Autoreduction of Copper in Zeolites: Role of Topology, Si/Al Ratio, and Copper Loading

Autoreduction of Cu-II-oxo species in the copper-exchanged zeolites MOR, MFI, BEA, and FAU was studied using temperature-programmed desorption of oxygen and in situ X-ray absorption spectroscopy. The zeolite topology and the Si/Al ratio have a significant effect on the onset temperature and rate of copper reduction, which originates from a different structure of the copper-oxo species, as dictated by the nature of the zeolite and the Si/Al ratio. In contrast, the autoreduction of copper hardly reacted to the variation in copper loading when the Si/Al ratio was constant, as shown for mordenite. Thus, there is a disparate effect of copper concentration on copper speciation and, consequently, on autoreduction, because of either the variation in the Si/Al ratio or incomplete exchange. Quantitative analysis revealed that the release of one oxygen molecule is accompanied by the reduction of four Cu-II ions to Cu-I.