MINI-CASCADE SISTEMACATALIZER
Field of the Invention The invention relates to a catalytic system which includes three separate catalytic blocks, arranged in series, within a passage of the exhaust gas of an internal combustion engine, the first coupling closest to the exhaust manifold of the engine and the second is closely adjacent to the first. Background of the Invention The low emission vehicle standards in California, E. U. A., which will be carried out in the future, will require a significant reduction in the emission levels of hydrocarbons and oxides of nitrogen in the exhaust pipe, currently acceptable. The standards will also become stricter in the performance of emissions during use and will require in the future compliance with up to 160.00 kilometers of vehicle service. Hydrocarbon (HC) emissions can occur during the cold start period of the engine, when the fuel-rich operation is necessary and the normal catalyst of the exhaust treatment has not reached the "on" temperature, necessary for its operation efficient. Until then, emissions of hydrocarbons from the engine, unburned or partially burned, can be released into the atmosphere.
A variety of techniques have been suggested to reduce HC emissions at cold start. One involves a cascade arrangement with an electrically heated catalyst, as described in (SAE 941042) entitled "Development of a Catalytic System, Electrically Heated, Energized by the Electric Generator". In cascade arrays, as described in this reference, the system consists of two blocks, a small block followed by another relatively large block. The small block, which is first contacted by the exhaust gases, is heated electrically during the cold start, to improve its efficiency during this time. However, electrical heating of the catalyst can be expensive and raise safety questions. Hydrocarbon traps have also been investigated to adsorb these hydrocarbons during cold start. Commonly available traps suffer from deficiency that are not able to adsorb low molecular weight hydrocarbons and trapping systems often suffer from sufficient durability and controllability. The narrow coupling of a catalyst block (ie, a catalyst carried on the porous support and coated on a substrate) seems to represent the least complex and costly of the solutions in the conversion of the HCs in the cold start. By placing the catalyst block close to the exhaust manifold, the temperatures in the catalyst block rise rapidly to the ignition temperature of the catalyst, when the catalyst reaches the ignition temperature, the carbon monoxide and the hydrocarbons begin to effectively convert in the desired inert gases. However, there are difficulties associated with a conventional narrowly coupled catalyst. For example, the "packing" of a large block of catalyst tightly coupled to the exhaust manifold presents difficulties due to the limited space available near the exhaust manifold. This type of closely coupled system also presents problems with the monitoring capability of the catalyst since, in narrowly coupled conventional systems, a large block is used, which does not provide the necessary sensitivity to detect a loss of catalyst activity of the block. Another problem with such systems is the lack of high temperature durability of many of the catalytic materials, when they are placed very close to the exhaust manifold. The present invention overcomes the deficiencies of the prior art systems and supplies a catalytic system which converts, quickly and efficiently, the hydrocarbons during the cold start of the engine, has an excellent durability and exceeds the packaging emissions associated with the use of a large block near the exhaust manifold, as in the design of conventional catalyst block, coupled in a narrow way. DISCLOSURE OF THE INVENTION The invention relates to a catalyst system for converting the hydrocarbons, carbon monoxide and nitrogen oxides contained in the exhaust gas, generated by an internal combustion engine. The catalytic system comprises: a first, second and third catalytic blocks, arranged in series in a passage of the exhaust gas of the internal combustion engine, downstream of a manifold of the exhaust gas. The first block has the smallest volume and is closely coupled to a manifold of the engine exhaust gas. This first block is placed adjacent to and spaced from the second block by a predetermined distance no greater than 25.4 cm. Preferably, the third block is larger in volume than the second catalytic block. Optimally, the second and third catalytic blocks are placed as close together as the package allows. Each catalyst block individually comprises a substrate coated with a metal catalyst, carried on a porous support; in which this metal catalyst carried on the first block comprises palladium.
optimally, the first catalyst block consists essentially of palladium and employs a wash coating devoid of cerium oxide. The last characteristic provides an excellent conversion of hydrocarbons, even in the cold start, and a good durability at high temperature. According to another aspect of the invention, it comprises a method for converting the exhaust gases, exposing them to the catalytic system described. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a modality of a mini-waterfall system according to the present invention. Figure 2 is another embodiment of a mini-cascade system according to the present invention, where the system is used with a motor having two exhaust manifolds. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention can be more easily understood with reference to one embodiment of the catalytic system (100) of the invention, shown schematically in Figure 1. This catalytic system comprises three catalytic blocks, labeled 10, 12 and 14, placed in series downstream of an exhaust manifold 16 of a motor 18 in a passage 20 of the exhaust gas. As will be evident, the exhaust gas generated by the engine will travel in sequence through the catalytic blocks 10, 12 and 14.
By "block or catalytic brick" is meant here a substrate made of, for example, a ceramic material, and which has a coating of a metal catalyst, such as palladium. During use, as is known in the art, the metallic catalyst will be optimally carried on a porous support material. These materials will be described in more detail below. The first catalytic block 10 has the least volume of the blocks and is narrowly coupled to the exhaust manifold 16 of the engine. The volume of the third catalytic block 14 is equal to or greater than that of the catalytic block 12. The distance between the first and second catalytic blocks is less than the distance between the second and third catalytic blocks. For this invention, the first and second blocks are placed together, so that the distance between them is minimal. In particular, the first block is placed adjacent to and spaced from the second block by a predetermined distance no greater than about 25.4 cm. Preferably, this predetermined distance is not greater than 15.24 cm. This distance is measured between the two adjacent faces 22, 24, respectively, of the first and second catalytic blocks. In a particular embodiment, the first catalyst block has a volume of about 295 to 623 cm 3, the second catalyst block has a volume of about 623 to 901 cm 3 and the third catalyst block has a volume of about 623 to 1508 cm 3. More particularly, according to a preferred embodiment, the respective volumes are 410, 623 and 1229 cm3. It is necessary that the blocks 10 and 12 be close to each other in order to promote all the exothermic energy created when the first block is turned on to transfer it substantially ("cascade") to the second block to assist in the ignition of this second block. Thus, as shown in Figure 1, the exit of the exhaust gas of the first block optimally will be carried directly into the inlet cone 26 of the second block, in order to minimize the loss of thermal energy outside the passages by thermal conduction. By placing the first and second catalytic blocks in this manner, the thermal energy of the exhaust gas is used more efficiently to heat the first and second blocks. Figure 2 schematically illustrates another embodiment of the mini-cascade catalyst system of the present invention. In this mode, forecasts are made for engines that have two exhaust manifolds, as in the V-6 and V-8 engines. As is well known in the art, for a useful application in an exhaust system, the catalyst is carried on a substrate of an electrically insulating material, stable at high temperature ("block or brick" material). Typical of such substrates are ceramics, such as cordierite, mulita, etc. The substrate can also be made of a metallic sheet comprised of materials, such as iron, chromium and aluminum. The block can be of any suitable configuration, often using a monolithic honeycomb structure, woven fibers, corrugated sheets or layered materials. Still other ceramic materials and configurations useful in this invention and suitable in the exhaust gas system will be apparent to those skilled in the art, in view of the present disclosure. It has been found that ideally, when the present block is a ribbed block of ceramic, the walls between the channels are thin, ie about 0.14 mm, in order to supply the optimum power of the vehicle, in view of the considerations of the retro -pressure, associated with the narrow coupling of a small block. In this thin-walled ceramic block, preferably the cell density of the substrate material is not greater than 54 cells per square centimeter, with the cells having a square cross-section. This provides larger cell sizes, compared to traditional substrate materials. This is in comparison with traditional ceramic substrates, which have larger wall thicknesses, for example of about 0.16 mm, and higher density of cells, for example of 62 cells per square centimeter (which supplies smaller cells). If a metal substrate is used in the first cell, rather than the ceramic, as discussed above, the metal cell may have a higher density of cells, since the preferred wall thickness of the comparative metal substrates is approximately 0.05 mm. , while providing similar performance. The second and third catalytic blocks do not need to have these preferred wall thicknesses or cell sizes, discussed, but can be made of materials with smaller cell sizes, for example of 62 cells per square centimeter. Particularly suitable metallic catalyst materials are precious metals, palladium, platinum, rhodium, and mixtures thereof. These catalysts are carried on a porous substrate. Generally, the catalytic support material is of high surface area, such as alumina, zirconia and cerium dioxide, with gamma-alumina being particularly preferred. Often such support materials are mixed with stabilizers and promoters, such as cerium dioxide, barium and nickel. The loading of one or more precious metals of catalyst on the support varies with the particular block, in which it is lifted. For example, optimally the first block is loaded more densely with the metal catalyst than any of the second or third blocks of the system of the invention.
A common way to supply the catalytic metal on the substrates as follows. First, the porous support material is impregnated with a solution of the precursor compounds for the metal catalyst, using incipient moisture techniques. The solution can be water-based or an organic solvent. For example, in order to load the platinum onto the gamma-alumina, this alumina can be impregnated with an aqueous solution of the hexachloroplatinic acid. After impregnation, the material is dried and calcined. If the porous support is different from cerium dioxide, for example gamma-alumina, it may also be convenient to incorporate by means of impregnation, some rhodium and / or cerium dioxide in the phase containing the platinum. The support material loaded with the catalytic metal can be coated by washing on the substrate, for example, the monolith. Alternatively, the porous support can first be coated by washing on the substrate and then, after drying and calcining, be impregnated with the desired metals, for example platinum. According to the preferred embodiment of the present invention, the metallic catalytic material provided on the first catalytic block consists essentially of palladium. More conveniently, in this embodiment, the first block will carry a load of at least 7.06 grams / liter of palladium, based on the porous support, more particularly of at least 7.95 grams / liter. In general, alumina is used as the porous support. Optimally, the porous support material of the first block does not include cerium dioxide, which is often present in conventional form in the catalytic support materials. It has been found that by using a support material without cerium dioxide, the catalytic materials, which consist essentially of palladium, result in a catalyst having excellent stability at high temperature. The use of the palladium-only combination without cerium dioxide will be more preferred in situations where the temperature of the first block exceeds 100 ° C during engine operation. At high temperatures, cerium dioxide tends to grow into large particles, which have a reduced surface area. As will be appreciated by those skilled in the art, the high surface area is a desirable attribute for catalyst support. The improvement in stability at high temperatures of the catalytic element means that the catalyst in the first block is capable of having an excellent hydrocarbon conversion activity for a prolonged period of time, which will be its primary function, rather than acting as a catalyst in three ways. In the above-described embodiment of the catalyst support only of palladium without cerium dioxide, optimally the second and third blocks will also use the formulation of the catalytic material only of palladium, but in the charges smaller than those provided on the first block. The second and third blocks, which conveniently have a lower load, should optionally have a minimum catalyst load of at least 3.53 grams / liter (catalyst / porous support) to maximize the ignition potential while Minimizes the cost of the catalyst. The second and third blocks will optimally contain the cerium dioxide in the porous support material of the washing coating and thus act as converters in three ways. The optimum loading of the metal catalysts is preferably individually selected from the precious metals and their mixtures in each of the three blocks, as will be apparent to those skilled in the art in view of the present disclosure. Your choice in type and quantity depends, for example, on the hydrocarbons in the exhaust gas and the handling of the exhaust gas energy at cold start. A particular advantage enjoyed by the mini-cascade catalytic system of the present invention is that the catalyst can be monitored to determine its efficiency. This can be done because the first block does not have a good capacity to store oxygen and the second block is small enough for the detection of high sensitivity of the catalyst inefficiency. By contrast, in a conventional cascade system, where the second block is very large, it is not possible to determine successfully if the system has become inefficient, due to the cushioning effect of the large block. Example A catalytic system was assembled, according to one embodiment of the present invention, as in Figure 1, and was obtained as follows: the first block comprises a block of 410 cm 3 with 7.95 grams / l of palladium only on a porous support of alumina without cerium dioxide. The substrate was made of ceramic and is of a thin wall block, where the wall thickness is 0.15 mm. The second block was made with ceramic substrates of 623 cm3 that carry 3.88 grams / liter of a palladium catalyst, in three ways, carried on a porous support typical of alumina plus cerium dioxide. The third block had a volume of 1229 cm 3 and was charged with 3.88 grams / liter of palladium, using a wash coating of the alumina plus the cerium dioxide on a ceramic substrate. The blocks were arranged in order to form a mini-waterfall system. This embodiment of the mini-cascade system of the present invention was installed in a motor vehicle using a 1.9-liter engine. The first block was 22.86 cm. from the escape door. The first and second blocks were arranged within 7.62 cm. each other and the third block at 25.4 cm. from the second block. The vehicle in this mode of the system was submitted to the Federal Trial Process 75. The emission levels of the exhaust pipe with non-aged (new) components were found to be excellent, hydrocarbons without methane (NMHC), 0.016; CO, 0.56; and NOx, 0.05. Although the results with the non-aging system do not illustrate the degradation of emissions under the actual operating conditions in the world, potential capacity is encouraged. These low levels were achieved, it is believed, due to the particular characteristics of the embodiment of the present invention, which results in the outstanding ignition of the mini-cascade system. The first block is small and has a small frontal area, so that the front face heats up very quickly, which also allows it to reach the ignition temperature very quickly. Once the exothermic reactions begin in this block, the subsequent blocks are heated as a result of this exothermic energy and also become catalytically reactive. These phenomena cause the entire system to become rapidly efficient, which greatly reduces the emissions levels of the start-up. This modality of the particular system was found to be 90% more efficient in the conversion of hydrocarbons in 32 seconds, after the cold start, during the federal test procedure 75 (FTP 75).