A new preparation method for porous silica particles was developed using activated silica sols which are called nano-silica solutions in this paper. Several kinds of organic and inorganic acids are employed to neutralize diluted sodium silicate solutions to form the nano-silica solutions: formic acid, acetic acid, propionic acid, oxalic acid, succinic acid, dl-malic acid, citric acid, and tricarballylic acid as carboxylic acids, and sulfuric acid and hydrochloric acid as inorganic acids. The effect of salts in the nano-silica solution is also studied. The products were investigated using a field emission scanning electron microscope, an X-ray diffractometer, the nitrogen adsorption technique, and a mercury porosimeter. Microporous silicas were produced when carboxylic acids were applied; the formation of micropores was influenced by the pH of the nano-silica solutions and molecular sizes of the carboxylic acids. Addition of a salt in a citric acid solution increased the mesopore volume. Macropores were formed when inorganic acids including salts were applied; the salt nanoparticles which were crystallized in silica spheres acted as templates. The anion types and salt concentrations in the nano-silica solutions affected the aggregation condition of silica nanoparticles, following the Schulze-Hardy rule.
Nanopore formation in silica, in which nanoparticles of metal compounds were embedded, was studied using analytical techniques such as SEM, XRD, XRF, XPS and nitrogen adsorption. Such metal compound nanoparticles formed between primary silica particles, providing micropores and mesopores caused by the formation of Si−O−M bonds (M = Al, Ni). Acid treatment dissolved the metal compound nanoparticles and destroyed the Si−O−M bonds, thereby increasing the micropore volume. The mesopore volume of the aluminium system increased on acid treatment, whereas that of the nickel system decreased.
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