Morphology control of nickel nanoparticles prepared in situ within silica aerogels produced by novel ambient pressure drying

Abstract

Silica aerogels are low density solids with high surface area and high porosity which are ideal supports for catalyst materials. the main challenge in aerogel production is the drying process, which must remove liquid from the pores of the wet gel while maintaining the solid network. in this work, the synthesis of silica aerogels and nickel-doped silica aerogels by a low energy budget process is demonstrated. Silica aerogels are produced by ambient drying using ammonium bicarbonate, rather than a conventional low surface tension solvent. Heating dissociates the ammonium bicarbonate, so generating co 2 and nH 3 within the pores of the wet gel which prevents pore collapse during drying. nickel-doped aerogels were produced by reducing nickel ions within pre-synthesised silica aerogels. the morphology of the resulting nickel particles-spheres, wires and chains-could be controlled through an appropriate choice of synthesis conditions. Materials were characterized using nitrogen adsorption/desorption isotherms, scanning electron microscopy, fourier-transform infrared spectroscopy, thermogravimetric analysis and X-ray diffraction. The surface area of undoped aerogel is found to increase with the concentration of ammonium bicarbonate salts from 360 to 530 m 2 g −1 , and that of nickel-doped silica aerogel varies from 240 to 310 m 2 g −1 with nickel doping conditions. Silica aerogels are ultra-low-density solids with the vast majority (typically > 90% 1) of their volume made up of voids consisting of mesopores and macropores. This unique structure leads to a variety of extraordinary properties such as extremely high specific surface area, low dielectric constant and low thermal conductivity, opening up potential uses including in catalysis, adsorption for pollution remediation, thermal super-insulation and in drug delivery systems 2-8. Practical application of aerogels has, however, been limited by high materials costs and laborious methods for drying. These issues have been solved only recently by the development of a low-cost ambient pressure drying (APD) approach 9 for aerogel production by Han and co-workers. In this method the reaction of sodium bicarbonate with HCl, generated from tetramethylchlorosilane (TMCS), was used to generate pore-supporting carbon dioxide within the wet gel during the drying process, so avoiding the need for low surface tension solvents. Nickel catalysts supported on silica have been employed for the reforming of carbon dioxide and methane to produce synthesis gas (syngas-typically a mixture of H 2 , CO and CO 2) 10 which are important intermediates for the production of synthetic natural gas and methanol. When immobilised within silica aerogels, nickel nanoparticles (NiNPs) and nanowires (NiNWs) have been shown to catalyse the CO 2 hydration reaction (CHR) which has significant potential for carbon capture, storage and utilisation (CCSU) to mitigate anthropogenic climate change 11. Similarly, CO 2 reforming of CH 4 has been demonstrated with nickel doped alumina aerogel catalysts 2. However...

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