Strong Ir–W interaction boosts CO-SCR denitration over supported Ir-based catalysts and influential mechanism of oxygen

Abstract

Selective catalytic reduction of NOx with CO (CO-SCR) is deemed to be prospective technology instead of NH3-SCR denitration, while poor CO-SCR performance in the presence of excess O2 profoundly hinders its further development. Herein, supported Ir catalysts with abundant active sites (Ir0 and Ir–WO3−x species) were prepared by ultrasonic-assisted impregnation, which achieved above 70 % NOx conversion and around 80 % N2 selectivity at 250 °C in the presence of 5 % O2. Also, NOx conversion maintained stable for more than 100 h due to the superior oxidation resistance of Ir-based catalysts that the ratio of Ir0/(Ir0 + Ir4+) only decreased by 1 % after the reaction. More importantly, a variety of characterization techniques including BET, XRD, XPS, H2-TPR, NO-TPD, and in situ DRIFTS measurements were employed. These results indicated that the highly dispersed Ir and WO3 species had contributed to the strong Ir-W interaction, bringing about more exposed Ir0 and Ir–WO3−x active sites, and enhancing the adsorption and dissociation of NO. Meanwhile, the influential mechanism of O2 on CO-SCR was studied. On the one hand, O2 accelerated the adsorption of NO to generate more NO-adsorbed species (NO3–, NO2–, NO2, and N2O4), especially for monodentate nitrates, when the reaction temperature was below 250 °C and the oxygen concentration was below 5 %. They acted as active intermediates to react with CO to form N2 and CO2, which was beneficial to CO-SCR performance. On the other hand, as the reaction temperature increased, the absorption of NO was inhibited because active sites were occupied by oxygen, thus promoting CO oxidation with O2 and inhibiting NO reduction with CO.