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| - Bis(μ-oxo)dicopper in Cu-ZSM-5 and Its Role in theDecomposition of NO: A Combined in Situ XAFS,UV−Vis−Near-IR, and Kinetic Study
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| - In situ XAFS combined with UV−vis−near-IR spectroscopy are used to identify the active sitein copper-loaded ZSM-5 responsible for the catalytic decomposition of NO. Cu−ZSM-5 was probed with insitu XAFS (i) after O2 activation and (ii) while catalyzing the direct decomposition of NO into N2 and O2. Acareful R-space fitting of the Cu K-edge EXAFS data is presented, including the use of different k-weightingsand the analysis of the individual coordination shells. For the O2-activated overexchanged Cu−ZSM-5 samplea Cu···Cu contribution at 2.87 Å 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 weakerband at 30 000 cm-1, while no corresponding EPR signal is detected. Comparison of these data with thelarge databank of well-characterized copper centers in enzymes and synthetic model complexes leads tothe identification of the bis(μ-oxo)dicopper core, i.e. [Cu2(μ-O)2]2+. After dehydration in He, Cu−ZSM-5shows stable NO decomposition activity and the in situ XAFS data indicate the formation of a large fractionof the bis(μ-oxo)dicopper core during reaction. When the Cu/Al ratio of Cu−ZSM-5 exceeds 0.2, both thebis(μ-oxo)dicopper core is formed and the NO decomposition activity increases sharply. On the basis ofthe in situ measurements, a reaction cycle is proposed in which the bis(μ-oxo)dicopper core forms theproduct O2 on a single active site and realizes the continuous O2 release and concomitant self-reduction.
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