Gamma-irradiation effects on the electrical conductivity of pure and Co-doped MgAl2O4 spinel

The electrical conductivity of pure and Co-doped MgAl₂O₄ spinels was measured at 700–1100 K. The results of both pure and doped spinels were found to be fitted with an exponential expression with two different activation-energy parameters, in two ranges of temperature. The effect of -irradiation on the electrical conductivity of pure and doped spinels was also studied. The kinetic parameters of the isothermal annealing of the induced irradiation damages in the spinels were determined. All the results were found to be consistent with the ionic conduction which was suggested to be due to the motion of the cation vacancy.     

Cobalt promoted Ni/MgAl2O4 catalyst in lean methane catalytic oxidation

In the present research, magnesium aluminate spinel was prepared as catalyst support using a novel, facile, and efficient mechanochemical method. The Co-promoted catalysts with 20 wt.% of Ni were fabricated using an impregnation route and the samples were analyzed by the X-ray diffraction (XRD), N₂ adsorption/desorption (BET), temperature-programmed reduction and desorption (H₂-TPR and O₂-TPD), and field emission scanning electron microscopy (FESEM) tests. The results confirmed that all samples have a mesoporous structure with a high specific surface area and the presence of cobalt caused complete CH₄ oxidation at low temperatures, and no side reactions were observed. The results indicated that the 3%Co-20%Ni/MgAl₂O₄ catalyst was the optimal sample among the prepared catalysts, owing to the improvement of reduction features and oxygen mobility. The 50 and 90% of methane conversion was obtained at 530 and 600 °C, respectively. Also, the influence of calcination temperature, GHSV, and feed ratio was determined on the catalytic activity. The obtained outcomes revealed that the calcination temperature has a significant effect on the textural properties and catalytic efficiency. The sample calcined at 700 °C showed the weakest performance, which was related to the sintering of particles at high temperatures. The catalytic stability showed that the 3%Co-20%Ni/MgAl₂O₄ has acceptable stability during 600 min time of reaction.

Graphical abstract

     

Kinetics modeling for the mixed reforming of methane over Ni-CeO2/MgAl2O4 catalyst

Kinetics model was developed for the mixed (steam and dry) reforming of methane, which is useful for the control of H2/CO ratio. The equilibrium constants of reaction rate were determined using the experimental equilibrium data at different reaction temperatures, while the forward reaction rate constants were estimated using the experimental data under non-equilibrium (high inert fraction and high space velocity) conditions. The comparison between calculated and experimental data clearly showed that the developed model described satisfactorily, and further analysis using the parametric sensitivity determined the wall temperature and CO2 fraction in the feed gas as effective parameters for the manipulation of CH4 conversion and H2/CO ratio of synthesis gas under the equilibrium condition. Meanwhile, the inert fraction, rather than the residence time, was selected as additional parameter under non-equilibrium condition.     

铈、铌改性镁铝尖晶石作为催化降烯烃助剂的研究

采用溶胶凝胶法制备Ce/MgAl2O4,Nb/MgAl2O4流化催化裂化（FCC）汽油降烯烃助剂,在小型固定床反应装置上考察了不同镁铝配比和Ce,Nb改性对助剂降烯烃性能的影响。并采用XRD和IR分析手段对助剂的降烯烃机制进行了分析。结果表明,经Ce改性后助剂的降烯烃活性较Nb改性高,镁铝比为1∶1时,添加20?元素的助剂可将烯烃的体积分数降低30%以上。XRD结果表明Ce,Nb的加入增强了镁铝尖晶石的氢转移活性,IR分析表明弱酸中心有利于助剂降烯烃。     

Effects of surface coordination chemistry on the magnetic properties of MnFe(2)O(4) spinel ferrite nanoparticles

To understand the influence of surface interactions upon the magnetic properties of magnetic nanoparticles, the surface of manganese ferrite, MnFe(2)O(4), nanoparticles have been systematically modified with a series of para-substituted benzoic acid ligands (HOOC-C(6)H(4)-R; R = H, CH(3), Cl, NO(2), OH) and substituted benzene ligands (Y-C(6)H(5), Y = COOH, SH, NH(2), OH, SO(3)H). The coercivity of magnetic nanoparticles decreases up to almost 50% upon the coordination of the ligands on the nanoparticle surface, whereas the saturation magnetization has increased. The percentage coercivity decrease of the modified nanoparticles with respect to the native nanoparticles strongly correlates with the crystal field splitting energy (CFSE) Delta evoked by the coordination ligands. The ligand inducing largest CFSE results in the strongest effect on the coercivity of magnetic nanoparticles. The change in magnetic properties of nanoparticles also correlates with the specific coordinating functional group bound onto the nanoparticle surface. The correlations suggest the decrease in spin-orbital couplings and surface anisotropy of magnetic nanoparticles due to the surface coordination. Such surface effects clearly show the dependence on the size of nanoparticles.     

Thermal dry reforming of methane over La2O3 co-supported Ni/MgAl2O4 catalyst for hydrogen-rich syngas production

Research on Chemical Intermediates - The excess emission of greenhouse gases (GHGs) such as CO2 and CH4 is posing an acute threat to the environment, and efficient ways are being sought to utilize...     

Preparation and characterization of Eu(3+) doped powder spinel phosphors (MgAl(1.8)Y(0.2-x)O(4))

The photoluminescence (PL) studies of powder phosphors are under rigorous study in view of the applications they have in the field of technology. Different methods are available for the preparation of rare earth ions doped in different host environment of powder phosphors. In the present work, a novel route known as sol-gel technique is employed to prepare spinel phosphor MgAl(1.8)Y(0.2-x)O(4):Eu(x) (x = 2-6 mol%). Then the studies have been carried out to optimize the dopant concentration in the host lattice with the help of photoluminescence spectra. These phosphors have displayed bright red color under UV source. The emission intensities were determined and the relative fluorescence intensities have been estimated. The richness of the red color has been verified by determining their chromaticity coordinates (X, Y) from the CIE standard charts. With the help of XRD, electron spin resonance (ESR), and photo-acoustic (PA) spectra of the samples prepared are also used for the confirmity of the host and analyzing of the data.     

Nanocast mesoporous MgAl2O4 spinel monoliths as support for highly active gold CO oxidation catalyst

Mesoporous MgAl2O4 spinel monolith was synthesized by the nanocasting pathway; high activity in CO oxidation was observed over gold catalysts based on such monoliths as support.     

Interaction of CO and NO with the spinel CuCr2O4 (100) surface: A DFT study

The characteristics of CO and NO molecules at Cu²⁺ and Cr³⁺ ion sites on the CuCr₂O₄ (100) surface have been studied by first principles calculations based on spin‐polarized density functional theory (DFT). The calculated results show that adsorption energies for X‐down(C, N) adsorption vary in the order: Cu²⁺‐CO>Cr³⁺‐NO≈Cr³⁺‐CO>Cu²⁺‐NO. CO molecules are preferentially adsorbed at Cu sites, whereas NO molecules adsorb favorably at Cu²⁺ and Cr³⁺ ion sites. The C‐O and N‐O stretching frequencies are red‐shifted upon adsorption. Combining the analysis of frontier molecular orbitals and Mulliken charge, for CO and NO X‐down adsorption systems, the 5σ orbitals donate electrons and the 2π^* orbitals obtain back‐donated electrons. Although for NO with O‐down adsorption systems, the NO‐2π^* orbitals obtain back‐donated electrons from substrates without 5σ‐donation. Coadsorption calculations show the CO/NO mixture adsorb selectively at the Cu²⁺ion site but simultaneously at the Cr³⁺ ion site, respectively. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Surface passivation of MgAl2O4 spinel powder by chemisorbing H3PO4 for easy aqueous processing

A stoichiometric MgAl 2O 4 spinel (MAS) powder was synthesized by heat treating at 1400 degrees C for 2 h a compacted mixture of alpha-Al 2O 3 and calcined caustic MgO, followed by crushing and milling. The surface of this powder was then passivated against hydrolysis with H 3PO 4 and Al(H 2PO 4) 3 in an ethanol solution. The as-passivated powder could then be dispersed in water using tetramethylammonium hydroxide (TMAH) and an ammonium salt of poly(acrylic acid) (Duramax D-3005) as dispersing agents and gelcast to form green consolidates with relatively high strength (>15 MPa). The good dispersing behavior of the passivated powder in water was confirmed by the low viscosity of its suspension containing 41-45 vol % solids, demonstrating the viability of replacing organic solvents by water in colloidal processing of MAS-based ceramics. The Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and energy dispersive X-ray (EDAX) studies revealed that only negligible amounts of phosphate ions at the surface are required to effectively protect the powder from reacting with water.     

甲烷部分氧化制合成气的La2O3助Ni/MgAl2O4催化剂

 用MgO与载体Al2O3在高温下焙烧成MgAl2O4尖晶石,防止了Ni在反应过程中与载体形成NiAl2O4, 促进了Ni在载体表面的分散. 应用CODEX软件优化了La2O3在Ni/MgAl2O4催化剂中的加入量和活化温度. La2O3助Ni/MgAl2O4在本文实验条件下经100 h反应后活性和选择性均未发生变化. 程序升温烧碳结果表明,催化剂表面仅存在一种较高温度下才可除去的碳物种,它可能是石墨碳. XRD和BET结果证实,催化剂具有较高的结构稳定性. 荧光分析结果表明,在100 h的反应过程中活性组分未发生明显流失. 根据脉冲反应结果对以Niδ+-(La2O4-x)δ-作为氧的活性位和Ni0作为甲烷的活性位的直接氧化反应机理进行了初步探讨.     

Effect of mono- (Cr) and bication (Cr, V) substitution on LiMn2O4 spinel cathodes

A study on the structural and electrochemical properties of LiCr_0.2Mn_1.8O₄ and LiV_0.2Cr_0.2Mn_1.6O₄ cathodes has been made with a view to understand the effect of mono- (Cr) and bication (Cr and V) substitution on LiMn₂O₄ spinel individually. Citric acid assisted modified sol–gel method has been followed to synthesize a series of LiMn₂O₄, LiCr_0.2Mn_1.8O₄, and LiV_0.2Cr_0.2Mn_1.6O₄ cathodes, and the corresponding lattice structure, surface morphology, and site occupancy of lithium in the spinel matrix are acknowledged using X-ray diffraction, scanning electron microscopy, and magic angle spinning ⁷Li nuclear magnetic resonance results. The site occupancy of Cr³⁺ in the 16d octahedral and that of V⁵⁺ in the 16d octahedral and 8a tetrahedral positions are understood. Electrochemical cycling studies of LiCr_0.2Mn_1.8O₄ cathode demonstrate an enhanced structural stability and better capacity retention (94%) resulting from the Cr³⁺ dopant-induced co-valency of Li-O-Mn bond. On the other hand, simultaneous substitution of Cr and V in LiV_0.2Cr_0.2Mn_1.6O₄ has failed to improve the electrochemical properties of native LiMn₂O₄ spinel cathode, mainly due to vanadium-driven cation mixing and the reduced lithium diffusion kinetics. Among the candidates chosen for the study, LiCr_0.2Mn_1.8O₄ qualifies itself as a better cathode for rechargeable lithium battery applications.     

Facile preparation of MgAl2O4/CeO2/Mn3O4 heterojunction photocatalyst and enhanced photocatalytic activity

Novel Mn₃O₄-promoted double p?n junction MgAl₂O₄/CeO₂/Mn₃O₄ heterojunction photocatalyst was constructed by one-step synthesis method and two-step synthesis method. The X-ray powder diffraction, Fourier transform infrared spectrum, X-ray photoelectron spectroscopy, optical and photoluminescence demonstrated that the MgAl₂O₄/CeO₂/Mn₃O₄ heterojunction photocatalyst was synthesized by the two-step synthesis method comprehends a high crystallinity, charge carrier migration and separation efficiency, and relatively low optical absorption coefficient. The MgAl₂O₄/CeO₂/Mn₃O₄ heterojunction photocatalysts were efficiently used as simulated sunlight-driven n-n and p-n double junction photocatalyst for the simultaneous degradation of methylene blue (MB) dye. The continuous double p?n junction MgAl₂O₄/CeO₂/Mn₃O₄ heterojunctions strengthened the function of single n-n or p-n junction and guided the charge carrier migration and separation direction; thus, the oxidation and reduction reactions occur at the active site of spatial separation and prevent the recombination of electrons and holes. The results suggest that the continuous double p?n junction MgAl₂O₄/CeO₂/Mn₃O₄ heterojunctions are very promising candidate material for enhancing the photocatalytic activity in the photocatalytic degradation of MB dye.     

Boosting Efficiency for the Synthesis of Dimethyl Oxalate over Pd/MgAl2O4: Effect of Different Preparation Methods for MgAl2O4 Supports

High active Pd-based catalyst with low loading capacity for CO oxidative coupling with methyl nitrite (MN) to dimethyl oxalate (DMO) has aroused great interest in the large-scale production of DMO for the next hydrogenation to bring forth ethylene glycol (EG). Here, spinel composite oxide support, namely MgAl₂O₄, had been prepared through three different methods: solid combustion method (MAO-SC), hydrothermal method (MAO-HT) and co-precipitation method (MAO-CP), and further Pd nanoparticles were anchored to the surface of MgAl₂O₄ through a simple wetness impregnation method. In the CO direct esterification to DMO reaction, the Pd/MAO-SC catalyst, which used MgAl₂O₄ support prepared by solid combustion method, exhibited the highest intrinsic activity with Turnover frequency (TOF) value of 2.39 s⁻¹ and DMO weight space-time yield (956.8 g⋅kg_cat⁻¹⋅h⁻¹) with 44.1 % CO conversion and 97.8 % DMO selectivity, and showed negligible attenuation of activity during the process of continuous reaction for 100 h. The characterization results confirmed that abundant oxygen defect sites occurred in MAO-SC synthesized by combustion method not only can help to anchor the Pd nanoparticles with appropriate size and dispersion, but also can promote the interfacial electron transfer from MgAl₂O₄ support to Pd nanoparticles, thus significantly enhancing the metal-support interaction (MSI) and providing more CO bridge-adsorption sites that is favorable for the CO oxidative coupling with MN to DMO reaction.     

Photoinduced charge transfer in ZnO/Cu(2)O heterostructure films studied by surface photovoltage technique

ZnO/Cu(2)O heterostructure films were prepared by a two-step electrodeposition method in aqueous solution on fluorine-doped tin oxide (FTO) substrates. Scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and UV-vis transmission measurements were utilized to characterize the films. Surface photovoltage (SPV) technique was used to investigate the process of photoinduced charge transfer. The results show that there is an electric field located at the interface between ZnO and Cu(2)O film and the photoinduced electrons in Cu(2)O film inject into ZnO under the effect of interfacial electric field with visible light irradiation. While under ultraviolet light illumination, the photoinduced electrons in Cu(2)O film accumulate at the surface of Cu(2)O film instead of injecting into ZnO under the action of surface built-in electric field of Cu(2)O film. The work function measurements confirm that the direction of interfacial electric field is from ZnO to Cu(2)O. These results are help to future design of high performance heterostructure photovoltaic devices.     

Investigations of zero-field splitting (ZFS) and local distortion parameters of Fe3+ ions doped into three oxi-spinels (ZnAl2O4, MgAl2O4, and ZnGa2O4) by theoretical analysis

The local environment around the paramagnetic centers formed by the Fe(3+) ions doped into three oxi-spinel crystals (ZnAl(2)O(4), MgAl(2)O(4), and ZnGa(2)O(4)) is investigated utilizing the fourth-order perturbation formula of the axial zero-field splitting parameter D on the basis of the dominant spin-orbit coupling mechanism. In order to fix a plausible cubic space group for B-sites located by Al(3+)/Ga(3+) cations, several modeling approaches are used and the results are discussed in detail.     

Defect processes in MgAl2O4 spinel

In perfect normal MgAl₂O₄ spinel the Mg²⁺ ions occupy tetrahedral 8a sites and Al³⁺ ions occupy octahedral 16d sites. In reality some cations are exchanged between the cation sublattices forming pairs of antisite defects and thus a degree of “inversion”. Here atomic simulation is used to investigate the influence that antisite defects have on the populations of other intrinsic defects, those associated with Schottky and Frenkel reactions. One consequence is that the total magnesium interstitial concentration is increased substantially over the aluminium interstitial concentration and the magnesium vacancy concentration is increased over the aluminium vacancy concentration but to a much smaller extent. The split structures of isolated interstitial defects and the stability of various defect clusters are also discussed.     

A hydrothermal investigation of the 1/2V2O5-H2C2O4/H3PO4/NH4OH system: synthesis and structures of (NH4)VOPO(4).1.5H2O, (NH4)0.5VOPO(4).1.5H2O, (NH4)2[VO(H2O)3]2[VO(H2O)][VO(PO4)2](2).3H2O, and (NH4)2[VO(HPO4)]2(C2O4).5H2O

The 1/2V2O5-H2C2O4/H3PO4/NH4OH system was investigated using hydrothermal techniques. Four new phases, (NH4)VOPO(4).1.5H2O (1), (NH4)0.5VOPO(4).1.5H2O (2), (NH4)2[VO(H2O)3]2[VO(H2O)][VO(PO4)2]2.3H2O (3), and (NH4)2[VO(HPO4)]2(C2O4).H2O (4), have been prepared and structurally characterized. Compounds 1 and 2 have layered structures closely related to VOPO(4).2H2O and A0.5VOPO4.yH2O (A = mono- or divalent metals), whereas 3 has a 3D open-framework structure. Compound 4 has a layered structure and contains both oxalate and phosphate anions coordinated to vanadium cations. Crystal data: (NH4)VOPO(4).1.5H2O, tetragonal (I), space group I4/mmm (No. 139), a = 6.3160(5) A, c = 13.540(2) A, Z = 4; (NH4)0.5VOPO(4).1.5H2O, monoclinic, space group P2(1)/m (No. 11), a = 6.9669(6) A, b = 17.663(2) A, c = 8.9304(8) A, beta = 105.347(1) degrees, Z = 8; (NH4)2[VO(H2O)3]2[VO(H2O)][VO(PO4)2]2.3H2O, triclinic, space group P1 (No. 2), a = 10.2523(9) A, b = 12.263(1) A, c = 12.362(1) A, alpha = 69.041(2) degrees, beta = 65.653(2) degrees, gamma = 87.789(2) degrees, Z = 2; (NH4)2[VO(HPO4)]2(C2O4).5H2O, monoclinic (C), space group C2/m (No. 12), a = 17.735(2) A, b = 6.4180(6) A, c = 22.839(2) A, beta = 102.017(2) degrees, Z = 6.     

Pt-Sn/MgAl2O4催化剂的TPR和H2-TPD研究

Pt Sn/MgAl 2O 4 catalysts prepared by co impregnation with nominal 0 35% platinum and varying content of tin (0～2 5%) were characterized by temperature programmed reduction (TPR) and temperature programmed desorption of hydrogen (H 2 TPD). TPR results showed that platinum catalyzed the reduction of tin. However, the average oxidation state of tin after reduction depended upon the concentration of tin on the catalysts. H 2 TPD data indicated that tin addition inhibited the inactivated adsorption of hydrogen but promoted the activated adsorption, implying that tin modified both the ensembles of platinum and metal support interface, thus increasing hydrogen mobility and promoting hydrogen spillover.     

Thermoluminescence and linear thermal expansion of MgAl2O4

The co-efficient of linear thermal expansion (α _av) and dose dependence of thermoluminescence (TL) in MgAl₂O₄(s) were measured. The change in length per unit length was recorded as a function of temperature between room temperature to 1,273 K at a heating rate of 8 K min^?1, in flowing argon atmosphere. The average of three measurements was quoted as the α _av for MgAl₂O₄(s). The linear thermal expansion was measured to an accuracy of ±3 %. The dose dependence of the TL was found to be super linear in the dose range of 0–10 kGy with a k value of 0.503 indicating that the MgAl₂O₄(s) ceramic is ideally suited for the dose estimation of self-irradiated inert matrix fuel in a once through fuel cycle for actinide burning.