Study On The Ce-based Mesoporous Low-temperature Oxidation Catalysts And Ba-based NSR Catalysts

Catalytic purification of automotive exhaust is one of the most efficient methods to improve the air quality. Since the conventional three-way catalysts can not effectively reduce the emissions of CO and hydrocarbons during the cold star period, as well as NOx from the lean-burn engines, it is necessary to develop highly active and low-cost oxidation catalysts, and highly sulfur-resistant NOx storage-reduction catalysts. In this dissertation, the preparation process of the noble metal-promoted CeO₂-based mixed oxide catalysts, the component interaction, and the catalytic mechanism for CO and C₃H₈ oxidation were systematically investigated, aiming at developing new catalyst system for the abatement of hazardous emissions during cold start. Meanwhile, the effect of Fe additive and preparation method on the microstructure of the Pt/Ba/Al₂O₃ NSR catalyst were carefully probed, and the structures were well correlated with the catalytic performance for NOx storage and desulfation.Firstly, mesoporous Co₃O₄-CeO₂ mixed oxide catalysts with high surface area were successfully synthesized by a surfactant-assisted method. The Co₃O₄ crystallites in these catalysts are encapsulated by nanosized CeO₂ with only a small fraction of Co ions exposing on the surface and strongly interacting with CeO₂. Such structure maximizes the interaction between Co₃O₄ and CeO₂ in three dimensions, resulting in unique redox properties. The results of activity measurements indicated that these catalysts exhibit excellent oxidation performance. By correlating the structure with the activity, it was found that the active site requirements for CO and C₃H₈ oxidation are different. CO oxidation preferentially occurs at the interface between Co₃O₄ and CeO₂, whereas C₃H₈ oxidation takes place on the neighboring surface lattice oxygen sites in Co₃O₄ crystallites. It was also found that the introduction of a small amount of Pd to Co₃O₄-CeO₂ can remarkably promote the CO oxidation, but can hardly alter the C₃H₈ oxidation. The different behaviors for CO and C₃H₈ oxidation are determined by their different reaction mechanisms and different rate-determining steps, on this basis, two totally different kinetic reaction pathways on molecular level for CO oxidation were proposed for Co₃O₄-CeO₂ and Pd/Co₃O₄-CeO₂ catalysts.In order to simplify the preparative procedures and maintain high surface area of the catalyst at the same time, a series of Pd-prmoted MOx-CeO₂ (M=Mn, Fe, Co, Ni, Cu) mixed oxide catalysts were synthesized by the surfactant-assisted method in one step. The aim of the study was to investigate the reaction mechanism from a broad point of view, and design new catalyst system. It was found that only trace amounts of Pd species are exposed on the surface, but there is a synergistic effect between these species and 3d transition metal oxides for CO oxidation. According to the in-situ DRIFTS results, the synergistic essential originates from the interaction-assisted generation of active oxygen species between Pd and MOx, which react readily with CO, forming bedentate carbonate (1587 and 1285 cm-1) as intermediates. The extent of synergism depends on the strength of interaction. Since a solid solution is formed between CeO₂ and MnOx or FeOx, very strong interaction between Pd and MOx is generated, resulting in the greatly enhanced CO oxidation activity. The light-off temperatures for Pd-doped Mn and Fe-containing catalysts, as compared with the Pd-free catalysts, are decreased by more than 70 and 100 oC, respectively. In particular, a CO conversion as high as 80% can be achieved at room temperature over the Pd-MnOx-CeO₂ catalyst. Whereas for C₃H₈ oxidation, the C-H bond activation, but not the oxygen activation, consists of the rate-determining step. The C-H bond activation ability is largely determined by the d-electron configurations of M cations, and a double-peak phenomenon can be derived with the 3d-transition metal oxides.The influence of the introduction of Fe on the structures, NOx storage and sulfur removal performance of the Pt/Ba/Al₂O₃ catalyst was studied. The results showed that although the introduction of Fe can greatly inhabit the growth of BaSO₄ particles, it is detrimental to the NOx storage and sulfur removal. Based upon the results of EXAFS, in-situ DRIFTS and repeated H2-TPR, it was found that the interaction between Pt and Ba species is of great importance for the NOx storage and sulfur removal. The Pt-Ba interaction not only accelerates the NOx spillover which is a key step during storage, but also facilitates the selective reduction of BaSO₄ into H2S, favorable to sulfur removal and catalyst regeneration. The introduction of Fe to the Pt/Ba/Al₂O₃ catalyst decreases the Pt-Ba interaction by encapsulation of Pt in the matrix of Fe/FeOx after repeated redox cycles, leading to the decrease of NOx storage capacity and sulfur removal ability.A mesoporous NSR catalyst Pt/BaCO₃-Al₂O₃ was synthesized by using tri-block copolymer P123 as template. Systematic comparative studies were performed on the structural and catalytic performance between this catalyst and the conventional impregnated one. The results of structural characterization show that the mesoporous catalyst exhibits high specific surface area, uniform pore size and high thermal stability. The Ba-containing species are highly dispersed in three-dimensions and strongly interacted with Al₂O₃, and all the BaCO₃ presents as LT-BaCO₃ (BaCO₃ with low thermal stability). Compared with the impregnated catalyst, the mesoporous sample possesses great advantages in serving as NSR catalysts, such as enhanced NOx trapping ability, lower sulfation degree and higher desulfation extent. In addition, after NOx and SOx sorption, no bulk phase of barium nitrates and sulfates are formed in the mesopotous catalyst.