| Safety, energy conservation, and environmental protection are three important topics in the development of today's automobile industry. In recent years, diesel engines have been widely used in the automobile industry because of their attractive characteristics such as high thermal efficiency, good fuel economy, high durability, and reliability. However, diesel exhaust produces severe environment pollution and causes harm to human health. Especially, soot particles are one of the most harmful pollutants emitted by diesel engines. Owing to the growing awareness of the effects of pollutants on the environment and human health as well as to the pressure to fulfill the more and more stringent emission legislation, development of an effective diesel-exhaust purification catalyst as the three-way catalyst for gasoline-exhaust purification has really become an extremely urgent subject.Temperatures in exhaust pipes from diesel engines are generally below 400℃while the temperatures, at which the soot particles spontaneously burn without catalyst, are about 550-600℃. Therefore, for the abatement of soot particles, the use of a catalytic trap that can filter and catalytically combust soot particles at low temperatures appears to be the most plausible post-treatment technology. It has been reported that the eutectic-salt composite Cs2SO4·V2O5 seemed to be a promising catalyst because of its high thermal stability and high catalytic activity for soot oxidation under a loose contact mode between catalyst and soot particles. The high catalytic activity of the eutectic-salt composite is attributed to an improved contact between soot particle and catalyst surface. However, a conventional method for catalyst preparation is not suitable for coating active species onto catalytic filter walls.A soft-chemistry method has been used to prepare Cs2SO4·V2O5-based monolithic catalyst (CV-M, the molar ratio of Cs2SO4 to V2O5 is 0.55:0.45) instead of the conventional preparation method. In addition, binary active monolithic catalyst (Pt-CV-AM) via first wash-coatingγ-Al2O3 on the cordierite monolith and then active species Pt and CS2SO4·V2O5 has been prepared. XRD and Raman techniques have been used to characterize the structure of the prepared catalysts and SEM technique has been used to observe the surface morphology of the catalysts. The oxidation of soot particles over the developed catalysts has been investigated by both temperature-programmed and thermostatic oxidation in a fixed-bed reactor. Additionally, the effects of NO, H2O, and SO2, which are present in diesel exhausts, on the catalytic soot oxidation have been studied in detail. Moreover, the mechanism of SO2 declining the catalytic activity has been addressed.The results show that the prepared Cs2SO4·V2O5-based monolith loaded with 10 wt.% Cs2SO4·V2O5 has a high activity and the ignition temperature of catalytic soot combustion is 320℃. The addition of NO and H2O into the feed stream does not affect the catalytic activity of soot oxidation. Although in the presence of SO2 the active species VⅤin the catalyst can partially be reduced VⅣspecies, leading to the reduction of catalytic activity to some extent, the catalyst still maintains a reasonable stability in terms of preventing SO2 poisoning.The higher catalytic activity of Pt-CV-AM catalyst for soot oxidation is attributed to the more oxidative NO2 produced in the presence of NO over the Pt catalyst and the ignition temperature of soot combustion is as low as 300℃. The presence of H2O can slightly enhance soot oxidation rate. However, in the presence of SO2, in addition to poisoning the active species VⅤ, SO2 can consume more oxidative NO2, leading to the reduction of catalytic activity. |
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