Study On The Co-based Low-temperature Oxidation Catalysts And The K-based Lean-burn NSR Catalysts

Catalytic purification is the most effective method to eliminate pollutants in vehicle exhaust. The conventional TWC is not efficient enough to remove CO and hydrocarbons released during cold-start period and NOx from lean-burn engines. For NOx reduction, the NOx storage-reduction (NSR) catalyst Pt/Ba/Al₂O₃ was proposed by Toyota, but the low sulfur-resisting ability limits its application. In order to solve these problems, Co-based mixed oxides are selected as the catalysts for low-temperature oxidation of CO and hydrocarbons. The compositions, structures, preparation methods of the catalysts, and reaction mechanisms are investigated. The acidic mixed oxides supported K-based NSR catalysts were prepared. The effect of additive Co or Ce on the catalyst Pt/K/TiO₂-ZrO₂ is studied. In addition, the support TiO₂-ZrO₂ was modified by Al₂O₃ doping. The existing state of the storage medium K was investigated and the distribution modes of K are proposed.A series of Co-based mixed oxides with high specific surface area were synthesized by dual surfactants-assisted method. The activities for CO and C₃H₈ oxidation are related to the mobility of Co-O bond. The dopant CeO₂ can promote the mobility of Co-O bond and then improve the activity. When La and Ce coexist in the catalyst, the activity and thermal stability can be further enhanced. The ultra-low content of noble metal Pd can promote the CO oxidation, but can hardly influence C₃H₈ oxidation. This promotion derives from oxygen spillover. The oxygen activation is the crucial step for CO oxidation while C₃H₈ oxidation depends on C-H bond activation.A series of Co-Ce-M (M=Cu, Fe, Ni or La) mixed oxides were prepared by surfactant-assisted method using co-polymer P123. The different CO oxidation mechanisms were revealed by DRIFTS. Dual active sites have formed on the Cu doped catalyst. The mobility of surface lattice oxygen is relevant with the CO oxidation activity. Cu doping improves CO oxidation activity of the catalyst calcined at 500oC or 650oC, while La doping effectively suppresses the sintering of the catalyst when it is calcined at a high temperature. Fe doping always decreases the mobility of surface lattice oxygen and then decreases the activity. Ni doping has altered CO oxidation mechanism and the weakly bound active oxygen replaces surface lattice oxygen to act as the main active oxygen species.The effect of additive Co or Ce on the storage and sulfur-resistance performance of the catalyst Pt/K/TiO₂-ZrO₂ was investigated. The results show that the Co or Ce addition can improve the storage performance. However, after sulfation and regeneration, the NOx storage capacity (NSC) of the modified catalyst is more or less lower than that of the unmodified one due to more stable sulfates are formed on the modified catalysts. The effect of Ce addition on Pt/K/TiO₂-ZrO₂ largely depends on the addition mode. The mechanically prepared Ce-promoted catalyst possesses considerable NOx storage capacity of 142μmol/g after sulfation and regeneration.The Al₂O₃ doping can obviously improve both the NOx storage capacity and sulfur-resistance performance of the catalyst Pt/K/Al₂O₃-TiO₂-ZrO₂. When the support is calcined at lower temperatures, K exists mainly in the form of–OK groups. After NOx adsorption, the main storage species are monodentate or bidentate nitrates. When the support is calcined at a higher temperature, K₂CO₃ is the dominating storage medium forming ionic free nitrates, which is more efficient for NOx storage than–OK groups. As a result, the catalyst with its support calcined at higher temperatures possesses higher NSC, however, K₂CO₃ also react with SO₂ more easily to form stable sulfates, determining its worse sulfur-resistance performance. With the increase of K loading, K₂CO₃ becomes the main storage medium with the NSC increasing, but with the sulfur-resistance decreasing.