Preparation of Cu-ion-exchanged Fe-PILCs for the SCR of NO by propene

Applied Catalysis B: Environmental 65 (2006) 175–184 www.elsevier.com/locate/apcatb

Preparation of Cu-ion-exchanged Fe-PILCs for the SCR of NO by propene Fernando Dorado b,*, Antonio de Lucas b, Prado B. Garcı´a b, Jose´ L. Valverde b, Amaya Romero a b

a Facultad de Ciencias Quı´micas/Escuela Te´cnica Agrı´cola, Spain Departamento de Ingenierı´a Quı´mica, Universidad Castilla-La Mancha, Campus Universitario s/n, 13004 Ciudad Real, Spain

Received 14 November 2005; received in revised form 18 January 2006; accepted 30 January 2006 Available online 15 March 2006

Abstract Cu-ion-exchanged iron-pillared interlayer clays (Fe-PILCs) were prepared under different pH conditions to analyze the influence on the distribution of the copper species over their structure, and on the catalytic performance for the selective catalytic reduction (SCR) of NOx by propene. It was observed that for those samples prepared without pH control, the copper was as isolated Cu2+ ions. When the samples were prepared under acid pH, the catalytic activity decreased and an appreciable CO production was observed, likely due to the low amount of Cu2+ cations in those catalysts. Finally, for the samples prepared under alkaline pH, the copper was as Cu2+ ions and CuO clusters. Their catalytic tests showed the best results for the SCR of NOx. The presence of CuO species led to an improvement in NOx yield to N2. With the catalytic tests and a study by in situ FTIR of SCR of NO, a reaction mechanism has been proposed, where the reaction intermediates are mainly acetates, organic nitro compounds and nitrous oxide species. # 2006 Elsevier B.V. All rights reserved. Keywords: Cu; NO-SCR; Iron-pillared clays; Infrared spectroscopy; TPD

1. Introduction Much research related to the selective catalytic reduction of NOx by hydrocarbons has been undertaken and reported in the literature due to its potential for the effective control of NO emissions in oxidative environment [1–11]. This reaction has been described as a method to remove NOx from natural-gasfuelled engines, such as lean-burn gas engines in cogeneration systems [12] and lean-burn gasoline and diesel engines, where the noble-metal three-way catalysts are not effective in the presence of excess oxygen [13]. Thus, hydrocarbons would be the preferred reducing agents over NH3 because of the practical problems associated to the ammonia use: handling and leakage through the reactor. Traa et al. [14] pointed out that the decomposition of nitrogen oxides into oxygen and nitrogen is thermodynamically favoured at temperatures below 900 8C, but no suitable catalyst for a practical application has been found yet. Only the Cu-ZSM-5

* Corresponding author. Tel.: +34 926295300; fax: +34 926295318. E-mail address: [email protected] (F. Dorado). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.01.008

system studied by Iwamoto et al. [15] and Held et al. [16] had presented high activity under this temperature. However, tests under more realistic conditions, i.e. with sufficiently low NO concentrations representative of real exhaust gas conditions, with 5 vol.% O2 in the reactant stream and with adequately high space velocities showed poor NO conversion. Reports on a large number of catalysts for the selective catalytic reduction (SCR) have appeared since 1990, being the majority of these catalysts ionexchanged zeolites [13,17]. Copper oxides supported on alumina have also been studied, e.g. CuO/Al2O3 [18]. Pillared interlayer clays (PILCs) represent a class of microporous solids with a wide range of potential applications in catalytic, adsorption and separation processes [19]. These materials present better hydrothermal stability than ZSM-5 in the SCR of NO by hydrocarbons. The preparation of PILCs involves the introduction of bulky inorganic or organic clusters into the interlayer region of the clay. Upon heating, the intercalated species are converted into the corresponding metal oxide clusters which are rigid enough not only to prevent the interlayer spaces from collapsing but also to generate micropores larger than those of conventional zeolites [20].

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Valverde et al. [21] recently reported a comparative study of SCR of NO by propene in presence of excess of oxygen on CuZSM-5 and Cu-Ti-PILC. In the presence of C3H6, the Cu2+ should be reduced to Cu+, then the Cu+ would be oxidized back to Cu2+ by NOx, thus completing the catalytic cycle. The results showed that the Cu-zeolite sample presented the maximum NOx conversion at 350 8C, whereas PILC-based catalysts did at 250 8C. As in the work of Yang and Li [22], Cu2+ ions on the pillared clay appear to be more catalytically active than on ZSM-5, probably due to the fact that the redox cycle is easier to occur on the pillared catalysts. The reaction mechanism is still the objective of many researches [5,23–26]. It is often claimed that the initial step for the selective reduction of NO is the formation of NO2 through the oxidation of NO [15]. Based on in situ IR results and the product analysis, a reaction pathway for NO decomposition on Cu2+-exchanged Al-PILC was proposed. The intermediates were assigned to be nitro, nitrous oxide and nitrate. In this mechanism, N2 can be generated from nitrate or nitrous oxide decomposition [24]. On the other hand, in situ IR studies of SCR of NO by propene over Cu2+-Ti-PILC, showed that reaction intermediates were mainly NO3
species, C3H7-NO2 and acetate. Cu2+OH groups reacted with the nitro group, thus forming nitrate [27]. The decomposition of nitrate species generated N2 and a small amount of N2O. The adsorption on the catalyst active sites was higher and stronger for C3H6 than for NO, and allowed the formation of hydrocarbon intermediates (C3H7-NO2 and acetate), which were responsible for the NO reduction. Despite of the vast amount of papers concerning pillared clays with pillars of Ti, Al or Zr, up to date a limited number of papers are devoted to the Fe-PILCs synthesis, their uses and catalytic applications. Due to the potential applications of iron-pillared clays for HC-SCR, attention has been focused recently on the performance and reaction mechanism over these catalysts

[28–30]. The present work was therefore devoted to study the behaviour of Cu2+ ion-exchanged Fe-PILC in the SCR of NOx by C3H6, with the aim of identifying some aspects of the reaction mechanism and, the nature and distribution of the copper species that are present. To reach these goals, Cu-FePILC catalysts containing either isolated Cun+ ions or CuO aggregates were prepared by ion-exchange using different pH values for the copper solution. 2. Experimental 2.1. Catalysts preparation The starting clay was a purified-grade bentonite (Fisher Company), with a particle size of <2 mm and a cationexchange capacity of 97 mequiv g
1 dry clay. Fe-PILCs were prepared as follows [31]. A FeCl36H2O solution was added to NaOH solutions to obtain the required OH/Fe molar ratio. In order to avoid precipitation of iron species, the pH was kept constant at 1.7. The mixture was aged for 4 h under stirring at room temperature. The pillaring solution was then added dropwise to an aqueous clay suspension. The mixture was kept under vigorous stirring for 12 h at room temperature. Finally, the solid was washed, dried and calcined for 2 h at 400 8C. Metal was introduced by conventional ion-exchange using 100 mL of metal aqueous solution per gram of iron-pillared clay. A range of copper-exchanged Fe-PILC samples were prepared with solutions of Cu(CH3COO)2H2O from 0.005 to 1.0 M, and the pH then adjusted by the addition of either aqueous ammonia or HCl to give the desired final pH. All catalysts were calcined for 2 h at 400 8C. Table 1 summarizes the catalysts prepared in this work. These catalysts are referred to as a function of the metal content and the pH of the ionexchanged solution. For instance, Cu3.7-5.6 corresponds to a catalyst ion-exchanged with copper, leading to a loading of this metal of 3.7 wt.% using a solution with a pH equal to 5.6.

Table 1 Composition and characteristics of the catalysts studied Cu solution (M)

SBET (m2/g) a

Sint (m2/g) a

Vp (cm3/g) b

Vup (cm3/g) b

Total acidity (mmol NH3/g)

–

–

285

247(86)

0.187

0.126

0.317

3.7 3.3 4.4 5.8 2.1

5.6 5.4 5.4 5.4 5.2

0.05 0.1 0.1 0.1 0.2

248 245 240 230 244

190 211 206 193 205

(76) (86) (85) (84) (84)

0.181 0.181 0.176 0.173 0.183

0.122 0.123 0.123 0.122 0.123

0.519 0.506 0.663 0.727 0.458

Cu0.48-1.0 Cu0.81-1.0 Cu0.83-1.0

0.48 0.81 0.83

1.0 1.0 1.0

0.1 0.4 1.0

188 123 137

133 (70) 57 (46) 63 (46)

0.184 0.151 0.168

0.097 0.047 0.053

0.228 0.201 0.211

Cu4.8-7.0 Cu6.2-9.0 Cu6.5-10.5

4.8 6.2 6.5

7.0 9.0 10.5

0.01 0.01 0.005

230 214 210

177 (77) 158 (74) 147 (70)

0.183 0.177 0.177

0.105 0.100 0.097

0.518 0.495 0.404

Set of experiments

Sample

Cu (wt.%)

–

Fe-PILC

–

1

Cu3.7-5.6 Cu3.3-5.4 Cu4.4-5.4c Cu5.8-5.4d Cu2.1-5.2

2

3

a b c d

pH

Total surface area obtained from the BET equation (SBET). Micropore area obtained from the t-plot method (Sint). Percentage of the total surface area in brackets. Micropore volume obtained from the t-plot method (Vup) and total pore volume at P/P0 = 0.99 (VP). Sample prepared with two ion-exchanged steps. Sample prepared with three ion-exchanged steps.

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2.2. Catalyst characterization X-ray diffraction (XRD) patterns were obtained with a Philips model PW 1710 diffractometer with nickel-filtered Cu Ka radiation. The XRD pattern for the parent clay calcined at 400 8C exhibited a main peak at 2u about 98 which it is commonly assigned to the basal (0 0 1) reflection (d(0 0 1)). Peak corresponding to the (0 0 1) reflection in Fe-PILC calcined at 400 8C appeared at smaller 2u angles (2u  48). This fact clearly indicates an enlargement of the basal spacing of the clay as consequence of the pillaring process, as explained in a previous work [31]. To quantify the total amount of metals incorporated into the catalyst, atomic absorption spectroscopy measurements were made, using a SPECTRAA model 220FS analyzer, with an error of 1%. The samples were previously dissolved in hydrofluoric acid and diluted to the interval of measurement. Surface area and pore-size distribution were determined by nitrogen adsorption at 77 K in a static volumetric apparatus (Micromeritics ASAP 2010 sorptometer). Pillared clays were outgassed prior to use at 180 8C for 16 h under a vacuum of 6.6  10
9 bar. Specific total surface areas were calculated using the Brunauer–Emmett–Teller (BET) equation, whereas specific total pore volumes were evaluated from the nitrogen uptake at a N2 relative pressure of P/P0 = 0.99. The t-plot method was used to determine the mesopore surface area and micropore volume. The Barret–Johner–Halenda (BJH) method was used to determine the mesopore size distribution [32]. The total acid-site density and acid-strength distribution of each of the catalysts were measured by temperatureprogrammed desorption of ammonia (TPDA), using a Micromeritics model TPD/TPR 2900 analyzer with a thermal conductivity detector (TCD). The samples were housed in a quartz tubular reactor and pretreated in a flow of helium while being heated at 15 8C/min to the calcination temperature of the sample. After a period of 30 min at this temperature, the samples were cooled to 180 8C and saturated for 15 min in a stream of ammonia. The catalyst was then allowed to equilibrate in a helium flow at 180 8C for 1 h. The ammonia then was desorbed using a linear heating rate of 15 8C/min to 600 8C. Temperature and detector signals were simultaneously recorded. Total acidity is defined as the total acid-site density, which is obtained by integration of the area under the curve. The average relative error in the acidity determination was <3%. Temperature-programmed reduction (TPR) measurements were carried out with the same apparatus as described above. After being loaded in the instrument, the sample was outgassed by being heated at 15 8C/min in an argon flow to the calcination temperature of the sample and kept constant at this temperature for 30 min. Next, the sample was cooled to room temperature and stabilized under an argon/hydrogen flow (99.9990% purity, 83/17 volumetric ratio). The temperature and thermal conductivity detector signals were then continuously recorded during heating at 15 8C/min to 450 8C. The liquids formed during the reduction process were retained by a cooling trap placed between the sample and the detector. TPR profiles were

177

reproducible, with standard deviations for the temperature of the peak maxima being 1%. The temperature-programmed desorption of NO or C3H6 or O2 was performed using the apparatus described above. The samples were housed in a quartz tubular reactor and pretreated in a flow of helium while being heated at 10 8C/min to the calcination temperature of the sample. After a period of 30 min at this temperature, the samples were cooled to 50 8C and saturated for about 30 min in a 4% NO/He or 4% C3H6/He or 99.999% O2 stream. The catalyst was then allowed to equilibrate in helium at 50 8C for 1 h. The NO or C3H6 or O2 was desorbed using a linear heating rate of 10 8C/min to 400 8C. Temperature and detector signals were simultaneously recorded. 2.3. Catalyst activity measurements Activity experiments were carried out at atmospheric pressure in a flow-type apparatus designed for continuous operation at atmospheric pressure. This apparatus consisted of a gas feed system for each component, with individual control by mass flowmeters, a fixed-bed downflow reactor, and an exit gas flowmeter. The reactor, a stainless steel tube with an internal diameter of 4 mm, was filled with the catalyst sample (0.25 g). A temperature programmer was used with a K-type thermocouple that was installed in contact with the catalyst bed. The products were analyzed simultaneously, using a chemiluminiscence analyzer (NO–NO2–NOx ECO PHYSICS) and a Fourier transform infrared (FTIR) analyzer (Perkin-Elmer Spectrum GX) that was capable of measuring the following species continuously and simultaneously: NO, NO2, N2O, CO2 and C3H6. The feed composition was as follow: 1000 ppm C3H6, 1000 ppm NO, 5% O2, 10% H2O (when used) and the balance He. The feed gases were mixed and preheated before entering the reactor. The space velocity (GHSV) was 15,000 h
1, and the flow rate was 125 mL/min. Before the reaction was started, the catalysts were preconditioned by being held at 400 8C under a flow of helium (125 mL/min) for 60 min. Then, the temperature was reduced to 200 8C. The reaction measurements for each temperature were carried out after 2 h to ensure that the steady state was reached. All experiments were tested for reproducibility with analytical repeatability, with an error in NO conversion of <5%. 2.4. In situ FTIR measurements In situ IR spectra were collected with a Perkin-Elmer FTIR Spectrum GX spectrometer, by accumulating 100 scans at a resolution of 4 cm
1. The focused wavenumber range was 4000–1000 cm
1. The sample was placed in the center of a high temperature reaction chamber with KBr windows (Harrick). The temperature was measured with a K-type thermocouple and controlled with an automatic temperature controller (Harrick). Prior to each experiment, 0.05 g of catalyst was heated at a rate of 10 8C/min from room temperature to 400 8C. After a

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period of 30 min at this temperature, the sample was later cooled to 200 8C at 10 8C/min in a He flow of 60 mL/min. After pretreatment, the background spectrum of a He flow of 25 mL/ min was collected at 200 8C. Then, the flow of a gas mixture was switched to the reaction chamber with a GHSV of 15,000 h
1. CO adsorption measurements were carried out in the same manner as described above in the case of in situ FTIR SCR of NO experiments. IR spectra were colleted during the exposure to the CO flow at 30 8C. Subsequently, the sample was purged for 15 min with helium. 3. Results 3.1. Metal loaded without pH control The composition and characteristics of the iron-pillared and Cu-exchanged iron-pillared clays are summarized in Table 1. The copper content of the catalysts was modified by two different ways: (a) changing the concentration of the aqueous Cu solution from 0.05 to 0.2 M (samples Cu3.7-5.6, Cu3.3-5.4 and Cu2.1-5.2) and (b) increasing the metal content with successive ion-exchange steps, keeping constant the concentration of the solution at 0.1 M (samples Cu3.3-5.4, Cu4.4-5.4 and Cu5.8-5.4). The pH of the solutions for the ion-exchange step was not controlled. Table 2 shows the temperature corresponding to the maximum NOx conversion (Tmax) and the NOx yield to N2, the C3H6 yield to CO2 and CO for all the samples tested in this work. In set 1 of experiments, it is clear that the catalytic activity increased with the metal loading. Sample Cu5.8-5.4 gave the maximum NOx conversion. The BET surface area and the pore volume for these catalysts are given in Table 1, set 1. An increase of the Cu loading led to a decrease of the surface area (mainly the micropore area) and the micropore volume of the catalyst. This may be consequence of a partial blocking of the pillaring matrix by the metal species located in the interlayer areas [33,34]. On the other hand, the acidity of ion-exchanged pillared clays depended on both the ion-exchanged transition metal and the metal loading, since the metallic cations are Lewis acid sites [35,36].

Fig. 1. H2-TPR profiles for samples (a) Cu3.7-5.6, (b) Cu3.3-5.4, (c) Cu4.4-5.4, (d) Cu5.8-5.4 and (e) Cu2.1-5.2.

Fig. 1 shows the TPR profiles for these samples. First of all, it should be noted that the objective of the TPR experiments is to analyze the Cu species. However, the parent Fe-PILC showed a small reduction peak at 350–370 8C due to the Fe3+ ! Fe2+ reduction process [31]. To avoid interferences, this peak was always accounted for without significant error by subtraction in the TPR profiles. Under these test conditions, these samples presented one reduction peak at around 250 8C, assigned to the reduction of Cu2+ to Cu+ [37,38]. It can be observed that this peak shifts towards lower temperatures as the metal content increases, due to a weaker metal-support interaction or a higher particle metal size, which makes easier the reduction of these species. 3.2. Metal loaded under acid pH For these experiments, the pH of the solutions for the ionexchange step was controlled and fixed at a value of 1.0 (Table 1, set 2 of experiments). In Table 2 (set 2), it can be observed that, in these samples, the NOx conversion decreases 15.5 points when compared to the catalysts synthesized without

Table 2 Temperatures and yields corresponding to the maximum NOx conversion Set of experimentsa

Sample

Tmax (8C)

Yield NOx to N2 (%)

Yield C3H6 to CO2 (%)

1

Cu3.7-5.6 Cu3.3-5.4 Cu4.4-5.4 Cu5.8-5.4 Cu2.1-5.2

280 280 280 280 290

35.9 36.2 37.2 38.5 31.0

81.0 81.5 82.0 89.8 88.2

0.0 0.0 0.0 0.0 0.0

2

Cu0.48-1.0 Cu0.81-1.0 Cu0.83-1.0

320 340 320

22.5 18.7 20.7

72.0 61.0 63.3

13.8 11.4 8.5

3

Cu4.8-7.0 Cu6.2-9.0 Cu6.5-10.5

260 260 280

43.0 53.9 45.2

81.2 86.7 95.6

0.0 0.0 0.0

a

Reaction conditions: NO = C3H6 = 1000 ppm, O2 = 5 wt.%, He = balance, catalyst = 0.25 g and total flow rate = 125 mL/min.

Yield C3H6 to CO (%)

F. Dorado et al. / Applied Catalysis B: Environmental 65 (2006) 175–184

Fig. 2. Evolution of the C3H6, CO2 and CO concentration with temperature during the SCR of NOx over the catalyst Cu0.48-1.0.

control of pH. The propene is not completely oxidized, as there is CO production. Fig. 2 shows the evolution of the C3H6, CO2 and CO concentration with temperature for the sample Cu0.481.0. The other catalysts showed similar evolution. First, the CO concentration increased with the temperature, then got a maximum, and finally decreased. The maximum CO production occurs at the same temperature than the maximum NOx conversion (320 8C). Independently of the metal content and concentration of the Cu solution used for the synthesis, the TPR profiles for these samples (not shown) showed one reduction peak centered at 350 8C, assigned to the reduction of Cu+ to Cu0 [35,37,38]. The textural characteristics and the total acidity for these catalysts are given in Table 1, set 2. All surface area, pore volume and acidity decrease when compared to the parent FePILC, being the higher decrease for the sample Cu0.83-1.0. It is possible that the synthesis conditions at so low pH produces the dehydroxylation of the iron pillars and damage in the clay structure.

179

Fig. 3. NOx conversion to N2 at 260 8C in the presence of water over the catalyst Cu6.2-9.0 vs. time on stream. Reaction conditions: NO = C3H6 = 1000 ppm, O2 = 5 wt.%, H2O = 10%, He = balance, catalyst = 0.25 g and total flow rate = 125 mL/min.

clusters, the surface area BET and pore volume decrease, because these CuO species block the pores [35,39]. It can also be observed that, in this set of experiments, the total acidity decreases when the pH is increased. Fig. 4 shows the TPR profiles for the samples Cu4.8-7.0, Cu6.2-9.0 and Cu6.5-10.5. Catalyst Cu4.8-7.0 shows two reduction peaks. The first peak, centered at 210 8C, is attributed to the overlapping of the reduction reaction of Cu2+ to Cu+ and CuO to Cu0. The second peak, less pronounced and about 350 8C, is assigned to the reduction of Cu+ to Cu0 [35,37,38]. Sample Cu6.2-9.0 shows a broad peak at 210 8C, attributed to the reduction of CuO to Cu0 [35,38,40,41], with a shoulder at 180 8C, assigned to the reduction of Cu2+ to Cu+. Finally, the sample Cu6.5-10.5 presents one peak centered at 240 8C, which is also attributed to the overlapping of the reduction reactions of Cu2+ to Cu+ and CuO to Cu0. It is clear that the total amount of copper introduced in the catalysts and the copper species formed over the structure depends on both the alkaline pH and concentration of the copper II acetate solution used to prepare the samples. Thus, it

3.3. Metal loaded under alkaline pH To prepare the last set of catalysts, the ion-exchange step with copper was carried out at a pH range from 7.0 to 10.5 (Table 1, set 3 of experiments). The maximum NOx and C3H6 yields and the Tmax for these samples are given in Table 2 (set 3). The more active sample was Cu6.2-9.0. For the most promising catalyst, a preliminary study about the influence of water presence in the feed on its performance was carried out. In Fig. 3 is shown the NOx conversion to N2 in the presence of water over Cu6.2-9.0 as a function of time at 260 8C (Tmax for this sample). The maximum activity of Cu6.29.0 under wet conditions appeared to be stable over extended periods of time at Tmax. The sample Cu6.5-10.5 shows a higher decrease of the surface area and pore volume than the other samples prepared under alkaline pH (Table 1), may be due to that the copper is mainly as CuO. Several authors have reported that when the main copper species in the structure of the catalysts are CuO

Fig. 4. H2-TPR profiles for samples: (a) Cu4.8-7.0, (b) Cu6.2-9.0 and (c) Cu6.5-10.5.

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will be possible, in the next section, to explain the catalytic results where its quite the contrary that observed in the experiments without control of pH. In this mentioned above case, it was observed that the catalytic activity increased with the metal loading. 4. Discussion Since Iwamoto et al. [15], in 1990, reported the activity of Cu-ZSM-5 for the catalytic decomposition of NOx, several authors have studied this reaction over different supports ionchanged with copper [13,35,36,41–44]. This fact indicates that the base catalyst plays an important role in the SCR reaction. Thus, these previous works showed the influence of the metal content, framework zeolite, Si/Al ratio, etc. Under reaction conditions, Cu shows a dynamic behaviour, being reduced by C3H6, and then, oxidized again by O2 and NO, allowing CO2 and N2 formation [18,27]. According to Delahay et al. [45], copper species in Cuexchanged catalysts can be as isolated (Cu2+/Cu+) ions or as oxygenated clusters (CuO). The reactions involved in the copper reduction process are: CuO þ H2 ! Cu0 þ H2 O Cu

2þ þ

þ

þ 0:5H2 ! Cu þ H 0

Cu þ 0:5H2 ! Cu þ H

fact points out that Cu species in this catalyst are isolated Cu2+ cations, which have been proposed as the real catalyst sites [18,27,29,47]. With regard to the influence of the concentration of the copper II acetate solutions, it was observed that the samples Cu3.7-5.6 and Cu3.3-5.4 had a similar catalytic performance (36% of NOx conversion at 280 8C). However, the sample Cu2.1-5.2 (prepared with a copper solution concentration of 0.2 M) showed different characterization and activity results compared to the others, as well a lower metal content (see Table 2). This fact could be justified according to the following copper aqueous behaviour: ½CuðH2 OÞx 2þ þ yAn $ ½CuðAÞy 2 þ yn þ xH2 O A high concentration of the aqueous Cu solution moves the equilibrium to the left, generating solvated Cu2+ cations with high ionic radius. They would have steric hindrance to enter into the interlayer spaces of the Fe-PILC and therefore to be ion-exchanged with the Na+ ions. Consequently, the lower density of isolated Cu2+ ions in this sample led to a lower NOx conversion (Table 2). 4.2. Metal loaded under acid pH

þ

þ

At low metal contents, the main specie over the catalyst is Cu2+, which can be reduced in two steps to Cu0. The reduction Cu2+ ! Cu+ occurs at lower temperatures, while the reduction Cu+ ! Cu0 would occur at higher temperatures [37,38]. When the copper content in the sample is higher, the copper may be also found as oxygenated clusters [35,38,40,41]. 4.1. Metal loaded without pH control TPD of NH3 and TPR tests (Fig. 1) have shown that when the copper content increases in the samples of the set 1 (Table 1), the total acidity increases and the copper species present in the catalysts are easy to reduce. This fact would indicate that Na+ ions, originally present in the Fe-PILC structure, have been ionexchanged by Cu2+ (Lewis acid cation), so that the Cu5.8-5.4 catalyst, ion-exchanged three times with copper II acetate solution, presents the highest total acidity value. In this sample, copper would occupy more accessible positions for NH3 molecules. These results suggest that, for all these samples, copper is present as isolated Cu2+, not as CuO. Moreover, several authors have reported that CuO clusters have a low participation in the total acidity as CuO clusters adsorb NH3 very weakly [44,46]. On the other hand, as it has been mentioned above, when the copper content increases the copper excess may be found as CuO. Thus, the sample Cu5.8-5.4, with the highest Cu loading, could present in its structure some CuO clusters. However, this assumption does not agree with the catalytic test, since Cu5.85.4 showed the highest NOx conversion (set 1, Table 2). This

The decrease of catalytic activity in the samples synthesized at pH equal to 1.0 and the appreciable CO production can be explained considering the low amount of the cation Cu2+ in these catalysts. Moreover, there is dehydroxylation of the iron pillars and damage in the clay structure due to the severe acid treatment, with a clear loss of surface area (Table 1). It is interesting to note that there was also CO production when a parent Fe-PILC was tested for the SCR of NO (result not shown). For these samples, there could be a higher competition between the NO reduction by propene and the oxidation of propene by oxygen on the Cu2+ sites. However, the oxidation of propene becomes dominant at higher temperatures, when CO production is not observed [13]. TPR profiles show one peak attributed to the reduction of Cu+ to Cu0. This phenomenon could be accounted for as the result of partial reduction of Cu2+ to Cu+ during the catalyst preparation (for the TPR experiments) or pretreatment (for the reactions tests) under He flow [43,46]. To justify this suggestion, they were carried out CO adsorption experiments by in situ FTIR on samples prepared in set 2 (Fig. 5). The spectra obtained after exposure of carbon monoxide at 30 8C were colleted over fresh and He-pretreated catalysts at 400 8C. It is observed that there are always bands in the C–O stretch region. The presence of Cu+ is confirmed by the characteristic adsorption bands of the n(CO) mode corresponding to linearly adsorbed CO species on a Cu+ site (bands at 2155 cm
1 ascribed to Cu+–CO and bands at 2170 and 2135 cm
1 attributed to [Cu+(CO)2]) [18,38,46,48]. However, the spectra of pretreated catalysts show bands with much higher intensity than fresh catalysts, due precisely to the pre-reduction of Cu2+ to Cu+ during the pretreatment with He. According to the literature [48], the appearance of bands at around 2180 and

F. Dorado et al. / Applied Catalysis B: Environmental 65 (2006) 175–184

Fig. 5. In situ FTIR adsorption CO on samples prepared in set 2. ( catalyst, (- - -) pretreated catalyst.

) Fresh

2140 cm
1, in spectra of fresh catalysts, can be interpreted as evidence for possible reduction of Cu2+ ions to Cu+, which could occur during the adsorption of CO on these samples. 4.3. Metal loaded under alkaline pH For the catalysts prepared in this set of experiments (set 3), it should be taken into account that the copper is not only present as Cu2+ ions, but also as CuO clusters. The presence of these oxygenated clusters can be explained considering the Cu solution behaviour under basic pH, when it is added NH3 to the acetate copper solution [38,49]: CuðOHÞ2 þ 4NH3 $ ½CuðNH3 Þ4 2þ þ 2OH
When it is added less than an equivalent amount of NH3, it is observed the precipitation of Cu(OH)2 and, on the other hand, the colour of solution turns to characteristic blue provided by the [Cu(NH3)4]2+ complex. After calcination, these compounds form CuO and isolated Cu2+, respectively. Thus, in Fig. 4, the first reduction peak (temperature from 180 to 250 8C) for the samples Cu4.8-7.0, Cu6.2-9.0 and Cu6.510.5 has been attributed to the reduction of both CuO and Cu2+ species. For the samples Cu4.8-7.0 and Cu6.5-10.5, the two peaks that could be expected for both reduction processes are unresolved. However, there is a better resolution for the sample Cu6.2-9.0, suggesting a higher proportion of Cu2+ ions. This result would be consistent with the catalytic activity results, since this catalyst showed the highest NOx conversion. For the sample Cu6.5-10.5, we suggest that the peak is mainly due to large CuO clusters, located at the outer surface of the catalysts. These clusters would be blocking the pores, explaining the textural characteristics of this sample (there is a clear loss of BET surface and micropore area for this catalyst, see Table 1).

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Typical results of NOx conversion versus temperature for the samples Cu4.8-7.0, Cu6.2-9.0 and Cu6.5-10.5 are shown in Fig. 6. First, the NOx conversion increased with the temperature, then reached a maximum, and finally decreased. This decrease cannot be related to the deactivation of the catalyst or collapse of the clay structure, since the results were repeatable and consistent in time [33]. According to Yang et al. [13], the decrease in NOx conversion at high temperature is due to the combustion of the hydrocarbon. The oxidation of hydrocarbon, which reduces the amount of reducing, becomes dominant at high temperature. Fig. 6 shows that the sample Cu6.2-9.0 gives the best activity for C3H6 combustion to CO2 at temperatures below 260 8C. This easier C3H6 oxidation favours the NOx reduction at low temperature, as it will be explained below. On the contrary, the sample Cu6.5-10.5, with similar Cu loading, shows a poor activity, suggesting again that the main copper specie in its structure is CuO. This specie is blocking the pores (see BET surface and micropore area in Table 1). Therefore, for the Cu6.5-10.5 sample, copper is not accessible as active site for SCR of NOx. Now, it is very interesting to compare samples prepared under alkaline pH with those prepared without pH control. The catalysts Cu6.2-9.0 and Cu5.8-5.4 have similar metal content, but part of the metal is as CuO clusters for the former and only as isolated Cu2+ ions for the later. If the catalytic performance was only related to the Cu2+ species, the later should be more active. However, the catalytic tests showed the opposite result (Table 1). The same discussion is applicable to the catalysts Cu4.8-7.0 and Cu4.4-5.4. As main conclusion, it should be remarked that the presence of CuO species in the samples Cu4.8-7.0 and Cu6.2-9.0 led to an improvement in NOx yield to N2, which is in agreement with their best activity for C3H6 combustion (see Fig. 7). Keeping the above results in mind, temperature-programmed desorption experiments of C3H6 and NO were performed over samples Cu4.8-7.0, Cu6.2-9.0, Cu6.5-10.5 and Cu4.4-5.4 (Fig. 8 and Table 3). Depending on the sample, two different peaks were obtained. Following to Wan et al. [43], the

Fig. 6. SCR of NOx by propene over the samples prepared in set 3. Reaction conditions: NO = C3H6 = 1000 ppm, O2 = 5 wt.%, He = balance, catalyst = 0.25 g and total flow rate = 125 mL/min.

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Fig. 9. TPD profiles of O2 adsorbed on Cu-Fe-PILC.

Fig. 7. Comparison of the catalytic behaviour on samples with similar metal content. Reaction conditions: NO = C3H6 = 1000 ppm, O2 = 5 wt.%, He = balance, catalyst = 0.25 g and total flow rate = 125 mL/min.

Fig. 8. TPD profiles of NO adsorbed on Cu-Fe-PILC.

peak at low temperature was ascribed to the desorption of NO from Cu2+ ions, and the second peak at higher temperature was associated to the decomposition of nitrate, nitrite or NO2+ adsorbed species. As expected, for the samples Cu4.4-5.4 (only isolated Cu2+ ions) and Cu6.2-9.0 (higher metal content with Table 3 Amount of NO and C3H6 desorbed per mmol of Cu and the temperature of the adsorption peaks Sample

NO/Cua (mmol/mmol)

Tmax (8C)

C3H6/Cu (mmol/mmol)

Tmax (8C)

Cu4.4-5.4 Cu4.8-7.0 Cu6.2-9.0 Cu6.5-10.5

5.55 1.65 8.12 0.92

200 167 222 200

1.65 4.50 1.81 1.10

169 175 148 164

a

Corresponds to the first NO adsorption peak.

high proportion of Cu2+ cations) only the first peak was found. Considering now the catalytic results, it is clear that a high quantity of Cu2+ cations may be useful for a good performance, but not enough. The second peak is characteristic of samples with big CuO clusters [43], and it was only observed for the sample Cu6.5-10.5 at 350 8C (see Fig. 8), confirming the hypothesis made during the discussion of the TPR results. On the other hand, TPD-C3H6 experiments reveal that propene can be adsorbed on Cu2+ ions and CuO species, but mainly over CuO, since sample Cu4.8-7.0 desorbs the highest quantity. In this point, it is necessary to remember that the CuO species present in sample Cu6.5-10.5 are blocking the structure, whereas for the samples Cu4.4-5.4 and Cu6.2-9.0 the copper is exclusively (the former) or in high proportion (the later) present as Cu2+, so that the sample with more accessible CuO is Cu4.87.0. To confirm our conclusion, the TPD profiles of O2 adsorbed over these last samples, normalized for the metal content, are shown in Fig. 9. The quantity of desorbed O2 is directly related to the quantity of CuO [50,51]. Therefore, both O2 and C3H6 are easily adsorbed by the CuO species, allowing their reaction to form reaction intermediates. At this point, we could propose a reaction mechanism of HC-SCR of NO over Cu-ion-exchanged Fe-PILC. At low temperatures, propene and oxygen are mainly adsorbed over the CuO aggregates. The CuO species promote the oxidation of propene, so at this time there are available CxHyOz intermediates to move on Cu2+ (real active site), where NO is anchored, to form other reaction intermediates (organic nitrite and organic nitro compounds) which lead to N2 and CO2 as main final products of SCR of NOx. According to this mechanism, the presence of accessible CuO clusters should be necessary. This also would explain why catalysts based on zeolites must be over exchanged with copper to be really active: after over exchanging, the formation of some CuO clusters is forced [38,43]. To verify this reaction mechanism, an in situ FTIR of SCR of NO was carried out. Fig. 10 shows the changes in the IR-spectra during exposure of a Cu-Fe-PILC sample to a gas mixture containing 1000 ppm NO, 1000 ppm C3H6 and 5 wt.% O2 in He, between 200 and 300 8C. It can be observed that the intensity of the bands at 2965 and 2884 cm
1, attributed to both C C and C–H stretching, decreases at higher temperatures. At lower temperatures, it can be seen bands at 1670, 1585, 1442

F. Dorado et al. / Applied Catalysis B: Environmental 65 (2006) 175–184

Fig. 10. Effect of temperature on surface species during in situ reaction of NO + C3H6 + O2 over Cu-Fe-PILC.

and 1363 cm
1, assigned to organic nitrito compound, bidentate nitrate, CH3COO
and organic nitro compound, respectively. At higher temperatures, there are bands at 1551, 1498 and 1415 cm
1 corresponding to bidentate nitrate, and C– H bending, respectively [18,26,27,52]. Therefore, the reaction intermediates are mainly acetates, organic nitro compounds and nitrous oxide species. These intermediates lead to N2 and CO2. Finally, it is interesting to compare this catalysts with others found in the literature. For example, Kubacka et al. [53] studied catalysts based on ferrierite with a Cu/framework Al ratio of 0.5, using ethene as reducing agent and 1% of oxygen in the feed. It was reached a NO conversion level of 78% at 375 8C. Zhang et al. [54] observed NO conversion of 81% at 450 8C and 97% at 700 8C over lanthanum ferrite after Cu substitution, using propene as reducing agent and 1% of O2 in the feed. Thus, the Cu6.2-9.0 sample, with an acceptable conversion at a relatively low temperature (53.9% at 260 8C), tested with 5% of oxygen in the feed, can be considered a promising catalyst. The preliminary test with 10% water in the feed (Fig. 3) is also promising. Even though there is loss of activity (from 53.9% to about 40%), NO conversion appeared to be stable over extended periods of time. This seems to be logic, as these catalysts do not suffer alteration of their structure by water presence, a major problem for zeolite-based catalysts. A complete study of the Cu-Fe-PILC activity under deactivation conditions (presence of H2O and SO2) will be carried out in a further work.

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these samples the total acidity and the catalytic activity increased with the metal content. When the samples were prepared under acid pH, the catalytic activity decreased and it was observed an appreciable CO production may be due to the low amount of the cation Cu2+ in these catalysts. Textural characteristics and total acidity values for these samples decreased because the synthesis conditions at so low pH produce the dehydroxylation of the iron pillars and damage in the clay structure. Finally, for the samples prepared under alkaline pH, it was observed that the copper was as Cu2+ ions and CuO clusters. Their catalytic tests showed the best results for the SCR of NOx. The presence of CuO species led to an improvement in NOx yield to N2, which is in agreement with the best activity for C3H6 combustion over these catalysts. A reaction mechanism was proposed, where the reaction intermediates are mainly acetates, organic nitro compounds and nitrous oxide species. Acknowledgments Financial support from the Ministerio de Ciencia y Tecnologı´a of Spain (Project CTQ-2004-07350-C02-O) and the Consejerı´a de Ciencia y Tecnologı´a de la Junta de Comunidades de Castilla-La Mancha (Proyect PBI-05-038) are gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

5. Conclusions

[14]

In this work, Fe-PILC-based catalysts ion-exchanged with copper were prepared under different pH conditions. It was proved that the copper species formed over the structure depends on both the pH and concentration of the copper II acetate solution used to prepare the samples. The influence of the nature and distribution of the copper species on the Cu-ionexchanged Fe-PILC for the SCR of NO by propene was also investigated. It was observed that samples prepared without pH control presented, in their structure, copper as isolated Cu2+ ions. In

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