Applied Catalysis B: Environmental 158–159 (2014) 48–59 Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb Evolution of unburnt hydrocarbons under “cold-start” conditions from adsorption/desorption to conversion: On the screening of zeolitic materials Alexandre Westermann a , Bruno Azambre a,∗ , Gisèle Finqueneisel a , Patrick Da Costa b , Fabien Can c a Université de Lorraine, Laboratoire de Chimie et Physique Approche Multi-échelle des Milieux Complexes (LCPA2MC), EA 4632, Institut Jean Barriol, Rue Victor Demange, 57500 Saint Avold, France b Sorbonne Universités, UPMC Univ. Paris 06, Institut Jean Le Rond d’Alembert, CNRS UMR 7190, 2 place de la gare de ceinture, 78210 Saint Cyr l’École, France c Institut de Chimie des Milieux et Matériaux de Poitiers IC2MP, UMR 7285 CNRS, Université de Poitiers, 4 rue Michel Brunet, 86022 Poitiers, France a r t i c l e i n f o Article history: Received 23 January 2014 Received in revised form 31 March 2014 Accepted 3 April 2014 Available online 13 April 2014 Keywords: Zeolites Emission control Diesel oxidation catalyst Acidity HC-trap a b s t r a c t The general purpose of this work is to examine the relative ability of some well-selected zeolitic materials for the reduction of HC emissions generated within the Diesel “cold-start” period, i.e. when the work temperature of the Diesel Oxidation Catalyst (DOC) has not been reached. More peculiarly, this study is focused on the chemical, textural and structural parameters of zeolites influent on the elimination, namely by adsorption, of unburnt HC (propene, toluene and decane) in presence of potential inhibitors (H2 O, CO, NO). Simulated “cold-start” conditions consisted in the rapid heating of the pre-treated zeolite sorbent/catalyst under the whole gas mixture from 35 to 530 ◦ C. The quantity of trapped HC and those converted to COx by oxidation were measured in function of the temperature, as well as the amount of NOx converted by the HC-SCR reaction. The interpretation of the HC emission profiles in close relation with the porous and acidic (through FTIR of adsorbed pyridine) properties of the corresponding zeolites allowed to gain insight onto the relative contributions of the pore topology, the pore size and the acid strength. For some selected zeolites, several consecutive cold-start cycles were performed in order to assess their stability. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Environmental regulations on unburnt hydrocarbons (HC) emitted from mobile sources are becoming worldwide increasingly demanding. In the case of diesel light vehicles, most of these HC emissions are effectively converted to harmless products by an on-board Diesel Oxidation Catalyst (DOC), when the temperature of exhaust gases exceeds its light-off temperature (typically 200–300 ◦ C) [1]. However, 80% of the residual HC pollution is still produced under “cold-start conditions”, i.e. during the first two minutes (or even for a shorter period if a close-coupled catalyst is present) of the driving cycle [1,2]. HC in exhaust gases include branched and linear alkanes, alkenes, oxygenates and aromatics of various molecular weights, ranging from C1 to C16 . ∗ Corresponding author. Tel.: +33 387939106; fax: +33 387939101. E-mail address: bruno.azambre@univ-lorraine.fr (B. Azambre). http://dx.doi.org/10.1016/j.apcatb.2014.04.005 0926-3373/© 2014 Elsevier B.V. All rights reserved. Further reduction of these HC emissions can be performed with the help of close-coupled or electrically heated catalysts but one of the most promising after-treatment technology consists in trapping the unburnt HC onto a sorbent at low temperatures. Within this concept, the HC-trap has to assist the Diesel Oxidation Catalyst (DOC) during the cold-start period i.e. when the exhaust temperature is below the light-off temperature of the DOC. Once this light-off temperature is reached, the trapped HC may eventually desorb and can be effectively oxidized by the DOC, so that the trap can be regenerated. For a practical application, the following requirements have to be fulfilled: (i) an effective trap is targeted to adsorb most of the HC present in the exhaust, without showing significant selectivity for a given class of compounds; (ii) adsorption has to be strong enough in order to prevent the release of some HC at too low temperatures; (iii) the HC-trap has to be resistant to potential inhibitors present in exhaust gases, namely water, but also COx , NOx and SOx ; (iv) the trapping material (in washcoated form) should be re-used for A. Westermann et al. / Applied Catalysis B: Environmental 158–159 (2014) 48–59 49 Table 1 Structural, textural and chemical characteristics of the used zeolites. Samples Structure Pore size (Å) Si/Al SBET (m2 g−1 ) Sext (m2 g−1 ) Smicro (m2 g−1 ) Acidity (␮mol g−1 )a,b Lewis Brönsted Total Ca5A HMOR-20 HZSM5-11 H␤-25 HY-100 HY-15 HY-5 Pt-3/Y Cu-9.5/Y LTA (5.0 × 5.0) MOR (6.5 × 7.0) MFI (5.3 × 5.6 5.1 × 5.5) *BEA (5.6 × 5.6 6.6 × 7.6) FAU (7.4 × 7.4) 2 20 11.5 25 100 15 5.1 430 525 310 650 735 835 756 660 615 301 170 5 215 36 30 61 64 63 129 355 305 435 699 805 695 596 542 n.d 24 45 146 8 43 137 126 543 n.d 277 273 120 16 71 172 143 82 n.d 301 318 266 24 114 309 269 625 a b Determined by pyridine adsorption followed by IR spectroscopy (evacuation at 150 ◦ C). Determination not possible for Ca5A zeolite. many adsorption/desorption cycles and should therefore present sufficient mechanical and heat resistance. About these last two issues, it has been shown for instance that the much studied Cu-ZSM-5 zeolite is not suitable for automotive applications due to structural damages induced by the presence of steam at elevated temperatures and loss of active copper component. Furthermore, the strong exotherm induced by the combustion of adsorbed hydrocarbons was also found to promote this instability. Nevertheless and owing to the possibility of finely tuning their structural, textural and chemical parameters, zeolites still represent an obvious choice for the design of such a HC-trap. Furthermore, zeolites loaded with transition or precious metals can be by themselves effective oxidation or HC-SCR catalysts. Literature studies addressing all these issues simultaneously are surprisingly missing and relevant data on the adsorption of HC mixtures in absence/presence of inhibitors are scarce or incomplete. The present study aimed to fill this gap. For that purpose, a screening of several zeolitic adsorbents was carried out under simulated “cold-start” conditions. These conditions consist in performing heating cycles under a well-defined gaseous mixture of HC (propene (C3 ), toluene (C7 ), decane (C10 )) and inhibitors (water, CO and NO) and measuring continuously the emissions of HC, COx and NOx at the reactor outlet. Several characteristics of the zeolitic materials were investigated: (i) the type of structure (FAU, LTA, *BEA, MOR, MFI were used); (ii) the Si/Al ratio (for the Faujasite structure); (iii) the nature of the compensating cation (Cu, Pt, for the Faujasite structure). 2. Experimental zeolites were carefully dried at 80 ◦ C and calcined under nitrogen flow at 500 ◦ C prior to characterization and cold-start tests. 2.2. Characterizations Porosimetric properties were obtained from N2 adsorption isotherms recorded at −196 ◦ C on an automated Autosorb-IQ sorptiometer supplied by Quantachrome. Prior to each adsorption measurement, samples were outgassed in situ in vacuum at 80 ◦ C for 3 h and then at 150 ◦ C for 12 h to remove any adsorbed impurities. Specific surface area (SBET ) was determined using the BET equation while microporous and external surface area were computed using the t-plot method [3]. Powder X-ray diffraction (PXRD) measurements were carried out using a Brüker-AXS diffractometer and the CuK␣ radiation (1.5405 Å). Powdered diffraction patterns were recorded between 5 and 50◦ (2) using increments of 0.01◦ and a counting time of 2 s. Acidic properties were characterized by pyridine (Py) adsorption monitored by infrared spectroscopy. IR spectra were recorded on a Nicolet Nexus spectrometer equipped with a DTGS detector and a KBr beamsplitter using a resolution of 4 cm−1 and 64 scans. Samples were progressively activated under nitrogen flow (30 mL min−1 ) up to 200 ◦ C (5 ◦ C/min) for 30 min and then to 450 ◦ C (5 ◦ C/min, 30 min). Pyridine was then adsorbed (200 Pa at equilibrium) at 25 ◦ C and further desorbed until 150 ◦ C. The total amount of Brönsted (BAS) and Lewis (LAS) Acid Sites were determined from the area of the ␯19b mode (at 1545 cm−1 for BAS and 1450 cm−1 for LAS), using their molar coefficient (εPyH+ = 1.8 cm ␮mol−1 and εPyL = 1.5 cm ␮mol−1 ) respectively [4]. Finally, the spectra obtained on the different samples were normalized to a disc of 10 mg cm−2 in order to obtain quantitative results. 2.1. Materials 2.3. Cold-start tests All the commercial zeolites used in this study (with different structures and framework molar Si/Al ratio ranging from 5 to 100, see Table 1 for further details) were provided by Zeolyst either in ammonium or protonated forms, excepted HY-100 and Ca5A, which were supplied by Degussa and Aldrich, respectively. Prior to use, the supports were calcined under air (4 L/h/g) with a heating rate of 5 ◦ C/min from room temperature to 200 ◦ C (plateau of 1 h) and then to 500 ◦ C (plateau of 4 h). This procedure allowed to obtain all the zeolites in protonated forms while preserving their structure from damages caused by desorbing water. Pt and Cu-modified zeolites (Table 1) were prepared by means of two different synthesis routes. Pt-3 wt%/Y zeolite was prepared by incipient wetness impregnation of HY-5 support using the adequate amount of a PtCl2 precursor salt. Cu-9.5 wt%/Y zeolite was prepared by three successive ionic exchanges of HY-5 support at 60 ◦ C for 2 h using a 0.05 M copper acetate (CuAc) solution at pH = 5 (allowing an exchange-degree of 140%). In both cases, the modified A cold-start test (CST) consists to a cycle performed under a representative gaseous mixture that is close to the real one experienced by an on-line HC-trap (presence of inhibitors and reactive gases) in an automobile [2]. Due to the rapid heating during the cycle, adsorption/desorption processes (with the gaseous mixture described below) take place simultaneously and the sorbent is never saturated with HC species (non-equilibrated adsorption). O2 and NOx being also present in the feed gas, HC oxidation and NOx reduction are also expected to occur during the heating, as it will be shown later on. More peculiarly, CST experiments were performed in a fixedbed reactor (with EUROTHERM 2408 temperature controller and a K-type thermocouple), using 0.2 g of zeolite loaded into a U-type (internal diameter = 6 mm) glass cell between 2 plugs of quartz wool. To ensure a reproducible feed at the reactor inlet, the concentration of each species was set-up using mass-flow controllers A. Westermann et al. / Applied Catalysis B: Environmental 158–159 (2014) 48–59 50 0 Physics CLD 700 AL chemiluminescence NOx analyzer (for NO and total NOx allowed to the simultaneous detection of NO and NOx (NO2 by difference). Two Ultramat 6 IR analyzers were used to monitor N2 O, CO and CO2 . A FID detector (Fidamat 5) was used to measure the total concentration of the HC compounds. Specific profiles corresponding to each HC were monitored through a Pfeiffer Vacuum GSD 301 Quadrupole Mass Spectrometer (MS) using the following signals: propene (m/z = 42), toluene (m/z = 91) and decane (m/z = 71). Because the stability of the sorbent material is an important issue for a practical application, some selected zeolites (HY-5, HY100, Pt-3/Y and Cu-9.5/Y) were selected to undergo 4 consecutive CST cycles. 3,0E-09 600 3. Results and discussion 2,5E-09 500 3.1. General description of the accessible pore structure for the investigated zeolites 14000 600 A 340 500 10000 400 8000 300 [C]0 6000 200 4000 Temperature (°C) [HC], [CO], [CO2] (ppm) 12000 100 2000 0 0 200 400 600 800 1000 1200 1400 1600 1800 B 2,0E-09 1,5E-09 400 300 m/z = 4 1,0E-09 200 5,0E-10 Temperature (°C) Ionic Current (a.u.) Time (s) 100 m/z = 91 m/z = 71 0,0E+00 0 0 200 400 600 800 1000 1200 1400 1600 1800 Up to 5 zeolite framework types have been used in this work (see Table 1). The structural data of our commercial zeolites, as deduced from their corresponding XRD patterns (not shown here), were found to be consistent with the expected ones [4–8]. Our general aim being to establish relationships between the pore topology, pore size and acidity on the one hand, and the CST results on the other hand, the different structures were arbitrarily classified in three groups depending on the pore topology accessible to HC molecules: Time (s) 600" 140" [C]0 120" 500" 100" 400" 62% 80" C 300" 60" 200" 40" 100" 20" 0" 0" 0" 200" 400" 600" 800" 1000" 1200" 1400" 1600" 1800" Fig. 1. (A) HC (FID signal) and COx emission profiles; (B) MS data and (C) NO, NO2 and NOx profiles monitored for HMOR-20 zeolite during a single CST cycle under a gaseous mixture composed by: 670 ppm propene, 280 ppm toluene and 200 ppm decane (the sum corresponding to the dotted line – 6000 ppm HC equiv. C1 ), 125 ppm NO, 250 ppm CO, 10% O2 and 3% H2 O (balance Ar). Temperature programme: 35–530 ◦ C (v = 20 ◦ C/min). (i) The LTA (Ca5A) and FAU (HY, Si/Al = 5, 15, 100) structures are composed by sodalite cages linked together by d4R units and hexagonal prisms, respectively. The particular arrangement of these sodalite cages forms ␣ cages (also called supercages due to their size, approximately 11.8 Å in diameter). Supercages are linked together through windows of 5 (LTA) or 7.4 Å (FAU), and this forms the accessible pore network (due to their molecular size, the HC used in this study can not diffuse through the empty spaces leading to the sodalite cages). (ii) The framework of *BEA (H␤-25) structure displays a system of two interconnected channels (straight and zigzag). These two channels have a pore diameter of 5.6 Å × 5.6 Å and 6.6 Å × 7.6 Å, respectively. The framework of HZSM5-11 zeolite (structure MFI) can be regarded as roughly similar to the one of the *BEA structure, but the pore size is smaller with 5.3 Å × 5.6 Å and 5.1 Å × 5.5 Å in diameter. (iii) The framework of MOR (HMOR-20 zeolite) can be considered as a one-dimensional pore system, with a pore size of 6.5 Å × 7.0 Å. 3.2. Textural characterization (Brooks, model 5850 TE), bubble towers set to well-defined temperatures (45 ◦ C for decane and water; 5 ◦ C for toluene), and heated transfer lines at 120 ◦ C. In each experiment, the gaseous mixture was constituted by: 6000 ppm equiv. C1 of HC (670 ppm propene, 280 ppm toluene and 200 ppm decane; each HC concentration being equal to 2000 ppm equiv. C1 ), 125 ppm NO, 250 ppm CO, 10% O2 and 3% H2 O (balance Ar). The total flow rate was maintained at 250 mL min−1 (GHSV = 20,000 h−1 ). Prior to apply the temperature programme (heating ramp from 35 to 530 ◦ C with v = 20 ◦ C/min) in presence of this gaseous mixture, the concentrations were allowed to equilibrate in a bypass. Hence, the initial (t0 ) points in the experiments presented in CST profiles (Figs. 1 and 2) correspond to the time when the feed gas was sent from the bypass to the adsorbent. The concentrations of the different compounds exiting the reactor were continuously monitored using different detectors. An Eco The porosimetric characteristics (SBET , Sext and Smicro ) summarized in Table 1 indicate that the specific surface area of the different zeolites depend namely on their structural type and spread between 310 m2 g−1 for HZSM5-11 and 835 m2 g−1 for HY15. Rather consistently, large-pore zeolites (HMOR-20, HY-15 and H␤-25) having 12 membered-ring channels or windows also display the highest SBET and Smicro . By contrast, zeolites with 8 or 10 membered-ring openings (HZSM5-11 and Ca5A) have lower specific surface area (Table 1). Only, the Ca5A, HMOR-20 and H␤-25 zeolites display significant contributions of their external surfaces (301, 170 and 215 m2 g−1 , respectively) to the overall specific surface area. Among the series constituted by HY zeolites with three different Si/Al ratios (5, 15 and 100), no specific order was observed about the A. Westermann et al. / Applied Catalysis B: Environmental 158–159 (2014) 48–59 12000 600 12000 600 Cu-9.5/Y HY-5 10000 400 [C]0 200 47 2000 900 1200 1500 4000 200 164 100 0 0 1800 300 600 12000 500 10000 400 160 [C]0 300 380 4000 200 2000 100 [HC], [CO], [CO2] (ppm) 8000 Temperature (°C) [HC], [CO], [CO 2] (ppm) 600 250 10000 6000 1200 1500 600 Pt-3/Y 500 8000 400 240 [C]0 6000 300 4000 200 2000 100 0 0 0 0 300 600 900 1200 1500 0 300 0 600 1800 [HC], [CO], [CO2] (ppm) 260 140 8000 12000 500 10000 400 [C]0 6000 300 375 4000 200 2000 100 0 [HC], [CO], [CO2] (ppm) HY-100 600 Temperature (°C) 12000 300 600 900 1200 1500 HZSM5-11 400 [C]0 6000 300 215 332 4000 600 12000 500 10000 400 230 6000 300 4000 200 2000 100 0 0 300 600 900 Time (s) 200 2000 100 0 0 1200 1500 1800 [HC], [CO], [CO 2] (ppm) [HC], [CO], [CO2] (ppm) 295 0 500 8000 1800 Temperature (°C) Hβ-25 [C]0 1800 300 600 900 1200 1500 1800 Time (s) 12000 8000 1500 600 Time (s) 10000 1200 0 0 0 900 Time (s) Time (s) 10000 1800 Time (s) Time (s) 12000 HY-15 900 Temperature (°C) 600 300 Temperature (°C) 300 6000 0 0 0 400 275 [C]0 2000 100 0 8000 600 Ca5A 500 8000 400 [C]0 6000 370 300 4000 200 2000 100 0 Temperature (°C) 4000 300 Temperature (°C) [HC], [CO], [CO2] (ppm) 8000 500 Temperature (°C) 500 265 [HC], [CO], [CO2] (ppm) 10000 6000 51 0 0 300 600 900 1200 1500 1800 Time (s) Fig. 2. HC (FID signal) and COx emission profiles monitored for the different studied zeolites during the first CST cycle under a gaseous mixture composed by: 670 ppm propene, 280 ppm toluene and 200 ppm decane (the sum corresponding to the dotted line – 6000 ppm HC equiv. C1 ), 125 ppm NO, 250 ppm CO, 10% O2 and 3% H2 O (balance Ar). Temperature programme: 35–530 ◦ C (v = 20 ◦ C/min). 52 A. Westermann et al. / Applied Catalysis B: Environmental 158–159 (2014) 48–59 evolution of the porosimetric characteristics. On the other hand, the specific surface area of HY-5 decreases by 13% after impregnation with PtCl2 (from 756 to 660 m2 g−1 ) and by 19% after successive ionic exchange with Cu2+ (from 756 to 615 m2 g−1 ), respectively. For the Pt-3/Y sample, this decrease has to be explained by to the partial blockage of the Faujasite porous network after impregnation, due to the accumulation of some metallic species at the 12 membered-ring windows or at the pore mouth [9]. Concerning the Cu-exchanged zeolite, the decrease of the SBET and Smicro may be caused by the presence of less dispersed species after the last exchange, as suggested by Diaz et al. [10]. When the exchange degree exceeds 100%, an accumulation of copper ions is expected, which is in line with the present experimental data. 3.3. Acidity studies from FTIR of adsorbed pyridine In zeolites, Brönsted acid sites correspond to the bridged SiOH-Al groups and their amount and strength depend on the Si/Al atomic ratio [11]. Thermal treatments and dealumination procedures used in the syntheses of the different zeolites can partially remove aluminium from the crystal framework. It is assumed that, the aluminium removed from the framework remains in the cavities (extra-framework aluminium, or EFAL) as nanoparticles whose surface induces Lewis acidity, which can be quantified by pyridine adsorption for instance. With the aim of measuring the acidity of the different zeolites both qualitatively and quantitatively, pyridine was preferred to ammonia as molecular probe because its dimensions (kinetic diameter equal to 5.8 Å [12]) better match with those of the adsorbed HC (propene = 4 Å [13], toluene = 5.8 Å [14] and decane = 4.3 Å [13]). Hence, only the sites potentially accessible to the HC were probed by FTIR of adsorbed pyridine (see part 2 for experimental details). In Table 1 are given the concentrations of Lewis and Brönsted acid sites as well as the total acidity (both in ␮mol g−1 ) for each zeolite [15]. Among the different structural types investigated (with Si/Al ratio in the 11–25 range), HMOR-20 and HZSM5-11 display the highest concentrations of Brönsted acid sites (Si(OH)Al hydroxyls) with ∼275 ␮mol g−1 , i.e. about twice those found for H␤-25 and four times more than for HY-15. These trends are overall consistent with those reported in the literature [15–19]. In addition, some Lewis acidity was observed for all samples. Its origin is presumably due to the presence of extra-framework aluminium debris brought by thermal or dealumination processes. Still within the series composed by the different structural types, the H␤-25 zeolite possesses the highest amount of Lewis acid sites ∼140 ␮mol g−1 , with the following order obtained: H␤-25 ≫ HY-15 ∼ HZSM5-11 > HMOR-20. Here, it is possible that the high Lewis acidity of the H␤-25 zeolite arises from the low crystallinity of this material and/or from the possible co-existence of several BEA* polymorphs (as deduced from the existence of broad peaks in its XRD pattern). On the other hand, the above classification could not be applied for zeolite Ca5A because of its pore size incompatible with pyridine and the presence of very strongly adsorbed water (that could not be evacuated) in the micropores. Among the series constituted by HY zeolites, it clearly appears that, the lower the Si/Al ratio (in the range 5–100), the higher the total amount of acid sites (both of Brönsted and Lewis types, Table 1). Moreover, it can be established that our HY-5 zeolite has acidic properties rather comparable to H␤-25 in terms of concentrations of Brönsted acid sites and aluminic extra framework debris (Lewis acid sites). As deduced from Table 1, the successive ion-exchange procedures of HY-5 with Cux+ cations led to a decrease of the Brönsted acidity by a factor 2 while strongly increasing the Lewis acidity (from 137 to 543 ␮mol g−1 for Cu-9.5/Y). The additional Lewis acid sites are due to the presence of Cu2+ and Cu+ cations in SI′ , SII and SII′ positions ([20–26], as also revealed by DRIFTS of adsorbed NO and CO, not shown here) and also possibly to very small clusters not detectable by XRD. By contrast, the impregnation of the Pt salt has a negligible effect on the acidic properties of the parent zeolite (Table 1), and metallic Pt0 particles located on the external surface of the crystallites were detected for the Pt-3/Y zeolite by XRD, confirming the results of Chakarova et al. [27]. 3.4. General interpretation of a cold-start test (CST): case of the HMOR zeolite Since many zeolites were used in this study, the in-depth presentation and interpretation of CST data will be limited to a single zeolite only, for the sake of brevity. In the later sections, only the differences of behaviour between the different zeolitic materials will be outlined and commented in detail. In Fig. 1A, are displayed the total HC and COx emission profiles in the case of the HMOR-20 zeolite. The specific temperature profiles corresponding to the emission of each individual HC (propene (m/z = 42), toluene (m/z = 91) and decane (m/z = 71)), as monitored from Mass Spectrometry, are given in Fig. 1B whereas Fig. 1C represents the evolution of N-containing products (NO, NO2 , N2 O and total NOx ). Starting from Room Temperature, a plateau in the FID profile corresponding to ca 2000 ppm equiv. C1 (Fig. 1A) is detected almost immediately. According to the SM data displayed in Fig. 1B, these HC emissions have to be assigned exclusively to propene, which is not adsorbed on most of the zeolites used in this study (including HMOR-20), with some exceptions that will be put forward later on. The non-adsorption of propene in presence of other HC and inhibitors (water, CO, NO) is in strong contrast with the results obtained by Lopez et al., who measured a significant propene adsorption capacity of 26 mg/g on a Na-MOR zeolite under singleadsorption conditions [1]. Hence, it can be hypothesized that either the presence of the heavier HC or the inhibitors may be responsible of these discrepancies. In that respect, some studies have reported that the mordenite structure is rather unselective for the adsorption of different HC present in a mixture [28]. Hence, it seems reasonable that the water molecules present in rather large amounts in the feed gas (3%) are the main inhibitors for propene, due to an adsorption competition onto the same acid sites [29–32]. Most of the “missing” HC (area below the [C0 ] line) at low temperatures (35–250 ◦ C, Fig. 1A) are therefore due to the storage of toluene and decane in the straight channels of the HMOR-20 zeolite. The simultaneous occurrence of desorption processes is visible from the increase of HC emissions, starting from ca 100 ◦ C, up to 340 ◦ C (maximum). MS data (Fig. 1B) reveal that toluene and decane are desorbed simultaneously on HMOR-20. The reported HC adsorption mechanism on zeolites of Mordenite-type, known also as “single-file diffusion” [28], is thought to involve first the strong adsorption of toluene in the straight channels of the unidimensional structure, which is promoted by the high acid strength of the HMOR-20 zeolite and the presence of many acid sites (see Table 1). Toluene adsorbed molecules act as plugs for the decane molecules intercalated between them. Hence, decane molecules cannot escape the channels until toluene has desorbed, both processes taking place simultaneously. Above 340 ◦ C, HC emissions progressively decline due to: (i) oxidation processes, as witnessed by the detection of significant amounts CO and CO2 in the gas phase (Fig. 1A); (ii) the occurrence of coking reactions (for a minor part) revealed by the darkening of the sample. Though the HC oxidation mechanism is beyond the scope of this study, it may involve the activation of molecular oxygen by Brönsted acid sites as a first step [33]. At 530 ◦ C, all the incoming or desorbing HC are converted to COx on HMOR-20 zeolite (CO2 /CO = 4, Table 2). A. Westermann et al. / Applied Catalysis B: Environmental 158–159 (2014) 48–59 53 Table 2 Desorption temperature maxima, amount of COx emitted, CO2 /CO ratio, percentage of hydrocarbon trapped, percentage of hydrocarbon eliminated, DeNOx window and NOx conversion to N2 determined from a single CST cycle. HC trappeda (%) HC eliminatedb (%) DeNOx window (◦ C) NOx conversion to N2 (%) 1.6 1.9 4.0 2.1 ∞ 4 36 56 62 18 20 47 36 64 29 2.1 11.0 32 43 53 52 63 81 12 25 47, 265 2.91 5.3 44 53 47, 240 164, 275 5.35 5.40 ∞ 5.8 43 69 71 75 260–500 160–500 315–500 120–500 340–450 110–250 390–490 165–270 350–450 120–300 300–530 Samples Desorption T (◦ C) nCOx (mmol/g) Ca5A H␤-25 HMOR-20 HZSM5-11 HY-100 120, 370 230, 295 340 215, 332 160, 240, 375 0.65 1.30 1.75 2.50 0.23 HY-15 160, 250, 380 HY-5 Pt-3/Y Cu-9.5/Y a b CO2 /CO 47 42 63 Net percentage of hydrocarbons trapped (%adsorbed − %desorbed ) onto the adsorbent between 35 and 250 ◦ C. Percentage of hydrocarbons eliminated as CO2 and/or coke between 35 and 530 ◦ C. To sum up the behaviour of HMOR-20 concerning its reaction with HC, it can be noted that the percentage of HC eliminated (the major part by oxidation and a minor part by coking) over the entire CST cycle (from 35 to 530 ◦ C) is 36% (Table 2 and Fig. 3B). More interestingly, the percentage of HC trapped by sorption from 35 to 250 ◦ C represents 56% of the total amount of HC sent on the catalyst over the same temperature range (Table 2 and Fig. 3A). We will refer to this parameter later on as “trapping efficiency”. The trapping efficiency is of particular significance for a practical implementation because the HC re-emitted at temperatures above 250 ◦ C can be completely oxidized by a DOC (for instance a Pt-supported material) placed downstream in the exhaust line. Another parameter of interest is the elimination of NOx during a CST cycle. In Fig. 1C, the NOx concentration at the reactor outlet remains overall equal to the C0 value below 300 ◦ C and then passes Fig. 3. (A) Trapping efficiencies corresponding to the fractions of HC trapped below 250 ◦ C; (B) fractions of HC eliminated over a whole CST cycle (from 35 to 530 ◦ C). The fraction eliminated as COx is indicated in blue. The term “missing HC” refers to the fraction of HC that was either deposited as coke or that could not be quantified. to a minimum at 450 ◦ C. In absence of any detected NO2 and N2 O, it can be calculated that 62% of the incoming NOx are reduced to N2 at this temperature, the “DeNOx ” window spreading between 315 and 500 ◦ C. The production of N2 occurs through HC-SCR reactions catalyzed by the HMOR-20 zeolite. It is expected that either the stored HC or their cracking products act as reducting agents for NOx , the active sites for NO reduction to N2 being the protons or still, some Al3+ defects [34]. 3.5. Comparison between the CST behaviour of the different zeolites In Fig. 2, are compared the total HC and COx (CO and CO2 ) emission profiles recorded during a CST cycle for the different zeolites investigated. The trapping efficiencies, defined thereafter as the total sum of HC stored below 250 ◦ C, are compared in Fig. 3A, whereas the fractions of HC eliminated (as COx and/or coke) over a whole CST cycle (between 35 and 530 ◦ C) are given in Fig. 3B. The temperature profiles of NO, NO2 , Total NOx and N2 O are also compared in Fig. 4. Complementary quantitative informations useful for a fine characterization of each zeolite behaviour are summarized in Table 2. In order to make the discussion easier, groups of zeolites were distinguished and analyzed separately, according to the Si/Al ratio, the nature of the cation and the type of structure. 3.5.1. Effect of the Si/Al ratio The first group is constituted by HY zeolites with increasing Si/Al ratio (5, 15 and 100). Whatever the Si/Al ratio, propene is not adsorbed in presence of inhibitors on HY zeolites, excepted slightly for the most acidic one, HY-5. By contrast with the CST profile of HMOR-20 (Fig. 1), the FID profile of HY-100 is more complex with the presence of two well-separated peaks (Fig. 2). As deduced from SM data (not shown here), the first one at 140 ◦ C is related to the emission of a weakly-held form of toluene, while the second at 260 ◦ C and the shoulder at 375 ◦ C are namely due to the desorption of decane. Oligomers and cracking products arising from secondary reactions of decane (and toluene disproportionation) with strong acid sites also contribute to the second (high-temperature) peak. Interestingly, it is worth noting that the temperature of the toluene desorption peak follows a trend opposite to that of the Si/Al ratio. Hence, the single broad and unresolved peak observed at 260 ◦ C for HY-5 (Fig. 2) corresponds both to the desorption of decane and a strongly held form of toluene. The HY-15 zeolite displays an intermediate behaviour, the separation between the low- and high-temperature peaks being still visible, though narrower than for HY-100. These evolutions can be rationalized by considering the existence of a higher amount of acid sites when the Si/Al ratio decreases (Table 1). For HY-5, toluene molecules have more chance A. Westermann et al. / Applied Catalysis B: Environmental 158–159 (2014) 48–59 54 180" 600" 180" HY-5 160" 150" "(ppm)" 500" 120" 400" 90" °C) " [C]0 600" Cu-9.5/Y 500" 140" 120" [C]0 400" 100" 300" 300" 80" 60" 200" 30" 100" 60" 200" 40" 100" 20" 0" 300" 0" 600" 900" 1200" 1500" 0" 0" 1800" 0" 300" 600" 900" 1200" 1500" 0" 1800" Time (s) 180" Pt-3/Y 600" 180" 500" 150" 120" 400" 120" 400" 90" 300" 90" 300" 60" 200" 60" 200" 30" 100" 30" 100" HY-15 150" 600" 500" [C]0 ," " [C]0 0" 300" 0" 180" 600" 900" 1200" 1500" HY-100 150" [C]0 0" 1800" 0" 0" 600" 180" 500" 150" 300" 600" 900" 1200" 1500" 0" 1800" 600" HZSM5-11 500" [C]0 400" 120" 90" 300" 90" 300" 60" 200" 60" 200" 30" 100" 30" 100" 400" ," 2 ," x 120" 0" 0" 300" 600" 900" 1200" 1500" 180" Hβ-25 150" 0" 1800" 0" 0" 600" 180" 500" 150" 300" 600" 900" 1200" 1500" 0" 1800" 600" Ca5A 500" [C]0 [C]0 120" 400" 120" 400" 90" 300" 90" 300" 60" 200" 60" 200" 30" 100" 30" 100" 0" 0" 300" 600" 900" 1200" 1500" 0" 1800" 0" 0" 0" 300" 600" 900" 1200" 1500" 1800" Fig. 4. NO, NO2 , total NOx and N2 O profiles monitored during a single CST cycle for the different studied zeolites under a gaseous mixture composed by: 670 ppm propene, 280 ppm toluene and 200 ppm decane (the sum corresponding to the dotted line – 6000 ppm HC equiv. C1 ), 125 ppm NO, 250 ppm CO, 10% O2 and 3% H2 O (balance Ar). Temperature programme: 35–530 ◦ C (v = 20 ◦ C/min). A. Westermann et al. / Applied Catalysis B: Environmental 158–159 (2014) 48–59 55 to interact strongly with acid sites because the density of these sites within the Y framework is considerably higher than for HY-100. In that respect, it is well known that the preferred siting of toluene consists in interactions between Lewis acid sites and the aromatic ring whereas hydrogens of the methyl group point towards the zeolite framework oxygens [35]. In the lack of available acid sites, dispersive (weak) interactions with the pore walls eventually occur and this interaction mode should predomine at the high Si/Al ratio. This explains the trends observed for the adsorption of toluene on the series constituted by HY zeolites. Still within the same series, it is worth noting that the global trapping efficiency below 250 ◦ C (Fig. 3A) decreases with the Si/Al ratio, passing from 44% for HY-5 to only 18% for HY-100. Interesting trends are also observed concerning the transient HC oxidation to COx (Fig. 3B) and NOx conversions to N2 (Fig. 4) Considering the catalyzed HC oxidation to COx first, CO2 is detected from 300 ◦ C for HY-5, whereas the onset of oxidation is delayed to 340 ◦ C for HY-15 and 350 ◦ C for HY-100 (Fig. 2). It is worth noting that the total sum of HC eliminated and COx produced over a CST cycle follow the same trend: HY-5 > HY-15 > HY-100 (Fig. 3B). Hence, it can be concluded that the ability of the HY zeolites to oxidize the HC depend on their relative acidities in the Si/Al range 5–100 (Table 1), the most acidic one (HY-5) having both the best trapping efficiency and also the best oxidizing properties. Coming now to the reduction of nitrogen oxides, two windows of NOx conversion to N2 are visible (Fig. 4). The first “DeNOx ” window is centred at ca 200 ◦ C (zeolites HY-5 and HY-15), whereas the second is at 430 ◦ C (all HY zeolites). From the results of Fig. 4, it seems clear that the amounts of NOx converted to N2 at these temperatures are somewhat correlated to the availability and strength of acid sites (Brönsted and Lewis) on the different zeolites through their Si/Al ratio. Here again, the following order is obtained: HY-5 > HY-15 > HY-100. Acid sites are not only involved in the dissociation of NO to N2 , but are also expected to contribute to some crucial steps of the DeNOx mechanism, such as the NO oxidation to NO2 , and/or the secondary reactions of the trapped HC with the NO2 formed [34]. About the latter, it has been reported that cracked products or molecular fragments (such as CHx entities from decane cracking) can often be considered as better reductants than their parent HC. 240 ◦ C (Fig. 2). Above this temperature, all the incoming HC and CO (present as 250 ppm in the feed) undergo a total (catalytic) oxidation to CO2 (and H2 O), these processes being catalyzed by the Pt0 crystallites located on the HY external surface. Hence, the Pt-3/Y can be considered to have a “DOC-like” behaviour. By contrast, the Cu-9.5/Y has less oxidizing properties, as witnessed for instance by the existence of residual CO emission at 530 ◦ C (end temperature of CST cycle). Nevertheless, the amounts of HC converted to COx on Cu-9.5/Y are much higher than for the parent HY-5 zeolite. Consistently with their enhanced oxidation properties, the Cuand Pt-containing Y zeolites display the highest fractions (about 70–75%) of HC eliminated over a whole CST cycle of all the zeolites investigated (Fig. 3B). As expected from their HC emission profiles, the Cu- and Ptcontaining zeolites displayed also NOx reduction features widely different from those observed for the parent HY-5 zeolite (Fig. 4). The Pt-3/Y zeolite is characterized by a broad peak of NOx conversion at low temperature (120–350 ◦ C) and centred at ca 240 ◦ C. Consistently, the peak temperature of NOx conversion also corresponds to the brutal oxidation of the stored HC, while N2 O is simultaneously detected. At higher temperatures, the NOx conversion quickly decline because all the incoming HC are fully converted to CO2 and there is no more HC available to reduce the NOx . This behaviour is typical of a supported oxidation catalyst containing metallic platinum particles [37]. By contrast, the Cu-9.5/Y zeolite exhibit very different NOx conversion profiles (Fig. 4): (i) a NO desorption peak is observed at low temperature (<100 ◦ C) assigned to Cux+ -NO nitrosyls [23]; (ii) the low-temperature DeNOx peak at 200 ◦ C observed for HY-5 becomes hardly visible; (iii) a broad DeNOx window is observed at medium-high temperatures (300–530 ◦ C) with a maximal NOx conversion to N2 of 63% at 500 ◦ C. In this study, it has been shown by FTIR of adsorbed pyridine that more than half of the protons were exchanged with Cux+ species and that the amount of Lewis acid sites increased by a factor 3 (Table 1). Therefore, the changes observed in the proportion and nature of active sites affect the DeNOx mechanism: the lowtemperature peak observed for HY-5 and HY-15 at 200 ◦ C can now be attributed to HC-SCR reactions on H+ sites, whereas the mediumhigh temperature peak involves DeNOx reactions on Cux+ sites, and to a lesser extent Al3+ defects. 3.5.2. Effect of the charge-compensating cation The incorporation of metallic species into the HY-5 zeolite by impregnation (Pt-3/Y) or ion-exchange (Cu-9.5/Y) induces some obvious changes on the CST behaviour (Fig. 2). Interestingly, the Cu-9.5/Y zeolite displays a zero HC emission profile during the first 200 s, meaning that all types of HC, including propene, are adsorbed on this catalyst. By comparison with the parent HY-5 zeolite (Fig. 2), a new emission peak is detected at 164 ◦ C, corresponding to the desorption of propene adsorbed at lower temperatures on Cux+ sites. Among the different zeolitic materials investigated, it is worth noting that only the copper-exchanged Y zeolite was able to trap propene significantly in presence of the other HC and the inhibitors. This seems consistent with the fact that Ag+ and Cu+ cations at exchangeable positions have the ability to efficiently activate double bonds of many types of molecules [35,36]. As summarized in Fig. 3A, the enhanced adsorption of unsaturated HC on Cu-9.5/Y accounts for its high trapping efficiency (69%, Fig. 3A), the highest of all the zeolites investigated. By contrast, the Pt-3/Y zeolite displays a trapping efficiency similar to that of the parent zeolite (44%). By contrast with HY-5, Cu-9.5/Y and more peculiarly Pt-3/Y display much less HC emissions in the 250–530 ◦ C range, due to the occurrence of oxidations reactions promoted by metal sites. For the Pt-3/Y material, the brutal combustion of the stored HC is responsible of both the exotherm and the very sharp CO2 peak visible at ca 3.5.3. Effect of the structure Some differences existing between the HY (5, 15, 100), H␤-25, HZSM5-11, HMOR-20, and Ca5A zeolites in terms of pore topology and relative acidity were already outlined in Sections 3.1 and 3.3. As expected, significant different behaviours are observed in Fig. 2, when comparing the CST profiles of these materials. The behaviour of both HY and HMOR zeolites during a coldstart test was already described in previous sections. Among all the zeolites investigated, the Ca5A zeolite displayed the highest HC emissions at low temperatures (Fig. 2), i.e. a very weak trapping efficiency (4%, Fig. 3A). In fact, the narrow windows in the LTA structure prevent the bulky toluene molecules to enter the porosity, as confirmed by MS data. Because propene is also weakly adsorbed, most of the missing HC at T < 350 ◦ C are solely due to the storage of decane, whose curved conformation in the gas phase is compatible with the pore size. For the Ca5A material, the existence of two broad emission peaks at ca 150 and 380 ◦ C reveal a dual adsorption, on the external and internal surfaces, respectively (Fig. 2). Decane oxidation only starts above 380 ◦ C and is rather limited, as witnessed both by the weak amounts of COx detected (Fig. 2) and the CO2 /CO ratio close to 1 (Table 2). Simultaneously to decane desorption/cracking, a small window of NOx conversion (to N2 and NO2 ) is detected around 400 ◦ C (Fig. 4). By comparison with Ca5A zeolite, the other zeolitic materials did not show phenomena of steric hindrance during exposition to the A. Westermann et al. / Applied Catalysis B: Environmental 158–159 (2014) 48–59 56 HY-5 10000 HC HC 8000 [HC] (ppm) [HC] (ppm) 8000 HY-100 10000 6000 4000 2000 6000 4000 2000 0 0 30 130 230 330 430 530 30 130 3000 2000 [CO] (ppm) [CO] (ppm) 430 530 430 530 430 530 430 530 CO 2500 1500 1000 500 2000 1500 1000 500 0 0 30 130 230 330 430 530 30 130 230 330 Temperature (°C) Temperature (°C) 12000 12000 CO2 10000 CO2 10000 8000 [CO2] (ppm) [CO2] (ppm) 330 3000 CO 2500 6000 4000 2000 8000 6000 4000 2000 0 0 30 130 230 330 430 530 30 130 230 330 Temperature (°C) Temperature (°C) 180 180 NOx 150 NOx 150 120 [NOx] (ppm) [NOx] (ppm) 230 Temperature (°C) Temperature (°C) 90 60 30 120 90 60 30 0 0 30 130 230 330 430 530 30 130 Temperature (°C) Cycle 1 230 330 Temperature (°C) Cycle 2 Cycle 3 Cycle 4 Fig. 5. Total HC, CO, CO2 and NOx emissions measured through 4 consecutive cold-start cycles for HY-5 (left) and HY-100 (right) zeolites. A. Westermann et al. / Applied Catalysis B: Environmental 158–159 (2014) 48–59 Pt-3/Y 10000 [HC] (ppm) [HC] (ppm) 6000 4000 6000 4000 2000 0 0 30 130 230 330 430 30 530 Temperature (°C) 3000 2500 2500 2000 2000 [CO] (ppm) [CO] (ppm) 130 1500 230 330 430 530 430 530 430 530 430 530 Temperature (°C) 3000 CO CO 1500 1000 1000 500 500 0 0 30 130 230 330 430 30 530 230 330 12000 CO2 10000 130 Temperature (°C) Temperature (°C) 12000 CO2 10000 8000 [CO2] (ppm) 8000 [CO2] (ppm) HC 8000 2000 6000 4000 6000 4000 2000 2000 0 0 30 130 230 330 430 30 530 130 Temperature (°C) 180 230 330 Temperature (°C) 180 NOx 150 NOx 150 [NOx] (ppm) 120 [NOx] (ppm) Cu-9.5/Y 10000 HC 8000 57 90 60 120 90 60 30 30 0 0 30 130 230 330 430 530 30 130 330 Temperature (°C) Temperature (°C) Cycle 1 230 Cycle 2 Cycle 3 Cycle 4 Fig. 6. Total HC, CO, CO2 and NOx emissions measured through 4 consecutive cold-start cycles for Pt-3/Y (left) and Cu-9.5/Y (right) zeolites. 58 A. Westermann et al. / Applied Catalysis B: Environmental 158–159 (2014) 48–59 HC mixture. Only H␤-25 and HZSM5-11 were able to trap propene, however in much smaller amounts than for Cu-9.5/Y (Fig. 2). More peculiarly, the HZSM5-11 zeolite displays an unusual behaviour with all types of HC being suddenly trapped together in the course of the heating, at ca 130 ◦ C (Fig. 2), whereas propene emissions were detected below this temperature. For the MFI structure, it can be recalled that toluene is preferentially adsorbed at the intersections between the straight and zigzag channels [38]. Its molecular size (5.6 Å) being rather comparable to those of the channels, its diffusion within the internal porosity is rather uncomfortable, especially at the lowest temperatures. Hence, it is possible that propene, which has been reported to adsorb mostly in the “zigzag” channels [38,39], is not able to enter them due to the blockage of intersections by toluene molecules. Overall, the CST behaviour of HZSM5-11 probably stands for an easier diffusion of the HC at the pore mouth or within the internal porosity of the MFI structure as the temperature increases. Another peculiarity of the HZSM5-11 zeolite lays in its ability to promote HC oxidation from ca 250 ◦ C, a temperature much lower than for the other protonated zeolites (in most cases around 300–350 ◦ C). As a result, only weak HC emission peaks are observed at 215 ◦ C (toluene + decane) and 332 ◦ C (decane, propene and cracking products) (Fig. 2). Also worth noting, the HZSM5-11 zeolite displays a strong NOx conversion to N2 of 81% at 220 ◦ C and a low-medium NOx conversion above 350 ◦ C (Fig. 4). Finally, the CST profile corresponding to H␤-25 displayed two overlapping HC peaks, at 230 and 295 ◦ C, respectively (Fig. 2). As deduced from MS data, these peaks have to be assigned mostly to decane, while the contributions of propene and toluene only slightly account for the second (high-temperature) peak. On this material, oxidation reactions start at 350 ◦ C, and the CO2 /CO ratio was one of the lowest of all the zeolites investigated. Moreover, this material was found to be medium-active for the transient reduction of NOx, as witnessed by the broad DeNOx window, spreading between 160 and 500 ◦ C, and the 52% conversion to N2 at 320 ◦ C (Fig. 4). The NOx conversions observed at high temperatures have probably to be put in relation with the high amount of Lewis acid sites measured for this zeolite (Table 1). In absence of steric limitations, the best trapping efficiencies were found for the most acidic zeolites, HMOR-20 and HZSM-11 with 56 and 62%, respectively, of HC trapped below 250 ◦ C (Fig. 3A and Table 1). The fractions of HC eliminated over a whole CST cycle (Fig. 3B) are less informative, because a significant part of these HC is not converted to COx but rather to coke or other products. 3.6. Stability tests Four zeolites were selected to undergo four consecutive CST cycles in order to assess their relative stability under cold-start conditions (Figs. 5 and 6). In the series of HY zeolites (Fig. 5), the hydrophobic and nonacidic HY-100 zeolite was found to be the most stable concerning both the HC, COx and NOx emission features, with no appreciable changes all along the different cycles. By contrast, an evolution was observed for the HY-5 zeolite (Fig. 5), namely between the first and second cycles (the material was found to be stable afterwards). During cycles 2–4: (i) propene is slightly better adsorbed; (ii) an increase of the amount of HC desorbed around 250 ◦ C is observed; (iii) less CO and more CO2 is produced at mediumhigh temperature; (iv) the NOx conversion to N2 at 200–220 ◦ C decreases significantly. It is possible that the origin of these different behaviours arise from coking reactions, which are more likely to take place on the acidic HY-5 zeolite during the first CST cycle. Coke deposits may indeed influence the adsorption and diffusion of HC in the vicinity of acid sites, and affect the activity of H+ sites in DeNOx reactions [40]. Also, they can slightly decrease the hydrophilicity of the HY-5 internal surface, this being a promoting parameter for the adsorption of propene in presence of water. The repetition of several CST cycles led to contrasted behaviours for the Pt-3/Y and Cu-9.5/Y zeolites (Fig. 6). Here also, the main changes are observed after the first cycle, the materials being rather stable afterwards. Starting with the Pt-3/Y zeolite, it can established that: (i) the propene trapped at low temperature increases after cycle 1; (ii) HC oxidation reactions to CO2 are promoted, resulting in lesser HC emissions; (iii) the NOx conversion increases due to an enhanced oxidation of NO to NO2 (above 230 ◦ C) and more intense N2 O emissions (below 230 ◦ C, not shown in Fig. 6). Thus, this reflects a change in the state of embedded platinum species, which occurs during the first CST cycle. Hence, the evolutions observed seem consistent with a further reduction and sintering of supported Pt particles, due to the presence of HC and/or water. On the Cu-9.5/Y zeolite, propene and NO adsorptions, which were strongly promoted by Cux+ sites at temperatures below 100 ◦ C during the first cycle, are hindered in cycles 2–4 (Fig. 6). Also, significantly less CO emissions were detected above 350 ◦ C and the NOx conversion to N2 is also negatively affected within the same temperature range. Though more investigations are needed to clarify the structural changes occurring in the catalyst, this probably points out a re-distribution and/or a partial reduction of Cux+ sites after the first cycle. 4. Conclusions In this study, a methodology was elaborated in order to investigate both qualitatively and quantitatively the behaviour of nine zeolitic materials under simplified conditions targeted to mimic those of diesel exhausts during the cold-start period (rapid heating from 35 to 530 ◦ C under a mixture composed by 670 ppm propene, 280 ppm toluene, 200 ppm decane, 125 ppm NO, 250 ppm CO, 10% O2 and 3% H2 O (balance Ar)). The selected zeolites were characterized according to their structural, textural and acidic properties. The effects of some chemical parameters such as the Si/Al ratio or the nature of the charge-compensating cation were further evaluated for Y zeolites with Faujasite-type structure. Namely, a “screening” of zeolites was performed according to their: (i) relative HC trapping properties at low temperatures; (ii) their ability in eliminating (namely by oxidation) the unburnt HC over a whole CST cycle; (iii) their performances in the Selective Catalytic Reduction (HC-SCR) of NOx ; (iv) their stability through several consecutive CST cycles. The results showed that: - The amounts of HC adsorbed under 250 ◦ C overall increases with the acidity of zeolites, even in presence of inhibitors such as water, CO or NO. This is due mainly to the strong affinity of Brönsted acid sites and/or metallic species (exchanged Cux+ cations, and Pt0 nanoparticles) for the adsorption of unsaturated HC, such as propene and toluene. By contrast, weakly-acidic and/or hydrophobic zeolites (such as HY-100) are more selective for decane adsorption and are less interesting for cold-start applications; - As expected, metal-loaded zeolites (namely with Pt) are the most efficient in oxidizing the trapped and/or incoming HC above 250 ◦ C. Hence, they display the highest fractions of HC eliminated over a whole cold-start cycle. The Pt-3/Y zeolite is stable after one single CST cycle and has the typical behaviour of a Diesel Oxidation Catalyst. Though the HZSM5-11 zeolite has non-negligible HC oxidizing properties, zeolites in protonated forms are overall less efficient for HC oxidation and more subjected to coking; A. Westermann et al. / Applied Catalysis B: Environmental 158–159 (2014) 48–59 - Among the zeolites investigated, the most acidic H+ /zeolites (such as HZSM5-11 and HMOR-20) and the Cu-9.5/Y zeolite display also the best transient NOx conversion to N2 . 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