The invention relates to the removal of nitrogen oxides (NOx) and ammonia from an exhaust gas of a fuel-lean combustion, with a focus on, but not limited to, exhaust gas from compression ignition engines in vehicles. In state-of-the-art systems, NO is reduced via the selective catalytic reduction (SCR) of NOx by NH3, which in oxidative atmosphere produces N2 and water efficiently. NH3 is usually provided by controlled injection of a urea solution in the exhaust gas stream. The selective catalytic reduction is usually performed with a slight excess of NH3, since the process then becomes more efficient. As a consequence, an NH3 slip is created, which has to be removed from the exhaust gas stream as well by catalytic oxidation of NH3 using the residual oxygen in the exhaust gas stream. Oxidation of NH3 with oxygen often leads to the formation of NO. In an exhaust system, it is of crucial importance to avoid this oxidation of ammonia to NO. A standard configuration of a modern exhaust gas aftertreatment system con- sists of an oxidation catalyst for the removal of CO and hydrocarbons, a filter to retain soot particles and an SCR - ammonia slip catalyst (SCR/ASC) system for the abatement of NOx and excess of NH3. The invention provides a catalyst article and a method with improved efficiency of the oxidation of NH3 and an increased yield of nitrogen.

The removal of NOx from the exhaust gas of fuel-lean combustion is based on the se- lective reduction of NOx by ammonia (NH3-SCR): 4 NO + 4 NH ₃ + 0 ₂ → 4 N ₂ + 6 H ₂ 0. The first type of catalysts for this reaction is base metal oxides or a combination of base metal oxides. The most commonly used SCR catalysts are based on vanadium oxide, such V2O5/T1O2, V2O5/WO3/T1O2, but other oxides from the metals in groups 3, 4, 5, 6 and 7 may be applied as well. The second type of SCR catalysts is based on ion- exchanged zeolites or zeotype materials. The most commonly used ions in such catalysts are Fe and Cu. Examples of known catalysts of this type are Cu- ^* BEA, Fe- ^* BEA Cu-MFI, Fe-MFI, Cu-CHA (Cu-SSZ-13), Cu-SAPO-34.

It is common practice to use a slight excess (0-20%) of ammonia to make the NH3-SCR process more efficient. The excess of ammonia leads to a small slip of ammonia from the SCR process. For abatement of this ammonia slip, a catalyst active for oxidation of ammonia with oxygen is used (oxidation catalyst). In principle, any material with activity for ammonia oxidation by oxygen could be used, but by far the most commonly applied catalysts are based on Pt, as these catalysts provide the lowest light-off temperature for ammonia oxidation and are already active at around 200 °C. The drawback of Pt is that oxidation of ammonia with oxygen produces larger amounts of NO, in particular at temperatures above 250 °C. To obtain a good selectivity towards N2, an oxidation cata- lyst, often Pt based, is combined with an SCR catalyst, to yield a bifunctional catalyst system which enables the NH3-SCR reaction to occur with the NO produced by oxidation of ammonia with the residual ammonia and oxygen in the gas stream, thus reducing the NH3 slip without compromising the NOx emission.

It is known that NH3 oxidation catalysts and NH3-SCR catalysts can be combined in dif- ferent ways to obtain a bifunctional catalyst system to remove the NH3 from an exhaust gas stream. US4188364 discloses a catalyst system comprising two catalyst beds in series in which the first catalyst bed contains an NH3-SCR catalyst and the second catalyst bed contains an oxidation catalyst, thus forming a simple serial arrangement of the two catalysts. Another possible configuration is to mix the oxidation catalyst and the SCR catalyst and apply the mixture on a monolith by a washcoating process.

JP3436567 discloses a layered arrangement of the oxidation and SCR catalysts in which the top layer contains the active SCR material, and the bottom layer contains the oxidation catalyst.

EP1992409 discloses a different layered structure, in which a first catalyst layer con- tains a mixture of a Pt based oxidation catalyst with a zeolite based material active for SCR, which is coated directly on the wall of the monolith, and a second layer on top of the first layer containing only a zeolite based SCR catalyst.

Another variation of a layered catalyst article is disclosed in US8524185, which describes a catalyst article with a first catalyst layer consisting of an oxidation catalyst ex- tending less than 100 % of the monolith length at the outlet length, and a second catalyst layer on top of the first layer extending over the full length of the monolith.

Instead of washcoating the layer containing the oxidation catalyst on the walls of a monolith, the oxidation catalyst or mixture of oxidation catalyst and SCR catalyst can also be impregnated in the walls of the monolith, after which the SCR active layer is ap- plied on the monolith walls by a washcoating process. Such a system is disclosed in JP3793283B2. Layered configurations of SCR and oxidation catalysts are known to result in an efficient removal of ammonia, without excessive NOx slip.

We have found that activity of SCR/ASC catalyst articles can further be improved in terms of oxidation efficiency of NH3 and increase the yield of nitrogen, when including in the catalyst article one or more SCR catalyst(s) in which at least one SCR catalyst has an average particle size or agglomerate size, as measured by light scattering, in the range of 4-40 μηη.

The term "SCR catalyst" as used hereinbefore and in the following description refers to catalysts with activity for NH3-SCR in the range 150-550 °C and also possess activity for the oxidation of ammonia by oxygen, typically at higher temperatures (> approximately 350 °C). The term "ammonia oxidation catalyst" refers to catalysts with a significantly higher activity for ammonia oxidation with oxygen below approximately 300 °C.

Pursuant to the above finding, this invention provides in a first aspect a catalyst article comprising a monolithic catalyst carrier substrate containing one or more oxidation cat- alysts, and one or more SCR catalysts, wherein at least one of the SCR catalysts has an average particle size or agglomerate size, as measured by light scattering, in the range of 4-40 μηη.

A second embodiment is a catalyst article comprising a monolithic catalyst carrier substrate comprising one coated layer containing a mixture of one or more oxidation cata- lysts and one or more SCR catalysts, wherein at least one of the SCR catalysts has an average particle size or agglomerate size, as measured by light scattering, in the range 4-40 μηη.

A third embodiment is a catalyst article comprising a monolithic catalyst carrier substrate with a first coated layer containing one or more oxidation catalysts, and a second layer containing one or more SCR catalysts, wherein at least one of the SCR catalysts has an average particle size or agglomerate size, as measured by light scattering, in the range 4-40 μηη.

A fourth embodiment is a catalyst article comprising a monolithic catalyst carrier substrate with a first coated layer containing one or more oxidation catalysts and further containing one or more SCR catalysts, and a second layer containing one or more SCR catalysts, wherein at least one of the SCR catalysts has an average particle size or agglomerate size, as measured by light scattering, in the range 4-40 μηη.

A fifth embodiment is a catalyst article with an inlet and an outlet end, in which the first catalyst layer containing one or more oxidation catalysts and optionally one or more SCR catalysts is applied at the outlet end in a range that extends less than 100% of the monolith length, in which at least one SCR catalyst has an average particle size or agglomerate size, as measured by light scattering, in the range of approximately 4-40 μηη.

A sixth embodiment is a catalyst article with an inlet and an outlet end, in which the first catalyst layer containing one or more oxidation catalysts and optionally one or more SCR catalysts is applied at the outlet end in a range that extends less than 100% of the monolith length, further containing a layer of one or more SCR catalysts, coated at the outlet end extending to the same range as the first catalyst layer, in which at least one SCR catalyst has an average particle size or agglomerate size, as measured by light scattering, in the range of approximately 4-40 μηη. A seventh embodiment is a catalyst article with an inlet and an outlet end, in which the first catalyst layer containing one or more oxidation catalysts and optionally one or more SCR catalysts is applied at the outlet end in a range that extends less than 100% of the monolith length, further containing a layer of one or more SCR catalysts, coated at the outlet end extending to the same range as the first catalyst layer, in which at least one SCR catalyst has an average particle size or agglomerate size, as measured by light scattering, in the range of approximately 4-40 μηη further containing a different SCR catalyst at the inlet end.

An eighth embodiment is a catalyst article with an inlet and an outlet end, in which the first catalyst layer containing one or more oxidation catalysts and optionally one or more SCR catalysts is applied at the outlet end in a range that extends less than 100% of the monolith length, further containing a layer of one or more SCR catalysts, coated at the outlet end extending to a larger range as the impregnated oxidation catalyst and a maximum of 100% of the monolith length, in which at least one SCR catalyst has an average particle size or agglomerate size, as measured by light scattering, in the range of approximately 4-40 μηη. In a ninth embodiment, the one or more ammonia oxidation catalysts in any of the previous embodiments are selected from the group of Pt, Ir, Pd, Rh and mixtures thereof.

In a tenth embodiment the one or more SCR catalysts in any of the previous embodiments comprise a zeolite or zeotype material containing Cu, Fe or combinations thereof.

In an eleventh embodiment related to the tenth embodiment, the zeolite or zeotype material is selected from the group having a framework type of AEI, AFX, CHA, KFI, ERI, LTA, IMF, ITH, MEL, MFI, SZR, TUN, ^* BEA, BEC, FAU, FER, MOR, LEV.

In a twelfth embodiment, the one or more SCR catalysts in any of the previous embodi- ments comprises an oxide selected from oxides of Mo, Cr, V, W, Ta, Nb, Ti, Ce and combinations thereof.

Various components and auxiliary agents, such as binders, can additionally be present in the catalyst article of the invention.

A second aspect of the invention is a method for the removal of ammonia and nitrogen oxides from an engine exhaust gas, comprising the step of contacting the exhaust gas with a catalyst article comprising a monolithic catalyst carrier substrate, containing one or more oxidation catalysts, and one or more SCR catalysts, wherein at least one of the SCR catalysts has an average particle size or agglomerate size, as measured by light scattering, in the range of 4-40 μηη. A second embodiment of the method for the removal of ammonia and nitrogen oxides from an engine exhaust gas comprises the step of contacting the exhaust gas with a catalyst article comprising a monolithic catalyst carrier substrate comprising one coated layer containing a mixture of one or more oxidation catalysts and one or more SCR catalysts, wherein at least one of the SCR catalysts has an average particle size or ag- glomerate size, as measured by light scattering, in the range 4-40 μηη.

A third embodiment of the method for the removal of ammonia and nitrogen oxides from an engine exhaust gas comprises the step of contacting the exhaust gas with a catalyst article comprising a monolithic catalyst carrier substrate with a first coated layer containing one or more oxidation catalysts, and a second layer containing one or more SCR catalysts, wherein at least one of the SCR catalysts has an average particle size or agglomerate size, as measured by light scattering, in the range 4-40 μηη.

A fourth embodiment of the method for the removal of ammonia and nitrogen oxides from an engine exhaust gas comprises the step of contacting the exhaust gas with a catalyst article with a first coated layer containing one or more oxidation catalysts, and a second layer containing one or more SCR catalysts, wherein at least one of the SCR catalysts has an average particle size or agglomerate size, as measured by light scattering, in the range 4-40 μηη.

A fifth embodiment of the method for the removal of ammonia and nitrogen oxides from an engine exhaust gas comprises the step of contacting the exhaust gas with a catalyst article with an inlet and an outlet end, in which the first catalyst layer containing one or more oxidation catalysts and optionally one or more SCR catalysts is applied at the outlet end in a range that extends less than 100% of the monolith length, in which at least one SCR catalyst has an average particle size or agglomerate size, as measured by light scattering, in the range of approximately 4-40 μηη.

A sixth embodiment of the method for the removal of ammonia and nitrogen oxides from an engine exhaust gas comprises the step of contacting the exhaust gas with a catalyst article with an inlet and an outlet end, in which the first catalyst layer containing one or more oxidation catalysts and optionally one or more SCR catalysts is applied at the outlet end in a range that extends less than 100% of the monolith length, further containing a layer of one or more SCR catalysts, coated at the outlet end extending to the same range as the first catalyst layer, in which at least one SCR catalyst has an average particle size or agglomerate size, as measured by light scattering, in the range of approximately 4-40 μηη. A seventh embodiment of the method for the removal of ammonia and nitrogen oxides from an engine exhaust gas comprises the step of contacting the exhaust gas with a catalyst article with an inlet and an outlet end, in which the first catalyst layer containing one or more oxidation catalysts and optionally one or more SCR catalysts is applied at the outlet end in a range that extends less than 100% of the monolith length, further containing a layer of one or more SCR catalysts, coated at the outlet end extending to the same range as the first catalyst layer, in which at least one SCR catalyst has an average particle size or agglomerate size, as measured by light scattering, in the range of approximately 4-40 μηη further containing a different SCR catalyst at the inlet end.

An eighth embodiment of the method for the removal of ammonia and nitrogen oxides from an engine exhaust gas comprises the step of contacting the exhaust gas with a catalyst article with an inlet and an outlet end, in which the first catalyst layer containing one or more oxidation catalysts and optionally one or more SCR catalysts is applied at the outlet end in a range that extends less than 100% of the monolith length, further containing a layer of one or more SCR catalysts, coated at the outlet end extending to a larger range as the impregnated oxidation catalyst and a maximum of 100% of the monolith length, in which at least one SCR catalyst has an average particle size or agglomerate size, as measured by light scattering, in the range of approximately 4-40 μηη.

In a ninth embodiment of the method for the removal of ammonia and nitrogen oxides from an engine exhaust gas, the one or more ammonia oxidation catalysts in any of the previous embodiments are selected from the group of Pt, Ir, Pd, Rh and mixtures thereof.

In a tenth embodiment of the method for the removal of ammonia and nitrogen oxides from an engine exhaust gas, the one or more SCR catalysts in any of the previous embodiments comprise a zeolite or zeotype material containing Cu, Fe or combinations thereof.

In an eleventh embodiment of the method for the removal of ammonia and nitrogen oxides from an engine exhaust gas, related to the tenth embodiment, the zeolite or zeotype material is selected from the group having a framework type of AEI, AFX, CHA, KFI, ERI, LTA, IMF, ITH, MEL, MFI, SZR, TUN, ^* BEA, BEC, FAU, FER, MOR, LEV.

In a twelfth embodiment of the method for the removal of ammonia and nitrogen oxides from an engine exhaust gas, the one or more SCR catalysts in any of the previous embodiments comprises an oxide selected from oxides of Mo, Cr, V, W, Ta, Nb, Ti, Ce and combinations thereof. Examples

EXAMPLE 1

This example highlights the performance of a catalyst item in which the oxidation catalyst is mixed with an SCR catalyst. The catalyst item was prepared by using a Cu-*BEA zeolite material with a particle size distribution, as measured by light scattering, given in Figure 1 . The average particle size is 1 1.9 μηη.

The catalyst was prepared by dispersion of 70 g of the Cu-*BEA material as specified above with 4.5 g of a 1wt% Pt/TiC>2 supported catalyst and 4.5 g binder (Levasil 200N, 30%) in 130 g water. The pH of the slurry was then adjusted to pH=9, using a solution of 33% ammonium hydroxide in water, and mixed thoroughly. Then, 50 g of a 1 % solution of ethyl cellulose (Ceravance 6800) was added to the slurry, 0.5 g Silfoam SE39 and finally water was added to obtain a washcoat slurry with a dry matter content of 21.5%. A glassfiber monolith coated with T1O2 (50x80 mm, ca. 260 cpsi) was immersed in the washcoat slurry and dried at room temperature. Then, water was added to the washcoat slurry to obtain a slurry with a dry matter content of 17.5% and the monolith was dipped once more in the washcoat slurry. The monolith was then dried at room temperature, and calcined for 3 h at 550 °C in air. Total washcoat loading after calcination was 170 g/l.

The performance measurement of the catalyst item was done by cutting a sample of 30x50 mm from the monoliths prepared as described above, and placing it in a reactor. Prior to the activity measurement, the catalyst was heated to 550 °C for 2 hours in the reaction feed gas consisting of 200 ppm NH3, 12% O2, and 4% H2O in N2, at a total flow of 15 m ³ /h, corresponding to a SV of 250,000 h ^"1 . In the activity measurement, the temperature was varied between 170°C and 550 °C, using the same reactor feed gas and flow. The concentrations of ammonia and NOx in the reactor exit gas were continuously monitored by an FTI R spectrometer, and the conversion of ammonia and the total yield of N2 were evaluated. Table 1 shows the measured ammonia conversion and total yield of nitrogen for this catalyst in the temperature range 250-550 °C. Table 1

Example 2

In this example, the performance of two monolith catalysts with a first washcoat layer of Pt/TiC>2 and a second washcoat layer containing a Cu- ^* BEA catalyst is shown, in which the agglomerate size of the Cu- ^* BEA particles is different. In the first catalyst (Catalyst A), the agglomerate size, measured with light scattering, of the Cu- ^* BEA material was 2.9 μηη; in the second catalyst (Catalyst B), the agglomerate size of the Cu- ^* BEA material was 9.3 μηη. The particle size distributions are shown in Figure 2. The monolith sub- strates used in catalysts A and B consisted of glassfiber coated with T1O2, with a channel density of about 260 cpsi, and were about 80 x 50 mm (height x diameter) in size.

The slurry for the Pt/TiC>2 washcoat for both Catalysts A and B was prepared as follows. A 1 wt% Pt/TiC>2 powder was prepared by impregnation of a T1O2 support at with the appropriate amount of Pt(NH ₃ )4(HCC>3)2 at pH=9 followed by drying at about 120 °C and calcination at 450 °C. To make the Pt/Ti0 ₂ slurry, 32.8 g of this Pt/Ti0 ₂ powder was mixed with 174 g of a solution of 0.23 wt% xanthan gum in water, and water was added to a total amount of 800 g slurry. The slurry was shaked in a paint shaker for 6 min. The dry matter content of the slurry was determined to be 4.1 %.

The Cu- ^* BEA material used in Catalyst A was prepared as follows. An aqueous solu- tion of 133 g Cu(N0 ₃ )2-3H ₂ 0. in 6500 g water was prepared. 1000 g of an H- ^* BEA zeolite with a Si/AI ratio of 15 was added to the solution and the mixture was stirred for ca. 1 hr at room temperature to perform the ion-exchange. The mixture was then dried at 120 °C and calcined to 450 °C in a rotary furnace to obtain a dry powder of Cu- ^* BEA with about 3.5 wt% Cu, with the particle size distribution for catalyst A as shown in Figure 1 . The slurry for the Cu- ^* BEA washcoat for Catalyst A (Slurry A) was prepared by mixing 100 g Cu- ^* BEA material with 16.5 g 30% Levasil 200 N solution, 90 g of a solu- tion of 0.26 wt% xanthan gum in water, a solution of NH4OH in water (33%) to pH=9, and water to a dry matter content of 24%.

Catalyst A was prepared by immersion of a monolith in the Pt slurry for 1 .5 min, followed by drying at room temperature and 250 °C. The uptake of slurry was determined to 1 1.6 g/ 1 monolith. Then, the monolith containing the Pt slurry was immersed three times in Slurry A for 1 .5 min, and dried at room temperature between the immersions. In the first immersion, the dry matter content of the Slurry A was adjusted to 20%; in the second immersion, the dry matter content was adjusted to 15.9% and in the third immersion the dry matter content was adjusted to 12%. After the final immersion, the monolith was calcined at 550 °C for 3 h. The total uptake was 175 g/l. The Cu- ^* BEA material used in Catalyst B was prepared as follows. An aqueous solution of 133 g Cu(N0 ₃ )2-3H ₂ 0. in 6500 g water was prepared. 1000 g of an H- ^* BEA zeolite with a Si/AI ratio of 15 was added to the solution. Furthermore, a solution of 30 % Levasil 200 N, to a total amount of 50 g on a dry matter basis, and a solution of polyvinyl alcohol (PVA), to a total amount of 50 g PVA, were added. The mixture was stirred for about 1 hr at room temperature to perform the ion exchange. The mixture was then dried at 120 °C and calcined at 550 °C for 3 h to obtain a dry powder of Cu- ^* BEA with about 3.5 wt% Cu, with the particle size distribution for catalyst B as shown in Figure 2. The Cu- ^* BEA slurry for catalyst B (Slurry B) was obtained by mixing 180 g of this Cu- ^* BEA material with 400 g of a 2 wt% solution of methyl cellulose (4000 cP) in water, and 30 g of a 30% solution of Levasil 200 N and a solution of NH4OH (33%) to adjust the pH to 9. Water was added until the dry matter content of the slurry was 18%.

Catalyst B was prepared by immersion of a monolith in the Pt slurry for 1 .5 min, followed by drying at room temperature and 250 °C. The uptake of slurry was determined to 1 1 .9 g/ 1 monolith. Then, the monolith containing the Pt slurry was immersed three times in the Slurry B for 1 .5 min, and dried at room temperature between the immersions. In the first immersion, the dry matter content of the Cu- ^* BEA slurry was adjusted to 16%; in the second immersion, the dry matter content was adjusted to 14.5% and in the third immersion the dry matter content was adjusted to 10%. After the final immersion, the monolith was calcined at 550 °C for 3 h. The total uptake was 165 g/l.

The performance measurements of Catalysts A and B were done by cutting a sample of 30 x50 mm from the monoliths prepared as described above, and placing it in a re- actor. Prior to the activity measurement, the catalysts were heated to 550 °C for 2 hours in the reaction feed gas consisting of 200 ppm NH3, 12% O2, and 4% H2O in N2, at a total flow of 15 m ³ /h, corresponding to a SV of 250,000 h ^"1 . In the activity measurement, the temperature was varied between 170°C and 550 °C, using the same reactor feed gas and flow. The concentrations of ammonia and NOx in the reactor exit gas were continuously monitored by an FTIR spectrometer. The conversion of ammonia and the total yield of N2 were evaluated

Table 2 shows the measured ammonia conversion and total yield of nitrogen for catalysts A and B in temperature range 250-550 °C. The results show a significantly improved conversion of NH3 and N2 yield in Catalyst B in the range 250-400 °C.

Table 2. Measured conversions of NH3 and yields of N2 for Catalysts A and B in the temperature range 250-500 °C.

Temperature NH3 converN ₂ yield (%)

(°C) sion (%)

Catalyst A Catalyst B Catalyst A Catalyst B

250 28.6 31.7 23.9 25.1

275 42.1 57.1 34.1 46.0

300 48.3 63.5 39.8 50.3

350 55.7 72.2 50.7 63.0

400 65.4 76.2 61.7 70.0

500 87.7 88.7 78.9 79.3

550 90.9 91.7 77.4 79.4