Amino-modified zirconia aerogels for the efficient filtration of NO2: effects of water on the removal mechanisms

Abstract

An amino-modified zirconia aerogel (AMZA) was prepared using a facile post-synthesis grafting strategy by introducing 3-aminopropyltriethoxysilane (APTES) into the zirconia sol–gel process and ambient pressure drying (APD). The AMZA shows a three-dimensional mesoporous network assembled by numerous sheet-like aggregates with polymolecular polysiloxane layers covering the surface. The AMZA has a density comparable to that of ASZM–TEDA impregnated activated carbon, which is approximately 65% less than that of porous zirconium hydroxide. NO₂ breakthrough tests show that the AMZA has excellent NO₂ filtration ability at different relative humidity (RH) levels, with a breakthrough time as long as 362 min and limited NO by-product evolution (≤8%). In particular, under high humidity conditions, the AMZA's adsorption performance against NO₂ is significantly superior to that of ASZM–TEDA carbon and porous zirconium hydroxide. Through meticulous post-breakthrough characterization, the impact of water molecules on the NO₂ adsorption mechanism was systematically investigated. At low RH, NO₂ and NO are mainly removed through redox reactions, with nitramine and nitrosamine compounds serving as the principal adsorption products. At high RH, NO₂ is mainly converted to HNO₃ by hydrolysis, which is then mitigated through acid–base neutralization to form nitrate species. When water molecules are pre-adsorbed on the AMZA surface, NO₂ elimination can be controlled by diffusion, and the aforementioned adsorption products show a similar formation trend.