The generation of sulfate species on Ir-based catalysts for boosting NO reduction with CO under the coexistence of O2 and SO2 atmosphere

[Display omitted] •NOx conversion and N2 selectivity are significantly enhanced in the presence of SO2.•The decrease in CO oxidation ability is the main reason for the improvement in NO reduction performance.•L-H reaction pathways for CO oxidation are inhibited by the formation of sulfate species.•The promotional mechanism of SO2 on CO-SCR reaction over Ir-based catalysts is proposed. Generally, sulfur poisoning is considered to be one of the main factors contributing to the deactivation of selective catalytic reduction of NOx by CO (CO-SCR) catalysts, while the promotional effect of SO2 on NO reduction over Ir/SiO2 is observed which is an interesting scientific phenomenon. After the introduction of 20 ppm SO2, NOx conversion increased from ∼ 40 % to ∼ 90 % at 275 °C, and N2 selectivity increased from ∼ 80 % to 100 % at 200 ∼ 300 °C. Furthermore, the promoting effect could remain unchanged after 24 h of continuous reaction. However, the temperature point for achieving complete conversion of CO increased from 225 °C to 275 °C after the introduction of SO2. Experimental characterization and theoretical calculation jointly proved that the inhibition of CO oxidation by the generation of sulfate was the main reason for promoting NO reduction. Under the coexistence of O2 and SO2, SO2 was firstly oxidized to SO3 on the iridium surface and generated sulfate species on surface hydroxyl groups of SiO2. Some active sites for O2 adsorption were covered by the generated surface sulfate, and adsorbed CO was hard to react with adsorbed O2, resulting in Langmuir–Hinshelwood (L-H) reaction pathways for CO oxidation being inhibited. Therefore, unoxidized CO reacted with NO adsorbed species and generated N2O to generate N2 and CO2, improving NO reduction. This new insight has implications for understanding the promotional effect of SO2 on NO reduction with CO in the presence of O2.