A review of NOx reduction on zeolitic catalysts under diesel exhaust conditions

ELSEVIER

Fuel Vol. 76, No. 6, pp. 507-515, 1997 :f: 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0016-2361/97/$17.00 + 0.00

PIh S0016-2361(96)00213-X

A review of NOx reduction on zeolitic catalysts under diesel exhaust conditions Patrick Gilot, Marc Guyon and Brian R. Stanmore* Laboratoire Gestion des Risques et Environnement, Eco/e Nationale Sup~rieure de Chimie de Mulhouse CNRS EP082, 25 rue de Chemnitz, 68200 Mulhouse, France * Department of Chemical Engineering, University of Queensland, Q 4072, Australia (Received November 1995; revised 15 August 1996)

The three-way catalysts currently used for controlling NOx gaseous pollutants from IC engines cannot be used under lean conditions such as diesel engine exhausts. Approaches to NOx reduction have therefore focused on direct decomposition of NO to its elements or selective catalytic reduction with hydrocarbons to N2. Metal-doped ZSM-5 catalysts are active in these reactions, with Cu-ZSM-5 receiving most attention. A survey of the literature regarding the performance, reaction mechanism and durability of these catalysts under engine conditions is presented. Although copper-based ZSM-5 has adequate catalytic properties, there remain doubts about its ability to withstand diesel exhaust conditions, since they deactivate at high temperature, due to dealumination or/and Cu migration. © 1997 Elsevier Science Ltd. (Keywords: N O x reduction; zeolites; diesel exhaust)

Diesel engines are chosen as the power source for large mobile applications, i.e. trucks and buses, because of their reliability and efficiency. In smaller sizes they are increasingly displacing spark ignition engines in automobiles. The diesel cycle, which operates at higher compression ratios and with leaner fuel mixtures, has the added advantage of producing lower carbon monoxide and hydrocarbon emissions. However, the higher operating temperatures and oxygen-rich environment lead to the production of increased amounts of nitrogen oxides in the form of NO. Typical values, relevant to a DI diesel engine, lie in the range 30 to 800ppmv, in the presence of 800 to 1500ppmv of unburnt hydrocarbons (HC), 7vo1.% water vapour and 10vol.% oxygen 1. The increased NO emissions are accompanied by particulate emissions (soot), which appear particularly during acceleration and when the engine wears. The legislative emission limits in force for NOx are continually being lowered. 1For instance, in California they were set at 2.0gmile- in 1975, but this had been reduced to 0.4gmile- 1 by 19892 . The current level for Europe 3 is 0.97 g km -1 (combined HC and N O 0 , which was due to fall to 0.7gkm -1 in 1996 and a provisional 0.5 g km -1 in 1999. These European regulations have to be met by the vehicle during an MVEG test cycle3. During this cycle, the temperatures of the exhaust gases are rather low. For example, for a 1.9L TDI diesel engine, this temperature ranges from 100 to 300°C 4. This test cycle is essentially a transient one. The first part of this cycle (Urbain ECE) lasts 820s with a maximum speed of 60 km h- 1. The second part of the cycle

(Extra-urbain EUDC) lasts 400 s with a maximum speed of 120kmh -1. Another test cycle, ECE R-49, is used in Europe for trucks and buses 5. This test cycle comprises 13 modes involving steady-state testing. Improved engine design is leading to better-quality exhaust emissions, but will probably not meet the legislative requirements outlined above. For this reason, post-combustion cleanup procedures are required, as for spark ignition engines. The three-way catalysts developed for SI engines are unsuitable for NOx reduction in diesel engine exhausts, as they must be operated under near-stoichiometric conditions 6. Nevertheless, monolithic flow through diesel oxidation catalysts was used for passenger cars from the 1993-model year in EFTA (European Free Trade Association) 7. The same technology currently exists in the USA for trucks 8. These precious-metal-based catalysts are able to oxidize CO, hydrocarbons and the soluble organic fraction (SOF) of particulates 9. The amount of soot emitted by diesel engines can be decreased by ~ 3 0 w t % . In the future, a particulate trap will be necessary to meet the more stringent regulations. New catalysts which are effective under high-oxygen conditions for NOx reduction are being investigated. These catalysts will have to be efficient under severe conditions in terms of space velocity SV (h- 1), defined as the ratio of volumetric flow rate to catalyst volume (powder form) or monolith volume (powders washcoated). Space velocities ranging from 30000 to lS0000h -1 are used for laboratory tests or enginebench evaluation. The catalysts will also have to work over a wide temperature range: low temperatures to meet

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A review of NOx reduction on zeofitic catalysts under diesel exhaust conditions." P. Gilot et al.

the emission regulations during an MVEG cycle (urbain part) and high temperatures during highway driving. Moreover, they have to exhibit high thermal stability as well as low activity for sulfate formation due to sulfur present in the fuel. Catalysts which have been examined for this application include copper Boralite or zirconate 1°'11, metals supported on silicates or alumina 12-15, supported platinum and rhodium 1637, and hydrogen- and metalexchanged zeolites (predominantly ZSM-5). This review will concentrate on the performance of copper-doped ZSM-5 catalysts which have been found to offer potential for NOx control, although other metal exchange options will be mentioned. THE METAL-ZSM-5 CATALYST ZSM-5 is a crystalline aluminosilicate or zeolite of form Mx/,[(AlO2)x(SiO2)y].H20 prepared from aqueous solution by templated crystallization is. The hydrated crystals are activated by calcination before use. The structure of the resulting crystal contains oxygen ion tetrahedra with either Si4+ or A13+ at the centre, forming regular channels of diameter ~-5.5A and cavities of dimension ~ 9 A. These dimensions allow straight-chain and smaller branched hydrocarbons to enter the channels. The BET surface area is typically 300-400 m2g -1. Charge neutrality demands an additional positive charge for every A13÷ in the lattice. The metal M is sodium during initial preparation, but this can be exchanged for H + or other metals by subsequent treatment with a solution of the appropriate cation. The cations in the calcined catalyst are thought to be located in the channels and cavities, and to be mobile within the lattice 19. Room-temperature e.p.r, studies 2° of activated Cu-ZSM-5 have detected the presence of Cu 2+ and autoreduced Cu ÷. Significant amounts of copper are exchanged on to silicalite samples (ZSM-5 structure) containing no A1, and the adsorption/reaction properties of Cu at the aluminium sites are different from those elsewhere 21. Measurements by i.r. indicate that although ion exchange introduces Brensted acid sites into the crystal, the copper sites are neither Bransted nor Lewis acid sites. For NOx control applications, high-silica versions are used, with typical Si/A1 ratios of 20 : 1 to 50 : 1. When an exchange metal is incorporated into the crystal, a substitution level of 100% is taken to be the loading when one M 2+ ion is exchanged for every two H + ions. Because of spatial considerations in high-silica zeolites, one of the charges on an M 2+ ion needs to be met by an anion other than from the lattice. The catalyst will be designated according to exchange metal, Si/A1 ratio and level of metal exchange, e.g. Cu-ZSM-5-23-100 denotes an Si/A1 ratio of 23 and a copper exchange of 100%. At copper exchange levels of ~ 100% as Cu(II) which are typical in practice, the copper represents 1-2% by mass of the catalyst. ZSM-5 catalysts are active in a range of reactions and find service in many commercial applications such as the methanol-to-gasoline process 22, C8 isomerization reactions 23 and as an additive to boost octane production in fluid catalytic cracking 24. In the field of air pollution control, they catalyse two different reactions which result in the formation of molecular nitrogen from NOx. Under ideal appropriate conditions the oxides NO and NO2 can

508

Fuel 1997 Volume 76 Number 6

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Figure 1 Influence of temperature on the catalytic decomposition of NO over Cu-ZSM-5-50-73 (ref. 25). ©, Decomposition of NO; @, product N2; [], product 02. Conditions: 4vo1.% NO, SV ~ 1000h-1, catalyst in powder form be decomposed directly into the elements N2 and 02. Alternatively, doped ZSM-5 can catalyse the reduction of NOx to N2 by light hydrocarbons or oxygenates. The latter reaction is targeted as the most promising diesel emission control technique, because it can proceed under a large excess of oxygen. As the reductant operates under oxidizing conditions, it is a form of selective catalytic reduction (SCR). DECOMPOSITION OF NITROGEN OXIDES OVER Cu-ZSM-5 Iwamoto et al. 25 showed in 1986 that Cu-ZSM-5 can directly decompose nitrogen oxides; Figure 1 shows the effect of temperature on the conversion of NO with a 73% exchanged sample. Around 80% conversion to N2 is achieved at temperatures of 500-600°C. Iwamoto and Yahiro 26 have since confirmed the activity of this catalyst. Li and Hall 27 have shown that the rate is controlled by the desorption of oxygen. The effect of copper content on conversion has been studied by Iwamoto and Hamada 28, who found that at 30% exchange the conversion was just over 20% at 450°C but rose to > 90% at 100% exchange. The Si/A1 ratio also has an influence. Zhang and FlytzaniStephanopoulos 29reported that the presence of co-cations enhanced the activity of the catalyst, and proposed that they stabilize the active copper sites. The order of reaction with respect to NO was found to be unity in the operating temperature range 320-550°C 27'30. Adequate residence times are required for the reaction to proceed 25. The residence time is usually specified as its inverse, namely the space velocity. The presence of water vapour has an inhibiting but reversible effect on the decomposition of NO 28,31,32 whereas sulfur dioxide totally poisons the catalyst 28'. Since oxygen is a product of the decomposition and is present in diesel exhaust, it inhibits the reaction 3~. Catalysts deactivated by water vapour can be restored to activity by heating to ,,~ 500°C 31. These deficiencies in the decomposition approach to NO removal over doped ZSM-5 have led to concentration on its ability to catalyse the reduction of NOx to N2 with suitable hydrocarbons. SELECTIVE CATALYTIC REDUCTION OF NOx TO N 2 OVER ZSM-5 In stationary combustion plant, selective catalytic

A review of NOx reduction on zeofitic catalysts under diesel exhaust conditions." P. Gilot et al.

reduction of NO to N 2 is practised with ammonia or hydrocarbons ('reburning'). Platinum is used for low temperatures, V205 for intermediate temperatures and zeolites for high temperatures 33. Although ammonia works well with doped zeolites 34 and has been proposed for use with diesel exhausts 35, the conditions are not suitable. Transport and injection of the reagent are commercially unacceptable. Hydrocarbons however are carried as fuel for mobile engines and could be injected to control both particulates and NOx 36. Since the hydrocarbon concentration is not high enough in the exhaust, controlled direct fuel injection into the exhaust to increase the hydrocarbon/NOx ratio seems to be a promising strategy. Only laboratory experiments have been described 377'38. Ratios of 1:1 to 10:1 (hydrocarbon expressed as C1) were used. An excellent review by Shelef of the catalyst and the mechanisms of reaction has recently appeared 39. Shelef points out that despite favourable thermodynamics, the most demanding part of the process is the pairing of nitrogen atoms into the dinitrogen molecule. This is best done with nitrogen-containing reductants, and is partly the reason for the success of ammonia injection as an NOx control technique. Hydrocarbon reductants are active in reducing NO but are catalytically oxidized by the excess oxygen present. For this reason, SCR reactions are carried out at low temperatures, generally < 400°C. (

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Figure 2 Effect of temperature on SCR conversion efficiency of Cu-ZSM-5-27-100 catalyst (ref. 45). ~, NO conversion;., N2 production; A, HC conversion. Conditions: 300 ppmv NO, 300ppmv C3H6, 10vol.% 02, SV 50000h -l, catalyst in powder form

A range of hydrocarbons have been tested as reductants. The mixture of hydrocarbons present in diesel exhaust is a better reductant than those used in synthetic mixtures 4°, probably because of a higher concentration of free radicals. Methane is not an efficient reductant in the SCR reaction over Cu-ZSM-541 , so most laboratory investigations have used propene as reductant, chosen because it displays greater reducing ability than other small saturated, unsaturated or aromatic hydrocarbons 42. It has been shown that most hydrocarbons have some activity4~ but that carbon monoxide is ineffective43. Because the soot generated in diesel engines has a large specific surface area, it should have some effect in reducing NO,. emissions. This was demonstrated by Gilot et al.36 using carbon black deposited in a particulate trap. The extent of reduction of 300 ppmv of NO in gas at 5000h ~ SV was in the region of 5-10% in the presence of 10vol.% oxygen. It was also shown by Cooper and Thoss 41 that NO2 could initiate soot combustion at lower temperatures. CATALYST P E R F O R M A N C E The huge size of the possible market for NOx reduction technology in the transport field has led to a plethora of investigations into Cu-ZSM-5 performance. Some of the more significant results are reported here. A typical plot of conversion efficiency against temperature for this type of catalyst, in this case Cu-ZSM-523-137, is shown in Figure 2 45. The feed gas, of composition 300ppmv NO, 300ppmv propene and 10vol.% oxygen in helium, was supplied at a space velocity of 50000h -l. The conversion of NO to N 2 below 250°C is negligible, but rises to a maximum of 38% at N 400°C and then declines again. The decline at higher temperatures is due to excessive oxidation of the propene, which is thought to take place on the outside of the pores 46, while the formation of nitrogen occurs within them. Products other than elemental nitrogen, mostly NO2, are also formed from the nitric oxide. Above 400°C the NO/NO2 ratio in the product gases is the same as that at thermodynamic equilibrium. The extent of conversion depends on the operating conditions, including extent of metal loading, reductant, gas composition and stoichiometry, temperature and residence time (or space velocity). The maximum conversions recorded under the operating conditions used by

Table 1 Maximum reduction of NO by Cu-ZSM-5 catalysts

Reductant

Reductant concn. (ppmv)

NO (ppmv)

02 (vol.%)

Space velocity (h 1)

Temp. of maximum (°C)

Maximum conv. to N 2 (%)

Ref.

Propene Diesel exh. Propene Propane Ethylene Propane

500 250 700 2500 250 1900

1000 1000 250 250 1000 670

2 10 5 5 2 4

10000 20000 20000 20000 15000 30000

300 400 400 400 250 430

57 28 30 50 40 58

47a 41b 48b 48b 49a 50a

a Catalyst in powder form b Catalyst wash-coated

Fuel 1997 Volume 76 Number 6

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A review of NO x reduction on zeofitic catalysts under diesel exhaust conditions: P. Gilot et al.

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a number of researchers 41'47-5° are listed in Table 1. The effects of the different variables will now be discussed. The influence of level of exchange is well demonstrated by the results of Sato et al. 49, who found for the conditions listed in Table I that a maximum efficiency lay at ,,~ 100% exchange (see Figure 3). There is a loss in efficiency by over-exchanging, and slight conversion is achieved even with zero copper. A recent study has demonstrated 51 that the sites responsible for selective reduction are different from those responsible for the direct decomposition discussed previously. Teraoka et al. 52 have shown that the inclusion of co-cations with copper increases the activity of the catalyst and extends the active temperature range. The effectiveness of a number of reductants has been tested by numerous investigators, e.g. by Engler et al. 42 and by K o n n o et al. 4°, who supplemented actual diesel exhaust. In the 300-450°C range propene and butene performed slightly better than ethylene, but methane was inferior. In the absence of water vapour and SO2, propane is a better reductant than propene 53. Two studies 54'55 have found that long-chain hydrocarbons favour the reaction and lower the effective temperature range. Oxygenates such as ketones, esters and alcohols will also act as reductants 56. Higher concentrations of hydrocarbon in the gas

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Figure 3 Influence of copper exchange level on SCR conversion at 250°C (ref. 49). Conditions: NO 1000ppmv, C2H4 250ppmv, 02 2vo1.%, SV ~20000h -1, catalyst in powder form

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Fuel 1997 Volume 76 Number 6

Figure 5 Effect of oxygen concentration on SCR efficiency over Cu-ZSM-5-23-150 catalyst at 500°C (ref. 58). Conditions: 500ppmv NO, 900ppmv C3H6, SV ~ 10000h -1, catalyst in powder form favour NO reduction 36'41. Figure 4 shows the effect of propene concentration on the reduction of NO to N2 over Cu-ZSM-5-27-10045. In a laboratory reactor, the extent of conversion with Cu-ZSM-5-23-137 was found to be unchanged below a space velocity of ~ 5 0 0 0 0 h -1, but to fall rapidly above that value 49. Over a Pt-exchanged zeolite, the conversion fell uniformly as space velocity increased and was a function of temperature 42. Water vapour, which is present in diesel exhausts at 7-10vo1.% concentration, has an inhibiting effect on the reaction. Held et al. 57 found that the conversion at 350°C fell from 49 to 23% when 16vo1.% water vapour was added to dry reactant gas. In contrast to its effect on the NO decomposition reaction, sulfur dioxide has only a slight inhibiting effect on the SCR reaction 42'58. The concentration of oxygen in the gas is an important variable, as oxygen competes with NO for the reductant. In Figure 5, which is taken from Iwamoto et al. 58 but is typical of others 28'53, there is zero conversion in the absence of oxygen, but additions of ,,~ 1-2 vol.% lead to a massive increase. Further addition up to 12vo1.% causes a steady decline in conversion. REACTION MECHANISMS Under atmospheric conditions Liu and Robota 59 showed that the oxidation state of the copper is II, but thermal treatment under reducing conditions transforms up to 70% of the Cu 2+ ions to Cu +. They believed6° from the evidence of XANES that the Cu(I) is present as [O-Cu-O]. In contrast, Kucherov et al. 61 maintained on the basis of e.s.r, measurements that no spontaneous reduction of Cu 2+ is achieved below 500°C in vacuum or under helium. A close correlation between the number of Cu(I) sites and the extent of catalytic decomposition of NO has been demonstrated 59. It is claimed by Lei et al. 62 that [ C u - O Cu] 2+ is the active species in this decomposition, formed by dehydration of the Cu(OH) 2 originally present. At high temperatures (500°C) under oxidizing conditions, all the copper ions are found as Cu(II), but the [ C u - O Cu] 2+ is readily converted 63 to Cu + under inert gas at this temperature. The addition of 1 vol.% NO to the inert gas at 500°C returns the copper to the Cu(II) state 59, probably under

A review of NOx reduction on zeofitic catalysts under diesel exhaust conditions." P. Gilot et al.

the action of oxygen released by NO decomposition6°. There is an interaction between NO and oxidized Cu-ZSM-5, as the e.s.r, signal from Cu(II) diminishes in proportion to the NO partial pressure 64. Thus NO adsorbs readily on Cu ion-exchanged sites, and it is postulated 62 that Cu+-NO + forms. The presence of even small amounts of hydrocarbons influences the activity of the catalyst. It was shown by Liu and Robota 65 that with 3200ppmv of propene in 7.5 vol.% oxygen, a significant portion of the copper in ZSM-5 can exist as Cu(I). They reported a close correlation between catalyst reduction activity towards NO and the fraction of copper in oxidation state I. They identified different, coexisting forms of copper whose relative concentrations were determined by temperature. The form which appeared in the temperature range 200-375°C differed from that which decomposed NO into its elements, so they postulated the intervention of a hydrocarbon molecule. A comprehensive study of NO adsorption on to zeolite-type catalysts was undertaken by Zhang et al. 66. In the absence of hydrocarbon and oxygen, Li and Armor 67 showed that NO is adsorbed on to activated catalyst even at 25°C, with an initial evolution of N 2 and N20. TPD (temperature-programmed desorption) of the saturated catalyst identified NO emission peaks at 90, 140, 210 and 360°C. N20 was evolved principally at ~ 90 and ,-~ 140°C, while oxygen was evolved at ,-~360°C. This suggests that there are different modes of adsorption, some of which involve dissociation. Under diesel exhaust conditions, the species Cu-NO is formed initially43, followed by the adsorption of a second molecule to give Cu-(NO)268. The decomposition of this species and further adsorption was thought 68 to lead to the formation of C u - O - C u bridges. The copper at aluminium sites also interacts strongly with propene 21, which is catalytically destroyed by 2vo1.% oxygen over ~^^o~45 Cu-ZSM-5 at > Juu ~ . During the catalytic reduction process, both NO and hydrocarbon are involved. Some of the early suggestions regarding reaction mechanism, e.g. refs 27, 28, were based on studies of NO decomposition. The promoting role of oxygen is ascribed to cleaning and regeneration of the active sites 46'53. A catalytic redox reaction on the catalyst surface is involved in this approach 69. The presence of NO2 was shown '° to be necessary for the reduction reaction, as 40% conversion efficiency was achieved with NO2 but only 7% with NO under the same conditions. The small conversion with NO was thought to be due to the presence of some NO 2 formed by disproportionation. With oxygen present, the behaviour of NO and NO2 is similar, leading to the conclusion that above 350°C these gases are in thermodynamic equilibrium71. This was also the conclusion of Shelef et al. 72. Inui et al. 54 proposed the formation of an intermediate

of hydrocarbon and oxygen which cleaned the catalyst surface. The concept of an intermediate was taken up by 73 Burch and Millington , who regarded it as a 'coke' deposit resulting from the partial oxidation of hydrocarbon. In this scheme, the oxygen and hydrocarbon maintain the copper in an oxidation state suitable for catalysing the direct decomposition of NO into its elements. Carbon monoxide is ineffective as a reductant74. The existence of a partly oxidized hydrocarbon

intermediate is now generally accepted by most authors 16-18'43'75. Kharas proposed that nitrogen was integrated into it in the form of species such as Cu-(NO)2-R, which are susceptible to direct decomposition 76. Using i.r. spectroscopy at 2240cm -l, a number of workers, e.g. refs 77, 79, have identified the isocyanate radical on the catalyst surface. The intensity of the isocyanate band was found 77 to be proportional to the reducing activity of the catalyst. Isocyanate was also detected on other related catalysts during the NO-hydrocarbon reaction 8°. NCO and CN radicals have been identified on the catalyst surface8~ and HCNO was found in the exit gases of a Cu-ZSM-5 reactor treating an olefin-NOx feed82. The nitrogen-containing organic species R-ONO (i.r. band 1655cm 1) and CH3NO2 (1565cm -~) have also been proposed as surface intermediates83. Nitriles are favoured by other authors, e.g. refs 13, 84, who believe that isocyanate will be hydrolysed immediately to amine in the presence of water vapour, and identify the 2240cm I i.r. band with the nitrile function. The ammonia formed in this way79 has been proposed as the active species during SCR reduction 34, and NH bands have recently been detected at low oxygen concentrations85. Some form of coked hydrocarbon was proposed as the intermediate by Obuchi et al. 86, who found by the e.s.r. technique that a carbon radical was attached to the aluminium in ZSM-5. In a systematic study of the mechanism, Ansell et al. 87 came to the same conclusion. Bethke et al. 8s proposed a scheme involving the formation of an adsorbed NOz-hydrocarbon species which can reduce both NO and NO 2. A examination of mechanisms led to the proposal that a nitroso intermediate is involved89. Guyon et al. 9° have recently demonstrated the formation of an unidentified intermediate. Although readily formed in the presence of propene and either NO or NO2, it subsequently reacted only with NO2. The presence of oxygen is therefore necessary to oxidize the NO to NO2. After the intermediate is formed on the surface, oxygen competes with NO2 as a parasitic reactant. No equivalent deposit was recorded with propene and oxygen alone. EXCHANGE METALS OTHER THAN COPPER IN ZSM-5 Although copper is the exchange metal chosen for most ZSM-5 investigations, many other metals have been tested, either alone or as co-cations. Cerium, gallium, platinum and cobalt have received most attention, while the others tested include most common and catalytically active metals, including Pd 91, Ag 26, Ni, Mn, Fe and Cr 92 , Z n a n d C o 26 '49 , Z n , In, S n a n d A 1 93 , a n d I n a n d Pb 47. Acid-exchanged catalyst, i.e. H-ZSM-5, has also been studied. Gallium-exchanged ZSM-5 was tested in SCR with 1000ppmv each of NO and propene by Yogo e t a [ . 93, who found that in its general behaviour regarding exchange level, gas concentrations, space velocity etc. it performed similarly to its copper homologue. However, it gave a superior SCR result to copper in the range 300-600°C. Not only was the conversion of NO to N2 much higher over this range, but the oxidation of propene was lower. Another important difference 9a,

Fuel 1997 Volume 76 Number 6

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A review of NOx reduction on zeofitic catalysts under diesel exhaust conditions." P. Gilot et al.

also noted by Li and Armor 95, was its ability to use methane as reductant. The superior performance of cerium over the copper, sodium and acid forms was noted by Yokahama and Misono47 in 1992. The incorporation of alkaline earth metals as co-cations further improved the level of reduction96, which they attributed to increased acidity at the active sites or to migration of the cerium ions97. The mechanical addition of manganese and cerium oxides also promoted catalyst activity98. The attraction of Pt-ZSM-5 is its resistance to the influence of water vapour and sulfur dioxide, which barely affect conversion99'1°°. Although it gives high SCR efficiencies, especially at low temperatures, a significant fraction of the product gas is N201°°'1°1. This raises doubts as to its usefulness in practical applications. The behaviour of cobalt-exchanged ZSM-5 has been studied extensively by Li and Armor 1°2'1°4, who found it to perform generally in a similar way to Cu-ZSM-5. In contrast to Cu-ZSM-5, it is active with methane as reductant and has unusually high thermal stability. The acid form H-ZSM-5 exhibits SCR performance inferior to that of its metal-exchanged equivalent under similar conditions 1°5.

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EFFECT OF DIESEL CONDITIONS ON CATALYST PERFORMANCE Most of the studies of catalyst performance reported above were carried out in the laboratory under idealized operating conditions. The gases were synthetic mixtures, the space velocities were often low (long residence times in the catalyst bed) and the operation was restricted to short time periods in the steady state. Under actual diesel exhaust conditions, the space velocity will be in the region of 50 000-100 000 h -~. The catalyst is generally coated onto the ceramic monolith. It may experience overheating during soot regeneration, be poisoned by sulfur or suffer both chemical and mechanical degradation. Heimrich and Deviney1°6 examined the performance of three copper-exchanged zeolites at a space velocity of 20 000 h -1 in the exhaust of a six-cylinder diesel engine run on low-sulfur fuel. A sample of Cu-mordenite was active above 200°C but became ineffective after --~ 15 min. With Cu-ZSM-5 at 350°C, a steady 52% conversion efficiency was achieved over a period of 20 min. However, over periods measured in hours the performance deteriorates significantly76. Long periods of successful operation of ZSM-5 zeolites doped with noble metals have been reported by Ansell et al. 17 ~ Engler et al. 42 , Iwamoto et al. 107 and Takami et al. 10 . After running engines for the equivalent of 100 000 km of driving, Takami et al. 1°8 found that the SCR activity of a Pt-exchanged catalyst had diminished by ,-~25%. Sulfate formation was found to be a problem with some types of catalyst1°9. The influence of overheating on catalyst performance was studied by Kharas et al. 11°. Samples of Cu-ZSM-530.5-207 were heated to temperatures ranging from 600 to 800°C for 1 h under exhaust conditions and then tested. The conversion efficiency as a function of operating temperature is depicted in Figure 6. The maximum efficiency falls and moves to higher temperatures as the pretreatment temperature rises. The loss of efficiency is not due simply to temperature, as a 6h calcination in air at 600°C did not induce deactivation.

512

Fuel 1997 Volume 76 Number 6

Figure 6 Influence of pretreatment temperature on deactivation of Cu-ZSM-5-30-207 catalyst in powder form (ref. 110). II, fresh; fT, 600°C; e, 700°C; ©, 750°C; A, 800°C

However, exposure to water vapour at high temperatures has been shown to reduce activity significantly111. The dealumination of ZSM-5 catalysts by water has been identified as a major cause of deactivation 112'113. Steaming of Cu-ZSM-5-80-147 at 410°C produced an activity loss which corresponded with a loss of aluminium and which was not sustained in dry gas Ill. Higher copper exchange levels and the presence of alkaline earth co-cations minimize the loss 111 ' 114 . A second effect of elevated temperatures is a reduction in the volume of micropores, which implies a loss of internal surface area for adsorption. The heat treatment referred to above 11°oproduced a decrease in microporous volume (pores < 9 A) o f ~ 30% at 800°C. Kharas et alJ 15 believed however that dealumination is not the cause of the decline in microporous volume, but rather that deposits of A1, Si or Cu block the entrances to the pores. Alternatively, the copper may migrate into smaller channels in the crystal which are inaccessible to gases 113. A decrease in crystallinity, as indicated by X-ray diffraction, accompanies these changes n°'m. Spectra from X-ray diffraction and EXAFS also suggest 11°'116 that copper (II) oxide, CuO, which is not present in fresh catalyst, forms during thermal treatment. The intensity of the signal is directly related to the temperature of treatment. The presence of Cu20 is indicated by XRD but not by EXAFS. Tanabe and Matsumoto 117 concluded that microcrystals of CuO form under these conditions, and this is supported by electron microscopy11°. CONCLUSIONS Metal-doped ZSM-5 catalysts are active in promoting the conversion of nitric oxide to molecular nitrogen

A review of NOx reduction on zeolitic catalysts under diesel exhaust conditions." P. Gilot et al.

under oxidizing conditions, both by direct decomposition and by selective catalytic reduction with hydrocarbons. The sensitivity of the catalysts to poisoning by water vapour and sulfur dioxide precludes their use in the direct decomposition route. Reduction by hydrocarbons occurs readily in the range 300-500°C, and is favoured by high NO and hydrocarbon concentrations. For Cu-ZSM-5 a copper exchange level of ,-~ 100% is optimal. The reaction proceeds via an intermediate between adsorbed NO2 and hydrocarbon formed on the surface. Oxygen is required to oxidize the NO to NO2, but also attacks the hydrocarbon and the intermediate. The Cu-based catalysts are slightly inhibited by water vapour and CuSO4, due to the prolonged exposure to sulfur, and suffer some deactivation at elevated temperatures under exhaust conditions. Morever, they are not active at low temperatures ( < 300°C). They are therefore not at a commercial stage of development. With noble metal catalysts such as Pt-ZSM-5, long lives have been demonstrated under practical conditions. However, a major product is N20, another undesirable emission. A possible strategy could be to associate a lowtemperature catalyst (Pt-ZSM-5 for example) with a high-temperature catalyst. The new catalysts under development must also avoid the oxidation of SO2 to SO3 and must be used with a support which interacts weakly with sulfur oxides J18. These requirements must be met with the current fuel sulfur level of 500ppmw. Platinum-containing catalysts are promising 118

13 14 15 16 17 18 19 20 21 22 23 24

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ACKNOWLEDGEMENTS This work was supported by the French automotive constructors Renault SA and PSA, as well as the Minist+re de l'Enseignement Sup6rieur et de la Recherche. The authors greatly appreciate discussions with the group working on the VPE project.

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