Patent Application
20090280978
The present invention relates to an exhaust gas purifying catalyst and a method of producing an exhaust gas purifying catalyst, and particularly relates to an exhaust gas purifying catalyst for purifying exhaust gas emitted from an internal combustion engine.
Since automobile emission restrictions have globalized, a three-way catalyst where a support such as Al2O3 (alumina) which is a porous carrier supports precious metal particles such as Pt (platinum), Pd (palladium), and Rh (rhodium) is used for the purpose of purify HC (hydrocarbon), CO (carbon monoxide), and NOX (nitrogen oxide) in exhaust gas.
Catalyst activity of the precious metal is almost in proportion to a surface area of the precious metal because a reaction using a precious metal is a contact reaction where the reaction progresses on the surface of the precious metal. Therefore, in order to obtain as much catalyst activity as possible from a small amount of precious metal, it is preferred to fabricate precious metal particles with a small particle size and a large specific surface area, and to disperse the particles uniformly onto a support while maintaining the particle size.
However, since precious metal particles with a particle size of under 10 nm has high catalyst activity but high surface reactivity and high surface energy, it the particles are very unstable. Also, when a particle size of precious metal particles becomes smaller than 5 nm, a melting point thereof is suddenly decreased (Reference: J. Phys. Chem. B, 107, pp 2719-2724 (2003)). Therefore, the precious metal particles move closer to each other and sinter together more easily. In particular, Pt sinters remarkably when heated, and even if Pt is dispersed uniformly on a support, Pt sinters due to heating and the particle size thereof increases. Hence, due to sintering of Pt caused by heating, a function of Pt as a catalyst, or conversion rate which is an indicator for purifying NOX is reduced. Because a catalyst for an automobile is usually exposed to high temperature of between 800 and 900° C., in some cases over 1000° C., it is difficult to prevent sintering of precious metal particles with a small particle size, maintain a particle size when fabricated, and maintain catalyst activity.
Meanwhile, in order to prevent precious metal particles from sintering, it is considered to reduce surface energy of the precious metal particles. However, to reduce the surface energy, it is necessary to have precious metal particles with a large particle size of approximately 50 to 100 nm, and in the case of such particle size, catalyst activity itself may be lost. Conventionally, in an above-described exhaust gas purifying catalyst using a precious metal, ceria was supported on alumina, a support, and further, a precious metal such as platinum was supported. In this exhaust gas purifying catalyst, platinum supported by ceria is sintered due to heat durability test. In the exhaust gas purifying catalyst after heat durability test, coarsened platinum is supported on ceria that is supported on alumina. In this case, since platinum is sintered and has a large particle size, catalyst activity is reduced. As just described, in a conventional exhaust gas purifying catalyst, even if a particle size of platinum is small, the particle size is not maintained when the catalyst is fabricated, and it is difficult to maintain catalyst activity.
Therefore, in Japanese Patent Laid-Open Publication No. H10-216517, an exhaust gas purifying catalyst is proposed where a catalyst active particle is supported on a support, and a material which is the same as or different from the support is adhered to the surface of the support.
However, even with the technology disclosed in the above patent document, sintering of catalyst active particles cannot be prevented sufficiently.
The present invention has been devised to solve such problems, and according to a first aspect of the invention, in summary, an exhaust gas purifying catalyst, comprises a composite compound in which a metal selected from among Al, Ce, La, Zr, Co, Mn, Fe, Mg, Ba and Ti is uniformly dispersed on an oxide selected from among Al2O3, ZrO2, and CeO2, and a precious metal selected from among Pt, Pd and Rh, supported on a compound of the metal, and covered with the composite compound.
Moreover, according to a second aspect of the invention, in summary, a method of producing an exhaust gas purifying catalyst, comprises the steps of preparing a dispersion system in which a second metal is uniformly dispersed on an oxide of a first metal, depositing a precious metal selectively on the second metal by introducing precious metal salt to the dispersion system and by adding a reducing agent, covering the precious metal deposited on the second metal with a mixture of salt of the first metal and salt of the second metal, and baking the dispersion system in which the precious metal is covered with the mixture.
a) is an explanatory view showing a state of an exhaust gas purifying catalyst in a reducing atmosphere.
a) is a HAADF-STEM image of an exhaust gas purifying catalyst obtained in Embodiment example 2 in an oxidizing atmosphere.
Details of an exhaust gas purifying catalyst according to the present invention, and a method of producing the exhaust gas purifying catalyst are described based on an embodiment.
An embodiment of an exhaust gas purifying catalyst according to the present invention is described.
As shown in
a) shows a state of an exhaust gas purifying catalyst 11 in which CeO2 13 is supported on Al2O3 12 which is a support, and, on top of that, Pt 14 is further supported, in a reducing atmosphere 6. Pt has a property that it is sintered in an oxidizing atmosphere, and is not sintered in a reducing atmosphere. Therefore, as shown in
Here, by repeating the states of
Therefore, in the exhaust gas purifying catalyst 1 according to the present embodiment, as shown in
Behaviors of Pt, Pd and Rh used as a precious metal are different in an oxidizing atmosphere and a reducing atmosphere, respectively. As described earlier, Pt sinters in an oxidizing atmosphere and does not sinter in a reducing atmosphere. Pd does not sinter in an oxidizing atmosphere and sinters in a reducing atmosphere. Rh does not sinter in an oxidizing atmosphere and sinter in a reducing atmosphere. Therefore, in a case where Pt is used, a metal in which Pt is dissolved in an oxidizing atmosphere is combined with Pt, and the precious metal is covered with a composite compound containing the metal. In a case where Pd or Rh is used, it is preferred to combine Pd or Rh with an element which is basically dissolved in a reducing atmosphere and maintains catalyst performance by being dissolved. For example, it is preferred to use Al for Rh. In particular, when Pt is used as a precious metal, it is preferred that CeO2 is used as a metal compound and Al2O3 is used as an oxide, in other words, a combination of Pt/CeO2/Al2O3 is preferred. In the case of Rh, a combination of Rh/Al2O3/ZrO2 is preferred, and in the case of Pd, a combination of Pd/Al2O3/Al2O3 is preferred. Also, when combining other element with a metal compound, for example, Ce—Zr-Ox, a dissolving speed of a precious metal is increased, and therefore sintering can be suppressed further. Note that, in
In the exhaust gas purifying catalyst of this embodiment, it is preferred that a precious metal is covered with the aforementioned composite compound in a range of 10 to 80% of the surface area of the precious metal. Normally, what functions effectively as a catalyst is a precious metal present on a catalyst surface. Therefore, when a percentage of covered precious metal is high, in other words, when the coverage ratio is high, the precious metal is stabilized, and a sintering suppressive ability is high, but because the precious metal cannot have sufficient contact with a reactant, sufficient catalytic activity cannot be obtained. On the other hand, when the coverage ratio of a precious metal is low, the initial activity of a catalyst is high, but since a precious metal supported on a support surface is sintered due to heating, durability is poor. Therefore, considering the balance of a sintering suppressive ability and catalyst performance, it is preferred that a precious metal is covered in a range of 10 to 80% of the surface area thereof. When the coverage ratio is in this range, sintering of a precious metal is suppressed, and an exhaust gas purifying catalyst having durability is obtained.
Here, how to calculate the coverage ratio is described. The coverage ratio is obtained as (100−exposure ratio) %. The exposure ratio is, as shown below, calculated from a ratio between a precious metal outer surface area (PMSA) calculated by a later-described CO (carbon monoxide) adsorption, and a theoretical particle surface area (TSA) to be calculated from particle sizes resultant from a transmission electron microscope (TEM) observation, and represents a ratio of a precious metal exposed on a surface of a composite compound out of a precious metal present in an exhaust gas purifying catalyst. With TEM, a precious metal which is not exposed on a surface of a composite compound can be observed. Therefore, if a precious metal is entirely exposed on a surface of a composite compound, an amount of gas adsorbed stoichiometrically in TSA is obtained, and TSA and PMSA become the same value. However, when a precious metal is supported on a composite compound surface while being covered, an amount of gas adsorbed stoichiometrically in the precious metal surface area obtained from a particle size of the precious metal cannot be obtained. Therefore, from the particle size of the precious metal observed with TEM and an amount of gas actually adsorbed in a sample, a ratio of the precious metal surface area exposed on the composite compound surface is calculated and used as exposure ratio.
PMSA is calculated from an expression (1) stated below.
The TSA is calculated by expressions (2) to (4) as follows. [D] is an average particle diameter of precious metal particles observed by the TEM. Letting [A] be the number of atoms of precious metal constituting a single [D], the number (n) of [D]'s contained in the catalyst is calculatable from the number [N] of precious metal atoms brought in during the preparation.
From the ratio between PMSA and TSA obtained above, exposure ratio is calculated.
Exposure ratio (%)=(PMSA)/(TSA)×100 (5)
Then, as shown in expression (6), coverage ratio is obtained by deducting exposure ratio (%) from 100.
Coverage ratio (%)=100−exposure ratio (6)
Note that, the following expression (7) is obtained by simplifying the calculation method of expressions (1) to (6) above. From this expression (7), coverage ratio (%) is obtained.
Wherein
α: Unit CO adsorption (cm3/g)
β: Atomic cross section (nm2)
γ: Stoichiometrical ratio (−)
δ: Precious metal supporting concentration (wt %)
ε: Supported precious metal concentration (g/ml)
D: TEM-observed particle size (nm)
In the exhaust gas purifying catalyst, it is preferred that the particle size of a metal compound is 10 nm or smaller. For producing an exhaust gas purifying catalyst, a precious metal is selectively deposited on a metal composite dispersed uniformed in the foregoing oxide. In this case, if the particle size of the metal compound contained in the oxide is large, the particle size of a precious metal deposited thereon becomes large as well. Therefore, it is preferred that the particle size of a metal compound uniformly dispersed on the oxide is 10 nm or smaller, and when the particle size is smaller than 10 nm or smaller, the particle size of a precious metal deposited thereon can be 10 nm or smaller.
Further, it is preferred that the particle size of a precious metal is 10 nm or smaller after an exhaust gas purifying catalyst is baked for three hours at 900° C. in the air. This is because, when the particle size of a precious metal is larger than 10 nm after three-hour baking at 900° C. in the air, in other words, after undergoing a heat durability test, catalyst performance is reduced. Note that, when the particle size of precious metal is 5 nm or smaller, catalyst performance is improved.
Also, it is preferred that the precious metal is Pt, the metal is Ce, and the oxide is Al2O3. In this case, Ce reacts with Al, easily forming Ce—Al2O4 as a composite compound. Then, when Pt is fixed on the surface of Ce—Al2O4, since Ce—Al2O4 has high heat durability and stable crystal structure compared with alumina, sintering of Pt can be prevented.
Further, it is preferred that the peak integrated intensity on the Ce (200) surface from X-ray diffraction analysis is larger than the peak integrated intensity on the Ce (111) surface by more than 0.6. The value within this range means that Ce is uniformly dispersed into alumina.
Also, when the precious metal is Pt, the metal is Ce, and the oxide is Al2O3, it is preferred that a ratio (IA/IB) between a spectral integrated intensity of Pt (IA) obtained from an energy-dispersive X-ray analysis (EDX) and a spectral integrated intensity of Ce (IB) after the exhaust gas purifying catalyst is baked for one hour at 400° C. in the air is 0.005 or larger. In this case, an amount of Pt selectively supported on the composite compound (Ce—Al2O4) is large.
In this exhaust gas purifying catalyst, it is preferred that a supporting concentration of Pt is 1.0 wt % or smaller. In this case, since an interparticle distance between Pt and other Pt is ensured, sintering of Pt can be prevented. Note that, when a supporting concentration of Pt is increased, Pt present on the composite compound surface without being able to be dissolved therein is sintered. Also, in a case where a supporting concentration of Pt is 0.01 wt % or smaller, when the exhaust gas purifying catalyst is to be applied to a honeycomb support or the like to be used for purifying automobile exhaust gas, a large amount of the exhaust gas purifying catalyst should be applied to the honeycomb support, which is poor in practicality.
As described above, in the exhaust gas purifying catalyst according to the embodiment of the present invention, by providing a composite compound in which a metal selected from among Al, Ce, La, Zr, Co, Mn, Fe, Mg, Ba and Ti is uniformly dispersed on an oxide selected from among Al2O3, ZrO2, and CeO2, and a precious metal selected from among Pt, Pd, and Rh, supported on a compound of the metal and covered with the composite compound, it becomes possible to obtain an exhaust gas purifying catalyst which suppresses a reduction of the dispersion rate of the precious metal, maintains a state where the particle size of the precious metal is small, and has excellent heat durability with a small amount of precious metal.
Next, an embodiment of a method of producing an exhaust gas purifying catalyst according to the present invention is described. The method of producing an exhaust gas purifying catalyst according to this embodiment is characterized by comprising the steps of: preparing a dispersion system where a second metal is dispersed uniformly in an oxide of a first metal; depositing a precious metal selectively on the second metal by introducing precious metal salt into the dispersion system and adding a reducing agent; covering the precious metal deposited on the second metal with a mixture of salt of the first metal and salt of the second metal; and baking the dispersion system where the precious metal is covered with the mixture. In this method of producing g an exhaust gas purifying catalyst, alkaline precious metal salt is adsorbed and supported selectively on the surface of a compound of the second metal such as CeO2. In this case, obtained is an exhaust gas purifying catalyst which is provided with a composite compound in which the second metal is uniformly dispersed on an oxide of the first metal, and a precious metal supported on a compound of the second metal and covered with a composite compound, and, in this exhaust gas purifying catalyst, the precious metal does not sinter in an oxidizing atmosphere because the precious metal is dissolved in the composite compound where the second metal is dispersed uniformly in the oxide of the first metal.
Here, as an example, a system of Pt/Ce/Al2O3, where the precious metal is Pt, the second metal is Ce, and the oxide of the first metal is Al2O3 is described. On Ce—Al2O3 where the peak integrated intensity on the Ce (200) surface by X-ray diffraction analysis is larger than the peak integrated intensity of Ce (111) surface by more than 0.6, Pt is selectively deposited on Ce by dinitrodiamine platinum salt as alkaline precious metal salt, and NaBH4 as a reducing agent. Thereafter, the deposited precious metal is covered with Al nitrate and Ce acetate, and dried. By these steps, Pt on Al2O3 where Ce is uniformly dispersed is covered with a composite compound containing Ce and Al.
As stated above, according to the method of producing an exhaust gas purifying catalyst of this embodiment, since the precious metal can be supported on the first oxide on which the second metal is dispersed uniformly while being covered with the oxide of the first metal which contains the second metal, it becomes possible to obtain an exhaust gas purifying catalyst which suppresses a reduction of the dispersion rate of the precious metal, maintains a state where the particle size of the precious metal is small, has excellent heat durability with a small amount of precious metal.
Note that, in producing the exhaust gas purifying catalyst, a step of preparing a dispersion system in which the second metal is dispersed uniformly in the oxide of the first metal, a step of depositing the precious metal selectively on the second metal by introducing precious metal salt into the dispersion system and adding a reducing agent, a step of covering the precious metal deposited on the second metal with a mixture of salt of the first metal and salt of the second metal, and a step of baking the dispersion system in which the precious metal is covered with the mixture may be incorporated to any preparation method and supported out. The preparation method is, for example, an inclusion method, a reversed micelle method, an impregnation method, or the like.
Hereinafter, the exhaust gas purifying catalyst according to the present invention is described more specifically by Embodiment example 1 to Embodiment example 19, Comparative example 1 to Comparative example 3, but the scope of the present invention is not limited thereto. These Embodiment examples are investigations for effectiveness of the exhaust gas purifying catalyst according to the present invention, and represent Embodiment examples of exhaust gas purifying catalysts prepared using different ingredients.
First of all, Ce acetate was introduced to alumina whish is dispersed in water so that CeO2 is 20 wt % to alumina. Thereafter, the sample was agitated for two hours, dried for a day at 120° C., and then baked for two hours at 600 in the air. After the agitation, the sample was dispersed into water, and tetraammine platinum hydroxide was introduced therein. Then, the sample was agitated for two hours, dried for a day at 120° C., and then baked for an hour at 400° C. in the air. The sample obtained after baking was dispersed into water, and Ce acetate and Al nitrate were introduced therein. Then, after two-hour agitation, the sample was dried for a day at 120° C. and baked for an hour at 400° C. in the air, and the target sample was obtained.
In Embodiment example 2, Ce—Al2O3 was used as alumina. First of all, CeO2 20%-Al2O3, where the peak integrated intensity of the Ce (200) surface was larger than the peak integrated intensity of the Ce (111) surface from X-ray diffraction analysis by more than 0.6, was dispersed in water. In this dispersion fluid, tetraammine platinum hydroxide was introduced. The fluid was agitated for two hours, dried for a day at 120° C., and then baked for an hour at 400° C. in the air. The sample thus obtained was dispersed in water, and Ce acetate and Al nitrate were introduced therein. Thereafter, the sample was agitated for two hours, dried for a day at 120° C., and baked for an hour at 400° C. in the air, and the target sample was obtained.
In Embodiment example 3, Ce—Al2O3 was used as alumina. First of all, CeO2 20%-Al2O3, where the peak integrated intensity of the Ce (200) surface was larger than the peak integrated intensity of the Ce (111) surface from X-ray diffraction analysis by more than 0.6, was dispersed in water. In this dispersion fluid, dinitrodiamine platinum salt was introduced, and NaBH4 for reducing Pt was further introduced, and then the fluid was agitated for two hours, dried for a day at 120° C., and then baked for an hour at 400° C. in the air. The sample thus obtained was dispersed in water, and Ce acetate and Al nitrate were introduced therein, and ammonia water was further introduced. Thereafter, the sample was agitated for two hours, dried for a day at 120° C., and baked for an hour at 400° C. in the air, and the target sample was obtained.
In Embodiment example 4, preparation was done similarly to Embodiment example 1 except that Ce acetate was changed to Ce nitrate, and tetraammine platinum hydroxide was changed to dinitrodiamine platinum salt.
In Embodiment example 5, preparation was done similarly to Embodiment example 1 except that the amounts of Ce acetate and Al nitrate were increased.
In Embodiment example 6, preparation was done similarly to Embodiment example 1 except that the amounts of Ce acetate and Al nitrate were reduced.
In Embodiment example 7, preparation was done similarly to Embodiment example 1 except that the amounts of Ce acetate and Al nitrate were increased.
In Embodiment example 8, first of all, in alumina which was dispersed in water, Ce acetate and Zr acetate were introduced so that CeO2 was 20 wt % and ZrO2 was 7 wt % to alumina. Thereafter, the sample was agitated for two hours, and dried for a day at 120° C. Thereafter, the sample was baked for two hours at 600° C. in the air. The sample obtained after baking was dispersed in water, and tetraammine platinum hydroxide was introduced therein. The sample was then agitated for two hours, dried for a day at 120° C., and baked for an hour at 400° C. The sample thus obtained was dispersed in water, and Ce acetate, Zr acetate and Al nitrate were introduced therein, and the sample was agitated for two hours, dried for a day at 120° C., baked for an hour at 400° C., and then the target sample was obtained.
First of all, in ceria which was dispersed in water, Al nitrate was introduced so that Al2O3 was 20 wt % to ceria. Thereafter, the sample was agitated for two hours, dried for a day at 120° C., and baked for two hours at 600° C. in the air. The sample obtained from baking was dispersed in water, and Pd nitrate was introduced therein. Then, the sample was agitated for two hours, dried for a day at 120° C., and baked for an hour at 400° C. The sample thus obtained from baking was dispersed in water, and Ce acetate and Al nitrate were introduced therein. The sample was then agitated for two hours, dried for a day at 120° C., baked for an hour at 400° C. in the air, and thus the target sample was obtained.
In zirconia which was dispersed in water, Al nitrate was introduced so that Al2O3 is 20 wt % to zirconia. Thereafter, the sample was agitated for two hours, dried for a day at 120° C., and then baked for two hours at 600° C. in the air. The sample obtained form baking was dispersed in water, and Rh nitrate was introduced therein. Thereafter, the sample was agitated for two hours, dried for a day at 120° C., and then baked for an hour at 400° C. in the air. The sample obtained from baking was dispersed in water, Zr acetate and Al nitrate were introduced, agitated for two hours, dried for a day at 120° C., baked for an hour at 400° C., and the target sample was obtained.
In Embodiment example 11, preparation was done similarly to Embodiment example 8 except that Zr acetate was changed to La acetate.
In Embodiment example 12, preparation was done similarly to Embodiment example 8 except that Zr acetate was changed to Co nitrate.
In Embodiment example 13, preparation was done similarly to Embodiment example 8 except that Zr acetate was changed to Mn nitrate.
In Embodiment example 14, preparation was done similarly to Embodiment example 8 except that Zr acetate was changed to Fe nitrate.
In Embodiment example 15, preparation was done similarly to Embodiment example 8 except that Zr acetate was changed to Mg acetate.
In Embodiment example 16, preparation was done similarly to Embodiment example 8 except that Zr acetate was changed to Ba acetate.
In Embodiment example 17, preparation was done similarly to Embodiment example 8 except that Zr acetate was changed to titanyl ammonium oxalate.
In Embodiment example 18, preparation was done similarly to Embodiment example 2 except that the Pt supporting concentration was changed to 0.5%.
In Embodiment example 19, preparation was done similarly to Embodiment example 2 except that Pt supporting concentration was changed to 1.0%.
In Comparative example 1, Pt was not covered with alumina which contained ceria. First of all, in alumina which was dispersed in water, Ce acetate was introduced so that CeO2 was 20 wt % to alumina, and agitated for two hours. Thereafter, the sample was dried for a day at 120° C., and baked for two hours at 600° C. in the air. The sample obtained from baking was dispersed in water, and tetraammine platinum hydroxide was introduced therein. Next, the sample was agitated for two hours, and the target sample was obtained.
In Comparative example 2, Pt was covered with alumina which did not contain Ce. First of all, tetraammine platinum hydroxide was introduced into alumina which is dispersed in water, agitated for two hours, dried for a day at 120° C., and then baked for an hour at 400° C. in the air. The sample obtained from baking was dispersed in water, Al nitrate was introduced therein, agitated for two hours, and dried for a day at 120° C. Thereafter, the sample was baked for an hour at 400° C. in the air, and the target sample was obtained.
In Comparative example 3, preparation was done similarly to Embodiment example 2 except that the Pt supporting concentration was changed to 3.0%.
Here, each of the samples obtained from the aforementioned sample preparations underwent a catalyst durability test through three-hour baking at 900° C. in a gas atmosphere in which H2 2%/He balance and O2 5%/He balance were changed by 10 seconds. In addition, the particle sizes were measured by TEM before and after the durability test. Further, Pt coverage ratio was calculated from the aforementioned expressions. Regarding Embodiment example 1, Embodiment examples 5 to 7, and Comparative example 1, 50% of conversion rate was obtained.
TEM-EDX measurements were supported out for the catalysts obtained from the aforementioned preparations and the catalysts after the baking. For TEM, Hf-2000 produced by Hitachi, Ltd. was used, and, the measurement was done with an accelerating voltage of 200 kv and the cutting condition was ambient temperature. For EDX, SIGMA manufactured by Kavex was used. The measurement method was that an embedding process was conducted to catalyst powder by epoxy resin, and after the epoxy resin was hardened, a super thin section was created from ultra microtome. By using this section, the dispersion state of each type of crystal particles was investigated by TEM. In the images obtained, the contrast (shadow) region were focused, the type of metal was specified, and the particle size of the metal was measured. Further, the samples obtained in Embodiment example 2 and Comparative example 1 were observed by using a high-angle annular dark field scanning transmission electron microscopy image (HAADF-STEM).
From the model gas shown in Table 1, 50% conversion rate temperature (T50) was obtained when temperature was increased from room temperature to 400° C. at 10° C./minute.
To obtain coverage ratio, a unit CO adsorption amount was measured. A metal dispersion rate measuring device BEL-METAL-3 produced by Bel Japan Inc. was used to measure a unit CO adsorption amount, and the measurement was supported out following the procedures below. The temperature of each sample was increased to 400° C. at 10° C./minute in a He 100% gas flow, and then oxidization treatment was conducted for 15 minutes at 400° C. in an O2 100% gas flow. Next, the sample was purged for 5 minutes in He 100% gas, and a reducing treatment was conducted for 15 minutes at 400° C. in H2 40%/He balance gas flow. Next, the temperature was decreased to 50° C. in a He 100% gas flow. Then, CO 10%/He balance gas was entered in a pulsing fashion, and the measurement was obtained.
For aforementioned Embodiment example 1 to Embodiment example 7, Comparative example 1 and Comparative example 2, Pt particle sizes and Ce particle sizes during production of the catalysts, as well as Pt particle sizes and Ce particle sizes after the durability test were obtained. Table 2 shows each particle size and Pt coverage ratio. Also,
Comparing Embodiment example 1 and Comparative example 1, in Comparative example 1 where the coverage ratio of Pt is 2%, since a speed of Pt being dissolved again after being released from the dissolved state into Ce—Al2O4 in a reducing atmosphere is slower than the sintering speed of Pt, sintering of Pt occurs when the atmosphere is changed to an oxidizing atmosphere. Therefore, a large difference was caused between Embodiment example 1 and Comparative example 1 in Pt particle size after the durability test. Comparing Embodiment example 1 and Comparative example 2, although the coverage ratio of Pt by alumina is 54% in Comparative example 2, since alumina which covers Pt does not contain CeO2, Pt is not dissolved in alumina and continue to sinter in an oxidizing atmosphere. Hence, in Comparative example 2, the particle size after the durability test became large. From the results obtained in Embodiment example 1 to Embodiment example 4, it is considered that there were differences in Pt particle size because, with a large particle size of CeO2 which is dispersed in alumina, an amount of Pt that is present thereon is increased, and sintering proceeds faster than dissolution. Further, as shown in
Next, Table 3 shows Pt particle sizes during production of catalysts, Pt particle sizes after the durability test, Pt coverage ratio, and 50% conversion rate temperature after the durability test for Embodiment example 1, Embodiment example 5 to Embodiment example 7, and Comparative example 1.
From the result stated in Table 3, it could be confirmed that, in Embodiment example 7 and Comparative example 1 where the coverage ratio was out of the range of 10 to 80%, 50% conversion rate temperature was high and the catalyst performance was reduced. In Embodiment example 7, although Pt sintering was suppressed as the coverage ratio was as high as 87%, 50% conversion rate temperature was considered high because contact with reactant gas was low. In addition, in Comparative example 1, it is considered that sintering of Pt could not be suppressed because coverage ratio was as low as 2%, the particle size of Pt after the durability test became large, and further, 50% conversion rate temperature was increased.
Next, Table 4 shows each constitutive element, particle sizes of precious metals during production of catalysts, particle sizes of the precious metals and coverage ratio of the precious metal after the durability test for Embodiment example 1, Embodiment example 8 to Embodiment example 17.
From the results shown in Table 4, comparing the values of Embodiment example 1, Embodiment example 8, and Embodiment example 11 to Embodiment example 17, in a case where an element other than Ce was contained in alumina, the particle size of Pt after the durability test was suppressed to about triple to quadruple of a size before the durability test compared to the case where only Ce was contained, and an effect by adding other metal was observed. In addition, where Pd or Rh was used as a precious metal, the particle size after the durability test could be kept small similarly to the case of Pt. Moreover, as shown in Embodiment example 8 and Embodiment example 11 to Embodiment example 17, it was found out that a similar level of effect could be obtained when any element of Zr, La, Co, Mn, Fe, Mg, Ba and Ti was used as other metal to be contained instead of Ce.
Next, regarding foregoing Embodiment example 2 to Embodiment example 4, Table 5 below shows the spectral integrated intensities of Pt (IA) obtained from EDX after 1-hour baking at 400° C., the spectral integrated intensities of Ce (IB), IA/IB, and particle sizes of Pt after the durability test. Also,
In Embodiment example 2 and Embodiment example 3, a good correlation was seen between Ce count and Pt count. In Embodiment example 2 and Embodiment example 3, the particle sizes of CeO2 were small, and the amount of Pt present per particle of CeO2 was small as shown in Table 2. Therefore, a sintering suppressive ability for Pt could be achieved. The sintering suppressive effect was obvious from the fact that the particle sizes of Pt after the durability test were small. Further, comparing Embodiment example 2 and Embodiment example 3, the IA/IB value for Embodiment example 2 was higher than Embodiment example 3. In this case, the amount of Pt selectively supported on Ce—Al2O4 was large, and the Pt sintering suppressive ability could be achieved. Compared to these results, no correlation was seen between Ce count and Pt count in Embodiment example 4. In Embodiment example 4, since the particle size of CeO2 dispersed in alumina was large, and the amount of Pt which was present thereon increased, it was considered that Pt sintering proceeded faster than PT being dissolved in Ce—Al2O4, and, as a result of this, the Pt sintering suppressive effect could not be achieved, and the particle size of Pt after durability test became large.
Next, Table 6 shows the Pt support densities and particle sizes of Pt after the durability test for Embodiment example 18, Embodiment example 19, and Comparative example 4. Moreover,
A in
Note that, because the melting point of a precious metal fine particle suddenly decreases when the particle size thereof becomes 5 nm or smaller, the precious metal particles easily move closer to each other and sinter together when the particle size becomes 5 nm or smaller. In particular, Pt sinters remarkably when heated, and, even when Pt is dispersed uniformly on a support, Pt sinters and the particle size thereof increases due to heating. Therefore, as shown in
Hereinbefore, the present invention has been described in detail based on embodiments of the invention with specific Embodiment examples. However, the present invention is not limited to the descriptions above, and may be modified or changed without departing from the idea of the present invention.
The entire contents of Japanese Patent Application No. 2004-372185 (filed on Dec. 22, 2004) and Japanese Patent Application No. 2005-21427 (Jan. 28, 2005) are incorporated herein by reference.
Since the exhaust gas purifying catalyst of the present invention can suppress a reduction of a dispersion rate of a precious metal, maintain a state where the particle size of the precious metal is small, and has excellent heat durability with a small amount of the precious metal, the catalyst can be used as a three-way catalyst for an automobile, and the like.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-372185 | Dec 2004 | JP | national |
| 2005-021427 | Jan 2005 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP05/19992 | 10/31/2005 | WO | 00 | 6/20/2007 |
