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Catalysis Today 197 (2012) 127– 136

Contents lists available at SciVerse ScienceDirect

Catalysis Today

jou rn al h om epage: www.elsev ier .com/ locate /ca t tod

he role of oxidation catalyst in dual-catalyst bed for after-treatmentf lean burn natural gas exhaust

reshit Gawade, Anne-Marie C. Alexander, Ryan Clark, Umit S. Ozkan ∗

epartment of Chemical and Biomolecular Engineering, The Ohio State University, 140 W. 19th Avenue, Columbus, OH 43210, United States

r t i c l e i n f o

rticle history:eceived 16 April 2012eceived in revised form 1 August 2012ccepted 2 August 2012vailable online 1 September 2012

eywords:Ox reduction

a b s t r a c t

A dual-catalyst bed composed of a reduction (Pd/SZ) and an oxidation (CoOx/CeO2) catalyst was investi-gated for selective catalytic reduction (SCR) of NO2 using hydrocarbons for lean-burn natural gas engines.The choice of a multifunctional oxidation catalyst was seen to play a crucial role to achieve the desirableNO2 reduction performance. A dual-catalyst bed optimization showed that different cobalt containingoxidation catalysts could be exploited to achieve desirable NOx and hydrocarbon conversions. Moreover,cyclic experiments in the presence of water vapor were conducted to understand its implications on dualcatalyst bed performance. Finally, both kinetic and in situ diffuse reflectance infrared Fourier transform

ulfated zirconiaalladiumobalt catalystineticsRIFTSffect of water

spectroscopy (DRIFTS) studies were undertaken to examine the behavior of the oxidation catalyst withrespect to hydrocarbon oxidation which consequently affect the overall NOx reduction activity.

© 2012 Elsevier B.V. All rights reserved.

ean gas engines

. Introduction

In the context of evermore stringent requirements for fuel effi-iency and CO2 emissions, natural gas-fired reciprocating enginesepresent an important and increasingly popular choice withinhe distributed energy market place. Lean burn natural gas recip-ocating engines offer a simple, well-understood technology andave several advantages over stoichiometric gasoline engines [1].ean burn conditions are associated with higher engine efficienciesnd significantly cleaner engine out emissions. However, despitemissions being greatly reduced, engine exhaust still contains con-iderable amounts of NOx species, carbon monoxide and un-burnedydrocarbons. Some NOx emissions can be reduced through theodification of fuels or by altering the combustion parameters,

owever in order to reach acceptable emission levels the need forffective after-treatment is evident.

Among current catalytic NOx reduction control technologies,hree way catalysts and ammonia- or urea-based selective catalyticeduction (SCR) are the most prevalent. Both methods are highlyffective for the current combustion technologies, which operate

ver a wide temperature window (200–400 ◦C), yet are unsuit-ble for the next generation of lean-burn engines. For example,hree way catalysts, which are mostly used in mobile applications,

∗ Corresponding author. Tel.: +1 614 292 6623; fax: +1 614 292 3769.E-mail address: ozkan.1@osu.edu (U.S. Ozkan).

920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.cattod.2012.08.009

quickly deactivate under lean conditions, while the use of ammo-nia in emission abatement poses hazards in itself, mainly related toammonia slip, direct ammonia oxidation, corrosion of equipmentdue to the formation of ammonium salts, and the additional issuesrelated to ammonia storage and handling [2].

The selective reduction of NOx species by hydrocarbons hasattracted significant attention as a promising alternative to conven-tional after-treatment technologies [3–5]. This type of applicationis particularly suited to natural gas engines, in which unburnedhydrocarbons, particularly methane, are already present in theexhaust stream and capable of acting as the reducing agent.Although methane, which is a major component of natural gas,is readily available, its effective use as a reducing agent inhydrocarbon-SCR systems is somewhat limited. This is mainly dueto the competition from the combustion of the hydrocarbon in thepresence of excess oxygen and the inherent difficulty of methaneactivation [6,7].

Among the catalysts which have been reported to date,palladium-based catalysts supported on zeolites, such as ZSM-5and mordenite, have shown the highest activity and selectivity forcatalytic NOx reduction under lean conditions [8]. Despite their highactivity and selectivity in HC-SCR, zeolitic catalysts suffer from poorhydrothermal stability [9–11]. Consequently dealumination of the

material may occur, resulting in the formation of PdO aggregates,which are thought to be active for methane combustion, and lossof metal dispersion [12]. Several authors have reported that Pdsupported on acidic materials, such as zeolites or zirconia show

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significant improvement in NO reduction [13–15]. The devel-pment of acidic non-zeolitic catalysts, such as sulfated zirconia,or the HC-SCR of NOx species, has therefore attracted much inter-st, mainly due to an increase in hydrothermal stability comparedo zeolitic counterparts. Studies by Resasco and co-workers havehown that acidic supports aid the stabilization of palladium in theorm of Pd2+ ions [16], the active sites for NOx reduction [17,18].urther studies by the same authors [19] have also shown that pro-ons associated with surface sulfate groups serve as anchoring sitesor the Pd2+ ions in Pd/SZ. Although there has been much debateegarding the mechanisms involved in NOx reduction with hydro-arbons, NO2 is thought to be a key intermediate species in NOx

eduction.Previously we have described an integrated oxidation and selec-

ive reduction catalytic system [20–24] where NO was first oxidizedo NO2, then reduced selectively with hydrocarbons. This methodombines separate oxidation and reduction catalyst componentsn order to perform three distinct catalytic functions, specificallyOx reduction, CO oxidation and hydrocarbon combustion. Theual-catalyst approach for lean-burn exhaust after-treatment takesdvantage of the stronger oxidizing potential of NO2 comparedo NO, which in turn helps to utilize the reducing capability ofnburned hydrocarbons in the exhaust. Despite the exothermicxidation of NO to NO2 being thermodynamically limited at highemperatures, the dual-catalyst mechanism helps to drive the NOxidation reaction in the forward direction, as a result of NO2 beingontinuously removed via NO2 SCR. Any gaseous NO that is presentn the system, a consequence of partially reduced NO2, can bee-oxidized by the oxidation catalyst. It is clear that the oxida-ion catalyst assumes a multi-functional role in the dual-catalystcheme. In addition to oxidizing NO or re-oxidizing partiallyeduced NO2 species, the oxidation catalyst also plays a role inatalyzing the combustion of un-burned hydrocarbons and the oxi-ation of carbon monoxide, which have not been consumed duringhe SCR reaction [25].

In this contribution, the multifunctional role of a metal oxide-upported cobalt oxidation catalyst, namely CoOx/CeO2, in aual-catalyst system is addressed. The effect of Co-loading in NO toO2 oxidation has been investigated through steady state exper-

ments. In order to optimize the dual bed composition, the ratiof reduction to oxidation catalyst was varied and investigated viateady state experiments under simulated engine exhaust condi-ions. Cyclic experiments were conducted in the presence of waterapor to understand the effect of different cobalt loadings on theverall hydrothermal stability of the dual-bed system. Both kineticnd in situ DRIFTS studies are reported with the aim of under-tanding the behavior of the oxidation catalyst with respect toydrocarbon oxidation, which consequently affects the overall NOx

eduction performance.

. Experimental

.1. Catalyst preparation

Palladium catalyst supported on sulfated zirconia with 0.3 wt%oading, which was employed as the NOx reduction catalyst forhe dual-catalyst system, was prepared using a one-pot sol–gel

ethod, details of which can be found elsewhere [20,21]. Cobaltatalysts supported on ceria were prepared through an incipi-nt wetness impregnation technique and were used as the NO

xidation catalyst component in the dual-catalyst system. Theanoparticle ceria support was prepared using a precipitationethod described previously [26]. The calcined ceria support was

mpregnated with aqueous cobalt nitrate hexahydrate so as to yieldatalysts with Co loadings of either 2 or 10 wt% [27].

ay 197 (2012) 127– 136

2.2. Catalyst characterization

Surface area measurements were obtained using a Micromerit-ics ASAP 2010/2020 accelerated surface area and porosimetryinstrument using a N2-physisorption method. Prior to analysis, thesamples (0.1 g) were degassed overnight at 130 ◦C under a vac-uum of 3 �mHg to remove any adsorbed moisture. The specificsurface areas of the catalysts, 0.3% Pd/SZ, 10% CoOx/CeO2, and 2%CoOx/CeO2, were determined by applying the Brunauer, Emmettand Teller (BET) method to the nitrogen physisorption isothermswhich were determined at liquid nitrogen temperatures (77 K) andfound to be 38 m2/g, 84 m2/g and 106 m2/g, respectively.

Palladium dispersion over sulfated zirconia was calculated byhydrogen chemisorption using a Micromeritics Autochem 2910instrument. The technique was based on the method developed byMaffucci et al. [28]. The experiment was conducted using 0.15-mlhydrogen pulses at 150 ◦C to avoid palladium hydride formation.Nitrogen was used as a carrier gas and hydrogen uptake was mon-itored using a thermal conductivity detector (TCD). The cobaltdispersion over ceria was calculated using N2O chemisorption. Thetechnique was based on the method developed by Jensen et al.[29]. The experiment was carried out at 40 ◦C in the presenceof 4% N2O/He. The nitrogen evolved during the experiment wasmonitored using an MKS-Cirrus mass spectrometer. The observedpalladium dispersion over zirconia was 60% whereas the cobaltdispersion over ceria support was around 20%, irrespective of themetal loading. The details of surface area analysis and dispersionstudies can be found in our previous articles [21,27].

X-ray diffraction experiments over the reduction and oxidationcatalyst were conducted using either Bruker D8 Advanced diffrac-tometer or Rigaku X-ray diffractometer both equipped with CuK� radiation source (� = 1.5418 A). Pd/SZ showed the presence ofboth monoclinic and tetragonal zirconia. The contribution from thet-ZrO2 was around 33% whereas, crystallize sizes of t-ZrO2 and m-ZrO2 were 11.9 nm and 13.7 nm, respectively [21]. X-ray diffractionanalysis over 10% CoOx/CeO2 and 2% CoOx/CeO2 showed the pres-ence of cerianite structure and cobalt was present in the form ofCo3O4 [27].

Diffuse Reflectance Infrared Fourier Transform Infrared Spec-troscopy (DRIFTS) was performed over individual reduction (0.3%Pd/SZ) and oxidation (2% CoOx/CeO2) catalysts and a dual-catalystbed (0.3% Pd/SZ: 2% CoOx/CeO2 = 8:1) during methane tempera-ture programmed desorption (CH4-TPD). The DRIFTS instrument(Thermo Nicolet 6700) was equipped with a liquid nitrogen-cooledmercury–cadmium–telluride (MCT) detector and a DRIFTS cham-ber with ZnSe window. The DRIFTS spectra were collected in themid-IR range with a resolution of 4 cm−1 averaged over 500 scans.Samples were pre-treated in situ at 500 ◦C in 10% O2/He for 30 minat a flow rate of 30 ccm, followed by helium flush at the same tem-perature for an additional 30 min. The background spectra werecollected while cooling, under a He flow, at regular temperatureintervals. Adsorption of 10% CH4/He was performed for 30 min at50 ◦C followed by He flush for 30 min at the same temperature, toremove physisorbed CH4. Finally, the spectra were acquired, in He,at 50 ◦C increments up to a maximum temperature of 500 ◦C.

2.3. Catalyst activity testing

Steady-state reaction experiments were conducted at ambientpressure in a ¼′′ O.D. stainless steel fixed bed reactor system. Aphysical mixture of reduction and oxidation catalysts were packedin desired quantities inside the reactor and held in place using

quartz wool plugs. The reactor bed temperature was monitoredand controlled using an Omega K-type thermocouple, which wasupstream of the catalyst bed and an Omega (model CS232) PID con-troller. The reactor was placed upstream in a resistively heated

sis Today 197 (2012) 127– 136 129

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omemade furnace. Brooks mass flow controllers (5850E) weresed to regulate the gas flows to the reactor system. Water vaporas introduced to the reactor by saturating a stream of helium

as through a heated water bubbler containing de-ionized water. Ahemiluminescence NOx analyzer (Thermo-Scientific 42i-HL) wasoupled with a micro-GC (Agilent 3000A) equipped with 0.32 mmol-sieve and PLOT Q columns with thermal conductivity detector

o analyze the product gases. NO2 yield and conversions of hydro-arbons (XCHx ) and NOx (XNOx ) were calculated using the followingelationships:

O2 yield = moles of NO2 producedmoles of NO fed

XCHx ) or (XNOx ) = [moles in] − [moles out]moles in

.3.1. NO oxidation over CoOx/CeO2The catalyst samples were pre-treated in 10% O2/He (40 ccm) for

0 min at 450 ◦C prior to catalytic activity testing. Steady-state NOxidation reactions were performed on CoOx/CeO2 with differentobalt loadings, namely 2 and 10% Co. These experiments were con-ucted in the temperature range of 200–400 ◦C and at a gas hourlypace velocity (GHSV) of 100,000 h−1. The feed composition wasade up of 1000 ppm NO, 10% O2 and balance He.

.3.2. NOx reduction over the dual catalyst bed: effect ofeduction-to-oxidation catalyst ratio

All the NOx reduction studies using hydrocarbon mixtures werearried out at a GHSV of 32,000 h−1 and under the following simu-ated engine exhaust composition: 180 ppm NO2, 1737 ppm CH4,08 ppm C2H6, 104 ppm C3H8, 650 ppm CO, 6.5% CO2, 10% O2,0–10%) water vapor and balance He, unless otherwise stated. Theffect of the ratio of reduction to oxidation catalyst in the dual-atalyst bed was evaluated in the temperature range of 300–500 ◦Cor NOx reduction performance. The ratio of reduction (0.3% Pd/SZ)o oxidation (10% CoOx/CeO2) catalyst was varied from 2:1, 4:1 and:1 (by wt) in the dual-catalyst bed. In order to keep the bed vol-me constant during reaction studies, quartz powder was mixedith the catalyst bed. The findings of these experiments (Section

.2) revealed that lower amounts of oxidation catalyst in the dual-ed assisted the overall NOx conversion. Therefore, the amount ofxidation catalyst in the dual-catalyst bed was further decreasedsing a lower cobalt-loading sample (2% CoOx/CeO2) and a sim-

lar experiment was performed under simulated engine exhaustonditions to evaluate its NOx reduction performance.

.3.3. Hydrothermal stability of the dual-catalyst bed: effect of Cooading

The hydrothermal stability of the dual-catalyst bed was evalu-ted in the presence of 7% water vapor at 450 ◦C under simulatedngine exhaust conditions as outlined above. Oxidation catalystsith different cobalt loadings (10% CoOx/CeO2 or 2% CoOx/CeO2)ere mixed with 0.3% Pd/SZ giving a reduction to oxidation cata-

yst ratio of 8:1 (by wt). The cyclic experiments were conducted inrder to understand the reversibility of the water effect on the dualatalyst bed.

.3.4. Kinetics of hydrocarbon oxidationThe oxidation kinetics of the hydrocarbon mixture (methane,

thane and propane) over 0.3% Pd/SZ, 2% CoOx/CeO2 and theual catalyst bed (Pd/SZ:2% CoOx/CeO2 = 8:1) was studied in the

ypical temperature range of 300–500 ◦C with10% O2. Methane,thane and propane concentrations were varied in the ranges of89–2583 ppm, 107–311 ppm and 53–155 ppm, respectively. Theeactor was treated either as a differential or an integral reactor

Fig. 1. NO2 yield during NO oxidation over 2% CoOx/CeO2 (�) and 10% CoOx/CeO2

(�). Reaction conditions: 1000 ppm NO, 10% O2; 1 atm; GHSV: 100,000 h−1, (—)thermodynamic equilibrium conversion of NO to NO2.

depending upon the fractional conversion of hydrocarbons. It isworthwhile to note that the presented oxidation kinetic study doesnot include extensive experiments to examine the inhibition effectof water. It is expected that the presence of significant amount ofwater vapor (7–10%) in engine exhaust would inhibit the hydrocar-bon oxidation. This inhibition phenomenon was observed on ourdual-catalyst bed in the separate study (not shown). In these find-ings, hydrocarbon oxidation was reversibly affected in the presenceof water vapor.

3. Results and discussion

3.1. NO oxidation over CoOx/CeO2

It is known that NO2 participates in the activation of hydrocar-bons [30,31], therefore the efficiency of the oxidation catalyst toperform oxidation of NO to NO2 is fundamental to the selectivereduction of NO to N2. NO may exist in the engine exhaust, but itcan also form through the partial reduction of NO2 during NO2 SCR,and it is necessary to convert it back into NO2. Therefore NO-to-NO2oxidation over CoOx/CeO2 with different Co loadings was evaluatedunder steady state conditions, as illustrated in Fig. 1. As a refer-ence, the thermodynamic equilibrium limitation (dashed line) forthe conversion of NO to NO2 is also shown for the chosen feed condi-tions. An increase in NO2 yield was observed with an increase in thereaction temperature upon going from 200 ◦C to 300 ◦C. Howevera further increase in temperature results in a decrease in the NO2yield, which may be due to thermodynamic limitation imposed bythe system at higher temperatures. As can be observed, both sam-ples reach equilibrium at approximately 300 ◦C, with a maximumconversion of c.a. 83%. At higher temperatures, conversions closelyfollow the equilibrium curve. These results suggest that the NO2yield is largely independent of cobalt loading on the ceria support.These results differ from those previously reported for cobalt load-ings on other supports, namely TiO2 and ZrO2, where it was foundthat increasing the cobalt concentration had a significant effect onthe NO2 yield [25]. The use of CeO2 as a support appears to enhancecatalytic activity for NO conversion with reduced cobalt loadings.

This may be a direct result of the high oxygen mobility in CeO2 atelevated temperatures [32] and [references within]. It should alsobe noted that the mixture of oxidation catalyst with reduction cata-lyst in a dual-catalyst bed is capable of pushing the thermodynamic

130 P. Gawade et al. / Catalysis Today 197 (2012) 127– 136

Fig. 2. (a) NOx conversion and (b) CH4 conversion over dual catalyst bed as a functionof Pd/SZ–10% CoOx/CeO2 ratios, 1:0 (�), 2:1( ), 4:1( ) and 8:1 ( ) under simulatedl[G

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Fig. 3. NOx (bold) and CH4 conversion (void) over Pd/SZ: CoOx/CeO2 = 8:1 at differ-ent cobalt loadings; 2% CoOx/CeO2 (�, �) and 10% CoOx/CeO2 (�, ©) under simulated

ean exhaust conditions. Reaction conditions: [NO2] = 180 ppm, [CH4] = 1737 ppm,C2H6] = 208 ppm, [C3H8] = 104 ppm, [CO] = 650 ppm, [CO2] = 6.5%, [O2] = 10%; 1 atm;HSV: 32,000 h−1.

quilibrium in the forward direction due to the continuous removalf NO2 during SCR with hydrocarbons.

.2. NOx reduction over the dual catalyst bed: effect ofeduction-to-oxidation catalyst ratio

In the previous section, it was found that NO oxidation appearso be independent of the oxidation catalyst employed. However,he oxidation catalyst not only plays a role in NO oxidation, butlso in hydrocarbon and carbon monoxide oxidation. The extent tohich these reactions occur over the oxidation component may

ventually affect the overall NOx reduction performance in theual bed scheme. In the dual-catalyst bed, the oxidation catalystlays multi-functional roles by removing carbon monoxide andn-burned hydrocarbons, which have not participated in the NOx

eduction. Also NO formed due to the partial reduction of NO2 cane re-oxidized using the oxidation catalyst [22]. Therefore the effectf the reduction-to-oxidation catalyst ratio in the dual-catalysted was investigated under the simulated lean exhaust condi-ions. Fig. 2a and b shows NOx and CH4 conversion as a functionf reaction temperature, with various dual-catalyst bed reduction-o-oxidation catalyst ratios. It is evident from these figures that the

owest NOx and CH4 conversions were obtained when the reductionatalyst (Pd/SZ) was tested in the absence of the oxidation cata-yst. This result clearly indicates the importance the presence ofoOx/CeO2 has in the envisioned dual catalyst bed system.

lean exhaust. Reaction conditions: 180 ppm [NO2] = 180 ppm, [CH4] = 1737 ppm,[C2H6] = 208 ppm, [C3H8] = 104 ppm, [CO] = 650 ppm, [CO2] = 6.5%, [O2] = 10%; 1 atm;GHSV: 32,000 h−1.

NOx conversion (Fig. 2a) was found to increase with increasingthe reduction-to-oxidation catalyst ratio (Pd/SZ:10% CoOx/CeO2),from 2:1 to 8:1 whereas, the opposite trend was observed for CH4conversion (Fig. 2b). These results would suggest that having theoxidation catalyst in smaller quantities in the dual catalyst bedcould favor the NOx conversion at the expense of hydrocarbonconversion. It may be expected that there is a lower hydrocarboncombustion rate over the 2% CoOx/CeO2 oxidation catalyst com-pared to the 10% CoOx/CeO2 sample, which ultimately results inthe presence of a higher hydrocarbon concentration in the reactionmedium to allow the NOx reduction to take place, consequentlyincreasing the NOx conversion. C2H6 and C3H8 conversion trendswere similar to that observed for CH4 conversions (not shown). Inspite of this, the combustion rates for C2H6 and C3H8 were muchhigher than CH4, reaching 100% conversion at temperatures above350 ◦C, indicating that the complete oxidation of C2H6 and C3H8was much easier than that of CH4. Furthermore, complete carbonmonoxide conversion was observed regardless of the dual-catalystbed ratio employed (not shown). These results indicate that the oxi-dation of CO to CO2 occurs readily over the oxidation catalyst andthe percentage of metal loading on the support does not appear tohave a significant effect on this process. A separate study was con-ducted, (not shown), to evaluate the effect of the presence of CO,CO2 and higher hydrocarbons on NO2 reduction, as well as hydro-carbon oxidation. However, the findings of this study are beyondthe scope of the current publication.

It was observed, from Fig. 2, that by lowering the amount ofoxidation catalyst (10% CoOx/CeO2) in the dual-catalyst bed, theNOx conversion was improved. The oxidation catalyst amount inthe dual-catalyst bed was further decreased by using the lowercobalt-loading sample (2% CoOx/CeO2). Fig. 3 shows NO2 SCR per-formance over the dual-catalyst bed (Pd/SZ:2% CoOx/CeO2 = 8:1). Inthis case, NOx conversion was further increased due to the lowerhydrocarbon oxidation and hence having a higher concentration ofhydrocarbons available to participate in the NO2 reduction.

3.3. Hydrothermal stability of the dual catalyst bed: effect of Coloading

In a previous study, we demonstrated that the presence of

water in the engine exhaust significantly affects NOx reductionperformance [20,21]. It was reported that the reduction catalyst,Pd/SZ, was considerably affected by the presence of water whereasthe inhibition effect on the oxidation catalyst (Co/ZrO2) was not

P. Gawade et al. / Catalysis Tod

Fig. 4. Reversibility of the effect of water vapor on (a) NOx and (b) CH4 con-version over Pd/SZ:CoOx/CeO2 = 8:1 at different cobalt loadings 2% CoOx/CeO2

(�, �) and 10% CoOx/CeO2 (�, ©) under simulated lean exhaust. Reaction con-ditions: [NO ] = 180 ppm, [CH ] = 1737 ppm, [C H ] = 208 ppm, [C H ] = 104 ppm,[3

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2 4 2 6 3 8

CO] = 650 ppm, [CO2] = 6.5%, [O2] = 10%, [H2O] = 0% or 7%; 450 ◦C; 1 atm; GHSV:2,000 h−1.

s apparent. Furthermore, temperature programmed desorptionTPD) studies, in the presence of water vapor, revealed that waterapor negatively affects the NO adsorption over Pd/SZ whereasO2 adsorption over Pd/SZ was unaffected. This suggests that thehoice of oxidation catalyst in a dual bed system could potentiallyeduce the negative effect of water vapor on the dual catalyst bed,o improve the overall NOx reduction performance [20,21].

In line with above discussion and the findings illustrated inigs. 2 and 3, the oxidation catalyst is likely to be associatedith improving the hydrothermal stability. Therefore, steady state

xperiments were conducted with different cobalt loadings in theual-catalyst bed at 8:1 (by wt) reduction-to-oxidation catalystatio. Simulated engine exhaust composition was used as a feed,s described in the experimental section, and the reaction experi-ents were performed at 450 ◦C in the presence of 7% water vapor.

ig. 4a and b shows NOx and CH4 conversion during the cyclic exper-ment in the presence and absence of water vapor. It was observedhat the effect of water vapor was reversible in both dual-catalysted samples, as the complete recovery of NOx and CH4 conversionsas observed upon the removal of water vapor from the feed.

For the dual-catalyst bed (Pd/SZ:10% CoOx/CeO2), upon goingrom dry to wet feeds streams the NOx and CH4 conversionsropped from 80% to 65% and 76% to 30–32%, respectively. Theomplete recovery of NO and CH conversion was observed upon

x 4emoval of the water vapor. A similar cyclic trend was observedver the Pd/SZ:2% CoOx/CeO2 dual-catalyst bed. NOx and CH4 con-ersions dropped from 94% to 73% and 53% to 22%, respectively.

ay 197 (2012) 127– 136 131

It is apparent that the dual-catalyst bed sample containing 2%CoOx/CeO2 showed better NOx conversion compared to that con-taining 10% CoOx/CeO2 whereas the hydrocarbon conversion trendwas opposite to NOx conversion. The higher NOx conversion andlower hydrocarbon conversion in 2% CoOx/CeO2 containing dual-catalyst bed compared to 10% CoOx/CeO2 containing dual-catalystbed could be due to lower hydrocarbon oxidation, and hence morehydrocarbons being available for NO2 reduction. A similar trendwas observed for higher hydrocarbons (not shown). Both C2H6 andC3H8 conversions were adversely affected, but also reversible, inthe presence of water vapor. However, conversions of both C2H6and C3H8 were more than 90% even the presence of water at 450 ◦C,while the CO conversion was 100% in the presence of water indi-cating that CO oxidation was unaffected during the wet cycle.

A noticeable decrease in CH4 conversion in the presence of waterindicated that CH4 combustion was significantly affected by watervapor. The inhibition effect of water on CH4 combustion is reportedin several studies in the past over Pd-based catalysts [33–35]. Itis believed that either the formation of Pd(OH)2 phase, which isinactive for CH4 combustion [33], or water inhibition during CH4oxidation [34,35] could result in a decrease of CH4 conversion inthe presence of water vapor.

It is worthwhile to note that time-on-stream experiments havebeen performed over the dual-catalyst bed in the presence of 10%water (not shown). The results show no significant activity loss afterthe ∼15–20 h under optimized conditions.

3.4. Kinetics of hydrocarbon oxidation

Hydrocarbon oxidation plays an indirect role in determiningNO2 SCR performance. It is desirable to remove hydrocarbons fromthe engine exhaust via combustion. However, only the remain-ing hydrocarbons, which have not reacted during NO2 SCR, shouldbe used for the oxidation process. Unselective direct oxidation ofhydrocarbons could result in a considerable drop in NO2 SCR due tolack of available hydrocarbons for the NO2 SCR reaction. In the dual-catalyst bed, it was expected that the oxidation catalyst would playa major role in determining the extent of hydrocarbon oxidation;thereby indirectly affecting the NO2 SCR. Therefore kinetic stud-ies of hydrocarbon oxidation were conducted to understand therole of the oxidation catalyst, individually and in a dual-catalystbed composition. Figs. 5–7 show the fractional hydrocarbon con-versions (CH4, C2H6 and C3H8) in the temperature range between300 ◦C and 500 ◦C over 2% CoOx/CeO2, Pd/SZ and dual-catalyst bed,respectively. The reactor was treated either as a differential or anintegral depending upon the fractional conversion of hydrocar-bons. It should be noted that kinetic parameters for C3H8 oxidationcould not be derived, due to the fact that almost 100% C3H8 con-version was achieved under the operating conditions regardless ofthe choice of the catalyst. This would indicate that the oxidation ofC3H8 occurs more readily compared to both CH4 and C2H6.

Fig. 5a shows rate of CH4 oxidation (mol/gcat. min) as a functionof CH4 concentration in the temperature range of 400–500 ◦C over2% CoOx/CeO2. Fractional conversion of CH4 (XCH4 ) was less than0.2, therefore differential reactor kinetics were employed and thereaction rate was calculated using

−rCH4 =XCH4 × FCH4,0

W

where W is the catalyst weight (g) and FCH4,0 is the initial CH4 flowrate (mol/min). The following Power-law model was assumed forthe kinetic analysis:

−rCH4 = k × (CCH4 )a × (CO2 )b (1)

where CCH4 and CO2 are concentrations (mol/cm3) of CH4 and O2,respectively.

132 P. Gawade et al. / Catalysis Today 197 (2012) 127– 136

Fig. 5. (a) Rate of methane oxidation vs. methane concentration and (b) fractionalemm

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Fig. 6. (a) Fractional methane conversion vs. W/FA0 and (b) fractional ethane con-

thane conversion vs. W/FA0 over 2% CoOx/CeO2. Inset: comparison of power-lawodel predictions for hydrocarbon oxidation rate against experimentally deter-ined rates over 2% CoOx/CeO2.

However, the oxygen concentration is approximately 100 timesigher than that of the methane concentration and hence the aboveower-law model could be reduced to

rCH4 = k∗ × (CCH4 )a where k∗ = k × (CO2 )b (2)

Using a linear regression analysis, the kinetic parameters forH4 oxidation over 2% CoOx/CeO2 were found as activation energyEa) = 79 ± 13 kJ/mol and reaction order “a” w.r.t. CH4 = 1.0 ± 0.3.he inset of Fig. 5a shows the parity plot comparing calculatedeaction rates (RateCal) vs. experimental reaction rates (RateExp).

For the kinetic analysis of C2H6 oxidation over 2% CoOx/CeO2,ntegral reaction kinetics were employed as the fractional conver-ion of C2H6 (XC2H6 ) was greater than 0.2. Fig. 6b shows the plot ofC2H6 vs. W/FC2H6,0 , where FC2H6,0 , is the initial flow rate of ethanemol/min). The reaction rate was calculated by applying the differ-ntial analysis as follows:

rC2H6 = dXC2H6

d(W/FC2H6,0)(3)

A reduced Power-law model (Eq. (2)) was used for data fitting

nd linear regression analysis was applied to obtain the kineticarameters for C2H6 oxidation over 2% CoOx/CeO2 as followed,ielding an activation energy (Ea) = 45 ± 3 kJ/mol and reaction ordera” w.r.t. C2H6 = 0.7 ± 0.08. The inset in Fig. 5b shows the parity

version vs. W/FA0 over Pd/SZ. Inset: comparison of power-law model predictions forhydrocarbon oxidation rate against experimentally determined rates over Pd/SZ.

plot comparing calculated reaction rates (RateCal) vs. experimentalreaction rates (RateExp). Kinetic analysis confirmed that the oxida-tion of C2H6 occurs more readily than the oxidation of CH4 over 2%CoOx/CeO2.

Fig. 6a and b illustrates the influence of W/FA0 on CH4 and C2H6conversions respectively, during oxidation reaction over only thereduction catalyst, Pd/SZ. The reactor was treated as an integralreactor and reaction rates were obtained using Eq. (3). A reducedPower-law model (Eq. (2)) was used for data fitting and linearregression analysis was performed to obtain the kinetic param-eters for CH4 and C2H6 oxidation over Pd/SZ. These results havebeen summarized in Table 1. Activation energies for CH4 andC2H6 oxidation over Pd/SZ were 95 ± 14 kJ/mol and 50 ± 23 kJ/mol,respectively, which was in agreement with results found by Ribeiroet al. [35] where the kinetic study of methane oxidation over aPd supported catalyst was reported. The higher activation energybarrier for hydrocarbon oxidation over Pd/SZ compared to 2%CoOx/CeO2 indicates that the direct oxidation of hydrocarbons overPd/SZ is difficult to achieve. As will be discussed in Section 3.5, Pd/SZtakes part primarily in the activation of CH rather than its direct

4oxidation. Here, CH4 is activated via hydrogen abstraction to form

CH3 or CH2 species over Pd/SZ, instead of being directly oxidizedto form CO2.

P. Gawade et al. / Catalysis Today 197 (2012) 127– 136 133

Table 1Summary of kinetic parameters during CH4 and C2H6 oxidation over oxidation, reduction and dual catalyst bed catalyst.

CH4 oxidation C2H6 oxidation

Ko a E (kJ/mol) Ko a E (kJ/mol)

8 79 3

9566

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3

C

Fvmm

2% CoOx/CeO2 2.76 × 10 1.0 ± 0.3

Pd/SZ 1.61 × 107 0.7 ± 0.19

Pd/SZ + 2% CoOx/CeO2 6.2 × 103 0.6 ± 0.06

Fig. 7a and b shows a similar kinetic study of CH4 and C2H6xidation over a dual-catalyst bed sample (Pd/SZ:2% CoOx/CeO2).he synergetic effect between reduction and oxidation catalystsignificantly decreases the activation energy barrier. The activa-ion energies for CH4 and C2H6 oxidation over dual catalyst bedere found to be 66 ± 5 kJ/mol and 35 ± 11 kJ/mol, respectively.

his observation is supported using DRIFTS data presented in Sec-ion 3.5, where it was observed that the oxidation of CH4 was morechievable over 2% CoOx/CeO2 after its initial activation by Pd/SZ.

.5. Investigation of surface species using DRIFTS during CH4-TPD

The investigation of surface species formed as a result ofH4 adsorption was examined using in situ DRIFTS. Fig. 8 shows

ig. 7. (a) Fractional methane conversion vs. W/FA0 and (b) fractional ethane con-ersion vs. W/FA0 over Pd/SZ:2% CoOx/CeO2 = 8:1. Inset: comparison of power-lawodel predictions for hydrocarbon oxidation rate against experimentally deter-ined rates over Pd/SZ:2% CoOx/CeO2 = 8:1.

± 13 6.2 × 10 0.7 ± 0.08 45 ± 3 ± 14 7.5 × 102 0.6 ± 0.17 50 ± 23± 5 1.49 0.4 ± 0.12 35 ± 11

in situ DRIFT spectra collected during methane temperaturedesorption (TPD) over pre-oxidized 2% CoOx/CeO2, Pd/SZ anda 1:8 dual-catalyst bed consisting of the two afore-mentionedcatalysts. Fig. 8a illustrates the higher wavenumber region of2000–3800 cm−1 while Fig. 8b depicts the lower wavenumberregion of 900–2000 cm−1. Band assignments for the respectivesamples are summarized in Table 2.

The DRIFT spectra obtained from both the dual-catalyst bed andreduction catalyst exhibited distinct bands, which were absent inthe 2% CoOx/CeO2 spectra. In both instances, a large negative peak isevident at 1392 cm−1, which is indicative of sulfated zirconia andis attributed to the asymmetric S O stretching mode [36,37]. Anadditional broad band at c.a. 1250 cm−1 can be assigned to S Ovibrations. Additional bands can also be attributed to sulfate specieson both Pd/SZ and dual-catalyst bed samples as shown by the nega-

tive band centered at c.a. 1175 cm−1 which becomes more negativewith increasing temperature. This band corresponds to the SO4

2−

ion and symmetric stretch of O S O [38,39]. In all catalysts a broad

Table 2Summary of DRIFTS study during CH4-TPD over oxidation, reduction and dual cat-alyst bed catalyst.

Wavelength (cm−1)

2%CoOx/CeO2

Pd/SZ Dual catalystbed

CH stretch 2839 28352-� (C H) 2704 2704CH2 stretch 2916 2918CH species from formate 2727 2727CH3 asymmetric stretch

Deformation mode29581448

Gas phase CH4 1302 1304

OH bands Type IType II

37203615

37303633

–3622

Physically adsorbed H2O 3600–3000 3600–3000 3600–3000Molecularly adsorbed

water3649 3700 3649/3701

3510 3510Bending vibration �(HOH) 1638 1620 1620Acidic terminal OH 2272

CO linear 2035 2021Bridged 1846 1846

1130 1130CO2 2339/2316 2343/2330 2343/2320O S O asymmetric 1392 1392

Symmetric 1176 1174v (S O) 1246 1252

Bidentate carbonates 15631363

Monodentate carbonates 1502 15061331 1326

Bidentate formates 1543 1540 15401336

Carboxylates 1563 155915181302 1304 1304

Hydrogen carbonate 12111417 14271049 1047 10471022 1024 1026

134 P. Gawade et al. / Catalysis Today 197 (2012) 127– 136

F /SZ an(

bibsabwp1id[ac

rawTstsmbadCi

ig. 8. In situ DRIFT spectra collected during CH4-TPD over (i) 2% CoOx/CeO2, (ii) Pdb) low wavenumber region.

and is apparent at around 1610–1630 cm−1, assigned to the bend-ng vibration of molecularly adsorbed water �(HOH). This bandecomes more intense over the Pd/SZ and the dual-catalyst bed,ignaling an increase in water formation with increasing temper-ture. This increase coincides with the S O and O S O featuresecoming more negative, which suggests a strong interaction ofater with the surface sulfate species on sulfated zirconia sup-ort, as reported previously [22–24,20,40–42]. A weak shoulder at302 cm−1 was observed on the CoOx/CeO2 oxidation catalyst. This

s a characteristic band of adsorbed methane, which is also evi-enced in the reduction catalyst (1304 cm−1), but to a lesser extent43]. This is perhaps not surprising in view of the fact that Pd speciesre known to promote H-abstraction, resulting in methane beingonverted to methyl species [44].

From Fig. 8a, it is evident that all three samples have wellesolved bands in the hydroxyl region. Two negative bands at 3720nd 3615 cm−1 for CoOx/CeO2 and at 3730 and 3633 cm−1 for Pd/SZ,ere identified as type I and type II OH groups, respectively [45,46].

hese bands were not as well resolved in the dual-catalyst bedample, however the negative band at 3622 cm−1 can be assignedo type II OH vibrations [45]. These negative bands may repre-ent interactions between surface hydroxyl species and adsorbedethane. In addition to the negative features, well resolved OH

ands, due to molecularly adsorbed water, were also observed at

round 3649 cm−1 and 3510 cm−1 in the case of the CoOx/CeO2,ual-catalyst bed samples and 3700 cm−1 in the Pd/SZ sample.hen et al. [43] have previously reported similar bands occuring

n the hydroxyl region due to the adsorption of methane on a

d (iii) 1:8 dual-catalyst bed 2% CoOx/CeO2–PdSZ; (a) high wavenumber region and

ZSM-5 catalyst. It was reported that the strong band at 3510 cm−1

may be due to a shift of the band at c.a. 3620 cm−1 as a result of per-turbation of surface bridging hydroxyl groups by adsorbed methane[43]. A broad positive band was observed, for all three sam-ples, in the range of 3600–3000 cm−1 owing to weakly H-bondedhydroxyl groups on the support and is characteristic of OH vibra-tions associated with physically adsorbed H2O [40]. Both Pd/SZ anddual-catalyst bed samples exhibited additional OH bands. The neg-ative peak evident at 2272 cm−1 can be attributed to terminal OHspecies interacting with surface lewis acid sites on the support.

Bidentate formate species were evidenced in both CoOx/CeO2and dual-catalyst bed samples. The band observed at approxi-mately 2835 cm−1 was attributed to surface CH species while theband at 2918 cm−1 could have contributions from CH2 stretch-ing vibrations and �as (OCO) vibrations. An additional small peakwas also observed at 2704 cm−1 due to the overtone vibration of2�(C H) [45,47,48]. These peaks were not observed in the DRIFTspectra of Pd/SZ; however bands arising from a CH3 asymmetricstretch and CH3 deformation modes were apparent at 2958 cm−1

and 1448 cm−1, respectively. A small weak feature at 2727 cm−1

was also evident in both the Pd/SZ and dual-catalyst bed samples(Fig. 8a(ii) and (iii)), which may be ascribed to the asymmetric C Hstretch of surface formate species [49,50].

The IR band observed at 2035 cm−1 in spectra over Pd/SZ and

the dual-catalyst bed has previously been reported as correspond-ing to bridged-bonded CO over reduced Pd (1 1 0) [51–53]. This peakdecreases with increasing temperature, however the developmentof additional IR bands at 1846 cm−1 and 1130 cm−1, above 250 ◦C,

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P. Gawade et al. / Cataly

s observed. These peaks may be attributed to multi-coordinatedorms of CO on Pd0 [42,54,55]. This type of carbonyl formationnd adsorption on Pd may possibly be explained by the reactionf gasous CH4 with the pre-oxidized surface of the Pd particles.d/SZ catalyst, and to a weaker extent, the dual catalyst bed showegative peaks around 2343 and 2320 cm−1, corresponding to CO2oublet. This suggests some adsorbed CO2 species on the surface,hich leads to negative bands due to background subtraction.

hese features become more positive over the dual catalyst bedollowing a similar trend that is observed with temperature overhe CoOx/CeO2, signaling formation of CO2. However no evidencef CO2 formation was observed in the CH4-TPD reaction over theeduction catalyst alone (Pd/SZ). These observations would sug-est that CH4 is only partially oxidized to CO in the presence of theeduction catalyst. However it may be postulated that at temper-tures above 250 ◦C the mobility of oxygen atoms from the CeO2upport, in both the dual catalyst bed and oxidation catalyst itself,s sufficiently high to enable re-oxidation of the catalyst surfacellowing the complete oxidation of CO and CH4 to occur.

The interaction between CO and/or CO2 with surface methylpecies leads to the formation of various adspecies including,ut not limited to, carbonate and formate type intermediateshemisorbed on the support. For this reason, the region between600 and 1300 cm−1 is particularly difficult to interpret, which

s perhaps more apparent in the spectra of CoOx/CeO2 and dual-atalyst bed samples. The most intense bands in this zone can bescribed to bidentate carbonates at c.a. 1563 and 1363 cm−1 whichre particularly evident in the lower temperature spectra, Fig. 8b(i).s the temperature is increased above 200 ◦C, the intensity of

hese bands decrease at the expense of bands at approximetaly540 cm−1 and 1336 cm−1. These features are characteristic ofidentate formate species [45] and correspond to asymmetric andymmetric v(oco) modes. The intensity of these bands increase withncreasing temperature and is most likely a result of the interac-ion between OH and CO species. These bands, however, are notbserved in the Pd/SZ or dual catalyst bed spectra (Fig. 8b(ii) andiii)), and are most likely masked as a result of the strong negativeand associated with sulfated zirconia. The positive peak centeredt approximately 1130 cm−1 in both Pd/SZ and dual-catalyst bedamples can be attributed to monodentate carbonate species. Thiss confirmed by the weak band at 1502 cm−1 and 1506 cm−1 forhe respective samples, which can also be assigned to monoden-ate carbonates [55]. The presence of carboxylate species over allamples cannot be ruled out due to the presence of a peak at c.a.500–1510 cm−1. This band is usually accompanied by a strongeak around c.a. 1560 cm−1, which is well evident in CoOx/CeO2.owever, the band at 1560 cm−1 is not well resolved for Pd/SZ or

he dual catalyst bed due to the presence of molecularly adsorbedater at 1620 cm−1.

The band observed at 1211 cm−1 in the spectra of CoOx/CeO2,long with those at c.a. 1420 cm−1, which are also evidenced inhe high temperature dual-catalyst bed spectra, are attributed toydrogen carbonate species. The presence of a strong band dueo adsorbed water makes it difficult to resolve a possible bandround 1608 cm−1, which would be expected in hydrogen carbon-te species. Bands at 1047 and 1024 cm−1 may also be attributed toydrogen carbonate species [45], however it may also be possiblehat these bands correspond to �S O stretching mode(s), particu-arly in the dual catalyst bed and Pd/SZ samples [38,56].

By studying the adsorption of CH4 on both the reduction andxidation catalysts independently, it is evident that each serves aifferent role in the mixed-bed system. From Fig. 8 it is clear that

n the spectra of both the oxidation catalyst and the dual cata-yst bed catalyst, bands attributed to surface CH and CH2 speciesre present. These bands are not, however, observed in the spec-ra obtained from the Pd/SZ reduction catalyst. What is apparent

[[

[

ay 197 (2012) 127– 136 135

though is the presence of CH3 stretching and deformation modes.This would suggest that the Pd reduction catalyst promotes the C Hbond dissociation in CH4, ultimately activitating CH4 and allowingfor its further oxidation. This can be illustrated in the spectra of thedual catalyst bed sample, Fig. 8a(i) and b(i). In these spectra there isagain no evidence of CH3 bands, but only CH and CH2, which wouldimply that the methyl species that are generated are immediatelyoxidized to either carbonate or formate species. These, in turn, arefurther oxidized to CO2, as evidenced by the CO2 doublet in spectraof both the dual catalyst bed and oxidation catalyst.

This study helps to elucidate the roles of both the oxidation andreduction components of a dual-catalyst system. It is also clearfrom this study that there is a synergystic effect in the dual cat-alyst bed, in that the Pd/SZ reduction catalyst dissociates a C Hbond in methane and these CHx species subsequently participatein NOx reduction or directly oxidized by the oxidation catalyst. Sec-ondly it is evident that formation of CO2 is over the CoOx/CeO2oxidation catalyst and the formation of CO over Pd/SZ may be aresult of surface hydroxyl species interacting with methane or otherhydrocarbons.

4. Conclusions

The investigated dual-catalyst system has been shown toachieve high NOx conversions during CHx-SCR in lean burn con-ditions, which can be further improved by optimizing the ratio ofreduction-to-oxidation catalyst in the bed composition. The choiceof cobalt loading in the oxidation catalyst is shown to play a crucialrole in determining the overall performance of the dual-catalystbed. The lower cobalt loading in the oxidation catalyst in a dual-catalyst bed led to a higher NOx conversion. This improvementin NOx conversion could be associated with lower hydrocarbonoxidation and hence increased availability of hydrocarbons forNOx reduction. A similar trend was also observed in the pres-ence of water vapor. Furthermore, the water inhibition effect wasreversible on NOx reduction and CHx oxidation regardless of thechoice of oxidation catalyst used. Both the kinetic and DRIFTS stud-ies confirmed that the reduction catalyst, Pd/SZ activated the CH4molecule via hydrogen abstraction to form CH3 and CH2 species,which subsequently either participate in NOx reduction or directlyoxidized by the oxidation catalyst.

Acknowledgements

The financial support provided by the US Department of Energyand Caterpillar Inc. is gratefully acknowledged. The authors alsothank Dr. Ronald Silver of Caterpillar Inc. for the invaluable insighthe provided through many detailed discussions.

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