Novel effect of SO2 on the SCR reaction over CeO2: Mechanism and significance

Ceria (CeO2) showed a poor activity for the selective catalytic reduction (SCR) of NO with NH3, while CeO2 showed an excellent SCR activity in the presence of SO2 at 300–500 °C. The promotion of SO2 on the SCR reaction over CeO2 was mainly due to the sulfation of CeO2. The SCR reaction over CeO2 and that over sulfated CeO2 both followed the Eley–Rideal mechanism (i.e. the reaction between activated NH3 with gaseous NO). Meanwhile, the catalytic oxidization of NH3 to NO could simultaneously happen during the SCR reaction, resulting in a drop of NOx conversion at high temperatures. The adsorption of NH3 on CeO2 was obviously promoted after the sulfation, resulting in an obvious promotion of the Eley–Rideal mechanism. Meanwhile, the adsorption sites for NH2 adsorption and the oxidization agents for NH2 oxidization on CeO2 were separated after the sulfation. Thus, the probability of the collision between NH2 and Ce4+ on sulfated CeO2 was much less than that on CeO2, resulting in an obvious repression of the catalytic oxidization of NH3 to NO. As a result, the SCR activity of sulfated CeO2 was much better than that over CeO2.

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► The promotion of SO2 on the SCR reaction was due to the sulfation of CeO2.
► The SCR reaction over CeO2 and sulfated CeO2 followed the E–R mechanism.
► The SCR reaction through the E–R mechanism was promoted after CeO2 sulfation.
► Catalytic oxidization of NH3 to NO over CeO2 was restrained after sulfation.