JP3550709B2 - Method for producing exhaust gas purifying catalyst - Google Patents

Description

【００３５】
上記表１によれば、蒸発乾固法ではトータル担持量の仕込み値からのずれが実施例（スプレードライ法）の約２倍、トータル担持量のばらつきについても実施例の約４倍になっている。このことから、実施例によれば、触媒活性種の担持量のばらつきを大幅に改善できることがわかる。
【００３６】
−粉末状担体の粒径の影響について−
粉末状担体として２次粒子径０．１μｍ以下の微小粉から６μｍ以上の粗大粉までが含まれるＨ型ＺＳＭ−５（ＳｉＯ₂／Ａｌ₂Ｏ₃＝７０〜８０）を用い、貴金属活性種についてはＰｔ：Ｉｒ：Ｒｈ＝３０：６：１（重量比）となり且つ該活性種の総量が触媒１リットル当たり４．５ｇになるように秤量して、先の例と同様の方法によってスラリーを調製した。なお、以下ではことわりがない限り、粒径はレーザ回折法によって測定した値である。
【００３７】
そして、先の場合と同じ条件でスラリーの噴霧乾燥を行なったところ、第１乃至第３の各触媒粉受け１３，１７，１８によって受けられた各触媒粉の粒度分布は図３乃至図５の各図の通りになった。そうして、各触媒粉受けの触媒の活性種担持量を調べたところ、表２に示す通りになった。
【００３８】
【表２】

【００３９】
上記図３乃至図５及び表２によれば、乾燥室３の下の第１触媒粉受け１３で受けられた触媒粉は、活性種担持量がその予定量ｘ（＝４．５ｇ／リットル）の±１５重量％の範囲内の担持量となっているから、略狙い通りものになっているということができ、その粒径は１〜５μｍの範囲にある。
【００４０】
これに対して、サイクロン５の下の第２触媒粉１７によって受けられた触媒粉は、粒径が３μｍ前後の値となっていてそのバラツキが小さいが、活性種担持量が予定の１．１〜５倍程度となっていて、バラツキが大きくなっているとともに高担持量のものになっている。また、バグフィルタ６の下の第３触媒粉受け１８によって受けられた触媒粉は活性種担持量が予定の０．２〜０．９倍であって少なく、また、粒径は０．４〜２μｍのものが全体の１／３程度を占め、比較的広い範囲に広がっている。
【００４１】
この場合、上記触媒粉の粒径は粉末状担体としてのＨ型ＺＳＭ−５の粒径に対応する。従って、以上のことから、担体の粒径が１μｍ以上あれば活性種担持量が狙い通りに、あるいは高担持量になる可能性が高いが、それよりも小径になると活性種担持量が少なくなる、ということができる。また、担体の粒径が６μｍを越える場合には、噴霧乾燥が難しくなるとともに活性種担持量も低くなる。
【００４２】
また、以上の結果から、第１乃至第３の各触媒粉受け１３，１７，１８に得られた各触媒粉を適当な割合で混合することにより、触媒性能をコントロールできることがわかる。すなわち、乾燥室下の第１触媒粉受け１３に得られた触媒粉のみによって排気ガス浄化用触媒（例えばハニカム触媒）を形成した場合に、ＨＣ浄化率が所期の値よりも低いときには、上記第１触媒粉受け１３で得られた触媒粉と、サイクロン下の第２触媒粉受け１７で得られた触媒粉とを後者の割合が１〜１０あるいは１〜５重量％となるように混合すると、当該触媒は熱処理後のＨＣ浄化率が向上する。一方、ＨＣ浄化率が所期の値よりも高いときには、上記第１触媒粉受け１３で得られた触媒粉と、バグフィルタ下の第３触媒粉受け１８で得られた触媒粉とを後者の割合が１〜１０重量％となるように混合すると、当該触媒は熱処理後のＨＣ浄化率が向上する。
【００４３】
−スラリーの粘度の影響について−
まず、貴金属活性種の濃度が異なる複数のスラリーを調製し、これらの粘度を測定した。すなわち、２価白金アンミン結晶と三塩化イリジウムと硝酸ロジウムとを用い、貴金属活性種の濃度が互いに異なる複数の活性種溶液（溶媒はいずれも水及びエタノール）を調製し、これらに同量のＨ型ＺＳＭ−５（ＳｉＯ₂／Ａｌ₂Ｏ₃＝７０〜８０）を加え、撹拌して各スラリーを得た。いずれのスラリーの場合も、貴金属活性種の重量比はＰｔ：Ｉｒ：Ｒｈ＝３０：６：１であり、また、上記ＺＳＭ−５粉末の量と溶媒量との割合は１：２である。また、この場合の貴金属活性種の濃度は、上記ＺＳＭ−５に担持させた場合の貴金属の総量が触媒１リットル当たり０〜７ｇとなる範囲で互いに異なるものにした。
【００４４】
結果は図６に示されている。なお、貴金属活性種の濃度については、触媒１リットル当たりの活性種担持量に換算して示されている。同図によれば、貴金属活性種の濃度が高くなるにつれてスラリーの粘度が高くなっている。これは貴金属活性種の濃度が高くなるにほどスラリー中の硝酸イオンの濃度が高くなるからである。従って、貴金属活性種の濃度を変えずにスラリーを所望の粘度のものにするには、硝酸の添加によって粘度調節を行なえばよい。
【００４５】
次に、上記ＺＳＭ−５と活性種溶液との量比を互いに異なるものとして各々の粘度を調節した数種類のスラリーを準備し、それぞれ上記スプレードライ装置によって噴霧乾燥させ、乾燥室３の下の第１触媒受け１３によって受けられた触媒粉の量を調べた。結果は図７に示されている。同図において、触媒粉収量が最も多い粘度３５〜４０ｃｐｓのところのスラリーは、上記ＺＳＭ−５粉末の量と溶媒量との割合が１：２になっている。
【００４６】
図７によれば、上記第１触媒粉受け１３の触媒粉収量は、スラリーの粘度３５〜４０ｃｐｓのところをピークとして該粘度が低すぎる場合及び高すぎる場合のいずれにおいても少なくなっており、当該粘度としては２０〜６０ｃｐｓ程度が良いこと、さらには３０〜５０ｃｐｓが良いことがわかる。
【００４７】
ここに、上記粘度が低い場合に触媒粉収量が少なくなっているのは、溶媒量が多くなっているため、噴霧されるスラリー中のＺＳＭ−５粉の割合が相対的に少なくなり、それが送風エネルギーによって浮游しサイクロン５の方へ飛散し易くなるためであると考えられる。一方、上記粘度が高い場合に触媒粉収量が少なくなっているのは、スラリー液滴の量が多くなるため送風の熱エネルギーが不足気味になり、スラリー液滴が完全に乾燥しないまま落下して乾燥室の内壁面に付着するためであると考えられる。
【００４８】
−スラリーのｐＨの影響について−
先にスラリー中の貴金属活性種の濃度、すなわち、硝酸イオンの濃度とスラリーの粘度との関係をみたように、ｐＨが粘度に影響することが当然に認められるが、スラリーのｐＨはさらに触媒粉の性能にも影響を及ぼす。すなわち、スラリーのｐＨを２，６，８の３種類に変え、他は同一の条件（粉末状担体；Ｈ型ＺＳＭ−５（ＳｉＯ2 ／Ａｌ2 Ｏ3 ＝７０），貴金属活性種の重量比；Ｐｔ：Ｉｒ：Ｒｈ＝３０：６：１，貴金属活性種の総量；４．５ｇ／触媒１リットル）として得られた各触媒粉の最高ＮＯｘ浄化率を調べたところ（使用ガス；Ａ／Ｆ＝２２相当のモデルガス，ＳＶ＝５５０００ｈ^-1）表３に示す通りのものになった。
【００４９】
【表３】

【００５０】
表３によれば、ｐＨ６において最も高いＮＯｘ浄化率になっており、当該浄化率として４０％以上を得ようとすれば、ｐＨを４〜８程度にすることが好適であることがわかる。
【００５１】
−λ＝１でのライトオフ特性について−
λ＝１（Ａ／Ｆ＝１４．７）でのライトオフ特性（触媒が活性を示すようになる温度特性）について検討した。すなわち、表４に示すＡ，Ｂ，Ｃの３種類の金属含有シリケートを用いてスプレードライ法により、Ｐｔ、Ｉｒ及びＲｈが重量比で３０：６：１であり且つそれらの総量が触媒１リットル当たり４．５ｇである触媒粉をそれぞれ調製し、これらに２００℃×１６時間の活性化処理（大気中）を施した。図８，９，１０は上記各金属含有シリケートＡ，Ｂ，ＣのＳＥＭ（走査型電子顕微鏡）による観察写真である。
【００５２】
そうして、得られた各触媒粉にバインダーとして水和アルミナを２０ｗｔ％加え、さらに、適量の水を加えてコーティング用スラリーを得た。これらのスラリーにコージェライト製ハニカム担体を浸漬し引上げ、余分なスラリーを吹き飛ばした後、５００℃×２時間の焼成処理（大気中）を行なった。各ハニカム担体への触媒粉の担持量はハニカム重量の３５〜４０ｗｔ％となるようにした。
【００５３】
【表４】

【００５４】
そして、各ハニカム触媒について、Ａ／Ｆ＝１４．７（λ＝１）相当のモデルガス、Ａ／Ｆ＝２２（リーン）相当のモデルガスを用い、それぞれＳＶ＝５５０００〜６００００ｈ^-1として、浄化率５０％が得られるときの触媒入口ガス温度を測定した。結果は表５に示されている。
【００５５】
【表５】

【００５６】
表５によれば、平均粒径が小さい金属含有シリケートを用いると、排気ガスがリーンのときのＮＯｘ浄化特性を損なうことなく、λ＝１のときに低い温度から触媒活性が得られ、排気ガスの浄化に有利になることがわかる。すなわち、金属含有シリケートＡ，Ｂを用いた場合は、ライトオフ温度が金属含有シリケートＣを用いた場合に比べて、ＨＣでは２５〜２８℃、ＣＯでは１４〜１７℃、ＮＯｘでは１８〜２５℃それぞれ低温側にずれている。
【００５７】
さらに、かかる観点からさらに実験を進めたところ、金属含有シリケートの平均粒径が０．５〜１．５μｍ、粒度分布が０．１〜５μｍのとき、さらには平均粒径が０．５〜１．０μｍ、粒度分布が０．１〜２．５μｍのときに好結果が得られることが判明した。
【図面の簡単な説明】
【図１】スプレードライ装置の構成図
【図２】雰囲気温度及びノズル回転数がＮＯｘ浄化率に及ぼす影響を示すグラフ図
【図３】乾燥室下の触媒粉受けに得られた触媒粉の粒度分布を示す図
【図４】サイクロン下の触媒粉受けに得られた触媒粉の粒度分布を示す図
【図５】バグフィルタ下の触媒粉受けに得られた触媒粉の粒度分布を示す図
【図６】スラリーの活性種濃度（貴金属担持量）と粘度との関係を示すグラフ図
【図７】スラリーの粘度と乾燥室下の触媒粉受けの触媒粉収率との関係を示すグラフ図
【図８】平均粒径０．８μｍの金属含有シリケート粒子構造を示す電子顕微鏡写真
【図９】平均粒径０．６μｍの金属含有シリケート粒子構造を示す電子顕微鏡写真
【図１０】平均粒径３．８μｍの金属含有シリケート粒子構造を示す電子顕微鏡写真
【符号の説明】
１ スラリータンク
２ チューブポンプ
３ 乾燥室
４ 電気ヒータ
５ 一次サイクロン
６ バグフィルタ
７ 排気ファン
８ アトマイザー
９ 熱風室
12 排気装置
13，17，18 触媒粉受け[0001]
[Industrial applications]
The present invention relates to a method for producing an exhaust gas purifying catalyst.
[0002]
[Prior art]
Exhaust gas purifying catalysts such as three-way catalysts, oxidation catalysts, and NOx purifying catalysts for purifying NOx in an oxygen-excess atmosphere are constituted by carrying an active species (catalytic metal) on an inorganic porous carrier.
[0003]
In carrying the active species on the carrier, an impregnation method in which the carrier is impregnated with a solution of the active species and dried, and an evaporative drying method in which a slurry obtained by dispersing the powdery carrier in the active species solution is evaporated to dryness Various methods such as an ion exchange method for exchanging active species with cations of a metal-containing silicate such as zeolite (a crystalline porous body having a large number of pores) have been adopted. For example, JP-A-4-243545 describes a method in which Pt or Rh as an active species is supported on a zeolite carrier by an ion exchange method.
[0004]
[Problems to be solved by the invention]
However, any of the impregnation method, the evaporation to dryness method, and the ion exchange method has a disadvantage that a relatively long time is required for carrying. In addition, since the properties of the slurry tend to be non-uniform due to the difference in specific gravity between the active species and its solvent, and the difference in specific gravity between the active species solution and the carrier, the carrier is not used in the above-described various methods that require a long time to support. Is difficult to carry uniformly. In particular, when a plurality of types of active species are supported on a carrier, if the types of solvents of the active species are different from each other, each active species solution can be easily separated into layers, and these can be uniformly supported on the carrier. Becomes more difficult.
[0005]
Further, in the evaporative drying method, when a plurality of types of active species are supported on a carrier, if the solvents of the respective active species have different boiling points from each other, the active species are sequentially loaded from the active species present in the solvent having a lower boiling point. It is difficult to uniformly disperse and carry a plurality of types of active species on a carrier. In addition, the thermal history of the active species (the thermal histories of the active species are different due to the different temperature distribution in the beaker when the slurry is heated and evaporated to dryness) affects the activity as a catalyst. There is a problem and therefore process management is difficult.
[0006]
In addition, there is a similar problem in the ion exchange method. When a plurality of types of active species are to be supported on a carrier at the same time, active species which are easily ion-exchanged are preferentially supported, and a plurality of types of active species can be supported at a desired ratio. It is difficult to carry the seed. In addition, when a plurality of active species are to be carried on ion exchange in order, the active species carried by ion exchange may be detached at the time of carrying out the next active species.
[0007]
That is, an object of the present invention is to enable active species to be supported on a carrier in a short time, and to allow a plurality of types of active species to be simultaneously and uniformly dispersed and supported.
[0008]
Means for Solving the Problems and Their Functions
In order to solve such a problem, the present invention is to support the active species on a carrier by bringing a slurry into a droplet state and contacting it with hot air.
[0009]
That is, the first means for solving the above problem (the invention according to claim 1) is:
A plurality of types of powdery pH4~8 slurry to adjust the Rutotomoni nitrate amount carrier allowed to disperse a 1~6μm is particle size mixed a solvent solution of the solvent containing the active species with water and an organic solvent The plurality of types of active species are supported on the powdery carrier by forming the slurry, making the slurry into a droplet state by spraying, and contacting with hot air to rapidly dry the slurry.
[0010]
The type of the active species is not particularly limited, and a precious metal such as Pt, Ir, and Rh, a transition metal other than a noble metal, or a typical element such as an alkaline earth metal can be applied. .
[0011]
The solvent of the active species may be a mixed solvent of water and an organic solvent such as alcohol.
[0012]
The particle size of the powdery carrier (here, the particle size is a secondary particle size) of 1 to 6 μm is used when the slurry has a large size exceeding 6 μm and is dried by contact with hot air. Or the spraying becomes difficult, so that the amount of the active species carried is reduced, and if it is less than 1 μm, the amount of the active species carried is reduced, and the recovery of the powder carrying the active species becomes difficult. It is.
[0013]
A disk atomizer, a pressure nozzle, a two-fluid nozzle, or the like can be used for spraying the slurry (spray drying method). Then, since the slurry is formed into fine droplets by spraying, more rapid drying can be achieved and the catalyst powder can be made finer.
[0014]
The contact between the hot air and the droplet state may be appropriately selected from a counter-current method, a co-current method, and a composite flow method.
[0015]
Thus, in this means, since the slurry containing multiple types of active species and the powdered carrier, and furthermore, the slurry containing water and the organic solvent comes into contact with hot air in the form of droplets, the solvent rapidly evaporates and the multiple types of The active species are uniformly dispersed and supported on the carrier, and a catalyst powder having a small particle size can be obtained in a short time. In this case, there is no need to place the active species solution and the powder carrier in a slurry state for a long time, so the difference in specific gravity does not matter, and it is not particularly necessary to separately heat the slurry itself. There is no problem of heat history.
[0016]
In addition, in this means, when forming the slurry, the amount of nitric acid is adjusted to pH 4 to 8, so that the exhaust gas purifying performance of the obtained exhaust gas purifying catalyst is improved.
[0017]
The second means for solving the above-mentioned problem (the invention according to claim 2) is the above-mentioned first means ,
It is characterized in that the viscosity of the slurry is 20～60Cps.
[0018]
That is, when the viscosity of the slurry is less than 20 cps, the powdery carrier and the active species solution are easily separated at the time of spraying, and the active species loading of the catalyst powder obtained by producing a small amount of the active species loading is made. The variation in the amount increases. The low viscosity of the slurry also means that the amount of the active species solution is large with respect to the powdery carrier, but this reduces the amount of powder that can be processed per unit time and reduces the amount of droplets or mist. Since the floating time of the boat becomes longer, it becomes difficult to recover the boat. On the other hand, when the viscosity of the slurry exceeds 60 cps, drying of the slurry by hot air becomes difficult, and there is a problem that the amount of powder adhering to the inner surface of the drying container increases (the amount of recovered powder decreases).
[0019]
For the above reasons, it is preferable to set the viscosity of the slurry to 20 to 60 cps, but it is more preferable to set the viscosity to 30 to 50 cps.
[0020]
Here, the viscosity of the slurry can be adjusted, for example, by changing the amount of nitric acid in the slurry. That is, if the viscosity is high, the amount of nitric acid may be increased, and if the viscosity is low, the amount of nitric acid may be reduced. Therefore, it is possible to obtain a desired viscosity by monitoring the pH of the slurry. However, as described above, this pH itself affects the performance (exhaust gas purification rate) of the obtained catalyst powder. It is preferable to adjust the viscosity in the range of pH 4 to 8.
[0021]
【The invention's effect】
According to the first means (the invention according to claim 1), a solution containing a plurality of types of active species and having a solvent of a mixed solvent of water and an organic solvent having a particle size of 1 to 6 μm is used. adjust the Rutotomoni nitrate amount to disperse the carrier to form a slurry of pH 4-8, the powder of the above plural kinds of active species by rapid drying in contact with hot air in the droplet state by spraying the slurry Since the carrier is supported on a carrier, it is possible to easily and easily obtain a catalyst powder for purifying exhaust gas in which a plurality of types of active species are uniformly dispersed and supported on the carrier. This is advantageous for miniaturization of the catalyst powder, and the particle size of the powdery carrier is set in the range of 1 to 6 μm. The amount of active species supported on the solid support It can be, yet you improve exhaust gas purification performance.
[0022]
According to the second means (invention of the second aspect), the viscosity of the slurry is set in the range of 20 to 60 cps. Increasing the amount of active species supported and reducing the variation in the amount of active species supported on the catalyst powder can be reduced.
[0023]
【Example】
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0024]
-Spray dry equipment-
The present embodiment is an example in which the present invention is implemented by a co-current spray drying method, and FIG. 1 shows the entire configuration of the apparatus. In the figure, 1 is a slurry tank, 2 is a tube pump, 3 is a drying chamber, 4 is an electric heater, 5 is a primary cyclone, 6 is a bag filter, and 7 is an exhaust fan.
[0025]
The slurry tank 1 is a tank for storing a slurry composed of an active species solution and a powder carrier. The slurry is sent by a tube pump 2 to an atomizer 8 at the top of a drying chamber 3. A hot air chamber 9 is formed on the ceiling of the drying chamber 3, and heating air is supplied from the electric heater 4 to the hot air chamber 9. The ventilation hole for supplying the above-mentioned heating air, that is, hot air, is opened around the atomizer 8 from the hot air chamber 9 to the drying chamber 3. The electric heater 4 takes in outside air from the lower part through the filter 11 and supplies hot air from the upper part.
[0026]
The drying chamber 3 includes an exhaust device 12 in the center of the inside thereof, and a first catalyst powder receiver 13 is provided below the exhaust device 12. The exhaust device 12 covers an exhaust hole opened above the first exhaust pipe 14 with an umbrella member. The first exhaust pipe 14 is connected to the upper side surface of the primary cyclone 5, and the top of the primary cyclone 5 and the side surface of the bag filter 6 are connected by a second exhaust pipe 16 having an air volume adjustment damper 15. A second catalyst powder receiver 17 is provided at a lower end of the primary cyclone 5, and a third catalyst powder receiver 18 is provided at a lower end of the bag filter 6. The exhaust fan 7 is provided in a third exhaust pipe 19 extending from an upper portion of the bag filter 6.
[0027]
-Preparation of slurry-
The divalent platinum ammine crystal, iridium trichloride and rhodium nitrate were weighed so that the weight ratio of Pt, Ir and Rh was 30: 10: 1. The divalent platinum ammine crystals and rhodium nitrate are dissolved in water (ion-exchanged water), and iridium trichloride is dispersed in ethanol. Thereafter, the two are mixed and further mixed therein. 5 powder was added and stirred at room temperature for 2 hours.
[0028]
-Spray drying-
The heating air from the electric heater 4 is introduced as hot air into the drying chamber 3 to maintain the atmospheric temperature of the drying chamber 3 at 160 ° C. In this state, the slurry is sent from the tank 1 to the atomizer 8 by the tube pump 2 and the drying chamber 3 is heated. Sprayed. The nozzle rotation speed of the atomizer 8 was 8000 rpm.
[0029]
-Collection of catalyst powder-
The minute droplets of the slurry are dried rapidly by contact with hot air (about 5 seconds), and a part thereof is collected in the first catalyst powder receiver 13 along the wall surface of the drying chamber 3. The floating catalyst powder in the drying chamber 3 is guided to the primary cyclone 5 through the first exhaust pipe 14 together with the hot air, and a part thereof is collected in the second catalyst powder receiver 17. Further, the floating catalyst powder of the primary cyclone 5 is guided to the bag filter 6 through the second exhaust pipe 16 and collected in the third catalyst powder receiver 18.
[0030]
-Conditions for spray drying-
The atmosphere temperature in the drying chamber 3 is preferably 140 to 190 ° C. as described above. If the number of revolutions of the nozzle decreases, the state becomes the same as that when the ambient temperature decreases, and the drying state of the fine droplets of the slurry deteriorates. The slurry supply speed related to the injection pressure is preferably 4 to 9 liters / hour if the capacity of the drying chamber is about 700 liters, because the spraying state deteriorates as the slurry supply speed increases. The exhaust air speed by the exhaust fan 7 is appropriately set in consideration of the fact that the higher the speed, the lower the yield of the catalyst powder.
[0031]
-Purification test-
FIG. 2 shows the NOx purification rate when the catalyst powder was obtained by changing the ambient temperature at a nozzle rotation speed of 8000 rpm, and the NOx purification rate when the catalyst powder was obtained by changing the nozzle rotation speed at an ambient temperature of 160 ° C. Show. Each catalyst powder was wash-coated on a 400 cell / inch 2 cordierite honeycomb carrier together with a binder (hydrated alumina) so that the total amount of Pt, Ir and Rh was 6 g per liter. As the exhaust gas, a model gas corresponding to A / F = 22 was used, and this gas was flowed from the upstream side to the downstream side of each of the above catalysts so as to have an SV of 55000 hr ^-1 .
[0032]
According to FIG. 2, the NOx purification rate exceeds 50% at a nozzle rotation speed of 8000 rpm, and the NOx purification ratio tends to decrease as the rotation speed decreases. On the other hand, in the case of the ambient temperature, the NOx purification rate decreases relatively significantly as the temperature decreases, and the ambient temperature is preferably set to 140 ° C. or higher. When the ambient temperature is increased, there is a fear of explosion of exhaust gas, but it is considered that there is no problem up to 190 ° C.
[0033]
−Comparison with evaporation to dryness−
After stirring the above slurry at room temperature for 2 hours, it is heated at 80 ° C. for about 3 hours to evaporate the liquid, and further dried in a thermostat at 150 ° C. for about 6 hours to make Pt, Ir, and Rh ZSM-5. A catalyst material supported on was obtained. The total amount of Pt, Ir, and Rh supported on ZSM-5 for each of the catalyst material obtained by the evaporation and drying method and the catalyst material of the above embodiment (atmospheric temperature: 160 ° C., nozzle rotation speed: 8000 rpm) The deviation from the charge value and the standard deviation of the variation were examined. The results are as shown in Table 1.
[0034]
[Table 1]

[0035]
According to Table 1 above, in the evaporative drying method, the deviation of the total supported amount from the charged value is about twice that of the embodiment (spray dry method), and the variation of the total supported amount is about four times that of the embodiment. I have. From this, it is understood that according to the example, the variation in the amount of the catalytically active species carried can be significantly improved.
[0036]
-Effect of particle size of powdered carrier-
H-type ZSM-5 (SiO ₂ / Al ₂ O ₃ = 70 to 80) containing fine powder having a secondary particle size of 0.1 μm or less to coarse powder having a secondary particle size of 6 μm or more is used as a powdery carrier. Is weighed such that Pt: Ir: Rh = 30: 6: 1 (weight ratio) and the total amount of the active species is 4.5 g per liter of the catalyst, and a slurry is prepared in the same manner as in the previous example. did. In the following, unless otherwise specified, the particle size is a value measured by a laser diffraction method.
[0037]
When the slurry was spray-dried under the same conditions as in the previous case, the particle size distribution of each catalyst powder received by each of the first to third catalyst powder receivers 13, 17, and 18 was as shown in FIGS. As shown in each figure. Then, when the amount of the active species carried on the catalyst of each catalyst powder receiver was examined, it was as shown in Table 2.
[0038]
[Table 2]

[0039]
According to FIGS. 3 to 5 and Table 2, the catalyst powder received in the first catalyst powder receiver 13 below the drying chamber 3 has the amount of active species carried that is the expected amount x (= 4.5 g / liter). Is within the range of ± 15% by weight, it can be said that the amount is almost as intended, and the particle size is in the range of 1 to 5 μm.
[0040]
On the other hand, the catalyst powder received by the second catalyst powder 17 below the cyclone 5 has a particle size of about 3 μm and a small variation, but the active species carrying amount is expected to be 1.1. Approximately 5 times, the dispersion is large, and the amount of the carrier is high. Further, the catalyst powder received by the third catalyst powder receiver 18 below the bag filter 6 has a small amount of active species carried 0.2 to 0.9 times the expected amount, and a particle size of 0.4 to 0.9. Those having a size of 2 μm occupy about ３ of the whole, and are spread over a relatively wide range.
[0041]
In this case, the particle size of the catalyst powder corresponds to the particle size of H-type ZSM-5 as a powdery carrier. Therefore, from the above, if the particle size of the carrier is 1 μm or more, the active species loading amount is likely to be targeted or high, but if the particle size is smaller than that, the active species loading amount will decrease. Can be said. On the other hand, when the particle size of the carrier exceeds 6 μm, spray drying becomes difficult and the amount of the active species carried becomes low.
[0042]
From the above results, it can be seen that the catalyst performance can be controlled by mixing the respective catalyst powders obtained in the first to third catalyst powder receivers 13, 17, and 18 at an appropriate ratio. That is, when an exhaust gas purifying catalyst (for example, a honeycomb catalyst) is formed only by the catalyst powder obtained in the first catalyst powder receiver 13 below the drying chamber, when the HC purification rate is lower than an expected value, When the catalyst powder obtained in the first catalyst powder receiver 13 and the catalyst powder obtained in the second catalyst powder receiver 17 under the cyclone are mixed such that the ratio of the latter becomes 1 to 10 or 1 to 5% by weight. In addition, the catalyst has an improved HC purification rate after the heat treatment. On the other hand, when the HC purification rate is higher than the expected value, the catalyst powder obtained in the first catalyst powder receiver 13 and the catalyst powder obtained in the third catalyst powder receiver 18 below the bag filter are combined with the latter. When the catalyst is mixed so that the ratio becomes 1 to 10% by weight, the HC purification rate of the catalyst after the heat treatment is improved.
[0043]
-Influence of slurry viscosity-
First, a plurality of slurries having different concentrations of the noble metal active species were prepared, and their viscosities were measured. That is, using a divalent platinum ammine crystal, iridium trichloride, and rhodium nitrate, a plurality of active species solutions having different concentrations of noble metal active species (solvents are water and ethanol) are prepared, and an equal amount of H is added thereto. type ZSM-5 to _{(SiO 2 / Al 2 O 3} = 70~80) was added, to obtain each slurry was stirred. In any of the slurries, the weight ratio of the noble metal active species is Pt: Ir: Rh = 30: 6: 1, and the ratio between the amount of the ZSM-5 powder and the amount of the solvent is 1: 2. In this case, the concentrations of the noble metal active species were different from each other in a range where the total amount of the noble metal when supported on the ZSM-5 was 0 to 7 g per liter of the catalyst.
[0044]
The results are shown in FIG. In addition, the concentration of the noble metal active species is shown in terms of the amount of active species carried per liter of the catalyst. According to the figure, the viscosity of the slurry increases as the concentration of the noble metal active species increases. This is because the higher the concentration of the noble metal active species, the higher the concentration of nitrate ions in the slurry. Therefore, in order to obtain a slurry having a desired viscosity without changing the concentration of the noble metal active species, the viscosity may be adjusted by adding nitric acid.
[0045]
Next, several kinds of slurries having different viscosities of the above-mentioned ZSM-5 and the active species solution were prepared by adjusting the respective viscosities, and each was spray-dried by the above-mentioned spray-drying device. The amount of catalyst powder received by one catalyst receiver 13 was examined. The results are shown in FIG. In the figure, the ratio of the amount of the ZSM-5 powder to the amount of the solvent is 1: 2 in the slurry where the yield of the catalyst powder is the highest at a viscosity of 35 to 40 cps.
[0046]
According to FIG. 7, the catalyst powder yield of the first catalyst powder receiver 13 peaks at a viscosity of 35 to 40 cps of the slurry, and decreases when the viscosity is too low or too high. It is understood that the viscosity is preferably about 20 to 60 cps, and more preferably 30 to 50 cps.
[0047]
Here, the reason why the yield of the catalyst powder is low when the viscosity is low is that the proportion of ZSM-5 powder in the slurry to be sprayed is relatively small because the amount of the solvent is large. This is considered to be due to the fact that the air is easily floated by the blast energy and scattered toward the cyclone 5. On the other hand, when the above-mentioned viscosity is high, the catalyst powder yield is low because the amount of slurry droplets is large, so that the thermal energy of the blast becomes insufficient, and the slurry droplets fall without being completely dried. This is considered to be due to adhesion to the inner wall surface of the drying chamber.
[0048]
-Influence of slurry pH-
It is naturally recognized that the pH influences the viscosity, as previously seen from the relationship between the concentration of the noble metal active species in the slurry, that is, the concentration of nitrate ions, and the viscosity of the slurry. Also affects the performance of That is, the pH of the slurry was changed to three types of 2, 6, and 8, and the other conditions were the same (powder carrier: H-type ZSM-5 (SiO2 / Al2 O3 = 70), the weight ratio of the noble metal active species; Pt: When the maximum NOx purification rate of each catalyst powder obtained as Ir: Rh = 30: 6: 1, total amount of noble metal active species; 4.5 g / liter of catalyst) was examined (used gas; A / F = 22 equivalent) Model gas, SV = 55000 h ^-1 ) was as shown in Table 3.
[0049]
[Table 3]

[0050]
According to Table 3, the highest NOx purification rate is obtained at pH 6, and it is understood that the pH is preferably set to about 4 to 8 in order to obtain 40% or more as the purification rate.
[0051]
-Light-off characteristics at λ = 1-
The light-off characteristics (temperature characteristics at which the catalyst becomes active) at λ = 1 (A / F = 14.7) were studied. That is, Pt, Ir and Rh were 30: 6: 1 in weight ratio and the total amount was 1 liter of the catalyst by the spray-drying method using three kinds of metal-containing silicates A, B and C shown in Table 4. Each of the catalyst powders was weighed 4.5 g, and was subjected to an activation treatment (in air) at 200 ° C. for 16 hours. FIGS. 8, 9 and 10 are photographs of the above metal-containing silicates A, B and C observed by SEM (scanning electron microscope).
[0052]
Then, 20 wt% of hydrated alumina was added as a binder to each of the obtained catalyst powders, and an appropriate amount of water was added to obtain a slurry for coating. A honeycomb carrier made of cordierite was immersed in these slurries, pulled up, and the excess slurries were blown off. Then, a baking treatment (atmosphere) at 500 ° C. × 2 hours was performed. The amount of catalyst powder carried on each honeycomb carrier was adjusted to 35 to 40 wt% of the honeycomb weight.
[0053]
[Table 4]

[0054]
Then, for each of the honeycomb catalysts, a model gas equivalent to A / F = 14.7 (λ = 1) and a model gas equivalent to A / F = 22 (lean) were used, and purification was performed by setting SV = 55000 to 60,000 h ⁻¹ . The catalyst inlet gas temperature when a rate of 50% was obtained was measured. The results are shown in Table 5.
[0055]
[Table 5]

[0056]
According to Table 5, when a metal-containing silicate having a small average particle size is used, catalytic activity can be obtained from a low temperature when λ = 1 without deteriorating the NOx purification characteristics when the exhaust gas is lean. It can be seen that it is advantageous for purification of wastewater. That is, when the metal-containing silicates A and B are used, the light-off temperature is 25 to 28 ° C. for HC, 14 to 17 ° C. for CO, and 18 to 25 ° C. for NOx as compared with the case where the metal-containing silicate C is used. Each is shifted to the low temperature side.
[0057]
Further experiments from this point of view show that when the average particle diameter of the metal-containing silicate is 0.5 to 1.5 μm and the particle size distribution is 0.1 to 5 μm, the average particle diameter is further 0.5 to 1 μm. It was found that good results were obtained when the particle size distribution was 0.1 μm and the particle size distribution was 0.1 to 2.5 μm.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a spray drying apparatus. FIG. 2 is a graph showing the effect of an ambient temperature and a nozzle rotation speed on a NOx purification rate. FIG. 3 is a particle size of a catalyst powder obtained in a catalyst powder receiver below a drying chamber. FIG. 4 is a diagram showing a distribution. FIG. 4 is a diagram showing a particle size distribution of catalyst powder obtained in a catalyst powder receiver under a cyclone. FIG. 5 is a diagram showing a particle size distribution of catalyst powder obtained in a catalyst powder receiver under a bag filter. FIG. 6 is a graph showing the relationship between the concentration of active species of the slurry (the amount of the noble metal carried) and the viscosity. FIG. 7 is a graph showing the relationship between the viscosity of the slurry and the yield of the catalyst powder in a catalyst powder receiver under a drying chamber. FIG. 8 is an electron micrograph showing the structure of metal-containing silicate particles having an average particle size of 0.8 μm. FIG. 9 is an electron micrograph showing the structure of metal-containing silicate particles having an average particle size of 0.6 μm. Electrons exhibiting a metal-containing silicate particle structure of 8 μm Micrograph DESCRIPTION OF SYMBOLS
Reference Signs List 1 slurry tank 2 tube pump 3 drying chamber 4 electric heater 5 primary cyclone 6 bag filter 7 exhaust fan 8 atomizer 9 hot air chamber
12 Exhaust system
13,17,18 Catalyst powder receiver

Claims (2)

A plurality of types of powdery pH4~8 slurry to adjust the Rutotomoni nitrate amount carrier allowed to disperse a 1~6μm is particle size mixed a solvent solution of the solvent containing the active species with water and an organic solvent Producing the catalyst for exhaust gas purification, wherein the plurality of types of active species are supported on the powdery carrier by forming the slurry into a droplet state by spraying and contacting with hot air to rapidly dry the slurry. Method. The method for producing an exhaust gas purifying catalyst according to claim 1 , wherein the viscosity of the slurry is 20 to 60 cps.

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