XAFS study of zirconia-supported copper catalysts for the NO–CO reaction: Deactivation, rejuvenation and stabilization of Cu species

Abstract

A Cu/ZrO₂ (1 wt.% Cu) catalyst shows high activity for the NO–CO reaction, even at low temperature (100–200 °C). However, the low-temperature activity is easily lost during prolonged reaction at 250 °C. The activity of the deactivated Cu/ZrO₂ catalyst is completely restored by NO treatment (1% in He) at >250 °C or O₂ treatment (5% in He) at 500 °C. The addition of iron (2 wt.%) to Cu/ZrO₂ causes not only a dramatic increase in the low-temperature activity but also high stability against deactivation. An X-ray absorption fine structure (XAFS) study has been conducted to reveal the local structure and chemical state of the Cu species in Cu/ZrO₂ and Cu–Fe/ZrO₂ during the reaction and rejuvenation. X-ray absorption near-edge structure (XANES) spectra and, in particular, their first derivatives are shown to be very effective for characterization of the Cu species. It is revealed that the deactivation of Cu/ZrO₂ is a consequence of reduction and sintering of highly dispersed Cu⁺ species, derived from Cu²⁺(Oct) species in an octahedral symmetry, to Cu metal particles. The Cu metal particles in a deactivated Cu/ZrO₂ catalyst are transformed into the original Cu²⁺(Oct) species by the NO treatment at 250 °C via highly dispersed Cu⁺ species. On the other hand, O₂ treatments bring about successive formation of Cu₂O at 150 °C and CuO at 250 °C. The Cu²⁺(Oct) species are reformed at >400 °C. The XAFS study of Cu–Fe/ZrO₂ suggests that the high stability of highly dispersed Cu⁺ species is a consequence of direct interaction of Cu species with iron oxide clusters. On the basis of kinetics and XAFS results, the catalytic synergies between Cu and Fe are inferred to be two-fold: stabilization of highly dispersed and catalytically active Cu⁺ species during the reaction and simultaneous participation of both Cu and Fe in the reaction.