Stability And Deactivation Mechanism Of Silver Based Soot Oxidation Catalysts

Soot emission from vehicle exhausts is one of the major sources of particulate matters.Catalytic oxidation of soot is the key technology of its emission control.Silver based catalysts are highly active for soot oxidation in the absence of NOx and are cheaper than those platinum group metal catalysts,which have attracted increasing attention.However,the stability and deactivation mechanisms of silver based catalysts remain to be solved.Based on the fact that the catalytic reaction occurs as a “solid-solid-gas” system,the interactions of silver active sites with support,soot and reaction atmosphere were studied to fully understand the deactivation mechanisms of catalysts and herein provide guidelines for designing high-stable catalysts.The study of silver-support interaction was focused on the anchoring effect of surface oxygen vacancies and surface acid sites.The anchoring effect inhibited migration,collision,and aggregation of silver particles during the thermal aging and soot oxidation reaction.However,excess surface oxygen vacancies can combine the oxygen species activated by silver,leading to reduced reactivity of the catalyst.On the other hand,acid sites on the support could promote the oxidation of soot and improved the catalyst activity.The dynamic silver-soot interaction was directly observed via an environmental transmission electron microscope.The silver aggregation as well as the redispersion were observed during and after the soot oxidation process,respectively.The oxidative atmosphere and the interaction with support induced the redispersion of silver,resulting in higher silver dispersion and higher soot oxidation activity,which was similar to the silver redispersion after the thermal ageing.The silver-NOx interaction was also studied and the formation of Ag NO3 was found on the surface of silver particles by the oxidative adsorption of NO.The oxygen activation capacity of Ag NO3 was less active,and its decomposition was complete at temperatures higher than 450 °C,which was different from that of platinum nitrate at 250 °C.Hence,the NOx-assisted effect was not found over silver based catalysts.Based on the systematic study of silver based soot oxidation catalysts,three design principles were presented.(1)The concentrations of surface oxygen vacancies and surface acid sites can be tuned to achieve a balance between the catalytic activity and stability.(2)The durability of catalysts can be improved by choosing those supports that would induce silver redispersion.(3)The NOx in the atmosphere would “poison” silver catalysts via the formation of less active silver nitrate,which can be decomposed by raising the catalyst temperature above 450 °C.These ideas may be utilized for developing silver industrial catalysts for soot oxidation.