XAFS Analysis Research Articles

Study of Capacity Loss Mechanism of Lithium Manganate Cathode during Electrochemical Cycles at High Temperature by <i>in situ</i> TXRF and XAFS Analyses

Study of Capacity Loss Mechanism of Lithium Manganate Cathode during Electrochemical Cycles at High Temperature by <i>in situ</i> TXRF and XAFS Analyses

The Role Of Ge In Cluster Formation In B And Bf2 Implanted Si Wafers After Ge Pre-Amorphization

ABSTRACTCluster formation in high dose B, BF2 implanted Si wafers is an important problem in silicon doping, since it is one of the leading causes of the electrical deactivation of the dopant. In this study, we used Ge pre-amorphized, ultra low energy B and BF2 implanted Si wafers in order to probe these clusters from a local structural point of view. Ge K-edge x-ray absorption spectroscopy (XAFS) is a powerful tool in obtaining local structural information around the Ge atom. The effects of different implant species with various implant doses and annealing conditions on the cluster formation are presented using Ge K-edge multi-shell XAFS analysis. The non-linear least-squares fits to the Ge K-edge Fourier Transformed (FT) XAFS data using calculated standards from multiple scattering simulations around the Ge atom reveal formed Ge-B clusters for the Rapid Thermal Processing (RTP) annealed wafers. The results also indicate the laser annealing process on the other hand is blocking the formation of these Ge induced Ge-B clusters.

Combined Application of XAFS Spectroscopy and Sequential Leaching for Determination of Arsenic Speciation in Coal

Both XAFS spectroscopy and sequential leaching have been used in previous studies to determine the modes of occurrence of arsenic and other elements in coal. However, a direct comparison of the two methods, carried out on identical fractions, has never been made. In this study, residues of coal extracted after each stage of the U. S. Geological Survey's leaching protocol (sequential extraction by solutions of ammonium acetate, HCl, HF, and HNO3) are examined by arsenic XAFS spectra. This procedure enables a rigorous evaluation to be made of the arsenic species leached at each stage of the sequential leaching method. We observed an approximate linear correlation (r2 > 0.98 for an Ohio bituminous coal, r2 > 0.90 for a North Dakota lignite) between the height of the edge step in the XAFS analysis and the concentration of arsenic in the leached residues measured by INAA. Data from the leaching protocol indicate that two major arsenic forms occur in both coals; one is removed by leaching with HCl and the other...

XAFS Analysis for Niobium Carbide Particle Growth on Silica Support During Preparation Process

SiO2-supported NbC catalysts were prepared by using temperature-programmed reactions (TPR). XAFS analysis confirmed that Nb2O5 was reduced to NbO2 in the first TPR stage and converted into NbC in the second TPR stage. Nb particles grew only in the second TPR stage. Formation of highly dispersed NbC particles on SiO2 surfaces was achieved.

Promotion Effect of CeO2 on Pd/SiO2 Catalyst for Methanol Decomposition.

The effects of CeO2 addition to supported Pd catalysts were investigated in the decomposition of MeOH to H2 and CO. The catalytic activity of SiO2-supported Pd was lower than those of Al2O3-, MgO- and ZrO2-supported Pd catalysts. Carbon dioxide and methane were produced in a temperature region higher than 350°C over Pd/Al2O3, Pd/MgO, and Pd/ZrO2 but not over Pd/SiO2. Addition of CeO2 promoted the reaction for all catalysts. The activity of Pd-CeO2/SiO2 was high as that of Pd-CeO2/ZrO2 without losing the high selectivity for CO and H2. XAFS analysis indicated that most of the Pd is metallic particles which are highly dispersed on CeO2/SiO2 and a small part of the Pd is in the oxidized state.

Promotion Effect of Ceria on Pd/Al2O3 Catalyst for the Hydrogenation of Carbon Monoxide to Dimethyl Ether.

Hydrogenation of CO was carried out over Pd catalyst supported on Al2O3, MgO, silica alumina, and silica gel supports. Addition of CeO2 increased the activity of Pd/Al2O3 and the main product was dimethyl ether (DME). DME formation proceeded via dehydration of methanol on the acid sites of the supports. However, the yield of DME was much higher over Al2O3 than silica alumina. Therefore, Al2O3 has an inherently high activity for methanol production. Temperature programmed reduction experiments showed that CeO2 on Al2O3 was reduced with H2, whereas CeO2 on MgO underwent little reduction as confirmed by a XAFS measurement. XAFS analysis indicated that Pd/CeO2/MgO contained Pd0 plus Pd2+ owing to the presence of the reduction-resistant CeO2 on MgO and the Pd on CeO2/Al2O3 was in a more reduced state although Pd2+ still remained. The results of CO adsorption measurement and XAFS and TEM observation revealed that an addition of CeO2 increased the dispersion of Pd metal particles on Al2O3. The intermediate strength interactions between Pd, CeO2, and Al2O3 seems to account for the higher activity of the Pd/CeO2/Al2O3 catalyst for O-containing compounds formation than Pd/CeO2/MgO.

XAFS Analysis Applied to Solutions

XAFS Analysis Applied to Solutions

Multiply Bonded Dimolybdenum Cation Immobilized in Mesoporous Silica: XAFS Analysis and Catalytic Activity in Cyclopentadiene Polymerization

MCM-41 derivatized with the cis-[Mo2(μ-O2CMe)2(MeCN)6]2+ cation has been characterized by means of XAFS spectroscopy and shown to be active as an initiator for the cationic polymerization of methylcyclopentadiene. The Mo-Mo quadruple bond is not disrupted during the fixation on the surface.

Multiply Bonded Dimolybdenum Cation Immobilized in Mesoporous Silica: XAFS Analysis and Catalytic Activity in Cyclopentadiene Polymerization

MCM-41 derivatized with the cis-[Mo2(μ-O2CMe)2(MeCN)6]2+ cation has been characterized by means of XAFS spectroscopy and shown to be active as an initiator for the cationic polymerization of methylcyclopentadiene. The Mo-Mo quadruple bond is not disrupted during the fixation on the surface.

XAFS in a single macrophage cell

In this paper, a novel approach using XAFS will be introduced for the study of the problem of cytotoxicity of metal ions. A micro beam from a SR source was used to measure the time-dependent distributions and chemical states of elements in mouse macrophages that were cultured in metallic solutions of different concentration. It is revealed that the concentrations and distributions of calcium, zinc and potassium in a single cell are closely related to the uptake of the metal. Using XAFS analysis, it is shown that metal elements absorbed by the cell interact with the cell and that the chemical state of the metal is changed.

Effect of the Promotion of Ceria on Pd/MgO Catalyst for Methanol Synthesis.

The hydrogenation of CO was carried out over Pd supported on MgO. The addition of various lanthanide oxides increased the activity of Pd/MgO to produce methanol. CeO2 exhibited the highest promoting effect at a temperature as low as of 250°C and the optimum molar ratio of Ce/Pd was about unity. TPR experiment showed that CeO2 on MgO (without Pd) suffered less reduction with hydrogen compared with that supported on Al2O3. Thus, the former retained the Ce(IV) state almost completely under the reducing condition, which was ascertained by XAFS measurement. XAFS analysis also indicated that Pd in Pd/MgO suffered reduction extensively while that in Pd/CeO2/MgO partly retained the oxidized state owing to the reduction-resistant CeO2 on MgO. The results of CO adsorption XPS, XAFS, and TEM measurement revealed that addition of CeO2 increased the dispersion of Pd metal particles. It was concluded that the high active of the catalyst resulted from the combination of highly dispersed Pd metal and partly oxidized Pd component with a Pd-O bond.

The Structure of Zirconium Oxide in Silica Matrix and its Epoxidation Activity

Zirconia-silica composite oxides were prepared by a sol-gel method, and the state of Zr in the silica matrix was analyzed in relation to its activity in the epoxidation of oct-1-ene with t-butyl hydroperoxide. XAFS analysis suggested that Zr in the composite oxides with low Zr content was in the coordinatively less saturated state compared with those in the oxides with higher Zr contents. Epoxidation of oct-1-ene proceeded effectively on the oxides with low Zr contents. The first step of the epoxidation was assumed to be the interaction of t-butyl hydroperoxide with the coordinatively unsaturated site of Zr.

Infrared and XAFS study on structure and transition behavior of zinc stearate

Structure and transition behavior of zinc(II) stearate crystal were investigated by infrared and XAFS spectroscopies. Structure of zinc stearate at room temperature was estimated as follows. From XAFS analysis, the coordination number of the carboxylate groups around the zinc atom was evaluated as 4 and the ZnO distance as 1.95 Å. Based on the infrared spectrum and a normal mode analysis, the conformation of the alkyl chain was confirmed as all- trans and the sub-cell packing was considered as parallel type, and also the coordination form of the carboxylate groups was determined as bridging bidentate type. As increasing temperature, zinc stearate has a solid–liquid phase transition at 130°C. At the transition, the alkyl chains goes into liquid like state as reported by Mesubi but the coordination structure was confirmed to be maintained.

Infrared and XAFS Study on Internal Structural Change of Ion Aggregate in a Zinc Salt of Poly(ethylene-co-methacrylic acid) Ionomer on Water Absorption

The internal structural change of an ion aggregate in a zinc salt of poly(ethylene-co-methacrylic acid) ionomer on the water absorption−desorption process was investigated by infrared and XAFS spectroscopies. In the infrared carboxylate antisymmetric stretch region, a prominent spectral change was found. In a wet state, the singlet at 1584 cm-1 appeared, while in a dry state the triplet at 1537, 1554, and 1625 cm-1 was observed. The spectral change was reversible with respect to the water content. From the XAFS analysis, the Zn−O distance and the coordination number were evaluated as 1.95 A and 4, respectively, in both the wet and dry states. The coordination form of the carboxylate groups around the zinc atom was considered as the bridging bidentate form in the two states. The large infrared spectral change was ascribed to a little reversible geometrical distortion of the tetrahedral coordination structure in the ion aggregate.

Theory on Extended X-Ray Emission Spectra

A simple theoretical approach is given to study XAFS-like spectra (extended X-ray emission fine structure, EXEFS) observed in X-ray fluorescence spectra based on one-step quantum mechanical formula derived by Almbladh and Hedin. Electronic relaxation and shake-off processes are crucial to explain the spectral features. Here the “shake-off” processes are different from those found in the typical XPS and XAFS spectra; the “shake-off” electrons are ejected during the fluorescence emission processes. Formal EXEFS formulas derived here are the same as the usually used XAFS formula. However, we should carefully use the phase shifts by taking account of the relaxation effects due to additional core hole production; they should be different from those used in the XAFS analyses. The radial dipole integral should be replaced by the corresponding monopole radial integral in the EXEFS analyses.

XAFS Theory at Zero and Finite Temperature

Many-body XAFS theory at zero and finite temperature is proposed based on the two different theories, scattering theory and nonequilibrium (Keldysh) Green's function theory. The latter has an advantage to describe finite temperature XAFS, and the former is flexible to include core-hole effects. In the Keldysh Green's function theory, the X-ray absorption intensity is described in terms of the reducible polarization propagator. By applying the skeleton diagram expansion, we obtain useful approximate formulas. Some important problems in XAFS analyses such as Debye-Waller and Franck-Condon factors, the subtle cancellation of loss structures are discussed by using these theoretical tools, and are closely compared with each other. In particular scattering Green's function including optical potential is compared with G > and G r , and the optical potential is compared with the self-energy in the Keldysh theory, Σ > , Σ r and Σ. Based on the nonequilibrium Green's function formula, we obtain the convolution XAFS...

XAFS Study on the Structure of Mo Species in NaY Zeolite Derived from Mo(CO)6

We carried out XAFS characterization on Mo species in the supercage which was prepared in the supercage of NaY zeolite by cycles of saturation adsorption of Mo(CO)6 at room temperature and subsequent thermal decomposition at 573 K. XAFS analysis revealed that Mo dimer with the Mo-Mo distance of 0.285 nm was formed in the supercage, the structure of which was unchanged during the saturation adsorption/thermal decomposition cycles.

Preparation of Titanium Oxide Photocatalysts Anchored on Porous Silica Glass by a Metal Ion-Implantation Method and Their Photocatalytic Reactivities for the Degradation of 2-Propanol Diluted in Water

Highly dispersed titanium oxide photocatalysts anchored onto transparent plates of porous silica glass were successfully prepared by metal ion implantation, and their photocatalytic reactivity for the liquid-phase photocatalytic degradation of an aqueous 2-propanol solution was compared with that in an aqueous TiO2 dispersion. The titanium ions implanted into the porous silica glass are found to be present on the surface layer as isolated tetrahedral titanium oxide moieties by diffuse reflectance absorption, SIMS, XPS, and XAFS analyses. The specific photocatalytic reactivity of the anchored catalyst was much higher than that for TiO2 powder, which may be attributed to the tetrahedrally coordinated titanium oxide moieties. Metal ion implantation is one of the novel and useful techniques to prepare highly efficient photocatalysts on glass plates.

Glass local structure and optical nonlinearities of oxide glasses

The origins of the nonlinear indices and of the generation of the second harmonic after poling in oxide glasses containing large proportions of TiO 2 have been investigated. Within this scope, the geometry of titanium oxygenated sites in glasses of TiO 2–NaPO 3–Na 2B 4O 7 and TiO 2–K 2O–SiO 2 systems were previously determined by XAFS analysis [T. Cardinal, E. Fargin, Y. Le Luyer, G. Le Flem, L. Canioni, P. Segonds, L. Sarger, F. Adamietz, A. Ducasse, Nucl. Instr. and Meth. B 97 (1995) 169]. Ab initio estimates of the first and second order microscopic polarizabilities for representative titanium oxygen clusters were obtained and then tentatively correlated to nonlinear optical properties of the glasses.

97/05132 Toxic substances from coal combustion: laboratory studies: Senior, C. L. et al. Proc. Annu. Int. Pittsburgh Coal Conf., 1996, 13, (2), 1344–1349

The Clean Air Act Amendments of 1990 identify a number of hazardous air pollutants (HAPs) as candidates for regulation. Should regulations be imposed on HAP emissions from coal-fired power plants, a sound understanding of the fundamental principles controlling the formation and partitioning of toxic species during coal combustion will be needed. PSI Technologies (PSIT) is teaming with researches from USGS, MIT, the University of Arizona, the University of Kentucky, the University of Connecticut, and Princeton University to develop a broadly applicable emissions model useful to regulators and utility planners. The new Toxics Partitioning Engineering Model (ToPEM) will be applicable to all combustion conditions including new fuels and coal blends, low-NO{sub x} combustion systems, and new power generation plants. Development of ToPEM will be based on PSIT`s existing Engineering Model for Ash Formation (EMAF). Extensive coal characterization and laboratory work will be conducted in order to develop and test new sub-models, including direct identification of the modes of occurrence of trace inorganic species in coal and ash using unique analytical techniques such as XAFS analysis and selective leaching and combustion testing too evaluate the formation and partitioning of inorganic toxic compounds as a function of mode of occurrence and combustion conditionsmore » at a range of combustion scales. Preliminary analysis of two American bituminous coals has focused on the forms of arsenic in both coal and ash. Trends in trace metal concentration in coal particles as a function of particle size have also been documented.« less