Inhibitory effect of SO2 on side reactions of NH3-SCR over olivine

Abstract

Olivine catalysts prepared by calcination achieved high N₂ selectivity and good NH₃-SCR activity at 150–450 °C. The existence of SO₂ significantly inhibited the formation of NO and N₂O in the NH₃-SCR reaction at the temperature range of 325–450 °C. This is mainly because of the reductive properties of SO₂ that could restrain the over-oxidization of NH₃. Therefore, both the NO_x conversion and N₂ selectivity were improved at high temperatures. Accordingly, the presence of SO₂ shifted the optimal temperature window of the olivine catalyst by about 75 °C towards high temperature. Both the Langmuir–Hinshelwood mechanism and the Eley–Rideal mechanism contributed to the SCR reaction over the olivine catalyst. Temperature-programmed desorption experiments show that excess ammonium sulfate formed in the presence of SO₂ suppressed the reaction of adsorbed NO_x species with adsorbed NH₃ species at low temperatures. Lots of activated NH₃ species (−NH₂) were formed by the decomposition of ammonium sulfate at high temperatures, promoting the reaction of adsorbed NH₃ species (−NH₂) with both adsorbed NO_x and gaseous NO_x. Preliminary results suggest that olivine is a potential catalyst for treating sulfur-containing exhausts.