Activity and stability of Ag–alumina for the selective catalytic reduction of NOx with methane in high-content SO2 gas streams

In this work, we investigated the activity and stability of Ag–alumina catalysts for the SCR of NO with methane in gas streams with a high concentration of SO2, typical of coal-fired power plant flue gases. Ag–alumina catalysts were prepared by coprecipitation–gelation, and dilute nitric-acid solutions were used to remove weakly bound silver species from the surface of the as prepared catalysts after calcination. SO2 has a severe inhibitory effect, essentially quenching the CH4-SCR reaction on this type catalysts at temperatures <600 °C. SO2 adsorbs strongly on the surface forming aluminum and silver sulfates that are not active for CH4-SCR of NOx. Above ∼600 °C, however, the reaction takes place without catalyst deactivation even in the presence of 1000 ppm SO2. The reaction light-off coincides with the onset of silver sulfate decomposition, indicating the critical role of silver in the reaction mechanism. SO2 is reversibly adsorbed on silver above 600 °C. While alumina sites remain sulfated, this does not hinder the reaction. Sulfation of alumina only decreases the extent of adsoption of NOx, but adsorption of NOx is not the limiting step. Methane activation is the limiting step, hence the presence of sulfur-free Ag–O–Al species is a requirement for the reaction. Strong adsorption of SO2 on Ag–alumina decreases the rates of the reaction, and increases the activation energies of both the reduction of NO to N2 and the oxidation of CH4, the latter more than the former. Our results indicate partial contribution of gas phase reactions to the formation of N2 above 600 °C. H2O does not inhibit the reaction at 625 °C, and the effect of co-addition of H2O and SO2 is totally reversible.