Influence of sulfur poisoning on NOx storage - reduction catalysts (NSR catalysts) was examined using both model gas and an actual vehicle. Deterioration of NSR catalysts is explained as the balance of sulfate formation in lean operating conditions and the amount of sulfate decomposed under rich operating conditions.
This study focused on sulfate decomposition characteristics of NSR catalysts. First, sulfate decomposition characteristics of an NSR catalyst were examined in a model gas test. It was found that the initial temperature of SOx release was higher than the sulfur poisoning temperature. Crystal growth of sulfate by increasing temperature was assumed, and hence suppressed SOx release. Second, various sulfur concentrations (8 - 500 ppm) in gasoline were used for vehicle durability. The duration of one durability cycle was 1,260 seconds, including a 60 second regeneration of sulfur poisoning (AFR 14.2, 700 °C). As a result, NOx conversion efficiency after the vehicle durability tests decreased with increasing sulfur concentration in gasoline. Sulfate particle size for vehicle aged catalysts was measured by X-Ray Diffraction (XRD). It was found that sulfate crystal growth was large for higher sulfur gasoline. Each vehicle-aged catalyst was regenerated under a rich condition, Air Fuel Ratio (AFR) 14, at 620 °C on an engine bench. The regeneration rate of aged catalysts was suppressed by increasing sulfur concentration in gasoline. It was assumed that the highly crystallized sulfate was not easily decomposed. Lowering the sulfur concentration in gasoline was effective at maintaining the NOx conversion efficiency of NSR catalysts.