Factors Controlling Activity and Selectivity for SCR of NO by Hydrogen over Supported Platinum Catalysts

The selective catalytic reduction of NO by H2 (H2-SCR) was carried out with Pt catalysts supported on various zeolites and nonzeolitic metal oxides. NO conversion and N2 selectivity were strongly dependent on the supports; a high NO conversion was obtained over Pt/SiO2-Al2O3 and Pt/SiO2, while the N2 selectivity was high on Pt/zeolites, particularly Pt/MFI. The strong effect of supports on the activity and selectivity for the H2-SCR was discussed based on Pt dispersion measured by the CO−H2 titration, the oxidation state of Pt estimated with Pt LIII-edge XAFS, and the acid properties of the supports. It was clarified that the SCR activity was controlled by the oxidation state of Pt (i.e., the less oxidized Pt showed the higher turnover frequency for the H2-SCR). On the other hand, the N2 selectivity was related to the acid strength of the supports. The formation of the NH4+ ion under the H2-SCR reaction was observed by in situ IR spectra, and the surface NH4+ concentration was higher on the more acidic supports. The transient reaction tests revealed that the NH4+ ion reacts with NO + O2 to produce N2. A bifunctional mechanism was proposed, which includes the NH3 formation through NO reduction by H2 on a Pt surface, followed by storage of NH4+ on the Brønsted acid site of acidic supports, and the selective N2 formation by the well-established NH3-SCR mechanism on the acid sites of the supports.