Catalysis Today 180 (2012) 96 104

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Catalysis Today

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Hydrog rmAg) nan

Ines A. C zutMauro Ga Centro Atmi es CienRo Negro, Argeb Department o ials, INof Trieste, Via L

a r t i c l

Article history:Received 20 DReceived in reAccepted 19 MAvailable onlin

Dedicated to Prof. Seran Bernal, anexemplary scientic guide, in occasion ofhis retirement.

Keywords:Ethanol steamHydrogen prodNanostructureCeria based m

, Pd, u/CeOood i

weregical

acterization techniques were used to evidence the modication of the samples induced by prolongedexposure to the reaction mixture. The promising performances of Ru/CeO2/YSZ were associated to thecombination of the positive action of the metal selectivity to syn-gas and of CeO2 in metal dispersionstabilization and coke deactivation prevention.

2011 Elsevier B.V. All rights reserved.

1. Introdu

In the laopment of increased ddependenceenergy demparticular aof active anversion efenergy neevery attractH2 based futal pollutionproblems reto the enormproblems intion [3]. Nevresources b

CorresponE-mail add

0920-5861/$ doi:10.1016/j. reforming (ESR)uctiond catalystsaterials

ction

st years, the research activities devoted to the devel-new energy vectors or alternative fuels have rapidlyue to the drivers for cleaner air and for reducing the

upon fossil fuels. In addition, the ever-growing worldand pushes for a diversication of energy sources withttention to renewables [1] and for the developmentd stable fuel cells (FC), able to guarantee high con-

ciency [2]. Among the various options to meet globalds, hydrogen, in combination with FCs, is considered aive energy vector. While, it is generally accepted thatel cells can contribute to preventing the environmen-

generated by conventional combustion engines, manymain unsolved for its large scale use. These latter relateous cost for the necessary infrastructure, technological

sustainable hydrogen production, storage and distribu-ertheless, the possibility to produce H2 from a variety ofy many different ways stimulates the research activities,

ding author. Fax: +39 040 5583903.ress: pfornasiero@units.it (P. Fornasiero).

with particular attention to the more sustainable options. At thepresent, steam reforming of natural gas and partial oxidation of coalor heavy hydrocarbons are the most commonly used and economi-cally competitive methods for large scale hydrogen production [4].There is a general consensus on the fact that any future energyscenario is based on a mix of energy sources with an increasedcontribution of the renewables ones. This applies also to hydrogenproduction, where its production from biomasses is a challengingtask [3].

Many different raw materials and reactions have been proposedfor hydrogen production [48]. Of these, great attention has beendedicated to ethanol conversion to syngas. In fact, large quantitiesof rst generation ethanol are already produced by fermentationof sugars and more sustainable second generation ethanol can beobtained from lignocellulosics [9,10] Literature reports reveal thatthe ethanol conversion and selectivity to hydrogen strongly dependon the type of metal catalyst used, type of precursor, preparationmethods, type of catalyst support, presence of additives, operat-ing condition and temperature. Catalysts consisting of Rh and Niloaded on different supports present so far the best activity andare the most commonly used for ethanol steam reforming (ESR)for hydrogen production [1116]. During ethanol steam reform-ing, Rh supported on Al2O3, MgO or TiO2 showed higher ethanol

see front matter 2011 Elsevier B.V. All rights reserved.cattod.2011.03.068en production from ethanol steam refoocomposites

arbajal Ramosa , Tiziano Montinib , Barbara Lorenrazianib, Paolo Fornasierob,

co Bariloche (CNEA) and Instituto Balseiro (UNCuyo), Consejo Nacional de Investigacionntinaf Chemical and Pharmaceutical Sciences, Center of Excellence for Nanostructured Mater. Giorgieri 1, 34127 Trieste, Italy

e i n f o

ecember 2010vised form 9 March 2011arch 2011e 5 May 2011

a b s t r a c t

M/CeO2/YSZ nanocomposites (M = Ruthe ethanol steam reforming (ESR). Rtion conditions. Pd/CeO2/YSZ shows gperformances of the Ag-based samplemicrostructural (HR-TEM), morpholo/ lo cate /ca t tod

ing on M/CeO2/YSZ (M = Ru, Pd,

b , Horacio Troiania , Fabiana C. Gennaria ,

tcas y Tcnicas (CONICET), (8400) S.C. de Bariloche, A. Bustillo km 9.5,

STM-Trieste Research Unit, CNR-ICCOM Trieste Research Unit, University

Ag) were prepared by successive impregnation and tested in2/YSZ demonstrated good activity and stability under reac-

nitial activity but suffers of signicant deactivation while the rather poor. A plethora of different structural (Powder XRD),(surface area, CO chemisorption) and thermal analysis char-

I.A.C. Ramos et al. / Catalysis Today 180 (2012) 96 104 97

conversion and hydrogen production than Ru, Pt and Pd [17]. Ptpromotes the Water Gas Shift Reaction (WGSR), but its activityin CC bond rupture was found to be limited. The hydrogen pro-duction of Ru-based catalyst is comparable to that of Rh only atrelatively hobserved foto form ethTherefore, scoke formanario, ceriawidely useddispersion olattice oxygof oxidationreducing thand dehydrthe catalystin Three-W[2331] (inH2 purica(SOFCs). In rate of oxidreaction beelectrolyte)and (iii) to [3740]. Manodes of SO[4143] or reforming opreliminary

In the prprising Ru, Pethanol steananocomposition simil

2. Experim

2.1. Catalys

CeO2 (1commerciaChemicals) priate amo99.9%, Aldricontinuousperature unfor 12 h. Th(heating rat

Pd, Ru anobtain a nomaqueous soFluka) or Rumetal precuthe CeO2 (1solvent wa120 C overat 400 C fo

2.2. Charac

CO chemperformed physisorptiature on 0.1

CO chemisorption experiments were performed at 35 C. Previ-ous to the chemisorption experiments, the calcined catalysts weresubjected to a cleaning pretreatment at 500 C for 1 h under O2(5%)/Ar ow followed by reduction at 150 C in H2 (5%)/Ar for 2 h

acuaampluctiod anms wr to

CeO dou

1:1 articlder

ips P54 n

rang

peraed

g of for

with minilize

min

etecemovse tointopulsrmoent

y, 10ed inbtai

C with resed

UT ersin

of th grid

talyt

alytic mic

ratiC for

Ar 200

an . All

to 12 of ore tretrein1

(40 ucede tem

h ate caisothigh Ru loading (5 wt%). While promising stability wasr Rh based systems, Ru induces dehydration of ethanolylene, leading to coke formation via polymerization.uitable promoters/additives must be added to preventtion for effective and stable operation [15]. In this sce-

can play a major role. In fact, CeO2-based materials are in catalysis since they increase and stabilize the metalf noble metals [18,19] and are able to provide reactiveen [20,21]. These latter factors increase the efciency

reactions, such those involved in reforming processes,e deposition of intermediates that, after polymerizationogenation, result in coke deposition and deactivation ofs. Accordingly, CeO2-based materials are largely presentay Catalysts [21,22], catalysts for reforming processescluding ethanol steam reforming [1416,32]) and fortion [3336] and electrodes for Solid Oxide Fuel Cellsthe latter application, CeO2 is used to (i) increase theation reaction, (ii) to avoid the undesired solid statetween Yttria-Stabilized Zirconia (YSZ the most used

and perovskitic materials used as electrode materialsincrease the length of the Triple Phase Boundary (TPB)oreover, CeO2-based materials are key components ofFCs fed directly with liquid fuels, such as hydrocarbons

alcohols (including ethanol) [44,45], operating internalr direct oxidation of the fuel without the necessity of a

reforming step to produce H2.esent work, the activity of heterogeneous catalysts com-d or Ag was evaluated with respect to H2 production bym reforming. The metals were deposited on a CeO2/YSZsite support in order to obtain a material with compo-ar to that of SOFCs anodes.

ental

t preparation

0 wt%)/YSZ support was prepared by impregnation ofl Yttria Stabilized Zirconia (YSZ, Y2O3 79 mol%, Melpreviously calcined at 750 C for 24 h. Briey, an appro-unt of cerium ammonium nitrate ((NH4)2Ce(NO3)6,ch) was dissolved in ethanol and YSZ was added under

stirring. After 2 h, the slurry was dried rst at room tem-der reduced pressure for 6 h and then in air at 120 C

e material was calcined in a static oven at 600 C for 5 he 1 C min1, cooling rate 4.5 C min1).d Ag were supported on the CeO2 (10%)/YSZ in order toinal metal loading of 2 wt% by wet impregnation using

lution of Pd(NO3)32H2O (puriss. Fluka), AgNO3 (puriss.(NO)(NO3)3 (ChemPur). An appropriate amount of thersor was dissolved in bidistilled water. After adding

0%)/YSZ powder, the suspension was stirred for 2 h. Thes removed at reduced pressure and the solid dried atnight. Finally, the samples were calcined in a static ovenr 5 (heating rate 1 C min1, cooling rate 4.5 C min1).

terization

isorption and BET surface area measurements wereusing a Micromeritics ASAP 2020 C analyzer. N2

on isotherms were collected at liquid nitrogen temper- g of sample, after evacuation at 350 C overnight.

and evaged sthe redused anisotherin ordetion onby theCO:M =nanop

Powa Phil( = 0.1the 2point.

Temperform0.25 350 Cpurged(40 mLto stabat 5 Ctivity DAr to rdecreaing O2the O2

TheInstrumUsualldesorbwere o1000

HigperformCM200by dispa dropcopper

2.3. Ca

Catquartzweight1300

into anModelobtainstreamheatedvalues

Befwere p10 C mfor 2 hintrodfurnacAfter 2rate. Thtested tion at 400 C for 4 h. In order to avoid coke removal fromes, the cleaning pretreatment was not applied and onlyn step was performed. Typically, 0.2 g of samples were

equilibration time of 10 min was employed. Adsorptionere measured in the low pressure range (220 Torr)

minimize the CO adsorption due to carbonate forma-2. The so-called reversible adsorption was eliminated

ble isotherms method. A chemisorption stoichiometryand a spherical geometry were assumed for the metales.XRD patterns of the samples were recorded withW 1710/01 diffractometer using Cu K radiationm). The data were collected with a 0.02 step size ine from 10 to 100, using a counting time of 10 s per

ture Programmed Reduction (TPR) experiments wereusing a Micromeritics AutoChem 2910 instrument onthe calcined materials. The samples were cleaned at1 h by pulsing of O2 in an Ar ow every 75 s, then

Ar at 350 C for 15 min and cooled to -70 C. H2 (5%)/Ar1) was admitted into the reactor and the ow allowed

for 30 min before increasing the temperature to 900 C1. H2 uptake was monitored using a Thermal Conduc-tor (TCD). After reduction, the ow was switched toe adsorbed hydrogen and the temperature allowed to

427 C. The reduced materials were re-oxidized by puls- the Ar ow at 427 C until a stable area is obtained fore (at least 40 pulses were injected into the ow).Gravimetric Analysis (TGA) were performed using a TAs TGA Q500 instruments in owing air (60 mL min1).

mg of the spent catalysts were loaded and water was owing air at 100 C for 20 min, until a constant weightned. After that, the temperature was increased up toh a heating rate of 10 C min1.olution-transmission electron microscopy studies wereusing a transmission electron microscopy (TEM Philipsoperating at 200 kV). Samples for TEM were preparedg a small amount of powder in hexane and depositinge resulting suspension on a commercial carbon coated.

ic tests

experiments were conducted in a U-shaped 4 mm IDroreactor. Typically 32 mg of catalyst, diluted in a 1:2o with high purity -Al2O3 (Grace Davison, calcined at

24 h), were used. EtOH/H2O 1:5 mixture were injectedow with a Hamilton Gastight syringe using an INSTECH0 syringe pump at a rate of 1.8 L min1, in order toethanol concentration of 1.0% by volume in the gasthe transfer lines between syringe, reactor and GC were0 C. Gas ow rates were 32 mL min1 to ensure GHSV

60,000 mL g1 h1.esting the catalytic activity, the calcined materialsated under O2 (5%)/Ar at 350 C for 1 h (40 mL min1,) and activated by reduction in H2 (5%)/Ar at 150 CmL min1, 5 C min1). The gaseous mixture was rst

in the reactor at 150 C for 1 h, before increasing theperature to 600 C with a heating rate of 1 C min1.

600 C, the furnace was cooled to 150 C at the sametalyst was subjected to two of these cycles before beingermally at selected temperatures. Stability tests were

98 I.A.C. Ramos et al. / Catalysis Today 180 (2012) 96 104

Fig. 1. Ethanethanol convRu/CeO2/YSZ (GHSV 60,000

performed for at least

On-line 5890 Seriesas carrier, wto analyze Hcarrier, wasization deteC balance w

3. Results

3.1. Catalyt

Fig. 1 shM/CeO2/YSneous reacwhile blankent of the chigh tempesion of 18

Ethanol investigatedgen yields adepending

Ag/CeO2reforming: temperaturwhile only

Ru/CeO2the ethanoproduction 5.2 molH2 mproduction by-productyields alwaabove 450

H2, CO and CO2 are detected in the gas phase, with a relative ratioin good agreement with the WGSR equilibrium.

Pd/CeO2/YSZ (Fig. 1d) converts completely ethanol above 350 Cproduction reaches the maximum of 4.8 molH2 molEtOH

1C. A-pro. Inn 20e obs

diffn be

ethvolve

subsjecte2, de

ed bcarboxidelenem rerom

the[47,4

acid a go

agrve ps [50ld beehyd

theed foto aning ts ofgh Hing reforehydol steam reforming activity on activated samples: overallersion (a) and molar yields of products for Ag/CeO2/YSZ (b),c) and Pd/CeO2/YSZ (d). Conditions: EtOH (1.0%) + H2O (5.0%) in Ar,

mL g1 h1.

exposing the samples at the reaction mixture at 550 C75 h.GC analysis was performed using a Hewlett Packard

II gas chromatograph. A Molsieve 5A column, with Aras connected to a thermal conductivity detector (TCD)2, O2, N2, CH4 and CO. A PoraPLOT Q column, with He as

connected in series to a methanator and to a ame ion-ctor (FID) to analyze the carbon-containing compounds.as always within 2%.

and discussion

and H2at 580ious bysamplebetweelene ar5%).

Theples cafor theway incan bebe suband COobtainand debasic oin ethyto steaagain frequirebonds by thestratesand, inas actigenatePd couacetaldstep inreportposed reformamounalthouincreaseasily acetaldic activity under ESR

ows the result of ethanol steam reforming on theZ nanocomposites. The contribution of the homoge-tion (evaluated with an empty reactor) is negligible

experiments performed using -Al2O3 alone (the dilu-atalysts) evidences only the formation of ethylene atrature (above 500 C) with a maximum ethanol conver-% at 600 C.conversion starts around 150 C, independently of the

catalyst (Fig. 1a). Signicant differences in the hydro-nd selectivity are however observed with temperatureon the nature of the metal phase./YSZ (Fig. 1b) shows poor activity in ethanol steamthe main products are H2 and acetaldehyde in all thee range investigated (yield up to 0.8 molH2 molEtOH

1)minor amounts of CO2 are produced above 400 C./YSZ (Fig. 1c) shows a progressive increase inl conversion up to 550 C. At the same time, H2continuously increases reaching a maximum ofolEtOH

1 at 580 C. Between 200 and 500 C, H2 and CO2is accompanied with the production of low amounts ofs, such as acetaldehyde, acetone and methane (molarys below 10 mol%). CO production is signicant onlyC. Above 500 C, when ethanol conversion is total, only

observed inalysts durinis reasonabethanol stedecompositon the metobtain syn-is the less to dissociatreact on ththe acid situct.

Prolongebased catalunder ethanduring stabinitial convCO2 and CHat the samedeactivatiooverall expPd/CeO2/YSmance, a prtime, a progCH4 was obt intermediate temperatures, the selectivity in the var-ducts is very different with respect to the Ru/CeO2/YSZ

fact, very large quantities of CH4 are produced0 and 580 C while acetaldehyde, acetone and ethy-erved only in very minor amounts (molar yields below

erences in the activity observed for the various sam- discussed considering the reaction network proposedanol steam reforming [16]. The most favorable path-s the dehydrogenation of ethanol to acetaldehyde, thatequently decomposed to CH4 and CO. Then, CH4 mustd to steam reforming, obtaining a mixture of H2, COpending from the WGSR equilibrium [16]. Acetone is

y a side reaction involving a condensation, oxidationxylation and is promoted by CeO2-based materials and

s [46]. On the other hand, dehydration of ethanol results production. Subsequently, ethylene can be subjectedforming producing H2, CO and CO2, depending oncethe WGSR equilibrium [16]. Dehydrogenation reactions

use of a metal catalyst to activate the CH and OH8] while dehydration to ethylene is usually promoted

sites of the oxide support [13]. Ru and Pd demon-od activity in the activation of CH and OH bonds

eement with this fact, many studies report their usehases for reforming of hydrocarbons [4951] or oxy-,52,53]. The differences in selectivity between Ru and

interpreted with a different catalytic behavior duringe decomposition, suggesting the possibility of a new

reaction mechanism. Pd follows the pathway usuallyr ethanol steam reforming: acetaldehyde is decom-

equimolar mixture of CH4 and CO, followed by steamof methane and WGSR. On Ru/CeO2/YSZ only minor

CH4 were detected in the medium temperature range,2, CO and CO2 amounts continuously increased withtemperature. This could be an indication that CH4 ismed on the Ru-based catalyst or CH4 is not formed bye decomposition. Usually, quite low conversions are

this temperature range (300500 C) on Ru-based cat-g methane reforming processes [5456]. Therefore, itle that CH4 is formed only in limited amounts duringam reforming and that the reaction proceeds throughion of acetaldehyde to CH3 and CHO groups adsorbedal surface, where they are easily reformed to nallygas (H2, CO and CO2). On the other hand, Ag/CeO2/YSZactive in ethanol reforming due to the inability of Age CH bonds of hydrocarbons [57]. Since it does note metal surface, ethanol is available for the reaction ofes of the support producing ethylene as major prod-

d stability tests were performed only on Ru- and Pd-ysts, since they demonstrate promising performancesol steam reforming. Fig. 2 presents the results obtained

ility tests performed for 75 h at 550 C. In both the cases,ersion and selectivity in the different products (H2, CO,4) are in excellent agreement with the values obtained

temperature during run-up experiments. No signicantn of the catalytic performances is observed during theeriment for Ru/CeO2/YSZ (Fig. 2a). On the other hand,Z shows, after an initial period of 40 h of stable perfor-ogressive decrease of ethanol conversion. At the sameressive decrease in the molar yields of H2, CO, CO2 andserved.

I.A.C. Ramos et al. / Catalysis Today 180 (2012) 96 104 99

Fig. 2. Stabilitethanol convepanel) for Ru/(5.0%) in Ar, G

3.2. CharacAg)

3.2.1. TempTempera

performed the best coalytic experwere perfolow temperformation.

CeO2/YSCeO2 (Fig. 3one (2505This TPR peCeO2 particnanocrystalthe reductio

The prepalladium of surface reduction oaffected by at low temreduction ovalues signcompositioPdO and Apresents twPd/CeO2/YStion with mat 65 C ascAg/CeO2/YSaround 70

Temp/YSZ (

the rnateivati.r redulse

literared fox pceriuy of ethanol steam reforming activity on activated samples: overallrsion (upper panel) and molar yields of products (central and lowerCeO2/YSZ (a) and Pd/CeO2/YSZ (b). Conditions: EtOH (1.0%) + H2OHSV 60,000 mL g1 h1, T = 550 C.

terization of M/CeO2/YSZ nanocomposites (M = Ru, Pd,

erature Programmed Reduction (TPR)ture Programmed Reduction (TPR) experiments wereto assess the reducibility of the materials and to select

Fig. 3. Ru/CeO2

range, impregthe actments

Afteby O2 pin the measuthe redof the nditions for the activation of the catalysts before cat-iments (complete reduction of the metal phases). TPRrmed from 70 C in order to gain information on theature reduction of palladium oxide and on its hydrides

Z support shows the typical TPR prole of supporteda). Two broad reduction peaks are observed. The rst50 C) is the convolution of various reduction processes.ak can be ascribed to the reduction of the surface ofles and/or to the reduction of highly dispersed CeO2s [58,59]. The second peak (550900 C) is ascribed ton of the bulk of CeO2 particles [60].

sence of a easily reducible metal oxide (ruthenium,or silver oxide) signicantly promotes the reductionceria/small ceria nanoparticles [61]. Vice versa, thef bulk ceriahigh temperature peakis not signicantlythe presence of supported metals. The H2 consumptionperature corresponds to the contribution of both thef supported metal oxide and of the surface of ceria. Theicantly differ among the samples due to the differentn of the metal oxides present after impregnation (RuO2,g2O, see below, XRD section). Ru/CeO2/YSZ (Fig. 3b)o sharp reduction peaks centered at 50 and 105 C.Z (Fig. 3c) shows a very sharp and intense H2 consump-aximum at 0 C, followed by a negative peak centeredribed to the decomposition of PdHx species. Finally,Z (Fig. 3d) presents a broad reduction peak centeredC. Despite the different behavior in the low temperature

than usuallbe related wKim et al. reat higher p(reported threspect to directing pconcentratian O2 consuO2 g1), suthese condmetal sintefor Ru/CeOhigher, 204same O2 corelated witaccount, a mRu- and Pdafter the setheir full remainly in tof an oxide

3.2.2. N2 phN2 phy

employed tthe samplesity experimerature Programmed Reduction (TPR) proles for CeO2/YSZ (a),b), Pd/CeO2/YSZ (c) and Ag/CeO2/YSZ (d).

eduction processes are completed at 150 C for all thed samples. Therefore, this temperature was selected foron of the catalysts before each catalytic activity experi-

uction, the amount of reduced species was evaluateds at 427 C, as a generally accepted procedure reportedture [20]. An O2 consumption of 112 mol O2 g1 wasor CeO2/YSZ. Assuming that only cerium is involved inrocess at these temperature, the data indicate that 77%m is reduced to Ce(III). This value is signicantly highery obtained for bulk CeO2 [20]. This discrepancy couldith the high interaction between CeO2 and YSZ. In fact,ported that CeO2 impregnated on YSZ remains reducedO ) compared to pure CeO [59]. Moreover, Putna et al.2 2at CeO2 lms on YSZ are much easily reducible withCeO2 lms of -Al2O3, as a result of the structure-roperties of zirconia surfaces which induce a highon of defects in CeO2 overlayers [62]. For Ag/CeO2/YSZ,mption close to that of CeO2/YSZ is obtained (109 molggesting that the re-oxidation of Ag is minimal underitions (high temperature pre-reduction, that leads toring and low temperature oxidation). On the other hand,2/YSZ and Pd/CeO2/YSZ the O2 consumption is much

and 168 mol O2 g1 respectively. Assuming that thensumption from reduced CeO2, the difference can beh the oxidation of the metal phases. Taking this intoolar O/M ratio of 0.46 and 0.30 can be calculated for

-based samples, respectively. This is an indication that,vere sinterization expected for the metals after the TPR,-oxidation is not possible and O2 consumption resultshe passivation of the metal surface or in the formation

layer on the surface of large metal cores.

ysisorptionsisorption at the liquid nitrogen temperature waso investigate the evolution of the textural properties of

from their preparation to the end of the catalytic activ-ents. YSZ support calcined at 750 C for 24 h presents a

100 I.A.C. Ramos et al. / Catalysis Today 180 (2012) 96 104

Table 1N2 physisorption results for M/CeO2/YSZ (M = Ru, Pd, Ag) after various treatments.

Sample SSAa (m2 g1) CPVb (mL g1) dMc (nm)

CalcinedYSZCeO2/YSZ Ru/CeO2/YSZPd/CeO2/YSZAg/CeO2/YSZActivated/redRu/CeO2/YSZPd/CeO2/YSZAg/CeO2/YSZRun-upRu/CeO2/YSZPd/CeO2/YSZAg/CeO2/YSZAfter stabilityRu/CeO2/YSZPd/CeO2/YSZAfter regenerRu/CeO2/YSZPd/CeO2/YSZ

a Specic Sub Cumulativ

tion isothermsc Relative m

the desorptiond After stabi

for 1 h to remo

physisorptibimodal disaround 10 slightly decsubsequentmodify the ties of the sreduction aup to 600 Cface area ansample afteperformed recovery ofsuggest thasurface of thdeactivationably, the teby the prolgesting thatrespect to Pperformanc

3.2.3. PowdPowder

vation (H2 r(both after terns are doP42/nmc) acination at phases are RuO2, PdO at 150 C (Fcan be obseunder ESR nanoparticlto estimatecomponentysis can und

owde/YSZ (

meta are

determined from Rietveld analysis, is very close to the nom-lues (2 wt% for the metals, 10 wt% for CeO2, 88 wt% for YSZ)the samples treated at high temperatures (after two run-upments at 600 C or after prolonged aging at 550 C). This sug-hat the XRD data can provide a representative description ofamples. On the other hand, after activation by reduction at47 0.17 11/2741 0.15 10/40

42 0.15 10/28 42 0.15 10/28

41 0.13 10/28uced

37 0.15 10/28 39 0.13 10/27

40 0.16 11/38

39 0.16 10/55 38 0.14 10/28

40 0.16 10/55 test

38 0.16 10/38 32 0.12 10/28ationd

39 0.15 10/39 36 0.14 10/38

rface Area from BET analysis.e Pore Volume determined from the desorption branch of physisorp-.axima of the pore distribution determined by the BJH analysis from

branch of the physisorption isotherms.lity tests, the catalysts were cleaned by oxidative treatment at 500 Cve carbonaceous deposits e reduced in H2/Ar at 150 C for 1 h.

on isotherm typical of a mesoporous material with atribution of the pores, with relative maxima centeredand 30 nm. The impregnation with CeO2 produces arease of the surface area and pore volume while the

impregnation with the metal precursors do not furthertexture on the materials. (Table 1) The textural proper-amples are marginally affected by the activation by H2t 150 C and after two consecutive run-up experiments

under ESR condition. A signicant decrease in the sur-d pore volume is observed only for the Pd/CeO2/YSZ

r stability tests. Finally, an oxidative treatment at 500 Con the spent Pd/CeO2/YSZ catalyst results in a partial

the initial surface area and pore volume. These resultst the deposition of carbonaceous compounds on the

Fig. 4. PAg/CeO2

when surfacephase,inal vafor all experigests tthose se Pd/CeO2/YSZ catalyst signicantly contributes to the process resulting in a partial pore blocking. Remark-xtural properties Ru/CeO2/YSZ are almost unaffectedonged exposure under ESR conditions at 550 C, sug-

the coke deposition takes place in a lower extent withd/CeO2/YSZ and does not signicantly affect the activitye.

er X-ray diffractionXRD patterns were collected on the samples after acti-eduction at 150 C) and after aging under ESR conditionrun-up experiments and stability tests). The XRD pat-minated by the reection of tetragonal YSZ (space groupnd cubic CeO2 (space group Fm3 m). Notably, after cal-400 C, very weak and broad reections of metal oxideobserved (data not shown), indicating the presence ofand Ag2O in the samples. After activation by reductionig. 4), very weak reections related to metal phasesrved. Their intensity signicantly increases after agingconditions, suggesting a partial sintering of the metales (Fig. 5). Rietveld analysis of the XRD patterns allows

the weight percentages and the cell parameters of each in the nanocomposite catalysts. Notably, Rietveld anal-erestimate the amount of metal active phase, especially

150 C, the nicant lowall the casedispersed foby XRD. Thoretical valextent by thples. Notablthe componof cation di

Table 2 different phnot affectedintensities nated on thcannot be cof the metaconditions tion (H2 redmetal nano

3.2.4. CO chCO chem

ity of the mr XRD patterns of CeO2/YSZ (a), Ru/CeO2/YSZ (b), Pd/CeO2/YSZ (c) andd) after activation by H2 reduction at 150 C for 2 h.

l nanoparticles are highly dispersed on mediumhigha supports. In the present study, the amount of eachamount of metal estimated by Rietveld analysis is sig-er with respect to the nominal value (about 1 wt% in

s), suggesting that part of the metal is present in a veryrm on the surface of the catalysts and hardly detected

e cell parameters are in good agreement with the the-ues. Moreover, they are not inuenced in a signicante thermal/chemical treatments subjected by the sam-y, this fact indicates that no solid state reaction betweenents (in particular, CeO2 and YSZ) takes place, as a resultffusion during prolonged high temperature treatment.summarizes the mean crystallite sizes obtained for theases. The mean crystallite sizes of YSZ and CeO2 are

by the various treatments of the materials. Since theof the reections of the oxides of the metals impreg-e CeO2/YSZ support are very low, their crystallite sizesalculate. On the other hand, the mean crystallite sizesl phases signicantly increases during tests under ESRwith respect to the correspondent values after activa-uction treatment), indicating a partial sintering of theparticles during ethanol reforming.

emisorptionisorption was employed to evaluate the accessibil-etal phases. Generally speaking, the selection of the

I.A.C. Ramos et al. / Catalysis Today 180 (2012) 96 104 101

Fig. 5. Detail Ru/CeO2/YSZ (tion at 150 C ESR conditiontions at 550 C

experimentof the fraclar, signicadeterminatfact, hydrogplace durin

Table 2Mean crystalliAg) after vario

Sample

CalcinedCeO2/YSZ Ru/CeO2/YSZPd/CeO2/YSZAg/CeO2/YSZActivated/ReRu/CeO2/YSZPd/CeO2/YSZAg/CeO2/YSZAfter run-upRu/CeO2/YSZPd/CeO2/YSZAg/CeO2/YSZAfter stabilityRu/CeO2/YSZPd/CeO2/YSZ

a Determineb Not calcul

metals impregcrystallite size

Table 3CO chemisorption results for M/CeO2/YSZ (M = Ru, Pd) after various treatments.

Sample CO chemisorption

CO/Ma Apparent particle Metal surface

Activated/redRu/CeO2/YSPd/CeO2/YSZAfter stabilitRu/CeO2/YSPd/CeO2/YSZAfter regenerRu/CeO2/YSPd/CeO2/YSZ

a Assumingb Calculatedc After stabi

for 1 h to remo

consumptioreduced ceity [63,64].the chemispartial presPdceria banot be applstoichiomesion of theadsorption

periunatepear

ibilitmentimizof the powder XRD patterns in the range 3250 for CeO2/YSZ (a),b), Pd/CeO2/YSZ (c) and Ag/CeO2/YSZ (d) after activation by H2 reduc-for 2 h (black lines), after two consecutive run-up experiments unders to 600 C (light grey lines) and after stability tests under ESR condi-

tion exunforttion apaccessexperito min for 75 h (dark grey lines).

al conditions is crucial to obtain a reliable estimationtion of atoms exposed to the gas phase. In particu-nt experimental problems can be encountered for the

ion of the metal dispersion of CeO2-based materials. Inen spillover from the metal to the support could take

g the chemisorption, resulting in a signicantly high gas

te sizes calculated for the phases present in M/CeO2/YSZ (M = Ru, Pd,us treatments.

Mean crystallite sizea (nm)

YSZ CeO2 Metal

17 10 18 10 N.C.b

18 10 N.C.b

17 9 N.C.b

duced 17 10 9

17 10 9 17 10 10

experiments 17 10 16

17 9 12 17 9 16

test 17 8 19

17 8 13

d applying the Scherrers formula to the main reection of each phase.ated: the intensities of reections from oxide phases related to thenated on the supports are too low for a reliable estimation of thes.

Ru/CeO2to Pd/CeO2sion of the mrespect to tmore sensially observinvolved inelectronic dples after s550 C for p

The app(Table 3), ismined by Xfraction of hing overestfraction of reection inthe very smdiffraction observed inat 550 C foboth the cabelow), sinoccurrencebelow). Thetreatment aCeO2.

3.2.5. High(HR-TEM)

Transmiinvestigatesizeb (nm) areab (m2 g1)

ucedZ 0.360 3.7 2.63

0.093 12.0 0.83y testZ 0.195 6.8 1.43

0.006 190 0.05ationc

Z 0.246 5.4 1.80 0.079 14.2 0.70

a CO/M stoichiometry of 1. assuming a spherical geometry of the metal particleslity tests, the catalysts were cleaned by oxidative treatment at 500 Cve carbonaceous deposits e reduced in H2/Ar at 150 C for 1 h.

n during the experiments [63]. In addition, presence ofria can lead to suppression of chemisorptions capabil-

Hydrogen spillover effects can be limited by reducingorption temperature or with a careful selection of thesure of the probe gas [64,65]. However, in the case ofsed materials, the low temperature chemisorption can-ied due to the formation of PdHx species with differenttry depending from the H2 pressure and the dimen-

metal nanoparticles. Furthermore, the kinetic of H2on Ru is very low at room temperature and chemisorp-ments must be performed at high temperature, wherely spillover is maximized. Therefore, CO chemisorp-s as the best option to simultaneously investigate they of both Ru- and Pd-based samples. Chemisorptions were conducted in a low pressure range (120 Torr)e the formation of carbonates on the CeO2 support./YSZ presents a higher metal accessibility with respect/YSZ (Table 3). This could be due to a different disper-

etal or to a different sensitivity of the two metals withhe electronic deactivation by reduced CeO2, being Pdtive than Ru. Notably, this latter deactivation is usu-ed after more severe reductive treatment than those

activation of the catalysts (150 C). The occurrence ofeactivation cannot be excluded in the case of the sam-tability tests, exposed to the reductive environment atrolonged time.arent Ru particle size estimated from chemisorption

lower with respect to the mean crystallite size deter-RD (Table 2), suggesting the presence of a considerableighly dispersed Ru nanoparticles. In fact, XRD broaden-

imates the mean crystallite dimensions when a relevanthighly dispersed metal nanoparticles are present: thetensity is mainly due to large, ordered particles whileall, disordered metal particles scarcely contribute to

of X-rays. On the other hand, a best agreement is

the case of Pd. After stability test under ESR conditionsr 75 h, the accessible surface area is strongly reduced fortalysts investigated, as a result of coke deposition (seetering, possible electronic deactivation or, but unlikely,

of metal particle decoration (no evidence by TEM, see accessibility of the metal is recovered after oxidativet 500 C, that removes carbon deposits and re-oxidize

resolution Transmission Electron Microscopy

ssion Electron Microscopy (TEM) was employed to the morphology of the samples and their possible mod-

102 I.A.C. Ramos et al. / Catalysis Today 180 (2012) 96 104

Fig. 6. TEM imnanoparticles

ications afthe YSZ andtheir morpcles in the large grainsCeO2 particsmaller, shaformly distrYSZ and CeOcalculated btive TEM imH2 reductiois quite simrespectivelyapplied on Yport grainscorners. Altand the metion, the decould be remetal phasponents (Cfraction of comparisonare local inspecic plaby TEM. Anidentied otative Ru padimension ages of samples after activation by H2 reduction at 150 C for 2 h for Ru/CeO2/YSZ (afor Ru/CeO2/YSZ (c) and Pd/CeO2/YSZ (d).

ter working under ESR conditions. TEM investigation of CeO2/YSZ supports (data not shown) allow to evidencehology, helping in the identication of metal parti-reduced samples. YSZ is composed by round-shaped,

(1520 nm), strongly interconnected within each other.les deposited on the YSZ support can be recognized asped particles with dimensions of 510 nm, that are uni-ibuted on the YSZ grains. The dimensions observed for2 are in good agreement with the mean crystallite sizesy XRD broadening (Table 2). Fig. 6 shows representa-ages of the Ru- and Pd-based samples after activation byn at 150 C. The morphology of both reduced catalystsilar (Fig. 6a and b for Ru/CeO2/YSZ and Pd/CeO2/YSZ,). Despite the prolonged thermal treatment at 750 CSZ in order to stabilize its textural properties, the sup-

present a rounded shape, with smoothed edged andhough the surface area of the materials is not very hightal loading should be reasonable for a TEM investiga-tection of metal nanoparticles was a hard task. Thisasonably related with the low contrast between thees (Ru or Pd) and the nanostructured support com-eO2 and YSZ) [64] or to the presence of a signicanthighly dispersed metal nanoparticles (as suggested by

of XRD and CO chemisorption data). TEM observations nature. The metal nanoparticles could be located onces this suggesting it is difcult to exactly nd themyway, a number of metal nanoparticles were clearlyn both the samples by TEM. Fig. 6c shows a represen-rticle composed by two crystalline nanodomains withof 10 nm each while Fig. 6d presents a representative

Pd nanopardeposited onar spacing(Fig. 6).

After agevidence sples. (Fig. 7were identalways in go(d = 0.335 nin the two ments, in asuch as Ni [activity demeven accessally grow mthe remainthe other hdeposits coThis result area evidenthe Pd nanoinhibiting s

3.2.6. ThermFig. 8 sho

ples, perforA complex cases, the wof the weig) and Pd/CeO2/YSZ (b) and representative HR-TEM images of metal

ticles of 2 nm supported on a spherical CeO2 particlen YSZ, as conrmed by the measurements of the pla-

and by the analysis of the morphology of the particles

ing under ESR condition for 75 h at 550 C, TEM imagesome modications on the morphology of the sam-), In particular, carbon deposits with graphitic natureied from the lattice spacing (0.351 nm) that areod agreement with the distance among graphite layers

m). The morphology of these deposits is very differentcases. On Ru/CeO2/YSZ, carbon ribbons grow as la-

similar way that previously observed for other metals,66,67]. This observation is in agreement with the goodonstrated by this catalyst and with the reduced but

ible metal surface area. In fact, carbon laments usu-oving from one facet of a metal nanoparticle, leaving

ing facets still available for adsorption of reagents. Onand, aged Pd/CeO2/YSZ shows the presence of carbonvering completely the surface of the metal particles.justies the strong decrease of the accessible surfaceced in this sample. After a short catalytic period of timeparticles appear to package themselves in carbon thisubsequent catalytic activity.

oGravimetric Analysis (TGA)ws the traces obtained by TGA analysis of the aged sam-med in order estimate the amount of carbon deposits.feature was obtained for both the samples. In both theeight changes are very limited, with an initial decreaseht followed by an increase above 500 C. The nal trend

I.A.C. Ramos et al. / Catalysis Today 180 (2012) 96 104 103

Fig. 7. Represat 550 C for 7carbon deposi

Fig. 8. Thermconditions for

in the weighbon removametal nanodominates important aof O2 acquiis generallyshould be c

bon residues deposited on the Pd/CeO2/YSZ is higher with respectto that present in the aged Ru/CeO2/YSZ.

clusions

anol mpohil

ver cnatiou an4. Con

EthnanocoSOFC. Wthe silCombiboth Rentative TEM images for the samples after aging under ESR conditions5 h: Ru/CeO2/YSZ (a) and Pd/CeO2/YSZ (b). Representative details ofts are shown in the insets.

oGravimetric Analysis (TGA) of the samples after aging under ESR 75 h: Ru/CeO2/YSZ (a) and Pd/CeO2/YSZ (b).

t of the materials is a combination of the loss due to car-l and of the gain due to reoxidation of reduced CeO2 andparticles. For both the materials, the rst process pre-at low temperatures while the second becomes moret higher temperatures. Considering that the amountsred during the re-oxidation of the reduced materials

quite similar, the weight increase due to re-oxidationomparable. This result suggests that the amount of car-

longed highon both catdepends onthe occlusiing to a procarbon is oit appears mallows a beWhile totaleffect, the pitively increespecially w

Acknowled

Dr. M. are acknowbilateral Itausing nobleFondo Triesable produsupport.

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Hydrogen production from ethanol steam reforming on M/CeO2/YSZ (M=Ru, Pd, Ag) nanocomposites1 Introduction2 Experimental2.1 Catalyst preparation2.2 Characterization2.3 Catalytic tests

3 Results and discussion3.1 Catalytic activity under ESR3.2 Characterization of M/CeO2/YSZ nanocomposites (M=Ru, Pd, Ag)3.2.1 Temperature Programmed Reduction (TPR)3.2.2 N2 physisorption3.2.3 Powder X-ray diffraction3.2.4 CO chemisorption3.2.5 High resolution Transmission Electron Microscopy (HR-TEM)3.2.6 ThermoGravimetric Analysis (TGA)

4 ConclusionsAcknowledgementsReferences