Lean NO_x reduction by CO at low temperature over bimetallic IrRu/Al₂O₃ catalysts with different Ir : Ru ratios

In this study, IrRu/Al2O3 bimetallic catalysts with various iridium : ruthenium (Ir : Ru) ratios were prepared for application in the reduction of NO with CO under lean conditions. The effect on the catalytic activity and the physicochemical properties of IrRu/Al2O3 catalysts caused by introducing Ru onto Ir with different amounts is investigated. Bimetallic IrRu catalysts were prepared by co-impregnation of iridium and ruthenium onto the alumina support, where the total amount of iridium and ruthenium combined was kept constant. IrRu bimetallic catalysts were analyzed using various characterization methods such as XRD, H2-TPR, CO chemisorption, H2-TPD, TEM, EDX-mapping, and XPS analyses. Activity results indicate that IrRu bimetallic catalysts led to a drastic enhancement of de-NOx activity in the low-temperature region, while monometallic Ir and Ru catalysts exhibited diminished or even zero activity toward NOx reduction. Detailed examination of XRD patterns and SEM/EDX mapping analyses implied the formation of an IrRu alloy after reduction. Hence, the synergetic effect between Ir and Ru is expected to originate from the intrinsic characteristics of the IrRu alloy phase rather than Ir and Ru acting separately as independent dual active sites. The utilization of bimetallic IrRu catalysts in NOx reduction using CO as a reductant under lean conditions is expected to enable the reduction of NOx with high efficiency at low temperatures without the need of using urea-based reductants even under oxidative conditions.