Study On Diesel Exhaust Purifying Catalyst

Diesel exhaust can cause greenhouse effect,photochemical smog and the haze weather,harm to human health,damage the ecological environment.Techniques for diesel exhaust control include the upgrading of diesel,internal transformation of engine and tail gas after-treatment technology,catalytic technology is the mainstream technology of tail gas after-treatment technology for diesel vehicle.At present,the main diesel exhaust purification catalyst is a noble metal catalyst with the major active component of platinum,palladium,rhodium.Precious metals palladium,platinum and rhodium have similar nature.Compared to Pt and Rh,Pd is much cheaper and more abundant.More cost-effective palladium catalyst is one of the main directions of diesel engine exhaust purification catalyst for development.In order to reduce costs and ensure the catalytic activity of catalyst,the following studies were carried out in this paper.Nano-Pd particles were prepared by means of combining physical and chemical oxidation method: a mixture of tetrachloropalladate hydrogen acid and sodium borohydride with certain ratio controlled by a peristaltic pump went through the honeycomb ceramic tube with nano membrane tube under ultrasound,and Pd nanoparticles were obtained.Finally,Pd nanoparticles as the active substance were loaded on activated alumina to obtain diesel vehicle exhaust purifying catalyst.Effects of the addition of tetrachloropalladate hydrogen acid,CTAB and ultrasound on the particle size were investigated.The platform for the test of diesel exhaust gas purification catalytic effect was set up.Effects of preparation methods,calcination temperature and loading amount of active components on catalyst activity were also studied.Moreover,the stabilities of Pd catalysts were also evaluated and the optimum catalyst was obtained.Finally,catalytic efficiencies for diesel exhaust purification over Pd catalysts under different reaction temperatures were also studied and the optimum reaction temperature was also obtained.In addition,effects of the optimum reaction temperature and the concentration of oxygen in the exhaust gas on catalytic efficiency were also analyzed.Compared to commercial catalysts,palladium nanoparticles prepared in this study are more uniform and well dispersed.Starting from the nanoscale of catalyst samples had higher activity in low cost.Under the experimental conditions,the activity of Pd catalyst increased firstly,then decreased with the increase in the calcination temperature,while catalytic activity increased with the increase in Pd loading.The catalyst with 0.3 wt% Pd loading and calcined at 550℃ was the optimal catalyst.The optimum reaction temperature was obtained at 238℃,purification efficiencies of CO,HC,NO_x and PM were 67%,83%,66% and 90%,respectively.In this study,the Pd catalyst prepared by impregnation method not only has high catalytic activity but also with low Pd loading,which reduces the cost of the catalyst to achieve high environmental and economic benefits.