Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
1995-12-21
2002-12-24
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C423S213700, C060S299000, C060S302000, C422S171000, C422S177000
Reexamination Certificate
active
06497851
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to apparatus for and a method of treating engine exhaust gases to reduce pollutants contained therein. More specifically, the present invention concerns apparatus containing catalysts of two different types, one of which may be a “close-coupled catalyst ” which is free of an oxygen storage component.
2. Related Art
Motor vehicle exhaust treatment devices such as catalytic converters have conventionally been located in an underfloor position in the vehicles. However, by the time engine exhaust gases travel through an exhaust pipe to an underfloor position, they cool significantly relative to the temperature at or near the engine outlet, so there is a significant period of low conversion activity before the exhaust gases heat the catalyst to its light-off temperature. Accordingly, during the cold-start period of engine operation there is a significant discharge of unconverted exhaust gas. Increasingly stringent governmental emissions standards require, however, that cold-start emissions be reduced. In particular, the California Resource Board (CARB) has announced new ultra-low emission vehicle standards that will prohibit vehicle emissions above 0.04 grams of non-methane hydrocarbons per mile, 1.7 grams carbon monoxide per mile and 0.2 grams NO
x
per mile. For most motor vehicles, a large portion (up to 80%) of the hydrocarbon emissions occurs during the first phase of the U.S. Federal Test Procedure (“FTP”), which encompasses the cold-start period of engine operation, and which requires simulation of cold-start, warm-up, acceleration, cruise, deceleration and similar engine operating modes over a specified time period. A variety of technologies are under development to reduce cold-start hydrocarbon emissions, including the use of close-coupled catalysts as disclosed, e.g., in Ball, D. J., “Distribution of Warm-Up and Underfloor Catalyst Volumes”, SAE 922338. It has been reported that close-coupled catalysts, especially Pd-containing catalysts, are effective for reducing HC emissions during cold-start of the FTP cycle.
The principal function of close-coupled catalysts, also referred to as “precat” and “warm-up” catalysts, is to reduce hydrocarbon emissions during cold-start. Cold-start is the period immediately after starting the engine from ambient conditions. The length of the cold-start period depends on the ambient temperature, the type of engine, the engine control system and engine operation. Typically, the cold-start period is within the first two minutes after the start of an engine at ambient temperature. FTP Test 1975 characterizes cold-start as the first bag (i.e., exhaust gas sample) of the FTP driving cycle which lasts for the first 505 seconds after starting an engine at ambient temperature, which is generally considered to be 25° C. In an exhaust apparatus comprising a close-coupled catalyst, at least part of the total exhaust system catalyst is positioned closer to the engine than a traditional “underfloor catalyst”. Specifically, the close-coupled catalyst is located in the engine compartment, i.e., beneath the hood and adjacent to the exhaust manifold. The close-coupled catalyst can constitute the entire catalyst mass of the exhaust treatment apparatus or it can be used in conjunction with an underfloor catalyst. The design option depends on the engine configuration, size and space available. Due to its proximity to the engine relative to the underfloor catalyst, the close-coupled catalyst receives exhaust gas at a higher temperature than the underfloor catalyst. Accordingly, the close-coupled catalyst attains its light-off temperature more quickly than an underfloor catalyst and therefore reduces emissions earlier relative to the cold-start period. On the other hand, a catalyst in a close-coupled position receives exhaust gas at operating temperatures, i.e., post-cold-start period temperatures, higher than those at which an underfloor catalyst receives the exhaust gas. As a consequence, the close-coupled catalyst must have high temperature stability, as discussed in Bhasin, M. et al, “Novel Catalyst for Treating Exhaust Gases For Internal Combustion and Stationary Source Engines”, SAE 93054, 1993.
A typical underfloor motor vehicle catalyst is a three-way conversion catalyst (“TWC”) which catalyzes the oxidation of the unburned hydrocarbons and carbon monoxide and the reduction of nitrogen oxides to nitrogen. TWC catalysts, which exhibit good activity and long life, typically comprise one or more platinum group metals (e.g., platinum or palladium, rhodium, ruthenium and iridium), optionally with one or more base metals, dispersed on a high-surface area, refractory oxide support, e.g., particles of high-surface area alumina, to form a catalytic material. The catalytic material is carried on a suitable carrier or substrate such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure, or refractory particles such as spheres or short, extruded segments of a suitable refractory material. High-surface area alumina support materials, also referred to as “gamma-alumina” (although it usually contains other phases of alumina in addition to gamma) or “activated alumina”, typically exhibit a BET surface area in excess of 60 square meters per gram (“m
2
/g”), often up to about 200 m
2
/g or more. It is known to utilize refractory metal oxides other than activated alumina as a support for at least some of the catalytic components in a given catalyst. For example, bulk cerium oxide, zirconium oxide, alpha-alumina and other materials are known for such use. Many of these other materials suffer from the disadvantage of having a considerably lower BET surface area than activated alumina, but that disadvantage tends to be offset by a greater durability of the resulting catalyst.
In a moving vehicle, exhaust gas temperatures can reach 1000° C., and such elevated temperatures cause the activated alumina (or other) support material to undergo thermal degradation caused by a phase transition with accompanying volume shrinkage, especially in the presence of steam, whereby the catalytic metal becomes occluded in the shrunken support medium with a loss of exposed catalyst surface area and a corresponding decrease in catalytic activity. It is a known expedient in the art to stabilize alumina supports against such thermal degradation by the use of materials such as zirconium oxide, titanium oxide, alkaline earth metal oxides such as barium oxide, calcium oxide or strontium oxide or rare earth metal oxides, such as cerium oxide, lanthanum oxide and mixtures of two or more rare earth metal oxides. For example, see C. D. Keith et al U.S. Pat. No. 4,171,288.
U.S. Pat. No. 4,504,598 discloses a process for producing a high temperature-resistant TWC catalyst. The process includes forming an aqueous slurry of particles of activated or gamma-alumina and impregnating the alumina with soluble salts of selected metals including cerium, zirconium, at least one of iron and nickel and at least one of platinum, palladium and rhodium and, optionally, at least one of neodymium, lanthanum, and praseodymium. The impregnated alumina is calcined at 600° C. and then dispersed in water to prepare a slurry which is coated on a honeycomb carrier and dried to obtain a finished catalyst.
SUMMARY OF THE INVENTION
The present invention relates to an engine exhaust treatment apparatus for abating pollutants contained in the exhaust stream of the engine. The apparatus defines a flow path for the exhaust and comprises an upstream catalyst member comprising an upstream catalytic material effective for catalyzing the oxidation of hydrocarbons and comprising a platinum group metal component dispersed on a refractory metal oxide first support. The upstream catalytic material is substantially free of oxygen storage component. There is also a downstream catalyst member comprising a downstream catalytic material which is effective at least for the oxidation of hydrocarbons and which comprises one or more catalyti
Amundsen Alan R.
Heck Ronald M.
Hu Zhicheng
Smaling Rudolf M.
Englehard Corporation
Medina Maribel
Negin Richard A.
Silverman Stanley S.
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