GB2470724A

GB2470724A - A catalyst for treating exhaust emissions

Info

Publication number: GB2470724A
Application number: GB0909364A
Authority: GB
Inventors: Annika C Kristoffersson; Fredrik B Ekstrom
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2009-06-01
Filing date: 2009-06-01
Publication date: 2010-12-08
Also published as: GB0909364D0

Abstract

A catalyst (1) for treating exhaust emissions from an automotive vehicle is disclosed which comprises a monolith (2) and a washcoat (4) covering the monolith (2). The washcoat (4) comprises at least one catalytically active metal (7) and a ceramic matrix (5) comprising a plurality of grains (6) having a grain size distribution (14, Fig. 5). The washcoat (4) further comprises a plurality of pores (8) having a diameter and a pore diameter distribution (16, Fig. 5). The grain size distribution (14) has a single peak (15, Fig. 5) and the pore diameter distribution (16) has two peaks (17, 18, Fig. 5). Also disclosed is a washcoat comprising at least one catalytically active metal, a ceramic, a fugitive material and a solvent. The metal may be a noble metal. The ceramic may be titania, alumina or zirconia.

Description

A catalyst for treating exhaust emissions from an automotive vehicle and a method of producing a catalyst The present application relates to a catalyst for treating ex-haust emissions from an automotive vehicle and a method of producing a catalyst.

A catalyst for the treatment of exhaust emissions from an automotive vehicle may include a monolith which acts as a sup-port for a coating comprising a catalytically active material which is selected so as to treat the exhaust emissions as de-sired.

The monolith may have a honeycomb structure so as to provide a large surface area through which the exhaust gases flow in or- der to increase the effectiveness of the treatment. The mono-lith may comprise metal or a ceramic. The coating including the catalytically active material is often referred to as a washcoat and typically comprises a ceramic carrier or matrix as well as the catalytically active material. The catalyti-cally active material may comprise a nobel metal such as Pt and/or Rh and Pd.

US 4,529,718 discloses a coating with macroporosity as well as microporosity with the aim of improving the catalyzation of very rapid reactions characterized by limited internal diffu-sion. The coating is fabricated by using a solution comprising a binder and a filler, both comprising alumina. The filler has a larger grain size than the binder.

However, further improvements to catalysts are desirable in order to further improve catalytic efficiency as well as to increase the stability and lifetime of the catalyst.

The application provides a catalyst for treating exhaust emis-sions from an automotive vehicle which comprises a monolith and a washcoat covering the monolith. The washcoat comprises at least one catalytically active metal and a ceramic matrix comprising a plurality of grains having a grain size distribu-tion. The washcoat further comprises a plurality of pores each having a diameter and a pore diameter distribution. The grain size distribution has a single peak and the pore diameter dis-tribution has two peaks.

The grain size distribution may be described by a graph of the number of grains of a particular size as a function of the grain size. A peak in the grain size distribution indicates that a large number of the grains have a grain size within a limited size range.

The pore diameter distribution may be described by a graph of the number of pores of a particular size as a function of pore size. A peak in the pore diameter distribution indicates that a large number of the pores have a pore diameter within a lim-ited diameter range.

The pore diameter distribution of the washcoat has two peaks spaced apart from one another. This indicates that there is a first group of pores having a pore diameter within a limited range of a first diameter and a second group of pores having a diameter within a limited range of a second different diame-ter.

The grain size distribution and the pore diameter distribution can be determined by measuring the pore diameters and grain sizes of a large number of grains and pores, respectively, and plotting the graphs defined above. The grain size and pore di-ameters may be measured from micrographs taken from one or more polished cross-sections of the washcoat.

The pore size distribution has two peaks whereas grain size distribution of the ceramic matrix has a single peak. The washcoat, therefore, can be said to comprise micropores or mi- croporosity as well as macrospores or macroporosity as repre- sented by the two separate peaks in the pore diameter distri-bution. The microporosity and macroporosity is independent of the grain size of the ceramic matrix since the grains size distribution has only a single peak. Two different types of grain size are no longer required in the ceramic matrix.

The macroporosity provides an increased surface area and de-creases the internal diffusion of the exhaust emissions within the washcoat which is required to treat the exhaust emissions satisfactorily. At temperatures above about 500°C, which are typically reached in catalytic converters of automotive vehi- cles, the catalytic conversion is rate limited by mass trans-fer to and into the surface of the washcoat. Since the surface area is increased as a result of the macropores, a higher ef-ficiency of catalytic conversion can be reached.

The increased geometric surface area can be provided without increasing the cell density of a honeycomb monolith due to the provision of macroporosity. Therefore, the increased back pressure, which may result as a result of increasing the cell density, is avoided.

Interfaces between large and small grains may suffer from re-duced sintering compared to interfaces between small grains.

This reduced sintering can lead to weak regions at these posi- tions within the ceramic matrix. Since the grain size distri-bution of the ceramic matrix has a single peak, the ceramic matrix has an improved mechanical strength since the matrix is uniform and homogeneous.

Additionally, the application of the washcoat is simplified since the ceramic particles of the washcoat have a particle size distribution having a single peak and therefore similar characteristics within the coating solution. The coating is easier to apply uniformly since sedimentation of larger grains can be avoided.

In an embodiment, one of the peaks of the pore diameter dis-tribution occurs at a diameter of 3 microns to 50 microns.

This first peak may be said to describe macroporosity. One of the peaks of the pore diameter distribution occurs at a diame- ter of 10 nm to 200 nm. This second peak may be said to pro-vide microporosity.

The pores having a diameter in the range of 3 microns to 50 microns have the advantage that they are stable at the condi-tions used to fabricate the coating, e.g. typical calcination temperatures of 400°C to 500°C. The pores having a diameter in this range are also stable at usage temperatures of up to ap-proximately 1000°C. Therefore, a high macroporosity in the washcoat can be reliably obtained.

Pore having a diameter in the range of 10 nm to 200 nm have the advantage that they can be more easily finely distributed throughout the ceramic carrier so that mechanically weak re-gions are avoided which may arise as the result of isolated larger pores.

In an embodiment, at least 80% of the pores having a diameter of 3 microns to 50 microns lie within � 50% of the peak maxi- mum of the first peak. At least 80% of the pores having a di-ameter of 3 microns to 50 microns may lie within � 50% of the peak maximum of the second peak.

However, the two peaks of the pore diameter distribution may not be spaced quite separately from one another in the pore size distribution, but may be superimposed. In an embodiment, the pore size distribution includes a single broad peak caused by the intrinsic porosity of the ceramic matrix and a second peak superimposed on the top of the first peak which is the result of the macropores.

The catalytically active metal may be one or more nobel metals such a platinum, palladium, rhodium and gold. The ceramic ma-trix may comprises alumina or titania or zirconia or ceria.

The monolith may comprise a ceramic or a metal. The monolith may be a honeycomb monolith comprising a plurality of cells.

The cells may have a cell width, breadth and length and the monolith may have a cell density.

The washcoat may have a thickness in the range of 10 microns to 100 microns. The thickness of the washcoat need not be uni-form throughout the monolith and may vary. For example, the thickness of the washcoat may be greater in corners of a square cell structure than in the centre of the walls defining the cells. This variation can arise as the result of surface tension effects and/or the wetting and drying characteristics of the washcoat solution.

The application also provides a washcoat for a catalyst for treatment of exhaust emissions from an automotive vehicle com-prising at least one catalytically active metal, at least one ceramic, a fugitive material and a solution.

As used herein fugitive material is used to denote a material which can be selectively removed from the applied washcoat whilst the ceramic and the catalytically active metal are sub-stantially not removed. By selectively removing the fugitive material, a plurality of pores can be formed in the ceramic matrix which have a pore diameter corresponding to the fugi- tive material removed. These pores have a pore diameter dis-tribution corresponding to one of the two peaks. Typically, the pores formed by selectively removing the fugitive material are larger than the pore of the intrinsic porosity formed be-tween the grains of the ceramic matrix.

In an embodiment, the fugitive material comprises starch. 1-low- ever, other fugitive materials which can be selectively re- moved may also be used, such as PTFE (polytetrafluoroethyl- ene), graphite, PVB (polyvinylbutyral), PMMA (polymethyl-methacrylate) that can be dispersed in a water-based slurry.

However, any organic or polymeric material may be used as a fugitive material that is insoluble in the liquid media and is available in particulate form having a suitable particle size.

Starch can be used in water-based solutions. water-based solu- tions are desirable as the problems associated with the han- dling and removal of organic-based solvents, such as the pro-vision of adequate ventilation, can be avoided.

The solution may be water-based or organic solvent-based and may include additional components such as dispersants, bind-ers, plasticizers and adhesion promoters.

The ceramic is typically provided in the washcoat in the form of a powder comprising particles having an average particle size about equal to or less than the grain size of the ceramic matrix of the washcoat covering the monolith in the finished catalyst.

The materials of the ceramic and the catalytically active metal can include those materials already described above in connection with the catalyst.

The application also provides a method for producing a cata- lyst for treatment of exhaust emissions from an automotive ve- hicle. A monolith is provided and coated with a washcoat corn-prising at least one catalytically active metal, at least one ceramic, a fugitive material and a solvent. The fugitive mate-rial is then selectively removed to produce a washcoat layer on the monolith comprising the at least one catalytically ac-tive metal, a ceramic matrix comprising a plurality of grains having a grain size distribution and pores each having a di-ameter and a pore diameter distribution. The pore diameter distribution has two peaks.

This method produces a washcoat having two different types of porosity, for example, microporosity as illustrated by a first peak in the pore diameter distribution and macroporosity as illustrated by a second different peak in the pore diameter distribution.

The first peak may be produced by the porosity occurring as a result of the packing and/or sintering of the ceramic grains of the ceramic matrix of the washcoat. The second peak is pro-duced by the selective removal of the fugitive material from the washcoat. The size of the pores created by the selective removal of the fugitive material can be pre-determined by pre- selecting the size of particles of the fugitive material in-cluded in the washcoat.

Furthermore, the density of the pores can be pre-determined by pre-selecting the amount of the fugitive material included in the washcoat. To increase the pore density of the second type of pores, which results in an increase the height of the sec- ond peak compared to that of the first peak in the pore diame-ter distribution, the proportion of the fugitive material is increased.

The grain size distribution may have a single narrow peak that a large number of grains have a grain size within a small range. However, in further embodiments, the grain size distri-bution has a general single broader peak or hump-type shape indicating that the grain size varies more widely.

The washcoat may be applied to the monolith by techniques such as dipping which easily enable the internal surfaces of the monolith to be coated, for example, the internal surfaces of the cells of a monolith with a cellular or honeycomb struc-ture. However, other techniques such as spraying may also be used.

In an embodiment, the washcoat is repeatedly applied onto the monolith. A number of layers of the washcoat is deposited in order to increase the total thickness of the washcoat on the monolith. Such a method may be used if it desired that the washcoat solution has a low viscosity, for example, as a low viscosity tends to result in the deposition of a thinner film when using, for example, dipping techniques.

After applying the washcoat to the monolith, the washcoat on the monolith may be further subjected to a drying treatment.

The drying treatment aims to remove solvent remaining in the applied layer and may be carried out by heating the monolith.

The solvent may be water or an organic solvent, for example.

In an embodiment, the fugitive material is selectively removed during the drying treatment. The fugitive material is, in this embodiment, selected so as to decompose at typical drying tern-peratures, such as 100°C to 250°C.

In a further embodiment, the washcoat on the monolith is sub-jected to a calcination treatment. The washcoat may first be subjected to a drying treatment and afterwards to a calcina-tions treatment. Both treatments can be carried out during a single heat treatment. Typically, a calcination heat treatment takes place at a higher temperature than a drying temperature, for example 350°C to 500°C, and is performed to remove com-pounds which are chemically bound with the ceramic particles of the washcoat. A calcination heat treatment may be carried out to remove carbonates and nitrates from the ceramic parti-

cles, for example.

In an embodiment, the fugitive material is selectively removed during the calcination treatment. The fugitive material is, in this embodiment, selected so as to decompose at typical calci-nation temperatures.

In a further embodiment, the catalyst is further treated with an acid or an alkali or a solvent to selectively remove the fugitive material. This method may be used for fugitive mate-rials which do not easily decompose or evaporate at typical drying and calcinations heat treatment temperatures and condi-tions.

The fugitive material may selectively removed to produce one of the peaks of the pore diameter distribution occurring at a diameter of 3 to 50 microns.

Embodiments will now be described with reference to the accom-panying drawings.

Figure 1 illustrates a schematic view of a catalyst compris-ing a washcoat according to the present invention, Figure 2 illustrates a portion of a monolith to be coated with a washcoat solution, Figure 3 illustrates the monolith of Figure 2 after the depo-sition of the washcoat solution, Figure 4 illustrates the selective removal of a fugitive ma-terial from the washcoat of Figure 3 to produce macropores in the washcoat, and Figure 5 illustrates a schematic view of the grain size dis- tribution of the ceramic matrix and the pore diarne-ter distribution of the washcoat of Figure 1.

Figure 1 illustrates a portion of a catalyst 1 for treating exhaust emissions from an automotive vehicle. The catalyst 1 may be used in a catalytic converter positioned in the exhaust system of an automobile. The catalytic converter may comprise several catalytic converters, each including a catalysts. For example, an automobile may include a primary catalytic con- verter located next to the engine and a main catalytic con-verter located in the underfloor area. The exhaust system may also include a particle filter, particularly in the case of an automotive vehicle with a diesel engine.

The catalyst 1 includes a monolith 2 having a honeycomb struc-ture defining a plurality of cells 3 which, in view of figure 1, each have a generally square cross-section of the same di-mensions. The monolith 2 comprises ceramic in this embodiment.

However, the monolith may comprise metal. The catalyst 1 also includes a washcoat 4 which coats the honeycomb structure of the monolith 2.

The structure of the washcoat 4 is illustrated in the detailed view of Figure 1. The washcoat 4 includes a ceramic matrix 5 comprising alumina made up of a plurality of grains 6 having a grain size and a grain size distribution 14. The washcoat 4 also includes at least one catalytically active metal 7 dis-tributed over the ceramic matrix 5. In this embodiment, the catalytically active metal 7 includes both platinum and rho-dium. However, other catalytically active materials such as nobel metals may also be used.

The grain size distribution 14 is described by a graph of the number of grains of a particular size as a function of the grain size and is schematically illustrated in Figure 5. The grain size distribution 14 of this embodiment has a single peak 15 illustrating that a large number of grains have a grain size within a small range. However, in further non- illustrated embodiments, the peak 15 is much broader indicat-ing that the grain size varies more widely.

The washcoat 4 also comprises a plurality of pores 8 each hay- ing a diameter. The pores 8 also have a pore diameter distri-bution 16 described by a graph of the number of pores of a particular size as a function of pore size. The pore diameter distribution 16 is illustrated schematically in Figure 5 and has two peaks 17, 18 spaced apart from one another in contrast to the grain size distribution 14 which has a single peak 15.

The first peak 17 is produced by the first plurality of macro-pores 9 having an average diameter in the range of 3 microns to 50 microns. The washcoat 4 also includes a second plurality of micropores 10 having an average diameter in the range of 10 nanometres to 200 nanometres. The second peak 18 in the pore diameter distribution 16 is produced by the micropores 10.

However, the two peaks 17, 18 of the pore diameter distribu-tion may not be spaced quite separately from one another in the pore size distribution, but may be superimposed. In a fur- ther embodiment, the pore size distribution 16 includes a sin- gle broad peak 17 caused by the intrinsic porosity of the ce-ramic matrix and a second peak 18 superimposed on the top of the first peak 17 which is the result of the macropores 10.

The micropores 10 occur as the result of pores forming between contiguous ceramic particles and, in the case of a heat treated ceramic matrix, between the ceramic grains 6. The mi- cropores can be thought of as being the intrinsic microporos-ity of the ceramic matrix 5.

The macropores 9 on the other hand can be thought of as ex-trinsic macroporosity since, according to present application, these are formed by the selective removal of a fugitive mate-rial from the washcoat 4 after the washcoat 4 is applied to the monolith 2.

The washcoat 4 has a thickness in the range of 10 microns to microns. The thickness may not be uniform throughout the monolith 2. As illustrated in Figure 1, in this embodiment, the thickness of the washcoat 4 is greater in the corners of the square cells 3 than in the centre of the straight sides defining the cell 3.

A fugitive material such as starch may be used which is easily selectively removed over the alumina ceramic matrix 5. The size of the macropores 9 as well as the density of these pores may be controlled by pre-selecting the proportion of fugitive material included in the washcoat 4 and the size of the parti- cles of fugitive material provided in the washcoat 4. By in-creasing the proportion of fugitive material the density of macropores 9 may be increased. Similarly, if larger pores are desired, larger particles of fugitive material may be pro-vided. The macropores 9 have an average pore diameter in the range of 3 microns to 50 microns. This size range enables the macropores to be retained during calcination of the ceramic matrix 5 so as to provide a stable macropores 9.

The microporosity and macroporosity provided by pores 9, 10 is independent of the grain size of the ceramic matrix 5. Two different types of grain size are no longer required. Since the grain size distribution of the ceramic matrix 5 has a sin-gle peak, the ceramic matrix 5 has an improved mechanical strength since the ceramic matrix is uniform and homogeneous.

The efficiency of the catalytic conversion of exhaust ernis- sions can be increased since the macropores 9 provide an in-creased surface area which promotes an improved mass transfer of the exhaust gases to be treated into the washcoat 4.

The catalyst 1 illustrated in figure 1 may be fabricated using the following method. Figure 2 illustrates a monolith 2 and a washcoat solution 11 which comprises at least one catalyti- cally active metal 7, least one ceramic 6, the least one fugi-tive material 12 and a solvent 13. The catalytically active metal 7, ceramic 6 and fugitive material 12 may be present in the form of discrete particles in the solution 11. The solu-tion 11 may also include further additives such as binders, dispersants and so on so as to provide a washcoat solution 11 having a uniform dispersion of the particle components. In this embodiment, the fugitive material 12 is starch, the solu- tion 13 is water, the ceramic 6 is alumina and the catalyti-cally active metal includes platinum and rhodium.

The monolith 2 is then coated with at least one layer of the washcoat solution 11, Figure 3, and afterwards, the fugitive material 12 is selectively removed from the deposited washcoat layer 4, schematically depicted by the arrows in Figure 4, to provide a plurality of pores 9 in the remaining washcoat layer 4 arranged in positions from which the fugitive material 12 has been selectively removed. After removal of the fugitive material 12, the wash coat layer 4 now comprises a ceramic ma-trix 5 comprising a plurality of grains 6 having grain size distribution 14 with a single peak 15 as well as pores 8 which have a pore diameter distribution 16 having two peaks 17, 18 corresponding to macropores 9 and micropores 10, respectively, illustrated in Figure 1.

In addition to the macropores 9 produced as a result of the selective removal of the fugitive material 12, the ceramic ma-trix 5 also includes micropores 10 arranged between adjacent grains 6 and arising as a result of the non-optimum packing and sintering of the ceramic particles 7.

After coating the monolith 2, the deposit washcoat layer 4 may be dried by heat treating the monolith at a temperature in the range of 1000 C to 200°C, for example. The washcoat layer 4 may also be calcined to remove carbonates and nitrates chemi-cally bound to the ceramic particles 6. Calcination may be carried out at temperatures in the range of 400°C to 600°C, for example. The fugitive material 12 may be removed in one or more of the drying treatment and calcination treatment.