Low-temperature synthesis of NH3via an alternate gas-switching NOx storage-reduction process using a BaO/Pt@mTiO2 nanocomposite catalyst

Abstract

Nitrogen oxides, NO_x, are harmful and should be eliminated to help the clean-up and/or maintenance of the global environment. In recent years, an opportunity to utilize the reactive nitrogen compound NO₂ in the synthesis of valuable chemicals, such as ammonia (NH₃), has been demonstrated by applying an alternate gas-switching NO_x storage-reduction (NSR) process. Herein, we provide significant knowledge to design nanocomposite catalysts that work very well for the selective synthesis of NH₃, even at low temperature. To ensure an abundant supply of hydrogen species during the steady-state reaction, we improved the aerosol-assisted synthesis of high-surface-area mesoporous titania (mTiO₂) in the presence of an amphiphilic triblock copolymer for the creation of a BaO/Pt@mTiO₂ type nanocomposite catalyst. After the storage of NO_x as Ba(NO₃)₂ at temperatures in the range 175 °C–300 °C, solid-state nitrate (NO₃⁻) species could be reduced by switching the inlet gas to hydrogen (H₂) at each temperature tested. Based on our previous report where we used BaO/Pt@mAl₂O₃, a rapid hydrogenation of the NO₃⁻ species near Pt nanoparticles leads to the formation of nitrogen (N₂) but an abundant supply of hydrogen species, that are spilled over and pooled at the TiO₂ surface after the dissociation of H₂ over Pt, is very advantageous for the conversion to NH₃. Via the rational design of slow hydrogenation at 175 °C with sufficient supply of dissociated hydrogen, the resulting NH₃ selectivity exceeded 90% when using BaO/Pt@mTiO₂.