Synthesis, physicochemical characterization, and catalytic evaluation of three-dimensionally ordered macroporous metal oxide catalysts and photocatalysts

Modifications of well-established sol-gel chemistry were adopted to improve the chemical, structural, and optical properties of three dimensionally ordered macroporous (3DOM) materials. The modified synthesis allowed for the control of the grain-size of the crystalline product, the chemical properties as in catalytic activity, the attainment of the desired crystalline phase, and the ability to produce complex materials such as 3DOM layered/tunneled titanates. One of the first successfully achieved syntheses was that of 3DOM zirconia and sulfate promoted zirconia. In this synthesis, grain sizes of less than 3–5 nm were achieved by stabilizing the zirconia precursor with either an acetate or nitrate group. This functionality in turn slowed down the rate of condensation of the zirconium complex, which led to the formation of small particles. In addition, sulfated latex-templated amorphous zirconia produced crystalline materials with even finer grains and relatively high surface area that possessed catalytic activity in n-butane isomerization reaction. Moreover, the simple synthesis adopted allowed for further improvement in the catalytic activity by the addition of a combination of different co-catalysts; platinum, iron, and tungsten. These co-catalysts enhanced the product yield and prolonged the lifetime of the catalyst on stream. The formation of zeolitic nano-crystalline, uniform and continuous layer increased the surface area of the catalysts and imparted the system with interesting and unique catalytic activity. The photonic stop band inherent in 3DOM structures was utilized in a TiO₂/SiO₂ nanocomposite photocatalyst to improve the photooxidation of a dye molecule in aqueous suspension. By positioning the wavelength of the stop band to overlap the red edge of the emission line of the catalytically active species in the catalyst, the rate of the photocatalytic oxidation was enhanced by increasing the emission efficiency of these active species. This was accomplished by making higher energy emission lines a less favorable de-excitation path by partially blocking them with the blue edge of the stop band.