JP4469164B2 - Method for producing exhaust gas purification catalyst - Google Patents

Description

  The present invention relates to a method for producing an exhaust gas purification catalyst for an internal combustion engine and an exhaust gas purification catalyst.

Conventionally, as an exhaust gas purifying catalyst for internal combustion engines such as automobiles, three-way purification is performed by simultaneously oxidizing carbon monoxide (CO) and hydrocarbons (HC) and reducing nitrogen oxides (NO _x ). A catalyst is used. As such a catalyst, for example, a porous layer made of a porous material such as γ-alumina is formed on the cell wall of a honeycomb monolith support made of heat-resistant inorganic ceramics made of cordierite, and the porous layer. A catalyst in which a noble metal catalyst such as Pt, Pd, or Rh is supported is known.

  Here, as a method for forming a porous layer on the monolith support or for supporting a catalyst component on the porous layer, a catalyst support such as a monolith support or a porous layer is immersed in a coating solution containing the catalyst component. A method of drying is known.

Patent Document 1 describes that when a catalyst component composed of a noble metal and an NO _x absorbent is supported on a porous layer, it is preferably dispersed uniformly. The NO _X absorbent is improved NO _X purification rate by causing disposed in the vicinity of the precious metal.
Japanese Patent Laid-Open No. 7-233204

  However, as a result of studies by the inventors of the present invention on a conventional method for producing an exhaust gas purification catalyst, it has been found that there is room for further uniform loading of the catalyst component on the catalyst carrier. That is, in the conventional manufacturing method, even if the catalyst component to be coated is prepared uniformly, the catalyst component is segregated on the catalyst carrier and it is difficult to uniformly disperse it.

  In view of the above circumstances, the present invention provides a catalyst for exhaust gas purification in which catalyst components are dispersed in a catalyst carrier at a higher level than before, and can provide a higher catalytic ability, and a method for producing such an exhaust gas purification catalyst. It is a problem to be solved.

  As a result of intensive studies conducted by the inventors of the present invention in order to solve the above-mentioned problems, it has been found that a drying step can be cited as one of the causes of segregation of catalyst components. Conventional catalyst components were dried with hot air or infrared rays using a drying oven. However, it was discovered that segregation can be eliminated by drying using electromagnetic waves having longer wavelengths than infrared rays such as microwaves.

  In hot air drying and infrared drying, energy transfer proceeds on the surface, so that heating and evaporation of moisture proceeds from the surface. Therefore, it is considered that segregation occurs due to the movement of moisture that diffuses and moves to the surface side as the moisture evaporates, and the catalyst component coated by this flow also moves. On the other hand, electromagnetic waves having a long wavelength such as microwaves can uniformly penetrate into liquids such as water, so that moisture can be heated uniformly. Therefore, it is considered that the flow accompanying the evaporation of moisture does not occur, and segregation of catalyst components does not occur. The present invention was completed based on this discovery.

That is, the method for producing an exhaust gas purifying catalyst of the present invention comprises a coating step of coating a catalyst carrier with a coating liquid containing a catalyst component,
A drying step of drying and removing the liquid medium contained in the coating liquid by electromagnetic wave irradiation having a wavelength of 10 ⁴ μm to 10 ⁸ μm,
The catalyst component supported on the catalyst carrier is selected from the group consisting of at least one first element selected from the group consisting of Pt, Pd, Rh, and Ir, and an alkali metal, alkaline earth metal, rare earth, and base metal. Consisting of at least one second element,
The coating liquid includes the first element as a salt, after the coating process, it has rows electromagnetic radiation having a wavelength of 10 ⁴ μm~10 ⁸ μm with heating,
The salt of the first element is thermally decomposed while being irradiated with electromagnetic waves .

  The “coating solution containing a catalyst component” in the present invention includes a solution that is solubilized as a solution in water as a liquid medium, such as a dinitroaminoplatinic acid aqueous solution, and a slurry containing cerium oxide or the like. Also includes suspensions of particulate materials. The “catalyst component” in this case is dinitroaminoplatinic acid or platinum in the former case, and cerium oxide or cerium in the latter case. That is, the present invention can be applied both when the slurry is coated (supported) and when the solution is coated. The present invention exerts an effect of improving the uniformity of dispersion of the catalyst component coated on the catalyst carrier by being applied to support of a liquid substance (including a coating liquid: including a solution and a slurry) on the catalyst body.

Here, it is more preferably pre-Symbol electromagnetic wave is a microwave.

  The catalyst component to be supported is at least one element selected from the group consisting of Pt, Pd, Rh and Ir, and at least one type selected from the group consisting of alkali metals, alkaline earth metals, rare earths and base metals. It is preferable to consist of an element. Since both can be supported in a uniformly mixed state, a synergistic effect of both can be expected.

  When the liquid medium contains water and / or alcohol, the liquid medium can be efficiently heated by the electromagnetic wave. The porous material is preferably composed of one or more heat-resistant inorganic oxides selected from the group consisting of alumina, silica, zirconia, titania and zeolite.

The manufacturing method of the exhaust gas purifying catalyst of the present embodiment includes a coating process and a drying process. Having the characteristic drying process is the main point of the manufacturing method of this embodiment. The production method of the present embodiment can be applied to general exhaust gas purifying catalysts such as a three-way catalyst, a diesel engine particulate filter (DPF), and a three-way catalyst having an NO _x absorber. The catalyst for exhaust gas purification produced by this production method has high uniformity of dispersion of the catalyst components even compared with the conventional exhaust gas purification catalyst.

(Coating process)
The coating step is a step of coating the catalyst support with a coating liquid. The method of coating is not particularly limited, and general methods such as a method of immersing the catalyst support in the coating liquid and a method of spraying the coating liquid onto the catalyst support can be employed.

  The coating liquid contains a catalyst component. Coating liquids include solutions and slurries. The solution is a solution in which a catalyst component is dissolved in a liquid medium. The slurry is a slurry in which a catalyst component is suspended in a liquid medium. The liquid medium is preferably a liquid having a high polarity, such as water or alcohol, which is easily heated by dielectric irradiation.

Catalyst components used as a solution include noble metals such as Pt, Rh, Pd, Ir, and Ru, alkali metals such as Li, K, and Na, alkaline earth metals such as Ba, Sr, and Ca, La, Y, and Ce. NO _X absorbent composed of a rare earth, such as base metals and the like. In particular, it contains at least one element selected from the group consisting of Pt, Pd, Rh and Ir, and at least one element selected from the group consisting of alkali metals, alkaline earth metals, rare earths and base metals. preferable. The elements contained in these catalyst components are made into salts, such as nitrates, acetates, sulfates, chlorides or complex salts, and dissolved in a liquid medium such as water or alcohol to form a coating solution.

  As the catalyst component used as the slurry, inorganic ceramics such as alumina, silica, titania, zeolite, silica alumina, and zirconia can be employed. Furthermore, compounds that do not dissolve in the liquid medium, such as oxides, are included even if they are the elements listed as the catalyst component used as the above solution. For example, Ce oxide.

The amount of the catalyst component made of a noble metal is preferably about 0.1 g / L to 10.0 g / L. When using 2 or more types together, the total is preferably about 0.1 g / L to 10.0 g / L. In particular, about 0.5 g / L to 3.0 g / L is preferable. The amount of the catalyst component composed of the NO _x absorbent is preferably about 0.1 mol / L to 1.0 mol / L. Even when using 2 or more types together, it is preferable to set it as about 0.1 mol / L-1.0 mol / L.

  As the catalyst carrier, a monolithic carrier made of heat-resistant ceramics such as cordierite, a metal carrier, or the like can be used. The shape of the catalyst carrier is not particularly limited, but a honeycomb shape (straight flow type, wall flow type for DPF, etc.), a pellet shape, and the like can be employed. The surface of the catalyst support can have a porous layer formed for the purpose of improving the surface area. The porous layer can be composed of inorganic ceramics such as alumina, silica, titania, zeolite, silica alumina, zirconia. The method for forming the porous layer on the catalyst surface is not particularly limited, and the porous layer can be formed by coating the slurry containing the inorganic ceramics and the like, followed by drying and firing. The production method of the present invention can also be applied to the formation of this porous layer. That is, it is preferable to irradiate the electromagnetic wave of the conditions mentioned later when drying (drying process) after slurry coating.

(Drying process)
The drying process is a process of drying and removing the liquid medium by irradiating an electromagnetic wave having a wavelength of 100 μm or more. Irradiation of electromagnetic waves is performed uniformly on the coated coating solution. Since the liquid medium contained in the coating liquid can be uniformly heated and evaporated by irradiating the coating liquid with electromagnetic waves uniformly, segregation of the catalyst component due to non-uniform evaporation of the liquid medium can be suppressed.

The wavelength of the electromagnetic wave to be irradiated is preferably 10 ⁴ μm to 10 ⁸ μm. In particular, the wavelength is more preferably 10 ⁵ μm to 10 ⁷ μm.

  Hot air or the like can be blown along with the drying process. Uniform drying can be achieved by quickly removing the liquid medium evaporated by the irradiation of electromagnetic waves. As a result, segregation of the catalyst component can be suppressed.

(Other processes)
There is no particular limitation after the drying step. For example, heating can be performed to perform pyrolysis of a salt contained as a catalyst component, firing of inorganic ceramics, and the like. However, since the liquid medium is removed by the drying process, there is little possibility of segregation of the catalyst component by subsequent heating. Accordingly, a general heating furnace or the like can be adopted as a heating method.

( Reference Example 1)
120 parts by mass of γ-alumina as a porous material, 10 parts by mass of cerium oxide and 10 parts by mass of acidic alumina sol and 150 parts by mass of ion-exchanged water as a liquid medium are mixed and pulverized for 6 hours as a coating liquid. A slurry was prepared.

DPF as a catalyst carrier (manufactured by NGK; cordierite, φ129 mm × length 150 mm, number of cells 46.5 cells · cm ⁻² (300 cells · in ⁻² ), porosity 60%, pore diameter 25 μm, volume 2 L) After dipping in the slurry, excess slurry was blown off by air blow. Excess water was dried by heating at 250 ° C. for 1 hour, and then calcined at 500 ° C. for 1 hour to obtain a porous catalyst carrier having a porous layer formed on the cell surface as the catalyst carrier. These drying and baking were performed in a heating furnace.

  A porous catalyst carrier is immersed in a dinitroaminoplatinic acid aqueous solution as a coating liquid containing Pt as a catalyst component, and Pt is supported at 2 g / L, and then heated at 250 ° C. for 1 hour to obtain Pt. The salt was decomposed to carry Pt metal.

  Further, a porous catalyst carrier supporting Pt metal was immersed in an aqueous solution as a coating solution containing Ba and K as catalyst components as Ba acetate and K nitrate. Excess liquid was blown away by air blow (coating process).

Water was dried by treatment for 10 minutes in a microwave drying apparatus with an output of 1200 kW and a wavelength of 10 ⁵ μm (drying step). The supported amounts of Ba and K were 0.1 mol / L and 0.05 mol / L, respectively. Further, a catalyst of Reference Example 1 was prepared by performing heat treatment at 350 ° C. in a heating furnace to decompose Ba acetate and K nitrate.

( Reference Example 2)
The same procedure as in Reference Example 1 was used except that a straight flow type catalyst support (φ129 mm × length 150 mm, cell number 93 cells · cm ⁻² (600 cells · in ⁻² )) was used instead of DPF. The catalyst of the reference example was prepared.

( Reference Example 3)
A catalyst of this reference example was prepared in the same manner as in Reference Example 1 except that oxalic acid V was used instead of nitric acid K.

( Reference Example 4)
A catalyst of this reference example was prepared in the same manner as in Reference example 1 except that ferric nitrate was used instead of nitric acid K.

( Reference Example 5)
A catalyst of this reference example was prepared in the same manner as in Reference Example 1 except that Ce nitrate was used instead of nitric acid K.

( Reference Example 6)
A catalyst of this reference example was prepared in the same manner as in Reference Example 1 except that a mixed aqueous solution of dinitroaminoplatinic acid and rhodium nitrate was used instead of the dinitroaminoplatinic acid aqueous solution. Platinum and rhodium were supported on the catalyst support in a mass ratio of 3: 1 and a total amount of 2 g / L.

( Reference Example 7)
A catalyst of this reference example was prepared in the same manner as in Reference Example 1 except that the treatment was performed for 10 minutes using a high-frequency drying apparatus having an output of 2000 kW and a wavelength of 10 ⁷ μm instead of the microwave drying apparatus.

( Reference Example 8)
A catalyst of this reference example was prepared in the same manner as in Reference example 1 except that warm air of about 80 ° C. was sent from below while drying with microwaves in a microwave drying apparatus and dried for 20 minutes.

(Example 1 )
The catalyst of this example was prepared in the same manner as in Reference Example 1 except that the Pt salt was decomposed at 250 ° C. after supporting Pt with an aqueous dinitroaminoplatinic acid solution and treated for 20 minutes in a microwave dryer. Prepared.

(Comparative Example 1)
A catalyst of this comparative example was prepared in the same manner as in Reference Example 1 except that it was dried for 20 minutes by an infrared drying apparatus having an output of 2000 kW and a wavelength of 10 μm instead of the microwave drying apparatus.

(Comparative Example 2)
Reference Example 1 except that the catalyst was placed on a mesh belt using a hot air type gas furnace instead of the microwave dryer and dried from room temperature to 250 ° C. at a heating rate of about 30 ° C./min. A catalyst of this comparative example was prepared in the same manner.

(Comparative Example 3)
The catalyst of this comparative example was prepared in the same manner as in Reference Example 1 except that it was dried from room temperature to 250 ° C. at a heating rate of about 30 ° C./min using a box type electric furnace instead of the microwave drying apparatus. Was prepared.

(Comparative Example 4)
The catalyst of this comparative example was prepared in the same manner as in Reference Example 2 except that it was dried from room temperature to 250 ° C. at a heating rate of about 30 ° C./min using a box-type electric furnace instead of the microwave drying apparatus. Was prepared.

(An endurance test)
The catalysts of each reference example, Example 1 and each comparative example were placed in a converter and 1 m downstream from the manifold. The catalyst temperature was set at 550 ° C. with a 2.0 L lean burn engine, and the A / F was alternately switched between 25 seconds for lean for 56 seconds and rich for 10 seconds for 4 seconds for durability. The amount of S in the fuel was 300 ppm. Next, the input gas temperature was set to 600 ° C., and the A / F was operated with stoichiometry (14.6) for 50 hours. The amount of S in the fuel at that time was 20 ppm.

(Performance evaluation test)
NO _X performance: After setting the exhaust gas temperature to 400 ° C using a lean burn engine, the A / F was alternately repeated for 30 seconds at 25 for lean and 0.5 seconds for 10 at rich to evaluate the NO _X purification performance. . The NO _x purification performance was evaluated by the following formula.

NO _x purification performance (%) = {(NO _x amount at engine outlet (g / test)) − (NO _x amount at catalyst outlet (g / test))} ÷ (NO _x amount at engine outlet (g / test)) ) X 100 (%)
Oxidation performance: A catalyst is attached to a 2.2 L direct injection common rail diesel engine, and the exhaust gas temperature is raised from idle (100 ° C.) to 400 ° C. at 50 ° C./min. Ignition temperature) was measured.

(result)
The results are shown in Table 1.

As is clear from Table 1, the catalysts of each reference example and Example 1 exhibited a higher NO _x purification rate than the catalysts of the respective comparative examples. Since the HC purification temperature and the CO purification temperature of the catalysts of each reference example and example 1 are lower than those of the catalysts of each comparative example, the catalysts of each reference example and example 1 have a higher catalytic capacity. It became clear. Since all of Reference Examples 1 to 6 showed a high NO _x purification rate, it was found that the effects of the production method of the present invention can be exhibited regardless of the type of the porous carrier and catalyst component to be supported.

Further, from the results of Reference Examples 1 and 7, it is possible to obtain a catalyst having high activity even if it is dried by either microwave or high frequency (at least in this wavelength range (10 ⁴ μm to 10 ⁸ μm)). I understood. Furthermore, from the results of Reference Examples 1 and 8, it was found that drying time can be shortened without impairing the effects of the invention by performing drying while sending warm air. Then, Example 1 of the catalyst of the catalyst capacity is higher than the catalyst of Example 1 from (NO _X purification rate is high, HC and purification temperature CO is low) that, after a noble metal catalyst in a micro wave It has been clarified that a higher catalytic ability can be exhibited by drying.

Here, the appearance of the catalyst of Reference Example 1 and the SEM photograph of the catalyst surface are shown in FIGS. 1 and 2, and the appearance of the catalyst of Comparative Example 3 and the SEM photograph of the catalyst surface are shown in FIGS. As shown in FIGS. 1 and 2, the catalyst of each reference example and Example 1 shows that the catalyst component and the porous carrier are uniformly supported, whereas the catalyst of this comparative example has a catalyst component. It can be seen that the porous carrier is segregated. It can be inferred that the difference in the loading of the porous carrier and the catalyst component affects the catalyst performance.

It is the figure which showed the external appearance of the reference example 1. FIG. 4 is a SEM photograph of the surface of Reference Example 1. FIG. 10 is a diagram illustrating an appearance of Comparative Example 3. 10 is a SEM photograph of the surface of Comparative Example 3.

Claims (4)

A coating step of coating a catalyst carrier with a coating liquid containing a catalyst component;
A drying step of drying and removing the liquid medium contained in the coating liquid by electromagnetic wave irradiation having a wavelength of 10 ⁴ μm to 10 ⁸ μm,
The catalyst component supported on the catalyst carrier is selected from the group consisting of at least one first element selected from the group consisting of Pt, Pd, Rh, and Ir, and an alkali metal, alkaline earth metal, rare earth, and base metal. Consisting of at least one second element,
The coating liquid includes the first element as a salt, after the coating process, it has rows electromagnetic radiation having a wavelength of 10 ⁴ μm~10 ⁸ μm with heating,
The method for producing an exhaust gas purifying catalyst, wherein the salt of the first element is thermally decomposed while being irradiated with electromagnetic waves . The method for producing a catalyst for exhaust gas purification according to claim 1, wherein the electromagnetic wave is a microwave. The method for producing a catalyst for exhaust gas purification according to claim 1 or 2, wherein the liquid medium contains water and / or alcohol. The exhaust gas-purifying catalyst according to any one of claims 1 to 3, wherein the catalyst component used as the slurry comprises one or more heat-resistant inorganic oxides selected from the group consisting of alumina, silica, zirconia, titania and zeolite. Manufacturing method.