Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
1999-07-01
2003-03-18
Griffin, Steven P. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C423S213200, C423S213500, C423S239100, C423S239200, C502S064000, C502S071000
Reexamination Certificate
active
06534439
ABSTRACT:
INTRODUCTION AND BACKGROUND
The present invention relates to a catalyst for reducing nitrogen oxides in oxidizing and reducing atmospheres. The catalyst contains iridium on a support material.
In a similar manner to diesel engines, it is now being attempted to lower the fuel consumption of modern gasoline engines by also operating them with lean air/fuel mixtures. Fuel savings of up to 25% are expected from so-called lean-burn engines, in particular those with direct gasoline injection, as compared with stoichiometrically operated internal combustion engines. However, lean-burn engines also have operating phases with stoichiometric or so-called rich air/fuel conditions. These types of conditions prevail after a cold-start, when accelerating and when under full load. Diesel engines, which are operated almost exclusively with lean air/fuel mixtures, also belong to the class of lean-burn internal combustion engines.
The catalytic removal of nitrogen oxides contained in the exhaust gas is a substantial problem in the case of lean-burn engines. Due to the high oxygen concentration in the exhaust gas from these engines, up to 15 vol. %, the nitrogen oxides (NO
x
) contained in the exhaust gas cannot readily be reacted with the hydrocarbons (HC) and carbon monoxide (CO) also contained in a lean exhaust gas on a conventional exhaust gas catalyst, because in this case the reductive components (HC and CO and also small amounts of hydrogen H
2
) are oxidized directly by oxygen.
Exhaust gas catalysts for the simultaneous conversion of hydrocarbons, carbon monoxide and nitrogen oxides, so-called three-way catalysts, require a stoichiometric composition of exhaust gas, with an oxygen concentration of about 0.7 vol. %, for the conversion to take place. The exhaust gas composition is usually described by the normalized air to fuel ratio &lgr;, which is defined as the air/fuel ratio normalized to stoichiometric conditions. The air/fuel ratio states how may kilograms of air are required for complete combustion of one kilogram of fuel. With conventional fuels, the stoichiometric air/fuel ratio has a value of 14.6, which corresponds to a normalized air/fuel ratio of 1.
Two alternative routes have been described for converting nitrogen oxides in lean exhaust gases. An attempt is made to store the nitrogen oxides in the form of nitrates during lean operation of the internal combustion engine, with the aid of so-called nitrogen oxide storage catalysts. Preferred storage materials for this purpose are, for example, alkaline earth metal oxides, in particular barium oxide. For storage purposes, the nitrogen oxides, between 50 and 90 vol. % of which consists of nitrogen monoxide depending on the type of engine and mode of operation of the engine, first have to be oxidized to nitrogen dioxide before they can form nitrates with the storage materials. Oxidation takes place mainly on the storage catalyst itself and this is provided, for example, with platinum as a catalytically active component for this purpose.
Depending on the driving conditions, the storage material has to be regenerated at certain intervals. For this, the internal combustion engines are operated for brief periods with rich air/fuel mixtures. Under the reductive exhaust gas conditions which then prevail, the nitrates are decomposed and the nitrogen oxides being released are converted into nitrogen, with simultaneous oxidation of the reductive components. The acceleration phases may sometimes be used for regeneration of the storage material. In addition, however, in the absence of acceleration phases, targeted regeneration is required and this has to be achieved by appropriate regulation of the engine. The fuel required for this reduces the theoretical fuel saving when using lean-burn engines.
Current storage catalysts still exhibit high sensitivity towards sulfur oxides contained in the exhaust gas from internal combustion engines. Sulfur oxides, after oxidation to sulfur trioxide on the storage catalyst, react with the storage material to form thermally very stable sulfates and continuously reduce the storage capacity for nitrogen oxides.
As an alternative to nitrogen oxide storage catalysts, catalysts have been developed which have a higher selectivity than conventional catalysts during the reaction of nitrogen oxides with hydrocarbons in an oxygen-rich exhaust gas. These include, for example, catalysts based on zeolites exchanged with copper or iron or iridium-containing catalysts. These catalysts enable permanent conversion of nitrogen oxides even in lean exhaust gases.
The activity of reduction catalysts generally depends on the oxygen concentration of the exhaust gas and on the temperature of the exhaust gas. Thus, Chajar et al. reported, in Catalysis Letters 28 (1994), 33-40, that a Cu-ZSM5 catalyst displays its optimum reduction activity with about 0.5 vol. % of oxygen in the exhaust gas, that is under slightly sub-stoichiometric conditions. If there is no oxygen in the exhaust gas, the conversion of NO on this catalyst is between 2% (at 250° C.) and 8% (at 500° C.), depending on the temperature of the exhaust gas.
In addition to depending on the oxygen concentration of the exhaust gas, reduction catalysts also exhibit a pronounced temperature dependence with regard to the conversion of nitrogen oxides. The light-off temperature for the reaction of nitrogen oxides in an oxygen-rich exhaust gas is about 350° C. The light-off temperature is understood to be the temperature at which the rate of conversion of a harmful substance reaches a specific value, usually 50%. As the exhaust gas temperature increases beyond this point, the conversion rate for nitrogen oxides initially increases, passes through a maximum at a specific temperature and then decreases again to almost zero at exhaust gas temperatures above 500° C.
Lean-burn gasoline engines, and in particular diesel engines, often achieve exhaust gas temperatures of less than 350° C. when operating under part loads. Therefore catalysts are required which develop their maximum rates of conversion at the lowest possible exhaust gas temperatures of less than 350° C., preferably less than 300° C.
EP 0 633 052 B1 describes a catalyst for the conversion of nitrogen oxides in oxygen-rich exhaust gases which consist of a crystalline iridium silicate with a Si/Ir atomic ratio of 50 to 800 and a Si/Al ratio of not less than 15. With an oxygen concentration of 3.5 vol. % in the exhaust gas, the maximum rates of conversion for this catalyst occur at exhaust gas temperatures of at least 430° C. and thus are not very suitable for the case described above. As a result of the method of preparation chosen for this catalyst, a defined compound of silicate and iridium is present, which leads to very homogeneous and atomic distribution of the iridium in this compound.
EP 0 832 688 A1 describes a catalyst which contains iridium, sulfur and optionally platinum as catalytically active substances. In this catalyst, iridium and sulfur can be applied to a common support material such as, for example, aluminum oxide. Alternatively, a metal sulfate may also be used as a support for the iridium. After impregnating the support material with iridium chloride, the material is dried and calcined at 500° C., so that the iridium is present as very fine particles on the support material. The catalyst is used to remove nitrogen oxides from oxidizing exhaust gases.
DE 196 19 791 A1 describes a catalyst which contains iridium, an alkali metal and at least one metal carbide and/or metal nitride as support. In that document, iridium and the alkali metal are applied to the support, for example, by simultaneous impregnation of the support material with soluble precursor compounds of iridium and the alkali metal. With an air/fuel ratio of 23, the temperature for maximum conversion of nitrogen oxides with this catalyst is about 350° C.
JP 07080315 A1 also discloses a catalyst for removing nitrogen oxides from oxidizing exhaust gases from lean-burn engines and diesel engines. The catalyst contains iridium as active component
Andorf Renato
Kreuzer Thomas
Leyrer Jürgen
Lox Egbert
Markert Norbert
Degussa-Huls Aktiengesellschaft
Griffin Steven P.
Kalow & Springut LLP
Strickland Jonas N.
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