Coating Structure Design, Action Mechanism And Application Performance Of High-efficiency Bifunctional ASC Catalysts

Ammonia(NH₃),as a typical harmful industrial waste gas,has a serious harm to human health and atmospheric environment.The ultra-low emission standard of diesel vehicles within/above the China VI emission legislation requires improving the injection amount of urea aqueous solution in SCR system,so as to achieve higher NOx purification efficiency and meet the ultra-low NOx emission requirement.However,increasing the injection of urea aqueous solution will lead to higher NH₃ slip,which will cause serious air pollution and become a new source of pollution.NH₃ oxidation catalyst(ASC),which can oxidize NH₃ to N₂,has become a necessary technology for diesel vehicle after-treatment within/above China VI legislation.Due to the poor selectivity of N₂,the application of pure precious metal catalysts(PGM)has been limited.The dual layer structure,coupled SCR and PGM catalyst,can significantly improve the selectivity of N₂.However,it still cannot solve the problem of NH₃ oxidation efficiency under low temperatures.Therefore,based on the different coupling modes of SCR and PGM catalyst,this paper developed various bi-functional ASC catalysts with different coating structures,and systematically studied the NH₃-SCO reaction performance,NH₃-SCO mechanism,thermal aging mechanism and its application on engine dyno.The main results are as follows:1.Pure SCR,pure PGM catalyst and bi-functional catalyst with different coating structure were designed and prepared.The NH₃-SCO performance of fresh and thermal aged catalysts were studied.The oxidation efficiency of NH₃,N₂ selectivity and by-products of catalysts with different coating types and coating structures were determined.ASC-MD catalyst with excellent NH₃-SCO and thermal aged performance that composite-layered structure was developed.The average NH₃ oxidation efficiency and N₂ selectivity of fresh catalyst were 85.66%and 89.25%,respectively.The average degradation of NH₃ oxidation efficiency and N₂ selectivity after thermal aging was only1.59%and 7.43%.The results showed that the high NH₃ oxidation efficiency of PGM catalyst and the high N₂ selectivity of SCR catalyst could be achieved by effective coupling together,but the coupling mode had a decisive influence on its NH₃-SCO performance and thermal aging resistance.2.The"5 mode test cycles"evaluation method was established to study the reaction path of catalysts under different atmosphere and temperatures,and to propose the NH₃-SCO reaction models of catalysts.It is found that the reaction mechanism of ASC catalyst is different under different temperatures.In the low temperature region,NH₃-SCO_PGM and NH₃-SCO_Inter are the key reactions to determine the oxidation efficiency of bi-functionnal catalysts,which is the main contribution of NOx.But the SCR reaction and interface type determine the concentration of NOx by-product of bi-functionnal catalysts.The N₂O mainly comes from PGM related reactions.3.The thermal aging mechanism of pure SCR and pure PGM catalyst were studied by means of XPS,BET,NH₃-TPD,SEM,EPMA,etc,and the effect of thermal aging on the NH₃-SCO reaction pathway was be clarifed.The thermal aging mechanism of bi-functional catalyst was explored based on the analysis of coating structures characteristics.The results showed that the thermal aging resulted in the formation of inactive Cu O and the decrease of acid sites,which lead to the significant degradation of SCR-Cu performance.The growth and surface migration of Pt particles in thermal aging process also enhanced the NH₃ oxidation efficiency of PGM-Pt catalyst.In the bi-functionnal catalyst,the performance degradation of SCR catalyst is an important factor for the decrease of oxidation efficiency and the increase of by-products.4.According to the test methods and equipment specified in the standards of?limits and measurement methods for emissions from diesel fueled heavy-duty vehicles?(China VI)(GB17691-2018),the emission performance of ASC-MD and commercial ASC-P catalysts were tested.The results showed that the NH₃ leakage of the engine equipped with the two catalysts was less than 1ppm whether the catalyst was fresh or aged,which fully meet the China VI emission standard of 10ppm.ASC-MD catalyst showed higher NH₃ oxidation efficiency and N₂ selectivity than commercial ASC-P catalyst in both fresh and aged state,which has a strong application prospect.