Control Method and Control Device for Exhaust Gas Control Apparatus

Abstract

It is an object of the invention to provide a technology for appropriately recovering a decreased HC oxidizing ability of rhodium (Rh), in an exhaust gas control apparatus including a catalyst that contains rhodium (Rh) and a particulate filter (5). In this exhaust gas control apparatus, rich-spike control is prohibited and a NOx storage reduction catalyst is placed in a reduction atmosphere during a period in which a temperature of the NOx storage reduction catalyst is equal to or higher than a predetermined temperature, in a course of decreasing the temperature of the NOx storage reduction catalyst after a PM trapping ability forcible recovery process of the particulate filter (5) is completed. Thus, the decreased HC oxidizing ability of rhodium (Rh) is recovered.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a control method and control device for an exhaust gas control apparatus including a catalyst that contains rhodium (Rh), and a particulate filter.

2. Description of the Related Art

In an exhaust gas control apparatus for an internal combustion engine, which is mounted in, for example, a vehicle, a catalyst that contains platinum (Pt) deteriorates with use. With the aim of addressing such a problem, there is a known method in which, when the catalyst deteriorates, the catalyst is placed in a rich atmosphere for a predetermined period, whereby the deteriorated catalyst is recovered. A technology related to such a method is disclosed in, for example, Japanese Utility Model Application Publication No. 63-128221.

As an exhaust gas control apparatus for a compression ignition internal combustion engine (i.e., diesel engine), an exhaust gas control apparatus is known which is formed by integrally or separately arranging a catalyst that contains rhodium (Rh) and a particulate filter for trapping particulate matter (hereinafter, referred to as “PM”) in an exhaust system.

In this type of exhaust gas control apparatus, while the PM trapping ability of the particulate filter is recovered, the catalyst is exposed to a high-temperature and lean atmosphere. If the catalyst that contains rhodium (Rh) is exposed to the high-temperature and lean atmosphere, rhodium (Rh) moves to the inside of a catalyst carrier, resulting in a decrease in the NOx reducing ability of the catalyst.

Such a decreased NOx reducing ability of the catalyst is recovered, when the catalyst is exposed to a rich atmosphere at a high temperature of 400° C. or higher. However, there is a problem that, since the temperature of the exhaust gas released from the compression ignition internal combustion engine is low, increasing the temperature of the catalyst to be 400° C. or higher decreases fuel efficiency.

SUMMARY OF THE INVENTION

The invention is made in light of the above-mentioned circumstances. It is therefore an object of the invention to provide a control method and control device which can appropriately recover a decreased reducing ability of a catalyst in an exhaust gas control apparatus for an internal combustion engine, the exhaust gas control apparatus being formed by integrally or separately arranging the catalyst that contains rhodium (Rh) and a particulate filter in an exhaust system of the internal combustion engine.

According to an aspect of the invention, there is provided a control method for an exhaust gas control apparatus formed by integrally or separately arranging a catalyst that contains rhodium (Rh) and a particulate filter in an exhaust system of an internal combustion engine, characterized in that the catalyst is placed in a reduction atmosphere in a course of decreasing a catalyst temperature after a PM trapping ability forcible recovery process for the particulate filter is completed.

In order to recover the PM trapping ability of the particulate filter, the temperature of the particulate filter is increased by forcibly increasing the temperature of exhaust gas and/or forcibly increasing the amount of reaction heat in the catalyst. Thus, the so-called PM trapping ability forcible recovery process is performed so as to oxidize and remove the PM trapped in the particulate filter.

When the PM trapping ability forcible recovery process is performed, the catalyst is exposed to a high-temperature and lean atmosphere together with the particulate filter. Accordingly, rhodium (Rh) moves to the inside of a catalyst carrier. If rhodium (Rh) moves to the inside of the catalyst carrier, the reducing ability of the catalyst is decreased.

When the catalyst is exposed to the reduction atmosphere at a high temperature, the rhodium (Rh), which has moved to the inside of the catalyst carrier, outcrops to a surface of the catalyst carrier.

Note that, if the temperature of the catalyst is forcibly increased only in order to recover the decreased reducing ability of the catalyst, the fuel efficiency may be considerably decreased.

In order to address such a problem, the catalyst is placed in the reduction atmosphere in a period, in which the temperature of the catalyst has been sufficiently decreased and re-heating of the catalyst need not be performed, in the course of decreasing the catalyst temperature after the PM trapping ability forcible recovery process is completed. Thus, the decreased the reducing ability of the catalyst can be recovered by using heat obtained during the PM trapping ability forcible recovery process. As a result, an extra temperature increasing process for recovering the decreased reducing ability of the catalyst need not be performed, and a decrease in the fuel efficiency is suppressed.

The decreased reducing ability of the catalyst is appropriately recovered, when the catalyst is exposed to the reduction atmosphere at a high temperature of approximately 400° C. or higher. Accordingly, the catalyst may be placed in the reduction atmosphere in a period, in which the catalyst temperature is approximately 400° C. or higher, in the course of decreasing the catalyst temperature after the PM trapping ability forcible recovery process is completed. In this case, an amount of reducing agent required to generate the reduction atmosphere can be minimized.

In the invention, a NOx storage reduction catalyst may be used as the catalyst that contains rhodium (Rh). Since the NOx storage ability of the NOx storage reduction catalyst is limited, the NOx storage ability needs to be recovered when required, in the exhaust gas control apparatus including the NOx storage reduction catalyst.

As a method for recovering the NOx storage ability of the NOx storage reduction catalyst, so-called rich-spike control is effective. In the rich-spike control, an air-fuel ratio of the exhaust gas flowing into the catalyst is made rich by supplying a reducing agent into the exhaust gas flowing upstream of the catalyst.

In the exhaust gas control apparatus including the particulate filter and the NOx storage reduction catalyst, the rich-spike control may be performed after the PM trapping ability forcible recovery process for the particulate filter is completed.

If the rich-spike control is performed when the NOx reducing ability of the catalyst has been decreased, although the NOx stored in the NOx storage reduction catalyst is released, the released NOx cannot be reduced sufficiently. Accordingly, the NOx may be released into the air without being reduced. In addition, with an increase in the amount of NOx that has not been reduced, the amount of reducing agent that has not reacted with NOx may increase.

Meanwhile, if the rich-spike control is performed after the PM trapping ability forcible recovery process is completed, the catalyst is exposed to the high-temperature and rich atmosphere. Accordingly, the decreased NOx reducing ability of the catalyst may be recovered. In the rich-spike control, however, the air-fuel ratio of the exhaust gas is made rich intermittently, and the length of each period in which the air-fuel ratio of the exhaust gas is rich is relatively short. It is therefore difficult to sufficiently recover the decreased the NOx reducing ability of the catalyst. Further, the conventional type of rich-spike control is performed without the characteristics of rhodium (Rh) taken into consideration. Accordingly, the catalyst is not always placed in the rich atmosphere when the temperature of the catalyst in an appropriate temperature range.

According to the invention, if the catalyst that contains rhodium (Rh) is the NOx storage reduction catalyst, the rich-spike control may be prohibited after the PM trapping ability forcible recovery process is completed, whereby the catalyst is placed in the reduction atmosphere.

When the catalyst is placed in the reduction atmosphere, preferably an air-fuel ratio of the exhaust gas is made higher than that when the rich atmosphere is generated by the rich-spike control for the following reason.

If the rich atmosphere similar to that generated by the rich-spike control is formed when the NOx reducing ability of the catalyst has been decreased, a relatively large amount of NOx is released from the NOx storage reduction catalyst, and therefore an amount of NOx released into the air without being reduced may increase.

Examples of a method for placing the catalyst in the reduction atmosphere include a method in which a small amount of reducing agent is supplied to the exhaust gas at intervals shorter than those in the rich-spike control, and a method in which an air-fuel ratio in the internal combustion engine is made low.

In the exhaust gas control apparatus including the NOx storage reduction catalyst and the particulate filter, a process for recovering the NOx storage reduction catalyst from sulfur poisoning (hereinafter, referred to as a “sulfur poisoning recovery process for the NOx storage reduction catalyst”) may be performed subsequent to the PM trapping ability forcible recovery process.

The control according to the invention is different from the sulfur poisoning recovery process in the following point. In the control according to the invention, the catalyst is placed in the reduction atmosphere without forcibly increasing the temperature of the catalyst and without forcibly maintaining the temperature of the catalyst. In contrast to this, in the sulfur poisoning recovery process, the catalyst is placed in the reduction atmosphere while the temperature of the catalyst is forcibly increased and maintained.

If the sulfur poisoning recovery process is performed, the catalyst is exposed to the high-temperature and rich atmosphere. Therefore the decreased NOx reducing ability of the catalyst can be recovered.

Accordingly, when the sulfur poisoning recovery process is performed subsequent to the PM trapping ability forcible recovery process, the control according to the invention is prohibited. On the other hand, when the sulfur poisoning recovery process is not performed subsequent to the PM trapping ability forcible recovery process, the control according to the invention is performed. In this case, the catalyst is prevented from unnecessarily being placed in the reduction atmosphere, and therefore the fuel efficiency is prevented from being decreased.

According to another aspect of the invention, there is provided a control device for an exhaust gas control apparatus including a particulate filter provided in an exhaust system of an internal combustion engine, and a catalyst that is provided integrally with or separately from the particulate filter in the exhaust system and that contains rhodium, the control device being characterized by including recovery means for increasing a temperature of the particulate filter and a temperature of the catalyst, thereby forcibly recovering a PM trapping ability of the particulate filter; and NOx reducing ability recovery means for placing the catalyst in a reduction atmosphere in a course of decreasing the temperature of the catalyst after the PM trapping ability of the particulate filter is forcibly recovered.

It is to be understood that “storage” used herein means retention of a substance (solid, liquid, gas molecules) in the form of at least one of adsorption, adhesion, absorption, trapping, occlusion, and others.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned embodiment and other embodiments, objects, features, advantages, technical and industrial significance of this invention will be better understood by reading the following detailed description of the exemplary embodiments of the invention, when considered in connection with the accompanying drawings, in which:

FIG. 1 is a view schematically showing a structure of an internal combustion engine to which the invention is applied;

FIG. 2 is a graph showing a temperature at which a NOx reducing ability of a NOx storage reduction catalyst is activated;

FIG. 3 is a flowchart showing a routine of catalyst's NOx reducing ability recovery control; and

FIG. 4 is a graph showing a concrete method for performing an exhaust gas enriching process.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

In the following description, the invention will be described in more detail in terms of exemplary embodiments.

An internal combustion engine 1 shown in FIG. 1 is a compression ignition internal combustion engine (i.e., diesel engine). The internal combustion engine 1 is provided with an intake passage 2 and an exhaust passage 3. An intake throttle valve 4 is provided in the intake passage 2. A particulate filter 5 is provided in the exhaust passage 3. The particulate filter 5 supports a NOx storage reduction catalyst that contains rhodium (Rh).

A reducing agent supply valve 6, which injects fuel from the internal combustion engine 1 as a reducing agent, is provided in the exhaust passage 3 at a position upstream of the particulate filter 5. An exhaust gas temperature sensor 7 is provided in the exhaust passage 3 at a position downstream of the particulate filter 5.

An EGR passage 8 permits communication between the intake passage 2 and the exhaust passage 3. An EGR valve 9 is provided in the EGR passage 8.

Each of the intake throttle valve 4, the reducing agent supply valve 6, the exhaust gas temperature sensor 7, and the EGR valve 9 is electrically connected to an ECU 10.

The ECU 10 performs known controls such as fuel injection control and EGR control based on operation states of the exhaust gas temperature sensor 7 and the internal combustion engine 1. The ECU 10 also performs catalyst's NOx reducing ability recovery control that is a main feature of the invention. Hereafter, the catalyst's NOx reducing ability recovery control will be described in detail.

Since the PM trapping ability of the particulate filter 5 is limited, the ECU 10 performs the PM trapping ability forcible recovery process before the limit of the PM trapping ability is reached. In the PM trapping ability forcible recovery process, the ECU 10 increases the temperature of the exhaust gas and/or increases the amount of reaction heat in the NOx storage reduction catalyst by performing post-injection and/or supplying fuel from the reducing agent supply valve 6 into the exhaust gas, thereby forcibly increasing the temperature of the particulate filter 5.

When the PM trapping ability forcible recovery process for the particulate filter 5 is performed, the NOx storage reduction catalyst supported by the particulate filter 5 is also exposed to the high-temperature and rich atmosphere. At this time, rhodium (Rh) contained in the NOx storage reduction catalyst moves to the inside of the catalyst carrier. As a result, the NOx reducing ability of the NOx storage reduction catalyst (especially, a hydrocarbon (HC) oxidizing ability) is decreased.

If the HC oxidizing ability of the NOx storage reduction catalyst is decreased, the NOx reducing ability of the NOx storage reduction catalyst is decreased. Namely, if the HC oxidizing ability of the NOx storage reduction catalyst is decreased, when the NOx storage ability of the NOx storage reduction catalyst is recovered, that is, when the rich spike control, in which fuel (hydrocarbon (HC)) is intermittently supplied from the reducing agent supply valve 6 into the exhaust gas, is performed, it becomes difficult for the hydrocarbon (HC) to transform into a reaction activated substance in the NOx storage reduction catalyst. Accordingly, the NOx released from the NOx storage reduction catalyst may be released into the air without being reduced, and the hydrocarbon supplied to the NOx storage reduction catalyst may be released into the air without reacting with NOx.

FIG. 2 is a graph showing the temperature at which the NOx reducing ability of the NOx storage reduction catalyst is activated. Before the PM trapping ability forcible recovery process for the particulate filter 5 is performed, the NOx reducing ability of the NOx storage reduction catalyst is activated at a temperature of approximately 300° C. In contrast to this, after the PM trapping ability forcible recovery process is performed, the NOx reducing ability of the NOx storage reduction catalyst is not activated until the temperature increases to be approximately 350° C. or higher.

The temperature of the exhaust gas released from the compression ignition internal combustion engine is approximately 300° C. at times other than a period in which the internal combustion engine is operated at high load. As described above, an increase in the temperature, at which the NOx reducing ability is activated, increases a possibility that the amounts of NOx and HC released into the air increase when the rich-spike control is performed.

Accordingly, when the PM trapping ability forcible recovery process is performed, the decreased HC oxidizing ability of the NOx storage reduction catalyst needs to be recovered. In order to recover the decreased HC oxidizing ability, rhodium (Rh), which has moved to the inside of the catalyst carrier, needs to outcrop to the surface of the catalyst carrier again.

Rhodium (Rh), which has moved to the inside of the catalyst carrier, outcrops to the surface of the catalyst carrier, when the catalyst is exposed to the reduction atmosphere at a high temperature of 400° C. or higher. Therefore, if the exhaust gas flowing in the NOx storage reduction catalyst is made rich after the temperature of the NOx storage reduction catalyst is increased to be 400° C. or higher, the decreased HC oxidizing ability can be recovered.

Examples of an effective method for increasing the temperature of the NOx storage reduction catalyst to be 400° C. or higher, that is, a temperature in a high temperature range, include a method in which the temperature of the exhaust gas is increased by performing post injection and a method in which the amount of reaction heat in the NOx storage reduction catalyst is increased by supplying fuel into the exhaust gas. However, there is a problem common to these methods, that is, a decrease in the fuel efficiency.

Accordingly, in the catalyst's NOx reducing ability recovery control according to the embodiment, the particulate filter 5 is placed in the reduction atmosphere (rich atmosphere) in the period in which the temperature of the NOx storage reduction catalyst is 400° C. or higher, in the course of decreasing the temperature of the catalyst after the PM trapping ability forcible recovery process is completed.

Hereafter, the catalyst's NOx reducing ability recovery control will be described with reference to FIG. 3. FIG. 3 is a flowchart showing the routine of the catalyst's NOx reducing ability recovery control. The catalyst's NOx reducing ability recovery control routine is stored in ROM of the ECU 10 in advance. The catalyst's NOx reducing ability recovery control routine is an interrupt routine that is performed by the ECU 10 when the PM trapping ability forcible recovery process is completed.

In the catalyst's NOx reducing ability recovery control routine, the ECU 10 initially determines in step S101 whether a PM trapping ability forcible recovery completion flag shows “1”. The PM trapping ability forcible recovery completion flag is stored in RAM or the like in advance. When the PM trapping ability forcible recovery process is completed, “1” is stored. When the catalyst's NOx reducing ability recovery control is completed, “0” is stored.

When it is determined in step S101 that the PM trapping ability forcible recovery completion flag shows “0”, the ECU 10 ends the routine. On the other hand, when it is determined in step S101 that the PM trapping ability forcible recovery completion flag shows “1”, the ECU 10 then performs step S102.

In step S102, the ECU 10 receives a signal Tout which indicates a temperature of the exhaust gas released from the particulate filter 5 (hereinafter, referred to as “an outflow exhaust gas temperature Tout”), and which is output from the exhaust gas temperature sensor 7.

In step S103, the ECU 10 determines whether the outflow exhaust gas temperature Tout received in step S102 is equal to or higher than a predetermined temperature Ts (e.g., 400° C.).

When it is determined in step S103 that the outflow exhaust gas temperature Tout is equal to nor higher than the predetermined temperature (Tout<Ts), the ECU estimates that a bed temperature of the NOx storage reduction catalyst is lower than the predetermined temperature Ts, and then performs step S110. In step S110, the ECU 10 changes the value of the PM trapping ability forcible recovery completion flag to “0”, and then ends the routine.

On the other hand, when it is determined in step S103 that the outflow exhaust gas temperature Tout is equal to or higher than the predetermined temperature Ts (Tout≧Ts), the ECU 10 estimates that the bed temperature of the NOx storage reduction catalyst is equal to or higher than the predetermined temperature T_s, and then performs step S104.

In step S104, the ECU 10 prohibits the rich-spike control.

In step S105, the ECU 10 performs an exhaust gas enriching process for making the exhaust gas flowing into the particulate filter 5 rich. In the exhaust gas enriching process, the ECU 10 controls the reducing agent supply valve 6 such that fuel is intermittently supplied into the exhaust gas.

At this time, the ECU 10 controls the reducing agent supply valve 6 such that the amount of fuel supplied from the reducing agent supply valve 6 during each supply become smaller than that in the rich-spike control, and the interval between the fuel supplies become shorter than that in the rich-spike control, as shown in FIG. 4.

The amount of fuel supplied from the reducing agent supply valve 6 during each supply is made smaller than that in the rich-spike control for the following reason. If the same amount of hydrocarbon (HC) as that in the rich-spike control is supplied to the NOx storage reduction catalyst when the HC oxidizing ability of the NOx storage reduction catalyst has been decreased, the amount of NOx released from the NOx storage reduction catalyst increases, and the amount of NOx released into the air without being reduced also increases.

The amount of fuel supplied from the reducing agent supply valve 6 during each supply is made smaller than that in the rich-spike control also for the following reason. If the same amount of hydrocarbon (HC) as that in the rich-spike control is supplied to the NOx storage reduction catalyst when the HC oxidizing ability of the NOx storage reduction catalyst has been decreased, the amount of hydrocarbon (HC) that is released into the air without reacting with NOx may increase.

The interval between the fuel supplies is made shorter than that in the rich-spike control for the following reason. The temperatures of the particulate filter 5 and the NOx storage reduction catalyst rapidly decrease after the PM trapping ability forcible recovery process is completed. Accordingly, if the fuel is supplied with the same intervals as those in the rich-spike control, the temperature of the NOx storage reduction catalyst may decrease to be the predetermined temperature Ts or lower, before the HC oxidizing ability is recovered.

In step S106, the ECU 10 receives the signal (i.e., outflow exhaust gas temperature) Tout output from the exhaust gas temperature sensor 7 again.

In step S107, the ECU 10 determines whether the outflow exhaust gas temperature Tout received in step S106 has decreased to be lower than the predetermined temperature Ts.

When it is determined in step S107 that the outflow exhaust gas temperature Tout has not decreased to be lower than the predetermined temperature Ts (Tout≧Ts), the ECU 10 determines that the bed temperature of the NOx storage reduction catalyst is still equal to or higher than the predetermined temperature Ts, and then performs step S105 and the following steps again.

On the other hand, when it is determined in step S107 that the outflow exhaust gas temperature Tout has decreased to be lower than the predetermined temperature Ts (Tout<Ts), the ECU 10 determines that the bed temperature of the NOx storage reduction catalyst has decreased to be lower than the predetermined temperature Ts, and then performs step S108.

In step S108, the ECU 10 ends the exhaust gas enriching process.

In step S109, the ECU 10 removes prohibition of the rich-spike control.

In step S110, the ECU 10 changes the value of the PM trapping ability forcible recovery process completion flag to “0”.

When the ECU 10 performs the catalyst's NOx reducing ability recovery control routine in the above-mentioned manner, the HC oxidizing ability of the NOx storage reduction catalyst can be recovered by using the heat obtained during the PM trapping ability forcible recovery process. As a result, a decrease in the fuel efficiency due to an increase in the temperature of the NOx storage reduction catalyst can be suppressed.

In the embodiment, the amount of fuel supplied into the exhaust gas during each supply is made smaller than that in the rich-spike control. Also, in the embodiment, the interval between the fuel supplies is made shorter than that in the rich-spike control. Accordingly, the HC oxidizing ability of the NOx storage reduction catalyst can be recovered in the period in which the bed temperature of the NOx storage reduction catalyst is equal to or higher than the predetermined temperature Ts. In addition, the amount of NOx released into the air without being reduced and the amount of hydrocarbon (HC) that are released into the air without reacting with NOx can be decreased.

If the exhaust gas enriching process is performed after the PM trapping ability forcible recovery process is completed, the bed temperature of the NOx storage reduction catalyst may be maintained at a temperature equal to or higher than the predetermined temperature Ts for a long time due to the heat generated by reaction of rhodium (Rh) and hydrocarbon (HC). In such a case, any one of the following methods may be employed; (1) the exhaust gas enriching process is completed when the performance time of the exhaust gas enriching process becomes equal to or longer than a predetermined time, (2) the temperature of the NOx storage reduction catalyst is gradually decreased by decreasing the fuel supply amount with an increase in the number of times of fuel supply, and (3) an interval is provided every time fuel supply has been performed a predetermined number of times such that the temperature of the NOx storage reduction catalyst is decreased in a stepwise manner.

In the embodiment, supplying fuel into the exhaust gas from the reducing agent supply valve 6 is employed as a concrete method for performing the exhaust gas enriching process. However, the air-fuel ratio of the exhaust gas released from the internal combustion engine 1 may be decreased by increasing the amount of the EGR gas.

In the embodiment, the particulate filter 5 and the NOx storage reduction catalyst are integrally provided in the exhaust passage 3. However, the particulate filter 5 and the NOx storage reduction catalyst may be separately provided in the exhaust passage 3.

For example, the particulate filter 5 and the NOx storage reduction catalyst may be provided in the exhaust passage 3 in series (preferably, the NOx storage reduction catalyst is provided upstream of the particulate filter 5). Note that, in this case, the reducing agent supply valve 6 needs to be provided upstream of the NOx storage reduction catalyst.

Hereafter, the other embodiments will be described.

When the amount of sulfur contained in the fuel used in the internal combustion engine 1 is large, sulfur poisoning (i.e., S poisoning) occurs in the NOx storage reduction catalyst. Accordingly, the sulfur poisoning recovery process may be performed subsequent to the PM trapping ability forcible recovery process.

In the sulfur poisoning recovery process, the catalyst is placed in the rich atmosphere while the temperature of the NOx storage reduction catalyst is maintained at a high temperature. Accordingly, the decreased HC oxidizing ability of the NOx storage reduction catalyst can be recovered.

Therefore, when performing the sulfur poisoning recovery process subsequent to the PM trapping ability forcible recovery process, the ECU 10 prohibits the catalyst's NOx reducing ability recovery control. On the other hand, when not performing the sulfur poisoning recovery process subsequent to the PM trapping ability forcible recovery process, the ECU 10 performs the catalyst's NOx reducing ability recovery control.

In this case, the catalyst's NOx reducing ability recovery control is prevented from being unnecessarily performed. Accordingly, fuel consumption due to the catalyst's NOx reducing ability recovery control can be suppressed.

Claims

1. A control method for an exhaust gas control apparatus formed by integrally or separately arranging a catalyst that contains rhodium and a particulate filter for trapping particulate matter, in an exhaust system of an internal combustion engine, comprising: performing a particulate matter trapping ability forcible recovery process for forcibly recovering a particulate matter trapping ability of the particulate filter by increasing a temperature of the particulate filter and a temperature of the catalyst, and placing the catalyst in a reduction atmosphere in a course of decreasing the temperature of the catalyst after the particulate matter trapping ability forcible recovery process is completed.

2. The control method for an exhaust gas control apparatus according to claim 1, wherein the catalyst is placed in a reduction atmosphere in a period in which the temperature of the catalyst is equal to or higher than a predetermined temperature.

3. The control method for an exhaust gas control apparatus according to claim 2, wherein the predetermined temperature is approximately 400° C.

4. The control method for an exhaust gas control apparatus according to claim 1, wherein: the catalyst is a NOx storage reduction catalyst, and a process for recovering a NOx storage reduction ability of the catalyst is prohibited from being performed, when the catalyst is placed in the reduction atmosphere.

5. The control method for an exhaust gas control apparatus according to claim 4, wherein an air-fuel ratio of exhaust gas when the catalyst is placed in the reduction atmosphere is made higher than an air-fuel ratio of the exhaust gas during the process for recovering the NOx storage reduction ability of the catalyst.

6. A control device for an exhaust gas control apparatus, comprising: recovery portion that forcibly recovers a particulate matter trapping ability of a particulate filter that is provided in an exhaust system of an internal combustion engine by increasing a temperature of the particulate filter and a temperature of a catalyst that is provided integrally with or separately from the particulate filter in the exhaust system and that contains rhodium; and NOx reducing ability recovery portion that places the catalyst in a reduction atmosphere in a course of decreasing the temperature of the catalyst after the particulate matter trapping ability of the particulate filter has been forcibly recovered.

7. The control method for an exhaust gas control apparatus according to claim 2, wherein: the catalyst is a NOx storage reduction catalyst, and a process for recovering a NOx storage reduction ability of the catalyst is prohibited from being performed, when the catalyst is placed in the reduction atmosphere.

8. The control method for an exhaust gas control apparatus according to claim 3, wherein: the catalyst is a NOx storage reduction catalyst, and a process for recovering a NOx storage reduction ability of the catalyst is prohibited from being performed, when the catalyst is placed in the reduction atmosphere.

9. The control method for an exhaust gas control apparatus according to claim 7, wherein an air-fuel ratio of exhaust gas when the catalyst is placed in the reduction atmosphere is made higher than an air-fuel ratio of the exhaust gas during the process for recovering the NOx storage reduction ability of the catalyst.

10. The control method for an exhaust gas control apparatus according to claim 8, wherein an air-fuel ratio of exhaust gas when the catalyst is placed in the reduction atmosphere is made higher than an air-fuel ratio of the exhaust gas during the process for recovering the NOx storage reduction ability of the catalyst.