Thermal deactivation of Pd/Al2O3–Cu/Al2O3-combined three-way catalysts via Cu migration and alloying

Abstract

A Cu oxide catalyst supported on γ-Al₂O₃ (Cu/A) is a promising candidate for substituting platinum group metals in automotive three-way catalysts (TWCs). Cu/A can be used by mixing with a Pd catalyst supported on γ-Al₂O₃ (Pd/A). In this study, the effect of thermal aging on the nanostructure and TWC performance of the physically mixed powder catalyst (Pd/A + Cu/A) was investigated under different environments, that is, simulated exhaust gas mixtures dynamically fluctuating among stoichiometric, fuel-lean, and fuel-rich (SLR) compositions, compared with static fuel-lean (L) and air at 600–900 °C. The observed thermal deactivation was strongly dependent on aging atmosphere, and it increased in the sequence air < L < SLR with increasing aging temperature. Characterization studies showed Pd–Cu alloying in most deactivated SLR-aged catalysts in contrast to air-aged catalysts, which consisted of Pd oxide dispersed on the Cu²⁺-incorporated γ-Al₂O₃ phase. During SLR aging, some of the incorporated Cu²⁺ species migrated to the surface, reduced to the metallic state, and immediately reacted with Pd metal to form a Pd–Cu random alloy with face-centered cubic structure. This phenomenon severely deteriorated the NO reduction activity because the Pd surface enriched with Cu lacks NO dissociation ability. Therefore, the observed deactivation was more evident for NO reduction than C₃H₆ oxidation but was nearly negligible for CO oxidation because Cu²⁺-incorporated γ-Al₂O₃ exhibited high CO oxidation activity superior to that of Pd catalysts. Deactivation via Pd–Cu alloying was inevitable even in air- and L-aged catalysts when repeating the TWC light-off experiment under a stoichiometric gas mixture.