The reduction of NO with H
2 in the presence of O
2 at 100-150
C over physical mixtures ofPt/ZrO
2 and secondary catalysts active for the selective catalytic reduction of NO
x with NH
3(NH
3-SCR) was investigated. For high N
2 selectivity, Pt/ZrO
2 particles must be in close contactwith the particles of the NH
3-SCR catalyst. The optimal Pt loading and calcination temperaturefor Pt/ZrO
2 were 1 wt % and 600
C, respectively. H-form zeolites exhibited higher N
2 selectivitythan V/TiO
2. Both H-ZSM-5 and H-mordenite were favorable as NH
3-SCR catalysts. The activityof a physical mixture of Pt/ZrO
2 and H-ZSM-5 was not strongly influenced by changes in the O
2or H
2 concentration or by the addition of H
2O to the feed gas. The addition of CO severelysuppressed the reaction owing to its strong adsorption on Pt/ZrO
2. Although the addition ofC
3H
8, C
3H
6, CO
2, or SO
2 negatively affected the activity of Pt/ZrO
2 for NH
3 production, thephysical mixture with H-ZSM-5 maintained high NO
x conversion, and only a small decrease inthe N
2 selectivity was observed upon the addition of C
3H
6 or SO
2.