WO2012127127A1 - Catalyst for reducing nox by means of hydrocarbons and oxygen compounds - Google Patents

Abstract

The present invention relates to a catalyst containing a mixture of alumina, titanium oxide, and silver, the silver content being greater than 3% and less than 5% of the total weight of the catalyst, to a device including such a catalyst deposited onto an integral part, to a method for reducing nitrogen oxides in a gaseous effluent containing nitrogen oxides and sulfur derivatives, and to a vehicle or boat, construction equipment, locomotive, or stationary facility including an engine and an exhaust line connected to the outlet of the engine, characterized in that the catalyst or the device is arranged in the exhaust line.

Description

 Catalyst for the reduction of NOx by hydrocarbons and oxygenates

The present invention relates to a catalyst or device for the reduction of nitrogen oxides in the presence of a reducing agent chosen from hydrocarbons, oxygenated compounds and their mixtures, in a gaseous effluent comprising sulfur oxides, and a process of preparing this catalyst and a method of reducing the corresponding nitrogen oxides.

The combustion of fossil fuel such as oil or coal in a combustion system, particularly in an engine, can result in the production of significant quantities of pollutants that can be discharged into the environment and cause damage to the environment. . Among these pollutants, nitrogen oxides, called NOx and mainly comprising NO and NO ₂ , can be cited, as they are suspected to be one of the factors contributing to the formation of acid rain and deforestation.

 The main known method for removing NOx is to selectively reduce these compounds by means of so-called selective catalytic reduction (SCR) catalysts in the presence of a reducing agent.

 This reducing agent may be according to a first variant the ammonia which is generated for example from urea, or according to a second variant of the hydrocarbons or oxygenated compounds.

 The first variant presents the risk of inducing leakage of ammonia, and thus aggravate the pollution phenomena instead of limiting them. In addition, the supply of ammonia in mobile applications remains problematic. These disadvantages, however, are not present in the context of the second variant.

The most studied catalytic systems for the selective catalytic reduction of NOx are zeolite catalysts and silver-on-alumina catalysts. By way of example of a process for treating gas reducing NOx emissions and using a catalyst based on zeolite, mention may be made of document EP 1 316 359, in which the catalyst used comprises a ferrierite exchanged with a rare earth chosen from cerium, praseodymium, samarium, terbium, europium and ytterbium.

 By way of example of a process for reducing NOx present in a gaseous effluent using a catalyst based on alumina on which silver has been deposited, mention may be made of documents US Pat. No. 5,772,973 and EP 0 526,099. processes cited in these two documents using hydrocarbons and / or oxygenates as reducing agent.

 The application FR 09/56491 also describes a process for reducing NOx in a gaseous effluent using a catalyst based on alumina and titanium oxide comprising silver, the reducing agent used in this process being hydrocarbons and or oxygenated compounds.

However, none of these documents deals with the problem of treating gaseous effluents containing, in addition to NOx, sulfur oxides and in particular S0 ₂ .

Indeed, the diesel used in diesel engines may contain significant amounts of sulfur derivatives, for example 1% by weight and more sulfur for gas oils used for boats, which is then in the exhaust gas under form of S0 ₂ at 400-500 ppm.

 However, the presence of sulfur oxides in the gaseous effluent to be treated can lead to a decrease in the activity of the catalyst with respect to the reduction of NOx over time.

 There is therefore a need to develop a catalyst for reducing the NOx content in a gaseous effluent also containing sulfur oxides, such as in an exhaust gas of a diesel engine.

The inventors have surprisingly discovered that it is possible to maintain the activity of a catalyst based on a mixture of alumina, titanium oxide and silver over time by increasing the silver content. of this catalyst, this content being between 3 and 10%, especially between 3 and 5%, such that> 3 and <5% by weight of the total weight of the catalyst, and preferably between 3.5 and 4.5%.

The invention therefore has for its first object a catalyst comprising a mixture of alumina, titanium oxide and silver, the silver content being between 3 and 10%, especially between 3 and 5%, such that greater than 3% and less than 5%, in particular between 3.5 and 4.5%, especially about 4%, by weight relative to the total weight of the support.

The silver in the catalyst is in particular in the form of Ag ⁺ and Ag _n ⁵⁺ clusters.

 Silver is the main active phase of this catalyst responsible for the reduction of NOx in the gaseous effluent.

Advantageously, the catalyst comprises predominantly alumina (Al ₂ O ₃ - aluminum oxide). Advantageously, the mass ratio of the amount of alumina to the amount of titanium oxide is between 50:50 and 99.99: 0.01, especially between 90:10 and 99.99: 0.01, advantageously between 95: 5 and 99.95: 0.05, preferably between 98: 2 and 99.9: 0.1. This alumina also contributes to the reduction of NOx in the gaseous effluent.

 The catalyst according to the invention may comprise in particular less than 1 wt.%, Advantageously less than 0.5 wt.%, Preferably less than 0.1 wt.%, And ideally will be essentially free of materials other than alumina, titanium oxide and silver. In other words, the catalyst consists or consists essentially of a mixture of alumina, titanium oxide and silver with impurities, the amounts of silver being as defined above.

 The catalyst will in particular be obtainable by a preparation process as described below.

 The catalyst will advantageously be prepared in powder form.

It can then be put in the form of pellets, extrusions, or beads or be deposited on a monolith. For this, the catalyst will be mixed with a conventional binder such as clay, alumina or silica gel. The present invention therefore also relates to a device comprising a catalyst according to the invention in the form of extrudates, pellets or balls.

 The present invention therefore also relates to a device comprising a catalyst as described above deposited on a monolith, in particular porous, in particular in the form of a honeycomb.

 The monolith may be a monolith of ceramics, the ceramics may be cordierite, or a metallic monolith.

 The monolith may include channels to allow the passage of the gaseous effluent to be treated through the monolith, the channels passing through the monolith from one side to the other.

The above catalyst, and therefore also the above device, is particularly suitable for use in the context of a selective catalytic reduction reaction of nitrogen oxides with hydrocarbons and / or oxygenated compounds, especially in a gaseous effluent comprising sulfur oxides, and advantageously S0 ₂ .

 The NOx selective reduction reaction can be carried out by simply bringing into contact a gaseous effluent containing NOx, as well as hydrocarbons and / or oxygenated compounds, with the above catalyst and at a reaction temperature that is more particularly understood. between 200 and 500 ° C, and preferably between 300 and 450 ° C.

 The catalyst above is particularly applicable in the context of the treatment of the exhaust gas of an engine, preferably diesel or lean burn gasoline, such as that of a vehicle. , a motor boat, a construction machine, or a locomotive, as well as for the treatment of gaseous effluents from all stationary installations emitting nitrogen oxides and sulfur oxides.

In the case of an engine, the catalyst or the device according to the invention will then be arranged in the exhaust line, at a level where the temperature will be adequate for good activity. The present invention also relates to a process for preparing a catalyst as defined above comprising the following successive stages:

(i) suspending alumina and titania in a solution of a silver salt in water,

 (ii) separating the compound obtained in step (i) above comprising a mixture of alumina, titanium oxide and silver, water, and drying the compound thus obtained,

 (iii) heat treating the compound obtained in the preceding step (ii) comprising a mixture of alumina, titanium oxide and silver, to give the catalyst, and

 (iv) hydrotreatment of the catalyst obtained in the preceding step (iii). Step (i):

 The alumina used in step (i) will advantageously be in powder form. Step (i) will preferably be carried out with stirring.

 The silver salt introduced into the aqueous solution will in particular be a salt of nitrate or of sulphate, and in particular a salt of nitrate.

 The titanium oxide may be obtained by introducing into the aqueous solution a titanium oxide mechanically mixed with alumina, or from a titanium salt such as titanium chloride, which hydrolyzes to the oxide of titanium. titanium with stirring in water.

 The water used will advantageously be a demineralized water whose conductivity is less than 14 μΞ.

 Step (ii):

 The separation may be carried out by evaporation of the water, in particular under vacuum at a temperature of 60 ° C., or possibly by filtration.

 The drying will advantageously be carried out at a temperature of between 80 and 100 ° C.

Step (iii): This heat treatment step, also called calcination, can be carried out at a temperature of between 200 and 600, and in particular between 400 and 500 ° C.

 Step (iv):

 The catalyst thus obtained may also be subjected to hydrotreatment under a stream of air charged with water, for example containing 10% water, at a temperature of between 600 and 750 ° C.

The method of preparation described above is an example is not limiting. We can also consider for example the preparation of such a catalyst by impregnation of silver from various precursors in various solvents, or by simple mechanical mixing of silver oxide, alumina and titanium oxide. The support may also be prepared by grafting ΤΊΟ ₂ from a flow of T1CI ₄ passing through a bed of alumina, followed by hydrolysis.

The present invention also relates to a method for preparing a device as defined above comprising the following successive steps:

(a) Successive soaking of the monolith in a slurry containing the catalyst according to the present invention, a binder and water. Advantageously, the slip contains 15 to 20% by weight of dry matter relative to the total weight of the slip, of which 70 to 90% by weight of the catalyst as defined above, advantageously 80%, and 10% to 30% by weight of binder, advantageously 20%, all being advantageously suspended in demineralized water. The binder in the sense of the present invention is a colloidal suspension to promote the adhesion of the catalyst on the monolith. This is for example an AI ₂ O ₃ pseudo-boehmite type.

(b) Then advantageously drying at 100 ° C. in an oven and heat treatment, advantageously carried out at a temperature of between 200 and 600 ° C. and in particular between 300 and 500 ° C., advantageously for about 4 hours, in order to obtain the device according to the invention. This heat treatment makes it possible to optimize the mechanical strength of the catalyst on the walls of the monolith. The present invention also relates to a process for reducing nitrogen oxides in a gaseous effluent also containing sulfur oxides, and preferably mainly S0 ₂ , by contacting said gaseous effluent with a catalyst as defined herein. above or a device as defined above in the presence of a reducing agent chosen from hydrocarbons, oxygenated compounds and their mixtures.

 Sulfur oxides mainly include sulfur dioxide

(S0 ₂ ).

 The gaseous effluent to be treated in the context of this process may be in particular an exhaust gas from an engine, preferably diesel or lean burn gasoline.

It will contain 200 to 4000 ppm of NOx. It may also contain 0.5 to several hundreds of ppm, especially up to 500 ppm of sulfur oxides such as S0 ₂ .

 The hydrocarbons comprise in particular saturated or unsaturated hydrocarbon derivatives (that is to say comprising at least one double or triple bond) with a linear or branched chain, especially comprising 1 to 12 carbon atoms and mixtures thereof. It may be in particular ethylene, propylene, acetylene, decane, propane, dodecane and their mixtures.

 The oxygenated compounds include in particular alcohols, aldehydes and ketones.

The alcohols consist of compounds comprising an OH function. In the context of the present invention, it will be in particular a compound of formula R 1 OH in which R 1 represents a linear or branched hydrocarbon chain, saturated or unsaturated (preferably saturated), comprising in particular 1 to 6, of preferably 1 to 4, carbon atoms. It may especially be methanol, ethanol, n-propanol, isopropanol and their mixtures, including denatured ethanol and fuels containing ethanol such as for example the fuel E85. It will be in particular ethanol and more particularly ethanol denatured, which may include isopropanol and fuels containing ethanol such as E85 fuel.

 For the purposes of the present invention, "denatured ethanol" is understood to mean ethanol to which a denaturant is added to render the mixture unfit for consumption. Among the denaturants, mention may in particular be made of isopropanol.

 E85 fuel is a fuel containing between 70 and 85% by volume of ethanol.

Aldehydes consist of compounds comprising a CHO function. In the context of the present invention, it will be in particular a compound of formula R ₂ -CHO in which R ₂ represents a linear or branched hydrocarbon chain, saturated or unsaturated (preferably saturated), including in particular 1 to 6, preferably 1 to 4, carbon atoms. This may include acetaldehyde.

Ketones consist of compounds comprising a C = O function. In the context of the present invention, it will be in particular a compound of formula R3-C (O) -R ₄ in which R ₃ and R ₄ each represent, independently of one another, a linear or branched hydrocarbon chain, saturated or unsaturated (preferably saturated), comprising especially 1 to 6, preferably 1 to 4, carbon atoms. This may include acetone.

 The oxygenated compound used in the context of this process may therefore be chosen from methanol, ethanol, n-propanol, isopropanol, acetaldehyde, acetone and their mixtures, including denatured ethanol and fuels. containing ethanol. The oxygenated compound will include ethanol and will be more particularly selected from denatured ethanol and E85 fuel.

The ratio reducing agent on nitrogen oxide (C / NOx) will advantageously be between 2 and 10, especially between 2 and 6. This ratio corresponds to the number of moles of "carbon atoms of the reducing agent" (number of moles of reducing agent x number of carbon atoms contained in the reducing agent) over the number of moles of nitrogen oxide For example, for a C / NOx ratio = 2, one mole of ethanol will be injected for 1 mole of nitrogen oxides. This process will also be advantageously carried out at a temperature of between 200 and 500 ° C., preferably between 300 and 450 ° C.

The present invention also relates to a vehicle, a boat, a construction machine or a locomotive, comprising an engine, preferably Diesel or lean burn gasoline, and an exhaust line connected in output of the engine, characterized in that the catalyst or the device according to the invention is disposed in the exhaust line.

 The catalyst or device according to the invention will then allow the treatment of the gases produced by the engine and containing oxides of nitrogen and oxides of sulfur.

The present invention also relates to a stationary installation, such as for example a cogenerator or a household waste incinerator, producing a gaseous effluent containing nitrogen oxides and sulfur oxides and comprising a catalyst or a device according to the invention. The invention is arranged so as to allow the treatment of gaseous effluent (i.e. reduction of nitrogen oxides).

The present invention will be better understood in light of the examples and non-limiting figures which follow.

FIGURES

Figures 1 to 6 show the conversion (in%) of ethanol (in black) and NOx (in gray) over time (in hours) under different catalyst conditions and composition of the gaseous effluent.

EXAMPLES I. Preparation of a catalyst and a device according to the invention A catalyst according to the invention was prepared according to the method by wet impregnation from Pural NG alumina marketed by SASOL, silver nitrate and titanium oxide.

In order to prepare 10 g of catalyst, 9.6 g of alumina and 19 mg of titanium oxide are premixed mechanically and then added to a solution in which 0.63 g of silver nitrate has been dissolved in water. 'Demineralized Water. The volume of the suspension was then adjusted so that it contained about 40% dry matter.

 After obtaining a homogeneous suspension, following stirring at 60 ° C, evaporation was carried out under vacuum at Rotavapor. The product thus obtained was oven-dried overnight at 80 ° C and then heat-treated in the oven for 5 hours at 500 ° C.

 After calcination at 500 ° C., the catalyst composition is 4% silver, on a support composed of 99.8% of alumina and 0.2% of titanium oxide.

The catalyst was then coated on a cordierite honeycomb of 400 cpsi by several successive quenchings in a slip containing 20% of dry matter, of which 80% of the catalyst obtained previously and 20% of binder, the latter corresponding to to a colloidal alumina.

 Once the catalyst was coated on the honeycomb, the assembly was again calcined at 400 ° C. for 2 hours.

IL Study of NOx reduction

In this study, the following catalysts were used:

the catalyst according to the invention obtained in the previous point I (catalyst A) and a catalyst according to the prior art comprising only 2% by weight of silver, obtained according to a process similar to that described in point I above (catalyst B) . These two catalysts were coated on a 400 cpsi cordierite honeycomb having the shape of a cylinder 1 "(2.54 cm) in diameter and 2" (5.08 cm) long, thus representing a volume 25 cm ³ , according to the process as described previously in point I.

The honeycombs thus coated were placed in a crossed-bed fixed reactor, surrounded by thermo-regulated heating shells.

 The gaseous effluent at the reactor inlet has the following composition:

 - NO: 400 ppm

- 0 ₂ : 4%

H ₂ 0: 7%,

S0 ₂ : variable, and

N ₂ representing the rest.

Ethanol was used as a reducing agent with C / NOx = 3 to 6. These values correspond to a good NOx reduction activity, for a surplus of acceptable reducing agent consumption.

The study was carried out at a constant temperature of 400 ° C and with a VVH of 30,000 h ^-1 .

 The composition of the effluent at the outlet of the reactor was continuously monitored by different methods of analysis:

NOx (NO and N0 ₂ ): by chemiluminescence, and

 ethanol: by a Flame Ionization Detector (FID).

The conversion of NOx and ethanol are shown in Figures 1 to 5 for the following different cases:

Figure C / NOx S0 ₂ (in Catalyst

 ppm) used

 1 3 0 B

2 3 5 B

3 3 5 A

4 3 15 A

5 3 - 4.5 - 6 50 A The "holes" in the curves shown in the figures correspond to acquisition interruptions, in particular to perform a rinsing of the analyzers, or to redo the levels of water and ethanol.

Comparing FIGS. 1 and 2, there is a very significant loss of catalyst activity comprising only 2% by weight of silver in the presence of 5 ppm of SO ₂ .

However, it can be seen from FIG. 3 that this loss of activity is not observed with a catalyst comprising 4% by weight of silver in the presence of 5 ppm of S0 ₂ which remains stable over time over at least 48 hours. .

When the content of S0 ₂ is increased (see FIGS. 3 to 5), there is a drop in initial activity, but the conversion of NOx remains stable over time. The C / NOx ratio was modified during the study with a catalyst comprising 4% by weight of silver and a SO ₂ content of 50 ppm (see FIG. 5).

The study started with a C / NOx = 3 ratio for 38 h, showing a decrease in initial activity of 80% (with 15 ppm S0 ₂ ) to about 65% (with 50 ppm S0 ₂ ) of conversion of NOx, activity that remains stable throughout the 38h. The C / NOx ratio was then increased to a value of 4.5 and then 6, which showed a significant increase in NOx conversion at 78% and 82.5% respectively.

A study was then carried out with the same catalyst comprising 4% silver on a pilot scale over a long operating period of about 400 hours, under the following conditions:

catalyst on 400cpsi cordierite support: diameter = 80 mm; height: 75 mm (ie a volume of 377 cm ³ ); active ingredient content (of which 20% binder) = 150 g / l. This is the mass of washcoat (catalyst + binder) conventionally deposited on such a cordierite support for an automotive application,

composition of the gaseous effluent: * NOx: 1,200 ppm

0 ₂ : 4%

* H ₂ 0: 7%

 *: 5 ppm

 represents the rest

 reducing agent: ethanol with C / NOx = 3

- VVH: 30,000 hr ^"1

The evolution of NOx and ethanol conversion over time is shown in Figure 6. It can be seen that the conversion of NOx remains stable throughout the study, this conversion being of the order of 90 %.

Claims

1. Catalyst comprising a mixture of alumina, titanium oxide and silver, the silver content being between 3 and 10%, advantageously between 3 and 5%, by weight relative to the total weight of catalyst. 2. Catalyst according to claim 1, characterized in that the mass ratio of the amount of alumina to the amount of titanium oxide is between 50:50 and 99.99: 0.01, especially between 90:10 and 99.99: 0.01, preferably between 95: 5 and 99.95: 0.05, preferably between 98: 2 and 99.9: 0.1. 3. Catalyst according to any one of claims 1 and 2, characterized in that it comprises less than 1% by weight, advantageously less than 0.5% by weight, preferably less than 0.1% by weight. materials other than alumina, titanium oxide and silver. 4. Device comprising a catalyst according to any one of claims 1 to 3 in the form of extrudates, pellets or balls. 5. Device comprising a catalyst according to any one of claims 1 to 3 deposited on a monolith preferably porous, especially in the form of honeycomb. 6. Device according to claim 5, characterized in that the monolith is a ceramic monolith or metal. 7. Process for the preparation of a catalyst according to any one of claims 1 to 3, comprising the following successive steps: (i) suspending alumina and titania in a solution of a silver salt in water, (ii) separating the compound obtained in the preceding step (i) comprising a mixture of alumina, titanium oxide and silver, with water and drying of the compound thus obtained,  (iii) heat treating the compound obtained in the preceding step (ii) comprising a mixture of alumina, titanium oxide and silver, to give the catalyst, and  (iv) hydrotreatment of the catalyst obtained in the preceding step (iii). 8. The method of claim 7, characterized in that step (iii) is carried out at a temperature between 200 and 600, and in particular between 400 and 500 ° C. 9. A method of preparing a device according to any one of claims 5 and 6 comprising the following successive steps:  (a) successive impregnation of the monolith in a slip containing the catalyst according to any one of claims 1 to 3, a binder and water,  (b) Then drying and heat treatment to obtain the device 10. A method for reducing nitrogen oxides in a gaseous effluent also containing sulfur oxides, preferably mainly S0 ₂ , by contacting said effluent with a catalyst according to any one of claims 1 to 3 or a Device according to any one of claims 4 to 6 in the presence of a reducing agent selected from hydrocarbons, oxygenates and mixtures thereof. 11. The method of claim 10, characterized in that the gaseous effluent.is an exhaust gas from a motor. 12. Method according to any one of claims 10 or 11, characterized in that the hydrocarbon is selected from ethylene, propylene, acetylene, decane, dodecane, propane and mixtures thereof; and the oxygenated compound is selected from methanol, ethanol, n-propanol, isopropanol, acetaldehyde, acetone and mixtures thereof, including denatured ethanol and fuels containing ethanol. 13. Method according to any one of claims 10 to 12, characterized in that the ratio reducing agent on nitrogen oxides (C / NOx) is between 2 and 10, in particular between 2 and 6 14. Vehicle, boat, construction machine, or locomotive comprising an engine, preferably diesel or lean gasoline, and an exhaust line connected at the output of the engine, characterized in that the catalyst according to any one of the claims. 1 to 3 or the device according to any one of claims 4 to 6 is disposed in the exhaust line. Stationary installation, such as for example a cogenerator or a household waste incinerator, producing a gaseous effluent containing oxides of nitrogen and oxides of sulfur and comprising a catalyst according to any one of claims 1 to 3 or a Device according to any one of claims 4 to 6 arranged so as to allow the treatment of the gaseous effluent.