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Applied Catalysis B: Environmental 106 (2011) 476– 487

Contents lists available at ScienceDirect

Applied Catalysis B: Environmental

journa l h o me pa ge: www.elsev ier .com/ locate /apcatb

utothermal reforming of low-sulfur diesel over bimetallic RhPt supported onl2O3, CeO2–ZrO2, SiO2 and TiO2

anthias Karatzasa,∗, Kjell Janssonb, Angélica Gonzáleza, Jazaer Dawodyc, Lars J. Petterssona

KTH – Royal Institute of Technology, Department of Chemical Engineering and Technology, Teknikringen 42, SE-100 44 Stockholm, SwedenStockholm University, Arrhenius Laboratory, Department of Materials and Environmental Chemistry, SE-106 91 Stockholm, SwedenVolvo Technology Corporation, Chalmers Science Park, SE-412 88 Göteborg, Sweden

r t i c l e i n f o

rticle history:eceived 28 March 2011eceived in revised form 31 May 2011ccepted 4 June 2011vailable online 12 June 2011

eywords:luminaeria–zirconiaiesel reforminghodium–platinumilicaitania

a b s t r a c t

The objective of this paper is to study and clarify the role of selected supports (both reducible and non-reducible) on the activity, selectivity and stability of RhPt-based catalyst for diesel reforming. Autothermalreforming (ATR) of low-sulfur diesel (S ∼6 ppm, C/H ∼6.43 (w/w)), H2O/C ∼2.5, O2/C ∼0.49, was tested atbench scale to detect differences in activity for catalysts consisting of 1 wt% Rh and 1 wt% Pt supportedon alumina, ceria–zirconia (17.5 wt% ceria), silica and titania. Promoters in the form of MgO, Y2O3, La2O3,CeO2 and ZrO2, ranging from 4 wt% to 10 wt%, were also added onto the supports to detect differencesin catalyst activity in terms of diesel conversion, CO2 selectivity, and hydrogen and ethylene production.All metals were added sequentially onto the support by the incipient wetness technique and washcoatedon 400 cpsi cordierite monolithic carriers with dimensions d = 17.8 mm, l = 30.5 mm.

The product gas analysis, using FTIR and NDIR, showed that RhPt/CeO2–ZrO2 was found to be mostactive for ATR of diesel since a fuel conversion close to 98% was obtained. Furthermore, the cata-lyst activity of the unpromoted samples, in terms of diesel conversion, increased in the followingorder: RhPt/SiO2 < RhPt/TiO2 < RhPt/Al2O3 < RhPt/CeO2–ZrO2. The addition of promoters was found to beinsignificant as well as having a negative impact on the catalyst performance in most cases, except forthe alumina-promoted sample. The addition of 10 wt% La2O3 on RhPt/Al2O3 was found to enhance dieselconversion, hydrogen productivity as well as lower the ethylene concentration from 3700 ppm to lessthan half that value. The latter observation was confirmed by O2-TPO analysis of aged powder sampleswhere lower loads of coke were present than on the La-promoted sample.

The morphology, surface and bulk properties of RhPt/CeO2–ZrO2 were closely examined in order toprovide a possible correlation between the activity and characterization results. N2-BET analysis showedthat the surface area of RhPt/CeO2–ZrO2 was ∼64 m2/g, while the silica samples exhibited the highest area,∼137–185 m2/g. Hence, the difference in the surface areas was not enough to explain the trends observedin the activity measurements. XRD analysis of RhPt/CeO2–ZrO2 showed crystalline phases characteristicof zirconia, most likely tetragonal. Also, the diffractogram did not reveal any Rh or Pt peaks indicatingthat the noble metal particles are highly dispersed on the support. In contrast, peaks ascribed to metallicPt (∼30–46 nm) were clearly visible on the XRD patterns taken from all the other supported samples.H2-TPR analysis of RhPt/CeO2–ZrO2 showed reduction peaks ascribed to RhiOx species as well as a minorhydrogen spillover effect on the support to be present at T = 120 ◦C and 450 ◦C, respectively. Also, thehydrogen consumption of the RhiOx species was the highest compared to the other supported RhPtsamples. TEM analysis performed on fresh RhPt/CeO2–ZrO2 showed that the RhiOx and Pt particles werehighly dispersed on the support, both with particle sizes in the vicinity of ∼5–15 nm. Rh species was found

on ceria and zirconia, while Pt was present mainly on the ceria layer possibly in the form of Pt–O–Ce bonds.H2-chemisorption analysis measured at T ∼40 ◦C shows similar Rh dispersion results.

To summarize, the higher activity results of RhPt/CeO2–ZrO2 for ATR of diesel, compared to othersupported catalysts, may be ascribed to the higher reducibility of RhiOx species as well as the superiorRh and Pt dispersion. Also, the support contribution, in particular ceria, is believed to promote watergas-shift activities as well as re

∗ Corresponding author. Tel.: +46 8 790 8236.E-mail address: xanthias@kth.se (X. Karatzas).

926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apcatb.2011.06.006

duce coke deposits on the catalyst surface.© 2011 Elsevier B.V. All rights reserved.

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. Introduction

Fuel cells (FC) by many deemed as an auspicious green powerechnology, have great potential to solve future energy demandsnd to contribute to a sustainable world [1,2]. One promising nichearket for fuel cell applications is as 5 kWe auxilliary power units

APU) onboard heavy-duty trucks [3–6]. In this scenario, a fuel cellan be employed to meet the growing electrical demands for com-ort units (e.g. A/C, radio and TV) as opposed to the conventionalay of using the vehicle’s main engine at standstill (aka idling)

o power the alternator/battery system to charge these particularlectronic units. By using fuel cells in heavy-duty trucks, smarteruel economy, higher energy and thermal efficiencies as well aslose to zero emissions can be accomplished compared to runninghe diesel truck’s main engine in idling mode [3–10]. Also, smalleight and volume can be achieved using FC-APU which is a posi-

ive feature enabling the APU unit to be easily fitted and installednboard trucks [3–10].

There are many different kinds of fuel cells that can be usedor power generation where polymer electrolyte fuel cells (PEFC)unning on hydrogen as energy feedstock is one of the main viableandidates [1,5,6]. Furthermore, market analysts predict PEFC-APUnits to reach the transport sector the fastest. This is due to sev-ral promising and winning features e.g. quick and versatile energyesponse, short start-up time, superior cell material stability andow fabrication cost [5,6].

To date, fueling mobile PEFC auxiliary power units is a bigechnical barrier. Ideally a PEFC should be fed with pure gaseousydrogen in order to ensure optimal performance and long dura-ility of the fuel cell. However, storing hydrogen gas can beroblematic due to the low energy density [1,2]. Hydrogen gastations can be used, however, the lack of infrastructure calls forther solutions. One possible solution is by catalytically reform-ng the transportation fuel, in this case diesel, into a hydrogen-richas that can be used by the fuel cell to generate electricity [1,2].owever, catalytic diesel reforming is at an early stage of commer-ialization in which further research is still needed. Most studiesave shown that autothermal reforming (ATR) is the most suit-ble reforming technology for hydrogen generation for a dieselEFC-APU system. The latter is based on the results derived fromeveral studies showing that ATR is the most feasible alternativen terms of acquiring high overall thermal and reforming efficien-ies for FC-APU applications [1,11–13]. Also, ATR is a dynamic andnergy-efficient process capable of handling the frequent start-upsnd shutdowns occurring both during transient and steady-stateperation of the diesel trucks [1,11–13]. However, to date, fur-her research and development is needed for finding promisingatalysts for ATR of diesel to be employed for a diesel PEFC-APUystem.

Heterogeneous catalytic materials implemented for dieseleforming can be divided into various groups and subgroups basedn their respective operating parameters, reaction temperaturesnd the type of diesel fuel that is employed. To date, low weightoadings of Rh, a typical reducing metal, is frequently used as the

ain active component in the diesel reforming catalyst compo-ition [14,15]. This is due to the superior activity, higher syngasormation and selectivity, as well as higher resistance towards car-on deposition and sulfur adsorption, compared to less expensivease metals such as Ni, Fe and Co [16,17]. A variety of supportsave been documented and used for the Rh-based catalyst e.g.lumina, zirconia, pyrochlores and magnesium oxide, among oth-rs [14,15,18–23]. Most of these studies use solely Rh as the main

ctive noble metal. However, it is a well-known fact that the maineactions of ATR are a combination of initial exothermic partial oxi-ation and subsequent endothermic steam reforming. Moreover,ide-reactions e.g. reverse water-gas-shift (WGS) and methanation

vironmental 106 (2011) 476– 487 477

may also take place [1,18,24–29]. Hence, both oxidative andreductive environments are present during ATR. Therefore, addingoxidation metals to alloy with Rh is considered a wise choice inorder to facilitate and promote the initial main oxidation reactions.Pt is an excellent oxidation metal frequently employed as TWC andas HCN synthesis catalyst [1,30]. In general, Pt alone is not usedfor diesel reforming as it can be easily deactivated by sintering,volatization and coke deposition [1,14]. Adding Rh to form RhPtalloys stabilizes and prevents sintering of Pt particles [1,14]. How-ever, a limited number of studies can be found where the effectof the support has been thoroughly investigated for RhPt catalystsemployed for diesel reforming [14,15]. Hence, it is interesting toascertain the activity, selectivity, stability and durability of selectedpromising supported RhPt catalysts, which is the scope of thisstudy.

For ATR of diesel, the support should exhibit several importantphysicochemical properties. For instance, the support should ingeneral [1,18,30–32]:

• provide high melting point and thermal stability especiallysince gas phase reactions usually take place in the vicinity of700–800 ◦C while surface reactions can occur at even higher tem-peratures, e.g. at 900–1100 ◦C;

• maintain the surface area and dispersion of the active compo-nents by reducing sintering and volatilization effects;

• interact closely with the loaded metals e.g. removing cokedeposits and sulfur adsorbents from active sites;

• provide a certain porosity in terms of pore shape and pore sizedistribution in order to facilitate the impregnation and dispersionof the active metals as well as aiding the reforming activities

• exhibit some catalytic activity, e.g. supplementary diesel conver-sion and hydrogen generation capability;

• provide a high surface area, mechanical strength and thermalstability, in particular if the supports are to be washcoated onmonolithic carriers for mobile applications.

In this study RhPt along with a selection of promis-ing promoters were added on irreducible supports such asAl2O3 and SiO2, and on reducible supports such as CeO2–ZrO2 and TiO2 to detect differences in catalytic activity.

Alumina, Al2O3, is a popular support material for reformingcatalysts due to its ability to provide high surface area, high melt-ing point (∼2000 ◦C) and thermal stability. Alumina can exist inseveral crystallographic forms, where the high surface area crys-talline structures of gamma (∼150 m2/g) and delta (∼100 m2/g)alumina are frequently employed as reforming catalyst supports[1,31–33]. In a previous work RhPt/Al2O3 (�-alumina) was testedfor ATR of diesel [15]. The results showed that the catalyst exhib-ited lower activity compared to Rh/Al2O3. Adding Ce and La to thebimetallic sample significantly improved the catalyst performanceas well as reduced the degree of coke deposits. It was assumedthat this effect was mostly due to positive Ce promotion, owing toPt–Ce interactions. However, the role of La was not fully under-stood and investigated. Some interesting data were found. Forinstance, TPR analysis showed that La improved the reducibilityof Ce. Also, XRD and XPS analyses showed that La was found inthe dispersed phase, in the form of La/Al2O3. In the present studyonly La was added to RhPt/Al2O3 to determine if La has a posi-tive effect on the catalyst performance. In general, La is known toimprove the thermal stability and inhibit adverse phase transitionsof alumina by forming stable textural structures with �-alumina[34–36].

Ceria–zirconia, CeO2–ZrO2, has been used extensively assupport for catalysts employed for TWC and methane reforming[9,30,37]. Ceria is a metal oxide with high surface area (∼270 m2/g),high melting point (∼2600 ◦C) and excellent oxygen storage capac-

478 X. Karatzas et al. / Applied Catalysis B: Environmental 106 (2011) 476– 487

Table 1Washcoat properties of fresh incipient wetness-prepared RhPt supported catalysts. The surface area and porosity were measured by N2-BET while the crystallite size of Ptwas determined by XRD. The Pt crystallite size was measured at 2� = 81.5◦ using the Scherrer equation.

Sample ID Catalyst compositiona (wt%) Surface area (m2/g) Pore volume (cm3/g) Pore diameter (Å) dp(Pt) (nm)

�-Al2O3 105 0.92 349 –RhPt/Al2O3 Rh(1.0)Pt(1.0)/Al2O3 94 0.59 250 30RhPt–La2O3/Al2O3 Rh(1.0)Pt(1.0)La2O3(10)/Al2O3 89 0.72 294 45

CeO2–ZrO2b 173 0.31 n.a. –

RhPt/CeO2–ZrO2 Rh(1.0)Pt(1.0)/CeO2–ZrO2 64 0.22 136 n.d.RhPt–MgO–Y2O3/CeO2–ZrO2 Rh(1.0)Pt(1.0)MgO(4.0)Y2O3(5.0)/CeO2–ZrO2 59 0.23 155 n.d.

SiO2 569 0.59 n.a. –RhPt/SiO2 Rh(1.0)Pt(1.0)/SiO2 137 0.30 87 46RhPt–CeO2–ZrO2/SiO2 Rh(1.0)Pt(1.0)CeO2(5.0)ZrO2(5.0)/SiO2 185 0.36 77 33

TiO2 217 0.33 n.a. –RhPt/TiO2 Rh(1.0)Pt(1.0)/TiO2 24 0.14 236 32RhPt–MgO/TiO2 Rh(1.0)Pt(1.0)MgO(5.0)/TiO2 12 0.06 205 36

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a The nominal weight loadings of the noble metals and the promoters on the supb Ceria content in the support, ∼17.5 wt% CeO2.

ty. The latter is based on ceria’s ability to switch between Ce4+

nder oxidizing conditions and Ce3+ under reducing conditions.lso, ceria can promote the dispersion and reduction of nobleetals, and enhance water gas-shift reactions. Zirconia is added

o ceria in order to improve the thermal stability, reducibility andulfur tolerance of ceria [9,30,37]. The surface, bulk and catalyticroperties of CeO2–ZrO2 vary depending on the weight percentagef CeO2 that is incorporated in the support. In general, 50–60 wt%eO2 is favorable for deNOx applications [38,39], while smallermounts of CeO2, less than 20 wt%, are frequently employedor reforming of e.g. methane and diesel [18,37]. In this study

gO–Y2O3 formulations were tested as promoters for CeO2–ZrO2.tudies have shown that magnesium oxide can improve thectivity of Rh [40–43] while yttrium can enhance the thermaltability of ZrO2 at reaction temperatures exceeding 700 ◦C [9,37].

Silica, SiO2, can also be used for TWC and reforming applications9,30,31]. An advantage with using silica as carrier is that it exhibitsigh tolerance towards sulfur poisoning. Furthermore, it can pro-ide high surface areas in the vicinity of 300–400 m2/g [9,30,31].owever, on the downside, silica exhibits some surface acidity due

minor water content which is bonded within the pores of theaterial. The latter ability makes silica inappropriate for applica-

ions in alkaline environment [31]. Also, the thermal stability ofilica is lower than that of alumina [9,31]. In this study, CeO2–ZrO2as used as promoter in order to improve the hydrogen generation

apability and thermal stability of the support material.Titania, TiO2, is an inert material, which just as silica also has

xcellent properties inhibiting potential sulfur deposits on the cat-lyst surface during reforming. Titania can exist in three crystalhases: as brookite, anatase and rutile. The crystallographies ofhese species differ significantly, brookite is rhombohedral, anatases tetragonal and rutile is close-packed tetragonal [31]. Typicallynatase is mostly used for high temperature catalytic applicationss it has a higher surface area (50–120 m2/g) and thermal stabilityompared to the other phases [9,31]. In this study, MgO was testeds promoter for the titania-supported RhPt catalyst. Several studiesave shown that the addition of MgO can improve the reformingctivity and selectivity of Rh [40–43].

In this study, autothermal reforming (ATR) of low-sulfur dieselS ∼6 ppm, C/H ∼6.43 (w/w)) was tested at bench scale to evaluateifferences in activity in terms of diesel conversion, CO2 selectivity,nd hydrogen and ethylene production for catalysts consisting of

wt% Rh and 1 wt% Pt supported on Al2O3, CeO2–ZrO2, SiO2 andiO2 with designated promoters. Fresh and aged powder samples

f the catalysts were characterized by N2-BET, H2 chemisorption,RD, H2-TPR, O2-TPO and TEM analyses. The aim of this study is toscertain the activity, selectivity, stability and durability of thesearticular bimetallic catalysts.

are given in the parentheses.

2. Experimental

2.1. Catalyst preparation

The supported RhPt catalysts were synthesized using the incip-ient wetness technique. The noble metal precursors used forthe preparation of the catalyst were Rh nitrate (Rh(NO3)3, Rh8–10%, w/w, Sigma–Aldrich) and Pt nitrate ((NH3)4Pt(NO3)2, Pt3–4%, w/w, Alfa Aesar) solutions. The promoters’ precursors usedwere Ce nitrate (Ce(NO3)3·6H2O, 99.99%, Alfa Aesar)), La nitrate(La(NO3)3·6H2O, 99.9%, Alfa Aesar), Mg nitrate (Mg(NO3)2·6H2O,98%, Alfa Aesar), Zr nitrate (ZrO(NO3)3·xH2O, 99%, Sigma–Aldrich)and Y nitrate (Y(NO3)3·6H2O, 99.8%, Sigma–Aldrich) solutions. Thesupports used were pretreated alumina (PURALOX HP-14/150,Sasol Germany GmbH) calcined at 1000 ◦C for 1 h to generate �-Al2O3, see also previous work [15], ceria–zirconia (XZO 1289/01,17.5 wt% CeO2, MEL Chemicals), silica (SiO2, 99.5%, Sigma–Aldrich),and titania (TiO2, anatase, 99.7%, Sigma–Aldrich), powders.

All supports were impregnated with the metals sequentially,starting with the promoters, in accordance with the nominal weightloadings presented in Table 1. The metal solution was diluted withMilli Q water, dripped onto the various supports and carefullymixed. This procedure was repeated twice, with a drying step at110 ◦C for 3 h in between. The resulting powders were then calcinedin air at 800 ◦C for 3 h.

After calcination, the powders were suspended in ethanol slurry(∼20 wt% powder), ball milled for 24 h and deposited via dip-coating procedure on 400 cpsi cordierite monoliths, d = 17.8 mm,l = 30.5 mm (Corning). The dip-coating procedure was repeateduntil catalyst loadings of 20 wt% of the total weight (monolith andactive material) were reached. The coated monoliths were thencalcined in air at 800 ◦C for 3 h. Table 1 exhibits the washcoat prop-erties of the monolithic catalyst used in this study. The promotersare expressed in the oxidized state.

2.2. Characterization

The catalyst powder samples in this study were characterized bythe following analytical tools and procedures which are describedin detail below.

Nitrogen adsorption at liquid N2 temperature (N2-BET) was usedto measure the surface area, pore volume and pore size distributionof the fresh samples. A Micromeretics ASAP 2010 instrument was

employed for this analysis. Prior to the analysis the sample, ∼0.4 g,was degassed in vacuum for 3 h at 250 ◦C.

X-ray diffraction (XRD) was used to determine the crystal phasesof the fresh samples. A Siemens Diffraktometer D5000 scanning 2�

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rom 10◦ to 90◦ in the scan mode (0.02◦, 1 s), using Ni filtered Cu-K�adiation was employed.

Temperature-programmed reduction (TPR) was performed toetermine the reducibility of the fresh samples. Approximately0.1 g sample was pre-treated in 5 vol.% O2 in He at 800 ◦C

or 30 min to remove potential impurities and to ensure that itemained in the oxidative state. The sample was then cooled tooom temperature in He and reduced in hydrogen, 5 vol.% H2 in Ar50 cm3/min), in the temperature range of 30–800 ◦C with a heat-ng rate set to 10 ◦C/min. A Micromeritics Autochem 2910 equipped

ith a thermal conductivity detector (TCD) was employed to recordhe hydrogen consumption.

Temperature-programmed oxidation (TPO) was performed toistinguish the type and amount of coke present on the agedamples. Approximately 0.1 g of aged sample was collected andeated in oxygen, 5 vol.% O2 in Ar (50 cm3/min), in the tempera-ure range of 30–1000 ◦C with a ramp speed set to 10 ◦C/min usinghe Micromeritics Autochem 2910.

Hydrogen chemisorption analysis was used to measure theispersion and the crystallite size of the Rh particles in monometal-

ic fresh samples (1 wt% Rh) using a Micromeritics ASAP 2020.he samples, ∼0.2 g/sample, were pre-reduced in hydrogen at aemperature set to 800 ◦C, helium-treated at 450 ◦C and finallyydrogen-treated and analyzed at 40 ◦C. The atomic stoichiometricalue of H/Rh = 1 was used for processing of the chemisorption data.

Transmission electron microscopy (TEM) was used to deter-ine the composition, morphology and particle size distribution

f Rh and Pt species of the fresh ceria–zirconia, silica and titania-upported powder samples. The samples were collected andransferred onto a carbon film containing holes supported by a 200

esh TEM grid of copper. The powders, ∼0.1 g/sample, were gen-ly ground using a pestle and mixed in a solvent containing ∼2–3roplets of n-butanol. A droplet of the solution was then added tohe TEM grid and dried. A JEM 2000FXII (JEOL) was used to imagehe particle size distribution (≥10 nm) on the supports. Imagesith high resolution were also analyzed using an energy disper-

ive X-ray (EDX) spectrometer, JED 2300 (JEOL).In this study, freshowder samples were analyzed by N2-BET, XRD, H2 chemisorption,2-TPR, and TEM analyses. The fresh powder samples were taken

rom the prepared catalysts prior to deposition on the monoliths.ged powder samples were taken from spent catalysts, after theeforming experiments were completed, and analyzed by O2-TPOnalysis. The aged powder samples were collected by cutting thepent monoliths in half and scraping off the interior washcoat using

scalpel.

.3. Low-sulfur diesel reforming

The diesel reforming experiments have been described in detailn previous work [15,19]. In short, in this study, the ATR exper-ments were carried out at the reaction conditions Tfeed = 650 ◦C,

2O/C ∼2.5, O2/C ∼0.49, TOS = 3 h, GHSV ∼13,000 h−1 and P = 1 atm. standard diesel fuel (S ∼6 ppm, C/H ∼6.43 (w/w)) was useds feedstock. The experiments were carried out in a verticallyounted stainless steel tubular reactor with ID = 23.7 mm equippedith a heating coil and three thermocouples to control the feed

nd reactor temperatures. The product gases were analyzed using Gasmet Cr-200 Fourier Transform Infrared Spectrometer (FTIR)nd a Maihak modular system S710 equipped with a non-dispersivenfrared sensor (NDIR) and TCD.

The catalytic activity was evaluated and expressed in terms of

iesel conversion, CO2 selectivity (expressed as CO2/(CO2 + CO)), asell as hydrogen and ethylene productivity. Details concerning the

alculations of the diesel conversion, using the FTIR instrument, cane found elsewhere [15].

vironmental 106 (2011) 476– 487 479

3. Results and discussion

3.1. Catalyst characterization, fresh powder samples

3.1.1. BETThe surface and porosity data of the different supports obtained

from the N2 sorption measurements are presented in Table 1. Asseen in the table, the addition of the promoters and noble metalson the supports, along with a final calcination temperature of 800 ◦Cfor 3 h, resulted in a decrease in surface area and pore volume. Forthe alumina samples, the pretreated alumina support had an ini-tial surface area of 105 m2/g. The BET surface areas for RhPt/Al2O3and RhPt–La2O3/Al2O3 were 94 and 89 m2/g. Hence, only a smalldegree of the initial surface area was lost, which is characteristicfor alumina [31,33].

For the other supports a much more significant drop of surfacearea loss was observed. For instance, for ceria–zirconia, the supporthad the initial surface area of 173 m2/g while for the impreg-nated samples RhPt/CeO2–ZrO2 and RhPt–MgO–Y2O3/CeO2–ZrO2the surface areas were reduced to almost 1/3. Similar trends werenoted for the silica and titania samples. For silica, the surfaceareas of RhPt/SiO2 and RhPt–CeO2–ZrO2/SiO2 were 137 m2/g and185 m2/g, respectively, which are the highest surface areas ofall impregnated samples. The lowest surface area was noted forthe titania samples RhPt/TiO2 and RhPt–MgO/TiO2, 24 m2/g and12 m2/g, respectively. Furthermore, the lowest pore volume wasnoted for RhPt–MgO/TiO2, ∼0.06 cm3/g.

Hence, to summarize, the surface area of RhPt (with and withoutpromoters) on the supports increased in the following order: tita-nia < ceria–zirconia < alumina < silica. Other groups have reportedsimilar surface area trends [40,44].

3.1.2. X-ray diffractionThe crystalline bulk phases obtained from the X-ray diffraction

measurements of the non-impregnated supports are displayed inFig. 1a–d. Furthermore, the diffractograms of the fresh powder sam-ples of RhPt/Al2O3, RhPt/CeO2–ZrO2, RhPt/SiO2 and RhPt/TiO2 withthe promoters are also shown in Fig. 1a–d. As seen in the figures,characteristic crystalline phases were detected for the bulk compo-sition of the non-impregnated alumina, ceria–zirconia and titaniasupports. In contrast, as expected, the XRD pattern of the non-impregnated silica did not exhibit any distinctive sharp diffractionpeaks as silica is amorphous.

In Fig. 1a, the XRD patterns showed that the �-phase of aluminais present for all alumina samples. Rh phases were not detectedby XRD for any impregnated samples indicating that the rhodiumparticles are small and well-dispersed on all supports. In addition,La species were not detected indicating that La is in the dispersedphase. Also, the addition of La was found to lower the intensityof the peaks. Similar results have been reported in previous work[15,19] and by others [45–47]. In this study, Pt phases ascribed tometallic Pt were observed at 2� = 40◦, 46.5◦, 81.5◦ and 86◦ on theimpregnated alumina samples [48,49]. This has also been noted inprevious studies [15,19]. In general, platinum oxides, e.g. PtO andPtO2, are unstable and can easily decompose and form metallic Ptat temperatures above 500 and 550 ◦C, respectively [50–52]. Also,Pt particles are known to sinter at temperatures exceeding 450 ◦C[1]. As seen in Table 1, the crystallite size of Pt on the alumina sam-ples was ∼30–45 nm. Interestingly, metallic Pt was detected on allsupports except on the ceria–zirconia samples, as seen in Fig. 1b.This observation indicates that Pt crystallites are well-dispersed onceria–zirconia.

As seen in Fig. 1b, for RhPt/CeO2–ZrO2 andRhPt–MgO–Y2O3/CeO2–ZrO2, only diffraction peaks from thesupport material were detected at 2� = 30.2◦, 35.2◦, 50.3◦ and60.1◦. These peaks can be ascribed to ZrO2 since reflections of

480 X. Karatzas et al. / Applied Catalysis B: En

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Fig. 1. XRD patterns for 2� between 10◦ and 90◦ at room temperature of (a) alu-mina samples, (b) ceria–zirconia samples, (c) silica samples and (d) titania samples.Reflections of (�) metallic Pt, (�) magnesium titanium oxide MgTiO3 and (+) rutilephase of TiO2. See Table 1 for catalyst formulations.

vironmental 106 (2011) 476– 487

typical fluorite-structured CeO2, normally present at 2� = 28.5◦,33.3◦, 47.5◦ and 56.4◦, were not detected [15,53,54]. Other groupshave reported similar results [55,56]. Regarding the crystallitestate of ZrO2, both cubic and tetragonal phases can be presentfor CeO2–ZrO2 with a CeO2 content less than 20 wt% [57,58]. Thebroadness of the ZrO2 peaks makes it difficult to distinguish andclarify the states of zirconia using XRD [55]. In general, lowerintensity peaks are noted for tetragonal ZrO2 [56]. In this study,the phase of ZrO2 is believed to be tetragonal. This observation isin good agreement with XRD results obtained from other studieswhere the same support and similar catalyst preparation havebeen employed [18,59]. In this study, the absence of Rh and Ptpeaks in the XRD patterns in Fig. 1b indicates that the noble metalsare finely dispersed on the ceria–zirconia support. TEM analysis,described in detail in Section 3.1.4, was performed on a fresh pow-der sample of RhPt/CeO2–ZrO2 in order to get a better overviewof the various Rh and Pt particle size distributions, dispersion andsurface states on the support.

For the silica samples, RhPt/SiO2 and RhPt–CeO2–ZrO2/SiO2,respectively, the X-ray diffraction measurements did not reveal anynew phases. Only diffraction peaks from metallic Pt crystallites,∼33–46 nm in size (Table 1), were observed as seen in Fig. 1c.

The XRD patterns of RhPt/TiO2 and RhPt–MgO/TiO2 are shownin Fig. 1d. As seen in the figure, peaks ascribed to anatase aswell as metallic Pt were detected by XRD. Furthermore, newpeak trends were observed on the MgO-promoted sample. Asseen in the figure, for RhPt–MgO/TiO2, a minor phase transitionfrom anatase-to-rutile took place and bulk Mg species integratedwith the titania support were also found. Small rutile peaks werepresent at 2� = 27.6◦ and 37.2◦ [60,61] while the main peaks ofthe magnesium species, MgTiO3, can be seen at 33.1◦, 35.7◦, 36.3◦,40.8◦, 49.4◦, 56.9◦, 62.4◦ and 64.1◦. The crystallite size of MgTiO3was approximately 48 nm, calculated by the Scherrer equationat 2� = 33.1◦. Finally, TiO7 crystallites or other suboxides phases,which have been reported by others for similarly prepared Rh ontitania samples [62], were not detected in this study using XRD.Interestingly, no signs of minor phase transition of anatase-to-rutilewere detected by XRD for the RhPt/TiO2 sample (calcined at 800 ◦C).This suggests that the addition of RhPt on titania could inhibitadverse phase transitions of the support. In particular since rutile isknown to have a very low surface area, <10 m2/g, and low thermalstability it is not suited for high-temperature applications such asdiesel reforming. In general, the anatase-to-rutile phase shift can becomplex and difficult to discern when it or if it occurs. For instance,reports have shown that the temperatures at which the transitiontakes place can differ significantly depending on the preparationroute and the crystallite sizes of the support [61,63,64]. The lowestphase transition temperature can be around 500 ◦C while the high-est can be up to 900 ◦C [61]. In general, both phases can co-exist inthe bulk for titania samples thermally treated at 800 ◦C, as shown inthe XRD analysis by Bakardieva et al. [65]. At higher temperaturesthan 800 ◦C rutile becomes the major phase present on titania [65].

3.1.3. Temperature-programmed reductionThe results from the H2-TPR analysis are shown in Fig. 2a–e.

In general, only peaks ascribed to RhiOx, the promoters and thesupport were observed by TPR. Peaks assigned to PtO were notpresent in the TPR profiles. Furthermore, other Rh species, e.g.rhodium aluminates, typically present at higher reduction temper-atures (T > 600 ◦C) were not observed by TPR.

It should be emphasized that all samples were hydrogen treatedonly once. It is a well-known fact that the chemisorption properties

of reducible supports can be suppressed when being exposed to ahigh number of H2 and CO cycles (TPR–TPO–TPR sequences), andhigh reduction temperatures [60]. The latter phenomenon is oftentermed strong metal–support interaction (SMSI) [40,62,66,67].

X. Karatzas et al. / Applied Catalysis B: Environmental 106 (2011) 476– 487 481

8007006005004003002001000

(a)

RhPt/Al2O3

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.u.

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8007006005004003002001000

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RhPt-MgO-Y2O3/Ce O2-ZrO2

Hyd

roge

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.u.

Temperat ure [°C]

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8007006005004003002001000

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.u.

Temperature [ °C]

(d)

RhPt-Ce O2-ZrO2/SiO2

RhPt/ SiO2

8007006005004003002001000

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.u.

Temperature [ °C]

(e)

RhPt- MgO /Ti O2

RhPt/Ti O2

F –zircoi

Stois

RiRh

ig. 2. TPR profiles of (a) unpromoted RhPt catalysts, (b) alumina samples, (c) cerias displayed as a function of temperature. See Table 1 for catalyst formulation.

MSI effects are known to alter the adsorptive and catalytic proper-ies of the metal and support [40,62,66,67]. Hence, in this study, inrder to fairly compare the overall hydrogen uptake abilities of therreducible (Al2O3 and SiO2) and reducible (CeO2–ZrO2 and TiO2)upports only one H2 reduction step was carried out.

In Fig. 2a, TPR analysis shows that the reducibility of

hiOx species, in the temperature region 40–250 ◦C, increases

n the following order: RhPt/Al2O3 < RhPt/SiO2 < RhPt/TiO2 <hPt/CeO2–ZrO2. The peaks at T ∼450 ◦C were ascribed to possibleydrogen spillover effect and reducibility of the supports. Similar

nia samples, (d) silica samples and (e) titania samples. The hydrogen consumption

TPR profiles for Rh on CeO2–ZrO2 have been reported by others[58,68]. In general, lower loadings of ceria, <20 wt%, on CeO2–ZrO2is considered to favor the reducibility of the RhiOx species at T∼140 ◦C as shown by Fornasiero et al. [68].

In Fig. 2b, the effect of La on RhPt/Al2O3 is depicted. As seen inthe figure, the addition of 10 wt% La improves the hydrogen uptake

of RhiOx species in the temperature interval 100–300 ◦C. Also,the reduction temperature of the RhiOx species is shifted from170 to 270 ◦C. Similar observations have been made by Ferrandonet al. [69,70] and Usmen et al. [71]. It is suggested that Rh forms

4 s B: Environmental 106 (2011) 476– 487

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Table 2TEM analyses results of fresh RhPt supported on ceria–zirconia, silica and titania. Theparticle size and distribution of the smallest Rh2O3, Pt and RhPt crystals observedand measured on the supports are given in the table. Close to 60 different particleswere analyzed for each sample.

Sample dp(Rh2O3) (nm) dp(Pt) (nm) dp(RhPt) (nm)

82 X. Karatzas et al. / Applied Catalysi

compound with the La promoter and as a result the interactionetween Rh and the alumina support becomes weaker. Hence, theddition of La decreases and inhibits Rh–Al interactions making thehiOx species more easily accessible for hydrogen reduction. Also,s seen in the figure, lanthanum oxide phases were not detected byPR indicating that La is in the dispersed phase. The latter confirmshe XRD results seen in Fig. 1a. Similar observations have beeneported in previous studies [15,19] and by others [45–47].

In Fig. 2c, the effect of MgO–Y2O3 on RhPt/CeO2–ZrO2 is dis-layed. As seen in the figure, the addition of the promoters causes

minor switch of the reduction temperature of the RhiOx speciesrom 140 ◦C to 170 ◦C. The hydrogen consumption of the RhiOx

pecies was similar for both the unpromoted and the promotederia–zirconia samples. Phases ascribed to magnesium and yttriumxides were not detected by TPR. This confirms the XRD results seenn Fig. 1b.

In Fig. 2d, the H2-TPR profiles of RhPt/SiO2 andhPt–CeO2–ZrO2/SiO2 are presented. For both samples, thewin peaks present in the initial part of the reduction graphs, therst peak at 110 ◦C and the second at 160 ◦C, can be ascribed toossible full reduction of RhiOx species, e.g. Rh2O3 and RhO2, intoetallic Rh. Similar TPR observations and Rh peak trends on silica

ave been reported by others [72,73]. Furthermore, in this study, aseen in the Fig. 2d, the addition of the CeO2–ZrO2 had a significantegative effect suppressing the reducibility of the RhiOx species.lso, the hydrogen spillover effect of the support at T ∼450◦ wasnhanced. In addition, for RhPt–CeO2–ZrO2/SiO2, reduction peaksscribed to cerium and zirconium oxides were not detected byPR. The latter confirms the XRD results seen in Fig. 1c.

Finally, in Fig. 2e, the reduction graphs of the titania sampleshPt/TiO2 and RhPt–MgO/TiO2 are shown. Again, the same trendan be noted as reported for TPR profiles of the silica samples, mean-ng that the addition of promoters, in this case MgO, inhibits theeducibility of RhiOx. As seen in the figure, the addition of 5 wt%gO completely hinders the hydrogen uptake of RhiOx species

t T ∼200 ◦C. This suggests that MgO affects the Rh–Ti interac-ion by making it stronger. Also, for RhPt–MgO/TiO2, a new peaks manifested at T ∼750 ◦C that might be ascribed to reduction of

gO species. It is possible that MgRhO2 species are reduced in thisemperature range as suggested by Ruckenstein and Wang [43].owever, in this study, it is unlikely that Rh species are reduced atigh temperature during TPR. In particular, since our XRD resultshow other kinds of Mg species to be present on the bulk fractionf the catalyst. According to the diffractogram of RhPt–MgO/TiO2see Fig. 1d) only reduced Mg species, e.g. MgTiO3, were detectednd present in the bulk of the support. Another possibility is thathe reduction peak at T ∼750◦ could be ascribed to reduction ofhe support material itself. Other groups have reported similarPR observations for TiO2 [74,75]. In general, titania can be partlyeduced to form TiO2−x by H2 at temperatures exceeding 500 ◦C,ee Eq. (1). Hence, to summarize, it was difficult to conclude, fromhe XRD and TPR analyses, the nature of the MgO as well as TiO2pecies that are reduced during the hydrogen reduction and heatreatment.

Reduction of titania at T > 500 ◦C

iO2 + xH2 → TiO2−x + xH2O (1)

.1.4. Transmission electron microscopyTEM images of RhPt/CeO2–ZrO2, RhPt/SiO2 and RhPt/TiO2 are

hown in Fig. 3a–e. Regarding RhPt/Al2O3, the result and discussiononcerning the dispersion, morphology and particle size distribu-

ion of the alumina sample using TEM analysis can be found in arevious study [76]. In this study, the TEM and EDX analyses con-rmed the presence of metallic Rh, oxide species RhiOx and metallict as shown in Fig. 3a–e. The presence of these species was observed

RhPt/CeO2–ZrO2 5–15 5–10 n.d.RhPt/SiO2 10–20 5–15 20RhPt/TiO2 5–15 5–15 20

on all supports. Also, the sizes of the Rh and Pt particles were simi-lar as seen in the figures and in Table 2. In general, the particle sizeand distribution of the smallest and single Rh and Pt crystals wasapproximately 5–20 nm and 5–15 nm, respectively. Furthermore,Rh and Pt particles were found both separately and in RhPt-alloys,as seen in Fig. 3a–e. Most of the single Rh particles discerned usingEDX analysis were oxide species e.g. Rh2O3, while metallic Rh par-ticles were mostly found in an RhPt-alloy. The particle size of thesmallest RhPt-alloy crystals observed was approximately 20 nm(see Table 2). Similar surface states, particle sizes and alloy for-mation of the Rh and Pt particles were noted for RhPt/Al2O3, asreported in a previous study [76].

In this study, for RhPt/CeO2–ZrO2, Rh species were found onceria and zirconia, while Pt was mainly found on the ceria layer pos-sibly in the form of Pt–O–Ce bonds. This Pt-bond type is consideredfavorable for high-temperature catalytic applications [77,78]. Fur-thermore, it is suggested that this particular Pt-bond type stabilizesPt by preventing sintering and promoting redispersion of agglom-erated Pt particles under an oxidative atmosphere, as shown byHatanaka et al. [77]. Also, low weight loadings of Pt are consideredto favor the formation of Pt–O–Ce bonds and thus the Pt dispersion[78]. In this study, according to the XRD analysis, the Pt dispersionon CeO2–ZrO2 was superior compared to the other supports (seeSection 3.1.2 and Fig. 1b). A possible explanation is that small Ptparticles are finely dispersed over the support. However, from theTEM analysis on RhPt/CeO2–ZrO2, it was difficult to see and con-firm whether the Pt particles were the smallest and most dispersedof all samples. Also, the occurrence of possible RhPt alloys on thesupport was difficult to verify for RhPt/CeO2–ZrO2. This was dueto multiple layers of ceria and zirconia crystals on top of the Rhand Pt particles making it challenging to scrutinize and measurethe particle size and distribution of the noble metals. In general,the size and distribution of the smallest and most visible Rh and Ptparticles observed were in the vicinity of ∼5–15 nm and 5–10 nm,respectively (see also Table 2).

The geometry and orientation of the Rh and Pt crystals onCeO2–ZrO2 were also difficult to determine using TEM due toextensive overlapping of ceria and zirconia crystals on the noblemetals. Most of the Rh species appeared rounded while differentshapes and sub layers of crystals where detected for the Pt par-ticles. Regarding Rh geometries, other groups have reported thatRh, present in oxidative environment e.g. in the form of Rh2O3,has a typical orthorhombic structure at T ∼500–900 ◦C [79]. Inthis study, possible orthorhombic structures of Rh2O3 crystals canbe seen on titania in Fig. 3d–e. Regarding the Rh and Pt ori-entation, typically (1 1 1) and (1 1 0) are reported for Rh and Ptcrystals, respectively, formed at T > 500 ◦C [80,81]. However, in thisstudy it was difficult to conclude the Rh and Pt orientation for allsupports.

On the subject of the support material, both cubic andtetragonal-shaped crystals were noted for ceria and zirconia, bothwith crystal sizes in the range of 10–20 nm, see Fig. 3a and b. For

silica, no crystal formations were noted for the support as silica isamorphous, depicted in transparent gray colour in the TEM imageshown in Fig. 3c. For titania, mostly tetragonal crystals ascribed toanatase, ∼ 30 nm in size, were present, as shown in Fig. 3d. Also,

X. Karatzas et al. / Applied Catalysis B: Environmental 106 (2011) 476– 487 483

Fig. 3. TEM images of dispersed Rh and Pt particles found in fresh powder samples of (a and b) RhPt/CeO2–ZrO2, (c) RhPt/SiO2 and (d and e) RhPt/TiO2.

484 X. Karatzas et al. / Applied Catalysis B: Environmental 106 (2011) 476– 487

Table 3H2 chemisorption results of fresh 1 wt% Rh supported on alumina, ceria–zirconia,silica and titania. The pre-reduction temperature was set at 800 ◦C and the finalhydrogen chemisorption analysis was carried out at 40 ◦C.

Sample H/Rh (%) dp(Rh) (nm)

Rh/Al2O3 47 2.4Rh/CeO2–ZrO2 54 2.2Rh/SiO2 n.a. n.a.

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Table 4Activity results of RhPt catalysts. The reaction condition for ATR of diesel wasTfeed = 650 ◦C, H2O/C ∼2.5, O2/C ∼0.49, TOS = 3 h, GHSV ∼17,700 h−1 and P = 1 atm.See Table 1 for catalyst formulations.

Sample Xdiesela (%) SCO2

b (%) H2 (vol.%) C2H4 (ppm)

RhPt/Al2O3 93.5 56 24 3700RhPt–La2O3/Al2O3 94.0 56 36 1700RhPt/CeO2–ZrO2 97.6 54 40 1000RhPt–MgO–Y2O3/CeO2–ZrO2 96.0 56 38 1800RhPt/SiO2 84.9 56 17 2400RhPt–CeO2–ZrO2/SiO2 82.1 58 21 1400RhPt/TiO2 92.3 53 29 2100RhPt–MgO/TiO2 85.4 49 29 2500

a Diesel conversion, calculated using FTIR analyses. Further details can be found

Rh/TiO2 8.3 13

o extensive Rh encapsulation tendencies, due to possible SMSIffects, were noted on the TEM images for the calcined RhPt/TiO2ample. In this study, possible SMSI effects on Rh particles onhe reducible supports were tested using high temperature H2-hemisoprtion analysis. The result and discussion of this analysisan be found in the following section.

.1.5. H2 chemisorptionIn general, H2 chemisorption analysis is not performed for

imetallic samples, e.g. RhPt, since it is difficult to interpret from theesults whether hydrogen adsorbs solely on Rh, Pt or on RhPt alloyarticles. In this study, a hydrogen chemisorption analysis was con-ucted for monometallic 1 wt% Rh samples in order to determinehe dispersion and crystallite sizes of Rh as well as to verify theccurrence of possible SMSI effects.

The hydrogen chemisorption results are presented in Table 3.s seen in the table, the highest Rh dispersion, ∼54%, was noted forh/CeO2–ZrO2 followed by the Rh/Al2O3 with a final value of ∼47%.he Rh crystallite sizes for both samples were comparable, in theicinity of ∼2 nm. For Rh/CeO2, similar chemisorption results haveeen reported by others [39,59].

In this study, Rh/SiO2 and Rh/TiO2 were also tested for H2-hemisorption at a pre-reduction temperature set at 800 ◦C.owever the results were, to some extent, inconclusive. For Rh/SiO2o accurate reading was manageable despite multiple trials ofhemisorption analyses. The failure to obtain correct measure-ents was due to minor pressure drop errors that occurred during

he analyses. For Rh/TiO2, the Rh dispersion was low, around 8%.owever, these results may be misleading as SMSI effects may haveffected the hydrogen uptake of Rh, for both samples, negatively.or instance, for Rh/TiO2, at these high reduction temperatures Rhurface sites on titania can be physically blocked by moieties ema-ating from the support material [62]. In general, typically suboxideroups of titania can migrate over Rh metal sites on the surfacef the catalyst [63,64]. Hence, this obstruction, aka Rh encapsula-ion, can suppress the hydrogen uptake of Rh. Furthermore, in thistudy, the inferior thermal stability of the silica and titania sup-orts at high temperatures may also have played a major role. For

nstance, for Rh/TiO2, a possible phase transition from anatase-to-utile could have taken place. This was to some extent noted bybserving the colour of the support as it changed from a white to aeep blue colour after the analysis was completed. Other groupsave made similar colour phase-switch observations for titania62].

Hence, to summarize, the hydrogen chemisorption resultshowed that the Rh dispersion of Rh/CeO2–ZrO2 was high ∼54%,nd moreover slightly higher than for Rh/Al2O3. These results sug-est that the Rh dispersion is well-maintained on ceria–zirconiand on alumina at high reduction temperatures. In contrast, SMSI

ffects and phase transitions may have affected the hydrogenptake for Rh/SiO2 and Rh/TiO2 negatively. It is possible that thesewo phenomena may also affect the catalyst activity negatively,s well. The results obtained from reforming experiments are dis-ussed in detail in the following section.

in previous work [15].b CO2 selectivity, expressed as (CO2/(CO2 + CO)), calculated using FTIR and NDIR

analyses.

3.2. Catalyst activity

In this study, the reaction temperature (measured at the outletgas stream of the monoliths) was equivalent to the feed tem-perature, T ∼650 ◦C, for all employed catalysts. Furthermore, thereaction temperatures were stable and no deviations were notedduring the 3 h time on stream. Hence, no significant temperatureeffects were observed for all employed catalysts that may explainthe differences noted in the activity calculations and measuredproduct gas concentrations shown in Table 4.

The activity results presented in Table 4 establish that lowweight loadings of rhodium–platinum formulations are highlyactive for ATR of diesel. The most promising diesel reforming cata-lyst was the unpromoted RhPt ceria–zirconia supported catalyst.As seen in the table, for RhPt/CeO2–ZrO2, the diesel conversionwas nearly complete, ∼98%, the H2 concentration was the highest∼40 vol.% and finally the ethylene concentration was the lowest,∼1000 ppm, out of all employed catalysts at the tested reac-tion conditions. The catalyst activity of the unpromoted samples,in terms of diesel conversion, increased in the following order:RhPt/SiO2 < RhPt/TiO2 < RhPt/Al2O3 < RhPt/CeO2–ZrO2. Again thistrend in activity confirms the tendencies noticed in the previouspapers; the reforming activity can be correlated with the reducibil-ity of RhiOx on the supports rather than the surface area [15]. In thisstudy, the highest RhiOx reducibility was noted for RhPt/CeO2–ZrO2(Fig. 2a) while the highest surface area was noted for RhPt/SiO2(Table 1). Another interesting trend is that RhPt/Al2O3 exhibitedthe highest ethylene formation, ∼3700 ppm. This indicates that ahigher degree of carbon deposition takes place on the active sites ofthe catalyst. The latter was confirmed by O2-TPO analysis discussedin Section 3.3.1.

The effect of the promoters on the catalyst activity can alsobe seen in Table 4. In general, the addition of promoters had aninsignificant or even negative impact on the catalyst performanceexcept for the promoted alumina sample. As seen in the table, theaddition of 10 wt% La2O3 on RhPt/Al2O3 was found to enhance thediesel conversion and hydrogen production, as well as lower theethylene concentration. For instance, the hydrogen concentrationwas increased by 50%, from 24 vol.% to 36 vol.%. Also, the ethyleneformation was reduced by more than half, to 1700 ppm. Again, theimproved activity was believed to be the result of higher reducibil-ity of RhiOx species at T ∼250 ◦C, as shown in the TPR-profile inFig. 2b. Other groups have reported similar results [14,82]. RhiOx

species as active Rh phases for diesel reforming are further dis-cussed in Section 3.4.

The promoter effects on the other supports were insignificantwith one minor exception; the addition of CeO2–ZrO2 on SiO2 wasfound to reduce the diesel conversion from 85 to 82%, while thehydrogen concentration was slightly increased by ∼4 vol.% and

s B: Environmental 106 (2011) 476– 487 485

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X. Karatzas et al. / Applied Catalysi

he ethylene formation was reduced to nearly half, ∼1400 ppm.gain, these results confirm previous observations, meaning that

he lower diesel conversion noted is most likely the result of lowerccessibility of RhiOx species on the support as seen in the TPRrofile in Fig. 2d. The same negative diesel conversion and H2-PR trends can also be seen for the titania-promoted sample (seeable 4 and Fig. 2e). Finally, for RhPtCeO2–ZrO2/SiO2 the improvedydrogen generation and ethylene reduction noted are most likelyhe result of the ceria contribution and promotion. This promotionccurs through ceria’s well-known and excellent WGS activity andoke removal ability [83–85].

.3. Catalyst characterization, aged powder samples

.3.1. Temperature-programmed oxidationIn diesel reforming, ethylene is a well-known coke precursor

hich can often lead to deactivation of the reforming catalysts dueo coke deposition on the active metal sites and on the support85–87]. In general, the higher the ethylene concentration in theroduct gas streams the greater the risk of getting severe carboneposits on the employed catalyst [88]. The extent of coke deposi-ion can be determined by performing an O2-TPO analysis. DuringPO, various carbon deposits are oxidized in specific temperaturentervals forming mainly CO and CO2 and measured by TCD. In gen-ral, the intensity of the TCD signals is an indicator of the amountf coke present on the samples.

In Fig. 4a, the TPO profiles of a fresh and an aged RhPt/Al2O3re depicted. As seen in the figure, significant differences may beoted, where several distinct and large peaks are present for theged sample in the temperature interval 40–1000 ◦C. This indicateshat coke species are present and being oxidized to the form CO andO2 during TPO.

The TPO peaks, a total of four, of the aged samples have beenoted and identified in previous work [15]. In general, the ini-ial small peak detected at ∼100–200 ◦C is ascribed to possibleoke deposits on the surface active metals, the second peak foundn the second interval between 250 ◦C and 400 ◦C may be due tooke deposits on bulk metallic centers, while the third peak in theicinity of ∼500–600 ◦C may be due to coke species on the sup-ort material [89,90]. Concerning the final peak at ∼1000 ◦C, it cane ascribed to slow oxidation of unconverted graphite that is stillresent on the samples [90–92].

In Fig. 4b, the TPO profiles of all aged unpro-oted RhPt samples are shown. As seen in the figure,

he peak intensities increased in the following order:hPt/SiO2 < RhPt/CeO2–ZrO2 < RhPt/TiO2 < RhPt/Al2O3. This indi-ates that larger amounts of coke deposits are present on aluminand titania than on ceria–zirconia and silica. Similar coke depositrends have been reported by Mizuno et al. [93]. For ceria–zirconia,n particular ceria’s excellent oxygen storage capacity and promo-ion of WGS activity are known to assist in oxidizing and removingarbon species from the metal surface area of the catalyst duringeforming [84,85]. Silica, however, is an inert and stable materialnown to exhibit high tolerance towards poisoning effects of e.g.ulfur [31].

In Fig. 4c and d, the TPO profiles of the aged promoted sam-les RhPt–La/Al2O3 and RhPt–MgO–Y2O3/CeO2–ZrO2 are shown.s seen in the figures, the addition of La as a promoter had a sig-ificant positive effect on reducing the amount of coke species.he latter positive effect was noted for both the bulk active met-ls in the temperature interval 250–350 ◦C and for the support at00–600 ◦C. Hence, La not only improves the catalyst activity, it

lso prevents extensive coke deposits, most likely prolonging theatalyst lifetime. For RhPt/CeO2–ZrO2, the addition of MgO–Y2O3ad the opposite effect resulting in more coke deposits, in par-icular on the active metals as seen in the graph at T ∼100 ◦C. In

moted RhPt catalysts, (c) alumina samples and (d) ceria–zirconia samples. In Fig. 3a,the fresh RhPt/Al2O3 sample was pre-reduced in H2 from room temperature to1000 ◦C, 10 ◦C/min, dwelling at the maximum temperature for 2 h, prior to theTPO analysis. The oxygen consumption is displayed as function of temperature. SeeTable 1 for catalyst formulation.

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86 X. Karatzas et al. / Applied Catalysi

eneral, MgO is known to have very low tolerance towards cokeeposits as shown by Mitzuno et al. [93]. For silica and titania sam-les, RhPt–CeO2–ZrO2/SiO2 and RhPt–MgO/TiO2, respectively, theesulting TPO graphs (not shown) were similar to the unpromotednes depicted in Fig. 4b. Hence, it was difficult to conclude whetherhe promoters had an overall positive or negative effect on reducingoke formation for silica and titania.

.4. Final discussion and remark: RhiOx as potential active phaseor ATR of diesel

Limited studies can be found in the open literature in whichctive Rh phases have been thoroughly investigated and discussedor diesel reforming [14,15]. In general, more research concerningotential active phases of Rh can be found for Rh catalysts employedor methane reforming [94–97]. In these studies, some researchersrgue that the metallic sites of Rh are the active phases for methaneeforming [94,95], while others favor the oxidized rhodium e.g.h2O3 [96]. In addition, intermediate phases between RhiOx and Rh,uch as partially oxidized Rh+ species are also proposed as activehases of rhodium [97].

In this study, it is evident that the reforming activity can-ot be linked to the surface area, as, for instance, the lowestiesel conversion, ∼82%, was noted for the sample, in this casehPt–CeO2–ZrO2/SiO2, with the highest surface area, ∼185 m2/g.urthermore, the Rh particle sizes measured through TEM analysis,oth in the present study and in previous work, showed that theh sizes were similar on all supports, in the vicinity of ∼5–20 nmsee Table 2). Hence, it was difficult to conclude if the Rh size affectshe diesel conversion. Also, Rh encapsulation tendencies were notbserved on any sample using TEM analysis. However, it should bemphasized that the TEM analyses were performed on fresh cal-ined samples. In ATR, both oxidative and reductive environmentsre present during reforming of HC fuels as shown in previous work15] and by others [1,18,24–29]. Therefore, the physicochemicalroperties of both the oxidizing and reducing capabilities of Rhn the selected supports should be considered and investigated. Inhis study, high temperature H2 chemisorption analysis was carriedut for 1 wt% Rh samples, at a pre-reduction temperature set at T800 ◦C. The chemisorption results showed the occurrence of SMSIffects taking place on Rh/SiO2 and Rh/TiO2; in particular, for theitania sample. Interestingly, this effect was not noted for Rh/Al2O3nd Rh/CeO2–ZrO2. Hence, the diesel reforming activity trendRhPt/SiO2 < RhPt/TiO2 < RhPt/Al2O3 < RhPt/CeO2–ZrO2) noted forhe unpromoted alumina and ceria–zirconia samples may be dueo the absence of SMSI effects on Rh. However, extensive researchnd discussion on potential SMSI effects are beyond the scope ofhis study and should be addressed in future studies. Instead, weropose that the reducibility of RhiOx species plays a major role inhe catalyst activity during diesel reforming.

In this study, a correlation between the diesel conversionnd the accessibility of reducible RhiOx species can be made. Aseen in the activity measurements for the unpromoted samplesTable 4), the highest diesel conversion, ∼98%, was noted for theample RhPt/CeO2–ZrO2, with highest hydrogen uptake measuredor RhiOx species at T ∼140 ◦C, as seen in Fig. 2a. Similar ten-encies were noted for the promoted samples. For instance, foramples RhPt–CeO2–ZrO2/SiO2 and RhPt–MgO/TiO2 the additionf the promoters on the supports was found to lower the hydro-en consumption of the RhiOx species, as shown in Fig. 2d–e, ands a result the diesel conversions were reduced, as seen in Table 4.ence, there is a strong possibility that the degree of accessible

hiOx species on the support can be correlated with the dieselonversion.

The higher reducibility of RhiOx noted for RhPt/CeO2–ZrO2 atow reduction temperatures, may be the result of a promoting

vironmental 106 (2011) 476– 487

effect caused by additional supply of oxygen originating from thesupport. Results from other studies have shown that during calcina-tion, Rh can be sturdily incorporated into the lattice of CeO2 in theCeO2–ZrO2 support [68,98,99]. Consequently, a strong interactionbetween Rh and CeO2 takes place and as a result the oxygen migra-tion from ceria to rhodium is facilitated [68,98,99]. Furthermore, inthis study, it is possible that weaker interactions between Rh andthe employed supports may also favor the formation of RhiOx andthus have a positive effect on the diesel conversion. Similar obser-vations and conclusions were made by Ferrandon et al. [69,70] forRh catalysts used for butane and gasoline reforming. Also, a similarobservation was made by Kim et al. [100] for Pd diesel-reformingcatalysts. In the study, the authors emphasize the importance ofgenerating a large amount of easily accessible PdO sites in order toenhance the activity of the Pd catalysts. Regarding active Pt phasesfor ATR of diesel, in this study, PtO sites were not present on theemployed catalysts, as shown in the characterization results. Onlymetallic Pt sites were observed, both separately and in RhPt alloyformation. Thus, the reducibility of PtO sites may play a minor rolewhen interpreting the activity results. One Pt trend was noted inprevious work in which Pt in an unpromoted RhPt–alumina sam-ple was found to lower the accessibility of RhiOx and thus the dieselconversion [19]. Also, in another study, possible Pt–Ce effects werefound favorable for diesel reforming [15]. However, despite thesetwo noted Pt trends, the role of Pt for diesel reforming is not fullyunderstood and thus more research is needed to fully clarify thepotential activity role of Pt.

4. Conclusions

The results presented in this study provide a deeper understand-ing regarding possible synergistic effects between the selectedsupports and RhPt that take place during ATR of diesel. The effects ofvarious promoters were also examined. The selected supports wereboth irreducible, Al2O3 and SiO2, and reducible such as CeO2–ZrO2and TiO2. The tested promoters were MgO, Y2O3, La2O3, CeO2 andZrO2.

To summarize, the most promising reforming catalyst for ATRof diesel was RhPt/CeO2–ZrO2. The diesel conversion was nearlycomplete, ∼98%, the H2 concentration was the highest, ∼40 vol.%,and finally the ethylene concentration was the lowest, ∼1000 ppm,out of all employed catalysts at the tested reaction conditions. Thehigher activity results noted for RhPt/CeO2–ZrO2 may be ascribedto the higher reducibility of RhiOx species as well as the superiorRh and Pt dispersion. The latter physicochemical properties wereconfirmed by TEM, H2 chemisorption, XRD, and H2-TPR analysesperformed on fresh powder samples of the catalyst. Finally, thesupport contribution, in particular ceria, is believed to promotehydrogen generation, through water gas-shift reactions, as well asreduce coke deposits on the catalyst surface. The latter was con-firmed by O2-TPO analysis performed on aged powder samples inwhich lower loads of carbon deposits were found to be presenton RhPt supported on ceria–zirconia than on RhPt supported onalumina.

Acknowledgements

The Foundation for Strategic Environmental Research (MIS-TRA) and the Swedish Energy Agency (Energimyndigheten) aregratefully acknowledged for financial support. The WallenbergFoundation is also acknowledged for financial support in the new

electron microscopy facilities at MMK, SU. Thanks also to CorningInc. for supplying cordierite substrates, to Sasol Germany GmbHfor providing the alumina and to MEL Chemicals for supplying theceria–zirconia.

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