Patent Issued for Non-Stoichiometric Perovskite Oxide Oxidation Catalyst for Oxidizing NO to NO.Sub.2

By a News Reporter-Staff News Editor at Life Science Weekly — From Alexandria, Virginia, NewsRx journalists report that a patent by the inventors Qi, Gongshin (Troy, MI); Li, Wei (Troy, MI); Wang, Xinquan (Tianjin, CN); Shen, Meiqing (Nankai District, CN), filed on May 25, 2012, was published online on July 1, 2014 (see also GM Global Technology Operations LLC).
The patent’s assignee for patent number 8765092 is GM Global Technology Operations LLC (Detroit, MI).
News editors obtained the following quote from the background information supplied by the inventors: “A hydrocarbon-fueled engine such as, for example, an internal combustion engine for a vehicle, may combust a mixture of air and fuel to drive mechanical equipment and perform work. The hot exhaust gas generated by the engine generally contains unwanted gaseous emissions and possibly some suspended particulate matter that may need to be converted to more innocuous substances before being discharged to the atmosphere. The gaseous emissions primarily targeted for abatement include carbon monoxide, unburned and partially burned hydrocarbons (HC’s), and nitrogen oxide compounds (NO.sub.X) comprised of NO and NO.sub.2 along with nominal amounts of N.sub.2O. An exhaust aftertreatment system that includes specially catalyzed flow-through components may be employed to dynamically treat a continuous exhaust flow with variable concentrations of these emissions. Many different exhaust aftertreatment system designs have been developed. But in general these systems seek to oxidize both carbon monoxide and HC’s (to carbon dioxide and water) and reduce NO.sub.X (to nitrogen and water). Suspended particulate matter, if present, is usually captured by a filter and burned off at regular intervals.
“The catalytic conversion efficiency of carbon monoxide, HC’s, and NO.sub.X over various types of catalysts depends largely on the air to fuel mass ratio of the mixture of air and fuel fed to the engine. A stoichiometric mixture of air and fuel (air to fuel mass ratio of about 14.7 for standard petrol-based gasoline) combusts to provide the exhaust flow with a reaction balance of oxidants (O.sub.2 and NO.sub.X) and reductants (CO, HC’s, and H.sub.2). This type of exhaust flow composition is generally the easiest to treat. A conventional three-way-catalyst (TWC) that includes a platinum group metal mixture dispersed on a base metal oxide support material, and which is close-coupled to the engine, can simultaneously reduce NO.sub.X and oxidize carbon monoxide and HC’s through various coupled catalytic reactions. But a stoichiometric mixture of air and fuel is not always maintained or even practical (i.e., a diesel engine). The engine may, for instance, combust a lean mixture of air and fuel (air to fuel mass ratio above 14.7 for standard petrol-based gasoline) to achieve more efficient fuel economy. The excess air contained in a lean mixture of air and fuel increases the concentration of uncombusted oxygen and decreases the concentrations of the various reductants in the exhaust flow. The catalytic reduction rate of NO.sub.X to N.sub.2 is slowed in such an oxidative environment over a conventional TWC and may require an entirely different system design or supplemental NO.sub.X treatment capacity to bring NO.sub.X concentrations within acceptable levels.
“The two most prevalent approaches, to date, for reducing NO.sub.X in an oxygen enriched exhaust flow are a selective catalytic reduction (SCR) system and a lean NO.sub.X trap (LNT). A SCR system introduces a reductant such as ammonia (or urea because it reacts to form ammonia) or a hydrocarbon into the exhaust flow which, in turn, reacts with NO.sub.X in the presence of oxygen over a reaction-specific SCR catalyst to form nitrogen. A LNT directs the exhaust flow over a NO.sub.X absorption catalyst that stores NO.sub.2 as a nitrate species until purged with a source of reductants that also converts the desorbed NO.sub.X into nitrogen over a NO.sub.X reduction catalyst. The overall NO.sub.X conversion efficiency for both practices can be enhanced by decreasing the molar ratio of NO to NO.sub.2 in the NO.sub.X gas constituency originally produced by the engine. A preferred NO:NO.sub.2 molar ratio for rapid NO.sub.X reduction in the SCR system is approximately 1.0 (equimolar). A preferred ratio of NO:NO.sub.2 for the LNT is much less. Most, if not all, of the NO present in the exhaust flow is preferably oxidized to NO.sub.2 to maximize the NO.sub.2 absorption selectivity of the NO.sub.X absorption catalyst.
“The NO.sub.X generated by the engine during combustion of a lean mixture of air and fuel generally constitutes greater than 90 mol \% NO and less than 10 mol \% NO.sub.2. An oxidation catalyst that can selectively oxidize NO to NO.sub.2 may be provided upstream of the SCR catalyst or the NO.sub.X absorption catalyst and, if desired, in close proximity to the hydrocarbon-fueled engine. The oxidation catalyst may be part of a diesel oxidation catalyst (DOC) or some other suitable two-way catalyst composition. The upstream oxidation catalyst oxidizes NO (to NO.sub.2) to achieve a more desirable NO:NO.sub.2 molar ratio and, additionally, may oxidize CO and HC’s to some extent. The lower NO:NO.sub.2 molar ratio boosts NO.sub.X reduction activity in the SCR system or the LNT and, in turn, enhances the overall NO.sub.X conversion efficiency of the exhaust aftertreatment system. The oxidation catalyst may also be intermingled within the SCR catalyst or the NO.sub.X absorption and/or reduction catalysts to further oxidize NO that may slip past the upstream oxidation catalyst to ensure near-complete conversion of NO.sub.X to N.sub.2. Other oxidation catalysts that are more selective towards CO and HC’s may be combined with the oxidation catalyst that affects NO to form a multi-functional catalyst material.
“The oxidation catalyst that has conventionally been used in an exhaust aftertreatment system to oxidize NO to NO.sub.2 is fine particles of platinum or a platinum-based metal alloy. But platinum and platinum-based alloys are rather expensive and tend to suffer from poor thermal durability. A better-performing, lower-cost, and more durable oxidation catalyst that exhibits a useful NO to NO.sub.2 oxidative activity would be a valuable contribution to those interested in NO.sub.X treatment because it could serve as partial or total substitute for platinum and platinum-based alloys in an exhaust aftertreatment system.”
As a supplement to the background information on this patent, NewsRx correspondents also obtained the inventors’ summary information for this patent: “An exhaust flow produced by a hydrocarbon-fueled engine may be exposed to immobilized fine particles of a non-stoichiometric perovskite oxide as part of an exhaust aftertreatment system. The particles of the non-stoichiometric perovskite oxide may be sized to have a diameter that ranges from about 10 nm to about 100 .mu.m. Intimate contact between the exhaust flow and the non-stoichiometric perovskite oxide particles oxidizes NO to NO.sub.2 and, as such, decreases the NO:NO.sub.2 molar ratio of the exhaust flow’s NO.sub.X content to a more desirable level. The non-stoichiometric perovskite oxide has the general formula La.sub.XMnO.sub.Y in which the molar ratio of lanthanum to manganese, which is represented as ‘X’ in the chemical formula, ranges from 0.85 to 0.95 (sometimes referred to as ‘La.sub.0.85-0.95MnO.sub.Y’). The oxygen content, which is represented as ‘Y’ in the chemical formula, fluctuates with variations in the molar ratio of lanthanum to manganese but generally falls somewhere between 3.0 and 3.30.
“This particular class of non-stoichiometric perovskite oxide particles (La:Mn molar ratio of 0.85 to 0.95) has a demonstrated NO oxidative activity that, in terms of NO to NO.sub.2 conversion and hydrothermal durability over the temperature range typically experienced in an exhaust aftertreatment system, is superior to the NO oxidative activity of similar perovskite oxide particles with a higher molar ratio of lanthanum to manganese. The enhanced NO oxidative activity of the La.sub.0.85-0.95MnO.sub.Y perovskite oxide particles is thought to be attributed to crystal lattice adjustments that are spurred by the decrease in the molar ratio of lanthanum to manganese away from stoichiometry. For instance, an increase in both the surface and bulk concentrations of Mn.sup.4+, as well as an increase in the available amount active oxygen, has generally been observed with decreasing values of the molar ratio of lanthanum to manganese. Setting the molar ratio of lanthanum to manganese (‘X’) between 0.85 and 0.95 in the non-stoichiometric perovskite oxide particles appears to coincide with a desirable balance of the Mn.sup.4+/Mn.sup.3+ atomic ratio, active oxygen availability, surface area, and thermal durability, all of which contribute to NO oxidative activity.
“The hydrocarbon-fueled engine that produces the exhaust flow may be a multi-cylinder, reciprocating, internal combustion engine that is supplied with a mixture of air and fuel (‘A/F mixture’) defined by an air to fuel mass ratio in known controlled fashion. The air to fuel mass ratio of the A/F mixture may be lean of stiochiometry either some of the time or all of the time. A lean A/F mixture, when combusted in the engine, typically affords the exhaust flow with a relatively high oxygen content in the range of 1.0 to 10 vol. \%, a NO.sub.X content of 50 ppmv to1500 ppmv, a partially burned or unburned hydrocarbon content of 250 ppmv to 750 ppmv, a CO content of 50 ppmv to 500 ppmv, maybe some suspended particulate matter, and the balance nitrogen, water, and carbon dioxide. The NO.sub.X content is usually comprised of greater than 90 mol \% NO and less than 10 mol \% NO.sub.2, which corresponds to a NO:NO.sub.2 molar ratio of about 9 or greater.
“The exhaust flow may be exposed to the immobilized La.sub.0.85-0.95MnO.sub.Y perovskite oxide particles within, for example, a catalyzed flow-through component that forms part of the exhaust aftertreatment system. The catalyzed flow-through component is installed in, and forms part of, a contained passageway through which the exhaust flow navigates. In terms of construction, the catalyzed flow-through component is a monolithic honeycomb support structure that includes a front end, a rear end, and a plurality of isolated channels that extend from the front end to the rear end. The isolated channels are defined by a wall surface over which the La.sub.0.85-0.95MnO.sub.Y perovskite oxide particles are deposited. A main function of the channels is to segregate and convey the exhaust flow through the catalyzed flow-through component while facilitating intimate contact between the segregated portions of the exhaust flow and the deposited La.sub.0.85-0.95MnO.sub.Y perovskite oxide particles.”
For additional information on this patent, see: Qi, Gongshin; Li, Wei; Wang, Xinquan; Shen, Meiqing. Non-Stoichiometric Perovskite Oxide Oxidation Catalyst for Oxidizing NO to NO.Sub.2. U.S. Patent Number 8765092, filed May 25, 2012, and published online on July 1, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=\%2Fnetahtml\%2FPTO\%2Fsrchnum.htm&r=1&f=G&l=50&s1=8765092.PN.&OS=PN/8765092RS=PN/8765092
Keywords for this news article include: Nitrogen, Chemistry, Lanthanum, Manganese, Chalcogens, Heavy Metals, Hydrocarbons, Carbon Dioxide, Carbon Monoxide, Organic Chemicals, Inorganic Carbon Compounds, Lanthanoid Series Elements, GM Global Technology Operations LLC.
Our reports deliver fact-based news of research and discoveries from around the world. Copyright 2014, NewsRx LLC