A review of selective catalytic reduction of nitrogen oxides with hydrogen and carbon monoxide

The selective reduction of NO with hydrogen (H2-SCR) and CO (CO-SCR) over platinum group metal catalysts in the presence of O2 is overviewed. In the case of H2-SCR, Pt and Pd show high activity at low temperatures. The acidity of the support material greatly affects NO reduction activity and selectivity to N2/N2O. Although the activity of Ir and Rh for H2-SCR is low, coexisting SO2 in the reaction gas considerably promotes NO reduction. The best support for Ir 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, Ir 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 H2-SCR and CO-SCR on Ir/SiO2 is to keep Ir in the form of the catalytically active Ir 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. Ir 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.

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► Supported Pt and Pd catalysts show high activity for SCR of NO with H2 (H2-SCR).
► The presence of SO2 promotes the catalytic activity of Ir/SiO2 and Rh/SiO2 for H2-SCR.
► Only Ir serves as an effective catalytically active metal for SCR of NO with CO (CO-SCR).
► Ir/SiO2 is active for CO-SCR in the presence of SO2, which keeps Ir in the active metal state.
► The activity of Ir/SiO2 is promoted by WO3 addition, for which Ir-WOx is the active species.