Structure and reducibility of a Fe/Al₂O₃ catalyst for selective catalytic reduction studied by Fe K-edge XAFS spectroscopy

EXAFS and pre-edge information from the Fe K-edge absorption spectra is used in this study to characterise the local environment and geometry of Fe-centres in a 1% Fe/Al2O3 model catalyst. The EXAFS results reveal clusters of 2-3 Fe oxo-moieties dispersed on the Al2O3-support. The Fe3+ centres are coordinated by 6 O-atoms in a strongly distorted octahedral geometry. This is supported by the pre-edge peak, which is far more intense than in α-Fe2O3 absorption data acquired for comparison. For preliminary investigations, catalytic tests for selective catalytic reduction (SCR) of NOx by ammonia and in situ XANES studies of Fe/Al2O3 are compared with previously published data for Fe/zeolite systems. The low SCR activity of Fe/Al2O3 (compared to Fe/zeolite catalysts) in which the Fe3+ species do not change their electronic state under SCR-relevant conditions (reference temperature 250°C in our case) correlates well to the significantly higher temperatures required to reduce these species to Fe2+. The difference in reducibility (and consequently in the SCR-activity) between the two systems probably results from differences in the structure and the electronic interaction of the Fe oxo-moieties and the particular catalyst support.