and geochemistry. Microporous clays exhibit an intricate atomic structure with different polar species able to interact with H₂. However, despite its numerous implications, the binding mechanism of H₂ has not been discussed in detail. In this work, a first-principles study of the structure, energetics, and chemical bonding of the H₂ in palygorskite clay is addressed. Total energy calculations including van der Waals interactions shown that, depending on water content, the structural channels of palygorskite clays offer different possibilities for the adsorption of H₂. The calculated binding energies range between 6 and 24 kJ/mol. The larger H₂ binding affinity corresponds to the case where the Mg²⁺ nearest to the inner surface of the palygorskite channel is exposed due to a partial loss of coordination water. The analysis of net atomic charges predicts a slight charge transfer from H₂ to Mg²⁺. Electronic structure and overlap population analysis reveal orbital interactions between the 3s and 3p states of Mg²⁺ with the σ orbitals of the H₂ molecule, explaining the charge transfer and large binding energy in this system (24 kJ/mol). In contrast, Al³⁺ does not induce charge transfer from H₂, while dispersion forces and electrostatic interactions dominate the binding mechanism. The results of the present study suggest that the controlled dehydration of coordinated water in Mg-rich palygorskite is a potential route to the creation of microporous materials with enhanced gas adsorption.     

Preparation and characterization of ceramic interconnect La0.8Ca0.2Cr0.9M0.1O3−δ (M = Al, Co, Cu, Fe) for IT-SOFCs

The lattice parameters, electrical conductivity, activation energy, mechanical properties, and microstructure of (La_0.8Ca_0.2)CrO_3−δ-based specimens were investigated systematically in this paper. The tolerance factors for (La_0.8Ca_0.2)CrO_3−δ-based specimens were all greater than 0.9, indicating the perovskite was not distorted with different cations (Al³⁺, Co³⁺, Cu²⁺, Fe³⁺) substitution for B site of (La_0.8Ca_0.2)CrO_3−δ. (La_0.8Ca_0.2)Cr_0.9Co_0.1O_3−δ specimen revealed the maximum electrical conductivity, σ_850 °C = 59.59 S/cm with minimum activation energy, E_a = 11.2 kJ/mol among (La_0.8Ca_0.2)CrO_3−δ-based specimens. The grain size seemed dependent on doping species and the grain sizes were distributed in the range of 2.4-5.6 μm for (La_0.8Ca_0.2)CrO_3−δ-based specimens. The rate of grain growth was proportional to the boundary mobility M_b, which was related to the diffusion coefficient of doping cation. (La_0.8Ca_0.2)CrO_3−δ-based specimens revealed variety in microhardness, in the range of 4.33-9.85 GPa and the fracture toughness were distributed in the range of 3.52-4.33 MPa m^1/2. Based on the results in terms of grain size and mechanical properties, we concluded that the microhardness and fracture toughness were dependent on the dopant ions. The (La_0.8Ca_0.2)Cr_0.9Co_0.1O_3−δ specimen shows high electrical conductivity and mechanical properties Consequently, it is a promising candidate as an interconnect material for intermediate temperature solid oxide fuel cell (IT-SOFC) applications.     

Hydrogen generation from the hydrolysis of sodium borohydride with new pyridinium dicationic salts containing transition metal complexes

Pyridinium based dicationic ionic salts [C₆(mpy)₂] containing transition metal halide anions [NiCl₄]²⁻ and [CoCl₄]²⁻ were synthesized and characterized by ¹H NMR, ¹³C NMR, LRMS-MS & TGA. The catalytic activities of dicationic ionic liquids were studied for the first time in the hydrolysis reaction of sodium borohydride. The reported work includes the full experimental details for the collection of a wealth of kinetic data to determine the activation energy (Ea = 56.36 kJ/mol) and effects of the amount of catalyst, amount of substrate and temperature on the rate for the catalytic hydrolysis of NaBH₄. The activation energy of the catalyst dicationic tetrachloronickelate (II) anion for hydrogen production is comparable to that of other metal halide catalysts.     

Investigation of crystallization kinetic of SrO-La2O3-Al2O3-B2O3-SiO2 glass and its suitability for SOFC sealant

Strontium-lanthanum-aluminoborosilicate (SrO-La₂O₃-Al₂O₃-B₂O₃-SiO₂) (SLABS) glass was prepared for sealant material in planar Solid Oxide Fuel Cells (pSOFC). Differential Thermal Analysis of the glass showed the formation of two crystalline phases, first one 807 °C and the second phase at 1021 °C. Crystallization kinetic of the first crystalline phase was investigated by differential thermal analysis (DTA) and using the JMA-kinetic model. The activation energy for crystallization was calculated to be 114.04 kJ/mol with frequency factor 5.9 × 10³. Coefficient of Thermal Expansion (CTE) of the glass was measured to be 9.72 × 10⁻⁶/°C. Optical band gap of the glass was measured to be 3.07 eV. Conductivity measured by Impedance spectroscopy found to be 5.68 × 10⁻⁷ S cm⁻¹ at 600 °C and it increased with temperature to 2.68 × 10⁻⁶ S cm⁻¹ at 800 °C. Activation energy for electrical conduction was measured to be 66.12 kJ/mol. Wetting behavior of the glass on a ferritic steel substrate was investigated under hot stage microscopy. It showed initial deformation temperature (IDT) 754 °C, softening temperature (ST) 840 °C, hemispherical temperature (HT) 1108 °C and flow temperature (FT) 1279 °C.     

Solar photocatalytic decomposition of Probenazole in water with TiO2 in the presence of H2O2

The photocatalytic decomposition of Probenazole in water using TiO₂/H₂O₂ under sunlight illumination is studied. The addition of H₂O₂ is effective for the improvement of photocatalytic decomposition of Probenazole with TiO₂. Furthermore, the operating conditions, such as photocatalyst dosage, temperature, pH, sunlight intensity and illumination time are also optimized. The kinetics of photocatalytic decomposition follow a pseudo–first–order kinetic law, and the rate constant is 0.129 min^?1. The activation energy (E_a) is 11.34 kJ/mol. The photocatalytic decomposition mechanism is discussed on the basis of molecular orbital (MO) simulation for frontier electron density.