FIELD OF THE INVENTION

The invention relates to a NOxadsorber catalyst comprising a composition comprising a neodymium-containing component, a method of making the neodymium-containing component, and emission systems for lean-burn engines comprising the NOx adsorber catalyst.

BACKGROUND OF THE INVENTION

Internal combustion engines produce exhaust gases containing a variety of pollutants, including nitrogen oxides (“NOx”), carbon monoxide, and uncombusted hydrocarbons, which are the subject of governmental legislation. Increasingly stringent national and regional legislation has lowered the amount of pollutants that can be emitted from such diesel or gasoline engines. Emission control systems are widely utilized to reduce the amount of these pollutants emitted to atmosphere, and typically achieve very high efficiencies once they reach their operating temperature (typically, 200° C. and higher). However, these systems are relatively inefficient below their operating temperature (the “cold start” period).

One exhaust gas treatment component utilized to clean exhaust gas is the NOxadsorber catalyst (or “NOxtrap”). NOxadsorber catalysts are devices that adsorb NOxunder lean exhaust conditions, release the adsorbed NOxunder rich conditions, and reduce the released NOxto form N2. A NOxadsorber catalyst typically includes a NOxadsorbent for the storage of NOxand an oxidation/reduction catalyst.

The NOxadsorbent component is typically an alkaline earth metal, an alkali metal, a rare earth metal, or combinations thereof. These metals are typically found in the form of oxides. The oxidation/reduction catalyst is typically one or more noble metals, preferably platinum, palladium, and/or rhodium. Typically, platinum is included to perform the oxidation function and rhodium is included to perform the reduction function. The oxidation/reduction catalyst and the NOxadsorbent are typically loaded on a support material such as an inorganic oxide for use in the exhaust system.

The NOxadsorber catalyst performs three functions. First, nitric oxide reacts with oxygen to produce NO2in the presence of the oxidation catalyst. Second, the NO2is adsorbed by the NOxadsorbent in the form of an inorganic nitrate (for example, BaO or BaCO3is converted to Ba(NO3)2on the NOxadsorbent). Lastly, when the engine runs under rich conditions, the stored inorganic nitrates decompose to form NO or NO2which are then reduced to form N2by reaction with carbon monoxide, hydrogen and/or hydrocarbons (or via NHxor NCO intermediates) in the presence of the reduction catalyst. Typically, the nitrogen oxides are converted to nitrogen, carbon dioxide and water in the presence of heat, carbon monoxide and hydrocarbons in the exhaust stream.

Typically, NOxadsorbent materials consist of inorganic oxides such as alumina, silica, ceria, zirconia, titania, or mixed oxides which are coated with at least one platinum group metal. PCT Intl. Appl. WO 2008/047170 discloses a system wherein NOxfrom a lean exhaust gas is adsorbed at temperatures below 200° C. and is subsequently thermally desorbed above 200° C. The NOxadsorbent is taught to consist of palladium and a cerium oxide or a mixed oxide or composite oxide containing cerium and at least one other transition metal.

PCT Intl. Appl. WO 2004/076829 discloses an exhaust-gas purification system which includes a NOxstorage catalyst arranged upstream of an SCR catalyst. The NOxstorage catalyst includes at least one alkali, alkaline earth, or rare earth metal which is coated or activated with at least one platinum group metal (Pt, Pd, Rh, or Ir). A particularly preferred NOxstorage catalyst is taught to include cerium oxide coated with platinum and additionally platinum as an oxidizing catalyst on a support based on aluminium oxide. EP 1027919 discloses a NOxadsorbent material that comprises a porous support material, such as alumina, zeolite, zirconia, titania, and/or lanthana, and at least 0.1 wt % precious metal (Pt, Pd, and/or Rh). Platinum carried on alumina is exemplified.

In addition, U.S. Pat. Nos. 5,656,244 and 5,800,793 describe systems combining a NOxstorage/release catalyst with a three way catalyst. The NOxadsorbent is taught to comprise oxides of chromium, copper, nickel, manganese, molybdenum, or cobalt, in addition to other metals, which are supported on alumina, mullite, cordierite, or silicon carbide.

At low temperatures (typically below about 200° C.), the NOxstorage function of these catalysts is inefficient and continues to be an area of catalyst development in need of improvement. It is also desirable for catalysts to be developed that have little or no NOxstorage properties at greater than a specific temperature, to allow control of when NOxis released for subsequent conversion by, for example, a further downstream catalyst. The deactivation of NOxadsorber catalysts by sulfur, which can be present in fuels or engine lubricating oil, is also a problem, particularly under lower temperature conditions at which it may be challenging to thermally desulfate the catalyst.

As with any automotive system and process, it is desirable to attain still further improvements in exhaust gas treatment systems. We have discovered a new NOxadsorber catalyst composition with improved low temperature NOxstorage characteristics, improved NOx release properties, and improved desulfation properties.

SUMMARY OF THE INVENTION

In a first aspect of the invention there is provided a NOxadsorber catalyst for treating emissions from a lean burn engine, said NOxadsorber catalyst comprising a first layer, said first layer comprising a composition comprising: a support material; one or more platinum group metals disposed on the support material; and a NOxstorage material; wherein the support material comprises alumina or a mixed oxide comprising alumina; the one or more platinum group metals comprise a mixture or alloy of platinum and palladium; the NOxstorage material comprises ceria; and the support material or the NOxstorage material comprises a neodymium-containing component.

In a second aspect of the invention there is provided a NOxadsorber catalyst as hereinbefore defined supported on a metal or ceramic substrate.

In a third aspect of the invention there is provided a NOxadsorber catalyst as hereinbefore defined, wherein the catalyst composition is extruded to form a flow-through or filter substrate.

In a fourth aspect of the invention there is provided a NOx adsorber catalyst composition comprising a support material, one or more platinum group metals disposed on the support material, and a NOx storage material; wherein the support material or the NOx storage material comprises a neodymium-containing component.

In a fifth aspect of the invention there is provided a NOxadsorber catalyst comprising the NOxadsorber catalyst composition as hereinbefore defined supported on a metal or ceramic substrate.

In a sixth aspect of the invention there is provided a NOxadsorber catalyst comprising the NOxadsorber catalyst composition as hereinbefore defined, wherein the catalyst composition is extruded to form a flow-through or filter substrate.

In a seventh aspect of the invention there is provided a method of making the NOxadsorber catalyst composition of as hereinbefore defined, comprising adding one or more precious group metals or precious group metal salts to a support material to form a PGM-support mixture, and adding a NOxstorage material to the PGM-support mixture.

In an eighth aspect of the invention there is provided a method of making a neodymium-containing material, comprising mixing a solution of a neodymium-containing salt with ceria particles, spray-drying the particles, and heating the spray-dried particles.

In a ninth aspect of the invention there is provided a NOx adsorber catalyst for treating emissions from a lean burn engine, said NOx adsorber catalyst comprising a first layer, said first layer comprising a composition comprising: a support material; one or more platinum group metals disposed on the support material; and a NOx storage material; wherein the support material comprises alumina or a mixed oxide comprising alumina; the one or more platinum group metals comprise a mixture or alloy of platinum and palladium; the NOx storage material comprises ceria; and the support material or the NOx storage material is obtainable by the method as hereinbefore defined.

In a tenth aspect of the invention there is provided an emission treatment system for treating a flow of a combustion exhaust gas comprising a lean-burn engine and the NOxadsorber catalyst as hereinbefore defined; wherein the lean-burn engine is in fluid communication with the NOxadsorber catalyst.

In an eleventh aspect of the invention there is provided a method of treating an exhaust gas from an internal combustion engine comprising contacting the exhaust gas with the NOxadsorber catalyst as hereinbefore defined, or the emission treatment system as hereinbefore defined.

