In situ XAFS combined with UV-vis-near-IR spectroscopy are used to identify the active site in copper-loaded ZSM-5 responsible for the catalytic decomposition of NO. Cu-ZSM-5 was probed with in situ XAFS (i) after O(2) activation and (ii) while catalyzing the direct decomposition of NO into N(2) and O(2). A careful R-space fitting of the Cu K-edge EXAFS data is presented, including the use of different k-weightings and the analysis of the individual coordination shells. For the O(2)-activated overexchanged Cu-ZSM-5 sample a Cu.Cu contribution at 2.87 A with a coordination number of 1 is found. The corresponding UV-vis-near-IR spectrum is characterized by an intense absorption band at 22 700 cm(-1) and a relatively weaker band at 30 000 cm(-1), while no corresponding EPR signal is detected. Comparison of these data with the large databank of well-characterized copper centers in enzymes and synthetic model complexes leads to the identification of the bis(mu-oxo)dicopper core, i.e. [Cu(2)(mu-O)(2)](2+). After dehydration in He, Cu-ZSM-5 shows stable NO decomposition activity and the in situ XAFS data indicate the formation of a large fraction of the bis(mu-oxo)dicopper core during reaction. When the Cu/Al ratio of Cu-ZSM-5 exceeds 0.2, both the bis(mu-oxo)dicopper core is formed and the NO decomposition activity increases sharply. On the basis of the in situ measurements, a reaction cycle is proposed in which the bis(mu-oxo)dicopper core forms the product O(2) on a single active site and realizes the continuous O(2) release and concomitant self-reduction.

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In-situ soft X-ray absorption spectroscopy (XAS) has been applied to study the iron redox behavior in overexchanged Fe/ZSM5. The Fe L 2,3 XAS and O K spectral shapes of the Fe/ZSM5 surface have been measured during heat treatments and reduction/oxidation cycles. Charge-transfer multiplet calculations provide a detailed understanding of the L 2,3 spectra of iron in Fe/ZSM5. The oxidized form of Fe/ZSM5 contains Fe III ions in an octahedral surrounding, with a total crystal field splitting of ∼1.0 eV. This value is significantly smaller than that for Fe 2 O 3 , which is indicative of a much weaker Fe-O bonding. The reduced form of Fe/ZSM5 has Fe II ions in a tetrahedral oxygen surrounding. The Fe L 2,3 spectra show that iron in calcined Fe/ZSM5 is reduced in 15 min to an average valence state of 2.65, under 10 mbar of pure helium at room temperature. This value has a relative uncertainty on the order of 0.01. Heating in helium up to 350°C under the same pressure further reduces the iron valence to 2.15. The oxygen spectra show that the autoreduction is accompanied by a loss of molecular oxygen and water. Reoxidation with 5% O 2 in helium yields a valence of >2.90 after 10 min.