Gallosilicate-Based Catalysts for NOx-SCR with CH4

Abstract

Ga-incorporated H-MFI (GaMFI) metallosilicate catalysts show robust activity for selective catalytic reduction of NO x (NO x -SCR) with CH 4 , although the reactivity of NO CH 4 is very low in the absence of O 2 . Over these catalysts, NO x CH 4 reaction proceeds selectively in the presence of O 2 . In contrast, FeMFI shows high activity for NO CH 4 reaction without O 2 , although no reactivity is shown in the presence of O 2 . In addition, transition-metal ion modification on FeMFI does not enhance NO x -SCR activity. From the results of XRD and XAFS characterization at Ga and Si K-edges, active Ga species are incorporated in H-MFI frameworks with T d symmetry. Local structure around Ga is very stable but distorted Si O 4 local structure can be formed after NO x -SCR reaction with CH 4 .Zeolites are well known as microporous materials, and have been applied widely to several engineering processes because of surface activity resulting from acidity. Zeolites are open framework aluminosilicates consisting of SiO 4 and AlO 4 , interconnected via oxygen atoms, and both Si and Al are present with T d symmetry in the framework. Distortion of T d sites (SiO 4 and AlO 4 ) is often brought about by application of several thermal processes or reactions. MFI zeolites are typical catalysts with unique activity for several useful reactions. In particular, Cu 2+ ion-exchanged H-MFI shows high deNO x activity with hydrocarbons. 1,2 For ion-exchanged zeolite catalysts, significant problems due to hydrothermal stability cannot be avoided in the case of modified MFI. On the other hand, metallosilicates are an appropriate material because active transition-metal ions are incorporated onto the framework. 3 For example, Fe 3+ -substituted MOR zeolites show high NO-SCR activity for methane. 4 But deactivation during the reaction is a major issue. We synthesized Fe-and Ga-MFI with various metal ratios by hydrothermal synthesis. Fe 3+ -and Ga 3+ -cosubstituted MFI (FeGaMFI) can be obtained. These materials show unique reactivity for NO x -SCR. 5 FeMFI with high Fe ratio shows high NO CH 4 reactivity, while no activity for NO x -SCR in the presence of O 2 . In contrast, GaO x is an appropriate component for NO x -SCR. It has been reported that Ga 2 O 3 Al 2 O 3 catalyst systems show high NO x -SCR activity. 6 Ga ion-exchanged H-MFI is also a widely-known catalyst for NO x -SCR with methane, as reported by Kikuchi et al. 7 and Inui et al. 8 Ga species on H-MFI show high and selective activity for NO x -SCR, but Ga modification brings about low acidity of H-MFI support because of coverage of non-framework Ga ions on the strong acid sites. 9 The NO x -SCR reactivity is likely to relate to the formation rate of NO 2 , and thus the acidity of Gamodified H-MFI is important to oxidize NO to NO 2 . In order to design useful and robust NO x -SCR catalysts, Ga-incorporated MFI metallosilicates (gallosilicates: GaMFI) are attractive because of stronger acidity than Ga ion-exchanged MFI. 9,10 Therefore, both the strong acidity and structural stabil...