Thick silica foam films through combined catalytic decomposition of H2O2 and sol-gel processes

A novel method for the preparation of thick silica foam films is reported that combines an alkoxide-based hydrolytic sol-gel process and in situ catalytic decomposition of hydrogen peroxide on a catalyst-coated support. A hydrogen peroxide/nitric acid aqueous solution was used to carry out acid-catalysed hydrolysis of tetramethoxysilane. Complete stability of H2O2 in the resulting silica sols over extended periods (>120 min) was demonstrated by in situ Raman spectroscopy. The H2O2-loaded sols were sprayed on MnO2-coated substrates, resulting in heterogeneous catalytic decomposition of H2O2 and effective foaming and simultaneous gel formation due to oxygen gas and water formation. Silica foam films with a well-defined closed-cell porosity were annealed at 600 degrees C without any damage to the closed-cell porous film morphology. Up to 530 mu m thick films were prepared with macropore sizes in the range of 29-47 mu m, exceptionally thin macropore wall thicknesses of 16-50 nm and a bulk density as low as 64 kg/m(3), comparable to that of the aerogels. The lowest measured thermal conductivity of the prepared foams was 0.018 +/- 0.001 W/(m*K), which is also similar to silica aerogels, enabling the prepared foams to be used as efficient thermal insulation materials. (C) 2016 Elsevier Ltd. All rights reserved.