Method for the manufacture of a catalytic filter

Abstract

A method for the manufacture of a catalytic filter for an exhaust gas treatment system comprises: i) providing a porous filter body; ii) providing a washcoat comprising: (a) one or more platinum group metals (PGM); (b) a support material; and (c) a surfactant; iii) coating the filter body with the washcoat to form a coated filter body; and iv) calcining the coated filter body to form the catalytic filter, and wherein the surfactant is a non-ionic, silicon-free, organic surfactant.

Description

[0001] METHOD FOR THE MANUFACTURE OF A CATALYTIC FILTER

[0002] The present invention relates to a method for the manufacture of a catalytic filter suitable for use in an exhaust gas treatment system, such as for an automobile. In particular, the method addresses issues arising from treatments now sometimes added to filter substrates to improve their filtration performance and operational lifetime, without compromising the benefits of those treatments.

[0003] The provision of filter components is well-known in an exhaust gas treatment system. These take the form of a so-called wall-flow filter. A wall-flow filter is comprised of a porous material substrate having a plurality of longitudinally extending channels therein. Typically, half of these are blocked at an inlet end and the other half are blocked at the outlet end. As a consequence, exhaust gases passing into the open inlet channels are forced to pass through the porous walls into the outlet channels to leave the filter. As a consequence, soot and other particulate matter can build up on the filter wall. The simplest diesel particulate filter (DPF) is formed simply from the porous material, but more modern advances have led to the provision of catalytic coatings on the filter body. These can help to catalyse soot combustion to regenerate the filter or can be present to have other catalytic benefits on components in the exhaust gases. Other well-known filter types include: catalysed soot filters (CSF), gasoline particulate filters (GPF) and selective catalytic reduction filters (SCRF). All of these are well known in the art and have characteristic formulations and components.

[0004] In recent years there have been some developments in providing coatings on filter substrates to improve their filtration performance and longevity. At least some of these coatings are applied to filter inlet channels to form a surface layer which enhance the filtration activity of the underlying porous channel walls. These coatings may comprise a silica-based material and may serve to reduce the interaction of the filter part with components in the exhaust gases.

[0005] WO2014137827 discloses catalyst washcoats with improved porosity. The process comprises incorporation of an oil-in-water macroemulsion into the catalyst slurry prior to washcoating the carrier substrate, and calcining the washcoated carrier substrate to remove the oil-in-water macroemulsion. The O / W emulsion is formed and stabilised with an organic surfactant which, like the oil, is calcined away in the final step. Removal of the oil by calcining helps to form the porous structure. US9687786 discloses a diesel particulate filter capable of removing soot from an exhaust gas while operating at low backpressure, the filter comprising (a) a wall-flow filter substrate having a mean pore size, an inlet side, an outlet side, and a porous interior between the inlet and outlet sides; and (b) a catalyst composition coated from the inlet side of the substrate. The catalyst composition has a D50 particle size distribution which is less than the mean pore size divided by 4.9 and the outlet side is substantially free of a catalyst coating.

[0006] DE102007002903 discloses the use the surfactant as foaming aids in a catalytic washcoat suspension for forming an automotive catalytic converter. The surfactant is then associated with gassing or degassing steps.

[0007] LIS2019 / 299139 discloses an exhaust gas purification filter that suppresses an increase in pressure loss associated with the formation of a catalyst layer and has a good PM burning quality.

[0008] US2004 / 176246 discloses catalysing filters and methods for placing catalyst onto filter media, which involve positioning the catalyst at locations on the filter media where materials to be catalysed will make contact during the filtering process.

[0009] JP2002361099 discloses a method of manufacturing an inorganic fiber catalyst which sufficiently exhibits catalyst activity by uniformly depositing a catalyst activity component on an inorganic fibre structure.

[0010] WO2012099868 discloses exhaust systems and components suitable for use in conjunction with gasoline engines to treat gaseous emissions such as hydrocarbons, nitrogen oxides, and carbon monoxides.

[0011] US2022 / 258137 discloses a catalyst article is provided including a substrate including a plurality of passageways, and further including a first and a second oxidation region including a first and a second subset of said plurality of passageways.

[0012] Applicant’s WO 2023 / 026022A1 discloses a method of forming an inorganic oxide coating on a monolith article. The coated monolith article is suitable for the treatment of an exhaust gas. The method comprises spraying, as a dry particulate aerosol, inorganic particles and a silicone resin to form a coating layer. There is a need to address issues with washcoat provision with porous substrates which have been treated with hydrophobic treatments, or to tackle problems associated with the prior art, or at least to provide an alternative thereto.

[0013] According to a first aspect there is provided a method for the manufacture of a catalytic filter for an exhaust gas treatment system, the method comprising: i) providing a porous filter body; ii) providing a washcoat comprising:

[0014] (a) one or more platinum group metals (PGM);

[0015] (b) a support material; and

[0016] (c) a surfactant; iii) coating the filter body with the washcoat to form a coated filter body; and iv) calcining the coated filter body to form the catalytic filter, and wherein the surfactant is a non-ionic, silicon-free, organic surfactant.

[0017] The present disclosure will now be described further. In the following passages different aspects / embodiments of the disclosure are defined in more detail. Each aspect / embodiment so defined may be combined with any other aspect / embodiment or aspects / embodiments unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. It is intended that the features disclosed in relation to the method may be combined with those disclosed in relation to the use and vice versa.

[0018] The inventors discovered that some of their products were having problems with an applied washcoat slurry failing to have sufficiently strong interaction with the substrate and this led to difficulties in scaling-up prototype production and poor catalytic performance of the final product. For example, there was a loss of washcoat slurry control, wherein a significant portion of the washcoat slurry dose ran along the length of the substrate channels without passing into the porous channel walls. On further investigation it was found that these problems were linked to the modern pre-treatments applied to the substrates discussed above, e.g. pre-coating filter inlet channels with a surface layer to enhance the filtration activity of the underlying porous channel walls. The inventors found that the pre-treatments had a consequential increase in the hydrophobicity of the part.

[0019] The inventors contemplated various solutions to the washcoat slurry application problems. It was considered whether the water content of the washcoat could be reduced to minimise the water interaction with the inherent hydrophobicity of the substrate. This led to a thicker, more viscous washcoat and a relative increase in washcoat solids. Similarly, they looked at the inclusion of rheology modifiers, such as gums, to increase the washcoat viscosity, without necessarily adjusting washcoat solids. While either approach could improve the ability to control the flow of the washcoat, and therefore its location, along the length of the substrate channels, the ability to infiltrate washcoat solids in-wall, i.e. laterally, was consequently reduced. The higher on-wall washcoat solids led to higher back-pressure and inhibited the desired location of PGMs in-wall.

[0020] Other solutions contemplated included additional processing of the substrate to reduce its apparent hydrophobicity. However, this was found to undermine the very advantages associated with the applied pre-treatment. One approach included a pre-calcining step but this was found to increase the expense of production and reduced capacity in the production plant as parts needed increased process steps to manufacture. Moreover, the resulting product had reduced performance in that the effectiveness of the initially applied pretreatment was compromised.

[0021] In the end the inventors found that careful selection of specific surfactants could be used to address the problem without necessarily adjusting washcoat solids. Although it is known to use some surfactants, it is not conventional to add surfactants to washcoat compositions. This is because it is highly preferred not to include additives in washcoat slurries that do not have technical benefit. Inclusion of unnecessary additives increases washcoat cost but also can negatively affect the final calcined washcoat catalytic activity, e.g. by influencing the location of the PGMs in the final product. Indeed, these adverse effects were seen for certain surfactants and certain surfactant concentrations in the inventors’ testing. It is therefore considered particularly unusual to use surfactants when preparing a PGM-containing washcoat for a filter.

[0022] The inventors have found that with careful selection of the surfactant it is possible to address the issues that have arisen. This meant that they could arrive at catalysed filters which benefitted both from the pre-treatment of the substrate to improve its filtration performance and at the same time maintaining the catalytic activity of the catalyst coatings applied.

[0023] The method relates to the manufacture of a catalytic filter for an exhaust gas treatment system. A catalytic filter is a component of an exhaust gas treatment system where the exhaust gases to be treated are passed through a filter body. The filter body serves to remove particulate matter. The catalytic filter has catalyst properties due to the application of the washcoat which contains PGM materials. The method comprises a step (i) of providing a porous filter body. Preferably the porous filter body is a wall-flow filter. The porous filter body can be a ceramic, e.g., silicon carbide, cordierite, aluminium nitride, silicon nitride, aluminium titanate, alumina, mullite, pollucite, or composites comprising segments of any two or more thereof. Preferably the porous filter body is composed of Silicon Carbide (SiC), Cordierite or Aluminium Titanate. Preferably the porous filter body has a hydrophobic coating applied thereto, and preferably the method further comprises applying a hydrophobic surface coating to the porous filter body before step (iii). It is preferred that the hydrophobic coating is only applied to the surface of the inlet channels of the wall-flow filter.

[0024] In order to determine whether the porous filter body would particularly benefit from the present method, it is useful to assess whether it has a hydrophobic nature or has had a hydrophobic treatment applied. Preferably the porous filter body provided in step (i) has a water contact angle of >90° when measured with the Washburn uptake method. The method is well known in the art and a good summary is available at https: / / www.kruss- scientific.com / en / know-how / glossary / washburn-method. In relation to the testing of the porous filter body, a core is taken from the substrate and held with a special clip to a force tensiometer, such as a Kruss Tensio, and comes into contact with a test liquid, the liquid is drawn up into the substrate and the change in mass is determined with respect to time. The first test liquid used is one with optimal wetting where the assumption is made that the contact angle is 0° (e.g., n-heptane) to determine the Washburn constant, c. Once the Washburn constant c has been found the contact angle of other test liquids, which have a known surface tension, viscosity and density, can be determined. Contact angles of 90° and above cannot be measured using this method as no wetting takes place, so the absence of a value for a contact angle for water on a substrate indicates the contact angle is > 90° and that the substrate is considered hydrophobic. This is not satisfied by conventional untreated filter substrates and, indeed, a conventional SiC or cordierite substrate wets so readily that it is difficult to measure the value which will be close to zero.

[0025] Preferably the hydrophobic surface coating is applied to inlet channels of the porous filter body, preferably only to the inlet channels. The washcoat is then applied to the inlet and / or outlet channels. The length and extent of each washcoat coating will depend on the catalyst performance that is desired. Applying the washcoat coating to the outlet channels helps to reduce the initial hydrophobic and repelling nature of the substrate as it encounters the washcoat coating. It should be noted that although the washcoat coatings are applied to the inlet or outlet channels, the material forming the washcoat coating may to some extent diffuse into and through the pores of the porous body. It is preferred that the total length of washcoat coatings applied to the inlet and / or outlet channels total at least 100% of a longitudinal length of the porous filter body. That is, if there is only an outlet coating then preferably it is 100% of the longitudinal length of the porous filter body.

[0026] A preferred embodiment provides an inlet washcoat coating that extends along 50 to 90% of the longitudinal length of the porous filter body from the inlet end thereof and an outlet coating that extends along 50 to 10% of the longitudinal length of the porous filter body from the outlet end thereof. A more preferred embodiment provides an inlet coating that extends along 70 to 80% of the longitudinal length of the porous filter body from the inlet end thereof and an outlet coating that extends along 30 to 20% of the longitudinal length of the porous filter body from the outlet end thereof.

[0027] The method comprises a step of (ii) providing a washcoat. Methods of forming and applying washcoats are well known in the art and any suitable method may be employed with this invention. Typically, the composition is an aqueous washcoat and the ingredients are added and dispersed before the coating step. The washcoat may contain conventional additives such as thickeners, acidity adjusters and the like. Preferably the surfactant is added as a final ingredient to minimise the opportunity for it to affect the PGMs in the washcoat.

[0028] The washcoat comprises one or more platinum group metals (PGM). Preferably the PGMs are present in a total amount such that the final coating contains at least 1 gft-3. Depending on the catalyst product being formed and the end application, the amount of PGMs will vary. Nonetheless, it is critical that there are PGMs present, since otherwise there would be no constraints on the addition of the surfactant as described herein. Preferably the washcoat contains the PGMs in a total amount such that the final coating contains from 1 gft-3to 100gft’3, more preferably 1 gft-3to 5gft-3especially for a CSF, and 25gft-3to 50gft'3especially for a GPF. The PGMs will be present in the washcoat as ions, such as nitrate ions. Suitable precursors of Pt and Pd include salts containing these metals, e.g., platinum nitrate and palladium nitrate.

[0029] The washcoat comprises a support material. Support materials are well known in the art and the specific nature of the support material is not critical, depending on the application. Suitable support materials include silica, alumina, ceria, ceria-zirconia and the like. Preferably the support material comprises alumina. One or more support materials may be present. The support such as alumina can be doped with a dopant. The dopant can be selected from the group consisting of La, Sr, Si, Ba, Y, Pr, Nd, Ce, and mixtures thereof. Preferably, the dopant is La, Ba, or Ce. Most preferably, the dopant is La. The dopant content in the inorganic oxide support can be from 1 to 30 wt%, preferably from 2 to 25 wt%, more preferably from 3 to 20 wt%.

[0030] The washcoat comprises a surfactant. The surfactant is a non-ionic, silicon-free, organic surfactant. The surfactant has to be non-ionic, since this minimises interactions with the other components of the washcoat. Non-ionic means that the surfactant contain no charge, as distinct from cationic or anionic surfactants. Commercial surfactants are clearly identified as to whether they are non-ionic or not.

[0031] It has to be silicon-free, since the inventors found that silane-containing surfactants have the potential to react adversely with the washcoat components, particularly affecting PGM distribution, affecting catalytic activity.

[0032] The surfactant has to be organic so that it is fully removed by a calcining step. That is, there are no inorganic parts to the surfactant that would remain as a residue or contaminant in the calcined catalyst.

[0033] Preferably the surfactant comprises one or more acetylenic diol surfactants. Preferably the one or more acetylenic diol surfactants are one or more alkoxylate acetylenic diol surfactants. More preferably the one or more alkoxylate acetylenic diol surfactants are one or more ethoxylated acetylenic diol surfactants. More preferably the one or more ethoxylated acetylenic diol surfactants are one or more ethoxylated tetramethyl dodecyne diol surfactants. It is most preferred that the surfactant comprises one or more ethoxylated acetylenic diol surfactant represented by the formula: wherein m and n are integers and the sum is from 1-15, preferably 4-12. Suitable surfactants and their structure and production are discussed in US5650543, the content of which is incorporated herein by reference.

[0034] Of the classes of surfactant discussed above, a preferred surfactant is Dynol-607™ which is a so-called superwetting surfactant. This meets the requirements outlined herein as preferred and overcame the problems with the washcoating process without compromising the properties of the hydrophobically-treated substrates. The term “superwetting” denotes a surfactant that achieves a low equilibrium surface tension (i.e. it is effective at reducing surface tension) and achieves a low dynamic surface tension (i.e. it works quickly and penetrates through the washcoat very quickly, even with limited mixing).

[0035] The inventors also considered other surfactants, such as Tego Twin 4200, but this has a silica functionality with possible interactions within the washcoat, e.g. during calcination. Another, non-ionic surfactant was BYK-349 which also contains siloxane functional groups but this turned the PGM washcoat a different colour; a sure sign that there is some kind of PGM-surfactant interaction taking place.

[0036] Desirably the surfactant is non-reducing with respect to the Pt and / or Pd ions present in the washcoat (and Rh ions, if Rh is present). That is, preferably the PGMs are included in the washcoat as ions and the surfactant is not capable of reducing those ions, in particular to form the metals. Desirably the surfactant has a reduction potential as measured by cyclic voltammetry, measured under standard conditions, which is not sufficient to reduce the Pt and / or Pd ions present in the washcoat. Methods of measuring reduction potentials using cyclic voltammetry are well known in the art and reference values are available for many materials. This ensures that there is no reaction between the surfactant and the PGMs.

[0037] Preferably the surfactant exhibits an equilibrium surface tension of less than 30 mN / m when measured using the Wilhelmy plate method (using a platinum plate) on a force tensiometer. A platinum plate is used due to its low surface energy (gives a contact angle of 0° with most liquids). The force tensiometer lowers the platinum plate into the liquid and the force is measured. The surface tension, o, of the liquid can be calculated according to the equation o = F / (L.cos0), where F = the force, L = the wetted length of the platinum plate and 0 = the contact angle of the liquid on the plate (which is known to be 0). The surface tension <30 mN / m is in comparison to water which is around 72.8 mN / m at 20 °C. This is a measure of the ability of the surfactant to improve the surface-wetting ability of the washcoat. Preferably the surfactant exhibits a dynamic surface tension of less than 30 mN / m (when measured by a bubble pressure tensiometer, e.g. a Kruss BP100). A gas bubble is formed through a capillary in the liquid under investigation and the maximum internal pressure of that bubble is measured. The dependence of surface tension on surface age can be measured by varying the speed at which the bubbles are produced. A dynamic surface tension of less than 30mN / m ensures that the surfactant quickly migrates to the surface of the washcoat / newly formed interfaces, improving the speed at which coating can be performed and the product consistency.

[0038] The amount of the surfactant will vary depending on the specific surfactant used, but it preferably from 0.01 to 0.5wt% of the washcoat, preferably 0.05 to 0.35wt%, preferably 0.1 to 0.2wt%. The amount of surfactant can be determined by testing and it is desirable to use as little as possible to avoid disrupting the PGMs. At the same time, as the amount is added there is generally a point where further addition does not have a further benefit which is typically just after the critical micelle concentration (CMC) of the surfactant has been achieved. The critical micelle concentration is defined as the concentration at which the surface (or the air-liquid interface) is completely saturated by surfactant molecules, such that micelles form in the bulk. This can be determined by a series of surface tension measurements at increasing surfactant concentration (using the Wilhelmy method on a force tensiometer). Above the CMC, the static surface tension becomes independent of the surface tension. It is desirable that the amount of surfactant used exceeds this threshold to ensure that the surfactant effect is consistent within and between coating applications, but it is equally desirable that it does not significantly exceed this point, since it would then risk PGM-interactions.

[0039] Preferably the washcoat consists of the components (a)-(c) and water, as well as less than 5wt% total of additives, preferably less than 2wt% total of additives. Preferably the additives comprise one or more of rheology modifiers, ammonia and pH adjusters. Rheology modifiers are discussed, for example, in LIS9144796. Suitable examples of rheology modifiers include polymers such as long chain polysaccharides, polyethylene glycol derivatives (PEGs) and acrylic polymers. The washcoat slurry may comprise an organic pore former. Examples of pore formers include cellulose, polyethylene, starch, graphite, polypropylene, polyaramides, polytetrafluoroethylene, polystyrene, cellulose fibres and polymethacryl-methacrylate, e.g., Arbocel, Vivapur, Mipelon PM-200, Propyltex, Orgasol and Remyrise.

[0040] The method comprises a step (iii) of coating the filter body with the washcoat to form a coated filter body. Preferably the washcoat is coated in step (iii) on and into outlet channels of the porous filter body, preferably only to the outlet channels. As discussed above, this has a synergistic benefit when a prior hydrophobic treatment has been applied to the inlet surface. The treatment on the inlet surface minimises interaction between the filter body and the exhaust gases (especially moisture) and enhances the filtration performance and longevity of the part. Meanwhile, coating the washcoat on the outlet channels provides the catalytic performance and, because of the addition of the surfactant, it can still penetrate into and adhere to the porous body.

[0041] The method comprises a step (iv) of calcining the coated filter body to form the catalytic filter. This serves to both dry and calcine the inorganic washcoat components. At the same time, the surfactant and any other carbon-based components are oxidised away. Calcining may be preceded by a drying step at a lower temperature (such as 100 to 200°C). Calcining is routine in the art and may be performed under usual conditions.

[0042] Preferably the catalytic filter is a Gasoline Particulate Filter (GPF) and wherein the washcoat further comprises an oxygen storage compound (OSC), preferably a ceria or ceria-zirconia OSC, and wherein the one or more PGMs comprise Pd and Rh. When forming a GPF, preferably the washcoat has a solids content of 25 to 40wt%.

[0043] Preferably the catalytic filter is a Catalysed Soot Filter (CSF) and wherein the one or more PGMs comprise Pt and optionally Pd. When forming a CSF, preferably the washcoat has a solids content of 1 to 10wt%. A CSF typically has a lower requirement for catalyst material than a GPF so the washcoat contains lower solids.

[0044] Preferably the method does not comprise forming an emulsion in the washcoat before coating in step (iii). The purpose of the surfactant is to change the surface tension of the washcoat and improve its adhesion, rather than to stabilise an emulsion.

[0045] Preferably the method does not comprise foaming the washcoat before coating in step (iii). The purpose of the surfactant is to change the surface tension of the washcoat and improve its adhesion, rather than to stabilise a foam. There is no foaming step since this would actively hinder the application of the washcoat into the pores of the porous body.

[0046] Preferably the method does not comprise degassing the washcoat before coating in step (iii). Degassing is a complex additional step which is not required in the present method. According to a further aspect there is provided the use of a non-ionic, silicon-free, organic surfactant to improve washcoat adhesion of a PGM-containing washcoat to a porous filter body which has a water contact angle of >90° when measured with the Washburn uptake method. All aspects of the method disclosed herein can also be applied to this use.

[0047] EXAMPLES

[0048] The invention will now be described further in relation to the following illustrative examples. A range of catalytic filters were produced and tested. The porous wall-flow filter body was a 143.8 mm diameter x 152.4 mm length dimension cylindrical part. All catalytic washcoat coatings were particulate alumina and 2g / ft3Pt only, i.e. no Pd included or (1 :0), at 0.1 g / in3total washcoat loading. A wall-flow filter body coated with a catalytic washcoat is known as a “catalysed soot filter” or “CSF”. The EFC coating coated at a 12 g / L loading is disclosed in Applicant’s WO 2023 / 026022A1 and is derived from a 50:50 mixture by weight of a particulate aluminosilicate CHA zeolite and a highly cross-linked ethoxylated poly(dimethyl siloxane) powder. The calcined EFC coating is hydrophobic.

[0049] Table 1 All calcination was done in air in a static oven at 500°C.

[0050] Laboratory Synthetic Catalytic Activity Test (SCAT) testing was then performed on 1 inch (2.54cm) diameter cores cut from the coated filter bodies. Testing was performed on an as- manufactured “fresh” product, a “conditioned” fresh product and an “aged” fresh product, under the following test conditions:

[0051] • “Fresh”: Ramped light-off (LO) test, without pre-conditioning. Each core was stabilised for 10 min at an inlet gas temperature of 150°C in the ramped light-off gas mix test described below before commencing the light-off temperature ramp itself at a ramp rate of 1°C per minute. The gases exiting the sample were analysed to determine conversion of the inlet constituents;

[0052] • “Conditioned”: Conditioning of a fresh sample was done on the SCAT apparatus itself by firstly exposing the sample to 100% N2 and then raising the inlet gas temperature at a rate of 30°C per minute to 500°C and then switching the inlet gas to the composition of the ramped LO gas mix described below for 15 minutes for 500°C. The “conditioned” core was then stabilised for 10 min at 150°C in the ramped light-off gas mix before commencing the light-off temperature ramp itself, as described for the “Fresh” sample above; and

[0053] • “Aged”: A “fresh” sample was aged at 500°C for 200hrs in air in a static oven. The “aged” sample was then conditioned and tested according to the same protocol described above for the “Conditioned” sample.

[0054] • Ramped light-off (LO) gas mix: Propene 10ppm (C3), Propane 10ppm (C3), Toluene 4.3ppm (C3), n-decane 3ppm (C3), CO 60 ppm, NO 500 ppm, H2O 5%, CO25%, O212%, Balance N2

[0055] • Space Velocity = 25000 IT1

[0056] NO oxidation activity for each sample is reported in Table 2, which clearly show differences in oxidation activity between the tested samples. A goal of this experimentation is to achieve oxidation activity for CSF coated filter bodies including an EFC coating that are as close as possible to the reference CSF-coated filter body sample without an EFC coating.

[0057] The catalytic performance of the hydrophobically-treated parts coated with 0.1wt% and 0.4wt% Dynol-containing washcoats was substantially identical to the reference part (non- EFC hydrophobic treatment). Performance of all parts was significantly better than the part where the hydrophobic EFC treatment was applied on top of the catalytic washcoated part (reference). It was found that the activity of the sample where the catalytic washcoat including the PVP additive applied on top of the hydrophobic EFC coating does not match that of the samples including the Dynol additive. The NO oxidation activity of the PVP example is slightly better than the surfactant-free comparative example under the fresh and the “conditioned” conditions and the substantially the same as when aged, but does not reach that of the reference without the EFC hydrophobic coating.

[0058] Table 2 The findings of the testing are summarised as follows:

[0059] • Coating of the hydrophobic EFC coating on top of a CSF coating (reference) has a big impact on oxidation activity, as illustrated by NO oxidation. CSF washcoat (WC) (not containing surfactant additive) coated on top of the EFC coating (comparative) shows lower NO2 / NOX than the non-EFC coated reference.

[0060] • A detrimental effect on oxidation activity is also seen for CO oxidation for the reference sample where the hydrophobic EFC coating is coated on top of the CSF WC relative to the non-EFC coated reference. Fresh CO oxidation for the CSF WC applied on top of the hydrophobic EFC coating (no-surfactant comparative example) is initially slightly worse than the non-EFC coated reference, but CO oxidation activity improves to become identical for both samples when both are aged.

[0061] • A detrimental effect on oxidation activity is also seen for HC oxidation for the reference sample where the hydrophobic EFC coating is coated on top of the CSF WC relative to the non-EFC coated reference. Fresh HC oxidation for the CSF WC applied on top of the hydrophobic EFC coating (no-surfactant comparative example) is initially slightly worse than the non-EFC coated reference, but HC oxidation activity improves to become identical for both samples when both are aged.

[0062] • Addition of PVP into the CSF washcoat coating improves the conditioned NO2 / NOXoxidation activity slightly relative to the no-surfactant comparative example, but still shows a gap in performance to the reference non-EFC coated CSF sample. CO oxidation improves with PVP addition from fresh to aged. No particular benefit is seen for PVP addition for HC oxidation activity, either fresh or aged.

[0063] • Addition of Dynol improves the NO2 / NOXto the level of the reference non-EFC coated sample. However, 0.4wt% Dynol shows slightly lower aged NO2 / NOXoxidation activity, possibly suggesting that there is too much Dynol present.

[0064] • Addition of Dynol results in CO oxidation activity which is very similar to the non-EFC coated reference. Again, the 0.4wt% Dynol coating shows slightly lower CO oxidation activity than the 0.1 wt% Dynol sample.

[0065] • Addition of Dynol, whether 0.1wt% addition or 0.4wt% addition, results in HC oxidation activity which is very similar to the non-EFC coated reference.

[0066] It should be appreciated that the testing of the Dynol addition shows that the catalytic oxidation performance of the part matches the non-EFC coated CSF reference part. This means that there is substantially no loss in oxidation activity performance and the parts according to the invention benefit from the additional filtration improvements provided by the EFC coating - namely improved lower back-pressure and better filtration performance.

[0067] Filtration efficiency testing

[0068] A filtration efficiency test was done using the apparatus and method described in EP1850068 and which is available for purchase from Cambustion®, Cambridge, UK as the “DPG Particulate Filter Testing System” (see https: / / www.cambustion.com / products / engine- exhaust-emissions / dpg-particulate-filter-testing-system), i.e.:

[0069] (i) an apparatus for generating and collecting particulate matter derived from combusting a liquid carbon-containing fuel, which apparatus comprising a fuel burner comprising a nozzle, which nozzle is housed in a container, which container comprising a gas inlet and a gas outlet, said gas outlet connecting with a conduit for transporting gas from the gas outlet to atmosphere, means for detecting a rate of gas flowing through the gas inlet and means for forcing an oxidising gas to flow from the gas inlet via the container, the gas outlet and the conduit to atmosphere, a station for collecting particulate matter from gas flowing through the conduit and means for controlling the gas flow-forcing means in response to a detected gas flow rate at the gas inlet, whereby the rate of gas flow at the gas inlet is maintained at a desired rate to provide sub-stoichiometric fuel combustion within the container, thereby to promote particulate matter formation; and

[0070] (ii) a method of generating and collecting particulate matter derived from combusting liquid carbon-containing fuel in an oxidising gas, which method comprising burning the fuel in a sub-stoichiometric quantity of oxidising gas in a fuel burner, said fuel burner comprising a nozzle, which nozzle being housed in a container, forcing an oxidising gas to flow from a gas inlet to the container to atmosphere via a gas outlet to the container and a conduit connected to the gas outlet, collecting particulate matter at a station located within the conduit, detecting a rate of oxidising gas flow at the gas inlet and controlling the rate of oxidising gas flow so that a desired rate of oxidising gas flow is maintained at the gas inlet.

[0071] The filter was inserted in the station for collecting particulate matter from gas flowing through the conduit. The fresh filter was first pre-conditioned at an air flow rate 80 kg / hr in a leanburn combustion stream using low sulphur diesel fuel (10 ppm S) to raise the filter inlet temperature to 650°C, a temperature that is typically used on a vehicle to regenerate a soot loaded filter. This pre-conditioning step temperature was well above the soot combustion temperature and was to ensure that the filter on test is clean at the outset. Pressure sensors disposed upstream and downstream of the station monitor the backpressure across the filter. The test was carried out at a filter inlet temperature of 250°C at an air flow rate of 180 kg / hour combusting low sulphur diesel fuel (10 ppm S).

[0072] Filtration efficiency testing was conducted on the DPG Particulate Filter Testing System as follows. At a constant flow rate, an initial test detected the second-by-second particulate number (PN) upstream of the filter using a PN counter. The upstream PN raw data was averaged (mean) across a 11 second detection period to create second-by-second moving 11 second average window to reduce the effect of any variability in PN generation, with time “t” corresponding to the midpoint of the 11 second window. Sample filters were then tested by detecting the post-filter PN, also using a PN counter. The instantaneous filtration efficiency at time “t” in the test is given by the following calculation:

[0073] FEt[%] = ([Mean upstream PN at time t - Downstream PN at time t] I [Average upstream PN at time t]) x 100% The results of the filtration efficiency analysis are presented in Table 3.

[0074] Table 3

[0075] This testing showed that the combination of the EFC (Hydrophobic treatment) and the inclusion of a suitable surfactant lead to the provision of an optimised part, i.e. a part that had a combination of both an improved filtration performance resulting from the treatment

[0076] (better with Dynol than with PVP); and improved catalytic oxidation performance (i.e. the effective use of the PGMs), which matched that of a leading reference product (i.e. CSF WC without the hydrophobic EFC coating).

[0077] All percentages herein are by weight unless otherwise specified. The foregoing detailed description has been provided by way of explanation and illustration and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art and remain within the scope of the appended claims and their equivalents. For the avoidance of doubt, the entire contents of all documents acknowledged herein are incorporated herein by reference.

Claims

CLAIMS:

1. A method for the manufacture of a catalytic filter for an exhaust gas treatment system, the method comprising: i) providing a porous filter body; ii) providing a washcoat comprising:(a) one or more platinum group metals (PGM);(b) a support material; and(c) a surfactant; iii) coating the filter body with the washcoat to form a coated filter body; and iv) calcining the coated filter body to form the catalytic filter, and wherein the surfactant is a non-ionic, silicon-free, organic surfactant.

2. The method according to claim 1 , wherein the porous filter body provided in step (i) has a water contact angle of >90° when measured with the Washburn uptake method.

3. The method according to claim 1 or claim 2, wherein the porous filter body is composed of Silicon Carbide, Cordierite or Aluminium Titanate, and wherein the method further comprises applying a hydrophobic surface coating to the porous filter body before step (iii).

4. The method according to claim 3, wherein the hydrophobic surface coating is applied to inlet channels of the porous filter body, preferably only to the inlet channels.

5. The method according to any preceding claim, wherein the washcoat is coated in step (iii) to inlet channels of the porous filter body, preferably to both inlet and outlet channels.

6. The method according to any preceding claim, wherein the support material comprises alumina.

7. The method according to any preceding claim, wherein the catalytic filter is a Gasoline Particulate Filter (GPF) and wherein the washcoat further comprises an oxygen storage compound (OSC), preferably a ceria or ceria-zirconia OSC, and wherein the one or more PGMs comprise Pd and Rh.

8. The method according to claim 7, wherein the washcoat has a solids content of 25 to 40wt%.

9. The method according to any preceding claim, wherein the catalytic filter is a Catalysed Soot Filter (CSF) and wherein the one or more PGMs comprise Pt and optionally Pd.

10. The method according to claim 9, wherein the washcoat has a solids content of 1 to 10wt%.

11. The method according to any preceding claim, wherein the washcoat consists of the components (a)-(c) and water, as well as less than 5wt% total of additives, preferably less than 2wt% total of additives.

12. The method of claim 11, wherein the additives comprise one or more of rheology modifiers, ammonia and pH adjusters.

13. The method according to any preceding claim, wherein the method does not comprise before coating in step (iii):(i) forming an emulsion in the washcoat;(ii) foaming the washcoat; or(iii) degassing the washcoat.

14. The method according to any preceding claim, wherein the surfactant:(I) is non-reducing with respect to Pt2+and Pd2+ions; and / or(II) exhibits an equilibrium surface tension of less than 30 mN / m; and / or(III) exhibits a dynamic surface tension of less than 30 mN / m; and / or(IV) forms 0.01 to 0.5wt% of the washcoat, preferably 0.05 to 0.2wt%.

15. The method according to any preceding claim, wherein the surfactant comprises one or more acetylenic diol surfactants, preferably one or more alkoxylate acetylenic diol surfactants.

16. The method according to any preceding claim, wherein the PGMs are present in a total amount sufficient to form a coating containing at least 1 gft-3of PGMs.

17. Use of a non-ionic, silicon-free, organic surfactant to improve washcoat adhesion of a PGM-containing washcoat to a porous filter body which has a water contact angle of >90° when measured with the Washburn uptake method.

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