铜对钌改性尖晶石催化剂结构和氧化性能的影响

通过湿化学法制备钌改性的纳米尖晶石MnFe1.95Ru0.05O4催化剂, 考察了添加助剂铜对该催化剂的影响.通过比表面积和孔径分布表征结果证实, 铜助剂的添加有效地增大了比表面积和孔容.XPS的测试结果表明, 钌的改性大大改变了尖晶石中各元素如锰和铁的化学环境；助剂铜的添加对于锰和铁等原子的影响不大, 但对钌原子的影响显著；EXAFS表征结果显示了铜助剂的导入影响了钌的周围环境, 形成了活性Ru=O键.以桂皮醇和苯甲醇的氧化反应为典型反应考察了添加助剂铜前后的影响.     

Effect of the ZnNi y Mn2– y O4 (0 ≤ y ≤ 1) spinel composition on electrochemical lithium insertion

The electrochemical insertion of lithium in the spinel-type manganite with the formula ZnNi _y Mn_{2– y} O₄ has been studied. The galvanostatic discharge curves show that the best performance is obtained for y = 0.25, where a tetragonal to cubic structural transformation occurs. The thermodynamics and kinetics of the process of insertion of the lithium into the tetragonal spinel Li _x ZnNi_0.25Mn_1.75O₄ (x = 0.05–1.3) have been studied. The molar thermodynamic quantities, such as enthalpy, entropy and free energy determined by EMF-T measurements, varied with the lithium concentration in the oxide structure, and a major variation was observed around x = 0.8. The chemical diffusion coefficient of lithium in these spinels was also determined. Structural analysis, degree of oxidation and magnetic susceptibility measurements were carried out for the lithiated oxides in order to obtain the cationic distribution as a function of x. It has been possible to demonstrate that, upon lithium insertion, Mn⁴⁺ ions on B sites are reduced to Mn³⁺ and then to Mn²⁺. A cooperative Jahn-Teller effect is present in these spinel manganese-nickel oxides. Received: 4 February 1997 / Accepted: 11 April 1997     

Electrochemical properties of niobium and phosphate doped spherical Li-rich spinel LiMn2O4 synthesized by ion implantation method

Spherical Li-rich lithium manganese oxide(LMO) spinel material was synthesized by an ion implanted method assisted by polyalcohol doped with Niobium and Phosphate simultaneously.The material was characterized by scanning electron microscopy,X-ray diffraction and BET specific surface area analysis.The electrochemical performances were investigated with galvanostatic techniques and cyclic voltammetry.The synthesis process was investigated with TG/DSC.The results show that the lithium ion can be immersed into the pore of manganese dioxide at a low temperature with the ion implanted method.The prepared materials have a higher discharge capacity and better crystallization than those prepared by solid phase method.The doped Nb can improve the capacity of the Li-rich LMO spinel and reinforce the crystal growth along(111) and(400) planes.The crystal grains show circular and smooth morphology,which makes the specific surface area greatly decreased.Phosphate-doped LMO spinel exhibits good high-rate capacity and structure stability.The prepared Li_(1.09)Mn_(1.87)Nb_(0.031)O_(3.99)(PO_4)_(0.021)delivers a discharge capacity of 119mAhg~(-1) at 0.2C(1C=148mAg~(-1)) and 112.8 mAhg~(-1) at 10 C,the discharge capacity retention reaches 98% at 1 ℃ after 50 cycles at 25 ℃ and 94% at 55 ℃.     

2-pyrrolidone - capped Mn<Subscript>3</Subscript>O<Subscript>4</Subscript> nanocrystals

Water-soluble Mn₃O₄ nanocrystals have been prepared through thermal decomposition in a high temperature boiling solvent, 2-pyrrolidone. The final product was characterized with XRD, SEM, TEM, FTIR and Zeta Potential measurements. Average crystallite size was calculated as ∼15 nm using XRD peak broadening. TEM analysis revealed spherical nanoparticles with an average diameter of 14±0.4 nm. FTIR analysis indicated that 2-pyrrolidone coordinates with the Mn₃O₄ nanocrystals only via O from the carbonyl group, thus confining their growth and protecting their surfaces from interaction with neighboring particles.      

Structure and electrical properties of single-phase cobalt manganese oxide spinels Mn3−xCoxO4 sintered classically and by spark plasma sintering (SPS)

Cobalt manganese oxide spinels Mn_3−xCo_xO₄ (with 0.98?x?3) were prepared by the thermal decomposition in air of oxalate precursors. The influence of the thermal treatments on the structure of these materials is emphasized. Single-phase ceramics were obtained after optimization of the sintering parameters. A precise phase diagram for the Co-Mn-O system is proposed according to thermal stability and structure of oxide powders. The electrical measurements on single-phase ceramics show that low values of resistivity can be achieved. The conduction could take place through jumps of polarons between Mn³⁺ and Mn⁴⁺ on octahedral sites. These compounds present interesting electrical characteristics for negative temperature coefficient (NTC) thermistor applications.     

Synthesis and Characterization of Spinel‐Type Gallia‐Alumina Solid Solutions

By precipitation with ammonia of ethanolic solutions containing the appropriate proportions of gallium and aluminium nitrate, following by calcination of the resulting gels at 773 K, mixed Ga₂O₃/Al₂O₃ oxides having Ga:Al ratios of 9:1, 4:1, 1:1, 1:4 and 1:9 were obtained. Powder X‐ray diffraction showed that these mixed metal oxides form a series of solid solutions having the spinel‐type structure; also shown by γ‐Al₂O₃ and γ‐Ga₂O₃. The specific surface area (determined by nitrogen adsorption at 77 K) was found to range from 160 m² g^?1 for the mixed oxide having Ga:Al = 9:1 up to 370 m² g^?1 for that having Ga:Al = 1:9. High resolution MAS NMR showed that Ga³⁺ and Al³⁺ ions occur at both tetrahedral and octahedral sites in the spinel‐type structure of the mixed metal oxides, although there is a preferential occupation of tetrahedral sites by Ga³⁺ ions. A proportion of penta‐coordinated Al³⁺ ions was also found. IR spectra of carbon monoxide adsorbed at 77 K showed that the mixed metal oxides have a considerable Lewis acidity, related mainly to tetrahedrally coordinated metal ions exposed at crystal surfaces. The characteristic infrared absorption band of coordinated (adsorbed) CO appears in the range 2205–2190 cm^?1, and its peak wavenumber is nearly independent of Ga:Al ratio in the mixed gallia‐alumina oxides.     

X-ray diffraction and Mössbauer spectroscopy studies of LiFe0.5Ti1.5O4 – A new primitive cubic ordered spinel

LiFe_0.5Ti_1.5O₄ was synthesized by solid-state reaction carried out at 900 °C in flowing argon atmosphere, followed by rapid quenching of the reaction product to room temperature. The compound has been characterized by X-ray powder diffraction (XRD) and ⁵⁷Fe Mössbauer effect spectroscopy (MES). It crystallizes in the space group P4₃32, a = 8.4048(1) Å. Results from Rietveld structural refinement indicated 1:3 cation ordering on the octahedral sites: Li occupies the octahedral (4b) sites, Ti occupies the octahedral (12d) sites, while the tetrahedral (8c) sites have mixed (Fe/Li) occupancy. A small, about 5%, inversion of Fe on the (4b) sites has been detected. The MES data is consistent with cation distribution and oxidation state of Fe, determined from the structural data.The title compound is thermally unstable in air atmosphere. At 800 °C it transforms to a mixture of two Fe³⁺ containing phases – a face centred cubic spinel Li_(1+y)/2Fe_(5−3y)/2Ti_yO₄ and a Li_(z−1)/2Fe_(7−3z)/2Ti_zO₅ – pseudobrookite. The major product of thermal treatment at 1000 °C is a ramsdellite type lithium titanium iron(III) oxide, accompanied by traces of rutile and pseudobrookite.     

Two-step process and fatigue in Li<Subscript><Emphasis Type="BoldItalic">x</Emphasis></Subscript>CrMnO<Subscript>4</Subscript> as positive electrode material for lithium ion batteries

The electrochemical behavior and structural changes of the positive electrode material LiCrMnO₄ are studied for different end-of-charge voltages. A potentiostatic intermittent titration technique (PITT) experiment performed up to 5.2 V shows three oxidative peaks. Cells charged to 4.88 V, which corresponds to the minimum between the second and the third oxidative peak, show 89% of capacity retention for the 60th cycle. Compared to that only 23% of capacity are preserved in the 60th cycle when the cell is charged to 5.2 V. The structural analysis by Rietveld refinement shows that for the former case, the amount of structural defects is low and their formation is reversible, while the defect amount is significantly higher for the latter case and the defect formation is only partially reversible. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007     

Surface properties, textural features and catalytic performance for NO + CO abatement of spinels MAl2O4 (M = Mg, Co and Zn) developed by reverse and bicontinuous microemulsion method

Three typical spinels of general formula MAl₂O₄ (M = Mg, Co and Zn) have been successfully prepared via a microemulsion method both in the reverse and bicontinuous state. The final solids were characterized by X-ray diffraction followed by Rietveld analysis, N₂ adsorption-desorption porosimetry and SEM. Pore connectivity (c) was also calculated with Seaton's method. The analysis of all these properties shown that spinels prepared via reverse microemulsion route have better surface and textural properties than bicontinuous ones. The spinels were tested for NO + CO reaction and reverse spinels shown better catalytic activity than bicontinuous ones while the full sequence of catalytic activity is: ZnAl₂O₄-r > ZnAl₂O₄-b > MgAl₂O₄-r > MgAl₂O₄-b > CoAl₂O₄-r > CoAl₂O₄-b. The Rietveld analysis helped us to give an explanation about the catalytic activity and shown that the configuration of inverse spinel phase is the critical factor for the catalytic behavior of final solids. The reactants NO and CO react in a 2:1 ratio at low temperature but they convert in a 1:1 ratio at high temperatures. From the kinetic analysis the heats of NO adsorption are estimated and are in full agreement with the results of catalytic activity.     

High surface area nickel aluminate spinels prepared by a sol–gel method

The oxide spinel NiAl₂O₄ and spinel-type solid solutions Al₂O₃–NiAl₂O₄ (at Ni/Al=1:4, and Ni/Al=1:8) were prepared by controlled hydrolysis of mixed metal alkoxides, followed by calcination of the resulting gels. Powder X-ray diffraction showed that all samples prepared were single phase cubic materials having the spinel-type structure. The cubic lattice parameter, a_o, was found to decrease gradually with increasing aluminium content of the mixed metal oxides. The specific surface area (determined by nitrogen adsorption at 77 K) was found to be in the range of 200–300 m² g⁻¹. The materials were found to be basically mesoporous, the most frequent pore radius being in the range 3.2–6.4 nm. IR spectroscopy of CO adsorbed at liquid nitrogen temperature gave a main band at 2186–2195 cm⁻¹, which was assigned to the C---O stretching vibration of surface Al³⁺CO adducts where coordinatively unsaturated Al³⁺ ions act as Lewis acid centres.