Praseodymium(Pr) was impregnated to CeO2-ZrO2 solid solution by an impregnation method.The as-obtained Pr modified CeO2-ZrO2 was impregnated with 1 wt.% Pd to prepare the catalysts.The structure and reducibility of the fresh and hydrothermally aged catalysts were characterized by X-ray diffraction(XRD),Raman,X-ray photoelectron spectroscopy(XPS),CO chemisorption and H2 temperature-programmed reduction(H2-TPR).The oxygen storage capacity(OSC) was evaluated with CO serving as probe gas.Effects of impregnated Pr on the structure and oxygen storage capacity of catalysts were investigated.The results showed that the aged Pr-impregnated samples had much higher OSC and better reducibility than the unmodified ones.The scheme of structural evolutions of the catalysts with and without Pr was also established.Partial of the impregnated Pr diffused into the bulk of CeO2-ZrO2 during ageing,which inhibited the sintering,and increased the amount of oxygen vacancies in CeO2-ZrO2 support.Furthermore,those impregnated Pr species which covered on the surface of the support obstructed the strong metal-support interaction between Pd and Ce so as to reduce the encapsulation of Pd as well as the back spill-over of the oxygen during the catalytic process.
A series of Ba/CeO2 catalysts with different Ba loading amounts were prepared by incipient wetness impregnation.Their NOx adsorption behaviors under NO and NO+O2 conditions were investigated by in situ DRIFTS.It was found that NOx was adsorbed and stored in the form of nitrites and nitrates on both Ba and Ce sites on the surface of the catalysts.The less thermally stable BaCO3 was suggested to be the main active phase for NOx trapping.Ceria served primarily as an oxygen supplier in the absence of O2,and the reaction from nitrites to nitrates on Ba sites was the key step in this case.In the presence of O2,however,gaseous O2became the main oxygen source.The NOx adsorption capacity of the catalyst was dominated by the Ba content.Moreover,the stability of nitrites and nitrates formed on Ce sites was found to be lower than those formed on Ba sites which existed in the form of the ionic barium nitrate species.
Rh/ZrO_2 catalyst was prepared by the impregnation method and then aged in N2 atmosphere with 10% steam at 1050°C for 12 h. The regeneration of the aged catalyst was examined by slow cooling the catalyst in O_2 following preheating at 950°C. The as-prepared, aged, and regenerated catalysts were characterized by X-ray diffraction, Brunauer–Emmett–Teller specific surface area analysis, and(CO+NO) reaction activity tests, CO chemisorption, and transmission electron microscopy. The results revealed that the Rh species on ZrO_2 that underwent severe sintering during the high-temperature aging process could be regenerated by the slow re-oxidation process employed herein. Owing to the remarkable increase in the degree of Rh dispersion after the regeneration process, the activity could be markedly recovered. Further investigations revealed that such a regeneration phenomenon, involving the re-dispersion of the Rh species during the slow cooling process in O_2, was strongly related to the phase transformation of the ZrO_2 support and the interaction between the Rh and tetragonal ZrO_2 species.
WAN JieCAO YiDanRAN RuiLI MinXIAO YiWU XiaoDongWENG Duan
The catalytic activity and durability of Rh/ZrO_2 catalyst were investigated compared with Rh/Al_2O_3 catalyst under diverse aging atmospheres, including lean, rich and lean–rich cyclic aging atmospheres, to simulate the real working conditions of three-way catalyst.Oxidation states and microstructures of rhodium species were investigated to correlate with the catalytic performance of the catalysts. The catalytic performance and durability of the Rh catalyst under diverse aging atmospheres were drastically enhanced by ZrO_2 support. ZrO_2 support was confirmed to be able to effectively inhibit rhodium sintering even under diverse aging conditions. It can also successfully keep Rh species in an active low-valence state on the surface of the catalyst. The superiority of ZrO_2 support compared to Al_2O_3 was verified by the Rh-based monolith catalyst.
