WO1997049481A1

WO1997049481A1 - Method for treating gas to reduce nitrogen oxide emission

Info

Publication number: WO1997049481A1
Application number: PCT/FR1997/001094
Authority: WO
Inventors: Philippe Barthe; Catherine Hedouin; Thierry Seguelong
Original assignee: Rhodia-Chimie
Priority date: 1996-06-21
Filing date: 1997-06-18
Publication date: 1997-12-31
Also published as: FR2750058B1; FR2750058A1; AU3349397A; ZA975465B

Abstract

The invention discloses a method for treating gas to reduce nitrogen oxide emission and a catalyst system for implementing this method. The method is characterised by the use of a catalyst composition essentially consisting of platinum or of platinum and manganese, deposited on a support, and which has been prepared by a method for depositing platinum on the support in which the platinum is brought in the form of a sol.

Description

GAS TREATMENT PROCESS FOR REDUCING EMISSIONS OF

P'AZQTE OXIDES

The present invention relates to a gas treatment process for the reduction of nitrogen oxide emissions and a catalytic system for the implementation of this process.

It is known that the reduction of emissions of nitrogen oxides (NOx) from the exhaust gases of automobile engines in particular is carried out using “three-way” catalysts which use the reducing gases present in the mixture stoichiometrically. Any excess oxygen results in a sudden deterioration in the performance of the catalyst.

However, certain engines such as diesel engines or gasoline engines operating in lean burn are fuel efficient but emit exhaust gases which permanently contain a large excess of oxygen of at least 5% for example. A standard three-way catalyst therefore has no effect on the NOx emissions of these engines. Furthermore, the limitation of NOx emissions is made imperative by the tightening of standards in automotive post combustion which now extend to this type of engine.

There is therefore a real need for an effective catalyst for reducing NOx emissions for this type of engine and, more generally, for the treatment of gases containing NOx.

The object of the invention is therefore to find a catalyst which can be used for the treatment of gases containing NOx and in particular those with a high oxygen content.

For this purpose, the process according to the invention for treating gases with a view to reducing the emissions of nitrogen oxides is characterized in that a catalytic composition is used comprising an active phase consisting essentially of platinum, deposited on a support, and which has been prepared by a process of depositing platinum on the support in which the platinum is provided in the form of a soil.

According to another embodiment of the invention, the method is characterized in that a catalytic composition is used comprising an active phase consisting essentially of platinum and manganese, deposited on a support, and which has also been prepared by a process for depositing platinum on the support in which the platinum is provided in the form of a soil. According to yet another embodiment of the invention, the process is characterized in that a catalytic composition is used comprising an active phase comprising platinum, deposited on a support, and which has also been prepared by a process for depositing platinum on the support in which the platinum is provided in the form of a sol, the size of the colloids of which is between 3 nm and 10 μm, more particularly between 3 nm and 8 nm.

Other characteristics, details and advantages of the invention will appear even more completely on reading the description which follows, as well as various concrete but nonlimiting examples intended to illustrate it. The catalytic composition used in the process of the invention firstly comprises an active phase. According to a first embodiment, this phase consists essentially of platinum. According to a second embodiment, the active phase consists essentially of platinum and manganese. According to a third embodiment, the active phase comprises platinum. By "consists essentially" is meant that the composition of the invention can have a catalytic activity in the absence in the active phase of any other element other than platinum or platinum and manganese, element of the precious metal or other metal type usually used in catalysis; or that the active phase contains, in addition to platinum or platinum and manganese, only catalytically inactive elements. The support of the composition will be chosen from thermally inert supports, that is to say capable of retaining a specific surface sufficient to keep a catalytic activity after exposure to high temperatures.

More particularly, this support can be chosen from alumina, titanium oxide, cerium oxide, zirconium oxide, mixtures of cerium oxide and zirconium oxide and mixed cerium oxides and zirconium, silica, spinels. The support can also consist of a mixture of at least two of these oxides.

In the case of alumina, it is also possible to use an alumina doped with a rare earth such as lanthanum or neodymium or with an alkaline earth such as barium.

By rare earth is meant, here and for the whole of the description, the elements of the group constituted by yttrium and the elements of the periodic classification with atomic number included inclusively between 57 and 71.

The support can also be titanium oxide further comprising at least one oxide chosen from tungsten oxide, niobium oxide, alkaline earth oxides, silica and rare earth oxides. As alkaline earth metal, there may be mentioned in particular barium and calcium. As rare earth, mention may be made of lanthanum and neodymium.

Cerium oxide can more particularly be of the type described in applications EP A 300852 and EP A 388567. It is also possible to use a support based on cerium oxide stabilized by an oxide of an element of the aluminum, silicon, thorium and zirconium type and as described in patent application EP A 369159.

It may also be advantageous to use mixed mixtures or solid solutions of cerium oxides and zirconium oxide and mention may be made on this subject of the compositions described in patent application EP A 605 274.

As spinels, it is possible to use the spinels described in patent application EP A 676232 of the type in particular ZnGa204, optionally with tin as dopant and in combination with alumina. In the case of the embodiment where the active phase comprises manganese, the support can more particularly be based on at least one oxide chosen from cerium oxide and zirconium oxide.

The respective proportions of cerium and zirconium in this support can be any. Finally, as another type of support which can be used, mention may also be made of zeolites, silicates, crystalline silicoaluminum phosphates, crystalline aluminum phosphates, these silicates or phosphates which may include metal substituents such as, for example, titanium, iron, magnesium, zinc, manganese, cobalt, gallium, lanthanum, copper, molybdenum, chromium, germanium or boron.

The platinum content of the composition can vary within wide limits. Usually, this proportion, expressed in weight of metal relative to the support weight is between 500 and 20,000 ppm, preferably between 2,500 and 15,000 ppm and even more particularly between 5,000 and 15,000 ppm. Manganese can be present in an amount which is preferably at most 50%. This quantity is expressed as the number of manganese atoms / sum of the atoms of elements other than the oxygen entering into the composition of the support, for example by the ratio number of manganese atoms / number of aluminum atoms in the case of a support in alumina or by the ratio number of manganese atom / number of atom of cerium and zirconium in the case of a support in mixed oxide of cerium and zirconium. This amount can more particularly be at most 20%. The minimum manganese content is usually at least 0.5%. Generally, the manganese content is between 5 and 20%.

According to a particular characteristic of the invention, the size of the platinum particles deposited on the support is between 1 and 50 nm, more particularly between 3 and 30 nm and even more particularly between 5 and 20 nm. The sizes given above relate to those of the platinum particles in the catalytic composition, once deposited on the support, after calcination. Generally, the active phase is predominantly or essentially present on the surface of the support and it is also mainly or essentially not encapsulated by the support.

The catalytic composition used in the process of the invention can be prepared by a method of the type in which a deposition of the active phase is carried out on the support and the characteristic of which is that the platinum is provided in the form of a sol. This way of preparing the composition gives it specific and clearly improved properties compared to a composition prepared by a conventional route in which the platinum is supplied in the form of a salt.

The platinum sol can be prepared in a known manner by heating a solution of hexachloroplatinic acid (l- ^ PtClβ) in the presence of a reducing agent. This reducing agent can be an alcohol such as methanol. It can also be sodium borohydride. The heating can be done in the presence of a stabilizing agent such as polyvinylpyrolidone. The platinum sol will preferably be chosen to have a particle or colloid size of at most 30 nm and, more precisely, a size which is between 3 nm and 10 nm and even more precisely between 3 nm and 8 nm.

The platinum is deposited on the support by bringing the support into contact with the ground. This contacting can be done using an impregnation or atomization method. After contacting, the composition obtained, that is to say the support with the deposited platinum, is calcined. The impregnation can be done by soaking the support in the platinum soil and removing the excess soil by draining or by passing through a rotary evaporator.

According to a particular variant, the impregnation is carried out "dry", that is to say that the total volume of soil used is approximately equal to the total pore volume developed by the support to be impregnated. Concerning the determination of this pore volume, it can be carried out according to the method known with a mercury porosimeter or by measuring the amount of water absorbed by a sample.

After the impregnation, the impregnated support can be dried to remove the water. The drying is most often carried out in air, at a temperature which can vary between 80 and 300 ° C. and preferably chosen between 100 and 150 ° C. Drying is continued until a constant weight is obtained. Generally, the drying time is between 1 and 24 hours.

Platinum deposition can also be done by atomization. In this case, the support is introduced into a platinum sol, for example in the form of a suspension, and the mixture thus formed is spray-dried. One can operate with a gas outlet temperature between 100 and 150 ° C.

The calcination of the support with the active phase deposited can be carried out at a temperature of at most 750 ° C., preferably at most 500 ° C. The duration of the calcination can, for its part, vary within wide limits, for example between 1 and 24 hours, from preferably between 2 and 10 hours. Calcination is generally carried out in air, but calcination carried out for example under inert gas is obviously not excluded.

The composition of the invention is particularly useful for its catalytic properties, in particular for the treatment of gases which may comprise nitrogen oxides in combination optionally with carbon oxides and / or hydrocarbons, with a view to reducing the emissions of the oxides nitrogen in particular.

The gases capable of being treated by the present invention are, for example, those originating from gas turbines, boilers of thermal power stations or even internal combustion engines. In the latter case, it may in particular be diesel engines or engines operating in a lean mixture.

The invention thus applies to the treatment of gases which have a high oxygen content and which contain nitrogen oxides, with a view to reducing the emissions of these oxides. By gas having a high oxygen content is meant gases having an excess of oxygen relative to the quantity necessary for the stoichiometric combustion of fuels and. more specifically, gases permanently having an excess of oxygen relative to the stoichiometric value λ = 1. The value λ is correlated with the air / fuel ratio in a manner known per se, in particular in the field of internal combustion engines. In other words, the invention applies to the treatment of gases from systems of the type described in the previous paragraph and operating continuously under conditions such that λ is always strictly greater than 1. In the case of gases having a content high in oxygen, the invention thus applies, on the one hand, to the treatment of engine gases operating in a lean burn mixture and which have an oxygen content (expressed by volume) generally between 2.5 and 5% and, on the other hand, to the treatment of gases which have an even higher oxygen content, for example gases from engines of the diesel type, that is to say at least 5% or more than 5% , more particularly at least 10%, this content can for example be between 5 and 20%.

The gases can contain hydrocarbons and, in such a case, one of the reactions which one seeks to catalyze is the reaction HC (hydrocarbons) + NO _x .

The hydrocarbons which can be used as a reducing agent for the elimination of NOx are in particular the gases or liquids of the families of saturated carbides, ethylenic carbides, acetylenic carbides, aromatic carbides and hydrocarbons from petroleum fractions such as for example methane , ethane, propane, butane, pentane, hexane, ethylene, propylene, acetylene, butadiene, benzene, toluene, xylene, kerosene and gas oil.

The gases can also contain, as reducing agent, organic compounds containing oxygen. These compounds can in particular be alcohols of the type for example saturated alcohols such as methanol, ethanol or propanol; ethers such as methyl ether or ethyl ether; esters such as methyl acetate and ketones.

The invention also applies to the treatment of gases containing no hydrocarbons or organic compounds as a reducing agent.

The invention also relates to a catalytic system for the treatment of gases with a view to reducing the emissions of nitrogen oxides, gases which may be of the type mentioned above. This system is characterized in that it comprises a catalytic composition as described above. In this system, the catalytic composition can be in various forms such as granules, beads, cylinders or honeycomb of variable dimensions.

The compositions can also be used in catalytic systems comprising a coating (wash coat) incorporating these compositions, the coating being arranged on a substrate of the type, for example metallic or ceramic monolith.

The systems are mounted in a known manner in the exhaust pipes of vehicles in the case of application to the treatment of exhaust gases.

Finally, the invention also relates to the process for the preparation of the abovementioned catalytic systems using a catalytic composition of the type described above.

Examples will now be given.

Examples ι • Synthesis of catalysts

- Raw materials: A platinum solution of the h ^ PtClg type (24.88% in Pt) and a platinum sol of colloid size of 6-7 nm are used.

The sol is obtained by carrying under reflux a solution comprising a mixture of polyvinylpyrrolidone, h ^ RCIβ ^and methanol.

Cerium oxide Ceθ2 calcined for 2 hours at 500 ° C. (specific surface area SBE = 150 m ² / g), a cerium and zirconium oxide (82% by weight of Ceθ2 and 18% by weight of Zrθ2 or 70% by weight of Ceθ2 ^{and 30% eπ} P ° ^{ids of Zr0} 2 ^of respective ^SBE of 85 and 65m ² / g) calcined for 2 hours at 750 ° C, a cerium oxide comprising manganese in a ratio of 15 atomic% and calcined 2h at 750 ° C (SBE = 45m ² / g), titanium oxide comprising 10% by weight of tungsten oxide (SBE = 40m ² / g), titanium oxide comprising 10% by weight lanthanum oxide (SBE = 80m ² / g), titanium oxide comprising 18% by weight of neodymium oxide (SBE = 45m ² / g) all these supports having been precalcined for 2 hours at 750 ° C., alumina precalcined 2h at 750 ° C (SBE = 145m ² / g), alumina stabilized with 11% lanthanum oxide and precalcined 2h at 750 ° C (SBE = 165m ² / g) and a support based on alumina and comprising 20% by weight of a spinel of formula Snrj _ι o25ZnGa2θ4 precalcined 2 hours at 750 ° C (SBE = 120m ² / g).

- Synthesis A:

The technique used is dry impregnation. It consists in impregnating the support considered with the platinum either in solution or in the form of a sol, the volume of solution or the volume of sol being equal to the pore volume of the support (determined with water). The operating protocol is as follows:

• Dry impregnation of the support by the platinum

• Drying in an oven (110 ° C. 2 h) • Calcination at 500 ° C for 2 h, rising to 5 ° C / min (1 ° C / min in the case of a titanium oxide support).

The specific surface area (SBE) of the products was determined by BET measurement at one point. The term BET specific surface is understood to mean the specific surface determined by nitrogen adsorption in accordance with standard ASTM D 3663-78 established on the basis of the BRUNAUER - EMMETT-TELLER method described in the periodical "The Journal of the American Society, βÛ, 309 (1938) ".

- Products obtained

With calcined alumina support for 2 hours at 750 ° C:

Product 1: 5000ppm of platinum supplied via the salt route, SBE = 150m ² / g Product 2: 5000ppm of platinum supplied via the ground route, SBE ≈ 151 m ² / g

With titanium oxide support comprising 10% by weight of tungsten oxide:

Product 3: 8000ppm of platinum supplied by the salt route, SBE = 83m ² / g

Product 4: δOOOppm of platinum supplied by the ground route, SBE = 83m ² / g

With titanium oxide support comprising 10% by weight of lanthanum oxide: Product 5: δOOOppm of platinum supplied by the salt route, SBE = 79m ² / g

Product 6: 10000ppm of platinum brought by the ground route, SBE = 82m ² / g With support in calcined cerium oxide 2h at 500 ° C: Product 7: 8000ppm of platinum brought by the salt route, SBE = 151m ² / g Product 8: δOOOppm of platinum supplied by the ground route, SBE = 148m ² / g With support in cerium and zirconium oxide (82% by weight of Ceθ2 and 18% by weight of Zrθ2) calcined for 2 hours at 750 ° C:

Product 9: δOOOppm of platinum brought by the ground route, SBE = 85m ² / g With support in cerium oxide comprising 15 atomic% of manganese, calcined 2h at 750 ° C: Product 10: 2500ppm of platinum brought by the salt route, SBE = 44m ² / g

Product 11: 2500ppm of platinum supplied by the ground route, SBE = 43m ² / g With support based on alumina and comprising 20% by weight of a spinel of formula Sn (0.025) ZnGa2θ4 _t precalcined 2 hours at 750 ° C: Product 12: δOOOppm of platinum supplied by the ground route, SBE = 118m ² / g

With alumina support stabilized with 11% lanthanum oxide and precalcined at 750 ° C:

Product 13: δOOOppm of platinum supplied by the ground route, SBE = 169m ² / g With support in titanium oxide comprising 16% by weight of neodymium oxide precalcined at 750 ° C:

Product 14: δOOOppm of platinum supplied by the ground route, SBE = 46m ² / g

- Summary B:

We use the atomization technique. A suspension is formed by mixing the platinum sol and the support. The mixture is atomized with a Buchi® atomizer. The dried product is calcined under the same conditions as those given for synthesis A. The specific surfaces are determined as above.

- Products obtained With titanium oxide support comprising 10% by weight of tungsten oxide:

Product 15: 5000ppm of platinum supplied via the ground, SBE = 37m ² / g With support in cerium and zirconium oxide (70% by weight of Ceθ2 and 30% by weight of Zrθ2 ^ ^•

Product 16: δOOOppm of platinum supplied by the ground route, SBE = 62m ² / g With support in titanium oxide comprising 10% by weight of lanthanum oxide:

Product 17: δOOOppm of platinum supplied by the ground route, SBE = 46m ² / g It • Catalvish tests

50 mg of the powdered catalyst are loaded into a quartz reactor. The powder used was previously compacted, then ground and sieved so as to isolate the particle size range between 0.125 and 0.250 mm.

The reaction mixture at the inlet of the reactor has the following composition (by volume):

- NO = 300 vpm

- C3H6 = 300 vpm - CO = 350 vpm

- O ₂ = 10% - Cθ2 = 10% - H2θ = 10% - N2 = qs 100% The overall flow is 30 Nl / h.

The WH is around 500,000 h ^"1 .

HC signals (C3H6). CO and NO _x (NO _x = NO + NO2) are permanently recorded as well as the temperature in the reactor. The HC signal is given by a BECKMAN total HC detector, based on the principle of flame ionization detection.

The NO _x signal is given by a NO _x ECOPHYSICS analyzer, based on the principle of chemistry-luminescence. The CO signal is given by a ROSEMOUNT infrared analyzer.

The catalytic activity is measured from the HC (C3H6), CO and NO _x signals as a function of the temperature during a programmed temperature rise from 150 to 700 ° C at a rate of 15 ° C / min and from the following relationships:

- The conversion rate of HC (THC) in% which is given by: T (HC) = 100 (HC ° -HC) / HC ° with HC ° signal of HC at time t = 0 which corresponds to the signal of HC obtained with the reaction mixture during the bypass of the catalytic reactor and HC is the signal of HC at time t.

- The CO conversion rate (TCO) in% which is given by:

T (CO) = 100 (CO ° -CO) / CO ° with CO ° CO signal at time t = 0 which corresponds to the CO signal obtained with the reaction mixture during the bypass of the catalytic reactor and CO is the CO signal at time t.

- The conversion rate of NO _x (TNO _x ) into% which is given by:

T (NO _x ) = 100 (NO _x ° -NO _x ) / NO _x ° with NO _x ° signal of NO _x at time t = 0 which corresponds to the signal of NO _x obtained with the reaction mixture during the by- pass of the catalytic reactor and NO _x is the signal of NO _x at time t.

Because the catalysts can activate under the conditions of the tests, the catalytic activity is given during the second consecutive passage in the test under the same conditions unless otherwise indicated.

EXAMPLES 1 (COMPARATIVE! AND 1 BIS ACCORDING TO THE INVENTION

Products 1 and 2 are used for Examples 1 and 1a respectively. The respective results are given in Tables 1 and 1a.

Table 1

It is found that the catalyst according to the invention has, with respect to the catalyst of the prior art, an identical activity vis-à-vis the hydrocarbon and a greater activity both on CO and on NO _x . EXAMPLES 2 COMPARATIVE AND 2 BIS ACCORDING TO THE INVENTION Products 3 and 4 are used for Examples 2 and 2a respectively. The respective results are given in Tables 2 and 2a.

Table 2

Table 2a

It is found that the catalyst according to the invention has a slightly greater activity with respect to the hydrocarbon and CO and a much greater activity with respect to NO _x .

It is further noted that this activity with respect to NO _x is obtained already during the first passage in catalytic test whereas in the case of the comparative composition, the maximum activity is only reached during the second passage in test .

EXAMPLES 3 COMPARATIVE AND 3 BIS ACCORDING TO THE INVENTION Products 5 and 6 are used for Examples 3 and 3a respectively. The results are given in Tables 3 and 3bis respectively.

Table 3

Table 3a

It is found that the catalyst according to the invention has a significantly greater activity both on CO, HC and on NO _x .

EXAMPLES N ° 4 COMPARATIVE AND 4 BIS ACCORDING TO THE INVENTION

Products 7 and 8 are used for Examples 4 and 4a respectively.

The respective results are given in Tables 4 and 4a.

Table 4

Table 4 bis

It is found that the catalyst according to the invention has a greater activity both on CO, HC and on NO _x .

EXAMPLE 5

We use product 9.

Table 5

EXAMPLES 6 COMPARATIVE AND 6 BIS ACCORDING TO THE INVENTION The products 10 and 11 are used for Examples 6 and 6 bis respectively, the respective results are given in Tables 6 and 6 bis.

Table 6