This invention relates to a method for regenerating the NOx catalyst in a NOx purifying system in which NOx (nitrogen oxides) in the exhaust gas of an internal combustion engine is purified by using a direct reduction type NOx catalyst, and to a NOx purifying system.
Various research efforts and proposals have been made concerning a catalyst-type exhaust gas purifying system for purifying the exhaust gas of an internal combustion engine of an automobile, a stationary type internal combustion engine or the like by reducing NOx. Especially, in order to purify the exhaust gas of automobiles or the like, a NOx occlusion reduction type catalyst, a three-way catalyst or the like have been used.
A regenerating operation is performed in the exhaust gas purifying system of an internal combustion engine provided with this NOx occlusion reduction type catalyst in its exhaust passage. This regenerating operation causes the NOx occlusion reduction type catalyst to occlude NOx when an air/fuel ratio of the exhaust gas flowing is lean. Moreover, when the NOx absorption capacity is almost saturated, the regeneration operation brings the air/fuel ratio of exhaust gas into the theoretical air/fuel ratio or rich state by reducing the oxygen concentration of the influent exhaust gas. Thus, the regenerating operation restores the NOx absorption capacity by discharging the occluded NOx, and also makes the discharged NOx reduce by a noble metal catalyst attached to the NOx purifying system.
This NOx occlusion reduction type catalyst is constituted with a noble metal catalyst such as platinum (Pt) and a NOx absorbent of an alkaline earth such as barium (Ba) on the catalyst support. Then, in the high oxygen concentration atmosphere, NO in the exhaust gas is oxidized into NO2 by the catalytic action of platinum, and this NO2 is diffused in the catalyst in a form of NO3− and absorbed in a form of nitrate.
Next, when the air/fuel ratio becomes rich and the oxygen concentration is reduced, the NO3− is discharged in a form of NO2. This discharged NO2 is reduced into N2 through a reducing agent such as unburned HC, CO or H2 contained in the exhaust gas by the catalytic action of platinum. This reducing action can prevent NOx from being discharged into the atmosphere.
This NOx occlusion reduction type catalyst entails a problem inasmuch as the absorbable NOx amount varies widely according to the temperature of the NOx absorbent, therefore, for example, the exhaust gas purifying system according to the Japanese Patent Laid-Open No. 102954/1995 is devised so as to be set to an optimal NOx absorption time by varying the NOx absorption time according to the exhaust gas temperatures.
On the other hand, separately from this NOx occlusion reduction type catalyst, there is a catalyst which directly reduces NOx (referred to hereafter as “direct reduction type NOx catalyst”). This direct reduction type NOx catalyst is the one provided with a metal such as rhodium (Rh), palladium (Pd), or the like as a catalyst component to be borne on a support such as β-zeolite. Moreover, cerium (Ce) is blended with the catalyst which contributes to maintaining the NOx reducing potential and to reduce the oxidation action of the metal, or a three-way catalyst is arranged in a lower layer in order to accelerate the oxidation-reduction reaction or especially the reducing reaction of NOx in a rich state, or iron (Fe) is added to the support in order to improve a purifying rate of NOx.
This direct reduction type NOx catalyst has the advantage of becoming less contaminated with sulfur poisoning. In a high oxygen concentration atmosphere, for example the exhaust gas of an internal combustion engine such as a diesel engine of which the air/fuel ratio is in a lean state, the NOx is directly reduced into N2. However, since O2 is adsorbed on the metal which is the active substance of the catalyst in the case of this reduction, the reducing ability is lowered.
For this reason, it is necessary to regenerate and activate the active substance of the catalyst by lowering the oxygen concentration in the exhaust gas almost to zero percent so that the air/fuel ratio of the exhaust gas becomes the theoretical air/fuel ratio or rich state. Still, this regeneration of the catalyst is speedily performed even at low temperatures (for example, 200° C. or higher) compared with the temperatures for other catalysts.
Therefore, in order to allow this direct reduction type NOx catalyst to demonstrate fully its NOx purifying performance in the NOx purifying system arranged in the exhaust passage of the engine, it is necessary to perform lean-condition control for normal driving and rich-condition control for catalyst regeneration by properly switching between them during engine operation.
However, even if rich-condition control is carried out, this direct reduction type NOx catalyst brings the following problem when the catalyst is in the high temperature range. Namely, if rich-condition control is performed for catalyst regeneration, the amount of NOx exhausted to the atmospheric air is in fact increased. Moreover, since the catalyst cannot be regenerated, the purifying performance is not recovered while at the same time fuel costs are increased.
Namely, unlike the NOx occlusion reduction type catalyst, this direct reduction type NOx catalyst does not occlude NOx by chemical combining. However, the catalyst exhibits a physical adsorption phenomenon of NOx. The relationship between this NOx adsorption quantity and the catalyst temperatures is shown in FIG. 3. Therefore, even under the rich condition, the NOx adsorption quantity is decreased due to the rising in the catalyst temperature when the catalyst temperature comes in the high temperature range. And, the portion of the NOx decreased in adsorption is exhausted. Thus, it is presumed that the NOx is increased in the exhaust quantity.
In FIG. 4, the NOx concentration at the outlet of the catalyst is shown when the engine is operated under a rich condition of the air/fuel ratio and the exhaust gas is raised in temperature at the catalyst outlet with time. According to this FIG. 4, even if a certain quantity of NOx is supplied to the NOx catalyst, the quantity of NOx at the catalyst outlet varies correspondingly to the rising in the exhaust gas temperature at the catalyst outlet. As the exhaust gas temperature rises at the catalyst outlet, the quantity of NOx more than that of NOx influent to the direct reduction type NOx catalyst from the inlet is discharged from the catalyst outlet. Especially, the discharge is remarkably increased at 420° C. or higher.