Effects of IrO2 nanoparticle sizes on Ir/Al2O3 catalysts for the selective catalytic oxidation of ammonia

Abstract

Gaseous ammonia (NH3), especially that emitted from selective catalytic reduction with NH3 (NH3–SCR) units in the atmosphere is potentially harmful to both human health and the environment. The noble metal catalytic oxidation technology placed behind the SCR unit has been considered to be a promising method for decreasing NH3 slip. In this study, we prepared Ir/Al2O3 catalysts using two different crystal structures of Al2O3 (α–Al2O3 or γ–Al2O3) as supported by an impregnation method and evaluated the performance of the selective catalytic oxidation of NH3 (NH3–SCO). We found that the Ir/γ–Al2O3 catalyst performed better than the Ir/α–Al2O3 catalysts, with 99% of NH3 converted at 260 °C. The X–ray absorption fine structure revealed that the active species on the Ir/Al2O3 catalysts are mainly oxidized IrO2. High–angle annular dark–field images of aberration–corrected scanning transmission electron microscopy revealed that IrO2 species over 1 nm in diameter are beneficial for activity. In situ diffuse reflectance for infrared Fourier transform spectroscopy studies suggested that the differences in NH3–SCO activity were caused by the activity of reactive oxygen species, and the formation of –NH2 may be the decisive step of the NH3 oxidation reaction.