BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic structure of a motor vehicle spark ignition engine at which an exhaust gas purification catalyst according to an embodiment of the present invention is mounted.
FIG. 2 is perspective and partly enlarged views showing the structure of the exhaust gas purification catalyst.
FIG. 3 is a graph showing the light-off temperatures of various kinds of catalyst materials.
FIG. 4 is a graph showing the high-temperature catalytic conversion efficiencies of the various kinds of catalyst materials.
FIG. 5 is a graph showing effects of the mass ratio of Rh carried on ZrLaO/Al2O3 on the light-off temperature.
FIG. 6 is a graph showing the results of examination of Rh 3d electron binding energy of Rh/ZrLaO/Al2O3 by x-ray photoelectron spectroscopy (XPS).
FIG. 7 is a graph showing the results of examination of Rh 3d electron binding energy of Rh/ZrO2/Al2O3 by XPS.
FIG. 8 is a graph showing effects of the mass ratio of Rh carried on ZrLaO/Al2O3 on the oxygen release capacity of an OSC.
FIG. 9 is a cross-sectional view showing part of a device for measuring the amount of oxygen release.
FIGS. 10A-10B schematically shows fuel-lean conditions and fuel-rich conditions for a catalyst in which Rh/ZrLaO/Al2O3 coexists with Rh/OSC.
DETAILED DESCRIPTION OF THE INVENTION
A description is given below of the best mode for carrying out the present invention with reference to the drawings.
FIG. 1 shows a schematic structure of a motor vehicle spark ignition engine 1 at which a three-way catalyst 11 is mounted as an exhaust gas purification catalyst according to this embodiment. The engine 1 has a plurality of cylinders 2 (only one shown in the figure). Air and fuel are supplied through an intake passage 3 and a fuel injection valve 4, respectively, to a combustion chamber 6 to form an air-fuel mixture. The air-fuel mixture explosively combusts in the combustion chamber 6 by spark ignition of an ignition plug 7 and the resultant exhaust gas is discharged through an exhaust passage 8 to the atmosphere. A catalytic converter 10 is disposed in the exhaust passage 8 and contains the three-way catalyst 11. Reference numeral 5 denotes a piston.
As shown in FIG. 2, the three-way catalyst 11 has a structure in which a catalytic coating 11b is formed on the walls of exhaust gas channels in a honeycomb support 11a made of cordierite. The catalytic coating 11b contains activated alumina particles coated with a Rh-carried ZrLa mixed oxide (Rh/ZrLaO/Al2O3) and a Rh-carried oxygen storage component (Rh/OSC). The ZrLa mixed oxide-coated activated alumina particles may be those in which a rare earth metal, such as La, is dissolved as a solid solution by about 3 to about 6 mole %.
The present invention imposes no special limitations on that the catalytic coating 11b contains additional one or more catalytic components or that the ZrLa mixed oxide-coated activated alumina particles and/or the oxygen storage component carry another one or more kinds of catalytic metals in addition to Rh. Alternatively, the three-way catalyst 11 may have a multilayered structure in which the catalytic coating 11b and another one or more catalytic coatings of different catalyst compositions are stacked one on another.
A detailed description is given below of examples of the catalytic coating 11b.
(Preparation of Rh/ZrLaO/Al2O3)
Activated alumina powder (γ-Al2O3) is dispersed in a mixed solution of zirconium nitrate and lanthanum nitrate. A specified amount of aqueous ammonia is added to the mixed solution to reach an alkaline pH, thereby forming a precipitate (coprecipitation). The precipitate is presumed to be activated alumina particles coated with a mixed oxide precursor (hydroxide of Zr and La). The obtained precipitate is filtered, rinsed, dried by keeping it at 200° C. for two hours and calcined by keeping it at 500° C. for two hours, thereby obtaining activated alumina particles whose surfaces are coated with ZrLa mixed oxide (ZrLaO/Al2O3).
The ZrLaO/Al2O3 is mixed with an aqueous solution of rhodium nitrate and then evaporated to dryness, thereby obtaining a Rh-carried ZrLaO/Al2O3 (Rh/ZrLaO/Al2O3).
(Formation of Catalytic Coating)
An oxygen storage component (OSC) is mixed with an aqueous solution of rhodium nitrate and evaporated to dryness, thereby obtaining a Rh-carried OSC (Rh/OSC). Then, the Rh/OSC, the Rh/ZrLaO/Al2O3 and a binder (ZrO2) are mixed and water and nitric acid are also added and mixed by stirring with a disperser, thereby obtaining a slurry. A honeycomb support 11a made of cordierite is immersed in the slurry and then picked up therefrom and surplus slurry is removed by air blow. This process is repeated until a specified amount of slurry is coated on the exhaust gas channel walls of the support 11a. Thereafter, the support 11a is heated from normal temperature to 450° C. at a constant rate of temperature increase in 1.5 hours and then kept at 450° C. for two hours (dried and calcined), thereby forming a catalytic coating 11b on the support 11a.
(La Ratio in ZrLa Mixed Oxide)
Four kinds of Rh-carried ZrLa mixed oxide-coated activated aluminas (Rh/ZrLaO/Al2O3) with different ZrO2 to La2O3 mass ratios of 20 to 1, 10 to 1, 5 to 1 and 1 to 1 were prepared according to the above preparation method. In addition to these, Rh-carried activated alumina (Rh/Al2O3) obtained by carrying Rh on activated alumina particles by evaporation to dryness and Rh-carried zirconia-coated activated alumina (Rh/Zr/Al2O3) obtained by using ZrO2 in place of ZrLaO in Rh/ZrLaO/Al2O3 (carrying Rh on ZrO2-coated activated alumina particles) were prepared. Then, these six kinds of catalyst materials were carried on their respective honeycomb supports, thereby preparing six samples. The amount of Rh carried per L of each support was 1.0 g/L.
Each of these samples was aged in an atmosphere of 2% O2 and 10% H2O at 1000° C. for 24 hours, then attached to a fixed-bed flow reactor and measured in terms of T50 (° C.) and C400 (%) which are indices for HC, CO and NOx conversion performance.
The simulated exhaust gas (including a mainstream gas and gases for changing the A/F ratio) used in the measurement had an A/F ratio of 14.7±0.9 and the flow rate of the simulated exhaust gas into each catalyst sample was 25 L/min. Specifically, a mainstream gas was allowed to flow constantly at an A/F ratio of 14.7 and a specified amount of gas for changing the A/F ratio was added in pulses at a rate of 1 Hz, so that the A/F ratio was forcedly oscillated within the range of ±0.9. O2 gas was used in changing the A/F ratio to a leaner value (15.6). H2 gas and CO gas were used in changing the A/F ratio to a richer value (13.8). The composition of the mainstream gas having an A/F ratio of 14.7 was as follows.
Mainstream Gas
Co2: 13.9%, O2: 0.6%, CO: 0.6%, H2: 0.2%, C3H6: 0.056%, NO: 0.1%, H2O: 10% and N2: the rest
T50 (° C.) is the gas temperature at the. catalyst entrance when the concentration of each exhaust gas component (HC, CO and NOx) detected downstream of the catalyst reaches half of that of the corresponding exhaust gas component flowing into the catalyst (when the conversion efficiency reaches 50%) after the temperature of the simulated exhaust gas is gradually increased (i.e., the light-off temperature), and indicates the low-temperature catalytic conversion performance of the catalyst.
C400 (%) is the catalytic conversion efficiency of each exhaust gas component (HC, CO and NOx) when the simulated exhaust gas temperature at the catalyst entrance is 400° C. and indicates the high-temperature catalytic conversion performance of the catalyst.
The measurement results for T50 (° C.) and the measurement results for C400 (%) are shown in FIGS. 3 and 4, respectively. In respect of C400 (FIG. 4), no substantial difference is recognized among the samples. Referring to T50 (FIG. 3), when the ZrO2 to La2O3 mass ratio was 20 to 1, 10 to 1 and 5 to 1, the effects of addition of La to the Zr-based mixed oxides were exhibited, i.e., the light-off temperatures were reduced. Furthermore, when the ZrO2 to La2O3 mass ratio was 20 to 1, the best results were exhibited. Therefore, it can be said that the ZrO2 to La2O3 mass ratio is preferably not smaller than 5 to 1 and more preferably about 20 to 1.
(Rh Distribution Ratio)
In view of the above results on the La ratio in ZrLa mixed oxide, a plurality of three-way catalysts having different Rh distribution ratios between ZrLaO/Al2O3 and the OSC were prepared using the ZrLa mixed oxide with a ZrO2 to La2O3 mass ratio of 20 to 1 according to the above-described preparation method. In other words, a plurality of three-way catalysts were prepared which have different mass ratios of the amount of Rh carried on ZrLaO/Al2O3 to the sum of the amount of Rh carried on ZrLaO/Al2O3 and the amount of Rh carried on the OSC.
Also in the cases (comparative examples) using ZrO2 in place of ZrLa mixed oxide, a plurality of three-way catalysts were also prepared which have different mass ratios of the amount of Rh carried on ZrO2/Al2O3 to the sum of the amount of Rh carried on ZrO2/Al2O3 and the amount of Rh carried on the OSC.
Both in the inventive examples and in the comparative examples, the sum of the amounts of Rh was 0.167 g/L and a CeZrNd mixed oxide of CeO2:ZrO2:Nd2O3=10:80:10 (mass ratio) was employed. Then, each three-way catalyst was aged in the same manner as described above and then measured in terms of T50 in the same manner. The measurement results are shown in FIG. 5. Note that the term “Rh on ZrLaO coated on alumina” in the figure means the amount of Rh carried on ZrLaO/Al2O3 and the term “Rh on OSC” means the amount of Rh carried on the OSC.
Referring to FIG. 5, the comparative examples using ZrO2 in place of ZrLa mixed oxide did not significantly change the T50 value even if the mass ratio of Rh carried on ZrO2/Al2O3 changed. In contrast, the inventive examples using ZrLa mixed oxides significantly changed the T50 value with changes in the mass ratio of Rh carried on ZrLaO/Al2O3 ranging from 33 mass % (the ratio of the amount of Rh carried on ZrLaO/Al2O3 to the amount of Rh carried on the OSC=1 to 2) to 80 mass %, both inclusive. It can be seen from FIG. 5 that the preferable range of the mass ratio of Rh carried on ZrLaO/Al2O3 is from 40 to 75 mass % both inclusive and the more preferable range is from 50 to 70 mass % both inclusive.
(Mechanism of Effect Development of Inventive Catalyst)
As described above, the catalyst according to the present invention improves the exhaust gas purification performance, particularly the low-temperature activity, owing to a combination of Rh/ZrLaO/Al2O3 and Rh/OSC. The reason for this is considered below.
FIG. 6 shows the results obtained by keeping Rh/ZrLaO/Al2O3 (an unaged fresh material having an amount of Rh carried of 1.0 g/L) in a fuel-lean atmosphere (having an A/F ratio of 15.0) at 400° C. for five minutes and then in a fuel-rich atmosphere (having an A/F ratio of 14.0) at 400° C. for five minutes, reducing the ambient temperature to room temperature in a N2 atmosphere and measuring the Rh 3d electron binding energy of the material. FIG. 7 shows the results obtained by subjecting Rh/Zr/Al2O3 (a fresh material having an amount of Rh carried of 1.0 g/L) using ZrO2 in place of ZrLaO of the former material, i.e., a comparative example, to the same process and measuring the Rh 3d electron binding energy of the material in both the fuel-lean and fuel-rich atmospheres. It is known that the peak value of the Rh 3d electron binding energy is 308.5 eV when Rh is in oxidized state, 307 eV when Rh is in reduced state and 310 eV when Rh is dissolved as a solid solution in alumina.
Referring to the comparative example of FIG. 7 (Rh/Zr/Al2O3), the peak value of the Rh 3d electron binding energy was in the vicinity of 308 eV in the fuel-lean atmosphere but was in the vicinity of 307 eV in the fuel-rich atmosphere.
On the other hand, as shown in FIG. 6, the material Rh/ZrLaO/Al2O3 in the catalyst according to the present invention did not have a significant difference between the eV characteristics in both the fuel-lean and fuel-rich atmospheres. More specifically, the Rh 3d electron binding energy peak of the catalyst in the fuel-rich atmosphere appeared at a higher energy point (closer to 308 eV) than that when Rh is in reduced state, 307 eV. This shows that Rh in Rh/ZrLaO/Al2O3 in the catalyst according to the present invention is not reduced so much but is kept appropriately oxidized even if the ambient atmosphere changes from fuel-lean to fuel-rich conditions.
The reason for this is believed to be that since the ZrLa mixed oxide contains La unlike ZrO2, La—O—Rh bonds are more likely to be formed between the ZrLa mixed oxide and Rh and, therefore, Rh becomes more likely to be kept oxidized.
On the other hand, Rh on the OSC is basically kept reduced not only under fuel-rich conditions but also under fuel-lean conditions because the OSC stores oxygen.
Therefore, the catalyst according to the present invention provides the coexistence of reduced Rh on the OSC and oxidized Rh on the ZrLa mixed oxide irrespective of whether the ambient atmosphere is fuel-lean or fuel-rich, thereby improving the exhaust gas purification performance. Specifically, if Rh on the mixed oxide is reduced, this will be disadvantageous to the oxidation of HC and CO. However, in the present invention, since Rh on the ZrLa mixed oxide is kept appropriately oxidized even if the ambient atmosphere becomes fuel-rich, HC and CO can be efficiently converted by oxidization. On the other hand, reduced Rh on the OSC effectively act to convert NOx by reduction. Furthermore, since HC and CO can be oxidized even if the ambient atmosphere becomes fuel-rich, NOx reduction progresses concurrently with the oxidization of HC and CO, which is advantageous in converting NOx by reduction.
FIG. 8 shows the results of examination of catalysts (fresh materials) of an inventive example and a comparative example (using ZrO2 in place of ZrLaO) in terms of the effect of the mass ratio of Rh carried on the above material ZrLaO/Al2O3 (and the above material Zr/Al2O3 in the comparative example) on the oxygen release capacity of the OSC. The sum of the amount of Rh on ZrLaO/Al2O3 (or Zr/Al2O3 in the comparative example) and the amount of Rh on the OSC was 0.167 g/L and a CeZrNd mixed oxide of CeO2:ZrO2:Nd2O3=10:80:10 (mass ratio) was employed as the OSC.
FIG. 9 shows the structure of an essential part of a test device for measuring the amount of oxygen release. The test device is configured to allow gas to flow through a sample 12 and has linear oxygen sensors 13, 13 disposed at the entrance and exit of the sample 12. In the measurement, gas containing 10% CO2 (and 90% N2) was first allowed to flow through the sample 12 heated up to 350° C. Then, a simulation was made by repeating the following cycle: oxygen was added to the gas for 20 seconds (to provide fuel-lean conditions); no gas was then added for 20 seconds (to provide stoichiometric conditions); CO was then added for 20 seconds (to provide fuel-rich conditions); and no gas was then added for 20 seconds (to provide stoichiometric conditions). While this cycle was repeated, the output difference between the entrance- and exit-side linear oxygen sensors, i.e., (the output at the catalyst entrance)—(the output at the catalyst exit), was measured. Under fuel-rich conditions, the sample releases oxygen so that the output difference assumes a negative value. The amount of oxygen release in each catalyst sample was obtained by summating the output differences under fuel-rich conditions in certain cycles.
Referring to FIG. 8, in the inventive example, the amount of oxygen release was large when the mass ratio of Rh carried on ZrLaO/Al2O3 was within the range from 33 mass % to 80 mass % both inclusive, and it was particularly large when the mass ratio was within the range from 40 mass % to 70 mass % both inclusive. On the other hand, in the comparative example using ZrO2 in place of ZrLaO, the amount of oxygen release did not significantly change even if the mass ratio of Rh carried on Zr/Al2O3 changed. Thus, it is believed that one of factors of the catalyst according to the present invention exhibiting high activity lies in that the amount of oxygen release from the OSC is increased when the ambient atmosphere becomes fuel-rich. Specifically, when the amount of oxygen release is increased, the catalyst enhances the capacity to convert HC and CO in exhaust gas by reduction and concurrently efficiently reduces NOx.
Now, consideration is made of reasons of why the OSC enhances its oxygen release capacity. FIG. 10A schematically shows the surface state of the catalyst under fuel-lean conditions when Rh/ZrLaO/Al2O3 coexists with Rh/OSC. Since Rh in Rh/ZrLaO/Al2O3 is oxidized under fuel-lean conditions, an active oxygen atom exists on the surface of Rh. On the other hand, Rh on the OSC remains reduced.
As shown in FIG. 10B, when the ambient atmosphere becomes fuel-rich, CO ions in the ambient atmosphere are attracted to active oxygen atoms on the surfaces of Rh particles on Rh/ZrLaO/Al2O3. Then, the reaction of CO+O→CO2 occurs at each Rh particle so that the oxygen atom on the Rh particle is removed. However, Rh in Rh/ZrLaO/Al2O3 is likely to be kept oxidized as is obvious from data in FIG. 6. Therefore, oxygen is actively supplied to Rh in Rh/ZrLaO/Al2O3 from Rh/OSC absorbing oxygen existing in the vicinity of Rh/ZrLaO/Al2O3. In other words, since Rh/OSC coexists with Rh/ZrLaO/Al2O3, oxygen release is promoted.
Patent Application
20070265160
