Catalytic NO–H2–O2 reaction over Pt/Mg–Al–O prepared from PtCl62−- and Pt(NO2)42−-exchanged hydrotalcites

The catalytic NO–H2–O2 reaction at low temperatures (50–130 °C) has been investigated over Pt-supported Mg–Al binary oxides, which were prepared from PtCl62− and Pt(NO2)42−–exchanged hydrotalcite(HT)-like compounds by heating at 600 °C in H2. Although the NOx conversion of both catalysts was similarly high (>80%) at around 70 °C, the Pt(NO2)42−–HT catalyst exhibited the higher selectivity to N2 (53% N2 and 47% N2O), compared to 12% N2 and 88% N2O for the PtCl62−–HT catalyst. The XANES/EXAFS spectroscopy showed that Pt prepared from PtCl62−–HT is partly oxidized due to the coordination of residual chlorides, whereas Pt(NO2)42−–HT yielded highly dispersed metallic Pt. Because of the presence of chloride strongly bound to Pt, the oxidative NO adsorption as NO2/NO3 was inhibited for PtCl62−–HT. The improved N2 selectivity with an increase of oxidative NO adsorption is in accord with the catalytic property of Na–Pt/ZSM-5 in our previous work [M. Machida, T. Watanabe, Appl. Catal. B: Environ. 52 (2004) 281.], suggesting that the N2 would be formed via NO2/NO3 intermediates.