Fuel 87 (2008) 2502–2511

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Hydrogen production for fuel cell by oxidative reforming of diesel surrogate:Influence of ceria and/or lanthana over the activity of Pt/Al2O3 catalysts

M.C. Alvarez-Galvan a,*, R.M. Navarro a, F. Rosa b, Y. Briceño c, M.A. Ridao d, J.L.G. Fierro a

a Instituto de Catálisis y Petroleoquímica (CSIC), c/ Marie Curie 2, Cantoblanco 28049, Madrid, Spainb Centro de Experimentación de ‘‘El Arenosillo” (CEDEA), Instituto Nacional de Técnica Aeroespacial (INTA), 21130 Mazagón-Moguer, Huelva, Spainc CIDAUT, Parque Tecnológico Boecillo, Valladolid, Spaind Escuela Técnica Superior de Ingenieros, Universidad de Sevilla, Sevilla, Spain

a r t i c l e i n f o

Article history:Received 25 September 2007Received in revised form 25 February 2008Accepted 4 March 2008Available online 3 April 2008

Keywords:HydrogenOxidative reformingDiesel surrogatePlatinum catalystsCeria and lanthana

0016-2361/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.fuel.2008.03.003

* Corresponding author. Fax: +34 91 5854760.E-mail address: c.alvarez@icp.csic.es (M.C. Alvarez

a b s t r a c t

A series of Pt catalysts supported on Al2O3 (Pt/A), Al2O3–CeO2 (Pt/A–C), Al2O3–La2O3 (Pt/A–L) and Al2O3–La2O3–CeO2 (Pt/A–L–C) have been prepared and tested in the oxidative reforming of diesel surrogate withthe aim of studying the influence of ceria and lanthana additives over the activity and stability towardhydrogen production for fuel cell application. Several characterization techniques, such as adsorption–desorption of N2, X-ray diffraction, X-ray photoelectron spectroscopy, temperature programmed reduc-tion, H2 chemisorption, and thermogravimetric analysis, have been used to define textural, structural,and surface properties of catalysts and to establish relationships with their behaviour in reaction. Thisphysicochemical characterization has shown that lanthana inhibits the formation of a phase in aluminasupport and decreases ceria dispersion. Activity results show a better performance of ceria-loaded cata-lysts, being the Pt/A–C sample the system that offers higher H2 yields after 8 h of reaction. The greater H2

production for ceria-loaded catalysts, particularly in the case of the system Pt/A–C, is attributed to the Pt–Ce interaction that may change the electronic properties and/or the dispersion of active metal phase.Also, the CeIII form of CeIV/CeIII redox pair enhances the adsorption of oxygen and water molecules, thusincreasing the catalytic activity and also decreasing coke deposition over surface active Pt phases. Stabil-ity tests showed that catalysts in which Pt crystallites are deposited on the alumina substrate covered bya lanthana monolayer, give rise to an increase in stability toward H2 production.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Hydrogen is considered as the most recognized clean energycarrier for the future. Its use in fuel cells fulfils the requirementsof being an efficient and environmentally-friendly source of power.Since the distribution and on-board storage of hydrogen presenttechnical problems, related to its very low volumetric energy den-sity, attention of many research groups has been focused on theconversion of more readily available and high energy density fuelsto hydrogen using on-board systems [1].

Hydrogen can be obtained directly from hydrocarbons by twomain processes: partial oxidation (Eq. (1)) and steam reforming(Eq. (2)). While partial oxidation offers exothermicity and a rapidresponse, steam reforming is endothermic and produces higherconcentrations of hydrogen that results in higher system efficien-cies. A third option combines the advantages of both approachesby co-feeding oxygen, steam and hydrocarbons simultaneouslythrough oxidative reforming process. The oxidative reforming

ll rights reserved.

-Galvan).

was chosen because this option appears most promising for con-verting complex fuels like diesel within the size and weight con-straints of many on-board reformer applications [2]

CnHm þ nH2O! nCOþ ðm=2þ nÞH2 DH > 0 ð1ÞCnHm þ n=2O2 ! nCOþm=2H2 DH < 0 ð2Þ

The requirements for the reforming catalysts are to present highyields of hydrogen and high selectivity for converting the carbonin the fuel to carbon oxides with minimum yields of residual hydro-carbons, as well as to be sulphur-tolerant and resistant to coke for-mation [3]. Catalysts formulations for heavy hydrocarbon reformingtypically comprise metals such as Ni or PGM (platinum group met-als) supported into carefully engineered oxide supports. AlthoughNi supported catalysts have shown excellent activity and selectivitytoward hydrogen with heavy hydrocarbons [4], platinum-based cat-alysts present better catalytic behaviour in the oxidative reformingof sulphur-containing hydrocarbon mixtures, as indicated by ourprevious studies. Textural modifiers added to supports can help tomaintain high metal and support surface areas at the high reform-ing temperatures, while structural promoters can be also added todecrease carbon formation, to increase sulphur tolerance or to

M.C. Alvarez-Galvan et al. / Fuel 87 (2008) 2502–2511 2503

improve noble metal dispersion [5]. Rare earth elements have beenextensively used as promoters in catalysts used in the reforming ofheavy hydrocarbons [6,7] and can play an important role in noblemetal dispersion [8]. Among them, ceria has been selected as astructural promoter due to its oxygen storage action that improves:(i) the reversibility of oxidation/reduction reactions of noble metalswith changes in redox potential and, (ii) the oxidation of cokedeposits over active phases [9,10]. Ceria can also create oxygenvacancies [11] that favour the adsorption of steam and oxygen inthe catalyst surface. The knowledge on the interaction between cer-ia and precious metals will be useful in the design of efficient cata-lysts for the reforming of hydrocarbons [12]. Trovarelli havesummarized a considerable amount of data that demonstrate thestrong interaction between precious metals with ceria at atomic le-vel [13]. Some authors propose that ceria may change the propertiesof the Group VIII metals through electronic interactions [14,15] andprevents the sintering of the metals, stabilizing their dispersed state[16]. Imamura et al. [17] have pointed that CeO2 transfers oxygen toprecious metal and then the oxygen is transferred to the reactants.Thus, the action of CeO2 strongly affects the state of precious metaland, also, may act as oxygen carrier [17]. The improvement in theadsorption and dissociation of steam in catalyst surfaces dopedwith ceria [18] will increase coke gasification.

On the other hand, as commented above, the high temperaturesrequired to reform heavy fuels makes necessary to introduce tex-tural promoters to decrease the loss of surface area in catalysts.Lanthana appeared to be one of the best additives for inhibitingthe sintering of high surface area Al2O3 [19]. The main role of Lais to stabilize Al2O3, decreasing the rate of surface diffusion thathinders sintering and loss of surface area [20]. Some authors[19,21] pointed out that La is inserted into the crystal lattice of alu-mina with a spinel structure while other thermal stabilizer, such asCe remains on the surface in the form of CeO2. This characteristicdifference explains why the modification by La was more effectivefor improving thermal stability of alumina.

The present work aimed to investigate the role of La2O3 and/orCeO2 additives in modifying the textural, structural and surfaceproperties of alumina supported platinum and how these proper-ties influence on the catalytic behaviour for hydrogen productionby oxidative reforming of diesel surrogate. Interaction betweenplatinum and ceria and its influence in catalytic activity and cokedeposition has been analyzed. The catalysts were characterizedby means of several techniques such as temperature-programmedreduction (TPR), X-ray diffraction, X-ray photoelectron spectros-copy (XPS) and thermogravimetric analysis (TGA). The optimizedcatalyst formulation has been successfully used in the scale-up ofa catalytic system included in the reformer unit of a diesel fuel pro-cessor coupled to a PEM fuel cell.

2. Experimental methods

2.1. Catalysts preparation

The c-Al2O3 support (aluminium oxide, gamma, bimodal,SBET = 237 m2/g, Johnson Matthey) was crushed and sieved to aparticle size between 0.20 and 0.425 mm and stabilized by calcina-tion at 1000 �C for 8 h. The lanthanum promoter was introducedinto alumina by incipient wetness pore volume impregnation withan aqueous solution of lanthanum nitrate (La(NO3)3 � 6H2O, 99.9%Johnson Matthey) [15] in the appropriate concentration to achieve1% mol La2O3. This concentration was selected taking into accountthat this is the optimal amount of La2O3 in alumina to produce thesmallest decrease in surface area by sintering at a calcination tem-perature of 1000 �C [22]. After impregnation, the lanthanum-con-taining support was dried at 110 �C and subsequently calcined at

1000 �C over 8 h. The incorporation of La into the alumina has beenrealized before the addition of CeO2, because this results in a high-er CeO2 dispersion and a greater range of reversible reducibility inthe ceria [23]. Thus, ceria was introduced into stabilized aluminaand lanthana-supported alumina by wetness impregnation in a ro-tary evaporator at 80 �C for 2 h, with the appropriate amount of anaqueous solution of cerium nitrate (Ce(NO3)3 � 6H2O, 99.9%, John-son Matthey) such as to obtain solids containing 10% wt. CeO2.After impregnation, the supports were dried at 110 �C for 8 h andthen calcined at 500 �C for 4 h. The four stabilized supports: alu-mina (A), alumina–ceria (A–C), alumina–lanthana (A–L) and alu-mina–lanthana–ceria (A–L–C) were subsequently used to preparethe platinum catalysts. For this purpose, a proper amount ofH2PtCl6 (41.29% Johnson Matthey) was used to obtain the sup-ported catalysts by wet impregnation. Platinum (0.75% wt.) wasintroduced using an aqueous solution (0.01 g/ml), in a rotary evap-orator at 80 �C during 2 h. All samples were dried at 110 �C for 4 hand calcined at 500 �C for 3 h.

2.2. Catalysts characterization

2.2.1. Atomic emission spectroscopyThe chemical composition of calcined catalysts was measured

by Inductively Coupled Plasma Atomic Emission Spectroscopy(ICP-AES) on a Perkin-Elmer optima 3300 DV device. Samples werefirst dissolved in acidic solutions (mixture of HF, HCl, HNO3, andH3PO4), micro-waved for 40 min and diluted to concentrationswithin the calibration range of the instrument.

2.2.2. X-ray diffractionX-ray diffraction patterns were recorded from calcined and re-

duced catalysts using a Seifert 3000P vertical diffractometer andnickel-filtered Cu Ka radiation (k = 0.1538 nm) under constantinstrument parameters. For each sample, Bragg angles between5� and 80� were scanned; a rate of 5 s per step (step size: 0.04�,2 � h) was used during a continuous scan in the above mentionedrange. Peak positions and widths were resolved by profile fitting.

2.2.3. Nitrogen adsorption–desorptionThe BET surface area and pore volume of prepared supports and

catalysts were calculated from the nitrogen adsorption–desorptionisotherms. These isotherms were determined at the liquid nitrogentemperature (�196 �C) using a Micromeritics ASAP 2100 automaticequipment. Prior to the measurements, samples were degassed un-der a residual vacuum of 2 � 10�5 mbar at 200 �C for 24 h to re-move the physically adsorbed moisture from the catalyst surface.

2.2.4. X-ray photoelectron spectroscopySurface analysis of samples was done by using a VG ESCALAB

200R electron spectrometer equipped with a hemisphericalelectron analyser and an Al Ka (h � m = 1486.6 eV, 1 eV = 1.6302 �10�19 J) 120 W X-ray source. The X-ray source was operated atlow X-ray fluxes to minimize X-ray induced reduction of Pt andCe species. The powder samples were pressed in 8 mm diameterstainless steel troughs mounted on a sample rod placed in thepre-treatment chamber and reduced in situ by hydrogen at350 �C prior to being transferred to the analysis chamber. The basepressure in the analysis chamber was maintained below4 � 10�9 mbar during data acquisition. The pass energy of the ana-lyser was set at 50 eV, for which the resolution as measured by thefull-width at half-maximum (FWHM) of the Au4f7/2core level was1.7 eV. The binding energies were referenced to the Al 2p peak ofthe alumina support at 74.5 eV. Data processing was performedwith the XPS peak program, the spectra were decomposed withthe least squares fitting routine provided with the software withGaussian/Lorentzian (90/10) product function and after subtract-

2504 M.C. Alvarez-Galvan et al. / Fuel 87 (2008) 2502–2511

ing a Shirley background, Atomic fractions were calculated usingpeak areas normalized on the basis of sensitivity factors providedby the manufacturer.

2.2.5. Temperature-programmed reductionTemperature-programmed reduction experiments were carried

out on a Micromeritics TPD/TPR apparatus. The catalyst (about30 mg) was pretreated under helium at 110 �C for 15 min. TheTPR profile was recorded by heating the sample from room tem-perature to 1000 �C at a rate of 10 �C/min under a H2/Ar (10% v/v) flow.

2.2.6. Hydrogen chemisorptionMetallic dispersion was evaluated by H2-pulse chemisorption in

a Micromeritics TPD/TPR 2900 instrument. Platinum surface areahas been determined by hydrogen chemisorption following amethodology previously described [24] to determine the specificarea of Rh and/or Pt supported on doped-alumina. Firstly, the sam-ple, ca. 0.5 g was degassed for 15 min at 100 �C, and then reducedin H2 flow (50 ml/min 10% H2/Ar) at 350 �C during 1 h. Afterwards,the hydrogen retained by catalyst was removed by purging the cellunder inert flow (Ar) during 15 min and then the sample wascooled down under Ar flow (50 ml/min) to �80 �C to minimizethe contribution of hydrogen spillover. Gas mixture of 10% H2/Arwas used as adsorbate. Pulses (0.1 ml each) of the H2/Ar mixturewere injected at 1 min intervals until no further gas uptake bythe catalyst was observed as indicated by constant peak areas ofthe last injections. The total amount of adsorbed gas was calculatedby adding the gas uptake observed in the series of gas injectionsuntil saturation was reached. In order to calculate the metal dis-persion an adsorption stoichiometry of Pt/H = 1 was assumed.

2.2.7. Thermogravimetric analysisThe amount of coke deposited on the catalysts was determined

by thermogravimetric analysis (TGA/SDTA 851e METTLER TOLEDO)measuring the weight change of the coked catalysts brought aboutduring oxidation at high temperature. The analysis was carried outby raising sample temperature from 25 �C to 1100 �C at a rate of5 �C min�1 under a 20% (vol) O2/N2 flow.

2.2.8. Transmission electron microscopyTEM images were recorded with a JEOL microscope mod. JEM-

4000 EX, working with an acceleration voltage of 400 kV. Speci-mens for TEM were prepared by ultrasonically dispersing somepowder sample in acetone and placing a droplet of the suspensionon a copper grid covered with a carbon film.

2.3. Activity tests

Model feed was blended from commercial n-hexadecane (n-C16H34 99% Merck) to surrogate paraffins, decaline (C10H18 for syn-thesis, >99%, Merck) to surrogate cycloparaffins, tetraline (C10H12

for synthesis >96%, Merck) to simulate aromatics in proportionssimilar to that in the diesel fraction (39, 45 and 16 %wt., respec-tively) and dibenzothiophene (50 ppm in S) as substitutive of sul-phur compounds. The oxidative reforming of the above model feedwas performed in an isothermal tubular fix bed (stainless steel)reactor. Activity tests were carried out using 100 mg of catalyst(sieved fraction, 200–450 lm) diluted with inert particles of SiCat a volume ratio (SiC/catalyst) of 6.5 in order to limit the thermalgradient.

High-pressure liquid chromatographic (HPLC) pumps were usedto inject fuel and water into the vaporizer, prior to being intro-duced into the reactor tube. A nitrogen stream (2 mlN/min) wasfed into the vaporizer (350 �C) to obtain a steady hydrocarbon/steam mixture. The steam/hydrocarbon mixture supply tube ended

directly in front of the catalyst bed to prevent homogeneous oxida-tion reactions. All gas flows were controlled by mass flow meters.Before activity measurement, catalysts were activated in hydrogenflow (10% H2/N2) during 2 h at 350 �C. Catalytic activities weremeasured at 800 �C, atmospheric pressure, gas hourly space veloc-ity (GHSV) of 20,000 h�1 and constant molar feed ratios of O2/C = 0.5 (stoichiometric) and H2O/C = 3 (higher than the stoichiom-etric value, equal to 1) to facilitate the gasification of carbon depos-ited on catalysts surfaces.

The product gas was analyzed every 45 min by means of an on-line gas chromatograph (HP 5890 Series II) equipped with a ther-mal conductivity detector (TCD) and programmed to operate underhigh-sensitivity conditions. The concentrations of the followinggases were determined: H2, O2, N2, CO, CH4, CO2, C2H4, C2H6, andH2O. Water and liquid hydrocarbons were separated from theproduct gas in a condenser.

Previous activity tests have been performed to analyze theinfluence of reaction temperature, gas hourly space velocity, H2O/C, and O2/C ratios over hydrogen production. The influence of gashourly space velocity has been analyzed at GHSV of 20,000,40,000 and 80,000 h�1 (800 �C, H2O/C = 3 and O2/C = 0.5). The influ-ence of reaction temperature has been studied at 750, 800 and850 �C (20,000 h�1, H2O/C = 3 and O2/C = 0.5) the effect of O2/C Cby performing tests at molar ratios of 0.2, 0.5 and 0.7 (H2O/C = 3,800 �C and 20,000 h�1) and the effect of H2O/C with tests at molarratios of 1, 3 and 5 (O2/C = 0.5, 800 �C and 20,000 h�1).

Conversion (%) is defined as

X ¼ hydrocarbonðinletÞ� � hydrocarbonðoutletÞ�

hydrocarbonðinletÞ�� 100

in which hydrocarbon* is expressed as molar flows. Yield towardhydrogen is expressed as hydrogen volume flow (mlNH2/min) oras the total mole amount of hydrogen produced by carbon atomin the feed, taking into account both the H2 production from oxida-tive reforming and the additional hydrogen generated in the subse-quent water–gas shift stage:

YðH2 þ COÞ ¼ Total mole amount of ðH2 þ COÞ ðoutletÞTotal mole amount of CðinletÞ

3. Results and discussion

3.1. Catalytic performance

The influence of space velocity, reaction temperature, H2O/Cand O2/C molar ratios over hydrogen production has been analyzedin a previous study (Fig. 1) with Pt/A–C catalyst. The results indi-cate that an increase in space velocity provokes a decrease in con-version and hydrogen yield which is attributed to diffusionlimitations, as it was concluded from an extensive kinetic studyperformed by our group. To avoid this, the space velocity used inactivity tests has been 20,000 h�1. No significant changes in prod-uct distribution have been observed for different space velocity asfound in literature for autothermal reforming of synthetic diesel[25]. An increase in reaction temperature leads to an increase inactivity and hydrogen production, being found higher values fora reaction temperature of 850 �C. However, the chosen reactiontemperature was 800 �C to keep activity at moderate levels, whichallow discriminating the activity of different catalysts. The lowesttemperature (750 �C) was rejected since the rate of coke formationis rather high, as we observed in our experiments and also found inliterature [3]. The effect of different O2/C molar ratios over hydro-gen production is also observed in Fig. 1. An increase in this ratiofrom 0.2 to 0.5 leads to a great increase in hydrogen flow, howeverfurther rising up to 0.8 results in a lower hydrogen selectivity and a

H2O/C=3 800ºC 20000 h-1

0

5

10

15

20

mlN

H2/m

in

O2/C=0.2O2/C=0.5O2/C=0.8

H2O/C=3 O2/C=0.5 20000 h-1

0

10

20

30

40

0 100 200 300 400 500 600Time on stream (min)

mlN

H2/m

in

750ºC800º C850º C

O2/C=0.5 800ºC 20000 h-1

0

10

20

30

0 100 200 300 400 500Time on stream (min)

mlN

H2/m

in

H2O/C=5H2O/C=3H2O/C=1

H2O/C=3 O2/C=0.5 800ºC

0

10

20

30

40

0 100 200 300 400 500Time on stream (min)

0 100 200 300 400 500Time on stream (min)

mlN

H2/m

in20000 h-1

40000 h-1

80000 h-1

O2 /C=0.2O2 /C=0.5O2 /C=0.8

1

750ºC800º C850º C

H2O/C=5H2O/C=3H2O/C=1

Fig. 1. Influence of gas hourly space velocity (GSHV), reaction temperature, O2/C and H2O/C ratios over hydrogen production (hydrogen volume flow, mlNH2/min) byoxidative reforming of synthetic diesel fuel over ceria doped Pt catalyst (Pt/A–C).

0

20

40

60

80

100

0 100 200 300 400 500 600

Time (min)

Con

vers

ion

(%)

Pt/A

Pt/A-L

Pt/A-C

Pt/A-L-C

Fig. 2. Catalytic activity (% conversion) for the oxidative reforming of syntheticdiesel fuel over Pt catalysts (T = 1073 K, P = 0.1 MPa, O2/C = 0.5, H2O/C = 3,GHSV = 20,000 h�1).

M.C. Alvarez-Galvan et al. / Fuel 87 (2008) 2502–2511 2505

parallel increase in selectivity to combustion. The influence of H2O/C has been studied performing activity tests with 1, 3, and 5 ratios.An increase in this ratio, in relation with the stoichiometric one isrecommended for heavy hydrocarbon reforming to facilitate thegasification of carbon, as previously commented in Section 2.3.An increase from 1 to 3 increases hydrogen flow by a factor oftwo, however, a H2O/C = 5 does not seem to modify substantiallyhydrogen production. Therefore, taking into account the steamproduction cost, the value used in activity tests was 3. An increasein H2O/C ratio rises the formation of synthesis gas by hydrocarbonreforming and slightly enhances the development of water–gasshift reaction, which is observed by an increase in CO2 and H2 con-centrations and decrease in CO, as it has been reported in the liter-ature for hexadecane oxidative reforming [26].

The oxidative reforming activities (conversion %) with time ofreaction of Pt/A, Pt/A–C, Pt/A–L and Pt/A–L–C catalysts are shownin Fig. 2. As observed, Pt/A and Pt/A–L systems had the highest ini-tial activity for oxidative reforming of diesel surrogate mixture at800 �C, with hydrocarbon conversion approaching to 90%. How-ever, the activity of these catalysts declined with reaction time toaround 70% conversion after 8 h of reaction. Initial activity of theother two catalysts, Pt/A–C and Pt/A–L–C was lower, around 70%,but is kept fairly stable indicating that introduction of CeO2 has apositive effect. The best result is obtained with the catalyst pro-moted only with ceria (Pt/A–C) with which the activity reachesup to 90% after 8 h under reaction. Hydrogen production for thissystem was around 1.6–1.7 mol/mol C (Fig. 3). The highest concen-tration of hydrogen in the reformate is also obtained by this systemas shown in Fig. 4. Rostrup–Nielsen [27] and Wang and Gorte [28]have proposed a mechanism to explain hydrocarbon steam reform-ing and the role of promoters, such as ceria, enhancing adsorptionand dissociation of steam. Moreover, ceria also promotes theadsorption and activation of oxygen acting as an oxygen supplier

for the reaction, thus contributing to a better catalytic performance[29]. The effect of both lanthana and ceria as promoters in pre-pared systems will be discussed in next paragraphs.

3.2. Effect of lanthanum addition

As commented above, the thermal stability of alumina supportcan be improved with the addition of lanthanum modifier. Theaddition of lanthanum raises the alumina phase change tempera-ture by reducing the number of surface sites where nucleation ispossible [22]. As a consequence, the sintering of alumina is re-duced, hindering the formation of a phase, and hence decreasingthe loss in surface area during thermal stabilization. This role of

0

0.5

1

1.5

2

Pt/A Pt/A-L Pt/A-C Pt/A-L-C

Y(H

2+C

O),

mol

/mol

C

Fig. 3. Hydrogen yield (including the potential obtained from water–gas shift ofCO) during the oxidative reforming of synthetic diesel over Pt catalysts (T = 800 �C,P = 0.1 MPa, O2/C = 0.5, H2O/C = 3, GHSV = 20,000 h�1).

0

5

10

15

20

25

30

35

H2

Con

cent

ratio

n, %

Pt/A

Pt/A-L

Pt/A-C

Pt/A-L-C

CO2CO

Fig. 4. Product distribution (gas phase, mole % dry basis) during the oxidativereforming of synthetic diesel over Pt catalysts (T = 800 �C, P = 0.1 MPa, O2/C = 0.5,H2O/C = 3, GHSV = 20,000 h�1).

Table 1Chemical composition (% wt) and BET surface area of calcined catalysts and Ptdispersion by H2 chemisorption in reduced samples

Sample % Pt % CeO2 % La2O3 BET surface area m2/g Pt dispersion (%)

Pt/A 0.7 – – 83 22Pt/A-L 0.8 – 2.7 108 13Pt/A–C 0.7 9.6 – 77 80Pt/A–L–C 0.8 9.9 3.1 101 55

0 200 400 600 800 1000

Pt/A-L

Pt/A-C

Pt/A

H2

cons

umpt

ion

(a.u

.)

Temperature (ºC)

Pt/A-L-C

Fig. 5. Temperature-programmed reduction profile for calcined Pt catalysts supportedMagnification: reduction peak related to platinum in close contact to ceria.

2506 M.C. Alvarez-Galvan et al. / Fuel 87 (2008) 2502–2511

lanthanum, producing a thermal stabilization of alumina, is evi-denced by XRD and texture results. As it is shown in Table 1, in-creases of 28% and 34% in BET surface areas are obtained for Pt/A–L and Pt/A–L–C, respectively, in relation to their Pt/A and Pt/A–C counterparts. As it was commented in a previous work [30],X-ray diffraction patterns revealed that pure alumina sample con-sisted of a and h phases, while the sample doped with 1.0 mol%La2O3 consisted only of h phase. However, as said in the previousparagraph, the observed increase in surface area is not accompa-nied by a direct activity increase toward hydrogen production.

3.3. Effect of cerium addition

CeO2 acts enhancing adsorption and activation of both water(steam) and oxygen and therefore, catalytic performance. Sincethese molecules can migrate via metal-ceria interface, the greaterdevelopment of metal-ceria interaction the greater catalytic activ-ity [31]. The enhanced adsorption of steam and oxygen reagents onceria could facilitate the removal of coke deposits [32]. In addition,ceria can increase metal dispersion and change the electronic prop-erties of active metals by electronic interactions. Thus, an attemptwill be made to analyze the effects of CeO2 in prepared platinumsystems and the influence over the performance as promoter inthe oxidative reforming of a diesel surrogate.

3.3.1. Platinum–ceria interactionThe promoter oxide (La2O3, CeO2) incorporated to the support

substrate tends to react under a reducing environment with thefirst atomic layers of alumina forming surface species. The reduced

Pt/A-L-C

Pt/A-C

Pt-Ce

Pt-Ce

100

100

300

300 200

200

on Al2O3 (Pt/A), La/Al2O3 (Pt/A–L), Ce/Al2O3 (Pt/A–C) and Ce-La/Al2O3 (Pt/A–L–C).

Table 2Binding energies of Ce 3d core electrons, surface atomic ratio for ceria promotedcatalysts and % u000 in Ce 3d XPS spectra for ceria promoted catalysts

Samples XPS

Calcined Reduced

Ce000 in Ce3d B.E. Ce3d5/2 (eV)

Ce/Al Ce000 in Ce3d B.E. Ce3d5/2 (eV)

Ce/Al

Pt/A–C 12.2 882.6 0.050 9.5 881.8 0.053Pt/A–L–C 12.1 882.5 0.036 11.8 882.0 0.046

M.C. Alvarez-Galvan et al. / Fuel 87 (2008) 2502–2511 2507

catalyst may then contain species in which zero-valent atoms ofactive noble metal may interact with the substrate-promoter sur-face species generated. This is particularly important in the Ce-pro-moted catalysts in which low-valent CeIII ions are formed underreducing conditions. The variation in the electronic properties ofmetal clusters resulting from the interaction with CeIII may inducechanges in catalytic activity [33,34].

Fig. 5 shows TPR profiles of Pt catalysts, whose description wasadvanced in detail in a previous work [30]. The reduction profile inthe 100–260 �C temperature interval includes two components:the first one, with maximum around 170 �C corresponds to thereduction of bulk oxychlorinated species and the second one,around 210 �C is related to Pt-induced surface reduction of ceria.This second contribution is greater for Pt/A–C catalyst, appearingonly as a small shoulder for Pt/A–L–C catalyst, which indicates agreater number of Pt–Ce interactive species for Pt/A–C catalyst.

The interaction between Pt clusters and reduced cerium oxidespecies has been also studied by XPS for Pt/A–C and Pt/A–L–C cat-alysts in reduced state. The Ce 3d spectra of the cerium-containingcatalysts consisted of several peaks resulting from either multipletsplitting or shake-up processes. The isolated peak in the Ce 3dspectra at ca. 917 eV (u000 using the nomenclature of Burroughs[35]) was used as identification of Ce4+ species and its relative peakarea contribution to total Ce 3d area as a quantitative measure ofthe amount of CeIV [36]. The fraction of the total Ce signal attribut-able to the u000 peak in both samples (Table 2) is lower than the13.4% reported for pure fully oxidized ceria implying the presenceof CeIII species. Taking into account that hydrogen is first dissoci-ated on Pt0 clusters and then spills over the ceria surface changingthe oxidation state of Ce atoms from CeIV to CeIII [37], the smallerproportion of CeIV in Pt/A–C system should indicate a greater inter-action between Pt and Ce in this catalyst compared to Pt/A–L–C. Onthe other hand, a greater dispersion of ceria on A–C support shouldincrease the interaction Pt–Ce, thus enhancing the activity of Pt/A–C system toward hydrogen production. Assuming the hypothesisthat ceria is equally dispersed in both A–C and A–L–C supports,surface Ce/Al ratio should be higher in A–L–C because surface alu-mina is partly covered by La. However, XPS results indicate thatsurface Ce/Al ratio is higher for A–C (0.053) as compared to A–L–C (0.046), which indicates a higher dispersion of ceria in A–C sup-port. This could be explained by a decrease of anchoring points ofCe over alumina–lanthana support as compared to bare alumina,during the preparation of A–L–C support, due to the presence ofsurface La ions. Thus, the higher ceria dispersion and its greaterinteraction with Pt contribute to the better performance of Pt/A–C system.

3.3.2. Platinum dispersionMetallic surface area determination is of great importance to

evaluate and compare catalytic performance of noble metal cata-lysts. Accordingly, metallic surface areas have been determinedto check whether the introduction of lanthanum and/or ceria inthe support increases metal dispersion.

It was claimed in several works [38–40] that the presence of Lamodifies the morphological features of the alumina support, pro-

viding an increased number of nucleation sites for metals duringthe first steps of deposition leading to an enhancement of disper-sion of the metallic phase. On the other hand, it is documented thatthe use of ceria increases the dispersion of a noble metal on c-Al2O3

[40] and prevents the agglomeration of dispersed Pt into discreteparticles. This is attributed to a Pt–CeO2 interaction which helpsto maintain Pt in an oxidized state thus hindering the sinteringof metallic platinum particles [36]. On the contrary, Vazquezet al. [41] compared platinum dispersion in Al2O3-, CeO2–Al2O3-and La2O3–Al2O3-supported platinum catalysts and found thatthe non-doped system presented higher dispersion. Chemisorptionresults of reduced catalysts (Table 1) show a decrease in Pt disper-sion for La-loaded alumina and La–Ce-loaded alumina in relation totheir counterparts without lanthana, in spite their higher surfacearea, which is attributed to a decrease in anchoring points of plat-inum anions when lanthana particles are over the alumina surface,in accordance with results obtained by Vazquez et al. [41].

The increase in active metal dispersion for ceria-containing sup-ports is explained in relation to the sharing of oxygen bonds [42]. Itis known that zero-valent metals are bound to the surface of oxidesupports primarily by van der Waals forces. Therefore, the activa-tion energy of migration of the metal atoms over the surface ofsuch supports is low. The low valent ions bound to the support sur-face may serve as an energetic wall for the migrating atoms or par-ticles of the metals. The formation of bonds between the atoms ofthe supported metal (Pt) and the surface atoms of the modifyingelement (Ce) may prevent the migration of the supported particlesof platinum [33]. Thus, the presence of ceria stabilizes Pt dispersionby a Pt–CeO2 interaction, most probably through the formation of asurface complex involving Pt2+–O2�–Ce4+ linkages [36]. Therefore,although ceria incorporation in both alumina and La-alumina-based catalysts produces a slight decrease in surface area, the greatincrease achieved in platinum dispersion is attributed to Pt–ceriainteraction that prevents the agglomeration of noble metal parti-cles. Taking into account the greater tendency of Pt to interact withCeO2 compared to alumina, the higher ceria dispersion in A–C sup-port compared to A–L–C one, could be responsible for the higher Ptdispersion in Pt/A–C system in relation to Pt/A–L–C counterpart.

3.3.3. Coke depositionCoke deposition on catalysts surfaces is a complex process in

which several reactions operate simultaneously:

CxHy¡Cx þ y=2H2

2CO¡Cþ CO2

ðCH2Þ0¡CþH2

The capability of ceria to inhibit coking processes by oxidation andto enhance steam reforming of hydrocarbons is rather known anddocumented [43,44]. The presence of Ce in the lower valence state(CeIII) may help the dissociative adsorption of water as O� or OH�

species which can then be transferred to carbon precursors, beinggasified to CO, CO2 and H2O [13]. Thus, ceria catalytically removesthe adsorbed hydrocarbon intermediate species [31].

Thermogravimetric analysis of the spent catalysts after 8 h ofreaction was carried out to estimate the amount of coke depositedas revealed by the weight loss due to the oxidation of coke in air. Asit is shown in Fig. 6, coke deposited on catalyst surfaces increasedin the order: Pt/A–C� Pt/A–L–C < Pt/A-L � Pt/A, being consistentlyhigher for the Ce-free catalyst. The greater Pt–Ce interaction in Pt/A–C catalyst may be associated to the greater resistance towardcoke deposition shown by this catalyst that presents the lowestamount of coke. Comparing Figs. 2 and 6, the deactivation of Ce-free catalysts could be related to the formation of greater amountsof carbon on the surface of metal particles. Although coke is formedon the surface of Pt particles for all the samples, the ability of ceria-

40

60

80

100

0 200 400 600 800 1000 1200

Temperature (º C)

Wei

ght l

oss

(%)

Pt/APt/A-LPt/A-CPt/A-L-C

Fig. 6. TGA profiles corresponding to the temperature programmed oxidation of Ptcatalysts used in the oxidative reforming of diesel surrogates.

Table 3Binding energies of Ce 3d core electrons and percentage of CeO2 in fresh reduced andaged ceria promoted catalysts (30 h of reaction)

Catalyst % u000 % CeO2

Fresh reduced Aged Fresh reduced Aged

Pt/A–C 9.5 0 65 0Pt/A–L–C 11.8 9.9 82 68

2508 M.C. Alvarez-Galvan et al. / Fuel 87 (2008) 2502–2511

doped systems to gasify carbon is higher, due the oxygen transferability, favouring the removal of carbon from metal surface [45].TPR results (Fig. 5) indicated that CeO2 near the metal particlesin Pt/A–C and in Pt/A–L–C catalysts is partially reduced takingplace an increase of the number of oxygen vacancies. As a conse-quence, the extent of adsorption of oxygen and/or water is en-hanced and then the resulting surface species react with carbonresidues to form carbon oxides and water [46].

3.4. Long-term stability tests

The stability of Ce-containing catalysts (Pt/A–C and Pt/A–L–C)under above conditions were also determined. Fig. 7 shows thehydrogen yield for a 30 h long-term test. It is clear from data inabove figure that the yield tends to decrease more slowly for Pt/A–L–C sample. The observed catalytic behaviour may be relatedwith some loss of the action of ceria as promoter under reactionconditions. The surface percentage of CeO2 for both Pt/A–C andPt/A–L–C systems has been calculated for both fresh reduced andaged states of the samples (Table 3). As it is shown in Table 3,CeO2species in aged Pt/A–C system completely disappear. How-ever, this value only decreases about a 17% for Pt/A–L–C. CeO2 ina reductive atmosphere can be reduced to Ce2O3 which further re-acts at the alumina interface to form CeAlO3. As the CeAlO3 phase israther stable, the redox ability of the catalyst becomes inhibited

0

0.5

1

1.5

2

0 5 10 15 20 25 30 35Time on stream (h)

Y(C

O+

H2)

, mol

/mol

C

Pt/A-L-C

Pt/A-C

Fig. 7. Stability tests (hydrogen yield) for the oxidative reforming of synthetic di-esel fuel on Pt/A–C and Pt/A–L–C catalysts (T = 800 �C, P = 0.1 MPa, O2/C = 0.5, H2O/C = 3, GHSV = 20,000 h�1).

once the CeAlO3 mixed oxide is developed. As suggested by Hum-bert et al. [47], the nucleation of CeAlO3 occurs by diffusion of Al3+

in the partially reduced CeO2 lattice, which is favoured by thesmaller ionic radii of Al3+ compared to Ce3+ ions. Moreover, thegeometrical arrangement between the three structures of CeO2,CeAlO3 and Al2O3 favours the formation of cerium aluminate inthe interface. Taking into account that reaction temperature is800 �C and temperatures above 550 �C favour the formation ofCeAlO3 is very likely that the action of ceria as promoter is dimin-ished by its loss via solid-state reaction with the few surface andsubsurface layers of the alumina substrate. In the A–L–C substrate,lanthana covers a fraction of the alumina surface with which it canreact to form a mixed oxide, i.e. LaAlO3 species, thus weakening theinteraction of Ce3+ ions with the alumina substrate. Taking into ac-count that the presence of lanthana in the catalyst surface de-creases the alumina–ceria interface, the better performance of Pt/A–L–C system for longer reaction time could be influenced byreducing the contact between ceria and alumina in this system.

To confirm this fact, a new catalyst formulation has been pre-pared increasing the La2O3 in a proportion close to that requiredto form a theoretical monolayer of La2O3 over the alumina surface[39,48] (12.8 lmol La atoms/m2, 15.3% La2O3), and with the samePt and Ce amount as Pt/A–L–C system. This new catalyst, with aBET surface area of 136 m2/g, has been called Pt/A–L–C (mono-layer). Stability test over this catalyst (Fig. 8) clearly indicate thatthe introduction of a monolayer of La on the alumina substrategreatly enhances catalyst stability. Product distribution shows veryhigh and stable yields toward hydrogen and conversion keeps closeto 100% during all the reaction. Only a small amount of methane(lower than 2% mol) is formed after 50 hours of reaction. Although,unfortunately, XPS analysis is not clear enough to allow the deter-mination of % CeO2 in aged Pt/A–L–C(monolayer) catalyst, takinginto that lanthana monolayer is physically layed out between bothceria and alumina, the interaction between both phases should bediminished, hindering the formation of CeAlO3. Additionally, thegreater proportion of ceria in this catalyst, with a proved coke gas-ification action, contributes to the observed performance of Pt/A–L–C(monolayer) catalyst for long reaction times.

The introduction of a monolayer of La on the alumina also influ-ences the dispersion of the metallic Pt phase. As it can be observedin Fig. 9, that show TEM micrographs of Pt/A–L–C and Pt/A–L–C(monolayer), mean particle size of platinum in Pt/A–L–C(mono-layer) is smaller, being obtained also a narrower distribution.TEM analysis of these catalysts showed that the sizes of the sup-ported Pt nanoparticles were significantly smaller for the systemwith a La monolayer. Thus, while more than 80% of Pt nanoparti-cles in Pt/A–L–C(monolayer) catalyst were under 2 nm, only 30%were observed under this value for Pt/A–L–C. The decrease of Ptparticle size may contribute to the high activity and stability ob-served for the Pt/A–L–C(monolayer) catalysts, once avoided theceria over reduction under reaction conditions, responsible of thedeactivation behaviour observed for the Pt/A–L–C sample.

Sulphur tolerance has been analyzed for the Pt/A–L–C(mono-layer) catalyst, performing activity tests with different concentra-tions of sulphur in the feed (0, 50 and 100 ppm S). Animprovement in sulphur tolerance has been observed when com-pared with a catalyst without promoters (Pt/A) (Fig. 10). High

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Pro

duct

dis

trib

utio

n

CH4

CO2

CO

H2

0 10 20 30 40 50 60 ConversionY(H2+CO)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Pro

duct

dis

trib

utio

n

CH4

CO2

COH2

0 10 20 30 40 50 60

0 10 20 30 40 50 60

Time on steam (h)

Time on steam (h)

0

20

40

60

80

100

Con

vers

ion

(%)

2.0

1.6

1.2

0.8

0.4

0.0

Y(H

2 +C

O), m

ol/mol C

Fig. 8. Stability test (conversion and hydrogen yield) for the oxidative reforming of synthetic diesel fuel over Pt/A–L–C (La monolayer) catalyst (T = 800 �C, P = 0.1 MPa, O2/C = 0.5, H2O/C = 3, GHSV = 20,000 h�1).

Fig. 9. Transmission electron micrographs of Pt/A–L–C and Pt/A–L–C(monolayer) fresh catalysts.

M.C. Alvarez-Galvan et al. / Fuel 87 (2008) 2502–2511 2509

and stable yields toward hydrogen production are observed duringcontinuous 30–35 h of reaction with a feed containing 50 ppm S.Even a feed with 100 ppm S decreases hydrogen yield lesser than5% with respect to a feed with 50 ppm S.

The good performance of the optimized catalyst formulation(Pt/A–L–C (monolayer)) has been corroborated with its use in a5 kW diesel fuel prototype to perform long time on stream tests.In this unit, the catalyst showed high hydrogen production for

0 5 10 15 20 25 30 350

1

2

3

4

Y(C

O+

H2),

mol

/mol

C

Time on stream (h)

Pt/A-L-C(monolayer) 0 ppm S Pt/A-L-C(monolayer) 50 ppm S Pt/A-L-C(monolayer) 100 ppm S Pt/A 0 ppm S Pt/A 50 ppm S

Fig. 10. Influence of sulphur concentration in hydrogen yield by oxidative reform-ing of synthetic diesel over Pt/A and Pt/A–L–C(monolayer) catalysts (T = 800 �C,P = 0.1 MPa, O2/C = 0.5, H2O/C = 3, GHSV = 20,000 h�1).

2510 M.C. Alvarez-Galvan et al. / Fuel 87 (2008) 2502–2511

more than 200 h time on stream with a deactivation degree lesserthan 5%.

4. Conclusions

Ceria and lanthana promoters had a significant influence overtextural, surface and structural properties of alumina-supportedPt catalysts and thus over their performance toward hydrogen pro-duction by oxidative reforming of diesel surrogate. This generalconclusion can be split in the following specific ones:

� The presence of lanthana in catalyst formulation inhibits the for-mation of the a-phase of the Al2O3 substrate by reducing its sin-tering, as demonstrated by textural properties derived from thenitrogen adsorption–desorption isotherms.

� Surface Ce/Al atomic ratios derived from quantitative XPS mea-surements for the two Ce-promoted catalysts indicate that add-ing La2O3 to the alumina surface leads to a drop of ceriadispersion, probably by decreasing the anchoring points of ceria.

� XPS and TPR results analysis indicate the development of Pt–Ceinteraction in ceria-promoted systems, this being greater in Pt/A–C catalyst. This fact could be responsible of the better perfor-mance of this catalyst by modifying the electronic properties ofPt. Moreover, ceria promoter enhances oxygen and wateradsorption. Since these molecules can migrate via metal–ceriainterface, the greater development of Pt–Ce interaction in Pt/A–C catalyst could also explain its higher activity and the smal-ler coke deposition on its surface as shown by XPS, TGA and ele-mental chemical analysis.

� Ceria promoter leads to an increase in Pt dispersion as it hasbeen obtained by H2 chemisorption, being higher for Pt/A–C cat-alyst. This could be due to interaction between Pt clusters andceria interface that could prevent the migration and sinteringof metal particles.

� Stability tests performed with the most active catalysts (Pt/A–Cand Pt/A–L–C) reveal a better performance of Pt/A–L–C whencompared to its La-free Pt/A–C counterpart. Surface analysis byXPS indicates a greater loss of ceria promoter by interaction withalumina support in the La-free catalyst. To avoid this phenome-non, a new catalyst containing a La monolayer was prepared andtested in the reaction, being stability markedly improved. Thesmaller size of Pt particles found in this catalyst should be alsocontributing to improve its catalytic performance.

� The catalyst containing a La monolayer has demonstrated agreat stability for activity tests with sulphur-containing syn-thetic diesel (50 ppm S).

Acknowledgements

This work was carried out in the frame of the Project ‘‘Develop-ment of a Diesel fuel processor” carried out by the Instituto de Téc-nica Aeroespacial (INTA), Centro de Investigación y Desarrollo enAutomoción (CIDAUT), the Instituto de Catálisis y Petroleoquímica(CSIC) and the Asociación de Investigación y Cooperación Internac-ional de Andalucía (AICIA). M.C.A.G. and R.M.N. gratefully acknowl-edge financial support from MCYT in the Ramon y Cajal researchprogramme. Thanks are due to Dr. M. Peña for TG measurements.

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