Selective catalytic reduction of nitrogen oxides over a modified silicoaluminophosphate commercial zeolite

JES-01081; No of Pages 7 J O U RN A L OF E N V I RO N ME N TA L S CI EN CE S X X (2 0 1 7 ) XX X–XXX

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Carolina Petitto, Gérard Delahay⁎

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Institut Charles Gerhardt Montpellier, UMR 5253 CNRS/UM/ENSCM, Matériaux Avancés pour la Catalyse et la Santé, 8 rue de l'Ecole Normale, 34296 Montpellier Cedex 5, France. E-mail: [email protected]

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Selective catalytic reduction of nitrogen oxides over a modified silicoaluminophosphate commercial zeolite

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AR TIC LE I NFO

ABSTR ACT

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Article history:

Nitrogen oxides (NOx: NO, NO2) are a concern due to their adverse health effects. Diesel 14

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Received 30 November 2016

engine transport sector is the major emitter of NOx. The regulations have been 15

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Accepted 6 March 2017

strengthened and to comply with them, one of the two methods commonly used is the 16

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Available online xxxx

selective catalytic reduction of NOx by NH3 (NH3-SCR), NH3 being supplied by the in-situ 17

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Keywords:

required. Durability is evaluated by hydrothermal treatment of the catalysts at temperature 19

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Diesel

above 800°C. In this study, very active catalysts for the NH3-SCR of NOx were prepared by 20

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Nitrogen oxides

using a silicoaluminophosphate commercial zeolite as copper host structure. Character- 21

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SCR process

izations by X-ray diffraction (XRD), scanning electron microscopy (SEM) and temperature 22

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Copper

programmed desorption of ammonia (NH3-TPD) showed that this commercial zeolite was 23

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Zeolite

hydrothermally stable up to 850°C and, was able to retain some structural properties up to 24

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Silicoaluminophosphate

950°C. After hydrothermal treatment at 850°C, the NOx reduction efficiency into NH3-SCR 25

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hydrolysis of urea. Efficiency and durability of the catalyst for this process are highly 18

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depends on the copper content. The catalyst with a copper content of 1.25 wt.% was the 26

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most active. The difference in activity was much more important when using NO than the 27 28

fast NO/NO2 reaction mixture.

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© 2017 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences.

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Introduction

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The selective catalytic reduction of NOx by NH3 (NH3-SCR) process, using urea as ammonia source, is now an automotive technology for removing NOx from Diesel exhaust gas (Brandenberger et al. 2008; Granger and Parvulescu 2011; Zhang et al. 2016a). Cu-, Fe-zeolite catalysts are commercially used for this technology since a few years (Zhang et al. 2016a; Kieffer et al. 2013). Among zeolite structures, zeolites with small pores attract more and more attention (Zhang et al. 2016a; Beale et al. 2015). Chabazite (CHA) structure is one of these and SAPO-34 zeolites are one of the possible choices. A very interesting property of silicoaluminophosphates is their high hydrothermal stability (Ishihara et al. 1997). Since SCR

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catalyst ageing is usually realized by hydrotreatment at high temperatures, the use of SAPO-34 as host structure allows preparing more stable Cu-SAPO-34 catalysts for the NH3-SCR process (Petitto and Delahay 2015; Wang et al. 2015). Moreover, the integration of SCR catalysts in the diesel particulate filter (DPF) should be considered and requires that SCR catalysts must be stable in increasingly drastic conditions. Hydrothermal stability of SAPO-34 zeolites depends on chemical composition. We have shown that among Cu-SAPO-34 catalysts, aged 5 hr at 850°C in a dynamic flow containing 10% H2O, the catalyst, with the lowest silica content (1.2 wt.%) exhibited the highest NH3-SCR of NOx activity by maintaining its initial structural properties (Petitto and Delahay 2015). More recently, Wang et al. (2012) using static hydrothermal conditions have reported that

⁎ Corresponding author. E-mail: [email protected] (Gérard Delahay).

http://dx.doi.org/10.1016/j.jes.2017.03.005 1001-0742/© 2017 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

Please cite this article as: Petitto, C., Delahay, G., Selective catalytic reduction of nitrogen oxides over a modified silicoaluminophosphate commercial zeolite, J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.03.005

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1. Experimental section

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The silicoaluminophosphate zeolithe (SBET (Brunauer Emmett Teller surface area) = 630 m2/g) with CHA structure (H-CZC) was graciously supplied by Clariant (Switzerland). The chemical composition given by Clariant is the following: 21.9 wt.% Al, 3.8 wt.% Si, 18.2 wt.% P and 1.97 wt.% Ti. According to the UV–visible spectroscopy spectrum of the H-CZC zeolite, the titanium possesses an octahedral coordination probably under the form of well-dispersed titanium oxide species. TiO2 is known for its activity in the hydrolysis of the urea (Lundström et al. 2011). The impregnation method, with an organic solvent, was used in order to introduce the adequate quantity of copper or iron. The use of an organic solvent instead of water was motivated by the possible negative effect of both pH and temperature during the evaporation phase (Parlitz et al. 1994). In a rotating evaporation flask, a desired amount of copper acetylacetonate or iron acetylacetonate was dissolved into 50 mL of acetonitrile and then 2 g of zeolite was added. Then, the flask was connected to the rotavapor and put in a bath heated at 60°C for 4 hr with a mild rotation speed. Then, a gentle evaporation, under vacuum, was made up to complete removal of acetonitrile. The final product was firstly dried in an oven at 80°C for 24 hr and then calcined overnight at 550°C in air. The catalysts were labelled Cu(x)- or Fe(x), x being the content in wt.%. Hydrothermal treatment (HDT) was performed in dynamic flow (55 mL/min of 20% O2/He containing 10% H2Og) using 0.5 g of catalyst deposited on a porous frit of a U tube quartz reactor. The solid was heated at 850°C (or 950°C) with a ramp of 6°C/min. The injection of H2O(liq.) (0.0041 mL/min), by a syringe-pump was started and kept at this temperature for 5 hr. The injection of water was stopped during the cooling of the solid once the temperature reached 450°C. Powder X-ray diffraction (XRD) data were obtained on a diffractometer apparatus (AXS D8, Bruker, United States) by using Cu Kα radiation and a Ni filter. The morphology of SAPO-34 catalysts was studied by scanning electron microscopy (SEM) (4800 S electron microscope, HITACHI, Japan). The acidity measurements were evaluated by temperature programmed desorption of ammonia (NH3-TPD) (AUTOCHEM 2910, Micromeritics, United States). NH3 adsorption was carried out at 100°C for 45 min using 5 vol.% NH3/He with a flow rate of 30 mL/min. After NH3 adsorption, the line was flushed with He (30 mL/min) during 45 min to remove physisorbed NH3. Finally, NH3-TPD was performed in helium flow (30 mL/min) from 100 to 550°C using a heating rate of 10°C/min. The catalytic tests were performed in a dynamic flow reactor operating at atmospheric pressure. Catalyst aliquots

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2. Results and discussion

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2.1. Characterization and hydrothermal stability of the zeolite 155 host structure 156

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(0.024 g) were activated in situ at 550°C in air and cooled to 180°C. The catalytic tests were carried out from 180 to 550°C at 5°C/min with a space velocity of 312,500 hr−1 using the following feed concentration: 0.1% NO (or 0.0542% NO +0.0458% NO2), 0.1% NH3, 8% O2, 3.5% H2O (v/v), and balance with He. The effluent composition was monitored continuously by sampling on line to a quadruple mass spectrometer (Omnistar, Pfeiffer Vacuum, Germany) equipped with Channeltron and Faraday detectors (0–100 amu). The possible occurrence of intraparticle mass transfer limitations was previously checked on copper Y zeolite using two aliquots being grinding and sieving, in the ranges 63–125 and 250–400 μm. These two aliquots were tested for the SCR of NO by NH3 in a temperature programmed surface reaction (TPSR) protocol (Kieger et al. 1999). The plots of NO conversion to N2 were quasi-superimposed for the two aliquots, which provides evidence that the intraparticle mass transfer is fast enough and will not screen the chemical processes. The grain size of the catalysts used for this study was in the same range. Therefore, the possible occurrence of intraparticle mass transfer limitations is unlikely since the catalytic study was performed in the same set up, with less drastic reaction conditions, and using catalysts in the same range of grain size.

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the stabilization of CHA structure is enhanced by the copper content. In the present work, the effect of copper content on a commercial silicoaluminophosphate chabazite has been investigated. The catalysts have been tested both after calcination at 550°C and hydrotreatment at 850°C in the NH3-SCR of NO and NO/NO2.

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Our goal was to obtain active and stable catalysts in SCR of NOx by NH3. Therefore, it was important to control the hydrothermal stability of the zeolite itself before the deposition of copper or iron. Thus, three different characterizations were carried out: XRD, SEM and NH3-TPD over calcined H-CZC sample and hydrotreated H-CZC sample (850 and 950°C).

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2.1.1. XRD

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Fig. 1 shows X-ray diffraction patterns of H-CZC after calcination at 550°C and hydrotreatment at 850 and 950°C. Fig. 1a–d shows typical powder diffraction patterns of a chabazite (CHA) structure but with traces of AFI structure (SAPO-5), pointed out by 2θ = 7.45° (Baerlocher et al. 2001; Nakhostin Panahi et al. 2015). However, the X-ray diffraction patterns of H-CZC after hydrotreatment showed important changes. The loss of crystallinity was evaluated close to 50%, after hydrotreatment at 850°C whereas 67% of loss of crystallinity is reached between calcination at 550°C and hydrothermal treatment at 950°C for 5 hr.

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2.1.2. SEM

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Zeolite crystals with an average size of 0.8–1.5 μm were observed for H-CZC calcined at 550°C (Fig. 2). As expected, the zeolite displays a cubic morphology, which is typical for SAPO-34 type zeolite (Xu et al. 2008). After HDT at 850°C, apparently no change of morphology was observed. On the other hand, amorphous phase was detected after hydrothermal treatment at 950°C (Fig. 2). It should be emphasized that the distribution, crystallized phase - amorphous phase, is not

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Please cite this article as: Petitto, C., Delahay, G., Selective catalytic reduction of nitrogen oxides over a modified silicoaluminophosphate commercial zeolite, J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.03.005

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Taking into account the porous diameter of the CHA structure but also the crystals size, the pore diffusion or intracrystalline diffusion, which involves diffusion of the reactants within the pores of the catalyst particles, cannot be excluded (Liu et al. 2016; Hu et al. 2016; Zhang et al. 2016b; Gao et al. 2013). But, considering the recent works reported by Hu et al. (2016) on Cu-SAPO-34 catalysts, potential pore diffusion impact may be ruled out since no significant change of activity was observed between Cu/SAPO-34 catalysts having crystal size between 1 and 10 μm. The commercial SAPO-34 used as host structure for copper, in this study, presented an average crystal size of 0.8–1.5 μm, the lower limit of the series studied for the crystal size effect by Hu et al. (2016). Moreover studying a series of Fe-Zeolite prepared by FeCl3 sublimation, Iwasaki et al. (2011) have pointed out that diffusivity of NH3 in the zeolite pores has a negligible impact on NH3-SCR over Fe/zeolite. Therefore, the catalytic activity measured, in this study, through the NOx conversion, is believed to be not controlled by diffusion problems.

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2.2.1. Catalytic activity of H-CZC

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H-CZC zeolite did not exhibit any activity for the standard SCR reaction (Fig. 4). On the contrary, with the fast SCR reaction mixture, a minimum of NOx conversion into N2 of 30% is obtained at low temperatures while a maximum of 65% at around 470°C (Fig. 5). This good activity in fast SCR conditions shows that H-CZC presents the adequate acidic properties. In the NH3-SCR of NO, the reaction rate is controlled by the NO oxidation into NO2 according to:

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2.2. NH3-SCR of NOx catalytic activity

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Fig. 1 – X-ray diffraction (XRD) patterns of H-CZC zeolites. H-CZC: the silicoaluminophosphate zeolithe with chabazite (CHA) structure.

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2.1.3. NH3-TPD

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The NH3-TPD profile of calcined H-CZC is constituted of two peaks (Fig. 3): one at low temperature, around 160°C, attributed to NH3 adsorbed onto OH sites, at zeolite external surface, like P-OH, Al-OH and Si-OH sites and the other one at higher temperature, above 260°C, attributed to NH3 adsorbed onto OH linked to structural Si (Petitto and Delahay 2015; Iwase et al. 2009). After HDT at 850°C, the NH3-TPD profile remained almost unchanged, exhibiting only a loss of desorbed NH3 of 7.4%. This small difference suggests that the structure of the zeolite was little affected by the hydrothermal treatment and thus, shows the high stability of this zeolite. On the other hand, the hydrothermal treatment at 950°C had a strong impact on the NH3 adsorption capacity. It is observed a huge decrease of intensity in the NH3 profile (Fig. 3, red profile) but with no change of its shape. By taking into account of this point, it was assumed that the structure was not entirely destroyed, which is in agreement with the SEM results. Therefore NH3-TPD technique is a very sensitive and adequate method to evaluate the structural change due to the hydrothermal treatment.

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uniform over the different images taken on this sample (not shown).

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2NO þ O2 ⇔ 2NO2

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2NO2 þ 2NO þ 4NH3 → 4N2 þ 6H2 O

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4NO2 þ 4NH3 þ O2 → 4N2 þ 6H2 Oðstandard reactionÞ

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Eq. (1) requires an oxidizing function in order to be implemented. This requirement explains the absence of activity in NH3-SCR of NO over H-CZC zeolite. On the other hand, with the fast reaction, where the gas mixture contains an equimolar mixture of NO and NO2, the oxidizing function is no more crucial since: 2NO þ 2NO2 þ 2NH3 → 2N2 þ 3H2 Oðfast reactionÞ

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2.2.2. Effect of copper content on catalytic activity

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NH3-SCR profiles of NO conversion over Cu(x)/H-CZC were reported into Fig. 4. On calcined samples, the efficiency in NO reduction at low temperature greatly increases with the copper content. For these catalysts, a peak is observed in the low temperature region. In order to check the permanence of this peak, NO conversion was measured, successively, into ascending and descending temperature ramp sequence over Cu(1.2)/H-CZC. From Fig. 4b, it is obvious that the peak of NO conversion occurring at around 250°C is not due to a transient phenomenon despite some differences between the profiles. After HDT at 850°C, this conversion peak disappeared completely revealing only a slight footprint. Considering the

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Please cite this article as: Petitto, C., Delahay, G., Selective catalytic reduction of nitrogen oxides over a modified silicoaluminophosphate commercial zeolite, J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.03.005

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Fig. 2 – Scanning electron microscopy (SEM) images of H-CZC zeolites calcined at 550°C (Calc. 550°C), hydrotreated at 850°C (HDT 850°C) and hydrotreated at 950°C (HDT 950°C). HDT: hydrothermal treatment; H-CZC: the silicoaluminophosphate zeolithe with chabazite structure.

recent work on Cu-SAPO-34 (Wang et al. 2013), it is possible to attribute this NO conversion peak, observed over the calcined samples, to the copper species on the zeolite external surface, since thermal treatment favours the migration of copper species from the external surface towards exchange sites present in the pore channels of the zeolite structure. On calcined catalysts, for temperatures below 450°C, the efficiency in the NO reduction by NH3 increases with the copper content of the sample (Fig. 4a, dotted line). Above 450°C, the NO conversion over Cu(2.40)/H-CZC starts to decrease due to the side reaction of oxidation of NH3 by O2 according to: 4NH3 þ 5O2 → 4NO þ 6H2 O

Fig. 3 – Temperature programmed desorption of ammonia (NH3-TPD) profiles obtained with H-CZC zeolites. H-CZC: the silicoaluminophosphate zeolithe with chabazite structure.

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or by 4NH3 þ 3O2 → 2N2 þ 6H2 O

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This drop of NO conversion observed on the catalyst containing the highest copper content is in line with the literature data (Centi and Perathoner 1995). After HDT at 850°C, the efficiency order, in the NO reduction by NH3, has changed as it is shown by Fig. 4a. A better conversion is obtained, at low temperatures, on Cu(1.25)/H-CZC and on Cu(0.57)/H-CZC while, a strong decrease of NO conversions is observed on Cu(2.40)/H-CZC in the overall temperature window. Introducing an high amount of copper species at exchange sites of a chabazite zeolite, by the aqueous ion

Please cite this article as: Petitto, C., Delahay, G., Selective catalytic reduction of nitrogen oxides over a modified silicoaluminophosphate commercial zeolite, J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.03.005

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Fig. 5 – . Reaction temperature dependence of NOx conversion (fast reaction) over H-CZC and Cu(x)/H-CZC. H-CZC: the silicoaluminophosphate zeolithe with chabazite structure.

weight percentage of Cu(II) ions (1 Cu for two structural Si) which can be stabilized by the structure is 1.33 wt.%. Thus, a change in order of efficiency of the catalysts in NH3-SCR of NO is found after HDT at 850°C. We assume that this difference of activity has to be related to the copper exchange capacity of the H-CZC, determined by NH3-TPD from the support alone, since the highest efficiency is obtained with the catalyst having the exchange level the close to the maximum capacity. An over exchange in copper (sample, Cu(2.40)/H-CZC) leads to a catalyst hydrothermally less stable and consequently to a loss of activity compared to the same sample calcined at 550°C. All these tendencies observed with the standard reaction on Cu(x)/H-CZC are also verified with the fast SCR reaction, but into a much less extent (Fig. 5). The smaller differences are due to the fact that the reaction of NO oxidation into NO2 does not govern, in this case, the overall SCR reaction.

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exchange process, is problematic since the kinetic diameter of [Cu(H2O)6]2+ (0.42 nm) (Tansel 2012) is superior to the channel diameter of the CHA structure; this zeolite structure having pore diameters of 0.38 nm. Secondly, the Cu(II) ion exchange capacity of H-CZC may be determined from NH3-TPD. The peak observed at high temperature in NH3-TPD, being attributable to NH3 bound to structural Si, allows to assume that the maximum

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Fig. 4 – (a) Reaction temperature dependence of NO conversion (standard reaction) over H-CZC and Cu(x)/H-CZC; (b) Catalytic test reproducibility over Cu(1.25)/H-CZC. H-CZC: the silicoaluminophosphate zeolithe with chabazite structure.

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2.2.3. Discussion on the superior activity of Cu(1.25)/H-CZC 314 after HDT 315 So, a very efficient and stable NH3-SCR NOx catalyst was prepared. This catalyst was prepared by impregnation of 1.25% Cu over a SAPO-34 commercial zeolite having a Si content of 3.8 wt.%. This result seems to contradict our earlier results (Petitto and Delahay 2015), which have shown that it was preferable to use a SAPO-34 with a low Si content for obtaining catalysts more hydrothermally resistant. Nevertheless, there is a great difference in morphology between the commercial zeolite of this study and the SAPO-34 prepared in our laboratory (Petitto and Delahay 2015), namely the average size of the crystals, which is two to three times greater. Large crystals would therefore be preferable for increasing the hydrothermal stability. Studying catalytic cracking of gas oil, Camblor et al. (1989) have reported, for zeolite Y, that smaller crystals are less hydrothermally stable than bigger crystals resulting a lost of activity and selectivity after HDT (or steaming) at 750°C. Using

Please cite this article as: Petitto, C., Delahay, G., Selective catalytic reduction of nitrogen oxides over a modified silicoaluminophosphate commercial zeolite, J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.03.005

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In conclusion, a very efficient and stable NH3-SCR NOx catalyst from a commercial SAPO-34 having large crystals was prepared. The hydrothermal stability of the copper catalysts depends on the copper content. A copper content exceeding the exchange capacity leads to a less efficient catalyst after hydrotreatment at 850°C. Catalysts with iron are less active but follow the same tendency that copper catalysts.

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Acknowledgments

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This work has been performed in the framework of the UREENOX project (ANR-11-VPTT-002) funded by the French ANR (L'Agence National de la Recherche). Dr. Carolina Petitto and Dr. G. Delahay gratefully acknowledge the French ANR for financial support. The authors wish to thank F. Can, X. Courtois and D. Duprez of The Institute of Chemistry of Poitiers (IC2MP) for the management of the UREENOX project.

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REFERENCES

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Baerlocher, C.H., Meier, W.M., Olson, D.H., 2001. Atlas of Zeolite Framework Types. 5th Rev. ed. Elsevier, Amsterdam, New York.

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The NO conversion profiles versus temperature of Fe(1.40)/ H-CZC and Fe(2.30)/HCZC are reported in Fig. 6 for both the standard and the fast reaction. In the NH3-SCR of NO, NO

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conversion over calcined and HDT Fe(x)/HCZC samples remains very low compared with the copper samples. It is well known that, with iron exchanged zeolites, the activity is really dependent on the NO/NO2 ratio in the NH3-SCR of NOx (Colombo et al. 2010). This much lower activity is attributed to the very low oxidation. On the other hand, when the reaction mixture has a NO/NO2 ratio close to 1.0, the NOx conversion into N2 obtained over Fe(1.40)/H-CZC is above 90% between 320 and 550°C. By increasing the amount of iron of 0.9 wt.%, a slightly better efficiency is reached. The hydrothermal treatment of these two samples lead to an opposite behaviour for the NH3-SCR of NO: the activity increases with respect to the calcined compounds for temperatures between 200 and 420/450°C but above 420/450°C, the activity is weaker. It is tempting to attribute this better activity at low temperatures to some redispersion of iron species inside the porous channels of the SAPO-34. For the fast reaction, Fe(1.40)/H-CZC is the more efficient among the Fe(x)/HCZC tested. As for copper catalysts, the hydrotreatment at 850°C does not imply a very significant change in efficiency. But it is interesting to emphasize that like for Cu/H-CZC catalysts, the best catalyst Fe/H-CZC, after hydrothermal treatment, is the sample whose iron content is close to the ion-exchange capacity of the zeolite. As for copper catalysts, the hydrotreatment at 850°C does not imply very significant changes in efficiency. But it is interesting to emphasize that, like for Cu/H-CZC catalysts, the Fe/H-CZC catalyst, with the best behaviour into NH3-SCR of NOx after hydrothermal treatment, is the sample whose iron content is close to the ion-exchange capacity of the zeolite.

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Beta zeolite, Panpranot et al. (2005) concluded to the same statement studying conversion of methanol to hydrocarbons. More recently, Iwasaki et al. (2011) and Feng et al. (2016) have proposed that the size of the crystals of the starting zeolite has a strong impact on the hydrothermal stability of the Fe-zeolite catalysts and thus of the maintenance of a higher NH3-SCR of NOx efficiency after hydrothermal treatment at high temperature. They concluded that this parameter is an important factor contributing to hydrothermal stability. In addition, Iwasaki et al. (2011) suggested that the hydrothermal stability of Fe/zeolite could be improved by increasing crystal size. Moreover, catalyst deactivation may also be caused by migration of Cu ions towards less accessible or less efficient sites (Yan et al. 2015). Nevertheless it is evident that the quantity of ions which migrate from the external surface to the chabazite cages must be lower or equal to the exchange capacity in order not to induce the formation of copper clusters. It is thus obvious the number of copper cations, able to be retained at exchange position, will be function on the quantity of Si inserted in the zeolite structure. Beside the size of the crystals, the copper exchange level of Cu(1.25)/H-CZC is close to the maximal capacity of the commercial SAPO-34 used in this study. Crystal size is probably an important factor but the formulation of the catalyst has also a strong impact on the efficiency of the catalyst after hydrothermal treatment.

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Fig. 6 – Reaction temperature dependence of NOx conversion over Fe(x)/H-CZC. H-CZC: the silicoaluminophosphate zeolithe with chabazite structure.

Please cite this article as: Petitto, C., Delahay, G., Selective catalytic reduction of nitrogen oxides over a modified silicoaluminophosphate commercial zeolite, J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.03.005

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Please cite this article as: Petitto, C., Delahay, G., Selective catalytic reduction of nitrogen oxides over a modified silicoaluminophosphate commercial zeolite, J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.03.005

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