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Catalysis Today xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Catalysis Today

j our na l ho me page: www.elsev ier .com/ locate /ca t tod

elective oxidation of ammonia on mixed and dual-layere-ZSM-5 + Pt/Al2O3 monolithic catalysts

achi Shresthaa, Michael P. Harolda,∗, Krishna Kamasamudramb, Aleksey Yezeretsb

Dept. of Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204-4004, USACorporate Research and Technology, Cummins Inc., 1900 McKinely Av., Columbus, IN 47201, USA

r t i c l e i n f o

rticle history:eceived 17 October 2013eceived in revised form 6 January 2014ccepted 7 January 2014vailable online xxx

eywords:mmonia oxidationMOXmmonia slip catalystSClatinum catalysteoliteelective catalytic reduction ;

a b s t r a c t

The development of improved Ammonia Slip Catalysts (ASCs) was pursued by changing the compositionand architecture of the critical functions, Selective Catalytic Reduction (SCR) and oxidation, needed inthe catalyst for high NH3 oxidation activity and selectivity to N2. Selected space velocities and reactantcompositions were used to probe chemical and physical processes that limit the performance of ASCs. Ona single component Pt/Al2O3 catalyst, at the lower space velocity (66k h−1) light-off was characterizedby a sharp increase in NH3 conversion to >98%, while at high space velocity (265k h−1) the transitionin conversion was more gradual and the limiting conversion was below 98%, indicative of transversetransport limitations. The light-off temperature of ammonia oxidation on Pt/Al2O3 catalyst decreasedwith Pt loading from 0.7 to 10 g Pt/ft3 monolith and, in general, the selectivity to N2 decreased while thatof N2O and NOx increased. A dual-layer ASC comprising a top layer of Fe-ZSM-5 and a bottom layer ofPt/Al2O3 resulted in higher selectivity and yield to N2, due to SCR reactions between the counter diffusingNOx formed in the Pt/Al2O3 and NH3 reactant in the Fe-zeolite layer. However, the diffusion resistanceprovided by the Fe-ZSM-5 layer inhibited the overall ammonia conversion at high space velocity. WhenFe-zeolite and Pt/Al2O3 particles were mixed and washcoated as a single layer, this led to an increase inNH3 conversion at high space velocity due to a decrease in the diffusion barrier that was observed with

the dual-layer structure. When SCR and oxidation catalyst particles were contiguous in the washcoatstructure as in single-layer mixed catalysts, the N2 yield was lower due to Pt-catalyzed NH3 oxidation,compared to the dual-layer ASC, which was especially apparent at low space velocity. The dual-layercatalyst was superior to the mixed layer catalyst at high temperatures, exhibiting lower NOx and higher N2

yields whereas the mixed catalyst out-performed the dual-layer catalyst at low temperature by exhibitinglower N2O and higher N2 yield.

. Introduction

Urea based selective catalytic reduction (SCR) of NOx is a widelyracticed technology for diesel engine vehicles to reduce NOx

NO + NO2) to N2 [1,2]. Urea is injected at a rate dictated by theO emission rate, which itself is a function of the engine operatingonditions, so that a nominal NH3:NO feed ratio of unity is achieveds required by the SCR stoichiometry. In order to meet the strin-ent EPA and European standards for NOx emissions, SCR catalystsequire occasional over-dosing of NH3, generated on-board by urea

Please cite this article in press as: S. Shrestha, et al., Selective oxidamonolithic catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.c

ydrolysis. Furthermore, under the dynamic conditions of vehicleperation, NH3 is stored on the Lewis and Bronsted acid sites of thee- or Cu-exchanged zeolites SCR catalysts [3,4] for NOx reduction

∗ Corresponding author.E-mail addresses: mharold@uh.edu (M.P. Harold),

rishna.kamasamudram@cummins.com (K. Kamasamudram).

920-5861/$ – see front matter © 2014 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.cattod.2014.01.024

© 2014 Elsevier B.V. All rights reserved.

reactions and also to minimize the concentration variations in NH3needed for NOx reduction i.e. adsorption sites acts as NH3 buffer.However, a sudden increase in the exhaust temperature may leadto NH3 desorption. SCR catalysts are generally poor NH3 oxidationcatalysts and NH3 over-dosing and desorption events could poten-tially lead to NH3 emissions to the environment, widely referredas NH3 slip. The “Ammonia Slip Catalyst” (ASC), positioned down-stream of the SCR catalyst, is used to oxidize such NH3 to preventits emissions in to environment [5,6].

For ASCs, Pt is the catalyst of choice given its high NH3 oxida-tion activity; however it also yields undesired products NOx andN2O whose selectivity depends on reaction temperature. In fact, Ptis the commercial catalyst for NH3 oxidation to NO as the first stepin nitric acid production [7], albeit at far more strenuous conditions

tion of ammonia on mixed and dual-layer Fe-ZSM-5 + Pt/Al2O3attod.2014.01.024

(T > 1000 ◦C, NH3 concentrations >10 vol.%) than those encoun-tered in the aftertreatment exhaust gas streams. NO and NO2 areobviously undesirable products of NH3 oxidation; their formationin the ASC decreases the overall effectiveness of the aftertreatment

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ARTICLEATTOD-8871; No. of Pages 11

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ystem whose function is to minimize the emission of NOx from theail pipe. N2O is a greenhouse gas (GHG) and its emission is subjecto emerging GHG regulations.

There is therefore a need to develop a very active, selective andompact NH3 oxidation catalyst, so that it takes up minimal spacen the aftertreatment system, and produces N2 as the main product.

oreover, this must be accomplished while reducing the cost of theSC by limiting the precious metal loading to «10 g/ft3 [8]. Severalpproaches have been taken by the exhaust aftertreatment com-unity to achieve the goals mentioned above. One such approach

nvolves the use of a bi-functional catalyst in which NOx that is pro-uced by NH3 oxidation on Pt based catalyst is selectively convertedo N2 with a second reduction catalyst component such as metalxchanged zeolite (SCR) catalysts. The bi-functional ASC can have

variety of architectures defined by the specific proximity of thet (oxidative function) and SCR (reductive function) components.n architecture that has been described in the recent literature is aual-layer ASC comprising a Pt/Al2O3 washcoat, itself coated with

Fe- or Cu-exchanged zeolitic catalyst [9,10].The basic working principle of the dual-layer ASC is described

s follows. NH3 in the flowing gas channel diffuses into the layeredatalyst, first encountering the top SCR catalyst layer. Some of theH3 is stored on this catalyst while the remaining diffuses to thenderlying Pt/Al2O3 layer. NH3 that reaches the Pt/Al2O3 layer maye oxidized by the excess O2 present to the various N-containingpecies through the following global reactions:

NH3 + 3O2 → 2N2 + 6H2O (R1)

NH3 + 2O2 → N2O + 3H2O (R2)

NH3 + 5O2 → 4NO + 6H2O (R3)

NH3 + 7O2 → 4NO2 + 6H2O (R4)

any studies have shown that at lower temperatures (R1) andR2) are dominant, while at higher temperature (R3) and (R4) areominant [11–19]. The NH3:O2 ratio is a second important param-ter with excess NH3 favoring N2 and N2O, and excess O2 favoringO and NO2 [11–13,15,16,20]. The products NO and NO2 that are

ormed in the bottom oxidation catalyst layer then diffuse throughhe top SCR catalyst layer where they may react with stored andounter-diffusing NH3 via selective catalytic reduction and oxida-ion chemistries described below:

NH3 + 4NO + O2 → 4N2 + 6H2O (R5)

NH3 + NO + NO2 → 2N2 + 3H2O (R6)

NH3 + 3NO2 → 3.5N2 + 6H2O (R7)

NH3 + 3O2 → 2N2 + 6H2O (R8)

NH3 + 4NO + 3O2 → 4N2O + 6H2O (R9)

The selective product N2 formed diffuses back to the flow chan-el and exits through the tail pipe, thus leading to the overallelective product conversion of NH3 to N2. Reactions (R5)–(R7)re called standard, fast and NO2 SCR, respectively. Several studiesave been conducted recently by different groups to understandhe effect of feed concentrations of NO and NO2 on rates of the SCReactions [4,21–25]. It has been determined that for an equimolarO and NO2 R6 is dominant, which is the fastest of all the SCR reac-

ions. Further, for higher NO2 content in the feed (R7) is dominant,nd at lower NO2 content (R5) is dominant. It has also been deter-ined that at higher temperature the direct NH3 oxidation, (R8), is

lso feasible on the SCR catalyst. By-products N2O may be formed

Please cite this article in press as: S. Shrestha, et al., Selective oxidamonolithic catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.

y reaction (R9).Most of the published studies reported to date on the ASC have

een modeling-focused[9,26–29], with a few notable exceptions.olombo et al. [27,30] reported experimental findings involving

PRESSday xxx (2014) xxx–xxx

powder catalysts in different configurations including the “DoubleBed” and “Mechanical Mixture”. These respectively mimicked dual-layer and mixed-catalyst configurations. Kamasamudram et al. [31]studied the behavior and the functionality of ASC by direct com-parison of ASC with diesel oxidation catalyst and SCR catalyst bythe means of accelerated progressive catalyst aging. Scheuer et al.[9] studied the effect of catalyst size and the thickness of the SCRlayer on NH3 oxidation through an experimentally-validated 2-Dmodel. In a recent study, our group reported data for the dual-layerASC comprising a bottom Pt/Al2O3 and Cu-ZSM-5 top layer sup-ported on a cordierite monolith [10]. That study provided for thefirst time experimental verification of the workings of the dual-layer architecture through the systematic variation of the SCR layerloading.

In this work we advance the understanding of the chemical andphysical processes occurring during ammonia oxidation on a seriesof synthesized ASCs of different catalyst architectures containingPt/Al2O3 and Fe-ZSM-5. Our objective is to examine the effect of thecomposition and architecture of the two critical ASC functions, SCRand oxidation, to achieve high NH3 oxidation activity and selectiv-ity to N2. Another objective of this study is to determine the catalystcomposition and architecture that maximizes the selectivity of NH3oxidation to N2 for a wide range of temperatures while minimizingthe Pt loading. To this end, the performance of a series of single com-ponent Pt/Al2O3 catalysts with the same overall washcoat loadingbut varied Pt loading are reported. These Pt/Al2O3 catalysts werethen modified with Fe-ZSM-5 catalyst. Two different structures,dual-layer and mixed, are compared to better understand the effectof proximity of the oxidation (of NH3) and reduction (of NOx) com-ponents. By keeping the loading of zeolitic catalyst and Pt/Al2O3catalyst on both dual and mixed ASC constant, this enables a directcomparison of structural configuration on NH3 conversion capabil-ity and its product selectivity. The findings are interpreted with aphenomenological model.

2. Experimental

2.1. Catalyst synthesis and characterization

Pt/Al2O3 catalysts with 0.03, 0.14 and 0.46% Pt loadings (massbasis) were prepared via the incipient wetness impregnationmethod. Aqueous slurry of �-Al2O3 was prepared by mixing 60 gof �-Al2O3 particles with 60 ml of deionized water. The Al2O3slurry was ball milled for approximately 20 h using an aluminaball milling machine. The pH of the slurry was adjusted to 3.5using acetic acid or ammonium hydroxide, which upon millinggave a particle size in the range 3–10 �m [32,33]. The slurry wasdried overnight at 120 ◦C, followed by calcination at 500 ◦C for5 h with a temperature ramp rate of 0.5 ◦C /min to yield Al2O3support. Pt precursor solution was prepared by mixing the appro-priate amount of chloroplatinic acid hexahydrate into deionizedwater, whose volume was equal to the manufacturer-provided porevolume of the �-Al2O3. The impregnation was carried out by drop-wise addition of Pt containing aqueous solution while constantlystirring the powder to obtain a uniform impregnation of Pt into�-Al2O3 particles. The resulting powder was dried overnight inan oven at 120 ◦C, followed by calcination at 500 ◦C for 5 h withthe temperature ramp rate of 0.5 ◦C/min to yield Pt/Al2O3 cata-lyst.

The Pt/Al2O3 slurry was prepared by mixing the Pt/Al2O3 pow-der, water and boehmite (AlOOH; 20 wt%) solution at a mass ratio of

tion of ammonia on mixed and dual-layer Fe-ZSM-5 + Pt/Al2O3cattod.2014.01.024

8:15:2. The pH of the slurry was adjusted to about 4 using acetic acidor ammonium hydroxide, and then ball milled for approximately20 h. Cordierite monolith cores of 400 cpsi provided by BASF (Iselin,NJ) were cut into 2 or 0.5 cm long and 0.8 cm diameter samples,

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oxidation.

Sampleno.

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monolith)Pt/Al2O3 wc loading(g/in3 monolith)

Pt loading (g Pt/100 gwashcoat)

Pt(10) 10.5 1.3 0.46

ARTICLEATTOD-8871; No. of Pages 11

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nd were washcoated with the catalyst by dipping the monolithnto the slurry solution for 30 s and blowing off the excess slurryor 10 s. To obtain as uniform a coating as possible, the washcoating

ethod was carried out by dipping the monolith from alternatingnds consecutively. The monolith was dried in an oven at 120 ◦Cor 3 h. The washcoating procedure was repeated until the desired

ass loading of Pt/Al2O3 was achieved. Finally, the monolith wasllowed to dry overnight at 120 ◦C and calcined at 500 ◦C for 5 hith a temperature ramp rate of 0.5 ◦C/min. The final mass load-

ng of the catalyst (in g washcoat/in3 of monolith) was determinedfter the calcination.

The Fe-ZSM-5 powder was provided by Sud Chemie (now Clari-nt Inc.). The Fe-ZSM-5 slurry was prepared by mixing Fe-ZSM-5owder, water and bohemite at the mass ratio of 4:10:5. The pHf the slurry was adjusted to ∼3.5 using acetic acid or ammo-ium hydroxide and ball milled for 20 h. A washcoating procedureimilar to the one described above was used to washcoat theatalyst onto the monolith. The catalyst was dried at 120 ◦C andalcined at 500 ◦C for 5 h with the temperature ramp rate of 0.5 ◦Cmin.

For the dual-layer catalyst, the zeolite catalyst was wash-oated onto the monolith that had been previously washcoatedith Pt/Al2O3. For the mixed catalyst, the requisite amount of

e-ZSM-5 powder and Pt/Al2O3 powder were physically mixedith a bohemite solution and water. The slurry pH was adjusted

o about 3.5–4, and was then ball milled for 20 h to ensure aniform mixing of the Pt- and Fe-containing catalysts. Similarashcoating, drying and calcination steps described above were

ollowed to obtain the appropriate loading of catalyst into theonolith. The freshly prepared catalyst consisting of Pt/Al2O3 was

educed at 500 ◦C for 30 min with 2% H2 and balance Ar at totalow rate of 1000 sccm. Finally, the catalysts were degreened at50 ◦C for 2 h with 5% O2 and balance Ar at the total flow rate of000 sccm.

The Pt dispersion (ratio of moles of exposed Pt to moles ofotal Pt) in the Pt/Al2O3 catalyst powder was measured by H2hemisorption with a Micromeritics Accelerated Surface Area andorosimetry System. The dispersion of Pt(10) catalyst was mea-ured as ∼35%. Energy Dispersive Spectroscopy (EDS) was used toetect any elemental Pt concentration in the top Fe-ZSM-5 layeror the dual layer catalyst to rule out Pt leaching from the bottomt/Al2O3 layer.

.2. Bench-scale monolith reactor system and steady-statexperiments

A detailed schematic and description of the reactor system cane found elsewhere [34]. The monolith catalysts used in the exper-

ment had 28 channels and were of two different lengths, 2 cm and.5 cm. FiberFraxTM ceramic paper was used to wrap the mono-

ith before mounting it to the heating furnace in order to seal theap between the monolith and tube walls. The analytical systemonsisted of a FTIR (Thermo-Nicolet, Nexus 470) and a quadrupoleass spectrometer (MKS Spectra Products; Cirrus LM99). The FTIRas used to measure concentration of effluent reactant and prod-ct gases such as NO, NO2, N2O, NH3, and H2O, and the QMS wassed to measure the N2 concentration in order to check the overall

balance; the nitrogen balance could be closed within 10%. The N2ield plots reported in this paper were obtained by mass balancenless otherwise indicated. The data acquisition and control sys-em comprised two PCs and an ADAM 5000 TCP (Advantech Ind.)

odule.

Please cite this article in press as: S. Shrestha, et al., Selective oxidamonolithic catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.c

To evaluate the steady state performance at different spaceelocities the flow rate of inlet gas was kept constant at 1000 sccmhich resulted in a space velocity (GHSV) of 66,000 h−1 for a 2 cm

ong monolith and 265,000 h−1 for a 0.5 cm long monolith. Before

Pt(3) 3.0 1.2 0.14Pt(0.7) 0.7 1.4 0.03

the activity measurements, the catalyst was pretreated in oxidizingenvironment with 5% O2 and balance Ar for 30 min at the tem-perature of 650 ◦C. Unless otherwise mentioned, the steady stateNH3 oxidation activity was measured with 500 ppm NH3, 5% O2in Ar. Ammonia oxidation was studied in the temperature range150–500 ◦C. The concentrations of effluent gas were measured bythe FTIR at the constant temperature and pressure of 140 ◦C and880 mmHg. The reaction was carried out for a sufficiently longperiod at each temperature until the effluent gases reached steadyvalues. Typically the lower the reaction temperature the longer ittook to reach steady state. In order to avoid the reactor contami-nation with Pt, different quartz tubes were used for each catalysttested.

In order to study the effect of Pt loading on the light-off behav-ior and product selectivity of the NH3 oxidation reaction, a series ofPt/Al2O3 catalyst samples were synthesized and washcoated ontothe monolith. The overall washcoat (wc) loading (g wc/in3 mono-lith) was kept constant while varying the concentration of Pt inthe washcoat, thus achieving the catalyst with constant washcoatthickness and varied Pt loading. The above-described approach wasfollowed to maintain the same washcoat thickness. The nomencla-ture for these catalysts was Pt(X) where X refers to the Pt loading(in g Pt/ft3 monolith).

To assess the effect of coupling a Pt/Al2O3 catalyst with a metalexchanged zeolite catalyst on the light-off behavior and productselectivity of NH3 oxidation reaction, a series of dual-layer andmixed catalysts were compared. The dual-layer catalyst compriseda bottom layer of Pt/Al2O3 and a top layer of Fe-ZSM-5. The wash-coat loading of Pt/Al2O3 was approximately 1.3 g/in3 for each ofthe catalyst whereas the loading of Fe-ZSM-5 catalyst was approx-imately 1.5 g/in3. For the mixed catalyst, both the Pt/Al2O3 andFe-ZSM-5 catalyst were mixed together and applied as a singlelayer. The total washcoat loading of the catalyst was approximately2.8 g/in3, and it was comprised of Pt/Al2O3 and Fe-ZSM-5 catalyst inthe ratio of 1.3:1.5. This way the dual-layer and mixed catalyst hadall the same loadings, etc, but different structural features. Thesecatalysts were named FeZ(XX)Pt(YY) where XX denoted the wash-coat loading of the Fe-ZSM-5 layer and YY denoted the Pt loadingin the Pt/Al2O3 layer.

3. Results and discussion

3.1. Effect of Pt loading on NH3 conversion and product selectivity

The chemical and physical rate processes and their interac-tions during NH3 oxidation were studied over a series of Pt/Al2O3catalysts with constant washcoat loading (g/in3) but different Ptloadings (wt%). As can be seen from Table 1, there were small dif-ferences in the washcoat loadings on the monoliths used. Such avariation is expected to be inconsequential to the thickness of thewashcoat layer. Moreover, the chemical and physical processes areprimarily affected by Pt loading. Figs. 1 and 2 show the NH3 conver-

tion of ammonia on mixed and dual-layer Fe-ZSM-5 + Pt/Al2O3attod.2014.01.024

sion and product yield as a function of temperature for the spacevelocities of 66k h−1 and 265k h−1, respectively, on the Pt-onlycatalysts. NH3 oxidation activity, expressed as conversion, andproduct selectivity (N atom basis) greatly depend on the operating

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4 S. Shrestha et al. / Catalysis Today xxx (2014) xxx–xxx

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onditions, particularly catalyst temperature, due to the occurrencef several parallel and consecutive reactions (R1)–(R4). In order ofncreasing temperature, the N-containing products are N2, N2O, andO. Furthermore, the formed NO may be oxidized further to NO2ith the extent of the reaction limited by reaction equilibrium.

The ammonia oxidation is a highly nonlinear function of tem-erature, particularly at higher Pt loadings and/or lower spaceelocities. Fig. 1(a) shows a sharp increase in the NH3 con-ersion with temperature, in agreement with previous works10,10,13,18,19,31,35]. The increase is most abrupt for the highestt loading catalyst, Pt(10), with the conversion increasing from ca.0% to 98% over a 15 ◦C increase in temperature. In contrast, the NH3onversion profile for the lowest Pt loading catalyst, Pt(0.7), is moreradual. Specifically, the light-off for Pt(10) has a narrow temper-ture window of 175–190 ◦C the light-off for Pt(0.7) occurred over

wider temperature window of 200–255 ◦C. The light-off temper-ture decreases with increasing Pt loading due to higher rate ofmmonia oxidation, also reported by others [36,37]. Beyond theight-off, the NH3 conversion is essentially complete for the 66k h−1

pace velocity for all three catalysts. A decrease in Pt loading in thet/Al2O3 leads to an increase in the light-off temperature.

Please cite this article in press as: S. Shrestha, et al., Selective oxidamonolithic catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.

At the higher space velocity for each of the Pt loadings the NH3ight-off was more gradual and also shifted to high temperaturesFig. 2(a)). The impact of the space velocity on the conversion can bexplained by using the transverse Peclet number, which provides

N2 yield, (c) N2O yield and (d) NOx yield. Reaction conditions: 500 ppm NH3, 5% O2

insight into the extent of external mass transport limitations. Thetransverse Peclet number is the ratio of the radial diffusion to axialconvection (flow) times (Pet = �d/�c), and is defined as

Pet = 〈u〉L

(R2

˝1

Df

)(1)

where R�1 is effective transverse diffusion length (defined as theratio of channel cross sectional area to the fluid–washcoat interfa-cial perimeter), 〈u〉 is average gas velocity in the monolith channel,L is the channel length, and Df is the diffusivity of the NH3 in the gasphase. At 66k h−1 and 300 ◦C, Pet ∼ 0.036. An increase in the spacevelocity to 265k h−1 gives Pet ∼ 0.14. This four-fold increase in theratio of the transverse diffusion time to flow time helps to explainwhy the ammonia conversion approaches an asymptotic conver-sion that is incomplete at 265k h−1. More details on this analysiswill be forthcoming in the modeling study of NH3 oxidation onbifunctional catalysts.

The effect of temperature on the product distribution, shownin Figs. 1(b)–(d) and 2(b)–(d), is consistent with previous studiesof Pt-catalyzed ammonia oxidation [10,19,20,30]. From Figs. 1(b)

tion of ammonia on mixed and dual-layer Fe-ZSM-5 + Pt/Al2O3cattod.2014.01.024

and 2(b) it can be seen that desired product N2 is a major productof NH3 oxidation in the temperature range 200–250 ◦C, exhibitinga maximum of 70% at ∼230 ◦C for Pt(10). For a fixed temperaturebeyond the maximum, the N2 yield decreases monotonically with

ARTICLE IN PRESSG ModelCATTOD-8871; No. of Pages 11

S. Shrestha et al. / Catalysis Today xxx (2014) xxx–xxx 5

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ig. 2. Steady state NH3 oxidation reaction for Pt/Al2O3 catalysts. (a) NH3 conversiond 265k h−1 GHSV.

ncreasing Pt loading. Above 255 ◦C the N2 yield is higher for theatalyst with the lowest Pt loading. From Fig. 2(b) it can be seenhat the N2 yield profile for a GHSV of 265k h−1 is similar to that for6k h−1, however, there is a decrease in maximum N2 yield for allhree Pt catalysts, due to a decrease in the overall NH3 conversion.or the lowest Pt loading the maximum N2 yield decreases in mag-itude while the temperature at which it occurs moves to higheralues.

Similar trends were obtained for the N2O yield as a function ofhe temperature and Pt loading (Figs. 1(c) and 2(c)). At both spaceelocities, N2O was detected over a wide temperature range of00–500 ◦C, exhibiting a maximum in the range 250–300 ◦C [19,30].he N2O yield is sensitive to the Pt loading, with Pt(10) giving aigher N2O yield than Pt(3) and Pt(0.7). Further, a comparison ofhe N2O yield trends for the catalyst with same Pt loading indicatesn overall decrease in N2O yield with space velocity.

Figs. 1(d) and 2(d) show the NO and NO2 yields. NO is the majorroduct of NH3 oxidation at high temperature and increases almost

inearly with temperature, a trend seen for both space velocities10,19,30]. NO2 is a secondary NOx product that emerges at a higheremperature than NO. The NO yield shows a monotonically increas-

Please cite this article in press as: S. Shrestha, et al., Selective oxidamonolithic catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.c

ng trend with temperature for all of the catalysts whereas the NO2ield exhibits a maximum at an intermediate temperature for thet(10) catalyst due to the fact that the oxidation of NO to NO2 isimited by thermodynamic equilibrium at higher temperature. The

N2 yield, (c) N2O yield and (d) NOx yield. Reaction conditions: 500 ppm NH3, 5% O2

NO2 fraction of the effluent decreases and that of NO increases asthe Pt loading is decreased. However, it is noted that the total NOx

yield at 66k h−1 for all three catalysts was quantitatively similardespite the differences in the Pt loading. At a GHSV of 265k h−1,there is a slight decrease in NOx yield with a decrease in Pt loadingof the catalyst (not shown). Compared to 66k h−1, the NO2 frac-tion is significantly lower at the higher space velocity for each ofthe three catalysts, which shows that the NO to NO2 oxidation islimited by the reaction kinetics.

The maximum in N2O yield at intermediate temperature(250–300 ◦C) is attributed to the balance between the rates of NH3and O2 adsorption/desorption (R10) and (R11), NH3 bond scission(R12), NO bond formation (R13), N2O formation (R14) and (R15),NO (R16) and N2O desorption (R17) [11,15]:

NH3 + ∗ ↔ NH3∗ (R10)

O2 + 2∗ ↔ 2O∗ (R11)

NHx∗ + 0.5xO∗ ↔ N ∗ + 0.5xH2O ∗ (x = 1–3) (R12)

N ∗ + O∗ ↔ NO ∗ + ∗ (R13)

tion of ammonia on mixed and dual-layer Fe-ZSM-5 + Pt/Al2O3attod.2014.01.024

N ∗ + NO∗ ↔ N2O ∗ + ∗ (R14)

2NO∗ ↔ N2O ∗ + O∗ (R15)

NO∗ ↔ NO + ∗ (R16)

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ARTICLEATTOD-8871; No. of Pages 11

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2O∗ ↔ N2O + ∗ (R17)

At low temperatures (<200 ◦C) the Pt surface is predominantlyccupied by N adatoms [11,13]. The generation of surface NO viaeaction R13 commences at about 200 ◦C. Once formed, surface NOay either dissociate (reverse of (R13)), react (R14) and (R15), or

esorb (R16). If the rate of NO desorption (R16) exceeds that of theum of rates of N2O formation (R14) and (R15), then the net ratef N2O formation will decrease. It is generally established that N2Oormation during the exposure of NO to Pt decreases with increas-ng temperature [9,19]. The decrease in N2O yield with decreasingt loading has been reported previously [37,38]. Pt mostly exists assolated atoms or small clusters for lower loading catalysts and asarger crystallites for higher loading catalysts. The larger Pt clustersre thought to be responsible for the higher N2O selectivity, and inurn the lower N2 selectivity; there is likely to be a larger fractionf clusters for Pt(10) than Pt(0.7).

The monotonic increase in NO yield with temperature for > 300 ◦C is due to the onset of NO desorption. Surface NO maylso react with O adatoms to form NO2:

R18)NO ∗ + O∗ ↔ NO2∗ + ∗

O2∗ ↔ NO2 + ∗ (R19)

he NO2 yield maximum for Pt(10) at low space velocity indicateshe onset of reaction equilibrium limitations (increasing rate of NO2ecomposition, reverse R18 reaction) [30]. The absence of a max-

mum in NO2 for the lower loading catalysts Pt(3.0) and Pt(0.7)ndicates that the formation of NO2 is kinetically-limited. At highpace velocity the decrease in NOx (NO + NO2) with decreasing Ptoading is attributed to a decrease in the NH3 conversion. Finally,he decrease in NO2 yield at high space velocity suggests that NOs formed in the front section of monolith followed by further oxi-ation downstream. The decreased contact time at higher spaceelocity results in less NO being oxidized.

.2. Effect of Pt loading on dual-layer catalyst performance

Figs. 3 and 4 show the NH3 conversion and product distributionuring NH3 oxidation at GHSV of 66 and 265k h−1, respectively, onual-layer catalysts having a fixed loading of Fe-ZSM-5 in the top

ayer and a varying Pt loading of Pt/Al2O3 in the bottom layer (refero Table 2). Some of the qualitative trends in the NH3 conversionersus temperature (Figs. 3(a) and 4(a)) for the three dual-layeratalysts are similar to those obtained for the single layer Pt/Al2O3atalyst (refer to Figs. 1(a) and 2(a)). For example, the light-off tem-erature increases with decreasing Pt loading and the conversionecreases with increased space velocity. The addition of the Fe-SM-5 top-layer attenuates the NH3 conversion (Figs. 3(a) and 4(a))s compared to the Pt-only catalyst (Figs. 1(a) and 2(a)) [9,10]. Theffect of a decreased Pt loading results in a higher temperature NH3ight-off, a trend which is more apparent for the dual-layer cat-lysts. Similarly, complete NH3 conversion is reached at relativelyower temperatures on the single component catalysts compared tohe dual-layer catalysts. These trends reflect the fact that the addedeolite layer serves as a barrier to the supply of NH3 to the under-ying Pt layer. Metkar et al. [4] established that washcoat diffusionimitations commenced at ∼300 ◦C for standard SCR on Fe-ZSM-5.he fact that NH3 oxidation on Pt/Al2O3 is very fast even at temper-tures as low as 200 ◦C makes it extremely sensitive to diffusionalransport of NH3 in the Fe-ZSM-5 top layer. Our findings are con-istent with those of Colombo et al. [39] who in their modeling

Please cite this article in press as: S. Shrestha, et al., Selective oxidamonolithic catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.

tudy of an ASC dual layer catalyst showed evidence for diffusionimitations caused by the top zeolite layer. At higher space veloc-ty the rate limitation posed by the transport of reacting specieshrough the top layer is more apparent. This is evident by the

PRESSday xxx (2014) xxx–xxx

significantly lower NH3 conversion compared to the single layerPt/Al2O3 catalyst [9,10]. The results highlight the practical findingthat the design of the dual-layer catalyst design must consider thetrade-off between achieving a high N2 selectivity and a high NH3conversion. We return to this point later.

The reduction in the high temperature (T > 250 ◦C) NH3 conver-sion for the dual-layer catalysts, especially apparent at the high SV(Fig. 4(a)), is attributed to a reduction in the mass transport of thelimiting NH3 reactant, due to the addition of Fe-ZSM-5 as a toplayer, to the Pt/Al2O3 surface [10]. If we assume the top layer isa porous inert layer, a modified transverse Peclet number (Pet,m)defined as a ratio of transverse effective diffusion time from bulkgas to the Pt/Al2O3 surface (sum of diffusion times from bulk gasto Fe-ZSM-5-gas interface and from Fe-ZSM-5-gas interface to Fe-ZSM-5-Pt/Al2O3 interface) to axial convection time, provides anestimate of the overall mass transfer limitation on the ammoniaconversion due to added layer of Fe-ZSM-5:

Pem,t = 〈u〉L

(R2

˝1

Df+ ı2

De

)= 〈u〉

LDf

(R2

˝1+ ı2

0.0125

)(2)

The second term represents the time constant for diffusionthrough the SCR washcoat layer were it to be inert. Clearly, theactual top layer is active so this analysis only approximates the dif-fusional effect. The effective diffusional effect for an active layerwould be expected to be more significant. Using De ∼ 0.0125Df [40]for diffusion through the zeolite layer and using measured valuesfor ı = 33 × 10−6 m, the second term in the brackets gives the valueı2/0.0125 = 0.09 × 10−6. This term augments the external transportterm (R2

˝1= 10−6m2). Thus, the addition of the SCR layer increases

the extent of the transport limitations. With the SCR layer not trulyinert this analysis is for qualitative instruction only.

The product selectivity data in Fig. 3 shows the benefit of addingthe SCR layer on top of the Pt layer. At higher temperatures N2becomes the main product compared to NOx in the case of thePt-only catalysts and such trend is not obvious at low tempera-tures. From Fig. 3(b) it can be seen that below 250 ◦C all threedual-layer catalyst have N2 yields that are slightly lower than thoseobtained for the Pt/Al2O3 single layer catalyst. The somewhat lowerN2 yield for the dual-layer catalysts is a result of the aforemen-tioned decrease in NH3 conversion caused by the Fe-ZSM-5 toplayer diffusion barrier. The N2 yield profile has a local minimumat about 250 ◦C, consistent with previous data for the dual-layerPt/Cu-ZSM-5 [10]. This feature is a result of the combined activitiesof the two layers. Below 250 ◦C the Fe-ZSM-5 is not a very activeoxidation catalyst (Fig. 5(a)). Therefore at lower temperatures theproduct N2 is mainly generated in the Pt/Al2O3 layer. Earlier weshowed that a maximum in N2 yield is achieved by the single layerPt catalyst at around 230 ◦C (Fig. 1(d)). A further increase in thetemperature leads to a decrease in the N2 yield from the Pt layerwhile the Fe-ZSM-5 activity increases. The net effect for these cat-alysts is the preservation of the local maximum (as in Fig. 1(b)) butthe emergence of a local minimum, brought about by the increasedactivity of the Fe-ZSM-5 top layer at a somewhat higher temper-ature. In effect, most of the N2 formed below 250 ◦C comes fromNH3 oxidation on the Pt/Al2O3 bottom layer, while the N2 formedabove 300 ◦C mostly comes from oxidation on the Fe-ZSM-5 toplayer. An important, additional contribution is the reaction of NOx

generated by the Pt/Al2O3 reacting with counter-diffusing NH3 inthe Fe-ZSM-5 layer.

To underscore these points, Fig. 5 reports the activity of the sin-gle layer Fe-ZSM-5 for NH3 oxidation (a), and standard SCR (b),

tion of ammonia on mixed and dual-layer Fe-ZSM-5 + Pt/Al2O3cattod.2014.01.024

involving an equimolar feed mixture of NH3 and NO (500 ppm each)in 5% O2. These data show that the Fe-ZSM-5 is active for bothammonia oxidation and the standard SCR reaction above 250 ◦C[4]. Thus, the high selectivity towards N2 observed at temperatures

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S. Shrestha et al. / Catalysis Today xxx (2014) xxx–xxx 7

F lysts.

5

atoh

lltmpft

TC

ig. 3. Steady state NH3 oxidation reaction for dual layer Fe-ZSM-5 and Pt/Al2O3 cata00 ppm NH3, 5% O2 and 66k h−1 GHSV.

bove 350 ◦C is due to the combination of NH3 oxidation by O2 inhe Fe-ZSM-5 layer, which is highly selective to N2, as well as NH3xidation by back diffusing NOx from Pt/Al2O3 layer, which is alsoighly selective to N2.

The N2 yield profiles for all three dual-layer catalysts at theower space velocity (Fig. 3(b)) indicate that the intermediate Ptoading catalyst FeZ(1.5)Pt(3) exhibits the highest N2 yield overhe entire temperature range of study (150–500 ◦C). In the low to

oderate temperature but higher range than the light-off tem-

Please cite this article in press as: S. Shrestha, et al., Selective oxidamonolithic catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.c

erature of Pt/Al2O3 (225–350 ◦C), the higher N2 yield obtainedor FeZ(1.5)Pt(3) catalyst as compared to FeZ(1.5)Pt(10) is dueo more selective NH3 oxidation, consistent with earlier results

able 2atalysts used to study the effect of Pt loading on performance of dual layer and mixed am

Sample no. Pt loading (g/ft3

monolith)Pt/Al2O3 wc Loadin(g/in3 monolith)

FeZ(1.5)Pt(10)a 10.5 1.3

FeZ(1.5)Pt(3) 3.0 1.3

FeZ(1.5)Pt(0.7) 0.7 1.2

FeZ(1.5)Pt(10)b 10.5 1.3

FeZ(1.5) 0 1.3c

a Dual layer catalyst.b Mixed catalyst.c No Pt content in the washcoat; Al2O3 only.

(a) NH3 conversion, (b) N2 yield, (c) N2O yield and (d) NOx yield. Reaction conditions:

shown in Fig. 1(b). The lower N2 yield obtained for FeZ(1.5)Pt(0.7)compared to other catalysts is attributed to an overall decreasein NH3 conversion, also in agreement with earlier experimentalfindings for the single layer Pt catalyst (Fig. 1(a)). At higher tem-peratures (>350 ◦C), all three catalyst are highly selective to N2,with FeZ(1.5)Pt(10) giving a slightly lower selectivity to N2 com-pared to other catalysts. In contrast, the N2 yield profile for thesame reaction at the higher space velocity of 265k h−1, Fig. 4(b),shows different trends compared to lower space velocity. At high

tion of ammonia on mixed and dual-layer Fe-ZSM-5 + Pt/Al2O3attod.2014.01.024

GHSV, the local maxima and minima for the N2 yield profile is notas apparent for FeZ(1.5)Pt(10), and is completely absent for theother two catalysts. Both FeZ(1.5)Pt(10) and FeZ(1.5)Pt(3) catalysts

monia slip catalysts.

g Pt loading (g Pt/100 gwashcoat)

Fe-ZSM-5 loading(g/in3 monolith)

0.46 1.50.14 1.40.03 1.50.46 1.50 1.5

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8 S. Shrestha et al. / Catalysis Today xxx (2014) xxx–xxx

F lysts.

5

stFcseli

adfZiaPtcctNie1d

ig. 4. Steady state NH3 oxidation reaction for dual layer Fe-ZSM-5 and Pt/Al2O3 cata00 ppm NH3, 5% O2 and 265k h−1 GHSV.

how comparable N2 yields at temperature above 225 ◦C evenhough FeZ(1.5)Pt(10) has a 10–15% higher NH3 conversion thaneZ(1.5)Pt(3). Thus the lower NH3 conversion for FeZ(1.5)Pt(3) isompensated by its higher selectivity towards N2. However, theame reasoning does not hold for FeZ(1.5)Pt(0.7) catalyst, whichxhibits a 20% decrease in the N2 yield compared to other two cata-ysts at temperatures above 225 ◦C. This is due to the sharp decreasen the overall NH3 conversion by 20–40% in this temperature range.

Figs. 3(c) and 4(c) show the N2O yield profile at GHSV of 66knd 265k h−1, respectively. A comparison between the single andual-layer catalysts, Figs. 1(c) and 3(c), reveal a decrease of N2Oormation for temperatures above 250 ◦C when a top layer of Fe-SM-5 is present. Such a decrease can be explained by (i) decreasen the NH3 conversion on the dual-layer catalysts, most apparentt the higher space velocity, and (ii) above 250 ◦C NOx generated ont/Al2O3 is consumed by the selective SCR reaction in the Fe-ZSM-5op layer thereby decreasing the overall NH3 flux to the Pt/Al2O3atalyst, and (iii) the SCR reaction in Fe-ZSM-5 also decreases theoncentration of NO, considered an intermediate for N2O forma-ion on the Pt/Al2O3 catalyst. At temperatures above 350 ◦C, the2O yield approaches zero for all three dual-layer catalysts. This

Please cite this article in press as: S. Shrestha, et al., Selective oxidamonolithic catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.

s in contrast to the three single layer Pt/Al2O3 catalysts whichxhibit a low but non zero N2O yield; e.g. the N2O yield is about0% at 400 ◦C for Pt(10) (Fig. 1 (c)). The reduced N2O yield may beue in part to the consumption of N2O via reduction by NH3, since

(a) NH3 conversion, (b) N2 yield, (c) N2O yield and (d) NOx yield. Reaction conditions:

Fe-ZSM-5 is active for N2O SCR in this temperature range[41]. Another contributing factor is N2O decomposition which isenhanced in the presence of NOx [41–43].

The formation of undesired NOx by the NH3 oxidation onPt/Al2O3 at higher temperatures can be mitigated by the doublelayer catalyst. At 66k h−1, the onset of NOx yield (Fig. 3(c)) occursat 250 ◦C which is consistent with the temperature at which NOx

formation commences on the Pt/Al2O3 catalyst (Fig. 1(d)). How-ever, in contrast to the Pt/Al2O3 catalysts, the NOx yield does notexhibit a monotonically increasing trend with temperature for thedual layer catalysts [9,30,31]. In fact, the NOx yield stays below11% for the entire temperature range. Moreover, a local maximumis observed in the temperature range 320–350 ◦C. The maximumis evidence that NOx generated in the underlying Pt/Al2O3 layerreacts with NH3 in the Fe-ZSM-5 top layer as the temperatureincreases above 300 ◦C. As shown in Fig. 5(b), the SCR activity ofFe-ZSM-5 increases considerably in this temperature ranges. Themore active is the Fe-ZSM-5 layer the less NH3 that is able to reachthe Pt layer due to its reaction with counter-diffusing NOx in thezeolite layer. Thus, the observed NOx concentration is determinedby the balance between the NOx generated by NH3 oxidation in

tion of ammonia on mixed and dual-layer Fe-ZSM-5 + Pt/Al2O3cattod.2014.01.024

the Pt/Al2O3 layer and the NOx consumed in the Fe-ZSM-5 layer.The decrease in NOx yield at high temperature explains the cor-responding increase in N2 yield (Fig. 3(b)). The increase in thedesired N2 yield at the expense of the NOx yield demonstrates the

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S. Shrestha et al. / Catalysis Today xxx (2014) xxx–xxx 9

Fig. 5. Steady state NH3 and NO conversion for Fe-ZSM-5 catalysts for various feedc5

wNZ[

hltthtanoslcr

a

Fig. 6. Comparison of activity and product selectivity of dual layer and mixed cata-

catalyst exhibit NH3 oxidation light-off at ∼230 ◦C, however, a few

ondition. (a) In absence of NO and (b) In presence of NO. Reaction conditions:00 ppm NH3, 500 ppm NO, 5% O2 and 66k h−1 GHSV.

orking principle of dual-layer catalyst: NOx that is formed throughH3 oxidation in the bottom Pt/Al2O3 layer diffuses to the Fe-SM-5 layer where it selectively reacts with counter diffusing NH39,10].

At the lower space velocity, the dual-layer catalyst with theighest Pt loading, FeZ(1.5)Pt(10), was the least effective cata-

yst in terms of the high temperature N2 yield. Fig. 3(b) showshat the intermediate loading dual-layer catalyst, FeZ(1.5)Pt(3), ishe most effective catalyst. The N2 yield for FeZ(1.5)Pt(3) is 3–5%igher than that of the other two catalysts except at very lowemperature. Moreover, NO2 was not observed for FeZ(1.5)Pt(3)nd FeZ(1.5)Pt(0.7), whereas the NO2 concentration was small butonzero for FeZ(1.5)Pt(10). This can be attributed to the higher NOxidation activity of the catalyst with higher Pt loading, as washown earlier (Fig. 1(d)). The very low yield of NO2 for the dual-ayer catalyst shows that the NO2 produced on the bottom Pt/Al2O3atalyst is consumed quickly at top SCR layer through the fast SCR

Please cite this article in press as: S. Shrestha, et al., Selective oxidamonolithic catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.c

eaction (R6).Unlike the results at the lower space velocity, the FeZ(1.5)Pt(10)

nd FeZ(1.5)Pt(3) have nearly equal effectiveness in converting

lyst comprising of Fe-ZSM-5 loading of 1.5 g/in3 and Pt loading of 10.5 g/ft3 for NH3

oxidation reaction. (a) Dual layer catalyst, (b) Mixed catalyst. Reaction conditions:500 ppm NH3, 5% O2 and 265k h−1 GHSV.

NH3 to N2 in the high temperature regime (Fig. 4(b) and (d)) athigher space velocity. The increased transverse diffusion limitationinhibits the NH3 supply which is detrimental to both conversionand N2 yield. This may also be attributed to the lower productionof NO2 by NO oxidation which would tend to decrease the extentof selective catalytic reduction.

3.3. Comparison between dual-layer and mixed catalystcomprising Fe-ZSM-5 and Pt/Al2O3

A comparison of dual-layer and mixed catalysts is shown inFig. 6. The mixed catalyst comprised of a physically-mixed Fe-ZSM-5 and Pt/Al2O3 washcoat applied onto the monolith as a single layer.Fig. 6(a) and (b) reports the NH3 conversion and product distri-bution for the dual-layer and mixed catalyst, respectively, at thehigher space velocity of 265k h−1. Both the dual-layer and mixed

tion of ammonia on mixed and dual-layer Fe-ZSM-5 + Pt/Al2O3attod.2014.01.024

key differences are apparent at temperatures above 250 ◦C. First,the mixed layer catalyst gives a ∼7% higher NH3 conversion thanthe dual-layer catalyst. For example, at 400 ◦C the NH3 conversion

IN PRESSG ModelC

1 sis Today xxx (2014) xxx–xxx

wTrSlol4licPflNNayaaIaft

idcdtNsFrtmcpr(ftlbFtlTftsmsdyPtFh

tottlAb

Fig. 7. Comparison of activity and product selectivity of dual layer and mixed cata-lyst comprising of Fe-ZSM-5 loading of 1.5 g/in3 and Pt loading of 10.5 g/ft3 for NH3

oxidation reaction. (a) Dual layer catalyst, (b) mixed catalyst. Reaction conditions:−1

ARTICLEATTOD-8871; No. of Pages 11

0 S. Shrestha et al. / Cataly

as 86% for the dual-layer and 93% for the mixed layer catalyst.he higher conversion for the mixed layer catalyst points to theeduced diffusion barrier for NH3 to reach the Pt crystallites [39].econd, while the N2O yield is similar below 300 ◦C, with both cata-ysts exhibiting maximum values of 38% at 275 ◦C, detectable levelsf N2O were not found at temperatures above 380 ◦C for the dual-ayer catalyst whereas a measurable N2O yield was obtained up to50 ◦C for the mixed catalyst. Thus it would appear that the mixed

ayer catalyst, with its shorter diffusion length, is not as effectiven reducing the N2O yield as was shown earlier for the dual-layeratalyst. That is, N2O formed during NH3 oxidation catalyzed byt in the mixed layer catalyst is able to diffuse more easily to theowing gas compared to the dual-layer catalyst. Third, similar to2O, not much difference is seen between the two catalysts for theO and N2 yield profiles at temperatures below 350 ◦C. However,t higher temperature the mixed catalyst has a slightly higher NOield and lower N2 yield than the dual-layer catalyst. For example,t 500 ◦C, the NO yield for the dual-layer and mixed catalysts arebout 11% and 22%, and N2 yields about 77% and 73%, respectively.n addition, above 350 ◦C the NO yield profile for the dual-layer cat-lyst is a moderately decreasing function of temperature whereasor the mixed catalyst the NO yield exhibits a moderate increasingrend.

The two catalysts were also compared at the lower space veloc-ty of 66k h−1. Fig. 7(a) and (b) gives the NH3 conversion and productistribution for NH3 oxidation reaction on the dual-layer and mixedatalyst, respectively. From these data it is seen that there is littleifference in the NH3 conversion. NH3 oxidation lights off at essen-ially the same temperature (210 ◦C) and complete conversion ofH3 is achieved at higher temperatures (T > ∼230 ◦C). However,

ome differences are seen in terms of the product distributions.or the dual-layer catalyst N2O was detected in the temperatureange of 200–380 ◦C with a maximum yield of 50% at 270 ◦C. Forhe mixed catalyst N2O was detected up to 440 ◦C and the maxi-

um yield was 40% at about 270 ◦C. The N2O yield for the mixedatalyst is slightly lower than that for dual-layer catalyst at tem-eratures below 350 ◦C. As discussed earlier, N2O is formed byeactions involving N and NO adspecies on the Pt/Al2O3 catalyst(R14) and (R15)). For the dual-layer catalyst, once the N2O isormed and desorbs from the Pt/Al2O3 layer, it can back diffusehrough the Fe-ZSM-5 layer unaffected since Fe-ZSM-5 has a veryow N2O reduction and decomposition activity at temperatureselow 350 ◦C. In contrast, for the mixed catalyst, the presence ofe-ZSM-5 catalyst in close proximity to Pt/Al2O3 catalyst facilitateshe migration of NO adspecies from Pt/Al2O3 to the Fe-ZSM-5 cata-yst, where NO can react selectively with adsorbed NH3 to give N2.hus, a fraction of the NO adspecies that would otherwise react toorm N2O is consumed, resulting in a decrease in N2O yield fromhe mixed catalyst compared to that of the dual-layer catalyst. Theituation changes at temperatures above 350 ◦C. In this regime theuch higher activity of the Fe-ZSM-5 catalyst for N2O decompo-

ition and N2O SCR with NH3 leads to the consumption of backiffusing N2O on the dual-layer catalyst, resulting in lower N2Oield [39]. However, for the mixed catalyst the presence of somet/Al2O3 catalyst on the gas-solid interface facilitates the forma-ion and desorption of N2O, without it having to interact with thee-ZSM-5 catalyst, and therefore resulting in higher N2O yield atigher temperature.

A comparison of the NO yields at low space velocity on bothhe dual-layer and mixed catalysts reveals trends similar to thosebtained at high space velocity. The only notable differences regardhe somewhat higher NO yield on the mixed catalyst compared to

Please cite this article in press as: S. Shrestha, et al., Selective oxidamonolithic catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.

he layered catalyst at high temperature. The NO yield for the dual-ayer and mixed catalysts are 5% and 15%, respectively, at 450 ◦C.

slight breakthrough of NO2 is noted at lower space velocity foroth the dual-layer and mixed catalysts, with the mixed catalyst

500 ppm NH3, 5% O2 and 66k h GHSV.

giving higher NO2 yield than the dual-layer catalyst. That said, theNO2 yield is well below 5% for both the catalysts. The differencesin the NOx and N2O yields of the two catalysts are reflected bydifferences in the N2 yields since NO and N2 are the two mainproducts. At low space velocity, the N2 yield for the dual layer andmixed catalyst is 45% and 55% at 275 ◦C, respectively, whereas at450 ◦C it is 95% and 80%, respectively. The mixed catalyst gives ahigher N2 yield at intermediate temperature due to its lower selec-tivity towards N2O which is the result of synergistic effect of havingPt crystallites and Fe-ZSM-5 catalyst in close proximity. At highertemperatures, the dual layer catalyst gives higher N2 yield due toits increased effectiveness in reducing NOx. At higher space veloc-ity, the N2 yield profiles for both dual layer and mixed catalyst arevery similar. Overall, the dual-layer is the more effective catalystat high temperature [39]. This again has to do with shortened dif-fusion length for the reacting species on the mixed catalyst. The

tion of ammonia on mixed and dual-layer Fe-ZSM-5 + Pt/Al2O3cattod.2014.01.024

shorter length is beneficial to NH3 conversion but detrimental toN2 selectivity.

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mann, Appl. Catal. B 111–112 (2012) 106–118.

ARTICLEATTOD-8871; No. of Pages 11

S. Shrestha et al. / Cataly

. Summary and conclusions

A systematic experimental study of ammonia oxidation reactionn combined Fe-ZSM-5 and Pt/Al2O3 monolith catalyst was carriedut to determine if a high ammonia conversion along with high2 selectivity could be sustained over a wider temperature range

han with individual Fe-ZSM-5 and Pt/Al2O3 catalysts. This studyxperimentally verifies the working principle of the dual-layer ASComprising a bottom layer of Pt/Al2O3 and a top layer of Fe-ZSM-5.

series of bi-functional dual-layer catalysts and mixed-layer cata-ysts give a high N2 selectivity and low NOx selectivity over a wideemperature range. To our knowledge, this is the first study focusedn determining whether the dual component ASC can achieve ade-uate levels of NH3 oxidation to N2 with a small Pt loading («10 gt/ft3). This study also demonstrates a comparative study betweenhe NH3 oxidation on a dual-layer and mixed catalyst washcoatedn a monolith.

The following concluding points can be made:

A rather low Pt loading of 3 g/ft3 is sufficiently effective in oxidiz-ing ammonia at temperatures exceeding 200 ◦C.The benefits of a dual layer catalyst are obvious as evidenced byan increased production of desired product N2 at intermediateand high temperatures. The addition of a Fe-zeolite SCR layer ontop of Pt/Al2O3 oxidation layer shifts the product selectivity fromN2O and NOx to N2. This shows that the N2O and NOx formed inthe bottom layer of Pt crystallites are selectively reduced by thetop SCR layer to N2.The zeolite top layer also inhibits ammonia oxidation in servingas an additional mass transfer resistance. While the zeolitic toplayer is effective in converting NOx to N2, the detrimental effectof an added mass transport barrier emerges at high temperaturesand space velocities.A comparison between the dual layer and mixed catalysts athigh space velocity shows higher NH3 oxidation capability of themixed catalyst due to the shorter diffusion length. Low spacevelocity experiments show the dual layer catalyst to be moreeffective than mixed catalyst at high temperature due to a higherselectivity towards N2 and lower selectivity towards NOx andN2O, whereas mixed catalyst proves more effective than duallayer catalyst at intermediate temperature due to its higher selec-tivity towards N2 and lower selectivity towards N2O

Finally, this study emphasizes the point that dual-layer cata-yst design is non-trivial and must consider the trade-off betweenmmonia conversion and N2 selectivity.

cknowledgements

The financial support of Cummins Inc is gratefully acknowl-dged. However, the ideas and viewpoints expressed in this paper

Please cite this article in press as: S. Shrestha, et al., Selective oxidamonolithic catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.c

o not reflect that of Cummins Inc. We would also like to acknowl-dge BASF Catalysts LLC for providing us with blank monolith corend Sud-Chemie (Clariant) for providing commercial Fe-ZSM-5 cat-lyst.

[

[

PRESSday xxx (2014) xxx–xxx 11

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