Preparazione e caratterizzazione di nanocompositi aerogel per applicazioni catalitiche

The research has focused on the synthesis, characterization and catalytic process applications of nanocomposite aerogels, which are materials made out of an active nanophase dispersed in a highly porous matrix. Nanocomposites made of nanoparticles dispersed in a inert matrix have unique properties which mainly depend on composition, size and distribution of the nanoparticles and on morphology and porosity of the matrix. The sol-gel route is a successful technique for the synthesis of highly porous nanocomposite aerogels. Highly porous MFe2O4-SiO2 (M=Co, Ni and Mn) and FeCo(Ni)-SiO2 nanocomposite aerogels were synthesized thanks to the development and optimization of a new two-step sol-gel method followed by supercritical drying1. The samples were characterized by a multitechnique approach through conventional (thermogravimetric analysis and differential thermal analysis, XRD, TEM, N2-physisorption at 77 K) and advanced (X-ray absorption spectroscopy, XAS) techniques. The FeCo(Ni)-SiO2 nanocomposite aerogels were successfully tested as catalysts for multiwall carbon nanotubes (MWCNT) production and for liquid fuels production by Fischer-Tropsch synthesis thanks to the collaboration with the Applied and Environmental Chemistry Department of the Szeged University (Hungary), the Chemistry Departiment of the “La Sapienza” Rome University and the Organic and Industrial Chemistry Departiment of the Parma University. Casula M.F. et al., Langmuir, 2007, 23, 3509-351

Growing CeO2_2 Nanoparticles within the Nano-Porous Architecture of the SiO2_2 Aerogel

In this study, new CeO$_2$-SiO$_2$ aerogel nanocomposites obtained by controlled growth of CeO$_2$ nanoparticles within the highly porous matrix of a SiO$_2$ aerogel are presented. The nanocomposites have been synthesized via a sol-gel route, employing cerium (III) nitrate as the CeO$_2$ precursor and selected capping agents to control the growth of the CeO$_2$ nanoparticles, which occurs during the supercritical drying of the aerogels. Samples with different loading of the CeO$_2$ dispersed phase, ranging from 5 to 15%, were obtained. The nanocomposites showed the morphological features typical of the SiO$_2$ aerogels such as open mesoporosity with surface area values up to 430 m$^2$·g$^{-1}$. TEM and XRD characterizations show that nanocrystals of the dispersed CeO$_2$ nanophase grow within the aerogel already during the supercritical drying process, with particle sizes in the range of 3 to 5 nm. TEM in particular shows that the CeO$_2$ nanoparticles are well distributed within the aerogel matrix. We also demonstrate the stability of the nanocomposites under high temperature conditions, performing thermal treatments in air at 450 and 900°C. Interestingly, the CeO$_2$ nanoparticles undergo a very limited crystal growth, with sizes up to only 7 nm in the case of the sample subjected to a 900°C treatment

graphene mediated surface enhanced raman scattering in silica mesoporous nanocomposite films

Highly performing mesoporous nanocomposite films with embedded exfoliated graphene and gold nanoparticles display a significant enhancement of G-SERS properties

Magnetic Study of CuFe2O4-SiO2 Aerogel and Xerogel Nano-composites

CuFe2O4 is an example of ferrites whose physico-chemical properties can vary greatly at the nanoscale. Here, sol-gel techniques are used to produce CuFe2O4-SiO2 nanocomposites where copper ferrite nanocrystals are grown within a porous dielectric silica matrix. Nanocomposites in the form of both xerogels and aerogels with variable loadings of copper ferrite (5 wt%, 10 wt% and 15 wt%) were synthesized. Transmission Electron Microscopy and X-Ray Diffraction investigations showed the occurrence of CuFe2O4 nanoparticles with average crystal size ranging from a few nanometers up to around 9 nm, homogeneously distributed within the porous silica matrix, after thermal treatment of the samples at 900°C. Evidence of some impurities of CuO and -Fe2O3 was found in the aerogel samples with 10 wt% and 15 wt% loading. DC magnetometry was used to investigate the magnetic properties of these nanocomposites, as a function of the loading of copper ferrite and of the porosity characteristics. All the nanocomposites show blocking temperature lower than RT and soft magnetic features at low temperature. The observed magnetic parameters are interpreted taking into account the occurrence of size and interaction effects in an ensemble of superparamagnetic nanoparticles distributed in a matrix. These results highlight how aerogel and xerogel matrices give rise to nanocomposites with different magnetic features and how the spatial distribution of the nanophase in the matrices modifies the final magnetic properties with respect to the case of conventional unsupported nanoparticles

Thermally Stable Surfactant-Free Ceria Nanocubes in Silica Aerogel

Surfactant-mediated chemical routes allow one to synthesize highly engineered shape- and size-controlled nanocrystals. However, the occurrence of capping agents on the surface of the nanocrystals is undesirable for selected applications. Here, a novel approach to the production of shape-controlled nanocrystals which exhibit high thermal stability is demonstrated. Ceria nanocubes obtained by surfactant-mediated synthesis are embedded inside a highly porous silica aerogel and thermally treated to remove the capping agent. Powder X-ray Diffraction and Scanning Transmission Electron Microscopy show the homogeneous dispersion of the nanocubes within the aerogel matrix. Remarkably, both the size and the shape of the ceria nanocubes are retained not only throughout the aerogel syntheses but also upon thermal treatments up to 900 °C, while avoiding their agglomeration. The reactivity of ceria is measured by in situ High-Energy Resolution Fluorescence Detected - X-ray Absorption Near Edge Spectroscopy at the Ce L3 edge, and shows the reversibility of redox cycles of ceria nanocubes when they are embedded in the aerogel. This demonstrates that the enhanced reactivity due to their prominent {100} crystal facets is preserved. In contrast, unsupported ceria nanocubes begin to agglomerate as soon as the capping agent decomposes, leading to a degradation of their reactivity already at 275 °C

Cation Distribution and Vacancies in Nickel Cobaltite

Samples of nickel cobaltite, a mixed oxide occurring in the spinel structure which is currently extensively investigated because of its prospective application as ferromagnetic, electrocatalytic, and cost-effective energy storage material were prepared in the form of nanocrystals stabilized in a highly porous silica aerogel and as unsupported nanoparticles. Nickel cobaltite nanocrystals with average size 4 nm are successfully grown for the first time into the silica aerogel provided that a controlled oxidation of the metal precursor phases is carried out, consisting in a reduction under H2 flow followed by mild oxidation in air. The investigation of the average oxidation state of the cations and of their distribution between the sites within the spinel structure, which is commonly described assuming the Ni cations are only located in the octahedral sites, has been carried out by X-ray Absorption Spectroscopy providing evidence for the first time that the unsupported nickel cobaltite sample has a Ni:Co molar ratio higher than the nominal ratio of 1:2 and a larger than expected average overall oxidation state of the cobalt and nickel cations. This is achieved retaining the spinel structure, which accommodates vacancies to counterbalance the variation in oxidation state

Aerogels Containing Metal, Alloy, and Oxide Nanoparticles Embedded into Dielectric Matrices

Aerogels are regarded as ideal candidates for the design of functional nanocomposites containing supported metal or metal oxide nanoparticles. The large specific surface area together with the open pore structure enables aerogels to effectively host finely dispersed nanoparticles up to the desired loading and to provide nanoparticle accessibility as required to supply their specific functionalities. The incorporation of nanoparticles as a way to increase the possibility of the use of aerogels as innovative functional materials and the challenges in the controlled preparation of nanocomposite aerogels are reviewed in this chapter

Surface Compositional Change of Iron Oxide Porous Nanorods: A Route for Tuning their Magnetic Properties

The capability of synthesizing specific nanoparticles (NPs) by varying their shape, size and composition in a controlled fashion represents a typical set of engineering tools that tune the NPs magnetic response via their anisotropy. In particular, variations in NP composition mainly affect the magnetocrystalline anisotropy component, while the different magnetic responses of NPs with isotropic (i.e., spherical) or elongated shapes are mainly caused by changes in their shape anisotropy. In this context, we propose a novel route to obtain monodispersed, partially hollow magnetite nanorods (NRs) by colloidal synthesis, in order to exploit their shape anisotropy to increase the related coercivity; we then modify their composition via a cation exchange (CE) approach. The combination of a synthetic and post-synthetic approach on NRs gave rise to dramatic variations in their magnetic features, with the pores causing an initial magnetic hardening that was further enhanced by the post-synthetic introduction of a manganese oxide shell. Indeed, the coupling of the core and shell ferrimagnetic phases led to even harder magnetic NRs

Using Ex Situ and In Situ HERFD-XANES to Reveal the Superior Oxidation and Reduction Cycling of Ceria Nanocubes Dispersed in Silica Aerogel

The oxygen storage capacity of ceria based catalytic materials are influenced by their size, morphology and surface structure, which can be tuned using surfactant-mediated synthesis. In particular, the cuboidal morphology exposes the most reactive surfaces, however, when the capping agent is removed, the nanocubes can agglomerate and limit the available reactive surface. Here, we study ceria nanocubes, lanthanum-doped ceria nanocubes and ceria nanocubes embedded inside a highly porous silica aerogel, by high-energy resolution fluorescence detected - X-ray absorption near edge spectroscopy at the Ce L3 edge. In-situ measurements showed increased reversibility of redox cycles of ceria nanocubes when embedded in the aerogel, demonstrating enhanced reactivity due to the retention of reactive surfaces. These aerogel nanocomposites show greater improvement of the redox capacity and increased thermal stability of this catalytic material compared to the surfactant capped nanocubes. Ex-situ measurements were also performed to study the effect of lanthanum doping on the cerium oxidation state in the nanocubes, indicating a higher proportion of Ce4+ compared to the undoped ceria nanocubes

NiFe2O4 Nanoparticles Dispersed in an Aerogel Silica Matrix: An X-ray Absorption Study

The formation of NiFe2O4 nanoparticles dispersed in an aerogel silica matrix was investigated as a function of calcination temperature by X-ray absorption fine structure and X-ray absorption near edge structure at both the Fe and Ni K-edges. In particular, nanocomposite aerogels containing a relative NiFe2O4 amount of 10 wt % and calcined at 450, 750 (1 h and 20 h), and 900 °C were studied. A quantitative determination of the relative occupancy of iron and nickel cations in the octahedral and tetrahedral sites of the spinel structure was obtained. It has been found that nickel ferrite prepared by sol?gel has the classical inverted spinel structure found in bulk materials with nickel(II) cations fully occupying the octahedra sites and iron(III) equally distributed between octahedra and tetrahedra sites