Phase changes in the Cu/Al₂O₃ catalyst during rapid and slow cooling after thermal aging at 900 °C under air

Abstract

The development of precious metal-free three-way catalysts presents numerous complex challenges, among which thermal aging is a significant hurdle. The effects of different thermal aging conditions, including varying heating and cooling rates, were studied using a Cu oxide catalyst supported on γ-Al₂O₃, prepared via a conventional wet impregnation method. Intriguing results emerged after subjecting the Cu/Al₂O₃ catalyst to rapid heating and cooling during thermal aging at 900 °C under 10 %H₂O/air. Notably, the integration of Cu²⁺ into γ-Al₂O₃ resulted in the preservation of a metastable low-crystalline solid with a notably higher surface area of 88 m² g⁻¹. This solid-state transformation facilitated the preferential generation of Cu⁺ species at tetrahedral Al sites during the CO and C₃H₆ oxidation process within the stoichiometric three-way catalytic reaction (NO–CO–C₃H₆–O₂). Consequently, a higher quantity of active sites for NO reduction was obtained, achieving a remarkable >70 % NO conversion at 600 °C. Conversely, subjecting the catalyst to slow heating and cooling (at a rate of 5 °C min⁻¹) during thermal aging at 900 °C resulted in a distinct outcome. This condition produced a thermodynamically favorable phase transition from the metastable phase to highly crystalline CuAl₂O₄ and α-Al₂O₃, leading to a drastic reduction in surface area (9 m² g⁻¹) and consequently, a notable decrease in NO reduction activity (<20 % NO conversion even at 600 °C). Furthermore, regardless of the heating rate employed during thermal aging, it was observed that a slow cooling rate of 5 °C min⁻¹ rendered the catalyst susceptible to phase transitions, resulting in lower surface areas and less active catalytic products.