Development of optimized Cu–ZSM-5 deNOx catalytic materials both for HC-SCR applications and as FCC catalytic additives

Applied Catalysis A: General 325 (2007) 345–352 www.elsevier.com/locate/apcata

Development of optimized Cu–ZSM-5 deNOx catalytic materials both for HC-SCR applications and as FCC catalytic additives Vasilis G. Komvokis a, Eleni F. Iliopoulou b, Iacovos A. Vasalos b, Kostas S. Triantafyllidis c,*, Christopher L. Marshall d b

a Department of Chemical Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece Chemical Process Engineering Research Institute, CERTH, P.O. Box 361 Thermi, 57001 Thessaloniki, Greece c Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece d Chemical Engineering Division, Argonne National Laboratory, Argonne, IL 60439-4837, USA

Received 20 July 2006; accepted 7 February 2007 Available online 3 March 2007

Abstract Cu-exchanged ZSM-5 zeolites finely coated with CeO2 nanoparticles were tested in this work as catalysts for the selective catalytic reduction (SCR) of NO with C3H6. The CeO2 coated Cu/ZSM-5 shows lower maximum NO conversion activity compared to the non-coated Cu/ZSM-5 catalyst at relatively high temperatures (ca. 450 8C) but the former catalyst is significantly more active at lower temperatures (ca. 350 8C), especially at lower space velocities, both under dry and wet feed conditions. Under simultaneous addition of both SO2 and water in the feed, the beneficial effect of the CeO2 coating at lower reaction temperatures was retained only at low space velocities. The same Cu/ZSM-5 based samples were evaluated as fluid catalytic cracking (FCC) catalytic additives for the in situ reduction of NOx formed during regeneration of the coked FCC catalyst. The amounts of NO and CO emitted during regeneration of the spent FCC catalyst at 700 8C in the presence of Cu/ZSM-5 based additives were compared with those obtained when a commercial CO promoter was used in the FCC catalyst inventory. All Cu/ZSM-5 additives exhibited significant NO reduction ability, which was further enhanced by increasing the Cu loading or the amount of additive in the FCC catalyst. The CeO2coated sample reached the highest deNOx performance (up to 78% NO reduction); however, all additives presented insufficient activity for CO oxidation. Simultaneous NO reduction and CO oxidation was achieved only when the CeO2-Cu/ZSM-5 additive was promoted with Rh or when the non-promoted additives were combined with a commercial CO promoter. Preliminary studies suggested that the Rh-promoted CeO2-Cu/ZSM-5 additives can be very effective for both NO reduction by CO and CO oxidation at certain O2 concentrations, such as in the O2-deficient zones of the FCC regenerator. Further studies are in progress in order to elucidate the reaction mechanism and optimize the additive’s formulation. # 2007 Elsevier B.V. All rights reserved. Keywords: Cu–ZSM-5; NO reduction; FCC regenerator; CO oxidation; Rh–Ce promotion

1. Introduction The selective catalytic reduction (SCR) of NOx by hydrocarbons has been proven to be a promising alternative technology for NOx abatement from oxygen-rich flue gases (‘‘lean burn’’ conditions) emitted both from stationary and mobile sources [1–4]. The catalysts should be highly active and selective to N2 formation at different space velocities (ranging usually from 10,000 to 150,000 h1), depending on the type of

* Corresponding author. Tel.: +30 2310 997730; fax: +30 2310 997730. E-mail address: [email protected] (K.S. Triantafyllidis). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.02.035

application (industrial flue gases or automotive emissions), in a wide temperature range (200–550 8C), under relatively high concentration of oxygen (up to ca. 10 vol.%), and in the presence of other ‘‘poisonous’’ gases, such as water vapor (up to ca. 12 vol.%) and sulphur dioxide (up to ca. 50 ppm). Furthermore, the selectivity towards N2O formation should be minimized and oxidation of CO (also included in the flue gases) towards CO2 should be maximized. Copper-exchanged ZSM-5 catalysts were shown to be active in the direct NO decomposition to N2 and O2 [5], being however sensitive to poisoning by SO2, H2O and O2. The addition of hydrocarbons as reducing agents in the oxygen-rich feed led to significant increase in the rate of the selective reduction of NO to N2, as it

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was first reported by Held et al. [6] and Iwamoto et al. [5,7]. Other metal-modified zeolites have also been studied [2] but Cu/ZSM-5 zeolites, as well as other Cu-based catalytic formulations [8], have attracted much attention. The major drawback of Cu/ZSM-5 based catalysts is their relatively poor activity in the presence of water, which has been previously attributed to changes in the coordination environment of Cu2+ ions or to their redistribution within the zeolite crystal, and to the formation of inactive CuO phases [9–11]. In addition, dealumination of the zeolite framework that occurs during long-term reaction under wet conditions, leads to the decrease of the Cu2+-exchange sites which are suggested as the active sites for NO reduction and also to decrease of the number of bridging hydroxyls (zeolitic Bro¨nsted acid sites), which are required for the activation of hydrocarbons [12]. Furthermore, the extra-framework Al2O3 phases formed during steaming dealumination of the zeolite may interact with copper, generating highly dispersed Cu ions in Al2O3 or as copper aluminate (CuAl2O4) compound [9]. The development of a hydrothermally durable deNOx catalyst, which can also operate in the presence of SO2 [3 and references therein], is of great importance since most combustion flue gases contain significant amounts of water and SO2. In the case of NO decomposition, bifunctional zeolitic catalysts prepared by co-exchanging calcium [13] or rare-earth metal ions [14] into Cu/ZSM-5 have been shown to improve the stability of the catalyst against steam. With regard to the NOx SCR process, mixtures of metal- or proton-exchanged zeolites with easily reducible oxides, such Mn2O3 [15] and CeO2 [16], have previously been proposed as potentially water-stable deNOx catalysts. Through a different approach, replacement of copper by cobalt enhanced the stability of ZSM-5 based catalysts under hydrothermal conditions [17] but did not resolve SO2 poisonous effect at reaction temperatures lower than 500 8C; however, at 550–600 8C the NO conversion activity was hardly affected by the presence of SO2 [18]. In addition to actual catalyst formulation, it has been also shown that the performance of the SCR catalysts under wet reaction conditions is strongly dependent on the type of reductant used [19]. Recently, a substantial enhancement of deNOx SCR activity in the presence of water has been reported by Argonne National Laboratory [20,21] via improvement of Ce–Cu/ZSM5 catalyst preparation method. The new catalyst formulation was based on Cu-exchanged ZSM-5 zeolites whose crystals were coated with fine nanoparticles of CeO2, leading to a promoted reducibility of Cu and Ce ions, both in the presence and absence of water, and also to increased resistance towards the destruction of the zeolite structure. However, the effect of SO2 on Cu/ZSM-5 deNOx catalysts activity still needs to be clarified, especially for industrial-stationary combustion applications where the production of significant amounts of SO2 can not be avoided, in contrast to the automotive emissions where the mandated 50 ppm and lower sulphur concentration in diesel will gradually reduce the SO2 emissions significantly and is expected to minimize their effects on catalytic converters. A very important industrial process, where clean-up of the flue gases from NOx is required, is the fluid catalytic cracking

(FCC) of gas–oil or of other heavy petroleum fractions for the production of gaseous and liquid fuels. During the cracking reactions the FCC catalysts are poisoned with coke which is being burnt off in the regenerator unit at high temperatures (ca. 700–750 8C) in the presence of air, resulting in the formation of NOx due to the nitrogen containing species in the coke (fuel NOx) [22]. The flue gas stream of the FCC regenerator contains O2, N2, CO, CO2, H2O, SOx and NOx. Typical NOx concentrations range between 50 and 500 ppm depending upon feed (gas–oil) nitrogen levels and regenerator conditions. NOx emissions from the regenerator contribute up to 50% of the total NOx emissions in modern refineries, which are forced worldwide to adopt more efficient deNOx technologies. Among different candidate NOx control technologies the use of catalytic additives is the most attractive one, as it is simple, cost-effective, and applicable in existing FCC units. Potentially effective deNOx additives should operate in the presence of the other flue gases (O2, N2, CO, CO2, H2O, SOx) and at the relatively high regeneration temperatures. Most modern refineries use CO promoters, which are Pt-based catalytic additives that accelerate the oxidation of CO to CO2 but unfortunately increase NO emissions. Therefore, novel NOx reduction additives should either operate in the presence of a conventional CO promoter or have the ability to simultaneously reduce NO and CO emissions. In the patent literature, mostly assigned to Grace Davison [23], several catalytic deNOx formulations have been described comprised mainly from acidic (other than zeolitic) metal oxide supports enriched with various alkali/alkaline earth, rare-earth, transition and noble metals/metal oxides; the active species are mainly the noble metal oxides while overall activity and stability can be fine-tuned by the redox properties of specific oxides, such as CeO2. Experimental tests of commercial deNOx FCC additives (DENOx and XNOx by Grace) showed that DENOx can be efficient in the presence of an oxidation promoter (CP-3) while XNOx can result in acceptable simultaneous reduction of NOx and oxidation of CO without the use of CP-3 [24]. More fundamental studies in the last 5 years have mainly focused on noble metals (Rh, Ru, Ir) or Ag supported on gAl2O3 or other mixed oxides [25–28]. Also, MCM-36 derivatives containing mixed oxide pillars (such as MgO– Al2O3, BaO–Al2O3, and others) have been shown to retain appreciable deNOx activity in the presence of commercial CO promoter co-existing in FCC catalysts [29]. Hydrotalcitederived Mg–Al mixed oxides promoted with Cu or Co or Ce have also been suggested to be active for the simultaneous removal of NOx, SOx and CO from the flue gases of the FCC regenerators, operating under controlled conditions [30,31]. The present paper, presents a systematic study of metalmodified ZSM-5 as potential FCC additive for the in situ reduction of NOx formed during regeneration of coked cracking catalysts. For almost 20 years ZSM-5-based FCC additives have been used by refiners to boost LPG olefins and/or FCC gasoline octane [32]. Combination of this ability with a significant deNOx performance would lead to the development of multifunctional FCC additives.

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2. Experimental 2.1. Materials A series of Cu/ZSM-5 catalysts were prepared by multiple ion-exchange of NH4+/ZSM-5 samples (Si/Al 28) with Cu(NO3)2 aqueous solution at pH 4 at 60 8C for 24 h for each exchange, followed by filtration, washing with distilled water and drying at 100 8C for 2 h. The Cu loading of the calcined exchanged samples ranged from 0.5 to 1.5 wt.%, which corresponds to Cu ion-exchange levels of 30–90% (based on the assumption that each Cu2+ ion interacts with two Al sites). A 1 wt.% Cu/ZSM-5 sample was also prepared via typical dry impregnation method. A ceria-modified Cu/ZSM-5 sample coated with CeO2 nanoparticles was prepared in Argonne National Laboratory by applying typical ion exchange methods for copper insertion followed by impregnation with a ceria colloidal sol [20,21]. Selected samples were further impregnated with small amounts of Rh using aqueous solutions of RhCl3. All catalytic materials were finally calcined at 500 8C for 4 h in air. For the evaluation of the Cu/ZSM-5 samples as FCC catalyst additives for the in situ reduction of NOx formed during regeneration of the coked catalyst, a spent (coked) FCC catalyst was supplied by Akzo. A conventional Pt-based CO promoter supplied by Grace GmbH, was also used in the evaluation of the catalysts as FCC additives. All samples that were tested in this work both as SCR deNOx catalysts and as FCC deNOx additives are summarized in Table 1. 2.2. Characterization All catalytic materials were characterized by: (a) ICP-AES for the determination of metal loading (wt.% of Al, Na, Ce, Cu), using a Plasma 400 (Perkin-Elmer) spectrometer, equipped with Cetac6000AT + ultrasonic nebulizer, (b) powder X-ray diffraction (XRD) for the determination of crystalline phases of the zeolites and the supported metal oxides, using a Siemens D500 X-ray diffractometer with Cu Ka radiation in the range of 5–658 2u at a scan rate of 18/min, (c) N2 adsorption/desorption experiments at 196 8C for the determination of surface area

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(micropore and meso/macropore), using an Autosorb-1 Quantachrome flow apparatus and applying the BET method for the total surface area and the V–t plot method for the micropore area, and (d) transmission electron microscopy (TEM) for the determination of the size and morphology of the supported metal oxides, using a Phillips CM120 instrument operating at 120 kV, equipped with a CCD camera (samples were mounted on holey carbon-coated copper grids). In situ Xray absorption spectroscopy (XAS) studies were performed at the Materials Research Collaborative Access Team (MR-CAT) beamline (10-ID) at the advanced photon source (APS), Argonne National Laboratory. Cu-K and Ce-L3 edge XAS spectra were collected in transmission mode, with the X-ray beam passing through ionization chambers filled with N2 before and after the sample cell. Further details of the experimental set-up are given elsewhere [21,33]. Before moving the sample cell onto the beamline, the samples were treated off-line in air (TPR) or H2 (4%) in He (TPO) up to 500 8C for at least an hour. After cooling to 50 8C, the samples in the cell were heated up to 600 8C with a temperature ramp of 2 8C/min under a 30 sccm flow of H2 (4%) in He (TPR) or dry air (TPO). Scans of all samples were recorded every 2–3 min during the reaction with 40–60 spectra per sample collected during each run. In addition, a Cu foil reference spectrum was collected for energy calibration using a third ionization chamber. 2.3. DeNOx experimental unit All catalytic experiments were performed in a bench-scale reaction unit that consists of the feed gas system, a fixed bed reactor (i.d. 1.8 cm), a three-zone furnace, controlled by PID controllers and the on-line gas analysis system which comprised a chemiluminescence’s NO/NO2/NOx analyzer (42C-HL, Thermo Environmental), a non-dispersive infrared CO and CO2 analyzer (NGA 2000, Rosemount), a magnetopneumatic O2 analyzer (MPA-510, Horiba), a N2O analyzer (VIA-510, Horiba) and a SO2 analyzer (NGA 2000, Rosemount). All selective catalytic reduction (SCR) experiments were carried out loading the fixed bed reactor with a mechanical mixture of 1.8 g SiC and 0.2 g of each ZSM-5 based catalyst in

Table 1 Preparation method, metal loading and surface area of Cu/ZSM-5 based deNOx catalytic additives Sample

Preparation method

Metal loadinga (wt.%) Cu

Al

Ce

Micropore

Meso/macropore

H-ZSM-5 Cu(0.5%)/ZSM-5 Cu(1%)/ZSM-5 Cu(1.5%)/ZSM-5 Cu(1%-impregn.)/ZSM-5 CeO2(8%)Cu(0.8%)/ZSM-5 Rh(0.05%)CeO2(8%)Cu(0.8%)/ZSM-5 Rh(0.05%)Cu(1 %)/ZSM-5 Rh(0.05%)Cu(1 %-impregn.)/ZSM-5

– Ion exchange Ion exchange Ion exchange Dry impregnation Ion exchange (Cu)/colloidal solution (CeO2) As sample above plus dry impregnation with Rh Cu(1%)/ZSM-5 plus dry impregnation with Rh Cu(1%-impregn.)/ZSM-5 plus dry impregnation with Rh

– 0.51 1.08 1.44 1.12 0.77 – – –

1.39 1.38 1.32 1.36 1.46 1.40 – – –

– – – – – 6.36 – – –

354 311 305 343 328 293 297 350 345

55 81 73 64 58 110 107 54 53

a b c

(31%)c (69%) (89%) (47%)

Surface areab (m2/g)

Weight % of metals was determined by ICP-AES chemical analysis. Micropore and meso/macropore surface area were determined by N2 adsorption experiments by the BET method and the V–t plot analysis. % ion-exchange level, assuming that each Cu2+ interacts with two Al sites.

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order to increase the size of the bed and avoid heat transfer effects. The total flow rate of the feed gas stream was 500 ml/ min with a composition of 1000 ppm NO, 1000 ppm C3H6, 2% O2, 0 or 10% H2O and 0 or 50 ppm SO2 in He. The contact time, W/F, defined as the ratio of zeolite catalyst weight to total flow rate was 0.024 g s/ml (corresponding GHSV ffi 59,000 h1). A second series of SCR experiments was performed at a total flow rate of 100 ml/min (corresponding GHSV ffi 11,800 h1) keeping all the other parameters constant, in order to study the effect of space velocity on the NO conversion activity of the catalysts. All catalysts were pre-treated at 550 8C with 5% H2/ He for 1 h, followed by 1 h at 250 8C under the reaction feed in order to approach pseudo-steady-state conditions. The activity of each catalyst in the SCR of NOx with C3H6 was examined in the temperature range of 250–600 8C and every measurement was taken after 30 min stabilization at each temperature. Evaluation of the Cu/ZSM-5 based materials as FCC additives for the in situ reduction of NOx formed during regeneration of the coked FCC catalyst in a full-burn mode were carried out in the same bench-scale reaction unit, following a protocol that was developed at CPERI and simulates the regenerator of an FCC unit [24]. Mechanical mixtures of spent FCC catalyst with candidate Cu/ZSM-5 additives were loaded in the fixed-bed reactor (total catalyst weight 2 g) and regeneration took place at 700 8C using 2% O2 in He feed, with a total flow rate of 500 ml/min. Mixtures of the spent FCC catalyst with a commercial CO oxidation promoter (99% catalyst + 1% CO promoter) were considered as the base (reference) case, since the CO promoters are usually included in total FCC catalyst inventory. The CO and NO emissions were measured by the above described on-line gas analyzers. A second series of experiments aiming at understanding of the reaction mechanism, were performed by loading the reactor with mechanical mixtures of equilibrium (regenerated) FCC catalyst with the candidate additives at 700oC using a feed of 1000 ppm NO, 1% CO, 0.3–0.7% O2, 10% H2O, 50 ppm SO2, in order to simulate the gases produced during burning of the coked FCC catalyst.

decrease of their microporosity, as was expected due to the high ceria loading (8 wt.%), but at the same time the total surface area remained high, possibly due to added meso/macropore area from the nanocrystalline ceria [20]. With regard to the metal phases present on the zeolite samples, no Cu crystalline phases could be detected on all calcined samples, indicating either the absence of crystalline Cu-oxides or the presence of very small (ca. <3 nm) crystallites. On the other hand, broad peaks attributed to CeO2 were present in the XRD patterns of the respective samples [20], indicating that bulk nanocrystalline ceria was present in the Cu-exchanged zeolite samples. Furthermore, TEM studies [20] have showed that ceria nanoparticles with sizes between 2 and 10 nm form a uniform coating covering the ZSM-5 zeolite crystals, while on the other hand Cu nanoparticles were not detected. The catalyst formulation described above consists of Cuexchanged ZSM-5 samples coated with fine CeO2 nanoparticles and was previously shown to possess remarkable redox properties, mainly due to the interaction between the Cu species and the ceria nanoparticles at the interface between the ceria coating and the surface of the zeolite crystals [21]. The variation of the Cu-K edge of Cu-ZSM-5 and CeO2/Cu-ZSM-5 as a function of temperature during H2-TPR, is shown in Fig. 1. The different oxidation states of copper have unique features in the XANES region (Fig. 1) [33]. Cu2+ shows a single peak just in the post-edge region with a very weak pre-edge feature at 8985 eV. A strong pre-edge feature dominates the Cu1+ XANES spectra at 8983 eV and Cu0 shows a doublet after the edge and some weak features. Visual inspection of these XANES traces easily shows the transformation of Cu2+ ! Cu1+ ! Cu0 for both samples. A comparison between

3. Results and discussion 3.1. Physicochemical characterization of the Cu/ZSM-5 based catalytic materials Physicochemical characterization data for all the Cu/ZSM-5 samples prepared in this work are given in Table 1. The Cu loading in the ion-exchanged or impregnated samples varied from 0.5 to 1.5 wt.%. In the Cu-exchanged samples these loadings correspond to ion-exchange levels of 30–90% (based on the assumption that each Cu2+ ion interacts with two Al sites). The Ce loading in the CeO2-impregnated samples was 6.4 wt.% Ce (8 wt.% CeO2), while the Rh content in the respective samples was 0.05 wt.%. All the samples were highly crystalline after calcination at 500 8C in air (XRD results, not shown) and retained their total surface area, as well as the ratio of micropore to meso/macropore area, almost unchanged (Table 1). The CeO2 coated ZSM-5 samples, showed a slight

Fig. 1. TPR of (a) Cu–ZSM-5 and (b) CeO2/Cu–ZSM-5 as detected by the CuK edge in XANES. The peak at 8983 eV corresponds to Cu1+.

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the waterfall plots indicates that, in the case of CeO2/Cu–ZSM5 (Fig. 1b), both Cu redox transitions (Cu2+ ! Cu1+ ! Cu0) occur at least at 50 8C lower temperatures, compared to the case of Cu–ZSM-5 sample (Fig. 1a). In situ XANES analysis allows the direct observation of the oxidation state of Cu during H2TPR. Using principle component analysis (PCA) the relative amounts of each Cu species can be estimated [33]. The results shown in Fig. 1 confirm a Ce–Cu interaction, which leads to enhanced reducibility of Cu. The redox transformations of Ce are also affected in the presence of Cu and Ce4+ reduces to lower mixed oxidations states at least at 100–125 8C lower temperatures in the case of CeO2/Cu–ZSM-5 sample compared to the CeO2/H–ZSM-5 sample, as it was previously shown based on XANES–TPR data [21]. The observed Ce–Cu chemical interactions could not be easily explained based on the formation of the fine CeO2 nanoparticles (2–10 nm) coating on the external surface of the zeolite crystals, as evidenced by TEM [20]. However, the CeO2 sol used for impregnation contains a finite amount of soluble, monomeric Ce3+ ions, which enter the ZSM-5 micropores during incipient wetness and reside close to the Cu ions at ion-exchange sites. Upon calcination of the impregnated samples the formation of mixed Cu–Ce oxide phases with enhanced interactions between the two metals could induce the increased reducibility of both metals, similar to Cu-doped ceria phases [21]. However, more detailed characterizations studies, i.e. with in situ FT-IR, are necessary for the identification of these phases within the ZSM5 micropores. Still, the ability of both Cu and Ce to be reduced at lower temperatures is a relevant catalyst benefit since the oxidative activation of NO and the hydrocarbon can take place at lower temperatures and thus shift the overall HC-SCR activity to lower temperatures, as it is shown in the next paragraph. 3.2. Evaluation of Cu/ZSM-5 based materials in C3H6-SCR of NO The catalytic results of the C3H6-SCR experiments with the different Cu/ZSM-5 samples are shown in Fig. 2, as NO conversion (to N2) versus temperature plots. The effects of Cu loading, of CeO2 coating, and of space velocity, under dry reaction conditions, are presented in Fig. 2a. It is clear that increasing the Cu loading of the ion-exchanged samples from 0.5 wt.% (30% ion-exchange) to 1.5 wt.% (90% ionexchange) results in a dramatic increase of both maximum NO conversion and of the active temperature window, mainly from 0.5 to 1 wt.% Cu (70% ion-exchange). The maximum NO conversion at 500 8C and at a relatively high GHSV of 59,000 h1 was 50% for the 1.5% Cu/ZSM-5 sample and 40% for the 1% Cu/ZSM-5 sample. At the same space velocity, the CeO2 coated Cu/ZSM-5 sample with 8 wt.% CeO2 and 0.8 wt.% Cu (corresponding to  50% ionexchange) presented a maximum NO conversion (33%) slightly lower than that of the 1% Cu/ZSM-5 sample but with a narrower active temperature window which was slightly shifted towards lower temperatures. This shift was more clearly shown at the lower space velocity experiments (GHSV = 11,800 h1),

Fig. 2. NO conversion (%) to N2 vs. temperature over various Cu/ZSM-5 catalysts under HC-SCR conditions—(a) feed: 1000 ppm NO, 1000 ppm C3H6, 2% O2 in He, (b) feed: 1000 ppm NO, 1000 ppm C3H6, 2% O2, 10% H2O in He, (c) feed: 1000 ppm NO, 1000 ppm C3H6, 2% O2, 10% H2O, 50 ppm SO2 in He. Dashed lines refer to GHSV = 11,800 h1. Solid lines refer to GHSV = 59,000 h1.

as can be seen in Fig. 2a. The whole active temperature window for the CeO2 coated Cu/ZSM-5 sample was shifted by 100 8C to lower temperatures compared to that of the Cu/ZSM-5 sample. With regard to the effect of space velocity on maximum NO conversion, both the CeO2–Cu/ZSM-5 and Cu/ZSM-5 samples showed higher maximum conversion, 53% (at 350 8C)

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from 33% (at 500 8C) for the former sample and 75% (at 450 8C) from 40% (at 500 8C) for the latter sample. This behaviour has been demonstrated also previously with various CeO2 loadings (from 2 up to 24 wt.%) [20,21] and can be rationalized on the basis of the above described specific interactions between Cu and Ce, which result in enhanced reducibility for both metals and consequently to lowertemperature activation of both NO and the hydrocarbon. Addition of 10% water to the feed (plots in Fig. 2b) had a minor effect on the maximum NO conversion and on the active temperature window for both the CeO2 coated and the noncoated Cu/ZSM-5 samples, compared to their dry feed performance. What is of high importance in the wet feed experiments, is that the temperature difference of the active temperature windows between the two samples is retained, and the CeO2 coated sample is considerably more active compared to Cu/ZSM-5 at lower temperatures. At 350 8C and GHSV = 11,800 h1, the former sample shows NO conversion activity of 50% against 14% with the latter sample, while at the same temperature and GHSV = 59,000 h1 the respective values for the two samples are 23% and 5%. When SO2 (50 ppm) was added to the feed, in the presence of water (Fig. 2c), the maximum NO conversion with the Cu/ ZSM-5 sample remained almost unaffected and the active temperature window was similar to that in the case of the experiment with only water addition in the feed. This was evident for both space velocities tested. In the case of the CeO2 coated Cu/ZSM-5 sample the maximum conversion also remained the same (compared to that in the absence of SO2) but the whole active temperature window was shifted to higher temperatures by 75 8C at GHSV = 11,800 h1 and by 125 8C at GHSV = 59,000 h1. It is obvious that the beneficial effect of the CeO2 coating has been counterbalanced by SO2, possibly due to the formation of CeSO4 and the consequent loss of the redox properties of CeO2. However, what is interesting to note is that the rate of loss of NO conversion activity with the CeO2 coated sample at relatively high temperatures (500 8C) is much lower at high space velocity compared to that with the Cu/ZSM-5 sample, indicating a potential enhanced performance of the former sample at high-temperature deNOx applications, not necessarily in the presence of hydrocarbons. This behaviour could be attributed to the CeO2 coating, which acts as a SO2 barrier protecting the Cu-sites within the zeolite crystals from forming CuSO4. Furthermore, the shift of the active temperature window towards lower temperatures observed under dry and wet feeds at low space velocity with the CeO2 coated sample is retained and this sample is significantly more active than the pure Cu/ZSM-5 at relatively lower temperatures.

total catalyst inventory decreased the CO emissions by approximately one order of magnitude, but, as expected, increased the NO emissions. As already mentioned in the experimental section, mixtures of the spent FCC catalysts with 1 wt.% CO promoter were considered as the base case for the evaluation of our new additives’ performance. In some experiments the combination of the spent FCC catalyst with the CO promoter and the new additives together was also tested. The total amounts of CO and NO emitted during regeneration of the spent FCC catalyst in the presence of 1 wt.% of each new additive or 1 wt.% of the commercial CO promoter (base case) are presented in Fig. 3. The Cu/ZSM-5 additives are effective for the in situ reduction of NO that are produced during burning of the coke, but at the same time they increase the CO emissions (Fig. 3a). Increasing the Cu loading of the ion-exchanged additives from 0.5 wt.% to 1 wt.% enhanced the NO conversion from 31% to 55%. The Cu(1%)/ZSM-5 additive prepared by dry impregnation showed slightly improved deNOx behaviour. When the CeO2 coated Cu/ZSM-5 sample was evaluated as FCC catalytic additive, a 58% NO reduction was achieved in spite of the lower Cu loading (0.8%), compared to the 1% Cu/ZSM-5 sample. Severe steaming pre-treatment (at 788 8C, for 5 h, with 100% steam) of the CeO2–Cu/ZSM-5 additive, in order to simulate the equilibrium state of the FCC catalysts after prolonged operation, seems to inhibit its deNOx ability. Still, a NO reduction of 20% could be achieved with the steamed additive. All previous regeneration experiments were repeated by increasing the amount of additive from 1 to 5 wt.% (Fig. 3b). In all cases the deNOx performance was improved achieving up

3.3. Evaluation of Cu/ZSM-5 based materials as deNOx FCC catalytic additives Regeneration experiments at 700 8C of pure spent FCC catalyst or of mechanical mixtures of the spent FCC catalyst with 1 wt.% of a commercial CO promoter were initially performed. The presence of a conventional CO promoter in the

Fig. 3. NO and CO emissions during the regeneration of spent FCC catalyst after the addition of 1% CO promoter (base case) or (a) 1% additive, and (b) 5 % additive. CatA: Cu(0.5%)/ZSM-5, CatB: Cu(1%)/ZSM-5, CatC: Cu(1%impregn.)/ZSM-5, CatD: CeO2(8%)Cu(0.8%)/ZSM-5.

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Fig. 4. NO and CO emissions during the regeneration of spent FCC catalyst using 1% CO promoter (base case) or 1% additive. CatA: Rh/ CeO2(8%)Cu(0.8%)/ZSM-5, CatB: Rh/Cu(1%-impregn.)/ZSM-5, CatC: Rh/ Cu(1%)/ZSM-5.

to 78% NO reduction in the case of the CeO2 coated additive and 38% NO reduction in the case of the respective steamed additive. In addition, the CO emissions were not increased as much as when 1% additive was used and they were close to those of the base case. The performance of the Cu/ZSM-5 additives was also tested in the presence of the commercial CO promoter in regeneration experiments with mechanical mixtures of 98% spent FCC catalyst-1% additive–1% CO promoter (not shown). In all cases CO emissions were at similar levels with the base case. However, the presence of the CO promoter inhibited the deNOx performance of all Cu-based additives. This effect was particularly dominant in the case of the low Cu loading additive. However, the CeO2 coated Cu/ZSM-5 exhibited again the optimum performance retaining a 50% NO reduction in the presence of the CO promoter. Another way to enhance additives’ combined performance for NO reduction and CO oxidation was to add a small amount of rhodium (Rh) in their formulation and use them in the absence of a CO promoter. As shown in Fig. 4, Rh presence resulted in a simultaneous combined performance both for NO reduction and CO oxidation. Up to 54% NO reduction and significantly lower CO emissions than those achieved in the base case experiment were measured. Interestingly, the combination of Rh with Ce resulted not only in high NO reduction activity but also in high CO oxidation activity, since

Fig. 5. Evaluation studies of Rh/CeO2/CuZSM-5 catalyst in NO reduction by CO under FCC regeneration conditions (at 700 8C, in the presence of 1000 ppm NO, 1% CO, 0.3–0.7% O2, 10% H2O, 50 ppm SO2).

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CO emissions were decreased by 40% compared to the base case. The reaction mechanism that leads to lower NO emissions when deNOx additives are mixed in the FCC inventory is not clearly understood. A simple explanation is that they promote the direct reaction of NO by CO. A more recently suggested explanation is the effect of these additives on the Nintermediates that lead to NO formation [24,34]. Thus, our next step was to test a mixture of an equilibrium (regenerated) FCC catalyst with 1% of the optimum Rh(0.05%)/CeO2(8%)/ Cu(0.8%)ZSM-5 additive in order to exclude the role of coke and it is involvement in NOx formation. More specifically, the additive’s ability in NO reduction by CO under various O2 concentrations, using a NO, CO, O2, H2O, SO2 containing feed, was tested. As shown in Fig. 5, a 100% NO reduction was achieved with up to 0.5% O2 (stoichiometrically required for 100% CO oxidation) in the feed. CO oxidation was continuously enhanced with increasing O2 concentration. As soon as there was excess of O2 in the feed, CO oxidation prevailed leaving no CO available for NO reduction. It seems that the tested Rh-promoted CeO2–Cu/ZSM-5 formulation can be very effective for NO reduction by CO mainly in the O2 deficient zones of the regenerator. In addition, there is an optimum CO–O2 balance where NO can be fully removed by CO and the remaining CO can be fully oxidized to CO2 by the additive, at least under the experimental conditions described in Fig. 5. It is expected that further improvement of the catalyst formulation in combination with optimization of process conditions mainly related to the balance between available oxygen and CO in the regenerator, will broaden the perspectives of the utilization of metal-modified ZSM-5 based catalytic materials as multifunctional FCC additives that can effectively reduce both NOx and CO emissions from the flue gases of the regenerator. 4. Conclusions The C3H6-SCR deNOx activity of Cu-exchanged ZSM-5 samples was significantly enhanced by increasing Cu loading from 0.5 to 1.5 wt.%, when tested under dry reaction conditions. A CeO2(8%)-coated Cu(0.8%)/ZSM-5 sample decreased maximum NO conversion but shifted the whole active temperature window by almost 100oC to lower temperatures, especially at relatively low space velocities (GHSV = 11,800 h1). At this space velocity, the CeO2 coated Cu/ZSM-5 showed 53% maximum NO conversion at 350 8C, compared to 13% conversion with the non-coated Cu/ZSM-5 sample. This enhancement was attributed to specific Cu–Ce interactions, which were also evidenced by the enhanced reducibility of both metals by XANES–TPR experiments. Addition of water in the feed had almost no effect on the performance of all additives, retaining the difference in maximum NO conversion activity at relatively low temperatures between the CeO2 coated and the non-coated Cu/ZSM-5 sample. Upon addition of SO2 in the feed, the beneficial effect of the CeO2 coating at lower reaction temperatures was retained only at low space velocities. At high space velocities

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the presence of SO2 counterbalanced the effect of the CeO2 coating, which however served as an SO2 barrier at relatively high reaction temperatures (500 8C), thus preventing deactivation of the Cu active sites via formation of CuSO4. All Cu/ZSM-5 samples were also tested as FCC catalytic additives for the in situ reduction of NO emissions formed during regeneration of coked FCC catalysts. Utilization of conventional CO promoters in the FCC catalyst (addition level 1%) was considered as the base (reference) case for the evaluation of additives’ performance. All Cu/ZSM-5 additives reduced NO and their deNOx ability was enhanced by increasing either the Cu loading of the ion-exchanged additives or the amount of additive used during regeneration, achieving up to 78% NO reduction in the case of the CeO2 coated Cu/ ZSM-5 additive. CO emissions were increased by the use of the Cu/ZSM-5 additives, compared to the base case FCC catalyst with the commercial CO promoter. Combined use of the Cu/ ZSM-5 additives with the commercial CO promoter resulted in similar CO emissions with the base case but inhibited the deNOx performance of all Cu-based additives. Still, the CeO2 coated Cu/ZSM-5 additive exhibited significant performance retaining a 50% NO reduction in the presence of the CO promoter. Further modification of the CeO2–Cu/ZSM-5 sample with small amounts of Rh resulted in significant simultaneous reduction of NO and decrease of CO in the absence of the CO promoter. Based on a mechanistic study it was shown that the tested metal-promoted Cu/ZSM-5 based additives can be very effective for NO reduction by CO mainly in the O2 deficient zones of the regenerator, while further improvement on the basis of catalyst formulation and regenerator process conditions can lead to the development of multifunctional additives for the simultaneous removal of NOx and CO from the flue gases of the FCC regenerator.

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This work was supported by the Greek General Secretariat for Research and Technology through the Greece-USA Bilateral Cooperation Programme of ‘‘EPAN’’ (contract USA-032). The work in Argonne National Laboratory was performed under contract number W-31-109-ENG-38. Additional funding support was provided by the BP Corporation. Work performed at MR-CAT was supported, in part, by funding from the Department of Energy under grant number DEFG0200ER45811.

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