Study On The Mechanical Stability Of Metallic Monolithic Catalysts

Metallic monolithic catalysts have a better application prospect in the field of some high-end automotive exhaust purification,high vibration industrial catalytic reaction,etc.However,in use the active materials may spall off from their substrate due to the poor washcoat adhesion,which prevents the development of metallic monolithic catalysts.In this work,?-A12O3 washcoated on the FeCrAl alloy is used as a model to examine the failure mechanism and mechanical preformance of metallic monolithic catalysts.Then,effects of pretreatments of metallic substrate on washcoat adhesion is systematically performed.Moreover,Effects of preparations and their preparation parameters are researched by experiments.Like the cordierite monolithic catalysts,the weight losses of active material on the metallic substrates has a large scater.The heterogeneity of the mechanical stability of monolithic catalysts is therefore an intrinsic property inherited from the fatigue failure of active material.Experiments and theoretical analyses show that mechanical failure of active material washcoated on the metallic substrates is a stress-induced fatigue damage process.The spallingof active washcoat layer for metallic monolithic catalysts is mainly adhesive failure,i.e.,failure occurs at the washcoat layer/metallic substrate interface.It is shown that the weight loss data follow a unimodal Weibull distribution well,which relevels that there is one spalling mode-interface spalling.Experiments of metallic substrate pretreatments indicate that pretreatments of metallic substrate greatly improve the mechanical stability of metallic monolithic catalysts.It is shown that chemical and thermal pretreatments produce different effects on the metallic surface morphology and phase,and different mechanisms are involved in their ways of improving the washcoat adhesion.In thermal pretreatment,AI surface segregation and oxidation increase the metallic surface roughness,change the washcoat-substrate interface characteristics,reduce the amounts of fatigue failure,and thus significantly improve the washcoat adhesion.In chemical pretreatment,metallic surface corrosion is the cause of the improved washcoat adhesion.Furthermore,combination of chemical and thermal pretreatments leads to the formation of a-alumina whiskers on corroded and rough metallic surface,thus giving the most satisfactory washcoat adhesion.Preparation of washcoat layer experiments show that preparation methods of washoat layer have great impacts on the washcoat adhesion.Slurry deposition leads to uneven washcoat layer,and some cracks appear on the washcoat surface.The washcoat prepared by sol deposition is uniform.But wide cracks will appear on the surface and the washcoat layers separate from the substrate during drying and calcination process if washcoating too times.For combinations of sol deposition and slurry deposition,a sol layer is used as an intermediate layer to improve the adhesion between the washcoat layer and metallic substrate.This method results in even layer and strong bonding ability of metallic substrate and washcoat layer.It is should be pointed out that mechanical failure of metallic monolithic catalysts has no relations to washcoat loading,and depends on properties of interface between washcoat layer and metallic substrate.Compared with slurry deposition method,the sol deposition method leads to good washcoat adhesion.However,sol deposition method can not lead to high washcoat loading.Different from the above two methods,sol-slurry deposition method not only results in high washcoat loading but also good washcoat adhesion,and is therefore recommended for use in the future preparation of metallic monolithic catalysts.