R. Franke

SELECTIVE OXIDATION OF GLUCOSE ON BISMUTH-PROMOTED PD-PT/C CATALYSTS PREPARED FROM NOCT4CL-STABILIZED PD-PT COLLOIDS

Charcoal-supported Pd-Pt catalysts based on Pd-Pt/NOct4Cl colloidal alloys have superior activity and selectivity in the oxidation of glucose to gluconic acid compared with industrial heterogeneous Pd-Pt catalysts. According to transmission electron microscopy, X-ray diffraction/Debye function analysis, X-ray photoelectron spectroscopy, X-ray absorption near edge structure, and extended X-ray absorption fine structure analysis the chemical coreduction of PdCl2 and PtCl2 in the appropriate ratio with NOct4BEt3H yielded the alloyed Pd-Pt colloids in organic solvents. They are screened by the lipophilic NOct4Cl surfactant layer from coagulation and poisoning. TEM showed colloids of particle sizes in the range from 1.5 to 3 nm.

X-ray absorption and photoemission electron spectroscopic investigation of crystalline and amorphous barium silicates

X-ray absorption near edge structure (XANES) measurements at the Si K-edge, X-ray photoelectron spectroscopy of the Si 2p and O 1s photoelectrons and X-ray excited Auger electron spectroscopy of the Si KLL Auger electrons are applied in order to investigate the degree of polymerisation (Qn) of the (SiO4)-polyhedra in barium silicate glasses with different BaO-contents. The spectra of the glasses with 33.3 and 37.0 mol% BaO are compared to those of the crystalline reference compounds: low quartz, BaSi2O5, BaSiO3 and Ba2SiO4. The analysis reveals a high average polymerisation between Q3 and Q4 in both glasses. The results are supported by calculations of the bound state transitions in Si K-XANES using the multiple scattering Xα method.

The determination of structural units in amorphous Si–B–N–C ceramics by means of Si, B, N and C K–XANES spectroscopy

Abstract X-ray absorption near edge structure (XANES) measurements at the Si, B, N and C K-edges of amorphous Si–B–N–C ceramics were performed to study the local atomic structure. Recorded spectra of samples synthesized by a Si-organopolymer synthesis route show the presence of (SiN 4 ), (SiN x C y ), (BN 3 ), (NSi x B y ), and (C=C) structural units. The quantitative content of these units depends on the chemical constitution of the precursor molecules and the reaction conditions.

Si and N K-XANES spectroscopic study of novel Si–C–N ceramics

Abstract X-ray absorption near edge structure (XANES) measurements at the Si and N K-edges of ternary Si–C–N compounds are applied to analyze the changes of structural units owing to the thermally induced ceramization of silicon dicarbodiimide Si(NCN) 2 at temperatures up to 1600°C. Samples synthesized below 800°C show an environment of silicon consisting of carbodiimide (NCN) groups. Further annealing leads to a crystalline phase, Si 2 CN 4 , which combines the structural units (Si–N–Si) and (Si–NCN). Recorded spectra of samples annealed at temperatures higher than 1200°C show the decomposition of Si 2 CN 4 . The material then transforms to a Si 3 N 4 –SiC composite material through an intermediate amorphous Si–C–N phase.

Local deformation in hexagonal boron nitride crystal near the B K-shell -excitation

Theoretical and experimental studies of the B K-shell absorption near the -excitation in hexagonal boron nitride were carried out. The absorption spectrum over the interval 191.5-193.5 eV was analysed in the framework of the quasi-atomic model with respect to its molecular simulation and for the features of its recombination. Our analysis points to the appearance of local deformations in the crystal lattice around a core-excited atom which are mainly created by the breaking of the ground-state symmetry due to out-of-plane displacements of core-excited boron atoms in a basal plane (0.16 A). The comparison with the experimental B K-shell absorption spectra for the metastable incoherent phase BN and defect-containing BN crystals supports this polaron mechanism. Methods for the description of the x-ray absorption in solids taking into account the related atomic rearrangement are discussed.