The selective reduction of NO with hydrogen (H-2-SCR) and CO (CO-SCR) over platinum group metal catalysts in the presence of O-2 is overviewed. In the case of H-2-SCR, Pt and Pd show high activity at low temperatures. The acidity of the support material greatly affects NO reduction activity and selectivity to N-2/N2O. Although the activity of Ir and Rh for H-2-SCR is low, coexisting SO2 in the reaction gas considerably promotes NO reduction. The best support for It and Rh is SiO2. Li and Zn additives improve the activity of Ir/SiO2 and Rh/SiO2, respectively, by maintaining the active reduced metal state. For CO-SCR, on the other hand, It is almost the only active metal species. Coexisting SO2 is also essential for CO-SCR on Ir/SiO2 to occur. The role of SO2 for both H-2-SCR and CO-SCR on Ir/SiO2 is to keep It in the form of the catalytically active It metal state. The additions of WO3 and Nb2O5 considerably promote the activity of Ir/SiO2 for CO-SCR, catalyzing CO-SCR even in the absence of SO2. It metal interacting strongly with W oxide is the active species on WO3-promoted Ir/SiO2. Furthermore, the addition of Ba improves the performance of Ir/WO3/SiO2 catalyst. (C) 2012 Elsevier B.V. All rights reserved.