Catalysis Communications 58 (2015) 108–111

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Catalysis Communications

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Short Communication

Global kinetic modelling of the NH3 oxidation on Fe/BEA zeolite

Christoph Hahn, Sven Füger, Matthias Endisch, Andreas Pacher, Sven Kureti ⁎Technical University of Freiberg, Institute of Energy Process Engineering and Chemical Engineering, Chair of Reaction Engineering, Fuchsmuehlenweg 9, D-09596 Freiberg, Germany

⁎ Corresponding author. Tel.: +49 3731 39 4482; fax: +E-mail address: kureti@iec.tu-freiberg.de (S. Kureti).

http://dx.doi.org/10.1016/j.catcom.2014.09.0121566-7367/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 July 2014Received in revised form 6 September 2014Accepted 9 September 2014Available online 16 September 2014

Keywords:Fe/BEAZeoliteNH3 oxidationSCRKinetic modelling

In the present paper, the kinetics of the NH3 oxidation on Fe/BEA zeolite catalyst was examined representing aside reaction of the selective catalytic reaction of NOx. The kinetic studies were performed in a gradient-freeloop reactor between 723 and 773 K varying the concentration of NH3 and O2. The NH3 oxidation was describedby a global kinetic model implying the adsorption/desorption of NH3 followed by its reactionwith O2. The imple-mented kinetic parameters were suitable for the reproduction and prediction of the experiments over a broadrange of NH3 and O2 concentrations evidencing the reliability of the model.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Stringent emission limits for diesel engines require after-treatmenttechnologies to efficiently reduce the output of hydrocarbons, carbonmonoxide, nitrogen oxides (NOx) and soot [1]. For NOx abatement, se-lective catalytic reduction (SCR) is applied in heavy duty vehicles,buses and passenger cars. SCR is a well-established process for fossilpower plants and implies the reduction of NOx by NH3 according tothe standard (4 NO+ 4 NH3 + O2 → 4 N2 + 6 H2O) and fast (2 NO +2 NO2 + 4 NH3 → 4 N2 + 6 H2O) SCR reactions. In automobiles, NH3

is produced from an aqueous solution of urea. The most common SCRcatalyst is V2O5/WO3/TiO2 due to its high activity and selectivity, where-as for automotive exhaust Fe and Cu zeolite catalysts are increasinglyconsidered to be associated with their improved thermal stability[2–10]. However, in SCR on zeolites the NH3 oxidation occurs as aside reaction potentially limiting the deNOx efficiency as reported forFe/MFI and Fe/HBEA [11–16]. Four reaction pathways are thermody-namically possible (Eqs. (1)–(4)):

NH3 þ 0:75O2→0:5N2 þ 1:5H2O ΔRH0 ¼ −317

kJmol

ð1Þ

NH3 þ O2→0:5N2Oþ 1:5H2O ΔRH0 ¼ −276

kJmol

ð2Þ

NH3 þ 1:25O2→NOþ 1:5H2O ΔRH0 ¼ −227

kJmol

ð3Þ

49 3731 39 4555.

NH3 þ 1:75O2→NO2 þ 1:5H2O ΔRH0 ¼ −300

kJmol

: ð4Þ

Eqs. (2) to (4) are notably critical to meet NOx emission limits, sinceNH3 is directly converted into nitrogen oxides. Contrary, the oxidationof NH3 slipped (Eq. (1)) is required downstream to the SCR catalyst toavoid output of NH3. For this purpose, precious metal catalysts areused in diesel vehicles [17].

Themechanism of the ammonia oxidation waswell examined for Ptcatalysts associated with the industrial production of nitric acid [18].But, only little is known on the NH3 oxidation on iron oxides. Somepapers discuss imide [19] and amide intermediates [20], whereas theoxidation of NHx to NOx surface species subsequently undergoing SCRis also suggested [21]. Thus, the NH3 oxidation is often described byglobal kinetic approaches, particularly within the modelling of SCR onFe zeolite and vanadia [4–6,8,9,22,23].

In this work, we investigated the kinetics of the NH3 oxidation onFe/BEA catalyst revealing marked SCR performance [2,3]. The experi-mental data were used for the construction of a global kinetic modelof the NH3 oxidation to support current models of SCR.

2. Experimental

2.1. Fe/BEA catalyst

The Fe/BEA zeolite (Si/Al= 12.5)was fromNanoScape and revealeda Fe content of 1 wt.%. N2 physisorption was measured with an ASAP2020 (Micromeritics); after pre-treatment at 623 K the N2 adsorptionisotherm was recorded at 77 K resulting in a BET surface area of657 m2/g. Diffuse reflectance UV–Vis analysis (Lambda 850, Perkin

Fig. 1.Effective rate of NH3 oxidation on Fe/BEA at various temperatureswhen varying the content ofO2 (left) andNH3 (right). Conditions: GHSV= 25,000/h,m=60 mg/l, andψ=83; inO2 variation: y(O2) = 0.5–15 vol.%; in NH3 variation: y(NH3) = 200–3000 vppm.

109C. Hahn et al. / Catalysis Communications 58 (2015) 108–111

Elmer) showed that the Fe oxo sites mainly exist in the form of isolatedspecies (70%) with minor presence of oligonuclear clusters (30%) [2,7].The crystalline structure of BEA zeolite was confirmed by X-raydiffraction (D8 Advanced, Bruker). The particle size was determinedwith a FEI Quanta FEG 250 scanning electronmicroscope predominatelyindicating agglomerates from 2 to 50 μm, while primary particles weresized between 100 and 500 nm.

2.2. Kinetic studies

For kinetic studies a commercial cordierite honeycomb (250 cpsi, d:9mm, l: 18mm)was coatedwith the catalyst. The substratewas dippedseveral times into a slurry of 0.5 g catalyst, 0.2 g hydroxyethyl cellulose(Natrosol) and 15mlH2O followed by heating at 723 K for 2 h in air. Theresulting load was 50 g/l corresponding to 60 mg catalyst.

Kinetic measurements were performed in a gradient-free loop reac-tor with external gas cycle [24]. The reactor corresponded to a continu-ous stirred tank reactor (CSTR) as derived from residence time analysis.The honeycombwasfixed in the quartz glass tube (i.d.: 9mm) by quartzwool and was pre-treated at 723 K in a N2 flow of 1 l/min for 30 min toremove possible impurities, e.g. adsorbed hydrocarbons. After this, the

Fig. 2. Effective rate of NH3 oxidation on Fe/BEA (black) and BEA (white). Conditions:y(O2) = 5 vol.% and y(NH3) = 200 vppm (squares) and y(O2) = 15 vol.% andy(NH3) = 500 vppm (circles), m = 60 mg, m(BEA) = 54 mg, GHSV = 25,000/h, andψ = 83.

feed consisting of NH3, O2 and N2 was added. The feeds were blendsof the pure components (Air Liquide) dosed from flow controllers(Bronkhorst). Total flow (F) was 500 ml/min (STP) with a recycle ratio(ψ = Floop / Fout) of 83 corresponding to a gas hourly space velocity(GHSV) of ca. 25,000/h. Temperature was monitored by two K-typethermocouples located directly in front of and behind the honeycombandwas varied from773 to 723K; the difference of inlet and outlet tem-peratures was below 10 K. Gas-phase was analysed with a FTIRMultiGas Analyzer 2030 (MKS Instruments), whereas O2 was checkedby a broadband lambda probe (LSU 4.9, Bosch). Reactor effluents wererecorded after reaching a steady state. For reference, a honeycomb coat-ed with an iron-free BEA zeolite (loading: 56 g/l) was also tested.

The effective rates of NH3 oxidation (reff) were calculated withEq. (5), where yin,out is the NH3 content at the reactor in- and outlet,Vm the molar volume (STP), and V the volume of the honeycomb,while TN refers to 273 K and Tref to the temperature of the FTIR cell(464 K). Inhibition by internal and external mass transport was exclud-ed by estimating Weisz–Prater and Mears criteria [25,26], i.e. internaland external diffusion is faster as referred to chemical kinetics.

reff ¼yin−youtð Þ � F � TNVm � V � Tref

ð5Þ

Furthermore, the SCR activity of the Fe/BEA-coated honeycomb waschecked between 523 and 673 K dosing a feed of 500 vppm NO,500 vppm NH3, 5 vol.% O2, 2 vol.% H2O and 10 vol.% CO2 (N2 balance);GHSV was 50,000/h (ψ = 42). The stationary SCR data indicated NOx

Table 1Kinetic parameters of the model of NH3 oxidation on Fe/BEA.

Parameter Valuea Unit Source

A1 0.87 [m/s] [10]E1 0 [kJ/mol] [10]A2 4.97 · 107 [mol/(m2 s)] [27]E2 140.2 [kJ/mol] [27]αNH3 55.32 [kJ/mol] [27]A3 1.9 · 106 ± 1.8 · 10−6 [mol0.5/(m0.5 s)] This workE3 199.1 ± 26.1 [kJ/mol] This workm1 2.0 ± 0.2 – This workm2 0.5 ± 0.03 – This work

a Tolerances correspond to standard deviation of Arrhenius plot.

Fig. 3. Comparison of modelled and experimental effective rate of NH3 oxidation on Fe/BEA upon variation of O2 (left) and NH3 (right) at temperatures from 723 to 773 K; dashedlines: ±20% deviation. Conditions are demonstrated in Fig. 1.

110 C. Hahn et al. / Catalysis Communications 58 (2015) 108–111

and NH3 conversion of 20% at 523 K [2,3]. At 673 K deNOx was 45% andNH3 consumption 55%. This difference was due to the competitive NH3

oxidation [2]; no N2O formed (Eq. 2).

3. Results and discussion

3.1. Kinetics of the NH3 oxidation on Fe/BEA

The kinetic data of the NH3 oxidation showed an increase in reactionrate with rising temperature as well as content of O2 (Fig. 1, left) andNH3 (Fig. 1, right). Since neither NOx nor N2O formed above the detectorlimits (b10 vppm), we assume that NH3 is selectively converted into N2

and H2O representing the thermodynamically favoured path (Eq. (1)).However, we cannot exclude completely that to some extent NH3 isoxidised into NO promptly undergoing SCR. The resulting decrease inthe NH3 content would lead to slight decline of the intrinsic NH3 oxida-tion rate, whereas this effect was not explicitly considered by the globalkinetic model developed below. Additionally, the BEA zeolite alsoshowed NH3 oxidation, but lower as compared to Fe/BEA (Fig. 2).Furthermore, NOx was produced on BEA with a selectivity of ca. 18%,while NO2 and N2O did not form. These results evidence that the Fesites of Fe/BEA influence the activity and selectivity in NH3 oxidation.

3.2. Kinetic modelling

For the modelling of the NH3 oxidation on Fe/BEA according toEq. (1), we postulate a two-stage reaction scheme. The first stage

Fig. 4. Comparison of modelled and experimental effective rate of NH3 oxidation on Fe/BEA at temperatures from 723 to 773 K (dashed lines: ±20% deviation). Conditions (●):y(NH3) = 200 vppm and y(O2) = 19.7 vol.%; conditions (○): y(NH3) = 3000 vppmand y(O2) = 19.2 vol.%.; remaining conditions are demonstrated in Fig. 1.

reflects the adsorption and desorption of NH3 on the catalyst (Eq. (6)),which is known to be a crucial step of the NH3 oxidation as well asSCR reaction [6,23], while the second one implies the reaction ofadsorbed NH3 with gaseous O2 to form N2 and H2O (Eq. (7)). In linewith previous studies [10,27], it is assumed that NH3 predominatelyadsorbs on Lewis acid sites of the zeolite (S) at the temperatures inves-tigated. This is associated with the thermal instability of the NH4

+

species originated from chemisorption of NH3 on Brønsted acid sitesof the zeolite. The Lewis acid sites are supposed to be equivalent andno distinction between the sites of zeolite and iron is made, whereasthat of the former clearly prevails [2].

NH3 gð Þ þ S→r1

←r2

H3N−S ð6Þ

H3N−Sþ 0:75O2 gð Þ→r3 0:5N2 gð Þ þ 1:5H2O gð Þ þ S ð7Þ

The kinetics of theNH3 adsorption and desorption is described by anelementary kinetic mean field approach implying coverage dependentactivation energy for desorption (Eqs. (8) and (9)) [10]; E1 is the activa-tion energy of adsorption,α considers the dependency of the activationenergy of NH3 desorption (E2) on the NH3 coverage θNH3

� �due to repul-

sive interactions of the surface species [28] and Ai is the respective pre-exponential factor.

r1 ¼ A1 � exp − E1R � T

� �� cNH3

� 1−θNH3

� �ð8Þ

r2 ¼ A2 � exp −E2−αNH3

� θNH3

R � T� �

� θNH3ð9Þ

Eq. (10) formally expresses the kinetics of the NH3 oxidation(Eq. (7)), while E3 is the apparent activation energy, m1 the reactionorder of adsorbed NH3 and m2 that of gaseous O2.

r3 ¼ A3 � exp − E3R � T

� �� θm1

NH3� cm2

O2ð10Þ

111C. Hahn et al. / Catalysis Communications 58 (2015) 108–111

For the modelling, the stationary CSTR mass balance was taken(Eq. (11)); Sact is the surface area of the catalyst derived from its massand BET surface area. θNH3 (Eq. 10) was calculated with Eqs. (8) and(9) supposing equilibrated adsorption/desorption, whereas the corre-sponding kinetic parameters were adopted from previous studies [10,27].

0 ¼ n�

i;in−n�

i;out þ Sact �X

υi � ri ð11Þ

A3, E3, m1 and m2 were obtained from Arrhenius plots using theexperimental data demonstrated in Fig. 1. Ln(reff) was plotted againstln θNH3

� �and ln θO2

� �to derive m1 and m2, respectively, while A3 and

E3 were obtained from plotting ln(reff) against 1 / T. All kinetic parame-ters of the model are shown in Table 1.

The parity plots (Fig. 3) show good agreement between calculatedand experimental data demonstrating the relevance of the global kineticmodel. Its reliability is also substantiated by satisfactory prediction ofexperiments referring to low as well as high NH3 proportion in excessof O2 (Fig. 4). Only for the slow rates, which refer to the NH3-rich feedat 723 and 733 K, some non-random deviation between predictionand experiment appears. It may be speculated that this effect is ascrib-able to the additional coverage of the Brønsted acid sites at high NH3

gas-phase content thus affecting the adsorption/desorption equilibriumof molecularly adsorbed NH3 by repulsive interactions.

The kinetic model shows a higher dependency of the NH3 oxidationrate on θNH3 (m1 = 2) than on O2 (m2 = 0.5) concentration, whereby itis more sensitive towards NH3. This may confirm that adsorbed NH3

does not react directly with O2 yielding N2 and H2O, i.e. formation ofintermediates as discussed in the literature [19–21] is very likely.Despite this, m1 is often assumed to be 1 or less [4,6,8,22]. The apparentactivation energy of the oxidation of adsorbed NH3 on Fe/BEA wascalculated to be 199 kJ/mol being slightly higher as compared to litera-ture data. For instance, the activation energy on Cu zeolitewas specifiedto be 162 kJ/mol [6], while for Fe/BEA catalysts values between 166 and180 kJ/mol were reported [22,23]. For standard SCR, activation energieswere found to be between 70 and 90 kJ/mol depending on the activecomponent [4,6,22]. The later values are consistent with the higheractivation energy and increased operation temperature of the NH3

oxidation. Furthermore, the kinetic model of NH3 oxidation impliessufficient availability of adsorbed NH3. The rate of NH3 adsorption(Eq. 6) is for the temperatures studied,which is more than 3 orders ofmagnitude faster than that of NH3 oxidation (Eq. 7). Finally, the NH3

adsorption/desorption is not known to be interfered by gaseous O2.

Acknowledgements

The authors thankfully acknowledge the financial support by theEuropean Social Fund (DynMo, SAB 100113147) and the Federal Minis-try of Education and Research (Nano-SCR, 03X0079D). We also thankNanoScape for supplying zeolites.

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