Zeolite-based materials for the selective catalytic reduction of NOx with hydrocarbons

Microporous and Mesoporous Materials 30 (1999) 3–41

Review

Zeolite-based materials for the selective catalytic reduction of NO with hydrocarbons x Yvonne Traa, Beate Burger, Jens Weitkamp * Institute of Chemical Technology I, University of Stuttgart, D-70550 Stuttgart, Germany

Abstract This review covers all aspects of the selective catalytic reduction of nitrogen oxides with hydrocarbons (HC-SCR) over zeolite catalysts, mainly from the viewpoint of their potential for a practical application in exhaust gases from lean-burn engines. Emphasis is placed on HC-SCR over metal-containing zeolites with the exception of copper zeolites. Other items which are addressed are the combination of various metals on zeolites, combined exhaust gas purification systems and mechanistic considerations. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Diesel exhaust; Hydrocarbons; Lean NO reduction; Selective catalytic reduction; Zeolite catalysts x

1. Introduction The main pollutants in exhaust gases from vehicle engines include carbon dioxide, carbon monoxide, hydrocarbons, nitrogen oxides (NO ), x sulfur dioxide and particulates. Presently, the most significant problem is the removal of nitrogen oxides which are known to contribute to the formation of acid rain, smog and ground-level ozone. Therefore, legislation requires that the emission of NO is strictly limited. x The catalytic methods for removing NO from x engine exhaust gases are usually classified into (i) non-selective reduction, (ii) selective reduction and (iii) decomposition. The three-way catalytic converters used for the purification of emissions from spark ignition engines are an example for non* Corresponding author. E-mail address: [email protected] (J. Weitkamp)

selective reduction: in an exhaust gas containing essentially no excess oxygen, nitrogen oxides are ‘non-selectively’ reduced by hydrocarbons, carbon monoxide and/or hydrogen, i.e. a simultaneous removal of the major pollutants is achieved [1]. In contrast to spark ignition engines, lean-burn engines operate under highly oxidizing conditions. The only true lean-burn engine in widespread use staying in the lean operating region under all engine conditions is the diesel engine [2]. The desire for improved fuel economy and, concomitantly, lower emissions of carbon dioxide is supposed to increase the demand for diesel engines throughout the world. It is thus of great importance to develop catalytic technologies that will allow NO reduction in lean environments. x However, the traditional three-way catalyst does not control NO emissions in oxygen-rich exhaust x gas. The reduction of the NO content under such x conditions can be accomplished by using hydrocarbons as ‘selective’ reducing agents which prefer-

1387-1811/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 9 ) 0 0 03 0 - X

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Fig. 1. NO conversion into N on Cu/ZSM-5 (n /n =0.8) after Iwamoto et al. [7]. 2 Cu Al

entially react with the nitrogen oxides rather than with oxygen. This process is the so-called hydrocarbon selective catalytic reduction (HC-SCR) of nitrogen oxides. Hydrocarbons are usually not available in sufficient quantities for the complete removal of the nitrogen oxides and have to be added to the exhaust gas stream. Therefore, the simplest and cheapest method for the removal of nitrogen oxides from exhaust gas streams, viz. their catalytic decomposition, continues to be the ultimate goal. The decomposition of nitrogen oxides into oxygen and nitrogen is thermodynamically favored at temperatures below 900°C, but no suitable catalyst for a practical application has been found yet. Iwamoto et al. [3] reported in 1986 that Cu/ZSM-5 has an exceptionally high activity for the decomposition of NO . However, tests under x more realistic conditions, i.e. with sufficiently low NO concentrations representative of real exhaust gas conditions, with 5 vol.% O in the reactant 2 stream and with adequately high space velocities showed only little NO conversion [2]. In addition, water and sulfur dioxide which are omnipresent in diesel exhaust gases were found to poison the catalyst [4]. For these reasons, many experts believe that Cu/ZSM-5 will fail to play any practical role as a catalyst for NO decomposition. Nor x do we so far perceive from the literature any other zeolitic system with a promising performance as a

catalyst in NO decomposition. This approach will x therefore not be treated further in this review. It was discovered independently by Held and coworkers [5,6 ] and Iwamoto et al. [7] (and already earlier by Ritscher and Sandner [8] but without receiving broad attention) that the selective catalytic reduction of NO in an excess of x oxygen can be achieved with hydrocarbons over zeolites containing copper. In the following paragraph, some general features of the copper zeolite system will be discussed, which were reported by many authors and are in a way characteristic for many other catalysts as well. Fig. 1 shows the NO conversion1 into N on 2 Cu/ZSM-5 as a function of the reaction temperature [7]. The temperature at which the NO conversion reaches its maximum corresponds to the temperature where (nearly) maximum oxidation of the hydrocarbon is achieved. The drop in NO conversion at higher temperatures is most probably due to the more rapid oxidation of the hydrocarbon. The temperature of the maximal NO conversion is dependent on the nature of the catalyst, the type and concentration of the hydrocarbon, 1 In a rigorous chemical reaction engineering sense, the quantity plotted is not the true conversion, but a yield. However, since it is sometimes difficult from the pertinent literature to clearly identify the definitions, we will mostly quote the terms used by the authors.

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the time on stream and the space velocity. The maximum NO conversion increases with increasing concentration of the hydrocarbon reducing agent. Whereas the decomposition of NO into the elements is inhibited by oxygen, the selective NO reduction is promoted by the presence of oxygen. In the absence of oxygen, the NO conversion is negligible. Adding 2 vol.% oxygen effects maximum NO conversion, at higher concentrations the NO conversion decreases again. Another parameter with considerable impact on the NO conversion is the copper loading: the catalytic activity increases with increasing copper exchange level up to a value n /n of 0.5 to 1. If more copper is Cu Al incorporated, the efficiency of the catalyst tends to drop again. So far, a vast number of papers dealing with HC-SCR on copper zeolites, mainly copper on zeolites ZSM-5, mordenite and beta, have been published. However, the experimental conditions employed were often far away from the ones prevalent in real exhaust gases. Although it is known that water inhibits the reaction and effects catalyst deactivation, most researchers keep examining the reaction in a dry atmosphere. The work of Ishihara et al. is an exception: the presence of 15 vol.% water in the reactant stream decreases the NO conversion on Cu/SAPO-34, but the catalytic activity is still high and there is a negligibly small deactivation over 70 h [9]. Deactivation of copper zeolite catalysts tends, however, to be a problem over the much longer lifetimes needed in a real operation in a vehicle. Even if these deactivation problems are overcome in the future, the temperature range of high NO conversion starting x at about 300°C or above is not favorable: the average temperature of diesel exhaust gases is well below 300°C [10] with emission of a significant NO mass already at 120°C [11]. x The majority of papers have been aimed at elucidating the mechanism of the HC-SCR on Cu/ZSM-5 by means of spectroscopy, in situ spectroscopy or computer modeling. Nevertheless, the nature of the active copper species continues to be a matter of debate, and the reaction mechanism has not been unambiguously understood. This might be due to the variety of the characterization techniques employed and the disparity in the experimental conditions, the last point being

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the major problem in the observation of the HC-SCR reaction: a large variety of space velocities, reactant concentrations and feed compositions has been applied, making the comparison of the results described in different publications nearly impossible. A first attempt to overcome this problem was made by Cho: he introduced a normalization parameter as a function of the temperature, the activation energy, the space velocity and the metal loading. Provided that the concentration ratio among the reactants remains constant, the activity of different catalysts of the same type under various experimental conditions can be rated [12]. Since copper zeolites have become so popular and advanced to a model system receiving far too much attention with regard to their lack of significance for commercial application, the focus of this review will be on zeolite systems containing metals other than copper. For details about the copper systems the reader is referred to previous review articles, e.g. Refs. [13–19], or to the original publications.

2. HC-SCR on platinum-containing zeolites Platinum operates at significantly lower temperatures than does copper, probably as a result of its high capability to oxidize hydrocarbons. Platinum-containing ZSM-5 zeolites were already mentioned by Held et al. [5], but since they were used at a temperature as high as 300°C, they showed only little NO conversion. Later, x Iwamoto’s group tested Pt/Na–ZSM-5 zeolites [20–24]. Some fundamental features of the HC-SCR reaction on platinum-containing zeolites were worked out in Ref. [25]. $ In a feed stream with propene as reducing agent, propene conversions at temperatures higher than the temperature of maximum NO reduction were near 100%. Propene was completely oxidized to carbon dioxide and water, with carbon monoxide or other partial oxidation products not being detected. $ The presence of oxygen was found to enhance significantly the NO reduction activity at low temperatures, but higher NO conversions were

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observed at higher temperatures in the absence of oxygen. The selectivity of the NO reduction towards N O in the presence of oxygen was 2 very high and increased with increasing temperature, reaching nearly 100% at temperatures above 350°C. Under these conditions, the hydrocarbon conversion was almost complete which might be the reason for the lack of any further reduction of N O to N . 2 2 $ The presence of NO retarded the light-off of the hydrocarbon by ca. 50 K, both with C H 2 4 and C H , suggesting that the NO molecules 3 6 are adsorbed on, and at low temperatures block, the sites responsible for the oxidation of the hydrocarbons. $ Above the temperature of the maximum NO x reduction, considerable amounts of NO were 2 present in the reactor effluent. Therefore, one should, in any case, be very careful with the comparison of different results unless the same definition of conversion is used, e.g. overall NO conversion into N , N O and NO cannot be 2 2 2 compared with NO conversion into N and/or x 2 N O. 2 Fig. 2 shows the NO conversion into N and 2 N O on Pt/Na–ZSM-5 with n /n =0.5 using a 2 Pt Al model exhaust gas [20]. Whereas the conversion into N did not change significantly by the intro2 duction of 8.6 vol.% of water vapor, the conversion

into N O decreased. At a reaction temperature of 2 473 K, the ethene conversion increased from 34% to 96% by the addition of water; concomitantly, the temperature of maximum NO conversion was lowered from 485 K to 473 K. Since the CO 2 concentration at the outlet of the reactor steeply increased by switching from a dry to a wet feed stream, and then gradually decreased again to the original value, it was proposed that the water addition resulted in the removal of coke or another residue on the catalyst [20]. When the catalyst was exposed to real diesel exhaust gas, the NO converx sion was again very similar for different water contents, though the temperature of the maximum NO conversion was considerably higher at higher x water concentration, and the ‘temperature window’ within which the material was capable of achieving effective NO conversion was much x wider than with model exhaust gas [21]. On a Pt/Na–ZSM-5 catalyst with n /n =0.04 the time Pt Al dependence of the activity was investigated using a model exhaust gas with 10 vol.% of water: after 150 to 200 h a steady state was reached and maintained for the rest of the experiment for a period of 800 h [22]. So altogether, the presence of water has little effect on the overall performance of platinum-containing ZSM-5 zeolites. In Fig. 3 the effect of sulfur dioxide is depicted: the maximum conversion into N did not change 2

Fig. 2. Effect of water on the NO conversion into N and N O on Pt/Na–ZSM-5 (n /n =0.49) after Shin et al. [20]. 2 2 Pt Al

Y. Traa et al. / Microporous and Mesoporous Materials 30 (1999) 3–41

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Fig. 3. Effect of sulfur dioxide on the NO conversion into N and N O on Pt/Na–ZSM-5 (n /n =0.49) after Shin et al. [20]. 2 2 Pt Al

by the addition of 300 vol. ppm of SO , but the 2 temperature window was widened significantly and shifted to a higher temperature. The maximum conversion into N O decreased by about 4% [20]. 2 Fig. 4 shows the NO conversion on catalysts with different ion exchange levels of platinum: while the maximum conversion into N increased 2 with decreasing platinum content, the maximum conversion into N O decreased, i.e. the selectivity 2 to N O decreased. The temperature at the maxi2 mum conversion was shifted to higher temper-

atures with decreasing degree of platinum ion exchange [20]. In contrast to these results, Schneider et al. reported that the selectivity to N O decreased with increasing Pt content of the 2 H–ZSM-5/SiO washcoat. They were using, how2 ever, catalysts supported on cordierite honeycombs and a feed gas with propene as reducing agent and 10 vol.% of water [26 ]. When testing the effect of the gas hourly space velocity on the catalyst performance, Iwamoto et al. observed at a reaction temperature of 483 K

Fig. 4. NO conversion into N and N O on Pt/Na–ZSM-5 with different Pt contents after Shin et al. [20]. 2 2

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a NO conversion of 49% at 50 000 h−1 and 39% x at 150 000 h−1 [22]. This gives only part of the information, though. Cho et al. reported that the light-off temperatures (i.e. the temperatures where 50% conversion is reached) for both ethene and NO conversion on a Pt/H–ZSM-5 catalyst increased with increasing space velocity. The maximum NO conversion, however, decreased not significantly with a three-fold increase of the space velocity [27]. By contrast, Wunsch et al. noticed, on Pt/ZSM-5 on cordierite honeycombs, a considerable decrease of the maximum NO converx sion and the selectivity to N O with increasing 2 space velocity with a slight increase of the temperature at the maximum NO conversion [28]. x In further experiments, Iwamoto et al. examined the effect of ethylene addition: at constant reaction temperature, the degree of NO removal was more x than doubled with a two-fold increase of the C/N ratio, i.e. the ratio of the number of carbon atoms in the exhaust (unburnt and added hydrocarbon) to the number of NO molecules included. A closer x look revealed that the conversion to N increased 2 with increasing ethene concentration. The conversion into N O also increased slightly, but leveled 2 off at a certain C/N ratio. Hence, the selectivity into N O decreased with increasing C H concen2 2 4 tration [22]. Iwasaki et al. observed on a Pt/ZSM-5 honeycomb catalyst in real exhaust gas with ethylene as reducing agent a pronounced increase of the NO reduction with increasing C/N ratio as x well. Using kerosene as reducing agent, however, this effect was much less significant, and nearly indiscernible with n-hexane. The selectivity into N O was virtually independent of the C/N ratio 2 with values of 80 to 90% for ethene, 60 to 70% for kerosene and ca. 40% for n-hexane. At 210°C reaction temperature with a C/N ratio of three and a space velocity of 70 000 h−1, the NO reducx tion was 49% with ethene, 19% with kerosene and only 4% with n-hexane [29]. In this context, Burch and Scire conveyed some general ideas of the use of hydrocarbon reducing agents: they observed that Pt/ZSM-5 was completely non-selective towards the NO reaction in the presence of oxygen with either methane or ethane, i.e. NO was not reduced at all. (In contrast, Pt/ZSM-5 had a high activity for NO reduction with methane or ethane

in the absence of oxygen.) However, methane or ethane conversion started to increase rapidly at temperatures higher than 400°C. Therefore, in the presence of oxygen it seemed that hydrocarbon combustion was the preferred reaction at high temperatures. When lower temperatures were sufficient to activate the used hydrocarbons, as in the case of ethene, propene and propane, NO reduction was the preferred reaction. This means that only reducing agents which can be activated at low temperatures are useful with platinum catalysts [30]. Based on experiments with Pt/ZSM-5, Pt/mordenite and Pt/ferrierite, showing each almost the same maximum conversion at similar temperatures, Yahiro et al. stated that the zeolite structure had little effect on the activity [23]. It should be borne in mind, however, that they were using zeolites with different amounts of platinum. Ishibashi et al. reported that Pt on zeolites ZSM-5, mordenite and Y in their H+ and Na+ form displayed similar NO conversions. They were comparing catalysts with equal Pt contents by weight, respectively, but unfortunately they did not specify their reaction conditions [31]. In contrast, Amiridis et al. observed a significant effect of the zeolite structure and zeolite preparation on the catalytic activity. They compared different Y zeolites and ZSM-5 loaded with 1.2 wt.% of platinum each. A steam-dealuminated, acid-washed Pt/Y zeolite with n /n =14 in the form of a monolith showed Si Al the best catalytic performance and high resistance to poisoning by water and sulfur dioxide: at a space velocity of 40 000 h−1 (based on the monolith volume corresponding to at least a 10 times higher space velocity on the basis of the washcoated layer) the introduction of 10 vol.% of H O and 20 vol. ppm of SO to a feed stream 2 2 consisting of 1850 vol. ppm NO, 300 vol. ppm C H , 100 vol. ppm C H and 1 vol.% O in N 3 6 3 8 2 2 resulted in a decrease of the NO reduction from x 23.5% to 22.5% only [25,32]. Another report deals with platinum on zeolite mordenite as well: Tamura et al. observed an NO reduction of ca. 50% on platinum supported on natural mordenite with propane or gas oil as reducing agents at a reaction temperature of 430°C, however, the space velocity was as low as 1200 h−1 [33]. Jentys et al.

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[34] as well as Long and Yang [35] describe the reduction of NO with propene on Pt/MCM-41 x catalysts achieving NO conversions higher than x on Pt/Al O . The catalysts remained stable by 2 3 adding water and sulfur dioxide to the feed stream, thus opening new prospects for the catalytic application of MCM-41 materials and for Pt-catalyzed HC-SCR. Ko¨nig et al. compared conventionally prepared (i.e. via aqueous ion exchange or impregnation) platinum zeolites with their analogues prepared via solid-state ion exchange. The catalysts made under dry conditions were claimed to show a higher NO reduction and a considerably wider x temperature window with NO conversions under x realistic conditions of 10% or higher at temperatures between 170°C and 340°C [36 ]. The state of platinum and its change during the HC-SCR reactions was investigated by Shin et al. [20] and Kharas et al. [37]. After pretreatment of Pt/Na–ZSM-5 in He or O , Shin et al. failed to 2 observe PtO [20]. Rather, metallic platinum with two different particle sizes was found, one around 3 nm and the other around 13 nm. Since the latter is much larger than the channel diameter of the zeolite, it was concluded that the Pt particles are located outside the zeolite pores. After exposure to reaction temperatures of up to 450°C, this size distribution did not change significantly, indicating little sintering during the HC-SCR reaction [20]. Kharas et al. were led to conclude that Pt in zeolite H–ZSM-5 was initially oxidized, but after 1 h of lean-burn catalysis unambiguously in the metallic state. A fraction of the Pt might have occurred as cations at exchange sites besides particles with a length of up to 10 nm. After 40 h catalytic aging at 700°C, the maximum temperature of diesel exhaust gas at full load, a nearly total deactivation of the catalyst was reported. While the initial deactivation was tentatively ascribed to a moderate sintering of the platinum to particle sizes of up to 50 nm, the subsequent progressive deactivation was accompanied by the formation of films on the platinum crystallites, which were claimed to consist of siliceous material derived from the zeolite and to effect geometric site blockage. Oddly enough, the authors measured a selectivity to N O of 10% 2 only [37].

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Rottla¨nder et al. studied the NO reduction by propane and propene over Pt/H–ZSM-5 by a transient pulse technique using a TAP (temporal analysis of products [38]) reactor. In the presence of gas-phase oxygen at 500 K, propene was found to be far more effective than propane, which agrees well with the results obtained under flow conditions. This was explained by the presence of propene-derived adsorbates and their chemical interaction with NO leading to N , N O and 2 2 CO . With propane, such intermediates were not 2 observed. A small activity was ascribed to NO decomposition on reduced Pt sites [39]. Xin et al. examined the adsorbates during NO reduction by propene on Pt/Na–ZSM-5 by using in situ DRIFT (diffuse reflectance infrared Fourier transform) spectroscopy. At temperatures below 473 K mainly nitro (R–NO ) and nitrito (R–ONO) 2 organic compounds were found, whereas above 473 K only carbonyl, carbonate and carboxylate species were detected. Referring to catalytic data from the literature, the authors concluded that nitro and nitrito species were not catalytically active intermediates, since they occurred only at temperatures below the catalytic region [40]. Guo et al. [21] determined the effect of the geometry of the reactor and the shape of the catalyst on the activity: they noticed that the NO reduction efficiency was much higher with a x long and thin reactor compared with a reactor with a wide cross-section. In further experiments they compared the catalytic performance of honeycomb and pellet catalysts at the same space velocity and found that the NO reduction efficiency of the x honeycomb catalyst was very much lower than that of the pellet catalyst. When the same honeycomb was crushed to pellets, the NO reduction x efficiency recovered to the level of the original pellets in the same reactor. This might be due to the smaller surface/volume value and the lower gas flow velocity on the surface in the honeycomb compared with the pellet [21]. Acke et al. examined the activity of Pt/H– ZSM-5 under transient (temperature ramps) and steady state conditions. They found that the selectivity to N was somewhat higher in steady state 2 experiments compared with the heating ramp experiments [41]. However, since they were calcu-

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lating the conversion into N using the nitrogen 2 balance without directly detecting N , this state2 ment is not very reliable. Several other studies [29,42–46 ] deal with HC-SCR under more realistic conditions. For example, Pt zeolites washcoated onto honeycombs were exposed to real diesel exhaust gas or fairly realistic model gases, and appropriate amounts of reducing agent were added. In some cases, particulate matter and SO were also present, which 2 generally tend to reduce the NO conversion. x Nevertheless, the NO conversions achieved were x non-negligible. Wunsch et al. described the use of a diesel particle trap coated with Pt/ZSM-5 zeolite. The basic idea was that removal of NO and particulate x matter in two separate units is unfavorable because the temperature of the exhaust gas will be too low in the second unit. Whereas the catalytic activity of this system was high in model exhaust gases free from particulates, in real diesel exhaust gas the conditions for a thermal regeneration of the particulate trap were unfavorable even if highly active fuel additives were used (the incorporation of catalytically active metal compounds in the deposited soot significantly reduces the ignition temperature of the trapped particles). It was presumed that the zeolite plugged the honeycomb micropores resulting in an unacceptable increase of the back-pressure. In the FTP ( Federal Test Procedure) vehicle test cycle only a NO converx sion of 10% could be achieved over the total test. Thus, the simultaneous removal of NO , hydrox carbons, CO and particulates in one unit, which could be called a four-way catalyst, is not as effective as the combination of two separate units [28]. Cho [47] observed that zeolite-based lean NO x catalysts can store substantial amounts of hydrocarbons even at temperatures up to 500°C and that the maximum catalytic activity for NO reducx tion is achieved during the transition between rich and lean conditions. Therefore, he applied a cyclic operation method for the enhancement of the catalytic activity with feed conditions alternating between rich and lean without changing the overall time-averaged feed composition (still highly lean). This operating condition was attained by periodi-

cally adding hydrocarbon pulses. The catalysts stored the hydrocarbons during the rich half-cycle and released them during the lean half-cycle, thus preventing the catalyst surface from deactivation due to adsorbed oxygen and widening the temperature window of the catalyst. With a Pt/ZSM-5 catalyst at 500°C an NO conversion to N of 72% 2 was achieved by adding alternate pulses of ethene plus NO in Ar (rich half-cycle) and oxygen ( lean half-cycle) compared with 2% under steady state operation [47]. The value of 72% seems to be exceptionally high and, to the best of our knowledge, no further record of these findings has since appeared in the literature, hence doubts exist as to the reliability of the data. So, at first glance, platinum-containing zeolites seem to be very promising catalysts. A closer inspection reveals, however, that they possess considerable deficits as well. Firstly, the deactivation problem has not been overcome, although it is not so serious as with Cu/ZSM-5. Secondly, the high selectivity to N O, being a strong greenhouse gas 2 and contributing to stratospheric ozone destruction, is a major problem. One starting point is the report of Wunsch et al. who found that the addition of alkali-earth or rare-earth oxides to the catalyst reduces the N O formation. By adding 2 CeO to a Pt/ZSM-5 catalyst (m /m #4) the 2 CeO Pt maximum NO conversion increased2 from 48% to x 59% without a change in temperature, whereas the selectivity to N O decreased from 62% to 54% 2 [28]. Another solution might be the work of Burch and Ottery who reported that, on a Pt/Al O 2 3 catalyst, no detectable amount of N O was formed 2 with toluene as reducing agent at any temperature [48]. It remains to be seen whether this favorable effect of toluene holds for zeolite-based catalysts as well.

3. HC-SCR on cobalt-containing zeolites The salient feature of cobalt-containing catalysts is their ability to reduce NO with methane x (CH -SCR). On the one hand, the use of methane 4 is normally restricted, because the activation of the strong C–H bond often necessitates high reaction temperatures. On the other hand, methane is

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abundantly and readily available in many parts of the world and, therefore, very desirable as a selective reducing agent [49]. Especially with stationary engines operating with natural gas, CH -SCR 4 would be very cost-effective, as methane is a major component of the exhaust gas anyway. However, the use of methane as a reducing agent is also viable for mobile lean-burn engines: for that, methane has to be stored or produced on board the vehicle and added to the exhaust gas stream. Alternatively, the engine can be powered by natural gas or other methane-containing fuels [50]. This would be especially favorable, since vehicles fueled with natural gas do not produce soot or sulfur compounds and emit less CO (due to the 2 higher H/C ratio of CH ), CO, unburned hydro4 carbons and NO [51]. x The first report on CH -SCR over cobalt4 exchanged zeolites was by Li and Armor: on Co/Na–ZSM-5 (n /n =0.7), they observed at a Co Al reaction temperature of 400°C increasing NO conversion into N with increasing CH concentration, 2 4 reaching 95% conversion at c /c =2.4, but at CH4 NO a gas hourly space velocity as low as 7500 h−1 [52]. In contrast to Pt-containing catalysts, where the main reduction product was N O, on the 2 Co-containing catalysts neither N O nor CO was 2 detected [53]. The NO conversion was enhanced by the presence of some oxygen and remained almost constant with increasing oxygen concentration [52]. Instead of oxygen itself, N O could 2 serve as an efficient source of oxygen, greatly enhancing the NO conversion in the absence of oxygen [54,55]. In contrast to these results, Burch and Scire reported that the NO reduction on Co/ZSM-5 with ethane or methane was higher in the absence of oxygen than in the presence of oxygen, even though at higher temperatures, suggesting the occurrence of different reaction paths with or without oxygen [30]. The maximum NO conversion decreased with increasing space velocity, and the temperature at which the maximum conversion was obtained shifted to a higher value. At the maximum NO conversion, the CH conversion was about 80%. 4 The overall catalytic activity was proportional to the number of exchanged Co cations in the zeolite. Co/H–ZSM-5 was active at higher temperatures

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than Co/Na–ZSM-5. The NO conversion was proportional to the NO adsorption capacity of the catalyst at room temperature [56 ]. Co on zeolites ZSM-5, ZSM-11, mordenite and beta was active, whereas the zeolites Y and L as well as Al O were 2 3 poor supports [57]. Co/ferrierite was even more active (although its maximum conversion was obtained only at a temperature as high as 500°C ) than the above-mentioned Co/zeolites, which was related to its low activity for methane combustion [58,59]. However, this holds only in model exhaust gases without H O and SO . Water inhibited the 2 2 reaction, and did so much more on Co/H–ferrierite than on Co/H–ZSM-5 [60,61]. This inhibition was reversible upon eliminating water from the system and was more severe at lower temperatures. Temperature-programmed desorption studies on Co/Na–ZSM-5 suggested that the competitive adsorption between H O and NO was the reason 2 for the inhibition by water [62]. Upon introduction of SO , the NO conversion on Co/H–ZSM-5 was 2 slightly diminished at a reaction temperature of 500°C, considerably increased at 550°C and even doubled at 600°C, which was proposed to result from SO poisoning preferentially the sites more 2 active for CH combustion. With the simultaneous 4 presence of H O and SO , the NO conversion was 2 2 decreased at lower temperatures, but still slightly higher at 600°C than without H O and SO . On 2 2 the other hand, the NO conversion on Co/H– ferrierite was greatly diminished by the addition of SO , and even more so in the presence of both 2 H O and SO . One possible reason for this might 2 2 be that the eight-ring channels of ferrierite were blocked by SO molecules [61]. A role of the eight2 ring channels was also invoked to explain the high activity of Co/H–ferrierite in the absence of H O 2 and SO : Co2+ ions in eight-rings were supposed 2 to be more selective for the use of CH than 4 Co2+ ions in 10-rings where, due to the enhanced CH combustion, the CH concentration for NO 4 4 reduction was low. Correspondingly, the 10-ring zeolite ZSM-5 and even more so the 12-ring zeolite mordenite were less active supports than ferrierite [54]. Osaka Gas Co. in cooperation with Eniricerche S.p.A. were concerned with NO reduction princix pally on Co-containing beta zeolites using propane

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as reducing agent under quite realistic conditions, e.g. with large quantities of water in the feed [63– 74]. In a comparative study, it was found that Co/Na–beta zeolite was much more active with propane than with methane at reaction temperatures of 400°C and 500°C [64]. Other zeolites, like ZSM-5, ZSM-11, Y, mordenite and ferrierite were less active supports, when propane was used as reducing agent [65,72,74]. The best results, i.e. high NO conversions already at temperatures as x low as 350°C, were obtained on Co/beta zeolites with some B or Ti in the zeolite framework [65]. Considerably increased NO conversions at lower x temperatures were also obtained, when Ni was introduced as a second metal into the Co/beta zeolite, possibly because the oxidation activity of the catalyst was suppressed [66,67]. The most important fact worked out in these papers is the influence of the reducing agent: normally, CH -SCR was not influenced by 4 intracrystalline diffusion, since the reaction rate was considerably lower than with propane, whereas in C H -SCR the NO conversion was 3 8 x found to be higher on catalysts with a smaller crystal size [70]. It was suggested that the superior activity of the large-pore zeolite Co/beta could be ascribed to the ease of diffusion of reactants, products and inhibitors such as water and SO in 2 its channels [68,74]. By contrast, the activity of Co/ferrierite was low in C H -SCR, probably 3 8 because the diffusion in its small pores was hindered [72,74]. Corresponding results were reported by Shichi et al.: on Co/H–mordenite, they observed that intercrystalline diffusion did not affect the NO x reduction, whereas intracrystalline diffusion had a significant effect: the reaction rate over largecrystallite Co/H–mordenite was lower than over the small-crystallite zeolite. However, this effect was again only pronounced with propane as reducing agent, but not with methane [75]. In their studies (which occurred, in fact, somewhat earlier than the above-discussed work), Hall and coworkers went one step further: they examined the NO reduction on Co/Na–ZSM-5 in x model gases containing NO, O and various hydro2 carbons including methane, propane, isobutane, npentane, 2,2-dimethylpropane (neopentane),

3,3-dimethylpentane, 2,2,4-trimethylpentane and 3,3-diethylpentane (neononane). The hydrocarbon concentrations were varied to maintain a constant flux of carbon, e.g. 0.8 vol.% of methane, but 0.2 vol.% of isobutane. The maximum NO conversion as a function of the reducing agent followed the order isobutane>methane>neopentane#npentane#2,2,4-trimethylpentane>propane#3,3dimethylpentane&neononane. Whereas the maximum conversion was reached at a temperature of 400°C with isobutane and propane and 450°C with methane, n-pentane and 3,3-dimethylpentane, it was only achieved at 500°C with neopentane and 550°C with 2,2,4-trimethylpentane and neononane. For all hydrocarbons, the NO conversion was almost the same at a given degree of hydrocarbon conversion into CO except for neononane, where 2 it was much lower (cf. Fig. 5). As neononane is too bulky to enter the zeolite pores, it was expected to be particularly effective as a pore blocker, thus slowing down the NO reduction. It was concluded that the formation of nitrogen (or nitrogen dioxide which would then react at the external surface or in the gas phase) must occur in the intrazeolitic channels [76,77]. Many papers were devoted to the role of NO : 2 Hall and coworkers mainly studied the reduction of NO with methane on Co/ZSM-5 catalysts using x model gases without water [78–82]. They compared the NO reduction and the NO reduction 2 with methane in an excess of oxygen with and without the catalyst. With catalyst, the obtained results were virtually the same in the two reactions and in the reaction of NO with CH in the absence 2 4 of oxygen, as long as enough oxygen could be supplied by NO , indicating the pivotal role of 2 NO in the reaction. Oxygen was suggested not to 2 participate directly in the methane oxidation, but to be important for the oxidation of NO into NO . Without catalyst, no N was formed, whereas 2 2 methane combustion could be observed at somewhat higher temperatures [78]. It was proposed that the NO reduction and the CH oxidation were 4 coupled reactions and initiated by the reaction of CH with NO , resulting in the formation of a 4 2 CH · radical [79,80]. This was confirmed by Cant 3 and coworkers who, when using CH and/or 4 CD , observed a strong deuterium kinetic isotope 4

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13

Fig. 5. Effect of the nature of the hydrocarbon on the NO reduction selectivity on Co/Na–ZSM-5 (n /n =0.18) after Witzel et al. [76 ]. Co Al

effect, suggesting that the rate-determining step was the breaking of a carbon–hydrogen bond [83,84]. Nitromethane could then be formed, and Sun et al. indeed got indications from in situ DRIFT spectroscopy for the presence of nitromethane on the catalyst surface [85]. Several other studies dealt with the reactions of nitromethane over Co/ZSM-5. By means of quantitative IR spectroscopy, NH , HCN, HNCO, N O, N and 3 2 2 CO were detected as reaction products. This led 2 to speculations about the reaction mechanism. However, a clear distinction between byproducts, spectator species and reaction intermediates could not be made [81,82,86,87]. Yan et al. found that the activity of a physical mixture of Co/Al O and H–zeolites is much higher 2 3 in CH -SCR than the sum of the activities of the 4 individual components. This synergistic effect was tentatively explained by the generation of NO on 2 Co2+ ions and the subsequent reduction of NO 2 by CH over the H+ ions [88]. 4 Chong and coworkers reported that the activity of Co/H–ZSM-5 for C H -SCR in a model gas 3 8 without water increased with cobalt content up to n /n =0.5. Exceeding this level favored the proCo Al pane combustion resulting in decreased NO conversion [89]. Catalysts containing only cobalt in ion exchange positions were found to be inactive in NO formation and to require higher temper2 atures for achieving considerable NO conversion.

With increasing Co loading up to n /n =0.5, Co Al cobalt oxide particles were partly formed, which turned out to be active in NO oxidation, thus strongly enhancing the NO reduction at lower temperatures. Whereas catalysts not active in the NO formation became dark after the reaction, 2 catalysts active in NO formation did not change 2 their color, indicating that NO could also serve 2 as a scavenger for coke precursors. In addition, it was presumed that NO activated hydrocarbons 2 for the NO reduction [90]. On catalysts not active in NO formation, the presence of oxygen pro2 moted the NO reduction, which was tentatively ascribed to the formation of species like Co–O where NO was not that strongly adsorbed [91]. Catalysts prepared by incipient wetness impregnation were far more active at lower temperatures than catalysts prepared by ion exchange, which was suggested to be due to the presence of some cobalt oxide in the impregnated samples despite the low overall cobalt loading [92]. The introduction of Ca, Sr and especially Ba into the impregnated or ion-exchanged catalysts significantly increased the NO conversion, probably as a result of suppressed propane oxidation [89,92]. By contrast, Kawai and Sekizawa observed no considerable change of the activity of Co/H–ZSM-5 catalysts by pre-exchange with Ba2+ ions [93]. The results discussed so far are supplemented by the findings of Eniricerche S.p.A. and Osaka

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Gas Co. that the formation of Co O in excessively 3 4 loaded Co/beta catalysts clogged the micropores and hence decreased the activity [67,71]. This refutes the assumptions that NO plays a key role 2 in the HC-SCR and that the presence of Co O is 3 4 important. It was indeed observed (in contrast to the other studies with feed streams containing 9 vol.% of H O!) that the NO conversion was 2 x higher with NO as reactant than with NO , 2 although the C H conversion became higher with 3 8 NO . This indicates that too much NO in the gas 2 2 phase decreases the selectivity for the NO reducx tion, since the hydrocarbon is consumed by the reduction of NO to NO [69]. In samples with 2 n /n #0.5, an additional Co species was Co Al observed in the Raman spectrum which was supposed to be a Co–O–Co species or a cluster with more than three Co atoms. Since these samples exhibited a higher NO reduction activity and x selectivity than any other sample with lower or higher Co loading, it was suggested that this additional Co species was especially active. However, the NO oxidation activity of catalysts with n /n #0.5 was low, which is another arguCo Al ment against a key role of NO in the HC-SCR 2 [73]. Again in a model gas without water, Campa et al. found that the catalytic activities of Co/H– ZSM-5 and Co/Na–ZSM-5 having similar Co contents were well comparable, thus excluding any important involvement of acid sites. Another intriguing observation was that the selectivity to N O 2 reached values up to 30%, for Co catalysts a high value which has nowhere else been reported [94]. On the other hand, Miller et al. examined the reduction of NO by methane at 400°C over Co/H– 2 mordenite and Co/Na–mordenite. They found that the activity of the acidic zeolite was much higher and, therefore, claimed that the formation of N 2 occurs on the acid sites of the support, whereas Co2+ ions non-selectively reduce NO to NO [95]. 2 This conclusion could be premature, because no temperature dependence was investigated, i.e. the maximum activity of Co/Na–mordenite and Co/H–mordenite could be the same at different temperatures. However, Montes de Correa and Luz Villa de P. concluded as well from studies of CH -SCR with NO on Co/H–zeolites and H– 4 2

zeolites that the reduction of NO to N occurred 2 2 on the support rather than on the Co sites and appeared to be proportional to the zeolite acidity [96 ]. Many groups conducted IR studies of adsorbed species on Co catalysts: Iwamoto et al. observed that Co/ZSM-5 irreversibly adsorbed the largest amount of NO among various metal ionexchanged zeolites [97,98]. The irreversibly adsorbed NO species were mainly attributed to two different dinitrosyl adsorbates according to two kinds of cobalt ions [99]. The weakly adsorbed dinitrosyl species desorbed via a transient formation of mononitrosyl species [100]. Li et al. and Zhu et al. reported also the existence of dinitrosyl and mononitrosyl species over Co/zeolites exposed to NO. At temperatures above 200°C in an oxygencontaining atmosphere, all adsorbed NO species were found to disappear and to be replaced by adsorbed NO species [101,102]. Bell and cowork2 ers conducted in situ IR investigations during CH -SCR over Co/Na–ZSM-5 zeolite and 4 observed, besides mono- and dinitrosyl species, Al3+-NCO and Co2+-CN species. They suggested that the cyanide species were reaction intermediates and preferentially reacted with NO , which 2 was readily formed from NO and O and was 2 more strongly adsorbed than NO [103–105]. By contrast, Sachtler and coworkers presumed that the prevailing adsorption complex in Co/Na– ZSM-5 was the nitrito complex Co–ONO exchanging its nitrogen atom spontaneously with gaseous NO [106–108]. Sun et al. found by EPR and XPS that, regardless of the pretreatment, cobalt species in Co/Na– ZSM-5 were in the +II oxidation state, stabilized in the zeolite matrix. Therefore, they suggested that electron transfer might not be necessary for achieving NO reduction [109]. x Based on the possibility of an oxygenated reaction intermediate, Vassallo et al. used methanol as reducing agent in comparison with methane over Co/mordenite. The NO conversion started to increase at similar temperatures for both reducing agents, the maximum conversion being more than twice as high with methanol. Without catalyst, no NO conversion was observed with CH , but 30% 4 conversion with CH OH at 600°C with or without 3

Y. Traa et al. / Microporous and Mesoporous Materials 30 (1999) 3–41

oxygen in the feed stream. They concluded that CH OH could be an intermediate in CH -SCR 3 4 [110,111]. Gutierrez et al. observed that the addition of 0.5 wt.% of Pt to a Co/mordenite catalyst with 2.0 wt.% of Co led to a more than two-fold increase of the NO conversion in CH -SCR, if the catalyst 4 was pretreated in hydrogen. This was explained by the fact that, as observed by temperatureprogrammed reduction, the presence of Pt facilitated partial reduction of Co [112]. Several authors were employing more unusual zeolites or zeolite-like materials: Inui and coworkers were studying the NO reduction mainly with x cetane (n-hexadecane) on H–[Co]silicates with MFI structure and Co incorporated into the silicate framework synthesized by the rapid crystallization method [113–122]. In a feed stream containing 1000 vol. ppm NO, 3000 vol. ppm n-C H , 10 vol.% O and 0 to 10 vol.% H O at 16 34 2 2 a space velocity of about 31 000 h−1 and a reaction temperature of 500°C, a constant NO conversion of about 85% was obtained irrespective of the H O concentration. The addition of up to 10 vol.% 2 of CO or 200 vol. ppm of SO to reactant streams 2 2 free of water did not significantly change the NO conversion either [115]. A comparison between H–[Co]silicate and Co/Na–ZSM-5 revealed a strong loss of activity for C H -SCR of Co/Na– 3 6 ZSM-5 after calcination at 1000°C for 2 h in air, whereas the performance of H–[Co]silicate did not considerably alter, showing the high thermal stability of the material [119]. When exposed to air containing 10 vol.% of steam for 2 h at 800°C, the activity of Co/H–ZSM-5 decreased to less than half of the original activity; by contrast, the maximum NO conversion on H–[Co]silicate was shifted to a higher temperature and became slightly higher, possibly because of partial eduction of Co from the framework, which, in the form of highly dispersed cobalt oxide clusters, enhanced the combustion activity of the catalyst to a favorable extent. Similar results were obtained by using n-octane as reducing agent, the only difference being that after steam treatment the maximum NO conversion on H–[Co]silicate was shifted to a lower temperature [120,122]. Corma et al. reported the use of Co/MCM-22 in the C H -SCR and arrived at the 3 8

15

conclusion that Co/MCM-22 was a stable catalyst in feed streams without water, but less active than Co/ZSM-5 and Co/beta [123]. Ishihara et al. observed that the aluminophosphate Co/SAPO-34 was active in C H -SCR again in a reactant stream 3 6 without water, but less active than Cu/SAPO-34 [124]. Howe and coworkers tested the effect of a hydrothermal treatment (24 h at 800°C in 15 vol.% H O in air) on the performance of Co/ZSM-5 and 2 observed dealumination of the zeolite framework, causing loss of cation exchange capacity and residual Brønsted acidity with a concomitant decrease in the catalytic activity. Pre-exchange of the zeolite with La3+ ions stabilized the catalysts by inhibiting dealumination [125,126 ]. Correspondingly, Park reported that the activity of an amorphous La– Co–oxide perovskite phase incorporated into ZSM-5 zeolite was much higher in C H -SCR over 3 6 a broad temperature range than that of CoO/ZSM-5 and Co/ZSM-5 [127]. Pre-exchange of Co/TSZ zeolites (similar to ZSM-5) with K+ or Cs+ ions likewise increased the durability of the catalysts during treatment with a model gas with 3 vol.% of water for 15 h at 800°C [128]. Several other studies focused on HC-SCR under more realistic conditions, i.e. using real exhaust gases and/or catalysts washcoated onto honeycombs. According to the foregoing results, Eshita et al. observed that the additional incorporation of alkaline-earth metals and/or rare-earth metals into Co/H–ZSM-5 zeolites could inhibit coke formation and Co aggregation by forming perovskitetype complex oxides with Co. A Co–La–Sr/H– ZSM-5 catalyst on a cordierite honeycomb maintained 93% of its initial NO conversion efficiency x of 60% after exposure to lean-burn engine exhaust corresponding to a distance of 30 000 km [129]. Ciambelli et al. tested Co/H–ZSM-5 with n /n =2.3 in the exhaust gas of a spark ignition Co Al heavy-duty engine using methane as fuel and achieved a maximum NO conversion of only 20% x at 400°C [130]. However, the Co content of their catalyst might have been unfavorably high. Co/beta proved to be durable over a test period of 4000 h, converting about 65% of NO at 400°C, x a gas hourly space velocity of 15 000 h−1, /c =3.3 and 9 vol.% of H O as well as c 2 C3H8 NO

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Y. Traa et al. / Microporous and Mesoporous Materials 30 (1999) 3–41

0.3 vol. ppm of SO in the feed (cf. Fig. 6). In the 2 exhaust gas of a lean-burning gas engine without addition of a reducing agent, a NO conversion of x about 30% was still reached at the same gas hourly space velocity [63]. Takeshima et al. observed an increase in NO conversion over a Co/ZSM-5 x catalyst in the exhaust of a lean-burn engine by sulfurizing the catalyst via H S treatment, thereby 2 reducing the oxidizing activity and enhancing the selectivity for NO reduction. Washing of the x sulfurized catalyst with glycol further increased the catalytic activity, probably because pore clogging cobalt sulfide (formed by H S treatment of cobalt 2 oxide) was removed [131]. Miyoshi studied the performance of a Co/ZSM-5 honeycomb catalyst in simulated exhaust gas and found, upon heat treatment up to 800°C, hardly any change in crystallinity, even though dealumination did occur [132]. Rak and Veringa examined the catalytic activity of Co/Na–ZSM-5 with an alumina binder on cordierite honeycombs. Unfortunately, they were using model exhaust gases without water and low gas hourly space velocities, therefore making a statement on the practical applicability of their catalysts unreasonable [133]. To conclude, cobalt-containing zeolites first appeared to be promising catalysts, because they were able to activate methane, which would be a very desirable reducing agent due to its availability.

However, the reaction temperatures necessary were too high for an application in diesel exhaust gas. ( For natural gas engines, the use of Co/zeolites might be possible because of the higher exhaust gas temperatures.) In addition, the catalysts were sensitive to water and sulfur dioxide and their activity was too low. Nevertheless, most studies were conducted under unrealistic conditions in model exhaust gases without water. The reader has to be very careful and should not compare results obtained in the absence of water with those collected in the presence of water.

4. HC-SCR on palladium-containing zeolites Palladium-containing zeolites can be used, inter alia, with methane as a reducing agent. Misono and coworkers reported a high activity of Pd/H– ZSM-5 (and Pd–Ce/H–ZSM-5) in the CH -SCR 4 without water in the feed stream, reaching a maximum NO conversion to N of ~70% at a temper2 ature around 475°C, c /c =2 and a gas hourly CH4 NO space velocity of 9000 h−1. The maximum conversion to N O was only 7%. With oxygen concen2 trations between 1 and 5 vol.%, very similar results were obtained, whereas without oxygen, the NO conversion was considerably lower but still sufficiently high [134]. By comparing the activity of

Fig. 6. Durability of Co/beta (n /n =0.49) and Co/ZSM-5 (n /n =0.53) after Ohtsuka et al. [63]. Co Al Co Al

Y. Traa et al. / Microporous and Mesoporous Materials 30 (1999) 3–41

Pd/H–ZSM-5 and Pd/Na–ZSM-5 with similar dispersions of palladium, it was demonstrated that protonic acidity is indispensable for the CH -SCR over Pd/zeolites, since Pd/Na–ZSM-5 4 was completely inactive. Pd/H–ZSM-5 samples with the same metal content but different degrees of metal dispersion displayed similar activities at lower temperatures; at higher temperatures, however, the sample with the higher dispersion was much more active [135]. In further studies, several reactions of NO and NO over Pd/H–ZSM-5 were 2 examined and the following reaction mechanism was derived: NO is oxidized to NO on metal or 2 acidic sites. NO reacts on acid sites with methane, 2 activated over palladium, forming nitrogen and carbon oxides [136 ]. Upon replacement of CH 4 by CD in the feed, a kinetic isotope effect was 4 observed which was, however, smaller than with Co/ZSM-5, suggesting that there are other ratedetermining steps in the Pd system besides the C– H dissociation, namely the NO oxidation [137]. By introduction of 3.5 vol.% of water into the reactant stream, the NO conversion was considerx ably reduced, possibly because of poisoning of the Brønsted acid sites by water. The addition of Rh to Pd/H–ZSM-5 led to a catalyst with high activity even in the presence of water [136 ]. Primet and coworkers compared the activity of Pd in CH -SCR on different H–zeolites, including 4 ZSM-5, mordenite and Y, using a model exhaust gas without water and relatively little oxygen (c /c =6.2). Pd/H–ZSM-5 and Pd/H–mordeO CH4 nite2 exhibited similar activities, whereas Pd/H–Y was hardly active. From the IR spectra, they concluded that, in zeolite Y, the formation of active nitrosyl complexes might be inhibited due to a migration of Pd into the small cavities where nitrosyl complexes could not be formed [138,139]. The catalytic activity of Pd/H–ZSM-5 and Pd/H– mordenite was found to be maximum at palladium contents of 0.5 and 0.8 wt.%, respectively. The selectivity of Pd/H–ZSM-5 to N O increased with 2 increasing oxygen content up to about 25% in the presence of 10 vol.% of O [51,140]. Steam aging 2 at 800°C in 10 vol.% of H O in N for 6 h resulted 2 2 in the total loss of the NO reduction activity of x Pd/H–ZSM-5 and Pd/H–mordenite, probably due to dealumination of the zeolite and the concomi-

17

tant disappearance of ion exchange sites. Therefore, the zeolite structure was deprived of its ability to anchor Pd ions, resulting in the formation of large Pd particles on the external surface of the zeolite crystals. Alternatively, the formation of Pd(OH ) was suggested which, not being attached 2 to the zeolite and hence highly mobile, would migrate to the outer surface of the zeolite and condense into PdO, which is unstable at high temperatures and transforms into Pd metal particles [138,141]. Whereas Co occurred in the +II oxidation state only, Pd could be obtained as Pd3+, Pd2+and Pd+. From IR and quantitative mass spectrometry studies, it was concluded that adsorption of NO onto Pd/H–ZSM-5 at room temperature results in the reduction of Pd2+ ions from the +II to the +I oxidation state with the concomitant oxidation of NO to NO . Subsequently, Pd(I )–NO com2 plexes are formed which can coordinate other molecules like H O and NO . So, as on Co cata2 2 lysts, NO is also formed on Pd catalysts, even 2 though not through oxidation by oxygen, but by reduction of Pd(II ) [142,143]. However, since all these studies were conducted in an oxygen-free atmosphere, the findings are not necessarily significant for strongly oxidizing conditions. Bell et al. suggested that in Pd/H–ZSM-5 with a palladium loading below 4 wt.%, Pd is present as PdO coordinated with two Brønsted sites, e.g. zeolite−H+(PdO)H+zeolite−, a complex which is highly selective for CH -SCR. At higher Pd 4 contents, PdO particles were formed being only active for the CH oxidation. With in situ IR 4 spectroscopy, they found little evidence for adsorbed nitrito or nitrate groups under reaction conditions, but they observed cyanide species as precursors for N and CO [103]. 2 2 On the basis of other studies, Pd2+ was claimed to be the active species: Adelman and Sachtler demonstrated by use of temperature-programmed reduction that NO promotes the conversion of 2 PdO particles to Pd2+ ions via a reaction with zeolitic protons, possibly by formation of small Pd(NO ) particles or Pd(NO )+ ions which can 32 3 diffuse faster through the zeolite channels than Pd2+ ions. Consistent with this view, exposure of a PdO-containing Pd/H–ZSM-5 catalyst to a reac-

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tion mixture containing CH , NO and O resulted 4 2 2 in a slowly increasing NO reduction rate due to 2 protonolysis of PdO to Pd2+ [144]. Resasco and coworkers observed with X-ray absorption that metallic Pd particles on acidic supports were transformed into Pd2+ ions upon exposure to a reaction mixture containing NO, CH and O , whereas on 4 2 non-acidic zeolites, Pd was oxidized to PdO clusters. They concluded that the presence of Pd2+ ions was a necessary but not sufficient condition for a high HC-SCR selectivity and proposed that the absence of PdO clusters was, at least partially, responsible for a high catalytic activity [145]. In a further study, they examined the influence of the acidity on the NO conversion: at first glance, Pd/H–ZSM-5 and Pd/Na–ZSM-5 had, at variance with the results of Nishizaka and Misono [135], similar activities [146 ]. However, Nishizaka and Misono pretreated their catalysts by calcination in air, whereas Loughran and Resasco reduced them in hydrogen, thereby introducing some acid sites into Pd/Na–ZSM-5 by reduction of the Pd precursor. After back-exchanging the reduced Pd/Na– ZSM-5 sample with a sodium acetate solution, the activity of the sample was strongly decreased, thus eventually confirming the results of Nishizaka and Misono. The highest NO conversion was achieved on Pd/H–ZSM-5 with a Pd content around 0.3 wt.%. On these catalysts with a low Pd loading, the presence of oxygen increased the NO conversion, whereas, e.g. on a catalyst with 0.8 wt.% Pd, the NO conversion was higher in the absence of oxygen. It was concluded that the most selective catalysts are those with the palladium sites inside the zeolite channels [146 ]. Niwa and coworkers tested the effect of chemical vapor deposition of silicon alkoxides on the catalytic performance of Pd/H–mordenite and Pd/H–ZSM-5 zeolites for CH -SCR. In the 4 absence of water, the NO conversion above 700 K was enhanced compared with the non-modified catalyst. In the presence of water, the maximum NO conversion was only slightly lowered, whereas on the non-modified catalyst the conversion was reduced to less than half of the initial conversion (cf. Fig. 7). The reason for this enhanced water tolerance of the modified samples was proposed to be the inactivation of the external surface sup-

pressing the adsorption of water and retarding the sintering of palladium [147,148]. Many authors studied the reduction on Pd-containing zeolites using reducing agents other than methane, e.g. C H [20,23,149], C H 2 4 3 6 [44,134,150], (CH ) O [151] and CH OH [144], 32 3 however, only NO conversions below 20% were achieved. By contrast, high and stable conversions were reported for C H -SCR on Pd/Na–Y zeolite 3 8 with the very high Pd content of 7 wt.% and a /c =5) [152]. high concentration of C H (c 3 8 C H8 NO Tamura et al. observed a high 3activity of Pd on natural mordenite in C H -SCR, but the space 3 8 velocity was as low as 1200 h−1 [33]. Uchida et al. achieved relatively high conversions on Pd/ mordenite with C H , C H and C H as reducing 2 6 3 8 4 10 agents, though with extremely high hydrocarbon concentrations (c /c #14). However, HC, C1 basis NO the conversions were lower than with the corresponding amount of CH as reducing agent, 4 because ethane, propane and butane were readily oxidized by oxygen [153]. Engler et al. reported the use of Pd/zeolite monoliths in a model gas containing H O, SO and a mixture of C H and 2 2 3 6 C H as reducing agent. The catalyst showed 3 8 almost no activity [42]. Ishibashi et al. employed Pd/zeolites on honeycombs, but they did not give their reaction conditions [31]. Ogura et al. observed that Pd–Co/H–ZSM-5 catalysts had a considerably higher activity in CH -SCR than the corresponding Pd/H–ZSM-5 4 and Co/H–ZSM-5 catalysts. Besides, the addition of 10 vol.% of water to the feed stream lowered the catalytic activity of Pd/H–ZSM-5 and even more so of Co/H–ZSM-5, whereas the performance of Pd–Co/H–ZSM-5 was hardly affected. The role of Co was attributed to the acceleration of the NO oxidation [154]. Kawai as well as Kagawa et al. achieved corresponding results with Pd–Co/H–ZSM-5 [155,156 ] and Hamon et al. with Pd–Co/H–mordenite [157]. In conclusion, the palladium system does not seem to have advantages over the cobalt system. A direct comparison between the two metals is, however, difficult to realize and was seldom performed, because the cobalt loading chosen was usually much higher than the palladium loading. Ogura et al. examined Pd/H–ZSM-5 and Co/H–

Y. Traa et al. / Microporous and Mesoporous Materials 30 (1999) 3–41

19

Fig. 7. Enhanced water tolerance of Pd/H–ZSM-5 by chemical vapor deposition of silica after Suzuki et al. [147].

ZSM-5 under the same conditions and found that Pd/H–ZSM-5 was superior to Co/H–ZSM-5, but since they did not give the metal contents of the catalysts, the comparison is not very informative [154]. By contrast, Kawai reported that Pd/H– ZSM-5 with a very low palladium content, though (n /n =0.05), was nearly inactive compared with Pd Al Co/H–ZSM-5 with n /n =0.7 [155]. In any case, Co Al studies on the time on stream behavior of Pd-containing zeolites in real exhaust gas would be very helpful for an evaluation of their potential for a practical application.

5. HC-SCR on gallium- or indium-containing zeolites Other metals facilitating the use of methane as reducing agent are gallium and indium. The first to report on HC-SCR over these metals were Kikuchi and coworkers. At the beginning, they were employing propane as reductant over several zeolites ion-exchanged with gallium. In a feed stream without water and with a relatively low space velocity, Ga/H–ZSM-5 and Ga/H–ferrierite displayed NO conversions higher than 60% in a wide range of reaction temperatures from 300°C to 600°C. Ga/H–mordenite was also highly active, but only at temperatures above 350°C. Ga/H–

USY was nearly inactive [158]. In further studies, ethene [159], ethane [160] and methane [160] were used as reducing agents over Ga/H–ZSM-5 zeolite. The salient feature on gallium-containing zeolites seems to be the extremely high hydrocarbon efficiency for NO reduction (compared with being burnt). Ga/H–ferrierite was claimed to be the most selective catalyst ever reported, e.g. at 300°C under appropriate conditions, three NO molecules could be reduced with one propane molecule [161]. A major disadvantage was, however, that CO was the primary reaction product, at least with methane as reductant. At high gas hourly space velocities virtually no CO was formed. In/H–ZSM-5 was 2 found to be highly active in CH -SCR as well, the 4 temperature of the maximum NO conversion being around 400°C, i.e. about 100 K lower than on Ga/H–ZSM-5. In contrast to Ga/H–ZSM-5, In/H– ZSM-5 yielded CO almost exclusively, with CO 2 being formed only at very high space velocities [162,163]. No N O was detected as reaction pro2 duct on either of these catalysts [164]. The selectivity for the NO reduction was slightly lower on In/H–ZSM-5 than on Ga/H–ZSM-5. The especially high selectivity of Ga/H–ZSM-5 was attributed to coordinatively unsaturated Ga cations which can efficiently coordinate NO and hydrox carbon species. By addition of 10 vol.% of water to a reactant stream containing NO, CH and O , 4 2

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the activity of Ga/H–ZSM-5 was totally suppressed; In/H–ZSM-5 still exhibited a maximum NO conversion of about 10%. If NO was used as 2 reactant instead of NO, however, an NO conversion of about 50% was achieved on In/H–ZSM-5 at 500°C (cf. Fig. 8). It was proposed that water is preferentially adsorbed on unsaturated gallium or aluminum sites, thus inhibiting the adsorption of NO and/or hydrocarbon species. The higher x resistance of In/H–ZSM-5 to steam was attributed to the smaller affinity of water to indium cations than to gallium cations [162,163]. Tabata and coworkers found that oxygen could not be reversibly adsorbed on Ga/H–ZSM-5, whereas methane was dissociatively adsorbed. This was suggested to be the reason for the extraordinarily high NO reduction selectivity of Ga/H– ZSM-5. Preadsorption of water considerably decreased the amount of methane adsorbed on Ga/H–ZSM-5, while only a small decrease was observed on In/H–ZSM-5. The authors therefore concluded, in line with Kikuchi and coworkers, that the better performance of In/H–ZSM-5 in a wet atmosphere compared with Ga/H–ZSM-5 was due to the competitive water adsorption which seems to inhibit methane adsorption much less on In/H–ZSM-5 than on Ga/H–ZSM-5 [165,166 ]. Based on the assumption of [GaO]+ cationic

species as gallium sites, molecular dynamics, computer graphics and quantum chemical calculations gave results which supported the view that the large nucleophilic space around gallium was capable of activating non-polar hydrocarbons. Water diminished the nucleophilic region, thus hindering the adsorption of other reactants, whereby the high sensitivity of Ga/H–ZSM-5 to water was explained [167–170]. Li and Armor observed that gallium impregnated onto Na–ZSM-5 had no activity at all for CH -SCR. By contrast, Ga/H–ZSM-5 was active 4 irrespective of the mode of preparation, i.e. ion exchange or impregnation. Apparently, the presence of H+ ions in the zeolite is vital which suggests a synergism between gallium species and H+ ions. TPD of ammonia showed that there was about the same amount of H+ ions in Ga/H– ZSM-5 as in H–ZSM-5. From this the authors concluded that most of the NH+ ions in 4 NH –ZSM-5 were not exchanged by gallium ions 4 but that gallium was mainly precipitated onto ZSM-5, presumably because the hydrated gallium is too bulky to enter the zeolite pores. This assumption was further supported by IR spectroscopy and pore volume measurements. It was, however, believed that a small amount of gallium does enter the zeolite pores, either during the preparation or

Fig. 8. Effect of the NO reactant on the NO conversion on In/H–ZSM-5 (n /n =0.31) and Ga/H–ZSM-5 (n /n =0.32) in a wet x x In Al Ga Al feed after Kikuchi and Yogo [162].

Y. Traa et al. / Microporous and Mesoporous Materials 30 (1999) 3–41

as a result of high temperature treatment, inducing a solid-state ion exchange between Ga O and 2 3 H+, and that this gallium inside the channels is catalytically more important [171]. Kikuchi and coworkers compared the catalytic activities of Ga/H–ZSM-5 and In/H–ZSM-5 prepared by ion exchange and of Ga O /H–ZSM-5 2 3 and In O /H–ZSM-5 prepared by physically 2 3 mixing H–ZSM-5 with Ga O and In O , respec2 3 2 3 tively. Ga/H–ZSM-5 was much more active in CH -SCR of NO than Ga O /H–ZSM-5, whereas 4 2 2 3 In/H–ZSM-5 and In O /H–ZSM-5 were equally 2 3 active. The same was true for In on H–mordenite and H–beta. Based on an IR study, these findings were interpreted in terms of a solid-state ion exchange occurring in the indium system: In O +2H+zeolite−2[InO]+zeolite−+H O 2 3 2 (1) [164,172,173]. Summarizing all their studies, the authors proposed that CH -SCR started with the 4 oxidation of NO by O to NO on acid sites of 2 2 the zeolite. Subsequently, NO would react with 2 CH and NO to N , H O and CO/CO on 4 2 2 2 [GaO]+ or [InO]+ sites. Oxidation of CO to CO could proceed on Ga O or In O , with 2 2 3 2 3 In O being much more active [164]. 2 3 Solid-state ion exchange was also observed by Zhang and coworkers on In/H–ZSM-5 prepared by impregnation of H–ZSM-5 with In(NO ) . At 33 variance to the results of Kikuchi and coworkers, catalysts prepared by mechanically mixing In O 2 3 and H–ZSM-5 were much less active than In/H– ZSM-5 [174,175]. Perhaps this inconsistency stems from the different metal loading and pretreatment conditions (4 wt.% In [164] vs. 10 wt.% In [174] and calcination at 540°C [164] vs. 700°C [174]), the different reactants (NO [164] vs. NO [174]) 2 or, since the reaction is diffusion-limited, different particle sizes [173]. Richter et al. reported that In/H–mordenite catalysts prepared by ion exchange or impregnation displayed similar activities. These authors moreover showed that In on ion exchange positions had strong Lewis-acid character [176 ]. Inui and coworkers were studying the NO x reduction again with n-hexadecane on H–

21

[Ga]silicates with Ga incorporated into the silicate framework. As on Ga/H–ZSM-5, CO was a major reaction product [113,114,117,121,177]. Kikuchi et al. observed a rather fast deactivation of H– [Ga]silicates in C H -SCR [178]. In similar studies, 3 6 Hayasaka and Kimura were employing galloaluminosilicates with MFI structure impregnated with various promoters and gallium impregnated onto zeolites Y and ZSM-5. At low gas hourly space velocities, high NO conversions could be observed [179]. Jacobs et al. used gallium-impregnated H– beta zeolite and achieved with propane and propene as reductants an NO conversion of 58% at 380°C, even though the feed stream contained 12 vol.% of H O and the space velocity was fairly 2 high [180]. Ga/EMT exhibited a maximum NO conversion of only 10% at 600°C [181]. Feeley et al. reported that sulfated Ga–Zr/H–ZSM-5 displayed among various similar catalysts the highest activity in NO reduction with CH or C H in the 4 3 8 presence of 50 vol. ppm SO and had a high hydro2 thermal stability. However, the presence of only 2 vol.% of water in the feed reversibly reduced the NO reduction to values below 10% [182]. x Misono and coworkers observed that In/Na– ZSM-5 was active for C H -SCR in the absence 3 6 of water [183,184]. Heinisch et al. achieved on an In/H–ZSM-5 honeycomb catalyst high NO conversions with methane or natural gas as reductants, which remained stable over 9.5 h. However, the gas hourly space velocity amounted to 8000 h−1 which is a relatively low value, and they were using a dry feed [185]. Based on the fact that the inhibiting effect of water vapor is stronger in the NO–CH –O system 4 2 than in the NO –CH –O system, because the NO 2 4 2 oxidation is hindered by H O molecules coordi2 nated to zeolite acid sites (vide supra), Kikuchi and coworkers were looking for a substitute for the acid sites, which would oxidize NO to NO 2 even in the presence of water. They found that In/H–ZSM-5 impregnated with Pt, Rh or Ir exhibited high NO reduction activity in feeds containing water. Interestingly, the formation of N O was 2 negligible on these catalysts, i.e. with respect to the N O selectivity, In dominated the catalyst 2 performance. In further studies, especially on Ir– In/H–ZSM-5, it was realized that Ir not only

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promotes the NO oxidation, but also enhances the NO adsorption. The authors proposed that x NO, which diffuses faster into the zeolite pores than NO , is oxidized on highly dispersed Ir species 2 inside the zeolite pores. The formed NO is then 2 believed to be reduced by CH on [InO]+ sites 4 [186–190]. Obuchi et al. employed an Ir–In/H– ZSM-5 honeycomb catalyst in CH -SCR with 4 7 vol.% of water in the feed. However, the reaction temperature was limited to 450°C, while the maximum NO conversion would have been reached at about 500°C [186 ], so the NO conversion was x below 10% [46 ]. To conclude, the NO conversions on Ga/ZSM-5 and Co/ZSM-5 are comparable, as long as a dry feed is employed. However, the temperature of the maximum NO conversion is about 100 K higher with Ga/ZSM-5 than with Co/ZSM-5. In addition, the gallium-containing zeolites are much more selective towards NO reduction than the cobaltcontaining zeolites, thus possibly facilitating the reaction with hydrocarbons present anyway without addition of a reductant [50,191]. However, in a wet feed stream, Co/zeolites perform much better than Ga/zeolites [49,191,192]. In/H–ZSM-5, especially if loaded with precious metals, is less sensitive to water vapor, but the temperature of maximum NO conversion is far too high for a practical application, as with gallium-containing zeolites.

6. HC-SCR on zeolites in their H-form The preceding sections already proved the importance of zeolitic acid sites in the HC-SCR. Hence, it seems reasonable to investigate the catalytic performance of zeolites just in their H-form. The first who reported on NO reduction with x hydrocarbons on H–zeolites were Hamada and coworkers: among various zeolites, they found H– mordenite to be the most active catalyst. H–ZSM-5 displayed a similar activity; H–Y and, for comparison, Na–ZSM-5 and silicalite were much less active. On H–mordenite, a maximum NO conversion of 65% at 400°C could be achieved with C H as reductant. However, the feed stream did 3 8 not contain water, and the gas hourly space veloc-

ity was very low. At most 9% of N O was formed 2 beside N , and CO was the prevailing oxidation 2 product beside CO [193–196 ]. 2 In many other publications the use of H–zeolites in HC-SCR was briefly mentioned, often in comparison with metal-exchanged zeolites, thus mostly not conveying fundamental information on the H– zeolite system. The most widely studied zeolites were H–mordenite [96,111,161,176,197,198], H– ZSM-5 [30,56,57,76,78,94,96,134–136,158,159, 161,171,174,178,184,199–205], H–Y [202,206 ], H– ferrierite [58,96,161], acid-treated natural mordenite [33,198], acid-treated natural clinoptilolite [33] and silicate with MFI structure [113,114]. Dependent on the reducing agent and how realistic the applied reaction conditions were, low to moderate NO reduction was achieved. However, H– x zeolites were generally found to be very sensitive, and a considerable loss of catalytic activity was often encountered, as will be demonstrated below. Iwamoto and coworkers reported that the NO conversion on H–ZSM-5 was reduced from 50% to 0% simply by increasing the gas hourly space velocity from 48 000 h−1 to 90 000 h−1 [203–205]. At temperatures above 450°C, Petunchi and Hall observed extensive dealumination of H–ZSM-5 zeolite during i-C H -SCR in a feed stream with4 10 out water and concomitantly a considerable loss of NO reduction activity [207]. During CH -SCR 4 at 650°C, H–mordenite deactivated already within 1 h on stream, which was suggested to be due to dealumination with a simultaneous decrease of the concentration of strong acid sites and partial pore blockage by extra-framework aluminum [208– 211]. The activity of H–ZSM-5 for CH -SCR was 4 also reduced on steaming [126 ]. Several authors reported a strong decrease in NO conversion upon introduction of water vapor to the reactant gas stream [62,162,211–214], which was generally proposed to be caused by the competitive adsorption between NO and H O and the 2 useless consumption of hydrocarbons in side reactions [212,213]. However, with methanol as reductant in the presence of 8 vol.% of water, an NO conversion as high as 56% could be obtained on H–ZSM-5, even though the space velocity was as high as 20 000 h−1 [214,215]. The HC-SCR activity of HNa–mordenite was also reduced by the pres-

Y. Traa et al. / Microporous and Mesoporous Materials 30 (1999) 3–41

ence of SO , probably due to a reduction of the 2 surface area and/or poisoning of the catalytic sites by sulfates [216 ]. Further papers deal with a closer investigation of the role of acidity in the HC-SCR. It was observed that the catalytic activity of different H– zeolites increased with decreasing n /n ratio Si Al [208–211,214,217]. Satsuma et al. compared several H–mordenite and H–ZSM-5 zeolites used for CH -SCR and C H -SCR by means of temper4 3 6 ature-programmed desorption of ammonia. On H– mordenites employed in C H -SCR, the amount 3 6 of acid sites was remarkably decreased, and the catalysts were darkened indicating a blockage of acid sites by carbonaceous material. The other samples did not display noticeable color change after the reaction. All in all, the activity was found to be dependent on the amount of zeolite acid sites and the type of zeolite, possibly due to the difference in the acid strength [217,218]. Correspondingly, excessively sodium ion-exchanged Na– mordenite was inactive; partly ion-exchanged NaH–mordenite was active, but surprisingly at lower temperatures than H–mordenite [219]. This was tentatively attributed to the high concentration of NO− species adsorbed on Na+ ions in NaH– 3 mordenite. These NO− species were suggested to 3 be reaction intermediates and to react with surface organic species in the rate determining step, i.e. the N formation [220]. By contrast, Stakheev 2 et al. reported that even the partial substitution of protons with sodium ions almost completely suppressed the activity of ZSM-5 zeolites in the oxidation of NO to NO and concomitantly strongly 2 decreased the HC-SCR activity [90]. Hall and coworkers proposed that on H–ZSM-5 as on Co/ZSM-5 (cf. Section 3) the NO reduction into N and the CH oxidation into CO were 2 4 x coupled and initiated by the reaction of CH with 4 NO . Co/ZSM-5 was reported to be much more 2 active than H–ZSM-5 [79]. However, in other publications of the same group, both catalysts displayed similar activities in CH -SCR [81,82]. 4 In any case, the hydrocarbon efficiency for NO reduction was better on H–ZSM-5 than on Co/ZSM-5, probably due to the stronger competition between O and NO for CH over Co/ZSM-5 2 2 4

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[79,80]. When nitromethane as a potential reaction intermediate was reacted with NO and O over x 2 H–ZSM-5, only N O and N were observed as 2 2 N-containing reaction products [81,82,87], which is at variance to the results on Co/ZSM-5 (cf. Section 3). Halasz et al. observed in C H -SCR on H– 3 8 /c <1, NO was selectively ZSM-5 that for c 2 C3H8 NO formed without reacting with C H to N . 3 8 2 Reduction of NO to N was only possible for 2 c /c ≥1 [221,222]. The NO reduction was C3H8 NO suggested to proceed via different reaction pathways below and above ca. 500°C: at low temperatures, the oxidation of NO to NO might be the 2 initial reaction step, because it was faster than other possible reactions. At higher temperatures, the combustion of propane probably initiated the NO reduction [221]. Active sites for the NO 2 formation were supposed to be Brønsted acidic bridging hydroxyl groups [223]. Kikuchi and coworkers observed considerably higher NO conversions on H–ZSM-5 in the x NO –O –CH system than in the NO–O –CH 2 2 4 2 4 system at high gas hourly space velocities. At lower space velocities, the differences leveled off. These results led the authors to assume that the first step of the reaction was the NO formation [224]. In 2 this context, several groups compared the performance of H–zeolites and different metal ionexchanged zeolites in the HC-SCR and the NO oxidation to NO . However, dependent on the 2 nature of the metal and the overall reaction conditions, the metal or the acid sites could prevailingly catalyze a certain step in the reaction path. So, e.g. for gallium or indium as metals, the NO 2 formation was proposed to take place on acid sites and the NO reduction to N on metal sites [162], 2 2 whereas for cobalt as metal, the NO oxidation was envisaged to be catalyzed by the metal and the NO reduction by the zeolite acid sites [88,95]. 2 To conclude, zeolites in their H-form are of limited attractiveness in practice, since their activity is low and they are very sensitive to water. However, the fundamental and mechanistic studies carried out on these catalysts produced interesting results. Unfortunately, even though water is well known to play a role in the reaction mechanism, most investigations were performed in dry feed

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streams, and this diminishes the significance of their results and the conclusions the authors arrived at considerably.

7. HC-SCR on cerium-containing zeolites Cerium-containing zeolites were mainly studied by Misono and coworkers. The best NO conversion to N of 84% at a temperature of 300°C was 2 achieved over Ce/Na–ZSM-5 in a feed containing 1000 vol. ppm NO, 500 vol. ppm C H and 3 6 2 vol.% O and with a gas hourly space velocity 2 of 10 000 h−1. The catalyst had a degree of cerium exchange of 60% and was prepared by ion exchange at 150°C and 4 bar in an autoclave. Zeolites prepared by conventional ion exchange had maximum exchange levels of about 25%, were less active and displayed their maximum NO conversion only at higher temperatures [183,184,199]. Usually, neither N O nor NO were formed as 2 2 reaction products [184]. Ce/Na–ZSM-5 was moderately active in the oxidation of hydrocarbons, but highly active in the activation of NO. Hence, the portion of the hydrocarbon consumed for the NO reduction was high, comparable with the values obtained with H–ZSM-5 [199]. When testing the effect of the zeolite structure, Ce/Na– ZSM-5 was found to be more active than Ce/Na– mordenite which, in turn, exhibited a far higher NO conversion than Ce/Na–Y, even though Ce/Na–ZSM-5 had the lowest Ce content of the samples [200]. Guyon et al. reported that the catalytic performance of Ce/ZSM-5 was much better than that of Ce/EMT [181]. Also, Ce/Na– ZSM-5 was much more active than La–, Pr– or Nd/Na–ZSM-5 with similar ion exchange levels [184,199]; Ce/Na–Y and Pr/Na–Y displayed similar, moderate NO conversions, whereas La/Na– Y, Sm/Na–Y and Tb/Na–Y were nearly inactive [206 ]. The addition of noble metal or copper ions, which have high oxidation activities, to Ce/Na– ZSM-5 decreased the NO removal activity because of the decrease in the reduction selectivity. On the other hand, the addition of alkaline-earth metals, especially Sr, enhanced the activity particularly above 350°C, possibly due to changing the

exchange site or the state of the Ce ions [183,184,199]. With methane as reducing agent, both Ce/H–ZSM-5 and Ce/Na–ZSM-5 showed only moderate NO conversions [134,225]. Further studies were devoted to the mechanism of C H -SCR on cerium-containing zeolites: an IR 3 6 study led to the assumption of organic nitro compounds being intermediates in the reaction [226 ]. Based on this result and on comparative experiments using different feed streams on Ce/zeolites and Na/zeolites, the authors proposed the following reaction mechanism: NO is oxidized on Ce to NO which subsequently reacts with propene 2 to organic compounds like nitro compounds. In this step, the carrier possibly plays an important role, and Ce is not needed. The organic nitro compounds are then, by reaction with NO and x oxygen on Ce, decomposed to N and CO , either 2 x directly or via HCN, C N or possibly HNCO 2 2 [199,200,227]. Cerium ions on ion exchange positions were suggested to be the active sites [199,200]. As another possibility, coordinatively unsaturated [CeO]+ was proposed, based on a molecular dynamics simulation [167]. In an attempt to check the mechanism outlined above, metal oxides which are virtually inactive in HC-SCR, but which can be expected to catalyze the NO formation more efficiently than Ce/Na– 2 ZSM-5, were mechanically mixed with Ce/Na– ZSM-5. Indeed, the addition of Mn O or CeO 2 3 2 to Ce/Na–ZSM-5 led to a considerably increased activity, particularly in the low to medium temperature region [228–230]. However, if Ce/Na– ZSM-5 and Mn O were applied together in a 2 3 honeycomb, the maximum NO conversion x obtained was below 10% [46 ]. This disappointingly poor performance could be due to difficulties in the preparation of the catalyst, the presence of SO and H O in the feed or the high space velocity. 2 2 In conclusion, Ce/zeolites were found to be more active in C H -SCR than Cu-, H- or 3 6 In/zeolites [183,184]. However, all the experiments of Misono and coworkers were performed in the absence of water. Hence, it is again hardly possible to rate the potential of these catalysts for a practical application, since studies under realistic conditions are very scarce.

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8. HC-SCR on iron-containing zeolites Until a poster at the 11th International Congress on Catalysis 1996 and the accompanying publication by Feng and Hall [231], little attention had been paid to iron-containing zeolites in HC-SCR. Feng and Hall reported on the exceptionally high activity and stability of Fe/Na– ZSM-5 in i-C H -SCR. At the relatively high gas 4 10 hourly space velocity of 42 000 h−1 in a feed stream containing 2000 vol. ppm NO, 2000 vol. ppm isobutane and 3 vol.% O , an NO conversion to N 2 2 of 95 to 100% was observed in the wide temperature range from 450°C to 550°C. By introduction of 20 vol.% of water and 150 vol. ppm of SO to 2 the reactant stream at a temperature of 500°C, this high conversion was maintained over a period of 2500 h with short-time excursions of the temperature up to 800°C [231,232]. It should be borne in mind, however, that it is not too meaningful to investigate the effects of various parameters on the catalytic performance at a conversion as high as 95 to 100%. The catalyst used was prepared in the absence of oxygen from a commercial Na–ZSM-5 sample with n /n =19 by ion exchange at room Si Al temperature for 24 h in a saturated solution of FeC O (ferrous oxalate). Thus, an Fe/Na–ZSM-5 2 4 zeolite with a high iron content (n /n #0.9) and Fe Al Fe mainly in form of Fe(II ) could be obtained. It was claimed that Brønsted acid sites were nearly absent which was suggested to stabilize the lattice and to prevent activity loss by dealumination [232]. In a later publication, however, the authors had to admit that their good results were confined to the above-mentioned catalysts made from one particular batch of a template-free synthesized commercial Na–ZSM-5 zeolite with n /n #1.2. Na Al The results could not be reproduced with an Na– ZSM-5 zeolite from a templated synthesis as starting material. It was suggested that the difference between the catalysts was caused by the amount of residual Brønsted acid sites in the parent zeolites. The authors assumed that the Na–ZSM-5 sample prepared with template possessed more Brønsted acid sites, because it had to be made from H–ZSM-5 resulting from the decomposition of the template, but unfortunately they did not try to obtain similar starting materials by performing

25

a proper sodium ion exchange with both commercial zeolites [233]. Chen and Sachtler also aimed at reproducing the results of Feng and Hall, but only obtained catalysts with a maximum iron content of n /n #0.5, having, as expected, a Fe Al comparatively low activity. For this reason, they prepared a second batch of catalysts by subliming FeCl into the cavities of H–ZSM-5. Irrespective 3 of the origin and the n /n ratio of the commercial Si Al Na–ZSM-5 zeolite used as starting material, catalysts with n /n =1 were achieved. With reaction Fe Al conditions corresponding to the ones applied by Feng and Hall, only maximum conversions of NO to N between 65% and 77% were reached on 2 these catalysts, but already at temperatures between 325°C and 350°C. CO was always produced in amounts similar to CO , which is far too 2 much for a practical application. In the presence of 10 vol.% of water, the N yield was at low 2 temperatures even slightly higher than in the absence of water, possibly because water hindered the formation of carbonaceous deposits. At high temperatures, the NO conversion was not affected. However, the oxidation of CO to CO was sup2 pressed by water. At variance to the results of Feng and Hall, the catalysts lost about 10% of their initial activity already after 100 h on stream [234]. By addition of promoters, the performance of the catalyst could be tuned: ion exchange with Ce shifted the temperature of the maximum NO conversion by 25 K to a lower value without a significant change in the N yield. La shifted the 2 temperature window by 25 K to a higher value, but considerably increased the N yield. Na 2 strongly diminished the N yield (cf. Fig. 9). The 2 activity of La–Fe/H–ZSM-5 was again decreased by about 10% after 100 h on stream in the presence of 10 vol.% of H O. This deactivation was revers2 ible: the initial activity could be restored by treatment with 10 vol.% of O in He at 500°C for 2 2 h [235]. Lee et al. prepared Fe/H–ZSM-5 by incipient wetness impregnation. In this way, they achieved an iron content of 1 wt.% and only a maximum NO conversion of 17% at 350°C in C H -SCR 3 8 [92]. Joyner and Stockenhuber obtained an Fe/H– ZSM-5 catalyst with 0.7 wt.% of Fe by conventional ion exchange at room temperature and

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Fig. 9. Effect of promoters on the N yield over Fe/H–ZSM-5 (n /n =1.0) after Chen and Sachtler [235]. 2 Fe Al

observed an NO conversion of 14% at 425°C in C H -SCR [236 ]. Fe/Na–ZSM-5 with n /n # 3 6 Fe Al 0.35 prepared by ion exchange exhibited relatively high NO conversions in C H -SCR [237] and 2 4 C H -SCR [238], but the space velocities were 3 6 quite low. So obviously, the preparation method and the degree of ion exchange are vital parameters which determine the catalytic performance of ironcontaining zeolites. The structure of iron incorporated into zeolites is a matter of debate: Joyner and Stockenhuber proposed iron–oxygen clusters with structures similar to those of ferredoxin II, i.e. Fe S , where iron 3 4 is readily interconvertible between the +II and +III oxidation states, but stable against reduction to metallic Fe even at 1250 K in hydrogen [236 ]. Feng and Hall suggested that in samples with a high iron content and n /n ratios between 11 Si Al and 25 the formation of [Fe–O–Fe]2+ bridges from [Fe(OH )]+ is possible [239]. Chen and Sachtler claimed that iron at ion exchange sites are the active species for HC-SCR, with an oxygen bridged binuclear iron complex as a plausible prototype for the active sites [234]. Another important parameter is the reducing agent: Li and Armor reported that Fe/H–ZSM-5 was a poor catalyst with methane as reductant [50,52,53]. Feng and Hall observed that Fe/Na– ZSM-5 was only satisfactorily active for CH 4

SCR at temperatures above 650°C [232]. However, if the first step of the reaction, the activation of methane, was overcome by starting with nitromethane, the reaction over Fe/Na–ZSM-5 proceeded at about the same rate as with Co/ZSM-5 [81,82]. Iwamoto and coworkers examined the influence of the zeolite framework on the activity of Fe/zeolites: the catalysts were prepared by conventional ion exchange of Na/zeolites and tested in C H -SCR. Fe/Na–mordenite with an ion 2 4 exchange level of 97% was found to be the most active catalyst with a maximum NO conversion into N of 20% at 250°C. Fe on zeolites ferrierite, 2 ZSM-5, Y and L was less active, but this does not necessarily have to be due to the zeolite structure, because the ion exchange degrees and the n /n Si Al ratios of the catalysts differed [240]. However, the introduction of 8.6 vol.% of water or the increase of the gas hourly space velocity to 144 000 h−1 completely suppressed the catalytic activity [23,24,241]. Kikuchi and coworkers reported on the high activity of H–[Fe]silicate with MFI structure in C H -SCR at a low space velocity and the stability 3 6 of the catalyst towards SO . Based on transient 2 response studies, the authors proposed a reaction mechanism in which a nitrogen-containing organic intermediate is formed from adsorbed C H and 3 6

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NO. N is then produced by decomposition of this 2 intermediate by reaction with NO or NO 2 [178,242,243]. Sadykov et al. concluded from temperature-programmed desorption of NO comx bined with activity measurements on Fe– Ca/ZSM-5 and other catalysts that nitrites and nitrates could be intermediates [244]. By contrast, Segawa et al. found from IR studies that isocyanate species were intermediates [238]. But since both studies were not performed under in situ conditions, the proposals remain speculative. Inui and coworkers used H–[Fe]silicates as well and, interestingly, observed only a small amount of CO as reaction product at lower temperatures [113,114,116,117,121,177]. After calcination at 1000°C for 2 h in air, the catalytic activities of both Fe/H–ZSM-5 and H–[Fe]silicate were drastically reduced, which was explained by sintering of Fe [119]. Ishihara et al. achieved a maximum NO conversion into N of 27% at 300°C on SAPO-34 2 impregnated with 3 wt.% of Fe [124]. Tamura et al. observed on zeolites Y, ferrierite, mordenite, ZSM-5, natural mordenite and natural clinoptilolite impregnated with Fe high NO conversions, but with extremely low space velocities [33]. Paul et al. used MCM-41 impregnated with an iron complex and [Pd(NH ) ]Cl under realistic conditions, i.e. 34 2 washcoated onto cordierite honeycombs in actual engine exhaust with a space velocity of 60 000 h−1. However, the maximum NO converx sion was at the most 10% at 270°C and the ligands used for complexing Fe were only stable up to 325°C [245–247]. To conclude, the major problem with ironcontaining zeolites is that samples with a low iron content display their maximum NO conversion sometimes at relatively low temperatures, but the height of these maxima is unsatisfactory, whereas samples with a high iron content do show high activities and a reasonable stability under realistic conditions, just the temperatures needed are too high. The amount of CO produced on iron-containing zeolites is far too high. Thus, for a practical application an Fe/zeolite either has to be combined with a CO oxidation catalyst in series, or it has to be promoted with a small amount of a noble metal like Pt. The latter route was followed by Ko¨gel et al. with Pt–Fe/H–ZSM-5 [248] and by Inui and

27

coworkers with H–[Pt–Fe]silicate [113,114], achieving total oxidation of CO to CO without a 2 considerable change of the NO removal x characteristics.

9. HC-SCR on silver-containing zeolites Zeolites containing silver only were usually reported to be not very active in HC-SCR, i.e. the maximum NO conversion was mostly below 30% [42,124,132,151,155,222,241,249]. This low activity was suggested to be due to the inability of Ag to act for NO production, with NO being probably 2 2 an initial reaction intermediate [222]. Iwamoto and coworkers observed a maximum NO conversion into N of 47% at 500°C on Ag/Na–ZSM-5 2 during C H -SCR [205]. This was inconsistent 2 4 with a later paper, though, where under the same conditions on the same catalyst the maximum NO conversion into N was only 18% and the active 2 temperature range was much smaller. The authors not only refrained from commenting on this, but even used the higher values of the earlier publication in the text [241]. On Ag/ZSM-5, Sato et al. achieved an NO conversion to N of about 50% 2 at 400°C in a feed stream with 8 vol.% of water. However, the metal content of the catalyst was as high as 8.5 wt.%, whereas the space velocity was quite low [250]. Nakano et al. examined the performance of Ag/Na–ZSM-5, Ag/H–ZSM-5 and Ba– Ag/H–ZSM-5 in C H -SCR under rather realistic 3 6 conditions. On Ba–Ag/H–ZSM-5 an NO conversion higher than 60% was achieved in the temperature range from 400°C to 500°C [251]. Jacobs and coworkers reported very remarkable results obtained over silver-containing zeolites: on Ag/H–beta and with a reactant stream containing 12 vol.% of water, they observed at a relatively high space velocity an NO conversion to N of 2 53% at 345°C. On Ag/H–beta promoted with Ga, even NO conversions to N of 60% or higher were 2 detected over the very wide temperature range from 150°C to 530°C [180]. From these data, one is tempted to rank Ag–Ga/H–beta zeolite as an almost ideal catalyst for diesel exhaust gas purification, and it remains to be seen whether this and other silver-containing zeolites attract a broader

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interest in the near future. In another paper the authors compared the N yield on Ag, Co, Cu and 2 Pt on H–ferrierite and Ag/H–mordenite in C H -SCR starting with NO . Ag/H–ferrierite dis3 6 2 played by far the best performance with NO 2 conversions to N higher than 60% between 250°C 2 and 450°C, although the space velocity was as high as 160 000 h−1 and the feed contained 12 vol.% of water. SO did not affect its good 2 NO reduction capability, but after 16 h on stream x at 660°C, the NO conversion had dropped by 2 20%. This relatively high stability of the catalyst was attributed to the thermal instability of silver oxide particles and the low affinity of silver for hydrolysis: sintered Ag particles would be redispersable by oxidation. It should be noted, however, that these high conversions could only be attained with NO as reactant. Since NO is the main 2 nitrogen-containing exhaust gas component and NO is hardly formed on Ag, Ag/H–ferrierite is 2 as such not very suitable for a practical application. Rather, it would have to be combined with an oxidation catalyst in front which transforms NO into NO [252]. 2 Another possibility for achieving a good performance of Ag-containing zeolites with NO as reactant is promoting the catalyst with a metal which has a high activity for NO formation: Masuda 2 et al. reported on the promotional effect of 0.01 wt.% of Pd on Ag/H–mordenite with 3 wt.% of Ag. Thus, in actual diesel engine exhaust gas with dimethylether as reductant and the catalyst washcoated onto honeycombs, a NO conversion x of 31% was reached at 325°C [151]. Li and Flytzani-Stephanopoulos used Ce as promoter for Ag/Na–ZSM-5 and observed in CH -SCR at a 4 space velocity of 30 000 h−1 and in the presence of 8.3 vol.% of water a maximum NO conversion to N of about 43% at 600°C. However, in the 2 presence of water the catalyst underwent irreversible structural changes decreasing the catalytic activity [225]. Relatively little has been published about silvercontaining zeolites up to now, but some results appear to be quite promising with respect to a practical application, and it is worthwhile to watch the pertinent literature carefully.

10. HC-SCR on nickel- and manganese-containing zeolites Most of the work concerning nickel- and manganese-containing zeolites was done by Li and Armor in the course of their investigations into Co/zeolites for CH -SCR. However, since Mn and 4 Ni on zeolites ZSM-5 and ferrierite were found to be less active and/or to exhibit their maximum NO conversions at higher temperatures than their corresponding Co counterparts, the systematic studies were predominantly performed on Co/zeolites (cf. Section 3), and the experiments with Mn- and Ni-containing zeolites did not supply essential new insights. Generally, no maximum in the NO conversions was observed with Mn- and Ni-containing zeolites, even up to a temperature of 550°C with CH as reductant, the exception being Ni/ZSM-5 4 with the maximum NO conversion already at 450°C [50,52,56–59,62]. Therewith, Mn/H–ferrierite displayed a considerably higher NO conversion than Mn/Na–ferrierite with a similar ion exchange level [58]. The addition of 2 vol.% of water to the feed stream considerably reduced the NO conversions on Mn/ZSM-5 and Ni/ZSM-5 at 450°C, but had no large effect at 500°C [62]. Besides Li and Armor, several other authors were reporting on the performance of Mn- and Ni-containing zeolites in CH -SCR [76,77,93,103, 4 125,155,253], C H -SCR [240], C H -SCR 2 4 3 6 [124,128,132,238,251], C H -SCR [33,92], 3 8 n-C H -SCR [113,114,177] and HC-SCR with 16 34 LPG [179]. However, the relevant information which remains to be mentioned is scant. Budi et al. found Mn/H–ZSM-5 which was initially less active than Co/H–ZSM-5 in CH -SCR at 500°C after steaming superior to 4 Co/H–ZSM-5 [125]. Kasahara et al. observed that a pre-exchange of Ni/zeolites with K or Cs enhanced the hydrothermal stability of the catalysts [128]. Eshita and Kasahara used different Ni/H–ZSM-5 zeolites and Ni/H–ZSM-5 promoted with rare-earth or alkaline-earth metals for C H -SCR under quite realistic reaction condi3 6 tions. The best catalyst, Ba–Ni/H–ZSM-5, still displayed a maximum NO conversion of 62% after 5 h catalytic aging at 800°C in a feed with 3 vol.% of water [254].

Y. Traa et al. / Microporous and Mesoporous Materials 30 (1999) 3–41

Bell and coworkers investigated the Mn/Na– ZSM-5 system more closely and detected N O as 2 a product of CH -SCR in amounts comparable 4 with N . From in situ IR studies they invoked a 2 reaction mechanism with cyanide intermediates and Mn changing between the +II and +III oxidation states [103,253]. So, apart from the fact that Mn/H–ZSM-5 was more resistant towards steaming than Co/H– ZSM-5 (vide supra), Mn- or Ni-containing zeolites do not seem to offer advantages over the Co system, especially since N O tends to be a main 2 reaction product over Mn/Na–ZSM-5.

11. HC-SCR on rhodium-containing zeolites Rhodium-containing zeolites give a better performance in the NO reduction without oxygen than in the HC-SCR [30,255]. Sullivan and Cunningham observed in C H -SCR on Rh/H– 2 4 ZSM-5 a maximum NO conversion to N of only 2 24% at 350°C [255]. Likewise, Iwamoto and coworkers reported for C H -SCR on Rh/Na– 2 4 ZSM-5 a maximum NO conversion of 15% into N and 9% into N O at 400°C [20,23]. Various 2 2 authors found the initial high conversion on Rh/zeolites irreversibly diminished through aging [30,31,42], and by introduction of only 2 vol.% of water, the NO reduction was reduced by one half [30]. With methane as reductant the maximum NO conversion was lower than with alkenes and shifted to a higher temperature [30,50,52,136,153]. In the C H -SCR on Rh/MCM-41, the NO 3 6 x conversion amounted to 41% at 290°C with a selectivity to nitrogen of 67%. However, the space velocity was only 11 000 h−1, and the feed stream was free of water [34]. The main conclusion from this work is that the use of Rh-containing zeolites will be limited to three-way catalysts working in a net reducing atmosphere.

12. HC-SCR on zeolites containing other metals Numerous other metals on zeolitic supports have been used for HC-SCR. Because of the lack

29

of systematic studies on these systems, they are only briefly enumerated here (see Table 1).

13. HC-SCR on zeolites containing more than one metal This topic can hardly be treated exhaustively, since nearly all metals of the periodic table of the elements have been used as promoters for HC-SCR catalysts. Various examples have already been quoted in the previous sections on the respective metals and will not be repeated here. Likewise, corresponding to the general scope of this review article, combinations of metals where copper is prevailing will not be treated. Rather, a few selected systems, which are important from the authors’ viewpoint, will be dealt with explicitly in the text, others will only be mentioned briefly. The mechanical mixture of Mn O , which had 2 3 a high activity for the NO formation in the 2 presence of water (cf. Section 7), and Sn/Na– ZSM-5 with about 36 wt.% of Sn (which means that a significant amount of tin was at the outer surface of the zeolite) had a much higher activity for C H -SCR in the presence of 5.7 vol.% of 3 6 water than in its absence (cf. Fig. 10). At a space velocity of 10 000 h−1 and c /c =2, a maxiNO C H6 mum NO conversion into N of 3about 75% was 2 achieved at 350°C. The positive effect of water was suggested to result from its slowing down the oligomerization or polymerization of the hydrocarbon and/or accelerating the decomposition of the polymerized products [229,230,258]. Researchers of Mazda Motor Corporation were investigating the combination of various precious metals with themselves and other metals. They developed a catalyst which met the Japanese emission standards and was used in a Mazda lean-burn vehicle in the Japanese domestic market. These catalysts consisted of Pt, Rh and Ir on H–ZSM-5 zeolite coated onto cordierite honeycombs. The best weight ratio was found to be Pt/Ir/Rh= 30/6/1. Further improving the catalyst by addition of CeO led to a NO conversion of nearly 70% 2 x in the FTP Mode Test. H O and SO did not exert 2 2 a considerable influence on the NO reduction x performance of the catalyst. Surprisingly, it was

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Table 1 Survey of metals on zeolitic supports for HC-SCR not further specified in the text Catalyst

References

H–[Al ]silicate Al/ZSM-5 Au/zeolite H–[B]silicate H–[B]ZSM-5 Ba/mordenite Ba/ZSM-5 Ca/mordenite Ca/ZSM-5 Cr/clinoptilolite Cr/ferrierite Cr/mordenite H–[Cr]silicate Cr/Y Cr/ZSM-5 Ir/zeolite Ir/ZSM-5 K/mordenite La/mordenite La/Y La/ZSM-5 Li/mordenite Li/ZSM-5 Mg/ZSM-5 Mo/mordenite Na/mordenite Na/ZSM-5 Nd/ZSM-5 Pb/ZSM-5 Pr/Y Pr/ZSM-5 Ru/ZSM-5 Sm/Y Sn/ZSM-5 Sr/mordenite Sr/ZSM-5 Ti/mordenite TS-1 V/ferrierite V/mordenite VS-1 V/Y V/ZSM-5 Zn/ferrierite Zn/mordenite Zn/Y Zn/ZSM-5 Zr/mordenite Zr/ZSM-5

[177] [158] [42] [178] [201] [217] [254] [217] [98,149,240] [33] [33] [33] [177] [5,33] [33,50,52,98,149,240] [42] [5,20,23,256,257] [219] [217] [206 ] [76,126,149,184,199,254] [219] [222] [238] [33] [219] [13,98,149,184,193,199–201,240] [184,199] [183,184,199] [206 ] [184,199] [20,23,134,136 ] [206 ] [158,229,230,258] [217] [184,199,227] [197,259] [260] [33] [33] [260] [5,33] [33] [33] [33] [33] [13,98,132,149,158,204,205,238,241] [259] [182]

claimed that the N O emission of the vehicle was 2 not increased with the use of this catalyst, although the main NO reduction product with Pt catalysts x is N O. It was suggested that the combination of 2 the three precious metals effected a small metal particle size and suppressed the growth of the particle size after thermal aging, thereby improving the activity of the composite catalyst compared with catalysts containing only one or two of these precious metals. However, severe aging still brought about a considerable worsening of the catalytic performance [256,257]. In earlier work already the combination of Pt and Ir with preferably one or more of the metals Rh, Tb, Ni, In, Sn, Co, Ce, Sb and Ga had been tested, but with less success than with the above-quoted system [261,262]. In later patents, the company favored catalysts with a basis of Pt or Pd and Ba on Al O and CeO coated onto cordierite honey2 3 2 combs and Pt as well as Rh on ZSM-5 as upper layer [263,264]. Much research effort has been devoted to the combination of Co with various metals, as has already been indicated in Section 3. In patents assigned to the Tosoh Corporation, the use of Co and an alkaline-earth or rare-earth metal as well as Mn, Ni, Cu, Zn, Rh, Pt and Ag on zeolite ZSM-5 in HC-SCR was reported [265,266 ]. In further work Co was combined with only an alkaline-earth and/or rare-earth metal [126,129], Ca [66 ], Sr [66,244], Ba [66 ], La [66,197], In [66 ], Nb [50,52], Cr [50,52], Mn [50,52,66 ], Ni [50,52,66 ], Cu [50,52] or Ag [50,52,66,155]. Examples with other metals comprise the combination of Pt with Mg [267], Ca [267], Sr [267], Ba [267], Ga [267], In [267], Sn [259,267], Pb [267], Y [267], La [267], Ce [267,268], Nd [267], Eu [268], Eu and Ni, Co or Cu [268], Ti [259,267], Ti and Ru [259], Zr [259,267], V [267], Mo [267,269], Fe [267], Rh [32], Ir [267], Au [270], Zn [267], the combination of Ti with V [197,259,260], Nb [259], Cr [259], Mo [259], W [259], Mn [259], Mn and La [197], Fe [259], Fe and La [197], Ru [259], Ru and Pd and Rh [259], Co [259], Co and La [197], Rh [259], Rh and Ag [259], Ni [259], Pd [259], Ag [259], Zn [259], the combination of Ru with Zr [259], Sn [259], the combination of Ga with Ir and Au [270], the com-

Y. Traa et al. / Microporous and Mesoporous Materials 30 (1999) 3–41

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Fig. 10. Enhancement of NO conversion on the mechanical mixture of Mn O and Sn/Na–ZSM-5 (n /n >2.5!) by water after 2 3 Sn Al Misono et al. [229,230].

bination of La with Cr [197], Ni [197], Sr, Mn and Co [197], Pt and Co [197], Pd and Co [197], the combination of V with Mn or Ni [33], the combination of Cr with Mn, Ni, V, Cu or Fe [33], the combination of Zn with Al and Cu [271] and finally the combination of Ag with Cu [272].

14. New concepts in diesel exhaust gas purification The simple HC-SCR concepts for diesel exhaust gas purification discussed so far all suffered from certain shortcomings. In this paragraph some more recent concepts are to be outlined and rated. The selective catalytic reduction of NO with x ammonia (SCR or NH -SCR), which is extensively 3 applied to stationary NO sources [273], would be x a very desirable method, because NH is generally 3 much more effective for the NO reduction than hydrocarbons. This can be attributed to an intrinsic high reactivity of NH towards NO species 3 x with a direct formation of the N–N bond and low activity towards a reaction with O compared with 2 hydrocarbons [274]. However, because of its toxicity and the possible emission of excessive amounts of the reductant, the use of ammonia for exhaust gases from mobile engines is hardly possible. In addition, side reactions often play an important

role in NH -SCR leading to the formation of 3 N O and NO, which is highly undesirable. 2 With the injection of an aqueous urea solution into the exhaust gas stream with in situ release of ammonia and Ce/zeolites as catalysts, van den Bleek and coworkers tried to overcome these problems of the NH -SCR technique [275,276 ]. With 3 a 30% excess of ammonia complete NO and NH 3 conversions were achieved in the broad temperature range between 300°C and 500°C in a model exhaust gas without water, though. Thereby, ammonia was exclusively oxidized to N with 2 N O and NO not being formed [274]. Ce/H– 2 ZSM-5 displayed a stable NO conversion under x quite realistic conditions in a laboratory test with NH as reductant, but the active temperature range 3 was shifted to 400°C to 600°C due to the presence of H O and SO [277], which is too high for a 2 2 practical application in diesel engine powered vehicles. Satokawa et al. reported the use of another ammonia-type reductant, namely ammonium acetate: on Fe/H–mordenite deposited on a cordierite honeycomb, a NO conversion higher than 80% x was detected between 350°C and 400°C in the presence of 10 vol.% of H O and with a gas hourly 2 space velocity as high as 60 000 h−1. However, as has already been reported with HC-SCR on Fe/zeolites (see Section 8), the CO present in the

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exhaust gas stream was hardly converted, thus prohibiting a practical application without further measures [278]. Iwamoto et al. developed the so-called IAR method, which stands for intermediate addition of reductant. On the basis of the mechanistic findings that the first step of the HC-SCR reaction is often the oxidation of NO to NO and the second step 2 the reduction of NO to N (see previous sec2 2 tions), the reducing agent C H was added between 2 4 an oxidation catalyst of NO and a reduction catalyst to N [279–282]. As oxidation catalyst 2 Pt/ZSM-5 was employed, as reduction catalyst numerous ions in ZSM-5. With Co, Cu, Mn, Ni, Fe and Pt as active metals of the reduction catalyst the NO reduction performance was hardly changed by applying the IAR method instead of using the respective metal on ZSM-5 without combination with an oxidation catalyst. With Zn, Ag, In, Ga, Mg, Ba, Ca, Na ions and protons on ZSM-5, i.e. metals not very active in C H -SCR on their own 2 4 and/or with NO as reactant (cf. e.g. Section 9), the NO conversion to N was considerably 2 increased and the temperature at the maximum NO conversion was shifted to lower values. However, the maximum NO conversion was still lower than on the above-mentioned simple HC-SCR systems, and the effect of water on the IAR system is yet unknown [281]. So, the sole benefit of the IAR system seems to be a slight widening of the active temperature window, but for this small advantage a complex system has to be installed, which is hardly an appropriate measure. The same technique was applied by Richter and Ko¨nig with a platinum-based oxidation catalyst and Ag–Ga/H–mordenite as reduction catalyst. Thus, under realistic conditions in the presence of water a NO conversion of 40% or x higher was achieved between about 200°C and 340°C [283]. Whereas the IAR system was characterized as site-resolved HC-SCR, NO storage and reduction x catalysts (NSR catalysts) represent a kind of timeresolved HC-SCR [282]. The NSR catalysts, mainly Pt–Ba/Al O , had been developed by 2 3 Toyota and Cataler Industrial Corporation for lean-burn engines with strongly changing air to fuel ratios. Thereby, NO was oxidized over Pt and

stored as nitrate by the Ba compound under oxidizing conditions. Under stoichiometric and reducing conditions the nitrate was decomposed and reduced to N on Pt [284]. Feeley et al. adopted 2 this method for diesel engines in a full lean environment. NO was stored in a non-zeolitic, x Pt-promoted trap over a temperature range where current lean NO catalysts were not active. The x trapped NO was periodically desorbed by hydrox carbon injection generating heat and reduced by a lean NO catalyst which could be zeolite-based. x Thus, a high percentage of NO removal was x achieved over a wide temperature range, but the trap material suffered from a permanent loss of NO adsorption capacity through sulfur oxides x [285,286 ]. A related concept was presented by Johnson Matthey Public Limited Company: unburnt fuel is adsorbed by zeolites, when the exhaust gas is relatively cool and there is little catalytic NO x conversion, and released again when the catalyst, which could as well be zeolite-based, is at a higher temperature to effectively reduce NO . It has been x claimed that the average NO conversion is much x improved by the use of the adsorbent [287]. In a similar way, as described in a patent assigned to Daimler-Benz AG, a mixture of the hydrocarbon adsorbent Ti/mordenite and a non-zeolitic NO x adsorbent brought about an acceptable NO x reduction [288]. Toyota developed a system comprising an NO oxidation catalyst, a diesel particulate filter and a zeolite-based NO reducing catalyst in series. x Thereby, the NO leaving the first component of 2 the system oxidizes the trapped carbon particles, and the NO formed in this way is reduced on the reducing catalyst, mainly during periodically established lower air to fuel ratios [289]. Another concept is the broadening of the active temperature window by combining two HC-SCR catalysts being active in different temperature regions. One example, again in a patent assigned to Toyota, is the combination of Co–Ba/H–ZSM-5 and Cu/H–ZSM-5: on a cordierite honeycomb on which the two catalysts were randomly distributed a high NO conversion over a wide temperature x range was observed [290]. Another example was described in a patent assigned to N.E. Chemcat

Y. Traa et al. / Microporous and Mesoporous Materials 30 (1999) 3–41

Corporation: a catalyst system consisting of an [Ir]silicate catalyst with zeolitic structure in a first zone and Pt/Al O in a second zone facilitated a 2 3 NO conversion of 40% or higher between 280°C x and 470°C under realistic conditions [291]. To conclude, with some of the new concepts only a more complex exhaust gas purification system was installed without achieving an exceptional NO reduction performance, whereas others x might open good prospects.

15. Oscillations in HC-SCR Several authors reported on fortuitously noticed oscillations of the NO concentration under certain x conditions during HC-SCR investigations [292– 294]. During C H -SCR on H–ZSM-5 at 380°C 3 8 to 450°C, Halasz et al. observed periodic oscillations and between 520°C and 600°C non-periodic oscillations together with chemiluminescence. At 600°C, oscillations and chemiluminescence occurred even without a catalyst, from which the authors concluded that the phenomena at higher temperatures were due to homogeneous processes in the gas phase, e.g. peroxy radicals could be formed during the combustion of propane and react with NO to excited NO radicals which could 2 generate the light. The periodic oscillations at

33

lower temperatures were suggested to be caused by a fast autocatalytic decomposition of radicals like nitroxy compounds. Interestingly, however, all these effects could only be observed at reactant concentrations 20 times higher than in usual HC-SCR experiments [292]. Cho et al. reported on oscillations during C H -SCR on Pt/ZSM-5 at temperatures between 2 4 195°C and 210°C. In this system the oscillations only occurred with 15NO as reactant and not with 14NO. Attempts to explain the oscillations remained speculative [293]. Traa et al. found oscillating NO concentrations x during C H -SCR on Pt–V/H–zeolites. In contrast 3 6 to the above mentioned studies, which were performed in dry model exhaust gases, quite realistic feed streams were used and the presence of relatively large amounts of water seemed to be essential for the occurrence of the oscillations, since the amplitude and frequency of the oscillation decreased with decreasing water concentration (cf. Fig. 11) [294]. These studies stress the complex nature of the HC-SCR system, which can react in a very sensitive way to small changes in the reaction conditions. For example, in the presence of water oscillating and in the absence of water stable NO concenx trations could be observed (see above). This indicates a change in the reaction mechanism. In

Fig. 11. Oscillation of the NO conversion on Pt–V/H–ZK-5 (n /n =0.03, n /n =0.07) after Ref. [294]. x Pt Al V Al

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Y. Traa et al. / Microporous and Mesoporous Materials 30 (1999) 3–41

addition, the reaction mechanism can be different with different catalysts, reductants, temperatures, etc. This is why mechanistic insight gained in a particular system can hardly be transferred to other systems. Therefore, the next section will deal with reaction mechanisms only in a schematic way.

16. Mechanistic considerations In the previous sections on the specific metalcontaining zeolites, reaction mechanisms have already been addressed. This brief section aims at combining all these studies and giving an overview of the classes of mechanisms discussed in the literature. Roughly, all mechanisms can be divided into three groups (cf. Fig. 12). The first group comprises mechanisms where the catalytic decomposition of NO to N and 2 adsorbed oxygen is an important step. Thereby, the hydrocarbon serves as a scavenger which liberates the catalyst surface from adsorbed oxygen and/or to reduce the active metal. This mechanism was propagated by Inui and coworkers as their so-called microscopic sequential reaction mechanism (cf. e.g. Ref. [113]). Burch et al. proposed this mechanism for C H -SCR on Pt/Al O and 3 6 2 3 could explain the formation of N O on Pt by the 2 reaction of adsorbed nitrogen with adsorbed nitric oxide [295].

The second group includes mechanisms where the oxidation of NO to NO is the vital reaction. 2 This applies to most of the mechanisms mentioned in the previous sections. In dependence on the reducibility of the metal, NO can be oxidized by the metal (e.g. Refs. [142,143]) and/or by oxygen (e.g. Refs. [88,164]) on acid sites of the zeolite (e.g. Refs. [136,164]) or on metal sites (e.g. Refs. [88,95,136,199]). The next step of the reaction sequence is then the reaction of NO with the 2 hydrocarbon to N , possibly via organic intermedi2 ates, e.g. nitro compounds [199]. This reaction can also take place on acid sites of the zeolite (e.g. Refs. [88,95,136 ]) or on metal sites (e.g. Refs. [164,199]). The third group embraces mechanisms where, in the first instance, the hydrocarbon is partially oxidized by oxygen or nitrogen oxides. In the second step, the oxygen- and/or nitrogen-containing organic intermediate, e.g. a cyanide [103–105] or nitro species [93,101], reacts with nitrogen oxides to nitrogen, carbon oxides and water. It should be kept in mind, however, that this mechanistic approach was schematic. Actually, the boundaries between the different types of mechanisms are not fixed and, in practice, combinations of all three categories might operate, especially combinations between category two and three, thus facilitating a change from one mechanism to another with changing reaction conditions.

Fig. 12. Schematic overview of reaction mechanisms in HC-SCR.

Y. Traa et al. / Microporous and Mesoporous Materials 30 (1999) 3–41

17. Outlook Numerous attempts to cope with the difficulties of diesel exhaust gas catalysis have been discussed, showing the complexity of the system. The catalyst of choice has to perform well at high flow rates over a wide range of temperatures and reactant mixtures. In addition, it should show reasonably high activity during the whole lifetime of a diesel engine without significant deactivation. There is a trend towards non-zeolitic catalysts, e.g. Pt/Al O , since with zeolites the deactivation 2 3 problem has not been overcome. In any event, a single catalyst will probably not be enough, because the active temperature window is too small. Thus, at least a combination of different metals has to be employed and/or a combination of different catalysts representing complex exhaust after-treatment systems. A completely different approach might be the use of synthetic fuels, e.g. dimethyl ether, which can meet the emission standards without a catalyst, but such fuels are considerably more expensive [296 ]. From today’s point of view, it is an open question whether the most complex and demanding requirements a catalyst for diesel exhaust gas purification must fulfil will be met satisfactorily with zeolite-based catalysts in the future.

Acknowledgements The authors gratefully acknowledge financial support by the German Science Foundation (Deutsche Forschungsgemeinschaft), Fonds der Chemischen Industrie, Max-Buchner-Forschungsstiftung and Dr. Leni Scho¨ninger-Stiftung.

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