WO2012160356A1

WO2012160356A1 - NOx REDUCTION CATALYST

Info

Publication number: WO2012160356A1
Application number: PCT/GB2012/051122
Authority: WO
Inventors: Valerie Marie HOUEL; Raj Rao Rajaram
Original assignee: Johnson Matthey Public Limited Company
Priority date: 2011-05-20
Filing date: 2012-05-18
Publication date: 2012-11-29
Also published as: DE102012208398A1; GB201111553D0; GB2491241A; GB2491241B; GB201208755D0

Abstract

A NOx reduction catalyst for treating lean exhaust gases comprises a combination of a silver-based lean NOx Catalyst, i.e. a catalyst that reduces NOx to N₂ using a hydrocarbon reductant, and a NOx storage component which does not contain a precious metal. The invention also comprises a method of chemically reducing the level of NOx in a lean exhaust gas using lean NOx catalysis, comprising the step of passing the exhaust gas over one or more catalyst components comprising a silver-based LNC and a NOx storage component which does not contain a precious metal, in the presence of hydrogen.

Description

NOx REDUCTION CATALYST

The present invention concerns improvements in the control of emissions, and more especially concerns improvements in the chemical reduction of oxides of nitrogen (NOx) in lean exhaust gases. The invention particularly relates to reducing low temperature NOx emission, which is difficult to achieve by current NOx reduction technologies such as NH₃-SCR and NOx storage-reduction (NSR) catalyst.

Almost all countries have now put in place regulations to control emissions from combustion devices such as vehicle engines, and these regulated emissions are generally carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NOx) and particulate matter (PM). Lean exhaust gases, that is exhaust gases in which there is an excess of oxygen over fuel components, are generally the product of compression ignition internal combustion engines (usually known as diesel engines), although there are certain gasoline-fuelled engines that are termed "lean-burn" engines, such as stratified charge designs. The control of CO and HC is carried out by catalysed oxidation reactions to form C0₂ and ¾0, but NOx presents a problem in lean exhaust gases because the desired chemical reaction to reduce quantities of NOx to innocuous materials involves the chemical reduction of NOx to N₂. Chemical reduction is much more difficult in the presence of excess oxygen, which is the case with lean exhaust gases.

Two routes to overcome the NOx problem are in commercial use. One involves the storage of NOx during normal lean operation conditions, as compounds within a catalyst composition. Thus a NOx storage catalyst contains components such as alkali metal or alkali earth metal compounds such as oxides or carbonates, which can convert NOx into nitrates or other compounds. The storage component in most common use is barium oxide or carbonate. If the engine operating conditions are changed, for example by pre-programmed engine management, or possibly under certain vehicle operating conditions, so that the exhaust gases change to rich conditions (net less than stoichiometric quantities of oxygen and other oxidising species), the NOx can be released from the catalyst component and can be catalytically reduced during the rich phase. That is, NOx storage catalysts (NSC), sometimes called NOx Storage and Reduction (NSR) catalyst, Lean NOx Traps (LNT), NOx traps or NOx Absorber Catalysts (NAC), are an established technique for lean NOx control.

An essential component of a NSC is a catalyst component, usually the expensive precious metal platinum. Platinum assists in the oxidation of NO to N0₂ which is the NOx component most readily stored on the surface of the catalyst by the alkali metal or alkaline earth metal storage component. During the rich phase of operation, the platinum catalyses the reduction of N0₂ to N₂ by HC and hydrogen. The second route involves the catalytic reduction of NOx using a suitable reductant. One example uses hydrocarbon reductant and is termed "lean NOx catalysis". The second uses nitrogenous reductant and is commonly referred to as "Selective Catalytic Reduction" (SCR). Lean NOx Catalysts (LNC) (also known as hydrocarbon SCR catalysts - see below), such as iron- or copper-based zeolite catalysts, are for conversion of NOx under lean conditions. There has been some interest recently in the possible use of Ag on Al₂0₃ as a LNC. Such LNCs, however, tend to have low conversions at lower exhaust gas temperatures.

It has also been suggested to add small quantities of hydrogen, at about 200ppm to 1000 ppm, to enriched exhaust gas for use in HC-SCR, to increase the deNOx activity of a fresh Ag/Al₂0₃. There have been several explanations for this increased activity, including activation of the Ag sites and the decomposition of surface nitrate groups.

SCR is being introduced commercially for heavy duty (trucks and buses) as well as light duty (private cars and light commercial vehicle). SCR involves the adding of a nitrogenous reductant to the exhaust gases before passing the mixture over an SCR catalyst, so that the NOx is reduced to the desired N₂. The most common nitrogenous reductant is ammonia, used in the form of a solution of urea, which can be injected into the hot exhaust gases before the SCR catalyst. SCR has been well established in stationary power sources, where conditions are steady state, unlike the much more challenging vehicular applications, where exhaust gas volumes, temperatures and composition can changes with changing operating conditions.

SCR techniques appear to offer good conversions of NOx when used under ideal conditions. However, the current systems require on-board storage of the reductant, and some form of infrastructure for reductant replenishment, even if replenishment is only carried out at servicing points. Since the urea solution is a source of ammonia, there is the danger of excess ammonia being released; ammonia is hazardous as well a pungent pollutant.

The use of hydrocarbons as a reductant in SCR has been proposed for some years, and although hydrocarbons are already carried on a vehicle in the form of the fuel, hydrocarbon SCR (HC-SCR) is generally less selective at reducing NOx than catalysts employing ammonia as reductant. It is believed that HC-SCR has not yet entered commercial use.

High-speed light duty diesel engines, which are fitted to about half the private cars in Europe, and an even higher proportion of the light commercial vehicles, are a challenge for effective catalytic aftertreatment to meet emission regulations. The main reason for this is that exhaust gas temperatures are very low, often in the range from 100°C to 200°C, which gives low conversions of known catalytic processes. Accordingly, the reduction of NOx at such low temperatures has been proving a considerable technical challenge.

WO 2005/016496 discloses a catalyst structure for treating exhaust gas from a lean burn internal combustion engine comprises a substrate monolith comprising a lean NOx catalyst composition associated with at least one partial oxidation catalyst (POC), wherein the LNC composition is selected from the group consisting of: (a) silver or a silver compound supported on alumina; and (b) at least one metal selected from the group consisting of copper (Cu), iron (Fe), cobalt (Co) and cerium (Ce) supported on at least one zeolite, and wherein the at least one POC is selected from the group consisting of: (i) a bulk oxide, a bulk composite oxide or a bulk mixed oxide comprising at least one metal selected from the group consisting of manganese (Mn), iron (Fe), cerium (Ce) and praseodymium (Pr); and (ii) at least one of rhodium (Rh) and palladium (Pd) disposed on at least one inorganic oxide support.

EP 1541219A1 discloses a method of low temperature NOx reduction without gaseous reductant under absolutely lean conditions using the combination of silver with some transition metals (Co, Ni, Cu, Mn, Fe) or metal oxides of Ce, Pr or Bi as a catalyst-adsorbent. The method comprises the steps of oxidising NO to N0₂ using the above catalyst at 220-350°C and then converting the N0₂ thus produced on the same catalyst from room temperature up to 200°C. The disclosure states that the catalyst is also a good adsorbent for both NO and N0₂, but the adsorbed NO and N0₂ is desorbed as N0₂. The N0₂ released can be used for simultaneous soot removal. The disclosure includes engine testing of a catalyst comprising Ag-Ce supported on alumina and deposited on diesel particulate. There remains a need for additional options to treat lean exhaust gases, especially at the lower temperatures.

Accordingly, the present invention provides a NOx reduction catalyst for treating lean exhaust gases, comprising a combination of a silver-based lean NOx catalyst and a NOx storage component which does not contain a precious metal.

"Precious Metal" as defined herein has its common meaning, i.e. the group consisting of platinum group metals, gold and silver. The catalyst of the present invention is different, therefore, from the disclosure in EP 1541219A1 at least in that the cerium component and the precious metal component (i.e. the silver) are disposed on the same support material (i.e. the alumina).

In embodiments, the silver based lean NOx Catalyst is separate and distinct from the NOx storage component.

The invention also provides a method of chemically reducing the level of NOx in a lean exhaust gas using HC-SCR (also known as lean NOx catalysis), comprising the step of passing the exhaust gas over one or more catalyst components comprising a silver-based LNC and a NOx storage component which does not contain a precious metal, in the presence of hydrogen.

In the case of the catalyst of the invention, the required components (i.e. the silver based LNC on the one hand and the NOx storage catalyst on the other) may be deposited, as is conventional, and using conventional technology, separately on separate catalyst substrates, in layers on a single substrate, in separate zones axially arranged on a single substrate, in admixture on a single substrate or in any other combination. Suitable catalyst substrates are high surface area ceramic or metal flow through monoliths of conventional form.

The preferred silver-based LNC is a well dispersed silver oxide on an aluminium oxide (i.e. alumina) support, such as a 50-200 m²g^_1 high surface area alumina such as boehmite, with a Ag loading from 0.5wt to 3wt .

The NOx storage component used in the present invention is preferably one that operates by adsorbing the NOx at lower temperatures, desirably below about 200°C, and desorbing the NOx at higher temperatures, desirably above about 200°C. It will be realised that this is different from the known commercial NOx storage components which absorb/react with NOx under lean conditions and release the NOx under rich conditions. Suitable NOx storage components for use in the present invention may be selected from a series of oxides or mixed oxides which can react with N0₂ to form nitrates of moderate stabilities. These oxides can include particulate high surface area oxides, typically between 40 to 200 m²g^_1 such as zirconia (Zr0₂), ceria (Ce0₂) or mixed oxides of these single oxides optionally including rare earth oxide stabilisers. Suitable zirconias, cerias and mixed oxides are commercially available.

Alternatively, the NOx storage component may comprise cerium oxide, zirconium oxide or a composite oxide of both cerium oxide and zirconium oxide dispersed on an inert (preferably non-ceria/non-zirconia-containing) oxide support material, optionally including rare earth oxide stabilisers.

Thus, the catalyst of the present invention may conveniently be a physical mixture of Ag/Al₂0₃ and a high surface area zirconia or ceria/zirconia mixed oxide. The catalyst may comprise more than one silver-based LNC and more than one NOx storage component. It may also contain other components providing these do not significantly adversely affect the desired low temperature activity of the catalyst. According to a further aspect, the invention provides an exhaust system for a lean burn internal combustion engine, which exhaust system comprising a catalyst according to the invention.

In one embodiment, the exhaust system comprises means for injecting a hydrocarbon reductant into exhaust gas upstream of the catalyst.

In a further aspect, the invention provides a lean burn internal combustion engine comprising an exhaust system according to the present invention. As has been described with reference to the method of the invention, it is necessary to operate in the presence of hydrogen. Suitable levels of hydrogen are from approximately lOOOppm by volume to approximately 5000ppm by volume, such as 1500ppm to 3000ppm, preferably approximately 2000ppm, of the total gas passing through the catalyst. The hydrogen may be supplied from any source, including compressed hydrogen, but it is convenient to provide hydrogen by reforming fuel and/or exhaust gas. It is not expected to be necessary to purify the reformate before use in the present invention.

Without wishing to be bound by any theory of operation, it is presently believed that the presence of hydrogen in the exhaust gas contacting the silver-based LNC assists in the formation of N0₂ from NO. The engine-out exhaust will contain a proportion of N0₂ in normal operation, as well as NO. Thus, the operation of the present invention with hydrogen obviates the need for Pt to convert NO to N0₂. It is believed that the N0₂ is the component that is adsorbed onto the NOx storage component. As the temperature of the exhaust gases increase and the N0₂ is released, it is reduced in contact with the silver-based LNC to N₂. The combination of a state of the art NSC, containing platinum, with the silver-based LNC causes non-selective oxidation of HC and interferes with effective HC-SCR. Although precious metals such as platinum or other platinum group metals are undesired components in the present invention, such catalytic components may be used in a downstream catalyst to remove any hydrocarbon that may slip past the silver-based catalyst. In order that the invention may be more fully understood and by way of illustration only the invention will now be described in the following Examples and with reference to the accompanying drawings, in which:

Figure 1 is a graph showing the effect of hydrogen or a mixture of both hydrogen and hydrocarbon on NO oxidation using a 2wt Ag/Al₂0₃ catalyst;

Figure 2 is a graph showing the effect of adding Zr0₂ on the lean NOx catalytic activity of a 2wt Ag/Al₂C>₃ catalyst; and Figure 3 is a graph showing the effect of adding Zr0₂ or Ce0₂ on lean NOx activity of a 2wt% Ag/Al₂0₃ catalyst.

EXAMPLES

Example 1

Figure 1 shows the formation of N0₂ in the presence of a synthetic exhaust gas containing 200ppm NO, 2000ppm H₂, 12% 0₂, 5% CO, 5% H₂0, 200ppm CO and 200ppm Ci hydrocarbon (US06 fuel), N₂ balance over an artificially aged 2wt% Ag/Al₂0₃ catalyst, either with the addition of hydrogen, or both HC (US06 diesel fuel) and hydrogen, at various temperatures. It is clear that in the presence of H₂, the Ag catalyst is very active at oxidising NO to N0₂ over a broad temperature range. Although the addition of HC as US06 fuel suppresses the N0₂ formation over the Ag catalyst, a high fraction of the NO is still being converted to N0₂ in the temperature range from 100°C to 200°C. At higher temperatures, the amount of N0₂ produced decreases, partly due to the HC-SCR reaction causing the reduction of NOx to N₂.

Example 2

Figure 2 shows the improvements possible at lower temperatures using the synthetic exhaust gas of Example 1, except in that the quantity of Ci hydrocarbon (US06 fuel) is varied, using the present invention. The graph shows the HC-SCR activity of an artificially- aged 2wt Ag/Al₂0₃ catalyst in the presence of 2000ppm H₂ and US06 fuel (in the legend, "LHA" stands for "lean hydrothermally aged". The catalyst is heated in a furnace at 700°C for 48 hours in a 10% water (i.e. steam)/air mixture). The HC:NOx ratio, plotted as the lowermost line and measured by the right-hand y-axis, is varied with increasing temperature to optimise the performance of the Ag catalyst and minimise any fuel penalty. It is clear that the Ag catalyst alone can achieve high lean NOx catalytic activity between 250°C to 400°C. The addition of a NOx storage component such as Zr0₂ further improves the NOx conversion at low temperature - at 200°C the conversion has increased from 30% to 50 or 60%. It is believed that the mechanism by which the improvement is achieved is via the adsorption of N0₂ on the Zr0₂.

Example 3

Figure 3 shows the effect lean NOx catalytic activity of combining Ce0₂ with a lean hydrothermally aged 2wt%Ag/Al₂0₃ catalyst compared with the catalyst of Example 2 in admixture with Zr0₂ at a 4:1 weight ratio of Ag/Al₂0₃:Zr0₂. The presence of the Ce0₂ improves the conversion at 200°C, but decreases the conversion at higher temperature as it promotes the parasitic consumption of the HC via direct oxidation with 0₂.

For the avoidance of any doubt, the entire contents of all prior art references cited herein are incorporated herein by reference in their entirety.

Claims

CLAIMS:

1. A NOx reduction catalyst for treating lean exhaust gases, comprising a combination of a silver-based lean NOx Catalyst (LNC) and a NOx storage component which does not contain a precious metal.

2. A NO_x reduction catalyst according to claim 1 , wherein the silver-based LNC is separate and distinct from the NOx storage component.

3. A catalyst according to claim 1 or 2, wherein the silver-based LNC is silver oxide dispersed on alumina, and having a Ag loading of 0.5 to 3 wt .

4. A catalyst according to claim 3, wherein the NOx storage component comprises a high surface area particulate zirconia, ceria or ceria/zirconia mixed oxide, optionally including one or more rare earth stabiliser.

5. A catalyst according to claim 3, wherein the NOx storage component comprises cerium oxide, zirconium oxide or a composite oxide of both cerium oxide and zirconium oxide dispersed on an inert oxide support material.

6. An exhaust system for a lean burn internal combustion engine, which exhaust system comprising a catalyst according to any preceding claim.

7. An exhaust system according to claim 6, comprising means for injecting a hydrocarbon reductant into exhaust gas upstream of the catalyst.

8. A lean burn internal combustion engine comprising an exhaust system according to claim 6 or 7.

9. A method of chemically reducing the level of NOx in a lean exhaust gas using HC-SCR, comprising the step of passing the exhaust gas over one or more catalyst components comprising a silver-based LNC and a NOx storage component which does not contain a precious metal, in the presence of hydrogen.

10. A method according to claim 9, wherein the quantity of hydrogen is from approximately lOOOppm to approximately 5000ppm of the total gas passing through the catalyst.

11. A method according to claim 9 or 10, applied to a light duty passenger or a light commercial vehicle.