Noc

I

a

b

a

ARRAA

KNDNCC

1

(Cahfa(ea

cfii

h0

Applied Catalysis A: General 500 (2015) 12–22

Contents lists available at ScienceDirect

Applied Catalysis A: General

jou rn al hom epage: www.elsev ier .com/ locate /apcata

i/CeO2–Al2O3 catalysts for the dry reforming of methane: The effectf CeAlO3 content and nickel crystallite size on catalytic activity andoke resistance

gor Luisettoa,∗, Simonetta Tutia,b, Chiara Battocchioa,b, Sergio Lo Mastroa, Armida Sodoa

Department of Sciences, University of Rome “Roma Tre”, Via della Vasca Navale 79, 00146 Rome, ItalyC.I.S.Di.C Center, University of Rome “Roma Tre”, Via della Vasca Navale 79, 00146 Rome, Italy

r t i c l e i n f o

rticle history:eceived 29 January 2015eceived in revised form 22 April 2015ccepted 1 May 2015vailable online 11 May 2015

eywords:

a b s t r a c t

The catalytic performances of Ni/CeO2–Al2O3 catalysts for the dry reforming of CH4 (DRM) were inves-tigated. Catalysts with different Ni dispersion and different amount of CeAlO3 species were prepared bydifferent methods and characterized by BET, XRD, XPS, Raman, TPR and TPO techniques. Catalytic activitywas studied during time on stream in the range 873–1073 K with a mixture of CH4:CO2:Ar = 40:40:20 vol.%and GHSV 90,000 cm3 g−1 h−1. The intrinsic catalytic activity increased with the increasing of Ni crystallitesize. Carbon was deposited as nano-fibres and graphite when catalysts worked at lower temperature, and

i/CeO2–Al2O3

ry reformingickel particle sizeeAlO3

arbon deposition

the largest amount was found on the catalyst with the largest Ni crystallite size. The formation of graphiticdeposits is limited by the presence of CeAlO3 species formed during catalyst activation. CA preparationmethod results particularly attractive because it allows to obtain catalysts with small Ni crystallite sizeand high content of CeAlO3 species, which both have a role in suppressing the carbon deposition andtherefore in obtaining stable catalytic performances.

© 2015 Elsevier B.V. All rights reserved.

. Introduction

The CO2 reforming of CH4 (Eq. (1)), also called dry reformingDRM), has been recently recognized as an efficient way for theH4 and CO2 valorization [1–4]. In fact, the produced syn-gas hasn H2/CO ratio equal to one suitable for the synthesis of oxygenatedydrocarbons and synthetic fuels. The feedstock that can be used

or the DRM, ranges from CO2-rich natural gas reserves to renew-ble biogas produced by anaerobic fermentation of waste sludgemainly composed by CH4 and CO2), offering thus the possibility tonlarge their utilization and to avoid the release of the CO2 in thetmosphere.

CH4 + CO2 → 2H2 + 2CO �H0298 K = 247 kJ mol−1 (1)

The DRM plays also an important role on determine the electro-hemical performances and the long-term stability of solid oxide

uel cells (SOFCs) fed by CH4 or biogas. In particular, the DRM occursnternally to the cell stack producing H2 used to feed the anode andt helpfully limits the carbon deposition [5–7].

∗ Corresponding author. Tel.: +39 0657333370; fax: +39 0657333390.E-mail address: igor.luisetto@uniroma3.it (I. Luisetto).

ttp://dx.doi.org/10.1016/j.apcata.2015.05.004926-860X/© 2015 Elsevier B.V. All rights reserved.

Due to its high endothermicity the DRM has also been proposedfor the energy storage and the energy transfer, for example in theconversion of the solar energy to chemical energy, which is referredto as solar reforming [8–10].

Regarding the application of the DRM for syn-gas production, todate its industrial implementation is impeded mainly by the fol-lowing issues: (i) the co-occurrence of the reverse water gas shiftreaction (RWGS) (Eq. (2)) that, consuming H2, lowers the H2/COratio;

H2 + CO2 → CO + H2O �H0298 K = 41 kJ mol−1 (2)

(ii) the catalyst deactivation and/or the reactor plugging due tocarbon deposits formed by the methane cracking (Eq. (3)) and theBoudouard reaction (Eq. (4)).

CH4 → C + 2H2 �H0298 K = 75 kJ mol−1 (3)

2CO → C + CO2 �H0298 K = −172 kJ mol−1 (4)

The DRM is operated at high temperatures (generally above

923 K) to achieve suitable CH4 and CO2 conversions. In that con-dition, the Boudouard reaction is thermodynamically unfavoured,the RWGS is suppressed by the low CO2 concentration, whereas, onthe contrary, the methane cracking is favoured [11].

lysis A

wmcsttcrsba

adSNcaspwiStDaL

ttottofioosCsagtCst

owproosbpaast

2

A

I. Luisetto et al. / Applied Cata

Therefore, the development of catalysts, selective for the DRM,ith high resistance towards carbon deposition and proper ther-al stability, is pivotal for the DRM implementation and is still a

hallenge. Catalysts containing noble metal such as Pt, Ru and Rhhow high activity and selectivity for the DRM reaction in additiono good stability towards the coke deposition [12–14]. Howeverheir high cost and low availability make them not economicallyompetitive in comparison with other transition metal based mate-ials. Among non-precious transition metals, Ni-based catalysts areo far the most active, but also highly prone to carbon formation,ecause, together with the ability to activate the C H bond, Ni has

high affinity to carbon.In the search of active and stable Ni-based catalysts, several

pproaches have been explored in order to suppress the carboneposition. The addition of a second metal, such as Co, Cu andn, results in the formation of less C-sensitive alloys [15–17]. Thei particle size has a strong effect on the carbon tolerance of theatalyst, and generally, particles smaller than 5 nm have low cat-lytic activity towards the C H cracking [11,18,19]. Therefore, thetabilization of small Ni nanoparticles at high temperatures is aromising way for the lifetime increase [20,21]. However, togetherith the properties of the active metal sites, the support choos-

ng is of great importance in designing high-performance catalysts.upports with basic character, such as MgO [22], and those con-aining lanthanum [23], showed limited carbon deposition duringRM because of their small number of Lewis acid sites, whichre involved in carbon formation, and because the presence ofa2O2CO3 that helps the gasification of carbonaceous deposits.

Among different supports, those containing cerium seem to behe most promising in limiting the deactivation by coking andherefore they are extensively investigated. CeO2 is known for itsxygen storing capacity due to the redox couple Ce4+/Ce3+. Owingo this property, during dry reforming the oxygen vacancies overhe CeO2-surface may adsorb the oxygen formed by the dissociationf CO2 on Ni sites, improving the reforming activity and the gasi-cation of coke [24]. However, the positive effect of CeO2 supportn carbon removal has been reported to be particularly dependentn the catalyst preparation method which may affect Ni disper-ion and metal support interaction [24,25]. The addition of ZrO2 toeO2 yields solid solution with high oxygen mobility and thermaltability [26], therefore the Ce(1−x)Zr(x)O2 family has been applieds support for robust DRM catalysts. Several authors have investi-ated the effect of the Ce/Zr ratio on carbon resistance. However,he results are not conclusive [27,28]. Moreover, as for the pureeO2 support, the catalytic performance of the transition metalsupported on Ce(1−x)Zr(x)O2 are strictly dependant on the prepara-ion methods [29].

CeO2 is used also as promoter of �-Al2O3 based catalysts becausef the combination of the large surface area and stability of �-Al2O3ith the oxygen storage and release capability of CeO2 [30,31]. Theromotion of the catalytic performances by CeO2 addition is alsoelated with the ability to increase the Ni dispersion [32,33]. More-ver, in reducing atmosphere at high temperature, CeO2 supportedn �-Al2O3 reacts to form CeAlO3-like species [34]. As reported byeveral authors these species play a key role in the removal of car-on residues [35]. A possible reaction mechanism has been recentlyroposed by Chen et al. [36]: the CeAlO3 formed during catalystctivation or in reaction condition, could react with CO2 to form COnd CeO2 (Eq. (5)). CeO2 oxidizes the CHx species located at the Ni-upport boundary, precursors of carbonaceous residues, restoringhe CeAlO3 sites (Eq. (6)).

CeAlO3 + CO2 → Al2O3 + 2CeO2 + CO (5)

l2O3 + 2CeO2 + CHx → CO + 2CeAlO3 + (x/2)H2 (6)

: General 500 (2015) 12–22 13

Despite recent advances, the role of CeO2–Al2O3 interaction andNi particle size in determining the catalyst performance and stabil-ity remains open.

Therefore, in the present work we studied Ni/CeO2–Al2O3 cat-alysts with different Ni crystallite size and Ce3+/Ce4+ ratio, withthe aim to better understand their interplay with regard to carbonformation and catalytic activity. Catalysts were prepared by co-precipitation, wet impregnation, sol–gel and citric acid methods,and characterized by several techniques. The effect of the prepara-tion method on the solid-state reaction between CeO2 and �-Al2O3to form CeAlO3 was studied by XRD, XPS and H2-TPR. Catalyticactivity was studied in the range 873–1073 K with high flow rateand during time on stream. Carbonaceous deposits were character-ized by XRD, Raman spectroscopy and O2-TPO.

2. Experimental

2.1. Catalyst synthesis

Catalysts with nominal composition of 10 wt.% Ni supported on20 wt.% CeO2 promoted �-Al2O3 have been prepared according tothe following procedures.

2.1.1. Co-precipitation method (CP)Stoichiometric amounts of nitrate salts (Ni(NO3)2 × 6H2O;

Ce(NO3)3 × 6H2O Al(NO3)3 × 9H2O) and ionic surfactantcetyl trimethyl ammonium bromide (CTAB) with molar ratioCTAB/(Ni2+ + Ce3+ + Al3+) = 0.4 were dissolved in water. The tem-perature was raised to 383 K and triethylamine (TEA) was rapidlyadded until pH 10. The precipitation of hydroxide instantaneouslyoccurred and the obtained mixture was aged for 20 h. The precipi-tated solid was filtered and washed with water and ethanol, driedat 393 K overnight and calcined at 873 K for 5 h.

2.1.2. Solution excess wet impregnation method (WI)Aqueous solution of Ni(NO3)2 × 6H2O and Ce(NO3)3 × 6H2O was

added to a commercial mesoporous alumina powder (Alfa Aesar)to form a slurry. The solvent was evaporated at 353 K under vigor-ous stirring. The impregnated alumina was further dried at 393 Kovernight and calcined at 873 K for 5 h.

2.1.3. Sol gel method (SG)Stoichiometric amounts of Ni(NO3)2 × 6H2O, Ce(NO3)3 × 6H2O

and aluminium-tri-sec butoxide were added to ethanol acidifiedwith a small amount of HNO3 (65 wt.%). The solution was mixedunder vigorous stirring at ambient temperature for 5 h, than thesolvent was evaporated at 333 K during 48 h. The dried gel wascalcined at 873 K for 5 h.

2.1.4. Citric acid method (CA)Stoichiometric amounts of nitrate salts (Ni(NO3)2 × 6H2O,

Ce(NO3)3 × 6H2O and Al(NO3)3 × 9H2O) and citric acid mono-hydrate (CA) with molar ratio CA/(Ni2+ + Ce3+ + Al3+) = 1.5 weredissolved in water. Then NH4OH solution (28 wt.%) was added untilpH 8. The solvent was evaporated at 393 K yielding a gel, and thenthe temperature was increased up to about 523 K to ignite the auto-combustion. The obtained dark powder was calcined at 873 K for5 h.

The chemical composition of samples was confirmed by energy

dispersive X-ray analysis (EDAX) using a SEM FEI XL30. Samplepowder was pressed in pellets at about 280 MPa. Several spots withsize of about 10 �m were performed and the results are reportedas average values in Table 1.

14 I. Luisetto et al. / Applied Catalysis A: General 500 (2015) 12–22

Table 1Chemical composition, textural properties (surface area, pore volume, main pore size) and Ni crystallite size of catalysts.

Catalyst Composition (wt.%) Surface area (m2 g−1) Pore volume (cm3 g−1) Pore size (nm) DNi (nm)b

Ni CeO2

CP 9.4 19.1 285 (147)a 1.143 (0.832)a 17.0 (17.0)a 22.6WI 10.0 20.2 200 (167) 0.720 (0.678) 9.8 (10.8) 11.1SG 9.3 22.7 71 (47) 0.159 (0.120) 4.1, 6.9, 11.8 (4.9, 13.5) 9.5CA 10.7 21.4 135 (130) 0.136 (0.128) 4.0 (3.3) 5.8

duced

2

2

ucebuvP6

2

Xt0sC

2

pS(atwkflw

2

leffmHIa

2

acecmXst

a In parenthesis are reported the BET, pore volume and pore size values of the reb Calculated from XRD by Scherrer’s equation of the Ni (2 0 0) reflection.

.2. Catalyst characterization

.2.1. BETN2 adsorption–desorption isotherms were obtained at 77 K

sing a Micromeritics Gemini V apparatus. The surface area wasalculated by the Brunauer–Emmett–Teller (BET) method in thequilibrium pressure interval 0.05 < P/P◦ < 0.5. The pore size distri-ution was obtained from the desorption branch of the isothermsing the Barrett–Joyner–Halenda (BJH) method and the total poreolume was calculated from the maximum adsorption point at/P◦ = 0.99. Prior to the N2 adsorption, the sample was treated at23 K in flowing He in order to remove adsorbed molecules.

.2.2. XRDPowder X-ray diffraction patterns were collected using a Scintag

1 diffractometer equipped with a Cu K� (� = 1.5418 A) source andhe Brag-Brentano �–� configuration in the 10–70 2� range, with.05◦ step size and 3 s acquisition time. The Ni0 and CeO2 crystalliteize were estimated by Scherrer’s equation from the Ni (2 0 0) andeO2 (1 1 1) reflection, respectively.

.2.3. H2-TPRTemperature programmed reduction (TPR) experiments were

erformed by a Thermo Scientific TPDRO1100 flow apparatus.ample (0.050 g) was pre-treated in a flow 5% O2/He mixture20 cm3 min−1) at 573 K for 30 min. The TPR was conducted flowing

5% H2/Ar mixture (10 cm3 min−1) starting at 313 K and heating upo 1273 or 1073 K, with a rate of 10 K min−1. The H2 consumptionas measured by a TCD detector, calibrated by the reduction of a

nown amount of CuO (99.99% purity from Sigma Aldrich). Beforeowing into the TCD detector, the H2O generated in the reductionas removed by a trap.

.2.4. O2-TPOTemperature programmed oxidation (TPO) of the used cata-

ysts were performed in the same apparatus used for the TPRxperiments. Prior to TPO analysis, the used catalyst was care-ully homogenized in an agate mortar. About 0.010 g was rampedrom 313 up to 1073 K, with a rate of 10 K min−1 flowing 5% O2/He

ixture (50 cm3 min−1). Before flowing into the TCD detector, the2O and CO2 generated by the oxidation were removed by a trap.

n identical experimental condition, CO was not observed by GCnalysis of the exhaust stream gas.

.2.5. XPSXPS analysis was performed in an instrument of our own design

nd construction, consisting of a preparation and an analysis UHVhamber, equipped with a 150 mm mean radius hemisphericallectron analyser with a four-elements lens system with a 16-hannel detector giving a total instrumental resolution of 1,0 eV as

easured at the Ag 3d5/2 core level. Al K� non-monochromatised-ray radiation (h� = 1486.6 eV) was used for acquiring core levelpectra of all samples (C1s, Ce3d, Ni2p, Al2p and O1s). The spec-ra were energy referenced to the C1s signal of aliphatic C atoms

catalysts.

having a binding energy BE = 285.00 eV, due to surface contam-ination, as expected for XPS measurements performed on solidsamples exposed to air. Atomic ratios were calculated from peakintensities by using Scofield’s cross-section values and calculated� factors [37]. Curve-fitting analysis of the C1s, Ce3d, Ni2p, Al2pand O1s spectra was performed using Gaussian profiles as fittingfunctions, after subtraction of a Shirley-type background [38].

2.2.6. RamanRaman measurements were performed by using a Labram

Micro-Raman spectrometer by Horiba, equipped with a He–Nelaser sources at 632.8 nm (nominal output power 18 mW). Theillumination and collecting optics of the system consists in a micro-scope in confocal configuration. The system achieves the highcontrast required for the rejection of the elastically scattered com-ponent by an edge filter. The backscattered light is dispersed by a1800 line/mm grating and the Raman signal is detected by a Peltiercooled (203 K) 1024 × 256 pixel CCD detector. Nominal spectral res-olution was about 1 cm−1. Spectral acquisitions (3 accumulations,30 s each, in the range 1000–2800 cm−1) were performed with along distance 20× objective (N.A. = 0.35).

2.3. Catalytic activity

The catalytic activity was measured in a fixed-bed quartz reactorat atmospheric pressure connected to a flow apparatus equippedwith mass flow controllers. The reactor was specially designed toremove the inner part containing the catalyst bed. Sample (0.050 g)was reduced in situ with 50% H2/Ar flow (30 cm3 min−1) increas-ing the reactor temperature from RT up to 1073 K with a rampof 10 K min−1 and isothermally kept at this temperature for 1 h.The dry reforming of methane was studied with a mixture ofCH4:CO2:Ar = 40:40:20 vol.% and flow rate of 75 cm3 min−1 (GHSV90,000 cm3 g−1 h−1). After reduction of the catalyst at 1073 K, thereactor was cooled to 873 K and purged with Ar flow (15 cm3 min−1)for 15 min, then the gas flow was switched to the reactant mixture.The catalytic run was performed in the temperature range from873 to 1073 K with 50 K temperature increments. Each temperaturestep was maintained for 5 h. Reaction stream was analyzed on line,at regular times, by a Agilent 7820 gas chromatograph equippedwith a Molecular Sieve X13 (for the H2, Ar, CO, CH4 separation) aHayesep Q (for CO2 separation) columns and a TCD detector. Aftereach catalytic run at specified temperature, the catalyst was cooledto RT in Ar flow (15 cm3 min−1) and the catalyst bed was weightedin order to verify the formation of massive carbon. Then, the cata-lyst was warmed up to the following temperature step in Ar flow(15 cm3 min−1) thus preventing the oxidation of catalyst surfacedue to the exposure to ambient atmosphere. CH4 and CO2 percentconversions (Xi %) were calculated according to Eq. (7) using Ar as

internal standard.

Xi(%) = 100 ×(

1 − Ci · C0Ar

C0i

· CAr

)(7)

lysis A: General 500 (2015) 12–22 15

w(s

t

S

wctgo

a

N

wt(

[

D

woc

t

r

wasa(

3

3

iasfCtscosistlm

Ftol

I. Luisetto et al. / Applied Cata

here C0i

and C0Ar are the inlet concentrations of the reactants

i = CH4 or CO2) and Ar respectively and Ci and CAr are the corre-ponding outlet concentrations.

The site time yield (STY) of hydrogen was calculated accordingo Eq. (8)

TYH2 = 2 ·F0 · C0

CH4· XCH4

22.414 · NNi(8)

here F0 is the inlet flow of reactants (in L s−1), C0CH4

is the inletoncentration of methane in the reactants mixture, XCH4 is the ini-ial CH4 conversion at 1073 K, 22.414 is the volume of one mole ofas at standard condition (L mol−1) and NNi is the number of molesf the Ni active sites.

The number of moles of the Ni active sites NNi was calculatedccording to Eq. (9)

Ni = g · WNi

MNi· DNi (9)

here g is the mass of catalyst, WNi is the weight fraction of Ni inhe sample as determined by EDAX, MNi is the molar mass of Ni58.71 g mol−1), DNi is the Ni0 dispersion.

The nickel dispersion DNi was estimated by the Vannice method39,40] (Eq. (10)):

M = 6 × 107 VM

AM· 1

d(nm)(10)

here DM is the metal dispersion, VM is the bulk atomic volumef the metal (cm3), AM is the atomic area (cm2), and d is the metalrystallite size (nm) from XRD.

The rate of carbon formation rc (h−1) was calculated accordinghe Eq. (11):

C (h−1) = �m

gcat.h(11)

here �m represents the difference between the mass of the cat-lyst bed at the start and at the end of the time on stream test at apecified temperature, gcat is the mass of the freshly charged cat-lyst (0.050 g) and h is the time in hours of the catalytic step run5 h).

. Results and discussions

.1. Structural and textural characterization

The XRD patterns of samples calcined at 873 K are reportedn Fig. 1A. The CA sample showed a broad and weak XRD peakt about 2� = 33◦ indicative of the amorphous phase. The otheramples showed peaks of fluorite CeO2 (JCPDS 81-0792) with dif-erent crystallinity, beside those of �-Al2O3. In particular the largesteO2 crystallite size of 7.0 nm was observed in WI sample, while inhe other samples, CeO2 was intimately dispersed in the �-Al2O3keleton as smaller and less crystalline particles. Indeed the CeO2rystallite size was 3.5 nm for SG and 4.8 nm for CP samples; more-ver the peaks of CeO2 of the SG sample were much less intenseuggesting that CeO2 was partially amorphous. Peaks correspond-ng to NiO cubic phase (JCPDS 78-0643) were observed only in SGample. The NiO absence in the other specimens was likely due tohe low size of particles or to the formation of NiAl2O4 spinel. Theatter species is hard to distinguish from the �-Al2O3 phase since

ost of their diffraction lines overlap.The XRD patterns of samples reduced at 1073 K are reported in

ig. 1B. All samples showed peaks at about 44.8◦ and 51.8◦ assignedo the (1 0 0) and (2 0 0) reflections of cubic Ni0 (JCPDS 87-0712)riginated from the reduction of NiO and Ni2+-species. The crystal-ite size of Ni0, calculated by Scherrer’s equation, ranges between

Fig. 1. XRD patterns of the catalysts calcined at 873 K (A) and reduced at 1073 K (B):CA (a); SG (b); WI (c); CP (d).

5.8 nm and 22.6, increasing in the sample order CA < SG ≈ WI < CP(Table 1). In CA sample no other peaks were observed, indicatingthat the oxide lattice remained amorphous even after reduction. Inthe other samples �-Al2O3, CeO2 and CeAlO3 phases were detected.In particular, peaks of cubic CeO2 (at 28.5◦; 33.0◦; 47.5◦) wereclearly observed on WI and SG samples and barely detected on CPsample, moreover peaks of CeAlO3 (at 23.6◦; 33.5◦; 41.5◦; 60.0◦)were observed only on SG sample. The Ce2O3, formed by H2 reduc-tion of CeO2 (Eq. (12)), was not observed due to the rapid oxidationof Ce3+ to Ce4+ upon exposure to ambient atmosphere.

The disappearance of CeO2 in CP sample and the presenceof CeAlO3 peaks in SG sample were due to the solid-state reac-tion between Ce2O3 and �-Al2O3 (Eq. (13)), generally observedabove 873 K under reducing condition. However on WI sample the

absence of modifications of CeO2 phase suggested that the largeand well crystallized CeO2 particles were only partially reduced toCe2O3 in H2 at 1073 K, and that a very small fraction of Ce3+ has been

1 lysis A

iwdCr

2

C

pTamaptHai(homwamtBptaditwacspcn

t0

3

tWh(wwacpIAtw(

t0

6 I. Luisetto et al. / Applied Cata

ncorporated into alumina as CeAlO3 species. These results agreeith the literature data. In fact Shyu et al. [34] reported that highlyispersed CeO2 nanoparticles supported on Al2O3 have becomeeAlO3 above 873 K and agglomerated CeO2 particles have beeneduced only at temperature higher than 1073 K.

CeO2 + H2 → Ce2O3 + H2O (12)

e2O3 + Al2O3 → 2CeAlO3 (13)

The textural properties (BET surface area, pore volume and mainore size) of calcined and reduced catalysts are summarized inable 1. According to the IUPAC classification [41], calcined CP, WInd CA catalysts belonged to IV type isotherms, characteristic ofesoporous materials, whereas the calcined SG sample showed

composite isotherm between type IV and type II, indicating theresence of mesoporous and macroporous structures (Fig. S1A inhe Supporting information). The hysteresis loop of SG samples was3-type, characteristic of aggregate particles with no uniform sizend shape, whereas on the other samples was H1-type, indicat-ng the presence of cylindrical mesopores. The PSD curve analysisFig. S1B in the Supporting information) showed that CA samplead the most uniform pore texture with small primary pore widthf 4.0 nm. The SG sample showed a broad pore size distributionainly in the mesoporous region (<50 nm) with a primary poreidth of 4.1 nm, with shoulders at 6.9 and 11.8 nm, and a broad

nd weak peak centred at 50 nm, suggesting the presence of someacropore. The WI and CP samples showed a broad pore size dis-

ribution with large mesopore of 9.8 and 17.0 nm, respectively. TheET surface area ranges from 71 to 285 m2 g−1 depending on thereparation method. The large surface area of CP sample was dueo the use of surfactant in the synthesis whereas the low surfacerea of SG sample was attributed to the rapid hydrolysis and con-ensation of the alkoxide precursor. The N2 adsorption–desorption

sotherms of reduced catalysts showed the same IUPAC classifica-ion of calcined catalysts, suggesting that the mesoporous structureas mostly retained after the thermal treatment at 1073 K. PSD

nalysis of CA sample showed the shrinkage of pore size; on theontrary a slight increase of pore size was observed for WI and SGamples. The latter showed a bimodal distribution in the meso-orous region confirming its heterogeneity. The CP sample did nothange the primary pore size, however its pore distribution becomearrow.

Supplementary material related to this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.apcata.2015.05.04.

.2. XPS

To get a deeper insight on the electronic and chemical proper-ies at the surface of the samples, XPS studies have been performed.

ith this aim, the C1s, Al2p, Ni2p and Ce3d core level spectraave been collected and analyzed. The core level binding energyBE) and the full width at half-maxima (FWHM) were analyzedith particular attention to the Ni2p and Ce3d signals components,hich are of major interest for rationalizing the observed catalytic

ctivity. The BE, FWHM and atomic percent values observed foralcined and reduced catalysts are collected in Table 1S in the Sup-orting Information. Al2p core level spectra were also investigated.

n agreement with the literature, the individual BE positions for thel2p signal contribution strongly depend on the preparation condi-

ion of the samples, thus Al O associated peaks are found in quiteide BE ranges. Al2p3/2 BE is observed between 71.8 and 74.2 eV

Table 1S). As an example, Al2p spectrum of CA is reported in Fig. 2c.Supplementary material related to this article can be found, in

he online version, at http://dx.doi.org/10.1016/j.apcata.2015.05.04

: General 500 (2015) 12–22

Ni2p spectra are made complicated by the presence of high BEsatellites adjacent to the main peaks; as an example, CA Ni2p spec-trum is reported in Fig. 2a. However, by following a peak fittingprocedure, a single pair of spin–orbit components was individu-ated for all calcined samples. The spin–orbit component of higherintensity 2p3/2 was taken as reference, and with the exception of SGsample, was found at about 856 eV, typical of oxidized nickel. Thisvalue was slightly higher than that reported for bulk NiO, charac-terized by a BE of about 854 eV, and was ascribed to the presence ofNi2+ in strong interaction with the support [42]. SG sample showedan intermediate situation with a BE value of 855 eV suggesting thepresence of Ni2+ in weak interaction with the support. The aboveassignments were supported by the XRD analysis detecting bulkNiO only on SG sample. For reduced samples, a second contributionwas observed at lower BE values (Ni2p3/2 = 853.0–853.5 eV), indica-tive of metallic Ni [25]. However, the spectral contribution arisingby Ni(OH)2 on Ni particle surface [43] was still intense (about 50% oftotal Ni2p signal), due to the sample exposure to the atmosphereduring preparation and prior the introduction into the UHV XPSmeasurement chamber.

Ce3d core level spectra were widely and deeply investigatedbecause Ce3+ ions in CeAlO3 species, formed under reaction condi-tion, are involved in the CO2 dissociative adsorption (Eq. (5)). Thisstep has been proposed to enhance the conversion of CHx interme-diates at the metal support interface, avoiding their accumulationas carbon (Eq. (6)). Therefore it is expected that samples with higherCe3+ amount are the most efficient. Ce3d spectra of all samples arereported in Fig. 2b. Ce3d spectra were extremely complicated, dueto the different components arising by Ce3+ and Ce4+ ions and byNi2p1/2 satellites superimposed to the first peaks. By following apeak-fitting procedure, five spin orbit pairs related to Ce3d wereindividuated, and the resulting components were associated to dif-ferent ions by comparison with literature data [34,44]. The smallpeak observed at higher BE values (nearly 916 eV BE), that is notobserved in pure CeAlO3 reference samples [34], is the 3d3/2 spinorbit component associated to the higher BE 3d5/2 peak (in pur-ple in Fig. 2b) and can be associated to the presence of Ce4+ ions[44]. By estimating the intensity percent of this peak, it was possi-ble to compare Ce4+ amounts in the calcined and reduced samples.The obtained values, indicative for oxidized Ce4+ cerium amounttrend, are collected in Table 2. On calcined samples, the intensityof the Ce4+ 3d3/2 peak was very similar to that of pure CeO2 for CP,WI and SG, whereas it was particularly less intense for CA. Afterreduction at 1073 K the intensity of the peak lowered, indicatingthat CeO2 was partially reduced to CeAlO3-like species, which arestable in the atmosphere during preparation and prior the intro-duction into the UHV XPS measurement chamber. The reductionextent depended by the preparation method. More in detail it ispossible to distinguish the reduced samples in two types: WI andSG with high Ce4+ content, namely less reducible, and CA and CPwith low Ce4+ content, namely more reducible.

3.3. Reducibility

From the literature it is known that CeO2 supported on �-Al2O3 is reduced in three main temperature regions. Peaks at lowtemperature (≈773–873 K) correspond to the reduction of surfaceCe4+ of small ceria crystallites. Peaks at intermediate tempera-ture (≈900–1100 K) correspond to the reduction of large and bulkceria crystallites. Peaks at high temperature (>1100 K) correspondto the reduction Ce4+ → Ce3+ of bulk related to the CeAlO3 for-mation. The position of these peaks is strongly dependent from

the CeO2 loading, the interaction with �-Al2O3 and the parti-cle size [32,34,45,46]. Regarding Ni2+ supported on �-Al2O3, fourspecies with increasing reduction temperatures have been recog-nized in the literature: bulk NiO, scarcely interacting with �-Al2O3

I. Luisetto et al. / Applied Catalysis A: General 500 (2015) 12–22 17

F : sampi is figu

((b(tr

rrsN

TX

a

ig. 2. XPS core level spectra of (a) Ni2p (sample CA); (b) Ce3d (from top to bottomn purple); (c) Al2p (sample CA). (For interpretation of the references to colour in th

Ni-� species), reduced in the temperature range of pure NiO573–753 K); NiO interacting with the support (Ni-�1 species), likei-dimensional NiO–AlOx monolayer, reduced at mild temperature773–873 K); non-stoichiometric spinel in strong interaction withhe surface (Ni-�2 species) and bulk NiAl2O4 (Ni-� species), botheduced at high temperatures (873–1153 K) [47–49].

The H2-TPR profiles of samples reduced up to 1273 K are

eported in Fig. 3. Several overlapped peaks corresponding toeduction processes described above were observed. All sampleshowed two weak peaks below 750 K ascribed to the reduction ofi-� and of Ce4+ located at the surface of CeO2 nanoparticles [50].

able 2PS Ce4+ diagnostic component percent (depth: 1–4 nm max); hydrogen consumption in

Catalyst XPS analysis TPR an

Ce4+ diagnostic peak % H2 con

Calcined Reduced at 1073 K Theore

CP 19.0 2.5 2.17

WI 26.7 10.0 2.29

SG 23.0 14.9 2.24

CA 9.3 1.6 2.45

a Theoretical H2 consumption for the reduction of Ni and Ce assuming as reduction reactnalysis used in calculation.b Hydrogen consumption in TPR experiment up to 1273 K.c Hydrogen consumption in TPR experiment up to 1073 K.d Percentage of Ce3+ calculated according to Eq. (14) (see text) using the hydrogen cons

le CP, WI, SG, CA. The spectral component associated with Ce3d5/2 signal of Ce4+ isre legend, the reader is referred to the web version of this article.)

In agreement with XRD and XPS analysis, SG sample showed Ni-� peak with the highest intensity. CP sample showed a large andasymmetric peak with maximum at 1090 K attributed to the reduc-tion of Ni-�2 and Ni-� species, and a very weak peak at 811 K due tothe reduction of Ni-�1. WI sample showed a main and broad peakat 1090 K ascribed to the reduction of Ni-�2 and Ni-�, a shoulder at846 K assigned to the reduction of Ni-�1, and a sharp peak at 1187 K

due to the reduction Ce4+ → Ce3+ with formation of CeAlO3. SG cat-alyst showed a similar TPR profile with broad and overlapped peaksat 864, 930 and 1043 K assigned to the reduction of Ni-�1, Ni-�2,Ni-� species, respectively, and a sharp peak at 1168 K indicative of

TPR experiments and Ce3+ percentage in calcined catalysts.

alysis

sumption (mmol g−1) % Ce3+d

ticala TPR 1273 Kb TPR 1073 Kc

2.04 1.94 5.852.19 1.70 4.272.17 1.65 2.652.13 2.12 12.9

ions Ni2+ + H2 → Ni0 + 2H+ and 2Ce4+ + H2 → 2Ce3+ + 2H+. Ni and Ce content by EDAX

umption in TPR experiment up to 1273 K.

18 I. Luisetto et al. / Applied Catalysis A

F

taicagCnpparub

rriticaps(

%

b

ig. 3. H2-TPR profiles of catalysts calcined at 873 K: CA (a); SG (b); WI (c); CP (d).

he CeAlO3 formation. CA sample showed a main reduction peakt 930 K attributed to the reduction of Ni-�1 and Ni-�2 and a lessntense peak at 1123 K assigned to the reduction of Ni-�. Among theatalysts, CA has the highest intensity of Ni-�1 and Ni-�2 peaks. Inll samples the weak peak of bulk CeO2 reduction was not distin-uished because superimposed to Ni reduction. Moreover in CP andA samples also the high temperature peak of CeAlO3 formation isot observed. However, as reported in the literature, the CeAlO3eak position may depends from many features like CeO2 loading,article size and support interaction. Furthermore, the quantitativenalysis of the hydrogen consumption (see below) suggests that theeduction of both cerium species occurred. These considerations lets to claim that also CeAlO3 peak in CP and CA catalysts was maskedy Ni reduction.

The hydrogen consumptions are reported in Table 2. The theo-etical hydrogen consumption for the Ni2+ → Ni0 and Ce4+ → Ce3+

eduction is calculated from the chemical composition. The exper-mental hydrogen consumption was slightly lower than theheoretical one for CP, WI and SG catalysts whereas it was signif-cantly lower for CA sample. Assuming that the Ni2+ species wereompletely reduced to Ni0, the difference between the theoreticalnd experimental hydrogen consumption (�[H2]) suggested thatart of cerium was present as Ce3+ after synthesis and the corre-ponding amount, was calculated as percentage according to Eq.14), where Ce(EDAX) is the cerium content obtained by EDAX.

3+ �[H2]

Ce = 100

2 · Ce(EDAX)(14)

The CA sample showed the largest amount of Ce3+ followedy CP, WI and SG samples. This finding was in line with the XPS

: General 500 (2015) 12–22

analysis of the Ce3d core level. In fact, CA sample showed the lowestintensity of the peak at 916 eV BE assigned to Ce4+.

With the aim of studying the effect of the activation treat-ment used in the DRM reaction on the oxidation state of thesample, H2-TPR experiments were also conducted up to 1073 Kand then maintained in isothermal step. During the isothermalstep at 1073 K, the hydrogen consumption rapidly decreased to thebaseline (Fig. S2 in the supporting information). The hydrogen con-sumption (Table 2) of CA and CP samples was comparable to thatobserved in the H2-TPR up to 1273 K, conversely, that of WI and SGsamples was significantly lower. Since the reduction temperatureof Ni-� species (the most difficult to reduce) in WI and SG sampleswas similar or lower than that observed in CP and CA (Fig. 3), andtaking into account the very similar hydrogen consumption in TPRup to 1273 K and up to 1073 K of CP and CA samples, it is reasonableto conclude that the Ni2+ species in all samples were completelyreduced. Therefore, the lower hydrogen consumption in reductionup to 1073 K of WI and SG samples may be attributed to the lowerreducibility of Ce4+ species, in agreement with XRD and XPS analy-sis. Indeed XRD diffraction lines of CeO2 were still observed on WIand SG samples after reduction up to 1073 K, whereas they almostdisappeared on CP catalyst (Fig. 1B). Moreover, XPS analysis of sam-ples reduced at 1073 K, showed that the intensities of the Ce4+ peaksof WI and SG samples were approximately six times higher thanthose observed on the other specimens, indicating larger amountof unreduced CeO2 (Table 2).

Supplementary material related to this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.apcata.2015.05.004

3.4. Catalytic activity

The methane and carbon dioxide conversions as a function oftemperature and time on stream are reported in Fig. 4a and b. Beingendothermic, the DRM reaction is thermodynamically and kinet-ically favoured at high temperatures, therefore, the conversionsincreased with the increasing of temperature in the studied range873–1073 K. The CO2 conversions were higher than methane con-versions at all temperatures, indicating the occurrence of the sidereverse water gas shift reaction (RWGS). Both methane and CO2conversions slightly decreased during 5 h of time on stream. Thedeactivation was mild at high temperatures, for example at 1073 Konly WI catalyst showed a 4% deactivation, whereas on CP and CAcatalysts the conversions remained almost constant and on SG cata-lyst they slightly increased. The catalytic activity strongly dependedon the preparation method. At 873 K the activity followed the sam-ple order SG < WI < CA < CP whereas at 1073 K the order changed inWI < SG � CA < CP. In particular, at 1073 K the highest methane con-version was obtained for CP and CA catalysts with similar values of75% and 70%, respectively. The H2/CO ratio as a function of tem-perature and time on stream is reported in Fig. 5. Consistently withthe RWGS occurrence, the H2/CO ratio was lower than unity at alltemperatures. However, its values increased with the increasingof temperature, suggesting a better selectivity towards the DRMat high temperatures. The highest H2/CO ratio value of ≈0.9 wasobserved for the most active CP and CA catalysts at 1073 K.

The mechanism of DRM reaction has been widely investigated[51–55]: CH4 dissociates over Ni0 sites leaving reactive carbon atomC*. This carbon atom may be converted to CO by reacting with oxy-gen deriving from CO2 activation on Ni0 or on support sites. Thedehydrogenation of CH4 on Ni0 sites has been recognized as therate determining step for the DRM reaction.

In order to compare the specific DRM activity of the cata-lysts, the STY of hydrogen at 1073 K was calculated (Table 3). Inthe DRM the only reactant containing hydrogen is CH4, howeverthe H2 produced by the DRM may further react with CO2 by the

I. Luisetto et al. / Applied Catalysis A: General 500 (2015) 12–22 19

Table 3Ni dispersion, active Ni site, STY and oxygen consumption of used catalysts.

Catalyst DNia Ni sites (mol × 106)b STY (s−1) Oxygen consumption (mmol g−1)

C� C�

CP 2.9 2.3 16.0 74.33 8.73WI 5.9 5.1 5.3 9.60 46.45SG 6.9 5.5 4.7 – 27.67CA 11.4 10.4 3.2 – 7.48

a Ni dispersion calculated by the Vannice method (Eq. (10)).b Active Ni sites calculated as DNi × Ni mol.

Fig. 4. Methane conversion (A) and carbon dioxide conversion (B), as a functiono♦C

Rrssastaf

f time on stream under different reaction temperatures: � CA; � SG; © WI; CP. Reaction condition: catalyst loading 0.050 g, reactant mixture compositionH4:CO2:Ar = 40:40:20 vol.%, GHSV 90,000 cm3 g−1 h−1.

WGS. Therefore, in order to exclude the H2 consumption by RWGSeaction, the H2 produced was calculated from the methane con-umption. The STY increased with the increasing of Ni crystalliteize. This behaviour is consistent with the observations of severaluthors [18,56] that reported the increase of TOF with Ni particle

ize. Moreover it is known that the rate of methane dehydrogena-ion depends on Ni particle size [57]. Thus, the trend of specific DRMctivity observed in the present study, was primarily due to the dif-erent ability of Ni nanoparticles to activate the C-H bond, which

Fig. 5. H2/CO ratio as a function of time on stream under different reaction tempera-tures: � CA; � SG; © WI; ♦ CP. Reaction condition: catalyst loading 0.050 g, reactantmixture composition CH4:CO2:Ar = 40:40:20 vol.%, GHSV 90,000 cm3 g−1 h−1.

is mainly related to the particle size. On the other hand, small Nicrystallite size provides a high number of active sites; therefore thesample CA, despite having the lowest STY, showed nearly the sameactivity of the CP sample having the highest STY value.

Differences in H2/CO ratio for the catalysts reflected the differ-ences in the catalytic activity and the lower reactants conversionyields a lower H2/CO ratio. In fact the RWGS, which is responsi-ble for the low H2/CO ratio, is reported to be near equilibrium inthe 700–850 K temperature range [4,58]. At high temperature theDRM is favoured and the CH4 and CO2 reactants are converted effi-ciently. Therefore the less amount of CO2 available for the RWGSgives H2/CO ratio closer to unity as expected when only the DRMreaction occurs.

Coke may be formed during DRM reaction by the conversion ofaccumulated C* atoms to less reactive carbon species, which mayencapsulate the surface of Ni0 or may dissolve in the nickel crys-tallite. The dissolved carbon diffuses through the nickel to nucleateat the metal-support interface forming carbon filaments [59]. Thecarbon formation rate (rc) and the amount of accumulated carbonduring 5 h of time of stream at different temperatures are reportedin Fig. 6a and b. On all catalysts the highest rc was observed at 873 K.The rc abruptly decreased with the increasing of temperature upto 973 K and it decreased less or do not vary at higher tempera-ture. The high rc observed on our catalysts at low temperature isattributed to a low rate of the C* intermediates oxidation and tothe occurrence of the CO disproportionation reaction. Moreover,

CP catalyst showed a slight increase of rc above 973 K, suggest-ing that on this catalyst the C* was gasified slower in comparisonwith the other catalysts. The rc increased in the following sampleorder CA � SG < WI � CP, namely it increased with Ni0 crystallite

20 I. Luisetto et al. / Applied Catalysis A: General 500 (2015) 12–22

Fig. 6. Carbon formation rate at different temperatures (A), and the correspond-ing cumulative carbon deposition (B), each temperature lasted for 5 h: CA (red), SG(i

sp

ipagcn

ow

3

s

band) and at 1576 cm−1 (G band), a shoulder at 1590 cm−1 (D′ band)and a less intense band at 2642 cm−1 (G′ band). Raman spectra ofCA sample shows two large bands at 1320 (D band) and 1593 cm−1

blue), WI (black), CP (green) catalysts. (For interpretation of the references to colourn this figure legend, the reader is referred to the web version of this article.).

ize, confirming the structure-sensitive nature of methane decom-osition reported by other authors [19].

The mass of carbon deposited on the surface, increased with thencreasing of temperature reaching a plateau (Fig. 6b). On all sam-les the much greater amount of carbon was accumulated at 873 K;t higher temperature the carbon increased by a small amount, sug-esting a better oxidation of C* carbon. The amount of depositedarbon strongly depends on catalyst preparation method beingearly negligible on CA and about 13 times greater on CP catalyst.

In order to shed light on the possible reasons of the stabilityf catalysts having different amount of coke, the deposited carbonas investigated by XRD, Raman and TPO analysis.

.5. Characterizations of used catalysts

The XRD patterns of the catalysts after catalytic tests (Fig. 7)howed a broad peak at about 2� = 26.1◦ corresponding to graphitic

Fig. 7. XRD patterns of the used catalysts: CA (a); SG (b); WI (c); CP (d).

carbon (JCPDS 41-1487) in addition to phases observed on thefreshly reduced catalysts. The peak intensity was in agreement withthe carbon amount weighted at the end of the catalytic test, beingthe highest for CP and the lowest for CA. Furthermore, the Ni0 crys-tallite size on used samples was nearly the same than on reducedsamples indicating that during the catalytic cycle, the metal sinter-ing was minimal.

Raman spectroscopy was used to investigate the order of thedeposited carbon on the used Ni catalysts (Fig. 8). For each sample,at least three Raman spectra were collected in different areas toassess the homogeneity of the investigated material. All the spec-tra collected on the same sample showed exactly the same featuresthus confirming the homogeneity of the carbon deposits. Ramanspectra of CP, SG and WI samples display two main bands at 1325 (D

Fig. 8. Raman spectra of the used catalysts: CA (a); SG (b); WI (c); CP (d).

I. Luisetto et al. / Applied Catalysis A

(iaidsp[udodCcad

ra

mCdctdwos

dIt

tttiCbI

[(2010) 7441–7453.

Fig. 9. O2-TPO profiles of the used catalysts: CA (a); SG (b); WI (c); CP (d).

D′ band). The G and G′ bands are related to the stretching vibrationn the aromatic layers of graphite (in-plane displacement of carbontoms in the hexagonal sheet) and they are the only bands presentn perfect crystalline graphite [60–62]. The D and D′ bands areefect bands that appear when disorder is introduced into graphitetructure and they have been assigned to the non-zone centredhonons associated to the disorder-induced vibration of C C bond63–65]. The relative intensity ratio of D and G bands (ID/IG) can besed to investigate the graphitization of carbon and the degree ofisorder in the structure [66]. An ID/IG ratio near zero indicates highrder whereas a ratio around 1 reveals high disorder due to abun-ant defects in the graphite structure. In our case, ID/IG it is 1.4 forP and WI samples and 1.3 for SG sample, indicating that graphiticarbon deposits have a high degree of disorder. The absence of Gnd G’ bands on CA sample indicate that the small amount of carboneposit, revealed by XRD, has a much higher degree of disorder.

O2-TPO experiments were carried out with the aim to study theeactivity of coke with the oxygen. TPO profiles are reported in Fig. 9nd the corresponding oxygen consumption in Table 3.

TPO profiles showed two peaks of oxygen consumption withaximum at about 813 and 945 K, assigned to the oxidation of the

� and C� carbon species, respectively. In agreement with literatureata [20], the C� species, corresponding to filamentous graphiticarbon, were oxidized at lower temperature in comparison withhe C� species, which are more stable moss like graphitic carboneposits. The C� species were the main on CP sample, whereas theyere found in low amount on WI and were not observed on the

ther samples. On the other hand, the C� species were the main onample WI and the only carbon species on SG and CA.

TPO analysis showed that the reactivity of carbon depositsepended by their morphology more then their disorder degree.

n fact, despite the similar ID/IG ratio observed on CP, WI and SG,hey present different distribution of C� and C� species.

The amounts of deposited carbon estimated from the consump-ion of oxygen in TPO experiment were in good agreement withhe increase of the weight of samples at the end of the catalyticest, and with the intensity of XRD peak assigned to graphite. It isnteresting to note that on samples CP and CA the content of the

� carbon species were similar, suggesting that the nature of car-onaceous residues is also influenced by the presence of CeAlO3.

ndeed samples WI and SG, with low Ce3+ content, have a greater

[[[[

: General 500 (2015) 12–22 21

quantity of C� compared to samples CP and CA, with high Ce3+ con-tent. Despite this positive effect, coke deposition is mainly drivenby the nickel crystallite size, in fact CP catalyst, having the largestNi crystallite, showed the greater carbon amount.

It is worth to note that CP catalyst showed high activity and sta-bility despite of the greater carbon content. This behaviour may beexplained considering the nature of the carbon deposits. In fact, thefilamentous carbon C� does not result in fast deactivation becausedoes not encapsulate Ni0 sites that remain accessible to the reac-tants.

4. Conclusions

The dry reforming of methane was studied on catalysts with10 wt.% Ni and 20 wt.% CeO2 supported on �-Al2O3 having differentNi crystallite size, Ce3+/Ce4+ ratio and textural properties.

The textural properties and thermal stability of catalysts dependon the preparation method. The highest stability of surface area andpore size, is showed by the sample prepared using the citric acidmethod that, allowing the mixing of cations at atomic level, yieldsa Ni Ce Al Ox mixed oxide with amorphous phase. By using theco-precipitation and citric acid methods, Al O Ce bonds areformed and CeO2−x results well dispersed within �-Al2O3 lattice.CeO2−x, strongly interacting with �-Al2O3, presents greater amountof Ce3+ in comparison with segregated CeO2.

The specific catalytic activity for DRM increased with theincreasing of Ni crystallite size, regardless of the textural propertiesand of the Ce3+/Ce4+ amount.

Carbon deposition is limited by operating at temperature higherthan 1023 K on catalysts having Ni crystallites smaller than ≈10 nm.The amount of less reactive carbonaceous deposits, graphitic mosscarbon, is lower on catalysts containing great amount of Ce3+ sta-bilized as CeAlO3 like species.

The citric acid method results particularly attractive becauseallows to obtain catalysts with very small Ni crystallite size (5.8 nm)and high content of CeAlO3, that have a beneficial role in increasingthe rate of carbon gasification.

The most active catalysts CA and CP are stable during time onstream: because CA catalyst accumulates a low amount of coke,whereas CP accumulates mainly filamentous carbon which doesnot encapsulate Ni0 active sites, causing only a slightly decreaseof the catalytic activity. However excessive carbon accumulationmay results in the reactor plugging and in the pressure increase,therefore the catalyst prepared by citric acid method, showing com-parable performances with minor carbon deposition, results themost suitable catalyst for long term operation.

References

[1] M.-S. Fan, A.Z. Abdullah, S. Bhatia, ChemSusChem 4 (2011) 1643–1653.[2] T.V. Choudhary, V.R. Choudhary, Angew. Chem., Int. Ed. Engl. 47 (2008)

1828–1847.[3] Z. Jiang, T. Xiao, V.L. Kuznetsov, P.P. Edwards, Philos. Trans. A Math. Phys. Eng.

Sci. 368 (2010) 3343–3364.[4] M.C.J. Bradford, M.A. Vannice, Catal. Rev. 41 (1999) 1–42.[5] I. Luisetto, E. Di Bartolomeo, A. D’Epifanio, S. Licoccia, J. Electrochem. Soc. 158

(2011) B1368–B1372.[6] J. Kirtley, A. Singh, D. Halat, T. Oswell, J.M. Hill, R.A. Walker, J. Phys. Chem. C 117

(2013) 25908–25916.[7] M. Pillai, Y. Lin, H. Zhu, R.J. Kee, S.A. Barnett, J. Power Sour. 195 (2010) 271–279.[8] C. Agrafiotis, H. von Storch, M. Roeb, C. Sattler, Renew. Sustain. Energy Rev. 29

(2014) 656–682.[9] T. Kodama, A. Kiyama, T. Moriyama, T. Yokoyama, K.I. Shimizu, H. Andou, N.

Satou, Energy Fuels 17 (2003) 914–921.10] N. Gokon, Y. Yamawaki, D. Nakazawa, T. Kodama, Int. J. Hydrog. Energy 35

11] Y. Li, D. Li, G. Wang, Catal. Today 162 (2011) 1–48.12] P. Djinovic, J. Batista, A. Pintar, Int. J. Hydrog. Energy 37 (2012) 2699–2707.13] J. Chen, C. Yao, Y. Zhao, P. Jia, Int. J. Hydrog. Energy 35 (2010) 1630–1642.14] S. Gaur, D.J. Haynes, J.J. Spivey, Appl. Catal. A 403 (2011) 142–151.

2 lysis A

[

[[[[[

[[

[

[[[

[

[

[

[

[

[[[[

[[

[[[[

[

[

[

[[

[[

[[[[[[

[

[

[

[[[[

[

[

[

2 I. Luisetto et al. / Applied Cata

15] I. Luisetto, S. Tuti, E. Di Bartolomeo, Int. J. Hydrog. Energy 37 (2012)15992–15999.

16] G. Bonura, C. Cannilla, F. Frusteri, Appl. Catal. B 121–122 (2012) 135–147.17] E. Nikolla, J. Schwank, S. Linic, J. Catal. 263 (2009) 220–227.18] K.O. Christensen, D. Chen, R. Lødeng, A. Holmen, Appl. Catal. A 314 (2006) 9–22.19] J.-H. Kim, D.J. Suh, T.-J. Park, K.-L. Kim, Appl. Catal. A 197 (2000) 191–200.20] K. Mette, S. Kühl, H. Düdder, K. Kähler, A. Tarasov, M. Muhler, M. Behrens,

ChemCatChem 6 (2014) 100–104.21] Z. Li, L. Mo, Y. Kathiraser, S. Kawi, ACS Catal. 4 (2014) 1526–1536.22] M. Yu, K. Zhu, Z. Liu, H. Xiao, W. Deng, X. Zhou, Appl. Catal. B 148–149 (2014)

177–190.23] Y. Kathiraser, W. Thitsartarn, K. Sutthiumporn, S. Kawi, J. Phys. Chem. C 117

(2013) 8120–8130.24] X. Du, D. Zhang, L. Shi, R. Gao, J. Zhang, J. Phys. Chem. C 116 (2012) 10009–10016.25] T. Odedairo, J. Chen, Z. Zhu, J. Phys. Chem. C 117 (2013) 21288–21302.26] Y. Madier, C. Descorme, A.M. Le Govic, D. Duprez, J. Phys. Chem. B 103 (1999)

10999–11006.27] S. Zeng, X. Zhang, X. Fu, L. Zhang, H. Su, H. Pan, Appl. Catal. B 136–137 (2013)

308–316.28] A. Kambolis, H. Matralis, A. Trovarelli, C. Papadopoulou, Appl. Catal. A 377

(2010) 16–26.29] P. Djinovic, I.G. Osojnik Crnivec, B. Erjavec, A. Pintar, Appl. Catal. B 125 (2012)

259–270.30] V. Gonzalezdelacruz, J. Holgado, R. Pereniguez, A. Caballero, J. Catal. 257 (2008)

307–314.31] N. Laosiripojana, W. Sutthisripok, S. Assabumrungrat, Chem. Eng. J. 112 (2005)

13–22.32] J. Ashok, S. Kawi, Int. J. Hydrog. Energy 38 (2013) 13938–13949.33] S. Wang, G.Q. Lu, Appl. Catal. B 19 (1998) 267–277.34] J.Z. Shyu, W.H. Weber, H.S. Gandhi, J. Phys. Chem. 92 (1988) 4964–4970.35] T.Y. Kim, S.M. Kim, W.S. Lee, S.I. Woo, Int. J. Hydrog. Energy 38 (2013)

6027–6032.36] W. Chen, G. Zhao, Q. Xue, L. Chen, Y. Lu, Appl. Catal. B 136–137 (2013) 260–268.37] P. Swift, D. Shuttleworth, M.P. Seah, Practical Surface Analysis by Auger and

X-ray Photoelectron Spectroscopy, John Wiley & Sons, 1983.38] D. Shirley, Phys. Rev. B 5 (1972) 4709–4714.39] J. Zhang, H. Wang, A.K. Dalai, Appl. Catal. A 339 (2008) 121–129.40] M.A. Vannice, Kinetics of Catalytic Reactions, Springer, US, 2005.

41] K.S.W. Singh, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T.

Siemieniewska, Pure Appl. Chem. 57 (1985) 603–619.42] C. Jiménez-González, Z. Boukha, B. de Rivas, J.J. Delgado, M.Á. Cauqui, J.R.

González-Velasco, J.I. Gutiérrez-Ortiz, R. López-Fonseca, Appl. Catal. A. 466(2013) 9–20.

[

[

: General 500 (2015) 12–22

43] A.V. Naumkin, A. Kraunt-Vass, S.W. Gaarenstroom, C.J. Powell, NIST X-rayPhotoelectron Spectroscopy Database, NIST X-ray Photoelectron SpectroscopyDatabase, Version 4. 1, National Institute of Standards and Technology, 2012,http://srdata.nist.gov/xps/.

44] G.P. Szijjártó, Z. Pászti, I. Sajó, A. Erdohelyi, G. Radnóczi, A. Tompos, J. Catal. 305(2013) 290–306.

45] H.C. Yao, Y.F.Y. Yao, J. Catal. 86 (1984) 254–265.46] A. Piras, S. Colussi, A. Trovarelli, V. Sergo, J. Llorca, R. Psaro, L. Sordelli, J. Phys.

Chem. B 109 (2005) 11110–11118.47] J.M. Rynkowski, T. Paryjczak, M. Lenik, Appl. Catal. A. 106 (1993) 73–82.48] R. Yang, X. Li, J. Wu, X. Zhang, Z. Zhang, Y. Cheng, J. Guo, Appl. Catal. A. 368

(2009) 105–112.49] F. Bentaleb, E. Marceau, Microporous Mesoporous Mater. 156 (2012) 40–44.50] W. Zheng, J. Zhang, Q. Ge, H. Xu, W. Li, Appl. Catal. B 80 (2008) 98–105.51] J. Wei, E. Iglesia, J. Catal. 225 (2004) 116–127.52] J. Wei, E. Iglesia, J. Catal. 224 (2004) 370–383.53] A. Donazzi, A. Beretta, G. Groppi, P. Forzatti, J. Catal. 255 (2008) 259–268.54] D. Pakhare, V. Schwartz, V. Abdelsayed, D. Haynes, D. Shekhawat, J. Poston, J.

Spivey, J. Catal. 316 (2014) 78–92.55] B. Bachiller-Baeza, C. Mateos-Pedrero, M.A. Soria, A. Guerrero-Ruiz, U. Rode-

merck, I. Rodríguez-Ramos, Appl. Catal. B 129 (2013) 450–459.56] S. Sokolov, E.V. Kondratenko, M.-M. Pohl, U. Rodemerck, Int. J. Hydrog. Energy

38 (2013) 16121–16132.57] V.Y. Bychkov, Y.P. Tyulenin, A.A. Firsova, E.A. Shafranovsky, A.Y. Gorenberg, V.N.

Korchak, Appl. Catal. A 453 (2013) 71–79.58] M.C.J. Bradford, M.A. Vannice, J. Catal. 173 (1998) 157–171.59] D.L. Trimm, Catal. Today 49 (1999) 3–10.60] R.A. DiLeo, B.J. Landi, R.P. Raffaelle, J. Appl. Phys. 101 (2007) 064307.61] M. Pimenta, A. Marucci, S. Empedocles, M. Bawendi, E. Hanlon, A. Rao, P.

Eklund, R. Smalley, G. Dresselhaus, M. Dresselhaus, Phys. Rev. B 58 (1998)R16016–R16019.

62] R. Saito, A. Grüneis, G.S. Ge, V.W. Brar, G. Dresselhaus, M.S. Dresselhaus, A.Jorio, L.G. Canc ado, C. Fantini, M.A. Pimenta, A.G.S. Filho, New J. Phys. 5 (2003)157.

63] F.F. de Sousa, H.S.A. de Sousa, A.C. Oliveira, M.C.C. Junior, A.P. Ayala, E.B.Barros, B.C. Viana, J.M. Filho, A.C. Oliveira, Int. J. Hydrog. Energy 37 (2012)3201–3212.

64] A.L. Pinheiro, A.N. Pinheiro, A. Valentini, J.M. Filho, F.F.d. Sousa, J.R.d. Sousa,

M.d.G.C. Rocha, P. Bargiela, A.C. Oliveira, Catal. Commun. 11 (2009) 11–14.

65] M. Matthews, M. Pimenta, G. Dresselhaus, M. Dresselhaus, M. Endo, Phys. Rev.B 59 (1999) R6585–R6588.

66] D. Geng, Y. Chen, Y. Chen, Y. Li, R. Li, X. Sun, S. Ye, S. Knights, Energy Environ.Sci. 4 (2011) 760–764.