Antonino Martorana

Particle size and metal-support interaction effects in pumice supported palladium catalysts

Abstract Pumice supported palladium catalysts with different metal loading and different metal particle sizes have been analyzed with X-ray photoelectron spectroscopy (XPS). The observed negative shift of the Pd 3d doublet binding energies of all the catalysts with respect to the corresponding binding energies of palladium metal powder suggested an increased electron density at the metal centers. Through the combination of the photoelectron and Auger shifts, yielding the Auger parameter shift, relative to the palladium metal, the charge distribution on the palladium atoms was estimated. The variation of that parameter with particle size is attributed to the interaction between the metal and the support rather than to an intrinsic size effect. The XPS intensity ratios of Pd 3d and Si 2p peaks were well reproduced by the Kerkhof and Moulijn model, especially at lower metal loading. Depletion of sodium, naturally present in pumice, has been observed in the catalysts with lower palladium content. An explanation of the catalytic behavior of these samples, in terms of electronic effects and surface elemental composition, is given.

SAXS/WAXS study of the annealing process in quenched samples of isotactic poly(propylene)

The structural rearrangement in samples of quenched isotactic poly(propylene) (iPP) submitted to different annealing treatments has been studied using simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS) at the synchrotron radiation source of DESY, Hamburg. From a quantitative analysis of the WAXS profiles the values of the α-monoclinic, mesomorphic, and amorphous mass fractions coexisting in the material were determined. It is demonstrated that the SAXS patterns were characterized by two different long-period values that are attributed to α- and mesomorphic periodicity, respectively. The related α- and mesomorphic volume phase fractions, calculated from the analysis of the SAXS data, are compared with the corresponding WAXS results. The rearrangement of the initial structure involves i) the thickening of the already existing α-phase lamellae at the expenses of the amorphous regions and ii) the structural rearrangement of the mesomorphic phase leading to its transformation into the α-monoclinic one.

Effect of sodium on the electronic properties of Pd/silica−alumina catalysts

Abstract The effect of the alkali ion content on the electronic properties of palladium, supported on natural pumice and on synthetic aluminosilicates, was studied by X-ray photoelectron spectroscopy (XPS) measurements. To this aim a series of catalysts of palladium supported on aluminosilicates with various amounts of sodium and potassium in the bulk structure were prepared and analysed. The Pd 3d 5/2 binding energy shifts of the different catalysts with respect to pure metal Pd 3d 5/2 binding energy were followed as a function of the Na/Pd atomic ratio. For reasons of comparison, a silica supported Pd catalyst whose surface had been doped with sodium was also studied. A correlation of the Pd 3d binding energy with the sodium content was found, regardless on whether sodium was present in the bulk structure or just at the surface of the catalyst. The effect was independent of the catalyst preparation procedure. The analysis of the intensity of the photoelectron peaks of Pd 3d and Si 2p of the lower surface area catalysts, using the Kerkhof-Moulijn model, yielded particle sizes in agreement with values obtained from small angle X-ray diffraction measurements (SAXS). For the large surface area supported catalysts a monolayer type of metal distribution was found. The intensity ratios of the Na 1s and Si 2p photoelectron peaks are in accordance with those estimated from the bulk Na/Si atomic ratios of the synthetic and natural pumice support. Therefore any segregation of the alkali ions on the palladium particle of the corresponding catalysts are ruled out. The electronic effect is thus discussed in terms of the formation of an electron donor composite formed by the metal, the oxygen and the alkali ion.

GISAXS study of Cu-Ni alloy clusters obtained by double ion implantation in silicate glasses

Sequential double ion implantation in silicate glasses has been used to realize copper-nickel alloy nanoclusters embedded in the host dielectric matrix. A grazing incidence small-angle X-ray scattering experiment has been performed on copper+nickel implanted silicate glasses, to obtain information on the cluster size and size distribution, as well as on the volume fraction. The potential of the technique in the study of these composite glasses is discussed.

Distorted f.c.c. arrangement of gold nanoclusters: a model of spherical particles with microstrains and stacking faults

The structures of two samples of gold nanoclusters supported on silica were studied by X-ray powder diffraction (XRD) and X-ray absorption spectroscopy. The data relative to both techniques were analysed by an approach involving simulation based on structural models and fitting. The XRD model concerned a distorted f.c.c. (face-centred cubic) arrangement, with microstrains and parallel stacking faults in approximately spherical particles; as an alternative possibility, a linear combination of ordered f.c.c. and noncrystalline (decahedral and icosahedral) particles was also taken into account. Both approaches gave calculated patterns closely resembling the experimental data. X-ray absorption spectra were fitted on the basis of f.c.c. and noncrystalline arrangements. The best results were obtained by the f.c.c. motif, while a simulation consisting in the superposition of f.c.c. and noncrystalline components in the relative amounts determined by XRD analysis gave a poor agreement with the experimental data. It was concluded that the good XRD fitting obtained by linear combination of lognormal size-distributed f.c.c. cuboctahedral, decahedral and icosahedral contributions was a result of the flexibility of the basis set of functions, but that the complementary analysis of X-ray absorption data did not confirm the presence of a noteworthy fraction of noncrystalline particles.

Crossing the boundary between face-centred cubic and hexagonal close packed: the structure of nanosized cobalt is unraveled by a model accounting for shape, size distribution and stacking faults, allowing simulation of XRD, XANES and EXAFS

The properties of nanostructured cobalt in the fields of magnetic, catalytic and biomaterials depend critically on Co close packing. This paper reports a structural analysis of nanosized cobalt based on the whole X-ray diffraction (XRD) pattern simulation allowed by the Debye equation. The underlying structural model involves statistical sequences of cobalt layers and produces simulated XRD powder patterns bearing the concurrent signatures of hexagonal and cubic close packing (h.c.p. and f.c.c.). Shape, size distribution and distance distribution between pairs of atoms are also modelled. The simulation algorithm allows straightforward fitting to experimental data and hence the quantitative assessment of the model parameters. Analysis of two samples having, respectively, h.c.p. and f.c.c. appearance is reported. Extended X-ray absorption fine-structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectra are simulated on the basis of the model, giving a tool for the interpretation of structural data complementary to XRD. The outlined structural analysis provides a rigorous structural basis for correlations with magnetic and catalytic properties and an experimental reference for ab initio modelling of these properties.

Structure of natural water-containing glasses from Lipari (Italy) and Eastern Rhodopes (Bulgaria) : SAXS, WAXS and IR studies

Abstract Two natural water-containing glasses with rhyolite composition originating from two different areas (perlite from Lipari, Italy, and expanded perlite from the Eastern Rhodopes, Bulgaria) have been characterised for chemical composition, atomic structure and specific surface area, with the ultimate goal of exploitation of volcanic glasses as carriers for heterogeneous metal catalysts. The two samples have similar chemical composition and total water content. The local structure, as determined by radial distribution function analysis of the wide angle X-ray scattering patterns, is similar, the medium-range structure being slightly more ordered for the Lipari glass. These results are confirmed by infra-red spectroscopy (IR) in the mid-IR range. Based on the small-angle X-ray scattering data we conclude that both glasses have a specific surface area of about 1 m 2 /g. The pore size distribution cannot be determined by our experiments because the two patterns are already asymptotic at the smallest attainable scattering angles.

Influence of metal–support interaction on the surface structure of gold nanoclusters deposited on native SiOx/Si substrates

The structure of small gold nanoclusters (around 2.5 nm) deposited on different silica-on-silicon (SiOx/Si) substrates is investigated using several characterization techniques (AFM, XRD, EXAFS and GISAXS). The grain morphology and the surface roughness of the deposited gold cluster layers are determined by AFM. The in-plane GISAXS intensity is modelled in order to obtain information about the cluster size and the characteristic length scale of the surface roughness. The surface morphology of the deposited clusters depends on whether the native defect-rich (n-SiOx/Si) or the defect-poor substrate obtained by thermal treatment (t-SiO2/Si) is used. Gold clusters show a stronger tendency to aggregate when deposited on n-SiOx/Si, resulting in films characterized by a larger grain dimension (around 20 nm) and by a higher surface roughness (up to 5 nm). The more noticeable cluster aggregation on n-SiOx/Si substrates is explained in terms of metal-support interaction mediated by the defects located on the surface of the native silica substrate. Evidence of metal-support interaction is provided by EXAFS, demonstrating the existence of an Au-O distance for clusters deposited on n-SiOx/Si that is not found on t-SiO2/Si.

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.

EXAFS study of ceria-lanthana-based TWC promoters prepared by sol-gel routes

Extended X-ray absorption fine structure (EXAFS) experiments at the Ce K- and La K-edges were performed on ceria–lanthana–alumina three-way catalysts promoters prepared by sol–gel routes, in order to investigate the effect of lanthanum doping on the ceria structure. The formation of Ce1−xLaxO2−x/2 solid solution, already observed by X-ray diffraction, was confirmed by EXAFS analysis, while no experimental evidence of a Ce–Al interaction was found. In presence of cerium and aluminum, lanthanum is involved in the formation of solid solution with CeO2 and of La–Al compounds. When the La:Al molar ratio is sufficiently high, the growth of a tridimensionally ordered LaAlO3 perovskite compound is observed. For increasing values of x/1−x in the solid solution Ce1−xLaxO2−x/2, the Ce–O distance decreases, while La–O distance remains nearly constant.