The kinetics and impact of steaming on the acid site concentration of zeolite beta were studied by 1H, 27Al, and 29Si MAS NMR spectroscopy as well as IR spectroscopy of adsorbed pyridine and temperature-programmed desorption of NH3. The main effects of steaming were the dealumination of the T3−T9 sites, the formation and migration of extra-framework Al species, and the healing of defect sites by condensation of silanol groups. These effects took place mainly in the first five hours of steaming, while after 14 h of steaming the system appeared to be stabilized. The concentration of framework Al atoms detected by 27Al MAS NMR spectroscopy is significantly higher than the concentration of Brønsted acid sites determined by 1H MAS NMR spectroscopy as well as by the sorption of basic probe molecules such as NH3 and pyridine. This shows conclusively that extra-framework Al oxide/hydroxide species act as cations balancing the framework charge. The concentration of extra-framework Al atoms matches the discrepancy between the concentration of framework Al atoms and the concentration of Brønsted acid sites indicating that each charge balancing entity contains only one aluminum.

We compare three methods for quantitatively distinguishing the location of noble metal (NM) particles in mesopores from those found on the external support surface. MCM-41 and SBA-15 with NM located in mesopores or on the external surface were prepared and characterized by TEM. 31 P MAS NMR spectroscopy was used to quantify arylphosphines in complexes with NM. Phosphine/NM ratios drop from 2.0 to 0.2 when increasing the probe diameter from 1.08 to 1.54 nm. The reaction between NM and triphenylphosphine (TPP) within 3.0 nm MCM-41 pores takes due to confinement effects multiple weeks. In contrast, external NM react with TPP instantly. A promising method is filling the pores by using the pore volume impregnation technique with tetraethylorthosilicate (TEOS). TPP loading revealed that 66 % of NMs are located on the external surface of MCM-41. The pore filling method can be used in association with any probe molecule, also for the quantification of acid sites.

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The type and location of Fe cations in zeolite BEA and their structural changes under NH 3 -SCR conditions are reported, as they are important catalysts for the selective catalytic reduction of NO x with NH 3 . Adsorption of basic probe molecules such as NH 3 and pyridine shows that additional Brønsted acid sites are formed at the expense of Lewis acidic octahedrally coordinated Fe cations during the aging cycle. The formation of new tetrahedrally coordinated Fe suggests that Fe cations are incorporated into the zeolite framework at tetrahedral positions. UV/vis, XAFS, and IR spectra indicate that these species, however, exist only when the samples are cooled to room temperature under NH 3 -SCR conditions. In situ EXAFS shows Fe cations forming bridging Fe−O−Fe species at cation exchange sites above 573 K under reaction conditions. This indicates that the Fe cations undergo a unique reversible transformation during temperature cycles under NH 3 -SCR conditions.

Structural changes to Fe 3+ cationic species in Fe-exchanged zeolite BEA during the selective catalytic reduction (SCR) of NO x with NH 3 were probed by UV−vis spectroscopy. The distribution between Fe 2+ and Fe 3+ species was characterized by IR spectroscopy of adsorbed CO. Upon heating to 723 K, some of the Fe cations formed Fe−O−Fe bonds that underwent reversible structural transformation under NH 3 −SCR conditions. The in situ formed Fe oxide clusters could be dissociated to isolated Fe cations at 423 K, while at higher temperatures O-bridged Fe clusters were again formed. The structure of the Fe cluster is related to the Al distribution in the zeolite probed by Co 2+ ion exchange. We propose here that two Fe cations bound within one six-membered ring containing an Al pair form hydroxylated dimeric Fe− O−Fe in the zeolite. This was supported by a structure simulation of a binuclear [HO-Fe(III)−O−Fe(III)−OH] model.