GB2491241A

GB2491241A - NOx reduction catalyst

Info

Publication number: GB2491241A
Application number: GB1208755.7A
Authority: GB
Inventors: Valerie Marie Renee Houel; Raj Rao Rajaram
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2011-05-20
Filing date: 2012-05-18
Publication date: 2012-11-28
Anticipated expiration: 2032-05-18
Also published as: GB201208755D0; WO2012160356A1; GB201111553D0; GB2491241B; DE102012208398A1

Abstract

A NOx reduction catalyst for treating lean exhaust gases comprises a combination of a silver-based lean NOx catalyst (LNC) and a NOx storage component which does not contain a precious metal. The silver based catalyst and the NOx storage component may be separate and distinct from one another. The catalyst may be silver oxide disperces on alumina and have a silver loading of 0.5-3 wt%. The NOx storage component may be a high surface area particulate zerconia, ceria or ceria/zirconia mixed oxide. The invention also discloses 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.

Ahuost 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 (IIC), 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-bum" engines, such as stratified charge designs. The control of CO and BC is carried out by catalysed oxidation reactions to form CO2 and HO, but NOx presents a problem in lean exbaust 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 preprogrammed engine management, or possibly under certain vehicle operating conditions, so that the exhaust gases change to rich conditions (net less than stoiehiometric 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 t) (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 NO2 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 NO2 to N2 by UC 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 cxhaust gas temperaturcs.

It has also been suggestcd 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 AgfAl203. There have been several explanations for this increased activity, including activation of the Ag sites and the decomposition of surface nitrate groups.

SCR is bcing 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 rcductant to the exhaust gases bcfore passing the mixture over an 5CR catalyst, so that the NOx is reduced to the desired N. The most common nitrogenous rcductant 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.

5CR techniques appear to offer good conversions of NOx when used under ideal conditions. However, the current systems require on-board storage of the reduetant, and some form of infrastructure for reduetant 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 reduetant in SCR has been proposed for some years, and although hydrocarbons are already carried on a vehicle in the form of the fuel, hydrocarbon 5CR (HC-SCR) is generally less selective at reducing NOx than catalysts employing ammonia as reduetant. 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 aftertrcatment 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 20051016496 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 hulk 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 l54l2i9Al discloses a method of low temperature NOx reduction without gaseous reductant under absolutely lean conditions using the combination of silver with some flnsition 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 NO2 using the above catalyst at 220-350°C and then converting the NO2 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 NO2, but the adsorbed NO and NO2 is desorbed as NO7. The NO7 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 l54l2l9Al 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 m2g1 high surface area alumina such as boehmite, with a Ag loading from 0.Swt% 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 NO2 to form nitrates of moderate stabilities. These oxides can include particulate high surface area oxides, typically between 40 to 200 m2g' such as zirconia (Zr07), ceria (CeO2) 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/A12O3 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 l000ppm by volume to approximately S000ppm by volume, such as iSOOppm 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 NO2 from NO. The engine-out exhaust will contain a proportion of NO2 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 NO2. It is believed that the NO2 is the component that is adsorbed onto the NOx storage component. As the temperature of the exhaust gases increase and the NO2 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/A1203 catalyst; Figure 2 is a graph showing the effect of adding ZrO2 on the lean NOx catalytic activity of a 2wt% Ag/A12O3 catalyst; and Figure 3 is a graph showing the effect of adding ZrO2 or CeO2 on lean NOx activity of a 2wt% Ag/Al203 catalyst.

EXAMPLES

Example I

Figure 1 shows the formation of NO2 in the presence of a synthetic exhaust gas containing 200ppm NO, 2000ppm H2, 12% 02, 5% CO, 5% H20, 200ppm CO and 200ppm C3 hydrocarbon (USO6 fuel), N2 balance over an artificially aged 2wt% Ag1A12O3 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 vety active at oxidising NO to NO2 over a broad temperature range.

Although the addition of HC as USO6 fuel suppresses the NO2 formation over the Ag catalyst, a high fraction of the NO is still being converted to NO2 in the temperature range from 100°C to 200°C. At higher temperatures, the amount of NO2 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 I, except in that the quantity of C hydrocarbon (USO6 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 USO6 fuel (in the legend, "LHA" stands for "lean bydrothermally aged". The catalyst is heated in a furnace at 700°C for 48 hours in a 10% water (i.e. steam)Iair 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 ZrO2 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 NO2 on the Zr02.

Example 3

Figure 3 shows the effect lean NOx catalytic activity of combining CeO2 with a lean hydrothermally aged 2wt%AgfAI2O3 catalyst compared with the catalyst of Example 2 in admixture with ZrO2 at a 4:1 weight ratio of Ag/Al203:ZrO2. The presence of the CeO2 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: I. 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 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 I 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 zireonia, 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. I0
10. A method according to claim 9, wherein the quantity of hydrogen is from approximately i000ppm 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.