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

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-Al 2 O 3 , prepared via a conventional wet impregnation method.Intriguing results emerged after subjecting the Cu/Al 2 O 3 catalyst to rapid heating and cooling during thermal aging at 900°C under 10%H 2 O/air.Notably, the integration of Cu 2+ into γ-Al 2 O 3 resulted in the preservation of a metastable low-crystalline solid with a notably higher surface area of 88 m 2 g -1 .This solid-state transformation facilitated the preferential generation of Cu + species at tetrahedral Al sites during the CO and C 3 H 6 oxidation process within the stoichiometric three-way catalytic reaction (NO-CO-C 3 H 6 -O 2 ).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 CuAl 2 O 4 and α-Al 2 O 3 , leading to a drastic reduction in surface area (9 m 2 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.