Kinetic and DRIFTS Studies of IrRu/Al2O3 Catalysts for Lean NOx Reduction by CO at Low Temperature

This study employs a series of bimetallic IrRu/Al2O3 catalysts with differing Ir:Ru compositions for lean NOx reduction by CO (CO-SCR). Catalytic activity assessments reveal that bimetallic IrRu/Al2O3 catalysts exhibit drastically enhanced de-NOx activities in the low-temperature region, compared with monometallic Ir/Al2O3 or Ru/Al2O3 catalysts. Furthermore, additional characterization and kinetic studies are used to elucidate the origin of the superior catalytic activity observed on bimetallic IrRu/Al2O3 catalysts. DRIFTS analyses show different adsorption affinities of the catalysts towards NO and CO, with CO for Ir and NO for Ru, respectively. The possibility of NCO intermediate formation is ruled out, which implies that the reduction of NOx proceeds through the dissociation of NO and the subsequent recombination of surface N species. Kinetic studies show that a monometallic Ir/Al2O3 catalyst favorably adsorbs CO, which acts as an inhibitor for facile NO adsorption and dissociation. However, bimetallic IrRu/Al2O3 catalysts exhibit higher affinity towards the adsorption of NO which, in terms of catalytic activities, would prove to be beneficial. Calculation of the activation energies for NO consumption and CO oxidation suggests that the rate determining step shifts from the dissociation of NO on a monometallic Ir/Al2O3 catalyst, to the removal of surface O with CO on bimetallic IrRu/Al2O3 catalysts. It is anticipated that the facile uptake of surface O through a CO oxidation reaction prevents the catalyst surface from being predominantly covered by CO or O species which could act as a poison, and leads to enhanced adsorption/dissociation of NO, resulting in a higher catalytic activity on the bimetallic IrRu/Al2O3 catalysts.