Preparation and characterization of CuO/SiO2 and NiO/SiO2 with bimodal pore structure by sol-gel method

CuO/SiO2 and NiO/SiO2 with bimodal pore structure were prepared by sol-gel reactions of Tetra-methoxysilane (TMOS) and the respective metal nitrate in the presence of poly (ethylene oxide) (PEO) with an average molecular weight of 10 000 and the catalyst of acetic acid. In this process, the interconnected macroporous morphology was formed when transitional structures of spinodal decomposition were frozen by the sol-gel transition of silica. The addition of copper and nickel into the silica-PEO system had a negligible effect on the morphology formation. In gel formation, it was found that NiO crystalline sizes in the samples increased with decreasing Si/Ni molar ratio. It was considered that PEO interacted with both silica and nickel cations. In the CuO/SiO2 with the presence of PEO, CuO crystalline sizes were larger than those of NiO/SiO2. It was considered that there was no obvious interaction between the Cu cation and PEO, most of the copper ions in wet silica gel were present in the outer solution. They easily aggregated as copper salts in the drying process of wet gel and decomposed into CuO particles in heating. While in the CuO/SiO2 with the absence of PEO, the Cu was selectively entrapped as small particles in the gel skeleton due to the interaction between Cu aqua complex and silica gel network.