3 Comparison of Catalytic Reduction of NO by Propene on Zeolite-Based and Clay-Based Catalysts Ion-Exchanged by Cu JOSE L. VALVERDE, FERNANDO DORADO, PAULA SA ´ NCHEZ, ISAAC ASENCIO, and AMAYA ROMERO University of Castilla–La Mancha, Ciudad Real, Spain I. INTRODUCTION Selective catalytic reduction (SCR) of NO with hydrocarbons has been a subject of extensive study due to its potential for the effective control of NO emission in oxidant environments [1–11]. Hydrocarbons would be the preferred reducing agents over NH 3 because the practical problems associated with its use: handling and slippage through the reactor. Although many types of catalysts have been studied for this purpose, only a few copper-loaded zeolites have been demon- strated as adequate, and among them Cu-ZSM-5 gives good yields and seems to be one of the most active ones [3,5,12–15]. The majority of these catalysts are ion-exchanged zeolites, including H ϩ forms. Metal oxides supported on alumina have also been studied. Shimuzu et al. [16,17] recently reported that Cu-aluminate catalysts, containing highly dispersed Cu 2ϩ ions in the aluminate phase, showed high de-NOx activity comparable to Cu-ZSM-5 and higher hydrothermal stabil- ity. The activity of copper-loaded zeolites was found to depend on the Cu content. Iwamoto et al. [18] observed that the activity of Cu-ZSM-5 increased with the increment of the Cu exchange level. On the other hand, the concentration of O 2 in the NO-containing stream plays an important role in the reaction rate and product selectivity of the SCR reaction. It has been suggested that the roles of the O 2 in this reaction are: to activate NO and hydrocarbons [19], to oxidize NO to NO 2 [20], to maintain a Cu ϩ /Cu 2ϩ site balance [21], and to react with carbona- ceous deposits on Cu-ZSM-5 [20,22]. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 40 Valverde et al. The majority of Cu ions in copper-exchanged zeolites are found as single cations located in structural cationic sites. Besides these atomically dispersed single Cu ions in cationic sites, dispersed CuO species were also detected [23– 25]. In any case, the facile reduction from Cu 2ϩ to Cu ϩ suggested that a possible redox mechanism could be involved. Shpiro et al. [26] suggested that copper in overexchanged Cu-ZSM-5 zeolites exists as small clusters of Cu-O or as isolated ions whose oxidation state (Cu 2ϩ or Cu ϩ ) depends on the atmosphere present. Corma et al. [27/38] observed that the most active Cu-beta zeolites for SCR of NOx were those in which the conversion between Cu 2ϩ and Cu ϩ was quickly produced under reaction conditions. This conversion was easier in overexchanged Cu-beta samples. Pillared clays (PILCs) are two-dimensional materials prepared by exchanging charge-compensating cations between the clay layers with large inorganic metal hydroxycations that are oligomeric and are formed by hydrolysis of metal oxides or salts. After calcination, the metal hydroxycations are decomposed into oxide pillars that keep the clay layers apart and create interlayer and interpillar spaces, thereby exposing the internal surfaces of the clay layers. The size of these oligo- mers appears to control the size of the pore opening in the pillared clays. It is known that the basal spacing of the natural clay is 9.6 A ˚ and that the insertion of these hydroxycations increases it to 18–20 A ˚ . In principle, any metal oxide or salt forming polynuclear species upon hydrolysis can be inserted as a pillar. Intercalated clays are usually natural smectites clays. Properties such as acidity, surface area, pore size distribution, and both thermal and hydrothermal stability depend on the method of synthesis as well as on the nature of the host clay. Most common ions used as pillaring agents prepared by hydrolysis of the correspond- ing salts in solution are polycationic species of Al, Zr, Fe, Cr, etc. Can ˜ izares et al. [28] recently reported a comparative study in which different PILCs with single-oxide pillers of Fe, Cr, and Zr and mixed-oxide pillars of these metals and Al were prepared from two different bentonites. One of the fields of applications of pillared clays is catalysis. More specifi- cally, PILC-based catalysts were found to be useful for the SCR reaction by NH 3 and hydrocarbons [29–31]. Using hydrocarbons, Cu-ion exchanged PILC yielded higher than those of Cu-ZSM-5-based catalysts; and their activity was only slightly decreased by H 2 O and SO 2 [32,33]. In an earlier paper, Yang et al. [34] tested different ion-exchanged pillared clays as catalysts for selective catalytic reduction of NO by ethylene. Cu-Ti-PILCs showed the highest activities at tem- peratures below 643 K, whereas Cu-Al-PILC was the most active at temperatures above 673 K. Ti-PILC was obtained using a procedure in which the pillaring solution of partially hydrolyzed Ti-polycations was prepared by first adding TiCl 4 into a HCl solution. In spite of these relevant results, Ti-PILCs have received considerably less attention than other pillared clays. As a result, few preparation methods for these TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. SCR of NO on Zeolite vs. Clay Catalysts 41 materials have been reported [34,35]. In our case, Ti metoxide was used as the source of Ti in the preparation of Ti-PILCs. The aims of this work are: 1. To compare the catalytic reduction of NO by propene on ZSM-5 based and Ti-PILC-based catalysts ion-exchanged by Cu. 2. To characterize all the catalysts here prepared in order to relate physical and chemical properties to the catalytic behavior of the two sets of catalysts. II. EXPERIMENTAL A. Preparation of Catalysts NaZSM-5 zeolite (Si/Al ratio of 20) was synthesized according to the method described elsewhere [36] using ethanol as the template. X-ray diffraction (XRD) confirmed that the product was 100% crystalline [37]. Cu was introduced by conventional ion exchange, using 25 mL of 0.1 M Cu(CH 3 COO) 2 ⋅4H 2 O aqueous solutions per gram of zeolite. The mixture was kept under agitation at the desired ion exchange temperature (303, 328, or 353K) for 14 h. Next, the suspension was filtered and thoroughly washed with deionized water in order to completely remove the occluded salt, and the solid was then air-dried at 393 K for 14 h. The whole procedure was repeated twice for some catalysts. Finally, the samples were calcined in air at 823 K for 4 h. Table 1 summarizes the zeolite-based catalysts here prepared. These catalysts were referred to as a function of the copper load- ing. For instance, CuZ-2.9 corresponds to a Cu-ZSM5 with a copper content of 2.9% by weight. Ti-PILC was prepared as follows. The starting clay was a purified montmoril- lonite (purified-grade bentonite power from Fisher Company), which has a parti- clesizeof2µm or less, a cation exchange capacity (CEC) of 97 eq/kg dry clay and a surface area of 44.7 m 2 /g. The pillaring solution of partially hydrolyzed Ti-polycations was prepared by first adding titanium metoxide to a 5 M HCl solution. The solution was aged for 3 h at room temperature. Then 1 gram of starting clay was dispersed in 1 L of deionized water for 3 h under stirring. The pillaring solution was slowly added with vigorous stirring into the clay suspen- sion until the amount of pillaring solution reached that required to obtain a Ti/ Clay ratio of 15 mM of Ti/g clay. The intercalation step took about 16 h. Subse- quently, the mixture was separated by vacuum filtration or centrifugation and washed with deionized water until the liquid phase was chloride free. The sample was dried at 393 K for 12 h and calcined at 773 K for 2 h. The basal spacing of the resulting sample, measured by XRD, was 23 A ˚ . One gram of the Ti-pillared bentonite was added to 200 mL of 0.05 M copper acetate solution. The mixture was stirred for 6 h at room temperature. The ion TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 42 Valverde et al. TABLE 1 Composition and Characterization of Zeolite-Based Catalysts a Ion exchange steps Cu Weak acid site Strong acid site and temperature content Ion exchange density density Surface area Micropore area Micropore Catalyst (K) (wt %) level (%) (mmol NH 3 /g) (mmol NH 3 /g) (m 2 /g) (m 2 /g) volume (m 3 /g) NaZSM-5 — 0 0 0.950 (579 K) Not detected 369.3 (100%) 360.0 (100%) 0.158 (100%) CuZ-2.4 1—303 2.4 94 0.881 (565 K) Not detected 345.4 (94%) 338.8 (94%) 0.140 (89%) CuZ-2.6 1—328 2.6 103 0.822 (573 K) Not detected — — — CuZ-2.9 2—303 2.9 116 0.907 (555 K) Not detected 326.4 (88%) 309.6 (86%) 0.130 (82%) CuZ-3.7 2—328 3.7 148 0.781 (573 K) Not detected — — — CuZ-4.4 1—353 4.4 175 0.477 (581 K) 0.440 (911 K) 321.0 (83%) 298.7 (83%) 0.126 (79%) a Temperatures corresponding to the maximum of the desorption peak are included in parentheses together with the acid sites density value. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. SCR of NO on Zeolite vs. Clay Catalysts 43 exchange product was collected by filtration or centrifugation, followed by wash- ing five times with deionized water. The obtained solid sample was first dried at 393 K in air for 12 h and then calcined at 773 K for 2 h. After this pretreatment, the sample was ready for further experiments. Table 2 summarizes the PILC- based catalysts here prepared. Pillared clay-based catalysts were referred to as a function of the copper loading. For instance, CuTi-7.4 corresponds to a Cu-Ti- PILC with a copper content of 7.4% by weight. B. Characterization Methods X-ray diffraction (XRD) patterns were measured with a Philips model PW 1710 diffractometer using Ni-filtered CuKα radiation. To summarize the (001) reflec- tion intensity in PILC samples, oriented clay-aggregate specimens were prepared by drying clay suspensions on a glass slide. Surface area and pore size distributions were determined by using nitrogen as the sorbate at 77 K in a static volumetric apparatus by using a micromeritics ASAP 2010 sorptometer. For this analysis, samples were outgassed at 453 K for 16 h under a vacuum of 5 ϫ 10 Ϫ6 torr. Specific total surface areas were calculated by using the Brunauer, Emmett, and Teller (BET) equation. The Horvath– Kawazoe method was used to determine microporous surface area and volume. Total acid site density of the samples was measured by a temperature pro- grammed desorption (TPD) of ammonia, by using a Micromeritics TPD-TPR analyzer. Samples were housed in a quartz tubular reactor and pretreated in flow- ing helium (99.999%) while heating at 15 K min Ϫ1 up to 773 K. After 0.5 H at 773 K, the samples were cooled to 453 K and saturated for 0.25 h in an ammonia (99.999%) stream. The sample was then allowed to equilibrate in a helium flow at 453 K for 1 h. Finally, ammonia was desorbed using a linear heating rate of 15 K min Ϫ1 . Temperature and detector signals were simultaneously recorded. The average relative error in the acidity determination was lower than 3%. Temperature programmed reduction (TPR) measurements were carried out with the same apparatus previously described. After loading, the sample was outgassed by heating at 20 K min Ϫ1 in an argon flow to 773 K. This temperature was constant for 30 min. Next, it was cooled to 298 K and stabilized under an argon/hydrogen (99.999%, 83/17 volumetric ratio) flow. The temperature and detector signals were continuously recorded while heating at 20 K min Ϫ1 . A cool- ing trap placed between the sample and the detector retained the liquids formed during the reduction process. The TPR profiles were reproducible with an average relative error in the determination of the reduction maximum temperatures lower than 2%. The metallic content (wt%) was determined by atomic absorption measure- ments by using a SpectrAA 220 FS analyzer. In all cases, calibrations from the corresponding patron solutions were performed. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 44 Valverde et al. TABLE 2 Composition and Characterization of PILC-Based Catalysts a Weak acid site Cu content Ion exchange level density Strong acid site Surface area Micropore area Micropore Catalyst (wt%) (%) (mmol NH 3 /g) density (mmol NH 3 /g) (m 2 /g) (m 2 /g) volume (m 3 /g) Ti-PILC 0 0 0.437 (579 K) 0.092 (691 K) 273.2 (100%) 224.5 (100%) 0.181 (100%) CuTi-4.6 4.6 149 0.108 (548 K) 0.362 (628 K) 241.7 (88%) 202.2 (90%) 0.153 (85%) CuTi-7.4 7.4 240 0.136 (543 K) 0.594 (623 K) 234.3 (86%) 189.1 (84%) 0.143 (79%) CuTi-9.0 9.0 292 0.156 (533 K) 0.738 (613 K) 201.8 (74%) 153.7 (68%) 0.126 (69%) a Temperatures corresponding to the maximum of the desorption peak are included in parentheses together with the acid sites density value. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. SCR of NO on Zeolite vs. Clay Catalysts 45 C. Reaction Studies The catalytic tests were carried out in a fixed-bed flow reactor. The standard reactant mixture was one constituted by NO (1000 ppm), C 3 H 6 (1000 ppm), O 2 (5%), and balance He at ambient pressure. The flow rates were controlled by calibrated Brooks flowmeters. The total flow rate was 125 mL/min. The space velocity of the feed was 15,000 h Ϫ1 (GHSV). The effluent stream was analyzed by a chemiluminescent NO/NOx analyzer (ECO PHYSICS NO-NO 2 -NOx ana- lyzer). III. RESULTS A. Characterization of the Catalysts Tables 1 and 2 list, for all the catalysts, the specific surface area, micropore area, and micropore volume, the weak and strong acid site density, and the copper content. The same tables also summarize the Cu ion exchange levels that were determined taking as a reference, in the case of zeolite-based catalysts, the num- ber of aluminum atoms contained in the structure and, in the case of PILC-based catalysts, the cation exchange capacity (CEC) of the clay [34]. It can be observed that, except for the CuZ-2.4 sample, all the catalysts presented more Cu content than that corresponding to 100% ion exchange. In fact, all the zeolite-based cata- lysts contained sodium ions. As expected, the loading of these decreased with increasing copper content in zeolite (0.16 wt% of Na for the CuZ-2.4 sample and 0.07 wt% of Na for the CuZ-4.4 sample. It is also observed for these catalysts that for the same number of ion exchange steps the copper loading increased with increasing ion exchange temperatures and the acid site density progressively diminished from the value corresponding to the parent Na/ZSM5 (0.95 mmol NH 3 /g) to 0.822 and 0.907 mmol NH 3 /g in the ZCu-2.6 and ZCu3.7 samples, respectively. In this case, it is clear that acidity is a combination of two effects: the presence of Cu and Na on the catalyst. The first Cu ions incorporated to zeolites would occupy hidden sites (small zeolite cages) [38]. These sites may not be very accessible to NH 3 molecules. Since multiple ion exchange steps are needed to achieve the complete filling of small cages, the increase of copper content would lead Cu species to incorporate at more accessible positions. As a consequence the acidity increases. Strong acid site density was observed only for zeolite-based catalysts with copper content higher than 4.4% by weight. In contrast, all the PILC-based catalysts presented strong acid sites. An increase of Cu content was accompanied in both sets of catalysts by a decrease of BET surface area and micropore volume, indicating that Cu intro- duced into the pillared matrix would preferentially occupy the interlayer area [39], whereas the Cu present in zeolite would cause the partial blocking of the TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 46 Valverde et al. zeolite channels. Temperature-programmed reduction can be used to identify and quantify the copper species in ion-exchanged samples. According to Delahay et al. [40] the reactions involved in the copper reduction process are: CuO ϩ H 2 → Cu 0 ϩ H 2 O Cu 2ϩ ϩ 1 2 H 2 → Cu ϩ ϩ H ϩ Cu ϩ ϩ 1 2 H 2 → Cu 0 ϩ H ϩ Some authors showed that, depending on the copper content, the reduction of Cu 2ϩ to Cu ϩ would occur at lower temperatures while the reduction of Cu ϩ to Cu 0 would occur at higher temperatures [41–43]. When the copper content in the sample is higher, the excess copper may be found as oxygenated clusters more easily reduced than the isolated copper species [24,34]. As shown in Table 3, low H 2 consumption was found in all the samples, indi- cating that Cu species are hard to reduce to lower valence. In the case of zeolite- based catalysts, this fact is justified considering the existence of metal ions in small zeolite cages [38]. In these small cages, Cu 2ϩ ions would be favorably coordinated to the framework oxygens. This bonding is generally much stronger for multivalent than for monovalents ions. Each ion would not be readily accessi- ble for H 2 molecules, and the activation energy for reducing such isolated metal ions would be rather high. These results are in good agreement with the measure- ments of acidity mentioned earlier. In PILC-based catalysts, the H 2 /Cu is slightly TABLE 3 Ratios of H 2 Consumption to Cu (H 2 /Cu, mol/mol, Measured by TPR Experiments) of Zeolite-and PILC-Based Catalysts a H 2 /Cu (mol/mol) H 2 /Cu (mol/mol) H 2 /Cu (mol/mol) H 2 /Cu (mol/mol) Catalyst CuO to Cu 0 Cu 2ϩ to Cu ϩ Cu ϩ to Cu 0 Total NaZSM-5 — — — — CuZ-2.4 Not detected 0.169 (517 K) 0.322 (643 K) 0.491 CuZ-2.6 Not detected 0.181 (503 K) 0.271 (676 K) 0.452 CuZ-2.9 Not detected 0.249 (503 K) 0.204 (693 K) 0.453 CuZ-3.7 Not detected 0.177 (450 K) 0.260 (653 K) 0.437 CuZ-4.4 0.251 (450 K) 0.134 (461 K) 0.216 (663 K) 0.601 Ti-PILC — — — — CuTi-4.6 0.405 (473 K) 0.172 (543 K) 0.046 (663 K) 0.623 CuTi-7.4 0.321 (431 K) 0.170 (507 K) 0.051 (685 K) 0.542 CuTi-9.0 0.395 (426 K) 0.107 (510 K) 0.036 (693 K) 0.538 a Temperatures corresponding to the maximum of the reduction peak are included in parentheses. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. SCR of NO on Zeolite vs. Clay Catalysts 47 higher than that of the zeolite, also showing that in these samples an important part of Cu species is not accessible for H 2 molecules. For the zeolite-based catalysts with a copper content lower than 4.4 wt%, the TPR profiles showed two reduction peaks (Table 3) which suggests a two-step reduction process of isolated Cu 2ϩ species. The peak at a lower temperature would indicate that the process of Cu 2ϩ to Cu ϩ occurred. The other peak at a higher temperature suggests that the produced Cu ϩ was further reduced to Cu 0 . For the sample CuZ-4.4, a peak existing at 450 K would be related to the presence of CuO aggregates. Due to the absence of a diffraction line of CuO species in XRD patterns of this sample, the occurrence of CuO aggregates larger than 3 mm can be ruled out [40]. According to the measurements of acidity, the presence of CuO aggregates would be related to the occurrence of strong acid sites. On the other hand, there is a clear shift to lower temperatures of the second peak (Cu 2ϩ to Cu ϩ ) that showed that Cu ϩ becomes more difficult to reduce as Cu content decreases. In the case of PILC-based catalysts, the peaks corresponding to the three reac- tions involved in the copper reduction are present. Again, there is a clear shift to a lower temperature of the second peak (Cu 2ϩ to Cu ϩ ) and the first (CuO to Cu 0 ) that showed that Cu 2ϩ and CuO get easier to reduce as Cu content increases. This fact would indicate that the higher the Cu content is, the lower CuO species dispersion that is observed. In a similar way as observed in zeolite-based cata- lysts, no diffraction of CuO species in XRD were detected. Figure 1 compares the TPR-profiles of the CuZ-2.9 and CuTi-7.4 samples. Figure 2 shows, for all the catalysts, the H 2 consumption for the Cu 2ϩ to Cu ϩ process as a function of the Cu loading. With an increase in Cu content, H 2 consumption increased, passing through a maximum, and then decreased at higher loadings. It can be verified that the maximum in the case of zeolite-based catalysts corresponds to the CuZ-2.9 sample and in the case of PILC-based cata- lysts to the CuTi-7.4 sample. B. NOx Reduction Activity The catalytic performance of the catalysts for the SCR reaction of NOx with propene as a function of the reaction temperature is summarized in Table 4. The presence of copper in the catalysts enhanced the catalytic activity. With an in- crease in reaction temperature, NOx conversion increased, passing through a maximum, and then decreased at higher temperatures. According to Yang et al. [34], the decrease in NOx conversion at higher temperatures was due to the com- bustion of propene. In general, all the Cu-zeolites samples presented the maxi- mum NOx conversion at the same temperature (623 K). Similar observations can be derived for all PILC-based catalysts, but in this case the corresponding maxi- mum appeared at 523 K. It can be observed that increasing the copper loading increased NOx conversion until the copper loading reached 116% ion exchange TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. 48 Valverde et al. FIG. 1 TPR profiles of CuTi-7.4 and CuZ-2.9. FIG. 2 Hydrogen consumption for the Cu 2ϩ to Cu ϩ reduction processes as a function of Cu loading. TM Copyright n 2003 by Marcel Dekker, Inc. All Rights Reserved. [...]... Performance of Zeolite- and PILC-Based Catalysts Ion Exchanged by Cu Catalyst Maximum NOx conversion Temperature for maximum NOx conversion (K) CuZ-2.4 CuZ-2.6 CuZ-2.9 CuZ -3 . 7 CuZ-4.4 CuTi-4.6 CuTi-7.4 CuTi-9.0 56.9 62.4 65.7 65.6 56.4 26.4 59 .3 36.7 6 23 6 23 6 23 6 23 6 23 5 23 5 23 5 23 Reaction conditions: [NO] ϭ [C3 H6 ] ϭ 1000 ppm, [O2 ] ϭ 5%, [He] ϭ balance; GHSV ϭ 15,000 hϪ1 FIG 3 Conversion of NOx for... 151: 135 –146, 1995 31 L.S Cheng, R.T Yang N Chen, J Catal 164:70–81, 1996 32 R.T Yang, W.B Li J Catal 155:414–417, 1995 33 W.B Li, M Sirihungren, R.T Yang App Catal 11 :34 7 36 3, 1997 34 R.T Yang, N Tharappiwattananon, R.Q Lay App Catal 19:289 30 4, 1998 35 H.L del Castillo, A Gil, P Grange Catal Lett 43: 133 – 137 , 1997 TM Copyright n 20 03 by Marcel Dekker, Inc All Rights Reserved 54 Valverde et al 36 M.A... suggested that in catalytic reactions proceeding according to the redox mechanism, the inplane bridging oxygens played an important role in electron transfer from the electron-accepting to the electron-donating sites at the catalytic surface According to Bahranowski et al [48], single doping of Al-PILC with copper introduced approximately 40% more than the CEC of the Al-PILC samples, suggesting an adsorption... According to Bahranowski et al [47], the most characteristic feature was the increase in covalency of the Cu-O in- plane σ-bonding due TM Copyright n 20 03 by Marcel Dekker, Inc All Rights Reserved SCR of NO on Zeolite vs Clay Catalysts 51 to the dehydration/dehydroxilation processes accompanying high-temperature treatment, resulting in the attachment of copper centers to the lattice oxygens, forming links... undefined CuO species in Cu-MFI-based catalysts resulted in a substantial increase in propane conversion as well selectivity to CO2 This fact would explain the decided decrease in NO conversion with increasing copper content in the CuTi-7.4 sample observed at reaction temperatures higher than that corresponding to the maximum of conversion This decided decrease was not observed in the CuZ-2.9 sample, with... same temperature (6 23 K) Similar observations for all PILCbased catalysts can be derived, but in this case the corresponding maximum appeared at 5 23 K It was observed that increasing the copper loading increased NOx conversion until the copper loading reached 116% ion exchange in the zeolite-based catalysts, the CuZ-2.9 sample, and 240% ion exchange in PILC-based catalysts, the CuTi-7.4 sample It seems... 8 Y.-H Yin, A Pisau, L Serventi, W.E Alvarez, D.E Resasco Catal Today 54: 419–429, 1999 9 D.K Captain, M.D Amiridis J Catal 184 :37 7 38 9, 1999 10 S Xie, J.P Rosynek, J.H Lunsford J Catal 188:24 31 , 1999 11 F.C Meunier, J.P Breen, V Zuzaniuk, M Olsson, J.R.H Ross J Catal 187:4 93 404, 1999 12 J Valyon, W.K Hall Stud Surf Sci Cat 75: 133 9– 135 0, 19 93 13 M Iwamoto, N Mizuno J Auto Eng 207: 23 37 , 19 93 14... According to Yang et al [34 ], Cu 2ϩ on the pillared clay is more active than that on ZSM-5 due to the fact that the redox cycle occurs more easily on the pillared catalysts On the other hand, it is also observed that the increase in Cu loading in the PILC-based catalysts resulted in an increase in CuO aggregates (Table 3) In a study of the reducibility and catalytic activity in propane oxidation [ 43] ,... showed the highest H 2 consumption for the Cu 2ϩ to Cuϩ process, taking as a reference the sets of zeolite-based and PILC-based catalysts, respectively Further increase in the copper loading resulted in a decrease in NOx conversion IV DISCUSSION In this work the parent ZSM-5 was in the sodium form According to TorreAbreu et al [15], Cu-MOR catalysts prepared from the sodium form exhibited much higher... Conversion of NOx for the SCR reaction on CuTi-7.4 and CuZ-2.9 Reaction conditions: [NO] ϭ [C3 H6 ] ϭ 1000 ppm, [O2 ] ϭ 5%, [He] ϭ balance, GHSV ϭ 15,000 hϪ1 TM Copyright n 20 03 by Marcel Dekker, Inc All Rights Reserved 50 Valverde et al in zeolite-based catalysts, CuZ-2.9 sample, and 240% ion exchange in PILCbased catalysts, CuTi-7.4 sample (Fig 3) Both CuZ-2.9 and CuTi-7.4 showed the highest H 2 consumption . conversion (K) CuZ-2.4 56.9 6 23 CuZ-2.6 62.4 6 23 CuZ-2.9 65.7 6 23 CuZ -3 . 7 65.6 6 23 CuZ-4.4 56.4 6 23 CuTi-4.6 26.4 5 23 CuTi-7.4 59 .3 5 23 CuTi-9.0 36 .7 5 23 Reaction conditions: [NO] ϭ [C 3 H 6 ] ϭ 1000. (94%) 33 8.8 (94%) 0.140 (89%) CuZ-2.6 1 32 8 2.6 1 03 0.822 (5 73 K) Not detected — — — CuZ-2.9 2 30 3 2.9 116 0.907 (555 K) Not detected 32 6.4 (88%) 30 9.6 (86%) 0. 130 (82%) CuZ -3 . 7 2 32 8 3. 7 148. (450 K) 0.260 (6 53 K) 0. 437 CuZ-4.4 0.251 (450 K) 0. 134 (461 K) 0.216 (6 63 K) 0.601 Ti-PILC — — — — CuTi-4.6 0.405 (4 73 K) 0.172 (5 43 K) 0.046 (6 63 K) 0.6 23 CuTi-7.4 0 .32 1 ( 431 K) 0.170 (507