Single-Phase Solid Solution (TiO2)x-(YSZ)1−x Mesoporous

Oct 28, 2010 - Mesoporous nanosized powders with single-phase solid solution fluorite structure in the ternary oxide zirconia−yttria−titania syste...
6 downloads 10 Views 2MB Size
Download PDF
J. Phys. Chem. C 2010, 114, 19365–19372

19365

Single-Phase Solid Solution (TiO2)x-(YSZ)1-x Mesoporous Nanoparticles with Catalytic Activity in the Oxidation of Methane Simona Somacescu,*,† Jose Maria Calderon Moreno,† Petre Osiceanu,† Bao-Lian Su,‡,§ and Viorica Parvulescu† Institute of Physical Chemistry, Romanian Academy, Spl. Independentei 202, Bucharest, Romania, Laboratory of Inorganic Materials Chemistry (CMI), UniVersity of Namur (FUNDP), 61 rue de Bruxelles, 5000 Namur, Belgium, and Laboratory of LiVing Materials at the State Key Laboratory of AdVanced Technology for Materials Synthesis and Processing, Wuhan UniVersity of Technology, 122 Luoshi Road, 430070, Wuhan, Hubei, China ReceiVed: June 24, 2010; ReVised Manuscript ReceiVed: October 4, 2010

Mesoporous nanosized powders with single-phase solid solution fluorite structure in the ternary oxide zirconia-yttria-titania system, with titania contents from 0 to 30 mol %, were synthesized by the selfassembling method associated with hydrothermal treatment using cetyltrimethylammonium bromide (CTAB) as the surfactant-directing agent and pore-forming agent. The resulting materials were characterized by X-ray diffraction (XRD), Raman spectroscopy (Raman), scanning and transmission electron microscopy (SEM and TEM), thermogravimetric analysis (TG-DTG), N2 adsorption/desorption isotherms (BET), and X-ray photoelectron spectroscopy (XPS). The as-synthesized mesoporous nanoparticles are single-phase crystallites with a crystal size and diameter between 6 and 12 nm and mean pore size around 2 nm. The specific surface area of the as-synthesized mesoporous samples after calcination at 550 °C remains as high as 140 m2/g. All spectroscopic methods clearly confirm the homogeneous incorporation of Ti4+ in the lattice of the nanocrystallites, forming a single-phase solid solution with fluorite structure and modifying the absorption spectra of the nanocrystallites. The mesoporous solid solution nanoparticles show significant activity in catalytic oxidation of methane in the range 650-750 °C. 1. Introduction Technological advances that characterized the 20th century have led to a dramatic growth of energy consumption; therefore, one of the most important tasks of the 21st century is to identify new energy sources and fuels. There is growing interest in solving some major needs in the field of energy production, such as reduction of the fuel cells operating temperature, by using nanomaterials. The ternary oxide system titania-yttria stabilized zirconia (TYZ), with fluorite-type structure, is investigated in view of their use as anodes in solid oxide fuel cells (SOFC).1,2 The physical, chemical, and structural characteristics of these nanomaterials are dependent on the preparation method. It was proved that using Ni- and zirconia-containing yttria and titania to performing cermet anodes causes reduction of the sintering effect of Ni particles due to the improved interfacial bonding at the ceramic/metal interface.1,3-5 Thus, the Ti-doped YSZ cermets are a useful material for the anode, which can be used especially as the anode-supported type cell, where the anode substrate provides the mechanical strength of the stack.6 Methods of synthesis for obtaining nanopowders with high surface area are the subject of many research studies. Nanostructured mesoporous ZrO2 and yttria-stabilized zirconia (YSZ) nanoparticles, with high crystallinity and high surface area, have been prepared by hydrothermal synthesis.7-9 Generally, TYZ ternary oxide is obtained from a high-purity commercial powder.6,10,11 Forming cubic or tetragonal structures * To whom correspondence should be addressed. E-mail: [email protected]. † Romanian Academy. ‡ Universite´ de Namur. § Wuhan University of Technology.

depends on the amount of TiO212,13 and the synthesis method as well. The phase diagram of Colomer et al.14 places the solid solution limit at 18 mol % TiO2, whereas the phase diagram of Feighery et al.15 locates the limit at only 7 mol % TiO2, in agreement with Traqueira.16 By introducing TiO2 in the YSZ lattice, the obtained nanomaterials have a high electronic conductivity, thermal stability, good compatibility with the electrolyte, and good catalytic activity.6 The catalytic properties of ZrO2 and YSZ in the partial oxidation of methane were investigated by some groups,17,18 and the conversion obtained was below 40%.19 In this work, we apply a self-assembling method associated with hydrothermal treatment to synthesize mesoporous singlephase solid solution TYZ nanocrystallites with compositions ZrO2-8%Y2O3-x%TiO2 (x ) 5, 10, 15, 20, and 30 mol %) using cetyltrimethylammonium bromide (CTAB) as a surfactantdirecting agent and pore-forming agent in a facile and reproducible way. Hydrothermal treatment applied is helpful to crystallize and stabilize the mesoporous structure. The obtained TYZ mesostructured nanoparticles show high catalytic activity on the combustion of methane exceeding that of previous studies. 2. Experimental Methods 2.1. Synthesis. In this study ZrO2 (Z), ZrO2-8%Y2O3 (YSZ), ZrO2-8%Y2O3-x%TiO2 (xTYZ), with x ) 5, 10, 15, and 30 molar ratio, were synthesized by the self-assembling method associated with hydrothermal treatment. Cetyltrimethylammonium bromide (CTAB, Aldrich, g98%) was used as the surfactant, zirconium(IV) oxide chloride octahydrtate puriss (ZrOCl2, Riedel de Hae¨n, g99.5%), yttrium nitrate hexahydrate (Y(NO3)36H2O, Merck, 99.8%), and titanium(IV) isopropoxide

10.1021/jp105834a  2010 American Chemical Society Published on Web 10/28/2010

19366

J. Phys. Chem. C, Vol. 114, No. 45, 2010

(Ti{OCH(CH3)2}4 (Fluka, purum) as inorganic precursors, urea (Alpha Aesar, 99.8%) as hydrolyzing agent, and tetramethylammonium hydroxide (TMAOH solutions 25 wt % in water, Merck) for pH adjusting. The molar ratio of metal/surfactant/ urea was 1:0.36:7. Subsequently, urea was added and the reaction proceeded at 80 °C for 4 h under stirring and refluxing conditions. The pH was adjusted at 9 using tetramethylammonium hydroxide (TMAOH), and the precipitate was loaded into a Teflon link steel autoclave and heated at 373 K for 3 days, knowing that the hydrothermal treatment is specific to preparative chemistry assisted by surfactants. All products were filtered, washed, dried at 373 K, and calcined in N2 flow and then in air at 823 K. The samples have been noted as follows: ZrO2 (Z), ZrO2-8%Y2O3 (YSZ), and ZrO2-8%Y2O3-x%TiO2 (xTYZ), with x ) 5, 10, 15, 20, and 30 molar ratio. 2.2. Characterization. Phase purity and crystal structure were determined using Raman spectroscopy and X-ray diffraction (XRD). XRD patterns were recorded with a standard diffractometer (model D5000 Siemens Diffractometer, Berlin, Germany) equipped with a graphite monochromator using Cu KR radiation (λ ) 1.5405 Å) operating at 40 mA and 40 kV, employing a scanning rate of 0.02° and counting time of 5 s per step in the (2θ) range from 4° to 100°. Raman spectra were recorded at room temperature, in the range from 30 to 1900 cm-1, by using a triple Jobin Yvon/ Atago-Bussan T-6400 spectrometer equipped with an Ar+ laser (λ) 514.5 nm) and liquid N2-cooled CCD detector. BET specific surface areas were determined by N2 adsorption/ desorption at liquid nitrogen temperature on a Tristar Micromeritics instrument. For TG-DTG, the evolution during thermal treatment until obtaining the final structure was measured using a Setaram B111 thermogravimetric analyzer. The samples were heated from room temperature to 650 °C in synthetic air with a heating rate of 10 °C min-1. The scanning electron microscopy (SEM) images were recorded with a Philips XL-20 microscope and energy-dispersive X-ray (EDX) measurements in a Jeol JSM-840 microscope equipped with an INCA Energy 250 EDX detector. The transmission electron microscopy (TEM) images were recorded with a HITACHI HT800 apparatus equipped with a Link EDX system operated at an accelerating voltage of 200 kV. The high-resolution TEM (HRTEM) micrographs were recorded with a JEOL 3011 microscope operated at 300 kV. The powders were ultrasonicated in ethanol, and a drop of the solution was dried on a carbon-coated microgrid before measurements. Surface analysis performed by X-ray photoelectron spectroscopy (XPS) was carried out on VG Scientific ESCA-3 equipment with a base pressure in the analysis chamber of 10-9 Torr. The X-ray source was Al KR radiation (1486.6 eV, nonmonochromatized), and the overall energy resolution is estimated at 1.1 eV by the full width at half-maximum (fwhm) of the Au4f7/2 line. In order to take into account the charging effect on the measured binding energies (BEs) the spectra were calibrated using the C1s line (BE ) 284.8 eV) of the adsorbed hydrocarbon on the sample surface. 2.3. Catalytic Activity Evaluation. Catalytic activity evaluation for methane oxidation was performed in a “U”-shaped tubular reactor made of quartz, operating at atmospheric pressure, loaded typically with 0.1 g of powder catalyst deposited between two quartz wool plugs. Before each catalytic

Somacescu et al.

Figure 1. (a) XRD pattern of sample Z. (b) Raman spectrum of sample Z.

run, the catalyst was flushed with air (100 mL/min) for 1 h at 600 °C in order to remove adsorbed species and decompose residual carbonaceous deposits at the surface and then cooled down to 200 °C. For catalytic activity evaluation, a mixture of 10% CH4 in N2 and air flowing at 100 mL/min (CH4/O2 ) 1/5) was admitted and the temperature was raised to 700 °C in steps of 50 °C. CO2 and hydrocarbons were checked as well using a Porapaq QS 80/100 and Molesieve 5A 80/100 column, mounted on a DANI GC 1000 chromatograph equipped with a TCD detector. 3. Results and Discussion 3.1. Physico-Chemical Characteristics. 3.1.1. XRD, EDX, and Raman. ZrO2. The XRD pattern of sample ZrO2 (Figure 1a) corresponds to a mixture of monoclinic and cubic zirconia phases.20 The wide diffraction peaks, due to small crystalline domain size, and the overlapping features do not allow us to determine the existence of tetragonal phase from peak splitting. The inset in Figure 1a shows the 400 peak, clearly a single peak. Characterization of nanosized zirconia with X-ray diffraction cannot be conclusive about the presence of tetragonal or cubic phases, because of the influence of highly broadened X-ray peaks. Since Raman spectroscopy is much more sensitive to the local rather than the long-range structure, rendering it sensitive to distinguish clearly zirconia polymorphs, we used this technique for phase analysis. The Raman spectra measured

Single-Phase Solid Solution (TiO2)x-(YSZ)1-x

J. Phys. Chem. C, Vol. 114, No. 45, 2010 19367

Figure 2. (a) XRD pattern of sample YSZ. (b) Raman spectrum of sample YSZ.

on sample ZrO2 (Figure 1b) display the characteristic features of the monoclinic and tetragonal phases.21,22 Born-von Karman formalism predicts six Raman-active modes (A1g + 2B1g + 3Eg) for tetragonal zirconia, which can be attributed to Zr-OI and Zr-OII stretching, to the coupling of OI (or OII)-Zr-OI (or OII) bending and to Zr-OI-Zr bending. The presence of the characteristic peaks of the tetragonal phase at ∼144, 266, and 310 cm-1, the shoulder at ∼460 and ∼600 cm-1 of the monoclinic peaks at ∼476 and ∼615 cm-1, respectively, and the increased intensity of the second peak of the monoclinic doublet at ∼638 cm-1 (overlapping with the tetragonal peak) compared to the 615 cm-1 peak demonstrate the presence of all the active modes for tetragonal zirconia. The well-defined and intense bands characteristic of monoclinic and tetragonal zirconia mask any contribution from domains of cubic symmetry. YSZ. The XRD pattern of sample YSZ (Figure 2a) corresponds to single-phase yttria-stabilized zirconia with cubic symmetry with a particle size of 6.2 nm, calculated by the Scherer’s method, and a cell parameter a ) 5.14 Å. The Raman spectrum corresponds to the features of cubic yttria-stabilized zirconia without traces of domains with noncubic symmetry. Addition of Y2O3 to ZrO2 causes formation of vacancies, which in turn induces internal shear deformations in the oxygen as well as zirconium sublattices of ZrO2, thus inducing a lowering of the phase transformation temperature. This finally leads to a stabilization of either a tetragonal or a cubic phase at room

Figure 3. (a) XRD patterns of samples Z, YSZ, and xTYZ (x ) 5, 15, 30). (b) Evolution of the lattice parameter in the equivalent cubic lattice with Ti addition. (c) Small-angle XRD patterns of samples xTYZ (x ) 10, 15, 20, 30).

temperature depending on the amount of Y2O3 added in the system.20 The Raman spectrum (Figure 2b) corresponds to the cubic fluorite structure of zirconia.23 The broad and asymmetric peak observed is related to the oxygen-ion vacancies introduced in the lattice by addition of Y2O3, distorting the cubic lattice. The T1u mode, which is normally Raman inactive, become active in the presence of sublattice distortions and may be responsible for the observed humps.24 3.1.2. Structural Effects of Adding Ti (TYZ). XRD. Addition of Ti results in similar single-phase XRD spectra (Figure 3a) with the same peaks and apparent crystal symmetry of YSZ, except for the sample 30TYZ, which clearly displays the peak splitting characteristic of tetragonal symmetry, in spite of the

19368

J. Phys. Chem. C, Vol. 114, No. 45, 2010

Somacescu et al.

TABLE 1: Lattice Parameter and Crystal Size of Samples YSZ and xTYZ (x ) 5, 15, 30) sample

Ti content (%)

a (Å)

crystal size (nm)

YSZ 5TYZ 15TYZ 30TYZ

0 5 15 30

5.140 5.133 5.120 5.090

6.2 6.8 6.5 10.1

small crystalline size. The effect of Ti addition is a progressive shifting of the peak positions toward higher angles, indicating incorporation of Ti into the cubic yttria-stabilized zirconia lattice and the associated reduction of the cell volume due to substitution of bigger ions Zr4+ and Y3+ by smaller Ti4+ ions. The lattice parameter for the equivalent cubic lattice decreases from 5.14 Å for YZ to 5.05 Å for 30TYZ (Table 1). The XRD results, namely, that the samples are all single phase and the continuous reduction of the lattice parameter with Ti addition, demonstrate incorporation of titanium in the lattice even at the higher titanium concentration. The crystallite sizes have been calculated by Scherrer’s method from the peak width of the main XRD feature, (111), at ∼30°, giving values of 6.8, 6.5, and 10.1 for 5TYZ, 15TYZ, and 30TYZ, respectively. The lattice parameter for the equivalent cubic lattice, a, defined as a ) 3d111/, where d111 ) λ/2 sin θ111, λ is the wavelength of the Cu KR radiation (λ ) 1.5405 Å), and θ111 is the position of the 111 peak (2θ111 ≈ 30°, Figure 3b), decreases from 5.140 Å for 5YZ to 5.090 Å for 30TYZ (Table 1). The XRD pattern of 30TYZ clearly displays the peak splitting characteristic of tetragonal symmetry in the (200) and (400) features at ∼35° and 75°, respectively (Supporting Information), in spite of the small crystalline size. The XRD results demonstrate all the samples are single phase and the progressive reduction of the lattice volume with Ti addition, evidence of incorporation of Ti atoms in the lattice for all samples, even in 30TYZ with the higher Ti concentration. Figure 3b illustrates the evolution of the equivalent cubic lattice a, indicative of the cell volume, obtained from d111, and the lattice parameter calculated from d200; there is an evident change to a tetragonal lattice in 30TYZ. In the small-angle range of the diffraction pattern the samples synthesized with different content of TiO2 exhibited a single peak (Figure 3c). Some authors attributed this type of diffraction behavior to a spongelike structure.25 Thus, the XRD measurements evidenced the important role of structure-directing agents in the mesoporosity formation. An increase of the XRD peak intensity in the order 20TYZ < 15TYZ < 10TYZ can be observed indicating an improvement of mesoporosity for the samples containing 5% and 10% TiO2 (5TYZ, 10TYZ). EDX measurements confirmed incorporation of Ti in the stabilized zirconia phase (Supporting Information). The Raman spectra (Figure 4) of Ti-containing samples show the growth of bands associated to tetragonal zirconia. In the case of tetragonal ZrO2, each Zr atom is surrounded by eight oxygen atoms, four of which formed an elongated tetrahedron and the remaining four a flattened tetrahedron. This brings a change in the Zr-O bond length and bond angle. As it can be seen in Figure 9, the Raman mode at 144 cm-1 does not shift and the Raman modes near 266 and 638 cm-1 continuously move to higher frequencies as the TiO2 content increases. The shifts of the 266 cm-1 mode were much greater than that of the 638 cm-1 one. These bands correspond to the Zr-OII and Zr-OI stretching modes; the evolution of those modes indicates that both the cation-OII and the cation-OI bond lengths are modified

Figure 4. Raman spectrum of samples xTYZ (x ) 10, 15, 20, 30).

Figure 5. (a) N2 adsorption/desorption isotherms and (b) pore size distributions for samples Z, YSZ, and xTYZ (x ) 5, 10, 15, 20, and 30).

with increasing TiO2 content, in agreement with the XRD observations of peak splitting associated to an increasing tetragonal distortion of the lattice with Ti content. The selective shift observed in the Raman modes indicates the occupation of the tetravalent ions Ti4+ in distorted sites in the tetragonal zirconia crystal lattice. The local environment of the tetravalent Ti4+ cations can be 6-fold coordinated to O2- in the ZrO2-Y2O3-TiO2 system, different from that of Zr4+ cations in the binary ZrO2, where cations are 8-fold coordinated. The ionic radius of Ti4+ for 6-fold coordination is 0.074 nm. 3.1.3. N2 Adsorption/Desorption. Before the start of the measurements, all samples were degassed at 150 °C. Figure 5a

Single-Phase Solid Solution (TiO2)x-(YSZ)1-x

J. Phys. Chem. C, Vol. 114, No. 45, 2010 19369

TABLE 2: Summary of the Properties of ZrO2, YSZ, and xTYZ (x ) 5, 10, 15, 20, 30) binding energy, eV SBET (m2/g)

relative pore volume, cm3/g-nm

O 1s

ZrO2

90

0.038

529.9

YSZ

105

0.028

5TYZ

109

0.061

10TYZ

110

0.088

15TYZ

105

0.060

20TYZ

140

0.049

30TYZ

65

0.015

529.9 531.2 529.8 531.1 530.0 531.3 529.9 531.1 530.0 531.2 529.9 531.1

sample

Ti 2p3/2, 2p1/2

458.3 463.6 458.2 463.5 458.1 463.5 458.2 463.6 458.1 463.5

stoichiometry

Y 3d5/2, 3d3/2

Zr 3d 5/2, 3d3/2

157.1 159.1 157.2 159.3 157.1 159.2 157.2 159.3 157.3 159.4 157.2 159.5

182.2 184.6 182.1 184.7 182.2 184.5 182.1 184.6 182.2 186.5 182.1 186.4 182.2 186.5

intended

experimental

Y0.15Zr0.85 OY

Y0.16Zr0.84 OY

Ti0.04Y0.15Y0.81

Ti0.04Y0.16Zr0.80

Ti0.09Y0.15Zr0.76 OY

Ti0.08Y0.16Zr0.76 OY

Ti0.14Y0.15Zr0.71 OY

Ti0.12Y0.14Zr0.74 OY

Ti0.18Y0.15Zr0.67 OY

Ti0.15Y0.13Zr0.72 OY

Ti0.28Y0.15Zr0.57 OY

Ti0.23Y0.13Zr0.64 OY

shows N2 adsorption/desorption isotherms representative of mesoporous Z, YSZ, and xTYZ. The N2 adsorption/desorption isotherm correspond to type B, H3, for sample ZrO2. The H3type hysteresis loop can be assigned to nitrogen condensation in open slit-type capillaries. This loop shape can be associated with a broad distribution of pore size and most of the small pores accessible through the larger ones. It is possible to form aggregates giving rise to lamellar pores. The N2 adsorption/ desorption isotherms corresponding to type-IV curves26 were obtained for the YSZ and TYZ samples. Large hysteresis loops with shapes that are intermediate between typical H1 and H2 type27 are observed for these samples. Incorporation of Y2O3 and TiO2 in ZrO2 lattice lead to enhanced mesoporosity. Thus, the species are formed and

polymerize in aqueous environment organized by the surfactant, leading to formation of crystalline walls and putting the pores in order. The increase of mesoposity for the samples with different content of TiO2 can be explained by the use alkoxide as precursor for Ti and metal chloride for Zr. These undergo various forms of hydrolysis and polycondensation reactions. Formation of a metal oxide involves connecting the metal centers with oxo (M-O-M) or hydroxo (M-OH-M) bridges, therefore generating metal-oxo or metal-hydroxo polymers in solution which are very well organized by the surfactant dispersed in aquous solution. Independent of the shape of the hysteresis loop, the presence of hysteresis is a characteristic marker of mesoporosity.27 The increase of TiO2 content above 20% results in texture destabilization, probably due to particle agglomeration. All samples have a high surface area (Table 2) and narrow pore size distribution (Figure 5b). ZrO2 shows a nonuniform pore size distribution in comparison with the others samples according to the N2 adsorption/desorption isotherm. 3.1.4. TG-DTG. Figure 6a represents the thermogravimetric analysis (TG-DTG) of the as-prepared 10TYZ powders. All TGDTG profiles represent the same behavior with three different stages of decomposition that can be described as follows. The first weight loss that occurred around 100 °C corresponds with desorption and physisorbed water and removal of chemisorbed hydroxyl groups. The second stage occurred before 300 °C corresponding to decomposition of the nitrates, and the third large stage is related to the oxidation process of the residual

Figure 6. (a) TG-DTG and (b) TG-HF analysis of sample 10TYZ.

Figure 7. SEM image of sample 10TYZ.

19370

J. Phys. Chem. C, Vol. 114, No. 45, 2010

Figure 8. TEM micrograph of sample YSZ.

organic materials derived from the surfactant and urea. The effect of urea increases the exothermicity of the reaction. Figure 6b evidences a large exothermic effect occurring in the range 300-600 °C, measured by heat flow (HF) rate, and completed formation of the porous structure around 600 °C. The color of the samples is white, demonstrating that formation of carbon deposition is not possible. Total weight loss was 13%. 3.1.5. SEM, TEM, and HRTEM. SEM. The morphology of synthesized materials, illustrated by SEM (Figure 7), is characteristic of mesoporous materials and consists of very small globular particles. The variation of the amount of titanium does not produce a significant modification of the powder morphology.

Somacescu et al. TEM. Figure 8 shows the TEM micrograph of powders obtained at 550 °C. A clear morphology of dispersed nanoparticles, with uniform grain size around 10 nm, is formed at 550 °C. Mean particle sizes of 11, 7, and 6 nm were calculated from over 200 nanoparticles in TEM micrographs for YSZ, 5TYZ, and 15TYZ. HRTEM. The agreement between grain sizes from TEM measurements and XRD crystalline domain sizes demonstrates that the nanoparticles observed by TEM are single-domain crystallites, as HRTEM measurements confirmed. An interesting aspect to take into account is that no significant particle growth was noticed on powders treated at a temperature of 700 °C; the particle size remained almost constant between 550 and 700 °C, a result observed in TEM micrographs as well as in the XRD-calculated crystalline size obtained by the Debye-Scherrer method. HRTEM was employed to obtain information about the internal structure of the produced TYZ nanocrystallites (Figure 9) and showed a good crystalline order inside the nanoparticles with a random orientation of the single crystallites. The YSZ nanoparticles exhibit clearly resolved lattice fringes with an interplanar spacing of 0.52 nm, assigned to the lattice parameter of the cubic structure, and 5TYZ and 15TYZ exhibit interplanar spacings of 0.36 nm, assigned to the (100) plane of the tetragonal structure, indicating formation of high-quality single-phase solid solution tetragonal TYZ nanocrystals, in agreement with the XRD patterns. The lattice images in Figure 9 show the detail of the lattice image of nanocrystallites. 3.1.6. XPS. XPS analysis was used to determine the chemical states of the elements present on the surface and, after quantitative analysis, to find the chemical state relative con-

Figure 9. HRTEM micrographs of 15TYZ showing the nanocrystalline grains and lattice image of a single tetragonal nanocrystallite.

Single-Phase Solid Solution (TiO2)x-(YSZ)1-x

J. Phys. Chem. C, Vol. 114, No. 45, 2010 19371

Figure 10. XPS deconvoluted photoelectron spectra of (a) C 1s and (b) O 1s and superimposed photoelectron spectra of (c) Ti 2p and (d) Zr 3d.

centrations. It must be emphasized that under XPS conditions the detected volume of our samples can be found in the first 20-25 monolayers, meaning that only 4-5 nm below the surface which obviously could be different from the bulk composition. From XPS spectra (Figure 10a), carbon was found on all surfaces with an average contribution of 35%, being roughly 85% in C-C (CHn) bondings, 9% as carbide, and about 6% in OH-C-O bondings. The relatively high percentage of carbon on the surface suggests the unavoidable hydrocarbon adsorption but also should be considered incomplete combustion of the organic part from the surfactants. By removing the carbon contribution from quantification and performing the corrections for the signals attenuation within this carbon layer we obtain the relative concentrations of the remaining elements O, Ti, Y, and Zr. As a result of the deconvolution procedure (Figure 10b) the O1s photoelectron line consists of two components as follows: BE ) 529.9 and 531.2 eV, which can be assigned to the oxygen bonded in oxides and OH adsorbed groups, respectively. A trace amount is found in the OH-C-O chemical state. FT-IR measurements carried out to reveal the presence of OH groups in the bulk powders failed to detect their presence. By investigating the chemical environment of Ti, Y, and Zr by their binding energies (BE’s) for the most prominent 2p and 3d XPS transitions we identified their fully oxidized states as TiO2, Y2O3, and ZrO2 in agreement with the reference data,28,29 and the intended chemical compositions were calculated. Figure 10c and 10d shows the superimposed Ti2p and Zr3d XPS spectra for the entire set of samples, exhibiting a very good agreement within the range (0.2 eV. The binding energies (BEs) of the most prominent XPS transitions (O1s, Ti2p, Y3d, and Zr3d) for the samples YSZ

and xTYZ (x ) 0, 5, 10, 15, 20, and 30) are presented in Table 2 together with the element relative concentrations leading to the experimental surface stoichiometries. We have to emphasize that the errors in our quantitative data are found in the range of (10%, while the accuracy for BEs assignments is (0.2 eV. The quantitative data processing leading to the surface stoichiometry reveals that the surface and the bulk stoichiometries are very close in samples with the lower content of titanium (x ) 0, 5, and 10). The other samples with a higher content of Ti show a decrease in Ti concentration on the surface and a corresponding increase of the Zr concentration, indicating that at higher Ti concentrations Zr tends to segregate from the bulk to the surface beyond the range of (10% mentioned above. The experimental relative concentrations of Y on the surface are found in good agreement with the intended ones within the range of quantification errors. 3.2. Catalytic Results. The catalytic activities of the samples were investigated by combustion of methane reaction in the temperature range 350-750 °C. All samples arecatalytically active; H2, CO, and CO2 were the detected products from oxidation of methane. It is observed from Figure 11 that addition of yttria and titania in the zirconia lattice increases the catalytic activities. The increase of titania content leads to an enhancement of the conversion of methane. By adding titanium in the YSZ lattice a significant increase of the conversion was observed in the temperature range 550-750 °C as follows: CZ < CYSZ < C5TYZ < C10TYZ. The activity of the catalysts can be related to the availability of oxygen species and the porosity in these samples. As it is well known that TiO2 is more easily reducible than ZrO2, the easier reduction of the titanium cation (Ti4+ to Ti3+) can lead to an enhancement of the oxygen mobility by creation of oxygen vacancies.30 On the other hand, the increase of the relative pore

19372

J. Phys. Chem. C, Vol. 114, No. 45, 2010

Somacescu et al. nanomaterials with complex composition and highly homogeneous structure. Preliminary results suggest promising properties for applications as anode materials in IT-SOFC. Supporting Information Available: Additional information added to show the evolution of the main diffraction features of the pseudo-cubic lattice, to evidence the incorporation of Ti into the YSZ lattice, and to exhibit the appearance of the nanocrystalline grains. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 11. Conversion of methane over Z, YZ, and xTYZ catalysts (x ) 5, 10, 15, and 20).

volume and a high dispersion of TiO2 lead to an enhancement in the conversion rate in 10TYZ. By increasing the content of TiO2 to 15% and 20% the conversion values exhibit a small decrease, correlated with the decrease of the relative pore volume. The sample with 30% TiO2 presented the lowest value of the conversion. This sudden decrease of the conversion is a consequence of the significant reduction of the specific area (60 m2/g) and relative pore volume. 4. Conclusions Mesoporous single-phase TYZ nanopowders with fluorite structure and titania content ranging from 0 to 30 mol % were synthesized and characterized. The self-assembling hydrothermal synthesis route allowed good control of the porous nanomaterials by using surfactants as directing agents, demonstrating complete formation of porous structure at a temperature of 550 °C. The synthesized nanopowders have values of the specific surface area as high as 140 m2/g, a narrow pore size distribution, and homogeneous particle sizes in the range of 6-10 nm. The high dispersion of Ti4+ incorporated into the YSZ lattice was evidenced by XRD and Raman spectroscopy, showing only the fluorite-type solid solution-phase characteristic features without any trace of TiO2, either anatase or rutile phase. The high dispersion of Ti4+ in the nanocrystallites has been proved by EDX and XPS compositional analysis, while HRTEM observations demonstrated the single-domain structure of the TYZ nanocrystallites as stabilized tetragonal domains. All samples were catalytically active in the methane oxidation reaction. Adding Ti4+ into the YSZ lattice increased the catalytic activity up to titania concentration of 10 mol % higher Ti4+ addition resulting in the progressive decrease of both the catalytic activity and the relative pore volume. The decrease in the relative pore volume at Ti4+ for concentrations higher than 10 mol % could explain the decreasing catalytic activity as a surface-sensitive process; additionally it is worth mentioning that XPS surface compositional analysis observed a trend of Ti4+ to diffuse from the surface to the bulk at the higher concentrations, accompanied by Zr segregation from the bulk to the surface. These results demonstrate that the self-assembling hydrothermal route permits direct preparation of TYZ mesoporous

(1) Skarmoutsos, D.; Nikolopoulos, P.; Tietz, F.; Vinke, I. C. Solid State Ionics 2004, 170, 153–158. (2) Mori, M.; Hiei, Y.; Itoh, H.; Tompsett, G. A.; Sammes, N. M. Solid State Ionics 2003, 160, 1–14. (3) Skarmoutsos, D.; Tietz, F.; Nikolopoulos, P. Fuel Cells 2001, 1, 243–248. (4) Tsoga, A.; Naoumidis, A.; Nikolopoulos, P. Acta Mater. 1996, 44, 3679–3692. (5) Tsoga, A.; Nikolopoulos, A.; Naoumidis, P. Ionics 1996, 2, 427– 432. (6) Primdahl, S.; Morgensen, M. Solid State Ionics 2002, 597, 152– 153. (7) Matos, J. M. E.; Anjos Junior, F. M.; Cavalcante, L. S.; Santos, V.; Leal, S. H.; Santos Junior, L. S.; Santos, M. R. M. C.; Longo, E. Mater. Chem. Phys. 2009, 117, 455–459. (8) Eriksson, E.; Rojas, S.; Boutonnet, M.; Fierro, J. L. G. Appl.Catal. A: Gen. 2007, 326, 8–16. (9) Zhu, J.; Van Ommen, G.; Lefferts, L. Catal. Today 2009, 117, 163– 167. (10) Tekeli, S.; Chen, T.; Nagayama, H.; Sakuma, T.; Mecartney, M. L. Ceram. Int. 2007, 33, 869–874. (11) Fagg, D. P.; Frade, J. R.; Mogensen, M.; Irvine, J. T. S. Solid State Ionics 2007, 180, 2371–2376. (12) Wakai, F.; Sagaguchi, S.; Matsuno, Y. AdV. Ceram. Mater. 1986, 1, 259–263. (13) Tao, S.; Irvine, T. S. J. Solid State Chem. 2002, 165, 12–18. (14) Colomer, M. T.; Dura’n, P.; Caballero, A.; Jurado, J. R. Mater. Sci. Eng. 1997, 229, 114–122. (15) Feighery, A. J.; Irvine, J. T. S.; Fagg., D. P.; Kaiser, A. J. Solid State Chem. 1999, 143, 273–276. (16) Traqueia, L. S. M.; Pagnier, T.; Marques, F. M. B. J. Eur. Ceram. Soc. 1997, 17, 1019–1026. (17) Zhu, J.; Van Ommen, J. G.; Bouwmeester, H. J. M.; Lefferts, L. J. Catal. 2005, 233, 434–441. (18) Zhu, J.; Van Ommen, J. G.; Lefferts, L. Catal. Today 2006, 112, 82–85. (19) Zhu, J.; Van Ommen, J. G.; Lefferts, L. J. Catal. 2004, 225, 388– 397. (20) Calderon-Moreno, J. M.; Yoshimura, M. Solid State Ionics 2001, 141-142, 343. (21) Ghosh, A.; Suri, A. K.; Pandey, M.; Thomas, S.; Rama Mohan, T. R.; Rao, B. T. Mater. Lett. 2006, 60, 1170–1173. (22) Phillippi, C. M.; Mazdiyasni, K. S. J. Am. Ceram. Soc. 1971, 54, 254–258. (23) Calderon-Moreno, J. M.; Yoshimura, M. Solid State Ionics 2002, 154, 125–133. (24) Calderon-Moreno, J. M.; Yoshimura, M. Solid State Ionics 2002, 154, 311–317. (25) Mori, Y.; Pinnavaia, T. J. Chem. Mater. 2001, 13, 2173–2178. (26) Di Renzo, F.; Galarno, A.; Quignard, F.; Valange, S.; Galbelica, Z. Adsorption and intrusion methods for the characterisation of mesoporous materials, 2nd ed.; Feza School: Paris, 2008. (27) Gregg, S. J.; Sing, K. S. Adsorption Surface Area and Porosity; Academic: London, 1982. (28) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-Ray Photoelectron Spectroscopy; Physical Electronics USA Inc.: Chanhassen, MN, 1995. (29) NIST X-Ray Photoelectron Spectroscopy Database, Version 4.0; National Institute of Standards and Technology: Gaithersburg, 2008. (30) Ferri, E. A. V.; Sczancoski, J. C.; Cavalcante, L. S.; Paris, E. C.; Espinosa, J. W. M.; de Figueiredo, A. T.; Pizani, P. S.; Mastelaro, V. R.; Varela, J. A.; Longo, E. Mater. Chem. Phys. 2009, 117, 192–198.

JP105834A