The slip ammonia from selective catalytic reduction (SCR) of NOx in coal-fired flue gas can cause degeneration of the utilities and environmental issues like aerosol. To achieve selective catalytic oxidation (SCO) of slip ammonia to N-2, novel IrO2 modified Ce-Zr solid solution catalysts were synthesized and tested under various conditions. It was found that IrO2/Ce0.6Zr0.4O2 (PVP) catalyst displayed outstanding catalytic activity for slip ammonia and the removal efficiency was higher than 98%. Interestingly, the effect of SO2 on NH3 oxidation was bifacial, which the presence of SO2 could result in slight deactivation of catalyst but also improve the N-2 selectivity of oxidized products to as high as 100% with coexistence of SO2 and NH3. The mechanism of NH3-SCO process over IrO2/Ce0.6Zr0.4O2 (PVP) was evaluated through various techniques, and the results demonstrated that NH3 oxidation could follow both ANH mechanism and internal SCR (iSCR) mechanism at different temperature regions. And the dominant pathway is the iSCR mechanism, in which adsorbed ammonia is firstly activated and reacts with oxygen atoms to form the -HNO intermediate. Then, the -HNO could be oxidized with atomic oxygen from O-2 to form NO species. Meanwhile, the formed/adsorbed NO could interact with -NH2 to N-2 with N2O as by-product, and the presence of SO2 can effectively inhibit the production of N2O and NO. Also, the mechanism of SO2 effects was also evaluated and determined reasonably. (C) 2015 Published by Elsevier Ltd.