Introduction of Cobalt Into Silica Aerogels, Metal Speciation and Reducibility

Abstract

Hydrophobic and amorphous silica aerogels with incorporated cobalt(II) in single-site was tried synthesized through the newly developed method of using sodium silicate as the silica precursor and achieving hydrophobicity by applying the ambient pressure drying method (APD). Two different variations of the sol-gel route was applied; the co-recursor method (CP)and the surface derivatization method (SD), where the cobalt precursor was added directly to the sol. Different parameter studies were performed to investigate if single-site cobalt were possible through this method. The parameters investigated ncluded; cobalt content, amount of surface modifcation agents, pH of the gel at point of gelation and cobalt precursor. A modifed version of the surface derivatization method was also tried, where gels were washed with ethanol and heptane to remove unreacted precursors and side products. Incorporation of cobalt into single-site was not possible to determine with certainty, and cobalt present in the gels were determined to be a mix of different cobalt species such as oxides and cobalt silicate. However, catalytically active silica aerogel-supported cobalt was successfully synthesized, but the catalytically active cobalt specie(s) was not possible to be determined. X-ray diffraction (XRD) was used to exclude the presence of crystalline phases, indicating unsuccessful incorporation of cobalt into single-site. ICP-MS was used to measure the uptake of cobalt in the silica aerogels, and nitrogen adsorption (BET/BJH) was used to measure the surface area and pore size. Furthermore, thermal stability was determined by heat treatment at 450 °C and 700 °C, and thermal events was monitored by thermal ravimetric analysis (TGA). Scanning electron microscopy was used to observe the topography, chemical environment was determined by x-ray absorption spectroscopy (XAS).