Cu-zeolite NH3-SCR catalysts for NOx removal in the combined NSR-SCR technology

2 Cu-zeolite NH3-SCR catalysts for NOx removal in the combined NSR–SCR technology Unai De La Torre, Beñat Pereda-Ayo, Juan R. González-Velasco* Departamento de Ingeniería Química, Facultad de Ciencia y Tecnología, Universidad del País Vasco, UPV/EHU, Campus de Leioa, P. O. Box 644, ES-48080 Bilbao, Bizkaia, Spain KEYWORDS: SCR, NSR, NSR-SCR, Diesel engine, Cu-zeolite *Corresponding author: [email protected] This is the accepted manuscript of the article that appeared in final form in Chemical Engineering Journal 207/208 : 10–17 (2012), which has been published in final form at https://doi.org/10.1016/j.cej.2012.06.092. © 2012 Elsevier under CC BY-NCND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) 3 ABSTRACT The challenge of efficient NOx removal from diesel and lean-burn engine exhaust gas by combining NSR and SCR catalyst is studied. Several Cu exchanged zeolites have been prepared, varying the preparation method (ion exchange and impregnation), the copper content (1–6%) and the zeolite (BETA and ZSM5). The prepared catalysts have been characterized, and acidity, surface area, crystallinity and metal reducibility have been compared. SCR experiments under 750 ppm NO, 750 ppm NH3 and 9.5% O2 (Ar to balance) discriminated low copper loading, prepared by ion exchange catalyst (Z-IE-1.4 and B-IE-2.1) as the most active for NOx conversion (>95%) in ample temperature range (280–450 ºC). These active SCR catalysts were placed downstream a monolith NSR Pt–BaO/Al O catalyst, running under cycled lean–rich conditions, and the improvement on NOx removal and selectivity to only N2 were determined. In an ample range of temperature, from 200 to 400 ºC, NOx conversion was increased in more than 30%, also notably increasing the production of nitrogen, and reducing production of ammonia and N2O below 3% and 2%, respectively, when comparing the combined NSR–SCR configuration versus the single NSR catalyst. 4 1. INTRODUCTION It is now well recognized that the use of diesel and lean burn engines decreases the fuel consumption and thereby reduces the CO2 emissions. However, conventional three way catalysts (TWCs) are not capable to reduce nitrogen oxides (NOx), due to the excess of oxygen in the environment. In the last decade, two main approaches towards NOx reduction have been proposed: the NOx storage and reduction (NSR) technology and the NOx selective catalytic reduction (SCR). SCR was originally developed for stationary emission sources, mainly power plants [1]. However, it soon turned out to be a promising technology for the NOx removal in automobile applications as well [2]. In 2005 it was introduced for commercial heavy-duty vehicles in Europe, and more recently also for passenger cars [3]. The NH3-SCR converter needs an external source of the selective reducing agent, e.g. urea. The urea solution is injected in a controlled way into the exhaust line, where it is thermally decomposed into NH3 and CO2. The ammonia then reacts selectively with NOx under lean (oxidizing) conditions, giving N2 as the final product [4,5]. Non-noble metals like Cu, Fe and Ce supported ZSM5 and BETA, are among the most active catalysts for the urea/NH3-SCR process [6–9]. The NSR catalysts (also called lean NOx traps, LNT) consist of a cordierite monolith washcoated with a porous alumina on which an alkali-earth oxide (e.g. BaO) and a noble metal (Pt) are deposited [10,11]. These catalysts operate alternatively under lean and rich conditions [12]. During the lean period, when oxygen is in excess, the platinum oxidizes NO to a mixture of NOx (NO + NO2), which is adsorbed (stored) on Ba as various species (nitrite, nitrate). Before an unacceptable amount of NOx slips through the catalyst, the engine switches to rich condition (reducing) for a short period where the stored NOx are released and reduced into N2 over Pt. Different types of reducing agents such as hydrocarbon, CO and H2 have been used in NSR catalyst studies [13], and hydrogen has been found to be the most effective. 5 On a commercial NSR system, the efficiency to transforming the emitted NOx to N2 should be as high as possible. This is remarkable, as the Pt itself is selective for the formation of N2 from NO and H2 only in a narrow range of NO to H2 ratio. Nova et al. [14] concluded that the ammonia formation over Pt/Ba/Al was dependent on the amount of stored NOx, temperature and hydrogen concentration. The reduction by H2 of nitrates stored proceeds according to a two-step mechanism in which the first step is the fast reaction of hydrogen with nitrates producing ammonia, followed by the slower reaction of the latter with nitrate species leading selectivity to N2 [15–17]. Accordingly a good tuning of the operating conditions of both the adsorption rate and the reduction phases can drive selectively to N2 and/or NH3 [18,19]. Consequently, LNTs generate NH3 during the fuel-rich purge period, and SCR catalysts, especially those based on zeolites store significant quantities of NH3 under reaction conditions [20]. Hence, combining the LNT with a downstream zeolite SCR catalyst offers a potential means of capturing NH3 generated by the LNT and using it to convert NOx that slips through the NSR catalyst. We are referring to these systems as combined NSR–SCR technology. Corbos et al. [7,21] showed that the NOx removal efficiency can be greatly improved under lean–rich atmosphere if a NSR model catalyst is physically mixed with CuZSM-5; this effect was ascribed to an increase in the formation of NCO species, their formation being promoted by Cu/ZSM-5 catalysts. The control of NOx storage and reduction in LNTs for designing combined NSR–SCR systems has been reported elsewhere [22]. We proposed the use of N2/ NH3 production surfaces in response to operational variables, including temperature and H2 concentration during the rich period, to run efficiently a combined NSR–SCR system with FeBETA zeolite catalyst placed downstream a Pt–BaO/Al2O3 monolith. However, only a 2% FeBETA catalyst was tested in [22]. Cu2+ ion-exchanged ZSM5 (Cu-ZSM5) zeolites were first showing high NO decomposition rates and NOx SCR activities [23,24]. More recently, Cu2+-exchanged beta zeolites (Cu-BETA) 6 have been shown to have good activity in the NH3-SCR of NOx, and metal-exchanged beta zeolites are generally found to have better hydrothermal stability than similar ZSM5 catalysts [25]. In this paper, several Cu exchanged ZSM5 and BETA zeolite catalysts have been prepared with copper loadings between 1 and 6 wt.% and their NH3-SCR behavior has been compared in relation to physico-chemical properties, including physical structure, redox properties and acidity. The powder catalysts were placed downstream of a Pt–BaO/Al2O3 monolith (previously synthesized and characterized elsewhere [10]) and the significant improvement in NOx removal efficiency to N2 without practical NH3 slip through the combined NSR–SCR system, running under cycled lean–rich atmosphere, is demonstrated. The H2 concentration during the rich period of the NSR cycle should be adequately tuned for the required intermediate production of ammonia. 2. EXPERIMENTAL 2.1. Catalysts preparation The SCR catalysts consisted of Cu-supported zeolites. Fresh zeolites were supplied by Zeolyst International, namely CP414E (BETA, Si/Al = 25) and CBV5524G (ZSM5, Si/Al = 50). Zeolites were first calcined at 550 ºC for 4 h to get the protonic form. The catalysts were prepared by two different conventional procedures, namely ion exchange (IE) and impregnation (IM). Metal ion exchange was carried out by dissolving the required amount of Cu(COOCH3)2 (Panreac, 98%) in water. Then, 12 g H-ZSM5 or H-BETA were added to 1.5 l of this solution and it was stirred for 24 h at 65 ºC. The ion exchanged samples were then filtered, washed twice in deionised water, dried during all night and calcined at 550 ºC for 4 h. 7 On the other hand, the impregnation method consisted in adding slowly the required amount of the precursor dissolved in water (1.5 wt.%) at 40 ºC and 3 mm Hg to some grams of H-beta or HZSM5, under continuous rotation until the solvent was evaporated. The samples were dried and later calcined at 550 ºC for 4 h. The actual amount of Cu in the prepared catalysts was determined by ICP-AES from the solid sample. All the catalysts were then pelletized, crushed and sieved to 0.3–0.5 mm to avoid mass transfer limitations, which was checked in some previous experiments carried out with different particle sizes. In order to characterize the SCR catalyst, several techniques were employed, such as BET surface area analysis, H2-TPR, NH3-TPD, and XRD. The prepared catalysts, with the precursor dissolution concentration and the actual copper content, are shown in Table 1. The Pt–BaO/Al2O3 NSR monolith catalyst was prepared according to our previously reported procedure [10]. In summary, a cordierite monolith, 20 mm in length and diameter, with a cell density of 400 cells per square inch and a wall thickness of 150 lm was washcoated with calumina (163 m2 g-1 after stabilization at 700 ºC, 4 h) by several immersions of the monolith into the alumina slurry until approx. 1g Al2O3 was deposited in the monolith structure. The incorporation of platinum was carried out by adsorption from tetraammine platinum (II) nitrate solution and the excess of liquid remaining in the channels was blown out with compressed air. After calcination in air (500 ºC, 4 h) and subsequent reduction of the metallic phase in a 5% H2/N2 stream (500 ºC, 1 h), the barium was incorporated by immersion of the monolith in a barium acetate solution. Finally, the catalyst was calcined again (500 ºC, 4 h). 8 Table 1. Characteristics of the prepared catalysts. Support Si/Al Metal incorporation methodology Cu initial conc. (ppm) Exchanged amount (%) Catalyst content Cu (wt.%) Nomination BETA 25 Ion exchange 160 75 1.5 B-IE-1.5 Ion exchange 320 53 2.1 B-IE-2.1 Ion exchange 640 36 2.9 B-IE-2.9 Ion exchange 960 38 4.5 B-IE-4.5 Ion exchange 2000 23 5.8 B-IE-5.8 Impregnation – – 1.3 B-IM-1.3 ZSM-5 50 Ion exchange 160 70 1.4 Z-IE-1.4 Ion exchange 320 40 1.6 Z-IE-1.6 Ion exchange 640 33 2.6 Z-IE-2.6 Ion exchange 960 28 3.4 Z-IE-3.4 Ion exchange 2000 20 4.9 Z-IE-4.9 Impregnation – – 1.2 Z-IM-1.2 9 2.2. Catalysts characterization 2.2.1. Ammonia temperature programmed desorption (NH3-TPD) Eighty milligrams sample (hydrate state) was placed in a Ushaped quartz reactor connected to a Micromeritics AutoChem 2910 instrument. The sample was pretreated in nitrogen flow at 550 ºC for 15 min, cooled down to 100 ºC and treated with helium for 60 min. Then, the sample was flushed with 10% NH3/He until saturation and the TPD was started using helium as carrier gas (500 ml/min, STP). The material was heated to 550 ºC at the rate of 10 ºC/min, while the NH3 desorption was continuously monitored with a TCD detector. The amount of ammonia desorbed at some given temperature range was taken as the acid site concentration, whereas the temperature range at which most of the ammonia was desorbed indicated the acid strength distribution. 2.2.2. Surface area The BET surface areas of the zeolite samples were determined by N2 adsorption–desorption at -196 ºC using a Micromeritics ASAP 2020 equipment. 2.2.3. XR diffraction The change in crystalline structure of the Cu modified zeolite samples were analyzed by XRD (Philips PW1710 diffractometer). The samples were finely ground and were subjected to Cu Ka radiation in continuous scan mode from 5º to 80º of 2h with 0.02º per second sampling interval. PANalytical X’pert HighScore specific software was used to data treatment. JCPDS database was used to confirm the spectrum. 2.2.4. Hydrogen temperature programmed reduction (H 2 -TPR) Reducibility of Cu in the catalyst was investigated by temperature-programmed reduction (TPR) using H2. The sample was pretreated in 30 ml/min of 10% O2/He mixture gas flow at 550 ºC for 45 min and then cooled down to 30 ºC and flushed with helium for 60 min. Then samples were heated from room temperature to 600 ºC with 10 ºC/min ramp in a 60 ml/min of 10 5% H2/Ar mixture gas flow. The water formed during reduction with H2 was trapped using a cold trap and the hydrogen consumption was continuously monitored with a TCD detector. 2.3. Activity tests 2.3.1. SCR experiments The SCR experiments were performed in a downflow stainless steel reactor. The reactor tube, with 1 g of 0.03–0.05 mm pelletized Cu-zeolite SCR catalyst inside, was located into a 3-zone tube furnace. The temperature was measured by a thermocouple at the top of the catalyst bed. The reaction temperature was varied from 100 to 500 ºC. The composition of the feed gas mixture was 750 ppm NO, 750 ppm NH3 and 9.5% O2 using Ar as the balance gas. Gases were fed via mass flow controllers and the total flow rate was set at 3000 ml min-1, which corresponded to a space velocity (GHSV) of 90,000 h-1. Previous experiments made with GHSV of 22,500 and 45,000 h-1 achieved almost 100% conversion in a wide range of temperatures (220–460 ºC) with all the catalysts prepared, which made difficult comparison of behavior and election of the best candidate for the double NSR–SCR configuration. The NO, NO2, NH3 and N2O concentration at the reactor exit were monitored every 40 ºC, once the analysis has been stabilized for at least 10 min, by online FTIR multigas analyzer (MKS 2030). The NOx and NH3 conversions were calculated as in out NO NO NO in NO 100 FF X F − = × (1) 33 3 3 in out NH NH NH in NH 100 FF X F − = × (2) and the N2, N2O, and NO2 selectivities were calculated as 2 2 33 out N Nin in NO NO NH NH 2F S FX F X =+ (3) 17 Acidity of the zeolite is one of the important parameter that determines the extent of NOx reduction with ammonia over zeolite based catalyst [33]. The acidity of all prepared catalysts was determined by NH3-TPD and the values are given in Table 3. All the samples exhibited two major desorption peaks; for ZSM5 based catalysts the first desorption peak was situated in the range 175– 210 ºC corresponding to weak acid sites, whereas the second desorption peak, corresponding to strong acid sites, was detected in the range 220–370 ºC. For BETA based catalyst, the low temperature desorption peak was coincident with that observed for ZSM5, but the high temperature desorption peak was observed in a much more narrow window, i.e. 250– 280 ºC. The high temperature desorption peak, or strong acid sites, can be related to the presence of Brønsted acid sites in zeolite catalysts [34–36]. The quantity of desorbed ammonia during TPD experiment can illustrate the number of acid sites in the sample, which generally increases with the copper content, the strong acidity in more extension. It can be also observed that the amount of desorbed ammonia at lower temperature is in general lower than the desorbed ammonia at higher temperatures, which means a superiority of Brønsted acid sites in comparison with Lewis acid sites. 18 Table 3. Acidity of Cu-zeolite catalysts, determined by NH3-TPD. Sample Weak acidity Strong acidity Total µmol NH 3 g-1 T (ºC) µmol NH 3 g-1 T (ºC) µmol NH 3 g-1 H-ZSM5 183 197 237 371 420 Z-IE-1.4 161 210 256 246 417 Z-IE-1.6 122 197 295 220 417 Z-IE-2.6 46 188 380 274 426 Z-IE-3.4 94 183 359 269 453 Z-IE-4.9 176 200 379 235 555 Z-IM-1.2 50 185 248 273 298 H-BETA 388 183 151 278 539 B-IE-1.5 304 183 307 264 611 B-IE-2.1 251 210 395 258 646 B-IE-2.9 205 194 464 255 669 B-IE-4.5 312 174 444 266 756 B-IE-5.8 328 191 746 257 1074 B-IM-1.3 174 182 433 257 607 For ZSM5 supported catalysts, the total acidity grows gradually with the copper content, from 420 µmol NH3 (g cat.)-1 corresponding to the bare zeolite to 555 µmol NH3 (g cat.)-1 for Z-IE4.9, when the copper was incorporated by L.I.E. On the other hand, the incorporation of copper by impregnation reduced the total acidity of the catalyst to 298 µmol NH3 (g cat.)-1 for Z-IM- 19 1.2, probably due to the blockage of the zeolite pores by copper aggregates which quantity is expected to be less for liquid ion exchange catalysts. Similar trends can be observed for CuBETA catalysts. The incorporation of cooper to the zeolite support also affected notably the surface area of the catalysts. The fresh ZSM5 zeolite presented a surface area of 398 m2 g-1 which gradually decreased with increasing cooper loading. In fact, when 4.9% of Cu (Z-IE-4.9) was incorporated through liquid ion exchange to the zeolite, the surface area decreased to 350 m2 g-1 (Table 4). On the other hand, the incorporation of Cu through wetness impregnation led to a much larger decrease in the exposed surface area (298 m2 g-1, ZIM-1.2), which was related to the presence of cooper aggregates blocking the pores. In the case of BETA supported catalysts, a similar trend was observed. The surface area of the bare zeolite (BETA, 585 m2 g-1) gradually decreased to 459 m2 g-1 (B-IE-5.8) when Cu was incorporated by liquid ion exchange, whereas the surface area of the B-IM-1.3 was reduced to 428 m2 g-1 when Cu was added by impregnation. Table 4. Surface area of the prepared SCR catalysts ZSM5 samples Surface area (m2 g-1) BETA samples Surface area (m2 g-1) H-ZSM5 398 H-BETA 585 Z-IE-1.4 385 B-IE-1.5 562 Z-IE-1.6 380 B-IE-2.1 493 Z-IE-2.6 361 B-IE-2.9 481 Z-IE-3.4 374 B-IE-4.5 463 Z-IE-4.9 350 B-IE-5.8 459 Z-IM-1.2 298 B-IM-1.3 427 20 3.2. Ammonia SCR activity tests The NOx selective catalytic reduction activity (SCR) tests of the prepared catalysts was carried out under a feedstream with the following composition: 750 ppm NO, 750 ppm NH3, 9% O2 and Ar to balance. The total flow rate was set at 3000 ml min-1, which corresponded to a space velocity (GHSV) of 90,000 h-1. This GHSV, higher than used in practice, allowed best comparing the behavior of the prepared catalysts, as makes the effect of temperature more relevant. The NO, NO2 NH3 and N2O concentrations at the reactor exit were monitored from 100 to 500 ºC, every 40 ºC, once the analysis had been stabilized for 10 min. Fig. 2a and b shows the conversions of NOx and NH3 as a func-tion of the reaction temperature. For each support, ZSM5 and BETA, three catalysts were chosen as representative of low, intermediate and high Cu content, prepared by ion exchange. The copper low content catalysts prepared by impregnation are also included in Fig. 2. The conversion trends are typical for NOx SCR reactions, reaching a maximum in activity for an intermediate temperature [37,38]. For the samples studied, the NOx conversion maximum was reached between 350 and 450 ºC for BETA supported catalysts and between 250 and 350 ºC for ZSM5 supported catalysts. The higher activity of ZSM-5 based catalysts at lower temperatures is probably related with the higher reducibility of Cu at low temperature observed in TPR experiments. 21 Fig. 2. Conversion of NOx (filled symbols) and NH3 (empty symbols) for (a) BETA and (b) ZSM5 supported catalysts for SCR reaction. Copper content: ■ low IE,  intermediate IE,  high IE and  low IM. Table 5 shows the maximum NOx conversion for each catalyst and the temperature at which it has been reached (three first columns). In the case of BETA supported catalysts, B-IE-2.1 showed the maximum NOx conversion of 95.8% at 420 ºC. For catalysts with higher copper loadings the maximum NOx conversion decreased to 87.3% and 82.7% for B-IE-4.5 and B-IE5.8, respectively, which instead were achieved at lower temperature, 340 ºC. It is suggested that the increase of Cu loading promotes the oxidation of NO to NO2 at lower temperature, which activates the fast SCR reaction (2NH3 + NO + NO2 → 2N2 + 3H2O), and consequently shifts the maximum NOx conversion to lower temperature. 22 Table 5. Maximum NO conversion and corresponding NH3 conversion and N2/N2O at this temperature. Sample Maximum NO x conversion Temperature window for X NO > 70% X NO (%) T (ºC) X NH3 (%) S N2 (%) S N2O (%) B-IE-2.1 95.8 420 98.6 98.4 1.3 280–520 ºC (240) B-IE-4.5 87.3 340 99.2 95.5 4.5 275–440 ºC (165) B-IE-5.8 82.7 340 98.4 93.5 6.5 250–395 ºC (145) B-IM-1.3 82.7 380 99.8 97.6 2.4 320–450 ºC (130) Z-IE-1.4 97.9 340 99.7 99.5 0.5 280–440 ºC (160) Z-IE-3.4 93.6 340 99.3 96.1 3.9 235–425 ºC (190) Z-IE-4.9 89.3 300 99.7 94.9 5.1 205–375 ºC (170) Z-IM-1.2 75.2 300 98.9 98.6 1.1 260–355 ºC (75) 23 The fourth column in Table 5 shows that ammonia conversion reached almost 100% for NOx maximum conversion. Only N2 and N2O (no NO2) were detected at the reactor exit as deduced from selectivities shown in columns fifth and sixth, which close the mole balance with N2 and N2O. It can be noted that higher copper loadings enhance selectivity to N2O, but always maintained below 6.5%. The last column in Table 5 indicates the amplitude of the temperature windows for maintaining NOx conversion higher than 70%. The wider amplitude was achieved with low copper content (B-IE-2.1) and it was decreased with the copper loading. Thus, it can be concluded that among the BETA catalysts prepared by ion exchange, the low Cu content BIE-2.1catalyst resulted the most active as it combines the highest NOx conversion and the wider temperature window. Although the increase in the copper content has a beneficial effect on the activity at low temperature, however the maximum NOx conversion and temperature window amplitude are significantly reduced due to some ineffective use of copper. The catalyst B-IM1.3, prepared by impregnation, achieved the lowest maximum conversion and the narrowest temperature window, as it was expected due to the low Cu dispersion obtained by this preparation method. In the case of ZSM5 supported catalysts prepared by ion exchange, as the copper content increases the NOx conversion curve shifted to lower temperatures whereas the catalyst achieved lower maximum NOx conversions. This trend is similar to that previously described for BETA catalysts; however, all Cu-ZSM5 catalysts presented very similar amplitudes of temperature windows. Thus, the best ZSM5 catalyst has to be chosen only based on the maximum NOx conversion, resulting in the low Cu loading Z-IE-1.4. Again, the impregnated Z-IM-1.2 can be disregarded due to its worse behavior, with 75.2% of maximum NOx conversion and narrow temperature window amplitude of 75 ºC. 3.3. NSR–SCR activity tests 24 The most active catalyst for SCR reaction, B-IE-2.1 and Z-IE-1.4, were used to accomplish the double NSR–SCRconfiguration, by adapting the SCR catalyst downstream the NSR catalyst. The monolith NSR catalyst used has been the same for all experiments [10]. The experiments in the double system were made with the following feedstream composition: 750 ppm NO, 9.5% O2 in Ar during the lean period (150 s); and 750 ppm NO, 4% H2 in Ar during the rich period (20 s). Concentrations of N2/NH3/N2O were monitored at the intermediate position, i.e. after NSR but before SCR, where production of ammonia as reactant for subsequent SCR should be important, and also they were monitored at the exit of the double NSR–SCR reactor. Fig. 3 shows the evolution of NOx conversion and the NH3 and N2 production with temperature, for the simple NSR configuration and for the combined NSR–SCR configuration, the latter with B-IE-2.1 and Z-IE-1.4 SCR catalysts. It can be observed that the addition of an SCR catalyst downstream the NSR catalyst improves significantly the NOx conversion, mainly for temperatures between 200 and 250 ºC. The maximum NOx conversion achieved with the single NSR resulted in 50%, and it was increased up to 76% with the double NSR–SCR configuration, independently of the SCR catalyst. In concordance with the SCR activity previously analyzed, the Z-IE-1.4 catalyst made the double NSR–SCR configuration more active at lower temperature (200 ºC) than the B-IE-2.1 (250 ºC). It is worthy to note that the combined NSR–SCR system achieved efficient NOx reduction at lower temperatures than NSR or SCR single systems with the aid if intermediate ammonia formed during the rich period of the NSR process in the SCR placed downstream, but no need of external NH3 feed as in the case of the single SCR system. 25 Fig. 3. 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