Hydrothermal synthesis and characterization of ...

Journal of Solid State Chemistry 203 (2013) 79–85

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Hydrothermal synthesis and characterization of zirconia based catalysts T. Caillot n, Z. Salama, N. Chanut, F.J. Cadete Santos Aires, S. Bennici, A. Auroux Université Lyon 1, CNRS, UMR 5256, IRCELYON, Institut de recherches sur la catalyse et l’environnement de Lyon, 2 Avenue Albert Einstein, F-69626 Villeurbanne, France

art ic l e i nf o

a b s t r a c t

Article history: Received 18 January 2013 Received in revised form 3 April 2013 Accepted 6 April 2013 Available online 12 April 2013

In this work, three equimolar mixed oxides ZrO2/CeO2, ZrO2/TiO2, ZrO2/La2O3 and a reference ZrO2 have been synthesized by hydrothermal method. The structural and surface properties of these materials have been fully characterized by X-ray diffraction, transmission electron microscopy, surface area measurement, chemical analysis, XPS, infrared spectroscopy after adsorption of pyridine and adsorption microcalorimetry of NH3 and SO2 probe molecules. All investigated mixed oxides are amphoteric and possess redox centers on their surface. Moreover, hydrothermal synthesis leads to catalysts with higher surface area and with better acid–base properties than classical coprecipitation method. Both Lewis and Brønsted acid sites are present on the surface of the mixed oxides. Compared to the other samples, the ZrO2/TiO2 material appears to be the best candidate for further application in acid–base catalysis. & 2013 Elsevier Inc. All rights reserved.

Keywords: Zirconia based catalysts Hydrothermal synthesis Adsorption microcalorimetry Acid–base properties Redox properties

1. Introduction Zirconia, due to its high mechanical and thermal stability, presents considerable interest in catalysis. Indeed, it is an important catalytic material with high Lewis acidity, significant basicity and weak Brønsted acidity used in many catalytic reactions in different research areas as for instance petrochemical industry [1,2], fuel cells [3], air remediation [4,5] or glycerol valorization [6]. Zirconia can be used directly as support or can be associated with other oxides which are active under reductive atmosphere [7–9]. For example, its association with ceria can lead to highly selective catalysts for the reaction of Knoevenagel [10], its association with titanium oxide can yield a mesoporous catalyst more active than classic TiO2 catalyst for the combustion of volatile organic compounds [11] and its association with lanthanum oxide can improve the yields of isomerization reactions used in the production of biodiesel [12]. The usual preparative procedures are based on precipitation with ammonia from a solution containing chloride (or oxychloride) precursors followed by a high temperature heat treatment. However, these oxide mixtures exhibit generally a too low surface area (often due to high temperature heat treatment) to obtain satisfactory catalytic conversions. The aim of this work is to develop a protocol to obtain these catalysts with the highest surface area possible. Among the wet chemical preparation methods, hydrothermal route has been recognized as energy and time saver with faster kinetics of crystallization than classic co-precipitation or sol–gel methods. Moreover, it is often used to synthesize nanoparticles of oxides [13–15]. This method has been applied to prepare zirconia based catalysts associated to ceria,

n

Corresponding author. Fax: +33 4 72 44 81 14. E-mail address: [email protected] (T. Caillot).

0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2013.04.005

zirconia and lanthanum oxide respectively. The physico-chemical properties, redox properties and the surface acidity and basicity of the catalysts were characterized with regard to the effect of the compound added to zirconia.

2. Experimental procedure 2.1. Sample preparation All the chemical reactants, ZrOCl2, 8H2O (Aldrich 98%), TiCl4 (Aldrich 99.9%), LaCl3, 6H2O (Aldrich 99.9%), CeCl3, 7H2O (Prolabo 99.9%) and ammonium hydroxide (NH4OH, Prolabo, Normapur, 28%) were reagent grade, and thus, used without further purification. To promote the germination stage, the solution containing the precursors was added dropwise to an ammonium hydroxide solution (pH¼10) under stirring. The pH of the solution was maintained at 10 using a second burette containing ammonium hydroxide. Then, the mixture was transferred to a stainless-steel autoclave equipped with a teflon liner and heated with two electrical resistances. The temperature and the pressure were controlled continuously with a thermocouple and a manometer, respectively. Thermal treatment was applied during 4 h at 220 1C and 25 bars under stirring. After several washes in distilled water in order to eliminate chloride ions, samples were dried in an oven at 120 1C and calcined at 500 1C in air for 10 h. The calcination temperature was chosen on the basis of thermogravimetric measurements. 2.2. Characterization methods After the preparation, the fresh samples were calcined with the aim of removing the chloride ions, water from the bulk and any

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T. Caillot et al. / Journal of Solid State Chemistry 203 (2013) 79–85

carbonaceous species present at the surface of the fresh catalysts. Thermogravimetry (TG–dTG, performed on a “Labsys-TG” instrument from Setaram) was used in order to determine the lowest temperature needed for calcination of fresh samples, at which no significant loss of mass occurred with further increase of temperature. The fresh samples (∼50 mg) were heated from 30 to 900 1C with a heating rate of 5 1C min−1 in a flow of air, which was chosen as a soft oxidizing agent for calcination. The chemical composition of the catalysts was determined using inductively coupled plasma–optical emission spectroscopy (ICP–OES) with an ACTIVA spectrometer from Horiba Jobin Yvon after they were digested in a mixture of inorganic acids (sulfuric and nitric acids). The surface areas, pore volumes and pore sizes were measured by low temperature nitrogen adsorption at 77 K on a Micromeritics Asap 2020 apparatus after heat pretreatment under vacuum for 2 h at a temperature of 350 1C. The Brunauer–Emmet–Teller (BET) method was used to derive surface areas from the resulting isotherms. XRD measurements were performed at room temperature using a Bruker (Siemens) D5005 powder diffractometer using Cu-Kα radiation source (0.15,418 nm) to determine the crystalline phases present on the samples. Diffractograms were recorded from 5 to 701 in 0.021 steps with 1 s per step. The applied voltage and current were respectively 50 kV and 35 mA. Transmission electron microscopy (TEM) experiments were performed in a JEOL JEM 2010 microscope operated at 200 kV. This microscope is equipped with LaB6 thermoionic electron gun, an ultra high resolution (UHR) pole-piece and an energy dispersive X-ray (EDX) spectrometer (Pentafet Link-Isis from Oxford Instruments). The point resolution is 0.196 nm and the information limit is 0.140 nm. Prior to the observation, the samples were diluted in ethanol and ultrasound-dispersed; a drop of the solution was then deposited onto an electron microscopy Cu grid (ϕ¼ 3.05 mm, 300 mesh) coated with a holey-carbon film. The morphology of the samples was characterized by conventional TEM [16–18] whereas local structural information was obtained by high resolution TEM (HRTEM) [18–20]; finally, the global (with de-condensed probes) and local (with condensed 5–25 nm probes providing sufficient current to make EDX analysis within acceptable times) composition was determined by EDX [21–24]. Surface composition was determined by XPS experiments which were performed using a KRATOS Axis Ultra DLD spectrometer equipped with an hemispherical electron analyzer and a monochromatic Al X-ray source (Kα¼1486.6 eV) powered at 150 W. The spot size aperture was 300 700 mm. The base pressure in the analysis chamber was better than 5 10−8 Pa. XPS spectra were measured at a pass energy of 20 eV and a charge neutralizer was used to control charge effects on powder samples. The peaks were referenced to the C–(C, H) components of the C 1s band at 284.6 eV. Shirley background subtraction and peak fitting to theoretical Gaussian–Lorentzian functions were performed using an XPS processing program (Vision 2.2.6 KRATOS). Temperature programmed reductions (TPR) were performed using a TPD/R/O-1100 instrument (ThermoFisher) to determine the samples redox properties. Prior to the TPR run, the fresh sample was treated in a O2/He stream (0.998% flowing at 20 mL min−1); the temperature ramp was set at 10 1C min−1 from 40 to 350 1C and maintained at the latter temperature for 60 min. Subsequently, samples were cooled down to 40 1C. The TPR measurements were carried out using H2/Ar (4.98%) as a reducing gas mixture, flowing at 20 mL min−1. The heating rate was 10 1C min−1 from 40 to 1000 1C. The acid–base properties were studied by adsorption microcalorimetry of NH3 and SO2, respectively. Experiments were performed at 80 1C in a heat flow calorimeter (C80 from Setaram)

linked to a conventional volumetric apparatus equipped with a Barocel capacitance manometer for pressure measurements. The samples (about 100 mg) were pretreated in a quartz cell by heating overnight under vacuum at 350 1C and then evacuated at the same temperature for 1 h prior to the measurements. This temperature was reached using a heating rate of 1 1C min−1. The differential heats of adsorption were measured as a function of coverage by repeatedly sending small doses of respective gas on to the sample until an equilibrium pressure of around 67 Pa was reached. The sample was then outgassed for 30 min at the same temperature, and a second adsorption run was performed at 80 1C until an equilibrium pressure of about 27 Pa was attained. The difference between the amounts adsorbed in the first and second adsorptions at 27 Pa represents the irreversibly adsorbed amount (Virr) which provides an estimation of the number of strong acidic/ basic sites. FTIR spectra after pyridine adsorption were recorded at room temperature with a Bruker Vector 22 FTIR spectrophotometer (DTGS detector) operating in the 4000–400 cm−1 range, with a resolution of 2 cm−1 and 100 acquisition scans. In each pyridine adsorption FTIR measurement, the self-supporting wafer (about 50 mg, 18 mm diameter) was first activated in situ at 400 1C in oxygen flow for 14 h, then evacuated at the same temperature for 2 h and then exposed to pyridine (Air Liquide, 99.8%, vapor pressure 3.3 kPa) at room temperature for 5 min. The desorption was carried out by evacuation for 30 min each at room temperature, 100, 200 and 300 1C, respectively. The spectra were recorded at room temperature after adsorption and desorption at each temperature.

3. Results and discussion Thermogravimetric analyses were carried out to provide information about any decomposition of the sample and also to help in determining the more accurate calcination temperature of the uncalcined samples. TG and dTG curves of the catalysts before calcination are shown in Fig. 1. All samples showed a first mass loss at 100 1C which can be attributed to the releasing of water molecules adsorbed on the catalysts. Between 100 and 500 1C, one large or two separate mass losses were observed for all samples. These mass losses correspond to desorption of physisorbed and, at for the contribution at higher temperature, chemisorbed hydroxyls groups. After 500 1C, no more weight losses were observed except for ZrO2/La2O3. For this sample, a peak at 800 1C is observed and corresponds to a structural change of the solid at high temperature; as evidenced hereafter by XRD and HRTEM/EDX chlorine remains within the sample after calcination at 500 1C leading to the formation of an oxychloride that only decomposes at around 800 1C yielding a mass loss at this temperature (due to the elimination of chlorine). Despite this result, in order to compare our samples after calcination and since the catalysts will not be used at very high temperature, 500 1C was chosen as standard calcination temperature. The values of the chemical analysis and the surface areas of catalysts are presented in Table 1 whereas chemical composition of the surface of catalyst (obtained by XPS) is given by Table 2. Chemical compositions are in agreement with the expected values at the exception of the ZrO2/TiO2 mixture in which a deficit in titanium oxide is observed. This deficit is probably due to the difficulty to manipulate the titanium chloride precursor which is very hygroscopic and decomposes instantaneously in air. Table 2 indicates that all samples are contaminated on their surface by fluorine (between 1 and 2%). This contamination is most certainly due to the Teflon liner of the autoclave [25]. The binding energy of the F1s peak (684.4 eV) is rather representative of a metal fluoride


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Table 1 Results of chemical analysis (ICP) of the investigated samples and BET measurements. Sample

ZrO2/atomic%

Surface area BET/m2 g−1

ZrO2 TiO2/ZrO2 CeO2/ZrO2 La2O3/ZrO2

100 59.7 49.2 50.3

65 209 96 77

Table 2 Surface composition of the samples obtained by XPS.

Fig. 1. TG and dTG curves of un-calcined samples: (a) ZrO2, (b) TiO2/ZrO2, (c) CeO2/ ZrO2 and (d) La2O3/ZrO2.

[26]. This pollution is however not prohibitive for the evaluation of the physicochemical properties of the samples. We must nevertheless prevent its presence on the catalysts in the case of the evaluation of their catalytic performances. The presence of carbon (mainly of graphitic type) is due to the contamination of samples by atmospheric pollutants (volatile

Sample

at% Zr

at% La

at% Ce

at% Ti

at% O

at% C

at% F


26.5 15.8 11.3 8.9

18.8

15.2 -

8.2 -

52.2 51.6 50.9 45.9

19.3 23.2 20.7 24.5

2.0 1.3 1.9 2.0

organic compounds dissociation). The decomposition of C1s peak shows not only the presence of graphitic carbon (284.6 eV) but also the presence of oxygen-bonded carbon with more or less strong biding energies (286.1 and 288.8 eV). These carbon pollutions are often seen on the surface of air-exposed fine powders [27] and are not due to the synthesis procedure. Surface and bulk compositions are different for all the mixed oxides. However, it can be noted that the surface of ZrO2/TiO2 is Zr-rich and Ti-poor when compared to the bulk composition. On the contrary, for the other samples, surface is Zr-poor and (La or Ce)-rich. It can be seen on Table 1 that the addition of lanthanum oxide, ceria or titanium oxide to zirconia increases the surface area of the mixed oxides compared with the zirconia reference. The mixture ZrO2/TiO2 presents a surface area more than two times higher (209 m2 g−1) than the other mixed oxides. In order to characterize the porosity of this sample, we performed adsorption and desorption isotherms under nitrogen (Fig. 2). The obtained curve (Fig. 2a) is characteristic of a mesoporous solid with pore sizes ranging between 2 and 50 nm (type IV isotherm [28]). Adsorption and desorption curves obtained are parallel on a large pressure range. This type of loop is typical of nearly-spherical agglomerated particles with uniform size. The BET surface area obtained (219 m2 g−1) is in agreement with the value obtained by the previous method (209 m2 g−1). The mesopore size varies from 2 to 22 nm with an average value of 13 nm (Fig. 2b). Diffractograms of the samples investigated in this work are presented in Fig. 3. It is visible that in the case of zirconia, a mixture of monoclinic (2θ¼28.3 and 31.61) and tetragonal (2θ¼30.21, 35.31, 49.81, 601) phases is obtained with higher contents of the former phase as often obtained during preparation of zirconia by coprecipitation [29,30]. For La2O3/ZrO2, the diffractogram shows monoclinic zirconia accompanied by another phase containing lanthanum which could be a lanthanum oxychloride (difficult to define exactly). This result suggests that no or little interaction has occurred between lanthanum and zirconium during precipitation and that the calcination temperature is too low to obtain lanthanum oxide. Moreover, calcination at higher temperature will most certainly lead to a mixture of monoclinic zirconia and La2O3. The diffractogram of the mixture CeO2/ZrO2 shows the formation of a cubic solid solution (CexZr1−xO2) already identified in the literature [31]. Finally, the mesoporous TiO2/ZrO2 is totally amorphous after calcination at 500 1C. Zirconia (monoclinic or tetragonal) or titanium oxide (anatase, rutile or brookite) characteristic of the two compounds [32] or mixed oxide phases are not observed by XRD. This type of amorphous phase has already

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Fig. 2. (a) Isotherms of adsorption and desorption of N2 at 77 K and (b) pore size and distribution for TiO2/ZrO2 mixed oxide.

Fig. 3. XRD diffractograms of the studied samples: (a) ZrO2, (b) TiO2/ZrO2, (c) CeO2/ ZrO2 and (d) La2O3/ZrO2.

been highlighted in the literature for equimolar compositions Zr/Ti [33]. Fig. 4 presents typical TEM images obtained for the mixed samples and for the reference. The latter is composed of facetted particles with particle sizes ranging from 10 to 20 nm (Fig. 4a). The surface is perfectly clean and the crystallographic planes and atomic steps are clearly resolved at the surface. The region presented in Fig. 4a is representative of the sample; the particle is oriented in the [1 0 −1] direction with resolved (1 −1 1), (0 2 0) and (1 1 1) interplanar distances. The measured distances (d(0 2 0) 0.260 nm, d(1 1 1) ¼ d(1 −1 1) 0.284 nm) and angles (angle between (1 1 1) and (0 2 0) 571, angle between (1 1 1) and (1 −1 1) 661) are very close to the values of monoclinic zirconia. Tetragonal zones are difficult to define. Nevertheless, they are detected on the surface of some grains (not shown). This result suggests that ZrO2 sample is composed of monoclinic phase in a large majority. CeO2/ZrO2 and La2O3/ZrO2 samples are heterogeneous with “cigar”-shaped large particles and agglomerates of smaller particles are observed (Fig. 4b and d). For CeO2/ZrO2 sample, “cigar”shaped particles are formed by an agglomeration of smaller particles. Conversely they are rather monolithic in the case of La2O3/ZrO2 sample. Nevertheless, in both cases, EDX analysis shows that these “cigar”-shaped particles (whether formed by the agglomeration of particles or in the monolithic form) are poor in zirconium. For La2O3/ZrO2, chlorine is detected in the “cigar”shaped particles indicating the presence of an oxychloride. This observation is in agreement with the XRD results and confirms that calcination temperature was too low to obtain lanthanum oxide for this system. The agglomerates of smaller particles coexisting with “cigar”-shaped particles are, in both cases (La2O3/ ZrO2 and CeO2/ZrO2), rather rich in zirconium.

For CeO2/ZrO2 sample, despite the very heterogeneous composition of the particles all grains (“cigar”-shaped particles or agglomerated particles) are identified as CexZr1−xO2 in cubic or in tetragonal system depending on the composition. Fig. 4c presents an example of a particle oriented in the [0 −1 1] direction with resolved (2 2 2), (−4 0 0) and (−2 2 2) interplanar distances. The measured distances (d(−4 0 0) 0.265 nm, d(−2 2 2) ¼d(2 2 2) 0.306 nm) and angles (angle between (2 2 2) and (−2 2 2) 70.51, angle between (−4 0 0) and (−2 2 2) 54.71) are in agreement with a cubic solid solution. TiO2/ZrO2 sample is essentially composed of amorphous grains (Fig. 4e) with some crystalline regions also present (Fig. 4f). The composition determined by EDX vary between 35% of zirconia (65% of titania) and 65% of zirconia (35% of titania). Crystalline regions observed were identified as monoclinic zirconia or as anatase titania depending on the regions. As an example of a crystalline region we present in Fig. 4f a region identified as monoclinic zirconia oriented in the [0 1 −1] direction with resolved (2 0 0), (−1 1 1) and (1 1 1) interplanar distances. The measured distances (d(−1 1 1) 0.316 nm, d(1 1 1) 0.284 nm and d(2 0 0) 0.254 nm) and angles (angle between (−1 1 1) and (1 1 1) 721, angle between (1 1 1) and (2 0 0) 501) are in agreement with the above mentioned phase of zirconia. These regions are however rare and rather small and as such are not detectable by XRD but can be locally observed by HRTEM. TPR profiles of the prepared samples as a function of temperature are presented in Fig. 5. Contrary to the reference, all mixed oxides present a reduction behavior. La2O3/ZrO2 and CeO2/ZrO2 samples present a large peak between 600 and 800 1C and between 500 and 700 1C, respectively. These large peaks are composed of overlapping peaks with the different components corresponding to the surface and to the bulk reduction [34] as well as to surface defects. For instance, the oxide surfaces can have imperfections such as steps, kinks or corners with O2− differently coordinated and yielding reduction peaks at different temperatures [35]. For TiO2/ZrO2, two different peaks are observed at 570 and 700 1C corresponding to the surface and to the bulk reduction of TiO2 respectively [36]. The adsorption of probe molecules was used to investigate the acid–base character by infrared spectroscopy and microcalorimetry methods. Since the principles of these two techniques are not the same the information obtained from these investigations is considered as complementary but cannot be directly compared. Adsorption microcalorimetry of ammonia or sulfur dioxide allows the precise determination of the concentration and the strength distribution of surface active (basic or acidic) sites whereas infrared spectroscopy measurement of adsorbed pyridine allows to differentiate the type of acidic sites (Lewis or Brønsted). Figs. 6 and 7 present the isotherms obtained for the adsorption of NH3 (right) and SO2 (left) and the differential heats of


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Fig. 4. TEM micrographs of the studied samples: (a) ZrO2, (b and c) CeO2/ZrO2, (d) La2O3/ZrO2 and (e and f) TiO2/ZrO2. 2D-FFTs are inserted in the HRTEM images (a, c, f); (see text for details).

Fig. 5. TPR profiles obtained for the studied samples: (a) ZrO2, (b) TiO2/ZrO2, (c) CeO2/ZrO2 and (d) La2O3/ZrO2.

Fig. 6. Volumetric isotherms of NH3 and SO2 adsorption on catalysts.

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adsorption (of NH3 and SO2) for the investigated samples and Table 3 compiles the data (initial heat of adsorption, total amount of adsorbed gases (Vtot) and amount of irreversible chemisorbed gases (Virr)) obtained from microcalorimetric measurements. In the case of NH3 adsorption, initial heats are larger than 150 kJ mol−1 for the reference ZrO2 and for TiO2/ZrO2 (Table 3). These values indicate that strong acid Lewis sites are present on the surface of these systems which is not the case for the other systems. For SO2 adsorption, all samples present high initial heats indicating the presence of strong basic chemisorptions sites on the surface of all samples. However, the differential heat curves (Fig. 7) show a decrease in energy as a function of coverage as usually observed for heterogeneous surfaces [37,38]. Different types of acidic and basic sites (Brønsted and Lewis) are also present on the surface of all samples. The NH3 adsorption isotherms (Fig. 6 right) clearly showed that the addition of lanthanum oxide has no effect on the amount of ammonia adsorbed. In contrast, the addition of cerium oxide and especially of titanium oxide leads to a larger adsorption of ammonia than for the reference ZrO2 (three times larger for TiO2/ZrO2). These results are confirmed and detailed in Table 3 where the total and irreversible adsorbed amounts are very important for TiO2/ZrO2. The SO2 adsorption isotherms (Fig. 6 left) demonstrated that the addition of lanthanum oxide, cerium oxide and especially of titanium oxide favors the adsorption of sulfur dioxide which is larger for the mentioned samples than for the ZrO2 reference (2.5 times larger for TiO2/ZrO2). It is important to note that, contrary to the acidity study, for the basicity study the amount of strong basic sites is important for the three mixed oxides. Finally, TiO2/ZrO2 is the only catalyst which presents high acidity and basicity properties simultaneously with strong acidic

Fig. 7. Differential heats of NH3 and SO2 adsorption as a function of adsorbed volume.

and basic chemisorptions sites. These surface properties can be directly related to the mesoporous structure and to the largest surface area measured for this sample. The relevant parts (1400–1700 cm−1) of collected infra-red spectra are shown in Fig. 8. All reported spectra were obtained by subtracting the spectrum of the fresh catalyst (without pyridine adsorption at room temperature) from those after pyridine adsorption. The spectra obtained for the sample containing lanthanum are saturated despite several acquisition attempts. This usually occurs when carbonates are present on the surface of the samples containing lanthanum [39]. The peaks at 1445 and at 1606 cm−1 are characteristic of pyridine coordinated to Lewis acid sites while the bands at 1637 and 1542 cm−1 are characteristic of pyridine ions bonded to Brønsted acid sites [40]. The bands around 1489 and 1575 cm−1 are associated simultaneously to both Brønsted and Lewis acid sites [41]. For TiO2/ZrO2 and for the reference ZrO2, acidic sites are mainly Lewis sites and a small amount of mixed sites (Lewis–Brønsted) is detected. On the contrary, for CeO2/ZrO2 and La2O3/ZrO2 samples, very few strong Lewis sites together with Brønsted and mixed sites are detected. These results are in agreement with the microcalorimetry results obtained for ammonia adsorption (high initial differential heats for ZrO2 and TiO2/ZrO2 and low initial differential heats for CeO2/ZrO2 and La2O3/ZrO2).

4. Conclusion Three equimolar mixtures ZrO2/CeO2, ZrO2/TiO2, ZrO2/La2O3 and a reference ZrO2 have been synthesized by the hydrothermal method and fully characterized. The addition of lanthanum to zirconia did not led to the formation of a mixed oxide but, instead, to a two-phase system containing oxychlorides. The increase of surface area is low and the proportion of strong acid sites on surface decreases by comparison to the ZrO2 reference. Moreover, the addition of lanthanum increased the basicity of the surface and, particularly the number of strong basic sites. This mixture can be more interesting than pure ZrO2 for applications requiring the presence of strong basic sites. The addition of cerium oxide to zirconia led to a heterogeneous solid solution CexZr1−xO2 with a surface area one and a half times larger than the reference. In addition, the proportion of acidic and basic sites is much more important than those of the reference ZrO2. However, these sites are numerous but are not strong Lewis sites but rather Brønsted sites in the majority. This mixture will be more suitable for applications requiring a large amount of Brønsted acidic sites type. Finally, the addition of titanium oxide to zirconia led to a mesoporous amorphous phase with a surface area three times larger than that of the reference ZrO2. In addition, this sample presents a surface with interesting acidic and basic properties by comparison to the reference and to the other mixed samples with a very important amount of strong Lewis acidic and basic sites. This mixture is also the most promising for further application in acid–base catalysis.

Table 3 Initial heat of adsorption, Vtot and Virr determined by microcalorimetry of NH3 and SO2 on the different materials. Sample


Acidity (NH3 adsorption)

Basicity (SO2 adsorption)

Qinit (kJ mol−1)

Vtot (mmol g−1)

Virr (mmol g−1)

Qinit (kJ mol−1)

Vtot (mmol g−1)

Virr (mmol g−1)

165.1 174.8 149.7 135.0

178.3 573.9 274.4 180.6

88.7 354.9 155.0 52.3

221.7 170.7 168.3 173.4

210.2 414.9 247.8 286.3

194.6 354.9 209.7 263.8


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Acknowledgments The authors wish to express their thanks to Mr. B. Beguin and Pr. A. Kaddouri for fruitful discussion and encouragement's, to Dr. S. Prakash for XPS experiments, to Mrs. N. Cristin for BET and isotherm measurements and all services of IRCELYON. References

Fig. 8. FTIR spectra for pyridine desorption on the investigated materials: (A) ZrO2, (B) TiO2/ZrO2, (C) CeO2/ZrO2 and (D) La2O3/ZrO2 at different temperatures: (a) 25 1C, (b) 100 1C, (c) 200 1C and (d) 300 1C.

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