Bis(í-oxo)dicopper in Cu-ZSM-5 and Its Role in the Decomposition of NO: A Combined in Situ XAFS, UV-Vis-Near-IR, and Kinetic Study

Abstract

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 O2 activation and (ii) while catalyzing the direct decomposition of NO into N2 and O2. 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 O2-activated overexchanged Cu-ZSM-5 sample a Cu***Cu contribution at 2.87 Å with a coordination number of 1 is found. The corresponding UV-visnear-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(í-oxo)dicopper core, i.e. [Cu2(í-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(í-oxo)dicopper core during reaction. When the Cu/Al ratio of Cu-ZSM-5 exceeds 0.2, both the bis(í-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(í-oxo)dicopper core forms the product O2 on a single active site and realizes the continuous O2 release and concomitant self-reduction.