NOx Storage Materials and Traps Resistant to Thermal Aging

Abstract

Nitrogen oxide storage materials and methods of manufacturing nitrogen oxide storage materials are disclosed. The nitrogen oxide storage materials can be used to manufacture catalytic trap disposed in an exhaust passage of an internal combustion engine which is operated periodically between lean and stoichiometric or rich conditions, for abatement of NO x in an exhaust gas stream which is generated by the engine. In one embodiment, the nitrogen oxide storage material comprises alkaline earth material supported on ceria particles having a crystallite size of between about 10 and 20 nm and the alkaline earth oxide having a crystallite size of between about 20-40 nm.

1 . A nitrogen oxide storage catalyst comprising:
 a coating on a substrate, the coating comprising a nitrogen oxide storage material comprising ceria particles having an alkaline earth oxide supported on the particles, the ceria having a crystallite size of between about 10 and 20 nm and the alkaline earth oxide having a crystallite size of between about 20-40 nm.   
     
     
         2 . The nitrogen oxide storage catalyst of  claim 1 , the coating further comprising at least one member of platinum group metals selected from the group consisting of platinum, palladium, rhodium, iridium and mixtures thereof supported on refractory oxide particles. 
     
     
         3 . The nitrogen oxide catalyst of  claim 2 , wherein the refractory oxide particles are selected from the group consisting of aluminum oxide, mixed aluminum oxide and zirconium oxide, mixed aluminum oxide and lanthanum oxide, mixed aluminum oxide and cerium oxide, mixed aluminum oxide and magnesium oxide, and alumina oxide mixed with one or more of zirconia and lanthana. 
     
     
         4 . The nitrogen oxide catalyst of  claim 2 , wherein the ceria particles have a particle size of between about 5 microns and about 50 microns and a BET surface area of between about 30 and 80 m 2 /g. 
     
     
         5 . The nitrogen oxide catalyst of  claim 4 , wherein the ceria particles have an average pore volume of about 0.3 to about 0.5 ml/g. 
     
     
         6 . The nitrogen oxide catalyst of  claim 5 , wherein the pores in the ceria particles have an average pore diameter of between about 3 nm and about 30 nm. 
     
     
         7 . The nitrogen oxide catalyst of  claim 2 , wherein the catalyst exhibits improved nitrogen oxide storage capacity after aging at 850° C. for 50 hours at a stoichiometric air fuel ratio compared with a catalyst having non-spray-dried ceria particles with baria supported on the ceria particles. 
     
     
         8 . A catalytic trap disposed in an exhaust passage of an internal combustion engine which operates periodically between lean and stoichiometric or rich conditions, for abatement of NO x  in an exhaust gas stream which is generated by the engine, comprising a catalytic trap material including a catalytic component effective for promoting the reduction of NO x  under stoichiometric or rich conditions supported on a refractory metal oxide and a NO x  storage material effective for adsorbing the NO x  under lean conditions and desorbing and reducing the NO x  to nitrogen under stoichiometric or rich conditions, the NO x  storage material comprising particles of ceria having an alkaline earth material supported on the ceria particles, the ceria having a crystallite size of between about 10 and 20 nm and the alkaline earth oxide having a crystallite size of between about 20-40 nm, and the catalytic trap material being disposed on a refractory carrier member. 
     
     
         9 . The catalytic trap of  claim 8 , the catalytic component comprising at least one member of platinum group metals selected from the group consisting of platinum, palladium, rhodium, iridium and mixtures thereof. 
     
     
         10 . The catalytic trap of  claim 9 , wherein the refractory oxide particles are selected from the group consisting of aluminum oxide, mixed aluminum oxide and zirconium oxide, mixed aluminum oxide and lanthanum oxide, mixed aluminum oxide and cerium oxide, mixed aluminum oxide and magnesium oxide, and alumina oxide mixed with one or more of zirconia and lanthana. 
     
     
         11 . The catalytic trap of  claim 9 , wherein the ceria particles have a particle size of between about 5 microns and about 20 microns. 
     
     
         12 . The catalytic trap of  claim 11 , wherein the ceria particles have an average pore volume of about 0.3 to about 0.5 ml/g. 
     
     
         13 . The catalytic trap of  claim 12 , wherein the pores in the ceria particles have an average pore diameter of between about 3 nm and about 30 nm. 
     
     
         14 . The catalytic trap of  claim 8 , wherein ceria particles are spray-dried particles. 
     
     
         15 . A method of making a nitrogen oxide storage material comprising mixing a solution of barium with ceria particles, spray drying the particles, heating the spray-dried particles, and coating the particles on a substrate. 
     
     
         16 . The method of  claim 15 , wherein the ceria particles have a surface area of between about 50 and about 150 m 2 /g prior to spray drying. 
     
     
         17 . The method of  claim 16 , wherein the ceria particles has a particle size of between about 5 microns and about 20 microns. 
     
     
         18 . The method of  claim 17 , wherein the ceria particles have an average pore volume of about 0.3 to about 0.5 ml/g. 
     
     
         19 . The method of  claim 15 , wherein the pores in the ceria particles have an average pore diameter of between about 3 nm and about 30 nm. 
     
     
         20 . The method of  claim 15 , wherein the nitrogen storage material exhibits an improved nitrogen oxide storage capacity after aging at 850° C. for 50 hours at a stoichiometric air fuel ratio compared with a catalyst having non-spray-dried ceria particles with baria supported on the ceria particles.