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

Reoxidation of Cu0 (3.1 wt%) in ZSM-5 (nSi/nAl=20) produced by over-exchange (nCu/nAl=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 (νasint) 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 CuII 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 (Eapps) 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.