Journal of Colloid and Interface Science 343 (2010) 256–262

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science

www.elsevier .com/locate / jc is

Oxide surface modification: Synthesis and characterizationof zirconia-coated alumina

Cleocir José Dalmaschio a,*, Valmor R. Mastelaro b, Pedro Nascente c, Jefferson Bettini d

José Luiz Zotin e, Elson Longo a, Edson Roberto Leite a

a LIEC, Department of Chemistry, Federal University of São Carlos, C. Postal 676, 13565-905 São Carlos, SP, Brazilb Institute of Physics of São Carlos, University of São Paulo, C. Postal 369, 13560-970 São Carlos, SP, Brazilc Department of Materials Engineering, Federal University of São Carlos, 13565-905 São Carlos, SP, Brazild Brazilian Synchrotron Light Laboratory, C. Postal 6192, 13084-971 Campinas, SP, Brazile PETROBRAS S.A./CENPES, R&D Center, Av. Horácio Macedo, 950 Cidade Universitária, Ilha do Fundão, 21941-915 Rio de Janeiro, RJ, Brazil

a r t i c l e i n f o

Article history:Received 17 September 2009Accepted 10 November 2009Available online 16 November 2009

Keywords:SurfaceCoatingDispersionCatalysisCeramics

0021-9797/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.jcis.2009.11.027

* Corresponding author. Fax: +55 1633518412.E-mail addresses: cleocir@ymail.com, derl@power.

a b s t r a c t

Four aluminas were used as supports for impregnation with a zirconium oxide with the aim to achieve acoating, without phase separation, between support and modifier. The supports were impregnated withdifferent concentrations of zirconium aqueous resin, obtained through the polymeric precursor method.After impregnation the samples were calcined and then characterized by XRD, which led to identificationof crystalline zirconia in different concentrations from each support used. Using a simple geometricmodel the maximum amount of surface modifier oxide required for the complete coating of a supportwith a layer of unit cells was estimated. According to this estimate, only the support should be identifiedbelow the limit proposed and crystalline zirconium oxide should be identified above this limit when acomplete coating is reached. The results obtained from XRD agree with the estimated values and to con-firm the coating, the samples were also characterized by EDS/STEM, HRTEM, XPS, and XAS. The resultsshowed that the zirconium oxide on the surface of alumina support reached the coating in the limitof 15 Zr nm�2, without the formation of the ZrO2 phase.

� 2009 Elsevier Inc. All rights reserved.

1. Introduction

Oxide coatings have been attracting a great deal of attentiondue to their applications as catalysts and as substrates for catalyz-ers in a wide variety of reactions [1–3]. Zirconium oxide presentsinteresting catalytic properties which are generally attributed tothe simultaneous presence of both acidic and basic sites at itssurface [4,5]. For example, it has been reported to selectively pro-duce 1-butene during 2-butanol dehydration [6], in contrast withacidic oxides which are selective for the 2-butenes or produce anequilibrated mixture of the olefins in this reaction. It also displaysremarkable selectivity in the hydrogenation of benzoic acid tobenzaldehyde [7]. It has been proven to have activity in the hydro-genation of conjugated dienes [8], in methanol synthesis [9], andas a photocatalyst [10]. When sulfate ion is present at its surface,zirconia is known to behave as a solid superacid [11,12]. As a cat-alytic support, it has been proven to be advantageous in severalreactions, such as hydrodesulfurization [13], complete oxidationof propane [14], Fischer–Tropsch synthesis [15], and methanolsynthesis [16].

ll rights reserved.

ufscar.br (C.J. Dalmaschio).

Some studies concerning the preparation and surface propertiesof ZrO2/SiO2 and ZrO2/Al2O3 binary oxides have been reported[17–21]. It has been shown that the standard impregnation of silicaor alumina with an aqueous solution of zirconium nitrate led to theformation of large ZrO2 particles which left a considerable part of thesupport uncoated [21,22]. One alternate method for avoiding theformation of large ZrO2 particles is the control of zirconium alkoxidehydrolysis by addition of acetylacetone molecules bound to the zir-conium atoms [18,23]. The reactivity of zirconium alkoxide towardhydrolysis is substantially reduced due to the formation of a com-plex compound with acetylacetone. The polymeric precursor meth-od, in which the metallic ion is complexed in a previous dispersionstep, can be employed to avoid the hydrolysis stage, thus achievinga better distribution of the modifier. The fact that zirconia is a rela-tively expensive material which has limited textural stability [21]has stimulated research on the properties of zirconia when sup-ported on other oxides, such as alumina [17,18] and silica [17,20]which do not suffer from the same disadvantages. Contributions thatlead to the obtainment of fine coating layers may broaden the pros-pects in the area of oxide surface modifications.

Aiming to achieve coating without phase separation betweensupport and modifier, a modified sol–gel route was used to coatalumina with zirconium oxide in different concentrations. Simple

C.J. Dalmaschio et al. / Journal of Colloid and Interface Science 343 (2010) 256–262 257

calculations based on a geometric model were used to estimate theamount of material required to completely coat a solid surface andto determine the critical concentration at which phase separationoccurs between the support and the modifier material.

2. Materials and methods

2.1. Preparation of zirconia-coated alumina

To synthesize the liquid precursor containing Zr4+ ions by thePechini method [24], a solution of 80% in mass of zirconium butox-ide in butanol (Aldrich) as the zirconium precursor source wasused. The preparation involved the following steps: first, 28.64 gof the zirconium butoxide solution was added to a beaker contain-ing 150 mL of deionized water. This solution was kept under con-stant stirring at a temperature of 80 �C for 2 h to obtain a Zr(OH)4

suspension. Citric acid anhydride (Synth) was then added to com-plex the metallic ion, establishing a citric acid:Zr4+ metallic ionmolar ratio of 3:1. The solution was kept under stirring at a tem-perature of 80 �C for 48 h to trigger the complexation process, afterwhich 51.72 g of ethyl glycol was added and the solution kept at80 �C to produce the esterification and polymerization reaction.Gravimetric analyses were carried out to determine the zirconiumconcentration in the resin.

A quantity of resin that would result in the desired ZrO2:Al2O3

mass ratio was added to a beaker containing 60 mL of deionizedwater to coat the alumina. This system, with viscosity similar tothat of water, was stirred for 15 min and after that 0.6 g of aluminawas added. The resulting suspension was kept under constant stir-ring for 3 h at room temperature. It was then transferred to around-bottomed flask, which was placed in a rotaevaporator toeliminate the water. The material obtained after evaporation ofthe water was calcined at 450 �C for 2 h with heating and coolingramp ups of 10 �C min�1. The following aluminas were used assupport: Alfa Aesar particulate with a surface area of 7.8 m2 g�1

(particle size estimate from BET data: 0.2 lm); Degussa aeroxideparticulate with a surface area of 62 m2 g�1 (particle size estimatefrom BET data: 26 nm); Degussa aeroxide particulate with a sur-face area of 111 m2 g�1 (particle size estimate from BET data:15 nm); and porous c-Al2O3 with a surface area of 190 m2 g�1, sup-plied by Petrobras.

2.2. Characterization

Surface areas were determined by nitrogen adsorption at 77 K,using a Micromeritics ASAP 2000 particle size analyzer. Beforethe measurements were taken, the samples were dried at 373 Kand degassed to a degassing pressure of less than 1 Pa. The surfaceareas were evaluated by the standard BET procedure. Zeta potentialmeasurements were carried out with a Brookhaven zetaplus ana-lyzer. For this analysis the ionic strength of a dilute suspensionwas maintained at 10�3 M using KCl. The Al2O3 sample was ultraso-nicated for 15 min before the measurements were taken. For thepure resin 40 g of resin dispersed in a 1 L of KCl solution at 10�3 Mwas used. The pH was adjusted by KOH and HCl 0.1 M solutions.

X-ray diffraction (XRD) analyses were carried out with a RigakuDmax 2500-PC X-ray diffractometer. This device emits X-raysthrough a copper anode irradiated by a 150 mA electron currentaccelerated by a 40 kV potential difference. The emitted radiationwas monochromatized with a nickel filter, resulting in radiationwith a predominant wavelength of 1.5456 Å corresponding to theCu Ka emission line. The routine conditions used in these analyseswere scanning at 2h between 20� and 75�, exposure time of 1 s, andan angular pass of 0.02�.

To prepare the samples for transmission electron microscopy(TEM), a small amount of the powder was dispersed in anhydrous

isopropanol using an ultrasonound. From this solution, a smallfraction was dripped onto a copper grid covered by a carbon film.The images presenting zirconium and aluminum concentrationmaps were analyzed by energy dispersive X-ray of the radiationlines emitted by these elements. To this end, we used a scanningtransmission electron microscope (STEM VG-603 with Cs correc-tion) with a 300 kV differential potential. The high resolutionimages were obtained with a transmission electron microscope(JEOL – 3010), using electron acceleration potential of 300 kV.

X-ray photoelectron spectroscopy (XPS) analyses were madeusing a Kratos XSAM HS surface spectromicroscope with energy res-olution better than 0.1 eV. The analyses were carried out in an ultra-high vacuum atmosphere, using magnesium Ka radiation as theexcitation source, which was obtained from a double Mg/Al anodewith energy of 1253.6 eV and 65 W power, provided by the 13 kVvoltage and 5 mA emission. A reference bonding energy of284.8 eV was used for the photoelectric line of the C 1 s associatedwith C–C and/or C–H. Peak adjustments were made using the pro-gram supplied by the manufacturer of the device, with Gaussianand Gaussian/Lorentzian curves (for the Zr 3d doublets), backgroundremoval by the Shirley method, and the least-square routine.

The EXAFS spectra were collected at the Zr K-edge using aSi(220) monochromator at the D04B-XAFS1 beam line of the Bra-zilian Synchrotron Light Laboratory (LNLS). The samples weredeposited on a polymer membrane and the incident and transmit-ted X-ray beams were detected by ionization chambers filled withAr gas. The energy range of the Zr K-edge EXAFS spectra was17,890–19,000 eV and was calibrated using a Zr foil. Data werecollected at room temperature using an integration time of 2 sand energy steps of 3 eV in a range of 17,890–17,980, 1 eV in arange of 17,980–18,050, and 2 eV in a range of 18,050–19,000.Three EXAFS spectra were recorded for each sample and the aver-age spectrum was used to analyze the data. The EXAFS data wereanalyzed with the MAX (Multiplatform Applications for XAFS)package according to standard international procedures for EXAFSspectrum analysis [25].

3. Results and discussion

The surface modification was carried out on four aluminas withsurface areas between 7.8 and 190 m2 g�1, upon which differentzirconium oxide contents were deposited. A resin obtained bythe polymer precursor method [24], which contained zirconiumcations in its chain, was used to disperse the zirconium. The advan-tage of this method compared to the traditional sol–gel one is thatthe solvent used is water, which precludes the problems of humid-ity control. When the traditional sol–gel route is used to dispersezirconium, the hydrolysis process requires an excellent control toachieve the best dispersion. This is a difficult step according tosome authors who reported the use of a complexant agent. Besidesthat, the attraction between the modifier and the support is notnormally evaluated in the organic solvent and the attraction is fun-damental for a good dispersion to occur. Then the first step per-formed was to analyze the support and the resin through zetapotential. The results shown in Fig. 1a indicate attractive interac-tion between alumina and resin for pH smaller than 5, where theopposite charge in the resin and in the alumina surface should im-prove the dispersion. Likewise, a better coating could be achievedusing pH � 3 in the resin dispersion. In this way, pH around 3was used in the dispersion process as described in the experimen-tal procedure. Fig. 1b represents the alumina particle coated withpolymer and after the calcination process when the coating withzirconium oxide is formed on the particle surface.

After dispersion, the material was calcined and then character-ized by XRD to evaluate the presence of crystalline zirconia on thesamples. The XRD results shown in Fig. 2 indicate that higher

2 3 4 5 6 7 8 9 10-40

-20

0

20

40

60 Al2O3 Resin

Zet

a po

tent

ial /

(m

V)

pH

a

b

Fig. 1. (a) Zeta potential analyzer of the Degussa Aeroxide alumina (not coated) andpure resin and (b) representation of coating process using pH lower than 5.

258 C.J. Dalmaschio et al. / Journal of Colloid and Interface Science 343 (2010) 256–262

surface area requires higher modifier quantities to form the secondphase, which can be expected once the complete coating is a sur-face area dependency. According to the data obtained, this functionindicated a rate of 15 Zr nm�2. Previous works involving surfacemodifications of alumina by zirconium detected the formation ofcrystalline ZrO2 at significantly lower concentrations than the val-ues obtained here. Márquez-Alvarez et al. [20], who used zirco-nium tetra-t-butoxide to impregnate zirconium on alumina,reported the formation of cubic zirconium crystallites on the alu-mina at concentrations starting at 4.9 Zr nm�2. Natio and Tanimoto[26] dispersed zirconium oxide on alumina, using a solution of zir-conium propoxide in hexane and, based on X-ray photoelectronspectroscopy (XPS) analyses, reported the formation of zirconiumcrystals at concentrations starting at 5.4 Zr nm�2. Using zirconiumpropoxide in an n-propanol solution, Damyanova et al. [17] dis-persed zirconium on alumina and identified the emergence of thecrystalline phase of zirconium oxide at concentrations starting at4.8 Zr nm�2. Kytökivi et al. [27] used successive alumina surfacesaturation reactions with ZrCl4 and H2O vapors and reported theformation of crystalline phase at concentrations starting at 4.2 Zrnm�2. Faro et al. [18] supported zirconia on alumina through mul-tiple impregnation steps, using a solution of zirconium acetylace-tonate in benzene. Using XRD, they identified a weak peakrelated to the formation of tetragonal zirconia phase in a samplecontaining a concentration of 5.5 Zr nm�2. The identification ofthe crystalline phase in the above-cited values may be related tothe dispersion method employed. Methods that use stages ofhydrolysis in the dispersion process do not allow for adequate con-trol, resulting in the formation of crystallites of the modifier mate-rial before the support is completely coated. The method weadopted, using the polymeric precursor, allowed for better disper-sion of the modifier material over the support, enabling us to reachthe limit of surface saturation of 15 Zr nm�2, above which phaseseparation between the support and the modifier begins. The value

of 15 Zr nm�2 may appear exceedingly high when compared withthe above-cited studies, but is understandable in view of the num-ber of monomeric units deposited. Tetragonal zirconium oxide has2 Zr per unit cell and a base area of 0.13 nm2, a rate 15 Zr nm�2,which is the value found in the XRD.

To offer a simplified description of the coating of a surface mod-ifier oxide layer, we propose an idealized structure in which thecoating is composed of a layer of unitary cells. Since the smallestunit in the solid is a unit cell, this cell was considered as the mono-mer in the coating. We also assume that the modifier is linked tothe support, thereby providing strong interaction between themand favoring a high dispersion of the modifier. In an idealized pro-cess of surface modification, the perfect coating condition prior tobeginning of the phase separation is attained through two-dimen-sional stacking of unitary cells. The process basically occurs inthree stages, which are represented schematically in Fig. 3a–c. Be-fore the complete coating depicted in Fig. 3a, the quantity of mod-ifier material is insufficient for the formation of its crystallinephase due to the high dispersion of the modifier. In the conditionof ultimate surface saturation shown in Fig. 3b, there is a two-dimensional stacking of unitary cells. Any quantity depositedabove this limit will cause phase separation between the coatingand the modifier material, a condition illustrated in Fig. 3c. This oc-curs because a solid composition of atoms arranged in a three-dimensional periodic pattern leads to the formation of the crystal-line lattice [28].

Assuming that the modifier monomer has much smaller dimen-sions than the particles to be coated, so that the support surfacecan be considered approximately flat compared with the unit tobe deposited, as indicated in Fig. 3a, it is possible to estimate thequantity of material required to completely coat the support. Tothis end, the mass of a unitary cell (l) of the modifier material asa function of the number of minimum formulas (Z) of the unit cell,the molar mass of the material (M), and the Avogadro number (NA)are determined. To coat a support having a given specific surfacearea a number of unitary cells is required, which can be given bythe ratio of the area of the support (A) to the area of the base ofa unitary cell (C). The product of the number of unitary cellsneeded for the complete coating by the cell mass (l) yields themass (m), as expressed in

m ¼ AC� Z �M

NAð1Þ

which is the mass necessary to completely coat the surface of 1 g ofsupport with a monolayer of unitary cells ideally arranged two-dimensionally.

The relation between the mass needed to coat the support,which is obtained by Eq. (1), and the surface area of the supportis linear, since the (Z�M/NA�C) terms are constant for a given mate-rial that is coating the support, so that it is possible to substitutethese terms with K, a constant that depends only on the materialand the phase that comprises the coating. With this substitutionto simplify Eq. (1), and rewriting this equation to determine themass percentage of the modifier material needed to coat the sup-port completely, one defines Eq. (2),

s ¼ K � A1þ K � A

� �� 100 ð2Þ

which establishes the ratio of the modifier material mass to the sys-tem’s total mass (support plus modifier material). Therefore, in Eq.(2), the term s represents the mass percentage needed to completecoating the support with a monolayer of the surface modifying unit.

Based on Eq. (2), it is possible to build the s � A diagrams pre-sented in Fig. 3d, which shows the curve for zirconium oxide coat-ing a support. The solid line represents the relation between the

20 30 40 50 60 70

10% ZrO2

20% ZrO2

30% ZrO2

35% ZrO2

40% ZrO2

Inte

nsi

ty (

a.u

.)

2θ / (degree)

ZrO2

45% ZrO2

Al2O

3

20 30 40 50 60 70

10% ZrO2

15% ZrO2

25% ZrO2

20% ZrO2

30% ZrO2

Inte

nsi

ty (

a.u

.)

2θ / (degree)

ZrO2

Al2O

3

20 30 40 50 60 70

2%ZrO2

Inte

nsi

ty (

u.a

.)

2θ / (degree)

Al2O

3

10%ZrO2

ZrO2

5%ZrO2

a b

dc

Fig. 2. X-ray diffractograms of four alumina covered with zirconium oxide at different concentrations. (a) Support surface area 190 m2 g�1. (b) Support surface area111 m2 g�1. (c) Support surface area 62 m2 g�1. (d) Support surface area 8 m2 g�1.

0 50 100 150 200 2500

10

20

30

40

50

Single phase

Two phase

Mas

s P

erce

nt

of

ZrO

2/(%

)

Surface Area Al2O

3/(m2 g-1)

unit cell

support

)c()b((a)

(d)

Fig. 3. Idealized representation of the coating process and diagrams built from the proposed. (a) Coating stage in which the coating mass is insufficient for the formation of amonolayer of unitary cells. (b) Ultimate stage of surface saturation without formation of the coating’s crystalline lattice, ideal two-dimensional stacking of unitary cells on thesurface of a support. (c) Stage of crystalline lattice formation of the modifier material in which stacking of unitary cells begins in the three axes. (d) Diagram for a coatingcomposed of tetragonal zirconium (ZrO2) oxide and experimental results obtained by XRD for the coating of aluminas with surface areas of 190, 111, 62, and 7.8 m2 g�1 coatedwith different zirconium oxide contents as indicated in Fig. 2.

C.J. Dalmaschio et al. / Journal of Colloid and Interface Science 343 (2010) 256–262 259

Fig. 4. EDS/STEM image of an alumina sample with a surface area of 111 m2 g�1 containing 10% in mass of ZrO2. (a) Map of concentrations of the element aluminum in theanalyzed region of the sample. (b) Map of concentrations of the element zirconium in the analyzed region of the sample. (c) Superposition of the concentration maps of theelements aluminum and zirconium.

Fig. 5. HRTEM images of coated alumina samples with a surface area of 111 m2 g�1.(a) Sample containing 20% in mass of zirconium oxide. (b) Sample containing 30% inmass of zirconium oxide.

260 C.J. Dalmaschio et al. / Journal of Colloid and Interface Science 343 (2010) 256–262

surface area to be coated and the quantity of coating required, inmass percentage of coating/total mass, for the formation of amonolayer of unitary cells on the support. According to the devel-opment depicted in the diagram of Fig. 3d, it can be inferred thatbelow the respective solid lines only the crystalline phase corre-sponding to the support should be identified. Above the respectivesurface saturation lines, the quantity of modifier material is suffi-cient to begin three-dimensional stacking of the modifying unit,which results in the emergence of the crystalline lattice of the sur-face modifier material. The XRD data presented in Fig. 2 show anexcellent correlation with the estimate proposed using the mono-layer, as can be seen in Fig. 3d, where the samples containing lowerzirconium contents than those foreseen by Eq. (2) presented onlythe crystalline phase of the support (alumina), which is repre-sented by the squares in the graph. In the samples containing apercentage of zirconium exceeding the value limited by the solidline, crystalline phases corresponding to the support and the mod-ifier material were identified, which are represented as stars in theFig. 3d.

Scanning transmission electron microscopy (STEM) with map-ping by energy dispersive X-ray of the zirconium and aluminumelements was used to confirm the coating and to obtain more de-tailed information about the coating. Fig. 4a–c present the maps ofthe concentrations of the aluminum and zirconium elements andtheir superposition in the sample containing 10% in mass of ZrO2

deposited on alumina with a surface area of 111 m2 g�1. Fig. 4cindicates that the dispersion method allowed for the depositionof zirconium on the alumina, without detectable zirconium-richregions where alumina is absent. Fig. 4c also shows regions ofthe support which are not coated with zirconium, identified by ar-rows, indicating incomplete coating of the support. This result isentirely congruous with the proposed, since the complete coatingof this support would only be achieved with a concentration ofmodifier above 25% in mass.

The two-dimensional arrangement and the strong interactionbetween support and coating can lead to a process of orientationof the first layer of unitary cells aligned with the crystallographicstructure of the support. For the alumina–zirconia system, this ori-entation was confirmed through high resolution transmission elec-tron microscopy (HRTEM) analyses. Fig. 5a, which shows a coatingcontent slightly below the value that results in the complete forma-tion of the modifier monolayer, indicates a monophasic material(alumina), with no vestige of amorphous phase. This finding sug-gests the occurrence of growth oriented by the support in the firstlayer of unitary cells. The image presented in Fig. 5a is compatiblewith the proposed orientation on the support, suggesting a two-dimensional arrangement of the modifier material, without detect-able zirconium agglomerates. As the image in Fig. 5b indicates, two

distinct lattice parameters are visible above the monolayer forma-tion percentage, one of which can be attributed to alumina andthe other to tetragonal zirconia, confirming the existence of thetwo crystalline phases detected by XRD in this sample. Thus, forconcentrations exceeding that foreseen by Eq. (2), at which the for-mation of three-dimensional unitary cells begins, the influence ofthe support is less intense, enabling the zirconium oxide to becomestructured in its own crystalline lattice. These results are consonantwith the STEM observations, since they indicate high modifier dis-persion. At modifier concentrations above the saturation line, if themodifier had formed agglomerates upon the support, the micro-scopic images would have revealed their existence.

The strong interaction was evaluated by X-ray photoelectronspectroscopy (XPS) analyses, which were carried out on the alu-mina samples with an area of 111 m2 g�1 coated with 20% of zirco-nia and the zirconia synthesized from the resin in 450 �C. The XPSspectra in Fig. 6 revealed strong support-coating interaction, witha displacement of the bonding energies of the zirconium’s 3d5/2electrons, indicating the existence of two distinct chemical environ-ments for the zirconium deposited on alumina compared with thezirconium oxide. One of these environments is related to the bond-ing energy of 182 eV, which is the value observed for these elec-trons in bulk zirconium oxide [17], while the other shows abonding energy of 183 eV, a value that is associated with zirconiumions bound to more electronegative species [29], which in this caseare Zr–O–Al-type bonds formed between the support and the mod-ifier material in response to the incorporation of the coating. TheXPS results confirm the strong interaction proposed by HRTEM,which lead the first layer assembling in the support structure.

The short-range order structure of the material was evaluated byX-ray absorption spectroscopy (XAS), which provided information

Fig. 6. XPS spectra in the energy range of the 3d orbital of Zr4+. (a) Alumina coatedwith 20% in mass of zirconium oxide and (b) zirconium oxide (ZrO2).

4 6 8 10 12 140,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

k χ (

k)

k / (A)-1

Al2O

3- 10% ZrO

2

Al2O

3- 20% ZrO

2

Al2O

3- 30% ZrO

2

ZrO2 Tetragonal

o

R / (A)o

0 1 2 3 4 5 60

5

10

15

20

25 Al

2O

3- 10% ZrO

2

Al2O

3- 20% ZrO

2

Al2O

3- 30% ZrO

2

ZrO2 Tetragonal

Zr-O

Zr-Zr

Zr-(Zr,Al)

Zr-O

Fo

uri

er t

ran

sfo

rm m

agn

itu

de

(u

.a.)

a

b

Fig. 7. EXAFS spectra at the Zr K-edge and their respective Fourier transform. (a)EXAFS spectra obtained from the alumina samples with surface area of 111 m2 g�1

containing 10%, 20%, and 30% in mass of ZrO2 and the spectrum of the tetragonalzirconium used as a reference compound and (b) Fourier transform curves of theEXAFS spectra presented in panel a.

C.J. Dalmaschio et al. / Journal of Colloid and Interface Science 343 (2010) 256–262 261

about the local structure around zirconium atoms on the sampleswith a surface area of 111 m2 g�1 coated with 10%, 20%, and 30%(m/m) of modifier. Extended X-ray absorption fine structure (EXAFS)studies revealed little resemblance among the coated alumina sam-ples and the tetragonal zirconia used as reference, indicated in Fig. 7.Applying the Fourier transform (FT) to the EXAFS spectra (Fig. 7a)yielded a partial radial distribution function that allowed us to asso-ciate the distance of the neighbors in relation to Zr atoms.

The FT results shown in Fig. 7b indicate that the first peak of thethree coated samples, related to the first Zr–O coordination shell,was similar in intensity and position. Compared with tetragonal zir-conia, the coated samples presented a smaller mean Zr–O distance. Asimilar effect was observed previously in a nanocomposite sample ofaluminum and zirconium oxides and is likely associated with theformation of Zr–O–Al bonds [30]. The FT curves also indicated thatthe most significant differences between the three analyzed sampleswere in the region of longer distances of the absorbing atom between2.5 and 4 Å. This region is related to the Zr–Zr distances in the tetrag-onal ZrO2 sample and the Zr–(Zr,Al) distances in the alumina sam-ples coated with zirconium oxide. In the samples containing 10%and 20% of zirconium, the position of the peak corresponding tothe Zr–(Zr,Al) distances occurred at smaller distances than that ofthe peak of the tetragonal zirconium oxide sample. In the samplecoated with 30% (m/m) of zirconium, the position of the second peakcoincided with that of the tetragonal zirconium sample, indicatingthe formation of the crystalline phase of tetragonal zirconium, whichwas congruent with the XRD results. In the samples in which zirconiamodified the surface of Al2O3, the intensity of the FT peak in the re-gion of 2.5 and 4 Å was significantly lower than that of the tetragonalZrO2 sample. This lower intensity has also been observed in nano-crystalline ZrO2 samples and is attributed to several factors, suchas the presence of amorphous material, a mixture of crystallinephases, and a high ratio of Zr on the particle surfaces [25]. As ob-served in the HRTEM images, the formation of amorphous materialwas not detected in the analyzed samples. Based on our XRD analy-ses and on the lattice parameters observed by HRTEM, no more thanone zirconia crystalline phase was detected. Therefore, the lowintensity of the FT second coordination sphere of the FT curves ob-served in our samples should be correlated with the high dispersionof zirconia on the support. Thus, it is this high dispersion that allowsthe modifier deposition on the support achieving the coating with-out the occurrence of phase separation.

4. Summary

In conclusion, this paper reports a synthesis process to achieveoxide coating using a sol–gel method. The attraction between resinand support allows a better dispersion resulting in later phase sepa-ration between support and surface modifier driven to improve thecoating. The XRD data indicated that the limit for alumina coatingwith ZrO2 is around 15 Zr nm�2; this value is very close to the esti-mate from the unit cell and all XRD data presented an excellent cor-relation with a simple model used to determine the limit. EDS/STEMmaps confirm the deposition of the modifier on the support surfaceand allow observation of the dispersion of the modifier. When themodifier is linked to the support the strong interaction between sup-port and modifier leads to a process of orientation of the first layer ofunitary cells aligned with the crystallographic structure of the sup-port, which could observed by HRTEM. Using XPS analyses the inter-action between zirconium and alumina with dislocation of the bondenergy 3d of the zirconium was confirmed. This interaction in shortrange could be observed in XAS spectra where the FT results of thefirst Zr–O coordination shell presented a smaller mean Zr–O dis-tance, which is associated with the formation of Zr–O–Al bonds.The low intensity of the signal of the Zr–(Zr,Al) coordination spherein the XAS spectra was related to the high dispersion of the surface

262 C.J. Dalmaschio et al. / Journal of Colloid and Interface Science 343 (2010) 256–262

modifier. Thus, the overall results lead to confirm the coating of sup-port with a limit of deposition of zirconium of 15 Zr nm�2.

Acknowledgments

The financial support of FAPESP and CNPq (both Brazilian agen-cies) and of Petrobras is gratefully acknowledged. The HRTEM andXAS facilities were provided by the LNLS (Brazilian Laboratory ofSynchrotron Light), Campinas, SP, Brazil.

References

[1] Y. Chen, L.F. Zhang, Catal. Lett. 12 (1992) 51.[2] Z. Wang, H.Q. Wan, B. Liu, X. Zhao, X.W. Li, H.Y. Zhu, X. Xu, F.Y. Ji, K.Q. Sun, L.

Dong, Y. Chen, J. Colloid Interface Sci. 320 (2008) 520.[3] S.C. Tsang, V. Caps, I. Paraskevas, D. Chadwick, D. Thompsett, Angew. Chem.,

Int. Ed. 43 (2004) 5645.[4] T. Yamaguchi, Catal. Today 20 (1994) 199.[5] K. Tanabe, Mater. Chem. Phys. 13 (1985) 347.[6] K. Tanabe, T. Yamaguchi, Catal. Today 20 (1994) 185.[7] T. Yokoyama, T. Setoyama, N. Fujita, M. Nakajima, T. Maki, K. Fujii, Appl. Catal.,

A 88 (1992) 149.[8] Y. Nakano, T. Yamaguchi, K. Tanabe, J. Catal. 80 (1983) 307.[9] R.G. Silver, C.J. Hou, J.G. Ekerdt, J. Catal. 118 (1989) 400.

[10] J.A. Navio, M.C. Hidalgo, G. Colon, S.G. Botta, M.I. Litter, Langmuir 17 (2001) 202.[11] S. Chokkaram, B.H. Davis, J. Mol. Catal. A: Chem. 118 (1997) 89.

[12] T. Yamaguchi, T. Morita, T.M. Salama, K. Tanabe, Catal. Lett. 4 (1990) 1.[13] M. Vrinat, M. Breysse, C. Geantet, J. Ramirez, F. Massoth, Catal. Lett. 26 (1994) 25.[14] H. Fujii, N. Mizuno, M. Misono, Chem. Lett. 11 (1987) 2147.[15] L. Bruce, J.F. Mathews, Appl. Catal. 4 (1982) 353.[16] I.A. Fisher, A.T. Bell, J. Catal. 178 (1998) 153.[17] S. Damyanova, P. Grange, B. Delmon, J. Catal. 168 (1997) 421.[18] A.C. Faro, K.R. Souza, V.L.D.L. Camorim, M.B. Cardoso, Phys. Chem. Chem. Phys.

5 (2003) 1932.[19] A.C. Faro, K.R. Souza, J.G. Eon, A.A. Leitao, A.B. Rocha, R.B. Capaz, Phys. Chem.

Chem. Phys. 5 (2003) 3811.[20] C. Márquez-Alvarez, J.L.G. Fierro, A. Guerreroruiz, I. Rodriguezramos, J. Colloid

Interface Sci. 159 (1993) 454.[21] P.D.L. Mercera, J.G. Vanommen, E.B.M. Doesburg, A.J. Burggraaf, J.R.H. Ross,

Appl. Catal. 78 (1991) 79.[22] A.C.Q.M. Meijers, A.M. Dejong, L.M.P. Vangruijthuijsen, J.W. Niemantsverdriet,

Appl. Catal. 70 (1991) 53.[23] Z. Dang, B.G. Anderson, Y. Amenomiya, B.A. Morrow, J. Phys. Chem. 99 (1995)

14437.[24] E.R. Leite, I.T. Weber, E. Longo, J.A. Varela, Adv. Mater. 12 (2000) 965.[25] A. Michalowicz, J. Phys. IV (7) (1997) 235.[26] S. Naito, M. Tanimoto, J. Catal. 154 (1995) 306.[27] A. Kytökivi, E.L. Lakomaa, A. Root, H. Osterholm, J.P. Jacobs, H.H. Brongersma,

Langmuir 13 (1997) 2717.[28] D.B. Cullity, Elements of X-Ray Diffraction, third ed., Prentice Hall, Upper

Saddle River, 2001.[29] M.K. Younes, A. Ghorbel, A. Rives, R. Hubaut, J. Sol–Gel Sci. Technol. 26 (2003)

677.[30] X. Yang, M. Dubiel, H. Hofmeister, W. Riehemann, XAFS13: 13, International

Conference on X-ray Absorption Fine Structure, Stanford, CA, vol. 882, 2007, p. 563.