Ir-based additives, developed to reduce NO and CO emitted during the regeneration of spent fluid catalytic cracking (FCC) catalysts were characterized to correlate physicochemical properties with catalytic performance. Support, metal loading and the state of the metal significantly affected the catalytic performance. Increasing the Ir loading or using a Ce-promoted gamma-alumina (CPBase) support results in the formation of larger Ir particles. Local reduction of iridium oxide surface in such particles leads to coexisting Ir and Ir2O phases being very beneficial for the catalytic activity. NO reduction and CO oxidation take place thermally at 700 degrees C. Increasing the O-2 concentration in the feed favors CO oxidation at the expense of NO reduction. With 500ppmIr/CPBase and 1000ppmIr/CPBase additives, complete NO reduction and CO oxidation is achieved in the presence of 40% excess oxygen. Higher oxygen excess, however, reduces or eliminates the NOx reduction activity of these materials. IR studies suggest that NO reduction by CO proceeds on Ir/alumina additives via the dissociative adsorption of NO, the formation of NCO species on Ir and their migration to the alumina support, where N-2 and CO2 are formed. IR spectroscopy indicates that Ce modifies the Ir surface enhancing the CO oxidation and enabling NO reduction via the NO2 formation. (c) 2005 Elsevier B.V. All rights reserved.