Zeolite proton-assisted reoxidation of Cu⁰ in over-exchanged Cu^II-ZSM-5 reduced by hydrogen. (1) Reoxidation in Ar stream: an FTIR study

Abstract

Reoxidation of Cu⁰ (3.1 wt%) in ZSM-5 (n_Si/n_Al=20) produced by over-exchange (n_Cu/n_Al=0.75) and a subsequent reduction with H₂ (773 K, 30 min) was studied in an Ar stream using FTIR spectroscopy. Above 573 K, the decrease in the intensity of the OH stretching band at 3611 cm^-1 raised the consideration that some Cu⁰ atoms were reoxidized to copper ions by zeolitic protons. The changes in the skeletal vibrations, namely in the internal (T–O–T) asymmetric stretching vibration (ν_as^int) in the range 1050–900 cm^-1, indicated that (i) mainly Cu⁺ ions formed (975 cm^-1); however, the reoxidation did not stop at this stage: a small amount of Cu^II species (937–925 cm^-1) was also produced. (ii) More or less, electronic interactions might occur between the different copper species inside the zeolite channels. The Arrhenius plots between 573 and 773 K based on the temperature-dependent decreases in the absorbance and integrated intensity of the 3611 cm^-1 band resulted in apparent activation energies (E_apps) of 42–46 kJ mol^-1 (≈10–11 kcal mol^-1) attributed the reoxidation of Cu⁰ caused in Ar by zeolitic H⁺. Basic processes and reasonable mechanisms related to the above reoxidation of Cu⁰ to Cu⁺ are also discussed.

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