Effects of support pre-calcination on the NOx storage and reduction performance of Pt–BaO/Al2O3catalysts

Abstract

Effects of the crystalline structure and surface property of the Al₂O₃ support on the NO_x storage and reduction (NSR) performance of Pt–BaO/Al₂O₃ catalysts were studied using catalysts prepared from a series of Al₂O₃ samples obtained by varying the calcination temperature of an Al(OH)₃ hydrogel in the range of 450–1000 °C (referred to as the pre-calcination temperature, PCT). The texture, crystalline structure and surface acidity of the Al₂O₃(PCT) supports were measured employing nitrogen adsorption–desorption, XRD, NH₃-TPD and IR spectroscopy of adsorbed pyridine, respectively. The Al₂O₃(PCT) samples showed a gradual ordering of the γ-Al₂O₃ phase when increasing the PCT from 450 to 800 °C, and a phase transition from the γ- to θ-Al₂O₃ phase upon further increasing the PCT to 1000 °C. The surface density of Lewis acid sites of the Al₂O₃(PCT) samples exhibited a maximum at PCT in the range of 800–900 °C. The NSR performance of Pt–BaO/Al₂O₃ catalysts derived from the Al₂O₃(PCT) samples was studied under cyclic lean/rich conditions. The numbers of NO_x stored and reduced on the Pt–BaO/Al₂O₃(PCT) catalysts showed similar volcano-type dependencies on PCT, peaking at PCT = 800 °C. The origins of the support PCT effect were discussed in the light of Pt particle size, the nature of BaO sites and Pt–BaO synergy. It was found that the increase in the crystallinity of the γ-Al₂O₃ phase and the surface acid site density of the supporting Al₂O₃ samples would result in improved proximity and synergy between Pt and BaO sites, leading to much more efficient NSR Pt–BaO/Al₂O₃ catalysts.