EP2288429A1

EP2288429A1 - Exhaust gas processing

Info

Publication number: EP2288429A1
Application number: EP09753859A
Authority: EP
Inventors: Gilbert Blanchard; Séverine ROUSSEAU; Philippe Bazin; Marco Daturi; Olivier Marie; Frédéric MEUNIER
Original assignee: Centre National de la Recherche Scientifique CNRS; Peugeot Citroen Automobiles SA
Current assignee: Centre National de la Recherche Scientifique CNRS; PSA Automobiles SA
Priority date: 2008-05-27
Filing date: 2009-05-25
Publication date: 2011-03-02
Also published as: FR2931700B1; FR2931700A1; WO2009144204A1

Abstract

The invention relates to a catalytic system that comprises palladium dispersed on a substrate made of zirconium oxide ZrO2 and praseodymium oxide Pr6O11, the mass ratio between the zirconium and praseodymium oxides being from 5 to 20%. The invention is particularly adapted for processing exhaust gases having a temperature of between 120°C and 400°C, and having a carbon monoxide concentration of between 2000 and 5000 ppm.

Description

TREATMENT OF EXHAUST GAS

Technical area

The present invention relates to the treatment of exhaust gases emitted by combustion engines, including those fitted to motor vehicles, and particularly compression ignition engines called diesel engines. More specifically, the invention relates to a catalytic converter for the treatment of these exhaust gases, an exhaust line of a motor vehicle comprising such a converter and a method of manufacturing this catalytic converter.

State of the art

Modern vehicles are equipped with different means to treat the exhaust and minimize air pollution due to carbon monoxide releases, nitrogen oxides and unburned hydrocarbons. Among these means figures a large number of catalytic converters.

These catalytic converters are said to be multifunctional when they eliminate the three main types of pollutants mentioned above. They are called oxidation when they essentially make it possible to transform unburned hydrocarbons and carbon monoxide into carbon dioxide. As a rule, they comprise a substrate on which is deposited a catalytic coating comprising one or more precious metals associated with one or more additional metals such as iron, nickel, zirconium, vanadium, chromium or cerium.

The substrate is typically an inert and rigid structure, also called monolith, usually ceramic or metal and forming a series of channels or ducts, for example in a honeycomb form.

Thus, for example, European Patent EP-27069 discloses a catalyst comprising a refractory oxide-based support and whose active catalytic phase. is composed of cerium, iron and at least one metal taken from the group consisting of iridium and rhodium, and a metal taken from the platinum and palladium group.

Reference may also be made to European Patent EP-54472 which describes a multifunctional catalyst comprising an inert honeycomb substrate coated with a layer or film of refractory oxide, the active phase being made of copper, cerium or iron of at least one metal selected from the group comprising platinum or palladium and at least one metal selected from the group consisting of iridium and rhodium. The active phase is deposited or impregnated uniformly on the surface of the monolithic support, by total immersion of the support in a precursor solution of the elements of the active phase.

[0007] More generally, compositions based on zirconium oxide or cerium are considered to be among the most promising for the treatment of exhaust gases from motor vehicles or the like.

[0008] Whatever they are, catalytic converters are effective only below a reference temperature, which varies according to the composition used. As a result, a great deal of work is aimed at accelerating the temperature rise of the elements of the exhaust line, in particular following a cold start.

In addition, in recent years, the tightening of emission standards tends to a particular effort for a drastic reduction of nitrogen oxides, produced notably by the engines operating with a fuel-oxidant mixture rich in oxygen, so in air, such as diesel engines.

Such non-stoichiometric mixtures notably make it possible to reduce fuel consumption, and hence carbon dioxide emissions, which, although not directly polluting, are directly involved in the problems of global warming and greenhouse effects. .

To reconcile low fuel consumption and lower nitrogen oxide emission rates, it has been proposed new engines which are characterized in particular by relatively low combustion temperatures, and therefore exhaust gases. less hot. Moreover, if these engines make it possible to reduce nitrogen oxide emissions at the source, this reduction is accompanied by an increase in emissions of unburned hydrocarbons and especially carbon monoxide. Carbon monoxide concentrations in the range of 2000 to 5000 ppm have been reported, significantly higher than for conventional conventional engines. In addition, these larger amounts are emitted in exhaust gases whose temperature can be between about 120 ° C and about 400 ⁰ C, is between 100 and 200 ⁰ C cooler than the gases emitted by conventional engines.

It is known that in order to increase the efficiency of a catalyst, in particular its effectiveness at low temperature, it is necessary to increase the amount of noble metal present in the catalyst. However, following the rise in recent years prices of many raw materials, including noble metals such as platinum and rhodium, the cost of treatment of exhaust fumes is already largely dictated by that of the noble metals present in the line. exhaust, so that significantly increase the amount of noble metals is difficult to envisage, especially for entry-level vehicles.

[0013] There is therefore a need for catalytic systems having relatively small amounts of precious metals but nevertheless effective with low temperature exhaust gases.

Brief description of the invention

According to the invention, it is proposed to use a catalytic system for the treatment of exhaust gas of an internal combustion engine comprising palladium dispersed by a support based on zirconium oxide ZrO ₂ , and praseodymium oxide Pr ₆ O, in a mass ratio between the zirconium oxide and praseodymium of between 5 and 20%.

In a variant, the weight ratio is more particularly between 10 and 15%, and preferably close to 12%, ie a support whose raw formulation is written Zr ₀ 92Pr ₀ OsO ₂ -O, δ corresponding to about 0.01 (and which can be calculated by recalling that the support is a mixture of ZrO ₂ and Pr ₆ On). Good results have been obtained with a support whose specific surface is of the order of 50 m ² g ^-1 .

The palladium impregnation may be carried out in such a way that its mass concentration (relative to the support) is at most 1.5%, preferably at most 1% and preferably still close to 0.8%. %, that is to say a relatively low content, leading to a reduction in cost.

The catalytic system according to the invention is particularly suitable for the treatment of exhaust gas whose temperature is between 120 ° C and 400 ⁰ C, and more particularly those whose carbon monoxide concentration is between 2000 and 5000 ppm, ie exhaust gases that are characterized by both a low temperature and a relatively high concentration of carbon monoxide.

Brief description of the figures

Other features, details and objects of the invention will appear more clearly in the detailed description given hereinafter with reference to the appended figures which describe:

Figure 1 and 2, the conversion rates of carbon monoxide, for different media, depending on the temperature of the gas, and depending on the noble metal used (palladium for Figure 1, platinum for Figure 2);

Figures 2 and 3, similar curves to the two previous curves, showing the hydrocarbon conversion rates:

Figure 5, a comparative curve of the conversion rates as a function of the temperature of the gases, depending on the concentration of palladium, for different media.

Detailed realization mode

Catalyst compositions based on predominant zirconium oxide and a rare earth oxide of the group of lanthanides, chosen from praseodymium, lanthanum and neodymium, are known from French Patent 2,866,871. This document notably describes in its example 2, a composition comprising 85% of zirconium and 15% of praseodymium (proportions expressed as mass percentages of the ZrO ₂ and Pr ₆ On oxides).

According to this example 2 essentially reproduced below, this composition is prepared by introducing into a beaker, with stirring, 708 ml of zirconium nitrate (in solution at 120 g / l) and 30 ml of praseodymium nitrate (in solution). at 500 g / l), and supplementing with distilled water to form a liter of nitrate solution. This nitrated solution is introduced in one hour into a reactor with constant stirring in which a solution comprising 220 ml of ammonia has also been prepared. The solution is placed under an autoclave to be heated at 150 ° C for hours, then filtered to recover a precipitate.

An ammonium laurate gel is prepared by introducing 250 g of lauric acid into 135 ml of ammonia 12 mol / l and 500 ml of distilled water. 21.4 g of gel are added to 100 g of precipitate and after mixing, the assembly is heated at 500 ^° C. for 4 hours in steps. After a calcination of 4 hours at 900 ^° C., an oxide is obtained whose specific surface area is 63 m 2 / g, and 41 m 2 / g for calcination at 1000 ^° C. for 10 hours.

The authors of the present invention have obtained different oxide-based catalyst supports: an Al ₂ O ₃ alumina support, a CeO ₂ ceria support, a mixed alumina-ceria Ce ₀ ₂ cerium oxide carrier. 50Zr ₀ 50O ₂ , a support based on a mixed oxide of the formula (ZrO ₂ ) ₀ 7125 (CeO ₂ ) O ₂ -17 (Nd ₂ O ₃ ) o o 5 (La ₂ O ₃ ) o 0205, which thereafter will be simply referenced by the expression "MO", and a support

The alumina support has a surface area of about 144 m ² g ^-1 , all other supports have a specific surface area of about 50 m ² g ^-1 .

Each support is subjected to calcination at 800 ⁰ C for 12 hours to stabilize the material before impregnation with a noble metal, impregnation carried out using nitrate salts as precursors. The following concentrations, measured by elemental analysis, were obtained:

The impregnated supports are then heated to 500 ^° C. for 12 hours and finally, each catalyst thus formed is subjected to a hydrothermal treatment at 700 ^° C. to simulate the aging of the catalyst, each sample being subjected to an air passage. hot saturated with water.

Powdered catalysts are then compressed to form supports and activated by heating at 400 ^° C. for 30 minutes, under an 8% oxygen flow in argon.

Finally, a reference gas comprising 0.1% CO, 300 ppm NO, 10% CO ₂ , 200 ppm H ₂ , 100 ppm decane, 8% O ₂ , 1% H ₂ O and 100 ppm was prepared. a 50/50 propane / propene mixture. Argon is used as the carrier gas and the catalysts are exposed to this reference gas, each time for 20 minutes, at temperatures of 100 ^° C., 125 ^° C., 150 ^° C., 200 ^° C. and 25 ^° C., respectively. ⁰ C, 300 ⁰ C, 350 ⁰ C and 400 ⁰ C.

The conversion rates of the various catalysts were thus measured, depending on the nature of the support and the noble metal used.

Figures 1 and 2 show the conversion rates of carbon monoxide as a function of temperature. The square marks correspond to praseodymium supports, triangular markings to alumina supports, crosses to ceria-zirconium oxides and MO oxide stars. In these figures, we have also used a solid line for measurements made with increasing temperatures (up or M) a broken line for measurements made down. These tests show that when the noble metal is palladium, in the case of a carrier containing praseodymium, a convention rate of 100% is obtained from 200 ⁰ C (in increasing temperatures), while this same rate is only obtained at 300 ^° C. for the mixed oxide OM and at even higher temperatures for the other supports.

With platinum, the classification of the supports is modified, the praseodymium support obtaining a conversion rate of the order of 95% for a temperature of 400 ⁰ C.

Figures 3 and 4 show the results with respect to hydrocarbon conversion rates. The same marks were used for the different media. It can be seen that the supports are much less differentiated, with the exception of the alumina support, which appears to be less efficient. In any case, the catalyst comprising praseodymium leads to quite acceptable performance.

In FIG. 5, the tests reported in FIG. 1 were repeated, but this time by comparing catalysts carrying respectively 0.8% (solid lines) or 2% (discontinuous lines) of palladium. From left to right, by placing on the points corresponding to a conversion rate of 60%, the first two curves concern the supported catalyst containing praseodymium (square marks), the two central curves (triangular marks), a support of alumina and the two right curves (round marks), an OM support. Remarkably, it is noted that in the case of the support containing praseodymium, the measured performances are higher in the case of the catalyst having only 0.8% of palladium, the conversion rate at 100% being even obtained about 100 ⁰ C lower.

The catalyst according to the invention is therefore remarkable in that it makes it possible to use reduced levels of palladium, without degrading performance (and in fact by increasing them), and with very good conversion rates at low levels. temperatures, which is particularly suitable

Claims

claims

1. Catalytic system for the treatment of the exhaust gases of an internal combustion engine comprising palladium dispersed on a support consisting of zirconium oxide ZrO ₂ and praseodymium oxide Pr ₆ On in a ratio of the zirconium and praseodymium oxides of between 5 and 20% by mass percentage.

2. Catalytic system according to claim 1, characterized in that said weight ratio is between 10 and 15%.

3. Catalyst system according to claim 1, characterized in that said oxide corresponds to the formulation Zr ₀ 92Pro osO ₂ -δ

4. Catalytic system according to claim 1, characterized in that the specific surface area of the support is approximately 50 m ² g ^-1 .

5. Catalyst system according to claim 1 or claim 2, characterized in that the mass percentage of palladium is at most 1, 5% relative to the mass of the support.

6. Catalytic system according to claim 5, characterized in that the mass percentage of palladium is at most 1% relative to the mass of the support.

7. Catalytic system according to claim 5, characterized by a mass percentage of palladium of about 0.8% relative to the mass of the support.

8. Catalyst system according to any one of the preceding claims, comprising a support formed of an oxide of formula Zr ₀ 92 Pro OsO ₂ -O ,, with a specific surface area of approximately 50 m ² g ^-1 , on which is dispersed palladium, in a weight ratio relative to the support of about 0.8%

9. Application of the system according to any one of claims 1 to 6, the treatment of exhaust gas whose temperature is between 120 ⁵ C eI ^ O ⁵ C.

10. Application according to claim 8, the treatment of exhaust gas whose carbon monoxide concentration between 2000 and 5000 ppm.