Exhaust gas catalyst, method for the production of carrier, method for the production of exhaust gas catalyst, and apparatus for treating exhaust gas

Abstract

An exhaust gas controlling catalyst includes zirconia particles; ceria particles which contact the zirconia particles, of which a mean particle size is smaller than a mean particle size of the zirconia particles; and an active metal that is supported on at least the ceria particles in a dispersed manner.

Claims

1. An exhaust gas controlling catalyst, comprising: zirconia particles; ceria particles which contact the zirconia particles, of which a mean particle size is smaller than a mean particle size of the zirconia particles; and an active metal that is supported on at least the ceria particles in a dispersed manner, wherein the active metal is Cu, wherein a mean particle size of the ceria particles is 1 to 9 nm, wherein a percentage of the ceria particles to the zirconia particles is 5 to 30% by weight, and a mean particle size of the zirconia particles is 5 to 30 nm.

2. An exhaust gas control apparatus, comprising: a first-stage base metal catalyst system that oxidizes HC and CO to harmless components, of which a conversion efficiency for HC is higher than a conversion efficiency for CO; and a second-stage base metal catalyst system that has the exhaust gas controlling catalyst according to claim 1 and reduces NOx to harmless components.

3. The exhaust gas controlling catalyst according to claim 1, wherein the ceria particles are supported on the surface of the zirconia particles in a dispersed manner.

4. An exhaust gas control apparatus according to claim 2, wherein the first-stage base metal catalyst system is a Fe/Al.sub.2O.sub.3 catalyst.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

(2) FIGS. 1A and 1B schematically illustrate the mechanisms by Which the conversion efficiency of a CeO.sub.2Cu catalyst is changed at low temperature and high temperature, respectively;

(3) FIG. 2 illustrates an example of the process of the production of a CeO.sub.2 particle-ZrO.sub.2 particle carrier powder according to Example of the present invention;

(4) FIG. 3 shows the NOx conversion efficiency of the catalyst that is composed of a CeO.sub.2 particle-ZrO.sub.2 particle carrier with Cu supported thereon according to Example of the present invention in comparison with that which was observed when a CeO.sub.2ZrO.sub.2 solid solution carrier according to Comparative Example 1 or a ZrO.sub.2 carrier according to Comparative Example 2 was used;

(5) FIGS. 4A and 4B show TEM images of the CeO.sub.2ZrO.sub.2 solid solution carrier according to Comparative Example 1 and CeO.sub.2 particle-ZrO.sub.2 particle carrier according to Example of the present invention, respectively;

(6) FIG. 5 shows XRD charts of the CeO.sub.2ZrO.sub.2 solid solution carrier according to Comparative Example 1 and CeO.sub.2 particle-ZrO.sub.2 particle carrier according to Example of the present invention;

(7) FIGS. 6A, 6B and 6C show the catalyst particle structures of a catalyst that is composed of the CeO.sub.2ZrO.sub.2 solid solution carrier (Comparative Example 1) with Cu supported thereon, a catalyst that is composed of the ZrO.sub.2 carrier (Comparative Example 2) with Cu supported thereon, and the CeO.sub.2 particle-ZrO.sub.2 particle carrier with Cu supported thereon according to the Example of the present invention;

(8) FIG. 7 shows the change in NOx conversion efficiency depending on the ratio of the amount of CeO.sub.2 to the amount of ZrO.sub.2; and

(9) FIG. 8 illustrates a function-separated controlling system to which a NOx controlling catalyst according to Example of the present invention is applied.

DETAILED DESCRIPTION OF EMBODIMENTS

(10) The reason why the NOx conversion efficiency increases at low temperature (250 C. or lower) but decreases at high temperature (500 C. or higher) when a base metal, such as Cu, is supported as an active metal on a carrier that uses ceria (CeO.sub.2) is discussed below.

(11) FIG. 1A schematically illustrates the mechanism by which a CeO.sub.2 carrier promotes formation of metal Cu at low temperature in a fuel rich atmosphere. Oxygen is released from CeO.sub.2 at the interface with CuO, and CeO.sub.2 at the surface is converted from Ce.sup.4+ to Ce.sup.3+. As a result, oxygen migrates from CuO to CeO.sub.2 to form metal Cu, and the metal Cu formed reduces NO to harmless nitrogen.

(12) FIG. 1B schematically illustrates a predicted mechanism at high temperature. Oxygen migrates from CeO.sub.2 in the bulk (inside of the carrier) to the surface and oxidizes Cu to CuO, resulting in a decrease in NOx reduction ability. Then, the CuO reacts with the gas phase to form metal Cu, restoring the reduction ability. In other words, the NOx conversion efficiency decreases at high temperature because formation of metal Cu is inhibited because of the release of oxygen from the bulk (inside of the carrier).

(13) Thus, in order to improve the purification performance of a carrier that uses CeO.sub.2 at high temperature, it is necessary to prevent release of oxygen from the bulk.

(14) Conventional CeO.sub.2ZrO.sub.2 solid solution carriers have high oxygen storage release capacity (OSC) and release a large amount of oxygen at high temperature. Thus, CuO is formed and the formation of metal Cu is less likely to occur.

(15) Thus, in Example of the present invention, the CeO.sub.2 and ZrO.sub.2 are not formed into a solid solution. Instead, CeO.sub.2 particles, are supported on the surfaces of ZrO.sub.2 particles in a finely dispersed manner. Therefore, release of oxygen from the bulk (inside of the carrier) is reduced and formation of metal Cu is accelerated at high temperature, leading to high NOx conversion efficiency. As the same time, a high NOx conversion efficiency, which originates from CeO.sub.2, is achieved at low temperature.

(16) Preferably, the ceria particles have a mean Particle size of 1 to 9 nm.

(17) Preferably, the zirconia particles have a mean particle size of 5 to 50 nm.

(18) Preferably, the percentage of the ceria particles to the zirconia particles is 5 to 30% by weight.

(19) According to the Example of the present invention, a CeO.sub.2 particle-ZrO.sub.2 particle carrier that is composed of ZrO.sub.2 particles, and CeO.sub.2 particles that are supported on the surfaces of the ZrO.sub.2 particles in a finely dispersed manner is produced. The procedure and conditions are shown in FIG. 2.

(20) [1] As ingredients, ZrO.sub.2La.sub.2O.sub.3 powder (2.50 g) and cerium nitrate hexahydrate (1.75 g) are prepared.

(21) [2] The ingredients are added to 500 ml of purified water, and the mixture is stirred in a beaker to form a suspension.

(22) [3] After the suspension is transferred into a 1 L separable beaker, a precipitant solution, a purified water solution of hexamethylenetetramine (HMT), was added to the separable beaker.

(23) [4] The solution that is obtained in step [3] is subjected to an aging treatment at 80 C. for one hour.

(24) [5] The solution that is obtained in step [4] is filtered under pressure and the residue is washed with 2 L of purified water.

(25) [6] The residue that is obtained in step [5] is thermally dried at 120 C. for 12 hours to obtain a ceria precursor that is supported on zirconia particles.

(26) Here, the amount of CeO.sub.2 is expected to be approximately 5 wt %. When necessary, the steps [1] to [6] are repeated. For example, to increase the amount of CeO.sub.2 to 20 wt %, the steps of [1] to [6] are repeated four times in total.

(27) [7] The zirconia particles and the ceria precursor that is supported on the zirconia particles are calcined at 600 C. for three hours. One hour is spent to increase the temperature to 600 C.

(28) In this way, a CeO.sub.2 particle-ZrO.sub.2 particle-La.sub.2O.sub.3 powder carrier is obtained.

(29) As Comparative Examples, conventional CeO.sub.2ZrO.sub.2 solid solution carrier (Comparative Example 1) and ZrO.sub.2 carrier (Comparative Example 2) were produced by conventionally known methods.

(30) Cu was deposited on each powder carrier in an amount of 5 wt % to prepare a catalyst.

(31) Using a model gas that had the following composition, the NOx conversion efficiency was measured at 250 C., 500 C. and 600 C. under the following conditions. The results are shown in FIG. 3.

(32) <Model gas composition> NO=0.3%, CO=0.9%, O.sub.2=0.3%, H.sub.2O=3%, balance=N.sub.2, gas flow rate=10 L/min, catalyst pellet=3 g

(33) When the CeO.sub.2ZrO.sub.2 solid solution carrier of Comparative Example 1 was used, the catalyst exhibited a high purification capacity from low temperature (250 C.), and showed a significant increase in the conversion efficiency at 500 C. However, the catalyst showed a decrease in the conversion efficiency at a high temperature of 600 C.

(34) When the ZrO.sub.2 carrier of Comparative Example 2 was used, the catalyst showed a low conversion efficiency at a low temperature (250 C.) but showed a high conversion efficiency at high temperatures of 500 C. to 600 C.

(35) When the CeO.sub.2 particle-ZrO.sub.2 particle carrier of Example of the present invention was used, the catalyst exhibited a high purification capacity from a low temperature (250 C.), and showed a high conversion efficiency even at high temperatures of 500 C. to 600 C.

(36) As described above, according to Example, a high NOx conversion efficiency can be achieved at both low temperature (250 C.) and high temperature (600 C.).

(37) In particular, the improvement in conversion efficiency at high temperature in Example, in spite of the fact that the carriers of Comparative Example 1 and Example are the same in that CeO.sub.2 and ZrO.sub.2 are combined, is believed to be due to the difference in particle structure between Comparative Example 1 and Example.

(38) FIGS. 4A and 4B show TEM images that show the particle structure of the carriers of Comparative Example 1 and Example, respectively.

(39) The carrier of Comparative Example 1, which is shown in FIG. 4A, has a conventional structure in which CeO.sub.2 and ZrO.sub.2 form a solid solution. The TEM image shows that only one type of particles is present.

(40) The carrier of Example, which is shown in FIG. 4B, has a structure in which CeO.sub.2 particles with a mean particle size of 5 nm are supported, in a dispersed manner, on the surfaces of ZrO.sub.2 particles with a mean particle size of 30 nm.

(41) FIG. 5 compares the XRD charts of the carriers of Example and Comparative Example 1. The peak position in Comparative Example 1 is shifted from that of ZrO.sub.2 because of a change in crystal structure due to the formation of solid solution with CeO.sub.2.

(42) On the contrary, in Example of the present invention, the peak position is consistent with the original peak position of ZrO.sub.2, which means that ZrO.sub.2 and CeO.sub.2 are present without forming a solid solution. The fact that ZrO.sub.2 is present in an undissolved form indicates that its counterpart CeO.sub.2 is not dissolved either. The reason why the peak of CeO.sub.2 cannot be distinguished is that the amount of CeO.sub.2 particles is so small (20% of the amount of ZrO.sub.2 particles) and the mean particle size of CeO.sub.2 particles is so small in this example that a clear peak does not appear. However, a small peak of CeO.sub.2 is present around the peak of ZrO.sub.2 and slightly broadens the peak of ZrO.sub.2.

(43) As described above, it is believed that, in Example of the present invention, because CeO.sub.2 particles are supported on the surfaces of ZrO.sub.2 particles in a highly dispersed manner, release of oxygen from inside of the carrier at high temperature is prevented and formation of metal Cu as an active metal is accelerated, leading to a high NOx conversion efficiency.

(44) FIGS. 6A, 6B and 6C schematically illustrate the particle structures of Comparative Example 1, Comparative Example 2, and Example of the present invention, respectively. The carrier of Comparative Example 1, which is shown in FIG. 6A, composed of particles of a solid solution of CeO.sub.2 and ZrO.sub.2 (CZ particles: 15 to 30 nm), and the carrier of Comparative Example 2, which is shown in FIG. 6B, is composed of ZrO.sub.2 particles (30 nm). In comparison, the carrier of Example of the present invention, which is shown in FIG. 6C, composed of ZrO.sub.2 particles (30 nm) and CeO.sub.2 particles (5 nm).

(45) Typically, the ZrO.sub.2 particles have a primary particle size of 5 to 30 nm (approximately 30 nm in average), and the CeO.sub.2 particles have a primary particle size of 1 to 9 nm (approximately 5 nm in average). The CeO.sub.2 particles do not have a shell structure that continuously covers the surfaces of the ZrO.sub.2 particles but are supported on surfaces of ZrO.sub.2 in a discretely dispersed (highly dispersed) manner. To obtain CeO.sub.2 particles that are supported in such a highly dispersed manner, the amount of CeO.sub.2 is at least 5% by weight but no more than 30% by weight, with respect to the amount of ZrO.sub.2 The amount of CeO.sub.2 is preferably approximately 20% by weight, with respect to the amount of ZrO.sub.2.

(46) FIG. 7 shows the change in NOx conversion efficiency depending on the ratio of the amount of CeO.sub.2 to the amount of ZrO.sub.2. A high NOx conversion efficiency is achieved when the weight ratio of CeO.sub.2 is in the range of 5 to 30%, and the NOx conversion efficiency has a peak when the weight ratio of CeO.sub.2 is 20%.

(47) FIG. 8 illustrates a typical example of an exhaust gas control apparatus (function-separated controlling system) to which the NOx controlling catalyst that uses the CeO.sub.2 particle-ZrO.sub.2 particle carrier of the Example of the present invention is applied.

(48) The apparatus that is shown in FIG. 8 includes a first-stage base metal catalyst system with a honeycomb structure: Fe/Al.sub.2O.sub.3 catalyst (=active metal/carrier, the same applies in the following), a second-stage base metal catalyst system with a honeycomb structure: Cu/(CeO.sub.2ZrO.sub.2) catalyst (the CeO.sub.2 particle-ZrO.sub.2 particle carrier of the present invention is used), and an additional third-stage base metal catalyst system with a honeycomb structure that is located on the downstream of the second-stage base metal catalyst system: Ag/Al.sub.2O.sub.3 catalyst. The first-stage base metal catalyst system to the third-stage base metal catalyst system are arranged in series in this order as shown from left to right in FIG. 8.

(49) The exhaust gas from the engine is controlled to be slightly richer (A/F=approximately 14) than the stoichiometric ratio (A/F=14.6).

(50) Under these conditions, HC and CO are oxidized to harmless components by O.sub.2 that remains in the exhaust gas in the first-stage Fe/Al.sub.2O.sub.3 catalyst system. At this time, the conversion efficiency for CO is higher than that for HC. CO that remains unoxidized or is produced here is reduced to harmless components when NOx is reduced to harmless components through the following CONO reaction in the second-stage Cu/(CeO.sub.2ZrO.sub.2) catalyst system.
CONO reaction: CO+NO.fwdarw.CO.sub.2+()N.sub.2
HC and CO that remains here is oxidized to harmless components in the third-stage Ag/Al.sub.2O.sub.3 catalyst system, which is additionally provided on the downstream side. To accelerate the oxidation, air is introduced as needed through an air introduction device that is provided between the second-stage catalyst system and the third-stage catalyst system.

(51) When the CeO.sub.2 particle-ZrO.sub.2 particle carrier according to Example of the present invention is used in an exhaust gas control apparatus that has the above structure, the temperature range of exhaust gas in which required performance is achieved can be increased.

(52) According to the present invention, there are provided an NOx controlling catalyst that uses a base metal as an active metal and that has purification capacity at both low temperature (250 C. or lower) and high temperature (500 C. or higher), a method for the production of a carrier, a method for the production of a catalyst, and an exhaust gas control apparatus.