Synthesis Of Silicon Carbide And Application In The CO And C₃H₆Oxidation Reaction

According to driving cycles test of New Europe, The emissions of CO and HC when vehicles started in the first200seconds achieved60%-80%of the total emissions. In order to solve the problem, close-coupled catalyst technology or installing a tightly coupled purification device in the engine exhaust manifold outlet was commonly used. But it maked the catalyst always to maintain the working temperature of900-1000℃, even higher temperature. Now γ-Al2O3is usually used as catalyst carrier coating. But γ-Al2O3easy turned into α-Al2O3in the high temperature, which declined the activity of catalyst as the decreased of specific surface area of γ-Al2O3could lead to sinter of noble metal. The way of preventing γ-Al2O3to turn into α-Al2O3was adding some alkaline-earth metals in the coating material. But a more effective way to solve this problem is looking for a new carrier with high stability and specific surface area in high temperature to replace γ-Al2O3. While silicon carbide (SiC) has excellent thermal stability, mechanical strength and chemical inertness, it is superior to conventional carrier as catalyst support under the reaction conditions of high temperature and makes it likely that SiC will become a potential new carrier of coating material in the automobile exhaust. However, the lower specific surface area has limited the application of SiC as catalyst support. By now, numerous studies have reported new approaches to produce high surface area SiC, including carbon template method, sol-gel method, polycarbosilane decomposition method and so on. In these ways, sol-gel method is suitable for large-scale production as lower cost. But this is only for relative to other methods, it also exist the problems of expensive raw material and complex control process.IN this paper, high specific surface area SiC was synthesized by a simple sol-gel process and carbothermal reduction method. In this method, the cheap sodium silicate and sucrose were used as silicon resource and carbon resource and HCl was used as acidity regulator while H2SO4acted as catalyst to prepare a carbonaceous silicon xerogel. The xerogel was treated by the carbothermal reaction at high temperature under argon gas condition. Then a series of palladium catalysts were prepared, using silicon carbide as catalyst support. The CO and C3H6oxidation activity of the catalysts was test under certain conditions, and the effects of the promoters on the Pd/SiC catalyst activity was also investigated in this study. The results were showed as follows:1. The results of the preparing of silicon carbide precursor and carbothermal reaction under inert gas condition showed:the specific surface area and pore volume of SiC, prepared from the precursor as n(C)/n(Si)/n(S)=3.00:1:0.44and calcined at1350℃for10h, were235m2/g and0.46cm3/g.2. Palladium catalysts were prepared using silicon carbide as catalyst support. The CO and C3H6oxidation activity of the catalysts was test under certain conditions, and the effects of the promoters on the Pd/SiC catalyst activity was also investigated. The result showed that Pd/SiC catalyst had low activity in the CO and C3H6oxidation because of the low dispersiveness of Pd on the SiC support.3. The promote behavior of CeO2on the Pd/SiC was also investigated in this study, the result showed:The addition of the CeO2could promoter the catalytic activity. When the catalyst was prepared by stepwise impregnation methods and the reduction temperature was200℃and the CeO2loading was2wt.%, the catalyst exhibited the highest catalyst activity. The lowest complete conversion temperature of CO was210℃and C3H6was215℃. The addition of the CeO2increased the Pd dispersion on SiC support and interacted with Pd in the same time, so that it improved the catalytic activity of the catalyst.4. The addition of cerium and zirconium could further improve the catalytic activity of the catalyst, as the solid solution of cerium and zirconium improved the redox of CeO2and pervented the sintering of CeO2. Compare with the catalyst of only CeO2added, The addition of cerium and zirconium further increased the Pd dispersion on SiC support and interacted with Pd, so that it immensely improved the catalytic activity of the catalyst. The lowest complete conversion temperature of CO was195℃and C3H6was200℃.