Multifunctional oxidation catalyst for suppressing n2o generation, exhaust gas treatment system, and method for suppressing n2o generation

Abstract

The present invention relates to a multifunctional oxidation catalyst for hydrocarbon oxidation, carbon monoxide oxidation, NO oxidation, ammonia oxidation, and selective catalytic reduction of nitrogen oxides (NOx), an exhaust gas treatment system for an internal combustion engine including same, and a method for suppressing N2O generation, wherein the multi-functional oxidation catalyst is a hybrid catalyst including an oxidation catalyst (OC) configuration and a selective catalytic reduction (SCR) catalyst configuration for NOx, the OC configuration includes one or more platinum group metals, and the SCR catalyst configuration includes a zeolite material containing copper and / or iron.

Description

Multifunctional oxidation catalyst for suppressing N2O generation, exhaust gas treatment system, and method for suppressing N2O generation

[0001] The present invention relates to a multi-functional oxidation catalyst (MFOC) for hydrocarbon (HC) oxidation, carbon monoxide (CO) oxidation, NO oxidation, ammonia (NH3) oxidation, and selective catalytic reduction (SCR) of nitrogen oxides (NOx), an exhaust gas treatment system for an internal combustion engine including the same, and a method for suppressing N2O generation, wherein the MFOC is a hybrid catalyst comprising an oxidation catalyst (OC) component and an SCR catalyst component, wherein the OC component comprises one or more platinum group metals, and the SCR catalyst component comprises a zeolite material containing copper and / or iron, and the exhaust gas treatment system comprises (i) means for injecting a nitrogenous reducing agent into an exhaust gas stream; (ii) an SCR module for the selective catalytic reduction of NOx contained in the exhaust gas stream; and (iii) a means for injecting the nitrogenous reducing agent, wherein the SCR module and the MFOC module are positioned in sequential order in the exhaust gas conduit from the internal combustion engine.

[0002] Technologies for removing NOx, a major cause of environmental pollution from automobile emissions worldwide, can be broadly classified into active DeNOx technology and passive DeNOx technology. Among active system technologies, selective catalytic reduction (SCR) is receiving attention for selectively converting NOx into N2 using reducing agents such as urea, ammonia, and hydrocarbons. Meanwhile, passive system technologies include four-way catalysts that simultaneously remove CO, HC, NOx, and PM, as well as technologies that remove NOx using effective gases contained in exhaust gases from diesel engines without using reducing agents.

[0003] Emission gas regulations are being strengthened, including the recent introduction of N2O regulations along with stricter NOx regulations. In particular, N2O is a new atmospheric environmental regulated substance with a global warming potential (GWP) 310 times that of CO2.

[0004] Among the various systems for responding to recently strengthened exhaust gas regulations, a close coupled system in which an SCR module using a nitrogenous reducing agent, particularly urea, is positioned close to the engine exhaust port is promising for increasing DeNOx performance, especially at low temperatures of 200 to 350°C; however, insufficient low-temperature DeNOx performance and the generation of N2O due to unexpected NH3 slip remain problems.

[0005] The present invention is proposed to address the need for improved DeNOx performance at additional medium / low temperatures (<350℃) and the problem of excessive N2O generation due to NH3 slip at the downstream end of the system in a proximity coupling system in which an SCR module utilizing elements is positioned close to the engine exhaust port.

[0006] The object of the present invention is to provide a novel catalyst capable of performing four functions in a multifunctional manner, particularly carbon monoxide oxidation, hydrocarbon oxidation, NH₃ oxidation, and selective catalytic reduction (SCR) of NOx. Furthermore, the present invention aims to design a novel post-treatment system. In particular, the object of the present invention is to provide an exhaust gas treatment system comprising a multifunctional catalyst, preferably a quaternary catalyst. In particular, the object of the present invention is to provide an exhaust gas treatment system for an internal combustion engine for suppressing N₂O generation, comprising: (i) means for injecting a nitrogenous reducing agent into an exhaust gas stream; (ii) an SCR module for selective catalytic reduction of NOx contained in the exhaust gas stream; and (iii) a novel multifunctional oxidation catalyst (MFOC) module, wherein the means for injecting the nitrogenous reducing agent, the SCR module, and the MFOC module are positioned in a sequential order in an exhaust gas conduit from the internal combustion engine. Another object of the present invention is to provide a method for suppressing N2O generation in an exhaust gas treatment system for an internal combustion engine, which comprises: (i) means for injecting a nitrogenous reducing agent into an exhaust gas stream; (ii) an SCR module for selective catalytic reduction of NOx contained in the exhaust gas stream; and (iii) a diesel oxidation catalyst (DOC) module in an exhaust gas treatment system comprising the steps of replacing the DOC module with a multifunctional oxidation catalyst module of the present invention, and treating exhaust gas emitted from an internal combustion engine, particularly a diesel engine.

[0007] According to the present invention, in a proximity coupling system in which an SCR module using an element is positioned close to the engine exhaust port, applying the multifunctional oxidation catalyst defined in the present invention improves DeNOx performance at additional medium / low temperatures (<350℃) and, above all, can suppress excessive N2O generation due to NH3 slip at the downstream end of the system.

[0008] Figure 1 illustrates an exhaust gas treatment system to which representative exhaust gas aftertreatment technologies are applied.

[0009] Figure 2 schematically illustrates a close coupled system in which an SCR module using urea is positioned close to the engine outlet to increase DeNOx performance at low temperatures of 200 to 350°C in response to recently strengthened exhaust gas regulations, and the unexpected N2O generation reaction process.

[0010] Figure 3 shows the experimental results of the hydrocarbon oxidation performance at 50% temperature (HC LOT T50) and the carbon monoxide oxidation performance at 50% temperature (CO LOT T50) according to the application of the catalysts of the embodiments and comparative examples of the present invention.

[0011] definition

[0012] In this document, the terms Diesel Oxidation Catalyst (DOC) and Oxidation Catalyst (OC) may be used interchangeably. In this document, when a catalyst is region-coated in a channel formed on a substrate or carrier, the front position of the substrate along the direction of exhaust gas flow refers to the exhaust gas inlet region, and the rear position of the substrate refers to the exhaust gas outlet region. The exhaust gas flows from upstream to downstream. Additionally, when describing a layer structure, the lower or bottom layer refers to the proximal region coated directly on the substrate, and the upper or top layer refers to the distal region from the substrate, that is, the region coated on top of the lower or bottom layer. In this document, the terms catalyst, module, or catalyst module are used interchangeably with the same meaning. The multifunctional oxidation catalyst according to the present invention is a composite catalyst and comprises two components, wherein the term component may be replaced with the expression part.

[0013] Since diesel engines operate at a high A / F (air-to-fuel) ratio under very lean fuel conditions, the conversion rates for hydrocarbons and carbon monoxide are excellent, especially when a diesel oxidation catalyst (DOC) is used; however, the emissions of nitrogen oxides (NOx) and particulate matter in diesel exhaust are particularly high. Both nitrogen oxides (NOx) and particulate matter are components of diesel exhaust that are difficult to convert into harmless substances. However, as a post-treatment technology to reduce particulate matter, a catalyzed diesel particulate filter (CDPF or DPF) is applied, and as a post-treatment technology to reduce NOx, a selective catalytic reduction (SCR) catalyst for NOx, in which urea is used as a nitrogenous reducing agent, is applied. Meanwhile, in SCR modules where urea is applied, an ammonia oxidation catalyst (AOC) is introduced to treat slipped ammonia, and a representative exhaust treatment system in which these post-treatment technologies are applied is illustrated in Fig. 1.

[0014] However, as emission gas regulations have recently been strengthened, including the introduction of N2O regulations along with stricter NOx regulations, a proximity coupling system is proposed to address this, particularly to increase DeNOx performance at low temperatures of 200–350°C, by positioning an SCR module using urea close to the engine exhaust port as shown in Fig. 2. While this proximity coupling system contributes to improving DeNOx performance at low temperatures, a new problem arises in which unexpected N2O is generated through the reaction between urea-derived slip ammonia (NH3) and oxygen, NO, and NO2 present or generated downstream, as indicated in Fig. 2.

[0015] (1) 4NH3+4O2-> 2N2O+6H2O

[0016] (2) 4NH3+4NO+3O2-> 4N2O+6 H2O

[0017] (3) 6NH3+8NO2-> 7N2O+9H2O

[0018] (4) 4NH3+4NO2+O2-> 4N2O+6 H2O

[0019] Therefore, the objective of the present invention is to provide an improved oxidation catalyst, an exhaust gas treatment system, and a method for reducing N2O, a new regulated substance. Surprisingly, the improved oxidation catalyst not only showed improvements in N2O reduction and NOx removal performance at low temperatures, particularly below 350°C, but also demonstrated hydrocarbon and carbon monoxide conversion rates equivalent to those of conventional oxidation catalysts. These unexpected technical effects were confirmed in a proximity coupling system in which an SCR module using urea is positioned close to the diesel engine exhaust outlet, and the oxidation catalyst according to the present invention can be referred to as a multifunctional oxidation catalyst in which various catalytic functions operate simultaneously.

[0020] The multifunctional oxidation catalyst for suppressing N2O generation according to the present invention is a hybrid catalyst comprising an oxidation catalyst (OC) component and a selective catalytic reduction (SCR) catalyst component for NOx, wherein the OC component comprises one or more platinum group metals and the SCR catalyst component comprises a zeolite material containing copper and / or iron.

[0021] The oxidation catalyst composition comprises a metal oxide supported by a platinum group metal, wherein the platinum group metal is one or more of Pt, Pd, and Rh, more preferably one or more of Pt and Pd, most preferably Pt and Pd, and the supporting metal oxide is one or more of alumina, zirconia-alumina, silica-alumina, titania, zirconia-titania, and ceria. More preferably, the metal oxide may be one or more of alumina, silica-alumina, and zirconia-alumina, more preferably zirconia-alumina. The metal oxide supporting the platinum group metal may be mixed with other additives and coated onto a substrate in the form of a washcoat, or mixed with the SCR catalyst composition described below and coated onto a substrate. Meanwhile, regarding the SCR catalyst composition, various zeolite materials may be applied when copper and / or iron are incorporated, preferably one of CHA, AEI, or BEA, more preferably one of CHA and / or AEI, and more preferably CHA, and may be coated onto a substrate in the form of a washcoat mixed with other additives or coated onto a substrate mixed with an OC composition. With respect to zeolite materials containing one or more of Cu and Fe, there are no restrictions on how one or more of Cu and Fe are incorporated into the zeolite material. Accordingly, one or more of Cu and Fe may be included in the zeolite material as skeletal structural elements or non-skeletal structural elements. In the present invention, the SCR catalyst composition is limited to zeolite materials, and vanadium-based (V-based) SCR catalyst components are intentionally excluded. The inventors have confirmed that if a vanadium-based SCR catalyst composition is included in an exhaust treatment system intended for a multifunctional oxidation catalyst, the vanadium component volatilizes, not only degrading catalyst performance but also causing the loss of vanadium components harmful to the human body.

[0022] As is obvious to those skilled in the art, the term "carrier" as described herein is also referred to as a carrier and is a ceramic type such as cordierite or a refractory metal type such as stainless steel. A typical carrier, schematically, is a flow-through type carrier having a front surface and a rear surface, and having a plurality of microchannels linearly connecting the front surface and the rear surface.

[0023] The multifunctional oxidation catalyst according to the present invention is a composite catalyst, wherein the OC composition and the SCR catalyst composition may be mixed with each other, form separate layers, or be coated in zones along the axial direction of the substrate, preferably stacked in an upper and lower layer structure, and more preferably the SCR catalyst composition may be located in the upper layer and the OC composition in the lower layer.

[0024] According to the present invention, a multifunctional oxidation catalyst is composed of two layers, namely an upper layer and a lower layer, wherein the upper layer is coated with a washcoat made of a zeolite material comprising one or more of Cu and Fe, and the lower layer is coated with a washcoat made of a platinum group metal supported on a metal oxide, wherein there are no limitations on the loading amounts of the zeolite and platinum group metal, but preferably, the zeolite material is contained in a range of 10 to 150 g / L, more preferably 30 to 120 g / L, and even more preferably 60 to 120 g / L, and the platinum group metal is contained in a loading amount in the range of 0.177 to 5.295 g / L, more preferably 0.353 to 3.177 g / L, and even more preferably 0.353 to 1.765 g / L.

[0025] Furthermore, according to the present invention, the multifunctional oxidation catalyst defined in the present invention is understood to be used as a 4-functional catalyst that simultaneously performs the selective catalytic reduction of NOx, the oxidation of ammonia, the oxidation of nitric oxide, and the oxidation of hydrocarbons in an exhaust gas treatment process, and surprisingly performs a function of reducing N2O generation. Accordingly, the present invention also relates to a method for reducing N2O generation while simultaneously performing the selective catalytic reduction of NOx, the oxidation of ammonia, the oxidation of nitric oxide, and the oxidation of hydrocarbons in exhaust gas treatment using the multifunctional catalyst defined herein. Specifically, the present invention provides a method for suppressing N2O generation in an exhaust gas treatment system for an internal combustion engine, comprising: (i) means for injecting a nitrogenous reducing agent into an exhaust gas stream; (ii) an SCR module for the selective catalytic reduction of NOx contained in the exhaust gas stream; and (iii) a diesel oxidation catalyst (DOC) module, and the step of replacing the DOC module with the multifunctional oxidation catalyst (MFOC) module of the present invention in an exhaust gas treatment system composed of the DOC module, and the step of treating the exhaust gas.

[0026] Furthermore, the present invention relates to an exhaust gas treatment system in fluid communication with an internal combustion engine, wherein the exhaust gas treatment system comprises a multifunctional catalyst as described above and defined herein, and the internal combustion engine is preferably a diesel engine. In relation to the exhaust gas treatment system of the present invention, in particular, the present invention is an exhaust gas treatment system for an internal combustion engine for suppressing N2O generation, comprising: (i) means for injecting a nitrogenous reducing agent into an exhaust gas stream; (ii) an SCR module for selective catalytic reduction of NOx contained in the exhaust gas stream; and (iii) a multifunctional oxidation catalyst module as defined herein, wherein the means for injecting the nitrogenous reducing agent, the SCR module, and the MFOC module are positioned in sequential order in an exhaust gas conduit from the diesel engine.

[0027] The exhaust gas treatment system of the present invention preferably has an SCR module for the selective catalytic reduction of NOx contained in the exhaust gas stream closely coupled to a lean-burn engine, and preferably, the lean-burn engine is a diesel engine. The close-coupled system is a promising system for increasing DeNOx performance at low temperatures of 200 to 350°C in response to recently strengthened exhaust gas regulations, by positioning an SCR module using urea close to the engine outlet, but unexpected N2O generation has been raised as a problem.

[0028] Preferably, a diesel particulate filter (DPF) module may be further included downstream of the multifunctional oxidation catalyst module according to the present invention for application in particulate removal. Furthermore, the exhaust gas treatment system according to the present invention may further include a second means for injecting a nitrogenous reducing agent downstream of the DPF module and a second SCR module for selective catalytic reduction of NOx, wherein an ammonia oxidation catalyst (AOC) module may be further included at the bottom of the SCR module. In this invention, the SCR module is distinguished from the SCR catalyst configuration included in the multifunctional oxidation catalyst defined in the present invention. The SCR module independently comprises a module with a means for injecting urea, which is a nitrogenous reducing agent, upstream, but in the multifunctional oxidation catalyst, the SCR catalyst configuration is inevitably composed as part of a composite catalyst together with the OC catalyst configuration.

[0029] According to the present invention, in a proximity coupling system in which an SCR module utilizing an element is positioned close to the engine exhaust port, it is confirmed that applying the multifunctional oxidation catalyst defined in the present invention improves DeNOx performance at additional medium / low temperatures (<350℃) and, above all, suppresses excessive N2O generation due to NH3 slip at the downstream end of the system. It goes without saying that in the exhaust treatment system according to the present invention, various components other than the mentioned component can be placed or replaced downstream of the multifunctional oxidation catalyst.

[0030]

[0031] Examples

[0032] Preliminary Example 1. Manufacturing of OC Composition

[0033] An OC part can be manufactured according to any widely known method. According to a preferred manufacturing method, a Pt / Pd supported OC catalyst component was prepared by supporting platinum (Pt) and palladium (Pd) in a 1:1 weight ratio of 0.7 wt% (Pt / alumina = 0.35 wt%, Pd / alumina = 0.35 wt%) on alumina powder, which serves as a support, and then dispersing the component in water to prepare a slurry. The OC composition was prepared by ball milling the slurry so that approximately 90% of the particles had a size of 8-10 µm.

[0034] Preliminary Example 2. SCR Configuration Manufacturing

[0035] A Cu-zeolite slurry was prepared by mixing and dispersing commercially available H-zeolite with CuO as a Cu-precursor, Zr-acetic acid as a binder, and additives. CuO was ion-exchanged by contact and ball milling with H-zeolite in a slurry state to prepare an SCR catalyst composition of Cu-containing zeolite.

[0036] Example 1.

[0037] 120 g / L of the OC slurry prepared in Preliminary Example 1 and 120 g / L of the SCR slurry prepared in Preliminary Example 2 were mixed and coated onto a cordierite carrier. After drying at 150°C to 160°C for about 10 minutes, the mixture was calcined at 650°C for 10 hours to prepare a multifunctional oxidation composite catalyst (MFOC “A”).

[0038] Example 2.

[0039] A multifunctional oxidation composite catalyst (MFOC “B”) was prepared by coating 120 g / L of the SCR slurry prepared in Preliminary Example 2 onto a cordierite carrier and drying it at 150°C to 160°C for about 10 minutes, then additionally coating 120 g / L of the OC slurry prepared in Preliminary Example 1 onto the top of the SCR composition, drying it at 150°C to 160°C for about 10 minutes, and then calcining it at 650°C for 10 hours.

[0040] Example 3.

[0041] A multifunctional oxidation composite catalyst (MFOC “C”) was prepared by coating 120 g / L of the OC slurry prepared in Preliminary Example 1 onto a cordierite carrier and drying it at 150°C to 160°C for about 10 minutes, then additionally coating 120 g / L of the SCR slurry prepared in Preliminary Example 2 onto the top of the OC composition, drying it at 150°C to 160°C for about 10 minutes, and then calcining it at 650°C for 10 hours.

[0042] Example 4.

[0043] A multifunctional oxidation composite catalyst (MFOC “D”) was prepared by processing in the same manner as in Example 3, except that 60 g / L of SCR slurry was applied.

[0044] Example 5.

[0045] A multifunctional oxidation composite catalyst (MFOC “E”) was prepared by processing in the same manner as in Example 3, except that 30 g / L of SCR slurry was applied.

[0046] Comparative Example 1.

[0047] A catalyst of only OC (Ref “A”) was prepared by coating a cordierite carrier with 120 g / L of the OC slurry prepared in Preliminary Example 1, drying it at 150°C to 160°C for about 10 minutes, and then calcining it at 650°C for 10 hours.

[0048] Comparative Example 2. /

[0049] A catalyst of only SCR (Ref “B”) was prepared by coating a cordierite carrier with 120 g / L of the SCR slurry prepared in Preliminary Example 2, drying it at 150°C to 160°C for about 10 minutes, and then calcining it at 650°C for 10 hours.

[0050] Example 6. Fabrication of an exhaust gas treatment system comprising a multifunctional oxidation composite catalyst in a proximity-coupled system

[0051] In the exhaust gas treatment system for an internal combustion engine of FIG. 2, a catalyst module manufactured in the examples or comparative examples is installed instead of the DOC module. The exhaust gas treatment system according to the present invention comprises (i) means for injecting urea, a nitrogenous reducing agent, into an exhaust gas stream; (ii) an SCR module for the selective catalytic reduction of NOx contained in the exhaust gas stream; and (iii) a multifunctional oxidation catalyst module according to the present invention, wherein the means for injecting the nitrogenous reducing agent, the SCR module, and the MFOC module are positioned in sequential order in the exhaust gas conduit from the internal combustion engine, optionally further comprising a diesel particulate filter (DPF) module downstream of the multifunctional oxidation catalyst module, and further comprising means for injecting urea, a nitrogenous reducing agent, and an SCR module for the selective catalytic reduction of NOx downstream of the DPF module. Additionally, to prevent ammonia slip caused by urea, an ammonia oxidation catalyst (AOC) module may be included at the bottom of the SCR module.

[0052]

[0053] Experimental Example

[0054] For the catalysts of Examples 1-6 and Comparative Examples, evaluations were conducted using a reactor to verify DeNOx performance (%) and N2O generation at low temperatures (200–350°C), and the evaluation results are shown in Tables 1 and 2. In this evaluation, only a multifunctional oxidation catalyst module was installed in the reactor; however, to simulate ammonia slipping in an actual proximity-coupled system, the evaluation was conducted while controlling the ANR (Ammonia NOx Ratio). That is, the evaluation was performed by controlling the amount of NH3 simultaneously introduced while maintaining the NOx concentration flowing upstream of the multifunctional catalyst installed in the reactor at a constant level. The catalysts subject to this evaluation were hydrothermally aged at 800°C for 16 hours, and the gas concentration and flow conditions inside the reactor were as follows: when ANR = 1.0, 500 ppm NO, 500 ppm NH3, 200 ppm CO, 5% CO2, 9.5% O2, 5% H2O, N2 balance, and linear velocity (SV). The velocity was 80,000 (1 / h), and the DeNOx performance and N2O generation were verified under conditions of holding for 10 minutes at temperatures of 200, 250, 300, and 350°C, respectively. Meanwhile, when ANR = 0.2, the evaluation was conducted identically to the conditions at ANR = 1.0, except that only the NH3 concentration was changed to 100 ppm. Table 1 shows the results from the evaluation at ANR = 1.0 to verify performance under conditions where ammonia slips 100% when urea is injected in a proximity-coupled system, and Table 2 shows the results from the evaluation at ANR = 0.2 to verify performance under conditions where ammonia slips 20% when urea is injected in a proximity-coupled system. The evaluation was conducted under conditions ranging from maximum NH3 slip (ANR = 1.0) to minimum discriminability (ANR = 0.2) when urea is injected in a proximity-coupled system.

[0055]

[0056]

[0057] As confirmed by the results in Tables 1 and 2, in the exhaust gas treatment system for an internal combustion engine of FIG. 2, the DeNOx performance of the examples was improved and the amount of N2O generated was significantly reduced compared to Comparative Example 1. Specifically, compared to the system equipped with the DOC module (Ref “ ”), the increase in DeNOx performance (%) and the reduction in N2O generation (ppm) in the system equipped with the multifunctional oxidation catalyst module (MFOC “ ” MFOC “ E”) according to the present invention were significantly improved in the low-temperature region under conditions of ANR=1 or ANR=0.2.

[0058] Meanwhile, Figure 3 illustrates the results of evaluating the oxidation performance of hydrocarbons and carbon monoxide according to temperature with the application of catalysts from comparative examples and examples. All catalysts subject to evaluation were hydrothermally aged at 800°C for 16 hours, and the gas concentration and flow conditions within the reactor were 1500 ppm THC [(C3H6 / C3H8 = 4 / 1, 200 ppm)+(C10H22, 1300 ppm)], 1800 ppm CO, 200 ppm NOx, 13% O2, 5% H2O, 5% CO2, N2 balance, linear velocity = 50,000 (1 / h), and were evaluated under a temperature increase of 10°C. It was confirmed that the hydrocarbon and carbon monoxide oxidation performance of the system to which the multifunctional oxidation catalyst according to the present invention was applied was nearly equivalent to that of a system equipped with a conventional DOC module (Comparative Example 1).

Claims

1. A multifunctional oxidation catalyst for suppressing N2O generation in an exhaust gas treatment system for an internal combustion engine, wherein the multifunctional oxidation catalyst comprises an oxidation catalyst (OC) composition and a selective catalytic reduction (SCR) catalyst composition for NOx, the OC composition comprises one or more platinum group metals, and the SCR catalyst composition comprises a zeolite material containing copper and / or iron.

2. A multifunctional oxidation catalyst according to claim 1, wherein the platinum group metal is one or more of Pt, Pd, and Rh, preferably one or more of Pt and Pd.

3. The zeolite material of claim 1 is a multifunctional oxidation catalyst having a skeletal structure of the CHA, AEI, or BEA type, more preferably the CHA or AEI type, more preferably the CHA type.

4. A multifunctional oxidation catalyst according to claim 1, wherein the OC composition and the SCR catalyst composition are mixed with each other or form separate layers, preferably stacked in an upper and lower structure, and more preferably the SCR catalyst composition is stacked in the upper layer and the OC composition in the lower layer.

5. A multifunctional oxidation catalyst, wherein the internal combustion engine is a diesel engine, in accordance with paragraph 1.

6. An exhaust gas treatment system for an internal combustion engine for suppressing N2O generation, comprising: (i) means for injecting a nitrogenous reducing agent into an exhaust gas stream; (ii) an SCR module for selective catalytic reduction of NOx contained in the exhaust gas stream; and (iii) a multifunctional oxidation catalyst module of claim 1, wherein the means for injecting the nitrogenous reducing agent, the SCR module, and the MFOC module are positioned in a sequential order in an exhaust gas conduit from the internal combustion engine.

7. An exhaust gas treatment system according to claim 6, further comprising a diesel particulate filter (DPF) module downstream of the multifunctional oxidation catalyst module.

8. An exhaust gas treatment system according to claim 7, further comprising a second means for injecting a nitrogenous reducing agent downstream of the DPF module and a second SCR module for selective catalytic reduction of NOx.

9. An exhaust gas treatment system in which, in paragraph 6, the internal combustion engine is a diesel engine.

10. An exhaust gas treatment system according to any one of claims 6 to 9, further comprising an ammonia oxidation catalyst (AOC) module at the bottom of the SCR module.

11. A method for suppressing N2O generation in an exhaust gas treatment system for an internal combustion engine, comprising: (i) means for injecting a nitrogenous reducing agent into an exhaust gas stream; (ii) an SCR module for selective catalytic reduction of NOx contained in an exhaust gas stream; and (iii) a step of replacing the DOC module with the multifunctional oxidation catalyst module of claim 1 in an exhaust gas treatment system comprising a diesel oxidation catalyst (DOC) module, and a step of treating the exhaust gas.

12. In paragraph 11, the method wherein the internal combustion engine is a diesel engine.

Images