WO2008152273A2

WO2008152273A2 - Catalytic monolith for exhaust gases from an internal combustion engine

Info

Publication number: WO2008152273A2
Application number: PCT/FR2008/050845
Authority: WO
Inventors: Gabriel Crehan
Original assignee: Peugeot Citroën Automobiles SA
Priority date: 2007-05-30
Filing date: 2008-05-16
Publication date: 2008-12-18
Also published as: FR2916656B1; FR2916656A1; EP2150344A2; WO2008152273A3

Abstract

The invention relates to a catalytic monolith for the exhaust gases from an internal combustion engine, of the type that comprises a plurality of channels (4) extending through the monolith from one transverse face to the other, the channels (4) including several walls (10), the monolith further including a coating catalytic material (14) applied onto the walls (10) of the channels (4), the catalytic material (14) having a catalytic surface (16) in contact with the exhaust gases flowing along the channels (4). The distance (d) between a point of the catalytic surface (16) and the closest point of the wall (10) on which the coating (14) is applied at this point varies along the longitudinal direction of the monolith so that the catalytic surface (16) has a geometry adapted to a non-laminar flow of the exhaust gases in the channels (4).

Description

Catalyst monolith for exhaust gases from an internal combustion engine

The present invention relates to a catalytic monolith for exhaust gases from an internal combustion engine, of the type comprising a plurality of channels passing through the monolith from one transverse face to the other, the channels comprising several walls, the a monolith further comprising a catalytic coating material applied to the walls of the channels, the catalytic material having a catalytic surface in contact with the exhaust gas flowing along the channels. In the automotive industry, the use of metal or ceramic catalytic monoliths is common. These monoliths comprise numerous channels, most generally rectilinear and parallel, traversing the monoliths from one transverse face to the other.

Under normal conditions, the exhaust gases flowing in the channels have a laminar flow that is parallel to the channel axis. In fact, the gases enter the channels with a turbulent flow but, because of the small diameter of the channels (0.5 mm to 1.0 mm) a laminar flow is established quickly after about 1 cm of gas flow in the canals.

As a result, the mass transfer of pollutants from the laminar flow of gases to the catalytic surface of the channels is by radial mass diffusion. However, the efficiency of a radial mass diffusion is less compared to that of the mass transfers resulting from a turbulent gas flow. The use of large amounts of coating materials such as platinum group metals (PGMs) is then necessary. There are monoliths with a metal structure for creating such a turbulent flow. These monoliths comprise for example a stack of alternately smooth and corrugated metal sheets. The corrugations of the corrugated sheets are, for example, formed of a plurality of segments connected and arranged one behind the other in the direction of flow of the gases, the segments being offset transversely to the flow direction, as described in document US Pat. 4665051.

However, these monoliths have a high cost and poorly support the addition of a catalytic coating material because of their high coefficient of thermal expansion.

To date, the configurations of these metal structures can not be reproduced on a ceramic monolith because the resulting monoliths would be too fragile. An object of the invention is to allow a reduction in the amount of coating materials applied to a monolith by establishing a turbulent flow inside the channels, and this by means less expensive and demanding in their application. implementation than the known means. For this purpose, the subject of the invention is a catalytic monolith of the aforementioned type, characterized in that the distance between a point on the catalytic surface and the point closest to the wall on which the coating rests at this point, varies according to a longitudinal direction of the monolith so that the catalytic surface has a geometry adapted to a non-laminar flow of the exhaust gases in the channels. According to particular embodiments, the monolith comprises one or more of the following characteristics, taken in isolation or in any technically possible combination:

the catalytic surface has, on at least one longitudinal portion of the channels, a normal forming, with a normal of the associated wall, an angle greater than or equal to 20 °,

the geometry of the catalytic surface is such that it describes, on at least one longitudinal portion of the channels, a pattern in bosses and recesses,

the amplitude of the pattern is between 50 μm and 200 μm, preferably approximately equal to 100 μm, the pattern is regular and has a periodic length of between 100 μm and 500 μm, preferably approximately equal to 200 μm,

the monolith has a length of between 5 cm and 100 cm, and a mean diameter of between 1 cm and 30 cm, the monolith having between 50 and 100 channels per cm 2, the monolith is made of a ceramic material.

The invention also relates to a method for applying a catalytic coating for exhaust gases from an internal combustion engine to a monolith comprising a plurality of channels passing through the monolith of a face transverse to the other, the channels comprising a plurality of walls, in which the monolith is immersed in a bath consisting of the catalytic coating material in the liquid state, characterized in that after removing the monolith from the bath, it is subjected to vibrations according to the longitudinal axis of the monolith so as to obtain a corrugated surface for the catalytic coating material. According to particular embodiments, the method comprises one or more of the following characteristics, taken separately or in any technically possible combination:

the vibrations have an amplitude of between 0.1 mm and 1 mm, preferably approximately equal to 0.5 mm and a frequency of between 10 Hz and 50 Hz, preferably approximately equal to 20 Hertz,

it furthermore comprises, during the vibration phase, a step of drying the catalytic coating, carried out by means of a flow of hot air passing through the monolith from one transverse face to the other. The invention will be better understood on reading the description which follows, given solely by way of example and with reference to the appended drawings, in which:

FIG. 1 is a diagrammatic perspective view of a catalytic monolith according to the invention;

- Figure 2 is a schematic side view in longitudinal section of a channel of the monolith of Figure 1;

- Figure 3 is a schematic side view of a vibratory machine for performing a method of applying a catalytic coating, according to the invention;

- Figure 4 is a sectional view along the line IV-IV of Figure 3;

- Figure 5 is a sectional view along the line V-V of Figure 3; and - Figure 6 is a graph illustrating the relationship between a vibration frequency and an amplitude of the corrugations.

FIG. 1 illustrates a catalytic monolith 2 for exhaust gases originating from an internal combustion engine, in particular from a motor vehicle. Monolith 2 is intended to be inserted into an exhaust line to reduce pollutant emissions to the atmosphere.

The monolith 2 is made of ceramic. It comprises a plurality of channels 4 arranged in rows and columns, and passing through the monolith 2 longitudinally from one transverse face 6 to the other.

The monolith 2 has, as illustrated in FIG. 1, a general cylinder shape extending along a longitudinal axis A. However, this cylindrical shape is not limiting and the monolith 2 can have any shape suitable for its insertion into the exhaust line and easy to manufacture, including parallelepiped.

With the exception of the few channels 4 located on the periphery of the monolith 2, each channel 4 comprises four partition walls 10 substantially flat and of the same dimensions. The walls 10 intersect at right angles and are all parallel to the axis A of the monolith 2. They comprise two walls 10 that can be described as "horizontal" and two walls 10 that can be described as "vertical". The walls 10 illustrated in Figure 2 are horizontal walls. Nevertheless, the walls 10 of each channel 4 are substantially identical in shape and dimensions in the example shown.

Each channel 4 further comprises a catalytic material 14 applied to each of the walls 10 over almost all of their surface. The material 14 has a catalytic surface 16, in contact with the exhaust gases flowing along the channels 4. The material 14 has a thickness at a point of the catalytic surface 16 equal to the distance "d" between this point of contact. the catalytic surface 16 and the point closest to the wall 10 on which the material 14 is deposited at this point.

The distance "d" between the surface 16 and the associated wall 10 varies continuously along the longitudinal axis of the channel 4, parallel to the longitudinal axis A of the monolith, so that the surface 16 has a geometry adapted to a non-flow. laminar exhaust gas in the channels 4. The distance "d" is substantially constant in a transverse direction.

The surface 16 has, opposite each of the horizontal and vertical walls 10 associated therewith, and in each channel 4, a similar geometry, in the example shown. Indeed, the catalytic surface 16 describes, in the longitudinal direction of the walls 10 and over the entire length of the channel 4, a regular pattern in bosses 20 and recesses 22.

The amplitude between the bumps 20 and the recesses 22 is defined by the distance "a" between the longitudinal line tangent to the bump 20 and the longitudinal line tangent to the adjacent recess 22. A periodic length is defined by the distance "b" between the transverse line passing through the top of a hump 20 and the transverse line passing through the top of an adjacent boss 20.

The amplitude "a" of a channel 4 according to the invention is advantageously between 50 μm and 200 μm, preferably about 100 μm. The periodic length "b" is advantageously between 100 μm and 500 μm, preferably about 200 μm.

Thus, the normal to the catalytic surface 16 forms a variable angle with the normal to the surface of the associated wall. Indeed, according to the invention, on at least a longitudinal portion of the channel 4, the surface 16 has a normal forming with a normal of the associated wall 10, an angle greater than or equal to 20 °. The monolith 2 has a length of between 5 cm and 100 cm and a cross section of the monolith 2 has a mean diameter of between 1 cm and 30 cm, the monolith having between 50 and 100 channels per cm 2.

The ceramic material forming the monolith 2 comprises, for example, the following compounds or mixtures thereof:

- Cordierite, SiC, B ₄ C, Si ₃ N ₄ , BN, AlN, Al ₂ O ₃ , ZrO ₂ , Y ₂ O ₃ , CeO ₂ , aluminum titanate, ZrB ₂ , Mullite, and Sialon®.

The catalytic coating material 14 comprises, for example, the following compounds or their mixtures: for a selective catalytic reduction (SCR) intended to treat NOx: Al ₂ O ₃ ,

TiO ₂ , ZrO ₂ , CeO ₂ , Y ₂ O ₃ , SiO ₂ , as well as typical catalytic elements such as precious metals (Pt, Pd, Rh, Ru, Re, Au, Ag), transition metals (Sc , Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn), perovskites or zeolites doped with Cu, Fe, or Mn;

for a NOx trap intended to treat NOx: Al ₂ O ₃ , TiO ₂ , ZrO ₂ , CeO ₂ , Y ₂ O ₃ , doped with Ba or Mn, with typical catalytic elements such as precious metals (Pt, Pd, Rh, Ru, Re, Au, Ag, U, Np, Pu, Am) or transition metals (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn);

for three-way catalysts for treating hydrocarbons, CO and NOx: Al ₂ O ₃ , TiO ₂ , ZrO ₂ , CeO ₂ , Y ₂ O ₃ with typical catalytic elements such as precious metals (Pt, Pd) , Rh, Ru, Re, Au, Ag) or transition metals (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn);

- for total oxidation catalysts to treat CO and hydrocarbons and form NO ₂ : Al ₂ O ₃ , TiO ₂ , ZrO ₂ , CeO ₂ , Y ₂ O ₃ with typical catalytic elements such as precious metals ( Pt, Pd, Rh, Ru, Re, Au, Ag) or transition metals (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn).

Figures 3 to 5 illustrate a vibratory machine 28 adapted for the implementation of a method of applying the material 14 to the monolith 2, according to the invention.

The machine 28 comprises a fixed support 30 adapted to carry and firmly tighten the monolith 2, a vibratory device 32 adapted to subject the monolith 2 to vibrations along its longitudinal axis A, and a control unit 34 of the device 32.

The support 30 has a frame 38 secured to the ground, two clamping and support members 40 extending vertically on either side of the monolith 2. The members 40 have vertical arms 41, each fixed rigidly in the support 30. The members 40 are adapted to clamp the monolith 2 and each have for this purpose a longitudinal jaw 42 rigidly attached to a respective arm 41 and whose shape matches the outer surface of the monolith 2.

The jaws 42 have a large surface so as to distribute the forces applied by the jaws 42 on the monolith 2.

The arms 41 are movable in translation relative to the frame 38 along a transverse axis of the monolith 2, so that the distance between the two jaws 42 is adjustable.

The arms 41 are weakly elastic, so as to hold the monolith 2 firmly in position while allowing a slight movement of the latter along its longitudinal axis A.

The jaws 42 are for example made of a hard and heat-resistant polymer material (such as teflon ® or woven fiberglass) or a soft metal (such as aluminum). The vibratory device 32 is composed of a frame 46 integral with the ground and a vibratory collar 48.

As illustrated in FIG. 5, the collar 48 is provided with two rigid vibrating arms 50 extending vertically on either side of the monolith 2, and with a rigid cylinder 52 surrounding the outer surface of the upstream end of the monolith. 2 and forming an edge pressing on the periphery of the transverse face 6 of this end. The two arms 50 are movably mounted in translation along the longitudinal axis A of the monolith 2, that is to say the right-left direction in FIG. 3, so that they can print a vibration (or a shock ) along this axis with for example an amplitude between 0.1 and 1.0 mm, preferably about 0.5 mm, and a frequency for example between 10 Hz and 50 Hz, preferably about 20 Hz.

The cylinder 52 is integral with the arms 50 so that the vibratory movement of the arms 50 is integrally transmitted to the monolith 2.

The cylinder 52 is for example made of a hard and heat-resistant polymer material (such as teflon ® or woven fiberglass) or a soft metal (such as aluminum).

The control unit 34 is adapted to slave the vibratory device 32 to a predetermined vibration amplitude value and vibratory frequency value.

The machine 28 further comprises a device 56 for ventilation and heating adapted to provide a flow of hot air passing through the monolith 2 of a transverse face 6 to the other, whose flow rate is between 0.1 and 2 l / s, preferably about 1 l / s, and whose output temperature of the device 56 is for example between 50 and 100 °, preferably about 80 ° C.

The method of manufacturing the monolith 2 comprises a first step of manufacturing the ceramic structure of the monolith 2. This first step is carried out according to any known technique and does not constitute in itself an innovative element of the invention.

Once a monolith 2 obtained naked, the monolith 2 is fully immersed in a liquid solution of the material 14, so as to impregnate the walls 10 of the channels 4 over almost their entire surface and with a uniform thickness, so that the distance "d" is uniform and between 10 microns and 150 microns, preferably about 50 microns.

The method then comprises, as soon as possible after the end of the previous step, a step of: - the establishment and tightening of the monolith 2 in the jaws 42 of the vibratory machine 28;

the control by the unit 34 of the vibratory device 32 so as to subject the collar 48 and the integral monolith 2 to the vibrations described above so as to effect a corrugation of the surface 16 of the coating material 14 to its drying;

during the vibration phase, a forced drying step carried out by means of the apparatus 56 described above.

The vibrations have for example an amplitude of between 0.1 mm and 1 mm, preferably between 0.4 and 0.6 mm, more preferably approximately equal to 0.5 mm, and a frequency of between 10 and 50 Hz, preferably between 15 and 30 Hz, preferably about 20 Hz.

The method finally comprises a heat treatment step to the final state of the monolith 2 and the material 14, in known manner, and a possible subsequent step of machining the monolith 2. As illustrated by the diagram of the figure 6, there exists for a given amplitude of vibrations, over a certain frequency range, substantially a relation according to an affine function (of type y = -cx + d) between the amplitude "a" of the pattern obtained on the surface 16 and the frequency of the vibrations printed at monolith 2. The frequency is inversely proportional to the amplitude "a". Overall, as the frequency increases, the amplitude "a" decreases. In Figure 6, this affine function is illustrated by a curve 60 of equation y = -4.8x + 246. This relation is determined experimentally. The frequency is represented on the horizontal axis 62 and the amplitude "a" on the vertical axis 64.

It is essentially the same for the relation (not shown) between the periodic length "b" of the pattern and the frequency of the vibrations.

In general, for a given amplitude of the vibrations, the amplitude "a" increases and the length "b" increases when the vibration frequency decreases.

The thickness of the catalytic coating 14 is uniform before the monolith 2 is vibrated so as to prevent the coating 14 from punctually losing contact with the surface of the wall 10 on which it rests, at the end of the application of the vibrations. it is preferable that the amplitude "a" does not exceed twice, preferably one and a half times the initial thickness of the catalytic coating 14.

Indeed, the profile of the corrugations formed by the vibrations is related to the existence of high pressure zones and low pressure zones in the coating 14. The material of the catalytic material 14 is pushed from the zones with high pressure towards the low pressure areas forming the undulations. If too much catalytic material is moved to the low pressure areas, the contact between the wall 10 and the material 14 may be locally broken. The attachment between the catalytic material 14 and the wall 10 is then deteriorated. Also, if one maximizes the geometric surface of the catalytic coating 14, molecular diffusion around the large specific surface is facilitated.

Thus, for a vibration amplitude of 0.5 mm, and a frequency of 20 Hz, an initial catalytic coating thickness 14 of 100 μm, the periodic length "b" will be approximately 1 mm and the amplitude "a" will be approximately 150 μm. For the same amplitude of the vibrations but at a frequency of 40 Hz, the periodic length "b" will be about 0.5 mm and the amplitude "a" will be about 50 microns. Finally, for the same amplitude of the vibrations but at a frequency of 50 Hz, the periodic length "b" will be about 0.4 mm and the amplitude "a" will be about 10 microns.

The duration of application of the vibrations by the vibratory device 32, to the monolith

2, is for example at least 5 s, preferably between 10 and 15 s .. This time is sufficient to establish a regular wave pattern of the catalytic surface 16 and ensure sufficient drying of the material 14, before a final heat treatment conventional coating impregnation 14.

Thus, with the invention, the catalytic efficiency is increased by the turbulence of the flow of exhaust gas passing through the channels 4. The increase in the turbulence of flux is obtained by the corrugated geometry of the catalytic surface 16 of the channels 4. This geometry does not harm the general flow of gas through the monolith 2.

Because of the improved performance of the catalytic monolith 2 according to the invention, it is possible to reduce its length and therefore the amount of catalytic coating material used compared to the catalytic monolith of the prior art.

The application of the invention is not limited to motor vehicles and the invention can be used to treat the exhaust gas of any internal combustion engine, even in fixed station.

In a variant, the monolith 2 is a monolithic brick intended to constitute part of a monolith 2.

Alternatively also, the monolith 2 is made of metal and / or metal oxide. Indeed, the invention makes it possible to produce turbulent flow metal monoliths in their channels, without it being necessary to impart a ripple to the walls of the monolith themselves, thus in an easy and economical manner.

Claims

1. Catalytic monolith (2) for exhaust gases from an internal combustion engine, of the type comprising a plurality of channels (4) passing through the monolith (2) from one transverse face (6) to the other , the channels (4) comprising a plurality of walls (10), the monolith (2) further comprising a catalytic coating material (14) applied to the walls (10) of the channels (4), the catalytic material (14) having a catalytic surface (16) in contact with the exhaust gases flowing along the channels (4), characterized in that the distance (d) between a point of the catalytic surface (16) and the nearest point of the wall (10) on which the coating (14) rests at this point, varies in a longitudinal direction (A) of the monolith (2) so that the catalytic surface (16) has a geometry adapted to a non-laminar flow of gases exhaust in the channels (4).

2. Monolith (2) according to claim 1, characterized in that the catalytic surface (16) has, on at least one longitudinal portion of the channels (4), a normal forming with a normal of the associated wall (10), a angle greater than or equal to 20 °.

3. Monolith (2) according to claim 1 or 2, characterized in that the geometry of the catalytic surface (16) is such that it describes, on at least a longitudinal portion of the channels (4), a bumps pattern ( 20) and recessed (22).

4. Monolith (2) according to claim 3, characterized in that the amplitude (a) of the pattern is between 50 microns and 200 microns, preferably about equal to 100 microns.

5. Monolith (2) according to claim 3 or 4, characterized in that the pattern is regular and has a periodic length (b) between 100 microns and 500 microns, preferably about 200 microns.

6. Monolith (2) according to any one of the preceding claims, characterized in that the monolith (2) has a length of between 5 cm and 100 cm, and a mean diameter of between 1 cm and 30 cm, the monolith having between 50 and 100 channels per cm2.

7. Monolith (2) according to any one of the preceding claims, characterized in that the monolith (2) is made of a ceramic material.

8. A process for applying a catalytic coating (14) for exhaust gases from an internal combustion engine to a monolith (2) comprising a plurality of channels passing through the monolith (2) of a face transverse (6) to the other, the channels (4) comprising a plurality of walls (10), in which the monolith (2) is immersed in a bath consisting of the catalytic coating material (14) in the liquid state, characterized in after taking out the monolith (2) from the bath, it is submitted to vibration along the longitudinal axis (A) of the monolith (2) so as to obtain a corrugated surface for the catalytic coating material (14).

9. A process according to claim 8, characterized in that the vibrations have an amplitude of between 0.1 mm and 1 mm, preferably approximately equal to 0.5 mm, and a frequency of between 10 Hz and 50 Hz, preferably about 20 Hertz.

10. A process according to claim 8 or 9, characterized in that it further comprises, during the vibration phase, a step of drying the catalytic coating (14), carried out by means of a flow of hot air through the monolith (2) from one transverse face (16) to the other.