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 catalyst, 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 different 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 contains 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 extending less than 100 % of the monolith length at the outlet length, and a second cata- lyst 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 applied 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 signifi- cantly 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, one or more ammonia oxidation catalyst impregnated in the body of the monolith, and and a layer containing one or more SCR catalysts coated on the walls of the monolith, wherein at least one of the SCR cat- alysts 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 with an inlet and an outlet end, in which the one or more oxidation catalysts are impregnated in the body of the monolith walls in a range at the outlet end that extends less than 100% of the monolith length, further con- taining a layer of one or more SCR catalysts, coated at the outlet end extending to the same range as the impregnated oxidation catalyst, 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 third embodiment is a catalyst article with an inlet and an outlet end, in which the one or more oxidation catalysts are impregnated in the body of the monolith walls in a range at the outlet end 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 impregnated oxidation catalyst, 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. A fourth embodiment is a catalyst article with an inlet and an outlet end, in which the one or more oxidation catalysts are impregnated in the body of the monolith walls in a range at the outlet end 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 fifth 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 sixth 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 a seventh embodiment related to the sixth 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 an eighth embodiment, 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.

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 substrate, one or more ammonia oxidation catalysts impregnated in the body of the monolith, and a layer containing one or more SCR catalysts coated on the walls of the monolith, 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 with an inlet and an outlet end, in which the one or more oxidation catalysts are impregnated in the body of the monolith walls in a range at the outlet end 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 im- pregnated oxidation catalyst, 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 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 with an inlet and an outlet end, in which the one or more oxidation catalysts are impregnated in the body of the monolith walls in a range at the outlet end 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 impregnated oxidation catalyst, in which at least one SCR catalyst has an average parti- cle 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.

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 an inlet and an outlet end, in which the one or more oxidation cata- lysts are impregnated in the body of the monolith walls in a range at the outlet end 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 meas- ured by light scattering, in the range of approximately 4-40 μηη.

In a fifth embodiment, 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, in which 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 sixth embodiment, 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, in which 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 a seventh embodiment related to the sixth embodiment, 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, in which 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 an eighth embodiment of the method for the removal of ammonia and nitrogen ox- ides from an engine exhaust gas comprises the step of contacting the exhaust gas with a catalyst article, in which 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

In this example, it is shown that the catalyst item of the invention improves the removal of ammonia from a gas stream, without compromising the selectivity to NOx. We compare two catalysts, both consisting of Pt impregnated on the monolith support and a washcoat based on a Cu-beta zeolite. In one catalyst (catalyst A), the Cu-beta zeolite had an average agglomerate size, as measured by light scattering, of 3.1 μηη. In the second catalyst (catalyst B), the same Cu-beta zeolite was used, but the particles were agglomerated to an average size of 8.4 μηη, as measured by light scattering as shown in Figure 1 .

The monolith substrates used in catalysts A and B were prepared by impregnating a monolith, consisting of glassfiber and T1O2 (ca. 260 cpsi), with Pt by impregnation of an aqueous solution of Pt(NH3)4HCC>3 to a final Pt loading of about 100 mg Pt/I monolith and calcined for 3 h at 550 °C.

The Cu-zeolite material used in catalyst A 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 zeo- lite 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 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 A as shown in Figure 1. This Cu- ^* BEA zeolite powder was then mixed with water to a dry matter content of 20 wt% and xanthan gum (Keltrol®) was added to a final concentration of 0.12 wt%, to obtain the washcoat slurry. A Pt-impregnated monolith (ca. 80 x 50 mm in size) was dipped in the washcoat slurry and dried at room temperature in flowing air for about 1 hour. Then, the monolith was dipped once more in the same slurry, dried for about 1 hr in flowing air and calcined for 3 hours at 550 °C to obtain catalyst A. The final load of Cu- ^* BEA in catalyst A was 141 g Cu- ^* BEA/I monolith.

The Cu-zeolite 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 and Levasil 200/30 were added to the solution. The amount of Levasil 200/30 corresponds to 0.05 g dry matter/g zeolite. 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 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 1. This Cu- ^* BEA zeolite powder was then mixed with water to a dry matter content of 30 wt% and Levasil 200/30 (0.02 g dry matter/g powder) and xanthan gum (Keltrol®) was added (0.06 wt%) were added, to obtain the washcoat slurry. A Pt-impregnated monolith (ca. 80 x 50 mm in size) was dipped in the washcoat slurry and dried at room temperature in flowing air for about 1 hour. Then, the monolith was calcined for 3 hours at 550 °C to obtain catalyst B. The final load of Cu- ^* BEA in catalyst B was 135 g Cu- ^* BEA/I monolilth. The performance measurements of catalysts A and B were 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 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 1 shows the measured ammonia conversion and total yield of nitrogen for catalysts A and B in temperature range 250-550 °C. These results show that the total yield of nitrogen for catalyst B has increased significantly, in particular in the range 250-400 °C, which is most relevant for diesel exhaust gas cleaning purposes.

Table 1

Temperature NH3 converN2 yield (%)

(°C) sion (%)

Catalyst A Catalyst B Catalyst A Catalyst B

250 31.1 50.1 26.1 33.5

275 46.2 64.2 38.0 46.4

300 54.9 72.9 44.7 53.3

350 62.9 79.0 52.5 60.5

400 69.9 82.0 61.3 67.7

500 81.4 87.7 72.1 75.3

550 86.1 90.2 72.9 74.3