Combatting air pollution

[0002] This application is the U.S. national-phase application of PCT International Application No. PCT/GB99/00090.

[0003] This invention relates to a method of combatting air pollution from the exhaust gas of an internal combustion engine, and to an internal combustion engine whose exhaust apparatus contains means therefor.

[0004] One of the pollutants in the exhaust gas of an internal combustion engine is NOx (oxides of nitrogen). NOx is produced by the thermal fixation of nitrogen in the combustion air, leading to thermal NOx, or by the conversion of chemically bound nitrogen in the fuel, leading to fuel NOx. The most abundant nitrogen species in an engine exhaust stream is NO, with low amounts of NO₂and N₂O. For example, the nitrogen oxide species in the exhaust stream of a typical diesel passenger car are about 95% NO, 4% NO₂and a little N₂O.

[0005] Although nitrogen monoxide (NO) is the most abundant nitrogen species in an engine exhaust stream, various processes for the treatment of the exhaust gas proceed more rapidly when the NOx species is NO₂rather than NO. For example, European patent specification 341832A discloses that in the regeneration of a diesel particulate trap at low temperature, NO₂is a more effective oxidant than NO or O₂. Again, to combat NOx emissions from lean-burn engines, it has been proposed to adsorb NOx by an adsorbent when the exhaust gas is lean (i.e. when there is a stoichiometric excess of oxygen) and release the adsorbed NO₂when the exhaust gas is rich, the exhaust gas being periodically made rich to release the adsorbed NOx; during the lean operation, NO is oxidised to NO₂which can then readily react with adsorbent surface to form nitrate—see for instance European patent specification 560991A. Yet again, in other strategies to combat NOx emissions from lean-burn engines, such as selective catalytic reduction (SCR), there are indications that the presence of NO₂may be beneficial, see for instance Applied Catalysis B: Environmental, 2 (1993) 81-100, Elsevier Science Publishers BV and Journal of Catalysis 171, 27-44 (1997), Academic Press.

[0006] It would be desirable to be able to oxidise NO to NO₂in engine exhaust gas at a lower temperature, for instance because the engine is operating at a lower temperature or while the exhaust apparatus is warming up after the engine has been started at ambient temperature.

[0007] In addition, it would be desirable to oxidise NO to NO₂in engine exhaust gas while not oxidising SO₂to SO₃. Fuels such as petrol (gasoline) or diesel contain small quantities of organo-sulphur compounds, e.g. thiophene. In the engine operation, these compounds are oxidised to SO₂. If the SO₂is oxidised to SO₃, the SO₃can readily react to form sulphuric acid or sulphate, which is undesirable. For instance, in diesel engines a small fraction of the SO₂, typically about 2%, is further oxidised to SO₃, which condenses with water in the exhaust gas as sulphuric acid, which is then adsorbed on carbonaceous particulates, hence increasing the mass of particulates. Increasing the amount of SO₃is thus undesirable. Again, in the technique of NOx adsorption discussed above, SO₃readily adsorbs on the NOx adsorbent to form base metal sulphate, which, since metal sulphates are generally much more stable than nitrates, deactivates the adsorbent and requires high temperature regeneration of it. Yet again, in the SCR technique discussed above, SO₃can react with reductant ammonia to form ammonium sulphate, which can poison the catalyst.

[0008] Catalysts which are active in the oxidation of NO to NO₂are also very active in the oxidation of SO₂to SO₃. The most effective, and most commonly used, catalyst for oxidising NO to NO₂is based on Pt, see for instance European specifications 560991A and 341832A cited above. Pt, however, is very active in the oxidation of SO₂to SO₃, as shown in the accompanying FIG. 1, which is a graph showing % conversion of NO or SO₂by oxygen on a 2% by weight platinum on alumina catalyst plotted against temperature. The graph shows also that the conversion of NO to NO₂at low temperature (e.g. below 250° C.) is very low.

[0009] Furthermore, the oxidation of NO to NO₂must be practicable in the environment of an engine exhaust gas. Although many catalysts, e.g. Cu, Co or Mn, oxidise NO to NO₂, many are not sufficiently active in an engine exhaust gas or are readily deactivated in this environment. Even Pt catalyst is inhibited by hydrocarbon and SO₂in this reaction.

[0010] The present invention provides a method of combatting air pollution from an internal combustion engine whose exhaust gas contains NO, which method comprises contacting the exhaust gas containing NO with ozone to react the NO with the ozone to produce NO₂and reducing the NO₂to nitrogen.

[0011] The invention provides also an internal combustion engine whose exhaust apparatus contains means to contact exhaust gas containing NO with ozone to react the NO with the ozone to produce NO₂and means to reduce the NO₂to nitrogen.

[0012] International patent application publication WO 97/22794 refers to an apparatus for reducing pollutants from the combustion of a fuel, the apparatus comprising:

[0013] a combustion chamber having a pre-combustion gas stream to the combustion chamber and a post-combustion gas stream of exhaust from the combustion chamber;

[0014] a catalytic converter for treating the exhaust to reduce the amount of at least one pollutant from at least one of incomplete combustion of the fuel and oxides of nitrogen; and

[0015] a device for adding ozone to at least one of the pre-combustion gas stream and the post-combustion gas stream to thereby further reduce the amount of at least one pollutant in the exhaust treated by the catalytic converter. There is no mention of reacting ozone with NO in the exhaust gas to produce NO₂. Indeed, while one can see that enhancing the oxidation of hydrocarbon or carbon monoxide in the exhaust gas might be beneficial, it is not apparent why one would wish to choose conditions so that NO in the exhaust gas is oxidised to NO₂for treatment with a catalytic converter.

[0016] It can be seen that the present invention does not rely on the use of a catalyst to oxidise the NO. Instead, the oxidation is achieved by use of a particular oxidant, ozone. We have demonstrated that ozone reacts with the NO to produce NO₂even at low temperature. In addition, we have shown that at low temperature ozone is an excellent selective oxidant, i.e. it converts all or much NO to NO₂while converting no or little SO₂to SO₃.

[0017] The internal combustion engine is usually in a vehicle. The engine is especially a lean-burn engine. The engine can be a diesel or petrol engine.

[0018] In the present invention, the ozone is usually injected into the exhaust gas containing NO. Accordingly, the engine usually includes injection means to inject the ozone into the exhaust gas containing NO.

[0019] The engine usually includes an ozone generator to generate the ozone. The ozone can be generated in ways known in themselves. For instance, the ozone generator can be a corona discharge tube through which passes air between two electrodes which are kept at a large potential difference. Alternatively, the ozone generator can be a high energy lamp to ionise oxygen in air to ozone.

[0020] Generally, at least 80% and preferably 100%, of the NO in the exhaust gas contacting the ozone is reacted with the ozone to produce NO₂. Sufficient ozone must be employed to carry out the required degree of conversion of the NO. The conversion is NO+O₃→NO₂+O₂. Generally, the exhaust gas containing NO is contacted with at least 80%, preferably at least 100%, of the amount of ozone required to react stoichiometrically with the NO to produce NO₂. The amount of ozone is usually 0.04-0.16% by volume of the exhaust gas with which it is contacted. The ozone generator is chosen accordingly. As an example, it has been found that for an exhaust gas flow rate of 7,500 liters per hour, containing 400 ppm NO, complete conversion to NO₂can be achieved if 1 liter of air per minute is fed to an ozone generator (Thermo Electron) operating at 60% capacity; if the operation is reduced to below 40% of capacity or the air flow is reduced below 250 ml per minute, the conversion efficiency decreases significantly.

[0021] Any ozone in excess over that used in forming the NO₂can be destroyed over a catalyst. However, excess ozone can be employed advantageously to promote the oxidation of hydrocarbon in the exhaust gas, especially at lower temperature.

[0022] The NO₂produced in the present invention can be employed in the exhaust gas in ways known in themselves, in particular in the control of particulate emissions from a diesel engine or NOx emission in oxygen-rich conditions.

[0023] In a preferred embodiment, the engine is a lean-burn, e.g. diesel, engine and NO₂produced is adsorbed by an adsorbent which adsorbs the NO₂when the exhaust gas is lean and releases the adsorbed NO₂when the exhaust gas is rich, and the exhaust gas is periodically made rich to release the adsorbed NO₂. Accordingly, the engine can be a lean-burn engine and include means to adsorb NO₂produced when the exhaust gas is lean and release the adsorbed NO₂when the exhaust gas is rich and means to make the exhaust gas rich periodically to release the adsorbed NO₂. This use of the NO₂produced can be as known in itself, see for instance the European specification 560991A cited above, whose contents are incorporated herein by reference. The adsorbent usually comprises at least one oxide selected from those of alkali metals (e.g. potassium, sodium, lithium and cesium), alkaline earth metals (e.g. barium, calcium, strontium and magnesium) and rare earth metals (e.g. lanthanum, yttrium and cerium). These oxides are capable of reacting with NO₂or SO₃to form the corresponding nitrate or sulphate respectively, but their reaction with NO or SO₂is very low. The advantage of the present invention in its ability to selectively oxidise the NO is clear. This advantage is apparent compared to the known use of Pt with such adsorbent to oxidise NO to NO₂. In the present invention, such adsorbent can be used advantageously with a catalyst, such as Pd or Rh, which does not oxidise NO or SO₂but which catalyses other reactions desired in such adsorbent technique. Such other reactions are the reductive release of the NO₂or reactions to oxidise CO and hydrocarbon contained in the exhaust gas. In a preferred embodiment, the adsorbent includes such catalyst, for instance the adsorbent can be Pd—Ba—Al₂O₃.

[0024] In a preferred embodiment, the NO₂produced is reduced to nitrogen by reaction with gaseous reductant, usually over a catalyst.

[0025] In a preferred embodiment, the engine is a lean-burn, e.g. diesel, engine and NO₂produced is selectively catalytically reduced to nitrogen. Accordingly, the engine can be a lean-burn engine and the means to reduce the NO₂to nitrogen comprise a SCR catalyst. This use of the NO₂produced can be as known in itself, see for instance the Applied Catalysis and Journal of Catalysis papers cited above, whose contents are incorporated herein by reference. A preferred SCR catalyst comprises a zeolite, especially ZSM5, carrying metal, particularly copper. A high conversion efficiency of NOx to nitrogen can be achieved when the exhaust gas contains a specific NO:NO₂ratio. Such ratio can be achieved, for instance over a Cu/ZSM5 catalyst, by means of the present invention, e.g. by injecting ozone into the exhaust stream to increase the NO₂concentration. In this way, significant improvement in the low temperature removal of NOx can be obtained. Thus, the engine can include means to introduce reductant into the exhaust gas to selectively reduce NO₂to nitrogen. Advantageously, the NO₂is reacted with ammonia to selectively catalytically reduce the NO₂to nitrogen. The engine can include a source of the reductant ammonia. Alternatively, the reductant can be hydrocarbon, e.g. propene. It has been found that while the presence of ozone improves the SCR of NOx by ammonia at low temperature (for instance below 250° C.), it can mar the conversion at higher temperatures, most probably because the stoichiometry required to achieve 100% conversion for the NO₂:NH₃reaction is higher than that for the NO:NH₃reaction. In such circumstance, it is advantageous to stop the reaction with ozone when the higher temperatures are reached. It has been found, however, that when the reductant is hydrocarbon, the presence of ozone can enhance the conversion of the NOx over the entire temperature window.

[0026] In a further preferred embodiment, the engine is a diesel engine, and NO₂produced reacts with particulates from the exhaust gas which have been deposited on a filter, to oxidise and hence remove them. Accordingly, the engine can be a diesel engine and the means to reduce the NO₂to nitrogen comprise a particulate filter on which particulates from the exhaust gas are deposited, which particulates are reacted with NO₂to oxidise and hence remove them. This use of the NO₂produced can be as known in itself, see for instance the European specification 341832A cited above, whose contents are incorporated herein by reference. That European specification describes the use of NO₂as an oxidant to combust diesel particulate deposited on a filter disposed in a diesel exhaust system. The filter can contain a catalyst which facilitates particulate combustion; the filter catalyst can comprise a base metal, for instance a combination of lanthanum, cesium and vanadium pentoxide. Soot or like carbonaceous particulate is combusted at a temperature usually less than 400° C., generally at a temperature of the order of 250-400° C. The most active catalysts for low temperature NO oxidation and therefore for combusting the diesel particulate, however, are also high sulphate producers. In such a system, generating the NO₂instead by means of the present invention is clearly advantageous. It has been found that when ozone is pre-mixed with NO to generate the NO₂, the oxidation of diesel soot is initiated at much lower temperature than when oxygen is pre-mixed with NO. Ozone itself can be used to oxidise particulates from diesel exhaust gas which have been deposited on a filter, and hence remove them. However, ozone is highly unstable; it readily decomposes on contact with various surfaces, for instance stainless steel pipe. Reacting the ozone with NO to produce NO₂in the present invention is advantageous for stability. Any ozone in excess over that used in forming the NO₂, though, can be used to promote particulate oxidation.

[0027] In the present invention, the NO is advantageously reacted with the ozone at a temperature from ambient temperature to 250° C., for instance from ambient temperature to 200° C. The engine can be adapted accordingly.

[0028] Advantage is also apparent when the exhaust gas contains SO₂, for instance 2-200 parts per million (ppm) by volume of SO₂. In a preferred embodiment, the exhaust gas contacting the ozone contains this amount of SO₂.

[0029] The NO is advantageously reacted with ozone in the absence of platinum. Hence, means in the engine to react the NO with the ozone advantageously does not contain platinum.

[0030] The present invention is advantageous even when the exhaust gas contacting the ozone has a relatively large gas hourly space velocity such as is employed in vehicles. The exhaust gas contacting the oxone preferably has a gas hourly space velocity of 30,000 to 50,000.

[0031] The present invention is advantageous even when the exhaust gas contacting the ozone contains water vapour. The exhaust gas contacting the oxone preferably contains 4-10% by volume water vapour.

[0032] The invention is illustrated by the accompanying drawings, which are graphs and in which:

[0033] FIG. 1 shows % conversion of NO and SO₂by oxygen on a Pt catalyst against temperature;

[0034] FIG. 2 shows % conversion of NO and SO₂by ozone without a catalyst against temperature;

[0035] FIG. 3 shows CO₂evolved from soot by NO—O₂or NO—O₃against temperature;

[0036] FIG. 4 shows % soot burnt off at 300° C. by NO—O₂or NO—O₃against time (minutes);

[0037] FIG. 5 shows the ppm of NOx left in gas on contact with Ba—Al₂O₃or Pt/Ba—Al₂O₃catalyst at 150° C. against time (minutes);

[0038] FIG. 6 shows in arbitrary units the SO₂concentration left in the gas phase on contact with the same catalyst at 650° C. against time (minutes);

[0039] FIG. 7 shows the ppm of NOx left in gas on contact with Ba—Al₂O₃catalyst at 150° C. when ozone is injected into the gas against time (minutes);

[0040] FIG. 8 shows the % conversion of NOx by SCR by ammonia at a space velocity of 50,000 per hour with and without ozone addition against temperature;

[0041] FIG. 9 shows the corresponding conversion with ozone addition which is stopped after the peak at 200° C.;

[0042] FIG. 10 shows the % conversion of NOx by SCR by C₃H₆at a space velocity of 50,000 per hour with and without ozone addition against temperature; and

[0043] FIG. 11 shows the % conversion of C₃H₆in SCR at a space velocity of 50,000 per hour with and without ozone addition against temperature.

[0044] FIG. 1 is discussed further above. The other Figures are discussed further below.

[0045] The invention is illustrated also by the following Examples, in which the ozone generator was a Thermo Electron ozone generator.

[0046] The effect of ozone on the oxidation of NO and SO₂was studied by injecting ozone, produced by passing air at a flow rate of 40 ml per minute through an ozone generator, into a mixture containing 400 ppm NO, 50 ppm SO₂, 4.5% H₂O, 4.5% CO₂, 0.5% CO and 400 ppm C₃H₆, the balance being nitrogen at a flow rate of 5 liters per minute. The NO and SO₂conversion is shown in FIG. 2. The complete oxidation of NO to NO₂is apparent at temperatures of 50° C. to 200° C. Above that temperature, the conversion decreases, mostly due to the decomposition of the ozone, and at high temperature to the thermodynamic instability of NO₂. The oxidation of SO₂to SO₃began at 250° C. and was then low, indicating the selectivity of ozone in oxidising NO and not SO₂.

[0047] This Example shows that when ozone is pre-mixed with NO to generate NO₂, the oxidation of diesel soot is initiated at much lower temperature than when NO is pre-mixed with O₂. The effect was demonstrated in a temperature programmed oxidation experiment in which a gas containing NO (400 ppm), SO₂(50 ppm), O₂(5%) and H₂O (4.5%) with the balance nitrogen at a flow rate of 5 liters per minute was mixed with O₃produced by passing air at a flow rate of 40 ml per minute through an ozone generator and the mixture passed over 1 g of diesel soot particulate, collected on a filter from a diesel engine exhaust, contained in a reactor in a furnace which was ramped at 10° C. per minute. FIG. 3 shows the rate of CO₂production formed by the particulate oxidation as a function of temperature. FIG. 4 shows the fraction of soot burned as a function of time at 300° C. when either NO and O₂or NO and ozone are used as the oxidant. It can be seen that the mixture of NO and O₃is active at lower temperature, and can achieve complete soot oxidation.

[0048] A proposal being considered to decrease NOx emission in high oxygen-containing exhaust gas is to incorporate an adsorbent material into the catalyst system, together with a catalytic element capable of oxidising nitric oxide to NO₂. The NO₂is then taken up by the adsorbent to form a nitrate. If the exhaust gas is then switched to reducing condition for a very short period, the surface complex nitrate becomes unstable and releases oxygen and NO₂which can be reduced. The first step is the oxidation of NO to NO₂and platinum is an important component of that system. However, Pt also catalyses the oxidation of SO₂to SO₃which can interact with the adsorbent.

[0049] FIG. 5 shows the NOx adsorption on Ba—Al₂O₃and Pt/Ba—Al₂O₃from a gas mixture containing 250 ppm NO, 8% O₂with the balance nitrogen by flowing 2 liters per minute of gas over 0.4 g of catalyst at 150° C. It is clear that in the presence of Pt, significant amounts of the NOx are being stored on the catalyst.

[0050] FIG. 6 shows the SO₂breakthrough from a mixture of 50 ppm and 8% O₂with the balance nitrogen on the same catalysts at similar mass hourly space velocity at 650° C. Similar to the NOx adsorption, in the absence of Pt the SO₂is not converted or adsorbed over the catalyst. The presence of Pt enhances these processes. In these adsorbent systems, the ability to selectively generate NO₂instead of SO₃would be important to decrease catalyst deactivation. Example 1 above demonstrates that ozone can selectively oxidise NO to NO₂and is not reactive to SO₂.

[0051] FIG. 7 demonstrates that the NO₂generated by the interaction of NO with ozone can readily be adsorbed on the storage material at low temperature (150° C.). The Figure shows the effect of injecting ozone produced from an ozoniser to a mixture of 250 ppm NO, 8% O₂with the balance nitrogen at 150° C. on the storage activity of Ba—Al₂O₃. The NOx absorption is significantly enhanced with the injection of ozone.

[0052] The effect of injecting ozone to a reactive mixture (200 ppm NO, 200 ppm NH₃, 12% O₂, 4.5% CO₂, 200 ppm CO, 4.5% H₂O, 20 ppm SO₂with the balance nitrogen on the NOx reduction activity of Cu/ZSM5 is shown in FIG. 8. The addition of ozone significantly improves the reduction of the NOx by NH₃at temperatures below 200° C., through the formation of the more active NO₂species. However, in the presence of ozone, the NOx conversion decreases at higher temperature, because most probably the stoichiometry required to achieve 100% conversion for the NO₂:NH₃reaction is higher than that for NO:NH₃reaction. However, if the ozoniser is switched off after the first peak conversion at 200° C., the high temperature conversion profile improves—FIG. 9. This ability to generate NO₂when required is highly desirable for SCR utility.

[0053] The selective reduction of NOx by hydrocarbon in an oxidising environment on Cu/ZSM5 catalyst is believed to occur by a dual function mechanism, with the acid sites promoting coke formation and the Cu sites catalysing NO₂formation. The reaction of the NO₂with the carbonaceous species is believed to lead to the reduction of NOx to nitrogen. It has been shown that if NO₂is present, the reactivity of the Cu/ZSM5 catalyst for the selective reduction of NOx be C₃H₆can be improved at low temperature. The effect of injecting ozone to a reactive mixture (200 ppm NO, 400 ppm C₃H₆, 12% O₂, 4.5% CO₂, 200 ppm CO, 4.5% H₂O, 20 ppm SO₂with the balance nitrogen) on the reduction activity over Cu/ZSM5 is shown in FIG. 10. FIG. 11 shows that the addition of ozone catalyses the hydrocarbon oxidation at a lower temperature.