EXHAUST GAS PURIFICATION SYSTEM

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to an exhaust gas purification system using a nitrogen oxide (below, sometimes abbreviated as "NO _x ") purification catalyst, more particularly relates to an exhaust gas purification system able to improve the NO _x purification performance by an exhaust gas composition including hydrocarbons (below, sometimes abbreviated as "HC") at the time of non-steady operation.

2. Description of the Related Art

[0002] In recent years, from the viewpoint of

protection of the global environment, emission controls have been toughened around the world. As one

countermeasure, in internal combustion engines, an exhaust gas purification catalyst is being used. In such an exhaust gas purification catalyst, to efficiently remove the HC (hydrocarbons) , carbon monoxide, and NO _x in exhaust gas, platinum, gold, rhodium, and other precious metals are being used as the catalytic ingredients. In vehicles using such a purification catalyst, for example, gasoline engine vehicles or diesel engine vehicles, various systems are being used to improve the catalytic activity and the fuel economy. For example, to ' improve the fuel economy, in steady state operation, fuel is burned under a lean air-fuel ratio (A/F) (oxygen rich) conditions, while to improve the catalytic activity, fuel is burned temporarily under stoichiometric

(stoichiometric air-fuel ratio, A/F=14.7) to rich (fuel excess) conditions.

[0003] This is because such conventional known

platinum, palladium, rhodium, and other precious metal catalysts have a low NO _x purification performance under low temperature and oxidizing conditions. If raising the purification catalyst to a high temperature to improve the purification performance, it is necessary add HC or carbon monoxide etc. to obtain a reducing atmosphere. Due to the effects on the catalytic activity, even during steady operation, it is not possible to increase the air- fuel ratio (A/F) . With such precious, metal catalysts, there is therefore a limit to the improvement of the fuel economy. In this way, in conventional known precious metal catalysts, it is necessary to lower the energy required for raising the purification catalyst to a high temperature so as to obtain the purification performance and the air-fuel ratio (A/F) of the engine. To improve the fuel economy of automobile engines and other internal combustion engines, a new purification catalyst able to exhibit NO _x purification performance at a low temperature and/or in an oxidizing atmosphere is being sought. On the other hand, the precious metal catalysts all have

problems of resource depletion. A purification catalyst which uses another metal, has a purification performance of an equal or better extent than a conventional precious metal catalyst, and can reduce the amount of catalyst or precious metal used has been sought. For this reason, various improvements have been tried out on purification catalysts.

[0004] For example, Japanese Patent Publication (A)

No. 10-216518 describes a gold alloy catalyst of gold and one or two metals of platinum, palladium, gold, copper, and nickel in which the weight ratio is made Au/M=l/9 to 9/1 and the amount of gold dissolved in the alloy is made 20 to 80 wt%. Further, the catalyst shown as a specific example in the publication is a catalyst comprised of gold and a metal M carried as a gold alloy with palladium or platinum on an A1 ₂ 0 ₃ carrier. Under a reducing

atmosphere, it exhibits a high NO _x purification

performance, but at a low temperature and/or in an oxidizing atmosphere, the N0 _X purification performance is low . [0005] Further, Japanese Patent Publication (A) No. 2001-239161 describes a low temperature harmful gas purification catalyst comprised of a carrier of a metal oxide or- carbonaceous material on which, using a high temperature, high pressure fluid, ultrafine particles of at least one type of metal selected from the group comprised of platinum, palladium, rhodium, ruthenium, iridium, osmium, gold, silver, copper, manganese, iron, and nickel is carried. Further, the catalyst shown as a specific example in the publication is a purification . catalyst on which one type of platinum, palladium, rhodium, ruthenium, nickel, or gold is carried and which exhibits N0 _X purification performance in a reducing atmosphere.

[0006] However, with exhaust gas purification systems having these known NO _x purification catalysts, it is difficult to reduce - the amount of use of precious metal and achieve an N0 _X purification performance at a low temperature and/or in an oxidizing atmosphere. This is because the NO _x reaction activity of the NO _x purification catalyst is affected by the composition in the exhaust gas .

SUMMARY OF THE INVENTION

[0007] Therefore, an object of the present invention is to provide an exhaust gas purification system which uses an NO _x purification catalyst reducing the amount of use of precious metal and able to exhibit an NO _x

purification performance at a low temperature and/or in an oxidizing atmosphere and which has an NO _x purification performance even by an exhaust gas composition containing HC and NO _x at the time of _/ non-steady operation etc.

[0008] The present invention relates to an exhaust gas purification system using an NO _x purification apparatus, provided with an NO _x purification catalyst provided in an exhaust gas passage and comprised of a carrier on which are carried nanoparticles including gold atoms and nickel atoms in a state of close proximity and a means for predicting the amount of exhaust of NO _x , which, when it is predicted that NO _x will be exhausted, operates means for . increasing carbon monoxide in exhaust gas fed to the NO _x purification catalyst and, when it is predicted that NO _x will not be exhausted, purifies exhaust gas exhausted from an engine by the NO _x purification catalyst without injection of excess fuel.

[0009] Summarizing the advantageous effects of the present invention, according to the present invention, it is possible to provide an NO _x purification catalyst reducing the amount of use of precious metal and able to exhibit an NO _x purification performance at a low

temperature and/or in an oxidizing atmosphere and to give an ΝΟχ purification performance even by an exhaust gas composition containing HC and NO _x at the time of non- steady operation etc.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] These and other objects and features of the present invention will become clearer from the following description of the preferred embodiments given with reference to the attached drawings, wherein:

FIG. 1 is a graph showing comparatively the NO purification characteristics of NO _x purification catalysts obtained by reference examples and comparative examples;

FIG. 2 is a graph showing comparatively the NO purification characteristics of exhaust gases of various gas ..compositions of NO _x purification catalysts obtained by the reference examples;

FIG. 3 is a schematic view of an exhaust gas

purification apparatus able to be used for an exhaust gas purification system of an embodiment of the present invention;

FIG. 4 is a schematic view of an exhaust gas

purification apparatus able to be used for an exhaust gas purification system of another embodiment of the present invention; and

FIG. 5 is a schematic view of an exhaust gas purification system of an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] In the present invention, the exhaust gas purification system has to be an exhaust gas purification system using an NO _x purification apparatus, provided with an ΝΟχ purification catalyst provided in an exhaust gas passage and comprised of a carrier on which are carried nanoparticles including gold atoms and nickel atoms in a state of close proximity and a means for predicting the amount of exhaust of NO _x , which, when it is predicted that ΝΟχ will be exhausted, operates means for increasing carbon monoxide, in exhaust gas fed to the NO _x purification catalyst and, when it is predicted that NO _x will not be exhausted, purifies exhaust gas exhausted from an engine by the NO _x purification catalyst without injection of excess fuel. Due to this, it is possible to provide an NO _x purification catalyst reducing the amount of use of precious metal and able to exhibit an NO _x purification performance at a low temperature and/or in an oxidizing atmosphere and to given an NO _x purification performance even by an exhaust gas composition containing HC and NO _x at the time of non-steady operation etc.

[0012] Below, referring to the drawings, ^' embodiments of the present invention will be explained. Referring to FIG. 1, an NO _x purification catalyst of an embodiment of the present invention comprised of carrier particles on which are carried nanoparticles including gold atoms and nickel atoms in a state of close proximity, compared with an ΝΟχ purification catalyst outside the scope of the present invention comprised of carrier particles on which nickel alone or gold alone is carried, exhibits an NO-CO catalytic activity in the temperature range of 300 to

500°C, in particular exhibits a high NO-CO catalytic activity even at a temperature of about 425°C or more.

Further, with an NO _x purification catalyst which, even if jointly using gold and nickel, is comprised of carrier particles on which is carried a simple mixture of these, not on which are carried nanoparticles including gold atoms and nickel atoms in a state of close proximity, the NO-CO catalytic activity at.500°C becomes conversely lower than with nickel alone.

[0013] Referring to FIG. 2, if using the NO _x

purification catalyst to treat exhaust gas of various compositions including C ₃ H ₆ at the time of non-steady operation, compared with an NO-CO gas composition

(stoichiometric) of a curve 1, in. a temperature range of about 425 to 500°C, with an NO-CO-0 ₂ -C ₃ H ₆ exhaust gas composition (stoichiometric) of a curve 2 and an NO-C ₃ H ₆ exhaust gas composition (stoichiometric) of a curve 3, in particular an NO-CO-0 ₂ -C ₃ H6 exhaust gas composition

(stoichiometric) of a curve 2, the NO purification rates are low and, further with an NO-CO-C ₃ H ₆ exhaust gas composition (rich) of a curve 4 and an NO-CO-0 ₂ -C ₃ H ₆

exhaust gas composition (rich) of a curve 5, the NO purification rates are slightly low, but high NO

purification rates are still obtained. That is, from FIG. 2, it will be understood that with an NO _x purification catalyst comprised of the above carrier particles on which are carried gold atoms and nickel atoms in a state of close proximity and at which at least one of the two atoms comprises primary particles, under stoichiometric control, with a gas composition comprised of the exhaust gas in which C ₃ H ₆ is copresent with NO, the NO

purification activity greatly falls, but under rich control, if a gas composition in which C ₃ H6 is included- copresent with NO and carbon monoxide, the NO

purification activity is high.

[0014] The present invention was made based on this discovery. An exhaust gas purification apparatus 10 able to be used for the exhaust gas purification system of an embodiment of the present invention, as shown in FIG. 3, is provided with, in an exhaust gas passage 3 from an engine 2, an NO _x purification catalyst 4 comprised of carrier particles on which are carried gold atoms and nickel atoms in a state of close proximity and at which at least one of the two atoms comprises primary particles and, at a position at a downstream side of the NO _x purification catalyst (also referred to as an "AuNi-NO _x reducing catalyst") 4, an NO _x sensor 5, A/F meters 6A, 6B, and an HC sensor 7.

[0015] Further, an exhaust gas purification apparatus 10 able to be used for the exhaust gas purification system of another embodiment of the present invention, as shown in FIG. 4, is provided with, in an exhaust gas passage 3 from an engine 2, an NO _x purification catalyst 4 comprised of carrier particles on which are carried gold atoms and nickel atoms in a state of close proximity and at which at least one of the two atoms comprises primary particles (also referred to as an "AuNi-NO _x reducing catalyst"), at a position at a downstream side of the NO _x purification catalyst, an NO _x sensor 5, A/F meters 6A, 6B, and an HC sensor 7, at the upstream side of the NO _x purification catalyst 4, an oxidation catalyst 8 which partially oxidizes the HC in the exhaust gas at a

position, and, at a position at the upstream side of the same, a fuel injection part 9.

[0016] An exhaust gas purification system 1 of an embodiment of the present invention, as shown in FIG. 5, uses the NO _x purification apparatus to predict at step 11 if NO _x will be exhausted to the outside, when it is predicted that NO _x will be exhausted, that is, when it is predicted and/or measured that the exhaust gas contains unreacted HC together with NO _x , at step 12, operates means for increasing the carbon monoxide in the exhaust gas fed to the NO _x purification catalyst, and, when it is

predicted that NO _x will not be exhausted, at step 13, purifies the exhaust gas exhausted from the engine by the exhaust purification catalyst in the present invention without injecting excess fuel. Whether or not the NO _x will be exhausted can be predicted from the relationship between the A/F value from the A/F meter 6A or 6B shown in FIG. 3 and FIG. 4, in particular the A/F value from the A/F meter 6B and the temperature estimated from a thermocouple (not shown) or the A/F value using a ^' NO concentration map prepared in advance. Further, it is possible to predict and/or measure if the unreacted HC is included by, for example, a device which measures the amount of exhaust of the unreacted HC, for example, by the HC sensor 7 shown in FIG. 3 and FIG. 4.

[0017] In an embodiment of the present invention, the means for increasing the carbon monoxide in the exhaust gas fed to the NO _x purification catalyst can control the engine by "fuel rich control". As the fuel rich control, as shown in FIG. 5, it is possible to inject fuel into the engine until an A/F value where no NO _x will be exhausted. Further, in another embodiment of the present invention, the means for increasing the carbon monoxide may be injection of fuel from a fuel injection part in the exhaust gas purification apparatus provided with an oxidation catalyst, provided in the exhaust gas passage at a position at an upstream side from the NO _x

purification catalyst, for partially oxidizing the HC in the exhaust gas and a fuel injection part at a position at an upstream side from the oxidation catalyst.

[0018] Further, according to the exhaust gas

purification system according to an embodiment of the present invention shown in FIG. 5, even when the exhaust gas exhausted from the engine is an exhaust gas

composition containing HC and NO _x in a stoichiometric ratio, by converting the air-fuel ratio from a

stoichiometric ratio to a rich ratio by injection of fuel to the engine or by injection of fuel at the upstream side of the oxidation catalyst, it is possible to use the engine or catalytic action of the oxidation catalyst at a low temperature to partially oxidize the HC and increase the carbon monoxide. The exhaust gas composition

introduced to the NO _x purification catalyst, as shown in curve 4 or curve 5 in FIG. 2, includes HC and NO plus the increased amount of carbon monoxide, so the NO _x

purification catalyst can be given a high NO _x purification performance. Further, in this embodiment of the present invention, when it is predicted that N0 _X will not be exhausted, for example, the air-fuel ratio can be

controlled to the lean side so as not to exhaust unburned HC from the engine.

[0019] The ΝΟχ purification catalyst of the present invention, in the above way, has to comprise carrier particles on which are carried nanoparticles including gold atoms and nickel atoms in a state of close

proximity. For this reason, the part where the atoms are in close proximity may include other metal atoms able to alloy with the two atoms, but an inert substance unable to alloy with the two atoms may be included only in a range able to secure a state where the two atoms are in close proximity. Therefore, the NO _x purification catalyst of the present invention can be obtained by for example using nanoparticles of the material forming the carrier as cores for obtaining nanoparticles in which the two metals are in close proximity. As other metal atoms able to alloy with both atoms of the gold atoms and nickel atoms, for example, tungsten (W) , which can improve the heat resistance of gold by alloying, may be mentioned. Further, as the carrier particles, AI2O3, Si0 ₂ , CeC>2, Ce0 ₂ - Zr0 ₂ , and other metal oxides particles may be mentioned.

[0020] The ΝΟχ purification catalyst in the present invention can be obtained by making carrier particles carry nanoparticles including gold atoms and nickel atoms in a state of close proximity. The nanoparticles

including the gold atoms and nickel atoms in a state of close proximity can, for example, be obtained by

reducing, in the presence of a polymer protective

material, a mixture of a gold salt and a nickel salt, by a reducing agent, for example, a polyol. This reduction reaction is preferably performed in a solution, preferably an aqueous solution, while stirring. After the end of the reduction reaction, the polymer protective material is separated and removed by any separating means, for example, centrifugal separation, extraction, etc., and the obtained colloid in which the gold atoms and nickel atoms are present in a state of close

proximity is uniformly mixed with the carrier so as to make the carrier carry nanoparticles which include gold atoms and nickel atoms in a state of close proximity. The size of the Au-Ni particles including the gold atoms and nickel atoms in a state of close proximity can be 0.2 to 100 nm, for example 1 to 20 nm or so.

[0021] As the gold salts, aurochloric acid (HAuCl ₄ ) , sodium chloroaurate, potassium chloroaurate, gold

trisodium disulfite, potassium trisodium disulfite, etc. may be mentioned. As the nickel salt, for example, nickel sulfate, nickel nitrate, nickel chloride, nickel bromide, nickel acetate, nickel hydroxide, etc. may be mentioned. As the polyols, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1,2- propanediol, dipropylene glycol, 1, 2-butanediol, 1,3- butanediol, 1, 4-butanediol, 2, 3-butanediol, 1,5- pentanediol, polyethylene glycol, etc. may be mentioned. To complete the reduction of the gold ions and nickel ions by the polyol, at the final stage of reduction, for example, boron dimethylamide, boron diethylamide, sodium borohydrate, borane, or another boron compound can be used as a reducing agent. As the polymer protective material, poly-N-vinylpyrrolidone, polyacrylamide, N- vinylpyrrolidone and acrylic acid copolymer,

polyvinylmethylketone, poly ( 4-vinylphenol) , oxazoline polymer, polyalkylene imine, and other polymers

containing functional groups may be mentioned.

[0022] The NO _x purification catalyst of the present invention is comprised of nanoparticles having gold and nickel as main ingredients. The composition of the gold and nickel is Au:Ni=7 to 91:93 to 9 (at%) , preferably 20 to 80:80 to 20 (at%), particularly preferably 40 to 60:60 to 40 (at%) . If the composition of gold and nickel in the solid is outside that range, the NO _x purification catalyst tends to drop in NO _x purification performance. The NO _x purification catalyst of the present invention combines gold and nickel and thereby has as a synergistic effect a superior NO _x purification performance unable to be

obtained by single ingredients and in particular has superior catalytic activity of NO _x purification even compared with Rh or other single precious metals.

Further, it is possible to determine the amount of the catalyst ingredient, that is, nanoparticles, or the carrier structure for purifying the NO _x in the exhaust gas to a sufficient level by the catalytic performance of the NO _x purification catalyst in the present invention.

[0023] In one embodiment in the present invention, as means for increasing the carbon monoxide (CO), it is possible to use fuel rich control to the engine or an oxidation catalyst for oxidizing the HC in the exhaust gas provided at a position at the upstream side from the

NO _x purification catalyst of the exhaust gas passage and fuel injection at a position at the upstream side of the oxidation catalyst. At the low temperature at which the exhaust gas from the engine contains HC, the HC in the exhaust gas can be used to produce carbon monoxide by the oxidation catalyst. The oxidation catalyst oxidizing the ^' HC is not particularly limited. For example, in general, a known catalyst used as an HC oxidation catalyst, for example, Pd/Ce0 ₂ , Ag/Al ₂ 0 ₃ , etc. may be mentioned.

[0024] According to the exhaust gas purification system of the present invention, an NO _x purification performance can be obtained even by an exhaust gas composition including HC and NO _x .

EXAMPLES

[0025] Below, examples of the present invention are shown. In the following examples, the obtained catalysts were evaluated by the methods of measurement shown below. 1. Measurement of Alloy Composition of Catalyst

Measurement method: Measurement of composition of bulk as a whole by XRD (X-ray diffraction)

Measurement apparatus: PHILIPS X'Pert MRD

2. Measurement of Particle Shape and Particle Size Distribution of Alloy Nanoparticles

Measurement method 1: Measurement by TEM

(transmission electron microscope)

TEM measurement apparatus: HITACHI HD-2000

Measurement method 2: Measurement by HRTEM (high resolution transmission electron microscope)

HRTEM Measurement apparatus: HITACHI HD2000

3. Measurement of Elemental Analysis of Alloy

Nanoparticles

Measurement method: Measurement of ratio of composition by TEM-EDS (EDS: energy dispersive X-ray spectroscopy) . . ..

TEM-EDS measurement apparatus: HITACHI HD2000

4. Measurement of Catalytic Activity

Catalyst pellets were packed into a glass reaction tube and fixed in place by glass wool. Premixed gas was run into the glass reaction tube. The gas temperature wa raised ^' at a temperature elevation rate of 20°C/min from 100°C to 500°C. The NO concentration was measured by an exhaust gas spectrometer (HORIBA MEXA7100H) or MS (mass spectrometry) . Note that, when running a gas not

containing H ₂ , the measurement was performed after hydrogen reduction at 500°C.

Reference Example 1

1. Synthesis of AuNi Nanoparticles

[0026] In a two-necked flask, 1.1 g of poly-n- vinylpyrrolidone (PVP) was added to 120 ml of anhydrous ethylene glycol. Into this mixture, 0.1404 g of nickel sulfate was added. The mixture was agitated at 80°C for hours to obtain a solution (solution 1) .

[0027] Separately, in a two-necked flask, 0.1809 g of NaAuCl ₄ was added to 50 ml of distilled water. The mixture was strongly agitated for 2 hours or more to cause dissolution and obtain a bright red colored solution (solution 2) .

[0028] The solution 1 was cooled by a cooling bath down to 0°C, then the solution 2 was poured into the solution 1 in the flask and the two were uniformly agitated. The mixed solution was adjusted by a 1M NaOH solution (about 5 ml) to give a pH of 9 to 10. The mixed solution was heated by an oil bath to 100°C and was held for 2 hours while being agitated. After this, the flask was lifted up from the oil bath and allowed to stand until the colloidal suspension was cooled to room

temperature. To completely reduce all of the ions in the flask, sodium borohydrate 0.038 g was added, then the suspension was allowed to stand for a while.

[0029] The produced nanoparticles were refined by treating a certain fraction including a predetermined amount of nanoparticles by a large amount of acetone. Due to this, the PVP polymer protective material was

extracted in the acetone phase, and the metal

nanoparticles coagulated. The supernatant was transferred (decanted) or centrifuged to obtain the colloid. The acetone phase was removed, then the refined colloid was gently stirred to disperse in pure ethanol .

2. Carrying of AuNi Nanoparticles on Carrier

[0030] In a 100 ml Schlenk flask, 1 g of the carrier (A1 ₂ 0 ₃ ) was inserted. The inside of the Schlenk flask was evacuated, then N ₂ was run into it to clean the piping and completely remove the air. The concentration of the suspension of the colloid previously synthesized (both the refined colloid and remaining solution) was

determined in advance, and a refined colloidal suspension containing Rh0.5wt% molar equivalents of amounts of gold and nickel metal was poured through a rubber septum into the Schlenk flask. The mixture was agitated at room temperature for 3 hours, then the solvent was removed by vacuum. After this, the remaining polymer protective material of the colloidal precipitate was removed and the result dried at 200 to 600°C by vacuum heating. The obtained catalyst powder was pressed to obtain pellets of approximately 2 mm size.

3. Evaluation of Catalyst

[0031] The obtained AuNi ( 50 : 50 ) /A1 ₂ 0 ₃ catalyst was measured for shape, particle size distribution, and elemental analysis of the alloy particles by TEM and TEM- EDS. The size of the nanoparticles was 3.75 nm±0.70 nm. Further, from a TEM-EDS. spectrum measured for an AuNi

(50:50) colloid on a copper coated grid, it is shown that all individual particles include gold and nickel.

[0032] Furthermore, the obtained AuNi ( 50 : 50 ) /A1 ₂ 0 ₃ catalyst was measured for NO purification characteristics under the following gas flow conditions.

Gas flow conditions

Gas composition: NO 1000 ppm, CO 1000 ppm, N ₂ bal/10 liter

Flow rate: 500 ml/min, pellets: 150 mg,

Space velocity: 3.3 liters/min-g

Ni, base metal concentrations: each 0.0486 mmol/g- cat

The results are shown together with other results in FIG. 1.

Comparative Example 1

[0033] Except for not using the solution 1, the same procedure was followed as in Example 1 to obtain an

Au/Al ₂ 0 ₃ catalyst. The obtained Au/Al ₂ 0 ₃ catalyst was measured for NO purification characteristics in the same way as in Example 1. The results are shown together with other results in FIG. 1.

Comparative Example 2

Synthesis of Ni nanoparticles

[0034] In a two-necked flask, 1.1 g of polyvinylpyrrolidone (PVP) was added to 120 ml of anhydrous ethylene glycol. Into this mixture, .0.1404 g of nickel sulfate was added, then the mixture was agitated at 80°C for 3 hours. The obtained solution was cooled down to 0°C, then was adjusted to a pH of 9 to 10. The solution was held at 2 hours while being agitated. After this, the flask was lifted up from the oil bath and allowed to stand until, the colloidal suspension was cooled to room temperature. The produced nanoparticles were refined by treating a certain fraction including a predetermined amount of nanoparticles by a large amount of acetone. Due to this, the protective PVP was extracted in the acetone phase, and the metal nanoparticles coagulated. The

supernatant was decanted or centrifuged to obtain the colloid. The acetone phase was removed, then the refined colloid was gently stirred to disperse in pure ethanol.

Carrying of Ni Nanoparticles on Carrier

[0035] In a 100 ml Schlenk flask, 1 g of the carrier (A1 ₂ 0 ₃ ) was inserted. The inside of the Schlenk flask was evacuated, then the piping was purged by N ₂ . The

concentration of the colloid suspension previously

synthesized (both the refined colloid and remaining solution) was determined in advance. A refined colloidal suspension containing Rh0.5wt% molar equivalents of an amount of nickel metal was poured into the Schlenk flask. The mixture was agitated at room temperature for 3 hours, then the solvent was removed by vacuum. After this, the remaining protective material of the colloidal

precipitate was removed and the result fired at 200 to 600°C in a vacuum or the air. The obtained catalyst powder was pressed to obtain Ni/Al ₂ 0 ₃ catalyst pellets of

approximately 2 mm size. The obtained Ni/Al ₂ 0 ₃ catalyst was measured for NO purification characteristic in the same way as Example 1. The results are shown together with other results in FIG. 1.

Comparative Example 3

[0036] Except for using, as the two types of metal salts, nickel sulfate and NaAuCl separately, the same procedure was followed as in Comparative Example 2 to cause metal to precipitate by evaporation of a gold and nickel mixed metal ion solution and thereby obtain

(Au+Ni) mixture/Al ₂ 0 ₃ catalyst pellets in which gold and nickel are not present in a state of close proximity. The obtained catalyst was measured for NO purification characteristic in the same way as Example 1, The results are shown together with other results in FIG. 1.

Reference Example 2

[0037] Except for changing the carrier particles from A1 ₂ 0 ₃ to Ce0 ₂ -Zr0 ₂ (CZ) , the same procedure was followed as in Reference Example 1 to obtain an AuNi ( 50 : 50 ) /CZ catalyst. The obtained AuNi ( 50 : 50 ) /Si0 ₂ catalyst was measured for shape, particle size distribution, and elemental analysis of the alloy particles by TEM and TEM- EDS. The size of the nanoparticles was 3.10 nm±1.42 nm. Further, from a TEM^EDS spectrum measured for an AuNi (50:50) colloid on a copper coated grid, it is shown that all individual particles include gold and nickel.

[0038] Furthermore, the obtained AuNi (5.0:50) /CZ catalyst was measured for catalytic activity under the following gas flow conditions.

Space Velocity (SV) : 100000 (0.6 g, 1 L/min)

All conditions are balanced by N ₂ .

The H ₂ treatment is done under 500°C before catalytic activity test

Gas · flow conditions:

(1) NO: 1500 ppm, CO: 1500 ppm (stoichiometric)

(2) NO: 1500 ppm, CO: 6500 ppm, 0 ₂ : 7000 ppm, C ₃ H ₆ : 1000 ppm (stoichiometric)

(3) NO: 1500 ppm, C ₃ H ₆ :167 ppm (stoichiometric)

(4) NO: 1500 ppm, CO: 1500 ppm, C ₃ H ₆ : 1000 ppm (rich)

(5) NO: 1500 ppm, CO: 1.55%, 0 ₂ : 7000 ppm, C ₃ H ₆ : 1000 ppm, (rich)

The obtained results are shown together with other results in FIG. 2.

Example 1 [0039] In the above device used for measurement of catalytic activity, the AuNi (50:50) /CZ catalyst obtained in. Reference Example 2 was used as an NO _x purification catalyst to prepare an exhaust gas purification

apparatus. It was predicted that NO would be exhausted if this exhaust gas purification apparatus were fed with gas of the gas composition (2) under stoichiometric control. For this reason, C ₃ H ₆ was added to change to rich control, and gas of the gas composition (6) was purified by the NO _x purification catalyst to obtain an NO purification

characteristic the same as the curve shown in curve 6 of FIG. 2. Next, when it was predicted that NO would not be exhausted, gas of the gas composition (1) was fed by the ΝΟχ purification catalyst for purification so as to obtain the same NO purification characteristic as the curve showing the curve 1 of FIG. 2.

[0040] Summarizing the industrial applicability, according to the exhaust gas purification system of the present invention, from the viewpoint of resource

depletion, use. is made of an NO _x purification catalyst using nickel - which is available to about the same extent as gold and copper. Using this, there is no need to raise the temperature of the catalyst for raising, the ΝΟχ purification activity to a high temperature like in the past, and a high NO _x purification performance can be obtained over a broad range of exhaust gas compositions.

[0041] While the invention has been described with reference to specific embodiments chosen for purpose of illustration, it should be apparent that numerous

modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention.