Catalytic Oxidation Of Soot Emitted From Diesel Engine Over Cobalt-based Complex Oxides

The boom of vehicles,especially for diesel engines,is due to their superior power and fuel economy.However,the PM emitted from diesel engines severely threatens both the environment and human health.Diesel Particulate filter,a possible solution to trap PM,is required to be regenerated timely to maintain its required efficiency for particle filtration.Catalytic Regeneration is a relatively feasible and economy strategy.Therefore exploration of a high efficient catalyst compatible with this route is on the schedule.Judging by the unique properties of Co₃O₄,Co–La,Co–Ce and Ag/Co–Ce composite oxides were prepared.Subsequently,the catalytic performance over these materials under O₂ and NOx atmospheres were investigated,respectively.On the basis of the above results,the effect of silver was also carefully analysed.Co–La complex oxides assist soot combustion,which can be ascribed to the strong interaction between Co₃O₄ and LaCoO₃.Co²⁺in Co₃O₄ partially replaces the La³⁺in LaCoO₃,which results in not only the shrink of LaCoO₃ lattice but the formation of Co⁴⁺and oxygen vacancies as well.This surface modification promotes the redox ability and oxygen adsorption capability,which enhances the effect of both MK mechanism and oxygen spillover mechanism.In Co–Ce mixed oxides system,cerium cations are partially incorporated into the lattice of Co₃O₄,which leads to the lattice expansion and the formation of Co–Ce–O solid solution at the interface between Co₃O₄ and CeO₂.The aforementioned surface modification consequently causes the promotion of redox ability and the improvement of the activity of lattice oxygen.In the air,the enhanced effects of both redox mechanism and oxygen spillover mechanism are involved due to the synergistic effect of the two oxides,Co₃O_4,and CeO₂.Also,reactions for yielding formate species are crucial steps for the soot–O₂ reaction.Ag loading shows a positive effect on soot combustion.In the air,the ignition temperature substantially depends on silver loading for Ag/Co–Ce system.However,there is no obvious distinction between the T₁₀0 value of0.2Ag/Co_0.93Ce_0.07 and 0.3Ag/Co_0.93Ce_0.07;their ignition temperature could be as low as 226 ^oC.The high activity of Ag–Co system can be ascribed to the electron transfer process between silver and Co₃O₄.The dissociation capability of oxygen,as well as the enhancement of redox ability via weakening the Co–O bond,is related to this complex mechanism.In O₂ atmosphere,AgO_x acts as an active site.The oxidized layer can provide its lattice oxygen to attack soot at low temperature followed by the active oxygen formation during its regeneration process.In NOx atmosphere,the redox ability and mobility of AgNO₃ seems to affect soot combustion process.Nevertheless,the inferior activity of AgNO₃ to that of Ag₂O brings about the differentiated catalytic performances under O₂ and NOx.