Effect of water on the autoreduction level of copper in excessively ion-exchanged Cu/ZSM-5. An FTIR study

Thermal treatment of a calcined Cu/ZSM-5 in inert gas (or in vacuum) is known as an aprotic autoreduction process when some of the Cu2+ species are reduced to Cu+ ions. If, however, an excessively ion-exchanged, calcined 3.1 % w/w Cu/ZSM-5 (Si/Al = 20, Cu/Al = 0.75) sample was heated in Ar flow up to 773 K, the intensity of the OH band at 3611 cm(-1), A(OH)(3611), increased relatively by 5.9 to 41.6% depending on the heating period and the amount of H2O added into the Ar line. On the other hand, it was not possible to go below 5-6% increase of A(OH)(3611) even under the most careful experimental conditions. An extended period of outgassing did not cause reduction to Cu-0, i.e. the Ar line was practically free from any reducing agent (H-2 or hydrocarbon). The framework vibrations revealed the autoreduction; the increasing sharp band at 2294 cm(-1) was attributed to the first overtone of superoxide anion in an asymmetrical complex as Cu2+ O-2(-)...Cu+. CO dose at 293 K about doubled the amount of Cui ions at the expense of superoxide while a strong CO band formed at 2157 cm(-1) (A(CO) = 1.764). (C) 1997 Elsevier Science B.V.