Antonella Balerna

A structural investigation on small gold clusters by EXAFS

Abstract A structural investigation using extended X-ray absorption fine structure (EXAFS) spectra, recorded on Au metal clusters whose average dimension ranges from 11 to 50 A, is reported. Data analysis performed on the first and second coordination shells, showed the presence of interatomic distance contractions, a lowering of the mean coordination number and an increasing EXAFS Debye-Waller factor at decreasing mean diameter of the clusters. No structural changes to icosahedra have been observed.

Supported Rhodium Nanoparticles in Catalysis: the role of stabilizers on catalytic activity and structural features

Abstract Rhodium nanoparticles supported on γ-Al 2 O 3 , derived from arene-solvated Rh atoms stabilized by trioctylamine (TOA), are valuable catalysts in hydrogenation and silylformylation reactions. They are largely more active than the analogous commercial catalyst, as well as of a sample similarly prepared in the absence of TOA. HRTEM measurements, IR studies on adsorbed CO species and extended X-ray absorption fine structure analyses evidenced the role of TOA in controlling the particle growth in the preparative process and in stabilizing the resulting Rh particles against erosion by CO and oxidation.

Time-resolved X-ray powder diffraction on a three-way catalyst at the GILDA beamline.

Time-resolved X-ray diffraction experiments carried out at the beamline BM08-GILDA of ESRF allowed a study of the structural modifications taking place in a Pt/ceria-zirconia catalyst while the CO oxidation reaction was in progress. The capillary tube in which the sample is stored acts effectively as a chemical microreactor that ensures homogeneity of the sample treatments and minimization of diffusion effects. During the flowing of the reactant CO/He mixture, the investigated catalyst undergoes a fast Ce(IV)-Ce(III) partial reduction that involves the release of one O atom for every two reduced Ce cations. Because Ce(III) has a larger ionic radius than Ce(IV), the structural modification produces an increase of the lattice constant of the ceria-zirconia mixed oxide, and this increase is monitored by the translating imaging-plate device implemented at GILDA. The CO(2) resulting from the oxidation of the fluxed CO is monitored by a quadrupole mass spectrometer during the recording of the time-resolved X-ray diffraction pattern. The chemical and structural information was combined to show that the CO(2) yield is nearly constant until the catalytic system can provide oxygen for the reaction, while the structural rearrangement of the catalyst is delayed with respect to the switching on of the CO/He flux. After this induction time, during which CO(2) is produced with no structural modification of the catalyst, a fast increase of the lattice constant takes place.

Nanostructure of Pd/SiO2 supported catalysts

Pd/SiO2 supported catalysts were investigated using anomalous small-angle X-ray scattering (ASAXS), wide-angle X-ray scattering (WAXS), transmission electron microscopy (TEM) and extended X-ray absorption fine structure spectroscopy (EXAFS). Samples containing about 3 wt.% and 6 wt.% of metal were obtained by impregnation, calcined at 773 K and reduced at 623 K. Two populations of nanoparticles were found in both samples. In the 3 wt.% sample, one population consisted of very small nanoparticles (average 3 nm), and the other of larger ones (average 13 nm). A higher metal load seems to cause a broadening of the size distribution of the first population and a consequent increase in its average dimension (6 nm), rather than to influence the larger nanoparticles. Despite the relatively high metal content and temperatures, the population of small particles was predominant in both samples: 80% in the 3 wt.% sample and 57% in the 6 wt.% one. A large number of the smallest particles (<3 nm) was present in the 6 wt.% sample as well, and the total metal surface available was about the same in the two samples in spite of the different amounts of metal. Anomalous scattering has proven to be essential to quantitative analysis.

Introduction to Synchrotron Radiation

Relativistic charged particles forced to move along curved trajectories by applied magnetic fields emit electromagnetic radiation called Synchrotron Radiation; today electron storage rings are routinely used to provide synchrotron radiation to users in a wide spectral range from infrared to hard X-rays. Thanks to its peculiar characteristics, synchrotron radiation is one of the more powerful tools for investigating the properties of matter in many different fields like molecular and atomic physics, cell biology, medical applications, nanotechnology, catalysis and cultural heritage. Up to now three generations of synchrotron radiation sources emitting radiation with increasing quality have been developed; the fourth generation, based on free-electron lasers, already produces high power and ultrafast pulses of highly coherent radiation. In the present contribution, the main characteristics and properties of the synchrotron radiation sources and of the produced radiation are introduced and explained using a simple approach.

A new cell for the study of in situ chemical reactions using X-ray absorption spectroscopy

An in situ cell for reductive and oxidative treatments at different temperatures that allows the possibility of recording data as a function of temperature has been designed and constructed for X-ray absorption experiments at the GILDA beamline BM08 of ESRF. The cell is linked to a mass quadrupole spectrometer providing control of the reaction gases and monitoring of the products. The apparatus allows measurements to be performed both in transmission and fluorescence geometry. The cell was tested by studying the CO oxidation reaction promoted by a Pt/ceria-zirconia-supported catalyst. The CO(2) yield is correlated with the structural results confirming the existence of a strong metal-support interaction between the Pt metal clusters and the ceria-zirconia support.

Anomalous Wide-Angle X-ray Scattering Apparatus on the GILDA Beamline at the ESRF

The experimental apparatus for anomalous wide-angle X-ray scattering (AWAXS) on the GILDA beamline at the ESRF is described. The main features are the high beam stability and reproducibility which allow anomalous scattering effects to be resolved also on dilute elements, the large spectral range which allows AWAXS experiments at the K edges of heavy elements, and the use of a high-efficiency detection system. The apparatus has been tested in extreme conditions by performing AWAXS experiments at the Eu K edge in Eu-doped Sr metaphosphate glasses.

Characterization of a Poly-4-vinylpyridine-Supported CuPd Bimetallic Catalyst for Sonogashira Coupling Reactions

CuPd bimetallic solvated metal atoms (SMA) synthesized by metal vapor synthesis (MVS) technique and supported on poly-4-vinylpyridine (PVPy) resin, showed significantly higher catalytic activity in Sonogashira-type carbon-carbon coupling reactions than the corresponding monometallic Cu and Pd systems as well as their physical mixture. The analysis of the bimetallic catalyst combining transmission electron microscopy techniques and X-ray absorption fine structure (XAFS) spectroscopy revealed the presence of small Pd nanoparticles (dm =2.5 nm) while the analysis of the X-ray absorption data, at the Cu K-edge suggests the formation of thin and incomplete Cu oxide layers around the Pd-rich cores.

EXAFS and XANES structural characterization of bimetallic AuPd vapor derived catalysts

Using an innovative procedure known as metal vapor synthesis (MVS) to prepare bimetallic catalysts, starting from Au and Pd vapors, [AuPd] co-evaporated and [Au][Pd] separately evaporated bimetallic catalysts were achieved. After being tested, the catalytic activity and selectivity of the [AuPd] catalyst turned out to be higher than the [Au][Pd] ones. Using EXAFS spectroscopy it was shown that, in the [AuPd] samples, small bimetallic AuPd nanoparticles were present, having an Au rich core surrounded by an AuPd alloyed shell while in the [Au][Pd] sample there was the presence of monometallic Au and Pd nanoparticles showing some alloying only in the boundary regions. The EXAFS results were also qualitatively confirmed by the XANES spectra.

EXAFS and XRD study of Pd–Ag bimetallic catalysts supported on pumice from organometallic precursors

Abstract Silver–palladium bimetallic catalysts were supported on pumice following different procedures, based on mild treatments of organometallic precursors. The experiments for the structural characterization of the synthesized samples, wide angle X-ray diffraction (XRD) and X-ray absorption spectroscopy, were carried out at the beamline GILDA of the European Synchrotron Radiation Facility. The analysis of the data demonstrated that alloying took place with a degree of homogeneity depending on the different preparation routes.