JP2001104804A - Method of manufacturing for catalyst for cleaning exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a catalyst for cleaning exhaust gas of NOx occlusion reduction type which is excellent in purifying ratio. SOLUTION: In a method for manufacturing a catalyst decomposing NOx in which particles of a metal selected from a group composed of Pt, Pd, Au, Ag, Rh and Ir are deposited on an inorganic carrier and particles of another metal selected from the group composed of Pt, Pd, Au, Ag, Rh and Ir are laminated on this metallic particles, the inorganic carrier having equipotential point of pH 3 to 10 is used and the metal is laminated on the metallic particles by reductive deposition method.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a manufacturing method of an exhaust gas purifying catalyst, in particular, improved the NO _X purification performance in a lean atmosphere, is Sekisho thereon a first catalytic metal particles the method of manufacturing of the NO _X purification catalyst with a second metal.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of protecting the earth, emission regulations and fuel consumption regulations have been strengthened worldwide every year. As a countermeasure, in an internal combustion engine, a lean burn engine has been developed as an effective means of improving fuel efficiency.
_The NO _X storage reduction three-way catalyst obtained by adding a function of occluding _X have been developed.

[0003] In a lean burn engine using this catalyst, fuel is normally burned under the condition that the air-fuel ratio (A / F) is lean (excessive oxygen), and the fuel is temporarily stored between stoichiometric (stoichiometric air-fuel ratio) and rich (excess fuel). Burn under the conditions of HC and CO in the exhaust gas is efficiently burned and removed by the action of an oxidizing atmosphere and the catalyst under lean conditions, whereas, NO _X is trapped by the occluding material under lean conditions, it temporary stoichiometric-rich It is released under conditions and reduced and purified by the action of the reducing atmosphere and the catalyst.

[0004] By the action of these combustion conditions and _the NO _X storage reduction three-way catalyst, as a whole, it is possible to HC simultaneously exhaust gas when fuel is increased, CO, NO _X can be efficiently purified. Lean burn system combining such air-fuel ratio control and _the NO _X storage material, and the exhaust gas compared with around a conventional three-way catalyst and the stoichiometric air-fuel ratio, improved fuel economy and CO, HC, total generation amount of the NO _X Has had some success with regard to the challenge of reducing.

[0005]

However, in order to further improve the fuel efficiency, the stoichiometric air-fuel ratio control is required.
It is necessary to increase the lean burn time by reducing the rich ratio. In this case, a certain amount of saturated N
Since O _X storage amount is a problem that it exceeds its saturation occlusion amount inevitably NO _X purification rate is lowered.

To solve this problem, the present inventors
From the idea of reducing the amount of adsorbed oxygen on metal particles under lean conditions to improve the NO _x purification performance of catalytic metal particles under lean conditions, see Japanese Patent Application Laid-Open No. H11-156193.
Japanese Patent Application Publication No. JP-A-2005-115139 proposes a catalyst in which another metal phase is accumulated on catalytic metal particles. According to this catalyst configuration,
A difference in electronic state of the catalytic metal particles and the metal phase, NO _X purification performance electronic state of the surface of the catalytic metal particles and the metal phase is changed is known to improve. An object of the present invention is to provide a method for stably and economically producing such a catalyst in which metal phases are accumulated.

[0007]

The object of the present invention is to support a metal particle selected from the group consisting of Pt, Pd, Au, Ag, Rh and Ir on an inorganic carrier, and put Pt, Pt, on the metal particle. pd, Au, Ag, an Rh and is selected from the group consisting of Ir and the supported production process of the NO _X purification catalyst for Sekisho the different metal metal particles, the metal onto the metal particles And the equipotential point of the inorganic carrier is pH 3-1.
The present invention is attained by a method for producing a catalyst for purifying NO _X , characterized by using an inorganic carrier of zero.

The term “accumulated phase” in the present invention means that another metal exists as a phase on a part of the surface of the metal particle, and an intermediate composition exists between the metal particle and the other metal. Although it may exist, it refers to a state in which both a metal particle phase and another metal phase exist on the metal particle as the metal phase. Preferably, a plurality of finer second metal particles are present on the first metal particles.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the method of the present invention, first, a suitable inorganic carrier having an equipotential point of pH 3 to 10 is selected.
The material of the inorganic carrier satisfying such requirements is a material that can withstand an exhaust gas purification atmosphere and has an equipotential point of pH 3 to 10. Specifically, among materials having appropriate durability such as inorganic oxides such as alumina, silica, zirconia, silica-alumina, zeolite, and inorganic non-oxides such as silicon carbide and silicon nitride, the equipotential point is pH
Three to ten materials are selected.

The equipotential point in the present invention means an equipotential point of an inorganic carrier, for example, JISR6129 (197)
It can be measured according to the method of 6). When the pH of the inorganic carrier is less than 3 or more than 10, the NO _x purification performance of the stacked phase catalyst of the present invention in a lean atmosphere becomes low.

Next, Pt, Pd,
The first metal particles selected from the group consisting of Au, Ag, Rh and Ir are supported. The method of supporting the first metal particles may be any method capable of dispersing the metal particles on the carrier, and is not limited, but is not limited to adsorption to an acid point / base point of the carrier, ion exchange. Method, reduction precipitation method and the like are desirable methods.

The particles of the first metal have an average particle diameter of 40 nm.
The following is preferred. An average particle diameter of 40 nm or less is preferable because the specific surface area is increased and the purification performance is improved. However, in forming the accumulated phase of the second metal on the first metal particles, Metal ions are reduced and precipitated at a site having a small curvature, that is, since there is a tendency that such fine metal particles are reduced and precipitated at the site as a nucleation site, a desirable metal phase is obtained when the average particle diameter is 40 nm or less. Some benefits are easy to obtain. More preferably,
It is 30 nm or less. The amount of the first metal carried on the inorganic carrier is not particularly limited, and may be the same as in the related art.

Next, on the first metal particles supported on the inorganic carrier, second metal particles other than the first metal particles selected from the group consisting of Pt, Pd, Au, Ag, Rh and Ir are placed. The two metals are phase-stacked. Changing the surface electronic states on catalytic metal particles by the product phase, improves NO _X purification performance. In the present invention, this step is performed by a reduction precipitation method. The reduction precipitation method referred to in the present invention is a method of adding a reducing agent to an aqueous solution of a metal salt that generates metal ions in an aqueous solution, and reducing the metal ions to make them insoluble, thereby precipitating a metal of the metal salt. Say.

Specifically, although not limited,
A water-soluble metal salt of a second metal such as Pt, Pd, Au, Ag, Rh or Ir and a reducing agent are added to water impregnated with the inorganic oxide carrier supporting the first metal particles. Can be performed by placing under gentle agitation.

The water-soluble metal salt includes, but is not limited to, Pd nitrate, Pd acetate, Pd chloride, Pd sulfate,
Rh nitrate, Rh chloride, Ir chloride, Ag nitrate, Ag chloride,
Examples include Ag sulfate, chloroplatinic acid, platinum dinitrodiammine, and chloroauric acid. As the reducing agent, sodium thiosulfate, potassium thiosulfate, ammonium thiosulfate, sodium metabisulfite, potassium metabisulfite, ammonium metabisulfite, NaBH ₄ is exemplified.

Preferably, the deposited second metal covers at least 10% or more of the surface of the first metal particles, and the weight ratio of the second metal to the first metal is , 0.1 to 15% by weight, more preferably 0.1 to 15% by weight.
5 to 5% by weight. If the amount of the second metal is less than 0.1% by weight, the effect of the accumulated phase is not obtained. If the amount of the second metal exceeds 15% by weight, the metal particles form giant particles and the purification performance is reduced.

Further, the reduction precipitation is carried out by adjusting the pH of the aqueous solution to 6
It is preferable to carry out in the range of from 9 to 9. When the pH is out of this range, the amount of metal ions adsorbed on the acid or base sites of the carrier or neutralized and precipitated increases, so that the amount of accumulated phase on the first metal particles decreases, and as a result, This is because the purification performance of the obtained alloy catalyst becomes small.

It has been found that the inorganic carrier needs to have an equipotential point of pH 3 to 10 during the reduction precipitation. Although the reason is not clear, the carrier itself affects the ionic atmosphere of the aqueous solution at the isopotential point of the carrier, so when the equipotential point pH of the inorganic carrier is in the range of 3 to 10, the water-soluble metal salt and the water-soluble It is considered that the metal accumulation phase by the ionic reaction of the reducing agent is performed stably.

The thus obtained catalyst comprising the first metal particles and the second metal deposited on the inorganic carrier on the inorganic carrier is formed into a monolithic or granular form deposited on a monolith. It can be used for exhaust gas purification as a pellet type catalyst.

[0020]

EXAMPLE 1 Example 1 Pt (NO_Two)_Two(NH_Three)_Two9.98 × 10^-2wt%
Γ-Al having an equipotential point of pH 8.0 was added to the dissolved aqueous solution._TwoO
_ThreeAfter adding the powder and stirring for 3 hours, in the air at 120 ° C.
Drying was performed for 24 hours. After drying, 300
C. x 2 hours heat treatment, 1.80 wt% Pt / Al_Two
O_ThreeA powder was obtained. This 1.80 wt% Pt / Al_TwoO_Threepowder
Powder in ion-exchanged water at 60 ° C.
AuCl_Four・ 4H_TwoO (2.04 × 10^-2wt%), Na
_TwoS_TwoO _Three・ 5H_TwoO (9.40 × 10^-2wt%), Na
_TwoS_TwoO_Three(2.41 × 10^-1wt%), C₆H₇NaO
₆(9.45 × 10^-1wt%) and stir for 24 hours.
The Au was reduced and precipitated while doing so.

After this reductive precipitation, the precipitate was filtered and treated at 60 ° C.
Washing was performed with exchanged water. Washed powder in air
Drying was performed at 0 ° C. × 2 hours. After drying, 50
A heat treatment at 0 ° C. × 2 hours is performed so that the carrier has an isoelectric point pH of 8.
Γ-Al of 0_TwoO_Three, Weight ratio Pt: Au = 9: 1, P
(Pt-Au) Al in which the total amount of t and Au is 2 wt%_TwoO _Three
Catalyst A was obtained.

Example 2 Catalyst A was used except that a composite oxide of SiO ₂ and Al ₂ O ₃ having an equipotential point of pH 3.9 (SiO ₂ / Al ₂ O ₃ molar ratio = 18.3) was used as a carrier. In the same manner, (Pt-Au) in which the carrier is a SiO ₂ / Al ₂ O ₃ composite oxide having an equipotential point of pH 3.9, the weight ratio is Pt: Au = 9: 1, and the total amount of Pt and Au is 2 wt% ) / SiO ₂ / Al ₂ O ₃ catalyst B was obtained.

Example 3 Pt (NO_Two)_Two(NH_Three)_TwoIs 1.74 × 10^-2wt%
Γ-Al having an equipotential point of pH 8.0 was added to the dissolved aqueous solution._TwoO
_ThreeAfter adding the powder and stirring for 3 hours, in the air at 120 ° C.
Drying was performed for 24 hours. After drying, 300
Heat treatment for 2 hours at 1.degree. C. and 1.99 wt% Pt / Al_Two
O_ThreeA powder was obtained. This 1.99 wt% Pt / Al_TwoO_Threepowder
Powder in 60 ° C. ion-exchanged water in which 4.76 wt%
AuCl_Four・ 4H_TwoO (9.99 * 10^-Fourwt%), Na
_TwoS_TwoO _Three・ 5H_TwoO (4.80 × 10^-3wt%), Na
_TwoS_TwoO_Three(1.22 × 10^-2wt%), C₆H₇NaO
₆(4.79 × 10^-2wt%), except that catalyst A
In the same manner as in the above, the carrier is γ-Al having an isoelectric point of pH 8.0.
_TwoO_Three, The weight ratio is Pt: Au = 995: 1, Pt and Au
(Pt-Au) Al with a total amount of 2 wt%_TwoO_ThreeCatalyst C
Obtained.

Example 4 A catalyst C was used except that a composite oxide of SiO ₂ and Al ₂ O ₃ having an equipotential point of pH 3.9 (SiO ₂ / Al ₂ O ₃ molar ratio = 18.3) was used as a carrier. In a similar manner, the carrier is a SiO ₂ / Al ₂ O ₃ composite oxide having an equipotential point of pH 3.9, the weight ratio is Pt: Au = 995: 5, and the total amount of Pt and Au is 2 wt.
Yield% of _{(Pt-Au) / SiO 2} / Al 2 O 3 catalyst D.

Example 5 A carrier was prepared in the same manner as in Catalyst C except that a composite oxide of TiO ₂ and SiO _{2 having} an equipotential point of pH 5.5 (SiO ₂ content = 4.1 wt%) was used. Has an equipotential point of pH 5.5
TiO ₂ / SiO ₂ composite oxide, weight ratio of Pt: A
u = 995: 5, and the total amount of Pt and Au is 2 wt% (Pt
-Au) / to give a TiO ₂ / SiO ₂ catalyst E.

[0026] Except for using a TiO ₂ powder isoelectric point pH6.2 (anatase) in Example 6 carriers, the catalyst C and similar methods, TiO _2, the weight ratio of carrier isoelectric point pH6.2 Is Pt: Au = 9
95: 5, the total amount of Pt and Au being 2 wt% (Pt-A
u) / TiO ₂ catalyst F was obtained.

COMPARATIVE EXAMPLE 1 Except that SiO ₂ powder having an equipotential point of pH 1.5 was used as the carrier, the carrier was treated with an isoelectric point of pH 1.5 in the same manner as in Catalyst A.
SiO ₂ , weight ratio Pt: Au = 9: 1, Pt and Au
(Pt-Au) / SiO ₂ catalyst G having a total amount of 2 wt% was obtained.

COMPARATIVE EXAMPLE 2 A carrier was used in the same manner as in Catalyst A except that MgO powder having an equipotential point of pH 12.5 was used as the carrier.
5, MgO with a weight ratio of Pt: Au = 9: 1, Pt and Au
(Pt-Au) / MgO catalyst H having a total amount of 2 wt% was obtained.

-Evaluation of exhaust gas purification performance- For the catalysts shown in Table 1, lean equivalent model gas (A /
F = 30) and the following gas composition and gas space velocity,
The temperature was evaluated from 500 ° C to 100 ° C, and the rate of temperature decrease was 1
The exhaust gas purification performance was evaluated at 0 ° C./min. Gas composition: 1000 ppm CO + 667 ppm C ₃ H ₆
+250 ppm NO + 7.3% O ₂ + 6.7% CO ₂ +
5% H ₂ O (residual: N ₂ ) Gas hourly space velocity: 150,000 h ^-1 Table 1 shows the maximum purification rate of NO _X defined by the following equation (1). Purification rate = [(incoming gas concentration-outgoing gas concentration) / incoming gas concentration] x 100 ... (1)

[0030]

[Table 1]

Based on the evaluation results shown in Table 1, Examples 1 to 6
Relative to exhibit good NO _X purification performance of any more than 50%, Comparative Example 7 and 8, quite inferior NO _X
This shows the purification performance. It is considered that these differences are due to differences in the equipotential points of the carriers.

[0032]

According to the method of the present invention, comprises a second metal that is Sekisho thereon a first catalytic metal particles, a good NO _X reduction catalyst purification rate, stabilized by reductive deposition method Can be manufactured.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between an equipotential point of a carrier and a NO _X purification rate.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/10 ZAB F01N 3/28 301C 3/28 301 301P B01D 53/36 102H F-term (Reference) 3G091 AA12 AA17 AB05 BA39 FB10 GA01 GA06 GA16 GB01W GB01X GB05W GB06W GB07W GB13X GB15X GB16X 4D048 AA06 AB03 BA03X BA06X BA07X BA30X BA31Y BA33Y BA34X BA41X BB01 4G069 AA03 AA08 AA09 AA12 BA01A BA01B BA03A BA03B BA04A BA04B BB02A BB02B BB04A BB04B BC32A BC33A BC33B BC71A BC72A BC74A BC75A BC75B CA02 CA03 CA10 CA13 DA06 EE01 FA02 FB45

Claims (1)

[Claims] 1. Pt, Pd, Au, A on an inorganic carrier
carrying metal particles selected from the group consisting of g, Rh and Ir, and having Pt, Pd, Au, Ag,
A is selected from the group consisting of Rh and Ir and the supported production process of the NO _X purification catalyst for Sekisho the different metal metal particles, reducing precipitation of the product phase of the metal onto the metal particles done by law, and the production method of the NO _X purification catalyst isoelectric point is characterized by using the inorganic carrier pH3~10 as the mineral support.