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Applied Catalysis A: General 479 (2014) 35–44

Contents lists available at ScienceDirect

Applied Catalysis A: General

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

xperimental evidences of the relationship between reducibility andicro- and nanostructure in commercial high surface area ceria

osé M. Gatica, Diana M. Gómez, Juan C. Hernández-Garrido, José J. Calvino,ustavo A. Cifredo, Hilario Vidal ∗

epartamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química Inorgánica, Universidad de Cádiz, Puerto Real 11510, Spain

r t i c l e i n f o

rticle history:eceived 21 February 2014eceived in revised form 10 April 2014ccepted 15 April 2014vailable online 23 April 2014

a b s t r a c t

Two commercial high BET surface areas CeO2 are characterized, as received and after calcination atdifferent temperatures, to better understand the relationship between reducibility and micro- and nano-structure. Combination of TPR data, Rietveld analysis of XRD diagrams and HREM/HAADF-STEM suggestthat the nano-particles are responsible for Ce4+ reduction in the moderate temperature range. Resultsobtained in this work show that previous models based on kinetic and theoretical analysis can be fully

eywords:eO2

eduction behaviouranostructureietveld analysisAADF-STEM.

supported by morphological characterization. This study unveils the key parameters that must be knownin order to select appropriate commercial ceria samples.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

From the pioneer use in three-way catalysts for automotiveollution control till the most recent application in hydrogen pro-uction reactions for fuel cell technology, the attractive redoxroperties of CeO2 has captured the attention of scientists [1,2]. The

iterature on these topics is so vast that it is really amazing how stilloday there is some controversy in the interpretation of ceria redoxehaviour, in particular the origin of the two reducibility thermalegions that a high surface area ceria uses to present. Classicallyeduction at low and high temperature have been associated tourface and bulk reduction respectively [3], the former based on ainear relationship between the peak area and surface area, which

as used to estimate the surface area of mixed materials [4]. Onhe contrary, other authors [5–7] have suggested differences in thentrinsic thermodynamic properties of ceria particles as a functionf their size as responsible for the appearance of two distinctiveeduction events.

Certainly, in the last 20 years an extraordinary progress has been

ade in the enhancement of ceria redox performance by appropri-

te doping, mainly with zirconia [8–11] and other rare earth oxides12–16]. It is also remarkable the great step forward achieved in the

∗ Corresponding author. Tel.: +34 956 012744; fax: +34 956 016288.E-mail address: hilario.vidal@uca.es (H. Vidal).

ttp://dx.doi.org/10.1016/j.apcata.2014.04.030926-860X/© 2014 Elsevier B.V. All rights reserved.

synthesis and characterization of ceria nanocrystals with controlledmorphology, structure and defect concentration, which is of sig-nificance for ceria-catalyzed oxidation reactions [17–21]. Recentlythere has been a lot of research aiming at the relation betweenthe nanostructure and the reducibility of ceria [22], in particu-lar theoretical studies [23]. Taking into account the use of H2 asconventional molecule probe for this kind of studies, numeroustheoretical investigations of both bulk and surface reduction ofCeO2 have been also reported, paying special attention to the inter-actions of atomic H with CeO2 (1 1 1) and (1 1 0) surfaces [24].Therefore it is very surprising that a similarly rigorous analysisis missing when dealing with commercial ceria powders widelyemployed. Although the importance of micro- and mesoporos-ity in determining the textural stability of CeO2 based materialshas been previously discussed [9], it is still often assumed thatany high surface area ceria sample will deliver good redox perfor-mance for catalytic applications and will behave in a similar way.This is so as scarce information about the thermal and chemicalhistory of the raw materials is usually provided by the suppli-ers. Thus, the novelty of this work is to evaluate the validity ofsuch assumption on the basis of experimental data. In particular,it aims at gaining a better understanding about the influence of

additional parameters, other than the BET specific surface area,on the reducibility of ceria, as the latter is a key property in thecatalytic reactions for which this material is usually employed[1–3,5,6].

3 alysis A: General 479 (2014) 35–44

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6 J.M. Gatica et al. / Applied Cat

. Experimental

.1. Materials

In this work two different ceria provided by the same manufac-urer, Rhodia (previously Rhône Poulenc) have been investigated.he samples were supplied at different times, 2001 and 2009,s a highly pure (≥99.5%) fine powder (5–15 �m), just labelleds high surface area cerium oxides without any additional infor-ation, except their respective batch numbers, 96067/01 and

3535201AC0. Selection of these samples was made as representa-ive of the evolution on the characteristics of the same product from

worldwide leading company, well-known in the rare earth oxidesarket. In order to investigate their thermal stability, both startingaterials were submitted to a heating treatment under static air

or 2 h at 500, 700 and 900 ◦C. Hereafter the starting, or fresh, sam-les will be referred to by the name CeO2 followed by the romanumerals I and II, in correspondence with the order of their deliv-ry. In the case of the calcined samples a second decimal digit (5, 7r 9) will be added to the name as a way to refer to the calcinationreatment used to prepare them. These numbers correspond to therst digit of the maximum temperature reached during calcination.

.2. Characterization techniques

Textural characterization of the samples was carried out by mea-uring the adsorption/desorption of N2 at −196 ◦C, employing aicromeritics ASAP 2020 device. The experiments were performed

sing 250 mg of sample. Before the measurement, all the samplesere submitted to a surface cleaning pre-treatment under high vac-um at (unless indicated) 200 ◦C for 2 h. The obtained isothermsere used to calculate the specific surface area (SBET) as well as theicro- and meso-porosity features of the studied samples.Reducibility of the samples in dynamic H2 atmosphere was

xamined by means of Temperature-Programmed Reduction (TPR)xperiments performed in a TA Instruments thermobalance, SDT600 model, working with 80–110 mg of sample. In all cases a0 cm3 min−1 flow of H2 (3%)/Ar was used, heating in a linear pro-ram at a rate of 10 ◦C min−1 from room temperature up to 900 ◦C.o ensure starting from a total oxidised state and a clean sur-ace, prior to each experiment the sample was pretreated with O25%)/He (60 cm3 min−1) at 500 ◦C for 1 h and further cooling downo 150 ◦C under the same atmosphere before switching to helium.he details of this pre-treatment were selected according to previ-us Temperature-Programmed Oxidation (TPO) experiments (nothown) aimed at defining the temperature necessary to removeny species that could further interfere in the calculation of theercentage of reduction from the weight losses recorded duringhe TPR study.

Induced Coupled Plasma spectroscopy (ICP) analysis of thehemical composition was performed using an IRIS Intrepid HRnstrument. Also X-ray Fluorescence (XRF) analysis in a Bruker S4ioneer spectrometer was carried out for the same purpose.

X-ray diffractograms (XRD) were obtained in a BRUKER D8DVANCE 500 powder diffractometer using Cu K� radiation (40 kV,0 mA). The diagrams were recorded in the 2� angle range from 2◦

o 132◦, using a step size of 0.03◦ and a counting time of 40 s. For aietveld refined estimate of the lattice parameter and crystal size,he BGMN software from Joerg Bergmann was used [25].

The granulometric study was carried out using a Mastersizer000 granulometer from Malvern Instrument, operating with laseriffraction. Typically, 100 mg of solid were dispersed in 20 ml of

ater and 1 ml of acetic acid. A few drops of the solution preparedere added to the sample chamber until getting 10% of obscura-

ion. To ensure reproducibility of the measurements, results werebtained in each case as the average of three independent runs.

Fig. 1. Thermogravimetric curves obtained by heating at 10 ◦C min−1 under a60 cm3 min−1 flow of H2 (3%)/Ar.

Secondary Electron Scanning Electron Microscopy (SEM)images, with nominal resolution about 3 nm, were obtained in aQUANTA-200 Scanning Electron Microscope.

Transmission electron microscopy studies were performed witha JEOL2010F microscope, working at 200 kV. This instrument hada structural resolution of 0.19 nm, and it was equipped with ahigh angle-annular dark field detector for scanning transmissionelectron microscopy (HAADF-STEM) technique. Electron diffractionpatterns were acquired using a 120 cm camera length.

3. Results and discussion

3.1. Reducibility

Fig. 1 shows the thermogravimetric traces related to theirreversible reduction process in dynamic 3%-H2 atmosphere of thetwo ceria samples investigated after the oxidizing pre-treatmentdescribed in Section 2. Preliminary studies (Fig. S1 of Supple-mentary Information) allowed discarding the presence of residualcarbonates after such pre-treatment so ensuring that only oxygenelimination with water production is taking place in these experi-ments [26]. To facilitate the analysis, percentage of ceria reductionwas estimated from the weight losses, their corresponding deriva-tive traces being shown in Fig. 2. Derivative curves obtained forboth fresh oxides (not represented) were similar to those obtainedin each case after calcination at 500 ◦C. In all plots depicted the

well reported peak of ceria reduction at high temperature (approx.900 ◦C) is observed. Nevertheless, the relevant result in this study isthat comparison between CeO2-I-5 and CeO2-II-5 reveals that theformer exhibit more intense peaks at 250 and 550 ◦C. Moreover,

J.M. Gatica et al. / Applied Catalysis A: General 479 (2014) 35–44 37

De

rivative T

GA

sig

nal (a

.u.)

Tempe rature (ºC)

200 300 40 0 500 60 0 70 0 80 0 90 0

CeO2-I-5

CeO2-I-7

CeO2-I-9

CeO2-II -5

CeO2-II -7

CeO2-II -9

0.06

10.2

19.9

37.7

2.2

5.7

22.0

0.1

1.7

17.4

2.97.5

22.2

3.87.2

22.7

1.2

3.4

18.0

Fig. 2. TPR curves obtained through derivation of thermogravimetric profiles. Addi-tionally, cumulative percentage of reduction reached at 475, 650 and 885 ◦C isift

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a

Quantity

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)

0

20

40

60

80

100

Quantity

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d (

cm

³/g S

TP

)

0

20

40

60

80

100

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1.00.80.60.40.20.0

Quantity

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d (

cm

³/g S

TP

)

0

20

40

60

80

P/P0

1.00.80.60.40.20.0

CeO2 CeO-I-5 2-II-5

CeO2 CeO-I-7 2-II- 7

CeO2 CeO-I-9 2-II-9

Fig. 3. Nitrogen physisorption isotherms (−196 ◦C). Adsorption (•) and desorption

ndicated in the graph. This is expressed as Ce3+/(Ce3+ + Ce4+) ratio, and estimatedrom weight loss associated with the irreversible reduction of ceria reached up tohe temperatures marked with a dotted line.

umulative reduction percentages amount up to around 10% and 3%at 475 ◦C) and approximately 38% and 22% (at 885 ◦C) for CeO2-I-5nd CeO2-II-5, respectively. In conclusion, CeO2-I-5 is clearly moreeducible at low/moderate temperatures. Furthermore, while theercentage of reduction drastically diminishes after calcination at00 ◦C in the case of CeO2-I, this decrease is only observed afteralcination at 900 ◦C for the CeO2-II sample.

.2. Chemical composition

Analysis of the chemical composition of the two ceria samplesas performed in order to clarify if this parameter could eventually

e at the roots of the differences observed in their reducibility. Athis respect, it is worth noting that no significant differences coulde detected between the two ceria samples either by ICP or XRF.lthough small amounts of different compounds (Fe2O3, SiO2, CuO

n CeO2-I; or CuO and ZrO2 in CeO2-II) seem to be present, as deter-ined by means of XRF analysis, they only appear at the level of

races (<0.1 wt.%). Moreover, the weight percentage of Ce measuredn both samples almost matched the nominal content of a pureeO2. In particular, it resulted to be 76.7 ± 3.0 and 79.6 ± 0.7 wt.%or CeO2-I and CeO2-II, respectively, as obtained by ICP technique.

.3. Textural properties

Textural study of the two ceria samples confirmed that bothre mesoporous with a slight contribution of microporosity in the

(o) branches are shown. The isotherms corresponding to the fresh samples (notrepresented) were very similar to those obtained after calcination at 500 ◦C.

case of CeO2-I, almost negligible in CeO2-II. All the N2 physisorptionisotherms (Fig. 3) were Type IV indicative of mesoporous materials[27], but their hysteresis loop, associated with capillary condensa-tion taking place in mesopores, changed as function of the sampleand/or treatment. Thus, while for CeO2-I the isotherms showedloops with intermediate characteristics between H3 and H4 types,usually related to aggregates of plate-like particles giving rise toslit-shaped pores, in the case of CeO2-II the loop changed from H2type, after calcination at 500 ◦C, to H1 type, in the sample calcined at900 ◦C. Although these types of loops are difficult to interpret, espe-cially the former, they are rather associated with porous materialsconsisting of agglomerates or compacts of approximately uniformspheres in fairly regular array, and hence having narrow distribu-tions of pore size.

As expected, both ceria samples showed a decrease of surfacearea and total pore volume with the corresponding increase ofaverage pore diameter upon the calcination treatments (Table 1).This evolution is reasonably attributable to a sintering effect dueto particle coarsening of ceria [28]. However, compared to CeO2-I, the CeO2-II sample showed higher stability upon calcinationtreatments. In fact, BET surface area drop was of 74% and 33%respectively after calcining at 700 ◦C. This effect can be related totheir different pore distribution in good agreement with previousstudies [9]. As shown in Fig. 4, CeO -I-5 presents a significant frac-

2tion of narrow pores (with diameter lower than 2 nm) which areabsent in CeO2-II-5 and that disappear after calcination at 700 ◦C.On the contrary it is particularly interesting the progressive shift

38 J.M. Gatica et al. / Applied Catalysis A: General 479 (2014) 35–44

Table 1Textural properties obtained from N2 physisorption at −196 ◦C over the studied ceria samples.

Sample Specific surface area(m2 g−1)a

Average pore diameter(nm)b

Total microporevolume (cm3 g−1)c

Total pore volume(cm3 g−1)d

CeO2-Ie 153.8 5.1 0.020 0.139CeO2-I-5e 143.0 5.0 0.007 0.135CeO2-I-7 39.6 8.4 0.001 0.092CeO2-I-9 5.8 22.9 0.001 0.039CeO2-II 117.9 5.6 0.000 0.172CeO2-II-5 114.8 5.7 0.000 0.167CeO2-II-7 78.3 8.3 0.003 0.166CeO2-II-9 25.8 23.1 0.002 0.132

a Calculated by BET method.b Determined by BJH method using the desorption branch.

er to g

tscppt

3

ustntem

c Calculated by t-method.d Determined from amount adsorbed at P/P0 = 0.99.e Subjected to a heating pretreatment under high vacuum at 350 ◦C for 4 h in ord

o higher sizes of the initially larger pores of CeO2-II-5, with aignificant development of pores with radii around 10 nm after cal-ination at 900 ◦C. This evolution denotes that during the sinteringrocess the pores grow partially producing pores of large size, thusroviding additional stability to the system for very high calcinationemperatures [9].

.4. Granulometric study

To improve the textural characterization of the samples, a gran-lometric study was also performed (Fig. 5). The estimated particleize distributions of the two ceria samples after the different pre-reatments are plotted against both the total volume and the

umber of particles. To understand the differences between the twoypes of granulometry graphs we should recall that the volume plotmphasizes the contribution of the largest particles, which accu-ulate much more volume. In any case, both graphs offer valuable

Po

re v

olu

me

(cm

3g

-1)

0.00

0.04

0.08

0.12CeO2-I-5

CeO2-I-7

CeO2-I-9

Pore dia meter (nm)

1 10 100

Po

re v

olu

me

(cm

3g

-1)

0.00

0.04

0.08

0.12

0.16

CeO2-II -5

CeO2-II -7

CeO2-II -9

1

Fig. 4. Cumulative (left) and dV/dD (right) pore size distribution curves obtained by

et access to microporosity.

and complementary information. First of all, the volume graph evi-dences more clearly that CeO2-I features a bimodal distribution, incontrast to CeO2-II for which the particle size distribution is uni-modal instead. A second interesting observation comes from theplots versus the number of particles. Notice how, although in bothceria samples particles with a size below 1 micron are predomi-nant, the increase in calcination temperature shifts the distributiontowards higher size values in the case of CeO2-I sample, whereas itleaves it rather unchanged in CeO2-II. Again these results depict ahigher textural stability for the latter, in very good agreement withthose obtained by N2 physisorption.

3.5. Scanning electron microscopy study

Direct observation of the samples by SEM has confirmed theoccurrence of significant microstructural differences between thetwo investigated ceria samples. As shown in Fig. 6, the aspect at

Pore di ameter (nm)

10 100

Po

re v

olu

me

(cm

3nm

-1g-1)

0.00 0

0.03 0

0.06 0

CeO2-II-5

CeO2-II-7

CeO2-II-9

Po

re v

olu

me

(cm

3nm

-1g-1)

0.00 0

0.01 5

0.030

CeO2-I-5

CeO2-I-7

CeO2-I-9

means of BJH analysis of the desorption branch of the N2 isotherms (−196 ◦C).

J.M. Gatica et al. / Applied Catalysis A: General 479 (2014) 35–44 39

after

mnfsamathfotogwp

3

gttstws

Fig. 5. Particle size distribution of the two ceria samples

icron-size level of CeO2-I is completely different to that of CeO2-II,o matter the temperature of the calcination treatment applied. The

ormer shows heterogeneities, both in terms of particle (aggregate)ize and shape, whereas the images of CeO2-II depict quite rounded,nd more homogeneous in size, particles, very close to the ceriaicrospheres prepared by other authors [13]. These observations

re in good agreement with results derived from N2 physisorp-ion, in particular with those derived from analyzing the isothermsysteresis loops (Fig. 3). We should also highlight the following

eatures of the SEM images. First, particularly evident in the imagef the CeO2-I-5 sample, the presence of very small particles onhe surface of what seems to be much larger aggregates. On thether hand, it is very important to state that the sizes of the aggre-ates observed under the microscope are, in both types of samples,ithin the ranges estimated by the granulometric study (Fig. 5), thisroviding stronger reliability to the data obtained by this technique.

.6. X-ray diffraction with Rietveld analysis

X-ray diffraction diagrams were recorded for the two investi-ated ceria samples both as fresh and after calcination at differentemperatures (Fig. S2 of Supplementary Information). In all cases,he only peaks observed were those assignable to the fluorite-type

tructure of CeO2. Also remarkable, no significant shift in the posi-ion of the different peaks, their relative intensities or their widthsas observable between the two CeO2 samples treated under the

ame conditions. As expected, a progressive narrowing of the peaks

the different pretreatments estimated by granulometry.

with increasing calcination temperature was found, clearly denot-ing a crystal size growth effect.

Since conventional X-ray diffraction analysis did not provide anyclue about the possible origin of the textural and redox differencesabove discussed, Rietveld analysis of the XRD data sets of CeO2-Iand CeO2-II was carried out in order to obtain fine details for thestructural characterization (Table 2). The most significant finding ofthis study was that fitting of the data set of CeO2-I and CeO2-I-5 XRDdiagrams required considering the contribution of two populationsof crystallites with different size and slightly different cell param-eters, i.e. a bimodal crystal size distribution, whereas in the case ofCeO2-II data set fitting could be perfectly accomplished consideringa unimodal distribution. This observation is illustrated for CeO2-I-5 in Fig. 7. The statistical goodness-of-fit parameters are very poorin the case of CeO2-I and CeO2-I-5 when a unimodal distributionof crystallites was used. These parameters improve considerablywhen a bimodal distribution is used with these two samples. Alsoa slight improvement can be noticed in the case of CeO2-I-7 andCeO2-I-9 when using the model with two types of particles. More-over, with the data set of CeO2-II samples it was not possible toimplant a bimodal model because of the instability introduced inthe program calculations. This observation resembles that made inthe granulometric study (Fig. 5) and it suggests a model accord-

ing to which there is a correspondence between the size of thecrystallites measured by X-ray Diffraction and that of the particlesresulting from their aggregation that are detected by granulome-try. In other words, the smaller the crystallites the smaller the size

40 J.M. Gatica et al. / Applied Catalysis A: General 479 (2014) 35–44

icrog

owTCtwct3i

Fig. 6. Scanning electron m

f the particles into which they aggregate, i.e. the microstructureould translate at a larger scale the features of the nanostructure.

hus, we may guess that the two types of particles observed ineO2-I are the result of a hierarchical organization in which thewo types of nanoparticles group into larger aggregates but in aay in which the ratios of the aggregate to nanoparticle size are

lose to each other. In fact, in the fresh CeO2-I sample the values ofhe aggregate-size/crystallite-size ratios are about 3 �m/3 nm and0 �m/21 nm for the smaller and larger particles respectively; i.e.

n both cases close to 1000.

raphs of the ceria samples.

Also worth noting in Table 2, the average size of the smallercrystals fraction in CeO2-I, absent in CeO2-II, increases significantlyupon calcination at 700 ◦C. This could be related to the textural andreducibility changes observed for this sample after such treatment.Note also how the average crystal size of CeO2-II (unique andaround 12 nm) only suffers a significant increase after calcination

at 900 ◦C, this pointing again at a higher textural stability of thiscerium dioxide. The evolution of the lattice parameters is alsoworth commenting. Thus, whereas in the case of CeO2-II it remainsalmost constant around 0.5410 nm, in agreement with data

J.M. Gatica et al. / Applied Catalysis A: General 479 (2014) 35–44 41

Table 2Results of Rietveld analysis of the XRD data collected on the ceria samples using BGMN software.

Sample Average crystal diameter (nm) Mass phase (%) Cell parameter (nm)a Rwp%/Rexp%* DW**

Unimodal Bimodal Bimodal Unimodal Bimodal Unimodal Bimodal Unimodal Bimodal

CeO2-I 9.(8) 21.3 58 0.5419(5) 0.5418(3) 1.20 1.03 1.37 1.833.2 42 0.542(3)

CeO2-I-5 12.(0) 24.0 57 0.5414(7) 0.5413(9) 1.16 1.04 1.55 1.905.1 43 0.5415(9)

CeO2-I-7 27.(3) 31.0 53 0.5411(1) 0.5411(8) 1.05 1.04 1.81 1.8026.0 47 0.5410(7)

CeO2-I-9 27.(5) 32.0 50 0.5410(7) 0.5412(0) 1.07 1.04 1.75 1.8028.0 50 0.5410(0)

CeO2-II 11.7 0.5410(0) 1.06 1.86CeO2-II-5 11.9 0.5410(5) 1.02 1.86CeO2-II-7 16.3 0.5410(9) 1.05 1.80CeO2-II-9 40.0 0.54104(2) 1.08 1.77

a Numbers affected by errors according to program statistics are expressed into brackets.* and 2.

pittipbIseeb

FteTc

/**Parameters that reflect the goodness of the refinement (better as closer to 1.00*

reviously reported for other ceria samples [29], significant highernitial lattice parameters are observed for CeO2-I, especially forhe fresh sample and that calcined at 500 ◦C. This modification ofhe lattice parameter following calcination temperature observedn sample CeO2-I is in agreement with Rietveld analysis reportedreviously for similar commercial ceria samples [30]. This mighte indicative of a higher residual content of Ce3+ in these samples.n that respect it must be recalled that the pre-treatment of theamples can induce a slight reduction of them. The fact that thisffect is only observed in the CeO2-I sample could be related to itsasier reduction. For CeO2-I-7 and CeO2-I-9 the lattice parameters

ecome progressively closer to 0.5410 nm.

Inte

nsity (

a.u

.)

0

500

1000

3000

3300a

Diffe

ren

ce

(a.u

.)

-200

0

200

400

c

2θ

20 30 40 50 60

Diffe

rence (

a.u

.)

-200

0

200

400

b

ig. 7. Rietveld adjustment of XRD data corresponding to the CeO2-I-5 sample usingwo different models for the crystal size distribution (a), and difference betweenxperimental and simulated spectra for the bimodal (b) and unimodal (c) fittings.he intensity of the modelled diagrams have been arbitrarily shifted for the sake oflarity in part (a) of the figure.

00** respectively).

3.7. Transmission electron microscopy studies

To get information at the nanoscale level, transmission electronmicroscopy studies were performed on the samples calcined at 500and 700 ◦C. Fig. 8 shows representative dark-field (DF) scanningtransmission electron microscopy (STEM) images for both CeO2-Iand CeO2-II samples after calcination at 500 ◦C. For the CeO2-I-5sample, we can observe the occurrence of both small crystallitesof c.a. 5 nm in size and larger crystallites of c.a. 30–40 nm in diam-eter, as illustrated on Fig. 8(a) in which these larger crystalliteshave been arrowed. Although the fraction of the larger particlesobserved within the observation field of these micrographs is smallwith respect to the smaller ones, we have to keep in mind that each40 nm particle would be the equivalent in volume to roughly 83, i.e.about 500, particles of those with diameters in the range of 5 nm.Therefore, the difference in population of the two types of parti-cles suggested by the bright and dark field micrographs could stillbe in good agreement with the results obtained from the Rietveldanalysis. It is in any case evident that STEM data confirm the het-erogeneity in the CeO2-I-5 sample, with two different particle sizesof ceria nanocrystals.

In the case of the CeO2-II-5 sample, the electron microscopystudies are also in good agreement with those summarized inTable 2. Nanometer sized particles with c.a. 10–15 nm in diameterare observed (see Fig. 8(b)) and, in contrast to the CeO2-I-5 sam-ple, the homogeneity in size is also evident from these analysis ofSTEM images. As shown in Fig. 9, these results were further con-firmed by the detailed analysis of SAED patterns recorded for thetwo samples.

Fig. 8(c and d) shows representative HREM micrographs for thesamples calcined at 700 ◦C. Two questions are worth commenting.First, note that the crystallites observed in both samples are nowquite similar both in shape and size, the latter lying roughly inthe range 20–30 nm. Additionally, the crystallites assemble intoagglomerates into which they keep oriented to each other. This isclearly observed in the Digital Diffraction Patterns (DDPs) includedas insets in Fig. 8(c and d), respectively. Note that both DDPs showmostly the reflections characteristic of a single [1 1 0] oriented CeO2crystal, in spite of corresponding to the whole areas imaged inFig. 8(c and d) respectively and which apparently show a high num-ber of distinct crystallites. This result indicates that the whole setof small 20–30 nm crystals observed in both images are necessarily

aligned to each other.

Further structural analysis of the two ceria samples was carriedout using selected area electron diffraction (SAED). Fig. 9 shows rep-resentative electron diffraction patterns for both samples calcined

42 J.M. Gatica et al. / Applied Catalysis A: General 479 (2014) 35–44

les st

asopsCtanpbentiodsn

atoo

Fig. 8. Transmission electron micrographs of different ceria samp

t 500 ◦C using a selection area aperture of the same size, at theame magnification and bringing into the aperture a similar amountf material. We should indicate first that the electron diffractionatterns confirmed the fluorite-like structure for both samples. Ashown in Fig. 9(a), the different reflections corresponding to theeO2-fluorite structure matched perfectly with those observed inhe experimental electron diffraction patterns. On the other hand,

simple comparison of the two diffraction patterns indicates sig-ificant differences between both samples. The electron diffractionattern for the CeO2-I-5 sample, see Fig. 9(a), is mainly dominatedy thick and diffuse diffraction rings. This is indicative of the pres-nce of a major fraction, in terms of number of crystals, of smallanocrystals which are spatially distributed at random orienta-ions. Once more, these results indicate that the CeO2-I-5 samples mainly constituted by small nanoparticles rather than largernes. In contrast, the diffraction pattern of the CeO2-II-5 sampleepicts a large number of well-defined and very thin diffractionpots rather than thick rings, this indicating the presence of largeanocrystals.

These features of the SAED patterns can be further confirmed

nalysing the radial intensity profiles corresponding to their rota-ional averages, Fig. 9(c and d). These profiles plot, as a functionf the radial distance, the change in the average of the intensitiesf the set of pixels which lie along the corresponding circle. These

udied: (a) CeO2-I-5, (b) CeO2-II-5, (c) CeO2-I-7, and (d) CeO2-II-7.

rotational averages can be considered therefore as a representationof the information contained in the whole pattern. By doing so, it isadditionally possible to improve the signal to noise ratio of the pat-tern as well as the accuracy of spacing measurements [31]. Fig. 9(cand d) show the corresponding radial intensity profiles for bothCeO2-I and CeO2-II samples calcined at 500 ◦C. Note that the onecorresponding to the CeO2-I-5 sample, Fig. 9(c), depicts both moreintense and broader peaks. These features are those expected fora large collection of small nanocrystals. In effect, the higher inten-sity of the peaks is due to the fact that in the sample with smallercrystals there is a larger fraction of the diffraction rings lighted up.Concerning the width, note than in the sample with smaller crys-tals, CeO2-I, some of the peaks, those at the lowest angles like the(1 1 1) and (0 0 2), even overlap. In any case the FWHM of the peaksin the CeO2-I profile are larger than those in the pattern of CeO2-II.In Fig. 9, we compare this parameter for the {2 2 0} reflection at1.91 A, resulting a FWHM for the CeO2-I-5 sample twofold that forCeO2-II-5.

Lets finally mention that TEM data corresponding to the sam-ples calcined at 900 ◦C (Fig. S3 of Supplementary Information) were

quite similar in terms of morphology and relative orientation ofthe crystallites to those just commented for the samples treated at700 ◦C. The higher crystal size observed after calcination at 900 ◦Cis consistent with the Rietveld analysis results of the XRD data.

J.M. Gatica et al. / Applied Catalysis A: General 479 (2014) 35–44 43

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Fig. 9. Electron diffraction patterns and radial int

. Conclusions

The relationship existing between ceria reducibility and micro-nd nanostructure has been proved by detailed characterization ofwo commercial high surface area ceria samples by means of H2-PR, N2 physisorption, granulometry, SEM, X-ray diffraction withietveld analysis, HREM and HAADF-STEM. To our knowledge forhe first time, experimental results support the model proposedy Trovarelli et al. through kinetic analysis [5]. CeO2 reducibilityt moderate temperatures appears to be related to the presencef a fraction of nanometer sized particles which suffer strong sin-ering after calcination at temperatures above 700 ◦C. In additionur findings are also consistent with the studies performed by Ney-an’s group who theoretically demonstrated the greatly facilitated

xygen vacancy formation in ceria nanocrystallites [23]. From aractical viewpoint, the results obtained in this work suggest that

commercial ceria sample with a relatively high BET value not nec-ssarily displays high reducibility at low/moderate temperatures.oreover, high reducibility and high textural stability appear as

wo properties which are difficult to have simultaneously in ceriaamples. In this sense, this study has helped defining the parame-ers that should be investigated to select ceria samples exhibitingither good redox properties or, instead, high textural stability.

cknowledgments

We thank financial support from the Ministry of Science

nd Innovation of Spain/FEDER Program of the EU (ProjectsAT2008-00889/NAN and CSD2009-00013). We also acknowledge

he electron microscopy facilities at the SCCyT of the University ofadiz.

[

[

s profiles of the ceria samples calcined at 500 ◦C.

Appendix A. Supplementary data

Supplementary material related to this article can be found,in the online version, at http://dx.doi.org/10.1016/j.apcata.2014.04.030.

References

[1] A. Trovarelli, Catalysis by Ceria and Related Materials, 1st ed., Imperial CollegePress, London, UK, 2002 (2013, 2nd ed.).

[2] A. Trovarelli, Catal. Rev. Sci. Eng. 38 (4) (1996) 439–520.[3] M.F.L. Johnson, J. Mooi, J. Catal. 103 (2) (1987) 502–505.[4] J. Kaspar, M. Graziani, P. Fornasiero, in: K.A. Gschneidner Jr., L. Eyring (Eds.),

Handbook on the Physics and Chemistry of Rare Earths: The Role of Rare Earthsin Catalysis, Elsevier, Amsterdam, 2000, pp. 159–267.

[5] F. Giordano, A. Trovarelli, C. DeLeitenburg, M. Giona, J. Catal. 193 (2000)273–282.

[6] E. Aneggi, M. Boaro, C. DeLeitenburg, G. Dolcetti, A. Trovarelli, J. Alloys Compd.408–412 (2006) 1096–1102.

[7] Y.-M. Chiang, E.B. Lavik, I. Kosacki, H.L. Tuller, J.Y. Ying, J. Electroceram. 1 (1)(1997) 7–14.

[8] E. Finocchio, M. Daturi, C. Binet, J.C. Lavalley, F. Fally, V. Perrichon, H. Vidal, J.Kaspar, M. Graziani, G. Blanchard, Stud. Surf. Sci. Catal. 121 (1999) 257–262.

[9] R. Di Monte, J. Kaspar, J. Mater. Chem. 15 (2005) 633–648.10] P. Fornasiero, R. Di Monte, G. Raga Rao, J. Kaspar, S. Meriani, A. Trovarelli, M.

Graziani, J. Catal. 151 (1995) 168–177.11] S. Bernal, G. Blanco, M.A. Cauqui, P. Corchado, J.M. Pintado, J.M. Rodríguez-

Izquierdo, H. Vidal, Stud. Surf. Sci. Catal. 116 (1998) 611–618.12] M.P. Yeste, J.C. Hernández-Garrido, D.C. Arias, G. Blanco, J.M. Rodríguez-

Izquierdo, J.M. Pintado, S. Bernal, J.A. Pérez-Omil, J.J. Calvino, J. Mater. Chem. A1 (2013) 4836–4844.

13] H. Li, G. Lu, Y. Wang, Y. Guo, Y. Guo, Catal. Commun. 11 (2010) 946–950.14] J.M. Dean, W. Chun, H.-W. Jen, D.A. Benson, R.W. McCabe, G.W. Graham, Appl.

Catal. A 207 (2001) 379–386.15] Benjaram, M. Reddy, L. Katta, G. Thrimurthulu, Chem. Mater. 22 (2010)

467–475.16] W.Y. Hernández, O.H. Laguna, M.A. Centeno, J.A. Odriozola, J. Solid State Chem.

184 (2011) 3014–3020.

4 alysis

[[[

[

[[[

[[

[

[

[

[29] G. Colón, M. Pijolat, F. Valdivieso, H. Vidal, J. Kaspar, E. Finocchio, M. Daturi, C.Binet, J.C. Lavalley, R.T. Baker, S. Bernal, J. Chem. Soc. Faraday Trans. 94 (1998)

4 J.M. Gatica et al. / Applied Cat

17] B. Choudhury, A. Choudhury, Mater. Chem. Phys. 131 (2012) 666–671.18] Z. Wu, M. Li, S.H. Overbury, J. Catal. 285 (2012) 61–73.19] R. Kostic, S. Askrabic, Z. Dohcevic-Mitrovic, Z.V. Popovic, Appl. Phys. A 90 (2008)

679–683.20] Z. Wu, M. Li, J. Howe, H.M. Meyer, S.H. Overbury, Langmuir 26 (21) (2010)

16595–16606.21] J. Strunk, W.C. Vining, A.T. Bell, J. Phys. Chem. C 115 (2011) 4114–4126.22] C. Sun, H. Li, L. Chen, Energy Environ. Sci. 5 (2012) 8475–8505.

23] A. Migani, G.V. Vassylov, S.T. Bromley, F. Illas, K.M. Neyman, Chem. Commun.

46 (2010) 5936–5938.24] H. Chen, Y.M. Choi, M. Liu, M.C. Lin, Chem. Phys. Chem. 8 (2007) 849–855.25] J. Bergmann, P. Friedel, R. Kleeberg, Commission of Powder Diffraction (Inter-

national Union of Crystallography) Newsletter 20 (1998) 5–8.

[[

A: General 479 (2014) 35–44

26] S. Bernal, J.J. Calvino, G. Cifredo, J.M. Rodríguez-Izquierdo, J. Phys. Chem. B 99(30) (1995) 11794–11796.

27] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquérol, T.Siemieniewska, Pure Appl. Chem. 57 (4) (1985) 603–619.

28] Q. Yu, X. Wu, C. Tang, L. Qi, B. Liu, F. Gao, K. Sun, L. Dong, Y. Chen, J. ColloidInterface Sci. 354 (2011) 341–352.

3717–3726.30] E. Aneggi, J. Llorca, M. Boaro, A. Trovarelli, J. Catal. 234 (1) (2005) 88–95.31] D.R.G. Mitchell, Microsc. Res. Tech. 71 (2008) 588–593.