XANES

About: XANES is a research topic. Over the lifetime, 7737 publications have been published within this topic receiving 188032 citations.

Three-coordinate aluminum in zeolites observed with in situ X-ray absorption near-edge spectroscopy at the Al K-edge: flexibility of aluminum coordinations in zeolites

Abstract: Application of in situ X-ray absorption near-edge spectroscopy (XANES) at the Al K-edge provides unique insight into the flexibilty of the aluminum coordinations in zeolites as a function of treatment or during true reaction conditions. A unique, previously not observed, pre-edge feature is detected in zeolites H-Mordenite and steamed and unsteamed H-Beta at temperatures above 675 K. Spectra simulations using the full multiple scattering code Feff8 identify the unique pre-edge feature as three-coordinate aluminum. The amount of three-fold coordinated aluminum is a function of temperature and pretreatment of a zeolite: a steamed zeolite Beta contains more three-coordinate aluminum than an unsteamed sample. No clear differences between zeolites H-Mordenite and H-Beta were observed. Octahedrally coordinated aluminum forms in zeolites H-Mordenite and H-Beta at room temperature in a stream of wet helium. This octahedrally coordinated aluminum is unstable at temperatures higher than 395 K, where it quantitatively reverts to the tetrahedral coordination.

Chemical interaction and imaging of single Co3O4/graphene sheets studied by scanning transmission X-ray microscopy and X-ray absorption spectroscopy

Abstract: Scanning transmission X-ray microscopy (STXM) has been used to investigate the chemical, electronic and structural nature of Co3O4 nanocrystals grown on single nitrogen-doped graphene sheets through spatially resolved X-ray absorption near edge structure (XANES) spectroscopy and chemical imaging. It has been found that Co3O4 nanocrystals grown on N-doped graphene were partially reduced via Co3+(Oh) to Co2+(Oh), and the reduction varies spatially on and among individual Co3O4 nanocrystal-graphene sheets. Nitrogen sites on graphene have been shown to be major and important anchoring sites for Co3O4 nanocrystals in addition to the carbon and possibly oxygen sites. Macroscopic XANES of Co L-edge and K-edge were also measured to confirm the localized STXM result that Co3+ was partly reduced in the hybrid material. These insights should account for the superior performance of the covalently coupled Co3O4/graphene hybrid in energy related applications.

Investigation of the Local Structure of the LiNi0.5Mn0.5 O 2 Cathode Material during Electrochemical Cycling by X-Ray Absorption and NMR Spectroscopy

Abstract: In situ X-ray absorption spectroscopy (XAS) of the Mn and Ni K-edges and 6 Li magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy have been carried out during the first charging and discharging process for the layered LiNi 0 . 5 Mn 0 . 5 O 2 cathode material. The Ni K-edge structure in the X-ray absorption near-edge structure (XANES) spectrum exhibits a rigid positive energy shift with increased Li deintercalation level, while the Mn XANES spectra do not show any substantial energy changes. The Ni edge shifts back reversibly during discharge. Further Li-ion intercalation at ∼1 V (vs. Li) could be accomplished by reduction of the Mn 4 + ions. The 6 Li MAS NMR results showed the presence of Li in the Ni 2 + /Mn 4 + layers, in addition to the expected sites for Li in the lithium layers. All the Li ions in the transition metal layers are removed on the first charge, leaving residual lithium ions in the lithium layers.

Transient Technique for Identification of True Reaction Intermediates: Hydroperoxide Species in Propylene Epoxidation on Gold/Titanosilicate Catalysts by X-ray Absorption Fine Structure Spectroscopy

Abstract: In situ ultraviolet−visible (UV−vis) diffuse reflectance spectroscopy was used in combination with in situ Ti K-edge X-ray absorption near-edge structure (XANES) to study the formation of Ti-hydroperoxo species during the gas-phase epoxidation of propylene with H2 and O2 at reaction conditions over a Au−Ba/Ti−SiO2 (Ti−TUD) catalyst. The in situ UV−vis measurements showed growth of a signal due to Ti-hydroperoxo species when the catalyst was put in contact with H2/O2/Ar (1/1/8) and C3H6/H2/O2/Ar (1/1/1/7) gas mixtures at 423 K and 0.1 MPa. Changes in the area of the pre-edge peak centered at 4968.9 eV present in the Ti K-edge XANES spectra of the catalyst were used to estimate the Ti-hydroperoxo species coverages (θ) under operating conditions. Transient Ti K-edge XANES experiments with H2/O2/Ar (1/1/8) and C3H6/H2/O2/Ar (1/1/1/7) gas mixtures allowed the estimation of the net epoxidation rate by a novel method involving the determination of dθ/dt. It is shown that the Ti-hydroperoxo species are true inter...