Silica-supported aminoxyls as reactive materials for NOx removal

Abstract

The design of new materials for gaseous NO_x (NO and NO₂) removal at ambient temperature using organic active sites is reported. The materials consist of unfunctionalized silica and silica modified by immobilized aminoxyls and function via sequential processes consisting of (i) NO oxidation to NO₂ and (ii) NO₂ storage. NO_x removal by physical mixtures of immobilized PTIO (2-phenyl-4,4,5,5-tetramethyl-imidazoline-3-oxide-1-oxyl) sites on silica as the NO oxidant and hydrated silica as the NO₂ trap occurs with significant degradation of the PTIO oxidant via undesired side reactions with NO₂ when NO₂ adsorption sites are fewer than required for its complete removal along the packed bed. The use of packed beds with sufficient NO₂ adsorption sites requires a large excess of unfunctionalized silica, because of its low surface density of geminal silanols, which are shown to be the relevant sites for NO₂ storage on silica at ambient temperature based on density functional theory calculations. This PTIO degradation is circumvented by the design of NO_x traps consisting of immobilized PTIO on silica as the NO oxidant and immobilized TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) on silica as an adsorbent with a high density of strong NO₂ binding sites. Packed beds consisting of a 2 : 1 molar mixture of PTIO and TEMPO sites consume NO_x as predicted by stoichiometry without detectable PTIO degradation and also without a contribution from geminal silanols as NO₂ storage sites. This result requires that PTIO and TEMPO sites on silica render geminal silanols as inactive towards NO₂ storage presumably because of the titration of these silanols by immobilized aminoxyls.