Infrared Characterization of Rh Surface States and Their Adsorbates during the NO−CO Reaction

Rh surface states and their adsorbates during the NO−CO reaction have been characterized by the in situ infrared (IR) coupled with temperature-programmed reaction (TPR) technique. The TPR profiles of adsorbates and CO2 show that Rh surface states and their adsorbates are governed by the redox reaction cycle of NO−CO. Adsorbed oxygen from dissociated NO oxidizes Rh0 to Rh+; adsorbed CO reduces Rh+ to Rh0. The extent of oxidation and reduction of Rh0/Rh+ is in part reflected in the intensity of the adsorbates residing on these sites (i.e., Rh+(CO)2, Rh0−CO, Rh−NO+, and Rh0−NO-). An increasing NO/CO ratio shifts the TPR profiles of Rh+(CO)2, NO conversion, and light-off to higher temperatures. The results reveal that a high NO/CO ratio or high concentration of oxidant enhances the extent of oxidation of Rh0 to Rh+, resulting in low catalyst activity for NO reduction. Keeping the Rh surface in the Rh0 state by a low NO/CO ratio decreases the Rh+(CO)2 intensity and shifts the light-off to a lower temperature. ...