Activity of an iron Colombian natural zeolite as potential geo-catalyst for NH3-SCR of NOx

Accepted Manuscript Title: Activity of an iron Colombian natural zeolite as potential geo-catalyst for NH3 -SCR of NOx Authors: John-Freddy Gelves, Ludovic Dorkis, Marco-A. ´ M´arquez, Andr´es-Camilo Alvarez, Lina-Mar´ıa Gonz´alez, A´ıda-Luz Villa PII: DOI: Reference:

S0920-5861(18)30033-6 https://doi.org/10.1016/j.cattod.2018.01.025 CATTOD 11215

To appear in:

Catalysis Today

Received date: Revised date: Accepted date:

26-9-2017 11-1-2018 21-1-2018

Please cite this article as: John-Freddy Gelves, Ludovic Dorkis, Marco-A.M´arquez, ´ Andr´es-Camilo Alvarez, Lina-Mar´ıa Gonz´alez, A´ıda-Luz Villa, Activity of an iron Colombian natural zeolite as potential geo-catalyst for NH3-SCR of NOx, Catalysis Today https://doi.org/10.1016/j.cattod.2018.01.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Activity of an iron Colombian natural zeolite as potential geo-catalyst for NH3-SCR of NOx John-Freddy Gelvesa, Ludovic Dorkisb, Marco-A. Márquezc, Andrés-Camilo Álvarezd, Lina-María Gonzálezd,*, Aída-Luz Villad a

Grupo de investigación competitividad y sostenibilidad para el desarrollo, Facultad de Ingeniería, Universidad Libre seccional Cúcuta, Cúcuta, Colombia b

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Grupo de investigación en Catálisis y Nanomateriales, Facultad de Minas, Departamento de Materiales y Minerales, Universidad Nacional de Colombia sede Medellín, Medellín, Colombia

Grupo de Mineralogía Aplicada y Bioprocesos, Facultad de Minas, Departamento de Materiales y Minerales, Universidad Nacional de Colombia sede Medellín, Medellín, Colombia d

Environmental Catalysis Research Group, Engineering Faculty, Chemical Engineering Department, Universidad de Antioquia UdeA, Calle 70 No. 52-21, Medellín, Colombia *

Corresponding author. Tel.: (+57 4) 2196509, (+57 4) 2198535

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Graphical Abstract

Highlights 

Iron Colombian geo-catalysts were active in NH3–SCR of NOx

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H-zeo-Fe3+ exhibited a higher catalytic activity than Nat-zeo and H-zeo-Fe2+ Fe3+ ions and the acidity in H-zeo-Fe3+ explains its activity in NO reduction

E-mail address: [email protected] (Lina-María González) Abstract

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The Selective Catalytic Reduction of NOx by ammonia (NH3–SCR of NOx) was studied over an iron Colombian natural zeolite (Nat-zeo) based geo-catalysts after several treatments (H-zeo-Fe3+ and H-zeo-Fe2+) under lean, dry and wet conditions. Nat-zeo from the Combia geological formation in Colombia (South America) was treated with NH4NO3, calcined in air for obtaining H-zeo-Fe3+ and then reduced with hydrogen to obtain the H-zeo-Fe2+ catalyst. Catalysts were characterized by XRD, FTIR, Mössbauer spectroscopy, SEM/EDX, XRF BET, H2–TPR, NH3–TPD, and NOx–TPD. H-zeoFe3+ showed better catalytic performance than Nat-zeo and H-zeo-Fe2+ in the NOx conversion. The presence of Fe3+ ions and the acidity in the catalyst explains partially the high activity towards NOx reduction. Water inhibits the NO and improves the NO2 adsorption on the catalyst H-zeo-Fe3+ surface according with the NOx-TPD analysis. After 30 h on-stream, under wet conditions, the Hzeo-Fe3+ catalyst showed a decreased in the NOx conversion. The decrease of activity could be related with the loss of catalyst surface acidity detected by NH3-TPD analysis after reaction and with the contraction of the zeolite channels due to the metaheulandite phase formation that was confirmed by XRD, BET and FTIR analysis. Keywords: Geo-catalysts; NH3-SCR; nitrogen oxides; selective catalytic reduction; iron catalysts

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1 Introduction

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Motor vehicles are the main emission source of nitrogen oxides (NOx) into the atmosphere, which reached dangerous levels and projected emission values above 1.6 Mt/year only in European light duty diesel vehicles [1]. NOx are known to cause the formation of acid rain, photochemical smog, and ground-level ozone [2,3]. Ground-level ozone is a very potent greenhouse gas and it has a direct warming effect on climate [3,4]. Several researches are addressed in decreased NOx vehicular emissions varying the engine type, operating conditions, and fuel used [5]. Notwithstanding, these efforts for reducing NOx emissions do not solve completely the problem, and even increase them [5,6]. In the treatment of NOx from exhausts, the three major technologies are lean NOx traps (LNT), ammonia (or urea) Selective Catalytic Reduction (NH3-SCR) and Hydrocarbon Selective Catalytic Reduction (HC-SCR) [7]. The leading technology is the NH3-SCR using zeolites with Fe and Cu ions, which have better hydrothermal stability compared with commercial V2O5 catalysts [8,9], and operates at a wider range of temperatures [8 – 10]. The ironbased catalysts have attracted attention owing to high NH3-SCR activity, low cost and lack of toxicity relative to vanadium [10]. Iron oxides catalysts have been used in the NH3-SCR, but they show low low-temperature activity (between 150 and 300 °C) due to the low surface area and acidity [9,10]. On the other hand, iron-based zeolites have high activity at temperatures above 300 °C and in comparison with copper-based zeolites, they produce fewer amounts of side products such as N2O and have better hydrothermal stability [11]. Volcanic rocks containing natural zeolites have been mined worldwide for more than 1,000 years for use as cements and building stone. Since the 1950’s, natural zeolites have found a variety of applications in adsorption, catalysis, building industry, agriculture, soil remediation, and energy

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[12,13]. The use of natural zeolites for environmental applications is gaining new research interests mainly due to their properties, and significant worldwide occurrence. A conversion of NOx above 80% in NH3-SCR was obtained over natural chabazite with iron, however low stability of the material was observed under hydrothermal conditions [14]. The presence of an iron-rich natural zeolite (around 12 wt% of Fe2O3) with heulandite (HEU) framework mixed with small amounts of the zeolites chabazite (CHA), phillipsite (PHI) and mordenite (MOR) was recently reported in the andesitic basalts from the Combia geological formation in Colombia (South America) [15]. This natural zeolite showed activity for the simultaneous NOx and odichlorobenzene removal (conversions of: NO around 5% and o-DCB around 25% at 550 °C) using methane as reducing agent under wet and lean conditions [16].

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The present study aim is to report the characterization and potential of an iron Colombian natural zeolite [15] as geo-catalyst for NH3-SCR of NOx under lean and hydrothermal conditions. The natural zeolite was tested in the reaction without any treatment and after the proton incorporation for obtaining an acid material. Because Fe3+ and Fe2+ species may have a different effect on NH3-SCR of NO reaction [18-20], this effect was also evaluated on the natural zeolite used as catalyst. The fresh catalysts were characterized by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Mössbauer spectroscopy, scanning electron microscopy with energy dispersive X-Ray spectroscopy (SEM/EDX), X-ray fluorescence spectroscopy (XRF), BET surface area, temperature-programmed reduction under hydrogen (H2-TPR), and temperatureprogrammed desorption of ammonia (NH3-TPD) and nitrogen oxides (NOx-TPD). The catalyst with the best performance in the NH3-SCR of NO reaction was tested during 30 h on-stream for evaluation of its durability; furthermore, the material was characterized after reaction by XRD, FTIR, BET, and NH3-TPD.

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2.1 Catalyst preparation

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2 Experimental

2.1.1 Extraction of natural zeolite

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Natural zeolite used in this work as Geo-catalyst was collected from the riverbed of the creek “La Sucia”, located north of the La Pintada municipality in the state of Antioquia-Colombia (South America) that is part of the volcanic member of the Combia geological formation [15]. The separation process of the zeolite from the rock was performed with dense liquids method [21] using LST heavy liquid from Central Chemical Consulting – Australia (density of 2.25 g cm-3 operating at room temperature) [22]. Initially the rock was manually grinded in an agate mortar, sieved in a Tyler 20 mesh (< 840 μm), and settled in the LST heavy liquid. The material rich in the zeolites (with a density less than 2.25 g cm-3) floated and was separated by filtration. Subsequently, it was cleaned in an ultrasonic bath with distilled water (Elma LC20/H) for 5 minutes in order to remove residues of LST bonded to the surface of the zeolites. Finally the solid was dried at 110 °C in an oven, milled and sieved to a diameter < 250 μm (120 Tyler mesh). This material was coded as Nat-zeo. 2.1.2 Synthesis of H-zeo-Fe3+ and H-zeo-Fe2+ materials In order to facilitate the ammonium ion exchange in the Nat-zeo material, the solid was treated with a 0.5 M solution of NaCl (Merck 99.5%) at 90 °C and 400 rpm magnetic agitation speed, with a

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ratio of (1 g of Nat-zeo) (10 mL of solution)-1. The process was performed for 5 days, renewing the solution every 24 h [17]; then the sample was washed with distilled water until negative presence of chlorine. Finally, the solid was filtrated and dried at 110 °C for 24 h. The sodium rich material obtained was then exchanged with ammonium nitrate (NH4NO3) (Merck 99 %) using a similar procedure as described above. The solid material was calcined in a muffle furnace at 360 °C during 6 h. The final material was coded H-zeo-Fe3+. H-zeo-Fe2+ catalyst was obtained by reduction of Hzeo-Fe3+ with 50 mL min-1 of 10% H2/Ar flow with a heating rate of 10 °C min-1 and at 600 °C for 2 h. 2.2 Catalyst characterization

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XRD patterns were performed at 5° ≤ 2θ ≤ 55º in a Rigaku Miniflex II diffractometer, with graphite monochromator using Cu Kα radiation at 30 KV and 15 mA, the scan step was 0.02°, and the counting time per step was 2 s. The software used for qualitative and semi-quantitative analysis (Rietveld refinement) was PANalytical Xpert Highscore Plus. The American Mineralogist Crystal Structure Database was used for identifying the crystalline phases in the material. The FTIR spectra were recorded in a Shimadzu 8400S spectrophotometer. Spectra were collected at room temperature, in KBr pellets ((1 mg sample) (100 mg KBr pressed at 10 t)-1), using 48 scans, at 4 cm-1 of resolution in transmittance mode, in the range of 4000 and 400 cm-1. The software used for the analysis was IR Solution v.1.4. Local iron ion configurations in the Nat-zeo were determined by Mössbauer spectroscopy. The Mössbauer spectra were collected in transmission geometry at room temperature, using a conventional WisselTM Mössbauer spectrometer, equipped with a 57Co (Rh) source working at constant acceleration. The Mössbauer 57Fe spectrum was calibrated by α-Fe as standard. The spectrum was adjusted and analyzed using the Recoil program in order to obtain the hyperfine parameters (hyperfine field, quadrupole and isomeric change). External morphology of the natural zeolite was evaluated with JEOL scanning electron microscopy, model JSM-5910LV. The chemical microanalysis was done using SEM coupled with EDX from INCA Oxford instruments (accelerating voltage of 15 kV, counting time of 80 s and working distance of 10 mm). The chemical composition of the natural zeolite was performed in an Axios X-ray fluorescence spectrometer from PANalytical, using the software WROXI. For the XRF analysis, the sample was compacted into a pellet at 20 t pressure. The surface area, pore volume and pore size distribution were determined by BET analysis in an ASAP 2020 V4.00 (V4.00 J) equipment from Micromeritics (sample degassed at 523 K for 12 h, and N2 adsorption at 77 K). H2–TPR and NH3–TPD experiments were carried out in an AutoChem II 2920 instrument (Micromeritics) equipped with a thermal conductivity detector (TCD). H2–TPR of fresh samples were obtained by flowing 50 mL min–1 of a mixture 10% H2/Ar and heating from 50 to 1050 ºC using a heating rate of 10 ºC min–1. Before H2– TPR analysis, the samples were treated under 50 mL min–1 of flowing argon at 350 ºC for 2 h. NH3– TPD was used to determine the total acidity of samples. Before analysis the samples were flushed with flowing He (50 mL min–1) for 2 h at 350 ºC, then the sample was cooled to 50 ºC. The saturation adsorption of ammonia was performed at 50 ºC by flowing 50 mL min–1 of a gas mixture 5% NH3/He for 60 min. In order to remove any physical adsorbed ammonia, the sample was flushed with 50 mL min–1 of He for 2 h. Desorption of ammonia was obtained by rising the temperature at 10 ºC min–1 up to 700 ºC. For NOx-TPD analysis two types of experiments were performed: NO-TPD and NO2/N2O-TPD in the presence or absence of water vapor. Prior to NOxTPD, 50 mg of the sample was flushed with 50 mL min-1 of an inert gas at 350 °C for 2 h. In the NO-

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TPD, the sample was saturated with 500 ppm of NO at 50 °C for 2 h in the presence of water vapor (0 – 5 % vol). The sample was flushed for 4 h and finally the temperature was raised to 550 °C at 5 °C min-1. A similar procedure was followed in the NO2/N2O-TPD, the sample was saturated with a mixture of 350 ppm NO2 and 20 ppm N2O at 50 °C for 2 h in the presence of water vapor (0 – 5 % vol). The gas concentration was monitored with an Antaris IGS, Thermo Scientific FTIR equipment, equipped with a MCT detector, KBr beamsplitter, 2 m pathlength gas cell, 200 mL gas cell volume and a ZnSe window. Data were collected with a spatial resolution of 0.5 cm–1 with 32 scans for each sampling. Gas cell was operated at atmospheric pressure and the temperature was fixed at 150 ºC in order to avoid condensation of the reactants inside the gas cell. A quantification model was developed by taking several spectrums of known gas mixtures and adjusting the absorbance and concentrations data by means of partial least square quantitative analysis, the software TQ Analyst 9.4.45 (Thermo Fisher Scientific Inc.) was used for this purpose. 2.3 Catalytic reactions

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 NO   NO in  100    N O  in 

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N O C o n v e r s io n  %

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Catalytic reactions were carried out in a quartz tube fixed–bed reactor (0.015 m i.d. x 0.4 m length) containing a frit to hold the powder catalyst samples. The reactor was operated at atmospheric pressure (0.84 atm) in steady state plug–flow mode. Reactor temperature was varied from 150 to 550 °C. The reactant gases were fed using electronic mass flow controllers (Sierra Instruments and Brooks 5850E) and mixed in line before entering the reactor. Ammonia was entered 100 mm above the bed to assure reaction just in the catalyst, and to minimize homogenous reactions. The total flow was 100 mL min-1 (GSHV = 30000 h-1). The gas composition of the reactant mixture was 445 ppm NOx (400 ppm NO, 45 ppm NO2), 400 ppm NH3, 8% O2, 0 - 5% H2O, and argon balance. Water vapor was introduced into the reaction system by means of a saturator at 60 ºC, using Ar as carrier gas. Gases were analyzed by an Antaris IGS, Thermo Scientific FTIR, as described above. NO and NH3 conversions were calculated according to Eq. (1) and Eq. (2), respectively.

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 in   N H 3  o u t  N H 3  in

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Durability test was carried out for 30 h of continues operation at the temperature where the highest conversion was obtained with the catalyst with the best performance.

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3 Results and discussion 3.1 Catalysts characterization 3.1.1 XRD X-ray diffraction (XRD) patterns of Nat-zeo, H-zeo-Fe3+, and H-zeo-Fe2+ catalysts are shown in Fig. 1. According with XRD analysis is possible to establish the presence of three crystalline phases in the Nat-zeo material. Heulandite/clinoptilolite and plagioclase feldspar (anortita and labradorite) are found as major phases, while celadonite appears as minority phase. By means of Rietveld

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refinement it can be established that the heulandite (heulandite + clinoptilolite) phase represents about 70 wt% of the composition of Nat-zeo material, followed by calcium plagioclase 25 wt% (equal amount of anortite and labradorite), and celadonite with 5 wt%. Similar composition in natural zeolites used as catalysts has been previously reported [23-25]. The XRD pattern of the acid catalyst H-zeo-Fe3+ shows much similarity with the diffraction pattern of the starting Nat-zeo material. However, there are two important changes in its pattern. The first one is associated with the decrease in the intensity of the reflection planes of the heulandite phase, Fig. 1. The second aspect is related with the presence the metaheulandite phase. The formation of this phase is associated with the transformation of the heulandite phase at temperatures above 300 °C [26], which in our case occur during the elimination of the NH4+ ions at 360 °C. This dehydrated heulandite exhibits a decrease in the unit cell parameters and in the size of the zeolitic channels [26]. Fig. 1

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In the case of the reduced catalyst H-zeo-Fe2+, the diffraction pattern (Fig. 1) shows that the intensity of the reflection plane (020) of metaheulandite (2θ around of 11°) increased in comparison with the intensity in H-zeo-Fe3+ material. The increment of this phase can be associated with the disappearance of the low stability phase of the heulandite structure during the reduction process at 600 °C; which is evidenced by the notorious decrease in the intensity of the peak at 10°.

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3.1.2 FTIR

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Infrared analysis corroborates the presence of the heulandite structure in the catalysts, Fig. 2. The "fingerprint" region of the heulandite phase in the FTIR spectra of the three catalysts present the characteristic vibrations of this zeolite, such as vibration of the pseudolattice rings at 604 cm-1, the asymmetric stretching vibration of the TO group at 1198 cm-1, and the stretching band of the TO4 group (between 1020 and 1060 cm-1) [27,28]. However, a widening in the band at 604 cm-1 and in the TO4 group bands is evidenced after the heat treatment in the Nat-zeo (calcination of ammonium ions and reduction treatment). This widening is associated with a decreasing in the crystallinity of the heulandite phase [27,28]. FTIR analysis also shows the symmetrical stretching of the OH groups at 3620 cm-1 and 3440 cm-1 [27,28]. The presence of calcite or dolomite in the catalysts is evidenced by the band at 1450 cm-1, which is associated with the asymmetric tension of the CO3 group [27]. Calcite concentration in the catalysts must be low since it was not identified by XRD. Fig. 2

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In the case of the H-zeo-Fe3+ catalyst, the band at 1405 cm-1 corresponds to the vibration of some NH4+ ions that remain as exchange cations in the zeolite [29]. The presence of celadonite and plagioclase is not very noticeable by FTIR, possibly due to the overlapping of the characteristic vibrations bands of the heulandite zeolite. However, the vibration band of the Mg-Fe3+ species of the celadonite at 3550 cm-1 was evidenced in the spectra of natural zeolite (Nat-zeo) and in the catalyst H-zeo-Fe3+ (Fig. 2) [27,28]. In the reduced material H-zeo-Fe2+ this band is no longer evident, possibly due to the transformation of the celadonite to a new ortoferrosilite phase (according to the result of XRD, see Fig. 1).

3.1.3 Mössbauer spectroscopy Mössbauer spectroscopy was used to elucidate the chemical environment and the coordination of the iron ions present in natural zeolite extracted (Nat-zeo). This technique allows checking the presence of the iron in the zeolite or in the minority phases such as celadonite (Fig. 3 and Table 1). Fig. 3

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According with Fig. 3, the Mössbuaer spectrum of Nat-zeo catalyst shows an asymmetric doublet with lower intensity in the left and high dispersion in the baseline, which suggest a low iron content in the sample. Based on these experimental results, it is possible to establish a model considering two sites for the iron in the natural zeolite. The values of the isomeric shift (IS), Table 1, lead us to conclude that the iron in the Nat-zeo material is present as Fe3+ in agreement with literature reports [30-32]. The data adjustment allows to identify the existence of quadrupole splitting in the material spectrum (Fig. 3). This fact implies that the iron in this material is not located in an isometric crystal structure. The distance between the peaks of the sites 1 and 2 (see Fig. 3) allows to corroborate that the iron is found as Fe3+ [30,31,33]. According with the isomeric shift (IS) and quadrupole splitting (ΔQ) we can establish that the iron in "site 1" is in a tetrahedral coordination, while in "site 2" is in octahedral coordination [30,34]. An approximate concentration of the sites 1 and 2 is 79% and 21%, respectively (Table 1).

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Table 1

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3.1.4. SEM/EDX

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The hyperfine parameters obtained for Fe3+ ions at site 2 are quite similar to those reported for the celadonite phase at octahedral sites. Fe3+ in this environment has been associated with the IR vibration band of (Fe3+-Mg)-OH at 3550 cm-1 observed in the FTIR analysis of the Nat-zeo, Fig. 2 [35-37]. Respecting to the site 1, this iron could be present in the structure (tetrahedral) of the zeolite and feldspar type plagioclase [35].

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Fig. 4 shows the morphology of the Nat-zeo material, which allows the recognition of heulandite zeolite crystals with laminar habit and predominance of the plane (020). This observation is in agreement with the high intensity of peak at 2 = 10° found in the XRD analysis (see Fig. 1). The predominant laminar habit in the material could be related with the presence of the potassium, Table 2, which acts as compensation cation in the structure [38]. The presence of celadonite film is observed like a coating the surface of the zeolite crystals, Fig. 4b. Fig. 4

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The chemical microanalysis displayed in Table 2 confirms the presence of heulandite phase due to the predominance of calcium and potassium which are their characteristic compensation cations [38]. However, the presence of calcium as compensating cation reduces the thermal stability of the heulandite structure [26,39]. In the case of the celadonite film, the composition found (Table 2) is typical for this material [15]. The content of Fe3+ present in the celadonite could generate a synergistic effect with the acidic sites suitable for its use in the selective catalytic reduction of NOx [18,23,24]. Table 2

3.1.5 XRF

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The chemical analysis by X-ray fluorescence, expressed as oxides, is presented in Table 3. The main elements present in the natural material are silicon and aluminum, in a Si/Al ratio of 3.76. This fact permits to identify the phases of the zeolite group as a silica heulandite with medium thermal stability [40]. Table 3

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The presence of calcium is associated with the existence of feldspathic and zeolitic phases according to the XRD analysis. Magnesium is directly related to the presence of celadonite and in a lesser extent to heulandite, as evidenced in EDX results. In the case of potassium, it can be associated to the clinoptilolite [41], celadonite [42], and in small amounts to heulandite [43] phases. The amount of iron (4.51 wt% as Fe2O3 or 3.15 wt% as Fe) present in the natural material (Nat-zeo) could be sufficient for being an active site in the NH3-SCR of NOx [18,23,24]. Traces of Ba and Sr in the Nat-zeo are associated to the heulandite phase [44]. Ti, Sr, Mn and other elements are also present; these elements may come from residual phases of basalt after the concentration process, which cannot be identified by FTIR and XRD, or be present as compensation cations in some zeolite crystals.

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3.1.6 BET analysis

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The adsorption/desorption isotherm for the natural material (Nat-zeo) is shown in Fig. 5a and the textural properties obtained from BET analysis of the Nat-zeo and the catalyst H-zeo-Fe3+ before and after reaction are presented in Table 4. The Nat-zeo material shows a typical adsorption/desorption isotherm type IV associated to mesoporous materials, with an hysteresis loop H2 that indicates pores with narrow mouths (ink-bottle pores), uniform channels and connectivity [40,45]. According with this BET analysis is also possible to establish that in Nat-zeo material there is a mixture of microporous (monolayer-multilayer adsorption in p/p0 lower than 0.6) and mesoporous (capillary condensation in mesoporous with p/p0 above 0.9). The mesoporosity in the material can be associated with the presence of the minor phases of phyllosilicates such as celadonite mica. Similarly, heulandite type zeolite with lamellar morphology can generate mesoporosity in the regions where the cleavage planes are given (see Fig. 4a) [46,47].

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An important change is observed in the Nat-zeo after the acid treatment and calcination to obtain H-zeo-Fe3+, Fig. 5b. BET analysis of H-zeo-Fe3+ displays a typical shape of a zeolitic material with an isotherm type I and the hysteresis loop of the type H4 [45,46]. In addition, it is observed a lower average pore size of H-zeo-Fe3+ in comparison with Nat-zeo material (Table 4). Fig. 5 Table 4

3.1.7

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In comparison with many of the synthetic zeolites [48], the surface area of natural materials can be considered low (Table 4); however, these values are typically reported for heulandite and celadonite materials [49,50]. The mesoporosity of the material is reflected by the average pore size of the material (193.8 Å) [49]. After the treatment for obtaining the proton form in catalyst Hzeo-Fe3+, both the pore size and the surface area decrease. This fact can be due to the changes in the crystallinity observed by XRD and FTIR (Fig. 1 and Fig. 2, respectively).

H2–TPR

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The H2–TPR profile of Nat-zeo, is presented in Fig. 6. In the natural zeolite, Nat-zeo, the reduction profile showed the presence of two peaks at 523 °C and 623 °C, associated with the reduction of cations or oxo-cations Fe3+ to Fe2+ ions in different sites of the zeolite [18]. In the case of Fe2+ sites detected in the Mössbauer analysis (Fig. 3), the TPR analysis showed the peaks at 698 °C and 794 °C may associated with the reduction of Fe2+ to Fe0 [18,51,52]. The peaks at 954 °C and 1021 °C may be associated with some residual iron species located in the more stable structure orthoferrosilite (observed in the XRD analysis) [18].

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3.1.8 NH3–TPD

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Fig. 7

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NH3–TPD profiles of Nat-zeo and H-zeo-Fe3+ fresh samples are shown in Fig. 7, and ammonia consumptions for each material are presented in Table 5. The acid–strength distribution is determined with the ammonia desorbed peaks above 100 ºC. Two peaks are observed at around 150 °C and 510 °C which are associated to weak and strong acid sites, respectively [53]. After the modification of the natural zeolite, by ion exchange, is evident the increment in the total acid concentration in the H-zeo-Fe3+ catalyst (Table 5). These changes are directly associated with the type of compensation cation present in the zeolite structure, since it modifies the electronegativity, the ability to form hydroxyl groups and the diffusion of the NH3 into the channels of the zeolite [54-56].

3.2 Activity measurements

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The conversions of NO and NH3 as a function of temperature over the natural zeolite and the modified materials (H-zeo-Fe3+ and H-zeo-Fe2+) are compared in Fig. 8 and Fig. 9, respectively. Fig. 8 Fig. 9 The Nat-zeo material showed a NO conversion around of 20 % in the temperature range between 300 °C and 450 °C. This activity can be associated with the presence of isolated Fe species on the zeolite framework [51], probably in tetrahedral positions, according with Mossbauer spectroscopy

(Fig. 3) [57]. In addition, iron species have high oxygen storage capacity and mobility based on their redox Fe2+/Fe3+ equilibrium [58].

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In our case, with H-zeo-Fe3+ and H-zeo-Fe2+ catalysts (Fig. 8a), the maximum NO conversion was attained at around 400 °C and then it decreased because of NH3 oxidation (Fig. 9). Fe3+ and Fe2+ are active species in the NOx reduction [59,60]. Here the catalyst H-zeo-Fe3+ showed the highest NO conversion. Fe3+ species improve the reactivity of the catalyst [11,29,59]. This fact can be associated with the high concentration of NO2 observed with this catalyst (Fig. 8b), that is the determining step in the SCR process over zeolites [60,61]. NOx can be converted according with the reactions 3 and 4, but in the presence of NO2 the reaction is more efficient due to the existence of a fast SCR reaction [9,59,61]. NO2 can be provided in the inlet gas or produced by the oxidation of NO [59]. 4𝑁𝑂 + 4𝑁𝐻3 + 𝑂2 → 4𝑁2 + 6𝐻2 𝑂

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𝑁𝑂 + 2𝑁𝐻3 + 𝑁𝑂2 → 2𝑁2 + 3𝐻2 𝑂

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Oxidized Fe3+ species interact with NO producing NO2; however, this reaction is controlled by the NO2 desorption [59] and in our case its presence in the gas-phase suggests a slow reduction rate. Regeneration of the Fe2+ sites takes place by the presence of the oxygen in the stream [58,59,62].

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Acidity of the materials also plays an important role in the NH3-SCR of NOx reaction; Wang and coworkers [9] reported than NH4+ ions located at the Brönsted acid sites are unstable and react with the NO2 species generated at the surface to form intermediate species, which further react with the NO to produce N2. Also, the key intermediates in the reaction are NH and NH2 species, which can be generated when NH3 is adsorbed on the Lewis acid sites [63].

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On the other hand, according with Fig. 8b, there is formation of N2O during the NO reduction over these catalysts and it is associated with the presence of NO2 and its reaction with ammonia, Equation 5 [61]. It is noticeable that over the Nat-zeo catalyst the formation of N2O is the highest (Fig. 8b). (5)

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2𝑁𝑂2 + 2𝑁𝐻3 → 𝑁2 𝑂 + 𝑁2 + 3𝐻2 𝑂

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NH3 conversion with the catalysts H-zeo-Fe3+ and H-zeo-Fe2+ (Fig. 9) was almost 100 % at temperatures above 400 °C. The conversion observed at low temperatures over the Nat-zeo and H-zeo-Fe2+ catalysts could be related with the NH3 adsorption at the beginning of the reaction (temperatures above 150 °C) than starts to desorb at about 200 °C. At temperatures above 400 °C, oxidation of NH3 to NO is evident in iron zeolites although ammonia preferentially participates in NO reduction [64]. Effect of water on the performance of the catalyst H-zeo-Fe3+ in the NO and NH3 conversion is shown in Fig. 10a. Water affected NO conversion in great extent and only good activity was observed between 400 and 450 °C. The effect of water at temperatures below 350 °C is because competitive adsorption between H2O and NH3, and NH3 adsorption is a key step of the NO reduction [65,66]. In the presence of water the formation of NO2 is improved and the N2O is avoided (Fig. 10b). NO2 can react with water to form a mixture of nitrous acid (HNO2) and nitric acid (HNO3) which likely

remain adsorbed on the active Fe sites required for the NO oxidation reaction [65]. Regarding to NH3 conversion, the presence of water decreases NH3 conversion until 500 °C, after this temperature the conversion remained almost constant (Fig. 10a). Fig. 10

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The NOx-TPD results over the catalyst H-zeo-Fe3+, Fig. 11, corroborate the facts described above. The NOx-TPD profiles of the catalysts show bands between 100 °C and 550 °C. This indicates the presence of weakly adsorbed NOx and the formation of different nitrogenated species on the catalyst surface [67-70]. In the NO-TPD (Fig. 11a), the peak at 103 °C could be attributed to physisorbed NOx; the peaks at 150 and 190 °C were due to the decomposition of monodentate nitrate species or iron nitrosyl species [71,72]; and the broad peaks above 250 °C were due to the decomposition of nitrate species (bridging and bidentate) with higher thermal stability [73,74]. The presence of water vapor significantly inhibited the NO adsorption as we expected (Fig. 11a) [73]. On the other hand, NO2 and N2O-TPD profiles (Fig. 11b) show the importance of water in their adsorption on the H-zeo-Fe3+ catalyst surface. NO2 presented two desorption broad peaks between 50 – 300 °C and 330 – 550 °C. The amount of NO2 desorbed is higher when the experiment was made in the presence of water, which corroborates the formation of HNO2 and HNO3 on the active Fe sites [65]. N2O desorption is observed at temperatures above 300 °C [74], and the improvement in the adsorption capacity of the catalyst H-zeo-Fe3+ in the presence of water can be related with the formation of Fe(OH)2 species in the catalyst, that can absorb N2O [75].

3.3 Catalyst stability

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Fig. 11

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A durability test of the catalyst H-zeo-Fe3+ at 400 °C under wet conditions is shown in Fig. 12. After the first 4 h of reaction the NO conversion decreased from 98% to 80% while the NH3 conversion increased from 83% to 97%. With 30 h on stream the NO conversion decreased until 65% and the NH3 conversion until 93%. However, the loss of conversion in time seems to be frequent in catalysts made from naturally occurring materials and in some zeolites [38,76,77]. Two causes could be associated to the blockage of active sites by the water that affects the NO oxidation reaction [65], or the deactivation of the NO/NH3 adsorption sites and the degradation of the structure by water effects and reaction time. Fig. 12

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In order to corroborate the previous approaches, infrared spectroscopy (Fig. 2), X-ray diffraction (Fig. 1), textural properties (Table 4) and TPD-NH3 analysis (Table 5) were performed to establish structural variations in the used H-zeo-Fe3+ catalyst. The XRD pattern of the H-zeo-Fe3+ catalyst before and after 30 h reaction in presence of water at 400 °C showed some structural variations in the catalyst, Fig. 1. Two aspects are evident in the material, the first of them is an increment in the metaheulandite phase (2θ = 11°), which is derived from the low thermal stable heulandite (as described in section 3.1.1). The second aspect is the decrease of the interplanar distance of the plane (020) of the heulandite from 9.07Å to 9.00 Å in the used catalyst (Fig. 1). These two aspects

suggest a decrease in the cross-sectional area of the zeolite channels, affecting also the diffusion of the reactants towards the active centers of the catalyst.

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The FTIR of the used catalyst did not show new bands, Fig. 2. The absence of the bands at 1387 cm-1 and 1242 cm-1 discards the formation of solid deposit of nitrate (NO3-) and nitrite (NO2-) ions in the used catalyst [24,57,78]. The band at 1405 cm-1 associated with the vibration of the ammonium ions in the H-zeo-Fe3+ catalyst remains after the durability test [78]. The presence of these ions in the fresh catalyst is associated with the cation exchange process and the low calcination temperature used (360 °C, 6 h), which is not sufficient for their total decomposition towards NH3. In the case of the material after reaction, a small increase in the intensity of the vibration of the NH4+ band is evidenced and it could be associated with the adsorption of NH3 on the Brönsted sites of the zeolite. Fig. 2 also shows changes in the vibration band associated to the TO4 group (T: Si or Al) of the heulandite structure in the used catalyst, observed between 1020 cm1 and 1070 cm-1 [28]. Mozgawa et al [28] correlated the displacement of this band with changes in the Si/Al ratio of the zeolite. It was established for the heulandite structure with high silicon content that there is a shift of the band in the direction of the near infrared, whereas an increase in the content of aluminum will lead to a displacement towards the far infrared [28]. The TO4 band in the used catalyst shifted from 1040 cm-1 to 1067 cm-1, compared with the material before reaction, which could be associated to dealumination of the catalyst. The dealumination process has been associated to the presence of water vapor and the high reaction temperature (500 °C). This fact may lead to the reduction of Brönsted acid sites, as well as the structural/textural degradation of the zeolite, which may affect catalyst activity [24,79,80]. However, the adsorption of water and ammonia during the reaction can also cause this shifted since the OH or NH groups near to the Si-O group generally lowers the stretching frequency due to the hydrogen bonding [81]. In addition, the degradation of the tetrahedral structure of the zeolite can be observed with the small widening of the TO4 band after reaction (Fig. 2), indicating a decrease in crystallinity [28]. This change in crystallinity was also observed by XRD (Fig. 1), but due to the formation of metaheulandite.

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The changes in the textural properties (Table 4) and in the acidity (Table 5) of the catalyst H-zeoFe3+ after the reaction were evaluated. A slight reduction in the surface area of H-zeo-Fe3+ after durability test is observed (Table 4 and Fig. 5b). This fact may be associated with the structural changes of the zeolite (according to the XRD and FTIR analysis) that make difficult the entrance of the N2 to the channels of the zeolite. Likewise, significant changes in the other textural parameters are observed in Table 4, showing an increment in the micropore area, average pore size and total pore volume. These changes could be associated with the dealumination process described above in the FTIR analysis. Finally, the changes in the acidity of the material presented in Table 5 allowed to reinforce the hypothesis that the dealumination by the breaking of the Si-OH-Al bonds, the decrease in the BET surface area due to the metaheulandite formation, and the loss of crystallinity contribute to decrease the acid centers of the H-zeo-Fe3+catalyst from 2576.7 to 821.3 mol g-1cat, which diminishes the amount of active sites for NH3 adsorption, affecting the efficiency of the catalyst.

Conclusions

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An iron Colombian natural zeolite composed mainly by heulandite and celadonite phases was successfully evaluated as a potential geo-catalyst for NH3-SCR of NOx. After the treatments for obtaining the acid material and the Fe3+ and Fe2+ species, the NO conversion increased with operating temperature window of 350 – 500 °C. Maximum conversion increased from 21% in the natural zeolite until 76% in the presence of Fe2+ species and to 86% with the Fe3+ species. The presence of Fe3+ favored the NO conversion, but with a 5% mol of water in the inlet stream narrowed the operating window to 400 – 500 °C obtaining a conversion near to 100% at 500 °C. NOx-TPD confirmed that water inhibits the NO adsorption on the catalyst H-zeo-Fe3+ surface, but improves that of NO2. At 500 °C under wet conditions after 30 h of reaction the NO conversion decreases to 35% over the catalyst H-zeo-Fe3+, which according with the FTIR, XRD and BET analysis can be associated with a dealumination process, the contraction of the zeolite channels due to the metaheulandite formation, and the acidity and crystallinity losses.

Acknowledgments

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JFG, LD and MAM acknowledge financial support to Universidad Nacional sede Medellín; ACA, LMG and ALV thank to Universidad de Antioquia (UdeA) and Colciencias for financial support through the project 1115-569-33782, and UdeA for project PURDUE 14-2-05.

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63. D. Zhang, R.T. Yang, NH3-SCR of NO over one-pot Cu-SAPO-34 catalyst: Performance enhancement by doping Fe and MnCe and insight into N2O formation, Appl. Catal. A: Gen. 543 (2017) 247–256. doi:10.1016/j.apcata.2017.06.021 64. P. Boron, L. Chmielarz, J. Gurgul, K. Latka, B. Gil, J.-M. Krafft, S. Dzwigaj, The influence of the preparation procedures on the catalytic activity of Fe-BEA zeolites in SCR of NO with ammonia and N2O decomposition, Catal. Today 235 (2014) 210– 225. doi:10.1016/j.cattod.2014.03.018 65. P.S. Metkar, N. Salazar, R. Muncrief, V. Balakotaiah, M.P. Harold, Selective catalytic reduction of NO with NH3 on iron zeolite monolithic catalysts: Steady-state and transient kinetics, Appl. Catal. B: Environ. 104 (2011) 110–126. doi:10.1016/j.apcatb.2011.02.022 66. R.Q. Long, R.T. Yang, Catalytic Performance of Fe–ZSM-5 Catalysts for Selective Catalytic Reduction of Nitric Oxide by Ammonia, J. Catal. 188 (1999) 332-339. doi:10.1006/jcat.1999.2674 67. F. Liu, H. He, Y. Ding, C. Zhang, Effect of manganese substitution on the structure and activity of iron titanate catalyst for the selective catalytic reduction of NO with NH3, Appl. Catal. B: Environ. 93 (2009) 194–204. doi:10.1016/j.apcatb.2009.09.029 68. F. Liu, H. He, Structure-Activity Relationship of Iron Titanate Catalysts in the Selective Catalytic Reduction of NOx with NH3, J. Phys. Chem. C 114 (2010) 16929– 16936. doi:10.1021/jp912163k 69. R. Gao, D. Zhang, X. Liu, L. Shi, P. Maitarad, H. Li, J. Zhang, W. Cao, Enhanced catalytic performance of V2O5–WO3/Fe2O3/TiO2 microspheres for selective catalytic reduction of NO by NH3, Catal. Sci. Technol. 3 (2013) 191-199. doi: 10.1039/c2cy20332d 70. A. Serrano-Lotina, A.C. Bueno, C. Goberna-Selma, P. Ávila, M.A. Bañares, NO adsorption and influence of the control of temperature over catalytic test results for NO oxidation, Catal. Today 297 (2017) 2–9. doi:10.1016/j.cattod.2017.06.031 71. V. Blasin-Aubé, O. Marie, J. Saussey, A. Plesniar, M. Daturi, N. Nguyen, C. Hamon, M. Mihaylov, E. Ivanova, K. Hadjiivanov, Iron Nitrosyl Species in Fe-FER: A Complementary Mössbauer and FTIR Spectroscopy Study, J. Phys. Chem. C 113 (2009) 8387–8393. doi:10.1021/jp900699m 72. X. Wang, W. Wu, Z. Chen, R. Wang, Bauxite-supported Transition Metal Oxides: Promising Low-temperature and SO2-tolerant Catalysts for Selective Catalytic Reduction of NOx, Sci. Rep. 5 (2015) 9766-9772. doi:10.1038/srep09766 73. L. Chen, Z. Si, X. Wu, D. Weng, Z. Wu, Effect of water vapor on NH3–NO/NO2 SCR performance of fresh and aged MnOx–NbOx–CeO2 catalysts, J. Environ. Sci. 31 (2015) 240 – 247. doi:10.1016/j.jes.2014.07.037 74. Z. Say, M. Dogac, E.I. Vovk,Y.E. Kalay, C.H. Kim, W. Li, E. Ozensoy, Palladium doped perovskite-based NO oxidation catalysts: The role of Pd and B-sites for NOx adsorption behavior via in-situ spectroscopy, Appl. Catal., B 154–155 (2014) 51–61. doi:10.1016/j.apcatb.2014.01.038 75. A. Heyden, B. Peters, A.T. Bell, F.J. Keil, Comprehensive DFT Study of Nitrous Oxide Decomposition over Fe-ZSM-5, J. Phys. Chem. B 109 (2005) 1857-1873. doi:10.1021/jp040549a 76. L. Chmielarz, M. Rutkowska, M. Jablonska, A. Wegrzyn, A. Kowalczyk, P. Boron, Z. Piwowarska, A. Matusiewicz, Acid-treated vermiculites as effective catalysts of

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high-temperature N2O decomposition, Appl. Clay Sci. 101 (2014) 237-245. doi:10.1016/j.clay.2014.08.006 77. I.M. Saaid, A.R. Mohamed, S. Bhatia, Activity and characterization of bimetallic ZSM-5 for the selective catalytic reduction of NOx, J. Mol. Catal. A: Chem. 189 (2004) 241-250. doi:10.1016/S1381-1169(02)00330-8 78. A.E.-A.A. Said, M.M.A. El-Wahab, M.N. Goda, Selective synthesis of acetone from isopropyl alcohol over active and stable CuO–NiO nanocomposites at relatively lowtemperature, Egypt. J. Basic Appl. Sci. 3 (2016) 357–365. doi:10.1016/j.ejbas.2016.08.004 79. P. Budi, E. Curry-Hyde, R.F. Howe, Steam deactivation of transition metal MFI zeolite catalysts for NOx reduction, Stud. Surf. Sci. Catal. 105 (1997) 1549-1556. doi:10.1016/S0167-2991(97)80798-7 80. P. Wang, H. Zhao, H. Sun, H. Yu, X. Quan, Porous metal–organic framework MIL100 (Fe) as an efficient catalyst for the selective catalytic reduction of NOx with NH3, RSC Adv. 4 (2014) 48912-48919. doi:10.1039/C4RA07028C 81. K. Byrappa, B.V. Suresh Kumar, Characterization of Zeolites by Infrared Spectroscopy, Asian J. Chem. 19 (2007) 4933-4935.

List of tables

IS (mm s-1)

ΔQ (mm s-1)

W (mm s-1)

Concentration (%)* 1 (doublet 1) 0.21 0.33 0.14 79 2 (doublet 2) 0.38 0.21 0.10 21 IS: isomer shift, ΔQ: quadruple splitting, W: width. Based on the peak area percentage calculated by the integration of the area under the curve

U

Site

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Table 1. Hyperfine parameters obtained from Mössbauer spectrum of Nat-zeo

N

Table 2. EDX-Microanalysis of heulandite and celadonite phases in Nat-zeo

TE

D

M

% Atomic 54.59 33.77 7.47 3.08 0.95 0.14 0.00

Celadonite Element O Si Al Ca K Mg Fe

% Atomic 52.84 26.06 5.45 0.84 2.16 4.86 7.77

EP

Element O Si Al Ca K Mg Fe

A

Heulandite

A

CC

Table 3. Chemical analysis of the Nat-zeo material by XRF

Oxide SiO2 Al2O3 CaO Fe2O3 K2O MgO BaO SrO TiO2 MnO

Weight % 63.31 14.90 5.48 4.51 1.98 1.30 0.18 0.11 0.07 0.06

SBETa (m2 g-1) 30.61 28.28 26.94

Material Nat-zeof H-zeo-Fe3+ fresh H-zeo-Fe3+ usedg

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SO3 0.04 CuO 0.03 As2O3 0.03 SnO2 0.02 V2O5 0.02 SeO2 0.010 Loss of ignition 7.96 Table 4. Textural parameters of Nat-zeo and H-zeo-Fe3+ catalysts before and after reaction

Amicroc (m2 g-1) 3.41 2.58 5.59

db (Å) 193.8 124.5 136.7

Vpe (cm3 g-1) 0.029 0.088 0.092

Specific surface area calculated by the Brunauer–Emmett–Teller (BET) method; b average pore size, derived from the BJH adsorption (4V/A); c micropore area calculated by t-plot method; d mesopore area calculated by t-plot method; e total pore volume estimated from the N2 adsorbed amount at p/p0 = 0.99; f analysis performed in a Tristar 3000 from Micromeritics; g after 30 h in reaction

A

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a

Amesod (m2 g-1) 20.51 25.70 21.35

M

Table 5. Ammonia consumption and total acidity of Nat-zeo and H-zeo-Fe3+ catalysts before and after reaction

Nat-zeo

T (°C)

160

EP

3+

Peak 2

Peak 3 T (°C)

Acidity (µmol g-1cat)

Total acidity (µmol g-1cat)

Acidity (µmol g-1cat)

T (°C)

Acidity (µmol g1 cat)

229.9

510

427.3

-

-

657.2

TE

Material

D

Peak 1

183

920.6

526

1609.9

665

46.17

2576.7

H-zeo-Fe3+ used

170

665.6

500

155.7

-

-

821.3

A

CC

H-zeo-Fe

List of figures

Fig. 1. XRD patterns of Nat-zeo, H-zeo-Fe2+, H-zeo-Fe3+ and H-zeo-Fe3+, U (U: used, reaction conditions: 400 ppm NO, 45 ppm NO2, 400 ppm NH3, 8% O2, 5% H2O, and balance Ar, GHSV = 30000 h-1, 400 °C, 30 h on stream). O: orthoferrosilite, H: heulandite/clinoptilite, C: celadonite, M: metaheulandite, P: plagioclase feldspar.

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Fig. 2. FTIR spectroscopy analysis of Nat-zeo, H-zeo-Fe2+, and H-zeo-Fe3+ catalysts before and after reaction (reaction conditions: 400 ppm NO, 45 ppm NO2, 400 ppm NH3, 8% O2, 5% H2O, and balance Ar, GHSV = 30000 h-1, 400 °C, 30 h on stream).

U

Fig. 3. Experimental (EXP) and fitted (FIT) room temperature Mössbauer spectrum of Nat-zeo catalyst. Spectrum was fitted using Recoil program.

N

Fig. 4. SEM analysis of Nat-zeo catalyst

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Fig. 6. H2–TPR of Nat-zeo catalyst

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Fig. 5. N2 adsorption/desorption isotherms of a) Nat-zeo catalyst and b) H-zeo-Fe3+ catalyst before (solid line) and after reaction (dotted line with symbol) (reaction conditions: 400 ppm NO, 45 ppm NO2, 400 ppm NH3, 8% O2, 5% H2O, and balance Ar, GHSV = 30000 h-1, 400 °C, 30 h on stream).

EP

Fig. 7. NH3–TPD profiles of Nat-zeo and H-zeo-Fe3+catalysts

CC

Fig. 8. Catalytic activity of Nat-zeo, H-zeo-Fe2+, and H-zeo-Fe3+ catalysts on a) NO conversion and b) NO2 (fully symbols) and N2O (open symbols) concentration. Reaction conditions: 400 ppm NO, 45 ppm NO2, 400 ppm NH3, 8% O2, and balance Ar, GHSV = 30000 h-1.

A

Fig. 9. NH3 conversion with Nat-zeo, H-zeo-Fe2+, and H-zeo-Fe3+ catalysts. Reaction conditions: 400 ppm NO, 45 ppm NO2, 400 ppm NH3, 8% O2, and balance Ar, GHSV = 30000 h-1.

Fig. 10. Effect of water with the H-zeo-Fe3+ catalyst on a) NO (fully symbols) and NH3 (open symbols) conversions and b) NO2 (fully symbols) and N2O (open symbols) concentration. Reaction conditions: 400 ppm NO, 45 ppm NO2, 400 ppm NH3, 8% O2, 5% H2O and balance Ar, GHSV = 30000 h-1.

Fig. 11. NOx desorption profiles from the H-zeo-Fe3+ catalyst after: (A) NO adsorption and (B) NO2/N2O adsorption at 50 °C under wet and dry conditions.

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Fig. 12. Durability test for H-zeo-Fe3+ catalyst. Reaction conditions: 400 ppm NO, 45 ppm NO2, 400 ppm NH3, 8% O2, 5% H2O, and balance Ar, GHSV = 30000 h-1, 500 °C, 30 h on stream.

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