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

• The nanoparticle size of IrO 2 influences NH 3 –SCO reaction activity. • Large IrO 2 nanoparticles contribute highly active reducible oxygen. • The formation of –NH 2 may be the decisive step of the NH 3 oxidation reaction. • NH 3 dehydrogenation by reducible oxygen facilitates to N 2 selectivity. Gaseous ammonia (NH 3 ), especially that emitted from selective catalytic reduction with NH 3 (NH 3 –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 NH 3 slip. In this study, we prepared Ir/Al 2 O 3 catalysts using two different crystal structures of Al 2 O 3 (α–Al 2 O 3 or γ–Al 2 O 3 ) as supported by an impregnation method and evaluated the performance of the selective catalytic oxidation of NH 3 (NH 3 –SCO). We found that the Ir/γ–Al 2 O 3 catalyst performed better than the Ir/α–Al 2 O 3 catalysts, with 99% of NH 3 converted at 260 °C. The X–ray absorption fine structure revealed that the active species on the Ir/Al 2 O 3 catalysts are mainly oxidized IrO 2 . High–angle annular dark–field images of aberration–corrected scanning transmission electron microscopy revealed that IrO 2 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 NH 3 –SCO activity were caused by the activity of reactive oxygen species, and the formation of –NH 2 may be the decisive step of the NH 3 oxidation reaction.