Oscillatory CO Oxidation Over Pt/Al2O3 Catalysts Studied by In situ XAS and DRIFTS
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X-ray absorption spectroscopy
Oxidation state
Noble metal
Particle (ecology)
Catalytic Oxidation
This work discussed the role of X-ray absorption spectroscopy (XAS) in determining the oxidation state, chemical fraction, and local atomic structure of the materials. These aspects of XAS were discussed by taking LiNiO2 and Mn3O4 as prototype materials. The oxidation state of metal ions in these oxides was estimated with the help of XAS spectra of the reference oxides such as NiO (in the case of LiNiO2), MnO, Mn2O3, and MnO2 (in the case of Mn3O4). Analysis of the oxidation state was performed from the main absorption edge which was estimated from half of the step height. This showed that the Ni K-edge absorption edge of LiNiO2 is slightly above that of NiO. In the case of Mn ions, the main absorption edges show a linear variation with the oxidation states. This estimates the presence of a mixed oxidation state (2.6+) of Mn ions in Mn3O4. Linear combination fitting results exhibit that almost 35% of ions are in a 2+ oxidation state. The remaining ions are in a 3+ oxidation state. Thus, XAS can determine the fractions of each oxidation state of a particular ion in a given material. Quantitative information on coordination number and bond distance of nearest neighbor for a given element of a material is another important use of this technique.
X-ray absorption spectroscopy
Oxidation state
Chemical state
Non-blocking I/O
Coordination number
XANES
Absorption edge
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Sulfur K-edge XAS data provide a unique tool to examine oxidation states and covalency in electronically complex Sbased ligands. We present sulfur K-edge X-ray absorption spectroscopy on a discrete redox-series of Ni-based tetrathiafulvalene tetrathiolate (TTFtt) complexes as well as on a 1D coordination polymer (CP), NiTTFtt. Experiment and theory suggest that Ni—S covalency decreases with oxidation which has implications for charge transport pathways. Finally, a characteristic peak for doubly oxidized TTFtt verifies this formal redox state in the CP, NiTTFtt.
X-ray absorption spectroscopy
Tetrathiafulvalene
Oxidation state
XANES
Linker
K-edge
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Combined X-ray absorption spectroscopy (XAS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were applied to investigate the interaction between reaction atmosphere, adsorbates and Pt oxidation state of Pt/Al2O3 model diesel oxidation catalysts under CO/NO oxidation conditions. The Pt oxidation state was correlated to the adsorbates on the catalyst's surface. Even at low temperature the reaction atmosphere had a strong impact on the oxidation state of the catalyst, and the oxidation state in turn strongly affected CO adsorption on the Pt particles.
X-ray absorption spectroscopy
Oxidation state
Diffuse reflection
Catalytic Oxidation
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A series of organometallic copper complexes in formal oxidation states ranging from +1 to +3 have been characterized by a combination of Cu K-edge X-ray absorption (XAS) and Cu Kβ valence-to-core X-ray emission spectroscopies (VtC XES). Each formal oxidation state exhibits distinctly different XAS and VtC XES transition energies due to the differences in the Cu Zeff, concomitant with changes in physical oxidation state from +1 to +2 to +3. Herein, we demonstrate the sensitivity of XAS and VtC XES to the physical oxidation states of a series of N-heterocyclic carbene (NHC) ligated organocopper complexes. We then extend these methods to the study of the [Cu(CF3)4]- ion. Complemented by computational methods, the observed spectral transitions are correlated with the electronic structure of the complexes and the Cu Zeff. These calculations demonstrate that a contraction of the Cu 1s orbitals to deeper binding energy upon oxidation of the Cu center manifests spectroscopically as a stepped increase in the energy of both XAS and Kβ2,5 emission features with increasing formal oxidation state within the [Cun+(NHC2)]n+ series. The newly synthesized Cu(III) cation [CuIII(NHC4)]3+ exhibits spectroscopic features and an electronic structure remarkably similar to [Cu(CF3)4]-, supporting a physical oxidation state assignment of low-spin d8 Cu(III) for [Cu(CF3)4]-. Combining XAS and VtC XES further demonstrates the necessity of combining multiple spectroscopies when investigating the electronic structures of highly covalent copper complexes, providing a template for future investigations into both synthetic and biological metal centers.
Oxidation state
XANES
X-ray absorption spectroscopy
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Solution V, Nb K-edge XANES (X-ray absorption near edge structure) analysis of molecular catalysis, high oxidation state vanadium(V), niobium(V) complexes containing both imido ligands (possessing metal–nitrogen double bond, NR) and mono anionic ancillary donor ligands (L) of type, M(NR)(L)X2 (X = Cl, Me), which catalyze ethylene dimerization/polymerization in the presence of Al cocatalysts, has been explored. The analysis method is highly useful to obtain the direct information of the active species (oxidation state, basic framework around the centered metal) in solution (in situ), and should thus become a powerful tool for better understanding the catalysis mechanism, basic coordination and organometallic chemistry.
X-ray absorption spectroscopy
XANES
Oxidation state
Organometallic Chemistry
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This is a short introduction of X-ray absorption spectroscopy (XAS) and its application to homogeneous transition metal compounds and catalysts. An XAS spectrum is composed of two regions, XANES and EXAFS, which provide element-specific information on formal oxidation state and local coordination environment, respectively. For molecules with similar environments, such as a common ligand set, the energy of the absorption edge can be calibrated from standards to obtain the formal oxidation state of unknown compounds. For structurally complex coordination environments, simulated EXAFS spectra obtained from XRD or DFT-modeled structures can be used to ascertain local structural information from an experimental EXAFS spectrum. With fast data acquisition at modern synchrotrons, it is also possible to follow the kinetic transformation of homogeneous compounds under realistic reaction conditions while gaining insight into the structural and electronic changes happening at the metal atom.
X-ray absorption spectroscopy
XANES
Oxidation state
Coordination number
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Sulfur K-edge XAS data provide a unique tool to examine oxidation states and covalency in electronically complex S-based ligands. We present sulfur K-edge X-ray absorption spectroscopy on a discrete redox-series of Ni-based tetrathiafulvalene tetrathiolate (TTFtt) complexes as well as on a 1D coordination polymer (CP), NiTTFtt. Experiment and theory suggest that Ni-S covalency decreases with oxidation which has implications for charge transport pathways.
X-ray absorption spectroscopy
Tetrathiafulvalene
Oxidation state
XANES
Linker
K-edge
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X-ray absorption spectroscopy
Oxidation state
Chemical state
X-Ray Spectroscopy
Characterization
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X-ray absorption spectroscopy
Oxidation state
X-Ray Spectroscopy
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