Autoreduction of Cu2+ species in Cu-ZSM-5 catalysts studied by diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, thermogravimetry and elemental analysis

The autoreduction process of Cu2+ species in Cu-ZSM-5 catalysts, with Si/Al=80 and copper exchange levels of 81 and 536% was studied by means of spectroscopic (in situ diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy) and chemical methods (thermogravimetry in N2, elemental analysis, adsorption-desorption of N2 at 77 K and adsorption of N2 at 77K). It appears that on the fresh catalysts (treated in air at 383 K) the majority of Cu2+ species may be reduced to Cu+, under vacuum or in a flow of inert gas at high temperature, by means of the carbonaceous deposits left in the zeolite synthesis. In the absence of the carbonaceous deposits (Cu-ZSM-5 treated in air at 823K) the autoreduction of Cu2+ speciesto Cu+ may occur only to a small extent in overexchanged Cu-ZSM-5 catalysts. The Cu+ ions so produced are peculiar in their ability to held N2 at 273 K. the amount of adsorbed N2, which resists evacuation at 273 K, indicates that about 2% of the Cu2+ ions may be reduced to Cu+ in overexchanged Cu-ZSM-5 (Sio/Al=80) catalysts pretreated in vacuum at 723 K.