Bis(μ-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

被引:301
作者
Groothaert, MH
van Bokhoven, JA
Battiston, AA
Weckhuysen, BM
Schoonheydt, RA
机构
[1] Katholieke Univ Leuven, Ctr Surface Chem & Catalysis, B-3001 Louvain, Belgium
[2] Univ Utrecht, Debye Inst, Dept Inorgan Chem & Catalysis, NL-3584 CA Utrecht, Netherlands
关键词
D O I
10.1021/ja029684w
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
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 (CuCu)-Cu-... contribution at 2.87 Angstrom with a coordination number of 1 is found. The corresponding UV-vis-near-IR spectrum is characterized by an intense absorption band at 22700 cm(-1) and a relatively weaker band at 30000 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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页码:7629 / 7640
页数:12
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