Influence of Co or Ce addition on the NOx storage and sulfur-resistance performance of the lean-burn NOx trap catalyst Pt/K/TiO2-ZrO2

The Pt/K/TiO₂-ZrO₂ catalysts promoted by Co or Ce were prepared by successive impregnation or mechanically mixing method. The influence of Co or Ce addition on the NOx storage and sulfur-resistance performance of the catalyst was investigated carefully. The techniques of XRD, FT-IR, in-situ DRIFTS, H₂-TPR and XPS were employed for catalyst characterization. The Co or Ce addition can greatly improve the NOx storage capacity of Pt/K/TiO₂-ZrO₂ due to the enhanced oxidation ability and the release of more K sites. Ce addition induces higher K/Ti atomic ratio and larger NOx storage capacity as compared with Co addition. After sulfation and regeneration, the promoted catalysts shows more or less decreased NSC than Pt/K/TiO₂-ZrO₂ due to the formation of more sulfates, especially for the Co-promoted catalysts, which possess better oxidation ability and facilitate the formation of large sulfates. The effect of Ce addition on Pt/K/TiO₂-ZrO₂ largely depends on the addition mode. The high oxidation ability and the high K/Ti ratio of the mechanically prepared Ce-promoted catalyst make it still possess considerable NOx storage capacity (NSC) of 142 μmol/g after sulfation and regeneration. With the decrease of sulfur content in fuels, the Co- and Ce-promoted catalysts possessing large NOx storage capacity, will be applicable to the purification of lean-burn NOx.     

Influence of preparation conditions to structure property, NOx and SO2 sorption behavior of the BaFeO3 − x perovskite catalyst

A series of the BaFeO_3 − x perovskite catalysts was synthesized by a sol-gel method using citric acid and/or EDTA as complexants with a purpose to improve their sulfur-resistance by forming a uniform perovskite structure at a low calcination temperature, i.e. 750 °C. The thermogravimetry results show that almost no carbonate was formed after calcination of the xerogel precursor with the complexants' molar ratio of CA/EDTA ≤ 1.5, which was convinced by the in situ DRIFT spectra results of the Ba-Fe-1 catalyst during the SO₂/O₂ sorption. It indicates that, after adding EDTA into the complexants, the metal ions of the raw material could be mixed homogeneously and react stoichiometrically by calcination at 750 °C to form a uniform perovskite structure. Accordingly, the obtained Ba-Fe-1 perovskite presented a performed sulfur-resistance. Moreover, the seriously damaged structure of the BaFeO_3 − x perovskite by reduction could be in situ regenerated by calcination under lean conditions at 400 °C, which is within the operating temperature zone of the aftertreatment system of diesel to meet the real commercial demands.     

NOx storage behavior and sulfur-resisting performance of the third-generation NSR catalysts Pt/K/TiO2–ZrO2

The NO_x storage and reduction (NSR) catalysts Pt/K/TiO₂–ZrO₂ were prepared by an impregnation method. The techniques of XRD, NH₃-TPD, CO₂-TPD, H₂-TPR and in situDRIFTS were employed to investigate their NO_x storage behavior and sulfur-resisting performance. It is revealed that the storage capacity and sulfur-resisting ability of these catalysts depend strongly on the calcination temperature of the support. The catalyst with theist support calcined at 500 °C, exhibits the largest specific surface area but the lowest storage capacity. With increasing calcination temperature, the NO_x storage capacity of the catalyst improves greatly, but the sulfur-resisting ability of the catalyst decreases. In situ DRIFTS results show that free nitrate species and bulk sulfates are the main storage and sulfation species, respectively, for all the catalysts studied. The CO₂-TPD results indicate that the decomposition performance of K₂CO₃ is largely determined by the surface property of the TiO₂–ZrO₂ support. The interaction between the surface hydroxyl of the support and K₂CO₃ promotes the decomposition of K₂CO₃ to form –OK groups bound to the support, leading to low NO_x storage capacity but high sulfur-resisting ability, while the interaction between the highly dispersed K₂CO₃ species and Lewis acid sites gives rise to high NO_x storage capacity but decreased sulfur-resisting ability. The optimal calcination temperature of TiO₂–ZrO₂ support is 650 °C.