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TiO2 catalysts prepared by polyol-mediated synthesis
Applied Catalysis A: General 325 (2007) 309–315 www.elsevier.com/locate/apcata
Chlorinated organics total oxidation over V2O5/TiO2 catalysts prepared by polyol-mediated synthesis S. Albonetti a,*, G. Baldi b, A. Barzanti b, A.L. Costa d, J. Epoupa Mengou a,c, F. Trifiro` a, A. Vaccari a a
Department of Industrial Chemistry and Materials, Viale Risorgimento 4, 40136 Bologna, Italy b CERICOL, Centro Ricerche Colorobbia, Via Pietramarina 53, 50053 Sovigliana Vinci, Italy c INCA, Interuniversity Consortium, Dorsoduro 2137, 30123 Venezia, Italy d CNR-ISTEC, Via Granarolo, 48018 Faenza, Italy Received 21 July 2006; accepted 7 February 2007 Available online 3 March 2007
Abstract Vanadia and titania/vanadia sols synthesized by the polyol method were used to prepare TiO2-supported catalysts. Both the modification of the V2O5 sol with a low amount of water and the use of a TiO2/V2O5 sol induced a reliable adhesion between the preformed active phases and the support, thus leading to stable catalysts. Despite the low amount of vanadium introduced in studied samples, V2O5/TiO2/WO3 catalysts were active in the o-DCB oxidation. The best results were obtained with 0.36-V2O5–TiO2 catalyst, prepared by direct impregnation with colloidal TiO2/V2O5mixed oxides. The electroacoustic analysis indicated that the support does not show any particular electrostatic affinity for any studied sol; therefore, the higher ability of TiO2/V2O5 sol to form stable vanadium active species may possibly be explained by a better chemical affinity with the support. # 2007 Elsevier B.V. All rights reserved. Keywords: Polyol-mediated synthesis; Catalytic total oxidation; V2O5/TiO2 catalysts; Preformed oxide sols
1. Introduction It is now accepted knowledge that for the preparation of supported catalysts, control at the atomic level is needed in order to design active sites, because the chemical and catalytic properties of atoms at terraces, corners, and edges of a metal crystallite are different [1]. Due to the fact that the preparation of nanoparticles is most controlled in the liquid phase, a promising way for the preparation of nanosized supported catalysts is the deposition of metal oxide sols – in which the particles are preformed – on the support surface [2]. Different preparations can be used to obtain preformed sols: among them is polyol-mediated synthesis, originally used to prepare nanosized metal and alloy particles, which allows both the accurate and reproducible control of the mean diameter of the particles in a broad size range and the mixing of the reactants at
* Corresponding author. Tel.: +39 051 2093681; fax: +39 051 2093680. E-mail address: [email protected] (S. Albonetti). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.02.031
the molecular level [3]. In this procedure, ‘‘polyol’’ stands as a general term for polyalcohols with high boiling temperature and sufficient ability to solve inorganic salts. For the preparation of metal particles the polyol method was applied because of its mild reducing properties, but polyol also has a chelating effect which avoids agglomeration of particles during preparation. The synthesis of mono or polymetal particles of cobalt, nickel, copper and noble metals in the submicrometer and nanometer size range has been reported [4–6], and materials obtained by this method show homogeneous phase composition, narrow particle distribution, and high specific surface area. Recently, this method has received considerable attention because of its simplicity and the advantage, over most other methods, of preparing highly pure mixed oxides [7] and a variety of other materials, including sulfides and phosphates, which are obtained under very similar experimental conditions [8,9]. In the case of oxide synthesis, it can be understood to be a sol–gel process carried out at high temperatures with an accurate control over particle growth [10]. In fact, polyol-mediated preparation of
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nanoscale oxides can be carried out by dissolving a suitable metal precursor (acetate, alcoholate, halogenide) in diethylenglycol (DEG) or other polyalcohols, and then adding a given amount of water. The mixture will then be rapidly heated at 180–240 8C and, at a given temperature, sudden precipitation of the oxide occurs. During this step, the surface of the growing particles will be immediately complexed by DEG as a chelating agent which limits grain growth. In our work, we used vanadia and titania/vanadia sols synthesized by the polyol method for preparing TiO2-supported catalysts, while checking the catalytic activity and stability of prepared samples in the total oxidation of chlorinated organic compounds [11]. 2. Experimental 2.1. Catalyst preparation Nanosized vanadia, titania and titania/vanadia suspensions were prepared via a polyol-mediated synthesis [12]. The preparation was performed by a controlled hydrolysis of vanadyl acetylacetonate and titanium isopropoxide in DEG at 160–180 8C. Fig. 1 shows the general flow chart for the preparation of the materials. The stoichiometric amount of suitable metal precursor was dispersed in DEG. The mixture was heated up to T1 (120–140 8C) with constant stirring, until a clear solution was obtained. An excess of water, as a hydrolysis agent, was then added and the emerging suspension was heated to T2 (160–180 8C) and left under vigorous stirring for 2 h under reflux. For the preparation of the TiO2/V2O5 system the synthesis was achieved in two steps: (i) the necessary amount of vanadyl acetylacetonate was added to the preformed titanium sol in the presence of a small quantity of water; (ii) the suspension was then left under reflux at 180 8C for 2 h. Prepared sols are listed in Table 1.
Table 1 Composition and particle size dimension (D50) of studied sols Sample
TiO2 content (wt.%)
V2O5 content (wt.%)
D50 (nm)
V2O5 sol TiO2 sol TiO2/V2O5 sol
0 6 6
0.9 0 0.5
147 22 28
To obtain the supported catalysts, Millennium Chemicals DT52 commercial powder (TiO2/WO3 = 90:10, w/w) was used as support. The suspensions were deposited onto TiO2/WO3 powder by incipient wetness impregnation, in order to verify the possibility of obtaining a highly dispersed active phase through an economic and well-established method. The obtained powders were dried in an oven for 15 h at 120 8C, and then calcined at 500 8C for 6 h. Prepared catalysts are listed in Table 2. 2.2. Characterization of nanoscaled suspensions The z-potentials of sols were measured by an electroacoustic method based on the measure of the electrokinetic sonic amplitude (ESA) signal, which is generated in organic suspensions when an alternating electric field is applied (AcoustoSizer II, Colloidal Dynamics, Warwick, USA) [13]. This technique is applicable to water and organic suspensions. This instrument determines the z-potential of the particles by fitting the volume average dynamic mobility over a range of 13 different frequencies (ranging from 1 to 18 MHz) in the set alternate electric field. The supporting powder suspension was prepared in water at an electrolyte strength of 10 mM KCl (Merck), with 4 wt.% solid concentration by ultrasonic mixing for 1 h. An automatic titration software was used to measure zpotential as a function of pH between natural pH and basic pH, in order to detect the point of zero z-potential, termed isoelectric point (pHiep). The catalyst DEG sols were measured in their current state. In this case, the attenuation in the glycol solvent was measured, and the result was subtracted from the sample measurement (background correction) in order to bring the particle contribution to the total attenuation. For all non-polar and polar solvent samples (samples not in water), the background correction is necessary because the attenuation of sound for the solvent significantly contributes to the attenuation spectrum of particles. The average particle sizes in suspension were measured by dynamic light scattering using a Malvern Zetasizer Nano S. Table 2 Vanadium oxide content, BET surface area and TiO2 anatase crystallite dimensions for studied catalysts
Fig. 1. Flow chart of the preparation method for materials used.
Sample
V2O5 content (wt.%)
Surface area (m2/g)
Crystallite size of TiO2 anatase (nm)
Support 3-V2O5 0.6-V2O5 0.6-V2O5–H2O 0.36-V2O5–TiO2
0 3 0.6 0.6 0.36
74 28 75 73 74
18 25 19 20 19
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2.3. Characterization of catalysts Surface areas were measured by N2 physisorption using a Carlo Erba Sorpty 1750 instrument after a vacuum pretreatment at 200 8C. The reduction behavior of vanadium oxide species was studied by TPR using a Termoquest TPDRO instrument. The reducing gas consisted of 5 vol.% of H2 in Ar. Before reduction, the catalysts were calcined at 500 8C for 1 h to achieve the V5+ state. X-ray diffraction patterns were obtained in the range of 10– 808 by a diffractometer Philips PW 1710, using Ni-filtered Cu ˚ ). The average crystallite sizes of Ka radiation (l = 1.5432 A the synthesized powders were determined from the full-width at half-maximum (FWHM) of the XRD diffraction peaks using the Debye–Scherrer equation. The Raman spectra were obtained by using the 514 nm line of an Ar+ ion laser, with a Renishaw Raman Spectroscopy System 1000 with Leica DMLM microscope. The laser power at the sample location was 50 mW; the experiments were carried out at ambient conditions. 2.4. Catalytic tests Catalytic experiments were carried out in a fixed-bed glass reactor under atmospheric pressure. Each run used approximately 350 mg of catalyst in the form of 30–60 mesh (250– 595 mm) particles, mixed with 1120 mg of corundum grains of similar size for best temperature control. The total volumetric flow through the catalyst bed was held constant at 140 ml/min (measured at atmospheric pressure and room temperature), 10 vol.% O2, 90 vol.% N2 and 1400 ppm of o-dichlorobenzene. Analysis of reactants and products was carried out as follows: the products in the outlet stream were scrubbed in cold acetone maintained at 25 8C by a mixture of dry ice and glycol. The amounts of reagent and products condensed during a reaction period of 15 min at steady-state conditions were analyzed with a GC (Perkin Elmer Autosystem XL) equipped with a PE-17 capillary column (30 m 0.25 mm, methylpolysiloxane series) and an electron capture detector (ECD) which uses 1,2dichloropropane as a standard reference. Additionally, the exit-flow of the reactor trapped in acetone was injected into a GC with a mass selective detector (Hewlett Packard G1800A) in order to confirm GC data and to verify the formation of nonchlorinated compounds. The CO and CO2 formed were separated on a capillary column Elite Plot Q (30 m 0.32 mm), attached to a methanizer and analyzed by FID. The C balance values always fell between 95% and 105%, calculated as the comparison between converted o-dichlorobenzene and the sum of the product yields.
Fig. 2. Particles size distribution of vanadia (dotted line) and titania/vanadia (solid line) particles in sols.
general, average particle diameter can be guided by adjusting temperature and duration of the reaction, as well as by changing the concentration of metal precursor and water amount. The increase in these parameters generally leads to enlarged particles [14–16]. V2O5 stable sol was more difficult to obtain, and the metal precursor concentration must be kept low in order to control the grain growth. Conversely, the particle size of TiO2 nanoparticles was very low even at high concentration, and the introduction of vanadium did not significantly increase it. If, however, the DEG suspension is mixed with water (DEG/ H2O = 1:10, v/v), the colloid breaks down and rapid agglomeration of the oxide particles occurs (Fig. 3), due to the replacement of the chelating agent on the particle surface with water. The results of the electroacoustic analysis (zpotential, conductivity) on oxide sols are reported in Table 3. In all the cases, nanoparticle surfaces showed a highly positive charge which is characteristic of oxide powders dispersed in organic solvents. The basic site of oxides leads to the deprotonation of DEG and the consequent formation of positive charges. The absolute value of z-potential ensures a good electrostatic stabilization in the DEG sols measured and, together with the monodisperse average particle diameters measured by laser diffraction, proved the surface role of DEG on particle growth and agglomeration. Considering that the electrostatic repulsion (Derjaguin equation) depends on both the product of solvent dielectric constant and the square of the surface potential, it can be
3. Results and discussion 3.1. Characterization of preformed nanoparticles Laser diffraction investigation of oxide particle suspensions indicated the presence of monodisperse particles (Fig. 2). In
Fig. 3. Particle size distribution of vanadium oxide particles in DEG (solid line), 300 after mixing with 10% water (- –) and 900 after mixing with water (- - -).
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Table 3 Results of electroacoustic analysis Sample
Zeta potential (mV)
Conductivity (S/m)
TiO2/WO3 support (in water) TiO2 DEG sol TiO2/V2O5 DEG sol V2O5 DEG sol
0.2 68 81 118
0.377 0.004 0.002 0.011
concluded that the surface potential in DEG (dielectric constant = 32) should be 1.6 times higher than in water (dielectric constant = 78) in order to provide the same repulsion. If 40 mV are requested in water, about 64 mV are required in DEG in order to achieve a good electrostatic repulsion [17]. The higher positive charge of the V2O5 surface in comparison with the TiO2 surface is probably due to the presence of acetylacetone as a reaction product which protonates the basic surface sites of the powder. The presence of acetylacetonate species is confirmed by the high value of conductivity of V2O5 sol in comparison with the other sols. The titania/vanadia sol has a positive charge higher than TiO2 sol as a consequence of the vanadium presence on the TiO2 surface. The great increase in zeta potential, which passes from 68 to 80 mV with the addition of only 5 wt.% of vanadium, indicates the prevailing contribution of vanadia surface to the zeta potential. This result confirms the complete adsorption of V2O5 powder onto the TiO2 surface. Nevertheless, the zeta potential value obtained (80 mV) is not as high as in the case of vanadia sol (117 mV) because of the low concentration of vanadium and, consequently, of the low amount of acetylacetone originated from the synthesis. In order to clarify the interactions between the support surface and the colloidal particles, the surface charges of supporting power (TiO2/WO3) were also examined. The zeta potential was measured in water, in order to simulate the charges developed into the surface when it is in contact with atmospheric humidity (Fig. 4). Nevertheless, the value of zeta potential close to zero at natural pH indicates that this surface is not able to show a particular electrostatic affinity for the impregnated sol. The oxide particles prepared by the polyol method can be investigated more closely after separation from the suspension
Fig. 4. Zeta potential of TiO2/WO3 commercial support as a function of pH.
Fig. 5. X-ray powder diffractograms of TiO2 sol dried and calcined at different temperatures. (a) T = 300 8C; (b) T = 500 8C; (c) T = 700 8C; (d) T = 900 8C.
Fig. 6. Conversion of o-dichlorobenzene as a function of reaction temperature for studied catalysts. TiO2/WO3 support (); 3-V2O5 (*); 0.6-V2O5 (^); 0.6V2O5–H2O (^).
by centrifugation. The resulting powders are poorly crystalline immediately after their synthesis at a maximum temperature of 180 8C. According to XRD data (Fig. 5), in the case of TiO2 and TiO2/V2O5 sols, the only crystallographic phase identified up to
Fig. 7. Conversion of o-dichlorobenzene as a function of reaction temperature for stable catalysts. TiO2/WO3 support (~); 0.6-V2O5–H2O (^); 0.36-V2O5– TiO2 (*).
S. Albonetti et al. / Applied Catalysis A: General 325 (2007) 309–315
700 8C was the anatase form of TiO2 and the crystallite dimension of anatase was calculated to be about 6 nm. Conversely, in the beginning, V2O5 nanoparticles were completely amorphous (data not reported) and only a thermal treatment at 600 8C led to a significant increase in crystallinity unlike what has been reported by Feldmann [14]. 3.2. Catalytic tests Recently, V2O5/TiO2-based catalysts, which are commercially employed for the reduction of NOx via NH3-SCR, have also been found to be active for the destruction of dioxins,
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furans and chlorinated compounds present in gaseous streams [18–22]. In a previous study of ours, we investigated the oxidation of 1,2-dichlorobenzene (o-DCB) over V-supported catalysts [23]. Thus, this experience was the basis for verifying the possible application of vanadia and titania/vanadia sols synthesized by polyol-mediated method for preparing stable and active TiO2-supported catalysts. The results of o-DCB conversion on catalysts prepared utilizing vanadium sol are summarized in Fig. 6, where the behavior of the pure support is also reported. The pure TiO2/ WO3 exhibited some activity in the reaction, mainly due to WO3, but the conversion value significantly enhanced with the
Fig. 8. (a–d) Raman spectra of studied catalysts. (1) Support; (2) 3-V2O5; (3) 3-V2O5 after catalytic test (4) 0.6-V2O5; (5) 0.6-V2O5–H2O; (6) 0.36-V2O5–TiO2; (7) 0.6-V2O5 after catalytic test.
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introduction of 3 wt.% of vanadia (3-V2O5 sample). Despite this high initial activity, the deactivation of the catalyst prepared by multiple impregnations of colloidal vanadia was very fast, thus suggesting poor adhesion of active species on the support surface. To avoid any possible interference due to multiple impregnation/drying steps, a new catalyst was prepared with lower vanadium content by a single impregnation (0.6-V2O5). However, in this case also, the rapid deactivation of the catalyst indicated a weak interaction between the support and vanadium oxide particles. Since the particle-size-distribution measurements indicated that mixing the DEG suspension with water caused a drop in colloidal stability, due to the replacement of the chelating agent on the particle surface, water addition was used to verify possible changes in the colloidal particle adhesion behavior. Supported catalysts prepared with this destabilized sol (0.6V2O5–H2O) showed higher stability in comparison to the analogous 0.6-V2O5 catalyst. High stability was also obtained by direct impregnation with colloidal TiO2/V2O5-mixed oxide (0.36-V2O5–TiO2): despite the low amount of vanadium introduced, this sample showed very interesting catalytic results (Fig. 7). Since the results of electroacoustic analysis indicated that the support does not show any particular electrostatic affinity for any of the impregnated sols, the higher ability of TiO2/V2O5 sol – if compared to V2O5 sol – to form stable vanadia-active species might be explained by a better chemical affinity of TiO2sol with the support. In fact, due to the presence of TiO2 nanoparticles, chemical bonding via hydrogen bridges, as well as direct Ti–O–Ti, can increase adhesion of TiO2/V2O5 colloidal particles on the TiO2/WO3 support surface. 3.3. Catalyst characterization In order to investigate the effect of preparation on the catalyst morphological properties, XRD and surface areas were measured and the results reported in Table 2. The data obtained clearly show that a significant loss of surface area and a growth of support crystallites occurred through multiple additions of V2O5 sol (catalyst 3-V2O5). Conversely, these parameters remain unchanged after single impregnation and following calcination, with all nanosized suspensions, thus indicating that the nanoparticles did not block the pore of the support. XRD analysis of all samples (not reported) detected only the lines due to the anatase polymorphic form of TiO2, thus indicating that the presence of vanadia nanoparticles did not destabilize the TiO2/WO3 support. In fact, in the case of structural degradation, the WO3, which is present in significant amounts, would be segregated. The Raman spectra for the various catalysts studied (Fig. 7a– d) show the bands of anatase at 632, 514, 390 cm1 associated with weak peaks at 965–980 and 770–800 cm1 for all the catalysts. The higher frequency Raman feature is attributable to superimposed W O and V O stretching of monomeric wolframyl and vanadyl species, while the band at about 780 cm1 consists of the overlapping contribution of a secondorder anatase feature, as well as of the W–O–W stretching
modes of structures similar to WO3 [24]. The addition of sols to the support gave rise to the appearance of a new Raman band at about 880 cm1, which could indicated the existence of some organic residues on the calcined powders [25,26] or to be related to the formation of tungsten–vanadia-mixed species. In fact, this band was earlier attributed, by the DFT calculations, to the stretching vibrations of tungstyls which substitute vanadyls in vanadia monolayer [27]. None of the spectra showed the presence of 995 and 694 cm1 bands, which are attributable to microcrystalline or amorphous V2O5 [28], thus proving both the effective spread of vanadium nanoparticles on support surface and the lack of significant agglomeration phenomena. As for general behavior, more stable samples (0.6V2O5–H2O and 0.36-V2O5–TiO2) showed a significant shift in the frequency of 965 cm1 band – assigned to W O and V O stretching mode – moving from this value to 982 and 978 cm1 for 0.6-V2O5–H2O and 0.36-V2O5–TiO2, respectively (Fig. 8b and c). This shift was previously reported for samples with increasing vanadium content [29] and suggests that different two-dimensional vanadium oxide species may be present in stable and non-stable samples. A similar shift was also observed for unstable samples after catalytic test, thus indicating that highly coordinated vanadia species are also formed in reaction conditions (Fig. 8d). Complementary to the Raman spectroscopic experiments, the measurement of the variation in the dispersed vanadium phase among supports can be provided by the TPR behavior. Unfortunately, in our samples, even in the presence of a very low vanadium amount in samples, two main reduction peaks were present, with a maximum at 505–540 8C and about 630–650 8C, respectively. In fact, as already reported [30], in WO3-containing samples, the exact determination of vanadium active species distribution from TPR analysis is rather difficult, due to the partial reduction of tungsta in the same region as vanadia. Nevertheless, our results, reported in Figs. 9–11, can provide some useful information. In particular, with respect to studied samples, the first peak – which is mainly due to highly dispersed vanadium strongly interacting with the support – can be analyzed. Stabilized samples 0.6-V2O5–H2O and 0.6-V2O5 after catalytic tests, showed the splitting of this first peak (Figs. 9 and 10), thus suggesting the formation of different
Fig. 9. H2-TPR of supported catalysts. (a) 0.6-V2O5; (b) 0.6-V2O5–H2O.
S. Albonetti et al. / Applied Catalysis A: General 325 (2007) 309–315
Fig. 10. H2-TPR of supported catalysts. (a) 0.6-V2O5; (b) 0.6-V2O5 after catalytic tests.
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preformed active phases and the support can be achieved, thus leading to stable catalysts. Despite the low amount of vanadium introduced in the studied samples, it was showed that the V2O5/ TiO2/WO3 catalysts obtained by impregnation of TiO2/WO3 with preformed sols were active in the o-DCB oxidation. The best results were obtained with 0.36-V2O5–TiO2 catalyst, prepared by direct impregnation with colloidal TiO2/V2O5mixed oxides. This sol shows an extremely interesting behavior, since the vanadium species seemed highly spread on the titanium nanoparticles, and could be used with very high surface area supports in order to maximize the total oxidation properties of the catalysts. Additional studies are in progress to assess the effect of different active phases and supports on the o-DCB oxidation. References
Fig. 11. H2-TPR of supported catalysts. (a) Support; (b) 0.6-V2O5–H2O; (c) 0.36-V2O5–TiO2.
vanadium oxide species, as already shown by Raman analysis. In particular, these different active phases were reducible at slightly a higher temperature than vanadia present in 0.6-V2O5 material after calcination. Moreover, the TPR of 0.36-V2O5–TiO2 catalyst, which was prepared by direct impregnation with colloidal TiO2/V2O5mixed oxides, suggests a higher availability of vanadium species in this sample and a very low reduction temperature for active phases present on its surface. This result seems to indicate that by using colloidal TiO2/V2O5-mixed oxides it is possible to maximize the availability and spreading of V2O5 and therefore the catalytic activity. 4. Conclusions The preparation of catalytic materials based on the polyolmediated synthesis has already been applied to nanoscale metal particles (such as Cu, Ag, Pd and Ni), but the wide applicability and the chance to produce nanosized oxides are a key feature for this method. Moreover, this technique allows the preparation of mixed oxide sols, with a very close contact between different active phases, such as requested in polyfunctional catalyst. Taking into account parameters such as polarity and charge of support surface, a reliable adhesion between the
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Chlorinated organics total oxidation over V2O5/TiO2 catalysts prepared by polyol-mediated synthesis S. Albonetti a,*, G. Baldi b, A. Barzanti b, A.L. Costa d, J. Epoupa Mengou a,c, F. Trifiro` a, A. Vaccari a a
Department of Industrial Chemistry and Materials, Viale Risorgimento 4, 40136 Bologna, Italy b CERICOL, Centro Ricerche Colorobbia, Via Pietramarina 53, 50053 Sovigliana Vinci, Italy c INCA, Interuniversity Consortium, Dorsoduro 2137, 30123 Venezia, Italy d CNR-ISTEC, Via Granarolo, 48018 Faenza, Italy Received 21 July 2006; accepted 7 February 2007 Available online 3 March 2007
Abstract Vanadia and titania/vanadia sols synthesized by the polyol method were used to prepare TiO2-supported catalysts. Both the modification of the V2O5 sol with a low amount of water and the use of a TiO2/V2O5 sol induced a reliable adhesion between the preformed active phases and the support, thus leading to stable catalysts. Despite the low amount of vanadium introduced in studied samples, V2O5/TiO2/WO3 catalysts were active in the o-DCB oxidation. The best results were obtained with 0.36-V2O5–TiO2 catalyst, prepared by direct impregnation with colloidal TiO2/V2O5mixed oxides. The electroacoustic analysis indicated that the support does not show any particular electrostatic affinity for any studied sol; therefore, the higher ability of TiO2/V2O5 sol to form stable vanadium active species may possibly be explained by a better chemical affinity with the support. # 2007 Elsevier B.V. All rights reserved. Keywords: Polyol-mediated synthesis; Catalytic total oxidation; V2O5/TiO2 catalysts; Preformed oxide sols
1. Introduction It is now accepted knowledge that for the preparation of supported catalysts, control at the atomic level is needed in order to design active sites, because the chemical and catalytic properties of atoms at terraces, corners, and edges of a metal crystallite are different [1]. Due to the fact that the preparation of nanoparticles is most controlled in the liquid phase, a promising way for the preparation of nanosized supported catalysts is the deposition of metal oxide sols – in which the particles are preformed – on the support surface [2]. Different preparations can be used to obtain preformed sols: among them is polyol-mediated synthesis, originally used to prepare nanosized metal and alloy particles, which allows both the accurate and reproducible control of the mean diameter of the particles in a broad size range and the mixing of the reactants at
* Corresponding author. Tel.: +39 051 2093681; fax: +39 051 2093680. E-mail address: [email protected] (S. Albonetti). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.02.031
the molecular level [3]. In this procedure, ‘‘polyol’’ stands as a general term for polyalcohols with high boiling temperature and sufficient ability to solve inorganic salts. For the preparation of metal particles the polyol method was applied because of its mild reducing properties, but polyol also has a chelating effect which avoids agglomeration of particles during preparation. The synthesis of mono or polymetal particles of cobalt, nickel, copper and noble metals in the submicrometer and nanometer size range has been reported [4–6], and materials obtained by this method show homogeneous phase composition, narrow particle distribution, and high specific surface area. Recently, this method has received considerable attention because of its simplicity and the advantage, over most other methods, of preparing highly pure mixed oxides [7] and a variety of other materials, including sulfides and phosphates, which are obtained under very similar experimental conditions [8,9]. In the case of oxide synthesis, it can be understood to be a sol–gel process carried out at high temperatures with an accurate control over particle growth [10]. In fact, polyol-mediated preparation of
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nanoscale oxides can be carried out by dissolving a suitable metal precursor (acetate, alcoholate, halogenide) in diethylenglycol (DEG) or other polyalcohols, and then adding a given amount of water. The mixture will then be rapidly heated at 180–240 8C and, at a given temperature, sudden precipitation of the oxide occurs. During this step, the surface of the growing particles will be immediately complexed by DEG as a chelating agent which limits grain growth. In our work, we used vanadia and titania/vanadia sols synthesized by the polyol method for preparing TiO2-supported catalysts, while checking the catalytic activity and stability of prepared samples in the total oxidation of chlorinated organic compounds [11]. 2. Experimental 2.1. Catalyst preparation Nanosized vanadia, titania and titania/vanadia suspensions were prepared via a polyol-mediated synthesis [12]. The preparation was performed by a controlled hydrolysis of vanadyl acetylacetonate and titanium isopropoxide in DEG at 160–180 8C. Fig. 1 shows the general flow chart for the preparation of the materials. The stoichiometric amount of suitable metal precursor was dispersed in DEG. The mixture was heated up to T1 (120–140 8C) with constant stirring, until a clear solution was obtained. An excess of water, as a hydrolysis agent, was then added and the emerging suspension was heated to T2 (160–180 8C) and left under vigorous stirring for 2 h under reflux. For the preparation of the TiO2/V2O5 system the synthesis was achieved in two steps: (i) the necessary amount of vanadyl acetylacetonate was added to the preformed titanium sol in the presence of a small quantity of water; (ii) the suspension was then left under reflux at 180 8C for 2 h. Prepared sols are listed in Table 1.
Table 1 Composition and particle size dimension (D50) of studied sols Sample
TiO2 content (wt.%)
V2O5 content (wt.%)
D50 (nm)
V2O5 sol TiO2 sol TiO2/V2O5 sol
0 6 6
0.9 0 0.5
147 22 28
To obtain the supported catalysts, Millennium Chemicals DT52 commercial powder (TiO2/WO3 = 90:10, w/w) was used as support. The suspensions were deposited onto TiO2/WO3 powder by incipient wetness impregnation, in order to verify the possibility of obtaining a highly dispersed active phase through an economic and well-established method. The obtained powders were dried in an oven for 15 h at 120 8C, and then calcined at 500 8C for 6 h. Prepared catalysts are listed in Table 2. 2.2. Characterization of nanoscaled suspensions The z-potentials of sols were measured by an electroacoustic method based on the measure of the electrokinetic sonic amplitude (ESA) signal, which is generated in organic suspensions when an alternating electric field is applied (AcoustoSizer II, Colloidal Dynamics, Warwick, USA) [13]. This technique is applicable to water and organic suspensions. This instrument determines the z-potential of the particles by fitting the volume average dynamic mobility over a range of 13 different frequencies (ranging from 1 to 18 MHz) in the set alternate electric field. The supporting powder suspension was prepared in water at an electrolyte strength of 10 mM KCl (Merck), with 4 wt.% solid concentration by ultrasonic mixing for 1 h. An automatic titration software was used to measure zpotential as a function of pH between natural pH and basic pH, in order to detect the point of zero z-potential, termed isoelectric point (pHiep). The catalyst DEG sols were measured in their current state. In this case, the attenuation in the glycol solvent was measured, and the result was subtracted from the sample measurement (background correction) in order to bring the particle contribution to the total attenuation. For all non-polar and polar solvent samples (samples not in water), the background correction is necessary because the attenuation of sound for the solvent significantly contributes to the attenuation spectrum of particles. The average particle sizes in suspension were measured by dynamic light scattering using a Malvern Zetasizer Nano S. Table 2 Vanadium oxide content, BET surface area and TiO2 anatase crystallite dimensions for studied catalysts
Fig. 1. Flow chart of the preparation method for materials used.
Sample
V2O5 content (wt.%)
Surface area (m2/g)
Crystallite size of TiO2 anatase (nm)
Support 3-V2O5 0.6-V2O5 0.6-V2O5–H2O 0.36-V2O5–TiO2
0 3 0.6 0.6 0.36
74 28 75 73 74
18 25 19 20 19
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2.3. Characterization of catalysts Surface areas were measured by N2 physisorption using a Carlo Erba Sorpty 1750 instrument after a vacuum pretreatment at 200 8C. The reduction behavior of vanadium oxide species was studied by TPR using a Termoquest TPDRO instrument. The reducing gas consisted of 5 vol.% of H2 in Ar. Before reduction, the catalysts were calcined at 500 8C for 1 h to achieve the V5+ state. X-ray diffraction patterns were obtained in the range of 10– 808 by a diffractometer Philips PW 1710, using Ni-filtered Cu ˚ ). The average crystallite sizes of Ka radiation (l = 1.5432 A the synthesized powders were determined from the full-width at half-maximum (FWHM) of the XRD diffraction peaks using the Debye–Scherrer equation. The Raman spectra were obtained by using the 514 nm line of an Ar+ ion laser, with a Renishaw Raman Spectroscopy System 1000 with Leica DMLM microscope. The laser power at the sample location was 50 mW; the experiments were carried out at ambient conditions. 2.4. Catalytic tests Catalytic experiments were carried out in a fixed-bed glass reactor under atmospheric pressure. Each run used approximately 350 mg of catalyst in the form of 30–60 mesh (250– 595 mm) particles, mixed with 1120 mg of corundum grains of similar size for best temperature control. The total volumetric flow through the catalyst bed was held constant at 140 ml/min (measured at atmospheric pressure and room temperature), 10 vol.% O2, 90 vol.% N2 and 1400 ppm of o-dichlorobenzene. Analysis of reactants and products was carried out as follows: the products in the outlet stream were scrubbed in cold acetone maintained at 25 8C by a mixture of dry ice and glycol. The amounts of reagent and products condensed during a reaction period of 15 min at steady-state conditions were analyzed with a GC (Perkin Elmer Autosystem XL) equipped with a PE-17 capillary column (30 m 0.25 mm, methylpolysiloxane series) and an electron capture detector (ECD) which uses 1,2dichloropropane as a standard reference. Additionally, the exit-flow of the reactor trapped in acetone was injected into a GC with a mass selective detector (Hewlett Packard G1800A) in order to confirm GC data and to verify the formation of nonchlorinated compounds. The CO and CO2 formed were separated on a capillary column Elite Plot Q (30 m 0.32 mm), attached to a methanizer and analyzed by FID. The C balance values always fell between 95% and 105%, calculated as the comparison between converted o-dichlorobenzene and the sum of the product yields.
Fig. 2. Particles size distribution of vanadia (dotted line) and titania/vanadia (solid line) particles in sols.
general, average particle diameter can be guided by adjusting temperature and duration of the reaction, as well as by changing the concentration of metal precursor and water amount. The increase in these parameters generally leads to enlarged particles [14–16]. V2O5 stable sol was more difficult to obtain, and the metal precursor concentration must be kept low in order to control the grain growth. Conversely, the particle size of TiO2 nanoparticles was very low even at high concentration, and the introduction of vanadium did not significantly increase it. If, however, the DEG suspension is mixed with water (DEG/ H2O = 1:10, v/v), the colloid breaks down and rapid agglomeration of the oxide particles occurs (Fig. 3), due to the replacement of the chelating agent on the particle surface with water. The results of the electroacoustic analysis (zpotential, conductivity) on oxide sols are reported in Table 3. In all the cases, nanoparticle surfaces showed a highly positive charge which is characteristic of oxide powders dispersed in organic solvents. The basic site of oxides leads to the deprotonation of DEG and the consequent formation of positive charges. The absolute value of z-potential ensures a good electrostatic stabilization in the DEG sols measured and, together with the monodisperse average particle diameters measured by laser diffraction, proved the surface role of DEG on particle growth and agglomeration. Considering that the electrostatic repulsion (Derjaguin equation) depends on both the product of solvent dielectric constant and the square of the surface potential, it can be
3. Results and discussion 3.1. Characterization of preformed nanoparticles Laser diffraction investigation of oxide particle suspensions indicated the presence of monodisperse particles (Fig. 2). In
Fig. 3. Particle size distribution of vanadium oxide particles in DEG (solid line), 300 after mixing with 10% water (- –) and 900 after mixing with water (- - -).
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Table 3 Results of electroacoustic analysis Sample
Zeta potential (mV)
Conductivity (S/m)
TiO2/WO3 support (in water) TiO2 DEG sol TiO2/V2O5 DEG sol V2O5 DEG sol
0.2 68 81 118
0.377 0.004 0.002 0.011
concluded that the surface potential in DEG (dielectric constant = 32) should be 1.6 times higher than in water (dielectric constant = 78) in order to provide the same repulsion. If 40 mV are requested in water, about 64 mV are required in DEG in order to achieve a good electrostatic repulsion [17]. The higher positive charge of the V2O5 surface in comparison with the TiO2 surface is probably due to the presence of acetylacetone as a reaction product which protonates the basic surface sites of the powder. The presence of acetylacetonate species is confirmed by the high value of conductivity of V2O5 sol in comparison with the other sols. The titania/vanadia sol has a positive charge higher than TiO2 sol as a consequence of the vanadium presence on the TiO2 surface. The great increase in zeta potential, which passes from 68 to 80 mV with the addition of only 5 wt.% of vanadium, indicates the prevailing contribution of vanadia surface to the zeta potential. This result confirms the complete adsorption of V2O5 powder onto the TiO2 surface. Nevertheless, the zeta potential value obtained (80 mV) is not as high as in the case of vanadia sol (117 mV) because of the low concentration of vanadium and, consequently, of the low amount of acetylacetone originated from the synthesis. In order to clarify the interactions between the support surface and the colloidal particles, the surface charges of supporting power (TiO2/WO3) were also examined. The zeta potential was measured in water, in order to simulate the charges developed into the surface when it is in contact with atmospheric humidity (Fig. 4). Nevertheless, the value of zeta potential close to zero at natural pH indicates that this surface is not able to show a particular electrostatic affinity for the impregnated sol. The oxide particles prepared by the polyol method can be investigated more closely after separation from the suspension
Fig. 4. Zeta potential of TiO2/WO3 commercial support as a function of pH.
Fig. 5. X-ray powder diffractograms of TiO2 sol dried and calcined at different temperatures. (a) T = 300 8C; (b) T = 500 8C; (c) T = 700 8C; (d) T = 900 8C.
Fig. 6. Conversion of o-dichlorobenzene as a function of reaction temperature for studied catalysts. TiO2/WO3 support (); 3-V2O5 (*); 0.6-V2O5 (^); 0.6V2O5–H2O (^).
by centrifugation. The resulting powders are poorly crystalline immediately after their synthesis at a maximum temperature of 180 8C. According to XRD data (Fig. 5), in the case of TiO2 and TiO2/V2O5 sols, the only crystallographic phase identified up to
Fig. 7. Conversion of o-dichlorobenzene as a function of reaction temperature for stable catalysts. TiO2/WO3 support (~); 0.6-V2O5–H2O (^); 0.36-V2O5– TiO2 (*).
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700 8C was the anatase form of TiO2 and the crystallite dimension of anatase was calculated to be about 6 nm. Conversely, in the beginning, V2O5 nanoparticles were completely amorphous (data not reported) and only a thermal treatment at 600 8C led to a significant increase in crystallinity unlike what has been reported by Feldmann [14]. 3.2. Catalytic tests Recently, V2O5/TiO2-based catalysts, which are commercially employed for the reduction of NOx via NH3-SCR, have also been found to be active for the destruction of dioxins,
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furans and chlorinated compounds present in gaseous streams [18–22]. In a previous study of ours, we investigated the oxidation of 1,2-dichlorobenzene (o-DCB) over V-supported catalysts [23]. Thus, this experience was the basis for verifying the possible application of vanadia and titania/vanadia sols synthesized by polyol-mediated method for preparing stable and active TiO2-supported catalysts. The results of o-DCB conversion on catalysts prepared utilizing vanadium sol are summarized in Fig. 6, where the behavior of the pure support is also reported. The pure TiO2/ WO3 exhibited some activity in the reaction, mainly due to WO3, but the conversion value significantly enhanced with the
Fig. 8. (a–d) Raman spectra of studied catalysts. (1) Support; (2) 3-V2O5; (3) 3-V2O5 after catalytic test (4) 0.6-V2O5; (5) 0.6-V2O5–H2O; (6) 0.36-V2O5–TiO2; (7) 0.6-V2O5 after catalytic test.
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introduction of 3 wt.% of vanadia (3-V2O5 sample). Despite this high initial activity, the deactivation of the catalyst prepared by multiple impregnations of colloidal vanadia was very fast, thus suggesting poor adhesion of active species on the support surface. To avoid any possible interference due to multiple impregnation/drying steps, a new catalyst was prepared with lower vanadium content by a single impregnation (0.6-V2O5). However, in this case also, the rapid deactivation of the catalyst indicated a weak interaction between the support and vanadium oxide particles. Since the particle-size-distribution measurements indicated that mixing the DEG suspension with water caused a drop in colloidal stability, due to the replacement of the chelating agent on the particle surface, water addition was used to verify possible changes in the colloidal particle adhesion behavior. Supported catalysts prepared with this destabilized sol (0.6V2O5–H2O) showed higher stability in comparison to the analogous 0.6-V2O5 catalyst. High stability was also obtained by direct impregnation with colloidal TiO2/V2O5-mixed oxide (0.36-V2O5–TiO2): despite the low amount of vanadium introduced, this sample showed very interesting catalytic results (Fig. 7). Since the results of electroacoustic analysis indicated that the support does not show any particular electrostatic affinity for any of the impregnated sols, the higher ability of TiO2/V2O5 sol – if compared to V2O5 sol – to form stable vanadia-active species might be explained by a better chemical affinity of TiO2sol with the support. In fact, due to the presence of TiO2 nanoparticles, chemical bonding via hydrogen bridges, as well as direct Ti–O–Ti, can increase adhesion of TiO2/V2O5 colloidal particles on the TiO2/WO3 support surface. 3.3. Catalyst characterization In order to investigate the effect of preparation on the catalyst morphological properties, XRD and surface areas were measured and the results reported in Table 2. The data obtained clearly show that a significant loss of surface area and a growth of support crystallites occurred through multiple additions of V2O5 sol (catalyst 3-V2O5). Conversely, these parameters remain unchanged after single impregnation and following calcination, with all nanosized suspensions, thus indicating that the nanoparticles did not block the pore of the support. XRD analysis of all samples (not reported) detected only the lines due to the anatase polymorphic form of TiO2, thus indicating that the presence of vanadia nanoparticles did not destabilize the TiO2/WO3 support. In fact, in the case of structural degradation, the WO3, which is present in significant amounts, would be segregated. The Raman spectra for the various catalysts studied (Fig. 7a– d) show the bands of anatase at 632, 514, 390 cm1 associated with weak peaks at 965–980 and 770–800 cm1 for all the catalysts. The higher frequency Raman feature is attributable to superimposed W O and V O stretching of monomeric wolframyl and vanadyl species, while the band at about 780 cm1 consists of the overlapping contribution of a secondorder anatase feature, as well as of the W–O–W stretching
modes of structures similar to WO3 [24]. The addition of sols to the support gave rise to the appearance of a new Raman band at about 880 cm1, which could indicated the existence of some organic residues on the calcined powders [25,26] or to be related to the formation of tungsten–vanadia-mixed species. In fact, this band was earlier attributed, by the DFT calculations, to the stretching vibrations of tungstyls which substitute vanadyls in vanadia monolayer [27]. None of the spectra showed the presence of 995 and 694 cm1 bands, which are attributable to microcrystalline or amorphous V2O5 [28], thus proving both the effective spread of vanadium nanoparticles on support surface and the lack of significant agglomeration phenomena. As for general behavior, more stable samples (0.6V2O5–H2O and 0.36-V2O5–TiO2) showed a significant shift in the frequency of 965 cm1 band – assigned to W O and V O stretching mode – moving from this value to 982 and 978 cm1 for 0.6-V2O5–H2O and 0.36-V2O5–TiO2, respectively (Fig. 8b and c). This shift was previously reported for samples with increasing vanadium content [29] and suggests that different two-dimensional vanadium oxide species may be present in stable and non-stable samples. A similar shift was also observed for unstable samples after catalytic test, thus indicating that highly coordinated vanadia species are also formed in reaction conditions (Fig. 8d). Complementary to the Raman spectroscopic experiments, the measurement of the variation in the dispersed vanadium phase among supports can be provided by the TPR behavior. Unfortunately, in our samples, even in the presence of a very low vanadium amount in samples, two main reduction peaks were present, with a maximum at 505–540 8C and about 630–650 8C, respectively. In fact, as already reported [30], in WO3-containing samples, the exact determination of vanadium active species distribution from TPR analysis is rather difficult, due to the partial reduction of tungsta in the same region as vanadia. Nevertheless, our results, reported in Figs. 9–11, can provide some useful information. In particular, with respect to studied samples, the first peak – which is mainly due to highly dispersed vanadium strongly interacting with the support – can be analyzed. Stabilized samples 0.6-V2O5–H2O and 0.6-V2O5 after catalytic tests, showed the splitting of this first peak (Figs. 9 and 10), thus suggesting the formation of different
Fig. 9. H2-TPR of supported catalysts. (a) 0.6-V2O5; (b) 0.6-V2O5–H2O.
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Fig. 10. H2-TPR of supported catalysts. (a) 0.6-V2O5; (b) 0.6-V2O5 after catalytic tests.
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preformed active phases and the support can be achieved, thus leading to stable catalysts. Despite the low amount of vanadium introduced in the studied samples, it was showed that the V2O5/ TiO2/WO3 catalysts obtained by impregnation of TiO2/WO3 with preformed sols were active in the o-DCB oxidation. The best results were obtained with 0.36-V2O5–TiO2 catalyst, prepared by direct impregnation with colloidal TiO2/V2O5mixed oxides. This sol shows an extremely interesting behavior, since the vanadium species seemed highly spread on the titanium nanoparticles, and could be used with very high surface area supports in order to maximize the total oxidation properties of the catalysts. Additional studies are in progress to assess the effect of different active phases and supports on the o-DCB oxidation. References
Fig. 11. H2-TPR of supported catalysts. (a) Support; (b) 0.6-V2O5–H2O; (c) 0.36-V2O5–TiO2.
vanadium oxide species, as already shown by Raman analysis. In particular, these different active phases were reducible at slightly a higher temperature than vanadia present in 0.6-V2O5 material after calcination. Moreover, the TPR of 0.36-V2O5–TiO2 catalyst, which was prepared by direct impregnation with colloidal TiO2/V2O5mixed oxides, suggests a higher availability of vanadium species in this sample and a very low reduction temperature for active phases present on its surface. This result seems to indicate that by using colloidal TiO2/V2O5-mixed oxides it is possible to maximize the availability and spreading of V2O5 and therefore the catalytic activity. 4. Conclusions The preparation of catalytic materials based on the polyolmediated synthesis has already been applied to nanoscale metal particles (such as Cu, Ag, Pd and Ni), but the wide applicability and the chance to produce nanosized oxides are a key feature for this method. Moreover, this technique allows the preparation of mixed oxide sols, with a very close contact between different active phases, such as requested in polyfunctional catalyst. Taking into account parameters such as polarity and charge of support surface, a reliable adhesion between the
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