Copper-Based Catalysts Supported on Highly Porous Silica for the Water Gas Shift Reaction

Magnetic Aerogels for Room-Temperature Catalytic Production of Bis(indolyl)methane Derivatives

The potential of aerogels as catalysts for the synthesis of a relevant class of bis-heterocyclic compounds such as bis(indolyl)methanes was investigated. In particular, the studied catalyst was a nanocomposite aerogel based on nanocrystalline nickel ferrite (NiFe2O4) dispersed on amorphous porous silica aerogel obtained by two-step sol-gel synthesis followed by gel drying under supercritical conditions and calcination treatments. It was found that the NiFe2O4/SiO2 aerogel is an active catalyst for the selected reaction, enabling high conversions at room temperature, and it proved to be active for three repeated runs. The catalytic activity can be ascribed to both the textural and acidic features of the silica matrix and of the nanocrystalline ferrite. In addition, ferrite nanocrystals provide functionality for magnetic recovery of the catalyst from the crude mixture, enabling time-effective separation from the reaction environment. Evidence of the retention of species involved in the reaction into the catalyst is also pointed out, likely due to the porosity of the aerogel together with the affinity of some species towards the silica matrix. Our work contributes to the study of aerogels as catalysts for organic reactions by demonstrating their potential as well as limitations for the room-temperature synthesis of bis(indolyl)methanes.

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.

Growing CeO2 Nanoparticles Within the Nano-Porous Architecture of the SiO2 Aerogel

In this study, new CeO₂-SiO₂ aerogel nanocomposites obtained by controlled growth of CeO₂ nanoparticles within the highly porous matrix of a SiO₂ aerogel are presented. The nanocomposites have been synthesized via a sol-gel route, employing cerium (III) nitrate as the CeO₂ precursor and selected surfactants to control the growth of the CeO₂ nanoparticles, which occurs during the supercritical drying of the aerogels. Samples with different loading of the CeO₂ dispersed phase, ranging from 5 to 15%, were obtained. The nanocomposites showed the morphological features typical of the SiO₂ aerogels such as open mesoporosity with surface area values up to 430 m²·g^-1. TEM and XRD characterizations show that nanocrystals of the dispersed CeO₂ 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 CeO₂ 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 CeO₂ 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.

Synthesis of Copper Nanoparticles in Ethylene Glycol by Chemical Reduction with Vanadium (+2) Salts

Copper nanoparticles have been synthesized in ethylene glycol (EG) using copper sulphate as a precursor and vanadium sulfate as an atypical reductant being active at room temperature. We have described a technique for a relatively simple preparation of such a reagent, which has been electrolytically produced without using standard procedures requiring an inert atmosphere and a mercury cathode. Several stabilizing agents have been tested and cationic capping agents have been discarded owing to the formation of complex compounds with copper ions leading to insoluble phases contaminating the metallic nanoparticles. The elemental copper nanoparticles, stabilized with polyvinylpyrrolidone (PVP) and sodium dodecyl sulphate (SDS), have been characterized for composition by energy dispersive X-ray spectroscopy (EDS), and for size by dynamic light scattering (DLS), and transmission electron microscopy (TEM), giving a size distribution in the range of 40-50 nm for both stabilizing agents. From a methodological point of view, the process described here may represent an alternative to other wet-chemical techniques for metal nanoparticle synthesis in non-aqueous media based on conventional organic or inorganic reductants.