Study On Eutectic Composite Catalysts Applied To The Abatement Of Soot Particles From Diesel Engines

Diesel engines have been widely used in passenger cars and trucks because of their attractive characteristics such as high thermal efficiency, fuel economy, and durability. Due to lower exhaust temperatures and higher air/fuel ratios as diesel engines powered, a catalytic system such as the TWC (three-way catalyst) converter, which is currently used in emission control as gasoline engines powered, is not suitable for soot abatement in diesel exhausts. The growing awareness of the effects of pollutants on the environment and human health has made the legislation of emission control more and more stringent. Therefore, it is a great challenge for the automotive industry to develop a reliable technology to abate the soot particles emitted from diesel engines.A temperature range in exhaust pipes from diesel engines is generally within 170-400℃, while the ignition temperature of soot particles is in a range of 550-600℃. Therefore, a catalytic system is required in order to combust soot particles at lower temperatures. The combination of traps and oxidation catalysts appears to be the most plausible post-treatment technique to abate soot particles. It has been reported that the eutectic-salt composite Cs₂SO₄·V₂O₅ seemed to be a promising catalyst for the oxidation of diesel soot. However, systematic studies on the application of the catalysts Cs₂SO₄·V₂O₅ to soot combustion have not been carried out and yet catalytic filters coated with Cs₂SO₄·V₂O₅ have not been developed for the abatement of diesel soot, because a conventional method to prepare the catalysts using a solid-solid mixing cannot be applied to coatings of the catalysts onto the filter walls.Cs₂SO₄·V₂O₅ with different ratios of Cs₂SO₄ to V₂O₅, eutectic salts, have been systematically studied in terms of their preparation, characterization, and catalytic properties as used in soot oxidation in the current work. A sol-gel method has been used to prepare the catalysts Cs₂SO₄·V₂O₅ instead of the conventional preparation method in order to find a way to coat Cs₂SO₄·V₂O₅ onto the walls of a filter. The ratios of Cs₂SO₄ to V₂O₅ and calcination temperature during the preparation of the eutectic salts Cs₂SO₄·V₂O₅ on their catalytic properties for soot oxidation have been systematically investigated. The Pt, K, Ce-doped Cs₂SO₄·V₂O₅ catalysts have been prepared and characterized as well. Additionally, the effects of NO_x and SO₂, which are present in diesel exhausts, on soot oxidation over the prepared catalysts have been studied in detail. The TG-DTA and XRD techniques have be,en used to characterize the structure of the catalysts and the SEM technique has been used to observe the morphology of the model soot particles and the catalysts. The TG technique and a fixed-bed reactor have been used to investigate the activity and stability of the prepared catalysts. Moreover, the relationship between melting point and catalytic activity of the eutectic salts Cs₂SO₄·V₂O₅ has been studied and the effects of a contact mode, either loose or tight contact, between catalyst and soot particles has been investigated.The results show that as a molar ratio of Cs₂SO₄ to V₂O₅ being equal to 0.55:0.45 and calcined at 600℃, the resulting catalyst has the best catalytic activity. The ignition temperature of soot combustion is 320℃in the presence of NO_x. The research results indicate that catalytic activity relates with not only molar ratios and gas compositions, but also the contact mode between catalysts and soot particles.The doped Pt, K and Ce into the Cs₂SO₄·V₂O₅ catalysts do not improve the catalytic activity for soot combustion. The Pt-doped Cs₂SO₄·V₂O₅ catalysts are unable to oxidize NO to NO₂. Although the K-doped Cs₂SO₄·V₂O₅ catalysts have the same activities as the undoped ones, they become unstable due to the decomposition of KNO₃ at the combustion temperature. In addition, ceria is a major form in the Ce-doped catalysts, and their catalytic activity decreases due to the sintering of CeO₂ at high temperatures.