JP2007084391A - Exhaust gas purifying device for automobile and hydrogen producing catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas purifying device having high NOx purifying performance at a low temperature, particularly at 150 to 250°C, and to provide a hydrogen producing catalyst that improves exhaust gas purifying performance at switching of lean/rich. <P>SOLUTION: The hydrogen producing catalyst is a catalyst for producing hydrogen from carbon monoxide in the exhaust gas of an internal-combustion engine and water, and includes one or more noble metals selected from Pt, Pd and Rh and a cerium oxide or a compound oxide of cerium, wherein the cerium oxide is one that has a ratio of the (111) plane diffraction peak intensity to the background of 4.3 or larger. And the exhaust gas purifying device has an above mentioned hydrogen producing catalyst and a catalyst that has a function for adsorbing or occluding nitrogen oxides. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an automobile exhaust gas purification device that efficiently purifies nitrogen oxides (hereinafter referred to as NOx) particularly at low temperatures, and a hydrogen production catalyst that produces hydrogen in automobile exhaust gas.

Reduction of CO (carbon monoxide), NOx (nitrogen oxide) and HC (hydrocarbon), which are harmful substances in automobile exhaust gas, and reduction of carbon dioxide, which is a greenhouse gas, for the purpose of preventing global warming and improving the environment It has been demanded. In order to reduce carbon dioxide from automobile exhaust, fuel efficiency must be improved. Therefore, it is necessary to operate the engine in a lean fuel atmosphere. A three-way catalyst is used for exhaust gas purification of a gasoline engine operating at a theoretical air fuel ratio (A / F = about 14.6), and CO and HC oxidation and NOx reduction are simultaneously performed using exhaust gas components. In an engine (lean burn engine) that operates at an air fuel ratio (hereinafter referred to as lean) that produces a leaner fuel atmosphere, the exhaust gas contains more oxygen than can be used in the three-way catalyst, so the reduction reaction does not proceed easily, and NOx Is difficult to purify. In order to solve these problems, NOx in the exhaust gas is adsorbed on the catalyst during lean, and lean-rich control is performed to temporarily make the air fuel ratio (hereinafter rich) of the fuel excessive from the stoichiometric air-fuel ratio. A lean NOx catalyst that reduces with HC, CO, H ₂ or the like in a rich gas has been put into practical use.

  Since the diesel engine is constantly operated lean, it is difficult to purify NOx in the exhaust gas. However, as with the lean burn engine of gasoline, NOx purification can be achieved by performing lean rich control using a lean NOx catalyst.

  A problem in applying an adsorption type lean NOx catalyst to exhaust gas treatment of a diesel engine is exhaust gas temperature. The exhaust gas temperature of the current diesel engine is 150 to 250 ° C., which is lower by about 100 ° C. than the gasoline lean burn engine. Therefore, the temperature of the catalyst heated by the exhaust gas is also lowered by about 100 ° C., and the catalytic activity is not sufficient. In particular, when performing lean / rich control, the catalytic activity is not sufficient when switching from lean to rich, so that the reduction of the adsorbed NOx is insufficient and the NOx purification rate decreases.

The rich gas contains reducing components such as HC, CO, and H ₂ . A method for reducing NOx using this reducing component has been proposed. Reactivity at a low temperature of 250 ° C. from 0.99 ° C. has a higher H ₂ than CO. Therefore, a method has been considered in which H ₂ is produced from CO and H ₂ O in exhaust gas, and NOx is reduced with the H ₂ (Japanese Patent Laid-Open No. 2001-300262 (Patent Document 1), etc.). The hydrogen production catalyst uses a noble metal as an active component, and the noble metal is supported on zirconium oxide (ZrO ₂ ), magnesium oxide (MgO), alumina (Al ₂ O ₃ ) -MgO composite oxide, TiBaO ₃ , spinel, etc. . The reaction for producing H ₂ from CO and H ₂ O is called a water gas shift reaction and is represented by the formula (1).

CO + H ₂ O → CO ₂ + H ₂ (1)
A combination of a catalyst that facilitates water gas shift reaction (hereinafter referred to as hydrogen production catalyst) and an adsorption-type lean NOx catalyst improves the reduction performance of rich NOx at low temperatures and performs lean / rich control to increase diesel engine exhaust gas. It becomes possible to purify with efficiency.

Japanese Patent Laid-Open No. 2001-300262

An object of the present invention is to provide a hydrogen production catalyst in which the water gas shift reaction is more likely to proceed and the H ₂ production capacity is improved, and an exhaust gas purification apparatus having high purification performance using the hydrogen production catalyst.

  A feature of the present invention that solves the above problems is a hydrogen production catalyst that contains one or more precious metals selected from Pt, Pd, and Rh and generates hydrogen from carbon monoxide and water, and includes cerium oxide or cerium. It is to use a composite oxide as a support.

  The used cerium oxide has high crystallinity, and specifically, the X-ray diffraction pattern has a (111) plane diffraction peak intensity / background ratio of 4.3 or more. Further, it is preferable to use a carrier obtained by adding a Ti oxide to the cerium oxide carrier. It is preferable that cerium oxide contains 28% or more of the total amount with Ti oxide.

  The hydrogen production catalyst preferably has a two-layer structure with a catalyst layer (NOx purification catalyst) having a function of adsorbing or occluding nitrogen oxides. In particular, it is preferable to coat the NOx purification catalyst on a catalyst support material (for example, cordierite, metal, honeycomb, etc.), and place the hydrogen production catalyst in contact with the NOx catalyst layer.

  Moreover, it is an exhaust gas purification apparatus for diesel engine automobiles, and includes the above-described hydrogen production catalyst and a NOx purification catalyst that purifies NOx under lean conditions. Although the respective catalysts may be mixed, it is preferable from the viewpoint of purification performance that a lean NOx catalyst is disposed after the hydrogen production catalyst. In addition, it is preferable in terms of efficiency that the hydrogen production catalyst and the lean NOx catalyst have a two-layer structure.

  According to the hydrogen production catalyst of the present invention, it is possible to provide a hydrogen production catalyst having a high hydrogen production capacity. Further, by combining the above-described hydrogen production catalyst and lean NOx catalyst with the exhaust gas purification device of a diesel engine that performs lean / rich control, it is possible to provide a purification device that efficiently performs rich NOx reduction at low temperatures. Further, it is possible to provide a diesel engine exhaust gas purification method that performs lean / rich control that can achieve high NOx purification performance.

  As a result, it is possible to provide a vehicle with low fuel consumption and less harmful substances in the exhaust gas.

  The inventors have developed a lean NOx catalyst suitable for exhaust gas treatment of a diesel engine. In the development evaluation of the lean NOx catalyst, the lean gas and rich gas simulating lean / rich control are switched to evaluate the NOx purification performance. The NOx purification performance after the durability test is greatly reduced when switching from lean to rich. The cause is considered to be deterioration of noble metals.

Since the activity of the catalyst increases with temperature, the activity at a low temperature is susceptible to the deterioration of the noble metal. Therefore, in order to obtain sufficient NOx purification performance even with deteriorated noble metals, the reducing components contained in the rich gas in the gas are utilized to purify NOx desorbed in the gas phase or from the lean NOx catalyst, thereby burdening the noble metals. I decided to reduce it. Therefore, a hydrogen production catalyst having a high ability to generate H ₂ from CO and H ₂ O in the rich gas is used. As a result, it was possible to improve the NOx purification performance when switching from lean to rich after the durability test.

  The aqueous shift reaction on cerium oxide is reported as equations (2) to (4).

CO + σ = COad (2)
H ₂ O + Ce ₂ O ₃ = 2CeO ₂ + H ₂ (3)
COad + 2CeO ₂ = CO ₂ + Ce ₂ O ₃ + σ (4)
ad: Adsorption, σ: Adsorption site of metal H ₂ is generated by (A) adsorption of CO to metal (formula (2)), (B) adsorption of H ₂ O on Ce ₂ O ₃ , and CeO ₂ And H ₂ (formula (3)), (C) oxidation of CO on the metal with CeO ₂ (formula (4)).

  In the present invention, cerium oxide having a high (111) plane peak height and background ratio is used as a support, and one or more precious metals selected from Pt, Pd, and Rh are supported. In particular, in the X-ray diffraction pattern, the ratio of the (111) plane diffraction peak intensity to the background is 4.3 or more, which is made of cerium oxide, or an oxide containing cerium and Ti oxide, and an oxide containing cerium oxide and Ti. A material in which the ratio of cerium oxide in the oxide was 28% or more was used. A high ratio between the diffraction peak and the background in the diffraction pattern indicates that the crystallinity is high. The state change of cerium oxide in the formulas (3) and (4) is continued by reacting oxygen in cerium oxide with CO on Pt. When the crystallinity is high, the movement of oxygen in the cerium oxide becomes easy, so the hydrogen production capacity seems to be high.

  As the noble metal, it was confirmed that when Pd, Pt, Rh is supported on cerium oxide, it has hydrogen production capability. These noble metals are stable even when used in automobile catalysts. The amount of precious metal is determined according to the required performance and cost. In the present invention, cerium oxide having a higher hydrogen production capacity when combined with the same amount of the same noble metal is combined. Of course, plural or all of Pd, Pt, and Rh may be supported.

  Usually, a lean NOx catalyst uses alumina having a high specific surface area as a carrier and carries a noble metal as a combustion active component, an alkali metal as a NOx adsorbent, cerium oxide, and the like. Sintering shrinkage of the carrier and aggregation of the noble metal can be suppressed by highly dispersing the noble metal in the alumina having a high specific surface area. In addition, cerium oxide takes oxygen from gas into cerium oxide in an oxygen-excess state and releases oxygen into the gas in an oxygen-deficient state, thereby leveling the gas composition and assisting oxidation / reduction on the noble metal.

In claim 2, the carrier is composed of an oxide containing cerium oxide and a Ti oxide, and a carrier having a ratio of cerium oxide in the oxide containing cerium oxide and Ti oxide of 28% or more is selected from Pdt, Pd, and Rh. It was decided to carry one or more precious metals. It has been found that even if the same cerium oxide contains Ti, the hydrogen production capability at low temperatures is improved. An improvement in hydrogen production capacity by addition of Ti was observed even when other oxides were included in cerium oxide, and was observed when the proportion of cerium oxide in the support was 28% or more. As will be described later, a catalyst in which Pt is supported on a TiO ₂ carrier has almost no hydrogen production capability. Therefore, it is presumed that Ti has a role of assisting the movement of oxygen during the shift reaction. A typical example of the case where cerium oxide contains another oxide is a Ce-Zr composite oxide. Other constituent oxides may include La, Pr, Mg, Ba and the like. The amount of precious metal is determined according to the required performance and cost. In the present invention, when the same amount is supported by the same noble metal, a material composition that increases the hydrogen production capability is described. Of course, a plurality or all of Pd, Pt, and Rh may be supported.

In Claim 3, the catalyst which combined the hydrogen production catalyst and the lean NOx catalyst was described. FIG. 2 shows the effect of hydrogen on the NOx purification performance of the lean NOx catalyst. This is the relationship between the CO conversion rate and the NOx purification rate of a two-layer catalyst in which a hydrogen production catalyst is overcoated on a certain lean NOx catalyst. The test is a rich average NOx purification rate when lean gas is circulated at 200 ° C. for 3 minutes and rich gas for 3 minutes. CO conversion is the ratio obtained by the conversion of CO in the rich gas to H _2. FIG. 2 shows that the NOx purification rate increases as the CO conversion rate increases. The hydrogen thus produced on the catalyst contributes to NOx purification of the lean NOx catalyst, and the effect becomes higher as the CO conversion rate is higher.

Nitrogen oxide adsorption / storage integrated hydrogen production catalyst is composed of a lean NOx catalyst and a hydrogen production catalyst in multiple layers in a honeycomb, a lean NOx adsorbent and a hydrogen production catalyst are mixed in a honeycomb, and a honeycomb and combustion material carrying the adsorbent Can be used in combination. In particular, the position of the lean NOx catalyst and the hydrogen production catalyst is preferably a two-layer type. By arranging the hydrogen production catalyst on the gas flow path side, CO in the rich gas is efficiently converted to H ₂ and desorbed from the lean NOx catalyst NOx to be purified can be efficiently purified.

As a method for supporting a noble metal on a Ce composite oxide (not limited to ZrO ₂ ) using the above-mentioned highly crystalline cerium oxide as a support or adding Ti in a range where cerium oxide is 28 mol% or more of the catalyst. , Impregnation and kneading methods can be used.

  As the shape of the exhaust gas purification catalyst, any of normal granular shape, extruded columnar shape, press-molded pellet shape, honeycomb-shaped honeycomb shape and the like can be used, but the honeycomb type is preferable from the viewpoint of pressure loss and the like.

  The lean NOx catalyst used together with the hydrogen production catalyst may be either an adsorption type or an occlusion type. The hydrogen production effect of the hydrogen production catalyst is useful for improving the NOx purification performance at low temperatures in any lean NOx catalyst.

An example of the production of a two-layer hydrogen oxide catalyst with integrated nitrogen oxide adsorption and storage function is shown. After forming a lean NOx catalyst layer on the cordierite honeycomb, a hydrogen production catalyst layer is formed. First, a method for producing a lean NOx catalyst layer will be described. A slurry in which alumina, alumina sol, and purified water were mixed was poured into a cordierite honeycomb and dried and fired to form an alumina layer.
An alkaline component such as Na and K was made into an aqueous solution and supported on the alumina layer by an impregnation method. When supporting an oxide having OSC (oxygen storage ability) such as cerium oxide, the alumina layer was impregnated into an aqueous solution using nitrate or the like. Then, an alumina layer was impregnated with an aqueous noble metal solution of Pd, Pt, and Rh to produce a lean NOx catalyst layer. In addition to the above impregnation method, it can also be produced by a kneading method. In the kneading method, a slurry in which an alkali component forming a lean NOx catalyst layer, an oxide containing OSC, and a noble metal are mixed is poured into a cordierite honeycomb to form a lean NOx catalyst layer. The hydrogen production catalyst was impregnated with cerium oxide with an aqueous solution of Pd, Pt, Rh, etc., and then calcined in air at 600 ° C. to obtain a hydrogen production catalyst powder. A slurry obtained by adding silica sol and purified water to a hydrogen production catalyst powder was poured into a honeycomb having a lean NOx catalyst layer to form a hydrogen production catalyst layer. In the case where the carrier of the hydrogen production catalyst is an oxide containing cerium oxide and a Ti oxide, the oxide containing cerium oxide is impregnated with TiO ₂ sol, calcined in air at 600 ° C., and then impregnated with a noble metal. And calcined to obtain a hydrogen production catalyst powder. that time,
Instead of the TiO ₂ sol, a method of mixing TiO ₂ powder in advance may be used.

  FIG. 1 shows an example of an aftertreatment system for automobile diesel engine exhaust gas. As a configuration, a three-way catalyst or an oxidation catalyst is placed in the exhaust gas passage of the engine, a diesel particulate filter (DPF) is installed in the subsequent stage to remove soot, and the nitrogen oxide of the present invention is further installed in the subsequent stage. Put hydrogen adsorption catalyst with integrated adsorption and storage function.

  The exhaust gas purification method using the catalyst of the present invention is to purify NOx discharged from the engine by passing the exhaust gas through the nitrogen oxide adsorption / occlusion function integrated hydrogen production catalyst. The exhaust gas that has passed through the hydrogen production catalyst integrated with the nitrogen oxide adsorption / storage function is characterized by being a three-way catalyst or an oxidation catalyst disposed upstream of the exhaust gas purification catalyst, and an exhaust gas that has already passed through the DPF. .

(Example)
Examples of the present invention are shown below. Note that the present invention is not limited to this example.

  In the hydrogen production catalyst integrated with the nitrogen oxide adsorption / storage function, the functions of both the lean NOx catalyst and the hydrogen production catalyst are combined to purify NOx. When the lean NOx performance is the same, it can be said that the higher the performance of the hydrogen production catalyst, the higher the NOx purification performance. In this example, the performance of the hydrogen production catalyst was compared.

  The compositions of Examples and Comparative Examples are shown in Table 2 below. Example catalyst 1 was prepared by the following procedure. As the cerium oxide, powder (manufactured by the first rare element) in which the ratio of the peak height of the (111) plane to the background was 9.5 in the XRD diffraction pattern was used. The cerium oxide powder was supported by Pt using a dinitrodiammine Pt aqueous solution to obtain a hydrogen production catalyst powder. The hydrogen production catalyst powder was prepared so that Pt was 2.7 g / L when 130 g / L of cerium oxide was supported on the honeycomb. 130 g / L of Pt-supported hydrogen production catalyst powder as cerium oxide was supported on the honeycomb by wash coating.

Example catalyst 2 used was cerium oxide (manufactured by 1st rare element) in which the ratio of the peak height of the (111) plane to the background was 9.5 in the XRD diffraction pattern of the cerium oxide powder. Ti was supported on CeO ₂ powder using a TiO ₂ sol aqueous solution. After drying and calcination, the cerium oxide powder was supported with Pt using a dinitrodiammine Pt aqueous solution to obtain a hydrogen production catalyst powder. The hydrogen production catalyst powder was prepared so that Ti in terms of element was 18.5 g / L and Pt was 2.7 g / L when 130 g / L of cerium oxide was supported on the honeycomb. On the honeycomb,
130 g / L of Pt-supported hydrogen production catalyst powder as cerium oxide was supported by a washcoat.

  Example catalyst 3 was produced in the same process as Example catalyst 2 with a Ti content of 32.7 g / L. Example catalyst 4 was produced in the same process as example catalyst 2 with the Ti content of 43.3 g / L.

In Example 5, a commercially available product (manufactured by Rhodia) was used as the Ce—Zr composite oxide. Ti was supported on the Ce—Zr composite oxide powder using an aqueous TiO ₂ sol solution. After drying and firing, Ce-
A dinitrodiammine Pt aqueous solution was used as the Zr composite oxide powder, and Pt was supported to obtain a hydrogen production catalyst powder. The hydrogen production catalyst powder was prepared so that Ti in terms of element was 5.6 g / L and Pt was 2.7 g / L when 130 g / L of Ce-Zr composite oxide was supported on the honeycomb. 130 g / L of a hydrogen production catalyst powder as a Ce—Zr composite oxide was supported on the honeycomb by wash coating.

  Example catalyst 6 was produced in the same process as example catalyst 5 with the Ti amount of 28.0 g / L. Example catalyst 7 was produced in the same process as in Example 5 with the Ti amount being 100 g / L.

Comparative Example Catalyst 1 was prepared in the same manner as Example Catalyst 1 using a commercially available cerium oxide powder having an XRD diffraction pattern with a (111) plane peak height to background ratio of 4.2. Comparative Examples 3 and 4 are cases where TiO ₂ and ZrO ₂ were used as carriers, respectively. Comparative Example 5 is a lean NOx catalyst.

  The hydrogen production capacity (reactivity of water gas shift reaction) of each hydrogen production catalyst was verified and evaluated in the following tests after endurance treatment (deterioration treatment). The gas composition of the hydrogen production catalyst evaluation test is shown in Table 1, the evaluation pattern is shown in FIG. 3, and the evaluation apparatus is shown in FIG.

In the evaluation, a fixed bed flow type reaction tube was used. The catalyst (17 mm × 17 mm × length 21 mm) was 6 ml. The evaluation was performed according to the following procedure.
(1) Stoichi temperature rise (pretreatment)
The stoichiometric gas shown in Table 1 (a) was circulated, and the temperature was increased from room temperature to 550 ° C. at a catalyst inlet temperature of 20 ° C./min. From 100 ° C., water fed by a pump was dropped into the reaction tube and added as water vapor. The SV was 30000 h ⁻¹ .
(2) Shift reaction evaluation After reaching 550 ° C., the hydrogen production catalyst evaluation gas shown in Table 1 (b) was switched to and maintained at 300 ° C., 250 ° C., 200 ° C., and 150 ° C. for 5 minutes while cooling down. The SV was set to 50000h- ¹ . After removing water vapor in the gas with a water trap at the catalyst outlet, it was led to a CO, CO ₂ analyzer (Horiba, VIA-510).

  Table 2 shows the CO conversion rates of the example catalyst and the comparative example catalyst. All were heat-treated at 700 ° C. for 20 hours. It is the result of having tested at 200 degreeC as a typical value of diesel exhaust gas temperature.

Here, the CO conversion rate is a ratio of CO in the gas converted to H ₂ . Since the gas contains 2% CO, for example, a CO conversion of 1% indicates that 1% of 2% CO has been converted to give 0.02% hydrogen. When a TiO ₂ or ZrO ₂ support was used, almost no water gas shift reaction was observed at a low temperature of at least 200 ° C. The CO conversion of the lean NOx catalyst was 0.7%.

  The CO conversion rates of Example 1 and Comparative Example 1 were 6.2 and 1.0%, respectively. Comparative Example 1 remained close to 0.7% of the lean NOx catalyst (Comparative Example 5). From these, in the XRD diffraction pattern, when cerium oxide having a high ratio of the (111) plane peak height to the background is used as the carrier, the CO conversion rate becomes high.

  The conversion rate of Examples 2 to 7 will be described. When Ti was added to cerium oxide or Ce—Zr composite oxide, the CO conversion increased. The CO conversion rate is 6.2% in Example 1, whereas 6.7 to 7.6% in Examples 2 to 4 when Ti is added to the cerium oxide support, 0.5 to 1.4. % Higher. Moreover, when adding Ti to Ce-Zr composite oxide with respect to 1.1% of Comparative Example 2, it becomes 1.2 to 2.4% of Examples 5 to 7, and 0.1 to 1.3%. It became high. That is, when the ratio of cerium oxide in the catalyst was 28% or more, the effect of increasing the CO conversion rate by adding Ti was recognized.

  It can be used for exhaust gas purification equipment for lean-burn vehicles including diesel engines.

The system diagram of diesel exhaust gas treatment. Relationship between hydrogen production capacity and NOx purification performance. Evaluation pattern of hydrogen production catalyst. Evaluation equipment for hydrogen production catalysts.

Claims (5)

A hydrogen production catalyst for producing hydrogen from carbon monoxide and water in an exhaust gas of an internal combustion engine,
The catalyst contains cerium oxide having a ratio of (111) plane diffraction peak intensity to background of 4.3 or more in an X-ray diffraction pattern as a support, and contains at least one of noble metals selected from Pt, Pd, and Rh. A hydrogen production catalyst. The hydrogen production catalyst according to claim 1,
A hydrogen production catalyst comprising the cerium oxide as a composite oxide. The hydrogen production catalyst according to claim 1 or 2,
A hydrogen production catalyst, wherein the support contains Ti oxide. A hydrogen production catalyst for producing hydrogen from carbon monoxide and water in an exhaust gas of an internal combustion engine,
The catalyst has a support containing an oxide containing cerium oxide and a Ti oxide, and a noble metal of at least one of Pt, Pd, and Rh supported on the support,
A hydrogen production catalyst, wherein a ratio of cerium oxide in a total amount of the oxide containing cerium oxide and Ti oxide is 28% or more. A hydrogen production catalyst according to any one of claims 1 to 4,
A hydrogen production catalyst characterized in that a nitrogen oxide adsorption / occlusion function catalyst having a function of adsorbing or occluding nitrogen oxides is attached to the downstream side of the exhaust gas flow path of the hydrogen production catalyst in a two-layer structure. .

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