XANES

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

In situ XANES characterization of the Cu oxidation state in Cu-ZSM-5 during NO decomposition catalysis

Abstract: The oxidation state of Cu in Cu-ZSM-5 has been investigated by the X-ray absorption near-edge structure (XANES) spectroscopic method during NO decomposition catalysis. We designed an in situ reactor system with which we can measure the relative NO decomposition rate while taking XANES spectra. We observed that the 1s→4p electronic transition of Cu(I) in Cu-ZSM-5 appears as a narrow, intense peak which is an effective measure of changes in the population of copper oxidation states. This transition is quite intense after Cu-ZSM-5 is activated in inert gas flow. However, its intensity decreases but by no means disappears after the admission of a NO/N2 gas mixture. We conducted the reaction in a temperature cycle around the optimum conversion temperature of 773 K and recorded the XANES at each temperature. We observed that the integrated intensity of the Cu(I) 1s→4p transition, which is proportional to the cuprous ion concentration in Cu-ZSM-5, was well correlated with the NO decomposition rate. This finding supports the conjecture that Cu(I) participates in a redox mechanism during catalyzed NO decomposition in Cu-ZSM-5 at elevated temperature.

Molecular mechanisms of electron-induced cross-linking in aromatic SAMs.

Abstract: When aromatic self-assembled monolayers (SAMs) are electron-irradiated, intermolecular cross-links are formed and the SAMs transform into carbon nanosheets with molecular thickness. These nanosheets have a very high mechanical stability and can withstand temperatures above 1000 K. In this report, we investigate the electron induced cross-linking of 1,1'-biphenyl-4-thiol (BPT) SAMs on gold by combining X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (NEXAFS), thermal desorption spectroscopy (TDS), and UV photoelectron spectroscopy (UPS). The experimental data were acquired as a function of electron dose and temperature and compared with quantum chemical calculations. Details of the intermolecular cross-linking, the microstructure of cross-linked films, and their structural transformations upon heating were obtained to derive a view of the mechanisms involved. Our analysis shows that room-temperature electron irradiation causes a lateral cross-linking via the formation of C-C linked phenyl species as well as a new sulfur species. The thermal stability of the BPT films increases with the electron dose and saturates at approximately 50 mC/cm2. Nevertheless, nonlinked fragments in the thermal desorption spectra indicate an incomplete cross-linking even at high doses, which can be attributed to steric reasons and quenching due to the reduced band gap of partially linked molecules. At temperatures above 800 K, all sulfur species are thermally desorbed, while the remaining film reveals an onset of carbonization.

Charge redistribution and electronic behavior in a series of Au-Cu alloys.

Abstract: A series of Au-Cu alloys of various stoichiometries and order have been studied using x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) Significant electronic changes are associated with alloying and with changes in the local environment The Au 5d electron charge depletion has been determined independently from XPS core-level and M\"ossbauer isomer shifts and from x-ray absorption near-edge structure (XANES) measurements Strong correlations were found using both methods in the parameters investigated here, especially in the elucidation of the charge-transfer mechanism It is found that at the Au site there is a loss of d charge, upon alloying, which increases as Au becomes more dilute in Cu This along with XANES evidence of d charge gain at the Cu site, conduction (primarily 6s) charge gain at the Au site, and overall charge gain at the Au site verifies the charge compensation model in which Au loses d charge but is overcompensated by a gain of conduction charge This observation is in line with electronegativity (Au is more electronegative than Cu) and electroneutrality (the overall gain of charge is small) arguments It is also found that there is a strong linear correlation between the 5d hole count at the Au site and (1) the M\"ossbauer isomer shift, (2) alloy d-band width, (3) ``Au'' apparent spin-orbit splitting in the alloy d band By comparing the ordered and disordered species (which have different local environments) it is found that the charge transfer depends primarily on the local environment (coordination number of like and unlike atoms) and not on atomic separation The charge transfer (d-band depletion) was larger in the ordered species and can be explained by the number of Cu and Au nearest neighbors as compared to the disordered phases

First principles calculations of the structure and V L-edge X-ray absorption spectra of V2O5 using local pair natural orbital coupled cluster theory and spin-orbit coupled configuration interaction approaches.

Abstract: A detailed study of the electronic and geometric structure of V2O5 and its X-ray spectroscopic properties is presented. Cluster models of increasing size were constructed in order to represent the surface and the bulk environment of V2O5. The models were terminated with hydrogen atoms at the edges or embedded in a Madelung field. The structure and interlayer binding energies were studied with dispersion-corrected local, hybrid and double hybrid density functional theory as well as the local pair natural orbital coupled cluster method (LPNO-CCSD). Convergence of the results with respect to cluster size was achieved by extending the model to up to 20 vanadium centers. The O K-edge and the V L2,3-edge NEXAFS spectra of V2O5 were calculated on the basis of the newly developed Restricted Open shell Configuration Interaction with Singles (DFT-ROCIS) method. In this study the applicability of the method is extended to the field of solid-state catalysis. For the first time excellent agreement between theoretically predicted and experimentally measured vanadium L-edge NEXAFS spectra of V2O5 was achieved. At the same time the agreement between experimental and theoretical oxygen K-edge spectra is also excellent. Importantly, the intensity distribution between the oxygen K-edge and vanadium L-edge spectra is correctly reproduced, thus indicating that the covalency of the metal–ligand bonds is correctly described by the calculations. The origin of the spectral features is discussed in terms of the electronic structure using both quasi-atomic jj coupling and molecular LS coupling schemes. The effects of the bulk environment driven by weak interlayer interactions were also studied, demonstrating that large clusters are important in order to correctly calculate core level absorption spectra in solids.