Effects of support pre-calcination on the NO_x storage and reduction performance of Pt-BaO/Al₂O₃ catalysts

Effects of the crystalline structure and surface property of the Al ₂O₃ 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-Al₂O₃ phase upon further increasing the PCT to 1000°C. The surface density of Lewis acid sites of the Al₂O ₃(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.