Hierarchical fe , cu- and co-beta zeolites obtained by mesotemplate free method synthesis and catalytic activity in n2 o decomposition

Microporous and Mesoporous Materials xxx (2014) xxx–xxx

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Microporous and Mesoporous Materials

journal homepage: www.elsevier .com/locate /micromeso

Hierarchical Fe-, Cu- and Co-Beta zeolites obtainedby mesotemplate-free method. Part I: Synthesisand catalytic activity in N2O decomposition

http://dx.doi.org/10.1016/j.micromeso.2014.10.0111387-1811/� 2014 Elsevier Inc. All rights reserved.

⇑ Corresponding author. Tel.: +48 126632096; fax: +48 126340515.E-mail address: [email protected] (M. Rutkowska).

Please cite this article in press as: M. Rutkowska et al., Micropor. Mesopor. Mater. (2014), http://dx.doi.org/10.1016/j.micromeso.2014.10.011

M. Rutkowska ⇑, Z. Piwowarska, E. Micek, L. ChmielarzJagiellonian University, Ingardena 3, 30-060 Kraków, Poland

a r t i c l e i n f o

Article history:Received 31 May 2014Accepted 6 October 2014Available online xxxx

Keywords:Zeolite bHierarchical zeolitesN2O decomposition

a b s t r a c t

Two series of BEA zeolites (Beta and Beta/meso) have been prepared. A first series of the samples wasobtained by a conventional aging of parent zeolite gel, while the second series (Beta/meso) was preparedby mesotemplate-free method. In this method Beta nanoparticles are aggregated under acidic conditionswith the formation of micro-mesoporous material. Both series (Beta and Beta/meso) were doped with Fe,Cu and Co by ion-exchange method and tested as catalysts of N2O decomposition. The Cu-Beta catalystwas found to be the most active in the process of N2O decomposition conducted in inert gas atmosphere.However, in the process performed under conditions similar to those prevailing in waste gases emittedfrom nitric acid plants (one of the main sources of N2O emission) higher reaction rate was found forthe Cu-Beta/meso catalyst.

� 2014 Elsevier Inc. All rights reserved.

1. Introduction

The emission of nitrous oxide (N2O) to the atmosphere is one ofthe main environmental problem, contributing to the greenhouseeffect and destruction of the ozone layer. The regulations draftedby European Council in 2009 assumed a decrease of greenhousegases emission, in the most developed countries, by 30% (withregard to emission levels from 1990) till 2020 [1]. Therefore, thereis a need of intensive studies focused on optimization of the exist-ing processes and the development of new technologies of N2Oemission abatement.

Nitrous oxide is one among six substances (CO2, CH4, N2O, HFCs,PFCs, SF6) approved in the Kyoto Protocol as the most dangerousgreenhouse gases [2]. Moreover, N2O contributes to the ozone layerdepletion [3]. One of the most important anthropogenic source ofN2O emission is industrial production of nitric acid (about 1% ofall greenhouse gases emission). Among several options of N2Oemission abatement its direct catalytic decomposition in the tailgas (about 523–773 K) is preferable from both application andoperation costs [4].

The concept of zeolites with the hierarchical pore structure(containing both micro- and mesopores) was proposed to over-come diffusion limitations characteristic for classical microporous

zeolites, which hinder the accessibility of active centers for bulkymolecules [5]. Development of a new type of materials combiningboth advantages of zeolites (e.g. strong acidity, ion-exchange prop-erties, hydrothermal stability) and mesoporous silica materials(favorable diffusion rates) is important due to possible optimiza-tion of a large number of catalytic processes [6].

The origin of mesoporosity in zeolites can be fundamentally dif-ferent, what greatly extends the areas of the synthesis methods.The most common methods are: (i) desilication [7], (ii) dealumina-tion [8], (iii) recrystallization of amorphous material [9,10], (iv)solid templating [11], (v) pillaring and delamination of layeredzeolites [12,13]. In the presented studies the novel way of the mes-oporosity generation in zeolites, called ‘‘mesotemplate-freemethod’’, was applied [14–17]. This method is based on the prep-aration of zeolite nanoparticles, followed by their controlled aggre-gation in acidic media, resulting in the formation of themesoporous interparticle structure. This method does not needany templates for the generation of mesopores, making it veryattractive from the economic and environmental issues.

Zeolites exchanged with transition metals are known as activecatalysts of various chemical processes. MFI, BEA, and FAU zeoliteswere widely studied in N2O decomposition e.g. [14,15,18]. Espe-cially interesting catalytic properties were reported for the sam-ples doped with Fe, Cu and Co. Liu et al. [19] studied thecatalytic performance of (Fe, Co, Cu)-BEA zeolites in N2O decompo-sition. Moreover, (Fe, Cu)-BEA zeolites were reported to be active

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and selective catalysts of N2O reduction by CO [20]. The results ofcatalytic tests over Fe-, Cu- and Co-exchanged Beta zeolites in theprocesses of N2O decomposition are very promising and therefore,in the presented work, were extended for Beta zeolites with thehierarchical porous structure.

2. Experimental methods

2.1. Catalysts preparation

The synthesis gel of zeolite Beta was prepared using the proce-dure described earlier [14]. Tetraethylammonium hydroxide (TEA-OH, 35%, Sigma–Aldrich) was used as a structure-directing agent,while fumed silica (Aerosil 200, Evonic) and NaAlO2 (Sigma–Aldrich) as silica and aluminium sources, respectively. The result-ing solution with the molar composition: SiO2: 0.024; Al2O3:0.612; TEAOH: 0.200; HCl: 21 H2O was divided into two partswhich where hydrothermally treated in autoclaves at 423 K for24 h and 8 days, respectively. The slurry after 24 h of aging (con-taining nanoseeds of Beta zeolite) was acidified in a proportionof 5 mL of concentrated HCl per 18 mL of the nanoseeds slurry.Subsequently, the acidified slurry was hydrothermally treated at423 K for 72 h, yielding micro-mesoporous Beta zeolite denotedas Beta/meso. Conventional microporous Beta zeolite, denoted asBeta, was obtained from the slurry aged for 8 days. After aging peri-ods the autoclaves were quenched and the samples were filtered,washed with distilled water, dried in ambient conditions and cal-cined at 823 K for 6 h.

The negative charge of the zeolite framework, in the samplesprepared by this method, was compensated by sodium cationswhich were replaced by protons in the next step of the catalystssynthesis (exchange details presented in [14]).

The H-forms of the obtained samples were modified with Fe, Cuand Co by ion-exchange method. Transition metals were intro-duced to the zeolite samples by stirring with 0.06 M solutions ofFeSO4�7H2O, Cu(CH3COO)2�4H2O or Co(CH3COO)2�4H2O (Sigma–Aldrich) for 6 h at 358 K (in case of iron salt) and at 353 K (in caseof copper and cobalt salts). In each ion-exchange procedure 250 mLof a solution of transition metal per 2 g of the sample was used.Iron was deposited in anaerobic atmosphere to avoid oxidationof Fe2+ to Fe3+. Then the samples were filtered, washed with dis-tilled water, dried in ambient conditions and finally calcined at823 K for 6 h. The codes of the catalysts are given in Table 1.

Table 1Textural properties of the samples determined from N2-sorption measurements at 77 K an

Sample code SBET

(m2 g�1)External surface area(m2 g�1)

Micropore area(m2 g�1)

Total p(p/p0 =

H-Beta 710 89 622 0.433H-Beta/meso 745 368 377 1.437Fe-Beta 720 95 625 0.446Fe-Beta/meso 551 273 278 1.013Cu-Beta 566 78 488 0.340Cu-Beta/meso 460 237 223 0.906Co-Beta 748 106 642 0.480Co-Beta/meso 512 261 251 1.064

Table 2Conditions of N2O decomposition.

Inlet composition Total flow (ml min�1) w= _nN2 O (g h mol�1)

N2O 50 746N2O + O2 50 746N2O + NO 50 746N2O + H2O 50 746N2O + H2O + O2 + NO 50 746

Please cite this article in press as: M. Rutkowska et al., Micropor. Mesopor. Ma

2.2. Catalysts characterization

The specific surface area (SBET) area of the samples was deter-mined by N2 sorption at 77 K using a 3Flex v1.00 (Micromeritics)automated gas adsorption system. Prior to the analysis, the sampleswere degassed under vacuum at 623 K for 24 h. The specific surfacearea (SBET) of the samples was determined using BET (Braunauer–Emmett–Teller) model according to Rouquerol recommendations[21]. The micropore volume (at p/p0 = 0.98) and specific surface areaof micropores were calculated using the Harkins and Jura model (t-plot analysis). The pore size distributions were determined from theadsorption branch of nitrogen isotherm by applying density func-tional theory (DFT). For calculations the method assuming nitrogenadsorption in cylindrical pores was used.

The X-ray diffraction (XRD) patterns of the samples wererecorded using a Bruker D2 Phaser diffractometer. The measure-ments were performed in the 2 theta range of 5–50� with a stepof 0.02�.

Thermogravimetric measurements were performed using aTGA/SDTA851e Mettler Toledo instrument. The samples wereheated in a flow of synthetic air (80 mL/min) with the ramping of10 K/min, in the temperature range of 303–1073 K.

IR measurements were performed using a Nicolet 6700 FT-IRspectrometer (Thermo Scientific) equipped with DRIFT (diffusereflectance infrared Fourier transform) accessory and DTGS detec-tor. The dried samples were grounded with dried potassium bro-mide powder (4 wt.%). The measurements were carried out in thewavenumber range of 400–4000 cm�1 with a resolution of 2 cm�1.

The transition metals content, as well as the Si/Al ratio in thesamples, were analyzed using a mass spectrometer with induc-tively coupled plasma (ICP-MS, ELAN 6100 Perkin Elmer).

Coordination and aggregation of transition metal species intro-duced into the obtained zeolitic materials were studied by UV–vis-DR spectroscopy. The measurements were performed using anEvolution 600 (Thermo) spectrophotometer in the range of 200–900 nm with a resolution of 2 nm.

Surface acidity (concentration and strength of acid sites) of thesamples was studied by temperature-programmed desorption ofammonia (NH3-TPD). The measurements were performed in a flowmicroreactor system equipped with QMS detector (VG Quartz).Prior to ammonia sorption, a sample was outgassed in a flow ofpure helium at 688 K for 30 min. Subsequently, microreactor wascooled to 343 K and the sample was saturated in a flow of gas

d the crystal sizes obtained using Schererr’s equation.

ore volume0.98) (cm3 g�1)

Micropore volume(cm3 g�1)

Meso + macropore volume(cm3 g�1)

dhkl

(nm)

0.242 0.191 190.159 1.278 250.244 0.202 180.116 0.897 260.189 0.151 160.093 0.813 240.253 0.227 200.105 0.959 24

CN2O (ppm) CO2 (ppm) CNO (ppm) CH2O (ppm)

1000 – – –1000 40,000 – –1000 – 200 –1000 – – 30,0001000 40,000 200 30,000

ter. (2014), http://dx.doi.org/10.1016/j.micromeso.2014.10.011


Fig. 1. Nitrogen adsorption–desorption isotherms (a) and pore size distributions calculated by DFT method (b) for the samples of Beta and Beta/meso series.

M. Rutkowska et al. / Microporous and Mesoporous Materials xxx (2014) xxx–xxx 3

mixture containing 1 vol.% of NH3 in helium for about 120 min.Then, the catalyst was purged in a helium flow until a constantbase line level was attained. Desorption was carried out with a lin-ear heating rate (10 K/min) in a flow of He (20 ml/min). Calibrationof QMS with commercial mixtures allowed recalculating detectorsignal into the rate of NH3 evolution.


2.3. Catalytic tests

Catalytic studies of N2O decomposition were performed in afixed-bed quartz microreactor. The experiments were done atatmospheric pressure and in the temperature range from 473 to823 K in intervals of 25 K. The composition of outlet gases was



Fig. 2. XRD patterns of the samples of Beta and Beta/meso series.


analyzed using a gas chromatograph (SRI 8610C) equipped withTCD detector. For each experiment 0.1 g of catalyst (particles sizesin the range of 0.160–0.315 mm) was placed on quartz wool plug


in microreactor and outgassed in a flow of pure helium at 823 Kfor 1 h. Then the appropriate gas mixture (total flow rate of50 ml/min) passed over the catalyst and the reaction proceeded



Fig. 3. DTG profiles of as-synthesized Na-Beta and Na-Beta/meso zeolites.


for about 1 h to stabilize the catalyst. The compositions of the reac-tion mixtures are summarized in Table 2. The space time (s) of N2Oin these conditions, defined as s ¼W= _nN2O (where: W is a catalystmass, and _nN2O is a molar flow of N2O in the inlet mixture), wasequal to 746 g h mol�1. The analysis of the outlet gases was per-formed 20 min after temperature stabilization and a steady stateregime was achieved.

3. Results and discussion

Textural parameters of the H-, Fe-, Cu- and Co-forms of Beta andBeta/meso series of the samples, determined by nitrogen sorptionmeasurements, are presented in Table 1.

The properties of H-Beta and H-Beta/meso (parent samples forion-exchanges) are significantly different. Both samples are charac-terized by relatively high BET surface area (about 700 m2/g), how-ever external surface area and volume of macro and mesopores areconsiderably greater in case of H-Beta/meso. Moreover, both vol-ume and surface area of micropores are lower in comparison tothe H-Beta sample. These effect could be explained by various crys-tallization conditions (which, in case of H-Beta/meso, were affectedby acidification). On the other hand, an increase in external surfacearea and volume of meso and macropores proves the successfulgeneration of mesopores in the H-Beta/meso sample.

Introduction of iron and cobalt into Beta zeolite by ion-exchange method resulted in slight increase in surface area, whilean opposite effect was observed for deposition of cobalt. Anincrease in surface areas of the samples could be related to the sec-ondary recrystallization of zeolites under hydrothermal conditions.A decrease of surface area observed in case of Cu-Beta, but also forall the ion-exchanged samples based on Beta/meso series, can berelated to a partial blocking of pores by metal oxide aggregates.

The nitrogen adsorption–desorption isotherms recorded for thesamples of Beta and Beta/meso series are shown in Fig. 1a. The iso-therm of type I (according to the IUPAC classification), characteris-tic of microporous structure, was obtained for the samples of Betaseries. While in case of Beta/meso series the adsorption–desorption isotherms form hysteresis loop of type H4, characteristicfor micro-mesoporous materials [22]. An increase in nitrogenadsorbed volume, observed in the range of low partial pressures,is smaller for the Beta-meso samples than for the Beta samples,what proves lower microporosity of this series. The shape of theisotherms reminded unchanged after deposition of transition


metals, although adsorbed volume of N2 decreased (especially incase of Beta/meso series, both at low and high partial pressures).

Classical methods used for characterization of the materialswith different pore structure morphologies, such as BET or BJH,failed [23]. Thus, the density functional theory (DFT) model forcylindrical pores was applied for calculation of pore size distribu-tions. The DFT pore size distributions (Fig. 1b) clearly show the dis-tinct pore widths present in the samples. In case of Beta seriesthree maxima below 2 nm (at about 1.0, 1.4 and 1.7 nm) are pres-ent. The pore size distribution in the micro-mesoporous samplesexhibits two types of pores – micropores of the same sizes as inBeta and mesopores in a broad range (between 10 and 35 nm) withfour maxima. It is worth to mention that the contribution of poresin the micropore range was changed after acidification. A maxi-mum at about 1 nm significantly decreased in the favor of the peakat about 1.4 nm. The peaks in the range below 2 nm slightly shiftedto higher pore values (especially in case of the Co-modified series)after transition metal disposition, what can be connected with apartial destruction of the zeolite matrix.

Analysis of nitrogen sorption measurements leads to the con-clusion that application of mesotemplate-free method resulted ingeneration of mesopores in Beta/meso series of the samples withpartial preserving of the Beta zeolite microporous matrix.

The XRD powder patterns of the H-, Fe-, Cu- and Co-forms ofBeta and Beta/meso samples are shown in Fig. 2. Reflections char-acteristic of the BEA topology are present in all diffractograms,what proves the zeolitic character of the samples of Beta/meso ser-ies and maintenance of this character in both series during calcina-tion and ion-exchange. The intensity of reflections obtained forBeta/meso series is lower in comparison to these recorded for thesamples of Beta series. The mesoporous samples are less crystal-line, and the intensity of the reflections decreased after ion-exchange, especially in case of the copper and cobalt modifiedsamples.

A narrow and intense (302) reflection at about 22.5� is shiftedin case of the Beta/meso sample to lower 2 theta angles indicatingthe relaxation of the Beta zeolite matrix [24]. The crystal sizesobtained using Schererr’s equation (k = 1, k = 0.154 nm) for (302)reflection are presented in Table 1. All these values are in the rangeof 16–26 nm, although it is worth to notice that in case of Beta/meso series the crystal sizes are slightly larger.

Thermal analysis curves (first derivative curves – DTG) of theas-synthesized Na-Beta and Na-Beta/meso samples are presentedin Fig. 3. The DTG curve of Beta zeolite exhibit four distinct peaks,which originate from the weight changes described in the litera-ture as follows [25,26]: (i) T < 400 K – desorption of zeolitic water(occluded in zeolite apertures); (ii) 400–625 K – thermal decompo-sition of tetraethylammonium hydroxide (removal of triethyl-amine and ethylene, which are the products of TEA+ degradation)occluded in zeolite pores, non-interacting with the zeolite frame-work; (iii) 625–750 K – the greatest weight loss related to decom-position of tetraethylammonium cations, interacting with thezeolite lattice (chemically bonded to Si–O–Al and Si–O–Si); (iv)T > 800 K – decomposition of residual TEA+ cations, strongly inter-acting with Al acidic sites.

The DTG profile obtained for the Na-Beta/meso sample consistsof four weight changes characteristic for Beta zeolite, proving zeo-litic character of the mesoporous sample. Although, what is impor-tant to notice, they significantly differ in intensity. The weightlosses in the first and second region are much significant, whatindicate greater amount of water and TEAOH molecules occludedin the micro-mesoporous structure. On the other hand, the bandsassociated to TEA+ cations interacting with the zeolite framework(III and IV region) are less intensive in comparison to conventionalBeta zeolite. These differences could be related to the disturbedzeolite structure (lower crystallinity) of the Na-Beta/meso sample.



Fig. 4. DRIFT spectra of the samples of Beta and Beta/meso series.




Table 3Transition metals content, Si/Al ratio and total NH3 uptake measured for the samplesfrom the Beta and Beta/meso series modified with Fe, Cu and Co.

Sample M* [%] Si/Al NH3 uptake [mmol/g]

Fe-Beta 2.1 27 0.977Fe-Beta/meso 1.6 42 0.508Cu-Beta 6.7 23 1.211Cu-Beta/meso 6.5 32 0.802Co-Beta 2.2 20 0.982Co-Beta/meso 2.2 37 1.641

* M = Fe, Cu or Co.

Fig. 6. NH3-TPD profiles of conventional and mesoporous Beta zeolites exchangedwith Fe, Cu and Co. Conditions: 10,000 ppm NH3 in He; gas flow 20 ml/min; weightof catalyst – 0.05 g.


Mesopores in this sample were created in favor of the zeoliteframework, thereby more unreacted TEAOH molecules reside inzeolite apertures.

The crystallinity of the samples of Beta and Beta/meso seriescan be determined beside the XRD also by IR-DRIFT method(Fig. 4). In the range of 500–600 cm�1 all the samples show twobands characteristic of five (525 cm�1) and six (575 cm�1) mem-bered rings present in the Beta zeolite structure [27]. Intensity ofthese bands is lower in case of Beta/meso series, what correspondswith the results of XRD analysis and indicates lower crystallinity ofthe micro-mesoporous samples. Intensity of these bands slightlydecreased after ion-exchange, especially in case of the samples ofBeta/meso series modified with Cu and Co. Additionally, the contri-bution of the zeolitic phase in the samples of Beta and Beta/mesoseries corresponds to intensity of the band located at 1230 cm�1

and attributed to the asymmetric stretching of strained siloxanebridges (with the same bond length) present in the zeolite lattice[28].

In the OH stretching region of the spectra, the band at3745 cm�1 is assigned to the terminal Si-OH groups present onthe external surface [29]. An increase in intensity of this band cor-responds to a decrease in the crystals sizes. In case of Beta/mesoseries it could be related to development of the external surfacethrough the generation of mesopores. The band at about3630 cm�1, assigned to hydroxyl stretching vibrations (Si–(OH)–Al), corresponds to Brønsted acidity in the zeolite framework.Intensity of this band is comparable for the Beta and Beta/mesosamples in H-forms. Deposition of Fe, Cu, and Co decreased inten-sity of this band for a series of the Beta/meso samples. The broadband in the range of 2000–3500 cm�1 is assigned to the presenceof internal OH groups (strong hydrogen bonds between neighbor-ing silanols). Intensity of this band is related to internal Si-OHdefects, whereby proving the looseness of the structure in case ofBeta/meso series. It could be a result of partial dealumination ofthe samples [30].

The content of transition metals introduced to the samples byion-exchange method, as well as the Si/Al ratio, was analyzed using

Fig. 5. UV–vis-DR spectra of conventional and mesoporous Beta zeolites exchanged with Fe (a), Cu (b) and Co (c).



Fig. 7. Temperature dependence of N2O conversion (a) and reaction rate (b) for Fe, Cu and Co modified Beta and Beta/meso. Conditions: 1000 ppm N2O; He as balancing gas;total flow rate – 50 ml/min; weight of catalyst – 0.1 g.


ICP method (Table 3). Transition metal content in the samples issignificantly different, despite the use of the same ion-exchangeconditions. The content of metals in the samples can be presentedin the following order: Cu P Co > Fe. The variations in particularmetals content are probably connected with differences in thehydrated radiuses of the exchanged cations, which are equal to4.19, 4.23 and 4.28 Å for Cu2+, Co2+ and Fe2+, respectively [31]. Itseems that for smaller hydrated cations the penetration of microp-ores is facilitated, thus they can reach more ion-exchange positionsand the resulting metal content is higher. The differences in metalcontents between Beta and Beta/meso series can be observed incase of the Fe and Cu modified samples, while in case of exchangewith cobalt the same amount was introduced to Beta and Beta/meso.

Smaller content of Fe and Cu in Beta/meso can be related to thehigher Si/Al ratios in these samples. Lower Al content in the micro-mesoporous samples (what was also evidenced by IR-DRIFT) canbe connected with disturbed and incomplete crystallization ofthe samples of Beta/meso series. It is worth to notice that the Si/Al ratios of the samples of conventional Beta zeolite series is closeto the expected ratio (�21). A slightly higher values of this ratiocan be assigned to partial dealumination of the samples duringtheir calcination.

Fig. 5 shows UV–vis-DR spectra of the samples of Beta and Beta/meso series, exchanged with Fe, Cu and Co. The samples modifiedwith iron (Fig. 5a) exhibit absorption bands in the range of 200–650 nm. The spectra were deconvoluted into four Gaussian sub-bands corresponding to monomeric Fe3+ ions in tetrahedral(k < 250 nm) or octahedral (250 < k < 300 nm) coordination, smalloligomeric FexOy species (300 < k < 400 nm) and Fe2O3 nanoparti-cles (k > 400 nm). The spectra of the both Fe-Beta and Fe-Beta/meso samples show four distinct absorption bands, although theydiffer in proportions between the particular iron forms. In case ofthe Beta/meso sample more iron was introduced in the form ofsmall oligonuclear FexOy species, while in case of the Beta samplerelatively greater amount of Fe2O3 nanoparticles was deposited,possibly on the outer surface of the sample. A wide bandwidth ofmaximum at about 380–400 nm indicates the presence of oligonu-clear iron clusters of various sizes and geometries [32]. Absorptionbelow 300 nm is related to CT transitions O ? isolated Fe3+ and theposition of this band depends on the iron coordination number. Itwas reported [32–34] that absorption band below 250 nm isassigned to tetrahedrally coordinated Fe3+ ions (in framework posi-tions or in other matrices), while the bands located between 250and 300 nm are assigned to octahedrally coordinated Fe3+ ions in


the extra framework positions. An appearance of the absorptionat about 225 nm in the spectra of the calcined samples is a veryinteresting result. A similar phenomenon was observed by Pérez-Ramiréz et al. [32] and explained by the release of water ligandsduring calcination, which resulted in a decrease of iron coordina-tion degree.

The UV–vis-DR spectra for the Cu-modified samples are pre-sented in fig. Fig. 5b. Both samples (Cu-Beta and Cu-Beta/meso)show the absorption below 400 nm, attributed to monomericCu2+ ions interacting with oxygen of the zeolite structure (maxi-mum at about 230 nm), and oligomeric [Cu2+–O2-–Cu2+] species(at about 280 nm) [35]. Moreover, the adsorption band, presentabove 500 nm, is related to hydrated Cu2+ cations in octahedralcoordination [35], what proves a very high dispersion of the depos-ited copper species.

Fig. 5c shows the UV–vis-DR spectra collected for the cobaltmodified samples. In both spectra of Co-Beta and Co-Beta/mesothe bands characteristic of Co3O4 spinel at about 250, 380 and670 nm appeared, while in case of the mesoporous sample theintensity of these bands is significantly higher [36–48]. Also inthe both calcined samples a triplet of absorption bands at about510, 590 and 650 nm, characteristic of tetrahedral Co2+ coordina-tion, appeared [36].

Temperature-programmed desorption of ammonia (NH3-TPD)was used to determine the surface acidity (surface concentrationand strength of acid sites) of the catalysts modified with Fe, Cuand Co (Fig. 6). NH3-TPD profiles with two desorption peaks wereobtained for all the examined samples. First of them could beattributed to NH3 bonded to weak acid sites, such as silanol groups,while the second one to ammonia interacting with framework Al[38]. In case of a series modified with iron the strength of acid cen-ters in Fe-Beta is higher than for Fe-Beta/meso, what is indicatedby the position of the peaks (at about 480 and 640 K for conven-tional zeolite Beta and at about 420 and 580 K for the mesoporoussample). Also the total NH3 uptake (Table 3) is higher in case of theFe-Beta sample. Higher acidity of conventional Beta zeolite can beconnected with a significantly larger amount of iron introduced tothis sample. Similar results were obtained in case of Cu-modifiedseries, however its worth to notice that the total NH3 uptake forthis series was higher in comparison to the Fe-exchanged samples.For Co series an opposite effect was observed. Higher concentra-tion of stronger acid sites was found for the micro-mesoporoussample. It is worth to notice that Co-Beta/meso possess strongeracidic properties despite the same Co content in the bothCo-modified samples and higher Si/Al ratio in this sample in



Fig. 8. Temperature dependence of N2O conversion for the Fe, Cu and Co modified samples from the Beta and Beta/meso series in the presence of different compositions ofreaction mixture. Conditions: 1000 ppm N2O and optionally: 40,000 ppm O2, 200 ppm NO, 30,000 ppm H2O; He as balancing gas; total flow rate – 50 ml/min; weight ofcatalyst – 0.1 g.


comparison to Co-Beta. Thus, the acidic properties could berelated to form of introduced cobalt, which is different in the bothsamples (Co-Beta/meso contains more cobalt in the form Co3O4

spinel). Among the examined samples the highest NH3 uptake1.641 mmol/g was found for the Co-Beta/meso sample.

The samples of Beta and Beta/meso series, modified with Fe, Cuand Co were tested as catalysts in the process of N2O decomposi-tion. The results of measurements performed in inert conditionsare presented in Fig. 7a. The samples activity depends on the intro-duced transition metal and the following order of their activation


effect: Cu > Co > Fe was observed. The N2O conversion over themost active Cu-modified sample started at about 598 K andreached 100% at about 723 K. Among the catalysts doped with Cuand Co more active were found to be the samples of Beta series,while in case of zeolites modified with Fe slightly higher conver-sion was obtained for Fe-Beta/meso.

Because the samples differ in surface area the most accuratemethod for comparison of their activity is the temperature depen-dence of the reaction rate (Fig. 7b). The reaction rate (understoodas a number of N2O molecules, which were converted on defined



Fig. 9. Temperature dependence of N2O conversion (a) and reaction rate (b) for Fe, Cu and Co modified Beta and Beta/meso. Conditions: 1000 ppm N2O, 40,000 ppm O2,200 ppm NO, 30,000 ppm H2O; He as balancing gas; total flow rate – 50 ml/min; weight of catalyst – 0.1 g.


surface area (1 m2) the catalyst during defined time (1 s)) was cal-culated basing on observed N2O conversion using the following

equation: r ¼_nN2O

m�SBET� X, where n – molar flow rate of N2O [mol/s],

m – catalyst weight [kg], SBET – BET surface area [m2/g] and X –N2O conversion. It can be seen that the reaction rate reached thehighest values over the copper and cobalt modified micro-meso-porous samples. Transition metal species present in Co-Beta andCu-Beta are more active than in the adequate samples of Beta/meso series (higher values of the reaction rate at lower tempera-tures). Whereas in case of the catalysts modified with iron theslight difference in activity (in favor of Fe-Beta/meso) wasobserved.

Thus, it could be concluded that Cu species, present in the Cu-Beta and Cu-Beta/meso samples, are the most active in N2O decom-position (in inert gas atmosphere). This high catalytic activity canbe attributed to the presence of Cu dimmers, bridged by two oxy-gen atoms, so called bis(l-oxo)dicopper species ([Cu2(l-O)2]2+).Groothaert et al. [39–41] investigated formation of such Cu-speciesin ZSM-5 by an operando optical fiber UV–vis spectroscopy and insitu XAFS combined with UV–vis-near-IR. The bis(l-oxo)dicopperspecies are able to storage the peroxy species generated fromN2O activation and release O2.

The mechanism of N2O decomposition is complex and despiteextensive studies still not clearly defined [42,43]. An influence ofadditional components of waste gases make the kinetic analysiseven more complicated. Thus, the studies of O2, NO and H2O (pres-ent in waste gases emitted from nitric acid plants) impact on theN2O decomposition mechanism were done. The results of thesestudies performed in the presence of different components of thereaction mixture over conventional and micro-mesoporous Betazeolites modified with Fe, Cu and Co are presented in Fig. 8.

In case of the iron and cobalt modified samples an addition ofoxygen to the reaction mixture only slightly influenced the N2Oconversion. In case of Cu-modified zeolites oxygen inhibitioneffect was observed. Transition metal species, which play a roleof catalytically active sites, can be oxidized by surface oxygen(O) species released upon N2O decomposition (Eq. (1)) or by theadsorptive dissociation of oxygen from gaseous phase (Eq. (2)).The second mentioned reaction is predominating at high oxygenpressures in the reaction inlet. The surface oxygen can beremoved by reaction with N2O molecule according to the Eley–Rideal mechanism (Eq. (3)) or by recombination of two surfaceoxygen (O) species according to the Langmuir–Hinshelwoodmechanism (Eq. (4)) [44].


N2Oþ� ! O� þ N2 ð1Þ

O2 þ 2� ! 2O� ð2Þ

O� þ N2O! N2 þ O2þ� ð3Þ

2O� ! O2 þ 2� ð4Þ

The presence of O2 in the reaction mixture could increase theefficiency of N2O conversion if the rate of reaction 3 is faster thanreaction 2. The inhibiting effect of oxygen, observed in case of thecopper modified samples, proves that the oxygen removal from thecatalyst surface is slower than the step described by Eq. (2). Lesssignificant influence of oxygen presence on the Fe and Co contain-ing samples can be explained by the comparable rates of the reac-tions 2 and 3, or the elementary steps, involving adsorption ofoxygen from the gas phase on active centers, are negligible. Theseresults are in agreement with the kinetic analysis of the N2Odecomposition over Cu and Co modified ZSM-5 zeolite carriedout by Kapteijn et al. [43].

An introduction of NO to the reaction mixture significantlyincreased the N2O conversion over the Fe-Beta and Fe-Beta/mesocatalysts. In case of the Cu and Co modified samples the presenceof NO did not influence the N2O decomposition rate. According toPirngruber et al. [42] and Pérez-Ramírez et al. [46] the role of NOin nitrous oxide decomposition over the ion-containing samplesis purely catalytic. Probably NO molecules adsorb on the catalystsurface and react with chemisorbed oxygen liberating active sitefor the next catalytic cycle (Eqs. (5), (6)).

NO� þ O� ! NO�2þ� ð5Þ

NO�2 þ O� ! 2� þ NOþ O2 ð6Þ

Opposite, strongly deactivating effect caused by the presence ofNO in a feed mixture was observed for noble metal based catalysts[47,48]. NO molecules adsorb on active centers making themunavailable for N2O activation and diffusive recombination of oxy-gen. The difference in the role of NO molecules observed for the Femodified samples can be connected with the presence of differentsites for NO and N2O (alpha-oxygen sites) adsorption [45,48].

The presence of water vapor in the reaction mixture signifi-cantly decreases the N2O conversion over all the studied samples(both conventional and micro-mesoporous materials). Probablythis effect is connected with occupying of active centers by H2Omolecules. The OH�surf species produced by water adsorption are



Fig. 10. Time dependence of N2O conversion (Stability tests: 50 h at 773 K) of the Fe,Cu and Co modified samples of Beta and Beta/meso series. Conditions: 1000 ppmN2O, 40,000 ppm O2, 200 ppm NO, 30,000 ppm H2O; He as balancing gas; total flowrate – 50 ml/min; weight of catalyst – 0.1 g.

Fig. 11. Time dependence of reaction rate (Stability tests: 50 h at 773 K) of: Fe-Beta(j), Fe-Beta/meso (h),Cu-Beta (d), Cu-Beta/meso (s), Co-Beta (N) and Co-Beta/meso (D). Conditions: 1000 ppm N2O, 40,000 ppm O2, 200 ppm NO, 30,000 ppmH2O; He as balancing gas; total flow rate – 50 ml/min; weight of catalyst – 0.1 g.


responsible for the selective blocking of active sites. Selmachowskiet al. [49,50], investigated the blocking of the sites present onMgO, Co-MgO and Co3O4 crystallite corners, edges and terraces byhydroxylation, using H2O-TPD, IR-measurements and DFT molecularmodeling. Their results show that the stability of adsorbed hydroxylspecies is much higher than that of peroxy species. The formedOH-

surf groups effectively block the active site, both for N2O activa-tion and diffusive recombination of oxygen. Piskorz et al. [51] stud-ied the water inhibition in N2O decomposition over Co3O4 insimultaneous presence of oxygen in the feed. Water adsorbed onactive center is stabilized by neighboring surface oxygen atoms,what strength the inhibiting effect of both molecules.


An influence of the simultaneous presence of O2, NO and H2O inthe reaction mixture on the N2O conversion is an imposition ofimpact of all components. Only in case of the iron modified sam-ples their catalytic activity measured in the presence of O2, NOand H2O was higher than under ambient gas conditions. The posi-tive influence of NO on the Fe-catalysts balanced with a surplus thewater inhibition. In case of Cu and Co series lower activity deter-mined in a test performed in the presence of all additional compo-nents of the nitric acid plants waste gases, was manly connectedwith a negative influence of H2O.

The comparison of the catalysts activity in the conditions simu-lating the composition of waste gases emitted from nitric acidplants is presented in Fig. 9. The profiles of the N2O conversioncurves (Fig. 9a) are more similar to each other than in case of inertconditions (Fig. 7a). The Fe-Beta sample was found to be the mostresistant for the process conditions, while the Co-Beta/meso sam-ple showed the lowest activity. Fig. 9b presents temperaturedependence of the reaction rate related to the surface areas ofthe samples. The reaction rate reached the highest values in thepresence of the Cu-Beta/meso catalyst. What proves, that underconditions prevailing in waste gases emitted from nitric acidplants, the highest activity among the tested transition metalsexhibits copper.

To examine stability of the catalysts in a flow of N2O, O2, NO andH2O, the extended isothermal catalytic tests (50 h at 773 K) weredone (Fig. 10). In case of the samples modified with Fe and Coany significant changes in the catalytic activity were observed.While, for the copper containing samples, the N2O conversiondecreased by about 20% during first 30 h of the test and thenreached nearly constant level. The comparison of time dependenceof the reaction rate is presented in Fig. 11. The most active and sta-ble species (under conditions simulating the composition of realgases emitted from nitric acid plants) were generated in Cu-Beta/meso.

4. Conclusions

The undertaken studies allowed the comparison of the catalyticperformance of different transition metal (Fe, Cu and Co) speciesintroduced to conventional and mesopore-modified Beta zeolitein N2O decomposition reaction. The results of the undertakenresearch can be analyzed from two points of view – physicochem-ical properties of the catalysts and their activity:




� Applied mesotemplate-free method resulted in the formation ofzeolitic materials (Beta zeolite) with the hierarchical micro-mesoporous structure (N2 sorption measurements). The pre-serving of BEA properties in the micro-mesoporous sampleswas confirmed by different techniques, such as XRD, TG andIR-DRIFT. Differences in the pore architecture between bothseries of the samples influenced the formation of different tran-sition metals species deposited on the catalyst surface (UV–vis-DRS).� All the studied catalysts were active in the process of nitrous

oxide decomposition. The following order of the transitionmetal activation effect was observed in tests performed withgas mixtures containing N2O diluted in helium: Cu > Co > Fe.The N2O conversion over the most active Cu-Beta catalyststarted at about 598 K. The highest rate of N2O decomposition(in a gas mixture containing O2, NO and H2O) was obtained overthe Cu-Beta/meso sample, what proves the generation of thesites, most active in this reaction, in the micro-mesoporoussample. The zeolite catalysts doped with Fe or Co were foundto be stable (50 h, 773 K) in the presence of typical gases emit-ted from nitric acid plants (O2, NO, H2O), while for the samplesmodified with copper a drop in N2O conversion by about 20%was measured. Detailed analysis of the influence of other com-ponents emitted (beside N2O) from nitric acid plants, as well asthe stability tests, enabled the selection of the most active cat-alyst Cu-Beta/meso. Despite similar forms of copper introducedto Cu-Beta/meso and Cu-Beta, the former catalyst present sig-nificantly higher activity, what could be explained by betteraccessibility of acid centers in the micro-mesoporous sample.On the other side the highest N2O conversion (under conditionssimulating the composition of gases emitted from nitric acidplants) was obtained over the Fe-Beta catalyst. This result,together with high hydrothermal stability (50 h, 773 K), makesthis catalyst the most interesting for the application in industryin a studied series of the samples.

Acknowledgements

M.R. acknowledges the financial support from the InternationalPhD-studies programme at the Faculty of Chemistry JagiellonianUniversity within the Foundation For Polish Science MPDProgramme co-financed by the EU European RegionalDevelopment Fund. The research was carried out with theequipment purchased thanks to the financial support of theEuropean Regional Development Fund in the framework ofthe Polish Innovation Economy Operational Program (contract no.POIG.02.01.00-12-023/08).

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Engineering

Hierarchical fe , cu- and co-beta zeolites obtained by mesotemplate free method synthesis and catalytic activity in n2 o decomposition