Influence of synthetic and processing parameters on the surface area, speciation, and particle formation in copper oxide/silica aerogel composites

A systematic study of the effects of pH, precursor salt, and temperature on the surface area, particle size, and copper speciation in 3 wt % CuO in silica aerogels and xerogels has been conducted. The materials were synthesized by copolymerization sol-gel reactions, followed by either supercritical solvent extraction using CO2 (aerogels) or conventional drying (xerogels) and annealing in air. Among the six different copper precursors examined, copper(II) acetate was found to be the best in terms of its stability, solubility, and binding capacity of copper in the silica matrix. Aerogels prepared with copper(II) acetate were found to have considerably higher surface areas than the corresponding xerogels, and the observed formation of CuO particles occurred at higher temperatures (900 vs 650 degreesC, by TEM analysis). X-ray diffraction of gels annealed at 900 degreesC likewise revealed the presence of nanocrystalline CuO, with larger particles observed in acid-catalyzed aerogels vs base-catalyzed aerogels (similar to26 vs similar to15 nm, respectively). Additionally, in 900 degreesC annealed acid- and base-catalyzed xerogels, the silica matrix crystallized as crystobalite and quartz, respectively. EPR data suggest that the Cu2+ environment goes through a series of structural changes upon annealing. These changes are significantly different in acid- vs base-catalyzed systems because of differences in the internal structures and pore distributions of the materials. In both cases, a broad isotropic component, attributed to CuO, is observed upon heating. By 900 degreesC, the hyperfine peaks originally attributed to dispersed Cu2+ are absent in base-catalyzed systems, but they are still present as a small fraction in acid-catalyzed systems, suggesting incomplete transformation in the latter case.