Multilayered catalyst compositions

#This invention pertains generally to layered catalysts used to treat gaseous steams containing hydrocarbons, carbon monoxide, and oxides of nitrogen. More specifically, this invention is directed to three-way conversion (TWC) catalysts having multiple layers, for example, three or more layers of catalytic material.

#Catalytic converters are well known for the removal and/or conversion of the harmful components of exhaust gases. They typically contain a three-way conversion (TWC) catalyst and are located in the exhaust gas line of internal combustion engines. Such catalysts promote the oxidation by oxygen in the exhaust gas stream of unburned hydrocarbons and carbon monoxide as well as the reduction of nitrogen oxides to nitrogen.

#Known TWC catalysts which exhibit good activity and long life comprise one or more platinum group metals (e.g., platinum, palladium, rhodium, rhenium and iridium) disposed on a high surface area, refractory metal oxide support, e.g., a high surface area alumina coating. The support 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. TWC catalysts can be manufactured in many ways. U.S. Pat. No. 6,478,874, for example, sets forth a system for catalytic coating of a substrate. Details of a TWC catalyst are found in, for example, U.S. Pat. Nos. 4,714,694 and 4,923,842. U.S. Pat. Nos. 5,057,483; 5,597,771; 7,022,646; and WO95/35152 disclose TWC catalysts having two layers with precious metals. U.S. Pat. No. 6,764,665 discloses a TWC catalyst having three layers, including a palladium layer having substantially no oxygen storage components.

#FIG. 1 is a schematic view showing a configuration of layers on a catalytic member of an exhaust gas treatment system; and

#FIG. 2 is a schematic view showing another configuration of layers on a catalytic member according to an embodiment of the present invention.

#The present invention relates to a layered catalyst composite of the type generally referred to as a three-way conversion catalyst having the capability of simultaneously catalyzing the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides. It has been found that the use of a trimetal catalyst having a palladium (Pd)-containing layer, a platinum (Pt)-containing layer, and a rhodium (Rh)-containing layer provides excellent conversion of NOx and hydrocarbons for SULEV applications where NOx and hydrocarbons emissions are tightly regulated. Without attempting to be bound by theory, it is thought that the Pt-containing layer promotes, for example, NOx reduction by hydrogen generated through water gas shift (WGS) as shown in Formula I: CO+H2OCO2+H2  (I), and assists, for example, hydrocarbon conversion by steam through steam reforming reaction (SRR). Another benefit is the availability of hydrogen to help Rh regeneration from much less active Rh+3 to active Rh0 so that Rh functionality can be maximized. The thermal-neutral nature of the WGS and the endothermic nature of the SRR help to reduce hydrothermal aging severity for the Pd- and Rh-containing layers where exothermic carbon monoxide (CO) and hydrocarbon oxidation can generate heat resulting in damage to precious metal components and to other materials of the catalyst layer.

#In one or more embodiments, the gas is located in an exhaust system of an automobile and is at a temperature of 1100 (or in other embodiments, 1000, or 900, or even 800)° C. or less.

#In a first aspect of the present invention, the structure of the layered catalyst composite of the present invention is designed to have three layers in conjunction with a carrier. The three layers are made up of three compositions: a first layer deposited on a carrier; a second layer deposited on the first layer; and a third layer deposited on the second layer. Among the three layers, there are three different precious metals, Pt, Pd, and Rh, one in each layer. Optionally, as needed, one or more additional precious metals can be added to each layer.