Method for production of a monolithic catalyst

Abstract: During the deposition of a catalytic component on a monolithic carrier, the position of the liquid level of a slurry containing the catalytic component is controlled with high accuracy and the adhesion of the slurry to the lateral face of the carrier is prevented. In the deposition of the liquid substance on the carrier by the introduction of the liquid substance into the carrier from the bottom face thereof, a water-detecting sensor detects the liquid level near the top face of the carrier.

Description: BACKGROUND OF THE INVENTION Field of the Invention: This invention relates to a method for producing a catalyst. More specifically, it relates to a method for producing a catalyst which comprises controlling the liquid level of the top face of a monolithic carrier by a water-detecting sensor, especially a method for producing an exhaust gas purifying catalyst. Description of Related Art: In the deposition of a catalytically active component on a monolithic carrier, when the expensive active component is deposited also on the lateral face of the carrier, such lateral face so deposited does not treat exhaust gases and it rather brings the disadvantage of heightening the price of produced catalysts. Thus, it is required a method for not depositing or adhering the active component to the lateral face. In this method, a slurry containing the active component is raised from the bottom face of carrier into the cells, and after the slurry has reached the top face of the carrier, the slurry in the cells is discharged from the lower side in a state not overflowing the top face. For preventing the slurry from overflowing the top face, it may cite a visual observation. It may use a laser or supersonic sensor in consideration of productivity. The method using such a sensor, however, has the problem of not satisfactorily corresponding to variations in the viscosity and the speed of forced penetration of the slurry because it is incapable of confirming the position of the slurry with accuracy till the slurry reaches the top face, and capable of detecting the liquid level only after the slurry has surpassed the top face. Thus, in order to coat the slurry to the carrier completely, it has entailed the problem of suffering the slurry to overflow the top face and come to adhere to the lateral face and the device being used for the deposition. Production methods prescribing to the preamble of appended claim 1 are known from the documents EP-A-0 428 990 and EP-A-1 136 462, published on 26.09.2001 and claiming the priority date of 23.03.2000. The former uses a not further specified detector to detect the liquid level, while the latter uses a capacitive sensor for this purpose. In the parallel application EP-A-1 180 398, of the same filing date and claiming the same priority date as the present application, a method prescribing to the preamble of appended claim 1 is claimed, which uses an image processing sensor to detect the liquid level. SUMMARY OF THE INVENTION We have pursued a diligent study of a method capable of accurately detecting the liquid level of the slurry at the proximity of the top face of the carrier while the slurry is ascending in the interior of the carrier, and have found that the position of liquid level of the slurry can be accurately detected by a water-detecting sensor. This invention has been perfected as a result. The object of this invention is accomplished by a method for the production of a catalyst, characterized by adopting a water-detecting sensor for detecting the liquid level at the top face of a monolithic carrier while a liquid substance containing a catalytic component to be deposited is being introduced into the carrier from the bottom face. According to the present invention, it can eliminate the problems of the conventional method such as the adhesion of the liquid substance overflowing the top face of the carrier to the lateral face and the device being used for the deposition and the consequent loss of the expensive liquid substance and spotting of the carrier and device used. It also permits a saving in the cost of production by ensuring safety of coating and decreasing the loss of liquid substances. The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments. BRIEF DESCRIPTION OF DRAWINGS The accompanying drawing incorporated in and forming a part of the specification, illustrates several aspects of the present invention, and together with the description serve to explain the principles of the invention. In the drawings: Fig. 1 is a partial cross section intended to describe the retain of a monolithic carrier in the case of deposition; Fig. 2 is a partial cross section of the impregnating device for depositing a slurry on a carrier; Fig. 3 is another partial cross section of the impregnating device; and Fig. 4 is a partial cross section of yet another example of the impregnating device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, this invention will be described in detail below. The monolithic carrier to be used in this invention is not restricted but required to be a refractory structure having a plurality of cells penetrate through the interior thereof. Monolithic honeycomb carriers and metal honeycomb carriers may be cited as examples. The monolithic carriers which are usable herein generally include honeycomb carriers using cordierite, mullite, α-alumina, zirconia, titania, titanium phosphate, aluminum titanate, alumino silicate, and magnesium silicate as raw materials and integral structures using such heat-resistant metals as stainless steel and Fe-Cr-Al alloys, for example. The monolithic carrier is produced by a method of extrusion molding, or reeling a sheet like element into a roll, or the like. The shape of the gas passages in the monolithic carrier (the shape of cells) may be hexagon, tetragon, triangle, or corrugation whichever best suits the occasion. The cell density {the number of cells per unit cross section, 6.45 cm 2 (1 square inch)} is generally in the range of 100 to 1500 cells, though variable with the kinds of exhaust gas such as unburnt hydrocarbon, carbon monoxide, and nitrogen oxides which occur in tunnels or emanate from plants and internal combustion engines such as automobile engines and diesel engines. Incidentally, the visible shape of the carrier is not discriminated but may be a triangle, circular, elliptic, or rectangular cross section. The monolithic carrier is put to use as coated in advance with a refractory inorganic oxide or as simultaneously coated with such an inorganic oxide and a catalytic component effective in treating an exhaust gas. This invention designates such inorganic oxide, catalytically active component, and mixture thereof as a "catalytic component." Examples of the refractory inorganic oxide may include γ, δ, η, and  activated alumina, ceriumoxide, zirconium oxide, tungsten oxide, titanium oxide, silicon oxide, zeolite, alkaline earth elements, and complex oxides thereof. The catalytic component is not particularly discriminated but required to be capable of disposing of harmful substances in the exhaust gas. Examples of the catalytic component may include noble metals such as rhodium, platinum, and palladium, base metals such as manganese, cobalt, chromium, nickel, and iron, and mixtures thereof. The catalytic component is treated by a known method such as a mill into slurry and then deposited on a monolithic carrier. The slurry, which is usable in this invention, is not restricted but required to be capable of ascending the interiors of the cells. The slurry can be produced for example by treating an inorganic oxide such as an alumina powder with an aqueous inorganic or organic acid solution or by treating an inorganic oxide such as alumina and a catalytic component such as the salt of platinum with an aqueous inorganic or organic acid solution. The liquid substance, which is used in this invention, embraces the aqueous solution of a catalytic component besides the slurry mentioned above. Further, this invention embraces also multilayer coating. The term "multilayer coating" as used herein means depositing coats of one same kind or different kinds in a multiple steps. In highly refractory and high performance catalysts, catalysts having a plurality of coats such as two are usual. Now, this invention will be described below with the drawings attached hereto. Fig. 1 is a partial cross section intended to describe the retention of a carrier. The method will be described with a cylindrical carrier as a typical example. In Fig. 1, a hollow frame 3 of a shape permitting insertion therein of a carrier 4 is prepared in advance and provided on the inner face thereof with at least one retainer 2 (two in Fig. 1) shaped like a swimming ring that is able to press the carrier 4 from the lateral side thereof. For the purpose of impregnating the carrier 4 with slurry (not shown), the retainer 2 like a swimming ring retains the carrier 4 and seals the lateral side of the carrier as well. The retainer 2 is made of an elastic material such as rubber or plastic or soft plastic because it is inflated by inserting a gas such as air and consequently enabled to retain the carrier 4. The gas is admitted into this retainer via a nozzle 5 that is disposed on the outer face of the retainer 2. Optionally, the retainer may be used at one point in the bottom face. Though the internal shape of the hollow frame 3 is not restricted but required to permit insertion therein of the carrier, it is preferred to bear similarity with the outer shape of the carrier. Specifically, when the carrier has a circular outer shape, the inner face of the frame is also a circular shape. The reason for constructing the frame as described above is that when the retainer 2 is inflated, it will uniformly hold the whole of the lateral face of the carrier 4 and seal the lateral face thereof as well. The carrier 4 is inserted into the interior of an impregnating device 1 from the above portion thereof. When the carrier 4 reaches a position with the bottom face of the retainer 2b, this motion is stopped. Then, the carrier 4 is fixed by inflating the retainer 2b. The retainer 2b so inflated is retained in the inflated state. Thereafter, with respect to the top face of the carrier, the retainer 2a is also inflated and fixed in the inflated state. Consequently, the carrier is fixed in the impregnating device. Now, the method for depositing the carrier with a slurry as a typical example will be described below. Fig. 2 is a partial cross section illustrating one example of the impregnating device serving to deposit a slurry on a carrier. In Fig. 2, a slurry 26 held inside a hollow frame 23 is ascended in the interior of the frame 23 by the action of a known liquid delivering device of the shape of a piston (not shown). The ascent of the slurry 26 in the interior of a monolithic carrier 24 is monitored and detected from above the carrier with a water-detecting sensor 27. The water sensor 27 is positioned above the monolithic carrier. Commendably, the distance of this sensor 27 from the carrier is suitably decided in consideration of factors such as the precision of the water sensor 27 and the color of the slurry. The water sensor 27 has a principal that it emits a light whose wave length can be absorbed by water (OH group) and receives the reflected light to detect the water based on the reduction of strength of the light. For example, it evaluates the reduction of strength of the light by emitting a light using an IR laser diode to expose nearby the top face of carrier to be deposited, receiving the reflected light by a photodiode. In the presence of water, the exposed light is absorbed by water, thereby the strength of the reflected light decreased. As a result, the water present can be detected. Thus, it can detect water and materials containing water. The position at which the water sensor is set may be directly above or oblique relative to the top face of the carrier 24 so long as the detection can be attained. Since the use of the water sensor 27 permits the position of the slurry 26 to be accurately detected, the slurry can be stopped without overflowing the top face of the carrier 24. After coating has been completed up to the top face, aspirating the excess slurry from the bottom face of the carrier or expelling the excess slurry by blowing a gas from above the carrier terminates it. The series of steps mentioned above may be performed either individually or continuously automatically, The carrier is removed from the impregnating device after the gas filled in the retainer has been withdrawn. Then, the removed carrier is dried and optionally calcined to complete a catalyst. Fig. 3 is another partial cross section of the impregnating device for depositing the slurry on the carrier. In Fig. 3, a slurry 36 is ascended in the interior of a hollow frame 33 made of a hard resin 33 by a known liquid-delivering device of the type of piston (not shown). When the slurry 36 so ascending happens to leak between the carrier 34 and the frame 33, the leaking slurry 36 is at a disadvantage in adhering to the carrier 34 and the frame 33 and inducing a loss of the expensive slurry. To avoid this leakage, a retainer 38 made of an elastic substance such as rubber or plastic or soft plastic is used to seal tightly the gap between the carrier 34 and the frame 33 and prevent the slurry 36 from leaking to the exterior. The slurry is deposited on the carrier by controlling the liquid level using the water sensor 37. The carrier coated with the slurry is dried and optionally calcined to complete a catalyst. Fig. 4 is a partial cross section of yet another example of the impregnating device for depositing the slurry on the carrier. In Fig. 4, since a carrier retainer 48 made of an elastic substance such as rubber or plastic or soft plastic retains a carrier 44, the carrier 44 can be retained in intimate contact with the retainer 48. If the slurry 46 has ascended to the site of this intimate contact, this slurry 46 will not leak from the gap between the carrier 44 and the retainer 48. Thus the slurry 46 that contains an expensive material can be used or reused without incurring any loss. The carrier retainer 48 is fixed to a frame 43 made of a material such as stainless steel, which is incapable of inducing an interaction with the slurry. Since the top of the frame 43 falls below the retainer 48, it is enabled to retain the carrier 44. The top shape of the frame 43 is not restricted but required to be capable of retaining the carrier and causing the slurry to flow inwardly. It may be inclined inwardly, for example. The slurry 46 is ascended in the interior of the retainer 48 by a known liquid-delivering device such as a piston (not shown). The slurry is deposited on the carrier by controlling the liquid level using the water sensor 47. The carrier coated with the slurry is dried and optionally calcined to complete a catalyst. The devices for retaining and impregnating the carrier may be selected from among known devices except for the water sensor. Methods for depositing the slurry on the carrier automatically by controlling the liquid level with the water sensor are embraced in the scope of this invention. EXAMPLES Now, this invention will be specifically described below with the examples. It should be noted that this invention is not limited to these examples. EXAMPLE 1 A monolithic carrier (400 cpsi/6 mil, available from Denso K.K. in Japan) was set in an impregnating device illustrated in Fig. 2. A water-detecting sensor was set above the carrier. Then, a slurry containing a Pd type catalytic component was forcibly introduced into the carrier from the bottom face thereof. This slurry had a specific gravity of 1.600 g/ml and a viscosity of 500 cps. The flow velocity of the slurry being introduced was 3,000 L/hr. When the slurry arrived at the top face of the carrier, the sensor detected the water in the slurry and then the supply of slurry was stopped. At this time, the slurry is completely coated to the top face without overflow on the lateral face. EXAMPLE 2 The procedure of Example 1 was repeated except that the flow velocity of the slurry was changed to 2,800 L/hr. At this time, the top face of the carrier was coated with the slurry except for the three cells next to the outermost periphery. EXAMPLE 3 The procedure of Example 1 was repeated except that the flow velocity of the slurry was changed to 1,000 L/hr. A size of about 5 mm from the top face of the carrier was left uncoated with the slurry. EXAMPLE 4 The procedure of Example 1 was repeated except that the viscosity of the slurry was changed to 1,000 cps. A size of about 5 mm from the top face of the carrier was left uncoated with the slurry. EXAMPLE 5 The catalyst prepared in Example 1 was set in the device illustrated in Fig. 2. Then, a slurry containing a Rh type catalytic component was introduced into the carrier with force from the bottom face of the carrier. This slurry had a specific gravity of 1.300 g/ml and a viscosity of 150 cps. The flow velocity of the slurry during the introduction was 2,500 L/hr. When the slurry arrived at the top face of the carrier, the sensor detected the water in the slurry and then the supply of slurry was stopped. As a result, the system could be stopped without suffering the slurry to overflow the top face of the catalyst and adhere to the lateral face. At this time, the top face of the catalyst was coated with the slurry except for three cells next to the outermost periphery. COMPARATIVE EXAMPLE A monolithic carrier (supra) was set in an impregnating device. Then, a slurry containing a Pd type catalytic component was introduced into the carrier with force from the bottom face thereof. This slurry had a specific gravity of 1.600 g/ml and a viscosity of 500 cps. The flow velocity of the slurry during the introduction was 3,000 L/hr. The liquid level of the slurry was detected with a laser sensor. Since the laser sensor only could detect the liquid level after the slurry had surpassed the top face of the carrier, the slurry overflowed and adhered to the lateral face of the carrier. As demonstrated above, this invention is to control the position for stopping the liquid level, the uncoated portion of the carrier, and the overflow by optimizing conditions such as the accuracy of water-detecting sensors, the viscosity of slurry, and the flow velocity of slurry by force, while the conventional laser sensor was incapable of controlling the position for stopping the liquid level of the slurry. It is possible to effect the deposition with high accuracy without inducing excess adhesion to the lateral face of the carrier by combining the water sensor and the retainer.

Claims: A method for producing a catalyst, comprising adopting a sensor (27, 37, 47) for detecting the liquid level near or at the top face of a monolithic carrier (24, 34, 44) while a liquid substance (26, 36, 46) containing a catalytic component to be deposited thereon is being introduced into the carrier from the bottom face, characterized in that said sensor is a water-detecting sensor. A method according to claim 1, wherein the cross section of the carrier has a shape selected from the group consisting of triangle, circle, ellipsis, and tetragon. A method according to claim 2 further comprising an impregnating device including a hollow cylinder (21, 31, 41) having an inner surface identical or similar in shape with or to the shape of the cross section and a larger inside diameter than the shape of the cross section. A method according to claim 3, wherein the cylinder has at least one retainer (22) of the shape of a swimming ring disposed on the inner face of the cylinder substantially horizontally across the entire circumference. A method according to claim 3, wherein a portion of the inner face of the cylinder contacting the carrier is an elastic substance (38). A method according to claim 3, wherein the inner face of the cylinder is made of an elastic substance and further a second hollow cylinder (43) disposed in contact with the inner side of the cylinder.

Inventors: HORIUCHI MAKOTO, KAKIMI RYUGO, TANIGUCHI SHIGEYOSHI, HORIUCHI, MAKOTO, KAKIMI, RYUGO, TANIGUCHI, SHIGEYOSHI
Applicants: ICT CO LTD, INT CATALYST TECHNOLOGY INC, NIPPON SHUKUBAI CO LTD, ICT CO., LTD., INTERNATIONAL CATALYST TECHNOLOGY, INC., NIPPON SHUKUBAI CO.,LTD.
Application year: 2001
Main classification: B01J37
IPC codes: B01J35, B01J37, F01N3, F01N13