Mihaela Popovici

Nanosized metallic particles embedded in silica and carbon aerogels as catalysts in the Mizoroki–Heck coupling reaction

We have prepared silica, organic, and carbon aerogels doped with metallic Ni and Pd through sol-gel processes. Silica aerogel nanocomposites have been synthesized by impregnation, with Ni(acac)2 or Pd(acac)2, of the silica wet gels followed by ethanol supercritical drying. Organic aerogels were prepared by sol-gel copolymerization of formaldehyde with the potassium salt of 2,4-dihydroxybenzoic acid, followed by ion exchange with either Ni(NO3)2·6H2O or Pd(OAc)2 and CO2 supercritical drying. No special reduction step was needed. Organic aerogels were further carbonized to obtain carbon aerogels. Pd organic and carbon aerogels containing 20–40% palladium nanoparticles are good catalysts for the Mizoroki–Heck reaction.

Fe2O3–SiO2 composites obtained by sol–gel synthesis

The present work deals with the synthesis of Fe2O3–SiO2 composites, obtained by embedding iron oxide particles in a silica matrix, through sol–gel method based on hydrolysis and condensation of a silicon alkoxide and thermal treatment. Methods employed in characterisation of the materials obtained at different iron concentration and at suitable thermal treatments are infrared spectroscopy, X-ray diffraction analysis and SEM analysis. In purpose to evaluate magnetic properties of these composites, static magnetic and Mossbauer spectra measurements were carried out.

Magnetic behaviour of iron oxide nanoparticles dispersed in a silica matrix

Abstract At room temperature the magnetic behaviour of non-interacting γ-Fe 2 O 3 nanoparticles dispersed in a silica matrix is superparamagnetic. At a lower temperature (77 K) hysteresis was observed. The appearance of the hysteresis loop is attributed to the Neel magnetic relaxation processes; the easy magnetisation axes of the nanoparticles being randomly oriented, the magnetic moments will rotate in an external magnetic field with a relaxation time τ , which depends on the effective anisotropy constant K eff . A supplementary anisotropy, e.g. stress anisotropy and the surface anisotropy leads to a total value ( K eff ), approximately 10 times higher than the bulk magnetocrystalline anisotropy K . At the surface of embedded nanoparticles a modification of the superexchange interaction takes place—due to specific particle-matrix interactions—on decreasing the temperature from 300 to 77 K; the magnetic moment of the particles increases 1.6 times. This behaviour can be attributed to the increase of the nanoparticles magnetic core volume of aligned spins and is not due to the variation of the spontaneous magnetisation with temperature.

Spherical (ZnδNi1-δFe2O4)γ nanoparticles in an amorphous (SiO2)1-γ matrix, prepared with the sol-gel method

Abstract By using the sol–gel method we have obtained spherical nanoparticles (NPs) of stoichiometric (Zn δ Ni 1- δ Fe 2 O 4 ) γ mixed ferrite ( δ =0.15; 0.35; 0.65 and γ =0.15), dispersed in an amorphous (SiO 2 ) 1- γ matrix (nanocomposites). The nanocomposites were investigated at room temperature by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), electron diffraction (ED), Mossbauer spectroscopy and quasi-static magnetic measurements. We have studied in detail the magnetic behavior of ferrite NPs in a silica matrix, both under quasi-static (50 Hz) and dynamic (20 MHz) conditions. The results show a pure superparamagnetic (SPM) behavior of the sample that has a concentration of the Zn ions of δ =0.15. At higher concentrations, there is a significant deviation from this behavior and this deviation increases with the increase of δ . We attribute the observed behavior to the Neel-type magnetic relaxation processes. After processing the TEM image and performing magneto-granulometric measurements, it has resulted that on the surface of NPs embedded in the silica matrix a layer is formed; this layer has a thickness in the size range of nanometers and it has no magnetic ordering.