Molecular Engineered Porous Nanocomposites of Metal Oxide and Clay Using Surfactants
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Molecular Engineered Porous Nanocomposites of Metal Oxide and Clay Using Surfactants Huai Y. Zhu and Gao. Q. Lu* NanoMaterials Centre and Department of Chemical Engineering, The University of Queensland St Lucia Qld 4072 Australia
ABSTRACT A novel synthesis route of metal oxide nanoparticles dispersed in a silicate framework is reported here. This composite nanostructure is highly thermally stable and porous, rendering large surface area and rich surface chemistry promising for catalytic applications. Aqueous solutions of metal salts were used as the precursors of the nanoparticles, and added in an aqueous dispersion of synthetic clay, laponite, in which the clay exists in exfoliated silicate sheets. Acid leaching of the clay sheets occurs in the reaction due to the strong acidity of the metal salt solution. Meanwhile, the metal hydrate ions polymerise because of the high pH of the clay dispersion and condense on the leached silicates. This mechanism is distinctly different from conventional pillaring process. The nanocomposites of various oxides and binary oxides were synthesised. By introducing polyethylene oxide surfactants, we obtained mesoporous nanocomposites with very large surface areas (400 –900 m2/g) and porosity. These nanocomposites are superior catalysts or catalyst supports over of microporous pillared clays [13] due to their structure and surface properties.
INTRODUCTION The nanoparticles of transition metal oxides, in several nanometers range, are very attractive materials for uses in catalysis, because these nanoparticles often exhibit superior properties and performance due to their large specific surface area. However, agglomeration of ultra-fine particles adversely affects their performance, and recovering such catalysts is difficult. These problems seriously limit their applications. A feasible approach is to disperse nanoparticles of metal oxide within an inorganic media, such as layered clays, meanwhile maintaining most of the surface of metal oxides accessible to reactant molecules. This approach is similar, in some aspects, to the synthesis of pillared layered clays developed in late 1970s [1-2]. It is well known that hydrate cations of many metal elements exist stably in acidic environment. If the pH increases, the metal hydrate ions hydrolyze, forming polymerized hydroxyl ions (oligomers), and finally precipitates [4]. Actually, there are layered clays, which can form well-dispersed suspensions at high pH values. For instance, laponite, synthetic clay that is iso-structural with the smectite clays, has a pH between 9.5 and 10. This pH is effective for inducing hydrolysis of various metal ions. The clay platelets are small, about 20-30 nm in diameter. In a dilute aqueous dispersion the clay exists as discrete plates [5]. Therefore, laponite is an ideal inorganic medium to form nanometer-scale composite structures with various metal hydroxyl species. Aqueous solutions of metal hydroxyl species or positively charged sol particles (precursor solutions) can be readily obtained from inorganic
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