Is There a Negative Thermal Expansion in Supported Metal Nanoparticles? An In-Situ X-ray Absorption Study Coupled with Neural Network Analysis

Interactions with their support, adsorbates and unique structural motifs are responsible for the many intriguing properties and potential applications of supported metal nanoparticles (NPs). At the same time, they complicate the interpretation of experimental data. In fact, the methods and approaches that work well for the ex situ analysis of bulk materials may be inaccurate or introduce artifacts in the in situ analysis of nanomaterials. Here we revisit the controversial topic of negative thermal expansion and anomalies in the Debye temperature reported for oxide-supported metal NPs. In situ X-ray absorption experimental data collected for Pt NPs in ultrahigh vacuum and an advanced data analysis approach based on an artificial neural network demonstrate that Pt NPs do not exhibit intrinsic negative thermal expansion. Similarly as for bulk materials, in the absence of adsorbates the bond lengths in metal NPs increase with temperature. The previously reported anomalies in particle size-dependent Debye temperatures can also be linked to the artifacts in the interpretation of conventional X-ray absorption data of disordered materials such as NPs

Continuous Cauchy wavelet transform analyses of EXAFS spectra: A qualitative approach.

To better understand the extended x-ray absorption fine structure (EXAFS) spectroscopic information obtained for complex materials such as those encountered in Earth sciences, we propose to use the Continuous Cauchy Wavelet Transform (CCWT). Thanks to this method, the EXAFS spectra can be visualized in three-dimension: the wavevector (k), the interatomic distance uncorrected for phase-shifts (R') and the CCWT modulus (corresponding to the continuous decomposition of the EXAFS amplitude terms). Consequently, more straightforward qualitative interpretations of EXAFS spectra can be performed, even when spectral artifacts are present, such as multiple-scattering features, multi-electronic excitations or noise. More particularly, this method provides important information concerning the krange of each EXAFS contribution, such as next nearest-neighbors identification. To illustrate the potential of CCWT analyses applied to EXAFS spectra, we present first the experimental and theoretical spectra obtained for well-crystallized minerals, thorite and zircon, at the Th LIII-, and Zr K-edges, respectively. Then, we present the CCWT analyses of EXAFS spectra collected for amorphous materials of geochemical and environmental interests, including a sodium trisilicate glass and an aqueous chloride solution, at the Mo K-, and Au LIII-edges, respectively

Continuous Cauchy wavelet transform of XAFS spectra.

The continuous Cauchy wavelet transform (CCWT) is applied to the analysis of XAFS spectra. Thanks to that method, XANES and EXAFS signals can be visualized in threedimension: the wavevector (k), the interatomic distance uncorrected for phase-shifts (R') and the CCWT modulus (corresponding to the continuous decomposition of the amplitude terms). Applied to EXAFS spectra, the CCWT analysis provides straightforward qualitative information related to the k-range of each “R'-EXAFS” contribution. Such information is particularly useful to perform next nearest-neighbors identification, despite the presence of spectral artifacts such as multiple-scattering features, multi-electronic excitations or noise. When applied to XANES spectra, the CCWT analysis helps highly to measure the “spectral limit” between XANES and EXAFS regions, as well as the energy range required to model properly next-nearest neighbors. To further illustrate the potential of CCWT analyses applied to XAFS spectra, we present examples related to: (1) a XANES spectrum collected at the Ti K-edge for titanite (CaTiSiO5); (2) an experimental Au LIII-edge EXAFS spectrum for gold sorbed on goethite (FeO(OH))

Investigating the role of Cu-oxo species in Cu-nitrate formation over Cu-CHA catalysts

The speciation of framework-interacting Cu(II) sites in Cu-chabazite zeolite catalysts active in the selective catalytic reduction of NO(x) with NH(3) is studied, to investigate the influence of the Al content on the copper structure and their reactivity towards a NO/O(2) mixture. To this aim, three samples with similar Cu densities and different Si/Al ratios (5, 15 and 29) were studied using in situ X-ray absorption spectroscopy (XAS), FTIR and diffuse reflectance UV-Vis during pretreatment in O(2) followed by the reaction. XAS and UV-Vis data clearly show the main presence of Z(2)Cu(II) sites (with Z representing a framework negative charge) at a low Si/Al ratio, as predicted. EXAFS wavelet transform analysis showed a non-negligible fraction of proximal Z(2)Cu(II) monomers, possibly stabilized into two 6-membered rings within the same cage. These sites are not able to form Cu-nitrates by interaction with NO/O(2). By contrast, framework-anchored Z[Cu(II)(NO(3))] complexes with a chelating bidentate structure are formed in samples with a higher Si/Al ratio, by reaction of NO/O(2) with Z[Cu(II)(OH)] sites or structurally similar mono- or multi-copper Z(x)[Cu(II)(x)O(y)] sites. Linear combination fit (LCF) analysis of the XAS data showed good agreement between the fraction of Z[Cu(II)(OH)]/Z(x)[Cu(II)(x)O(y)] sites formed during activation in O(2) and that of Z[Cu(II)(NO(3))] complexes formed by reaction with NO/O(2), further confirming the chemical inertia of Z(2)Cu(II) towards these reactants in the absence of solvating NH(3) molecules

Annual Report 2005 - Institute of Radiochemistry

The Institute of Radiochemistry (IRC), one of the six institutes of the Forschungszentrum Rossendorf (FZR), performs application-oriented research in the fields of radiochemistry and radioecology. Motivation and background of our research are environmental processes relevant for the installation of nuclear waste repositories, for remediation of uranium mining and milling sites, and for radioactive contaminations caused by nuclear accidents and fallout. Due to their high radiotoxicity and long half-life the actinides are of special interest. Hence our research focuses on the chemical behavior of actinides at the molecular level in order to predict the relevant macroscopic processes in the environment. Within this framework, special emphasis is on the interface between geological and biological systems. In the last year our research topics were as follows: # Aquatic chemistry of actinides # Actinides in bio-systems # Interaction of actinides with solid phases # Reactive transport of actinides About 60 scientists, technicians and PhD students are employed in the Institute of Radiochemistry. We have achieved a wide range of new scientific results in the past year, which are presented in this Annual Report. Among them only a few can be highlighted here in this preface. For the first time it was possible to determine uranium speciation in situ in drinking and mineral waters e.g. by a dedicated fluorescence spectrometer at lowest µg/L concentrations. This methodical progress is an important prerequisite to study the uranium toxicity and its dependence on chemical speciation. We were very successful in the determination of formation pathways and structure of various actinide complexes, e.g., the surface complexes of uranium (VI) onto mica and iron hydroxides over a wide range of pH and carbonate concentration. These results contribute to a better understanding of actinide speciation in geo- and bio-systems, especially with respect to the chemical processes on the interfaces. Studies to the interaction of uranium with biofilms, green algae and bacteria coming from extreme habitats extended our research on the field of bio-systems. Major progress in the structural analysis of multiple uranium species has been achieved by applying Monte Carlo simulations and iterative transformation factor analysis to EXAFS spectroscopy. Furthermore, our new radiochemical experimental station at the Free Electron Laser of the Rossendorf accelerator ELBE is now in full operation. We have started first experiments on the uranium and neptunium complexation on selected mineral surfaces