State-Sensitive Monitoring of Active and Promoter Sites. Applications to Au/Titania and Pt-Sn/Silica Catalysts by XAFS Combined with X-Ray Fluorescence Spectrometry
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Abstract:
State‐sensitive XAFS was enabled combined with high‐energy‐resolution (ΔE = 0.3 eV@5.5 keV) X‐ray fluorescence spectrometry and applied to Au sites of An/TiO2 and Sn promoter sites of Pt‐Sn/SiO2. Each state of interfacial Au sites located on Ti/O atoms and negatively/positively charged Aun clusters was discriminated. Feasibility of more direct information of on‐site catalysis via frontier orbital‐sensitive XAFS was demonstrated.Keywords:
X-Ray Fluorescence
Oxidation state
XANES
Chemical state
Sulfur (S) is an essential macronutrient for all living organisms. A variety of organic and inorganic S species with oxidation states ranging from -2 to +6 exist. Today few spectroscopic and biochemical methods are used to investigate sulfur oxidation state and reactivity in biological samples. X-ray absorption near edge spectroscopy (XANES) is a very well suited spectroscopic technique to probe the oxidation state and the surrounding chemical environment of sulfur. Microspectroscopy beamlines, operating at almost all synchrotron facilities, allow the combination of XANES with X-ray fluorescence mapping (µXRF). Using this approach distribution maps of S in complex biological samples (intact parts of tissue, or individual cells) can be obtained using µXRF and its oxidation state can be probed in-situ (µXANES). Moreover, µXRF mapping at specific energies enables for chemical contrast of S at different oxidation states without the need of staining chemicals. This review introduces the basic concepts of synchrotron µXRF and µXANES and discusses the most recent applications in life science. Important methodological and technical issues will be discussed and results obtained in different complex biological samples will be presented.
XANES
Oxidation state
Chemical state
X-ray absorption spectroscopy
X-Ray Fluorescence
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XANES
X-ray absorption spectroscopy
X-Ray Spectroscopy
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X-Ray absorption near-edge structure (XANES) spectroscopy was used to monitor the oxidation state of cobalt following treatment of CoIII complexes with reducing agents such as ascorbate and cysteine. It was established that the XANES spectra of mixtures of CoII and CoIII complexes can be used to calculate proportions of the two oxidation states by monitoring the height of the Co K-edge. The relationships developed were used to estimate proportions of each complex in solutions of CoIII complexes treated with reducing agents.
XANES
Oxidation state
X-ray absorption spectroscopy
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XANES
X-ray absorption spectroscopy
X-Ray Spectroscopy
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A series of Cr complexes varying in oxidation state, ligand and geometry were studied with Cr K‐edge XANES. The main absorption edge energy shift for an oxidation state change from Cr0 to Cr6+ is found to be similar to that for a series of Cr3+ complexes with different ligands. Theoretical XANES and density of states calculations using FEFF8.0 provided detailed insights in the origin of the XANES features for the series of distorted octahedral CrCl3L complexes. The geometry of the CrCl3L complex governs the position of the main absorption edge. Hard versus soft donor effects are overruled by the chlorine ligand for complexes with a facial geometry, whereas the chlorine ligand does not play a significant role in meridional geometry. The combined results call for a redefinition of generally used concepts like oxidation state.
XANES
Oxidation state
Octahedral molecular geometry
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XANES
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The palladium oxidation state of an SiO2-supported palladium catalyst was quantitatively determined by Pd L3-edge XANES (X-ray absorption near-edge structure) analysis. By changing the time of CO-reduction pre-treatment at 673 K, a series of 5 wt% Pd-loaded SiO2 catalysts (PdOx/2/SiO2) containing different amounts of the Pd metal and PdO phases were prepared, and the average oxidation number (x) was estimated from the number of CO2 molecules formed during the CO-reduction treatment. L3-edge XANES spectra of these samples and a reference sample (Pd powder) were recorded, and the white line area of the spectra was evaluated. A good linear relationship was confirmed between the white line area intensity and the average oxidation number (x), indicating that the oxidation state of Pd in structurally unknown Pd samples could be quantitatively determined by a simple XANES analysis. To demonstrate the utility of this method in a catalytic study, the effect of the oxidation number (x) on the CO oxidation activity of PdOx/2/SiO2 was also examined, and metallic Pd0 sites in PdOx/2/SiO2 were shown to be active species.
XANES
Oxidation state
Catalytic Oxidation
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XANES
Oxidation state
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The sections in this article are
Introduction
Physical Principles of X-Ray Adsorption and Electron Scattering
Absorption of X-Rays
Scattering of the Photoelectron
Isolating the Oscillatory Part (χAX + χEX) from the Absorption Data
Pre-Edge Subtraction and Determination of Eedge
Post-Edge Background Removal and Normalization
EXAFS
The EXAFS Equation
EXAFS Data Analysis
Fourier Transformation
Fourier Filtering
Phase Shifts and Backscattering Amplitudes Derived from EXAFS Data Obtained from Reference Compounds
Obtaining R, N and Δσ2, k-Space and R-Space Fitting
The Use of Theoretical References
Difference File Technique
Structural Analysis with EXAFS of a Heterogeneous Single-Site Cr/SiO2 Ethylene Trimerization Catalyst. An Illustrative Example
Introduction
Raw XAS Data of New Cr/SiO2 Catalysts and CrCl3–TAC Reference Compound
Fabrication and Calibration of Ab Initio Phase Shifts and Backscattering Amplitudes
EXAFS Data Analysis
Conclusions: EXAFS Data Analysis
Atomic XAFS (AXAFS)
Introduction
Physical Principles of AXAFS
The Field and Inductive Interactions
Separation of AXAFS from EXAFS Data
The Interatomic Potential Model for Metal–Support Interaction
ΔXANES Technique
Introduction
Calculations of Pt L3 ΔXANES Using the FEFF8 Code
Experimental ΔXANES Data from Pt/H-USY and Pt/NaY
Experimental Information
ΔXANES Spectra
Relevance for Pt-Catalyzed Hydrogenolysis/Hydrogenation Reactions
General Applicability of the ΔXANES Technique
Acknowledgments
Keywords:
physical principles of XAS;
EXAFS data-analysis;
atomic XAFS;
delta XANES
XANES
X-ray absorption spectroscopy
X-Ray Spectroscopy
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