Preparation and Structure of In−ZSM-5 Catalysts for the Selective Reduction of NO by Hydrocarbons

The influence of the preparation route on the microstructure of indium species in In−ZSM-5 catalysts for the selective reduction of NO by hydrocarbons has been investigated. Samples were prepared by aqueous ion exchange, by precipitation with or without previous ion exchange step, by reductive solid-state ion exchange of indium precipitates in flowing hydrogen or at the expense of NH4 ions in the zeolite, and by sublimation or solid-state ion exchange with InCl3. These materials have been studied by XPS, X-ray absorption spectroscopy, representative samples also by FTIR, and transmission electron microscopy. It was found that in aqueous exchange undesired precipitation of extra-zeolite aggregates can be avoided only at low pH, where the indium enters the zeolite voids only in low amounts (maximum exchange degree ≈10% (at In/Al = 1)). At near-neutral pH, extra- and intra-zeolite indium species are formed, and the coordination spheres of the latter differs from those of intra-zeolite indium species obtained at low pH. Sublimation or solid-state ion exchange routes based on InCl3 provide large amounts of intra-zeolite indium species, with concomitant formation of InO x phases being observed only near the theoretical exchange capacity of the zeolite. The indium interacts both with Brønsted sites and extraframework Al−OH groups and carries chlorine ligands, which are only partially removed by washing. Reductive solid-state ion exchange in H2 is suited for the preparation of intra-zeolite In oxo sites even in overstoichiometric amounts. Excess indium is deposited in oligomeric species, probably polynuclear In-oxo cations. It was confirmed that reductive solid-state ion exchange is also induced by NH4 ions in the zeolite lattice. Because of the short reductant supply, the penetration depth of the indium is low and the reproducibility is insufficient.