Comparative study of kinetic modeling for the oxidative coupling of methane by genetic and marquardt algorithms

Overall kinetic studies on the oxidative coupling of methane, OCM, have been conducted in a tubular fixed bed reactor, using perovskite titanate as the reaction catalyst. The appropriate operating conditions were found to be: temperature 750-775 ℃, total feed flow rate of 160 ml/min, CH4/O2 ratio of 2 and GHSV of 100 min-1. Under these conditions, C2 yield of 28% was achieved. Correlations of the kinetic data have been performed with lumped rate equations for C2 and COx formation as functions of temperature, O2 and CH4 partial pressures. Six models have been selected among the common lumped kinetic models. The selected models have been regressed with the experimental data which were obtained from the Catatest system by genetic algorithm in order to obtain optimized parameters. The kinetic coefficients in the overall reactions were optimized by different numerical optimization methods such as: the Levenberg-Marquardt and genetic algorithms and the results were compared with one another. It has been found that the Santamaria model is in good agreement with the experimental data. The Arrhenius parameters of this model have been obtained by linear regression. It should be noted that the Marquardt algorithm is sensitive to the first guesses and there is possibility to trap in the relative minimum.     

B-site ordered perovskite LaSrMnNbO6: Synthesis, structure and antiferromagnetism

LaSrMnNbO₆ has been synthesized by high temperature solid state reaction under 1% H₂/Ar dynamic flow. The structure is determined by Rietveld refinement of the powder X-ray diffraction data. It crystallizes in the monoclinic space group P2₁/n with the unit cell parameters: a=5.69187(12), b=5.74732(10), c=8.07018(15) Å and β=90.0504(29)°, which were also confirmed by electron diffraction. The Mn²⁺ and Nb⁵⁺ ions, whose valence states are confirmed by X-ray absorption near-edge spectroscopy, are almost completely ordered over the B-site (<1% inversion) of the perovskite structure due to the large differences of both cationic size (0.19 Å) and charge. The octahedral framework displays significant tilting distortion according to Glazer’s tilt system a⁻b⁻c⁺. Upon heating, LaSrMnNbO₆ decomposes at 690 °C under O₂ flow or at 775 °C in air. The magnetic susceptibility data indicate the presence of long-range antiferromagnetic ordering at T_N=8 K; the experimentally observed effective paramagnetic moment, μ_eff=5.76 μ_B for high spin Mn²⁺ (3d⁵, S=5/2) is in good agreement with the calculated value (μ_calcd=5.92 μ_B).     

Degree of order and redox balance in B-site ordered double-perovskite oxides, Sr2MMoO6−δ (M=Mg, Mn, Fe, Co, Ni, Zn)

We have investigated a series of double-perovskite oxides Sr₂MMoO_6−δ (M=Mg, Mn, Fe, Co, Ni, Zn) for redox stability, oxygen content and crystal structure. Phases with M=Co, Ni and Zn were found to be oxygen-stoichiometric and stable under oxidizing conditions, whereas those with M=Mn and Fe were oxygen-deficient and stable under reducing conditions. The M=Mg phase is stable both under reducing and oxidizing conditions, showing variable oxygen contents within 0.00≤δ≤0.04 depending on the annealing conditions. Structural data indicate somewhat depressed values for the degree of M/Mo cation order and also evidence of electron transfer from M^II to Mo^VI for M=Mn, Fe and Co.     

Hydrogen Production by Water Dissociation in Surface‐Modified BaCoxFeyZr1−x−yO3−δ Hollow‐Fiber Membrane Reactor with Improved Oxygen Permeation

A porous perovskite BaCo_xFe_yZr_0.9?x?yPd_0.1O_3?δ (BCFZ‐Pd) coating was deposited onto the outer surface of a BaCo_xFe_yZr_1?x?yO_3?δ (BCFZ) perovskite hollow‐fiber membrane. The surface morphology of the modified BCFZ fiber was characterized by scanning electron microscopy (SEM), indicating the formation of a BCFZ‐Pd porous layer on the outer surface of a dense BCFZ hollow‐fiber membrane. The oxygen permeation flux of the BCFZ membrane with a BCFZ‐Pd porous layer increased 3.5 times more than that of the blank BCFZ membrane when feeding reactive CH₄ onto the permeation side of the membrane. The blank BCFZ membrane and surface‐modified BCFZ membrane were used as reactors to shift the equilibrium of thermal water dissociation for hydrogen production because they allow the selective removal of the produced oxygen from the water dissociation system. It was found that the hydrogen production rate increased from 0.7 to 2.1 mL H₂ min^?1 cm^?2 at 950 °C after depositing a BCFZ‐Pd porous layer onto the BCFZ membrane.     

Effect of Copper Substitution and Preparation Methods on the LaMnO3 Structure and Catalysis of Methane Combustion and CO Oxidation

 LaMn1-xCuxO3刡冊 perovskite oxides (x = 0, 0.2, 0.4, 0.6, 0.8, 1) were prepared by two different methods, the Pechini and sol-gel methods. The catalysts were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray fluorescence spectroscopy, N2 adsorption, and temperature-programmed reduction. Their catalytic activity in the oxidation of methane and CO was evaluated. EDS and SEM results showed that the Pechini samples had more homogeneity and smaller particles (higher specific surface area). The catalytic activity for methane combustion was highest for x = 0.2. In CO oxidation, the oxides with x = 0.2 and x = 0.4 were the most active. The Pechini samples had higher activity and stability than the sol-gel samples.     

Structural and ac electrical properties of a newly synthesized single phase rare earth double perovskite oxide: Ba2CeNbO6

A single phase rare earth double perovskite oxide Ba₂CeNbO₆ (BCN) is synthesized by solid-state reaction technique for the first time. The X-ray diffraction pattern of the sample at room temperature shows monoclinic structure, with the lattice parameters, a=5.9763 Å, b=5.975 Å and c=8.48 Å and β=90.04°. Impedance spectroscopy is used to study the ac electrical behavior of this material as a function of frequency (10²-10⁶ Hz) at various temperatures (30-450 °C). A relaxation is observed in the entire temperature range. Conduction mechanism is investigated by fitting the complex impedance data to Cole-Cole equation. Complex impedance plane plots show only one semicircular arc, indicating only the grain contribution of dielectric relaxation. The scaling behavior of imaginary part of electric modulus (M″) and imaginary part of electrical impedance (Z″) suggests that the relaxation describes the same mechanism at various temperatures. The frequency dependence of conductivity is interpreted in terms of the jump relaxation model and is fitted to Jonscher's power law. The values of dc conductivities extracted from the Jonscher power law varies from 2.79×10⁻⁷ to 3.5×10⁻⁵ Sm⁻¹ with the increase in temperature from 100 to 450 °C. The activation energies (0.37 eV) extracted from M″(ω) and Z″(ω) peaks are found to follow the Arrhenius law.     

SrRuO3样品的制备及性能研究

利用化学共沉淀法制备了SrRuO3样品.X射线衍射确定样品为单相正交畸变钙钛矿结构.扫描电子显微镜观察了颗粒形貌基本成球形,颗粒尺寸分布在150~300 nm范围之间,SrRuO3样品在161 K温度以下具有铁磁性,SrRuO3样品没有表现出金属导电性.     

Thermal Behaviour of Some Polyhydrocarboxylic Coordination Compounds with Neodium

New Nd-Co based polynuclear coordination compounds containing as ligand polyhydrocarboxylic acid as tartaric, malic and gluconic acids were prepared, namely: [NdCo(tart)₃]·4H₂O, (NH₄)[NdCo_0.5Cu_0.5(tart)₃]·4H₂O, (NH₄)[NdCo(malic)₃]·4H₂O, (NH₄)[NdCo_0.5Cu_0.5(malic)₃]·4H₂O, [NdCo(gluc)₄]·4H₂O and [Nd₂CoCu(gluc)₇]·5H₂O. A comparison between the thermal behaviour of the studied polynuclear coordination compounds concerning thermal stability and thermal decomposition stoichiometry was inferred. Oxalic and malonic intermediates were identified at about 300°C in the thermal decomposition of tartaric and malic compounds. In all the decomposition processes at about 400°C the presence of oxocarbonate is shown. The residual products are mixed oxides of perovskite type. This revised version was published online in August 2006 with corrections to the Cover Date.