Catalyst for purifying an exhaust gas

Abstract: A catalyst for purifying an exhaust gas includes a composite oxide including gold (Au) and having crystalline unit cells in every one of which an Au ion is disposed. Since the Au exists in a state of an ion, the catalyst exhibits a good catalytic activity. Since the Au is present in such a manner that it is surrounded by the other elements, it is inhibited from moving. As a result, the catalyst is improved in terms of the heat resistance.

Description: The present invention relates to a novel catalyst for purifying an exhaust gas, catalyst which comprises Au. Description of the Related Art As an exhaust-gas-purifying catalyst for purifying an exhaust gas emitted from an automobile engine, a catalyst has been used widely which includes a porous oxide support, such as alumina, loaded with a noble metal, such as Pt, Rh and Pd. Recently, a research has been developed on a catalyst in which Au is used instead of a noble metal, such as Pt, Rh and Pd. For example, there is a catalyst, in which Au particulates are loaded on a support, such as alumina. The catalyst exhibits a good oxidizing activity for an inflammable gas. However, the catalyst has a problem in that the Au particulates exhibit low heat resistance. Hence, a variety of measures are taken in order to improve the heat resistance. For instance, in Japanese Unexamined Patent Publication (KOKAI) No. 4-235,742, as a catalyst for decomposing aldehyde in an exhaust gas at a low temperature, there is disclosed an aldehyde-decomposing catalyst for purifying an exhaust gas, catalyst which includes: a composite oxide expressed by a chemical formula, Ce 1-x Zr x Cu 1-y Au y O 3-z , in which "x" is 0.1 or 0.2, "y" falls in the range of from 0.02 to 0.04 and "z" is 1.0 or less; an activated alumina containing cerium; and an oxide having an oxygen storage ability. It is therefore an object of the present invention to improve the heat resistance of Au and to inhibit the purifying performance after a durability test from decreasing by making a structure of a catalyst different from that of the catalyst disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 4-235,742. SUMMARY OF THE INVENTION The object of the invention is achieved by the combination of the features defined in claim 1. Preferable embodiments of the invention are set forth in the dependent claims 2 and 3, respectively. The exhaust-gas-purifying catalyst of the present invention is good in terms of the durability of the catalytic activity of Au, and can maintain high purifying performance from an initial stage to a final stage after a durability test. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Having generally described the present invention, a further understanding can be obtained by reference to the specific preferred embodiments which are provided herein for the purpose of illustration only and not intended to limit the scope of the appended claims. The present exhaust-gas-purifying catalyst comprises the composite oxide which includes Au, and has crystalline unit cells in every one of which an Au ion is disposed. Namely, since the Au exists in a state of the ions, the present exhaust-gas-purifying catalyst exhibits a good catalytic activity. Further, this exhaust-gas-purifying catalyst is extremely superb in terms of the heat resistance. It is believed that these advantages result from the arrangement that the Au is present in such a manner that it is surrounded by the other elements, and that it is accordingly inhibited from moving. Note that, in the conventional catalyst expressed by the chemical formula, Ce 1-x Zr x Cu 1-y Au y O 3-z , in which "x" is 0.1 or 0.2, "y" falls in the range of from 0.02 to 0.04 and "z" is 1.0 or less, the Au content is so insufficient that the Au ion is not disposed in every one of the crystalline unit cells. Thus, the conventional catalyst is structurally different from the present exhaust-gas-purifying catalyst, and operates differently from the present exhaust-gas-purifying catalyst in which the Au mainly purifies an exhaust gas. In the aforementioned present exhaust-gas-purifying catalyst a metal is selected from the group consisting of alkaline-earth metals. When an alkaline-earth metal (M) is composited in the composite oxide, the structure, which is turned into the crystal including an Au ion in every one of the crystalline unit cells, is expressed by a chemical formula, Au 2 M 5 O 8 . As for the alkaline-earth metal, which is used preferably in the present exhaust-gas-purifying catalyst, Be, Mg, Ca, Sr and Ba can be exemplified, and it is possible to composite at least one of these elements. Among them, Sr and Ba are especially preferred. The present exhaust-gas-purifying catalyst can be used by loading it on a porous oxide, such as activated alumina, alumina, zirconia, silica and titania. The present exhaust-gas-purifying catalyst can be produced by mixing the powders of the aforementioned metals or metallic compounds in a solid phase and burning them. Alternatively, oxide precursors can be precipitated from a mixture solution including the aforementioned metals by the co-precipitation method or the sol-gel process, and they can be burned to make a composite oxide. Hereinafter, the present invention will be described in detail with reference to examples and comparative examples. Example No. 1 4.1 g (0.01 mol) of an HAuCl 4 · 4H 2 O powder and 5.4 g (0.025 mol) of an Sr(CH 3 COO) 2 · 1/2H 2 O powder were mixed uniformly in a solid phase by using a ball mill. The mixture was calcined in air at 300 °C for 3 hours. The resulting precursor was pulverized uniformly, and was thereafter burned in air at 1,000 °C for 50 hours, thereby preparing a composite oxide powder whose composition was Au 2 Sr 5 O 8 . According to the XRD analysis, the resulting powder was found to be a composite oxide. According to the x-ray diffraction, the crystalline structure of this composite oxide was found to be the type F222. Therefore, in this composite oxide, an Au ion was included in every one of the crystalline unit cells. 1 part by weight of the resulting composite oxide powder and 99 parts by weight of a γ-Al 2 O 3 powder were mixed by using a ball mill, thereby making a catalyst of Example No. 1. The content of Au in the catalyst was 0.4% by weight. Example No. 2 2.8 g (0.01 mol) of an AuSr alloy powder and 4.0 g (0.015 mol) of an Sr(CH 3 COO) 2 · 1/2H 2 O powder were mixed uniformly in a solid phase by using a ball mill. The mixture was burned in air at 650 °C for 100 hours, thereby preparing a composite oxide powder whose composition was Au 2 Sr 5 O 8 . According to the XRD analysis, the resulting powder was found to be a composite oxide. According to the x-ray diffraction, the crystalline structure of this composite oxide was found to be the type F222. Therefore, in this composite oxide, an Au ion was included in every one of the crystalline unit cells. 1 part by weight of the resulting composite oxide powder and 99 parts by weight of a γ-Al 2 O 3 powder were mixed by using a ball mill, thereby making a catalyst of Example No. 2. The content of Au in the catalyst was 0.4% by weight. Comparative Example No. 1 Ion exchanged water was prepared, included a γ-Al 2 O 3 powder in an amount of 0.51% by weight, and exhibited a temperature of 60 °C. Into the ion exchanged water, the following were solved so that an HAuCl 4 · 4H 2 O powder was solved in an amount of 0.022% by weight, Na 2 S 2 O 3 · 5H 2 O was solved in an amount of 0.106% by weight, Na 2 SO 3 was solved in an amount of 0.269% by weight, and C 6 H 7 NaO 6 was solved in an amount of 1.06% by weight. The ion exchanged water was held at 60 °C, and was stirred for 24 hours, thereby reducing and precipitating Au on the γ-Al 2 O 3 in a particulate manner. Thereafter, the ion exchanged water was filtered, and the resulting <Test and Evaluation> Each of the resulting catalysts was pelletized by an ordinary method, and was disposed in a testing and evaluation apparatus in a predetermined amount. Then, an evaluation model gas as set forth in Table 1 below was flowed at a space velocity of 150,000 hr -1 . Then, the conversions of C 3 H 6 were measured substantially continuously while decreasing the inlet gas temperature from 500 °C to 100 °C by a temperature decrement rate of 10 °C/minute. Note that the conversions were calculated by the following equation: Conversion = 100 x {(C 3 H 6 Concentration in Inlet Gas) -(C 3 H 6 Concentration in Outlet Gas)}/(C 3 H 6 Concentration in Inlet Gas) From the resulting data, 50% conversion temperatures of C 3 H 6 were determined, and the results are set forth in Table 2 below as initial values. Subsequently, a durability test was carried out, in which a durability model gas as set forth in Table 1 below was flowed in each of the pelletized catalysts at an inlet gas temperature of 800 °C for 5 hours. Then, on each of the pelletized catalysts subjected to the durability test, the conversions of C 3 H 6 were measured in the aforementioned manner, and the results are set forth in Table 2 below as after-durability-test values. CO (ppm) C 3 H 6 (ppmC) NO (ppm) O 2 (%) CO 2 (%) H 2 O (%) N 2 For Evaluation 1000 670 250 7.3 6.7 5 Balance For Durability 1000 670 500 6.5 10 10 Balance Catalyst Composition Temp. at 50% Conversion Chemical Formula Au Content (% by Weight) Initial (°C ) After Durability Test (°C) Ex.#1 Au 2 Sr 5 O 8 0.4 345 346 Ex.#2 Au 2 Sr 5 O 8 0.4 340 355 C.E.#1 Au/ γ -Al 2 O 3 2.0 378 433 The following are apparent from Table 2. Although the Au contents of the catalysts of the examples were smaller than that of Comparative Example No. 1, the catalysts of the examples were good in terms of the initial purifying performance, and were hardly degraded in terms of the conversion after the durability test. On the other hand, the catalyst of Comparative Example No. 1 was not only inferior to the catalysts of the examples in terms of the initial purifying performance, but also the purifying performance was deteriorated sharply after the durability test. Namely, the catalysts of the examples exhibited high purifying performance initially and even after the durability test. It is apparent that this advantage resulted from the arrangement that the composite oxides had the compositions recited in the claims.

Claims: A catalyst for purifying an exhaust gas, comprising: a composite oxide including gold (Au) and at least one metal "M" selected from the group consisting of alkaline-earth metals and expressed by a chemical formula, Au 2 M 5 O 8 , and having crystalline unit cells in every one of which an Au ion is disposed. The catalyst according to claim 1, wherein the metal "M" is at least one member selected from a group consisting of Be, Mg, Ca, Sr and Ba. The catalyst according to claim 2, wherein the metal "M" is at least one member selected from the group consisting of Sr and Ba.

Inventors: MIYAKE YOSHIHARU, TSUJI SHINJI, MIYAKE, YOSHIHARU, TSUJI, SHINJI
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 2000
Main classification: B01D53
IPC codes: B01J23, B01D53