Characterization of Structural and Surface Properties of Nanocrystalline TiO2−CeO2 Mixed Oxides by...

Characterization of Structural and Surface Properties of Nanocrystalline TiO2-CeO2 MixedOxides by XRD, XPS, TPR, and TPD

Shingo Watanabe, Xiaoliang Ma, and Chunshan Song*Clean Fuels and Catalysis Program, EMS Energy Institute, and Department of Energy and MineralEngineering, The PennsylVania State UniVersity, 209 Academic Project Building, UniVersity Park,PennsylVania 16802

ReceiVed: December 14, 2008; ReVised Manuscript ReceiVed: May 6, 2009

This work focuses on characterizing the structural and surface properties of TiXCe1-XO2 (TiO2-CeO2) mixedoxides using XRD, XPS, BET, H2-TPR, and NH3-TPD techniques. The TiXCe1-XO2 mixed oxides synthesizedby the urea coprecipitation method showed unimodal nanoporous structure with pore sizes increasing from3.7 nm for X ) 0.9 to 5.3 nm for X ) 0.1. Concomitant with their higher surface area and pore volume, themixed oxides were nanocrystalline, about 4.0 nm in crystallite size when X ) 0.9, and 4.8-5.4 nm when X) 0.1- 0.3, which are significantly smaller than TiO2 and CeO2 single oxides prepared by the same method(8.1 to 8.4 nm). A dominant anatase phase was detected by XRD when X was 0.9 or higher while a cubicfluorite phase was dominant when X was 0.3 or lower. Lattice parameters were changed by incorporating Ceinto TiO2, and Ti into CeO2, respectively. This change indicates distortion of structure and was attributed toreduction of Ti4+ to Ti3+, and Ce4+ to Ce3+. XPS (Ce 3d, Ti 2p, O 1s) and H2-TPR revealed that the oxidationstate of surface cations decreased, and oxygen deficiency of the surface was significantly enhanced byintroducing Ce into TiO2, and Ti into CeO2. The structural and surface modification by introducing Ce intoTiO2 increased the reducibility of mixed oxides in H2-TPR. NH3-TPD showed that increasing Ti content inTiXCe1-XO2 enhanced surface acidity. Furthermore, H2O and N2 formation from NH3 was detected by massspectrometry, which was attributed to the oxidation activity of the TiXCe1-XO2 mixed oxides. The highestNH3 oxidation activity was observed when X ) 0.9. The present study clearly established that the structural(crystal phase, crystal size, nanoporosity, pore size) and surface properties (reducibility, oxygen deficiency,acidity, oxidation activity) of the TiXCe1-XO2 mixed oxides can be tailored by controlling their compositionby the urea coprecipitation procedure.

1. Introduction

Titania (TiO2) and ceria (CeO2) have been extensively studiedfor various applications.1-4 TiO2 is well-known for photoca-talysis5 and environmental catalysis.4,6 CeO2 is known as anexcellent support for base metals and noble metals in a varietyof catalytic processes because of its high oxygen storagecapacity, redox properties, and metal-support interactions.7-9

In addition to the excellent catalytic properties, CeO2-basedmaterials have also been developed in recent years for theremoval of SOx from flue gas, and H2S from natural gas andcoal-derived gas.10 CeO2 has been evaluated as a high temper-ature regenerable desulfurization sorbent, which yields elementalsulfur upon regeneration. In addition to pure TiO2 and CeO2,modified TiO2-based and CeO2-based mixed oxides haveattracted much interest for catalytic applications because of theirimproved properties. Several studies on TiO2-CeO2 mixedoxides have shown improvements regarding redox,11,12 textural,and structural properties.13 These improvements have beenattributed to modification of the structure, electronic property,or thermal stability by doping.

CeO2- and TiO2-based materials may also be used for theremoval of thiophenic sulfur compounds present in the hydro-carbon fuels by selective adsorption.14-18 Our recent studyrevealed that CeO2 mixed with TiO2 oxide show superiorperformance than either oxide alone for adsorptive desulfur-

ization of liquid fuels such as jet fuels, and TiO2-CeO2 mixedoxides synthesized by the urea precipitation method are potentialoxidatively regenerable adsorbent materials.19 However, thereare very few studies on either the structure or surface propertiesof TiO2-CeO2 mixed oxides. Little has been published on bothstructure and surface chemistry of TiO2-CeO2 mixed oxidessynthesized by the urea precipitation method.

The present work focuses on characterization of TiO2-CeO2

(TiXCe1-XO2) mixed oxides synthesized by the urea precipitationmethod, with emphasis on the effect of mixing TiO2 and CeO2

in the synthesis stage on the structural and surface propertiesof TiO2-CeO2 mixed oxides. Nitrogen adsorption-desorptionand X-ray diffraction (XRD) were conducted to characterizethe textural and structural properties of the oxides. The surfaceproperties of TiO2-CeO2 oxides were investigated by X-rayphotoelectron spectroscopy (XPS), temperature-programmeddesorption of ammonia (NH3-TPD), and temperature-pro-grammed reduction in hydrogen (H2-TPR). The results providednew insight on how mixing Ti and Ce in the synthesis stagecan impact the characteristics of TiO2-CeO2 mixed oxides withrespect to their nanocrystallite structure, crystallite size, porestructure, surface area, and surface chemical properties includingoxygen deficiency, reducibility, acidity, and oxidation activity.

2. Experimental Section

2.1. Synthesis of Oxides. Cerium(IV) oxide, titanium(IV)oxide, and their mixed oxides were synthesized by applying a* E-mail: [email protected].

J. Phys. Chem. C 2009, 113, 14249–14257 14249

10.1021/jp8110309 CCC: $40.75 2009 American Chemical SocietyPublished on Web 07/15/2009

urea precipitation/gelation method reported by Wang andFlytzani-Stephanopoulos20 for CeO2 synthesis and by extendingthe procedure further to TiO2-CeO2 mixed oxides synthesis.Prior to urea precipitation, aqueous solutions were preparedusing cerium ammonium nitrate (NH4)2Ce(NO3)6 (Aldrich,99.99%), titanium oxysulfate TiOSO4 · xH2SO4 · xH2O (Aldrich,Ti: 18.5 wt %), and urea CO(NH2)2 (Aldrich 99+%). The totalamount of cerium ammonium nitrate and titanium oxysulfatewas 0.075 mol, but the Ti:Ce mole ratio was varied between1:0 to 0:1. In a typical preparation, the prescribed amounts ofcerium ammonium nitrate and titanium oxysulfate were dis-solved in 200 mL of distilled water, respectively. Ammoniumcerium nitrate aqueous solution was added to the titaniumsolution. The mixed aqueous solution was then added to 800mL of aqueous solution containing 70 g of urea and stirredvigorously by a magnetic stirrer. The solution was heated to90-95 °C and held there for 8 h, while the total liquid volumewas kept at 1000 mL. The resulting precipitant was filtered anddried overnight in an oven at 110 °C. The dried precipitant wascrushed into powder and calcined in static air by heating at arate of 1.5 °C/min from room temperature to 450 °C and keptat 450 °C for 6 h in a muffle furnace.

2.2. Characterization of Oxides. The Brunauer, Emmett,and Teller (BET) surface area and pore size distribution of metaloxides were obtained from 49-point nitrogen adsorption/desorption analysis with a Quantachrome Autosorb 1. Allsamples were outgassed at 220 °C prior to the adsorption-desorption measurements. Micropore surface area and volumewas calculated using the t-method. The pore size distributionwas calculated by applying the Barrett-Joyner-Halenda (BJH)method on the desorption branch of isotherm curve.

X-ray powder diffraction (XRD) patterns for the samples werecollected at a scan rate of 1°/min in the 2θ range of 20-65°using a Scintag-I XRD instrument equipped with Cu KRradiation. The working voltage of the instrument was 35 kV,and the current was 35 mA. Separately, a silicon standard wasused for 2θ corrections. The mean crystallite size of sampleswas calculated from peak broadening by the Scherrer equationusing Jade 6.5 software, where the Scherrer constant K (particleshape factor) was taken as 0.85.

X-ray photoelectron spectroscopy (XPS) spectra were re-corded on a Kratos Analytical Axis Ultra spectrometer withmonochromatic aluminum (1486.6 eV). The X-ray sourceoperated at 14 kV and 20 mA. The sample powders were pressedinto 5 mm × 5 mm 3 M double-sided tape using a mortar andpestle and visualized by a stereomicroscope to ensure completecoverage and powder uniformity over the tape. Prior to theanalysis, oxide samples were dried overnight in an oven at 120°C under atmospheric pressure. After drying, all samples wereimmediately stored in a container and mounted for analysis.Sample height positions were set from O 1s signal at 529 eVfollowing changing of lateral coordinates such that the measuredsignal from the sample powders were maximized, thus minimiz-ing any possible signal from the 3 M double-sided tape. The 3M tape was examined independently, and the characteristicshape of the C 1s line was not found when compared to the C1s line collected from these sample powders. As a reference,we used the C 1s signal of the adventitious carbon (carbon ofany surface adsorbed), which we fixed at 285 eV. A surveyscan with analyzer pass energy of 80 eV was initially recordedfor the sample to identify elements present. The compositionand chemical states were determined from the charge-correctedhigh resolution scans with analyzer pass energy of 20 eV. Anestimation of the amount of Ce(III)21,22 can be obtained from

the intensity of the v0 (u0) and v′ (u′) lines, according to theformula:

Temperature-programmed reduction (TPR) analysis wasconducted on a Micrometritics 2910 Autochem analyzer. In atypical TPR experiment, about 0.2 g of the powder sample wasloaded in a U-shaped quartz cell above a small amount of quartzwool. The TPR profile of sample was recorded between 30 and750 °C at a heating rate of 10 °C/min in 4.93% hydrogen inargon with a flow rate of 50 mL/min. Hydrogen uptake wasmonitored by TCD detector.

Temperature-programmed desorption (TPD) of ammonia wasconducted in a flow apparatus on a Micrometrics 2910 Au-tochem analyzer. In a typical NH3-TPD experiment, about 0.2 gof the sample was loaded in U-shaped quartz cell above a smallamount of quartz wool. The sample was treated under argonflow at 300 °C for 60 min. After the treatment, the temperaturewas cooled to 110 °C. For NH3 adsorption, thermally treatedmetal oxide samples were saturated with 20 pulses of 1% NH3/He mixture at 110 °C. After pulse NH3 adsorption, the samplewas flushed in ultra-high-purity grade He flow at 110 °C forremoval of physically adsorbed NH3 on the oxides. An NH3-TPD profile of the sample was recorded by increasing thetemperature from 110 to 700 °C at a heating rate of 10 °C/minunder 50 mL/min of UHP He flow. The desorbed products wereanalyzed by a mass spectrometer (AMETEK Dycor DM200M).The desorbed ammonia, water, nitrogen, and oxygen wereidentified on the basis of the intensity of the mass fragmentswith m/e ) 16, 18, 28, and 32, respectively. The peak m/e )16 was used for ammonia because the parent peak, m/e ) 17,was influenced by the desorbed water.

3. Results and Discussion

3.1. BET Analysis. The results of N2 adsorption-desorptionanalysis are summarized in Table 1 for surface area and porevolume and in Figure 1 for pore size distribution of TiO2-CeO2

mixed oxides along with those of TiO2 and CeO2 single oxides.The surface area of CeO2 and TiO2 prepared by urea precipita-tion was 101.9 and 223.6 m2/g, respectively. The surface areaof TiO2 was twice as high as that of CeO2. As shown in Table1, these single oxides exhibit significantly higher surface areasthan those of corresponding commercial CeO2 and TiO2. MixingTiO2 and CeO2 in the synthesis stage led to a synergetic increaseof surface area. In particular, Ti0.7Ce0.3O2 and Ti0.9Ce0.1O2

exhibited the highest surface areas among all the mixed andsingle oxides.

TABLE 1: Physical Properties of TiO2-CeO2 Oxides

surface area(m2/g)

pore volume(cm3/g)

average poresize (Å)

CeO2 101.9 0.169 63Ti0.1Ce0.9O2 138.7 0.216 53Ti0.3Ce0.7O2 119.2 0.365 73Ti0.5Ce0.5O2 183.4 0.278 46Ti0.7Ce0.3O2 241.0 0.253 38Ti0.9Ce0.1O2 249.3 0.460 37TiO2 223.6 0.246 37

Commercial OxidesCeO2 2.1 0.022 38TiO2 12.8 0.132 19

CeIII(%) )v0 + v′ + u0 + u′

∑ (v + u)(1)

14250 J. Phys. Chem. C, Vol. 113, No. 32, 2009 Watanabe et al.

The pore size distribution presented in Figure 1 shows thatthese oxides possess a unimodal nanoporous structure withnarrow pore size range between 3 to 7 nm. The pore sizedistribution ranges of TiO2-CeO2 mixed oxides tend to bepositioned between those of TiO2 and CeO2 single oxides.Increasing Ce content shifted the peak pore size from around3.7 nm for Ti0.9Ce0.1O2 to around 5.3 nm for Ti0.1Ce0.9O2.

The increase of surface area and pore volume can be attributedto the effect of mixing TiO2 and CeO2 precursors. As reportedin the literature, the urea coprecipitation method also permitsthe synthesis of ZrO2-TiO2 mixed oxides with high surfacearea.23,24 The resulting high surface area of the mixed oxideswas attributed to the inhibition of the individual crystallizationduring precipitation.23 Similarly, mixing TiO2 and CeO2 precur-sors in the urea coprecipitation process could lead to theinhibition of their individual crystallization thus leading tosmaller crystallites in nanometer (nm) size, as confirmed byXRD analysis (see below); thus, mixing TiO2 and CeO2 canincrease surface area and pore volume and change the pore sizerange in the nanoporous structure.

3.2. XRD. Figure 2 shows the XRD spectra of the mixedoxides along with the synthesized TiO2 and CeO2 as well asthe commercial TiO2 and CeO2 samples. XRD analysis revealedthe crystallographic structure and crystallite size of TiO2-CeO2

oxide samples. Structures of CeO2 and Ti0.1Ce0.9O2 andTi0.3Ce0.7O2 oxides were identified as cubic fluorite phase.Anatase structure was shown by the XRD patterns of TiO2 andTi0.9Ce0.1O2. The intensities for these peaks corresponding tolaboratory-synthesized CeO2 and TiO2 single oxides are sig-nificantly broader than those of the corresponding commercialCeO2 (Aldrich) and TiO2 (Degussa). The peak intensities of Ti-containing CeO2 and Ce-containing TiO2 were even lower thanthose of CeO2 and TiO2 alone. In the case of Ti0.5Ce0.5O2 andTi0.7Ce0.3O2, their XRD patterns exhibited very broad diffractionpeaks centered at around 2θ ) 28°. The intensity differenceand peak broadening were attributed to a further reduction ofcrystallite size.

Table 2 shows the mean crystallite sizes of the synthesizedoxides calculated using the Scherrer equation. The meancrystallite sizes of CeO2 and TiO2 prepared by urea precipitationwere significantly smaller than those of the correspondingcommercial ones. Furthermore, mixed TiO2-CeO2 oxidesshowed even smaller crystallite sizes than those of laboratory-prepared CeO2 and TiO2 single oxides. Introducing a smallamount of TiO2 into CeO2 significantly decreased the crystallitesize from 84 to 48 Å for Ti0.1Ce0.9O2 and to 54 Å for Ti0.3Ce0.7O2.Similarly, introducing CeO2 to TiO2 decreased the crystallitesize from 81 to 40 Å for Ti0.9Ce0.1O2. When the value of X inthe mixed oxides TiXCe1-XO2 is in the range of 0.5 to 0.7, thecrystallite sizes were too small to be detected clearly by XRDso the calculation of crystallite sizes was difficult. Consequently,it became clear that mixing TiO2 and CeO2 oxides in the TiX-Ce1-XO2 synthesis led to a significant reduction of crystallitesizes.

Changes of lattice parameters in the series of TiO2-CeO2

oxides were observed along with the mixing TiO2 and CeO2.In the fluorite structure, addition of Ti into CeO2 increased thelattice parameter of the a-axis from 5.217 to 6.168 Å. The ionicradius of Ti4+ is 0.61 Å, which is smaller than that of Ce4+

(0.97 Å). Thus, it is clear that this expansion of the latticeparameter did not occur by a direct substitution of Ce4+ by Ti4+.On the other hand, the ionic radius of Ce3+ (1.14 Å) is largerthan that of Ce4+. Thus, the addition of Ti into CeO2 contributedto an increase in the lattice parameter, which may be caused byCe cation reduction from tetravalent to trivalent. The reductionof Ce cation could be attributed to a decrease in the O 2p-Ce4f hybridization. This result concurs with the report from acalculation by Skorodumova et al.25 In contrast to the Tiaddition, Zr and Ca addition into CeO2 resulted in a decreaseof the lattice parameter.26,27 Thus, the addition of Ti into CeO2

is comparatively unique.

Figure 1. Pore size distributions of TiO2, Ti0.9Ce0.1O2, Ti0.5Ce0.5O2,Ti0.1Ce0.9O2, and CeO2 prepared by urea precipitation.

Figure 2. XRD patterns of laboratory-prepared TiO2, CeO2, andTiO2-CeO2 oxides: (a) TiO2, (b) Ti0.9Ce0.1O2, (c) Ti0.7Ce0.3O2, (d)Ti0.5Ce0.5O2, (e) Ti0.3Ce0.7O2, (f) Ti0.1Ce0.9O2, and (g) CeO2. CommercialTiO2 and CeO2 samples are shown for comparison.

TABLE 2: Crystalline Structure, Lattice Parameter, andCrystallite Size of TiO2-CeO2 Mixed Oxides

lattice parameter (Å)crystallitestructure a c

meancrystallitesize (Å)

TiO2 anatase 3.726 10.478 81Ti0.9Ce0.1O2 anatase 3.808 9.661 40Ti0.7Ce0.3O2 N/Da - - -Ti0.5Ce0.5O2 N/Da - - -Ti0.3Ce0.7O2 fluorite 6.168 - 54Ti0.1Ce0.9O2 fluorite 5.477 - 48CeO2 fluorite 5.217 - 84

ReferencesTiO2: (P-25) Degussa anatase + rutile - 311, 519CeO2: Aldrich fluorite 5.402 - >1000TiO2

30 anatase 3.785 9.514CeO2

30 fluorite 5.411 -

a Not determined because of a very weak and broad XRD peak.

Nanocrystalline TiO2-CeO2 Mixed Oxides J. Phys. Chem. C, Vol. 113, No. 32, 2009 14251

Anatase structure was identified by XRD for TiO2 andTi0.9Ce0.1O2 (Figure 2). Upon Ce addition into TiO2, an increaseof the lattice parameter in the a-axis was observed with adecrease of that in c-axis (Table 2). Lattice parameter reductionof the c-axis in anatase structure corresponds to the shorteningof metal-oxygen bond.28 This bond shortening resulted fromlocalization where the electron density of the Ti cations shiftstoward empty lattice sites, which involves formation of oxygenvacancy and reduction of the Ti cation.29 Consequently, theexpansion of the lattice parameter in the a-axis was attributedto not only the substitution of titanium Ti4+ (0.61 Å) by Ce4+

(0.97 Å)30 but also the reduction of Ti. In summary, mixingTiO2 and CeO2 results in significant reduction of crystallite sizeand the distortion of crystal structure as reflected by changesin lattice parameters.

3.3. XPS. XPS analysis was conducted to understand thesurface chemical state of Ce, Ti, and O in TiXCe1-XO2 oxides.Ce 3d spectra of CeO2 and TiO2-CeO2 mixed oxides are shownin Figure 3. The labels used in identifying Ce 3d XPS peaks inFigure 3 were established by Burroughs et al.,31 where V andU indicate the spin-orbit coupling 3d5/2 and 3d3/2, respectively.The peaks referred to as v, v′′, and v′′′ are contributed by CeO2,and assigned to a mixture of Ce IV (3d94f2) O (2p4), Ce IV(3d94f1) O (2p5) and Ce IV (3d94f 0) O (2p6), respectively. Thesame peak assignment is applied to u structures. The peaks vo

and v′ are assigned to a mixture of Ce III (3d94f2) O (2p5) andCe III (3d94f1) O (2p6), respectively. Ce 3d spectrum of CeO2

(Figure 3a) shows six peaks at 916.2, 907.2, 900.8, 898.0, 888.9,and 882.1 eV. These peaks represent the presence of Ce4+. Theintensity of these six peaks gradually decreased with the increaseof Ti concentration. On the contrary, the intensity at 903.9 and885.4 eV increased with the increase of Ti content. Thesechanges in the Ce 3d XPS spectra point to the increase of Ce3+

concentration of total Ce in oxides with increasing Ti concentra-tion. The curve fitting procedure of Ce 3d XPS spectra isillustrated in the Supporting Information (SI-1). Curve-fittingfor the XPS spectra indicated that the surface concentration ofCe3+ of the TiO2-CeO2 oxides varied with the amount of Tiaddition from 15.1% in CeO2 to 40.7% in Ti0.9Ce0.1O2. The Ce3+

concentration shows an essentially linear relationship with theTi concentration, as shown in Figure 4. Thus, introducing TiO2

into CeO2 causes the partial reduction of Ce tetravalent cation.Concomitant with the change in CeO2 structure upon Ti

concentration, the chemical state of Ti also changed with Ceconcentration. Figure 5 shows the Ti 2p1/2 and Ti 2p3/2 spectraof TiO2-CeO2 mixed oxides. The Ti 2p3/2 peak shifted from459.4 eV in TiO2 to 457.9 eV in Ti0.1Ce0.9O2. This shifting

represents an intermediate oxidation state of Ti from tetra- totrivalent. The curve-fitting of Ti 2p XPS is shown in theSupporting Information (SI-2). In Figure 6, the binding energyof Ti 2p3/2 and the oxidation state of Ti were correlated usingindependent literature data,32-35 and the correlation shows alinear relationship between the binding energy of Ti 2p3/2 andthe oxidation state of Ti. On the basis of this relationship, theoxidation state of Ti in the series of TiO2-CeO2 oxides wasestimated as lower than 4+, and Ti was found to be significantlyreduced with higher Ce content, as can be seen from Figure 7.The reduction of Ti and Ce in the series of TiO2-CeO2 oxideswas observed for Ce and Ti, respectively. This reductionbehavior is consistent with what was discussed in the above

Figure 3. XPS Ce 3d spectra for (a) CeO2, (b) Ti0.1Ce0.9O2, (c)Ti0.3Ce0.7O2, (d) Ti0.5Ce0.5O2, (e) Ti0.7Ce0.3O2, and (f) Ti0.9Ce0.1O2.

Figure 4. Correlation between the surface Ce3+ concentration asderived from XPS analysis and the concentration of Ce in TiO2-CeO2

oxides.

Figure 5. XPS Ti 2p spectra for (a) TiO2, (b) Ti0.9Ce0.1O2, (c)Ti0.7Ce0.3O2, (d) Ti0.5Ce0.5O2, (e) Ti0.3Ce0.7O2 and (f) Ti0.1Ce0.9O2.

Figure 6. Correlation between binding energy of Ti 2p3/2 and oxidationstate of Ti from the literature data.32-35


XRD analysis. Thus, the Ti and Ce metal cations influence eachother to change their oxidation states both on surface and inthe bulk.

Figure 8 shows O 1s spectra of TiO2-CeO2 mixed oxides.Binding energy of O 1s for surface oxygen of these oxidesdecreased from 530.4 to 529.0 eV with increasing Ce concentra-tion. Moreover, O 1s peaks corresponding to TiO2 and CeO2

did not individually appear in the spectra of TiO2-CeO2 mixedoxides. Thus, the binding energy of the surface oxygen wassensitive to the concentrations of Ti and Ce in the TiO2-CeO2

mixed oxides. This O 1s XPS peak shift suggests that Ti andCe chemically interact with each other in the TiO2-CeO2 mixedoxides. In other words, the mixed oxides are not mixtures oftwo different oxides but rather a uniform solid solution in whichTi and Ce have chemical interactions.

Figure 9 compares the measured and calculated surfaceoxygen concentration of TiO2-CeO2 oxides. The measuredsurface oxygen concentration was obtained by a curve-fittingof O 1s XPS. The peak deconvolution of O1s XPS spectra isillustrated in Supporting Information (SI-2) for NH3-TPDprofiles of TiO2, Ti0.9Ce0.1O2, and Ti0.7Ce0.3O2. The calculatedsurface oxygen concentration was indicated by the oxidationstate of surface Ce and Ti. Both surface oxygen concentrationswere described by stoichiometric values. As shown in Figure9, for the cases with TiO2 alone and CeO2 alone, the measuredoxygen concentration was higher than the calculated value.However, for most of the mixed oxides including Ti0.3Ce0.7O2,Ti0.5Ce0.5O2, Ti0.7Ce0.3O2, and Ti0.9Ce0.1O2, the measured oxygenconcentration was lower than the calculated value. This

comparison suggests that mixing TiO2 and CeO2 significantlyenhanced the oxygen defects or oxygen deficiency on thesurface, especially for Ti0.7Ce0.3O2 and Ti0.9Ce0.1O2. This featureof the mixed oxides may be important for adsorptive interactionof gas-phase oxygen with the surface. Since these oxides werecalcined and thermally treated overnight in an oven before XPSanalysis, their reduced centers are remarkably stable. Thisstability may be attributed to the localization of 4f orbital forCe and 3d orbital for Ti, and those localized orbitals partiallyincrease electron density toward empty oxygen sites.36-39 Thus,XPS analysis revealed the higher level of oxygen deficiencyand significant reduction of Ce and Ti due to the mixing CeO2

and TiO2.3.4. H2-TPR. Figure 10 shows the H2-TPR profiles of the

series of TiO2-CeO2 oxides. A peak appeared at 410 °C witha shoulder around 510 °C on the higher temperature side inTPR profile of CeO2. This peak and the shoulder correspond tothe reduction of surface oxygen and bulk oxygen, respectively.40

Increasing the Ti content in TiO2-CeO2 oxides caused anincrease of reduction temperature to 580 °C for Ti0.1Ce0.9O2,600 °C for Ti0.3Ce0.7O2 and 670 °C for Ti0.5Ce0.5O2. The TPRpeak intensity of mixed oxides also increased with the increaseof Ti content up to Ti0.5Ce0.5O2. A shoulder on the lowertemperature side of the reduction peak was observed for theseoxides, which corresponds to reduction of surface oxygen. Thus,the major TPR peak for Ti0.1Ce0.9O2, Ti0.3Ce0.7O2, andTi0.5Ce0.5O2 corresponds to the significant reduction of bulk

Figure 7. Calculated average oxidation state of Ti in TiO2-CeO2

oxides based on XPS.

Figure 8. XPS O 1s spectra for (a) TiO2, (b) Ti0.9Ce0.1O2, (c)Ti0.7Ce0.3O2, (d) Ti0.5Ce0.5O2, (e) Ti0.3Ce0.7O2 and (f) Ti0.1Ce0.9O2, and(g) CeO2.

Figure 9. Comparison of the TiO2-CeO2 composition with stoichi-ometry of oxides. The measured surface lattice oxygen concentrationwas obtained from XPS analysis, and calculated lattice oxygenconcentration was derived from concentrations of surface Ce4+, Ce3+,Ti4+, and Ti3+.

Figure 10. H2-TPR profiles of synthesized TiO2, CeO2, andTiO2-CeO2 mixed oxides.


oxygen. The XRD patterns of Ti0.1Ce0.9O2, Ti0.3Ce0.7O2, andTi0.5Ce0.5O2 showed an increase of lattice parameter with Tiaddition in fluorite structure. This increase of lattice parametercontributed to the structure distortion, and as a consequence alarger amount of oxygen was removed from the mixed oxidesupon H2 reduction.

Reduction of TiO2 alone is more difficult, and essentially noTPR peak was observed from 50 to 750 °C. However, areduction peak appeared at 545 °C for Ti0.9Ce0.1O2. A higherreduction peak at 645 °C with a shoulder around 513 °Cappeared by further Ce addition up to Ti0.7Ce0.3O2. The distortionof the structure, as revealed by XRD study, could contribute tothis change of reduction patterns. By a comparison of the TPRpeak intensities, it is clear that the reduction of Ti0.7Ce0.3O2 andTi0.9Ce0.1O2 was not as significant as that of other mixed oxides.The reason is possibly due to the higher oxygen deficiency asdiscussed in the XPS study. Nevertheless, mixing TiO2 and CeO2

clearly modified the reduction property of CeO2 and TiO2

significantly.H2 consumption during TPR was calculated and plotted in

Figure 11 versus measured surface oxygen concentration ofTiO2-CeO2 oxides obtained from the XPS study. H2 consump-tion for reduction of Ti0.1Ce0.9O2 was remarkably large. Thislarge H2 consumption is attributed to the high surface oxygenconcentration as measured by XPS. Although measured surfaceoxygen concentrations of CeO2 and TiO2 single oxides werehigh, H2 consumption of those oxides was not comparable.While the measured surface oxygen concentration of mixedoxides was moderately low for the TiO2-CeO2 mixed oxides,large H2 consumption indicated the significant reduction of theTiO2-CeO2 mixed oxides. This difference in reduction behaviorsuggests that mixing CeO2 and TiO2 could significantly enhancethe level of reduction for the mixed oxides. This fact suggeststhat reduction with the bulk oxygen was enhanced by mixingTiO2 and CeO2. This property also corresponds to oxygenstorage capacity. As discussed in the XRD study, distortion ofthe structure induced by mixing TiO2 and CeO2 promoted thereducibility, including oxygen storage capacity.

3.5. NH3-TPD. Temperature-programmed desorption of ad-sorbed ammonia has been employed to investigate the acidicand catalytic properties with simultaneous mass spectrometricanalysis of desorbed molecules. NH3-TPD for the series ofTiXCe1-XO2 oxides was conducted by NH3 adsorption at 110 °C,and desorption from 110 to 700 °C at a heating rate of 10 °C/min under UHP He flow. The detected response of desorbedNH3 was normalized by the surface area of the TiXCe1-XO2

oxides. The acquired NH3-TPD profiles of TiXCe1-XO2 oxidesare shown in Figure 12. NH3 desorption was observed at170-680 °C. The maximum NH3 desorption temperature forTiXCe1-XO2 oxides shifted from 235 to 260, 260, 260, 265, and270 °C with increasing Ti content, and these peaks correspondto weak acid. This desorption temperature of NH3 suggests thatthe surface of these oxides is predominantly weakly acidic. Inaddition, some strong acidity was identified for the mixed oxidesby deconvolution of the peaks. Peak deconvolution is shownin the Supporting Information (SI-3) for NH3-TPD profiles ofTiO2 (left), Ti0.9Ce0.1O2 (middle) and Ti0.7Ce0.3O2 (right).

Figure 13 shows the area obtained by curve fitting in theweaker acid region for TiO2-CeO2 mixed oxides. The corre-sponding area for the TPD peak around 235-270 °C increasedwith decreasing Ce content. A linear correlation for weak acidityversus Ce concentration (or Ti:Ce ratio) was found. Conse-quently, the results suggest that acid sites, especially weak acid,increased with increasing Ti concentration and decreased withincreasing Ce concentration. NH3 desorption peaks for theTiXCe1-XO2 oxides showed a shoulder on the higher temperatureside. The desorption at higher temperature corresponds to astronger acid site, which increased with the increase of Ticoncentration. This may be attributed to the NH3 adsorption onthe Lewis acid as indicated by Tsyganenko et al.41

Furthermore, apart from NH3, several products including H2O,N2, and O2 were also detected by mass spectrometer (MS) duringNH3-TPD. For comparison, TPD was also conducted withoutNH3 preadsorption but under otherwise identical conditions forthe same samples. Selective ion monitoring by MS during TPD

Figure 11. Measured surface oxygen concentration and H2 consump-tion in TPR of TiO2-CeO2 oxides.

Figure 12. NH3-TPD profiles for (a) CeO2, (b) Ti0.1Ce0.9O2, (c)Ti0.3Ce0.7O2, (d) Ti0.5Ce0.5O2, (e) Ti0.7Ce0.3O2, (f) Ti0.9Ce0.1O2, and (g)TiO2.

Figure 13. Correlation between the area of the NH3 desorption peakaround 235-270 °C and the concentration of Ce in TiO2-CeO2 oxides.


reveals that NH3 shows peaks at m/e ) 16 and 17 while H2Oshows peaks at m/e ) 17 and 18. Thus TPD-MS peaks withm/e ) 16 and 18 were selected for NH3 and H2O, respectively.NH3, H2O, N2, and O2 TPD profiles with and without NH3

adsorbed on Ti0.9Ce0.1O2, Ti0.5Ce0.5O2, and Ti0.1Ce0.9O2 areplotted in Figures 14-16.

Figure 14 shows the desorption profiles of NH3 (m/e ) 16),H2O (m/e ) 18), N2 (m/e ) 28), and O2 (m/e ) 32) forTi0.9Ce0.1O2 with and without preadsorbed NH3. Desorption ofH2O and N2 appeared at 150 °C along with NH3 for NH3-adsorbed Ti0.9Ce0.1O2. Although NH3 desorption did not appearat temperatures higher than 500 °C, desorption of H2O and N2

Figure 14. TPD profiles of NH3, H2O, N2, and O2 for Ti0.9Ce0.1O2 sample with NH3 (left) and without NH3 preadsorbed (right). The sample wastreated at 300 °C for 1 h and cooled to 110 °C, NH3 adsorption was conducted at 110 °C, and desorption was conducted under He flow at a heatingrate 10 °C/min to 700 °C.




continued to 590 and 700 °C, respectively. These desorptionprofiles suggest that strongly adsorbed NH3 are oxidizedeffectively at higher than 500 °C. In comparison with thedesorption profiles for Ti0.9Ce0.1O2 without preadsorbed NH3,the intensities for desorption of H2O and N2 were significantlyhigher for the NH3-adsorbed one. This intensity differenceclearly suggests that H2O and N2 were produced by oxidationof NH3. A trace amount of O2 (<0.1%) in He flow was consumedto oxidize NH3 to produce H2O and N2. When NH3 desorptionbegan, O2 uptake was observed which was followed by H2Oand N2 desorption, and when the N2 desorption peak diminished,O2 uptake also decreased, as can be seen from Figure 14.

Desorption profiles of NH3, H2O, N2, and O2 for Ti0.5Ce0.5O2

and Ti0.1Ce0.9O2 with and without preadsorbed NH3 are shownin Figure 15 and Figure 16, respectively. Desorption of H2Oand N2 appeared at 150 °C along with that of NH3 for NH3-adsorbed Ti0.5Ce0.5O2 and Ti0.1Ce0.9O2 similar to the trendsobserved for Ti0.9Ce0.1O2 in Figure 14. The maximum temper-ature of H2O desorption shifted from 430 to 385 °C withdecreasing Ti content. H2O desorption was observed along withN2 desorption and O2 uptake. These desorption products areattributed to oxidation of NH3 with uptake of trace O2 presentin the He flow during NH3-TPD.

It should be noted that the H2O desorption peak was observedin the TPD profiles of both non-NH3 adsorbed NH3-adsorbedmixed oxides. However, for non-NH3 adsorbed mixed oxides,H2O desorption occurred at a much higher temperature, startingat 300 °C with lower intensity (than that with NH3-adsorbed),and there is little N2 desorption. Since H2O desorption was notobserved at temperatures lower than 300 °C for the non-NH3

adsorbed sample, oxidation of NH3 is considered to take placealong with NH3 desorption. The difference of intensity for H2Odesorption peaks suggests that oxidation of NH3 also occurredabove 300 °C. Interestingly, significant O2 uptake was onlyobserved in the TPD profiles of NH3 adsorbed oxides. Thus,these TiO2-CeO2 mixed oxides could function as oxidationcatalysts which is related to their oxygen storage capacity.

On the basis of the difference in the H2O desorption betweenNH3-adsorbed and nonadsorbed TiO2-CeO2 mixed oxidesshown in Figures 14-16, the difference in intensities of H2Odesorption peaks was plotted versus Ti concentration in Figure17. This difference represents the extent of H2O formation fromNH3 oxidation on TiO2-CeO2 mixed oxides. It is clear that theTi:Ce molar ratio influences the oxidation activity. Increasing

Ti in TiO2-CeO2 mixed oxides increased the NH3 oxidationactivity. Oxidation of NH3 during NH3-TPD occurred at thehighest level over Ti0.9Ce0.1O2. It should be mentioned thatamong the various TiXCe1-XO2 mixed oxides, Ti0.9Ce0.1O2 hasbeen found to show the highest capacity as adsorbent in selectiveadsorption for removing sulfur compounds from aviation jetfuels at room temperature.

4. Conclusions

The following conclusions may be drawn from the investiga-tion on structural and surface characteristics of a series of TiX-Ce1-XO2 oxides synthesized by urea precipitation.

1. The TiXCe1-XO2 mixed oxides showed unimodal nanoporousstructure with pore sizes increasing from 3.7 nm for X ) 0.9 to5.3 nm for X ) 0.1. Their pore size ranges generally varybetween those of TiO2 and CeO2 single oxides prepared by thesame method.

2. The TiXCe1-XO2 mixed oxides were nanocrystalline, about4.0 nm in crystallite size when X ) 0.9, and 4.8-5.4 nm whenX ) 0.1- 0.3, which are significantly smaller than TiO2 andCeO2 single oxides (8.1 to 8.4 nm), as determined by XRDanalysis. Mixing TiO2 and CeO2 precursors in the synthesis stagecontributed to decreasing the crystallite size of the resultingmixed oxides.

3. Change in XRD patterns and binding energy in XPS spectrarevealed important changes in crystal structures upon mixingTiO2 and CeO2. Introducing a small amount of Ce into TiO2

caused a structural distortion of anatase phase; introducing asmall amount of Ti into CeO2 caused a structural distortion offluorite phase. A dominant anatase phase was detected by XRDwhen X was 0.9 or higher while a cubic fluorite phase wasdominant in XRD when X was 0.3 or lower in TiXCe1-XO2.

4. When the value of X in the mixed oxides TiXCe1-XO2 wasin the range of 0.5-0.7, the XRD peaks were too weak and toobroad, which did not permit the determination of crystallinephase and crystallite sizes. This was because their crystallitesizes were too small to be detected clearly by XRD.

5. Surface oxygen deficiency and H2 reducibility wereenhanced by mixing TiO2 and CeO2, but the surface cationoxidation state, reduction temperature, and level were stronglydependent on the composition of TiXCe1-XO2, as revealed byXPS and H2-TPR.

6. Acid sites were identified on surfaces of TiXCe1-XO2 oxidesby NH3-TPD. These surface acid sites increased with increasingconcentrations of Ti and decreased with increasing Ceconcentration.

7. Oxidation of NH3 along with H2O and N2 formation anddesorption was observed during NH3-TPD for TiXCe1-XO2 oxides.This reactive desorption took place together with uptake of O2

present in a trace amount in the He carrier gas. The catalyticoxidation of ammonia during TPD occurred at increasing levelwith increasing Ti content in the mixed oxides. The highestNH3 oxidation activity was observed when X ) 0.9 forTi0.9Ce0.1O2. Thus, it became clear that TiO2-CeO2 mixedoxides can act as oxidation catalysts whose activity can beadjusted by changing Ti content.

8. The present study clearly established that the structural(nanoporosity, pore size, surface area, crystalline phase, crys-tallite size) and surface properties (reducibility, oxygen defi-ciency and oxygen storage capacity, acidity, and oxidationactivity) of the TiXCe1-XO2 (TiO2-CeO2) mixed oxides can betailored by controlling their composition during synthesis bythe urea coprecipitation procedure.

Figure 17. Correlation between H2O formation during NH3 desorptionand Ti concentration in TiO2-CeO2 oxides. The value in the Y axiswas obtained from the difference intensities between H2O desorptionof NH3 and non-NH3 adsorbed samples.


Acknowledgment. The authors gratefully acknowledge thesupport of this work in part by the U.S. Office of NavalResearch and by US Environmental Protection Agency-USNational Science Foundation through the NSF-EPA joint TSEprogram. We thank the ONR Navsea program managerDonald Hoffman and John Heinzel for their support andencouragement. We also thank Bob Hengstebek, Vince Bojan,and Magda N. Salama of Penn State for their technicalsupport for analytical instrumentation.

Supporting Information Available: SI-1 shows the curvefitting procedure of Ce 3d XPS spectra, which is applicable tocurve fitting of other XPS spectra. The curve-fitting of Ti 2pXPS is shown in the Supporting Information (SI-2). Peakdeconvolution is shown in the Supporting Information (SI-3)for NH3-TPD profiles of TiO2 (left), Ti0.9Ce0.1O2 (middle) andTi0.7Ce0.3O2 (right). Experimental details. This material isavailable free of charge via the Internet at http://pubs.acs.org.

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