Studies in Surface Science and Catalysis vol:154 pages:2449-2457
Novel spectroscopic methods were elaborated to tackle the intriguing question of the active site in Cu-ZSM-5 catalyzing the decomposition of NO. First, a DFT/ab initio approach was developed, allowing to assign the experimental EPR spectra of Cu-ZSM-5 to representative Cu-zeolite structures. Second, an optical fiber UV-vis set-up was optimized, permitting to monitor the events taking place on the catalyst under reaction conditions. The computational study showed that both EPR signals result from bare Cu(II) ions, i.e. without coordinated extra-lattice oxygen ligands. Studying the NO decomposition activity in function of the Cu/Al ratio of the samples, indicated a sharp increase in TOF between Cu/Al = 0.2 and 0.3. Concomitantly, at the latter Cu/Al ratio, an EPR silent species is formed that is characterized by an intense band at 22700 cm(-1) in UV-vis. EXAFS identified it as a dimeric Cu species with Cu --- Cu distance of 2.87 angstrom. Combining all spectroscopic data and comparing them with the well-characterized copper centers in enzymes and synthetic model complexes led to the identification of the bis(mu-oxo)dicopper core, i.e. [CU2(mu-O)(2)](2+). The operando UV-vis approach assigned the bis(mu-oxo)dicopper core as a key intermediate in the NO decomposition reaction, allowing the smooth formation and desorption of O-2.