FTIR study on the interaction of O-2 and CO with Cu+ ions in zeolite ZSM-5 - Formation and reactivity of superoxide

FTIR spectroscopy was used to study the interaction of Cu+ ions with O-2 and CO. Cu+ ions were produced by outgassing of a calcined, ion-exchanged 3.1 wt% Cu-ZSM-5 sample (Si/Al = 20, Cu/Al = 0.75) in Ar flow at 773 K. This was accompanied by the increase of a sharp band at 2294 cm(-1) which disappeared in O-2 flow at 293 K and reversible recovered in Ar. A Cu2+O2-... Cu+ complex ion was suggested to interpret the results having three characteristic IR bands: at 2294 cm(-1) (first overtone of superoxide anion), 971 cm(-1) (perturbation of the internal asymmetric T-O-T vibration by Cu+ ion weakly interacted with O-2(-) anion) and 930-925 cm(-1) (relating to Cu2+ ion in the complex above). CO addition at 293 K irreversibly decomposed the superoxide structure: Cu2+O2-... Cu+ + 2CO = 2Cu(+) - CO + O-2 (bands at 2157 and 966 cm(-1)). A part of isolated Cu+ ions easily reacted with O-2 at 293 K, first probably to superoxide: Cu+ + O-2 = Cu2+O2- (band at 930-925 cm(-1)), while another part only weakly interacted: Cu+ + O-2 = Cu+... O-2 (band at 947 cm(-1)). Preadsorbed CO on Cu+ ions hindered the reaction of Cu+ with O-2 at 293 K, since it reacted only above 473 K. (C) 1997 Elsevier Science B.V.