Catalyst for cleaning up the exhaust gases of a diesel engine

Details Catalyst for cleaning up the exhaust gases of a diesel engine Catalyst for cleaning up the exhaust gases of a diesel engine

1999-11-18

2004-01-13

Nguyen, Cam N. (Department: 1754)

Catalyst, solid sorbent, or support therefor: product or process

Zeolite or clay, including gallium analogs

And group viii containing

C502S327000, C502S329000, C502S330000, C502S331000, C502S339000, C502S326000, C502S527120, C502S066000, C502S077000, C502S078000, C502S079000

Reexamination Certificate

active

06677264

ABSTRACT:

INTRODUCTION AND BACKGROUND
The present invention relates to a catalyst for cleaning up the exhaust gases of a diesel engine. With the catalyst according to the invention the nitrogen oxides in the oxygen-rich exhaust gas of a diesel engine can be converted with optimal utilization of the reductive constituents contained in the exhaust gas.
Diesel engines emit exhaust gases that contain, besides unconsumed oxygen and harmless combustion products such as water and carbon dioxide, additional substances that are harmful and which endanger human health and pollute the environment. These include carbon monoxide CO, non-combusted hydrocarbons HC, nitrogen oxides NO
x
and particles. The nitrogen oxides are formed from the nitrogen of the combustion air during combustion or arise out of nitrogenous compounds in the fuel. Depending on the operating point of the engine, they consist of nitrogen monoxide NO in a proportion amounting to about 50 to 90 vol.-%. Furthermore, depending on the sulfur content of the fuel, the exhaust gas of internal combustion engines also contains sulfur dioxide.
With a view to keeping the air clean, certain upper limits for the emission of these harmful substances have been prescribed by the legislature. The limits are revised downwards from time to time, in accordance with the technical possibilities, in order to lessen the emissions of harmful substances. With a view to checking the conversion of harmful substances by exhaust gas emission control systems in a way approximating to practical reality, various test cycles have been established which simulate frequently occurring driving conditions.
The test cycle that is recognized in Europe in respect of a vehicle is designated as the MVEG-A cycle (Motor Vehicle Emission Group) and consists of an urban driving portion (ECE) and an extra-urban portion (EUDC). In this connection the vehicle to be examined covers the appropriate driving cycle on a roller dynamometer. The centrifugal-mass flywheels of the roller dynamometer constitute a collective load and simulate the weight of the vehicle. During the driving cycle the gaseous harmful substances which are emitted are measured and added up by means of standard analytical processes (HC: with a flame ionization detector (FID), CO: with infrared spectroscopy (IR), NO
x
: with a chemiluminescence detector (CLD)), so that the emissions of harmful substances of the entire cycle are available in the form of grams of harmful substance per kilometer travelled. These emissions can be directly compared with the limits specified by legislation.
With respect to their exhaust gas composition the present day internal combustion engines can be subdivided into two classes. On the one hand there are the conventional Otto (internal combustion) engines, the exhaust gas of which is composed substantially stoichiometrically, and on the other hand there are the diesel engines and lean burn Otto engines, the exhaust gas of which contains an excess of oxygen.
The exhaust gas of conventional Otto engines also exhibits, besides the stated harmful substances, an oxygen content of about 0.7 vol.-%. It is stoichiometrically composed; that is to say, oxidative and reductive components of the exhaust gas balance one another and can be converted simultaneously and almost completely with so-called three-way catalysts into the harmless components carbon dioxide, water and nitrogen. By way of catalytically active components, three-way catalysts contain on high surface area carrier oxides, in addition to platinum and/or palladium, also rhodium, which particularly favours the selective reduction of the nitrogen oxides to nitrogen by making use of carbon monoxide and non-combusted hydrocarbons as reducing agents. A prerequisite for simultaneous conversion of the three harmful substances by means of a three-way catalyst is the regulation of the air/fuel ratio supplied to the engine to the stoichiometric value. The latter amounts to 14.6 in the case of conventional fuels. Accordingly, 14.6 kilograms of air are needed for complete combustion of 1 kilogram of fuel.
In contrast, the exhaust gas of diesel engines and of lean burn Otto engines contains a high oxygen content amounting to about 6 to 20 vol.-%, since these engines are operated with lean air/fuel ratios. A major problem with these engines is constituted by the emission of the nitrogen oxides. On account of the high oxygen content of the exhaust gas it is not possible to convert the nitrogen oxides in accordance with the established three-way process using carbon monoxide and hydrocarbons as reducing agents. Instead, the oxidation of carbon monoxide and hydrocarbons in the oxygen-rich exhaust gas is preferred.
Frequently, therefore, the exhaust gases of these engines are only purified oxidatively with so-called oxidation catalysts; that is to say, carbon monoxide and hydrocarbons are oxidized on a catalyst by the oxygen in the exhaust gas to form water and carbon dioxide. Such a catalyst is described in DE 39 40 758 C1, for example. It is distinguished by a high catalytic activity in respect of the oxidation of carbon monoxide and hydrocarbons, whereas further oxidation of nitrogen monoxide to nitrogen dioxide and of sulfur dioxide to sulfur trioxide is largely suppressed. By this means, the formation of sulfates which could contaminate the catalysts that are used is also lessened. By way of catalytically active components the catalyst contains, on high surface area carrier oxides such as aluminum oxide, titanium oxide and silicon dioxide, platinum and/or palladium, which are modified in their catalytic activity as a result of additions of vanadium oxide in such a way that scarcely any further oxidation of nitrogen monoxide and sulfur dioxide is to be observed.
There have been various suggested solutions with a view to lessening the nitrogen oxides in the exhaust gas of these engines. In the case of so-called selective catalytic reduction (SCR) the nitrogen oxides in the exhaust gas are selectively reduced on an SCR catalyst by addition of reducing agents to the exhaust gas. The fuel itself can be used by way of reducing agent. Optimal results are achieved with ammonia as reducing agent, which, for example, can be generated on board the vehicle by hydrolysis of urea. Such a process is described in DE 42 03 807 A1. A suitable SCR catalyst is mentioned in EP 0 410 440 B1. It consists of an intimate mixture of the oxides of titanium, iron, vanadium, molybdenum, tungsten and various other additives.
Selective catalytic reduction can be employed economically only in the case of large diesel engines in trucks. Therefore the use of so-called nitrogen oxide storage catalysts has been proposed in recent years for the reduction of the nitrogen oxides in the exhaust gas of lean burn Otto engines. In this case the nitrogen oxides are oxidized to a higher state of oxidation on the storage catalyst by platinum group metals to form nitrogen dioxide and are stored in the form of nitrates by a basic storage material. After the storage capacity of the catalyst has been exhausted, it has to be regenerated. Regeneration is initiated by switching the operation of the engine from a lean air/fuel mixture to a rich, that is to say reducing, air/fuel mixture. In the reducing exhaust gas atmosphere the stored nitrates are decomposed, releasing nitrogen oxides which are converted into nitrogen under the reducing exhaust gas conditions on the platinum group metals. Accordingly, with this process, as also with the SCR process, reducing agents in the form of additional fuel are actively employed. To this end, suitable engine electronics are required which switch over periodically from lean running mode to rich running mode.
EP 0 669 157 A1 describes such a system. By way of storage material for the nitrogen oxides, use is made of basic materials such as alkali metal oxides, alkaline earth metal oxides and rare earth oxides. The storage catalyst additionally contains platinum and/or palladium on a high surface area carrier oxide.
With a view to improving the purification of e

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