Applied Catalysis A: General 249 (2003) 165–174

Preparation of alumina-supported palladium catalystsfor complete oxidation of methane

K. Perssona,∗, P.O. Thevenina, K. Janssonb, J. Agrella,S.G. Järåsa, L.J. Petterssona

a Department of Chemical Engineering and Technology, KTH—Royal Institute of Technology,Chemical Technology, Teknikringen 42, SE 100 44 Stockholm, Sweden

b Arrhenius Laboratory, Department of Inorganic Chemistry, University of Stockholm,S. Arrhenius väg 12, SE 106 91 Stockholm, Sweden

Received 5 September 2002; received in revised form 26 February 2003; accepted 27 February 2003

Abstract

Alumina-supported palladium catalysts (Pd/Al2O3) have been prepared by incipient wetness (IW), grafting (G) and mi-croemulsion techniques (ME). Two slightly different microemulsion methods have been used (ME1) and (ME2). The catalystshave been calcined at 1000◦C for 4 h. The catalysts have then been examined with respect to their activity for the combus-tion of methane, during heating and cooling ramps. Pd/Al2O3-IW and Pd/Al2O3-ME2 exhibit the highest activity whereasPd/Al2O3-ME1 and Pd/Al2O3-G are the less active. There is not much of a difference in activity between Pd/Al2O3-IW andPd/Al2O3-ME2, despite strong differences in palladium particle size observed by TEM analysis. TPO experiments show thatPd/Al2O3-IW and Pd/Al2O3-ME2 present a higher PdO/Pd ratio than the other samples exhibit. The preparation techniqueaffects the reoxidation ability of palladium during cooling, hence the combustion activity of the fresh catalyst, as PdO is moreactive than Pd for the complete oxidation of methane under lean conditions.© 2003 Elsevier B.V. All rights reserved.

Keywords: Catalytic combustion; Methane; Palladium; Alumina; Preparation method; Incipient wetness; Grafting; Microemulsion; TPO; TEM

1. Introduction

In the mid 1950s, emissions from combustionsources were identified to cause atmospheric pollution[1]. The main concern was and is carbon monoxide(CO), hydrocarbons (HC) and nitrogen oxides (NOx).The implementation of more stringent regulations withrespect to the release into the atmosphere of the above-

∗ Corresponding author. Tel.:+46-8790-82-81;fax: +46-810-85-79.E-mail addresses: katper@ket.kth.se, katarina.persson@ket.kth.se(K. Persson).

mentioned pollutants has entailed the development ofenvironmentally friendly catalytic processes[2].

Concerning gas turbine applications, catalyticallystabilized combustion has proven to be a suitable alter-native to conventional flame combustion for heat andpower production[3]. The combustion temperature isreduced to about 1400◦C, the oxidation reaction iscomplete and it is possible to achieve a stable com-bustion of fuel/air mixtures outside the flammabilitylimits [4]. The result is that almost no thermal NOx

is formed, and ultra-low levels of carbon monoxideand hydrocarbons can be observed as well, typicallybelow 5 ppm[5].

0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0926-860X(03)00193-5

166 K. Persson et al. / Applied Catalysis A: General 249 (2003) 165–174

Natural gas, or methane which is its main compo-nent, provides an attractive source of energy since it isin abundant supply. It has found application in firinggas turbine combustion chambers and more recentlyhas been reported as an alternative fuel for automotiveapplications[6,7].

Methane is the most difficult hydrocarbon to oxi-dize since it contains only C–H bonds[8]. Therefore,much of the combustion catalyst developments havefocused on noble metal catalysts[9–11] as they ex-hibit the highest activity per active site for the com-plete oxidation of hydrocarbons[12]. Moreover, theyare more resistant to volatilization and sulfur poison-ing, when compared to transition metal oxides. Palla-dium has been reported to be the most active speciesfor the combustion of methane when operating in ex-cess of oxygen[13].

The purpose of the present paper is to compare aconventional preparation deposition technique, incip-ient wetness (IW), with microemulsion and prepara-tion by grafting (G). The catalysts are compared withrespect to their surface properties and activities forthe combustion of methane. Temperature-programmedoxidation (TPO) provides information concerning thetransition between the metallic and oxide states of pal-ladium. In addition, BET surface area measurement,X-ray diffraction (XRD) and transmission electron mi-croscopy have been used to characterize the catalysts.

2. Experimental

2.1. Preparation of the catalysts

Palladium was deposited onto�-Al2O3 (Condea,Puralox HP 14–150, 144 m2 g−1) by four differenttechniques: incipient wetness, grafting and two vari-ants of microemulsion. For each method catalysts con-taining approximately 5 wt.% Pd on�-Al2O3 wereprepared. All catalyst samples were first calcined at500◦C for 3 h and then at 1000◦C for 4 h to avoid anythermal deactivation during reaction.

The catalysts were then coated onto the outer sur-face of mullite tubes (outer diameter 4 mm) to test theircatalytic activity in combustion of methane. The pow-ders were first dispersed in a solution of HNO3 andwater (0.675 mmol HNO3 per gram H2O), whereuponthe slurry was ball-milled for 24 h to obtain a homo-

geneous suspension, as described elsewhere[14]. Themullite tubes were immersed in HCl solution at roomtemperature for 2–3 min to obtain a rougher superfi-cial surface, which improves the adherence of the cat-alyst slurry to the surface of the tube. Subsequently,the tubes were immersed in a HNO3 solution at 80◦Cfor 5 min to favor the formation of superficial oxides.The tubes were then dipped into the catalyst slurry anddried at room temperature over night. The length ofthe catalyst layer was 30 mm. By weighing the mul-lite tubes before and after coating, the weight of thecatalyst layer was established to ca. 10 mg. The thick-ness of the catalyst layer on the mullite tubes wasestimated to a value around 8�m by means of SEMimages using a Zeiss DSM 940. The tubes were cal-cined at 1000◦C for 4 h. To make sure that the catalystlayer on the mullite tube and the catalyst powder con-sists of equal weight percent of Pd the samples wereexamined by EDX and SEM using a JEOL JSM 840microscope. No diversity of the palladium concentra-tion was observed between the layer on mullite tubesand the powders.

2.1.1. Incipient wetnessWhen depositing palladium on�-Al2O3 by incipi-

ent wetness impregnation, a Pd(NO3)2 solution (AlfaAesar, 15.19 (w/w)) was used as a metal precursor, asdescribed elsewhere[15]. The Pd(NO3)2 solution wasdissolved in deionised water in such a manner that thevolume of the impregnation solution equals the porevolume of the support (1 cm3 g−1). The Pd-containingsolution was mechanically mixed with the aluminasupport. Hereafter, the catalyst prepared by the in-cipient wetness impregnation will be called Pd/Al2O3-IW.

2.1.2. GraftingThe palladium was deposited on the surface of

�-Al2O3 by using palladium(II) 2,4-pentanedionate(Alfa Aesar, Pd 34.7% Assay) as metal precursor. Thepalladium salt and�-Al2O3 were dissolved separatelyin n-butanol before mixing. The grafting reaction wasperformed for 6 h in an inert atmosphere at the boilingpoint of butanol (117◦C). After the reaction, the cat-alytic powder was recovered by filtration and washedwith n-butanol. The powder was dried over night at120◦C. Henceforward, the catalyst prepared by usingthe grafting method will be called Pd/Al2O3-G.

K. Persson et al. / Applied Catalysis A: General 249 (2003) 165–174 167

2.1.3. MicroemulsionBy means of the microemulsion technique (ME),

a catalyst with small metal particles of narrow sizedistribution can be obtained[16]. This method differsfrom previously described methods in the sense thatthe metal particles were prepared without having con-tact with the support. The microemulsion is obtainedby mixing a surfactant, an oil-phase and water to aternary system in given concentration. The microemul-sion consists of small aggregates called reversed mi-celles, which have an aqueous core where it is possibleto dissolve a metal precursor[17]. After reduction ofthe metal precursor, the support and a destabilizationagent are added to the microemulsion to obtain smallmetal particles on the surface of the support. The sizeof these particles depends on the size of the micelles,which in turn is influenced by the ratio between waterand the surfactant. The palladium particle size gener-ally increases with increasing water-to-surfactant ra-tio. In this study, a ratio of 8.5 was chosen.

The oil-phase used was cyclohexane (J.T. Baker,>99.0%), the surfactant was nonylphenolethoxylateNP-5 (Akzo Nobel Surface Chemistry, Berol 02)and the metal precursor was a diluted solution ofPd(NO3)2 (Alfa Aesar, Pd 15.19 (w/w)). Reduction ofpalladium(II) was accomplished by adding hydrazinemonohydrate (Fluka, purum, >99.0%) under vigorousstirring. The �-Al2O3 was added to the palladiumsuspension and the slurry was destabilised by addingtetrahydrofuran (J.T. Baker) under stirring until thesolution became clear. The catalyst will be calledPd/Al2O3-ME1.

Additionally, a catalyst called Pd/Al2O3-ME2 inthe paper, also consisting of Pd/�-Al2O3 with awater-to-surfactant ratio of 8.5, was prepared withanother microemulsion method than the previouslydescribed one. Instead of reducing the microemul-sion apart from the�-Al2O3, the alumina was firstadded to the microemulsion whereupon the mixturewas reduced. For this microemulsion technique adestabilizing agent was not necessary.

2.2. Activity tests

Activity tests were performed on the Pd/�-Al2O3catalysts at atmospheric pressure in an annular reac-tor, as described elsewhere[10,18]. The lab-scale testfacility consists of a mullite tube coaxially placed in-

side a quartz reactor, which in turn is located inside an800 mm long electric furnace. The catalyst was coatedon the external surface of the mullite tube, as describedpreviously. In order to measure the temperature of thecatalyst during the combustion experiment, a thermo-couple was inserted inside the ceramic tube.

The feed consisted of 2 vol.% methane in air andthe gas mixture was injected at a total flow velocityof 850 cm3 min−1 at atmospheric pressure. The tem-perature of the furnace was varied in repeated cyclesbetween 250 and 950◦C at a rate of 10◦C min−1.

During the transient experiment, the compositionof the product was monitored by on-line gas chro-matography (GC Varian 3800) equipped with a ther-mal conductivity detector. The conversion calculationsare based on the concentration of CH4 and corrobo-rated by yields of CO2 and CO.

2.3. Catalyst characterization

The surface area and the pore size distribution ofthe different catalysts were determined by nitrogenadsorption at the liquid N2 temperature in a Micro-metrics ASAP 2010 instrument. The surface area wasdetermined according to the Brunauer–Emmett–Tellertheory. The samples were degassed in vacuum for atleast 4 h at 250◦C before analysis.

The crystal phases of the various catalysts weredetected by X-ray diffraction, by using a SiemensDiffraktometer D5000. The operation parameterswere: Cu K� radiation, Ni filter, 30 mA, 40 kV, 2θscanning from 20 to 80◦ with a step size of 0.02.Crystallite size estimation was made using the Funda-mental Parameter Approach application of the TOPASv2.0 software and phase identification was performedusing the reference database (JCPDS-files) suppliedwith the equipment.

The morphology and size of the palladium parti-cles on the alumina washcoat were studied using atransmission electron microscope (TEM – JEOL 2000FX) operated at 200 kV. The microscope was equippedwith an energy dispersive X-ray spectroscopy (EDS)detector (LINK AN 10000).

The redox properties of the catalysts were mea-sured by means of temperature-programmed oxida-tion experiments on a Micromeritics AutoChem 2910,equipped with a thermal conductivity detector (TCD).The analysis was performed with a continuous flow

168 K. Persson et al. / Applied Catalysis A: General 249 (2003) 165–174

of 5% O2/He over the sample. Approximately 100 mgof calcined powder was used for each measurement.The temperature was increased from 300 to 900◦C ata rate of 10◦C min−1.

3. Results

3.1. Activity tests

Activity tests were carried out for the various cat-alysts in an annular reactor, as described above. Foreach sample, the CH4 combustion experiment was per-formed twice and the obtained combustion profiles ofthe two cycles exhibit a good reproducibility. Resultsof the activity test over the various catalysts are dis-played inFig. 1 for the heating ramp and inFig. 2 forthe cooling ramp.

The ignition of the catalysts, defined here as the tem-perature (T10) at which 10% conversion is achieved,occurs between 520 and 600◦C, depending on thedeposition method. The CH4 conversion increasesgradually with temperature up to around 800◦C,where the conversion reaches a maximum. There-

Fig. 1. Combustion activity of alumina-supported palladium cata-lysts prepared by different techniques. Microemulsion, Pd/Al2O3-ME1 (�) and Pd/Al2O3-ME2 (�), grafting Pd/Al2O3-G (�) andincipient wetness Pd/Al2O3-IW (�). The activity is measuredunder heating conditions.

Fig. 2. Combustion activity of alumina-supported palladium cata-lysts prepared by different techniques. Microemulsion, Pd/Al2O3-ME1 (�) and Pd/Al2O3-ME2 (�), grafting Pd/Al2O3-G (�) andincipient wetness Pd/Al2O3-IW (�). The activity is measured un-der cooling conditions.

after, the conversion decreases until the temperaturereaches 870–910◦C, where Pd starts to become ac-tive and the conversion increases sharply and reaches100% at 950◦C. The previous described increase ofconversion is both due to catalytic activity and homo-geneous combustion. According to previous reports[10,19], the decrease in combustion activity observedat about 800◦C is due to the decomposition of PdOinto the less active Pd. During the cooling ramp, aconversion as low as 2% is observed at temperaturesbetween 700 and 750◦C. The activity of the cata-lysts then increases rapidly due to Pd reoxidation upto a maximum at 650–680◦C, followed by a slowdecrease in conversion down to 0%.

The influence on the activity of the different depo-sition techniques has been investigated and the resultsare presented inFigs. 1 and 2. The comparison ismade between catalysts which have been preparedwith different deposition techniques. A significantvariation in the combustion profiles between the dif-ferent catalysts was observed. The highest activityis obtained for Pd/Al2O3-IW as to ignition tempera-ture T10 and conversion. However, Pd/Al2O3-IW and

K. Persson et al. / Applied Catalysis A: General 249 (2003) 165–174 169

Table 1Surface properties and nomenclature of the different alumina-supported palladium catalysts

Sample name Preparation method BET surfacearea (m2 g−1)

TEM PdO particlesize (nm)

XRDa PdO crystallitesize (nm)

Ignition temperatureT10

b (◦C)ρ-valuec

Pd/Al2O3-IW Incipient wetness 86 30–80 18 520 1.2Pd/Al2O3-G Grafting 100 40–100 22 580 1.5Pd/Al2O3-ME1 Microemulsion 1 90 100–1000 50 600 1.9Pd/Al2O3-ME2 Microemulsion 2 103 60–220 27 530 1.4

a Estimated by TOPAS—Fundamental Parameter Approach.b Catalyst temperature required to reach 10% conversion.c The value of the area of the decomposition peaks divided by the area of the reoxidation peak shown in the TPO profile.

Pd/Al2O3-ME2 exhibit similar combustion behaviors,even though Pd/Al2O3-IW is slightly more active. Incomparison with Pd/Al2O3-IW and Pd/Al2O3-ME2,Pd/Al2O3-G is less active for the methane combus-tion. The least active sample tested is Pd/Al2O3-ME1.

T10 of Pd/Al2O3-IW is at about 520◦C, whereasthe sample prepared with ME1 reaches 10% conver-sion first at 600◦C. The large temperature window in-dicates that the deposition method may influence theignition of the catalyst.T10 of Pd/Al2O3-ME2 andPd/Al2O3-G are at 530 and 580◦C, respectively.

As shown inFig. 1, the conversion at the maximumvaries markedly between the different techniques.Pd/Al2O3-IW exhibits the highest activity with 52%,whereas Pd/Al2O3-ME1 only presents a conversionof 28% at the same temperature. A comparable resultis achieved for maximum conversion during the cool-ing ramp, where Pd/Al2O3-IW reaches a considerablyhigher value compared to Pd/Al2O3-ME1.

3.2. Catalyst characterization

3.2.1. BET surface area measurementWhen alumina is subjected to elevated temperature,

the support undergoes changes in its crystal structureaccompanied by a sharp loss of surface area[20]. Thepalladium-based samples were calcined at 1000◦C for4 h before the BET analyses were carried out.

All Pd-based catalysts presented a BET surface areaof around 100 m2 g−1 regardless of the synthesis, asseen inTable 1. Concerning the surface area of thesupport, a decrease of around 60 m2 g−1 is observedupon calcination of the freshγ-Al2O3 support. There-fore, the phase transformation which occurs at ele-vated temperatures seems to affect the surface areamore than the choice of deposition technique.

3.2.2. X-ray diffraction (XRD)As reported in previous studies[21], when alumina

is exposed to a temperature as high as 1000◦C, atwhich all our catalysts are calcined, a shift of crystalphase occurs. This phase transformation is confirmedby XRD (seeFig. 3), which shows that all catalystsconsist ofδ-Al2O3 andθ-Al2O3 instead of the initialγ-phase of alumina.

Furthermore, the XRD-diffractograms show thatall samples contain PdO. The characteristic peak forPdO at 2θ = 34◦ varies in sharpness and intensity be-tween different catalysts possibly due to differences

Fig. 3. X-ray diffraction pattern of alumina-supported palla-dium catalysts prepared by microemulsion (Pd/Al2O3-ME1 andPd/Al2O3-ME2), grafting (Pd/Al2O3-G) and incipient wetness(Pd/Al2O3-IW). Reflections of (�) PdO, (�) Pd0, (�) δ-Al2O3

and (�) θ-Al2O3.

170 K. Persson et al. / Applied Catalysis A: General 249 (2003) 165–174

in particle size and amount of palladium in the crys-tal phase. In fact, Pd/Al2O3-G exhibits the mostintense PdO peak of all catalysts. Pd/Al2O3-IW andPd/Al2O3-ME2 display similar XRD diffractions pat-terns, whereas Pd/Al2O3-ME1 considerably differsfrom the rest of the samples with the less intense PdOpeak. However, the amount of metallic Pd seems to

Fig. 4. TEM picture of Pd/Al2O3-G (top left), Pd/Al2O3-ME1 (top right), Pd/Al2O3-IW (bottom left) and Pd/Al2O3-ME2 (bottom right).

be slightly lower for Pd/Al2O3-G compared with theother samples.

In order to estimate the size of the PdO crystal-lite from the XRD diffractograms, the FundamentalParameter Approach application of the TOPAS soft-ware was used. As shown inTable 1, the diametersof the crystallites lay in the same range between

K. Persson et al. / Applied Catalysis A: General 249 (2003) 165–174 171

18 and 27 nm for Pd/Al2O3-IW, Pd/Al2O3-G andPd/Al2O3-ME2. On the contrary, Pd/Al2O3-ME1presents a crystallite diameter of 50 nm, which istwice as large as for the rest of the catalysts.

3.2.3. Transmission electron microscopy (TEM)The results from the microscopy analyses show

that the Pd particle size depends on the depositionmethod. Samples prepared by incipient wetness andgrafting consist of Pd particles with diameters from30 to 100 nm (seeFig. 4). An even dispersion of Pdon the support material is observed for both methods.

On the contrary, Pd/Al2O3-ME1 exhibits a poor dis-persion of the Pd particles. The Pd particles appear tohave agglomerated into larger particles in the range of100–1000 nm.

Pd/Al2O3-ME2 consists of particles in the range of60–220 nm, which is markedly smaller than the par-ticles on Pd/Al2O3-ME1. However, the foggy TEMimage suggests that the Pd particles are located in thebulk phase, instead of at the surface. When compar-ing Pd/Al2O3-IW and Pd/Al2O3-ME2 it can be estab-lished that the dispersion of palladium on the formercatalyst is higher.

3.2.4. Temperature-programmed oxidation (TPO)In order to investigate the behavior of supported

Pd catalysts under oxidative conditions, TPO wasused. During the heating ramp, two and sometimesthree positive peaks are observed in the range of700–900◦C (cf. Fig. 5), associated with differentstages of PdO decomposition[22]. The first two peakscorrespond to the major amount of PdO species,whereas the third peak, when observed, correspondsto a very small amount. The first decomposition peakappears at around 740◦C and the second peak ap-pears at around 790◦C, but a slight variation in thepeak temperatures between the catalysts is discerned.Concerning the third peak, the high thermal stabilitysuggests that this species has strong interactions withthe support, possibly consisting of a support-Pd oxidecomplex[22].

During the cooling ramp, a single negative peakwas observed for all samples, representing Pd reoxi-dation. The O2 consumption starts at around 650◦Cand ends at around 500◦C. Thus, there is a hysteresisin temperature between decomposition to palladiumand reformation into palladium oxide. This tempera-

Fig. 5. TPO profile of alumina-supported palladium catalysts pre-pared by microemulsion (Pd/Al2O3-ME1 and Pd/Al2O3-ME2),grafting (Pd/Al2O3-G) and incipient wetness (Pd/Al2O3-IW).

ture difference is in accordance with previous reports[10,19].

On comparison between catalysts prepared with thefour different deposition methods, but with similar Pdcontent, appreciable distinct TPO results are observed.The TPO profiles for the different samples are dis-played inFig. 5.

Concerning sample Pd/Al2O3-IW, three decompo-sition peaks are observed in the TPO profile. Theintensity of the first peak is much smaller in compari-son with the second one, but the third peak is the leastintense one. In contrast, the Pd/Al2O3-G catalyst ex-hibits only two decomposition peaks and the first peakis much more intense than the second peak.

Catalysts prepared with ME1 present a TPO profilesimilar to those of the grafting sample, where the firstpeak is more intense than the second. However, theO2 response is less intense compared to the results ofthe grafting samples. Pd/Al2O3-ME2 presents a TPOprofile with two peaks, which exhibit almost the samearea.

As shown inFig. 5, no pronounced differences inthe temperatures where the maximum of the first andthe second decomposition peak occur are discerned be-tween the different techniques. On the contrary, the in-cipient wetness sample is the only sample with a TPO

172 K. Persson et al. / Applied Catalysis A: General 249 (2003) 165–174

profile showing a third decomposition peak, appear-ing at 869◦C. The onset temperature of the reoxida-tion is around 650◦C for all samples. The reoxidationcoefficient,ρ, defined as the ratio between the totalarea of the decomposition and the reoxidation peaks,varies markedly between the preparation methods. Asshown inTable 1, Pd/Al2O3-IW displays the lowestvalue of 1.2, meaning that the ability to reoxidize isthe highest of the series of catalysts tested. Thereafter,the Pd/Al2O3-ME2 exhibits a value of 1.4, followedby Pd/Al2O3-G with 1.5 and last Pd/Al2O3-ME1 with1.9.

The variation between the different catalysts withrespect to their TPO profiles can therefore be attributedto the different deposition techniques.

4. Discussion

A comparative study between four deposition tech-niques has been made with respect to catalytic activ-ity of samples containing approximately 5 wt.% Pd/Al2O3. To correlate activity variations with the na-ture of the catalysts, the samples were characterizedby means of BET, TEM, XRD and TPO.

As the results of the CH4 combustion experimentsindicate, Pd/Al2O3-ME1 exhibits the lowest activ-ity of all catalysts tested under the given conditions.Pd/Al2O3-ME1 presents aT10 value at around 80◦Chigher than Pd/Al2O3-IW and its conversion is con-siderably lower compared to the other catalysts tested.Both TEM images and XRD diffractograms con-firm that the PdO particle size of the Pd/Al2O3-ME1sample is considerably larger than for the samplesprepared with other techniques. However, the influ-ence of the particle size on the combustion activity isstill an issue of debate[23,24]. The difference in ac-tivity cannot be explained by the particle size alone,while other factors may influence the catalyst perfor-mance. A systematic difference between the valuesdetermined by TEM and XRD can be observed inTable 1. The size determined by XRD analysis is theaverage size of the PdO crystallite. This value is notinfluenced if the PdO particles consist of agglomer-ates. In contrast, the TEM images show particles thatcould be agglomerates of small particles or crystal-lites. The size of the agglomerates could be estimatedby TEM, but the crystallite size is more difficult to

determine unless it is well dispersed and in a verysmall size range. Hence, the disparity between XRDand TEM may account for that XRD measures the av-erage crystallite size and in the TEM images the sizeof particles. Furthermore, the low catalytic activityobserved for Pd/Al2O3-ME1 may be ascribed to thepoor dispersion of Pd/PdO displayed by TEM images.

Pd/Al2O3-IW, which is the most active of the cat-alysts, displays a reverse TPO profile as comparedwith the Pd/Al2O3-ME1 profile. In the former one,the first decomposition peak is less intense than thesecond one, whereas the reverse trend is observed forthe Pd/Al2O3-ME1. As suggested in previous reports,each decomposition peak represents a particular PdOspecies with inherently different activity[17], and therelative extent of the second peak increases with pa-rameters which favour reoxidation of Pd metal[22].In addition, a third decomposition peak is observedeven though the intensity is small.

Furthermore, the difference in intensity of the de-composition peaks suggests that more PdO could beaccessible for CH4 combustion in the Pd/Al2O3-IWcatalyst than in the Pd/Al2O3-ME2.

According to the TPO profiles, the two first decom-position peaks occur within a wider temperature win-dow for Pd/Al2O3-IW than for the other techniques.Moreover, the decomposition starts at a slightly highertemperature for the sample prepared by incipient wet-ness. The size of the temperature window is larger forPd/Al2O3-IW than for the less active Pd/Al2O3-ME2,suggesting that it may be a correlation between activ-ity and size of the temperature window.

The reoxidation coefficient,ρ, indicates how wellthe catalysts are reoxidized into PdO. A trend is ob-served for the different catalysts where the most ac-tive catalyst has the lowest�-value and the less activecatalyst presents the highestρ-value.

Considering the cooling ramp of the activity teststhe Pd/Al2O3-IW and Pd/Al2O3-ME2 samples exhibithigher conversion than Pd/Al2O3-G and Pd/Al2O3-ME1. This result is supported by the TPO results.

The results from the BET analysis indicate that thedecrease in the surface area due to calcination is largerthan the decrease depending on the choice of depo-sition technique. The pronounced decrease of surfacearea due to thermal treatment at elevated temperatureis a consequence of phase transformation of Al2O3[20]. This phase transformation is confirmed by XRD

K. Persson et al. / Applied Catalysis A: General 249 (2003) 165–174 173

analysis. All catalysts consist of bothθ-Al2O3 andδ-Al2O3 instead of the initialγ-Al2O3. Therefore, thevariation of activity between the catalysts cannot beascribed to loss of surface area.

In spite of the fact that Pd/Al2O3-ME1 andPd/Al2O3-ME2 are prepared under the same condi-tions and with the same constituents, except for thedestabilization agent, the activity between the cat-alysts differs considerably. As confirmed by TEMand XRD analyses, Pd/Al2O3-ME1 consists of largerPdO particles than Pd/Al2O3-ME2, which may bereflected by the catalytic activity of the samples. TheTPO profiles indicate that PdO is poorly dispersed inthe Pd/Al2O3-ME1 compared to the Pd/Al2O3-ME2.

Pd/Al2O3-IW and Pd/Al2O3-ME2 catalysts displaysimilar activities for methane combustion. From an in-dustrial point of view, incipient wetness is a well es-tablished technique for the preparation for supportedcatalysts. Since no considerable increase in combus-tion activity is achieved for catalyst prepared withME2, Pd/Al2O3-IW is preferable for preparing cata-lysts consisting of Pd on alumina for methane com-bustion.

However, a number of researchers have reportedthat the properties of the catalysts prepared by mi-croemulsion technique are very sensitive to the synthe-sis condition[25,26]. Many different parameters canbe varied, such as nature of surfactant and micelle size,in order to obtain catalysts with the desired properties.Hence, the results of this study do not rule out the pos-sibility that microemulsion could be a good techniquefor preparing an ignition catalyst for methane com-bustion, provided that the proper synthesis conditionsare established.

5. Conclusions

Incipient wetness, grafting and two different mi-croemulsion deposition techniques have been com-pared with respect to CH4 combustion activity forsamples consisting of Pd/Al2O3. Under the given con-ditions, the sample prepared by incipient wetness ex-hibited the lowest ignition temperature. Moreover, theconversion was observed to be higher for the incip-ient wetness catalyst in comparison to the catalystsprepared by other deposition techniques. In contrast,catalysts prepared by one of the microemulsion tech-

niques exhibit a considerably lower catalytic activityunder the given conditions.

As confirmed by TPO, the decomposition of PdOinto metallic and less active Pd occurs during a widertemperature window and starts at a higher tempera-ture for the more active catalyst. Furthermore, the in-tensity of the decomposition peaks in the TPO profilesuggests that more PdO is accessible for CH4 com-bustion for the sample prepared by incipient wetnessthan for the microemulsion catalysts. The variation be-tween the different deposition techniques may also beascribed to ability of reoxidation as pinpointed by thereoxidation coefficient, hence to the combustion activ-ity. According to the TPO results, the catalyst preparedby incipient wetness and the first microemulsion tech-nique may consist of different PdO species, resultingin variations in activity.

Due to the high calcination temperature needed forthis catalyst, the support undergoes phase transforma-tion into phases which have lower surface area. There-fore, the variation between the deposition techniquesas to surface area is negligible compared to the losscaused by alumina phase transformation.

Acknowledgements

STEM—The Swedish Energy Agency—is acknow-ledged for financially supporting this work. Thanks aredue to Condea GmbH for providing the alumina sup-ports. The authors would also like to thank Dr. SergioRojas for enlightening discussions and help with themanuscript and Marcus Sjöström for the help with theexperiments.

References

[1] J.H. Seinfeld, Atmospheric Chemistry and Physics of AirPollution, Wiley, New York, 1986.

[2] J.N. Armor, Appl. Catal. A 194–195 (2000) 3.[3] W.C. Pfefferle, J. Energy 2 (1978) 142.[4] E.M. Johansson, D. Papadias, P.O. Thevenin, A.G. Ersson, R.

Gabrielsson, P.G. Menon, P.H. Björnbom, S.G. Järås, in: J.J.Spivey (Ed.), Catalysis—Specialists Periodical Reports, vol.14, Royal Society of Chemistry, Cambridge, 1999, p. 183.

[5] R.A. Dalla Betta, J.C. Schlatter, D.K. Yee, D.G. Loffler, T.Shoji, Catal. Today 26 (1995) 329.

[6] S.R. Bell, SAE Paper, vol. 931829, 1993, p. 29.

174 K. Persson et al. / Applied Catalysis A: General 249 (2003) 165–174

[7] F. Klingstedt, A.K. Neyestanaki, R. Byggningsbacka, L.-E. Lindfors, M. Lundén, M. Petersson, P. Tengström, T.Ollonqvist, J. Väyrynen, Appl. Catal. A 209 (2001) 301.

[8] W.S. Epling, G.B. Hoflund, J. Catal. 185 (1999) 5.[9] G. Groppi, W. Ibashi, M. Valentini, P. Forzatti, Chem. Eng.

Sci. 56 (2001) 831.[10] J.G. McCarty, Catal. Today 26 (1995) 283.[11] M. Lyubovsky, L. Pfefferle, Appl. Catal. A 173 (1998) 107.[12] R. Burch, M.J. Hayes, J. Mol. Catal. A 100 (1995) 13.[13] R.B. Anderson, K.C. Stein, J.J. Feenan, L.J.E. Hofer, Ind.

Eng. Chem. 53 (1961) 809.[14] M. Valentini, G. Groppi, C. Cristiani, M. Levi, E. Tronconi,

P. Forzatti, Catal. Today 69 (2001) 307.[15] P.O. Thevenin, E. Pocoroba, L.J. Pettersson, H. Karhu, I.J.

Väyrynen, S.G. Järås, J. Catal. 207 (2002) 139.[16] M. Kishida, K. Umakoshi, J.-I. Ishiyama, H. Nagata, K.

Wakabayashi, Catal. Today 29 (1996) 355.[17] E. Pocoroba, L.J. Pettersson, J. Agrell, M. Boutonnet, K.

Jansson, Top. Catal. 16–17 (2001) 407.

[18] D. Papadias, U. Würtenberger, L. Edsberg, P. Björnbom,Chem. Eng. Sci. 57 (2002) 749.

[19] R.J. Farrauto, M.C. Hobson, T. Kennelly, E.M. Waterman,Appl. Catal. A 81 (1992) 227.

[20] J.S. Church, N.W. Cant, D.L. Trimm, Appl. Catal. A 101(1993) 105.

[21] P.O. Thevenin, A. Ersson, H.J.M. Kusar, P.G. Menon, S.G.Järås, Appl. Catal. A 212 (2001) 189.

[22] G. Groppi, C. Cristiani, L. Lietti, P. Forzatti, Stud. Surf. Sci.Catal. 130 (2000) 3801.

[23] T.V. Choudhary, S. Banerjee, V.R. Choudhary, Appl. Catal.A 234 (2002) 1.

[24] Y.H. Chin, D. Resasco, in: J.J. Spivey (Ed.), Catalysis—Specialists Periodical Reports, vol. 14, The Royal Society ofChemistry, Cambridge, 1999, p. 1.

[25] M. López-Quintela, J. Rivas, J. Colloid Interface Sci. 158(1993) 446.

[26] M. Boutonnet, J. Kizling, P. Stenius, Colloids Surf. B 5 (1982)209.