Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
2001-02-22
2004-02-03
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C502S064000, C502S066000, C502S073000, C502S074000
Reexamination Certificate
active
06685900
ABSTRACT:
INTRODUCTION AND BACKGROUND
The present invention relates to a catalyst for purifying the exhaust gases of diesel engines, which catalyst contains at least one zeolite and, additionally, at least one support oxide selected from the group consisting of aluminum oxide, silicon dioxide, titanium dioxide and aluminum silicate and at least one noble metal selected from the group consisting of platinum, palladium, rhodium, iridium, gold and silver.
The exhaust gas of diesel engines contains carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NO
x
), sulfur dioxide (SO
2
) and carbon black particles as atmospheric pollutants. The unburned hydrocarbons include paraffins, olefins, aldehydes and aromatic compounds. In comparison with the exhaust gases of gasoline engines, diesel exhaust gases contain a substantially higher proportion of not readily oxidizable, long-chained paraffins, which are for a large part condensed onto the carbon black particles as so-called VOF's (VOF=volatile organic fraction) and hence increase the particulate load of the exhaust gas. The sulfur dioxide contained in the exhaust gas originates from the sulfur content of the diesel fuel. By oxidation to sulfur trioxide, sulfates may form which likewise accumulate on the carbon black particles and increase the mass of the particles.
Moreover, diesel exhaust gases are leaner than stoichiometric in composition, which means that their oxygen content is higher than would be necessary for complete combustion of all combustible constituents of the exhaust gas. The oxygen concentration in diesel exhaust gas is usually from 3 to 10 vol. %, whereas in stoichiometrically composed exhaust gases of gasoline engines it is only about 0.7 vol. %.
The high oxygen concentration of diesel exhaust gases is based on the fact that diesel engines are operated with a high air/fuel ratio (kg of air/kg of fuel) of over 18. By contrast, stoichiometrically operated gasoline engines work with an air/fuel ratio of 14.6, which permits stoichiometric combustion of the hydrocarbons.
A further peculiarity of diesel exhaust gases is their substantially lower temperature as compared with the temperature of gasoline engines. In part-load operation, the exhaust gas temperature of modern diesel engines is in the range from 120 to 250° C., and it reaches a maximum temperature of from 550 to 650° C. only in full-load operation.
The actual composition of the exhaust gas of a diesel engine depends on the type of engine in question. Moreover, the development of diesel engines in the last 15 years has led to a continual change in the composition of diesel exhaust gas. Important developmental steps in this connection were the introduction of exhaust gas recirculation (EGR) and the continual further development of fuel injection systems such as “unit injector” and “common-rail”. As a result of such developments it has been possible to reduce still further the nitrogen oxide emissions of diesel engines, which were already low as compared with gasoline engines, and exhaust gas temperatures have constantly been lowered further. Modern diesel engines for motor-cars exhibit nitrogen oxide emissions of less than 100 vol.-ppm in the majority of operating states of the engine.
The described particularities of diesel exhaust gases have made it necessary to introduce special exhaust gas purification systems for diesel engines. Successes in engine development and ever more stringent legal requirements as regards permissible emissions demand the continual further development of existing exhaust gas purification systems for diesel engines.
As well as reducing the particulate emission of diesel engines by introducing suitable diesel soot filters, the fundamental question initially was to reduce emissions of hydrocarbons by introducing suitable oxidation catalysts. Since the nitrogen oxide emissions of diesel engines were markedly higher about 10 years ago than they are today, it was important when developing such oxidation catalysts also to inhibit the further oxidation of nitrogen monoxide contained in the exhaust gas to nitrogen dioxide and the further oxidation of sulfur dioxide to sulfur trioxide.
Diesel oxidation catalysts having a reduced tendency to oxidize nitrogen monoxide and sulfur dioxide are described, for example, in patent specifications DE 39 40 758 C2, DE 42 13 018 C1 and U.S. Pat. No. 5,911,961.
A further step was the development of so-called “lean-NO
x
” catalysts. Such catalysts are capable of reducing nitrogen oxides even in oxygen-rich exhaust gases. The unburned hydrocarbons still present in the diesel exhaust gas serve as the reducing agent. If the content of such hydrocarbons in the exhaust gas is not sufficient for the reduction of the nitrogen oxides, it can be increased accordingly by means of suitable measures in engine control or by the separate injection of diesel fuel. Of course, that also leads to a higher fuel consumption.
Lean-NO
x
catalysts are described in DE 196 14 540 A1, in EP 0 427 970 A2, in EP 0 920 913 A1 and in U.S. Pat. No. 5,897,846.
There have also become known catalysts that are said to improve the conversion of hydrocarbons and also of nitrogen oxides by storing the hydrocarbons at low exhaust gas temperatures and releasing them at higher exhaust gas temperatures. Such catalysts are described in U.S. Pat. No. 5,849,255, in WO 94/22564 and in WO 96/39244.
In the case of the first-mentioned oxidation catalysts, the reduced tendency to oxidize nitrogen monoxide and sulfur dioxide is achieved by additions of tungsten, antimony, molybdenum, nickel, vanadium, manganese and others. Vanadium is preferably used. Accordingly, the active component of the catalyst according to DE 39 40 758 C2 consists of platinum, palladium, rhodium and/or iridium in contact with vanadium or with an oxidic vanadium compound. The active component is deposited on finely divided aluminum oxide, titanium dioxide, silicon dioxide, zeolite and mixtures thereof. In order to prepare the catalyst, the oxidic support materials are first applied in the form of a dispersion coating to an inert carrier body. The dispersion coating is then impregnated with the active components. If mixtures of the various support oxides are used for the dispersion coatings, then all the constituents of the dispersion coating are coated uniformly with the active components in the subsequent impregnation.
DE 42 13 018 C1 also describes the use of aluminum oxide, titanium dioxide, silicon dioxide and zeolite as supports for the catalytically active components, which are present in the form of noble metals platinum, palladium, rhodium and/or iridium doped with vanadium or in contact with an oxidic vanadium compound.
The oxidation catalyst according to U.S. Pat. No. 5,911,961 contains on a first support oxide platinum and/or palladium in conjunction with at least one metal selected from the group tungsten, antimony, molybdenum, nickel, manganese, iron, bismuth and others. The catalyst additionally contains further oxides selected from the group aluminum oxide, silicon dioxide, titanium dioxide, zirconium dioxide, aluminum silicate and, inter alia, also zeolites. The additional oxides are not coated with the catalytically active components.
The lean-NO
x
catalyst according to DE 196 14 540 A1 contains one or more zeolites and at least one platinum group metal. The catalyst additionally contains one or more support oxides selected from the group aluminum silicate, aluminum oxide and titanium dioxide. The catalytically active noble metals of that catalyst are deposited only on the additional support oxides.
EP 0 427 970 A2 describes a lean-NO
x
catalyst for decreasing the nitrogen oxides in an oxidizing exhaust gas having an air/fuel ratio of 22. The catalyst contains at least one zeolite having a molar ratio SiO
2
/Al
2
O
3
of greater than 10 and pore diameters of from 0.5 to 1 nm. Noble metals are deposited on the zeolites; for each platinum group metal, the weight ratio of the metal to the zeolite must not fall below a minimum value if good rates of conver
Domesle Rainer
Klein Harald
Kreuzer Thomas
Lox Egbert
dmc
Kalow & Springut LLP
Silverman Stanley S.
Strickland Jonas N.
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